Understanding of Oxygen Reduction Reaction on Perovskite-Type

Article Cite This: J. Phys. Chem. C XXXX, XXX, XXX−XXX

pubs.acs.org/JPCC

Understanding of Oxygen Reduction Reaction on Perovskite-Type Ba0.5Sr0.5Fe0.91Al0.09O3‑δ and Ba0.5Sr0.5Fe0.8Cu0.2O3‑δ Using AC Impedance Spectroscopy Genetic Programming Alon Oz,†,‡ Kalpana Singh,§ Danny Gelman,‡ Venkataraman Thangadurai,*,§ and Yoed Tsur*,†,‡ †

The Nancy and Stephen Grand Technion Energy Program, TechnionIsrael Institute of Technology, Haifa 3200003, Israel Department of Chemical Engineering, TechnionIsrael Institute of Technology, Haifa 3200003, Israel § Department of Chemistry, University of Calgary, Calgary, Alberta, T2N 1N4, Canada Downloaded via UNIV OF CALIFORNIA SANTA BARBARA on June 28, 2018 at 12:23:18 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

‡

ABSTRACT: Fundamental understanding of the oxygen reduction reaction (ORR) mechanism in electrochemical cells (e.g., solid oxide fuel cells (SOFCs)) is rather challenging because of several processes, which occur with similar time constants. Also, it is very difficult to elucidate ORR reaction steps using a conventional equivalent circuit modeling in ac impedance spectroscopy. There is no unique model to fully explain the ORR mechanism, especially in high temperature SOFCs. In this study, attempt has been made using impedance spectroscopy genetic programming (ISGP) technique to describe SOFC cathode ORR processes. Using ISGP, we analyzed the electrochemical performance of Ba0.5Sr0.5Fe0.91Al0.09O3‑δ (BSFAl) and Ba0.5Sr0.5Fe0.8Cu0.2O3‑δ (BSFCu) cathodes with oxide ion conducting La0.8Sr0.2Ga0.8Mg0.2O3‑δ (LSGM) and proton conducting Ba0.5Sr0.5Ce0.6Zr0.2Gd0.1Y0.1O3‑δ (BSCZGY) electrolytes. The ORR mechanism is explained by finding the distribution function of relaxation time (DFRT) with the help of ISGP. By monitoring the changes in the DFRT models at different temperatures and using both oxide ion and proton conducting electrolytes, we are able to deconvolute the ORR to its several polarization subprocesses. Using the present approach, it is possible to gain additional information, which may be convoluted and therefore undetected in conventional impedance analysis. The analysis procedure results in a direct and unambiguous DFRT model, with a distinct physical meaning. Therefore, making it especially beneficial in comparative studies, as demonstrated in this work, where it was found that BSFCu-LSGM cathode showed better charge-transfer properties than BSFAl-LSGM cathode, due to higher conductivity of BSFCu phase. In both BSCZGY-BSFCu and BSCZGY-BSFAl cathodes, the rate-limiting step was found to be the charge-transfer process, owing to low electrical (ionic) conductivity of BSCZGY.

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INTRODUCTION

leads to low structural stability, which hampers the performance of SOFCs.13 Recently, several Co-free cathodes, based on Fe-based perovskite-type such as Sr 1−x A xFeO 3‑δ (A = Ba, Pr), Pr 0 . 5 Sr 0 . 5 Fe 1 − x Cu x O 3 ‑ δ , Sm 0 . 5 Sr 0 . 5 Fe 0 . 8 Cu 0 . 2 O 3 ‑ δ , and La0.6Sr0.4Fe0.8Cu0.2O3‑δ showed rather promising ORR activity in the IT range.12,14−19 Perovskite-type oxides can sustain large amount and various dopants, which have helped material chemists to develop cathodes with improved physical and chemical properties.20 Substituting Sr2+ (1.44 Å) for Ba2+ (1.61 Å) in A-site results in enhanced electrochemical activity due to the increased free volume in the perovskite structures.21 Perovskite-type Ba0.5Sr0.5Fe1−xAlxO3‑δ showed high thermal stability up to 1350 °C. Al3+ doping was done in order to improve the redox stability by reducing the nonstoichiometric oxygen variation.22 Lou et al. showed that 9% Al-doped Ba0.5Sr0.5Fe1−xAlxO3‑δ cathode showed the lowest area specific

Intermediate temperature (IT, 400−700 °C) solid oxide fuel cells (SOFCs) offer advantages of allowing use of economical sealants and interconnect and also avoid material degradation due to high temperature operation.1,2 However, reduced operating temperature hampers the cathode process as oxygen reduction reaction (ORR) becomes too slow and thus leads to high polarization resistance, especially in conventional Mnbased cathodes such as La1−xSrxMnO3‑δ, resulting in high ORR overpotential.3−6 On the other hand, Co-based perovskite-type oxides such as Sm0.5Sr0.5CoO3‑δ, La0.6Sr0.4Co0.2Fe0.8O3‑δ, and Ba0.5Sr0.5Co0.8Fe0.2O3‑δ offer high electrochemical properties for oxygen reduction reaction.7−9 Compared to Mn- and Fe-based cathodes, Co-containing cathodes exhibit lower resistance for ORR due to mixed valence Co4+/Co3+ and associated better redox properties leading to higher electronic and oxide ion conductivities.10 However, Co-based perovskite cathodes induce a high thermal coefficient of expansion (TCE) that may lead to mismatches with the solid oxide ion electrolytes.11,12 Also, “CoO” evaporation at elevated temperatures © XXXX American Chemical Society

