Gadolinium Doped Ceria as Nickel–Free Fuel Electrode in High Temperature CO₂‐Electrolysis

Abstract

In this work, the performance, electrochemical processes, and stability of GDC (Ce0.8Gd0.2O1.9) fuel electrode containing single cells (GDC/8YSZ/GDC/LSCF) were examined in CO2 electrolysis conditions. The current density and area specific resistance is determined for different operation conditions. The examined cells showed a higher performance compared to state‐of‐the‐art Ni−YSZ fuel electrode (Ni‐YSZ/8YSZ/GDC/LSCF) based single cells with a similar production route. Furthermore, the cells were examined by means of electrochemical impedance spectroscopy (EIS) and the recorded data were evaluated by the distribution of relaxation times (DRT) analysis. An equivalent circuit model (ECM) consisting of four time constants and a Gerischer impedance (LR−RQ1−RQ2−G−RQ4−RQ5) was established to gain further insight into the individual processes in the cells. For instance, the low–frequency process P5 referring to RQ5 is the rate‐determining step in CO2 electrolysis and is assigned to a surface reaction including the charge transfer and possibly the gas diffusion in the GDC fuel electrode. A long–term stability test was performed in CO2 electrolysis conditions at 900 °C with a constant current load of −0.5 A ⋅ cm−2 for up to 1200 h. A degradation rate of 62 mV ⋅ kh−1 was observed. By EIS analysis, a similar contribution of the ohmic and polarization resistances to the degradation was determined. The low–frequency process P5 and thus the surface reaction including the charge transfer in the GDC fuel electrode is contributing most to the increase in polarization resistance. After the long–term stability test, scanning electron microscopy (SEM) analysis was carried out and a coarsening of GDC particles in the fuel electrode was observed.

Electrochemical Characterization of Nickel/Gadolinia Doped Ceria Fuel Electrodes under H2/H2O/CO/CO2-Atmospheres

“…For all temperatures it can be seen, that the surface reaction resistance is lowest in the range y y + ⩾ 70%. Generally, higher activation losses under CO/CO 2 -atmospheres compared to H 2 /H 2 O atmospheres are a known phenomenon for GDC based electrodes 3,6,40,[45][46][47] as well as for Ni/YSZ based electrodes, 2,48,49 thus the observed result is in accordance with literature.…”