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Performance of Ethane/Oxygen Fuel Cells Using Yttrium-Doped Barium Cerate as Electrolyte at Intermediate Temperatures Shouyan Wang, Jing-Li Luo,* Alan R. Sanger, and Karl T. Chuang Department of Chemical and Materials Engineering, UniVersity of Alberta, Edmonton, Alberta T6G 2G6, Canada ReceiVed: October 11, 2006; In Final Form: January 27, 2007
Fuel cells having proton conducting membranes comprising doped barium cerate ceramic BaCe0.85Y0.15O3-R (BCY15) as electrolyte and platinum as both anode and cathode electrodes performed well for conversion of ethane to ethylene at 650 and 700 °C. No deterioration in performance occurred during 20 days of operation at 700 °C. The electrolyte was stable, and the membrane remained a single perovskite phase after long-term testing. Current density at maximum power density increased with temperature from 650 to 700 °C at ethane flow rates of 100-200 mL/min. A better seal and consequently a better performance were obtained when the sealant was under compression, achieved by operating the cell in vertical attitude. Power density of 120 mW/cm2 was attained at current densities in the range 300-650 mA/cm2 at 700 °C and ethane flow rate 100 mL/min. There was good agreement between current and change in gaseous composition across the anode. The protonic current arose in two ways: by conversion of hydrogen arising from dehydrogenation of ethane, and by electrochemical conversion of ethane. At 650 °C the former is the major reaction path, while at 700 °C the latter is the major path.
Introduction High-temperature solid electrolyte fuel cells have inherent advantages over other fuel cell types. Higher reaction rates can be achieved at higher operating temperatures, leading to higher current and power densities. Also, this type of cell co-generates electrical power and high-quality heat. As a result, the overall fuel utilization efficiency is enhanced. Several oxide or metal catalysts are available for activation of hydrocarbons at high temperatures. Thus, when hydrocarbons are fed to fuel cells, a further advantage of operation at high temperatures is that there is no requirement to use expensive noble metal catalysts. There have been several approaches to use of hydrocarbons as fuels for high-temperature fuel cells using oxide ion conductive solid electrolytes such as yttrium stabilized zirconia1-5 and other oxides.6-9 Direct hydrocarbon fuel cells generate power by conversion of hydrocarbon fuel to carbon oxides and water over the anode catalyst.3,5 Attempts also have been made to develop oxide ion conducting fuel cells for selective conversion of hydrocarbons to value-added products.10,11 However, to date no oxide ion conducting fuel cell system has shown sufficiently strong performance to be economically viable for industrial production of value-added products, e.g., conversion of ethane to ethylene. When electrochemical membrane reactors using three different kinds of ionic conductors (protonic, oxide ion, and mixed ionic conductors) were compared for electrochemical oxidative dehydrogenation of ethane to ethylene at 700 °C, it was found that the protonic conductor had the highest current efficiency.12 Protonic conductors have advantages over oxide ion conductors when used as electrolytes in solid oxide fuel cells (SOFCs). When a protonic conductor is used instead of an oxide ionic conductor, water vapor is evolved at the cathode side where it mixes with excess air. Excess air ensures that there is no large change in oxygen concentration resulting from water vapor production at the cathode, and so it does not significantly lower the cell voltage. * Corresponding author. E-mail:
[email protected]. Tel.: 1-780492-2232. Fax: 1-780-492-2881.
Fuel cells having proton conducting solid electrolytes which can be operated at intermediate temperature (500-700 °C) also offer significant potential advantages over oxide ion systems for selective conversion of hydrocarbons.11 When ethane is used as fuel for a proton conducting cell, hydrogen and catalyst surface bonded H are generated and then consumed at the anode side by routes exemplified by reactions 1-8 [* indicates a catalyst surface bound species].13-19 In contrast to oxide ion conducting fuel cells, ethylene so formed cannot be oxidized by oxygen to carbon oxides in a proton conducting cell. When a hydrocarbon fuel cell is operated under humid conditions,20 the hydrocarbon reacts with water over the anode catalyst to generate carbon oxides (steam reforming), but no such reaction is possible under dry conditions.21,22 Therefore, a proton conducting fuel cell operated under anhydrous conditions can potentially generate both electrical power and ethylene by consuming ethane directly as a fuel (eqs 2, 3, and 8).
