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Effects of discharge mode and fuel treating temperature on the fuel utilization of direct carbon solid oxide fuel cell
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Effects of discharge mode and fuel treating temperature on the fuel utilization of direct carbon solid oxide fuel cell Chaoqi Wang a, Zhe Lu¨ a,*, Chaoxiang Su a, Jingwei Li a, Zhiqun Cao b, Xingbao Zhu a, Yanyan Wu a, Huan Li a a b
Department of Physics, Harbin Institute of Technology, Harbin, PR China College of Chemistry and Environmental Engineering, Shenzhen Univ, Shenzhen, PR China
article info
abstract
Article history:
Direct carbon solid oxide fuel cells (DC-SOFCs) with the novel perovskite La0.3Sr0.7Fe0.7
Received 15 September 2018
Ti0.3O3ed (LSFT) electrodes are evaluated by using biochar derived from some one-time
Received in revised form
toothpicks made of wood as fuel in different discharging modes. The constant current
8 November 2018
(CC) mode and constant resistance (CR) mode are compared with same fuel cell configu-
Accepted 10 November 2018
ration and fuel loading. The results show that the fuel discharged in CR mode possesses
Available online xxx
larger fuel utilization (39.7%) than that in CC mode (34.5%). The biochar fuels obtained from the wood pyrolysis in Ar atmosphere for 1 h at 400 C, 500 C, 600 C and 700 C present the
Keywords:
efficiencies of fuel utilization are 26.3%, 34.1%, 38.6% and 39.6%, respectively. A special
Direct carbon solid oxide fuel cell
discharging mode is employed in this paper for DC-SOFC to test cell performance and
(DC-SOFC)
improve fuel utilization simultaneously as well.
Fuel utilization
© 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Discharging mode Fuel treating temperature
Introduction Due to the combustion of fossil fuels present not only inefficient in energy conversion, but also causing pollution problems, efficient and clean energy conversion devices begin to receive widespread attention. Direct carbon fuel cell (DCFC) is not limited by Carnot cycle, which has high energy conversion efficiency. The theoretical conversion efficiency is attain to DG is slightly more than 1 in DCFC field. 100% since the value of DH Environmentally friendly characteristics of DCFC can avoid polluting problems caused by fossil fuel direct burning [1e3]. Depending on the electrolytes, DCFCs have many types, which
consist of molten hydroxide fuel cell (MHFC) [4,5], molten carbonate fuel cell (MCFC) [6,7], hybrid direct carbon fuel cell (HDCFC) [8,9], solid oxide fuel cell (SOFC) [10,11] and so on. Direct carbon solid oxide fuel cell (DC-SOFC) is a real solid state fuel cell composed of solid electrolyte, electrodes and operated with solid carbon fuel. DC-SOFC can directly convert the chemical energy of carbon fuel into electricity with high efficiency and avoid the environmental pollution emission of combustion during usual thermal power generation [12e16]. Meanwhile, DC-SOFC does not require any external gas fuel during operation and the work of DC-SOFC relies on the coupling of CO and CO2 [1]. DC-SOFCs can use biochar, derived from plants which convert solar energy into chemical energy
* Corresponding author. Department of Physics, Harbin Institute of Technology, 92 Xi Dazhi Street, Harbin 150001, PR China. E-mail address: [email protected] (Z. Lu¨). https://doi.org/10.1016/j.ijhydene.2018.11.073 0360-3199/© 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article as: Wang C et al., Effects of discharge mode and fuel treating temperature on the fuel utilization of direct carbon solid oxide fuel cell, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2018.11.073
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through photosynthesis, as a renewable carbon source [17e20]. The main internal reactions in DC-SOFC were found and proved as follow [3,11]. CO þ O2 /CO2 þ 2e
(1)
C þ CO2 /2CO
(2)
The cycle of two reactions realizes the generation and consumption of CO gas to maintain the function of DC-SOFC. Reaction (1) is an electrochemical oxidation reaction of CO to CO2 on the anode. Reaction (2) is reverse Boudouard reaction on the carbon fuel, and the rate of this reaction affects the cell performance. The reactant CO2 of reaction (2) is primarily produced from reaction (1) and the product CO of reaction (2) can be used as fuel for DC-SOFC. Meanwhile, the constant current (CC) mode is often used to test the discharging performance of DC-SOFC [11,18e21]. The loss of the specific surface area of solid carbon fuel cause the reduction of CO production rate to decrease the discharge voltage of the cell [18e24]. When the CO gas production rate can not support the large current density discharge, the voltage of the cell will drop to 0 V quickly to stop the discharging process, although there is still some residual carbon fuel in the cell, causing incomplete fuel utilization [19,24]. Therefore, it is necessary to find a more suitable discharge mode for DC-SOFC. Tang et al. fabricated a YSZ supported symmetrical Ag-GDC cell, which achieved 13% and 18% fuel utilization by activated carbon and 20% Fe-loaded activated carbon in CC mode [22]. Bai et al. fabricated a YSZ tubular cone-shaped Ni-based anode supported solid oxide fuel cells, which delivered 28% fuel utilization by 5% Fe-loaded activated carbon in CC mode [24]. Cai et al. fabricated a YSZ supported symmetrical Ag-GDC cell, which achieved 18.2% fuel utilization by 5% Ca-loaded activated carbon in CC mode [25]. These kind of cells using activated carbon have lower fuel utilization less than 30% in CC mode. Up to now, commercial activated carbon is still a usual carbon fuel for DCFC in most work despite of its relative high price [26e34]. At the same time, wastes or bi-products from industry, agriculture and forestry were also used as carbon fuel resources [16e19,35e39]. Jiang et al. used a waste product of medium density fibreboard treated at 400 C in powder with 20% Li2CO3eNa2CO3 (62:38 mol) as fuel, which had a fuel
utilization of 21.5% in 13 h CC mode discharging process [16]. Zhu et al. reported a La0.75Sr0.25Cr0.5Mn0.5O3-d impregnated porous YSZ anode supported cell, which generated 1000 min stable discharging process with fuel of carbonated willow leaves powder by U-tube. However, the surface reunion of carbon fuel made replacement with massive carbon, causing the uncertainty of fuel utilization [20]. Zhou et al. operated a symmetrical Ag-GDC cell fueled by pyrolysis con corb powder treated at 700 C and got a fuel utilization of 38% in CC mode. After discharging test of the cell, residual carbon had been sintered into bulk, which was not completely converted into electric energy [19]. In this paper, disposable toothpicks from daily life are employed to simulate usual woodiness carbon source since they can still maintain a certain mechanical strength in a strip structure after pyrolysis technology at high temperature, which is more convenient for filling and re-filling carbon fuel than powder carbon fuel. The influence of heat treating temperature on the utilization of carbon fuel in DC-SOFC is also investigated. The effect of the CC and constant resistance (CR) mode on the fuel utilization is explored. On the basis of the CC and CR mode, a mixed discharge mode is proposed.
