Examine Operando Generated Ni-Based Alloy Nanomaterials as Fuel Electrode in Solid Oxide Cells
Authors: Pukhova Alice , He Shuai , Zhang Nuoxi , Yue Xiangling 
Published: July 2021

Solid oxide cells are highly efficient electrochemical device for energy conversion. They can convert chemical energy to electrical energy in solid oxide fuel cells (SOFCs) mode and vice versa in solid oxide electrolysis cells (SOECs) mode. Constructing nanostructured electrodes have been demonstrated as an effective approach to enhance performance and durability for solid oxide cells, primarily due to significantly extended triple phase boundaries, where the active sites for electrochemical reactions locate.
[1, 2]
Exsolution, an innovative method of generating uniformly distributed fine particles through redox treatment, is of particular interest in recent years, as it offers well embedded nanoparticles bringing greatly boosted catalytic properties, thermal stability, and coking resistance. A wide range of nanomaterials have been fabricated by this manner, including metals
[3, 4]
, alloys
[5, 6]
and oxides
[7, 8]
.

Through exsolution processing, catalytically active materials can be produced either in-situ by exposing to a H2-containing atmosphere in which nanoparticles form on the surface upon reduction, or operando in an SOEC through electrochemical reduction when a high potential is applied. It was reported that exsolution happened almost instantly at 2.0 V in steam electrolysis operation, generating sufficiently high amount of nanocatalysts pinned on the surface of perovskite.
[9]
In contrast, it takes hours even days by chemical reduction, yet with less impressive performance. This indicates that electrochemical reduction by applying suitable potential or current can serve as an efficient method to produce electrocatalysts in nanoscale. However, potential-driven exsolution has not been extensively studied, especially in CO2 electrolysis via SOEC.

In this work, we have been focusing on examine titanate perovskites with exsolved Ni-based alloy nanocatalysts as fuel electrode of solid oxide cells, particularly on generating nanocatalysts via electrochemical reduction in CO2 electrolysis conditions. A series of titanate perovskites were prepared, including La0.43Ca0.37Ni0.06Ti0.94O3-δ, La0.43Ca0.37Ni0.03Fe0.03Ti0.94O3-δ, and La0.43Ca0.37Ni0.03Co0.03Ti0.94O3-δ. These compositions represent metallic Ni, Ni-Fe alloy, and Ni-Co alloy as nanocatalysts to be exsolved respectively. X-ray diffraction (XRD) and DC conductivity measurement were conducted to determine material crystal structure and conductivity properties, respectively. SOCs with scandium stabilized zirconia (ScSZ) electrolyte, standard (La, Sr)MnO3 (LSM)/ScSZ air electrode and the above prepared titanates fuel electrodes were fabricated for electrochemical characterisation. The fuel electrodes were firstly activated by applying varying potential/current in pure CO2 gas, prior to performance evaluation. In-situ impedance spectroscopy and polarization curves were recorded, and post-mortem microstructure analysis was carried out to investigate the exsolution behaviour on different electrodes. Results obtained so far showed that nanoparticles induced by electrochemical reduction formed at potentials higher than 2V in CO2 electrolysis conditions, leading to improved cell performance towards CO2 reduction as well as H2 oxidation. Higher performance was observed on the cells with alloy nanocatalysts compared to that with sole Ni nanoparticles.

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