Process Investigation of a Solid Carbon-Fueled Solid Oxide Fuel Cell

Nov 10, 2015 - Fuel cells are a type of energy conversion device that can allow for the direct conversion of chemical energy stored in the fuel into e...
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Process Investigation of a Solid Carbon-Fueled Solid Oxide Fuel Cell Integrated with a CO2‑Permeating Membrane and a SinteringResistant Reverse Boudouard Reaction Catalyst Yijun Zhong,† Chao Su,*,‡ Rui Cai,*,† Moses O. Tadé,‡ and Zongping Shao‡ †

State Key Laboratory of Materials-Oriented Chemical Engineering, College of Chemistry and Chemical Engineering, Nanjing Tech University, Number 5 Xin Mofan Road, Nanjing, Jiangsu 210009, People’s Republic of China ‡ Department of Chemical Engineering, Curtin University, Perth, Western Australia 6845, Australia ABSTRACT: The process of a new type of carbon−air battery based on a solid oxide fuel cell (SOFC) integrated with a CO2permeable membrane was investigated systematically. Solid carbon fuel was modified with an excellent reverse Boudouard reaction catalyst, FemOn (active component)−K2O (promoter). Al2O3 as a sintering inhibitor was also introduced to the catalyst−carbon mixture. Two preparation techniques for catalyst-loaded carbon fuel were tried. One technique was the direct mix of all catalyst components and carbon (method A), and the other technique was the premix of all catalyst components before introducing carbon (method B). The performance of carbon−air batteries with different catalyst−carbon mixing techniques was studied by an I−V polarization test. Both techniques produced a good maximum power density of approximately 300 mW cm−2 at 850 °C. The sintering of the catalyst at high temperatures was prohibited, for the most part, using an Al2O3 support (i.e., method B). The carbon−air battery could operate continuously for 314 min at 750 °C with a specific capacity up to 672 mAh g−1 (on the basis of the solid carbon loaded into the SOFC) and a high fuel conversion of 98.7%. This work optimized the operation of carbon−air batteries and further demonstrated the feasibility of this new type of electrochemical energy device. much higher energy density than the electrodes in LIBs.18−20 On the basis of the overall cell reaction, C + O2 = CO2, solid carbon showed a theoretical energy density of 8930 mAh g−1 as a fuel for fuel cells compared to 372 mAh g−1 as the lithiumintercalation material in LIBs. Recently, we proposed a novel carbon−air battery (CAB) based on a solid oxide fuel cell (SOFC)-type reactor integrated with a CO2-permeating membrane, which has a theoretical energy density more than 10 times the state-of-the-art commercial LIBs.21 A catalyst is applied with solid carbon fuel to allow for the in situ conversion of carbon to CO through the reverse Boudouard reaction, C(s) + CO2(g) → 2CO(g), and CO serves as the direct fuel for electric power generation, while the CO2 reactant is from the electrochemical oxidation of CO. The main innovation of CABs is the integration of the high-temperature CO2-permeating membrane composed of a carbonate mixture and an O2− conducting phase, which can in situ separate CO2 from the anode chamber based on the equilibrium, CO2 + 1/2O2 + 2e− ⇌ CO32−, while keeping CO inside the anode chamber as the fuel. Therefore, the CAB can easily be minimalized for portable application, showing great potential. However, the preliminary results suggest that the CAB failed to generate power after continuous operation of 2 h. Upon verification by dissembling the CAB, the catalyst was found to have sintered with carbon, resulting in the deactivation

