Oxygen without Oxides: Photo- and Electrocatalytic Water Oxidation with a non-oxidic Cobalt Catalyst

Debora Ressnig^a, Menny Shalom^a, Markus Antonietti^a, Fabio Evangelisti^b, Greta R. Patzke^b

^aMax Planck Institute of Colloids and Interfaces - Potsdam, Research Campus Golm, D-14424 Potsdam, Germany

^bUniversity Zurich, Winterthurerstrasse, 190, Zürich, Switzerland

Proceedings of International Conference on New Advances in Materials Research for Solar Fuels Production (SolarFuel14)

Montréal, Canada, 2014 June 25th - 26th

Organizer: Thomas Hamann

Poster, Debora Ressnig, 020
Publication date: 16th April 2014

Efficient, low-cost and stable heterogeneous catalysts are essential for solar fuel generation. The targeted design of water oxidation catalysts (WOCs) via structure-mechanism relationships is much more demanding for heterogeneous, complex metal oxides or electrodeposited catalysts than for molecular catalysts. This also applies for efficient and economic cobalt based WOCs, which attract increasing interest due to their regenerative performance in a broad operating window in neutral and alkaline conditions. Cobalt based WOCs are currently in the focus of intense debates about their operational stability and consequently for the true catalytic species at the interface of heterogeneous and homogeneous catalysts. Most recently non-oxidic Prussian Blue-type [1] electrocatalysts have emerged as exceptionally stable alternatives to cobalt oxide catalysts and they inspired us to further investigate the widely unexplored potential of heterogeneous early transition metal WOCs with well-defined nitrogen based matrices.
We introduce a new-type, non-oxidic WOC with high, dual photo- and electrocatalytic performance. The visible-light-driven activity was evaluated within the [Ru(bpy)₃]²⁺ photosensitizer, S₂O₈^2- sacrificial electron acceptor standard system. The superior performance over the benchmark Co₃O₄ WOC suggests the anionic matrix to be an all-important host for the catalytically active Co centers. The electrocatalytic characteristics were assessed in neutral or alkaline solution. The WOC showed a promising performance in terms of overpotential and current densities and the evaluations underlined the fundamental difference between the WOC and common CoO_x catalysts.[2] Furthermore we explored tuning options of the clear-cut structure trough Ni doping, which resulted in increased current densities at low substitution levels. Long-time electrolysis experiments and post catalytic characterization of the catalyst - after photo- and electrocatalytic experiments - strongly corrugates the bulk stability of the WOC.
[1] S. Pintado, S. Goberna-Ferrón, E. C. Escudero-Adán, J. R. Galán-Mascarós, JACS, 2013, 135, 13270-13273.
[2] J. B. Gerken, J. G. McAlpin, J. Y. C. Chen, M. L. Rigsby, W. H. Casey, R. D. Britt, S. S. Stahl, JACS, 2011, 133, 14431-14442.

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