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Photovoltaic devices in hydrogen production
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 4 ) 1 e6
Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.elsevier.com/locate/he
Photovoltaic devices in hydrogen production Xiaoniu Peng, Chao He, Xi Fan, Qingyun Liu, Jun Zhang, Hao Wang* Hubei Collaborative Innovation Center for Advanced Organic Chemical Materials, Faculty of Physics and Electronic Science, Hubei University, Youyi Avenue 368#, Wuhan 430062, PR China
article info
abstract
Article history:
Hydrogen is an ideal fuel for the increasing human demand of abundant, clean and
Received 19 October 2013
renewable energy. Various technologies are developed for the production of the hydrogen
Received in revised form
through water splitting, and in particular the photovoltaic device is widely regarded as an
6 March 2014
opportunity for the high efficiency, low cost and simple configuration devices. Three main
Accepted 3 April 2014
systems are briefly reviewed in this article by demonstrating the typical devices of them,
Available online xxx
including the IIIeV group compound semiconductor, the multi-junction Si structure and the dye-sensitized solar cells. Although considerable disadvantages and limitations are
Keywords:
obvious in performance, the challenging and competing criteria are outlined for the im-
Photovoltaic device
provements. Moreover, the introduction of plasmonic metal nanostructures provides an
Hydrogen production
intriguing prospect of extensive applications of the photovoltaic devices. Copyright ª 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Energy consumption outlines the development of human technologies and civilizations [1]. Most of the energy the humans consumed every year directly comes from the combustion of fossil fuels, which leads to the potentially drastic effects of increased atmospheric CO2 concentrations on global climates, as well as the depletion of the reserve coal, natural gas, and petroleum [2,3]. Therefore, the access to a renewable, carbon neutral energy source is expected to be a key factor in sustaining population growth. The solar energy, as the clean and most abundant resource from the sun, satisfies the current and future human energy demand [4,5]. The solar energy form the sunlight strikes our planet on an annual basis at a rate of approximately 120,000 TW, which vastly exceeds the energy use of the human society every year, w15 TW [6e8]. A general agreement is proposed that photosynthesis is an efficient energy generation and storage system for the solar energy, especially in biology. The light reactions in
photosynthesis provide photons and electrons for the conversion from the carbon dioxide to the organic molecules, including living organisms, food and fossil fuels [7]. Splitting water into oxygen and hydrogen (2H2O þ hv / O2 þ 4Hþ þ 4e, DG0 ¼ 1.23 eV) is the heart process of photosynthesis. The hydrogen is ideal energy storage medium for the solar fuel, and the freely available energy from the sunlight is captured and turned into the valuable and strategic power in the form of H2 molecules. The photovoltaic water splitting is a promising technology to produce clean hydrogen. The idea of generating electric power and chemical fuels from the sunlight captures the most intensive attentions of the scholars ever since the photoelectric effect is firstly demonstrated by the French scientist Edmond Becquerel [9]. Among various kinds of ways to facilitate the water splitting reaction, the photoelectrochemical (PEC) cells are the most efficient and economical selection. A basic photoelectrochemical water splitting cell can be constructed in the form of the Schottky type or the tandem (Z-scheme) type
* Corresponding author. Tel.: þ86 27 88663390. E-mail address: [email protected] (H. Wang). http://dx.doi.org/10.1016/j.ijhydene.2014.04.030 0360-3199/Copyright ª 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Please cite this article in press as: Peng X, et al., Photovoltaic devices in hydrogen production, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/j.ijhydene.2014.04.030
2
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 4 ) 1 e6
Fig. 1 e Basic configurations of (a) the Schottky type and (b) the tandem or Z-scheme type of the photoelectrochemical cells [Reprinted from Ref. [10] with permission].
(Fig. 1) [10]. In the photoanodeecathode configuration, the majority carriers are transferred to the anode/cathode for carrying out the photoelectrolysis, while the minority charge carriers (electrons in p-type semiconductor photocathodes and holes in n-type semiconductor photoanodes) are driven to the semiconductor/aqueous solution interface by the electric field formed at the solid/liquid contact. In the configuration of the photoanode/photocathode, both the holes and the electrons are utilized for the water splitting reactions at their respective solid/liquid interfaces. The typical PEC cell consists of a photoanode and a photocathode, which is firstly reported by Fujishima and Honda in their classic work of Pt w TiO2 [11]. The electrons are extracted from the water using solar irradiation as the energy source in the photoanode, and the reductive electrons are utilized for the hydrogen-generation reaction in the photocathode. Different semiconductors and structures are studied for the PEC cells in water splitting with the request of the wide absorption range of the solar irradiation and the sufficient band gap energy for the H2 and O2 evolution reactions [8,12]. To describe the hydrogen production efficiency in a watersplitting reaction, the solar-to-fuel conversion efficiency of the PEC device, upon illumination with 1 sun (100 mW/cm2) of air mass (AM) 1.5 simulated sunlight, can be calculated using the equation [13], hð%Þ ¼ j$DE=S$100% ¼ J mA cm2 $1:23 V 100 mW cm2 $100%
where S is the total incident intensity of the solar irradiance, j is the short-circuit current density at zero bias, and DE is the thermodynamic voltage required for water splitting reaction. The science of the PEC cells is dominated by the study for the improvement of efficiency in the devices.
