Photovoltaic electrolysis: Hydrogen and electricity from water and light

Int. J. l'(vdrogen Energy. Vol. 9. No. 7, pp. 557-561,1984.

0360-319~/8J, $3.00 + 0.00 Pergamon Press Ltd. 1984 International Association for Hydrogen Energy.

Printed in Great Britain.

PHOTOVOLTAIC ELECTROLYSIS: HYDROGEN AND ELECTRICITY FROM WATER AND LIGHT O. J. MURPHY and J. O'M. BOCKRIS Department of Chemistry, Texas A&M University, College Station, TX 77843, U.S.A. (Received for publication 3 November 1983)

Abstract--Two .photovoltaic couples, consisting of n on p and p on n gallium arsenide, respectively, have been converted into a water splitting device. Light is allowed to fall on the p part of one couple, which is in contact with air, and on the n side platinum is plated, which contacts the solution. On the other couple, the n side is in contact with air, while on the p side ruthenium dioxide is plated, which is in contact with the solution. Such a device gives a performance (8% conversion efficiency of solar light to hydrogen) better than that of known photoelectrolysis devices operating without battery assistance. Comparison with a coupled photovoltaic-distant water electrolyzer shows, under certain circumstances, some advantages for the present device.

INTRODUCTION In recent years, a number of publications have appeared concerning tlre interaction of light with solids at semiconductor/solution interfaces [1]. One aim of such work has been the fixing of solar light in the form of a fuel, e.g. hydrogen [2-10]. Hitherto, however, devices using solar light alone have yielded only about 1% conversion efficiency [11-15]. In the present paper, a new approach is described. A one-unit photovoltaic electrolyzer was constructed, incorporating two n/p-GaAs junctions, arranged in series and coated on the dark sides with electrocatalyst layers, contact to the solution being made via these catalyst layers. Irradiation takes place at the air/semiconductor interfaces.

Eppley precision pyranometer, model PSP (Eppley Laboratory, Rhode Island, U.S.A.). The photocurrent was recorded as a potential drop across a standard resistor (0.5 Q, Central Scientific Co., Chicago, Illinois, U.S.A., model No. 82821C decade resistor), using a multimeter (Keithley, model 177). Each photocurrent value was recorded after a time lapse of 3 min when steady state was reached. The corresponding electrolysis cell potentials were measured by attaching external copper wire leads, in direct contact with the back surface of the titanium and platinum foils, to a Keithley multimeter. The photocurrents were varied by varying the value of a second resistor (a load resistor) in series with the one-unit photovottaic electrolysis cell (Fig. 1). RESULTS

EXPERIMENTAL The p/n-GaAs junction was covered with Pt foil and the n/p junction with Ti/RuO2, the latter being prepared as described previously [16, 17]. The electrocatalyst materials were attached to the ohmic contact layers on the dark GaAs surfaces (as received) by means of conducting silver-filled epoxy (E-Solder No. 3021, Acme Chemicals, Connecticut, U.S.A.). The area of the GaAs cells and electrodes was I cm 2 for the n/p and 4 cm 2 for the p/n. The electrocatalyst-coated cells were mounted on polyethylene holders by means of epoxy cement (E. P O X . E 5, Loctite Corp., Cleveland, OH 44128, U.S.A.), the holders were capable of being fitted into ground-glass joints in the cell wall, exposing only the electrocatalyst layers to the solution. The cell was of the H-shaped configuration, and the arrangement employed is shown schematically in Fig. 1. Prior to irradiation, the electrolyte in the cell (5 M H2SO4) was flushed with pure nitrogen for 30 min. Irradiation was achieved by means of a solar simulator (Oriel, model 6730/6742), fixed with an Air Mass One filter. Light intensities were measured using a standardized

