FeNC Oxygen Reduction Electrocatalyst with High Utilization Penta‐Coordinated Sites

Abstract

Atomic Fe in N-doped carbon (FeNC) electrocatalysts for oxygen (O 2 ) reduction at the cathode of proton exchange membrane fuel cells (PEMFCs) are the most promising alternative to platinum-group-metal catalysts. Despite recent progress on atomic FeNC O 2 reduction, their controlled synthesis and stability for practical applications remains challenging. A two-step synthesis approach has recently led to significant advances in terms of Fe-loading and mass activity; however, the Fe utilisation remains low owing to the difficulty of building scaffolds with sufficient porosity that electrochemically exposes the active sites. Herein, we addressed this issue by coordinating Fe in a highly porous nitrogen doped carbon support (~3295 m 2 g -1 ), prepared by pyrolysis of inexpensive 2,4,6triaminopyrimidine and a Mg 2+ salt active site template and porogen. Upon Fe coordination, a high electrochemical active site density of 2.54×10 19 sites g FeNC -1 and a record 52% FeN x electrochemical utilisation based on in situ nitrite stripping was achieved. The Fe single atoms are characterised pre-and post-electrochemical accelerated stress testing by aberration-corrected high-angle annular dark field scanning transmission electron microscopy, showing no Fe clustering. Moreover, ex situ X-ray absorption spectroscopy and low-temperature Mössbauer spectroscopy suggest the presence of penta-coordinated Fe sites, which were further studied by density functional theory calculations. catalysts [7,8] or introducing axial ligands. Different FeN x active site axial ligands have been recently proposed following in situ Mössbauer, x-ray absorption spectroscopy, nuclear inelastic scattering or electron paramagnetic resonance. [9,10] Some of them bearing close resemblance to biological systems, [11] such as N axially coordinated FeN 4 sites resembling heme. [12] However, spectroscopic discernibility is often challenging in these typically heterogeneous FeNC catalysts, [13] therefore experimental structure-activity correlations are hard to conclude. To overcome experimental limitations, the effect of O axial ligands on model FeNC systems has been calculated by density functional theory, [14,15] although the effect of other possible axial ligands on different Fe sites (pyridinic and pyrrolic) has not been fully considered. [16] Alternatively, to improve catalyst performance, the number of active sites can be increased, an approach which has shown significant progress in recent years. [17] To selectively form a high density of atomic Fe sites and avoid undesired Fe-induced carbothermal reduction, Fellinger and coworkers first identified that the high temperature pyrolytic step (800-1000ºC) should be decoupled from the Fe loading, by using a suitable N x site template. [17][18][19][20] The decoupled two-step synthetic approach to prepare FeNC O 2 reduction catalysts has led to remarkable progress; Mehmood et al. recently showed bulk FeN x site density (SD Mössbauer, , Eq. 1-2) up to 7.4×10 20 sites g FeNC -1 . The reported in situ nitrite strippin...

Reaching the Fundamental Limitation in CO2 Reduction to CO with Single Atom Catalysts

“…6H 2 O at 900 °C ( Figure ). [ 23 ] This process allows TAP to self‐organize by hydrogen bonding between the amine groups and the water molecules of MgCl 2 . 6H 2 O before polymerizing in a molten state.…”

“…[ 21,22 ] However, very recently we reported a synthetic protocol to prepare highly electrochemically accessible FeNC single atom catalysts (FeN x electrochemical utilization >50%) employing a decoupled two‐step synthesis that entailed low temperature metalation of a templated micro‐ and mesoporous catalyst. [ 23 ]…”

“…We build on our previously reported synthetic protocol that provides MNC materials with optimum utilization that allows us to maximize TOF and reach the highest values reported in the state‐of‐the‐art (6.8 e − site −1 s −1 at −0.59 V vs reversible hydrogen electrode, RHE). [ 23 ] The active site structure of the prepared catalysts is elucidated by means of XAS, the atomic dispersion by high angle annular dark field transmission electron microscopy (HAADF‐TEM), and the effect of axial ligands in the binding energy of reaction intermediates is estimated by DFT calculations. Additionally, the stability of the catalyst is assessed through time‐of‐flight secondary ion mass spectrometry (ToF‐SIMS) pre and post extended measurements.…”

Co-Doped Ni3V2O8 Nanofibers Achieving d–d Orbital Coupling for Electrocatalytic Oxygen Reduction

“…Consequently, it is very essential to explore alternate non-PGM electrocatalysts for boosting ORR performance in fuel cells. 6 Transition-metal vanadium-based blended metal oxides, especially for Ni 3 V 2 O 8 , have been of great interest as an alternative candidate for PGM components in electrocatalysts because of their plentiful valence states, abundant reserves, and low prices. 7,8 But Ni 3 V 2 O 8 has always been limited by low conductivity and sluggish kinetics.…”

“…At present, platinum (Pt)-group metal (PGM) components are improving the sluggish cathodic ORR kinetics and exhibit acceptable electrocatalytic ORR performance, , albeit at a high cost and low natural reserves. Consequently, it is very essential to explore alternate non-PGM electrocatalysts for boosting ORR performance in fuel cells …”