Double-Layered Perovskite Anode with - American Chemical Society

Dec 21, 2015 - (Pr0.4Sr0.6)3(Fe0.85Mo0.15)2O7 (DLP-PSFM) with uniformly dis- persed in situ exsolution of Co−Fe alloy nanoparticles was prepared...
0 downloads 0 Views 5MB Size
Download PDF
Research Article pubs.acs.org/acscatalysis

Double-Layered Perovskite Anode with in Situ Exsolution of a Co−Fe Alloy To Cogenerate Ethylene and Electricity in a Proton-Conducting Ethane Fuel Cell Subiao Liu, Karl T. Chuang, and Jing-Li Luo* Department of Chemical and Materials Engineering, University of Alberta, Edmonton, Alberta T6G 1H9, Canada ABSTRACT: A new proton-conducting ethane fuel cell (PCEFC) anode material comprised of double-layered perovskite (Pr0.4Sr0.6)3(Fe0.85Mo0.15)2O7 (DLP-PSFM) with uniformly dispersed in situ exsolution of Co−Fe alloy nanoparticles was prepared by annealing cubic perovskite Pr0.4Sr0.6Co0.2Fe0.7Mo0.1O3−δ in a 10% H2/N2 reducing atmosphere at 900 °C. The BaCe0.7Zr0.1Y0.2O3−δ electrolyte-supported PC-EFC single cell fabricated with the new DLP-PSFM anode material has achieved a maximal output power density of 496.2 mW cm−2 in H2 and 348.84 mW cm−2 in C2H6 at 750 °C. In the meantime, a high ethylene yield, increasing from 13.2% at 650 °C to 41.5% at 750 °C with a remarkable ethylene selectivity over 91% and no CO2 emission, was achieved because of the considerably efficient catalysis of in situ Co−Fe alloy nanoparticles that were homogeneously distributed on the DLPPSFM backbone. Furthermore, a single cell under a constant current load of 0.65 A cm−2 reached a stable power output at 750 °C in C2H6 during the 100 h stability test. This indicates an excellent coking resistance, which is also supported by Raman spectra, X-ray diffraction patterns, and scanning electron microscopy image analyses. The results clearly indicate that the DLPPSFM anode material possesses high ethane partial dehydrogenation activity, enhanced electrocatalytic activity, and good stability. On the basis of its remarkable performance in cogeneration of electricity and ethylene in PC-EFC, DLP-PSFM ceramic material is an attractive anode for a directly hydrocarbon-fueled solid oxide fuel cell. KEYWORDS: Co−Fe alloy, anode material, ethane, partial dehydrogenation, ethylene

1. INTRODUCTION Ethylene is a versatile petrochemical intermediate that plays a crucial role in modern society. The gas ethane and petroleum distillates (naphtha) are the main feedstocks for ethylene production.1,2 Currently, tube furnace steam cracking is the dominant technology in ethylene production, and ∼99% of global ethylene production employs the tube furnace pyrolysis method.3 However, cracking reactions are highly endothermic, reversible, and severely limited by thermodynamic equilibrium.4 Moreover, large amounts of CO and CO2 are formed in the steam cracking process because of the existence of oxygen sources, and the high operation temperature produces lowvalue byproducts.5 According to the mechanism of ethane dehydrogenation, if one of the products, hydrogen, can be selectively removed from the reaction system, the conversion is no longer limited by thermodynamic equilibrium, allowing the ethane conversion rate to increase at a lower temperature and, thus, making ethylene production more economical. In response to the critical need for a cleaner energy technology, the solid oxide fuel cell (SOFC) has emerged as a very promising candidate.6 Recently, the proton-conducting solid oxide fuel cell (PC-SOFC) has been developed to convert fuel to desired chemicals and generate cleaner energy simultaneously.7−9 Ethane has been studied for the cogenera© XXXX American Chemical Society

tion of ethylene and electricity with low or even no CO2 emission in a proton-conducting fuel cell.10−12 However, current anode materials used for PC-SOFC have not met the requirements of excellent electrochemical performance and high ethylene yield. Therefore, it is a great challenge to develop a new class of SOFC anode materials having good coking tolerance, excellent catalytic activity, and mixed electronic and ion conductivity.13 As previously reported, directly fueled hydrocarbon SOFCs employing the perovskite oxides La0.75Sr0.25Cr0.5Mn0.5O314 and Sr2Fe1.5Mo0.5O6−δ15 as the anode materials have shown different levels of enhanced coking resistance but power densities lower than that of SOFCs with Ni-based anode materials.16 This poor performance is ascribed to inadequate conductivity and/or low catalytic activity. Recently, double-layered perovskites have been investigated as anode materials because of their good stability and high mixed ionic and electronic conductivity for the partial oxidation of hydrocarbons.17−19 Also, it is known that a Co−Fe bimetallic alloy is an excellent electrochemical catalyst and has been widely used as a catalyst in fuel cell anode materials.20,21 Received: October 13, 2015 Revised: December 16, 2015