Systems of photoelectrochemical cell The lack of an efficient, stable and energetic system poses huge challenges to direct photoelectrolysis of water [14]. The oxides with wide absorption range are poor in semiconductor characteristics, although they are most photochemical stable in aqueous solution [10]. IIIeV group compound materials have better performance with respect of oxides. P-Type indium phosphide semiconductor with better solid-state characteristics has been proved to be stable in strong acid under illumination [15,16]. A multi-junction solar cell based on the GaInP2/GaAs p/n, p/n tandem cell is reported by Khaselev and Turner (Fig. 2) [17]. Multi-junction structures are set for the absorption of both visible and near-infrared portion of the solar spectrum. Electrons are transferred to the illuminated surface and holes are transferred to the ohmic contact under illumination. The cathode is protected in the electrolyte by the H2 produced at the semiconductor electrode. The top junction of the cell is the p/Schottky layer, whereas it is p/n type in the fully photovoltaic tandem cell. Although the efficiency limit for this combination of band gaps is 34%, the actually
Fig. 2 e (A) Schematic structure and (B) idealized energy level diagram of the GaInP2/GaAs multi-junction solar cell [Reprinted from Ref. [17] with permission]. Please cite this article in press as: Peng X, et al., Photovoltaic devices in hydrogen production, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/j.ijhydene.2014.04.030
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 4 ) 1 e6
conversion efficiency in this system reaches 12.4% under the incident light intensity of 1190 mW/cm2. Single-junction GaAs solar cell is also employed for the water-splitting reaction and the solar-to-fuels efficiency increases to an impressive 18.2% with the theoretical limitation efficiency of 19.7% [18]. Although efficient reactions of hydrogen production are achieved in the IIIeV materials, high costs and poor stability are the main restriction for the applications. Another approach to the water splitting system is the multiple structure of silicon in PEC cells [19e21]. A stack of amorphous Si or SieGe alloys comprise the typical cells operating in basic electrolyte. The indium tin oxide (ITO) passivates the p-side of the structure and provides marginal protection of the Si from the extreme corrosion of the solution, thus resulting in relatively low hydrogen production efficiency with respect of IIIeV semiconductor. A simple PEC system consisted of triple junction amorphous silicon cell and a separate catalytic anode is reported by Rocheleau et al. [20]. The NiFexOy catalyst is deposited onto the ITO layer on one surface of Si wafer forming the oxygen-evolving complex (OEC), and the CoMo catalyst is assembled on steel plate in contact with the other side of Si plate for the hydrogen evolution reaction (HER). Charge is separated in the Si cell under illumination of sunlight. The negative electrons flow towards the HER catalyst for the reduction of protons into hydrogen, while the positive holes flow towards the OEC catalyst for the oxidation into oxygen. Although not fully optimized, the solar-to-fuel conversion efficiency in this multi-junction Si system reaches record high of 7.8% with steady-state operation. A similar structure is reported by the Nocera and coworkers more recently, except CoPi and NiMoZn being employed instead of the catalyst used before [22]. The nearneutral pH electrolyte is applied, and the performance of the PEC cell with wire connecting is compared against that of the wireless one (Fig. 3). Higher energy conversion efficiency, 4.7%, is observed in the wired configuration and a theoretical maximum, greater than 10%, is proposed for this system. Nevertheless, the shadow effects of the incident light may become a possible disadvantage of this structure, which is
3
caused by the catalyst deposited on top of the front layer of the solar cell. Crystalline Si is also applied in the single junction PEC cell, as shown in Fig. 4 [23]. The n-side and pþ-side are deposited on the Si junction for the protection from oxidation. Improved performance is demonstrated by the nppþ-Si structure for the fact that the thin space-charge region acts as a tunneling layer in the pþ-side layer. Comparing with the thermodynamic potential of water splitting of the formal normal hydrogen electrode, the onset potential in this nppþ-Si structure significantly decreases. The photovoltage generated by this single junction cell is sufficient to drive the water oxidation. Moreover, the threshold of the external applied potential needed for the reaction is further reduced after connecting an additional nppþ-Si solar cell to nppþSi-ITO-Co-OEC structure in series. Based on these results, the external supply may be eliminated with the fact that sufficient potential is provided by numbers of Si solar cell connected in tandem. The photovoltaic cell composed of multi-junction Si and the Earth’s abundant catalyst provides an inexpensive way to engineer a wireless efficient photoelectrochemical device to convert sunlight into hydrogen under neutral condition. This strategy significantly broadens the commercial applications of the robust artificial photosynthetic system. An alternative simple strategy for ideal photoelectrochemical cell for water splitting is the dye-sensitized solar cells (DSCs). The wide-band gap metal-oxide semiconductor are combined with visible light absorbing dyes in the structure, and the absorption characteristics can be turned by molecular design to cover different parts of the solar spectrum. In DSCs, the incident light is absorbed by sensitive dyes and charge separation takes place. The photoinduced electrons inject into the conduction band of the semiconductor from the excited dye, and the holes at the dye ground state are further regenerated through reduction by the hole-transport material. The PEC systems of the dyes combined with semiconductor absorbers and the tandem arrangements with multiple dyes have been demonstrated with efficient performance [24e27].
