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High performance solid oxide fuel cells with Co1.5Mn1.5O4 infiltrated (La,Sr)MnO3-yittria stabilized zirconia cathodes
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 4 0 ( 2 0 1 5 ) 3 3 3 2 e3 3 3 7
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High performance solid oxide fuel cells with Co1.5Mn1.5O4 infiltrated (La,Sr)MnO3-yittria stabilized zirconia cathodes Xiaomin Zhang a,b,1, Li Liu a,b,1, Zhe Zhao a,1, Lei Shang a,b,1, Baofeng Tu a,1, Dingrong Ou a,1, Daan Cui a,1, Mojie Cheng a,* a
Division of Fuel Cells, Dalian National Laboratory for Clean Energy, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian, 116023, China b University of Chinese Academy of Sciences, Beijing, 100049, China
article info
abstract
Article history:
Solid oxide fuel cells with nano-sized Co1.5Mn1.5O4 (CMO) crystals infiltrated LSM-YSZ
Received 10 November 2014
cathodes have been investigated using XRD, SEM, EIS and cell performance measure-
Received in revised form
ments. 20~30 nm nanocrystals of Co1.5Mn1.5O4 are present on the surfaces of LSM and YSZ
15 December 2014
particles. The infiltrated cells display more than 2 times higher power density than the
Accepted 11 January 2015
non-infiltrated cell under 0.7 V at the same temperature in 600e700 C. The Co1.5Mn1.5O4
Available online 31 January 2015
infiltration reduces both ohmic resistance and polarization resistance in the cells. The distribution of relaxation times (DRT) analysis of the EIS data depicts that oxygen reduction
Keywords:
process is greatly accelerated on the infiltrated cathode, which is attributed to high cata-
LSM-YSZ cathode
lytic activity of nano-sized CMO crystals.
Co1.5Mn1.5O4
Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Distribution of relaxation times Oxygen reduction activity Three phase boundaries (TPBs)
Introduction Solid oxide fuel cell (SOFC) is a device that directly converts chemical energy of fuels into electricity with high efficiency and low emission level [1,2]. The lanthanum strontium manganate (LSM) and yittria-stablized zirconia (YSZ) composite is the classical cathode for high temperature SOFCs [3e6], but its catalytic activity for oxygen reduction becomes quite low when operation temperature is below 700 C [7,8]. In order to enhance oxygen reduction activity at low
temperatures, various methods, such as new processes for LSM-YSZ composites [9,10], new structure cathodes [11] and introducing active particles into LSM-YSZ cathode through infiltration [12,13] have been investigated. (Co,Mn)3O4 (CMO) spinel oxides exhibit good chromium retention capability, reasonable electronic conductivity and compatible coefficient of thermal expansion (CTE) with other SOFC components, which are primarily used as efficient protection coating for alloy interconnects in SOFC [14e17]. Recently, CMO oxides have also been adopted as SOFC cathodes for oxygen reduction reaction [18e20]. Liu et al. employed
* Corresponding author. Tel./fax: þ86 411 84379049. E-mail address: [email protected] (M. Cheng). 1 Tel./fax: þ86 411 84379049. http://dx.doi.org/10.1016/j.ijhydene.2015.01.040 0360-3199/Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
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 4 0 ( 2 0 1 5 ) 3 3 3 2 e3 3 3 7
Mn1.5Co1.5O4 (MCO) spinel oxide as SOFC cathode and got a peak power density of 386 mW cm2 at 800 C [19]. The infiltration of nano-sized Mn1.5Co1.5O4 crystals into YSZ scaffold attained a polarization resistance of 0.43 U cm2 at 800 C [20]. Although high catalytic activity was expected on the Mn1.5Co1.5O4 oxide, the cell with a Mn1.5Co1.5O4 cathode performance is lower than LSM-YSZ cathode [7]. This might come from the lower electronic conductivity of Mn1.5Co1.5O4 oxide than that of LSM [19,21]. Recently, a ternary cathode concept was raised on a nanosized Ce0.9Mn0.1O2d infiltrated LSM-YSZ cathode for illustrating the excellent performance at 600 C [22]. That is, LSM acts as the electron conductor, YSZ acts as the oxide ion conductor and the infiltrated catalyst located at three phase boundary takes the main responsibility for oxygen reduction reaction. The catalytic activity of infiltrated catalyst is critical for enhancing the cathode performance. In this work, nanosized Co1.5Mn1.5O4 (CMO) particles are introduced into the prepared LSM-YSZ cathode by infiltration method. The infiltrated cathodes can take advantage of the high catalytic activity of CMO nano-particles for oxygen reduction reaction and the high electronic conductivity of LSM for delivering electrons. The cells are investigated using XRD, SEM, EIS and cell performance measurement.
