Enhanced Oxygen Reduction Activity and Solid Oxide Fuel Cell

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Enhanced Oxygen Reduction Activity and Solid Oxide Fuel Cell Performance with a Nanoparticles-Loaded Cathode Xiaomin Zhang, Li Liu, Zhe Zhao, Baofeng Tu, Ding Rong Ou, Daan Cui, Xuming Wei, Xiaobo Chen, and Mojie Cheng Nano Lett., Just Accepted Manuscript • Publication Date (Web): 16 Feb 2015 Downloaded from http://pubs.acs.org on February 17, 2015

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Enhanced Oxygen Reduction Activity and Solid Oxide Fuel Cell Performance with a

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Nanoparticles-Loaded Cathode

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Xiaomin Zhang1,2, Li Liu1,2, Zhe Zhao1,2, Baofeng Tu1, Dingrong Ou1, Daan Cui1, Xuming Wei3,

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Xiaobo Chen4,*, Mojie Cheng1,*

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Division of Fuel Cells, Dalian National Laboratory for Clean Energy, Dalian Institute of

Chemical Physics, Chinese Academy of Sciences, Dalian, 116023, China. 2

University of Chinese Academy of Sciences, Beijing, 100049, China.

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State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of

Sciences, Dalian, 116023, China. 4

Department of Chemistry, University of Missouri – Kansas City, Kansas City, Missouri, 64110,

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USA.

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*Correspondence to: [email protected]; [email protected].

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1 ACS Paragon Plus Environment

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ABSTRACT

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Reluctant oxygen-reduction-reaction (ORR) activity has been a long-standing challenge limiting

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cell performance for solid oxide fuel cells (SOFCs) in both centralized and distributed power

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applications. We report here that this challenge has been overcome with co-loading of

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(La,Sr)MnO3 (LSM) and Y2O3 stabilized zirconia (YSZ) nanoparticles within a porous YSZ

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framework. This design dramatically improves ORR activity, enhances fuel cell output (200 –

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300 % power improvement), and enables superior stability (no observed degradation within 500

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h’s operation) from 600 C to 800 C. The improved performance is attributed to the intimate

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contacts between nanoparticulate YSZ and LSM particles in the three-phase boundaries in the

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cathode.

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KEYWORDS: Solid oxide fuel cells, oxygen reduction reaction, lanthanum strontium

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manganite - yittria-stablized zirconia, nanostructure, cathode

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Fuel cell technologies are widely expected to improve security of electricity supply,

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enable a ‘hydrogen economy’, reduce dependence on fossil fuels, and lessen CO2 and noxious

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pollutant emissions.1-3 Intermediate-temperature (600  800C) solid oxide fuel cells (IT-SOFCs)

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have been intensively investigated for both centralized and distributed power generations in the

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past decades.1-3 Unfortunately, high-performance and reliable IT-SOFCs still remain a big

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challenge, mainly due to the sluggish oxygen surface kinetics at the electrolyte surface and

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reluctant oxygen-reduction-reaction (ORR) activity in the cathode. Yittria-stablized zirconia

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(YSZ) is one of the most common electrolyte materials for SOFCs.3 However, with the decrease


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of the operation temperature the sluggish oxygen surface kinetics at YSZ electrolyte surface

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restrain the performance of SOFCs. To enhance the oxygen incorporation kinetics at YSZ

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surface, a thin YSZ layer was deposited onto the YSZ electrolyte surface by atomic layer

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deposition (ALD) and the results indicated the oxygen exchange coefficient achieved a 5-fold

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increase for the modified YSZ electrolyte,4,5 the cell performance was thus improved. The most

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studied cathode material, lanthanum strontium manganite (LSM) and yittria-stablized zirconia

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(YSZ) composite, requires high operation temperatures ( 800 C), and only has a limited

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performance (e.g. maximum values of 1.46 Acm-2 and 1.02 Wcm-2 for current density and power

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density, under an output voltage of 0.7 V at 800 C).6 It barely produces any useable output at

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lower operation temperatures (< 700 C),7 and has a poor stability, i.e. under high current

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density.8 Alternative cathode materials, such as La1-xSrxCo1-yFeyO3-δ,9 Ba1-xSrxCo1-yFeyO3-δ,10

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La0.6Sr0.4CoO3-δ11 and Sm0.5Sr0.5CoO3-δ,12 have been thus investigated to overcome the poor

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performance of LSM at lower temperatures. However, their practical applications are limited by

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the fast performance degradation due to their chemical incompatibility with YSZ electrolyte and

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their susceptibility to sintering and carbon dioxide.13-15

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In the cathode, LSM and YSZ particles are normally interconnected for good ion

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conduction and electronic conduction, respectively. Low ORR activity is commonly observed for

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traditional LSM-YSZ cathodes, i.e. when operated at temperatures lower than 700 C.6-8 In those

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cathode designs, LSM and YSZ particles of micrometer sizes are used to fabricate the cathodes.

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This causes a limited interfacial area of the three-phase boundary (TPB) between the air,

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electrode and electrolyte, and leads to a highly reluctant ORR reaction on the LSM catalyst.

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Increasing the interfacial area at TPB with nanostructured cathodes enhances the ORR activity

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and the cathode’s performance, by either mixing16 or co-precipitating17 of LSM/YSZ


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nanoparticles. However, ORR activity hasn’t been significantly raised due to particle

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coarsening.17 Recent studies have demonstrated reasonable performance improvements by direct

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loading of LSM nanoparticles onto a porous YSZ scaffold.18-21 The improvement was which was

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attributed the extension of three-phase boundaries (TPBs) at the interfaces between LSM

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nanoparticles and YSZ mainframe in the LSM-infiltrated cathode.19 However, no exciting results

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have been obtained due to their size increase at high temperatures.22 Here, we report that an

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innovative design of LSM-YSZ cathode with co-loaded LSM and YSZ nanoparticles within the

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porous YSZ framework can overcome these problems and achieve excellent cell performance

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with greatly enhanced ORR activity and high stability, even when operated at low temperature.

