A series of six exemplary cobalt-polyoxometalate (Co-POM) precatalysts have been examined to determine if they are molecular water-oxidation catalysts (WOCatalysts) or if, instead, they actually form heterogeneous, electrode-bound CoO_x as the true WOCatalyst under electrochemically driven water-oxidation catalysis (WOCatalysis) conditions. Specifically, WOCatalysis derived from the following six Co-POMs has been examined at pH 5.8, 8.0, and 9.0: [Co₄(H₂O)₂(PW₉O₃₄)₂]^10– (Co₄P₂W₁₈), [Co₉(H₂O)₆(OH)₃(HPO₄)₂(PW₉O₃₄)₃]^16– (Co₉P₅W₂₇), [ββ-Co₄(H₂O)₂(P₂W₁₅O₅₆)₂]^16– (Co₄P₄W₃₀), [Co(H₂O)PW₁₁O₃₉]^5– (CoPW₁₁), [α₁-Co(H₂O)P₂W₁₇O₆₁]^8– (α₁-CoP₂W₁₇), and [α₂-Co(H₂O)P₂W₁₇O₆₁]^8– (α₂-CoP₂W₁₇). The amount of Co(II)_aq in 500 μM solutions of each Co-POM was measured after 3 h of aging as well as from t = 0 for pH = 5.8 and 8.0 by μM sensitive Co(II)_aq-induced ³¹P NMR line broadening and at pH = 9.0 by cathodic stripping. The amount of detectable Co(II)_aq after 3 h for the six Co-POMs ranges from ∼0.25 to ∼90% of the total cobalt initially present in the Co-POM. For 12 out of 18 total Co-POM and different pH cases, the amount Co(II)_aq detected after 3 h forms heterogeneous CoO_x able to account for ≥100% of the observed WOCatalysis activity. However, under 0.1 M NaPi, pH 5.8 conditions for CoPW₁₁ and α₁-CoP₂W₁₇ where ∼1.5% and 0.25% Co(II)_aq is detectable, the measured Co(II)_aq cannot account for the observed WOCatalysis. The implication is that these two Co-POMs are primarily molecular, Co-POM-based, WOCatalysts under electrochemically driven, pH 5.8, phosphate-buffer conditions. Even for the single most stable Co-POM, α₁-CoP₂W₁₇, CoO_x is still an estimated ∼76× faster WOCatalyst at pH = 5.8 and an estimated ∼740× faster WOCatalyst at pH = 8.

Energy production and consumption without the use of fossil fuels are amongst the biggest challenges currently facing humankind and the scientific community. Huge efforts have been invested in creating technologies that enable closed carbon or carbon neutral fuel cycles, limiting CO₂ emissions into the atmosphere. Formic acid/formate (FA) has attracted intense interest as a liquid fuel over the last half century, giving rise to a plethora of studies on catalysts for its efficient electrocatalytic oxidation for usage in fuel cells. However, new catalysts and catalytic systems are often difficult to compare because of the variability in conditions and catalyst parameters examined. In this review, we discuss the extensive literature on FA electrooxidation using platinum, palladium and non-platinum group metal-based catalysts, the conditions typically employed in formate electrooxidation and the main electrochemical parameters for the comparison of anodic electrocatalysts to be applied in a FA fuel cell. We focused on the electrocatalytic performance in terms of onset potential and peak current density obtained during cyclic voltammetry measurements and on catalyst stability. Moreover, we handpicked a list of the most relevant examples that can be used for benchmarking and referencing future developments in the field.