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H2 and CO oxidation process at the three-phase boundary of Cu-ceria cermet anode for solid oxide fuel cell
Journal of Power Sources 345 (2017) 165e175
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H2 and CO oxidation process at the three-phase boundary of Cu-ceria cermet anode for solid oxide fuel cell Minghao Zheng, Shuang Wang, Mei Li, Changrong Xia* Key Laboratory of Materials for Energy Conversion, Chinese Academy of Sciences, Department of Materials Science and Engineering & Collaborative Innovation Center of Suzhou Nano Science and Technology, University of Science & Technology of China, No. 96 Jinzhai Road, Hefei, Anhui Province, 230026, PR China
h i g h l i g h t s Cu is a synergistic catalyst for H2/CO oxidation on doped ceria catalyst. H2/CO oxidation takes place at Cu-ceria-gas 3 PB rather than ceria-gas 2 PB. Much faster kinetics is demonstrated for CO than H2 oxidation on Cu-ceria. Rate limiting steps are proposed for H2/CO oxidation at Cu-ceria-gas 3 PB.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 24 October 2016 Received in revised form 5 January 2017 Accepted 29 January 2017
Cu-ceria cermets have been widely investigated as the anode materials for solid oxide fuel cells (SOFCs) that operated with hydrocarbon fuels. However, the anode reaction processes are not clear yet, especially those at the ceria-Cu-gas three phase boundary (3 PB). This work investigates samaria-doped ceria (SDC)-Cu-gas 3 PB reaction kinetics for the oxidation of H2 and CO, the products from hydrocarbons via external and internal reforming. Electrochemical conductivity relaxation measurement demonstrates that Cu is a synergistic catalyst that can significantly increase the reaction rate. The reaction at 3 PB contributes 81.3/66.8% of H2/CO oxidation when 5.4% SDC surface is covered with Cu particles. Combining with AC impedance analysis, elementary steps are proposed for the reaction at 3 PB. Water vapor combining to oxygen vacancy and carbon monoxide transforming to carbonate are the ratedetermining steps for the oxidation of H2 and CO, respectively. Cu-SDC has shown much higher catalytic activity, i.e. about fivefold reaction rate, for the oxidation of CO than H2. In addition, Cu-SDC electrodes exhibit lower interfacial polarization resistance and lower activation energy for the electrochemical oxidation of CO than H2. Consequently, CO is easier to be oxidized than H2 when the Cuceria anode is fueled with syngas, the reforming product from hydrocarbons. © 2017 Elsevier B.V. All rights reserved.
Keywords: Solid oxide fuel cell Anode reaction kinetics Three phase boundary Cu-ceria cermet Hydrogen oxidation CO oxidation
1. Introduction Solid oxide fuel cells (SOFCs) are gaining great momentum towards commercialization because of their distinguished advantages of high energy conversion efficiency and great fuel flexibility. The electrochemical oxidation via oxygen ions at the anode side makes it feasible to feed SOFCs with hydrocarbons such as petroleum, coal, and biogas. When using hydrocarbons as the fuel, it is usually assumed that SOFCs must be operated on syngas, a mixture
* Corresponding author. E-mail address: [email protected] (C. Xia). http://dx.doi.org/10.1016/j.jpowsour.2017.01.127 0378-7753/© 2017 Elsevier B.V. All rights reserved.
of CO and H2 produced by external reforming. Meanwhile, internal reforming, which is conducted by feeding the hydrocarbons directly to the anode side, is applicable considering the relatively high SOFC operating temperatures up to 900 C [1]. Compared with the external reforming, the internal reforming offers advantages of system simplified, cost reduction and efficiency improvement. Different with the external reforming, the internal reforming could result in various cracked products comprising carbon, hydrogen, chain hydrocarbons, etc. when it is conducted by feeding the hydrocarbons at ambient humidity (ca. 3 vol% H2O). Nevertheless, CO and H2 are the main products, so the fuel flexibility depends critically on the catalytic performance for electrochemical oxidation of CO/H2 at the anodes,
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CO þ O2 ¼ CO2 þ 2e
(1)
H2 þ O2 ¼ H2 O þ 2e
(2)
Thus, a right physical and chemical understanding of CO/H2 oxidations that happen at the fuel electrode, is significant for defining cell performance and finding lifetime limiting factors as well as for building new advanced electrodes. The reactions take place either at the three-phase boundary (3 PB) between gas phase, oxygen conductor, and electron conductor, or at the two-phase interface (2 PB) of the gas phase and the anode electrocatalyst. The state-of-the-art anodes are Ni-based cermets, typically NiYSZ (yttria-stabilized zirconia) [2,3]. Lots of efforts have thus gone into understanding the kinetics processes of H2-H2O and CO-CO2 reactions at Ni-YSZ anodes [4e7]. The general conclusion is that charge transfer reaction is the rate-dominating step and probably achieves with a H or CO spillover mechanism. However, Ni-based anodes are unstable in hydrocarbons because Ni is a catalyst for the formation of carbon fibers which can reduce the electrochemical reaction sites and decrease the gas permeability eventually degrading the cell performance and limiting the application of SOFCs with hydrocarbons fuels [8,9]. Cu does not catalyze the formation of carbon fibers in the way that Ni does, it has been used for the direct utilization of hydrocarbons since the pioneer work of Fagg et al. in 2003 [10]. However, Cu is not as good as Ni in catalyzing the anode reaction. Ceria is thus added because of its catalytic activity for the electrochemical oxidation reaction in addition to its higher oxygen ionic conductivity than YSZ at intermediate temperatures [11,12]. The Cu-ceria composites such as Cu-CeO2-YSZ and Cu-CeO2-samaria doped ceria anodes have been demonstrated with stable operation for hydrocarbons and make a great contribution for SOFCs operating directly with carbonaceous fuels [13e15]. However, the kinetics processes of H2-H2O and CO-CO2 reactions at Cu-ceria cermets are not as clear as those at Ni-YSZ. The reaction processes at Cu-ceria anode must be different with Ni-YSZ since ceria (doped ceria) is a mixed electronic-ionic conductor while YSZ is regarded as a pure ionic conductor in the anode reducing atmospheres. The reaction at Ni-YSZ anodes takes place only at 3 PB, where the oxygen ions from YSZ, electrons from Ni, and gas meet. At Cu-ceria cermets, the reaction could occur on 2 PB, i.e. the ceria-gas two-phase interface, in addition to the Cuceria-gas 3 PB. In order to further understand the elementary kinetics of H2-H2O and CO-CO2 reactions on Cu-ceria, here we perform electrical conductivity relaxation (ECR) method to investigate the relationship between the surface reaction kinetics and the 3 PB length. Further, a three-electrode set-up is used to measure the interfacial polarization resistance under a range of gas components. Moreover, comparisons are carried out for the H2-H2O and CO-CO2 reactions to further understand the difference of electrochemical reaction between CO oxidation and H2 oxidation. Cu-SDC (Sm0.2Ce0.8O1.9) system is selected to study the reaction kinetics of Cu-ceria cermets considering the high ionic conductivity and excellent catalyst activity of ceria doped with 20 mol.% samaria [16]. 2. Experimental 2.1. Sample fabrication SDC (Sm0.2Ce0.8O1.9) powder was prepared using the carbonate co-precipitation method with cerium nitrate hexahydrate (Ce(NO3)3$6H2O, 99.9%), samarium nitrate hexahydrate (Sm(NO3)3$6H2O, 99.9%) and ammonium carbonate ((NH4)2CO3, 99.9%) as the precursors [17]. All the chemicals are from
Sinopharm Chemical Reagent Co. Ltd. The precipitates were heated at 600 C for 2 h to form the fluorite structured SDC. Then the powder was uniaxially dry-pressed under 320 MPa into rectangular bars, which were subsequently annealed in air at 1500 C for 5 h with a ramping rate of 3 C min1. The SDC bars were about 30.0 mm long, 5.10 mm wide, and 0.50 mm thick. Their relative density was 97.9% of the theoretical value as determined using the Archimedes method. Cu particles are formed using a sputter-heating process. The sputtering was conducted to deposit Cu film on the SDC bar surface with a copper target (4 N purity, Kejing Materials Technology Co. Ltd) using sputter coater (JFC-1600, JEOL) at 20 mA and under the vacuum of 8 Pa. Afterwards, the sample was heated at 800 C for 2 h with a ramping rate of 3 C min1 in reducing atmosphere (10% H2 with Ar as the carrier gas) to convert the Cu film to Cu particles. The deposition time was set to be 10e60 s to vary the film thickness and consequently, the amount of Cu particles per unit area. For comparison, Ni particles and Ni-Cu alloy particles were also deposited on SDC surface. Ni was sputtered for 10, 30, and 40 s while the NiCu alloy was formed by subsequently sputtering Cu and Ni for 20 s. The surface microstructures were revealed using scanning electron microscopy (SEM, JSM-6700F, JEOL). The area covered by the Cu particles and the length of Cu-SDC boundary (the Cu-SDC-gas 3 PB) were statistically determined using ImageJ. 2.2. Electrical conductivity relaxation measurement Electrical conductivity relaxation (ECR) method was utilized to characterize the H2-H2O and CO-CO2 reactions at 800 C with a digital multimeter (2001-785D, Keithley) using the four-point technique [16]. For H2 oxidation, H2eAr gas mixtures were used and the gas atmosphere was changed from flowing 5% H2 to 10% H2. The gas was humidified using a moisture bottle, resulting in about 3% water vapor. For CO oxidation, CO-CO2 mixtures were applied and the CO concentration was changed from 50% to 66.7%. The gas atmosphere was changed by increase the content of H2 or CO, so that oxidation reactions occur by combining H2 or CO with oxygen ions from SDC, the same reaction for the anode processes [16,18]. The gas flowing rate was 200 cm3 min1. 2.3. Electrochemical measurement YSZ electrolyte substrates (12 mm in diameter, 0.4 mm in thickness) were formed by pressing YSZ powder (Sichuan China) uniaxially under 320 MPa, followed by sintering at 1450 C for 5 h in air. CuO powder was prepared by glycine-nitrate combustion method with copper nitrate trihydrate (Cu(NO3)2$3H2O, 99.9%). The working electrode (WE) (~5 mm) with an active area of 0.2376 cm2 was fabricated on the YSZ substrate by screen-printing method using SDC and CuO mixed powder at a weight ratio of 2:3 dispersed in terpilenol containing 6% ethyecellulose, and heated at 1000 C for 2 h in air. Platinum paste (PE-Pt-7840, Sino-Platinum Metals Co. Ltd) was symmetrically painted on the opposite of YSZ as counter electrode (CE). Reference electrode (RE) was also used with Platinum ink beside WE. The distance of WE and RE is 2.8 mm as shown in Fig. S1. The external Ag wires were attached to the electrode with Ag conducting glue (Shanghai Research Institute of Synthetic resins) as binder. AC impedance plots were measured and the frequency ranged from 106 to 102 Hz with amplitude of 10 mV under a fixed overpotential of þ0.3 V. The measurements were carried out under a series of gas components. For H2-H2O atmosphere, the percent of H2O ranged from 3% to 50%, and for CO-CO2 atmosphere, the percent of CO ranged from 25% to 50%. The interfacial polarization resistances at different temperature (600e800 C) were also obtained in both H2-H2O and CO-CO2
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atmospheres. 3. Results and discussion 3.1. Kinetics by electrical conductivity relaxation 3.1.1. Cu-SDC-gas 3 PB of the ECR samples Fig. 1 is the SEM micrographs for the surface microstructures of the Cu-SDC bars used for the ECR measurements. Fig. 1a shows that the SDC bar is fully dense, no holes or cracks are observed. The relative density is 97.9% of the theoretical value as determined using the Archimedes method. The density is high enough for the
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ECR test [19]. The average grain size is about 2.5 mm. The small particles on the SDC grains in Fig. 1bef are copper as formed in the sputtering-heating processes, in which the sputtering results in thin copper films while the heating turns the films into particles. The copper particles are evenly distributed, not connected to each other. The bar conductivity should not be affected since no connections are formed among these copper particles which shown in Fig. S2 and Fig. S3. The particle number per unit area increases with the sputtering time while the particle size, which is averagely 55 nm, is not obviously different. It is possible that the particle size is determined by the same heating temperature of 800 C since the particles are heating derived. Table 1 lists the 3 PB line density
Fig. 1. SEM surface micrographs of the SDC bars with Cu particles deposited in (a) 0s, the bare SDC, (b) 10s, (c) 20s, (d) 40s, (e) 50s, and (f) 60s.
