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Development of a tubular molten carbonate direct carbon fuel cell and basic cell performance
Journal of Power Sources xxx (xxxx) xxx
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Development of a tubular molten carbonate direct carbon fuel cell and basic cell performance Akifumi Ido *, Makoto Kawase Central Research Institute of Electric Power Industry (CRIEPI), 2-6-1, Nagasaka, Yokosuka, Kanagawa 240-0196, Japan
H I G H L I G H T S
� A novel tubular type molten carbonate direct carbon fuel cell was fabricated. � Stable continuous power generation was achieved with high power density. � The higher the operating temperature, the higher the power density obtained. � Most of the generated gas was CO during a continuous power generation test. � TMC-DCFC can effectively generate both electric power and CO gas. A R T I C L E I N F O
A B S T R A C T
Keywords: Direct carbon fuel cell Molten carbonate Tubular type Biomass Waste to energy
Direct carbon fuel cells (DCFCs) are expected to be useful devices that effectively utilize carbon resources. In this study, a tubular and molten carbonate electrolyte type DCFC (TMC-DCFC) was conceived and its basic perfor mance was investigated. Continuous power generation tests were conducted using a fuel mixture consisting of activated carbon and Li/Na carbonate powders such that stable power generation of about 100 mW cm 2 for 24 h was achieved. The temperature dependency of the I–V curves showed that the performance of the TMC-DCFC increased as the operating temperature increased. Analysis of the gas composition at the outlet of the anode indicated that most of the generated gas was CO. It is considered that CO was generated by the reverse Bou douard reaction between the solid carbon in the fuel and the CO2 generated at the anode. As a result of calculation of energy balance for the TMC-DCFC, the electrical efficiency was 20.4% and the heat value of generated CO was 79.6%. This result suggests that TMC-DCFC can generate both electric power and CO with high effective utilization of carbon.
1. Introduction To mitigate the effects of global warming and resolve energy issues, novel technologies which enables us to more effectively utilize carbon resources such as biomass and waste are needed. Examples of power generation systems using solid carbon resources as fuel include direct combustion power plants and gasification power plants. These power systems are restricted by Carnot efficiency and require large-scale fa cilities because they convert the heat energy of combustion into elec trical energy. In contrast, direct carbon fuel cells (DCFCs) convert the chemical energy of solid carbon directly into electrical energy. The total reaction is expressed as follows: C(s) þ O2(g) → CO2(g)
(1)
The DCFCs do not combust fuel, and construction of a compact sys tem is therefore relatively straightforward. At the same time, high electrical efficiency may be achieved. Given that DCFCs directly utilize solid carbon as fuel, the energy density of the fuel is high, and good transportability and storability of the fuel can be expected. Coal, carbonized biomass, and carbonized waste are candidates for DCFC fuels. When utilizing biomass as fuel for DCFCs, the power generation system will be a carbon-neutral system. In addition, when introducing CO2 capture, utilization, and storage (CCUS) facilities, the system will be a carbon-negative system. The first DCFC was built and patented by W. W. Jacques [1]. This DCFC system adopted molten hydroxide as an electrolyte. However, DCFCs have never been put into practical use to date. The DCFCs studied in recent years can be divided into solid oxide electrolyte types
* Corresponding author. E-mail address: [email protected] (A. Ido). https://doi.org/10.1016/j.jpowsour.2019.227483 Received 21 June 2019; Received in revised form 14 November 2019; Accepted 17 November 2019 0378-7753/© 2019 Elsevier B.V. All rights reserved.
Please cite this article as: Akifumi Ido, Makoto Kawase, Journal of Power Sources, https://doi.org/10.1016/j.jpowsour.2019.227483
A. Ido and M. Kawase
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(SO-DCFCs) [2–4] and molten carbonate electrolyte types (MC-DCFCs) [5–7]. Both types are composed of nearly the same electrode materials as gas-fueled SOFCs and MCFCs, respectively. The SOFCs are generally composed of perovskite-type oxide cathodes, such as lanthanum stron tium manganite (LSM) and lanthanum strontium cobalt ferrite (LSCF); solid oxide electrolytes, such as yttria-stabilized zirconia (YSZ) and gadolinium-doped ceria (GDC); and Ni-YSZ anodes. SO-DCFCs that use a mixture of carbon and carbonate as fuel are called hybrid DCFCs (HDCFCs) and are currently receiving much attention [8–10]. The cathode and anode reactions that occur with complete oxidation of carbon in the SO-DCFC are expressed as follows [11]: Cathode: O2(g) þ 4e → 2O2
(2)
Anode: C(s) þ 2O2 → CO2(g) þ 4e
(3)
