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Investigation and optimization of nanostructured TiO2 photoelectrode in regard to hydrogen production through photoelectrochemical process
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International Journal of Hydrogen Energy 28 (2003) 1089 – 1094 www.elsevier.com/locate/ijhydene
Investigation and optimization of nanostructured TiO2 photoelectrode in regard to hydrogen production through photoelectrochemical process P.R. Mishra, P.K. Shukla, A.K. Singh, O.N. Srivastava∗ Department of Physics, Banaras Hindu University, Varanasi-221005, India
Abstract This paper reports the investigations on the optimization of nanostructured TiO2 with respect to optimum photoelectrode area for modular design of photoelectrolysis cell. This was done for determining the electrode area for optimum electrical output and hydrogen production rate. The nanostructured TiO2 has been formed by hydrolysis of titanium tetraisopropoxide Ti[OCH(CH3 )2 ]4 followed by the deposition with spin on technique. The photoelectrochemical cell having nanostructured TiO2 photoanode of several geometric areas, namely, 0.21, 0.50, 0.72, 1.47 and 1:85 cm2 , were fabricated and characterized. It has been found that the photoanode area corresponding to optimum electrical output and hydrogen production rate corresponds to ∼ 0:5 cm2 . ? 2003 International Association for Hydrogen Energy. Published by Elsevier Science Ltd. All rights reserved. Keywords: Water photoelectrolysis; Photoelectrochemical solar cell TiO (ns); Modular PEC cells
1. Introduction The perennial problem related with the pollution and depletion of the state-of-the-art commercial energy (electricity and fuel) has given rise to the urgent need of fostering development and demonstration e>ects in the area of renewable energies which are inexhaustible as well as non-polluting. There is an ever-increasing interest in solar photon conversion devices called solar cells. Out of the two varieties, namely the dry and wet solar cells, the latter type which is also called photoelectrochemical (PEC) solar cell has several advantages [1–7]. It can not only lead to the conversion of solar energy into electrical energy but also to store the same in the form of chemical fuel. One such more interesting conversion corresponds to the hydrogen production through the photo cleavage of water employing photoelectrochemical solar cell. During last two decades various oxide and non-oxide semiconductors have been
∗ Corresponding author. Tel.: +91-542-368468; fax: +91-542368468. E-mail address: [email protected] (O.N. Srivastava).
tested [1–4] with the aim of developing stable high eDciency water electrolysis cell. But none of them could meet all the requirements of eDcient light absorption, stability and matching of the semiconductor band-edges with H2 and O2 evolution reactions. The semiconductors with appreciable photo response (such as non-oxide semiconductors, e.g., Si, InP, CdTe, GaAs, etc.) were found to be unstable while stable semiconductors (usually high band gap oxide semiconductors, e.g., WO3 ; TiO2 ; SrTiO3 , etc.) can respond only for a limited portion of solar spectrum [6–10]. These obstacles have kept the photochemists from reaching the eventual goal of developing eDcient and commercially viable water photoelectrolysis systems. However, recent developments on the nanostructured photoelectrode and decoration with the dyes coupled with the new type of electrolytes, e.g. solid polymer electrolyte, has raised the hope of realizing commercial high eDciency photoelectrochemical solar cells [11–16]. The developments concerning the photoelectrodes have received much attention recently. In the nanostructured photoelectrode of oxide semiconductors (e.g., TiO2 ) possessing very high e>ective surface area and with suitable surface treatment with dyes, the incident photon-to-current
0360-3199/03/$ 30.00 ? 2003 International Association for Hydrogen Energy. Published by Elsevier Science Ltd. All rights reserved. PII: S 0 3 6 0 - 3 1 9 9 ( 0 2 ) 0 0 1 9 7 - 0
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conversion eDciency is found to be in excess of 80% [12–16]. One important aspect, which has not been widely investigated, relates to the determination of e>ective photoelectrode area that will lead to high electrical and hydrogen production output. This would help in making modular PEC cell conIgurations for hydrogen production. For the investigation of photoelectrochemical solar cells with modular conIguration leading to “Hydrogen Production Reactor”, estimation of optimum photoelectrode geometric area forms an essential ingredient. In this paper, we have dwelt upon this issue and determined the optimum photoelectrode area for high electrical/hydrogen production output.