Received: March 30, 2018 Revised: June 3, 2018

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DOI: 10.1021/acs.jpcc.8b03036 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C resistance (ASR) values, 0.26 Ω cm2 at 800 °C and ∼1.9 Ω cm2 at 700 °C.23 Sm0.5Sr0.5Fe0.8Cu0.2O3‑δ cathode on Sm0.2Ce0.8O1.9 electrolyte showed a very promising ASR of 0.085 Ω cm2 at 700 °C.18 Ba0.5Sr0.5Fe0.8Cu0.2O3‑δ (BSFCu) coupled with proton conducting SOFC, BSFCu-BaZr0.1Ce0.7Y0.2O3‑δ/ BaZr0.1Ce0.7Y0.2O3‑δ/Ni-BaZr0.1Ce0.7Y0.2O3‑δ (H-SOFCs) exhibited power density of 430 mW cm−2 at 700 °C.17 Thus, Both Ba0.5Sr0.5Fe1−xAlxO3‑δ and Ba0.5Sr0.5Fe0.8Cu0.2O3‑δ look promising as IT-cathodes for O-SOFCs and H-SOFCs. Recently, a novel proton conducting Ba0.5Sr0.5Ce0.6Zr0.2Gd0.1Y0.1O3‑δ (BSCZGY) electrolyte has been developed by us, which showed an excellent chemical stability under CO2 and water vapor conditions.24,25 As recorded ASR values for ORR activity are not low enough for getting high performance during full cell operations on BSCZGY electrolyte, developing highly catalytic active and chemically stable cathode material is underway.26 Thus in the current report, we have employed above-mentioned advantageous Ba0.5Sr0.5Fe1−xAlxO3‑δ and Ba0.5Sr0.5Fe0.8Cu0.2O3‑δ cathodes with IT oxide ion conducting La0.8Sr0.2Ga0.8Mg0.2O3‑δ (LSGM) and proton conducting BSCZGY electrolytes.

Ba0.5Sr0.5Fe0.8Cu0.2O3‑δ (BSFCu) as IT-cathodes for LSGM and BSCZGY electrolytes in air, through combined electrochemical impedance spectroscopy and ISGP that were reported in conference proceedings.34,35 ISGP utilizes genetic algorithm to find the most suitable distribution function of relaxation times (DFRT). Almost no a priori knowledge of the system is required when using ISGP. eq 1 shows the correlation between the measured impedance and the DFRT:36 ∞

∫‐∞

Z(ω) = R ∞ + R pol

Γ(log(τ )) d(log(τ )) 1+iωτ

(1)

Here Z is the impedance, R∞ is the series resistance, Rpol is the total polarization resistance, Γ is the DFRT, τ is the relaxation time, and ω is the angular frequency. eq 1 is an ill-posed inverse problem; the DFRT cannot be extracted from the measured impedance data without the use of some advanced numeric techniques.37,38 ISGP is a MATLAB program developed to find the most suitable DFRT using evolutionary algorithm. It starts with a set of initial DFRT models, each comprised of a linear combination of known mathematical functions. The DFRT models are graded using a “fitness function”, based on their compatibility to the measured data and other factors that ensure the most physical sound model is chosen.38,39 The highest graded models serve as the basis for the next generation, where new sets of models are generated from the pre-existing ones, albeit with minor changes (or “mutations”). Those changes can be addition, removal or replacement of a peak in the model. Each new DFRT model is then graded and again, the highest graded models survive. Once the end criteria have reached, the final best model is chosen. ISGP is designed to find a model that has the best compatibility to the measured data while ensuring it has the least amount of free parameters, to avoid overfitting.27 The outcome of the analysis is a DFRT model comprised of a linear combination of mathematical peaks, i.e., Gaussians, Lorentzians etc. Figure 1 shows a typical DFRT model plotted as a function of log (τ). Each peak is characterized by its relaxation time (or