760

DOI: 10.1021/acscatal.5b02296 ACS Catal. 2016, 6, 760−768

Research Article

ACS Catalysis

located on the bottom, while the cathode, placed on the top, was exposed to air. PC-EFC testing was performed with electrochemical techniques after the furnace had reached a stable temperature. The polarization resistance of the PC-EFC was determined from electrochemical impedance spectroscopy (EIS) measured by applying an ac potential with a frequency range of 1 MHz to 0.1 Hz and an amplitude of 5 mV at the stable open circuit voltage (OCV). 2.2. Characterization and Testing. The crystalline structures of the synthesized powders were identified by powder X-ray diffraction (XRD) with Rigaku Rotaflex Cu Kα radiation, and the raw data were analyzed using JADE version 6.5. Thermogravimetric analyses (TA SDT Q600) were performed from 20 to 1200 °C with a heating/cooling rate of 20 °C min−1 in air to characterize the thermophysical properties. Microstructures in the MEA were determined with a Vega-3 (Tescan) scanning electron microscope (SEM) equipped with an EDX detector (INCA, Oxford Instruments) and a high-resolution Zeiss Sigma FE-SEM equipped with an EDX detector and an EBSD detector. P-PSCFM and DLPPSFM powders were analyzed using a JEOL JEM 2100 transmission electron microscope (TEM) at an accelerating voltage of 200 kV. The powders were first dispersed in 2propanol and then floated onto carbon-coated copper TEM grids. The electrochemical performances of the single cells were measured by employing a four-probe method with Au wires as the leads. The electrochemical measurements were conducted with a Solartron 1255 frequency response analyzer and a Solartron 1286 electrochemical interface instrument. The temperature of a single cell was slowly increased to 800 °C, and a 10% H2/N2 reducing gas flow was continuously pumped into the anode compartment. The temperature was held for 2 h to complete the further reduction of the anode material, and then the temperature was decreased to 750, 700, and 650 °C for electrochemical measurements. A stability test in an ethane atmosphere was performed under a constant current load (0.65 A cm−2) at 750 °C. The outlet gases from the anode compartment were analyzed using a Hewlett-Packard model HP5890 gas chromatograph equipped with a packed bed column (Porapak QS) operated at 80 °C with a thermal conductivity detector and a flame ionization detector.

Moreover, both Co and Fe have been utilized as effective alloying elements to enhance the performance of anode materials, and the combination of a Co−Fe alloy catalyst22−24 favors the formation of C2−C4 alkenes. In this work, Co−Fe alloy nanoparticles were in situ exsolved and uniformly distributed on the double-layered perovskite (Pr0.4Sr0.6)3(Fe0.85Mo0.15)2O7 (DLP-PSFM) anode backbone by reducing the cubic perovskite Pr0.4Sr0.6Co0.2Fe0.7Mo0.1O3−δ (PPSCFM) in a 10% H2/N2 atmosphere at 900 °C. Meanwhile, this DLP-PSFM material was fabricated as the anode in a single BaCe0.7Zr0.1Y0.2O3−δ (BCZY) electrolyte-supported protonconducting ethane fuel cell (PC-EFC). The redox stability, catalytic activity, and electrochemical performances of DLPPSFM toward the partial dehydrogenation of ethane to ethylene were systematically investigated.

2. EXPERIMENTAL PROCEDURES 2.1. Preparation of Materials and Cell Fabrication. Pr0.4Sr0.6Co0.2Fe0.7Mo0.1O3−δ (P-PSCFM) polycrystalline powders were prepared with a soft chemistry method. 25 Stoichiometric amounts of Pr(NO3)3·5H2O, Sr(NO3)3, Co(NO3)2·6H2O, Fe(NO3)3·9H2O, and (NH4)6Mo7O24·4H2O were first dissolved in citric acid containing several drops of nitric acid. The solution was stirred thoroughly and heated on a hot plate, leading to the formation of organic resins that contained the homogeneously distributed cations involved because of the slow evaporation of the solvent. The obtained gel was dried at 150 °C and decomposed at 600 °C for 2 h to remove the organic components and the nitrates. The precursor powders were fired at 1050 °C for 10 h in air to achieve the expected pure single P-PSCFM phase and then heated in a tubular furnace at 900 °C for 10 h in a 10% H2/N2 reducing gas flow, forming the double-layered perovskite with in situ exsolution of Co−Fe alloy nanoparticles. BaCe0.7Zr0.1Y 0.2 O 3−δ (BCZY) 26 and (La 0.60 Sr 0.40 ) 0.95 Co 0.20 Fe 0.80 O 3−δ (LSCF)27 were fabricated with a conventional solid state reaction method. Gd0.2Ce0.8O2−δ (GDC) powders were synthesized with a combustion method described elsewhere.28 BCZY electrolyte discs were dry pressed uniaxially under a force of 5 tons in a stainless steel die to form dense substrates and subsequently sintered in air at 1550 °C for 10 h. Asprepared discs were ground and polished to a thickness of ∼300 μm. The single cells investigated in this work were BCZY electrolyte-supported. The GDC slurry was screen-printed on both anode and cathode sides of the BCZY electrolyte disc and co-sintered at 1300 °C for 4 h to form dense GDC buffer layers with a thickness of ∼2 μm. The anode material slurry was prepared by combining DLP-PSFM or NiO and BCZY (weight ratio of 1:1) with a glue containing 1-butanol, benzyl butyl phthalate (BBP), ethyl cellulose, and α-terpineol. The weight ratio of catalyst and glue was 1.7:1. A cathode electrode comprised of LSCF and BCZY was prepared with the same method as the anode material slurry. Both the porous PPSCFM anode material slurry and LSCF cathode material slurry were screen-printed onto the corresponding surfaces of the BCZY disc to form a membrane electrode assembly (MEA) with a circular area of 0.2 cm2. The MEA was sintered at 1000 °C for 2 h in air. Gold paste was painted onto the surfaces of both the anode and the cathode to form current collectors. The PC-EFC was built by fixing the MEA between coaxial pairs of alumina tubes with a sealant, which was fastened in a vertical tubular furnace (Thermolyne F79300). Dry ethane was fed to the PC-EFC at a flow rate of 100 mL min−1 via the anode tube