C2H6 f C2H4 + H2
(1)
C2H6 f C2H5* + H*
(2)
C2H5* f C2H4 + H*
(3)
C2H5* f 2C + 5H*
(4)
C2H4 f CH4 + C
(5)
C + 4H* f CH4
(6)
H2 f 2H*
(7)
H* f H+ + e-
(8)
In this paper, we report the performance of ethane/oxygen fuel cells using yttrium doped barium cerate (BCY) protonic membranes at 650 and 700 °C and the relationship between cell performance and ethane flow rate. We will show that there
10.1021/jp066690w CCC: $37.00 © 2007 American Chemical Society Published on Web 03/13/2007
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are two mechanisms for proton generation and that the predominant mechanism depends on temperature. Experimental Section Proton conducting membranes were constructed using yttrium doped barium cerate (BaCe0.85Y0.15O3-R, BCY15, SCI engineered materials, Inc.) as the electrolyte. The main procedures used for membrane fabrication are summarized as follows. BCY15 powder was calcined in air at 1400 °C for 10 h. The compound was ground in isopropanol using a planetary ball mill with a zirconia mill container and zirconia balls at 100 rpm for at least 24 h, and then it was dried in a fume hood. The crystalline structure of the powders was determined using X-ray diffraction (XRD) analysis. The powder was uniaxially pressed at 30 MPa into disks approximately 2.54 cm in diameter and 2 mm thick, which were then sintered at 1550 °C for a further 15 h to obtain high-density membranes. The disk surfaces were polished using abrasive papers with different mean particle sizes until the thickness was reduced to about 0.8 mm. Platinum paste (Heraeus Inc., CL11-5100) was used to prepare both anode and cathode electrodes. Following deposition of the electrode layers, the membrane electrode assembly (MEA) was dried overnight at 150 °C. Platinum wires and meshes were used as output terminals and current collectors, respectively, at both electrodes. The anode and cathode gas chambers were set up by placing the MEA between concentric pairs of alumina tubes, as described previously.23,24 Each chamber was sealed by bonding the outer tube of each chamber to the MEA using ceramic sealant (Aremco 503). The sealed cell was situated in a tubular furnace. To cure the sealant, the furnace temperature was increased at 0.5 °C/min to 110 °C, where it was kept for 2 h before being increased to 650 °C for further heat treatment. The cell was heated to operating temperature with nitrogen flowing in each chamber. After the cell had stabilized at the selected temperature, the anode and cathode feeds were switched from nitrogen to ethane and oxygen, respectively. The furnace temperature was changed to the selected value at the rate of 1.5 °C/min where it was kept at least 0.5 h to stabilize performance before conducting measurements at selected temperatures. The fuel cell voltage and conductivity were measured using a Solartron 1287 electrochemical interface together with 1255B frequency response analysis instrumentation. Current-voltage characteristics were determined potentiodynamically with the electrochemical systems using a scanning rate of 5 mV/s. AC impedance spectra were measured for frequencies between 0.01 Hz and 1 MHz to determine the conductivity of each membrane. Oxygen and pure dry ethane were metered using mass flow controllers. The outlet gases from the anode chamber were analyzed using a Hewlett-Packard model HP5890 GC with a packed bed column (OD: 1/8 in.; length: 2 m; Porapak QS) at 80 °C by a thermal conductivity detector. The furnace temperature of the zone containing membranes under test was calibrated in both horizontal and vertical configurations using a thermocouple to ensure consistency of readings in each orientation. Results and Discussion Structure of BCY15 Electrolyte. A typical cross-sectional SEM micrograph of a sintered membrane is shown in Figure 1. The fired density of the specimen was greater than 90% of the theoretical density. No visible holes or cracks in the membrane were found using SEM. Figure 2 shows the XRD pattern of
Figure 1. Cross-sectional SEM of BCY15 electrolyte after sintering at 1550 °C in air for 10 h.
Figure 2. XRD pattern of sintered BCY15 electrolyte powder using Cu KR radiation.
