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A miniature solar device for overall water splitting consisting of series-connected spherical silicon solar cells.

Kageshima Y, Shinagawa T, Kuwata T, Nakata J, Minegishi T, Takanabe K, Domen K - Sci Rep (2016)

Bottom Line: Impacts of the configuration on the PV module performance were carefully analyzed, revealing that a drastic increase in the photocurrent (≈20%) was attained by the effective utilization of a reflective sheet.Separate investigations on the electrocatalyst performance showed that non-noble metal based materials with reasonably small sizes (<0.80 cm(2)) exhibited substantial currents at the PV working voltage.By combining the observations of the PV characteristics, light management and electrocatalyst performance, solar-driven overall water splitting was readily achieved, reaching solar-to-hydrogen efficiencies of 7.4% (3PVs) and 6.4% (4PVs).

View Article: PubMed Central - PubMed

Affiliation: Department of Chemical System Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo, 113-8656, Japan.

ABSTRACT
A novel "photovoltaics (PV) + electrolyzer" concept is presented using a simple, small, and completely stand-alone non-biased device for solar-driven overall water splitting. Three or four spherical-shaped p-n junction silicon balls were successfully connected in series, named "SPHELAR." SPHELAR possessed small projected areas of 0.20 (3PVs) and 0.26 cm(2) (4PVs) and exhibited working voltages sufficient for water electrolysis. Impacts of the configuration on the PV module performance were carefully analyzed, revealing that a drastic increase in the photocurrent (≈20%) was attained by the effective utilization of a reflective sheet. Separate investigations on the electrocatalyst performance showed that non-noble metal based materials with reasonably small sizes (<0.80 cm(2)) exhibited substantial currents at the PV working voltage. By combining the observations of the PV characteristics, light management and electrocatalyst performance, solar-driven overall water splitting was readily achieved, reaching solar-to-hydrogen efficiencies of 7.4% (3PVs) and 6.4% (4PVs).

No MeSH data available.


Related in: MedlinePlus

Solar irradiance at different depth of pure water (22 °C) calculated from the reported absorption coefficient (Supplementary Fig. S1).AM1.5G (reported by NREL), solar spectra at 1, 5, and 100 cm depth of water are illustrated in red, yellow, blue, and black line, respectively. The integrated portions of photon flux (380–1100 nm) are designated in brackets as the percentage to pristine AM1.5G.
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f3: Solar irradiance at different depth of pure water (22 °C) calculated from the reported absorption coefficient (Supplementary Fig. S1).AM1.5G (reported by NREL), solar spectra at 1, 5, and 100 cm depth of water are illustrated in red, yellow, blue, and black line, respectively. The integrated portions of photon flux (380–1100 nm) are designated in brackets as the percentage to pristine AM1.5G.

Mentions: When the SPHELAR module is placed in water, the surrounding water markedly changes the refractive index optimized for the configuration in air, which in turn affects its light absorption due to reflection, scattering, and absorption by the water. Nevertheless, light-absorbing semiconductors may have to reside in water for a stand-alone water splitting system. The absorption coefficient of pure water was investigated both theoretically49 and experimentally5051, which revealed that photons were absorbed not only in the infrared region but also in the visible light region to a different degree, as shown in Supplementary Fig. S1. The solar spectra at different depths of pure water were calculated with the absorption coefficient of water and are illustrated in Fig. 3, where the integrated values of the photon flux at each water depth are summarized as a percentage relative to AM1.5G. Above approximately 500 nm, the presence of water significantly reduced the irradiance inside the water. In the infrared region, only a water depth of 5 cm was required to absorb most of the photons (0.07 decreased to approximately 0.01 mW cm−2 nm−1), which would decrease the available amount of photon for the PVs in water. Particularly in the case of using silicon as a light absorber, light absorption by water seems to be a crucial issue because silicon utilizes infrared light. Although the absorption coefficient of a liquid depends on the identity of the electrolytes5253, light absorption by water is very critical in the scale-up development of stand-alone water splitting devices when light absorbers are placed in water, such as integrated PEC systems. In such cases, the depth of water should be adequately designed to be as low as possible to lower the light absorption by electrolytes, or wherever possible, it is desirable to locate the PV above water while maintaining the connections with the electrodes that are immersed in water. Notably, these effects of light condensation by a reflector and photon loss due to water can be regarded as one of the most crucial factors to establish an efficient STH production system. In a “PV + electrolyzer” system, the photon collection efficiency can be easily improved when the light absorbers and catalysts are separated. This aspect is a great advantage over other solar hydrogen production systems. In the following section, various configurations (PVs fixed in/above water) are investigated.


