Transition metals in Ni/GDC for the reversible solid oxide cell operation: Optimization of the Mo-Au-Ni synergy and further enhancement via substitution of Mo with Fe

Abstract

The study focused on the elucidation of the synergistic interaction of different wt.% loadings of Mo and Au in Ni/GDC, with the objective to find their optimum content for enhanced rSOC performance. In addition, the promoting effect of Fe in Ni/GDC was stabilized via co-deposition with Au and there is for the first-time comparison among the transition metal modifications of Mo-Au-Ni vs Fe-Au-Ni. A series of samples was prepared, where the content of Au varied between 1 and 3 wt.% and between 0.4 and 1 wt.% for Mo. Physicochemical characterization took place with XRF, BET, SEM, H2-TPR, TPH2O and in-situ H2O-XPS. The effect of the transition metals interaction on the performance of SoA Ni/GDC was investigated via electrochemical I-V and EIS measurements in rSOC operation, by applying different pH2O/pH2 mixtures in the temperature range of 900 – 800 °C. It was found that high Mo and/or Au wt.% content, results in larger Mo-Au-Ni particles and affects differently the bulk and surface interaction of the samples with H2O, as well as the Rohm and Rpol values of the fuel electrodes. The modified samples were less prone to bulk oxidation by H2O, while they exhibited higher surface oxidation of Ni and higher amount of Ce3+, compared to Ni/GDC. On the surface of the optimum 0.4 wt.% Mo-1 wt.%Au-Ni/GDC: (i) Ni was less oxidized, (ii) Au0 enrichment was more stable and (iii) Mo was more oxidized, overall affecting positively the Rpol versus the electrode with the higher loadings of 1 wt.% Mo-3 wt.% Au. The main degradation factor of the latter cell was ascribed to the increased Rohm. Finally, a new ternary 0.5 wt.% Fe-3 wt.% Au-Ni/GDC electrode is presented, which exhibited superior electrochemical rSOC performance and stability, compared to the optimum Mo-Au-Ni/GDC, highlighting the need for further research in the transition metals effect on SOCs.