Fig. 3 e (A) Schematic structure of the wired a-Si PEC cell. (B) solar-to-fuel efficiency of the wired structure in 1 M potassium borate (pH 9.3, black) and 0.1 M KOH (pH 13, red) [Reprinted from Ref. [22] with permission]. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.) Please cite this article in press as: Peng X, et al., Photovoltaic devices in hydrogen production, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/j.ijhydene.2014.04.030
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Fig. 4 e (A) Schematic structure, (B) CeV curves and (C) onset potential of water oxidation of the single junction nppDSi-ITOCo-OEC cell. The curves in (B) represent the sample immersed in KPi electrolyte in dark (black) and illuminated with Xe lamp (green). The blue line corresponds to the dark CeV curve when the potential is applied. The curves in (C) represent the samples measured in dark (black), at 100 mW/cm2 (green), and 1000 mW/cm2 (orange) illumination [Reprinted from Ref. [23] with permission]. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
A dual-absorber solar cell for water splitting is reported by Sivula and co-workers, as shown in Fig. 5 [28]. The structure consists of the tungsten oxide (WO3) or iron oxide (Fe2O3) photoanode in tandem with the dye-sensitized solar cell. Although the WO3 (2.6 eV) and Fe2O3 (2.0 eV) have sufficient Eg energy for solar energy conversion, the energy of the electron
at their conduction band is insufficient for the reduction of the hydrogen, and therefore the additional energy is provided by the DSCs for conducting the splitting reaction. The incident light is absorbed by the metal-oxide semiconductor before the underlying photovoltaic cell, and the photoanode and DSC are carefully selected to exploit a substantial part of the solar
Fig. 5 e (A) General schemes and energy diagrams and (B) JeV curves of the WO3/DSC (left in A and upper in B) and Fe2O3/ DSC (right in A and lower in B) tandem cells [Reprinted from Ref. [28] with permission]. Please cite this article in press as: Peng X, et al., Photovoltaic devices in hydrogen production, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/j.ijhydene.2014.04.030
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 4 ) 1 e6
spectrum. The band gap of WO3 only allows absorption form ultraviolet to blue, whereas the absorption range of the Fe2O3 is extended up to yellow wavelength. Although the state-ofthe-art strategies of surface catalysis and passivation are used to reduce the overpotential needed for water oxidation, the late onset of the photocurrent in the photoelectrode is the critical limiting factor in Fe2O3/DSC. In the case of the WO3/ DSC system, the main limitation is the low photocurrent in the photoanode as a result of the poor absorption capability in the visible region of the solar spectrum. The solar-tohydrogen conversion efficiency in the Fe2O3/DSC and WO3/ DSC system is 1.17% and 3.10% respectively. The clear advantage of the WO3 over Fe2O3 in terms of the onset potential of the photocurrent makes the relative better performance in the WO3/DSC tandem cell. The main reason for the low quantum yield of water splitting in the dye-based PEC system is the fact that the charge recombination reaction is faster than the catalytic four-electron oxidation of water to oxygen. This limitation can be great improved by incorporating a redox mediator into the DSC, which is demonstrated by Mallouk’s group recently (Fig. 6) [29]. The IrOx$nH2O colloid is bond to the BiP mediator, and co-absorbed a Ru polypyridine sensitizer onto a TiO2 electrode without direct linkage. Comparing with the mediator free catalyst, the sustained and relatively higher photocurrent is measured in the mediator capped catalyst, and the internal quantum efficiency of the photoelectrochemical water splitting is approximately 2.3% with the Faradaic efficiency (the electron efficiency in converting to hydrogen) is close to unity. Furthermore, an analysis of the kinetics of bleaching recovery of the sensitizer is presented and the multi-exponential processes are verified in this kinetically complex system. The dye-sensitized PEC cell is a relatively new area of the researches in water splitting. Although low conversion efficiency is obtained in the current performance, considerable progress has been made in advancing the concept of the
5
structure design, which has significant promise for the development of simple, low-cost and efficient solar fuel systems. Comparing with other ways of producing hydrogen, the photovoltaic devices have its unique superiority. Currently, three mature methods are mainly employed for the hydrogen production such as the steam reforming of the methane, the coal gasification of the fossil fuels, and the thermochemical or biological processes of the biomass. A large amount of energy is consumed in the production process from the natural gas, coal, or biomass, with the substantial amount of CO2 that is generated as the by-product. Comparing to these more established methods of hydrogen production, the photovoltaic devices take advantages of the ultrafast initial charge separation and the much slower recombination, which promotes the efficient cleavage of the water. Reasonable solar-tohydrogen efficiency could be optimized after further improvement. The bifacial configuration and the transparent versions could also be set to capture the solar energy from all angles in different wavelength. In addition, the properties of the low system cost, the simple configuration, the environment friendly process, and the separation of the hydrogen and oxygen evolution during the reaction also make the photovoltaic devices more preferable for the sustained utilization of the H2 energy. The opportunity to exploit solar energy in the form of hydrogen with the photoelectrochemical devices offer a huge reward, but also poses huge challenges in the practical application. Efficient materials are needed for the photocurrents of 8 mA/cm2 in “1 Sun” (100 mW/cm2) illumination with the conversion efficiency reaching or exceeding 10%. The bias requirement between the anode to cathode should be optimized to reduce the cost of external supply. In addition, the durability of the devices should provide the efficient lifetime of at least 15 years, or better 20 years. Most importantly, the limitation of the system cost should not exceeding $160/m2, including the external supplement devices. Although the photoelectrochemical devices provide an intriguing prospect of hydrogen production, but significant improvements of them are still required to meet the practical demands.