1. INTRODUCTION The development of innovative energy storage devices that show high energy and power density, good safety, low price, low self-discharge rate, and environmental friendliness has received remarkable attention during the past few decades as a result of the significant advances in electronics requiring better power systems, electric vehicles, and smart grids.1−3 Electrochemical energy storage plays an important role in the various types of energy storage systems.4,5 Lithium-ion batteries (LIBs) are one type of the most successful electrochemical energy storage devices that have been widely used in personal electronics.6,7 However, LIBs experience a bottleneck with the further increase in energy density, which is mainly limited by the electrode materials.8,9 The state-of-the-art electrodes are graphite (anode) and LiCoOx (cathode), which are lithiumintercalation materials showing a limited reversible capacity of 372 and 140 mAh g−1, respectively.10,11 Although considerable research efforts have been conducted to develop alternative electrode materials for LIBs, no commercial products with significantly improved energy density are available as a result of the complexity of LIBs.7,12 Fuel cells are a type of energy conversion device that can allow for the direct conversion of chemical energy stored in the fuel into electric power without the limitation posed by the Carnot cycle, thus offering a high efficiency up to 100%.13−15 Kobayashi et al. reported a rechargeable proton exchange membrane fuel-cell battery (PEM-RFCB) that employed partially oxygenated carbon as the anode material and air as the cathode material.16,17 However, the energy density of 2.5− 13.8 Wh kg−1 for the PEM-RFCBs still could not meet the need of practical applications. In comparison to LIBs, fuel cells show a much higher energy density because most of the fuels show © XXXX American Chemical Society

Special Issue: 5th Sino-Australian Symposium on Advanced Coal and Biomass Utilisation Technologies Received: September 23, 2015 Revised: November 10, 2015

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Sm0.2Ce0.8O1.9 (SDC) buffer layer was coated on the YSZ electrolyte surface. A SDC (synthesized by a hydrothermal method) suspension in absolute ethanol prepared by ball milling was sprayed onto the YSZ surface using a modified spraying gun. The cell was then sintered at 1300 °C under an air atmosphere for 5 h with a heating rate of 5 °C min−1. To deposit the cathode layer, the anode of tubular SOFCs with the YSZ electrolyte and SDC buffer layer was first reduced in a hydrogen atmosphere at 700 °C for 2 h. A high-temperature plastic mask was used during the wet powder spraying process to obtain a geometric surface area of 10 cm2 for the cathode. The cathode suspension prepared in the same way as the SDC suspension was sprayed onto the SDC buffer layer, and then the complete tubular SOFC was sintered under an Ar atmosphere at 1100 °C for 2 h with a heating rate of 5 °C min−1. 2.2.2. CO2-Permeating Membrane. A SDC/molten carbonate dualphase membrane was integrated into the tubular cell for in situ CO/ CO2 separation also by the method reported in our previous work.21 The carbonate mixture was composed of Li2CO3, Na2CO3, and K2CO3 (Sinopharm Chemical Reagent Co., Ltd., Shanghai, China; analytical grade) at the weight ratio of 1.0:1.1:1.1, which was mixed with the SDC powder [synthesized by an ethylenediaminetetraacetic acid (EDTA)−citrate complexing method] in an absolute ethanol medium at a weight ratio of 4:1 by ball milling at 400 rpm for 40 min to obtain a slurry. The slurry was then dried in the oven at 60 °C to form a solid powder. The as-obtained mixture powder was packed into a stainlesssteel mold (Ø, 20 mm) and pressed into disk-shape tablets under a hydraulic pressure of 200 MPa. The disks were then sintered under an air atmosphere at 900 °C for 5 h. During this process, most of the carbonate ran out from the SDC tablet but left abundant pores. Finally, the mixture of Li2CO3, Na2CO3, and K2CO3 was introduced to the sintered porous SDC tablet again, followed by sintering at 520 °C for 30 min to absorb molten carbonate to form the SDC/molten carbonate dual-phase membrane. 2.2.3. CAB Assembly. Each tubular SOFC was loaded with 1−2 g of FeK/C + Al or FeK/Al + C catalyst−carbon fuel into the anode chamber. Then, the SDC/molten carbonate dual-phase membrane was sealed onto the open top of the tubular SOFC using silver paste and dried at 180 °C for 30 min. A CAB with a dual-phase membrane was obtained. The silver paste as the current collector was drawn as a mesh-like pattern onto the surface of the cathode. The total weight of the CAB is approximately 15 g, containing the weight of the cell, the catalyst-loaded carbon fuel, the SDC/molten carbonate membrane, and the silver current collector. 2.3. Characterization. The morphologies of the as-prepared catalysts and carbon fuels were analyzed by a field emission scanning electron microscope (FESEM, Hitachi S-4800). The specific surface area of the active carbon was measured using a nitrogen adsorption− desorption instrument (BELSORP-mini, Japan) at 77 K and calculated using the Brunauer−Emmett−Teller (BET) equation. For the electrochemical performance test, the CAB was located vertically in a split tubular furnace, which was programmatically heated to 850 °C at a heating rate of 10 °C min−1. During the test, the cathode of the tubular SOFC was exposed to the ambient atmosphere. The I−V polarization measurement was performed from 850 to 725 °C with a scan rate of 50 mA per step using a digital source meter (Keithley 2440) with a four-probe configuration. To obtain the t−V curves of the CAB, it was tested by a continuous polarization at 750 °C under a certain current (between 0 and 1600 mA) until a stable voltage response was achieved.