Experimental La0.3Sr0.7Fe0.7Ti0.3O3-d (LSFT) electrode material was synthesized via conventional solid state reaction method, and the detailed fabrication process of the symmetrical cell was described in our previous papers [40,41]. Stoichiometric amounts of La2O3, SrCO3, Fe2O3 and TiO2 were weighted and ball milled for 24 h and dried under the baking lamp for 1 h. Then, the mixed powder was calcined at 1400 C for 5 h in air and cooled down to room temperature to obtain LSFT powder. The 8 mol% yttria-stabilized-zirconia (8YSZ) powder was pressed into pellets under the pressure of ~250 MPa with a die of 13 mm in diameter and sintered at 1400 C for 4 h in air. After sintering, the dense electrolyte was ~400 mm in thickness. The buffer layer of Sm0.2Ce0.8O1.9 (SDC) was prepared by spin coating on the surface of 8YSZ electrolyte pellets and cosintered at 1300 C for 2 h to prevent harmful reaction. The electrode powder was ground with binder of ethyl cellulose in terpineol. The LSFT slurry was coated on the two sides of the electrolyte and co-sintered at 1200 C for 2 h to obtain the
Fig. 1 e Pictures of one-time toothpicks (a) in untreated state and (b) after high temperature treatment of 700 C in Ar gas for 1 h. Please cite this article as: Wang C et al., Effects of discharge mode and fuel treating temperature on the fuel utilization of direct carbon solid oxide fuel cell, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2018.11.073
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symmetrical cell with LSFT electrode. The effective areas of electrode were 0.12 cm2. Toothpicks made of wood, as shown in Fig. 1a , were respectively heat treated at 400, 500, 600 and 700 C for 1 h in a quartz tube with Ar (99.9%) as purging gas After cooling down to room temperature with the protection of Ar, biochars are obtained (BC-400, 500, 600, 700). The char from heat treating at 700 C was shown in Fig. 1b. The DC-SOFC test system is shown in Fig. 2. Carbon fuel bars were loaded in the anode chamber of DC-SOFC and stationary air was used as oxidant at cathode. Electrochemical performance of the DC-SOFC was characterized by an electrochemical workstation (Ivium Stat) and the cell was stabilized at 750, 800 and 850 C for 15 min prior to test. The IeVeP curves were measured from open circuit voltage (OCV) to 0 V at the scanning rate of 20 mV/s and the AC impedance spectrum was measured at OCV over a frequency range of 400 kHz to 10 mHz with an AC amplitude of 10 mV. The microstructure of electrodes and carbon fuel was revealed by a scanning electron microscope (SEM, ZEISS SIGMA VP). Thermogravimetry analysis (TGA) and differential thermal analysis (DTA) of carbon were tested by SDT2960 (TA instruments) in air from room temperature to near 1000 C with a heating rate of 10 C/ min.
Results and discussion Characterization of DC-SOFC Fig. 3a and b show the output performance for the DC-SOFC fueled by BC-700. OCVs are 1.00 V, 0.98 V and 0.94 V at 850, 800 and 750 C, respectively. 1.00 V is slightly lower than the theoretical OCV for 1.06 V calculated by Nernst equation at 850 C. According to the Nernst equation, the OCV is related to CO ), illustrating that higher CO partial pressure means log(PPCO 2 higher OCV, while CO partial pressure is related to reverse Boudouard reaction. The carbon source with larger surface area is more favorable for gas-solid reaction on its surface,
Fig. 2 e Testing device of DC-SOFC with symmetrical LSFT electrode fueled by biochar.
Fig. 3 e Electrochemical measurements of DC-SOFC fueled by biochar at different temperatures (a) AC impedance spectra of whole cell; (b) IeVeP curves.
leading to higher CO concentration and OCVs as well. Otherwise, Liu group added catalyst into activated carbon, such as Fe2O3 and CaO, which promoting the reaction rate of the reverse Boudouard reaction, also causing higher OCVs [22,25]. Fig. 3a shows the AC impedance spectra of the cell with the working temperature from 750 to 850 C. The impedance spectroscopy includes two depressed arcs and it can be fitted by Zview software using the equivalent circuit as shown in Fig. 3a. The fitting result of Rh (high frequency arc) are 0.15 U cm2, 0.19 U cm2, 0.29 U cm2, and Rl (low frequency arc) are 0.5 U cm2, 0.4 U cm2, 0.59 U cm2 at 850, 800 and 750 C, respectively. The Rh represents the charge transfer resistance and it decreases with elevated operating temperature. The Rl is related to the process of gas adsorption, dissociation and diffusion. Therefore, it is related to the gas concentration in the anodic gas chamber. When the temperature rises from 750 to 800 C, the concentration of CO increases and the low frequency decreases. When the temperature further rise, Rl increases to 0.5 U cm2. Fig. 3b shows the IeVeP curves of the cell with the working temperature from 750 to 850 C. The maximum power densities (MPD) are 223.0 mW/cm2, 138.6 mW/cm2, 74.2 mW/cm2 at 850, 800 and 750 C, respectively. The MPD of DC-SOFC were lower than the SOFC operated with hydrogen fuel as widely reported in the previous paper [18,21].
