Optimized Synthesis of Fe/N/C Cathode Catalysts for PEM Fuel Cells: A Matter of Iron–Ligand Coordination Strength

Abstract

Hydrogen-air polymer-electrolyte-membrane fuel cells (PEMFCs) show promise for the replacement of gasoline internal-combustion engines for vehicle propulsion and other applications. However, the high cost of components, which is largely due to the use of platinum-based catalysts for the O 2reduction reaction (ORR), remains an impediment. [1] For a production of 500 000 PEMFC stacks a year, electrocatalysts alone were estimated to account for nearly half the cost of a stack. [2] Recent studies on pyrolyzed Fe/nitrogen/carbon and Co/ nitrogen/carbon catalysts for the ORR have increased their initial performance close to the level reached by platinumbased catalysts, and other studies have demonstrated promising durability. [3] We reported the use of a Zn II zeolitic imidazolate framework (ZIF) as a microporous support for ferrous acetate (Fe II Ac 2 ) and 1,10-phenanthroline to prepare a catalyst precursor which, after pyrolysis in Ar and then in NH 3 , resulted in unprecedented activity and power performance. [4] The investigated ZIF, referred to as ZIF-8, was a commercial product (Basolite Z1200 from BASF). ZIFs are a subclass of metal-organic frameworks (MOFs), which were first used for the preparation of platinum-free catalysts by Liu and co-workers. [5] MOFs are now actively investigated for electrochemical applications. [6] Herein, we describe our investigations on the replacement of 1,10-phenanthroline (phen) with 2,4,6-tris(2-pyridyl)-striazine (TPTZ) in our synthesis with ZIF-8. The use of TPTZ was investigated previously by Zhang and co-workers, who used a high-surface-area carbon material as a host. [7] However, the current density in the resulting PEMFC was only approximately 0.1 A cm À2 at 0.6 V, [7c] as compared to the value of 1.2 A cm À2 observed with Fe/phen/ZIF-8 precursors. [4] We show herein that a high performance can also be reached with TPTZ by the use of an appropriate synthesis procedure based on an improved understanding of the coordination chemistry of the Fe II /ligand/ZIF-8 catalyst precursor.We first prepared an Fe/TPTZ/ZIF-8 catalyst precursor of weight composition 1:10:90 (see the Supporting Information) by wet impregnation followed by drying and planetary ball milling. This composition results in a TPTZ/Fe molar ratio of about 2:1. The blue color characteristic of [Fe II (TPTZ) 2 ] was immediately observed when Fe II Ac 2 and TPTZ were dissolved. ZIF-8 was then dispersed in the solution, whose color slowly changed to gray-blue and then ochre. The absorption peak at 596 nm characteristic of [Fe II (TPTZ) 2 ] [8] was no longer observed in the UV/Vis spectrum after 2 h (Figure 1 a). Thus, [Fe(TPTZ) 2 ] had reacted with ZIF-8. We expected 2methyl-imidazole (2-MeIm), the structuring ligand of ZIF-8, to compete with TPTZ for ferrous cations. Indeed, the absorption peak of [Fe(TPTZ) 2 ] also vanished after the addition of 2-MeIm (see Figure S1 in the Supporting Information). In contrast, this competition for Fe II cations between 2-MeIm of ZIF-8 and the phen ligand was not observed for th...

Fe/N/C hollow nanospheres by Fe(iii)-dopamine complexation-assisted one-pot doping as nonprecious-metal electrocatalysts for oxygen reduction

“…Figure 6D shows that compared with the samples without hol lo w cor e ( F e/N / C N Ss -750 w ith ou t t e mpl a t e an d Fe/N/C@SiO 2 -750 without removing the template) and the sample with thicker shell (Fe/N/C HNSs-750-2L), Fe/N/C HNSs-750 shows more positive onset and half-wave potentials, and higher current density over the whole potential range. 6 The hollow, porous and thin-shell morphology with high specific surface area allows more exposure of the active sites to the electrolyte, better utilization of the active materials, and more efficient transport and To further investigate the pathway and kinetic process of the ORR, polarization curves of the Fe/N/C HNSs-750 catalyst were recorded using RDE technique at different rotation rates (100, 400, 900, 1600 and 2500 rpm) in O 2 -saturated solution with a scan rate of 10 mV/s ( Figure 6E).…”

“…Owing to the sluggish kinetics of cathodic oxygen reduction reaction (ORR) in these devices, efficient ORR electrocatalysts are demanded for practical applications. [3][4][5][6][7] A promising approach is to introduce nonprecious metals, such as Fe, Co or Mn, or their compounds into nitrogen-doped carbon materials, e.g., N-doped carbon nanotubes, graphene and amorphous activated carbon, 3,[8][9][10][11][12] because N-doped carbon also exhibits good electrocatalytic activities towards ORR 13 and may work synergistically with the metal species, providing significantly improved catalyzing efficiency. 2 However, high cost, scarcity, methanol crossover effect and poor durability of the Pt-based catalysts hinder their largescale applications.…”

Cobalt and Iron Complexes with N-heterocyclic Ligands as Pyrolysis Precursors for Oxygen Reduction Catalysts

“…The precursor materials were prepared following a similar procedure to previous reports [26,29,33]. In general, 40 mg of 6 ] 2+ solution thus obtained, a four-fold molar excess of the mono dentate ligands (i.e.…”

“…The presence of the metal would facilitate the incorporation of N-atoms during the pyrolysis in the proper conformation [28]. Some authors synthetized metal-free catalysts employing metal complexes or, more recently, metal organic frameworks (MOFs), followed by acid leaching, but traces of metal may be still present in the final product [29][30][31]. Moreover, in many cases, the acid treatment performed after the pyrolysis to remove unwanted or unreactant metallic particles, produces a reduction of the ORR activity.…”

A resin-based methodology to synthesize N-doped graphene-like metal-free catalyst for oxygen reduction

“…Even so, the Fe (Co)-N-C catalysts are regarded as the best alternative non-precious metal catalysts (NPMCs) at the moment. To synthesize Fe(Co)-N-C catalysts, different nitrogen-containing precursors like cyanamide [7], melamine [9], tripyridyltriazine [10] and many others have been investigated.…”