Highly Efficient Sunlight‐Driven Seawater Splitting in a Photoelectrochemical Cell with Chlorine Evolved at Nanostructured WO₃ Photoanode and Hydrogen Stored as Hydride within Metallic Cathode

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. Fabrication 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 electrode (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 materials as anodes in batteries employing oxygen reduction cathodes.