difficulty supplying solid fuel continuously. We previously developed a method for fabricating a tubular MCFC (TMCFC) [15,17]. If this method can be applied to the DCFC, the problems mentioned above may be solved and DCFCs may be put into practical use. In this study, a tubular type MC-DCFC (TMC-DCFC) was fabricated and its basic performance was investigated. 2. Experimental 2.1. Fabrication of tubular MC-DCFC In the TMC-DCFC, inner layers were fastened by sintering the anode Ni particles on the outer surface of the tubular cell in a manner similar to that used with the TMCFC. It becomes possible to continuously supply solid fuel by using tubular-type cells. In this study, tubular MC-DCFC electrodes were fabricated based on TMCFC electrodes. In DCFCs, the carbon powder fuel has electronic conductivity; therefore, it is necessary to prevent shorting between the anode and cathode via the carbon powder. For this reason, a closed-end structure was adopted for the TMC-DCFC. Fig. 1 shows the fabrication process for the TMC-DCFC. In addition, Table 1 shows the cell component properties of the two cells (cell 1 and cell 2) fabricated in this study. Hong et al. reported that when alumina was added to the Ni anode of MCFC, the wettability and durability was enhanced [18]. In cell 2, alumina particles were added to the anode to improve durability against excessive sintering and wetta bility of the anode by molten carbonate. First, a part of a SUS316L pipe (outer diameter 9.6 mm, inner diameter 6.0 mm), of which one end was closed, was shaved and holes were punched in the pipe. The outer diameter of the punched part was 8.0 mm and the length was 33 mm. Second, a cathode slurry was applied to the punched part and where
However, SOFCs suffer problems of poor robustness and durability in the presence of impurities such as sulfur and chlorine [12,13]. DCFCs need to be durable in the presence of such impurities to effectively utilize solid carbon resources such as coal, biomass, and waste. On the other hand, MCFCs have higher durability with respect to sulfur and chloride compared with SOFCs [14,15]. MCFCs are typically composed of NiO cathodes, molten carbonate electrolytes, LiAlO2 electrolyte matrices, and Ni anodes. The cathode and anode reactions that occur with complete oxidation of carbon in the MC-DCFC are expressed as follows [16]: Cathode: O2(g) þ 2CO2(g) þ 4e → 2CO23
(4)
Anode: C(s) þ 2CO23 → 3CO2(g) þ 4e
(5)
However, existing MC-DCFCs have planar structures and have
Fig. 1. Fabrication method for TMC-DCFC. 2
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Table 1 Characteristics of cell components. Material Cell 1 Cell 2
Cathode Electrolyte matrix Anode Cathode Electrolyte matrix Anode
Ni–3%MgO α-LiAlO2 Ni–2%AlCr Ni–3%MgO α-LiAlO2 Ni–2%AlCr–10%Al2O3
Thickness 0.8 1.4 1.0 0.8 1.5 1.9
mm mm mm mm mm mm
adhesive tape had been wrapped. After that, the cathode was sintered at 900 � C for 1 h in a N2/H2 (90/10%) atmosphere. The adhesive tape was volatilized during this process. Next, an electrolyte matrix slurry was applied on the cathode and the closed end. After drying the electrolyte matrix, an anode slurry was applied over the electrolyte matrix. After that, the anode was sintered at 900 � C for 3 h in a N2/H2/H2O (63/7/30%) atmosphere. Finally, Li/Na carbonate powders dispersed in isopropyl alcohol were applied to the anode surface and the powders were impregnated into the electrodes and the electrolyte matrix. Impregnation was undertaken at 650 � C in a N2/H2 (90/10%) atmo sphere. The quantity of the impregnated electrolyte was decided based on the electrolyte packing ratios (volume of pores filled with electrolyte relative to total pore volume) of the anode, cathode, and electrolyte matrix, which were 50%, 50%, and 100%, respectively.
Fig. 3. I–V curves for the TMC-DCFC for cell 1 at specified temperatures.
density, the electrode area was defined as the contact area between the punched part of the cathode and the electrolyte matrix. 3. Results and discussion 3.1. Temperature dependence of I–V curves The performance of the TMC-DCFC as a function of temperature was investigated. The I–V curves for cell 1 at 700–850 � C are presented in Fig. 3. During the experiment the current density rose 10 mA cm 2 per minute. The results show that the higher the temperature rises, the larger the negative slope becomes. In other words, the higher the tem perature, the higher the cell performance of the TMC-DCFC. In general, cell performance for high-temperature hydrogen fuel cells (H2–FCs) reaches a plateau at a certain temperature. The total reaction for a H2-FC is expressed as
2.2. Power generation test apparatus The apparatus used for the power generation test is shown in Fig. 2. A tubular cell was inserted, and a 40 g mixture of activated carbon powder (activated charcoal, ~20 μm; Kanto Kagaku) and the same carbonate powder as the electrolyte (carbon/carbonate: 80/20 wt%) was placed in an alumina crucible. Only activated carbon is the fuel for DCFCs and carbonate in the fuel mixture is not the fuel. The alumina crucible was set in a nickel alloy holder and heated in an electric furnace. The cathode gas was introduced inside the tubular cell and the anode gas was introduced outside the alumina crucible, respectively. The cathode gas was N2/O2/CO2 (31/23/46%) with a flow rate of 150 mL min 1. The anode gas was N2/H2 (96/4%) with a flow rate of 150 mL min 1. The anode gas included a small amount of H2 to prevent oxidation of the anode. The anode outlet gas composition was analyzed by a gas chro matograph (3000 Micro GC; Infinicon). For calculation of the current
(6)
2H2(g) þ O2(g) → 2H2O(g) Also, the theoretical EMF of fuel cells is calculated from E� ¼
ΔG� f nF
(7)
where E� [V] is the theoretical electromotive force (EMF), ΔG� f [J mol 1] is the standard Gibbs free energy of formation for the total re action, n is the number of electrons transferred in the reaction and F is the Faraday constant 96485 C mol 1. Table 2 shows the theoretical EMF of a H2-FC and a DCFC at each temperature. Although the internal resistance and the reaction overpotential decrease, the theoretical EMF also decreases as the operating temperature increases for the H2-FC. A decrease in the theoretical EMF means a decrease in the OCV. Hence the performance of the H2–FCs reaches a plateau at a certain temperature. However, the theoretical EMF hardly changes as the operating temper ature increases in the DCFC, so it is expected that the cell performance of the DCFC will increase as the operating temperature rises.
Table 2 Theoretical electromotive force E� for the H2 FC and the DCFC at specified temperatures.
