Energy & Fuels 1998, 12, 3-10
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High-Efficiency Photoelectrochemical Hydrogen Production Using Multijunction Amorphous Silicon Photoelectrodes Richard E. Rocheleau,* Eric L. Miller, and Anupam Misra Hawaii Natural Energy Institute, School of Ocean and Earth Science and Technology, University of Hawaii at Manoa, Honolulu, Hawaii 96822 Received July 29, 1997. Revised Manuscript Received October 14, 1997X
Photoelectrochemical solar-to-hydrogen conversion efficiencies as high as 7.8% (based on the lower heating value of hydrogen) have been demonstrated in outdoor testing using a photocathode fabricated from triple junction amorphous silicon-solar cells and a separate catalytic anode. The tests were conducted in a specially designed Teflon-sealed reactor in 1 N KOH with a photoactive area of 0.27 cm2 and anode and cathode areas of 1 cm2. The hydrogen production rates, inferred from direct measurement of the anodic/cathodic currents and confirmed by independent volumetric and gas chromatographic measurements of the evolved hydrogen, were in excellent agreement with the rates expected from the measured solid-state JV behavior of the solar cell and the overpotentials of the thin-film catalysts. The thin-film catalysts, CoMo hydrogen catalysts deposited by sputtering from a compound target and NiFeyOx oxygen catalysts deposited from nickel-iron Permalloy target by reactive sputtering, have, in separate tests, shown no degradation after over 7200 h of operation in 1 N KOH electrolyte. During outdoor testing, the solar-tohydrogen conversion efficiency decreased in the late afternoon as the blue portion of the spectrum decreased, a result of the spectral sensitivity of the solar cell used to construct the photoelectrode. Detailed modeling of the multijunction amorphous silicon cells is being conducted to identify structures that are better load-matched to the catalyst performance and that could yield higher hydrogen production efficiencies. Future work to advance this technology includes development of improved thin-film catalysts and development of transparent protective coatings that will allow complete immersion of the active electrode into the electrolyte.
Introduction High-efficiency photoelectrochemical systems to produce hydrogen directly from water using sunlight as the sole energy source have been the focus of numerous research efforts ever since Fujima and Honda1 reported that n-TiO2 photoelectrodes in aqueous electrolyte could provide a significant fraction of the energy needed for direct water splitting. However, progress in the development of such liquid junction systems has been slow, limited by the high voltage required to dissociate water and corrosion of the semiconductors when exposed to electrolyte. To date, the only single photon processes successfully splitting water without external bias have employed very wide bandgap semiconductor materials.2,3 Unfortunately, these materials effectively absorb only the UV part of the solar spectrum, resulting in very low conversion efficiencies. The difficult materials issues associated with developing stable high-efficiency interfaces in aqueous electrolyte, and the significant advances in multijunction photovoltaic devices since the mid-1980s, have led to a X Abstract published in Advance ACS Abstracts, December 1, 1997. (1) Fujishima, A.; Honda, K. Nature 1972, 238, 37. (2) Kraeutler, B.; Bard, A. J. J. Am. Chem. Soc. 1978, 100, 4317. (3) Wrighton, M. S.; Wolczanski, P. T.; Ellis, A. B. J. Solid State Chem. 1977, 22, 17.
new approach: photoelectrodes fabricated from solidstate multijunction devices. The series-connected multiple junction devices typically operate at higher voltages than the single junction devices and can be designed to operate at voltages optimized for direct water splitting while still using a large fraction of the available solar spectrum. Since the electrical field in the multijunction photoelectrodes is internally generated at the solid-state junctions, the need for direct contact between the semiconductor and electrolyte is eliminated and the very difficult criteria requiring the electrolyte interface to simultaneously act as both a high-quality diode and a low kinetic barrier for charge transfer into the electrolyte are removed. The result is increased flexibility in the photoelectrode design including a wider choice of semiconductors, protective coatings, and catalysts. In contrast to conventional photoelectrochemical cells that require an intimate semiconductor-electrolyte interface, the semiconductor in the multijunction configuration, shown schematically in Figure 1, is physically isolated from the electrolyte by a conductive and catalytic film. Although the system shown in Figure 1 retains some elements of a PVelectrolyzer system, there are no wires or cell interconnections and the cell stacks are individually matched for water splitting. Thus, we believe this integrated system is more accurately classified as a photoelectro-
S0887-0624(97)00134-5 CCC: $15.00 © 1998 American Chemical Society Published on Web 01/12/1998
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Figure 1. Schematic of a multijunction photoelectrode in a photoelectrochemical cell using solar energy to split water for direct hydrogen production.
