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[Display omitted] • Papermaking nanofibers-derived N -doped carbon aerogels were constructed with Cd crosslinking and volatilization. • Fe-SAC@N/CA-Cd with single-atom Fe-N 4 structure was developed via a CVD strategy. • The Fe-SAC@N/CA-Cd exhibited superb ORR activity due to plentiful active species, ultra-high S BET and hierarchical pore architecture. • The assembled LES-ZAB and ASS-ZAB provided encouraging performances. Single-atom catalysts (SACs), with precisely controlled metal atom distribution and adjustable coordination architecture, have gained intensive concerns as efficient oxygen reduction reaction (ORR) electrocatalysts in Zn-air batteries (ZAB). The attainment of a monodispersed state for metallic atoms anchored on the carbonaceous substrate remains the foremost research priority; however, the persistent challenges lie in the relatively weak metal-support interactions and the instability of captured single atom active sites. Furthermore, in order to achieve rapid transport of O 2 and other reactive substances within the carbon matrix, manufacturing SACs based on multi-stage porous carbon substrates is highly anticipated. Here, we propose a methodology for the fabrication of carbon aerogels (CA)-supported SACs utilizing papermaking nanofibers, which incorporates advanced strategies for N -atom self-doping, defect/vacancy introduction, and single-atom interface engineering. Specifically, taking advantages of using green and energy-efficient feedstocks, combining with a direct pore-forming template volatilization and chemical vapor deposition approach, we successfully developed N -doped carbon aerogels immobilized with separated iron sites (Fe-SAC@N/CA-Cd). The obtained Fe-SAC@N/CA-Cd exhibited substantially large specific surface area (S BET = 1173 m2/g) and a multi-level pore structure, which can effectively mitigate the random aggregation of Fe atoms during pyrolysis. As a result, it demonstrated appreciable activity and stability in catalyzing the ORR progress (E 1/2 = 0.88 V, E onset = 0.96 V). Furthermore, the assembled liquid electrolyte-state Zn-air batteries (LES-ZAB) and all-solid-state Zn-air battery (ASS-ZAB) also provides encouraging performance, with a peak power density of 169 mW cm−2 for LES-ZAB and a maximum power density of 124 mW cm−2 for ASS-ZAB. [ABSTRACT FROM AUTHOR]

[Display omitted] • Cellulose nanofibers carbon aerogel based Co single-atom catalyst is obtained. • Cd assists the Co-N 4 atomic dispersion in the cross-linked porous carbon aerogel. • Co SAs/NCNA allows accessible active sites and rapid electron/mass transfer channels. • Co SAs/NCNA delivers superior ORR activity and Zn-air batteries performance. Single-atom catalysts (SACs) have opened up unprecedented possibilities for expediting oxygen reduction reaction (ORR) kinetics owing to their ultrahigh intrinsic activities. However, precisely controlling over the atomically dispersed metal-N x sites on carbon support while fulfilling the utmost utilization of metal atoms remain the key obstacles. Here, atomically distributed Co-N 4 sites anchored on N -doped carbon nanofibers aerogel (Co SAs/NCNA) is controllably attained through a direct pyrolysis of metal-chelated cellulose nanofibers (TOCNFs-Cd2+/Co2+) hydrogel precursor. The usage of Cd salt assists the assembly of cross-linked aerogel, creates a large number of interior micropores and defects, and favors the physical isolation of Co atoms. The hierarchically porous biomass carbon aerogel (2265.1 m2/g) offers an advantageous platform to facilitate accessibility of the catalytic centers, also renders rapid mass diffusion and electron-transfer paths throughout its 3D architecture. Notably, Co SAs/NCNA affords a paramount ORR activity and respectable durability when integrated into zinc-air battery devices. [ABSTRACT FROM AUTHOR]

[Display omitted] • Fe-SA@PNC was developed via a novel chemical vapor deposition strategy. • 3D honeycomb aerogel was prepared employing CNF, ANF and GO through directional freeze-casting and Cd volatilization. • Fe-SA@PNC possess atomically dispersed Fe-N 4 sites and hierarchical pore structure. • Fe-SA@PNC exhibit remarkable ORR activity in Zn-air batteries. Fe-N x C single-atom catalysts have been immensely explored as the most promising ORR catalysts for replacing noble metals. Nevertheless, how to precisely introduce Fe atoms to optimize their spatial distribution and coordination environment in carbonaceous matrices while constructing the hierarchical porous structures to facilitate the mass transport process and ensure the accessibility of Fe-N x sites remain significant challenges. Here, we propose a strategy to prepare porous N -doped carbon aerogels anchored with single atomic Fe sites (Fe-SA@PNC) as highly efficient ORR electrocatalysts via directional freeze-casting and chemical vapor deposition. Fe-SA@PNC exhibit remarkable ORR activity and stability due to the highly stable honeycomb morphology and hierarchical pore structure, as well as the atomically dispersed Fe-N 4 sites in the carbon support. Furthermore, the practical application of Fe-SA@PNC catalysts as cathode materials in Zn-air batteries was demonstrated with encouraging performance. This study provides a promising approach for constructing high-efficiency ORR single-atom catalysts. [ABSTRACT FROM AUTHOR]

[Display omitted] • TOCNFs-Cd2+ aerogel is constructed via the electrostatic and ionic interactions. • Cd metal acts as the thermally sacrificial template to generate ultralight N-CNA-Cd. • N -CNA-Cd has abundant micro-/meso-pores, rich defects, huge SBET and fibrous network. • An impressive ORR electrocatalytic efficacy was obtained on N-CNA-Cd sample. Carbon aerogel (CA) emerges as a kind of burgeoning material platform for electrocatalysis with merits of the 3D porous and interconnected architecture to facilitate the low-resistant mass diffusion and charge transport. There still remain great challenges in fine-tuning the intrinsic chemical nature and creating hierarchically porous microstructure in CA through a sustainable synthesis route. Here, we employed TEMPO-mediated oxidized cellulose nanofibers (TOCNFs) and a thermally sacrificial template Cd metal with low boiling point (767 °C) to assemble TOCNFs-Cd2+ hydrogel/aerogel, which is then subsequently converted to ultralight N -doped carbon nanofibers aerogel (N -CNA-Cd) upon pyrolysis at a moderate temperature. Thanks to the presence of abundant interior micro- and meso-pores, rich defects, huge BET specific surface area (1782 m2 g−1) and interconnected fibrous network, the as-obtained N -CNA-Cd samples demonstrate an impressive electrocatalytic efficacy towards oxygen reduction reaction in terms of the positive half-wave potential (0.84 V), large electrochemical active surface area and considerable stability under both alkaline and acidic conditions. This study provides a versatile strategy for designing and engineering carbonaceous aerogels that can be suited for electrocatalysis and energy storage applications from renewable plant cellulose resources. [ABSTRACT FROM AUTHOR]