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The rechargeable all‐solid‐state Na–O2 battery is one of the most promising candidates for next‐generation energy storage devices owing to its high theoretical specific energy, safety, electrochemical stability, and abundant Na resources. However, the practical implementation of current all‐solid‐state Na–O2 batteries is still limited by low‐resistance interfaces, low energy efficiency, and poor cycle life. Herein, an all‐solid‐state Na–O2/H2O battery that can sustain highly reversible cycling with a low overpotential is reported. Using a customized silver–polymer composite cathode, this battery can be operated under ≈7% relative humidity (RH) at 80 °C and cycled for more than 100 times with a low overpotential (≈75 mV at 100th cycle) and high round trip efficiency >97% at 100th cycle) at an energy density of 20 mA g−1. Furthermore, mechanistic insight that the RH intimately controls the type and hydration state of the discharge product is also provided, and thereby the charge kinetics and battery performance are modulated. It is also revealed that silver catalyst can efficiently reduce the reaction barrier of NaOH decomposition. With the further optimization, this battery potentially can be implemented in real‐world applications and confer practical applicability that can extend to other energy‐storage systems, such as other metal–air batteries. [ABSTRACT FROM AUTHOR]

An ionomer strategy is introduced to deal with two major performance‐limiting issues for the proton exchange membrane fuel cells (PEMFCs) operating under low platinum (Pt) and low humidity. Specifically, the highly hydrophilic multiple‐valence Anderson‐type polyoxometalate (POM) cluster, [MnMo6O18(OH)6]3−, is chosen to covalently graft the perfluorinated polymer, substituting for sulfonate groups as proton carriers in perfluorosulfonic acid (PFSA). The structural and electrochemical characterizations indicate that this POM cluster, with large size, charges delocalization among multiple terminal oxygen atoms, and high hygroscopicity, can prevent the ionomer adsorption, which is believed to induce dense backbone crystallization to hinder O2 permeation at Pt/PFSA interface, and lead to ionomer assemblies with large and well‐connected hydrophilic domains, which are recognized as the channels for efficient oxygen transport as well as proton conduction. Besides, the involvement of redox Mn ions in POM clusters makes the ionomer have capability to scavenge peroxides, which are known as the major chemical agents to degrade the MEA components. Consequently, fuel‐cell cathodes using the POM‐grafted ionomers exhibit significantly lower local O2 transport resistance and higher proton conductivity as compared with the PFSA‐based electrodes; and accordingly, the fuel cells exhibit much‐increased output performance at 50% relative humidity and impressive performance stability. [ABSTRACT FROM AUTHOR]