Conversion of electrodeposited Co(OH)2 to CoOOH and Co3O4, and comparison of their catalytic activity for the oxygen evolution reaction

Cobalt hydroxide, β-Co(OH)2, was electrodeposited cathodically from an alkaline solution of tris(ethylenediamine)cobalt(III). The Co(OH)2 crystals grew into a microcone morphology and the surface became CoOOH at anodic potentials. The Co(OH)2 film was fully converted to CoOOH by electrochemical oxidation at 95 °C in 1 M KOH and to Co3O4 by thermal decomposition at 300 °C in air. The Co(OH)2 and CoOOH films grew with a [001] orientation on Au(111), and the converted Co3O4 films had a [111] orientation. The overall morphology was retained upon conversion. However, an increase in roughness was observed on the surface of the CoOOH and Co3O4 microcones. The electrochemically active area of the films was estimated from double layer capacitance measurements in acetonitrile with tetrabutylammonium hexafluorophosphate electrolyte. The CoOOH had a roughness factor of 9.4, and the Co3O4 had a roughness factor of 63.6. The catalytic activities of the films for the oxygen evolution reaction (OER) were compared by Tafel analysis in an O2 saturated 1 M KOH at room temperature. The CoOOH and Co3O4 films appeared to exhibit different Tafel behavior with Co3O4 being the superior catalyst when the geometric area was used in the Tafel analysis. However, when the observed current densities were corrected for the measured electrochemically active areas, the linear regions of the two Tafel plots fell on the same line. The results suggest that the active species, likely Co(IV), for the OER is the same on both materials. Both CoOOH and Co3O4 had a Tafel slope of 60 mV dec−1 and an exchange current density of 6.1 × 10−11 A cm−2. The conversion method outlined in the paper provides a means to produce Co3O4 with a large electrochemically-active area.