No abstract

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.

The confinement effect of h-BN shells helps to maintain active metallic Ni cores and strengthen the HOR processes occurring at h-BN/Ni interfaces.

The Pt-catalyzed hydrogen oxidation reaction (HOR) for alkaline polymer electrolyte fuel cells (APEFCs) has been one of the focus subjects in current fuel-cell research. The Pt catalyst is inferior for HOR in alkaline solutions,a nd alloying with Ru is an effective promotion strategy.A PEFCs with Pt-Ru anodes have provided ap erformance benchmark over 1Wcm À2 at 60 8 8C. The Pt anode is nowfound to be in fact as good as the Pt-Ru anode for APEFCs operated at elevated conditions.A t8 08 8Cw ith appropriate gas back-pressure,t he cell with aP ta node exhibits ap eak power density of about 1.9 Wcm À2 ,w hich is very close to that with aP t-Ru anode. Even by decreasing the anode Pt loading to 0.1 mg cm À2 ,over 1.5 Wcm À2 can still be achieved at 80 8 8C. This finding alters the previous understanding about the Pt catalyzedH OR in alkaline media and casts an ew light on the development of practical and high-power APFEC technology.

Hydrogen fuel cells operated in alkaline media enable the use of abundant nonprecious 3d metal oxides to replace Pt to catalyze the sluggish oxygen reduction reaction (ORR). Herein, we describe Co–Mn spinel oxide electrocatalysts with metal oxide loadings of up to 80 wt % on carbon supports. Despite little variation in ORR activity derived from rotating disk electrode (RDE) measurements, practical membrane electrode assembly (MEA) tests exhibited significant enhancement in performance when loadings increased from 40 to 80 wt %. This was ascribed to the enhanced mass transport through the thin catalyst layer at 80 wt % (<10 μm). This work highlights the importance of incorporating MEA tests, even in early-stage catalyst development. A benchmark peak power density (PPD) of 1.2 W/cm2 at 2.6 A/cm2 was achieved using MnCo2O4 (80 wt %). CoMn2O4/C also achieved a PPD of 1.1 W/cm2 under the same conditions, indicating that MEA performance of Co–Mn oxides is generally high and tolerant to compositional variations that may occur in larger-scale production.