Cu1-xNixWO4 as a Selectivity-Enhancing Catalytic Overlayer for WO3 in Photoelectrochemical Water Oxidation

James Brancho^a, Charles Lhermitte^a, Bart Bartlett^a

^aUniversity of Michigan, 930 N University, Ann Arbor, 0, United States

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, James Brancho, 042
Publication date: 16th April 2014

Semiconductor-semiconductor heterojunctions have long been a topic of interest in photovoltaics research. Due to the intrinsic electric field that builds up between semiconductors of differing electron concentration, photoexcited electrons and holes are more easily separated physically as they move in opposite directions across the junction. It is also possible to use a semiconductor heterojunction to improve catalysis at the solution interface of an electrode. We seek to combine these two principles in the study of a CuWO₄/WO₃ bulk heterojunction (BHJ) anode for photoelectrochemical (PEC) water oxidation. WO₃ is a well-known photoanode material for PEC water oxidation but is limited by its instability in neutral electrolytes and its propensity to oxidize electrolyte components in efficiency-sapping side reactions.^1,2 Photoexcited WO3 can drive oxidations indiscriminately due to its extremely oxidizing valence band potential of +3.1 V vs. NHE. We hypothesize, therefore, that introducing an overlayer of CuWO₄ will provide an energetic filter for high-energy holes since its valence band maximum sits at approximately +2.6 V vs. NHE.³ We predict that this strategy will increase the selectivity for water oxidation on the BHJ electrode compared to WO₃. Additionally, we hypothesize that doping Ni²⁺ into the CuWO4 overlayer will raise the overlayer conduction band and further improve kinetics.
WO₃ electrodes 2 μm in thickness were fabricated by a known sol-gel method from an H₂WO₄ suspension.⁴ Cu_1-xNi_xWO₄ (x = 0, 0.05) overlayers were introduced by treating the WO₃ electrodes with solutions of metal nitrates, which partially reacted with WO₃ at 550 ºC after drying (adapted from a published procedure).⁵ The resulting BHJ electrodes were characterized by powder X-ray diffraction and scanning electron microscopy, revealing a perfect two-phase mixture with insignificant changes in electrode thickness. PEC water oxidation was carried out in borate electrolyte under 1-sun illumination, and a significant increase in the Faradaic efficiency over WO₃ was noted after the application of the Cu_1-xNi_xWO₄ overlayer (49.8% WO₃, 58.7% CuWO₄/WO₃, 75.6% Cu_0.95Ni_0.05WO₄/WO₃). The measured linear dependence of steady-state photocurrent on photon flux for all three electrode compositions indicates that junction recombination plays a minor role in overall electrode kinetics.⁶ To investigate the origins of the selectivity increase, photoelectrolysis was carried out in the presence of the spin trapping agent α-(1-oxo-4-pyridyl)-N-tert-butylnitrone, which has been used previously to provide evidence of ^●OH generation on WO₃ powders.⁷ The results of these studies will be presented.

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