Advanced Fuel Cell Based on New Nanocrystalline Structure Gd0

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Energy, Environmental, and Catalysis Applications

Advanced Fuel Cell Based on New Nanocrystalline Structure Gd0.1Ce0.9O2 Electrolyte Gang Chen, Wenkang Sun, Yadan Luo, Yang He, Xuebai Zhang, Bin Zhu, Wenyuan Li, Xingbo Liu, Yushi Ding, Ying Li, Shujiang Geng, and Kai Yu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b20454 • Publication Date (Web): 22 Feb 2019 Downloaded from http://pubs.acs.org on February 22, 2019

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ACS Applied Materials & Interfaces

Advanced Fuel Cell Based on New Nanocrystalline Structure Gd0.1Ce0.9O2 Electrolyte Gang Chen*a , Wenkang Suna, Yadan Luoa, Yang Hea, Xuebai Zhanga, Bin Zhu*b,c, Wenyuan Lid, Xingbo Liu*d, Yushi Dinga, Ying Lia, Shujiang Genga, Kai Yua a

Liaoning Key Laboratory for Metallurgical Sensor and Technology, School of

Metallurgy, Northeastern University, Shenyang, Liaoning 110819, P.R. China b

Hubei Collaborative Innovation Center for Advanced Materials, Faculty of Physics and Electronic Science, Hubei University, Wuhan, Hubei 430062, P.R. China c

Department of Aeronautical & Automotive Engineering, Loughborough University, Loughborough, Leicestershire LE113TU, United Kingdom d

Department of Mechanical & Aerospace Engineering, West Virginia University, Morgantown, WV 26505, USA

Keywords: Solid oxide fuel cell, Gd0.1Ce0.9O2, electrolyte, nanocrystalline, interfacial conduction, transference number

*Corresponding author. E-mail:

[email protected]

(G.

Chen),

[email protected]

[email protected] (X. Liu)

ACS Paragon Plus Environment

(B.

Zhu),

ACS Applied Materials & Interfaces 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

Abstract Lowering the operating temperature is a universal R&D challenge for the development of low-temperature (< 600 °C) solid oxide fuel cells (SOFCs) that meets the demands of commercialization. Regarding the traditional electrolyte materials of SOFCs, bulk diffusion is the main ionic conduction mechanism, which is primarily affected by the bulk density and operating temperatures. In this study, we report a new mechanism for the Ce0.9Gd0.1O2-δ (GDC) electrolyte based on a nanocrystalline structure with surface or grain boundary conduction, exhibiting an extremely high ionic conductivity of 0.37 S·cm-1 at 550 °C. The fuel cell with the nanocrystalline structure GDC electrolyte (0.5 mm in thickness) can deliver a remarkable peak power density of 591.8 mW·cm-2 at 550 °C, which is approximately 3.5 times higher than that for the cell with the GDC electrolyte densified at 1550 °C. An amorphous layer enriched by oxygen vacancies was found at the surface of the nano-GDC particles in the fuel cell test atmosphere, which was attributed to the ion conduction channel of the grain boundary diffusion. The ionic conduction at the interfaces between the particles was discovered to be the dominant conduction mechanism of the nanocrystalline structure GDC electrolyte. Oxygen ions and protons were determined to be the charge carriers in this interfacial conduction phenomenon, and the conduction of oxygen ions is dominant.


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1. Introduction A popular consensus in fuel cell community is that reducing the operational temperature of solid oxide fuel cell (SOFC) will accelerate its commercialization. However, the intrinsic temperature dependence of the conventional SOFC electrolyte material, i.e., the decrease in the conductivity with decreasing temperature, leads to a contradiction between reducing the operational temperature and maintaining high electrochemical cell performance.1 The three most commonly used traditional SOFC electrolyte materials are stabilized ZrO2, Gd/Sm-doped ceria (GDC/SDC) and Sr- and Mg-doped La Ga (LSGM).2-7 The oxygen ion conductivity of the classical 8 mol.% Y2O3stabilized ZrO2 (YSZ) electrolyte material is 0.1 S·cm-1 at 1000 °C, while at 800 °C, the conductivity drops to 0.03 S·cm-1 and then decreases further to 0.0011 S·cm-1 at 600 °C.56

