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1. XPS analysis.-- 2. X-ray diffraction and scanning electron microscopy.-- 3. Peak Current Analysis.-- 4. Cyclic Voltammetry in Acid Solution.-- 5. Randles-Sevčik Analysis.-- 6. Cell Voltage vs. Time Plots

An increasing number of studies focus on organic flow batteries (OFBs) as possible substitutes for the vanadium flow battery (VFB), featuring anthraquinone derivatives, such as anthraquinone-2,7-disulfonic acid (2,7-AQDS). VFBs have been postulated as a promising energy storage technology. However, the fluctuating cost of vanadium minerals and risky supply chains have hampered their implementation, while OFBs could be prepared from renewable raw materials. A critical component of flow batteries is the electrode material, which can determine the power density and energy efficiency. Yet, and in contrast to VFBs, studies on electrodes tailored for OFBs are scarce. Hence, in this work, we propose the modification of commercial carbon felts with reduced graphene oxide (rGO) and poly(ethylene glycol) for the 2,7-AQDS redox couple and to preliminarily assess its effects on the efficiency of a 2,7-AQDS/ferrocyanide flow battery. Results are compared to those of a VFB to evaluate if the benefits of the modification are transferable to OFBs. The modification of carbon felts with surface oxygen groups introduced by the presence of rGO enhanced both its hydrophilicity and surface area, favoring the catalytic activity toward VFB and OFB reactions. The results are promising, given the improved behavior of the modified electrodes. Parallels are established between the electrodes of both FB technologies.

El desarrollo de la tecnología de producción de energía eléctrica a partir de fuentes renovables para reducir la emisión de gases de efecto invernadero, implica un sistema de almacenamiento acorde a sus prestaciones que evite problemas de estabilidad en la red eléctrica. Las baterías de flujo redox de vanadio (BFRV) son muy prometedoras en almacenamiento de energía debido a la rapidez de carga, gran capacidad y seguridad en la operación. Los electrodos de la BFRV, donde ocurren las reacciones químicas, juegan un papel crucial para aumentar el rendimiento del sistema y disminuir costes. Los materiales que se utilizan comercialmente como electrodos son fieltros de carbono, que se pueden modificar con diferentes estructuras metálicas, carbonosas u orgánicas para optimizar su comportamiento [1]. Diversos estudios en literatura han demostrado una mayor actividad y eficiencia de fieltros modificados con estructuras carbonosas. No obstante, los estudios en batería de flujo completa no son tantos. Por ello, en este trabajo se han investigado fieltros de carbono modificados con nanofibras de carbono y con óxido de grafeno, como electrodos de una batería de flujo redox de vanadio de 25 cm2

La reducción electroquímica de CO2 (CO2RR) destaca como una solución prometedora para la mitigación del cambio climático. Sin embargo, esta reacción presenta desafíos debido a la alta estabilidad de la molécula de CO2 y la difícil selectividad hacia diferentes productos, así como la competencia con la reacción de evolución de hidrógeno, haciendo necesario el desarrollo de catalizadores activos y selectivos. Actualmente, los catalizadores atómicamente dispersos se posicionan como una solución prometedora para modular la selectividad de esta reacción hacia CO. Entre ellos, los catalizadores M-N-C presentan sitios activos basados en átomos de un metal de transición coordinado con nitrógeno en una matriz carbonosa, lo que permite obtener catalizadores con alta porosidad, buena conductividad eléctrica y bajo coste. En cuanto a la identificación de los productos, si bien técnicas exsitu (cromatografía, espectrometría de masas), son comúnmente utilizadas, estas presentan ciertas limitaciones en términos de desfase temporal y detección de productos en concentraciones muy bajas. Para abordar este inconveniente, el uso de un electrodo rotatorio de disco anillo (RRDE) se propone como una alternativa rápida y factible para la identificación de los productos generados durante la CO2RR. En este método, los catalizadores se depositan en el disco del electrodo, donde tiene lugar la CO2RR, mientras que los productos generados son simultáneamente oxidados en el anillo. Este trabajo presenta el estudio de catalizadores Cu-N-C basados en una matriz carbonosa de xerogel de carbono dopado con nitrógeno. Se analizó la influencia del precursor de cobre y de la concentración de metal durante la síntesis de los catalizadores en su comportamiento electroquímico para la CO2RR mediante el uso de RRDE.

The electrochemical CO2 reduction reaction (CO2RR) to controllable chemicals is considered as a promising pathway to store intermittent renewable energy. Herein, a set of catalysts based on copper-nitrogen-doped carbon xerogel (Cu-N-C) are successfully developed varying the copper amount and the nature of the copper precursor, for the efficient CO2RR. The electrocatalytic performance of Cu-N-C materials is assessed by a rotating ring-disc electrode (RRDE), technique still rarely explored for CO2RR. For comparison, products are also characterized by online gas chromatography in a H-cell. The as-synthesized Cu-N-C catalysts are found to be active and highly CO selective at low overpotentials (from −0.6 to −0.8 V vs. RHE) in 0.1 M KHCO3, while H2 from the competitive water reduction appears at larger overpotentials (−0.9 V vs. RHE). The optimum copper acetate-derived catalyst containing Cu-N4 moieties exhibits a CO2-to-CO turnover frequency of 997 h−1 at −0.9 V vs. RHE with a H2/CO ratio of 1.8. These results demonstrate that RRDE configuration can be used as a feasible approach for identifying electrolysis products from CO2RR.

Platinum-group-metal free catalysts have been widely studied in the last decade as an alternative to Pt, employed in fuel cells. Several carbon precursors have been investigated as carbon matrix for iron-nitrogen-carbon (Fe–N–C) catalysts, characterized by a large amount of micropores, fundamental to create Fe-Nx-C active sites. Nonetheless, it is also acknowledged that wider pores are needed to facilitate mass transfer of oxygen/water (reagent/product of fuel cells) towards/from the active sites. Organic xerogels are known as easily tunable materials, that allow doping with a variety of heteroatoms. The present manuscript presents an investigation on the use of organic xerogels combined with iron and nitrogen in a single-step to obtain Fe–N-CXG electro-catalysts for the oxygen reduction reaction (ORR) of proton exchange membrane fuel cells. Several features of both organic xerogels and catalysts on the ORR activity are assessed: the textural properties of the organic xerogel, the iron loading of the Fe–N–C catalysts and the effect of acid-leaching of the catalysts. The present study proves the feasibility of using organic xerogels as carbon precursor to obtain PGM-free catalysts in an easy manner and scalable single-step synthesis. Catalysts obtained from organic xerogels synthesized at mildly acid pHs (5.8 and 6), with a nominal iron loading of 2 wt%, and subjected to two sets of acid leaching/thermal treatments, present enhanced catalytic activity towards the ORR.

