Search

Proton exchange membrane fuel cells (PEMFCs) and alkaline membrane fuel cells (AEMFCs) have received great attention as energy devices of the next generation. Accelerating oxygen reduction reaction (ORR) kinetics is the key to improve PEMFC and AEMFC performance. Platinum‐based catalysts are the most widely used catalysts for the ORR, but their high price and low abundance limit the commercialization of fuel cells. Non‐noble metal‐nitrogen‐carbon (M−N−C) is considered to be the most likely material class to replace Pt‐based catalysts, among which Fe−N−C and Co−N−C have been widely studied due to their excellent intrinsic ORR performance and have made great progress in the past decades. With the improvement of synthesis technology and a deeper understanding of the ORR mechanism, some reported Fe−N−C and Co−N−C catalysts have shown excellent ORR activity close to that of commercial Pt/C catalysts. Inspired by the progress, regulation strategies for Fe−N−C and Co−N−C catalysts are summarized in this Review from 5 perspectives: (1) coordinated atoms, (2) environmental heteroatoms and defects, (3) dual‐metal active sites, (4) metal‐based particle promoters, and (5) curved carbon layers. We also make suggestions on some challenges facing Fe−N−C and Co−N−C research. [ABSTRACT FROM AUTHOR]

The single‐atom Fe−N−C is a prominent material with exceptional reactivity in areas of sustainable energy and catalysis research. It is challenging to obtain the dense Fe‐N4 site without the Fe nanoparticles (NPs) sintering during the Fe−N−C synthesis via high‐temperature pyrolysis. Thus, a novel approach is devised for the Fe−N−C synthesis at low temperatures. Taking FeCl2 as Fe source, a hydrogen environment can facilitate oxygen removal and dichlorination processes in the synthesis, efficiently favouring Fe‐N4 site formation without Fe NPs clustering at as low as 360 °C. We shed light on the reaction mechanism about hydrogen promoting Fe‐N4 formation in the synthesis. By adjusting the temperature and duration, the Fe‐N4 structural evolution and site density can be precisely tuned to directly influence the catalytic behaviour of the Fe−N−C material. The FeNC‐H2‐360 catalyst demonstrates a remarkable Fe dispersion (8.3 wt %) and superior acid ORR activity with a half‐wave potential of 0.85 V and a peak power density of 1.21 W cm−2 in fuel cell. This method also generally facilitates the synthesis of various high‐performance M−N−C materials (M=Fe, Co, Mn, Ni, Zn, Ru) with elevated single‐atom loadings. [ABSTRACT FROM AUTHOR]

Iron‐nitrogen‐carbon (Fe‐N‐C) catalysts for the oxygen reduction reaction (ORR) in proton exchange membrane fuel cells (PEMFCs) have seriously been hindered by their poor ORR performance of Fe‐N‐C due to the low active site density (SD) and site utilization. Herein, we reported a melamine‐assisted vapor deposition approach to overcome these hindrances. The melamine not only compensates for the loss of nitrogen caused by high‐temperature pyrolysis but also effectively etches the carbon substrate, increasing the external surface area and mesoporous porosity of the carbon substrate. These can provide more useful area for subsequent vapor deposition on active sites. The prepared 0.20Mela‐FeNC catalyst shows a fourfold higher SD value and site utilization than the FeNC without the treatment of melamine. As a result, 0.20Mela‐FeNC catalyst exhibits a high ORR activity with a half‐wave potential (E1/2) of 0.861 V and 12‐fold higher ORR mass activity than the FeNC in acidic media. As the cathode in a H2‐O2 PEMFCs, 0.20Mela‐FeNC catalyst demonstrates a high peak power density of 1.30 W cm−2, outstripping most of the reported Fe‐N‐C catalysts. The developed melamine‐assisted vapor deposition approach for boosting the SD and utilization of Fe‐N‐C catalysts offers a new insight into high‐performance ORR electrocatalysts. [ABSTRACT FROM AUTHOR]