Conclusion and outlook
Fig. 6 e Electron transfer reactions of the mediator-based dye-sensitized TiO2 photoanode [Reprinted from Ref. [29] with permission].
Although the materials and structures for the water photoelectrolysis reaction have been greatly developed over the past several decades, the photoelectrochemical hydrogen production is not yet a productive solution for the current and future energy demand of the humans. Basic composition improvements are required, together with robust system implementation. More active materials are needed not only for the high energy conversion efficiency but also for the wide range absorption of solar spectrum. The bias requirements should be reduced for the absence of the external supply. Simple configuration and low cost with high durability of the system are also the fundamental factors for the wide applications. Furthermore, the integration of plasmonic metal nanostructures to water splitting is an intriguing and potentially rich area for future exploration. The plasmonic effects of the near-field absorption and light scattering could be
Please cite this article in press as: Peng X, et al., Photovoltaic devices in hydrogen production, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/j.ijhydene.2014.04.030
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employed for the improvement of photon absorption and utilization, which suggests significant strategies for further efficiency enhancements in photoelectrochemical devices of hydrogen production.
Acknowledgments This work is supported in part by the National Nature Science Foundation of China (Nos. 51372075, 51311130312). Research Fund for the Doctoral Program of Higher Education of China (RFDP, No. 20124208110006).
references
[1] The Human Development Index, published by the United Nations Development Programme, is based on factors such as life expectancy, education level, and GDP per capita as defined by the United Nations Human Development Report. http://hdr.undp.org/en/. [2] Solomon S, Plattner G-K, Knutti R, Friedlingstein P. Irreversible climate change due to carbon dioxide emissions. Proc Natl Acad Sci U S A 2009;106:1704e9. [3] Chiari L, Zecca A. Constraints of fossil fuels depletion on global warming projections. Energy Policy 2011;39:5026e34. [4] Lewis NS, Nocera DG. Powering the planet: chemical challenges in solar Energy utilization. Proc Natl Acad Sci U S A 2006;103:15729e35. [5] Lewis NS, Crabtree G, Nozik A, Wasielewski M, Alivisatos P. Basic research needs for solar energy utilization. Washington, DC: US Department of Energy; 2005. [6] Hoffert MI, Caldeira K, Benford G, Criswell DR, Green C, Herzog H, et al. Advanced technology paths to global climate stability: energy for a greenhouse planet. Science 2002;298:981e7. [7] Barber J. Photosynthetic energy conversion: natural and artificial. Chem Soc Rev 2009;38:185e96. [8] Blankenship RE, Tiede DM, Barber J, Brudvig GW, Fleming G, Ghirardi M, et al. Comparing photosynthetic and photovoltaic efficiencies and recognizing the potential for improvement. Science 2011;332:805e9. [9] Becquerel AE. Recherches sur les effets de la radiation chimique de la lumie`re solaire, au moyen des courants e´lectriques. C R Acad Sci 1839;9:145e9. [10] Walter MG, Warren EL, McKone JR, Boettcher SW, Mi Q, Santori EA, et al. Solar water splitting cells. Chem Rev 2010;110:6446e73. [11] Fujishima A, Honda K. Electrochemical photolysis of water at a semiconductor electrode. Nature 1972;238:37e8. [12] Shakhnazaryan GE, Sarkisyan AG, Arutyunyan VM, Arakelyan VM, Turner JA, Vartanyan RS. Study of quantum efficiency of doped Fe2O3 ceramic photoelectrodes. Russ J Electrochem 1994;30:610e4.
