ELECTRON PARAMAGNETIC RESONANCE SPECTROELECTROCHEMISTRY OF TRANSITION METAL SOLAR FUELS CATALYSTS
R. Adam Kinney and Veronika A. Szalai
Solar energy is the only renewable energy source capable of meeting the rapidly expanding global demand for energy (15 TW demand; 120,000 TW theoretical maximum capacity). One of the most promising conversion strategies is hydrogen fuel production, wherein the solar energy is stored as stable chemical bonds. Solar-driven water oxidation (artificial photosynthesis), which splits water into oxygen and hydrogen, produces dihydrogen from an abundant natural resource. However, the realization of an efficient water splitting system depends on the catalysts used to facilitate the oxygen and hydrogen evolution reactions at the anode and cathode, respectively. For many catalysts, including the widely used heterogeneous metal oxides (e.g. α-Fe2O3, WO3, cobalt phosphate), the chemical environment of the active catalyst species is poorly understood, due in large part to the difficulty of making the appropriate spectroscopic measurements on the microscopic materials in situ.
To investigate the mechanism of water oxidation on a wide variety of catalysts, we have designed a spectroelectrochemical cell for in situ electron paramagnetic resonance (EPR) spectroscopy. The cell permits simultaneous photochemical and electrochemical excitation of both homogeneous and heterogeneous catalyst samples. Additionally, our cell provides a method to directly assess how nanostructured electrodes influence the metal oxide chemistry. We present preliminary EPR measurements of trapped intermediates, generated photo- and/or electrochemically under working conditions, for several transition metal water oxidation catalysts, including iron oxide, iridium oxide, and cobalt phosphate (CoPi). Our results have demonstrated the applicability of EPR spectroscopy to the study of transition metal solar fuels catalysts on real electrodes capable of splitting water.