of the catalyst for the gasification of solid carbon to CO. The mismatch in the CO2 formation rate over the anode, the CO2 consumption rate of the catalyst, and the membrane permeation flux could have a significant impact on the performance and operational stability of the CAB. In this study, we aimed at investigating the process of this innovative CAB during operation. The performance of the CAB was optimized by the improvement of the catalyst, the catalyst− carbon mixing technique, and the operational mode. Under the optimized conditions, the CAB can be operated continuously for more than 10 h.

2. EXPERMENTAL SECTION 2.1. Preparation of Catalyst-Loaded Carbon Fuels. Catalystloaded carbon fuels were prepared by two different methods in this study. One method was named “FeK/C + Al”, prepared by “method A”, which was similar to our previous work.21,22 Amorphous Al2O3 was introduced as a sintering inhibitor to improve the ability of the catalyst to resist thermal deactivation during battery operation at high temperatures. A mass ratio of Fe2O3/Al2O3 = 1:8 was chosen in this study, while the mass ratio of Fe2O3/C was in accordance with our previous study.21,22 For a typical preparation, 9.7 g of Fe(NO3)3·9H2O and 2.43 g of KNO3 were first dissolved in 200 mL of deionized water and then 15.33 g of Al2O3 and 20 g of active carbon (Sinopharm Chemical Reagent Co., Ltd.) were added to the solution, followed by vigorous stirring for 12 h at room temperature. The as-obtained slurry was dried in an electric oven at 60 °C for 24 h and then ground with an agate mortar. The powder was further calcined at 700 °C for 2 h under Ar at a flow rate of 200 mL min−1 to obtain FeK/C + Al. The other method was denoted as “FeK/Al + C”, prepared by “method B”. Amorphous Al2O3, FemOn, and K2O were used as the catalyst support, active component, and promoter, respectively. Typically, the same amount of Fe(NO3)3·9H2O, KNO3, deionized water, and Al2O3 were mixed well to form a slurry, followed by the drying and grinding process, as mentioned above. Then, the powder was calcined at 700 °C for 2 h under an air atmosphere to obtain the pristine FeK/Al catalyst without active carbon. The FeK/Al catalyst was then mixed with 20 g of active carbon in absolute ethanol with the help of a high-energy ball miller (Pulverisette 6, Fritsch, Germany) at 400 rpm for 40 min and dried at 60 °C for 12 h. The as-prepared FeK/ Al + C was ready for later tests. For a reasonable comparison, both the FeK/C + Al and FeK/Al + C catalyst−carbon fuels contain the same amount of active carbon (52.12 wt %) and catalyst components (47.88 wt %). 2.2. Fabrication of CABs. 2.2.1. One-End Tubular SOFC Reactor. One-end tubular anode-supported SOFCs were fabricated by the method reported in our previous work.21 The anode support was fabricated using a conventional slip casting method. Typically, NiO (Chengdu Shudu Nano-Science Co., Ltd., China) and 8 mol % yttriastabilized zirconia (YSZ, Tosoh, Japan) were mixed at the weight ratio of 60:40 with deionized water to obtain a slurry. A solid NiO + YSZ layer was formed slowly on the inner wall of a plaster mold after adding the slurry to the mold. The required thickness of the NiO + YSZ wall was reached when the excess slurry was poured out. Then, the mold was put into an electric oven at 60 °C for 6 h. The green tubular anode support separated from the mold spontaneously during the drying process. Finally, the tubular anode support was sintered at 1100 °C for 2 h in air. To prepare the electrolyte layer, YSZ powder, triethanolamine (TEA), polyvinyl butyral (PVB), and absolute ethanol were mixed using the high-energy ball mill at 250 rpm for 1 h. Then, the asprepared tubular anode support was dipped into the YSZ suspension for 10 s, followed by drying at room temperature. The dip-coating process was repeated 5 times. Then, the anode support with the YSZ electrolyte was sintered under an air atmosphere at 1400 °C for 5 h with a heating rate of 5 °C min−1 to obtain a dense YSZ electrolyte. To avoid the interfacial reaction between the YSZ electrolyte and Ba0.95La0.05FeO3−δ (BLF) cathode layers at high temperatures, a

