Low Site Density Research Articles

Candied Haws-Like Fe–N–C Catalysts with Broadened Carbon Interlayer Spacing for Efficient Zinc–Air Battery

As efficient nonprecious metal catalysts for oxygen reduction reaction (ORR), Fe-N-C materials are one of the most promising alternatives to Pt-based catalysts for fuel cells and metal-air batteries. However, the intrinsically low density of key active sites like FeN4 moieties hampers their commercial applications. Herein, we provide a smart strategy to construct a candied haws-like Fe-N-C catalyst (CH-FeNC) with broadened carbon interplanar spacing (>4 Å), starting with trehalose as a structure-built brick coupled with a zinc-zeolite imidazole framework (ZIF-8) and polyaniline (PANI) and then followed by copyrolysis carbonization of them. The obtained CH-FeNC exhibits half-wave potentials of 0.92 and 0.90 V (vs RHE) before and after 10,000 cycles in 0.1 M KOH, which are superior to the 0.90 and 0.85 V obtained by commercial Pt/C for ORR. The power density of a homemade zinc-air battery equipped with the catalyst is up to 131 mW cm-2, greater than that of Pt/C (124 mW cm-2). The extended X-ray absorption fine structure (EXAFS) results and density functional theory (DFT) theoretical calculations reveal that there exists enriched zigzag or armchair edge-hosted FeN4 active sites, located at the abundant interface between carbon components in this composite. Furthermore, the unique broadened carbon interlayer spacing plays a key role in deciding the ORR rate in alkaline but not in acidic environments because there exists a fifth ligand of active Fe in the form of FeN4 centers coupled with SO42- and ClO4- from acids.

Porous Cobalt-nickel phosphides prepared from Al-doped NiCo-LDH precursors for supercapacitor and electrocatalysis applications

The relatively low density of active sites and poor ions transport of electrode materials are the two main barriers hindering the applications of supercapacitors and electrocatalysts. Herein, ultrafine porous nickel cobalt phosphides (NiCoP-P) nanosheets are constructed by using aluminum (Al)-doped NiCo layered double hydroxides as precursors with followed removal of Al species and subsequent phosphorization. Benefiting from the Al-induced rich active sites, ultrafine porous nanosheet structure and fast ions transport, the optimized NiCoP-P electrode delivers a high specific capacity of 1059C·g−1 at 1 A·g−1 and possesses 71 % capacitance retention even at 50 A·g−1. An asymmetric supercapacitor achieves a high energy density of 51.7 Wh·kg−1 and long-term cyclic stability (99 % capacity retention after 10,000 cycles) in KOH/PVA sol electrolyte. The optimized NiCoP-P electrocatalyst also shows great enhancement in the hydrogen evolution reaction (HER) with an onset potential of −10 mV and a Tafel slope of 60 mV·dec−1 in alkaline electrolyte. Impressively, the HER performance can still be maintained at 93.1 % after 100 h stability test, while the commercial Pt/C catalyst reduces to 71.7 % in 40 h. Our results highlight that Al doping in precursors plays key roles in creating porous nanostructured electrode materials with rich active sites, while the residual metal oxides and Al species in the pores may provide abundant hydrophilic hole surface for fast ions transport, which are essential to heighten the property of various energy-related storage and catalysis.

Ru monolayer island doped MoS2 catalysts for efficient hydrogen evolution reaction

Developing highly-efficient hydrogen evolution reaction (HER) catalysts is significantly important for hydrogen generation by water splitting. In this work, Ru monolayer islands (MIs)-doped MoS2 catalysts are synthesized controllably by high-gravity technology. Ru MIs are successfully anchored on the MoS2 nanosheets via the coordination of Ru and S species in the MoS2 matrix to form monolayer island structure, which not only provides high-density active centers for boosting the HER activity, but also exhibits the maximum atomic utilization. The prepared Ru MIs-doped MoS2 catalysts exhibit a remarkably low overpotential of 17 mV at the current density of 10 mA cm−2 (η10) in alkaline conditions. Density functional theory (DFT) results reveal that multi-Ru monolayer doping can efficiently tailor the electronic structure and improve the adsorption of hydrogen on MoS2, and thereby boost the catalytic activity of HER. This work provides a Ru MIs doping approach to enhance the density of active sites of electrocatalysts, which would overcome the bottleneck of low active site density of single-atom catalysts to design high-efficient electrocatalysts with maximum atomic utilization.

