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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

Current-voltage properties for SPHELARs and electrocatalysts.(a) 3PVs and (b) 4PVs were performed in various configurations, where SPHELAR was fixed at approximately 1 cm depth of water, above water, and above water with MCPET illustrated in green, blue, and red lines, respectively. As representative choices of electrocatalysts, I–V curves for 0.31 cm2 of NiFe – NiMo in alkaline solution for 3PVs and 0.80 cm2 of NiCo – NiMo in near neutral pH solution or 0.07 cm2 of NiFe – Ni in alkaline for 4PVs were also plotted. The second y-axis corresponds to the “current density” for each SPHELAR (3PVs and 4PVs), which means photocurrent generated by SPHELAR divided by projected surface area of SPHELAR. The voltage of SPHELAR was swept from open-circuit to short-circuit at 10 mV s−1, and the voltage of electrocatalysts was swept at 10 mV s−1 cathodically. As a light source, solar simulator adjusted to AM1.5G was used.
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f4: Current-voltage properties for SPHELARs and electrocatalysts.(a) 3PVs and (b) 4PVs were performed in various configurations, where SPHELAR was fixed at approximately 1 cm depth of water, above water, and above water with MCPET illustrated in green, blue, and red lines, respectively. As representative choices of electrocatalysts, I–V curves for 0.31 cm2 of NiFe – NiMo in alkaline solution for 3PVs and 0.80 cm2 of NiCo – NiMo in near neutral pH solution or 0.07 cm2 of NiFe – Ni in alkaline for 4PVs were also plotted. The second y-axis corresponds to the “current density” for each SPHELAR (3PVs and 4PVs), which means photocurrent generated by SPHELAR divided by projected surface area of SPHELAR. The voltage of SPHELAR was swept from open-circuit to short-circuit at 10 mV s−1, and the voltage of electrocatalysts was swept at 10 mV s−1 cathodically. As a light source, solar simulator adjusted to AM1.5G was used.

Mentions: The current-voltage profiles of one SPHELAR (3PVs or 4PVs) module in various configurations, i.e., at a water depth of 1 cm, above water, and above water with MCPET, are shown in Fig. 4 and Supplementary Table S1. The 3PVs generated approximately 1.8 V of open-circuit voltage (VOC) and approximately 1.5 V of power maximum voltage (VPM), and the 4PVs exhibited 2.3 V for VOC and 1.9 V for VPM. The voltage differences observed in 3PVs and 4PVs originated from the number of connected silicon cells. When placed above the electrolyte, one SPHELAR module generated approximately 1.0 mA of photocurrent at the plateau. With the MCPET reflector, the photocurrent was improved to >1.2 mA. In the SPHELAR module, there is a transparent resin surrounding spherical silicon balls as seen in Fig. 1, which is aimed at maximizing the absorption of light by the Si SPHELAR while suppressing the light reflection from the device. The difference in observed photocurrents with/without the MCPET implies that the total absorption of photons failed with a single path, and that the reflected light by the MCPET was further absorbed by the SPHELAR. In contrast, the SPHELAR module placed in the electrolyte exhibited a significantly smaller photocurrent of approximately 0.7–0.8 mA. Although a water depth of 1 cm absorbed only 6% of the photons in solar irradiance (Fig. 3), the photocurrent of SPHELAR in water decreased by approximately 20–30% compared with that placed above water. This might be due to not only the light absorption by water but also a decrease in the lens effect of the transparent polymer resin molding of SPHELAR. The light refracting angle from the water to the resin is expected to be smaller than that from the air to the resin because the refractive indices of air, water, and the resin are approximately 1.0, 1.3, and 1.5, respectively545556. These observations quantitatively revealed the importance of the PV module location. The current module system was capable of generating a maximum photocurrent of approximately 1.2 mA even at a plateau region (up to 1.5 V and 1.9 V for 3PVs and 4PVs, respectively) corresponding to a STH efficiency higher than 7%, assuming that all the photocurrents were converted into hydrogen and oxygen.


