Kinetic modelling of catalytic reactions in solid oxide cells operating under pressure in co-electrolysis mode

摘要

One of the solutions to limit the global temperature rise is to increase the part of the renewable energies in the energy mix, which implies massive energy storage. One of the most efficient solutions seems to be the chemical storage via H2 produced by water electrolysis. The High Temperature Electrolysis (HTE), based on Solid Oxide Cell, is very attractive as it consumes less electrical energy than low temperature electrolysis thanks to lower energy demand due the vaporization of water into steam [1]. HTE can also directly convert a mix of water and CO2 into syngas, which may be converted into CH4 or other hydrocarbons. Moreover, operating this type of co-electrolyzer under pressurized conditions is energetically favored and expected to increase the process efficiency [2]. Pressurized co-electrolysis tests were performed at the single cell level integrating Ni-xYSZ (ZrO2 stabilized with Y2O3 in Ni) as standard H2 electrode between 973 and 1073 K at P_(atm) and 3.7 bar with the flowing gas molar composition: CO2/H2O/H2: 25/65/10 with a total flow rate of 16 NmL.min-1.cm~(-2)_(cell). The results show that at low current (|i|<0.3 mA.cm~(-2)), the cell performance was better at P_(atm) since the open circuit voltage (OCV) increase according to the Nernst law. In contrast, at high current, the pressure has a positive effect due to a better gas diffusion in the electrode. Analysis of outlet gas revealed that the CO2 conversion is favored with the temperature increase. It can also be noticed that at the OCV, a part of H2 is slightly consumed in these gas conditions and that at low temperature and high current some CH4 is produced. These results can be explained by catalytic reactions occurring on the nickel present in the H2 electrode. Indeed, Ni° is known to catalyze the CO and CO2 methanation and the reverse water gas shift reaction [3]. In order to model the cells operation under pressure, it is important to determine the catalytic reactions kinetics. In this view, catalytic experiments have been carried out with cermet powder under pressure and gas conditions close to those tested with complete cell. Thanks to these experimental data, catalytic reactions kinetics models have been determined and then implemented in the electrochemical models [2]. Finally a comparison between simulations and experimental data allowed to experimentally validate the model for the co-electrolysis mode under pressure.