Inert Gas Step Addition Research Articles

Overpotential Behavior of Electrolysis and Fuel Cell Modes of Molten Carbonate Cells

Molten carbonate is an high operating temperature electrolyte for the fuel cell (MCFC) and electrolysis cells (MCEC). In particular, the liquid molten carbonates allow the reversible operation between fuel cell and electrolysis cells. The electrode kinetics and mass transfer characteristics of MCFC have been rather extensively investigated compared with those of MCEC. In this work, the mass transfer behaviors between MCFC and MCEC have been focused by experimental way with 100 cm2 class bench cells.The electrodes and matrix in the cell were supplied by KIST in Korea. The cells were operated at 923K under atmospheric pressure. The hydrogen electrode was supplied by a mixture of H2:CO2=1:1, 2:1, 1:2 ratios at 20% or 40% H2O. The oxygen electrode was fed by Air:CO2=0.7:0.3 atm. The steady state polarization (SSP) from 0 to 150 mA cm-2, inert gas step addition (ISA), and reactant gas addition (RA) methods were employed. The ISA adds inert gas to the hydrogen or oxygen electrodes, then the addition varies reactants flow rate and partial pressures. Consequently, the variations are reflected to the cell voltage at open circuit and polarization states. The voltage shift by the flow and partial pressure change represents overpotential behaviors at the electrodes [1]. The RA method uses a reactant gas instead of inert gas in the ISA. Therefore, the RA provides the overpotential information of the added reactant species [2].From the experimental investigations, the total voltage loss at the FC is larger than that of EC mode. In addition, the hydrogen electrode has significant gas phase mass transfer resistance due to the H2, CO2, and H2O species at the FC and EC modes. A similar overpotential was observed at the hydrogen electrode. On the other hand, the oxygen electrode has much less overpotential at the EC mode than the FC mode. Since the oxygen reduction at the FC mode contains O2 dissolution and mass transfer through the carbonate electrolytes, liquid phase mass transfer resistance is dominant. However, the oxidation of carbonate ions to oxygen does not contain liquid phase mass transfer process at the EC mode. Therefore, much less overpotential was observed at the oxygen electrode at the EC mode than the FC mode. This can be a reason of smaller total voltage loss at the MCEC than the MCFC.[1] Samuel Koomson, Choong-Gon Lee, J. Electroanalytical Chemistry, 925, 116896 (2022).[2] C.-G. Lee, H.-C. Lim, J. Electrochem. Soc., 152, A219-A228 (2005).

Effect of temperature on the electrode overpotential of molten carbonate electrolysis and fuel cells with inert-gas step addition method

This study explores molten carbonate electrolysis and fuel cell behaviour across temperatures from 823 to 973 K using steady-state polarization (SSP), electrochemical impedance spectroscopy (EIS), and inert-gas step addition (ISA) techniques. The findings reveal that total voltage loss decreases with rising temperature in fuel cell (FC) and electrolysis (EC) modes. The ohmic loss similarly declines as temperature increases. The ISA method determines the gas-phase mass transfer-induced overpotential, which increases as temperature increases in both modes at the hydrogen electrode (HE). This indicates that pore diffusion resistance strongly affects the HE in both modes. The liquid-phase mass transfer-induced overpotential at the oxygen electrode (OE) exhibits a much smaller value in EC mode than in FC mode at all temperatures. The increased O2 and CO2 partial pressures and decomposition of sufficient amounts of carbonate electrolytes to generate O2 in EC mode would reduce the liquid-phase resistance and its induced overpotential at the OE in the temperature range.