Please cite this article as: Wang C et al., Effects of discharge mode and fuel treating temperature on the fuel utilization of direct carbon solid oxide fuel cell, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2018.11.073
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The performance and fuel utilization of the cell under CR and CC discharge mode are compared in Fig. 4a. The discharge voltage of the CR and the CC mode is stable during the whole discharging process. The fuel utilization increases from 34.5% to 39.7%. In CC mode, the inside CO gas may be not enough to maintain the cell's consumption, in other words, the sudden drop of discharge voltage is due to the inadequacy of the internal CO gas supply in the cell. Meanwhile, there is still carbon fuel containing inside the cell. According to Ohm's law, the cell discharging can be expressed as I¼
ε rþR The maximum current is
Imax ¼
ε r
ε is cell electrodynamic force (EMF), r is internal resistance of cell, R is load resistance of external circuit and I is discharge current in the electric circuit. The discharging voltage drop is mainly due to two reasons, decreasing EMF or increasing r. EMF is related to the concentration of CO gas. r is related to the performance of electrodes, electrolyte and the concentration
of CO gas. Decreasing EMF is mainly due to the consumption of carbon and decrease of surface active sites, r is also influenced by these two factors. In CC mode, as discharging time goes by, if r increase, Imax decrease. When Imax is less than the discharge current set in CC mode, the discharge voltage drops suddenly to stop working. Nevertheless, the discharge current changes with the terminal voltage in CR mode. If there is an EMF, there will be a discharge current, therefore, the carbon fuel inside the cell chamber can be fully converted into electricity to improve fuel utilization. Many factors may affect fuel utilization. Firstly, a small amount of O2 may leek into the cell chamber through the sealing region or the end of tube, which will also cause quick consumption of carbon and the discharge voltage drop. Secondly, a small amount of carbon can react with the residual O2 in chamber to produce CO2. Thirdly, CO gas could go out of the cell chamber while discharging. Finally, the ash of carbon fuel can not be used for discharging, but it is assumed to fuel during the calculation of fuel utilization. Fig. 4b illustrates the discharge tests of BC-400, 500, 600, 700 in the CR mode. It is found that the voltage of the cells keep steady during the continual discharging process for 16.33, 21.87, 24.30 and 26.33 h, respectively. According to the Faraday's law, the fuel utilization could be calculated as follow. Q ¼ nzF h¼
Q1 Q2
Q is capacity, n is molar mass, z is the number of transfer electron, F is Faraday constant, Q1 is the discharging capacity and Q2 is the total capacity. Discharge capacities are 1691 C, 2194 C, 2483 C and 2554 C, which corresponding to the fuel utilization of 26.3%, 34.1%, 38.6% and 39.7%, respectively. It is obvious that the higher temperature treatment for carbon, the higher fuel utilization and the longer discharging time for cell. After higher temperature treatment, biochar will have a higher carbonization degree. Carbon which was treated at relatively lower temperature will leave over some volatiles that run away during heating up process of the DC-SOFC before operation at high temperature. This may cause lower fuel utilization. Table 1 compares the fuel utilization data of different fuels. It can be seen that this work using BC-700 as fuel and discharging in CR mode presents a high fuel utilization near 40%. Compared with other carbon sources, this result is quite excellent. Subsequent research may aimed at searching for carbon sources with lower ash content and higher heat treating temperature for higher fuel utilization.
A mixed discharging mode
Fig. 4 e Discharging curves of DC-SOFCs fueled by 0.2 g biochar at 850 C (a) discharging cures with CR (25U) mode and CC (24 mA) mode; (b) discharging cures with CR (25U) mode fueled by biochar with different treating temperatures.
A DC-SOFC fueled by 0.2 g BC-400 was tested in a current stepup mode as Fig. 5a. In this mode, the discharge current was increased step by step from 20 mA to 80 mA with an interval of 10 mA, and each current stage was kept for 10 min, then rested at open circuit condition for 15 min. Subsequently, the cell was discharged for 10 h with a current of 30 mA, and after 15 min stand-by, the cell was discharged with a constant resistance load of 25 U until the voltage of the cell drop to 0 V. As
Please cite this article as: Wang C et al., Effects of discharge mode and fuel treating temperature on the fuel utilization of direct carbon solid oxide fuel cell, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2018.11.073
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Table 1 e Comparisons of utilization of carbon fuel. The type of fuel
Discharging mode
Treating temperature ( C)
Fuel utilization (%)
Ref.