3. Fabrication of PEC solar cell
2. Experimental techniques The nanostructured TiO2 (ns-TiO2 ) Ilms were prepared through sol–gel route. For preparing sol–gel, Ti[OCH(CH3 )2 ]4 solution was added slowly to propanol drop by drop. Deionized water was slowly added under vigorous stirring conditions for a duration of 10 min. During the addition, a white precipitate was formed; then 1 ml of 70% HNO3 was added to the mixture. The mixture was then stirred for 15 min at 80◦ C. The propanol, together with some water, was allowed to evaporate during this time. In this way, stable TiO2 colloidal solution was obtained. This TiO2 solution was then concentrated by evaporation of water in vacuum at 25◦ C, until a viscous liquid was obtained. Carbowax M-20000 (40% by weight of TiO2 ) was added and a viscous dispersion was obtained. The chemical process can be represented as Hydrolysis
Ti[OCH(CH3 )2 ]4 −−−◦−→ TiO2 80 C
ined by X-ray di>ractometry (Philips, PW1710 equipped with graphite monochromator). The microstructural feature of the Ilms was investigated under transmission electron microscope (Philips, EM-CM-12) to explore the formation of porous nanocrystalline structure. The photoelectrochemical characterization of the ns-TiO2 Ilm for di>erent photoelectrode areas was carried out by standard photoelectrochemical techniques employing a computer-controlled scanning Potentiostat/Galvanostat (Princeton Applied Research, model 263A), Oriel light source (400 W Hg–Xe lamp) and other related accessories.
(sol–gel):
(1)
Titanium sheet was used as conducting substrate. Titanium sheets (∼ 0:3 mm thickness) of di>erent areas were taken and mechanically polished with emery papers of various grades. Finally, these substrates were ultrasonically cleaned in acetone for 15 min. The spin on technique using Photoresist Spinner was used for thin Ilm deposition because it is advantageous to deposit a nearly uniform thin Ilm. Another aspect of spin on technique is that one can deposit a nanostructured matrix layer by layer. It has been observed that nanostructured version of the material has a large surface area resulting in high quantum yield. The nearly uniform Ilms were deposited by placing ultrasonically cleaned substrate over vacuum interlocked spinner holder. One drop of sol was spread over the substrate rotating at 3000 rpm. The Ilms so obtained were dried in an air oven for 15 min at 80◦ C and then Ired at 450◦ C for 30 min [17,18]. This process was repeated four or Ive times to increase Ilm thickness. Finally, samples were annealed in argon atmosphere at 550◦ C for 4 h. Electrodes thus obtained (having ∼ 5 m, ns-TiO2 layer over Ti) were then subjected to structural and photoelectrochemical characterization. The gross structure of the ns-TiO2 Ilm was exam-
A rectangular photoelectrochemical cell was fabricated with perspex having a quartz window for illumination. The photoelectrodes were Ixed over separate perspex mounts, having a central hole of predeIned area, using a chemically inert epoxy resin. The ohmic contact was made on the back of Ti surfaces through Cu wire glued with the help of silver paint and sealed using epoxy resin. The part of the cell between quartz window and the photoelectrode was Illed with electrolyte solution (viz. 1 M NaOH). The I –V measurements were carried out by employing standard three-electrode conIguration. A platinum sheet (5 cm2 ) was used as counter and saturated calomel electrode (SCE) with KCl bridge as reference. All the chemicals used were of analytical grade (Qualigens ExcelR, unless otherwise indicated) and electrolyte was prepared with deionized distilled water. The conventional three-electrode conIguration along with other cell components for controlled potential measurement is schematically shown in Fig. 1. For the water photoelectrolysis experiments, identical cell as described above, have been used. The only di>erence was that Pt-wire counter electrode was used in place of Pt sheet. Besides the potentiostatic mode of electrolysis (three-electrode conIguration), we have also employed photovoltaic power for biasing the PEC cell (two-electrode conIguration). A separate illumination source (Tungsten Halogen Lamp) with standard AM2 irradiance was used to activate a 5 V Si-PV panel. The output power from PV panel was controlled through a potentiometer. Hydrogen production rates for both the cases have been measured by collecting gas liberated over cathode in a predeIned duration with the help of an inverted burrette as shown in Fig. 1. 4. Results and discussion Fig. 2 shows the XRD pattern for one of the samples. All the observed X-ray peaks have been indexed appropriately on the basis of standard ASTM values. It has been observed that the deposited TiO2 Ilm corresponds to anatase phase Q and c = 9:514 A). Q As can be seen (tetragonal: a = 3:785 A that, besides the X-ray peaks corresponding to anatase phase,
P.R. Mishra et al. / International Journal of Hydrogen Energy 28 (2003) 1089 – 1094
Fig. 1. Schematic representation of a photoelectrolysis cell showing conventional three-electrode controlled potential measurement system along with various components. (CE: counter electrode terminal, RE: Reference electrode terminal, WE: working electrode terminal, 1: TiO2 working electrode, 2: Pt CE fused in Pyrex glass, 3: saturated calomel electrode, 4: Inverted burrettes for H2 & O2 collection, 5: quartz window, 6: Electrolyte, 7: Perspex cell.)