Fig. 2. Apparatus for the power generation test. 3
Temperature/� C
Theoretical electromotive force, E� /V H2 FC
DCFC
600 700 800 900
1.04 1.01 0.98 0.95
1.03 1.03 1.03 1.03
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3.2. Continuous power generation test
represent the TG curves and the dotted lines represent the DTA curves. The weight loss and endothermic reaction for activated carbon below 100 � C are derived from the evaporation of water. The endothermic reaction for Li/Na carbonate around 500 � C is derived from the melting of carbonate. A weight loss and an endothermic reaction were shown for the mixture of activated carbon and Li/Na carbonate from 750 � C to 850 � C, although these changes were not shown for either activated carbon or Li/Na carbonate. In the continuous power generation test, the generated gas at the anode before starting power generation was 1.86 � 10 4 mol min 1 CO and 0.06 � 10 4 mol min 1 CO2. It is presumed that CO and CO2 were generated by the chemical reaction in the fuel mixture considering the TG-DTA results. As expressed by reaction (5), CO2 is ideally expected to be generated at the anode in the DCFC. However, it has been reported that a signifi cant quantity of CO is present in the gas generated at the anode in the DCFC [5]. This CO generation is attributed to the reverse Boudouard reaction between CO2 generated at the anode and solid carbon in the fuel as expressed by
Continuous power generation tests were conducted for the TMCDCFC. Fig. 4 shows the cell voltage transient curves for cell 1 and cell 2 during power generation tests at 800 � C and 100 mA cm 2. Each cell continuously generated stable electric power and a high power density of about 100 mW cm 2 for 24 h was achieved in cell 2. Although cell 2 was expected to show better performance than cell 1, the difference of the cell performance between cell 1 and cell 2 could not be observed. The effect of alumina addition should be investigated more detail. From the results, it is expected that a higher power density can be achieved by operating the cells at a higher current density. The flow rate of H2(g) at the anode outlet gas before and 3 h after starting power generation were both 6.3 mL min 1, which was same as that at the anode inlet gas. These results show that H2(g) was not consumed during the power generation tests. The open circuit voltages (OCVs) were 1.44 V for cell 1 and 1.33 V for cell 2, which were higher than the 1.03 V standard oxidationreduction potential of reaction (1) at 800 � C. From the beginning of the power generation tests, the cell voltage decreased gradually over 60–90 min and then stabilized. These high OCVs and the transition of the cell voltage are considered to reflect a transition of the gas compo sition near the anode. From equations (4) and (5), the OCV for the MCDCFC is expressed as follows: E ¼ E� þ
2 RT pCO2ðCaÞ pO2ðCaÞ ln 4F p3CO2ðAnÞ
C(s) þ CO2(g) → 2CO(g)
(9)
The standard Gibbs free energy of formation of the reverse Bou douard reaction ΔG� f(R.B.) at 800 � C is 17.5 � 103 J mol 1, so reaction (9) proceeds spontaneously. Electrochemical oxidation of CO is pre sumed to occur at the anode in the DCFC as in the following reaction:
(8)
CO(g) þ CO23 → 2CO2(g) þ 2e
(10)
where E [V] is the OCV, R is the gas constant 8.314 J K 1 mol 1, T [K] is the temperature and p [atm] is the partial pressure of each gas. The partial pressure of CO2 near the anode was sufficiently low prior to beginning the power generation test but gradually increased as a result of continuous power generation, and the gas atmosphere attained a steady-state after 60–90 min.
There are some reports on fuel cells using this reaction with dry gasification of carbon [21,22]. In this experiment, the anode outlet gas composition 3 h after the start of power generation was 11.6% CO and 0.86% CO2; that is, most of generated gas was CO. Therefore, it is considered that, in this study, reactions (9) and (10) occurred in addition to reaction (5) during the power generation test.
3.3. Gas generation in the DCFC
3.4. Calculation of energy balance
Gas generation for cell 2 was calculated based on the anode inlet flow rate and the anode outlet gas composition. Carbon reacts with molten alkali carbonate and generates CO and CO2 above 700 � C [19,20]. Hence, CO and CO2 generation was investigated. A mixture of activated carbon and Li/Na carbonate was used as fuel in this study, therefore CO and CO2 were presumed to be generated even when there was no power generation. Thermal analysis studies on activated carbon powder, Li/Na (60/40) carbonate powder, and an 80/20 (wt%) mixture of these pow ders was undertaken using thermogravimetry differential thermal analysis (TG-DTA) equipment (Mac Science, TG-DTA 2000s). The ana lyses were performed in a N2 atmosphere with a N2 flow rate of 50 mL min 1 and a heating rate of 10 K min 1 from room temperature up to 1000 � C. Fig. 5 shows the results of the TG-DTA where the solid lines
The energy balance of the TMC-DCFC was calculated from the gas generation for cell 2. It should be noted that activated carbon, which contains much more fixed carbon than carbonized biomass or waste, was used as fuel in this study. Power from auxiliary equipment was not considered in the calculation of electrical efficiency. Furthermore, at this time, the MCFC is not commercially operated using only CO as fuel.
Fig. 5. TG and DTA curves for (a) activated carbon, (b) Li/Na (60/40) car bonate, and (c) 80/20 (wt%) mixture of activated carbon and Li/Na carbonate.
Fig. 4. Cell voltage transient curve during continuous power generation. 4
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Mass generation per unit time by the anode reaction is given by the following expression: X¼
I � 60 nF
2XCð22;expÞ ¼
xI � 60 4F
(12)
XCO2ð5Þ ¼
3xI � 60 4F
(13)
XCO2ð10Þ ¼
XCOð10Þ ¼
xÞI
ð1 F ð1
’ XCðexpÞ ¼
xÞI � 60 2F
2XCOðexpÞ þ XCO2ðexpÞ 3
XCOð22;expÞ ¼ 2
5:68 � 10
4
� � mol min 1 (24)
(25)
The calories of generated CO per unit time Q’CO(exp) [J s ] are calculated from the standard enthalpy of formation of reaction (25) where ΔH� f(25) ¼ 2.823 � 105 J mol 1 at 800 � C and all CO generation X’CO(exp) is calculated as
(15)
yðXCO2ð5Þ þ XCO2ð10Þ Þ
Q’COðexpÞ ¼ ΔH � f ð25Þ �
Xi ¼ Xið5Þ þ Xið9Þ þ Xið10Þ
ði : C; CO or CO2 Þ
(16)
� � 3I � 60 ¼ 9:33 � 10 4 mol min 1 2F
(18)
(19)
when Q(heat) [J s 1] is defined as the heat input to the system, the following equation holds from the heat balance and equations (26)–(28). Q’CðexpÞ þ QðheatÞ ¼ PðDCFCÞ þ Q’COðexpÞ � � QðheatÞ ¼ 1:14 J s 1
(20)
(29)
The system received heat from the electric furnace in this experi ment. From the above, assuming that there was no heat loss in the DCFC, the electrical efficiency η1(DCFC) [%] and the calories of generated CO ηCO (DCFC) [%] based on the total input energy are calculated as follows.
