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Pr2NiO4eAg composite cathode for low temperature solid oxide fuel cells with ceria-carbonate composite electrolyte Liangdong Fan a,b, Mingming Chen a,*, Chengyang Wang a, Bin Zhu b,** a
Key Laboratory for Green Chemical Technology of Ministry of Education, School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, PR China b Department of Energy Technology, Royal Institute of Technology, Stockholm S-100 44, Sweden
article info
abstract
Article history:
Pr2NiO4eAg composite was synthesized and evaluated as cathode component for low
Received 22 May 2011
temperature solid oxide fuel cells based on ceria-carbonate composite electrolyte. X-ray
Received in revised form
diffraction analysis reveals that the formation of a single phase K2NiF4etype structure
6 September 2011
occurs at 1000
Accepted 22 September 2011
composite electrolyte. Symmetrical cells impedance measurements prove that Ag displays
Available online 5 November 2011
acceptable electrocatalytic activity toward oxygen reduction reaction at the temperature
Keywords:
electrochemical performances than those of Ag-free cells. An improved maximum power
Low temperature solid oxide fuel cells
density of 695 mW cm2 was achieved at 600 C using Pr2NiO4eAg composite cathode, with
Composite electrolyte
humidified hydrogen as fuel and air as the oxidant. Preliminary results suggest that
C and Pr2NiO4eAg composite shows chemically compatible with the
range of 500e600 C. Single cells with Ag active component electrodes present better
K2NiF4-type structure cathode
Pr2NiO4eAg composite could be adopted as an alternative cathode for low temperature
Ag nano-particle
solid oxide fuel cells.
Impedance spectroscopy
Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
Solid oxide fuel cells (SOFCs), one of the most efficient energy conversion and low environmental impact devices, enable the conversion of chemical energy into electrical energy directly [1,2]. In the last several decades, great effort has been made to lower the working temperature of SOFCs to increase their applicability and competitiveness [3,4]. However, the electrolyte ohmic loss and electrode polarization resistance increase significantly as the decrease of working temperature, which cause substantial performance decline. Recently, novel ceriacarbonate composites have attracted great attentions and have shown high capabilities as functional electrolytes for SOFCs [5e13]. The applications of ceria-carbonate composite
electrolyte have realized the operation of SOFCs at low temperatures (300e600 C) without compromising fuel cell performances because of their impressive ionic conductivity. In the previous work, the most commonly used cathode materials are lithiated NiO (LiNiOx) for composite electrolytebased SOFCs. LiNiOx has shown excellent electro-catalytic activity toward oxygen reduction reaction (ORR) between 400 and 600 C [8,9,12]. However, the potential dissolution of NiO into the molten carbonate may cause the structural degradation and the short circuit [14,15]. Effort is therefore demanded to explore high electrochemical activity and structural compatible cathode materials for composite electrolyte-based SOFCs. Layered perovskite oxides with a K2NiF4-type structure have been extensively studied at recent years as cathode
* Corresponding author. Tel.: þ86 2227890481. ** Corresponding author. Tel.: þ46 87907403. E-mail addresses:
[email protected],
[email protected] (M. Chen),
[email protected] (B. Zhu). 0360-3199/$ e see front matter Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2011.09.124
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 7 ( 2 0 1 2 ) 1 9 3 8 8 e1 9 3 9 4
candidates for intermediate and low temperature SOFCs given their interesting transport and catalytic properties, thermochemical stability and compatibility with other cell components [16e21]. Among these layered oxides, Pr2NiO4 has been reported to possess the highest oxygen diffusion and surface exchange coefficient, and the largest amount of interstitial oxygen [22,23]. Therefore, an integration of these specific features and the effectively prohibited dissolution of Ni in perovskite lattice would give rise to a high and stable performance, when Pr2NiO4 is employed as cathode for low temperature SOFCs with composite electrolyte. However, Huang et al. [20] reported that the peak power density of SOFCs using ceria-carbonate composite electrolyte and La2NiO4 cathode was 325 mW cm2 at 600 C, which is much inferior to the LiNiOx cathode. In addition, previous studies showed that intermediate temperature SOFCs