relatively strong X-ray reSections belonging to rutile phase Q and c = 2:959 A) Q and the substrate (tetragonal: a = 4:593 A Ti are also appearing in the X-ray di>ractogram. Though the relatively strong X-ray peaks corresponds to rutile phase
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of TiO2 , the deposited Ilm consists of pure anatase nanoparticles. This has been conIrmed through electron microscopic observation discussed in the next paragraph. It should be noted that the more stable polycrystalline rutile Ilm was formed over Ti substrate during Iring and Inal annealing process of the deposited sol [17]. The XRD patterns of the samples thus reveal the formation of nanocrystalline anatase Ilm over polycrystalline rutile Ilm on the Ti substrate. Fig. 3a shows the typical TEM micrograph of the as-synthesized TiO2 Ilm bringing out microstructure of the Ilm. The observed microstructural feature reveals a Ine-grained network of nanoparticles suggesting the formation of nanocrystalline Ilm with average grain size ∼ 2 nm. The nanocrystalline form leads to increase in effective surface area of the Ilm due to the attainment of a higher roughness factor which, in a suitable case, can be a few hundred times the geometric surface area. The electron di>raction patterns were also taken for the phase identiIcation of the material. Fig. 3b depicts the selected area (from region shown in Fig. 3a) electron di>raction pattern, which conIrms the formation of nanosized anatase crystallites joined together. The current–voltage (I –V ) characteristics of the cell under dark and illumination with di>erent geometric area of photoelectrodes have been drawn. Fig. 4 represents variation in photocurrent as a function of electrode potential vs. SCE for Ive di>erent photoelectrode areas. It is observed that the photocurrent starts rising at around −0:92 V vs. SCE. Further, the rate of change of photocurrent with increase in anodic bias, in the charge transfer limited photocurrent region, decreases for increase in the electrode area. The rapid
Fig. 2. X-ray di>raction pattern for ns-TiO2 Ilm deposited over Ti substrate after initial Iring and annealing.
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Fig. 3. (a) Transmission electron micrograph and (b) corresponding selected area electron di>raction pattern for spin deposited ns-TiO2 Ilm. The oxide Ilm layer scrapped from the substrate was used for the TEM analysis.
Fig. 4. Current–potential curves for ns-TiO2 electrodes for Ive di>erent photoanode areas. The illumination intensity used is ∼ 0:85 W cm−2 .
increase in photocurrent with small anodic bias is indicative of higher Ill factor and hence the higher photoconversion eDciency of the solar cell. From the observed variation in photocurrent as a function of electrode potential, the photocurrent values at selective anodic bias (viz. 0:0 V vs. SCE) for di>erent photoelectrodes have been extracted and plotted against electrode area (Fig. 5a). It has been observed that the photocurrent density Irst decreases rapidly on increas-
ing the electrode area and then saturates. Such a behavior of photocurrent is known to be due to increase in surface states, originating mostly from grain boundaries/surface defects, acting as recombination centre for charge carriers. The volume of hydrogen gas evolved under potentiostatic photo assisted mode of photoelectrolysis with cell conIguration: Ti=ns-TiO2 ==1M-NaOH==Pt under illumination was also measured for di>erent area of photoelectrodes. The volume of H2 gas evolved has been normalized to represent it as hydrogen production rate (HPR). Fig. 5b depicts the variation of hydrogen production rate with photoanode area. It can be noted that the highest value of photocurrent density and hydrogen production rate corresponds to the smallest electrode area and start decreasing for increasing electrode area (Fig. 5a and b). The observed decrease in HPR is slow beyond a speciIc area viz., 0:72 cm2 , in the present case. Since the modular PEC cells would require high photocurrent density and hydrogen production rate leading to adequate hydrogen production, and at the same time workable adequate photoelectrode area to reduce the complexity. It would be appropriate to choose the electrode area that is in between the smallest and the one beyond which, decrease in photocurrent density and hydrogen production rate nearly saturates. The observed relative decrease in photocurrent density, while going from an electrode area of 0.2–0:5 cm2 and from 0.5 to 0:72 cm2 is 24.6% and 25.7%, respectively, while the corresponding decrease in the hydrogen production rates are 21.4% and 6.5%. In the light of this, for the present case, the appropriate photoelectrode area for modular cell design for hydrogen production can be taken as ≈ 0:5 cm2 .