(21)
X’ [mol min 1] is defined as material generation per unit time including that by the reaction not related to power generation. The experimental values were X’CO(exp)¼8.26 � 10 4 mol min 1, X’CO2(exp) ¼ 0.61 � 10 4 mol min 1, and X’CO(exp) þ 2X’CO2(exp) ¼ 9.48 � 10 4 mol min 1. The difference between the theoretical value XCO(exp) þ 2XCO2 (exp) and the experimental value X’CO(exp) þ 2X’CO2(exp) is considered to be due to gas generation by the chemical reaction between carbon and molten carbonate as mentioned in section 3-4. This reaction is not the consumption of electrolyte but the consumption of molten carbonate in the fuel mixture. Gas generation by this reaction decreased compared with that during no power generation. This result suggests that the re action between carbon and molten carbonate is suppressed during power generation because the partial pressure of CO or CO2 in the anode chamber is increased by power generation. Assuming that the entire reaction between carbon and molten carbonate proceeds as in equation (22) and only CO is generated, then gas generation via power generation would be XCO(exp) ¼ 8.11 � 10 4 mol min 1 and XCO2(exp) ¼ 0.61 � 10 4 mol min 1. C(s) þ M2CO3(l) → 2CO(g) þ M2O(s) (M ¼ Li, Na)
(26)
The calories of consumed carbon per unit time Q’C(exp) [J s 1] are calculated from the standard enthalpy of formation of reaction (1) ΔH� f 5 1 at 800 � C and all carbon consumption X’C(exp) (1) ¼ 3.948 � 10 J mol as follows: � � � X’CðexpÞ Q’CðexpÞ ¼ ΔH � f ð1Þ � (28) ¼ 3:74 J s 1 60
(17)
From the deformation of equation (12)–(19), the following equations hold during power generation at 1 A. 2XCO þ XCO2 3
� � X’COðexpÞ ¼ 3:88 J s 1 60
The electric power output of the DCFC in this experiment P(DCFC) [J s 1] is calculated from the electric current, I ¼ 1 A, and the cell voltage, V ¼ 0.992 V, after 3 h from the start of power generation � � (27) PðDCFCÞ ¼ I � V ¼ 0:992 J s 1
Consumption or generation for each material is expressed as the following equation by considering the equations (5), (9) and (10).
XCO þ 2XCO2 ¼
(23)
CO(g) þ 1/2 O2(g) → CO2(g)
(14)
� 60
XCOð9Þ ¼ 2yðXCO2ð5Þ þ XCO2ð10Þ Þ
XC ¼
� � mol min 1
1
yðXCO2ð5Þ þ XCO2ð10Þ Þ
XCO2ð9Þ ¼
4
In this experiment, the reverse Boudouard reaction [reaction (9)] was presumed to have occurred because a significant amount of CO was generated. The reverse Boudouard reaction is an endothermic reaction, so it is possible that this DCFC system was deficient in heat to the point that the temperature could not be maintained from waste heat origi nating from the DCFC alone. Therefore, heat input to the DCFC system was calculated. The oxidation of CO is expressed as
When y is defined as the ratio of CO2 converted to CO by reaction (9) to all CO2 generated by the anode reaction, then carbon consumption, CO2 consumption, and CO generation by reaction (9) are respectively expressed as follows: XCð9Þ ¼
0:15 � 10
All carbon consumption per unit time during the power generation test is calculated from equations (20) and (23).
(11)
where X [mol min 1] is material generation or consumption per unit time as a result of power generation and I [A] is electric current. It should be noted that the value of X is negative when there is con sumption. Assuming that all electric current was generated by reaction (5) or (10), when x is defined as the ratio of reaction (5) to both reactions (5) and (10), then carbon consumption, CO2 generation, and CO gen eration by each reaction are respectively expressed as follows: XCð5Þ ¼
XCOð22;expÞ ¼
η1ðDCFCÞ ¼
PðDCFCÞ � 100 ¼ 20:4 ½%� Q’CðexpÞ þ QðheatÞ
ηCOðDCFCÞ ¼
Q’COðexpÞ ’ QCðexpÞ þ QðheatÞ
� 100 ¼ 79:6 ½%�
(30)
(31)
Based on the calorie of consumed carbon, the electrical efficiency η2 [%] is calculated as follows.
(DCFC)
η2ðDCFCÞ ¼
PðDCFCÞ � 100 ¼ 26:5 ½%� Q’CðexpÞ
(32)
Fig. 6 shows the energy balance of the TMC-DCFC in this study. Values in brackets is based on the calories of consumed carbon. Although heat input to the cell was needed in this experiment due to proceeding of reverse Boudouard reaction a lot, heat self supporting can be achieved by burning some of generated CO. Although the cell per formance of a TMC-DCFC is declined, Reverse Boudouard reaction is suppressed and the electrical efficiency will be improved by decreasing operating temperature. Further study to optimize operating conditions
(22)
5
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Fig. 6. Energy balance of the TMC-DCFC in this experiment.
of TMC-DCFCs is necessary. As above, TMC-DCFCs generate both electricity and CO by using solid carbon as fuel. Since heat generation of the cell is utilized for heat self supporting of the cell and reverse Boudouard reaction, it is expected that heat loss is considerably low and effective utilization of carbon is almost 100%. Various use of CO generated in DCFCs can be considered such as additional power generation by MCFC, fuel synthesis or pro ducing chemical products.
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4. Conclusion Tubular and molten carbonate type DCFCs (TMC-DCFCs) to which fuel can be continuously supplied were fabricated. The results of continuous power generation tests indicated that the TMC-DCFC can generate a stable supply of electric power with high power density. The I–V curves for the TMC-DCFC were temperature dependent and showed that cell performance increased as the operating temperature increased. Based on analysis of the anode outlet gas, it was found that most of the gas generated during power generation was CO. The CO was considered to be generated by the reverse Boudouard reaction which involved the reaction of the solid carbon in the fuel and the CO2 generated at the anode. The electrical efficiency was 20.4% and the calories of generated CO was 79.6% based on the total input energy. The electrical efficiency based on the calories of consumed carbon was 26.5%. It is revealed that TMC-DCFCs effectively utilize solid carbon resources and generate both electricity and CO which can be utilized in various ways. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. References [1] W.W. Jacques, Method of Converting Potential Energy of Carbon into Electrical Energy, US Patent 555511, 1896. [2] S.L. Jain, Y. Nabae, B.J. Lakeman, K.D. Pointon, J.T.S. Irvine, Solid state electrochemistry of direct carbon/air fuel cells, Solid State Ion. 179 (2008) 1417–1421. [3] M. Dudek, P. Tomczyk, Composite for direct carbone fuel cell, Catal. Today 176 (2011) 388–392.