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This structure is somehow similar to the pomegranate-like structure that has successfully

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improved the electrode performance in lithium rechargeable batteries.23 Compared to the LSM-

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infiltrated cathode, the new design in this letter would possess the following benefits. First, the

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intimate interaction between the LSM and YSZ nanoparticles can effectively suppress the growth

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of LSM nanoparticles in the LSM-YSZ co-infiltrated cathode. Second, additional large amounts

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of TPBs could be created at the interfaces of the LSM and YSZ nanoparticles besides the

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interfaces of LSM nanoparticles and YSZ mainframe. Third, the YSZ of LSM-YSZ

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nanoparticles would construct additional paths for oxygen ion transportation. With an output

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voltage of 0.8 V, the LSM-YSZ nanoparticles-loaded cell (new cell) had power densities 200 %,

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250 %, 310 % times of the conventional LSM-YSZ cell (old cell) with values of 1.88, 0.96 and

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0.34 W cm-2 at 800, 700 and 600 oC respectively. Meanwhile, the new cell can deliver the similar

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output (voltage, current and power density) at an operation temperature at 100 oC lower than the

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old cell. Moreover, the new cell has shown unprecedentedly stable performance when operated

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at 600 C or 800 C during the 500 h test, with no apparent degradation observed.


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In order to prepare the pomegranate LSM-YSZ cathode, a porous YSZ nanoparticulate

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mainframe (Figure 1a) was prepared first using a tape-casting method, and then a LSM-YSZ

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nanoparticle sol (see supporting information Figure S1) was filled into the pores under vacuum,

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followed by annealing at high temperatures to desired crystallized LSM-YSZ nanoparticles and

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the good contact between these LSM-YSZ nanoparticles and the YSZ mainframe (see supporting

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information). In this case, increased interfacial area of TPB was achieved with the LSM-YSZ

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nanoparticles24 and a highly continuous pathway for the ionic conduction was formed as well

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with the YSZ mainframe (Figure 1b). Figure 1a shows a scanning electron microscopy (SEM)

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image of the porous YSZ nanoparticulate scaffold. The average size of the primary YSZ particles

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was around 100  200 nm in diameter. All the particles were interconnected with each other to

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form a continuous backbone for good ionic conductivity. Figure 1b and Figure S2 show the SEM

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image of the porous YSZ framework filled with LSM-YSZ nanoparticles. The size of LSM-YSZ

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nanoparticles was around 20  40 nm (see supporting information Figure S2). The LSM phase

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and YSZ phase crystallized separately from the as-prepared LSM-YSZ sol during sintering from

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the X-ray diffraction patterns of the LSM-YSZ, LSM and YSZ powder samples (see supporting

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information 1D and Figure S3). And the high-resolution transmission electron microscopy

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(HRTEM) image of the loaded LSM-YSZ nanoparticles in the new cathode shown in Figure S4

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of the supporting information and Figure 1c also indicated the highly crystallized LSM particles

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and YSZ particles were clearly distinguished and contacted intimately with each other in the

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loaded LSM-YSZ nanoparticles.17 These LSM-YSZ nanoparticles filled the YSZ pores very well

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and connected with each other to form another continuous network for good electronic and ionic

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conductivity. Meanwhile, a small portion of pores was still available for air penetration in this

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3D network. Together, this structure largely enhanced the TPB interfaces between the air, LSM


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nanoparticles (electronic conductor) and YSZ nanoparticles or YSZ mainframe (oxygen-ion

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conductor).24 For comparison, Figure 1d shows a typical SEM image of the old LSM-YSZ

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electrode obtained with a conventional slurry method (see supporting information). A network

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made of large particles and large voids with the sizes of 200  1000 nm was obviously seen. This

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limited the available TPB interfaces for the ORR reaction. The corresponding Brunauer-Emmett-

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Teller (BET) surface areas of the new and old LSM-YSZ cathode were 3.48 and 0.79 m2/g

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respectively (see supporting information Table S1). This indicated much more TPBs were

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created in the new LSM-YSZ cathode. The BET surface areas of the YSZ mainframe before and

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after LSM-YSZ infiltration are 4.76 and 3.48 m2/g, respectively. Both were much higher than

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that mentioned in reference 18. The highly crystallized nature of both electrodes was seen from

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the strong diffraction peaks of the X-ray diffraction (XRD) patterns as well (see supporting

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information Figure S5). The wider peaks’ widths of the new LSM-YSZ electrode indicated its

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nanocrystalline nature, which was consistent with the observations from SEM and HRTEM. In

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addition, we also prepared LSM-infiltrated cathode for comparison. We found that the addition

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of YSZ nanoparticles to the LSM nanoparticles effectively suppressed the growth of the LSM

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nanoparticles. In the absence of the YSZ nanoparticles, the average size of the LSM

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nanoparticles increased to 50 – 100 nm in diameter (supporting information Figures S6a and

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S7a) after sintered at 950 C for 1 h. In the presence of the YSZ nanoparticles, their average size

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was only between 20 – 40 nm (supporting information Figures S6b and S7b).

