Energy Focus: Earth-abundant photocorrosion-resistant material used for solar water splitting

A simple and low-temperature antimony (Sb) deposition method has been used to generate a new photocorrosion-resistant material for water-splitting applications. Research led by David Tilley of the University of Zürich made the conventional protective layer in a solar water-splitting application an obsolete requirement by introducing a combination of Sb₂Se₃ and MoS_x, which functions without the need of any protective layer. Their research was published in a recent issue of the Journal of Materials Chemistry A (doi:10.1039/c7ta08993g).

The Sb₂Se₃ thin films were fabricated through selenization of simple, low-temperature electrodeposited Sb films. Current state-of-the-art photocathodes need a protective layer of titanium dioxide (TiO₂). Initially, Tilley’s group used Sb₂Se₃ with the TiO₂ protective layer and a platinum (Pt) catalyst. However, they soon realized the stable nature of Sb₂Se₃ and found low-cost amorphous MoS_x to replace scarce and expensive Pt.

Tilley says, “We considered that the Sb₂Se₃ might be stable without protective layers, and we also wanted to explore earth-abundant alternatives to state-of-the-art HER catalysts like Pt and RuO_x. One such candidate which came to our mind was amorphous MoS_x as it could be prepared by a simple electrodeposition method which was low cost and fast.” The photocorrosion-resistant behavior of Sb₂Se₃ in combination with MoS_x eliminates the need for any such coating. Sb₂Se₃–MoS_x is one of the few materials that is immune to photocorrosion while providing the high photocurrent that is required for water-splitting applications.

(a) Schematic representation of Sb₂Se₃–MoS_x photocathode. (b) Current density-potential characteristics of Sb₂Se₃–MoS_x (non-sulfurized) and Sb₂Se₃–MoS_x–S (sulfurized) photocathode in 1 M H₂SO₄ under simulated 1 sun illumination (100 mW cm⁻²). The bare Sb₂Se₃ is under light chopping (i.e., light was blocked as the device was measured under dark conditions). Reproduced from J. Mater. Chem. A 5 (2017), p. 23139, with permission from the Royal Society of Chemistry.

A sulfurization process became the key factor in increasing the photocurrent of the device. A high photocurrent density of 5.2 and 13 mA cm^–2 at 0 V versus RHE was recorded for Sb₂Se₃–MoS_x and Sb₂Se₃–MoS_x–S (sulfurized photocathode coupled with the catalyst), respectively. The sulfurization only affects the surface of the MoS_x layer as indicated in elemental mapping performed through energy-dispersive x-ray spectroscopy.

Kazuhiro Takanabe of the Physical Science and Engineering Division at King Abdullah University of Science and Technology (KAUST), highlights the importance of this publication in the field of solar water splitting. “Chalcogenide semiconductors have been long considered as photoelectrochemical material, but its stability in water under illumination remains an issue. Ramanujam et al. (this study) exhibited a way of the successful decoration of Sb₂Se₃ with a hydrogen evolution catalyst (MoS_x) to solve the photocorrosion path. This methodology can be applied to various materials, and thus it is an important finding,” Takanabe says.

Tilley and his research team are now looking to gain a deeper understanding of recombination mechanisms in the Sb₂Se₃–MoS_x–S photocathode to increase the photovoltage.