Fabrication of CuOx nanowires@NiMnOx nanosheets core@shell-type electrocatalysts: crucial roles of defect modification and valence states for overall water electrolysis

Abstract

Combining oxides with bimetallic elements may be an efficient route to modify the electronic distribution for improving the intrinsic activity of electrocatalysts, and further engineering the defect structure and adjusting the valence state could provide more adsorption sites as well as active sites for the decomposition of reactants and/or intermediates. Herein, NiMnO_x nanosheets with rich defects were electrochemically deposited on the surface of Cu(OH)₂ nanowires followed by a thermal treatment process to form a CuO_x nanowires@NiMnO_x nanosheets (CuO_x NWs@NiMnO_x NSs) electrocatalyst with a core–shell configuration. Structural characterization indicated that NiMnO_x nanosheets had numerous vacancies with scattered defects and dislocations with partial cracking of the inert basal planes, resulting in extra active edge sites and a coordination-unsaturated spinel crystal. As such, the valence states of Ni and Mn elements became unstable in the defect modified NiMnO_x materials under the applied potential. Compared with the initial NiMnO_x, more Ni³⁺ and Mn⁴⁺ were formed during the oxygen evolution reaction (OER) whereas they were converted to Ni²⁺ and Mn³⁺ during the hydrogen evolution reaction (HER). These controllable valence state changes of metal elements endowed the NiMnO_x with high ability for catalyzing overall water electrolysis. As a result, the CuO_x NWs@NiMnO_x NSs bifunctional electrocatalysts exhibited high performances in both HER and OER in an alkaline electrolyte. Meanwhile, by using the same electrocatalysts in a neutral electrolyte, overpotentials at 10 mA cm⁻² were also as low as 80.7 and 390 mV with Tafel slopes of 77.6 and 101.6 mV dec⁻¹ for the HER and OER, respectively. A two-electrode overall water electrolysis system using this bifunctional electrocatalyst exhibited low cell voltages of 1.62 and 1.75 V in alkaline and neutral electrolytes respectively at the standard current density of 10 mA cm⁻² with long-term stability, indicating that this CuO_x NWs@NiMnO_x NSs core@shell-type material should be an effective bifunctional electrocatalyst in both alkaline and neutral pH media.