CO2 by high-pressure coelectrolysis in tubular solid oxide cells

Applied Energy 212 (2018) 759–770

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Applied Energy journal homepage: www.elsevier.com/locate/apenergy

Production of syngas from H2O/CO2 by high-pressure coelectrolysis in tubular solid oxide cells

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Muhammad Taqi Mehrana,1, Seong-Bin Yua,1, Dong-Young Leea, Jong-Eun Honga, ⁎ Seung-Bok Leea,b, Seok-Joo Parka,b, Rak-Hyun Songa,b, Tak-Hyoung Lima,b, a b

Fuel Cell Research Laboratory, Korea Institute of Energy Research (KIER), 152 Gajeong-ro, Yuseong-gu, Daejeon 34129, Republic of Korea Department of Advanced Energy and Technology, Korea University of Science and Technology (UST), 217 Gajeong-ro, Yuseong-gu, 34113 Daejeon, Republic of Korea

H I G H L I G H T S of syngas from H O/CO by high-pressure coelectrolysis in tubular solid oxide cells. • Production dependency of the electrochemical characteristics on the tubular SOC cell was studied. • Pressure pressure operation resulted into significantly high OCV and less polarization resistance. • High • No significant degradation in the microstructure of the SOC cell. 2

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A R T I C L E I N F O

A B S T R A C T

Keywords: Tubular solid oxide cell Coelectrolysis Syngas Pressure Electrochemical impedance spectroscopy

The conversion of CO2 and steam into syngas in a pressurized solid oxide coelectrolysis (SOC) cell is considered one of the most promising pathways towards the production of sustainable fuels. In this study, a high pressure tubular SOC system was designed and developed that can efficiently convert a mixture of steam and CO2 into valuable syngas fuel. Tubular SOC cells based on a Ni-yttria stabilized zirconia (Ni-YSZ) fuel electrode, scandia stabilized zirconia (ScSZ) electrolyte, and La0.6Sr0.4Co0.2Fe0.8O3 (LSCF)-Ce0.8Gd0.2O1.9−δ (GDC) composite air electrode were fabricated and tested at various high pressure conditions to determine the electrochemical and syngas production characteristics. The pressurized tubular SOC cell was first operated at the ambient pressure for various inlet gas conditions and the electrochemical performance of the tubular SOC was studied by currentvoltage curves combined with electrochemical impedance spectroscopy at different H2O and CO2 mole% in the inlet gas. The pressurized SOC cell was then operated between 1 and 8 bar pressure at 800 °C in both fuel cell and coelectrolysis modes. In the fuel cell mode, the SOC showed a 44.2% increase in the maximum power density to with a pressure increase of 1–8 bar. The increase in the performance of the cell in the fuel cell mode was attributed to the higher open circuit voltage (OCV) and reduced polarization resistance of the electrodes at higher pressures. In the coelectrolysis mode, the pressure dependency of the electrochemical characteristics on the tubular SOC cell was studied and the relation between different parameters of the system and the pressure conditions was derived. It was found that the higher open circuit voltage (OCV) and the reduced polarization resistance resulted in a significant improvement in the performance of the pressurized tubular SOC cell for the production of syngas. A post-test material characterization by electron microscopy did not show any significant degradation in the tubular SOC cell microstructure during the high pressure operation at 8 bar.

1. Introduction Hydrocarbon fuels can be produced from water, CO2, and renewable electricity by electrochemical conversion in an electrolysis cell. With recent advances in renewable energy technology, researchers are looking for efficient and large-scale energy storage methods that will

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make it possible to cope with the inherent fluctuations of the wind and solar energy supply [1]. The conversion of CO2 into high-value hydrocarbon fuels by using a coelectrolysis cell is considered a viable energy storage method [2]. The high-temperature conversion of CO2 and steam into syngas (H2 + CO) in a CO2 and steam co-electrolysis cell is easy to control, modular, efficient, and scalable [3]. Employing CO2

Corresponding author at: Fuel Cell Research Laboratory, Korea Institute of Energy Research (KIER), 152 Gajeong-ro, Yuseong-gu, Daejeon 34129, Republic of Korea. E-mail address: [email protected] (T.-H. Lim). Equally contributing authors.

https://doi.org/10.1016/j.apenergy.2017.12.078 Received 22 September 2017; Received in revised form 1 December 2017; Accepted 19 December 2017 0306-2619/ © 2017 Published by Elsevier Ltd.