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THEORETICAL ASPECTS The ORR catalytic performance of symmetrical cells was studied by analyzing electrochemical impedance spectroscopy (EIS) using impedance spectroscopy genetic programming (ISGP), in order to achieve a more detailed analysis of the overall cathode and electrolyte mechanisms.27,28 The ISGP analysis results in a f unctional form of the distribution function of relaxation times (DFRT), a unique model comprised of peaks in the time-domain that can be correlated with different processes in the overall polarization resistance.29 Its main advantages over other known EIS analysis methods are as follows. (1) It finds the correct number of independent free parameters as part of the genetic process, thus avoids overfitting (or less-than-ideal compatibility between the model’s result and the data). This is done using a discrepancy-complexity approach, and monitored graphically.30 (2) It is less prone to finding a model that its main influence is out of the measured range. (3) As compared to other DFRT analysis methods, it does not use any filter or Lagrange coefficients for regularization that could result in either too smooth solution or artifact peaks. (4) It finds an analytic function that is relatively easy to work with, e.g., to separate convoluted phenomena and to follow the behavior of each peak separately. Several studies of ORR activity using EIS have been reported, the vast majority using equivalent circuit modeling (ECM), which may require presumptions regarding the tested system.14,15,19 Also, other studies involve the use of distribution of relaxation times (a.k.a. DRT), to identify the number of different processes only to construct a corresponding equivalent circuit model and correlate the different peaks with different circuit elements (R,Q).31−33 Utilizing our analysis approach, we are able on the one hand to obtain a unique and unambiguous model of the system and, on the other hand, to assign physical meaning to the different peaks in the model without “translating” them back to ECM, resulting in a straightforward and efficient analysis. Our approach is ideal for comparative studies, where a method to carefully screen different cathode and electrolyte assemblies is presented here, in order to find the optimal parameters of the systems in an objective and thorough manner. We further analyze and discuss the ORR performance of Co-free perovskite-type Ba0.5Sr0.5Fe0.91Al0.09O3‑δ (BSFAl) and

Figure 1. (a) Typical EIS Nyquist plot and (b) its corresponding DFRT model containing two peaks, each account for an arc in the Nyquist plot.

central frequency), height and width. Ideally, each peak can be correlated with a polarization process. Calculating its area and multiplying by Rpol yields the polarization resistance of the corresponding process. For a large enough bandwidth, the total sum of all the peak areas in the DFRT model equals the total polarization resistance, Rpol. Because of the nature of the genetic algorithm, the chosen model is unique in terms of number, position, and area of the peaks; no ambiguity of models exists in B


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The Journal of Physical Chemistry C our analysis approach in that respect. This can be verified by running the program several times for each set of data. Correlating peaks with different polarization processes allows us to identify and quantify the effect of each one on the overall performance of the tested sample. The obtained DFRT can be regarded as the “fingerprint” of the sample. Unlike other timedomain DRT analysis techniques, which result in a point-bypoint solution, the DFRT has a functional form that assists in identifying the different processes and working directly in the time domain. Our approach can be especially beneficial in the case of monitoring the ORR activity, since several processes (i.e., surface-exchange, surface and bulk diffusion, chargetransfer) are usually convoluted and therefore hard to adequately monitored using other common analysis techniques, notably equivalent circuit modeling (ECM). In addition, ISGP is more flexible than ECM; it does not impose that the measured data will be fitted to a predetermined set of lumped and distributed circuit elements, but simply finds the best model as discussed above, without further presumptions. In that manner, the overall cathode performance can be evaluated adequately.40−42

scanning electron microscope (SEM) was used for obtaining the micrographs of the symmetrical cells. Half-Cell Preparation and Electrochemical Evaluation. Cathode composite slurries were prepared by mixing electrode and electrolyte powders in 1:1 weight ratio and milling with organic components (α-terpineol, butanol, butyl benzyl, and ethyl cellulose) for proper mixing. Then the slurries were screen printed on both sides of the electrolyte and sintered at 1000 °C for 3 h at heating and cooling rate of 2°/min. Gold current collector paste (Heraeus) was brush painted on both sides of the half-cells, which were cured for 30 min at 800 °C. 2-probe AC impedance spectroscopy (Solartron 1260, 0.1 Hz-1 MHz, 100 mV AC amplitude) was used for the electrochemical evaluation of the half-cells. The samples were measured between 400 and 900 °C in air. To attain thermal stability, the samples were kept for at least 45 min before each impedance reading. Half-cells BSFCu-LSGM/LSGM/BSFCuLSGM, BSFCu-BSCZGY/BSCZGY/BSFCu-BSCZGY, BSFAlBSCZGY/BSCZGY/BSFAl-BSCZGY, and BSFAl-LSGM/ LSGM/BSFAl-LSGM were constructed and investigated for their ORR performances. The symmetrical cell dimensions are shown in Table 1.