BCY15 powder after sintering at 1400 °C. The XRD peaks for powders calcined at 1400 °C for 10 h are very sharp and are attributable to a single perovskite phase for BCY15 with a cubic unit cell, and additional smaller peaks are attributable to small amounts of BaCO3 added before sintering to compensate for evaporation of BaO during high-temperature calcination. Stability of BCY15 under Fuel Cell Testing Conditions. Many barium containing compounds react with CO2 at temperatures below 1100 °C to form BaCO3. Intense studies have been conducted into the stability of this class of materials in a high surface area powder form at high temperatures in a pure CO2 atmosphere.25-29 Three approaches have been shown to be effective for improving the stability of barium containing compounds in CO2 containing atmospheres: (i) preparation of slightly Ba-deficient compounds,25,30 (ii) partial substitution of Ce for Zr in Ba-Zr-O composites,31,32 and (iii) synthesis of mixed cation BaB′0.5B′′0.5O3-R perovskites, where B′ and B′′ are represented trivalent and pentavalent ions.33,34 In the present application, the electrolyte comprised a dense material in an atmosphere containing a relatively low partial pressure of CO2. Consequently, it was anticipated that reaction with CO2 to form BaCO3 would be slow, would occur primarily at the surface, and required cation transport within the BCY15 matrix to form substantial amounts of BaCO3 and that such cation migration would be considerably slower than either proton or oxide ion transport.33,34
Ethane/Oxygen Fuel Cells Using BCY as Electrolyte
Figure 3. SEM of used electrolyte showing formation of barium carbonate on the membrane surface exposed to anode gases for 20 days under fuel cell operating conditions.
Small white crystals formed over 20 days at exposed portions of the membrane anode surface under anode gases under fuel cell operating conditions (Figure 3), and XPS showed that these were barium carbonate. However, there was no similar change in surface composition at places where the electrolyte was covered with sealant or catalyst, or at the cathode side. Membrane cross-sectional SEM showed that there were no changes to the bulk BCY15 perovskite phase. Thus dense BCY15 membranes were very stable when using anhydrous ethane as fuel. In our testing conditions, no oxygen was present at the anhydrous anode. However, there was a low concentration of carbon oxides at the anode, arising from oxidation of anode fuel by trace amounts of oxygen that diffused through the sealant, which became slightly porous after it was sintered. Although there was a very small contribution to total ionic current from oxide ion conductivity at 700 °C,35 this was not a significant source of CO2 since there was no BaCO3 formed at the anode electrode area. It is unknown whether stability and performance of the electrolyte would be as good under high CO2 concentrations, such as those arising when either oxygen or steam is present in the anode feed. The unaffected nature of the interior of the electrolyte layer and the stability of membrane performance indicated that the BaCO3 crystals at the surface hindered migration of CO2 into the matrix, and so protected the interior of the membrane from further reaction. There was no extensive reaction of BCY15 with CO2, and so this reaction was not of serious concern when the material was densely packed to form a contiguous mass and not loosely packed powder. The absence of extensive reaction with CO2, and thus the absence of consequent serious effects on the electrolyte, was reflected in the stability of performance of the membranes. Figure 4 shows that the conductivity of the Pt/BCY15/Pt membrane configuration was stable during fuel cell testing at 700 °C for 20 days. After each set of measurements during the first 14 days, the anode feed was switched to standby under nitrogen and the furnace temperature was reduced to 650 °C. Then continuous testing was conducted under ethane feed during the remaining 6 days. There was no deterioration in conductivity during the periods of either intermittent or continuous testing (Figure 4). Performance was particularly steady during continuous operation. Relationship of Cell Performance to Ethane Flow Rate at 650 and 700 °C. The cell performance (Figures 5-7), the
J. Phys. Chem. C, Vol. 111, No. 13, 2007 5071
Figure 4. Conductivity as a function of exposure time under open circuit conditions at 700 °C with ethane flow rate of 150 mL/min and oxygen flow rate of 120 mL/min.
Figure 5. Current density-voltage and power density curves of the ethane/oxygen fuel cell at different ethane flow rates with BCY15 electrolyte and platinum electrodes at 650 °C and oxygen flow rate of 120 mL/min.
Figure 6. Current density-voltage and power density curves of the ethane/oxygen fuel cell at different ethane flow rates with BCY15 electrolyte and platinum electrodes at 700 °C and oxygen flow rate of 120 mL/min in all cases (with horizontal furnace assembly).
conversion of ethane, and the selectivity to ethylene (Tables 1 and 2) were each dependent on both temperature and ethane flow rate. It also was found that the attitude of the cell had an effect on performance. The performance of the cell (Figure 7) improved
5072 J. Phys. Chem. C, Vol. 111, No. 13, 2007
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Figure 7. Current density-voltage and power density curves of the ethane/oxygen fuel cell at different ethane flow rates with BCY15 electrolyte and platinum electrodes at 700 °C and oxygen flow rate of 120 mL/min (with vertical furnace assembly).
