Different catalytic behavior of amorphous and crystalline cobalt tungstate for electrochemical water oxidation

Abstract

Amorphous and crystalline cobalt tungstate (CoWO₄) particles, synthesized by solution co-precipitation and subsequent heat treatment, were examined on rotating disk electrodes as water oxidation catalysts. In close to neutral electrolyte (0.2 M Na₂WO₄, pH 8.0), amorphous CoWO₄ exhibited much higher catalytic activity than its crystalline counterpart over an overpotential range of 0.25 to 0.45 V. Calculated Tafel slopes are ∼60 mV per decade for amorphous and ∼110 mV per decade for crystalline electrodes, while the exchange current density of crystalline catalysts is two orders of magnitude higher than that of amorphous. Elemental analysis did not identify substantial composition changes in either amorphous or crystalline films on indium tin oxide (ITO) electrodes, after being used for multiple cyclic voltammetry and bulk electrolysis tests. However, X-ray photoelectron spectroscopy (XPS) revealed that the oxidation state of the surface Co atoms on amorphous catalyst film changed from +2 to +3, although no difference was found in crystalline CoWO₄ . The characteristics of the amorphous catalyst closely resemble the well known self-repairing cobalt phosphate catalytic film (Co–Pi), in which the cooperative interaction of adsorbants on neighboring Co sites plays an important role in its high activity. In the case of crystalline CoWO₄, density functional theory (DFT) calculations showed that its most stable surface should be along the (010) plane, featuring Co atoms located at least 4.69 Å apart, which is significantly larger than the reported 2.82 Å in the cubane structure of Co–Pi. We believe this longer interatomic span hinders the adsorbant interaction and thus results in a different mechanism for oxygen evolution on the surface of crystalline CoWO₄.