Tuning And Performance Of Transition Metal/Nitrogen-Doped Carbon-Based Catalyst Materials

Under the condition of the“double carbon”goal（Goal of“peak carbon dioxide emission and carbon neutrality”）,the pursuit of environmental protection and sustainable energy has become more and more urgent,which brings new challenges and opportunities to the electrochemical energy storage technologies.Metal-air batteries have the advantages of high energy density,environmental friendliness,and low cost,and have become one of the alternative energy storage technologies for lithium ion battery（LIB）.Among them,zinc-air batteries（ZABs）shows promising prospects due to their sufficiently high energy density(1350 Wh kg^-1,much better than LIBs),low cost,high safety（without organic electrolytes）,and environmentally friendly.However,its poor reaction kinetics and high overpotential usually need noble metal-based catalysts to catalyze,which usually face the problems of scarcity,high cost,and poor stability.Transition metal nitrogen-doped carbon（M-N-C,M=Fe,Co,Cu,etc.）has shown better catalytic performance than the noble metal catalysts,which has been a promising alternative to noble metal catalysts such as Pt/C.However,the stability and low actual activity still need to be improved.Therefore,this paper focuses on the improvement of the ORR performance by precursor regulation and structure optimization.Details are as follows:（1）The Co and N co-doped nitrogen-rich Co-N-C-900 catalyst was prepared by pyrolysis of platanus barks after immersion in cobalt salt solution using ethylenediamine vapor-assisted nitridation strategy.The optimized catalyst（Co-N-C-900）exhibits comparable half-wave potential(E_1/2,0.836 V)and lower Tafel slope(69.55 m V dec^-1)than Pt-C-20%.When applied to ZAB,it showed a higher open circuit potential and a peak power density up to 186.17 m W cm^-2(Pt-C-20%,54.46m W cm^-2),a fully discharged specific capacity of 770.2 m Ah g^-1(Pt-C-20%,760.5m Ah g^-1).During the stability test in 100 cycles（16.7 h）,the voltage gap increased by only 4.2%,which is significantly better than the Pt/C-20%-based ZAB.When applied to flexible ZABs,it exhibits higher full discharge specific capacity（783.24 m Ah g-1）and cycling stability（1.7%increase in charge-discharge voltage gap）than liquid devices.Cyclic charge-discharge test was carried out under different bending angles,and the charge and discharge curves did not change significantly,showing high flexibility and practicability.（2）The black fungus rich in Fe and N elements was selected as the precursor,immersed in a cobalt/zinc salt mixed solution,and then pyrolyzed to obtain the Fe,N co-doped porous biomass carbon-derived catalyst（Fe-N-PC1R）.The optimal catalyst showed an E_1/2 0.896 V,which was 53 m V higher than Pt-C-20%with a lower Tafel slope(69.56 m V dec^-1).When used in ZAB,it exhibited a higher open circuit potential and a peak power density as high as 185.3 m W cm^-2,a full discharge specific capacity of 772.98 m Ah g^-1,and high durability.The flexible ZAB based on Fe-N-PC1R exhibits a higher full discharge specific capacity(789.29 m Ah g^-1),showing high flexibility and high value in use.（3）Two-dimensional MXene（Ti₃C₂T_x）sheet with strong negative charges was used to pre-attracting the initiator(Fe³⁺)and dispersed in the pyrrole（py）monomer solution to produce the uniformly coated metal–Ppy precursor on the MXene by confined in-situ oxidative polymerization.And then pyrolyzed to obtain Fe₃N/Co-NC@MXene catalyst with hyperdispersed Fe₃N and Co.The optimized catalyst showed an E_1/2 of 0.871 V,which was 28 m V higher than that of Pt-C-20%,and a lower Tafel slope(52.56 m V dec^-1).When applied to ZAB,it exhibits higher open circuit potential（1.41 V）,a peak power density up to 189.16 m W cm^-2,full discharge specific capacity(778.54 m Ah g^-1),and excellent cycling stability.When applied to the flexible ZAB,it exhibits a higher fully discharged specific capacity(780.32 m Ah g^-1),excellent stability in different scenes（bending and twisting）and shows high application potential in wearable devices.（4）The confined oxidative polymerization strategy was used to produce the uniformly metal-polypyrrole precursor on the C₃N₄ sheets,which was then pyrolyzed to obtain a sheet-like N-doped porous carbon-supported Fe₃N catalyst（Fe₃N-N-C）.The C₃N₄ template was completely decomposed into a gas containing C and N,which can not only remove the template and pore formation but also enhance the N-doping.The optimal Fe₃N-N-C-50 catalyst showed an E_1/2 of 0.885 V and a lower Tafel slope of64.73 m V dec^-1.When applied to ZAB,it exhibits a higher open circuit potential,a peak power density as high as 191.85 m W cm^-2,a fully discharged specific capacity(794.39 m Ah g^-1),and excellent cycle stability for 210 hours.The flexible ZAB based on Fe₃N-N-C-50 exhibits a higher full discharge specific capacity(794.04 m Ah g^-1)and can adapt to different bending scenarios,showing a high use value.（5）Polystyrene（PS）spheres were used as hard templates to obtain uniform spherical metal（iron,zinc）-polypyrrole precursors,and a series of Fe and N co-doped porous hollow carbon spheres（Fe-N-HPCSs）were prepared by pyrolysis.During the pyrolysis process,the PS spheres were directly decomposed and removed,which simplifies the conventional template synthesis process and improves the operability.The optimized Fe-N-HCPSs catalyst shows an E_1/2 of 0.896 V,which exceeds that of the commercial Pt-C-20%catalyst and a lower Tafel slope of 61.83 m V dec^-1.When applied to ZAB,it exhibits a higher open circuit potential,a power density as high as164.13 m W cm^-2,a fully discharged specific capacity(762.54 m Ah g^-1),and performing excellently for 370 h.The flexible ZAB exhibits a full discharge performance comparable to that of liquid ZAB and exhibits high flexibility.