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To validate our proposed approach, we tested the performance of the complete fuel cells

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made with the new and old cathodes. NiO and YSZ were used as the anode and electrolyte

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materials, respectively (see supporting information for preparation of entire fuel cells, the testing

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procedure and setup Figure S8). Figures 2a and 2b show the cross-section SEM images of the


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SOFC cells made with the new and old LSM-YSZ electrodes. Figures 2c and 2d show the I-V

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and I-P curves of the new and old LSM-YSZ cells. Dramatic increase in cell performance was

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achieved with the new cell. Under voltage output of 0.8 V, the new LSM-YSZ cell delivered

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current (and power) densities of 2.30 (1.88), 1.19 (0.96) and 0.43 A cm-2 (0.34 W cm-2) at 800,

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700 and 600 oC respectively, about 200%, 250%, 310% times of the old LSM-YSZ cell.

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Meanwhile, the new LSM-YSZ cell operated at 600 oC or 700 oC gave a power density

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comparative to that of the old LSM-YSZ cell operated at 700 oC or 800 oC, respectively. This

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suggests that the new LSM-YSZ cell delivered the similar outputs of the old LSM-YSZ cell but

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at 100 oC lower operation temperature. In addition, the performances of the new and old LSM-

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YSZ cells under H2/Air conditions were also evaluated at various operating temperatures (see

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supporting information Figure S9). Similar performance improvement of the new LSM-YSZ cell

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was observed compared with the old LSM-YSZ cell. Under an output voltage of 0.8 V, the new

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LSM-YSZ cell delivered current (and power) densities of 1.61 (1.28), 0.83 (0.66) and 0.30 A cm-

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(0.24 W cm-2) at 800, 700 and 600 oC respectively, about 200 %, 260 %, 300 % times of the

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old LSM-YSZ cell. Under an output voltage of 0.7 V at 800 C, the new LSM-YSZ cell yielded

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2.43 A cm-2 and 1.68 W cm-2 for current density and power density, respectively. Apparently, the

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new LSM-YSZ cell outperformed (170 %) the best available LSM-YSZ cells in previous studies

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which showed maximum values of 1.46 A cm-2 and 1.02 W cm-2 for current density and power

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density, under an output voltage of 0.7 V at 800 C.6

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Moreover, we compared the performances of LSM-infiltrated YSZ cathode and the new

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LSM-YSZ cathode with the conventional LSM-YSZ cathode (see supporting information Figure

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S10). The new LSM-YSZ cathode displayed the best performance and the LSM-infiltrated YSZ

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cathode showed better performance than the conventional LSM-YSZ cathode. For example,


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under an output voltage of 0.8 V, the power densities of the new LSM-YSZ cell at 600 C is 0.34

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W cm-2, 150 % and 310 % times of that of the LSM infiltrated cell and old LSM-YSZ cell,

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respectively.

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Figure 3a shows the Nyquist AC impedance plots of the new and old LSM-YSZ cell

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measured at various operation temperatures of 600 C, 700 C and 800 C. The highest

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frequency intercept on the real axis of a Nyquist plot represents the total specific ohmic

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resistance (RΩ), and the distance between the lowest and the highest frequency intercepts

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corresponds to the total area specific polarization resistance (Rp) from both anode and cathode.

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The new LSM-YSZ cell showed drastically reduced Rp values: 0.270, 0.450 and 1.08 Ω cm2 at

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800, 700 and 600 °C, only as 62 %, 51 % and 37 % of the old LSM-YSZ cell (0.429, 0.860 and

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2.911Ω cm2 at 800, 700 and 600°C). This suggests that the total area specific polarization

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resistance (Rp) from both anode and cathode was largely reduced in the new cell. Meanwhile, the

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new LSM-YSZ cell had a RΩ value of 0.054, 0.087 and 0.19 Ω cm2 at 800, 700 and 600 °C

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respectively, lower than those of the old LSM-YSZ (0.075, 0.132 and 0.293 Ω cm2 at 800, 700

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and 600 °C). This suggests a better interfacial contact between the electrolyte and cathode.

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In order to identify the contribution of each reaction process in the cell, the impedance

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spectra were fitted with an extended equivalent circuit model and analyzed through calculating

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the distributions of relaxation times (DRT).25,26 Each reaction process can be simulated with a

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component (P) in the equivalent circuit model. This component is made of a capacitor (C) and

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resistor (R) to represent the adsorption, diffusion and reaction steps, respectively. The extended

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equivalent circuit model and the fitting results for the impedance spectra of the new and old cells

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operated at 600 C are shown in Table 1. In this model, P1C, P2C, P3C and P3C’ represent four

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cathode processes, and P1A and P2A represent two anode processes. P1A represents for


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hydrogen charge transfer reaction,27,28 P2A for hydrogen gas diffusion in the anode supported

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layer.25,26,29 P1C represents for transport of oxygen anions across LSM/YSZ interfaces and

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through YSZ network in the cathode,26,27,30 P2C for ORR at TPBs,27,29 P3C for oxygen gas

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diffusion in cathode layer,26 and P3C’ for the oxygen species diffusion to the TPBs. From the

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fitting results listed in the table, we can see that the resistances in most components were

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reduced. Remarkably, the resistances for P1C and P2C were reduced in the new cell to less than

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35% of the old cell. The resistance of P1C for the transport of oxygen anions was reduced to

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0.084  cm2 in the new cell from 0.233  cm2 in the old cell, the resistance of P2C for the ORR

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at TPBs was reduced to 0.331  cm2 from 1.970  cm2, the resistance of P3C for oxygen gas

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diffusion in cathode layer was changed to 0.068  cm2 from 0.055  cm2, and the resistance of

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P3C’ for oxygen species diffusion to the TPBs was changed to 0.220  cm2 from 0.256  cm2.

