A Proton-Conductive Co(II)-Polyoxometalate Acts as a Precatalyst for Efficient Electrocatalytic Water Oxidation

A cobalt(II)-containing polyoxometalate, [H₃O]₅[{Co(H₂O)₄}₃{Na(H₂O)₄}W₁₂O₄₂]·3H₂O (Co-POM), has been isolated in a one-step facile aqueous synthesis and characterized unambiguously using single-crystal X-ray crystallography along with routine spectral analysis. The paratungstate cluster anion [W₁₂O₄₂]^12- coordinates with {Co^II(H₂O)₄}²⁺ and {Na(H₂O)₄}⁺ complex cations resulting in the formation of the water-insoluble Co-POM compound having three-dimensional (3-D) extended structure. Motivated by the protonated water molecules existing as the counter cations in Co-POM, herein, we demonstrate the detailed proton conductivity studies of the Co-POM, reaching a value of 1.04 × 10^-2 S cm^-1 at 80 °C and 98% relative humidity (RH). The temperature- and humidity-dependent proton conductivity in Co-POM is governed by Grotthus mechanism with E_a = 0.25 eV. In addition, we examined the electrochemical behavior of Co-POM, in an alkaline borate buffer where it is found to be electrochemically unstable and acts as a precatalyst (and not a true catalyst) for oxygen evolution reaction (OER). We also discuss the "post-mortem" analysis of the postelectrolysis sample to identify the active species which turns out to be a cobalt oxide material (Co₃O₄) incorporating small amounts of tungsten. Thus, in the present electrocatalysis work, the Co-POM molecule transforms into an efficient water oxidation catalyst (WOC).