Operation protocols to improve durability of protonic ceramic fuel cells

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Operation protocols to improve durability of protonic ceramic fuel cells Ka-Young Park, You-Dong Kim, John-In Lee, Muhammad Saqib, Ji-Seop Shin, Yongho Seo, Jung Hyun Kim, Hyung Tae Lim, and Jun-Young Park ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b04748 • Publication Date (Web): 10 Dec 2018 Downloaded from http://pubs.acs.org on December 13, 2018

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ACS Applied Materials & Interfaces

Operation protocols to improve durability of protonic ceramic fuel cells

Ka-Young Parka, You-Dong Kima, John-In Leea, Muhammad Saqiba, Ji-Seop Shina, Yongho Seoa, Jung Hyun Kimb, Hyung-Tae Limc, Jun-Young Parka,*

aHMC,

Department of Nanotechnology and Advanced Materials Engineering, Sejong

University, Seoul 05006, Korea bDepartment

of Advanced Materials Science and Engineering, Hanbat National University,

Daejeon, 34158, Korea cSchool

of Materials Science and Engineering, Changwon National University, Changwon

51140, Korea

* Corresponding author: Tel.: +82-2-3408-3848; Fax: +82-2-3408-4342 E-mail address: [email protected] (J.-Y. Park)

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Abstract To develop reliable and durable protonic ceramic fuel cells (PCFCs), the impacts of the operation protocols of PCFCs on the cell durability are investigated through analyses of the main degradation mechanisms. We herein propose three appropriately designed control protocols, including cathode air-depletion, shunt current, and fuel cell/electrolysis cycling, to fully circumvent the operating-induced degradation of PCFCs. For this purpose, anodesupported cells, comprised of a NiO-BaCe0.7Zr0.1Y0.1Yb0.1O3-δ anode, BaCe0.7Zr0.1Y0.1Yb0.1O3δ

electrolyte, and NdBa0.5Sr0.5Co1.5Fe0.5O5+δ-Nd0.1Ce0.9O2-δ composite cathode, are prepared,

and their long-term performances are evaluated under a galvanostatic condition of 0.5 A·cm–2 at 650℃. The cell voltages of the protected cells using the operation protocols to prevent performance degradation are stably maintained under the applied current density for more than 1,200 h without any noticeable degradation, whereas the performance of the unprotected cell gradually decreased with time, and the decay ratio was 14.9% over 850 h. The significant performance decay of the unprotected cell is strongly associated with the cathode degradation phenomenon, which was caused by the water vapor continuously produced during the electrochemical reactions. Hence, the performance recovery of the PCFCs with the operation protocols is achieved by incrementally decreasing the cathode potential (close to a value of zero) to minimize the effect of high 𝑃𝐻2𝑂 and 𝑃𝑂2 during the PCFC operations.

Keyword: protonic ceramic fuel cells, operation protocols, degradation mechanisms, degradation prevention method, durability.


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1. Introduction During the past few decades, the protonic ceramic fuel cell (PCFC), which is a new class of fuel cell, has been developed based on ceramic electrolyte materials that exhibit the property of high protonic conductivity at intermediate temperatures (500–700°C).1-3 PCFCs have the attractive feature of high intrinsic fuel utilization, because the water vapor is produced at the cathode, whereas the water vapor in solid oxide fuel cells (SOFCs) is produced at the anode, where the fuel is diluted.4 In addition, PCFCs are ideal for use with hydrocarbon fuels, because the protons are transported from the anode to the cathode through the electrolyte at elevated temperatures.4,5 Among a large number of ceramic materials, perovskite crystal structure materials primarily demonstrate high protonic conductivity at intermediate temperatures. In particular, yttrium-doped BaZrO3 (BZY) and BaCeO3 (BCY) proton conductors have been extensively studied for use as electrolytes in PCFCs because of their high proton conductivity and low activation energy at an intermediate temperature.6-9 However, the BCY materials have low chemical stability against CO2 and H2O, and BZY materials require high-temperature heat treatment to fabricate dense electrolytes. To overcome these technical issues, BaCe0.8xZrxY0.2O3-δ

(BCZY, with 0.0 ≤ x ≤ 0.8) proton conductors were designed to achieve a

synergistic effect by combining the high proton conductivity of BCY and good chemical stability of BZY. In addition, promising performance was observed for the BaCe0.3Zr0.5Y0.2O3δ

(BCZY) electrolyte, with a relatively high electrical conductivity and PCFC performance.10-

12

However, the stability of BCZY was still insufficient to meet the practical requirement for

the

electrolyte

material

of

PCFCs.