Platinum group metal-free catalysts have been intensively investigated in the last decades as an alternative to platinum with the aim of lowering the cost of polymer electrolyte fuel cells. In particular, iron-nitrogen-carbon (Fe-N-C) has proved to be the most active towards the oxygen reduction reaction. For practical application, a hierarchical pore structure is required, with micropores favouring the creation of active sites and larger pores (meso- and macropores) facilitating the mass transport. In this work, carbon xerogels are investigated to hosting iron and nitrogen species obtained by a template-free method. The introduction of nitrogen in a one-step polymerization of urea with resorcinol and formaldehyde is investigated for the first time in this field by varying the relative content of reactants. The urea/resorcinol ratio greatly influences the pore structure of the Fe-N-C catalyst and the ORR electrocatalytic activity thereof. The ORR activity is favored for a balanced urea/resorcinol ratio where porosity is well developed and relatively high iron and nitrogen contents are incorporated to the carbon xerogel. In acid (0.5 M H2SO4), the onset potential is 0.82 V vs. RHE, with a number of exchanged electrons very close to 4 (i.e. full conversion to water) and low Tafel slope of 71 mV dec−1, for the most active catalyst of the series, possessing the best compromise of iron and nitrogen active sites. Fuel cell tests corroborate that the catalyst with the most developed porous structure shows the best performance.

El hidrógeno es el vector energético que nos permitirá realizar la transición hacia nuevas energías limpias. Para ello es necesario encontrar catalizadores activos y de bajo coste para la producción y uso del hidrógeno en tecnologías, como las pilas de combustible regenerativas en una unidad (URFCs), evitando el uso de metales nobles (Pt, Ir, Ru). En este dispositivo, la reacción de reducción de oxígeno (ORR) y la reacción de evolución de oxígeno (OER) desempeñan roles vitales en el electrodo de oxígeno [1]. Se ha realizado un estudio de materiales compuestos basados en combinaciones de Cu y Co con una matriz de carbono dopada con nitrógeno como catalizadores bifuncionales activos para OER y ORR para su uso en URFC [2]. El objetivo de este trabajo ha sido optimizar la cantidad de metales en los composites manteniendo la misma actividad y estabilidad de los electrocatalizadores.

Table S1. Raman shift (cm-1) of the G, D, D’ and D’’ from Raman spectra deconvolution and ID/IG intensity ratio. Table S2. Surface carbon, oxygen, nitrogen, cobalt and titanium contents and C/N, Co/N and Ti/N ratios calculated by XPS of the electrocatalysts and the relative content (%) of the deconvoluted peaks for N1s, Co2p and Ti2p. Figure S1. High-resolution C1s core level of NGC, Ti/NGC, Co/NGC and TiCo/NGC composites. Figure S2. LSV at 0.005 V s−1 in an O2-saturated 0.1 M NaOH solution, recorded using different electrode rotating rates indicated in the legend (rpm). (A) NGC, (B) Ti/NGC, (C) Co/NGC, (D) TiCo/NGC. Figure S3. Koutecky−Levich diagrams obtained between 0.3 and 0.56 V vs RHE for each composite. (A) NGC, (B) Ti/NGC, (C) Co/NGC, (D) TiCo/NGC. Figure S4. Koutecky−Levich diagrams obtained between 0.6 and 0.78 V vs RHE for each composite. (A) NGC, (B) Ti/NGC, (C) Co/NGC, (D) TiCo/NGC.

Three-dimensional N-doped graphene cryogels (NGC) modified with Ti and Co nanoparticles are investigated as bifunctional oxygen electrocatalysts for potential application in unitized regenerative fuel cells (URFCs). These composites have been successfully produced via a combined solvothermal, freeze-drying, and thermal treatment procedure. Crystallographic structure, surface properties, and morphology are analyzed to study the influence of each metal and the bimetallic combination in forming active sites for oxygen reduction (ORR) and oxygen evolution (OER) reactions. The intrinsic catalytic activity of the composites was determined by rotating disk electrode and further assessed in a gas diffusion electrode to test them in more realistic conditions for URFCs in 6 M KOH. Results revealed the remarkable bifunctional activity of Co/NGC and bimetallic TiCo/NGC composites. The synergistic effect of the metallic cobalt and surface atomic composition of Co3O4 distributed on the N-doped graphene cryogel in Co/NGC is responsible for the high bifunctional performance. Moreover, the partial substitution of cobalt for a less critical raw material, such as titanium, allows the electrocatalyst to maintain great bifunctional activity and stability. The nitrogen doping of the graphene cryogel structure can influence the anatase/rutile titania ratio that forms during pyrolysis. As a result, defects, oxygen vacancies, and Ti3+ species are formed, which, when combined with Co nanoparticles, contribute to the outstanding performance and stability of the TiCo/NGC composite. The excellent performance of N-doped graphene cryogel-based electrocatalysts in GDE conditions indicates their potential use in electrochemical processes, which involve gas-phase reactions.

Dye-sensitized solar cells (DSSCs) rely heavily on the counter electrode for their performance, which is responsible for collecting and transferring electrons generated at the photoanode. While platinum (Pt) has traditionally been used as a counter-electrode material, its cost, limited availability, and environmental concerns make it an unsuitable option for large-scale implementation. Iron–nitrogen––carbon (Fe–N–C) catalysts are receiving increasing attention due to their high catalytic activity and low cost. This study aims to investigate the performance of Fe–N–C materials as counter electrodes in DSSCs and assess their potential as a sustainable alternative to currently used platinum. Two different Fe–N–C-based materials have been synthesized using different carbon and nitrogen sources, and their electrochemical behavior has been assessed using current–voltage curves and impedance spectroscopy. The catalyst comprised a higher amount of iron and nitrogen shows higher efficiency and lower charge-transfer resistance due to improved iodide reaction kinetics and proper stability under potential cycling. However, this catalyst shows lower stability under a passive ageing procedure, which requires further clarification. Results provide new insights into the performance of Fe–N–C-based materials in DSSCs and aid in the further development of this promising technology.

Fig. S1. STEM (left) and TEM (right) images of Cu-N-C-Ac2 acquired using a Tecnay F30 microscope (300 kV). Fig. S2. XRD pattern of Cu-N-C-Ac2-3h obtained using the same experimental details than for Cu-N-CAc2 but with a lixiviation treatment of 3 hours. Fig. S3. N1s XPS spectra of the Cu-N-C. The material Cu-N-C-Ac1-BPT before the two-step post-treatment (acid leaching and 2nd carbonization) is also included for comparison purposes. Fig. S4. Effect of the two-step post-treatment (acid leaching and 2nd carbonization) on the nitrogen adsorption-desorption isotherms for Cu-N-C-Ac1. Fig. S5. Correlation between the Cu oxidation state and the energy position of the XANES spectrum, determined as the first maximum of the first derivative spectrum of Cu-N-C-Ac0.5 (red circle) and different copper reference compounds (black circles).Fig. S6. Variation with time of pH of the electrolyte (0.1 M KHCO3 aqueous solution) as a function of CO2 partial pressure. Balance with an inert gas for the equilibrium CO2 (aq) + H2O ↔ HCO3– + H+, pKa = 6.4, room temperature, 1 atm total pressure. Fig. S7. Oxidation of formic acid and methanol at the Pt ring of an RRDE, disc turned off, 1600 rpm. Fig. S8. Comparison of the CV in 0.1 M KHCO3 saturated with N2 (blue curve), methane oxidation voltammogram after saturation of the electrolyte with CH4 at the open circuit potential (red curve) and CH4 stripping voltammogram (Ead = 0.4 V vs. RHE, green curve) at the Pt ring of an RRDE, disc turned off, 1600 rpm. CV conditions from 0.05 to 1.6 V vs. RHE, 50 mV s-1. Room temperature. Fig. S9. CVs of the disc at 50 mV s-1 and 1600 rpm in CO2-saturated 0.1 M KHCO3 of Cu-N-C catalysts. Fig. S10. Chronoamperometric experiments at the disc for the Cu-N-C catalysts. Electrolyte: Fig. S11. CVs at the platinum ring for disc potentials reported in the legend. Table S1. Bulk chemical composition (wt. %) from EA and ICP of Cu-N-C electrocatalysts before and after the two-step post-treatment (acid leaching and 2nd

