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Effect of hydrogen and carbon dioxide on the performance of methane fueled solid oxide fuel cell
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Effect of hydrogen and carbon dioxide on the performance of methane fueled solid oxide fuel cell Zhiyuan Chen a,c, Liuzhen Bian a,b, Lijun Wang a,b,*, Ning Chen c, Hailei Zhao c, Fushen Li c, KuoChih Chou a,b a
State Key Laboratory of Advanced Metallurgy, University of Science and Technology Beijing, Beijing 100083, PR China b School of Metallurgical and Ecological Engineering, University of Science and Technology Beijing, Beijing 100083, PR China c School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing 100083, PR China
article info
abstract
Article history:
Recycling and reusing of exhaust gas from solid oxide fuel cell (SOFC) could enhance the fuel
Received 19 March 2015
utilization of methane. In the present work, the effects of two main components of exhaust
Received in revised form
gas, viz. hydrogen and carbon dioxide, on the performance of exhaust gas reforming methane
28 February 2016
fueled cell were investigated. The cell with Ni/8 mol% Y2O3 doped zirconia (Ni/YSZ) as the
Accepted 12 March 2016
anode was exposed to H2eCO2eCH4 atmospheres at 800 C. DC techniques and impedance
Available online 10 April 2016
spectroscopy was used for characterization. Two kinds of cells with different fuel utilization
Keywords:
density/voltage with current density. The results indicated that electrooxidation of H*
Methane
dominated the electrochemical reaction on the anode of the cell at low Uf value, while
Solid oxide fuel cell
electrooxidation of C* becomes important at high Uf value. In the case of CO2eH2 mixtures,
factors (Uf) were used in experiments, which represented different relationship of power
Hydrocarbon fuels
CO2 improved the exchange current density, raised the value of Uf, and depressed anodic
Carbon dioxide reforming
activation polarization. As for H2eCH4eCO2 system, H2, partly replacing CH4, reduced the
Exhaust gas reforming
heat loss in dry reforming, improved the exchange current density, raised Uf value, and
Anode-off gas
depressed the activation and concentration polarization resistances of Ni/YSZ anode. But it had limited effect on the power density of the cell with high Uf value. Copyright © 2016, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Introduction The increasing interest in developing solid oxide fuel cell fed (SOFC) with hydrocarbon such as methane (CH4) is originating due to the possibility of using a wider energy source for clean electricity generation and effective reducing gas emissions.
Nickel/yttrium-stabilized zirconia (Ni/YSZ) porous ceramic is the most widely used material for SOFC anode. But the problem in this case is the dissociation of methane on the surface of nickel particles can cause carbon deposition through the following reaction: CH4 /C þ 2H2
(1)
* Corresponding author. School of Metallurgical and Ecological Engineering, University of Science and Technology Beijing, Beijing 100083, PR China. Tel.: þ86 10 6233 3622; fax: þ86 6233 2570. E-mail address: [email protected] (L. Wang). http://dx.doi.org/10.1016/j.ijhydene.2016.03.090 0360-3199/Copyright © 2016, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
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which results in poor performance and eventual failure of SOFC. The other two carbon producing reactions (Boudouard reaction and CO oxidization by H2) have little effects on carbon deposition, because the standard Gibbs free energies of the two reactions were positive above ~700 C [1,2]. Therefore, CH4 is the major source of deposited carbon at high temperatures. To inhibit carbon deposition, several reforming methods have been investigated, including steam reforming (SR) [3,4], partial oxidation (POx) [5,6], autothermal reforming (ATR) [7,8] and dry reforming (DR). Among these methods, carbon dioxide (dry) reforming in the SOFC system [9e12] is an environment friendly process. CO2 can be recycled from the engine exhaust or anode-off gas in practical system [13]. In recent years, the exhaust gas recirculation of engine, especially natural gas fueled engine has been widely studied [14,15]. Researchers have been paying attention to engine/ anode-exhaust gas reusing with SOFC, with main focus on the design of catalyst, blueprint of recycling system and reforming optimization [16e21]. It has been proved that the exhaust gas reforming is the most preferable way for SOFC fuel feeding from a thermodynamic view point [22]. Kasraet et al. [23] reported that a proper level of gas recycling would increase current density of the SOFC. But researchers preferred to study the effect of steam [18,19,24] and partial oxidation reforming [20] in exhaust gas recycling system. Since recycling gas usually contains a significant percentage of steam [25], steam reforming was always treated as primary strategy. On the contrary, CO2 was considered to be a negative factor in the reusing of the exhaust gas [19,24]. However, it was noticed that the electric efficiency in the steamcontaining anode-off gas recycling mode was higher than in SR and POx without gas recycling [25,26]. In the former mode, CO2, H2 and CO were introduced into the anode, which could be beneficial to the electrochemical reaction. Effort also was made to improve the efficiency of CO2 reforming in reusing process, where proton conductor was used [27], several systems were designed [28] towards this end. Furthermore, the present work is focused to provide an approach to examine the effect of CO2 on exhaust gas reforming process. In order to conveniently understand the effect of each component in off-gas system, the strategy here was to simplify gas mixture compositions. Thus only CO2 and H2 were introduced in this experiment rather than complex compositions gas injection in the earlier works [25,29,30]. Moreover, considering that high content of steam in anode-off gas accelerates the corrosion of the interconnector materials [31e33] and sintering of Ni particles in the anode [34,35], the present authors suggest that steam should be removed before reusing of the exhaust gas. As Fig. 1 shows, CO2 and H2 in fuel cell exhaust gas are recycled into the anode chamber and reused. The reforming reaction in anodic chamber in Fig. 1 is as following: CH4 þ CO2 ¼ 2CO þ 2H2 ;
DHQ 298 ¼ 247 kJ=mol
(2)
The presence of carbon dioxide is beneficial to suppressing the carbon deposition as shown in Eq. (3). But the carbon dioxide reforming usually decreased open circuit voltage (OCV) [36] and the open power density of the cell [13]. Staniforth and Ormerod [11] suggested that the power production reached
the maximum at 45 vol% CH4 in dry reforming fuel, with a maximal production of CO and H2 through internal reforming at the same time. It shows a potential of optimizing fuel gas composition to improve the electrochemical performance. C þ CO2 ¼ 2CO
(3)
Besides, the concentration of hydrogen in the recycled exhaust gas lies in a wide range [20,37,38]. Here, hydrogen is one of the products in reforming rather than an additional supplement from other sources, so it is a promising reusing power source without extra cost. Reusing hydrogen in methane fuel cell has a potential to improve cell's performance. For instance, hydrogen can suppress carbon formation and growth over the Ni/YSZ anodes by promoting the reverse of Eq. (1) [38e42]. Moreover, Nikooyeh et al. [38] reported that the Ni/YSZ anode exposed to methane in the presence of hydrogen, rather than oxygen, would result in less damage to the anode microstructure in the elimination of deposited carbon. Yet the disadvantage is that hydrogen inhibits the reforming rate via Eq. (2) [43,44]. Studies are needed to evaluate the effect of CO2 and H2 in recycled exhaust gas to the fuel cell's performance, which can promote exhaust gas to be a potential fuel resource for SOFC. As mentioned above, CO2 and H2 are the main components in the exhaust gas of SOFC. And both of them were expected to be beneficial to the electrochemical oxidation of methane. In order to observe the interaction of CO2 and H2, the present work was carried out to examine the electrochemical properties of the SOFC which was fed with H2eCH4eCO2 at 800 C.