3. RESULTS AND DISCUSSION 3.1. Cell Initialization. The electrochemical oxidation of fuel in the fuel cell proceeded mainly over the triple-phase boundary (TPB), where the fuel, the anode, and the electrolyte meet. By applying mixed conducting electrodes, the active zone for the fuel oxidation was located typically in the approximately 15 μm thick anode layer nearest the electrolyte interface.23−25 Because of the special configuration of the fuel cell reactor used B

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Energy & Fuels in this study, as shown in Figure 1, the carbon fuel was far from the active zones of the anode layer for direct electro-oxidation

Figure 2. Temperature dependence of the OCV for CAB 1 fueled by the FeK/C + Al catalyst−carbon fuel.

catalytic activity of the catalyst toward the carbon conversion. An OCV of 0 V was observed for the current CAB below 370 °C. With the increase in the temperature, a sharp increase in the OCV was exhibited and the first peak OCV of approximately 1.0 V was reached at approximately 490 °C. Then, a slight decrease in the OCV of 0.75 V was noted with the further increase in the temperature to 590 °C. An increase in the OCV then occurred again, reaching approximately 1.0 V at 800 °C. By flowing carbon with CO2 gas, Liu et al. observed a similar trend in the OCVs,34 suggesting that the reaction mechanism is similar in both cell configurations; i.e., the oxidation of carbon for power generation occurs via the indirect way of CO. Because no CO2 was introduced externally into the CAB reactor in the current study and the CAB was assembled in an ambient air atmosphere, the initial PO2 in the atmospheres at both the cathode and anode sides should be the same. Thus, the OCV of 0 V at temperatures lower than 370 °C suggests that the catalyst had no catalytic activity for carbon oxidation. The sharp increase in the OCV at temperatures higher than 370 °C indicates that the catalyst started to be activated, and the residual oxygen in the anode chamber probably reacted with the solid carbon, leading to a reduced PO2 inside the anode chamber. The slight decrease in the OCV in the temperature range of 490−590 °C possibly implies the partial reduction of FemOn in the catalyst, resulting in an increase in the CO2 content in the anode chamber (thus, an oxygen partial pressure). At temperatures higher than 600 °C, the catalyst started to exhibit improved catalytic activity for the gasification of carbon with the formation of CO; therefore, an increase in the OCV appeared as a result of the reduced PO2. In comparison to the results of Liu et al.,30,34 the introduction of Al2O3 into the catalyst did not seem to have an obvious detrimental effect on the catalytic performance. 3.2. Power Generation. At elevated temperatures and under polarization, continuous oxygen flow from the cathode to the anode is achieved, and then the electrochemical oxidation of CO over the anode with the formation of CO2, the gasification of solid carbon to gaseous CO, and the permeation of partial CO2 from the anode chamber to the surrounding atmosphere occur simultaneously. The power generation of the CAB is strongly related to the above reactions, especially the catalytic activity of the catalyst for carbon gasification, which could greatly affect the CO formation rate and, thus, the CAB performance. To demonstrate the importance of the reverse Boudouard reaction in the current CAB, two different methods

Figure 1. Schematic diagram of the CAB.

