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Ni modified Ce(Mn, Fe)O2 cermet anode for high-performance direct carbon fuel cell
Electrochimica Acta 232 (2017) 174–181
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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Ni modified Ce(Mn, Fe)O2 cermet anode for high-performance direct carbon fuel cell Jia Liua , Jinshuo Qiaoa,* , Hong Yuana , Jie Fenga , Chao Suia , Zhenhua Wanga,b , Wang Suna , Kening Suna,b,* a Beijing Key Laboratory for Chemical Power Source and Green Catalysis, School of Chemical Engineering and Environmental, Beijing Institute of Technology, Beijing, 100081, People’s Republic of China b Innovation Center of Electronic Vehicles in Beijing, No. 5 Zhongguancun South Avenue, Haidian District, Beijing, 100081, People’s Republic of China
A R T I C L E I N F O
Article history: Received 17 January 2017 Accepted 22 February 2017 Available online 24 February 2017 Keywords: Direct carbon fuel cell Ce0.6Mn0.3Fe0.1O2 cermet oxide Ni additive high electrochemical performance
A B S T R A C T
Ni modified Ce0.6Mn0.3Fe0.1O2 (Ni-CMF) material is evaluated as an anode material for hybrid direct carbon fuel cell (HDCFC). The Temperature Programmed Reaction (TPR) and GC test results show that the Ni additive significantly promotes the oxidation of carbon and consequently accelerates the formation rate of CO. Moreover, the Ni-CMF material is found to show a high electrical conductivity compared to pure CMF in HDCFC operating conditions. Taking advantage of the excellent catalytic activity and high conductivity, the electrolyte-supported HDCFC with Ni-CMF anode shows a high maximum power density of 580.7 mW cm2 at 800 C, when activated carbon is used as the fuel. Furthermore, the cell displays good stability under galvanostatic operation at current density of 50 mA cm2 at 750 C. These results indicate that the designed Ni-CMF composite material has the potential to be developed as an alternative anode for HDCFCs. © 2017 Elsevier Ltd. All rights reserved.
1. Introduction Direct carbon fuel cell (DCFC) has gained tremendous attention as a power generation device that directly converts chemical energy of solid carbon to electrical energy with high efficiency, low environmental pollution, and good fuel availability [1–3]. Generally, DCFCs can be classified as molten hydroxide, molten carbonate, solid oxide (SO-DCFC), or hybrid direct carbon fuel cells (HDCFCs), based on the type of electrolyte used [4–7]. Amongst these, HDCFCs have attracted more attention due to the number of benefits they offer [8]. HDCFCs, which employ two different electrolytes, viz. molten carbonate and solid oxide, combine the technologies of molten carbonate carbon fuel cell and solid oxide carbon fuel cell by dispersing solid carbon particles into molten carbonate as anode [9,10]. Such a technology prevents the possible corrosion of cathode and alleviates the need for CO2 circulation, required in traditional molten carbonate carbon fuel
* Corresponding authors at: Beijing Key Laboratory for Chemical Power Source and Green Catalysis, School of Chemical Engineering and Environmental, Beijing Institute of Technology, Beijing, 100081, People’s Republic of China. Tel./Fax: +86010-6891-8696. E-mail addresses: [email protected] (J. Qiao), [email protected] (K. Sun) . http://dx.doi.org/10.1016/j.electacta.2017.02.135 0013-4686/© 2017 Elsevier Ltd. All rights reserved.
cells [11,12]. Furthermore, such a cell configuration can extend the electrochemically active reaction zone from a two-dimensional electrolyte/anode interface to three-dimensional carbon/carbonate slurry at high temperature, and can thus kinetically favor the oxidation of carbon [13–15]. However, despite the advantages it offers, most of the reported HDCFCs generate much lower power densities than cells with gas fuels. The lowered cell performance can be attributed to insufficient electrical conductivity as well as the poor catalytic activity of anodes. More recently, doped ceria-based anodes have attracted considerable attentions in DCFCs owing to their high oxygen ionic conductivity, good catalytic activity and excellent resistance to coking and sulfur poisoning [16–18]. GDC infiltrated (La0.75Sr0.25)0.97Cr0.5Mn0.5O3 (GDC-LSCM) was evaluated as the HDCFC anode with two different carbon fuels [19]. Although the availability of different carbon fuels in HDCFCs was demonstrated, the power density achieved was poor, due to the low electrical conductivity of GDC-LSCM anode in reducing atmosphere. In comparison, Ce0.6Mn0.3Fe0.1O2 (CMF) material was reported to have high conductivity, and the cell with this anode demonstrated a competitive power generation property and oxidation tolerance [20]. In addition, transition metals proved to be highly active catalysts, and could be added to anode materials to effectively
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improve cell performance [21–23]. Amongst these, conductive Ni particles as intensively investigated catalysts exhibit prominent catalytic activity for the oxidation of solid carbon and demonstrate excellent electrochemical performance in SO-DCFCs [24]. Furthermore, adding Ni into the carbon-molten carbonate reduces the apparent activation energy, and significantly improves the electrooxidation activity at low temperatures. The current density on activity of oxidized graphite felt with the addition of Ni is 7-times higher than that without Ni at 750 C [25]. The present work demonstrated that the Ni modified CMF (NiCMF) composite material could be directly applied as a potential HDCFC anode for the first time. For comparison, the performance of the cell with pure CMF anode was also tested. The influence of Ni additive on the catalytic activity, electrical conductivity, and anode performance of CMF oxide was systematically investigated. The steps for the reactions at the anode were also proposed in a scheme. Finally, the electrochemical performance and the stability of Ni-CMF anode were further evaluated by fabricating the LSGM electrolyte-supported HDCFCs. 2. Experimental 2.1. Preparation of materials and pre-treatment of fuel The dopant CMF materials were synthesized by a modified combustion method [26]. Stoichiometric amounts of Ce (NO3)26H2O, Fe(NO3)29H2O and Mn(NO3)24H2O (Sinopharm Chemical Reagent Co., Ltd, AR) were used as metal precursors. Glycine (Tianjin Fuchen chemical reagents factory, AR) and citric acid (Beijing Chemical Works, AR) were added to promote combustion. The resultant ash was then calcined at 1000 C for 6 h in air atmosphere to obtain pure CMF powder. Next, CMF and commercial NiO (Japan Tosoh Co., AR) in a mass ratio of 1:1 were mixed by ball milling in ethanol and dried overnight at 80 C to obtain the NiO-CMF composite. Moreover, commercially available activated carbon was passed through a 1000-mesh standard sieve and then used as the fuel. Lithium carbonate (Li2CO3) and potassium carbonate (K2CO3) (Aladdin, China) were pretreated prior to use as the secondary electrolytes in the cell device. The pre-treatment was carried out according to a previous procedure [27] as follows. Li2CO3 and K2CO3 were dried at 300 C in air and then mixed together in a molar ratio of 62:38 by ball milling in ethanol and then dried overnight at 80 C. 2.2. Cell design and preparation Electrolyte-supported single cells were fabricated using La0.9Sr0.1Ga0.8Mg0.2O3 electrolyte (LSGM, Fuel Cell), La0.6Sr0.4Co0.2Fe0.8O3 cathode (LSCF, Fuel Cell), and Ni-CMF composite anode. The LSGM electrolyte (340 mm in thickness) was prepared by dry pressing method carried out uniaxially using LSGM powder under 200 MPa, and then sintered at 1450 C for 6 h in air atmosphere [28]. Then, an interlayer of Ce0.8Sm0.2O2 (SDC), which prevented a chemical reaction between Ni and LSGM electrolyte in reduced atmosphere, was screen-printed onto the LSGM electrolyte and calcined at 1400 C for 5 h. Later, slurries of anode and cathode were screen-printed onto the SDC interlayer and LSGM electrolyte surface, respectively, and co-calcined at 1100 C for 2 h. In addition, a single cell with CMF anode was also fabricated by the same procedure for comparison. Prior to electrochemical testing, NiO in the composite was reduced in situ to metallic nickel by solid carbon or by the generated CO at 800 C for 2 h. Fig. 1 shows the schematic diagram of the cell design. For current collection, Ag wire was connected on the electrodes with Ag paste (Sino-Platinum Metals Co., Ltd) and then co-calcined at 750 C for 30 min. The single cell was attached on one end of an
Fig. 1. Schematic diagram of the tested cell.
