Co_<i>x</i>(VO)_<i>y</i>O_<i>z</i> Nanocrystal-Integrated Covalent Organic Polymers as a Highly Active and Durable Catalyst for Electrochemical Water Oxidation: An Untold Role of the VO²⁺/VO₂⁺ Redox Couple

Abstract

Water oxidation is a crucial half-cell reaction in water splitting, metal–air batteries, and CO2 reduction. In this work, cobalt- and vanadium-containing mixed oxides [Cox(VO)yOz] are synthesized, and further, a unique composite of mixed oxide nanocrystals with a covalent organic polymer [Cox(VO)yOz@COP] is prepared. A high increase in activity and stability is exhibited by the Cox(VO)yOz@COP in comparison to its independent oxide counterparts. Higher activity is attributed to the presence of the VO2+/VO2+ couple, which helps in the facile oxidation of CoOOH to CoO2 and enhances the oxygen evolution reaction activity. The optimized composite material Cox(VO)yOz@COP(1:1) shows a low overpotential of 265 and 298 mV for the current densities of 10 and 30 mA cm–2, respectively. The composite shows a low Tafel slope (43 mV/dec), high turnover frequency (3.6 s–1 at 1.58 V), and high durability (tested for 14 h continuous oxygen evolution at 1.53 and 1.60 V). The durability is further supported by (i) chronopotentiometry (10,000 s at 25 mA cm–2), (ii) negligible variation in the linear sweep voltammetry responses and electrochemically active surface area values before and after 1000 cyclic voltammetry cycles, (iii) negligible dissolution of cobalt during catalysis observed from inductively coupled plasma mass spectroscopy of the electrolyte, and (iv) insignificant change in the catalyst surface composition observed from post-catalysis X-ray photoelectron spectroscopy. To the best of our knowledge, this Cox(VO)yOz@COP(1:1) material shows a higher activity in comparison to previously reported crystalline/amorphous cobalt–vanadium oxides. In addition, the increase in activity and stability from bare oxides to composite suggests that the COP shall work as a reliable catalytic support for future applications.