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18th International Symposium on Solid Oxide Fuel Cells (SOFC-XVIII), Boston, USA, USA, 28 May 2023 - 2 Jun 2023; ECS transactions 111(6), 1657 - 1665 (2023). doi:10.1149/11106.1657ecst, Published by Pennington, NJ

RSC Advances 13(21), 14565 - 14574 (2023). doi:10.1039/D3RA02476H, The identification of fundamental relationships between atomic configuration and electronic structure typically requires experimental empiricism or systematic theoretical studies. Here, we provide an alternative statistical approach to gauge the importance of structure parameters, i.e., bond lengths, bond angles, and dihedral angles, for hyperfine coupling constants in organic radicals. Hyperfine coupling constants describe electron–nuclear interactions defined by the electronic structure and are experimentally measurable, for example, by electron paramagnetic resonance spectroscopy. Importance quantifiers are computed with the machine learning algorithm neighborhood components analysis using molecular dynamics trajectory snapshots. Atomic–electronic structure relationships are visualized in matrices correlating structure parameters with coupling constants of all magnetic nuclei. Qualitatively, the results reproduce common hyperfine coupling models. Tools to use the presented procedure for other radicals/paramagnetic species or other atomic structure-dependent parameters are provided., Published by RSC Publishing, London

The study of phase formation in the U-Te-O systems with mono and divalent cations under high-temperature high-pressure (HT/HP) conditions has resulted in four new inorganic compounds: K2 [(UO2) (Te2O7)], Mg [(UO2) (TeO3)2], Sr [(UO2) (TeO3)2] and Sr [(UO2) (TeO5)]. Tellurium occurs as TeIV, TeV, and TeVI in these phases which demonstrate the high chemical flexibility of the system. Uranium VI) adopts a variety of coordinations, namely, UO6 in K2 [(UO2) (Te2O7), UO7 in Mg [(UO2) (TeO3)2] and Sr [(UO2) (TeO3)2], and UO8 in Sr [(UO2) (TeO5)]. The structure of K2 [(UO2) (Te2O7)] is featured with one dimensional (1D) [Te2O7]4- chains along the c-axis. The Te2O7 chains are further linked by UO6 polyhedra, forming the 3D [(UO2) (Te2O7)]2- anionic frameworks. In Mg [(UO2) (TeO3)2], TeO4 disphenoids share common corners with each other resulting in infinite 1D chains of [(TeO3)2]4- propagating along the a-axis. These chains link the uranyl bipyramids by edge sharing along two edges of the disphenoids, resulting in the 2D layered structure of [(UO2) (Te2O6)]2-. The structure of Sr [(UO2) (TeO3)2] is based on 1D chains of [(UO2) (TeO3)2]∞2− propagating into the c-axis. These chains are formed by edge-sharing uranyl bipyramids which are additionally fused together by two TeO4 disphenoids, which also share two edges. The 3D framework structure of Sr [(UO2) (TeO5)] is composed of 1D [TeO5]4− chains sharing edges with UO7 bipyramids. Three tunnels based on 6-Membered rings (MRs) are propagating along [001] [010] and [100] directions. The HT/HP synthetic conditions for the preparation of single crystalline samples and their structural aspects are discussed in this work.

Eighteenth International Symposium on Solid Oxide Fuel Cells (SOFC-XVIII) SOFC XVIII, Boston, USA; Pennington, NJ : IOP Publishing Bristol 111, (2023). doi:10.1149/11106.0937ecst, Published by IOP Publishing Bristol, Pennington, NJ

Catalysts 13(5), 903 - (2023). doi:10.3390/catal13050903 special issue: "CO2 Catalytic Conversion and Utilization", SnO2 nanoparticles have frequently been reported as effective electrocatalysts for CO2 electroreduction to formate. However, in the literature, there is little knowledge of SnO2 nanoparticles that guarantee superior electrocatalytic performance. Hence, in this study, several SnO2 nanoparticles are compared with respect to their material properties, and correlations to the electrocatalytic performance are established. For comparison, three custom-made SnO2-electrocatalysts were prepared, reproducing frequently cited procedures in literature. Based on the comparison, it is found that hydrothermal, sol-gel, and solid-state synthesis provide quite different electrocatalysts, particularly in terms of the particle size and crystal lattice defect structure. Desirably small nanoparticles with a comparatively high number of lattice defects are found for the nanoparticles prepared by hydrothermal synthesis, which also provide the best electrocatalytic performance in terms of Faradaic efficiency for the electroreduction of CO2 to formate. However, despite the considerably smaller surface area, the commercial reference also provides significant electrocatalytic performance, e.g., in terms of the overall produced amount of formate, which suggests a surprisingly high surface area-specific activity for this material that is low on defects. Thus, defects do not appear to be the preferred reaction site for the CO2 electroreduction to formate on SnO2 in this case., Published by MDPI, Basel

ACS sustainable chemistry & engineering 11(2), 679-688 (2023). doi:10.1021/acssuschemeng.2c05539, Published by American Chemical Society Publ., Washington, DC

Electrochimica acta 414, 140231 (2022). doi:10.1016/j.electacta.2022.140231 special issue: "Electrochemistry of Electroactive Materials-2020 / Edited by Mikhail Vorotyntsev, Sotiris Sotiropoulos", Published by Elsevier, New York, NY [u.a.]

Functional materials letters : FML (2022). doi:10.1142/S1793604722400021, Published by World Scientific, Singapore [u.a.]

In situ transmission electron microscopy (TEM) research has enabled better understanding of various battery chemistries (Li-ion, Li–S, metal–O2, Li, and Na metal based, etc.), which fueled substantial developments in battery technologies. In this review, we highlight some of the recent developments shedding new light on battery materials and electrochemistry via TEM. Studying battery electrode processes depending on the type of electrolytes used and the nature of electrode–electrolyte interfaces established upon battery cycling conditions is key to further adoption of battery technologies. To this end, in situ/ operando TEM methodologies would require accommodating alongside correlation microscopy tools to predict battery interface evolution, reactivity, and stability, for which the use of x-ray computed tomography and image process via machine learning providing complementary information is highlighted. Such combined approaches have potential to translate TEM-based battery results into more direct macroscopic relevance for the optimization of real-world batteries.

Chemical communications : ChemComm 10(19), 3130-3133 (2022). doi:10.1039/D1CC07172F, Published by Royal Society of Chemistry, Cambridge

Functional materials letters 15(05), 2240001 (2022). doi:10.1142/S179360472240001X, Published by World Scientific, Singapore [u.a.]

Electrochimica acta 430, 141077 (2022). doi:10.1016/j.electacta.2022.141077, Published by Elsevier, New York, NY [u.a.]

Lithium titanium phosphate LiTi2(PO4)3 is an electrode material for lithium-ion batteries with a specific capacity of 138 mA h g−1. Owing to its potential of 2.5 V vs. Li/Li+ it provides an electrochemically stable interface when used as an anode in all-solid state batteries with NASICON type lithium aluminium titanium phosphate electrolyte. High performance has been identified for in situ carbon coated LiTi2(PO4)3 synthesized via a hydrothermal route, resulting in micro-scaled spindle shaped particles consisting of nano-scaled sub-particles. To elucidate the internal microstructure of these spindle-like particles in three dimensions we applied tomographic Focused Ion Beam – Scanning Electron Microscopy. For more detailed chemical analysis we performed electron-energy loss spectroscopy and energy dispersive X-ray spectroscopy in the scanning electron microscope as well as high resolution (scanning) transmission electron microscopy for structural insight. It could be clearly shown that the spindle-like particles mainly are made up of LiTi2(PO4)3 sub-particles in the 100 to 400 nm range. Additionally, two types of secondary phase materials were identified. LiTiOPO4, which shows different surface morphology, as a volume component of the spindles and TiO2 nanoparticles (anatase), which are not only present at the particle surface but also inside the spindle, were detected. Reconstruction from tomography reveals the nanoparticles form a three-dimensionally interconnected network even though their phase fraction is low.

Journal of molecular liquids 322, 114934 (2021). doi:10.1016/j.molliq.2020.114934, Published by Elsevier, New York, NY [u.a.]

Hybrid electrolytes are developed to meet the requirements of safety, performance, and manufacturing for electrolytes suitable for Li-ion batteries with Li-anodes. Recent challenges—in addition to these key properties—emphasize the importance of sustainability. While compromising between these three objectives, the currently available materials are still well below the targeted goals. Three important issues for the design of hybrid electrolytes are (i) the role of the morphology and surface state of the ceramic particles in the polymer matrix, (ii) the dependence of salt concentration and ionic conductivity and, (iii) the effects of substituting part of the polyethylene oxide (PEO), with biopolymers. Electrolyte films were prepared from PEO, lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), Li6.5La3Zr1.5Ta0.5O12 (LLZO:Ta), and biopolymers with varying contents of these components by a solution casting method. The films were analyzed with respect to structural and microstructural characteristics by DSC, Raman spectroscopy, and SEM. Ionic conductivity was evaluated by electrochemical impedance spectroscopy. Most interesting, when comparing films with LLZO:Ta versus without, the content of LiTFSI required for the maximum conductivity in the respective systems is different: a higher LiTFSI concentration is required for the former type. Overall, addition of LLZO:Ta as well as partial substitution of PEO by chitosan mesylate or cellulose acetate decrease the ionic conductivity. Thus—at least in the present approaches—a loss in performance is the drawback from attempts to enhance the safety by LLZO:Ta additions and sustainability by biopolymer blending of hybrid electrolytes.

ACS applied energy materials 3(10), 9509-9515 (2020). doi:10.1021/acsaem.0c01803, Published by ACS Publications, Washington, DC

ACS applied energy materials 3(6), 5101-5106 (2020). doi:10.1021/acsaem.0c00543, Published by ACS Publications, Washington, DC

Journal of applied electrochemistry 50, 93-109 (2019). doi:10.1007/s10800-019-01372-5, Published by Springer Science + Business Media B.V, Dordrecht [u.a.]

International audience; Poly-acrylonitrile (PAN)-derived carbon fibres were characterised as air electrode frameworks for aqueous-alkaline metal–air batteries, focussing on the influence of the carbonisation temperature on the structure and electrochemical properties. Elemental composition, (atomic) structure, electrical conductivity, and electrochemical performance related to the oxygen reduction were investigated for electrodes carbonised in the range from 300 °C to 1400 °C. Chemical and structural properties were analysed using elemental analysis, XPS, SEM, and Raman spectroscopy; electrical conductivities of the fibre networks were examined by four-point probe measurements. Electrochemical properties were evaluated using linear sweep voltammetry in 6 M KOH by the open circuit potentials, the cathodic current densities at given overpotentials, and required overpotentials at given current densities. The highest current density was obtained from fibres carbonised at 850 °C. The connection between the fibre characteristics and electrochemical properties are discussed, highlighting the importance of the nitrogen bonding state. The results provide a base for thedevelopment of high performance air electrodes.

