A Highly Active and Redox-Stable SrGdNi0.2Mn0.8O4Â±Î´ Anode with

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A Highly Active and Redox Stable SrGdNi0.2Mn0.8O4±# Anode with In-situ Exsolution of Nanocatalysts Kyeong Joon Kim, Manasa Kumar Rath, Hunho H. Kwak, Hyung Jun Kim, Jeong Woo Han, Seung-Tae Hong, and Kang Taek Lee ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b03669 • Publication Date (Web): 02 Jan 2019 Downloaded from http://pubs.acs.org on January 2, 2019

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Title: A Highly Active and Redox Stable SrGdNi0.2Mn0.8O4±δ Anode with In-situ Exsolution of Nanocatalysts

Kyeong Joon Kim,†# Manasa K. Rath,†# Hunho H. Kwak,† Hyung Jun Kim,§ Jeong Woo Han,‡ Seung-Tae Hong† and Kang Taek Lee*,†

†Department

of Energy Science and Engineering, Daegu Gyeongbuk Institute of Science and

Technology (DGIST), Daegu 42988, Republic of Korea Department of Chemical Engineering, University of Seoul, Seoul 02504, Republic of Korea

§

Department of Chemical Engineering, Pohang University of Science and Technology

‡

(POSTECH), Pohang, Gyeongbuk 37673, Republic of Korea

* Corresponding Author: Prof. Kang Taek Lee Address: Daegu Gyeongbuk Institute of Science and Technology (DGIST), 333, Techno Jungang Daero, Hyeongpung-Myeon, Dalseong-Gun, Daegu, 42988, Republic of Korea Email: [email protected] Tel: +82-53-785-6430 Fax: +82-53-785-6409

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Abstract Layered perovskite SrGdNixMn1-xO4±δ phases were evaluated as new ceramic anode materials for use in solid oxide fuel cells (SOFCs). Hydrogen temperature programmed reduction (H2-TPR) analysis of the SrGdNixMn1-xO4±δ (x=0.2, 0.5, and 0.8) materials revealed that significant exsolution of Ni nanoparticles occurred in SrGdNi0.2Mn0.8O4±δ (SGNM28) in H2 at over 650 oC. Consistently, the SGNM28 on the LSGM electrolyte showed low electrode polarization resistance (1.79 Ω cm2) in H2 at 800 °C. Moreover, after 10 redox cycles at 750 °C, the electrode area specific resistance of the SGNM28 anode in H2 increased only 0.027 Ω·cm2 per cycle (1.78% degradation rate), indicating excellent redox stability in both reducing and oxidizing atmospheres. An LSGM-electrolyte supported SOFC employing an SGNM28-Gd-doped ceria anode yielded a maximum power density of 1.26 W cm-2 at 850 °C, which is the best performance among the any SOFCs with Ruddlesden-Popper based ceramic anodes to date. After performance measurement, we observed that metallic Ni nanoparticles (~ 25 nm) were grown in situ and homogeneously distributed on the SGNM28 anode surface. These exsolved nanocatalysts are believed to significantly enhance the hydrogen oxidation activity of the SGNM28 material. These results demonstrate that the SGNM28 material is promising as a high catalytically active and redox-stable anode for SOFCs.

Keywords : solid oxide fuel cells, layered perovskite, ceramic anode, exsolution, redox stable


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1. Introduction Solid oxide fuel cells (SOFCs) are electrochemical devices that can convert chemical energy directly into electricity. SOFCs have become a viable technology for energy supply owing to their high energy conversion efficiency and fuel flexibility, allowing the use of hydrocarbon fuels as well as hydrogen. [1-3] Ni-based cermet anodes have been utilized as state-of-the-art SOFC anode materials, because Ni provides excellent electronic conductivity, catalytic activity in H2 oxidation, and good chemical compatibility with other SOFC components. [4-6] However, one of the major problems with Ni-based anodes is deterioration of the anode performance upon long-term operation due to Ni coarsening and re-oxidation. In particular, repeated reduction and oxidation of Ni to NiO causes irreversible chemical expansion of Ni, leading to a large volume change and cracking or failure either of the anode cermet or the electrolyte layer. This redox cycling of the anode likely happens in a commercial SOFC system due to regular on-off cycles, emergency shutdown, fuel supply failure, or non-uniform distribution of fuel flow in stacks. To address the above issues, perovskite-based oxide materials with various dopants (e.g. Cr, Mo, and Ti) have been developed for SOFC anodes due to their reasonable electrochemical catalytic activity and high redox stability.[7, 18-20] Recently, La2NiO4+δ with a RuddlesdenPopper structure (space group I4/mmm or F4/mmm) has been intensively studied due to its rapid surface exchange reaction and bulk ion diffusion.[8, 9] In its crystal structure, perovskite (ABO3) and rock salt (AO) layers are alternately stacked along the c-axis. Interstitial oxygen is preferentially localized at the pseudo-tetrahedron of La in the rock salt, and has fast ionic conduction paths through the AO layers, thus promoting ionic conductivity [10, 11] and electronic conductivity [12]. To improve the performance of the nickelate materials, Sr doping


