Effect of Oxygen Partial Pressure on Solid Oxide Electrolysis Cells

HOU Quan , GUAN Chengzhi , XIAO Guoping , WANG Jianqiang ,＊, ZHU Zhiyuan

1Department of Molten Salt Chemistry and Engineering, Shanghai Institute of Applied Physics, Chinese Academy of Sciences,Shanghai 201800, P. R. China.

2 Key Laboratory of Interfacial Physics and Technology, Chinese Academy of Sciences, Shanghai 201800, P. R. China.

3 University of Chinese Academy of Sciences, Beijing 100049, P. R. China.

Abstract: High-temperature (700-900 °C) steam electrolysis based on solid oxide electrolysis cells (SOECs) is valuable as an efficient and clean path for large-scale hydrogen production with nearly zero carbon emissions,compared with the traditional paths of steam methane reforming or coal gasification. The operation parameters, in particular the feeding gas composition and pressure, significantly affect the performance of the electrolysis cell. In this study, a computational fluid dynamics model of an SOEC is built to predict the electrochemical performance of the cell with different sweep gases on the oxygen electrode. Sweep gases with different oxygen partial pressures between 1.01 × 103 and 1.0 × 105 Pa are fed to the oxygen electrode of the cell, and the influence of the oxygen partial pressure on the chemical equilibrium and kinetic reactions of the SOECs is analyzed. It is shown that the rate of increase of the reversible potential is inversely proportional to the oxygen partial pressure. Regarding the overpotentials caused by the ohmic, activation, and concentration polarization, the results vary with the reversible potential. The Ohmic overpotential is constant under different operating conditions. The activation and concentration overpotentials at the hydrogen electrode are also steady over the entire oxygen partial pressure range. The oxygen partial pressure has the largest effect on the activation and concentration overpotentials on the oxygen electrode side, both of which decrease sharply with increasing oxygen partial pressure. Owing to the combined effects of the reversible potential and polarization overpotentials, the total electrolysis voltage is nonlinear. At low current density, the electrolysis cell shows better performance at low oxygen partial pressure, whereas the performance improves with increasing oxygen partial pressure at high current density. Thus, at low current density, the best sweep gas should be an oxygen-deficient gas such as nitrogen, CO2, or steam. Steam is the most promising because it is easy to separate the steam from the by-product oxygen in the tail gas, provided that the oxygen electrode is humidity-tolerant. However, at high current density, it is best to use pure oxygen as the sweep gas to reduce the electric energy consumption in the steam electrolysis process. The effects of the oxygen partial pressure on the power density and coefficient of performance of the SOEC are also discussed. At low current density, the electrical power demand is constant, and the efficiency decreases with growing oxygen partial pressure, whereas at high current density, the electrical power demand drops, and the efficiency increases.

Key Words: Solid oxide electrolysis cell; Interface; Oxygen partial pressure; Theoretical model; Computational fluid dynamics

1 Introduction

A solid oxide electrolysis cell (SOEC) (Fig. 1) is an electrochemical energy conversion cell that transforms electrical energy from passing electrons into a chemical fuel1. A typical SOEC consists of a three-layer solid structure (composed of porous cathode, electrolyte and porous anode) and an interconnect plate2-6. The reactions, which are segregated by the electrolyte, occur in the anode and cathode, thus SOEC can produce both pure hydrogen and oxygen7-9. SOEC operates in a temperature range of 600-1000 °C at atmospheric or elevated pressures10-12. Solid oxide steam electrolysis at elevated temperatures might offer a solution by consuming less electricity than that is required at ambient conditions through favorable thermodynamics and kinetics13. High-temperature can achieve higher electrical efficiencies than low temperatures, as illustrated in Fig. 2a14. Although part of electrical demand is replaced by more heat demand in high-temperature SOECs, electricity is still the major energy consumption. The potential of furthering reducing electric energy demand by changing the voltage efficiency is often mentioned. The voltage efficiency is defined as the net voltage (cell equilibrium voltage minus the irreversible losses) divided by the maximum voltage. The irreversible voltage losses are attributed to polarization losses that primarily originate from three sources: activation, concentration, and Ohmic polarizations. The anode activation overpotential is significant, whereas the polarization due to the cathode activation remains significantly limited13. Thermodynamics indicate that, assuming ideal-gas behavior of H2, H2O, and O2,the enthalpy change involved during the electrolysis reaction is dependent of the partial pressure of the gases15. Fig. 2b shows the influence of oxygen partial pressure on the energy demand of H2O electrolysis. Additionally, the heat of high-temperature heat sources can be directly integrated in the process via the anode side by a heated oxygen sweep stream. Such a strategy may potentially be applied in the case of a SOEC, in which the introduction of air flow through the cell could both force the O2product out of the anode channel and aid in controlling the temperature16.