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EXPERIMENTAL SECTION Cathode and Electrolyte Preparation. Ba0.5Sr0.5Fe0.91Al0.09O3‑δ (BSFAl) and Ba0.5Sr0.5Fe0.8Cu0.2O3‑δ (BSFCu) were synthesized through sol−gel method.17,23,43 Analytical grade Ba(NO3)2, Sr(NO3)2, Fe(NO3)3, Al(NO3)3, and Cu(NO3)2 were mixed in stoichiometric amounts and dissolved in water. The nitrate solutions were heated and stirred on a hot plate. Another solution, containing citric acid monohydrate and ethylene glycol in water, was mixed with the nitrates solution. The molar ratio of citric acid:ethylene glycol:total nitrates was 1:4:1. The mixed solution was heated up to ∼190 °C and stirred; the pH of the solution was adjusted to ∼9 with dropwise addition of concentrated ammonium hydroxide solution. After a while, the solution turned into a viscous gel. The viscous gel then turned into ash while releasing small sparks and flames. The resulting ash-like material was heated again in an oven at 300 °C for 3 h, in order to burn out remaining unreacted contents. The powders were then calcined at 700 and 1050 °C (10 h) to form crystalline phases. Dense Ba0.5Sr0.5Ce0.6Zr0.2Gd0.1Y0.1O3‑δ (BSCZGY) electrolyte pellets were synthesized via solid-state method as reported elsewhere.24 Commercial La0.8Sr0.2Ga0.8Mg0.2O3‑δ (LSGM) electrolyte powder from fuel cell materials was pressed into pellets and was sintered at 1450 °C for 5 h. Phase, Thermal, and Microstructural Analysis. Phase identification and purity of the cathode and electrolyte powders were investigated by using powder X-ray diffractometer (PXRD, Bruker D8) with Cu−Kα X-ray radiation (40 kV; 40 mA, step-size 0.015° and step-time 6 s). Phase stability of the cathode powders at different temperatures were studied by recording the high-temperature PXRD patterns with (Cu−Kα X-ray radiation, 40 kV; 40 mA, step-size 0.02° and step-time 3 s) between 30 and 800 °C. The cell parameters of BSFAl and BSFCu were obtained by Le Bail refinement by using the general structure analysis system (GSAS) with the graphical user interface EXPGUI. Thermogravimetric analysis (TGA, Mettler Toledo TGA/DSC/HT1600 instrument) of the cathode powders was measured in air for three consecutive heating and cooling cycles between 25 and 900 °C with heating and cooling rate of 5 °C/min. Zeiss sigma VP field-emission

Table 1. Dimensions of Four Cell Assemblies Analyzed in Current Work symmetrical cell

electrode area (cm2)

electrode thickness (μm)

electrolyte thickness (μm)

BSFCu-LSGM BSFAl-LSGM BSFCu-BSCZGY BSFAl-BSCZGY

0.37 0.86 0.50 0.27

∼20 ∼25 ∼34 ∼21

∼450 ∼800 ∼500 ∼500

Electrochemical Impedance Spectroscopy Analysis. The analysis procedure using ISGP involves subtracting the series resistance from the real part of the impedance data, Rs. It is correlated with Ohmic resistance of the electrolyte, the leads and the current collector.44 In that manner, the Ohmic resistance does not obscure the other polarization resistances, which can be much smaller at elevated temperatures. Therefore, the DFRT model is comprised of only polarization processes within the cathode and its interfaces with the gas-phase and the electrolyte. ISGP can find Rs at a first iteration, or Rs can be inferred from the high frequency impedance.

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RESULTS AND DISCUSSION Phase and Thermal Analysis. Figure 2 shows the PXRD patterns of as-prepared perovskite-type BSFAl and BSFCu powders, which crystallizes in the Pm-3m (#221) space group. The lattice constant of BSFCu (3.9406 (6) Å) is lower than the BSFAl (3.9577 (3) Å) even though Cu2+ (0.73 Å) has a larger ionic radius than Al3+ (0.535 Å) in 6-fold coordination.45 This might be due to the presence of higher fractions of Fe3+ (0.645 Å, HS) than Fe4+ (0.585 Å) due to the Al doping at the Fe site, as reported by Martynczuk et al. and Lou et al.22,23 Another possible influence on the lattice parameter might be the oxygen vacancy concentration in the bulk, that is expected to be much higher in the case of BSFCu as compared to BSFAl due to selfcompensation.46 Figure 3a,b shows the high temperature PXRD patterns of BSFAl and BSFCu, with systematic increase and decrease in lattice constant values with temperature changes, both in heating and cooling, along with reproducible lattice constant values as expected due to thermal expansion. The C


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The Journal of Physical Chemistry C ⎛ a − a 0 ⎞⎛ 1 ⎞ α(T − T0) = ⎜ ⎟⎜ ⎟ ⎝ a0 ⎠⎝ T − T0 ⎠

(2)

where a is the lattice parameter at a specific temperature, and a0 is the lattice parameter at room temperature, T0. The line of best fit of the graph a − a0 vs temperature gave the average a0

lattice thermal expansion (α). The lattice expansion in the current phases can also be chemical in nature, due to the formation of oxygen vacancies during heating and repulsion between mutually exposed cations, and the reduction of transition metals.47,48 TCE of BSFAl (35.4 × 10−6 K−1) was found to be higher than that of BSFCu (25.8 × 10−6 K−1), which may be due to their difference in oxygen uptake capacity during thermal cycling. Figure 4a,b shows the TGA of BSFAl and BSFCu in air for three consecutive heating and cooling cycles. Initially both

Figure 2. Room temperature powder X-ray diffraction patterns for asprepared Ba0.5Sr0.5Fe0.91Al0.09O3‑δ (BSFAl) and Ba0.5Sr0.5Fe0.8Cu0.2O3‑δ (BSFCu).