The conductivity of LSGM drops from 0.17 S·cm-1 at 800 °C to 0.046 S·cm-1 at

600 °C.7-8 Compared to YSZ, the doped ceria electrolyte has higher conductivity at low temperature, for example with the conductivity of 0.019-0.011 S·cm-1 obtained for Ce0.9Gd0.1O2-δ (GDC) at 600-500 °C.4 However, the electrolyte conductivity of 10-2 S·cm-1 is not sufficiently high for developing high performance low-temperature SOFC (LTSOFC). Therefore, it is imperative to either explore new methods for improving the conductivity of existing materials or to develop new electrolyte materials. It is well-known that most of electrolyte conductivity results are based on Pt electrodes, e.g., GDC electrolyte conductivity of 0.019 S·cm-1 at 600°C.4 In our previous studies, we found that the low-temperature ionic conductivity of GDC measured in a single cell prepared by using 0.5 mm thick GDC as electrolyte and Ni foamNi0.8Co0.15Al0.05LiO2 (NCAL) composite as symmetrical electrode was significantly



increased.9 It is found that Ni foam-NCAL electrodes have excellent catalytic activity for O2 reduction reaction (ORR) and H2 oxidation reaction (HOR).9 In addition, many studies have found that two-phase composite electrolytes can significantly enhance the ionic conductivity of the materials.10-12 Zhu et al. fabricated a composite electrolyte with SDC and carbonate, showing a conductivity of 0.1 S·cm-1 at 500°C.10 Unlike the conventional high-temperature densified electrolytes, the composite electrolyte has no need to undergo a high-temperature sintering process. The conduction mechanism of the composite electrolyte is also different from that of the traditional electrolytes (bulk and grain boundary diffusions). For example, carbonate and SDC formed a composite or core-shell structure with the amorphous carbonate shell and SDC core. The particles interact with each other and form a high-speed ion transport channel at the interface.11-15 Interfacial conduction is considered to play the dominant role in the composite structure.11-15 Recent studies have found that the conductivity of two-phase composite electrolytes without the carbonate material reached very high values.13-18 The specific mechanism of ionic conductivity enhancement is still not clear. Moreover, the interfacial conduction phenomenon and the ionic conduction of the traditional electrolytes have not been compared in the same cell structure for obtaining a comprehensive understanding. In addition, in recent years, the ionic conductivities of many SOFC electrolyte materials have been greatly improved in nanocrystalline structure or in nanoscale thin film heterostructure.19-24 Takamura et al. reported the enhancement in the ionic conductivity of CeO2 nanoparticles prepared under high pressure, for example, obtaining 3×10−3 0.1 S·cm-1 at 300 °C under 5 GPa for 6 mol% SDC nanoparticles.20 The ionic conductivity of cubic zirconia, ceria and titania with nanocrystalline structures increased


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significantly compared to the conventional polycrystalline and single crystalline materials.19 Garcia-Barriocanal et al. reported a high lateral ionic conductivity, with up to eight orders of magnitude enhancement near room temperature in YSZ/STO heterostructures.21 However, the phenomena that give rise to these dramatically enhanced ionic conductivities have not been demonstrated in practical applications. To investigate the ion conduction mechanism of the GDC electrolytes in a nanocrystalline structure, two types of SOFC devices were constructed. GDC electrolyte of one cell was densified at 1550 °C, while in the other one the electrolyte was made by pressing loose powders under 360 MPa without high temperature densification. The GDC electrolyte without densification formed the nanocrystalline structure by maintaining the original GDC nanoscale structure due to the absence of grain growth during hightemperature sintering. Comparing the electrochemical properties of the two GDC structures, we found that the ionic conductivity of nanocrystalline GDC electrolyte reaches approximately 0.34 and 0.37 S·cm-1 at 500 and 550 °C, which is nearly an order of magnitude higher than that of the high-temperature sintered GDC electrolyte. This finding was accompanied by the much higher power output that was more than three times higher than that of the high-temperature sintered GDC electrolyte device. The interface and/or surface conduction in the nanocrystalline GDC electrolyte are the origin of the dramatic increase in the ionic conductivity. Oxygen ions and protons both act as the charge carriers in the nanocrystalline GDC electrolyte. The ionic conduction mechanism of the nanocrystalline GDC electrolytes was investigated in detail.