Fe−N−C catalysts are an interesting option for polymer electrolyte fuel cells due to their low cost and high activity towards the oxygen reduction reaction (ORR). Since Fe−N−C active sites are preferentially formed in the micropores of the carbon matrix, increasing the microporosity is highly appealing. In this work, carbon xerogels (CXG) were activated by physical and chemical methods to favor the formation of micropores, used as carbon matrices for Fe−N−C catalysts, and investigated for the ORR. The catalysts were characterized by solid-state techniques to determine chemical composition and pore structure. Physical activation increased microporosity up to 2-fold leading to catalysts with a larger density of active sites (more than twice iron and nitrogen uptake, pyridinic N and Nx−Fe). This entailed a higher ORR intrinsic activity determined in a 3-electrode cell (80 mV better half-wave potential). At the cathode of a fuel cell, the catalysts based on activated carbon materials showed 26 % lower power density ascribed to a more hydrophilic surface, causing a larger extent of flooding of the electrode that counterbalances the higher intrinsic activity. Interestingly, a more stable behavior was observed for the activated catalysts, with up to 2-fold better relative power density retention after 20-hour operation.

Figure S 1. SEM images of (a) CoFe/NC10% , (b) CoFe/NC20% , and (c) CoFe/NC40%. Figure S 2.TEM images of (a) CoFe/NC10% , (b) CoFe/NC20% , and (c) CoFe/NC40%. Figure S 3. Particle size distribution of the various electrocatalyst: (a) CoFe2O4, (b) CoFe/NC10% , (c) CoFe/NC20% ,(d) CoFe/NC30% , and (e) CoFe/NC40% . Table S 1. Amount of CoFe alloys and oxides in the composite samples, determined from the XRD analysis. Figure S 4. XPS high resolution spectra of (a) Fe 2p, (b) Co 2p, for all electrocatalysts. Table S 2. XPS analysis results of the electrocatalysts. Table S 3. Elemental composition of the prepared electrocatalysts. Table S 4. Specific capacitance and ECSA values for the various eletrocataysts. Table S 5. Charge transfer resistance (Rct) and solution resistance (Rs) determined from EIS measurement. Figure S 5. ECSA-normalized polarization curve.

Designing and developing affordable, high-performance, and stable electrocatalysts for oxygen evolution reaction (OER) is decisive for pragmatic water electrolysis to produce green hydrogen energy. In this work, we report cobalt and iron incorporated in phosphorus and nitrogen co-doped carbon foam (CF) derived from petroleum pitch as a promising electrocatalyst for alkaline OER. The P, N heteroatoms co-doped carbon foam (PN-CF) was first synthesized via thermo-chemical treatment of low-cost petroleum pitch in the presence of melamine (N source) and sodium hypophosphite (P source) precursors, followed by carbonization. Then, mono and bimetals of Co and Fe were impregnated into the as-prepared composite carbon foam (PN-CF) substrate, followed by further carbonization. Among the different catalysts, the bimetallic CoFe integrated with the PN-CF (CoFe@PN-CF) reveals an outstanding electrocatalytic activity (320 mV overpotential at j = 10 mA·cm-2), low Tafel slope (48 mV·dec-1), and excellent durability during OER measurement in 1 M KOH aqueous solution. The superb performance of the CoFe@PN-CF catalyst stems from the synergetic effect of the bimetals confined on phosphorus and nitrogen co-doped carbon foam support with high specific surface area, highly porous structure, and formation of graphitic domains, which enhances the electrical conductivity. This work sheds light on the potential for valorizing petroleum pitch and provides a facile synthesis approach to synthesizing a low-cost, high-performance, and durable electrocatalyst for alkaline OER.

Nanocarbon materials present sp2-carbon domains skilled for electrochemical energy conversion or storage applications. In this work, we investigate graphene oxide nanofibers (GONFs) as a recent interesting carbon material class. This material combines the filamentous morphology of the starting carbon nanofibers (CNFs) and the interlayer spacing of graphene oxide, and exhibits a domain arrangement accessible for fast transport of electrons and ions. Reduced GONFs (RGONFs) present the partial removal of basal functional groups, resulting in higher mesoporosity, turbostratic stacking, and surface chemistry less restrictive for transport phenomena. Besides, the filament morphology minimizes the severe layer restacking shown in the reduction of conventional graphene oxide sheets. The influence of the reduction temperature (140–220 °C) on the electrochemical behaviour in aqueous 0.5 M H2SO4 of RGONFs is reported. RGONFs present an improved capacitance up to 16 times higher than GONFs, ascribed to the unique structure of RGONFs containing accessible turbostratic domains and restored electronic conductivity. Hydrothermal reduction at 140 °C results in the highest capacitance as evidenced by cyclic voltammetry and electrochemical impedance spectroscopy measurements (up to 137 F·g−1). Higher temperatures lead to the removal of sulphur groups and slightly thicker graphite domains, and consequently a decrease of the capacitance.

Highly efficient, low-cost and stable bifunctional catalysts for oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) are highly desirable for the development of green electrochemical energy storage and conversion devices. Here, we report the synthesis of sulfur-doped carbon nanofibers (CNF) as support for tantalum oxide nanoparticles. Carbon nanofibers and tantalum oxides represent a durable choice for oxygen electrocatalysis, but an improvement of catalytic activity is required. Upon doping, sulfur is found to be mainly bonded to carbon as C–S–C species (more than 80%). The results show that the effective incorporation of sulfur in the support has a clear positive effect on the electroactivity of the tantalum oxide catalysts. It causes a decrease of OER/ORR overpotential of 80 mV with respect to the undoped counterpart, with special improvement in the ORR. The new catalysts have shown an interesting bifunctional behavior for the OER and ORR, as well as a good stability through time.

In recent years, renewable energies have played an essential role in the technological development to meet energy demand and replace fossil fuels. At the same time, new challenges have arisen in order to implement clean energy production. The problem to store renewable energy that is obtained intermittently in the long term has led the development of fuel cells, electrolyzers and unitized regenerative fuel cells (URFC) to use hydrogen as an energy vector. URFCs can be used for both electric power production and hydrogen generation by adapting to the energy needs of the power grid. The most recent studies using an anion exchange membrane as electrolyte have opened up a range of possibilities for synthesizing new catalysts by replacing noble metals such as Pt, Ir or Ru with cheaper transition metals in combination with N-doped carbon structures [1] This work focuses on the development of noble metal-free bifunctional electrocatalysts for the oxygen electrode of unitized regenerative alkaline fuel cell (UR-AFC), where oxygen evolution (OER) and reduction (ORR) reactions take place [2]. Composites based on non-precious metals such as Ti, Mn, Fe, Co, Ni or Zn, and nitrogen-doped carbonaceous nanostructures obtained from polydopamine have been prepared.