CR CC CC CC CC CC CC CC CC
700 700 e e e e 700 700 e
39.7 34.5 18.2 12.6 9.9 7.2 38 28.4 17
This work This work [25] e e [26] [19] [18] [21]
Toothpicks 5% Ca-loaded active carbon 5% Fe-loaded active carbon Activated carbon Active carbon with FemOn-MxO (M ¼ Li, K, Ca) Corn cob char Leaf char 5% Fe-loaded activated carbon
can be seen from Fig. 5a and Table 2, when the cell is discharged with 20 mA or 30 mA, the voltage platforms of the cell increase slightly, the ramp values are 3.9 and 13.4 mV/s, respectively. When the cell discharge current is higher than 30 mA, the voltage platforms of the cell attenuate with ramp values of 6.0, 19.3, 24.5, 30.6 and 34.8 mV/s, respectively. The higher discharge current causes rapid voltage degradation. When discharging with a larger current, more O2 and CO react on anode zone as reaction (1), however, the rate of the reverse Boudouard reaction is slow and can not follow the consuming rate of CO on the anode, thus the voltage decreases with time. When the discharge current is smaller, the reverse Boudouard reaction can provide enough CO gas as fuel. Therefore, the discharge voltage platforms can exhibit increases. A mixed discharging mode is shown in Fig. 5b, the discharge voltage of CC mode possesses a little increase, which illustrating the cell voltage decreasing in different current discharging process is caused by the test condition rather than the cell degradation. The cell voltage decrease is due to the test mechanism. In future, we could firstly use the discharging mode of the multiplying rate to find the maximum current that the cell can sustain, and then the cell can be tested with a stable discharging voltage platform. If the cell is required for specific situation, the discharging current need to be maintained in long term work. Considering the fuel efficiency, the CC and CR mode can be used for discharging of the cell together. At the end of the discharging, the CR mode is used to make the internal carbon fuel fully converted and improve the fuel utilization.
Microstructure of the cell Fig. 5 e Discharging curves of 0.2 g biochar at 850 C (a) discharging mode with current from 20 mA to 80 mA; (b) mixed mode discharge with step-up CC mode, long time CC mode and end with CR mode.
Fig. 6a is the cross section of the cell after testing. The 8YSZ electrolyte is dense and it can keep good gas tightness. The thickness of the SDC buffer layer is ~20 mm and the thickness of the LSFT electrode is ~25 mm. Fig. 6b is the surface of the prepared electrode. The particles on the surface of the electrode are about hundreds of nanometer, and the connectivity
Table 2 e Fitting slope of step-up current discharging. Discharging Current (mA) Fitting Slope (mV/s)
20 mA 3.9
30 mA 13.4
40 mA 6.0
50 mA 19.3
60 mA 24.5
70 mA 30.6
80 mA 34.8
Please cite this article as: Wang C et al., Effects of discharge mode and fuel treating temperature on the fuel utilization of direct carbon solid oxide fuel cell, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2018.11.073
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Fig. 6 e SEM images (a) cross section of LSFTjSDC buffer layerjYSZ; (b) surface of LSFT before test; (c) and (d) surface of LSFT anode and cathode after test.
Fig. 7 e SEM images (a) and (b) cross section and surface of toothpick; (c) and (d) cross section and surface of BC-700. Please cite this article as: Wang C et al., Effects of discharge mode and fuel treating temperature on the fuel utilization of direct carbon solid oxide fuel cell, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2018.11.073
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of the particles is good. These pores ensure the adsorption and diffusion of the gas in the electrode. Fig. 6c and d are the anode and cathode after test. The particles of the anode are aggregated and there is no obvious carbon deposition on the surface. The agglomeration of the cathode is not obvious.
Characterization of biochar As shown in Fig. 7a and b, the cross section of the toothpick are hollow tubes with diameter in the order of ten microns, they are surrounded by multi layers, and the surface has a long striped texture. The cross section and surface structure of the BC-700 are shown in Fig. 7c and d, the cross section still maintain a hollow tube structure, but walls become thinner and the multilevel structure disappears. Meanwhile, because of the cracking during thermal treatment, the shrinkage of the biochar leads to a ridge surface. The thermogravimetric curve of BC-400, 500, 600, 700 is shown in Fig. S1. The specific data is arranged in Tab S1. With the increase of ambient temperature, the quality of carbon decreases gradually. The small amount of weight loss for carbon is mainly due to the internal water desorption before 200 C. The dehydration losses are 0.71%, 0.75%, 1.18% and 1.73%, respectively. When the ambient temperature is above 400 C, the quality of carbon decreases obviously due to the temperature reaches up to the burning point of carbon, and weight losses are 96.16%, 95.97%, 94.92% and 94.83%, respectively. Tests of the fracture force and impact resistance on BC-700 are also evaluated in this paper. The fracture force values of five samples with heat treating temperature of 700 C are tested with device as Fig. S2. Force curve of sample 1 is shown in Fig. S3. As shown in Table S2, the average fracture force of three point bending test is 1.751 N. Free falling test to Al2O3 board from 0.5 m to 2.0 m is applied for testing the impact resistance of biochar and the biochar keep original shape after 2 m drop as shown in Fig. S4a and b. However, the charcoal is in broken after 2 m drop as shown in Fig. S4c and d. Toothpicks are not only easily obtained from daily life, but also have a certain mechanistic strength after high temperature treatment. Comparing with powdery carbon, it can be filled in the cell chamber easily and avoid dispersing.
Conclusions DC-SOFCs with a symmetrical configuration using LSFT electrodes were operated with BC-700. The MPD is 223.0 mW/cm2 at 850 C. The fuel utilization of CR mode is 5.2% higher than CC mode. The fuel utilization of carbon under different heat treating temperatures is compared. The fuel utilization of carbon after 700 C heat treatment is the highest, reaching up to 39.7%, and the discharging capacity is 2554 C. The cell remained stable during the 26.33 h discharging process. A mixed discharging mode is presented with step-up CC discharging, long time CC discharging and final CR discharging processes, the cell shows different voltage platforms at small and large discharge current because of the difference between electrochemical reaction rate and reverse Boudouard reaction rate.
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Acknowledgements This work was supported by the National Natural Science Foundation of China (21773048, 51872067).
Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.ijhydene.2018.11.073.