chemical cell. Direct hydrogen production has been demonstrated with a number of multijunction systems including amorphous silicon,4-6 crystalline-amorphous silicon,7 and crystalline group III-V alloys.8 However, these have, in general, yielded low conversion efficiency, poor stability, and in some instances, unusual gas evolution characteristics. We recently conducted a systematic analysis9 to compare the potential hydrogen production rates of different semiconductor materials and different configurations of single and multijunction devices. The elements of this model, based on a lumped circuit model of a photocell in series with a current dependent electrochemical load, are shown in Figure 2 for a triple junction photocell. In this analysis, the dark diode characteristics were estimated from the performance of high-quality devices reported in the literature. The light JV characteristics of the photocells were calculated assuming superposition with light-generated currents and complete absorption of photons with energies above the optical gap and complete transmission of those below it. The electrochemical model (shown on the right-hand side of Figure 2) used the Butler-Volmer relation to describe the current dependent overpotentials due to charge-transfer kinetics at the electrode surfaces and included additional resistances for the potential drop due to ion transport through the electrolyte. Kinetic parameters used in the model were based (4) Lin, G. H.; Kapur, M.; Kainthla, R. C.; Bockris, J. O’M. Appl. Phys. Lett. 1989, 55, 386. (5) Matsumura, A. J.; Sakai, Y.; Sugahara, S.; Nakato, Y.; Tsubomura, H. Sol. Energy Mater. 1986, 12, 57. (6) Rocheleau, R. E.; Zhang, Z.; Miller, E. L.; Gao, Q. Proceedings of the 1994 U.S. Department of Energy/National Renewable Energy Laboratory Hydrogen Program Review; U.S. DOE and NREL: Livermore, CA, 1994; p 335. (7) Sakai, Y.; Sugahara, S.; Matsumura, M.; Nakato, Y.; Tsubomura, H. Can. J. Chem. 1988, 66, 1853. (8) Kocha, S.; Peterson, M.; Hilal, H.; Arent, D.; Turner, J. Proceedings of the 1994 U.S. Department of Energy/National Renewable Energy Laboratory Hydrogen Program Review; U.S. DOE and NREL: Livermore, CA, 1994; p 301. (9) Rocheleau, R. E.; Miller, E. L. Int. J. Hydrogen Energy 1997, 22, 771.
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Figure 2. Analytical model for photoelectrolysis based on a solid-state photogenerator (shown for a triple-junction device) driving an electrochemical load.
Figure 3. Load-line analysis showing the operating point of a photoelectrochemical cell as the intersection of the JV curve of the photocell and the load curve of the electrochemical components.
on values achieved in our thin-film sputter-deposited catalysts.10,11 The decomposition potential (Veq) along with system resistances (Rec) were taken from the literature.12,13 The output from this analysis consisted of the JV curve of the photocell and the load curve of the electrochemical cell. The intersection of these two lines defines the operating point of the system, as illustrated in the load-line analysis plot in Figure 3, which specifically shows the measured JV of a triplejunction amorphous silicon solar cell provided by Solarex Thin Film Division and an electrochemical load based on our thin-film catalysts. For each semiconductor (10) Miller, E. L.; Rocheleau, R. E. J. Electrochem. Soc. 1997, 144, 1995. (11) Miller, E. L.; Rocheleau, R. E. J. Electrochem. Soc. 1997, 144, 3072. (12) Lobo, V. M. M.; Quaresma, J. L. Electrolyte Solutions: Literature Data on Thermodynamic and Transport Properties; Departamento de Quimica da Universidade de Coimbra, 1981; pp 254-262. (13) Bard, A. J.; Parsons, R.; Jordan, J. Standard Potentials in Aqueous Solutions; M. Dekker: New York, 1985; pp 41-54.