Materials and methods Anode-supported cells with a diameter of 12 mm were used in the present experiments. Cells with different fuel utilization factor (Uf) were used in the experiments. The first one was with low fuel utilization factor, of which the electrolyte was 8 mol% Y2O3 doped zirconia (YSZ) with a thickness of about 35 mm. The porous anode with a thickness of about 400 mm had a Ni:YSZ molar ratio of 1:1. The porosity of the anode was 25.08%. The porous cathode of area 0.3 cm2 and a thickness of about 100 mm was made of 8 mol% Y2O3 doped zirconia & Sm0.5Sr0.5CoO3 (YSZ/SSC). After structure optimization, the other cell was with relatively high fuel utilization factor, of which the area of the Gd0.1Ce0.9O2d/Bao.5Sr0.5Co0.8Fe0.2O3d (GDC/BSCF) porous cathode was 0.26 cm2. A GDC barrier was between the cathode and YSZ electrolyte. The cell was mounted at the end of an alumina tube. Ag paste was painted on the anode edge and fired at 600 C, which was used as current collector and seal. A ceramic paste seal was used to cover the silver paste layer. Silver wires were fixed with a drop of silver paste on the top of the cathode and used as current collectors. The anode was exposed to the fuel gas, while the cathode was in contact with flowing air. The total fuel flow was maintained at 100 ml/min. The fuel gases were introduced into the cell and their flow rates were controlled using Alicat mass flow controllers. The compositions of the fuel gas are shown in Table 1. The evolve gas of the internal carbon dioxide reforming SOFC was analyzed by mass spectra system
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Fig. 1 e Scheme of exhaust gas reforming.
(Hiden HPR20 Research Grade System for Continuous Sampling Gas/Vapour Analysis). The electrochemical characterization of the cell was carried out by electrochemical impedance spectroscopy (EIS) using an electrochemical workstation (Princeton PARSTAT 2273). Measurements were recorded in the frequency range of 2 MHze0.1 Hz using voltage amplitude of 5 mV at a temperature of 800 C. The currentevoltage characteristic of the cell was also measured using linear sweep voltammetry at a sweep rate of 1 mV$s1. After measurement, the cell microstructure was studied by scanning electron microscopy (SEM, CARL ZEISS EVO MA 10/ LS 10 JS) with energy dispersive spectrum (EDS, Thermo NORAN System). The used cell was tested by temperature programing oxidation (TPO) and Raman spectrum (Horyba LabRAM HR Evolution), to detect whether carbon deposited on the anode in the operation. The same mass spectra system was employed to analyze evolved gas in TPO test.
Results and discussions
A comparison of the present experimental results to the theoretical values is helpful to the analysis, even though equilibrium is hardly achieved in practice [25,30]. Fig. 2 and Table 1 showed the thermochemical calculation results obtained from FactSage 6.2 with the free energy minimization method. Table 1 showed that the oxygen partial pressures of the fuel gas, which was related to the theoretical electromotive force (EMF) of the cell, decreased with the increasing contents of H2 and CH4 in the fuel. The oxygen partial pressure decreased with the decreasing hydrogen concentration in methane-containing fuel, since the oxidation potential of methane is higher than hydrogen at 800 C. The electromotive force of the cell was calculated out from the oxygen partial pressure of fuels by the following equation: EMF ¼
RT 1 ln fuel 4F PO2
! (4)
where R, P and F are the gas constant, partial pressure of gas and Faraday constant, respectively. T is the operation temperature.
Cell with low fuel utilization factor Low utilization factor implied that electrochemical reactions will not significantly change the composition of fuel gas. Usually, Uf was calculated from the molar flow rate of fuel at inlet and outlet [45]. In our study, it was calculated by the following equation [37,46]: Uf ¼
I nel n_FI F
where, I is appointed to be the current value at 0.7 V. F is faraday constant, nel is the molar flow of electrons, and n_FI is the fuel molar flow at the cell's inlet. The calculation results shows that the value of Uf at this situation was around 0.06e0.63%.
Fig. 2 e Chemical equilibrium diagram of CH4eH2eCO2 system at 800 C.
Table 1 e Compositions of 100 mol feeding fuel gas in equilibrium state. No.
1 2 3 4 5 6
Fed gas (mol)
Gas composition in equilibrium (mol)
CH4
CO2
H2
H2
CH4
H2O
CO
CO2
O2
0 0 0 15 30 40
0 40 60 60 60 60
100 60 40 25 10 0
100.00 36.46 16.53 36.53 56.85 39.16
0.00 0.03 0.00 0.05 0.25 0.37
0.00 23.48 23.46 18.38 12.64 4.88
0.00 23.42 23.46 48.28 72.14 48.91
0.00 16.55 36.54 26.67 17.61 6.69
0 1.81$1017 8.79$1017 1.44$1017 3.45$1018 6.78$1019
EMF (V) 0.9778 0.9977 0.9611 1.009 1.047 1.074
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The value of DH can be used to estimate the thermal effect in the methane reforming process. Fig. 3 showed the enthalpy change of gas from the preset composition to thermodynamic equilibrium state. The value of enthalpy change per mole of fuel gas linearly dropped with the increase of hydrogen content in carbon dioxide reform methane. It could be noticed that feeding CO2 in hydrogen fuel gas also could cool down the cell slightly. The enthalpy change of the relaxation process implied that hydrogen was effective in the depression of the temperature gradient in SOFC stack. The possibility of carbon deposition in a certain ambient atmosphere can be predicted from thermodynamics calculations. As Fig. 2 shown, compositions of fuel gas were controlled within the non-coking zone at 800 C. Thus, there should be no coking on anode of SOFC. Fig. 2 shows that No. 5 fuel is the nearest composition to the carbon deposition boundary. After operation in No. 5 fuel, the cell was probed by SEM/EDS and TPO. Fig. 4(a) and (b) showed the SEM photos of the anode of the cells before and after the test at 800 C. The structure of the cell had not obviously changed. Significant carbon deposition was not found by EDS detection yet. Besides, the TPO result shown in Fig. 4(c) indicated that no carbon was detected in the used SOFC fragment. CO2 and H2 in the atmosphere inhibited the deposition of carbon and this is in conformity with thermodynamic analysis. Fig. 5 illustrated voltageecurrent and power outputcurrent data of the SOFC at 800 C. All of the voltages linearly decreased with increasing current density of cell. It implied that the dominant polarization loss step of electrochemical process in the cell kept the same within all the scope. The power density could be significantly improved by substituting some CH4 to H2. And it was found that the power density of cell did not obviously depress with 40 vol% CO2 feeding into H2. Even the depression of the power density in 60 vol% CO2 containing hydrogen was limited. It implied that dilution of hydrogen with certain amount of CO2 could not affect the power output of the SOFC with low Uf value. Fig. 6 was obtained by plotting the OCV and maximum of power density for the SOFC as function of hydrogen concentration. It
Fig. 3 e DH of the relaxation process of 1 mol fed fuel gases listed in Table 1.
was found that the change of OCV in H2eCO2 atmosphere did not follow the predicted value in Table 1. Here, the cell reaction can be represented as
Fig. 4 e Cross-sectional SEM images of the SOFC with low Uf (a) before and (b) after operation in No. 5 fuel gas at 800 C, and (c) CO2 partial pressure of evolved gas in the TPO test of the SOFC fragment after operation.