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Please cite this article as: Wang C et al., Effects of discharge mode and fuel treating temperature on the fuel utilization of direct carbon solid oxide fuel cell, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2018.11.073
Available online at www.sciencedirect.com
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Effects of discharge mode and fuel treating temperature on the fuel utilization of direct carbon solid oxide fuel cell Chaoqi Wang a, Zhe Lu¨ a,*, Chaoxiang Su a, Jingwei Li a, Zhiqun Cao b, Xingbao Zhu a, Yanyan Wu a, Huan Li a a b
Department of Physics, Harbin Institute of Technology, Harbin, PR China College of Chemistry and Environmental Engineering, Shenzhen Univ, Shenzhen, PR China
article info
abstract
Article history:
Direct carbon solid oxide fuel cells (DC-SOFCs) with the novel perovskite La0.3Sr0.7Fe0.7
Received 15 September 2018
Ti0.3O3ed (LSFT) electrodes are evaluated by using biochar derived from some one-time
Received in revised form
toothpicks made of wood as fuel in different discharging modes. The constant current
8 November 2018
(CC) mode and constant resistance (CR) mode are compared with same fuel cell configu-
Accepted 10 November 2018
ration and fuel loading. The results show that the fuel discharged in CR mode possesses
Available online xxx
larger fuel utilization (39.7%) than that in CC mode (34.5%). The biochar fuels obtained from the wood pyrolysis in Ar atmosphere for 1 h at 400 C, 500 C, 600 C and 700 C present the
Keywords:
efficiencies of fuel utilization are 26.3%, 34.1%, 38.6% and 39.6%, respectively. A special
Direct carbon solid oxide fuel cell
discharging mode is employed in this paper for DC-SOFC to test cell performance and
(DC-SOFC)
improve fuel utilization simultaneously as well.
Fuel utilization
© 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Discharging mode Fuel treating temperature
Introduction Due to the combustion of fossil fuels present not only inefficient in energy conversion, but also causing pollution problems, efficient and clean energy conversion devices begin to receive widespread attention. Direct carbon fuel cell (DCFC) is not limited by Carnot cycle, which has high energy conversion efficiency. The theoretical conversion efficiency is attain to DG is slightly more than 1 in DCFC field. 100% since the value of DH Environmentally friendly characteristics of DCFC can avoid polluting problems caused by fossil fuel direct burning [1e3]. Depending on the electrolytes, DCFCs have many types, which
consist of molten hydroxide fuel cell (MHFC) [4,5], molten carbonate fuel cell (MCFC) [6,7], hybrid direct carbon fuel cell (HDCFC) [8,9], solid oxide fuel cell (SOFC) [10,11] and so on. Direct carbon solid oxide fuel cell (DC-SOFC) is a real solid state fuel cell composed of solid electrolyte, electrodes and operated with solid carbon fuel. DC-SOFC can directly convert the chemical energy of carbon fuel into electricity with high efficiency and avoid the environmental pollution emission of combustion during usual thermal power generation [12e16]. Meanwhile, DC-SOFC does not require any external gas fuel during operation and the work of DC-SOFC relies on the coupling of CO and CO2 [1]. DC-SOFCs can use biochar, derived from plants which convert solar energy into chemical energy
* Corresponding author. Department of Physics, Harbin Institute of Technology, 92 Xi Dazhi Street, Harbin 150001, PR China. E-mail address: [email protected] (Z. Lu¨). https://doi.org/10.1016/j.ijhydene.2018.11.073 0360-3199/© 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article as: Wang C et al., Effects of discharge mode and fuel treating temperature on the fuel utilization of direct carbon solid oxide fuel cell, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2018.11.073
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through photosynthesis, as a renewable carbon source [17e20]. The main internal reactions in DC-SOFC were found and proved as follow [3,11]. CO þ O2 /CO2 þ 2e
(1)
C þ CO2 /2CO
(2)
The cycle of two reactions realizes the generation and consumption of CO gas to maintain the function of DC-SOFC. Reaction (1) is an electrochemical oxidation reaction of CO to CO2 on the anode. Reaction (2) is reverse Boudouard reaction on the carbon fuel, and the rate of this reaction affects the cell performance. The reactant CO2 of reaction (2) is primarily produced from reaction (1) and the product CO of reaction (2) can be used as fuel for DC-SOFC. Meanwhile, the constant current (CC) mode is often used to test the discharging performance of DC-SOFC [11,18e21]. The loss of the specific surface area of solid carbon fuel cause the reduction of CO production rate to decrease the discharge voltage of the cell [18e24]. When the CO gas production rate can not support the large current density discharge, the voltage of the cell will drop to 0 V quickly to stop the discharging process, although there is still some residual carbon fuel in the cell, causing incomplete fuel utilization [19,24]. Therefore, it is necessary to find a more suitable discharge mode for DC-SOFC. Tang et al. fabricated a YSZ supported symmetrical Ag-GDC cell, which achieved 13% and 18% fuel utilization by activated carbon and 20% Fe-loaded activated carbon in CC mode [22]. Bai et al. fabricated a YSZ tubular cone-shaped Ni-based anode supported solid oxide fuel cells, which delivered 28% fuel utilization by 5% Fe-loaded activated carbon in CC mode [24]. Cai et al. fabricated a YSZ supported symmetrical Ag-GDC cell, which achieved 18.2% fuel utilization by 5% Ca-loaded activated carbon in CC mode [25]. These kind of cells using activated carbon have lower fuel utilization less than 30% in CC mode. Up to now, commercial activated carbon is still a usual carbon fuel for DCFC in most work despite of its relative high price [26e34]. At the same time, wastes or bi-products from industry, agriculture and forestry were also used as carbon fuel resources [16e19,35e39]. Jiang et al. used a waste product of medium density fibreboard treated at 400 C in powder with 20% Li2CO3eNa2CO3 (62:38 mol) as fuel, which had a fuel
utilization of 21.5% in 13 h CC mode discharging process [16]. Zhu et al. reported a La0.75Sr0.25Cr0.5Mn0.5O3-d impregnated porous YSZ anode supported cell, which generated 1000 min stable discharging process with fuel of carbonated willow leaves powder by U-tube. However, the surface reunion of carbon fuel made replacement with massive carbon, causing the uncertainty of fuel utilization [20]. Zhou et al. operated a symmetrical Ag-GDC cell fueled by pyrolysis con corb powder treated at 700 C and got a fuel utilization of 38% in CC mode. After discharging test of the cell, residual carbon had been sintered into bulk, which was not completely converted into electric energy [19]. In this paper, disposable toothpicks from daily life are employed to simulate usual woodiness carbon source since they can still maintain a certain mechanical strength in a strip structure after pyrolysis technology at high temperature, which is more convenient for filling and re-filling carbon fuel than powder carbon fuel. The influence of heat treating temperature on the utilization of carbon fuel in DC-SOFC is also investigated. The effect of the CC and constant resistance (CR) mode on the fuel utilization is explored. On the basis of the CC and CR mode, a mixed discharge mode is proposed.