Effect of temperature on the electrode reactions in a planar solid oxide fuel cell

Understanding the temperature dependence of electrode reactions is attempted with a commercial 100 cm2 class planar solid oxide fuel cell. Experimental investigations are conducted using a specially designed setup, i.e., inert gas step addition (ISA) and reactant gas step addition (RA) methods, to measure the electrode overpotential behaviour at temperatures ranging from 923 to 1073 K. The ISA reveals that the gas-phase mass transfer resistance at the anode is more substantial and becomes immensely larger with increasing temperature due to the effect of pore diffusion resistance (PDR). On the other hand, the gas-phase mass transfer resistance of the cathode shows an unclear dependence on temperature, indicating little resistance in the cathode. The RA method represents that the H2-inducing mass transfer resistance increases with increasing temperature due to the PDR effect, while the O2-inducing mass transfer resistance decreases with increasing temperature because of the reducing solid-phase mass transfer resistance of oxide ions.

Comparison of gas phase transport effects between fuel cell and electrolysis cell modes of a 100 cm2 class molten carbonate cell

• Gas phase transport effects are compared between electrolysis and fuel cell modes with 100 cm 2 class cells. • Symmetric overpotential behaviours are obtained between electrolysis and fuel cell modes. • Severe gas phase mass transfer resistance exists in the hydrogen electrode of electrolysis cell. • Negligible gas phase resistance is observed in the oxygen electrode of electrolysis cell. The gas-phase mass transfer effects of 100 cm 2 class molten carbonate cells (MCCs) were investigated in both the fuel cell and electrolysis modes using steady-state polarization (SSP) and inert-gas step addition (ISA) methods. The SSP results showed symmetric and linear behaviours between fuel cell (FC) and electrolysis cell (EC) modes without showing activation polarization in both modes. The ISA results showed that the hydrogen electrode (HE) has significant gas-phase mass transfer resistance in both modes, and the overpotential shift pattern of HE by the flow rate change is symmetric in both modes. On the contrary, a small gas-phase mass transfer inducing overpotential was observed at the OE in the FC mode compared to the HE, while there was no gas-phase mass transfer-induced overpotential in the EC mode. Therefore, the EC mode of MCCs comprises mainly gas-phase mass transfer controlling processes at HE and negligible at OE. In addition, it can be inferred that reversible operation in both modes is possible, and the reaction mechanisms are symmetrical between FC and EC modes.

Experimental analysis of internal leakage current using a 100 cm2 class planar solid oxide fuel cell

This work studies the effect of the electronic conductivity of a commercially used yttria-stabilised zirconia (YSZ) electrolyte on solid oxide fuel cell performance. For the first time, an experimental method is used to measure the electronic conductivity of YSZ in commercially manufactured and operated single cells. An inert-gas step addition (ISA) method is employed to measure the electronic conductivity in the form of voltage changes at the open-circuit state. Since the electronic conductivity allows oxide ion transfer and water generation at the anode, a reduction in open-circuit voltage (EOCV) occurs due to the increased water partial pressure. The step increase in the anode flow rate in the ISA method reduces water partial pressure and raises EOCV. The EOCV change is then converted into internal leakage currents and electronic resistivities. The YSZ's electronic resistivity obtained in this work is comparable to the results obtained using the Hebb-Wagner polarisation technique and the oxygen permeation method.

