Significant Zirconium Substitution Effect on the Oxygen Reduction

Jun 2, 2019 - To analyze the reasons for the enhancement of ORR activity, we further analyzed the impedance spectrum data. The Arrhenius corves of ...
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Cite This: ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

Significant Zirconium Substitution Effect on the Oxygen Reduction Activity of the Cathode Material NdBaCo2O5+δ for Solid Oxide Fuel Cells Chengzhi Sun,† Yu Kong,† Lin Shao,§ Qi Zhang,† Xian Wu,† Naiqing Zhang,*,†,‡ and Kening Sun*,†,‡ †

Downloaded by ALBRIGHT COLG at 16:46:34:448 on June 10, 2019 from https://pubs.acs.org/doi/10.1021/acssuschemeng.9b01486.

State Key Laboratory of Urban Water Resource and Environment, School of Chemistry and Chemical Engineering, Harbin Institute of Technology, Harbin 150001, P. R. China ‡ Academy of Fundamental and Interdisciplinary Sciences, Harbin Institute of Technology, Harbin 150001, P. R. China § College of Materials Science and Engineering, Shenzhen University, Shenzhen 518060, P. R. China ABSTRACT: NdBaCo2O5+δ (NBCO) based double perovskite is an attractive cathode material with many advantages, yet its electrochemical performance still cannot meet the requirements. We first design and prepare Zr cation doping NdBaCo1.95Zr0.05O5+δ and systematically study the effects of Zr-doping on the oxygen kinetics, redox, and electrical properties of NdBaCo1.95Zr0.05O5+δ as cathode material for oxygen conduction SOFCs. NdBaCo1.95Zr0.05O5+δ show rapid oxygen bulk diffusion coefficients and surface exchange coefficients through zirconium cations doping, reaching 5.827 × 10−5 cm2·s−1 and 2.878 × 10−4 cm·s−1, respectively, at 700 °C, enabling the improved performance of oxygen reduction, and the polarization impedance is as low as 0.024 Ω·cm2.

KEYWORDS: SOFC cathode, Oxygen surface exchange, Bulk diffusion coefficients, Lattice free volume

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INTRODUCTION

Particularly, cobalt-based layered perovskite oxides, NdBaCo2O5+δ(NBCO), propose faster oxygen ion diffusion and surface exchange kinetics, resulting in low area-specific resistance (ASR).10 In this regard, Kim et al.11,12 demonstrated the higher performance of the NBCO cathodes, with Sr or Fe dopants. He et al.13 reported that partial replacement of Cu and Fe for cobalt in NBCFC establishes good chemical compatibility with electrolytes while maintaining its electrochemical performance. Although considerable research has been carried out to enhance the performance of cobalt based double perovskite cathode material through an ion doping method, how to rationally select an appropriate dopant kind and the effect of the doping are still ambiguous and unclear. It is urgent to develop a rational strategy to design the high-performance cathode material and give a deep understanding of the doping effect. In this work, we first designed and prepared Zr doping NdBaCo1.95Zr0.05O5+δ (NBCZrO) and systematically studied the effects of Zr-doping on oxygen kinetics, electrical properties, and redox properties of NdBaCo1.95Zr0.05O5+δ as a cathode material for oxygen conduction SOFCs. We assume that the larger lattice parameter would bring about a large free volume for oxygen ion diffusion, increase oxygen ion mobility, and decrease activation energy. As expected, NdBa-