with K2NiF4-type electrodes always displayed high area specific resistances (ASR) even though their total conductivities were satisfactory [21]. According to suggestion of Tao [24], suitable cathode materials should display not only high electrical conductivity but also satisfactory electro-catalytic activity for ORR. Therefore, the high ASR values of K2NiF4-type electrode materials in intermediate/low temperature SOFCs may be the result of the inadequate electro-catalytic activity. Recently, many studies proved that performances of mixed ionic and electronic conductor cathodes can be improved by modified with small amount of high activity catalysts (Ag, Pt, Pd and Cu) [25e30]. Among various candidates, Ag has been extensively studied for its superior electronic conductivity, high catalytic activity, substantial oxygen solubility and mobility and more affordable than the cost-prohibitive Pd and Pt [31,32]. For example, Li et al. [29] studied the electrochemical performance of La1.6Sr0.4NiO4eAg composite as cathode for SOFCs and found that the polarization resistance was reduced from 3.0 U cm2 to 0.21 U cm2 at 700 C when 5 vol. % of Ag was added, and the active energy was correspondingly reduced from 1.45 eV to 1.22 eV. Zhou et al. [30] also demonstrated the charge-transfer process of ORR was significantly improved in an Ag promoted Ba0.5Sr0.5Co0.8Fe0.2O3 cathode. Although Ag modified perovskites show a positive effect, previous works are focusing on the high temperature operation (>600 C). The sintering and volatilization problems at high temperature retard its prospective application. In this report, combining the advantages of Pr2NiO4 (referred as PNO in this paper) and Ag, PNOeAg composite was employed as cathode component for low temperature SOFCs. The electrochemical performance of the PNOeAg composite electrode on the ceria-carbonate composite electrolyte was studied compared with the PNO electrode.
2.
Experimental
2.1.
Powder synthesis
The raw materials in this study are from SigmaeAldrich except Pr2O3 (99%) from Tianjin Guangfu Fine Chemicals of China. All chemicals were used as received without any further purification. Ce0.8Sm0.2O1.9e(Na/Li)2CO3 composite electrolyte (LNSDC) was prepared as described in our previous
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publication [11]. PNO powders were synthesized by a coprecipitation technique with Na2CO3 as a precipitant. In Fig. 1, a general preparation procedure for PNO and PNOeAg powders is presented. In a typical experiment, stoichiometric of Pr2O3 and Ni(NO3)2.6H2O were dissolved in distilled water with HNO3 to form a 0.5 M homogeneous solution. The above solution was then added drop-wise to a Na2CO3 solution (0.5 M) at a molar ratio of cations (Pr3þ þ Ni2þ) to CO2 3 as 1 : 2. NaOH solution (1 M) was used to adjust the pH value of the mixed solution to approximately 13. A green precipitant was then collected by filtration, rinsed with distilled water for three times and air-dried in an oven at 100 C for overnight. It was further sintered at 1000 C for 3 h to get a blank powder. After that, the blank powder was dispersed into AgNO3 solution (0.5 M) and dried at 100 C. The as-obtained powder was heat-treated at 450 C for 10 min to get the final product. The weight percentage of Ag to PNO oxide was fixed at 10 wt. %.
2.2. Cells fabrications and electrochemical measurements Electrolyte supported symmetrical cells and complete single cells were produced by a dry-pressing and co-firing technique. The electrode materials were prepared by mixing Ag, PNO, PNOeAg with LNSDC at a mass ratio of 2:1, respectively. Single cells with the structure of NieLNSDC/LNSDC/PNOeLNSDC or PNOeAgeLNSDC were fabricated. The anode, electrolyte, cathode powders were loaded into a die layer by layer and copressed under 300 MPa to form a sandwich pellet (13 mm in diameter and 0.8e1.0 mm in total thickness). The green pellets were then sintered in the air at 600 C for 30 min. Silver coated metallic nickel mesh was used as a current collector. All the electrochemical measurements were performed on the same equipment as reported in detail elsewhere [33,34] and the active area was 0.785 cm2. The currentevoltage data of single cells were recorded by a fuel cell test instrument (SM102, San Mu Corp., China) between 500 C and 600 C, using humidified H2 (3 vol. % H2O) and air as fuel and oxidant, respectively. The flow rate of fuel was kept at 80e100 mL min1 under 1 atm pressure. The symmetrical cells impedance spectra were performed on a computerized Versa
Fig. 1 e Schematic procedure for PNO and PNOeAg powders synthesis.