TABLE 1: Conversion of Ethane and Selectivity to Ethylene at Different Ethane Flow Rate and Temperatures Using the Horizontal Fuel Cell ethane conversion (%)a/ethylene selectivity (%)b ethane flow rate (mL/min) temp (°C)
100
150
200
650 700
6.6/84 24.1/94
5.9/85 20.2/95
17.6/97
Conversion ) [(nfeedin - nfeedout)/nfeedin] × 100 (%), where nfeedin is moles of ethane in the feed and nfeedout is moles of ethane in the effluent. b Selectivity ) [nethylene/(nfeedin - nfeedout)] × 100 (%), where nethylene is moles of ethylene produced. a
TABLE 2: Conversion of Ethane and Selectivity to Ethylene at Different Ethane Flow Rates and Temperatures Using the Vertical Fuel Cell ethane conversion (%)/ethylene selectivity (%) ethane flow rate (mL/min) temp (°C)
100
150
200
650 700
9.0/98 27.9/95
6.8/98 23.7/96
5.5/98 20.5/96
significantly when the assembly was situated so that the axis of the chambers was vertical, when compared with the same cell oriented horizontally (Figure 6), and a compressible glass sealant was used. The composition of the effluent gas streams suggested that there was minor crossover of air into the anode compartment in the horizontal alignment, which was absent when using the vertical alignment. Similar results were obtained using a variety of sealants for the horizontal cell. The leakage is attributed to formation of leaks resulting from torsional strain at the interface between the membrane and electrode chambers. Thus it appeared that compression of sealant, achieved in vertical format, was necessary to ensure a good seal at the membrane. When there were no leaks ethylene was the only product as there was no reaction with leaked oxygen to form water. Reactions of hydrocarbons over platinum catalysts are more rapid at higher temperatures. However, coke formation can be a serious problem for operation of hydrocarbon activation catalysts at high temperatures (eqs 4 and 5). Consequently it was necessary to operate the fuel cell at temperatures sufficiently low to prevent or minimize pyrolysis and catalytic reactions leading to detrimental formation of carbon, yet high enough to maintain high electrochemical reaction rates and hence high
current density. Thus the present series of experiments was conducted at 650 °C and 700 °C. Ethane/oxygen fuel cell current-voltage performance was a function of ethane flow rate at both 650 °C (Figure 5) and 700 °C (Figures 6 and 7). The oxygen flow rate was held constant at 120 mL/min in all cases. Current density, open circuit voltage (OCV), and cell performance each decreased rapidly when the ethane flow rate was increased, especially the cell performance at 700 °C. Further, increasing the ethane flow rate had the effect of shifting the entire current-voltage curve downward at both temperatures. Products of ethane reactions in the anode chamber were detected using online GC (Figure 8). The major products in the effluent were hydrogen, ethylene, and methane. Table 1 compares the conversion of ethane and selectivity to ethylene at 650 °C and 700 °C for different ethane flow rates. The conversion was higher at 700 °C for all flow rates. The conversion of ethane was reduced as the flow rate increased, and the selectivity to ethylene increased slightly. Thus the residence time affects the composition of the effluent stream. As hydrogen, ethylene, and methane were detected in subequilibrium amounts, the residence time was insufficient to attain equilibrium between ethane and the thermal reaction products. Potentially, a longer residence time would allow more extensive thermal reactions, which would have resulted in lower selectivity to ethylene. When the flow rate of ethane was increased, the residence time of ethane in the anode chamber decreased, which resulted in rapid removal of products. Consequently, there was a decrease in concentration of hydrogen in the anode compartment, and this was shown by the drop in OCV with increase in flow rate: the OCV dropped from 0.952 V at ethane flow rate 100 mL/min to 0.897 V at 150 mL/min at 650 °C. Similarly, at 700 °C, the OCVs were 0.976, 0.956, and 0.948 V at ethane flow rates 100, 150, and 200 mL/min, respectively. Consequently, this range of ethane flow rates was sufficiently low to ensure good conversion, but sufficiently high to avoid extensive side reactions, thus maintaining good selectivity to ethylene. It can also be seen from Figures 6 and 7 that cell performance at 700 °C was superior to that at 650 °C. Much higher power density and current density were obtained using the vertical cell (Figure 7) than the horizontal cell (Figure 6). For the vertical cell, maximum power density over 120 mW/cm2 was obtained over a wide