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Performance and Durability of the Molten Carbonate Electrolysis Cell (MCEC) and the Reversible Molten Carbonate Fuel Cell (RMCFC) Lan Hu, Göran Lindbergh, and Carina Lagergren J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b04417 • Publication Date (Web): 09 Jun 2016 Downloaded from http://pubs.acs.org on June 10, 2016

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The Journal of Physical Chemistry C is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

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The Journal of Physical Chemistry

Performance and Durability of the Molten Carbonate Electrolysis Cell (MCEC) and the Reversible Molten Carbonate Fuel Cell (RMCFC) Lan Hu, Göran Lindbergh, Carina Lagergren* Applied Electrochemistry, Department of Chemical Engineering and Technology, KTH Royal Institute of Technology, SE-100 44 Stockholm, Sweden

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ABSTRACT The molten carbonate electrolysis cell (MCEC) provides the opportunity for producing fuel gases, e.g. hydrogen or syngas, in an environmentally friendly way, especially when in combination with renewable electricity resources such as solar, wind and/or hydropower. The evaluation of the performance and durability of the molten carbonate cell is a key for developing the electrolysis technology. In this study, we report that the electrochemical performance of the cell and electrodes somewhat decreases during the long-term test of the MCEC. The degradation is not permanent, though, and the cell performance could be partially recovered. Since conventional fuel cell materials consisting of Ni-based porous catalysts and carbonate electrolyte are used in the MCEC durability test, it is also shown that the cell can alternatingly operate as an electrolysis cell for fuel gas production and as a fuel cell for electricity generation, i.e. as a so-called reversible molten carbonate fuel cell (RMCFC). This study reveals that the cell performance improves after a long period of RMCFC operation. The stability and durability of the cell in long-term tests evidence the feasibility of the electrolysis and reversible operations in carbonate melts using a conventional fuel cell set-up, at least in lab-scale.


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1. INTRODUCTION In recent years, hydrogen has been considered to be a promising energy carrier because of its clean, storable and transportable characteristics.1-3 However, the leading industrial processes for hydrogen production are largely dependent on the consumption of fossil fuels, such as in steam reforming, partial oxidation of heavy hydrocarbons and coal gasification.4,5 When considering limited fossil fuel resources and environmental issues, water electrolysis could be a suitable and feasible approach for producing hydrogen. With increased production of renewable electricity, there is also a need for energy storage capabilities. To use the surplus from renewable energy sources, e.g. solar, wind, tidal and hydropower, as power supply for the electrolysis is a very attractive possibility. The commercially available electrolysis technology is mainly developed for operation at low temperatures, such as in alkaline water electrolysis and proton exchange membrane water electrolysis.6-9 From the perspective of the thermodynamic and kinetic properties, the overall efficiency will be improved by increasing the operating temperature in the electrolysis cell.10, 11 Additionally, a fraction of the electrical energy demand could be reduced and balanced by the thermal energy in the high-temperature electrolysis.12 These benefits make the high-temperature electrolysis technology, e.g. the molten carbonate electrolysis cell (MCEC) and the solid oxide electrolysis cell (SOEC), attract an increasing interest. The MCEC operates at temperatures ranging from 600 to 700 °C, thus avoiding the need for the noble catalysts such as platinum used at lower temperatures and the advanced ceramic materials for the higher temperatures. The molten carbonate electrolysis cell provides a promising option for producing fuel gas such as hydrogen via water electrolysis, i.e. Reaction 1. In the MCEC, carbon dioxide should be present in the cell for producing carbonate ions, the ionic conductor in the electrolyte. Also direct CO2 electrolysis (Reaction 2) generating CO 3 ACS Paragon Plus Environment


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may take place.13 However, the kinetics of this reaction is much slower compared to water electrolysis on nickel-based electrodes.14, 15 Despite that, carbon monoxide could be generated through the reverse water-gas shift reaction, i.e. Reaction 3. If the electrolysis cell is operated under pressurized conditions or has very high conversion, the production of CH4 through the methanation process (Reactions 4 and 5) may also take place. 16

