Solid oxide fuel cells play a key role in the transition to the green economy. They use the chemical energy of the fuel, for instance, hydrogen to produce electricity in a clean way generating only heat and water in the process.
Increasing the durability of solid oxide fuel cells is one of the main goals for achieving wider industrial application. The quality of the electrode plays a major role in the performance and durability of a fuel cell. In this work, we study the reduction mechanism of NiO which is a part of NiO/YSZ fuel electrode during manufacturing of a full cell. Upon exposure to hydrogen and heat during cell or stack commissioning NiO reduces to Ni needed for operation, leading to a change in volume and appearance of the crystallographic defects. It’s known from commissioning that e.g. reduction temperature and H2 pressure influence the initial performance of the fuel electrode by governing Ni particle size and shape.
With in-situ TEM we are able to observe the reduction of NiO in real time while exposing the sample to hydrogen gas and heat. Using in-situ TEM atmosphere system from Protochips we studied the electrode reduction at the H2 pressures up to 1 atmosphere and temperatures up to 850 °C which fit the real working condition of a solid oxide cell. Beforehand, electron transparent sample was prepared from a bulk fuel cell with a FIB and placed on a MEMs chip for TEM analysis. With this set-up we can observe the initial steps of NiO reduction, local change in oxidation state of Ni and formation of defects in Ni grains. In-situ results go in a good agreement with ex-situ results obtained from a bulk cell reduced in a test bench described in Ref. [1].
[1] D. Klotz et al., Electrochim. Acta, 227, 110 (2017).