Experimental La0.3Sr0.7Fe0.7Ti0.3O3-d (LSFT) electrode material was synthesized via conventional solid state reaction method, and the detailed fabrication process of the symmetrical cell was described in our previous papers [40,41]. Stoichiometric amounts of La2O3, SrCO3, Fe2O3 and TiO2 were weighted and ball milled for 24 h and dried under the baking lamp for 1 h. Then, the mixed powder was calcined at 1400 C for 5 h in air and cooled down to room temperature to obtain LSFT powder. The 8 mol% yttria-stabilized-zirconia (8YSZ) powder was pressed into pellets under the pressure of ~250 MPa with a die of 13 mm in diameter and sintered at 1400 C for 4 h in air. After sintering, the dense electrolyte was ~400 mm in thickness. The buffer layer of Sm0.2Ce0.8O1.9 (SDC) was prepared by spin coating on the surface of 8YSZ electrolyte pellets and cosintered at 1300 C for 2 h to prevent harmful reaction. The electrode powder was ground with binder of ethyl cellulose in terpineol. The LSFT slurry was coated on the two sides of the electrolyte and co-sintered at 1200 C for 2 h to obtain the
Fig. 1 e Pictures of one-time toothpicks (a) in untreated state and (b) after high temperature treatment of 700 C in Ar gas for 1 h. Please cite this article as: Wang C et al., Effects of discharge mode and fuel treating temperature on the fuel utilization of direct carbon solid oxide fuel cell, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2018.11.073
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symmetrical cell with LSFT electrode. The effective areas of electrode were 0.12 cm2. Toothpicks made of wood, as shown in Fig. 1a , were respectively heat treated at 400, 500, 600 and 700 C for 1 h in a quartz tube with Ar (99.9%) as purging gas After cooling down to room temperature with the protection of Ar, biochars are obtained (BC-400, 500, 600, 700). The char from heat treating at 700 C was shown in Fig. 1b. The DC-SOFC test system is shown in Fig. 2. Carbon fuel bars were loaded in the anode chamber of DC-SOFC and stationary air was used as oxidant at cathode. Electrochemical performance of the DC-SOFC was characterized by an electrochemical workstation (Ivium Stat) and the cell was stabilized at 750, 800 and 850 C for 15 min prior to test. The IeVeP curves were measured from open circuit voltage (OCV) to 0 V at the scanning rate of 20 mV/s and the AC impedance spectrum was measured at OCV over a frequency range of 400 kHz to 10 mHz with an AC amplitude of 10 mV. The microstructure of electrodes and carbon fuel was revealed by a scanning electron microscope (SEM, ZEISS SIGMA VP). Thermogravimetry analysis (TGA) and differential thermal analysis (DTA) of carbon were tested by SDT2960 (TA instruments) in air from room temperature to near 1000 C with a heating rate of 10 C/ min.
Results and discussion Characterization of DC-SOFC Fig. 3a and b show the output performance for the DC-SOFC fueled by BC-700. OCVs are 1.00 V, 0.98 V and 0.94 V at 850, 800 and 750 C, respectively. 1.00 V is slightly lower than the theoretical OCV for 1.06 V calculated by Nernst equation at 850 C. According to the Nernst equation, the OCV is related to CO ), illustrating that higher CO partial pressure means log(PPCO 2 higher OCV, while CO partial pressure is related to reverse Boudouard reaction. The carbon source with larger surface area is more favorable for gas-solid reaction on its surface,
Fig. 2 e Testing device of DC-SOFC with symmetrical LSFT electrode fueled by biochar.
Fig. 3 e Electrochemical measurements of DC-SOFC fueled by biochar at different temperatures (a) AC impedance spectra of whole cell; (b) IeVeP curves.
leading to higher CO concentration and OCVs as well. Otherwise, Liu group added catalyst into activated carbon, such as Fe2O3 and CaO, which promoting the reaction rate of the reverse Boudouard reaction, also causing higher OCVs [22,25]. Fig. 3a shows the AC impedance spectra of the cell with the working temperature from 750 to 850 C. The impedance spectroscopy includes two depressed arcs and it can be fitted by Zview software using the equivalent circuit as shown in Fig. 3a. The fitting result of Rh (high frequency arc) are 0.15 U cm2, 0.19 U cm2, 0.29 U cm2, and Rl (low frequency arc) are 0.5 U cm2, 0.4 U cm2, 0.59 U cm2 at 850, 800 and 750 C, respectively. The Rh represents the charge transfer resistance and it decreases with elevated operating temperature. The Rl is related to the process of gas adsorption, dissociation and diffusion. Therefore, it is related to the gas concentration in the anodic gas chamber. When the temperature rises from 750 to 800 C, the concentration of CO increases and the low frequency decreases. When the temperature further rise, Rl increases to 0.5 U cm2. Fig. 3b shows the IeVeP curves of the cell with the working temperature from 750 to 850 C. The maximum power densities (MPD) are 223.0 mW/cm2, 138.6 mW/cm2, 74.2 mW/cm2 at 850, 800 and 750 C, respectively. The MPD of DC-SOFC were lower than the SOFC operated with hydrogen fuel as widely reported in the previous paper [18,21].