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system: amorphous silicon, group III-V crystalline cells, and hybrid crystalline-amorphous silicon cells, different combinations of bandgaps and electrochemical parameters were examined to identify designs with the highest expected hydrogen production rates. Even with highly optimistic assumptions for the diode characteristics (i.e., small reverse saturation currents), the analysis showed that semiconductor bandgaps greater than 1.9 eV are required to generate voltages sufficient for direct water splitting when single-junction photoelectrodes are considered. The analysis also showed that any loss in junction or catalyst performance in such systems would result in large performance losses, requiring even higher bandgaps to provide a reasonable probability of stable operation. The poor absorption of the solar spectrum with such high bandgaps results in solar-to-hydrogen efficiencies that are only about half of what can be achieved using multijunction devices with the same diode characteristics.9 Of the materials evaluated, multijunction photoelectrodes based on crystalline group III-V heterojunctions showed the highest theoretical efficiencies. However, because of their good voltage match, solar-to hydrogen conversion efficiencies for the much lower cost multijunction amorphous silicon and amorphous-crystalline silicon hybrid structures were predicted to be as high as 50-75% of those calculated for the group III-V heterojunctions. Based on the cost reductions predicted by the PV industry for the thin-film technologies, development efforts in our laboratory have been focused on the latter materials, primarily on triple-junction amorphous silicon. This paper describes recent progress in the development and outdoor testing of prototype photoelectrodes fabricated using sputter-deposited thin-film catalysts and smallarea triple-junction solar cells. Results obtained for solar-to-hydrogen efficiency and for electrochemical stability provide reason to be optimistic about the future development of this technology. Experimental Section Reactor Design. The primary goals of this research have been the development of highly active and stable thin film OER (oxygen evolution reaction) and HER (hydrogen evolution reaction) catalysts and the demonstration of high-efficiency hydrogen-producing photoelectrodes fabricated from triplejunction amorphous silicon solar cells. To facilitate catalyst and photoelectrode testing, the Plexiglas reactors shown schematically in Figure 4 were designed and constructed. The catalyst test cell shown in Figure 4a is used for static and dynamic testing of the sputter-deposited thin-film HER and OER catalysts, while the prototype reactor shown in Figure 4b is used for testing the efficiency and stability of the integrated photoelectrodes. Important features in the reactor designs include (1) thin Teflon seals for isolating the active areas of the electrodes, (2) a corrosion resistant Plexiglas body, (3) electrical leads for potentiodynamic and potentiostatic measurements, (4) ports for installation of reference electrodes close to either electrode, (5) gas collection systems for both hydrogen and oxygen, and (6) easy replacement of catalytic films and solar cells for testing different photoelectrode structures. The single-piece construction and Teflon sealing system eliminate the need for epoxies and minimize the possibility of electrolyte leakage. In the work discussed in this paper, electrochemical testing was carried out using 1 N KOH. The thin-film catalyst development has focused on sputterdeposited nickel-iron oxide (NiFeyOx) as an alkaline-based
Figure 4. Schematic of Plexiglas test reactors fabricated for (a) electrochemical testing of hydrogen and oxygen catalyst films and (b) outdoor testing of multijunction amorphous silicon photoelectrodes. OER catalyst,10,11 and a sputter-deposited CoMo alloy14,15 as a HER catalyst. For both materials, numerous experiments were conducted to determine the sputtering conditions that yield minimum overpotentials with maximum stability. To evaluate the catalysts, test electrodes were fabricated by depositing the thin films onto nickel foil. These test coupons were then mounted onto opposite ends of the catalyst test cell (Figure 4a). As detailed more fully in the following Testing section, potentiodynamic tests of film surfaces (vs SCE reference electrodes) were used to measure catalytic activity, while long-term potentiostatic tests were used to evaluate chemical stability. (14) Fan, C.; Piron, D. L.; Sleb, A.; Paradis, P. J. Electrochem. Soc. 1994, 141, 382. (15) Miller, E. L.; Rocheleau, R. E. To be published.
