Non-Precious ORR Catalyst for PEMFC Application Using Iron, Nitrogen and Sulfur Co-Doped Few Layer Graphene

Abstract

The sluggish kinetics of oxygen reduction reaction (ORR) at the cathode of proton exchange membrane fuel cells (PEMFC) demand a high loading of precious platinum (Pt) or Pt alloy electrocatalysts [1]. The extensive implementation of fuel cell technologies for stationary and vehicular applications, however, demands the replacement of high cost and less abundant Pt with low cost and natural abundant electrocatalysts. Recent studies show the use of various non-precious metals, metal alloys, metal oxides and nitrogen doped carbon as alternative catalysts for ORR [2, 3]. Even though, these non-precious metal catalysts experience severe dissolution and large agglomeration under vigorous PEMFC operational conditions and subsequent catalyst performance degradation. In this work, we developed a facile method for making non-precious ORR catalyst using nitrogen (3.7 at. %), sulfur (2.4 at. %) and iron (1.3 at. %) co-doped porous graphene (Fe-NSG). The ORR activity and durability of catalyst in acidic media was investigated by half (Fig. 1) and full cell electrochemical measurements. RDE measurements of the Fe-NSG catalyst confirm a four electron transfer ORR process with high current density. PEMFC full cell measurements of the catalyst give a maximum power density of 225 mW cm-2 at 80 °C with good stability [4]. The high ORR activity and good stability of Fe-NSG electrocatalyst can be ascribed due to, (a) the comparatively higher electronegativity of doped N and S atoms (N: 3.04, S: 2.58) with respect to carbon (C: 2.55), which induces more charged sites within the graphene lattice favorable for oxygen adsorption and reduction, (b) presence of the large amount of pyridinic nitrogen, which can coordinate with iron cations and increase the density of ORR active Fe-Nx centers, (c) an optimum amount of sulfur within the non-precious catalyst suppress the iron carbide formation, and increases the formation of Fe-N4 ORR active species, (d) the unique confined morphology of Fe nanoparticles within the graphene layers suppress the agglomeration/dissolution of metal particles and increase their interfacial contact and durability. References F. Jaouen, E. Proietti, M. Lefevre, R. Chenitz, J.-P. Dodelet, G. Wu, H. T. Chung, C. M.Johnston and P. Zelenay, Energy & Environmental Science, 2011, 4, 114-130.Z. Chen, D. Higgins, A. Yu, L. Zhang and J. Zhang, Energy & Environmental Science, 2011, 4, 3167-3192.M. Lefèvre, E. Proietti, F. Jaouen and J.-P. Dodelet, Science, 2009, 324, 71-74.B. P. Vinayan, T. Diemant, R. J. Behm and S. Ramaprabhu, RSC Advances, 2015, 5, 66494-66501. Figure 1