(the anode layer has a thickness of approximately 1.5 mm). Therefore, the direct electrochemical oxidation of carbon to CO2 for power generation is less possible. Instead, the carbon is first converted to gaseous CO, which diffuses to the active zone to perform as the direct fuel for generating electricity.26,27 The successful operation of the CAB is thus strongly dependent upon the catalytic activity of the catalyst for the reverse Boudouard reaction as well as the properties of carbon fuel. Here, an activated carbon fuel with a high specific surface area of 1598 m2 g−1 was used. Previously, we have used a typical FemOn−MxO (M = Li, K, and Ca) mixture as a catalyst to promote the reverse Boudouard reaction.22 However, the catalyst was easily sintered during the operation and lost the catalytic activity quickly before the full conversion of the solid carbon, thus greatly reducing the cell lifetime.28−30 Therefore, we introduced Al2O3 as a sintering inhibitor to the FemOn−K2O catalyst to increase the stability of the catalyst. As mentioned, the cell performance is closely related to the reverse Boudouard reaction over the catalyst. The cell can be initialized successfully only when the catalyst shows activity for the gasification of carbon. The cell initialization was investigated by examining the open circuit voltage (OCV) response of the CAB with respect to the operating temperature, as shown in Figure 2. For comparison, the theoretical OCVs of SOFCs operating on direct solid carbon fuel and pure CO fuel were calculated on the basis of the Nernst equation (eq 1),31−33 and the theoretical OCVs were 1.09 and 1.04 V at 800 °C, respectively 2 PCO PO RT E=E + ln 2 2 4F PCO2 0

(1) 0

where E is the OCV of SOFCs, E is the standard electromotive force, R is the gas constant (8.314 J mol−1 K−1), T is the Kelvin temperature, F is Faraday’s constant (96485 C mol−1), and PCO, PO2, and PCO2 represent the partial pressures of CO, O2, and CO2, respectively. The OCV of a fuel cell is determined by the oxygen partial pressures (PO2) of both the cathode and anode chambers. In the current CAB, the PO2 of the anode was strongly related to the C

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Figure 3. I−V polarization curves of (a) CAB 1 fueled by FeK/C + Al and (b) CAB 2 fueled by FeK/Al + C.

appeared at temperatures lower than 825 °C, while for CAB 1, such concentration polarization appeared only at the operating temperature of 725 °C (the lowest temperature investigated). On the basis of the initial I−V curves with the low current density and the linear response of the cell voltage with respect to the polarization current, the calculated CAB resistances were 1.17, 0.90, 0.87, 0.80, 0.72, and 0.65 Ω cm2 at 725, 750, 775, 800, 825, and 850 °C, respectively, for CAB 1.39 As a comparison, the resistances for CAB 2 were 1.61, 1.28, 0.98, 0.80, 0.73, and 0.68 Ω cm2 at the corresponding temperatures, demonstrating that both CABs have comparable resistances; therefore, the difference in electrochemical performance should be contributed mainly by the performance of the different catalyst−carbon mixtures. A further improvement in the catalytic activity of the Boudouard catalyst is critical to increase the power output at reduced temperatures. The reverse Boudouard reaction is thermodynamically favored at higher operating temperatures. CO2 formed from the electrochemical oxidation of CO over the anode could react quickly with solid carbon to convert into CO.40 Because 1 M CO2 would create 2 M CO, a quick buildup of pressure inside the anode chamber could be experienced. At a critical point, a crack may be formed within the soft silver paste part or the ceramic membrane to release the pressure; therefore, the fuel could be partially lost. Actually, during the performance test, a red hot point was indeed observed over the sealing part of the fuel cell at a current density of 300 mA at 850 °C, which became brighter at a larger current. A flame was even formed at a high current, and the size of the flame became larger with the increase in the polarization current. However, once the current polarization was stopped, the flame progressively reduced until it disappeared. Such a flame was clearly related to the release of CO from the anode chamber through the cracks formed over the reactor during the operation, with CO burning to form a strong flame when it met the air atmosphere. As shown in Figure 4, a hole appeared in the silver paste after the operation. Obviously, the formation rate of CO over the catalyst was too quick, resulting in the serious accumulation of CO in the anode chamber and then the increase in gas pressure. Therefore, to achieve a stable cell performance, a more efficient ceramic membrane is needed to separate CO2 to the ambient atmosphere swiftly or the polarization current of the CAB should be tailored to reduce the formation of CO2, so that the formation of CO over the catalyst matches the oxidation of CO over the SOFC anode. This suggests that reduction of the polarization current may be a good strategy.

