Photoelectrochemical Reaction in an Electric Cell with a Porous

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C: Surfaces, Interfaces, Porous Materials, and Catalysis

Photoelectrochemical Reaction in an Electric Cell with a Porous Carbon Anode Toshiyuki Kaizu, Yousuke Kawajiri, Masahito Enomoto, Takashi Uchino, Minoru Mizuhata, Yuichi Ichihashi, Keita Taniya, Satoru Nishiyama, Masakazu Sugiyama, Masami Ueno, and Takashi Kita J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.9b01731 • Publication Date (Web): 24 Jul 2019 Downloaded from pubs.acs.org on July 25, 2019

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The Journal of Physical Chemistry

Photoelectrochemical Reaction in an Electric Cell with a Porous Carbon Anode Toshiyuki Kaizu,1* Yousuke Kawajiri,1 Masahito Enomoto,1 Takashi Uchino,2 Minoru Mizuhata,1 Yuichi Ichihashi,1 Keita Taniya,3 Satoru Nishiyama,1 Masakazu Sugiyama,4 Masami Ueno, 5 and Takashi Kita1 1

Graduate School of Engineering, Kobe University, 1-1 Rokkodai, Nada, Kobe 657-8501, Japan Tel.: +81-78-803-6402, E-mail: [email protected] 2 Graduate School of Science, Kobe University, 1-1 Rokkodai, Nada, Kobe 657-8501, Japan 3 Organization for Advanced and Integrated Research, Kobe University, 1-1 Rokkodai, Nada, Kobe 657-8501, Japan 4 Graduate School of Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan 5 Faculty of Agriculture, University of the Ryukyus, 1 Senbaru, Nishihara-cho, Nakagami-gun 903-0129, Japan Abstract A photoelectrochemical cell utilizing porous carbon derived from Binchotan charcoal as the anode is demonstrated. Visible pulse laser irradiation of the Binchotan charcoal anode induces the ablation of carbon cations into a NaNO3 aqueous solution. Electrons remaining in the anode reduce the potential of the anode with respect to a platinum cathode. As the electrodes are shortcircuited, photogenerated electrons are transported from the anode to the cathode as a diffusion current. The photoinduced charge density at the anode exhibits superlinear and linear excitation-laser power density dependences at low and high laser power densities, respectively. This difference in excitation-power dependence is caused by the valences of the carbon cations generated by absorbing two-photon energy. The revealed reaction mechanism for electron generation at the Binchotan charcoal anode in the photoelectrochemical cell suggests that the sacrificial reagent for the anode can be not only Binchotan charcoal but also any material capable of being ionized by laser ablation. In particular, porous materials that easily ablates cations are a promising sacrificial reagent for the anode of the photoelectrochemical cell.

1. Introduction Carbon has a variety of allotropes such as diamond, graphite, carbon nanotubes, fullerenes, 1 ACS Paragon Plus Environment

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and amorphous carbon. These carbon allotropes have different physical and chemical properties depending on the hybridization states and atomic arrangements. Porous carbon, which is a type of amorphous carbon, is utilized in various fields such as gas separation and storage,1–3 water purification,4 electrocatalyst supports in fuel cells,5–7 and electrochemical energy storage and conversion.8,9 Additionally, the photoinduced reactions of porous carbon have attracted interest in the degradation of organic pollutants10-12 and the production of hydrogen13,14 because porous carbon exhibits photoactivity under not only ultraviolet light irradiation but also visible light irradiation. It has been reported that TiO2 coated with porous carbon exhibits an enhancement of the photocatalytic activity under visible light irradiation,10 while the porous carbon without the semiconductor exhibits photocatalysis by itself.11,12 Recently, hydrogen production in a photoinduced reaction using biomass charcoal with a porous structure has been reported.13,14 Upon irradiation with a visible pulse laser light, carbon atoms/ions are ablated from the surface of Binchotan charcoal powder dispersed in pure water to act as a sacrificial reagent for producing hydrogen and carbon monoxide gases. It is known that the conversion of absorbed photon energy into thermal energy causes a rapid increase in material temperature which accompanies an explosive expansion of the material. Indeed, it has been demonstrated that the laser-ablated charcoal surface attains a temperature above 3800 K.15 Such a high-temperature condition is similar to that used for coal gasification in practical plants.16 The hydrogen production efficiency, determined by the ratio of the product energy to the input energy,12 is lower than the hydrogen production efficiency of coal gasification14 as well as that of water splitting using a GaN semiconductor photocatalyst under ultraviolet light irradiation.17,18 Generally, methanol enhances hydrogen production in water splitting using semiconductor photocatalysts because it acts as a sacrificial scavenger of photogenerated holes on the photocatalyst anode surface to produce carbon dioxide, and simultaneously provides protons that react with photogenerated electrons transferred from the photocatalyst anode to the cathode.19–21 Similar effects enhancing the hydrogen production efficiency have been 2 ACS Paragon Plus Environment

