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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

Time courses of overall water splitting into H2 (☐) and O2 (○) by 4PVs.Overall water splitting were demonstrated by (a) 4PVs fixed in 0.5 M KOH aqueous solution as an electrolyte, (b) 4PVs fixed above the electrolyte, and (c) 4PVs with MCPET fixed above the electrolyte under irradiation of solar simulator (AM1.5G). As the electrocatalysts, combination of NiFeOx (anode) and Ni (cathode) was used with geometric surface area of approximately 0.14 cm2 and 0.19 cm2 for 4PVs (a,b) and 4PVs with MCPET in air (c), respectively. The reactions were measured in a Pyrex top-irradiation flow cell at room temperature.
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f7: Time courses of overall water splitting into H2 (☐) and O2 (○) by 4PVs.Overall water splitting were demonstrated by (a) 4PVs fixed in 0.5 M KOH aqueous solution as an electrolyte, (b) 4PVs fixed above the electrolyte, and (c) 4PVs with MCPET fixed above the electrolyte under irradiation of solar simulator (AM1.5G). As the electrocatalysts, combination of NiFeOx (anode) and Ni (cathode) was used with geometric surface area of approximately 0.14 cm2 and 0.19 cm2 for 4PVs (a,b) and 4PVs with MCPET in air (c), respectively. The reactions were measured in a Pyrex top-irradiation flow cell at room temperature.

Mentions: In this section, overall water splitting with our SPHELAR devices, composed of (NiFe, Pt/Ni) for 3PVs and (NiFe, Ni) for 4PVs in 0.5 M KOH, are demonstrated as model cases. The electrode surface areas were 0.45 cm2 for 3PVs with MCPET and 0.19 cm2 for 4PVS with MCPET (see Supplementary Fig. S5 for calculation details). Various SPHELAR configurations were evaluated, i.e., the body of SPHELAR was fixed in the electrolyte, above the electrolyte, and above the electrolyte with MCPET. Figs 6 and 7 show the measured gas production rate by devices with 3PVs and 4PVs, respectively, where the corresponding STH efficiencies calculated with the PV module projected surface area are also noted. The measured hydrogen evolution rates and STH efficiencies are summarized in Supplementary Table S3. In all cases, steady evolutions of hydrogen and oxygen were observed during light irradiation. The light was turned off after 2.0 h, and no further formations of hydrogen and oxygen were observed, thus confirming that gas productions were induced by harvesting the light. Additionally, it was confirmed that there was no consumption of evolved hydrogen and oxygen in the circulation system after the light was turned off (shown in Supplementary Fig. S8), indicating a negligible reverse reaction of products forming water in the dark. In both measurements in the flow reactor (Figs 6 and 7) and in the batch reactor (Supplementary Fig. S8), the ratio of evolved hydrogen over oxygen was approximately 2, which agreed with the stoichiometry of the water splitting reaction. The devices with MCPET showed the highest STH efficiencies of 7.4% (3PVs) and 6.4% (4PVs). Importantly, the STH efficiency was calculated with the projected surface area of the SPHELAR device and the same area of the incident photons. Therefore the STH efficiency improved with the MCPET reflector, which successfully utilized the photons unused unless it is present, as demonstrated in the previous section. Without MCPET as the reflector, smaller efficiencies were obtained: 6.1% (3PVs) and 4.8% (4PVs) with SPHELAR above the electrolyte and 4.9% (3PVs) and 3.9% (4PVs) with SPHELAR in the electrolyte. The differences in the STH efficiencies due to device location can be attributed to light absorption by the electrolyte and the altered lens effect, as shown in Figs 3 and 4 545556. The hydrogen and oxygen production rates in Figs 6 and 7 agreed with the expected photoelectric and electric currents (see Fig. 4), revealing that an approximately 100% of Faradic efficiency was attained. Indeed, a spontaneous gas evolution from both the anode and the cathode could be seen in the supplementary Movie S1 using a representative configuration of 3PVs located above the electrolyte with MCPET. As clearly seen in the supplementary movie, this SPHELAR based device could split water only by solar energy and work stand-alone without any bias or extra circuit. As Fig. 1(a) demonstrates the actual photograph of SPHELARs, the total volumes for SPHELARs were approximately 5.6 × 10−2 and 7.2 × 10−2 cm3 for 3PVs and 4PVs, respectively. Since the present SPHELAR devices could split water in the completely stand-alone configuration, it is remarkable that such a small volume (less than 0.1 cm3) of the present PV + electrolyzer system achieves the high STH efficiencies. Additionally, a potential method for the scale-up of the SPHELAR device, where three 3PVs are placed in parallel with MCPET and shared the electrocatalyst, was demonstrated. The picture of this configuration is shown in Fig. 8(a), which can also show how the reflector was integrated in the SPHELAR device. The projected surface area of the whole device was approximately 0.6 cm2, which was three times larger than that of one 3PV module (0.20 cm2). The amount of evolved gas upon irradiating the photon is compiled in Fig. 8(b). Indeed, an approximately three times larger gas production rate was observed in the configuration compared with one 3PV module. The observed hydrogen production rate corresponded to approximately 6.8% of STH efficiency. This STH efficiency was slightly smaller than that of one 3PV (ca. 7.4%), which was likely due to a scramble of the scattered lights by the MCPET between the SPHELAR modules. Nonetheless, the demonstrated device with three parallel SPHELARs showed a quite promising scale-up capability with a STH efficiency higher than 5%.


