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In this review, we discuss the use of binary and multi-component metal oxides as independent electrocatalysts, co-catalysts and supports for various anode oxidation and cathode reduction reactions in polymer electrolyte fuel cells.

aThe electrochemical kinetics of battery electrodes at the single-particle scale are measured as a function of state-of-charge, and interpreted with the aid of concurrent transmission X-ray microscopy (TXM) of the evolving particle microstructure. An electrochemical cell operating with near-picoampere current resolution is used to characterize single secondary particles of two widely-used cathode compounds, NMC333 and NCA. Interfacial charge transfer kinetics are found to vary by two orders of magnitude with state-of-charge (SOC) in both materials, but the origin of the SOC dependence differs greatly. NCA behavior is dominated by electrochemically-induced microfracture, although thin binder coatings significantly ameliorate mechanical degradation, while NMC333 demonstrates strongly increasing interfacial reaction rates with SOC for chemical reasons. Micro-PITT is used to separate interfacial and bulk transport rates, and show that for commercially relevant particle sizes, interfacial transport is rate-limiting at low SOC, while mixed-control dominates at higher SOC. These results provide mechanistic insight into the mesoscale kinetics of ion intercalation compounds, which can guide the development of high performance rechargeable batteries. Broader contextThe performance of high performance Li-ion batteries, central to both electric transportation and grid scale storage, is ultimately reliant on the performance of critical components such as their cathode and anode compounds. For ease of manufacturing, the prevailing technological forms of these materials are secondary particles of nearly spherical morphology containing many nanocrystallites. The electrochemical kinetics at this critical length scale have been difficult to assess; particle-level behavior has primarily been deduced from macroscale cell measurements. Thus the microelectrode technique developed in this work, combined with state-of-the-art TXM imaging, allows for the first time the direct measurement of electrochemical kinetics of particles as they are charged and discharged. Surprising behavior is revealed -interfacial charge transfer kinetics are found to vary greatly with state-of-charge and cycling history, both for intrinsic chemical reasons (in NMC333) and because of massively damaging ''electrochemical shock'' (in NCA). Moreover, a thin coating of polymer binder is found to ameliorate fracture damage. These results, and the techniques demonstrated, provide a bridge between macroscopic battery function and microscale electrode kinetics as influenced by electrochemomechanical stress and charging history.