Design of Active Site at Heterointerface between Brownmillerite Type

Article Cite This: ACS Appl. Energy Mater. 2019, 2, 5183−5197

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Design of Active Site at Heterointerface between Brownmillerite Type Oxide Promoter and Fluorite Cubic ZrO2 in Anode of Intermediate Temperature SOFCs Shigeharu Ito,†,‡ Toshiyuki Mori,*,† Akira Suzuki,† Hiroshi Okubo,† Shunya Yamamoto,§ Takaya Sato,‡ and Fei Ye∥,#

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†

Global Research Center for Environmental and Energy Based on Nanomaterials (GREEN), Center for Green Research on Energy and Environmental Materials, National Institute for Materials Science (NIMS), 1-1 Namiki Tsukuba, Ibaraki 305-0044, Japan ‡ Department of Creative Engineering, Chemical and Biological Course, National Institute of Technology, Tsuruoka College, 104 Sawada, Inoka, Tsuruoka, Yamagata 997-8511, Japan § Takasaki Advanced Radiation Research Institute, National Institute for Quantum and Radiological Science and Technology (QST), 1233 Watanuki, Takasaki, Gunma 370-1292, Japan ∥ School of Material Science and Engineering, Dalian University of Technology, 2 Linggong Road, Dalian 116024, China # Department of Materials Science and Engineering, Southern University of Science and Technology, No. 1088, Xueyuan Road, Shenzhen 518055, Guangdong, China S Supporting Information *

ABSTRACT: New hopping site for mobile oxygen (i.e., active site) on cubic ZrO2 surface in the anode was designed along the guide of previously published first-principles simulation. To design it, the authors proposed a heterointerface between brownmillerite type oxide and fluorite type cubic ZrO2 (“BF heterointerface”) as model active site. Small amount (0.2 wt %) of brownmillerite type oxide promoter made new hopping site on YSZ and conspicuously improved both anode performance and its stability at 700 °C. The combination work of XPS analysis and surface atomistic simulation suggests the formation of Frenkel type defect clusters on BF heterointerface. Also, this combination work was useful to conclude why the optimum content of brownmillerite type oxide for promotion of anode reaction was around 0.2 wt %. Finally, it is found that “BF heterointerface” designed by combination of the modeling, surface microanalysis, and fabrication can solve the trade-off relation between high anode performance and its long-time stability for development of state-of-the-art IT-SOFC. KEYWORDS: IT-SOFC, anode, three-phase boundary, active heterointerface, brownmillerite type oxide promoter, surface atomistic simulation temperature of SOFCs.6−10 Also, the influence of defect structure formation in the solid electrolyte11−13 and in the interface of the anode/solid electrolyte14−22 on the conducting property was investigated by using analytical TEM to improve the performance of IT-SOFC devices. Among them, the microanalysis works of anode/solid electrolyte interface of ITSOFC11−22 especially highlighted that the functional interface as new active site in the anode layer of IT-SOFCs has to be designed. However, it is necessary to realize compatibility between the high performance of IT-SOFC and its good stability. In the anode layer of SOFCs, the proton conductors such as Sr(Y, Ce)O3 or Ba(Y, Ce)O3 perovskite oxide promotor which reveals the mixed conducting property at 800 °C enhanced the

1. INTRODUCTION Cermet materials based on composite of Ni and 8 mol % yttria stabilized zirconia (YSZ) have been widely used in the anode layer of solid oxide fuel cells (SOFCs).1−5 The operation temperature of conventional SOFCs is over 800 °C. This high temperature operation of SOFCs gives rise to the serious demerits such as short lifetime, thermal expansion coefficient mismatch among cell components, expensive production cost by using ceramics components, and so on. Nowadays, development of intermediate temperature (from 700 to 750 °C) SOFCs (IT-SOFCs) has attracted much attention for use of stainless steel as stack cell component. For development of “state-of-the art IT-SOFC” with the quality of exceeding the limit of conventional IT-SOFC, the design of new active site at interface between electrolyte and electrode of SOFCs is key. The interface between cathode materials and solid electrolytes such as YSZ or YSZ with Sm doped CeO2 interlayer has been characterized and modified for lowering the operation © 2019 American Chemical Society

Received: May 3, 2019 Accepted: June 26, 2019 Published: June 26, 2019 5183

DOI: 10.1021/acsaem.9b00864 ACS Appl. Energy Mater. 2019, 2, 5183−5197

Article

ACS Applied Energy Materials

Figure 1. Cubic ZrO2(111) surface (a); cubic BaInO2.5(111) surface (b); overlap surface area between cubic BaInO2.5(111) and cubic ZrO2(111) (i.e., BF heterointerface) (c-1); model BF heterointerface with disordered oxygen vacancies as new active site (c-2).

anode performance.23−26 It is widely accepted that both charge migration and proton diffusion through the Sr(Y, Ce)O3 or Ba(Y, Ce)O3 perovskite proton conductor promoted the anode reaction at three-phase boundaries (TPBs) in the anode layer of SOFCs. The content of Sr(Y, Ce)O3 or Ba(Y, Ce)O3 perovskite oxide promoter in the anode layer is 4−5 wt % in whole anode component (i.e., Ni and YSZ). Small amount of Sr(Y, Ce)O3 or Ba(Y, Ce)O3 perovskite promoter contributes to the enhancement of anode reaction at TPBs at 800 °C. Also, the promotion effect of precious metal (i.e., Pt, Ru, Pd, and so on) loaded mixed conductor Sm doped CeOx(SDC) was examined for improvement of anode performance of SOFC.27,28 The loading amount of precious metal on SDC is approximately 0.25 wt %. This work suggests that the promotion effect of precious metal on SDC comes up less than 0.5 wt % which is much lower than the content of Sr(Y, Ce)O3 or Ba(Y, Ce)O3 perovskite oxide promoter. The TPBs activated by the precious metal loaded SDC promoter show higher promotion effect and lower overpotential on the anode at 800 °C. Most recently, trace amount of Pt (approximately 9 ppm) conspicuously improved the anode performance of SOFC at 700 °C by formation of unique defect interface structure as active site on partially oxidized Ni surface.29 Previously published effect of oxide promotor additives23−28 suggests that the design of active interfaces in the anode layer

of IT-SOFCs is one of promising challenges to see new innovation in the fuel cell field. Also, all those applications are largely attributed to their high mixed conducting property (i.e., electron migration and ion diffusion property) for enhancement of anode reaction at TPBs. And it is widely accepted that mixed conducting properties of both of oxide and noble metal doped oxide are dominated by the oxygen defect structure in the oxide. Therefore, we believe that design of interface between oxide promoter and a cermet composition anode at TPBs provides us great opportunity to fabricate the state-ofthe-art IT-SOFCs. However, the promotion effect of mixed conductor did not reach a sufficient level for fabrication of the SOFCs operated around 700 °C. To maximize the promotion effect, the active interface between high quality mixed conductor and anode has to be designed along the guide by theoretical analysis. Recently, first-principles simulation in combination with already available data predicted the great opportunity to make innovative breakthroughs on the anode in SOFCs.30 This firstprinciples simulation work30 suggests that the surface oxide ion diffusion on YSZ in the cermet anode is the rate-determining step for the formation of water molecules in the fuel cell anode reaction, although the dissociation rate constant of H2 on Ni is known to be high. Also, this first-principles simulation clearly indicates that the formation reaction of water molecules on the 5184