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)

Time courses of overall water splitting into H2 (☐) and O2 (○) by 4PVs.Overall water splitting were demonstrated by (a) 4PVs fixed in 0.5 M KOH aqueous solution as an electrolyte, (b) 4PVs fixed above the electrolyte, and (c) 4PVs with MCPET fixed above the electrolyte under irradiation of solar simulator (AM1.5G). As the electrocatalysts, combination of NiFeOx (anode) and Ni (cathode) was used with geometric surface area of approximately 0.14 cm2 and 0.19 cm2 for 4PVs (a,b) and 4PVs with MCPET in air (c), respectively. The reactions were measured in a Pyrex top-irradiation flow cell at room temperature.
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Related In: Results  -  Collection

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

f7: Time courses of overall water splitting into H2 (☐) and O2 (○) by 4PVs.Overall water splitting were demonstrated by (a) 4PVs fixed in 0.5 M KOH aqueous solution as an electrolyte, (b) 4PVs fixed above the electrolyte, and (c) 4PVs with MCPET fixed above the electrolyte under irradiation of solar simulator (AM1.5G). As the electrocatalysts, combination of NiFeOx (anode) and Ni (cathode) was used with geometric surface area of approximately 0.14 cm2 and 0.19 cm2 for 4PVs (a,b) and 4PVs with MCPET in air (c), respectively. The reactions were measured in a Pyrex top-irradiation flow cell at room temperature.
Mentions: In this section, overall water splitting with our SPHELAR devices, composed of (NiFe, Pt/Ni) for 3PVs and (NiFe, Ni) for 4PVs in 0.5 M KOH, are demonstrated as model cases. The electrode surface areas were 0.45 cm2 for 3PVs with MCPET and 0.19 cm2 for 4PVS with MCPET (see Supplementary Fig. S5 for calculation details). Various SPHELAR configurations were evaluated, i.e., the body of SPHELAR was fixed in the electrolyte, above the electrolyte, and above the electrolyte with MCPET. Figs 6 and 7 show the measured gas production rate by devices with 3PVs and 4PVs, respectively, where the corresponding STH efficiencies calculated with the PV module projected surface area are also noted. The measured hydrogen evolution rates and STH efficiencies are summarized in Supplementary Table S3. In all cases, steady evolutions of hydrogen and oxygen were observed during light irradiation. The light was turned off after 2.0 h, and no further formations of hydrogen and oxygen were observed, thus confirming that gas productions were induced by harvesting the light. Additionally, it was confirmed that there was no consumption of evolved hydrogen and oxygen in the circulation system after the light was turned off (shown in Supplementary Fig. S8), indicating a negligible reverse reaction of products forming water in the dark. In both measurements in the flow reactor (Figs 6 and 7) and in the batch reactor (Supplementary Fig. S8), the ratio of evolved hydrogen over oxygen was approximately 2, which agreed with the stoichiometry of the water splitting reaction. The devices with MCPET showed the highest STH efficiencies of 7.4% (3PVs) and 6.4% (4PVs). Importantly, the STH efficiency was calculated with the projected surface area of the SPHELAR device and the same area of the incident photons. Therefore the STH efficiency improved with the MCPET reflector, which successfully utilized the photons unused unless it is present, as demonstrated in the previous section. Without MCPET as the reflector, smaller efficiencies were obtained: 6.1% (3PVs) and 4.8% (4PVs) with SPHELAR above the electrolyte and 4.9% (3PVs) and 3.9% (4PVs) with SPHELAR in the electrolyte. The differences in the STH efficiencies due to device location can be attributed to light absorption by the electrolyte and the altered lens effect, as shown in Figs 3 and 4 545556. The hydrogen and oxygen production rates in Figs 6 and 7 agreed with the expected photoelectric and electric currents (see Fig. 4), revealing that an approximately 100% of Faradic efficiency was attained. Indeed, a spontaneous gas evolution from both the anode and the cathode could be seen in the supplementary Movie S1 using a representative configuration of 3PVs located above the electrolyte with MCPET. As clearly seen in the supplementary movie, this SPHELAR based device could split water only by solar energy and work stand-alone without any bias or extra circuit. As Fig. 1(a) demonstrates the actual photograph of SPHELARs, the total volumes for SPHELARs were approximately 5.6 × 10−2 and 7.2 × 10−2 cm3 for 3PVs and 4PVs, respectively. Since the present SPHELAR devices could split water in the completely stand-alone configuration, it is remarkable that such a small volume (less than 0.1 cm3) of the present PV + electrolyzer system achieves the high STH efficiencies. Additionally, a potential method for the scale-up of the SPHELAR device, where three 3PVs are placed in parallel with MCPET and shared the electrocatalyst, was demonstrated. The picture of this configuration is shown in Fig. 8(a), which can also show how the reflector was integrated in the SPHELAR device. The projected surface area of the whole device was approximately 0.6 cm2, which was three times larger than that of one 3PV module (0.20 cm2). The amount of evolved gas upon irradiating the photon is compiled in Fig. 8(b). Indeed, an approximately three times larger gas production rate was observed in the configuration compared with one 3PV module. The observed hydrogen production rate corresponded to approximately 6.8% of STH efficiency. This STH efficiency was slightly smaller than that of one 3PV (ca. 7.4%), which was likely due to a scramble of the scattered lights by the MCPET between the SPHELAR modules. Nonetheless, the demonstrated device with three parallel SPHELARs showed a quite promising scale-up capability with a STH efficiency higher than 5%.

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