Electrode kinetics of the NiO porous electrode for oxygen production in the molten carbonate electrolysis cell (MCEC)

The performance of a molten carbonate electrolysis cell (MCEC) is to a great extent determined by the anode, i.e. the oxygen production reaction at the porous NiO electrode. In this study, stationary polarization curves for the NiO electrode were measured under varying gas compositions and temperatures. The exchange current densities were calculated numerically from the slopes at low overpotential. Positive dependency on the exchange current density was found for the partial pressure of oxygen. When the temperature was increased in the range 600-650 degrees C, the reaction order of oxygen decreased from 0.97 to 0.80. However, there are two different cases for the partial pressure dependency of carbon dioxide within this temperature range: positive values, 0.09-0.30, for the reaction order at lower CO2 concentration, and negative values, -0.26-0.01, with increasing CO2 content. A comparison of theoretically obtained data indicates that the oxygen-producing reaction in MCEC could be reasonably satisfied by the reverse of oxygen reduction by the oxygen mechanism I, an n = 4 electron reaction, assuming a low coverage of oxide ions at high CO2 content and an intermediate coverage for a low CO2 concentration.QC 20151202</p

Electrode kinetics of the NiO porous electrode for oxygen production in the molten carbonate electrolysis cell (MCEC)

The performance of a molten carbonate electrolysis cell (MCEC) is to a great extent determined by the anode, i.e. the oxygen production reaction at the porous NiO electrode. In this study, stationary polarization curves for the NiO electrode were measured under varying gas compositions and temperatures. The exchange current densities were calculated numerically from the slopes at low overpotential. Positive dependency on the exchange current density was found for the partial pressure of oxygen. When the temperature was increased in the range 600-650 degrees C, the reaction order of oxygen decreased from 0.97 to 0.80. However, there are two different cases for the partial pressure dependency of carbon dioxide within this temperature range: positive values, 0.09-0.30, for the reaction order at lower CO2 concentration, and negative values, -0.26-0.01, with increasing CO2 content. A comparison of theoretically obtained data indicates that the oxygen-producing reaction in MCEC could be reasonably satisfied by the reverse of oxygen reduction by the oxygen mechanism I, an n = 4 electron reaction, assuming a low coverage of oxide ions at high CO2 content and an intermediate coverage for a low CO2 concentration.QC 20151202</p

Performance Recovery after Contamination with Nitrogen Dioxide in a PEM Fuel Cell

While the market for fuel cell vehicles is increasing, these vehicles will still coexist with combustion engine vehicles on the roads and will be exposed to an environment with significant amounts of contaminants that will decrease the durability of the fuel cell. To investigate different recovery methods, in this study, a PEM fuel cell was contaminated with 100 ppm of NO2 at the cathode side. The possibility to recover the cell performance was studied by using different airflow rates, different current densities, and by subjecting the cell to successive polarization curves. The results show that the successive polarization curves are the best choice for recovery; it took 35 min to reach full recovery of cell performance, compared to 4.5 h of recovery with pure air at 0.5 A cm&minus;2 and 110 mL min&minus;1. However, the performance recovery at a current density of 0.2 A cm&minus;2 and air flow 275 mL min&minus;1 was done in 66 min, which is also a possible alternative. Additionally, two operation techniques were suggested and compared during 7 h of operation: air recovery and air depletion. The air recovery technique was shown to be a better choice than the air depletion technique

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.QC 20160419</p

Performance and durability of the molten carbonate electrolysis cell (MCEC) and the reversible molten carbonate fuel cell (RMCFC)

The molten carbonate electrolysis cell (MCEC) provides the opportunity for producing fuel gases, e.g. hydrogen or syngas, in an environmentally friendly way, especially when in combination with renewable electricity resources such as solar, wind and/or hydropower. The evaluation of the performance and durability of the molten carbonate cell is a key for developing the electrolysis technology. In this study, we report that the electrochemical performance of the cell and electrodes somewhat decreases during the long-term test of the MCEC. The degradation is not permanent, though, and the cell performance could be partially recovered. Since conventional fuel cell materials consisting of Ni-based porous catalysts and carbonate electrolyte are used in the MCEC durability test, it is also shown that the cell can alternatingly operate as an electrolysis cell for fuel gas production and as a fuel cell for electricity generation, i.e. as a so-called reversible molten carbonate fuel cell (RMCFC). This study reveals that the cell performance improves after a long period of RMCFC operation. The stability and durability of the cell in long-term tests evidence the feasibility of the electrolysis and reversible operations in carbonate melts using a conventional fuel cell set-up, at least in lab-scale.QC 20160419</p

Performance and durability of the molten carbonate electrolysis cell (MCEC) and the reversible molten carbonate fuel cell (RMCFC)

The molten carbonate electrolysis cell (MCEC) provides the opportunity for producing fuel gases, e.g. hydrogen or syngas, in an environmentally friendly way, especially when in combination with renewable electricity resources such as solar, wind and/or hydropower. The evaluation of the performance and durability of the molten carbonate cell is a key for developing the electrolysis technology. In this study, we report that the electrochemical performance of the cell and electrodes somewhat decreases during the long-term test of the MCEC. The degradation is not permanent, though, and the cell performance could be partially recovered. Since conventional fuel cell materials consisting of Ni-based porous catalysts and carbonate electrolyte are used in the MCEC durability test, it is also shown that the cell can alternatingly operate as an electrolysis cell for fuel gas production and as a fuel cell for electricity generation, i.e. as a so-called reversible molten carbonate fuel cell (RMCFC). This study reveals that the cell performance improves after a long period of RMCFC operation. The stability and durability of the cell in long-term tests evidence the feasibility of the electrolysis and reversible operations in carbonate melts using a conventional fuel cell set-up, at least in lab-scale.QC 20160419</p