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STAT-4 electrochemical workstation (Princeton Applied research, USA) between 500 C and 600 C in air under open circuit voltage (OCV) condition. The applied frequency range is 100 kHze0.01 Hz with the amplitude of 20 mV. The obtained impedance data was fitted with an equivalent circuit using ZsimpWin software. The equivalent circuit contains RQ(C) element in series, which is suggested to fit the ORR process in porous mixed ionic and electronic conducting electrodes [35].
2.3.
Other characterizations
Thermo-gravimetric analysis (TGA) on the PNO precursor was carried out on a Mettler Toledo TGE/SDTA 851e in air from 50 C to 900 C under temperature raising rate of 5 C min1. Xray diffraction (XRD) powder patterns were recorded at room temperature using a D/max 2500 v/pc instrument (Rigaku Corp. Japan) with Cu Ka radiation, 40 kV and 200 mA. To study the compatibility between PNO-based (PNO and PNOeAg) electrodes and the LNSDC composite electrolyte, mixtures of PNO/LNSDC and PNOeAg/LNSDC were sintered at 650 C for 5 h, respectively. Powder microstructures were recorded by a Zeiss Ultra 55 Field-Emission Scanning Electron Microscopy (SEM) equipped with Energy-Dispersive X-ray Spectroscopy (EDS).
3.
Results and discussion
3.1.
Phase analysis
The TGA curve of the PNO precursor is shown in Fig. 2. Clearly, there exists a continuous weight loss with the increase of temperature. The total weight loss is about 19 wt% between 50 and 900 C. Loss of crystalline water takes place before 270 C. Besides, Three other major weight-loss regions (270e380 C, 380e680 C and 680e900 C), corresponding to the decomposition of hydroxide and carbonate intermediates, the release of lattice oxygen and formation of oxygen vacancy which lead to form the final desired phase, respectively, are observed. To find out the accurate formation temperature of the K2NiF4type crystal, the precursor was sintered at 800, 900 and 1000 C in series for XRD measurements. As can be seen from Fig. 3,
Fig. 2 e TG curve of the PNO precursor.
for the sample of 1000 C, the observed major peaks are fully indexed to the pattern of JCPDS data (file No. 86-0870) corresponding to Pr2NiO4. Conversely, the samples sintered at 800 and 900 C still present a few other phases. In addition, a close look at Fig. 3 shows that the peak intensity increases as the sintering temperature increase, and the average crystallite size of PNO sample sintered at 1000 C calculated by the Scherrer equation is approximately 69 nm. Fig. 4 also shows the XRD patterns of single phase and mixed phase materials. It can be seen that LNSDC shows the pattern of cubic fluorite-type of CeO2 and the carbonate is an amorphous state [11]. After sintered at 650 C for 5 h, all peaks for the PNO/LNSDC and PNO-Ag/LNSDC composites reflect the characteristics of their single-phase structures; and no any other peaks or peaks shifts can be observed. These demonstrated that PNO, LNSDC and Ag show chemically compatible with one another at 650 C. Considering that all operational temperatures in the following research are lower than this value, the cathode/electrolyte interfacial stability is assured. Also, it is interesting to observe the peaks of PNO in the composites become wide after calcinations. The similar observation was also reported by Zhang [36]. It was attributed to the molten salt effect, which needs further experiment to approve. On the other hand, improved electro-catalytic activity is desired for composite electrode material with smaller particle size.
3.2.
Powder microstructures
Fig. 5 presents the SEM images of PNO and PNO-Ag powders. Microscale features on the order of 0.5e1.5 mm were observed with spherical particles for PNO sample. The apparent average particle size is not matched with the XRD analysis on crystallite size, suggesting that PNO particles suffer a severe aggregation when sintered at 1000 C. For the PNO-Ag powders, nano-scale Ag particle with an average size of 150 nm was observed to load on the surface of the PNO substrate as shown in Fig. 5b. The nano-particle will increase triple phase boundary (TPB) length and supply a higher catalytic activity toward ORR. As a consequence, improved
Fig. 3 e XRD patterns of PNO precursors calcined at 800, 900, 1000 C for 3 h, respectively.