range of current densities from 300 to 650 mA/ cm2 at ethane flow rate 100 mL/min. At 150 mL/min the power density peaked at 95 mW/cm2 with a current density of 250 mA/cm2. Comparing results using the horizontal cell, at 650 °C the maximum power densities were 20.5 and 16.9 mW/cm2 at ethane flow rates 100 and 150 mL/min, respectively. The corresponding current densities were 58 and 42 mA/cm2. At 700 °C, current densities at maximum power density were enhanced to 93, 131, and 164 mA/cm2 at ethane flow rates 200, 150, and 100 mL/min, respectively. We will now show that the difference in performance at the two temperatures is attributable to different predominant mechanisms of proton formation. Mechanism of Reaction in Ethane Fuel Cells. Inlet and outlet anode gas streams were analyzed using online GC with helium and argon as the carrier gases, respectively, during operation of the cell at 650 and 700 °C and constant ethane flow rate 150 mL/min. The GC was calibrated for determination of all gases detected in the outlet gas. Of these, C2H6 and C2H4 were present in highest amounts, with lesser amounts of H2, CH4, CO, and CO2. We did not detect any C3H6 and C4H8, though these may have been produced in minor amounts at these
Ethane/Oxygen Fuel Cells Using BCY as Electrolyte
Figure 8. Hydrogen and ethylene concentration changes in anode chamber effluent at (a) 650 °C and (b) 700 °C using fuel cells with BCY15 as electrolyte and platinum as electrodes at ethane flow rate of 150 mL/min (with horizontal furnace assembly).
temperatures.36 The composition of the outlet gases was dependent on both temperature and flow rate. The variations showed that dissociation of ethane to hydrogen and ethylene (eq 1) accounted for only part of the products, as follows. When dissociative adsorption of ethane takes place on platinum surfaces, C-H bonds are broken and coadsorbed C2Hx (0 e x e 6) and H species are formed (eqs 2-4).14,15 Pyrolysis of ethane produces H2 and C2H4 (eq 1), each of which also forms surface bonded H (eq 7) and C2Hx species. At 650 °C the ethylene concentration increased only a little with increasing current (Figure 8a). The amount of the decrease in hydrogen concentration with increasing current was much larger than the amount of the increase in ethylene concentration. Thus the increase in total current passing through the membrane was attributable mainly to increased hydrogen oxidation and, only to a lesser degree, was a result of increased direct electrochemical dehydrogenation of ethane. At lower hydrogen partial pressures, the active sites at the surface of the catalyst were primarily hydrocarbon covered.16 The activity of the anode catalyst for ethane activation, and so for very rapid direct electrochemical dehydrogenation of ethane (eqs 2, 3, and 8), was lower than that for hydrogen activation at 650 °C. Thus the cell reaction was still in the region of activation polarization, and there was insufficiently rapid production of ethylene and protons to maximize the anode reaction rate. That is, the anode catalyst was insufficiently active to furnish protons at a rate high enough to maximize proton conduction through the membrane. Hydrogen produced by thermal dehydrogenation (eq
J. Phys. Chem. C, Vol. 111, No. 13, 2007 5073 1) and a small amount of catalytic dehydrogenation of ethane (eqs 2 and 3) was the main source of protons at this temperature, and dehydrogenation of ethane was a significant source of ethylene. The dependence of the current-voltage curves on ethane flow rate at 650 °C was consistent with this mechanism (Figure 5). When the flow rate of ethane was increased, the residence time of ethane in the anode chamber decreased, and the exchange rate of product and reagent increased, which resulted in a decrease in hydrogen concentration. Consequently, the current-voltage curve shifted to lower values when the ethane flow rate was increased. In contrast, the outlet ethylene concentration increased greatly with current at 700 °C (Figure 8b). The concurrent decrease in concentration of hydrogen was much smaller. These data are consistent with a mechanism in which there was more rapid catalytic dehydrogenation via ethane dissociative adsorption (eq 2) at this temperature, and adsorbed ethyl radicals undergo further dehydrogenation leading to ethylene formation (eq 3).17 Therefore the anode catalyst was sufficiently active for the ethane dissociative adsorption and dehydrogenation reaction at 700 °C to generate a higher flux of protons for conduction through the membrane than at 650 °C. Thus, while both the thermal reaction (eq 1) and the direct electrochemical conversion of ethane (eqs 2, 3, and 8) contributed