The development of active and inexpensive non-precious metal catalysts is a necessary and essential requirement to replace currently used Pt-based catalysts, in order to reduce the cost of polymer electrolyte fuel cells(PEFC). [1] Catalysts based on the Fe-N-C structure develop high electroactivity towards the oxygen reduction reaction (ORR), the limiting reactionin fuel cells. To increase the catalysts activity, Fe must be dispersedin acarbon material with a high surfacearea.In this context, carbon xerogels are excellent candidates, as their main properties can be easily tailored: porosity, electrical conductivity and surface chemistry. [2] The meso/macroporosity of carbon xerogels can be designed by modifying the conditions of the synthesis process, while microporosity can be generated by subsequent carbonization/activation treatments [3].In the present work, on the basis of carbon xerogels with an optimal ratio of micro/meso/macroporosity, we investigate the effect of two different methods of nitrogen-doping in the catalytic activity of Fe-N-C catalysts. Catalysts are evaluated both in half-cell and single-cell configurations and their activity is correlated to their physical-chemical features.

Electrochemical water splitting is an environmentally benign technology employed for H2 production; however, it is critically hampered by the sluggish kinetics of the oxygen evolution reaction (OER) at the positive electrode. In this work, nitrogen-doped carbon-coated CoFe electrocatalysts were synthesized via a three-step route comprising (1) hydrothermal reaction, (2) in-situ polymerization of dopamine and (3) carbonization. The effect of carbonized polydopamine on the overall physicochemical properties and electrochemical activity of CoFe catalysts was systematically studied. By controlling and optimizing the ratio of CoFe2O4 and dopamine contents, a transformation of the CoFe2O4 structure to CoFe alloy was observed. It was found that CoFe/NC30% (prepared with 30% dopamine) exhibits an excellent catalytic activity towards OER. A small overpotential of 340 mV was required to generate a current density of 10 mA cm-2 in a 1.0 M KOH electrolyte. More importantly, the CoFe/NC30% catalyst reflected exceptional durability for at least 24 h. This research sheds light on the development of affordable, highly efficient, and durable electrocatalysts for OER.

Las pilas de combustible de membrana polimérica son una de las opciones más prometedoras para la conversión fiable y eficiente de energía limpia que van a ejercer un papel fundamental en la nueva transición energética del siglo XXI. Sin embargo, uno de los mayores obstáculos que presentan estos dispositivos es la relativa baja actividad de la reacción de reducción de oxígeno (ORR) que tiene lugar en el cátodo, y el alto coste de los catalizadores basados principalmente en Pt. El objetivo de este trabajo es la elaboración de electrocatalizadores sostenibles y competitivos en la reacción de reducción de oxígeno (ORR) para una pila de combustible de membrana polimérica intercambiadora de aniones. Para ello se han preparado composites formados por una matriz carbonosa dopada con nitrógeno y utilizando el Cu como metal activo. Este tipo de catalizadores ha sido poco estudiado en la bibliografía en comparación con otros metales, que cada vez están más cotizados para otros fines y que proceden de cadenas de suministro cuestionables en países subdesarrollados.

High performance, durable and affordable electrocatalyst for Oxygen Evolution Reaction (OER) is of great importance for tenable hydrogen production via water electrolysis. Although spinel oxide-based electrocatalysts (AB2O4, A, B = metal) represent a class of promising candidates for OER, their intrinsically poor electrical conductivity impacts their electrochemical performance. To overcome these bottlenecks, heteroatom-doped carbon structures can be used to improve the conductivity, adjust the electronic structure, durability and also boost the charge transfer, leading to enhanced electrochemical water oxidation [1]. In this work, a selection of nitrogen doped carbon decorated NiFe2O4 composites derived of polydopamine for OER in alkaline medium have been investigated.

Actualmente, el dióxido de carbono (CO2) es uno de los gases de mayor contribución al calentamiento global, lo que impolica la necesidad de realizar grandes esfuerzos para descarbonizar el sector energético en particular y la economía en general. En este escenario de transición, la reacción de electro-reducción de CO2 (CO2RR) se presenta como una tecnología prometedora para la utilización de corrientes concentradas de CO2 con el fin de producir productos químicos e industriales de alto valor añadido, como alcoholes o hidrocarburos, utilizando electricidad renovable como fuente energética. Sin embargo, la alta estabilidad de la modécula de CO2 bajo condiciones ambientales implica un elevado aporte de energía para producir cantidades razonables de productos finales. Además, la CO2RR tiene lugar involucrando un complejo mecanismo de reacción con diferentes rutas paralelas y múltiples etapas de transferencia de electrones, compitiendo a su vez con la reacción de evolución de hidrógeno (HER), lo que limita el proceso. Por todo esto, el diseño de catalizadores estables, eficientes y selectivos para la reducción de CO2 es actualmente uno de los principales retos de esta tecnología. Una solución prometedora es el uso de catalizadores bimetálicos combinando un metal noble (Pt), con el fin de reducir el sobrepotencial de la reacción, y cobre (Cu), que regula la selectividad hacia la formación de hidrocarburos y alcoholes. Además, la dispersión de la fase activa en un soporte con adecuada textura y química superficial mejora la utilización del metal, reduciendo por tanto el coste de los electrodos. Este trabajo estudia el efecto del soporte carbonoso para la CO2RR en catalizadores bimetálicos CuPt/C (20% en peso de cobre y ratio atómico Cu:Pt de 95:5). Como soportes carbonosos se utilizaron xerogeles de carbono (CXG), óxidos de grafeno reducido (RGO), carbones mesoporosos ordenados (OMC) y un negro de carbono comercial (Vulcan XC-72R).

Las celdas solares sensibilizadas con colorante (DSSCs), o celdas de Grätzel, son dispositivos fotovoltaicos prometedores para la generación de energía eléctrica, debido a su bajo coste y su simplicidad de fabricación. Las DSSCs están formadas por un fotoánodo basado en una película de colorante y óxido de titanio, un electrolito que contiene pares redox yoduro/triyoduro y un contraelectrodo basado en una película delgada de platino. El rol del contraelectrodo es catalizar la reacción de reducción de yodo a yoduro del electrolito. Los materiales carbonosos se han postulado como un sustituto prometedor del platino, gracias a su elevada área superficial y conductividad eléctrica, lo que permitiría reducir el coste de fabricación de las DSSC. En este trabajo se han investigado xerogeles de carbono (CXGs) dopados con nitrógeno (N-CXG) o azufre (S-CXG) como contraelectrodos de DSSCs y se ha evaluado su comportamiento electroquímico.

La reacción elctroquímica de reducción de dióxido de carbono (CO2RR) representa una tecnología atractiva de almacenamiento de energía renovable (intermitente) en combustibles químicos y otras materias primas. En este proceso, el CO2 es reducido en la superficie de un catalizador y, por ende, la naturaleza de la fase activa resulta clave en la actividad/selectividad de la reacción. Entre los diferentes sistemas catalíticos estudiados, aquellos basados en cobre han despertado un gran interés por su tendencia única para producir hidrocarburos y alcoholes. Sin embargo, la mayoría de los electrodos reportados presentan una baja eficiencia energética y selectividad, lo que desalienta su implementación a nivel industrial. Recientemente, la alteración del entorno químico del cobre con un metal noble se ha propuesto como una estrategia prometedora para reducir el sobrepotencial de reacción y favorecer la CO2RR., Este trabajo se centra en la reducción electroquímica de CO2 usando catalizadores CuPt soportados en un negro de carbono comercial (CuPt/C, 20% en peso de cobre). Los catalizadores se sintetizaron variando el pH y con diferentes relaciones atómicas Cu:Pt, con el fin de estudiar su influencia en la actividad y selectividad hacia la CO2RR.