Recently,

several

groups

reported

that

BaCe0.7Zr0.1Y0.1Yb0.1O3-δ (BCZYYb) demonstrated a higher proton conductivity than BCZY, with significant chemical stability, even in CO2, H2O, and H2S atmospheres, indicating the feasibility of using BCZYYb as an electrolyte material for PCFCs.13-15 3 ACS Paragon Plus Environment


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For the cathode materials of PCFCs, Co- and Sr-containing perovskites, such as Ba0.5Sr0.5Co0.8Fe0.2O3–δ (BSCF), La0.6Sr0.4Co0.2Fe0.8O3–δ (LSCF), and Sm0.5Sr0.5CoO3–δ (SSC), have been considered because of their excellent electrocatalytic activities for oxygen reduction reactions (ORRs).15-18 More recently, significant progress in terms of the ORR activity and electrical

property

has

been

achieved

with

layered

LnBaCo2O5+δ

(LBC)

and

LnBa0.5Sr0.5Co1.5Fe0.5O5+δ (Ln = Pr, Nd, and Gd) (LBSCF) double perovskite materials.19-21 Employing composite cathode materials consisting of double perovskite materials (e.g., LBC, LBSCF) and proton-conducting electrolytes (e.g., BCZY, BCZYYb) has led to high performances in PCFCs. In particular, these composite cathodes offer the simultaneous migration of protons, oxygen ions, and electrons, resulting in the high performance of PCFCs at intermediate temperatures.22 While much progress has been achieved to improve the performance of PCFCs with these advanced cathode and electrolyte materials, any significant further gains in long-term durability have apparently not been demonstrated experimentally. In particular, the chemical stability of Sr-containing electrocatalysts should be carefully considered to determine the feasibility of applying them as the cathode materials of PCFCs, because these materials can be easily degraded with increased polarization resistances and even decomposed into secondary phases (e.g., Sr segregation) during long-term operations by the water vapor produced at the cathode.2328

Lee et al.28,29 reported that the single perovskite materials (e.g., BSCF) were extremely

degraded during the cycling operation of dry and highly humidified air in an accelerated degradation test for the cathodes in SOFCs. Predominantly, severe SrO-like phases were observed as the main degradation phenomenon in the post-mortem analysis. In addition, the cathode film was catastrophically delaminated and removed from the electrolyte in the anodesupported cell structure as a result of hydration by the wet air. This condition may be similar to that of cathode of a PCFC, because the water vapor is produced at the cathode of PCFCs, 4 ACS Paragon Plus Environment

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1 according to the overall reaction ( O2( g ) + 2 H + + 2e - → H 2 O( g ) ), and the amount of water 2 produced depends on the applied current by the electrochemical reactions at elevated temperatures.30 Hence, the reaction with steam (hydration) is of paramount importance in the cathodes of PCFCs31 and, theoretically, the proton conductivity is facilitated by the hydration of

oxygen

vacancies

in

a

humid

atmosphere,

following

the

reaction

[ H 2 O( g ) + VO•• + OO× → 2OH O• ].32 Particularly, the double perovskite NBSCF-cell appeared to be stable during several wet/dry cycling conditions at the cathode side of the SOFCs.23 Furthermore, several publications33,34 recently proposed that double perovskite-structured materials (e.g., PrBa0.5Sr0.5Co2O5+δ, NdBa0.5Sr0.5Co1.5Fe0.5O5+δ) have significant protonic conductivity. Thus these materials can function as a mixed conducting material that exhibits electronic, oxygenion, and protonic conductivity. However, even though these double perovskite-structured materials have better hydration-dehydration properties35 with great electrochemical performances compared to those of single perovskite materials, the prolonged exposure under the condition of highly humidified air may significantly influence the long-term stability of the materials.29,36 Moreover, only a few publications on PCFCs have reported long-term endurance test measurements under galvanostatic/potentiostatic profiles. Information is needed on the prolonged influence of the highly humidified air of cathode materials on the durability of anodesupported PCFCs based on an understanding of the main degradation mechanisms.28,37 In this work, to develop durable PCFCs, reliability testing is carried out on NiO-BCZYYb anode supported cells with a BCZYYb electrolyte and an NBSCF-Nd0.1Ce0.9O2-δ (NDC) composite cathode because of their high electrical conductivity and chemical stability. In the reaction of protons and oxygen-ions in PCFCs, the oxygen-ions should diffuse through the extension of the electrochemically active sites to the entire surface of the cathode.36 Hence, employing 5 ACS Paragon Plus Environment


composite cathode materials consisting of double perovskite materials (e.g., NBSCF), oxygenion conductors (e.g., NDC), and proton-conducting electrolytes (e.g., BCZYYb) may provide the simultaneous migrations of protons, oxygen ions, and electrons, resulting in high performances for PCFCs at intermediate temperatures, as previously mentioned. We also report several solutions (properly simulated control operation protocols: cathode air-depletion, shunt, and fuel cell/electrolysis cycling) to minimize the performance degradation of the cathode during the PCFC operation, because the degradation phenomenon of solid-state electrochemical devices such as SOFCs and PCFCs strongly depends on the operation conditions.38