H O g, Cat + CO g, Cat → H g, Cat + O g, An + CO g, An

(1)

2CO g, Cat → CO g, Cat + O g, An + CO g, An

(2)

H O g + CO g ⇄ H g + CO g

(3)

CO g + 3H g → CH g + H O g

(4)

CO g + 4H g → CH g + 2H O g

(5)

The electrolysis in molten carbonate salts has been studied by a number of researchers, mainly focusing on the electrolytic reduction of carbonate melts or carbon dioxide into solid carbon or gaseous carbon monoxide.13, 16-26 However, most experiments were carried out on planar electrodes, which differ from porous electrodes with regard to the electrode surface and masstransfer properties. Our previous study27 showed that it is feasible to operate the electrolysis cell in molten carbonate salts by using conventional fuel cell catalysts (i.e. Ni-based porous electrodes). Additionally, lower polarization losses were found for the electrolysis cell than for the fuel cell at operating temperatures of 600–675 °C. The viability of electrolysis operation using fuel cell catalysts illustrates that it is possible to run the molten carbonate cell reversibly. That is, the molten carbonate cell can alternatingly operate as an electrolysis cell to produce fuel gas and as a fuel cell to generate electricity. The so-called reversible molten carbonate fuel cell (RMCFC) may increase the usefulness of the overall system and probably improve the economic benefits. Furthermore, the molten carbonate fuel cell (MCFC) technology has evolved to current megawatt-scale commercial 4 ACS Paragon Plus Environment

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power plants.28, 29 In terms of scale of installed power generation units, the MCFC is the leader among all fuel cell technologies.29, 30 Thus, the possibility of operating the electrolysis cell in the commercial fuel cell set-up will be beneficial from a cost perspective. Although promising performance for the molten carbonate electrolysis cell is shown in the electrochemical measurements, little is known about the durability of the cell in electrolysis and/or reversible operations in the carbonate melts. In this study we test the durability of the MCEC and the RMCFC and investigate the change that takes place in performance of the full cell and individual electrodes during long-term operations. Electrochemical studies determine the factors that affect the cell performance in the durability tests, providing information for the development of the electrolysis and reversible cells in carbonate melts. 2. EXPERIMENTAL SECTION Experimental data were obtained from a laboratory cell unit with a geometrical area of 3 cm2, where the cell components were provided by Ansaldo Fuel Cells in Italy. The cell has a porous Ni-Cr alloy as hydrogen electrode, and a porous nickel, oxidized and lithiated in situ, as oxygen electrode. The electrodes are separated by a porous LiAlO2 matrix, which also supports the electrolyte, a eutectic mixture of 62/38 mol% Li2CO3/K2CO3. Two reference electrodes, each consisting of gold (Au) wires in equilibrium with a gas mixture of 33/67% O2/CO2, are placed in separate chambers filled with the same electrolyte as that in the cell. These chambers are connected to the cell through a capillary with a gold plug. The schematic drawing of the cell set-up has been shown in Ref. 27. The MCEC durability test was performed galvanostatically by running the cell under a constant electrolysis current density of -0.16 A·cm-2 for 2165 h, while the long-term test of the RMCFC was done by operating the cell alternatingly in fuel cell mode (i.e. under a 5 ACS Paragon Plus Environment


