Water oxidation electrocatalysis in acidic media with Co-containing polyoxometalates

A series of Cobalt-polyoxometalates (Co-POM) have been incorporated into carbon paste electrodes as insoluble salts of Cs⁺ or Ba²⁺. Specifically, we included the lacunary Keggin-type derivatives [Co(H₂O)(PW₁₁O₃₉)]^5– (CoPW₁₁), [Co(H₂O)(SiW₁₁O₃₉)]^6– (CoSiW₁₁), [Co(H₂O)(CoW₁₁O₃₉)]^7– (CoCoW₁₁), [Co₄(H₂O)₂(PW₉O₃₄)₂]^10– (Co₄P₂W₁₈), [Co₄(H₂O)₂(SiW₉O₃₄)₂]^12– (Co₄Si₂W₁₈), and [Co₉(H₂O)₆(OH)₃(HPO₄)₂(PW₉O₃₄)₃]^16– (Co₉); and the lacunary Dawson-Wells derivative [Co₄(H₂O)₂(P₂W₁₅O₅₆)₂]^16– (Co₄P₄W₃₀). The activity and stability of these composite electrodes towards water oxidation in acidic media (pH < 1) was examined by electrochemical methods and post-catalytic characterization. Our results indicate the stability of these catalysts in these conditions depends on composition, since only those species containing P^V as heteroatom appeared to be stable. All Si^IV and Co^III derivates showed rapid decomposition, regardless their nuclearity or molecular structure. All stable derivatives exhibited consistent activity, favored by high nuclearity and overall total charge, with Keggin derivatives being intrinsically more active than the corresponding Dawson-Wells analogues.