(Invited) Impact of Pore Morphology and Surface Hydrophobicity of the Carbon Matrix on the Macrokinetics of the Oxygen Reduction Reaction Performance for Atomically Dispersed Fe-N-C Catalysts

Atomically dispersed transition metal catalysts are an important family of non-precious metal electrocatalysts for the oxygen reduction reaction (ORR) in proton exchange membrane fuel cells. The iron-nitrogen-carbon (Fe-N-C) catalyst, wherein the iron metal centers are coordinated by nitrogen atoms (FeNx) on carbonaceous supports, exhibits outstanding ORR activity in both rotating disk electrode (RDE) and membrane electrode assembly (MEA) configurations.[1] The pore structure[2] and surface hydrophobicity[3] of the carbon matrix are of particular significance to the MEA performance, where the high current throughput requires good mass transport capability and controlled water flooding on the electrodes. In this work, we employed two widely used Fe-N-C catalysts – one pyrolyzed from metal organic framework (MOF-derived)[4] and the other produced by using the sacrificial support method (sacrificial templated)[1]. Specifically, nitrogen physisorption and water vapor physisorption were used to study the pore morphology and the specific surface hydrophobicity, respectively. As shown in Figure 1, the sacrificial templated Fe-N-C catalyst showed a significant mesoporosity as compared to the MOF-derived counterpart, while its surface was much more hydrophobic, as indicated by the less water uptake through whole pressure range (Figure 1f). Such difference of carbon morphology was further correlated to their macrokinetic performance in a MEA configuration, to reveal the impact of the morphological and hydrophilic/hydrophobic balance on these two types of Fe-N-C catalysts. Reference [1] T. Asset and P. Atanassov, Joule, 2020, 4, 33. [2] Y. Huang, Y. Chen, M. Xu, T. Asset, P. Tieu, A. Gili, D. Kulkarni, V. de Andrade, F. de Carlo, H. S. Barnard, A. Doran, D. Y. Parkinson, X. Pan, P. Atanassov, and I. Zenyuk, Materials Today, 2021, 47, 53. [3] L. Liu, S. J. Tan, T. Horikawa, D. D. Do, D. Nicholson and J. Liu, Adv Colloid Interface Sci, 2017, 250, 64. [4] K. Kumar, T. Asset, X. Li, Y. Liu, X. Yan, Y. Chen, M. Mermoux, X. Pan, P. Atanassov, F. Maillard and L. Dubau, ACS Catalysis, 11, 484. Figure 1. XRD (a) and AC-STEM (b-d) images of the Fe-N-C catalyst produced by the sacrificial support method. Nitrogen (e) and water vapor (f) physisorption isotherms of MOF-derived and sacrificial templated Fe-N-C catalysts. (g) Pore size distribution of the two catalysts by the BJH method based on the N2 adsorption branch. Figure 1