for loading the catalyst with carbon were tried in this study. For method A, all catalyst components and carbon were mixed directly to form FeK/C + Al. For method B, the three catalyst components were premixed and then carbon was introduced to produce FeK/Al + C. Figure 3 shows the I−V polarization curves of the CABs fueled by different catalyst−carbon fuels. The operating temperatures of 725−850 °C were selected because the reverse Boudouard reaction is typically favored at temperatures higher than 700 °C.35 For the CAB that is fueled by FeK/C + Al (CAB 1), an OCV of 0.98 V was observed at 725 °C and the OCV increased slightly with the increase in the operating temperature, reaching 1.02 V at 850 °C (Figure 3a). Those values are comparable to the theoretical OCV values for the cell operating on CO fuel (1.04 V) and suggest the favorable catalytic activity for the gasification of carbon to CO at temperatures higher than 725 °C. At the temperature of 775 °C or higher, a linear response of the CAB voltage with respect to the polarization current was observed, up to the current density of 360 mA cm−2, which is the maximum current density that we can draw as a result of the large surface area of the CAB and the current limitation of the source meter. Although the peak power density (PPD) was still not reached, a maximum power density of 292 mW cm−2 was achieved at 850 °C, suggesting the good electrochemical performance of the CAB. The PPDs at different temperatures were then estimated on the basis of the slopes and the intercepts of the I−V polarization lines in the low polarization current range, where the concentration polarization did not appear. Therefore, the effect of the reverse Boudouard reaction on the cell performance was negligible. The PPDs of CAB 1 were 200, 267, 292, 327, 362, and 410 mW cm−2 at 725, 750, 775, 800, 825, and 850 °C, respectively, comparable to the values reported in the literature for applying CO fuel.36−38 Such high power outputs demonstrated the excellent electrochemical activity of the CAB for power generation operating on carbon fuel. At the lower temperature of 725 °C, however, serious concentration polarization appeared at the current density higher than 200 mA cm−2, suggesting that the formation rate of gaseous CO from the catalytic reverse Boudouard reaction was likely not sufficiently fast to meet the rate of CO electrochemical oxidation over the SOFC anode needed for producing the required amount of electricity. Thus, the concentration of CO in the anode chamber was substantially reduced, which caused an obvious decrease in cell voltage. For the CAB that was fueled by FeK/Al + C (CAB 2), as shown in Figure 3b, a serious concentration polarization D

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300 s at a large current of 1400 mA. Steady-state power generation was reached when the CO concentration inside the anode chamber reached equilibrium, as determined by the rate of CO electro-oxidation over the anode, the reverse Boudouard reaction rate, and the CO2 permeation rate of the ceramic membrane (1.79 mL min−1 cm−2 at 750 °C).21 The polarization current is directly related to the CO electrochemical oxidation rate over the anode, while the OCV is directly related to the CO concentration within the anode chamber. Assuming that the electrode polarization resistance was not changed with respect to the CO concentration inside the anode chamber, the decrease in the OCV under polarization implies a decrease in the CO concentration inside the anode chamber. In other words, the reverse Boudouard reaction was not sufficiently fast to match the CO consumption over the anode or the CO2 permeation flux of the ceramic membrane was not sufficiently high to maintain the high CO concentration inside the chamber; therefore, the CO concentration inside the anode chamber decreased, while the CO2 concentration increased, leading to the increased CO formation rate until a new balance was reached. At a current of 1600 mA, it seems that the CO formation rate over the catalyst did not meet the CO electro-oxidation rate needed to obtain the required amount of current. As a result, the CAB voltage dropped to zero, further demonstrating that the enhanced catalytic activity of the catalyst for the reverse Boudouard reaction is key to achieving the increase in power output at reduced temperatures. 3.3. Capacity Evaluation. Figure 6 shows typical voltage responses of CABs with respect to time at a constant small polarization current of 100 mA at 750 °C. As shown in Figure 6a, CAB 1 fueled by FeK/C + Al was operated stably at a voltage of 0.98 V for approximately 2 h and then a new cell voltage of approximately 0.8 V was established, implying the buildup of a new reaction equilibrium within the anode chamber. The voltage of CAB 1 was sustained for another 4.5 h and then dropped to zero within approximately 1 h, suggesting that the time of the continuous power generation for CAB 1 was 7.5 h in total. For CAB 2 fueled by FeK/Al + C, as shown in Figure 6b, the power generation was sustained for approximately 5 h at the same constant current of 100 mA. The specific capacity has often been used as an evaluation criterion for primary or secondary batteries. In this work, the specific capacity of the two CABs fueled by different catalyst−

