Role of Physicochemical Features toward Bifunctional Redox Activity of Transition-Metal Molybdates

In recent years, the search for non-noble metal-basedbi-functionalelectrode materials with excellent activity and stability for overallwater splitting has led the energy field research toward transition-metal(TM)-based materials that are abundant and comparatively stable overa wide range of pH. Herein, a series of late first-row TM molybdates(TMMo, TM = Co, Ni, Cu, and Zn) were studied for alkaline water splitting,where the materials were synthesized through a surfactant confinementreaction via the hydrothermal process. The microscopic analyses showedvaried morphology like, fusiform, forest, disks, and rugby ball withmultivalency and monoclinic/triclinic crystal structures of the TMMomaterials. Among the molybdates, CoMo, with a larger number of activesites with an inequivalent atomic position in the cluster and upshiftedd/p bands showed the best activity as both cathode and anode materialswith the overpotentials of 280 and 408 mV, respectively, to obtaina current density of 100 mA cm(-2). CoMo exhibitedfaster reaction kinetics over other molybdates as it had lower charge-transferresistance with significant stability. Furthermore, a two-electrodesystem with CoMo as the cathode and anode provided a lower cell voltageof 1.86 V at 100 mA cm(-2) current density over commercialelectrode materials, indicating CoMo can be an excellent commercialalternative electrocatalyst for overall water splitting. This work explores the bifunctional redoxbehavior of multivalentcobalt molybdate nanofusiforms with crystal nonequivalence and unsaturationfor water electrolysis.