alumina tube using a ceramic paste (Aremco 552). 2.0 g of carboncarbonate mixture (mass ratio of 4:1) was filled into the anodic chamber at room temperature. Ar gas (20 mL min1) was fed into the anodic chamber to prevent combustion of carbon fuel, and the cathode was exposed to ambient air. 2.3. Structure and characteristics The phase purities and crystal structures of NiO-CMF samples, before and after reduction, were determined by X-ray diffraction (XRD) (X’ Pert PRO MPD) using Cu Ka radiation. The microstructures and distribution of elements in the NiO-CMF composite samples were studied using scanning electron microscope (SEM, FEI QUANTA-250), equipped with energy dispersive spectrometer (EDS). The electronic conductivity measurements were performed by a four-probe DC technique (Keithley 2400, USA) in Ar and CO atmosphere. For conductivity measurements, 0.8 g of NiO-CMF or CMF powder was pressed uniaxially into rectangular bars and then sintered at 1200 C for 5 h. Temperature programmed reaction (TPR) experiments were carried out on a thermal analyzer (TA/DTA6200, Seiko) at a heating rate of 5 C min1 from room temperature to 800 C in N2 atmosphere. The experiments were conducted by mixing activated fuel with CMF or NiO-CMF samples (mass ratio of 1:1) in an alumina pan and then placing this pan in a tubular flow reactor. The off-gases from the experiments were analyzed by gas chromatography (GC-2014, Shimadzu), equipped with a column packed with 5A molecular sieves and a thermal conductivity detector (TCD). For GC analysis, helium was used as carrier gas for the detection of argon, carbon monoxide, and carbon dioxide. The percentages of gaseous outputs were determined and converted to gas volumes by external standard methods. Furthermore, the cell performance was evaluated using an Arbin Instruments’ tester (Fuel Cell Test System, FCTS) at 700– 800 C. The electrochemical resistance was measured by electrochemical impedance spectroscopy (EIS, PARSTAT 2273, USA), using a 10 mV DC signal in the frequency range of 10 mHz–100 kHz. The impedance data was fitted and analyzed using ZSimpWin software.
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Fig. 2. (a) XRD patterns of pure CMF powder, NiO-CMF powder before and after reduction at 800 C for 20 h in CO atmosphere; (b) SEM image and elemental distributions of NiO-CMF composite materials.
3. Results and discussion 3.1. XRD and SEM analyses XRD patterns of pure CMF powder, NiO-CMF powder, and NiOCMF powder after reduction in CO atmosphere are presented in Fig. 2(a). Pure CMF powder shows a single-phase fluorite structure (PDF#43-1002) without any impure phases, which indicates that Mn and Fe elements are completely doped into the lattice of CeO2. In case of NiO-CMF sample, characteristic peaks of CMF and NiO can be prominently seen. After reduction, although only a few diffraction peaks of MnxOy phase can be seen, the diffraction peaks are mainly assigned to CMF. In addition to the diffraction peaks of CMF, peaks of metallic Ni phase are also present, indicating that NiO is effectively reduced to Ni by carbon monoxide. This reduced metallic Ni can significantly improve the conductivity of the anode material. Fig. 2(b) shows the microstructures and elemental distribution maps of NiO-CMF composite materials. It is obvious that NiO particles are evenly distributed in the composite materials
without agglomerates, which forms an interconnected porous structure. Such an interconnected porous structure not only provides sufficient number of channels for diffusion of produced CO, but also provides more number of active sites for the oxidation of CO, consequently improving the cell performance. 3.2. TPR and GC study Ni plays an important catalytic role in Ni-based anode for the oxidation of CO or solid carbon [29,30]. Temperature programmed reaction (TPR) measurements are carried out to investigate the improvement in catalytic activity of Ni-CMF anode for activated carbon fuels. Fig. 3(a) and (b) show the TPR curves of CMF and NiOCMF samples, respectively. Fig. 3(a) shows two distinct peaks of CMF sample. The first peak (about 680 C) could possibly be attributed to the reaction of CMF with activated carbon [31]. The second peak at about 815 C can be attributed to the reverse Boudouard reaction. On addition of NiO (Fig. 3(b)), the first peak shifts to a higher temperature of around 700 C, and the TPR curve
Fig. 3. TPR curves of the mixture of activated carbon and the samples: (a) CMF and (b) NiO-CMF; (c) Plots of flow rates of gaseous products at 500–800 C under Ar atmosphere. In Fig. 3(c), the solid line represents the off-gases of activated carbon mixed with NiO-CMF samples and the dashed line indicates the off-gases of activated carbon mixed with CMF samples, respectively.
