Inert nickel doping unlocking aqueous proton storage of hydrated copper hexacyanoferrate as a competitive cathode for proton pseudocapacitors

Abstract

Proton pseudocapacitors (PPCs), utilizing the smallest charge carrier of H+ to achieve ultrafast kinetics through the Grotthuss mechanism, exhibit attractive advantages of high safety, environmental compatibility, and abundant resource availability. However, the development of PPCs with high-energy and high-power density, and long service life has been limited seriously by the constraints of cathode materials. Herein, the inert hetero-atomic Ni doping strategy is smartly designed to optimize the copper hexacyanoferrate (CuHCF) toward PPCs as a competitive cathode. With the purposeful Ni2+ doping, the abundant defects are introduced along with the optimized framework structure, which enhances the hydrogen-bonding network within the framework, facilitating the H+ transport and subsequently outstanding rate capability. Moreover, the doped Ni as the structural anchor mitigates ion displacement, favoring the enhanced cycling stability of electrodes. As a result, compared with the pristine CuHCF, the optimized cathode CuHCF-3 (i.e., Ni0.14Cu0.86[Fe(CN)6]0.61·□0.39·4.35H2O) exhibits exception high-rate capacitances and cycling property. More impressively, the assembled asymmetric PPCs achieves a high energy density of 25.1 Wh kg−1 at 20 kW kg−1, and approximately 100% capacitance retention after 20,000 cycles at 10 A g−1. Our contribution here provides new insights into rational design of reliable cathode platforms for efficient aqueous proton storage.