Low-Cost,\nEfficient, and Durable H₂ Production by Photoelectrochemical\nWater Splitting with CuGa₃Se₅ Photocathodes

Abstract

Photoelectrochemical\n(PEC) water splitting is an elegant method of converting sunlight\nand water into H₂ fuel. To be commercially advantageous,\nPEC devices must become cheaper, more efficient, and much more durable.\nThis work examines low-cost polycrystalline chalcopyrite films, which\nare successful as photovoltaic absorbers, for application as PEC absorbers.\nIn particular, Cu–Ga–Se films with wide band gaps can\nbe employed as top cell photocathodes in tandem devices as a realistic\nroute to high efficiencies. In this report, we demonstrate that decreasing\nCu/Ga composition from 0.66 to 0.31 in Cu–Ga–Se films\nincreased the band gap from 1.67 to 1.86 eV and decreased saturated\nphotocurrent density from 18 to 8 mA/cm² as measured by\nchopped-light current–voltage (CLIV) measurements in a 0.5\nM sulfuric acid electrolyte. Buffer and catalyst surface treatments\nwere not applied to the Cu–Ga–Se films, and they exhibited\npromising stability, evidenced by unchanged CLIV after 9 months of\nstorage in air. Finally, films with Cu/Ga = 0.36 (approximately stoichiometric\nCuGa₃Se₅) and 1.86 eV band gaps had exceptional\ndurability and continuously split water for 17 days (∼12 mA/cm² at −1 V vs RHE). This is equivalent to ∼17 200\nC/cm², which is a world record for any polycrystalline\nPEC absorber. These results indicate that CuGa₃Se₅ films are prime candidates for cheaply achieving efficient and durable\nPEC water splitting.