2. Experimental 2.1 Fabrication of fuel cell



The GDC powder was prepared by a sol-gel method.9 Two types of cells were constructed in a symmetrical configuration of Ni-foam-NCAL/GDC/NCAL-Nifoam. Cells of GDC with and without the high-temperature sintering process is denoted as sintered-GDC cell and nanocrystalline-GDC cell, respectively. Commercial NCAL powder and terpineol were mixed into a slurry, and the slurry was coated onto the Ni-foam to form the electrode pellet9. For sintered-GDC cell, the GDC electrolyte pellet was made by the same method as in previous literature9. The GDC pellets was polished down to 0.5 mm. This GDC pellet was sandwiched by two pieces of NCAL-coated Ni-form to form a full cell. The cell was clamped with a stainless-steel cell performance test fixture, and two nickel mesh sheets placed inside the stainless-steel fixture were used as electrode supports. The foam Ni-NCAL electrode and the GDC electrolyte were clipped together with the test fixture. For nanocrystalline-GDC cell, a piece of the NCAL-coated Ni-form electrode was placed in the mold, then the GDC electrolyte powder was spread evenly, and then another NCAL-coated Ni-form electrode was placed on the GDC powder. The NCAL-coated Ni-form electrodes and GDC electrolyte were pressed into a single cell under the 360 MPa. The effective electrode area of the cell was 0.64 cm2. 2.2 Material characterizations SEM measurements were carried out on a FEI Quanta FEG 250 Field Emission Scanning Electron Microscope (FE-SEM). The surface characteristics of the GDC powder before and after the performance test were analyzed by X-ray


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photoelectron spectroscopy (XPS). The bonding energy and oxidation state of the oxygen can be obtained by high-resolution scanning. 2.3 Electrochemical measurements Two symmetrical cells were respectively fixed in the performance-testing device. H2 and air were supplied as the fuel and the oxidant, respectively. The flow rates of both H2 and air were 100 ml·min-1. The electrochemical performance and the electrochemical impedance spectra (EIS) of the cells were measured at 500 and 550 °C using a VersaSTAT3 electrochemical system (Princeton Applied Research). EIS of the cells were measured between 0.1 Hz and 1 MHz with the ac voltage of 10 mV amplitude under open-circuit conditions. 3. Results and Discussion Fig. 1 show the SEM images of the two cells with different GDC electrolyte microstructures. The cross-sectional SEM images and the magnified views of half-cell and the GDC electrolyte for sintered-GDC cell and nanocrystalline-GDC cell are shown in Fig. 1(a)-(c) and Fig. 1 (d)-(f), respectively. From Fig. 1(a) and (d), it is observed that both GDC electrolyte were approximately 0.5 mm thick. The silvery-white phase in the Ni-NCAL electrode was foam Ni, and the dark gray phase was NCAL. The Ni-NCAL electrode in sintered-GDC cell shows a porous structure and is very closely integrated with the GDC electrolyte. The porosity of the Ni-NCAL electrode in nanocrystallineGDC cell is not very high due to the co-press preparation of the cell. An NCAL and GDC material mixing layer is present at the electrolyte/electrode interface in cell with nanocrystalline GDC electrolyte, as shown in Fig. 1(e). Fig. 1(c) and (f) shows the magnified views of the two GDC electrolytes with different microstructures. It is