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the results show that chemical kinetics and diffusion play important roles in the H2O–CO2 coelectrolysis process. However, in a recent paper, the conversion of CO2/H2O into syngas was reported by using tubular cells with Ni-YSZ/Ni-SSZ/SSZ/GDC/LSCF-GDC configuration and at 750 °C, good performance of the SOC was achieved [25]. Microtubular solid oxide electrolysis cells (MT-SOECs) have recently been extensively studied for H2O electrolysis, CO2 electrolysis, and H2O–CO2 co-electrolysis [26,27]. The co-electrolysis of CO2–H2O to methane by using a micro-tubular SOC cell was also reported by Lei et al. [5]. Most studies reported the utilization of tubular cells for the production of syngas or subsequent conversion to methane and hydrocarbons. At high pressures, the increased diffusion rates of the reactants to reduce the ASR of the SOC cell can be advantageous in tubular cells. In this study, we systematically investigated high temperature and pressure coelectrolysis using a tubular solid oxide cell to produce syngas and report that by utilizing a tubular cermet-supported SOC cell, high selectivity and conversion of steam and CO2 can be achieved, even when the cell is operated at a high current density. We employed, for the first time, tubular SOCs for pressurized operation to produce syngas from H2O–CO2. An in-house high-pressure tubular SOC cell test station was designed and fabricated with capability of SOC cell characterization up to 8 bar pressure and 850 °C at various operating conditions. This study reports the detailed performance characteristics of a pressurized tubular SOC cell with an extensive discussion of the related factors influencing the syngas production from H2O–CO2. The impact of high-pressure testing on the microstructural properties of the fuel and air electrodes is also investigated in this study.

and steam and co-electrolysis to produce syngas using nuclear or renewable electricity and waste heat in the energy cycle is a promising way of reusing CO2. A high temperature solid oxide coelectrolyzer (SOC) cell includes a cathode (fuel electrode), an anode (air electrode), and a dense electrolyte (O2− ion conductor), where CO2 and steam are fed into the fuel electrode and the oxide ions (O2−) are conducted across the dense electrolyte from the fuel electrode to the anode. Finally, O2 is evolved at the anode and syngas is produced at the fuel electrode [4]. The syngas that is produced is an effective energy carrier beyond electricity that can be used for large-scale energy storage [5]. It can also be further processed to generate chemicals or liquid fuels by the Fischer–Tropsch (FT) process [6–8]. High pressure water splitting for the production of hydrogen is beneficial as it effectively reduces the cost of hydrogen compression at hydrogen station which accounts for 53% of the total hydrogen refueling cost [9,10]. However, due to the lower strength of the PEM cell, membrane creep causes severe degradation issues during high pressure electrolysis [11]. Using SOCs for high-temperature and high-pressure electrolysis eliminates the membrane creep problems. It also offers simultaneous conversion of both CO2 and H2O to syngas. Also, many studies have reported that increasing the pressure of the SOC can improve the performance of the coelectrolysis cell [7–16]. Ni et al. developed an electrochemical model for hydrogen production in a fuelelectrode supported SOC and showed that increasing the pressure leads to a decrease of the diffusion overpotential and predicted higher performance of the pressurized SOC [12]. In an experimental study at high pressure on a LSM (La0.8Sr0.2)0.98MnO3 based SOC cell, oxygen electrodes exhibited a decrease in the polarization resistance with increasing pressure of air at the anode side [13]. The pressurized SOC test results show that when the pressure was increased from 1 bar to 10 bar, the total area specific cell resistance (ASR) at the open circuit voltage (OCV) declined by ∼20%, when the cell was operated at 750 °C [14]. With Ni being a known catalyst for the synthesis of methane, nickelyttria stabilized zirconia (Ni-YSZ) based SOC cells at high pressure can also produce CH4 directly from the syngas in a single cell [15,16]. Jensen et al. [17,18] studied the pressurized operation of a planar 11cell solid oxide stack in fuel cell and electrolysis modes at high pressure from 1.2 bar to 25 bar with 50% H2 + 50% H2O. The stack was tested as a steam electrolyzer for ∼200 h at 10 bar. The integration of solid oxide electrolyzer and Fischer-Tropsch (FT) process is a sustainable pathway for production of synthetic fuels [19,20]. The syngas produced in the SOC can be utilized for downstream processing, including FT conversion to hydrocarbons [6,21] and direct combustion in a combined cycle [1]. In a recent paper, Chen et al. [19] modeled the effect of pressure on the methanation process in a SOC-FT reactor and predicted that it is feasible to operate a pressurized SOC at a lower temperature for CH4 production with improved catalyst activity [8]. The high-pressure generation of syngas thus offers direct integration with high overall efficiency and decreased process costs. Through modeling and experimental studies, Bernadet et al. [22] explained that at a high steam ratio in the input gas and high steam conversion rates, the effect of pressurization on the OCV is balanced by lower ASR values for an air electrode and high performance of electrolysis is expected. They also noted that at high pressure, the cell performance is less sensitive to changes in the microstructural properties of the cermet-support. Gas diffusion plays a crucial role in electrolysis mode. Furthermore, an optimized microstructure of the anode support under high pressure leads to an increase in current density during the co-electrolysis in SOC [23]. The tubular design of a SOC cell offers inherent benefits of easier sealing, better thermal properties, and high strength at pressurized conditions as compared to planar cells. In a previous study by our group, we reported that using a tubular cell for coelectrolysis is beneficial [24] and very good selectivity of CO2 can be achieved in tubular cells for the production of syngas. The tubular cells used by Lee et al. [24] were based on the conventional Ni-YSZ/YSZ/LSM-YSZ design and