During steam electrolysis, high oxygen partial pressures occur at the oxygen electrode/electrolyte interface. Thus, the delamination of oxygen electrodes becomes one of the major degradation issues in SOECs17-24. An excessive amount of oxygen is generated as the result of the distribution of vacancies in the oxygen electrode and electrolyte interface and leads to peeling of the oxygen electrode and an increase of polarization resistance17,18,20,25. The presence of excess oxygen near the oxygen electrode degraded regions is associated with the high PO2at the electrolyte-electrode interface, in accord with Virkar’s model23, producing an irreversible degradation of the electrolyte due to YSZ electro-reduction and, in some cases, the delamination of the oxygen electrode. Improvement of oxygen electrodes can help to reduce these losses and increase electricalefficiency.

Fig. 1 diagram of a typical SOEC cell 1.

Fig. 2 (a) Temperature depends on thermodynamics of water electrolysis 14; (b) oxygen partial pressure dependency of thermodynamics of water electrolysis at a temperature of 800 °C assuming a gas molar fraction of XH2 = 0.2 and XH2O = 0.8 14.

The aim of this paper is to theoretically analyze the influence of oxygen partial pressure on the performance of a SOEC cell.Computational fluid dynamics (CFD) simulations are carried out in an oxygen partial pressure range between 1.01 × 103and 1.0 ×105Pa. The effects of oxygen partial pressure on the reversible voltages, overpotentials and the total electrolysis efficiency are discussed.

2 Model and simulation

2.1 Electrochemical model

The method of electrochemical modeling is to describe the limiting processes mentioned in the preceding section by introducing the concept of overpotentials. The cell potentials (V)can be expressed as the sum of the reversible potential (Vrev),activation overpotential (ηact,aand ηact,c), concentration overpotentials (ηcon,aand ηcon,c), and Ohmic overpotentials (ηOhm)in the SOEC model,

The reversible potential of the SOEC model can be expressed by the Nernst equation considering the pressure dependence26-28:

where ΔG is the change of Gibbs free energy, F is the Faraday constant (9.6485 × 104C·mol-1), R is the universal gas constant(8.3145 J·mol-1·K-1), T is the absolute temperature (K), and PH2,PH2Oand PO2are the partial pressures of H2O, H2and O2on the electrode surfaces, respectively.

The activation overpotentials of the SOEC demonstrate the activity of the electrodes. They are commonly expressed by the Butler-Volmer (B-V) equation. For the cathode electrode, the BV equation for electrochemical reduction of H2O takes the form as29:

For the O2 electrode, the B-V equation for oxygen production can be expressed as29:

where i is current, i0the exchange current density, ηactthe activation overpotential, F the Faraday constant, T the temperature, and β the asymmetric charge-transfer coefficient.The exchange current densities (io,H2and io,O2) represent the readiness of the electrode to proceed with the electrochemical reaction, and depend on the operating temperature as30,31:

where γ is the pre-exponential, Eact is the activation energy, and Pstd is the operating pressure. The values used for modeling are summarized in Table 131,32.