observed from Fig. 1(c) that the GDC electrolyte sintered at 1550 °C shows very typical polygonal grains with very clear grain boundaries. Only a few minute pores were observed in the GDC electrolyte of sintered-GDC cell. The relative density of the sintered GDC is 95.1% measured using a standard Archimedes method. The GDC electrolyte in the nanocrystalline-GDC cell shows a dense packing powder structure that was different from the GDC microstructure found in the sintered GDC. It was found that no large grains and corresponding long grain boundaries are observed in the nanocrystalline GDC electrolyte due to the absence of a high temperature sintering process. Fig. 2 shows that the open circuit voltages (OCVs) changes with time after H2 and air were supplied for both sides of the cells at 550 °C. The OCV of the two cells increased rapidly to more than 1 V in approximately 200 sec and then stabilize at approximately 1.075 V after 400 sec. These OCV values are close to the OCV of SOFC reported in literature, indicating that the electrolyte of the cells is gastight enough, and no obvious internal leakage phenomenon occurs.25 Oxygen ions are the charge carriers of the GDC electrolyte in sintered-GDC cell, whereas the identity of the charge carriers for cell in nanocrystalline GDC electrolyte is unclear. Fig. 3 (a) and (b) shows the IV/IP curves of sintered-GDC cell and nanocrystallineGDC cell at 500 and 550 °C, respectively. At 500 and 550 °C, the maximum power density of the sintered-GDC cell reaches 97.1 and 167.3 mW·cm-2, but that of the nanocrystalline-GDC cell was 386.5 and 591.8 mW·cm-2, respectively. The maximum power density of cell with nanocrystalline GDC electrolyte is 3.5 times that of the sintered-GDC cell. To further study the electrochemical mechanism in the two cells, EIS analysis was


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carried out. Fig. 4 shows the impedances spectra of the sintered-GDC cells and nanocrystalline-GDC cell under the conditions of open circuit conditions during fuel cell performance test at 500 and 550 °C, respectively. The high-frequency intercept on the real axis represents the ohmic resistance (Ro) that includes the resistance of the electrolyte, the ohmic resistance of the electrodes and the contact ohmic resistance associated with the interfaces. The difference between the high- and low-frequency intercepts on the real axis represents the electrode polarization resistance (Rp). The Rp are composed of several overlapping depressed arcs reflecting the physical and /or chemical processes associated with the anode and cathode reactions.26-27 From Fig. 4(a), it is observed that Ro for sintered-GDC cell was 1.472 and 1.369 Ω·cm2, and Rp was 0.227 and 0.167 Ω·cm2 at 500 and 550 °C, respectively. Ro for nanocrystalline-GDC cell were 0.147 and 0.135 Ω·cm2, and the Rp values were 0.434 and 0.204 Ω·cm2 at 500 and 550 °C, respectively. Since both cells use Ni-NCAL electrodes, the intrinsic property of these electrodes should be the same. The minor difference between their Rp most likely is due to the assembling method. As shown in Fig. 1(b) and (e), the preparation methods of these two cells are different; thus, the combined conditions of the electrode and the electrolyte and the electrode porosity will be different for the two cells. In contrast, Ro of the cell with sintered-GDC is approximately 10 times higher than that of the nanocrystalline-GDC cell, indicating that GDC compacted in nanocrystalline form shows ionic conductivity remarkably higher than that of the traditional sintered one. It was reported that NCAL is an electron conductor with the conductivity of approximately 9.8 S·cm-1 at 500-550 °C (hole type in the air), and its conductivity should be higher when it is combined with the nickel foam into Ni-NCAL.28 Therefore, the



ohmic resistances of sinterd-GDC and nanocrystalline-GDC cells should both be dominated by the GDC electrolyte. Thus, we derived the ionic conductivity of GDC electrolyte from the ohmic resistance of the cells. Fig. 5 shows the Arrhenius-type plot of the total conductivities of the two GDC electrolytes with different microstructures. At 500 and 550 °C, the ionic conductivity for sintered-GDC was 0.034 and 0.036 S·cm-1, for nanocrystalline-GDC was 0.34 and 0.37 S·cm-1, respectively. The conductivity for the former is comparable to the reported value 0.011-0.025 S·cm-1 in the literature at 500600 °C.4, 29-30 The activation energies can be calculated as 0.883 eV, which is also close to the activation energy of GDC electrolyte reported in the literature, indicating that the sintered-GDC is an oxygen ion conductor. On the other hand, the Arrhenius activation energy is only 0.249 eV for the nanocrystalline GDC electrolyte, which is much lower than that of the conventional high-temperature densified electrolytes with oxygen ion conduction or proton conduction.4, 31 It was reported that the activation energy of GDC electrolyte sintered at 1450 °C is approximately 0.93 eV, and the activation energy of BZY with proton conduction is approximately 0.5 eV.4, 31 The lower activation energy and higher ionic conductivity of the nanocrystalline GDC indicate that the ionic conduction mechanism is different from the traditional mechanisms of proton and oxygen ion conduction. It is well-known that the high temperature sintering of GDC results in large grains and obvious grain boundaries that can be observed in Fig. 1(c), and the oxygen ions must migrate through the oxygen vacancies channels of these grains and grain boundaries.4,

