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The oxygen evolution reaction (OER) plays a paramount role in a variety of electrochemical energy conversion devices, and the exploration of highly active, stable, and low-cost electrocatalysts is one of the most important topics in this field. The exfoliated black phosphorus (EBP) nanosheet with a two-dimensional (2D) layered structure has high carrier mobility but is limited by excessive oxygen-containing intermediate absorption and fast deterioration in air. We here report the fabrication of nanohybrids of amorphous CoFeB nanosheets on EBP nanosheets (EBP/CoFeB). The 2D/2D heterostructure, thanks to the electronic interactions and oxygen affinity difference between EBP and CoFeB nanosheets, is capable of balancing the oxygen-containing intermediate absorption to an optimal status for facilitating the OER process. While the crystalline EBP contributes to the improved conductivity, the amorphous coating protects EBP and thus ensures the catalytic stability. The EBP/CoFeB electrocatalyst shows excellent OER performance with an ultralow overpotential of 227 mV at 10 mA cm-2 with an ultrasmall Tafel slope of 36.7 mV dec-1 with excellent stability. This study may inspire more researches to develop heterostructured nanohybrid electrocatalysts for a diversity of electrochemical reactions.

Reversible Solid Oxide Cells (r-SOCs) are an attractive electrochemical energy conversion technology that can act as both a Solid Oxide Fuel Cell (SOFC) for power generation, and a Solid Oxide Electrolysis Cell (SOEC) for steam electrolysis. Unfortunately, Ni-zirconia cermet, which is widely used in SOFCs, has the problem that the electronically conductive phases, Ni, agglomerates due to redox reactions and breaks the conductive path, resulting in performance degradation. In this study, we fabricated an alternative fuel electrode with a stable conductive structure using both an ionic conductor Ce0.9Gd0.1O2 (GDC) and an electronic conductor Sr0.9La0.1TiO3 (LST), and we co-impregnate Ni and GDC to the fuel electrode using only Ni as an electrocatalyst. The durability of the conventional fuel electrode and our alternative fuel electrode was compared and evaluated, and the potential of the alternative fuel electrode material was demonstrated.

The hybrid battery integrating a typical Zn redox battery and a Zn-air battery is a promising green technology for energy storage, and the cathode integrating the redox reaction and electrocatalytic oxygen reduction is a key point for efficient electrochemical energy conversion. Herein, we report a scalable strategy to fabricate nanoporous Ag2Al intermetallic compound as a self-standing cathode for the hybrid Zn battery. The abundant surface oxygen species, the Ag-Al intermetallic interaction and the np-Ag2Al@AgAlOx interface cooperatively contributed to the catalytic ORR activity. The electrode endows efficient catalytic oxygen reduction (a Tafel slope of 38.0 mV/dec and an onset potential of 0.998 V) and regulated redox activity as compared with Ag. The nanoporous channels allow efficient ion transport, interface charge exchange and gas molecular diffusion. Significantly, the assembled hybrid Zn-Ag2Al/air battery delivers a high capacity of 3.23 mAh/cm2 as compared with recent reports. As far as we know, this is the first exploration for the electrochemical property of Ag2Al, and it would inspire more exploration in developing multifunctional materials and robust hybrid batteries for practical applications.

The development of non-precious metal catalysts to replace scarce and costly Pt-based catalysts is crucial for oxygen reduction reaction (ORR), which involves various electrochemical energy conversions, such as fuel cells and metal-air batteries. However, it is still a challenge to devise a simpler method to obtain high-performance non-noble metal catalyst, which can be used for large-scale production. Herein, Cu and N co-doped mesocellular carbon foam (n-Cu/NC) has been developed by strong electrostatic adsorption (SEA) method, which inherits the original morphology of carbon foam, exhibiting high ORR performance with a half-wave potential of 0.825 V vs. RHE and excellent long-term durability. The outstanding performance of n-Cu/NC is attributed to the high specific surface area, hierarchical porous carbon matrix doped by multiple N atoms and uniform Cu2O/CuO active sites induced by the SEA method. The strategy of SEA method is applied to the preparation of carbon-supported catalysts, which shows great potential in the design and large-scale production of M and N co-doped carbon materials.

The oxygen reduction/evolution reactions (ORR/OER) are a key electrode process in the development of electrochemical energy conversion and storage devices, such as metal-air batteries and reversible fuel cells. The search for low-cost high-performance nanocarbon-based metal-free and non-precious metal bifunctional electrocatalysts for ORR/OER alternatives to the widely-used noble metal-based catalysts is a research focus. This review aims to outline the opportunities and available options for these nanocarbon-based bifunctional electrocatalysts. Through discussion of some current scientific issues, we summarize the development and breakthroughs of these electrocatalysts. Then we provide our perspectives on these issues and suggestions for some areas in the further work. We hope that this review can improve the interest in nanocarbon-based metal-free and non-precious metal bifunctional electrocatalysts for ORR/OER.

Characterization and electrochemical evaluation of novel metal-free electrocatalysts obtained by solar pyrolysis is reported. Carbon-based electrocatalysts were prepared from agave bagasse waste, using a sustainable process based on concentrated solar energy as heat source. Agave was processed in a spherical borosilicate glass solar reactor using a heating rate of 30 °C min−1 to a target of 500, 700, or 900 °C, and maintaining temperature for 1 h under inert atmosphere. The structure and composition of the prepared electrocatalysts were influenced by pyrolysis temperature. In addition, electrocatalytic activity towards the oxygen reduction reaction in 0.1 M KOH solution was explored. The electrocatalyst obtained at 500 °C showed the highest activity among all pyrolyzed samples due to its moderate surface area, but mostly due to its higher oxygen content. The metal-free electrocatalysts reported in this work are promising eco-friendly alternative as cathode materials for anion-exchange membrane fuel cells. This study provides a sustainable approach to use agricultural biomass waste to produce valuable materials for electrochemical energy devices.