Typical cyclic voltammograms for the RuO2 electrocatalyst layer in 5 M H20, obtained in the dark by means of external wire contacts to the titanium support and the platinum foil coating are shown in Fig. 2. (A saturated calomel reference electrode and Luggin probe attachment were inserted into the cell for this measurement.) Large background currents and approximately equal anodic and cathodic charges can be observed [17]. Integration of the anodic charge for the cyclic voltammogram obtained using the narrower limits in Fig. 2 and utilizing reported charge-surface area (BET) correlations obtained under similar conditions [17] reveals the true surface area of the RuO2 layer to be of the order of 840 cm -2 (roughness factor of ca 840). The first result obtained for the splitting of water using the one-unit photovoltaic electrolyzer described above is presented in Fig. 3(a). A current density of more than 10 m A cm 2 can be obtained (based on the geometric area of the limiting n/p-GaAs cell) at an insolation of one sun (100mWcm-2). This rate of hydrogen evolution is one order of magnitude greater than those observed with photoelectrolysis cells. A

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Fig. 1. Diagrammatic sketch of electrochemical photovoltaic cell capable of splitting water into hydrogen and oxygen gases, using only solar energy as input.

similar result for the evolution of hydrogen and chlorine from chloride ion-containing solution can be seen in Fig. 3(b). The dependence of the activity of the device upon

light intensity is linear, up to at least 120 mW cm-2 (Fig. 4). Rates of hydrogen evolution corresponding to those of advanced water electrolyzers (0.5-l .OA cm-‘) should be readily obtainable [18,19]. By varying the load resistor (Fig. l), it is possible simultaneously to withdraw both chemical and electrical power from the cell. The power/load graph can be split into chemical and electrical components. The latter passes through a maximum as in Fig. S(a) [13], when the potential drop across the external load is 0.2 V. The

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maximum efficiencies of solar energy conversion to hydrogen and electricity (Fig. 5b) are 7.8 and l%, respectively. These values may be varied, by varying the value of the external resistance.

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DISCUSSION At present, four related solar-based systems, having distinctive features, are being investigated, with the view of achieving solar energy conversion to hydrogen. These are: (i) photoelectrolysis, requiring only solar energy as input; (ii) photo-aided electrolysis, necessitating both solar and electrical energies as inputs; (iii) photovoltaic array and a separate water electrolyzer, giving rise to two plants, and (iv) colloidal semiconductor systems which operate on solar energy alone as input. Whereas systems (i)-(iii) yield well-separated gaseous decomposition products from water, system (iv) gives rise to a mixture resulting in severe losses from hydrogen and oxygen recombination and can be used successfully only when an anodic depolarizer, such as (consumable) H2S, is incorporated into the system to suppress the oxygen evolution reaction. Systems (i), (ii) and (iv) suffer from the fact that direct irradiation of optimum bandgap semiconductor materials in contact with aqueous solutions often give rise to photostimulated corrosion [20-22] and passivation [23, 24] processes. The maximum efficiency obtained from photoelectrolysis devices is ca 1% [11-15], while photo-aided electrolysis gives around 10% [4--6], but involves the use of costly electrical energy. Photovoltaic arrays, coupled to separate water electrolyzers, have efficiencies, at present, of 4-7% [25-28]. A more meaningful comparison of the present device with that of the others being researched would be on the basis of the cost of hydrogen produced by such devices, rather than on energy conversion efficiencies. While photoelectrolysis cells are simpler and do not require the complex manufacturing steps necessary in the formation of p/n and n/p photovoltaics, the

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of load potential drop (V) across a load, placed in series with the cell p/n-GaAs(Pt)15 M H2SO4[n/p-GaAs(RuO2) irradiated by one sun of simulated solar irradiation at room temperature. (b) Variation of solar conversion efficiency, r/, (%) as a function of load potential drop (V). r/ch~r,ic,i = ip V,e,./2I, where ip is the photocurrent density, Vrcvis the reversible potential for the reaction: H2(g) + ½02(g) = H20(l), and I is the light intensity per unit electrode area. Since the functioning of the device involves two photons per electron, a value of twice I is used in the denominator of the efficiency expression. Similarly, the conversion to electrical energy is given by: r/electrical = ip Vlo,O/21.

extremely low efficiencies (high land-space and semiconductor requirements), unknown cell lifetimes (expected to be low) and difficult materials problems (hydrogen embrittlement, etc.), make it not feasible at present to carry out a detailed economic assessment of photoelectrolysis devices. It suffices to state that there is still a wide gap in net efficiency between the results of photoelectrolysis [12-15] and those of hydrogen from photovoltaic arrays coupled to distant electrolyzers.