Synthesis of very small molybdenum disulfide nanoflowers for hydrogen evolution reaction

Among various transition metal dichalcogenides, molybdenum disulfides such as molybdenum disulfide nanoflowers (MoS2 NFs) can effectively catalyze a hydrogen evolution reaction (HER) because of the abundance, ease of processing, and high catalytic activity of MoS2. The main disadvantage of using MoS2 NFs for HER on the industrial scale is their low density and number of active sites. Herein, we propose for the first time a facile, inexpensive, and scalable route for fabricating extremely small MoS2 NFs (SNFs). The size of the synthesized MoS2 SNFs (50–90 nm) is much lower than that of conventional MoS2 NFs (900–1500 nm), which significantly increases the number of catalytically active sites. In addition, the in situ doping of N atoms considerably enhances the catalytic activity of the prepared MoS2 SNFs. MoS2 SNFs exhibit superior electrocatalytic activity toward HER with a low Tafel slope 49 mV·dec−1, an overpotential of 270 mV at a current density of 50 mA·cm−2, a large surface area of 98 m2·g−1, and very high stability in an acidic environment. The obtained results indicate that MoS2 SNFs can be potentially used for energy storage and electrochemical applications.

The Understanding of the Improved Catalytic Performance of Steam-Activated Fe-N-C Catalyst for Alkaline Hydrazine Fuel Cell

Fe-N-C materials are currently the most promising non-noble-metal-based ORR electrocatalyst, but their low density of surface active sites compared with Pt/C necessitates improving their utilization via porous structures. Previously, we studied modified Fe-N-C catalyst for the oxygen reduction reaction (ORR) in the cathodes of alkaline hydrazine liquid fuel cells (AHFC). Fe-N-C with magnetite particles and hierarchical pore structure was prepared by steam activation. The steam activation process significantly improves the power performance of the AHFC as indicated by the lower IR and activation voltage losses. The steam activation transformed the Fe3C nanoparticles in Fe-N-C into larger Fe3O4 particles, which caused the electrode surface to become more hydrophilic. Here, elimination of the Fe3O4 nanoparticles by acid leaching was conducted to investigate the effect of Fe3O4 nanoparticles, resulting in enhancing power performance for alkaline hydrazine liquid fuel cells.

General Carbon-Supporting Strategy to Boost the Oxygen Reduction Activity of Zeolitic-Imidazolate-Framework-Derived Fe/N/Carbon Catalysts in Proton Exchange Membrane Fuel Cells.

The oxygen reduction reaction (ORR) activity of the Fe/N/Carbon catalysts derived from the pyrolysis of zeolitic-imidazolate-framework-8 (ZIF-8) has been still lower than that of commercial Pt-based catalysts utilized in the proton exchange membrane fuel cells (PEMFCs) due to low density of accessible active sites. In this study, an efficient carbon-supporting strategy is developed to enhance the ORR efficiency of the ZIF-derived Fe/N/Carbon catalysts by increasing the accessible active site density. The enhancement lies in (i) improving the accessibility of active sites via converting dodecahedral particles to graphene-like layered materials and (ii) enhancing the density of FeNx active sites via suppressing the formation of nanoparticles as well as providing extra spaces to host active sites. The optimized and efficient Fe/N/Carbon catalyst shows a half-wave potential (E1/2) of 0.834 V versus reversible hydrogen electrode in acidic media and produces a peak power density of 0.66 W cm-2 in an air-fed PEMFC at 2 bar backpressure, outperforming most previously reported Pt-free ORR catalysts. Finally, the general applicability of the carbon-supporting strategy is confirmed using five different commercial carbon blacks. This work provides an effective route to derive Fe/N/Carbon catalysts exhibiting a higher power density in PEMFCs.