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Fig. 5 e Influence of methane concentration of H2eCO2eCH4 mixed gases on terminal voltage and power density of the cell with low Uf at 800 C. 1 H2 þ O2 ¼ H2 O 2
(5)
As the hydrogen was diluted with carbon dioxide (fuel gas No. 1 to 3 in Table 1), the voltage decreased according to the relationship in the form of Nernst equation: E ¼ EðH2 Þ
RT PH O ln 1=2 2 2F PO2 PH2
! (6)
where, EðH2 Þ is the theoretical potential of reaction (5) which is 0.9778 V at 800 C. Moreover, the water vapor in the anode chamber at the open state is supposed to be in the standard state. It is admitted that the present analysis is based on some assumptions which could form the first level of approximation. In fact, limited fuel utilization in a SOFC would reduce the Nernst voltage of the practical system. And equation (6) did not consider the partial pressure of carbon dioxide. Fig. 6 showed that the OCV consistent with Nernst equation (6). It indicated that carbon dioxide cannot obviously affect the OCV of the cell with low Uf value. Although the theoretic potential of methane oxidation is higher than that of hydrogen oxidation at 800 C, Fig. 6 showed that the OCV under methane-containing atmosphere was not
Fig. 6 e Open circuit voltage (OCV) and maximum power density of the cell with low Uf at 800 C using H2eCO2eCH4 fuel. The curve was the fitting result of Eq. (6).
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higher than the extended dashed line of Equation (6). This was not consistent with the order of EMF values of these fuel gases in equilibrium shown in Table 1. The low OCV under methane containing atmosphere may be attributed to be the difference of electrocatalytic activity of Ni/YSZ anode with respect to the hydrogen and methane [47e49]. Furthermore, this relatively low electrocatalytic activity implied that the oxidation of absorbed C* was difficult within the three-phase boundary (TPB) zone. Evolve gas composition which was cooled to room temperature was analyzed using mass spectrum at open circuit (Fig. 7). In this analysis, the molecular fragment of the particle was approximately regarded as the molecular itself. H2O was in equilibrium state at room temperature, and the peak corresponding to N2 gas which accompanied H2 as impurity was the same as CO (mass 28). The invisible peak of O2 could confirm that there was no obvious gas leakage in the fuel cell seals. The spectrum shows the gas composition did not agree well with the equilibrium prediction given in Table 1. For example, CH4 did not decrease to the extent shown in Table 1. And the detected CO content was below the predicted value. Thus, the fuel gas was not in an equilibrium state, which was possibly because of the flow geometry of the SOFC test [50] and the slow catalytic rate of CH4 reforming. The detected CO in H2eCO2 atmosphere implied that the reverse reaction of water gas shift occurred (As Eq. (7) shown, H2O and CO were the products of this reversible reaction [51]). The equilibrium constant of water gas shift is equal to 1.07 (kPa)0 at 800 C [1], which is close to unity. Hence, the reverse reaction could be possible at 800 C is possible according to thermodynamic analysis. CO þ H2 O ¼ CO2 þ H2
(7)
In summary, the mass spectra result in Fig. 7 indicated that CO and H2O were produced by the reverse reaction of the water gas shift reaction. However, carbon based compositions had limited effect on the performance of the cell. It implied that the contribution of electrooxidation of absorbed C* on anode was limited to the power generation. The electrooxidation of H* dominated the electrochemical reaction on anode.
Fig. 7 e Exhaust gas compositions of the cell with low Uf detected by mass spectrometer.
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Fig. 8 showed the impedance spectra measured at open circuit situation. It was found that the impedance arc expanded significantly when CH4 was introduced to the anode. But it was almost identical in case of hydrogen diluted only with CO2. The real-axis intercept, Rs, at high frequencies represents the ohmic resistance of the electrolyte and lead wires. The difference between the high and low frequency intercepts with the real axis, Rp, is the polarization resistance of the cell, which is the sum of Rp1 and Rp2. Rp1 at higherfrequency part of the arc corresponds to the charge transfer, Rp2 at the lower-frequency part corresponds to the mass transfer [52,53]. The analysis result of the spectra and the corresponding equivalent circuit model were drawn in Fig. 9. In this model, CPEs are constant phase elements with admittance, and L1 is associated with lead inductances [54e56]. The result showed that the values of Rs were approximately the same in all types of fuel components, which suggested that the ionic resistance of the electrolytes was fuel composition independent. As for Rp, it slightly increased when CO2 was fed into H2. The ratio of Rs/Rp decreased from 2.7 to 1.4 with the decreasing hydrogen concentration in fuel. Such high value of Rs is suggested to be an important reason of low Uf of the cell. Rp1 was about 4.5e15 times of Rp2. According to the analysis of Fig. 5, it is suggested that the activation polarization was dominant in the full-scale of current densities. If CH4 was introduced to the fuel system, Rp1 was enlarged. The visible increase of Rp1 in CH4eCO2eH2
Fig. 9 e Effect of hydrogen contents in gas on the resistances of the cell with low Uf at 800 C. The fitting equivalent circuit model was at the top in the figure.
ambience could be partly attributed to the limited activity of the anode towards methane [57]. The above results proved that the addition H2 reduced the polarization resistance of the cell using dry reforming methane. Fig. 10 showed the Tafel profiles of the cell and the corresponding exchange current density i0. It was indicated that feeding H2 into CO2 þ CH4 could improve i0 of the cell linearly. The electrochemical oxidization of methane to CH3 on nickel is the slowest step of absorbed hydrogen production from methane based on the data in the work of Hecht et al. [58]. And the research of Kanle et al. [39] indicated that addition of hydrogen into dry reforming methane could modify the reaction pathway of the conversion of methane: more CH3 was produced. As a result, the activation polarization resistance of anode reaction could be suppressed. And it is interesting that diluting hydrogen with CO2 could also improve i0 of the cell. The activation polarization DVact is a function of i0, which is as follows [59]: DVact fln
i i0
(8)
This implies that CO2 could reduce the activation polarization of the electrochemical oxidation of fuel gases. The experiments [60e62] and model analysis results [63] suggested that water had activating effect on the anode kinetics due to the equilibrium-potential effect. Water vapor formed through the reverse reaction of water gas shift before the electrooxidation step according to the former analysis result of Fig. 7. As a result, the generated water vapor raised the exchange current density.
Cell with high fuel utilization
Fig. 8 e Impedance plots of the cell with low Uf at 800 C for various fuel gases atmospheres.
The value of Uf was optimized to be around 1.9e7.6% for this cell, which was about 10 times of the former cell. Fig. 11 showed the voltageecurrent and power output-current data of the cell at 800 C. The shape of voltageecurrent curve implied the dominant polarization step transferred with the
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Fig. 10 e (a) Tafel plots and (b) exchange current densities of the cell with low Uf using different fuel gas at 800 C. increase of current densities, which was different from that in Fig. 5. Furthermore, concentration polarization should be dominant at high current densities, when the mass-transfer from the anode chamber to the TPB zone became a limiting step for the anodic process. Also, the change of power density with varied hydrogen concentration in fuel gas was different from the cell with low fuel utilization. The addition of CO2 into hydrogen obviously depressed the power density of the cell. Besides, the gradual replacement of hydrogen with methane had limited influence on the cell power generation. Bruce suggested that high current density improved the oxidation of hydrocarbon to CO2 and H2, of which CO2 production can only increase sharply when the lattice oxygen stoichiometry of anode increased to a certain value [64]. His result implied that the oxidation of absorbed C* could contribute to the power generation only if the cell's Uf was elevated high enough. That was to say, oxidation of H* which was from CH4 and H2 dominated the electrochemical reaction on TPB of anode at low Uf value. Plots of the maximum of power density and OCV for the SOFC were shown in Fig. 12. The calculated EMF in Table 1 was also presented in this figure. The trends of EMF and OCV were similar, except for the values in pure hydrogen. The results implied that OCV of cell was depressed more or less in methane or carbon dioxide containing fuels. On the other
Fig. 11 e Fuel utilization (Uf) of two kinds of cells used in this work at 0.7 V.