2. Experimental 2.1. Fabrication of a tubular SOC cell State-of-the-art Ni-YSZ cermet–support based tubular SOC cells were fabricated by following the conventional ceramic processing route. The detailed fabrication process of the cermet-support and thin electrode and electrolyte layers has been reported elsewhere [28]. The schematics and an actual picture of the tubular cell used for high pressure operation are shown in Fig. 1(a and b). Fig. 1(c) shows the assembly of the tubular cell, with all attached connectors and sensors. Stainless steel connectors were used for the supply of gas at high pressure and the current collection from the fuel electrode side was also integrated, as shown in Fig. 1(d and e). Fig. 1(e) presents details of the thickness and materials of different layers of the SOC cell after sintering. The ScSZ (scandia-stabilized zirconia) electrolyte and GDC (Ce0.8Gd0.2O1.9−δ) buffer layer were 6.15 and 3.67 µm thick, respectively, and the composite air electrode was based on La0.6Sr0.4Co0.2Fe0.8O3 (LSCF)-(GDC) with an average thickness of ∼16 µm. The cell had a diameter of 1.0 cm, a length of 5.0 cm, and an effective electrode area of 3.14 cm2 (electrode length: 1 cm). The SOC cell was sealed with a glass sealant (sealing paste, MiCo, South Korea) and a mica sheet was inserted for insulation between the tubular SOC cell and the metal caps of the cell [24]. 2.2. High-pressure SOC cell testing facility Fig. 2(a) schematically illustrates the high-pressure SOC cell testing facility developed in our lab. The pressure vessel or the pressurized chamber was designed in such a way that it can provide heating capability up to 850 °C to the tubular SOC cell and an easy sealing system for the cell. The pressure inside the pressurization chamber was monitored and controlled by differential pressure transmitters (DPTs). Furthermore, to eliminate the risk of an accident that can be caused by the rapid pressure increase, a rupture disk was set up inside the pressurized chamber. The pressure chamber has provisions for air and fuel supply, current collection, and temperature and pressure data monitoring. The inlet at the fuel electrode side was connected to a pump with a vaporizer in order to 760

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Fig. 1. (a and b) Schematic illustration and actual photograph of tubular solid oxide coelectrolysis cell to be used for pressurized operation, (c) overall configuration of the tubular cell connected to sensors and stainless steel connectors for gas supply, (d) fuel electrode current collection, (e) SEM micrograph of the cross section of the tubular cell showing various layers of the cell (f) fuel electrode current collection.

supply steam continuously and a MFC (mass flow controller) for H2, CO2, and N2 was installed to control the gas flow rate. Moreover, to control the pressure difference by monitoring the pressure values through DPTs, an electronic pressure controller (EPC) was installed, as shown in Fig. 2(b). The test station for the pressurized operation of the tubular cells was also equipped with an online gas-chromatograph to determine the composition of the outlet gas from the SOC cell. The test station can be controlled by a remote computerized program from which the gas flow rate, pressure, and other operating conditions can be adjusted as per the requirements of the experiment.

gases in the heating coil, and finally fed to the fuel electrode side of the tubular SOC cell, as shown in Fig. 2(a). In order to avoid oxidation of the Ni based cathode of the tubular SOC, at least 10% H2 was supplied as a safety gas in all the experiments. The current-voltage electrochemical performance of the SOC cell under each set of operating conditions was measured using an Agilent DAQ-system (34970 A, USA) combined with a load device (DP30-03TP, Toyotech, South Korea). All of the data, temperature, voltage, mass flow, etc., were continuously monitored, acquired, stored, and controlled by a computer through the Agilent DAQ system. The AC impedance (EIS) was measured by impedance measuring equipment (SP 240, Bio-Logic Science Instruments SAS, France) in a frequency range from 1 MHz to 10 mHz, with AC amplitude of 10 mV, and 10 data points per decade of frequency. Finally, for analysis and quantification of the syngas production, we used an online gas chromatograph (PerkinElmer Clarus 580 GC) equipped with one capillary column with He as a carrier for the detection of CO, CO2, H2, and CH4. The columns were calibrated with standard gas compositions, and for each measurement, several readings were taken. The values reported were averages of the gas composition. The microstructures of the SOC cell before and after the pressurized operation were obtained by using SEM (SEM, Hitachi X-4800).

