Atomically Dispersed Fe‐N_x/C Electrocatalyst Boosts Oxygen Catalysis via a New Metal‐Organic Polymer Supramolecule Strategy

Abstract

the-state-of-the-art ORR catalysts but their uses are largely restricted by the prohibitive cost and limited activity/stability. [3][4][5][6][7] In this regard, the development of non-Pt group metal (non-PGM) catalysts derived from earth-abundant elements for ORR is the fundamental solution for the widespread applications of PEMFCs. [8][9][10] Among various non-PGM ORR catalysts developed in last decade, transition metalnitrogen-carbon (M-N-C) catalysts with M-N x coordination active sites embedded in the basal planes of carbon matrixes were the most promising ones due to their decent activity in both acidic and alkaline media and ease of scale-up production. [11][12][13][14][15] The ORR performance of M-N-C electrocatalysts in alkaline electrolyte has been well demonstrated notwithstanding, [9,[16][17][18][19][20] their performance in acidic environment is still deficient and often degrades rapidly due to the etching of metal species and/or decomposition of active sites. [21,22] Generally, the M-N-C catalysts are prepared via high-temperature (T > 800 °C) pyrolysis process of transition metal (e.g., Fe, Co, Ni), nitrogen, and carbon precursors, during which the metal atoms are very easy to agglomerate into large particles. The aggregated metal and metal oxide/carbide particles will hinder the accessibility of M-N x /C active sites and lower the utilization of M atoms seriously, thus compromising their ORR activity. Furthermore, metal aggregates will be easily etched away in acid, leading to The development of high-performance oxygen reduction reaction (ORR) catalysts derived from non-Pt group metals (non-PGMs) is urgent for the wide applications of proton exchange membrane fuel cells (PEMFCs). In this work, a facile and cost-efficient supramolecular route is developed for making non-PGM ORR catalyst with atomically dispersed Fe-N x /C sites through pyrolyzing the metal-organic polymer coordinative hydrogel formed between Fe 3+ and α-L-guluronate blocks of sodium alginate (SA). High-angle annular dark field scanning transmission electron microscopy (HAADF-STEM) and X-ray absorption spectroscopy (XAS) verify that Fe atoms achieve atomic-level dispersion on the obtained SA-Fe-N nanosheets and a possible fourfold coordination with N atoms. The best-performing SA-Fe-N catalyst exhibits excellentORR activity with half-wave potential (E 1/2 ) of 0.812 and 0.910 V versus the reversible hydrogen electrode (RHE) in 0.5 m H 2 SO 4 and 0.1 m KOH, respectively, along with respectable durability. Such performance surpasses that of most reported non-PGM ORR catalysts. Density functional theory calculations suggest that the relieved passivation effect of OH* on Fe-N 4 /C structure leads to its superior ORR activity to Pt/C in alkaline solution. The work demonstrates a novel strategy for developing high-performance non-PGM ORR electrocatalysts with atomically dispersed and stable M-N x coordination sites in both acidic and alkaline media.

Edge‐Rich Fe−N4 Active Sites in Defective Carbon for Oxygen Reduction Catalysis

“…[ 12 ] The typical 4e − transfer pathway toward ORR on these two models is given in Figure S1, Supporting Information, summarized as five sequential steps: (1) adsorption of molecular O 2 on Fe active sites; (2) one proton reacts with adsorbed O 2 for form OOH*; (3) OOH* transforms into O*; (4) O* reacts with one proton to form OH*; (5) further combination with one proton and electron converts OH* to H 2 O. [ 13 ] Gibbs free energy diagrams at different equilibrium potential (U) are further calculated to theoretically predict the ORR activity difference between c‐ND−Fe and e‐ND−Fe (Figure 1c,d). At U = 0 V (ideal electrode potential for ORR), all the elementary steps are thermodynamically downslope, indicating the process is exothermic under this condition.…”

Densely Populated Single Atom Catalysts

“…Electrochemical energy storage and conversion technology have been widely studied to produce sustainable and renewable energy . Many advanced clean energy technologies, such as fuel cells, metal air batteries, and water splitting, etc., require highly active catalysts to lower the energy barrier and increase the reaction rate with an efficient and stable route.…”

Synthesis and Active Site Identification of Fe−N−C Single‐Atom Catalysts for the Oxygen Reduction Reaction

“…For example, Manthiram and co‐workers reported an iron‐engaged PDA‐derived coating approach to embed the FeN x moiety into a carbon nanotube (Figure B) . Li and co‐workers developed a supramolecular route to fabricate Fe−N−C SAC through pyrolyzing a coordinate of iron‐sodium alginate (rich with hydrophilic groups, e. g., −COOH, −OH) with cyanamide as nitrogen source (Figure C). In addition, N‐bearing small molecules, such as ethylenediaminetetraacetic acid (EDTA), dicyandiamide (DCDA) and glucosamine, were also used as chelating agents to guarantee the homogeneous dispersion of Fe ions in N and C components at atomic scale.…”