3. RESULTS AND DISCUSSION P-PSCFM and DLP-PSFM samples were obtained as wellcrystallized powders. XRD patterns have been identified for the as-prepared cubic P-PSCFM sintered first in air at 1050 °C and then annealed in a 10% H2/N2 gas mixture at 900 °C to prepare DLP-PSFM. As shown in Figure 1A, no impurity phases were detected in the as-prepared P-PSCFM (Figure 1A1). However, the XRD pattern clearly shows a doublelayered perovskite structure after the reduction of the PPSCFM to DLP-PSFM in a 10% H2/N2 gas mixture at 900 °C for 2 h, as indicated in Figure 1A2. The two weak peaks labeled with stars are assigned to the Co−Fe alloy.29 The XRD pattern with the appearance of the starred peaks in Figure 1A2 supports the hypothesis that the cubic P-PSCFM is phase-changed to DLP-PSFM under reducing conditions. Cobalt together with iron can undoubtedly be in situ exsolved in the form of alloy. In the meantime, the powders of DLP-PSFM with Co−Fe alloy nanoparticles were analyzed using EDX line scans by crossing two nanoparticles to quantitatively describe the variations in the 761

DOI: 10.1021/acscatal.5b02296 ACS Catal. 2016, 6, 760−768

Research Article

ACS Catalysis

Figure 2. Thermogravimetric analysis testing. (A) Weight loss and differential thermal analyses of cubic pervoskite P-PSCFM in a 10% H2/N2 reducing atmosphere. (B) Redox cycling ability test of cubic pervoskite P-PSCFM, conducted first in a 10% H2/N2 reducing atmosphere in the ascending temperature range and then switched to air in the descending temperature range.

Figure 1. (A) X-ray diffraction patterns of cubic P-PSCFM material (A1) before and (A2) after sintering at 900 °C in a 10% H2/N2 atmosphere for 2 h. (B) EDX line scan of the exsolved Co−Fe alloy nanoparticles.

a reducing atmosphere at 900 °C for 2 h changed to a doublelayered perovskite structure (Pr0.4Sr0.6)3(Fe0.85Mo0.15)2O7,19 which is in accordance with the chemical formula A(n+1)BnO(3n+1) (A3B2O7, when n = 2). The DLP-PSFM was exactly built of a perovskite structure layer of [(Pr0.4Sr0.6) (Fe0.85Mo0.15)O3−δ]2 and a rock salt layer of (Pr0.4Sr0.6)O, and the surface termination of the DLP-PSFM with Co−Fe alloy nanoparticles was a (Pr0.4Sr0.6)O layer. Druce et al. showed that the A-site cation preferentially located at the surface with segregation of the AO acceptor substituent in the doublelayered perovskite, suggesting that AO surface termination is energetically favorable for double-layered perovskite-structured materials and the AO surface shows an overwhelming tendency to terminate in an AO plane.31 Recently, it has been determined by Meilin Liu et al., with X-ray absorption fine structure (XAFS) experiments, that cobalt ion is initially reduced to metal, where the Co:Fe mole ratio in the alloy is 59:41.29 To further identify the surface concentration of the Co−Fe alloy, the powders of DLP-PSFM with Co−Fe alloy nanoparticles were analyzed using EDX spot scans to quantitatively measure the concentrations of Co and Fe. Results of EDX spot analysis of the selected zone of the Co−Fe alloy nanoparticle showed that the Co:Fe surface concentration (mole ratio) in the alloy is 44:56. The thermogravimetric analyses showed that the phase change of single cubic P-PSCFM to DLP-PSFM under

concentration of each element (Pr, Sr, Co, Fe, and Mo), and the results are presented in Figure 1B. As shown in the figure, the magnitudes of the Co and Fe signals increased sharply over those of the two selected nanoparticles, indicating that the exsolved particles should be metallic Co−Fe alloy. Thermogravimetric tests of the cubic P-PSCFM were conducted in different gas atmospheres, as indicated in Figure 2. To detect the phase change, the weight losses were measured and the differential thermal analysis of cubic pervoskite PPSCFM was performed in a 10% H2/N2 reducing atmosphere with a temperature range of 300−900 °C, as shown in Figure 2A. The phase change of single cubic P-PSCFM to DLP-PSFM was confirmed by differential thermal analysis, which clearly shows the presence of a sharp exothermic peak (★) in Figure 2A when annealing was conducted at ∼490 °C. The phase change process continued until the temperature reached ∼570 °C where a second exothermic peak was produced. It is wellknown that double-layered perovskite possesses a particular layered structure comprised of m (m = 1, 2, 3, ...) layers. ABO3 cubic perovskite units are sandwiched by rock salt AO layers and can be expressed with the chemical formula A(n+1)BnO(3n+1) (n = 1, 2, 3, ...).30 The phase structure of P-PSCFM sintered in 762