P.R. Mishra et al. / International Journal of Hydrogen Energy 28 (2003) 1089 – 1094
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Table 1 All-solar hydrogen production data observed for PV assisted water electrolysis (anode area: 0:72 cm2 ) Applied PV bias (V)
HPR with dark anodea (1 h−1 m−2 )
HPR with illuminated anodeb (1 h−1 m−2 )
0.5 0.8 1.2 1.5 2.0 2.5 3.0 5.0
No electrolysis No electrolysis No electrolysis No electrolysis No electrolysis Non-measurable 0.20 2.12
Non-measurable 2.06 2.64 3.17 5.94 9.04 11.3 25.0
a Dark electrolysis: The alkaline water (viz., 1 M NaoH) electrolysis using ns-TiO2 Ilm as anode and Pt wire as cathode in the cell. b PEC cell: cell same as a but with illuminated anode.
photocurrent density, Jph , will be Jph Eapp . However, in a practical system, water splitting reaction at the observed rate Jph also demands certain over-voltage, Eover-voltage , that had to be supplied by the photoelectrochemical system. Thus, for a practical system 0 Eonset = (Erev + Eover-voltage ):
Fig. 5. Variation of (a) photocurrent density, (b) hydrogen production rate, and (c) photoconversion eDciency with photoanode area at 0:0 V vs. SCE.
The photoconversion eDciency, , of light to chemical energy in the presence of an applied potential, Eapp , has been calculated following the relation [9]: % = [(total power output − electrical power input) =(light power input)] × 100 = [Jph (Eonset − Eapp )=I0 ] × 100;
(2)
0 and Eapp is the external bias applied to where Eonset = Erev drive the cell. Since the power produced is in terms of a chemical product, H2 , the maximum voltage that can be used in an eDciency calculation is Ixed by the chemical potential of hydrogen (viz., 1:23 V vs. NHE). Since the electron aDnity (the property that decides the band edge position) is too high for TiO2 to overcome this chemical potential, an external bias must be applied to drive the cell reaction. Thus, the external power required to drive the cell at the observed
(3)
Combining Eqs. (2) and (3) and substituting I0 = 85 mW cm−2 and a minimum over-voltage Eover-voltage = 0:5 V vs. NHE, for observed Jph in the present system, the conversion eDciency for di>erent area photoanodes have been calculated and presented in Fig. 5c. It can be seen that here again the optimum photoelectrode area for adequate eDciency is ∼ 0:50 cm2 . It should be stressed here that for modular cell design the two criteria, namely ease of cell fabrication and eDciency, should be fulIlled simultaneously. Even though the eDciency increases on decreasing photoelectrode area, but it will make the cell design complex. On the other hand, larger area of individual electrode reduces complexity of the modular design due to decrease in the number of individual cells. Therefore, a compromise between eDciency and electrode area should be made. To test the hydrogen production through all solar system conIguration, the above PEC cell with photoanode area (0:72 cm2 ) was biased through a photovoltaic panel of 5 V. The hydrogen production rates for di>erent applied biases have been measured for illuminated and dark conditions of photoanode and are presented in Table 1. The load characteristic for this panel was determined separately for one Sun simulated light source (AM 2). It was found that for PEC cell under PV bias the hydrogen production rate was higher as compared to dark electrolysis for the same applied external bias.
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5. Conclusion
References
The photoelectrochemical response of ns-TiO2 photoelectrode for Ive di>erent electrode areas has been measured to explore the e>ect of electrode area on the output power in a chemical fuel (i.e., H2 ) producing PEC cell. Based on the present studies, it has been found that the optimum TiO2 photoelectrode area for adequate photoconversion eDciency and hydrogen production rate corresponds to ∼ 0:50 cm2 . From PV biased PEC cell experiments it can be concluded that PV assisted photoelectrochemical water electrolysis may be beneIcial over dark PV powered water electrolysis, since only small biasing potential is needed to start water electrolysis in this cell. Further, such a water electrolysis system represents an all-solar powered device.
[1] Memming R. Topics in current chemistry. Berlin: Springer, 1988. p. 79 –112. [2] Bard AJ, Fox MA. Acc Chem Res 1995;28:141. [3] Nozik AJ, Memming R. J Phys Chem 1996;100:13061. [4] Pandey RN, Chandra Babu KS, Srivastava ON. Prog Surf Sci 1996;52:125. [5] Lewis NS. J Phys Chem B 1998;102:4843. [6] Tributsch H. Chem Ind 1999;53:410. [7] Salvador P. J Phys Chem B 2001;105:6128. [8] Bockris JO’M. Int J Hydrogen Energy 1999;24:1. [9] Khan SUM, Akikusa J. J Phys Chem B 1999;103:7184. [10] Majumdar SA, Khan SUM. Int J Hydrogen Energy 1994;19:881. [11] O’Regan B, Gratzel M. Nature 1991;353:737. [12] Nazeeruddin MK, Kay A, Rodicio I, Hmphry-Baker R, Muller E, Liska P, Vhachopoulos N, Gratzel M. J Am Chem Soc 1993;15:6382. [13] Hagfeldt A, Gratzel M. Chem Rev 1995;95:49. [14] Bach U, Lupo D, Comte P, Moser JE, Weisso F, Salbeck J, Spreitzer H, Gratzel M. Nature 1998;39:585. [15] Hagfeldt A, Gratzel M. Acc Chem Res 2000;33:269. [16] Gratzel M. Nature 2001;414:338. [17] Hagfeldt A, Bjorksten, Gratzel M. J Phys Chem 1996;100:8045. [18] Karn RK, Srivastava ON. Int J Hydrogen Energy 1999;24:27.
