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

Abstract

The purpose of this study was to elucidate the kinetics of a porous nickel electrode for hydrogen production in a molten carbonate electrolysis cell. Stationary polarization data for the Ni electrode were recorded under varying gas compositions and temperatures. The slopes of these iR-corrected polarization curves were analyzed at low overpotential, under the assumption that the porous electrode was under kinetic control with mass-transfer limitations thus neglected. The exchange current densities were calculated numerically by using a simplified porous electrode model. Within the temperature range of 600-650 • C, the reaction order of hydrogen is not constant; the value was found to be 0.49-0.44 at lower H 2 concentration, while increasing to 0.79-0.94 when containing 25-50% H 2 . The dependence on CO 2 partial pressure increased from 0.62 to 0.86 with temperature. The reaction order of water showed two cases as did hydrogen. For lower H 2 O content (10-30%), the value was in the range of 0.47-0.67 at 600-650 • C, while increasing to 0.83-1.07 with 30-50% H 2 O. The experimentally obtained partial pressure dependencies were high, and therefore not in agreement with any of the mechanisms suggested for hydrogen production in molten carbonate salts in this study. Electrolysis in molten carbonate salts at high temperatures is a promising method for hydrogen and/or syngas (H 2 +CO) production, especially when combining it with renewable electricity resources such as solar energy and wind power. Due to the favorable thermodynamic and kinetic conditions, high-temperature electrolysis will attain higher overall efficiency and require lower applied voltage when compared to low-temperature electrolysis.1 Electrolysis in molten carbonates has been evidenced by some authors, mainly by converting CO 2 into CO.2-5 Peelen et al. 2 studied the electrochemical reduction of CO 2 into CO on a gold flag electrode in 62/38 mol% Li/K carbonate mixture in the temperature range 575-700• C. Kaplan et al.3-4 observed the conversion of CO 2 to CO by using a cell with a molten electrolyte consisting of lithium carbonate and lithium oxide, in which the electrodes were graphite (anode) and titanium (cathode), and the working temperature was 850-900• C. Chery et al. 5 presented thermodynamic calculations and experimental measurements on the reduction of CO 2 into CO on a gold flag or planar disk electrode with Li/K and Li/Na carbonate eutectics at temperatures from 575 to 650• C. However, most experiments were carried out on flag electrodes, which are different from porous electrodes when considering electrode surface and mass-transfer limitations. In our previous study 6 a cell operating at 650• C, with conventional molten carbonate fuel cell (MCFC) materials (Ni-based porous electrode and molten carbonate electrolyte), was investigated also for electrolysis. The cell was found to give lower polarization losses in MCEC mode than in MCFC mode, mainly due to the NiO electrode performing much better as anode in the electrolysis cell. Reversing...

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

“…Concerning the effect of carbon dioxide or water, it is unexpectedly found that in gases 2-5 an increase of the CO2 or H2O concentration from 24.5 to 49.5% does not significantly decrease the charge-transfer polarization. This is in contrast to the previous conclusion [21] that the electrode kinetics of hydrogen production is strongly dependent on the partial pressure of carbon dioxide or water in the MCEC. But it is necessary to point out that in previous study the inlet gas has a higher amount of hydrogen, being constant at 25%.…”

“…On the basis of the polarization data in Fig. 2, the exchange current densities are determined using the same theory and method as discussed in the previous study [21]. When analyzing the polarization data for a porous electrode, it is important to take the current distribution into account.…”

“…In addition, the effective conductivity of the Ni electrode is much higher than that of the electrolyte phase, then a simplified expression (Eq. 1) is obtained for the polarization resistance of the Ni electrode [21,22].…”

“…The latter factor is presumably very likely, since the roles played by hydrogen, carbon dioxide and water in the electrolysis cell are complex and most probably affect each other. Despite the previous research to elucidate the electrode kinetics, the reaction mechanism of the Ni electrode for hydrogen production in the MCEC is still not determined [21].…”

“…In our previous studies [20,21] the gases used for the Ni electrode still contain a relatively high amount of hydrogen in the MCEC. If the electrochemical cell is mainly applied for fuel gas production, the inlet gases should contain low amounts of hydrogen or even no hydrogen.…”