Effect of Gas Leakage on the Performance of Planar Solid Oxide Fuel Cells

Solid oxide fuel cell (SOFC) is a promising power source with its strong advantage being the use of solid oxide electrolyte. This allows long time operation of the cell without electrolyte depletion. Due to the solid nature of SOFCs, sealing of the cell to the peripheral attachments becomes an issue, especially in planar SOFC type. This defect in gas sealing causes gas-leakage to the outside, as well as, gas-crossover through the peripheral areas of the cell. These leakages lead to a reduction in the open-circuit voltage (OCV) and the overall cell performance. Most previous reports made on gas crossover were only based on H2 crossover through the thin electrolyte membrane. In this work, the performance of a gas-leaked cell was compared with one with a gas-tight sealing. Both cells were 100 cm2 planar SOFCs operated at 750 and supplied with H2 and air gases at the anode and cathode respectively. The cells were supplied by a commercial SOFC producer in South Korea. Electrochemical Impedance Spectroscopy (EIS) and steady-state polarization (SSP) techniques were used in comparing the performance of the cells. An inert gas step addition (ISA) method [1] was also employed in determining quantitatively the partial pressure of water formed at the anode and its resulting internal short current. The ISA was first used to investigate the flow and partial pressure effect on the performance of a molten carbonate fuel cell (MCFC). The method adds nitrogen gas to the anode gas flow and the addition simultaneously increases the reactant gas flow until the added nitrogen arrives at the cell. The nitrogen addition port is placed ca. 5 m ahead of the anode electrode and is supplied through a 1/4-inch stainless tube, thus several seconds are required by the N2 gas to reach the cell. The SSP behavior showed that the OCV and the polarization voltages recorded were higher in the gas-tight cell than in the gas-leaked cell. The impedance spectra, on the other hand, showed a remarkable decrease in the gas-leaked cell and was attributed to the decrease in the water-induced mass transfer resistance of the cell. Through the ISA method, the partial pressure of water formed at the anode as a result of the gas leak was quantitatively determined and was observed to be significantly higher in the case of the gas-leaked cell. Similarly, the computed internal short current was greater in the case of the gas-leaked cell.[1] C.-G. Lee, B.-S. Kang, H.-K. Seo, H.-C. Lim, J. Electroanal. Chem., 540, 169–188 (2003).

Effect of gas-phase transport in a planar solid oxide fuel cell

Reaction mechanisms and performance improvement in solid oxide fuel cells (SOFCs) have been the major research areas in this field. There are several discrepancies associated with the kinetics of SOFCs due to the contradicting theories proposed by researchers in this area. This work aims at determining the overpotential due to the gas-phase mass transfer resistance at the various electrodes and the possible electrode reaction characteristics using the inert gas step addition (ISA) method proposed by this research group. The ISA method can quantitatively measure the overpotentials due to the mass transfer resistance at each electrode. The ISA method shows an inverse relationship between the overpotential shift and the square root of the reactant gas flow rate according to the boundary layer theory. The overpotential due to the gas-phase mass transfer resistance is expected to be much higher at the anode than at the cathode, indicating the presence of considerable gas-phase mass transfer resistance at the anode.

Effect of Short Current on the Open Circuit Voltage in a Solid Oxide Fuel Cell

The solid oxide fuel cell (SOFC) is a promissing power source with its strong point of solid oxide electrolyte. It allows long time operation without electrolyte supplying. But the electroylte in the SOFC must be very thin to reduce the internal resistance. Thinner electrolyte provides better performance. In my laboratory, a 100 cm2 class SOFC has been operated. It was supplyed from a SOFC maker in Korea. Since it has several tenth micrometer of electrolyte thickness, it showed voltage behavior of short current when the inert gas step addition (ISA) method [1] was employed. The ISA was firstly used to investigate the flow and partial pressure effect on the performance of molten carbonate fuel cell (MCFC). The method adds nitrogen gas to the anode flow, simultaneously it raises reactant gas flow until the added nitrogen arrives at the cell. Indeed, the nitrogen addition port places about 5 m ahead of the anode cell with 1/4 inch stainless tube, thus several seconds are required to reach to the cell. In the MCFC, there was no voltage change during the period of seveal seconds at open circuit state. Since there was no applied current, it has no doubt. But the SOFC showed clear voltage increase at the tiem period by the nitrogen addition although it was at open circuit state. It is clearly compared with that behavior of MCFC. In this presentation, the difference will be investigated in terms of short current at the SOFC. [1] C.-G. Lee, B.-S. Kang, H.-K. Seo, H.-C. Lim, J. Electroanal. Chem., 540, 169–188 (2003).