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)

Current-voltage properties for SPHELARs and electrocatalysts.(a) 3PVs and (b) 4PVs were performed in various configurations, where SPHELAR was fixed at approximately 1 cm depth of water, above water, and above water with MCPET illustrated in green, blue, and red lines, respectively. As representative choices of electrocatalysts, I–V curves for 0.31 cm2 of NiFe – NiMo in alkaline solution for 3PVs and 0.80 cm2 of NiCo – NiMo in near neutral pH solution or 0.07 cm2 of NiFe – Ni in alkaline for 4PVs were also plotted. The second y-axis corresponds to the “current density” for each SPHELAR (3PVs and 4PVs), which means photocurrent generated by SPHELAR divided by projected surface area of SPHELAR. The voltage of SPHELAR was swept from open-circuit to short-circuit at 10 mV s−1, and the voltage of electrocatalysts was swept at 10 mV s−1 cathodically. As a light source, solar simulator adjusted to AM1.5G was used.
© Copyright Policy - open-access
Related In: Results  -  Collection

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

f4: Current-voltage properties for SPHELARs and electrocatalysts.(a) 3PVs and (b) 4PVs were performed in various configurations, where SPHELAR was fixed at approximately 1 cm depth of water, above water, and above water with MCPET illustrated in green, blue, and red lines, respectively. As representative choices of electrocatalysts, I–V curves for 0.31 cm2 of NiFe – NiMo in alkaline solution for 3PVs and 0.80 cm2 of NiCo – NiMo in near neutral pH solution or 0.07 cm2 of NiFe – Ni in alkaline for 4PVs were also plotted. The second y-axis corresponds to the “current density” for each SPHELAR (3PVs and 4PVs), which means photocurrent generated by SPHELAR divided by projected surface area of SPHELAR. The voltage of SPHELAR was swept from open-circuit to short-circuit at 10 mV s−1, and the voltage of electrocatalysts was swept at 10 mV s−1 cathodically. As a light source, solar simulator adjusted to AM1.5G was used.
Mentions: The current-voltage profiles of one SPHELAR (3PVs or 4PVs) module in various configurations, i.e., at a water depth of 1 cm, above water, and above water with MCPET, are shown in Fig. 4 and Supplementary Table S1. The 3PVs generated approximately 1.8 V of open-circuit voltage (VOC) and approximately 1.5 V of power maximum voltage (VPM), and the 4PVs exhibited 2.3 V for VOC and 1.9 V for VPM. The voltage differences observed in 3PVs and 4PVs originated from the number of connected silicon cells. When placed above the electrolyte, one SPHELAR module generated approximately 1.0 mA of photocurrent at the plateau. With the MCPET reflector, the photocurrent was improved to >1.2 mA. In the SPHELAR module, there is a transparent resin surrounding spherical silicon balls as seen in Fig. 1, which is aimed at maximizing the absorption of light by the Si SPHELAR while suppressing the light reflection from the device. The difference in observed photocurrents with/without the MCPET implies that the total absorption of photons failed with a single path, and that the reflected light by the MCPET was further absorbed by the SPHELAR. In contrast, the SPHELAR module placed in the electrolyte exhibited a significantly smaller photocurrent of approximately 0.7–0.8 mA. Although a water depth of 1 cm absorbed only 6% of the photons in solar irradiance (Fig. 3), the photocurrent of SPHELAR in water decreased by approximately 20–30% compared with that placed above water. This might be due to not only the light absorption by water but also a decrease in the lens effect of the transparent polymer resin molding of SPHELAR. The light refracting angle from the water to the resin is expected to be smaller than that from the air to the resin because the refractive indices of air, water, and the resin are approximately 1.0, 1.3, and 1.5, respectively545556. These observations quantitatively revealed the importance of the PV module location. The current module system was capable of generating a maximum photocurrent of approximately 1.2 mA even at a plateau region (up to 1.5 V and 1.9 V for 3PVs and 4PVs, respectively) corresponding to a STH efficiency higher than 7%, assuming that all the photocurrents were converted into hydrogen and oxygen.

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