Electrochemical energy technologies such as fuel cells, supercapacitors, and batteries are some of the most suitable energy storage and conversion devices to meet our needs proving the future generation’s equitable opportunity to meet their own needs. For this purpose, an earth-abundant precursor such as biomass is the best candidate for the synthesis of the next generation of low-cost and green electrode materials. This review summarizes the most recent progress in biomass-derived carbons for use in fuel cells, supercapacitors and lithium-ion batteries, the physical-chemical properties, desired features, performances, and limitations for electrochemical energy technologies. Several thermochemical treatments such as chemical activation, template methods, doping and hydrothermal treatments have been reviewed. Finally, we provide the reader with comprehensive information of the challenges, future research efforts, advantages, limitations and opportunities which will be a fundamental insight for the future design of biomass-derived carbon electrode materials for electrochemical storage and conversion systems.

The synthesis of nonprecious electrocatalysts for oxygen electrocatalysis is of considerable interest for the development of electrochemical energy devices. Herein, we demonstrate a facile approach for the synthesis of bamboo-like nitrogen-doped carbon nanotube-encapsulated Co0.25Ni0.75 alloy electrocatalyst (Co0.25Ni0.75@NCNT) and its bifunctional oxygen electrocatalytic performance toward oxygen reduction and oxygen evolution reactions. The Co0.25Ni0.75 alloy wrapped with NCNT is obtained by a one-step carbothermal reduction approach using dicyandiamide and NiCo-MOF precursors. Dicyandiamide acts as a nitrogen source, and the in situ generated Co0.25Ni0.75 alloy nanoparticles catalyze the growth of bamboo-like NCNTs. The hollow NiCo-MOF plays a sacrificial role in providing a suitable environment for the controlled growth of Co0.25Ni0.75 alloy and NCNT. Co0.25Ni0.75@NCNT efficiently catalyzes both the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER) at a favorable overpotential. It shows a low potential gap (ΔE) of ∼0.8 V between the two reactions, and it qualifies for the development of air cathode in metal-air batteries. The enhanced bifunctional activity and excellent durability stem from the chemical composition and the synergistic effect between Co0.25Ni0.75 alloy and encapsulating NCNT. The original phase and morphology of the catalyst is preserved after an extensive durability test. Aqueous rechargeable Zn-air battery (ZAB) is fabricated using a Co0.25Ni0.75@NCNT-based air cathode. The battery has high open-circuit voltage (1.53 V) and a maximum peak power density of 167 mW cm-2 with only 1.6% loss in the voltaic efficiency after 36 h charge-discharge cycles. As a proof-of-concept demonstration, the as-fabricated ZAB is successfully used for the electrochemical water splitting in alkaline solution.

Bipolar membranes (BPMs) are gaining interest in energy conversion technologies. These membranes are composed of cation- and anion-exchange layers, with an interfacial layer in between. This gives the freedom to operate in different conditions (pH, concentration, composition) at both sides. Such membranes are used in two operational modes, forward and reverse bias. BPMs have been implemented in various electrochemical applications, like water and CO2 electrolyzers, fuel cells, and flow batteries, while BPMs are historically designed for acid/base production. Therefore, current commercial BPMs are not optimized, as the conditions change per application. Although the ideal BPM has highly conductive layers, high water dissociation kinetics, long lifetime, and low ion crossover, each application has its own priorities to be competitive in its field. We describe the challenges and requirements for future BPMs, and identify existing developments that can be leveraged to develop BPMs toward the scale of practical applications.

Oxygen evolution reaction (OER), as the primary anodic reaction, plays a critical role in many electrochemical energy conversion processes. As the state-of-the-art OER catalysts, iridium-based materials are largely hindered from practical applications mainly due to the extreme scarcity of iridium. Here we demonstrate the successful fabrication of boron-doped amorphous iridium oxide (IrOx-B) via a facile boric acid-assisted method, which realizes an ultrahigh OER mass activity of 2779 A g Ir −1 at 300 mV overpotential, representing one of the best acidic OER catalysts reported so far. It is found that boric acid can not only facilitate the exposure of Ir, but also dope the amorphous IrOx with a form of metaborate, which could further modify the electronic and local ligand structure of Ir for the improved intrinsic activity. Interestingly, the reported strategy is universal that can be applied to improve other metal oxide OER catalysts, highlighting a versatile strategy for creating high-performance electro-catalysts with ultrahigh mass activity for OER and beyond.

Hierarchical 3-dimensional (3D) carbon nanostructures have potential applicability in several electrochemical energy applications own to their sizeable effective surface, specific energy density, cycling stability and flexibility. Herein, combining electrospinning with in situ chemical vapor deposition (CVD) technologies as an efficient and secure procedure is applied for constructing 3D carbon nanostructure-based on carbon fibers (CFs) and electrospun carbon nanofibers (ECNFs). Affect the catalysis type, immersion solution, carbon sources and treat temperature are experimentally demonstrated. Cactus-like 3D carbon nanomaterials are constructed via growth forest-like carbon nanofibers (CNFs) directly on both CNFs and CFs at 900 °C using Ni as a catalyst, cellulose acetate (CA) as carbon source and ethanol-urea as an immersions solution. The Ni/CNFs/ECNFs mat exhibits a large surface area of 342.3 m2 g−1, while just 19.3 m2 g−1 is recorded to Ni/CNFs/CFs. Based on the nano-nonwoven structure and forest-like grown CNFs nanostructure, Ni/CNFs/ECNFs exhibit a favorable hydrogen evolution reaction (HER) performance in an alkaline medium with a low overpotential of 88 mV to deliver 10 mV cm−2 current density and Tafel slope of 170 mV dec−1. This work proves the synthesis parameters of 3D hierarchical carbon nanostructures and their enticing to apply as an advanced substrate for electrochemical applications.