As an efficient electrochemical device, solid oxide fuel cells (SOFCs) can transform fuel energy into practicable electricity with high conversion efficiency and low pollutant emissions. For the conventional SOFCs, a high operating temperature is necessary to meet the required performance. This high operating temperature results in a high rate of interface interactions between the components, the cell performance degradation, and the limited material selection. Therefore, it is preferred for SOFCs running under the operating temperature in an intermediate temperature (IT) range (600−800 °C) instead of the conventional high temperature range (1000 °C). With the degradation of operating temperature, the lifetime of the materials can be extended significantly, the costs of the SOFCs system can be reduced, and the choice of materials becomes wide open. However, for the traditional hightemperature cathode materials, the decrease in operating temperature results in slower oxygen reduction reaction (ORR) activity and larger polarization loss. Therefore, it is essential to develop novel types of cathode materials with great ORR activity under intermediate temperature, resulting in improved electrochemical performance.1−4 Recently, mixed ionic and electronic conductors (MIECs) were considerably studied as a series of cathode materials for SOFCs due to their higher concentration of oxygen vacancies and ionic conductivities. The study of layered perovskite oxides LnBaMO5+δ have attracted much attention, due to their higher surface exchange coefficients and chemical diffusion.5−10 © XXXX American Chemical Society

Received: March 16, 2019 Revised: May 10, 2019 Published: June 2, 2019 A

DOI: 10.1021/acssuschemeng.9b01486 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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Figure 1. (a) XRD patterns of NdBaCo1.95Zr0.05O5+δ, NdBaCo1.9Zr0.1O5+δ, and NBCO samples; (b) XRD patterns of NBCZrO, SDC, and NBCZrO-SDC mixtures sintered at 1050 °C for 4 h; Rietveld refined XRD pattern for the NdBaCo1.95Zr0.05O5+δ (c) and NBCO (d); (e) crystal structure, and (f) TEM image for NdBaCo1.95Zr0.05O5+δ. electrolyte disks were fabricated by a tape casting process, and the dense electrolyte substrate was obtained by sintering at 1450 °C for 10 h in air. The thickness of the SDC symmetric disk is about 100 μm. The cathode slurries for screen printing were prepared by mixing NBCZrO samples with the organic solvent (80% terpineol, 10% ethyl cellulose, and 10% corn starch), and the prepared slurries were deposited onto both sides of SDC electrolyte pellet by screen printing process, and then sintered at 1050 °C for 2 h. The prepared cathode thickness was 25 μm, and its effective area was about 0.196 cm2. The diluted silver paste was then deposited onto the electrode as current collectors, with sintering at 750 °C for 30 min. The nanostructures and surface microstructures of the cathode materials were characterized by using transmission electron microscopy (TEM, Tecnai G2F30, FEI) and a scanning electron microscope (SEM, SU8010, Hitachi). An X-ray diffractometer (XRD, X’Pert PRO with Cu Kα radiation) was utilized to characterize synthesized powders for the phase structure and purity with a step size of 0.02° over the 2θ range of 10° to 90° under room temperature. The surface chemistry of the samples was examined with the utilization of the X-ray photoelectron spectroscopy (XPS). The surface area of the prepared samples was measured through the Brunauer−Emmett− Teller (BET) analysis method by using an ASAP 2020 (Micromeritics). The stoichiometry of oxygen vacancies was measured with the iodometric titration technique.14 The electrochemical impedance spectra (EIS) of the symmetrical cells were measured over the frequency range of 0.01 Hz−1 MHz with the signal amplitude of 10 mV, using an electrochemical workstation (PARSTAT 2273) at 600−800 °C in air. The electrical conductivity measurements were carried out by a Keithley 2400 SourceMeter as a function of temperature from 50 °C to 850 °C in air with a four

Co1.95Zr0.05O5+δ shows rapid oxygen surface exchange (kchem) and bulk diffusion coefficients (Dchem) caused by the larger lattice parameter and the larger free volume due to the substitution of Co3+/Co4+ by larger Zr4+ cations, resulting in improved performance of the oxygen reduction. Additionally, the polarization impedance reached as low as 0.024 Ω·cm2 at 700 °C.

