Intra-Lattice Inverse Charge Transfer in Bimetallic Electrocatalysts

Abstract

The prevalence of intermetallic charge transfer is a marvel for fine-tuning the electronic structure of the active centers in the electrocatalysts. Although, Pauling electronegativity is the primary deciding factor for the direction of charge transfer, we report an unorthodox intra-lattice ‘inverse’ charge transfer from Mo to Ni in two systems, Ni73Mo alloy electrodeposited on Cu nanowires and NiMo-hydroxide (Ni:Mo = 5:1) on Ni foam. The inverse charge transfer deciphered by X-ray absorption fine structure studies and X-ray photoelectron spectroscopy has been understood by the Bader charge and projected density of state analyses. The undercoordinated Mo-center pushes the Mo 4d-orbitals close to the Fermi energy in the valence band region while Ni 3d-orbitals lie in the conduction band. Since, electrons are donated from the electron-rich Mo-center to the electron-poor Ni-center, the inverse charge transfer effect navigates the Mo-center to become positively charged and vice versa. The reverse charge distribution in Ni73Mo accelerates the electrochemical hydrogen evolution reaction in alkaline and acidic media with 0.35 and 0.07 s-1 turnover frequency at -33 and -54 mV versus reversible hydrogen electrode, respectively. The mass activities are 12.5 and 67 A g-1 at 100 mV overpotential, respectively. Anodic potential oxidizes the Ni-center of NiMo-hydroxide for alkaline water oxidation with 0.43 O2 s-1 turnover frequency at 290 mV overpotential. This extremely durable homologous couple achieves water and urea splitting with cell voltages of 1.48 and 1.32 V, respectively at 10 mA cm-2.