Figure 4. Digital photo of CAB 1 after the electrochemical test.

The I−V curves reported above were performed under fast polarization, which represents the non-equilibrium condition. To obtain information about the steady-state CAB power output, CAB 2 after the I−V polarization curve test was then polarized continuously at a constant current until a stable voltage response was achieved. As shown in Figure 5, the larger

Figure 5. t−V curves of CAB 2 under different polarization currents at 750 °C.

the polarization current, the longer the time needed to reach the steady state. For example, only 50 s was required at a small current of 200 mA, while the time increased to approximately

Figure 6. Voltage responses of (a) CAB 1 fueled by FeK/C + Al and (b) CAB 2 fueled by FeK/Al + C as a function of time under a constant polarization current of 100 mA at 750 °C. E

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Energy & Fuels Table 1. Carbon Consumption and Fuel Conversion of the CABs CABs

catalyst−carbon mixture

total mass before test (g)

carbon mass before test (g)

carbon mass after test (g)

carbon consumption (g)

fuel conversion (%)

CAB 1 CAB 2

FeK/C + Al FeK/Al + C

2.89 1.49

1.51 0.78

0.33 0.01

1.18 0.77

78.14 98.72

carbon mixtures was calculated and used to assess the conversion rate of carbon fuel within the CABs. Under the same polarization current of 100 mA, the discharge capacities of CAB 1 and CAB 2 were 504 and 672 mAh g−1, respectively, indicating that the conversion rate of carbon fuel for CAB 2 is much superior to that for CAB 1. This result can be further supported by the different carbon consumptions evaluated by the mass variation before and after tests, as listed in Table 1. On the basis of the catalyst weights before and after the test, the carbon conversion of CAB 2 was 98.72%, much higher than the carbon conversion of CAB 1 (78.14%). This result suggests that method B for mixing the catalyst and carbon is beneficial for obtaining a high specific capacity of the CAB and is much more suitable for the practical application. Both CABs after the capacity test were disassembled and characterized. Figure 7 shows digital photos of the CABs and Figure 8. SEM images of (a) FeK/Al + C residual and (b) FeK/C + Al residual after the lifetime test and (c) pristine Al2O3 and (d) fresh FeK/C + Al before the test.

Figure 7. Digital photos of (a) CAB 1 and FeK/C + Al residual and (b) CAB 2 and FeK/Al + C residual after the capacity investigation.

compact covering of the carbon surface seriously reduced the active surface of carbon, and therefore, the contact between carbon and CO was blocked. As a result, the carbon conversion of FeK/C + Al was greatly inferior to the carbon conversion of FeK/Al + C.