Frontiers in Chemistry 7, 800 (2019). doi:10.3389/fchem.2019.00800, Published by Frontiers Media, Lausanne

RSC Advances 9(11), 6267 - 6277 (2019). doi:10.1039/C8RA10491C, Published by RSC Publishing, London

Journal of the Electrochemical Society 164(12), A2310-A2320 (2017). doi:10.1149/2.0301712jes, Published by Electrochemical Society, Pennington, NJ

RSC Advances 7(41), 25276-25284 (2017). doi:10.1039/C7RA01622K, Published by RSC Publishing, London

Perovskite-type LaVO2.78N0.10 powder was synthesized by thermal ammonolysis of the oxide precursor LaVO4. By X-ray, neutron, and electron diffraction an orthorhombic crystal structure with space group Pnma was identified. XANES spectra showed that the oxidation state of vanadium changes from 5+ in LaVO4 to approximately 3+ in LaVO2.78N0.10. The temperature dependence of the electrical conductivity revealed an Arrhenius-type behavior with an activation energy of 0.103 eV in the temperature range of 119–302 K indicating that the conduction process is thermally activated band transition. Based on the positive Seebeck coefficient, holes were identified as the dominant charge carriers in the temperature range of 100–302 K. Both the Seebeck coefficient and the thermal conductivity showed an anomaly at 138 K, which is attributed to the Néel temperature for antiferromagnetic ordering according to magnetic susceptibility measurements.

Dissertation, RWTH Aachen University, 2023; Aachen : RWTH Aachen University 1 Online-Ressource : Illustrationen, Diagramme (2023). = Dissertation, RWTH Aachen University, 2023, In the composite cathode of solid-state Li-metal batteries (SSLMBs), the high interfacial resistance and unstable interphase between the cathode active material (CAM) and solid-state electrolyte (SSE) are two of the main reasons for the low energy density in current SSLMBs. Matching the physical/(electro)chemical properties of the CAM and SSE is vital to obtaining a stable interface/interphase in the composite cathode. Li1.3Al0.3Ti1.7(PO4)3 (LATP) is a promising candidate as a Li+ conductive component in all-phosphate-based composite cathode produced by a co-firing method due to its good thermal chemical stability against phosphate-based CAMs. Herein, with the effort to optimize the synthesizing and sintering process of LATP, highly-conductive LATP is obtained at a low sintering temperature. Consequently, the phosphate-based CAM/LATP interface in composite cathode is stabilized, which significantly improves the energy density of SSLMBs. Specifically, a high-density, fully phosphate-based composite cathode is prepared by co-firing LiFePO4 (LFP) and LATP. In this way, an ion-conductive and redox-active Li3-xFe2-x-yTixAly(PO4)3 (LFTAP) interphase is formed at the interface between LFP and LATP after heating, which not only improves the adhesion of materials but also provides additional capacity. The structure of the fabricated composite cathode is studied in detail. The electrochemical performance and the influence of the electrochemically active LFTAP interphase on the corresponding SSLMBs composed of the co-fired LFP/LATP composite cathodes, PEO-based solid polymer electrolytes and Li metal anodes are investigated.The active interphase in co-fired LFP/LATP composite cathode enables the SSLMBs to achieve high areal capacities (2.0 ~ 3.0 mAh cm-2). Whereas, the adoption of PEO-based SSE restricts the batteries to be operated under a low current density at a relative high temperature (eg. 36 µA cm-2, 60 °C). To enhance the Li+ kinetic in SSE at room temperature (RT), single Li+ conducting polymer-in-ceramic free-standing hybrid electrolyte membranes are prepared for solid-state Li metal batteries. The hybrid electrolyte membrane is composed of 3D interconnected LATP ceramic and a conductive polymer matrix of cross-linked poly[bis(2-(2-methoxyethoxy)ethoxy)-phosphazene] (MEEP) and lithium(4-styrenesulfonyl)-(trifluoromethanesulfonyl)imide (LiSTFSI). Attributed to the synergistic effects of LATP nanofibers and the single-ion conducting polymer, the hybrid electrolyte exhibits improved electrochemical performance, leading to enhanced rate capability and stable cycling performance of the LFP|hybrid electrolyte|Li battery., Published by RWTH Aachen University, Aachen

Dissertation, RWTH Aachen University, 2022; Aachen : RWTH Aachen University 1 Online-Ressource : Illustrationen (2022). = Dissertation, RWTH Aachen University, 2022, Perovskite materials of the type ABO3 are generally known for their flexibility regarding changes in their defect chemistry with the help of A- or B-site doping, by which it is possible to influence their structural and catalytic properties. Changes in the defect chemistry involve both modifications in the oxygen nonstoichiometry δ and in the B-site oxidation state within the material. They are strongly dependent on the applied synthesis methods and atmospheric conditions, as well as on the operating temperatures. For the use of such demand-optimized materials, e. g. as air electrodes in solid oxide cells, significant improvements in efficiency and performance can be achieved in the field of energy storage and conversion systems. In the scope of this thesis, the lanthanum-based compositions La1−xFeO3−δ (x = 0 − 0.1) and La1−xCaxMnO3+δ (x = 0 − 0.5) were synthesized using the conventional solid state synthesis method in air and subsequently characterized. The impact of lanthanum-substoichiometry and calcium content on the respective B-site oxidation state (Fe/Mn), as well as on the oxygen nonstoichiometry was investigated. With the help of powder X-ray diffraction solely orthorhombic structures for La1−xFeO3−δ were determined. However, the increasing introduction of lanthanum-substoichiometry lead to an additional formation of the secondary phase Fe2O3 next to stoichiometric LaFeO3. This could be verified using various microscopic investigation methods. Thermogravimetric investigations on the La1−xFeO3−δ series in pure oxygen atmosphere did not reveal significant mass changes in dependence on the lanthanum content or temperature, which suggested that no significant oxygen uptake or release occured. It has been suspected that instead of lanthanum-substoichiometry rather the basic perovskite LaFeO3 and Fe2O3 are formed. LaMnO3 on the other hand showed a great structural diversity from a rhombohedral phase up to two differently distorted orthorhombic phases. With increasing calcium content, the less distorted orthorhombic modification was stabilized. Investigations of the oxygen nonstoichiometry of the La1−xCaxMnO3+δ series revealed a strong dependence on the calcium content, temperature and atmosphere. Unlike in LaFeO3, significant and reversible mass changes could be observed, which were correlated with oxygen uptake and release processes. The δ-values of the initial La1−xCaxMnO3+δ compositions were obtained from thermogravimetric measurements in reducing atmosphere. The determination of the manganese oxidation state in the La1−xCaxMnO3+δ series was performed indirectly by using the obtained thermogravimetric data and literature models and directly by X-ray absorption near edge spectroscopy experiments. In this way, the influence of intrinsic and extrinsic effects on the manganese oxidation state of the differently calcium-doped samples was revealed. Subsequent current–voltage and electrochemical impedance spectroscopy measurements in dependence on the calcium content and temperature enabled the determination of the samples’ electrochemical behavior and catalytic activity. Thus, up to temperatures of 850 °C, high calcium contents in the samples showed the highest measured currents under the influence of voltage, as well as the smallest polarisation resistances. The highest current, in combination with a small polarisation resistance, was obtained at 900 °C at medium calcium contents. It could be distinguished between a surface process and a process taking place at the triple phase boundary, and their activation energies could be determined. This allowed for drawing careful correlations between the defect chemistry and electrochemical or electrocatalytical properties observed within the sample series., Published by RWTH Aachen University, Aachen

Dissertation, RWTH Aachen University, 2022; Aachen : RWTH Aachen University 1 Online-Ressource : Illustrationen, Diagramme (2022). = Dissertation, RWTH Aachen University, 2022, Li-ion batteries suffer from degradation processes arising particularly during long-term usage or operation outside of recommended cycling conditions. Similarly, degradation hinders the safe implementation of ‘beyond lithium’ batteries, which promise significant performance enhancements. To mechanistically understand degradation and test counter-measures, a variety of analytical techniques have been applied. However, certain degradation phenomena such as the morphology evolution of metallic lithium deposits and the behavior of dissolved transition metal ions in solution are hard to address by established analytics, and are therefore still largely unexplored. In this thesis, pulse Electron Paramagnetic Resonance (pEPR) is demonstrated to contribute a complementary perspective on battery degradation, addressing metallic lithium formation and evolution as well as transition metal complexation and precipitation. In the first part, pEPR of conduction electrons in metallic lithium that has formed at the battery anode is discussed, where the skin effect and electron mobility are key characteristics. The measurement of relaxation times and electron spin nutations enables semi-quantitative approximation of the lithium morphology, which is irregular and microstructured when deposited electrochemically. The dynamics of lithium deposits are studied during operation with a maximum time resolution of 100 ms. At lithium metal anodes, pEPR reveals the continuation of morphology changes for several seconds, even after the current flow terminated, which is attributed to surface diffusion leading to a smoothened morphology. At graphite anodes operated at C-rates of up to 18C, detrimental lithium plating is monitored and pEPR provides details on the plating onset, the time-dependent plating rate, the partial intercalation following the charging process, and the relative amounts of dead lithium. In the second part, electron-nuclear spin interactions in transition metal complexes are exploited to derive solvation preferences, using pEPR and Density Functional Theory (DFT). Experiments were performed at cryogenic temperatures to enable spin manipulation and detection. First, dissolved Mn2+ ions from cathode dissolution and Cu2+ ions from current collector dissolution are modelled by dissolving salts in premixed electrolyte solutions. Mn2+ and Cu2+ ions are both found to be mainly coordinated by cyclic carbonates. However, if stored at 35 °C for 24 h, their behavior diverges. Mn2+ is selectively coordinated by fluorophosphate ligands that formed via heat-induced decomposition of the electrolyte salt LiPF6 with trace H2O. In contrast, relaxation and hyperfine data of Cu2+ species indicate partial precipitation and unaltered complexation of Cu2+ in solution. To investigate dissolved transition metal ions in operating batteries, V2O5 cathodes are used, which exhibit severe dissolution already during the first discharge, liberating vanadyl ions (VO2+). Among others, glycol dianion ligands formed through cyclic carbonate decomposition are found to selectively coordinate to VO2+. Additionally, chemisorption at conductive carbon surfaces can be postulated based on measurements with deuterated electrolyte solvents. If stored at 45 °C for a week, fluorophosphate and fluorophosphite ligands with phosphorus oxidation states +V and +III are determined. The obtained complexes exhibit remarkably large distributions of hyperfine coupling constants over around 40 MHz that allow the analysis of spin delocalization pathways., Published by RWTH Aachen University, Aachen