30

The proton conductivity values of proton conductors such as

BaCe0.9Y0.1O3–δ and BaZr0.9Y0.1O3–δ are also dominated by the grains and grain boundaries that are significantly affected by the sintering characteristics of the pellets. 32-


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33

. In the present case, high-temperature sintering leads to tightly connected polygonal

grain and grain boundaries due to the grain growth of GDC as shown in Fig. 1(f). But the nanocrystalline GDC sample shows a loose connection with tremendous interfaces and free surfaces. Since interfaces and/or surface conduction have been reported as beneficial ionic channels, resulting in very high conductivities and low activation energies, it is speculated that the interfaces and/or surface of GDC particles in this nanocrystalline GDC electrolyte may be the main paths for high speed ionic motion.11-12 To determine the charge carriers of conduction in nanocrystalline GDC electrolyte, a densified GDC pellet and a densified proton conductor SrCe0.95Y0.05O3-δ (SCY) with high proton transference number were used as the oxygen ions and proton filter, coupled with nanocrystalline GDC, respectively.34 SCY was synthesized via a sol-gel method.35 The results of the characterizations of the phase and structure of SCY are shown in Figs. S1 and S2. Fig. 6(a) shows the schematic illustrations of these cells. It should be noted that the thicknesses of the densified GDC and SCY were approximately 0.8 mm, and the thickness of the nanocrystalline GDC is 0.5 mm. One foam Ni-NCAL pellet and GDC powder were pressed under 360 MPa to form a half of the cell. A foam Ni-NCAL pellet, a densified GDC or SCY pellet, and the half-cell were then sandwiched together to form the double electrolyte cells. The cell structure was Ni-NCAL/nanocrystalline GDC/GDC or SCY/NCAL-Ni. Fig. 6 (b) shows the IV-IP curves of the cells with the bilayer nanocrystalline GDC/GDC electrolyte and nanocrystalline GDC/SCY double layer electrolyte obtained at 550°C. The maximum power densities of the cells with GDC pellet and SCY pellet double-layer electrolytes were 50.4 and 71.5 mW·cm-2, respectively. As mentioned above, the ionic conductivity of GDC pellet sintered at 1550



o

C was below 0.036 S·cm-1 at 550 °C, corresponding to approximately 1/10 of the

conductivity of the nanocrystalline GDC electrolyte and indicating that the performance of the cell with double layer electrolyte should be determined by the densified GDC pellet or SCY pellet electrolyte. It is well-known that densified GDC is a predominant oxygen ionic conductor, while SCY is a proton conductor with a very high proton transference number. The results obtained from the double layer cells shown in Fig. 6(b) indicate that the nanocrystalline GDC layer must allow for both O2- and H+ transport to deliver the power (current) output, i.e. nanocrystalline GDC electrolyte should possess a hybrid conduction of oxygen ion and proton. In order to clarify the transference numbers of protons and oxygen ions in this nanocrystalline GDC electrolyte, a Pt/nanocrystalline GDC/Pt concentration cell was prepared. The nanocrystalline GDC electrolyte pellet was pressed under 360 MPa and sintered at 600oC for 1 h. The thickness of nanocrystalline GDC electrolyte pellet was 1mm. Pt paste is coated on two sides of the nanocrystalline GDC electrolyte as electrodes. The voltage of hydrogen concentration cell and oxygen concentration cell were measured in 3%H2+97%Ar/pure H2 and Air/pure O2 atmospheres, respectively. The theoretical voltage of two kinds of concentration cell and the transference numbers of protons and oxygen ions in nanocrystalline GDC electrolyte were calculated according to Eq. (1) and Eq. (2).36 Fig. 7(a) and (b) show the curves of the observed voltages and the calculated theoretical voltages of the Pt/nanocrystalline GDC/Pt cell tested in hydrogen and oxygen concentration cell atmospheres, respectively. The theoretical voltages are calculated by Eq. (1). Fig. 7(c) shows the relationship between temperature and the transference numbers of protons and oxygen ions in nanocrystalline GDC electrolyte