Concentration overpotentials are caused by mass-transport limitations of reactants and products at the TPBs of the electrolyte, porous electrode, and gas species at which the reaction takes place. These concentration overpotentials at the cathode and anode can be determined from the Nernst equation,which are expressed by33,34:

where dc is the thickness of the cathode electrode, da the thickness of the anode electrode, and Deffthe effective diffusion coefficient, which is dependent on porosity, tortuosity, and temperature, and which can be evaluated by Eqs. (9)-(11)31,33,34:

where ξ/ε is the ratio of tortuosity to porosity of porous electrodes, rpis the radius of pores, and Dijis the binary diffusion coefficient of species i and j, which can be expressed by31,33:

Table 1 Values of pre-exponential factor γ and activation energy Eact for anode and cathode 31,32.

The electrical connecting plates and electrodes generally have much higher electrical conductivity than the electrolyte11.Neglecting the resistance of the electrodes and electrical connecting, the Ohmic overpotential of the cell can be computed by

where Reis electronic resistance of electrolyte, which is given by

where deis the thickness of the electrolyte, and σeis the electrolyte conductivity in SI units of S·m-1, which varies as a strong function of temperature as:

here, Eel is the activation energy for ion transport (80.0 kJ·mol-1),and σ0is the pre-exponential factor (3.6 × 105S·cm-1)35.

2.2 Compactional scheme

The flow field is solved using the commercial CFD code a commercial software package, ANSYS-FLUENT 14.5, which solved the continuity, momentum, energy, and species continuity equations coupled with the current continuity equations, and the electrochemical reactions. However, the source terms and fluxes appearing in Eqs. (16)-(18), and the electrochemistry model are implemented using defined functions (UDF). During each iteration, the thermodynamic state variables and the species concentrations are accessed from the solver, which are in-turn used to evaluate the UDF returned values. Velocity inlet boundary conditions and pressure outlet boundary conditions are used for the calculations. At the cathode-electrolyte interface, electrochemical reactions take place and thus the flux of H2O and H2 can be related to the current densities as28:

At the anode side, air is usually used as a sweep gas. The oxygen molecules produced at the TPB of anode transport to the anode surface and get collected. The flux of O2is related to the current density as28:

3 Results and discussion

3.1 Model validation

In this subsection, the modeling results of current-potential characteristics are compared with experimental data for model validation. In the literature, the work of Liu et al.25. on hydrogen production by steam electrolysis provides details of the experimental setup and operating conditions, such as the gas composition at the inlet and outlet, and the thickness of the SOEC components. In their study, the current density-voltage(I-V) characteristics of H2O electrolysis by a hydrogenelectrode-supported planar SOEC are measured. The thickness of the Ni-YSZ (yttrium-stabilized zirconia) cathode, LSCFGDC (GDC, Ce0.9Gd0.1O1.95; LSCF, La0.6Sr0.4Co0.2Fe0.8O3-δ)anode, and YSZ electrolyte are 410, 33, and 10 μm, respectively.The overall single cell area for testing was 10 cm × 10 cm and the active area was 62 cm2. The cell was placed in a furnace and heated to 800 °C at the rate of 2 K·min-1from room temperature.After maintaining the temperature at 800 °C for 2 h, N2 gas was used to purge and protect the stack module from oxidization during the heating process. At an operating temperature of 800 °C, the molar fractions at the inlet of the cathode were 90%H2O and 10% H2, and air were introduced to the anode electrode.These operating conditions and structural parameters from the experiments are used as input parameters in the theoretical simulation. The model tuning parameters are summarized in Table 2. Fig.3 shows good agreement between numerical simulation results and experimental data. The numerical model over-predicts the potential at open-circuit conditions. Owing to this fact, the model under-predicts the current density at a specified voltage for the latter fuel composition. Nevertheless,the model can qualitatively reproduce the experimental data.

3.2 Effects of the oxygen partial pressure on the performances of SOEC

As the amount of sweep gas is very large, the influence of the oxygen produced by electrolysis steam on the oxygen partial pressure in the sweep gas can be ignored. The results of the parameter study are presented below for the different cases outlined in Table 2.

