About 3 μm thick tungsten trioxide film electrodes consisting of partly sintered, 40-80 nm in diameter, particles
deposited on conducting glass substrates exhibit high photon-to-current conversion efficiencies for the photooxidation of
water, exceeding 70% at 400 nm. This is facilitated by a ca. 40% film porosity resulting in high contact area with the
electrolyte. It is shown that the activity of the WO3 electrodes towards photooxidation of water is enhanced by addition
of even small amounts of halide (Cl-, Br-) ions to the acidic electrolyte. Photoelectrolysis experiments performed either
in acidic electrolytes containing chloride or bromide anions or in a 0.5 M NaCl solution, under simulated 1.5 AM solar
illumination, demonstrated long term stability of the photocurrents. Oxygen remains the main product of the
photoanodic reaction even in a 0.5 M NaCl solution, a composition close to the sea water, with chlorine accounting for
ca. 20% of current efficiency.
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