To achieve the Fe−N−C materials with both high activity and durability in proton exchange membrane fuel cells, the attack of free radicals on Fe−N4 sites must be overcome. Herein, we report a strategy to effectively eliminate radicals at the source to mitigate the degradation by anchoring CeO2 nanoparticles as radicals scavengers adjacent (Scaad‐CeO2) to the Fe−N4 sites. Radicals such as ⋅OH and HO2⋅ that form at Fe−N4 sites can be instantaneously eliminated by adjacent CeO2, which shortens the survival time of radicals and the regional space of their damage. As a result, the CeO2 scavengers in Fe−NC/Scaad‐CeO2 achieved ∼80 % elimination of the radicals generated at the Fe−N4 sites. A fuel cell prepared with the Fe−NC/Scaad‐CeO2 showed a smaller peak power density decay after 30,000 cycles determined with US DOE PGM‐relevant AST, increasing the decay of Fe−NCPhen from 69 % to 28 % decay. [ABSTRACT FROM AUTHOR]

Metal-nanoparticles (M-NPs) can adjust the electronic structure of the singly atomic MN 4 to promote ORR reactivity effectively. However, the synergy effect of M-NPs and MN 4 is influenced in a multifactorial way, and is difficult to investigate their separate effects on promoting ORR. Herein, we establish a new strategy to synthesize catalysts containing highly active MN 4 and M-NPs composite site and study their separate effects on ORR specifically. The MN 4 can be controlled in high-density, and meanwhile the M-NPs can be regulated precisely at very fine nanoscale. Our studies reveal that the MN 4 density shall be high enough so that the ORR reactivity can depend on the M-NPs size. The catalyst with clusters, i.e. H -(M Cs -(FeCo)N 4), can show the optimal ORR activity (j m @0.85 V) ~1.5 times as high as commercial Pt/C. This work not only develop a high-performance ORR catalyst, but also propose a reliable strategy for the MN 4 /M-NPs composite site catalyst synthesis. [Display omitted] • MN 4 and M-NPs composite site was elaborately constructed as efficient ORR catalyst. • High-density MN 4 is a prerequisite for the construction of M-MN 4 composite site. • M-NPs size of M-MN 4 composite site could effectively influence the catalytic ORR. • M-NPs size of M-MN 4 composite site at cluster level show the optimal ORR activity. [ABSTRACT FROM AUTHOR]

Inadequate performance of oxygen reduction reaction (ORR) hinders the commercialization of proton exchange membrane fuel cells (PEMFCs). Herein, we report an ORR catalyst consisting of intermetallic PtCo nanoparticles and atomically dispersed CoNC sites, exhibiting outstanding performance in the RDE test (E 1/2 = 0.906 V vs. RHE, 7 mV loss after 20,000 cycles, 0.63 A mg−1 Pt of mass activity at 0.9 V). The enhancement of activity and durability is attributed to the unique structure that Pt atoms are modified by the smaller transition metal atoms Co to form PtCo intermetallic and further regulated by CoNC, the dual electronic engineering results appropriate binding energy between Pt and oxygen species. In addition, enhanced PtCo–CoNC interaction impedes the agglomeration of nanoparticles, which enables the formation of sub-5 nm PtCo intermetallic and enhances the stability. This synthesis method provides an idea for further regulating the electronic structure of Pt alloy and synthesize uniform and small intermetallic particles. [Display omitted] • Development of PtCo intermetallic supported on atomically dispersed CoNC. • CoNC enables the formation of highly dispersed sub-5 nm PtCo intermetallic. • PtCo–CoNC exhibits excellent ORR activity and enhanced stability. • Pt atoms are modified by Co atoms in PtCo and further regulated by Co2+ in CoNC. • The dual electronic engineering is confirmed by XPS and in situ FTIR spectroscopy. [ABSTRACT FROM AUTHOR]