6
Contents lists available at ScienceDirect
Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour
Development of a tubular molten carbonate direct carbon fuel cell and basic cell performance Akifumi Ido *, Makoto Kawase Central Research Institute of Electric Power Industry (CRIEPI), 2-6-1, Nagasaka, Yokosuka, Kanagawa 240-0196, Japan
H I G H L I G H T S
� A novel tubular type molten carbonate direct carbon fuel cell was fabricated. � Stable continuous power generation was achieved with high power density. � The higher the operating temperature, the higher the power density obtained. � Most of the generated gas was CO during a continuous power generation test. � TMC-DCFC can effectively generate both electric power and CO gas. A R T I C L E I N F O
A B S T R A C T
Keywords: Direct carbon fuel cell Molten carbonate Tubular type Biomass Waste to energy
Direct carbon fuel cells (DCFCs) are expected to be useful devices that effectively utilize carbon resources. In this study, a tubular and molten carbonate electrolyte type DCFC (TMC-DCFC) was conceived and its basic perfor mance was investigated. Continuous power generation tests were conducted using a fuel mixture consisting of activated carbon and Li/Na carbonate powders such that stable power generation of about 100 mW cm 2 for 24 h was achieved. The temperature dependency of the I–V curves showed that the performance of the TMC-DCFC increased as the operating temperature increased. Analysis of the gas composition at the outlet of the anode indicated that most of the generated gas was CO. It is considered that CO was generated by the reverse Bou douard reaction between the solid carbon in the fuel and the CO2 generated at the anode. As a result of calculation of energy balance for the TMC-DCFC, the electrical efficiency was 20.4% and the heat value of generated CO was 79.6%. This result suggests that TMC-DCFC can generate both electric power and CO with high effective utilization of carbon.
1. Introduction To mitigate the effects of global warming and resolve energy issues, novel technologies which enables us to more effectively utilize carbon resources such as biomass and waste are needed. Examples of power generation systems using solid carbon resources as fuel include direct combustion power plants and gasification power plants. These power systems are restricted by Carnot efficiency and require large-scale fa cilities because they convert the heat energy of combustion into elec trical energy. In contrast, direct carbon fuel cells (DCFCs) convert the chemical energy of solid carbon directly into electrical energy. The total reaction is expressed as follows: C(s) þ O2(g) → CO2(g)
(1)
The DCFCs do not combust fuel, and construction of a compact sys tem is therefore relatively straightforward. At the same time, high electrical efficiency may be achieved. Given that DCFCs directly utilize solid carbon as fuel, the energy density of the fuel is high, and good transportability and storability of the fuel can be expected. Coal, carbonized biomass, and carbonized waste are candidates for DCFC fuels. When utilizing biomass as fuel for DCFCs, the power generation system will be a carbon-neutral system. In addition, when introducing CO2 capture, utilization, and storage (CCUS) facilities, the system will be a carbon-negative system. The first DCFC was built and patented by W. W. Jacques [1]. This DCFC system adopted molten hydroxide as an electrolyte. However, DCFCs have never been put into practical use to date. The DCFCs studied in recent years can be divided into solid oxide electrolyte types
* Corresponding author. E-mail address: [email protected] (A. Ido). https://doi.org/10.1016/j.jpowsour.2019.227483 Received 21 June 2019; Received in revised form 14 November 2019; Accepted 17 November 2019 0378-7753/© 2019 Elsevier B.V. All rights reserved.
Please cite this article as: Akifumi Ido, Makoto Kawase, Journal of Power Sources, https://doi.org/10.1016/j.jpowsour.2019.227483
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(SO-DCFCs) [2–4] and molten carbonate electrolyte types (MC-DCFCs) [5–7]. Both types are composed of nearly the same electrode materials as gas-fueled SOFCs and MCFCs, respectively. The SOFCs are generally composed of perovskite-type oxide cathodes, such as lanthanum stron tium manganite (LSM) and lanthanum strontium cobalt ferrite (LSCF); solid oxide electrolytes, such as yttria-stabilized zirconia (YSZ) and gadolinium-doped ceria (GDC); and Ni-YSZ anodes. SO-DCFCs that use a mixture of carbon and carbonate as fuel are called hybrid DCFCs (HDCFCs) and are currently receiving much attention [8–10]. The cathode and anode reactions that occur with complete oxidation of carbon in the SO-DCFC are expressed as follows [11]: Cathode: O2(g) þ 4e → 2O2
(2)
Anode: C(s) þ 2O2 → CO2(g) þ 4e
(3)
difficulty supplying solid fuel continuously. We previously developed a method for fabricating a tubular MCFC (TMCFC) [15,17]. If this method can be applied to the DCFC, the problems mentioned above may be solved and DCFCs may be put into practical use. In this study, a tubular type MC-DCFC (TMC-DCFC) was fabricated and its basic performance was investigated. 2. Experimental 2.1. Fabrication of tubular MC-DCFC In the TMC-DCFC, inner layers were fastened by sintering the anode Ni particles on the outer surface of the tubular cell in a manner similar to that used with the TMCFC. It becomes possible to continuously supply solid fuel by using tubular-type cells. In this study, tubular MC-DCFC electrodes were fabricated based on TMCFC electrodes. In DCFCs, the carbon powder fuel has electronic conductivity; therefore, it is necessary to prevent shorting between the anode and cathode via the carbon powder. For this reason, a closed-end structure was adopted for the TMC-DCFC. Fig. 1 shows the fabrication process for the TMC-DCFC. In addition, Table 1 shows the cell component properties of the two cells (cell 1 and cell 2) fabricated in this study. Hong et al. reported that when alumina was added to the Ni anode of MCFC, the wettability and durability was enhanced [18]. In cell 2, alumina particles were added to the anode to improve durability against excessive sintering and wetta bility of the anode by molten carbonate. First, a part of a SUS316L pipe (outer diameter 9.6 mm, inner diameter 6.0 mm), of which one end was closed, was shaved and holes were punched in the pipe. The outer diameter of the punched part was 8.0 mm and the length was 33 mm. Second, a cathode slurry was applied to the punched part and where
