A Carbon-Free Ag–Co3O4 Composite as a Bifunctional Catalyst for Oxygen Reduction and Evolution: Spectroscopic, Microscopic and Electrochemical Characterization

Preparation and Performance of Nickel-Doped LaSrCoO3-SrCO3 Composite Materials for Alkaline Oxygen Evolution in Water Splitting

Perovskites exhibit catalytic properties on the oxygen evolution reaction (OER) in water electrolysis. Elemental doping by specific preparation methods is a good strategy to obtain highly catalytical active perovskite catalysts. In this work, La0.5Sr0.5Co1-xNixO3-δ perovskite materials doped with different ratios of nickel were successfully synthesized by the sol-gel method. The electrochemical measurement results show that for OER in 1 M KOH solution, La0.5Sr0.5Co0.8Ni0.2O3-δ prepared by the sol-gel method requires only a low overpotential of 213 mV to reach 10 mA cm-2, which is significantly lower than that of La0.5Sr0.5Co0.8Ni0.2O3-δ prepared by the hydrothermal method for the increasing about 45.24% (389 mV at 10 mA cm-2). In addition, La0.5Sr0.5Co0.8Ni0.2O3-δ by the sol-gel method can be kept stable in an alkaline medium tested for 30 h without degradation. This indicates that the prepared La0.5Sr0.5Co0.8Ni0.2O3-δ has better OER performance. The X-ray diffraction (XRD) results show that SrCO3 is the main phase formed, which is a disadvantage of this method. The performance improvement may be affected by the carbonate phase. The scanning electron microscopy (SEM) results show that layer structured La0.5Sr0.5Co0.8Ni0.2O3-δ by the sol-gel method has more surface pores with a pore diameter of about 0.362 μm than spherical granular structured La0.5Sr0.5Co0.8Ni0.2O3-δ by the hydrothermal method. X-ray photoelectronic spectroscopy (XPS) results reveal that the crystal lattice of La0.5Sr0.5Co0.8Ni0.2O3-δ by nickel doping is lengthened, and the electronic configuration of Co is also changed by the sol-gel preparation process. The improved electrocatalytic performance of La0.5Sr0.5Co0.8Ni0.2O3-δ may be attributed to the pore structure formed providing more active sites during the sol-gel process and the improved oxygen mobility with Ni doping by the sol-gel method. The doping strategy using the sol-gel method provides valuable insights for optimizing perovskite catalytic properties.

Tungsten Carbide/Tungsten Oxide Catalysts for Efficient Electrocatalytic Hydrogen Evolution

Catalyzing hydrogen evolution reaction (HER) is a key process in high-efficiency proton exchange membrane water electrolysis (PEMWE) devices. To replace the use of Pt-based HER catalyst, tungsten carbide (W2C) is one of the most promising non-noble-metal-based catalysts with low cost, replicable catalytic performance, and durability. However, the preparation access to scalable production of W2C catalysts is inevitable. Herein, we introduced a facile protocol to achieve the tungsten carbide species by plasma treatment under a CH4 atmosphere from the WO3 precursor. Moreover, the heterogeneous structure of the tungsten carbide/tungsten oxide nanosheets further enhances the catalytic activity for HER with the enlarged specific surface area and the synergism on the interfaces. The prepared tungsten carbide/tungsten oxide heterostructure nanosheets (WO3-x-850-P) exhibit exceptional HER catalytic activity and stable longevity in acid electrolytes. This work provides a facile and effective method to construct high-performance and non-precious-metal-based electrocatalysts for industrial-scale water electrolysis.

Recent Advances in Bimetallic Catalysts for Methane Steam Reforming in Hydrogen Production: Current Trends, Challenges, and Future Prospects

As energy demand continues to rise and the global population steadily grows, there is a growing interest in exploring alternative, clean, and renewable energy sources. The search for alternatives, such as green hydrogen, as both a fuel and an industrial feedstock, is intensifying. Methane steam reforming (MSR) has long been considered a primary method for hydrogen production, despite its numerous advantages, the activity and stability of the conventional Ni catalysts are major concerns due to carbon formation and metal sintering at high temperatures, posing significant drawbacks to the process. In recent years, significant attention has been given to bimetallic catalysts as a potential solution to overcome the challenges associated with methane steam reforming. Thus, this review focuses on the recent advancements in bimetallic catalysts for hydrogen production through methane steam reforming. The review explores various aspects including reactor type, catalyst selection, and the impact of different operating parameters such as reaction temperature, pressure, feed composition, reactor configuration, and feed and sweep gas flow rates. The analysis and discussion revolve around key performance indicators such as methane conversion, hydrogen recovery, and hydrogen yield.

