Surface thin conductor layers enhance sunlight driven hydrogen evolution from water using Cu(In,Ga)Se2photocathodes in a concentrated phosphate buffer solution.
Pulverization followed by annealing treatment improved the activity of BiVO 4 although the annealing treatment had a negative impact on the non-milled sample.
Reduction of CO2 producing high-energy compounds using water as an electron donor and sun light as an energy source has been investigating as useful technology for solving both depletion of the fossil resources and the global warming problem. Our group has successfully developed several types of hybrid photocatalysts consisting of semiconductors and metal complexes, which have both efficient CO2 reduction ability supplied by the metal-complex unit and strong oxidation power of semiconductors. In this paper, our recent progresses of the visible-light-driven hybrid photocatalytic and photoelectrochemical systems for CO2 reduction are introduced: (1) hybrids consisting C3N4 and Ru(II) mononuclear complexes, (2) semiconductors, e.g., TaON, and a Ru(II)-Ru’(II) binuclear complex, (3) a photoelectrochemical cell comprising a photocathode of the Ru(II)-Re(I) binuclear complex immobilized on p-type semiconductor NiO and a CoOx/TaON photoanode. First two systems can photocatalyze CO2 reduction using methanol as an electron donor, and the third photoelectrochemical system can reduce CO2 using water as a reductant.
Mixed-anion compounds (e.g., oxynitrides and oxysulfides) are potential candidates as photoanodes for visible-light water oxidation, but most of them suffer from oxidative degradation by photogenerated holes, leading to low stability. Here we show an exceptional example of a stable, mixed-anion water-oxidation photoanode that consists of an oxyfluoride, Pb2Ti2O5.4F1.2, having a band gap of ca. 2.4 eV. Pb2Ti2O5.4F1.2 particles, which were coated on a transparent conductive glass (FTO) support and were subject to postdeposition of a TiO2 overlayer, generated an anodic photocurrent upon band gap photoexcitation of Pb2Ti2O5.4F1.2 (λ <520 nm) with a rather negative photocurrent onset potential of ca. -0.6 V vs NHE, which was independent of the pH of the electrolyte solution. Stable photoanodic current was observed even without loading a water oxidation promoter such as CoOx. Nevertheless, loading CoOx onto the TiO2/Pb2Ti2O5.4F1.2/FTO electrode further improved the anodic photoresponse by a factor of 2-3. Under AM1.5G simulated sunlight (100 mW cm-2), stable water oxidation to form O2 was achieved using the optimized Pb2Ti2O5.4F1.2 photoanode in the presence of an applied potential smaller than 1.23 V, giving a Faradaic efficiency of 93% and almost no sign of deactivation during 4 h of operation. This study presents the first example of photoelectrochemical water splitting driven by visible-light excitation of an oxyfluoride that stably works, even without a water oxidation promoter, which is distinct from ordinary mixed-anion photoanodes that usually require a water oxidation promoter.
Abstract The front cover artwork is provided by the Ishitani and Maeda group at the School of Science at the Tokyo Institute of Technology (Japan). The image shows CO 2 reduction and H 2 production coupled to water oxidation by a photoelectrochemical system based on a Ta/N codoped rutile TiO 2 anode under sunlight. Read the full text of the Article at 10.1002/cptc.201800157 .
Photocathodes prepared using p-type semiconductor photocatalyst powders of copper gallium selenides (CGSe) were investigated for visible-light-driven photoelectrochemical water splitting. The CGSe powders were prepared by solid-state reaction of selenide precursors with various Ga/Cu ratios. The CGSe photoelectrodes prepared by the particle transfer method showed cathodic photocurrent in an alkaline electrolyte. Pt modification was conducted for all the photoelectrodes by photoassisted electrodeposition. CGSe particles with a Ga/Cu ratio of 2, consisting of the CuGa3Se5 phase and an intermediate phase between CuGaSe2 and CuGa3Se5, yielded the largest cathodic photocurrent. By surface modification with a CdS semiconductor layer, the photocurrent density and onset potential clearly increased, indicating enhancement of charge separation caused by the formed p-n junction with appropriate band alignment at solid–liquid interfaces. A multilayer structure on the particles was confirmed to be beneficial for enhancing the photocurrent, as in the case of thin-film photoelectrodes. A Pt/CdS/CGSe electrode (Ga/Cu = 2) was demonstrated to work as a photocathode contributing stoichiometric hydrogen evolution from water for 16 h under visible light irradiation.