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into A-site of La2NiO4+δ has been conducted, demonstrating a positive effect on the electrical conductivity as well as the catalytic activity. In addition, Gd doping at the A-site of the layered perovskite significantly enhanced the oxygen diffusivity as well as the electrical conductivity.[13, 14] The catalytic activity can be further manipulated by B-site cation doping. Li et al. reported that B-site substitution with Mn can facilitate the slow surface exchange step of La2NiO4 by promoting surface adsorption activity.[15] Considering the previous studies, Gd and Mn co-doped layered perovskite anode materials can be rationally designed to promote the electrocatalytic activity as well as mixed conductivity with high stability. In this study, we prepared a layered perovskite SrGdNixMn1-xO4±δ (referred to as SGNM) as a new ceramic anode for SOFC applications. To optimize the performance of the SGNM electrode, we systematically changed the Ni/Mn ratio and investigated the phase stability and thermochemical and electrochemical properties. The redox stability of the SGNM anode materials with the optimal composition was evaluated through cycling tests. Moreover, the electrochemical performance of an SOFC employing a SGNM anode was measured. Finally, the correlation between the performance of the SGNM anode and characteristic microstructural features was investigated to demonstrate high feasibility of the SGNM material as an alternative and promising SOFC anode.

2. Experimental 2.1 Materials synthesis SrGdNixMn1-xO4±δ (x = 0.2, 0.5 and 0.8, denoted as SGNM28, SGNM55, and SGNM82) powders were prepared using a glycine nitrate process. Stoichiometric amounts of Sr(NO3)2 (99.99%, Sigma Aldrich), Gd(NO3)2·6H2O (99.9%, Sigma Aldrich), Ni(NO3)2·6H2O (99.99%,


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Alfa aesar), and Mn(NO3)2·6H2O (99.99%, Alfa aesar) with glycine were dissolved in distilled water by stirring for 2 h. Subsequently the reactant mixture was evaporated at 120 °C and heated to 250 °C for combustion to form a dark brown ash. The resultant powders were subjected to a calcination process at 1000 °C for 10 h to remove the residual impurities. The calcined powders were uniaxially pressed to form a disk and then sintered at 1200 °C for 12 h to obtain a SGNM single phase.

2.2 Cell fabrication For the preparation of the electrode ink, a series of SGNM powders were mixed with a binder system (ESL 441, ESL Electro Science) at a ratio of 50:50 wt.%. The viscosity of the solution was

controlled

by

the

addition

of

ethanol.

For

symmetric

cells

with

SGNM|La0.8Sr0.2Ga0.8Mg0.2O3-d (LSGM, Kceracell, Korea)|SGNM configuration, the dense LSGM substrates were prepared by uniaxial pressing with subsequent sintering at 1450 °C for 10 h. The SGNM ink was screen-printed on both sides of the sintered LSGM pellet and heated at 1300 °C for 2 h. For an SOFC test, La0.6Sr0.4Co0.2Fe0.8O3-δ (LSCF, Fuel Cell Materials, USA)Gd0.1Ce0.9O1.95(GDC, Rhodia, USA) composite (50:50 wt.%) was used as the cathode material on the LSGM electrolyte support. The SGNM28-GDC (30:70 wt%) and LSCF-GDC cathode were screen printed on both sides of the LSGM pellet. The anode and the cathode were continuously fired at 1150 °C for 3 h and at 1050 °C for 3 h, respectively. The active cathode area for the SOFC was ~1 cm2. As a current collector, an Ag mesh (52 mesh, Alpha Aesar, USA) was attached to each electrode using Pt paste (ESL Electro Science, USA).