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demonstrated in the laser irradiation of Binchotan charcoal powder by using ethanol.14 Thus, electrons photogenerated on the surface of porous carbon play an important role in hydrogen production. In this study, we demonstrated a photoelectrochemical reaction in a cell structure using porous carbon as an anode and platinum as a cathode. We fabricated a photoelectrochemical cell utilizing porous carbon derived from Binchotan charcoal as the anode and demonstrated electric power generation under visible pulse laser irradiation. Here, Binchotan charcoal was laser-ablated to act as not only an anode but also a sacrificial reagent. Carbon cations are optically ablated from the surface of the anode, and electrons remaining in the anode are transported to the cathode as a diffusion current. The reaction mechanism is discussed in detail as a function of the excitation-laser power density.

2. Experimental Section Figure 1 shows a schematic diagram of a photoelectrochemical cell utilizing porous carbon composed of Binchotan charcoal and platinum electrodes. Binchotan charcoal (Hayakawa, Wakayama, Japan), obtained by carbonizing ubame oak, was cut into a specimen with dimensions of 15 mm × 20 mm × 5 mm for the anode, which was attached to a copper plate by an electrically conductive paste (KAKEN TECH Co., Ltd., TK paste CR-2800). Coiled platinum (Nilaco Corp., PT-35951, 99.7%) was used as the cathode. The electrodes were fixed using connecting clips to soak in 0.1 mol L−1 NaNO3 aqueous solution. The current flows from the platinum cathode to the charcoal anode through short-circuiting between the electrodes. The current was amplified with a transimpedance of 103 V A−1 in a variable-gain high-speed current amplifier (Opto Science, Inc., DHPCA-100) and measured using an oscilloscope (TEXIO Tech. Corp., DCS-4605) with a temporal resolution of 4 ns. The current was calculated by dividing the voltage measured using the oscilloscope by the transimpedance. On the other hand, the voltage was measured directly by using the oscilloscope under an open-circuit condition. The 3 ACS Paragon Plus Environment


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light source used for excitation was a 532 nm Q-switched Nd:YAG pulse laser (Spectra-Physics, INDI 40) with a pulse duration of 8 ns and a repetition rate of 10 Hz. The diameter of the laser beam was 8 mm. The oscilloscope was synchronized with a trigger signal generated by the pulse laser light source, by which the temporal evolutions of the photoinduced short-circuit current and open-circuit voltage during the irradiation of four-cycle laser pulses were recorded. The Binchotan charcoal used for laser-induced gasification was crushed to particles of sub100 μm size. Binchotan charcoal particles and 10 mL of pure water were placed in sample bottles along with a Teflon-coated stirring bar. The sample bottles were covered with a silicone rubber cap. The air dissolved in the pure water and present in the sample bottles was replaced by purging the bottles with pure Ar gas for 15 min. The light source used for excitation was the same Nd:YAG pulse laser as that used for the excitation of the photoelectrochemical cell. The excitation light was irradiated on the Binchotan charcoal powder dispersed in pure water while stirring using a magnetic stirrer. After the pulse laser was irradiated for 30 min, 0.15 mL of the sampling gas was collected from the sample bottles by piercing a syringe through the silicone rubber caps. The produced gas component and its volume were analyzed by gas chromatography (SHIMADZU Corp., GC-8A).