6 Energy & Fuels, Vol. 12, No. 1, 1998 Photoelectrode development has involved coupling the sputter-deposited catalysts with multijunction amorphous silicon pinpinpin solar cells. Triple-junction a-Si:H cells are currently available in two basic configurations: the glass/TCO superstrate structure pioneered by Solarex Thin Film Division and the metal substrate structure developed by Energy Conversion Devices. In both configurations, light enters the cell through the p layer. Photogenerated electrons drift under the influence of the internal fields toward the back of the cell (n layer) where, if the surface is appropriately catalyzed, they can participate in the hydrogen evolution reaction. Similarly, holes drift toward the p layer for collection in the transparent conductive oxide (TCO) and can drive the oxygen reaction. Although both types of cells are suitable for development of high-efficiency photoelectrochemical systems for water splitting, results reported in this paper were all achieved using triple junction (glass/TCO/pinpinpin/metal) amorphous silicon solar cells provided by Solarex Thin Film Division. Photocathodes were fabricated by mounting the triplejunction photovoltaic cell against a Ni/CoMo cathode in the prototype reactor (Figure 4b). Indium dots were used to ensure good electrical contact. External wires connect the front surface TCO to a separate Ni/FeNiOx anode. Use of the separate rather than an integrated anode allows the insertion of instrumentation to monitor the current-voltage behavior of both the solar cell and the electrochemical load during photoelectrolysis, yielding important information for the loadline analysis. In this study, the Solarex cells mounted on the Ni/CoMo cathode had a photoactive area of 0.27 cm2 and solarto-electric efficiencies at the maximum power point between 10.0 and 10.3% under AM1.5 conditions. An external aperture was also placed over the photoelectrode to prevent light-piping into the cell. The anode and cathode catalytic surfaces were 1 cm2 each. The approximately 4:1 ratio between the cell and catalyst areas is comparable to that which could be expected for operation of a system under mild solar concentration. Testing. Electrochemical and photoelectrochemical characterizations were performed using a specially designed instrumentation/data-acquisition system comprised of a Keithley source-measurement unit (SMU) and a digital multimeter (DMM) interfaced to a Macintosh computer running Labview software. Test software for four protocols has been developed: (1) potentiodynamic characterization of the thinfilm HER and OER catalysts; (2) monitoring catalyst overpotentials during long-term testing at a constant current; (3) potentiodynamic characterization of the integrated photoelectrodes (combined solar cell and electrochemical load) under incident light; (4) monitoring of hydrogen current during steady-state photoelectrolysis. For potentiodynamic catalyst testing (protocol 1), the SMU provides a staircase sweep of current biases to the electrochemical load (i.e., the Ni|CoMo||KOH||NiFeyOx|Ni cell) while measuring the resulting voltage response vs the SCE reference electrode. Long-term catalyst stability tests (protocol 2) are conducted by having the SMU provide a constant 20 mA/cm2 current density to the electrochemical load while monitoring the total-cell and SCE voltages. Potentiodynamic photoelectrode testing (protocol 3) can be used to chart the JV behavior of either the solid-state pinpinpin cell or of the integrated photoelectrochemical cell. This procedure is similar to procedure 1 except voltage bias is swept while current levels are monitored. For steady-state testing of hydrogen production rates from the integrated photoelectrodes (protocol 4), the SMU monitors hydrogen current under a 0 V bias. In hydrogen production experiments, this latter procedure is conducted outdoors to ensure natural sunlight spectra. As discussed in the Results and Discussion section, small changes in the incident spectra can have significant effects on the performance of the multijunction cells used in these tests and hence, on the efficiency of the hydrogen production process.