2.3 Characterization


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The powder XRD measurement was performed using an X-ray diffractometer (Rigaku, Miniflex 600) over the 2θ range of 10°−80° with monochromatized Cu Kα radiation (λ = 1.5418 Å). The crystal structures were refined using the powder profile refinement program GSAS[16], The initial structural model of Sr0.33Gd1.67NiO3.8 was adopted from the literature[17]. The refinement parameters were scale factors, background, and unit cell parameters. Hydrogen temperature programmed reduction (H2-TPR) was performed using a BELCAT-M instrument (BEL, Japan inc.). All samples were treated with helium gas at 1000 °C for 1 h before H2-TPR measurement. The flow rate and the temperature ramping rate were 10% H2/Ar at 40 mL min−1 and 10 °C min−1, respectively. The oxidation behavior of reduced powders was analyzed by thermo-gravimetric analysis (TG8120, Rigaku, USA) under constant air flow in the temperature range of 30–900 °C at a heating rate of 2 °C min-1. The microstructure of the electrode surface and cross-section was observed by field emission scanning electron microscopy (FE-SEM, S-4800, Hitachi). Highresolution microstructural analysis was carried out using a transmission electron microscope (TEM) (Hitachi, HF-3300, JAPAN) that was equipped with an energy dispersive X-ray spectroscopy (EDS) system (Bruker Co., model QUANTAX‐200). X-ray photoelectron spectroscopy (XPS, ESCALAB 250, VG SCIENTIFIC) analysis was performed to identify the oxidation states of the elements in SGNM powders. The electrode polarization impedance of the prepared symmetric cells (~ 2 mm thick LSGM electrolyte with ~ 30 𝜇m thick SGNM electrodes) was measured at the temperature range of 600 to 800 °C using a potentiostat (VMP-300, Bio-Logic). In addition, redox cycling tests of the SGNM electrodes were performed at 750 °C with the following steps: (Step 1) flushing with 100 sccm of dry N2 for 1 h to completely remove the H2; (Step 2) shutting off the anode gas and


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exposing the electrode to 100 sccm of air for 1 h; (Step 3) refluxing with 100 sccm of N2 for 1 h to fully evacuate the air; (Step 4) reduction in 100 sccm of H2 for 1h. For electrochemical performance measurements, the SOFC samples were loaded in a fuel cell test station. The SOFC edge was sealed to the testing alumina tube using Ceramabond TM 517 (Aremco). The Ag mesh and Pt wire were connected on the SOFC electrode surface with Pt paste as the current collector. I-V-P characteristics of the SOFCs were measured using a potentiostat (VMP-300, Bio-Logic) with dry air (200 sccm) and 3% humidified hydrogen (200 sccm) delivered to the cathode and the anode side, respectively. After the I-V-P test, electrochemical impedance spectroscopy analysis was performed under open circuit conditions with an AC voltage amplitude of 50 mV in the frequency range of 1 MHz to 10 mHz.

3. Results and Discussion 3.1 Phase analysis The refined XRD patterns of the SGNM82, SGNM55, and SGNM28 powders are depicted in Figure 1a-c. The calculated refined lattice parameters are summarized in Table 1. The SGNM powder had a tetragonal crystal system and I4/mmm space group of Ruddlesden-Popper-type phase. The weighted profile R-factor (Rwp), which is defined as the mean deviation in accordance with the calculated model, was used to check the agreement of the observed and calculated data. The Rwp values for SGNM82, SGNM55, and SGNM28 were 0.112, 0.107 and 0.130, respectively, indicating that the refinement results were acceptable. Although the XRD patterns of SGNM28, and SGNM82 indicated a minor NiO impurity, the intensities of these peaks were less than 1 % compared with the intensity of the strongest (013) reflection of the Ruddlesden-


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Popper -type phase. Based on Rietveld refinement, the cell volumes of SGNM82, SGNM55, and SGNM28 were estimated as 175.200, 176.373, and 177.343 Å3, respectively. With the increase in nickel, a and c parameters decreased, and the unit cell volume also decreased probably due to the different ionic size of Ni2+ (0.69 Å) compared with that of Mn2+ (0.67 Å), Mn3+ (0.58 Å), and Mn4+ (0.53 Å), respectively. Furthermore, the presence of Ni2+ in the B-site can promote the change from Mn3+ to Mn4+ (6-coordination) to maintain charge balance.[18] Based on the calculated refinement result in Table 1, the crystal structures with perfective polyhedra of the SGNM layered perovskite structures (An+1BnO3n+1) were drawn using the program VESTA software. (Insets of Figure 1a-c) Here, Sr and Gd co-existed as a solid solution in the A-site, and Ni and Mn are co-existed in the B-site. For the SGNM layered perovskite structure, 2D layers of BO6 (B= Ni and Mn) corner-sharing octahedra were joined along the c direction and separated by rock-salt AO (A= Sr and Gd ) layers. TEM images of prepared SGNM82, SGNM55, and SGNM28 samples are shown in Figure 1d-f. The bright-field TEM images (Figure 1d-f) shows that the average particle size of SGNM powders was less than 200 nm. The insets in Figure 1d-f show the SAED patterns consisting of the concentric rings, which indicate the polycrystalline nature of the SGNM powders. Furthermore, the dominant diffraction patterns coincide with the (002), (013), and (011) planes for SGNM82, SGNM55, and SGNM28, respectively, which are matched well with the Rietveld analysis from XRD results. Figure S1a-c shows the HR-TEM images of SGNM82, SGNM55, and SGNM28 samples; the insets show fast Fourier transform (FFT) simulated lattice fringes. The calculated d-spacing values from the fringes were 0.192, 0.157, and 0.278 nm, corresponding to the (hkl) plane values of (020), (123),


