Dottorato in PROCESSI CHIMICI PER L'INDUSTRIA E PER
L'AMBIENTE

Titolo della tesi: Hydrogen production through steam electrolysis in molten carbonate cells

Molten carbonate cells, widely used industrially as fuel cells (MCFC), can also operate in reverse mode (MCEC), representing a valid solution for large-scale power-to-gas applications and H2 or syngas production. Since a molten carbonate cell can operate in both electrolysis and fuel cell modes, it provides a cost-effective solution for integrating this technology into existing energy infrastructures. However, the study of MCECs is still relatively recent, and several aspects require further investigation and optimization before industrial application can be fully realized. This thesis aims to analyze the electrochemical performance, operational stability, and hydrogen production efficiency of MCECs under varying conditions through experimental and numerical approaches. For the experimental campaign, two different setups were used: i) a button cell, with an active area of approximately 3 cm2, and ii) a planar single cell, square-shaped with an active area of 100 cm2. The effects of systematic variations in temperature and hydrogen electrode composition were evaluated in both setups. On the one hand, the button cell was employed to investigate the hydrogen (Ni) electrode kinetics, as this setup allows for individual electrode measurements. On the other hand, the planar single cell was used to assess electrochemical performance and to carry out gas analysis of the hydrogen electrode output. Polarization curves and impedance spectra obtained from the button cell were analyzed to investigate the hydrogen production mechanism occurring at the Ni electrode. Notably, the impedance data were deconvoluted using the Distribution of Relaxation Times, marking a novel approach in MCEC research. By combining these findings with the overpotential measurements, the potential rate-determining step of the reaction was identified, indicating that water reduction at the Ni electrode is under mixed kinetic-diffusion control. As for the planar cell, both experimental and numerical results, including a 2D model used to evaluate thermal effects, have demonstrated that by carefully adjusting the gas composition and operating temperature, the output gas can be tailored to produce either pure hydrogen or syngas. Under certain conditions, a significant amount of CO is produced, underscoring the need for further investigation into CO2 electrolysis. Additionally, a second experimental campaign conducted using this setup demonstrated the stable operation of an MCEC coupled with a variable power supply. This result highlights the feasibility of using such technology for large-scale applications in which the electrolyzer can be directly integrated with renewable energy sources. Furthermore, the study includes an analysis of process integration, assessing how molten carbonate electrolyzers could be incorporated within an industrial framework for hydrogen production. Finally, fuel-assisted electrolysis mode has been tested on a molten carbonate button cell, demonstrating its feasibility. However, further investigation is needed to validate these findings and identify the most suitable fuel to achieve energy savings for the electrolyzer. The obtained results provide an in-depth picture of the main factors affecting the performance of MCECs, highlighting the significance of optimizing operational conditions and electrode kinetics to facilitate their industrial applications.