Synthesis of CoMoO4 Nanofibers by Electrospinning as Efficient Electrocatalyst for Overall Water Splitting

To improve the traditional energy production and consumption of resources, the acceleration of the development of a clean and green assembly line is highly important. Hydrogen is considered one of the most ideal options. The method of production of hydrogen through water splitting constitutes the most attractive research. We synthesized CoMoO4 nanofibers by electrospinning along with post-heat treatment at different temperatures. CoMoO4 nanofibers show a superior activity for hydrogen evolution reaction (HER) and only demand an overpotential of 80 mV to achieve a current density of 10 mA cm-2. In particular, the CoMoO4 catalyst also delivers excellent performances of oxygen evolution reaction (OER) in 1 M KOH, which is a more complicated process that needs extra energy to launch. The CoMoO4 nanofibers also showed a superior stability in multiple CV cycles and maintained a catalytic activity for up to 80 h through chronopotentiometry tests. This is attributed mainly to a synergistic interaction between the different metallic elements that caused the activity of CoMoO4 beyond single oxides. This approach proved that bimetallic oxides are promising for energy production.

Electrodeposited Cobalt Nanosheets on Smooth Silver as a Bifunctional Catalyst for OER and ORR: In Situ Structural and Catalytic Characterization

Developing earth-abundant, cost-effective, and active bifunctional electrocatalysts for oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) is key to boosting sustainable energy systems such as electrolyzers and lithium-air batteries. However, the performance of promising cobalt-based materials is impaired by the external effects of binders and carbon additives as well as inhomogeneous electrode fabrication. In this work, binder- and carbon-free flower-like Co-decorated Ag catalytic nanosheets were in situ-synthesized via a simple electrodeposition approach. The morphology, composition, and structure of Co/Ag before and after OER were characterized using scanning electron microscopy (SEM), energy-dispersive X-ray (EDX), X-ray photoelectron spectroscopy (XPS), and X-ray diffraction (XRD). Co/Ag thin film electrodes with various Co contents exhibited a bifunctional activity toward ORR and OER due to a synergistic effect. XPS analysis suggested the formation of Co₃O₄ as the main active species for OER. In particular, Co (83%)/Ag surface revealed a 60 mV lower ORR overpotential than a pure Ag surface and even lower than drop-casted Co₃O₄ nanoparticles on Ag surface. Only 1.5% peroxide was generated, suggesting a four-electron transfer ORR. In addition, the OER onset potential on Co/Ag is 60 mV less than Co₃O₄. Tafel slopes of 71 and 75 mV dec^-1 were obtained for ORR and OER, respectively. Importantly, the three-dimensional (3D) growth mechanism of a cobalt layer (∼1 nm) on a well-defined atomic smooth Ag surface is unraveled by in situ electrochemical scanning tunneling microscopy (EC-STM). EC-STM suggests that Co prefers to nucleate at the step edges of Ag and grows in a 3D, forming nanoparticles, where the deposition/dissolution process of the Co adlayer on Ag is reversible. This investigation may provide insights into design strategies of efficient oxygen electrocatalysts.

A cost-effective alkaline polysulfide-air redox flow battery enabled by a dual-membrane cell architecture

With the rapid development of renewable energy harvesting technologies, there is a significant demand for long-duration energy storage technologies that can be deployed at grid scale. In this regard, polysulfide-air redox flow batteries demonstrated great potential. However, the crossover of polysulfide is one significant challenge. Here, we report a stable and cost-effective alkaline-based hybrid polysulfide-air redox flow battery where a dual-membrane-structured flow cell design mitigates the sulfur crossover issue. Moreover, combining manganese/carbon catalysed air electrodes with sulfidised Ni foam polysulfide electrodes, the redox flow battery achieves a maximum power density of 5.8 mW cm−2 at 50% state of charge and 55 °C. An average round-trip energy efficiency of 40% is also achieved over 80 cycles at 1 mA cm−2. Based on the performance reported, techno-economic analyses suggested that energy and power costs of about 2.5 US$/kWh and 1600 US$/kW, respectively, has be achieved for this type of alkaline polysulfide-air redox flow battery, with significant scope for further reduction.