3. Results and Discussion 3.1. Photoelectrochemical Cell with Porous Carbon Anode Under a dark condition, the charcoal electrode has a lower potential than the platinum electrode. By short-circuiting between the charcoal and platinum electrodes, Binchotan charcoal is oxidized to act as an anode because of a lower oxidation-reduction potential. As biomass charcoals generally increase the pH in water, the NaNO3 aqueous solution in the photoelectrochemical cell with the charcoal anode is weakly alkaline. Therefore, the following reactions proceed at the anode and cathode. Anode: 2OH－ → H2O + 1/2 O2 + 2e－,

(1) 4

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Cathode: 2H2O + 2e－ → H2 + 2OH－.

(2)

Figures 2(a) and 2(b) show typical temporal evolutions of the short-circuit current density and open-circuit voltage under visible pulse laser excitation, respectively. They were obtained by subtracting the values of the short-circuit current density and open-circuit voltage under the dark condition from the temporal evolutions extracted during the irradiation of the four-cycle laser pulses shown in the insets. The excitation power density per pulse was 99.5 mJ cm−2. As soon as the surface of the charcoal anode is irradiated with the pulse laser, the short-circuit current density rapidly increases and then decays exponentially with a time constant of approximately 20 ms, which accompanies the deposition of laser-ablated charcoal particles at the bottom of the NaNO3 aqueous solution. When the electrodes are short-circuited, the photogenerated electrons are transported to the cathode, mainly as a diffusion current, to react with protons originating from the dissociation of water in the NaNO3 aqueous solution. The decay time of the short-circuit current density is determined by the rate of electron discharge due to the reaction with protons on the cathode. Conversely, the open-circuit voltage slightly increases. Electrons photogenerated in the charcoal anode reduce the potential of the anode with respect to the platinum cathode, which results in the slight increase in open-circuit voltage. The open-circuit voltage exhibits an extremely long decay time of approximately 900 ms, reflecting the rate of electron discharge at the anode. The electrons photogenerated in the anode are mainly discharged by reacting with protons attracted to the anode, while hardly any reaction with the carbon cations ablated from the anode surface occurs because the carbon cations are consumed by their reaction with the hydroxide ions in the NaNO3 aqueous solution to produce carbon monoxide, as represented by Eq. (4) later. The Binchotan charcoal anode attracts very few protons as compared with the platinum cathode because of its lower oxidation-reduction potential, which results in a reduction in the rate of electron discharge at the surface of the anode. The decay time longer than the laser plus repetition time causes the base level of the opencircuit voltage to rise, as shown in Fig. 2(b). 5 ACS Paragon Plus Environment


Figure 3(a) shows the photoinduced charge densities derived by integrating the decay profiles plotted against the excitation-laser power density. At laser power densities below 50 mJ cm−2, the photoinduced charge density increases with the square of the laser power density, while at laser power densities above 50 mJ cm−2, it increases proportionally to the laser power density. This change in the slope of the laser power density dependence is explained by the difference in carbon cation species ablated from the anode surface. It is known that the number of carbon cations increases superlinearly with the laser power density because laser ablation is caused by the absorption of multiphoton energy.22 On the other hand, the valence of carbon cations generally increases with the laser power density to generate multiple electrons,23 which leads to the linear dependence of the number of photogenerated electrons on the laser power density at a high laser power density. The changes in the carbon cation species with respect to the laser power density are discussed in detail later. Moreover, the increase in the number of electrons generated in the anode affects the decay time of the photoinduced current. The excitation-laser power density dependence of the decay time of the photoinduced current density is shown in Fig. 3(b). The decay time increases with the laser power density because the electron inflow from the anode to the cathode exceeds the electron discharge due to the reaction of electrons with protons on the cathode surface. The external quantum efficiency (EQE) of the electric cell with the Binchotan charcoal anode was estimated from the photoinduced charge density shown in Fig. 3(a). The EQE defined as the ratio of the number of electrons collected as photoinduced current to the number of incident photons, was 1.1×10-5 at 99.5 mJ cm−2. Generally, in conventional electric cells, hydrogen is generated at the cathode because electrons transported from the anode to the cathode react with protons in the aqueous solution. However, in our photoelectrochemical cell, little hydrogen gas was generated on the platinum cathode, even at the maximum laser power density, whereas it was clearly generated on the Binchotan charcoal anode. This suggests that only part of the photogenerated electrons in the Binchotan charcoal anode are transported to the platinum cathode. We considered that 6 ACS Paragon Plus Environment

