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

Abstract

A cobalt(II)-containing polyoxometalate, [H 3 O] 5 [{Co(H 2 O) 4 } 3 {Na(H 2 O) 4 }W 12 O 42 ]•3H 2 O (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 [W 12 O 42 ] 12− coordinates with {Co II (H 2 O) 4 } 2+ and {Na(H 2 O) 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 (Co 3 O 4 ) incorporating small amounts of tungsten. Thus, in the present electrocatalysis work, the Co-POM molecule transforms into an efficient water oxidation catalyst (WOC).