Operating the nickel electrode with hydrogen-lean gases in the molten carbonate electrolysis cell (MCEC)

If a molten carbonate electrolysis cell (MCEC) is applied for fuel gas production it is important to know the polarization of the nickel electrode when operated at low concentration of hydrogen. Thus, the electrochemical performance of the Ni electrode was investigated under hydrogen-lean gases containing 1/24.5/24.5/50%, 1/49.5/24.5/25%, 1/24.5/49.5/25% and 1/49.5/49.5/0% H2/CO2/H2O/N2 in the temperature range of 600-650 °C and was then compared to the reference case with 25/25/25/25% H2/CO2/H2O/N2. The electrochemical measurements included polarization curve coupled with current interrupt, and electrochemical impedance spectroscopy. Polarization resistances of the Ni electrode obtained by the two different techniques agreed well. For the inlet gases containing low amounts of hydrogen the Ni electrode exhibited higher polarization losses than when using the reference case in the electrolysis cell. The electrochemical impedance measurements showed that both charge-transfer and mass-transfer polarizations were higher for hydrogen-lean gases at all measured temperatures. Except under the condition with 1/49.5/49.5% H2/CO2/H2O at 650 °C, the Ni electrode exhibited lower mass-transfer polarization when compared to the reference case. Furthermore, the mass-transfer polarization was strongly dependent on temperature under H2-lean gases, differing from the reference case when the temperature has almost no effect on mass-transfer polarization. The activation energy for hydrogen production was calculated to be in the range of 69-138 kJ mol−1 under all measured gases, indicating that the Ni electrode is under kinetic and/or mixed control in the MCEC.