hand, the increase of OCV with increasing CH4 concentration in fuel gas indicated methane was relatively electrochemical active compared with the former kind of cell. Although the two kinds of cells were different from each other, the experimental results (Fig. 13) indicated that both of feeding with carbon dioxide into hydrogen and replacement of methane with hydrogen were beneficial to improving fuel utilization of them. Raman spectrograph was employed to detect the cell anode after operation. As Fig. 14 shown, there were two weak bands at 1580 cm1 and 1350 cm1 which were corresponding to the G and D bands of carbonaceous materials [65e68]. It was indicated that a few of carbon deposited on the anode. The deposited carbon can raise the impedance of anodic reaction [69]. But it was not found any unstable electrochemical performance during the operation, which implied that the deposited carbon has limited influence on the cell. In fact, slight deposited carbon could be electro-oxidized in the operation [70]. A further observation found that the value of band intensity ratio of D to G bands, I(D)/I(G), was about 0.85. D3 band around 1500 to 1550 cm1, the broad band between D and G bands, indicated numbers of amorphous sp2-bonded forms were contained in the deposited carbon. And the obvious low-wavenumber-tail of D band (D4 band at
Fig. 12 e Influence of methane concentration of H2eCO2eCH4 mixed gases on terminal voltage and power density of the cell with high Uf at 800 C.
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Fig. 13 e Open circuit voltage (OCV), theoretical electromotive force (EMF) and maximum power density of the cell with high Uf at 800 C using H2eCO2eCH4 fuel.
Fig. 14 e Raman spectrum of the cell with high Uf after operation at 800 C.
1150 cm1) appears only in very poorly organized carbon materials [67,68]. It implied that the deposited carbon was structural disordered [65,66] which formed at low temperature. Experimental results of Chao et al. [71] showed carbon with the similar structures formed at 600 C rather than 800 C. It was suggested that a closer components of fuel gas to the carbon deposition boundary at a decreased temperature [72] could result an overstep of it to coking zone after operation. A difference of the impedance spectra of the cells with different Uf could also be noticed from Figs. 8 and 15. The values of intercepts of the spectra in Fig. 15 were obviously smaller than the former. Similarly, Takano et al. [46] suggested that mass-transfer in SOFC depended on fuel utilization: the value of corresponding resistance became smaller when Uf increase from 10 to 50% in hydrogen fuel atmosphere. Usually, two arcs or one arc with a low frequency tail were included in Fig. 15. It could be noticed that the impedance
Fig. 15 e Impedance plots of the cell with high Uf at 800 C for various fuel atmospheres.
became larger as hydrogen concentration approached either 0 or 100 vol%, and reached a minimum value in the fuel atmosphere with the highest CO2 concentration. The higherfrequency and lower-frequency parts were simulated by the model in Fig. 16, the same simulation as it in Fig. 9. It can be observed from Fig. 16 that Rs was unaffected by the variation of fuel gas compositions. The unchanged Rs in both the two kinds of cells confirmed that no carbon deposited in operation, since carbon deposition always result increasing Rs [69,73]. The values of Rp1 were similar to Rp2, which varied from 0.2 to 1.5 U. By the way, a missing data of Rp2 at 10 vol% hydrogen concentration is attributed to a large error of it in the simulation. The values of Rp1 and Rp2 both increased when the hydrogen concentration decreased from 40 vol% to 0, which kept consistent with Fig. 9. Usually, high hydrogen concentration improves the mass transfer of fuel due to the small molecule diameter, which will depress the concentration polarization resistance as it did in the range of 0e40 vol%. However, both Rp1 and Rp2 became larger when the hydrogen
Fig. 16 e Effect of hydrogen contents in gas on the resistances of the cell with high Uf at 800 C. The fitting equivalent circuit model was at the top in the figure.
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Fig. 17 e (a) Tafel plots and (b) exchange current densities of the cell with high Uf using different fuel gas at 800 C. concentration increased from 40 to 100 vol%. The result suggested that the composition of No. 3 fuel was beneficial to suppressing polarization of anode. The reason of activation polarization depression in CO2 diluted H2 was expounded as before. It could be noticed that in the cell with high Uf value, concentration polarization at anode was obviously depressed by CO2 feeding. According to the following equation [74]: p0H RT 1þ 0 2 Rca ð0Þ ¼ 2ias F pH2 O
2)
! (9)
where, Rca(0) is the anodic concentration polarization at OCV, ias is the anode-limiting current density, p0H2 and p0H2 O are the partial pressure of hydrogen and water vapor outside of anode, respectively. Eq. (9) suggested that the moister the feeding hydrogen gas, the higher was the Rca(0). Thus, the improvement of mass transfer in H2eCO2 fuel gas was also contributed to the water vapor which was generated from the reaction of H2 and CO2. The Tafel profiles of the cell and the corresponding i0 were plotted in Fig. 17. The trend of the i0 value to hydrogen concentration was similar to Fig. 10: both of feeding with carbon dioxide into hydrogen and replacement of methane with hydrogen were beneficial to a high value of i0. But the variation of i0 of the cell with high Uf was more obvious. It kept consistent with the significant change of Rp1 in Fig. 16. Comparing with Fig. 11, it could be noticed that i0 increased with the increasing Uf in one cell. Moreover, both the mass transfer and charge transfer on the anode were improved with the increasing Uf.
Conclusion We studied the effects of CO2 and H2, two of the principal compositions in the anode-off gas of SOFC, to the performance of simulative exhaust gas reforming methane SOFC in the current study. The results are as following: 1) Performance of SOFC with Ni/YSZ porous anode was measured when using CO2 diluted H2 fuels, and H2 diluted dry reforming CH4 fuels. Cells with different fuel utilization factor (Uf)wereemployedintheexperiments.PowerdensityandOCV of the dry reforming methane SOFC with low Uf value could be enhanced by H2 feeding. But H2 has limited effect on the power
3)
4)
5)
density of the cell with high Uf value. It was suggested that absorbed C* on anode rarely participated electrooxidation process at low Uf value, which resulted the inhibition of the electron transfer from oxygen ions to methane. The enthalpy of relaxation of gas composition to the equilibrium state was linearly depressed with increasing H2 concentration in dry reforming methane, which implied the addition of H2 could mitigate the temperature gradient of dry reforming methane fueling SOFC stack. Regard of the Uf value of SOFC, the mass transfer, charge transfer, and the exchange current density of the cell were improved with the increasing hydrogen concentration in CO2 reforming methane. A small molecular size of H2 result the relative low concentration polarization. The suppression of activation polarization could be contributed to be two reasons: relatively high activity of H2 in electrooxidation comparing to methane and the increasing concentration of CH3 with the help of H2. The cell performance decreased by the CO2 dilution in the H2eCO2 fuels. The mass transfer, charge transfer on the anode and the exchange current density of the cell could be improved by diluting hydrogen with CO2. The evolve gas result suggested that the reverse reaction of water gas shift produced water vapor, which could depress the polarization resistances of anode reaction. The mass transfer, charge transfer on the anode and the exchange current density of the cell were all improved with the increasing value of Uf in one cell.
Acknowledgments The authors gratefully acknowledge the financial support of National Program on Key Basic Research Project of China (Grant No. 2012CB215400), China Postdoctoral Science Foundation (No. 2015M570036), and National Nature Science Foundation of China (No. 51474141). The authors thank Ms. Dan-Tang and Dr. Teng-Long Zhu for material supply.
Appendix A. Supplementary data Supplementary data related to this article can be found at http://dx.doi.org/10.1016/j.ijhydene.2016.03.090.