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Table 1 Cu coverage q, 3 PB density L3PB (mm1) and Chemical surface exchange coefficient in different gas atmospheres for the SDC samples. L3PB (mm1)
q ()
SDC Cu10 Cu20 Cu30 Cu40 Cu50 Cu60
0 0.0075 0.0212 0.0285 0.0379 0.0464 0.0542
± ± ± ± ± ±
0.0009 0.0028 0.0016 0.0018 0.0077 0.0068
0 0.3345 0.7703 0.8939 1.4445 1.9369 2.2724
± ± ± ± ± ±
0.0251 0.1177 0.0663 0.0144 0.0353 0.0404
H2-H2O
CO-CO2
keff 105 cm s1
kCu 105 cm s1
keff 105 cm s1
kCu 105 cm s1
1.42 2.58 3.63 3.95 5.01 5.95 7.15
0 1.17 2.24 2.57 3.65 4.6 5.81
12.2 13.4 16.1 18.8 25.5 30.0 34.9
0 1.24 4.11 6.90 13.7 18.3 23.3
(L3PB), i.e. the length of Cu-SDC-gas boundaries per unit surface area and the surface coverage of the Cu particles (q), the surface area which is occupied by the Cu particles per unit surface area. L3PB and q are statistically analyzed with ImageJ from the SEM micrographs. Both L3PB and q increase with the sputtering time. Basically, L3PB increases linearly with the increasing of q showing a linearlydependent-coefficient of 0.9663.
3.1.2. Kinetics for hydrogen oxidation Fig. 2a shows the normalized conductivity curves measured at
800 C when the gas is changed from humidified 5%H2 to 10%H2. The re-equilibrium time is about 7800 s for the bare SDC sample. It is consistent with our previous results about 7000 s [20] while different with that reported by Gopal and Haile, about 15000 s [18] due to the difference in oxygen partial pressure gradient and sample thickness. After Cu is deposited, the re-equilibrium time is obviously reduced. It is 4000 s when the sputtering is conducted for 10 s. And the re-equilibrium time decreases to the lowest of 1800 s with the increase of sputtering time up to 60 s. The reduced reequilibrium time demonstrates enhanced oxygen transport
Fig. 2. The surface reaction kinetics in H2-H2O mixtures. (a) normalized conductivity profiles versus relaxation time, (b) kCu versus 3 PB length, (c) kCu versus Cu coverage and (d) 3 PB contribution to the total H2 oxidation.
M. Zheng et al. / Journal of Power Sources 345 (2017) 165e175
kinetics, i.e. the surface reaction and/or the bulk diffusion rates. Since the bulk diffusion process could not be changed by Cu particles on the surface, the enhanced kinetics must be attributed to increased surface reaction rate as a result of Cu deposition. Therefore, the Cu particles have catalytic effect and/or synergistic catalytic effect on H2 oxidation reaction, i.e. ceria reduction reaction. It is reported that copper is inert to hydrogen oxidation and its primary function in the Cu-ceria anodes is to provide the path for electronic conduction while ceria serves as the electrocatalyst [21]. The enhancement in catalytic activity is quantitatively characterized using the chemical surface exchange coefficient, kchem, which is determined from the relaxation profile shown in Fig. 2a using Eq. (3) that has been reported in details elsewhere [22,23]. The formula is only applicable when the diffusion dimension is much smaller than the characteristic thickness L ¼ kchem =Dchem , in which Dchem is the chemical bulk diffusion coefficient.
sðtÞ sð0Þ k t ¼ 1 exp chem a sð∞Þ sð0Þ
(3)
where s(0) and s(∞) represent the equilibrated conductivities at the initial and final time of the relaxation curve. a is a constant, numerically equals half of the sample thickness, and represents the diffusion length. Since the minimum dimension of the SDC sample is 0.05 cm, which is only 5% of L z1 cm, the relaxation data is fitted using Eq. (3) to obtain kchem. It is 1.42 105 cm s1 for the bare SDC, comparable with 1.8 105 cm s1, measured also with ECR method at the same temperature [18]. When Cu is deposited, the re-equilibrium time is reduced. It shows clearly that Cu increases the surface reaction rate, which can be presented using the effective surface exchange coefficient, keff, which is obtained by fitting the relaxation data with Eq. (3). keff is 2.58 105 cm s1 when the sputtering is conducted for 10 s. It is much higher than that without Cu. It increases with the sputtering time to 7.15 105 cm s1 in the experimental conditions as shown in Table 1. Consequently, when Cu presents, the surface reaction rate increases up to fivefold, clearly demonstrates that Cu is an excellent electrocatalyst and/or a synergistic electrocatalyst for H2 oxidation reaction on doped ceria. Physically, keff is contributed by the reaction on the SDC surface and the reaction related to Cu, kCu. That is
kCu ¼ keff kchem ð1 qÞ
(4)
kCu represents the Cu catalytic and/or synergistic catalytic activity for H2 oxidation reaction. It is the surface exchange rate associated with the reactions taken place at the SDC-Cu-gas 3 PB and/or on the Cu surface. If it is taken place on Cu surface, kCu should be proportional to the surface area. In this case, Cu should have catalytic activity for the oxidation reaction. Otherwise, the reaction occurs at 3 PB, kCu should be proportional to 3 PB length, and Cu must be a synergistic catalyst. To vary the two effects, kCu is plotted as a function of L3PB and q in Fig. 2b and c. It can be seen that kCu increases linearly with L3PB with a linearly-dependent-coefficient of 0.9899 but the kCu-q curves derives much from the linear relation. If it is linearly fitted, kCu-q has a linearly-dependent-coefficient of 0.9653, much lower than kCu-L3PB, suggesting that the reaction rate is limited by the step occurred at/near the SDC-Cu-gas 3 PB. Consequently, Cu has synergistic catalytic effect that can improve the H2 oxidation reaction on SDC. The 3 PB contribution l3PB, which is defined as the ratio of H2 oxidized at 3 PB to the total amount of reacted H2, is presented in Fig. 2d versus L3PB. l3PB increases with L3PB. It is 81.3% when L3PB is 2.27 mm1 (length per unit area).
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3.1.3. Kinetics for CO oxidation Fig. 3a shows the ECR profiles measured in CO-CO2 mixtures at 800 C when the CO:CO2 ratio is changed from 1:1 to 2:1. The reequilibrium time is reduced obviously with Cu particles and it decreases with the increase of sputtering time. For bare SDC, the reequilibrium time is about 1200 s. It is only 400 s when the sputtering is conducted for 60 s, demonstrating that Cu particles have catalytic effect and/or synergistic catalytic effect on CO oxidation just as H2 oxidation. The chemical surface exchange coefficient kchem calculated with Eq. (3) is 1.22 104 cm s1 for the bare SDC in CO-CO2 atmospheres. And the effective surface exchange coefficient, keff, is 3.49 104 cm s1 when the sputtering time is conducted for 60 s. Fig. 3b and c shows kCu-L3PB and kCu-q curves. Better linear relationship is observed with kCu-L3PB, showing a linearly-dependent-coefficient of 0.9888, suggesting that the reaction rate is limited by the step occurred at/near the SDC-Cu-gas 3 PB, just like the H2 oxidation reaction. And also, Cu particles have the synergistic catalytic activity on CO oxidation rather than catalytic activity, which is supposed to occur on Cu surface. Fig. 3d shows that the 3 PB contribution increases with L3PB. It is 66.8% for L3PB ¼ 2.27 mm1, suggesting that the reaction at 3 PB dominates the total CO oxidation at a Cu-SDC electrode. The re-equilibrium time in CO-CO2 atmospheres is much lower than that in H2-H2O, compared with Figs. 2a and 3a. For the bare SDC, it is 1200 s, about 15% of 7800 s as observed in H2-H2O atmospheres. The result is consistent with previous work by Gopal and Haile [18], who have also reported much lower re-equilibrium time in CO-CO2 atmospheres. The ECR results suggest favorable thermo-chemical reaction for SDC anode operating with CO-CO2 even without any metal catalysts. When Cu particles present, the re-equilibrium time is also much smaller in CO-CO2 mixtures. Consequently, the effective surface exchange coefficient in CO-CO2 atmospheres is much higher than that in H2-H2O atmospheres in Figs. 2b and 3b. Precisely, the value of keff for CO oxidation is about 5 times of that for H2 reaction, revealing a remarkable advantage of surface reaction rate when exposing to the CO-CO2 mixtures for the Cu-SDC cermet anodes.