2. Experimental procedures 2.1. Preparations of anode-supported cells Anode-supported single cells were fabricated using the uniaxial pressing method and slurry coating technique. For the anode substrates, NiO and BaCe0.7Zr0.1Y0.1Yb0.1O3-δ (BCZYYb) powders (Kceracell Co. Ltd, Korea) were mixed with 10 wt.% corn starch as a pore-former in ethanol by ball-milling for 24 h and then dried in an electric oven. The NiO-BCZYYb mixtures were placed into a cylinder-type mold and pressed at 18 MPa for 1 min. The green NiOBCZYYb disks (25 mm diameter and 1 mm thickness) were pre-sintered at 900℃ for 2 h. A slurry of the anode functional layer (AFL) was prepared by mixing NiO and BCZYYb powders in ethanol with Solsperse (SG24000, Lubrizol), polyvinyl butyral (PVB, B-98, Butvar), and Din butyl phthalate (DBP, 99% purity, Deajung Chemicals & Metals) for 48 h. The AFL slurry was coated on the pre-sintered NiO-BCZYYb substrates and heat-treated at 400℃ for 2 h. For the electrolyte, the BCZYYb powders were ball-milled with Solsperse, PVB, and DBP for 48 h. The electrolyte slurry was also coated onto the AFL, and the tri-layered substrates were then co-sintered at 1450℃ for 4 h. For the cathode materials, NdBa0.5Sr0.5Co1.5Fe0.5O5+δ (NBSCF) powders were synthesized 6 ACS Paragon Plus Environment

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using the glycine-nitrate process (GNP). The starting materials of Nd(NO3)3·6H2O (99.9% purity, Alfa Aesar), Ba(NO3)2 (99.95% purity, Alfa Aesar), Sr(NO3)2 (99% purity, Alfa Aesar), Co(NO3)2·6H2O (98% purity, Alfa Aesar), and Fe(NO3)3·9H2O (99.99% purity, Sigma-Aldrich) were dissolved in deionized (DI) water with glycine as a fuel. The suspension was stirred at 150 rpm and heated at 280℃ until the nitrates changed to black ash. The ash-like products were pre-calcined at 600℃ for 4 h and then ball-milled in ethanol for 24 h. The ground NBSCF powders were re-calcined at 900℃ for 5 h to obtain a double perovskite structure. The Nd0.1Ce0.9O2-δ (NDC) powders were also synthesized by GNP and calcined at 400℃ for 2 h. The cathode ink was prepared by mixing the NBSCF and NDC powders at a 3:2 weight ratio. The composite NBSCF-NDC powders were ground and mixed with α-terpineol, DBP, and PVB in ethanol using a mortar and pestle. The composite NBSCF-NDC ink was screen-printed onto the electrolyte of the cells and then sintered at 1150℃ for 2 h.

2.2 Characterizations and electrochemical measurements The phases of the synthesized powders were investigated by X-ray diffraction (XRD, D/MAX 2500, Rigaku, Japan) using Cu-Kα radiation in the 2θ range varying from 20° to 80°. The microstructures of the anode-supported cells were analyzed by field emission-scanning electron microscopy (FE-SEM, S-4700, Hitachi High Tech, Japan) and electron probe microanalysis (EPMA, JXA-8530F, JEOL Ltd) with wavelength dispersive X-ray spectroscopy (WDX, JEOL, Ltd). After the durability test, the cells were sectioned to investigate the crosssection and surfaces of the cells in a post-mortem analysis and compared with those for the reference cell. For electrochemical measurements, Au wires were attached to both the anode and cathode sides of the prepared cells using Pt paste (Heraeus, Germany) and then fired at 900℃ for 30 min. As-prepared anode-supported cells were installed in an alumina reactor, in which a high7 ACS Paragon Plus Environment


temperature ceramic adhesive (Ceramabond™ 569, Aremco) was used to seal the cell. The operation was controlled by a fuel cell test station (NARA Cell Tech Co., Korea) equipped with an electrical load (PLZ-164WA, KiKusui Electronics Co., Japan). Humidification of hydrogen gas was accomplished by bubbling air through a water bottle filled with DI water. The current– voltage (I-V) characteristics were measured at 650℃ with 3 vol.% humidified H2 and ambient air with a flow rate of 200 sccm. The long-term operations of the Ni-BCZYYb anode-supported cells with the NBSCF-NDC composite cathode were carried out using a constant current mode of 0.5 A·cm-2 at 650℃. To investigate the influences of the various methods used to prevent performance degradation, the cathode air-depletion, shunt, and fuel cell/electrolysis cycling protocols were tested in the anode-supported cells. The electrical properties of the anode-supported cells were determined using electrochemical impedance spectroscopy (EIS) with a potentiostat/galvanostat instrument (SP240, BioLogic, France) using a frequency range of 1 MHz to 1 Hz. The signal amplitude was 10 mV under open-circuit voltage (OCV) conditions. An equivalent circuit model was fitted to the impedance spectra using the EC-Lab software to separate the components. The 3-electrode configuration (working, counter, and reference electrodes) was used to independently measure the potentials of the anode and cathode versus the reference electrode.39,40 The working and counter electrodes had the same area of 0.2827 cm2 at the same position. A Pt reference electrode was attached to the surface at the cathode side. The distance between the working electrode and reference electrode was 8 mm. A multimeter (Agilent model 34401a) was used to measure the electrode potentials.