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EXPERIMENTAL SECTION

The NdBaCo1.95Zr0.05O5+δ nanoparticles were synthesized through a citrate EDTA complex sol−gel method. Here, stoichiometric amounts of neodymium nitrate (Nd(NO3)3·6H2O), barium nitrate (Ba(NO3)2), cobalt nitrate (Co(NO3)2·6H2O), and zirconyl nitrate (ZrO(NO3)2) were used as the raw materials for the cation sources. Subsequently, the 1.5 times mol EDTA acid and equivalent mol of citric acid were applied to the total cations as complexing agents. The pH value of the resulting solution was adjusted to about 6−7 applied by additional ammonium hydroxide. Water was evaporated, and the gel precursors were ignited to dispose of organic compounds, and the resulting compound was then ground to form a fine powder. The obtained ash after combustion was calcined at 600 °C for 4 h and 950 °C for 4 h to obtain the powders in air. Then, the NdBaCo2O5+δ (NBCO) and NdBaCo1.9Zr0.1O5+δ powders were synthesized in the same way. To evaluate the electrochemical performance of the prepared samples for ORR, the electrochemical polarization impedance was measured by applying the symmetrical electrochemical cell with NBCZrO/SDC/NBCZrO configurations. Ce0.8Sm0.2O1.9 (SDC) B

DOI: 10.1021/acssuschemeng.9b01486 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

Research Article

ACS Sustainable Chemistry & Engineering Table 1. Refined Structural Parameters of NBCO and NBCZrO Derived from XRD NBCO NBCZrO

a (Å)

b (Å)

c (Å)

V (Å3)

Rwp (%)

Rp (%)

3.8992 3.9064

3.8992 3.9064

7.6200 7.6430

115.853 116.632

4.99 5.71

3.77 4.3

Figure 2. (a) Typical impedance spectra of symmetric cells measured under an open-circuit condition; (b) comparison of Nyquist plots for the NBCO and NBCZrO symmetrical cell at 600 and 700 °C; (c) area specific resistances plotted versus inverse temperature for NBCZrO and NBCO cathode; (d) the corresponding fitted impedance spectra at 600 °C using a two-process equivalent circuit model; (e) effects of temperatures on the DRT functions for NBCO and NBCZrO cathodes; and (f) DRT analysis of the area specific resistances for NBCO and NBCZrO cathodes at 600 °C. probing DC technique on sintered NBCZrO and NBCO samples. The die-pressing films used for conductivity measurements were sintered at 1000 °C for 2 h. The BT2000 instrument (Arbin) was used to monitor the single-cell performance under humid H2 and ambient air.

amount, traces of one impurity phase was detected and identified as Ba2NdZrO5.5 (PDF #47−0387), suggesting that a reliable solid solubility limit of zirconium does not exceed 0.05 in the NdBaCo2−xZrxO5+δ series significantly. To assess the compatibility of NBCZrO with SDC electrolyte, NBCZrO cathode was mixed with SDC electrolyte in 50:50 wt % and sintered at 1050 °C for 2 h in the air. Figure 1b shows the XRD contrast patterns of the NBCZrO-SDC composites after heat treatment. There are no other diffraction peaks except those characteristic of the two components, NBCZrO and SDC, indicating a good chemical compatibility between NBCZrO and SDC under the processing condition, even up to the cathode fabrication temperature of 1050 °C.

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RESULTS AND DISCUSSION Figure 1a shows the XRD patterns of NdBaCo2−xZrxO5+δ (x = 0.05, 0.06, 0.08, 0.1, and 0.15) sintered at 950 °C for 4 h, and all of the peaks in the XRD patterns can be indexed using tetragonal structure (P4/mmm) for the as-prepared NdBaCo1.95Zr0.05O5+δ powders. For the x = 0.06 sample (referred to as NdBaCo1.94Zr0.06O5+δ) and the samples with a higher doping C

DOI: 10.1021/acssuschemeng.9b01486 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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ASR values of this single element Zr-doping NBCZrO cathode are even lower than those of Sr, Fe codoping and Cu, and Fe codoping cathodes, which are 0.081, and 0.073 Ω·cm2 at 700 °C, respectively.12,13 It should be noted that the surveyed cathode powders showed approximately specific surface areas in this work (8.3762 m2/g for NBCO, 7.852 m2/g for NBCZrO) and exhibited approximate particle size as shown in Figure 3a,b. On the basis of this, we could draw a conclusion