Acknowledgements The authors are grateful to Prof. M.V.C. Sastry, A.R. Verma, V. Venkatraman, G.V. Subba Rao, K.V.C. Rao and P.N. Dixit for helpful discussions. The present work was Inancially supported by the Ministry of Non-conventional Energy Sources, Government of India.
International Journal of Hydrogen Energy 28 (2003) 1089 – 1094 www.elsevier.com/locate/ijhydene
Investigation and optimization of nanostructured TiO2 photoelectrode in regard to hydrogen production through photoelectrochemical process P.R. Mishra, P.K. Shukla, A.K. Singh, O.N. Srivastava∗ Department of Physics, Banaras Hindu University, Varanasi-221005, India
Abstract This paper reports the investigations on the optimization of nanostructured TiO2 with respect to optimum photoelectrode area for modular design of photoelectrolysis cell. This was done for determining the electrode area for optimum electrical output and hydrogen production rate. The nanostructured TiO2 has been formed by hydrolysis of titanium tetraisopropoxide Ti[OCH(CH3 )2 ]4 followed by the deposition with spin on technique. The photoelectrochemical cell having nanostructured TiO2 photoanode of several geometric areas, namely, 0.21, 0.50, 0.72, 1.47 and 1:85 cm2 , were fabricated and characterized. It has been found that the photoanode area corresponding to optimum electrical output and hydrogen production rate corresponds to ∼ 0:5 cm2 . ? 2003 International Association for Hydrogen Energy. Published by Elsevier Science Ltd. All rights reserved. Keywords: Water photoelectrolysis; Photoelectrochemical solar cell TiO (ns); Modular PEC cells
1. Introduction The perennial problem related with the pollution and depletion of the state-of-the-art commercial energy (electricity and fuel) has given rise to the urgent need of fostering development and demonstration e>ects in the area of renewable energies which are inexhaustible as well as non-polluting. There is an ever-increasing interest in solar photon conversion devices called solar cells. Out of the two varieties, namely the dry and wet solar cells, the latter type which is also called photoelectrochemical (PEC) solar cell has several advantages [1–7]. It can not only lead to the conversion of solar energy into electrical energy but also to store the same in the form of chemical fuel. One such more interesting conversion corresponds to the hydrogen production through the photo cleavage of water employing photoelectrochemical solar cell. During last two decades various oxide and non-oxide semiconductors have been
∗ Corresponding author. Tel.: +91-542-368468; fax: +91-542368468. E-mail address: [email protected] (O.N. Srivastava).
tested [1–4] with the aim of developing stable high eDciency water electrolysis cell. But none of them could meet all the requirements of eDcient light absorption, stability and matching of the semiconductor band-edges with H2 and O2 evolution reactions. The semiconductors with appreciable photo response (such as non-oxide semiconductors, e.g., Si, InP, CdTe, GaAs, etc.) were found to be unstable while stable semiconductors (usually high band gap oxide semiconductors, e.g., WO3 ; TiO2 ; SrTiO3 , etc.) can respond only for a limited portion of solar spectrum [6–10]. These obstacles have kept the photochemists from reaching the eventual goal of developing eDcient and commercially viable water photoelectrolysis systems. However, recent developments on the nanostructured photoelectrode and decoration with the dyes coupled with the new type of electrolytes, e.g. solid polymer electrolyte, has raised the hope of realizing commercial high eDciency photoelectrochemical solar cells [11–16]. The developments concerning the photoelectrodes have received much attention recently. In the nanostructured photoelectrode of oxide semiconductors (e.g., TiO2 ) possessing very high e>ective surface area and with suitable surface treatment with dyes, the incident photon-to-current
0360-3199/03/$ 30.00 ? 2003 International Association for Hydrogen Energy. Published by Elsevier Science Ltd. All rights reserved. PII: S 0 3 6 0 - 3 1 9 9 ( 0 2 ) 0 0 1 9 7 - 0
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conversion eDciency is found to be in excess of 80% [12–16]. One important aspect, which has not been widely investigated, relates to the determination of e>ective photoelectrode area that will lead to high electrical and hydrogen production output. This would help in making modular PEC cell conIgurations for hydrogen production. For the investigation of photoelectrochemical solar cells with modular conIguration leading to “Hydrogen Production Reactor”, estimation of optimum photoelectrode geometric area forms an essential ingredient. In this paper, we have dwelt upon this issue and determined the optimum photoelectrode area for high electrical/hydrogen production output.
