Topochemical Copper Nitroprusside Cathode Enables High Capacity and Cyclability Aqueous Proton Batteries

Aqueous proton batteries (APBs) are emerging as transformative candidates for grid-scale renewable energy storage, leveraging the unique properties of protons: light weight, small radius, and versatile bonding capabilities. However, the development of full-cell APBs is still limited by inadequate capacity, energy density, and long-term cyclability. Here, we introduce APBs integrating Cu–Fe nitroprusside that exhibits topochemical proton storage behavior, driven by the presence of strong π-acceptor nitrosyl ligands that also induce a high charge density on the metal centers, which enables the accommodation of lattice water molecules. This unique structure enables rapid (de)insertion kinetics coupled with Grotthuss-type proton conduction and delivers a high capacity of 122 mAh g–1 at 10 A g–1 in the half-cell and retains ∼77% capacity up to 4500 cycles. The assembled full cell achieves a capacity of 80 mAh g–1 at 10 A g–1 with 84% capacity retention over 6000 cycles. Thus, this work demonstrates a scalable pathway toward next-generation aqueous proton batteries for sustainable energy storage.