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Fig. 4. Temperature dependence of electronic conductivity of the sample: (a) NiO-CMF; (b) CMF. Inset in (a) shows the expanded electronic conductivity of NiO-CMF sample in Ar.
becomes rough. This indicates that, in addition to the reaction between activated carbon and CMF, the reduction of bulk NiO occurred at this temperature. Moreover, the peak for the reverse Boudouard reaction shifts to a lower temperature of about 797 C and the intensity of the second peak increases. This suggests much lower activation energy for the reverse Boudouard reaction (Fig. S1) and an enhanced catalytic activity of Ni-CMF for the oxidation of activated carbon. To further establish the catalytic activity of the Ni-CMF anode, off-gases of the mixtures of activated carbon and CMF or Ni-CMF (denoted in the following as CMF/C or Ni-CMF/C) are quantified by GC analysis. Before GC experiments, the NiO-CMF samples are reduced to Ni-CMF at 800 C for 2 h under CO atmosphere. Fig. 3(c) shows the off-gases of CMF/C (dash lines) and Ni-CMF/C (solid line) under Ar atmosphere. CO2 would be the product of the reaction between activated carbon and CMF and CO would be produced in the reverse Boudouard reaction (shown in TPR curves). The formation rate of CO2 for Ni-CMF/C is much higher than that for
CMF/C mixture. Moreover, as the temperature exceeds 700 C, the formation rate of CO for Ni-CMF/C is significantly increased. Accordingly, the rate of reverse Boudouard reaction is higher (because the slope of curve for the formation rate of CO is much steeper). These results further demonstrate that the addition of Ni significantly improves the catalytic activity for the oxidation of activated carbon. 3.3. Conductivity In an attempt to simulate the operating conditions of the anode of HDCFC, the conductivities of NiO-CMF and CMF samples without reduction are determined in CO and Ar atmosphere at 500–800 C. The densities of the NiO-CMF and CMF bars, for conductivity measurements, are about 92% and 93%, respectively. Fig. 4(a) and (b) illustrate the temperature dependence of electrical conductivity of NiO-CMF samples and CMF samples, respectively. In CO atmosphere, the NiO-CMF sample shows higher electrical
Fig. 5. Electrochemical performance of SO-DCFCs with Ni-CMF anode, operating at different temperatures (solid dots) and with CMF anode at 800 C (open dots) in Ar. (a) I-V and I-P curves; (b) impedance spectra tested under open circuit conditions.
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Fig. 6. Schemes for the reactions at the Ni-CMF composite anode.
conductivity than that in Ar atmosphere (Fig. 4(a)). The observed electrical conductivity is as high as 433.41 S cm1 at 800 C. This high conductivity is attributed to the reduction of NiO to conductive metallic Ni in CO. Notably, the conductivity of NiOCMF sample is much higher than that of CMF (Fig. 4(b)) in both Ar and CO atmospheres. In conclusion, the presence of NiO (metallic Ni) has a significant positive effect on the anode conductivity. 3.4. Anode performance Based on TPR and conductivity tests, both the catalytic activity and conductivity improves on the addition of Ni. In order to further understand these results, SO-DCFCs with Ni-CMF anode and CMF anode are tested using activated carbon without carbonate as fuel. They are prepared by the same procedure as that of HDCFC, mentioned in Section 2.2. Fig. 5(a) shows the current-voltage (I–V) and current-power density (I-P) curves of the SO-DCFC with NiCMF anode (solid dots) measured at 700–800 C. The maximum power densities (MPDs) for the cell are about 44.5, 96.3, and 183.2 mW cm2 at 700, 750, and 800 C, respectively. These values are much higher than those of the cell with CMF anode (open dots) tested under similar conditions. For example, the MPD at 800 C for
Fig. 7. I-V and I-P curves of the HDCFC with Ni-CMF anode (solid dots) tested at 700–800 C in Ar. The open dots in Fig. 7 show the I-V and I-P curves of the HDCFC with CMF anode at 800 C.
the Ni-CMF anode is 183.2 mW cm2, which is 2.7-fold higher than 68.0 mW cm2 for the CMF anode. The high performance of the SODCFC with Ni-CMF anode could be attributed to the improved catalytic activity and conductivity, due to the addition of metallic Ni catalyst. The corresponding impedance spectra of the cells measured under open circuit conditions are shown in Fig. 5(b) and the expanded view of impedance spectra are all shown in Fig. S2. The intercept with the real axis at high frequency in Fig. 5(b) represents the ohmic resistance arising from the electrode, electrolyte and collecting wires. In addition, two semicircular arcs are obvious at all temperatures (from Fig. S2). The relevant process for the low frequency arc is probably due to mass transportations. And the semicircular arc at the high frequency is due to the charge transfer at the interface between the electrolyte and electrode, which reflects the catalytic activity of the electrode for carbon oxidation. Ni-CMF anode displays much smaller ohmic resistance and polarization resistance than CMF anode at the same temperature. This suggests the incredible improvement of electronic conductivity and catalytic activity due to the addition of Ni. Additionally, for the Ni-CMF anode, cell resistance decreases with the increase in temperature, which is in good agreement with the increased power densities. The influence of temperature on resistance is specifically studied and described in Section 3.6.1. 3.5. Anode reaction schemes Scheme in Fig. 6 illustrates the possible reactions at the Ni-CMF anode. On one hand, during cell operation, the porous anode extends the length of the triple phase boundaries and provides more active sites for the oxidation of carbon along the surface of CMF. Simultaneously, CMF, as an efficient catalyst for carbon oxidation, is reduced by activated carbon to a lower cerium valence. This is subsequently oxidized back to ceria by carbon dioxide or oxygen ion [29], thereby completing the process of regeneration of the cerium catalyst. Such cycles of cerium catalysts contribute to the indirect electrochemical oxidation of solid carbon and accelerate the rate of formation of CO, produced by reverse Boudouard reaction. On the other hand, due to the existence of conductive Ni, the activation energy for oxidation of carbon is
Fig. 8. Impedance spectra of HDCFC with Ni-CMF anode tested under OCV conditions. The inset shows expanded view of the impedance spectra at different temperatures.
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efficiently reduced (shown in TPR analysis). Moreover, the generated electrons can be rapidly conducted through Ni to the external circuit. Therefore, due to the synergistic effects of high catalytically active CMF materials and conductive Ni, the anode catalytic activity is significantly improved, which contributes to the improvement of cell performance in DCFCs.