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2. EXPERIMENTAL SECTION

anode will be conspicuously enhanced if we design the new active site for surface oxide ion diffusion on cubic ZrO2. As other state-of-the-art fuel cell works, the active sites on none-precious metal doped carbon cathode in polymer electrolyte fuel cells were clarified by combining first-principles simulation with STEM-EELS analysis.31 Also, the combination of in situ/operando Raman spectroscopy and simulation work successfully characterized coaking and sulfer poisoning phenomena on Ni surface in the anode of SOFC.32 Those latest state-of-the-art fuel cell works clearly suggest that the simulation of surface defect structure combined with characterization of defect surface will provide us good opportunity for fabrication of superior quality IT-SOFC device which has good balance between high performance and longtime stability at low or intermediate temperature region. On the basis of recent published works,30−32 the authors concluded that the combination work among modeling, microanalysis, and fabrication provides us with a great chance to fabricate the state-of-the-art IT-SOFC device. In order to tailor the activity of water molecule formation on the anode, we compared the surface structure of cubic barium indium oxide BaInO2.5 (BIO) (111) as high-quality mixed conductor with that of cubic ZrO2 (111) in advance. BIO which consists of orthorhombic brownmillerite-type structure has been examined for electrical conductivity as fast ion conductor in dry atmosphere above 930 °C and high temperature (i.e., above 300 °C) proton conductor in wet atmosphere. Also, it is well-known that high electric conducting phenomenon in BIO appeared around the transition temperature (Tt = 930 °C) by an order−disorder transition of oxygen vacancies.33−38 Recently, we fabricated BIO based compound Ba2(In1−x(Zr0.5, Zn0.5)x)2O5 (BIZZO, x = 0.1, 0.2, and 0.3) which reveals the top level electrical conductivity around 700 °C as compared with previously reported BIO based systems.39 We considered that BIZZO is one of the good candidates for tailoring active interface on the anode. To develop our design concept for fabrication of the active site which can promote the surface oxide ion diffusion on the cubic ZrO2 in the anode, the surface structure models of cubic ZrO2 (111) and cubic BIO (111) on X−Y plane and Y−Z plane in the two-dimensional coordinate system are demonstrated in Figure 1a and Figure 1b, respectively. Since the unite cell size of cubic ZrO2 (111) surface is smaller than that of cubic BIO (111) surface on both X−Y plane and Y−Z plane, it turns out that the positions of atoms on the cubic BIO (111) surface do not lie directly over the atoms on the cubic ZrO2 (111) surface (refer to Figure 1c-1). Figure 1c-1 suggests that we can make unique heterointerface between cubic BIO and cubic ZrO2, and this unique “brownmillerite/fluorite heterointerface” (“BF heterointerface”) would play a key role in order to design the hopping sites for surface diffusion of oxide ion on cubic ZrO2 in the anode (refer to Figure 1c-2). In the present work, the authors fabricated the “BF heterointerface” to improve the cell performance in a temperature ranging from 700 to 800 °C. Also, the effect of formation of BF heterointerface as active site on improvement of anode performance was concluded on the basis of the combination work among modeling, surface microanalysis, and fabrication.

2.1. Preparation of Promoter. Fine BIZZO powder was prepared in a stepwise process. Nondoped BIO fine powder was synthesized by a hot ammonium carbonate precipitation method. The commercially available Ba(NO3)2 (99% up, Wako Pure Chemical, Japan), and In2(NO3)3·4.7H2O (99.99%, Kojundo Chemical, Japan), (NH4)2CO3 (Wako Pure Chemical Industries, Ltd., Japan) powders were used as the starting materials. To prepare the fine precipitation, a (NH4)2CO3 aqueous solution was heated at 45 °C in a thermostatic chamber. The mixed aqueous solution of Ba(NO3)2 (5.8804 g dissolved into 300 mL of water) and In2(NO3)3·4.7H2O (5.7539 g dissolved into 300 mL of water) was dropped into the (NH4)2CO3 (43.2 g dissolved into water) aqueous solution for 1 h period, and this mixture was continuously stirred using a magnetic stirrer at 45 °C for 24 h. After filtration and rinsing, the precipitate was dried at room temperature in a N2 gas flow for 2 days. The dried powders were calcined at 450 °C for 2 h in an O2 gas flow. In the last step of the preparation process, the fine BIZZO powder, the commercially available ZnO powder (purity, 99.999%, Kojundo Chemical Laboratory, Co. Ltd., Japan), and the ZrO2 powder (0Y grade, TOSOH Company, Japan) were mixed and wet ball-milled in ethanol for 12 h. After drying and rinsing, the dried cake was lightly crushed with agate motor and pestle. Then, the powder mixture was calcined at 1000 °C for 1 h in air. 2.2. Sample Characterization. The crystal phases of nondoped BIO and BIZZO were identified by using power XRD analyzer (RINT-ULTIMA III, Rigaku, Japan). Details of the refinements are given in the explanation of “detailed measurement condition of Rietveld analysis of XRD” of Supporting Information (refer to explanation of Figure S2). The surface chemical states of prepared samples were characterized by XPS (VersaProbe II, ULVAC-PHI PHI, Japan). XPS spectra were observed by using a standard laboratory Al Kα X-ray source (hν = 1486.6 eV). Binding energies of Ba 3d, In 4f, Zr 4f, Zn 3d, Ni 2p, and O 1s core level peaks were corrected using the binding energy of C 1s core level peak as 284.5 eV. The morphology of particles in anode layer was observed using field emission microspore (FE-SEM, SU8230, Hitachi, Japan). Composition analysis was performed by FE-SEM equipped with an energy-dispersive X-ray spectrometer (EDX). Also, the particle size distribution observed for the cermet anode was examined by using linear intercept technique. The number of intercepts between the test lines and grain boundaries was counted by using 400 grains in SEM pictures taken from the cross section of the reduced anode layer of SOFC single cells. 2.3. Electrochemical Measurement. Webb−Wagner direct current (dc) polarization measurement was performed for estimation of ratio of oxide ionic conductivity and n-type electronic conductivity in BIZZO. For this electrochemical measurement, the dense sintered sample whose density was more than 93% of theoretical was used (refer to Figure S1 of Supporting Information). The process details for fabrication of dense sintered sample were given in our previously published paper.39 Webb−Wagner dc polarization measurement requires one ion-blocking electrode and the other to be reversible electrode (i.e., nonblocking electrode). Pt plate (thickness, 0.1 mm) which is the ion-blocking electrode was put on the BIZZO dense pellet (diameter, 13 mm; thickness, 0.5 mm). On the other side of the BIZZO pellet, a thin Pt paste was applied and calcined at 900 °C for 1 h at a heating rate of 10 °C min−1. This process is for fabrication of porous nonblocking electrode on BIZZO dense pellet. The oxygen partial pressure (PO2) dependence of dc conductivity measurement was performed at 700 °C. The PO2 was controlled by humidified hydrogen at room temperature from 10−17 to 10−20 atm, and this was checked by online zirconia oxygen sensor. The direct conductivity was measured by four-point probe method. The steady state current in the potentiostatic mode was measured. The potential was increased in 0.1 V steps from 0.05 to 1.2 V. Each step was maintained at fixed time to collect the steady-state current data at each Po2. 5185