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photogenerated electrons distribute in the Binchotan charcoal anode, as the porous structure in which micropores of various sizes are formed randomly, leading to the potential distribution in the anode. The electrons distributed in the lower potential area of the anode are hardly transported to the cathode, and thus, react with protons in the aqueous solution on the anode surface.

3.2. Mechanism of Electron Generation by Laser Ablation of Porous Carbon The laser ablation of the porous carbon anode is a key process in the operation of the photoelectrochemical cell. As discussed above, electrons remaining in the anode after ablating carbon cations by the pulse laser are transported to the cathode. The change in ablated carbon cation species with respect to the excitation-laser power density strongly affects the amount of photoinduced current. On the other hand, the ionization of porous carbon by laser ablation triggers the reactions that produce hydrogen and carbon monoxide in laser-induced gasification. Such laser-induced gasification has been reported for not only Binchotan charcoal but also various biomass charcoals such as bagasse charcoal and activated carbon,24 from which any porous carbon derived from biomass charcoal is a promising sacrificial reagent for the anode of the photoelectrochemical cell. Here, the mechanism underlying the electron generation is clarified by investigating the volumes of gasified hydrogen and carbon monoxide in detail. Figure 4(a) shows the volume of hydrogen produced per 0.1 mL of the sampling gas collected after pulse laser irradiation to Binchotan charcoal powder dispersed in pure water plotted against the laser power density. The carbon concentration was 0.83 mol L−1. The hydrogen production volume increases superlinearly with the laser power density, which is similar to the behavior of the photoinduced charge density at low laser power densities shown in Fig. 3(a). However, the slope of the laser power density dependence of the hydrogen production volume is steeper than that of the photoinduced charge density. Moreover, the change in slope shown in Fig. 3(a) does not appear. In laser-induced gasification, hydrogen production is affected by 7 ACS Paragon Plus Environment


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the processes of electron generation occurring at the charcoal powder and the reaction of the generated electrons with protons. From the extremely long decay time of the open-circuit voltage shown in Fig. 2(b), the reaction rate of photogenerated electrons with protons at the charcoal surface is surmised to be considerably low, which mainly causes the superlinearity of the hydrogen production volume with respect to the laser power density. Figure 4(b) shows the volume ratio of the hydrogen and carbon monoxide gases generated from the Binchotan charcoal powder plotted against the excitation-laser power density. At low laser power densities, a volume ratio close to 0.5 was obtained and the reaction proceeded as follows: C → C+ + e－,

(3)

2C+ + 2OH－→ 2CO + H2.

(4)

Charcoal-dispersed water is weakly alkaline before laser irradiation, which means that surplus hydroxide ions are present. The monovalent carbon cations ablated from the charcoal surface into pure water react with hydroxide ions to produce carbon monoxide and hydrogen, while electrons are left in the charcoal. At low laser power densities, the reaction rate of electrons with protons in the weakly alkaline charcoal-dispersed water is low because of the low electron and proton densities. Therefore, the reaction process represented by Eq. (4) mainly determines the volume ratio of the hydrogen and carbon monoxide gases. As the laser power density is increased, the volume ratio gradually increases and approaches 1. This implies that the reaction mechanism changes from that given by Eq. (4) at high laser power densities. This can be attributed to the difference in the carbon cation species ablated from the charcoal surface at low and high laser power densities. Generally, a high laser power density results in the ablation of multivalent carbon cations along with monovalent cations.23 The ratio of multivalent cations to monovalent cations is high. Therefore, the following reactions are considered to occur: C → C2+ + 2e－,

(5)

C2+ + H2O → CO + 2H+,

(6) 8 ACS Paragon Plus Environment

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2H2O + 2e－ → H2 + 2OH－.

