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High-Performance Protonic Ceramic Fuel Cells with Thin-Film Yttrium-Doped Barium Cerate−Zirconate Electrolytes on Compositionally Gradient Anodes Kiho Bae,†,‡ Sewook Lee,§ Dong Young Jang,† Hyun Joong Kim,† Hunhyeong Lee,∥ Dongwook Shin,*,∥,⊥ Ji-Won Son,*,‡,§ and Joon Hyung Shim*,† †
School of Mechanical Engineering, Korea University, 145 Anam-ro, Seongbuk-gu, Seoul 02841, Republic of Korea High-Temperature Energy Materials Research Center, Korea Institute of Science and Technology (KIST), 5, Hwarang-ro 14-gil, Seongbuk-gu, Seoul 02792, Republic of Korea § Nanomaterials Science and Engineering, Korea University of Science and Technology (UST), KIST Campus, 5, Hwarang-ro 14-gil, Seongbuk-gu, Seoul 02792, Republic of Korea ∥ Division of Materials Science and Engineering and ⊥Department of Fuel Cells and Hydrogen Technology, Hanyang University, 222 Wangsimni-ro, Seongdong-gu, Seoul 04763, Republic of Korea ‡
S Supporting Information *
ABSTRACT: In this study, we used a compositionally gradient anode functional layer (AFL) consisting of Ni−BaCe0.5Zr0.35Y0.15O3−δ (BCZY) with increasing BCZY contents toward the electrolyte−anode interface for high-performance protonic ceramic fuel cells. It is identified that conventional homogeneous AFLs fail to stably accommodate a thin film of BCZY electrolyte. In contrast, a dense 2 μm thick BCZY electrolyte was successfully deposited onto the proposed gradient AFL with improved adhesion. A fuel cell containing this thin electrolyte showed a promising maximum peak power density of 635 mW cm−2 at 600 °C, with an opencircuit voltage of over 1 V. Impedance analysis confirmed that minimizing the electrolyte thickness is essential for achieving a high power output, suggesting that the anode structure is important in stably accommodating thin electrolytes. KEYWORDS: protonic ceramic fuel cells, gradient anode functional layer, thin-film electrolytes, yttrium-doped barium cerate−zirconate, low-temperature performance
1. INTRODUCTION Protonic ceramic fuel cells (PCFCs) have attracted attention as promising alternatives to low-temperature solid oxide fuel cells (LT-SOFCs) for several reasons. First, most proton-conducting ceramics have relatively high ionic conductivities, with lower activation energies than those of oxide-ion-conducting ceramics.1−3 For example, the bulk conductivity of yttriumdoped barium zirconates (BZY), one of the most popular proton ceramics, is about 25 times greater than that of yttriastabilized zirconia (YSZ), which is the most common electrolyte for commercial SOFCs.2 The conductivity of BZY is about eight times greater than those of strontium- and magnesium-doped lanthanum gallate, which is the bestperforming oxide-ion conductor at 500 °C. For this reason, proton-conducting ceramics are expected to have the potential to replace current oxide ion-conducting electrolytes and to enable operation of ceramic fuel cells at an intermediate temperature regime near or below 600 °C. Second, PCFCs generate water at the cathode side, unlike SOFCs, and there is no dilution of the fuel at the anode side. This enables fuel recycling without the requirement of an additional device such © 2016 American Chemical Society
as a dehydrator. It is also possible to supply a conformal fuel concentration to the entire PCFC stack without the Nernst voltage drop caused by the water production in anode during the stack operation. These allow the augmentation of fuel utilization in PCFC systems, resulting in the enhancement of the electrical efficiency in relation to the intrinsic thermodynamic energy of the fuel.4,5 Lastly, operation of PCFCs using hydrocarbon fuels is more favorable than that of SOFCs at higher fuel conversion levels because of direct proton removal from the fuels and higher carbon-coking resistance.5 The characteristics of PCFCs vary depending on their constituent materials. PCFCs based on barium cerate give promising performances but have poor chemical stabilities against water vapor, and the presence of carbon dioxide leads to fast degradation under fuel-cell operating conditions.6−8 In contrast, barium zirconate based PCFCs show excellent chemical stabilities, but their poor sinterabilities and low Received: January 14, 2016 Accepted: March 22, 2016 Published: March 22, 2016 9097
DOI: 10.1021/acsami.6b00512 ACS Appl. Mater. Interfaces 2016, 8, 9097−9103
Research Article
ACS Applied Materials & Interfaces
Figure 1. Schematic diagrams of PCFCs with thin BCZY electrolytes on (a) S- and (b) G-AFLs.