Electrocatalysis in Alkaline Media and Alkaline Membrane-Based Energy Technologies

Hydrogen energy-based electrochemical energy conversion technologies offer the promise of enabling a transition of the global energy landscape from fossil fuels to renewable energy. Here, we present a comprehensive review of the fundamentals of electrocatalysis in alkaline media and applications in alkaline-based energy technologies, particularly alkaline fuel cells and water electrolyzers. Anion exchange (alkaline) membrane fuel cells (AEMFCs) enable the use of nonprecious electrocatalysts for the sluggish oxygen reduction reaction (ORR), relative to proton exchange membrane fuel cells (PEMFCs), which require Pt-based electrocatalysts. However, the hydrogen oxidation reaction (HOR) kinetics is significantly slower in alkaline media than in acidic media. Understanding these phenomena requires applying theoretical and experimental methods to unravel molecular-level thermodynamics and kinetics of hydrogen and oxygen electrocatalysis and, particularly, the proton-coupled electron transfer (PCET) process that takes place in a proton-deficient alkaline media. Extensive electrochemical and spectroscopic studies, on single-crystal Pt and metal oxides, have contributed to the development of activity descriptors, as well as the identification of the nature of active sites, and the rate-determining steps of the HOR and ORR. Among these, the structure and reactivity of interfacial water serve as key potential and pH-dependent kinetic factors that are helping elucidate the origins of the HOR and ORR activity differences in acids and bases. Additionally, deliberately modulating and controlling catalyst-support interactions have provided valuable insights for enhancing catalyst accessibility and durability during operation. The design and synthesis of highly conductive and durable alkaline membranes/ionomers have enabled AEMFCs to reach initial performance metrics equal to or higher than those of PEMFCs. We emphasize the importance of using membrane electrode assemblies (MEAs) to integrate the often separately pursued/optimized electrocatalyst/support and membranes/ionomer components. Operando/in situ methods, at multiscales, and ab initio simulations provide a mechanistic understanding of electron, ion, and mass transport at catalyst/ionomer/membrane interfaces and the necessary guidance to achieve fuel cell operation in air over thousands of hours. We hope that this Review will serve as a roadmap for advancing the scientific understanding of the fundamental factors governing electrochemical energy conversion in alkaline media with the ultimate goal of achieving ultralow Pt or precious-metal-free high-performance and durable alkaline fuel cells and related technologies.

Single Co3O4 Nanocubes Electrocatalyzing the Oxygen Evolution Reaction: Nano-Impact Insights into Intrinsic Activity and Support Effects

Single-entity electrochemistry allows for assessing electrocatalytic activities of individual material entities such as nanoparticles (NPs). Thus, it becomes possible to consider intrinsic electrochemical properties of nanocatalysts when researching how activity relates to physical and structural material properties. Conversely, conventional electrochemical techniques provide a normalized sum current referring to a huge ensemble of NPs constituting, along with additives (e.g., binders), a complete catalyst-coated electrode. Accordingly, recording electrocatalytic responses of single NPs avoids interferences of ensemble effects and reduces the complexity of electrocatalytic processes, thus enabling detailed description and modelling. Herein, we present insights into the oxygen evolution catalysis at individual cubic Co₃O₄ NPs impacting microelectrodes of different support materials. Simulating diffusion at supported nanocubes, measured step current signals can be analyzed, providing edge lengths, corresponding size distributions, and interference-free turnover frequencies. The provided nano-impact investigation of (electro-)catalyst-support effects contradicts assumptions on a low number of highly active sites.

The Oxygen Reduction Reaction in Ca2+ -Containing DMSO: Reaction Mechanism, Electrode Surface Characterization, and Redox Mediation*

In this study the fundamental understanding of the underlying reactions of a possible Ca-O2 battery using a DMSO-based electrolyte was strengthened. Employing the rotating ring disc electrode, a transition from a mixed process of O2 - and O2 2- formation to an exclusive O2 - formation at gold electrodes is observed. It is shown that in this system Ca-superoxide and Ca-peroxide are formed as soluble species. However, there is a strongly adsorbed layer of products of the oxygen reduction reaction (ORR) s on the electrode surface, which is blocking the electrode. Surprisingly the blockade is only a partial blockade for the formation of peroxide while the formation of superoxide is maintained. During an anodic sweep, the ORR product layer is stripped from the electrode surface. With X-ray photoelectron spectroscopy (XPS) the deposited ORR products were shown to be Ca(O2 )2 , CaO2 , and CaO as well as side-reaction products such as CO3 2- and other oxygen-containing carbon species. It is shown that the strongly attached layer on the electrocatalyst, that was partially blocking the electrode, could be adsorbed CaO. The disproportionation reaction of O2 - in presence of Ca2+ was demonstrated via mass spectrometry. Finally, the ORR mediated by 2,5-di-tert-1,4-benzoquinone (DBBQ) was investigated by differential electrochemical mass spectrometry (DEMS) and XPS. Similar products as without DBBQ are deposited on the electrode surface. The analysis of the DEMS experiments shows that DBBQ- reduces O2 to O2 - and O2 2- , whereas in the presence of DBBQ2- O2 2- is formed. The mechanism of the ORR with and without DBBQ is discussed.