(7)

The electrons densely localized at the charcoal surface after ablating divalent carbon cations attract protons in the charcoal-dispersed water. Additionally, the relative proton density increases because of the consumption of hydroxide ions in the reaction process given by Eq. (4). Therefore, the reaction of electrons with protons for hydrogen production readily occurs at the charcoal surface. As the laser power density is increased, the reaction processes given by Eqs. (6) and (7) proceed to produce carbon monoxide and hydrogen, respectively, along the pathway given by Eq. (4) and then become dominant. It has been reported that the pH of porous carbon-dispersed water after laser-induced gasification gradually decreases from 7.9 and then saturates at ~7.5 as the excitation-laser power density is increased.24 This result indicates that the hydroxide ions in porous-carbon-dispersed water are consumed during the reaction with the monovalent carbon cations ablated from the charcoal surface at low laser power densities, as represented by Eq. (4). At high laser power densities, on the other hand, as the reaction processes given by Eqs. (6) and (7) are predominant, protons are also consumed along with the hydroxide ions. In the photoelectrochemical cell, the ablation process of carbon cations given by Eqs. (3) and (5) occurs at the anode, whereas the reaction for hydrogen production given by Eq. (7) proceeds at the cathode. The reactions given by Eqs. (4) and (6) proceed in the NaNO3 aqueous solution. In the laser ablation of porous carbon, the photon energy is absorbed to generate one electron (two electrons) along with a monovalent (divalent) carbon cation at a low (high) power density. The fact that the electronic charge density is proportional to the square of the laser power density at a low power density and the laser power density at a high power density, as shown in Fig. 3(a), indicates that the laser ablation is caused by the absorption of two-photon energy.

3.3. Photoelectrochemical Cell with Porous Silicon Anode 9 ACS Paragon Plus Environment


The reaction mechanism revealed by investigating the photoelectrochemical cell with the porous carbon anode suggests that the sacrificial reagent for the anode can be any material capable of being ionized by laser ablation. In particular, a porous structure with a large specific surface area is necessary to efficiently generate electrons. To confirm that such a condition is required for the anode, we fabricated a photoelectrochemical cell utilizing porous silicon for the anode instead of porous carbon. The porous silicon anode was fabricated by anodizing a silicon substrate.25 Figure 5 shows the temporal evolution of the short-circuit current density gained by the visible pulse laser excitation. The excitation power density per pulse was 99.5 mJ cm−2. As the surface of the porous silicon anode was irradiated with a pulse laser, the shortcircuit current density increased and then exponentially decayed with a time constant of approximately 29 ms. Such a behavior is similar to the temporal evolution of the short-circuit current density observed in the photoelectrochemical cell with the porous carbon anode in Fig. 2. The photoinduced charge density derived by integrating the decay profile was approximately 6.4 × 102 μC cm−1 at 99.5 mJ cm−2, which is comparable to that obtained in the photoelectrochemical cell with the porous carbon anode. Interestingly, it was observed that the photoelectrochemical reaction suddenly stopped when the porous surface was completely consumed by laser ablation. Thus, a porous material that easily ablates cations is a promising sacrificial reagent for the anode of a photoelectrochemical cell. Detailed analysis of the characteristics and reaction mechanism for the photoelectrochemical cell with the porous silicon anode is ongoing.