Rocheleau et al. Experiments to measure hydrogen production rates using the prototype reactor (Figure 4b) were conducted outdoors in natural sunlight at different times of the day and under different weather conditions. During these tests, the SMU was configured as a low-impedance ammeter to monitor current between the anode and cathode (e.g., 0 V bias), while the DMM concurrently monitored the short-circuit current from a crystalline silicon solar cell mounted close to and in the same plane as the photoelectrode. The crystalline cell, used as the secondary standard for light intensity, exhibits a short-circuit current that is directly proportional to light intensity over the range of interest (based on calibration under ELH light). It is estimated that current densities under natural solar radiation could vary from the ELH calibration by as much as 5%. Prior to initiating the outdoor testing of hydrogen production, the load-line (current-voltage) behavior of the electrochemical elements of the system (thin film anode and cathode in the actual test configuration) and the illuminated JV response of the solar cell under different lighting conditions were measured using test protocol 3. This allowed direct comparison of the measured hydrogen-conversion efficiencies with predictions based on the load-line analysis. Outdoor testing was conducted with the photoreactor tilted toward the sun with periodic adjustment of the orientation to maintain near-normal incidence of the sunlight. Analysis. During outdoor testing, the current passing between the photocathode and anode was monitored for extended periods of time with no external bias applied. The theoretical hydrogen gas production was calculated from the measured current using Faraday’s law combined with the ideal gas law:
dV RT dN RT ) ηj ) ηj I dt P dt nFP
(1)
where dV/dt is the volume rate of gas production (m3/s), I is the measured driving current (A), N is the number of moles of gas, n is the number of electrons exchanged in electrolyzing 1 mole of gas, P is pressure (Pa), T is absolute temperature (K), R is the molar gas constant (8.314 J mol-1 K-1), and F is the Faraday constant (96485 C/equiv). For the hydrogen evolution reaction, two electrons are exchanged for every mole of H2 generated, so n ) 2. The factor ηj is the reaction current efficiency, which will be 1 if the current drives only the HER and OER reactions. As summarized earlier (Figure 2) and described in more detail in a recent publication,9 the operation of a photoelectrolysis reactor can be analytically modeled as a photovoltaic generator driving an electrochemical load. Similarly, the independent measurements of the solar cell and electrochemical components can be compared to the operation of the complete photoelectrochemical reactor to confirm the behavior of the system and identify unexpected losses. The operating current (hydrogen production rate) at fixed illumination should correspond to the intersection of the solar cell’s JV response curve and the electrochemical components load-line under comparable illumination. This was illustrated in Figure 3, which shows the JV curve of the triple-junction cell used in these studies along with the load-line of the electrochemical elements under operation in our reactor. The hydrogen conversion efficiency,16 ηhc, expressing the ratio of power available in recombining the hydrogen and oxygen produced by the system to the incident solar power used to split the water, is calculated from the operating current at the intersection point according to (16) Bellobono, I. R.; Bonardi, M.; Castellano, L.; Selli, E.; Righetto, L. J. Photochem. Photobiol. A 1992, 66, 253.