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and (013), respectively. This result suggests that these SGNM materials were ideally indexed according to the spatial group (I4 / mmm). Figure 2a shows XRD patterns of SGNM materials after reduction at 850 °C in 10%H290%Ar for 10 h. All three SGNM samples exhibited good phase stability with the pristine Ruddlesden-Popper structure, while with the exception of the SGNM28 composition, SGNM55 and SGNM82 had some secondary peaks consistent with Gd2O3. Moreover, in SGNM28 and SGNM55 samples, we observed additional peaks at 44 and 52°, which are assigned to metallic Ni phase. Figure 2b shows the XRD patterns of the SGNM28 and GDC (and LSGM) mixtures annealed at 1300 oC for 10 h, indicating no impurity peak and demonstrating excellent chemical compatibility of the SGNM material for GDC and LSGM electrolytes.

3.2 XPS analysis XPS analysis was carried out to confirm the elemental composition and valence states of the reduced SGNM samples. Figure 3 shows the core level XPS spectra of Ni 2p, Mn 2p, and O 1s. Prior to XPS peak deconvolution of each SGNM sample, the C1s peak of carbon, which was set as a reference and baseline, was subtracted. Subsequently, each element spectra of the SGNM samples was analyzed using XPSPEAK 4.1 software. For Ni 2p spectra of SGNM samples, the peaks at 855.0 and 872.0 eV are assigned to Ni 2p1/2 and Ni 2p3/2, respectively (Figure 3a). The Ni 2p1/2 spectrum from the SGNM series could be fitted into two or three peaks. The binding energies (EB) of these sub-peaks were detected at 852.3, 854.4, and 856.0 eV, which can be assigned to metallic Ni0, Ni2+, and the satellite peak, respectively.[47, 48] With exception of the SGNM82, the peaks for metallic Ni were observed in both SGNM28 and SGNM55 samples as


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shown Figure 3a. In addition, the observed peaks at 871.1 and 879.0 eV correspond to the Ni3+ 2p1/2 and its satellite peak, respectively.[48] Figure 3b shows the XPS spectra of the O 1s core levels of the reduced SGNMs. The O 1s signal can be deconvoluted into three peaks at 529.0, 531.3 and 533.5 eV, corresponding to lattice oxygen (OL), adsorbed oxygen (OA) and moisture oxygen (OH O), respectively.[25] The 2

additional peak at 536.0 eV also corresponds to moisture oxygen (OH O).[46] 2

Figure 3c shows the high-resolution spectrum of Mn 2p for the SGNM series after reduction. All SGNM compositions exhibited apparently two EB values for Mn 2p3/2 and Mn 2p1/2 spectra at 642.4 eV and 654.0 eV, respectively. [45] After the Gaussian fitting method, the Mn 2p3/2 spectrum from the SGNM series were deconvoluted into two or three peaks. The theoretical EB values of Mn 2p3/2 are 639.5, 641.4 and 643.0 eV for Mn2+, Mn3+, and Mn4+, respectively, which coincide with the deconvoluted peaks from the Mn 2p3/2 series. [21, 22] The Mn 2p1/2 spectrum could be fitted into one or two peaks. The EB of 648.8 eV is assigned to a satellite peak of Mn2+.[51] In addition, the theoretical EB values of Mn 2p1/2 at 654.0 and 654.7 eV are assigned to Mn2+ and Mn3+, respectively. [52, 53] The additional peak of Mn2+ was detected at 656.8 eV.[54] In addition, XPS spectra of Mn 2p3/2 and Mn 2p1/2 for SGNM samples before reduction (referred to as ‘pristine’ samples) were analysis in the same manner (Figure S2). It is known that the reduction of Mn facilitates the formation of surface oxygen vacancies when Mn(n+1)+ (higher oxidation state) is converted to Mnn+ (lower oxidation state) according to the following defect reaction. [23, 24]

∙

𝑋

1

+ 1) + 2(𝑀𝑛(𝑛 ) + 𝑂𝑋𝑂 →2(𝑀𝑛𝑛𝑀𝑛+𝑛 + ) + 𝑉𝑂∙∙ + 2𝑂2 𝑀𝑛𝑛 +


(1)

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To assess the amount of oxidation state transition of Mn ions in SGNMs, the proportion of Mn ions with different oxidation states of pristine and reduced SGNM series were calculated by integration of corresponding peak regions. The resultant values were summarized in Table 2. Based on these values, total amount of Mn ion conversion of the SGNM during reduction can be estimated by summing the change from Mn4+ to Mn3+ (Δ[Mn4 + → Mn3 + ]) and the change from Mn3+ to Mn2+ (Δ[Mn3 + → Mn2 + ]), which can be calculated by following equations.

