Graphitic Layer Encapsulated Iron Based Non‐precious Catalysts for the Oxygen Reduction Reaction

Research output: ResearchPh.D. thesis – Annual report year: 2017

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Proton exchange membrane fuel cells (PEMFCs) are highly efficient energy conversion devices, which can be in combination with hydrogen fuel providing a clean energy technology to produce electricity. One crucial challenge for this technology is the large cathodic overpotential due to the sluggish oxygen reduction reaction (ORR) kinetics. Carbon supported platinum (Pt/C) is the stateof-the-art benchmarking catalyst for PEMFCs since it exhibits the highest activity. However, the high cost and low abundance of noble metals have limited large-scale commercialization of the technology. Current efforts are made to develop non-precious metal catalysts (NPMCs) as a replacement to the Pt/C electrocatalysts. In this thesis, a new type of NPMCs is synthesized by means of a dry autoclave with volatile ferrocene and cyanamide as precursors. The catalysts are morphologically featured by porous microspheres consisting of uniform metallic nanoparticles encapsulated in graphitic layers. The thesis work is conducted aiming at three major objectives: further optimization of the pyrolysis to achieve improved performance of catalysts, investigation of the complex Fe-containing components, and exploration of the possible active sites.

By systematic investigation of pyrolytic parameters i.e. temperature and duration, the best performance is achieved at 700 oC and 75 minutes, exhibiting a high catalytic activity in acid media (0.1 M HClO4) with an onset potential of 0.85 V at 0.1 mA cm-2 and a mass specific kinetic current of 7.84 A g-1 at 0.7 V vs. RHE. A good stability with 25 mV potential losses after 10,000 cycles of potential scan between 0.6 and 1.0 V has also been demonstrated.

The featuring morphology of the catalysts, i.e. the porous microspheres consisting of the graphitic layer encapsulated metal-containing nanoparticles, is essentially maintained during the pyrolysis of varied durations and temperatures. The metal-containing nanoparticles showed changes in the iron phases and their contents, as characterized by 57Fe Mössbauer spectroscopy. The iron containing components include reduced metals (α-Fe and γ-Fe), oxide (γ-Fe2O3), carbide (Fe3C) as well as a minor paramagnetic component due to Fe3+ (high spin) and/or possibly Fe2+ (low spin), likely coordinated with nitrogen (FeNx/C) as well identified for the Fe/N/C type catalysts in the literatures.

Quantitative determination of these metal containing components by low temperature 57Mössbauer spectra shows that the content of the reduced metal component is steadily increasing with the pyrolytic time and temperature while the content of iron oxide is nearly constant. The most interesting finding is that the Fe3C content shows a peak in both the temperature-varying and the duration-varying series of samples. The possible FeNx/C coordination phase, however, varies to a very limited extent for the studied samples.

The catalytic activities and mechanisms for ORR are evaluated by rotating disk electrode (RDE) and rotating ring-disk electrode (RRDE) voltammetry. In terms of the mass specific kinetic current density and half-wave potential, a strong correlation of the catalytic activity is established with the Fe3C content within the entire composition range from 1.1 wt% to 4.5 wt% as well as with the FeNx/C content in a narrow range from 0.5 wt% to 0.85 wt%. Other iron containing components, i.e. α-Fe, γ-Fe and Fe2O3, showed no association with the ORR activity. It is concluded that, for the present catalysts, the recognized encapsulated iron carbide is most likely contributing to the ORR catalysis, in addition to the well identified N-coordinated Fe species.

More evidences are found from the catalyst synthesized from nitrogen free precursors. This catalyst, consisting of only carbon encapsulated iron-based nanoparticles, shows some, though low, ORR activity, which is enhanced by the post heat treatment in an ammonia atmosphere, indicating the contribution of the nitrogen functionalities.

Two anions in the electrolyte are used to probe the iron containing active sites towards the ORR, cyanide (CN-) in alkaline and thiocyanate (SCN-) in acidic medium, which seem supporting the above conclusions. These findings provide new insights to the encapsulation structure of Fe based nanocatalysts and therefore options for further development of NPMCs.
Original languageEnglish
PublisherDepartment of Energy Conversion and Storage, Technical University of Denmark
Number of pages114
StatePublished - 2016
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