Constructing Transition Metal Carbon-Based Oxygen Electrocatalyst Via Structure Design For Zn-Air Batteries

Ever-increasing demands of miniaturization and long durability in electronic devices have sparked global interest in exploring high energy storage systems.Rechargeable Zn-air batteries have been regarded as promising candidates for next generation advanced energy devices on account of high theoretical energy density,eco-friendliness and safety.However,sluggish reaction kinetics and weak durability on the air electrode still severely impede their practical implementation.To date,noble metal-based materials（Pt/C,Ru O₂,etc.）have been demonstrated to be the most effective catalysts for oxygen reduction reaction（ORR）and oxygen evolution reaction（OER）.Unfortunately,the exorbitant cost and low durability of noble metal catalysts have motivated research communities to discover affordable alternatives,preferably consisting of non-noble metals.Therefore,the rational design and preparation of non-noble metal-based bifunctional catalysts with high catalytic activity and considerable stability towards ORR and OER is the key factor for the development of high-performance Zn-air batteries.Nowadays,there are generally two strategies to improve the activity of an electrocatalyst system:（i）increasing the intrinsic activity of each active site or（ii）increasing the number of active sites by adjusting the morphology structure of catalysts.Inspired by these,the above two strategies were organically coupled to effectively improve the catalyst activity via synergistic effect in this paper.Firstly,the catalysts with unique morphologies were obtained based on structural design,and then the inactive metal active cites of catalysts were transformed into metallic compounds with high catalytic activity via a modified strategy.Finally,the catalyst synthesis is further extended to the preparation of air electrode.A novel porous carbon nanotube network was used to construct self-supported composite air electrode with homogeneous distribution of catalyst,thus achieving enhancement in electrochemical performance of air electrode.The main research contents and results were displayed as follows:（1）Folic acid/cobalt nanotube（FACo/NTs）precursors were prepared using a simple template-free self-assembly strategy,and then transformed into nitrogen-doped carbon nanotubes embedded with Co nanoparticles（Co-NC/NTs）after high temperature pyrolysis.Finally,Co-NC/NTs-derived Co₉S₈-NC/NTs materials can be obtained by a sulfurization process.During the synthesis process of precursors,the morphologies of folic acid as carbon/nitrogen source changed from block structure to nanotube structure in the presence of hydrazine hydrate and Co²⁺.The precursors-derived Co-NC/NTs possessed abundant Co-N_xsites and nitrogen species,which were the main active sites for oxygen reduction reaction（ORR）.While Co₉S₈ in Co₉S₈NC/NTs originated from the vulcanization of Co-NC/NTs was the main active site of oxygen evolution reaction（OER）.Therefore,the as-prepared Co₉S₈NC/NTs nanotubes displayed excellent bifunctional catalytic activities due to the highly coupled and synergistic effects of Co-N_x and Co₉S₈.As expected,the as-prepared Co₉S₈-NC/NTs catalyst exhibited a high half-wave potential of 0.83 V,and low OER overpotential of 320 m V at 10 m A cm^-2.The Zn-air battery with the Co₉S₈-NC/NTs catalyst delivered a higher discharge capacity of 753.7 m Ah g^-1,power density of 116.6 m W cm^-2 and more remarkable stability（140 h）than the Pt/C/Ru O₂.（2）A template-assisted strategy was designed to prepare core-shell structure Co Zn ZIF@PS precursors consisting of polystyrene（PS）cores and bimetallic MOF composite shells via a self-assembling,followed by a thermal treatment and further sulfurization process to obtain hollow stacked carbon spheres（Co₉S₈/NHSC-T）with a high catalytic activity.By adjusting the ratios of Co/Zn in the precursors,it was found that the addition of Zn was beneficial to prevent the agglomeration of Co nanoparticles and increase the specific surface area of material.Meanwhile,the hierarchical porous structure of hollow stacked carbon spheres was conducive to facilitate electrolyte infiltration and expose more active sites.When the ratio of Co/Zn is 0.5,the obtained Co₉S₈/NHSC-0.5 material had abundant Co-N_x,Co-S bonds and N species,resulting in excellent bifunctional catalytic activity.As a result,the Co₉S₈/NHSC-0.5 had the best ORR and OER performance,with an ORR half-wave potential of 0.85 V and OER(10 m A cm^-2)overpotential of 320 m V,which were comparable to commercial Pt/C and Ru O₂ catalysts.The assembled Zn-air battery based on Co₉S₈/NHSC-0.5catalyst showed a high peak power density of 116 m W cm^-2)and a stable rechargeability over200 h.（3）3D crosslinked freestanding carbon nanotubes films（Co/N@CNTs@CNMF）with high catalytic activity were obtained by in-situ catalytic formation of cobalt nanoparticles embedded in nitrogen-doped carbon nanotubes（Co/N@CNTs）grown in the carbon nanotube microfilm（CNMF）.The CNMF network with porous structure can promote the uniform growth of Co/N@CNTs on the surface and form strong interface coupling,which was conducive to improve the stability of electrode.Benefiting from the high nitrogen content,strong interfacial coupling and synergetic effects,the Co/N@CNTs@CNMF can deliver a half-wave potential of 0.86 V for ORR and an overpotential of 310 m V for OER at a current density of 10 m A cm^-2,which is better than the commercial Pt/C and Ru O₂ catalysts.The aqueous Zn-air batteries assembled by Co/N@CNTs@CNMF deliver a maximum power density of 133 m W cm^-2,and the battery performance had no obvious degregation after continuous 190 h of charge-discharge cycling.Furthermore,the flexible Zn-air batteries show a maximum power density of 26.5 m W cm^-2 and exhibit an impressive flexibility and stability at different bending operation.