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calculated from Eq. (2). From Fig. 7(c), one can see that the transference numbers of oxygen ions in nanocrystalline GDC electrolyte were basically over 75%, and the proton transference numbers were less than or equal to 20%. This shows that oxygen ion conduction is dominant in this nanocrystalline GDC electrolyte. 𝑅𝑇

𝑝1

ENernst=𝑛𝐹ln(𝑝2)

(1)

𝐸𝑜𝑏𝑠

(2)

tion=𝐸

𝑁𝑒𝑟𝑛𝑠𝑡

Since the nanocrystalline GDC electrolyte does not undergo a high-temperature densification process, its grains do not grow through particle diffusion but rather still maintain the grain size of the original powder, as shown in Fig. 1(f). The size of the particles of this sample is below 100 nanometers. Tuller considered such solids with nanometer-sized grains to be nanocrystalline and concluded that the ionic conductivity of the materials in nanocrystalline solids is completely different from the traditional polycrystalline solids.19 In recent years, there have been many reports that the grain size of the material has a significant influence on its ionic conductivity.19, 21-22, 32, 37-38 When the grain size is in the nanoscale range, the ionic conductivity of the material will increase significantly.39-42 The increases in the conductivity of approximately one order of magnitude were reported for YSZ thin films (0.4-0.7 μm thick).38 In the epitaxial YSZ/STO heterostructure, the YSZ conductivity of the interface with the nanoscale thickness increased by several orders of magnitude.21,

43

Bera et al. found that under

relatively low-temperature operation, GDC with a thin film structure has significant improvement in oxygen ion conductivity compared to the polycrystalline YSZ and GDC.39 Despite the high number of reports on the enhancement of ionic conductivity in



nanocrystalline solids or nanocrystalline films, the origin of this ionic conductivity increase is still unclear, particularly for the single-phase materials that can also form the heterostructure due to the particle surface state. In the present case, the change in the surface of the nanocrystalline GDC particles before and after the fuel cell measurements may help us to understand its high ionic conductivity. Fig. 8(a) and (b) shows the O1s XPS spectra of the as-prepared GDC powder and the GDC powder scraped from the nanocrystalline GDC electrolyte cell after the performance test in H2, respectively. The measured O1s peaks were fitted with two peaks using fitting software. The binding energy of the fitted peaks and the area and area ratio of the fitted peaks were shown in Table 1. As shown in Fig. 8(a) and (b), the peak center located at approximately 528.7 eV corresponds to the lattice oxygen in the GDC cubic structure (Position2). The peak center located at approximately 531.5 eV is attributed to oxygen neighbored by oxygen vacancy and the O-H bonds of the adsorbed water molecules (Position1).44 Examination of Fig. 8(a) and (b) shows that many oxygen vacancies and O-H bonds are already present on the surface of raw GDC powders and that the concentrations of oxygen vacancies and O-H bonds were significantly increased after the performance test in the fuel cell conditions. Fig. 9 shows the HRTEM images of nanocrystalline GDC powder in cell before and after the performance test, where Fig. 9(a) and (b) shows the images of the GDC raw material, and Fig. 9(c) and (d) is for the GDC powder scraped and ground from the GDC electrolyte in the cell with nanocrystalline GDC electrolyte after the measurement. It was observed from Fig. 9(a) and (c) that the raw GDC powders have some agglomeration, but the GDC powder particle size before and after the fuel cell measurement does not change


