Molten Carbonate Fuel Cells for Electrolysis

The molten carbonate fuel cell (MCFC) has evolved to current megawatt-scale commercial power plants. When using the fuel cell for electrolysis (MCEC), it provides a promising option for producing fuel gases such as hydrogen, via water electrolysis, and syngas, via co-electrolysis of water and carbon dioxide. The molten carbonate cell can thereby operate reversibly as a dual energy converter for electricity generation and fuel gas production. The so-called reversible molten carbonate fuel cell (RMCFC) will probably increase the usefulness of the system and improve the economic benefits. This work has investigated the performance and durability of the cell in electrolysis and reversible operations to demonstrate the potential viability of the MCEC and the RMCFC. A lower polarization loss is found for the electrolysis cell than for the fuel cell, mainly due to the NiO oxygen electrode performing better in the MCEC. The stability of the cell in long-term tests evidences the feasibility of the MCEC and the RMCFC using a conventional fuel cell set-up, at least in labscale. This study also elucidates the electrode kinetics of hydrogen production and oxygen production to better understand the two important reaction processes that determine the performance of the electrolysis cell. The experimentally obtained partial pressure dependencies for hydrogen production are high, and they do not reasonably satisfy the reverse pathways of the hydrogen oxidation mechanisms. The reverse process of an oxygen reduction mechanism in fuel cell operation is found to suitably describe the oxygen production in the MCEC. To evaluate the effect of the reverse water-gas shift reaction and the influence of the gas phase mass transport on the porous Ni electrode in the electrolysis cell, a mathematical model based on the Maxwell-Stefan diffusion equations is applied in this study. When the humidified inlet gas compositions enter the current collector the decrease of the shift reaction rate increases the electrode performance. The model well describes the polarization behavior of the Ni electrode when the inlet gases have low contents of reactants. The experimental data and modeling results are consistent in that carbon dioxide has a stronger effect on the gas phase mass transport than other components, i.e. water and hydrogen.