3.2.1 Reversible potential

The thermodynamics indicates that, assuming ideal-gasbehavior of H2, H2O, and O2, the enthalpy change involved during the electrolysis reaction is dependent of the partial pressure of the gases at the conditions of constant temperature and isopiestic pressure. The reversible potential, also known as the open-circuit voltage of the SOEC, is usually calculated using Eq. (2). Fig. 4 shows the effect of oxygen partial pressure on the reversible potential. At oxygen partial pressure less than 0.2 ×105Pa, the reversible potential increases strongly with increasing oxygen partial pressure. The reversible potential increase 0.27 V from 0 to 0.2 × 105Pa, and the growth rate is more than 45%.However, the reversible potential increases only 0.04 V from 0.2 ×105to 1 × 105Pa.

Table 2 Cell parameters and properties used for model validation/analysis.

Fig. 3 Comparison of simulation results with experimental data for model validation.

3.2.2 Polarization overpotential

Apart from the reversible potential, cell performance is greatly influenced by Ohmic, activation, and concentration overpotentials36.The effects of oxygen partial pressure on the ohmic, activation and concentration overpotentials in the SOEC are discussed in this section. Ohmic overvoltage is almost a constant over the entire investigated oxygen partial pressure range at the same current density and the conditions of constant temperature and isopiestic pressure, as shown in Fig. 5. The ohmic overvoltage is caused by the electrical resistance of the cell. Electrical resistance is determined by the material and connection of cell. Therefore, it is not directly influenced by oxygen partial pressure.

Fig.4 Influence of oxygen partial pressure on open-circuit voltage.

The activation overpotential and concentration overpotential at the cathode electrode are almost a constant over the entire investigated oxygen partial pressure range at the same current density and the conditions of constant temperature and isopiestic pressure, as presented in Fig. 5. At the cathode electrode, the activation overpotential and concentration overpotential are calculated by Eqs. (3) and (7), respectively. The activation overpotential and concentration overpotential at the cathode electrode are caused by the current density, the gas compositions of steam and hydrogen, and the temperature. No evidence indicates that the activation overpotential and concentration overpotential are directly influenced by oxygen partial pressure.

Fig. 5 also shows that the activation overpotential and concentration overpotential at the anode electrode decreases with increasing oxygen partial pressure. At oxygen partial pressure less than 0.2 × 105Pa, the concentration overpotential at the anode electrode decreases strongly with increasing oxygen partial pressure. The concentration overpotential at the anode electrode decreases 0.052 V from 0.1 × 104to 0.2 × 105Pa,respectively. However, the concentration overpotential at the anode electrode decreases only 0.018 V from 0.2 × 105to 1.0 ×105Pa, respectively. The higher gradient of pressure can improve the gas diffusion37,38. The pressure gradient of oxygen decreases with the increase of oxygen partial pressure, thus oxygen partial pressure effects become less pronounced at higher oxygen partial pressure. At oxygen partial pressure less than 0.2 × 105Pa, the activation overpotential at the anode electrode decreases strongly with increasing oxygen partial pressure. The activation overpotential at the anode electrode decreases 0.057 V from 0.1 ×104to 0.2 × 105Pa, respectively. However, the activation overpotential at the anode electrode decreases only 0.013 V from 0.2 × 105to 1.0 × 105Pa, respectively. The concentration of oxygen strongly influences the desorption rates of the oxygen at the anode electrode surface39.

3.2.3 The electrolysis voltage

The current-voltage (I-V) curves are illustrated in Fig. 6 for different oxygen partial pressures. The reversible potential increases with oxygen partial pressure, whereas polarization overpotentials decrease. At low current density, the cellperformance decreases with increasing oxygen partial pressure.The reason is the synthetic effects due to reversible potential and polarization overpotentials, which affect by oxygen partial pressure. At intermediate current density, hardly any oxygen partial pressure influence is visible, because the positive influence of oxygen partial pressure on polarization overpotentials compensates for the negative effect of that on reversible potential. Because at high current density the positive influence polarization overpotentials of oxygen partial pressure is more than the negative effect of that on reversible potential,the cell performance increases with increasing oxygen partial pressure.