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significantly, ranging from 20 to 50 nm. Fig. 9(b) and (d) shows the HRTEM images at larger magnification of the GDC powder in cell with nanocrystalline GDC electrolyte before and after the performance test. A lattice disorder layer with the thickness of more than 1 nm was observed for the GDC after test in Fig. 9(d) that was identified as an amorphous layer. No amorphous layer was observed on the surface of the raw GDC powder, as observed in Fig. 9(b). The structural changes of the GDC surface shown in Fig. 9(b) and (d) explain the changes in the O1s XPS spectra of the GDC in Fig. 8. Combining with the XPS results of Fig. 8, it can be understood that the O-H bonds concentrations and oxygen vacancies in the amorphous layer of GDC scraped from the cell after testing should be significantly increased. Based on the dependence of the oxygen vacancies and O-H bonds concentrations on the GDC particles and the microstructure of the nanocrystalline GDC electrolyte, we conclude that the interface and/or surface diffusion are the main ionic conduction mechanisms of the nanocrystalline GDC electrolyte. The electrical conduction properties of dense epitaxial and nanocrystalline ceria thin films were investigated by Gregori et al. in dry and wet atmospheres at temperatures below 500°C.45 Compared to the fully dense epitaxial ceria thin film, a significant enhancement in the conductivity was observed for the nanocrystalline porous films. It was reported that the observed proton conduction can arise not only from the bulk water adsorbed in the open pores but also from the space charge zones on the water side of the water/oxide interface. In the present case, the large number of oxygen vacancies and O-H groups generated on the surface and interface of the GDC nanoparticles play an important role in the conduction of oxygen ions and protons.



Fig. 10 shows the proposed schematic of the ion conduction mechanism of the nanocrystalline GDC. It is observed from Fig. 1(c) and (f), that the grain boundaries of the GDC sample with a nanocrystalline structure are quite different from the grain boundaries of the conventionally sintered sample. The grain boundaries of the GDC electrolyte with a nanocrystalline structure can also be regarded as the interfaces of the GDC nanoparticles. Therefore, the grain boundary diffusion of the nanocrystalline structure GDC electrolyte should be different from that of the traditional sintered GDC electrolyte. In the fuel cell test atmosphere, the amorphous layer with a large amount of O-H bonds and oxygen vacancies produced at the GDC nanoparticle interface was observed, to be considered as the ion conduction channel of the grain boundary diffusion of the nanocrystalline GDC. The interfacial conduction of ions may be the dominant ion conduction mechanism for the nanocrystalline structure GDC electrolyte. The ion carriers of this interfacial conduction phenomenon have been determined to contain oxygen ions and protons, as described above. The specific migration mechanism of oxygen ions and proton in nanocrystalline GDC electrolyte still needs further study. Fig. 11 shows the short-term stability test result for the nanocrystalline-GDC cell performed at 550 °C in H2/Air. The voltage of the cell remained at approximately 1 V for the first 9 hours under the current density of 76 mA·cm-2, showing almost no degradation and possibly demonstrating the ionic conduction mechanism and working principle. The cell voltage began to decline from the ninth hour, and this stability issue will be the focus of our future work. Further engineering efforts are needed to enhance the device durability that is beyond the scope for the present research project. We will perform furthers research on the performance degradation mechanism and on the engineering efforts to enhance the


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device stability that will be reported in the future.

4. Conclusions To investigate the ion conduction mechanism of GDC electrolytes in a nanocrystalline structure, two types of cells with different GDC electrolyte microstructures, GDC sintered at 1550 oC and nanocrystalline GDC, were prepared with or without the high-temperature sintering. The peak power density of the nanocrystallineGDC cell reached 591.8 mW·cm-2 at 550 °C in H2, which is approximately 3.5 times higher than that for sintered-GDC cell. The ionic conductivity of the nanocrystalline GDC was approximately 10 times higher than that of the sintered GDC. In addition, the ion conduction activation energy of the former is much lower than that of the latter. It was found that the ion carriers of the nanocrystalline-GDC were both oxygen ions and protons, in which the transference number of oxygen ion is higher than that of proton. The microstructure and surface properties of the nanocrystalline-GDC electrolyte characterized by SEM, HRTEM and XPS indicate that the ionic interface conduction or surface diffusion is the main ionic conduction mechanism. These properties give rise to the much higher power output of the nanocrystalline-GDC cells compared to that of the sintered GDC cells.