3.1.4. Comparison with Ni-SDC for CO oxidation ECR measurement demonstrates that Ni-SDC has extreme advantage for H2 oxidation over Cu-SDC. But, the result for CO oxidation is absolutely different. Fig. 4a shows the normalized conductivity curves for Ni-SDC samples in CO-CO2 atmospheres. Contrast to Cu, when the SDC surface is deposited with Ni the reequilibrium time is obviously increased, from ~1200 s for the bare SDC to ~3000 s for the 10 s sputtered sample. And the reequilibrium time increases with the increase of sputtering time, revealing a negative effect on the surface reaction of CO oxidation. Fig. 4b compares the effect of Ni, Cu and Cu-Ni alloy, which are deposited on SDC surface by 40 s sputtering. Comparing with the bare SDC, the re-equilibrium time is reduced by Cu, increased by Ni, and almost not varied by Cu-Ni alloy. Although Ni is an excellent catalyst for H2 oxidation, it has negative effect for CO oxidation. Fig. 4c and d show the micrographs of Ni-SDC sample before and after the ECR test. Carbon filaments are clearly found, which could reduce the surface reaction sites, thus impedes the CO oxidation reaction. The carbon might be formed by cracking CO due to the high catalyst activity of Ni [8]. So, the ECR results clearly demonstrate the advantages of using Cu-SDC instead of Ni-SDC in the CO oxidation reaction.
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Fig. 3. The surface reaction kinetics in CO/CO2 mixtures. (a) normalized conductivity profiles versus relaxation time, (b) and kCu versus 3 PB density, (c) kCu versus Cu coverage, and (d) 3 PB contribution to the total CO oxidation.
3.2. Electrochemical performance 3.2.1. Electrochemical performance for H2 oxidation at Cu-SDC electrode Fig. 5a shows the impedance spectra for the Cu-SDC composite electrode measured under þ0.3 V overpotential, at 800 C and with different H2-H2O contents. The resistance corresponding to the electrolyte and lead wires is subducted to clearly comparing the electrode performance. It is obviously that the interfacial polarization resistance, Rp, decreases with the increase of vapor content, from 29.5 U cm2 for 6% H2O to 18.5 U cm2 for 42%. For a composite electrode with average particle size of 0.3 mm, the 3 PB length per unit volume is about 6.2 mm2 [24], corresponding to 2.48 mm1, the 3 PB length per unit area. And 3 PB length increases with the decrease of particle size. As ECR results shown, the contribution of 3 PB to the total reaction is 81.3% for H2 oxidation and 66.8% for CO oxidation when L3PB is 2.27 mm1. So, for a typical SDC-Cu composite electrode, the contribution of 3 PB is higher than these values, and the reaction takes place at 3 PB with minor contribution from 2 PB. Considering the vapor effect on electrode reaction and the significant contribution of 3 PB to H2 oxidation, elementary steps are suggested for the electrode reaction at Cu-SDC-gas 3 PB,
listed in Table 2. By kinetic derivation, a relationship between Rp and vapor partial pressure can be obtained. Eq. (5) and Eq. (6) are the general form for the reaction rate r (mol,s1,cm2) of an elementary chemical reaction and a charge transfer step [25]:
r ¼ kf
Y
cpmm kr
m2Rf
Y
cqmm
(5)
m2Rr
anF h Y pm ð1 aÞnF h cm kCT;f exp rCT ¼ kCT;f exp RT RT m2Rf Y q cmm m2Rr
(6) Here, kf and kr represent the rate constants of the forward and reverse reaction, respectively. cm is the area-specific concentration of species m (mol,cm2). pm and qm are stoichiometric coefficient of species m in the forward and reverse reactions. n is the number of electrons transferred in the electrochemical reaction. a is the transfer coefficient. h is the overpotential, and F is the Faraday constant. Assuming that other reactions reach the equilibrium state
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Fig. 4. Ni-SDC performance in CO-CO2 atmospheres. (a) normalized conductivity profiles measured at 800 C when Ni is sputtered for 0 (bare SDC), 10, 20, and 30 s, (b) comparison of relaxation profiles for Cu, Ni and Cu-Ni alloy, which are all prepared by 40 s sputtering, (c) surface micrograph for Ni-SDC before the ECR test, and (d) surface micrograph for NiSDC after the ECR test.
very fast, as a result, the rate of the forward reaction is equal to the reverse reaction, and the global rate can be considered to be zero except the rate limiting reaction. So, according to iterative method, the relationship can be derived for faradaic current and partial pressure when an elementary reaction is in the process of rate control [26]. The faradaic current produced by a charge transfer reaction is expressed as
i ¼ nFr
(7)
Further, Rp can be obtained as
Rp ¼ h=i
(8)
Therefore, Rp is a function of vapor pressure, pH2 O , and hydrogen pressure, pH2 , as shown in Table 12. For example, Rp ap1 H2 O when step 3 H-3 is rate limiting while Rp apH24O pH24 is for H-4. Since Rp decreases with the increase of pH2 O , H-1, H-2 and H-5 are not the rate limiting step. When step H-3 and/or H-4 are the rate limiting ones, Rp decreases with the increase of vapor pressure, consistent with the experimental results shown in Fig. 5a. To verify the rate limiting step, Rp is plotted as a function of pH2 O . Fig. 5b presents a much better linear relationship than Fig. 5c, demonstrating H-3 is the rate limiting step. Fig. 5c shows a complicated relation between Rp and
1
3
pH24O pH24 , suggesting H-4 is not the limiting step in the experimental conditions. For an electrode consisting of ceria and Cu, the reaction could take place at both 2 PB and 3 PB. The reactions via 2 PB, the ceria-H2 interface, have been proposed in the literature. Zhang et al. [27] have shown that the charge transfer is accompanied with hydrogen atoms on the surface of ceria while Feng et al. [28] have stated that the surface oxygen vacancy is fully saturated with hydroxyl, which facilitates the charge transfer reaction. For the reaction at Cu-SDC-gas 3 PB, we propose the charge transfer step, H-3, in which water vapor combining with oxygen vacancy to form hydroxyl is the rate-limiting reaction. 3.2.2. Electrochemical performance for CO oxidation at Cu-SDC electrode Fig. 6a shows the impedance spectra for the Cu-SDC composite electrode measured under þ0.3 V overpotential in different CO-CO2 atmospheres. Rp decreases with the increase of CO content from 17.6 U cm2 for 15% CO to 4.4 U cm2 for 40% CO. The elementary reaction processes are thus proposed for CO oxidation at 3 PB, as 0 shown in Table 3, regarding the intermediate carbonate ðCO3 ÞO , as suggested by Feng and Yu [29,30] for the reactions via 2 PB, the ceria-CO interface. Increasing CO content could promote the for0 mation of adsorbates ðCO3 ÞO , thus increases the reaction rate, i.e. reduces the interfacial polarization resistance.
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Fig. 5. Impedance spectra measured at 800 C in H2-H2O atmosphere (a) and interfacial polarization resistance Rp as a function of the partial pressure for (b) H-3 and (c) H-4.
Table 2 Elementary steps for the electrode reaction at Cu-SDC-gas 3 PB in H2-H2O atmospheres and the Rp dependence on vapor pressure,pH2 O , and hydrogen pressure,pH2 . Step code
Elementary equation
Rpf
H-1: Hydrogen adsorption
H2 4H2ðCuÞ
p1 H2
H-2: Hydrogen dissociation and migration
H2ðCuÞ 42Hð3PBÞ ,, , H2 OðgÞ þ O O þ VO 42HOOð3PBÞ
p1 H2
H-3: Water dissociation H-4: Charge transfer
Hð3PBÞ þ HO,Oð3PBÞ 4H2 O,, Oð3PBÞ þ e
H-5: Water desorption
,, H2 O,, Oð3PBÞ 4H2 OðgÞ þ VO
Fig. 6b and c compare the Rp dependence on gas composition. A 1 3 good linear relationship is observed for Rp-pCO4 2 pCO4 , demonstrating that C-3 is the rate limiting step. Meanwhile, C-4 is not the limiting step in the experimental conditions 3since Fig. 6c shows a compli 1 cated relation between Rp and pCO4 2 pCO4 . Table 4 compares the mechanisms for H2 and CO oxidation via 2 PB and 3 PB between literature and our work. With the present work, the H2 and CO oxidation mechanisms for a Cu-ceria electrode, through both 2 PB and 3 PB, can be fully understood. It is noted that Rp for CO oxidation is much lower than that for H2. The result is consistent with the ECR measurement. It should be mentioned that for the Ni-YSZ electrodes, Rp for CO is higher than H2 [31]. Fig. 7 compares the Arrhenius plots of Rp for H2 and CO
p1 H2 O 1
3
pH24 O pH24 pH2 O
oxidation measured from 600 to 800 C. In addition to Rp, lower activation energy is also achieved for CO oxidation. The difference in activation energy and Rp indicates distinction in reaction mechanism, e.g. elementary reaction or rate determining step, and demonstrates an advantageous electrochemical reaction in CO-CO2 atmosphere, that is, CO is easier to be oxidized than H2 over CuSDC. 4. Conclusion Cu-ceria cermets can efficiently catalyze the oxidation of H2 and CO, the products from hydrocarbons via external and internal reforming for solid oxide fuel cells. In addition to electronic
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Fig. 6. Impedance spectra measured at 800 C in CO/CO2 atmosphere (a) and interfacial polarization resistance Rp as a function of the partial pressure for the charge transfer processes of (b) C-3 and (c) C-4 for CO oxidation.