Engineering of nanostructured materials with a unique and uniform morphological design is considered to be a competent candidate for a diverse range of electrochemical energy applications. However, the construction of ball-in-ball transition metal sulfide via the vaporization process remains a big challenge of today's research community. In this work, carbon encapsulated nickel sulfide (Ni3S2@C) with a complex hollow interior was synthesized and further studied with nitrogen-doped carbon encapsulated nickel sulfide (Ni3S2@NC) for comparison basis. We used metal salt (nickel nitrate) and organic linker (trimesic acid, TMA) as a precursor to synthesize Ni-TMA via a solvothermal method. Next, the sulfidation process was done under 5% H2 balanced argon gas in a tubular furnace to convert into Ni3S2 with encapsulation of carbon. The prepared hybrid material showed ball-in-ball morphology and heazlewoodite mineral phase structure. Furthermore, the prepared materials were examined for electrochemical energy storage properties owing to their well-known faradaic dominant (battery-type) charge storage features and key positive electroactive material for today's growing hybrid electrochemical capacitor. The nitrogen-doped carbon encapsulated nickel sulfide has demonstrated good electrochemical features like a specific capacity of 442.45 F g−1 at 5 mV s−1 of scan rate and 373.52 F g−1 at 2 A g−1 of current density. Moreover, the material showed excellent capacitance retention of 81% and 70% after 5000 and 10,000 cycles run with 100% of coulombic efficiency at 8 A g−1.

Global increasing demand in the need of energy leads to the development of non-conventional, high power energy sources. Supercapacitors (SCs) are one of the typical non-conventional energy storage devices which are based on the principle of electrochemical energy conversion. SCs are promising energy storage devices for better future energy technology. Increasing progress has been made in the development of applied and fundamental aspects of SCs. Manganese oxide electrode materials have been well studied; however, their capacitive performance is still inadequate for practical applications. Recent research is mainly focused on enhancing manganese oxide capacitive performance through the incorporation of electrically conductive materials and by controlling its morphology to reveal a more active surface area for redox reactions. In this review, progress in the applications of manganese oxide carbon-based materials towards the development of highly effective SCs is briefly discussed. In this regard, manganese oxide carbon-based nanocomposites synthesis methods and techniques used to approximate the capacitance of electrode materials are discussed.

Photoelectrochemical properties have been investigated for flexible photoelectrodes containing 310 nm thick ZnO film on spin-coated ITO/PET. The high crystalline structure of ZnO was studied using x-ray diffraction pattern. A value of 3.4 eV has been estimated for optical band gap from its absorption spectra. The flexible ZnO photoelectrode was demonstrated to generate photoelectrochemical current. The photocurrents are enhanced by 4% whereas flat-band potential is shifted by 8 V due to the illumination. Values of 1.022 and 0.714 AW−1 were found to be for photo switching and photoresponsivity, respectively. ZnO/ITO/PET can be used as a substrate for making flexible hybrid PEC devices to generate solar power and solar fuels.

Energy conversion and storage in a Li-ion cell involves multiple closely coupled transport processes, such as species diffusion through solid and solution phases in the electrode. Mathematical modeling of these processes is critical for fully understanding and optimizing the performance of a Li-ion cell. While a number of analytical and numerical models have been presented for solution phase diffusion, most of such work is based on the assumption of a constant current density. This paper presents analytical modeling of solution phase diffusion in a separate–electrode composite for a generalized, time-dependent current density. An analytical solution for the concentration field in a separator–electrode composite in such conditions is derived using the method of eigenfunction expansion. Good agreement with past work as well as numerical simulations is shown. Results for linear, periodic and step-function boundary conditions are discussed. The theoretical analysis presented here may help accurately model realistic processes where the applied current changes over time, for example, cyclic charge and discharge in an electric vehicle, or sudden changes in the battery load. Results presented here contribute towards the fundamental understanding of solution phase diffusion in Li-ion cells, and provide a basis for improving electrochemical energy conversion and storage processes.

Nitrogen-doped carbon dots (N-CDs), derived from the biomass (anthocyanin), are the novel additive to the nanocarbon materials, which is expected to bring a wide spectrum of novel applications. Moreover, metallic oxides are emerging for their unique potential for electrocatalysis. Herein, we report the synthesis of N-CDs for the selective detection of Fe3+ with a limit of detection of 2.57 μM in the range of 5–60 μM using ethylenediamine and H2O2 by a hydrothermal method. The obtained N-CDs displayed a spherical morphology with a particle size range of 2–7 nm and emitted blue luminescence at 394 nm under excitation at 319 nm. Meanwhile, we have demonstrated the fabrication of cost-efficient electrocatalysts for oxygen evolution reaction (OER) in an alkaline medium, employing N-CDs. Owing to the successful incorporation of N-CDs into NiO nanospheres, the resulting N-CDs/NiO with large surface areas, fast charge transfer, and increased conductivity vastly improved the catalytic activity. Remarkably, the optimal of N-CDs/NiO composite requires the overpotential of only 380 mV at a current density of 10 mA cm−2 and a relatively low Tafel slope of 57.96 mV dec−1 compared with pure NiO. These results open up a facile route for the application of N-CDs and offer prospects for CD-metal hybrids as high OER catalysts in electrochemical energy devices.

The conductive metal-organic frameworks (MOFs) are suggested as the ideal electrocatalysts for hydrogen evolution reaction (HER) because of the high utilization of metal atoms. Rational design and facile synthesis of MOFs with large specific surface area, proper metals as center, and tunable chemical components is still full of challenges. Herein, we report the facile synthesis three types of porous MOFs by regulating metal center using benzene-1,3,5-tricarboxylic acid (H3BTC) as organic ligand and have successfully synthesized the rhombic octahedral Cu-BTC, rod-shaped Co-BTC and spherical Ni-BTC materials with large specific surface area ranged in 350-500 m2 g−1. These as-prepared MOFs materials exhibit high performance of HER in 0.5 M H2SO4. Ni-BTC material exhibits the lowest overpotential of 53 mV at 10 mA cm-2 and the smallest Tafel slope of 62 mV dec−1 than those of Cu-BTC (270 mV, 155 mV dec−1) and Co-BTC (123 mV, 100 mV dec−1), which are much superior to these previously reported MOFs catalysts. In addition, the fast catalytic kinetic of Ni-BTC was confirmed by the smaller charge transfer resistance (Rct) value of 0.9 Ω and larger electrochemical active surface area (ECSA) of 35.5 cm2 than those of Cu-BTC (8.2 Ω, 22.5 cm2) and Co-BTC (1.9 Ω, 27.7 cm2). Because of the structural advantage and large ECSA, the turnover frequency (TOF) value of Ni-BTC reaches up to 0.041 s−1 at 120 mV overpotential, which is 20.5 and 2.6 times greater than that of Cu-BTC (0.002 s−1) and Co-BTC (0.016 s−1). Besides, these three types of MOFs exhibited excellent durability over 12 h. This study unfolds diverse insights into the design and facile synthesis of MOFs for electrochemical energy conversion system.