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The higher resistance of the oxygen diffusion process (P3C) of the new cell was due to the

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reduced porosity and pore diameter, based on the based on the isotherm linear plots shown in

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Figure S11 in the supporting information. On the other hand, the resistances for the hydrogen

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reactions on the anode P1A (0.171  cm2) and P2A (0.221  cm2) were only slightly reduced.

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The P2C had the largest resistance (1.970  cm2) in old cell. In the new cell, this value was

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largely reduced (0.331  cm2). The resistance of the component is related to the apparent

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activation energy of the corresponding process. The higher the resistance, the larger the apparent

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activation energy is. This suggested that apparent activation energies for oxygen reaction were

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effectively reduced in the new cell. Similar conclusions were drawn on the impedance spectra

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when the cells were operated at 700 and 800 C as well (see supporting information Figures S12

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& S13 and Table S2).

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Meanwhile, the adsorption steps (C) were also affected in the new cells. The frequency (f)


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corresponds to the central reaction rate of the process, representing the overall reaction rate after

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taking into consideration of the contribution of the adsorption and activation energy barrier, as f

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= 1/(2RC). The higher value the f, the faster the corresponding reaction rate is. In the old cell,

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the ORR reaction was the most reluctant. In the new cell, the reaction rate of the ORR increased

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14 times and was not the most reluctant anymore; instead, the hydrogen diffusion in the anode

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(P2A) became the rate-limiting step. The detailed CNLS (complex non linear least-squares) fits

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of inductance corrected impedance spectra of the new and old cells at different temperatures and

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the corresponding Bode plots are shown in Figure S12 and S13 in the supporting information. It

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can be seen that the key rate determining process changes from P2A to P2C with decreasing

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temperature on both cells, and P2C is much smaller on the new cell than on the old cell at each

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temperature.

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Figure 3b shows the DRT plots for the new and old LSM-YSZ cells at 600 C. Six peaks,

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corresponding to the six processes in the extended equivalent circuit model, were identified on

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the new and old LSM-YSZ cells from 600 C to 800 C in Figure S14 and S15 of the supporting

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information. The area under each DRT peak corresponded to the importance of the resistance of

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that process. Apparently, for both cells, P2A was the key rate-determining process at 800 C,

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whereas P2C was the key rate-determining process at 600 C. P1A was only apparent below 700

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C and was not the key rate-determining step at all temperatures. P1C and P3C had small

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contributions and temperature dependences. P2C occurred at much faster frequencies at all the

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operation temperatures in the new cell than in the old cell. This suggested that the new design

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facilitated the ORR reaction at the TPBs. Meanwhile, it had the largest temperature dependence

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for both cells. At 800 C, its contribution on the rate-limiting was small for both cells, but

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increased dramatically with the decrease of the operation temperature, and finally predominated


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at 600 C for both cells. When the operation temperature was lowered to 600 C, P2C proceeded

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at a faster rate than the P2A in the new cell; however, P2C was slower than the P2A in the old

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cell. Apparently, upon the temperature change, the new cathode structure improved the reaction

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rate of P2C, the ORR at TPBs on the cathode.

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The ORR on LSM-YSZ cathode can be expressed as

1 O2  g  + 2e' +VO  OOX , where 2

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O2  g  , OOX , e' and VO are gas-phase oxygen, oxygen in a regular lattice site in the YSZ

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electrolyte, an electron, and an oxygen vacancy, respectively. The ORR is believed to occur on

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the active centers of pairs of surface oxygen vacancies and nearby Mn3+ ions at the TPB of LSM-

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YSZ cathode.31 The steps of ORR on the oxygen vacancies in the LSM-YSZ cathode can be

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measured from the oxygen desorption profile with a temperature-programmed desorption

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technique.32 As shown in Figure 3c, the low temperature band below 600 C was from oxygen

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molecules adsorbed on surface oxygen vacancies related to the interfacial interaction between

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LSM crystals and YSZ crystals.32 On the old cathode, this band was centered around 410 C with

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an onset of 250 C. On the new cathode, it centered around 310 C with an onset of 150 C, 100

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C lower than on the old cathode. Apparently, the activation energy of the adsorption and

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diffusion of oxygen molecules was lowered on the surface of the new LSM-YSZ cathode. The

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high temperature oxygen desorption band around 770 C was associated with ORR by the

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reduction of Mn4+ ions to Mn3+ ions and the formation of oxygen vacancies at the TPB in the

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cathode.32 The new cathode showed an onset temperature around 540 C, about 80C lower than

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that of the old cathode. Thus the activation energy of the ORR reaction was successfully lowered

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on the new LSM-YSZ cathode as well. This observation was consistent with the results from

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impedance and DRT analysis. This improvement was likely due to the larger number of available


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oxygen vacancies in the larger TPB and lower surface chemical bond strength due to the smaller

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sizes of the nanoparticle of the new LSM-YSZ cathode.

