Preparation Of Nanocellulose-based Metal Single-atom Carbon Materials And Their Application In Zn-air Batteries

The new requirements of global sustainable development for clean energy and a healthy environment impel us to become increasingly concerned with efficient and clean electrochemical storage and conversion devices.Zn-air battery(ZAB)is considered as energy storage devices with broad application prospects due to its advantages such as excellent theoretical energy density,relatively cost-effectiveness,environmental compatibility,and reliable safety.However,the sluggish oxygen reactions of the air electrode,namely the oxygen reduction reaction(ORR)and oxygen evolution reaction(OER),are responsible for the poor overall performance of ZAB.Therefore,it is important to develop efficient electrocatalysts to promote ORR and OER for the development of ZAB.Althoilgh noble metal catalysts are considered to be the most ideal ORR/OER electrocatalysts,their application and development have been constrained by their prohibitive cost and limited availability of resources.Recently,metal single-atom loaded N-doped carbon catalysts(M-N-C SACs)have attracted attention due to their maximum atomic utilization,homogeneous morphology and quantum size effect.However,the carbon-based materials used in these catalysts are usually obtained through tedious and hazardous processes,which hinders the development of M-N-C SACs.As a novel type of nanomaterial with abundant sources,low cost,non-pollution,and renewability,nanocellulose has a high specific surface area,high surface activity,and good mechanical strength.Therefore,these excellent properties endow nanocellulose displaying great advantages as a carbon-based matrix for M-N-C SACs.Herein,this paper uses nanocellulose as a carbon carrier to prepare M-N-C electrocatalysts with metal sizes ranging from nano/atomic coexistence to atomic level by compounding with active substances.The effect of carbon carriers on the uniform dispersion of metal atoms was revealed,and the mechanism of high catalytic activity of M-N-C SACs was elucidated to meet the practical application and commercialization of Zn-air batteries.Details are as follows:(1)One-dimensional nanocellulose filaments(NFCs)were used as biological templates to construct nanocellulose fibril-loaded Co-adenine complexes(Co-Ad/NFC)precursors by in situ coordination.Subsequently,the nano/atomic-scale Co coexisting Co@N-C composites(Co-Ad/NFC-700)were synthesized by pyrolysis as ORR electrocatalysts.In accordance with the characterization results,the one-dimensional structure of NFC ensured the uniform loading of active Co nanoparticles and high-density Co-Nx active material after pyrolysis.In addition,a large number of micropores and mesopores in Co-Ad/NFC-700 exposed more active sites and reduced the mass transfer resistance of ORR.Therefore,the prepared Co-Ad/NFC-700 exhibited excellent ORR catalytic performance.The Zn-air battery(Co-Ad/NFC-700 as air cathode with 2 mmol of Co)was further assembled,which exhibited an outstanding maximum power density of 167 mW cm-2,a specific capacity of 739 mA h g-1,and a cycle durability of more than 40 h.(2)In order to further improve the atomic utilization,Fe single-atom anchored N-doped carbon spheres(FeSA/N-PSCS)were synthesized as ORR electrocatalysts by a direct pyrolysis method(volatilization of Zn)using Fe2+ and Zn2+competitive ligand self-assembly strategy to precisely regulate the dual metal supramolecular complex precursors.According to the experimental results,FeSA/N-PSCS contained highly dispersed Fe-N4 sites,peripheral N-enriched microstructures,high specific surface area and abundant multistage pore structures.Therefore,the prepared FeSA/N-PSCS has excellent ORR catalytic performance.Meanwhile,the assembled Zn-air battery(FeSA/N-PSCS as air cathode with 0.004 mmol of Fe)was made up and exhibited a maximum power density of 164.5 mW cm-2,a specific capacity of 725.3 mA h g-1 and a cycling stability of more than 150 h.(3)Based on the above two studies,the one-dimensional nanocellulose crystals(CNCs)were used as carbon carriers to assist in situ the ligand self-assembly of metal ions with adenine,and then carbonized at high temperature to obtain the Fe/Co bi-metal single-atom loaded N-doped carbon nanofibers(Fe/Co-NC)with ORR/OER bifunctional catalytic activity.According to the experimental results,one-dimensional CNC sacrificial template,and the Zn ions sites act as "spacers" to enlarge the spatial distance between Fe and Co atoms.This ensured the uniform loading of Fe and Co atoms on the carbon skeleton,and formed an ultra-high specific surface area and abundant multi-level pore structure.Meanwhile,the synergistic effect of adjacent Fe-N4 and Co-N4 active centers and the coupling effect of a small amount of Fe-Co bonds enable the Fe-Co-NC composites exhibit excellent ORR and OER catalytic performance.The assembled Zn-air batteries(Fe/Co-NC as air cathode,both Fe and Co contents were 0.004 mmol)exhibited a maximum power density of 159 mW cm-2,a specific capacity of 767 mA h g-1,and an excellent charge/discharge cycle efficiency of more than 370 h.