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Scientists explore alternatives to platinum-based fuel cells


Researchers led by CNRS have found that alkaline fuel cells made of transition metals still perform after being artificially aged and despite changes in their chemical composition. The results are published in Applied Catalysis B: Environmental.

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Fuel cells convert the chemical energy of a fuel (often hydrogen) and an oxidizing agent into electricity through chemical reactions.

One of the most widely-known applications of fuel cells is electric cars that are powered by electricity generated by hydrogen instead of having to plug them into charging stations. Despite some advances and companies like Toyota and Hyundai producing fuel-cell powered cars, there are still many hurdles for fuel cells to become mainstream in transportation and other uses.

The most crucial drawback for the use of fuel cells is the cost of manufacturing them, as they contain precious metals, such as platinum that is used in acidic fuel cells, and these are very expensive. Scientists have been looking for alternative materials for several years now, and transition metals (such as iron, cobalt, tin) embedded in a carbon matrix and doped with nitrogen have shown promising results. Iron-nitrogen-carbon catalysts efficiently electrocatalyse the oxygen reduction reaction. So much so, that they could replace platinum in commercial fuel cells. 

When using an acid electrolyte, the fuel cells initially have a lot of power, but iron-nitrogen-carbon catalysts corrode faster than desired and lead to decreasing electric power of the fuel cells. Conversely, in an alkaline medium, iron-nitrogen-carbon materials don’t only perform better but also experience less damage.  However, scientists still don't know which is the most resistant site structure within iron-nitrogen-carbon catalysts.

viktoriialow.jpg (ID26)

Viktoriia Saveleva on beamline ID26. Credits: S. Candé

Now scientists from the CNRS, the Université Grenoble Alpes (UGA) and the ESRF have carried out extensive experiments, both at UGA and at the facility, to analyse the stability of iron-nitrogen-carbon catalysts in an alkaline medium. At the ESRF, the team used X-ray absorption spectroscopy on beamline ID26 and wide-angle X-ray scattering on ID31. “The high detection efficiency of the ID26 spectrometer and the increased photon flux thanks to EBS enabled the study of thin catalyst layers with low Fe loading” says Pieter Glatzel, scientist in charge of ID26 and co-author of the publication. The scientists artificially aged catalysts that comprise iron either as single iron atoms or iron carbide nanoparticles supported on nitrogen-doped carbon matrix catalysts and then observed how their composition and structure are altered. “The atomically-dispersed iron catalyst undergoes irreversible changes by losing 50% of the iron sites.”, explains Viktoriia Saveleva, post-doctoral researcher at ID26 and co-author of the publication. Specifically, 15 % of the iron atoms dissolve in the electrolyte and 35 % redeposit as iron oxide nanoparticles, which are also catalytically active. The iron carbide nanoparticles transform into iron carbide core@iron oxide shell nanoparticles. “Despite these changes in structure and composition, the catalysts continue to work correctly”, she adds.

“The technique of X-ray absorption and emission spectroscopy on ID26 is a unique tool to investigate the active iron site of the catalyst”, explains Frédéric Maillard, director of research at the CNRS and corresponding author of the publication. “This provides us with clues about the steps to take to tweak the catalyst to make it more performing”, he adds. For example, the results show that iron-nitrogen-carbon materials based on zero-valent iron nanoparticles should be designed so that all iron nanoparticles are protected by a defect-free graphite shell, so that they last longer in time.

Reference :

Sgarbi, R., et al, Applied Catalysis B: Environmental, 5 April 2022.

Text by Montserrat Capellas Espuny