[13] Gra¨tzel M. Photoelectrochemical cells. Nature 2001;414:338e44. [14] Gerischer H. Topics in applied physics. In: Seraphin BO, editor. Solar energy conversion, vol. 31. Berlin: SpringerVerlag; 1979. pp. 115e72. [15] Aharon-Shalom E, Heller A. Efficient p-InP (Rh-H alloy) and p-InP (Re-H alloy) hydrogen evolving photocathodes. J Electrochem Soc 1982;129:2865e6. [16] Heller A. Hydrogen-evolving solar cells. Science 1984;223:1141e8. [17] Khaselev O, Turner JA. A monolithic photovoltaicphotoelectrochemical device for hydrogen production via water splitting. Science 1998;280:425e7. [18] Winkler MT, Cox CR, Nocera DG, Buonassisi T. Modeling integrated photovoltaiceelectrochemical devices using steady-state equivalent circuits. Proc Natl Acad Sci U S A 2013;110:E1076e82. [19] Kelly NA, Gibson TL. Design and characterization of a robust photoelectrochemical device to generate hydrogen using solar water splitting. Int J Hydrogen Energy 2006;31:1658e73. [20] Rocheleau RE, Miller EL, Misra A. High-efficiency photoelectrochemical hydrogen production using multijunction amorphous silicon photoelectrodes. Energy Fuels 1998;12:3e10. [21] Yamane S, Kato N, Kojima S, Imanishi A, Ogawa S, Yoshida N, et al. Efficient solar water splitting with a composite “n-Si/p-CuI/n-i-p a-Si/n-p GaP/RuO2” semiconductor electrode. J Phys Chem C 2009;113:14575e81. [22] Reece SY, Hamel JA, Sung K, Jarvi TD, Esswein AJ, Pijpers JJH, et al. Wireless solar water splitting using silicon-based semiconductors and earth-abundant catalysts. Science 2011;334:645e8. [23] Pijpers JJH, Winkler MT, Surendranath Y, Buonassisi T, Nocera DG. Light-induced water oxidation at silicon electrodes functionalized with a cobalt oxygen-evolving catalyst. Proc Natl Acad Sci U S A 2011;108:10056e61. [24] Basham JI, Mor GK, Grimes CA. Fo¨rster resonance energy transfer in dye-sensitized solar cells. ACS Nano 2010;4:1253e8. [25] Shalom M, Albero J, Tachan Z, Martı´nez-Ferrero E, Zaban A, Palomares E. Quantum dotedye bilayer-sensitized solar cells: breaking the limits imposed by the low absorbance of dye monolayers. J Phys Chem Lett 2010;1:1134e8. [26] Du¨rr M, Bamedi A, Yasuda A, Nelles G. Tandem dyesensitized solar cell for improved power conversion efficiencies. Appl Phys Lett 2004;84:3397e9. [27] Guo X-Z, Zhang Y-D, Qin D, Luo Y-H, Li D-M, Pang Y-T, et al. Hybrid tandem solar cell for concurrently converting light and heat energy with utilization of full solar spectrum. J Power Sources 2010;195:7684e90. [28] Brillet J, Yum J-H, Cornuz M, Hisatomi T, Solarska R, Augustynski J, et al. Highly efficient water splitting by a dualabsorber tandem cell. Nat Photonics 2012;6:824e8. [29] Zhao Y, Swierk JR, Megiatto Jr JD, Sherman B, Youngblood WJ, Qin D, et al. Improving the efficiency of water splitting in dye-sensitized solar cells by using a biomimetic electron transfer mediator. Proc Natl Acad Sci U S A 2012;109:15612e6.
Please cite this article in press as: Peng X, et al., Photovoltaic devices in hydrogen production, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/j.ijhydene.2014.04.030
Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.elsevier.com/locate/he
Photovoltaic devices in hydrogen production Xiaoniu Peng, Chao He, Xi Fan, Qingyun Liu, Jun Zhang, Hao Wang* Hubei Collaborative Innovation Center for Advanced Organic Chemical Materials, Faculty of Physics and Electronic Science, Hubei University, Youyi Avenue 368#, Wuhan 430062, PR China
article info
abstract
Article history:
Hydrogen is an ideal fuel for the increasing human demand of abundant, clean and
Received 19 October 2013
renewable energy. Various technologies are developed for the production of the hydrogen
Received in revised form
through water splitting, and in particular the photovoltaic device is widely regarded as an
6 March 2014
opportunity for the high efficiency, low cost and simple configuration devices. Three main
Accepted 3 April 2014
systems are briefly reviewed in this article by demonstrating the typical devices of them,
Available online xxx
including the IIIeV group compound semiconductor, the multi-junction Si structure and the dye-sensitized solar cells. Although considerable disadvantages and limitations are
Keywords:
obvious in performance, the challenging and competing criteria are outlined for the im-
Photovoltaic device
provements. Moreover, the introduction of plasmonic metal nanostructures provides an
Hydrogen production
intriguing prospect of extensive applications of the photovoltaic devices. Copyright ª 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Energy consumption outlines the development of human technologies and civilizations [1]. Most of the energy the humans consumed every year directly comes from the combustion of fossil fuels, which leads to the potentially drastic effects of increased atmospheric CO2 concentrations on global climates, as well as the depletion of the reserve coal, natural gas, and petroleum [2,3]. Therefore, the access to a renewable, carbon neutral energy source is expected to be a key factor in sustaining population growth. The solar energy, as the clean and most abundant resource from the sun, satisfies the current and future human energy demand [4,5]. The solar energy form the sunlight strikes our planet on an annual basis at a rate of approximately 120,000 TW, which vastly exceeds the energy use of the human society every year, w15 TW [6e8]. A general agreement is proposed that photosynthesis is an efficient energy generation and storage system for the solar energy, especially in biology. The light reactions in
photosynthesis provide photons and electrons for the conversion from the carbon dioxide to the organic molecules, including living organisms, food and fossil fuels [7]. Splitting water into oxygen and hydrogen (2H2O þ hv / O2 þ 4Hþ þ 4e, DG0 ¼ 1.23 eV) is the heart process of photosynthesis. The hydrogen is ideal energy storage medium for the solar fuel, and the freely available energy from the sunlight is captured and turned into the valuable and strategic power in the form of H2 molecules. The photovoltaic water splitting is a promising technology to produce clean hydrogen. The idea of generating electric power and chemical fuels from the sunlight captures the most intensive attentions of the scholars ever since the photoelectric effect is firstly demonstrated by the French scientist Edmond Becquerel [9]. Among various kinds of ways to facilitate the water splitting reaction, the photoelectrochemical (PEC) cells are the most efficient and economical selection. A basic photoelectrochemical water splitting cell can be constructed in the form of the Schottky type or the tandem (Z-scheme) type
* Corresponding author. Tel.: þ86 27 88663390. E-mail address: [email protected] (H. Wang). http://dx.doi.org/10.1016/j.ijhydene.2014.04.030 0360-3199/Copyright ª 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Please cite this article in press as: Peng X, et al., Photovoltaic devices in hydrogen production, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/j.ijhydene.2014.04.030
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i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 4 ) 1 e6
Fig. 1 e Basic configurations of (a) the Schottky type and (b) the tandem or Z-scheme type of the photoelectrochemical cells [Reprinted from Ref. [10] with permission].