Biomass-derived activated carbon has attracted much attention in electrochemical energy systems, and its great potential can be further unlocked for achieving high-performance by tuning its composi...

Strain engineering is widely employed to manipulate the intrinsic relationship of activity and the crystal structure, while the mechanism and rational strategy toward high-performance devices are still under investigation. Here straining engineering is utilized to manipulate a series of a typical perovskite structures via introducing different types of heteroions (Bi1-xMxFeO3, M = Ca2+ or Y3+ ion). The space group R3c in BiFeO3 perovskites is found to be maintained with substituting a certain amount of heteroions at Bi3+ sites ( 5%). Such a transformation is linked with the mismatched crystal strain induced by the heteroions substituted at Bi3+ sites, while the activity, stability, and energy storage capability of Bi1-xMxFeO3 have been essentially varied. The results offer a strategy for manipulating stability and activity of perovskites in electrochemical energy conversion and storage.

Herein, synthesis of graphitic carbon nitride (g-C3N4) and its nanocomposites with monometallic nickel (Ni-g-C3N4), and bimetallic zirconium-nickel (ZrNi-g-C3N4) is reported. New materials for high storage performance are investigated eagerly in electrochemical energy applications. With this aim, the electrochemical method is used for the evaluation of hydrogen storage capacity (HSC) of the samples in the present research work. Synthesis of samples were carried out using a combination of thermal and hydrothermal methods. The evaluation of electrochemical hydrogen sorption and storage capability of g-C3N4 and its nanocomposites was done after sample characterization. Characterization techniques followed were powder X-ray diffraction (XRD), Nitrogen adsorption and desorption isotherms measurements at 77 K, thermo-gravimetric analysis (TGA), Fourier transform infrared spectroscopy (FTIR), Field-emission scanning electron microscopy (FESEM) and energy dispersive X-ray spectrometry (EDS). Cyclic voltammetric and chronopotentiometry measurements carried out in an alkaline medium helped to calculate the HSC. The discharge capacity was measured to be 82 mAhg−1 for g-C3N4, 182 mAhg−1 for Ni-g-C3N4 and 380 mAhg−1 for ZrNi-g-C3N4 at 1 mAg−1 of current density. Catalytic properties of the metal nanoparticles augment the storage capability of the g-C3N4 nanocomposites. These nanocomposites help to store hydrogen which serves as safe carrier in fuel cell applications.

The development of efficient and low-cost electrocatalysts toward the oxygen evolution reaction (OER) is critical for improving the efficiency of several electrochemical energy conversion and storage devices. Here, we report an elaborate design and synthesis of porous Co-based trimetallic spinel oxide nanoboxes (NiCo2-x Fex O4 NBs) by a novel metal-organic framework engaged strategy, which involves chemical etching, cation exchange, and subsequent thermal oxidation processes. Owing to the structural and compositional advantages, the optimized trimetallic NiCo2-x Fex O4 NBs (x is about 0.117) deliver superior electrocatalytic performance for OER with an overpotential of 274 mV at 10 mA cm-2 , a small Tafel slope of 42 mV dec-1 , and good stability in alkaline electrolyte, which is much better than that of Co-based bi/monometallic spinel oxides and even commercial RuO2 .

Major research efforts are being carried out for the technological advancement to an energetically sustainable society. However, for the full commercial integration of electrochemical energy storage devices, not only materials with higher performance should be designed and manufactured but also more competitive production techniques need to be developed. The laser processing technology is well extended at the industrial sector for the versatile and high throughput modification of a wide range of materials. In this work, a method based on laser processing is presented for the fabrication of hybrid electrodes composed of graphene nanowalls (GNWs) coated with different transition-metal oxide nanostructures for electrochemical capacitor (EC) applications. GNW/stainless steel electrodes grown by plasma enhanced chemical vapor deposition were decorated with metal oxide nanostructures by means of their laser surface processing while immersed in aqueous organometallic solutions. The pseudocapacitive nature of the laser-induced crystallized oxide materials prompted an increase of the GNW electrodes’ capacitance by 3 orders of magnitude, up to ca. 28 F/cm3 at 10 mV/s, at both the positive and negative voltages. Finally, asymmetric aqueous and solid-state ECs revealed excellent stability upon tens of thousands of charge–discharge cycles., The authors are grateful for the financial support of the Spanish Ministry of Economy, Industry, and Competitiveness under the projects ENE2017-89210-C2-1-R and ENE2017-89210-C2-2-R and support from AGAUR of Generalitat de Catalunya through projects 2017 SGR 1086. ICMAB acknowledges the financial support from the Spanish Ministry of Economy and Competitiveness, through the ‘Severo Ochoa’ Programme for Centres of Excellence in R&D (CEX2019-000917-S). Y.E.B acknowledges the financial support from CONACyT—Mexico through the postdoctoral scholarship granted 740661. P.G.L thanks the financial support of the Spanish Ministry of Economy, Industry, and Competitiveness through the grant BES-2017-081652 for the formation of scientific researchers. The authors acknowledge the support of the publication fee by the CSIC Open Access Publication Support Initiative through its Unit of Information Resources for Research (URICI).