DOI: 10.1021/acscatal.5b02296 ACS Catal. 2016, 6, 760−768

Research Article

ACS Catalysis

Figure 3. SEM images of perovskite P-PSCFM powders (A) before and (B) after reduction in a 10% H2/N2 atmosphere at 900 °C for 2 h. Brightfield TEM images of cubic P-PSCFM powders (C) before and (D) after reduction in a 10% H2/N2 atmosphere at 900 °C for 2 h. High-resolution TEM images and corresponding fast Fourier-transformed patterns of (E) P-PSCFM powders and (F) DLP-PSFM powders.

ramping heating process in air, and no weight loss occurred when the temperature was continuously increased from 250 to 450 °C. The weight loss of P-PSCFM occurring in a single sharp step in the range of 490−570 °C under reducing conditions can be attributed to oxygen vacancy formation or a decrease in oxygen content.32 The weight loss of ∼2.72 wt % between 490 and 900 °C agrees well with the nonstoichiometric oxygen value (δ = 0.37) of the phase transformation. In addition, the sharp exothermic peak in Figure 2A marked with a star further confirms that oxygen vacancies were formed in P-PSCFM under reducing conditions. After the temperature had reached 900 °C, the gas was switched to air and the temperature was then decreased to measure the oxidation ability of DLP-PSFM. Apparently, the DLP-PSFM anode material shows a prodigious weight gain during the cooling process, especially at temperatures from 900 to 490 °C, which may be attributed to the reoxidation of the DLP-PSFM to the P-PSCFM accompanied by an increase in oxygen content. Furthermore, the weight change percentage versus temperature of the DLP-PSFM under oxidizing conditions is well in line with the weight change percentage versus temperature of P-PSCFM under reducing conditions below 490 °C, which indicates a good redox cycling ability. SEM images of the as-obtained porous cubic P-PSCFM before and after the reduction in a 10% H2/N2 atmosphere at 900 °C for 2 h are shown in panels A and B of Figure 3, respectively. It is well-known that a uniform coating of nanoscale metal or alloy particles on the surface of the anode material backbone is very crucial for the anode catalytic activity and coking tolerance in SOFCs with hydrocarbon as the fuels.33 Figure 3B shows that after exposure in 10% H2/N2 gas at 900

Table 1. Thermodynamic Data of (A) Nonoxidative Ethane Dehydrogenation and (B) Oxidative Ethane Dehydrogenation from HSC Software (A) Nonoxidative Dehydrogenation: C2H6(g) = C2H4(g) + H2(g) T (°C)

ΔH (kJ)

ΔG (kJ)

equilibrium constant K

600 143.721 19.894 700 143.857 13.184 750 143.952 6.467 (B) Oxidative dehydrogenation: C2H6(g) + 0.5O2(g) =

7.49 × 10−2 1.96 × 10−1 4.68 × 10−1 C2H4(g) + H2O(g)

T (°C)

ΔH (kJ)

ΔG (kJ)

equilibrium constant K

reversible potential E (V)

650 700 750

−103.497 −103.706 −103.938

−177.05 −181.03 −184.99

1.04 × 1010 5.22 × 109 2.79 × 109

0.918 0.938 0.959

reducing conditions with in situ formation of the Co−Fe alloy was quite consistent with the XRD pattern. Moreover, the cubic pervoskite P-PSCFM redox cycling ability was tested. First, the test was conducted in a 10% H2/N2 reducing atmosphere in the ascending temperature range from 20 to 900 °C and then switched to air in the descending temperature range from 900 to 20 °C. It clearly showed that P-PSCFM could be phaseconverted first to DLP-PSFM and then oxidized back to PPSCFM after being exposed to air in the descending temperature range, indicating an excellent cyclic redox ability of this anode material. Meanwhile, the oxygen content decrease in the cubic pervoskite P-PSCFM under reducing conditions was estimated according to Figure 2B. The weight loss (∼0.32%) below 250 °C can be ascribed to the desorption of H2O under the 763

DOI: 10.1021/acscatal.5b02296 ACS Catal. 2016, 6, 760−768

Research Article

ACS Catalysis

Figure 4. Electrochemical performances of the PC-SOFC with DLP-PSFM anode material. (A) Current−voltage curves and corresponding power densities with C2H6 as fuel at different temperatures. (B) Comparison of the current−voltage curves and the corresponding power densities with C2H6 and H2 as fuel at 750 °C. Experimental and simulated EIS of a PC-EFC with (C) C2H6 and (D) H2 as fuel at 650, 700, and 750 °C. The flow rate of ethane was 100 mL min−1, and the cathode was exposed to air. The filled symbols reflect measured results, and the lines represent the simulated results using the equivalent circuit inserted in the plot.

