Grain Boundary Conductance Mechanisms of Ultra- ne Grained CeO2/BaCeO3 Based Electrolytes Fabricated by a Two-step Sintering Process

Aiming to clarify the grain boundary conductance mechanism of CeO2/BaCeO3 based electrolytes suitable for solid oxide fuel cells (SOFCs), Sm, Bi codoping CeO2/BaCeO3 (80 wt.% Ce0.8Sm0.1Bi0.1O2-δ 20 wt.% BaCe0.8Sm0.1Bi0.1O3-δ, BiSDC-BCSBi) electrolytes with ultra-fine grained (110-220 nm) and micron (1-1.8 μm) structures were prepared by the two step sintering and conventional sintering method, respectively. Both electrolytes have pure phases corresponding to CeO2 and BaCeO3 without other purities. In the ultra-fine grained structure, apparent grain boundary conductivities measured at 350 oC and 400 oC are 1-2 orders of magnitude higher than micron structures, thus resulting in dramatically enhanced electrical performances. This grain boundary effect can be attributed to two aspects. One is the decrease of space charge potential Δφ(0) (0.165 V for ultrafine-fine grained ones, 0.396 V for micron ones). The other is the dilution of impurities (the impurity blocking term ω/dg is 0.94 for ultrafine-fine grained ones, and 0.53 for micron ones). In the ultra-fine grained electrolytes, no extra electronic conduction is introduced, and the ion migration number of O2is higher than that of H+. Finally, the ultra-fine grained BiSDC-BCSBi electrolytes maintain a good long-term stability in the operating condition of SOFCs at 600 oC for 100 h.


Introduction
Solid oxide fuel cells (SOFCs) are regarded as one of the most promising energy conversion devices due to their advantages of high efficiency and environmental friendliness [1][2][3][4][5]. Aiming to avoid issues of cell packaging and electrode interface reactions caused by high operating temperatures, the current tendency of SOFCs is to decrease the operating temperature, but the consequent problem is deceased ionic conductivities of electrolytes [6]. Therefore, it is urgent to develop high conductive electrolytes suitable for intermediate-temperatures.
CeO2 and BaCeO3 based electrolytes have attracted much attentions due to their excellent oxygen ion conductivity and proton conductivity, respectively [7][8][9]. However, Ce 4+ in CeO2-based electrolytes is easy to be partially reduced to Ce 3+ in the reducing atmosphere, leading to undesirable electronic conductions [8]. And the poor chemical stability in CO2 and H2O hinders the application of BaCeO3-based electrolytes [7,9].
One is the effect of space-charge layers [16]. The other is the enrichment of impurities on grain boundary regions [17]. 3 Several researches confirm that the grain size has important effects on electrical properties for polycrystalline electrolytes [18][19][20][21][22][23][24]. For oxygen ion conductors, the commonly accepted view is that, the ionic conductivity can be dramatically enhanced with the decrease of grain sizes [25][26][27], which may be attributed to the promtion of ionic conductions due to the delocalized hopping, and the dilution of impurities along grain boundaries. But for Proton conductors, ones require larege-grained or grainboundary-free structures because grain boundaries own high barrier potential for proton conductions [28][29][30]. CeO2/BaCeO3-based electrolytes are mixtures of oxygen ion and proton conductors, the effect of grain sizes on electrical conductive behavior is still unclear.
Herein, micron and ultra-fine grained CeO2/BaCeO3-based electrolytes were fabricated, and carrier conduction behaviors effected by grain sizes were clarified. This work proved the promotion of ionic transportations benefiting from the decrease of grain sizes, which pointed in a direction for the development of such composite electrolytes.

Fabrication of electrolytes
Based on our previous work [11], Sm2O3 and Bi2O3 co-doped CeO2/BaCeO3 based electrolytes (80 wt.% Ce0.8Sm0.1Bi0.1O2-δ -20 wt.% BaCe0.8Sm0.1Bi0.1O3-δ, BiSDC -BCSBi) were taken as the research object due to their optimized conductivity. Micron and ultra-fine grained BiSDC-BCSBi electrolytes were fabricated by the conventional sintering (CS) and two-step sintering (TSS) process, respectively. For the CS process, the pellets were heated to 1250 o C at 3 o C min -1 and kept for 3 h, thus obtaining micron electrolytes. For the TSS process, the pellets were heated to 1100 o C at 3 o C min -1 and kept for 10 min. Subsequently, the sintering temperature was rapidly lowered to 1050 o C and kept for 10 h, thus achieving ultra-fine grained electrolytes.

Characterizations
Crystal structures were analyzed by the X-ray diffractometer (XRD, D8 Advance).
Microstructures were observed by the scanning electron microscopy (SEM, Sirion 200).
Oxidation states of electrolytes were analyzed by the X-ray photoelectron spectroscopy 4 (XPS, Thermo Scientific ESCALAB 250XI XPS system). The electrochemical impedance spectroscopy (EIS) was tested using the electrochemical workstation (Zennium Pro) at the range from 0.1 Hz to 1 MHz According to Eq. (1), the resistance value obtained from the EIS spectrum was converted to the conductivity (σ).

=
(1) Where L and S represent the thickness of electrolytes (cm) and area (cm 2 ) of the electrolyte surface, respectively. R represents the resistance value (Ω).
The blocking electrode method [31] was applied to confirm whether there is electronic conduction. The Keithley 2400 was used to test the I-t curves at a polarization voltage of 0.8 V. The electronic conductivity can be obtained from Eq. (2).
Where I and E are the steady-state current (μA) and the polarization voltage (V), respectively. L, S are the same as in Eq. (1).

Cell assembly and measurement
Firstly, electrolyte pellets were ground to be a thickness of 0. The ionic transference number was obtained by the design of concentration cells. The details have been reported in our previous work [32]. For the measurement of oxygen ion transference number, the atmosphere on both sides of electrolytes are air and pure O2, respectively. For the measurement of proton transference number, the atmosphere on one side is pure H2, and the other side is the mixture of 97 vol.% Ar and 3 vol.% H2.
The long -term stability of the single cell was measured at 600 °C for 100 h, taking humid hydrogen (containing 3 vol.% H2O, 40 mL min -1 ) and ambient air as fuel and oxidant, respectively. grained BiSDC-BCSBi electrolytes, respectively. It can be seen that both electrolytes are in a dense state without obvious pores. The Nano Measurer software was used to carry out the grain size statistics. Grain sizes of micron electrolytes are roughly distributed in 1-1.8 μm, while ultra-fine grained electrolytes are in 110-220 nm. In the TSS process, the dominate grain growth mechanism is grain boundary diffusion instead of grain boundary migration while maintaining the escape of pores, thus achieving high grain boundary ratios.  Compared with the standard cards of CeO2 and BaCeO3, it can be seen that both electrolytes are pure phases with only the diffraction peaks of CeO2 and BaCeO3, no impurity peaks or extra reactions are formed. The sintering process has no effect on the phase composition of electrolytes.

Grain boundary conductance mechanism
The EIS spectra of both electrolytes measured in humid air is shown in Fig. 3 Where Ri represents resistance values, and CPEi represents ideal capacitance values of Ri.    Table 2 The fitting data and characteristic capacitance values of ultra-fine grained BiSDC-BCSBi electrolytes measured at different temperatures. It can be considered that R0 is the Rgi. Since C1 is in the range of 10 -6 -10 -8 F, it is judged that C1 is the constant phase angle element of the grain boundary (CPEgb) polarization process, thus R1 represents Rgb. Similarly, since C2 is 10 -5 F, C2 is considered as the constant phase angle element of the electrode polarization process (CPEp), thus R2 is the polarization resistance (Rp) [33][34][35][36]. Ultra-fine grained electrolytes have both lower Rgi (R0) and Rgb (R1) than micron ones, among which the decrease of Rgb is more significant. For example, Rgb of micron electrolytes is higher than Rgi at 350 o C, but Rgb of ultra-fine grained electrolytes is just 30% of Rgi. According to Eq. (1), the grain-interior (σgi) and macroscopic grain boundary conductivity (σgb) of electrolytes can be obtained, as shown in Table. 3. Ultra-fine grained BiSDC-BCSBi electrolytes have slightly higher σgi and significant higher σgb than micron ones, thus obtaining higher total electrical conductivities.
Where δgb is the thickness of a grain boundary layer, and dg is the average grain size.
When the dielectric constant εgb of the space charge layer is approximately equal to the 8 dielectric constant εgi, δgb can be expressed as Eq. (5). Therefore, the calculation formula of is converted into the Eq. (6) related to capacitance. It can be seen that the correlation between δgb and is relatively small. In order to facilitate calculation and comparison, δgb is taken as 5 nm [22,37]. values of micron and ultrafine grained electrolytes are listed in Table. 3. of the ultrafine grained electrolytes is 1 to 2 orders of magnitude higher than that of micron electrolytes. The space charge layer and impurity blocking effects are usually used to explain the behavior of grain boundary conduction, which can be discussed use Eq. (7) [21].
Where kB is Boltzmann's constant, and ω 2 is the direct contact area of grain to grain, ∆ (0) is the space charge potential. Defined ω 2 / 2 as the impurity blocking term. The higher the value, the weaker the influence of impurities such as SiO2 on the conduction path. Among them, impurities mainly come from the high-temperature volatilization of the furnace material, or from the preparation process. ω 2 / 2 represents the clean part in the grain boundary region that is not covered by impurities. The value of ω 2 / 2 is determined by the purity and grain size of the sample, and its value is 1 when there are no any impurities in the grain boundaries. The space charge potential can be calculated by Eq. (8) [17].
Where k is the slope of the straight line drawn with 1000/T as the abscissa and ln (T/σgi) 9 as the ordinate, representing the chemical reaction rate constant.
Values of k, ∆ (0) and ω/dg are obtained from Eq. (7)(8), and listed in Table. 4. It is obvious that the ultra-fine grained sample has lower space charge potential and higher ω/dg. That is, with the increase of grain boundary ratios, the ionic conduction is easier to get over barriers, and the impurity blocking effect is suppressed due to the impurity dilution at grain boundaries [21]. Table 4 Values of k, ∆ ( ) and ω/dg for micron and ultra-fine grained BiSDC-BCSBi electrolytes.
Meanwhile, ultra-fine grained ones do not introduce extra electronic conduction.
Therefore, it can be concluded that the improved electrical conductivity does not derive from electronic conduction.

Long-term stability
In order to verify whether the increase of grain boundaries damage the stability of electrolytes, electrolyte-supported single cells were prepared to test the long-term stability using humid H2 as fuels and air as oxidants at 600 o C, which is shown in Fig.   13 8. Both ultrafine-grained and micron BiSDC-BCSBi electrolyte-supported cells maintain stable OCV values for testing of 100 h. And ultrafine-grained ones show higher OCV, due to their better electrical performance. Fig. 8 The long-term stability of BiSDC-BCSBi electrolyte-supported single cells.

Conclusion
Ultra-fine grained CeO2/BaCeO3 based electrolytes were successfully fabricated by the TSS method, introducing more grain boundaries. Such electrolytes show much higher electrical conductivities than micron ones, due to in essence their decreased apparent unit grain boundary resistance. The beneficial grain boundary effect can ascribe two aspects including the decrease of space charge potential and the dilution of impurities. In the ultra-fine grained structure, there is no generation of additional electronic conduction. The good electrical performance comes from both enhanced transport of O 2and H + caused by the relatively high concentration of O-H groups and oxygen vacancies, during which the oxygen ion transference number is higher than the proton transference number. Finally, the electrolyte shows excellent long-term stability in SOFC operating conditions.