Effect of steam content on nickel nano-particle sintering and methane reforming activity of Ni–CZO anode cermets for internal reforming SOFCs

In this work, the reforming activity of Ni–Ce0.75Zr0.25O2 (Ni–CZO) solid oxide fuel cell (SOFC) cermet anodes for steam reforming of methane has been investigated in the absence of electrochemical effects and the results were compared with Ni–Ce0.9Gd0.1O2 (Ni–CGO) cermet anodes. From X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy/energy dispersive X-ray spectroscopy (TEM/EDS) and CHN analysis on the spent Ni–CZO anode cermets, it was found that a decrease in reforming activity at higher steam-to-carbon ratios (S/C) was due to sintering of nickel nano-particles rather than re-oxidation or sintering of bulk nickel and carbon formation. Time-on-stream analysis demonstrated a higher reforming activity for Ni–CZO cermet anodes compared to Ni–CGO cermet anodes. While the initial sizes of nickel nano-particles were similar (5–10 nm) for both cermet anodes, after the operation under steam-rich conditions (S/C = 1.5) the nickel nano-particles on Ni–CGO were found to be significantly larger (20–40 nm) than those on Ni–CZO (15–25 nm). Under steam-lean conditions (S/C = 0.5), the size of nickel nano-particles remained below 10 nm for both cermet anodes during the operations, indicating that the effect of sintering of Ni nano-particles on the performance was negligible. The reforming activity is still higher for Ni–CZO cermet anodes than Ni–CGO cermet anodes, which could be attributed to higher concentration of ex-solved nickel nano-particles as well as lower Ce4+/Ce3+ reduction energy of CZO than CGO support.

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► ISRM over Ni–CZO anode cermets of SOFCs has been investigated.
► Steam rich conditions showed high reforming activity which decreased and stabilized with time.
► Steam lean conditions showed a moderate and constant reforming activity.
► Excess steam content is the primary cause for nickel nano-particle sintering and resulted in decrease of reforming activity.
► TEM/EDS analysis manifested the nickel nano-particle sintering.