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.