560

O. J. MURPHY AND J. O'M. BOCKRIS Table 1. Principal capital investments involving a photovoltaic array coupled to a water electrolyzer Cost ratio of electrolyzer to photovoltaic array

Reference

Photovoltaic array

Water electrolyzer

Silicon photovoltaic array. Maximum power output: 8 kW. Cost: $11500/kW

Alkaline bipolar electrolyzer. Maximum power input: 7.7 kW. Cost: $2600/kW

23%

[26]

Silicon photovoltaic array. Maximum power output: 4 kW. Cost: $12000/kW

Alkaline bipolar electrolyzer. Maximum power input: 4 kW. Cost: $3750/kW

31%

[27]

Silicon photovoltaic array. Maximum power output: 18000 kW. Cost: $4000/kW

Water electrolyzer. Maximum power input: 18000 kW. Cost: $375/kW

9.4%

[25]

A more realistic approach would be to compare the projected economics of the present device with those of the latter. A number of economic analyses concerning these have appeared [25-28]. Principal component costs for present-day non-mass produced items are given in Table 1. The electrolyzer module has a significant cost in relation to the photovoltaic array. It is anticipated that the present approach of attaching thin electrocatalyst layers (ca O. 1-1 ~tm [29]) directly onto the dark side of photovoltaic cells, every two cells being coupled together forming an electrolysis cell, has the potential for significant overall cost reduction by considerably lowering the dollar value figures in the second column of Table 1. Without scale-up of an optimally designed one-unit photovoltaic electrolyzer, it is not possible to state how low these figures will be. Economic benefits are inherent to the present oneunit device. As electrolyzers are low-voltage, highcurrent devices, series-parallel arrangements of the photovoltaic array are necessary, so as to take maximum advantage of the output of the array. In addition to wiring and busbar costs, a defective cell or broken electrical contact in such arrays can lead to significant energy losses and raises manufacturing standards and costs considerably [29, 30]. In the present approach, every two photovoltaic cells form a unit, independent of any other photo-cell; hence, there is only one interconnection needed--the electrolyte completes the circuit. A faulty photo-cell just removes one local unit in a scaled-up device. Also, the one-unit approach does not need power conditioning or other ancillary equipment required for optimum operation, when interfacing a water electrolyzer to a photovoltaic array [25]. In constructing a similar-sized plant by both approaches, the greater simplicity, flexibility, lower materials requirements and production standards, as well as lower maintenance and replacement needs of the former, would yield a lower installed cost for the

photovoltaic array [29]. In fabricating the latter, costs in addition to those of producing the photovoltaic cells would include interconnects, power conditioning and storage. Excluding storage, the latter costs would range from 15 to 39¢/Wp (2-4.5¢/kWhr), depending on the scale and efficiency of the array [31]. If storage is included, these figures translate to $1/Wp (9¢~kWhr) for battery storage and approximately 35¢/Wp (4¢/kWhr) for fuel-cell systems [31]. A cost study of an earlier one-unit approach (cf. [29]), involving photovoltaic cells associated with the splitting of HBr, storage of the resulting hydrogen and bromine for later recombination in a fuel cell, revealed an overall cost of the fuel cell-derived electricity of 4.75¢/kWhr in 1983 money. In the absence of further economic data in this area, the lower final installed cost of the photovoltaic array, associated with the one-unit approach, can be used in negating the costs of plating thin electrocatalyst layers on photovoltaic cells, as well as those arising from plastic containers and tubing needed for electrolyte containment and for collection of the evolved gases, respectively. To a first approximation, the costs involved in amortization of the electrolyzer disappear, i.e. those in column two of Table 1. Thus, taking the average costs of the electrolyzers and photovoltaic arrays from Table 1, the electrolyzers represent about 20% of the total, and this would be, therefore, a measure of the saving involved by utilizing the present approach. Another estimate of the cost associated with the two approaches can be obtained from the cost of production of 1 MBTU of hydrogen, using the relation [32, 33]: $/MBTU of H2 = 2.29 Ec + x