Table 3 Elementary steps for CO oxidation at Cu-SDC-gas 3 PB and Rp dependence on the partial pressure of CO, pCO , and CO2, pCO2 . Step code
Elementary equation
Rpf
C-1: CO adsorption
COðgÞ 4COðCuÞ
1 p1 CO pCO2
C-2: CO migration
COðCuÞ 4COð3PBÞ
C-3: Charge transfer
,, COð3PBÞ þ 2O Oð3PBÞ 4ðCO3 ÞOð3PBÞ þ VOð3PBÞ þ e
1 p1 CO pCO2 0
1
3
34
1
pCO4 2 pCO4
0
C-4: Charge transfer
ðCO3 ÞOð3PBÞ 4CO2ð3PBÞ þ O Oð3PBÞ þ e
pCO2 pCO4
C-5: CO2 desorption
CO2ð3PBÞ 4CO2ðgÞ
p1 CO2
Table 4 The charge transfer reactions for H2 and CO oxidations at 2 PB and 3 PB over Cu-ceria electrodes. H2 oxidation via 2 PB, Zhang et al. [27]
, HðceriaÞ þ O O 4OHOðceriaÞ þ e , HðceriaÞ þ OHOðceriaÞ 4H2 OðgÞ þ VO,, þ e
H2 oxidation via 2 PB, Feng et al. [28]
, HðSDCÞ þ O O 4OHOðSDCÞ þ e , 2OHOðSDCÞ 4H2 OðgÞ þ VO,, þ O O
H2 oxidation via 3 PB, present work
,, , H2 OðgÞ þ O O þ VO 42HOOð3PBÞ Hð3PBÞ þ HO,Oð3PBÞ 4H2 OðgÞ þ VO,, þ e
CO oxidation via 2 PB, Feng & Yu [29,30]
,, COðSDCÞ þ 2O OðSDCÞ 4ðCO3 ÞOðSDCÞ þ VOðSDCÞ þ e
0
0
ðCO3 ÞOðSDCÞ 4CO2ðSDCÞ þ O OðSDCÞ þ e
CO oxidation via 3 PB, present work
0
,, COð3PBÞ þ 2O Oð3PBÞ 4ðCO3 ÞOð3PBÞ þ VOð3PBÞ þ e 0
ðCO3 ÞOð3PBÞ 4CO2ð3PBÞ þ
O Oð3PBÞ
þ e
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M. Zheng et al. / Journal of Power Sources 345 (2017) 165e175
kCu kchem
L Dchem sðtÞ sð0Þ sð∞Þ a
l3PB Rp pH2 O pH2 pCO pCO2 r kf kr cm pm qm n
a h Fig. 7. The Arrhenius plots of interfacial polarization resistance for Cu-SDC electrodes measured in different gas mixtures.
conductivity, Cu can effectively enhance the catalytic activity. For example, when Cu particles are formed by sputtering for 60 s, the effective surface exchange coefficients were increased from 1.42 105 cm,s1 to 7.15 105 cm s1 and from 1.22 104 cm,s1 to 3.49 104 cm s1 for H2 and CO oxidation, respectively. The reactions take place at both SDC-gas 2 PB and CuSDC-gas 3 PB. With the increasing amount of Cu particles, the surface reaction rate increases. 3 PB rather than 2 PB is the main reaction site for both H2 and CO oxidation. For instance, the reaction at 3 PB contributes 81.3% of the total H2 oxidation and of 66.8% of CO oxidation when 3 PB length is 2.27 mm1. Elementary reactions at 3 PB were proposed combining with AC impedance measurements. For H2 oxidation at 3 PB, the rate limiting step is the formation of hydroxyl from oxygen vacancy and water vapor while the transformation of carbon monoxide to carbonate adsorbate limits CO oxidation via 3 PB. When the oxidation reactions are compared, it is found that Cu-SDC has exhibited much higher catalytic performance for CO oxidation than H2 as follows: about fivefold high in reaction rate, lower interfacial polarization resistance and smaller activation energy for electrode reaction. Therefore, we believe that the CO oxidation at Cu-ceria anode is the preferential reaction when hydrocarbons are served as the fuel. Acknowledgment We gratefully acknowledge the financial supports from the National Nature Science Foundation of China (51372239 and 91645101). Glossary L3PB
q
keff
triple-phase boundary line density surface coverage of Cu particles effective surface exchange coefficient
F i
Cu catalytic and/or synergistic catalytic activity for H2 oxidation reaction chemical surface exchange coefficient characteristic thickness chemical bulk diffusion coefficient conductivity at time t conductivities at the initial time conductivities at the final time constant, numerically equals half of the sample thickness 3 PB contribution to the total reaction interfacial polarization resistance vapor pressure partial pressure of H2 partial pressure of CO partial pressure of CO2 elementary reaction rate rate constant of the forward reaction rate constant of the reverse reaction area-specific concentration of species m stoichiometric coefficient of species m in the forward reaction stoichiometric coefficient of species m in the reverse reaction electrons transferred in the electrochemical reaction transfer coefficient overpotential Faraday's constant faradaic current
Appendix A Elementary steps for the electrode reaction at Cu-SDC-gas 3 PB in H2-H2O atmospheres are shown below
H2 4H2ðCuÞ
(A1)
H2ðCuÞ 42Hð3PBÞ
(A2)
,, , H2 OðgÞ þ O O þ VO 42HOOð3PBÞ
(A3)
Hð3PBÞ þ HO,Oð3PBÞ 4H2 O,, Oð3PBÞ þ e
(A4)
,, H2 O,, Oð3PBÞ 4H2 OðgÞ þ VO
(A5)
The general form for the reaction rate r (mol,s1,cm2) of an elementary chemical reaction and a charge transfer step are
r ¼ kf
Y
cpv m kr
m2Rf
Y
q
cmm
(A6)
m2Rr
anF h Y pm ð1 aÞnF h cm kCT;f exp rCT ¼ kCT;f exp RT RT m2Rf Y q cmm m2Rr
(A7) Here, kf and kr represent the rate constants of the forward and reverse reaction, respectively. cm is the area-specific concentration of species m (mol,cm2). pm and qm are stoichiometric coefficient of species m in the forward and reverse reactions. n is the number of electrons transferred in the electrochemical reaction. a is the transfer coefficient. h is the overpotential, and F is the Faraday
M. Zheng et al. / Journal of Power Sources 345 (2017) 165e175
constant. Therefore, the reaction rate for each elementary step can be expressed as:
h i r1 ¼ k1 pH2 k1 H2ðCuÞ
(A8)
h i h i2 r2 ¼ k2 H2ðCuÞ k2 Hð3PBÞ
(A9)
Rp ¼ h=i
175
(A18)
Therefore,
h i h i2 ,, , r3 ¼ k3 pH2 O O O VO k3 HOOð3PBÞ
Rp fp1 H2
(A19)
Similarly, we can get the relationship between interfacial polarization resistance Rp and partial pressure when each elementary step dominates the total reaction rate for H2 and CO oxidation reaction.
(A10)
ih i h i Fh exp r4 ¼ k4 Hð3PBÞ HO,Oð3PBÞ exp k4 H2 O,, Oð3PBÞ 2RT Fh 2RT
Appendix B. Supplementary data
h
(A11) r5 ¼
h
k5 H2 O,, Oð3PBÞ
i
k5 pH2 O VO,,
(A12)
The total reaction rate is equal to the slowest elementary reaction rate, that is, other elementary reactions reach the equilibrium state very fast. As a result, the rate of the forward reaction is equal to the reverse reaction and the global rate can be considered to be zero except the rate limiting reaction. If the elementary step (A1) is the rate limiting reaction, the total reaction rate r is equal to r1 , and r2 ¼ r3 ¼ r4 ¼ r5 ¼ 0. So, according to iterative method, the total reaction rate can be written as
r ¼ r1 ¼ k1 pH2
k1 k2 k3 k24 k25 VO,, 2F h h i pH2 O exp RT k2 k3 k24 k25 O O (A13)
At equilibrium state, r ¼ 0. And the concentration of oxygen vacancy and the lattice oxygen ions are considered to be constant. Then, we can obtain the value of h.
h¼
RT pH2 O ln þ const: 2F pH2
(A14)
The total reaction rate is turned to be
r ¼ r1 ¼ k1 pH2
k1 k2 k3 k24 k25 VO,, h i pH2 k2 k3 k24 k25 O O
A(15)
The faradaic current produced by a charge transfer reaction is expressed as
i ¼ nFr
(A16)
Besides, at equilibrium state, the faradaic currents in forward and reverse reaction are the same value, and the total current r ¼ rf rr ¼ 0.
rf ¼ rr fpH2 Further, Rp can be obtained as
(A17)
Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.jpowsour.2017.01.127. References [1] N. Laosiripojana, S. Assabumrungrat, J. Power Sources 163 (2) (2007) 943e951. [2] H. Koide, Y. Someya, T. Yoshida, T. Maruyama, Solid State Ionics 132 (3e4) (2000) 253e260. [3] E.S. Hecht, G.K. Gupta, H.Y. Zhu, A.M. Dean, R.J. Kee, L. Maier, O. Deutschmann, Appl. Catal. a-General 295 (1) (2005) 40e51. [4] A. Utz, A. Leonide, A. Weber, E. Ivers-Tiffee, J. Power Sources 196 (17) (2011) 7217e7224. [5] M. Mogensen, J. Hogh, K.V. Hansen, T. Jacobsen, Solid Oxide Fuel Cells 10 (Sofc-X), Pts 1 2 7 (1) (2007) 1329e1338. [6] A. Bieberle, L.J. Gauckler, Solid State Ionics 146 (2002) 23e41. [7] M. Vogler, A. Bieberle-Hutter, L. Gauckler, J. Warnatz, W.G. Bessler, J. Electrochem. Soc. 156 (5) (2009) B663eB672. [8] T. Chen, W.G. Wang, H. Miao, T.S. Li, C. Xu, J. Power Sources 196 (5) (2011) 2461e2468. [9] J.H. Koh, Y.S. Yoo, J.W. Park, H.C. Lim, Solid State Ionics 149 (3e4) (2002) 157e166. [10] D.P. Fagg, G.C. Mather, J.R. Frade, Ionics 9 (3e4) (2003) 214e219. [11] M.F. Liu, D. Ding, Y.H. Bai, T. He, M.L. Liu, J. Electrochem. Soc. 159 (6) (2012) B661eB665. [12] C.R. Xia, W. Rauch, F.L. Chen, M.L. Liu, Solid State Ionics 149 (1e2) (2002) 11e19. [13] S.W. Jung, C. Lu, H.P. He, K.Y. Ahn, R.J. Gorte, J.M. Vohs, J. Power Sources 154 (1) (2006) 42e50. [14] C. Lu, W.L. Worrell, R.J. Gorte, J.M. Vohs, J. Electrochem. Soc. 150 (3) (2003) A354eA358. [15] R.J. Gorte, S. Park, J.M. Vohs, C.H. Wang, Adv. Mater. 12 (19) (2000) 1465e1469. [16] Y.L. Wang, Y. Wang, C.R. Xia, J. Electrochem. Soc. 159 (9) (2012) F570eF576. [17] D. Ding, B.B. Liu, Z. Zhu, S. Zhou, C.R. Xia, Solid State Ionics 179 (21e26) (2008) 896e899. [18] C.B. Gopal, S.M. Haile, J. Mater. Chem. A 2 (7) (2014) 2405e2417. [19] J.A. Lane, J.A. Kilner, Solid State Ionics 136 (2000) 997e1001. [20] Y.L. Wang, Z.Y. Zhu, C.R. Xia, Electrochem. Commun. 36 (2013) 10e13. [21] S.P. Jiang, S.H. Chan, J. Mater. Sci. 39 (14) (2004) 4405e4439. [22] I. Yasuda, T. Hikita, J. Electrochem. Soc. 141 (5) (1994) 1268e1273. [23] T. Ishihara, J.A. Kilner, M. Honda, N. Sakai, H. Yokokawa, Y. Takita, Solid State Ionics 113 (1998) 593e600. [24] Y.X. Zhang, M. Ni, C.R. Xia, F.L. Chen, J. Am. Ceram. Soc. 97 (8) (2014) 2580e2589. [25] Y.X. Shi, C. Li, N.S. Cai, J. Power Sources 196 (13) (2011) 5526e5537. [26] W.F. Yao, E. Croiset, J. Power Sources 248 (2014) 777e788. [27] C.J. Zhang, Y. Yu, M.E. Grass, C. Dejoie, W.C. Ding, K. Gaskell, N. Jabeen, Y.P. Hong, A. Shayorskiy, H. Bluhrn, W.X. Li, G.S. Jackson, Z. Hussain, Z. Liu, B.W. Eichhorn, J. Am. Chem. Soc. 135 (31) (2013) 11572e11579. [28] Z.L.A. Feng, F. EI Gabaly, X.F. Ye, Z.X. Shen, W.C. Chueh, Nat. Commun. 5 (2014). [29] Z.L.A. Feng, M.L. Machala, W.C. Chueh, Phys. Chem. Chem. Phys. 17 (18) (2015) 12273e12281. [30] Y. Yu, B.H. Mao, A. Geller, R. Chang, K. Gaskell, Z. Liu, B.W. Eichhorn, Phys. Chem. Chem. Phys. 16 (23) (2014) 11633e11639. [31] C. Graves, C. Chatzichristodoulou, M.B. Mogensen, Faraday Discuss. 182 (2015) 75e95.