Figure 4. TGA of (a) Ba0.5Sr0.5Fe0.91Al0.09O3‑δ (BSFAl) and (b) Ba0.5Sr0.5Fe0.8Cu0.2O3‑δ (BSFCu) in air/N2 (50:50 volume ratio) for three consecutive heating and cooling cycles. Solid lines represent the heating and dotted lines represent the cooling stages.34

samples absorb oxygen around 400 °C and then loses lattice oxygen as seen by a continuous weight loss in TGA. The first cooling cycle did not follow the first heating curve, but during the subsequent second and third heating and cooling cycles, all TGA curves were reversible, indicating uptake of oxygen. This is the characteristic of oxygen storage and release materials.49 The onset temperature of continuous weight loss is slightly higher in BSFCu with lower Fe content than BSFAl with higher Fe content. This is in accord with the assumption that BSFCu contains initially more oxygen vacancies than BSFAl due to self-

Figure 3. PXRD patterns at variable temperature in air for (a) Ba0.5Sr0.5Fe0.91Al0.09O3‑δ (BSFAl) and Ba0.5Sr0.5Fe0.8Cu0.2O3‑δ (BSFCu) powders.

average lattice thermal expansion coefficient (TCE, α) was calculated from the linear expansion region for both BSFAl and BSFCu using the expression: D


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The Journal of Physical Chemistry C

Figure 5. SEM micrograph of the electrode−electrolyte interface of (a) BSFCu+LSGM/LSGM/BSFCu+LSGM, and (b) the magnification of the selected area SEM, along with corresponding EDX mapping of symmetrical-cells after the electrochemical measurements in air. Inset in part a shows the surface SEM micrograph of BSFCu+LSGM symmetrical cell and exhibits the porous nature of Au current collector.

compensation. Another related observation in that regard is the fact that below 400 °C the weight, i.e., the oxygen content, is pinned in the BSFCu sample. Microstructure and Electrochemical Analysis of Symmetrical Cells. Figures 5a,b and 6a,b show the electrode-oxide ion electrolyte interface SEM micrographs of the BSFCu +LSGM, and BSFAl+LSGM symmetrical cells, respectively after the electrochemical measurements in air. SEM shows that the investigated composite electrodes exhibit good adherence to the LSGM electrolyte with thickness of about 20 μm. Moreover, surface SEM micrograph of BSFCu+LSGM sym-

metrical cell shows the porous nature of the Au current collector, as seen from the inset of Figure 5a. In addition, energy dispersive X-ray (EDX) mappings in Figures 5b and 6b suggests that both (LSGM and cathode) phases are homogeneously distributed in the composite cathode layer. Similar microstructure was also shown by BSFCu+BSCZGY, and BSFAl+BSCZGY symmetrical cells. Figure 7 shows Nyquist plots of the four examined systems, BSFCu and BSFAl with electrolytes LSGM and BSCZGY. The Ohmic resistance of each system was subtracted in order to focus on the cathode and cathode/electrolyte processes, E


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Figure 6. SEM micrograph of the electrode−electrolyte interface of (a) BSFAl+LSGM/LSGM/BSFAl+LSGM and (b) the magnification of the selected area SEM, along with corresponding EDX mapping of symmetrical cells after the electrochemical measurements in air.

ASR of BSFAl/BSCZGY at this temperature drops to 1.78 Ω cm2, which is higher than BSFCu with the same electrolyte even at lower temperature, 0.87 Ω cm2 at 700 °C. The ASR serves as the sum of the polarization resistances taking place at the cathode and its interface with the gas and electrolyte. The following DFRT analysis can shed light on the origin of different ASR values. ORR involves several steps: gas diffusion at the porous electrode; adsorption and ionization of oxygen gas on the cathode surface; diffusion of oxygen ions at the cathode surface and in the bulk; and charge-transfer of ions at the cathodeelectrolyte interface.50−52 The latter process, the chargetransfer, is said to be faster than the previous ORR steps,

therefore the low frequency intercept of the arc with the real axis is its ASR of the two cathodes (the values are divide by 2, to account for one cathode).48 A small ASR characterizes a well-performing cell, which is a common measure to evaluate electrochemical systems. It is evident that as temperature increases, the ASR of each system decreases. At 600 °C, the cathodes coupled with LSGM exhibit lower ASR than the ones with BSCZGY; 1.48 and 2.48 Ω cm2 for BSFAl/LSGM and BSFCu/LSGM, respectively. BSFCu/BSCZGY shows 8.30 Ω cm2 and BSFAl/BSCZGY has the highest ASR values of 17.59 Ω cm2. The trend of better performance at higher temperatures for the LSGM system remains, as BSFAl/LSGM and BSFCu/ LSGM exhibit 0.15 and 0.22 Ω cm2, respectively at 725 °C. The F


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Figure 7. Experimental (symbols) and fitted (lines) Nyquist plots at temperature range 600−825 °C of (a) BSFCu+LSGM (b) BSFAl+LSGM (c) BSFCu+BSCZGY, and (d) BSFAl+BSCZGY. Inset shows the expanded view.

