Electrode Kinetics of the Ni Porous Electrode for Hydrogen Production in the Molten Carbonate Electrolysis Cell (MCEC)

There is a growing interest in electrolysis cell technology for producing hydrogen that is today considered as an important fuel in the future systems. Electrolysis in molten carbonate salts at high temperature is a promising method for hydrogen- and/or syngas (H₂+CO₂) production. Due to the favorable thermodynamic and kinetic conditions, high-temperature electrolysis will gain higher overall efficiency and require lower applied voltage when compared to low-temperature electrolysis. Electrolysis in molten carbonates has been investigated by some authors mainly by converting CO₂into CO [1-3]. In our previous study, the feasibility of running the molten carbonate fuel cell reversibly with conventional state-of-the-art cell components is evidenced [4]. However, the Ni hydrogen electrode shows slightly higher polarization losses in water electrolysis than in hydrogen oxidation. For the development of molten carbonate electrolysis cells it is important to figure out the activity of the Ni electrode for water electrolysis in molten carbonates.    

In the present study the kinetics and reaction mechanism of hydrogen production in the Ni porous electrode in a molten carbonate electrolysis cell is investigated. For this purpose the electrochemical reaction orders of hydrogen, carbon dioxide and water were determined on basis of the steady-state polarization data. Within the temperature range of 600-650 °C the electrochemical reaction order of hydrogen is not constant; the value was found to be 0.49-0.44 at lower H₂ concentration, while increasing to 0.79-0.94 when containing 25-50% H₂, i.e. not strongly depending on the temperature. On the other hand the partial pressure dependence of CO₂ exhibits a larger influence by temperature increasing from 0.62 to 0.86 when the temperature rose from 600 to 650 °C. The reversed water-gas shift reaction has little or almost no impact on the electrochemical reaction order of hydrogen and carbon dioxide. The electrochemical reaction order of water shows two cases as does hydrogen. At lower water concentration, 10-30%, the reaction order is in the range of 0.47-0.67 while it increases to 0.83-1.07 with 30-50% water at 600-650 °C. When taking the shift equilibrium into account, the reaction order of water is constant and the value is approaching to that of higher water content. The activation energy of the Ni porous electrode for hydrogen production is in the range of 55-100 kJ·mol^-1, which indicates that the Ni electrode is under mixed-control.

Reference

[1] W.H.A. Peelen, K. Hemmes, and J. H. W. De Wit, Electrochim. Acta, 43, 763 (1997).

[2] V. Kaplan, E. Wachtel, K. Gartsman, Y. Feldman, and I. Lubomirsky, J. Electrochem. Soc., 157, B552 (2010).

[3] D. Chery, V. Albin, V. Lair, and M. Cassir, Int. J. Hydrogen Energy, 39, 12330 (2014).

[4] L. Hu, I. Rexed, G. Lindbergh, and C. Lagergen, Int. J. Hydrogen Energy, 39, 12323 (2014).