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Effect of hydrogen and carbon dioxide on the performance of methane fueled solid oxide fuel cell Zhiyuan Chen a,c, Liuzhen Bian a,b, Lijun Wang a,b,*, Ning Chen c, Hailei Zhao c, Fushen Li c, KuoChih Chou a,b a
State Key Laboratory of Advanced Metallurgy, University of Science and Technology Beijing, Beijing 100083, PR China b School of Metallurgical and Ecological Engineering, University of Science and Technology Beijing, Beijing 100083, PR China c School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing 100083, PR China
article info
abstract
Article history:
Recycling and reusing of exhaust gas from solid oxide fuel cell (SOFC) could enhance the fuel
Received 19 March 2015
utilization of methane. In the present work, the effects of two main components of exhaust
Received in revised form
gas, viz. hydrogen and carbon dioxide, on the performance of exhaust gas reforming methane
28 February 2016
fueled cell were investigated. The cell with Ni/8 mol% Y2O3 doped zirconia (Ni/YSZ) as the
Accepted 12 March 2016
anode was exposed to H2eCO2eCH4 atmospheres at 800 C. DC techniques and impedance
Available online 10 April 2016
spectroscopy was used for characterization. Two kinds of cells with different fuel utilization
Keywords:
density/voltage with current density. The results indicated that electrooxidation of H*
Methane
dominated the electrochemical reaction on the anode of the cell at low Uf value, while
Solid oxide fuel cell
electrooxidation of C* becomes important at high Uf value. In the case of CO2eH2 mixtures,
factors (Uf) were used in experiments, which represented different relationship of power
Hydrocarbon fuels
CO2 improved the exchange current density, raised the value of Uf, and depressed anodic
Carbon dioxide reforming
activation polarization. As for H2eCH4eCO2 system, H2, partly replacing CH4, reduced the
Exhaust gas reforming
heat loss in dry reforming, improved the exchange current density, raised Uf value, and
Anode-off gas
depressed the activation and concentration polarization resistances of Ni/YSZ anode. But it had limited effect on the power density of the cell with high Uf value. Copyright © 2016, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Introduction The increasing interest in developing solid oxide fuel cell fed (SOFC) with hydrocarbon such as methane (CH4) is originating due to the possibility of using a wider energy source for clean electricity generation and effective reducing gas emissions.
Nickel/yttrium-stabilized zirconia (Ni/YSZ) porous ceramic is the most widely used material for SOFC anode. But the problem in this case is the dissociation of methane on the surface of nickel particles can cause carbon deposition through the following reaction: CH4 /C þ 2H2
(1)
* Corresponding author. School of Metallurgical and Ecological Engineering, University of Science and Technology Beijing, Beijing 100083, PR China. Tel.: þ86 10 6233 3622; fax: þ86 6233 2570. E-mail address: [email protected] (L. Wang). http://dx.doi.org/10.1016/j.ijhydene.2016.03.090 0360-3199/Copyright © 2016, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
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which results in poor performance and eventual failure of SOFC. The other two carbon producing reactions (Boudouard reaction and CO oxidization by H2) have little effects on carbon deposition, because the standard Gibbs free energies of the two reactions were positive above ~700 C [1,2]. Therefore, CH4 is the major source of deposited carbon at high temperatures. To inhibit carbon deposition, several reforming methods have been investigated, including steam reforming (SR) [3,4], partial oxidation (POx) [5,6], autothermal reforming (ATR) [7,8] and dry reforming (DR). Among these methods, carbon dioxide (dry) reforming in the SOFC system [9e12] is an environment friendly process. CO2 can be recycled from the engine exhaust or anode-off gas in practical system [13]. In recent years, the exhaust gas recirculation of engine, especially natural gas fueled engine has been widely studied [14,15]. Researchers have been paying attention to engine/ anode-exhaust gas reusing with SOFC, with main focus on the design of catalyst, blueprint of recycling system and reforming optimization [16e21]. It has been proved that the exhaust gas reforming is the most preferable way for SOFC fuel feeding from a thermodynamic view point [22]. Kasraet et al. [23] reported that a proper level of gas recycling would increase current density of the SOFC. But researchers preferred to study the effect of steam [18,19,24] and partial oxidation reforming [20] in exhaust gas recycling system. Since recycling gas usually contains a significant percentage of steam [25], steam reforming was always treated as primary strategy. On the contrary, CO2 was considered to be a negative factor in the reusing of the exhaust gas [19,24]. However, it was noticed that the electric efficiency in the steamcontaining anode-off gas recycling mode was higher than in SR and POx without gas recycling [25,26]. In the former mode, CO2, H2 and CO were introduced into the anode, which could be beneficial to the electrochemical reaction. Effort also was made to improve the efficiency of CO2 reforming in reusing process, where proton conductor was used [27], several systems were designed [28] towards this end. Furthermore, the present work is focused to provide an approach to examine the effect of CO2 on exhaust gas reforming process. In order to conveniently understand the effect of each component in off-gas system, the strategy here was to simplify gas mixture compositions. Thus only CO2 and H2 were introduced in this experiment rather than complex compositions gas injection in the earlier works [25,29,30]. Moreover, considering that high content of steam in anode-off gas accelerates the corrosion of the interconnector materials [31e33] and sintering of Ni particles in the anode [34,35], the present authors suggest that steam should be removed before reusing of the exhaust gas. As Fig. 1 shows, CO2 and H2 in fuel cell exhaust gas are recycled into the anode chamber and reused. The reforming reaction in anodic chamber in Fig. 1 is as following: CH4 þ CO2 ¼ 2CO þ 2H2 ;
DHQ 298 ¼ 247 kJ=mol
(2)
The presence of carbon dioxide is beneficial to suppressing the carbon deposition as shown in Eq. (3). But the carbon dioxide reforming usually decreased open circuit voltage (OCV) [36] and the open power density of the cell [13]. Staniforth and Ormerod [11] suggested that the power production reached
the maximum at 45 vol% CH4 in dry reforming fuel, with a maximal production of CO and H2 through internal reforming at the same time. It shows a potential of optimizing fuel gas composition to improve the electrochemical performance. C þ CO2 ¼ 2CO
(3)
Besides, the concentration of hydrogen in the recycled exhaust gas lies in a wide range [20,37,38]. Here, hydrogen is one of the products in reforming rather than an additional supplement from other sources, so it is a promising reusing power source without extra cost. Reusing hydrogen in methane fuel cell has a potential to improve cell's performance. For instance, hydrogen can suppress carbon formation and growth over the Ni/YSZ anodes by promoting the reverse of Eq. (1) [38e42]. Moreover, Nikooyeh et al. [38] reported that the Ni/YSZ anode exposed to methane in the presence of hydrogen, rather than oxygen, would result in less damage to the anode microstructure in the elimination of deposited carbon. Yet the disadvantage is that hydrogen inhibits the reforming rate via Eq. (2) [43,44]. Studies are needed to evaluate the effect of CO2 and H2 in recycled exhaust gas to the fuel cell's performance, which can promote exhaust gas to be a potential fuel resource for SOFC. As mentioned above, CO2 and H2 are the main components in the exhaust gas of SOFC. And both of them were expected to be beneficial to the electrochemical oxidation of methane. In order to observe the interaction of CO2 and H2, the present work was carried out to examine the electrochemical properties of the SOFC which was fed with H2eCH4eCO2 at 800 C.