Experimental The NiO (from J.T.Baker)-YSZ (8% yttria stabilized zirconia; TZe8Y, Tosoh Corp., Tokyo, Japan) anode substrates were prepared by a tape-casting method. NiO and YSZ at a weight ratio of 45:55 were mixed thoroughly by ball milling. Then suitable organic binders, n-butanol solvent and graphite were added and grinded to form the NiO/YSZ slurry which was fabricated into anode substrate by tape-casting. The green anode substrate was cut into discs with a diameter of 25 mm and sintered at 1000 C for YSZ electrolyte preparation. Then the YSZ slurry was prepared onto the pre-sintered discs through slip-coating and a followed co-sintering at 1295 C for 4 h. The thickness of the obtained NiO-YSZ substrate and YSZ electrolyte are ~600 mm and ~10 mm respectively. The LSM-YSZ cathode is composed of a LSM-YSZ composite functional layer and a pure LSM current collecting layer. The (La0.8Sr0.2)0.9MnO3þd (LSM) powders of the LSM-YSZ functional layer were synthesized by citric acid ammonium assisted Pechini-type method [23] using nitrate salts as raw materials and calcined at 1100 C for 2 h to get the perovskite phase. Large size LSM powders of the current collecting layer with a composition of La0.8Sr0.2MnO3þd were prepared by solid state reaction using La2O3, SrCO3 and MnO2 as raw materials and sintered at 1400 C for 6 h to get pure perovskite phase. The cathodes were prepared on YSZ electrolyte through slurrycoating. The functional layer slurry, consisting of LSM and YSZ powders with a weight percentage of 60:40, organic binders and solvent binders, was coated on the YSZ electrolyte and sintered at 900 C. Then pure La0.8Sr0.2MnO3þd slurry was applied onto the LSM-YSZ functional layer and sintered at 1100 C for 2 h as current collector. The total thickness of the cathodes is 70e80 mm. The obtained cell is referred to LSMYSZ cell in the following.
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The Co1.5Mn1.5O4 infiltrated LSM-YSZ cathodes were prepared as follows. First, a precursor solution was prepared using Co(NO3)2.6H2O and Mn(NO3)2 (49e51%) as raw materials and glycine as complex reagent with a total metal ion (Co2þ and Mn2þ) concentration of 2 M. Then, the precursor solution was infiltrated into the LSM-YSZ cathodes under vacuum conditions and calcined at 500 C for 1 h in air to decompose the nitrate and organics. Different CMO loading was obtained through controlling the infiltration times. Lastly, the infiltrated cells were calcined at 700 C for 1 h in air to get the CMO spinel phases. The CMO loading in cathode was calculated through weighting up the cells before and after infiltration. The infiltrated cathodes with 2.3wt% and 4.8wt% CMO were prepared, and the corresponding cells were denoted as CMO2.3 cell and CMO-4.8 cell respectively. The X-Ray diffraction patterns of LSM-YSZ cathode and CMO infiltrated cathodes were collected in a 2q range of 20e80 on a Rigaku RINT D/ ˚ ) at 40 kV and Max-2500 with Cu Ka radiation (l ¼ 1.54 A 200 mA. The electrochemical performance of fuel cells were measured on a home-made device using humidified H2 (100 ml min1) as fuel and pure O2 (100 ml min1) as oxidant. The electrochemical impedance spectra (EIS) were measured in 600e700 C at open circuit voltage (OCV) with AC amplitude of 10 mV on a Solartron 1287 potentiostat combined with a 1260 frequency response analyzer. The frequency ranged from 1 MHz to 0.08 Hz. The microphotographs of the tested cells were taken on a JSM 7800F scanning electron microscope equipped with a field emission gun at 5 kV.
Results and discussion Fig. 1 shows the XRD patterns of LSM-YSZ cathode and the Co1.5Mn1.5O4 infiltrated LSM-YSZ cathodes. For the LSM-YSZ cathode, most diffraction peaks are assigned to LSM perovskite phase (2q ¼ 32.5 , 32.7 and 46.7 , PDF #401100) and YSZ fluorite phase (2q ¼ 30.1 , 50.1 and 59.7 , PDF #301468). A
Fig. 1 e XRD patterns of the (a) LSM-YSZ cathode, (b) CMO2.3 cathode and (c) CMO-4.8 cathode.