very glabrous and devoid of attached particles. In Figure 3F, local regions b and c of DLP-PSFM are magnified to investigate the interplanar spacing of nanoparticles. The lattice space between the two parallel planes of the Co−Fe alloy nanoparticles is 0.212 nm (Figure 3F), which is closely in accordance with the calculated value of 0.202 nm in (1 1 0) planes of the Co−Fe alloy in space group Pm3̅m (221). Meanwhile, for the DLP-PSFM backbone, the distance between the two parallel planes is 0.288 nm, which corresponds to the lattice constant of (1 0 3) planes with an angle of 15.7°, consistent with the value of 0.284 nm determined by XRD calculation. In the partially enlarged drawing (a) of the PPSCFM substrate shown in Figure 3E, the interplanar spacing between two parallel planes is 0.376 nm, much closer to the value of 0.386 nm for the lattice constant of (1 0 0) planes calculated from the XRD pattern with Bragg’s law. Currently, steam cracking technology still plays a vital role in ethylene production. Data for thermodynamic parameters of (A) nonoxidative and (B) oxidative ethane dehydrogenation to ethylene using enthalpy, entropy, and heat capacity (HSC) software are listed in Table 1. As mentioned earlier, steaming cracking suffers from several problems. (1) The reaction is reversible, highly endothermic, and severely limited by thermodynamic equilibrium.4 (2) Large amounts of CO and CO2 are released in the steam cracking process because of the presence of oxygen sources. (3) The readily produced acetylene

Table 2. Results of an Equivalent Circuit Analysis Based on the EIS of a Single Cell (PC-EFC) with H2 and C2H6 as Fuel at Different Temperatures fuel gas H2

C2H6

temperature (°C)

Rs (Ω cm2)

R1 (Ω cm2)

R2 (Ω cm2)

RP (Ω cm2)

650 700 750 650 700 750

0.573 0.479 0.392 0.661 0.569 0.423

0.128 0.074 0.045 0.212 0.137 0.060

0.326 0.144 0.097 0.553 0.271 0.139

0.454 0.218 0.142 0.765 0.408 0.199

°C for 2 h, the morphology of the P-PSCFM had changed, and nanoscale Co−Fe alloy particles were exsolved in situ and uniformly coated on the DLP-PSFM substrate. Bright-field TEM images of the cubic P-PSCFM powders before and after the reduction are presented in panels C and D of Figure 3, respectively. The diameter of Co−Fe alloy nanoparticles is ∼20 nm. The morphologies of P-PSCFM and DLP-PSFM associated with in situ exsolved Co−Fe alloy nanoparticles were further studied with a high-resolution TEM together with fast Fourier transformation. Figure 3D shows one of the Co−Fe alloy nanoparticles that were distributed on the DLP-PSFM backbone. In contrast, the P-PSCFM substrate (Figure 3C) is 764

DOI: 10.1021/acscatal.5b02296 ACS Catal. 2016, 6, 760−768

Research Article

ACS Catalysis

hydrogen was then selectively oxidized to proton at elevated temperatures. The peak power densities of the DLP-PSFM/ BCZY/LSCF single cells with DLP-PSFM anode material in C2H6 reached 208.9 and 348.8 mW cm−2 with corresponding current densities of 0.38 and 0.66 A cm−2 at 700 and 750 °C, respectively. In the meantime, the electrochemical performances of the PC-EFC with DLP-PSFM anode material using H2 and C2H6 as fuels were also compared, as shown in Figure 4B. The open circuit voltages (OCVs) of the PC-EFC were 1.005 and 0.962 V at 750 and 650 °C, respectively. They were slightly higher than the calculated values of the thermodynamic reversible potentials [E°(T) = 0.959 V at 750 °C and 0.918 V at 650 °C] of the ethane electrochemical dehydrogenation (eq 1). This can be attributed to the second term in eq 2 where different partial pressures of gas species result in a negative value because the ethane dehydrogenation is an expansion reaction. Values of E°(T) were determined from ΔG°(T) (eq 3), which was obtained from HSC software. C2H6(g) +

1 O2 (g) = C2H4(g) + H 2O(g) 2

E = E°(T ) −

E°(T ) = −

⎛ ⎞ RT ⎜ PH2OPC2H4 ⎟ ln⎜ 2F ⎝ PC H PO 1/2 ⎟⎠ 2 6 2

ΔG°(T ) 2F

(1)

(2)

(3)

Figure 4B shows that the obtained typical peak power densities with C2H6 as the fuel are slightly lower than those with H2 as the fuel, indicating a slower C2H6 oxidation kinetics compared with that of H2 on the DLP-PSFM anode material. This may be because the protons ionized from C2H6 traveling to the bulk proton-conducting electrolyte encounter a transportation resistance larger than that of the protons ionized from H2.34 EIS results of the PC-EFC with DLP-PSFM anode material and LSCF cathode material in C2H6 or H2 fuel under a stable OCV condition at different temperatures, together with simulated results based on the inserted equivalent circuit, are shown in panels C and D of Figure 4, respectively. It can be seen that the measured results are inconsistent with the simulated ones, indicating that the employed equivalent circuit model is suitable for the PC-EFC. At different temperatures, the total resistance in the PC-EFC with DLP-PSFM employing C2H6 as the fuel (Figure 4C) was slightly larger than the resistance when H2 was used (Figure 4D); this agrees well with the hypothesis that oxidation kinetics of C2H6 is slower than that of H2 on the DLP-PSFM anode material.34 Table 2 summarizes the values of the parameters of the equivalent circuit, obtained by simulating the circuit with EIS data at the different temperatures. It is well-known that the first loop corresponds to the R-CPE model of the anode activation kinetics (R1) while the second loop corresponds to the R-CPE model of the cathode activation kinetics (R2). Obviously, R1 decreases significantly from 0.212 to 0.060 Ω cm2 with an increase in temperature, whereas the low-frequency arc in Figure 4C shows that R2 decreases more dramatically from 0.553 Ω cm2 at 650 °C to 0.139 Ω cm2 at 750 °C. In the meantime, the real-axis intercept corresponds to the ohmic resistance of the fuel cell mainly contributed by the resistance from ionic transportation in the electrolyte, which is denoted as Rs. Table 2 shows that Rs changes slightly from 0.661 to 0.423

