Affecting the H2O electrolysis process in SOECs through modification of NiO/GDC; experimental case of Au-Mo-Ni synergy

Abstract

This is a study on how the Solid Oxide Cell H2O electrolysis can be affected, through modification of commercial NiO/GDC powder and the experimental case focuses on the effect of Au/Mo doping. Physicochemical characterization is presented, in which the samples were examined both in the form of powders and as half cells with BET, HR-TEM/EDS, SEM, in-situ H2-XPS and specific re-oxidation TG analysis in the presence of H2O. Furthermore, there are comparative electrocatalytic measurements, under various pH2O/pH2 ratios (from 1 to 9) in the temperature range of 800–900 °C. The latter include I-V measurements and Electrochemical Impedance Spectra (EIS) analysis of single electrolyte supported cells with Ni/GDC, 3 wt% Au-Ni/GDC, 3 wt% Mo-Ni/GDC and 3 wt% Au – 3 wt% Mo-Ni/GDC, as the steam/hydrogen electrode. In particular, the cell comprising the ternary cathode performed significantly better, compared to the (Au-, Mo-) binary electrodes and Ni/GDC. The superior performance of the ternary sample is primarily ascribed to the enrichment of the surface with Au and of the bulk phase with Mo, through the formation of Ni-Au-Mo solid solution. The involved elements act in synergy and modify the physicochemical properties of the electrode, improving the: (i) H2O re-oxidation rate, (ii) electronic conductivity and (iii) electrochemical electrode/electrolyte interface. Indicatively, the polarization resistance (Rpol) terms of the ternary electrode, exhibited the lowest values and most importantly showed negligible degradation, by increasing the pH2O/pH2 ratio. It is worth mentioned that the presented study corroborates our recently published approach in which H2O, apart from the reactant of the process, is also acknowledged as a potential poisoning agent. Hence, proper modification of Ni/GDC should aim to the selectively weak interaction of H2Oads with the electrode, towards the inhibition of the re-oxidation rate of Ni in favor of the direct electrochemical splitting of H2O.