+ + ] ―[𝑀𝑛4𝑟𝑒𝑑𝑢𝑐𝑡𝑖𝑜𝑛 Δ[𝑀𝑛4 + → 𝑀𝑛3 + ] = [𝑀𝑛4𝑝𝑟𝑖𝑠𝑡𝑖𝑛𝑒 ]

(2)

+ + ]) ―[𝑀𝑛3𝑟𝑒𝑑𝑢𝑐𝑡𝑖𝑜𝑛 Δ[𝑀𝑛3 + → 𝑀𝑛2 + ] = (Δ[𝑀𝑛4 + → 𝑀𝑛3 + ] + [𝑀𝑛3𝑝𝑟𝑖𝑠𝑡𝑖𝑛𝑒 ]

(3)

Considering the molar fractions of Mn ions in the SGNM lattice (i.e., 0.2, 0.5, and 0.8 for SGNM28, SGNM55, and SGNM82, respectively), the total amount of Mn ion transitions during reduction for SGNM82, SGNM55, and SGNM28 were calculated to be 0.114, 0.240, and 0.392, respectively, in molar fraction. This result indicates that, the amount of surface oxygen vacancies can be increased in order of SGNM82 < SGNM55 < SGNM28 under reducing condition, which implies that the catalytic activity on hydrogen oxidation reaction is possibly highest in SGNM28.

3.3 H2-TPR studies The H2-TPR technique was conducted to investigate the reducibility of SGNM series, and the resultant TPR plots are shown in Figure 4. For all three SGNM samples, a major peak for H2 consumption at ~ 450 oC was detected. This peak can be identified as α-peak due to the reduction of Ni phase from Ni3+ to Ni2+ ions.[49, 50] In SGNM55 and SGNM28, we observed additional 11 ACS Paragon Plus Environment

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H2 consumption peaks at ~ 600 ℃ that is associated with reduction of Ni2+ to Ni0 as displayed in Figure 4b-c, whereas SGNM82 did not occurred second peak as shown in Figure 4a. This result indicates that Ni-metal was detected in reducing condition for the SGNM55 and SGNM28, which coincides to previous XRD and XPS results. Based on the TPR result, we observed surface morphologies of SGNM materials after reduction of 10% H2/Ar at 350 °C and 700 °C for 10 h. As shown in Figure 4d-f, at 350 oC all three samples showed only smooth surface without any exsolved particles. At 700 °C, SGNM82 still maintained smooth surface (Figure 4g), while surface morphologies of SGNM28 (Figure 4h) and SGNM55 (Figure 4i) were significantly changed with decoration of nanoparticles, indicating occurrence of Ni-phase exsolution. These results are in good agreement with above TPR analysis (Figure 4a-c). Moreover, it is clearly shown that the population of the exsolved nanoparticles of SGNM28 was much higher than that of SGNM55.

3.4 Electrochemical performance Figure

5a-c

show

the

impedance

spectra

of

symmetrical

cells

consisting

of

SGNM|LSGM|SGNM measured in the range of 700 to 800 °C under open circuit conditions in H2. For direct comparison of area specific resistances (ASRs) with different electrodes, the ohmic resistance at the high-frequency intercept with the real axis of each Nyquist plot was subtracted. The difference between the high and low frequency intercepts at the real axis was designated as the ASR, which reflects the catalytic activity of the electrodes in the reducing atmosphere. As intuitively shown in Figure 5a-c, the ASR of the SGNM was dramatically decreased when the Mn content was increased (i.e., the Ni content was decreased) in the SGNM composition.


