Electrochemical Water Oxidation Catalysis Beginning with Co(II) Polyoxometalates: The Case of the Precatalyst Co₄V₂W₁₈O₆₈^10–

The question is addressed of whether the cobalt polyoxometalate (Co-POM) precatalyst Co₄V₂W₁₈O₆₈^10– (hereafter <b>Co</b>_<b>4</b><b>V</b>_<b>2</b><b>W</b>_<b>18</b>) is a stable, homogeneous water oxidation catalyst under electrochemically driven conditions and in 0.1 M pH 5.8 and 8.0 NaPi buffer as well as pH 9.0 sodium borate (NaB) buffer. This question is of considerable interest since <b>Co</b>_<b>4</b><b>V</b>_<b>2</b><b>W</b>_<b>18</b> has been reported to be highly stable and a 200-fold faster water oxidation catalyst than its P congener Co₄P₂W₁₈O₆₈^10– (hereafter <b>Co</b>_<b>4</b><b>V</b>_<b>2</b><b>W</b>_<b>18</b>), for reasons that were not specified. The nature of the true water oxidation catalyst with <b>Co</b>_<b>4</b><b>V</b>_<b>2</b><b>W</b>_<b>18</b> as the starting material is of further fundamental interest because a recent report reveals that the ⁵¹V NMR peak at ca. −507 ppm assigned by others to <b>Co</b>_<b>4</b><b>V</b>_<b>2</b><b>W</b>_<b>18</b> and used to argue for its solution stability is, instead, correctly assigned to the highly stable <i>cis</i>-V₂W₄O₁₉^4–, in turn raising the question of the true stability of <b>Co</b>_<b>4</b><b>V</b>_<b>2</b><b>W</b>_<b>18</b> under water oxidation catalysis conditions. A battery of physical methods is used to address the questions of the stability and true water oxidation catalyst with <b>Co</b>_<b>4</b><b>V</b>_<b>2</b><b>W</b>_<b>18</b> as the precatalyst: ³¹P line-broadening detection of Co­(II) present in solution from leaching or as a counterion impurity; a check of those Co­(II) concentration results by the second method of cathodic stripping; the O₂ yield (and, hence, Faradaic efficiency) of electrocatalytic water oxidation; electrochemical, SEM, EDX, and XPS characterization of CoO_<i>x</i> films produced on the electrode; and multiple controls and other experiments designed to test alternative hypotheses that might explain the observed results. The collective evidence provides a compelling case that Co­(II) derived from <b>Co</b>_<b>4</b><b>V</b>_<b>2</b><b>W</b>_<b>18</b> forms a CoO_<i>x</i> film on the electrode which, in turn, carries all the observed, electrochemically driven water-oxidation catalysis current within experimental error. A list of seven main findings is provided as a summary