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Table 2 Resistances from LSCF/LSGM//LSCF electrode in a symmetric cell at 700–800 C in air. Operation temperature ( C)
800
750
700
Cathode polarization resistance (V cm2)
0.07
0.12
0.24
3.6. Cell performance of HDCFC 3.6.1. Electrochemical testing To further investigate the electrochemical performance of NiCMF anode, the discharge power and the corresponding resistance of HDCFC with this anode are measured at different temperatures. Fig. 7(a) shows the I-V and I-P curves of HDCFC with Ni-CMF anode (solid dots) tested at 700 C–800 C in Ar. The open circuit voltages (OCVs) for all cells are very close to the theoretically expected potential of 1.02 V, which indicates good sealing and dense electrolyte packing for the cells. The cell performance is enhanced as the operating temperature increased, reaching MPDs of 226.5, 478.4, and 580.7 mW cm2 at 700 C, 750 C and 800 C, respectively. Notably, the cell with Ni-CMF anode shows a 3.38 fold increase in MPD compared to the cell using CMF anode (open dots), due to the electro-catalytic function of Ni-CMF anode as mentioned in Section 3.5. Furthermore, comparing to the SO-DCFC (in Fig. 5(a)), the HDCFC with Ni-CMF anode shows a much better cell performance. This significant increase in cell performance is mainly attributed to the extension of the reaction zones in HDCFCs. Fig. 8 shows typical impedance spectra of integrated cells under OCV conditions. Obviously, the impedance spectrum mainly consists of two semicircles. The intercept on the real axis at high frequency corresponds to the ohmic resistance (RV). The semicircular distance on the real axis at high frequency represents the charge-transfer resistance (RH), whereas the distance between the two arcs on the real axis represents the diffusion resistance (RL). They are related to the activation over-potential and the mass transfer over-potential, respectively. The polarization resistance (RP) is equal to the sum of RH and RL (RP = RH + RL). Then the equivalent circuit LR (QRH) (QRL) is used to fit the impedance spectra and is also shown in Fig. 8. As can be seen in Fig. 8, the HDCFC using Ni-CMF (solid dots) anode shows a much lower RP compared to that with CMF anode (open dots). For example, the RP at 800 C for the Ni-CMF is 0.37 V cm2, which is much lower than 0.51 V cm2 for the CMF anode. Such a decrease in RP results in a great enhancement of cell performance of the HDCFC with Ni-CMF anode. In addition to this, Table 1 shows the fitted resistance data for HDCFC using Ni-CMF anode. As the temperature increased, the total resistance and polarization resistance decrease. The values of RP are 0.37, 0.52, and 0.76 V cm2 at 800, 750, and 700 C, respectively. Notably, RL significantly increases from 0.35 V cm2 to 0.69 V cm2 with decrease in temperature, whereas RH shows a slight increase from 0.02 V cm2 to 0.07 V cm2. This shows that temperature has a great effect on the mass transfer processes. To further distinguish the anode polarization resistance (Ra) and cathode polarization resistance, the cathode polarization
resistances are measured using a symmetric cell LSCF/LSGM/LSCF, which is prepared using the same procedure shown in Section 2.2. The impedance data of the symmetric cell was collected from 700– 800 C in air and is presented in Table 2. The anodic polarization resistance is obtained by subtracting the cathode polarization resistance from overall polarization resistance, and the results are presented in Table 1. From the Tables, the calculated values of Ra are 0.52, 0.40, and 0.30 V cm2 at 700, 750, and 800 C, respectively, which are significantly higher than the respective cathodic values of 0.24, 0.12, and 0.07 V cm2. This indicates that the oxygen reduction reaction at cathode is much faster than the oxidation reaction of solid carbon or CO at the anode in HDCFCs. 3.6.2. Microstructure analysis The morphology of the as-prepared composite anode before and after electrochemical testing is characterized by SEM as shown in Fig. 9. After the testing, the microstructure of the electrode becomes loosely porous and relatively homogeneous with the breakdown of larger grains into smaller particles. This morphological change is ascribed to the reduction of NiO and the decomposition of CMF material [32]. Due to the finer microstructure after testing, surface area of the anode increases, and consequently provides more reaction sites for anodic oxidation. These results provide a good explanation of the excellent electrochemical performance of HDCFC with Ni-CMF anode. 3.6.3. Fuel cell stability test The stability of the cell fabricated with Ni-CMF composite anode is investigated under a constant current density of 50 mA cm2 at 750 C. As shown in Fig. 10, there is no obvious decrease in voltage even after testing consecutively for 6 hours. Subsequently, the voltage decreases slowly along with the depletion of activated carbon. But the OCV still remains above 0.896 V after stability measurements, which is an indication that the cell assembled with Ni-CMF composite anode has excellent stability. In order to further understand the good performance of Ni-CMF anode, the fuel utilization is systematically calculated using the following equation: It
h ¼ nc electricity nc total 100% ¼ zF m 100% ¼ M
ItM 100% zFm
ð1Þ
In this equation, I is the current, t is the testing time, F is the Faraday constant; m and M are the mass and the molecular weight of carbon, respectively, and z is the number of electrons transferred. For complete or incomplete electrochemical oxidation of carbon, z = 4 or z = 2, respectively.
Table 1 Results of resistances of the HDCFC fabricated with Ni-CMF anodes at 700–800 C. Temperature ( C)
RV (V cm2)
RP (V cm2)
RH (V cm2)
RL (V cm2)
Total resistance (V cm2)
Ra (V cm2)
800 750 700
0.29 0.31 0.40
0.37 0.52 0.76
0.02 0.05 0.07
0.35 0.47 0.69
0.66 0.83 1.16
0.30 0.40 0.52
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Fig. 9. SEM images of NiO-CMF anode (a) before and (b) after the electrochemical measurements.
Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j. electacta.2017.02.135. References
Fig. 10. The stability of HDCFC with Ni-CMF anode at a constant current density of 50 mA cm2 at 750 C.
Therefore, according to Eq. (1), the calculated maximum fuel utilization is 14.32%, which is an indication the Ni-CMF composite could perform well as the anode for HDCFC. 4. Conclusions The Ni modified Ce0.6Mn0.3Fe0.1O2 (Ni-CMF) composite is successfully prepared and employed as an anode material for HDCFC. The TPR and GC test results demonstrate that the catalytic activity of CMF anode for solid carbon is significantly improved by Ni additive. Moreover, due to the presence of Ni, the conductivity of the composite material is significantly increased. Meanwhile, the performances of SO-DCFC with Ni-CMF anode (183.2 mW cm2) and CMF (68.0 mW cm2) at 800 C are also studied. This further demonstrates that Ni additives promote both, the catalytic activity and conductivity. As a result, HDCFC fabricated with this Ni-CMF anode shows a high maximum power density of 580.7 mW cm2 at 800 C, using activated carbon as fuel. Furthermore, the cell also displays a good stability under constant current density of 50 mA cm2 at 750 C. All these results indicate that Ni-CMF composite has good prospects to be used as an anode for HDCFCs. Acknowledgments This work was supported by the National Natural Science Foundation of China (grant numbers 21576028, 21506012).
