Hydrous Nickel-Iron Turnbull's Blue as a High-Rate and Low-Temperature Proton Electrode.

Proton batteries are emerging as a promising solution for energy storage; however, their development has been hindered by the lack of suitable cathode materials. Herein, a hydrous Turnbull's blue analogue (TBA) of Ni[Fe(CN)6]2/3·4H2O has been investigated as a viable proton cathode. Particularly, it shows an extremely high rate performance up to 6000 C (390 A g-1) at room temperature and delivers good capacity values at a low temperature of -40 °C in an aqueous electrolyte. The excellent rate capability is also amenable to high mass loadings of 10 mg cm-2. Such fast and low-temperature rate behavior likely stems from the fast proton conduction that is afforded by the Grotthuss mechanism inside the TBA structure. Furthermore, advanced characterization, including in operando synchrotron X-ray diffraction (XRD), and X-ray absorption near-edge structure (XANES) were employed to understand the changes of crystal structures and the oxidation-states of metal elements of the electrodes.