Figure 5. Product analysis of the PC-EFC with DLP-PSFM anode material. (A) Ethane conversion, ethylene selectivity, and ethylene yield tested at the corresponding peak current density load vs temperature. (B) Comparison of ethylene yield with and without corresponding peak current density load vs temperature. The flow rate of ethane is 100 mL min−1, and the cathode is exposed to air.

poisons the catalysts. (4) The high-temperature operation results in low-value byproducts. The oxidative dehydrogenation of ethane to ethylene with a PC-SOFC, on the other hand, can prevent the problems mentioned above. In a PC-SOFC, the maximal nonmechanical work (electrical work) done by the system is equal to the change in the Gibbs free energy. The data for the thermodynamic parameters with ethane as the fuel in a PC-EFC are listed in Table 1B, which make it possible to evaluate the performance of a single cell. A PC-EFC with a high electrocatalytic composite anode material is not only an energy conversion device that directly makes use of the “‘hydrogen energy”’ stored in C2H6 gas without CO2 emission but also a highly efficient reactor for coproducing ethylene with high selectivity and yield.11,12 To study the electrocatalytic activity of the DLP-PSFM anode material, the electrochemical performance of a PC-EFC with DLP-PSFM anode material was investigated, and the results are shown in Figure 4. Figure 4A presents the typical current−voltage curves and the corresponding power densities at different temperatures of a single BCZY electrolytesupported PC-EFC with DLP-PSFM anode material and LSCF cathode material, when C2H6 as the fuel was fed through the anode compartment at a flow rate of 100 mL min−1 and the cathode was exposed to ambient air. Ethane at the anode compartment was partially dehydrogenated to ethylene, and 765

DOI: 10.1021/acscatal.5b02296 ACS Catal. 2016, 6, 760−768

Research Article

ACS Catalysis

Figure 6. (A) Short-term stability of the PC-EFC with DLP-PSFM anode material under a constant current load of 0.65 A cm−2 at 750 °C; the flow rate of ethane is 100 mL min−1, and the cathode is exposed to air. (B) SEM image of the DLP-PSFM/GDC/BCZY interface after the stability test. (C) SEM image of the anode cross section. (D) Raman spectra collected from (E1) the Ni/BCZY anode surface and (E2) DLP-PSFM with a Co− Fe alloy/BCZY anode surface after the stability test. (E) XRD patterns for DLP-PSFM with a Co−Fe alloy/BCZY anode surface after the stability test, DLP-PSFM with Co−Fe alloy powders, and BCZY powders.

Ω cm2 when the temperature increases from 650 to 750 °C. Both the high peak power density and the low polarization resistances are due to the in situ exsolved Co−Fe alloy nanoparticles that were homogeneously coated on the porous DLP-PSFM backbone after exposure of P-PSCFM in a 10% H2/N2 atmosphere at 900 °C for 2 h. The products collected in the anode compartment of the PCEFC with the DLP-PSFM catalyst were analyzed with on-line GC after the reaction became stable. Figure 5A displays the variations of ethane conversion, ethylene selectivity, and ethylene yield in the PC-EFC at various temperatures. It reveals that ethane conversion significantly increased from 13.5

to 45.4%, in accordance with the ethylene yield that increased from 13.2 to 41.5% as the temperature increased from 650 to 700 °C and then 750 °C. However, the corresponding selectivity of ethylene slightly decreased from 97.8 to 91.4% because the side reactions resulted from the increased temperature. The main byproduct detected in the anode exhaust stream of the PC-EFC with DLP-PSFM anode material was methane, and no CO2 greenhouse gas was generated. However, a trace amount of CO was detected in the gas mixture when the temperature reached >700 °C, a result of the slight oxide ion conductivity in the BCZY electrolyte.35,36 In the meantime, the ethylene yield was increased slightly when a peak 766

DOI: 10.1021/acscatal.5b02296 ACS Catal. 2016, 6, 760−768

Research Article

ACS Catalysis

DLP-PSFM with a Co−Fe alloy is stable at 750 °C in an ethane atmosphere.