Greenhouse gas emissions and following global climate change problems are the most important human-being facing issues. The electrochemical CO 2 reduction reaction (CO 2 RR) is one of the most promising processes to obtain value-added hydrocarbons from carbon dioxide (CO 2 ) and water (H 2 O), and the process is being actively investigated as an alternative way to conventional hydrocarbon production by fossil resources. Among the electrocatalysts for CO 2 RR, copper (Cu) species are unique and most studied material group because of their capability of producing C 2+ species, such as ethylene (C 2 H 4 ), ethanol (C 2 H 5 OH) and propanol, through forming C-C bonding. Especially C 2 H 4 is the most highly demanded and profitable product [1] . Porous structure of electrodes is reported to be advantageous to CO 2 RR because it enables enrichment of reactants, CO 2 molecules, and intermediates [2–5] . Polytetrafluoroethylene (PTFE), a hydrophobic polymer, has been reported to be beneficial to increase the FE towards C 2+ products [6] and suppressing competitive reaction, hydrogen production [7,8] . In this study, we prepared PTFE modified porous Cu electrodes and investigated the influence of each factor and their synergistic effect on CO 2 RR activity. As a result, the introduction of porous structure increased FE to C 2 H 4 (FE(C 2 H 4 )) and suppressed both carbon monoxide (CO) and methane (CH 4 ) production. Further modification by PTFE enhanced FE(C 2 H 4 ) and improved durability significantly. Porous Cu electrode was prepared by de-alloying process [9] . First, Cu and Al metals were co-sputtered to form CuAl alloy on gas-diffusion layer (GDL, MFK-A, Mitsubishi Chemical) coated with micro porous layer (MPL). The samples were etched using 5 wt.% hydrochloric acid for approximately 20 minutes to remove Al from the alloy and obtained porous Cu electrodes. CO 2 RR test was carried out in flow cell with gas-diffusion electrode (GDE), the electrocatalyst coated GDL, as a cathode under applied current density of 300 mA cm -2 . Both gas phase products and liquidous products were quantified by gas chromatography (GC). Structural characterizations using scanning electron microscope (SEM) confirmed that porous Cu structure was successfully formed, and porous Cu electrodes exhibited higher FE(C 2 H 4 ) of 48%, and lower FE toward CO (FE(CO)) of 6% and FE toward CH 4 (FE(CH 4 )) of less than detection limit, while smooth Cu electrode showed FE(C 2 H 4 ) of 45%, FE(CO) of 12%, FE(CH 4 ) of 0.6%, respectively. However, FE(C 2 H 4 ) started to decrease 6 hours after the start of the reaction and decreased to 12% after 24 hours reaction. This could be caused by GDE soaking to electrolyte. To overcome these issues, we next investigated PTFE added porous Cu electrodes. PTFE was introduced into the porous Cu layer by electrostatic spray [10] .The introduction of PTFE further improved FE(C 2 H 4 ) from 48% to 55% and demonstrated over 80% of FE towards valuable C 2+ products at 300 mA cm -2 . PTFE introduction enhanced durability and resulted in FE(C 2 H 4 ) of 55% for over 24 hours. References [1] S. Nitopi, E. Bertheussen, S. B. Scott, X. Liu, A. K. Engstfeld, S. Horch, B. Seger, I. E. L. Stephens, K. Chan, C. Hahn, J. K. Nørskov, T. F. Jaramillo, I. Chorkendorff, Chem. Rev. 2019 , 119 , 7610–7672. [2] E. G. Derouane, J.-M. André, A. A. Lucas, Chem. Phys. Lett. 1987 , 137 , 336–340. [3] J.