Co/Ni-polyoxotungstate photocatalysts as precursor materials for electrocatalytic water oxidation

Abstract

An open-core cobalt polyoxometalate (POM) [(A-α-SiW₉O₃₄)Co₄(OH)₃(CH₃COO)₃]⁸⁻ Co(1) and its isostructural Co/Ni-analogue [(A-α-SiW₉O₃₄)Co_1.5Ni_2.5(OH)₃(CH₃COO)₃]⁸⁻ CoNi(2) were synthesized and investigated for their photocatalytic and electrocatalytic performance. Co(1) shows high photocatalytic O₂ yields, which are competitive with leading POM water oxidation catalysts (WOCs). Furthermore, Co(1) and CoNi(2) were employed as well-defined precursors for heterogeneous WOCs. Annealing at various temperatures afforded amorphous and crystalline CoWO₄- and Co_1.5Ni_2.5WO₄-related nanoparticles. CoWO₄-related particles formed at 300 °C showed substantial electrocatalytic improvements and were superior to reference materials obtained from co-precipitation/annealing routes. Interestingly, no synergistic interactions between cobalt and nickel centers were observed for the mixed-metal POM precursor and the resulting tungstate catalysts. This stands in sharp contrast to a wide range of studies on various heterogeneous catalyst types which were notably improved through Co/Ni substitution. The results clearly demonstrate that readily accessible POMs are promising precursors for the convenient and low-temperature synthesis of amorphous heterogeneous water oxidation catalysts with enhanced performance compared to conventional approaches. This paves the way to tailoring polyoxometalates as molecular precursors with tuneable transition metal cores for high performance heterogeneous electrocatalysts. Our results furthermore illustrate the key influence of the synthetic history on the performance of oxide catalysts and highlight the dependence of synergistic metal interactions on the structural environment.