Effect of sulfur on the cell performance in a molten carbonate fuel cell

This study evaluated the performance characteristics of a molten carbonate fuel cell (MCFC) under sulfur poisoning condition. A 100 cm2 bench-scale MCFC was used for the performance test. The performance of the cell at the normal operation condition was measured at 620 °C under atmospheric pressure. To evaluate the sulfur poisoning effect, 50 ppm H2S/Balanced N2 gas mixture at a flow rate of 30 ml/min was injected into the anode. The cell performance was analyzed via steady-state polarization (SSP) and inert gas step addition (ISA) methods. The overpotential of the cell was observed to have been changed by the injection of H2S gas, and an overpotential increase of ca. 40% was recorded after about 100 hours from the point of addition of the H2S gas. A voltage reduction rate of −2.62×10−4 mV/s was recorded at a current density of 150mA/cm2 after about 240 hours. The steady-state polarization characteristics of the cell showed that the sulfur poisoning was relatively slow. In addition, the effect of H2S gas on the cell performance was quantitatively measured by the ISA method.

Temperature effect on the electrode reactions in a molten carbonate fuel cell

A temperature effect on the electrode reactions was investigated in terms of activation energy of overpotential with 100cm2 class molten carbonate fuel cells. The overpotential is attributed to the resistance in the electrochemical reactions on electrodes, thus the activation energy reflects reaction characteristics of the electrodes. The activation energy was measured with electrochemical impedance spectroscopy (EIS) and inert gas step addition (ISA) methods. These methods revealed that the anodic overpotential had positive activation energies showing larger overpotential at higher temperature, whereas the cathodic overpotential had negative activation energies. In particular, the positive activation energy of anodic overpotential is a unique behavior which has not been treated before. The gas phase diffusion resistance in the porous electrode, calling pore diffusion resistance (PDR), would be responsible for the positive activation energy. On the other hand, the temperature dependence of reactant gas dissolution to the carbonate melts and transport through the carbonate film leads to negative activation energy because the cathode is covered by the relatively thick electrolyte film. Insignificant PDR effect is observed at the cathode through the activation energy of gas phase mass transfer resistance at the cathode.

Influence of temperature on the anode reaction in a molten carbonate fuel cell

Anode reaction characteristics in a molten carbonate fuel cell (MCFC) have been investigated in terms of overpotential by varying the temperature for 100cm2 class single cells. The temperature range was from 823 to 973K under atmospheric conditions. Anodic overpotential was measured with the methods of electrochemical impedance spectroscopy (EIS), inert gas step addition (ISA), and reactant gas addition (RA). The cells successfully showed anodic overpotential at the very extreme temperatures of 823 and 973K. The results of EIS and ISA revealed that anodic overpotential rose when the temperature increased. This is attributed to the fact that mass-transfer resistance through the porous electrode, that is, pore diffusion resistance, was increased at higher temperature. When using the RA method, it was also found that enlarged mass-transfer resistance of the reaction species, H2, CO2, and H2O, was responsible for the increase in overpotential at higher temperatures.

Effect of temperature on the cathodic overpotential in a molten carbonate fuel cell

The effect of temperature on the cathodic overpotential in a molten carbonate fuel cell (MCFC) has been investigated with a 100cm2 class single cell. The temperature range was from 823K to 973K under atmospheric conditions. Cathodic overpotential was measured using the inert gas step addition (ISA) and reactant gas addition (RA) methods. The cell successfully showed the overpotential at the very extreme temperatures of 823K and 973K. The results of ISA revealed that the cathode did not show clear dependence of gas-phase-transport-induced overpotential on the temperature. However, overpotential due to diffusion through the liquid electrolyte was significantly reduced by the temperature increase. The RA method revealed that most of the reduction could be attributed to the increase in the mass transfer of O2 species.

Overpotential Behavior of Carbon Monoxide Fuel in a Molten Carbonate Fuel Cell

AbstractThe overpotential of carbon monoxide (CO) fuel was analyzed with a 100‐cm2 class molten carbonate fuel cell. The overpotential at the anode was measured using the steady state polarization, inert gas step addition, and reactant gas addition methods. Then, the overpotential was compared between normal hydrogen fuel (H2:CO2:H2O = 0.69:0.17:0.14 atm, inlet composition) and CO fuels (CO:CO2:H2O = 0.5:0.5:0 atm and 0.43:0.43:0.14 atm, inlet compositions). The CO fuel without H2O showed a much greater overpotential at 150 mA cm–2 than the CO fuel with H2O. This implies that the water‐gas‐shift reaction prevails at the anode and humidification of CO fuel is an efficient way to reduce anodic overpotential. The anodic overpotential with CO:CO2:H2O = 0.43:0.43:0.14 atm was about 73% of that of the H2 fuel at 150 mA cm–2. The anode showed gas‐phase mass‐transfer limitations with CO fuels.