3. Results and discussion 3.1. Crystal structural and microstructural analysis of cell components In order to confirm the phase identification of the cell components, the as-prepared powders 8 ACS Paragon Plus Environment

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were investigated using XRD analysis. Fig. 1 shows the XRD patterns of the BCZYYb (for the electrolyte), NiO-BCZYYb (for the anode), and NBSCF-NDC (for the cathode) powders. The XRD peaks confirm that the BCZYYb powder used for the electrolyte had a single phase with the perovskite structure without any undesirable phases, which was similar to the peak for BCY (JCPDS 81-1386).41 The peaks of the NiO-BCZYYb anode powder were separately composed of NiO and BCZYYb phases, indicating that no chemical reactions occurred between the powders during the high-temperature fabrication process. The NBSCF cathode powder formed a double perovskite structure of the orthorhombic Pmmm space group (No. 47), which was in agreement with the results of previous studies. Yoo et al.42 and Kuroda et al.43 demonstrated that double perovskite materials showed a structural variation from the single perovskite to the orthorhombic and tetragonal structures based on the contents of both the A-site and B-site dopants. The XRD patterns of the NDC powders after calcination showed that the samples were composed of a single fluorite phase. The cross-sectional microstructure of the NiO-BCZYYb anode-supported cell with the NBSCF cathode was examined using FE-SEM and is shown in Figure 2. It was clearly observed that the NiO-BCZYYb was composed of the anode supporting layer and anode functional layer (AFL) (Figure 2a). The microstructures of these layers were heterogeneous and porous, with macro- and micro-pores due to the addition of the pore former (corn starch) (Figure 2b). Additionally, the NiO and BCZYYb particles were evenly distributed in the anode supporting layer. At a thickness of 20 μm, the AFL showed a uniformly porous microstructure, which could not only facilitate the electrochemical reactions by expanding the area of the triple phase boundary (TPB), but could also mitigate the mechanical stress caused by the mismatch in thermal expansion coefficients between the anode and electrolyte layers. The 15 μm thick BCZYYb electrolyte layer was entirely dense without any pores or pinholes, and was continuously attached to the area between the AFL and composite NBSCF-NDC cathode layer. 9 ACS Paragon Plus Environment


Figure 2c shows the detailed microstructure of the composite NBSCF-NDC cathode layer. The cathode layer showed a sufficiently porous and homogeneous distribution of fine NBSCF and NDC particles.

3.2. Long-term operation of anode-supported PCFCs under constant current condition To investigate the degradation behaviors of PCFCs, their durability was examined under a constant current density of 0.5 A·cm-2 at 650℃. Dry air (as an oxidant) and 3 vol.% humidified H2 (as fuel) were supplied to the cathode and anode sides, respectively. Figure 3 shows the change in the operating voltage under the constant current operation of the PCFCs with time. The cell voltage gradually decreased from 0.878 to 0.747 V, and the degradation ratio of the cell was 14.9% after 850 h of operation. In order to evaluate the cell degradation at the recurrence intervals, current-voltage-power (I-V-P) polarization curves were obtained before and after the PCFC operation using the same test conditions as those of the durability test. The I–V–P characteristics clearly indicated a gradual decay in the performance at the periodic intervals of the durability test (Figure 3b). The maximum power density decreased from 0.719 to 0.595 Wcm-2 at 650℃ after 850 h of operation. The open-circuit voltages (OCVs) of the cells decreased slightly, indicating that significant cell delamination was not occurred. The decay in the overall performance during continuous operation mainly occurred at the high current density region (> 1.25 A·cm-2) with increasing time, mainly due to the mass transport limitations (concentration polarization loss) on the cathode side.44 This phenomenon will be discussed in the post-mortem analysis section. Providing further support to reveal the primary degradation mechanisms, electrochemical impedance spectroscopy (EIS) measurements were performed under the OCV state at 650℃, and the results are shown in Figures 3c and 3d. The total resistance (Rtot) increased from 0.300 to 0.407 Ω·cm2. In particular, the ohmic resistance (Rohmic) slightly increased from 0.115 to 0.135 Ω·cm2. On the other hand, the polarization 10 ACS Paragon Plus Environment