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constant load of 0.16 A·cm-2) and electrolysis cell mode (i.e. under a constant current density of -0.16 A·cm-2) for 1019 h. The value of 0.16 A·cm-2 applied in this study is the current density typically used for operating the molten carbonate fuel cell. Since here using the molten carbonate fuel cell for electrolysis, we choose the same current density of 0.16 A·cm-2 when studying the durability of the cell during long-term electrolysis and reversible operations. Both durability tests were undertaken at 650 °C. The gas containing 15/30/55% O2/CO2/N2 was introduced to the NiO oxygen electrode throughout the experiments. The gas mixtures consisting of 25/25/25/25% H2/CO2/H2O/N2 and 64/16/20% H2/CO2/H2O were used for the Ni hydrogen electrode in long-term tests of the MCEC and the RMCFC, respectively. The water content in the gas mixtures was controlled by bubbling at a set temperature. The gas flow rates for the Ni and NiO electrodes were approximately 150 mL·min-1, while the reference gas had a flow rate of 20 mL·min-1. All the gases used in the experiments are certified gas mixtures from AGA Gas AB, Sweden. The long-term tests of the MCEC for 2165 h and the RMCFC for 1019 h were performed continuously, only interrupted by shorter periods of complementary electrochemical measurements. At each break the electrochemical performance of the full cell and individual electrodes was measured by stationary polarization curves and electrochemical impedance spectroscopy (EIS). The polarization curves were performed in potentiostatic steps by using a Solartron Interface SI1287 supported by CorrWare software. The current-interrupt technique was used to distinguish the ohmic loss from other types of polarization. For electrochemical impedance measurements of the cell and electrodes, the frequency spectra were recorded at three different operating conditions, including open circuit voltage (OCV), fuel cell and electrolysis cell modes. A potentiostatic offset is applied to simulate fuel cell and electrolysis cell modes. At the overpotential employed in impedance measurements, the corresponding current density is about 0.16 A·cm-2 in fuel cell mode. The same overpotential is also 6 ACS Paragon Plus Environment

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employed for electrolysis operation. EIS was measured potentiostatically in the frequency range between 10 kHz and 10 mHz with a potential perturbation of 10 mV. Solartron frequency response analyzer 1255 and Solartron Interface SI1287 were used for impedance measurements. 3. RESULTS AND DISCUSSION 3.1. Performance and Durability of the MCEC Figure 1 shows the cell voltage recorded under a constant electrolysis current density of -0.16 A·cm-2 at 650 °C for 2165 h. The open circuit voltage of the cell is about 0.995 V when the gas containing 25/25/25/25% H2/CO2/H2O/N2 is used for the Ni electrode. The interruptions observed in the figure are due to complementary electrochemical measurements for the cell and electrodes. During the long-term MCEC operation, the cell voltage initially shows a gradual increase and then levels off or even decreases slightly, instead of increasing continuously. The cell voltage eventually reached under the constant electrolysis conditions ranges from 1.155 to 1.170 V. By evaluating the voltage change for each period (e.g. 0–60 h, 60–150 h, 150–252 h, 252–340 h, etc.), the increase rate of the cell voltage is 0.2–0.6 mV/h. Despite interruptions and small disturbances taking place in the long-term test, the change trend of the cell voltage is not strongly affected. The small variation in cell voltage during 2165 h of MCEC operation indicates that it is viable and durable to operate the electrolysis cell for fuel gas production using conventional fuel cell components.



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Figure 1. The cell voltage recorded under electrolysis operation for 2165 h at 650 °C. During the MCEC durability test the ohmic losses (RΩ) and polarization losses (Rp) of the cell at three operating modes were obtained from impedance measurements, see Figure 2. An overpotential versus OCV of -0.15 V and +0.15 V was applied on the cell to simulate MCFC and MCEC mode, respectively. The ohmic loss is evaluated from the intercept with X-axis at the high frequency of the impedance spectra, while the polarization resistance is calculated as the difference between the low- and high-frequency X-axis intercept. The ohmic losses (RΩ) are varied in the same way in all measured modes, but slightly lower values are shown in MCEC mode (Figure 2a). When running up to 760 h (before the electrolyte is added to the cell), the ohmic loss increases from 0.27 to 0.34 Ω·cm2 in OCV and MCFC modes but rises only to 0.32 Ω·cm2 in MCEC mode. The increase of the internal resistance may probably be attributed to electrolyte loss during long-term operation, while redistribution of the electrolyte when running the electrolysis cell may explain the deviant value for that mode. The cell works at the temperature of 650 °C, and the electrolyte loss is mainly caused by evaporation and corrosion in the open atmospheric system.31, 32 Also the oxygen electrode, formed by in situ oxidation and lithiation of nickel, consumes part of the electrolyte during startup. In order to keep the optimal amount of electrolyte, 0.034 g electrolyte was added after 760 h of operation. As expected ohmic losses of the cell are decreased. With the second addition of 0.090 g electrolyte, the ohmic resistances continue to decrease to the interval 0.24–0.25 8 ACS Paragon Plus Environment