Materials and methods Anode-supported cells with a diameter of 12 mm were used in the present experiments. Cells with different fuel utilization factor (Uf) were used in the experiments. The first one was with low fuel utilization factor, of which the electrolyte was 8 mol% Y2O3 doped zirconia (YSZ) with a thickness of about 35 mm. The porous anode with a thickness of about 400 mm had a Ni:YSZ molar ratio of 1:1. The porosity of the anode was 25.08%. The porous cathode of area 0.3 cm2 and a thickness of about 100 mm was made of 8 mol% Y2O3 doped zirconia & Sm0.5Sr0.5CoO3 (YSZ/SSC). After structure optimization, the other cell was with relatively high fuel utilization factor, of which the area of the Gd0.1Ce0.9O2d/Bao.5Sr0.5Co0.8Fe0.2O3d (GDC/BSCF) porous cathode was 0.26 cm2. A GDC barrier was between the cathode and YSZ electrolyte. The cell was mounted at the end of an alumina tube. Ag paste was painted on the anode edge and fired at 600 C, which was used as current collector and seal. A ceramic paste seal was used to cover the silver paste layer. Silver wires were fixed with a drop of silver paste on the top of the cathode and used as current collectors. The anode was exposed to the fuel gas, while the cathode was in contact with flowing air. The total fuel flow was maintained at 100 ml/min. The fuel gases were introduced into the cell and their flow rates were controlled using Alicat mass flow controllers. The compositions of the fuel gas are shown in Table 1. The evolve gas of the internal carbon dioxide reforming SOFC was analyzed by mass spectra system
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Fig. 1 e Scheme of exhaust gas reforming.
(Hiden HPR20 Research Grade System for Continuous Sampling Gas/Vapour Analysis). The electrochemical characterization of the cell was carried out by electrochemical impedance spectroscopy (EIS) using an electrochemical workstation (Princeton PARSTAT 2273). Measurements were recorded in the frequency range of 2 MHze0.1 Hz using voltage amplitude of 5 mV at a temperature of 800 C. The currentevoltage characteristic of the cell was also measured using linear sweep voltammetry at a sweep rate of 1 mV$s1. After measurement, the cell microstructure was studied by scanning electron microscopy (SEM, CARL ZEISS EVO MA 10/ LS 10 JS) with energy dispersive spectrum (EDS, Thermo NORAN System). The used cell was tested by temperature programing oxidation (TPO) and Raman spectrum (Horyba LabRAM HR Evolution), to detect whether carbon deposited on the anode in the operation. The same mass spectra system was employed to analyze evolved gas in TPO test.
Results and discussions
A comparison of the present experimental results to the theoretical values is helpful to the analysis, even though equilibrium is hardly achieved in practice [25,30]. Fig. 2 and Table 1 showed the thermochemical calculation results obtained from FactSage 6.2 with the free energy minimization method. Table 1 showed that the oxygen partial pressures of the fuel gas, which was related to the theoretical electromotive force (EMF) of the cell, decreased with the increasing contents of H2 and CH4 in the fuel. The oxygen partial pressure decreased with the decreasing hydrogen concentration in methane-containing fuel, since the oxidation potential of methane is higher than hydrogen at 800 C. The electromotive force of the cell was calculated out from the oxygen partial pressure of fuels by the following equation: EMF ¼
RT 1 ln fuel 4F PO2
! (4)
where R, P and F are the gas constant, partial pressure of gas and Faraday constant, respectively. T is the operation temperature.
Cell with low fuel utilization factor Low utilization factor implied that electrochemical reactions will not significantly change the composition of fuel gas. Usually, Uf was calculated from the molar flow rate of fuel at inlet and outlet [45]. In our study, it was calculated by the following equation [37,46]: Uf ¼
I nel n_FI F
where, I is appointed to be the current value at 0.7 V. F is faraday constant, nel is the molar flow of electrons, and n_FI is the fuel molar flow at the cell's inlet. The calculation results shows that the value of Uf at this situation was around 0.06e0.63%.
Fig. 2 e Chemical equilibrium diagram of CH4eH2eCO2 system at 800 C.
Table 1 e Compositions of 100 mol feeding fuel gas in equilibrium state. No.
1 2 3 4 5 6
Fed gas (mol)
Gas composition in equilibrium (mol)
CH4
CO2
H2
H2
CH4
H2O
CO
CO2
O2
0 0 0 15 30 40
0 40 60 60 60 60
100 60 40 25 10 0
100.00 36.46 16.53 36.53 56.85 39.16
0.00 0.03 0.00 0.05 0.25 0.37
0.00 23.48 23.46 18.38 12.64 4.88
0.00 23.42 23.46 48.28 72.14 48.91
0.00 16.55 36.54 26.67 17.61 6.69
0 1.81$1017 8.79$1017 1.44$1017 3.45$1018 6.78$1019
EMF (V) 0.9778 0.9977 0.9611 1.009 1.047 1.074
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The value of DH can be used to estimate the thermal effect in the methane reforming process. Fig. 3 showed the enthalpy change of gas from the preset composition to thermodynamic equilibrium state. The value of enthalpy change per mole of fuel gas linearly dropped with the increase of hydrogen content in carbon dioxide reform methane. It could be noticed that feeding CO2 in hydrogen fuel gas also could cool down the cell slightly. The enthalpy change of the relaxation process implied that hydrogen was effective in the depression of the temperature gradient in SOFC stack. The possibility of carbon deposition in a certain ambient atmosphere can be predicted from thermodynamics calculations. As Fig. 2 shown, compositions of fuel gas were controlled within the non-coking zone at 800 C. Thus, there should be no coking on anode of SOFC. Fig. 2 shows that No. 5 fuel is the nearest composition to the carbon deposition boundary. After operation in No. 5 fuel, the cell was probed by SEM/EDS and TPO. Fig. 4(a) and (b) showed the SEM photos of the anode of the cells before and after the test at 800 C. The structure of the cell had not obviously changed. Significant carbon deposition was not found by EDS detection yet. Besides, the TPO result shown in Fig. 4(c) indicated that no carbon was detected in the used SOFC fragment. CO2 and H2 in the atmosphere inhibited the deposition of carbon and this is in conformity with thermodynamic analysis. Fig. 5 illustrated voltageecurrent and power outputcurrent data of the SOFC at 800 C. All of the voltages linearly decreased with increasing current density of cell. It implied that the dominant polarization loss step of electrochemical process in the cell kept the same within all the scope. The power density could be significantly improved by substituting some CH4 to H2. And it was found that the power density of cell did not obviously depress with 40 vol% CO2 feeding into H2. Even the depression of the power density in 60 vol% CO2 containing hydrogen was limited. It implied that dilution of hydrogen with certain amount of CO2 could not affect the power output of the SOFC with low Uf value. Fig. 6 was obtained by plotting the OCV and maximum of power density for the SOFC as function of hydrogen concentration. It
Fig. 3 e DH of the relaxation process of 1 mol fed fuel gases listed in Table 1.
was found that the change of OCV in H2eCO2 atmosphere did not follow the predicted value in Table 1. Here, the cell reaction can be represented as
Fig. 4 e Cross-sectional SEM images of the SOFC with low Uf (a) before and (b) after operation in No. 5 fuel gas at 800 C, and (c) CO2 partial pressure of evolved gas in the TPO test of the SOFC fragment after operation.