protons to the ionic current, the direct electrochemical conversion reaction of ethane to ethylene in a simple faradic process was a much more significant source of protons for the ionic current at 700 °C than at 650 °C. Catalytic dehydrogenation of ethane to ethylene (eq 1) was the source of the lesser amount of hydrogen detected in the effluent stream, which contributed in a proportionally lesser degree to the overall proton flux at 700 °C than at 650 °C. The combination of higher concentration of available hydrogen and catalyst surface bonded hydrogen with the increased conductivity of the membrane at 700 °C resulted in much higher protonic current density than at 650 °C. When the flow rate of ethane was decreased for the fuel cell operated at 700 °C, more methane and a small amount of carbon were produced in the anode chamber, which indicated that reactions 4 and 5 became more significant at low feed flow rates. Thus there was more catalyst surface bonded hydrogen produced for the anode reaction, which consequently increased cell electrical performance at the cost of enhanced conversion of ethane and reduced selectivity to ethylene (Tables 1 and 2). Further, build up of carbon at the anode will ultimately limit cell lifetime. The measured protonic current correlated closely with predicted values calculated from the combined changes in hydrogen and ethylene concentrations at both 650 and 700 °C (Figure 8). These results showed that the electrochemical dehydrogenation of ethane and subsequent oxidation of adsorbed hydrogen were the predominant reactions. There was only a small contribution from reactions to byproducts. The results showed unequivocally that BCY15 ceramic membranes were good and stable proton conductors for use in ethane fuel cells at 650 and 700 °C. Conclusions Dense membranes having doped barium cerate ceramic BaCe0.85Y0.15O3-R (BCY15) as electrolyte have high and stable proton conductivity at 650 °C and 700 °C. Membranes comprising dense BCY15 as electrolyte and platinum as anode and cathode catalysts are active for use in ethane/oxygen fuel cells for direct conversion of ethane to ethylene with high selectivity
5074 J. Phys. Chem. C, Vol. 111, No. 13, 2007 at these temperatures. The conductivity of the fuel cell was stable during 20 days operation. Small amounts of crystals of barium carbonate were formed only at positions on the surface where it was exposed directly to anode gases, by reaction of BCY15 in the presence of traces of CO2, was localized at the electrolyte surface and away from anode catalyst, and did not affect fuel cell performance. There was no corresponding change in surface composition where the electrolyte was covered with sealant or by catalyst on the anode side, nor were there any changes to the bulk BCY15 perovskite phase. Thus dense BCY15 membranes are stable and good proton conductors for use under fuel cell operating conditions where CO2 concentrations are low. High performance was achieved using a vertically oriented cell, with power density over 120 mW/cm2 over the range of current densities from 300 mA/cm2 to at least 650 mA/cm2. Lower power and current densities were attained using a horizontal cell, the power density increasing strongly from 42 mA/cm2 at 650 °C to 131 mA/cm2 at 700 °C at a constant ethane flow rate of 150 mL/min. The superior performance in vertical orientation is attributed to an improved seal achieved by compression of the sealant. The protonic current arose from both hydrogen from thermal dehydrogenation of ethane and from direct electrochemical conversion of ethane. The former reaction was proportionally more important at 650 °C, and the latter reaction was proportionally stronger at 700 °C. There was a good correlation between the current generated and ethane consumed during cell discharging, as determined from the concentrations of major (C2H6, C2H4) and minor components (H2, CH4, CO, and CO2) in the anode effluent. Acknowledgment. This work was supported by the COURSE program of Alberta Energy Research Institute and NOVA Chemicals. References and Notes (1) Costa-Nunes, O.; Vohs, J. M.; Grote, R. J. J. Electrochem. Soc. 2003, 150, 858-863. (2) Sasaki, K.; Teraoka, Y. J. Electrochem. Soc. 2003, 150, 878-884. (3) Park, S.; Vohs, J. M.; Gorte, R. J. Nature 2000, 404, 265-267. (4) Murray, E. P.; Tsai, T.; Barnett, S. A. Nature 1999, 400, 649651. (5) Atkinson, A.; Barnett, S.; Gorte, R. J.; Irvine, J. T. S.; Mcevoy, A. J.; Mogensen, M.; Singhal, S. C.; Vohs, J. Nat. Mater. 2004, 3, 17-27. (6) Zha, S. W.; Moore, A.; Abernathy, H.; Liu, M. L. J. Electrochem. Soc. 2004, 151, 1128-1133.
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