Defective Ru-doped α-MnO2 nanorods enabling efficient hydrazine oxidation for energy-saving hydrogen production via proton exchange membranes at near-neutral pH

Abstract

Proton exchange membrane water electrolysis (PEMWE) showes substantial advantages over the conventional alkaline water electrolysis (AWE) for power-to-hydrogen (PtH) conversion, given the faster response and wider dynamic current range of the PEMWE technology. However, PEMWE is currently still expensive due partly to the high voltage needed to operate at high current densities and inevitable usage of precious iridium/ruthenium-based catalysts to expedite the slow kinetics of the oxygen evolution reaction (OER) and to ensure sufficient durability under strongly acidic conditions. Herein, we report that ruthenium doped α-manganese oxide (Ru/α-MnO2) nanorods show outstanding electrocatalytic performance toward the hydrazine (N2H4) oxidation reaction (HzOR) in near-neutral media (weak alkaline and weak acid), which can be used to replace the energy-demanding OER for PEMWE. The as-prepared Ru/α-MnO2 is found to comprise abundant defects. When used to catalyze HzOR in the acid-hydrazine electrolyte (0.05 M H2SO4 + 0.5 M N2H4), it can deliver an anodic current density of 10 mA cm−2 at a potential as low as 0.166 V vs. reversible hydrogen electrode (RHE). Moreover, Ru/α-MnO2 exhibits remarkable corrosion/oxidation resistance and remains electrochemically stable during HzOR for at least 1000 h. Theoretical calculations and experimental studies prove that Ru doping elongates the Mn–O bond and produces abundant cationic defects, which induces charge delocalization and significantly lowers material’s electrical resistance and overpotential, resulting in excellent HzOR catalytic activity and stability. The introduction of N2H4 significantly reduces the energy demand for hydrogen production, so that PEMWE can be accomplished under remarkably low voltages of 0.254 V at 10 mA cm−2 and 0.935 V at 100 mA cm−2 for a long term without notable degradation. This work opens a new avenue toward energy-saving PEMWE with earth-abundant OER catalysts.