New, Dual-Anion Mechanism for Molten Carbonate Fuel Cells Working as Carbon Capture Devices

Abstract

Molten Carbonate Fuel Cells (MCFCs) are commercially employed in MW-scale power production, and recently are being developed also for carbon capture. Past experiments showed that MCFC performance with wet cathode feeding was higher than with dry cathode feeds at otherwise similar conditions. This was ascribed to a mechanism that predicted the water increasing the apparent CO 2 diffusion rate. However, recent tests performed at low CO 2 cathode feed concentrations, as in carbon capture service, showed the emergence of a different water effect. Namely, there seems to be an electrochemical reaction path attributable to water, involving hydroxide ions that runs parallel with the main path involving CO 2 . This results in lower CO 2 transfer from the cathode to the anode than what can be calculated from the electrical current. For the first time, here, a theoretical analysis will be presented to introduce a kinetic expression for MCFCs working under this dual-ion regime. Focus will be given to the expression of CO 2 and water polarization to assess the ratio between the current due to the two anions. Simulation and experimental results will be discussed providing a reliable and effective basis for the performance optimization of the MCFCs both in power and in carbon capture applications.

Experimental and Modeling Investigation of CO3=/OH– Equilibrium Effects on Molten Carbonate Fuel Cell Performance in Carbon Capture Applications

“…In previous studies (Audasso et al, 2020a;Audasso et al, 2020b;Rosen et al, 2020) we had not directly observed the presence of OH − ions in the electrolyte melt. We developed the dual-anion model construct based on global mass balances that indicated the cathode-anode migration of not only CO 2 , but also that of H 2 O.…”

“…Experimental data also showed that the branching ratio between the two anion paths is determined by the reacting cathode gas composition (Rosen et al, 2020) and that, to favor the carbonate path, it is necessary to enhance CO 2 diffusion (Rosen et al, 2020;Audasso et al, 2020b). We have defined this simultaneous migration of carbonate and hydroxide ions as a "dual-anion" mechanism (Audasso et al, 2020a).…”

“…Considering these new findings and based on previous studies (Audasso et al, 2016;Arato et al, 2016), we developed the first semi-empirical dual-anion model to evaluate MCFC performance under these working conditions (Audasso et al, 2020a;Audasso et al, 2020b) using the circuit represented in Figure 1.…”

Significance of Molten Hydroxides With or Without Molten Carbonates in High-Temperature Electrochemical Devices

“…Although the presence of molten hydroxides is likely in molten carbonates due to the presence of water, it is only until recently that their significant role has been demonstrated in the literature. Indeed, new studies show that these ions also contribute to overall MCFC performance in some conditions (Audasso et al, 2020;Rosen et al, 2020). For CO 2 -lean wet atmospheres, very high current densities, and/or high CO 2 usage, the MCFC can also operate using hydroxide as charge carrier, along with carbonates.…”

“…Figure 5 recapitulates the reactions taking place at both anode and cathode side for both possible charge carriers. Audasso et al (2020) then proposed a dual-anion model to more accurately describe MCFC performance. In FIGURE 4 | Plots of measured CO 2 utilization rate versus expected, for various current densities, water contents, and CO 2 contents (Rosen et al, 2020).…”