A miniature solar device for overall water splitting consisting of series-connected spherical silicon solar cells.

Kageshima Y, Shinagawa T, Kuwata T, Nakata J, Minegishi T, Takanabe K, Domen K - Sci Rep (2016)

Solar irradiance at different depth of pure water (22 °C) calculated from the reported absorption coefficient (Supplementary Fig. S1).AM1.5G (reported by NREL), solar spectra at 1, 5, and 100 cm depth of water are illustrated in red, yellow, blue, and black line, respectively. The integrated portions of photon flux (380–1100 nm) are designated in brackets as the percentage to pristine AM1.5G.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4834539&req=5

f3: Solar irradiance at different depth of pure water (22 °C) calculated from the reported absorption coefficient (Supplementary Fig. S1).AM1.5G (reported by NREL), solar spectra at 1, 5, and 100 cm depth of water are illustrated in red, yellow, blue, and black line, respectively. The integrated portions of photon flux (380–1100 nm) are designated in brackets as the percentage to pristine AM1.5G.
Mentions: When the SPHELAR module is placed in water, the surrounding water markedly changes the refractive index optimized for the configuration in air, which in turn affects its light absorption due to reflection, scattering, and absorption by the water. Nevertheless, light-absorbing semiconductors may have to reside in water for a stand-alone water splitting system. The absorption coefficient of pure water was investigated both theoretically49 and experimentally5051, which revealed that photons were absorbed not only in the infrared region but also in the visible light region to a different degree, as shown in Supplementary Fig. S1. The solar spectra at different depths of pure water were calculated with the absorption coefficient of water and are illustrated in Fig. 3, where the integrated values of the photon flux at each water depth are summarized as a percentage relative to AM1.5G. Above approximately 500 nm, the presence of water significantly reduced the irradiance inside the water. In the infrared region, only a water depth of 5 cm was required to absorb most of the photons (0.07 decreased to approximately 0.01 mW cm−2 nm−1), which would decrease the available amount of photon for the PVs in water. Particularly in the case of using silicon as a light absorber, light absorption by water seems to be a crucial issue because silicon utilizes infrared light. Although the absorption coefficient of a liquid depends on the identity of the electrolytes5253, light absorption by water is very critical in the scale-up development of stand-alone water splitting devices when light absorbers are placed in water, such as integrated PEC systems. In such cases, the depth of water should be adequately designed to be as low as possible to lower the light absorption by electrolytes, or wherever possible, it is desirable to locate the PV above water while maintaining the connections with the electrodes that are immersed in water. Notably, these effects of light condensation by a reflector and photon loss due to water can be regarded as one of the most crucial factors to establish an efficient STH production system. In a “PV + electrolyzer” system, the photon collection efficiency can be easily improved when the light absorbers and catalysts are separated. This aspect is a great advantage over other solar hydrogen production systems. In the following section, various configurations (PVs fixed in/above water) are investigated.

Bottom Line: Impacts of the configuration on the PV module performance were carefully analyzed, revealing that a drastic increase in the photocurrent (≈20%) was attained by the effective utilization of a reflective sheet.Separate investigations on the electrocatalyst performance showed that non-noble metal based materials with reasonably small sizes (<0.80 cm(2)) exhibited substantial currents at the PV working voltage.By combining the observations of the PV characteristics, light management and electrocatalyst performance, solar-driven overall water splitting was readily achieved, reaching solar-to-hydrogen efficiencies of 7.4% (3PVs) and 6.4% (4PVs).

View Article: PubMed Central - PubMed

Affiliation: Department of Chemical System Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo, 113-8656, Japan.

ABSTRACT
A novel "photovoltaics (PV) + electrolyzer" concept is presented using a simple, small, and completely stand-alone non-biased device for solar-driven overall water splitting. Three or four spherical-shaped p-n junction silicon balls were successfully connected in series, named "SPHELAR." SPHELAR possessed small projected areas of 0.20 (3PVs) and 0.26 cm(2) (4PVs) and exhibited working voltages sufficient for water electrolysis. Impacts of the configuration on the PV module performance were carefully analyzed, revealing that a drastic increase in the photocurrent (≈20%) was attained by the effective utilization of a reflective sheet. Separate investigations on the electrocatalyst performance showed that non-noble metal based materials with reasonably small sizes (<0.80 cm(2)) exhibited substantial currents at the PV working voltage. By combining the observations of the PV characteristics, light management and electrocatalyst performance, solar-driven overall water splitting was readily achieved, reaching solar-to-hydrogen efficiencies of 7.4% (3PVs) and 6.4% (4PVs).

No MeSH data available.


Related in: MedlinePlus