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Fig. 4 e XRD patterns of PNO, LNSDC, PNOeLNSDC and PNOeAgeLNSDC mixture, respectively.
electrochemical performance is expected. However, because of the capillarity effect of the solution in the scaffold, the Ag nano-particles are not uniformly distributed. Instead, isolated Ag clusters are formed. The EDS elements mapping analysis (see Fig. 5c and d) also prove this observation.
3.3.
Impedance analysis
The symmetrical cell impedance spectra with different electrode materials, i.e. Ag, PNO and PNO-Ag mixed with LNSDC respectively, are shown in Fig. 6aec. The impedance spectra of
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PNO-based electrodes (after subtracting the ohmic resistance) were evaluated by fitting to an equivalent circuit with a configuration of L (R1Q1) (R2Q2) (see Fig. 6d) using a Zsimpwin software. In this circuit, L is an inductance element arising from the stainless steel reactor and instrument leads. R1 and R2 are the polarization resistances at high frequency and low frequency, respectively, and Q1(Q2) is a constant phase element. The fitted parameters of R1 and R2 at different temperatures are detailed listed in the Table 1. The ASR calculated by the following equation: ASR ¼ Rp S/2 is also included, in which S is the electrode active area and the Rp is the whole electrode polarization resistance: Rp ¼ R1 þ R2. As expected, all the impedances decrease with the increase of temperature. The Rp of Ag electrode is clearly smaller than that of both PNO and PNOeAg electrodes at the whole measurement temperature range, suggesting that Ag electrode shows a higher catalytic activity at or below 600 C. The ASR of Ag-electrode is 0.6 U cm2 at 600 C, which is similar to the result reported by Xiao [37], where mesoporous nanostructure SDC and Ag nano-particle were prepared. These results prove that Ag electrode on the ceria-carbonate composite electrolyte shows acceptable electrochemical activity toward ORR. Besides, the ASR of PNOeAg electrode is smaller than that of PNO electrode, but larger than Ag electrode, which is different from Sholklapper’s result [38]: the impedance of LSMeAg composite is smaller than both of its single component electro-catalysts (Ag and LSM). And the composite effect was attributed to the unique nano-structural electrode prepared by a complex infiltration technology. Hence, the preparation process for the PNOeAg composite in our case is needed to optimize to obtain a homogeneous distribution microstructure. In addition, the impedance
Fig. 5 e SEM images of PNO (a) and PNOeAg (b), and corresponding quantitative EDS element mapping of Ni (c) and Ag (d) derived from (b).
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Fig. 6 e Nyquist curves of symmetrical cells with a) Ag, b) PNO and c) PNOeAg electrode in air at 600 C and d) the applied equivalent circuit model.
spectra shape of Ag electrode with two clear semi-arcs is also visibly different from PNO-based electrodes, suggesting that various ORR mechanisms/processes are existed. The ASRs of PNO electrode are 5.23, 2.80 and 2.10 U cm2 at 500, 550 and 600 C as shown in Table 1. They are reduced to 4.11, 2.59 and 1.88 U cm2 when 10 wt. % Ag nano-particle is added, suggesting the ORR activity is improved after the introduction of Ag. The improvement becomes more significant at low temperature compared with at high temperature, such as a 27% improvement at 500 C, while only 12% at 600 C. It can be seen from Fig. 6b and c that the impedance spectra with PNO-based electrodes possess two overlapped arcs. The small depressed arc on the left is associated with a high frequency process and the large arc on the right corresponds to a low frequency process, indicating there are at least two different processes occurring in ORR. According to references [39e41], the resistance (R1) at high frequency arc is related to charge (ion and electron) transfer resistances occurring at the current collector/electrode and the electrolyte/cathode interfaces. The resistance (R2) at low frequency zone corresponds to the non-charge transfer resistance, including resistances of oxygen dissociation and diffusion at the gas-cathode surface. It can be observed from Fig. 6b and c that the arc on the right is much bigger than that on the left, which suggests that oxygen diffusion and dissociation processes determinate the ORR rate. Further, it can be seen from Table 1, both high frequency and low frequency resistances of PNO-Ag electrode are smaller than those of PNO electrode, indicating that the addition of Ag not only reduces the charge transport resistance but also
hastens the oxygen diffusion and dissociation processes. These are attributed to the improved electronic conductivity, oxygen reduction catalytic activity and oxygen ionic mobility in PNOeAg electrode. Finally, their impedance spectra of these two cells (Fig. 6b and c) have a similar shape, indicating that impregnated Ag does not change the PNO electrode reaction processes significantly, which may be the result of small volume content of Ag in the composite electrode.