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simulated air (10 mL min−1 of O2 + 40 mL min−1 of N2) to the cathode side for 3.5 h at 800 °C. During the measurement of the electrode performance, the humidified hydrogen (80 mL min−1) and oxygen (80 mL min−1) flowed into the anode and cathode sides, respectively. The IR-free polarization characteristics (i.e., cell potential (IR-free) curves) observed for the single cells were evaluated by a current interrupt method at 700 and 800 °C. The potentiostat/galvanostat (HAL3001A, Hokutodenko, Japan) as a fast solid-state switch was used for this measurement. Data were collected using a digital storage oscilloscope (DS5612A, IWATSU, Japan). To conclude the correct anode overpotential values, a cathode overpotential was measured using reference electrode Pt (diameter, 0.2 mm). Also, the IR included polarization characteristics (i.e., I−V curves and I−P curves) recorded from the single cell were examined using the potentiostat/galvanostat (HAL3001A, Hokutodenko, Japan) and the electrochemical measurement system HZ-5000 (Hokuto Denko, Japan). For measurement of cell voltages between OCV and zero, the current was varied by adjusting the resistance of the external load. In order to obtain data at higher current densities, an external voltage was applied to the cell using function generator (HB305, Hokuto Denko, Japan). And all I−V curves were obtained under the steady state condition. 2.6. Surface Atomistic Simulation. To conclude the formation of BF heterointerface which is induced by small amount of BIZZO additives in the prepared cermet anodes, the surface atomistic simulation was performed. The surface cluster energy is calculated by a two-region approach coded in the General Utility Lattice Program (GULP),40 which is incorporated within the functionality of the code MARVIN. In this approach, the lattice for energy minimization is partitioned into two regions: a spherical inner region I at the center where the defects are introduced and an outer region II, which extends to infinity. To ensure a smooth transition between region I and region II, an interfacial region IIa is introduced. The surface of cubic ZrO2 (111) is stacked by repeating units of O−Zr layers. In the present work, the size of the supercell containing the cubic ZrO2 surface is 7 × 7. Region I consists of approximately 10 repeating units with thicknesses about 22.9 Å. Region II consists of approximately nine repeating units with thicknesses of 21.7 Å (refer to Figure 2).

In general, PO2 dependence of electrical conductivity measured in the mixed conductor which involves Frenkel type defects is expressed by the following two equations. In the low PO2 region (i.e., in anodic condition), the reduction reaction which is shown in eq 1 can be considered in anodic condition.

OO x → Vo•• + 2e′ + (1/2)O2

(1)

The equilibrium constant Ke for this reduction reaction is determined by eq 2.

[Vo••][e′]2 PO21/2/[OO x ] = Ke

(2)

When [e′] (i.e., electron concentration) corresponds to [Vo••] (i.e., oxygen vacancy concentration), eqs 3 and 4 are obtained from eq 2. [e′]3 PO21/2/[OO x ] = Ke

(3)

[e′] ∝ PO2−1/6

(4)

Then, the slope of PO2 dependence of electrical conductivity is −1/6 in the anodic condition. In addition, the fraction of the total conductivity which is carried by electronic and ionic defects is termed the “transference number of ion, ti”,and it is estimated by eq 5, t i = (ionic conductivity)/(total conductivity) = 1 − (electric conductivity measured by blocking electrode method/total conductivity measured by nonblocking electrode method)

(5)

The semiconducting property of BIZZO was characterized on the basis of eq 4 and eq 5. To examine another evaluation index for performance of both the conventional anode and present sample with 0.2 wt % BIZZO, the impedance spectra were measured near OCV using full cell. The impedance spectra were obtained, with a frequency range from 0.1 Hz to 100 kHz and a 10 mV ac perturbation, using Solartron SI 1260 impedance/gain-phase analyzer in conjunction with electrochemical measurement system HZ-5000 (Hokuto Denko, Japan). 2.4. SOFC Single Cell Preparation. SOFC single cells were prepared by screen printing anode and cathode materials on both sides of YSZ solid electrolyte disk (diameter, 13 mm; thickness, 0.5 mm; relative density, ∼95% of the theoretical). A slurry containing the mixture of nickel oxide (99.9%, Wako Laboratory Chemicals, Japan) and YSZ powder (grade name; TZ-8Y, TOSOH, Japan) with the mass ratio of NiO:YSZ = 4:1 was used as the cermet anode material. To examine the promotion effect of BIZZO in the anode, the prepared fine BIZZO powder (from 0.1 to 0.4 wt %), the cermet anode powder, and small amount of ethanol (i.e., several drops of ethanol) were mixed by using agate motor and pestle for 30 min. The anode slurry was printed on the solid electrolyte disk in a five step repeated process. The coated material was calcined at 1200 and 1300 °C for 1 h. The thickness of anode layer after the calcination was approximately 40 μm. For the cathode, the La0.85Sr0.15MnO3 slurry (LSM20-1, Fuel Cell Materials, USA) was used. In a process similar to that described previously, the cathode material was applied in five-step process and then calcined at 1100 °C for 1 h. The thickness of cathode layer was approximately 50 μm. The area of electrodes was 6 mm in diameter. 2.5. SOFC Single Cell Performance. Electrode performance of the prepared SOFC single cells was examined by a SOFC evaluation device (FC-400H series, CHINO, Japan) with gas sealer cement. To minimize the interfacial contact resistance, a platinum mesh was attached to both cathode and anode surfaces. To complete the circuit, two platinum wires (diameter, 500 μm) for current supply and potential probe were connected to the platinum mesh. Prior to the measurement, each SOFC cell was subjected to a reducing atmosphere which was supplied by a diluted hydrogen mixture gas (5 mL min−1 of H2 + 45 mL min−1 of N2) to the anode side and

Figure 2. Cubic ZrO2(111) 7 × 7 surface model (a) and its enlarged surface model (b). The atomistic interactions are based on the energy minimization within the framework of the Born model of an ionic crystal, which includes a long-range term to represent Coulombic interaction and a short-range term to describe the electronic attractive van der Waals force and electronic repulsion between electron clouds. The shortrange term is described by a Buckingham potential, described in the eq 6: E(rij) = Aij exp(− rijρij−1) − Cijrij−6

(6)

where rij is the distance between ions and Aij, ρij, and Cij are three adjustable parameters (i.e., short-range pair potential parameters) depending on interacting ions. Those short-range potential parameters are listed in Table 1.41−43 The interactions arising from the polarizations of O2−, Zr4+, Ba2+, In3+, and Zn2+ are described by the shell model. The short-range interactions are set to zero beyond a cutoff of 20 Å. The polarized 5186


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From eq 7, it can be noticed that a positive ΔEb implies a preference of the formation of surface defect clusters over its individual components. Also, the calculated ΔEb, ∑Eisolated, and Ecluster are positive values indicating that the calculations are reasonable for the defect cluster model proposed.

