Ni/CGO-Based Electrolyte-Supported Cells During Co-Electrolysis: The Role of Feed Gas Impurities

Abstract

The electrochemical generation of syngas via simultaneous co-electrolysis of steam and CO2 in high temperature solid oxide cells and its consecutive catalytic upgrade has attracted growing interest over recent years since this pathway offers a sustainable route to the production of synthetic fuels. However, most solid oxide cell technologies have been developed for durable and efficient fuel cell operation and thus, their use in electrolysis mode leads to the emergence of different lifetime-limiting degradation mechanisms. For instance, strongly cathodic polarization of the cermet fuel electrode leads to irreversible Ni migration away from the electrode/electrolyte interface. Furthermore, carbon deposition can become an issue when local non-equilibrium, carbon monoxide-rich gas phase compositions are formed. All of these mechanisms have been shown to be particularly harmful in Ni/YSZ-based fuel-electrode supported cells. Ni/Gadolinium-doped ceria (CGO) electrodes offer a potentially promising alternative to alleviate these issues due the intrinsic high electro-catalytic activity and carbon mitigating effect of the ceria phase. Furthermore, the higher operating temperatures commonly used for electrolyte-supported cell (ESC) architectures may mitigate carbon deposition and impurity adsorption due to thermodynamic considerations. In addition, the thinner electrodes employed in ESC could help to avoid the formation of CO-rich gas phases at the electrode/electrolyte interface due to enhanced porous diffusion and thus, further prevent coking. With regards to durability of the reactor, the purity of the feed gases steam and CO2 becomes of great importance.In the present study durability aspects of industrial Ni/CGO-based ESC are investigated in co-electrolysis mode under various operating conditions by means of current-voltage characteristics and electrochemical impedance spectroscopy. Durability was evaluated at different current densities for times of >=1500 h. Post-mortem analysis was carried out with scanning electron microscopy (SEM) to correlate the observed degradation phenomena with microstructural changes, particularly Ni migration. It is shown that the steam inlet gas has to be carefully purified to avoid the deposition of silica in the fuel electrode. Furthermore, electrochemical measurements performed in dry CO2 electrolysis demonstrate that sulfur impurities in the CO2 feed gas must be removed to achieve low degradation rates.