Analysis of Flow Rate Inducing Voltage Loss in a 100 cm2Class Molten Carbonate Fuel Cell

This work focuses on the behavior of the overpotential increase due to a utilization rise in a molten carbonate fuel cell. The behavior is generally explained by Nernst loss, which is a kind of voltage loss due to the thermodynamic potential gradients in a polarization state due to the concentration distribution of reactant species through the gas flow direction. The evaluation of Nernst loss is carried out with a traditional experimental method of constant gas utilization (CU). On the other hand, overpotential due to the gas-phase mass-transport resistance at the anode and cathode shows dependence on the utilization, which can be measured using the inert gas step addition (ISA) method. Since the Nernst loss is assumed to be due to the thermodynamic reasons, the voltage loss can be calculated by the Nernst equation, referred to as a simple calculation (SC) in this work. The three values of voltage loss due to CU, ISA, and SC are compared, showing that these values rise with increases in the utilization within acceptable deviations. When we consider that the anode and cathode reactions are significantly affected by the gas-phase mass transfer, the behavior strongly implies that the voltage loss is attributable not to thermodynamic reasons, namely Nernst loss, but to the kinetic reason of mass-transfer resistance in the gas phase.

용융탄산염형 연료전지에서 과전압에 미치는 전극두께의 영향

본 연구에서는 용융탄산염형 연료전지의 연료극 전극두께가 과전압에 미치는 영향을 <TEX>$100\;cm^2$</TEX> 급 단위전지를 사용하여 검토하였다. 용융탄산염형 연료전지에서의 수소 산화속도는 충분히 빨라 전극면적이 성능에 크게 영향을 미치지 않을 수 있어, 본 연구에서는 전극의 기하학적 면적의 크기가 과 전압에 미치는 영향에 대해 연구하였다. 평가는 정상분극법과 비활성가스 계단형 첨가법 (ISA)와 반응물 첨가법 (RA)를 사용하여 연료극 두께 0.77 mm와 0.36 mm에 대해 수행하였다. 평가결과 두 전지에서 연료극 과전압이 거의 동일하게 관찰되어 연료극 두께에 의한 과전압의 차이는 발생하지 않았다. This work investigated the effect of anode thickness on the anodic overpotential with <TEX>$100\;cm^2$</TEX> class MCFC single cells. The hydrogen oxidation rate in the molten carbonate is sufficiently high, which may lead to weak relation of overpotential with anode geometrical area. The relation of anode surface area and overpotential was analysed in terms of anode thickness in this work. Steady state polarization, inert gas step addition (ISA), and reactant gas addition (RA) methods were employed to the two cells with 0.77 mm and 0.36 mm thickness of anode. The result represented that the anodic overpotential at the cells were identical. It implied that the anodic overpotential was independent on the electrode thickness within the tested range.

Overpotential analysis with various anode gas compositions in a molten carbonate fuel cell

The effect of anode gas composition on the overpotential in a 100cm2 class molten carbonate fuel cell is investigated. A total of five different gas compositions are used. They are applied to cross-check the effect of flow rate and composition, e.g., a given composition with different gas flow rates and a total flow rate with different gas compositions. The overpotential at the anode is analyzed via steady-state polarization, inert gas step addition and reactant gas addition methods. The analyses reveal that the anodic overpotential depends on the flow rate of the reactant species rather than the composition. Two identical gas compositions show less overpotential when a larger flow rate is applied. Compositions with large flow rates of CO2 and H2O also yield less overpotential due to the gas species. Overpotential analyses show that the three measurements have complementary relationships.