Dissertation, RWTH Aachen University, 2022; Aachen : RWTH Aachen University 1 Online-Ressource : Illustrationen, Diagramme (2022). = Dissertation, RWTH Aachen University, 2022, A solid oxide fuel cell (SOFC) converts the chemical energy of a fuel gas directly into electrical energy. The SOFC is thus a promising technology for power generation with high efficiency. One of the possible SOFC system configurations that offers the highest potential for DC electrical efficiency of up to 65% is a SOFC system with anode exhaust gas recirculation (AEGR). For its safe and durable operation, the oxygen-to-carbon ratio and fuel utilization as characteristic parameters must not exceed stack- and reformer-specific limits. The determination and control of these characteristic parameters is therefore of crucial importance. However, this is a challenging task, especially for SOFC systems with AEGR, due to increased system complexity. This thesis deals with studies on sensor concepts for the online determination of characteristic parameters in SOFC systems. Six different basic concepts were developed for this purpose. These include the use of automotive exhaust gas sensors and the application of the fuel cell diagnostic methods electrochemical impedance spectroscopy, total harmonic distortion analysis as well as the wavelet transformation on the stack voltage and current. Other concepts include the use of the characteristic U-i curve of a fuel cell and the application of a data-driven soft sensor. In the evaluation of the concepts, the oxygen or lambda sensors as representative of the automotive exhaust gas sensors and the data-driven soft sensor showed the highest potential and were consequently investigated in more detail as part of this thesis. The oxygen sensors are used directly in the fuel gas path at the anode inlet and outlet of the SOFC stack. Analytical correlations of the measured value of oxygen sensors to characteristic parameters were derived. Based on representative natural gas compositions, simulation studies were used to demonstrate the basic suitability of oxygen sensors as gas sensors in SOFC systems. Subsequently, experimental studies of oxygen sensors in representative gas compositions for the anode inlet and outlet of a SOFC system with AEGR were done. For this purpose, an appropriate test rig was constructed to emulate the operating conditions at the anode inlet and outlet of a SOFC stack. Various oxygen sensors from Robert Bosch GmbH were tested. These were switch-type oxygen sensors of the type LSF 4.2 and LSF Xfour as well as wide-range oxygen sensors of the type LSU 4.9 and LSU ADV. For operation of the LSU 4.9, a novel type of amplifier circuit was also added to enable operation in the rich gas mixtures at the anode inlet. It is shown that switch-type oxygen sensors are generally only suited for use at the anode outlet and therefore qualifies for the determination of the system-specific fuel utilization. The LSF Xfour turned out to be the more advantageous variant in this respect due to its integrated temperature control. The wide-range oxygen sensors can be used both at the anode inlet and outlet. This allows the stack-specific fuel utilization and the recirculation ratio to be determined in addition to the system-specific fuel utilization. While the LSU 4.9 has a higher resolution but requires the additional amplifier circuit, the LSU ADV does not require the amplifier circuit but owns a lower resolution. It was also found that for a sufficient determination of the characteristic parameters with high accuracy by the wide-range oxygen sensors, the hydrogen-to-carbon ratio(H/C) of the supplied natural gas must generally be known. The soft sensor is a two-step approach in which hybrid models were developed for components of a SOFC system being in steady-state. The physics-based part of the hybrid model is an ideal energy balance of a component which is adapted to the real behavior by a data-driven Gaussian process. In the first step, hybrid models were set up for the hotbox and the tailgasburner of the SOFC system, which were set up in sole dependence of the H/C-ratio in natural gas and solved in this respect. The inert gas components in natural gas (CO2, N2) were also estimated using separate Gaussian processes. In the subsequent second step, hybrid models were set up and solved for the reformer and the stack as a function of the recirculation ratio. It was shown on the basis of measured data from a real SOFC system that both the tailgasburner and the hotbox are suited for determining the H/C-ratio. In contrast, for the second step, only the reformer model was shown to be suited for determining the recirculation ratio on basis of the available data. In summary, this thesis shows that a combination of a soft sensor and wide-range oxygen sensors is most effective for determining and controlling the characteristic parameters of a SOFC system. In steady-state operation of a SOFC system, the soft sensor can be applied to determine the H/C-ratio of the supplied natural gas in step one and thus the system-specific fuel utilization. In its second step the soft sensor shows a good potential to determine the recirculation ratio and thus the stack-specific fuel utilization. Based on the steady-state H/C-ratio from step one of the soft sensor, wide-range oxygen sensors can be optimally deployed directly in the fuel gas path at the anode inlet and outlet of a SOFC stack. It enables the dynamic determination of characteristic parameters and its usage in system control., Published by RWTH Aachen University, Aachen

Dissertation, RWTH Aachen University, 2021; Aachen : RWTH Aachen University 1 Online-Ressource : Illustrationen (2021). = Dissertation, RWTH Aachen University, 2021, High-temperature co-electrolysis is the simultaneous electrochemical conversion of H2O and CO2 in the temperature range of about 700 ��� 900 ��C. In this thesis solid oxide cells with ceramic electrolyte are used. The cells consist of a cathode made from a cermet (ceramic metal) of nickel and yttria-stabilized zirconia (YSZ), a YSZ electrolyte and an anode made of lanthanum strontium cobalt ferrite (LSCF) or lanthanum strontium cobaltite (LSC) as well as a barrier layer made of gadolinium-doped ceria (GDC) between electrolyte and anode. In the thesis, the performance and underlying processes were analyzed by direct and alternating current measurements in CO2 and co-electrolysis as well as the boundary in between. The aim was the identification of the processes especially with regard of the role of electrochemical conversion of CO2 in order to optimize the co-electrolysis process in future applications. It was shown that the performance increases with increasing H2O partial pressure between 0 and 30 %. In this range H2O and CO2 are electrolyzed simultaneously. Above 30% the performance stayed more or less constant and equals that of pure steam electrolysis. Pure H2O electrolysis is dominating and CO2 is converted in the reverse water gas shift reaction (RWGS). The impedance spectroscopy measurements showed, that the largest losses were caused by diffusion processes in the cathode. These losses increase with decreasing H2O:CO2 ratio, that is with predominating CO2 electrolysis. This could be explained by the significantly larger diffusion coefficient of CO2 compared to H2O. The unique selling point of high-temperature co-electrolysis is the ability to tailor the syngas ratio in one single process step. The tailoring was investigated by varying temperature, gas composition and flow rate. The temperature only showed a small influence on the H2:CO ratio, which is caused by the water-gas shift equilibrium. The ratio of H2:CO in the output gas is especially dependent on the H2O:CO2 ratio of the input gas. The relation is about linear, causing the input H2O:CO2 ratio to react into H2:CO of about the same value. The decrease of flow rate results in an increased gas utilization, however, it is also responsible for a decreased current density at the same voltage. Thus, the space-time conversion is reduced. Although the percental conversion is increased (gas utilization), the current density determines the absolute conversion. High-temperature co-electrolysis is a very versatile method to utilize and store energy from renewable sources, while at the same time CO2 is recycled and valorized., Published by RWTH Aachen University, Aachen

Dissertation, RWTH Aachen University, 2021; Aachen : RWTH Aachen University 1 Online-Ressource : Illustrationen (2021). = Dissertation, RWTH Aachen University, 2021, Materials with NASICON structure are a promising class of Solid-State ionic conductors for application in electrochemical storage. The crystal structure enables high stability and flexibility and a wide range of compositions. An analysis of literature data of lithium ion conducting NASICON materials revealed the highest ionic conductivities for compositions of the formula Li1+xMx(III)Ti2-x(PO4)3 with 0.3 ��� x ��� 0.5 and hexagonal R��3c structure. In addition, the great influence of microstructure and phase chemistry on conductivity was shown. Li1+xMx(III)Ti2-x(PO4)3(M(III)= Al, Cr, Fe; 0.5 ��� x ��� 2.0) was prepared using two current synthesis routes and the samples were characterized in terms of density, stoichiometry and phase chemistry. The focus of the work is on investigations of the conductivity of the polycrystalline material using electrochemical impedance spectroscopy. The scalable synthesis routes allow comparable material quality to previously known methods, which are categorically limited to laboratory scale or not cost effective on a large scale. Additionally, the question of possibly increased oxygen mobility in NASICON was investigated. For this purpose, 18O oxygen exchange experiments followed by depth profiling by secondary ion mass spectrometry were performed on Li1.5Al0.5Ti1.5(PO4)3 and Na3.4Zr2(PO4)0.6(SiO4)2.4. This revealed new, material-specific challenges with regard to the chosen method. The results allow a first classification and show no unusually high oxygen mobility in the dedicated alkali ionic conductors., Published by RWTH Aachen University, Aachen

Dissertation, RWTH Aachen University, 2021; Aachen : RWTH Aachen University 1 Online-Ressource : Illustrationen (2021). = Dissertation, RWTH Aachen University, 2021, Electrospun Polyacrylonitrile-based carbon fibers are a powerful and tunable material for CO2 capture applications. In this work, unmodified carbon nanofibers carbonized at various temperatures are presented. Surface chemistry and morphology of these fibers are characterized using scanning electron microscopy, elemental analysis and X-Ray photoelectron spectroscopy as well as argon and CO2 sorption measurements. It is shown that the surface chemistry as well as the formation of ultramicropores depend significantly on the applied carbonization temperature, enabling a tailoring of the material for specific gas separation applications. To evaluate some of these applications, additional adsorptives are measured and give evidence that a molecular sieve effect is the major driver of the gas adsorption properties. In these measurements, an unusually high affinity to CO2, water and ammonia is found in comparison to other carbon materials, which is attributed to the extremely narrow pore width. The tailorable ultramicropore structure inducing the molecular sieve effect allows to evaluate concepts for the determination of molecular dimensions that play an important role in the discussion of molecular sieves, for example the kinetic diameter. Furthermore, the molecular sieve effect in the carbon nanofibers could be tailored for any gas separation application, given that their difference in molecular size is sufficient. An excellent adsorption selectivity is predicted from static sorption isotherms by the ideal adsorbed solution theory for the separation of CO2 and N2 as well as CO2 and CH4. The separation performance of CO2 and N2 is analyzed in greater detail using dynamic sorption methods that confirm the suitability of the carbon nanofibers predicted by static results. Furthermore, the adsorption kinetics are analyzed to evaluate possible limitations of the CO2 sorption rate in narrow ultramicropores. To increase the adsorption capacity of the carbon nanofibers, various concepts to introduce additional porosity are evaluated. While the introduction of mesopores with polymer-blend spinning does not enhance the CO2 sorption capacity, physical activation with CO2 and chemical activation with KOH significantly improve the high-pressures sorption performance, however at the expense of sorption selectivity and low-pressure performance. Overall, PAN-based carbon nanofibers turn out to be a well-performing material for CO2 adsorption and separation applications. This thesis provides a versatile toolbox to tailor electrospun carbon nanofibers to specific gas separation applications and process conditions., Published by RWTH Aachen University, Aachen