Fig. 5 Influence of oxygen partial pressure on activation,concentration, and Ohmic overpotentials for the cathode (hyd) and anode (oxy) electrode.

Fig. 6 Cell voltage curves at different oxygen partial pressures and different current densities.

3.2.4 Efficiency analysis

The effects of oxygen partial pressure on the power density and coefficient of performance in the SOEC are discussed in this section. The electrical power demand over oxygen partial pressure at three different current densities is shown in Fig. 7a.At a current density of 0.1 A·cm-2, the electrical power demand is almost constant over oxygen partial pressure. The differences of applied cell voltage do not have a strong influence on power demand, because the current density is low. However, power density is lower at low oxygen partial pressure, because oxygen partial pressure has a strongly effect on reversible potential at low oxygen partial pressure. The highest power demand appears at low oxygen partial pressure at an intermediate current density of 0.4 A·cm-2. At blow 0.5 × 105Pa, power demand decreases with the increase of oxygen partial pressure. At above 0.5 × 105Pa, power demand slightly increases again with a further increase in oxygen partial pressure. At this current density, the positive influence of oxygen partial pressure on polarization overpotentials compensates for the negative effect of that on reversible potential and the overall oxygen partial pressure influence is very small. At a high current density of 0.7 A·cm-2,because the decrease of polarization overpotentials with the increase of oxygen partial pressure is dominant, power demand decreases with oxygen partial pressure over the investigated oxygen partial pressure range. However, the influence of oxygen partial pressure on reversible potential is still significant, and overall oxygen partial pressure effects are small.

Fig. 7 Influence of oxygen partial pressure on (a) electrical power demand and (b) electrical efficiency.

Fig. 7b shows the electrical efficiency εe of the electrolysis over oxygen partial pressure at three different current densities.The efficiency of electrical is calculated by25

with the molar hydrogen production rate ˙; the reaction enthalpy of hydrogen oxidation,at standard conditions (101325 Pa and 20 °C); and electrical power Pe. At less than 0.5 × 104Pa, the electrical efficiency for four different current densities increases with increasing oxygen partial pressure. It can be explained by that the positive influence on polarization overpotentials of oxygen partial pressure is less than the negative effect of that on reversible potential at less than 0.5 ×104Pa. At more than 0.5 × 104Pa, the electrical efficiency for current density of 0.1 A·cm-2decreases with increasing oxygen partial pressure. The electrical efficiency for current density of 0.4 A·cm-2and 0.7 A·cm-2increase with increasing oxygen partial pressure. However, at more than 0.5 × 105Pa, the electrical efficiency for current density of 0.4 A·cm-2slightly decreases again with a further increase in oxygen partial pressure. Due to the required reaction enthalpy is partly provided as the thermal energy via furnace, the electrical efficiency is more than 1at current density of 0.1 A·cm-2, and electrical efficiency is more than 1.27 within the over oxygen partial pressure range. At a high current density of 0.7 A·cm-2, because the applied cell voltage is higher than the thermo-neutral voltage,the electrical efficiency drops below 1.

4 Conclusions

The comparative numerical investigations on a planar SOEC with H2O electrolysis and the effect of oxygen partial pressure on cell performance have been presented and discussed. The experimental measurements for I-V curve were conducted for model validation. Based on the numerical simulations, the spatial variation-characteristics of reversible potential,overpotential, power demand and coefficient of performance in SOEC were analyzed and compared in detail. It should be noted that the findings based on the simulations are only applicable under the specified operating conditions. A few conclusions are summarized as follows:

(1) At high current density, the higher oxygen partial pressure,the better performance of SOECs can be obtained. The reversible potential increases with increasing oxygen partial pressure,whereas overpotentials are reduced. Thus, using oxygen gas to sweep gas at high current density can not only increase the performance of SOEC, but also make it easy to recycle oxygen gas from the sweep gas.

(2) At low current density, the less oxygen partial pressure, the better performance of SOECs can be obtained. Thus, using steam water to sweep gas at low current density can not only increase the performance of SOEC, but also make it easy to recycle oxygen gas from the sweep gas.