(Fig. 1) [10]. In the photoanodeecathode configuration, the majority carriers are transferred to the anode/cathode for carrying out the photoelectrolysis, while the minority charge carriers (electrons in p-type semiconductor photocathodes and holes in n-type semiconductor photoanodes) are driven to the semiconductor/aqueous solution interface by the electric field formed at the solid/liquid contact. In the configuration of the photoanode/photocathode, both the holes and the electrons are utilized for the water splitting reactions at their respective solid/liquid interfaces. The typical PEC cell consists of a photoanode and a photocathode, which is firstly reported by Fujishima and Honda in their classic work of Pt w TiO2 [11]. The electrons are extracted from the water using solar irradiation as the energy source in the photoanode, and the reductive electrons are utilized for the hydrogen-generation reaction in the photocathode. Different semiconductors and structures are studied for the PEC cells in water splitting with the request of the wide absorption range of the solar irradiation and the sufficient band gap energy for the H2 and O2 evolution reactions [8,12]. To describe the hydrogen production efficiency in a watersplitting reaction, the solar-to-fuel conversion efficiency of the PEC device, upon illumination with 1 sun (100 mW/cm2) of air mass (AM) 1.5 simulated sunlight, can be calculated using the equation [13], hð%Þ ¼ j$DE=S$100% ¼ J mA cm2 $1:23 V 100 mW cm2 $100%
where S is the total incident intensity of the solar irradiance, j is the short-circuit current density at zero bias, and DE is the thermodynamic voltage required for water splitting reaction. The science of the PEC cells is dominated by the study for the improvement of efficiency in the devices.
Systems of photoelectrochemical cell The lack of an efficient, stable and energetic system poses huge challenges to direct photoelectrolysis of water [14]. The oxides with wide absorption range are poor in semiconductor characteristics, although they are most photochemical stable in aqueous solution [10]. IIIeV group compound materials have better performance with respect of oxides. P-Type indium phosphide semiconductor with better solid-state characteristics has been proved to be stable in strong acid under illumination [15,16]. A multi-junction solar cell based on the GaInP2/GaAs p/n, p/n tandem cell is reported by Khaselev and Turner (Fig. 2) [17]. Multi-junction structures are set for the absorption of both visible and near-infrared portion of the solar spectrum. Electrons are transferred to the illuminated surface and holes are transferred to the ohmic contact under illumination. The cathode is protected in the electrolyte by the H2 produced at the semiconductor electrode. The top junction of the cell is the p/Schottky layer, whereas it is p/n type in the fully photovoltaic tandem cell. Although the efficiency limit for this combination of band gaps is 34%, the actually
Fig. 2 e (A) Schematic structure and (B) idealized energy level diagram of the GaInP2/GaAs multi-junction solar cell [Reprinted from Ref. [17] with permission]. Please cite this article in press as: Peng X, et al., Photovoltaic devices in hydrogen production, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/j.ijhydene.2014.04.030
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conversion efficiency in this system reaches 12.4% under the incident light intensity of 1190 mW/cm2. Single-junction GaAs solar cell is also employed for the water-splitting reaction and the solar-to-fuels efficiency increases to an impressive 18.2% with the theoretical limitation efficiency of 19.7% [18]. Although efficient reactions of hydrogen production are achieved in the IIIeV materials, high costs and poor stability are the main restriction for the applications. Another approach to the water splitting system is the multiple structure of silicon in PEC cells [19e21]. A stack of amorphous Si or SieGe alloys comprise the typical cells operating in basic electrolyte. The indium tin oxide (ITO) passivates the p-side of the structure and provides marginal protection of the Si from the extreme corrosion of the solution, thus resulting in relatively low hydrogen production efficiency with respect of IIIeV semiconductor. A simple PEC system consisted of triple junction amorphous silicon cell and a separate catalytic anode is reported by Rocheleau et al. [20]. The NiFexOy catalyst is deposited onto the ITO layer on one surface of Si wafer forming the oxygen-evolving complex (OEC), and the CoMo catalyst is assembled on steel plate in contact with the other side of Si plate for the hydrogen evolution reaction (HER). Charge is separated in the Si cell under illumination of sunlight. The negative electrons flow towards the HER catalyst for the reduction of protons into hydrogen, while the positive holes flow towards the OEC catalyst for the oxidation into oxygen. Although not fully optimized, the solar-to-fuel conversion efficiency in this multi-junction Si system reaches record high of 7.8% with steady-state operation. A similar structure is reported by the Nocera and coworkers more recently, except CoPi and NiMoZn being employed instead of the catalyst used before [22]. The nearneutral pH electrolyte is applied, and the performance of the PEC cell with wire connecting is compared against that of the wireless one (Fig. 3). Higher energy conversion efficiency, 4.7%, is observed in the wired configuration and a theoretical maximum, greater than 10%, is proposed for this system. Nevertheless, the shadow effects of the incident light may become a possible disadvantage of this structure, which is