Figure 8. DFRT plots (normalized by Rpol) at 600−825 °C of (a) BSFCu+LSGM, (b) BSFAl+LSGM, (c) BSFCu+BSCZGY, and (d) BSFAl +BSCZGY.35

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can arise from the relatively low electrical conductivity of BSFAl at 600−750 °C, which is lower than 15 S cm−1, compared to 57 S cm−1 of BSFCu at the same temperature range.23,57 Furthermore, the central frequency of peak I in BSFCu shifts toward higher frequencies as the temperature increases, indicating that diffusion and/or oxygen incorporation at the cathode becomes faster. This is not the case for BSFAl (Figure 8b), as for all analyzed temperatures, the central frequency of peak I remains roughly between 1 and 10 Hz. Thus, indicating that BSFCu coupled with LSGM has better cathode activity than BSFAl with the same electrolyte, once the temperature is higher than 600 °C. Between the two systems with BSCZGY (Figure 8c,d), it appears that below 700 °C the kinetics of the ORR is pretty slow, since peak I in both systems exhibits its central frequency near the lowest edge of the measured bandwidth (0.1 Hz). It can arise from the relatively high electrolyte resistance when compared to LSGM, according to the series resistance, Rs: at 600 °C the Al and Cu-containing systems with LSGM, Rs values are 6.9 and 4.0 Ω cm2 respectively; Al and Cu compounds with BSCZGY showed Rs of 39.8 and 28.9 Ω cm2, respectively, at the same temperature. This order of magnitude difference is attributed to the fact that BSCZGY serves mainly as proton conductor and thus conducts oxygen ions relatively poor in air when compared with LSGM. As the temperature rises above 700 °C, peak II becomes more prominent in both systems, indicating the charge-transfer is the rate-limiting step in systems containing BSCZGY as their electrolyte. Figure 9 shows the variation of the area specific resistance of each peak in the DFRT models as a function of temperature. All four systems show an Arrhenius-type behavior, demonstrated by a linear correlation which enables calculating the activation energy of each peak in the DFRTs. Peak II in the BSFCu-LSGM (Figure 9a) system is the only one who differs from this linear behavior; its value fluctuates around a relatively low value, reiterating again that the charge transfer losses are negligible in respect to the other ORR processes in this system. It accounts for less than 8.5% of the total polarization resistance at all analyzed temperatures. On the other hand, peak II in the Al-containing system (Figure 9b) becomes larger than peak I above 700 °C. Peak II accounts for 25% of the total polarization resistance at 600 °C and rises to above 75% at 750 °C. As mentioned, this difference arises from the relatively low conductivity of BSFAl. When BSFAl is coupled with LSGM, which conducts oxygen ions fairly well, the charge-transfer at the electrode−electrolyte interface becomes limiting as temperature increases. For the BSCZGY cases (Figure 9c,d) the notion that below 700 °C the ORR activity is relatively slow is evident by the resistance values of both Al and Cu-containing systems, especially peak I, which exhibits resistance values of 12.5 and 30.6 Ω cm2 at 600 °C for BSFCu and BSFAl, respectively. Both systems exhibit the same activation energy of peak I, indicating that the symmetrical cell assembly with BSCZGY influences the surface exchange and diffusion of oxygen more than the different doping element at the perovskite B-site. As in the BSFAl-LSGM, it can also be seen here that peak II in both systems serves as the rate-determining step as temperature increases, since its resistance value decreases more moderately than peak I. It is evident from our comparative study that the cell assembly, which exhibits the best ORR activity, is BSFCuLSGM. The advanced EIS analysis using ISGP enables us to

which will assist in identifying the peaks in the DFRT model. Figure 8 shows the DFRT models of the four systems, which gives us more detailed information on the governing processes in the ORR mechanism. For convenience, it is plotted as a function of frequency instead of τ (f ≡1/(2πτ)), which leads to a similar representation as the Bode diagram in terms of left and right. All DFRT models exhibit two peaks (except for BSFCu +BSCZGY, which has an additional peak at high frequencies at temperatures lower than 650 °C), which indicate at least two distinguished processes. Peak I, located at low frequency, is correlated with the oxygen surface-exchange and diffusion at the cathode, whereas peak II is correlated with charge-transfer at the electrode/electrolyte interface.23 The correlation of the peaks to the different subprocesses is done according to their frequency range: peak II is located at higher frequencies (lower relaxation time) than peak I, which indicates that the process peak II accounts for is faster. Thus, peak II is assigned to polarization losses originating from the charge transfer of oxygen ions at the electrode−electrolyte interface. Furthermore, the effective capacitance of each peak can be easily calculated by dividing the central relaxation time by the area of each peak, according to τ Ceff , i = i Ri (3) where Cef f,i is the effective capacitance of peak i, τi is the relaxation time of peak i and Ri is the area of peak i, which accounts for its resistance. The typical capacitance values associated with the charge transfer are 10−4−10−5 F, where 10−1−10−2 F is typically associated with the chemical capacitance, which originates from O2− diffusion and surface exchange at the gas-cathode interface of the MIEC.14,53,54 Table 2 shows the calculated effective capacitance of each peak in the Table 2. Calculated Effective Capacitance of Each Peak of the Four Cell Assemblies symmetrical cell