2.3. Electrochemical performance evaluation for the pressurized SOC cell The testing of the pressurized SOC cell was conducted for the coelectrolysis and fuel cell mode. Before testing the cells at high pressure, a dummy cell was installed and the suitability of the test facility for testing at high pressures was evaluated. The fuel electrode was fed with different CO2/H2O compositions by controlling the CO2 flow with a digital flow meter and deionized water flow with a peristaltic pump (Model EMS-200S, EMS Tech, error < ± 2%). The water was evaporated to steam by using heating tape and mixed with incoming H2/CO2

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Fig. 2. (a) Schematic illustration of the setup used for testing the pressurized tubular SOC. (b) Actual photograph of the experimental setup for pressurized testing. The inset image shows the pressure chamber.

3. Results and discussion

observed at 1.8 V when the cell was operated at 750 °C. The clear improvement at high temperature was due to the improved conductivity of the ScSZ based electrolyte and higher conversion of CO2 to CO by the reverse water-gas shift reaction (RWGS) [29]. Fig. 3(c) shows the effects of increasing the steam content in the fuel electrode inlet gas. At 20% H2O, the cell showed very low performance and the voltage increased rapidly. This shows that the trend of increased losses was related to the concentration polarization. By increasing the steam amount from 20% to 60% in the inlet gas, cell voltage of 1.8 V was achieved at −500 and −900 mA/cm2, respectively. To obtain current density of −500 mA/cm2, cell voltage of 1.6, 1.49, and 1.38 V was required at 20, 40, and 60% H2O, respectively. Increasing the steam amount in the inlet gas provided a sufficient supply of the reactants for coelectrolysis. It has been reported in the literature that the coelectrolysis process is dominated by H2O electrolysis due to the higher adsorption rate of H2O as compared to that of carbon dioxide. Kim et al. explained that by increasing the steam content, the performance of the SOC cell is increased because the steam electrolysis reaction (H2O + 2e− = H2 + O2−) becomes dominant [30]. Fig. 3(d) shows the composition of syngas produced during the

3.1. Performance of the tubular SOC cell at atmospheric pressure At atmospheric pressure conditions, the performance of the tubular SOC cell was investigated to obtain reference data for the pressurized operation. The polarization (i–V) curves of the tubular SOC cell for steam electrolysis and CO2/H2O coelectrolysis are presented in Fig. 3(a) at 850 °C. 10% H2 was supplied at the fuel electrode side as a safety gas. The cells were operated in coelectrolysis mode at a H2O/CO2 ratio of 2. The results show that the absolute current densities increased with increasing voltage. The tubular SOC cell performance was better in the case of coelectrolysis of steam and CO2 than that of steam electrolysis. It was also noted that increasing the steam amount from 40 to 60% decreased the cell voltage from 1.61 V to 1.42 V at -800 mA/cm2 current density. Fig. 3(b) shows the effect of temperature on the performance of the SOC cell when the fuel electrode gas was supplied at 60% H2O, 30% CO2, and 10% H2. As expected, a clear improvement of performance was observed at 800 °C. The SOC cell showed 1.75 V at −800 mA/cm2 at 800 °C while the maximum current density of 760 mA/cm2 was 762

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Fig. 3. Performance of the tubular SOC at atmospheric pressure (a) Polarization (i-V) curves for different condition of inlet gases, (b) effect of temperature on i-V characteristics of the tubular SOC, (c) effect of steam amount in the inlet gas, and (d) composition of H2 and CO produced at various polarization conditions at 750 and 800 °C.

1.6

operated at a higher current density. It can be noted from Fig. 3(d) that at the OCV, when no current was applied, the amount of H2 in the syngas decreased upon increasing the temperature from 750 °C to 800 °C. This is due to the WGS (water gas shift reaction, CO + H2O ↔ CO2 + H2, ΔH800°C = 36.82 kJ/mol) being favorable in the forward direction at lower temperature and in the reverse direction at higher temperature. Lowering the temperature thus promotes hydrogen production while decreasing CO formation [30]. The formation of CO from the electrochemical reaction and thermochemical reaction during high temperature coelectrolysis is a competitive reaction. By applying high current, there is not a substantial increase in CO concentration while the H2 amount reached 40 mol%. This indicates that during the coelectrolysis, the conversion of H2O is more effective as compared to CO2 electrochemical conversion. This behavior is in accordance with findings from the Mogensen group [31]. The maximum mole fractions of 43% H2 and 13% CO were observed at 800 °C. The high H2 concentration syngas produced during coelectrolysis in a tubular SOC cell at atmospheric conditions showed that the cell is capable of converting H2O/CO2 into good quality syngas, and the these results are comparable with those of earlier studies [32].