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Based on the Nyquist plots, the Arrhenius plots of the calculated ASRs of all the samples were plotted as a function of inverse temperature (Figure 5d). The calculated Ea values of the SGNM28, SGNM55, and SGNM82 anodes were 1.05, 1.09, and 1.13 eV, respectively, and decreased with decreasing Ni/Mn ratio in the electrode material. With the same tendency as the Ea, the ASR of the SGNM electrode decreased in the order of SGNM82 >> SGNM55 > SGNM28 at all measured temperatures. For example, the ASR of SGNM28 in H2 at 750 °C (1.01 Ω cm2) was 2 and even 12 times lower compared to that of SGNM 55 (2.09 Ω cm2) and SGNM82 (12.39 Ω cm2), respectively, at the same measurement conditions. Based on the previous XRD (Figure 2), XPS (Figure 3), and H2-TPR (Figure 4) analysis, we believe that the Ea and ASR values (thus, catalytic activity) of SGNM series were reflected synergistically by surface oxygen vacancy formation and Ni-exsolution. However, considering similar tendency of drastic change in both ASR values (Figure 5b) and amounts of Ni-exsolution (Figure 4g-i) with SGNM compositional change, it can be reasonably suggested that the hydrogen oxidation activity of SGNM materials was strongly dominated by the amount of in situ exsolved Ni nanoparticles. The anode stability under redox cycling is one of the most critical features for the assessment of anode materials for SOFCs. To evaluate the redox behavior of the SGNM28 anode, the ASR of the symmetrical cell (SGNM28|LSGM|SGNM28) was monitored in situ by EIS measurement in alternating atmospheres shifting of 100% H2 and air with a flow rate of 200 ml min−1 at 750 °C. Figure 6a plots the resultant ASR values over a total of 10 cycles. The ASR of SGNM28 in H2 at 750 °C was initially 1.74 Ω·cm2 and reached 2.14 Ω·cm2 after 10 cycles, showing a negligible increase in ASR per cycle (~ 0.021 Ω·cm2). Compared to recently reported ceramic anodes, such as La0.8Sr0.2Cr0.8Mn0.2O3-δ-GDC-Ni (7.81% degradation rate) [26] and


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La0.85Sr0.15Cr0.9Ni0.1O3 (21.08% degradation rate) [27], the SGNM28 exhibited excellent redox stability (1.78% degradation rate). In addition, the redox stability of SGNM55 anode was investigated at the same testing condition, resulting in similar redox stability (2.03 % degradation rate) with higher ASR value (~ 4 Ω·cm2) in H2 at 750oC compared to the SGNM28. For better understanding of redox stability, the microstructural evolution of the SGNM28 anode during the redox cycle (oxidation  reduction  reoxidation) was observed by SEM (Figure 6b). Initially, there was no observable particles on the surface of SGNM28 (left in Figure 6b), while it is clearly shown that significant amounts of the nanoparticles were formed on the SGNM28 surface after exposure in H2 for 0.5 h (middle in Figure 6b). However, upon subsequent reoxidation of the reduced SGNM28 anode, the majority of the exsolved nanoparticles on the surface disappeared (right in Figure 6b), indicating dissolution of the Ni nanoparticles into the host SGNM lattice in an oxidizing atmosphere. This result suggests that these nanometer-sized Ni particles on the SGNM28 surface can sustain without agglomeration by reversible exsolution and dissolution during the redox cycles. From the all the prior results, the SGNM28 had the best electrode performance among all the tested samples, it was selected as an electrode for the applied as an SOFC anode. Figure 7a shows the temperature dependence of the I-V-P performance of an LSGM electrolyte supported SOFC with the LSCF-GDC cathode and SGNM28-GDC anode in the temperature range of 700 to 850 °C. In addition, Figure S4 shows the testing results of SOFCs with various SGNM-GDC anode compositions at 850 °C. The open circuit voltage (OCV) was closed to the theoretical value of 1.1 V derived by the Nernst equation, indicating the highly dense electrolyte as well as the gas-tight sealing. The maximum power density (MPD) of the SOFC employing the SGNM28-GDC anode at 850, 800, 750, and 700 °C was 1.26, 1.01, 0.67,