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Ni modified Ce(Mn, Fe)O2 cermet anode for high-performance direct carbon fuel cell Jia Liua , Jinshuo Qiaoa,* , Hong Yuana , Jie Fenga , Chao Suia , Zhenhua Wanga,b , Wang Suna , Kening Suna,b,* a Beijing Key Laboratory for Chemical Power Source and Green Catalysis, School of Chemical Engineering and Environmental, Beijing Institute of Technology, Beijing, 100081, People’s Republic of China b Innovation Center of Electronic Vehicles in Beijing, No. 5 Zhongguancun South Avenue, Haidian District, Beijing, 100081, People’s Republic of China
A R T I C L E I N F O
Article history: Received 17 January 2017 Accepted 22 February 2017 Available online 24 February 2017 Keywords: Direct carbon fuel cell Ce0.6Mn0.3Fe0.1O2 cermet oxide Ni additive high electrochemical performance
A B S T R A C T
Ni modified Ce0.6Mn0.3Fe0.1O2 (Ni-CMF) material is evaluated as an anode material for hybrid direct carbon fuel cell (HDCFC). The Temperature Programmed Reaction (TPR) and GC test results show that the Ni additive significantly promotes the oxidation of carbon and consequently accelerates the formation rate of CO. Moreover, the Ni-CMF material is found to show a high electrical conductivity compared to pure CMF in HDCFC operating conditions. Taking advantage of the excellent catalytic activity and high conductivity, the electrolyte-supported HDCFC with Ni-CMF anode shows a high maximum power density of 580.7 mW cm2 at 800 C, when activated carbon is used as the fuel. Furthermore, the cell displays good stability under galvanostatic operation at current density of 50 mA cm2 at 750 C. These results indicate that the designed Ni-CMF composite material has the potential to be developed as an alternative anode for HDCFCs. © 2017 Elsevier Ltd. All rights reserved.
1. Introduction Direct carbon fuel cell (DCFC) has gained tremendous attention as a power generation device that directly converts chemical energy of solid carbon to electrical energy with high efficiency, low environmental pollution, and good fuel availability [1–3]. Generally, DCFCs can be classified as molten hydroxide, molten carbonate, solid oxide (SO-DCFC), or hybrid direct carbon fuel cells (HDCFCs), based on the type of electrolyte used [4–7]. Amongst these, HDCFCs have attracted more attention due to the number of benefits they offer [8]. HDCFCs, which employ two different electrolytes, viz. molten carbonate and solid oxide, combine the technologies of molten carbonate carbon fuel cell and solid oxide carbon fuel cell by dispersing solid carbon particles into molten carbonate as anode [9,10]. Such a technology prevents the possible corrosion of cathode and alleviates the need for CO2 circulation, required in traditional molten carbonate carbon fuel
* Corresponding authors at: Beijing Key Laboratory for Chemical Power Source and Green Catalysis, School of Chemical Engineering and Environmental, Beijing Institute of Technology, Beijing, 100081, People’s Republic of China. Tel./Fax: +86010-6891-8696. E-mail addresses: [email protected] (J. Qiao), [email protected] (K. Sun) . http://dx.doi.org/10.1016/j.electacta.2017.02.135 0013-4686/© 2017 Elsevier Ltd. All rights reserved.
cells [11,12]. Furthermore, such a cell configuration can extend the electrochemically active reaction zone from a two-dimensional electrolyte/anode interface to three-dimensional carbon/carbonate slurry at high temperature, and can thus kinetically favor the oxidation of carbon [13–15]. However, despite the advantages it offers, most of the reported HDCFCs generate much lower power densities than cells with gas fuels. The lowered cell performance can be attributed to insufficient electrical conductivity as well as the poor catalytic activity of anodes. More recently, doped ceria-based anodes have attracted considerable attentions in DCFCs owing to their high oxygen ionic conductivity, good catalytic activity and excellent resistance to coking and sulfur poisoning [16–18]. GDC infiltrated (La0.75Sr0.25)0.97Cr0.5Mn0.5O3 (GDC-LSCM) was evaluated as the HDCFC anode with two different carbon fuels [19]. Although the availability of different carbon fuels in HDCFCs was demonstrated, the power density achieved was poor, due to the low electrical conductivity of GDC-LSCM anode in reducing atmosphere. In comparison, Ce0.6Mn0.3Fe0.1O2 (CMF) material was reported to have high conductivity, and the cell with this anode demonstrated a competitive power generation property and oxidation tolerance [20]. In addition, transition metals proved to be highly active catalysts, and could be added to anode materials to effectively
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improve cell performance [21–23]. Amongst these, conductive Ni particles as intensively investigated catalysts exhibit prominent catalytic activity for the oxidation of solid carbon and demonstrate excellent electrochemical performance in SO-DCFCs [24]. Furthermore, adding Ni into the carbon-molten carbonate reduces the apparent activation energy, and significantly improves the electrooxidation activity at low temperatures. The current density on activity of oxidized graphite felt with the addition of Ni is 7-times higher than that without Ni at 750 C [25]. The present work demonstrated that the Ni modified CMF (NiCMF) composite material could be directly applied as a potential HDCFC anode for the first time. For comparison, the performance of the cell with pure CMF anode was also tested. The influence of Ni additive on the catalytic activity, electrical conductivity, and anode performance of CMF oxide was systematically investigated. The steps for the reactions at the anode were also proposed in a scheme. Finally, the electrochemical performance and the stability of Ni-CMF anode were further evaluated by fabricating the LSGM electrolyte-supported HDCFCs. 2. Experimental 2.1. Preparation of materials and pre-treatment of fuel The dopant CMF materials were synthesized by a modified combustion method [26]. Stoichiometric amounts of Ce (NO3)26H2O, Fe(NO3)29H2O and Mn(NO3)24H2O (Sinopharm Chemical Reagent Co., Ltd, AR) were used as metal precursors. Glycine (Tianjin Fuchen chemical reagents factory, AR) and citric acid (Beijing Chemical Works, AR) were added to promote combustion. The resultant ash was then calcined at 1000 C for 6 h in air atmosphere to obtain pure CMF powder. Next, CMF and commercial NiO (Japan Tosoh Co., AR) in a mass ratio of 1:1 were mixed by ball milling in ethanol and dried overnight at 80 C to obtain the NiO-CMF composite. Moreover, commercially available activated carbon was passed through a 1000-mesh standard sieve and then used as the fuel. Lithium carbonate (Li2CO3) and potassium carbonate (K2CO3) (Aladdin, China) were pretreated prior to use as the secondary electrolytes in the cell device. The pre-treatment was carried out according to a previous procedure [27] as follows. Li2CO3 and K2CO3 were dried at 300 C in air and then mixed together in a molar ratio of 62:38 by ball milling in ethanol and then dried overnight at 80 C. 2.2. Cell design and preparation Electrolyte-supported single cells were fabricated using La0.9Sr0.1Ga0.8Mg0.2O3 electrolyte (LSGM, Fuel Cell), La0.6Sr0.4Co0.2Fe0.8O3 cathode (LSCF, Fuel Cell), and Ni-CMF composite anode. The LSGM electrolyte (340 mm in thickness) was prepared by dry pressing method carried out uniaxially using LSGM powder under 200 MPa, and then sintered at 1450 C for 6 h in air atmosphere [28]. Then, an interlayer of Ce0.8Sm0.2O2 (SDC), which prevented a chemical reaction between Ni and LSGM electrolyte in reduced atmosphere, was screen-printed onto the LSGM electrolyte and calcined at 1400 C for 5 h. Later, slurries of anode and cathode were screen-printed onto the SDC interlayer and LSGM electrolyte surface, respectively, and co-calcined at 1100 C for 2 h. In addition, a single cell with CMF anode was also fabricated by the same procedure for comparison. Prior to electrochemical testing, NiO in the composite was reduced in situ to metallic nickel by solid carbon or by the generated CO at 800 C for 2 h. Fig. 1 shows the schematic diagram of the cell design. For current collection, Ag wire was connected on the electrodes with Ag paste (Sino-Platinum Metals Co., Ltd) and then co-calcined at 750 C for 30 min. The single cell was attached on one end of an
Fig. 1. Schematic diagram of the tested cell.
