Water splitting is a promising approach to the efficient and cost-effective production of renewable fuels, but water oxidation remains a bottleneck in its technological development because it largely relies on noble-metal catalysts. Although inexpensive transition-metal oxides are competitive water oxidation catalysts in alkaline media, they cannot compete with noble metals in acidic media, in which hydrogen production is easier and faster. Here, we report a water oxidation catalyst based on earth-abundant metals that performs well in acidic conditions. Specifically, we report the enhanced catalytic activity of insoluble salts of polyoxometalates with caesium or barium counter-cations for oxygen evolution. In particular, the barium salt of a cobalt-phosphotungstate polyanion outperforms the state-of-the-art IrO₂ catalyst even at pH < 1, with an overpotential of 189 mV at 1 mA cm^–2. In addition, we find that a carbon-paste conducting support with a hydrocarbon binder can improve the stability of metal-oxide catalysts in acidic media by providing a hydrophobic environment.

A major roadblock in realizing large-scale production of hydrogen via electrochemical water splitting is the cost and inefficiency of current catalysts for the oxygen evolution reaction (OER). Computational research has driven important developments in understanding and designing heterogeneous OER catalysts using linear scaling relationships derived from computed binding energies. Herein, we interrogate 17 of the most active molecular OER catalysts, based on different transition metals (Ru, Mn, Fe, Co, Ni, and Cu), and show they obey similar scaling relations to those established for heterogeneous systems. However, we find that the conventional OER descriptor underestimates the activity for very active OER complexes as the standard approach neglects a crucial one-electron oxidation that many molecular catalysts undergo prior to O–O bond formation. Importantly, this additional step allows certain molecular catalysts to circumvent the “overpotential wall”, leading to enhanced performance. With this knowledge, we establish fundamental principles for the design of ideal molecular OER catalysts.

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.