The unsatisfactory cycle life of nickel-based cathodes hinders the widespread commercial usage of nickel–zinc (Ni–Zn) batteries. The most frequently used methods to improve the cycle life of Ni-based cathodes are usually complicated and/or involve using organic solvents and high energy consumption. A facile process based on the hydrolysis-induced exchange of the cobalt-based metal-organic framework (Co-MOF) was developed to prepare aluminum (Al)-doped cobalt–nickel double hydroxides (Al-CoNiDH) on a carbon cloth (CC). The entire synthesis process is highly efficient, energy-saving, and has a low negative impact on the environment. Compared to undoped cobalt–nickel double hydroxide (Al-CoNiDH-0%), the as-prepared Al-CoNiDH as the electrode material displays a remarkably improved cycling stability because the Al-doping successfully depresses the transition in the crystal phase and microstructure during the long cycling. Benefiting from the improved performance of the optimal Al-CoNiDH electrode (Al-CoNiDH-5% electrode), the as-constructed aqueous Ni–Zn battery with Al-CoNiDH-5% as the cathode (Al-CoNiDH-5%//Zn) displays more than 14% improvement in the cycle life relative to the Al-CoNiDH-0%//Zn battery. Moreover, this Al-CoNiDH-5%//Zn battery achieves a high specific capacity (264 mAh g−1), good rate capability (72.4% retention at a 30-fold higher current), high electrochemical energy conversion efficiency, superior fast-charging ability, and strong capability of reversible switching between fast charging and slow charging. Furthermore, the as-assembled quasi-solid-state Al-CoNiDH-5%//Zn battery exhibits a decent electrochemical performance and satisfactory flexibility.

Developing earth-rich and high-efficiency nonprecious metal catalysts is critical but extremely challenging for oxygen evolution reaction (OER). Herein, a simple room-temperature sulfuration method was developed for in situ synthesis of walnut kernel-like iron-cobalt-nickel sulfide nanosheets on nickel foam (FeCoNiSx/NF). The unique nanosheets exposed abundant active sites and provided large electrochemically active surface area. The as-built FeCoNiSx/NF exhibited high OER performances with the small overpotentials of 231 and 268 mV to afford 10 and 50 mA cm−2 in 1.0 M KOH, respectively, coupled with a small Tafel slope of 55 mV dec−1. Furthermore, the FeCoNiSx/NF acted as the anode towards overall water electrolysis with acceptable results, where commercial Pt/C dropped on the NF worked as the cathode. This study provides some valuable insights for rational construction of nonprecious electrocatalysts in electrochemical energy technologies.

Bi2O3 is a strong candidate as the optimal aqueous battery anode material, but suffers from its poor stability and rate performance (a result of fragile structure and poor conductivity). Herein, two self-derived Bi species, interstitial-Bi liberated from Bi2O3 unit cell and colonized surface nano-Bi, function synergistically in the Bi2O3 by creating a new electronic state in the energy gap and tuning the Bi/Bi2O3 interface coordination. As a result, electronic conductivity, ionic diffusion, and structural stability are fully enhanced, leading to a package of optimal electrochemical performances. Our self-enhanced strategy may thereof serve in design of functional oxide complexes for electrochemical energy conversion and storage.

Cobalt oxide has been widely investigated among potential transition metal oxides for the electrochemical energy conversion, storage, and water splitting. However, they have inherently low electronic conductivity and high corrosive nature in alkaline media. Herein, we propose a promising and facile approach to improve the conductivity and charge transport of cobalt oxide Co3O4 through chemical coupling with well-dispersed multiwall carbon nanotubes (MWCNTs) during hydrothermal treatment. The morphology of prepared composite material consisting of nanosheets which are anchored on the MWCNTs as confirmed by scanning electron microscopy (SEM). A cubic crystalline system is exhibited by the cobalt oxide as confirmed by the X-ray diffraction study. The Co, O, and C are the only elements present in the composite material. FTIR study has indicated the successful coupling of cobalt oxide with MWCNTs. The chemically coupled cobalt oxide onto the surface of MWCNTs composite is found highly active towards oxygen evolution reaction (OER) with a low onset potential 1.44 V versus RHE, low overpotential 262 mV at 10 mAcm-2 and small Tafel slope 81 mV dec-1. For continuous operation of 40 hours during durability test, no decay in activity was recorded. Electrochemical impedance study further revealed a low charge transfer resistance of 70.64 Ohms for the composite material during the electrochemical reaction and which strongly favored OER kinetics. This work provides a simple, low cost, and smartly designing electrocatalysts via hydrothermal reaction for the catalysis and energy storage applications.

Temperature and humidity measurements in electrochemical energy devices are essential for maximizing their overall performance under different operating conditions and avoiding hazardous consequences that may arise from the malfunction of these systems. Using sensors for in situ measurements of temperature and relative humidity (RH) is a promising approach for continuous monitoring and management of electrochemical power sources. Here, we report on the feasibility of using thread-based sensors for in situ measurements of temperature and RH in proton exchange membrane fuel cells (PEMFCs) as an example of electrochemical energy devices. Commodity threads are low-cost and flexible materials that hold great promise for the creation of complex three-dimensional (3D) circuits using well-established textile methods such as weaving, braiding, and embroidering. Ex situ and in situ characterization show that threads can be introduced in the gas diffusion layer (GDL) structure to inscribe water highways within the GDL with minimal impact on the GDL microstructure and transport properties. Fluorinated ethylene propylene (FEP) is coated on thread-based sensors to decouple the response to temperature and humidity; the resulting threads achieve a linear change of resistance with temperature (−0.31%/°C), while RH is monitored with a second thread coated with poly(dimethylsiloxane) (PDMS). The combination of both threads allows for minimally invasive and dynamically responsive monitoring of local temperature and RH within the electrode of PEMFCs.