The cross-sectional SEM images were taken from the NBCO and NBCZrO electrodes after testing. The electrode cross sectional structure of NBCO appeared identical to that of the NBCZrO electrode, and there was no evidence of delamination or interfacial reactions between the cathode and the SDC electrolyte after testing. Shown in Figure 1e is a schematic diagram of the crystal cell structure for NBCZrO as confirmed by XRD Rietveld refinement (Figure 1c) with the GASS program and listed in Table 1, with a stacking sequence of BaO/Co(Zr)O2/NdO/ Co(Zr)O2/BaO. The unit cell parameters a = 3.8992 Å, b = 3.8992 Å, and c = 7.6200 Å determined for NBCO are in agreement with previous studies of materials. The unit cell parameters for tetragonal structure NBCZrO are a = 3.9064 Å, b = 3.9064 Å, and c = 7.6430 Å, and the cell volume is 116.632 Å3, which are larger than those of the NBCO. As expected, the unit cell volume of NdBaCo1.95Zr0.05O5+δ increases with Zrdoping content due to the larger size of Zr4+(0.72 Å) than Co3+/Co4+ (0.55 Å /0.53 Å) as depicted in Figure 1e. Shown in Figure 1f is the TEM image of a NBCZrO grain, recorded with the electron beam along the [100] direction, which confirms the double perovskite structure of NBCZrO with Nd and Ba cations ordered in alternating layers along the c axis, i.e., with a stacking sequence of [BaO]− [Co(Zr)O2]− [NdO]−[Co(Zr)O2]−[BaO]. The distance between NdO or BaO planes is about 0.395 nm, the result is nearly consistent with the XRD analysis. Such a layered structure is vital to rapid transport of oxygen vacancies and fast surface exchange of oxygen.15−19 Dissociated oxygen species may diffuse through the oxygen vacancy channels within the NBCZrO bulk phase.17 Generally, the exploitation of new cathode materials with great oxygen reduction activity under lower temperature condition is of great importance for the SOFCs. We determined the ORR activity of NBCO and NBCZrO cathode materials using symmetrical cells configuration range from 600 °C to 800 °C by EIS method. The cathode ASR is the key variable evaluating the cathode performance, calculated from the intercept difference of EIS impedance with the real axis, with low ASR values indicating high activity. To facilitate the comparison of the resistance data, we normalized the impedance data. As shown in Figure 2a, the ASR values of the symmetrical cell reduced observably with increasing temperature, under open circuit voltage conditions. The polarization impedance values of the NBCZrO cathode on the SDC electrolyte reached 0.006, 0.012, 0.024, 0.057, and 0.189 Ω·cm2 at 800, 750, 700, 650, and 600 °C, respectively. Figure 2b shows the typical impedance spectra of NBCO and NBCZrO cathodes on the SDC electrolyte. Clearly, the developed NBCZrO cathode material revealed observably increased ORR activity relative to the NBCO cathode under similar conditions. Minor introduction of Zr element into the NBCO double-perovskite can remarkably accelerate the diffusion rates and charge transfer rates of oxygen ions at the electrode interface and bulk, thus resulting in an improved electrochemical performance. These ASR values of NBCZrO are substantially lower than that of the NBCO double perovskite cathode with SDC electrolyte. As an excellent cathode for IT-SOFCs, this figure is far less than the widely accepted target value of 0.10 Ω•cm2 at 700 °C. In particular, the ASR values of the Zr-doping NBCO cathode are also lower than those of Fe, Cu and Sr single doping cathodes from previous reports.11,20,21 In addition, the

Figure 3. SEM images of the microstructure for (a) NBCO and (b) NBCZrO cathode powder materials sintered at 950 °C for 4 h; the fracture cross sectional SEM images for (c) NBCO and (d) NBCZrO electrodes after testing.