Effect of Anode Area on the Cell Performance in a Molten Carbonate Fuel Cell

The effect of the geometrical anode area of a molten carbonate fuel cell on the cell performance was investigated with class single cells. A cell with a area anode (cell I) and another cell with a area anode (cell II) were used. The cathode area of the cells was . The cell with anode area was prepared by filling the edge area of the class cell with 100 meshes of Ni, which was wrapped by thick Al foil. Other conditions for the preparation of the cells were identical. Cell performance was analyzed with steady-state polarization, inert gas step addition, and reactant gas addition methods. The analyses revealed that cell II showed an larger anodic overpotential than cell I. This implies that the anode performance is less dependent on its geometrical area. Cathodic overpotential was also larger for cell II than for cell I, likely due to higher electrolyte contents at the cathode in cell II.

Electrode Reaction Characteristics under Pressurized Conditions in a Molten Carbonate Fuel Cell

This work examines the overpotential behavior of two types of single cells with (cell I) and (cell II) anodes while under varying pressures up to . Inert gas step addition (ISA) and steady-state-polarization methods were employed. These methods revealed that cell I showed a lower anodic and cathodic overpotential ascribed to the gas-phase mass-transfer process compared to cell II. The results of the Hg porosimetry show that the anode has a more uniform pore structure with a smaller amount of small-size pores compared to the anode, which implies that the anode of cell II has a lower electrolyte content, resulting in a larger overpotential due to the increased diffusion resistance inside the pores. Both cells showed enlarged gas-phase-resistance induced overpotential at of pressure. Theoretically, reduced gas-phase diffusivity and flow velocity under pressurized conditions increase gas-phase mass-transfer resistance and its overpotential. However, pressurization reduces the overpotential of the liquid-phase mass-transfer resistance, which results in a total overvoltage decrease under pressurization.

Effect of Anodic Gas Compositions on the Overpotential in a Molten Carbonate Fuel Cell

Anodic overpotential has been investigated with gas composition changes in a <TEX>$100cm^2$</TEX> class molten carbonate fuel cell. The overpotential was measured with steady state polarization, reactant gas addition (RA), inert gas step addition (ISA), and electrochemical impedance spectroscopy (EIS) methods at different anodic inlet gas compositions, i.e., <TEX>$H_2:CO_2:H_2O=0.69:0.17:0.14\;atm\;and\;H_2:CO_2:H_2O=0.33:0.33:0.33\;atm$</TEX>, at a fixed <TEX>$H_2$</TEX> flow rate. The results demonstrate that the anodic overpotential decreases with increasing <TEX>$CO_2\;and\;H_2O$</TEX> flow rates, indicating the anode reaction is a gas-phase mass-transfer control process of the reactant species, <TEX>$H_2,\;CO_2,\;and\;H_2O$</TEX>. It was also found that the mass-transfer resistance due to the <TEX>$H_2$</TEX> species slightly increases at higher <TEX>$CO_2\;and\;H_2O$</TEX> flow rates. EIS showed reduction of the lower frequency semi-circle with increasing <TEX>$H_2O\;and\;CO_2$</TEX> flow rate without affecting the high frequency semi-circle.

Characteristics of a novel method, inert gas step addition, for the investigation of gas-phase mass-transfer effects in a molten carbonate fuel cell

This work illuminated characteristics of the inert gas step addition (ISA) method with various time ranges of reactant-flow shift in a 100 cm2 class molten carbonate fuel cell. The flow shift caused by the volume between the addition port for the inert gas and the cell was controlled by the length of 0.6 cm tubes in a range from 0.8 to 4.8 m at the anode as well as the cathode. Anodic voltage peaks provoked by the inert gas addition depended on the tube length and became blunt at longer tube lengths, indicating that the anodic overpotential is a function of reactant flow rate. The anodic overpotential estimated by the ISA depended on the tube length, but it became a constant at longer tubes. The cathode showed a flat peak at the moment of inert gas addition regardless of the tube length, indicating that the cathode has a relatively small gas-phase mass-transfer resistance. The cathodic overpotential with ISA showed an insignificant dependence on tube length.