Energy Storage/electrocatalytic Properties Of Seaweed Fiber@ZIF-67/ZIF-8 Derived Materials

Because of the shortage of fossil energy and the serious damage to the environment,it is very important to fabricate economic and environmentally friendly energy conversion technologies,including zinc-air batteries and supercapacitors.Oxygen reduction reaction（ORR）is a key reaction on the cathode of zinc-air batteries,which with slow reaction kinetics can decide the final performance of the device.In order to improve the reaction rate of ORR,it is necessary to develop efficient electrocatalysts.Usually,Pt-based catalysts as excellent catalysts can boost ORR activity,but they can’t be applied on a large scale limited by the shortcomings:high cost,low reserves and low tolerance.Hence,it is very important to fabricate non-noble metal ORR catalysts with high-performance and low-cost.Transition metal-nitrogen-carbon（TM-N-C）materials are considered to be the most promising non-noble metal catalysts due to their low cost and comparable catalytic activity with Pt.Moreover,TM-N-C materials fit for electrode materials of electric double layer capacitors（EDLCs）due to abundant pore structure and high surface area.Metal-organic frameworks（MOFs）are a class of crystal materials with porous structure.It possesses the large specific surface area and rich porosity,adjustable pore channel and site modification.Zeolite imidazolate frameworks（ZIFs）are an important branch of MOFs,which are a three-dimensional network composed of nitrogen-containing organic ligands and metal ions.Rich TM,C,and N atoms in ligands can be used to prepare porous TM-N-C composites.In addition,ZIFs materials have the advantages of low cost,easy synthesis and strong stability.However,poor conductivity of ZIFs derived materials limits their activity.Therefore,loading ZIFs on carbon materials is an effective method to improve conductivity and activity.Porous carbon materials extracted from various biomasses have the advantages of unique structure and low cost.For example,alginate is a polysaccharide,containing rich carboxyl and hydroxyl groups,which can chelate with metal ions(Zn²⁺、Co²⁺and Ni²⁺)to form an"egg-box"structure.Metal ions in alginate fibers（AFs）can provide growth sites for loading ZIFs.And then,AFs@ZIFs composites were prepared.After carbonization at high temperature,the cobalt AFs（Co-AFs）can be converted into Co carbon fibers（Co-CF）,which increases the conductivity and catalytic activity of the material.In this paper,Co/Zn-ZIF-67 was grown on AFs as carriers.After the carbonization,Co/Zn-N-C composites were prepared.Their ORR activity and supercapacitor properties were studied.The details are as follows:（1）The Ca²⁺in Ca-AFs is replaced by Co²⁺by simple ion exchange,forming Co-AFs.Using Co²⁺as the reaction site,ZIF-67 particles are uniformly loaded on the surface of the Co-AFs through regular solvothermal reaction.The Co-AFs@ZIF-67precursor was carbonized in the Ar atmosphere at 900°C.ZIF-67 was pyrolyzed at high temperature to form porous Co-N-C polyhedra.Meanwhile,AFs was pyrolyzed to CFs at high temperature.The final product is the Co-N-C particles loaded on the CFs（CoNC-CF-900）.Among the prepared samples at different pyrolysis temperatures,CoNC-CF-900 showed the best ORR performance in 0.1 M KOH electrolyte with the half-wave potential of 0.833 V（vs.RHE）,onset potential of 0.926 V（vs.RHE）,and limited current density of-5.56 m A cm^-2.The CoNC-CF-900 exhibited excellent stability with high current retention of 94.8%after 20000 s（the current retention of 20%Pt/C catalyst after 20000 s is about 80.9%）.The excellent performance is attributed to the Co and N catalytic active sites produced by ZIF-67 and the CFs.The CFs can enhance their conductivity,which can enhance proton and electron transfer during the ORR.（2）Replacement of Ca²⁺in Ca-AFs with Co²⁺/Zn²⁺through simple ion exchange,Co/Zn-AFs were prepared.Using Co²⁺/Zn²⁺as the reaction site,Co/Zn-ZIF-67 particles are uniformly loaded on the surface of Co/Zn-AFs.The Co/Zn-AFs@ZIF-67 precursor was carbonized in Ar atmosphere at 800°C.ZIF-67 was pyrolyzed to form porous Co/Zn-N-C polyhedra at high temperature.At the same time,AFs were pyrolyzed to CFs at high temperature.The final product is Co/Zn-N-C particles loaded on the CFs（Co/Zn NC-CF-800）.When used as electrode material for symmetric supercapacitors,Zn NC-CF-800-KOH sample exhibits the maximum specific capacitance of 155 F g^-1 at current density of 0.5 A g^-1 due to its largest surface area of 1202.4 m² g^-1.