Transition metal atom M (M = Fe, Co, Cu, Cr) doping and oxygen vacancy modulated M–Ni5P4–NiMOH nanosheets as multifunctional electrocatalysts for efficient overall water splitting and urea electrolysis reaction

It is significant to develop reasonable and efficient hydrogen evolution reaction catalysts to alleviate the energy crisis, yet challenging to produce hydrogen through the electrolysis of water and urea. In this work, the dual control strategy of doping and vacancy creation was used to improve the electrocatalytic performance of the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) for the design of a multifunctional catalyst. A series of M-doped-Ni5P4/M-doped Ni(OH)2 (M = Fe, Co, Cu, Cr) hierarchical materials with abundant oxygen vacancies was constructed for the first time by hydrothermal and partial phosphating methods. The Co-doped-Ni5P4/Co-doped-Ni(OH)2 (Co-Ni5P4-NiCoOH) exhibited superior performance in HER, OER and urea oxidation reaction (UOR). Moreover, the electrode couple is fitted with two Co-Ni5P4-NiCoOH (C-NP-NCOH) electrodes to drive the current density of 10 mA cm-2; the necessary cell voltage was 1.57 V in 1.0 M KOH with 0.5 M urea for urea electrolysis and water electrolysis required a 1.6 V cell voltage in 1.0 M KOH electrolyte, which is one of the best catalytic activities reported so far. The experimental results suggest that the co-action of Co-doping and oxygen vacancies increases the specific surface area of the material, enhances the electronic conductivity and promotes the exposure of more active sites, thus improving the water splitting and urea electrolysis performances of the catalyst. Density functional theory analysis suggests that Co-Ni5P4-NiCoOH displays optimal adsorption energy of water and electrical conductivity, thus optimizing the adsorption/desorption of intermediates.