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The operation stability was evaluated by monitoring the output voltage under constant

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current density. The new cell displayed excellent performance and stability when operated at 600

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C or 800 C during a 500 h run (Figure 3d). When operated under a constant current density of

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0.40 Acm-2, the new cell outputted a voltage of 0.61 V and a power density of 0.24 Wcm-2 from

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the beginning and remained constant during entire 500 h run. Under similar conditions, the old

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cell only had 0.15 V and 0.06 Wcm-2 with an apparent degradation. The new design improved

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the output 400% over the old design and had superior stability. In order to further test the

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stability of the performance of the new design, the cell was tested under constant current density

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of 0.92 Acm-2 at 800 C. Generally, a current density of 0.75Ａcm-2 was chosen to evaluate

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accelerated degradation of the cell performance for the LSM-YSZ cell.8 Here we chose a much

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higher current density in order to exaggerate any possible performance degradation. Meanwhile,

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testing at higher temperature would reveal if there was any performance degradation from

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sintering of LSM-YSZ nanoparticles. The new cell steadily outputted a stable voltage of 0.81 V

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and a stable power density of 0.75 Wcm-2 during entire 500 h run. Apparently, this new LSM-

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YSZ cell overcame the commonly seen long-term stability problem encountered for

18

nanostructured cathodes22,33 and the poor output of old LSM-YSZ design. After the tests, the

19

structure of the pomegranate-structured cathode was analyzed with SEM, and no apparent size of

20

the LSM-YSZ nanoparticles or structural change of the cathode was observed (see supporting

21

information Figure S16 and Figure S17). It can be seen that the size of the LSM-YSZ

22

nanoparticles were kept in range of 20-40 nm before and after the stability test (Figure S17). The

23

possible explanation for the prevention of the coarsening of the nanoparticles in the new cathode


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Nano Letters

1

could be the co-loading of LSM particles and YSZ particles, as seen from Figure S7b. This

2

accounted for the high stability of the cell under operation over time, due to the intimate

3

interaction between nanoparticles of YSZ and LSM. On the other hand, the particle size of LSM

4

nanoparticles in the old cathode increased dramatically after the stability test, as seen from

5

Figure S18. The apparent particle growth in the old cathode was possibly due to poor heat budget

6

due to large polarization.22,34,35 In the new cathode, the co-loading of YSZ nanoparticles with

7

LSM nanoparticles may lower the polarization on the cathode and help the heat dissipation to

8

prevent the growth of the LSM nanoparticles (Figure S16 and S17). In addition, we found

9

different LSM:YSZ weight ratios in the cathode apparently affected the cell performance and

10

impedance. The cathode with 60 wt% LSM nanoparticles displayed the best performance and the

11

lowest impedance among the three cathodes loaded with 50 wt%, 60 wt%, 70 wt% LSM

12

nanoparticles, as shown in Figure S19 in the supporting information.

13

In summary, we demonstrated here enhanced battery performance can be achieved with a

14

nanoparticle-loaded SOFC cathode. This new cathode dramatically improves ORR activity,

15

enhances fuel cell output, and enables superior stability from 600 C to 800 C. The improved

16

performance is likely due to the higher TPB densities from higher surface enhancement,

17

improved LSM-YSZ particle-to-particle contact, and lowering onset temperature of Mn4+ to

18

Mn3+ in the new cathode. The improved stability is likely due to the high-temperature stability of

19

the loaded LSM nanoparticles where the co-loaded YSZ nanoparticles successfully suppressed

20

the particle growth of the LSM nanoparticles, the intimate contacts between the co-loaded LSM-

21

YSZ nanoparticles and with the YSZ framework and better heat budget in the new cathode. This

22

study thus provides a promising approach in obtaining stable and high-performance at various

23

operation temperatures for IT-SOFCs.


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Page 14 of 24

1 2

ASSOCIATED CONTENT

3

Supporting Information

4

Experimental details, Figures S1-S19, Table S1 and S2. This material is available free of charge

5

via the Internet at http://pubs.acs.org.

6 7

AUTHOR INFORMATION

8

Corresponding Authors

9

Email: (M.C.) [email protected];

10

Email: (X.C.) [email protected].

11

Author Contributions:

12

M. C. and X. C. conceived the idea. X. Z. and L. L. prepared the cells and conducted

13

electrochemical measurements. Z. Z. and B. T. conducted the TPD characterization of the

14

cathodes. D. C. analyzed the impedance spectra. D. O. and X. W. conducted the SEM and TEM

15

measurements and analysis of the results. M. C. and X. C. wrote the manuscript with

16

contributions from all authors; everyone participated in discussions and analysis of the results.

17

Notes

18

The authors declare no competing financial interest.

19

ACKNOWLEDGMENTS

20

M.C. thanks the financial support from the National Science Foundation of China (No.21376238,

21

21306189, 21076209, 51101146/E011002) and the Ministry of Science and Technology

22

(No.2010CB732302, 2012CB215500 and 2011AA050704) is gratefully acknowledged. X. C. 14 ACS Paragon Plus Environment

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1

acknowledges the support of this research from the College of Arts and Sciences, University of

2

Missouri  Kansas City (UMKC) and University of Missouri Research Board.

3 4

REFERENCES

5

1. Wachsman, E. D.; Lee, K. T. Lowering the temperature of solid oxide fuel cells. Science 2011,

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334, 935–939.

7

2. Steele, B. C. H.; Heinzel, A. Materials for fuel-cell technologies. Nature 2001, 414, 345–352.

8

3. Brett, D. J. L.; Atkinson, A.; Brandon, N. P.; Skinner, S. J. Intermediate temperature solid

9 10 11 12 13

oxide fuel cells. Chem. Soc. Rev. 2008, 37, 1568–1578. 4. Chao, C.-C.; Kim, Y. B.; Prinz, F. B. Surface Modiﬁcation of Yttria-Stabilized Zirconia Electrolyte by Atomic Layer Deposition. Nano Lett. 2009, 9, 3626–3628. 5. Chao, C.-C.; Park, J. S.; Tian, X.; Shim, J. H.; Gür, T. M.; Prinz, F. B. Enhanced Oxygen Exchange on Surface-Engineered Yttria-Stabilized Zirconia. ACS Nano 2013, 7, 2186–2191.