3. Fabrication of PEC solar cell
2. Experimental techniques The nanostructured TiO2 (ns-TiO2 ) Ilms were prepared through sol–gel route. For preparing sol–gel, Ti[OCH(CH3 )2 ]4 solution was added slowly to propanol drop by drop. Deionized water was slowly added under vigorous stirring conditions for a duration of 10 min. During the addition, a white precipitate was formed; then 1 ml of 70% HNO3 was added to the mixture. The mixture was then stirred for 15 min at 80◦ C. The propanol, together with some water, was allowed to evaporate during this time. In this way, stable TiO2 colloidal solution was obtained. This TiO2 solution was then concentrated by evaporation of water in vacuum at 25◦ C, until a viscous liquid was obtained. Carbowax M-20000 (40% by weight of TiO2 ) was added and a viscous dispersion was obtained. The chemical process can be represented as Hydrolysis
Ti[OCH(CH3 )2 ]4 −−−◦−→ TiO2 80 C
ined by X-ray di>ractometry (Philips, PW1710 equipped with graphite monochromator). The microstructural feature of the Ilms was investigated under transmission electron microscope (Philips, EM-CM-12) to explore the formation of porous nanocrystalline structure. The photoelectrochemical characterization of the ns-TiO2 Ilm for di>erent photoelectrode areas was carried out by standard photoelectrochemical techniques employing a computer-controlled scanning Potentiostat/Galvanostat (Princeton Applied Research, model 263A), Oriel light source (400 W Hg–Xe lamp) and other related accessories.
(sol–gel):
(1)
Titanium sheet was used as conducting substrate. Titanium sheets (∼ 0:3 mm thickness) of di>erent areas were taken and mechanically polished with emery papers of various grades. Finally, these substrates were ultrasonically cleaned in acetone for 15 min. The spin on technique using Photoresist Spinner was used for thin Ilm deposition because it is advantageous to deposit a nearly uniform thin Ilm. Another aspect of spin on technique is that one can deposit a nanostructured matrix layer by layer. It has been observed that nanostructured version of the material has a large surface area resulting in high quantum yield. The nearly uniform Ilms were deposited by placing ultrasonically cleaned substrate over vacuum interlocked spinner holder. One drop of sol was spread over the substrate rotating at 3000 rpm. The Ilms so obtained were dried in an air oven for 15 min at 80◦ C and then Ired at 450◦ C for 30 min [17,18]. This process was repeated four or Ive times to increase Ilm thickness. Finally, samples were annealed in argon atmosphere at 550◦ C for 4 h. Electrodes thus obtained (having ∼ 5 m, ns-TiO2 layer over Ti) were then subjected to structural and photoelectrochemical characterization. The gross structure of the ns-TiO2 Ilm was exam-
A rectangular photoelectrochemical cell was fabricated with perspex having a quartz window for illumination. The photoelectrodes were Ixed over separate perspex mounts, having a central hole of predeIned area, using a chemically inert epoxy resin. The ohmic contact was made on the back of Ti surfaces through Cu wire glued with the help of silver paint and sealed using epoxy resin. The part of the cell between quartz window and the photoelectrode was Illed with electrolyte solution (viz. 1 M NaOH). The I –V measurements were carried out by employing standard three-electrode conIguration. A platinum sheet (5 cm2 ) was used as counter and saturated calomel electrode (SCE) with KCl bridge as reference. All the chemicals used were of analytical grade (Qualigens ExcelR, unless otherwise indicated) and electrolyte was prepared with deionized distilled water. The conventional three-electrode conIguration along with other cell components for controlled potential measurement is schematically shown in Fig. 1. For the water photoelectrolysis experiments, identical cell as described above, have been used. The only di>erence was that Pt-wire counter electrode was used in place of Pt sheet. Besides the potentiostatic mode of electrolysis (three-electrode conIguration), we have also employed photovoltaic power for biasing the PEC cell (two-electrode conIguration). A separate illumination source (Tungsten Halogen Lamp) with standard AM2 irradiance was used to activate a 5 V Si-PV panel. The output power from PV panel was controlled through a potentiometer. Hydrogen production rates for both the cases have been measured by collecting gas liberated over cathode in a predeIned duration with the help of an inverted burrette as shown in Fig. 1. 4. Results and discussion Fig. 2 shows the XRD pattern for one of the samples. All the observed X-ray peaks have been indexed appropriately on the basis of standard ASTM values. It has been observed that the deposited TiO2 Ilm corresponds to anatase phase Q and c = 9:514 A). Q As can be seen (tetragonal: a = 3:785 A that, besides the X-ray peaks corresponding to anatase phase,
P.R. Mishra et al. / International Journal of Hydrogen Energy 28 (2003) 1089 – 1094
Fig. 1. Schematic representation of a photoelectrolysis cell showing conventional three-electrode controlled potential measurement system along with various components. (CE: counter electrode terminal, RE: Reference electrode terminal, WE: working electrode terminal, 1: TiO2 working electrode, 2: Pt CE fused in Pyrex glass, 3: saturated calomel electrode, 4: Inverted burrettes for H2 & O2 collection, 5: quartz window, 6: Electrolyte, 7: Perspex cell.)
