Direct atomic-level insight into the active sites of a high-performance PGM-free ORR catalyst

Abstract

Platinum group metal-free (PGM-free) metal-nitrogen-carbon catalysts have emerged as a promising alternative to their costly platinum (Pt)-based counterparts in polymer electrolyte fuel cells (PEFCs) but still face some major challenges, including (i) the identification of the most relevant catalytic site for the oxygen reduction reaction (ORR) and (ii) demonstration of competitive PEFC performance under automotive-application conditions in the hydrogen (H)-air fuel cell. Herein, we demonstrate H-air performance gains achieved with an iron-nitrogen-carbon catalyst synthesized with two nitrogen precursors that developed hierarchical porosity. Current densities recorded in the kinetic region of cathode operation, at fuel cell voltages greater than ~0.75 V, were the same as those obtained with a Pt cathode at a loading of 0.1 milligram of Pt per centimeter squared. The proposed catalytic active site, carbon-embedded nitrogen-coordinated iron (FeN), was directly visualized with aberration-corrected scanning transmission electron microscopy, and the contributions of these active sites associated with specific lattice-level carbon structures were explored computationally.

Rational Design and Synthesis of Low-Temperature Fuel Cell Electrocatalysts

“…Recently, Zelenay et al [104] reported a direct observation of FeN 4 moiety with aberration-corrected scanning transmission electron microscopy and high-resolution electron energy loss spectroscopy (EELS) (Fig. 10a, Fig.…”

“…9 Illumination of active sites in carbon-based catalysts 2) in a few-layer graphene sheet. [104]. b EEL spectra of the N K-edge (N k ) and Fe L-edge (Fe L ) acquired from single atoms (1 and 2) and few-layer graphene (3), demonstrate the presence of N around Fe atoms.…”

Morphology-Controlled Nitrogen-Containing Polymers as Synthetic Precursors for Electrochemical Oxygen Reduction Fe/N/C Cathode Catalysts

“…Figure 1 shows the most commonly proposed active sites in ORR catalysts. One particularly convincing model involves a catalytic center based on metal-N coordination, which was proposed based on experimental evidence from time-of-flight secondary ion mass spectrometry (TOF-SIMS), high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM), and extended X-ray absorption fine structure (EXAFS) [12][13][14]. Simultaneously, metal-free N-doped carbon species with pyridinic and graphitic nitrogen have also been suggested to be responsible for the observed ORR activity based on experimental [15][16][17] and theoretical studies [18][19][20].…”

Direct Observation of Fe-N4 Species as Active Sites for the Electrocatalytic Oxygen Reduction

“…6 Aniline was employed together with cyanamide (CM) as the N and C sources. Fe precursor was added at the same step of dissolution of aniline and CM.…”