Heating and electrical in situ STEM of titanium oxynitride support for iridium catalyst nanoparticles

Session

PSA.5 - Nanoparticles & Catalysts

Authors

Gorazd Koderman Podboršek (2, 3), Marjan Bele (2), Luka Suhadolnik (1), Goran Dražić (2, 3)

Affiliations

1. Jožef Stefan Institute
2. National Institute of Chemistry
3. Jožef Stefan International Postgraduate School

Keywords

4-probe conductivity measurement, in situ heating, PEM fuel cells, STEM, Titanium Oxynitride

Abstract text

The main objective of this work was in situ thermal stability and electrical conductivity study of titanium oxynitride (TiON) nanotubes and nanofibres used as an iridium catalyst nanoparticles support. 

Electrolyzers and proton exchange membrane (PEM) fuel cells can be seen as a viable alternative technology in our pursuit of eliminating dependence on fossil fuels as a source of energy. One of the major research topics at the present is to find a way to extend the lifetime of PEM electrochemical reactors, where catalyst layer stability is one of the bottlenecks. Currently, precious metal nanoparticles are dispersed on carbon-based supports, but these are known to degrade in the harsh operating conditions of PEM electrochemical reactors. A promising alternative to carbon supports is TiON [1]. Beside that the properties of TiON can be tuned changing the N/O concentration ratio.

TiON nanotubes were prepared with anodic oxidation of titanium foil to obtain TiO₂ nanotubes and subsequent nitridation in ammonia gas at high temperature. TiON nanofibres were prepared with electrospinning of an organic precursor, which was calcined at high temperature to obtain TiO₂ and subsequently nitrided in ammonia gas at high temperature. Ir catalyst nanoparticles were grown on the supports from Ir salt using the wet impregnation method and subsequent heat treatment in a reductive atmosphere. 

Samples were analysed with a probe Cs corrected STEM using in-situ heating and biasing sample holder which enables conductivity measurements at atomic level. Using EELS the oxygen to nitrogen ratio was determined.  

The 4 point conductivity measurements showed specific conductivities which are comparable to the specific conductivity of bulk TiN and much better than amorphous carbon. Quantitative EELS measurement revealed a stable N/O ratio throughout the heating range - up to 900 °C, confirming high thermal stability. At elevated temperature, Ir nanoparticles start to coalesce and grow. 

Based on the results, we can conclude that TiON supports are a viable alternative to carbon-based supports. They have high conductivity, which is very important for charge transport in the catalyst layer and they show high thermal stability, which is one of the properties needed for long term stability of the catalyst system. 

References

[1] M. Bele et al, ChemCatChem (2019), 11, p. 5038.