However, SOFCs suffer problems of poor robustness and durability in the presence of impurities such as sulfur and chlorine [12,13]. DCFCs need to be durable in the presence of such impurities to effectively utilize solid carbon resources such as coal, biomass, and waste. On the other hand, MCFCs have higher durability with respect to sulfur and chloride compared with SOFCs [14,15]. MCFCs are typically composed of NiO cathodes, molten carbonate electrolytes, LiAlO2 electrolyte matrices, and Ni anodes. The cathode and anode reactions that occur with complete oxidation of carbon in the MC-DCFC are expressed as follows [16]: Cathode: O2(g) þ 2CO2(g) þ 4e → 2CO23
(4)
Anode: C(s) þ 2CO23 → 3CO2(g) þ 4e
(5)
However, existing MC-DCFCs have planar structures and have
Fig. 1. Fabrication method for TMC-DCFC. 2
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Table 1 Characteristics of cell components. Material Cell 1 Cell 2
Cathode Electrolyte matrix Anode Cathode Electrolyte matrix Anode
Ni–3%MgO α-LiAlO2 Ni–2%AlCr Ni–3%MgO α-LiAlO2 Ni–2%AlCr–10%Al2O3
Thickness 0.8 1.4 1.0 0.8 1.5 1.9
mm mm mm mm mm mm
adhesive tape had been wrapped. After that, the cathode was sintered at 900 � C for 1 h in a N2/H2 (90/10%) atmosphere. The adhesive tape was volatilized during this process. Next, an electrolyte matrix slurry was applied on the cathode and the closed end. After drying the electrolyte matrix, an anode slurry was applied over the electrolyte matrix. After that, the anode was sintered at 900 � C for 3 h in a N2/H2/H2O (63/7/30%) atmosphere. Finally, Li/Na carbonate powders dispersed in isopropyl alcohol were applied to the anode surface and the powders were impregnated into the electrodes and the electrolyte matrix. Impregnation was undertaken at 650 � C in a N2/H2 (90/10%) atmo sphere. The quantity of the impregnated electrolyte was decided based on the electrolyte packing ratios (volume of pores filled with electrolyte relative to total pore volume) of the anode, cathode, and electrolyte matrix, which were 50%, 50%, and 100%, respectively.
Fig. 3. I–V curves for the TMC-DCFC for cell 1 at specified temperatures.
density, the electrode area was defined as the contact area between the punched part of the cathode and the electrolyte matrix. 3. Results and discussion 3.1. Temperature dependence of I–V curves The performance of the TMC-DCFC as a function of temperature was investigated. The I–V curves for cell 1 at 700–850 � C are presented in Fig. 3. During the experiment the current density rose 10 mA cm 2 per minute. The results show that the higher the temperature rises, the larger the negative slope becomes. In other words, the higher the tem perature, the higher the cell performance of the TMC-DCFC. In general, cell performance for high-temperature hydrogen fuel cells (H2–FCs) reaches a plateau at a certain temperature. The total reaction for a H2-FC is expressed as
2.2. Power generation test apparatus The apparatus used for the power generation test is shown in Fig. 2. A tubular cell was inserted, and a 40 g mixture of activated carbon powder (activated charcoal, ~20 μm; Kanto Kagaku) and the same carbonate powder as the electrolyte (carbon/carbonate: 80/20 wt%) was placed in an alumina crucible. Only activated carbon is the fuel for DCFCs and carbonate in the fuel mixture is not the fuel. The alumina crucible was set in a nickel alloy holder and heated in an electric furnace. The cathode gas was introduced inside the tubular cell and the anode gas was introduced outside the alumina crucible, respectively. The cathode gas was N2/O2/CO2 (31/23/46%) with a flow rate of 150 mL min 1. The anode gas was N2/H2 (96/4%) with a flow rate of 150 mL min 1. The anode gas included a small amount of H2 to prevent oxidation of the anode. The anode outlet gas composition was analyzed by a gas chro matograph (3000 Micro GC; Infinicon). For calculation of the current
(6)
2H2(g) þ O2(g) → 2H2O(g) Also, the theoretical EMF of fuel cells is calculated from E� ¼
ΔG� f nF
(7)
where E� [V] is the theoretical electromotive force (EMF), ΔG� f [J mol 1] is the standard Gibbs free energy of formation for the total re action, n is the number of electrons transferred in the reaction and F is the Faraday constant 96485 C mol 1. Table 2 shows the theoretical EMF of a H2-FC and a DCFC at each temperature. Although the internal resistance and the reaction overpotential decrease, the theoretical EMF also decreases as the operating temperature increases for the H2-FC. A decrease in the theoretical EMF means a decrease in the OCV. Hence the performance of the H2–FCs reaches a plateau at a certain temperature. However, the theoretical EMF hardly changes as the operating temper ature increases in the DCFC, so it is expected that the cell performance of the DCFC will increase as the operating temperature rises.
Table 2 Theoretical electromotive force E� for the H2 FC and the DCFC at specified temperatures.
