Hydroxylated Polyoxometalate with Cu(II)- and Cu(I)-Aqua Complexes: A Bifunctional Catalyst for Electrocatalytic Water Splitting at Neutral pH

A sole inorganic framework material [Li(H₂O)₄][{Cu^I(H₂O)_1.5} {Cu^II(H₂O)₃}₂{W^VI₁₂O₃₆(OH)₆}]·N₂·H₂S·3H₂O (1) consisting of a hydroxylated polyoxometalate (POM) anion, {W^VI₁₂O₃₆(OH)₆}^6-, a mixed-valent Cu(II)- and Cu(I)-aqua cationic complex species, [{Cu^I(H₂O)_1.5}{Cu^II(H₂O)₃}₂]⁵⁺, a Li(I)-aqua complex cation, and three solvent molecules, has been synthesized and structurally characterized. During its synthesis, the POM cluster anion gets functionalized with six hydroxyl groups, i.e., six W^VI-OH groups per cluster unit. Moreover, structural and spectral analyses have shown the presence of H₂S and N₂ molecules in the concerned crystal lattice, formed from "sulfate-reducing ammonium oxidation (SRAO)". Compound 1 functions as a bifunctional electrocatalyst exhibiting oxygen evolution reaction (OER) by water oxidation and hydrogen evolution reaction (HER) by water reduction at the neutral pH. We could identify that the hydroxylated POM anion and copper-aqua complex cations are the functional sites for HER and OER, respectively. The overpotential, required to achieve a current density of 1 mA/cm² in the case of HER (water reduction), is found to be 443 mV with a Faradaic efficiency of 84% and a turnover frequency of 4.66 s^-1. In the case of OER (water oxidation), the overpotential needed to achieve a current density of 1 mA/cm² is obtained to be 418 mV with a Faradaic efficiency of 80% and turnover frequency of 2.81 s^-1. Diverse electrochemical controlled experiments have been performed to conclude that the title POM-based material functions as a true bifunctional catalyst for electrocatalytic HER as well as OER at the neutral pH without catalyst reconstruction.