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 molecularlevel 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 highperformance and durable alkaline fuel cells and related technologies.

Electrocatalysis has been the cornerstone for enhancing energy efficiency, minimizing environmental impacts and carbon emissions, and enabling a more sustainable way of meeting global energy needs. Elucidating the structure and reaction mechanisms of electrocatalysts at electrode–electrolyte interfaces is fundamental for advancing renewable energy technologies, including fuel cells, water electrolyzers, CO2 reduction, and batteries, among others. One of the fundamental challenges in electrocatalysis is understanding how to activate and sustain electrocatalytic activity, under operating conditions, for extended time periods and with optimal activity and selectivity. Although traditional ex situ methods have provided a baseline understanding of heterogeneous (electro)­catalysts, they cannot provide real-time interfacial structural and compositional changes under reaction conditions, which calls for the use of in situ/operando methods. Herein, we provide a selective review of in situ and operando characterizations, in particular, the use of operando synchrotron-based X-ray techniques and in situ atomic-scale scanning transmission electron microscopy (STEM) in liquid/gas phases to advance our understanding of electrode–electrolyte interfaces at macro- and microscopic levels, which dictate the charge transfer kinetics and overall reaction mechanisms. The use of scanning electrochemical microscopy (SECM) enables direct probing of the local activity of electrocatalysts at the nanometer scale. In addition, differential electrochemical mass spectrometry (DEMS) and the electrochemical quartz crystal balance (EQCM) enable the simultaneous identification of multiple reaction intermediates and products for mechanistic studies of electrocatalyst selectivity and durability. We anticipate that continuous advances of in situ/operando techniques and probes will continue to make significant contributions to establishing structure/composition-reactivity correlations of electrocatalysts at unprecedented atomic-scale and molecular levels under realistic, real-time reaction conditions.

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We have developed a facile and effective electrochemical dealloying strategy to enhance the electrocatalytic activity of Pd/M (M = Ni, Mn) nanoparticles for the oxygen reduction reaction (ORR) in alkaline media. The enhancement arises from the higher atomic concentration of electrochemically active Pd exposed on the surface of the nanoparticles as a result of the electrochemical dealloying process, which was verified by electron energy loss spectroscopy elemental mapping, X-ray photoelectron spectroscopy, and CO stripping. These dealloyed catalysts also showed a negligible decrease in the ORR activity after 4000 potential cycles in alkaline media. In addition, after the degradation that resulted from extended potential cycles in alkaline media, the ORR activity of the catalysts could be restored by repeating the dealloying procedures, with only a 14 mV decrease in the half-wave potential after 16,000 cycles for dealloyed PdNi/C. Similar results were obtained for PdMn/C. These findings provide insights for the rational design of the composition and structure of electrocatalysts with higher atomic utilization of active materials and enhanced electrocatalytic activity, based on postsynthesis modification methods.

Water-in-salt electrolytes are an appealing option for future electrochemical energy storage devices due to their safety and low toxicity. However, the physicochemical interactions occurring at the interface between the electrode and the water-in-salt electrolyte are not yet fully understood. Here, via in situ Raman spectroscopy and molecular dynamics simulations, we investigate the electrical double-layer structure occurring at the interface between a water-in-salt electrolyte and an Au(111) electrode. We demonstrate that most interfacial water molecules are bound with lithium ions and have zero, one, or two hydrogen bonds to feature three hydroxyl stretching bands. Moreover, the accumulation of lithium ions on the electrode surface at large negative polarizations reduces the interfacial field to induce an unusual “hydrogen-up” structure of interfacial water and blue shift of the hydroxyl stretching frequencies. These physicochemical behaviours are quantitatively different from aqueous electrolyte solutions with lower concentrations. This atomistic understanding of the double-layer structure provides key insights for designing future aqueous electrolytes for electrochemical energy storage devices.