Please cite this article as: Wang C et al., Effects of discharge mode and fuel treating temperature on the fuel utilization of direct carbon solid oxide fuel cell, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2018.11.073
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The performance and fuel utilization of the cell under CR and CC discharge mode are compared in Fig. 4a. The discharge voltage of the CR and the CC mode is stable during the whole discharging process. The fuel utilization increases from 34.5% to 39.7%. In CC mode, the inside CO gas may be not enough to maintain the cell's consumption, in other words, the sudden drop of discharge voltage is due to the inadequacy of the internal CO gas supply in the cell. Meanwhile, there is still carbon fuel containing inside the cell. According to Ohm's law, the cell discharging can be expressed as I¼
ε rþR The maximum current is
Imax ¼
ε r
ε is cell electrodynamic force (EMF), r is internal resistance of cell, R is load resistance of external circuit and I is discharge current in the electric circuit. The discharging voltage drop is mainly due to two reasons, decreasing EMF or increasing r. EMF is related to the concentration of CO gas. r is related to the performance of electrodes, electrolyte and the concentration
of CO gas. Decreasing EMF is mainly due to the consumption of carbon and decrease of surface active sites, r is also influenced by these two factors. In CC mode, as discharging time goes by, if r increase, Imax decrease. When Imax is less than the discharge current set in CC mode, the discharge voltage drops suddenly to stop working. Nevertheless, the discharge current changes with the terminal voltage in CR mode. If there is an EMF, there will be a discharge current, therefore, the carbon fuel inside the cell chamber can be fully converted into electricity to improve fuel utilization. Many factors may affect fuel utilization. Firstly, a small amount of O2 may leek into the cell chamber through the sealing region or the end of tube, which will also cause quick consumption of carbon and the discharge voltage drop. Secondly, a small amount of carbon can react with the residual O2 in chamber to produce CO2. Thirdly, CO gas could go out of the cell chamber while discharging. Finally, the ash of carbon fuel can not be used for discharging, but it is assumed to fuel during the calculation of fuel utilization. Fig. 4b illustrates the discharge tests of BC-400, 500, 600, 700 in the CR mode. It is found that the voltage of the cells keep steady during the continual discharging process for 16.33, 21.87, 24.30 and 26.33 h, respectively. According to the Faraday's law, the fuel utilization could be calculated as follow. Q ¼ nzF h¼
Q1 Q2
Q is capacity, n is molar mass, z is the number of transfer electron, F is Faraday constant, Q1 is the discharging capacity and Q2 is the total capacity. Discharge capacities are 1691 C, 2194 C, 2483 C and 2554 C, which corresponding to the fuel utilization of 26.3%, 34.1%, 38.6% and 39.7%, respectively. It is obvious that the higher temperature treatment for carbon, the higher fuel utilization and the longer discharging time for cell. After higher temperature treatment, biochar will have a higher carbonization degree. Carbon which was treated at relatively lower temperature will leave over some volatiles that run away during heating up process of the DC-SOFC before operation at high temperature. This may cause lower fuel utilization. Table 1 compares the fuel utilization data of different fuels. It can be seen that this work using BC-700 as fuel and discharging in CR mode presents a high fuel utilization near 40%. Compared with other carbon sources, this result is quite excellent. Subsequent research may aimed at searching for carbon sources with lower ash content and higher heat treating temperature for higher fuel utilization.
A mixed discharging mode
Fig. 4 e Discharging curves of DC-SOFCs fueled by 0.2 g biochar at 850 C (a) discharging cures with CR (25U) mode and CC (24 mA) mode; (b) discharging cures with CR (25U) mode fueled by biochar with different treating temperatures.
A DC-SOFC fueled by 0.2 g BC-400 was tested in a current stepup mode as Fig. 5a. In this mode, the discharge current was increased step by step from 20 mA to 80 mA with an interval of 10 mA, and each current stage was kept for 10 min, then rested at open circuit condition for 15 min. Subsequently, the cell was discharged for 10 h with a current of 30 mA, and after 15 min stand-by, the cell was discharged with a constant resistance load of 25 U until the voltage of the cell drop to 0 V. As
Please cite this article as: Wang C et al., Effects of discharge mode and fuel treating temperature on the fuel utilization of direct carbon solid oxide fuel cell, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2018.11.073
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Table 1 e Comparisons of utilization of carbon fuel. The type of fuel
Discharging mode
Treating temperature ( C)
Fuel utilization (%)
Ref.