Poly (2-acetyl pyrrole) P (2-APy) has been prepared chemically using ammonium persulfate (APS) as an initiator under high stirring at room temperature and pH (3.5). The formed P(2-APy) has been fully characterized via Fourier transform infrared (FTIR) spectroscopy, scanning electron microscopy (SEM), transmission electron microscopy (TEM) as well as UV–VIS spectroscopy. The morphology of the formed polymer has shown core–shell of a ball-like structure in the range of nanoscale. The optical energy gap Eopt and the electrochemical energy gap Eg for the resultant polymer have been determined via UV–Vis spectroscopy and cyclic voltammetry, respectively. The ac conductivity measurements have been also investigated. The results showed enhanced optical as well as electrical properties for the formed polymer which can be further exploited for optoelectric devices.

Developing efficient catalysts toward the oxygen evolution reaction(OER) is important for water splitting and rechargeable metal-air batteries. Although NiFe oxides are considered as potentially applicable catalysts in the alkaline media, there are still a limited numbers of researches working on membrane electrode assembly(MEA) fed with pure water due to their poor electrical conductivity. In this work, antimony doped tin oxide(ATO) has been employed as conductive supports where NiFe layered double hydroxide uniformly dispersed[named NiFe-LDH(layered double hydroxide)/ATO]. The catalysts have been synthesized by a one-step co-precipitation method, and then NiFe-LDH/ATO-air plasma was obtained through mild air plasma treatment. According to XPS analysis, binding energies of Ni2p and Fe2p were shifted negatively. Moreover, a new signal of low oxygen coordination appeared on O1s spectrum after air plasma treatment. These XPS results indicated that oxygen vacancies(Ov) were generated after air plasma treatment. Electrochemical measurement indicated that the vacancy-rich NiFe-LDH/ATO-air plasma exhibited better performance than NiFe-LDH/ATO not only in 1 mol/L KOH solutions but also in an alkaline polymer electrolyte water electrolyzer(APEWE) fed with deionized water. This work provides a feasible way to design practical catalysts used in electrochemical energy conversion systems by choosing corrosion resistance supports and defect engineering.

Solid oxide fuel cells (SOFCs) have attracted great interest as an alternative potential way to become the most efficient and cleanest electrochemical energy conversion system. The commercialization of SOFC technology is hindered by the degradation of component materials. The durable and high performing cathode materials is of immense importance in the durability improvement of SOFCs. Cobaltite type perovskite-based oxides have shown remarkable results but cation migration and formation of the insulating phases within and near the interface between cathode and electrolyte is often observed, which impacts greatly on the electrochemical performance and durability. Therefore, the reaction barrier layer (interlayer) typically made of doped ceria is required between cathode and electrolyte. The stability of this layer due to cations cross-migration between cathode and electrolyte and interdiffusion with electrolyte during fabrication and operation is presently one of the foremost issues (motivation) in the SOFC industry. The chemical and structural disparity associated with the cations migration and interdiffusion could affect the stability and functionality of different layers of SOFC. Understanding the formation of secondary phases and their evolution during the operating lifespan is thought-provoking because of the complexity of the system and the occurrence of numerous other processes simultaneously. In this review paper, the recent progress and advancement in this extent are presented, emphasizing the key driving forces, kinetics, analysis techniques at the micro- and nano-scale levels, and cations migration in extensively studied perovskite-based materials. An insightful understanding of the interdiffusion phenomenon taking place in the cathode/electrolyte/interlayer of SOFCs and control measures are then highlighted which is important to achieve the rational design of highly efficient SOFC with outstanding stable performance.

Water splitting via an electrochemical process to generate hydrogen is an economic and green approach to resolve the looming energy and environmental crisis. The rational design of multicomponent materials with seamless interfaces having robust stability, facile scalability, and low-cost electrocatalysts is a grand challenge to produce hydrogen by water electrolysis. Herein, we report a superhydrophilic homogeneous bimetallic phosphide of Ni2P-CuP2 on Ni-foam-graphene-carbon nanotubes (CNTs) heterostructure using facile electrochemical metallization followed by phosphorization without any intervention of metal-oxides/hydroxides. This bimetallic phosphide shows ultralow overpotentials of 12 (HER, hydrogen evolution reaction) and 140 mV (OER, oxygen evolution reaction) at current densities of 10 and 20 mA/cm2 in acidic and alkaline mediums, respectively. The excellent stability lasts for at least for 10 days at a high current density of 500 mA/cm2 without much deviation, inferring the practical utilization of the catalyst toward green fuel production. Undoubtedly, the catalyst is capable enough for overall water splitting at a very low cell voltage of 1.45 V @10 mA/cm2 with an impressive stability of at least 40 h, showing a minimum loss of potential. Theoretical study has been performed to understand the reaction kinetics and d-band shifting among metal atoms in the heterostructure (Ni2P-CuP2) that favor the HER and OER activities, respectively. In addition, the catalyst demonstrates an alternate transformation of solar energy to green H2 production using a standard silicon solar cell. This work unveils a smart design and synthesizes a highly stable electrocatalyst against an attractive paradigm of commercial water electrolysis for renewable electrochemical energy conversion.

Metal-organic frameworks (MOFs), featuring porous crystalline structures with coordinated metal nodes and organic linkers, have recently found increasing interest in diverse applications. By virtue of their versatile and highly tunable compositions and structures, constructing hollow architectures will further endow MOFs with enhanced properties and designability, exceeding the molecular scale. MOFs could be considered as promising building units to fabricate complex hollow nanocomposites with faster mass transport, multiple active components, more exposed active sites, and better compatibility than bulk MOFs. To construct a promising blueprint for hollow pristine MOFs, this review provides a comprehensive overview for structural design strategies and applications of hollow pristine MOFs. We will highlight the merits, challenges and future potential by structuring and applying MOFs in sensing, separation, storage, catalysis, environmental remediation, photochemical and electrochemical energy conversion. This review might pave a new avenue for future development of novel pristine hollow MOFs.