T (°C)

BSFCu-LSGM BSFAl-LSGM BSFCu-BSCZGY BSFAl-BSCZGY

650 650 650 725

peak I capacitance [F] 2.9 7.0 4.1 2.8

× × × ×

10−2 10−2 10−2 10−2

peak II capacitance [F] 4.7 7.6 5.6 1.9

× × × ×

10−3 10−3 10−5 10−5

DFRT of the four symmetrical cell assemblies. The latter process has been treated in ECM by incorporating a Gerischer element (GE).55 Therefore, peak I in our model and GE are both intertwined with the same process in the ORR mechanism, of oxygen surface exchange and solid state diffusion.56 The four systems are quite different in their impedance at a given temperature and therefore it is hard to find a single temperature in which they can be compared. Thus, the three better (more conductive) systems are brought in Table 2 at a single temperature and the values of the worst one, the BSCZGY-BSFAl system, are shown at a higher temperature. When comparing the two cathodes with LSGM, it is evident that peak II in the Cu-containing cathode (Figure 8a) is much smaller than peak II in the Al system (Figure 8b), and at 725 °C, peak II no longer appears in the model, suggesting its resistance becomes so small that is it is undetected. On the other hand, as temperature increases, peak II at the Alcontaining cathode becomes more prominent. This difference H


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Figure 9. Specific resistances of the DFRT peaks in (a) BSFCu+LSGM, (b) BSFAl+LSGM, (c) BSFCu+BSCZGY, and (d) BSFAl+BSCZGY.

exchange of oxygen and its diffusion at the cathode, and the high frequency one is ascribed to the charge-transfer process at the electrode−electrolyte interface. It was found that for BSFCu coupled with LSGM, the charge-transfer is negligible, and the vast majority of the ASR arises from the oxygen incorporation and diffusion. For BSFAl/LSGM system, the charge-transfer is much more significant. As for the BSCZGY systems, both with BSFCu and BSFAl, they exhibit low ORR activity below 700 °C and the charge-transfer process is detected as the limiting step, since it has very mild temperature dependence, unlike the other ORR subprocess. The comparative study was based on analyzing the EIS data using ISGP to obtain a DFRT models. This approach bears inherent advantages over other common analysis techniques, since it allows seeing straightforward which system exhibits the best ORR activity and helps in understanding the reasons for it. It presents a direct and effective method to screen through fourcell configuration to determine the optimal one. It also carries the potential to do so in other systems, which require an efficient method to shed light on the best parameters.

deconvolute the total polarization into its two main subprocesses and directly compare them in a manner not readily available in other analysis techniques. The main challenge using our approach, as in any other approach, is assigning physical meaning to the different peaks in the model.58 Once we successfully tackled that issue, the advantages of the current approach become apparent: Plotting the DFRT as a function of frequency allows us to better understand how the contributions at each temperature evolve relative to each other and how the peaks shift in frequency. In the LSGM containing assemblies, we can straightforwardly see how peak I shifts to higher frequencies in the Cu system while its frequency remains unchanged in the Al one; we can also precisely monitor and compare the evolution of peak II in these two systems, where one becomes more prominent and the other disappears. In addition, as mentioned in the introduction, the genetic algorithm combined with the “fitness function” also ensures obtaining a unique model, which is the best-fitted model with the least amount of free parameters.

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CONCLUSIONS The phase structure, thermal behavior, and mainly the electrochemical performance of Co-free cathodes with two types of electrolytes were examined in symmetrical-cell configuration. In-depth analysis of the ORR subprocesses was done using ISGP in order to find DFRT models that depict the data adequately. The DFRT models comprised, in most parts, two peaks. The low frequency peak is ascribed to surface-

