Proceedings of nanoGe September Meeting 2017 (NFM17)
Publication date: 20th June 2016
For improving the efficiency of converting solar energy to H2 fuel using a photoelectrochemical tandem cell system, it is important to clarify the internal resistances. We have investigated the internal resistances of this system with a GaN-based photoanode by AC impedance analysis. When light estimated as AM 1.5 was irradiated on the surface of the photoanode, the efficiency of converting solar energy to H2 fuel was about 0.2%. The resistance of the whole system was 220 ohms and main resistance was related to the water oxidation reaction at the GaN-based photoanode [1]. In this study, we investigated the rate-determining step of the water oxidation reaction with the GaN-based photoanode.
A NiO/Al0.1Ga0.9N/n-GaN electrode was used as the photoanode and Pt wire was used as a cathode and a reference electrode. Nafion® 117 (Sigma-Aldrich) was used as a proton exchange membrane. The photoanode and the reference electrode were immersed in NaOH solution and the cathode was immersed in KHCO3 solution (pH = 9.0). A NiO cocatalyst was formed on the Al0.1Ga0.9N surface by the metal organic decomposition method [2]. The photoelectrochemical properties between the photoanode and the reference electrode were evaluated under various light intensities (2.2, 3.3, and 4.4 mW/cm2) and NaOH pH (12.0 and 13.0) by AC impedance measurement around the open circuit voltage. As a light source, an Xe lamp (Asahi Spectra, MAX-303 with SHX360 filter) was used. The impedance spectrum was fitted to an R-C parallel equivalent circuit consisting of resistance R1 and R2 as ohmic resistance and charge transfer resistance and capacitance C.
The typical impedance spectrum of the GaN-based photoanode was dominated by one arc with a peak frequency of several kHz. The area specific conductivity which was defined as the inverse of R2 increased with increasing light intensity, while that showed no remarkable change when the NaOH pH was changed. Light energy (λ≦365 nm) produces electron-hole pairs in the Al0.1Ga0.9N/n-GaN layers. The light intensity is related to the numbers of electrons and holes generated in the photoanode. Therefore, the rate-determining step with the GaN-based photoanode was considered to be due to the water oxidation reaction process caused by charge transport.
[1] Y. Uzumaki et al., Book of Abstract, International Conference on Artificial Photosynthesis, P4–16 (2017).
[2] S. H. Kim et al., Applied Surface Science 305, 638–641 (2014).
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