Dissertation, RWTH Aachen University, 2021; Aachen : RWTH Aachen University 1 Online-Ressource : Illustrationen (2021). = Dissertation, RWTH Aachen University, 2021, In the scope of this PhD thesis, mechanisms taking place on primary metal-air battery cell anodes, which employ pure Al and Al-Si alloys as their negative electrodes, were investigated to gain insights about the correlation between the microstructure and the anode performance. The primary cells were operated with a room temperature ionic liquid as their electrolytes, which is composed of 1-ethyl-3methylimidazolium cation and µ-fluoro-bis(fluorohydrogenate), µ3-fluoro-tris(fluorohydrogenate) anions. The pure Al samples were used as the negative electrode of the batteries as received. Vacuum arc melting is employed as the processing method of Al-Si alloys from pure Al and Si raw materials, also as for the microstructure modification for the pure Al. Processed samples of the alloys were subjected to microstructure characterization for calculating the quantities of the alloy micro-constituents. The grain size of the fine-grained pure aluminium was increased by processing with arc melting process from 10-75 µm to 200-1500 µm, which increased the polarization resistance of aluminium. It was found that the fine-grained aluminium is more prone to corrosion, but also can deliver higher gravimetric capacities. Moreover, the discharge potential is found higher than the coarse-grained aluminium under same discharge conditions. As a result of discharge experiments with various current densities, it was observed that the pure Al anode batteries can deliver capacities up to 2200 math/g which corresponds to the 73% utilization of the theoretical capacity of Al, when discharged with highest current density of 1.5 mA/cm2 A reduction of the discharge current density resulted in low capacity discharges. The open circuit and discharge potentials of the cells were reduced with the increasing Si content of the anodes. The reduction of the open circuit potentials of the cells with increasing Si content was also confirmed by the linear polarization measurement in half-cells vs. gel-based reference electrodes. The electron microscopy study of the Al-Si anode surface morphologies after discharge experiments revealed that, for all alloy anodes, the discharge results mostly from the oxidation of the aluminium rich phase. The corrosion potentials, corrosion currents and the Tafel slopes of the electrodes were obtained by using the Tafel fitting on the linear polarization curve for each electrode composition as well as for the Si reference materials. In addition, mixed potential theory was employed under different scenarios based on the microstructural properties to investigate the galvanic coupling of the alloy components. The results were discussed in terms of Evans diagrams; thereby approaches based on alternative scenarios for the galvanic coupling were examined. It was shown that by employing an extended approach in mixed potential calculations in terms of accepting the physical properties of eutectic-Si and primary-Si different, the potentials and corrosion currents can be estimated for a metal-semiconductor galvanic couple from the electrochemical properties of the parent components of the alloy. Corrosion rates of the pure Al and the Al-Si alloys were investigated via gravimetric measurements in combination with calculations over corrosion currents, which were obtained from linear polarization measurements. The corrosion rates of the anodes were first increased as the aluminium alloyed with silicon up to 50 wt% with Si, then decreased with further alloying. The electrolyte compositions of the batteries operated with Al-Si alloy anodes were analysed with inductively coupled plasma optical emission spectroscopy method after corrosion and discharge experiments. The data indicated that the total mass lost by the anode of the batteries after can be found as dissolved species in the electrolytes, which may lead to the saturation of the electrolyte and consequent termination of cell discharge. In addition, for the cells operated with the anodes containing 75 wt.% and more Si in their compositions, the amount of Si detected in the electrolyte was higher than that of Al. The electron microscopy observations of these anode surfaces after discharge revealed pitting type dissolution patterns on the primary-Si surfaces which may be the proof of the active contribution of primary-Si to the discharge., Published by RWTH Aachen University, Aachen

Dissertation, RWTH Aachen University, 2021; Aachen : RWTH Aachen University 1 Online-Ressource : Illustrationen, Diagramme (2021). = Dissertation, RWTH Aachen University, 2021, Hybrid electrolytes are electrolytes for lithium ion batteries (LIB) with lithium anode meeting the requirements of safety, performance and processing. Hybrid electrolytes are defined as composites that combine two or more ionically conducting materials. In the scope of this thesis an organic polymer electrolyte poly(ethylene oxide) with lithium bis(trifluoromethanesulfonyl)imide (PEO with LiTFSI) and an inorganic solid-state electrolyte Li7-xLa3Zr2-xTaxO12 (Ta-LLZO) were used. Cubic Ta-LLZO possesses a higher ionic conductivity than the tetragonal phase of Ta-LLZO and was synthesized via a classical all-solid-state and a sol-gel route. For the application of Ta-LLZO in hybrid electrolytes, the powder was remilled after calcination reducing the average particle size. The composition of the hybrid electrolyte was optimized regarding the ionic conductivity. Influencing parameters such as Ta-LLZO particle properties, LiTFSI salt concentration, Ta-LLZO weight fraction and substitution with biopolymers were investigated. In comparison to a pure polymer electrolyte PEO12LiTFSI a higher ionic conductivity was measured for a hybrid electrolyte with 20 wt.-% Ta-LLZO in a PEO6LiTFSI polymer matrix. The substitution of PEO with biopolymers, specifically chitosan mesylate or cellulose acetate, was an approach for more sustainable materials in LIB, but resulted in a loss of performance. The applicability of hybrid electrolytes in Li-LiFePO4 (Li-LFP) cells was confirmed. LFP cathode sheets were adapted to the requirements of the hybrid electrolytes by using PEO6LiTFSI as binder and lithium ion conductor. Compared to the polymer electrolyte, an increase in the electrochemical stability of the hybrid electrolyte was obtained. The substitution of PEO by cellulose acetate further increased the electrochemical stability. The interfacial layer at the organic-inorganic interface was characterized on microscopic level using Infrared Spectroscopy and Electrochemical Strain Microscopy (ESM). Interactions of polymer and ceramic lead to an increase of mobile charge carriers. Moreover, ESM measurements visualized the possible formation of an interfacial layer in the polymer phase near the interface with the ceramic. This supports the assumption that interfacial percolation substantially influences the ionic conductivity of hybrid electrolytes on macroscopic level., Published by RWTH Aachen University, Aachen

Dissertation, RWTH Aachen University, 2020; Aachen 1 Online-Ressource (X, 167 Seiten) : Illustrationen, Diagramme (2020). doi:10.18154/RWTH-2020-12361 = Dissertation, RWTH Aachen University, 2020, In this dissertation the potential of catalytic hydrogen release from different chemical hydrogen storage systems for a sustainable hydrogen cycle was investigated. For this purpose, borohydride and formic acid were selected as favorable hydrogen storage media. For the dehydrogenation of these hydrogen storage media, different catalysts were tailor-made, and their activity and selectivity were evaluated in both batch and continuous operation. In the first part of the work borohydride was coupled with various ionic liquids. 1-propyl-3-methylimidazolium BH4- and 1-ethyl-3-methylimidazolium BH4- were identified as particularly promising due to low melting points and high hydrogen capacities. Subsequently, the hydrogen release was optimized for a micro fuel cell by catalyzed hydrolysis with metal catalysts and acid initiators, first in batch operation and later in a semi-continuous process. Hydrogen was completely released by the continuous addition of 1 M HCl to 1-propyl-3-methylimidazolium BH4- and the hydrogen release rate could be adjusted by the acid addition rate. The relationship between hydrolysis and acid addition was followed by 11B-NMR spectroscopy. Based on these results, a mechanism for the hydrolysis of 1-propyl-3-methylimidazolium BH4- with acids was proposed. In the second part formic acid was used as a liquid hydrogen storage material and the catalyzed dehydrogenation was investigated. For this purpose, supported catalysts known from literature were compared with immobilized catalysts in terms of activity and selectivity in batch operation. Immobilized Ru and Ir species on tailor-made polymers and covalent triazine networks were identified as preferential catalysts and used in continuous operation in liquid and gas phase in various catalyst test plants. In gas phase dehydrogenation of formic acid, the polymer-based Ru catalyst and Ir catalyst stabilized by a triazine network were most active and the Ir catalyst showed stable activity over 72 h. In proof of concept studies, the potential of 1-propyl-3-methylimidazolium BH4- and formic acid vapor by coupling the catalytic hydrogen release with a micro fuel cell was evaluated. Based on the demonstration of electricity production from the catalytically released hydrogen from 1-propyl-3-methylimidazolium BH4- and formic acid steam, the feasibility of the two chemical hydrogen storage systems was confirmed., Published by Aachen

Dissertation, RWTH Aachen University, 2020; Aachen 1 Online-Ressource(xiii, 106, XLI Seiten) : Illustrationen, Diagramme (2020). = Dissertation, RWTH Aachen University, 2020, In this thesis, the application of electrospun polyacrylonitrile-derived carbon-fibre mats as electrodes for aqueous-alkaline metal--air batteries is investigated. Three main aims are pursued: First, a deeper understanding of the morphological properties of the fibres and their transformation during carbonisation. Secondly, establishing a method of activity evaluation that allows for an analysis in near-application scenarios. Lastly, relating structural information and activity.After outlining current challenges in the field of global energy storage, the history and working principles of the aqueous-alkaline metal--air battery systems is presented. The main reactions on the air-electrode, oxygen reduction and evolution are presented and discussed in detail. Current approaches to catalyse these kinetically challenging reactions are focussed on, considering mainly nitrogen-doped carbons and co-doped carbons using transition metal catalysts. Polyacrylonitrile is a feasible base material, because of its inherent nitrogen content and behaviour during carbonisation. It was further enhanced by adding cobalt and nickel, promising oxygen electro-catalysts. A variety of methods was employed to investigate the materials, including Raman spectroscopy, XPS, XRD, and linear sweep voltammetry. The structure and composition of polyacrylonitrile-derived carbon-fibres are discussed in wide range of carbonisation temperatures. The nitrogen content was found to be more than halved between 600 °C and 1000 °C. Also, the nitrogen bonding-types evolve from mainly pyridinic nitrogen to graphitic nitrogen. This was found to influence the electrochemical performance of the material, especially the limiting current and overpotentials of the oxygen reduction reaction. The overall activity maximum was found for samples carbonised at 800 °C.The structure of the carbon material is modified in presence of cobalt and nickel. In both cases, turbostratic carbon is formed and the nitrogen removal is shifted towards lower temperatures. The nitrogen bonding is also affected, with the metals inducing a higher content of pyridinic nitrogen. The changes affect activity in terms of oxygen reduction current densities and overpotentials. While nickel was found to influence the morphology more, cobalt made a stronger impact on electrochemical activity. Neither metal, significantly influenced the oxygen evolution activity. While the presented materials are shown to function as an air-electrode and the introduced key parameters of performance description allow for a sufficient correlation of structural properties and electrochemical activity, more research is required to enhance the performance of the electrodes to a competitive level., Published by Aachen

