Iron-nitrogen-carbon catalysts have the potential to replace the more expensive platinum catalysts currently used in fuel cells. This is shown by a study conducted by researchers from the Helmholtz-Zentrum Berlin (HZB), Physikalisch-Technische Bundesanstalt (PTB) and universities in Tartu and Tallinn, Estonia. At BESSY II, the team observed the formation of complex microstructures within various samples. They then analysed which structural parameters were particularly important for fostering the preferred electrochemical reactions. The raw material for such catalysts is well decomposed peat.
Fuel cells convert the chemical energy of hydrogen directly into electrical energy, producing only water. Fuel cells could be an important component in a climate-neutral energy system. The greatest potential for improvement lies in the reduction of costs via the replacement of the electrocatalysts, which are currently based on the precious metal platinum.
A 'nano-labyrinth' for molecules
Carbon-based catalysts containing iron and nitrogen are a promising option for this purpose and can be used in anion exchange membrane fuel cells. This combination can be found in well-decomposed Estonian peat, for example. Carbon-based materials have remarkable properties, some of them are highly porous, with interconnected pores of different sizes that resemble passages in an ant colony. Hydrogen and oxygen atoms can migrate through these passages until they reach the catalytically active sites where the desired reactions actually can take place. The end product, water, is also transported away in this way. 'By changing the hierarchical structure of the catalyst, the size and thickness of the pore walls, we can produce materials with very different properties,' says Rutha Jäger, first author of the study from the University of Tartu.
Looking for the best structures
Eneli Härk, an electrochemist and small-angle scattering scientist at HZB, outlines the research question as follows: 'We sought to understand why one of the Fe-N-C electrocatalysts exhibited exceptional efficiency and selectivity, with performance comparable to the best non-noble metal catalyst, while other Fe-N-C-samples did not perform as well'. With the technique of small-angle X-ray scattering at BESSY II they investigated the key structural characteristics: hierarchical porosity, structural disorder, and the interaction distance between active centres within the pores. 'Small-angle X-ray scattering provides detailed and quantitative information on pore curvature and the ratio between pore size and pore wall thickness – parameters that are difficult to measure directly by other methods,' Eneli Härk explains.
Rather than relying on trial and error, the team designed a systematic study. Five samples were synthesized concurrently, at different synthesis temperatures from 800 to 1000°C, and using different pore modifiers to systematically vary the pore and pore wall structure. These samples, along with a commercial catalyst, were characterized at BESSY II using anomalous small-angle X-ray scattering (ASAXS) and conventional SAXS to determine their pore structure and active centres distribution. The materials were subsequently tested as oxygen reduction reaction catalysts to empirically correlate structural features with electrochemical performance. From the X-ray investigations, the team derived 13 structural parameters that influence the catalytic performance, including porosity, disorder, and pore curvature. 'Small-angle scattering provides us with a precise map of the anthill, so to speak, while the electrochemical behaviour of the catalyst shows us how the "ants", i.e. the molecules, move within it,' explains Eneli Härk. One result is that, with a pore curvature of at least three nanometres, oxygen reduction to water works best, and the formation of troublesome hydrogen peroxide is also minimised.
Outlook – a pathway to cut costs
'We knew how the materials work electrochemically in principle and that the hierarchical porosity of the material is important, but why one of them works better remained a mystery. Now, however, we have finally been able to uncover the structural nuances that promote the reaction,' says Rutha Jäger. Since Fe-N-C can be synthesized from well decomposed peat, the material is truly environmentally friendly. 'Estonian deposits offer a promising resource for producing high-tech functional materials', Rutha Jäger adds. The findings demonstrate a viable pathway from peat to functional fuel cells, potentially cutting system costs and enhancing sustainability.
