Hydrogen Generation for 500 hours by Photoelectrolysis of Water using GaN
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Hydrogen Generation for 500 hours by Photoelectrolysis of Water using GaN W. Ohara, D. Uchida, T. Hayashi, M. Deura, and K. Ohkawa Department of Applied Physics, Tokyo University of Science, 1-3 Kagurazaka, Shinjuku, Tokyo 162-8601, Japan
ABSTRACT We confirmed that GaN photocatalyst with NiO cocatalyst (GaN-NiO) continuously produced hydrogen from water for 500 hours without any extra bias. The GaN-NiO photocatalyst was hardly etched and 184-mL hydrogen gas was produced from the electric charge of 1612 coulombs, the Faradic efficiency of which was 89.2%. The conversion efficiency from incident light energy to hydrogen chemical energy was 0.98% in average for 500 h. The incident photon-to-current conversion efficiency (IPCE) was 50% at 300 nm and 35% at 350 nm after the experiment, which was much higher than those of other semiconductor-based photocatalysts. INTRODUCTION Since no CO2 is discharged after its combustion, hydrogen is promising as the new energy source instead of fossil fuels. As a method of H2 generation, photoelectrolysis of water using photocatalysts is attracting because only water and solar light energy are necessary [1-5]. We have focused on III-nitrides, especially GaN, as the photocatalyst [6-14]. Their band edge potentials are comparatively higher than those of conventional oxides used as photocatalysts and straddle the oxidation-reduction level of water. In fact, we have realized H2 generation using GaN photocatalyst without any extra bias. Moreover, they can absorb not only ultraviolet but also visible light using InGaN alloys by changing the group-III content. However, most of the photocatalysts have insufficient durability for practical use. Although GaN is chemically stable essentially, GaN layer itself is etched during photocatalytic reaction due to holes generated by light illumination and accumulated on the surface of the GaN layer. This etching reduces the durability of the photocatalyst. We have found that deposition of NiO cocatalyst on the GaN layer avoids the etching [15]. Furthermore, the amount of H2 generated for GaN photocatalyst with NiO (GaN-NiO) increased more than four times compared to that for GaN (GaN w/o NiO). In this study, we investigated the stability of hydrogen generation from water using GaN-NiO for 500 hours. EXPERIMENT We used a 3-μm-thick n-type GaN layer on a sapphire substrate grown by metalorganic vapor-phase epitaxy, and NiO was deposited with around 1.2% coverage on the GaN surface. The film thickness of GaN was 3.1μm, and its room temperature carrier density and electron
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mobility were 1.2×1017 cm-3 and 580 cm2/V·s, respectively. This GaN-NiO working electrode connected to the Pt counterelectrode was dipped into a NaOH solution with the concentration of 1 mol/L. The irradiated area of the GaN-NiO electrode was adjusted to 1.0 cm2 by covering the remaining area with epoxy resin. A Xe lamp with the energy density of 100 mW/cm2 was used as the light source and the photocurrent density was measured using a potentiostat. All of the experiments were perform
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