Table 1. Short Range Potential Parameters interaction

A (eV)

ρ (Å)

C (eV Å6)

ref

Zr −O O2−−O2− Ba2+−O2− In3+−O2− Zn2+−O2−

985.87 22764.3 931.8 1495.9 690.04

0.376 0.149 0.3873 0.3601 0.2986

0.0 27.89 0.0099 2.182 0.099

41 41 42 42 43

4+

2−

3. RESULTS AND DISCUSSION Recently, we reported on the high electrical conductivity induced by design of Frenkel defect structure in the BIO lattice.39 In this material, the formation of Frenkel defect clusters in BIO lattice contributed to the increase of local disordering of oxygen vacancies and maximization of electrical conductivity in BIO system. As supporting evidence for formation of the local disordering area in BIZZO, the peak intensity of superlattice peaks of XRD profile observed for BIZZO was at quite a small level as compared with conventional BIO (refer to Figure S2 of Supporting Information). That result indicates that the aforementioned local disordering area of oxygen vacancy corresponds to formation area of the microdomain which consists of pseudo cubic BIO lattice at room temperature. In the present work, a design of high quality active site which promotes both surface oxide ion diffusion and water molecules formation on cubic ZrO2 around TPBs in anode is required. In previously published work, the oxygen electrode performance of water-electrolysis was conspicuously improved by formation of “effective reaction zone” which consists of mixed conductor such as samarium doped ceria particles.44 This suggests that “BF heterointerface” in the present work can be “effective reaction zone” in nanoscale on the cubic ZrO2. To fabricate “effective reaction zone” in nanoscale on YSZ using “BF heterointerface”, the authors expected that BIZZO promotes both oxide ion diffusion and electron migration as promoter for anode reaction. Figure 3 presents PO2 depend-

ions in the present model are described by a massive core with charge X|e| connected by a massless shell with charge Y|e|, resulting in the overall ion charge of (X + Y)|e|. The core and shell are connected by an isotropic harmonic spring with the force constant k of O2−. The shell parameters are listed in Table 2.41−43

Table 2. Shell Parameters species

Y |e|

k (eV Å−2)

ref

Zr4+ O2− Ba2+ In3+ Zn2+

1.35 −2.077 1.46 −6.1 0.0

169.617 27.290 14.78 1680.0 8.57

41 41 42 42 43

To investigate the preference and stability of the surface defect clusters, the binding energy (ΔEb) is calculated using eq 7: ΔE b =

∑ Eisolated − Ecluster

(7)

where ∑Eisolated is the sum of the formation energy for isolated defects and Ecluster is the formation energy of a cluster. ΔEb is normalized by the total number of defects in the clusters. The calculation step details are as follows. First, lattice energy of cubic ZrO2 (111) at 7 × 7 region was calculated using GULP. Calculated lattice energy of cubic ZrO2 (111) was −253 263.2 eV. In the second step, the isolated defect energies for VO••, vacancy of Zr4+ sublattice site (VZr⁗), Ba2+ interstitial defect (Bai••), In3+ interstitial defect (Ini•••), Zn2+ interstitial defect (Zni••), and oxygen interstitial defect (Oi″) were calculated by following eq 8.

isolated defect energy = [lattice energy of cubic ZrO2 ] − [surface energy of each defect]

(8)

Note that the surface energy of each defect was calculated by GULP in advance. One example for calculation of the isolated defect energy of VO•• is as follows: Figure 3. Oxygen partial pressure dependence of electrical conductivity observed for BIZZO sintered pellets at 700 °C: nonblocking electrode method (○); blocking electrode method (●).

isolated defect energy of VO•• = −253263.2 eV − (−253665.1 eV) = 401.9 eV The calculated energies of isolated point defects are summarized in Table 3.

ence of electrical conductivity of BIZZO sintered pellets which is measured by nonblocking method and blocking method at 700 °C. The relationship between the logarithm of conductivity and logarithm of PO2 has slopes of −1/6 in the anodic condition. The measurement result in Figure 3 basically agreed with eq 4 in steady state condition. This result indicates that BIZZO promotes electron migration in anodic condition. Also, the transference number of ion (ti) was examined using electrical conductivity in anodic condition as follows:

Table 3. Calculated Isolated Point Defect Energies on ZrO2(111) Surface defect

isolated defect energy (eV)

VO•• VZr⁗ Bai•• Ini••• Zni•• Oi″

401.9 −1027.6 341.1 543.8 428.6 −233.9 5187


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promoted by addition of small amount of BIZZO (0.2 wt %) to anode layer as shown in Figure 4a. To characterize the influence of small amount of BIZZO addition (0.2 wt %) on the charge transfer process in the activation overpotential region and concentration overpotential region, Tafel lines derived from the cell potential (IR-free) data were compared at 700 °C in Figure 4b. Tafel lines as shown in Figure 4b have curvatures. One region whose current density is from 2 mA cm−2 to 17 mA cm−2 corresponds to the activation overpotential region. In this region, fast charge transfer on the anode is the rate-determining step in the anode reaction. If high quality active sites were created on the anode surface, the Tafel line in the activation overpotential region conspicuously shifts to higher cell potential direction. The cell potential (IRfree) at 8 mA cm−2 (cell current density) observed for the present anode with BIZZO (1.09 V) was approximately 0.18 V higher than the conventional anode without BIZZO promoter. This result strongly suggests that high quality active site for promotion of water molecule formation is created by addition of small amount of BIZZO promoter into the anode. Another region of Tafel line is termed the concentration overpotential region in which the observed current density is from 17 to 200 mA cm−2 in Figure 4b. In that region, the diffusion process is rate-determining step. The cell potential (IR-free) is strongly affected by the gas diffusion at the interface of anode layer. As shown in Figure 4b, the cell potential (IR-free) at 30 mA cm−2 (cell current density) observed for the present work anode with BIZZO (0.97 V) was approximately 0.23 V higher than that of the conventional anode. It suggests that the diffusion of oxide ion on the cubic ZrO2 surface was enhanced and the anode activity was conspicuously improved by formation of active site around TPBs in the anode of ITSOFC. Also, the promotion effect of BIZZO was clearly observed at 800 °C by comparison between the conventional anode and the present work anode with 0.2 wt % BIZZO (refer to cell potential (IR-free) data, as shown in Figure S3 of Supporting Information). In the present work, we used same cathode material with same amount on the electrolyte. However, small level gas leak through the cement gas sealer of the single cell and small fluctuation of cathode content would affect the observed cell potential (IR-free) value. As a consequence of this, we examined the cathode overpotential observed for conventional anode and our anode with 0.2 wt % BIZZO. Also we concluded the influence of fluctuation of cathode overpotential on the observed cell potential (IR-free). The small difference (approximately 0.03 V at 30 mA cm−2) between two cathode overpotential lines in concentration overpotential region was observed, as shown in Figure 5a. Also, this small difference was seen in comparison of two Tafel lines (refer to Figure 5b). Aforementioned small difference in the comparison of cathode overpotential would be attributable to the small level gas leak through the cement gas sealer of SOFC single cell. As demonstrated in the small subwindow of Figure 5b (i.e., small Figure 4b in Figure 5b), the fluctuation level of cell potential (IR-free) was negligible as compared to the enough level difference in Figure 4b (i.e., 0.23 V at 30 mA cm−2) in the present work. This clearly indicates that the big improvement of current density observed for the present work anode at 0.8 V cell potential (i.e., Figure 4a) is attributable to the improvement of fast charge transfer phenomenon which corresponds to the

t i in anodic condition (9.2 × 10−22 atm) = 1 − (0.01883 (S cm−1)/0.07599 (S cm−1)) = 0.75 (9)

