www.advenergymat.de 1903213 (1 of 8) © 2019 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim COMMUNICATION Highly Efficient Sunlight-Driven Seawater Splitting in a Photoelectrochemical Cell with Chlorine Evolved at Nanostructured WO 3 Photoanode and Hydrogen Stored as Hydride within Metallic Cathode Michal Jadwiszczak, Katarzyna Jakubow-Piotrowska, Piotr Kedzierzawski, Krzysztof Bienkowski,* and Jan Augustynski* DOI: 10.1002/aenm.201903213 semiconductor photocatalytic particles, pure water is generally used and both reduction and oxidation reaction prod- ucts, H 2 and O 2 , are formed in the same cell compartment and should be sub- sequently separated. [2,3] On the other hand, employing a photoelectrolysis cell with separated anodic and cathodic com- partments imposes use of a supporting electrolyte with ionic conductivity large enough to avoid excessive Ohmic losses within the cell. [4,6] The choice of the appro- priate electrolyte is even more important when relatively thick nanoporous film photoelectrodes with high internal photo- active surface area are employed, where too low conductivity of the electrolyte may adversely affect the amount of collected photocurrent due to uneven current dis- tribution across the semiconductor film. [7] Consequently, in addition to pure water, also the chemicals required to prepare electrolyte for the photoelectrolysis cell will potentially constitute a nonnegligible part of the operational cost of larger scale photoelectrochemical (PEC) water splitting devices. The fact that cannot at all be neglected—extensive utilization for elec- trolysis of fresh water would put heavy pressure on vital water resources. Since the seawater is a free and widely abundant electro- lyte, there were various attempts to use it in the PEC [8–13] and conventional electrochemical [14] devices to produce hydrogen. From the practical viewpoint, the photoelectrolysis of seawater requires at first identification of photoanode materials stable in the presence of chloride ions under highly oxidizing condi- tions. Only few among works reported on the PEC seawater splitting describe experiments conducted under visible light irradiation. An electrode consisting of molybdenum-doped bis- muth vanadate (Mo-BiVO 4 ) reached, under simulated AM 1.5G (100 mW cm −2 ) illumination, a photocurrent of 2.2 mA cm −2 at 1 V versus RHE (reversible hydrogen electrode). [9] Loading the Mo-BiVO 4 electrode with precious metal RhO 2 catalyst was effective in limiting to ≈10% the drop of the photocurrent over a 5 h long stability test. Analysis of the photoelectrolysis A seawater splitting photoelectrochemical cell featuring a nanostructured tungsten trioxide photoanode that exhibits very high and stable photocurrents producing chlorine with average 70% Faradaic efficiency is described. Fabrica- tion of the WO 3 electrodes on fluorine-doped tin oxide substrates involves a simple solution-based method and sequential layer-by-layer deposition with a progressively adjusted amount of structure-directing agent in the precursor and a two-step annealing. Such a procedure allows tailoring of thick, highly porous, structurally stable WO 3 films with a large internal photoactive surface area optimizing utilization of visible light wavelengths by the photoanode. With the application of an anodic potential of 0.76 V versus Ag/AgCl reference elec- trode (0.4 V below the thermodynamic Cl 2 /Cl − potential) in synthetic seawater, the designed WO 3 photoanodes irradiated with simulated 1 sun AM 1.5G light reach currents exceeding 4.5 mA cm −2 . Photocurrents close to 5 mA cm −2 are attained in the case of fresh water splitting using 1 M methane–sulfonic acid supporting electrolyte with oxygen evolved at the WO 3 photoanode. The amount of formed hydrogen is determined by discharging the palladium sheet electrode employed as a cathode. Collection of hydrogen in the form of a hydride opens, more generally, the prospect of subsequently using such mate- rials as anodes in batteries employing oxygen reduction cathodes. M. Jadwiszczak, K. Jakubow-Piotrowska Centre of New Technologies University of Warsaw S. Banacha 2c, 02-097 Warsaw, Poland Dr. P. Kedzierzawski Institute of Physical Chemistry of Polish Academy of Sciences 01-224 Warsaw, Poland Dr. K. Bienkowski, Prof. J. Augustynski Centre of New Technologies University of Warsaw S. Banacha 2c, 02-097 Warsaw, Poland E-mail: k.bienkowski@cent.uw.edu.pl; jan.augustynski@unige.ch The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/aenm.201903213. Photocatalytic water splitting is widely investigated as a pro- mising means for sustainable fuel generation using sunlight. [1–5] When the splitting process is conducted via suspensions of Adv. Energy Mater. 2019, 1903213