Una de las principales desventajas de las pilas de combustible (PEMFCs) es su elevado coste, debido principalmente a los catalizadores de Pt empleados. Se han investigado numerosos materiales como posibles alternativas al Pt, siendo los catalizadores basados en metales como el hierro coordinado con nitrógeno y carbono (Fe-N-C), los más activos para la reacción de reducción de oxígeno (ORR), la reacción limitante en las PEMFCs. Generalmente, estos catalizadores emplean negros de carbono microporosos como matrices carbonosas. Un estudio detallado de la micro/meso/macroporosidad de esta estructura carbonosa es esencial para el desarrollo de catalizadores de Fe-N-C. En este trabajo se ha modificado la microporosidad de estos xerogeles de carbono mediante diferentes tratamientos de activación y se ha estudiado su efecto en la actividad de los catalizadores Fe-N-CXG para la ORR.

Las pilas de combustible regenerativas en celda unidad (URFC) están llamadas a tener un papel importante en el diseño de nuevos planes energéticos que permitan abandonar el uso de combustibles fósiles. Sin embargo, esta tecnología se encuentra limitada por la baja cinética de las reacciones que ocurren en el electrodo de oxígeno (reacciones de evolución (OER) y reducción (ORR) de oxígeno) y del alto coste de los catalizadores empleados en su operación. Una forma de incrementar la cinética de estas reacciones es el uso de electrolitos alcalinos. Este tipo de medio disminuye la corrosión de los materiales empleados como catalizadores, permitiendo el uso de metales no nobles, así como soportes basados en carbono. El objetivo de este trabajo es la síntesis de materiales nanoestructurados, basados en criogeles de grafeno dopados con nitrógeno y nanopartículas metálicas de Ti y Co, y el estudio de su actividad como catalizadores para la ORR y la OER en medio alcalino.

Ceria-iron oxide mesoporous materials with Fe:Ce molar ratio of 5:5 and 9:1 were synthesized by hydrothermal method using CTAB as a template and subsequently modified with NiO (molar ratio Ni:Fe = 1:2) by incipient wetness impregnation technique. In order to increase the electro-capacitive properties and reduce the intrinsic impedance of the metal oxides, the samples were consecutively modified by reduction in hydrogen to obtain highly dispersed Ni–Fe alloys into ceria matrix. By exploiting the high permeability of carbon inside ferrous alloys, the metal phase has been further modified into ferrous carbides and metal alloys encapsulated within carbon nanofibers. For this purpose, a reaction, already widely studied for the production of hydrogen, was used, that is the decomposition of methanol vapors. In fact, this decomposition, in addition to producing syn-gas and methane, changes the catalysts in use through a chemical vapor deposition-carbon coating process. This fact, has been used by us to demonstrate how the newly obtained metal-carbon nanocomposites can be used for electro-catalytic purposes. The modified phases of the two molar ratios of the Fe–Ni–Ce catalysts were tested in the Oxygen Evolution Reaction (OER) in an alkaline environment (1 M KOH), showing a satisfactory and progressive increase in activity and a surprising decrease in the overpotential at 10 mA/cm2 of current density. The morphological, textural and physicochemical properties of the samples were characterized in details by XRD, N2-physisorption, TG-TPO, TEM, EDX, FTIR, XPS, Raman and Moessbauer spectroscopies.

Las reacciones de reducción y de evolución de oxígeno (ORR, OER) son de especial interés en el desarrollo de dispositivos energéticos como pilas de combustible regenerativas o baterías metal-aire. El diseño de catalizadores bifuncionales capaces de catalizar eficientemente ambas reacciones es un tema de gran actualidad. Algunos óxidos metálicos de los grupos IV y V de la tabla periódica, como el tantalio, son catalíticamente activos y además presentan una muy buena estabilidad, pero su conductividad eléctrica y su área superficial son generalmente muy bajas. En este trabajo se ha investigado la utilización de nanofibras de carbono como soporte de óxidos de tantalio para mejorar dichas características.

High performance, durable and inexpensive electrocatalyst for oxygen evolution reaction (OER) is of great importance for tenable hydrogen production via water electrolysis. Although spinel oxides (AB2O4, A, B = metal) represent a class of promising candidates for OER, their intrinsically poor electrical conductivity impacts their electrochemical performance. Herein, we employed a facile approach to transform an intrinsically low active NiFe2O4 into nitrogen-doped carbon decorated Ni3Fe@Fe3O4 catalyst with improved activity and stability for alkaline OER. Initially, a pristine NiFe2O4 octahedron-like structure was synthesized by a hydrothermal route. Then, series electrocatalysts were prepared by incorporating the pristine NiFe2O4 with different dopamine concentrations via in-situ polymerizations of dopamine followed by carbonization. The morphology, crystalline structure, and chemical composition of the catalysts were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and inductively coupled plasma (ICP). The OER electrocatalysis performance was measured in a standard three-electrode system. The effect of the carbonized dopamine on the electrocatalytic activity and structure of the NiFe2O4 precursor was systematically investigated. Among several NiFe electrocatalysts, the one with 10 wt% of dopamine (NiFe/NC10%) exhibited a relatively higher catalytic activity for OER tested in 1.0 M KOH; unveiled low overpotential (350 mV at 10 mAcm−2 current density), a low Tafel slope (56 mVdec−1), low charge transfer resistance, relatively higher electrochemically active surface area. Most prominently, it remained stable for at least 12 h. This work provides a new perspective for functionalizing metal oxides and affords a facile synthesis approach, low-cost, high-performance, and robust electrocatalyst for alkaline OER electrodes.

The stainless steel mesh (SSM) has received remarkable attention for hydrogen and oxygen evolution reactions. It was demonstrated that the SSM exhibits admirable performance towards oxygen evolution reaction (OER) electrocatalysis, while its catalytic activity for hydrogen evolution reaction (HER) remains quite low. This obstructs the utilization of SSM-based catalysts for sustainable complete water electrolysis. In this study, a facile hydrothermal route followed by a phosphorization process was adopted to transform commercially available SSM materials into high-performance and stable electrocatalysts for alkaline HER. We report an interconnected NiCoP-CoP sponge-like structure on SSM substrate without polymer binder. Benefiting from the 3D construction with high exposed surface area, close contact between electroactive species and conductive surface, and facilitated infiltration of electrolyte, the as-prepared NiCoP@SSM electrocatalyst brought an improved catalytic activity for HER, required a low overpotential of 138 mV to derive a current density of 10 mAcm−2 in 1.0 M KOH aqueous solution. The high performance of the NiCoP@SSM catalyst has also unveiled fast reaction kinetics (presents a small Tafel slope of 74 mV/dec), a relatively large electrochemical active surface area (ECSA), and small charge transfer resistance. Furthermore, the NiCoP@SSM electrode also presented excellent stability during long-term measurements, making it one of the most encouraging HER electrodes to date. This research study paves the way for the development of HER-active electrocatalysts made from SSMs that are commercially available, low-cost, and highly active.