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caused by the catalyst deposited on top of the front layer of the solar cell. Crystalline Si is also applied in the single junction PEC cell, as shown in Fig. 4 [23]. The n-side and pþ-side are deposited on the Si junction for the protection from oxidation. Improved performance is demonstrated by the nppþ-Si structure for the fact that the thin space-charge region acts as a tunneling layer in the pþ-side layer. Comparing with the thermodynamic potential of water splitting of the formal normal hydrogen electrode, the onset potential in this nppþ-Si structure significantly decreases. The photovoltage generated by this single junction cell is sufficient to drive the water oxidation. Moreover, the threshold of the external applied potential needed for the reaction is further reduced after connecting an additional nppþ-Si solar cell to nppþSi-ITO-Co-OEC structure in series. Based on these results, the external supply may be eliminated with the fact that sufficient potential is provided by numbers of Si solar cell connected in tandem. The photovoltaic cell composed of multi-junction Si and the Earth’s abundant catalyst provides an inexpensive way to engineer a wireless efficient photoelectrochemical device to convert sunlight into hydrogen under neutral condition. This strategy significantly broadens the commercial applications of the robust artificial photosynthetic system. An alternative simple strategy for ideal photoelectrochemical cell for water splitting is the dye-sensitized solar cells (DSCs). The wide-band gap metal-oxide semiconductor are combined with visible light absorbing dyes in the structure, and the absorption characteristics can be turned by molecular design to cover different parts of the solar spectrum. In DSCs, the incident light is absorbed by sensitive dyes and charge separation takes place. The photoinduced electrons inject into the conduction band of the semiconductor from the excited dye, and the holes at the dye ground state are further regenerated through reduction by the hole-transport material. The PEC systems of the dyes combined with semiconductor absorbers and the tandem arrangements with multiple dyes have been demonstrated with efficient performance [24e27].
Fig. 3 e (A) Schematic structure of the wired a-Si PEC cell. (B) solar-to-fuel efficiency of the wired structure in 1 M potassium borate (pH 9.3, black) and 0.1 M KOH (pH 13, red) [Reprinted from Ref. [22] with permission]. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.) Please cite this article in press as: Peng X, et al., Photovoltaic devices in hydrogen production, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/j.ijhydene.2014.04.030
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Fig. 4 e (A) Schematic structure, (B) CeV curves and (C) onset potential of water oxidation of the single junction nppDSi-ITOCo-OEC cell. The curves in (B) represent the sample immersed in KPi electrolyte in dark (black) and illuminated with Xe lamp (green). The blue line corresponds to the dark CeV curve when the potential is applied. The curves in (C) represent the samples measured in dark (black), at 100 mW/cm2 (green), and 1000 mW/cm2 (orange) illumination [Reprinted from Ref. [23] with permission]. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
A dual-absorber solar cell for water splitting is reported by Sivula and co-workers, as shown in Fig. 5 [28]. The structure consists of the tungsten oxide (WO3) or iron oxide (Fe2O3) photoanode in tandem with the dye-sensitized solar cell. Although the WO3 (2.6 eV) and Fe2O3 (2.0 eV) have sufficient Eg energy for solar energy conversion, the energy of the electron
at their conduction band is insufficient for the reduction of the hydrogen, and therefore the additional energy is provided by the DSCs for conducting the splitting reaction. The incident light is absorbed by the metal-oxide semiconductor before the underlying photovoltaic cell, and the photoanode and DSC are carefully selected to exploit a substantial part of the solar
Fig. 5 e (A) General schemes and energy diagrams and (B) JeV curves of the WO3/DSC (left in A and upper in B) and Fe2O3/ DSC (right in A and lower in B) tandem cells [Reprinted from Ref. [28] with permission]. Please cite this article in press as: Peng X, et al., Photovoltaic devices in hydrogen production, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/j.ijhydene.2014.04.030
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spectrum. The band gap of WO3 only allows absorption form ultraviolet to blue, whereas the absorption range of the Fe2O3 is extended up to yellow wavelength. Although the state-ofthe-art strategies of surface catalysis and passivation are used to reduce the overpotential needed for water oxidation, the late onset of the photocurrent in the photoelectrode is the critical limiting factor in Fe2O3/DSC. In the case of the WO3/ DSC system, the main limitation is the low photocurrent in the photoanode as a result of the poor absorption capability in the visible region of the solar spectrum. The solar-tohydrogen conversion efficiency in the Fe2O3/DSC and WO3/ DSC system is 1.17% and 3.10% respectively. The clear advantage of the WO3 over Fe2O3 in terms of the onset potential of the photocurrent makes the relative better performance in the WO3/DSC tandem cell. The