Open Circuit Voltage (V)

1.4 1.2 1.0

Theory Experiment

0.8 0.6 0.4

Active area = 3.14 cm2 Temperature = 800 oC

0.2

Cathode flowrate = 300 cc/min(Air) Anode flowrate = 200 cc/min(H2)

0.0

1

2

3

4

5

6

7

8

Pressure (bar)

3.2. Performance of the pressurized tubular SOC cell

Fig. 4. Effect of pressurization on the OCV of the tubular SOC measured at 800 °C in H2. The theoretical values of the OCV were calculated from the Nernst Equation.

3.2.1. Effect of pressurization on the OCV of tubular SOC cell The effect of pressurization on the open circuit voltage (OCV) of the tubular SOC cell at 800 °C is shown in Fig. 4. The theoretical OCV was calculated from the Nernst equation. The observed OCV increased with increasing pressure from 1 to 8 bar. However, the experiment OCV

coelectrolysis. The exhaust gases of the tubular SOC cell were analyzed by online GC during testing at different operating conditions. CO and H2 were present in higher mole fractions when the tubular SOC cell was 763

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Fig. 5. Electrochemical performance characteristics of the pressurized tubular SOC in SOFC mode (a) Polarization (i-V) curves (b and c) electrochemical impedance spectroscopy (EIS) at different pressure conditions.

experimental OCV values. It is also important to note that by increasing the pressure from 1 to 8 bar, the OCV of the tubular SOC cell is increased up to 7.3%. Because pressurization may enhance the gas-phase diffusion in the porous electrode, this effect can increase the supply of fuel at the reaction sites, resulting in higher OCV values. The higher OCV of the tubular SOC cell can improve the electrochemical performance for coelectrolysis and fuel cell reactions, similar to the results reported by other groups [36].

values were slightly lower than the OCV calculated from the theory. The maximum difference of the theoretical and experimental values was about 3% at 8 bar pressure. This might be due to increased gas leakage in the cell at higher pressure [33]. The electrolyte of the SOC cell is sintered at high temperature (1400 °C) and it was verified that it is fully dense. However, during the fabrication process, there is a possibility that small pinholes might occur in the electrolyte microstructure. When high pressure (8 bar) gas is passed through the tubular SOC cell fuel electrode, some leakage of the gas might occur from these pinholes, resulting in an experimental OCV lower than the theoretical value [22,34]. Rasmussen et al. explained that the OCV difference at lower pressure (1 bar) might be due to electric leakage while at higher pressure (e.g., 8 bar) it results from electric leakage as well as gas diffusion leakage through the pinholes and cracks in the seals [35]. However, in the tubular SOC cell, the leakage through the seals is minimal because the cylindrical cells are easily sealed. Therefore, we can observe that the difference in the theoretical and experimental OCV is significantly less than the values reported by Hashimoto et al. [33]. Thus, the tubular SOC cells offer better sealing properties and higher

3.2.2. Characteristics of the pressurized tubular SOC in fuel cell mode The electrochemical performance of the pressurized tubular SOC cell was studied in both fuel cell and co-electrolysis modes. Fig. 5(a) shows the electrochemical performance of the SOC cell in fuel cell mode under different pressure conditions. The tubular SOC cell was operated with humidified H2 as fuel and ambient air as an oxidant at 800 °C. The fuel flow rate was 200 cc/min and the air was kept at 300 cc/min. The OCV of the cell increased from 1.14 V to 1.23 V due to the increase in the pressure of the H2 gas from 1 to 8 bar. The maximum power density of the cell at 1 bar was 586 mW/cm2. By increasing the pressure up to 764

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Fig. 6. Electrochemical performance characteristics of the pressurized tubular SOC in SOEC mode (a) Polarization (i-V) curves (b and c) electrochemical impedance spectroscopy (EIS) at different pressure conditions.

and 100–150 Hz for low frequency and high frequency arcs, respectively (Fig. 5(c)). The low-frequency arc is usually attributed to the diffusion processes in the anode and anode support of the SOFC [37] and the high frequency arc is considered to be the result of activation polarization of the fuel electrode and anode [36,38]. From the high frequency and low frequency intersects of the EIS plots, the ohmic resistance (Rs) and polarization resistance (Rp) of the tubular SOFC under pressurized conditions were determined. In SOFC mode operation of the pressurized cells, the Rs values remained almost constant while the Rp decreased from 1.294 to 0.597 Ω cm2. The gas conversion resistance at the SOFC electrode depends on the flux of the reactant molecules at the TPB [39]. Due to the pressurization, the flux perhaps increased, causing