In this work, carbon xerogels (CXGs) doped with nitrogen or sulfur have been investigated as DSSC counter electrodes. CXGs have been prepared by a sol–gel method from resorcinol and formaldehyde and subsequent carbonization. Nitrogen doping has been carried out by introducing melamine into the synthesis process along with resorcinol and formaldehyde, while sulfur has been incorporated by direct reaction of the carbon material with elemental sulfur. The counter electrodes for DSSCs have been prepared by airbrushing on conductive glass (fluorine-doped tin oxide, FTO), and their electrochemical behavior has been evaluated, observing that the introduction of heteroatoms such as nitrogen or sulfur leads to an improvement in efficiency compared to the undoped material thanks to a decrease in charge transfer resistance.

En los últimos años, la reducción de CO2 por vía electroquímica ha emergido como una estrategia para la obtención de compuestos de alto valor añadido (alcoholes o hidrocarburos) con aplicaciones energéticas o como moléculas plataforma para la síntesis de otros productos químicos. Si bien el CO2 es una fuente de carbono abundante, la alta estabilidad de este compuesto dificulta su conversión en otros productos, ya que implica un aporte de energía significativo. Además, la reacción de electro-reducción de CO2 (CO2RR) tiene lugar a partir de un mecanismo de reacción complejo con diferentes rutas paralelas y con múltiples etapas de transferencia de electrones. En este contexto, la utilización de catalizadores activos y selectivos que aumenten la eficiencia de la reacción y modulen la formación del producto deseado resulta primordial. Los sistemas catalíticos basados en cobre han atraído mucha atención por su tendencia única a producir hidrocarburos y alcoholes, aunque requieren de altos sobrepotenciales para activar la molécula de CO2. Una solución prometedora es el uso de catalizadores bimetálicos entre este elemento y un metal noble (Pt), de forma que el Cu regule la selectividad y el Pt reduzca el sobrepotencial. En este trabajo se estudia el efecto del ratio atómico Cu:Pt en el comportamiento electroquímico para la CO2RR de catalizadores bimetálicos soportados sobre negro de carbono (CuPt/C, 20 % de cobre en peso). La síntesis consistió en la preparación de nanopartículas de Cu/C por impregnación y reducción con NaBH4. Posteriormente, el Pt se incorporó mediante desplazamiento galvánico, generando partículas bimetálicas CuPt/C [1]. Los electrocatalizadores CuPt/C fueron analizados mediante diversas técnicas (XRD, ICP, TEM), confirmando cargas metálicas similares a las nominales, así como la correcta deposición de las nanopartículas en la superficie del material carbonoso. Por otro lado, la actividad de los catalizadores hacia la CO2RR se evaluó por voltamperometría

Active and durable electrocatalysts for the oxygen evolution reaction (OER), capable of replacing noble metal catalysts, are required to develop efficient and competitive devices within the frame of the water electrolysis technology for hydrogen production. In this work, we have investigated tantalum based catalysts supported on carbon nanofibers (CNF) for the first time. The effect of CNF characteristics and the catalyst annealing temperature on the electrochemical response for the OER have been analyzed in alkaline environment using a rotating ring disc electrode (RRDE). The best OER activity and oxygen efficiency were found with a highly graphitic CNF, despite its lower surface area, synthesized at 700 °C, and upon a catalyst annealing temperature of 800 °C. The ordering degree of carbon nanofibers favors the production of oxygen in combination with a low oxygen content in tantalum oxides. The most active catalyst exhibited also an excellent durability.

Biomass from agricultural by-products is gaining increasing interest as cheap and abundant precursor in the development of active materials for efficient and environmentally friendly devices like fuel cells. Herein, we investigated iron and nitrogen co-doped nanoporous carbons derived from aronia, peach stones and coal tar pitch/furfural as electrocatalysts for the electrochemical oxygen reduction reaction (ORR) in alkaline media. Urea was used as nitrogen precursor and two annealing steps with intermediate acid leaching served to activate the catalysts. Within the series, the peach stone-derived catalyst exhibited a catalytic activity for the ORR close to the benchmark Pt/C, with a 60 mV dec−1 Tafel slope upon the incorporation of 0.57 wt% Fe and proper combination of N-Fe species (20%) with pyridinic/pyridonic moieties (49%). We concluded that the microporosity and a certain content of meso/macro-pores of the activated carbon, together with the creation of graphitic domains result in a high relative amount of Fe-N4 and nitrogen functionalities, which determine the electrocatalytic performance.

Graphene oxide (GO) is reduced by Joule heating using in-situ transmission electron microscopy (TEM). The approach allows the simultaneous study of GO conductivity by electrical measurements and of its composition and structural properties throughout the reduction process by TEM, electron diffraction and electron energy-loss spectroscopy. The small changes of GO properties observed at low applied electric currents are attributed to the promotion of diffusion processes. The actual reduction process starts from an applied power density of about 2 × 1014 Wm−3 and occurs in a highly uniform and localized manner. The conductivity increases more than 4 orders of magnitude reaching a value of 3 × 103 Sm−1 with a final O content of less than 1%. We discuss differences between the reduction by thermal annealing and Joule heating.

In the pursuit of good performing, low cost, and scalable anion-exchange membranes (AEM), a series of blended electrolytes based on cross-linked quaternized poly epichlorohydrin (qPECH) and polyvinylidene fluoride (PvDF) were prepared to evaluate their suitability for AEM fuel cell application. The thermo-mechanical and swelling analyses revealed that the blending of these two macromolecules produces robust and heat-resistant microphase-segregated membranes with good dimensional stability. By varying the blend ratio, the ion-exchange capacity (IEC) and transport properties of the resulting membrane can be easily adjusted and optimized with clear impact on its electrochemical performance. At 67:33 wt % blend ratio, high hydroxide conductivity (i.e., 56.3 mS cm–1 at 80 °C) and quite reasonable alkaline stability were achieved. The single H2–O2 fuel cell using the qP-67 membrane yielded a beginning-of-life maximum power density of 32 mW cm–2 and an open circuit voltage (OCV) of 1.03 V at 50 °C without optimization. These preliminary results demonstrate that qPECH/PvDF blended membranes can be potentially applied in AEMFCs.

Bifunctional oxygen electrocatalysts are essential in the development of low-temperature unitized regenerative fuel cells (URFCs), as a promising alternative for storing energy via hydrogen. TiO2, as a semiconductor material, is commonly not established as an active electrocatalyst for oxygen reduction and oxygen evolution due to its poor electrical conductivity and low reactivity. Here, we demonstrated that composites composed of TiO2 and N-doped graphene can be active in oxygen reduction and evolution reactions in an alkaline environment. Combination factors such anatase/rutile interaction, N-doping graphene, and the presence of Ti3+/Ti–N species raise the active sites and improve the electrochemical activity. Our results may afford an opportunity to develop a non-noble and promising electrocatalyst in energy storage technology.

