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D6.07 - Understanding the Effect of Calcination Temperature and Composition on Cobalt Titanium Oxides as Catalysts for the Oxygen Evolution Reaction 
April 24, 2014   11:15am - 11:30am

Photoelectrochemical water splitting can be used to store solar energy in the form of hydrogen. However, the efficiency of the overall water splitting reaction is severely limited by the high overpotential costs required for the oxygen evolution half reaction. Furthermore, there exists a need for a nonprecious metal catalyst to drive the oxygen evolution reaction (OER) at low overpotentials to make the process more economical as well as more efficient. As an alternative to the best known precious metal OER catalysts like RuO2 and IrO2, previous work has focused on manganese oxide and cobalt oxide catalysts.[1-5] We have identified amorphous CoTixOy as a novel, active, nonprecious metal catalyst for OER. Using a simple and scalable sol gel synthesis, thin films (50 nm - 200 nm) of CoTixOy can be deposited on a number of different conductive substrates. This work begins by characterizing the structure and morphology of this material and studying the effect of varying the heat treatment temperature and metals composition on catalytic activity and material stability. Furthermore, we characterize the effect of preparation route and electrochemical testing on the oxidation state of cobalt using ex situ L-edge x-ray absorption spectroscopy (XAS). We have verified that crystalline CoTiO3 is formed at calcination temperatures 550°C and above, however the lower temperature calcinations which result in an amorphous CoTixOy material produce better catalysts for the OER. A calcination temperature of 150°C for a sample with equal amounts of cobalt and titanium produces a catalyst with a relatively low overpotential (~440 mV) to reach 10 mA/cm2 and loses only 26% of its initial activity over 10,000 oxidation/reduction cycles. All samples with 55% or greater cobalt in the metals composition have high activity for the OER. Furthermore, adjusting the metal composition of this material tunes the redox peak potential that is observed for cobalt on the first oxidative electrochemical sweep. Ex situ XAS reveals that both calcination temperature and metal composition have an effect on the cobalt oxidation state. It is found that the cobalt is in a mixed 2+/3+ state under OER conditions but that achieving this state via adjusted material composition or electrochemical oxidation results in a more active catalyst than via thermal means.1. Gorlin, Y. & Jaramillo, T.F. J Am Chem Soc 132, 13612-13614 (2010).2. Brimblecombe, R., Koo, A., Dismukes, G.C., Swiegers, G.F. & Spiccia, L. J Am Chem Soc 132, 2892-2894 (2010).3. Yeo, B.S. & Bell, A.T. J Am Chem Soc 133, 5587-5593 (2011).4. Esswein, A.J., Surendranath, Y., Reece, S.Y. & Nocera, D.G. Energy Environ. Sci. 4, 499-504 (2011).5. Liang, Y. Li, Y. Wang, H. Zhou, J. Wang, J. Regier, T. & Dai, H. Nature Mater. 10, 780-786 (2011).

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