In situ Reduction and Oxidation of Nickel from Solid Oxide Fuel Cells in a Transmission Electron Microscope

Abstract

Environmental transmission electron microscopy was used to characterize in situ the reduction and oxidation of nickel from a Ni/YSZ solid oxide fuel cell anode support between 300-500°C. The reduction is done under low hydrogen pressure. The reduction initiates at the NiO/YSZ interface, then moves to the center of the NiO grain. At higher temperature the reduction occurs also at the free NiO surface and the NiO/NiO grain boundaries. The growth of Ni is epitaxial on its oxide. Due to high volume decrease, nanopores are formed during reduction. During oxidation, oxide nanocrystallites are formed on the nickel surface. The crystallites fill up the nickel porosity and create an inhomogeneous structure with remaining voids. This change in structure causes the nickel oxide to expand during a RedOx cycle.

Operando observation of nickel/ceria electrode surfaces during intermediate temperature steam electrolysis

“…It is well-known that reduction of NiO to Ni is followed by a significant volume contraction (up to ca. 70%) [58], [59]. On the contrary, reduction of CeO2 to CeO1.5…”

“…4-9%) [60], [61], due to the fact the Ce 3+ has a larger atomic radius as compared to Ce 4+ [62]. The reverse tendency is anticipated upon re-oxidation, even if differences from the original volume due to coarsening might occur [58]. fig.…”

How the surface state of nickel/gadolinium-doped ceria cathodes influences the electrochemical performance in direct CO2 electrolysis

“…In particular, the Ni 0 -shell/NiO core morphology of nickel particles during CO 2 electrolysis revealed by NAP-XPS, seems as an optimum structural configuration, since surface Ni 0 helps to maintain the high electronic conductivity, whereas the NiO core preserves the volume of nickel particles thus sustaining the microstructure of the electrode and the contact between the particles. This is related to the volume expansion of oxidized nickel as compared to metallic Ni 0 [56,57], which helps to sustain the contact areas between the cermet particles, therefore significantly increasing the electrons pathways and compensating the decrease in electron conductivity due to NiO in the inner layers. Long term tests (not presented here), showed that in direct CO 2 electrolysis conditions the performance decreases progressively within a time period of four hours.…”

Metal-Insulator-Metal Single Electron Transistors with Tunnel Barriers Prepared by Atomic Layer Deposition

“…indicate that the NiO layer is about 2 nm thick. Fortunately, NiO can be reduced to Ni by annealing in hydrogen at moderately high temperatures ≥300 °C [35][36][37]. We have found that a forming gas anneal (FGA) at 400 °C for 2 min in 5% H2-95% Ar causes the conductance of the cross-tie structures with two cycles of PEALD SiO2 to increase from G ≈ 5 nS to G > 600 μS [11], while a 30 min FGA is required to increase the conductance of the Ni nanowire covered by ~1 nm of SiO2 back to that of the as-deposited nanowires [11,38].…”