8 bar, a 44.2% increase in the maximum power density of the SOFC was observed. The increase in the performance due to pressurization, as observed from the maximum power output, is caused by an increase in the OCV and a decrease in the polarization resistance of the cell. The influence of the pressurization can be further elaborated from the electrochemical impedance spectroscopy (EIS), as shown in Fig. 5(b). The serial resistance (Rs) of the cells remained the same when the pressurization was applied whereas the polarization resistance, Rp, markedly decreased. The shape of the EIS Nyquist plots corresponded with that of a typical anode supported SOFC. The low and high frequency arcs in the EIS plots were changed by increasing the operating pressure. The characteristic frequencies were found to be around 0.1–1 765

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Fig. 7. Effect of pressurization on the polarization resistance of the tubular SOC (a) Ohmic polarization (Rs) as a function of pressure, (b) electrode polarization (Rp) as a function of pressure, (c) effect of pressurization on the activation polarization during SOFC and SOEC mode.

electrolysis is decreased at higher pressure. The advantage of increasing the pressure is therefore reduced due to the high OCV value. Nevertheless, the performance of the SOC cell under pressurized conditions is significantly higher than that under ambient pressure conditions, which is in accordance with the results from other groups [12,13]. Fig. 6(b and c) shows the EIS data for the SOC cell in coelectrolysis mode. The impedance spectra depict that by increasing the pressure, the low frequency arc is reduced significantly, but the ohmic resistance is not affected by the pressurization. The summit frequencies of the low and high frequency arcs were around 0.5–1 Hz and 100–150 Hz, respectively. The summit frequency of the gas conversion arc (low-frequency arc) decreases with an increase in the pressure. The EIS results also show that the ohmic resistance was constant at 0.14 Ω cm2, and the polarization resistance was significantly decreased to 0.09 Ω cm2 at 1 bar and 0.04 Ω cm2 at 8 bar, respectively. Fig. 6(c) shows that the high frequency arcs also were changed due to high pressure. The summit frequency of the high frequency arcs was also influenced by increasing pressure but comparing to the low frequency arcs, the effect was smaller. This might be because the low frequency arcs belong to the diffusion related polarization whereas the high frequency arcs indicate the influence of activation polarization. The high pressure condition strongly influences the diffusion of gases at the TPB, and therefore the effect is visible in the EIS spectra [33]. Comparing the arcs for 1 bar and 8 bar, it can be noted that the size of the arc is significantly reduced in

a significant reduction in the polarization resistance and increased performance of the pressurized tubular SOFC. The EIS trends observed in the tubular SOFC are similar to those of anode-supported planar SOFCs, as reported by [18,36,40]. 3.2.3. Characteristics of the pressurized tubular SOC cell in coelectrolysis mode The electrochemical performance of the tubular SOC cell was investigated in the coelectrolysis mode to determine the influence of the various pressurization parameters. Fig. 6(a) presents i-V polarization curves for the pressurized tubular SOC cell during coelectrolysis. The inlet gas composition at the fuel electrode was H2O – 60%, CO2 – 30% and H2 – 10% at 800 °C and the air was supplied to the anode (air electrode). The SOC cell was operated from 1 to 8 bar, and from Fig. 6(a), it can be noted that the slope of i–V decreased with an increase of the pressure due to higher OCV and lower ASR values. A similar trend in the i–V curves was also reported by Jensen et al. [17,18]. At 8 bar pressure, a current density of 970 mA/cm2 was achieved at an applied voltage of 1.38 V while the same current density was achieved at 1.58 V at the ambient pressure conditions. The benefit of increasing the pressure on the electrochemical performance of the SOC cell in coelectrolysis mode is smaller as compared to the fuel cell mode. This is due to the fact that the OCV is increased by increasing the pressure, and the maximum voltage required at a specific current density for 766

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(1)), compared to CO2 electrochemical conversion [43–45]. At high pressures, it is observed that the concentration of reactions (1)–(3) is reduced; this might result in the formation of methane, which occurs due to the result of methanation reactions (4) and (5). The formation of higher CH4 amounts at high pressure was also predicted by Sun et al. [15], who reported that decreased concentrations of H2 and CO when the pressure is increased might be due to reactions (4) and (5). However, due to experimental limitations, we could not determine the exact amount of methane formed during the high pressure operation. In the literature, many studies report the direct generation of CH4 during the coelectrolysis process [42,46,47]. Since our system was tested up to a maximum of 8 bar, the higher pressure might result in greater CH4 amounts in the outlet gas. By employing the integrated approach of SOC–FT reactors in the pressurized conditions, this might be an easier way to obtain value added gas-to-liquid conversion [6]. In future experiments, we will modify the pressurization test station and plan to conduct detailed experiments on the effect of pressurization on the concentrations of H2, CO, and methane. The design of an electricity-toliquid fuel system, made of a pressurized SOC stack working in coelectrolysis and a Fischer-Tropsch reactor to produce the middle-distillates can be envisaged by using tubular pressured SOC cells [19].

