Non-Precious Metal Catalysts Based on Activated Biochar for the Oxygen Reduction Reaction in High Temperature Proton Exchange Membrane Fuel Cell

Kurzfassung

The high-temperature proton exchange membrane fuel cell (HT-PEMFC) plays an essential role for energy conversion of green hydrogen to electricity contributing to the target of CO2 neutral energy supply. However, the high platinum catalyst loading which is necessary in HT-PEMFC due to partial poisoning of the catalysts by phosphates originating from the phosphoric acid-doped membrane, is one major contribution to the material costs. Therefore, the reduction but also replacement of Pt catalysts in HT-PEMFCs is in focus of research. In case of the oxygen reduction reaction Fe-N-C catalysts are promising candidates to replace Pt-based catalysts, since they show no poisoning by phosphates and activities close to Pt/C. However, this catalyst class suffers from low volumetric activity and stability mainly due to carbon corrosion. In this work, activated lignocellulosic biomasses are investigated as novel sustainable carbon supports with native heteroatom doping in Fe-N-Cs for application in HT-PEMFC gas diffusion electrodes (GDE). Two chemical activation procedures are used for generation of activated biomasses with different porosities. Pyrolysis and KOH activation for rye straw is done for generation of a microporous high surface area carbon. H3PO4 activation of rye straw and coconut shells is carried out to yield a mesoporous carbon. Implementation of the three biomass-based supports and two common oxidised carbon blacks Vulcan® and Black Pearls® in Fe-N-C synthesis, reveals a homogeneous incorporation of Fe and N in the H3PO4 activated biomasses and oxidised Black Pearls. In the case of oxidised Vulcan and the KOH activated biochar iron containing particles like iron carbide are found. Comparison of the physical parameters of the supports and Fe-N-Cs reveals that a surface area higher than 800 m² g-1, presence of mesopores and low amounts of amorphous carbon are beneficial for Fe-Nx site incorporation. The atomically dispersed Fe-N-Cs reveal high mass activity whereas low mass activities for catalysts containing iron particles are shown. A 50 % higher stability in terms of mass activity loss for the H3PO4 activated biomass-based Fe-N-Cs against carbon corrosion is observed. This is attributed to the presence of P species which inhibit electron withdrawal from the carbon network. GDEs are fabricated using ultrasonic spray coating and doctor blade coating. The GDEs fabricated by ultrasonic spray coating display thinner catalyst layers that lead to low performances in GDE half-cell measurements in conc. H3PO4 at 140 °C due to suspected H3PO4 flooding of the GDE which was not the case in HT-PEMFC for the Black Pearl-basedand commercial Fe-N-C. Biomass-based GDEs display poor performance in GDE half-cell measurement and HT-PEMFC independent of fabrication method. This is attributed to a rough catalyst layer surface due to larger catalyst agglomerate sizes and a higher hydrophilic character which also can induce phosphoric acid flooding. A 90 h constant load HT-PEMFC test shows fast activity decay for all tested catalysts within the first 24 h of operation which is attributed to carbon corrosion and deactivation of active sites according to electrochemical characterisation and post-mortem analysis. In conclusion, the suitability of H3PO4 activated lignocellulosic biomasses as sustainable support for Fe-Nx sites is shown, whereas KOH activated biochars are not suitable. A higher stability against carbon corrosion due to the presence of phosphor species and higher amounts of nitrogen species in the biomass-based Fe-N-Cs compared to Black Pearl-based Fe-N-C is revealed. However, the agglomerate size of the biomass-based Fe-N-Cs and hydrophilic character has to be adjusted to overcome challenges like sedimentation of catalyst ink or distribution of hydrophobic binder during GDE fabrication to increase the performances in HT-PEMFC.