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Ω·cm2 at 1865 h in these three operating modes. However, before the long-term MCEC operation ceased at 2165 h, an increase of the internal resistance is shown in OCV and MCFC modes and in MCEC mode.

Figure 2. The (a) ohmic losses and (b) polarization losses of the cell as function of operating time evaluated from impedance spectra in OCV, MCFC (OCV-0.15 V) and MCEC (OCV+0.15 V) modes during the long-term test of the MCEC. In Figure 2b, the polarization resistances (Rp) have fluctuations between 0.50 and 0.63 Ω·cm2 in OCV and MCFC modes during 760 h of electrolysis operation, while a small increase from 0.42 to 0.48 Ω·cm2 is shown in MCEC mode. For the operation period of 760–1195 h with first addition of 0.034 g electrolyte, there is an increase of Rp, but then the value decreases back to 0.63 Ω·cm2 in OCV and MCFC modes. However, in MCEC mode the polarization resistance is almost constant at 0.47 Ω·cm2 during this period. At 1195 h a relatively high amount of electrolyte (0.090 g) is added to the cell. This results in a largely increased 9 ACS Paragon Plus Environment


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polarization resistance of the cell in OCV and MCFC modes but a less significant rise in MCEC mode. The performance of the fuel cell is strongly dependent on the degree of electrolyte filling, which affects the effective conductivity in the electrolyte, the effective diffusion path in the electrolyte and gas pores, as well as the active surface area for electrochemical reactions.33,

34

As the cell continues to operate for water electrolysis, the

polarization resistance gives the value of 0.71 Ω·cm2 in OCV and MCFC modes before the durability test stops. In MCEC mode the value of the polarization losses still remains at 0.47 Ω·cm2, indicating that in comparison with the fuel cell, the performance of the electrolysis cell is less influenced by the electrolyte filling degree. Figure 3 shows polarization losses of the electrodes obtained from the impedance spectra (Supporting Information) during 2165 h of MCEC operation. The overpotential versus OCV of +0.05 V and -0.05 V was applied on the Ni electrode to simulate fuel cell and electrolysis cell modes, respectively, while the overvoltage of -0.10 V and +0.10 V was applied on the NiO electrode. In Figure 3a, the Ni electrode has slightly higher polarization resistances in electrolysis cell mode than in OCV and fuel cell modes. During the long-term test, a small increase in polarization losses of the Ni electrode is shown and the values vary in the ranges of 0.15–0.17, 0.16–0.18 and 0.17–0.19 Ω·cm2 in OCV, fuel cell and electrolysis cell modes, respectively. It can be noticed that there is no significant change in the electrode polarization after the second addition of 0.090 g electrolyte. This indicates that the performance of the Ni porous electrode is not strongly affected by the amount of electrolyte in the cell. Therefore, the change in the polarization losses of the cell is largely caused by the NiO oxygen electrode, as shown in Figure 3b. Before the electrolyte addition at 760 h, the polarization resistances in OCV, fuel cell and electrolysis cell modes are almost constant, at values of about 0.35, 0.38 and 0.25 Ω·cm2, respectively. Concerning the effect of the added electrolyte on the polarization losses during the durability test, the NiO electrode is more 10 ACS Paragon Plus Environment

