A Humidity‐Induced Nontemplating Route toward Hierarchical Porous Carbon Fiber Hybrid for Efficient Bifunctional Oxygen Catalysis

Abstract

Hierarchical porous carbons (HPCs) are highly efficient supports for various remarkable catalytic systems. However, templates are commonly utilized for the preparation of HPCs, and the postremoval of the templates is uneconomical, time‐consuming, and harmful for the environment in most cases. Herein, a new humidity‐induced nontemplating strategy is developed to prepare 1D HPC with rich topologies and interconnected cavities for catalysis and energy storage applications. Porous electrospun nanofibers as calcination precursors are prepared via a humidity‐induced phase separation strategy. A nitrogen‐doped hierarchical porous carbon nanofiber (HPCNF), loading Co/Co3O4 hetero‐nanoparticles as exemplary nonprecious‐metal active substance (Co/Co3O4@HPCNF), is fabricated through the subsequent hydrothermal and pyrolysis treatment. The internal mesopore and cavity structure can be simply controlled by varying environment humidity during the electrospinning process. Benefiting from the unique topology, Co/Co3O4@HPCNF exhibits superior bifunctional activity when being used as electrocatalysts for oxygen reduction/evolution reactions. Moreover, the hybrid catalyst also demonstrates a remarkable power density of 102.5 mW cm−2, a high capacity of 748.5 mAh gZn−1, and long cycle life in Zinc–air batteries. The developed approach offers a facile template‐free route for the preparation of HPCNF hybrid and can be extended to other members of the large polymer family for catalyst design and energy storage applications.

A Review on the Impact of Humidity during Electrospinning: From the Nanofiber Structure Engineering to the Applications

“…elaborated, from a three steps process (Figure 13a), Co/Co 3 O 4 @HPCNF carbon nanofibers having a hierarchical porous structure and embedding Co/Co 3 O 4 noble-metalfree hetero nanoparticles for oxygen reduction/evolution electrocatalytic reactions occurring during the charging and discharging processes of rechargeable batteries. [72] In the first step of the process, PI nanofibers were fabricated by electrospinning under various RH leading to ultraporous fibers having a porous density increasing with the humidity. Then, the PI nanofibers were immersed in a cobalt acetate solution and further Aghasiloo et al [181] took advantage of the control of RH during electrospinning in order to fabricate TiO 2 nanofibers with optimized properties for photocatalytic applications.…”

Recent developments of nanocarbon based supports for PEMFCs electrocatalysts

“…Considering that micropores and mesopores can increase the specific surface area, macropores can reduce the mass transfer resistance. A support material with a rational pore size can promote the metal utilization efficiency and electrocatalytic activity of the catalysts [ 64,65].…”

“…When the pore size of the carbon support is >50 nm, the diffusion of O2 molecules is scarcely affected by the pore size, but the small specific surface area of the carbon support is not sufficient for the fine dispersion of metal particles. Therefore, coordinating the advantages and disadvantages of different pore sizes of porous carbons, when the pore size of the carbon support is 20-30 nm, not only provides sufficient anchoring sites for the catalyst nanoparticles, but also ensures that there is no obstacle to the diffusion process [65]. Furthermore, to obtain an excellent catalytic performance for the supported metals by taking advantage of the different pore sizes of the carbon supports, hierarchical porous carbons with micropores, mesopores, and/or macropores could be a good choice for PEMFC electrocatalysts [72,73].…”

Hierarchical Carbon/Metal Nanostructure with a Combination of 0D Nanoparticles, 1D Nanofibers, and 2D Nanosheets: An Efficient Bifunctional Catalyst for Zinc‐Air Batteries

“…[14] To this end, various carbon and non-precious transition metal-based materials (e. g., oxides, sulfides, and nitrides) have been extensively explored as bifunctional oxygen catalysts recently. [15][16][17][18][19][20][21][22][23][24] In principle, hierarchically porous structure, high specific surface area, and good electronic conductivity are essential parameters for electrocatalysts since they are beneficial to exposing more active sites and promoting electrolyte diffusion and mass transmission. Therefore, various nanomaterials and/or nanostructures including 0D metal single-atoms/ clusters/nanoparticles, [25][26][27] 1D nanowires/nanotubes/nanofibers (e. g., carbon nanotubes, carbon nanofibers), [28][29][30] and 2D nanosheets (e. g., graphene, carbon nanosheets) [31,32] have been rationally designed for oxygen catalysts.…”

“…To this end, various carbon and non‐precious transition metal‐based materials ( e. g ., oxides, sulfides, and nitrides) have been extensively explored as bifunctional oxygen catalysts recently [15–24] . In principle, hierarchically porous structure, high specific surface area, and good electronic conductivity are essential parameters for electrocatalysts since they are beneficial to exposing more active sites and promoting electrolyte diffusion and mass transmission.…”