Fig. 2. Apparatus for the power generation test. 3
Temperature/� C
Theoretical electromotive force, E� /V H2 FC
DCFC
600 700 800 900
1.04 1.01 0.98 0.95
1.03 1.03 1.03 1.03
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3.2. Continuous power generation test
represent the TG curves and the dotted lines represent the DTA curves. The weight loss and endothermic reaction for activated carbon below 100 � C are derived from the evaporation of water. The endothermic reaction for Li/Na carbonate around 500 � C is derived from the melting of carbonate. A weight loss and an endothermic reaction were shown for the mixture of activated carbon and Li/Na carbonate from 750 � C to 850 � C, although these changes were not shown for either activated carbon or Li/Na carbonate. In the continuous power generation test, the generated gas at the anode before starting power generation was 1.86 � 10 4 mol min 1 CO and 0.06 � 10 4 mol min 1 CO2. It is presumed that CO and CO2 were generated by the chemical reaction in the fuel mixture considering the TG-DTA results. As expressed by reaction (5), CO2 is ideally expected to be generated at the anode in the DCFC. However, it has been reported that a signifi cant quantity of CO is present in the gas generated at the anode in the DCFC [5]. This CO generation is attributed to the reverse Boudouard reaction between CO2 generated at the anode and solid carbon in the fuel as expressed by
Continuous power generation tests were conducted for the TMCDCFC. Fig. 4 shows the cell voltage transient curves for cell 1 and cell 2 during power generation tests at 800 � C and 100 mA cm 2. Each cell continuously generated stable electric power and a high power density of about 100 mW cm 2 for 24 h was achieved in cell 2. Although cell 2 was expected to show better performance than cell 1, the difference of the cell performance between cell 1 and cell 2 could not be observed. The effect of alumina addition should be investigated more detail. From the results, it is expected that a higher power density can be achieved by operating the cells at a higher current density. The flow rate of H2(g) at the anode outlet gas before and 3 h after starting power generation were both 6.3 mL min 1, which was same as that at the anode inlet gas. These results show that H2(g) was not consumed during the power generation tests. The open circuit voltages (OCVs) were 1.44 V for cell 1 and 1.33 V for cell 2, which were higher than the 1.03 V standard oxidationreduction potential of reaction (1) at 800 � C. From the beginning of the power generation tests, the cell voltage decreased gradually over 60–90 min and then stabilized. These high OCVs and the transition of the cell voltage are considered to reflect a transition of the gas compo sition near the anode. From equations (4) and (5), the OCV for the MCDCFC is expressed as follows: E ¼ E� þ
2 RT pCO2ðCaÞ pO2ðCaÞ ln 4F p3CO2ðAnÞ
C(s) þ CO2(g) → 2CO(g)
(9)
The standard Gibbs free energy of formation of the reverse Bou douard reaction ΔG� f(R.B.) at 800 � C is 17.5 � 103 J mol 1, so reaction (9) proceeds spontaneously. Electrochemical oxidation of CO is pre sumed to occur at the anode in the DCFC as in the following reaction:
(8)
CO(g) þ CO23 → 2CO2(g) þ 2e
(10)
where E [V] is the OCV, R is the gas constant 8.314 J K 1 mol 1, T [K] is the temperature and p [atm] is the partial pressure of each gas. The partial pressure of CO2 near the anode was sufficiently low prior to beginning the power generation test but gradually increased as a result of continuous power generation, and the gas atmosphere attained a steady-state after 60–90 min.
There are some reports on fuel cells using this reaction with dry gasification of carbon [21,22]. In this experiment, the anode outlet gas composition 3 h after the start of power generation was 11.6% CO and 0.86% CO2; that is, most of generated gas was CO. Therefore, it is considered that, in this study, reactions (9) and (10) occurred in addition to reaction (5) during the power generation test.
3.3. Gas generation in the DCFC
3.4. Calculation of energy balance
Gas generation for cell 2 was calculated based on the anode inlet flow rate and the anode outlet gas composition. Carbon reacts with molten alkali carbonate and generates CO and CO2 above 700 � C [19,20]. Hence, CO and CO2 generation was investigated. A mixture of activated carbon and Li/Na carbonate was used as fuel in this study, therefore CO and CO2 were presumed to be generated even when there was no power generation. Thermal analysis studies on activated carbon powder, Li/Na (60/40) carbonate powder, and an 80/20 (wt%) mixture of these pow ders was undertaken using thermogravimetry differential thermal analysis (TG-DTA) equipment (Mac Science, TG-DTA 2000s). The ana lyses were performed in a N2 atmosphere with a N2 flow rate of 50 mL min 1 and a heating rate of 10 K min 1 from room temperature up to 1000 � C. Fig. 5 shows the results of the TG-DTA where the solid lines
The energy balance of the TMC-DCFC was calculated from the gas generation for cell 2. It should be noted that activated carbon, which contains much more fixed carbon than carbonized biomass or waste, was used as fuel in this study. Power from auxiliary equipment was not considered in the calculation of electrical efficiency. Furthermore, at this time, the MCFC is not commercially operated using only CO as fuel.
Fig. 5. TG and DTA curves for (a) activated carbon, (b) Li/Na (60/40) car bonate, and (c) 80/20 (wt%) mixture of activated carbon and Li/Na carbonate.
Fig. 4. Cell voltage transient curve during continuous power generation. 4
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Mass generation per unit time by the anode reaction is given by the following expression: X¼
I � 60 nF
2XCð22;expÞ ¼
xI � 60 4F
(12)
XCO2ð5Þ ¼
3xI � 60 4F
(13)
XCO2ð10Þ ¼
XCOð10Þ ¼
xÞI
ð1 F ð1
’ XCðexpÞ ¼
xÞI � 60 2F
2XCOðexpÞ þ XCO2ðexpÞ 3
XCOð22;expÞ ¼ 2
5:68 � 10
4
� � mol min 1 (24)
(25)
The calories of generated CO per unit time Q’CO(exp) [J s ] are calculated from the standard enthalpy of formation of reaction (25) where ΔH� f(25) ¼ 2.823 � 105 J mol 1 at 800 � C and all CO generation X’CO(exp) is calculated as
(15)
yðXCO2ð5Þ þ XCO2ð10Þ Þ
Q’COðexpÞ ¼ ΔH � f ð25Þ �
Xi ¼ Xið5Þ þ Xið9Þ þ Xið10Þ
ði : C; CO or CO2 Þ
(16)
� � 3I � 60 ¼ 9:33 � 10 4 mol min 1 2F
(18)
(19)
when Q(heat) [J s 1] is defined as the heat input to the system, the following equation holds from the heat balance and equations (26)–(28). Q’CðexpÞ þ QðheatÞ ¼ PðDCFCÞ þ Q’COðexpÞ � � QðheatÞ ¼ 1:14 J s 1
(20)
(29)
The system received heat from the electric furnace in this experi ment. From the above, assuming that there was no heat loss in the DCFC, the electrical efficiency η1(DCFC) [%] and the calories of generated CO ηCO (DCFC) [%] based on the total input energy are calculated as follows.