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sensitive to the electrolyte filling degree than the Ni electrode. For example, 0.090 g electrolyte added after 1195 h of MCEC operation, the values of Rp for the NiO electrode drastically increase by about 0.25 Ω·cm2 in OCV and fuel cell modes in the interval 1195– 1350 h, while an increase of 0.07 Ω·cm2 is shown in electrolysis cell mode. This reveals that the polarization behavior of the NiO electrode for oxygen production in the MCEC is less strongly affected by the degree of the electrolyte filling than for oxygen reduction in the MCFC. The electrode performance is partially recovered as the long-period electrolysis operation completed, and the polarization losses of the NiO electrode are found to be 0.39, 0.44 and 0.25 Ω·cm2 in OCV, fuel cell and electrolysis cell modes, respectively. Scanning electron microscopy (SEM) micrographs of the Ni and NiO electrodes after the MCEC durability test were studied, and no obvious change is seen on the catalyst particles of both electrodes when comparing to the case with only fuel cell operation. This indicates that the long-term electrolysis operation has a minor effect on the microstructure of electrode catalysts.



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Figure 3. The polarization losses of (a) Ni and (b) NiO electrodes obtained from impedance spectra in the OCV, fuel cell and electrolysis cell modes during the long-term MCEC test. The ohmic losses and polarization resistances analyzed above reveal that the long-term MCEC operation shows some degradation on the performance of the cell and electrodes. Especially when performing measurements in fuel cell mode, the decrease of the cell performance takes place more obviously. In Figure 4, the electrochemical impedance spectra in Nyquist diagrams are presented to acquire detailed information on the polarization behavior of the cell. The Nyquist diagram shows the existence of two loops, but they are not easily distinguishable. The separation of the arcs could be based on the frequency regions, i.e. the high-frequency arc at about 20 Hz–10 kHz and the low-frequency arc at about 0.01 Hz–20 Hz. The size of the high-frequency arc corresponds to the charge-transfer polarization and the size of the low-frequency arc corresponds to mass-transfer limitations in the cell.35 The cell impedance spectra are more or less shifted to the right in all measured modes in the MCEC durability test. An increase in the size of the high-frequency arc is observed in OCV and MCFC modes, implying that the long-term electrolysis operation increases the chargetransfer polarization of the cell. At the same time, an apparently more increased size is found for the low-frequency arc, see Figure 4a and 4b. These results support that the change in polarization losses of the fuel cell in the durability test is primarily attributable to masstransfer limitations. However, the contribution from the charge-transfer polarization cannot be excluded. Since the gas composition and flow rate are kept constant throughout the experiment, the mass transport in the gas phase will only have a small effect on the cell performance, at least in OCV and MCFC modes. Additionally, the influence of gas-phase mass-transfer limitations is small for standard gas compositions and normal operation current density.36 However, operating the cell in electrolysis mode may affect the electrolyte loss and cause redistribution of the carbonates, that in turn may influence the effective diffusion path 12 ACS Paragon Plus Environment

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in the electrolyte and the gas pores. This may be a reason for the variation in mass-transfer limitations in fuel cell mode. When measuring the cell impedance in MCEC mode, both the charge-transfer and masstransfer polarizations show only very small changes in the durability test, see Figure 4c. This demonstrates that the electrolysis cell in carbonate melts exhibits relatively stable performance for fuel gas production during this long-period operation.

Figure 4. Nyquist diagrams of cell impedance spectra in the three operating modes during the long-term MCEC test. (a) OCV, (b) MCFC (OCV-0.15 V) and (c) MCEC (OCV+0.15 V). The solid symbols refer to some specific frequencies.



