(369b) Effect of Gas Composition on Co-Electrolysis Performance of Solid Oxide Electrolyzer Cells

Solid oxide electrolysis cells (SOECs) are attractive electrochemical energy conversion devices with high conversion efficiencies because of their high operating temperatures of around 700°C. They can convert electrical energy into chemical energy by electrochemically reducing steam and carbon dioxide to produce hydrogen and carbon monoxide, which enables large-scale energy storage. The mixture of hydrogen and carbon monoxide is also an important precursor to produce hydrocarbon fuels through methanation and the Fischer-Tropsch processes. Since the electrolysis performance of the SOECs depends on operation conditions such as temperature and gas compositions, their effects have been investigated experimentally in the literature. In particular, the electromotive force (EMF) of the cells and the electrochemical activity of the fuel electrode are essential to understand the cell performance. They are also desired to be formulated into mathematical models to be used for the numerical simulation of SOECs.

The EMF is usually estimated using the Nernst equation based on the gas compositions on the electrodes, namely, the anode and cathode. However, unlike the steam electrolysis operation, several redox couples co-exist in the cells under co-electrolysis operation, inhibiting the simple use of the Nernst equation. When steam and carbon dioxide are supplied to the fuel electrode of SOECs, the reverse-gas-shift reaction produces a gas mixture consisting of hydrogen, steam, carbon monoxide, and carbon dioxide, and its composition is not always at equilibrium. Therefore, evaluating the EMF is not a trivial problem and requires experimental and theoretical investigation.

The electrochemical reaction in the fuel electrode is also considered to depend on gas compositions. It is reported that carbon dioxide is less actively reduced in the co-electrolysis operation; hence steam is preferentially reduced. In this case, the reverse-gas-shift reaction is the main route for converting carbon dioxide to carbon monoxide. However, comprehensive analyses have not been conducted to understand the dependence of the electrochemical activity of the fuel electrode on gas compositions.

Therefore, in this study, the EMF and the electrochemical activity of the fuel electrode are experimentally investigated under various gas compositions with a particular focus on the effect of the reverse-gas-shift reaction. A catalyst pellet is inserted near the fuel electrode to offer a larger reaction site and control the progress of the reverse-gas-shift reaction in the supplied gas. The EMF, current-voltage, and impedance characteristics are measured under various gas compositions.

The electrolyte-supported SOEC cells are fabricated as follows and used in this study. A yttria-stabilized zirconia disk (8-YSZ, Tosoh Corp., Japan) with a diameter of 30 mm and a thickness of 500 μm is used for the electrolyte. Ni-YSZ anode is fabricated using screen printing. NiO powder (Fujifilm Wako Pure Chemicals Corp., Japan) and YSZ powder (TZ-8Y, Tosoh Corp., Japan) are mixed by planetary ball milling with ethanol. The mixture ratio is determined to achieve Ni: YSZ = 50: 50 vol.%. After evaporating ethanol, the mixture powder is sieved to remove agglomerate and mixed with a binder (VEH, Nexceris LLC, USA) at NiO-YSZ: binder = 75: 25 wt.% to form anode slurry. The obtained slurry is screen printed on the electrolyte surface using a mask with a thickness of 60 μm and sintered at 1400°C for 5h. On the other side of the anode, a barrier layer consisting of GDC (gadolinia-doped ceria, GDC-10, Shin-Etsu Astech Co. Ltd., Japan) is formed by spin coating. GDC powder is mixed with binder at GDC: binder = 25: 75 wt.% and dropped onto the YSZ disk rotating at 8000rpm. Sintering is performed at 1300°C for 10h. On the GDC barrier layer, LSCF (LSCF-6428-N, Kceracell Co. Ltd., Korea)-GDC cathode is screen printed using a mask of 60 μm and sintered at 950°C for 5h. In addition, to increase the catalytic activity of the fuel electrode for the reverse-gas-shift reaction, a Ni-YSZ pellet is inserted near the fuel electrode to promote the reverse-gas-shift reaction. The pellet is fabricated using a tape-casting process. Details of the fabrication process are found elsewhere (Seo et al., Fuel Cells, 2020).

The electrochemical performance of the cell is evaluated at 800°C with several gas compositions. For the co-electrolysis experiment, H₂: H₂O: CO₂: N₂ = 10: x: 40-x: 50 (x = 0, 10, 20, 30, 40) on the fuel electrode, and pure oxygen on the oxygen electrode is used. Note that the total amount of steam and carbon dioxide is kept constant. Hydrogen is added to avoid the oxidation of nickel. For the reference experiment, steam electrolysis and carbon oxide electrolysis are also performed under H₂: H₂O: N₂ = 10: x: 90-x (x=10, 20, 30, 40), and H₂: CO₂: N₂ = 10: x: 90-x (x=10, 20, 30, 40), respectively. The open-circuit voltage (OCV) and current-voltage and impedance characteristics are measured using a Solartron 1455A frequency response analyzer and a Solartron 1470E electrochemical interface (Solartron Analytical, UK). The outlet gas compositions are measured using a gas chromatograph (990 Micro GC System, Agilent, USA).

First, OCV is measured with and without inserting the Ni-YSZ catalyst near the fuel electrode (Fig. 1). Overall, OCV increases as the amount of carbon dioxide increases in the supplied gas. The OCV obtained from the cell without the catalyst is found to be closer to the EMF obtained using the Nernst equation considering the hydrogen and steam redox couple. On the other hand, the OCV obtained from the cell with the catalyst is lower than in the other case and closer to the EMF obtained using the Nernst equation with chemical equilibrium of the reverse-gas-shift reaction. These indicate that the reverse-gas-shift reaction is not always at the chemical equilibrium in the gas phase and that the hydrogen-steam redox couple has a more significant influence on the OCV. Subsequently, the mixed potential theory is employed to develop a model for the EMF. The theoretical EMF values obtained using the Nernst equation from the hydrogen-steam redox couple and the carbon monoxide-carbon dioxide redox couple are weighted averaged. The surface coverages of the gas species are calculated from the adsorption equilibrium and used for the weights. By calibrating the equilibrium constants within a physicochemically reasonable range, the developed model successfully reproduces the trend of EMF in the experiment. In addition, the obtained surface coverage of the gas species indicates that the electrochemical reaction site is mainly covered with hydrogen and steam, particularly when the catalyst is not used, which is why the OCV is closer to the value obtained from the hydrogen-steam redox couple.

Next, the current-voltage and impedance characteristics are measured under co-electrolysis operation and compared with those under steam electrolysis and carbon dioxide electrolysis. The overpotential under carbon dioxide electrolysis is found to be larger than that under steam electrolysis. The overpotentials under co-electrolysis operation are found to fall between those of the steam and carbon dioxide electrolysis but are closer to those under steam electrolysis. These indicate that the carbon dioxide is less electrochemically active in the electrolysis. A similar trend is observed in the impedance measurement. The polarization resistances obtained from the equivalent circuit fitting (Fig. 2) are larger in the carbon dioxide electrolysis compared with the steam electrolysis, and the values in the co-electrolysis are closer to those in the steam electrolysis.

In summary, during the co-electrolysis operation of SOECs, it is found that the EMF and polarization resistance are mainly determined from the hydrogen-steam redox couple because of the higher surface coverage of hydrogen and steam near the reaction site.