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( ) ( )
ηjJ ηhc )
mA 1.229(V) cm2 mW Pin cm2
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(2)
where J is the steady-state operating current density, 1.229 V is the thermodynamic potential for electrolysis based on the lower heating value of H2,17 Pin is the integrated intensity of the sunlight and ηj has been left in to allow for parasitic reactions that could take place. It is interesting to note that the solar-to-hydrogen conversion efficiency calculated from eq 2 depends only on the operating current J, in contrast to power efficiency in solid-state solar cells, which is maximized at the peak JV product.18
Results and Discussion Catalysts Development. A series of iron-doped nickel oxide films (NiFeyOx) films were deposited by reactive sputtering from elemental and alloy targets in a 20% oxygen/argon atmosphere and were characterized for use as oxygen evolution catalysts.11 As shown in Figure 5, the incorporation of iron reduced the overpotential required for oxygen evolution by as much as 300 mV at a current density of 100 mA/cm2 compared to undoped nickel oxide deposited under similar conditions. Detailed analysis reported previously11 showed that the Tafel slopes were reduced from 95 mV/decade for the undoped NiOx, to less than 40 mV/decade for films containing between 1.6 and 5.6 mol % Fe, indicating a change in the rate-limiting step. The catalyst film chosen for extended stability testing and use in the prototype photoreactor was sputtered-deposited from a Ni0.81Fe0.19 Permalloy target at a rate of 1 Å/s with a residual Ar/O pressure of 10 mTorr. The 1 µm thick film contained 5.6 mol % Fe (with a film composition of NiFe0.19O2.2) and demonstrated an oxygen overpotential of 0.31 V at 20 mA/cm2 in 1 N KOH (line in Figure 5). A series of CoMo films were sputter-deposited from a Co0.7Mo0.3 alloy target or co-sputtered from independent Co and Mo targets in argon and were evaluated as hydrogen evolution catalysts.15 A 1 µm thick film deposited from the alloy target in 30 mTorr of Ar at 1 Å/s was chosen for the prototype reactor. This film, composed of Co0.73Mo0.27, with a HER overpotential of 0.20 V at 20 mA/cm2, was chosen largely on the convenience of using the alloy sputtering target. Some of the cosputtered films, however, have demonstrated significantly lower overpotentials and are being further developed for the next-generation system. A critical issue for any commercial electrochemical or photoelectrochemical reactor is the long-term chemical stability of the catalysts. This was particularly true for our catalysts because of the relatively thin films used. We addressed this issue by conducting extended life testing of the NiFe0.19O2.2 and Co0.73Mo0.27 films. Figure 6 shows the results of the more than 7200 h of lifetesting to date, conducted in 1 N KOH at a constant current density of 20 mA/cm2. Included in Figure 6 are the anodic half cell potential of the NiFeyOx OER catalyst and the total cell potential of the NiFeyOx|1 N (17) Rivkin, S. L. Thermodynamic Properties of Gases; Hemisphere Publishing: New York, 1988; pp 239-267. (18) Green, M. A. Solar Cells: Operating Principles, Technology and System Applications; University of New South Wales: Kensington NSW, 1992; p 81.
Figure 5. JV catalysis plots for sputtered NiFeyOx films showing the effect of iron content on the oxygen overpotential in 1 N KOH.
Figure 6. Long-term stability test results for sputtered NiFe0.19O2.2 and Co0.73Mo0.27 catalyst films at constant current of 20 mA/cm2 in 1 N KOH, showing the NiFe0.19O2.2 voltage vs an SCE reference (Vsce) and the total cell potential (Vcell) over a 7200 h period.
KOH|CoMo configuration. (The relatively high total cell potential results from the large anode/cathode separation in the catalyst test cell.) Visible in Figure 6 are many small variations in the total and half cell potentials, attributed to changes in the electrolyte concentration, pH, and conductivity caused by evaporation, hydrogen production, and mechanical removal of electrolyte from stirring in the open reactor. In all instances, periodic electrolyte replacement, as indicated in the figure, reestablished the low-overpotential performance of the catalysts so that no significant degradation has been apparent during this period. Repeated polarization scans at random intervals during the 7200 h of steady-state operation showed less than a 1% change in the OER and HER Tafel slopes compared with the original values, a further indication of catalyst stability. These results are very encouraging for longterm operation of photoelectrochemical reactors with sputtered film catalysts, but further investigation, including characterization of even thinner films, is warranted. Another important issue for commercial photoelectrolysis is the electrolyte purity-level required for stable operation, a factor that affects both the direct electrolyte
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Figure 7. Photoelectrode current density (O) and solar-tohydrogen conversion efficiency (top inset) during a 2 h outdoor test on a clear afternoon (measured insolation shown by line in lower plot).