relatively strong X-ray reSections belonging to rutile phase Q and c = 2:959 A) Q and the substrate (tetragonal: a = 4:593 A Ti are also appearing in the X-ray di>ractogram. Though the relatively strong X-ray peaks corresponds to rutile phase
1091
of TiO2 , the deposited Ilm consists of pure anatase nanoparticles. This has been conIrmed through electron microscopic observation discussed in the next paragraph. It should be noted that the more stable polycrystalline rutile Ilm was formed over Ti substrate during Iring and Inal annealing process of the deposited sol [17]. The XRD patterns of the samples thus reveal the formation of nanocrystalline anatase Ilm over polycrystalline rutile Ilm on the Ti substrate. Fig. 3a shows the typical TEM micrograph of the as-synthesized TiO2 Ilm bringing out microstructure of the Ilm. The observed microstructural feature reveals a Ine-grained network of nanoparticles suggesting the formation of nanocrystalline Ilm with average grain size ∼ 2 nm. The nanocrystalline form leads to increase in effective surface area of the Ilm due to the attainment of a higher roughness factor which, in a suitable case, can be a few hundred times the geometric surface area. The electron di>raction patterns were also taken for the phase identiIcation of the material. Fig. 3b depicts the selected area (from region shown in Fig. 3a) electron di>raction pattern, which conIrms the formation of nanosized anatase crystallites joined together. The current–voltage (I –V ) characteristics of the cell under dark and illumination with di>erent geometric area of photoelectrodes have been drawn. Fig. 4 represents variation in photocurrent as a function of electrode potential vs. SCE for Ive di>erent photoelectrode areas. It is observed that the photocurrent starts rising at around −0:92 V vs. SCE. Further, the rate of change of photocurrent with increase in anodic bias, in the charge transfer limited photocurrent region, decreases for increase in the electrode area. The rapid
Fig. 2. X-ray di>raction pattern for ns-TiO2 Ilm deposited over Ti substrate after initial Iring and annealing.
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Fig. 3. (a) Transmission electron micrograph and (b) corresponding selected area electron di>raction pattern for spin deposited ns-TiO2 Ilm. The oxide Ilm layer scrapped from the substrate was used for the TEM analysis.
Fig. 4. Current–potential curves for ns-TiO2 electrodes for Ive di>erent photoanode areas. The illumination intensity used is ∼ 0:85 W cm−2 .
increase in photocurrent with small anodic bias is indicative of higher Ill factor and hence the higher photoconversion eDciency of the solar cell. From the observed variation in photocurrent as a function of electrode potential, the photocurrent values at selective anodic bias (viz. 0:0 V vs. SCE) for di>erent photoelectrodes have been extracted and plotted against electrode area (Fig. 5a). It has been observed that the photocurrent density Irst decreases rapidly on increas-
ing the electrode area and then saturates. Such a behavior of photocurrent is known to be due to increase in surface states, originating mostly from grain boundaries/surface defects, acting as recombination centre for charge carriers. The volume of hydrogen gas evolved under potentiostatic photo assisted mode of photoelectrolysis with cell conIguration: Ti=ns-TiO2 ==1M-NaOH==Pt under illumination was also measured for di>erent area of photoelectrodes. The volume of H2 gas evolved has been normalized to represent it as hydrogen production rate (HPR). Fig. 5b depicts the variation of hydrogen production rate with photoanode area. It can be noted that the highest value of photocurrent density and hydrogen production rate corresponds to the smallest electrode area and start decreasing for increasing electrode area (Fig. 5a and b). The observed decrease in HPR is slow beyond a speciIc area viz., 0:72 cm2 , in the present case. Since the modular PEC cells would require high photocurrent density and hydrogen production rate leading to adequate hydrogen production, and at the same time workable adequate photoelectrode area to reduce the complexity. It would be appropriate to choose the electrode area that is in between the smallest and the one beyond which, decrease in photocurrent density and hydrogen production rate nearly saturates. The observed relative decrease in photocurrent density, while going from an electrode area of 0.2–0:5 cm2 and from 0.5 to 0:72 cm2 is 24.6% and 25.7%, respectively, while the corresponding decrease in the hydrogen production rates are 21.4% and 6.5%. In the light of this, for the present case, the appropriate photoelectrode area for modular cell design for hydrogen production can be taken as ≈ 0:5 cm2 .