power outputs hinder their practical use.9−13 Recently, significant improvements in the chemical stabilities and sintering behaviors have been achieved using solid solutions such as yttrium-doped barium cerate−zirconate (BaCe1−x−yZrxYyO3−δ, BCZY).7,14−21 Acceptable chemical stabilities were achieved when a Zr ion molar fraction of at least 0.3 was included. On the basis of these results, stable and reliable fuel-cell performances have been reported from BCZY PCFCs at reduced operating temperatures, under 700 °C.22−25 The introduction of thin-film deposition techniques for the fabrication of BCZY electrolytes is expected to enable the operating temperature to be further reduced, resulting in enhanced performances. However, the physical instability caused by reducing the electrolyte thickness on a conventional powder-processed anode surface, with pore sizes on the micrometer scale, makes the use of thin (less than a few micrometers) BCZY electrolytes difficult. A novel approach is therefore required to improve the structural stabilities of thinfilm electrolytes. We describe an effective way of adopting thin BCZY electrolytes, with thickness of about 2 μm, with enhanced structural stabilities and electrochemical performances using a gradient anode structure. Pulsed laser deposition (PLD) and electrostatic slurry spray deposition (ESSD) were used to fabricate the thin BCZY electrolytes and the gradient anode functional layers (AFLs), respectively. A comparison of the microstructures and fuel-cell performances of cells with a thin BCZY electrolyte over differently structured anodes, shown schematically in Figure 1a,b, confirmed that a gradient anode enhances the physical stability of the thin electrolyte while the fuel cell is in operation and allows the PCFC to achieve a high power output with decreased ohmic resistance.
fabricate a gradient AFL, which was NiO-rich on the anode support surface and BCZY-rich close to the top of the AFL. The former and latter AFLs are referred to as a single AFL (S-AFL) and gradient AFL (G-AFL), respectively. The anode support and ALFs were cosintered at 1500 °C for 2 h. Details of the powder synthesis and anode fabrication processes are fully described in our previous report.23 Thin BCZY electrolytes of various thicknesses from 2 to 4 μm were deposited by PLD on each type of AFL. A highly dense BCZY pellet, which was sintered at 1500 °C for 10 h following uniaxial pressing, was used as the PLD target. The substrate temperature and background oxygen pressure were kept at 700 °C and 6.7 Pa, respectively, during deposition. A 2 μm thick porous LaSr0.6Co0.4O3−δ (LSC) cathode was also deposited by PLD at room temperature with a background oxygen pressure of 13.3 Pa, followed by postannealing at 650 °C for 1 h. For both PLDs, the target-to-substrate distance and fluence of the KrF excimer laser (λ = 248 nm) were set at 5 cm and ∼2.5 J cm−2, respectively. X-ray diffraction (XRD; Rigaku D/MAX-2500/PC) was used to examine the crystallinities of the synthesized BCZY powder and the PLD BCZY film. The microstructures of the anode and the fabricated PCFCs were observed using scanning electron microscopy (SEM; FEI Inspect F50). The composition of the PLD BCZY film was checked using SEM−energy dispersive spectroscopy (EDS). The fuel cell tests were conducted using air and wet H2 (3% H2O) as the oxidant and fuel, respectively, at operating temperatures of 450−600 °C. Prior to the cell test, anode reduction was performed at 600 °C by varying the wet H2 content from 10% to 100% with the balance N2. The gas flow rate in both the fuel and air sides was maintained at 100 sccm during the reduction and cell operation. Current−voltage (I−V) and electrochemical impedance spectroscopy (EIS; Gamry Instruments Inc., Gamry Reference 3000 potentiostat/galvanostat/ZRA) data were collected at intervals of 50 °C to investigate the performances of the PCFCs.