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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 4 0 ( 2 0 1 5 ) 3 3 3 2 e3 3 3 7
minor Mn3O4 phase is detected due to the excessive Mn of (La0.8Sr0.2)0.9MnO3þd employed in cathode. When 2.3 wt% CMO is introduced into the LSM-YSZ cathode, diffraction peaks at 2q values of 35.8 and 36.1 appear for MnCo2O4 cubic phase (PDF #840482) and Mn2CoO4 tetragonal phase (PDF #770471) respectively. The two diffraction peaks become stronger at 4.8wt% CMO loading. Thus, the CMO oxide infiltrated into the LSM-YSZ cathode is a mixture of cubic and tetragonal phases, which is consistent with the previous report [19]. Fig. 2 shows the cross-sectional SEM images of (a) the LSMYSZ cell and the cells infiltrated with (b) CMO-2.3 and (c) CMO4.8. It is observed from Fig. 2 (a) that the LSM-YSZ cathode displays a porous structure. LSM and YSZ particles with a size of 200e1000 nm are well connected with each other to construct continuous electron and oxygen ion conductive paths, which is important for the oxygen reduction reaction. The pore size ranges from several hundred nanometers to several micrometers. At the 2.3wt% CMO loading, nanocrystals of 20e30 nm distribute on the surfaces of LSM and YSZ particles as shown in Fig. 2 (b). At 4.8wt%CMO loading, almost one layer of nano-crystals covers on the whole surface of the LSM-YSZ cathode (Fig. 2 (c)). Fig. 3 shows the currentevoltageepower density (IeVeP) curves of the three cells at different temperatures. The high open circuit voltage of ~1.14 V at 700 C indicates that the YSZ electrolyte films are dense in all the tested cells. The power density of the LSM-YSZ cell is 414 mW cm2 at 700 C and 0.7 V, which increases to 861, 986 mW cm2 for CMO-2.3 cell and CMO-4.8 cell respectively. The increment in power density is more significant at low temperatures. At 600 C, the power densities of the CMO-2.3 cell and CMO-4.8 cell are 372 and 401 mW cm2 at 0.7 V respectively, which are 2.67 and 2.86 times as high as that of the LSM-YSZ cell (140 mW cm2). The electrochemical impedance spectra (EIS) of LSM-YSZ cell, CMO-2.3 cell and CMO-4.8 cell at different temperatures are shown in Fig. 4. The high frequency intercept of EIS on X (Z0 ) axis represents the overall ohmic resistances Rohm from the contributions of electrolyte, electrode, connection wires, and contact resistance between electrode and electrolyte. The distance between the highest and lowest frequency intercepts of EIS with X (Z0 ) axis corresponds to the total electrodes polarization resistances Rp. The ohmic resistances of CMO-2.3 cell and CMO-4.8 cell at 700 C are 0.145, and 0.136 U cm2 respectively, smaller than 0.196 U cm2 of the LSM-YSZ cell. One reason is that electronic conductive CMO particles increase the electronic conductivity in cathode and improve the cathode and YSZ electrolyte interfacial contact. Another reason is that the CMO particles coated on the LSMYSZ cathode surface reduce the contact resistance between cathode and current collector. The introduction of CMO nano-particles leads to a pronounced reduction in polarization resistance. The polarization resistances of CMO-2.3 cell and CMO-4.8 cell are 1.49 and 1.27 U cm2 at 600 C, only 45% and 39% of the LSM-YSZ cell (3.28 U cm2). As the same anode employed in the cells, the reduction in polarization resistance is attributed to the decreased cathodic polarization. Several processes usually contribute to the measured impedance spectra and overlap with each other. In order to elucidate the promoting effect of CMO on oxygen reduction, a method based on calculating the distribution of relaxation
times (DRT) is used to separate electrode processes with different time constants from the transformed EIS data [24]. The area under each DRT peak reflects the corresponding resistance of individual electrode process. The detailed process of DRT calculation strictly followed the procedures stated in Liu et al. [25] using software package Ftikreg. Fig. 5 (aec) shows the DRT plots of the three tested cells at different temperatures and the inserts of (a) and (c) present the transformation of DRT plots as a function of PO2 for the LSM-
Fig. 2 e SEM images of cross-sections of tested cells: (a) LSM-YSZ cell, (b) CMO-2.3 cell and (c) CMO-4.8 cell.