Equation 9 suggests that BIZZO has the mixed conducting property, and the electrical current which is carried by electron in the anodic condition is lower than that carried by oxide ion diffusion. Therefore, it is concluded that BIZZO can be useful promoter in the present work. On the basis of measurement results in Figure 3, we assumed that high quality active site on YSZ can be fabricated by using BIZZO as promoter. Cell potential (IR-free) values observed for the conventional cermet anode (i.e., without BIZZO) and anode with 0.2 wt % BIZZO are shown in Figure 4a. The cell potential (IR-free) value (125 mA cm−2 at 0.8 V) observed for anode with 0.2 wt % BIZZO in SOFC single cell was conspicuously higher than that of conventional anode without BIZZO (approximately 25 mA cm−2 at 0.8 V) at 700 °C. The anode activity was clearly

Figure 4. IR-free polarization characteristics (I−V curves) observed for the conventional cermet anode without BIZZO (blue closed circle) and present work anode with BIZZO (red closed circle) (a); Tafel lines derived from IR-free polarization characteristics (I−V curves) observed for the conventional cermet anode without BIZZO (blue closed circle) and the present work anode with 0.2 wt % BIZZO (red closed circle) (b). Cermet anode: NiO-8YSZ (mass ratio NiO (4):8YSZ (1)). Cathode: La0.85Sr0.15MnO3. Electrolyte: 8YSZ sintered pellet (thickness, 0.5 mm). Cathode side: O2 gas flow (80 mL min−1). Anode side: humidified H2 gas (+3% H2O) flow (80 mL min−1). Operation temperature: 700 °C. Dashed line in (a) indicates the level of 54% of power generation efficiency. Green arrows indicate comparison points in activation overpotential region and in concentration overpotential region in the present work. 5188


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Figure 6. Cell potential (IR included) and power density vs current density. Closed symbols: power density vs current density. Open symbols: cell potential (IR included) vs current density (i.e., I−V curve). Thickness of dense YSZ electrolyte: 500 μm. Operation temperature: (a) 800 °C and (b) 700 °C. Anode gas: humidified H2 (80 mL min−1). Cathode gas: O2 (80 mL min−1).

Figure 5. Cathode overpotential (IR-free) observed for the conventional cermet anode without BIZZO (blue open circle) and the present work anode with BIZZO (red open circle) (a). Tafel lines derived from cathode overpotential (IR-free) observed for conventional cermet anode without BIZZO (blue closed circle) and the present work anode with 0.2 wt % BIZZO (red closed circle) (b). Subwindow in (b) shows Tafel line in Figure 4b with cathode overpotential level observed at 30 mA cm−2. Cermet anode: NiO8YSZ (mass ratio NiO (4):8YSZ (1)). Cathode: La0.85Sr0.15MnO3. Electrolyte: 8YSZ sintered pellet (thickness, 0.5 mm). Cathode side: O2 gas flow (80 mL min−1). Anode side: humidified H2 gas (+3% H2O) flow (80 mL min−1). Operation temperature: 700 °C. Green arrows indicate comparison points in activation overpotential region and in concentration overpotential region in the present work.

According to previously published data, the reported Pmax values at 800 °C were 15 mW cm−2 (thickness of YSZ electrolyte, 60 μm) with conventional anode),45 62 mW cm−2 (thickness of YSZ electrolyte, 60 μm) with anode including 10 wt % CeO2),45 300 mW cm−2 (YSZ thickness, 60 μm) with anode including 45 wt % La0.3Sr0.7TiO3(LST), 5 wt % CeO2, and 0.5 wt % Pd, calcination temperature of anode, 1200 °C).46 Taking the thickness of dense YSZ electrolyte (i.e., 500 μm) into account, Pmax value observed for our sample with 0.2 wt % BIZZO promoter was at a high level. Also, the dashed lines in Figure 6a and Figure 6b indicate that the cell potential (IR included) at 700 °C observed for the present anode with 0.2 wt % BIZZO is comparable to the cell potential (IR included) at 800 °C observed for conventional anode. This indicates that the promotion effect by BIZZO is clear in the comparison of the cell potential (IR included) data. This comparison of observed Pmax clearly suggests that addition of small amount of BIZZO is effective to enhance the power density at lower temperature such as 700 °C. To examine another evaluation index for performance of both the conventional anode and the present sample with 0.2 wt % BIZZO, the impedance spectra were measured near OCV at 700 °C using full cell. As shown in Figure 7, the ohmic resistance which is determined by the high-frequency intercept with the real axis observed for the present anode with 0.2 wt % BIZZO promoter clearly shifted to lower impedance side as compared to the conventional anode. Also, the two semicirclers which correspond to the nonohmic loss region observed for present anode became really small. This suggests

formation of high quality active site on the anode and promotion of water molecule reaction in the fuel cell anode reaction. On the basis of the experimental results of Figure 4b and Figure 5b, it is concluded that the diffusion of oxygen on anode surface is conspicuously promoted on the high quality active site which is formed at BF heterointerface. Also, the unique effective reaction zone in nanoscale around TPBs in the anode would be formed at BF heterointerface in the anode layer. To characterize the anode performance, the authors examined I−P curves using cell potential (IR included) data which are measured at 800 and 700 °C (refer to Figure 6a and Figure 6b). The maximum power density (Pmax) values observed for dense YSZ (thickness, 500 μm) with conventional anode (thickness, 20 μm) at 800 and 700 °C were approximately 130 mW cm−2 and 30 mW cm−2, respectively. Also, the Pmax observed for dense YSZ (thickness, 500 μm) with the present anode including 0.2 wt % BIZZO (anode thickness, 20 μm; calcination temperature of anode, 1300 °C) at 800 and 700 °C were 180 mW cm−2 and 62 mW cm−2, respectively. 5189


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Figure 7. Impedance spectra observed for full cells with conventional anode (□) and present anode with 0.2 wt % BIZZO (○) at 700 °C.