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H2 and CO oxidation process at the three-phase boundary of Cu-ceria cermet anode for solid oxide fuel cell Minghao Zheng, Shuang Wang, Mei Li, Changrong Xia* Key Laboratory of Materials for Energy Conversion, Chinese Academy of Sciences, Department of Materials Science and Engineering & Collaborative Innovation Center of Suzhou Nano Science and Technology, University of Science & Technology of China, No. 96 Jinzhai Road, Hefei, Anhui Province, 230026, PR China
h i g h l i g h t s Cu is a synergistic catalyst for H2/CO oxidation on doped ceria catalyst. H2/CO oxidation takes place at Cu-ceria-gas 3 PB rather than ceria-gas 2 PB. Much faster kinetics is demonstrated for CO than H2 oxidation on Cu-ceria. Rate limiting steps are proposed for H2/CO oxidation at Cu-ceria-gas 3 PB.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 24 October 2016 Received in revised form 5 January 2017 Accepted 29 January 2017
Cu-ceria cermets have been widely investigated as the anode materials for solid oxide fuel cells (SOFCs) that operated with hydrocarbon fuels. However, the anode reaction processes are not clear yet, especially those at the ceria-Cu-gas three phase boundary (3 PB). This work investigates samaria-doped ceria (SDC)-Cu-gas 3 PB reaction kinetics for the oxidation of H2 and CO, the products from hydrocarbons via external and internal reforming. Electrochemical conductivity relaxation measurement demonstrates that Cu is a synergistic catalyst that can significantly increase the reaction rate. The reaction at 3 PB contributes 81.3/66.8% of H2/CO oxidation when 5.4% SDC surface is covered with Cu particles. Combining with AC impedance analysis, elementary steps are proposed for the reaction at 3 PB. Water vapor combining to oxygen vacancy and carbon monoxide transforming to carbonate are the ratedetermining steps for the oxidation of H2 and CO, respectively. Cu-SDC has shown much higher catalytic activity, i.e. about fivefold reaction rate, for the oxidation of CO than H2. In addition, Cu-SDC electrodes exhibit lower interfacial polarization resistance and lower activation energy for the electrochemical oxidation of CO than H2. Consequently, CO is easier to be oxidized than H2 when the Cuceria anode is fueled with syngas, the reforming product from hydrocarbons. © 2017 Elsevier B.V. All rights reserved.
Keywords: Solid oxide fuel cell Anode reaction kinetics Three phase boundary Cu-ceria cermet Hydrogen oxidation CO oxidation
1. Introduction Solid oxide fuel cells (SOFCs) are gaining great momentum towards commercialization because of their distinguished advantages of high energy conversion efficiency and great fuel flexibility. The electrochemical oxidation via oxygen ions at the anode side makes it feasible to feed SOFCs with hydrocarbons such as petroleum, coal, and biogas. When using hydrocarbons as the fuel, it is usually assumed that SOFCs must be operated on syngas, a mixture
* Corresponding author. E-mail address: [email protected] (C. Xia). http://dx.doi.org/10.1016/j.jpowsour.2017.01.127 0378-7753/© 2017 Elsevier B.V. All rights reserved.
of CO and H2 produced by external reforming. Meanwhile, internal reforming, which is conducted by feeding the hydrocarbons directly to the anode side, is applicable considering the relatively high SOFC operating temperatures up to 900 C [1]. Compared with the external reforming, the internal reforming offers advantages of system simplified, cost reduction and efficiency improvement. Different with the external reforming, the internal reforming could result in various cracked products comprising carbon, hydrogen, chain hydrocarbons, etc. when it is conducted by feeding the hydrocarbons at ambient humidity (ca. 3 vol% H2O). Nevertheless, CO and H2 are the main products, so the fuel flexibility depends critically on the catalytic performance for electrochemical oxidation of CO/H2 at the anodes,
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M. Zheng et al. / Journal of Power Sources 345 (2017) 165e175
CO þ O2 ¼ CO2 þ 2e
(1)
H2 þ O2 ¼ H2 O þ 2e
(2)
Thus, a right physical and chemical understanding of CO/H2 oxidations that happen at the fuel electrode, is significant for defining cell performance and finding lifetime limiting factors as well as for building new advanced electrodes. The reactions take place either at the three-phase boundary (3 PB) between gas phase, oxygen conductor, and electron conductor, or at the two-phase interface (2 PB) of the gas phase and the anode electrocatalyst. The state-of-the-art anodes are Ni-based cermets, typically NiYSZ (yttria-stabilized zirconia) [2,3]. Lots of efforts have thus gone into understanding the kinetics processes of H2-H2O and CO-CO2 reactions at Ni-YSZ anodes [4e7]. The general conclusion is that charge transfer reaction is the rate-dominating step and probably achieves with a H or CO spillover mechanism. However, Ni-based anodes are unstable in hydrocarbons because Ni is a catalyst for the formation of carbon fibers which can reduce the electrochemical reaction sites and decrease the gas permeability eventually degrading the cell performance and limiting the application of SOFCs with hydrocarbons fuels [8,9]. Cu does not catalyze the formation of carbon fibers in the way that Ni does, it has been used for the direct utilization of hydrocarbons since the pioneer work of Fagg et al. in 2003 [10]. However, Cu is not as good as Ni in catalyzing the anode reaction. Ceria is thus added because of its catalytic activity for the electrochemical oxidation reaction in addition to its higher oxygen ionic conductivity than YSZ at intermediate temperatures [11,12]. The Cu-ceria composites such as Cu-CeO2-YSZ and Cu-CeO2-samaria doped ceria anodes have been demonstrated with stable operation for hydrocarbons and make a great contribution for SOFCs operating directly with carbonaceous fuels [13e15]. However, the kinetics processes of H2-H2O and CO-CO2 reactions at Cu-ceria cermets are not as clear as those at Ni-YSZ. The reaction processes at Cu-ceria anode must be different with Ni-YSZ since ceria (doped ceria) is a mixed electronic-ionic conductor while YSZ is regarded as a pure ionic conductor in the anode reducing atmospheres. The reaction at Ni-YSZ anodes takes place only at 3 PB, where the oxygen ions from YSZ, electrons from Ni, and gas meet. At Cu-ceria cermets, the reaction could occur on 2 PB, i.e. the ceria-gas two-phase interface, in addition to the Cuceria-gas 3 PB. In order to further understand the elementary kinetics of H2-H2O and CO-CO2 reactions on Cu-ceria, here we perform electrical conductivity relaxation (ECR) method to investigate the relationship between the surface reaction kinetics and the 3 PB length. Further, a three-electrode set-up is used to measure the interfacial polarization resistance under a range of gas components. Moreover, comparisons are carried out for the H2-H2O and CO-CO2 reactions to further understand the difference of electrochemical reaction between CO oxidation and H2 oxidation. Cu-SDC (Sm0.2Ce0.8O1.9) system is selected to study the reaction kinetics of Cu-ceria cermets considering the high ionic conductivity and excellent catalyst activity of ceria doped with 20 mol.% samaria [16]. 2. Experimental 2.1. Sample fabrication SDC (Sm0.2Ce0.8O1.9) powder was prepared using the carbonate co-precipitation method with cerium nitrate hexahydrate (Ce(NO3)3$6H2O, 99.9%), samarium nitrate hexahydrate (Sm(NO3)3$6H2O, 99.9%) and ammonium carbonate ((NH4)2CO3, 99.9%) as the precursors [17]. All the chemicals are from
Sinopharm Chemical Reagent Co. Ltd. The precipitates were heated at 600 C for 2 h to form the fluorite structured SDC. Then the powder was uniaxially dry-pressed under 320 MPa into rectangular bars, which were subsequently annealed in air at 1500 C for 5 h with a ramping rate of 3 C min1. The SDC bars were about 30.0 mm long, 5.10 mm wide, and 0.50 mm thick. Their relative density was 97.9% of the theoretical value as determined using the Archimedes method. Cu particles are formed using a sputter-heating process. The sputtering was conducted to deposit Cu film on the SDC bar surface with a copper target (4 N purity, Kejing Materials Technology Co. Ltd) using sputter coater (JFC-1600, JEOL) at 20 mA and under the vacuum of 8 Pa. Afterwards, the sample was heated at 800 C for 2 h with a ramping rate of 3 C min1 in reducing atmosphere (10% H2 with Ar as the carrier gas) to convert the Cu film to Cu particles. The deposition time was set to be 10e60 s to vary the film thickness and consequently, the amount of Cu particles per unit area. For comparison, Ni particles and Ni-Cu alloy particles were also deposited on SDC surface. Ni was sputtered for 10, 30, and 40 s while the NiCu alloy was formed by subsequently sputtering Cu and Ni for 20 s. The surface microstructures were revealed using scanning electron microscopy (SEM, JSM-6700F, JEOL). The area covered by the Cu particles and the length of Cu-SDC boundary (the Cu-SDC-gas 3 PB) were statistically determined using ImageJ. 