the catalyst−carbon mixture after the disassembly. The FeK/C + Al residual (Figure 7a) showed some sintered blocks in gray, which, however, can easily be pulverized. The nature of the residual suggests that the introduction of Al2O3 indeed suppressed the sintering of the FemOn−K2O catalyst. For the FeK/Al + C residual, as shown in Figure 7b, the form of loose power was well-maintained, indicating that the catalyst sintering was prohibited completely with the help of Al2O3. Meanwhile, the color of the FeK/Al + C residual resembled that of the pure FeK/Al catalyst, signifying that the carbon fuel was fully converted and further demonstrating that the CAB with the catalyst−carbon mixture prepared by method B could achieve high fuel conversion. The morphologies of the residuals were further observed by the scanning electron microscope (SEM), as shown in Figure 8. For comparison, the SEM images of pristine Al2O3 and a fresh FeK/C + Al catalyst−carbon mixture before the electrochemical test are also provided. For the FeK/Al + C residual (Figure 8a), massive amounts of carbon could hardly be found. The FemOn active component and the K2O promoter were still well-loaded on the Al2O3 support, indicating the high conversion of carbon and the stable morphology of the FeK/ Al catalyst. For the FeK/C + Al residual (Figure 8b), the catalyst was clearly adhered compactly onto the surface of the massive carbon, which was totally different from the fresh FeK/ C + Al, as shown in Figure 8d, where the catalyst was welldispersed throughout the microporous carbon fuel. The

4. CONCLUSION In summary, we conducted a comprehensive process investigation of the new type of CAB, which is composed of the solid carbon-fueled SOFC, the CO2-permeating membrane, and the reverse Boudouard reaction catalyst. The sintering of the typical FemOn−K2O catalyst with carbon fuel was controlled perfectly by introducing an Al2O3 inhibitor, especially for the preparation technique that involved the premixing of FemOn, K2O, and Al2O3 catalysts before mixing with carbon (method B) to create FeK/Al + C, which can be proven by the still loose form of the catalyst powder after the whole test. Both of the CABs with catalyst−carbon mixtures prepared by the different mixing methods delivered the desired power output with the maximum power density of approximately 300 mW cm−2 at 850 °C. Furthermore, after the I−V and t−V curve tests, the CAB fueled by FeK/Al + C could still generate power continuously for another 5 h. Therefore, the total time of power generation for the CAB exceeded 10 h. More importantly, a remarkable fuel conversion of 98.7% at 750 °C was achieved, corresponding to the high specific capacity of up to 672 mAh g−1 (on the basis of the solid carbon loaded into the SOFC), which is superior to that of the CAB with the catalyst−carbon mixture prepared by method A (504 mAh g−1). Our investigation emphasized the superiority and feasibility of optimizing the composition of the reverse Boudouard reaction catalyst and the mixing mode with carbon fuel for the operation of the CAB. A further optimization of the ceramic membrane for CO2 separation and the CAB configuration may result in a F

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Energy & Fuels

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further increase in the power density and working stability of CABs.

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AUTHOR INFORMATION

Corresponding Authors

*Telephone: +61-8-92663761. Fax: +61-8-92662681. E-mail: [email protected]. *Telephone: +86-25-83172256. Fax: +86-25-83172242. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Key Projects in the Nature Science Foundation of Jiangsu Province under Contract BK2011030, the Major Project of the Educational Commission of Jiangsu Province of China under Contract 13KJA430004, and the Priority Academic Program Development of Jiangsu Higher Education Institutions. The authors thank the Australia Research Council for supporting the project under Contracts DP150104365 and DP160104835.

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REFERENCES

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DOI: 10.1021/acs.energyfuels.5b02198 Energy Fuels XXXX, XXX, XXX−XXX

Article

Energy & Fuels (38) Liu, Y.; Bai, Y.; Liu, J. Carbon monoxide fueled cone-shaped tubular solid oxide fuel cell with (Ni0.75Fe0.25-5%MgO)/YSZ anode. J. Electrochem. Soc. 2013, 160, F13−F17. (39) Shi, H.; Su, C.; Yang, G.; Ran, R.; Hao, Y.; Tadé, M. O.; Shao, Z. Fabrication and operation of flow-through tubular SOFCs for electric power and synthesis gas cogeneration from methane. AIChE J. 2014, 60, 1036−1044. (40) Liu, R.; Zhao, C.; Li, J.; Zeng, F.; Wang, S.; Wen, T.; Wen, Z. A novel direct carbon fuel cell by approach of tubular solid oxide fuel cells. J. Power Sources 2010, 195, 480−482.

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DOI: 10.1021/acs.energyfuels.5b02198 Energy Fuels XXXX, XXX, XXX−XXX