main reason for the low quantum yield of water splitting in the dye-based PEC system is the fact that the charge recombination reaction is faster than the catalytic four-electron oxidation of water to oxygen. This limitation can be great improved by incorporating a redox mediator into the DSC, which is demonstrated by Mallouk’s group recently (Fig. 6) [29]. The IrOx$nH2O colloid is bond to the BiP mediator, and co-absorbed a Ru polypyridine sensitizer onto a TiO2 electrode without direct linkage. Comparing with the mediator free catalyst, the sustained and relatively higher photocurrent is measured in the mediator capped catalyst, and the internal quantum efficiency of the photoelectrochemical water splitting is approximately 2.3% with the Faradaic efficiency (the electron efficiency in converting to hydrogen) is close to unity. Furthermore, an analysis of the kinetics of bleaching recovery of the sensitizer is presented and the multi-exponential processes are verified in this kinetically complex system. The dye-sensitized PEC cell is a relatively new area of the researches in water splitting. Although low conversion efficiency is obtained in the current performance, considerable progress has been made in advancing the concept of the
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structure design, which has significant promise for the development of simple, low-cost and efficient solar fuel systems. Comparing with other ways of producing hydrogen, the photovoltaic devices have its unique superiority. Currently, three mature methods are mainly employed for the hydrogen production such as the steam reforming of the methane, the coal gasification of the fossil fuels, and the thermochemical or biological processes of the biomass. A large amount of energy is consumed in the production process from the natural gas, coal, or biomass, with the substantial amount of CO2 that is generated as the by-product. Comparing to these more established methods of hydrogen production, the photovoltaic devices take advantages of the ultrafast initial charge separation and the much slower recombination, which promotes the efficient cleavage of the water. Reasonable solar-tohydrogen efficiency could be optimized after further improvement. The bifacial configuration and the transparent versions could also be set to capture the solar energy from all angles in different wavelength. In addition, the properties of the low system cost, the simple configuration, the environment friendly process, and the separation of the hydrogen and oxygen evolution during the reaction also make the photovoltaic devices more preferable for the sustained utilization of the H2 energy. The opportunity to exploit solar energy in the form of hydrogen with the photoelectrochemical devices offer a huge reward, but also poses huge challenges in the practical application. Efficient materials are needed for the photocurrents of 8 mA/cm2 in “1 Sun” (100 mW/cm2) illumination with the conversion efficiency reaching or exceeding 10%. The bias requirement between the anode to cathode should be optimized to reduce the cost of external supply. In addition, the durability of the devices should provide the efficient lifetime of at least 15 years, or better 20 years. Most importantly, the limitation of the system cost should not exceeding $160/m2, including the external supplement devices. Although the photoelectrochemical devices provide an intriguing prospect of hydrogen production, but significant improvements of them are still required to meet the practical demands.
Conclusion and outlook
Fig. 6 e Electron transfer reactions of the mediator-based dye-sensitized TiO2 photoanode [Reprinted from Ref. [29] with permission].
Although the materials and structures for the water photoelectrolysis reaction have been greatly developed over the past several decades, the photoelectrochemical hydrogen production is not yet a productive solution for the current and future energy demand of the humans. Basic composition improvements are required, together with robust system implementation. More active materials are needed not only for the high energy conversion efficiency but also for the wide range absorption of solar spectrum. The bias requirements should be reduced for the absence of the external supply. Simple configuration and low cost with high durability of the system are also the fundamental factors for the wide applications. Furthermore, the integration of plasmonic metal nanostructures to water splitting is an intriguing and potentially rich area for future exploration. The plasmonic effects of the near-field absorption and light scattering could be
Please cite this article in press as: Peng X, et al., Photovoltaic devices in hydrogen production, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/j.ijhydene.2014.04.030
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employed for the improvement of photon absorption and utilization, which suggests significant strategies for further efficiency enhancements in photoelectrochemical devices of hydrogen production.
Acknowledgments This work is supported in part by the National Nature Science Foundation of China (Nos. 51372075, 51311130312). Research Fund for the Doctoral Program of Higher Education of China (RFDP, No. 20124208110006).
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Please cite this article in press as: Peng X, et al., Photovoltaic devices in hydrogen production, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/j.ijhydene.2014.04.030