-J. Lv, M. Jouny, W. Luc, W. Zhu, J.-J. Zhu, F. Jiao, Adv. Mater. 2018 , 30 , e1803111. [4] T. T. H. Hoang, S. Ma, J. I. Gold, P. J. A. Kenis, A. A. Gewirth, ACS Catal. 2017 , 7 , 3313–3321. [5] W. Tang, A. A. Peterson, A. S. Varela, Z. P. Jovanov, L. Bech, W. J. Durand, S. Dahl, J. K. Nørskov, I. Chorkendorff, Phys. Chem. Chem. Phys. 2012 , 14 , 76–81. [6] J. Pellessier, X. Gong, B. Li, J. Zhang, Y. Gang, K. Hambleton, C. Podder, Z. Gao, H. Zhou, G. Wang, H. Pan, Y. Li, J. Mater. Chem. A Mater. Energy Sustain. 2023 , 11 , 26252–26264. [7] F. Huq, I. Sanjuán, S. Baha, M. Braun, A. Kostka, V. Chanda, J. R. C. Junqueira, N. Sikdar, A. Ludwig, C. Andronescu, ChemElectroChem 2022 , 9 , DOI 10.1002/celc.202101279. [8] P. An, L. Wei, H. Li, B. Yang, K. Liu, J. Fu, H. Li, H. Liu, J. Hu, Y.-R. Lu, H. Pan, T.-S. Chan, N. Zhang, M. Liu, J. Mater. Chem. A Mater. Energy Sustain. 2020 , 8 , 15936–15941. [9] M. Zhong, K. Tran, Y. Min, C. Wang, Z. Wang, C.-T. Dinh, P. De Luna, Z. Yu, A. S. Rasouli, P. Brodersen, S. Sun, O. Voznyy, C.-S. Tan, M. Askerka, F. Che, M. Liu, A. Seifitokaldani, Y. Pang, S.-C. Lo, A. Ip, Z. Ulissi, E. H. Sargent, Nature 2020 , 581 , 178–183. [10] Z. Xing, L. Hu, D. S. Ripatti, X. Hu, X. Feng, Nat. Commun. 2021 , 12 , 136.
Abstract The photoelectrochemical (PEC) properties of particulate CuGaSe2 (CGSe) and CuIn0.7Ga0.3Se2 (CIGS) photocathodes were evaluated in an acetonitrile electrolyte containing iron(III) acetylacetonate (Fe(acac)3) under simulated sunlight illumination, and compared to that in a typical aqueous electrolyte. The particulate CGSe and CIGS photocathodes can generate higher photovoltages, which is a more positive onset potential than the hydrogen evolution in an aqueous electrolyte possibly due to the facile one-electron reduction of Fe(acac)3, while the cathodic photocurrent decreased due to light shielding by the colored nonaqueous electrolyte. Indeed, the incident-photon-to-current conversion efficiencies (IPCEs) of the photocathode evidently decreased in the wavelength region of 400–600 nm, where the Fe(acac)3 acetonitrile electrolyte shows an intense light absorption. The CIGS photocathode generates a higher cathodic photocurrent than the CGSe during hydrogen evolution from the aqueous electrolyte, while the CGSe exhibits superior PEC performances to CIGS in the nonaqueous electrolyte, which can be explained by the energy level of the conduction band minimum (CBM) of CGSe and CIGS compared to the reduction potential for Fe(acac)3. Finally, the two-electrode PEC-voltaic (PECV) cell consisting of the CGSe photocathode and Pt anode demonstrated a stable generated photovoltage by a one-step photoexcitation process.
Binary hydroxyl ammonium nitrate (HAN) aqueous mixtures have been prepared. Four series of ternary mixtures have been synthesized with methanol and ethanol as fuels: two series with HAN excess and two other series with stoichiometric fuel contents. Thermal and catalytic decomposition of the prepared solutions have been analyzed. For binary HAN solutions, the thermal decomposition starts only once water has been fully vaporized and the oxidizer is in the liquid state. The influence of the fuel depends strongly on the oxidizer. Methanol and ethanol are vaporized before the decomposition, leading to results close to those observed for binary mixtures. HAN and HAN-fuel-based solutions display the highest catalytic effect with a temperature decrease of about 100° C.
