Single Atomic Iron Catalysts for Oxygen Reduction in Acidic Media: Particle Size Control and Thermal Activation

Abstract

It remains a grand challenge to replace platinum group metal (PGM) catalysts with earth-abundant materials for the oxygen reduction reaction (ORR) in acidic media, which is crucial for large-scale deployment of proton exchange membrane fuel cells (PEMFCs). Here, we report a high-performance atomic Fe catalyst derived from chemically Fe-doped zeolitic imidazolate frameworks (ZIFs) by directly bonding Fe ions to imidazolate ligands within 3D frameworks. Although the ZIF was identified as a promising precursor, the new synthetic chemistry enables the creation of well-dispersed atomic Fe sites embedded into porous carbon without the formation of aggregates. The size of catalyst particles is tunable through synthesizing Fe-doped ZIF nanocrystal precursors in a wide range from 20 to 1000 nm followed by one-step thermal activation. Similar to Pt nanoparticles, the unique size control without altering chemical properties afforded by this approach is able to increase the number of PGM-free active sites. The best ORR activity is measured with the catalyst at a size of 50 nm. Further size reduction to 20 nm leads to significant particle agglomeration, thus decreasing the activity. Using the homogeneous atomic Fe model catalysts, we elucidated the active site formation process through correlating measured ORR activity with the change of chemical bonds in precursors during thermal activation up to 1100 °C. The critical temperature to form active sites is 800 °C, which is associated with a new Fe species with a reduced oxidation number (from Fe to Fe) likely bonded with pyridinic N (FeN) embedded into the carbon planes. Further increasing the temperature leads to continuously enhanced activity, linked to the rise of graphitic N and Fe-N species. The new atomic Fe catalyst has achieved respectable ORR activity in challenging acidic media (0.5 M HSO), showing a half-wave potential of 0.85 V vs RHE and leaving only a 30 mV gap with Pt/C (60 μg/cm). Enhanced stability is attained with the same catalyst, which loses only 20 mV after 10 000 potential cycles (0.6-1.0 V) in O saturated acid. The high-performance atomic Fe PGM-free catalyst holds great promise as a replacement for Pt in future PEMFCs.

Towards High‐Performance Electrocatalysts for Oxygen Reduction: Inducing Atomic‐Level Reconstruction of Fe‐Nx Site for Atomically Dispersed Fe/N‐Doped Hierarchically Porous Carbon

“…%), the Fe-N-HPC-A possessed al ower total N content of 3.2 at. [9,33,41] Based on the existence of isolated single Fe atomic sites, as identified by HAADF-STEM image, combined with the different configuration of pyridinic Na nd graphitic N, we do speculate that the pyridinicNoffers coordination sites to Fe atoms to form the Fe-N x moieties for increasing the density of active sites. The subsequent heat-treatment led to the total decomposition of unsta- Chem.E ur.J.…”

“…%), N( 1.1 at. [9,33] Electrochemical measurement of ORR activity Further rotating ring-diske lectrode (RRDE) measurements indicated that Fe-N-HPC-AH also possessedahigh transferred electron number (> 3.9, highert han those of Fe-N-HPCa nd Fe-N-HPC-A and similar with the commercial Pt/C) with av ery low percentage (< 5%)o fp eroxide yield (% HO 2 À )i nt he whole measured potentialr ange, suggestingt hat the ORR catalysed by Fe-N-HPC-AH progressedt hrough an almost ideal 4-electron path (Figure 3c). %), but the Fe element was hardly detected (Table S1 and Figure S4 in Supporting Information).…”

“…[4] Great efforts have been devotedt ot he development of lowcost and high-performance non-precious metal electrocatalysts (NPMCs) for ORR, including transition metal-nitrogen co-doped carbon (TM-N-C, TM:F e, Co, Cu, Ni,e tc. [9] The deliberate investigation of TM-N-C electrocatalysts can be traced back to 1964 when Jasinski found that cobalt phthalo-cyanine exhibited ORR activity in alkaline electrolyte. [5][6][7][8] Amongt hem, the TM-N-C electrocatalysts are regardeda so ne kind of the most promising NPMCs for ORR, duet ot heir relativelyh igh activity and durability.…”

Bottom‐Up Fabrication of a Sandwich‐Like Carbon/Graphene Heterostructure with Built‐In FeNC Dopants as Non‐Noble Electrocatalyst for Oxygen Reduction Reaction

“…In this context, the development of efficient non-noble catalysts for ORR has been the focus of many researchers aroundt he world. [4] Of those strategies been reported, [5] using metal-organic frameworks (MOFs) has been proved as an effectivem ean to immobilize highly dispersing MN x active sites in resulting carbonized materials because of their highly ordered structure and uniformly distributed metal struts. [3] It has been revealed that highly dispersing MN x sites embedded in carbon matrix could render the resulting catalystw ith high catalytic activity that rival conventionalP t-based catalysts.…”

Modulierung der elektronischen Strukturen anorganischer Nanomaterialien für eine effiziente elektrokatalytische Wasserspaltung

“…[11,12,19,72] Die Elektronenpolarisierung an der Dotierungsstelle kçnnte die Elektronenaffinitäts owohl der dotierenden Metallatome als auch der ursprünglichen Nichtmetallatome abstimmen und hierdurch die Reaktionskinetik des Ladungstransfers in Nachbarschaft zur Dotierungsstelle verbessern. [74][75][76] Daher kann man durch Dotierung mit Metallatomen wegen deren unvollständiger Koordination nicht nur die Anzahl der aktiven Zentren erhçhen, sondern auch die benachbarten Nichtmetallatome durch Elektronenpolarisierung aktivieren. [66,73] Währenddessen haben die üblicherweise eingesetzten, Kohlenstoff-basierten Nichtmetall-Trägermaterialien typischerweise große Oberflächen und hervorragende Leitfähigkeiten.…”