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3.2. Performance and Durability of the RMCFC The long-term MCEC operation somewhat decreases the performance of the cell and electrodes, and, as shown above, to a larger extent when measuring the electrochemical performance in fuel cell mode. However, the feasibility of operating the cell reversibly in carbonate melts for long time is still interesting. In order to demonstrate the viability of producing fuel gas and generating electricity in a single device, it is crucial to understand the performance and durability of the cell under reversible conditions. In Figure 5, the cell voltage is recorded during 1019 h of RMCFC operation, i.e. reversing the electrolysis cell (0.16 A·cm-2) and the fuel cell (0.16 A·cm-2) back and forth, at 650 °C. The open circuit voltage of the cell is obtained to be about 1.060 V when introducing the gas composition 64/16/20% H2/CO2/H2O to the Ni electrode. A gradual increase in the cell voltage is initially found under electrolysis condition, i.e. in periods 1, 3, 5 and 7, and the cell voltage then becomes constant or even decrease a little. This shows the same phenomenon as discussed in the MCEC durability test above. Following each of these intervals, the cell is put in fuel cell mode and the cell voltage, under a current load of 0.16 A·cm-2, stabilizes at about 0.9 V. This implies stability of the cell when performing reversible operation in molten carbonate salts for long time.


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Figure 5. The cell voltage recorded under reversible operation for 1019 h at 650 °C. Figure 6 shows the polarization curves of the cell and electrodes before and after the RMCFC durability test. No discontinuity takes place when shifting from fuel cell to electrolysis cell modes. Furthermore, the performance of the cell improves after running the reversible operation for 1019 h. The total resistances of the fuel cell and the electrolysis cell, obtained from the slopes of the curves in Figure 6a, are initially 0.98 and 0.91 Ω·cm2, respectively. After the long-term RMCFC operation, lower values are obtained in both MCFC (0.93 Ω·cm2) and MCEC (0.80 Ω·cm2) modes. The polarization resistances of the Ni and NiO electrodes are calculated from the slopes of the iR-corrected curves in Figure 6b and 6c. Operating the cell reversibly for 1019 h gives only a slight influence on the polarization behavior of the Ni electrode as anode in fuel cell mode, while decreasing the polarization loss from 0.27 to 0.22 Ω·cm2 for water electrolysis. As regards the NiO electrode, the long-period reversible operation results in an improvement of the electrode performance for both oxygen reduction (MCFC) and oxygen production (MCEC). The polarization losses decrease from 0.48 to 0.41 Ω·cm2 in fuel cell mode and from 0.36 to 0.30 Ω·cm2 in electrolysis cell mode.



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Figure 6. (a) Polarization curves without iR-correction for the cell, and that with iRcorrection for (b) Ni and (c) NiO electrodes in both fuel cell and electrolysis cell modes, before and after the RMCFC durability test. Figure 7 shows the ohmic loss and polarization resistance of the cell, obtained from impedance spectra, during the long-term RMCFC test. An overvoltage of -0.20 V and +0.20 V was applied on the cell to simulate MCFC and MCEC modes, respectively. Despite small


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fluctuations, ohmic losses of the cell are almost constant at 0.30 Ω·cm2 for these three operating modes in the RMCFC durability test. A roughly decreasing tendency for the polarization resistances of the cell is shown in all measured modes. The values of Rp are initially 0.70, 0.67 and 0.60 Ω·cm2 in OCV, MCFC and MCEC modes, respectively, and are reduced to 0.61, 0.61 and 0.51 Ω·cm2 after 1019 h of RMCFC operation. The impedance measurements agree with the results from the polarization curves discussed above, confirming that the cell polarization is decreased after the long-term reversible operation.

Figure 7. The ohmic and polarization losses of the cell evaluated from the impedance spectra in OCV, MCFC (OCV-0.20 V) and MCEC (OCV+0.20 V) modes during the long-term RMCFC operation. The polarization losses of the electrodes evaluated from the impedance spectra (Supporting Information) in the RMCFC durability test are shown in Figure 8. The polarization resistances of the Ni electrode are evaluated to be 0.20, 0.19 and 0.23 Ω·cm2 in OCV, fuel cell and electrolysis cell modes, respectively, keeping almost constant during the long-term RMCFC test. It indicates that the performance of the Ni electrode is more or less unaffected by running the cell reversibly for long period. Thus, the NiO electrode plays an important role in the variation of the cell performance in the durability test, see Figure 8b. The overpotential versus OCV of -0.15 V and +0.15 V was applied on the impedance measurements to simulate fuel cell and electrolysis cell modes, respectively. It is visible that in all measured modes the 17 ACS Paragon Plus Environment


