Structure Design And Electrochemical Performance Of Bacterial Cellulose-derived Carbon Aerogels

Carbon aerogel has excellent properties,such as high electrical conductivity and thermostability,making it widely used in the fields of supercapacitors,electrocatalysis,batteries and thermally insulating.However,the fracture of carbon aerogel seriously limits its further applications.Based on that,effective methods to make it flexibility enchancement and structure design,such as precursor surface chemical modification,hierarchical porous structure control,heteroatom or metal doping,which can endow carbon aerogel with a synergistic effect of mechanical and electrochemical properties,so that they can meet the requirements of energy devices for the potential wearable performance,high-performance of cycle ability and energy density.The main contents are as follows:（1）The precursor of bacterial cel ulose derived carbon aerogel exhibites poor mechanical strength and disordered state.We use the chemical modification of the bacterial cel ulose to enable the monodispersion and aligning of nanofibers in water and thus beneficial to the reservation of a robust carbon nanofiber aerogel with unique 3D network structure after carbonization.The as-prepared carbon aerogel had abundant welded joints between nanofibers with enhanced interber connections for desired stable structures and excellent mechanical properties.Moreover,as self-supporting carbon materials,the as-constructed solid-state supercapacitors could be robust against the different bending times and bending states without the decay of specific capacitance.This work demonstrates that the chemical modification of the bacterial cellulose derived carbon aerogel exhibits flexibility and shape-controllability.As the self-supporting electrode,it avoids the process complexity of traditional coating methods and the decrease in electrochemical performance based on addiction of conductive agents and binders,which shows the great potential to be applied as flexible energy storage devices.（2）In order to enchance the specific capacitance and energy density of carbon aerogels,the fabrication of carbon aerogels with the existence of rich defects and creating micropores as well as enlarging mesopores by improving the wettability and surface utilization is reported,which is based on carbonization of bacterial cel ulose（BC）assisted by the soft template of Zn（NO₃）₂-1,3,5-Benzenetricarboxylic acid.Specifically,evaporation of Zn at high temperatures etches pores,enabling the creation of more microporous pores and enlargement of mesoporous pores increasing the specific surface area and adding a certain degree of defects,thus yielding greater specific capacitance and energy density.The doping of additional metal/nonmetal and heteroatom and the complicated process of conventional coating method as well as the addition of conductive agent and binder are avoided,thus improving the cycle stability.This work demonstrates that according to rationanl designed hierarchical porous structures and defects,the carbon aerogels deliver a fundamentally enchanced electrochemical properties superior to commercial activated carbons,representing the great potential to be applied in commercial markets.（3）In order to enchance the number and variety of active sites,we developed a carbon nanofiber aerogel by one-pot in-situ coupling of uniform Fe and N active sites;abundant surface carboxylate groups create on the nanofibers surface can act as the anchoring sites for Fe ions,thus facilitating the high dispersion of metal sites within melamine molecules;The results show that the catalysts have a hierarchical porous structure with a large number of accessible active sites（N-C,Fe₃C nanoparticles and Fe-N_x）and fast mass transport,enhancing the ORR and OER electrocatalytic performance.According to the excellent catalytic intralayer mass transfer and electron transfer capabilities of carbon aerogels,comparable to the state-of-the-art Pt/C-Ru O₂catalyst,as-assembled zinc–air batteries with self-supporting Fe NCFs air electrode exhibited better performance;as the material was subjected to 50 cycles of stress–strain test,the specific capacity of the corresponding device still maintained.This work demonstrates that robust freestanding cathode catalyst that consists of highly dispersive Fe-N-C active sites can be successful synthesized based on unique network structure and surface chemical properties of bacterial cel ulose.（4）In order to enchance the activity and stability,this paper offers a new strategy to build flexible air catalyst that has highly dispersive Co nanoparticles and Co@N active sites within 3D interconnected carbon nanofibers networks,via the in-situ growth and followed calcination of bimetallic zinc/cobalt-based zeolitic imidazolate framework within bacterial cel ulose skeleton.Particularly,the addition of inactive Zn can stabilize pyridinic-N,enrich mesopores and defects,and restrict growth of Co clusters in carbon nanofibers.In addition,Co@N-induced defect sites in the carbon backbone can change the electronic properties of adjacent carbon sites,which can positively affect the catalytic performance of ORR and OER with smaller reversible oxygen electrode index（0.76 V）than commercial Pt/C-Ru O₂ and most Co-based electrocatalysts ever reported.Moreover,the as-assembled quasi-solid-state zinc–air batteries using such self-supporting cathode represented excellent mechanical flexibility and outstanding cycle performance because of the synergy of its cladding and three-dimensional network-like structure,regardless of being serviced under extremely bending conditions,the voltage gap between discharge and charge had no obvious change.This work demonstrates that metal nanoparticles are evenly loaded on the fibrous network with highly uniform active species,underscoring their promising applications as durable bifunctional cathode for portable zinc-air batteries.