2.2. Electrical conductivity relaxation measurement Electrical conductivity relaxation (ECR) method was utilized to characterize the H2-H2O and CO-CO2 reactions at 800 C with a digital multimeter (2001-785D, Keithley) using the four-point technique [16]. For H2 oxidation, H2eAr gas mixtures were used and the gas atmosphere was changed from flowing 5% H2 to 10% H2. The gas was humidified using a moisture bottle, resulting in about 3% water vapor. For CO oxidation, CO-CO2 mixtures were applied and the CO concentration was changed from 50% to 66.7%. The gas atmosphere was changed by increase the content of H2 or CO, so that oxidation reactions occur by combining H2 or CO with oxygen ions from SDC, the same reaction for the anode processes [16,18]. The gas flowing rate was 200 cm3 min1. 2.3. Electrochemical measurement YSZ electrolyte substrates (12 mm in diameter, 0.4 mm in thickness) were formed by pressing YSZ powder (Sichuan China) uniaxially under 320 MPa, followed by sintering at 1450 C for 5 h in air. CuO powder was prepared by glycine-nitrate combustion method with copper nitrate trihydrate (Cu(NO3)2$3H2O, 99.9%). The working electrode (WE) (~5 mm) with an active area of 0.2376 cm2 was fabricated on the YSZ substrate by screen-printing method using SDC and CuO mixed powder at a weight ratio of 2:3 dispersed in terpilenol containing 6% ethyecellulose, and heated at 1000 C for 2 h in air. Platinum paste (PE-Pt-7840, Sino-Platinum Metals Co. Ltd) was symmetrically painted on the opposite of YSZ as counter electrode (CE). Reference electrode (RE) was also used with Platinum ink beside WE. The distance of WE and RE is 2.8 mm as shown in Fig. S1. The external Ag wires were attached to the electrode with Ag conducting glue (Shanghai Research Institute of Synthetic resins) as binder. AC impedance plots were measured and the frequency ranged from 106 to 102 Hz with amplitude of 10 mV under a fixed overpotential of þ0.3 V. The measurements were carried out under a series of gas components. For H2-H2O atmosphere, the percent of H2O ranged from 3% to 50%, and for CO-CO2 atmosphere, the percent of CO ranged from 25% to 50%. The interfacial polarization resistances at different temperature (600e800 C) were also obtained in both H2-H2O and CO-CO2
M. Zheng et al. / Journal of Power Sources 345 (2017) 165e175
atmospheres. 3. Results and discussion 3.1. Kinetics by electrical conductivity relaxation 3.1.1. Cu-SDC-gas 3 PB of the ECR samples Fig. 1 is the SEM micrographs for the surface microstructures of the Cu-SDC bars used for the ECR measurements. Fig. 1a shows that the SDC bar is fully dense, no holes or cracks are observed. The relative density is 97.9% of the theoretical value as determined using the Archimedes method. The density is high enough for the
167
ECR test [19]. The average grain size is about 2.5 mm. The small particles on the SDC grains in Fig. 1bef are copper as formed in the sputtering-heating processes, in which the sputtering results in thin copper films while the heating turns the films into particles. The copper particles are evenly distributed, not connected to each other. The bar conductivity should not be affected since no connections are formed among these copper particles which shown in Fig. S2 and Fig. S3. The particle number per unit area increases with the sputtering time while the particle size, which is averagely 55 nm, is not obviously different. It is possible that the particle size is determined by the same heating temperature of 800 C since the particles are heating derived. Table 1 lists the 3 PB line density
Fig. 1. SEM surface micrographs of the SDC bars with Cu particles deposited in (a) 0s, the bare SDC, (b) 10s, (c) 20s, (d) 40s, (e) 50s, and (f) 60s.
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Table 1 Cu coverage q, 3 PB density L3PB (mm1) and Chemical surface exchange coefficient in different gas atmospheres for the SDC samples. L3PB (mm1)
q ()
SDC Cu10 Cu20 Cu30 Cu40 Cu50 Cu60
0 0.0075 0.0212 0.0285 0.0379 0.0464 0.0542
± ± ± ± ± ±
0.0009 0.0028 0.0016 0.0018 0.0077 0.0068
0 0.3345 0.7703 0.8939 1.4445 1.9369 2.2724
± ± ± ± ± ±
0.0251 0.1177 0.0663 0.0144 0.0353 0.0404
H2-H2O
CO-CO2
keff 105 cm s1
kCu 105 cm s1
keff 105 cm s1
kCu 105 cm s1
1.42 2.58 3.63 3.95 5.01 5.95 7.15
0 1.17 2.24 2.57 3.65 4.6 5.81
12.2 13.4 16.1 18.8 25.5 30.0 34.9
0 1.24 4.11 6.90 13.7 18.3 23.3
(L3PB), i.e. the length of Cu-SDC-gas boundaries per unit surface area and the surface coverage of the Cu particles (q), the surface area which is occupied by the Cu particles per unit surface area. L3PB and q are statistically analyzed with ImageJ from the SEM micrographs. Both L3PB and q increase with the sputtering time. Basically, L3PB increases linearly with the increasing of q showing a linearlydependent-coefficient of 0.9663.
3.1.2. Kinetics for hydrogen oxidation Fig. 2a shows the normalized conductivity curves measured at
800 C when the gas is changed from humidified 5%H2 to 10%H2. The re-equilibrium time is about 7800 s for the bare SDC sample. It is consistent with our previous results about 7000 s [20] while different with that reported by Gopal and Haile, about 15000 s [18] due to the difference in oxygen partial pressure gradient and sample thickness. After Cu is deposited, the re-equilibrium time is obviously reduced. It is 4000 s when the sputtering is conducted for 10 s. And the re-equilibrium time decreases to the lowest of 1800 s with the increase of sputtering time up to 60 s. The reduced reequilibrium time demonstrates enhanced oxygen transport
Fig. 2. The surface reaction kinetics in H2-H2O mixtures. (a) normalized conductivity profiles versus relaxation time, (b) kCu versus 3 PB length, (c) kCu versus Cu coverage and (d) 3 PB contribution to the total H2 oxidation.
M. Zheng et al. / Journal of Power Sources 345 (2017) 165e175
kinetics, i.e. the surface reaction and/or the bulk diffusion rates. Since the bulk diffusion process could not be changed by Cu particles on the surface, the enhanced kinetics must be attributed to increased surface reaction rate as a result of Cu deposition. Therefore, the Cu particles have catalytic effect and/or synergistic catalytic effect on H2 oxidation reaction, i.e. ceria reduction reaction. It is reported that copper is inert to hydrogen oxidation and its primary function in the Cu-ceria anodes is to provide the path for electronic conduction while ceria serves as the electrocatalyst [21]. The enhancement in catalytic activity is quantitatively characterized using the chemical surface exchange coefficient, kchem, which is determined from the relaxation profile shown in Fig. 2a using Eq. (3) that has been reported in details elsewhere [22,23]. The formula is only applicable when the diffusion dimension is much smaller than the characteristic thickness L ¼ kchem =Dchem , in which Dchem is the chemical bulk diffusion coefficient.
sðtÞ sð0Þ k t ¼ 1 exp chem a sð∞Þ sð0Þ
(3)
where s(0) and s(∞) represent the equilibrated conductivities at the initial and final time of the relaxation curve. a is a constant, numerically equals half of the sample thickness, and represents the diffusion length. Since the minimum dimension of the SDC sample is 0.05 cm, which is only 5% of L z1 cm, the relaxation data is fitted using Eq. (3) to obtain kchem. It is 1.42 105 cm s1 for the bare SDC, comparable with 1.8 105 cm s1, measured also with ECR method at the same temperature [18]. When Cu is deposited, the re-equilibrium time is reduced. It shows clearly that Cu increases the surface reaction rate, which can be presented using the effective surface exchange coefficient, keff, which is obtained by fitting the relaxation data with Eq. (3). keff is 2.58 105 cm s1 when the sputtering is conducted for 10 s. It is much higher than that without Cu. It increases with the sputtering time to 7.15 105 cm s1 in the experimental conditions as shown in Table 1. Consequently, when Cu presents, the surface reaction rate increases up to fivefold, clearly demonstrates that Cu is an excellent electrocatalyst and/or a synergistic electrocatalyst for H2 oxidation reaction on doped ceria. Physically, keff is contributed by the reaction on the SDC surface and the reaction related to Cu, kCu. That is
kCu ¼ keff kchem ð1 qÞ
(4)
kCu represents the Cu catalytic and/or synergistic catalytic activity for H2 oxidation reaction. It is the surface exchange rate associated with the reactions taken place at the SDC-Cu-gas 3 PB and/or on the Cu surface. If it is taken place on Cu surface, kCu should be proportional to the surface area. In this case, Cu should have catalytic activity for the oxidation reaction. Otherwise, the reaction occurs at 3 PB, kCu should be proportional to 3 PB length, and Cu must be a synergistic catalyst. To vary the two effects, kCu is plotted as a function of L3PB and q in Fig. 2b and c. It can be seen that kCu increases linearly with L3PB with a linearly-dependent-coefficient of 0.9899 but the kCu-q curves derives much from the linear relation. If it is linearly fitted, kCu-q has a linearly-dependent-coefficient of 0.9653, much lower than kCu-L3PB, suggesting that the reaction rate is limited by the step occurred at/near the SDC-Cu-gas 3 PB. Consequently, Cu has synergistic catalytic effect that can improve the H2 oxidation reaction on SDC. The 3 PB contribution l3PB, which is defined as the ratio of H2 oxidized at 3 PB to the total amount of reacted H2, is presented in Fig. 2d versus L3PB. l3PB increases with L3PB. It is 81.3% when L3PB is 2.27 mm1 (length per unit area).