4. Conclusion We have demonstrated a photoelectrochemical cell with a porous carbon anode derived from Binchotan charcoal and a platinum cathode. As the porous carbon anode was irradiated with the visible pulse laser, carbon cations were ablated into NaNO3 aqueous solution. The electrons remaining in the anode reduce the potential of the anode with respect to the platinum 10 ACS Paragon Plus Environment

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cathode. The photogenerated electrons are transported to the cathode as a diffusion current by short-circuiting the electrodes, which react with protons originating from the dissociation of water in the NaNO3 aqueous solution. At a low excitation-laser power density, the photoinduced charge density exhibited a superlinear increase with respect to the laser power density, whereas at a high laser power density, it exhibited a linear increase. This difference in laser-power dependence is due to a change in the valence of carbon cations ablated by absorbing two-photon energy. The reaction mechanism for electron generation revealed in the photoelectrochemical cell with the porous carbon anode suggests that the sacrificial reagent for the anode can be any material capable of being ionized by laser ablation. Furthermore, we demonstrated a photoelectrochemical cell utilizing porous silicon for the anode instead of porous carbon, from which any porous material that easily ablates cations is a promising sacrificial reagent for the anode of the photoelectrochemical cell.

Acknowledgements We deeply thank Professor M. Fujii of Kobe University and Dr. K. Aoki of Kobe University (present affiliation: National Institute of Information and Communications Technology) for fabricating porous silicon anode used in this work.

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(4) Choi, W. C.; Woo, S. I.; Jeon, M. K.; Sohn, J. M.; Kim, M. R.; Jeon, H. J. Platinum Nanoclusters Studded in the Microporous Nanowalls of Ordered Mesoporous Carbon. Adv. Mater. 2005, 17, 446-451. (5) Ding, J.; Chan, K. Y.; Ren, J. W.; Xiao, F. S. Platinum and Platinum–ruthenium Nanoparticles Supported on Ordered Mesoporous Carbon and Their Electrocatalytic Performance for Fuel Cell Reactions. Electrochim. Acta 2005, 50, 3131-3141. (6) Joo, J. B.; Kim, P.; Kim, W.; Kim, J.; Yi, J. Preparation of Mesoporous Carbon Templated by Silica Particles for Use as a Catalyst Support in Polymer Electrolyte Membrane Fuel Cells. Catal. Today 2006, 111, 171-175. (7) Antolini, E. Nitrogen-doped Carbons by Sustainable N- and C-containing Natural Resources as Nonprecious Catalysts and Catalyst Supports for Low Temperature Fuel Cells. Renew. Sustain. Energy Rev. 2016, 58, 34-51. (8) Simon, P.; Gogotsi, Y. Capacitive Energy Storage in Nanostructured Carbon Electrolyte Systems. Acc. Chem. Res. 2013, 46, 1094-1103. (9) Yang, S.; Bachman, R. E.; Feng, X.; Müllen, K. Use of Organic Precursors and Graphenes in the Controlled Synthesis of Carbon-Containing Nanomaterials for Energy Storage and Conversion. Acc. Chem. Res. 2013, 46, 116-128. (10) Toyoda, T.; Yoshikawa, T.; Tsumura, T.; Inagaki, M. Photoactivity of Carbon-coated Anatase for Decomposition of Iminoctadine Triacetate in Water. J. Photochem. Photobiol. A, Chem. 2005, 171, 167-171. (11) Velasco, L. F.; Lima, J. C.; Ania, C. Visible-Light Photochemical Activity of Nanoporous Carbons under Monochromatic Light. Angew. Chem. Int. Ed. 2014, 53, 4146-4148. (12) Velasco, L. F.; Maurino, V.; Laurenti, E.; Ania, C. Light-induced Generation of Radicals on Semiconductor-free Carbon Photocatalysts. Appl. Catal. A 2013, 453, 310-315. (13) Akimoto, I.; Maeda, K.; Ozaki, N. Hydrogen Generation by Laser Irradiation of Carbon Powder in Water. J. Phys. Chem. C 2013, 117, 18281-18285. 12 ACS Paragon Plus Environment