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6. Haanappel, V. A. C.; Mertens, J.; Rutenbeck, D.; Tropartz, C.; Herzhof, W.; Sebold, D.; Tietz,

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F. Optimisation of processing and microstructural parameters of LSM cathodes to improve the

16

electrochemical performance of anode-supported SOFCs. J. Power Sources 2005, 141, 216–

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226.

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7. Liu, L.; Zhao, Z.; Zhang, X. M.; Cui, D. A.; Tu, B. F.; Ou, D. R.; Cheng, M. J. A ternary

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cathode composed of LSM, YSZ and Ce0.9Mn0.1O2-δ for the intermediate temperature solid

20

oxide fuel cells. Chem. Commun. 2013, 49, 777–779.

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8. Hagen, A.; Liu, Y. L.; Barfod, R.; Hendriksena, P. V. Assessment of the cathode contribution

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to the degradation of anode-supported solid oxide fuel cells. J. Electrochem. Soc. 2008, 155,

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B1047–B1052.


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9. Tu, H. Y.; Takeda, Y.; Imanishi, N.; Yamamoto, O. Ln0.4Sr0.6Co0.8Fe0.2O3-δ (Ln = La, Pr, Nd,

2

Sm, Gd) for the electrode in solid oxide fuel cells. Solid State Ionics 1999, 117, 277–281.

3

10. Shao, Z. P.; Haile, S. M. A high-performance cathode for the next generation of solid-oxide

4 5 6 7 8

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fuel cells. Nature 2004, 431, 170–173. 11. Yamamoto, O.; Takeda, Y.; Kanno, R.; Noda, M. Perovskite-type oxides as oxygen electrodes for high temperature oxide fuel cells. Solid State Ionics 1987, 22, 241–246. 12. Xia, C. R.; Rauch, W.; Chen, F. L.; Liu, M. L. Sm0.5Sr0.5CoO3 cathodes for low-temperature SOFCs. Solid State Ionics 2002, 149, 11–19.

9

13. Simner, S. P.; Anderson, M. D.; Engelhard, M. H.; Stevenson, J. W. Degradation

10

Mechanisms of La–Sr–Co–Fe–O3 SOFC Cathodes. Electrochem. Solid-State Lett. 2006, 9,

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A478–A481.

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14. Yan, A. Y.; Cheng, M. J.; Dong, Y. L.; Yang, W. S.; Maragou, V.; Song, S. Q.; Tsiakaras, P.

13

Investigation of a Ba0.5Sr0.5Co0.8Fe0.2O3−δ based cathode IT-SOFC I. The effect of CO2 on the

14

cell performance. Appl. Catal. B-Environ. 2006, 66, 64–71.

15

15. Zhao, Z.; Liu, L.; Zhang, X. M.; Wu, W. M.; Tu, B. F.; Ou, D. R.; Cheng, M. J. A

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comparison on effects of CO2 on La0.8Sr0.2MnO3+δ and La0.6Sr0.4CoO3-δ cathodes. J. Power

17

Sources 2013, 222, 542–553.

18

16. Song, H. S.; Kim, W. H.; Hyun, S. H.; Moon, J.; Kim, J.; Lee, H.-W. Effect of starting

19

particulate materials on microstructure and cathodic performance of nanoporous LSM–YSZ

20

composite cathodes. J. Power Sources 2007, 167, 258–264.

21

17. Kim, J.-H.; Song, R.-H.; Kim, J.-H.; Lim, T.-H.; Sun, Y.-K.; Shin, D.-R. Co-synthesis of

22

nano-sized LSM – YSZ composites with enhanced electrochemical property. J. Solid State

23

Electrochem. 2007, 11, 1385–1390.


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1 2 3 4 5 6 7 8 9 10

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18. Huang, Y. Y.; Vohs, J. M.; Gorte, R. J. Characterization of LSM-YSZ Composites prepared by impregnation methods. J. Electrochem. Soc. 2005, 152, A1347–A1353. 19. Vohs, J. M.; Gorte, R. J. High-Performance SOFC Cathodes Prepared by Inﬁltration. Adv. Mater. 2009, 21, 943–956. 20. Sholklapper, T. Z.; Lu, C.; Jacobson, C. P.; Visco, S. J.; De Jonghe L. C. LSM-infiltrated solid oxide fuel cell cathodes. Electrochem. Solid-State Lett. 2006, 9, A376–A378. 21. Zhi, M. J.; Mariani, N.; Gemmen, R.; Gerdes, K.; Wu, N. Q. Nanofiber scaffold for cathode of solid oxide fuel cell. Energy Environ. Sci. 2011, 4, 417–420. 22. Wang, W. S.; Gross, M. D.; Vohs, J. M.; Gorte, R. J. The stability of LSF-YSZ electrodes prepared by infiltration. J. Electrochem. Soc. 2007, 154, B439–B445.

11

23. Liu, N.; Lu, Z. D.; Zhao, J.; McDowell, M. T.; Lee, H.-W.; Zhao, W. T.; Cui, Y. A

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pomegranate-inspired nanoscale design for large-volume-change lithium battery anodes. Nat.

13

Nanotechnol. 2014, 9, 187–192.

14

24. Zhu, W.; Ding, D.; Xia, C. R. Enhancement in Three-Phase Boundary of SOFC Electrodes

15

by an Ion Impregnation Method: A Modeling Comparison. Electrochem. Solid-State Lett.

16

2008, 11, B83–B86.

17

25. Sonn, V.; Leonide, A.; Ivers-Tiffée, E. Combined deconvolution and CNLS fitting approach

18

applied on the impedance response of technical Ni/8YSZ cermet electrodes. J. Electrochem.