Electrode materials with high conductivity and high mass transport rate are highly desirable for a variety of electrochemical energy devices but face a grand challenge to be readily prepared yet. Here, we propose the design and preparation of a nanohybrid of V2O3 nanoparticles embedded in a multichannel carbon nanofiber (V2O3@MCNF) network with high conductivity and high mass transport. We demonstrate the V2O3@MCNF shows superior capability for sodium storage with an excellent capacity of 214.3 mA h g-1 even at 5 A g-1, thanks to its high conductivity for electron transfer and facilitated mass transportation endowed by the one-dimensional conductive multichannel fiber structure. Such favorable structures and properties in V2O3@MCNFs enable them to be applied as high-performance anodes of sodium-ion hybrid capacitors (SIHCs), successfully addressing the critical kinetics imbalance between Faradaic anodes and capacitive cathodes for application of SIHCs, which show impressively high energy/power densities along with impressive cycling performance over 10,000 cycles.

Nickel (Ni) and its (oxy)hydroxide species are an important class of materials that attract significant attention from the scientific community due to their application in alkaline electrochemical energy technologies. Despite their increasing importance, Ni electrochemistry and electrocatalysis are not very well understood. In this article, we contribute to the understanding of the interfacial electrochemical phenomena occurring at polycrystalline Ni electrodes by conducting the first ever systematic analysis of the electrochemical reduction of β-Ni(OH)2. The procedure for the reduction of surface β-Ni(OH)2 is as follows. First, a cyclic voltammetry (CV) measurement is performed in the potential region of β-Ni(OH)2 formation to grow a reproducible hydroxide layer, followed by a linear sweep voltammetry (LSV) measurement conducted between an upper limit potential of Eu = 0.00 V and a lower limit potential (El) ranging between −0.15 V and −0.60 V. The LSV measurement is carried out using very low potential scan rates (s = 0.10, 0.20, 0.50, and 1.0 mV s−1). The analysis of the electrochemical reducibility of the surface β-Ni(OH)2 layer is accomplished by comparing the CV transient of a freshly polished Ni electrode and the CV transient recorded after the LSV experiment that is designed to reduce the β-Ni(OH)2 layer. The influence of different experimental parameters of the above-described procedure is analyzed independently, namely, (i) the lower potential limit of the LSV measurement, (ii) the potential scan rate of the LSV measurement, (iii) the direction of the LSV measurement (anodic or cathodic), (iv) the thickness of the surface layer of β-Ni(OH)2, and (v) the cathodic polarization of the electrode for different polarization times. Our findings demonstrate that, contrary to an existing belief, the reduction of β-Ni(OH)2 can be achieved electrochemically. Features characteristic of the formation and reduction of α-Ni(OH)2 are observed in the CV transients acquired after employing the procedure for the electrochemical reduction of β-Ni(OH)2, indicative that a metallic Ni surface is regenerated. The analysis of the charge density value of the anodic peak in the CV transient also points to the electrochemical reduction of β-Ni(OH)2 to metallic Ni. These findings lead to an updated version of the Bode diagram and might have significant implications for alkaline electrochemical energy technologies using Ni-based materials.

Electrochemical energy systems such as fuel cells and metal–air batteries can be used as clean power sources in the field of electric transportation and possess great potential in the reduction of various energy and environmental issues. In these systems, the oxygen reduction reaction (ORR) at the cathode is the rate-determining factor for overall system performance, and up to now, platinum group metals supported on carbon materials, especially Pt, remain the highest performing and the most practical ORR electrocatalysts. However, corresponding carbonaceous catalyst supports are extremely susceptible to corrosion under electrochemical operation, and therefore, the extensive exploration of alternative stable materials for ORR electrocatalysts with both high electrochemical stability and catalytic performance is essential. Here, noncarbon materials with high corrosion resistance have been explored to substitute traditional carbon supports or even act directly as low-cost non-noble metal electrocatalysts, and based on this, this review will present a comprehensive overview and deep analysis of the recent progress in noncarbon materials, including metals, oxides, nitrides, carbides, sulfides, and so on. Overall, general attributes associated with noncarbon materials include high corrosion resistance, strong metal–support interaction, and impressive porous structure retention. However, major drawbacks include low electrical conductivity, insufficient chemical stability in acidic or alkaline media, and poor electrochemical stability at ORR electrode potentials. To overcome these challenges, this review will also summarize efficient strategies such as combining with highly conductive materials, introducing dopants and forming vacancies to result in promising electrocatalytic ORR performances. Finally, this review will propose possible research directions to facilitate future research and development toward the practical application of noncarbon-based ORR electrocatalysts.

Transition metal hydroxides (TMHs) nanosheets have attracted wide attention in electrochemical energy storage and conversion because of their superior surface area, highly tunable composition, and low cost. Moreover, the self-supported electrode has been extensively studied for electrochemical devices due to its fast electron transfer and mass transport, resulting in enhanced stability and electrode performance. Hence, reviewing the recent advances in self-supported TMHs nanosheets is crucial for developing high-performance electrodes for electrochemical devices. In this review, we first introduce the fundamental properties of TMHs in terms of layered single metal hydroxides (LSHs) and layered double hydroxides (LDHs). Then, we review various synthetic approaches utilized to construct self-supported TMHs nanosheets with tunable compositions and structures. Afterwards, the electrode performance and durability of self-supported TMHs nanosheets in various electrochemical applications (water electrolysis, zinc-air battery and supercapacitor) are comprehensively summarized. Finally, the further perspectives on current challenges and research directions of self-supported TMHs nanosheets towards electrochemical energy storages and conversion applications are proposed.