3.4.
Single cell performances
The various electrodes were further investigated in single cells. Fig. 7a and b show the IeV and IeP curves of the fuel cells with PNO-based electrodes between 500 and 600 C. The OCVs for the cells with PNO electrode are 1.04, 0.98 and 0.97 V at 500, 550 and 600 C, respectively. The values are close to the thermodynamic theoretical values by Nernst equation, indicating the composite electrolyte layer is acceptable gas-
Table 1 e The fitted resistance parameters of symmetrical cells with PNO-based electrodes. Temp. ( C)
500 550 600
PNO
PNOeAg
R1 (U)
R2 (U)
ASR (U cm2)
R1 (U)
R2 (U)
ASR (U cm2)
0.29 0.11 0.07
13.09 7.04 5.28
5.23 2.80 2.10
0.07 0.04 0.02
10.39 6.51 4.47
4.11 2.59 1.88
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composite electrolyte. The PNOeAg composite electrode exhibits reduced electrode polarization and improved electrochemical performance compared with pure PNO cathode. The impregnation of nano-Ag particle on PNO surface improves not only charge transfer but also the dissociation and diffusion process during oxygen reduction reaction, hence raises fuel cell performance. Peak power densities of 695 and 652 mW cm2 were achieved using PNO-based cathode with and without Ag component at 600 C, respectively. All the results suggest that the PNOeAg composite is a potential cathode for low temperature SOFCs. Further work will concentrate on Ag loading and its homogeneous distribution to achieve a higher electrochemical performance.
Acknowledgements Funding from the Swedish Governmental Agency for Innovation Systems (VINNOVA) is acknowledged. The work has been also partial supported by the Program of Introducing Talents to the University Disciplines (file No. B06006) and the Innoenergy project via the Division of Heat and Power Technology (Royal Institute of Technology (KTH), Sweden). The assistance from Xiaodi Wang and Ying Ma on the SEM/EDX characterizations is greatly appreciated. L. Fan is grateful for a scholarship from the Chinese Scholarship Council (No. 2010625060). Fig. 7 e IeV and IeP characteristics of the single cells with (a) PNO and (b) PNOeAg composite cathodes.
tightness and presents negligible electronic conduction. The peak power densities of 652 and 465 mW cm2 were reached at 600 and 500 C for the cell with PNO electrode, while increased to 695 and 556 mW cm2 for the cell with PNOeAg electrode. The performances reported here are slight lower than those of the cells with LiNiOx cathodes [10,42], but much higher than those of La2NiO4 cathode with the similar fuel cell configuration [20]. The corresponding OCV values are also enhanced after the addition of Ag. The results clearly demonstrate that the Ag modified PNO cathode can improve the cell performance, especially at a lower temperature. For SOFCs with nano-structural components, special attention is needed to concern on the long-term stability. The grain growth and sintering of nano-particle at raised temperature would decrease the number of electrochemical reaction active sites, and then lead to the electrochemical performance degradation [43,44]. In this study, the nanoparticle Ag contained composites are applied at low temperatures (£ 600 C), and thus it is expected to have a long-term electrocatalytic effect. Investigation of long-term stability of the SOFC with PNO-Ag composite cathode is under way, and the results will be presented in the future.
4.
Conclusions
In this study, Ag modified Pr2NiO4 was employed as the cathode for low temperature SOFCs with ceria-carbonate
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