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Table 1 All-solar hydrogen production data observed for PV assisted water electrolysis (anode area: 0:72 cm2 ) Applied PV bias (V)
HPR with dark anodea (1 h−1 m−2 )
HPR with illuminated anodeb (1 h−1 m−2 )
0.5 0.8 1.2 1.5 2.0 2.5 3.0 5.0
No electrolysis No electrolysis No electrolysis No electrolysis No electrolysis Non-measurable 0.20 2.12
Non-measurable 2.06 2.64 3.17 5.94 9.04 11.3 25.0
a Dark electrolysis: The alkaline water (viz., 1 M NaoH) electrolysis using ns-TiO2 Ilm as anode and Pt wire as cathode in the cell. b PEC cell: cell same as a but with illuminated anode.
photocurrent density, Jph , will be Jph Eapp . However, in a practical system, water splitting reaction at the observed rate Jph also demands certain over-voltage, Eover-voltage , that had to be supplied by the photoelectrochemical system. Thus, for a practical system 0 Eonset = (Erev + Eover-voltage ):
Fig. 5. Variation of (a) photocurrent density, (b) hydrogen production rate, and (c) photoconversion eDciency with photoanode area at 0:0 V vs. SCE.
The photoconversion eDciency, , of light to chemical energy in the presence of an applied potential, Eapp , has been calculated following the relation [9]: % = [(total power output − electrical power input) =(light power input)] × 100 = [Jph (Eonset − Eapp )=I0 ] × 100;
(2)
0 and Eapp is the external bias applied to where Eonset = Erev drive the cell. Since the power produced is in terms of a chemical product, H2 , the maximum voltage that can be used in an eDciency calculation is Ixed by the chemical potential of hydrogen (viz., 1:23 V vs. NHE). Since the electron aDnity (the property that decides the band edge position) is too high for TiO2 to overcome this chemical potential, an external bias must be applied to drive the cell reaction. Thus, the external power required to drive the cell at the observed
(3)
Combining Eqs. (2) and (3) and substituting I0 = 85 mW cm−2 and a minimum over-voltage Eover-voltage = 0:5 V vs. NHE, for observed Jph in the present system, the conversion eDciency for di>erent area photoanodes have been calculated and presented in Fig. 5c. It can be seen that here again the optimum photoelectrode area for adequate eDciency is ∼ 0:50 cm2 . It should be stressed here that for modular cell design the two criteria, namely ease of cell fabrication and eDciency, should be fulIlled simultaneously. Even though the eDciency increases on decreasing photoelectrode area, but it will make the cell design complex. On the other hand, larger area of individual electrode reduces complexity of the modular design due to decrease in the number of individual cells. Therefore, a compromise between eDciency and electrode area should be made. To test the hydrogen production through all solar system conIguration, the above PEC cell with photoanode area (0:72 cm2 ) was biased through a photovoltaic panel of 5 V. The hydrogen production rates for di>erent applied biases have been measured for illuminated and dark conditions of photoanode and are presented in Table 1. The load characteristic for this panel was determined separately for one Sun simulated light source (AM 2). It was found that for PEC cell under PV bias the hydrogen production rate was higher as compared to dark electrolysis for the same applied external bias.
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5. Conclusion
References
The photoelectrochemical response of ns-TiO2 photoelectrode for Ive di>erent electrode areas has been measured to explore the e>ect of electrode area on the output power in a chemical fuel (i.e., H2 ) producing PEC cell. Based on the present studies, it has been found that the optimum TiO2 photoelectrode area for adequate photoconversion eDciency and hydrogen production rate corresponds to ∼ 0:50 cm2 . From PV biased PEC cell experiments it can be concluded that PV assisted photoelectrochemical water electrolysis may be beneIcial over dark PV powered water electrolysis, since only small biasing potential is needed to start water electrolysis in this cell. Further, such a water electrolysis system represents an all-solar powered device.
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Acknowledgements The authors are grateful to Prof. M.V.C. Sastry, A.R. Verma, V. Venkatraman, G.V. Subba Rao, K.V.C. Rao and P.N. Dixit for helpful discussions. The present work was Inancially supported by the Ministry of Non-conventional Energy Sources, Government of India.