My participation to BRISK2 took place in POLITO on the POL2 facility for the electro-chemical characterization of SOCs (Solid oxide fuel cells). On this test rig, I performed a series of experiments on single cells, quantifying the effects of different inlet compositions on the electro-chemical performance of solid oxide cells, especially in electrolysis mode. In particular, I tested the effect of carbon dioxide and methane on the polarization curve of some electrolysis cells. Two different cells were tested: a non-commercial anode supported cell manufactured at the Ceramic Department (CEREL) of the Institute of Power Engineering in Poland, and a commercial cell ASC4 produced by H.C. Starck Ceramics GmbH.
The cells were firstly characterized in fuel cell mode, meaning that they were fed with pure hydrogen to produce electricity.
Then, the same cells were fed with different mixtures of hydrogen (always at least 20% of the molar composition to maintain a reducing environment around the cell), water, carbon dioxide and biogas (simulated as 60% CH4 and 40% CO2). The different curves obtained in the aforementioned experiments are reported in Figure 7.
Finally, a small quantity of sulfur was added to the cell’s cathodic stream to quantify the degradation of the performance in the electrolysis mode, due to the presence of this contaminant in the biogas.
This work is part of the research on the direct use of biogas in electrolysis cells, in order to convert the CO2 into methane via co-electrolysis and, thus, enhancing the calorific value of biogas.
The results showed that the presence of methane increases the internal resistance of the cell (called Area Specific Resistance – ASR – and measure in Ohm*cm2). The effects on the two cells are similar. The average ASR increases by 30% between the case of water electrolysis and the case with 25% biogas in the feed.
The electrolysis mode appears to suffer more than the fuel cell mode from the presence of sulfur. Concentrations of 1-2 ppm seem to worsen the polarization curve irremediably. The performance degradation is irreversible and the cell does not seem to recover even partially its functionality, as instead happens when sulfur is fed in fuel cell mode.
The experiments were performed at atmospheric pressure. The inlet volume flows of the gases were set through an appropriate interface, that included also a data acquisition system.
This experimental campaign was very useful to increase the practical knowledge on the processes of co-electrolysis that occur on solid oxide cells. The interaction with the members of the BRISK team at POLITO, who are experts in the field of high-temperature electrochemical devices and biogas conversion processes, has been fruitful towards a deeper understanding of the effects of different inlet mixtures in co-electrolysis processes.