alumina tube using a ceramic paste (Aremco 552). 2.0 g of carboncarbonate mixture (mass ratio of 4:1) was filled into the anodic chamber at room temperature. Ar gas (20 mL min1) was fed into the anodic chamber to prevent combustion of carbon fuel, and the cathode was exposed to ambient air. 2.3. Structure and characteristics The phase purities and crystal structures of NiO-CMF samples, before and after reduction, were determined by X-ray diffraction (XRD) (X’ Pert PRO MPD) using Cu Ka radiation. The microstructures and distribution of elements in the NiO-CMF composite samples were studied using scanning electron microscope (SEM, FEI QUANTA-250), equipped with energy dispersive spectrometer (EDS). The electronic conductivity measurements were performed by a four-probe DC technique (Keithley 2400, USA) in Ar and CO atmosphere. For conductivity measurements, 0.8 g of NiO-CMF or CMF powder was pressed uniaxially into rectangular bars and then sintered at 1200 C for 5 h. Temperature programmed reaction (TPR) experiments were carried out on a thermal analyzer (TA/DTA6200, Seiko) at a heating rate of 5 C min1 from room temperature to 800 C in N2 atmosphere. The experiments were conducted by mixing activated fuel with CMF or NiO-CMF samples (mass ratio of 1:1) in an alumina pan and then placing this pan in a tubular flow reactor. The off-gases from the experiments were analyzed by gas chromatography (GC-2014, Shimadzu), equipped with a column packed with 5A molecular sieves and a thermal conductivity detector (TCD). For GC analysis, helium was used as carrier gas for the detection of argon, carbon monoxide, and carbon dioxide. The percentages of gaseous outputs were determined and converted to gas volumes by external standard methods. Furthermore, the cell performance was evaluated using an Arbin Instruments’ tester (Fuel Cell Test System, FCTS) at 700– 800 C. The electrochemical resistance was measured by electrochemical impedance spectroscopy (EIS, PARSTAT 2273, USA), using a 10 mV DC signal in the frequency range of 10 mHz–100 kHz. The impedance data was fitted and analyzed using ZSimpWin software.
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Fig. 2. (a) XRD patterns of pure CMF powder, NiO-CMF powder before and after reduction at 800 C for 20 h in CO atmosphere; (b) SEM image and elemental distributions of NiO-CMF composite materials.
3. Results and discussion 3.1. XRD and SEM analyses XRD patterns of pure CMF powder, NiO-CMF powder, and NiOCMF powder after reduction in CO atmosphere are presented in Fig. 2(a). Pure CMF powder shows a single-phase fluorite structure (PDF#43-1002) without any impure phases, which indicates that Mn and Fe elements are completely doped into the lattice of CeO2. In case of NiO-CMF sample, characteristic peaks of CMF and NiO can be prominently seen. After reduction, although only a few diffraction peaks of MnxOy phase can be seen, the diffraction peaks are mainly assigned to CMF. In addition to the diffraction peaks of CMF, peaks of metallic Ni phase are also present, indicating that NiO is effectively reduced to Ni by carbon monoxide. This reduced metallic Ni can significantly improve the conductivity of the anode material. Fig. 2(b) shows the microstructures and elemental distribution maps of NiO-CMF composite materials. It is obvious that NiO particles are evenly distributed in the composite materials
without agglomerates, which forms an interconnected porous structure. Such an interconnected porous structure not only provides sufficient number of channels for diffusion of produced CO, but also provides more number of active sites for the oxidation of CO, consequently improving the cell performance. 3.2. TPR and GC study Ni plays an important catalytic role in Ni-based anode for the oxidation of CO or solid carbon [29,30]. Temperature programmed reaction (TPR) measurements are carried out to investigate the improvement in catalytic activity of Ni-CMF anode for activated carbon fuels. Fig. 3(a) and (b) show the TPR curves of CMF and NiOCMF samples, respectively. Fig. 3(a) shows two distinct peaks of CMF sample. The first peak (about 680 C) could possibly be attributed to the reaction of CMF with activated carbon [31]. The second peak at about 815 C can be attributed to the reverse Boudouard reaction. On addition of NiO (Fig. 3(b)), the first peak shifts to a higher temperature of around 700 C, and the TPR curve
Fig. 3. TPR curves of the mixture of activated carbon and the samples: (a) CMF and (b) NiO-CMF; (c) Plots of flow rates of gaseous products at 500–800 C under Ar atmosphere. In Fig. 3(c), the solid line represents the off-gases of activated carbon mixed with NiO-CMF samples and the dashed line indicates the off-gases of activated carbon mixed with CMF samples, respectively.