En los últimos años, las energías renovables han cobrado un papel esencial en el desarrollo tecnológico para satisfacer la demanda energética creciente y sustituir a los combustibles fósiles. Al mismo tiempo, han surgido nuevos retos para poder implementar estas vías de obtención de energía limpia, la incógnita de cómo almacenar a largo plazo la energía renovable que se obtiene de manera intermitente ha favorecido el desarrollo de pilas de combustible, electrolizadores o pilas regenerativas en una unidad (URFC) para utilizar el hidrógeno como un vector energético. Estas últimas se pueden utilizar tanto para la producción de energía eléctrica como para la generación de hidrógeno adaptándose a las necesidades energéticas de la red eléctrica. Los estudios más recientes utilizando una membrana de intercambio aniónico como electrolito, han abierto un abanico de posibilidades a la hora de sintetizar nuevos catalizadores sustituyendo los metales nobles, como el Pt, Ir o Ru, por otros metales de transición más baratos combinándolos con materiales de carbono, como nanotubos o grafeno, óxidos metálicos o estructuras organometálicas (MOFs). Este trabajo se centra en el desarrollo de electrocatalizadores bifuncionales libres de metales nobles, para el electrodo de oxígeno de una URFC alcalina, donde tienen lugar las reacciones de evolución (OER) y reducción (ORR) de oxígeno, las cuales poseen una cinética de reacción lenta. Se han preparado composites basados en metales no preciosos de Co, Fe, Mn, Ni y Zn y nanoestructuras carbonosas dopadas con nitrógeno, obtenidas a partir de precursores poliméricos.

Actualmente, uno de los mayores retos en la tecnología de pilas de combustible de electrolito polimérico (PEFCs) es la disminución del coste de los sistemas catalíticos empleados en el ánodo y en el cátodo. Una de las estrategias más exploradas es la de sustituir los catalizadores basados en Pt por otros basados en metales de transición coordinados con nitrógeno y carbono (Me-N-C, donde Me = Fe, Co y/o Mn), siendo los catalizadores Fe-N-C los más activos para la reacción de reducción de oxígeno (ORR), la reacción limitante en las pilas de combustible. Generalmente, estos catalizadores emplean negros de carbono microporosos como matrices carbonosas. En este trabajo se han sintetizado diferentes catalizadores de hierro usando como estructura carbonosa los xerogeles de carbono, previamente dopados con nitrógeno (N-CXG), con el fin de usar una matriz carbonosa con una mezcla adecuada de microporos y meso/macroporos, necesarios para una correcta difusión de especies en la pila de combustible.

La reducción de CO2 por vía electroquímica es una alternativa novedosa con altas expectativas de cara a la reutilización de corrientes concentradas de CO2, con el fin de obtener productos de alto valor añadido (como alcoholes e hidrocarburos) con aplicaciones energéticas o para la síntesis química. Sin embargo, uno de los principales inconvenientes de la reacción de reducción de CO2 (CO2RR) es la alta estabilidad de esta molécula bajo condiciones ambientales, lo que implica la necesidad de utilizar una gran cantidad de energía para obtener cantidades razonables de dichos productos. Además, la reducción de CO2 viene inevitablemente acompañada de la evolución de hidrógeno cuando se lleva a cabo en electrolitos acuosos. En este contexto, la utilización de catalizadores altamente activos y selectivos hacia los compuestos de interés se torna esencial para aumentar la eficiencia de la reacción. Entre los diferentes sistemas catalíticos, los catalizadores basados en cobre son los más prometedores para la electro-reducción de CO2 debido a la actividad única de este metal a la formación de hidrocarburos, alcoholes y aldehídos. Sin embargo, se requieren altos sobrepotenciales para alcanzar densidades de corriente sustanciales. Una posible solución es el uso de catalizadores bimetálicos usando cobre y un metal noble (por ejemplo, Pt). De este modo, la presencia del metal noble es de utilidad para disminuir el sobrepotencial de la reacción mientras que el cobre permite modular la selectividad a compuestos derivados de la reducción de CO2. Además, el uso de un soporte adecuado con alta área superficial permite dispersar la fase activa (normalmente en forma de nanopartículas), mejorando la utilización del metal y la distribución del CO2 en la superficie del catalizador y, por tanto, favoreciendo el proceso y disminuyendo el coste del electrodo. Por ello, el uso de nanopartículas metálicas soportadas en materiales de carbono representa una vía prometedora para la electro-reducción

Las baterías de flujo rédox (RFB) se han postulado en los últimos años como unas de las tecnologías más prometedoras para el almacenamiento de energía a gran escala debido a su diseño modular que favorece el escalado y el diseño separado de energía y potencia. Los electrodos de las RFB contribuyen al rendimiento de la batería condicionando la densidad de potencia y la eficiencia energética, pues es sobre la superficie electroactiva de éstos, y gracias a su capacidad de transporte electrónico, donde se lleva a cabo la reacción electrocatalítica. Además, el uso de especies de vanadio en los dos electrolitos presenta unas ventajas únicas como son: la eliminación de la contaminación cruzada debida a la difusión de iones a través de la membrana, un tiempo de respuesta corto y una vida útil prolongada. En este trabajo se ha llevado a cabo la modificación de fieltros comerciales de carbono de poliacrilonitrilo (PAN) carbonizado (GFD 4.6 IW1) suministrados por Sigracell, con el fin de mejorar sus propiedades como electrodos, que actualmente son los más empleados a gran escala en la fabricación de baterías de flujo redox. Para ello se ha diseñado un método de modificación de la superficie del fieltro con óxido de grafeno (GO) y polietilenglicol (PEG), seguido de una pirolisis en atmósfera inerte. Se ha estudiado la influencia del contenido en GO mediante una deposición progresiva de distintas cantidades de GO y PEG. El objetivo de la modificación con GO y PEG es el de obtener una superficie funcionalizada con oxígeno gracias al poder oxidante del PEG y aumentar la actividad catalítica de los fieltros hacia las reacciones de reducción y oxidación del vanadio, por acción del óxido de grafeno.

In direct alcohol fuel cells (DAFCs), the oxidation of alcohols happens at the expenses of the oxygen reduction reaction at the cathode. DAFCs' cathodes use a significant amount of platinum that is an expensive critical raw material. Moreover, platinum oxidizes alcohols, a fact that combined with alcohol crossover, decreases significantly the performance of the cells. The use of Fe-based (Fesingle bondNsingle bondC) platinum group metal–free (PGM-free) cathodes is a convenient strategy to overcome these limitations. This review analyzes the application of PGM-free cathodes to DAFCs. The discussion focuses on acidic systems and covers the following subjects: (i) the breakdown of DAFC potential in its components, (ii) the analysis of the advantages from the use of the PGM-free cathode, and (iii) a review of the performance and durability of DAFCs. The review closes with a view of the authors of the future perspective for the research.

This review paper presents the most recent research progress on carbon-based composite electrocatalysts for the oxygen evolution reaction (OER), which are of interest for application in low temperature water electrolyzers for hydrogen production. The reviewed materials are primarily investigated as active and stable replacements aimed at lowering the cost of the metal electrocatalysts in liquid alkaline electrolyzers as well as potential electrocatalysts for an emerging technology like alkaline exchange membrane (AEM) electrolyzers. Low temperature electrolyzer technologies are first briefly introduced and the challenges thereof are presented. The non-carbon electrocatalysts are briefly overviewed, with an emphasis on the modes of action of different active phases. The main part of the review focuses on the role of carbon–metal compound active phase interfaces with an emphasis on the synergistic and additive effects. The procedures of carbon oxidative pretreatment and an overview of metal-free carbon catalysts for OER are presented. Then, the successful synthesis protocols of composite materials are presented with a discussion on the specific catalytic activity of carbon composites with metal hydroxides/oxyhydroxides/oxides, chalcogenides, nitrides and phosphides. Finally, a summary and outlook on carbon-based composites for low temperature water electrolysis are presented.

