Investigation of Pt/Fe-N-C Hybrids Towards ORR in Acidic Environment

Abstract

Polymer electrolyte membrane fuel cells (PEMFCs) are recognized as one of the renewable energy sources in portable, automobile, and stationary applications. Currently, both low temperature (LT) and high temperature (HT) PEMFCs use platinum group metal (PGM) based catalysts for oxygen reduction reaction (ORR) containing usually Pt nanoparticles on carbon black. To reduce the total cost of PEMFC stack, worldwide researchers have considerable attention to find an inexpensive alternative catalyst. Recently, metal-nitrogen-carbon (M-N-C) compounds such as Fe-N-Cs are the most promising PGM-free catalysts for ORR. However, they suffer from insufficient volumetric activity and electrochemical stability in PEMFCs. [1,2] Xiao et al. have proven an improved electrochemical stability of Pt-Fe-N-C electrocatalysts consisting of atomically dispersed Pt and Fe atoms or Pt-Fe alloy nanoparticles in comparison with Fe-N-C only. [3] Mechler et al. have reported that the addition of 1-2 wt.% Pt in hybrid Pt/Fe-N-C catalyst performs well with an increased stability in LT-PEMFCs. [4] Liao et al. have recently shown better ORR activity of Pt/Fe-N-C than Pt/C. [5] With the overriding goal of reducing LT- and HT-PEMFC production costs, Pt/Fe-N-C activity, selectivity and stability with systematically reducing the Pt-content has not been investigated yet. In this study, Pt/Fe-N-C hybrids are synthesized using PMF-011904 from Pajarito Powder (USA) as catalyst support and wet-chemically precipitated Pt nanoparticles with targeted Pt-contents of 40, 5, 1 and 0 wt.%. First, ICP mass spectrometry is used for Pt quantification along all catalysts, and transmission electron microscopy is carried out to investigate morphology and Pt nanoparticle diameter and distribution on the Fe-N-C catalyst. Second, ORR activity and selectivity is investigated by rotating ring disc electrode (RRDE) experiments in 0.1 mol L-1 HClO4, and electrochemical surface area of Pt is calculated by hydrogen underpotential deposition and CO stripping voltammetry. Last, accelerated stress testing (AST) is performed with 5,000 triangle potential cycles between 0.6–1.5 VRHE to evaluate the catalyst stability. Figure 1 shows the initial ORR curves during the RRDE experiments. The mass activities of 40, 5 and 1 wt.% Pt/Fe-N-C determined at 0.9 VRHE are 222.5 ± 41.2 A gPt -1, 170.8 ± 80.4 A gPt -1 and 49.0 ± 8.6 A gPt -1, respectively. Thus, the mass activity depends on Pt-content in the hybrid catalyst strongly. In addition, the other electrochemical characterization results are also going to be discussed in terms of catalyst selectivity, stability and the correlation with morphological aspects during the presentation. Figure 1 ORR curves in O2-saturated 0.1 mol L-1 HClO4 electrolyte with rotation speed of 1,600 rpm and scan rate of 5 mV s-1 (averaged results of three separate electrodes per each catalyst).[1] Y. He, S. Liu, C. Priest, Q.Shi , G. Wu, Chem. Soc. Rev. 2020, 11, 3484–3524.[2] T. Reshetenko, A. Serov, M. Odgaard, G. Randolf, L. Osmieri, A. Kulikovsky, Electrochem. Commun. 2020, 118, 106795.[3] F. Xiao, G.-L. Xu, C.-J. Sun, I. Hwang, M. Xu, H.-W. Wu, Z. Wei, X. Pan, K. Amine, M. Shao, Nano Energy 2020, 77, 10592.[4] A. K. Mechler, N. R. Sahraie, V. Armel, A. Zitolo, M. T. Sougrati, J. N. Schwämmlein, D. J. Jones, F. Jaouen, J. Electrochem. Soc. 2018, 165, F1084.[5] W. Liao, S. Zhou, Z. Wang, F. Liu, H. Pan, T. Xie, Q. Wang, ChemCatChem 2021, 13, 23, 4925-4930. Figure 1