Acknowledgments The authors thank the financial support of the funds below. Dr.Gang Chen received funding for the financial support from National Natural Science Foundation of China (No. 51302033) and the Fundamental Research Funds for the Central Universities (No. N172504025). Prof. Bin Zhu received funding the financial support from National Natural Science Foundation of China (No. 51772080). Prof. Ying Li received funding the



financial support from National Natural Science Foundation of China (No. 51834004). The authors thank Mr. Yu Dong at New Materials Technology Research Institute of Northeastern University for assistance with the STEM measurements. G.C. and B.Z. conceived and conducted the study and wrote the manuscript. W.S. fabricated the fuel cells and performed the tests. Y.L., Y.H., X.Z. and K.Y. synthesized powder material and performed the XPS, XRD and SEM measurements. All authors discussed the results and commented on the manuscript.

Supporting Information Fig. S1 XRD pattern of the SCY powders after sintering at 1000 °C in air for 2 h Fig. S2 SEM image of the surface of an SCY pellet sintered at 1600 °C

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Table 1 Binding energy of the center of the fitted peaks and the area and area ratio of the fitted peaks Position Sample GDC raw powder Nanocrystalline GDC after test

Position1

Position2

Area1

Area2

531.61 531.53

528.76 528.74

76533.1 80640.5

204031.1 41034.66


Area ratio [1/(1+2)] 0.273 0.663

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Figure 1 (a)-(f) SEM images of two cells with different GDC electrolyte microstructures, (a) Cross-sectional SEM image of cell with GDC electrolyte sintered at 1550 oC , (b) Magnified view of half-cell with GDC electrolyte sintered at 1550 oC , (c) Magnified view of the GDC electrolyte of cell with GDC electrolyte sintered at 1550 oC , (d) Crosssectional SEM image of cell with nanocrystalline GDC electrolyte, (e) Magnified view of half-cell with nanocrystalline GDC electrolyte, (f) Magnified view of the GDC electrolyte of cell with nanocrystalline GDC electrolyte



Figure 2 OCV changes with time after pure H2 and air were flowed into both sides of the two cells


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Figure 3 I-V/I-P curves of the two cells operated at 500 and 550 °C in H2/air, (a) cell with GDC electrolyte sintered at 1550 oC, (b) cell with nanocrystalline GDC electrolyte



Figure 4 Impedance spectra (under open circuit conditions) of cells with (a) GDC electrolyte sintered at 1550 oC and (b) nanocrystalline GDC electrolyte operated at 500 and 550 °C in H2, respectively


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Figure 5 Arrhenius-type plot of the total conductivity of two GDC with different microstructures



Figure 6 Schematic illustration of the cells with (a) the nanocrystalline GDC/GDC and (b) nanocrystalline GDC/SCY bilayer electrolyte, (c) IV-IP curve of the cell with the nanocrystalline GDC/GDC or SCY bilayer electrolyte


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Figure 7 The observed OCV and theoretical OCV from (a) an oxygen concentration cell and (b) a hydrogen concentration cell with nanocrystalline GDC electrolyte, (c) the transfer number of O2- and H+ of the nanocrystalline GDC electrolyte



Figure 8 O1s XPS spectra of (a) the raw GDC powder, and (b) the GDC powder scraped from the nanocrystalline GDC electrolyte cell after the fuel cell measurements


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Figure 9 TEM images of the GDC powder in cell with nanocrystalline GDC electrolyte before and after the performance test, (a) and (b) the HRTEM images of GDC raw powders, (c) and (d) the TEM images of the GDC powder scraped and ground from the GDC electrolyte in cell with nanocrystalline GDC electrolyte after the performance test



Figure 10 A schematic diagram of the proposed ion conduction mechanism of the GDC electrolyte with the nanocrystalline structure


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Figure 11 Short-term stability test performed at 550 °C for the cell with the nanocrystalline structure GDC electrolyte



174x71mm (96 x 96 DPI)


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Advanced Fuel Cell Based on New Nanocrystalline Structure Gd0

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