Mechanistic Investigation of Electrochemical Water Oxidation
Ki Tae Nam
Department of Materials Science and Engineering, Seoul National University

Water splitting is regarded as a promising step towards environmentally sustainable energy schemes. The oxygen evolution reaction (OER) requires extremely high overpotential due to its slow reaction kinetics. The water oxidizing cluster in photosystem II, in the form of cubical Mn4CaO5 cluster, efficiently catalyzes water oxidation. Specific questions that we intensively focus for further applications include how to translate the underlying principles in Mn4CaO5 cluster into synthetic heterogeneous catalysts. Toward this vision, we have been developing a new catalytic platform based on sub-10 nm-sized Mn oxide nanoparticles to bridge the gap between atomically defined biological catalysts. In this approach, the local atomic geometry is controlled by the surface modification of the specific ligand and the heterogeneous atom doping, that enhance the catalytic activity and selectivity. Furthermore, we detected key intermediate species, Mn(IV)=O, based on comprehensive electrokinetic and in-situ spectroscopic analysis. We revealed unique water oxidizing mechanism mediated by Mn-oxide nanocatalysts different from bulk counterparts.
We further conducted electrochemical impedance spectroscopy (EIS) analysis to understand various electrochemical processes in film-type OER electrocatalysts. The transmission line model with Havriliak-Negami capacitors and Warburg element was newly established for accurate fitting analysis and well-defined kinetic parameters for Mn-oxide nanocatalysts. From EIS analysis, protons are involved in the electron transport process across the nanoparticle film, which directly proves the ongoing hypothesis of the oxo-hopping mechanism. Also, from the correlation between k and TOF, the ratio (∼0.001) of total Mn sites on the surface to the actual number of active sites could be evaluated.
Also, we focused on the entropic contribution for water oxidizing electrocatalysis. The activation enthalpy and entropy was measured from temperature dependent analysis for well-known Co-oxide catalyst (Co-Pi). The high negative entropic contribution in heterogeneous water oxidation was confirmed. The entropy effect on OO bond formation by both the AB and RC mechanisms was studied. We anticipate that the unfavorable entropic effect in heterogeneous system could be improved by a hydrogen-bond network stabilizing the active site controlling the transportation of water molecules to the electrode surface in an ordered way.