Hiromu Kumagai

Stable Hydrogen Evolution from CdS-Modified CuGaSe2 Photoelectrode under Visible-Light Irradiation

The photoelectrochemical properties of CuGaSe2 modified by deposition of a thin CdS layer were investigated. The CdS layer formed a p-n junction on the surface of the electrode, improving its photoelectrochemical properties. There was an optimal CdS thickness because of the balance between the charge separation effect and light absorption by CdS. CdS-deposited CuGaSe2 showed high stability under the observed reaction conditions and evolved hydrogen continuously for more than 10 days.

Efficient solar hydrogen production from neutral electrolytes using surface-modified Cu(In,Ga)Se2 photocathodes

The effects of a phosphate buffer electrolyte and surface modification with thin conductor layers on the photoelectrochemical properties of CdS and Pt-modified polycrystalline Cu(In,Ga)Se2 (CIGS) photocathodes were investigated. The photocurrent obtained from Pt/CdS/CIGS electrodes, in which the CIGS layer was fabricated by co-evaporation using a three stage method, clearly increased in a phosphate buffer electrolyte solution as a result of promotion of the hydrogen evolution reaction. The half-cell solar-to-hydrogen efficiency (HC-STH) of this device reached a maximum of 5.4% at 0.30 VRHE even under neutral conditions. Furthermore, significant enhancement of the hydrogen evolution reaction on a CIGS photocathode by surface modification with thin conductor layers was observed. The enhancement was due to the promoted charge transfer between the underlying photocathode and water through the Pt catalyst. The HC-STH of a CIGS photocathode modified with a conductive Mo/Ti layer (Pt/Mo/Ti/CdS/CIGS) was as high as 8.5% at 0.38 VRHE, a value that exceeds those previously reported for photocathodes based on polycrystalline thin films.

Photoelectrochemical Reduction of CO2 Coupled to Water Oxidation Using a Photocathode with a Ru(II)–Re(I) Complex Photocatalyst and a CoOx/TaON Photoanode

Photoelectrochemical CO2 reduction activity of a hybrid photocathode, based on a Ru(II)-Re(I) supramolecular metal complex photocatalyst immobilized on a NiO electrode (NiO-RuRe), was confirmed in an aqueous electrolyte solution. Under half-reaction conditions, the NiO-RuRe photocathode generated CO with high selectivity, and its turnover number for CO formation reached 32 based on the amount of immobilized RuRe. A photoelectrochemical cell comprising a NiO-RuRe photocathode and a CoOx/TaON photoanode showed activity for visible-light-driven CO2 reduction using water as a reductant to generate CO and O2, with the assistance of an external electrical (0.3 V) and chemical (0.10 V) bias produced by a pH difference. This is the first example of a molecular and semiconductor photocatalyst hybrid-constructed photoelectrochemical cell for visible-light-driven CO2 reduction using water as a reductant.

Selective CO production by Au coupled ZnTe/ZnO in the photoelectrochemical CO2 reduction system

A gold-coupled ZnTe/ZnO-nanowire array is a new photocathode for selective CO2 reduction to CO. At −0.7 VRHE under simulated 1 sun illumination, its photocurrent (−16.0 mA cm−2) and incident photon-to-current conversion efficiency (97%) represent the highest among reported ZnTe photocathodes for CO2 reduction and dramatic enhancement from those of a bare electrode (−7.9 mA cm−2, 68%). In addition, the Au nanoparticles convert mainly-hydrogen-producing bare ZnTe/ZnO-nanowires into mainly-CO-producing photocathodes in photoelectrochemical CO2 reduction. The remarkable effects of the Au co-catalyst originate from the formation of a Schottky junction with ZnTe to improve charge separation and to provide reaction centers for CO2 reduction suppressing competing water reduction.

Solar-driven Z-scheme water splitting using tantalum/nitrogen co-doped rutile titania nanorod as an oxygen evolution photocatalyst

A visible-light-driven water-splitting system that involves two-step photoexcitation (Z-scheme) was constructed using rutile TiO2 nanorod doped with Ta and N (TiO2:Ta/N) as an O2 evolution photocatalyst. The Ta-doped TiO2 nanorods, prepared by a solvothermal synthesis, underwent nitridation to possess visible-light absorption under mild conditions, even at 623 K under an ammonia flow. The TiO2:Ta/N powders modified with a RuO2 cocatalyst were active under visible light up to 540 nm for water oxidation for producing O2 in the presence of reversible electron acceptors (IO3− or Fe3+), while TiO2:N exhibited negligible activity. The results of time-resolved infrared absorption spectroscopy indicated that co-doping Ta with N into TiO2 prolonged the lifetime of photogenerated free electrons, leading to high photocatalytic activity. Simultaneous H2 and O2 evolution via water splitting was achieved using a combination of RuO2-modified TiO2:Ta/N, Ru-loaded SrTiO3:Rh and an Fe3+/Fe2+ redox couple under visible-light irradiation (λ > 420 nm) and under AM 1.5G simulated sunlight.

Correction: Selective CO production by Au coupled ZnTe/ZnO in the photoelectrochemical CO2 reduction system

Correction for ‘Selective CO production by Au coupled ZnTe/ZnO in the photoelectrochemical CO2 reduction system’ by Youn Jeong Jang et al., Energy Environ. Sci., 2015, 8, 3597–3604.

Photoelectrochemical CO2 Reduction Using a Ru(II)–Re(I) Supramolecular Photocatalyst Connected to a Vinyl Polymer on a NiO Electrode

A Ru(II)-Re(I) supramolecular photocatalyst and a Ru(II) redox photosensitizer were both deposited successfully on a NiO electrode by using methyl phosphonic acid anchoring groups and the electrochemical polymerization of the ligand vinyl groups of the complexes. This new molecular photocathode, poly-RuRe/NiO, adsorbed a larger amount of the metal complexes compared to one using only methyl phosphonic acid anchor groups, and the stability of the complexes on the NiO electrode were much improved. The poly-RuRe/NiO acted as a photocathode for the photocatalytic reduction of CO2 at E = -0.7 V vs Ag/AgCl under visible-light irradiation in an aqueous solution. The poly-RuRe/NiO produced approximately 2.5 times more CO, and its total Faradaic efficiency of the reduction products improved from 57 to 85%.

Inorganic assembly catalysts for artificial photosynthesis: general discussion

Hiromu Kumagai, Leif Hammarström, Dong Ryeol Whang, Yuki Shinohara, Jose Martinez, Joshua Karlsson, Peter Summers, Christopher D. Windle, Masanori Kodera, Richard Cogdell, Kristine Rodulfo Tolod, Dogukan Hazar Apaydin, Etsuko Fujita, Alexander Kibler, Fengtao Fan, Elizabeth A. Gibson, Hisanao Usami, Akihide Iwase, Haruo Inoue, Akihiko Kudo, Devens Gust, Kazunari Domen, Flavia Cassiola, Katsuhiko Takagi, Sang Ook Kang, Akira Yamakata, Can Li, Licheng Sun, Hyunwoong Park, Young Soo Kang, Rengui Li, Fabio Di Fonzo, Tohru Setoyama and Osamu Ishitani