Highly Accessible Atomically Dispersed Fe‐N_x Sites Electrocatalyst for Proton‐Exchange Membrane Fuel Cell

Abstract

Atomically dispersed transition metal‐Nx sites have emerged as a frontier for electrocatalysis because of the maximized atom utilization. However, there is still the problem that the reactant is difficult to reach active sites inside the catalytic layer in the practical proton exchange membrane fuel cell (PEMFC) testing, resulting in the ineffective utilization of the deeply hided active sites. In the device manner, the favorite structure of electrocatalysts for good mass transfer is vital for PEMFC. Herein, a facile one‐step approach to synthesize atomically dispersed Fe‐Nx species on hierarchically porous carbon nanostructures as a high‐efficient and stable atomically dispersed catalyst for oxygen reduction in acidic media is reported, which is achieved by a predesigned hierarchical covalent organic polymer (COP) with iron anchored. COP materials with well‐defined building blocks can stabilize the dopants and provide efficient mass transport. The appropriate hierarchical pore structure is proved to facilitate the mass transport of reactants to the active sites, ensuring the utilization of active sites in devices. Particularly, the structurally optimized HSAC/Fe‐3 displays a maximum power density of up to 824 mW cm−2, higher than other samples with fewer mesopores. Accordingly, this work will offer inspirations for designing efficient atomically dispersed electrocatalyst in PEMFC device.

Bifunctional Single-Atom Cobalt Electrocatalysts with Dense Active Sites Prepared via a Silica Xerogel Strategy for Rechargeable Zinc–Air Batteries

“…The porosity of the carbon support critically influences the mass transport between the catalyst surface and the bulk solution [ 42 , 43 ]. The N 2 adsorption/desorption isotherm and type-IV adsorption isotherm for Co-N-C SAC are illustrated in Figure 3 c. The Co-N-C SAC presented a steep increase in V ads at a relatively low N 2 pressure (P/P 0 = 0–0.015) and a well-defined hysteresis loop at a higher N 2 pressure (P/P 0 = 0.4–0.9), indicating the coexistence of micro and mesopores [ 43 , 52 ].…”

“…Apart from the stabilization of Co, the porosity of the carbon matrix is another key factor for the OER and ORR activity, which exerts an important impact on hosting active sites and the accessibility of reactants to the three-phase boundary. In particular, micropores mainly host the Co-N x active sites, and mesopores can facilitate the mass transport of reactants [ 42 , 43 , 44 ]. Thus, it is necessary to select a suitable substrate to assist the synthesis of densely well-defined SACs with optimized pore distribution.…”

Stabilizing Fe–N–C Catalysts as Model for Oxygen Reduction Reaction

“…[10c] Therefore, the construction of Fe-N-C structures with sufficiently exposed TPB and high-density active sites is of great significance for optimizing the catalyst structure and increasing the catalyst mass activity. [103] However, it is difficult to realize, because Fe tends to agglomerate during high-temperature sintering. [104] As shown in Figure 14a, Sun's team proposed a carboxylate-assisted strategy to increase the density of active sites.…”

In‐Situ Silica Xerogel Assisted Facile Synthesis of Fe‐N‐C Catalysts with Dense Fe‐Nx Active Sites for Efficient Oxygen Reduction

“…In general, micropores mainly host the FeN x active sites, and mesopores can facilitate the mass transport of reactants. [30][31][32] The excessive micropore structure could favor the formation of FeN x sites, though, it narrows the smooth diffusion channels for efficient reactant exchange, causing the ineffective utilization of the deeply hided active sites. [30,31] More specifically, the high site density (SD) and turnover frequency (TOF) of accessible FeN x sites are intensely desired for the performance of ORR electrocatalysts.…”

“…[30][31][32] The excessive micropore structure could favor the formation of FeN x sites, though, it narrows the smooth diffusion channels for efficient reactant exchange, causing the ineffective utilization of the deeply hided active sites. [30,31] More specifically, the high site density (SD) and turnover frequency (TOF) of accessible FeN x sites are intensely desired for the performance of ORR electrocatalysts. [32,33] A microporous catalyst usually features large SD values paired with low TOF, while the mesopore-dominated sample displays oppositely.…”