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polarization losses of the NiO electrode change in the same way as the cell does. The polarization resistance varies in the range of about 0.37–0.26 Ω·cm2 in electrolysis cell mode, while higher values are obtained in OCV and fuel cell modes (about 0.57–0.40 Ω·cm2). In comparison with only long-period MCFC operation, there is no distinguishable change in the microstructure of the nickel and nickel oxide catalyst particles after the RMCFC durability test. This is also confirmed by the stable electrochemical performance of the Ni and NiO electrodes during the long-term reversible operation.

Figure 8. The polarization losses of (a) Ni and (b) NiO electrodes obtained from impedance spectra in the OCV, fuel cell and electrolysis cell modes during the long-term RMCFC operation.


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In Figure 9, the Nyquist diagrams shows that the RMCFC durability test has a minor impact on the size of the high-frequency arc (about 20 Hz–10 kHz) in all measured modes. It implies that the electrode kinetics is almost unaffected by 1019 h of RMCFC operation. The influence of the long-period reversible operation is mainly seen on the size of the low-frequency arc (about 0.01 Hz–20 Hz) in these three operating modes, which is consistent with the case for the long-term MCEC operation discussed above. Therefore, in the durability tests the masstransfer limitations are responsible for a stronger effect on the variation of the cell performance than other contributions, e.g. the charge-transfer polarization or ohmic losses. The long-term operations of the RMCFC or the MCEC may redistribute the electrolyte, and thus influence the effective diffusion path in the electrolyte and gas pores. However, in order to interpret the polarization behavior of the cell more thoroughly and to determine the factors relevant to the cell performance variation in the long-term tests, further modeling studies of the electrochemical impedance spectra will be needed.



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Figure 9. Nyquist diagrams of cell impedance spectra in three operating modes during the long-term RMCFC test. (a) OCV, (b) MCFC (OCV-0.20 V) and (c) MCEC (OCV+0.20 V). The solid symbols refer to some specific frequencies.

4. CONCLUSIONS Through long-term test of a cell in electrolysis operation in carbonate melt, we show that it is feasible and durable to develop the MCEC for fuel gas production using conventional fuel cell components, at least in the lab-scale. Although a decrease in the performance of the cell and electrodes is shown after the MCEC durability test, especially when performing measurements in fuel cell mode, the degradation is not permanent and the cell performance 20 ACS Paragon Plus Environment

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can be partially recovered. The durability of the cell in reversible operation, i.e. alternatingly operating as an electrolysis cell for producing fuel gases and as a fuel cell for generating electricity, is also investigated in this study. The electrochemical measurements evidence that the performance of the cell and electrodes improves after 1019 h of RMCFC operation. In the durability tests of the MCEC and the RMCFC, the change of polarization behavior of the cell is largely attributable to mass-transfer limitations. Running long-term operations has small impact on the particles of electrode catalysts, and this may be the reason that the chargetransfer polarization slightly influences the cell performance during the durability tests. ASSOCIATED CONTENT Supporting Information The electrochemical impedance spectra of Ni and NiO electrodes measured in open circuit voltage (OCV), fuel cell and electrolysis cell modes during long-term MCEC and RMCFC operations. AURTHOR INFORMATION Corresponding Author *E-mail: [email protected]. Phone: +46 (0)8 790 6507. Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest.



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ACKNOWLEDGEMENTS The financial support of the China Scholarship Council (CSC) and of StandUp (Stockholm and Uppsala) for Energy is appreciated. The cell components were provided by Ansaldo Fuel Cells in Italy.


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