the latter case. At high pressure, the mass transfer limitations at the electrodes are reduced, and consequently the electrochemical performance is improved [22]. Fig. 7(a–c) presents a comparison of the ohmic and electrode polarization of the pressurized tubular SOC cell, respectively. The ohmic resistance of the cell did not increase significantly by increasing the pressure in both the fuel cell and electrolysis modes. However, as can be seen from Fig. 7(b and c), the electrode polarization was reduced significantly by increasing the pressure. The decrease in the electrode polarization resistance due to the increase in pressure is presented in Fig. 7(d). It can be noted that there is around a 60% reduction in the electrode polarization by increasing the pressure up to 8 bars. In both modes, the values of electrode polarization resistance are decreased in the same fashion, and shifting the mode of operation from fuel cell to coelectrolysis does not significantly change the pressure effects on the electrochemical performance. This means that a pressure increase greatly influences the parameters related to the diffusion and availability of the fuel at the reaction sites [22,23]. 3.3. Outlet gas composition from the pressurized SOC cell Fig. 8 shows the effect of pressure on the outlet composition of syngas at 800 °C in the tubular SOC cell. The inlet gas composition was chosen as 10% H2 + 30% CO2 + 60% H2O. The gas composition was determined when the cell was operated at −800 mA/cm2 current density for all pressure conditions. When the current density is gradually increased, the H2 mole fraction increases significantly more than the CO mole fraction does. At all current density values, the H2O electrolysis is easier (due to faster reaction kinetics) than CO2 electrolysis, because the voltage required for the electrolysis of H2O is lower than that of CO2 electrolysis [41]. The following reactions occur during the pressurized coelectrolysis operation in a tubular SOC cell [42]:

2H2 O= 2H2 + O2

(steam electrolysis)

(1)

2CO2 = 2CO + O2

(CO2 electrolysis)

(2)

CO2 + H2 = CO + H2 O (reverse water gas shift reaction, RWGS)

(3)

CO2 + 4H2 = CH 4 + 2H2 O (methanation reaction)

(4)

CO + 3H2 = CH 4 + H2 O (methanation reaction)

(5)

3.4. Effect of pressurized coelectrolysis on microstructure of the tubular SOC cell The microstructure of the SOC cell before and after the pressurized operation was studied by SEM and the results are presented in Fig. 9. Fig. 9(a) shows a cross-section SEM micrograph of the tubular cell before the operation. Fig. 9(b) and (c) shows the microstructure after the pressurized operation at 1 bar and 8 bar, respectively. We can see from Fig. 9 (b) that the anode side of the cell was delaminated after the operation. The delamination of the anode layers during the coelectrolysis operation has been reported by many researchers [48,49]. However, for the case of operation at 8 bar, it was observed that the electrolyte had small cracks as well as delamination on both the fuel electrode and anode sides. The cracking of the electrolyte might occur by the following mechanism. The tubular SOC cell fabrication process consists of deposition of the electrolyte layer by vacuum slurry coating [50]. In some cases, during the fabrication, impurities result in the formation of micron-sized pinholes in the electrolyte, as can be seen in Fig. 9(a). It is hence speculated that those small pinholes might have experienced high pressure and caused crack propagation at high pressure of up to 8 bar. Some researchers pointed out that at high pressure, during coelectrolysis, carbon formation might also occur due to the Boudouard reaction (2CO2(g) = C(s) + CO(g)). Therefore, we analyzed the high-resolution SEM microstructures of the fuel electrode functional layers of the cells before and after the operation at high pressure, as shown in Fig. 9(d and e). However, no traces of carbon deposition were detected from the after operation test samples. In their modeling study, Sun et al. [39] calculated the carbon formation possibility on a Ni surface at a pressure above 25 bars, and our results confirmed that carbon deposition was not found at high-pressure operation up to 8 bar. The degradation of the SOC cell performance during long-term operation is one of the biggest issues in the high-temperature electrolysis community, and high-pressure operation increases the stress on the cell and the system, leading to long-term performance loss. However, there is a need for a systematic and detailed study to investigate the specific degradation that occurs when the tubular cells are operated at high pressure. Ebbesen et al. [51] explained that the operation of Ni-YSZ based solid oxide cells in fuel cell and coelectrolysis modes is different, and in the electrolysis mode, higher degradation of the performance is observed. The diffusion limitation in the fuel and air electrodes during the electrolysis operation can easily be overcome by high-pressure operation of the tubular cells, as shown in the EIS analysis of the tubular SOC cells in Fig. 6. It was also noted by Hauch et al. [49] that the degradation contribution of the fuel electrodes during the coelectrolysis