Multiple Surface Site Three-Dimensional Structure Determination of a Supported Molecular Catalyst.

The structural characterization of supported molecular catalysts is challenging due to the low density of active sites and the presence of several organic/organometallic surface groups resulting from the often complex surface chemistry associated with support functionalization. Here, we provide a complete atomic-scale description of all surface sites in an N-heterocyclic carbene based on iridium and supported on silica, at all stages of its synthesis. By combining a suitable isotope labeling strategy with the implementation of multinuclear dipolar recoupling DNP-enhanced NMR experiments, the 3D structure of the Ir-NHC sites, as well as that of the synthesis intermediates were determined. As a significant fraction of parent surface fragments does not react during the multistep synthesis, site-selective experiments were implemented to specifically probe proximities between the organometallic groups and the solid support. The NMR-derived structure of the iridium sites points to a well-defined conformation. By interpreting EXAFS spectroscopy and chemical analysis data augmented by computational studies, the presence of two coordination geometries is demonstrated: Ir-NHC fragments coordinated by a 1,5-cyclooctadiene and one Cl ligand, as well as, more surprisingly, a fragment coordinated by two NHC and two Cl ligands. This study demonstrates a unique methodology to disclose individual surface structures in complex, multisite environments, a long-standing challenge in the field of heterogeneous/supported catalysts, while revealing new, unexpected structural features of metallo-NHC-supported substrates. It also highlights the potentially large diversity of surface sites present in functional materials prepared by surface chemistry, an essential knowledge to design materials with improved performances.

Zinc-assisted MgO template synthesis of porous carbon-supported Fe-Nx sites for efficient oxygen reduction reaction catalysis in Zn-air batteries

Atomically dispersed iron-nitrogen-carbon catalysts offer great potential in oxygen reduction reaction (ORR), yet the poor exposure and low density of Fe-Nx sites causes relatively low ORR activity. Herein, a zinc-assisted MgO template strategy is reported to construct porous carbon-supported Fe-N4 sites (Fe-N-C). Iron atoms surrounded by zinc species are converted to abundant Fe-N4 sites rather than Fe containing nanoparticles. Meanwhile, both the zinc species and the MgO template can effectively produce porous structure so as to increase the utilization of Fe-N4 sites. Fe-N-C achieves superior ORR performance and stability in alkaline medium. Theoretical calculations manifest that Fe-N4 sites can narrow the energy barrier for ORR. Moreover, finite element simulation exhibits the porous framework in Fe-N-C could significantly accelerate the diffusion of O2. Therefore, Fe-N-C provides a high peak power density and superior discharge ability toward Zn-air batteries.

Host-guest interactions promoted formation of Fe-N4 active site toward efficient oxygen reduction reaction catalysis

FeNC is the most promising material to replace the noble metal catalyst for cathodic oxygen reduction reaction in proton exchange membrane fuel cells (PEMFCs). However, the practical performance of FeNC catalyst is significantly limited by its low active site (Fe-N4) density. Herein, we propose to promote the formation of Fe-N4 active sites in FeNC catalyst by strengthening the interaction of N precursors and Fe precursors during the carbonization synthesis. In our approach, ionic liquid (IL, [EMIM][NTf2]) with high nitrogen content and good thermal stability is caged in the pores of Fe-ZIF-8 through the host–guest interactions. These interactions are critical for the preservation of Fe and N species and formation of active sites during the synthesis. The optimal catalyst developed with this approach (Fe0.05NC/10) has a high density of accessible Fe-N4 sites (1.88*1019 sites g−1). Therefore, in both acidic and alkaline media, Fe0.05NC/10 showed excellent ORR activity comparable to commercial Pt/C catalyst. Moreover, PEMFC performance with a peak power density of 300 mW cm−2 was demonstrated with Fe0.05NC/10 under H2/O2 conditions. The synthetic approach reported herein may be used for tailoring of advanced catalyst with high intrinsic activity.