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(14) Maeda, K.; Ozaki, N.; Akimoto, I. Alcohol Additive Effect in Hydrogen Generation from Water with Carbon by Photochemical Reaction. Jpn. J. Appl. Phys. 2014, 53, 05FZ03. (15) Akimoto, I.; Yamamoto, S.; Maeda, K. White-light Emission from Solid Carbon in Aqueous Solution during Hydrogen Generation Induced by Nanosecond Laser Pulse Irradiation. Appl. Phys. A 2016, 122, 675. (16) Rosen, M. A.; Scott, D. S. Comparative Efficiency Assessments for a Range of Hydrogen Production Processes. Int. J. Hydrogen Energy 1998, 23, 653-659. (17) Fujii, K.; Karasawa, T.; Ohkawa, K. Hydrogen Gas Generation by Splitting Aqueous Water Using n-Type GaN Photoelectrode with Anodic Oxidation. Jpn. J. Appl. Phys. 2005, 44, L543-L545. (18) Ohkawa, K.; Ohara, W.; Uchida, D.; Deura, M. Highly Stable GaN Photocatalyst for Producing H2 Gas from Water. Jpn. J. Appl. Phys. 2013, 52, 08JH04. (19) Khan, S. U. M.; Sultana, T. Photoresponse of n-TiO2 Thin Film and Nanowire Electrodes. Sol. Energy Mater. Sol. Cells 2003, 76, 211-221. (20) Yeh, T. -F.; Syu, J. -M.; Cheng, C.; Chang, T. -H.; Teng, H. Graphite Oxide as a Photocatalyst for Hydrogen Production from Water. Adv. Funct. Mater. 2010, 20, 2255-2262. (21) Bowker, M. Sustainable Hydrogen Production by the Application of Ambient Temperature Photocatalysis. Green Chem. 2011, 13, 2235-2246. (22) Gaumet, J. J.; Wakisaka, A.; Shimizu, Y.; Tamori, Y. Energetics for Carbon Clusters Produced Directly by Laser Vaporization of Graphite: Dependence on Laser Power and Wavelength. J. Chem. Soc. Faraday Trans. 1993, 89, 1667-1670. (23) Constantinescu, R. C. ; Hunsche, S.; van Linden van den Heuvell, H. B.; Muller, H. G.; LeBlanc, C.; Salin, F. Highly Charged Carbon Ions Formed by Femtosecond Laser Excitation of C60: A Step Towards an X-ray Laser. Phys. Rev. A 1998, 58, 4637-4646. (24) Enomoto, M.; Kawajiri, Y.; Kaizu, T.; Uchino, T.; Ichihashi, Y.; Taniya, K.; Nishiyama, S.; Mizuhata, M.; Sugiyama, M.; Ueno, M.; Kita, T. International Symposium on Novel Energy 13 ACS Paragon Plus Environment


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Nanomaterials, Catalysts and Surfaces for Future Earth, The University of ElectroCommunication, Tokyo, Oct. 28-30, 2017. (25) Bisia, O.; Ossicinib, S.; Pavesi, L. Porous Silicon: a Quantum Sponge Structure for Silicon Based Optoelectronics. Surf. Sci. Rep. 2000, 38, 1-123.

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Figure 1. Schematic illustration of photoelectrochemical cell with porous carbon anode composed of molded Binchotan charcoal and platinum cathode.



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Figure 2. Temporal evolutions of (a) short-circuit current density and (b) open-circuit voltage under visible pulse laser excitation for photoelectrochemical cell with porous carbon anode. The temporal evolutions during the irradiation of the four-cycle laser pulses are shown in the insets. The pulse laser was generated at 0 ms. The broken lines represent the single exponential functions used as fitting curves.


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Figure 3. (a) Photoinduced charge density (ρ) derived by integrating the decay profiles and (b) decay time of short-circuit current density plotted against excitation-laser power density (P).



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Figure 4. Excitation-laser power density (P) dependence of laser-induced gasification for Binchotan charcoal powder dispersed in pure water. (a) Volume of hydrogen (VH2) produced per 0.1 mL of sampling gas. (b) Ratio of the production volume of hydrogen to that of carbon monoxide. The broken and dotted lines denote ratios of 0.5 and 1, respectively.


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Figure 5. Temporal evolution of short-circuit current density gained by the visible pulse laser excitation for photoelectrochemical cell with porous silicon anode. The pulse laser was generated at 0 ms. The broken lines represent the single exponential functions used as fitting curves.



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Photoelectrochemical Reaction in an Electric Cell with a Porous

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