169
3.1.3. Kinetics for CO oxidation Fig. 3a shows the ECR profiles measured in CO-CO2 mixtures at 800 C when the CO:CO2 ratio is changed from 1:1 to 2:1. The reequilibrium time is reduced obviously with Cu particles and it decreases with the increase of sputtering time. For bare SDC, the reequilibrium time is about 1200 s. It is only 400 s when the sputtering is conducted for 60 s, demonstrating that Cu particles have catalytic effect and/or synergistic catalytic effect on CO oxidation just as H2 oxidation. The chemical surface exchange coefficient kchem calculated with Eq. (3) is 1.22 104 cm s1 for the bare SDC in CO-CO2 atmospheres. And the effective surface exchange coefficient, keff, is 3.49 104 cm s1 when the sputtering time is conducted for 60 s. Fig. 3b and c shows kCu-L3PB and kCu-q curves. Better linear relationship is observed with kCu-L3PB, showing a linearly-dependent-coefficient of 0.9888, suggesting that the reaction rate is limited by the step occurred at/near the SDC-Cu-gas 3 PB, just like the H2 oxidation reaction. And also, Cu particles have the synergistic catalytic activity on CO oxidation rather than catalytic activity, which is supposed to occur on Cu surface. Fig. 3d shows that the 3 PB contribution increases with L3PB. It is 66.8% for L3PB ¼ 2.27 mm1, suggesting that the reaction at 3 PB dominates the total CO oxidation at a Cu-SDC electrode. The re-equilibrium time in CO-CO2 atmospheres is much lower than that in H2-H2O, compared with Figs. 2a and 3a. For the bare SDC, it is 1200 s, about 15% of 7800 s as observed in H2-H2O atmospheres. The result is consistent with previous work by Gopal and Haile [18], who have also reported much lower re-equilibrium time in CO-CO2 atmospheres. The ECR results suggest favorable thermo-chemical reaction for SDC anode operating with CO-CO2 even without any metal catalysts. When Cu particles present, the re-equilibrium time is also much smaller in CO-CO2 mixtures. Consequently, the effective surface exchange coefficient in CO-CO2 atmospheres is much higher than that in H2-H2O atmospheres in Figs. 2b and 3b. Precisely, the value of keff for CO oxidation is about 5 times of that for H2 reaction, revealing a remarkable advantage of surface reaction rate when exposing to the CO-CO2 mixtures for the Cu-SDC cermet anodes.
3.1.4. Comparison with Ni-SDC for CO oxidation ECR measurement demonstrates that Ni-SDC has extreme advantage for H2 oxidation over Cu-SDC. But, the result for CO oxidation is absolutely different. Fig. 4a shows the normalized conductivity curves for Ni-SDC samples in CO-CO2 atmospheres. Contrast to Cu, when the SDC surface is deposited with Ni the reequilibrium time is obviously increased, from ~1200 s for the bare SDC to ~3000 s for the 10 s sputtered sample. And the reequilibrium time increases with the increase of sputtering time, revealing a negative effect on the surface reaction of CO oxidation. Fig. 4b compares the effect of Ni, Cu and Cu-Ni alloy, which are deposited on SDC surface by 40 s sputtering. Comparing with the bare SDC, the re-equilibrium time is reduced by Cu, increased by Ni, and almost not varied by Cu-Ni alloy. Although Ni is an excellent catalyst for H2 oxidation, it has negative effect for CO oxidation. Fig. 4c and d show the micrographs of Ni-SDC sample before and after the ECR test. Carbon filaments are clearly found, which could reduce the surface reaction sites, thus impedes the CO oxidation reaction. The carbon might be formed by cracking CO due to the high catalyst activity of Ni [8]. So, the ECR results clearly demonstrate the advantages of using Cu-SDC instead of Ni-SDC in the CO oxidation reaction.
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Fig. 3. The surface reaction kinetics in CO/CO2 mixtures. (a) normalized conductivity profiles versus relaxation time, (b) and kCu versus 3 PB density, (c) kCu versus Cu coverage, and (d) 3 PB contribution to the total CO oxidation.
3.2. Electrochemical performance 3.2.1. Electrochemical performance for H2 oxidation at Cu-SDC electrode Fig. 5a shows the impedance spectra for the Cu-SDC composite electrode measured under þ0.3 V overpotential, at 800 C and with different H2-H2O contents. The resistance corresponding to the electrolyte and lead wires is subducted to clearly comparing the electrode performance. It is obviously that the interfacial polarization resistance, Rp, decreases with the increase of vapor content, from 29.5 U cm2 for 6% H2O to 18.5 U cm2 for 42%. For a composite electrode with average particle size of 0.3 mm, the 3 PB length per unit volume is about 6.2 mm2 [24], corresponding to 2.48 mm1, the 3 PB length per unit area. And 3 PB length increases with the decrease of particle size. As ECR results shown, the contribution of 3 PB to the total reaction is 81.3% for H2 oxidation and 66.8% for CO oxidation when L3PB is 2.27 mm1. So, for a typical SDC-Cu composite electrode, the contribution of 3 PB is higher than these values, and the reaction takes place at 3 PB with minor contribution from 2 PB. Considering the vapor effect on electrode reaction and the significant contribution of 3 PB to H2 oxidation, elementary steps are suggested for the electrode reaction at Cu-SDC-gas 3 PB,
listed in Table 2. By kinetic derivation, a relationship between Rp and vapor partial pressure can be obtained. Eq. (5) and Eq. (6) are the general form for the reaction rate r (mol,s1,cm2) of an elementary chemical reaction and a charge transfer step [25]:
r ¼ kf
Y
cpmm kr
m2Rf
Y
cqmm
(5)
m2Rr
anF h Y pm ð1 aÞnF h cm kCT;f exp rCT ¼ kCT;f exp RT RT m2Rf Y q cmm m2Rr
(6) Here, kf and kr represent the rate constants of the forward and reverse reaction, respectively. cm is the area-specific concentration of species m (mol,cm2). pm and qm are stoichiometric coefficient of species m in the forward and reverse reactions. n is the number of electrons transferred in the electrochemical reaction. a is the transfer coefficient. h is the overpotential, and F is the Faraday constant. Assuming that other reactions reach the equilibrium state
M. Zheng et al. / Journal of Power Sources 345 (2017) 165e175
171
Fig. 4. Ni-SDC performance in CO-CO2 atmospheres. (a) normalized conductivity profiles measured at 800 C when Ni is sputtered for 0 (bare SDC), 10, 20, and 30 s, (b) comparison of relaxation profiles for Cu, Ni and Cu-Ni alloy, which are all prepared by 40 s sputtering, (c) surface micrograph for Ni-SDC before the ECR test, and (d) surface micrograph for NiSDC after the ECR test.
very fast, as a result, the rate of the forward reaction is equal to the reverse reaction, and the global rate can be considered to be zero except the rate limiting reaction. So, according to iterative method, the relationship can be derived for faradaic current and partial pressure when an elementary reaction is in the process of rate control [26]. The faradaic current produced by a charge transfer reaction is expressed as
i ¼ nFr
(7)
Further, Rp can be obtained as
Rp ¼ h=i
(8)
Therefore, Rp is a function of vapor pressure, pH2 O , and hydrogen pressure, pH2 , as shown in Table 12. For example, Rp ap1 H2 O when step 3 H-3 is rate limiting while Rp apH24O pH24 is for H-4. Since Rp decreases with the increase of pH2 O , H-1, H-2 and H-5 are not the rate limiting step. When step H-3 and/or H-4 are the rate limiting ones, Rp decreases with the increase of vapor pressure, consistent with the experimental results shown in Fig. 5a. To verify the rate limiting step, Rp is plotted as a function of pH2 O . Fig. 5b presents a much better linear relationship than Fig. 5c, demonstrating H-3 is the rate limiting step. Fig. 5c shows a complicated relation between Rp and
1
3
pH24O pH24 , suggesting H-4 is not the limiting step in the experimental conditions. For an electrode consisting of ceria and Cu, the reaction could take place at both 2 PB and 3 PB. The reactions via 2 PB, the ceria-H2 interface, have been proposed in the literature. Zhang et al. [27] have shown that the charge transfer is accompanied with hydrogen atoms on the surface of ceria while Feng et al. [28] have stated that the surface oxygen vacancy is fully saturated with hydroxyl, which facilitates the charge transfer reaction. For the reaction at Cu-SDC-gas 3 PB, we propose the charge transfer step, H-3, in which water vapor combining with oxygen vacancy to form hydroxyl is the rate-limiting reaction. 3.2.2. Electrochemical performance for CO oxidation at Cu-SDC electrode Fig. 6a shows the impedance spectra for the Cu-SDC composite electrode measured under þ0.3 V overpotential in different CO-CO2 atmospheres. Rp decreases with the increase of CO content from 17.6 U cm2 for 15% CO to 4.4 U cm2 for 40% CO. The elementary reaction processes are thus proposed for CO oxidation at 3 PB, as 0 shown in Table 3, regarding the intermediate carbonate ðCO3 ÞO , as suggested by Feng and Yu [29,30] for the reactions via 2 PB, the ceria-CO interface. Increasing CO content could promote the for0 mation of adsorbates ðCO3 ÞO , thus increases the reaction rate, i.e. reduces the interfacial polarization resistance.
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Fig. 5. Impedance spectra measured at 800 C in H2-H2O atmosphere (a) and interfacial polarization resistance Rp as a function of the partial pressure for (b) H-3 and (c) H-4.