that excess overpotential on the present anode was decreased by the formation of BF heterointerface as active site in the anode layer. This big difference of impedance spectra recorded from our anode and conventional anode agreed with the observation results of I−P performance in Figure 6b well. As expected in the introduction part, the anode activity was conspicuously improved by the formation of active site on the “BF heterointerface”. To characterize the new active site on the cubic ZrO2 surface, the surface chemical composition of YSZ was observed by means of XPS analysis. Zr 3d profiles and O 1s profiles were observed for the conventional anode (Figure 8a-i) and the present anode with 0.2 wt % BIZZO promoter (Figure 8a-ii) which were reduced in a hydrogen gas flow at 800 °C. The Zr 3d spectra in Figure 8a show two spin−orbital splitting doublets Zr 3d (3/2)−3d (5/2). The peaks at 185.4− 186.1 eV and 183.0−183.7 eV are attributed to Zr 3d (3/2) and Zr 3d (5/2), respectively. According to previously published works,47,48 the oxidized ZrO2 surface showed the doublets at 184.8 eV (Zr 3d (3/2)) and 182.4 eV (Zr 3d (5/ 2)). Also, another set of doublet peaks at 184.2 eV (Zr 3d(3/ 2) and 182.0 eV (Zr 3d(5/2)) observed for the reduced ZrO2−x surface were reported. The XPS analysis on YSZ surface suggests that the big shift of doublet peak taken from the present anode occurred by formation of the additional oxygen defects in BF heteointerface on ZrO2 surface. In addition, the O 1s peak observed for the anode samples resolves into four distinguishable peaks labeled by α (535.0 eV), β (533.0 eV), γ (532.0 eV), and δ (531.1 eV).48−51 The α state centered at 535.0 eV and the β state centered at 533.0 eV are nonlattice oxygen species. The α state is attributable to adsorbed oxygen and/or hydroxylic species on YSZ. The β state is the oxidic contribution from the YSZ surface. Both α and β states are also observed on active metal unloaded YSZ surface. Also, the state γ centered at 532.0 eV can be attributed to atomically chemisorbed oxygen. Their peak positions would be shifted by the existence of surface oxygen on active metal such as Ni which is in three phase boundary region. On the other hand, the state δ peak centered at 531.1 ± 0.2 eV indicates the oxygen vacancy or ionic oxygen which appears in YSZ, δ-Bi2O3, and interface between Ni and YSZ.49,51 Since the

Figure 8. (a) Zr 3d profiles observed for the conventional anode (i) and the present work anode with 0.2 wt % BIZZO (ii). (b) O 1s profiles observed for the conventional anode (i) and the present work anode with 0.2 wt % BIZZO (ii). (c) Cross-section image by SEM and element analysis by EDX observed for the conventional anode (i) and the present work anode with 0.2 wt % BIZZO (ii).

peak intensity of state δ is increased by formation of oxygen vacancy or ionic oxygen at “BF heterointerface”, the peak position of state δ would be observed within a previously reported region. The δ peak (i.e., oxygen vacancy49,50 or ionic oxygen51) was clearly observed from white square area in cross section image of SEM (refer to Figure 8b-ii and Figure 8c-ii) which is near the BIZZO promoter. In contrast, the intensity of δ peak (i.e., oxygen vacancy49,50 or ionic oxygen51) in O 1s profile observed for white area in the cross-section image taken from the conventional anode without promoter BIZZO (refer to Figure 8b-i and Figure 8c-i) was much lower than the present anode. It suggests that oxygen vacancy site or ionic oxygen was created on YSZ by a small amount of BIZZO additives. From a 5190


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(In terms of all measurement data at 700 °C, refer to Figure S5a and Figure S5b of Supporting Information). When the printing temperature of anode on the electrolyte was 1200 °C, it was hard to observe the improvement of cell potential in both activation overpotential and concentration overpotential regions as demonstrated in Figure 9a and Figure 9b (refer to the open white circle plots in both graphs). In contrast, the observed overpotential was conspicuously improved by addition of BIZZO promoter in both activation overpotential and concentration overpotential regions when the printing temperature was 1300 °C, although the overpotential observed for the conventional anode without BIZZO promoter became low level due to the grain growth of Ni particles (refer to the closed black circles in Figure 9a and Figure 9b). Those results indicate that the new hopping site for surface mobile oxygen was formed on YSZ (i.e., cubic ZrO2) surface by diffusion of BIZZO through the grain boundaries of anode layer at 1300 °C. Then, it is concluded that the “BF heterointerface” promoted the water molecule formation on the cubic ZrO2 surface and enhanced the current density observed for the present anode (refer to Figure 10). In addition, the optimum amount of BIZZO promoter was observed around 0.2 wt %. Especially, the anode activity observed for the sample with 0.2 wt % BIZZO was conspicuously higher than all examined samples when the printing temperature is 1300 °C. Also, Figure 9a and Figure 9b suggested that the anode activity was depressed by introduction of a lot of surface defects. To conclude why we can see the optimum composition in Figure 9a and Figure 9b, we performed the surface atomistic simulation whose source cord is GULP. For development of our model about new hopping site for mobile oxygen formed by “BF heterointerface”, we compared the surface structure model of cubic ZrO2 in X−Z plane (refer to Figure 11a-1) and surface structure model of C-type rare earth structure which is cubic ZrO2 related structure in X-Z plan (refer to Figure 11a-2) in advance. Since fluorite phase and C-type rare earth phase coexist in the same phase diagram, C-type rare earth like defect structure can be formed on YSZ surface. Also, if the positions of oxygen vacancies on “BF heterointerface” and C-type rare earth structure model overlapped at many positions, the surface diffusion of oxide ion on YSZ would be depressed and the anode activity would be at low level. On the basis of our BF heterointerface model which is shown in Figure 1c-2 and surface chemical state analysis which is shown in Figure 8, we assumed two defect cluster formation reactions in the present work (refer to eq 10 and eq 11).

comparison of parts a, b, and c of Figure 8, we concluded that new hopping site for surface mobile oxygen on YSZ was created around BIZZO promoter due to formation of “BF heterointerface” in the present work. (Note that no peak shift of Ni 2p was observed for the conventional anode and the present work anode with BIZZO, as presented in Figure S4 and Table S1 of Supporting Information.) Then, it is concluded that the new hopping site on the surface of YSZ which is induced by the formation of “BF heterointerface” contributed to the enhancement of anode activity. To estimate the optimum content of BIZZO promoter in the anode and conclude why the promotion effect of BIZZO was observed in the present work, the observed cell potential in activation overpotential region (i.e., at 8 mA cm−2) and in concentration overpotential region (i.e., at 30 mA cm−2) as a function of BIZZO promoter content and printing temperature of anode on the electrolyte was examined at 700 °C (refer to Figure 9a and Figure 9b).

cubic ZrO2

BaO + In2O3 ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→ 2VZr⁗ + Ba i•• + 2In i••• + 4Oi″ + 4VO••

(10)

cubic ZrO2

ZnO + In2O3 ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→ 2VZr⁗ + Zn i•• + 2In i••• + 4Oi″ + 4VO•• −2

Figure 9. Cell potential (IR-free) at 8 mA cm (a) and at 30 mA cm−2 (b) observed for the anode samples. Content of BIZZO: 0−1.0 wt %. Cermet anode: NiO-8YSZ (mass ratio NiO (4):8YSZ (1)). Cathode: La0.85Sr0.15MnO3. Electrolyte: 8YSZ sintered pellet (thickness, 0.5 mm). Cathode side: O2 gas flow (80 mL min−1). Anode side: humidified H2 gas (+3% H2O) flow (80 mL min−1). Operation temperature: 700 °C. Printing temperature: (○) 1200 °C and (●) 1300 °C.

