Influence of Vanadate Structure on Electrochemical Surface Reconstruction and OER Performance of CoV2O6 and Co3V2O8

Abstract

The increase in global population and modernization of the world created a surge in the energy demand. Currently, carbon-based fossil fuels like coal, petroleum, natural gases etc., are the most widely used energy sources. However, they are limited in resources, and the burning of these fuels produces greenhouse gases which create a harmful impact on the environment. Wind, solar, geothermal energies are considered efficient, clean energy sources, but their intermittent nature restricts their real-life applications [1]. In this regard, Hydrogen is increasingly envisaged as an alternative fuel due to its highest gravimetric energy density (142 MJ/kg) and environmentally benign nature. Electro-catalytic water splitting is a promising method to produce hydrogen in a sustainable manner. For efficient water splitting, the oxygen evolution reaction (OER) at the anode acts as a bottleneck due to its four-electron/proton-coupled mechanism. To facilitate OER, catalysts are generally employed, but most are based on expensive metals, such as Ir, Rh etc. This has enthused researchers to develop cheaper transition metal based catalysts. Although many transition metal based catalysts have been developed, cobalt vanadate systems as OER catalysts are less explored [2]. This enthused us to synthesize pure phase Co3V2O8 and CoV2O6 by thermal co-precipitation followed by calcination. We could able to get these two different phases of cobalt vanadate by altering the reaction conditions slightly. Among these materials, CoV2O6 showed higher geometric electrocatalyst activity towards OER. The overpotential requirement to attain the benchmark current density of 10 mA/cm2geo was 360 mV, whereas Co3V2O8 required 510 mV overpotential to reach 10 mA/cm2geo current density. In addition, we have investigated the factors responsible for the superior phase-dependent electrocatalytic OER activity of two different crystalline cobalt vanadate phases. Vanadates as a class of oxometallates can exist as discrete as well as polymeric ions. For example, while orthovanadate is a discrete [VO4]3− ion, metavanadate ([VO3]n)n− can be visualized as a polymeric network of VO3 units. Earlier reports on cobalt vanadate as OER catalyst have ascribed activity difference to optimal metal-oxygen bond strength. In contrast, we found out that the etching of vanadate moieties from the precatalyst during the electrochemical activation plays a pivotal role [3]. Further, we observed that the polymerization of vanadate ions significantly affects the etching process. CoV2O6 has a polymeric structure consisting of chains of [VO3]−, formed via corner-shared VO4 tetrahedra, corner-shared VO6 octahedra, and edge-shared VO6 octahedra, whereas in Co3V2O8 vanadate ions only exist as discrete [VO4]3- tetrahedral. Due to the polymeric structure of metavanadate, its etching from the material would tear off a long chain of vanadate, thereby aiding the transformation of the material to its active phase. In contrast, the etching of orthovanadate is structurally constrained and, thus, much slower in nature. The resulting higher surface reconstruction exposes a higher number of catalytically active cobalt sites that otherwise lay inactive due to their position in bulk. Cobalt vanadates with polymeric vanadates (CoV2O6) showed better OER over monomeric orthovanadates (Co3V2O8) due to higher vanadate etching. Further, this work validates that etching can happen even from lattice positions, where bonding between constituents are strong.