that the differences in electrochemical reaction activity not caused by the contribution of the three-phase boundary (TPB), and the unique lattice structure as mentioned above could be the critical factor affecting the activity of the ORR. And, there were indeed slight differences in the particle size and cathode porosity for NBCO and NBCZrO as shown in the Figure 3a,b. By testing the specific surface area of the cathode powder, it is found that NBCO and NBCZrO had an approximate specific surface area. We speculated that the effect of these slight differences on the cathode polarization performance is insignificant. To analyze the reasons for the enhancement of ORR activity, we further analyzed the impedance spectrum data. The Arrhenius corves of polarization impedance values for the NBCO and NBCZrO cathode materials on SDC electrolytes shown in Figure 2c. The calculated activation energies of NBCO and NBCZrO for the oxygen reduction process are 132.425 and 129.108 kJ/mol, respectively. It is clear that activation energy value after doping is slightly lower than that of the undoped samples. These results indicate that the activation energy would not significantly affected by the Zrdoping. The cathode reciprocal resistances show different oxygen partial pressure dependence at high and low-frequencies, which is weaker at high-frequencies (HF) than that at low frequencies (LF).22 On the basis of the relationship between the oxygen partial pressure dependencies and rate-determining-steps discussed by other researchers, the high-frequency polarization resistance may be associated with charge transfer, while at lowfrequencies corresponding to the noncharge transfer step, which is mainly thanks to the oxygen adsorption or dissociation at the cathode surface and oxygen ions diffusion through the cathode bulk.23 In the charge transfer process formula (O2,ads + 2e′ + V..O ↔ O×O), O2,ads represents the oxygen molecule adsorbed on the D

DOI: 10.1021/acssuschemeng.9b01486 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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Figure 4. XPS spectra of NBCO and NBCZrO cathode materials obtained for (a) O 1s and (b) Co 2p.

surface of cathode, e′ is an electron, V..O is an oxygen vacancy and O×O is an oxygen. Then, the impedance spectra of NBCO and NBCZrO cathodes was fitted into an equivalent circuit model consisting of two main reaction processes, with an example fitting presented in Figure 2d, for the purpose of measuring the contribution of diverse electrode reaction processes to the oxygen reduction reaction, which has the characteristics of two semicircular arcs. Compared with the NBCO cathode, NBCZrO shows a significant decrease of the ASRs in the noncharge transfer process, and in the charge transfer process, its impedance is about half of that for the NBCO cathode. Next, the distribution of relaxation time (DRT) of the area specific resistances were examined in an effort to get a better understanding of electrochemical process kinetics on the cathode, which is in combination with the peaks in the DRT plot. DRT analysis is a technique for analyzing impedance data related to complex electrochemical processes, helping quantificationally isolate each electrochemical processes in the electrode reaction.31 Figure 2e,f shown the DRT plots of electrochemical processes for NBCO and NBCZrO cathode under different temperatures in air. Each curve was resolved into high and low frequencies, and the area of each peak represents the impedance of the corresponding process. Since the frequency response characteristic of the characteristic peaks in the high frequency region is close to the pure resistance, these characteristic peaks may be related to the oxygen ion charge transport at the interface. In addition, since the oxygen diffusion process has a longer relaxation time and shorter characteristic frequency than the electrochemical process, the impedance characteristic peak in the low-frequency region should be related to the oxygen diffusion process. Apparently, it had large temperature dependence for NBCO cathode, the oxygen adsorption process represented by the low frequency impedance is a rate-determining step, and the oxygen ion transport process represented by the high frequency impedance has a relatively large value to that of NBCZrO. The peaks were almost nonexistent at large frequencies at all the operation temperatures in the NBCZrO cathode than in the NBCO cathode, which realized more efficient performance for oxygen ionic conduction process in the bulk. The area of the low frequency peak for NBCZrO is more than 50% less than that of NBCO cathode, indicating the NBCZrO cathode shows faster surface oxygen exchange. Therefore, we can assume that the zirconium doping into the NBCO cathode leads to a significant increase in the rate of each electrochemical step, wherein, it does not change the nature of rate-