(11)

where the Kröger−Vink notation is used for simple explanation of both defect formation and solid solution reactions. In both defect cluster models, we assumed that four interstitial sites such as Ba2+ interstitial defect (Bai••), Zn2+ interstitial defect (Zni••), In3+ interstitial defect (Ini•••), oxygen interstitial defect (Oi″) sites are formed on the cubic ZrO2 surface by diffusion of BIZZO promoter through the 5191


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Figure 10. Schematic diagram for comparison of water molecule formation reaction in the conventional anode30 (a) and the present work anode with BIZZO promoter (b).

Figure 11. Defect surface models, conventional cubic ZrO2(111) (a- 1), C type rare earth structure (111) (a-2), lattice defect configuration of model defect cluster A (eq 10) or B (eq 11) on cubic ZrO2 (111) (b), another lattice defect configuration of model defect cluster A′ (eq 10) or B′ (eq 11) on cubic ZrO2 (111) (c), and large combination of defect cluster on cubic ZrO2(111) (d).

Also, we calculated another type defect cluster which is shown in the enlarged defect cluster model Figure 9c-1. The calculated ΔEb values of model defect cluster A′ (i.e., 2 VZr⁗+ Bai•• + 2Ini••• + 4Oi″ + 4 VO••) and model defect cluster B (i.e., 2 VZr⁗+ Zni•• + 2Ini••• + 4Oi″+ 4 VO••) are 2.7 and 2.89 eV, respectively. Those positive calculated ΔEb values also indicate that the model defect clusters (i.e., A, A′, B, and B′) which are demonstrated in Figure 11b-1 and Figure 11c-1 are stably formed on the cubic ZrO2 (111) surface. In addition, the model defect clusters (i.e., A, A′, B, and B′) would make large combination defect clusters as shown in

grain boundaries in the anode layer. Also, oxygen vacancy (VO••) is generated for keeping electrical neutrality. On the basis of ideal model which is shown in Figure 1c-2 of the introduction part, we put the lattice defects on cubic ZrO2 (111) surface model as demonstrated in the enlarged Figure 9b-1. The calculated ΔEb values of model defect cluster A (i.e., 2 VZr⁗+ Bai•• + 2Ini••• + 4Oi″ + 4 VO••) and model defect cluster B′ (i.e., 2 VZr⁗+ Zni•• + 2Ini••• + 4Oi″+ 4 VO••) are 2.9 and 3.9 eV, respectively (refer to Table S2 and Table S3 for explanation of “the detailed calculation steps” of Supporting Information). 5192


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In the case of design of anode performance for state-of-theart IT-SOFC device, we must overcome one more serious problem such as rapid anode performance degradation. In general, Ni particle coarsening in anode layer is considered a major reason for anode performance degradation. In previously reported phase field simulation study about relationship between Ni coarsening and local structure modification around TPBs,52,53 the TPB density in Ni-YSZ anode was sensitively changed by the local morphological modification. This simulation work indicates that anode performance is also sensitive to local defect structure modification by addition of small size promoter. On the basis of the suggestion of phase field simulation work,52 we assumed that our BIZZO promoter contributes to both formation of new hopping site of oxide ion on YSZ surface and good pinning effect of Ni grain growth to maintain its high anode performance. Figure 12 compared anode performance degradation observed for the conventional anode with the present anode with BIZZO at 8 mA cm−2 (i.e., at activation overpotential region, refer to Figure 12a) and at 30 mA cm−2 (i.e., at concentration overpotential region, refer to Figure 12b) up to 300 h. As we mentioned in the section of Figure 5, the observed cell potential (IR-free) was affected by fluctuation of cathode overpotenatial due to gas leak through the cement sealer, although the fluctuation level of cathode overpotential is not so much. Since the cement sealer of our single cell sometimes got damaged during long time operation and the observed cell potential (IR-free) was inflated by fluctuation of cathode overpotential, our observed data included experimental error in Figure 12. We compared the lines of performance degradation in consideration of experimental error which appeared by the fluctuation of cathode overpotential (refer to small size Figure 5b in Figure 12) In the activation overpotential region (i.e., at 8 mA cm−2), the cell potential (IR-free) observed for the present anode with BIZZO slightly decreased by small grain growth of Ni particles, but the observed cell potential (IR-free) after 300 h operation was still higher than the conventional anode. It indicates that new hopping sites created on the cubic ZrO2 surface are quite

Figure 11b-2 and Figure 11c-2. As representative example, we calculated the ΔEb for the formation of combination cluster between the model defect cluster A and model defect cluster B′ as shown in Table 4. (refer to Figure S6a−e for confirmation of the positions of combined clusters on ZrO2 (111) surface.) Table 4. Distance between Model Defect Cluster A and Model Defect Cluster B′ as a Function of Calculated Binding Energy of Combination of Two Clusters on ZrO2(111) Surface distance of two clusters (Å) ΔEb (eV)

3.59 6.18

7.17 6.17

10.8 5.65

14.3 3.93

17.9 1.62

Table 4 indicates that the combined defect cluster which consists of the model defect cluster A and B′ (or model defect cluster A′ and B) is stably formed within the length of lattice constant of cubic ZrO2 unit cell. Note that lattice constant of cubic ZrO2 is 5.07 Å and the length of diagonal line of cubic ZrO2 unit cell is 7.17 Å. If the distance between two model defect clusters is longer than the lattice constant of cubic ZrO2 (111) surface, the combination of model defect clusters on cubic ZrO2 (111) surface will be suppressed. Namely, our surface atomistic simulation clearly suggests that the surface diffusion of oxide ion on the cubic ZrO2 is conspicuously encouraged by formation of new hopping site of oxide ion (i.e., combined defect clusters) on “BF heterointerface”. Then, the performance of anode in SOFC cell is promoted. In contrast, the ordering of oxygen vacancies which is seen in C-type rare earth structure (refer to Figure 11a-2) comes up if the combined defect clusters are too many on cubic ZrO2 surface (refer to Figure 11d). In this case, the surface diffusion of oxide ion and activity of anode will be suppressed. That is why we observed the sharp optimum peak in Figure 9. On the basis of the experimental results of Figure 9 and Figure 11, it is concluded that the optimization of fabrication condition along the guide of surface atomistic simulation around TPBs is key for the design of high quality active sites on the anode in SOFC.