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Fig. 4. Temperature dependence of electronic conductivity of the sample: (a) NiO-CMF; (b) CMF. Inset in (a) shows the expanded electronic conductivity of NiO-CMF sample in Ar.
becomes rough. This indicates that, in addition to the reaction between activated carbon and CMF, the reduction of bulk NiO occurred at this temperature. Moreover, the peak for the reverse Boudouard reaction shifts to a lower temperature of about 797 C and the intensity of the second peak increases. This suggests much lower activation energy for the reverse Boudouard reaction (Fig. S1) and an enhanced catalytic activity of Ni-CMF for the oxidation of activated carbon. To further establish the catalytic activity of the Ni-CMF anode, off-gases of the mixtures of activated carbon and CMF or Ni-CMF (denoted in the following as CMF/C or Ni-CMF/C) are quantified by GC analysis. Before GC experiments, the NiO-CMF samples are reduced to Ni-CMF at 800 C for 2 h under CO atmosphere. Fig. 3(c) shows the off-gases of CMF/C (dash lines) and Ni-CMF/C (solid line) under Ar atmosphere. CO2 would be the product of the reaction between activated carbon and CMF and CO would be produced in the reverse Boudouard reaction (shown in TPR curves). The formation rate of CO2 for Ni-CMF/C is much higher than that for
CMF/C mixture. Moreover, as the temperature exceeds 700 C, the formation rate of CO for Ni-CMF/C is significantly increased. Accordingly, the rate of reverse Boudouard reaction is higher (because the slope of curve for the formation rate of CO is much steeper). These results further demonstrate that the addition of Ni significantly improves the catalytic activity for the oxidation of activated carbon. 3.3. Conductivity In an attempt to simulate the operating conditions of the anode of HDCFC, the conductivities of NiO-CMF and CMF samples without reduction are determined in CO and Ar atmosphere at 500–800 C. The densities of the NiO-CMF and CMF bars, for conductivity measurements, are about 92% and 93%, respectively. Fig. 4(a) and (b) illustrate the temperature dependence of electrical conductivity of NiO-CMF samples and CMF samples, respectively. In CO atmosphere, the NiO-CMF sample shows higher electrical
Fig. 5. Electrochemical performance of SO-DCFCs with Ni-CMF anode, operating at different temperatures (solid dots) and with CMF anode at 800 C (open dots) in Ar. (a) I-V and I-P curves; (b) impedance spectra tested under open circuit conditions.
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Fig. 6. Schemes for the reactions at the Ni-CMF composite anode.
conductivity than that in Ar atmosphere (Fig. 4(a)). The observed electrical conductivity is as high as 433.41 S cm1 at 800 C. This high conductivity is attributed to the reduction of NiO to conductive metallic Ni in CO. Notably, the conductivity of NiOCMF sample is much higher than that of CMF (Fig. 4(b)) in both Ar and CO atmospheres. In conclusion, the presence of NiO (metallic Ni) has a significant positive effect on the anode conductivity. 3.4. Anode performance Based on TPR and conductivity tests, both the catalytic activity and conductivity improves on the addition of Ni. In order to further understand these results, SO-DCFCs with Ni-CMF anode and CMF anode are tested using activated carbon without carbonate as fuel. They are prepared by the same procedure as that of HDCFC, mentioned in Section 2.2. Fig. 5(a) shows the current-voltage (I–V) and current-power density (I-P) curves of the SO-DCFC with NiCMF anode (solid dots) measured at 700–800 C. The maximum power densities (MPDs) for the cell are about 44.5, 96.3, and 183.2 mW cm2 at 700, 750, and 800 C, respectively. These values are much higher than those of the cell with CMF anode (open dots) tested under similar conditions. For example, the MPD at 800 C for
Fig. 7. I-V and I-P curves of the HDCFC with Ni-CMF anode (solid dots) tested at 700–800 C in Ar. The open dots in Fig. 7 show the I-V and I-P curves of the HDCFC with CMF anode at 800 C.
the Ni-CMF anode is 183.2 mW cm2, which is 2.7-fold higher than 68.0 mW cm2 for the CMF anode. The high performance of the SODCFC with Ni-CMF anode could be attributed to the improved catalytic activity and conductivity, due to the addition of metallic Ni catalyst. The corresponding impedance spectra of the cells measured under open circuit conditions are shown in Fig. 5(b) and the expanded view of impedance spectra are all shown in Fig. S2. The intercept with the real axis at high frequency in Fig. 5(b) represents the ohmic resistance arising from the electrode, electrolyte and collecting wires. In addition, two semicircular arcs are obvious at all temperatures (from Fig. S2). The relevant process for the low frequency arc is probably due to mass transportations. And the semicircular arc at the high frequency is due to the charge transfer at the interface between the electrolyte and electrode, which reflects the catalytic activity of the electrode for carbon oxidation. Ni-CMF anode displays much smaller ohmic resistance and polarization resistance than CMF anode at the same temperature. This suggests the incredible improvement of electronic conductivity and catalytic activity due to the addition of Ni. Additionally, for the Ni-CMF anode, cell resistance decreases with the increase in temperature, which is in good agreement with the increased power densities. The influence of temperature on resistance is specifically studied and described in Section 3.6.1. 3.5. Anode reaction schemes Scheme in Fig. 6 illustrates the possible reactions at the Ni-CMF anode. On one hand, during cell operation, the porous anode extends the length of the triple phase boundaries and provides more active sites for the oxidation of carbon along the surface of CMF. Simultaneously, CMF, as an efficient catalyst for carbon oxidation, is reduced by activated carbon to a lower cerium valence. This is subsequently oxidized back to ceria by carbon dioxide or oxygen ion [29], thereby completing the process of regeneration of the cerium catalyst. Such cycles of cerium catalysts contribute to the indirect electrochemical oxidation of solid carbon and accelerate the rate of formation of CO, produced by reverse Boudouard reaction. On the other hand, due to the existence of conductive Ni, the activation energy for oxidation of carbon is
Fig. 8. Impedance spectra of HDCFC with Ni-CMF anode tested under OCV conditions. The inset shows expanded view of the impedance spectra at different temperatures.
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efficiently reduced (shown in TPR analysis). Moreover, the generated electrons can be rapidly conducted through Ni to the external circuit. Therefore, due to the synergistic effects of high catalytically active CMF materials and conductive Ni, the anode catalytic activity is significantly improved, which contributes to the improvement of cell performance in DCFCs.