Table 2 Elementary steps for the electrode reaction at Cu-SDC-gas 3 PB in H2-H2O atmospheres and the Rp dependence on vapor pressure,pH2 O , and hydrogen pressure,pH2 . Step code
Elementary equation
Rpf
H-1: Hydrogen adsorption
H2 4H2ðCuÞ
p1 H2
H-2: Hydrogen dissociation and migration
H2ðCuÞ 42Hð3PBÞ ,, , H2 OðgÞ þ O O þ VO 42HOOð3PBÞ
p1 H2
H-3: Water dissociation H-4: Charge transfer
Hð3PBÞ þ HO,Oð3PBÞ 4H2 O,, Oð3PBÞ þ e
H-5: Water desorption
,, H2 O,, Oð3PBÞ 4H2 OðgÞ þ VO
Fig. 6b and c compare the Rp dependence on gas composition. A 1 3 good linear relationship is observed for Rp-pCO4 2 pCO4 , demonstrating that C-3 is the rate limiting step. Meanwhile, C-4 is not the limiting step in the experimental conditions 3since Fig. 6c shows a compli 1 cated relation between Rp and pCO4 2 pCO4 . Table 4 compares the mechanisms for H2 and CO oxidation via 2 PB and 3 PB between literature and our work. With the present work, the H2 and CO oxidation mechanisms for a Cu-ceria electrode, through both 2 PB and 3 PB, can be fully understood. It is noted that Rp for CO oxidation is much lower than that for H2. The result is consistent with the ECR measurement. It should be mentioned that for the Ni-YSZ electrodes, Rp for CO is higher than H2 [31]. Fig. 7 compares the Arrhenius plots of Rp for H2 and CO
p1 H2 O 1
3
pH24 O pH24 pH2 O
oxidation measured from 600 to 800 C. In addition to Rp, lower activation energy is also achieved for CO oxidation. The difference in activation energy and Rp indicates distinction in reaction mechanism, e.g. elementary reaction or rate determining step, and demonstrates an advantageous electrochemical reaction in CO-CO2 atmosphere, that is, CO is easier to be oxidized than H2 over CuSDC. 4. Conclusion Cu-ceria cermets can efficiently catalyze the oxidation of H2 and CO, the products from hydrocarbons via external and internal reforming for solid oxide fuel cells. In addition to electronic
M. Zheng et al. / Journal of Power Sources 345 (2017) 165e175
173
Fig. 6. Impedance spectra measured at 800 C in CO/CO2 atmosphere (a) and interfacial polarization resistance Rp as a function of the partial pressure for the charge transfer processes of (b) C-3 and (c) C-4 for CO oxidation.
Table 3 Elementary steps for CO oxidation at Cu-SDC-gas 3 PB and Rp dependence on the partial pressure of CO, pCO , and CO2, pCO2 . Step code
Elementary equation
Rpf
C-1: CO adsorption
COðgÞ 4COðCuÞ
1 p1 CO pCO2
C-2: CO migration
COðCuÞ 4COð3PBÞ
C-3: Charge transfer
,, COð3PBÞ þ 2O Oð3PBÞ 4ðCO3 ÞOð3PBÞ þ VOð3PBÞ þ e
1 p1 CO pCO2 0
1
3
34
1
pCO4 2 pCO4
0
C-4: Charge transfer
ðCO3 ÞOð3PBÞ 4CO2ð3PBÞ þ O Oð3PBÞ þ e
pCO2 pCO4
C-5: CO2 desorption
CO2ð3PBÞ 4CO2ðgÞ
p1 CO2
Table 4 The charge transfer reactions for H2 and CO oxidations at 2 PB and 3 PB over Cu-ceria electrodes. H2 oxidation via 2 PB, Zhang et al. [27]
, HðceriaÞ þ O O 4OHOðceriaÞ þ e , HðceriaÞ þ OHOðceriaÞ 4H2 OðgÞ þ VO,, þ e
H2 oxidation via 2 PB, Feng et al. [28]
, HðSDCÞ þ O O 4OHOðSDCÞ þ e , 2OHOðSDCÞ 4H2 OðgÞ þ VO,, þ O O
H2 oxidation via 3 PB, present work
,, , H2 OðgÞ þ O O þ VO 42HOOð3PBÞ Hð3PBÞ þ HO,Oð3PBÞ 4H2 OðgÞ þ VO,, þ e
CO oxidation via 2 PB, Feng & Yu [29,30]
,, COðSDCÞ þ 2O OðSDCÞ 4ðCO3 ÞOðSDCÞ þ VOðSDCÞ þ e
0
0
ðCO3 ÞOðSDCÞ 4CO2ðSDCÞ þ O OðSDCÞ þ e
CO oxidation via 3 PB, present work
0
,, COð3PBÞ þ 2O Oð3PBÞ 4ðCO3 ÞOð3PBÞ þ VOð3PBÞ þ e 0
ðCO3 ÞOð3PBÞ 4CO2ð3PBÞ þ
O Oð3PBÞ
þ e
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M. Zheng et al. / Journal of Power Sources 345 (2017) 165e175
kCu kchem
L Dchem sðtÞ sð0Þ sð∞Þ a
l3PB Rp pH2 O pH2 pCO pCO2 r kf kr cm pm qm n
a h Fig. 7. The Arrhenius plots of interfacial polarization resistance for Cu-SDC electrodes measured in different gas mixtures.
conductivity, Cu can effectively enhance the catalytic activity. For example, when Cu particles are formed by sputtering for 60 s, the effective surface exchange coefficients were increased from 1.42 105 cm,s1 to 7.15 105 cm s1 and from 1.22 104 cm,s1 to 3.49 104 cm s1 for H2 and CO oxidation, respectively. The reactions take place at both SDC-gas 2 PB and CuSDC-gas 3 PB. With the increasing amount of Cu particles, the surface reaction rate increases. 3 PB rather than 2 PB is the main reaction site for both H2 and CO oxidation. For instance, the reaction at 3 PB contributes 81.3% of the total H2 oxidation and of 66.8% of CO oxidation when 3 PB length is 2.27 mm1. Elementary reactions at 3 PB were proposed combining with AC impedance measurements. For H2 oxidation at 3 PB, the rate limiting step is the formation of hydroxyl from oxygen vacancy and water vapor while the transformation of carbon monoxide to carbonate adsorbate limits CO oxidation via 3 PB. When the oxidation reactions are compared, it is found that Cu-SDC has exhibited much higher catalytic performance for CO oxidation than H2 as follows: about fivefold high in reaction rate, lower interfacial polarization resistance and smaller activation energy for electrode reaction. Therefore, we believe that the CO oxidation at Cu-ceria anode is the preferential reaction when hydrocarbons are served as the fuel. Acknowledgment We gratefully acknowledge the financial supports from the National Nature Science Foundation of China (51372239 and 91645101). Glossary L3PB
q
keff
triple-phase boundary line density surface coverage of Cu particles effective surface exchange coefficient
F i
Cu catalytic and/or synergistic catalytic activity for H2 oxidation reaction chemical surface exchange coefficient characteristic thickness chemical bulk diffusion coefficient conductivity at time t conductivities at the initial time conductivities at the final time constant, numerically equals half of the sample thickness 3 PB contribution to the total reaction interfacial polarization resistance vapor pressure partial pressure of H2 partial pressure of CO partial pressure of CO2 elementary reaction rate rate constant of the forward reaction rate constant of the reverse reaction area-specific concentration of species m stoichiometric coefficient of species m in the forward reaction stoichiometric coefficient of species m in the reverse reaction electrons transferred in the electrochemical reaction transfer coefficient overpotential Faraday's constant faradaic current
Appendix A Elementary steps for the electrode reaction at Cu-SDC-gas 3 PB in H2-H2O atmospheres are shown below
H2 4H2ðCuÞ
(A1)
H2ðCuÞ 42Hð3PBÞ
(A2)
,, , H2 OðgÞ þ O O þ VO 42HOOð3PBÞ
(A3)
Hð3PBÞ þ HO,Oð3PBÞ 4H2 O,, Oð3PBÞ þ e
(A4)
,, H2 O,, Oð3PBÞ 4H2 OðgÞ þ VO
(A5)
The general form for the reaction rate r (mol,s1,cm2) of an elementary chemical reaction and a charge transfer step are
r ¼ kf
Y
cpv m kr
m2Rf
Y
q
cmm
(A6)
m2Rr
anF h Y pm ð1 aÞnF h cm kCT;f exp rCT ¼ kCT;f exp RT RT m2Rf Y q cmm m2Rr
(A7) Here, kf and kr represent the rate constants of the forward and reverse reaction, respectively. cm is the area-specific concentration of species m (mol,cm2). pm and qm are stoichiometric coefficient of species m in the forward and reverse reactions. n is the number of electrons transferred in the electrochemical reaction. a is the transfer coefficient. h is the overpotential, and F is the Faraday
M. Zheng et al. / Journal of Power Sources 345 (2017) 165e175
constant. Therefore, the reaction rate for each elementary step can be expressed as:
h i r1 ¼ k1 pH2 k1 H2ðCuÞ
(A8)
h i h i2 r2 ¼ k2 H2ðCuÞ k2 Hð3PBÞ
(A9)
Rp ¼ h=i
175
(A18)
Therefore,
h i h i2 ,, , r3 ¼ k3 pH2 O O O VO k3 HOOð3PBÞ
Rp fp1 H2
(A19)
Similarly, we can get the relationship between interfacial polarization resistance Rp and partial pressure when each elementary step dominates the total reaction rate for H2 and CO oxidation reaction.
(A10)
ih i h i Fh exp r4 ¼ k4 Hð3PBÞ HO,Oð3PBÞ exp k4 H2 O,, Oð3PBÞ 2RT Fh 2RT
Appendix B. Supplementary data
h
(A11) r5 ¼
h
k5 H2 O,, Oð3PBÞ
i
k5 pH2 O VO,,
(A12)
The total reaction rate is equal to the slowest elementary reaction rate, that is, other elementary reactions reach the equilibrium state very fast. As a result, the rate of the forward reaction is equal to the reverse reaction and the global rate can be considered to be zero except the rate limiting reaction. If the elementary step (A1) is the rate limiting reaction, the total reaction rate r is equal to r1 , and r2 ¼ r3 ¼ r4 ¼ r5 ¼ 0. So, according to iterative method, the total reaction rate can be written as
r ¼ r1 ¼ k1 pH2
k1 k2 k3 k24 k25 VO,, 2F h h i pH2 O exp RT k2 k3 k24 k25 O O (A13)
At equilibrium state, r ¼ 0. And the concentration of oxygen vacancy and the lattice oxygen ions are considered to be constant. Then, we can obtain the value of h.
h¼
RT pH2 O ln þ const: 2F pH2
(A14)
The total reaction rate is turned to be
r ¼ r1 ¼ k1 pH2
k1 k2 k3 k24 k25 VO,, h i pH2 k2 k3 k24 k25 O O
A(15)
The faradaic current produced by a charge transfer reaction is expressed as
i ¼ nFr
(A16)
Besides, at equilibrium state, the faradaic currents in forward and reverse reaction are the same value, and the total current r ¼ rf rr ¼ 0.
rf ¼ rr fpH2 Further, Rp can be obtained as
(A17)
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