Low-Cost, Efficient, and Durable H2 Production by Photoelectrochemical Water Splitting with CuGa3Se5 Photocathodes

Photoelectrochemical (PEC) water splitting is an elegant method of converting sunlight and water into H₂ fuel. To be commercially advantageous, PEC devices must become cheaper, more efficient, and much more durable. This work examines low-cost polycrystalline chalcopyrite films, which are successful as photovoltaic absorbers, for application as PEC absorbers. In particular, Cu-Ga-Se films with wide band gaps can be employed as top cell photocathodes in tandem devices as a realistic route to high efficiencies. In this report, we demonstrate that decreasing Cu/Ga composition from 0.66 to 0.31 in Cu-Ga-Se films increased the band gap from 1.67 to 1.86 eV and decreased saturated photocurrent density from 18 to 8 mA/cm² as measured by chopped-light current-voltage (CLIV) measurements in a 0.5 M sulfuric acid electrolyte. Buffer and catalyst surface treatments were not applied to the Cu-Ga-Se films, and they exhibited promising stability, evidenced by unchanged CLIV after 9 months of storage in air. Finally, films with Cu/Ga = 0.36 (approximately stoichiometric CuGa₃Se₅) and 1.86 eV band gaps had exceptional durability and continuously split water for 17 days (∼12 mA/cm² at -1 V vs RHE). This is equivalent to ∼17 200 C/cm², which is a world record for any polycrystalline PEC absorber. These results indicate that CuGa₃Se₅ films are prime candidates for cheaply achieving efficient and durable PEC water splitting.