determining steps, indicating that ORR kinetics has been boosted. Since existence of oxygen vacancies is crucial to the chargetransfer process, the rapid kinetics of charge transfer can be partially attributed to the high oxygen vacancy content from the introducing of zirconium. The iodometric titration method was employed to assess oxygen vacancy concentration for the NBCO and NBCZrO samples at ambient temperature. With the doping of Zr content, the oxygen nonstoichiometry (5+δ) of NBCZrO decreased comparing with that of NBCO (5.48 for NBCZrO, 5.76 for NBCO24), and the cobalt cations at Bsite were reduced to a lower oxidation state correspondingly as will be discussed later. According to these results, with the loss of lattice oxygen in NBCZrO, some Co4+ cations were gradually reduced to Co3+ cations accordingly as shown in Figure 4b, which is similar to other cobalt-based perovskite oxides.25 In addition to crystal structure defects, the surface chemical state of the cathode, such as the electronic structure, can also largely impact on the dynamics of oxygen reduction process.15 The XPS was then measured on NBCO and NBCZrO cathode materials, and the resulting spectra were presented in Figure 4. The O 1s spectra exhibited two distinct peaks at 525.2−527.2 eV and 527.6−532.5 eV, respectively. In general, the lattice oxygen (OL) is associated with the lower binding energy peak, whereas the adsorbed or loosely bonded oxygen (OA) is assigned to the peak located at higher binding energy peak, in this case, the high bonding energy peak is corresponded to the surface oxygen vacancies.15 There are clear results of the proportion of surface oxygen vacancies is higher respect to the lattice oxygen (OL) for NBCZrO samples as listed in Table 2. Table 2. Quantitative Analysis of the OL and OA on the Surface of NBCO and NBCZrO Cathode Materials Zr

area of OL

area of OA

OA/(OA+OL) [%]

0 0.05

17053.48 15721.33

66638.28 71878.17

79.62 82.05

The general consensus is that these surface oxygen defects are normally under higher energy states and may provide important active sites for the O2 adsorption/dissociation.16 Remarkably, NBCZrO had a higher concentration of surface oxygen vacancies than that of NBCO, making it a higher electrochemically active cathode. This conclusion was also supported by the electrical conductivity relaxation method measurement later. E

DOI: 10.1021/acssuschemeng.9b01486 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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Figure 5. (a) Temperature dependence of electrical conductivity for NBCZrO and NBCO and (b) Arrhenius plots of electrical conductivity for NBCZrO and NBCO.

Figure 6. (a) Oxygen tracer diffusivity (Dchem) and (b) surface-oxygen-exchange coefficient (kchem) for NBCO and NBCZrO.

trical conductivities of NBCZrO composite material were studied by the DC four-terminal technique. Figure 5a presents the variation curve of the electrical conductivities of the NBCO and NBCZrO samples with the temperature. The NBCZrO sample behaves as a typical semiconductor conduction behavior below nearly 200 °C, which is then converted into metallic-like conduction behavior at the following temperatures. The electron conductivities of the Zr doping samples gradually increase with the increase of temperature and reach the maximum value of ∼900 S·cm−1 at ∼200 °C, indicating a typical semiconductor characteristic. The electrical conductivity decreases with the further increase in temperature due to increasing resistance to the electrons by violent self-motion of metal atoms in NBCZrO. This changing trend is consistent with that of the NBCO sample,24 while the electronic conductivities of NBCZrO are lower than those of NBCO. The formation of more oxygen vacancies in NBCZrO decreases the carrier concentration and resulting in a slight decrease in the electrical conductivity. However, the average conductivities of NBCZrO are still comparable to those of other double-perovskite materials previously reported,24 reaching 350−500 S·cm−1 at 600−800 °C, which meets the requirements of cathode materials for intermediate-temperature SOFC. The Arrhenius plots of conductivities are shown in Figure 5b for the NBCO and NBCZrO samples, respectively. As a small polaron conduction mechanism,28 we can use the Arrhenius equation to represent the activation energy.

