Defective Ru-Doped Α-Mno2 Nanorods Enabling Efficient Hydrazine Oxidation for Energy-Saving Hydrogen Production in Acidic Media

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 acidic medium, 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 (0.05MH2SO4+0.5MN2H4), it can deliver an anodic current density of 10mAcm-2 at a potential as low as 0.166V vs. reversible hydrogen electrode (RHE). Moreover, Ru/α-MnO2 exhibits remarkable corrosion/oxidation resistance and remains electrochemically stable during HzOR in acid for at least 1000 hours. 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 not only significantly reduces the energy demand for hydrogen production, but more importantly prevents the deactivation of the Ru/α-MnO2 in the strong acid. Consequently, PEMWE can be accomplished under remarkably low voltages of 0.254V at 10mAcm-2 and 0.935V at 100mAcm-2 for a long term without notable degradation. This work opens a new avenue toward energy-saving PEMWE with earth-abundant OER catalysts.

Keywords: cationic defect, Ru doping, PEM water electrolysis, hydrazine oxidation reaction, Electrocatalysis