Water Oxidation Electrocatalysis in Acidic Media with Fe-Containing POMs/Carbon Composites

Iron-based catalysts are very appealing in terms of applications due to the low cost of Fe and to its abundance in the Earth's crust. In the field of water oxidation, unfortunately, iron oxides cannot match the activity of Co or Ni oxides, much less than the activity of noble metal oxides (IrO₂). The activity of transition metals to promote the oxygen evolution reaction (OER) can be tuned and enhanced by their incorporation into polyoxometalate frameworks (POMs). In comparison with metal oxides, POMs offer a controlled, discrete structure and a tailor-made environment. Fe-POMs still show a low OER activity in neutral or basic media when compared to Co-POMs. When moving to highly acidic media, we have found an unexpected electrochemical response in carbon paste electrodes containing salts of the [Fe₄^III(H₂O)₂(PW₉O₃₄)₂]^6- (Fe₄) polyanion. In oxidative conditions, these electrodes showed lower onset potentials and higher current densities than their Co-based analogues, contrary to computational expectations. Careful analyses have shown the excellent stability of the Fe₄ in these pH < 1 conditions, but a poor selectivity. CO₂ is the dominant product, in addition to O₂. The capability of Fe₄ to oxidize amorphous carbon under acidic conditions appears to be unique since it is not found in Fe oxides or simple Fe salts. Thus, Fe-POMs, in acidic conditions, are still modest OER catalysts, but exhibit a unique performance when electrochemically oxidizing carbon.