Overall water splitting using a photoelectrochemical (PEC) cell composed of a photocathode and photoanode connected in a series is an attractive method to produce hydrogen from water under sunlight. Because driving forces of two photoelectrodes are combined for the water splitting reaction, narrow-gap materials can be used to achieve high solar-to-hydrogen conversion efficiency (STH). However, the STHs obtained from a PEC cell without any external bias voltage reported so far have been less than 1%. This is because of insufficient onset potentials and photocurrent values of photoelectrodes. For the sake of overcoming the drawbacks, (ZnSe)_0.85(CuIn_0.7Ga_0.3Se₂)_0.15 ((ZnSe)_0.85(CIGS)_0.15) thin film photocathodes have recently been developed and reported to show a high onset potential of 0.89 V_RHE and long absorption edge of 850-900 nm.² These properties are suitable for use in the PEC cell.

(ZnSe)_0.85(CIGS)_0.15 thin films are prepared by co-evaporation onto Mo-coated soda-lime glass substrates. Subsequently, CdS, a binary of Mo/Ti and Pt are deposited onto the photocathode surface as a buffer layer, surface conductor and hydrogen evolution reaction (HER) catalyst, respectively.³ The surface-modified (ZnSe)_0.85(CIGS)_0.15 photocathodes show a relatively high photocurrent value of 12 mA cm^-2 at 0 V_RHE and sufficient stability in a span of hours at potentials of more negative than 0.5 V_RHE under simulated sunlight in a neutral potassium phosphate buffer solution.⁴ However, at potentials more positive than 0.5 V_RHE, the photocurrent value attributed to HER dramatically decreases by half in just one hour mainly due to self-oxidation of the surface sulfide layer, accompanying detachment of the Mo/Ti and Pt at the surface. The poor stability at the positive potentials has made it difficult to construct durable PEC cell using the photocathodes. In this work, effects of surface modifications onto PEC properties of (ZnSe)_0.85(CIGS)_0.15 photocathodes are investigated.

For the sake of suppressing the surface corrosion, the CdS layer was passivated with In₂S₃, which was formed by using chemical bath deposition (CBD).⁵ Figure 1 shows the current-time curves of the surface-modified (ZnSe)_0.85(CIGS)_0.15 photocathodes at 0.6 V_RHE under simulated sunlight. Without the CBD treatment, the photocurrent value decreased by 50% in one hour under light irradiation. On the other hand, the In₂S₃-modified photocathode showed relatively stable PEC HER and the decline of the photocurrent value in one hour was decreased to 25%. It is highly possible that the improvement of stability originates from low solubility of indium oxide or hydroxide generated by the self-oxidation process, while the cadmium oxide and hydroxide are relatively soluble, which can cause corrosion of the surface.

Furthermore, effects of oxide-coating onto the stability of the (ZnSe)_0.85(CIGS)_0.15 photocathode during PEC hydrogen evolution have also been investigated. Among various kinds of coating processes, direct formation of the oxide layer by photoelectrodeposition without annealing the photocathode is an effective method without exerting a bad influence on the underlying sulfide and selenide materials. In the presentation, the details of preparation conditions and PEC properties will be discussed.

References

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