Dissertation, RWTH Aachen University, 2019; Aachen 1 Online-Ressource (VI, 113 Seiten) : Illustrationen (2019). = Dissertation, RWTH Aachen University, 2019, Due to the ever-growing demand for higher energy density of lithium ion batteries, intensive research is underway to identify and develop the new cathode materials with high intercalation voltage over 4.5 V (vs. Li/Li+). LiCoPO4 is a promising candidate for cathode materials of high-voltage lithium ion batteries to obtain high energy, due to its high theoretical capacity (167 mAh g-1), high operating voltage (4.7 V vs. Li/Li+) and the thermal stability thanks to P-O covalent bonding. LiCoPO4 materials have not been widely used in practical applications, since there are several problems hindering its utilization, including low electronic conductivity, poor Li+ diffusion and limited stability of electrolytes at high voltage. To solve these critical problems and improve the electrochemical performance of LiCoPO4 cathode in high-voltage lithium ion batteries, this thesis uses solvothermal methods to synthesize various LiCoPO4-based materials, mainly focuses on three parts: carbon coating strategies, Fe substitution and particle morphologies control. The details are described as followed: Carbon free, ex-situ carbon coated and in-situ carbon coated olivine polymorph LiCoPO4 materials were prepared by solvothermal and a subsequent annealing process. With the addition of citric acid in the solvothermal reaction, a carbon layer was coated via an in-situ approach. To systematically compare the different carbon coating routes, the structure and morphology of the LiCoPO4 materials were investigated by XRD, Raman, and SEM. HAADF-STEM combined with EDX was applied to analyze the homogeneity of the carbon layer and corresponding antisite defects. Electrochemical properties were analyzed by half-cells measuring cyclic-voltammograms, charge/discharge cycling behavior stability and rate-capability. It was found that the in-situ carbon coated LiCoPO4/C exhibited a superior electrochemical performance due to the relatively uniform and complete surface-layer formation. As a result, an appropriate carbon layer improves the electronic and ionic transport properties, ensures fast electron-transfer kinetics at the electrode particle surfaces and suppresses unwanted side reactions with the electrolyte. Carbon coated olivine Pnma LiCoPO4 (LCP/C) and Fe-substituted LiCo0.8Fe0.2PO4 (LCFP/C) were synthesized by a solvothermal method and their structural features and electrochemical properties were investigated. The electrochemical performance of LCFP/C was better than that of LCP/C, owing to the partial substitution of Co by Fe which efficiently suppresses the increment of antisite exchange between Li+ and Co2+ ions within the structure during cycling, although the Li-Co antisite exchange amount in pristine LCP/C and LCFP/C was similar. Furthermore, direct visualization of Co in Li sites in the pristine samples and after 50 cycles was achieved through IIhigh-resolution scanning transmission electron microscopy for both LCP/C and LCFP/C. It was found that LCP/C locally formed a new cation-ordered structure after cycling due to the Li-Co antisite exchange, while the structure of LCFP/C remains almost the same. This study provides direct evidence that Fe substitution reduced the Li-Co exchange and improved the electrochemical cycling life of the LiCoPO4 cathode for high-voltage lithium ion batteries. Various LiCoPO4 materials have been synthesized by solvothermal methods and a subsequent annealing process. By adding citric acid, PVP or CTAB, different morphologies of LiCoPO4 materials were achieved, including unstructured nanoparticle, nanosheet, nanorod and microrod shape. Electrochemical analysis showed that the controllable morphology has an influence in electronic and ionic pathways, thus affects the electrochemical performance. The nanosheet shape LiCoPO4 shows the largest discharge capacity and the best rate capability, while the nanorod shape LiCoPO4 displays the relatively better cycling stability. Furthermore, the apparent Li+ diffusion coefficients of LiCoPO4 samples were determined to investigate the influence of particle size and shape on the Li+ migration., Published by Aachen

Dissertation, RWTH Aachen University, 2019; Aachen 1 Online-Ressource (III, 104 Seiten) : Illustrationen, Diagramme (2019). = Dissertation, RWTH Aachen University, 2019, LiMn$_{2}$O$_{4}$ (LMO) is a candidate for the cathode material in the next generation of lithium ion batteries. To specifically tailor and optimize the material, knowledge of the defect chemistry, microstructure, diffusion and the effect of doping is required. In this study oxygen and lithium diffusion of dense, undoped and Al-doped LMO-pellets is investigated using isotope tracer diffusion experiments. The analysis is carried out by secondary ion mass spectrometry (SIMS). The pellet samples are prepared by spray drying and subsequent field assisted sintering technique (FAST). To investigate the oxygen diffusion in undoped and Al-doped LMO pellets oxygen exchange experiments in 18O2-enriched atmosphere are performed and analyzed by means of SIMS. Due to an occurring phase transition at the surface during the experiments in the temperature range of 400 °C to 700 °C and a pO2 of 200 mbar no results are obtained without the knowledge of the transition kinetics. The lithium diffusion in the undoped and Al-doped LMO pellets is investigated using an amorphous 6Li enriched thin-film, deposited on the pellets by pulsed laser deposition (PLD). Diffusion anneals in air in a temperature range of 20 °C to 210 °C were carried out to introduce tracer diffusion profiles between thin-film and pellet. The resulting diffusion profiles show several features assigned to film, bulk and grain boundary diffusion. To obtain tracer diffusion coefficients from the experimental profiles two two-dimensional numeric models are used and compared. The first consisting of infinite parallel grain boundaries and grains and the second being the brick-layer model. The modelling yields lithium tracer diffusion coefficients for the amorphous film, the bulk material, and the grain boundaries. The results show a fast grain boundary diffusion being orders of magnitude faster than bulk diffusion. Results obtained from the numerical models are compared to Harrison’s type A and B regimes of classic diffusion kinetics in polycrystalline samples. Activation energies of the corresponding diffusion processes are obtained by temperature dependent experiments. In this study, bulk and grain boundary diffusion in LMO are d for the first time using one experiment only., Published by Aachen

Dissertation, RWTH Aachen University, 2019; Aachen 1 Online-Ressource (iv, 125 Seiten) : Illustrationen (2019). = Dissertation, RWTH Aachen University, 2019, Lithium-ion battery (LIB) is one of the potential solutions in near future for stationary storage of renewable energy and power supply for electric vehicles. LIB technology has led to a massive advance of its performance over the last few decades. Its development is recently accelerated by automobile industry in order to adapt to the requirements of the electric cars. Large quantities of research have focused on cathode materials because they work as the reservoir of lithium and limit the energy density of LIB. Spinel-type lithium manganese oxides, especially Li1+xMn2-xO4 with 0 < x < 1/3 have been commercialized as cathode materials. The stoichiometry factor x, with respect to the atomic ratio between Mn4+ and Mn3+, has an impact on the electrochemical performance of this material. Furthermore, trace amounts of Li2MnO3 usually occur as a secondary phase in synthesized Li1+xMn2-xO4 in the common high temperature of solid-state synthesis, affecting the overall Li-Mn stoichiometry in the spinel phase and thereby the electrochemical performance. This PhD work focuses on investigating the relationship between secondary-phase Li2MnO3, stoichiometry and electrochemistry of Li1+xMn2-xO4.The formation of secondary-phase Li2MnO3 in trace amounts during the synthesis of lithium manganese spinel has a considerable influence on the stoichiometry of the spinel host. The detection and quantitative analysis of trace amounts of Li2MnO3 in spinel host is challenging. In Chapter 3, an efficient methodology is developed to analyze quantitatively trace amounts of Li2MnO3 formed in lithium manganese spinel hosts by exploiting the different line shapes between Li2MnO3 and lithium manganese spinel in Electron Paramagnetic Resonance (EPR) spectrum. The superior sensitivity of EPR enables a detected Li2MnO3 mass-fraction less than 10-2 wt.%. A successful separation of the EPR signals belonging to Li2MnO3 and lithium manganese spinel is achieved. According to the quantified Li2MnO3 mass-fraction, the changes in the stoichiometry of lithium manganese spinel materials are determined. The result indicates that even Li2MnO3 exists in trace amount, its impact on the stoichiometry factor x in Li1+xMn2-xO4 should be considered. By resolving the EPR spectra of Li1+xMn2-xO4 from this methodology, a narrowed EPR linewidth along the increase of stoichiometry factor x is identified due to the enhanced exchange interaction inside the spinel lattice. In Chapter 4, study is focused on the reaction between Li1+xMn2-xO4 and secondary phase Li2MnO3 along with heat treatments at temperatures between 773 K and 873 K. At moderately high temperature, Secondary-phase Li2MnO3 can react with the spinel phase during heat treatment. This reaction appears to be clear that two educts namely LMO spinel and Li2MnO3 produce a new spinel phase with higher Li-content. But the mechanism and the kinetics of this reaction are challenging to be investigated. The physical and chemical processes during the reaction have not yet been clarified. A strategy is established to monitor experimentally the change in Li2MnO3 amount in the synthesized LMO spinel materials by implementing Electron Paramagnetic Resonance (EPR) spectroscopy. The observation of the reaction process is succeeded by recording the subtle decrease in Li2MnO3 at different temperatures along varied reaction times. It is shown that the kinetic behavior could be assigned to either pseudofirst-order or second-order depending on the microstructure of Li2MnO3 as well as the Li-content of the educt spinel phase. The product spinel phase which is transformed originally from Li2MnO3 phase has been visualized by SEM, presenting a distinct morphology. At last, the reaction mechanism has been described in detail including lithium diffusion, oxygen exchange and rearrangement of atom positions. It addresses an interpretation of how monoclinic Li2MnO3 does integrate into lithium manganese spinel hosts. According to the studies in previous chapters, the exact stoichiometries of synthesized LMO spinels have been determined, where the stoichiometry factor x varies from 0.000 to 0.182. Those synthesized LMO spinel materials are casted into electrodes and assembled into half cells. The electrochemical data from Chapter 5 provides a comparison of varied electrochemical performances along with different stoichiometries of the samples. The obtained experimental capacities of synthesized Li1+xMn2-xO4 with x > 0 coincide quantitatively with their theoretical capacities after correcting the stoichiometry factors x according to Li2MnO3 mass-fractions from EPR analysis. Moreover, ex-situ EPR present a systematic change of linewidths with different states of charge. The linewidths decrease during the solid-solution reaction region and increase during the two-phase reaction region., Published by Aachen