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Fig. 5 e Influence of methane concentration of H2eCO2eCH4 mixed gases on terminal voltage and power density of the cell with low Uf at 800 C. 1 H2 þ O2 ¼ H2 O 2
(5)
As the hydrogen was diluted with carbon dioxide (fuel gas No. 1 to 3 in Table 1), the voltage decreased according to the relationship in the form of Nernst equation: E ¼ EðH2 Þ
RT PH O ln 1=2 2 2F PO2 PH2
! (6)
where, EðH2 Þ is the theoretical potential of reaction (5) which is 0.9778 V at 800 C. Moreover, the water vapor in the anode chamber at the open state is supposed to be in the standard state. It is admitted that the present analysis is based on some assumptions which could form the first level of approximation. In fact, limited fuel utilization in a SOFC would reduce the Nernst voltage of the practical system. And equation (6) did not consider the partial pressure of carbon dioxide. Fig. 6 showed that the OCV consistent with Nernst equation (6). It indicated that carbon dioxide cannot obviously affect the OCV of the cell with low Uf value. Although the theoretic potential of methane oxidation is higher than that of hydrogen oxidation at 800 C, Fig. 6 showed that the OCV under methane-containing atmosphere was not
Fig. 6 e Open circuit voltage (OCV) and maximum power density of the cell with low Uf at 800 C using H2eCO2eCH4 fuel. The curve was the fitting result of Eq. (6).
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higher than the extended dashed line of Equation (6). This was not consistent with the order of EMF values of these fuel gases in equilibrium shown in Table 1. The low OCV under methane containing atmosphere may be attributed to be the difference of electrocatalytic activity of Ni/YSZ anode with respect to the hydrogen and methane [47e49]. Furthermore, this relatively low electrocatalytic activity implied that the oxidation of absorbed C* was difficult within the three-phase boundary (TPB) zone. Evolve gas composition which was cooled to room temperature was analyzed using mass spectrum at open circuit (Fig. 7). In this analysis, the molecular fragment of the particle was approximately regarded as the molecular itself. H2O was in equilibrium state at room temperature, and the peak corresponding to N2 gas which accompanied H2 as impurity was the same as CO (mass 28). The invisible peak of O2 could confirm that there was no obvious gas leakage in the fuel cell seals. The spectrum shows the gas composition did not agree well with the equilibrium prediction given in Table 1. For example, CH4 did not decrease to the extent shown in Table 1. And the detected CO content was below the predicted value. Thus, the fuel gas was not in an equilibrium state, which was possibly because of the flow geometry of the SOFC test [50] and the slow catalytic rate of CH4 reforming. The detected CO in H2eCO2 atmosphere implied that the reverse reaction of water gas shift occurred (As Eq. (7) shown, H2O and CO were the products of this reversible reaction [51]). The equilibrium constant of water gas shift is equal to 1.07 (kPa)0 at 800 C [1], which is close to unity. Hence, the reverse reaction could be possible at 800 C is possible according to thermodynamic analysis. CO þ H2 O ¼ CO2 þ H2
(7)
In summary, the mass spectra result in Fig. 7 indicated that CO and H2O were produced by the reverse reaction of the water gas shift reaction. However, carbon based compositions had limited effect on the performance of the cell. It implied that the contribution of electrooxidation of absorbed C* on anode was limited to the power generation. The electrooxidation of H* dominated the electrochemical reaction on anode.
Fig. 7 e Exhaust gas compositions of the cell with low Uf detected by mass spectrometer.
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Fig. 8 showed the impedance spectra measured at open circuit situation. It was found that the impedance arc expanded significantly when CH4 was introduced to the anode. But it was almost identical in case of hydrogen diluted only with CO2. The real-axis intercept, Rs, at high frequencies represents the ohmic resistance of the electrolyte and lead wires. The difference between the high and low frequency intercepts with the real axis, Rp, is the polarization resistance of the cell, which is the sum of Rp1 and Rp2. Rp1 at higherfrequency part of the arc corresponds to the charge transfer, Rp2 at the lower-frequency part corresponds to the mass transfer [52,53]. The analysis result of the spectra and the corresponding equivalent circuit model were drawn in Fig. 9. In this model, CPEs are constant phase elements with admittance, and L1 is associated with lead inductances [54e56]. The result showed that the values of Rs were approximately the same in all types of fuel components, which suggested that the ionic resistance of the electrolytes was fuel composition independent. As for Rp, it slightly increased when CO2 was fed into H2. The ratio of Rs/Rp decreased from 2.7 to 1.4 with the decreasing hydrogen concentration in fuel. Such high value of Rs is suggested to be an important reason of low Uf of the cell. Rp1 was about 4.5e15 times of Rp2. According to the analysis of Fig. 5, it is suggested that the activation polarization was dominant in the full-scale of current densities. If CH4 was introduced to the fuel system, Rp1 was enlarged. The visible increase of Rp1 in CH4eCO2eH2
Fig. 9 e Effect of hydrogen contents in gas on the resistances of the cell with low Uf at 800 C. The fitting equivalent circuit model was at the top in the figure.
ambience could be partly attributed to the limited activity of the anode towards methane [57]. The above results proved that the addition H2 reduced the polarization resistance of the cell using dry reforming methane. Fig. 10 showed the Tafel profiles of the cell and the corresponding exchange current density i0. It was indicated that feeding H2 into CO2 þ CH4 could improve i0 of the cell linearly. The electrochemical oxidization of methane to CH3 on nickel is the slowest step of absorbed hydrogen production from methane based on the data in the work of Hecht et al. [58]. And the research of Kanle et al. [39] indicated that addition of hydrogen into dry reforming methane could modify the reaction pathway of the conversion of methane: more CH3 was produced. As a result, the activation polarization resistance of anode reaction could be suppressed. And it is interesting that diluting hydrogen with CO2 could also improve i0 of the cell. The activation polarization DVact is a function of i0, which is as follows [59]: DVact fln
i i0
(8)
This implies that CO2 could reduce the activation polarization of the electrochemical oxidation of fuel gases. The experiments [60e62] and model analysis results [63] suggested that water had activating effect on the anode kinetics due to the equilibrium-potential effect. Water vapor formed through the reverse reaction of water gas shift before the electrooxidation step according to the former analysis result of Fig. 7. As a result, the generated water vapor raised the exchange current density.
Cell with high fuel utilization
Fig. 8 e Impedance plots of the cell with low Uf at 800 C for various fuel gases atmospheres.
The value of Uf was optimized to be around 1.9e7.6% for this cell, which was about 10 times of the former cell. Fig. 11 showed the voltageecurrent and power output-current data of the cell at 800 C. The shape of voltageecurrent curve implied the dominant polarization step transferred with the
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Fig. 10 e (a) Tafel plots and (b) exchange current densities of the cell with low Uf using different fuel gas at 800 C. increase of current densities, which was different from that in Fig. 5. Furthermore, concentration polarization should be dominant at high current densities, when the mass-transfer from the anode chamber to the TPB zone became a limiting step for the anodic process. Also, the change of power density with varied hydrogen concentration in fuel gas was different from the cell with low fuel utilization. The addition of CO2 into hydrogen obviously depressed the power density of the cell. Besides, the gradual replacement of hydrogen with methane had limited influence on the cell power generation. Bruce suggested that high current density improved the oxidation of hydrocarbon to CO2 and H2, of which CO2 production can only increase sharply when the lattice oxygen stoichiometry of anode increased to a certain value [64]. His result implied that the oxidation of absorbed C* could contribute to the power generation only if the cell's Uf was elevated high enough. That was to say, oxidation of H* which was from CH4 and H2 dominated the electrochemical reaction on TPB of anode at low Uf value. Plots of the maximum of power density and OCV for the SOFC were shown in Fig. 12. The calculated EMF in Table 1 was also presented in this figure. The trends of EMF and OCV were similar, except for the values in pure hydrogen. The results implied that OCV of cell was depressed more or less in methane or carbon dioxide containing fuels. On the other
Fig. 11 e Fuel utilization (Uf) of two kinds of cells used in this work at 0.7 V.