AUTHOR INFORMATION

Corresponding Authors

*(V.T.) E-mail: [email protected]. *(Y.T.) E-mail: [email protected];. ORCID

Venkataraman Thangadurai: 0000-0001-6256-6307 I


Article

The Journal of Physical Chemistry C Notes

Intermediate-Temperature Solid Oxide Fuel Cells. Chem. Mater. 2011, 23, 5037−5044. (17) Zhao, L.; He, B.; Ling, Y.; Xun, Z.; Peng, R.; Meng, G.; Liu, X. Cobalt-Free Oxide Ba0.5Sr0.5Fe0.8Cu0.2O3‑δ for Proton-Conducting Solid Oxide Fuel Cell Cathode. Int. J. Hydrogen Energy 2010, 35, 3769−3774. (18) Ling, Y.; Zhao, L.; Lin, B.; Dong, Y.; Zhang, X.; Meng, G.; Liu, X. Investigation of Cobalt-Free Cathode Material Sm0.5Sr0.5Fe0.8Cu0.2O3‑δ for Intermediate Temperature Solid Oxide Fuel Cell. Int. J. Hydrogen Energy 2010, 35, 6905−6910. (19) Zhou, Q.; Xu, L.; Guo, Y.; Jia, D.; Li, Y.; Wei, W. C. J. La0.6Sr0.4Fe0.8Cu0.2O3‑δ Perovskite Oxide as Cathode for IT-SOFC. Int. J. Hydrogen Energy 2012, 37, 11963−11968. (20) West, A. R. Inorganic Functional Materials: Optimization of Properties by Structural and Compositional Control. Chem. Rec. 2006, 6, 206−216. (21) Zhang, J.; Wang, J.; Yang, J.; Wang, Y.; Chan-Park, M. B. ThreeDimensional Macroporous Graphene Foam Filled with Mesoporous Polyaniline Network for High Areal Capacitance. ACS Sustainable Chem. Eng. 2014, 2, 2291−2296. (22) Martynczuk, J.; Liang, F.; Arnold, M.; Šepelák, V.; Feldhoff, A. Aluminum-Doped Perovskites as High-Performance Oxygen Permeation Materials. Chem. Mater. 2009, 21, 1586−1594. (23) Lou, Z.; Qiao, J.; Yan, Y.; Peng, J.; Wang, Z.; Jiang, T.; Sun, K. Synthesis and Characterization of Aluminum-Doped Perovskites as Cathode Materials for Intermediate Temperature Solid Oxide Fuel Cells. Int. J. Hydrogen Energy 2012, 37, 11345−11350. (24) Kannan, R.; Singh, K.; Gill, S.; Fürstenhaupt, T.; Thangadurai, V. Chemically Stable Proton Conducting Doped BaCeO3-No More Fear to SOFC Wastes. Sci. Rep. 2013, 3, 2138. (25) Naeem Khan, M.; Savaniu, C. D.; Azad, A. K.; Hing, P.; Irvine, J. T. S. Wet Chemical Synthesis and Characterisation of Ba0.5Sr0.5Ce0.6Zr0.2Gd0.1Y0.1O3‑δ Proton Conductor. Solid State Ionics 2017, 303, 52−57. (26) Singh, K.; Baral, A. K.; Thangadurai, V. Electrochemical Studies of Gd0.5Pr0.5BaCo2O5+δ (GPBC) Cathode for Oxide Ion and Proton Conducting Solid Oxide Fuel Cells. Solid State Ionics 2016, 288, 351− 356. (27) Hershkovitz, S.; Baltianski, S.; Tsur, Y. Electrochemical Impedance Analysis of SOFC Cathode Reaction Using Evolutionary Programming. Fuel Cells 2012, 12, 77−85. (28) Hershkovitz, S.; Baltianski, S.; Tsur, Y. Harnessing Evolutionary Programming for Impedance Spectroscopy Analysis: A Case Study of Mixed Ionic-Electronic Conductors. Solid State Ionics 2011, 188, 104− 109. (29) Oz, A.; Gelman, D.; Goren, E.; Shomrat, N.; Baltianski, S.; Tsur, Y. A Novel Approach for Supercapacitors Degradation Characterization. J. Power Sources 2017, 355, 74−82. (30) Baltianski, S.; Tsur, Y. Analysis of Impedance Spectroscopy Data − Finding the Best System Function. J. Electroceram. 2003, 10, 89−94. (31) Im, H. N.; Choi, M. B.; Singh, B.; Lim, D. K.; Song, S. J. Investigation of Oxygen Reduction Reaction on La0.1Sr0.9Co0.8Fe0.2O3‑δ Electrode by Electrochemical Impedance Spectroscopy. J. Electrochem. Soc. 2015, 162, F728−F735. (32) Papurello, D.; Menichini, D.; Lanzini, A. Distributed Relaxation Times Technique for the Determination of Fuel Cell Losses With an Equivalent Circuit Model to Identify Physicochemical Processes. Electrochim. Acta 2017, 258, 98−109. (33) Carpanese, M. P.; Clematis, D.; Bertei, A.; Giuliano, A.; Sanson, A.; Mercadelli, E.; Nicolella, C.; Barbucci, A. Understanding the Electrochemical Behaviour of LSM-Based SOFC Cathodes. Part I -Experimental and Electrochemical. Solid State Ionics 2017, 301, 106− 115. (34) Singh, K.; Sahoo, A.; Thangadurai, V. Oxygen Reduction Reaction Properties of Cobalt-Free Perovskites for SOFCs. ECS Trans. 2017, 78, 479−488. (35) Oz, A.; Gelman, D.; Tsur, Y.; Singh, K.; Thangadurai, V. Evolutionary Programming Based Approach for SOFC Cathode

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was undertaken thanks in part to funding from the Canada First Research Excellence Fund (CFREF) at the University of Calgary as well as being supported by the Grand Technion Energy Program (GTEP) and the second Israel National Research Center for Electrochemical Propulsion (INREP 2). The authors also thank Dr. Neta Shomrat for the illustrations and Dr. Suresh Mulmi for advice on powder Xray analysis.

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Understanding of Oxygen Reduction Reaction on Perovskite-Type

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