(544gz) Probing the (Photo)Electrochemical Stability of Atomic Layer Deposited Coatings for Solar-Driven Hydrogen Evolution

A key remaining challenge for the demonstration of a technologically relevant solar-driven water splitting device is achieving durability that translates to years of standalone operation. While highly-efficient (up to 16% solar-to-hydrogen efficiency) systems have been developed, these have only been shown to function continuously for several days. Since intrinsic stability has yet to be demonstrated for an efficient light-absorbing material, the field has investigated a number of coatings to protect these materials from contact with the aqueous electrolyte. Given the requirement for completely isolating the light absorber from the electrolyte, atomic layer deposition (ALD) has emerged as a preferred technique for depositing conformal and thin films in this context.¹

In this work, we investigate the (photo)electrochemical stability of several canonical metal oxide and chalcogenide protective coatings deposited by ALD, including titania (TiO₂) and molybdenum disulfide (MoS₂). Experiments are conducted in both standalone electrochemical testing and photoelectrochemical testing when combined with thin film chalcopyrite (CuGa_xSe_y) photocathodes. Through x-ray photoelectron spectroscopic (XPS) interrogation of as-prepared samples and of those that have undergone electrochemical testing, some surprising results emerge. While ALD-deposited TiO₂ has imparted good stability to photoanodes under oxygen-evolving potentials in basic conditions,^1,2 our ultra-thin film (5 nm) amorphous TiO₂ coatings completely degrade on the order of hours at hydrogen-evolving potentials in acidic conditions, and over a couple of days at hydrogen-evolving potentials in near-neutral conditions. Similarly, amorphous molybdenum trioxide (MoO₃)-derived MoS₂ coatings were shown to be dramatically less stable than Mo metal-derived MoS₂ coatings under hydrogen-evolving potentials in acidic conditions. High temperature anneals were shown to have a marked stabilizing influence on the hydrogen-evolving performance of the ALD-deposited coatings; for TiO₂, the initial degradation rate was slowed by at least a factor of five. Finally, promising next-generation oxide and chalcogenide coatings for hydrogen evolution were also examined and their results are presented herein.

¹ Chen, Y. W., Prange, J. D., Dühnen, S., Park, Y., Gunji, M., Chidsey, C. E. D., & McIntyre, P. C. (2011). Atomic layer-deposited tunnel oxide stabilizes silicon photoanodes for water oxidation. Nature Materials, 10(7), 539–544.

² Hu, S., Shaner, M. R., Beardslee, J. A., Lichterman, M., Brunschwig, B. S., Lewis, N. S., … Wallrapp, F. (2014). Amorphous TiO₂ coatings stabilize Si, GaAs, and GaP photoanodes for efficient water oxidation. Science (New York, N.Y.), 344(6187), 1005–9.