hand, the increase of OCV with increasing CH4 concentration in fuel gas indicated methane was relatively electrochemical active compared with the former kind of cell. Although the two kinds of cells were different from each other, the experimental results (Fig. 13) indicated that both of feeding with carbon dioxide into hydrogen and replacement of methane with hydrogen were beneficial to improving fuel utilization of them. Raman spectrograph was employed to detect the cell anode after operation. As Fig. 14 shown, there were two weak bands at 1580 cm1 and 1350 cm1 which were corresponding to the G and D bands of carbonaceous materials [65e68]. It was indicated that a few of carbon deposited on the anode. The deposited carbon can raise the impedance of anodic reaction [69]. But it was not found any unstable electrochemical performance during the operation, which implied that the deposited carbon has limited influence on the cell. In fact, slight deposited carbon could be electro-oxidized in the operation [70]. A further observation found that the value of band intensity ratio of D to G bands, I(D)/I(G), was about 0.85. D3 band around 1500 to 1550 cm1, the broad band between D and G bands, indicated numbers of amorphous sp2-bonded forms were contained in the deposited carbon. And the obvious low-wavenumber-tail of D band (D4 band at
Fig. 12 e Influence of methane concentration of H2eCO2eCH4 mixed gases on terminal voltage and power density of the cell with high Uf at 800 C.
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Fig. 13 e Open circuit voltage (OCV), theoretical electromotive force (EMF) and maximum power density of the cell with high Uf at 800 C using H2eCO2eCH4 fuel.
Fig. 14 e Raman spectrum of the cell with high Uf after operation at 800 C.
1150 cm1) appears only in very poorly organized carbon materials [67,68]. It implied that the deposited carbon was structural disordered [65,66] which formed at low temperature. Experimental results of Chao et al. [71] showed carbon with the similar structures formed at 600 C rather than 800 C. It was suggested that a closer components of fuel gas to the carbon deposition boundary at a decreased temperature [72] could result an overstep of it to coking zone after operation. A difference of the impedance spectra of the cells with different Uf could also be noticed from Figs. 8 and 15. The values of intercepts of the spectra in Fig. 15 were obviously smaller than the former. Similarly, Takano et al. [46] suggested that mass-transfer in SOFC depended on fuel utilization: the value of corresponding resistance became smaller when Uf increase from 10 to 50% in hydrogen fuel atmosphere. Usually, two arcs or one arc with a low frequency tail were included in Fig. 15. It could be noticed that the impedance
Fig. 15 e Impedance plots of the cell with high Uf at 800 C for various fuel atmospheres.
became larger as hydrogen concentration approached either 0 or 100 vol%, and reached a minimum value in the fuel atmosphere with the highest CO2 concentration. The higherfrequency and lower-frequency parts were simulated by the model in Fig. 16, the same simulation as it in Fig. 9. It can be observed from Fig. 16 that Rs was unaffected by the variation of fuel gas compositions. The unchanged Rs in both the two kinds of cells confirmed that no carbon deposited in operation, since carbon deposition always result increasing Rs [69,73]. The values of Rp1 were similar to Rp2, which varied from 0.2 to 1.5 U. By the way, a missing data of Rp2 at 10 vol% hydrogen concentration is attributed to a large error of it in the simulation. The values of Rp1 and Rp2 both increased when the hydrogen concentration decreased from 40 vol% to 0, which kept consistent with Fig. 9. Usually, high hydrogen concentration improves the mass transfer of fuel due to the small molecule diameter, which will depress the concentration polarization resistance as it did in the range of 0e40 vol%. However, both Rp1 and Rp2 became larger when the hydrogen
Fig. 16 e Effect of hydrogen contents in gas on the resistances of the cell with high Uf at 800 C. The fitting equivalent circuit model was at the top in the figure.
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Fig. 17 e (a) Tafel plots and (b) exchange current densities of the cell with high Uf using different fuel gas at 800 C. concentration increased from 40 to 100 vol%. The result suggested that the composition of No. 3 fuel was beneficial to suppressing polarization of anode. The reason of activation polarization depression in CO2 diluted H2 was expounded as before. It could be noticed that in the cell with high Uf value, concentration polarization at anode was obviously depressed by CO2 feeding. According to the following equation [74]: p0H RT 1þ 0 2 Rca ð0Þ ¼ 2ias F pH2 O
2)
! (9)
where, Rca(0) is the anodic concentration polarization at OCV, ias is the anode-limiting current density, p0H2 and p0H2 O are the partial pressure of hydrogen and water vapor outside of anode, respectively. Eq. (9) suggested that the moister the feeding hydrogen gas, the higher was the Rca(0). Thus, the improvement of mass transfer in H2eCO2 fuel gas was also contributed to the water vapor which was generated from the reaction of H2 and CO2. The Tafel profiles of the cell and the corresponding i0 were plotted in Fig. 17. The trend of the i0 value to hydrogen concentration was similar to Fig. 10: both of feeding with carbon dioxide into hydrogen and replacement of methane with hydrogen were beneficial to a high value of i0. But the variation of i0 of the cell with high Uf was more obvious. It kept consistent with the significant change of Rp1 in Fig. 16. Comparing with Fig. 11, it could be noticed that i0 increased with the increasing Uf in one cell. Moreover, both the mass transfer and charge transfer on the anode were improved with the increasing Uf.
Conclusion We studied the effects of CO2 and H2, two of the principal compositions in the anode-off gas of SOFC, to the performance of simulative exhaust gas reforming methane SOFC in the current study. The results are as following: 1) Performance of SOFC with Ni/YSZ porous anode was measured when using CO2 diluted H2 fuels, and H2 diluted dry reforming CH4 fuels. Cells with different fuel utilization factor (Uf)wereemployedintheexperiments.PowerdensityandOCV of the dry reforming methane SOFC with low Uf value could be enhanced by H2 feeding. But H2 has limited effect on the power
3)
4)
5)
density of the cell with high Uf value. It was suggested that absorbed C* on anode rarely participated electrooxidation process at low Uf value, which resulted the inhibition of the electron transfer from oxygen ions to methane. The enthalpy of relaxation of gas composition to the equilibrium state was linearly depressed with increasing H2 concentration in dry reforming methane, which implied the addition of H2 could mitigate the temperature gradient of dry reforming methane fueling SOFC stack. Regard of the Uf value of SOFC, the mass transfer, charge transfer, and the exchange current density of the cell were improved with the increasing hydrogen concentration in CO2 reforming methane. A small molecular size of H2 result the relative low concentration polarization. The suppression of activation polarization could be contributed to be two reasons: relatively high activity of H2 in electrooxidation comparing to methane and the increasing concentration of CH3 with the help of H2. The cell performance decreased by the CO2 dilution in the H2eCO2 fuels. The mass transfer, charge transfer on the anode and the exchange current density of the cell could be improved by diluting hydrogen with CO2. The evolve gas result suggested that the reverse reaction of water gas shift produced water vapor, which could depress the polarization resistances of anode reaction. The mass transfer, charge transfer on the anode and the exchange current density of the cell were all improved with the increasing value of Uf in one cell.
Acknowledgments The authors gratefully acknowledge the financial support of National Program on Key Basic Research Project of China (Grant No. 2012CB215400), China Postdoctoral Science Foundation (No. 2015M570036), and National Nature Science Foundation of China (No. 51474141). The authors thank Ms. Dan-Tang and Dr. Teng-Long Zhu for material supply.
Appendix A. Supplementary data Supplementary data related to this article can be found at http://dx.doi.org/10.1016/j.ijhydene.2016.03.090.
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