[EN] The interest in hydrogen technologies has grown in recent years, mainly because an economy based on hydrogen can help to solve important challenges related to the global economy of the future: energy security and climate change. Taking advantage of this momentum, more and more countries are implementing a growing number of policies related to hydrogen. Indeed, the European Hydrogen Strategy establishes hydrogen as essential drivers for the total decarbonization of the current energy system in order to achieve the EU’s commitment related to carbon neutrality by 2050. However, the successful development of the hydrogen technologies requires the collaboration of the public and private sectors to accelerate its deployment and make more competitive its implementation at large-scale. The research groups that take part of the line of work dedicated to hydrogen technologies, within the CSIC Interdisciplinary Thematic Platform PTI Mobility 2030, work in this regard, developing their investigations in several important areas related to the hydrogen technologies such as hydrogen generation, storage, distribution and uses., [ES] El interés por las tecnologías del hidrógeno ha crecido en los últimos años, principalmente porque una economía basada en el hidrógeno puede dar respuesta a los grandes desafíos de la economía global del futuro: seguridad energética y cambio climático. Aprovechando este impulso, cada vez son más los países que están implementando un número creciente de políticas en favor del hidrógeno. Prueba de ello es la Estrategia Europea del Hidrógeno que establece al hidrógeno como un elemento esencial en la descarbonización total del actual sistema energético para alcanzar el compromiso de la UE con la neutralidad de carbono en 2050. No obstante, el desarrollo exitoso de las tecnologías del hidrógeno requiere que todos los actores, incluidos los sectores público y privado, aumenten sus esfuerzos para acelerar su despliegue y hacer que su implantación a gran escala resulte competitiva. Los grupos de investigación que forman parte del área de trabajo de tecnologías del hidrógeno, dentro de la Plataforma Temática Interdisciplinar PTI Mobility 2030 del CSIC, trabajan en este sentido, desarrollando su labor en áreas tan diversas como la generación, el almacenamiento, la distribución y los usos del hidrógeno.

Polymer electrolyte fuel cells with alkaline anion exchange membranes (AAEMs) have gained increasing attention because of the faster reaction kinetics associated with the alkaline environment compared to acidic media. While the development of anion exchange polymer membranes is increasing, the catalytic layer structure and composition of electrodes is of paramount importance to maximize fuel cell performance. In this work, we examine the preparation procedures for electrodes by catalyst-coated substrate to be used with a well-known commercial AAEM, Fumasep® FAA-3, and a commercial ionomer of the same nature (Fumion), both from Fumatech GmbH. The anion exchange procedure, the ionomer concentration in the catalytic layer and also the effect of membrane thickness, are investigated as they are very relevant parameters conditioning the cell behavior. The best power density was achieved upon ion exchange of the ionomer by submerging the electrodes in KCl (isopropyl alcohol/water solution) for at least one hour, two exchange steps, followed by treatment in KOH for 30 min. The optimum ionomer (Fumion) concentration was found to be close to 50 wt%, with a relatively narrow interval of functioning ionomer percentages. These results provide a practical guide for electrode preparation in AAEM-based fuel cell research.

In order to decrease the cost of direct methanol fuel cells (DMFCs), allowing the market penetration of these devices, it is necessary to reduce the costs of the components, in particular the cathode catalysts based on platinum. Another issue of these fuel cells is related to methanol crossover through the polymeric membrane, which creates a mixed potential at the cathode and decreases the overall efficiency. For this reason the development of methanol-tolerant cathode catalysts that are cheaper than Pt are of great importance in this field. In this chapter a short review analysis on Pt-free catalysts, mainly based on Pd and platinum group metal-free compounds with the general formula M–N–C (M being a transition metal such as Fe or Co) for the cathode, with good methanol-tolerance characteristics, has been carried out. Preparation procedures for the catalysts and physicochemical characteristics are reported and discussed in correlation with the DMFC performance.

Nanocarbon materials present sp2-carbon domains skilled for electrochemical energy conversion or storage applications. In this work, we investigate graphene oxide nanofibers (GONFs) as a recent interesting carbon material class. This material combines the filamentous morphology of the starting carbon nanofibers (CNFs) and the interlayer spacing of graphene oxide, and exhibits a domain arrangement accessible for fast transport of electrons and ions. Reduced GONFs (RGONFs) present the partial removal of basal functional groups, resulting in higher mesoporosity, turbostratic stacking, and surface chemistry less restrictive for transport phenomena. Besides, the filament morphology minimizes the severe layer restacking shown in the reduction of conventional graphene oxide sheets. The influence of the reduction temperature (140–220 °C) on the electrochemical behaviour in aqueous 0.5 M H2SO4 of RGONFs is reported. RGONFs present an improved capacitance up to 16 times higher than GONFs, ascribed to the unique structure of RGONFs containing accessible turbostratic domains and restored electronic conductivity. Hydrothermal reduction at 140 °C results in the highest capacitance as evidenced by cyclic voltammetry and electrochemical impedance spectroscopy measurements (up to 137 F·g−1). Higher temperatures lead to the removal of sulphur groups and slightly thicker graphite domains, and consequently a decrease of the capacitance.

Carbonaceous materials have always played an important role in history of electrochemical processes, from the fabrication of solid electrodes to the development of porous matrixes in the conversion and storage of energy and chemicals. This book chapter provides a general overview of emerging carbon nanostructures potentially used in electrochemistry. One-dimensional carbon nanofilaments, such as carbon nanotubes and carbon nanofibers, two-dimensional graphene, and three-dimensional materials, such as carbon gels and ordered mesoporous carbons, are reviewed. The chapter focuses on the physicochemical properties and synthesis methodologies of these carbon nanostructures together with their application to cutting-edge technologies for electrochemical energy conversion and storage, such as fuel cells, electrolyzers, supercapacitors, or batteries. The major challenges and directions for future research are also discussed to guide the design of efficient carbon-based electrodes and catalysts for electrochemical devices.

Alkaline unitized regenerative fuel cells (URFCs) could well be one of the energy sources in the next future. This technology has the potential to provide power to portable devices, transportation and stationary sectors. The bifunctional oxygen electrode plays a central role in catalytic activity and stability due to its slow kinetics under extremely oxidizing or reducing environments. With the aim to solve the principal catalytic problems at the bifunctional electrocatalysts, carbonaceous materials including carbon nanofibers (CNFs), graphitized mesoporous carbon (gCMK-3) and hydrazine-reduced graphene oxide (H-rGO) were synthesized, characterized and tested toward the oxygen evolution reaction (OER) and the oxygen reduction reaction (ORR) and compared to commercial carbon black (Vulcan), graphite, and glassy carbon (GC). Physicochemical characterization was conducted by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), elemental analysis and Raman spectroscopy, meanwhile rotating ring-disk electrode (RRDE) was employed to determine the electrochemical activity and stability. Main results indicate that CNFs can act as a feasible catalyst for URFC applications.