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and 0.40 W cm−2, respectively. To the best of our knowledge, these results are the best performance ever reported to date among all SOFCs using Ruddlesden-Popper phase-based anodes.(Figure 7b) For example, a similar structured SOFC employing the state-of-the-art La1.2Sr0.8Mn0.4Fe0.6O4-GDC anode exhibited 0.82 W cm−2 in MPD at 850 °C [19], which is 35% lower than that of our SGNM28-GDC cell (1.26 W cm−2 at 850 °C), demonstrating the high catalytic activity of the SGNM28 material for SOFC anode applications. Detailed SOFC configurations shown in Figure 7b are summarized in Table 3. Figure 7c shows the EIS results of the SGNM28-GDC cell at 700 - 850 °C under OCV conditions. From these Nyquist plots, the total (Rt), and ohmic (RΩ) resistances were calculated from the low and high frequency intercepts at the real axis, respectively. The (non-ohmic) electrode polarization resistance (Rp) was estimated from the difference between the Rt and RΩ. Figure 7d plots Rt, RΩ, and Rp as a function of temperature. Although we used a ~ 300 μm thick LSGM electrolyte in the SOFC, at 850 oC the Rp accounted for 45 % of the Rt value. Moreover, as the temperature decreased, the Rp portion of the Rt gradually increased to 61% at 700 oC (e.g., 0.55 of 0.90 Ω cm2). These results imply that the major contribution of the resistance still came from the electrodes. In this case, the Rp was attributed to both the cathodic and anodic polarization resistances at the electrode/electrolyte interface. Thus, further enhancement of the performance of the SGNMGDC anode based SOFC can be achieved by applying recently reported high performance oxygen electrodes (e.g., double perovskite- and bismuth oxide-based cathodes [28, 29]). In addition, we evaluated the performance of the SOFC using SGNM28-GDC anode in CH4 (3% H2O) at 800 °C. (Figure S5) The SOFC achieved a MPD of 0.26 W cm−2, which is comparable to that of the Ruddlesden-Popper anode-based SOFCs in the literature. [36, 42-44]


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Figure 8 shows the microstructure of the as-prepared and reduced SGNM28-GDC anodes. The as-prepared SGNM28-GDC anode exhibited highly porous cermet structures with an average feature size of ~300 nm (Figure 8a). Conversely, after exposure in H2 at 850 °C, we observed a homogeneously distribution of nanoparticles on the surface of the reduced SGNM phase (Figure 8b). The bright-field TEM image confirmed that the exsolved nanoparticles with a ~25 nm diameter were well adhered to the SGNM28 surface (Figure 8c). Figure 8d shows an HRTEM image with a higher magnification at the nanoparticle/SGNM interface. The interplanar distance of each region was calculated by fast Fourier transformation (FFT), followed by masking and filtering steps using the Gatan Digital Micrograph software. The estimated lattice space in the substrate region was 0.128 nm, which identified the (022) plane of the Ruddlesden-Popper structure (space group I4/mmm (139)), suggesting the SGNM28 phase was stable under the reducing atmosphere. The lattice spacing of the exsolved nanoparticle was 0.280 nm, which corresponded to the lattice constant of the (013) plane of the metallic Ni. In Figure 8e-f, the elemental distribution analysis (Ni and O) in the same region of Figure 8c confirmed that the nanoparticles existed as the metallic Ni, not in the NiO form. This result was supported by the fact that the reduction of the NiO phase into a Ni phase is thermodynamically favorable, with a negative ∆Gr in humidified H2 at 1000 K (Equation (3)), which is a similar to the SOFC operating conditions in this study (H2 with 3% H2O at ~ 800 °C ).

NiO(s)

1,000𝐾, 𝐻2

Ni(s) + H2O(g)

∆Gr = - 43. 48 kJ mol-1 (3)

The Ni metal is well-known for high electrocatalytic activity for H2 oxidation. Therefore, in reducing condition, the in-situ growth of Ni metal nanoparticles on the SGNM surface is


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expected to facilitate rapid dissociation of H2 and to effectively enlarge the hydrogen adsorption sites of the SGNM [30-32]. Subsequently, surface-absorbed protons would diffuse and spill over onto the anode surface for the subsequent electro-oxidation reaction at the triple phase boundary as illustrated in Figure 9. Therefore, we believe that the in-situ exsolution of nano-catalysts on SGNM28 significantly contributed to the high performance of the SGNM28 anode.

4. Conclusions In this study, we developed novel SrGdNixMn1-xO4±δ (x = 0.2, 0.5, 0.8) materials with a Ruddlesden-Popper structure and used them as SOFC anodes. The XPS result showed that the SGNM28 exhibited ~ 3 times higher in Mn ion transition fraction during reduction than that of SGNM82, suggesting a high amount of oxygen vacancies on the surface of the SGNM28 material. In addition, the H2-TPR study revealed in situ formation of the Ni phase on the surface of SGNM28 at over 650 oC, which was confirmed by SEM microstructure analysis. The redox cycling test of a symmetrical cell with SGNM28 electrodes resulted in only a small amount of change in the ASR per cycle (~ 0.021 Ω·cm2) even after 10 cycles, indicating remarkable reduction-oxidation stability at SOFC operating conditions. The LSGM electrolyte-supported SOFC using the SGNM28-GDC anode demonstrated an MPD of 1.26 W cm−2 at 850 °C, which is a record-high performance among any SOFCs with Ruddlesden-Popper based ceramic anodes to date. After the electrochemical performance testing, it was observed that metallic Ni nanoparticles (~25 nm) were well distributed on the surface of the SGNM28 anode. Considering high electrocatalytic activity of Ni on the hydrogen oxidation reaction, these in situ exsolved Ni nanoparticles on SGNM promoted H2 chemisorption and oxidation process during the SOFC operation. We believe that our novel SGNM materials with in-situ growth of metallic


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nanoparticles are highly promising SOFC anodes due to their outstanding electrocatalytic activity and redox stability.