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 4 0 ( 2 0 1 5 ) 3 3 3 2 e3 3 3 7
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Fig. 3 e IeVeP curves of the (a) LSM-YSZ cell, (b) CMO-2.3 cell and (c) CMO-4.8 cell at different temperatures.
YSZ cell and CMO-4.8 cell at 700 C. Six peaks as labeled in the DRT plots are distinguished, which indicates at least six electrode processes are the rate-determined steps. P1C with summit frequency at ~10000 Hz, which dose not vary with PO2, is attributed to the oxygen ions transfer through the cathode/electrolyte interface and incorporation into the YSZ electrolyte [25e28]. P2C with summit frequency at 200e120 Hz and P3C with summit frequency at 10~2 Hz on
Fig. 4 e Electrochemical impedance spectra of (a) LSM-YSZ cell, (b) CMO-2.3 cell and (c) CMO-4.8 cell at different temperatures.
the non-infiltrated cell and at 100~10 Hz on the CMO infiltrated cells, showing a strong temperature dependency and PO2 dependency, are attributed to cathodic processes. Based on the characteristic frequency, the peaks P2C and P3C represent the surface diffusion process of the charged
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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 4 0 ( 2 0 1 5 ) 3 3 3 2 e3 3 3 7
dependency and is attributed to the anodic charge transfer process [24,29,31]. P2A is small at all temperatures and overlaps with P1A at low temperatures, which may be related to hydrogen dissociation and hydrogen atoms diffusion on nickel. P3A varies little with temperature and is attributed to gas diffusion in anode [24,29e31]. As shown in Fig. 5, P3C is the main rate determining step on the three cells, and changes significantly after the infiltration of CMO nano-particles. A very large P3C peak overlapped with P3A peak indicates that oxygen reduction process is the main rate-limiting step on the LSM-YSZ cell. After the introduction of the CMO nano-particles, P3C moves to higher frequency, and the corresponding peak areas are dramatically reduced. This reflects that oxygen reduction process is significantly accelerated. Concurrently, as indicated from the reduced P2C peak, the surface diffusion proecss of charged oxygen intermediate species is also accelerated after the CMO nanoparticles introduction due to the increased surface area and TPBs. Thus, the CMO nano-particles play the main role in catalyzing oxygen reduction reaction at the infiltrated cathodes. Liu et al. have proposed that MyCe1xyMnxO2d (M ¼ La, Gd) solid solutions, with huge amounts of oxygen vacancies on surface and easy Ce4þ/Ce3þ redox cycle and Mn4þ/Mn3þ redox cycle, displays excellent oxygen reduction activity [32]. Yang et al. and Joy et al. have found that the addition of Co oxide is beneficial to the oxidation of Mn3þ to Mn4þ [33,34]. Zhang et al. reported that the CMO oxide has the Mn4þ/Mn3þ redox couple in octahedral coordination [18]. The improvement in oxygen reduction of the CMO-infiltrated cathode may be due to high catalytic activity of CMO oxide. On the other hand, nano-sized CMO particles can break Sr-layer coverage on LSM particles and expose much more active sites for oxygen reduction.
Conclusions
Fig. 5 e DRT plots of (a) LSM-YSZ cell, (b) CMO-2.3 cell, (c) CMO-4.8 cell at different temperatures and oxygen partial pressure at 700 C for LSM-YSZ cell (insert of (a)) and CMO4.8 cell (insert of (c)).
The electrochemical performance of the LSM-YSZ cathode is greatly improved through infiltrating Co1.5Mn1.5O4 (CMO) nano-particles of 20~30 nm on the surface of the LSM-YSZ cathode. The infiltrated cell displays more than 2 times higher power density than the LSM-YSZ cell at the test temperatures. The distribution of relaxation times (DRT) analysis of the EIS results shows the accelerated oxygen reduction process and surface diffusion process of charged oxygen intermediate species on the CMO infiltrated cathode, which can be attributed to high catalytically oxygen reduction activity of CMO with easy Mn4þ/Mn3þ redox cycle and the increased surface area and TPBs.
