Hierarchically Ordered Porous Carbon with Atomically Dispersed FeN₄ for Ultraefficient Oxygen Reduction Reaction in Proton‐Exchange Membrane Fuel Cells

Abstract

The low catalytic activity and poor mass transport capacity of platinum group metal free (PGM‐free) catalysts seriously restrict the application of proton‐exchange membrane fuel cells (PEMFCs). Catalysts derived from Fe‐doped ZIF‐8 could in theory be as active as Pt/C thanks to the high intrinsic activity of FeN4; however, the micropores fail to meet rapid mass transfer. Herein, an ordered hierarchical porous structure is introduced into Fe‐doped ZIF‐8 single crystals, which were subsequently carbonized to obtain an FeN4‐doped hierarchical ordered porous carbon (FeN4/HOPC) skeleton. The optimal catalyst FeN4/HOPC‐c‐1000 shows excellent performance with a half‐wave potential of 0.80 V in 0.5 m H2SO4 solution, only 20 mV lower than that of commercial Pt/C (0.82 V). In a real PEMFC, FeN4/HOPC‐c‐1000 exhibits significantly enhanced current density and power density relative to FeN4/C, which does not have an optimized pore structure, implying an efficient utilization of the active sites and enhanced mass transfer to promote the oxygen reduction reaction (ORR).

Recent Advances in Single-Atom Electrocatalysts for Oxygen Reduction Reaction

“…MEA provided high current density up to 700 mA cm -2 at 0.6 V and 120 mA cm -2 at 0.8 V. SAECs prepared by pyrolyzing Fe-doped ZIF-8 show comparable ORR activity to commercial Pt/C, but the micropores limit rapid mass transfer. Qiao et al prepared single-atom Fe catalysts (FeN 4 /HOPC) by embedding atomically dispersed FeN 4 sites in an ordered hierarchical porous structure [150]. The optimal FeN 4 /HOPC-c-1000 catalyst exhibits outstanding performance with a half-wave potential of 0.80 V in 0.5 M H 2 SO 4 solution, only 20 mV lower to that of commercial Pt/C (0.82 V).…”

“…Besides noble metal-based SAECs, Fe-based SAECs also showed high ORR activity in acidic electrolyte [ 150 ]. However, Fe-based SAECs show undesirable properties for application in PEMFCs; in that, Fe 2+ or Fe 3+ ions derived from Fe-based SAECs could react with H 2 O 2 and produce hydroxyl and hydroperoxyl radical species.…”

“…Since the pioneering work reported by Liang and coworkers that silica nanoparticles could be used as the template to prepare single-atom Co-N-C catalysts by adsorption and pyrolysis of Vitamin B 12 [ 174 ], various hard templates such as self-assembled polystyrene sphere (PS) [ 150 , 175 ], silica colloidal crystal [ 153 , 176 – 178 ], silica particles with a hollow core and a mesoporous shell [ 114 ], self-assembled Fe 3 O 4 nanocubes [ 179 ], and ordered mesoporous silica [ 145 ] have been used to construct SAECs with atomically dispersed active sites embedded in the carbon framework with hierarchical pores from micropores to mesopores and macropores to increase the accessible active-site numbers and enhance the mass and electron transports. Lee and coworkers further systematically investigated the influence of a porous carbon structure on the ORR activity by preparing three types of N-doped carbon catalysts with different pore size distributions but similar Brunauer-Emmett-Teller (BET) surface area and active-site concentration [ 173 ].…”

Recent Advances in Phosphorus‐Coordinated Transition Metal Single‐Atom Catalysts for Oxygen Reduction Reaction

“…The EXAFS provides information about the coordination number and bond distance of SACs [56] . Fourier transformations (FT) of the k 3 ‐weighted EXAFS of FeNPc (Fe, N and P tri‐doped porous carbon electrocatalyst) and reference compounds are shown in Figure 8c.…”

Deep-Breathing Honeycomb-like Co-Nx-C Nanopolyhedron Bifunctional Oxygen Electrocatalysts for Rechargeable Zn-Air Batteries

“…X-ray absorption near-edge structure spectra were collected to further confirm the chemical state of the elements ( Ravel and Newville, 2005 ). As shown in the Co K-edge absorption spectra ( Figure 4 D), the position of absorption edge for Honeycomb-600 is close to that of reference CoO, implying a higher chemical valence than cobalt foil, which is ascribed to the strong coupling between cobalt and carbon through Co-N x coordination ( Qiao et al., 2019 ). More importantly, the peak area of Honeycomb-600 is much lower than that of CoO, revealing that it is more electron rich compared with CoO.…”