(Invited) Bioinspired Water Oxidation and CO2 Reduction Electrocatalysts

Abstract

Water splitting is regarded as a promising step towards environmentally sustainable energy schemes because electrolysis produces only hydrogen and oxygen, without any by-products. Within the overall water splitting process, the oxygen evolution reaction (OER), an anodic half-cell reaction that generates an oxygen molecule from two water molecules, requires extremely high overpotential due to its slow reaction kinetics. In this regard, development of cost-effective and robust catalyst has been demanding challenge to solve the modern energy crisis. In nature, there exists a water oxidation complex (WOC) in photosystem II (PSII) comprised of the earth-abundant elements Mn and Ca. The WOC in PSII, in the form of a cubical Mn4CaO5 cluster, efficiently catalyzes water oxidation under neutral conditions with extremely low overpotential value (~160 mV) and a high turnover frequency (TOF) number (~25,000 mmolO2 mol-1 Mn s-1). Inspired from the asymmetric geometry and flexible ligation of the biological CaMn4O5 cluster, we designed new manganese based water oxidation catalysts, Mn3(PO4)2-3H2O and Li2MnP2O7.We first identified a new crystal structure, Mn3(PO4)2-3H2O, and demonstrated its superior catalytic performance at neutral pH. From the combined ex-situ spectroscopic analysis and DFT calculations, we revealed that structural flexibility can stabilize Jahn-Teller distorted Mn(III), and thus facilitate Mn(II) oxidation during catalytic cycle. We also studied a new pyrophosphate based Mn compound, Li2MnP2O7 for water oxidation catalysis. We verified the influence of oxidation state of Mn and asymmetric Mn geometry on water oxidation catalysis using Li2MnP2O7 and its derivatives. We believe higher Mn(III) portion and asymmetric arrangement of Mn atoms which can be seen in the WOC, enhance catalytic water oxidation reaction. Specific questions that our group intensively focus for the further applications include 1) how we can translate the underlying principles in Manganese Calcium Clusters into the synthetic heterogeneous catalysts and 2) how we can mimic the redox molecule involved biological dark reaction for the CO2 reduction. Toward this vision, we have been developing a new catalytic platform based on sub-10 nm uniform nanoparticles to bridge the gap between atomically defined biological catalysts or their metalloenzyme counterparts and the scalable, electrode depositable heterogeneous catalysts. In this approach, the local atomic geometry is controlled by the nitrogen containing graphitic carbon and the heterogeneous atom doping, that enhance the catalytic activity and selectivity. Additional surface modification by the specific ligand allow for the atomic scale tunability to realize the unique electronic hybridization.