Nesta tese são deduzidas equações por meio das quais é possível simular as curvas características de supercapacitores eletroquímicos simétricos à base de carbono e eletrólito orgânico por meio da análise no domínio do tempo dos circuitos elétricos 2RC, 2RLC e 2R(C+kUC(t)), os quais, em acordo com o tipo de curva a ser simulada, podem ser carregados (alimentados) por uma fonte de potencial elétrico constante no tempo ε, ou por uma fonte de corrente contínua I, ou então por uma fonte de potencial elétrico triangular ε(t). Também são deduzidas nesta tese equações por meio das quais é possível simular os efeitos da indutância de entrada L nos diagramas de Bode do módulo da impedância complexa Z, de Bode de -θ e de Nyquist gerados pela avaliação de um supercapacitor eletroquímico simétrico por meio da técnica de espectroscopia de impedância eletroquímica, sendo tais equações deduzidas por meio da análise no domínio do tempo considerando-se que o supercapacitor eletroquímico simétrico pode ser representado pelos circuitos elétricos 2RC, 2RLC, 2R(C+kUC(t)) e 2RL(C+kUC(t)), cada qual alimentado por uma fonte senoidal ε(t) que gera entre seus terminais o potencial elétrico ε sen(ωt) (V) com diferentes valores de frequência angular ao longo do tempo. In this Thesis, equations are presented that make it possible to simulate the characteristic curves of symmetrical electrochemical supercapacitors based on carbon and organic electrolyte through the analysis in the time domain of the 2R(C+kUC(t)) electrical circuit, which, in accordance with the type of curve to be simulated, can be powered by a source of constant electric potential in time ε, or powered by a source of direct current I, or powered by a source of electromotive force ε(t) that generates between its terminals a time-varying electric potential, which is governed by a triangular signal (for the latter case, the 2RC electric circuit was also used for the case in which the capacitance index dependent on the electric potential k is null). Equations are also presented in this thesis that make it possible to simulate the effects of the input inductance L in the Nyquist diagram, the Bode diagram of the complex impedance module and the Bode diagram of the negative of the angle θ of the angular lag between the intensity of the electric current i1(t) present in the terminals of a symmetrical electrochemical supercapacitor based on carbon and organic electrolyte and the electric potential U(t) existing between its terminals when its evaluation by means of electrochemical impedance spectroscopy technique, being such equations obtained through the analysis in the time domain of the electrical circuit 2RL(C+kUC(t)) powered by a sinusoidal source ε(t) that presents different frequency values over time.

Nickel (Ni) is an important catalytic metal that finds application in electrochemical energy technologies, such as alkaline water electrolyzers (AWEs) and alkaline fuel cells (AFCs). An essential property that determines nickel’s applicability in AWE and AFC technologies is its stability under operating conditions, i.e., performance at extreme potential (E) conditions and in the presence of reactive gases. However, the electrochemical and electrocatalytic behavior of nickel is not as well understood as that of noble electrocatalytic metals such as platinum. This lack of knowledge needs to be addressed prior to dedicating significant efforts to the design, fabrication, and characterization of Ni-based materials for electrochemical energy technologies. We report experimental data on the catalytic activity and corrosion behavior of polycrystalline Ni in the presence of different dissolved gases. Cyclic voltammetry (CV) and potentiodynamic polarization (PDP) measurements are conducted at room temperature in 0.10 M aqueous NaOH solution saturated with either N2(diss), H2(diss), or O2(diss). CV measurements are performed using different potential scan rates in the regions of α-Ni(OH)2 and NiOOH formation/reduction. PDP measurements are conducted in the same electrolyte and in the presence of different dissolved gases but over a much broader potential range (−0.40 V ≤ E ≤ 2.20 V) and at a very low potential scan rate (s = 0.10 mV s−1) to achieve steady-state conditions. The influence of the state of the electrode’s surface (metallic versus oxidized) on the catalytic activity and corrosion behavior of nickel is also investigated by applying two different types of conditioning. The results demonstrate that the electrochemical behavior of Ni changes depending on the nature of the dissolved gas. The corrosion behavior is shown to depend on the polarization direction, thus the surface state of the electrode, and the nature of the dissolved gas. A graph, which is an extension of the Pourbaix diagram for Ni, summarizes the main interfacial and faradaic processes occurring at the surface of polycrystalline Ni in relation to the potential. The results and their analysis are expected to benefit renewable electrochemical energy technologies, such as alkaline water electrolyzers and fuel cells. In addition, they will serve as standards in electrochemical and electrocatalytic characterization of monocrystalline Ni materials and Ni-based nanomaterials.

Oxygen electrocatalysis at transition metal oxides is one of the key challenges underpinning electrochemical energy conversion systems, involving a delicate interplay of the bulk electronic structure and surface coordination of the active sites. In this work, we investigate for the first time the structure-activity relationship of A2RuMnO7 (A = Dy3+, Ho3+, and Er3+) nanoparticles, demonstrating how orbital mixing of Ru, Mn, and O promotes high density of states at the appropriate energy range for oxygen electrocatalysis. The bulk structure and surface composition of these multicomponent pyrochlores are investigated by high-resolution transmission electron microscopy, X-ray diffraction, X-ray absorption spectroscopy, X-ray emission spectroscopy (XES), and X-ray photoemission spectroscopy (XPS). The materials exhibit high phase purity (cubic fcc with a space group Fd3¯ m) in which variations in M-O bonds length are less than 1% upon replacing the A-site lanthanide. XES and XPS show that the mean oxidation state at the Mn-site as well as the nanoparticle surface composition was slightly affected by the lanthanide. The pyrochlore nanoparticles are significantly more active than the binary RuO2 and MnO2 toward the 4-electron oxygen reduction reaction in alkaline solutions. Interestingly, normalization of kinetic parameters by the number density of electroactive sites concludes that Dy2RuMnO7 shows twice higher activity than benchmark materials such as LaMnO3. Analysis of the electrochemical profiles supported by density functional theory calculations reveals that the origin of the enhanced catalytic activity is linked to the mixing of Ru and Mn d-orbitals and O p-orbitals at the conduction band which strongly overlap with the formal redox energy of O2 in solution. The activity enhancement strongly manifests in the case of Dy2RuMnO7 where the Ru/Mn ratio is closer to 1 in comparison with the Ho3+ and Er3+ analogs. These electronic effects are discussed in the context of the Gerischer formalism for electron transfer at the semiconductor/electrolyte junctions.