Comparison of the Electrochemical and Degradation Behaviour of Ni-YSZ and Ni-GDC Electrodes Under Steam, Co- and CO2 Electrolysis
Authors: Unachukwu Ifeanyichukwu Daniel , Vibhu Vaibhav , Uecker Jan , Vinke Izaak C. , Eichel Rudiger-A , de Haart L.G.J. (Bert) 
Published: September 2023

Abstract

The mixed ionic and electronic conductive property of Ni-GDC has ensured that the electrochemical reaction zones of the electrode extend beyond the triple phase boundary of Ni/GDC/fuel gas to the double phase boundary of the GDC and the fuel gas [1, 2]. In addition, ceria has shown good carbon suppression properties when operated in carbon-containing fuels[3, 4]. For these reasons, Ni-GDC has emerged as a possible replacement for the conventional Ni-YSZ electrode. However, a direct comparison of the performance and long-term degradation of Ni-GDC with literature values of the Ni-YSZ would be ambiguous. On one hand, different reports utilize different fuel gas compositions, operating temperatures as well as current densities. On the other hand, the fabrication of fuel electrode-supported Ni-GDC is still a challenge due to the well-known inter-diffusion [5, 6] between the YSZ and the GDC oxide phase at a high sintering temperature (1400 °C) of YSZ electrolytes. Hence, most of the electrode fabrication has remained on electrolyte support. Thus, a direct comparison of electrolyte-supported Ni-GDC with the conventional fuel electrode-supported Ni-YSZ is ineffective due to different degradation behaviour.
Therefore, this present study aims to investigate and compare the long-term stability of Ni-GDC and Ni-YSZ under three different electrolysis modes; steam-electrolysis, co-electrolysis and CO2-electrolysis. Firstly, electrolyte-supported single cells of Ni-GDC (NiO-GDC//8YSZ//GDC//LSCF) and Ni-YSZ (NiO-GDC//8YSZ//GDC//LSCF) were fabricated and investigated using electrochemical impedance spectroscopy (EIS) from 750-900 °C temperature range. Furthermore, the impedance data were also recorded under polarization (0.7 to 1.4V) as well as at OCV by varying the partial pressure of steam, CO2 and oxygen (, pH20, pCO2 and pO2). Finally, stability tests of the single cells were carried out under steam electrolysis (H2O: H2, 50:50) and co-electrolysis (H2O: CO2:H2, 40:40:20) conditions at 900 °C with 0.5 A/cm2 current density for 500 h [7]. For the CO2-electrolysis (CO2:CO, 80:20), the stability test was performed up to 1000 h. The post-test characterization of the operated cells was carried out using both SEM-EDX as well as FIB-SEM. The results reveal that Ni-GDC exhibits higher current density than Ni-YSZ in all the electrolysis modes. In the post-test analysis, loss of GDC percolation was observed and the Ni particles were observed to be covered by the GDC oxide phase. Furthermore, both electrodes demonstrate that Ni is migrating away from the electrolyte in all the electrolysis modes. The detailed electrochemical and microstructural properties of the operated cells will be presented and discussed.

References

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