Transition metal tailored δ-MnO2 with optimized charge compensation for enhanced hydrogen evolution

Abstract

Layered manganese dioxide (δ-MnO2) exhibits potential in hydrogen evolution reaction (HER) due to its multivalent redox properties and high specific surface area. However, its poor intrinsic conductivity and insufficient active sites limit the electron transfer efficiency and adsorption/desorption of *H intermediates. Herein, we constructed an X-MO/NF (X=Ni, Cu, Zn) electrocatalytic system by in situ growth of δ-MnO2 on the surface of nickel foam (NF) by a one-step hydrothermal method and introduction of transition metal atoms. It has been shown that the transition metals triggered the charge compensation mechanism by changing the O coordination environment, inducing the formation of oxygen vacancies and unsaturated Mn3+ sites in MnO2 lattice, thus balancing the charge distribution and optimizing the electronic structure. Ni-MO/NF, Cu-MO/NF, and Zn-MO/NF exhibit low overpotentials of 145 mV, 131 mV, and 115 mV at 10 mA cm-2 in 1 M KOH, respectively. Theoretical calculations confirmed that Ni, Cu, and Zn doping was able to reduce the hydrogen adsorption-free energy and accelerate the surface reaction kinetics. Nevertheless, the diversity of their orbital contributions to the electronic structure of MnO2 led to different redistribution of the electron cloud density, thus showing variation in the degree of enhancement of the catalytic performance. This work reveals the regulation rule of transition metal atoms on the coordination environment of δ-MnO2, which provides a new perspective and theoretical guidance for optimizing the catalytic performance of transition metal oxides.