Dissertation, RWTH Aachen University, 2019; Aachen 1 Online-Ressource (VI, 124 Seiten) : Illustrationen, Diagramme (2019). = Dissertation, RWTH Aachen University, 2019, Lithium aluminium titanium phosphate Li$_{1+x}$Al$_{x}$Ti$_{2-x}$(PO$_{4}$)$_{3}$ is one of the materials under consideration as a solid electrolyte in future all solid-state lithium-ion batteries. In this work, I study the evolution of the microstructure of LATP ceramic samples, sintered between 900 and 1100 $^{o}$C from Li$_{1.3}$Al$_{0.3}$Ti$_{1.7}$(PO$_{4}$)$_{3}$ powders prepared via a sol-gel route, by scanning electron microscopy (SEM) and confocal laser scanning microscopy (CLSM). LATP ceramics are highly brittle and they degrade under water; therefore, an all-oil-based grinding and polishing route was developed that does not alter the microstructure during sample preparation that allows differentiation of different microstructural components. Sintering temperature dependent evolution of grain size, morphology, and connectedness as well as secondary phase content are tracked by CLSM and SEM. In addition to that, transmission electron microscopy (TEM) was used for the determination of grain boundary width and to identify the amorphous structure of the secondary phase. Goal of the work is to correlate the microstructure with total, grain and grain boundary resistance as extracted from electrochemical impedance spectroscopy via means of distribution of relaxation times (DRT). The latter showed a grain conductivity almost three order of magnitude larger than that of the grain boundaries. An increase of total conductivity with grain size is observed, which correlates with the grain size. However, at 1100 $^{o}$C, total resistance increases with excess amount of secondary phase and crack formation comes into account. In ceramic processing and for final macroscopic conductivity, the presence of secondary phases and porosity play an important role. In presence of more than one secondary phase and pores, image analysis must tackle the difficulties about distinguishing between these microstructural features. Therefore, I come up with a novel image segmentation and reconstruction method. In this work, we also study the phase evolution of LATP ceramic samples by the image segmentation method based on elemental maps acquired in the scanning electron microscope combined with quantitative analysis of LATP grains. We find aluminium phosphate AlPO$_{4}$ and another phosphate phase containing an only little amount of aluminium. It may contain lithium, which is not detectable by energy dispersive X-ray spectroscopy (EDS). The amount of these phases changes with sintering temperature which may influence the ionic conductivity of the whole material. First: As the grains act as an aluminium source for AlPO$_{4}$ formation, the aluminium content decreases decreasing also the intragranular conductivity. Second: Also, the amount of secondary phase changes from more (Li$_{x}$)P$_{y}$)O$_{z}$ at 950°C to mainly AlPO$_{4}$ at 1100°C sintering temperature, which in addition may influence the grain boundary conductivity., Published by Aachen

Dissertation, RWTH Aachen University, 2019; Aachen 1 Online-Ressource (XIX, 138 Seiten) : Illustrationen, Diagramme (2019). = Dissertation, RWTH Aachen University, 2019, In the present work, various mononuclear manganese and iron complexes were investigated in the context of water oxidation catalysis. The aim of the work was to understand how a change of the electronic properties of the ligand influences the fundamental electrochemical water oxidation behavior. In addition, the formation of the respective catalytically active species and the mechanism of water oxidation were studied in detail. At the beginning, mononuclear manganese-bipyridine complexes were used. However, they did not show catalytic water oxidation neither when using chemical oxidants nor electrochemically. In the second part of the work the dpaq ligand system was used, which has stronger σ-donor properties due to an anionic carboxamide group. Starting from the known [FeIII(dpaqH)(OH2)](ClO4)2-WOC, the influence of different substituents in the 5-position of the ligand on water oxidation was investigated. It was shown that electron-withdrawing substituents increase the catalytic current, but only with a simultaneous increase of the overpotential, while electron-donating substituents reduce the catalytic activity. An increased catalytic current with a concomitant decreased overpotential was finally achieved by the introduction of a pyrene group into the ligand framework which led to an increased π-conjugation. In addition to the Fe complexes, the analogous [MnII(dpaqR)](ClO4) complexes were investigated and also showed a catalytic current. The influence of the different substituents in the ligand framework on the overpotential followed the same trend. In contrast to the Fe complexes, however, no reversible reduction waves were visible, so that a fundamental structural change of the starting complexes must be assumed under the measurement conditions. In the third part of the work, the tetradentate dpqma ligand was used to synthesize a new mononuclear [MnII(dpqma)Br2] complex, which showed a catalytic current in CV measurements in a borate buffer. However, when using Cerium (IV) ammonium nitrate (CAN) as the oxidant, no evolution of oxygen could be detected. Using EPR spectroscopy, it was shown that the dpqma ligand presumably dissociates after the addition of CAN, followed by the formation of a μ-oxo bridged MnIIIMnIV dimer. After a longer reaction time, the EPR measurements indicated the formation of MnOx compounds. In addition, the formation of MnO4- ions was detected by UV/Vis spectroscopy. This showed that the [MnII(dpqma)Br2] complex is not a suitable water oxidation catalyst when using CAN as the oxidant due to the poor stability under the acidic oxidative reaction conditions., Published by Aachen

The investigation of fundamental mechanisms and especially aging processes in lithium ion and lithium-oxygen batteries is the overarching topic of this PhD thesis. For a comprehensive understanding of the battery aging processes it is necessary to connect macroscopic observations ("capacity fading") with the underlying microscopic changes. The main goal of this thesis is the development of novel spectro-electrochemical techniques for the time resolved observation of processes occurring during battery charge and discharge. Das grundlegende Thema dieser Doktorarbeit ist die Untersuchung der Reaktionsmechanismen und insbesondere der Alterungsprozesse in Lithium-Ionen und Lithium-Sauerstoff Batterien. Für ein tiefgreifendes Verständnis der Batteriealterung ist es notwendig, die makroskopischen Beobachtungen ("abnehmende Kapazität") mit den zugrunde liegenden mikroskopischen Veränderungen zu verknüpfen. Das Ziel dieser Arbeit ist die Entwicklung neuartiger spektroelektrochemischer Methoden, um die ablaufenden Prozesse zeitaufgelöst direkt während des Zyklierens zu untersuchen.

Dissertation, RWTH Aachen University, 2018; Aachen 1 Online-Ressource (xiii, 138 Seiten) : Illustrationen, Diagramme (2018). = Dissertation, RWTH Aachen University, 2018, The understanding and development of electrochemical systems, i.e. batteries, electrolysis and fuel cells, is a key link for electromobility. Since most systems substantially suffer from several problems, the magnetic properties of battery and fuel cell materials were targeted by electron paramagnetic resonance (EPR) spectroscopy. In this work, the in operando mode of investigation of electrochemical systems with EPR spectroscopy was developed for high temperature polymer electrolyte fuel cells and lithium-ion batteries. Furthermore, influences of direct currents to in operando EPR measurements were analyzed and EPR imaging was developed for metallic lithium. Battery and fuel cell setups compatible with the EPR microwave were developed since commercial cells do not fulfill requirements. A first approach was done with a 3D printed fuel cell showing the capabilities and possibilities of rapid prototyping techniques and moreover, clarifying the requirements for electrochemical systems in EPR spectroscopy. In a next step, a reusable battery cell setup with high reproducibility, electrochemical functionality and minimum interactions with the EPR microwave was developed from quartz glass. With this cell setup, spinel as well as layered oxide cathode materials for lithium-ion batteries were analyzed by in operando EPR spectroscopy, i.e. during battery operation. A LiNi0.5Mn1.5O4 (LNMO) spinel cathode showed the impact of Mn3+ on the Li+ motion inside the spinel. Moreover, state of charge dependent linewidth variations confirm the formation of a solid solution for slow cycling, which is taken over by mixed models of solid solution and two-phase formation for fast cycling due to kinetic restrictions and overpotentials. Long-term measurements for 480 h showed the stability of the investigated LNMO, but also small amounts of cathode degradation products became visible. The results point out how local, exchange mediated magnetic interactions in cathode materials are linked with battery performance and can be used for material characterization. Iron substituted spinel structure LiFe0.4Mn1.6O4 (LFMO) was analyzed showing a manganese oxidation in the low voltage regime while the high voltage process was identified as Fe3+ / Fe4+ redox couple. The long-term change of magnetic properties exhibits an electrochemical inactive iron species after 523 h of battery cycling giving hints towards the local magnetic structure, i.e. a change from an exchange coupled system to a dipolar broadened system. After electrolyte soaking, lithium cobalt oxide (LCO) battery cathode material showed a considerable amount of paramagnetic Co2+ that was oxidized during the formation cycle. The absence of an EPR signal during subsequent battery cycles, irrespective of the state-of-charge, shows that no EPR active Co4+ is formed. This suggests the presence of an oxygen redox mechanism in the active material while the cobalt oxidation state remains constant at Co3+. LiNi0.18Co0.1Mn0.59O2 (NCM) cathode material showed a nickel oxidation in combination with oxygen oxidation. On a long run, battery degradation showed an altered EPR linewidth and intensity depending on the state-of-charge that give information about the capacity and voltage fading. Both are effects of an increasing amount of inactive Ni2+ and a slightly growing amount of nickel that cannot be oxidized to Ni2+ anymore. As in operando measurements come along with currents inside the EPR resonator, the EPR signal for metallic lithium has been investigated with direct currents applied to the sample. Currents up to 900 mA change the signal intensity and signal shape. Experimental as well as theoretical considerations show that the observations are an effect of interactions between DC induced magnetic fields, the static magnetic field B_0 and the interaction field B_1. Especially in the case of microwave shielding due to the skin effect in conductors, DC currents enable to mirror the whole sample properties by EPR spectroscopy. Finally, conduction EPR imaging (CEPRI) was developed for metallic lithium. It was found to be a sensitive characterization method to image the microstructure of lithium deposits in lithium-ion battery components. The versatility of the method is demonstrated for both, imaging surface-patterns of thick lithium metal anodes, as well as obtaining high-resolution images of lithium dendrites formed inside a separator with several micrometre pixel size. The determined spatial distribution of dendrites may then serve as an indicator of the current density distribution inside battery cells., Published by Aachen

