In situ transmission electron microscopic observations of redox cycling of a Ni–ScSZ cermet fuel cell anode

Abstract

In situ transmission electron microscopy (TEM) observations of a Ni(O)-Sc2O3-stabilized ZrO2 (ScSZ; 10 mol% Sc2O3, 1 mol% CeO2, 89 mol% ZrO2) anode in a solid oxide fuel cell (SOFC) have been performed at high temperatures under a hydrogen/oxygen gas atmosphere using an environmental transmission electron microscope (ETEM); the specimens were removed from cross-sections of the real SOFC by focused ion beam milling and lifting. When heating the NiO-ScSZ anode under a hydrogen atmosphere of 3 mbar in ETEM, nano-pores were formed at the grain boundaries and on the surface of NiO particles at around 400°C due to the volume shrinkage accompanying the reduction of NiO to Ni. Moreover, densification of Ni occurred when increasing the temperature from 600 to 700°C. High-magnification TEM images obtained in the early stages of NiO reduction revealed that the (111) planes of Ni grew almost parallel to the (111) planes of NiO. In the case of heating Ni-ScSZ under an oxygen atmosphere of 3 mbar in ETEM, oxidation of Ni starting from the surface of the particles occurred above 300°C. All Ni particles became polycrystalline NiO after the temperature was increased to 800°C. Volume expansion/contraction by mass transfer to the outside/inside of the Ni particles in the anode during repeated oxidation/reduction seems to result in the agglomeration of Ni catalysts during long-term SOFC operation. We emphasize that our in situ TEM observations will be applied to observe electrochemical reactions in SOFCs under applied electric fields.

Silica: ubiquitous poison of metal oxide interfaces

“…While typically operated under ultra-high vacuum conditions, sophisticated sample holders now allow in-situ examination under controlled gas exposures. 59,60 A large number of reviews on the use of microscopy for metal oxide based electrochemical systems are available. 61,62 Scanning probe microscopy offers information about surface morphology and ex situ scanning tunnelling microscopy (STM) is extensively used to study the surface chemistry and band structure of e.g.…”

Insights into Heterogeneous Catalysts under Reaction Conditions by In Situ/Operando Electron Microscopy

“…The operation of a solid oxide fuel cell (SOFC) depends on an effective anode catalyst, Ni(O)‐Sc 2 O 3 ‐stabilized ZrO 2 (ScSZ), which is examined by an ETEM study of the redox cycling of the Ni(O)‐Sc 2 O 3 cermet fuel cell anode under H 2 /O 2 gas atmosphere at high temperatures. [ 244 ] Upon heating the NiO‐ScSZ anode under H 2 at 400 °C, the formation of nano‐pores at the grain boundaries and on the NiO particles is observed. There is a shrinkage of NP volume along with the reduction of NiO to Ni, showing densification of Ni upon further heating to 700 °C.…”

In Situ TEM Investigation of Structural Changes in Ni Nanoparticle Catalysts under Gas Atmospheres: Implications for Catalyst Degradation

“…14 The oxidation and reduction of the Ni catalyst particle during redox cycling in SOFC anodes are accompanied by volumetric changes and mass transfer, which is thought to result in coarsening and the agglomeration of the Ni particles. 15,16 18 However, in situ TEM observations indicated that carbon deposition did not result in carbide formation on the surface of Ni catalysts in a mixture of ammonia and acetylene gas. 18,19 Yoshida et al conducted in situ TEM to observe the growth of CNTs in an acetylene and hydrogen gas mixture.…”

“…Many researchers have employed in situ transmission electron microscopy (TEM) to monitor microstructural changes in Ni catalysts. − Chenna et al investigated Ni activation for partial methane oxidation using an environmental transmission electron microscope (ETEM). They reported that the Ni catalyst transformed into NiO at approximately 300 °C in a CH 4 and O 2 mixture before any significant catalytic reaction occurred .…”

“…Jeangros et al performed electron energy loss spectroscopy (EELS) to evaluate the activation energy for NiO reduction in pure hydrogen gas . The oxidation and reduction of the Ni catalyst particle during redox cycling in SOFC anodes are accompanied by volumetric changes and mass transfer, which is thought to result in coarsening and the agglomeration of the Ni particles. , Lawrence et al studied the impact of the interactions between the Ni catalyst and CeO 2 support on catalyst activity …”