(21)
X’ [mol min 1] is defined as material generation per unit time including that by the reaction not related to power generation. The experimental values were X’CO(exp)¼8.26 � 10 4 mol min 1, X’CO2(exp) ¼ 0.61 � 10 4 mol min 1, and X’CO(exp) þ 2X’CO2(exp) ¼ 9.48 � 10 4 mol min 1. The difference between the theoretical value XCO(exp) þ 2XCO2 (exp) and the experimental value X’CO(exp) þ 2X’CO2(exp) is considered to be due to gas generation by the chemical reaction between carbon and molten carbonate as mentioned in section 3-4. This reaction is not the consumption of electrolyte but the consumption of molten carbonate in the fuel mixture. Gas generation by this reaction decreased compared with that during no power generation. This result suggests that the re action between carbon and molten carbonate is suppressed during power generation because the partial pressure of CO or CO2 in the anode chamber is increased by power generation. Assuming that the entire reaction between carbon and molten carbonate proceeds as in equation (22) and only CO is generated, then gas generation via power generation would be XCO(exp) ¼ 8.11 � 10 4 mol min 1 and XCO2(exp) ¼ 0.61 � 10 4 mol min 1. C(s) þ M2CO3(l) → 2CO(g) þ M2O(s) (M ¼ Li, Na)
(26)
The calories of consumed carbon per unit time Q’C(exp) [J s 1] are calculated from the standard enthalpy of formation of reaction (1) ΔH� f 5 1 at 800 � C and all carbon consumption X’C(exp) (1) ¼ 3.948 � 10 J mol as follows: � � � X’CðexpÞ Q’CðexpÞ ¼ ΔH � f ð1Þ � (28) ¼ 3:74 J s 1 60
(17)
From the deformation of equation (12)–(19), the following equations hold during power generation at 1 A. 2XCO þ XCO2 3
� � X’COðexpÞ ¼ 3:88 J s 1 60
The electric power output of the DCFC in this experiment P(DCFC) [J s 1] is calculated from the electric current, I ¼ 1 A, and the cell voltage, V ¼ 0.992 V, after 3 h from the start of power generation � � (27) PðDCFCÞ ¼ I � V ¼ 0:992 J s 1
Consumption or generation for each material is expressed as the following equation by considering the equations (5), (9) and (10).
XCO þ 2XCO2 ¼
(23)
CO(g) þ 1/2 O2(g) → CO2(g)
(14)
� 60
XCOð9Þ ¼ 2yðXCO2ð5Þ þ XCO2ð10Þ Þ
XC ¼
� � mol min 1
1
yðXCO2ð5Þ þ XCO2ð10Þ Þ
XCO2ð9Þ ¼
4
In this experiment, the reverse Boudouard reaction [reaction (9)] was presumed to have occurred because a significant amount of CO was generated. The reverse Boudouard reaction is an endothermic reaction, so it is possible that this DCFC system was deficient in heat to the point that the temperature could not be maintained from waste heat origi nating from the DCFC alone. Therefore, heat input to the DCFC system was calculated. The oxidation of CO is expressed as
When y is defined as the ratio of CO2 converted to CO by reaction (9) to all CO2 generated by the anode reaction, then carbon consumption, CO2 consumption, and CO generation by reaction (9) are respectively expressed as follows: XCð9Þ ¼
0:15 � 10
All carbon consumption per unit time during the power generation test is calculated from equations (20) and (23).
(11)
where X [mol min 1] is material generation or consumption per unit time as a result of power generation and I [A] is electric current. It should be noted that the value of X is negative when there is con sumption. Assuming that all electric current was generated by reaction (5) or (10), when x is defined as the ratio of reaction (5) to both reactions (5) and (10), then carbon consumption, CO2 generation, and CO gen eration by each reaction are respectively expressed as follows: XCð5Þ ¼
XCOð22;expÞ ¼
η1ðDCFCÞ ¼
PðDCFCÞ � 100 ¼ 20:4 ½%� Q’CðexpÞ þ QðheatÞ
ηCOðDCFCÞ ¼
Q’COðexpÞ ’ QCðexpÞ þ QðheatÞ
� 100 ¼ 79:6 ½%�
(30)
(31)
Based on the calorie of consumed carbon, the electrical efficiency η2 [%] is calculated as follows.
(DCFC)
η2ðDCFCÞ ¼
PðDCFCÞ � 100 ¼ 26:5 ½%� Q’CðexpÞ
(32)
Fig. 6 shows the energy balance of the TMC-DCFC in this study. Values in brackets is based on the calories of consumed carbon. Although heat input to the cell was needed in this experiment due to proceeding of reverse Boudouard reaction a lot, heat self supporting can be achieved by burning some of generated CO. Although the cell per formance of a TMC-DCFC is declined, Reverse Boudouard reaction is suppressed and the electrical efficiency will be improved by decreasing operating temperature. Further study to optimize operating conditions
(22)
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Fig. 6. Energy balance of the TMC-DCFC in this experiment.
of TMC-DCFCs is necessary. As above, TMC-DCFCs generate both electricity and CO by using solid carbon as fuel. Since heat generation of the cell is utilized for heat self supporting of the cell and reverse Boudouard reaction, it is expected that heat loss is considerably low and effective utilization of carbon is almost 100%. Various use of CO generated in DCFCs can be considered such as additional power generation by MCFC, fuel synthesis or pro ducing chemical products.
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4. Conclusion Tubular and molten carbonate type DCFCs (TMC-DCFCs) to which fuel can be continuously supplied were fabricated. The results of continuous power generation tests indicated that the TMC-DCFC can generate a stable supply of electric power with high power density. The I–V curves for the TMC-DCFC were temperature dependent and showed that cell performance increased as the operating temperature increased. Based on analysis of the anode outlet gas, it was found that most of the gas generated during power generation was CO. The CO was considered to be generated by the reverse Boudouard reaction which involved the reaction of the solid carbon in the fuel and the CO2 generated at the anode. The electrical efficiency was 20.4% and the calories of generated CO was 79.6% based on the total input energy. The electrical efficiency based on the calories of consumed carbon was 26.5%. It is revealed that TMC-DCFCs effectively utilize solid carbon resources and generate both electricity and CO which can be utilized in various ways. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. References [1] W.W. Jacques, Method of Converting Potential Energy of Carbon into Electrical Energy, US Patent 555511, 1896. [2] S.L. Jain, Y. Nabae, B.J. Lakeman, K.D. Pointon, J.T.S. Irvine, Solid state electrochemistry of direct carbon/air fuel cells, Solid State Ion. 179 (2008) 1417–1421. [3] M. Dudek, P. Tomczyk, Composite for direct carbone fuel cell, Catal. Today 176 (2011) 388–392.
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