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Table 2 Resistances from LSCF/LSGM//LSCF electrode in a symmetric cell at 700–800 C in air. Operation temperature ( C)
800
750
700
Cathode polarization resistance (V cm2)
0.07
0.12
0.24
3.6. Cell performance of HDCFC 3.6.1. Electrochemical testing To further investigate the electrochemical performance of NiCMF anode, the discharge power and the corresponding resistance of HDCFC with this anode are measured at different temperatures. Fig. 7(a) shows the I-V and I-P curves of HDCFC with Ni-CMF anode (solid dots) tested at 700 C–800 C in Ar. The open circuit voltages (OCVs) for all cells are very close to the theoretically expected potential of 1.02 V, which indicates good sealing and dense electrolyte packing for the cells. The cell performance is enhanced as the operating temperature increased, reaching MPDs of 226.5, 478.4, and 580.7 mW cm2 at 700 C, 750 C and 800 C, respectively. Notably, the cell with Ni-CMF anode shows a 3.38 fold increase in MPD compared to the cell using CMF anode (open dots), due to the electro-catalytic function of Ni-CMF anode as mentioned in Section 3.5. Furthermore, comparing to the SO-DCFC (in Fig. 5(a)), the HDCFC with Ni-CMF anode shows a much better cell performance. This significant increase in cell performance is mainly attributed to the extension of the reaction zones in HDCFCs. Fig. 8 shows typical impedance spectra of integrated cells under OCV conditions. Obviously, the impedance spectrum mainly consists of two semicircles. The intercept on the real axis at high frequency corresponds to the ohmic resistance (RV). The semicircular distance on the real axis at high frequency represents the charge-transfer resistance (RH), whereas the distance between the two arcs on the real axis represents the diffusion resistance (RL). They are related to the activation over-potential and the mass transfer over-potential, respectively. The polarization resistance (RP) is equal to the sum of RH and RL (RP = RH + RL). Then the equivalent circuit LR (QRH) (QRL) is used to fit the impedance spectra and is also shown in Fig. 8. As can be seen in Fig. 8, the HDCFC using Ni-CMF (solid dots) anode shows a much lower RP compared to that with CMF anode (open dots). For example, the RP at 800 C for the Ni-CMF is 0.37 V cm2, which is much lower than 0.51 V cm2 for the CMF anode. Such a decrease in RP results in a great enhancement of cell performance of the HDCFC with Ni-CMF anode. In addition to this, Table 1 shows the fitted resistance data for HDCFC using Ni-CMF anode. As the temperature increased, the total resistance and polarization resistance decrease. The values of RP are 0.37, 0.52, and 0.76 V cm2 at 800, 750, and 700 C, respectively. Notably, RL significantly increases from 0.35 V cm2 to 0.69 V cm2 with decrease in temperature, whereas RH shows a slight increase from 0.02 V cm2 to 0.07 V cm2. This shows that temperature has a great effect on the mass transfer processes. To further distinguish the anode polarization resistance (Ra) and cathode polarization resistance, the cathode polarization
resistances are measured using a symmetric cell LSCF/LSGM/LSCF, which is prepared using the same procedure shown in Section 2.2. The impedance data of the symmetric cell was collected from 700– 800 C in air and is presented in Table 2. The anodic polarization resistance is obtained by subtracting the cathode polarization resistance from overall polarization resistance, and the results are presented in Table 1. From the Tables, the calculated values of Ra are 0.52, 0.40, and 0.30 V cm2 at 700, 750, and 800 C, respectively, which are significantly higher than the respective cathodic values of 0.24, 0.12, and 0.07 V cm2. This indicates that the oxygen reduction reaction at cathode is much faster than the oxidation reaction of solid carbon or CO at the anode in HDCFCs. 3.6.2. Microstructure analysis The morphology of the as-prepared composite anode before and after electrochemical testing is characterized by SEM as shown in Fig. 9. After the testing, the microstructure of the electrode becomes loosely porous and relatively homogeneous with the breakdown of larger grains into smaller particles. This morphological change is ascribed to the reduction of NiO and the decomposition of CMF material [32]. Due to the finer microstructure after testing, surface area of the anode increases, and consequently provides more reaction sites for anodic oxidation. These results provide a good explanation of the excellent electrochemical performance of HDCFC with Ni-CMF anode. 3.6.3. Fuel cell stability test The stability of the cell fabricated with Ni-CMF composite anode is investigated under a constant current density of 50 mA cm2 at 750 C. As shown in Fig. 10, there is no obvious decrease in voltage even after testing consecutively for 6 hours. Subsequently, the voltage decreases slowly along with the depletion of activated carbon. But the OCV still remains above 0.896 V after stability measurements, which is an indication that the cell assembled with Ni-CMF composite anode has excellent stability. In order to further understand the good performance of Ni-CMF anode, the fuel utilization is systematically calculated using the following equation: It
h ¼ nc electricity nc total 100% ¼ zF m 100% ¼ M
ItM 100% zFm
ð1Þ
In this equation, I is the current, t is the testing time, F is the Faraday constant; m and M are the mass and the molecular weight of carbon, respectively, and z is the number of electrons transferred. For complete or incomplete electrochemical oxidation of carbon, z = 4 or z = 2, respectively.
Table 1 Results of resistances of the HDCFC fabricated with Ni-CMF anodes at 700–800 C. Temperature ( C)
RV (V cm2)
RP (V cm2)
RH (V cm2)
RL (V cm2)
Total resistance (V cm2)
Ra (V cm2)
800 750 700
0.29 0.31 0.40
0.37 0.52 0.76
0.02 0.05 0.07
0.35 0.47 0.69
0.66 0.83 1.16
0.30 0.40 0.52
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Fig. 9. SEM images of NiO-CMF anode (a) before and (b) after the electrochemical measurements.
Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j. electacta.2017.02.135. References
Fig. 10. The stability of HDCFC with Ni-CMF anode at a constant current density of 50 mA cm2 at 750 C.
Therefore, according to Eq. (1), the calculated maximum fuel utilization is 14.32%, which is an indication the Ni-CMF composite could perform well as the anode for HDCFC. 4. Conclusions The Ni modified Ce0.6Mn0.3Fe0.1O2 (Ni-CMF) composite is successfully prepared and employed as an anode material for HDCFC. The TPR and GC test results demonstrate that the catalytic activity of CMF anode for solid carbon is significantly improved by Ni additive. Moreover, due to the presence of Ni, the conductivity of the composite material is significantly increased. Meanwhile, the performances of SO-DCFC with Ni-CMF anode (183.2 mW cm2) and CMF (68.0 mW cm2) at 800 C are also studied. This further demonstrates that Ni additives promote both, the catalytic activity and conductivity. As a result, HDCFC fabricated with this Ni-CMF anode shows a high maximum power density of 580.7 mW cm2 at 800 C, using activated carbon as fuel. Furthermore, the cell also displays a good stability under constant current density of 50 mA cm2 at 750 C. All these results indicate that Ni-CMF composite has good prospects to be used as an anode for HDCFCs. Acknowledgments This work was supported by the National Natural Science Foundation of China (grant numbers 21576028, 21506012).
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