Preparation And Electrocatalytic Performance Of Multi-Stage Porous Wood-Derived Carbon-Supported Single-Atom Catalyst

With the continuous development of the global economy,the demand for energy is increasing rapidly.However,as traditional fossil fuels continue to deplete and cause significant environmental damage,the global energy crisis is becoming more urgent and severe.Therefore,researching and developing new clean energy technologies has become an important challenge facing countries around the world.Zinc-air batteries,as a new renewable energy technology,have great advantages in terms of energy density and storage capacity.However,the slow oxygen reduction reaction(ORR)at the air electrode in zinc-air batteries hinders the performance and cycle life of the batteries.Although precious metal catalysts can effectively catalyze the ORR,the development of zinc-air batteries is hindered by the drawbacks of limited reserves and high cost.In recent years,single-atom catalysts(SACs)have received increasing attention due to their excellent ORR catalytic performance and good stability.However,their synthesis conditions are relatively stringent,and the complex process is not conducive to large-scale commercialization.Wood,as a common biomass material,has obvious cost advantages when used to prepare ORR catalysts.The porous structure of wood can provide a larger active surface area and facilitate the reaction.In addition to this,the catalytic performance of the catalyst can be further improved by modifying the structure and chemical properties of the wood.Therefore,we prepared single-atom catalysts(SACs)by combining non-precious metals with ligands in a straightforward manner.Furthermore,we integrated the SACs with wood,utilizing the structure and properties of wood itself,to fabricate self-supported single-atom catalysts.In this paper,the morphological structure and electrochemical properties of the catalysts are analyzed,and the mechanism of enhancing the electrochemical properties is discussed,and finally their practical application in zinc-air batteries is investigated.The main contents are as follows:(1)Adenine-Zn/Co Supramolecular Coordination Microspheres(Ad-Zn/Co SCMS)were successfully prepared by a competitive coordination strategy.Adenine,as a nitrogen-rich ligand(51.8 wt%),provided a large number of reaction sites for metal ions.By adjusting the Zn/Co-N coordination and Zn/Co ratio in the Ad-Zn/Co SCMS precursor,a single-dispersed cobalt active site(Co-N4)was ensured to form on the nitrogen-rich porous carbon microspheres(CoSA/N-PCMS)after pyrolysis.At the same time,zinc was eliminated during the pyrolysis process,producing porous carbon microspheres with a high specific surface area of 1151.6 m2 g-1,which maximized the exposure of the Co-N4 site and facilitated charge transfer during the ORR process.The highly doped nitrogen(18.66 at%)promoted the effective utilization of the Co single atom,which significantly improved the ORR activity.CoSA/N-PCMS exhibited an onset potential(Eonset)of 0.99 V and a half-wave potential(E1/2)of 0.87 V in an O2-saturated 0.1 M KOH solution.Additionally,CoSA/N-PCMS was used as a cathode for zinc-air batteries(ZABs)showed high open-circuit voltage(1.47 V),good power density(168.7 mW cm-2),large specific capacity(796 mAh g-1),and excellent charge/discharge cycle stability(stable operation for 90 h).(2)Using natural and renewable wood as a carbon substrate,the porous wood-loaded Fe single-atom catalyst(MPWC-FeSA)was successfully prepared by using the pore structure of the wood and the adenine metal complex to achieve metal and nitrogen doping.During the preparation process,two different metal ions,Fe and Zn,react with adenine coordination,and the volatilization of Zn during pyrolysis both prevents the aggregation of Fe elements and creates a large number of micropores.After pyrolysis,the catalyst could maintain its complete shape and be directly used as a solid-state air cathode for batteries.MPWC-FeSA has a rich porous structure of wood and independently dispersed Fe-Nx active sites,exhibiting excellent ORR catalytic activity.Under alkaline conditions,the Eonet of MPWC-FeSA was 0.988 V,and the E1/2 was 0.85 V,surpassing commercial Pt/C and exhibiting better stability.The battery assembled based on MPWC-FeSA showed a discharge power density of 152 mW cm-2 and a capacity density of 765 mAh g-1,both superior to the Pt/C+RuO2-assembled zinc-air battery.In addition,during the 225-hours charge-discharge stability test,MPWC-FeSA exhibited excellent stability,far exceeding the zinc-air battery assembled with precious metal catalysts.MPWC-FeSA was directly used as electrodes to assemble solidstate batteries,which exhibited a maximum power density of 80.6 mW cm-2 and good stability.