[Display omitted] • A plasma-assisted approach is developed to improve the SD and site utilization of Fe N C catalyst. • The plasma-assisted approach can give a 2.81-fold SD improvement and 1.98-fold higher site utilization, simultaneously. • The carbon defects near the Fe-N 4 sites can regulate the electronic structure and promoted the intrinsic activity of Fe-N 4 sites. • The hierarchical pore structure increased the abundance of edge sites and enhance the mass transport performance of Fe N C. Enhancing the density and utilization of FeN 4 sites can serve as a viable approach to enhance the catalytic efficacy of iron nitrogen carbon (Fe N C) catalysts for oxygen reduction reaction (ORR). Herein, we present a plasma-assisted method for enhancing the porosity of nitrogen-doped carbon. Our findings indicate that the ideal ratio of mesopore to micropore area is 0.463. This ratio not only promotes the diffusion of Fe3+ but also creates additional active sites for Fe3+ loading, leading to an increase in the number of available FeN 4 sites in Fe N C electrocatalysts during pyrolysis. The density (76.5 μmol g−1) and utilization (21.08 %) of d -FeNC-30 are significantly higher than those of FeNC without plasma treatment, with a 2.8-fold and 2-fold increase, respectively. Remarkably, it displays outstanding performance, evidenced by a half-wave potential of 0.835 V (vs. RHE) in a 0.1 M HClO 4 solution and a power density of 0.860 W cm−2 in proton exchange membrane fuel cells (PEMFCs). The developed plasma-assisted approach for improving the site density (SD) and utilization of FeN 4 provides a new perspective for high-performance ORR Fe N C catalysts. [ABSTRACT FROM AUTHOR]

[Display omitted] • The Fe–Co bimetallic sites were confirmed in FeCo–N–C by one-step pyrolysis. • The as-prepared catalyst achieves high ORR performance in acidic electrolytes. • The bimetallic site can result in facile cleavage of the O–O bond and thus a direct 4e− ORR process. • The FeCoNC-10 exhibits remarkable performance in DMFCs with superior methanol tolerance. The cost and efficiency issues of cathodic electrocatalyst for oxygen reduction reaction (ORR) hinder a broad application of fuel cells. Therefore, it is necessary to develop highly efficient and sustainable ORR electrocatalysts of platinum group metal-free (PGM-free). The atomically dispersed FeCoNC-10 electrocatalysts were constructed by incorporating Fe phthalocyanine (FePc) molecules into bimetallic metal–organic frameworks (BMOF) followed by one-step pyrolysis. It showed remarkable ORR activity in acidic medium compared with FeNC and CoNC catalysts. The Fe–Co bimetallic sites were confirmed in FeCo–N–C, together with abundant isolated FeN 4 and CoN 4 sites. Theoretical study revealed that the bimetallic site can enhance O 2 adsorption and elongate O–O bond length rather than FeN 4 and CoN 4. This can result in facile cleavage of the O–O bond and thus a direct 4e− ORR process. FeCoNC-10 catalyst showed enhanced ORR stability, due to very weak Fenton-type reaction related to Co species. Membrane electrode assemblies (MEAs) consisted of the FeCoNC-10 was applied in the cathode. It showed a peak power density of 100 mW cm−2 in a direct methanol fuel cells (DMFCs) with superior methanol tolerance. This work sheds light on the mechanism of atomically dispersed Fe/Co dual-doped carbon in enhancing ORR activity and stability, which enables the cost-effective ORR electrocatalyst design to achieve robust fuel cell performance. [ABSTRACT FROM AUTHOR]

• Fe-N/C‑meso-evap with unique porous structure is prepared by doping sulfate. • Fe-N/C‑meso-evap achieves high Fe-N X density by a second FeCl 2 evaporation method. • Fe-N/C‑meso-evap shows superior ORR activity with the E 1/2 of 0.835 v in acid media. • Rich mesopores in Fe-N/C‑meso-evap increase the number of accessible active sites. • Doped s facilitates the absorption of O 2 and the breakage of O O bond. Due to the existence of highly active Fe-N X coordination moieties, iron-nitrogen/carbon (Fe-N/C) materials have been considered as potential electrocatalysts toward oxygen reduction reaction (ORR). Increasing the density of Fe-N X sites is supposed to be a useful strategy to enhance the ORR performance of Fe-N/C catalysts. Herein, we report a stepwise synthesis strategy to construct Fe-N/C catalyst possessing a high ratio of mesopores and a high concentration of accessible Fe-N X active sites. Integrated with the properties of composition and morphology, the synthesized catalyst, Fe-N/C‑meso-evap, achieves remarkable ORR activity with a half potential of 0.835 V and a kinetic current density of 31.02 mA cm−2 at 0.8 V in 0.1 M HClO 4. This work provides new insight for designing single-atom catalysts towards ORR. [Display omitted] [ABSTRACT FROM AUTHOR]