Acknowledgment oxygen intermediate species and oxygen reduction process, respectively [25,29,30]. P1A in 8000~2000 Hz, P2A in 1500~1000 Hz and P3A with summit frequency at 5 Hz, which do not change with cathode composition and PO2, are three anodic processes. P1A displays a strong temperature
Financial support from the National Science Foundation of China (No.21376238, 21306189 and 21076209) and the Ministry of Science and Technology (No.2010CB732302 and 2012CB215500) is gratefully acknowledged.
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Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.elsevier.com/locate/he
High performance solid oxide fuel cells with Co1.5Mn1.5O4 infiltrated (La,Sr)MnO3-yittria stabilized zirconia cathodes Xiaomin Zhang a,b,1, Li Liu a,b,1, Zhe Zhao a,1, Lei Shang a,b,1, Baofeng Tu a,1, Dingrong Ou a,1, Daan Cui a,1, Mojie Cheng a,* a
Division of Fuel Cells, Dalian National Laboratory for Clean Energy, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian, 116023, China b University of Chinese Academy of Sciences, Beijing, 100049, China
article info
abstract
Article history:
Solid oxide fuel cells with nano-sized Co1.5Mn1.5O4 (CMO) crystals infiltrated LSM-YSZ
Received 10 November 2014
cathodes have been investigated using XRD, SEM, EIS and cell performance measure-
Received in revised form
ments. 20~30 nm nanocrystals of Co1.5Mn1.5O4 are present on the surfaces of LSM and YSZ
15 December 2014
particles. The infiltrated cells display more than 2 times higher power density than the
Accepted 11 January 2015
non-infiltrated cell under 0.7 V at the same temperature in 600e700 C. The Co1.5Mn1.5O4
Available online 31 January 2015
infiltration reduces both ohmic resistance and polarization resistance in the cells. The distribution of relaxation times (DRT) analysis of the EIS data depicts that oxygen reduction
Keywords:
process is greatly accelerated on the infiltrated cathode, which is attributed to high cata-
LSM-YSZ cathode
lytic activity of nano-sized CMO crystals.
Co1.5Mn1.5O4
Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Distribution of relaxation times Oxygen reduction activity Three phase boundaries (TPBs)
Introduction Solid oxide fuel cell (SOFC) is a device that directly converts chemical energy of fuels into electricity with high efficiency and low emission level [1,2]. The lanthanum strontium manganate (LSM) and yittria-stablized zirconia (YSZ) composite is the classical cathode for high temperature SOFCs [3e6], but its catalytic activity for oxygen reduction becomes quite low when operation temperature is below 700 C [7,8]. In order to enhance oxygen reduction activity at low
temperatures, various methods, such as new processes for LSM-YSZ composites [9,10], new structure cathodes [11] and introducing active particles into LSM-YSZ cathode through infiltration [12,13] have been investigated. (Co,Mn)3O4 (CMO) spinel oxides exhibit good chromium retention capability, reasonable electronic conductivity and compatible coefficient of thermal expansion (CTE) with other SOFC components, which are primarily used as efficient protection coating for alloy interconnects in SOFC [14e17]. Recently, CMO oxides have also been adopted as SOFC cathodes for oxygen reduction reaction [18e20]. Liu et al. employed
* Corresponding author. Tel./fax: þ86 411 84379049. E-mail address: [email protected] (M. Cheng). 1 Tel./fax: þ86 411 84379049. http://dx.doi.org/10.1016/j.ijhydene.2015.01.040 0360-3199/Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
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 4 0 ( 2 0 1 5 ) 3 3 3 2 e3 3 3 7
Mn1.5Co1.5O4 (MCO) spinel oxide as SOFC cathode and got a peak power density of 386 mW cm2 at 800 C [19]. The infiltration of nano-sized Mn1.5Co1.5O4 crystals into YSZ scaffold attained a polarization resistance of 0.43 U cm2 at 800 C [20]. Although high catalytic activity was expected on the Mn1.5Co1.5O4 oxide, the cell with a Mn1.5Co1.5O4 cathode performance is lower than LSM-YSZ cathode [7]. This might come from the lower electronic conductivity of Mn1.5Co1.5O4 oxide than that of LSM [19,21]. Recently, a ternary cathode concept was raised on a nanosized Ce0.9Mn0.1O2d infiltrated LSM-YSZ cathode for illustrating the excellent performance at 600 C [22]. That is, LSM acts as the electron conductor, YSZ acts as the oxide ion conductor and the infiltrated catalyst located at three phase boundary takes the main responsibility for oxygen reduction reaction. The catalytic activity of infiltrated catalyst is critical for enhancing the cathode performance. In this work, nanosized Co1.5Mn1.5O4 (CMO) particles are introduced into the prepared LSM-YSZ cathode by infiltration method. The infiltrated cathodes can take advantage of the high catalytic activity of CMO nano-particles for oxygen reduction reaction and the high electronic conductivity of LSM for delivering electrons. The cells are investigated using XRD, SEM, EIS and cell performance measurement.
