COMMUNICATION 1700111 (1 of 7) © 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.advsustainsys.com Efficient Photoelectrochemical O 2 and CO Production Using BiVO 4 Water Oxidation Photoanode and CO 2 Reduction Au Nanoparticle Cathode Prepared by In Situ Deposition from Au 3+ Containing Solution Zaki N. Zahran,* Eman A. Mohamed, Ashraf Abdel Haleem, and Yoshinori Naruta* DOI: 10.1002/adsu.201700111 Reduction of CO 2 particularly to CO is an essential reaction for developing alter- nate sources of fuels and for reducing the global warming resulted from the green- house effect of CO 2 . [1,2] The reduction (Equation 1), however, requires a protons and electrons source. For sustainable and large scale CO 2 to CO conversion, water via its oxidation (Equation 2) is the one and only source. [3,4] The combination of CO 2 reduction and water oxidation to pro- duce CO and O 2 (Equation 3) is, however, thermodynamically and kinetically a chal- lenging process demanding selective and efficient catalysts for both CO 2 reduction and water oxidation half reactions par- ticularly in aqueous solutions. An ideal approach, which is known as artificial photosynthesis, is the utilization of solar energy to drive reaction (1) in a photo- electrochemical (PEC) cell. Different PEC cell designs have been reported to achieve reaction (1). These include PEC cell with a single p-type semiconductor photocathode (p/PEC), PEC cell with a single n-type semiconductor photoanode (n/PEC), p/n PEC, photovoltaic (PV)/PEC, and PV/electrolyzer devices. [5–8] Such PEC cell mimics the photosynthesis process within a leaf that converts the solar energy into chemical energy by oxida- tion of water to produce O 2 and reduction of CO 2 to produce carbohydrate [9]  CO 2H 2e CO HO 0.52 V, pH 7.0 2 2 CO /CO 0 2 E ( ) + + + =- + - (1)  HO 1 2 O 2H 2e 0.82 V,pH 7.0 2 2 O /H O 0 2 2 E ( ) + + = + - (2)  CO CO 1 2 O 1.34 V 2 2 0 E ( ) + Δ = (3) As one of the most efficient water oxidation photoanodes, scheelite-monoclinic bismuth vanadate (s-m BiVO 4 ) modified with water oxidation catalysts (WOCs) and supported on fluo- rine doped tin oxide (FTO) electrode (FTO/s-m BiVO 4 /WOCs) The cathodic polarization of various conducting electrodes in a CO 2 -saturated DMF/5% H 2 O solution containing AuCl 3 (DMF = N,N-dimethylformamide) results in in situ deposition of a highly selective and efficient CO 2 to CO conversion Au nanoparticle electrocatalyst. These electrodes include glassy carbon, fluorine doped tin oxide (FTO), and Au plate. The efficiency of the Au nanoparticles is enhanced by introducing 1-ethyl-3-methylimidazolium trifluo- romethanesulfonate ionic liquid (EMI OTf IL, 10%) to the AuCl 3 -containing DMF/5% H 2 O deposition solution. In addition to the simplicity and novelty of the preparation method, the resultant in situ deposited Au nanoparticle cathode shows superior activity and selectivity for CO 2 to CO conversion compared to Au nanoparticles prepared by other techniques. Moreover, the in situ deposited Au nanoparticle electrode is used as a cathode in an n-type photoelectrochemical cell with a water oxidation BiVO 4 photoanode modified with three-layer water oxidation catalysts [FTO/EMI BiVO 4 /FeOOH/NiOOH/ Co(bpy-P) 2 , bpy-P = 2,2′-bipyridine-4,4′-diphosphonic acid] to achieve efficient solar-assisted CO and O 2 production with 0.78% solar to CO conversion efficiency. The resultant 0.78% efficiency is the highest efficiency obtained toward CO 2 to CO conversion in PEC devices without any use of photovoltaic cell as a tandem. Dr. Z. N. Zahran, Dr. E. A. Mohamed, Dr. A. A. Haleem, Prof. Y. Naruta Center for Chemical Energy Conversion Research Institute for Science and Technology Research Chubu University 1200 Matsumoto-Cho, Kasugai, Aichi 487-8501, Japan E-mail: znzahran@isc.chubu.ac.jp; naruta@isc.chubu.ac.jp Dr. Z. N. Zahran Chemistry Department Faculty of Science Tanta University Tanta 31111, Egypt Dr. A. A. Haleem Department of Engineering Mathematics and Physics Faculty of Engineering Fayoum University Fayoum 63514, Egypt The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/adsu.201700111. Artificial Photosynthesis Adv. Sustainable Syst. 2017, 1700111