Abstract
As an electrocatalyst for water electrolysis, nickel oxide (NiO) has received significant attention due to its cost-effectiveness and high reactivity among non-noble-metal-based catalytic materials. However, NiO still exhibits poor alkaline hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) kinetics compared to conventional noble metal-based catalysts. This is because NiO has a strong interaction with protons for the HER and too low free energy of the OH* state, resulting in slower rate-determining step (RDS) kinetics for the OER. To address these issues, adding a dopant is suggested as an efficient method to modify the electron structure of the NiO electrocatalyst favorably for each reaction kinetics. In this context, we demonstrate that Bismuth (Bi), due to its higher electronegativity than that of Nickel (Ni), induces a positive charge on Ni sites. This enhances the catalytic activity by reducing the number of excessive cation interactions with the NiO electrocatalyst. Moreover, as the Bi ratio increases, the Ni reaction sites in NiO become more positively charged, and these changes in the electronic structure directly impact the free energy of the reaction mechanism. Particularly, it is confirmed that for the HER, Bi additives increase the proton-adsorbed free energy toward a near-zero value and, additionally, decrease the free energy difference of the second step considered as the RDS in the OER, as calculated by density functional theory. The positive effects of Bi in both the HER and the OER are demonstrated in practical electrochemical evaluations of half/single cells. Notably, the Bi-containing catalysts Bi05:NiO and Bi02:NiO exhibit remarkable alkaline HER and OER kinetics, showing performance improvements of 97.0% and 21.9%, respectively.