(Invited) Photoelectrochemical Investigation of Rate Limiting Processes and Mechanistic Understanding of Solar Water Oxidation with CuWO4 Photoanodes

Monday, 29 May 2017: 10:00
Grand Salon A - Section 6 (Hilton New Orleans Riverside)
T. Hamann (Michigan State University)
Copper tungstate (CuWO4) is a promising material for solar water oxidation that has received limited attention, with many fundamental processes which control the efficiency of water oxidation not fully elucidated. In order to address this, we developed a modified atomic layer deposition method to prepare CuWO4 thin film electrodes for photoelectrochemical (PEC) water oxidation. Application of a suite of systematic PEC measurements were applied to reveal several interesting aspects of this promising material. The hole collection length was quantified by studying the current density-voltage (J-V) behavior and via comparisons of the incident-photon-to-current efficiencies (IPCE) for electrodes with different thicknesses. The CuWO4 thin film electrodes show severe recombination in its depletion region so that the charge separation is limited by the drift length of holes. We will also show that use of H2O2 as a hole scavenger is problematic as it gives rise to current doubling with CuWO4. Sulfite was found to be an ideal hole scavenger which was employed to quantify the hole collection efficiency of water oxidation at the CuWO4 surface. This was compared with results of intensity-modulated photocurrent spectroscopy (IMPS) measurements which showed essentially quantitative hole collection efficiency at 1.23 V vs RHE. The photocurrent onset of water oxidation is shifted about 200 mV positive compared with sulfite oxidation, suggesting surface recombination still limits the water oxidation performance somewhat. Electrochemical impedance spectroscopy (EIS) measurements were employed to understand the role of surface states of CuWO4 which indicated the surface state is not a permanent state in the electronic structure of CuWO4 as proposed by previous literature, but a photogenerated water oxidation intermediate. These combined results will be compared to other well-studied materials, such as hematite, and general operational principles discussed.