Experimental The NiO (from J.T.Baker)-YSZ (8% yttria stabilized zirconia; TZe8Y, Tosoh Corp., Tokyo, Japan) anode substrates were prepared by a tape-casting method. NiO and YSZ at a weight ratio of 45:55 were mixed thoroughly by ball milling. Then suitable organic binders, n-butanol solvent and graphite were added and grinded to form the NiO/YSZ slurry which was fabricated into anode substrate by tape-casting. The green anode substrate was cut into discs with a diameter of 25 mm and sintered at 1000 C for YSZ electrolyte preparation. Then the YSZ slurry was prepared onto the pre-sintered discs through slip-coating and a followed co-sintering at 1295 C for 4 h. The thickness of the obtained NiO-YSZ substrate and YSZ electrolyte are ~600 mm and ~10 mm respectively. The LSM-YSZ cathode is composed of a LSM-YSZ composite functional layer and a pure LSM current collecting layer. The (La0.8Sr0.2)0.9MnO3þd (LSM) powders of the LSM-YSZ functional layer were synthesized by citric acid ammonium assisted Pechini-type method [23] using nitrate salts as raw materials and calcined at 1100 C for 2 h to get the perovskite phase. Large size LSM powders of the current collecting layer with a composition of La0.8Sr0.2MnO3þd were prepared by solid state reaction using La2O3, SrCO3 and MnO2 as raw materials and sintered at 1400 C for 6 h to get pure perovskite phase. The cathodes were prepared on YSZ electrolyte through slurrycoating. The functional layer slurry, consisting of LSM and YSZ powders with a weight percentage of 60:40, organic binders and solvent binders, was coated on the YSZ electrolyte and sintered at 900 C. Then pure La0.8Sr0.2MnO3þd slurry was applied onto the LSM-YSZ functional layer and sintered at 1100 C for 2 h as current collector. The total thickness of the cathodes is 70e80 mm. The obtained cell is referred to LSMYSZ cell in the following.
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The Co1.5Mn1.5O4 infiltrated LSM-YSZ cathodes were prepared as follows. First, a precursor solution was prepared using Co(NO3)2.6H2O and Mn(NO3)2 (49e51%) as raw materials and glycine as complex reagent with a total metal ion (Co2þ and Mn2þ) concentration of 2 M. Then, the precursor solution was infiltrated into the LSM-YSZ cathodes under vacuum conditions and calcined at 500 C for 1 h in air to decompose the nitrate and organics. Different CMO loading was obtained through controlling the infiltration times. Lastly, the infiltrated cells were calcined at 700 C for 1 h in air to get the CMO spinel phases. The CMO loading in cathode was calculated through weighting up the cells before and after infiltration. The infiltrated cathodes with 2.3wt% and 4.8wt% CMO were prepared, and the corresponding cells were denoted as CMO2.3 cell and CMO-4.8 cell respectively. The X-Ray diffraction patterns of LSM-YSZ cathode and CMO infiltrated cathodes were collected in a 2q range of 20e80 on a Rigaku RINT D/ ˚ ) at 40 kV and Max-2500 with Cu Ka radiation (l ¼ 1.54 A 200 mA. The electrochemical performance of fuel cells were measured on a home-made device using humidified H2 (100 ml min1) as fuel and pure O2 (100 ml min1) as oxidant. The electrochemical impedance spectra (EIS) were measured in 600e700 C at open circuit voltage (OCV) with AC amplitude of 10 mV on a Solartron 1287 potentiostat combined with a 1260 frequency response analyzer. The frequency ranged from 1 MHz to 0.08 Hz. The microphotographs of the tested cells were taken on a JSM 7800F scanning electron microscope equipped with a field emission gun at 5 kV.
Results and discussion Fig. 1 shows the XRD patterns of LSM-YSZ cathode and the Co1.5Mn1.5O4 infiltrated LSM-YSZ cathodes. For the LSM-YSZ cathode, most diffraction peaks are assigned to LSM perovskite phase (2q ¼ 32.5 , 32.7 and 46.7 , PDF #401100) and YSZ fluorite phase (2q ¼ 30.1 , 50.1 and 59.7 , PDF #301468). A
Fig. 1 e XRD patterns of the (a) LSM-YSZ cathode, (b) CMO2.3 cathode and (c) CMO-4.8 cathode.
