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 IrO catalyst even at pH < 1, with an overpotential of 189 mV at 1 mA cm. 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.

The polyanion of formula {Co(9)(H(2)O)(6)(OH)(3)(HPO(4))(2)(PW(9)O(34))(3)}(16-) (Co(9)) contains a central nonacobalt core held together by hydroxo and hydrogen phosphate bridges and supported by three lacunary Keggin-type polyphosphotungstate ligands. Our data demonstrate that Co(9) is a homogeneous catalyst for water oxidation. Catalytic water electrolysis on fluorine-doped tin oxide coated glass electrodes occurs at reasonable low overpotentials and rates when Co(9) is present in a sodium phosphate buffer solution at neutral pH. We carried out our experiments with an excess of 2,2'-bipyridyl as the chelating agent for free aqueous Co(II) ions, in order to avoid the formation of a cobalt oxide film on the electrode, as observed for other polyoxometalate catalysts. In these conditions, no heterogeneous catalyst forms on the anode, and it does not show any deposited material or significant catalytic activity after a catalytic cycle. Co(9) is also an extremely robust catalyst for chemical water oxidation. It is able to continuously catalyze oxygen evolution during days from a buffered sodium hypochlorite solution, maintaining constant rates and efficiencies without any significant apparition of fatigue.

An insoluble salt of the water oxidation catalyst [Co9(H2O)6(OH)3(HPO4)2(PW9O34)3](16-) (Co9) has been used to modify amorphous carbon paste electrodes. The catalytic activity of this polyoxometalate is maintained in the solid state. Good catalytic rates are reached at reasonable overpotentials. As a heterogeneous catalyst, Co9 shows a remarkable long-term stability in turnover conditions. The oxygen evolution rate remains constant for hours without the appearance of any sign of fatigue or decomposition in a large pH range, including acidic conditions, where metal oxides are unstable.