Generating TON zeolites with reduced [0 0 1] length through combined mechanochemical bead-milling and porogen-directed recrystallization with enhanced catalytic property in hydroisomerization

Mechanochemical bead-milling of micron-meter sized zeolites to nanometers provides an effective route to enhance their diffusion and catalytic properties. This post-synthetic method is particularly important to reduce crystals of unidimensional zeolites preferentially along their channel direction, as direct synthesis is difficult to achieve since they habitually grow into rod-like crystals. The milling process is often escorted by surface amorphorization and structure degradation, hence, a subsequent recrystallization is often entailed to restore crystallinity. Herein, taken ZSM-22 as an example, it is demonstrated that secondary crystal growth and coalescence during recrystallization, a process that often offset the effect of milling, could be suppressed by the introduction of porogens in the post-milling recrystallization process. The combined bead-milling and porogen-directed recrystallization method allows the preparation of highly crystalline TON crystals with an axis length of largely 100–300 nm, which is record short in axis [001] channel length. The Si/Al ratio could be manipulated between 40 and 300, permitting the influence of acidity and diffusion to be evaluated. The catalytic assessments in n-heptane hydroisomerization demonstrate that isomer yields can be enhanced from 56% for parent material to up to 74% for an optimized catalyst with Si/Al of 200. Correlating of zeolite structure, kinetic studies and catalytic behavior discloses that low acid site density and improved diffusion property are critical factors to decrease cracking probability and improve isomerization selectivity. This study opens new ways to tailor diffusion properties of unidimensional zeolites, and are potentially extendable to other zeolites.

Single-Atom Pt Boosting Electrochemical Nonenzymatic Glucose Sensing on Ni(OH)2/N-Doped Graphene.

Conventional nanomaterials in electrochemical nonenzymatic sensing face huge challenge due to their complex size-, surface-, and composition-dependent catalytic properties and low active site density. In this work, we designed a single-atom Pt supported on Ni(OH)2 nanoplates/nitrogen-doped graphene (Pt1/Ni(OH)2/NG) as the first example for constructing a single-atom catalyst based electrochemical nonenzymatic glucose sensor. The resulting Pt1/Ni(OH)2/NG exhibited a low anode peak potential of 0.48 V and high sensitivity of 220.75 μA mM-1 cm-2 toward glucose, which are 45 mV lower and 12 times higher than those of Ni(OH)2, respectively. The catalyst also showed excellent selectivity for several important interferences, short response time of 4.6 s, and high stability over 4 weeks. Experimental and density functional theory (DFT) calculated results reveal that the improved performance of Pt1/Ni(OH)2/NG could be attributed to stronger binding strength of glucose on single-atom Pt active centers and their surrounding Ni atoms, combined with fast electron transfer ability by the adding of the highly conductive NG. This research sheds light on the applications of SACs in the field of electrochemical nonenzymatic sensing.

Conversion of glycerol over vanadium supported beta zeolite: Role of acidity and alkali cations

Different vanadium supported beta zeolites modified with Cs and K, were used as catalysts for the conversion of glycerol, in gas phase, without any external reductant. The catalytic data show that all the catalysts studied are active in the conversion of glycerol and that the presence of vanadium oxide species promotes the formation of allyl alcohol. Moreover, modification of the vanadium-based catalysts with Cs or K changes the vanadium species and puts in evidence the importance of the acidity in this process. A low density of acid sites favors the formation of allyl alcohol, in detriment of acrolein one. The data also indicate that the formation of these compounds is closely related and could be explained using the same mechanistic path. • Vanadium oxide species promote the direct formation of allyl alcohol from glycerol. • The catalytic behavior depends on the acidity and presence of alkaline metals. • Formation of allyl alcohol and acrolein is intimately related