Figure 8. Photoelectrode current density (b) and solar-tohydrogen conversion efficiency (top inset) during a 2 h outdoor test on a partly cloudy afternoon (measured insolation shown by line in lower plot).
cost and requirements for the construction materials. The results of Figure 6 were obtained using very highpurity, high-cost deionized water. Use of less expensive, commercially available drinking water is preferable but demands that the catalysts show stability and satisfactory performance in an electrolyte that does not meet the high-purity requirement normally specified for electrochemical systems. To this end, long-term testing of the electrochemical stability of the same NiFeyOx and CoMo films has been performed in 1 N KOH prepared using ACS grade KOH and drinking water distributed by a local water-purification company. In addition to impurities in the water, the ACS grade of KOH contains a variety of other chemicals, namely, K2CO3 (e2%), Na (e0.05%), NH4OH (e0.02%), Cl- (e0.01%), SO4-2 (e0.003%), N compounds (e0.001%), etc. Normally, ppm amounts of impurities can cause a significant effect in the electrochemical properties of a catalyst by acting as inhibitors. However, the stability test in drinking water, which has been conducted for over 6500 h to date, has indicated that both NiFeyOx and CoMo films are extremely stable even in the high-impurity, low-cost electrolyte. No significant degradation in the overpotentials or Tafel slopes has been observed in either material over the duration of the test. Photoreactor Performance. Hydrogen production rates, inferred from the reaction current, were measured outdoors under various conditions using photoelectrodes fabricated from the Solarex (glass/TCO/pinpinpin/metal) amorphous silicon solar cells with NiFe0.19O2.2 and Co0.73Mo0.27 thin-film catalysts. Although hydrogen was collected during these outdoor tests, the small photoelectrode area and relatively short test times made quantification difficult. Independent testing conducted indoors under an ELH lamp for longer periods of time using the same catalysts and photoelectrode structures yielded current efficiencies exceeding 95%. We believe that the small difference between the actual gas collection and that expected based on the current between
the anode and cathode surfaces can be attributed to problems inherent in effectively collecting all the gas (e.g., gas dissolution back into the KOH, small gas bubbles clinging to the sides of the burets, and escape of some gas around the collection buret). The gas collected from the cathode was confirmed by gas chromatography to be >99% hydrogen, providing further evidence for high water-splitting efficiency in the prototype reactor. This high gas purity also represents one of the significant advantages of photoelectrolysis reactors over other hydrogen production technologies. Figure 7 shows the result of a 2 h outdoor test performed in the early afternoon on a clear day. The line shows the insolation (right ordinate) as a function of time based on the measured short-circuit current from our secondary silicon standard. The circles show the measured current density (left ordinate) between the anodic and cathodic surfaces of the photoreactor. The inset at the top of Figure 7 shows the net conversion efficiency of the photoelectrode as a function of time based on eq 2 assuming a reaction current efficiency ηj equal to 1. The hydrogen currents observed during the test, ranging from 1.6 to 1.75 mA (over an active area of 0.27 cm2), are consistent with predictions from the load-line analysis based on the independent JV measurements of the illuminated solar cell and the electrochemical elements (shown in Figure 3). The corresponding solar-to-hydrogen efficiencies, between 7.5 and 7.8%, to our knowledge represent the highest one-sun efficiencies reported to date for direct photoelectrolysis of water. Figure 8, which has the same format as Figure 7, shows the results of another 2 h test, this one conducted on a partly cloudy day with considerable short-term variability in the insolation. The hydrogen production efficiency (top inset), calculated at each point using the measured insolation, was essentially constant at 7.5% despite the very large variation in insolation levels during the test, from a high of 130 mW/cm2 to a low of 30 mW/cm2. We attribute insolation values exceeding
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Figure 11. Solar cell efficiency measured at the operating photoelectrochemical cell voltage as a function of time showing a decrease in performance due to spectral mismatch.