CR CC CC CC CC CC CC CC CC
700 700 e e e e 700 700 e
39.7 34.5 18.2 12.6 9.9 7.2 38 28.4 17
This work This work [25] e e [26] [19] [18] [21]
Toothpicks 5% Ca-loaded active carbon 5% Fe-loaded active carbon Activated carbon Active carbon with FemOn-MxO (M ¼ Li, K, Ca) Corn cob char Leaf char 5% Fe-loaded activated carbon
can be seen from Fig. 5a and Table 2, when the cell is discharged with 20 mA or 30 mA, the voltage platforms of the cell increase slightly, the ramp values are 3.9 and 13.4 mV/s, respectively. When the cell discharge current is higher than 30 mA, the voltage platforms of the cell attenuate with ramp values of 6.0, 19.3, 24.5, 30.6 and 34.8 mV/s, respectively. The higher discharge current causes rapid voltage degradation. When discharging with a larger current, more O2 and CO react on anode zone as reaction (1), however, the rate of the reverse Boudouard reaction is slow and can not follow the consuming rate of CO on the anode, thus the voltage decreases with time. When the discharge current is smaller, the reverse Boudouard reaction can provide enough CO gas as fuel. Therefore, the discharge voltage platforms can exhibit increases. A mixed discharging mode is shown in Fig. 5b, the discharge voltage of CC mode possesses a little increase, which illustrating the cell voltage decreasing in different current discharging process is caused by the test condition rather than the cell degradation. The cell voltage decrease is due to the test mechanism. In future, we could firstly use the discharging mode of the multiplying rate to find the maximum current that the cell can sustain, and then the cell can be tested with a stable discharging voltage platform. If the cell is required for specific situation, the discharging current need to be maintained in long term work. Considering the fuel efficiency, the CC and CR mode can be used for discharging of the cell together. At the end of the discharging, the CR mode is used to make the internal carbon fuel fully converted and improve the fuel utilization.
Microstructure of the cell Fig. 5 e Discharging curves of 0.2 g biochar at 850 C (a) discharging mode with current from 20 mA to 80 mA; (b) mixed mode discharge with step-up CC mode, long time CC mode and end with CR mode.
Fig. 6a is the cross section of the cell after testing. The 8YSZ electrolyte is dense and it can keep good gas tightness. The thickness of the SDC buffer layer is ~20 mm and the thickness of the LSFT electrode is ~25 mm. Fig. 6b is the surface of the prepared electrode. The particles on the surface of the electrode are about hundreds of nanometer, and the connectivity
Table 2 e Fitting slope of step-up current discharging. Discharging Current (mA) Fitting Slope (mV/s)
20 mA 3.9
30 mA 13.4
40 mA 6.0
50 mA 19.3
60 mA 24.5
70 mA 30.6
80 mA 34.8
Please cite this article as: Wang C et al., Effects of discharge mode and fuel treating temperature on the fuel utilization of direct carbon solid oxide fuel cell, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2018.11.073
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Fig. 6 e SEM images (a) cross section of LSFTjSDC buffer layerjYSZ; (b) surface of LSFT before test; (c) and (d) surface of LSFT anode and cathode after test.
Fig. 7 e SEM images (a) and (b) cross section and surface of toothpick; (c) and (d) cross section and surface of BC-700. Please cite this article as: Wang C et al., Effects of discharge mode and fuel treating temperature on the fuel utilization of direct carbon solid oxide fuel cell, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2018.11.073
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of the particles is good. These pores ensure the adsorption and diffusion of the gas in the electrode. Fig. 6c and d are the anode and cathode after test. The particles of the anode are aggregated and there is no obvious carbon deposition on the surface. The agglomeration of the cathode is not obvious.
Characterization of biochar As shown in Fig. 7a and b, the cross section of the toothpick are hollow tubes with diameter in the order of ten microns, they are surrounded by multi layers, and the surface has a long striped texture. The cross section and surface structure of the BC-700 are shown in Fig. 7c and d, the cross section still maintain a hollow tube structure, but walls become thinner and the multilevel structure disappears. Meanwhile, because of the cracking during thermal treatment, the shrinkage of the biochar leads to a ridge surface. The thermogravimetric curve of BC-400, 500, 600, 700 is shown in Fig. S1. The specific data is arranged in Tab S1. With the increase of ambient temperature, the quality of carbon decreases gradually. The small amount of weight loss for carbon is mainly due to the internal water desorption before 200 C. The dehydration losses are 0.71%, 0.75%, 1.18% and 1.73%, respectively. When the ambient temperature is above 400 C, the quality of carbon decreases obviously due to the temperature reaches up to the burning point of carbon, and weight losses are 96.16%, 95.97%, 94.92% and 94.83%, respectively. Tests of the fracture force and impact resistance on BC-700 are also evaluated in this paper. The fracture force values of five samples with heat treating temperature of 700 C are tested with device as Fig. S2. Force curve of sample 1 is shown in Fig. S3. As shown in Table S2, the average fracture force of three point bending test is 1.751 N. Free falling test to Al2O3 board from 0.5 m to 2.0 m is applied for testing the impact resistance of biochar and the biochar keep original shape after 2 m drop as shown in Fig. S4a and b. However, the charcoal is in broken after 2 m drop as shown in Fig. S4c and d. Toothpicks are not only easily obtained from daily life, but also have a certain mechanistic strength after high temperature treatment. Comparing with powdery carbon, it can be filled in the cell chamber easily and avoid dispersing.
Conclusions DC-SOFCs with a symmetrical configuration using LSFT electrodes were operated with BC-700. The MPD is 223.0 mW/cm2 at 850 C. The fuel utilization of CR mode is 5.2% higher than CC mode. The fuel utilization of carbon under different heat treating temperatures is compared. The fuel utilization of carbon after 700 C heat treatment is the highest, reaching up to 39.7%, and the discharging capacity is 2554 C. The cell remained stable during the 26.33 h discharging process. A mixed discharging mode is presented with step-up CC discharging, long time CC discharging and final CR discharging processes, the cell shows different voltage platforms at small and large discharge current because of the difference between electrochemical reaction rate and reverse Boudouard reaction rate.
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Acknowledgements This work was supported by the National Natural Science Foundation of China (21773048, 51872067).
Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.ijhydene.2018.11.073.
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Please cite this article as: Wang C et al., Effects of discharge mode and fuel treating temperature on the fuel utilization of direct carbon solid oxide fuel cell, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2018.11.073