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Acknowledgement This work was supported by the Global Frontier R&D on Center for Multiscale Energy System funded by the National Research Foundation under the Ministry of Science, ICT & Future Planning, Korea (2014M3A6A7074784). This work was also supported by “Human Resources Program in Energy Technology” of the Korea Institute of Energy Technology Evaluation and Planning (KETEP), granted financial resource from the Ministry of Trade, Industry & Energy, Republic of Korea. (20174030201590).

Supporting Information Supporting information with regard to characterization and electrochemical performance of SGNMs; TEM images of high-resolution lattice fringe images of SGNMs; XPS spectra of Mn 2p of pristine SGNMs; cyclic stability test of SGNM55|LSGM|SGNM55 at 750 oC; I-V-P characteristics of SOFCs with the SGNM28-GDC anode with different weight percent ratios; IV-P characteristics of an SOFC consisting of SGNM28-GDC anode in wet CH4 at 800 °C. (Figures S1-S5)

Author Contributions # (K.J.K.,

M.K.R.) These authors contributed equally to this work.

Notes The authors declare no competing financial interest.


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Table 1. Unit cell parameters of the as-prepared SGNMs.

Material

a, b [Å]

c [Å]

V [Å3]

Rwp

χ2

SGNM82

3.778

12.276

175.200

0.112

1.613

SGNM55

3.789

12.285

176.373

0.107

1.346

SGNM28

3.795

12.315

177.343

0.133

1.040


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Table 2. Summary of Mn valence state in percentage of SGNMs by XPS analysis.


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Table 3. Comparison of the performance of various Ruddlesden-Popper and perovskite anode SOFCs in this study and the reported in the literature.


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Figure 1. Observed and calculated PXRD profiles for the materials: (a) SGNM82, (b) SGNM55, and (c) SGNM28 with calculated unit cell structures (insets). TEM images and SAED patterns (inset) of (d) SGNM82, (e) SGNM55, and (f) SGNM28.


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Figure 2. XRD patterns of (a) SGNM samples reduced at 850 °C for 10 h in H2/Ar, and (b) powder mixtures of SGNM28 and GDC (and LSGM) annealed at 1300 oC for 10 h.


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Figure 3. XPS spectra of reduced SGNMs: (a) Ni 2p, (b) O 1s, and (c) Mn 2p


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Figure 4. H2-TPR result of SGNMs: (a) SGNM82, (b) SGNM55, and (c) SGNM28. SEM images of (d) SGNM82, (e) SGNM55, and (f) SGNM28 reduced at 350 °C in 10% H2/Ar for 10 h, and (g) SGNM82 (h) SGNM55, and (i) SGNM28 reduced at 700 °C in 10% H2/Ar for 10 h.


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Figure 5. Ohmic-corrected Nyquist plots of SGNM symmetrical electrodes in reducing conditions at (a) 700, (b) 750, and (c) 800 °C and and (d) comparison of the ASR vs. temperature (Arrhenius plots) for the SGNM electrodes.


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Figure 6. (a) Cyclic stability of the LSGM-supported symmetrical cells with the SGNM28 electrode at 750 ℃ versus the time of stream after 10 cycles, in which the anode gas was switched between H2 and air. (b) Cross-sectional SEM images of SGNM28 anode during a redox cycle (oxidation  reduction  reoxidation). 35 ACS Paragon Plus Environment

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Figure 7. (a) Cell voltages and power densities versus current densities of SGNM28-GDC anode at 700-850 ℃ and (b) comparison of the maximum power densities at various temperatures. (open square : Ruddlesden-Popper structure anodes, open triangle : Perovskite structure anodes). (c) Impedance spectra of SGNM28-GDC anode at 700-850 ℃ and (d) total, electrode, and ohmic ASRs of fuel cells at tested temperatures calculated from the impedance spectra


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Figure 8. FE-SEM images of the (a) as-prepared and (b) reduced SGNM28-GDC anode. (c) Bright field and (d) high-resolution TEM images of the reduced SGNM28-GDC anode. Elemental distribution of (e) Ni and (f) O in (c). 37 ACS Paragon Plus Environment

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Figure 9. Schematic illustration of the as-prepared and reduced SGNM28-GDC anode.


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Graphical abstract


A Highly Active and Redox-Stable SrGdNi0.2Mn0.8O4Â±Î´ Anode with

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