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

Abstract

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 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 CO 2 concentration, and negative values, À0.26-0.01, with increasing CO 2 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 ¼ 4 electron reaction, assuming a low coverage of oxide ions at high CO 2 content and an intermediate coverage for a low CO 2 concentration. IntroductionThere is a growing interest in high-temperature electrolysis cell technology for producing hydrogen and/or syngas, since these fuels are considered important for future energy systems. Electrolysis in molten carbonate salts at high temperature (typically 650 C) is a promising option, especially when in combination with renewable electricity resources such as wind power and/or solar energy. Some researchers have evidenced the possibility of electrolysis in molten carbonates, but mainly by converting CO 2 into CO. 1-4 However, most experiments were performed on ag electrodes, which differ from porous electrodes with regards to the electrode surface and mass-transport properties. Our previous study 5 showed that it is feasible to run the molten carbonate fuel cell reversibly with conventional Ni- View Article OnlineView Journal | View Issue based porous electrodes at 650 C. A lower polarization loss has been found for the electrolysis cell compared to the fuel cell, mainly due to the NiO electrode performing much better as an anode in MCEC mode. 5 The activity of the NiO electrode provides a dominant contribution to the performance of the cell, regardless of fuel cell or electrolyzer operation. Thus, it is essential to elucidate the kinetics at the NiO porous electrode for oxygen reduction in the fuel cell, as well as for oxygen production in the electrolysis cell.However, the kinetics and reaction mechanism at the NiO porous anode for oxygen production in MCEC mode are still unknown. A feasible path to overcome this is to understand whether the reversed process of oxygen reduction takes place at the NiO electrode or whether there are some other electrochemical/chemical reaction pathways in the electrolysis operation. Several reaction mechanisms for the oxygen reduction reaction in fuel cell mode have been...

Green Synthetic Fuels: Renewable Routes for the Conversion of Non-Fossil Feedstocks into Gaseous Fuels and Their End Uses

“…Though the kinetics of CO 2 electrolysis is slower than water splitting, CO could be produced through the CO 2 reduction (Equation 28). At the anode, CO 3 2− ions oxidize, generating CO 2 and O 2 (Equation 29) [217].…”

Degradation of MCFC Materials in a 81 cm2 Single Cell Operated Under Alternated Fuel Cell/Electrolysis Mode

“…Their results indicate that in the temperature range 600-675 • C the overall MCFC performance in electrolysis mode is slightly better than in fuel cell operation mode (Hu et al, 2014), which could open on the possibility of using conventional MCFC electrodes as efficient bi-functional electrode catalysts. Lower polarization losses of the porous NiO oxygen electrode were identified as the main cause for the better performance in electrolysis mode (Hu et al, 2014(Hu et al, , 2015b. In particular, it was found that gas production at the NiO oxygen electrode [reaction (2)] takes place under a dominant charge transfer control (Hu et al, 2015b), being the role of mass transfer polarization negligible even at higher current densities, which is a striking difference as compared to a MCFC running in the conventional fuel cell mode.…”

Carbon Species Solvated in Molten Carbonate Electrolyser Cell from First-Principles Simulations

“…Moreover, because MCFC consume CO 2 at the cathode side and release CO 2 at the anode side, they have been proposed to combine this electricity production capability with the capture and the concentration of CO 2 out of exhaust gas from steel mills [3][4][5]. Alternatively, the high solubility of CO 2 in molten carbonates is a possible path to perform carbon capture and utilization (CCU) by electroreduction of CO 2 in molten carbonate electrolyser cells (MCEC) [6][7][8][9][10]. The reduction of CO 2 in MCEC into amorphous carbon has been well documented and the conversion of CO 2 to CO or even CH 4 has also been evidenced recently [11][12][13], thus adding another opportunity for carbon capture and utilisation using carbonate melts.…”