Bioinspired Water Oxidation Using a Mn-Oxo Cluster Stabilized by Non-Innocent Organic Tyrosine Y161 and Plastoquinone Mimics

Abstract

The complexity and energy demand of the H2O oxidation half-reaction represents the bottleneck for the development of sustainable, environmentally friendly H2 economies using H2O as energy feedstock. In Nature, photosynthetic H2O oxidation processes occur in photosystem-II (PS-II) and are facilitated by the oxygen evolving complex (OEC), a manganese-oxo cluster {Mn4CaO5} with cubane-like topology. In recent years, the use of manganese-based H2O oxidation catalysts has attracted significant scientific attention, not only to mimic and understand naturally occurring processes, but also due to the low toxicity and high abundancy of Mn in the Earth’s crust. Here we report the catalytic H2O oxidation activity at pH 7.2 of a high-nuclearity manganese-oxo cluster. The species, which contains multiple cubane motifs and which is stabilized by redox-active aromatic organic ligands, gives rise to an onset overpotential as low as 255 mV when dispersed in a carbon paste matrix, achieving high current densities of 10 mA cm–2 and even 100 mA cm–2 at η = 482 and 654 mV, respectively. The electrodes show good stability under turnover conditions for 7 h. Additionally, direct light-induced O2 evolution measurements confirm a reaction rate of 0.72 s–1 and turnover number (TON) of up to 55. The outlined experimental concept demonstrates how a synergistic effect between non-innocent, redox-active organic ligands and bioinspired Mn oxo-clusters resembling the natural {Mn4CaO5} unit, which are dispersed in a conductive carbon matrix and protected by a Nafion membrane, can facilitate remarkably high catalytic activity under neutral, environmentally friendly pH conditions.