Figure 9. Photoelectrode current density (O) and solar-tohydrogen conversion efficiency (top inset) during a 2 h outdoor test conducted in the late -afternoon (measured insolation shown by line in lower plot).
Figure 12. Ratio of hydrogen conversion efficiency to solar cell efficiency as a function of the time of the day, showing a constant value of 74.5%, which indicates that the early-evening drop in hydrogen production is a direct result of the spectralmismatch effect on solar cell performance.
Figure 10. Current density normalized with respect to measured insolation for triple-junction solar cell as a function of time in the late afternoon/early evening, indicating the effect of spectral mismatch on cell performance.
100 mW/cm2 to effects of reflections from nearby clouds. The almost constant efficiency despite the more than 4-fold change in insolation suggests that the photo-
reactor could be successfully operated at higher solar intensities with the help of concentrators, if desired. The performance of the photoreactor was also tested on a clear day during the late afternoon at a time when both insolation and spectral distribution would be changing. As shown in the top inset in Figure 9, the solar-to-hydrogen efficiency was 7.5% initially (3:30 PM), decreasing slowly to around 7% by 4:30 PM, and then dropping rather sharply to as low as 5.5% over the next hour. We attribute this effect to a reduction in the solid-state efficiency resulting from spectral sensitivity of the multijunction solar cell. In particular, the loss of blue light that occurs late in the day can affect the current matching from the top cell and decrease photovoltaic efficiency. Figure 10 shows the JV curves of the photovoltaic cell at different times during the test, normalized to the measured incident power. Figure 11, based on the curves in Figure 10, shows the efficiency of the solar cell calculated at the voltage required to drive the electrolysis reaction (as the insolation decreases, the required operating voltage decreases slightly; this has been accounted for in developing Figure 11). The loss in solar cell efficiency due to the spectral
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change is clearly evident in this last figure. Figure 12 shows the ratio of the hydrogen production efficiency (Figures 7 and 9 ) to the solar cell efficiency at the operating point (Figure 11) as a function of the time of day. Despite the large change in both the hydrogen production and solar cell efficiencies, this ratio remains essentially constant at 74.5 ( 0.8%, confirming that there has been no degradation of the photoelectrode or of the catalyst activity. Conclusions A reactor for the direct photoelectrolysis of water to produce hydrogen has been developed using triplejunction amorphous silicon solar cells catalyzed with sputter-deposited thin films of NiFeyOx and CoMo as the photoelectrode. Solar-to-hydrogen efficiencies of 7.8%, based on the lower heating value of hydrogen, have been achieved in outdoor tests. Independent life-testing of the catalysts in 1 N KOH for more than 7200 h showed no degradation of the catalyst activity even when lowpurity KOH and drinking water were used to make the electrolyte. These experiments used “off-the-shelf” research cells provided by Solarex Thin Film Division. Higher hydrogen production efficiency can be expected by adjusting solar cell structures to maximize the
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electrochemical load-matching and to minimize spectral sensitivity. The operation of the catalysts and photoreactor for extended periods of time has provided new insights into the remaining problems and provides strong evidence that the U.S. Department of Energy goals of a low-cost 10% efficient direct photoelectrolysis system can be developed. The integrated reactor system discussed in this work uses solar energy to split water and when fully developed will not use electricity as an intermediate energy carrier between the photocell and electrochemical components. Ultimately, a full engineering cost analysis comparing various configurations of the integrated and nonintegrated reactor types will be necessary to determine the most cost-effective method of producing hydrogen. Acknowledgment. We thank the U.S. Department of Energy for support of this work under Grant DEFG04-94AL85804. We also thank Dr. Liu Yang of the Solarex Thin Film Division for the amorphous silicon samples used in this work and Jim Phillips at the Institute of Energy Conversion, University of Delaware for calibration of the Si cells. EF9701347