3. RESULTS AND DISCUSSION The XRD patterns of the synthesized BCZY powder and the BCZY film fabricated on a Si wafer using PLD are shown in Figure 2. The preferred orientation is confirmed from the XRD pattern of the BCZY film, as shown in Figure 2b, which is commonly observed in thin films grown by PLD.26−28 It is because that film growth occurs along the most stable lattice orientation during atomic nucleation under sufficiently energetic deposition conditions such as high temperatures and low background pressures.28 Although it has been reported that intermittent phases containing nonstoichiometric compositions are easily extruded in BCZY synthesis,20,29,30 the XRD patterns of both the prepared film and powder perfectly matched that of perovskite BCZY, with no secondary phase. The stoichiometric composition of the PLD BCZY film was
2. EXPERIMENTAL SECTION The base BCZY powder, of composition BaCe0.5Zr0.35Y0.15O3−δ, was synthesized by the citric nitrate method. Ba(NO3)2, Ce(NO3)3·6H2O, Zr(NO3)2·xH2O, and Y(NO3)3·6H2O (Sigma-Aldrich) were used as precursors. Fully crystalline and stoichiometric BCZY powder was obtained after calcination at 1200 °C for 2 h. Commercial NiO powder (J. T. Baker) was used as the anode catalyst. The porous anode support was fabricated from a NiO−BCZY mixture (NiO:BCZY weight ratio 6:4); 7 wt % carbon black was added as a pore former. The powder mixture was uniaxially pressed to form the green body. AFLs with different NiO:BCZY mixing ratios were deposited on the green body using ESSD. BCZY and NiO were uniformly sprayed to form an AFL on one anode support. For another anode support, the flow rates of BCZY and NiO slurries were sequentially varied to 9098
DOI: 10.1021/acsami.6b00512 ACS Appl. Mater. Interfaces 2016, 8, 9097−9103
Research Article
ACS Applied Materials & Interfaces
Figure 4. OCV profiles of the fabricated PCFCs with thin BCZY electrolytes, obtained during the anode-reduction step at 600 °C with varying H2 contents from 10% to 100%, with balance N2. Figure 2. XRD patterns of (a) synthesized BCZY powder and (b) BCZY thin film deposited by PLD, indexing to perovskite BaCe0.5Zr0.35Y0.15O3−δ.
Table 1. Relative Atomic Ratios of BaCe0.5Zr0.35Y0.15O3−δ Thin Film Fabricated by PLD, Obtained Using SEM-EDSa atomic content [M/(A + B)]
Ba
Ce
Zr
Y
1.01
0.49
0.35
0.16
a
Data are calculated by dividing each cation atomic concentration (M) by sum of those of all A- and B-site cations (standard deviation: ± 0.01).
Figure 5. I−V−P comparison of PCFCs with 2 and 4 μm thick BCZY electrolytes on (a) S- and (b) G-AFLs, obtained at 600 °C with air and wet H2 reactants.
Figure 3a,b shows the surface morphologies of S-AFL and GAFL, respectively; they consist of BCZY (light) and NiO (dark). The G-AFL surface content of BCZY is larger and the particle size of NiO exposed on the surface is smaller than in the case of S-AFL. This shows that each AFL was wellstructured with different compositions, as intended. The material contents on both surfaces were estimated using ImageJ software from at least five SEM images per AFL (Figure
Figure 3. Surface SEM images of NiO−BCZY (a) S- and (b) G-AFLs fabricated by electrostatic slurry spray deposition and sintering at 1500 °C for 2 h.
confirmed using SEM-EDS; the results are listed in Table 1. These results indicate that BCZY films grown under the fabrication conditions are appropriate for use as electrolytes. 9099
DOI: 10.1021/acsami.6b00512 ACS Appl. Mater. Interfaces 2016, 8, 9097−9103
Research Article
ACS Applied Materials & Interfaces
Figure 6. SEM images after fuel cell tests of PCFCs with 2 μm thick BCZY electrolytes: (a) surface and (b) cross-sectional SEM images of 2S-PCFC, and (c) high- and (d) low-magnification cross-sectional SEM images of 2G-PCFC.
