Photo-assisted electrolysis of water by Si photoelectrodes

The photoelectrolysis of water in NaOH and NaCl electrolytes with Si semiconductor photoelectrodes has been investigated. External bias voltage Vb is applied between the p-type Si cathode and Pt anode. In the absence of Vb, the potential of the Si electrode Ve (vs SCE) and the short circuit current between Si and Pt electrodes increase by illumination, dissolving the Si surface. This reaction as an electric cell can be stopped by applying Vb. When Ve exceeds about −1.5 V, the photocurrent increases steeply, compared with the dark current, with obvious gas evolution from the electrodes. The decomposition voltage, deduced from the present study, for 0.1 M NaOH is about 1.4 V and less than about 1.9 V for 0.1 M NaCl, where the power conversion efficiency, which is defined as the ratio of hydrogen energy produced to the total optical and electrical energies supplied, is higher in the former case. The maximum efficiencies under the illumination of 161 mWcm−2 and 82 mWcm− are about 13.4 and 16.6%, respectively. An Si electrode coated with SnO2 can be used as a stable anode. Its threshold potential, above which the photocurrent flow is high, is smaller than that of the uncoated sample, but the photocurrent saturates at higher current level.