Additionally, it is believed that the redox properties of the Bsite ions in perovskite cathode materials play an important role to the oxygen reduction activity.17,18 Therefore, the chemical state of Co was also analyzed by the XPS spectra. The main peaks of Co 2p located at 775.25 and 777.05 eV are obviously distinguishable from each other, representing Co4+ and Co3+. Figure 4 also depicts that satellite peaks of Co 2p are at 790.52 and 792.34 eV, and the valence state of cobalt ion decreases, which is assigned to the production of oxygen vacancies in the perovskite structure. The mixture valence of Co3+ and Co2+ exists in these oxides. Because Co3+ (0.62 Å, HS) is larger than Co4+ (0.53 Å), the reduction of Co valence also causes the explanation with the slight expansion of the c-axis.26 Through the above analysis, Zr-doping into the perovskite structure brings about the optimization of crystal structure and surface chemical state and then causes the improvement of electrochemical properties, this can also be understood by considering the material electronegativity, which was regarded to affect the ORR activity.27 From the perspective of the electronegativity, the Pauling electronegativity for Co3+/Co4+ (1.88) is larger than that of Zr4+ (1.33). Therefore, the overall electronegativity for NBCZrO has been lowered via suitable doping amount of Zr, which would increase the charge transfer rate during the process of oxygen reduction. Simultaneously, it is further confirmed that the expansion of lattice results from the substitution of Co ions by larger Zr ions through the XPS analysis. However, the formation of excessive oxygen vacancies through introducing of zirconium will inevitably affect the electronic conductivity of materials. High-temperature elecF

DOI: 10.1021/acssuschemeng.9b01486 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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Figure 7. I−V and power density curves of single cells with (a) NBCO and (b) NBCZrO cathode at 600−800 °C.

σ=

i −E y A expjjj a zzz T k KT {

1−

(1)

where K is the Boltzmann constant, Ea is the activation energy, T is the absolute temperature, and A is the pre-exponential factor. The activation energies for the NBCO cathode resulted from the slope of the fitting line from the curve (ln σT vs 1000/T) are 2.827 and 5.687 kJ·mol−1 in the relatively high and low temperature ranges, respectively. While for NBCZrO cathode, they are 3.367 and 3.824 kJ·mol−1, respectively. For more quantitative analysis, the oxygen surface exchange and bulk diffusion kinetics were investigated under the operating condition through electrical conductivity relaxation technique (ECR) for the entire temperature range and represented in Figure 6, resulted from the relaxation profiles of different NBCO and NBCZrO films, exhibiting the transient behavior of the conductivity with a rapid change of the oxygen partial pressure in the surrounding atmosphere. This process consists of two consecutive steps, including the adsorption of oxygen at the film surface and the bulk oxygen ions diffusion. In order to obtain a solution to Fick’s second law, we consider the effect of the surface reaction as the boundary condition. Specifically, the control of the oxygen transport process can be obtained by judging the relationship between the minimum dimension size of the samples and the critical thickness (Lc = Dchem/kchem). According to the literature results, the critical thickness of double perovskite materials was generally about 1 mm, so we selected the strip samples with the thickness of 0.7 mm and 2 mm, respectively, and then used simple single parameter equation to fit the experimental data, so that the fitting process becomes simple and reliable.5,32−34 When the thickness of NBCO and NBCZrO films are far less than the critical thickness, the oxygen phase transmission distance is relatively short, and the oxygen transfer process is controlled by surface reaction.30 As a result, the surface oxygen exchange coefficient (kchem) can be calculated by equation as follows: i k y σ(t ) − σ(0) = 1 − expjjjj− chem t zzzz σ(∞) − σ(0) L { k

ij D π 2t yz σ(t ) − σ(0) 8 = 2 expjjj− chem2 zzz j σ(∞) − σ(0) π 4L z{ k

(3)