(1)

where E is the electrolysis cell voltage necessary for water decomposition (1.6V), c is the cost of photovoltaic-derived electrical power in ¢/kWhr and x is a variable which takes into account the amortization

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561

7. A. Heller, E. Aharon-Shalom, W. A. Bonner and B. Miller, J. Am. chem. Soc. 104, 6942 (1982). 8. F.-R. F. Fan, R. G. Keil and A. J. Bard, J. Am. chem. Soc. 105, 220 (1983). 9. M. Szklarczyk and J. O'M. Bockris, Appl. Phys. Lett. 42, 1035 (1983). 10. M. Szklarczyk and J. O'M. Bockris, Appl. Phys. Commun. 2, 295 (1982-1983). 11. A. J. Nozik, Appl. Phys. Lett. 29, 150 (1976). 12. K. Ohashi, J. McGann and J. O'M. Bockris, Nature, Lond. $/MBTU of H2 = (2.29 x 1.6 × 6.5) + 3 266, 610 (1977). = $27. (2) 13. K. Ohashi, J. McCann and J. O'M. Bockris, Energy Res. 1,259 (1977). To a first approximation, the cost pertaining to a one- 14. C. Leygraf, M. Hendewerk and G. A. Somorjai, J. CataL 78, 341 (1982). unit photovoltaic electrolyzer would be: 15. C. Leygraf, M. Hendewerk and G. A. Somorjai, Proc. $/MBTU of H2 = (2.29 x 1.6 × 6.5) natn. Acad. Sci. 79, 5739 (1982). 16. L. D. Burke, O. J. Murphy, J. F. O'Niell and S. Ven= $24. (3) katesan, J.C.S. Faraday Trans. I, 73, 1659 (1977). The approach described here may be compared with 17. L. D. Burke and O. J. Murphy, J. electroanal. Chem. 96, 19 (1979). that [29, 30, 39] in which a closed loop involving HBr/ 18. P. E. Gregory, P. G. Borden, M. J. Ludowise, C. B. H2,Br2 is used. The system effectively allows, only the Cooper and R. R. Sexana, 15th IEEE Photo. Spec. Conf. production of electricity, whereas the present approach 147, (1981). allows a continuous production of hydrogen. 19. G. W. Turner, J. C. C. Fan, R. L. Chapman and R. P. Gale, 15th IEEE Photo. Spec. Conf. 151 (1981). Acknowledgements--We acknowledge Dr S. Kamath, Hughes 20. K. W. Frese, M. J. Madou and S. R. Morrison, J. phys. Research Laboratories, Malibu, California; Dr A. Shuskus, Chem. 84, 3127 (1980). United Technology Research Center, Hartford, Connecticut; 21. K. W. Frese, M. J. Madou and S. R. Morrison, J. elecand Dr A. E. Blakeslee, Solar Energy Research Institute, trochem. Soc. 128, 1527 (1982). Golden, Colorado; for donating GaAs photovoltaic cells. 22. J. R. Wilson and S.-M. Park, J. electrochem. Soc. 129, 149 Financial support from the Center of Energy and Mineral (1982). Resources, Texas A&M University and the Hydrogen 23. A. B. Bocarsly, E. G. Walton and M. S. Wrighton, J. Am. Research Center, Texas A&M University (National Science chem. Soc. 102, 3390 (1980). Foundation, Atlantic Richfield, Standard Oil of Ohio, Kop- 24. A. Q. Contractor and J. O'M. Bockris Electro-chimica pets, Teledyne Energy Systems and Chapparal Minerals) and Acta (submitted for publication). the Hampton Robinson grant, Texas A&M University, is 25. D. Esteve, C. Ganibal, D. Steinmetz and A. Vialaron, Int. gratefully acknowledged. The authors are grateful to Mrs Lynn J. Hydrogen Energy 7, 711 (1982). McCartney-Murphy for her editing and typing of this 26. A. Koubouvinos, V. Lygerou and N. Koumoutros, Int. J. manuscript. Hydrogen Energy 7,645 (1982). 