High-Capacity F-Doped Na0.7MnO2.05 with Balanced Voltage Distribution for Decoupled Water Electrolysis

Decoupled water electrolysis, which utilizes redox mediators to separate the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) in space and time, is considered a potential method for producing high-purity green hydrogen. However, there are some key challenges in decoupled water electrolysis using solid-state redox mediators, such as redox potentials, voltage distribution, capacity limitations, and material stability. Here, F-doped Na_0.7MnO_2.05 (NMOF) with an appropriate redox potential, high capacity, and stability was synthesized by a simple sol–gel method. The redox peak pair of NMOF was located at −0.064 V/–0.314 V (vs Hg/HgO), which is located between the onset potentials of the HER and OER. By F-doping, F–Mn bonds significantly inhibited the dissolution of Mn²⁺ in the electrolyte, thereby reducing the Jahn–Teller effect and improving the cycling stability of Na-ion insertion and removal in Na_0.7MnO_2.05. NMOF prepared by adding 5 mol/% NaF at 850 °C (named NMOF2) exhibited excellent electrochemical performance, with a discharge capacity of 114.3 mAh/g at a current density of 0.5 A/g. Using NMOF2 for decoupled water electrolysis, voltage balance distribution was achieved, and hydrogen and oxygen production was achieved at such low voltages (0.85 V for the HER process and 0.89 V for the OER process) at a current density of 5 mA/cm². These suggest that NMOF2 could be a promising material for decoupled water electrolysis.