19

Soc. 2008, 155, B675–B679.

20

26. Liu, B.; Muroyama, H.; Matsui, T.; Tomida, K.; Kabata, T.; Eguchi, K. Analysis of

21

impedance spectra for segmented-in-series tubular solid oxide fuel cells. J. Electrochem. Soc.

22

2010, 157, B1858–B1864.

23

27. Barfod, R.; Mogensen, M.; Klemensø, T.; Hagen, A.; Liu, Y.-L.; Hendriksen, P. V. Detailed


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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

1

characterization of anode-supported SOFCs by impedance spectroscopy. J. Electrochem.

2

Soc. 2007, 154, B371–B378.

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28. Ramos, T.; Hjelm, J.; Mogensen, M. Towards quantification of relations between electrode polarisation and microstructure. J. Electrochem. Soc. 2011, 158, B814–B824. 29. Kornely, M.; Menzler, N. H.; Weber, A.; Ivers-Tiffée, E. Degradation of a high performance SOFC cathode by Cr-poisoning at OCV-conditions. Fuel Cells 2013, 13, 506–510.

7

30. Murray, E. P.; Tsai, T.; Barnett, S. A. Oxygen transfer processes in (La,Sr)MnO3/Y2O3 -

8

stabilized ZrO2 cathodes: an impedance spectroscopy study. Solid State Ionics 1998, 110,

9

235–243.

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31 Jiang, Y.; Wang, S. Z.; Zhang, Y. H.; Yan, J. W.; Li, W. Z. Kinetic study of the formation of

11

oxygen vacancy on lanthanum manganite electrodes. J. Electrochem. Soc. 1998, 145, 373–

12

378.

13

32. Zhang, M.; Yang, M.; Liu, B.; Hou, Z. F.; Dong, Y. L.; Cheng, M. J. La0.4Ce0.6O1.8–

14

La0.8Sr0.2MnO3–8 mol% yttria-stabilized zirconia composite cathode for anode-supported

15

solid oxide fuel cells. J. Power Sources 2008, 175, 739–748.

16

33. Shah, M.; Voorhees, P. W.; Barnett, S. A. Time-dependent performance changes in LSCF-

17

infiltrated SOFC cathodes: The role of nano-particle coarsening. Solid State Ionics 2011,

18

187, 64–67.

19 20 21 22

34. Bernardi, D.; Pawlikowski, E.; Newman, J. A general energy balance for battery systems. J. Electrochem. Soc.1985, 132, 5–12. 35. Jørgensen, M. J.; Holtappels, P.; Appel, C. C. Durability test of SOFC cathodes. J. Appl. Electrochem. 2000, 30, 411–418.

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1

FIGURE CAPTIONS

2

Figure 1. The microstructural properties of the electrodes. Cross-section scanning electron

3

microscopy (SEM) images of (a) YSZ mainframe, (b) the new pomegranate electrode with LSM-

4

YSZ nanoparticles loaded on YSZ electrode framework (new electrode), (c) high-resolution

5

transmission electron microscopy (HRTEM) image of the loaded LSM-YSZ nanoparticles, and

6

(d) a typical SEM image of conventional LSM-YSZ electrode (old electrode).

7

Figure 2. The microstructural properties and performances of the SOFC cells. Cross-

8

section scanning electron microscopy (SEM) images of (a) new and (b) old LSM-YSZ cells, (c)

9

I-V and (d) I-P curves of new and old LSM-YSZ cells at operation temperatures of 600 C, 700

10

C and 800 C with H2/O2 gases.

11

Figure 3. The electrochemical and stability properties of the SOFC cells. (a) EIS curves of

12

new and old LSM-YSZ cells at operation temperatures of 600 C, 700 C and 800 C with H2/O2

13

gases. (b) The plots of the distributions of relaxation times for new and old LSM-YSZ cells

14

operated at 600 C. (c) O2-TPD profiles of new and conventional LSM-YSZ cathodes. (d)

15

Comparison of the stability curves of the new and conventional LSM-YSZ cells operated at 600

16

and 800 C with H2/air gases.

17 18

TABLE CAPTIONS

19

Table 1. The equivalent circuit analysis results of the new and old LSM-YSZ cells operated

20

at 600 C. The equivalent circuit used to fit the EIS spectra is drawn on the top, the results are

21

listed in the table in the middle, and the corresponding electrochemical processes are written on

22

the bottom.


Nano Letters

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Page 20 of 24

1

c

2 3

Figure 1. The microstructural properties of the electrodes. Cross-section scanning electron

4

microscopy (SEM) images of (a) YSZ mainframe, (b) the new pomegranate electrode with LSM-

5

YSZ nanoparticles loaded on YSZ electrode framework (new electrode), (c) high-resolution

6

transmission electron microscopy (HRTEM) image of the loaded LSM-YSZ nanoparticles, and

7

(d) a typical SEM image of conventional LSM-YSZ electrode (old electrode).

8


Page 21 of 24

1

2

c

d 1.0

o

800 C o 700 C o 600 C

0.9 0.8 0.7 0

1

2

3 2

3

Current density / A/cm

2.4

Red: new LSM-YSZ cell Black: old LSM-YSZ cell

2

Power density (W/cm )


1.1

Voltage / V

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Nano Letters

1.8

1.2 o

800 C o 700 C o 600 C

0.6

0.0

0

1

2

3 2

Current density / A/cm

4

Figure 2. The microstructural properties and performances of the SOFC cells. Cross-

5

section scanning electron microscopy (SEM) images of (a) new and (b) old LSM-YSZ cells, (c)

6

I-V and (d) I-P curves of new and old LSM-YSZ cells at operation temperatures of 600 C, 700

7

C and 800 C with H2/O2 gases.