Optimization of Low Site Density Area for 4G Network in Urban City

The development of telecommunications is currently growing rapidly, especially in urban areas. To obtain optimal data services for users, the performances of 4G network services must continue to be optimized. It is known that many users are scattered in urban areas, but sometimes it is not balanced with an even distribution of the site in this area. This condition occurs because the distribution of the site is not evenly optimal distributed, either due to licensing constraints, limited land access for site development, or in terms of plans that have not been made. Balanced with the requirement of a 4G network, which is required, this "empty space" condition or low site density condition must find a solution or optimize it. Many optimization methods can optimize the area with low site density possibility. This study will optimize the area by adding a new site proposal based on coverage planning. We need to analyze Reference Signal Received Power (RSRP) coverage signal distribution using Atoll Planning Software. After optimization, the RSRP level below or equal -80 dBm increased from 75.195% to 94.08%. Furthermore, the percentage calculation for inadequate coverage (below -80 dBm) decreased from 24.816% to 5.931%. This RSRP signal level also shows that the condition after optimization with a new site can improve the signal level condition from areas with low site density possibility.

Acylation of m-cresol with acetic acid supported by in-situ ester formation on H-ZSM-5 zeolites

Friedel-Crafts acylation of phenolic compounds with carboxylic acids catalyzed by Brønsted-acid zeolites is an interesting reaction that could find application in the production of fuels and chemicals from renewable feedstocks. The mechanism of direct acylation of m-cresol with acetic acid over protonic zeolites and the role that the confining environment may have on reaction rate and product selectivity are topics of current interest. Here, we evaluate this reaction at varying conditions over H-ZSM-5 catalysts with different acid site densities. By decoupling the effect of parasitic phenomena – diffusion and catalyst deactivation – we have been able to evaluate the true kinetics of this reaction. We have found that, in the absence of a solvent, the reaction over H-ZSM-5 zeolites under commonly studied conditions (Si/Al = 40) at 250 °C is often influenced by the diffusion of reactants to active sites. By further decreasing active site density and operating under diffusion-free conditions, the true activation barrier for acetophenone (2-hydroxyl-4-methylacetophenone) formation obtained on H-ZSM-5 under surface-saturation conditions is relatively low (41 kJ/mol). This value is significantly lower than the barrier associated with the dehydration of the carboxylic acid to form surface acyl species when not assisted by the formation of intermediate ester species. Based on reaction kinetic analysis, it is proposed that the formation of acylium ion from adsorbed m-tolyl acetate is the rate-limiting step. However, feeding the intermediate ester – i.e., m-tolyl acetate – to act as acylating agent, results in reduced rates compared to those in which the acylating agent is acetic acid. This result reveals that diffusion of m-tolyl acetate is slow and that the kinetically relevant ester is formed within the confines of the zeolite pores. By co-feeding water with m-tolyl acetate, we reveal that the bulky ester is hydrolyzed to form smaller molecules that diffuse faster inside zeolite pores and subsequently enhance the reaction rate. Vapor phase acylation of m-cresol with acetic acid over low acid site density H-Beta zeolite confirms the role of the active intermediate ester in accelerating the rate of acylated product formation. Catalyst deactivation during reaction increases with the partial pressure of acetic acid, even under the zero-order kinetic regime, during which the partial pressure of produced gaseous ketene remains constant. This result would indicate that this usual coke precursor is not the main cause of deactivation, but rather bulkier compounds formed by subsequent esterification of acetic acid with the acylated product (e.g., 2-hydroxyl-4-methylacetophenone).

Facet-Dependent selectivity of CeO2 nanoparticles in 2-Propanol conversion

CeO2 nanoshapes, cubes with dominant (100) facets and octahedra with dominant (111) facets, were synthesized to investigate the influence of surface structure on acid-base properties. An optimization of calcination temperatures, coupled with Raman and TEM, was employed to minimize the intrinsic (Frenkel-type) defect sites and their potential complications on the facet studies involved in this work. The acid-base properties of these CeO2 nanoshapes were characterized with in situ pyridine and CO2 adsorption using infrared spectroscopy and quantified using pyridine, ammonia, and CO2 temperature-programmed desorption (TPD). The (100) facet displayed weaker acid sites with lower site density than the (111) facet which had a similar density of base sites but those on the (100) facet were stronger. A strong correlation was observed between 2-propanol conversion and each facet’s acid-base properties. CeO2 cubes exhibited greater base-site catalyzed dehydrogenation to acetone while the more acidic CeO2 octahedra were more active in dehydration to propene.