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resistance (Rpol) increased from 0.185 to 0.272 Ω·cm2 after operation for 850 h, indicating that the change in Rpol was one of the main degradation factors during the normal operation of PCFCs. This result concurs with the I-V curve shown in Figure 3b. Rpol was contributed from the electrochemical reaction, such as the diffusion and charge-transport resistances in the anode and cathode.45 A post-mortem analysis was performed to clarify the degradation mechanisms related to the increase in electrode resistance values. Figure 4a shows SEM images of the interfacial NBSCF-NDC cathode and BCZYYb electrolyte boundary layers after the long-term durability test for 850 h. No significant change is observed in the BCZYYb electrolyte and NiO-BCZYYb anode compared to the as-fabricated cells (Figure 2a). On the other hand, significant NBSCF particle coarsening was observed within the devastation vicinity of the three-phase boundaries, whereas the microstructural features of NDC were not affected in the NBSCF-NDC composite cathode (Figure 4b). This observation agrees with the results of the I-V-P and EIS analyses (Figures 3b and 3c). During normal PCFC operations, performance degradation intrinsically proceeds because the cell is exposed to continuous redox reactions at high temperatures. As the main degradation mechanisms of PCFCs, numerous failure sources such as the growth and delamination of the cathode, and the growth and oxidation of Ni particles to NiO in the anode, can be considered, similar to those of SOFCs. However, unlike the SOFCs, the PCFCs produce water on the cathode side, because protons transport from the anode to the cathode through the electrolyte. In particular, it was reported that the electrochemical activity and thermal expansion coefficient of Sr-containing perovskite materials (e.g., LSCF, BSCF, and NBSCF) were strongly dependent on the Sr content under a cathode humidified condition.23,28 In other words, the coarsening of NBSCF during long-term operation might have resulted from the Sr surface segregation (probably SrO-like phases) by the water produced on the cathode side, in agreement with the results of the WDX analysis shown in Figure 4c. This indicates that the strong 11 ACS Paragon Plus Environment


electroactivity of SrO with H2O would be the major cause of the observed NBSCF cathode degradation, because the degradation by the presence of the Sr enriched layer is accelerated at much higher water vapor concentrations, as reported in previous studies.23,28 In addition, Sr segregation and the surface coverage by SrO-like insulating phases at the perovskite surface obstruct the electron transfer and oxygen exchange, leaving a dopant-deficit subsurface region. Accordingly, drastic changes in the surface composition of perovskite particles by the Sr segregation, and the formation by SrO-like insulating phases (i.e. Sr(OH)2) on the perovskite surface are expected to decrease the porosity of the electrode with the coarsening of NBSCF.

3.3. Operating logics to prevent performance degradation in anode-supported PCFCs Various strategies, such as using durable materials and degradation prevention methods against H2O in the cathode, need to be developed to minimize the performance degradation of cell components that occurring during the PCFC operation. We herein propose three properly designed control protocols, including cathode air-depletion, shunt current, and fuel cell (FC)/electrolysis cycling, to completely circumvent the operating-induced degradation of PCFCs. During long-term PCFC operation, maintaining the cathode layer at a high electrical potential level is unfavorable in terms of durability because it causes oxidation reactions in the surface layer of the cathode (surface oxide formation), as shown in Figure 5a. Additionally, the water production (high 𝑃𝐻2𝑂) caused by the electrochemical reaction of PCFC leads to an increase in the oxide species (high 𝑃𝑂2), including the formation of the SrO-like phases, which is normally controlled by the H2/H2O equilibrium reaction.23,28 Increasing the electrode potential with the operation time makes it more difficult to reduce the oxide materials, which lead to an irreversible performance loss in PCFCs. Hence, the performance recovery method is associated with the simulated low cathode potential level (a value of nearly zero) via the cathode air-depletion and shunt conditions (Figure 5b). 12 ACS Paragon Plus Environment

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First, the cathode air-depletion protocol is used under a periodic air-starvation condition by significantly decreasing the air flow rate in the cathode, which can temporarily create a reducing condition on the cathode side through a drop in the cathodic potential level. Second, the shunt current is applied to the cell to remove the newly formed oxide films in the cathode via the water production caused by the electrochemical reaction of the PCFC under the longterm operation (shunt protocol). Third, the FC/electrolysis cycling protocol is used with a periodic electrolysis process in a short period of time during the FC operation. This protocol can remove the water or steam produced at the cathode during the FC operation via electrolyzing the water in the electrolysis mode, which increases the cell durability. The results of long-term durability tests under the proposed three protocols are discussed with information on the electrochemical characteristics in the next section.