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minor Mn3O4 phase is detected due to the excessive Mn of (La0.8Sr0.2)0.9MnO3þd employed in cathode. When 2.3 wt% CMO is introduced into the LSM-YSZ cathode, diffraction peaks at 2q values of 35.8 and 36.1 appear for MnCo2O4 cubic phase (PDF #840482) and Mn2CoO4 tetragonal phase (PDF #770471) respectively. The two diffraction peaks become stronger at 4.8wt% CMO loading. Thus, the CMO oxide infiltrated into the LSM-YSZ cathode is a mixture of cubic and tetragonal phases, which is consistent with the previous report [19]. Fig. 2 shows the cross-sectional SEM images of (a) the LSMYSZ cell and the cells infiltrated with (b) CMO-2.3 and (c) CMO4.8. It is observed from Fig. 2 (a) that the LSM-YSZ cathode displays a porous structure. LSM and YSZ particles with a size of 200e1000 nm are well connected with each other to construct continuous electron and oxygen ion conductive paths, which is important for the oxygen reduction reaction. The pore size ranges from several hundred nanometers to several micrometers. At the 2.3wt% CMO loading, nanocrystals of 20e30 nm distribute on the surfaces of LSM and YSZ particles as shown in Fig. 2 (b). At 4.8wt%CMO loading, almost one layer of nano-crystals covers on the whole surface of the LSM-YSZ cathode (Fig. 2 (c)). Fig. 3 shows the currentevoltageepower density (IeVeP) curves of the three cells at different temperatures. The high open circuit voltage of ~1.14 V at 700 C indicates that the YSZ electrolyte films are dense in all the tested cells. The power density of the LSM-YSZ cell is 414 mW cm2 at 700 C and 0.7 V, which increases to 861, 986 mW cm2 for CMO-2.3 cell and CMO-4.8 cell respectively. The increment in power density is more significant at low temperatures. At 600 C, the power densities of the CMO-2.3 cell and CMO-4.8 cell are 372 and 401 mW cm2 at 0.7 V respectively, which are 2.67 and 2.86 times as high as that of the LSM-YSZ cell (140 mW cm2). The electrochemical impedance spectra (EIS) of LSM-YSZ cell, CMO-2.3 cell and CMO-4.8 cell at different temperatures are shown in Fig. 4. The high frequency intercept of EIS on X (Z0 ) axis represents the overall ohmic resistances Rohm from the contributions of electrolyte, electrode, connection wires, and contact resistance between electrode and electrolyte. The distance between the highest and lowest frequency intercepts of EIS with X (Z0 ) axis corresponds to the total electrodes polarization resistances Rp. The ohmic resistances of CMO-2.3 cell and CMO-4.8 cell at 700 C are 0.145, and 0.136 U cm2 respectively, smaller than 0.196 U cm2 of the LSM-YSZ cell. One reason is that electronic conductive CMO particles increase the electronic conductivity in cathode and improve the cathode and YSZ electrolyte interfacial contact. Another reason is that the CMO particles coated on the LSMYSZ cathode surface reduce the contact resistance between cathode and current collector. The introduction of CMO nano-particles leads to a pronounced reduction in polarization resistance. The polarization resistances of CMO-2.3 cell and CMO-4.8 cell are 1.49 and 1.27 U cm2 at 600 C, only 45% and 39% of the LSM-YSZ cell (3.28 U cm2). As the same anode employed in the cells, the reduction in polarization resistance is attributed to the decreased cathodic polarization. Several processes usually contribute to the measured impedance spectra and overlap with each other. In order to elucidate the promoting effect of CMO on oxygen reduction, a method based on calculating the distribution of relaxation
times (DRT) is used to separate electrode processes with different time constants from the transformed EIS data [24]. The area under each DRT peak reflects the corresponding resistance of individual electrode process. The detailed process of DRT calculation strictly followed the procedures stated in Liu et al. [25] using software package Ftikreg. Fig. 5 (aec) shows the DRT plots of the three tested cells at different temperatures and the inserts of (a) and (c) present the transformation of DRT plots as a function of PO2 for the LSM-
Fig. 2 e SEM images of cross-sections of tested cells: (a) LSM-YSZ cell, (b) CMO-2.3 cell and (c) CMO-4.8 cell.