Effect of Nickel Active Site Density on the Deactivation of Ni-Beta Zeolite Catalysts during Ethene Dimerization

Isolated Ni(II) sites supported on zeolites and other porous materials transform in situ during alkene dimerization to form active Ni(II)-alkyl centers, and their density influences the kinetic orders and mechanisms of deactivation of Ni-Beta zeolites during ethene dimerization (453 K). Ni-Beta containing high Ni site densities shows deactivation rates that are second-order in Ni, consistent with a dual-site deactivation mechanism involving the formation of unreactive Ni-alkyl-Ni intermediates, as confirmed by DFT calculations. Under the same reaction conditions, by contrast, Ni-Beta containing low Ni site densities shows deactivation rates that are first-order in Ni, consistent with a single-site deactivation mechanism reflecting inhibition by strongly bound intermediates derived from heavier alkene oligomers. On Ni-Beta containing low Ni site densities, cofeeding H2 along with ethene results in a higher number of Ni(II)-alkyl intermediates formed at initial reaction times and a concomitant change to deactivation kinetics that become second-order in Ni. These findings reveal the strong influence of the density of active Ni(II)-alkyl centers in porous supports, which depends both on material properties and reaction conditions that generate active centers in situ, on the kinetics and mechanisms of deactivation during alkene oligomerization.

Modulation of CD22 Antigen Density Improves Efficacy of CD22 Chimeric Antigen Receptor (CAR) T Cells Against CD22lo B-Lineage Leukemia and Lymphoma

While overall survival for acute lymphoblastic leukemia (ALL) in children treated with risk-adapted, multi-agent chemotherapeutic regimens is greater than 85%, relapsed ALL remains a major challenge. Chimeric antigen receptor (CAR) T cell therapy targeting CD19 provides these patients with a remission induction rate of 70-90% across multiple clinical trials, but loss of the targeted epitope results in relapse following CD19 CAR T cells. In the setting of these relapses, our group conducted a CD22 CAR T cell trial, which induces remission in approximately 70% percent of patients with relapsed/refractory CD22-positive pre-B cell ALL, including those relapsing after CD19 CAR therapy (NCT02315612). However, relapse associated with reduced CD22 site density has shortened remission duration in some patients. To test the impact of low CD22 site density on CD22 CAR T cell functionality, we modified Nalm6 ALL cell lines using CRISPR/Cas9 to eliminate the CD22 gene, followed by lentiviral transduction of full-length CD22 and single-cell cloning based on CD22 expression levels. CD22 site density was assessed with the Quantibrite-PE Assay Kit and four cell lines were identified for a spectrum of CD22 expression: CD22 negative (CD22neg); CD22 low (CD22lo) - site density comparable to that seen in ALL patients relapsing following CD22 CAR therapy; CD22 normal (Nalm6) - site density comparable to ALL CD22 expression; and CD22 high (CD22hi) - site density comparable to CD19 levels. As CD22 site density decreases, CD22 CAR T cells produce incrementally less interferon-gamma and do not produce IL-2 against CD22lo ALL. CD22 CAR delayed, but was unable to prevent, progression of CD22lo ALL in murine xenografts. We next evaluated two potential approaches to overcoming low site density antigen escape: improving CAR activity or increasing tumor sensitivity. To improve CAR activity, we generated a high-binding affinity CD22 CAR (CD22 CARH1) using an affinity-enhanced version of the scFv from the CD22 CAR construct in our ongoing trial. Despite higher affinity, the CD22 CARH1 T cell did not improve in vitro killing or in vivo clearance of CD22lo ALL. To increase tumor sensitivity, we sought to increase CD22 site density using the PKC inhibitor Bryostatin1, a drug previously shown to increase CD22 expression in CLL. Bryostatin1 increases CD22 site density on a panel of pre-B cell ALL and diffuse large B cell lymphoma (DLBCL) cell lines. Interestingly, the CD22 increase is durable for at least 1 week after exposure to Bryostatin1. Bryostatin1-mediated CD22 increased site density improves cytokine production by CD22 CAR T cells. To evaluate the mechanism of Bryostatin1-mediated upregulation of CD22 expression, RNAseq was performed on Bryostatin1-exposed ALL. Preliminary analysis demonstrates that Bryostatin1 does not directly affect CD22 mRNA expression levels, but modulates early transcription factors within the B-cell receptor pathway. Based on these results, we are further evaluating pathways affected by Bryostatin1 through protein expression analysis and assessing in vivo function of Bryostatin1 in the setting of a patient-derived CD22 CAR-resistant xenograft model. In conclusion, these results demonstrate a clear threshold antigen site density, below which the CD22 CAR is unable to clear CD22-expressing ALL. We further demonstrate that increased CD22 CAR affinity does not seem to improve CD22 CAR activity against CD22lo ALL. Bryostatin1-mediated CD22 modulation can improve CD22 CAR response against leukemia and lymphoma providing a potential rational combination to rescue CD22 CAR activity and remission durability. DisclosuresNo relevant conflicts of interest to declare.