are shown in Figure 5. As shown in Figure 5a, the reduced OCV of 2S-PCFC resulted in a significantly lower power output. The 4S-PCFC gave a reliable peak power density (PPD) of 409 mW cm−2 at 600 °C. A similar power output was obtained with 4G-PCFC, with a PPD of 405 mW cm−2 at 600 °C, as shown in Figure 5b. The power outputs from 2G-PCFC are much higher than those from PCFCs with 4 μm thick electrolytes. The PPD of 2G-PCFC, which was the maximum power density in this study, was 653 mW cm−2 at 600 °C. The SEM images of the PCFCs after the fuel cell tests are shown in Figure 6. The differences between the performances of the cells with 2 μm thick electrolytes on each AFL can be explained based on the observed microstructures. The surface SEM image of 2S-PCFC in Figure 6a shows that some of the SAFL is exposed in the middle of the LSC cathode surface, different from fully covered microstructure before the test in Figure S2. This is because for 2S-PCFC, the electrolyte and cathode layers became detached from the S-AFL in some places during the fuel cell test. This is the main cause of the poor OCV and power output shown in Figure 5a. The crosssectional SEM image in Figure 6b shows that the 2 μm thick BCZY electrolyte delaminates from the Ni surface of S-AFL, with formation of vertical cracks, indicating that adhesion between the electrolyte and the AFL was sustained only by BCZY−BCZY connections at the anode and electrolyte. Figure S3 shows that this microstructural instability was not observed for 4 μm thick BCZY electrolytes, so we conclude that 2 μm thick electrolytes are unable to withstand the mechanical stress and large pore formation that occurred during anode reduction and collapse (i.e., the thinner BCZY electrolyte was too weak and thin to maintain its mechanical integrity on S-AFL). In contrast, a stable microstructure was observed for 2G-PCFC after the fuel cell test, as shown in Figure 6c,d, and Figure S4 shows full surface coverage by the cathode and electrolyte. The enhanced physical stability of this thin electrolyte can be
Figure 7. Comparison of AC impedance spectra obtained from 4S-, 4G-, and 2G-PCFCs under OCV condition at 600 °C.
S1). The resulting area fraction of BCZY for S-AFL was 46.3%, with a standard deviation of ±2.11%; for G-AFL, the value was 82.9 ± 1.13%. The amount of BCZY on the G-AFL surface was about twice that on the S-AFL surface. Thin BCZY electrolytes of thicknesses of 2 and 4 μm were fabricated on each AFL by PLD to prepare four combinations of PCFCs. The PCFCs are denoted by the combination of thickness and AFL type; for example, 4G-PCFC means the PCFC with a 4 μm thick electrolyte on G-AFL. Figure 4 shows the open-circuit voltage (OCV) profiles obtained from the fabricated PCFCs during the anode reduction step at 600 °C prior to the fuel cell tests. As shown in Figure 4, a high OCV of 1.03 V was obtained with 4S-PCFC, but an irreversible OCV drop was observed with 2S-PCFC before changing to 80% H2. In contrast, stable and high OCVs were achieved with both GAFL-based PCFCs, namely 1.02 V for 2G-PCFC and 1.03 V for 4G-PCFC. The fuel cell performances of the PCFCs at 600 °C 9100
DOI: 10.1021/acsami.6b00512 ACS Appl. Mater. Interfaces 2016, 8, 9097−9103
Research Article
ACS Applied Materials & Interfaces
Figure 8. I−V−P curves obtained from (a) 2G- and (b) 4G-PCFCs as a function of operating temperature from 450 to 600 °C. (c) Ohmic area specific resistance and peak power density of each PCFC. (d) Comparison of PCFCs in terms of maximum power output. Blue triangles represent performances of reference PCFCs other than those of Duan et al.38 and this work.
responses at similar frequency points, implying that the cells have similar charge-transfer and transport kinetics during cell operation.34−36 A comparison of the 4 μm thick electrolyte PCFCs (squares and triangles in Figure 7) shows slightly better ohmic resistance for the G-AFL PCFC, which suggests that the improved integrity between the electrolyte and G-AFL reduced interfacial resistance. The ohmic resistance shown by 2G-PCFC (circles in Figure 7) is better than those of the 4 μm thick electrolyte PCFCs; this is a result of the reduced electrolyte thickness. A lower electrolyte thickness of