8


Nano Letters

1

a

b

1.0


old LSM-YSZ cell new LSM-YSZ cell

5.0Hz

0.8

Rp /cm

2

1.2

2

0.9

-Z" / cm

0.6 o

800 C

0.3

600oC 0.6 0.4

63 Hz 8912Hz 125 Hz

0.2

o

700 C

1122 Hz

o

600 C

0

1

0.0

2

3

-2

0

2

2

Z' / cm

2

6

d 0.8 new LSM-YSZ cathode old LSM-YSZ cathode

770C

Voltage / V

310C

410C

0.6 o

-2

800 C new cell 0.92 A cm current density

0.4

o

-2

600 C new cell 0.40 A cm current density o

-2

600 C old cell 0.40 A cm current density

0.2

200

400

600

800

0.0

0

o

3

4

log f / Hz

c Intensity / a.u

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 24

Temperature / C

100

200

300

400

500

Time / h

4

Figure 3. The electrochemical and stability properties of the SOFC cells. (a) EIS curves of

5

new and old LSM-YSZ cells at operation temperatures of 600 C, 700 C and 800 C with H2/O2

6

gases. (b) The plots of the distributions of relaxation times for new and old LSM-YSZ cells

7

operated at 600 C. (c) O2-TPD profiles of new and conventional LSM-YSZ cathodes. (d)

8

Comparison of the stability curves of the new and conventional LSM-YSZ cells operated at 600

9

and 800 C with H2/air gases.

10


Page 23 of 24

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Nano Letters

1 2

Table 1. The equivalent circuit analysis results of the new and old LSM-YSZ cells operated

3

at 600 C. The equivalent circuit used to fit the EIS spectra is drawn on the top, the results are

4

listed in the table in the middle, and the corresponding electrochemical processes are written on

5

the bottom. R0

R1C

R2C

R3C

R3C‘

R1A

R2A

C1C

Q2C

C3C

Q3C‘

C1A

Q2A

P1C

P2C

P3C

P3Cʹ

P1A

P2A

6

7 8

Element R0Old cell New cell R1C C1C R2C Q2C-T Old cell New cell Q2C-P R3C Old cell C3C New cell R3C‘ Q3C‘ R1A C1A R2A Q2A-T Q2A-P

R0

Freedom (Ω cm2) Free(+) 0.321 0.189 Free(+) Free(+) Fixed(X) Free(+) Fixed(X) Free(+) Free(+) Fixed(X) electrode Free(+) contact Free(+) resistance Fixed(X) Fixed(X) Fixed(X) Fixed(X)

Data File: Circuit Model File:

Mode: Maximum Iterations: Optimization Iterations: Type of Fitting: Type of Weighting:

R1C R2C R3C Value 2) (Ω cm (Ω cm2) Error (Ω cm2) 0.12387 1.970 N/A 0.055 0.233 0.084 0.053863 0.331 N/A 0.068 Q2C-Y0 0.00017948 N/A C3C C1C (Ω-1 cm-2 s-2 -2 (F cm0.039 ) N/A (F cm ) n) 0.0013176 7.765E-5 2.956E-2N/A 2.236E-2 2.349E-4 9.713E-3N/A 3.700E-2 1 f/Hz 0.10934 f/Hz N/A f/Hz 8912 125 0.0030746 4.4 N/A 8912 63 56

R3Cʹ

Error % (Ω cm2) N/A 0.256 N/A 0.220 N/AC3Cʹ N/A(F cm-2) N/A 4.892E-3 4.508E-3 N/A N/A f/Hz N/A 141 0.043 N/A N/A 158 oxygen 0.030916oxygen N/A oxygen N/Aoxygen species anions gas 0.42097reductionN/A N/A transport reaction at diffusion diffusion to 0.022 N/A N/A the TPBs in YSZ In cathode the TPBs 0.20835 N/A N/A 0.155 N/A N/A 0.96 N/A N/A

C:\Documents and Settings\liul\桌面 \yy\ 蒤 700conventional\700-LSMmixYSZ-6arcs-0. 5times.mdl Run Fitting / All Data Points (1 - 1) 100 0 Complex Calc-Modulus


R1A (Ω cm2) 0.185 0.171

9.642E-4 8.537E-4 f/Hz 891 1122

R2A (Ω cm2) 0.265 0.221 Q2A-Y0 (Ω-1 cm-2 sn) 0.148 0.168 f/Hz 5.0 5.0

hydrogen charge transfer reaction

hydrogen gas diffusion in anode

C1A (F cm-2)

Nano Letters

1

TOC

0.8

1.0 New LSM-YSZ cell

0.9 0.8 0.7 Old LSM-YSZ cell 0

2

1

Voltage / V

1.1

Voltage / V

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 24

800oC new cell 0.92 A cm-2 current density

0.6

600oC new cell 0.40 A cm-2 current density

0.4 0.2 600oC old cell 0.40 A cm-2 current density

2

3

Current density / A/cm2

0.0

0

100

200

300

400

500

Time / h

3

Loading nanoparticles of (La,Sr)MnO3 (LSM) and Y2O3 stabilized zirconia (YSZ) within the

4

porous YSZ framework dramatically improves the cathode’s ORR activity (14 times rate

5

enhancement), enhances fuel cell output (200 – 300 % power improvement), and enables

6

superior stability (no observed degradation within 500 h’s operation) from 600 C to 800 C.


Enhanced Oxygen Reduction Activity and Solid Oxide Fuel Cell

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