27. O. G. Hancock, Final Report on Contract No. NAS 10-10544, NASA, Kennedy Space Center, Florida, July 15 REFERENCES (1983). 28. G. Strickland and G. A. Schoener, An Integrated Test Bed 1. For recent reviews in this field, see: (a) R. H. Wilson, for Advanced Hydrogen Technology: Photoooltaic Solid State Mater. Sci. 10, 1 (1980); (b) M. A. Butler and Array/Electrolyzer System, under Contract No. DED. S. Ginley, J. Mater. Sci. 15, 1 (1980); (c) A. Heller, ACO2-76CHOO16, Brookhaven National Laboratory, Acc. Chem. Res. 14, 154 (1981); (d) H. Tributsch, Structure Upton, Long Island, NY 11973, U.S.A. (July 1982). and Bonding 49, 127 (1982); (e) A. Aruchamy, G. Ara29. E. L. Johnson, in Electrochemistry in Industry, New Direcvamudam and G. V. S. Rao, Bull. Mater. Sci. 4, 483 tions (U. Landau, E. Yeager and D. Kortan, eds). Plenum (1982); (f) J. Kiwi, K. Kalyanasundaram and M. Gratzel, Press, New York (1982). Structure and Bonding 49, 37 (1982); (g) S. U. M. Khan 30. J. S. Kilby, J. W. Lathrop and W. A. Porter, U.S. Patent and J. O'M. Bockris, in Modern Aspects of Electrochem4,021,323 (3 May 1977). istry (J. O'M. Bockris, B. E. Conway and R. E. White, 31. Z. J. Kiss, Sunworld 6, 63 (1982). eds), Vol. 14, p. 151. Plenum Press, New York (1982). 32. J. O'M. Bockris, Energy Options, Australia-New Zealand 2. W. Kautek, J. Gobrecht and H. Gerischer, Ber. Bunsenges. Book Company, Sydney, Australia (1980). Phys. Chem. 84, 1034 (1980). 33. J. O'M. Bockris, Int. J. Hydrogen Energy 6, 223 (1981). 3. F.-R. F. Fan, B. Reichman and A. J. Bard, J. Am. chem. 34. Z. J. Kiss, Chemical Week p. 44. (20 July 1983). Soc. 102, 1488 (1980). 35. Z. J. Kiss, Solar Digest, p. 2. (May 1983). 4. A. Heller and G. Vadimsky, Phys. Rev. Lett. 46, 1153 36. Solar Energy Digest, p. 5. (May 1983). (1981). 37. R. Dahlberg, Int. J. Hydrogen Energy 7, 121 (1982). 5. A. B. Bocarsly, D. C. Bookbinder, R. N. Dominey, N. 38. J. E. Perrolt, Economic Assessment of Solar Energy, p. S. Lewis and M. S. Wrighton, J. Am. Chem. Soc. 102, 49. ISES, Royal Inst., London (1977). 3683 (1980). 39. W. R. McKee, K. R. Carson and J. D. Levine, Proc. 16th 6. R. N. Dominey, N. S. Lewis, J. A. Bruce, D. C. Book1EEE Photol. Spec. Conf., p. 257. San Diego, California, binder and M. S. Wrighton, J. Am. chem. Soc. 1114,457 U.S.A. (27-30 September 1982). (•982). of the electrolyzer. In the 1984-1990 time frame, c is likely to be of the order of 4-9¢/kWhr [31, 34-38], taking into account mass production (this may not hold for GaAs photovoltaics, but will for other high-voltage output cells, such as amorphous silicon). For an interest rate of 12-15%, x = 3. Thus, for a photovoltaic array, coupled to a distant water electrolyzer: