Investigation on samarium and yttrium co-doping barium zirconate proton conductors for protonic ceramic fuel cells

Ceramics International 45 (2019) 19289–19296

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Investigation on samarium and yttrium co-doping barium zirconate proton conductors for protonic ceramic fuel cells

T

Zhiwen Zhu∗∗,1, Shuai Wang∗,1 Key Laboratory of Processing and Testing Technology of Glass & Functional Ceramics of Shandong Province, School of Materials Science and Engineering, Qilu University of Technology (Shandong Academy of Sciences), Jinan, Shandong, 250353, PR China

A R T I C LE I N FO

A B S T R A C T

Keywords: Protonic ceramic fuel cells Barium zirconate Co-doping Sinterability Electrical conductivity

Trivalent rare-earth cation (R3+) and Y3+ co-doped BaCeO3 materials show an enhanced proton conductivity than single Y doped BaCeO3 material. For the same purpose, Sm3+ and Y3+ as paired ions are incorporated into BaZrO3 perovskite structure. The crystal structure, electrical conductivity and sinterability of Sm3+ and Y3+ codoped BaZr0.8Y0.2-xSmxO3-δ (BZYS, x = 0, 0.05, 0.1, 0.15, 0.2) materials are investigated for the potential electrolyte application of protonic ceramic fuel cells. Powder XRD diffraction shows BZYS with the cubic perovskite structure and a decrease of lattice constants with increasing Sm3+ content indicates Sm3+ doped into perovskite B site. Although the sinterability enhances by Sm3+ introduction, the grain interior conductivity, specific grain boundary conductivity and total conductivity all cannot be improved. This proves that, unlike BaCeO3 proton conductors, Sm3+ and Y3+ co-doping strategy cannot improve the charge transport properties of BaZrO3 proton conductors, which is the main contribution of this paper. Through comprehensively considering sinterability and electrical conductivity, the BZYS5 (BaZr0.8Y0.15Sm0.05O3-δ) is considered as the potential electrolyte material and the corresponding protonic ceramic fuel cell shows the peak power density of 180 mW cm−2 at 700 °C.

1. Introduction In the last past decades, the protonic ceramic fuel cells (PCFCs) have attracted extensive attentions due their potential in the aspect of intermediate-to-low temperatures solid oxide fuel cell (SOFC) [1–3]. Protonic ceramics have a low apparent activation energy of proton transport and high ionic conductivity in intermediate-low temperature range. Besides, water generated at cathode avoids the dilution of fuel gas for PCFCs [4,5]. The PCFC electrolyte need to meet two primary requirements of high proton conductivity and good chemical stability. Up to now, the study on PCFC electrolyte has pay great attention to BaCeO3 and BaZrO3 system. BaCeO3 electrolyte materials have a high proton conductivity, but the deficiency of chemical instability against CO2 and water vapor restricts its PCFC application [6,7]. The main approaches of stability improvement are to partially substitute for host Ce4+ ions using high electronegative elements [8–10]. Zhao et al. [9] reported BaCe0.7Ti0.1Sm0.2O3-δ (BCST) showed a certain degree of resistivity to CO2 and water vapor corrosion than BaCe0.8Sm0.2O3-δ but the electrical

conductivity decreased from 1.66 × 10−2 S cm−1 for BCS to 2.24 × 10−3 S cm−1 for BCST. BaCeO3 and BaZrO3 complex oxides with any desired ratio can form solid solutions and the stability increases with increasing Zr content [11]. However, the Zr4+ heavydoped barium cerate materials show a decreasing proton conductivity and still decompose when suffering from high concentrations CO2 corrosion [12,13]. Therefore, the improvement of BaCeO3 stability in cost of electrical conductivity cannot be a desirable approach. Barium zirconate material is substantially stable against CO2 and water vapor and considered as the promising proton conducting electrolyte material. The unsatisfying conductivity resulting from high grain boundary resistance becomes one of the biggest obstacles for PCFCs application [14–16]. Through trivalent rare-earth cation (R3+) and Y3+ co-doping strategy, the conductivity enhancement can be realized for BaCeO3 material system. Wang et al. [17] demonstrated that Nd3+ and Y3+ codoped BaCe0.8Nd0.05Y0.15O3-δ showed a higher conductivity than that of BaCe0.8Y0.2O3-δ. The report from Shi et al. [18] also confirmed the same fact that BaCe0.8Y0.1Sm0.1O3-δ materials exhibited a higher proton

∗

Corresponding author. Corresponding author. E-mail addresses: [email protected] (Z. Zhu), [email protected] (S. Wang). 1 These authors contribute equally to this work. ∗∗

https://doi.org/10.1016/j.ceramint.2019.06.179 Received 21 May 2019; Received in revised form 17 June 2019; Accepted 18 June 2019 Available online 20 June 2019 0272-8842/ © 2019 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

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conductivity compared to single-doped BaCe0.8Y0.2O3-δ, 2.37 × 10−2 S m−1 vs 2.04 × 10−3 S cm−1 at 600 °C. There are the positive effects of R3+ and Y3+ co-doping on conductivity that also exist in BaCeO3–BaZrO3 complex oxides. Lyagaeva et al. [19] studied the influence of Y and Yb co-doping on charge transport property of BaCe0.5Zr0.3Yb0.2-xYxO3-δ and the material with BaCe0.5Zr0.3Yb0.1Y0.1O3δ composition possessed the highest ionic conductivity. For BaZrO3 material system, some studies of two trivalent ions codoped BaZrO3 focused on non-rare-earth cation (e.g. In3+, Sc3+, Ga3+) and Y3+ as paired ions [20,21]. Sun et al. [22] found that BaZr0.8Y0.15In0.05O3-δ exhibited the highest total electrical conductivity among of BaZr0.8Y0.2-xInxO3-δ (x = 0–0.2) compositions. In contrast, the investigation from Ito et al. ascertained that BaZr0.8Y0.05(In, Ga)0.05O3-δ samples could not provide the improved electrical conductivity through their sinterability were better than BaZr0.9Y0.1O3-δ material [20]. So far there are few reports about trivalent rare-earth cation (R3+) and Y3+ co-doped BaZrO3 materials. In this paper, we investigated the co-doping effect using Sm3+ and Y3+ as doped ion pairs on phase structure, sinterability, thermal shrinkage, chemical stability and transport properties of BaZrO3 materials.

2. Experimental 2.1. Powder synthesis A citrate-nitrate combustion process was used for the synthesis of BaZr0.8Y0.2-xSmxO3-δ(BZYS, x = 0, 0.05, 0.1, 0.15, 0.2) powders. According to Sm3+ doping level, the powders were designed as BZY, BZYS5, BZYS10, BZYS15 and BZS, respectively. Firstly, the stoichiometric amounts of Sm2O3, Y2O3, BaCO3 and Zr(NO3)4·5H2O were dissolved into a diluted HNO3 solution. Next, the citric acid as complex agent were added and the molar ratio of citric acid to total metal ions was 1.5:1. Finally, a pH value of ∼7 was achieved by adding ammonium hydroxide. After stirring for 24 h at 60 °C thermostatic water bath for fully complexing, the solution was transferred to an evaporating basin. The gel was formed and ignited under heating, producing the black ash. BZYS powders with cubic perovskite structure formed after calcining at 1250 °C for 3 h. SmBaCo2O5+δ-Ce0.8Sm0.2O2-δ (SBC-SDC) composite cathode powders (weight ratio of 7:3) were synthesized using the same route. SBCSDC cathode powders were fired at 1000 °C for 3 h. The anode powders were obtained by mixing NiO, starch and BZYS5 in a weight ratio of 65:35:20.

2.2. Sample preparation BZYS powders were dry-pressed into pellets with 1.3 mm thickness and 15 mm diameter under 200 MPa and a rectangular bar with 25 mm × 7 mm × 2 mm dimension under 350 MPa, respectively. In order to suppress the Ba evaporation at high temperature, the pellet was putted into an alumina boat, covered using the same powders, and sintered at 1600 °C for 5 h under static air. The sintered pellet was employed for the measurement of electrical conductivity and relative density, and microstructure observation. The rectangular bar was used to study the sintering shrinkage. BZYS5|NiO-BZYS5 bi-layer half cells were prepared using a copressing and co-firing method. Fine BZYS5 powders were distributed on the pre-prepared NiO-BZYS5 support, co-pressed under 200 MPa and then co-sintered at 1450 °C for 10 h. SBC-SDC composite cathode slurry was brushed on the electrolyte membrane surface of a sintered half cell and then fired for 3 h at 1000 °C. The anode-supported PCFC with BZYS5 electrolyte and 0.237 cm2 cathode area was used for the next measurement of electrochemical performance.

2.3. Characterization The phase composition and chemical stability of BZYS powders were detected using an X-ray diffractometer (Shimadzu XRD-6100, Cu K1 radiation, λ = 0.15418 nm) with a working voltage of 40 kV and tube current of 30 mA. The samples were scanned with a speed of 6° min−1 and 2 theta range from 20 to 80°. The lattice parameters were calculated using Unitcell and Powder X software. The morphologies of the sintered pellet and tested cell were observed by a ZEISS G500 scanning electron microscope. In order to obtain the average grain sizes of BZYS samples sintered 1600 °C, the horizontal sizes of the well-developed and defects-free 50 grains in SEM images were measured using Nano Measurer software. The shrinkage behaviors of green rectangular bars were measured in flowing air using a DIL 402 C dilatometer. For the electrical conductivity measurement, Ag slurry was brushed on both sides of the polished BZYS pellet and then sintered at 700 °C for 1 h to burn off the organics and make porous Ag electrode firmly adhered to dense ceramic pellets. The electrical conductivity of BZYS pellets were tested in humidified hydrogen (3 vol% H2O) and humidified air (3 vol% H2O) using AC impedance analyzer (Zahner) with sweeping frequency from 1 MHz to 0.1 Hz. The electrochemical impedance spectroscopy (EIS) was fitted by LR (QR) (QR) or LR (QR) (QR) (QR) equivalent circuits. L, R and Q are the inductance element, resistance and constant phase element, respectively. The grain interior, grain boundary and electrode resistances were determined according to the fitting values of capacitances. The grain interior conductivity (σgi), grain boundary conductivity (σgb) and total conductivity (σt) were calculated according to the following equation:

σ=

l RS

where l and S are the sample thickness, and cross-sectional area, respectively. The specific grain boundary conductivity (σSgb), was determined according to the following equation:

σSgb = σgb

(Cgi) (Cgb)

,

where Cgi and Cgb are the capacitances corresponding to grain interior and grain boundary, respectively [23]. The single PCFC was measured by a home-made device as described in previous reports [24]. Ag slurry and paste were used as a current collector and sealant, respectively. The static air and flowing wet hydrogen (3 vol% H2O, 20 mL min−1 flow rate) were used as the oxidant and fuel gas. Current-Voltage (I–V) and Current-Power density (I–P) curves of the single PCFC were obtained using a DC Electronic Load (IT8511). The electrochemical impedance spectroscopy (EIS) of the single PCFC was swept under open circuit condition from 100 KHz to 0.1 Hz using an AC impedance analyzer (Zahner). 3. Results and discussion 3.1. Phase composition The room-temperature XRD patterns of BZYS powders calcined at 1250 °C for 3 h in air are illustrated in Fig. 1. BZYS powders all show the cubic perovskite phase with space group Pm-3m, indicating that the same symmetry of crystal structure. No other supererogatory phase is found. As shown in the magnified XRD pattern of 29-31°, the characteristic peaks shift toward the low-angle direction with increasing the Sm3+ concentration, due to the larger radii of Sm3+ (RVI = 0.958 Å) than Y3+ (RVI = 0.90 Å) [25]. The cell parameters of all powders were calculated according to XRD data, shown in Table 1. As Sm doping level increases, the lattice constant and unit cell volume gradually increase. The diffraction peak shift direction and lattice parameter variation clarify that the Sm3+ dopant mainly incorporate into perovskite B site, not excluding the possibility that the little Sm3+ ions are introduced

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Fig. 1. (a) XRD patterns of BZYS powders fired at 1250 °C for 3 h. (b) Enlarged XRD pattern with 2 theta ranging from 29-31°.

into perovskite A site and grain boundary area. The tolerance factor is very close to unity for all the compositions, which is in accord with the cubic structure from the XRD analysis [26]. The preparation of electrolyte membrane with ideal proton conductivity and good densification is difficult for BaZrO3 material due to its refractory nature. In some literature [18,27,28], the incorporation of Sm3+ into BaCeO3-materials can enhance the thermal shrinkage. In order to study the sintering shrinkage behavior of the BZYS, the dilatometric measurements were implemented using a dilatometer. Fig. 2 displays the sintering behaviors of BZYS rectangular bars in the temperature range of 50–1550 °C. No evident variation for the initial shrinkage temperature indicates that the sintering mechanism keeps unchanged after Sm3+ doping. The shrinkage percentage after sintering at 1500 °C reaches to 5% for BZY, 7.6% for BZYS5, 8.5% for BZYS10, 9.8% for BZYS15 and 12.4% for BZS, respectively. It is evident that the sintering shrinkage of BaZrO3 materials significantly enhances with an increase of Sm3+ doping level, which is consistent with some reports [29,30]. SEM images of the cross-sectional morphology of the sintered BZYS pellets are presented in Fig. 3. One can see that the porosity reduces and the grain size increases with increasing of Sm3+ incorporation concentration, further indicating that the sintering ability promotes after Sm3+ incorporation. The average grain sizes (Dg) of BZYS are shown in Table 1, Dg increases from 0.28 μm for BZY to 0.79 μm for BZS. The relative densities of BZYS pellets measured using the traditional Archimedean methods are also cataloged in Table 1. The relative density of BZS is up to 91.36%, while that of BZY is only 79.32%. These can be verified from the result reported by Gilardi et al. [29] that Sm3+ doping could boost the sintering activity of barium zirconate. The improved sinterability of barium zirconate with Sm incorporation can be explained in two aspects. Firstly, Sm incorporation can enhance the free lattice volume, which increases the mobility of cations, especially at the grain boundary, and thus boosts the grain growth rate [27]. Secondly, the segregated barium oxide resulting from the substitution of Sm3+ for Ba2+ might form a liquid phase and then promote the grain growth of barium zirconate [29,31]. Good electrolyte sinterability favors the

Fig. 2. Sintering shrinkage curves of BZYS rectangular bars measured in range of 50–1550 °C.

Fig. 3. Cross-section morphologies of the sintered BZYS samples at 1600 °C for 5 h.

PCFC low-temperature preparation and the compatibility of between electrolyte and electrode. 3.2. Electrical conductivity For BZYS electrolyte materials, the oxygen vacancies can create in the form of Sm3+ and Y3+ acceptor doping (equation (1)). The proton defects produce by two ways (equations (2) and (3)) and transports with assistance of vacancies Besides, the electronic holes generate when oxygens dissolve into oxygen vacancies (equation (4)). As results, the total electrical conductivity in wet hydrogen circumstance is predominated by proton transportation and while in wet air the BZYS

Table 1 The cell parameters, tolerance factor and relative density of BZYS. Compositions

System

Cell Parameters a (Å)

BZY BZYS5 BZYS10 BZYS15 BZS

Cubic Cubic Cubic Cubic Cubic

4.197 4.199 4.201 4.203 4.205

Tolerance Factor b (Å)

(9) (8) (8) (7) (4)

4.197 4.199 4.201 4.203 4.205

c (Å) (9) (8) (8) (7) (4)

4.197 4.199 4.201 4.203 4.205

Relative Density (%)

3

V (Å ) (9) (8) (8) (7) (4)

73.976 74.077 74.183 74.283 74.374

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Average Grain Sizes (Dg) (μm)

(9) (4) (2) (9) (1)

0.987 0.985 0.984 0.983 0.981

(1) (8) (5) (2) (9)

79.32 86.76 87.09 89.22 91.36

0.28 0.41 0.49 0.63 0.79

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Fig. 4. Total electrical conductivities of BZYS pellets in (a) wet hydrogen, (b) wet air and (c) the total electrical conductivities of BZYS5 in two different atmospheres.

expresses a mixed protonic, oxygen-ionic and p-type electronic conduction [32,33].

(Y, Sm)2 O3

BaZrO3 × → 2(Y, Sm)′Zr + V ″o + 3OO

Table 2 The apparent activation energy (Ea) of charge transport for BZYS samples in wet hydrogen and wet air.

(1)

′

(2)

× H2 O+ V′O′ + OO → 2OH⋅O

(3)

× O2 + 2V′O′ → 2OO + 4h⋅

(4)

H2 +

× 2OO

→

2OH⋅O

+ 2e

Fig. 4a and b depicts the total electrical conductivity of BZYS in humidified hydrogen and humidified air. As can be seen, in the two different atmospheres the total conductivities of BZYS samples are significantly dependent on the Sm3+ doping level and decrease with the increasing of Sm3+ concentration. This indicates that there is no synergistic effect appearing in Sm3+ and Y3+ co-doped BaZrO3 material system. The similar phenomenon also was found in In and Y, Sc and Y, and Ga and Y co-doped BaZrO3 proton conductors [20,21]. However, the conductivity enhancement effect of co-doping could be observed for BaCe0.8Y0.2-xSmxO3-δ, BaCe0.8Y0.2-xNdxO3-δ and BaCe0.8Y0.2-xYbxO3-δ material systems [17,18,34,35]. The reason for decreasing conductivity with increasing Sm doping level will be unraveled in details further below. The apparent activation energy (Ea) was calculated according to the linear relationship between logarithmic conductivity and reciprocal temperature, shown in Table 2. Ea values of BZYS are in range of 0.28–0.31 eV in wet hydrogen, smaller than 0.94–0.97 eV in wet air. The lower Ea values of BZYS samples in wet hydrogen indicate protons as the major current carriers, and while the higher Ea values in wet air

Atmospheres

Wet H2 Wet air

Temperature range

700–300 °C 700–400 °C

Ea (eV) BZY

BZYS5

BZYS10

BZYS15

BZS

0.28 0.96

0.30 0.94

0.31 0.96

0.30 0.97

0.29 0.96

indicate an extra contribution of oxygen ions and electronic holes to total conductivity. These results are in accord with previous literature [22,36]. The total conductivities for BZYS5 in wet air and wet hydrogen are shown in Fig. 4c. In the higher temperatures (> 600 °C), due to the appearance of p-type electronic conduction (equation (4)), the total conductivity of BZYS5 in wet air is higher than that of in wet hydrogen. The apparent activation energy of proton conduction in wet hydrogen is lower than those of hole hopping and oxygen ion conduction in wet air. Therefore, in lower temperatures (< 600 °C) BZYS5 shows a higher conductivity in wet hydrogen than in wet air. The electrochemical impedance spectroscopy (EIS) was analyzed in order to distinguish the conductivity contribution of grain interior and grain boundary to the total conductivity. A typical EIS of BZYS5 measured at 400 °C in wet hydrogen is shown in Fig. 5. The medium-frequency arc with a capacitance in the order of ∼10−9 F cm−1 is characteristic of grain boundary conductivity, while the low-frequency arc with a capacitance in the order of ∼10−6 F cm−1 is connected with

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Fig. 5. The electrochemical impedance spectroscopy of BZYS5 in wet hydrogen under 400 °C.

electrode processes [37]. The complete grain-boundary arc only appears in EIS measured in 300–500 °C and hence the grain interior and grain boundary conductivities in this temperature range are shown in Fig. 6. The grain interior conductivity decreases with an increase in the Sm level, as shown in Fig. 6a. Several reasons maybe result in this phenomenon. As reported from Han et al. [38], the proton concentration of the sample with Y dopant is higher than that of the sample with Sm, which may be lead to a decreasing conductivity. According to first principle density functional theory calculations from Björketun et al., the ionic radius of the dopant in BaZrO3 affects the proton occupation: for the dopants with small ionic radii, the protons bonded to oxygens at the first neighbor of dopants is the most stable, while for the dopants with large ionic radii, the most stable proton position locates at the oxygen-site at the second neighbor of dopants [29,39]. The partial substitution of Sm3+ for Y3+ possibly causes the alteration of the stable proton position and thereby increases the proton-dopant association energy restricting the proton mobility. The Ba evaporation resulting from high sintering temperature will induce the Sm3+ incorporation into perovskite A site. After Sm3+ substitution for Ba2+, the oxygen vacancies will be consumed for charge compensation according to the following equation (5) [29,38,40]. × Ba×Ba + V′O′ + Sm2 O3 → BaO + 2Sm⋅Ba + 3OO

Fig. 7. XRD patterns of BZYS5 powders treated at 700 °C for 10 h in PCFCs operating atmospheres. (a) untreated, treated in (b) wet H2 (3 vol% H2O), (c) dry H2, (d) wet air (3 vol% H2O) and (e) wet CO2 (3 vol% H2O).

proton mobility, and consequently decrease proton conductivity. BaZrO3 refractory nature encourages that the studies focus on the enhancement of sintering activity. Most of the studies demonstrated that, after the improvement of sintering activity, the grain boundary conductivity increased due to the decreasing grain boundary density [26,41,42]. The specific grain conductivities of BZYS samples measured in humidified hydrogen are shown in Fig. 6b and decreases with increasing Sm3+ concentration. The decreasing conductivity accompanied by the improved sinterability suggests that the chemical composition variation in grain boundary region caused by Sm3+ doping is dominant for the specific grain boundary conductivity of BZYS samples. The experimental study from Gilardi et al. [29] found that the specific grain boundary conductivity decreased for dopant with larger ionic radius than Y. The chemical composition analysis of grain boundary for Sc3+ and Y3+ doped BaZrO3 proton conductor confirms the trivalent ions enrichment in grain boundary region which has a significant effect on proton conductor [23,43]. In addition, Ea values for specific grain boundary conductivity are larger compared to those for grain conductivity, indicating a higher proton migration resistance through grain boundary.

(5)

The decreasing vacancies will reduce the proton concentration and

Fig. 6. The grain interior (a) and grain boundary (b) conductivities of BZYS pellets measured in wet H2. 19293

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Fig. 8. (a) I–V and I–P curves and (b) EIS under open circuit condition of the single PCFC at operating temperatures of 600–700 °C.

Table 3 Summary of the single PCFCs performance with doped BaZrO3 electrolytes reported in the literature and comparison with the current study. Electrolyte

Pmax (mW cm−2)

OCV (V)

Rp (Ω cm2)

Ro (Ω cm2)

Film thickness (μm)

Film conductivity (σFilm) (10−3 S cm−1)

Ref.

BZYS5 BaZr0.8Pr0.1Y0.2O3-δ BaZr0.8Sn0.1Y0.2O3-δ BaZr0.8Y0.2O3-δ BZYS5 BaZr0.7Nd0.1Y0.2O3-δ BaZr0.75In0.2Bi0.05O3-δ BaZr0.8Y0.15In0.05O3-δ BaZr0.8In0.2O3-δ

92 (600 °C) 82 (600 °C) 214 (600 °C) 70 (600 °C) 180 (700 °C) 142 (700 °C) 340 (700 °C) 379 (700 °C) 85 (700 °C)

1.01 0.9 0.98 1.01 0.92 0.9 0.96 0.93 0.95

1.4 1.3 0.41 1.3 0.18 0.23 0.18 0.07 0.68

1.84 1.33 0.46 1.4 1.01 1.07 0.4 0.34 1.84

25 20 12 20 25 30 12 12 20

1.36 1.5 2.61 1.43 2.48 2.8 3 3.51 1.09

This work [26] [44] [45] This work [46] [42] [22] [47]

Fig. 9. SEM images of the electrolyte membrane surface (a) and the cross-section (b) morphologies of the single PCFC.

3.3. Chemical stability

3.4. PCFC performance

By the comprehensive comparison of electrical conductivity and sinterability, BZYS5 material with an improved sinterability and a comparable electrical conductivity with BZY is considered as a potential electrolyte candidate. Next, the chemical stability of BZYS5 material against PCFCs operating atmospheres was investigated and the XRD patterns before and after treatment were shown in Fig. 7. The treated BZYS5 powders in humidified H2, air and CO2 and dry H2 display the same cubic perovskite with the as-prepared powders and no other phases appear. The peak positions of BZYS5 remain no significant movement before and after treatment. These phenomena reflect that BZYS5 possesses enough tolerance to H2O and CO2 and Sm3+ doping could not lead to the deterioration of BZY chemical stability. The good stability makes it suitable to utilize as the PCFCs electrolyte materials.

The single PCFC with BZYS5 electrolyte was tested at 600–700 °C using the static air as the oxidant and the wet H2 as the fuel gas. Fig. 8a shows I–V and I–P curves of the single PCFC. The Open Circuit Voltages (OCVs) values are 0.92, 0.97 and 1.01 V at 700, 650 and 600 °C, respectively. High OCV are comparable with the other cells with BaZrO3 based electrolyte reported in literature (Table 3) and consistent with the dense electrolyte membrane in SEM images (Fig. 9). The peak power densities (Pmax) at 700, 650 and 600 °C are 180, 147 and 92 mW cm−2, respectively. And Pmax values are higher than the reported values for the single PCFCs with BaZr0.8Pr0.1Y0.2O3-δ, BaZr0.7Nd0.1Y0.2O3-δ and BaZr0.8In0.2O3-δ electrolytes, but lower than the reported values for the single PCFCs with BaZr0.8Sn0.1Y0.2O3-δ, BaZr0.75In0.2Bi0.05O3-δ and BaZr0.8Y0.15In0.05O3-δ electrolytes (Table 3). In order to explain the comparison results of peak power density, the polarization resistance (Rp) and ohmic resistance (Ro) were obtained

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from EIS analysis of the single cell measured under open circuit condition in Fig. 8b. Ro and Rp of the single PCFC with BZYS5 electrolyte were calculated according to the high-frequency intercept with the real axis and the difference between high-frequency and low-frequency intercept with the real axis, respectively. Ro and Rp are 1.01 and 0.18 Ω cm2 at 700 °C, 1.45 and 0.53 Ω cm2 at 650 °C, and 1.84 and 1.4 Ω cm2 at 600 °C, respectively. As summarized in Table 3, the film conductivity of BZYS5 electrolyte is as the same level compared with other reported electrolyte. The reported high Pmax values contribute to the small Rp, and therefore the research of the high-performance BaZrO3-based PCFC should be focused on the development of cathode materials suitable to BaZrO3-based electrolytes, and the optimization of electrode microstructures and electrolyte/electrolyte interfaces. Fig. 9 shows the surface morphology of BZYS5 electrolyte membrane and the cross-sectional microstructure of the measured single cell. As shown in Fig. 9a, the BZYS5 electrolyte membrane is very dense and no visible pores can be found, and the grain size is about 200–500 nm. In addition to blocking the diffusion of anode and cathode gases across electrolyte membrane, the high-quality BZYS5 membrane certainly enhances fast proton transport of electrolyte and hence decreases Ro value of the single cell. Fig. 9b further ascertains the electrolyte membrane density and gives the film thickness of about 25 μm. Two electrode layers of the single cell firmly adhere to the BZYS5 electrolyte film, forming good the electrolyte/electrodes interfaces. Remarkably, the well assembly favors the rapid mass and charge transport during cell operation and therefore acquires high electrochemical performance. l film Noting: σ= R S ,L is the film thickness, S is the area of the cathode. o

4. Conclusions

[6]

[7] [8]

[9]

[10]

[11]

[12] [13]

[14] [15]

[16]

[17]

[18]

[19]

The main contribution of this paper is to prove that, unlike BaCeO3 system, the Sm3+ and Y3+ co-doping strategy cannot improve the charge transport properties of BaZrO3 proton conductor. In this work, Sm and Y co-doping BaZr0.8Y0.2-xSmxO3-δ proton conductors were studied with respect to phase structure, sinterability and electrical conductivity. The results show that the sintering activity increases and electrical conductivity decreases as Sm3+ doping level increases. A comprehensive comparison of sinterability and electrical conductivity of BZYS materials determined BZYS5 as potential PCFC electrolyte candidate. BZYS5 shows remain good stability against water vapor and CO2. The single PCFC based BZYS5 electrolyte gives the peak power density of 180 mW cm−2 at 700 °C. Acknowledgements

[20]

[21]

[22]

[23]

[24]

[25]

This work is kindly supported by the National Natural Science Foundation of China (51408582). We also gratefully acknowledge the Incubation Program of Universities' Preponderant Discipline of Shandong Province (No. 03010304), Shandong Provincial Natural Science Foundation (ZR2019PEE029) and Key Research and Development Program of Shandong Province (2017GGX20122).

[26]

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