current load was applied at all different temperatures tested (Figure 5B), indicating that an applied peak current load during PC-EFC testing can improve the ethane partial dehydrogenation ability, and electrochemical reaction of the single cell can contribute to a relatively higher ethylene yield, in contrast to the one without peak current load. To assess the coking resistance and short-term stability of the PC-EFC with DLP-PSFM anode material, the cell potential of a single cell was recorded as a function of time under a constant current load of 0.65 A cm2 in C2H6 at 750 °C, as described in Figure 6A. Carbon deposition normally builds up on a conventional Ni-based catalyst in the form of carbon fibers in a PC-EFC directly fed with hydrocarbon fuels.37 However, under the applied peak current load for >100 h, the potential of the single cell PC-EFC with DLP-PSFM anode material showed no obvious degradation when C2H6 was supplied to the anode compartment, indicating the excellent resistance of the anode to the carbon deposition. Figure 6B depicts the interfacial microstructure of the porous DLP-PSFM anode material and the dense BCZY electrolyte after a 100 h stability test at 750 °C. It can be seen that the thickness of the porous DLP-PSFM anode is ∼15 μm, and the adhesion of the anode and the electrolyte is strong and stable after this short-term stability test. Meanwhile, the anode cross section of the PCEFC with the DLP-PSFM anode material was analyzed with a high-resolution SEM. As we can see, some particles shown in Figure 6C are Co−Fe nanoparticles exsolved from the DLPPSFM backbone, and the particles without the exsolved Co−Fe alloy are BCZY, because the anode material slurry was prepared by combining DLP-PSFM and BCZY in a weight ratio of 1:1 to further increase triple-phase boundaries. Besides, there is no visible carbon deposition on the surface of anode materials. To further verify if there was any carbon deposition formed during the process of ethane partial dehydrogenation to ethylene, the surfaces of the DLP-PSFM with a Co−Fe alloy/ BCZY anode and a Ni/BCZY anode, after being operated at a constant current load of 0.65 A cm−2 in C2H6 fuel at 750 °C for 100 h, were analyzed via ex situ Raman spectroscopy. Figure 6D2 shows that the Raman spectrum for the DLP-PSFM with a Co−Fe alloy/BCZY anode surface after the stability test did not have any notable carbon peaks. However, for the Ni/BCZY anode surface, two typical carbon features located at 1338.37 and 1568.22 cm−1 were observed in the Raman spectra (Figure 6D1) after the stability test under the same condition. The G band centered around 1568.22 cm−1 corresponds to C−C stretching that is common to all sp2-bonded carbons, while the D band centered around 1338.37 cm−1 is a defect-induced Raman feature. The results confirm that the DLP-PSFM with a Co−Fe alloy anode has an excellent coking resistance toward ethane conversion and can be potentially utilized as the anode for direct hydrocarbon-fueled SOFCs. To investigate the chemical stability of the DLP-PSFM with a Co−Fe alloy in an ethane atmosphere, the DLP-PSFM anode surface of the single cell has been characterized using XRD after the PC-EFC was operated under a constant current load of 0.65 A cm−2 in C2H6 at 750 °C for 100 h. As shown in Figure 6E, the XRD patterns for the DLP-PSFM anode surface (Figure 6E1) matched well with those of DLP-PSFM with Co−Fe alloy powders (Figure 6E2) and BCZY powders (Figure 6E3). This clearly suggests that DLP-PSFM with a Co−Fe alloy/BCZY anode remained unchanged under a constant current load of 0.65 A cm−2 in C2H6 fuel at 750 °C for 100 h, implying that

4. CONCLUSIONS Double-layered perovskite (Pr0.4Sr0.6)3(Fe0.85Mo0.15)2O7 (DLPPSFM) with uniformly dispersed in situ exsolution of Co−Fe alloy nanoparticles was prepared by annealing porous cubic perovskite Pr0.4Sr0.6Co0.2Fe0.7Mo0.1O3−δ (P-PSCFM) in a 10% H2/N2 reducing atmosphere at 900 °C. The P-PSCFM was transformed first to DLP-PSFM under reducing conditions, and the DLP-PSFM was then oxidized back to the starting material (P-PSCFM) after being exposed to air at elevated temperatures. This indicates that P-PSCFM possesses an excellent redox cycling ability. A proton-conducting ethane fuel cell (PC-EFC) with the DLP-PSFM anode demonstrated the high ethane partial dehydrogenation activity, enhanced electrocatalytic activity, and good stability without engendering CO2 emission. In the meantime, a single cell reached high output power densities of 496.2 mW cm−2 in H2 and 348.84 mW cm−2 in C2H6 at 750 °C. Ethylene yields of 13.2% at 650 °C and 41.5% at 750 °C with a high ethylene selectivity of >91% were achieved. Moreover, a single PC-EFC under a constant current load of 0.65 A cm−2 reached a stable power output at 750 °C in C2H6, indicating an excellent coking resistance. A PC-EFC with DLP-PSFM anode material successfully demonstrated the cogeneration of electricity with high power output and ethylene with a high yield in a directly ethane-fueled proton-conducting SOFC.

■

AUTHOR INFORMATION

Corresponding Author

*Telephone: +1 780 492 2232 . E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■ ■

ACKNOWLEDGMENTS This work was supported by the Natural Sciences and Engineering Research Council of Canada. REFERENCES