Ionic Liquid-Stabilized Single-Atom Rh Catalyst Against Leaching

Ionic Liquid-Stabilized Single-Atom Rh Catalyst Against Leaching

FeNC Electrocatalysts with Densely Accessible FeN4 Sites for Efficient Oxygen Reduction Reaction

AbstractThe development of iron and nitrogen co‐doped carbon (FeNC) electrocatalysts for the oxygen reduction reaction (ORR) in proton‐exchange membrane fuel cells (PEMFCs) is a grand challenge due to the low density of accessible FeN4 sites. Here, an in situ trapping strategy using nitrogen‐rich molecules (e.g., melamine, MA) is demonstrated to enhance the amount of accessible FeN4 sites in FeNC electrocatalysts. The melamine molecules can participate in the coordination of Fe ions in zeolitic imidazolate frameworks to form FeN6 sites within precursors. These FeN6 sites are then converted into atomically dispersed FeN4 sites during a pyrolytic process. Remarkably, the FeNC/MA exhibits a high single‐atom Fe content (3.5 wt.%), a large surface area (1160 m2 g−1), and a high density of accessible FeN4 sites (45.7 × 1019 sites g−1). As a result, FeNC/MA shows a much enhanced ORR activity with a half‐wave potential of 0.83 V (vs the reversible hydrogen electrode) in a 0.5 m H2SO4 electrolyte solution and a good performance in a PEMFC system with an activity of 80 mA cm−2 at 0.8 V under 1.0 bar H2/air. This work offers a promising approach toward high‐performance carbon‐based ORR electrocatalysts.

Chemical vapour deposition of Fe-N-C oxygen reduction catalysts with full utilization of dense Fe-N4 sites.

Replacing scarce and expensive platinum (Pt) with metal-nitrogen-carbon (M-N-C) catalysts for the oxygen reduction reaction in proton exchange membrane fuel cells has largely been impeded by the low oxygen reduction reaction activity of M-N-C due to low active site density and site utilization. Herein, we overcome these limits by implementing chemical vapour deposition to synthesize Fe-N-C by flowing iron chloride vapour over a Zn-N-C substrate at 750 °C, leading to high-temperature trans-metalation of Zn-N4 sites into Fe-N4 sites. Characterization by multiple techniques shows that all Fe-N4 sites formed via this approach are gas-phase and electrochemically accessible. As a result, the Fe-N-C catalyst has an active site density of 1.92 × 1020 sites per gram with 100% site utilization. This catalyst delivers an unprecedented oxygen reduction reaction activity of 33 mA cm-2 at 0.90 V (iR-corrected; i, current; R, resistance) in a H2-O2 proton exchange membrane fuel cell at 1.0 bar and 80 °C.