3.3.1. Cathode air-depletion protocol The cathode air-depletion protocol was conducted under a constant current density of 0.5 A·cm-2 at 650℃. The humidified H2 and air (200 sccm) were typically fed into the cell during the constant current density operation for 3 h (for Case I) and 10 h (for Case II), and the cathode air was shut off to 0 sccm for 1.5 min (Figure 6a). As soon as the cell voltage decreased close to 0 V, the normal flow rate of air was resupplied to the cathode. These cycles were repeated several times to obtain reproducible results. The performance degradation prevention tests included Case I (every 3 h) and Case II (every 10 h) to investigate the effect of the air-depletion time period. The results of the Case I and II tests are shown in Figure 6b. The operating voltage of the cell was very stable at 0.790 V during the test for both Case I and Case II. The cell voltage swiftly decreased over time after the air supply was stopped in the cathode. When the normal flow rate of air was resupplied to the cathode, the cell voltage increased remarkably to 0.851 V, and then reached a steady-state value under a constant current operation. In order to evaluate 13 ACS Paragon Plus Environment


the effect of the cathode air-depletion protocol on the cell performance, polarization curves were obtained before and after the PCFC operation using the same test conditions as those of the durability test (Figure 6c). The OCV of the cell was 1.063 V in the initial state and negligibly changed to 1.077 V after Cases I and II, indicating a lack of cell interface delamination under the operation of the cathode air-depletion protocol. It can also be seen that the maximum power densities for Cases I and II were obviously higher than those before the test, suggesting that the cell performance was recovered after the Case I and II protocols. To evaluate the relative contributions of the cathode and anode in the cell, a further investigation was performed using an electrode polarization experiment with the threeelectrode configuration. Cathodic polarization (𝜂𝑐) curves were first measured, and the anodic polarization (𝜂𝑎) was calculated from 𝜂𝑐 after the IR drop was compensated, because the difference between the cathodic and anodic potentials was equal to the observed cell voltage. In Figure 6d, the 𝜂𝑎 and 𝜂𝑐 values of the cell are shown as a function of the electrode current density. It is apparent that 𝜂𝑐 overwhelms 𝜂𝑎 in the PCFCs because of the sluggish kinetics of the oxygen reduction reaction at 650℃. Furthermore, 𝜂𝑐 decreased at an equivalent current density after the durability test with the cathode air-depletion protocol, indicating that the performance recovery in the anode-supported PCFC was mainly due to the curing of the cathode side. Figure 6e shows the EIS results measured under the OCV condition and 650℃ before and after the cathode air-depletion protocol. The area specific resistances (ASRs) calculated from the EIS analysis are shown in Figure 6f. After the performance degradation prevention tests (Cases I and II), the Rpol of the cell decreased considerably (0.333 → 0.225 Ω·cm2 and 0.261 Ω ·cm2, respectively), while the Rohmic of the cell showed negligible change, indicating that the Rpol was the main factor affecting the electrical performance of the PCFCs in the durability test. These results demonstrated that the cathode air-depletion protocol was very effective in mitigating cell degradation, and the cell performance was remarkably increased. 14 ACS Paragon Plus Environment

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3.3.2. Shunt current protocol The shunt current protocol was also proposed to minimize the performance degradation during PCFC operations, as shown in Figure 7a. To implement the shunt current protocol, a significant electrical load was applied to the cell, resulting in air starvation at the cathode, while the flow of air and H2 through the cathode and anode was maintained. To apply this protocol to actual FC systems, the shunt current protocol must electrically connect the FC stack and shunt resistor using an additional relay switch. During the durability test, the shunt current protocol was automatically controlled by the software in the fuel cell test station. The shunt current protocol was conducted under a constant current of 0.5 A·cm-2 at 650℃ at 3 h intervals. As soon as the current density increased from 0.5 to 2.0 A·cm-2, the cell voltage greatly decreased to around 0 V and was maintained for 1.5 min. These cycles were repeated 15 times. Figure 7b shows the operating voltage behavior as a function of time under the shunt current protocol. The cell voltage appeared to be slightly increased (0.706 → 0.743 V) at 0.5 A·cm-2 during the initial few cycles, and it was finally saturated. It was also observed that the maximum power density of the cell in the polarization curves slightly increased before the test (Figure 7c). The I-R-corrected electrode polarization curves of the cells before and after the durability test with the shunt current protocol were analyzed at 650℃ in order to further validate these results. Figure 7d shows the difference between the 𝜂𝑐 and 𝜂𝑎 values in the cell, and the NBSCF cathode shows two distinct curves for the 𝜂𝑐 values of the cell as a function of the electrode current density before and after the durability test with the shunt current protocol. On the other hand, the Ni-BCZYYb anode had very close 𝜂𝑎 values. In the EIS measurements (Figures 7e and 7f), the total ASR of the cell decreased from 0.530 to 0.483 Ω·cm2, with a significant decrease in Rp (0.290 → 0.253 Ω·cm2). It is evident that the shunt current protocol decreased 𝜂𝑐, in agreement with the I-V and EIS measurements. In other words, the shunt current protocol 15 ACS Paragon Plus Environment


played an important role in alleviating the degradation of the cell performance by the sudden decrease in cell potential to approximately 0 V, similar to the cathode-air depletion protocol. This protocol could contribute to recover the clean cathode surface from the oxidized one at a high 𝑃𝑂2 with the water production (high 𝑃𝐻2𝑂) caused by the electrochemical reaction during the continuous PCFC operation, as shown in Figure. 5b.

3.3.3. FC/electrolysis cycling protocol The FC/electrolysis cycling protocol was designed to prevent the performance degradation of the cell by removing the reaction products (water or steam) produced at the cathode side of the PCFC. PCFCs generate water or steam at the cathode side by electrochemical reactions during normal operation. The presence of water molecules can force the cathode materials to readily oxidize and decompose into unexpected phases (e.g., Sr segregation), leading to a permanent performance degradation. Hence, we propose the operation protocol to prevent the degradation of cathode materials via the periodic elimination of the produced water. This can be accomplished by applying frequent and short-term electrolysis reactions during the PCFC operations. Figure 8a shows a schematic diagram of the FC/electrolysis cycling protocol. The FC/electrolysis cycling protocol was performed under a constant current density of ±0.5 A·cm-2 at 650℃. The positive and negative signs for the current densities refer to the direction of electron flow. In the fuel cell mode, a positive current density of 0.5 A·cm-2 was applied to the cell for 3 h, and the direction of the current density was suddenly altered to a negative current density of ― 0.5 A·cm-2 for 1.5 min for the electrolysis mode. These cycles were repeated 15 times. Figure 8b shows the operating voltage behavior as a function of time under the FC/electrolysis cycling mode. In the FC mode of 0.5 A·cm-2, the operating voltage of the cell was very stable and increased from 0.737 to 0.740 V with the cycles. Further, the cell voltage increased sharply to 1.3 V for 1.5 min in the electrolysis mode (at ― 0.5 A·cm-2), as shown in 16 ACS Paragon Plus Environment

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the inset of Figure 8b. Figure 8c–8f present the I-V curves, electrode polarization results, and EIS analysis results before and after the FC/electrolysis cycling protocol at 650℃. It can be seen that the maximum power densities, ASRs, and electrode polarizations showed no significant differences before and after the test, suggesting that the cell performance was negligibly degraded during the FC/electrolysis cycling protocol.

3.4. Long-term durability tests with operation protocols to prevent cell performance degradation To verify the ability of the operation protocols to prevent cell performance degradation, long-term durability tests were carried out at 650℃ with the very effective mitigation approaches of the cathode air-depletion and shunt current protocols for more than 1,000 h. The operating voltage behaviors of the protected cells at 0.5 A·cm-2 using the cathode air-depletion and shunt current protocols are shown in Figure 9 and are compared to those of the unprotected cell under the same conditions. The operating voltage of the unprotected cell gradually decreased over time, and the decay ratio was 14.9% for 850 h. On the other hand, the cell voltages of the protected cells were stably maintained for more than 1,200 h without any noticeable degradations, even though the initial voltages of the three cells were similar before the test. I-V-P polarization curves and the results of EIS measurements of the protected cells are presented in Figure 10. It is interesting to note that the maximum power density of the protected cell using the shunt protocol increased from 0.678 to 0.739 W·cm-2 with the decreased ASRs (0.364 → 0.321 Ω·cm2) for 1,200 h (Figures 10a and 10b). In particular, the concentration losses at the low frequency range were effectively suppressed using the shunt current protocol, which improved the cell performance. The OCVs of the cell were also maintained at 1.085 V for 1,200 h without any noticeable reduction. Figures 10c and 10d show the electrochemical results of the protected cell with the cathode air-depletion protocol during the long-term 17 ACS Paragon Plus Environment


durability test. Similarly, the values of the OCVs, maximum power densities, and ASRs showed no significant differences after operation for 1,500 h, indicating that the cathode air-depletion protocol effectively prevented the performance degradation of the cells. A post microstructure analysis of the anode-supported PCFC was carried out using SEM observations. After operation for 1,500 h with the cathode air-depletion protocol, the NBSCF cathode showed cell microstructures that almost identical to those in the initial state, as presented in Figure 2. As a result, it was confirmed that these properly designed control protocols were a significant part of the operating logic for the prevention of SOFC performance degradation without any noticeable microstructural changes.

4. Conclusion In order to design robust PCFCs, Ni-BCZYYb anode supported cells were fabricated with a BCZYYb electrolyte and an NBSCF-NDC composite cathode. Reliability testing of these cells was then carried out under a continuous constant current density of 0.5 A·cm-2 at 650℃. Without any protection protocols, the operating voltage of the cell gradually decreased, and the decay rate of the cell was 14.9% over 850 h. This significant decay in the overall performance during the continuous operation mainly occurred in the high current density region in the I-V polarization curves. In addition, significant NBSCF particle coarsening was observed in the SEM and EPMA/WDX analysis results, within the devastation vicinity of the three-phase boundaries. This result was due to the formation of SrO-like phases by the water produced on the cathode side. In other words, the presence of water molecules could force the cathode materials to readily oxidize and decompose into unexpected phases, leading to the permanent performance degradation of the PCFCs. Hence, to prevent the degradation of the cathode in the cell components, three performance recovery methods (cathode air-depletion, shunt current, and FC/electrolysis cycling) were proposed, which were associated with a simulated low 18 ACS Paragon Plus Environment

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cathode potential level (a value of nearly zero). To verify the influence of these operation protocols in preventing cell performance degradation, long-term durability tests were carried out at 650℃ for more than 1,200 h. It was found that the cell voltages and maximum power densities of the protected cells were stably maintained without any noticeable degradations.

Acknowledgments This research was supported by the Mid-career Researcher through the National Research Foundation of Korea (NRF-2017R1A2A2A05069812) and by the Industrial Technology Innovation R&D Program of the Korea Institute of Energy Technology Evaluation and Planning (KETEP), which has been granted financial resources from the Ministry of Trade, Industry & Energy, Republic of Korea (No. 20173010032290 and No. 20153010031940).

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Figure

Figure 1. XRD patterns of components of anode-supported cells: BCZYYb electrolyte, NiOBCZYYb anode substrate, and NBSCF-NDC composite cathode.

Figure 2. SEM micrographs of anode-supported cell before durability test: (a) cross-sectional view, (b) Ni-BCZYYb anode, and (c) NBSCF-NDC cathode surface view. 25 ACS Paragon Plus Environment


Figure 3. Long-term stability test under galvanostatic condition of 0.5 A·cm-2 at 650℃ and electrochemical analysis at periodic intervals: (a) change in operating voltage under constant current operation of PCFC with time, (b) current–voltage polarization behaviors, (c) EIS measurements under OCV state, and (d) total, ohmic, and electrode area specific resistance (ASR) values after operation for 850 h.


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Figure 4. SEM images and EPMA/WDX analyses of anode-supported cell after long-term stability test for 850 h at 650℃: (a) cross-sectional SEM view, (b) NBSCF-NDC cathode surface SEM view (up and down refer to before and after stability test, respectively), and (c) EPMA/WDX analyses of composite NBSCF-NDC cathode (particularly focused on NBSCF particles).



Figure 5. Schematic diagram of (a) electrochemical potential levels of both electrodes during PCFC operations and (b) curing process of degraded cathode surface by operation protocols to prevent performance degradation. The water vapor is produced at the cathode in PCFCs, whereas in SOFCs water vapor is produced at the anode. In addition, always maintaining the cathode layer at a high electrical potential level is unfavorable in terms of durability during the long-term PCFC operation, because it causes oxidation reactions in the surface layer of the cathode. Hence, the performance recovery method is associated with the simulated low cathode potential level to remove the newly formed oxide films in the cathode via the water production.


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Figure 6. Short-term stability test under cathode air-depletion operating protocol and electrochemical analysis: (a) schematic illustration of Cases I and II air-depletion operating protocols, (b) time-voltage behaviors under galvanostatic condition of 0.5 A·cm-2 at 650℃, (c) I–V polarization curves, (d) cell overpotential as function of electrode current density, (e) EIS measurements at OCV state, and (f) total, ohmic, and electrode ASR values.



Figure 7. Short-term stability test under shunt current operating protocol and electrochemical analysis: (a) schematic illustration of shunt current operating protocol, (b) time-voltage behaviors under galvanostatic condition of 0.5 A·cm-2 at 650℃, (c) I–V polarization curves, (d) cell overpotential as function of electrode current density, (e) EIS measurements at OCV state, and (f) total, ohmic, and electrode ASR values.


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Figure 8. Short-term stability test under FC/electrolysis cycling operating protocol and electrochemical analysis: (a) schematic illustration of FC/electrolysis cycling operating protocol, (b) time-voltage behaviors under galvanostatic condition of 0.5 A·cm-2 at 650℃, (c) I–V polarization curves, (d) cell overpotential as function of electrode current density, (e) EIS measurements at OCV state, and (f) total, ohmic, and electrode ASR values.



Figure 9. Long-term stability test results for unprotected and protected cells using operation protocols with cathode air-depletion and shunt current under constant current density of 0.5 A·cm-2 at 650℃.


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Figure 10. Electrochemical analysis results for protected cells using operation protocols with cathode air-depletion and shunt current after long-term stability test: (a) I–V–P characterizations and (b) EIS measurements of protected cells using shunt current operation protocol, (c) I–V–P characterizations, and (d) EIS measurements of protected cells using cathode air-depletion operation protocol.



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Operation protocols to improve durability of protonic ceramic fuel cells

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