(1) True, W. R. Oil Gas J. 2013, 111, 90−95. (2) True, W. R. Oil Gas J. 2012, 110, 78−84. (3) Ren, T.; Patel, M.; Blok, K. Energy 2006, 31, 425−451. (4) Azizi, Y.; Petit, C.; Pitchon, V. J. Catal. 2008, 256, 338−344. (5) Towfighi, J.; Sadrameli, M.; Niaei, A. J. Chem. Eng. Jpn. 2002, 35, 923−937. (6) Stambouli, A. B. Renewable Sustainable Energy Rev. 2011, 15, 4507−4520. (7) Alcaide, F.; Cabot, P. L.; Brillas, E. (eBooK), Trends in electrochemistry and corrosion at the beginning of the 21st century, University of Barcelona, Barcelona, 1a part; 2004, 141−165. (8) Alcaide, F.; Cabot, P. L.; Brillas, E. J. Power Sources 2006, 153, 47−60. (9) Wiyaratn, W. Eng. J. 2010, 14, 1−14. (10) Fu, X. Z.; Luo, J. L.; Sanger, A. R.; Xu, Z. R.; Chuang, K. T. Electrochim. Acta 2010, 55, 1145−1149. (11) Fu, X. Z.; Luo, X. X.; Luo, J. L.; Chuang, K. T.; Sanger, A. R.; Krzywicki, A. J. Power Sources 2011, 196, 1036−1041. (12) Fu, X. Z.; Lin, J. Y.; Xu, S. H.; Luo, J. L.; Chuang, K. T.; Sanger, A. R.; Krzywicki, A. Phys. Chem. Chem. Phys. 2011, 13, 19615−19623. (13) McIntosh, S.; Gorte, R. Chem. Rev. 2004, 104, 4845−4866. (14) Tao, S.; Irvine, J. T. Nat. Mater. 2003, 2, 320−323. (15) Liu, Q.; Dong, X. H.; Xiao, G. L.; Zhao, F.; Chen, F. C. Adv. Mater. 2010, 22, 5478−5482. 767

DOI: 10.1021/acscatal.5b02296 ACS Catal. 2016, 6, 760−768

Research Article

ACS Catalysis (16) Ding, D.; Liu, Z. B.; Li, L.; Xia, C. R. Electrochem. Commun. 2008, 10, 1295−1298. (17) Nuansaeng, S.; Yashima, M.; Matsuka, M.; Ishihara, T. Chem. Eur. J. 2011, 17, 11324−11331. (18) Du, X.; Zou, G. J.; Zhang, Y.; Wang, X. L. J. Mater. Chem. A 2013, 1, 8411−8416. (19) Zhang, L.; Yang, C. H.; Frenkel, A.; Wang, S. W.; Xiao, G. L.; Brinkman, K.; Chen, F. L. J. Power Sources 2014, 262, 421−428. (20) Mirzaei, A. A.; Galavy, M.; Youssefi, A. Fuel Process. Technol. 2010, 91, 335−347. (21) Lu, Z. G.; Zhu, J. H.; Bi, Z. H.; Lu, X. C. J. Power Sources 2008, 180, 172−175. (22) de la Peña O'Shea, V. A.; Á lvarez-Galván, M. C.; CamposMartín, J. M.; Fierro, J. L. G. Appl. Catal., A 2007, 326, 65−73. (23) Tihay, F.; Roger, A. C.; Kiennemann, A.; Pourroy, G. Catal. Today 2000, 58, 263−269. (24) Cabet, C.; Roger, A. C.; Kiennemann, A.; Läkamp, S.; Pourroy, G. J. Catal. 1998, 173, 64−73. (25) Martínez-Coronado, R.; Alonso, J. A.; Aguadero, A.; FernandezDiaz, M. T. J. Power Sources 2012, 208, 153−158. (26) Zuo, C. D.; Zha, S. W.; Liu, M. L.; Hatano, M.; Uchiyama, M. Adv. Mater. 2006, 18, 3318−3320. (27) Yang, L.; Liu, Z.; Wang, S. Z.; Choi, Y. M.; Zuo, C. D.; Liu, M. L. J. Power Sources 2010, 195, 471−474. (28) Yang, Z. B.; Yang, C. H.; Jin, C.; Han, M. F.; Chen, F. L. Electrochem. Commun. 2011, 13, 882−885. (29) Yang, C. H.; Li, J.; Lin, Y.; Liu, J.; Chen, F. L.; Liu, M. L. Nano Energy 2015, 11, 704−710. (30) Du, X. R.; Zou, G. J.; Zhang, Y.; Wang, X. L. J. Mater. Chem. A 2013, 1, 8411−8416. (31) Dulli, H.; Dowben, P. A.; Liou, S. H.; Plummer, E. W. Phys. Rev. B: Condens. Matter Mater. Phys. 2000, 62, R14629−R14632. (32) Sengodan, S.; Choi, S.; Jun, A.; Shin, T. H.; Ju, Y. W.; Jeong, H. Y.; Shin, J.; Irvine, J. T. S.; Kim, G. Nat. Mater. 2014, 14, 205−209. (33) Neagu, D.; Tsekouras, G.; Miller, D. N.; Menard, H.; Irvine, J. T. S. Nat. Chem. 2013, 5, 916−923. (34) Liu, S. B.; Behnamian, Y.; Chuang, K. T.; Liu, Q. X.; Luo, J. L. J. Power Sources 2015, 298, 23−29. (35) Ricote, S.; Bonanos, N.; Marco de Lucas, M. C.; Caboche, G. J. Power Sources 2009, 193, 189−193. (36) Suksamai, W.; Metcalfe, I. S. Solid State Ionics 2007, 178, 627− 634. (37) Yang, L.; Wang, S. Z.; Blinn, K.; Liu, M. F.; Liu, Z.; Cheng, Z.; Liu, M. L. Science 2009, 326, 126−129.

768

DOI: 10.1021/acscatal.5b02296 ACS Catal. 2016, 6, 760−768