Figure 12. Comparison of anode performance degradation observed at activation overpotential region (i.e., at 8 mA cm−2) (a) and at concentration overpotential region (i.e., at 30 mA cm−2) (b). Operation temperature: 700 °C. Open red symbols: anode with 0.2 wt % BIZZO. Open blue symbols: conventional anode without BIZZO. 5193


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Figure 13. Grain size distribution observed for conventional anode and present anode with BIZZO promoter before long time operation test: conventional anode (a) and the present work anode with BIZZO (b). The printing temperature of anode on electrolyte was 1300 °C. Both samples were reduced in wet hydrogen. Reducing temperature was 800 °C.

Figure 14. Grain size distribution observed for conventional anode and present anode with BIZZO promoter after 300 h operation test at 700 °C: conventional anode (a); the present work anode with BIZZO (b). The printing temperature of anode on electrolyte was 1300 °C.

included) and operation time up to 300 h (refer to Figure S7 in the Supporting Information). The data of stability test suggest that the new hopping site for mobile oxygen on the cubic ZrO2 surface which is induced by formation of BF heterointerface is also quite active and stable in the concentration overpotential region during 300 h operation of fuel cell. In other words, we successfully provided

active and stable in the activation overpotential region as compared with the conventional cubic ZrO2 surface. In concentration overpotential region, the degradation level observed for the conventional anode (−0.16 V) became larger than that of the present anode with BIZZO promoter (i.e., −0.05 V) (refer to Figure 12b). The authors observed similar results in the relationship between the cell potential (IR 5194


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Figure 15. Schematic diagram for comparison of anode activity degradation level observed: conventional anode (a); present work anode with BIZZO (b). Symbol P in (b): oxide promoter BIZZO.

for mobile oxygen on the cubic ZrO2 while Ni coarsening occurred in the anode (refer to Figure 15b). This suggests that the design of defect cluster area on the cubic ZrO2 without heavily ordering of oxygen vacancies will show us good balance between high performance of IT-SOFC and its long-term stability. Also, the present concept for design of “BF heterointerface” would be used for development of high quality anode by using other BF heterointerface (i.e., brownmillerite type mixed conductor, CaFeO2.5 and SrFeO2.5 based compounds; fluorite type ZrO2, Sc2O3 stabilized ZrO2 based compound including sufficient amount of cubic ZrO2).

the surface of YSZ the useful defect structure modification to increase the TPB density in the present anode. Also, the classical pinning effect of grain growth of Ni particles by BIZZO small particles contributes to minimize the degradation of anode performance in the present work. To confirm the validity of our idea for active BF heterointerface, the grain size distribution observed for the conventional anode and the present anode with BIZZO promoter was examined before and after long operation test up to 300 h at 700 °C. Before long time operation test, the main peak of grain size distribution observed for cross-section image taken from the preset anode with BIZZO shifted to smaller grain size region as compared with the conventional anode, as shown in Figure 13 (refer to cross-section SEM images taken from the reduced conventional anode and present anode with BIZZO promoter in Figure S8 of Supporting Information). Since the printing temperature of anode on electrolyte was 1300 °C, the gain growth of cermet anode particles was observed in Figure 13. The grain growth of anode particles was slightly depressed by classical grain growth pinning effect of BIZZO in the anode layer. On the other hand, the difference of observed grain size distribution data between the conventional anode and the present anode with BIZZO became clear after 300 h operation test at 700 °C (refer to Figure 14). Since it was reported that Ni particle coarsening in cermet anode was fast and the particle size distribution observed for YSZ particles did not show pronounced change in comparison to that of Ni particles at 1000 °C up to 4000 h in the SOFC operation condition,51 the difference of grain size distribution after 300 h operation test in Figure 14 is attributable to the difference of grain growth speed of Ni particles (not YSZ) in the conventional anode without BIZZO and the present work anode with BIZZO. The comparison of Figure 14a and Figure 14b indicates that the classical grain growth pinning effect of BIZZO works in the anode layer. This agrees with the conclusion part of Figure 12 well. In addition, we can highlight one more important effect which is expected by formation of new hopping site for mobile oxygen on the cubic ZrO2 around BF heterointerface. As mentioned in the conclusion part for Figure 12, the activity of water molecule formation on the cubic ZrO2 in the conventional anode was conspicuously decreased by the grain growth of Ni during long operation of SOFC device (refer to Figure 15a). In contrast, the degradation of performance became small by formation of new hopping site

4. CONCLUSION The high quality active site on YSZ in the cermet anode of SOFC was designed at heterointerface between brownmillerite type oxide promoter and fluorite cubic ZrO2 (“BF heterointerface”) along the guide of previously published first-principles simulation. The high quality active site (i.e., new hopping site for mobile oxygen) which consists of defect clusters was formed at BF heterointerface and promoted both surface oxide ion diffusion and charge transfer on anode (i.e., high anode activity and high performance of IT-SOFC), as expected in advance. Finally, it is confirmed that the combination work among the surface defect structure modeling, surface microanalysis, and fabrication provides us with the opportunity to fabricate the state-of-the-art IT-SOFC with good balance between performance and longtime stability.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsaem.9b00864. Bulk density, and relative density of BIZZO vs sintering temperature, Rietveld analysis of XRD taken from BIO and BIZZO, current density vs cell potential (IR-free) at 800 °C, Tafel plot derived from electrochemistry data observed for the present work anode with BIZZO at 800 °C, Ni 2p spectra observed for the anode samples, Ni 2p positions observed for the anode samples, influence of printing temperature of anode on the relationship between cell potential (IR-free) and current density, coordination points of lattice defect in the model of defect cluster based on eqs 10 and 11, calculation process details of binding energy of defect clustered 5195


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based on eqs 10 and 11, binding energy as a function of model defect cluster A and model defect cluster B′, relationship between operation time and cell potential (IR included) which are observed at steady state condition at 700 °C, cross-section images observed for conventional anode and present work anode with BIZZO (PDF)

AUTHOR INFORMATION

Corresponding Author

*Telephone: +81-29-860-4395. Fax: +81-29-860-4712. E-mail: [email protected]. ORCID

Toshiyuki Mori: 0000-0003-3199-2498 Funding

The present work was partially supported by the open laboratory program of Global Research Center for Environmental and Energy based on the Nanomaterials Science (GREEN), National Institute for Materials Science (NIMS), Japan, and NIMS joint research hub program and Grant-in-Aid for Young Scientists (Grant JP 19K15684). Also, our work was partially supported by the research fund of National Institute of Technology, Tsuruoka College, Japan. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors greatly appreciate the useful advice regarding conclusion of experimental results by Professor Michihisa Koyama (Shinshu University and NIMS, Japan), Professor Takashi Morinaga, Professor Toshio Kamijo, and Associate Professor Ryo Shomura (National Institute of Technology, Tsuruoka College, Japan). Also, the authors are grateful for surface and bulk microanalysis support from Dr. Hiroyuki Okazaki (Takasaki Advanced Radiation Research Institute, National Institute for Quantum and Radiological Science and Technology), Noriko Isaka (TEM station, National Institute for Materials Science), Fumitaka Sato, and Kaito Honma (National Institute of Technology, Tsuruoka College, Japan).

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Design of Active Site at Heterointerface between Brownmillerite Type

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