When the oxygen partial pressure suddenly changes from 0.05 to 0.21 atm, the oxygen surface exchange coefficients (kchem) were determined ranging from 2.74 × 10−4 to 3.51 × 10−4 cm·s−1 for NBCZrO and are clearly higher than those of NBCO under the temperature range from 600 to 800 °C, which indicating the surface reaction rate increases as a largesized dopant was added, and oxygen vacancies are necessary for the oxygen transport and exchange reaction process. It is the effects of Zr-doping on oxygen surface exchange process and the microstructure effect of introducing Zr into the Co site that promoted the oxygen transport process of composite materials jointly. Alternatively, the diffusion coefficients of NBCZrO are also higher, ranging from 5.05 × 10−5 to 6.19 × 10−5 cm2·s−1 at a temperature range from 600 to 800 °C. The NBCZrO mass equilibrates faster than NBCO, indicating that NBCZrO possesses better oxygen surface exchange properties under the same conditions. The oxygen diffusion coefficients of the NBCZrO mass increase by nearly a factor of 2 compared to that on NBCO, since partially substituting cobalt cations into the NBCO lattice with Zr cations may create fast transport paths with very low migration barrier energies level for oxygen ion. The cathode oxygen kinetics can be enhanced by introducing a large number of oxygen vacancies, resulting in faster oxygen bulk diffusion and surface exchange.29 We have proved the existence of more oxygen vacancies in the material by iodometric titration method and XPS surface chemical state analysis. Thus, higher concentration of the oxygen vacancies as a result of Zr-doping into the Co-site contribute to better electrochemical performance and faster oxygen kinetics. The electrochemical properties of NBCO and NBCZrO cathode materials were tested by an anode supported single cell with scandia-stabilized zirconia (SSZ) electrolyte and NiSSZ anode. Figure 7 is the current−voltage curve under the temperature range from 600 °C to 800 °C, and wet hydrogen is used as fuel, and the ambient atmosphere is cathodic oxidant. It can be seen that the maximum power density of NBCZrO cathode at 800 °C is 1012 mW·cm−2, which is better than that of the undoped material NBCO, further indicating that the introduction of Zr elements is conducive to increasing the electrochemical properties of the cathode.

(2)

where σ(t) and L represent the electrical conductivity at time t and the thickness of the film, respectively. Similarly, when the thickness of the NBCO and NBCZrO films are larger than the critical thickness, the oxygen phase transport distance is relatively long, the surface reaction is fast relatively, and the oxygen transport process is under control of the bulk phase diffusion. Then, the bulk diffusion coefficients (Dchem) can be calculated by the equation as follows:

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CONCLUSIONS Zr-doping NdBaCo2−xZrxO5+δ double perovskites were synthesized through a citrate EDTA complexing sol−gel method. G

DOI: 10.1021/acssuschemeng.9b01486 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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ACS Sustainable Chemistry & Engineering

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Larger cations radius Zr doping at the Co sites resulted in larger lattice parameters and more oxygen vacancies. The valence state of Co decreased, and the loosely bonded oxygen quantity increased in NBCZrO cathode materials. NBCZrO shows rapid oxygen bulk diffusion coefficients and surface exchange coefficients through zirconium cations doping, reaching 5.827 × 10−5 cm2·s−1 and 2.878 × 10−4 cm·s−1 respectively at 700 °C. The polarization of NBCZrO cathodes decreased to 0.024 Ω·cm2 at 700 °C. It is worth noting that this work may open the door toward rationally designing advanced electrode materials for the SOFCs cathode.

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AUTHOR INFORMATION

Corresponding Authors

*Tel: +451 86412153. Fax: +86-451-86412153. E-mail: [email protected]. *Tel: +451 86412153. Fax: +86-451-86412153. E-mail: [email protected]. ORCID

Naiqing Zhang: 0000-0002-9528-9673 Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Funding

This work was supported by the National Natural Science Foundation of China (no. 21646012); the State Key Laboratory of Urban Water Resource and Environment, Harbin Institute of Technology (no. 2016DX08); and China Postdoctoral Science Foundation (no. 2016M600253). Notes

The authors declare no competing financial interest.

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ABBREVIATIONS SOFCs, solid oxide fuel cells; MIECs, mixed ionic and electronic conductors; ECR, electrical conductivity relaxation technique.

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REFERENCES

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DOI: 10.1021/acssuschemeng.9b01486 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acssuschemeng.9b01486 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX