Preparation Of ZIF-Derived Carbon-Supported Cobalt Composites And Their Catalytic Performance For The Electrooxidation Of NaBH₄

Direct sodium borohydride fuel cell（DBFC）is an energy supply device using NaBH₄ as fuel（reducing agent）and O₂ or H₂O₂ as oxidant.Transition metal cobalt has a strong ability to promote the cleavage of B-H bonds and a low price,which has great application potential in catalyzing NaBH₄ electrooxidation（BOR）.However,its catalytic performance is still poor compared with the precious metal catalyst of traditional electrode materials.Reasonable design of the structure can make it play a high catalytic activity comparable to precious metals.Zeolitic imidazolate framework（ZIF）is a kind of metal-organic framework（MOF）material based on imidazole acid and transition metal.It has the advantages of abundant pores,high adjustability,and low price.The cobalt-based catalyst obtained by pyrolysis of ZIF as precursor has a porous nitrogen-doped carbon structure and a large specific surface area.However,the metal in a single ZIF-derived carbon material is easy to agglomerate,resulting in a decrease in catalytic performance.In this paper,zinc-based ZIF materials,graphene oxide,and nickel foam were introduced as two-dimensional or three-dimensional skeletons to prepare three nitrogen-doped carbon composites with metal Co atoms uniform dispersion,which were used as BOR catalysts.The specific research contents are as follows:Using ZIF-8 as a precursor,a two-dimensional porous N-doped carbon nanosheet containing metal Co（Co-NCNS）was prepared by hydrolysis in Co²⁺aqueous solution and further pyrolysis.The presence of metal Zn can inhibit the agglomeration of Co and make it evenly distributed on the surface of the nanosheets,exposing more active sites.At a pyrolysis temperature of 950°C,metal Zn escapes forming a porous structure.The specific surface area of the material is 487.96 m²·g^-1.The maximum current density is 653.4 m A·cm^-2 in 2.0 mol·L^-1 Na OH and 0.5 mol·L^-1 NaBH₄ solutions.The electrode has good stability at different voltages.On the Co-NCNS electrode,the activation energy is 15.36 k J·mol^-1,and the number of transferred electrons n is 6.1.In order to improve the weak conductivity of ZIF materials,two-dimensional graphene oxide（GO）was introduced and further pyrolyzed to prepare carbon-coated hexahedral cobalt composites loaded on two-dimensional sheets.Due to the presence of carbon,cobalt agglomeration was reduced.The graphene oxide and cobalt-based material precursors were completely reduced after pyrolysis.Compared with the pyrolysis product of ZIF-67,it has an ultra-thin lamellar structure and better electrical conductivity.By changing the calcination temperature,it is found that when the calcination temperature is 700°C,the obtained catalyst has the best catalytic BOR performance.In 2.0 mol·L^-1 Na OH and 0.5 mol·L^-1 NaBH₄ solution,the maximum current density is 759.0 m A·cm^-2,and the activation energy is 11.93 k J·mol^-1,and the number of transferred electrons n is 6.8.The use of nickel foam as a matrix to prepare three-dimensional materials avoids the use of binders in powdered catalysts.At the same time,three-dimensional materials have a larger specific surface area than two-dimensional materials,exposing more catalytic active sites.A Co-NC/NF precursor was prepared by impregnation and pyrolysis,and the effects of different Co²⁺and 2-methylimidazole ratios on the material were investigated.Changing the ratio can accurately control the morphology of the catalyst.When the ratio is 1:8,the prepared precursor Co-NC/NF is a triangular sheet structure and has the best performance.In the 1.0 mol·L^-1 Na OH and 0.4 mol·L^-1 NaBH₄ solution,the maximum current density is 621.6 m A·cm^-2.Co LDH@ZIF-67/NF was prepared by electrodeposition using the precursor ZIF-67/NF with the ratio of 1:8 as a template.Nanosheet stacked porous hydrangea Co@Co-NC/NF was obtained after pyrolysis,and the sheet layer was more dense and lighter.The maximum oxidation current density is 1049.5 m A·cm^-2 in 2.0 mol·L^-1 Na OH and 0.3 mol·L^-1 NaBH₄solution;The number of transferred electrons is 7.9,which is very close to the theoretical number of transferred electrons（8 electrons）of BOR.The activation energy is 10.23 k J·mol^-1,which is comparable to that of precious metal catalysts.