Effects of heteroatom doping on hydrogen uptake in tungsten oxide

Abstract

Redox-active transition metal oxides (TMOs) that can undergo proton-insertion coupled electron transfer (PICET) are promising candidates for catalyzing molecular conversion reactions which require the transfer of hydrogen atoms (or the thermochemical equivalent, H⁺, e⁻). Herein, we studied the effects of isovalent (Mo⁶⁺) and aliovalent (V⁵⁺ and Nb⁵⁺) heteroatom doping on the electrochemical PICET behavior of monoclinic tungsten oxide (WO₃). Cyclic voltammetry in aqueous acidic electrolytes shows that the addition of redox-active heteroatoms (Mo⁶⁺ and V⁵⁺) leads to systematic shifts in redox couple half-wave potentials (E_1/2), broadening, and an overall decrease in the current response. Conversely, the non-redox active heteroatom (Nb⁵⁺) only reduces the current response with no observable peak-current broadening. This broadening is attributed to changes in the proton binding affinities of oxygen in different chemical environments, i.e., bridging different pairs of redox-active transition metal cations. We determined the hydrogen bond dissociation free energy (H BDFE) values to elucidate the thermodynamic effect of heteroatom substitution. Density functional theory calculations reveal a differentiation in the hydrogen binding and oxygen vacancy formation energies between heteroatom doped structures. The PICET-induced structural phase transitions of the pristine and doped samples were further probed with operando electrochemical X-ray diffraction (EC-XRD) and with ex situ chemical reduction. The broadening of the potential-dependent current response with increased heteroatom doping manifests in the operando EC-XRD results as prolonged structural regions where multiple hydrogen bronze phases exist and the appearance of cubic bronze phases at lower degrees of reduction compared to pristine WO₃.