≈0.8 V versus RHE, for the α-Fe 2 O 3 .[9] Consequently, to operate as water splitting photoanodes, both WO 3 and α-Fe 2 O 3 require application of an external bias.These early observations have led to the concept of a tandem water splitting system. [12,13] One of the proposed configurations combined a photoelectrolysis cell, featuring a WO 3 photoanode and a metallic cathode, with a dye-sensitized solar cell (DSSC). [14] This concept has been recently demonstrated in dual absorber tandem cell in which the WO 3 photoanode was associated with a single-junction DSSC placed behind the photoelectroanalyzer. [15] In such a configuration, the semitransparent WO 3 film, supported on a conductive glass substrate, absorbs the blue wavelengths of solar beam and transmits its yellow and red parts that are captured by the DSS photovoltaic cell that acts as a photodriven bias. The solar-to-hydrogen (STH) efficiency measured over steady-state operation of the device was 3.1%, corresponding to 2.52 mA cm −2 water oxidation photocurrent density reached at a WO 3 photoanode under ≈1 V bias. Due to its onset potential, much more positive than that of WO 3 , two times lower photocurrent was recorded under such conditions for an α-Fe 2 O 3 photoanode. [15] Given that at 0.9-1 V the DSSC can deliver currents exceeding 5 mA cm −2 , it is clearly the photo anode that is the bottleneck of the tandem device.Attempts to improve the water splitting performance of the semiconductor photoanodes involve the use of OER catalysts. In contrast with the "dark" electrolysis of water where sluggish kinetics of oxygen evolution manifests itself in significant overvoltages, in the PEC oxidation of water at n-type semiconducting photoanodes it results in charge recombination losses, becoming apparent through a positive shift of the photo current onset potential and a slow subsequent rise of the photo current. In fact, although the potential at which positive holes are photogenerated in metal oxide semiconductors (TiO 2 , WO 3 , BiVO 4, Fe 2 O 3 ) is by far sufficiently positive to afford the OER, the slow charge transfer to the solution species (i.e., OH − ions or H 2 O molecules) leads to enhanced electron-hole (e − -h + ) recombination.Common approach to reduce recombination losses involves deposition of electrocatalysts on the photoelectrode surface. Either metals or metal compounds such as mixed cobalt phosphate-oxide (Co-Pi) [16][17][18] or, more recently, oxyhydroxides of nickel and iron [19,8] were coated on the surface of α-Fe 2 O 3 or of another metal oxide photoanode BiVO 4 . Notwithstanding its high activity for the OER, the application of the Co-Pi in PEC devices is however restricted by its filtering of the incident light. [20] The Co-Pi and Ni 1−x Fe x OOH deposits are stable in Electrolysis of water is an attractive approach to produce hydrogen fuel, however the conventional process is energetically expensive requiring ≈1.7-1.8 V that includes 0.5-0.6 V overvoltage losses due principally to oxygen-evolution half-reaction. [1,2] Water s...