According to Fig. 7, it is evident that the concentration of H2 is significantly higher than that of CO in the outlet syngas. The higher concentration of H2 is due to the fast reaction of steam electrolysis (Eq. 70 H2

Mole fraction (%)

60

CO

50 40 30 20 10 0

1

2

3

4

5

6

7

8

Pressure (bar) Fig. 8. . Effect of pressure on outlet syngas composition at 800 °C in the tubular SOC. The inlet gas composition was chosen as 10% H2 + 30% CO2 + 60% H2O. The gas composition was determined when the cell was operated at −800 mA/cm2 current density for all pressure conditions.

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Fig. 9. Cross-section SEM micrographs of the tubular SOC before and after operation at pressurized conditions (a) before test, (b) after the test at 1 bar, (c) after the test at 8 bar pressure, (d) high-resolution SEM of the fuel electrode functional layer of tubular SOC before the pressurized operation, (e) high-resolution SEM of the fuel electrode functional layer of tubular SOC after pressurized operation at 8 bar.

operating process for converting syngas into liquid fuel in the same cell. The gas-to-liquid (GTL) conversion is more effective and efficient if the FT reaction occurs simultaneously in the same cell. In other words, we can combine highly specialized reactors that apply pressurized processes, the syngas synthesis process and the FT synthesis process - into a single integrated process. Considering the factors outlined above, the pressurized tubular SOC technology provides an attractive route to significantly reduce CO2 emissions and permit energy storage at a very large scale.

might be related to diminishing active electrode sites. Nevertheless, the operation of the SOC cell at high pressure shows the promise of obtaining value added syngas. Also, the tubular SOC cell can easily be scaled up into a stack with minimal leakage and a FT-reactor can also be easily integrated. The production of syngas by high temperature CO2/H2O co-electrolysis at elevated pressures using SOCs thus has remarkable potential for reduced costs and improved efficiency. The syngas (H2 + CO) that is produced during the pressurized coelectrolysis process is a very effective energy carrier beyond electricity that can be used for large-scale energy storage. Further processing of the syngas can produce chemicals or liquid fuels via Fischer–Tropsch (F–T) synthesis. Similarly, the fuels (syngas) produced by pressurized coelectrolysis can be used to generate power within the same pressurized SOEC device in the reversible mode. Using the tubular SOC cells for pressurized conversion of CO2-H2O requires very few seals as compared to a flat-plate type cell where the sealing portion is quite large. In the pressurized operation process, there are many parts where the sealing portion is relatively more significant than the other parts. Also, there is possibility to use the pressurized

4. Conclusions This study examines the performance of pressurized tubular solid oxide coelectrolysis cells (SOCs) for the production of syngas from CO2 and steam. The operation of the SOC cell at ambient pressure at the fuel inlet and temperature conditions confirmed that the tubular SOC cell is suitable for producing a good quality syngas. The effect of pressurization on the electrochemical performance of the tubular SOC cell in the fuel cell and coelectrolysis mode shows that by increasing the pressure 768

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of the system, we can obtain higher performance in terms of the maximum power density of the fuel cell and current density at a specific voltage for the coelectrolysis case. The OCV of the pressurized system increased from 1.14 V at 1 bar to 1.23 V at 8 bar. The maximum power density in the fuel cell mode of the tubular SOC cell increased from 586 mW/cm2 to 845 mW/cm2 when the pressure was increased up to 8 bar. In the coelectrolysis mode, the voltage obtained for the pressurized tubular cells at −800 mA/cm2 was 1.46 and 1.28 V at 1 and 8 bar, respectively. The impedance spectroscopy analysis showed that at higher pressure, there is a significant decrease in the polarization resistance of the electrodes in the tubular SOC cell, which contributes to increased performance of the pressure SOC cells.

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Acknowledgements This work was supported by Korea CCS R&D Center (KCRC) grant (No. 2014M1A8A1049298) funded by the Korean government (Ministry of Science, ICT & Future Planning). This work was also supported by the Korea Institute of Energy Technology Evaluation and Planning (KETEP) and the Ministry of Trade, Industry & Energy (MOTIE) of the Republic of Korea (No. 20163030031850).

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