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Fig. 3 e IeVeP curves of the (a) LSM-YSZ cell, (b) CMO-2.3 cell and (c) CMO-4.8 cell at different temperatures.
YSZ cell and CMO-4.8 cell at 700 C. Six peaks as labeled in the DRT plots are distinguished, which indicates at least six electrode processes are the rate-determined steps. P1C with summit frequency at ~10000 Hz, which dose not vary with PO2, is attributed to the oxygen ions transfer through the cathode/electrolyte interface and incorporation into the YSZ electrolyte [25e28]. P2C with summit frequency at 200e120 Hz and P3C with summit frequency at 10~2 Hz on
Fig. 4 e Electrochemical impedance spectra of (a) LSM-YSZ cell, (b) CMO-2.3 cell and (c) CMO-4.8 cell at different temperatures.
the non-infiltrated cell and at 100~10 Hz on the CMO infiltrated cells, showing a strong temperature dependency and PO2 dependency, are attributed to cathodic processes. Based on the characteristic frequency, the peaks P2C and P3C represent the surface diffusion process of the charged
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dependency and is attributed to the anodic charge transfer process [24,29,31]. P2A is small at all temperatures and overlaps with P1A at low temperatures, which may be related to hydrogen dissociation and hydrogen atoms diffusion on nickel. P3A varies little with temperature and is attributed to gas diffusion in anode [24,29e31]. As shown in Fig. 5, P3C is the main rate determining step on the three cells, and changes significantly after the infiltration of CMO nano-particles. A very large P3C peak overlapped with P3A peak indicates that oxygen reduction process is the main rate-limiting step on the LSM-YSZ cell. After the introduction of the CMO nano-particles, P3C moves to higher frequency, and the corresponding peak areas are dramatically reduced. This reflects that oxygen reduction process is significantly accelerated. Concurrently, as indicated from the reduced P2C peak, the surface diffusion proecss of charged oxygen intermediate species is also accelerated after the CMO nanoparticles introduction due to the increased surface area and TPBs. Thus, the CMO nano-particles play the main role in catalyzing oxygen reduction reaction at the infiltrated cathodes. Liu et al. have proposed that MyCe1xyMnxO2d (M ¼ La, Gd) solid solutions, with huge amounts of oxygen vacancies on surface and easy Ce4þ/Ce3þ redox cycle and Mn4þ/Mn3þ redox cycle, displays excellent oxygen reduction activity [32]. Yang et al. and Joy et al. have found that the addition of Co oxide is beneficial to the oxidation of Mn3þ to Mn4þ [33,34]. Zhang et al. reported that the CMO oxide has the Mn4þ/Mn3þ redox couple in octahedral coordination [18]. The improvement in oxygen reduction of the CMO-infiltrated cathode may be due to high catalytic activity of CMO oxide. On the other hand, nano-sized CMO particles can break Sr-layer coverage on LSM particles and expose much more active sites for oxygen reduction.
Conclusions
Fig. 5 e DRT plots of (a) LSM-YSZ cell, (b) CMO-2.3 cell, (c) CMO-4.8 cell at different temperatures and oxygen partial pressure at 700 C for LSM-YSZ cell (insert of (a)) and CMO4.8 cell (insert of (c)).
The electrochemical performance of the LSM-YSZ cathode is greatly improved through infiltrating Co1.5Mn1.5O4 (CMO) nano-particles of 20~30 nm on the surface of the LSM-YSZ cathode. The infiltrated cell displays more than 2 times higher power density than the LSM-YSZ cell at the test temperatures. The distribution of relaxation times (DRT) analysis of the EIS results shows the accelerated oxygen reduction process and surface diffusion process of charged oxygen intermediate species on the CMO infiltrated cathode, which can be attributed to high catalytically oxygen reduction activity of CMO with easy Mn4þ/Mn3þ redox cycle and the increased surface area and TPBs.
Acknowledgment oxygen intermediate species and oxygen reduction process, respectively [25,29,30]. P1A in 8000~2000 Hz, P2A in 1500~1000 Hz and P3A with summit frequency at 5 Hz, which do not change with cathode composition and PO2, are three anodic processes. P1A displays a strong temperature
Financial support from the National Science Foundation of China (No.21376238, 21306189 and 21076209) and the Ministry of Science and Technology (No.2010CB732302 and 2012CB215500) is gratefully acknowledged.
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