Dissertation, RWTH Aachen University, 2018; Aachen 1 Online-Ressource (VIII, 107 Seiten) : Illustrationen, Diagramme (2018). = Dissertation, RWTH Aachen University, 2018, Interest on metal–air batteries has been emerging due to increased demand on resource efficient battery technologies and development of materials. By possessing high theoretical specific energies as well as using low cost, safe and abundant electrode materials, metal–air batteries are promising energy storage devices which could fulfill the current demands. A recent concept of metal–air batteries, Si–air, possesses a good potential with 8470 Wh/kg theoretical specific energy to play a major role (at least) in the primary battery market due to utilizing the second most abundant element in the Earth’s crust as an anode material. The aim of this thesis is to investigate and obtain new insights into understanding of the mechanisms taking place within the Si–air batteries which, eventually, could lead to further developments. In this regard, the first part of this work considers the non-aqueous Si–air batteries independently from the rest which focus on the aqueous alkaline electrolyte. The non-aqueous Si–air batteries employing fluorohydrogenated room temperature ionic liquid (EMIm(HF)2.3F) are investigated in order to understand the influence of Si anodes specifications on the battery performance. The study is focused on three different types of dopants namely, As, Sb, and B, as well as with and crystal orientations in each case. Discharge experiments are performed at various current densities and corrosion rates are obtained by mass loss calculations in combination with potentiodynamic polarization experiments. The results confirmed that there are at least two different corrosion mechanisms existing and potentiodynamic polarization experiments are not sufficient alone to quantify them. Furthermore, the most suitable Si anode type is discussed by considering the mass conversion efficiencies and specific energies while taking the consumed anode mass into account. For aqueous alkaline Si–air batteries, at first, the corrosion behavior of highly As-doped oriented silicon wafers are investigated with respect to (i) time dependence, (ii) influence of KOH concentration, and (iii) chemical vs. electrochemical corrosion. Corrosion rates are found to exhibit stable time profiles for immersion times longer than 8 h. With respect to concentration dependence, three ranges of KOH concentrations were identified; within each range, the corrosion behavior is governed by similar mechanisms, but different limiting factors. Potentiodynamic experiments showed that large part of the corrosion is chemical in nature. As a first step for an understanding of the corrosion during discharge, the impact of the OH- concentrations on the anodic currents are investigated by means of cyclic voltammograms in half-cells. The current state-of-the-art alkaline Si–air battery is limited in discharge time to few minutes when a Si anode with a plane surface is employed. Thereby, in the second part of the investigations for aqueous alkaline Si–air batteries, the discharge behavior of Si–air cells with KOH electrolyte is reconsidered and a new cell setup is designed to put forward the discharge process until the complete anode is exhausted. Discharging Si–air cells enhanced the corrosion rates depending on the current densities and KOH concentrations. Along with the corrosion and discharge of Si in KOH, condensation of silicate structures in the electrolyte has been observed. Both effects accelerate electrolyte loss in the cell; therefore, appropriately balancing the electrolyte supply of the Si–air battery is a precondition for ongoing discharge. Specifically, cells with As-doped Si anodes with 0.6 mm and 3.0 mm thickness were discharged in 5 M KOH electrolyte at current densities up to 0.05 mA/cm2 for 260 and 1100 hours, respectively. Although a considerable fraction of the anode material is not transformed to electrical energy owing to corrosion, specific energies up to 140 Wh/kg (for 1100 h) related to the total anode mass loss are realized., Published by Aachen

Dissertation, RWTH Aachen University, 2018; Aachen, 1 Online-Ressource (65, xxix Seiten) : Illustrationen, Diagramme (2018). = Dissertation, RWTH Aachen University, 2018, Electrochemical impedance spectroscopy (EIS) has been successfully applied for a long time among various research and development fields, however, it is still subject to further improvements. Mainly these regard the development of sophisticated data analysis algorithms, which can extract previously hidden information. In this thesis, a two-dimensional regularized impedance data analysis algorithm was developed and applied to lithium-ion batteries with liquid and solid electrolytes to gain insights into their ion dynamics. Mathematical modeling of the physico-chemical processes occurring in electrochemical systems by equivalent circuit models works well if the system is known and the processes’ time constants are well separated, i.e., the processes take place on different time scales. However, it suffers from ambiguity if detailed a priori knowledge of the system is not available or the impedance contributions of two or more processes are strongly overlapping. In this case, it is hardly possible to obtain physically meaningful results. One approach to mitigate this problem is to analyze the processes, which caused the impedance, by their distribution of relaxation times (DRT). In this thesis, an evolution of DRT analysis using generalized Tikhonov regularization with a uniform penalty and an RC kernel was developed. By conducting the transformation without discriminating or constraining the sign of the distribution and by using both the imaginary and the real part of the RC kernel it could be distinguished between impedance contributions of RQ-like processes, pure ohmic resistances and inductances in the system under investigation. Furthermore, regularization could be simultaneously applied in a second, non-inverted dimension (2D-DRT), which provided an increased resolution of the spectrum. The 2D-DRT was used to enhance the impedance data analysis to gain more insights into the battery processes occurring on time scales from hours to microseconds. In liquid electrolyte lithium-ion batteries, the kinetics is mostly determined by the low-frequency solid-state diffusion, as well as the mid-frequency charge transfer and solid-electrolyte interphase layer (SEI) processes. For the quantification of the diffusion process, the use of complex and time consuming low-frequency impedance spectroscopy techniques, such as time domain measurements (TDM), was previously required. In contrast, the developed 2D-DRT analysis could infer the low-frequency kinetics already from millihertz impedance data, which are easily accessible by standard EIS. In the mid-frequency range, the charge transfer and SEI processes at the electrode-electrolyte interface take place on a similar time scale. While this makes them hardly distinguishable in an impedance spectrum, the higher resolution of the 2D-DRT analysis enabled separation and quantification of both. Furthermore, by combination of EIS with nuclear magnetic resonance (NMR) spectroscopy, the processes could be attributed to the anode and cathode, respectively. In solid-state batteries, in addition to the charge transfer kinetics, the ionic conductivity of the solid electrolyte is crucial for the performance and is so far a limiting factor. For improvements, a better understanding of the high-frequency grain and grain boundary ion transport properties are necessary. However, similar time constants again prevent an easy distinction of both so that measurements well below 0°C are necessary for their separation. In contrast, in this thesis a Li1.3Al0.3Ti1.7(PO4)3 (LATP) solid electrolyte was analyzed under battery operation temperatures between 10°C and 50°C by EIS in combination with a 2D-DRT analysis. The electrode-electrolyte interface charge transfer kinetics were studied at a monolithic solid-state battery. For this, the LATP was combined with a phosphate-based electrode pair of Li3V2(PO4)3 (LVP) and LiTi2(PO4)3 (LTP). Since the charge transfer kinetics at both electrodes showed similar time constants, the quantification was done at symmetrical cells consisting of LATP with LVP or LTP electrodes on both sides, respectively. The results were then used to simulate and characterize the impedance behavior of the solid-state battery., Published by Aachen

Dissertation, RWTH Aachen University, 2018; Aachen 1 Online-Ressource (VI, 166, xvii) : Illustrationen (2018). = Dissertation, RWTH Aachen University, 2018, Fe-air batteries provide a most promising, resource-efficient battery concept with superior power densities compared to state-of-the-art Li-O2- and potentially superior specific energies compared to Li-ion batteries. However, although alkaline Fe-air batteries have been under repeated consideration over the past five decades, they still require considerable research and development. Among other things, the limited rechargeability of the iron electrode imposes a considerable bottleneck for the broad implementation of commercial cells. Contributing to the development of rechargeable Fe-air batteries, in this dissertation the electrochemistry of model-like and applicable, i.e., plane and porous iron electrodes in concentrated alkaline electrolyte is investigated. At this, under-standing charge-transfer reactions at the anode-electrolyte interface as well as fundamental electrode processes for porous iron-anodes is the key to develop high-performance cells. Over the course of this thesis it is established that the performance of iron electrodes in concentrated alkaline electrolyte hinges on the existence and the continuance of the solute intermediate reaction products, which, eventually, produce a thin but essential redox-layer on the iron electrode surface. With respect to this, it is shown by the application of chelating agents for iron ions in aqueous media that the electrochemistry of iron becomes irreversible as soon as the intermediate reaction products are removed from the equilibrium between electrode and electrolyte. Furthermore, by a dedicated in-situ electrochemical atomic force microscopy (EC-AFM) study it is shown that the evolution of the precipitating redox-layer proceeds discontinuously on preferential precipitation sites during the oxidation and reduction of the iron electrode. The latter particularly explains the extraordinarily increasing discharge capacity of pristine iron electrodes upon repeated galvanostatic cycling, which is investigated for both plane and porous iron anodes. Given the repeated dissolution and precipitation of iron in concentrated alkaline electrolyte, the surface area of iron electrodes gradually increases in every cycle and may repeatedly be discharged, if the previous recharge step was complete. Due to that, comparatively simple, surface reaction-confined iron electrodes may store tremendous amounts of energy already (here: up to 225 mAh·gFe-1)) and may reversibly provide decent specific power of up to 280 mW·gFe-1., Published by Aachen

