Low electronegativity Mn bulk doping intensifies charge storage of Ni2P redox shuttle for membrane-free water electrolysis

Abstract

A high efficiency electrochemical energy storage and high areal capacitance solid-state mediator can act as an energy vector to solve the key problem in mixtures of H₂ and O₂ in electrically driven water splitting architectures. It is believed that introducing foreign metal atoms into a monometallic compound could promote the energy storage capacity of the mediator. Herein, based on the theoretical prediction that partial substitution of low electronegativity Mn in the Ni₂P structure could enhance the local charge density around Ni and lower the rate-determining Gibbs free energy difference (ΔG_OOH–ΔG_O) compared to Fe substitution or the pure compound, a supercapacitor-type transition metal phosphide (TMP) mediator Ni–Mn–P with an areal capacitance of 12.25 F cm⁻¹ is fabricated. Benefitting from the enhanced energy storage activity and high exposure of active sites, the charge storage in the H₂ generation step in a 1 cm² Ni–Mn–P mediator electrode when coupled with Ni₂P/NF bifunctional electrodes maintains >500 s O₂ evolution (10 mA), ∼4 times longer than that of an unsubstituted Ni–P mediator. In addition, the superior stability for >20 000 s cycle operation at 20 mA further demonstrates its operational flexibility and potential for practical application.