Materials and methods
Materials: Experiments were carried out with a photoanode of sputtered WO3 on fluorine doped tin oxide (FTO) (WO3/FTO/glass, d WO3 = 1 µm) or with a sputtered Cu(Ga,Se)(S,Se)2 (CIGS) photocathode (CIGS/Mo/glass, d CIGS = 0.25 µm). 1.0 M HClO4 and 0.1 M H2SO4were used as electrolytes and were purchased from Carl Roth (Rotipuran p.a.).
Methods: Illumination (spectrum AM1.5G) was provided by A+AA LED solar simulator (SINUS 270, Wavelabs). Intensity calibration to 100 mW cm–2 was carried out with a monocrystalline Si reference solar cell (ReRa Solutions). The spectral distribution was measured with a Zeiss MCS UV-NIR spectrometer. The spatial non-uniformity was determined with a Homogeneity Tester (Fraunhofer CSP). The temporal stability was recorded with a Si reference solar cell.
The system for temperature control was constructed with a 65 W Peltier element with two heat sinks, a 12 V peristaltic pump (Grothen), silicon tubes (d = 3 mm) and an electrolyte reservoir of 20 ml.
Gas measurements were carried out with a »Mobile GC2« (ECH Elektrochemie Halle), which was optimized by the manufacturer for the measurement of H2 and O2 in photoelectrochemical water splitting experiments. A sampling and analysis loop was implemented that enables automated acquisition and evaluation of the Faraday and STH efficiency.
Photoelectrochemical measurements were carried out in a commercial PEC cell (PECC2, Zahner Elektrik). The working electrode had a fixed diameter of 1.8 cm (A = 2.54 cm2). An Ag|AgCl electrode was used as reference electrode and a coiled platinum wire as counter electrode. The Faraday and the solar to hydrogen efficiencies were measured in a Porto cell [12] with a platinum mesh as counter electrode. Bias was provided by a photovoltaic module (5 cSi cells in series, A = 6.0 cm2), mounted parallel to the absorber.
Current-voltage curves were recorded in the dark or under constant illumination at a scan rate of 100 mV s–1 between 0.6 VRHE and 2.7 VRHE with a maximum current of 5 mA. Before each measurement, the samples were equilibrated at 0.6 VRHE for 60 s. The potentials versus the reversible hydrogen electrode (RHE) were calculated by
E RHE = E AgAgCl +E 0AgAgCl + 0.059 x pH
with E 0AgAgCl = 0.209 V. Onset potentials were determined as intercept of a tangent with the abscissa.