Dissertation, RWTH Aachen University, 2018; Aachen 1 Online-Ressource (xi, 121 Seiten) : Illustrationen, Diagramme (2019). = Dissertation, RWTH Aachen University, 2018, Due to the growing interest in renewable energies and the reduction of the use of fossil fuels, the interest in reliable energy storage is growing. One contemplated storage system besides fuel cells and others are batteries. Conventional lithium-ion batteries use toxic, flammable electrolytes containing lithium salts and organic solvents that pose safety hazards. One approach to solve these problems are solid electrolytes. This thesis is concerned with the production of these electrolytes and characterization by means of X-ray diffractometry, thermogravimetry, dilatometry and scanning electron microscopy. The material classes of garnet-type or garnet-related compounds offer a high electrochemical stability in addition to a high lithium-ion conductivity among the potential solid electrolyte materials. Based on the classical solid state synthesis, phase pure sintered ceramics for aluminum substituted garnet-type and iron substituted garnet-related compounds were obtained. This was also achieved using a nitrate citrate stabilized sol gel method, which was applied to Li7-3xFexLa3Zr2O12 for the first time. For the differently synthesized materials it was also possible to show the sintering behavior by means of in situ dilatometer measurements for the first time. It turned out that the densification is completed for the solid state syntheses after a dwell of 2 h and for the sol gel syntheses after a dwell of 20 min, at an optimized temperature of 1225 °C, regardless of the substituent. Prolonged sintering times or increasing sintering temperature to 1250 °C lead to secondary phase formation accompanied by an increase in crystallite sizes, which was shown by the solid state synthesis of Li6.4Fe0.2La3Zr2O12. Li7-3xFexLa3Zr2O12 ceramics produced by the solid state synthesis and sintered for 2 h at 1225 °C have crystallite sizes in the order of magnitude of 10 μm. In comparison pellets of the sol gel synthesis have an order of magnitude larger crystallites, sintered under the same conditions. This shows that with the choice of synthesis, as well as the sintering conditions the crystallite size of sintered ceramics of these materials is controllable. Relative densities obtained after sintering were about 10 percentage points higher in the sol gel synthesis than in the solid state synthesis. In addition, iron improves the densification of the material during sintering, which could be shown by changing the iron content in Li7-3xFexLa3Zr2O12 for the solid state synthesis. Therefore, the density of Li6.4Fe0.2La3Zr2O12 is about 10 percentage points higher than that of unsubstituted or aluminum substituted Li7La3Zr2O12 and Li6.4Al0.2La3Zr2O12. Whereas only partially cubic stabilized powders calcined at 950 °C for 12 h were obtained for the solid state synthesis due to diffusion problems, the use of the sol gel synthesis enabled the production of phase pure powders for both substituents. For the first time, the morphology of calcined powders of Li7-3xAlxLa3Zr2O12 can be controlled via the pH. This is accompanied by a pH-dependent pyrolysis behavior of the dried gels. In the presence of iron in the sol gel synthesis of Li7-3xFexLa3Zr2O12, the pH shows a markedly reduced effect on pyrolysis. Therefore, no apparent change in the microstructure of calcined powders was observable. Nonetheless, the pH shows an influence on the sintering behavior and the microstructure of the sintered pellets, which could be demonstrated by thermal etching. In addition, a reaction with water was found for Li7-3xFexLa3Zr2O12, which, although known for other lithium-ion conducting garnet compounds, was previously unknown for the iron substituted garnet-related compounds., Published by Aachen

Dissertation, RWTH Aachen University, 2018; Aachen 1 Online-Ressource (XIV, 99 Seiten) : Illustrationen, Diagramme (2018). = Dissertation, RWTH Aachen University, 2018, Solid Oxide Fuel Cells which represent highly efficient energy conversion concepts, can be considered as alternative technologies for de-/centralized power plants or in the automotive sector as auxiliary power units. However, chrome poisoning accompanied with cathode degradation has been identified as a major cause for reduced SOFC stack/component lifetimes. As in-depth knowledge of the mechanism has been obtained for (La,Sr)MnO3 (LSM) based cathode materials, it was long thought that (La,Sr)(Co,Fe)O3- (LSCF) cathodes were less susceptible towards chrome poisoning, because of other mechanisms, i.e. mainly the formation of less conductive SrCrO4-phases on the cathode surface. Recently, however, similar poisoning effects as seen for LSM have been observed for LSCF as well, inducing an additional in-depth investigation of the chrome poisoning of non-manganiferous cathode materials. Here, it should be identified whether interactions of chrome and LSCF cathode materials can be attributed to a chemical or electrochemical reaction mechanism. To realize this objective, anode supported SOFCs (full cells) and symmetrical electrolyte supported half cells were investigated under various different operating conditions, such as the working temperature, cathode gas humidity, applied current densities and in absence/presence of a Cr source. An interplay of electrochemical in operando direct- and alternating-current techniques, i.e. current-voltage characteristics and electrochemical impedance spectroscopy, as well as several pre/post test characterization methods showed enhanced Cr related cell degradation especially at high operating temperatures and current densities in humid cathode gas atmospheres. It was shown, that besides the formation of SrCrO4-phases (Cr(VI)), also Cr(III) species were formed during operation. Here, the formation of Cr(VI) species could be solely confirmed on the LSCF cathode surface whereas Cr(III) has been detected on top of the electrode and at the triple phase boundary between electrode, electrolyte and gas phase. With the implementation of the new 2 D DRT method, the presence of four different processes as well as a fifth process that was strongly superimposed by anodic contributions, had been confirmed. This knowledge facilitated the adaption of a suitable equivalent circuit model for the electrochemical description of anode excluding half cells with La0.58Sr0.4Co0.2Fe0.8O3--, La0.58Sr0.4FeO3-- and La0.58Sr0.4CoO3--cathodes. In comparison to the analysis of full cells, half cells showed enhanced degradation tendencies and enabled a more detailed analysis, also with regard to the B-site stoichiometry of the perovskite cathode material. The results showed that, in fact, the deposition of Cr(VI)-species on the cathode surface can be affected with the application of higher current densities (i.e. higher cathodic overpotentials). This, however, does not confirm a direct electrochemical degradation mechanism as the formation can also be indirectly induced by disassembly of the material, leading to a chemical degradation mechanism. As measurements at (continuously) high overpotentials did not reveal an increased formation of Cr(III) at the cathode surface compared to measurements at lower overpotentials, it is assumed to be chemically induced. However, the presence of Cr(III) species at the triple-phase boundary can be identified as solely electrochemically induced as no signal has been detected for the occurrence of Cr(VI) in this electrochemical highly active region., Published by Aachen

Dissertation, RWTH Aachen University, 2018; Aachen, 1 Online-Ressource (xv, 139 Seiten) : Illustrationen (2018). = Dissertation, RWTH Aachen University, 2018, Metal-air batteries have attracted plenty of attentions because of their high theoretical energy densities. They are considered to be the next generation of energy storage devices for the electric vehicles. However, right now there still some challenges restrict the practical application of rechargeable metal-air batteries, the main challenges is the intrinsic sluggish reaction kinetics of oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) during discharge and charge, resulting in a limited practical energy densities. Because extra energy required overcoming the high-activation barriers, which limit the energy conversion in an electrochemical process, the extent of the barrier is defined by the overpotential. The overpotentials from both ORR and OER significantly lead to the low power output and poor round-trip efficiency of rechargeable metal-air batteries. Therefore, development of efficient bifunctional and cost effective electrocatalysts for ORR and OER is the critical issue for rechargeable metal-air batteries. Transition metal oxides and transition metal oxide-carbon hybrids are found to be the promising candidate as bifunctional electrocatalysts among plent of catalysts. The electrocatalytic activity of the catalyst generally depends on the electronic structure properties of the catalyst, which in turn is impacted by the defect structure of the material. In this thesis, a series of MnOx/C composites and perovskite structured (La0.65Sr0.3)xFeO3-δ catalysts with different A-site stoichiometry are designed and prepared at different temperatures, their properties are investigated by several characterization methods. The influence of the preparation temperature on the properties of the MnOx/C composites, and the influences of the preparation temperatures and A-site stoichiometry on the properties of the perovskite structured (La0.65Sr0.3)xFeO3-δ catalysts are studied in detail. The MnOx/C 120°C composite performs a better ORR and OER activity than the other two composites, because of its smaller particle size, larger electrochemical active surface area (ECSA), more content of reactive active sites and more efficient electron transfer pathway. The ORR and OER mechanisms on the surfaces of the MnOx/C composites are investigated by rotating ring disk electrode (RRDE) technique. Most of the O2 is reduced through a direct 4-electron pathway catalytic by the MnOx/C composites; only a small part of O2 is reduced through an indirect 2×2-electron pathway with the generation of intermediate H2O2. The OER catalyzed by the MnOx/C composites are through direct 4-electron pathway. For the LSF catalysts with the same A-site stoichiometry, the higher ORR and OER activity are obtained at lower calcination temperature; because the content of reaction active site and the content of oxygen vacancy are decreasing with the increasing of the calcination temperature. For the LSF catalysts with same calcination temperature, the best ORR activity is obtained at moderate content of oxygen vacancy. While the best OER activity is obtained at highest content of oxygen vacancy. Their ORR and OER mechanisms are also investigated by RRDE technique. All of the LSF catalysts possess two ORR potential ranges. The first ORR range is in the low potential values, and the O2 is reduced through the indirect 2×2-electron pathway. In the second ORR range with high potential values, the O2 is reduced through the direct 4-electron pathway. The selection of the two ORR mechanisms on the surface of the transition metal oxide is mainly decided by the electron transport efficiency and the O2 adsorption model on the active site. The OER catalytic by LSF catalysts are through 4-electron pathway. This direct 4-electron pathway on the surface of the transition metal oxide is occurred under the synergistic effect of two-reaction active sites of M3+. The electrocatalytic activities of the LSF catalysts towards ORR and OER are affected seriously by the conductivities, adding some highly conductive carbon into the LSF catalysts can improve their ORR and OER activities, because the electron transport during the ORR and OER is important for the catalytic activity of the material. In this work, the influence factors of the transition metal oxide and transition metal oxide/carbon hybrid towards ORR and OER are found, and their mechanisms of ORR and OER in alkaline media are studied., Published by Aachen

Dissertation, RWTH Aachen University, 2017; Aachen, 1 Online-Ressource (124, XXV Seiten) : Illustrationen, Diagramme (2018). = Dissertation, RWTH Aachen University, 2017, The development of bulk-type solid-state lithium-ion batteries highly requires reliable approaches to address the solid-solid interfacial issues from the viewpoints of both materials and architecture of batteries. Herein, a concept of monolithic bulk-type all-phosphate solid-state lithium-ion battery is developed and demonstrated, based on a phosphate backbone that (electro-)chemically and structurally circumvents the interphase issues. Specifically, LiTi2(PO4)3 is used as anode material, Li3V2(PO4)3 is used as cathode material and a dense Li1.3Al0.3Ti1.7(PO4)3 pellet serves as solid electrolyte. The choice of materials was primarily based on the (electro-)chemical and mechanical matching of the components instead of a solely focusing on high-performance. Thus the battery utilized a phosphate backbone in combination with tailored morphology of the electrode materials to ensure a good interfacial match for a durable mechanical stability. Moreover, the phosphate backbone leads to a high chemical stability. The operating voltage range of the electrodes matched with the intrinsic electrochemical window of the electrolyte which also resulted in a high electrochemical stability of the battery. The analysis of interfacial charge transfer kinetics confirmed the structural and electrical properties of the electrodes and their interfaces with the electrolyte, as evidenced by the excellent cycling performance of the monolithic all-phosphate solid-state battery. These interfaces have been studied via impedance analysis with subsequent distribution of relaxation times (DRT) analysis. Moreover, the prepared solid-state battery could be operated in air atmosphere owing to the low oxygen sensitivity of the phosphate materials. The analysis of electrolyte/electrode interfaces after cycling demonstrates that the interfaces remained stable during cycling. Furthermore, a prospect of potential application of the all-phosphate batteries presented in this thesis is to develop an integrated photovoltaic-battery device without any blocking diode or added electronics. The idea is proposed and confirmed by the help of a Li4Ti5O12//LiFePO4/C battery system which exhibits a similar operating potential with here presented all-phosphate batteries., Published by Aachen