Preparation And Performance Study Of Bimetallic ZIF-67 (Co/Zn) Derived Electrocatalyst

With the increase of the world population and the improvement of the economic level,traditional fossil fuels,such as coal and petroleum,have been continuously consumed.As a result,a series of global environmental problems have arisen.Energy shortage and environmental pollution have become two major challenges.Therefore,it is necessary to develop green energy and efficient energy conversion devices to solve these challenges.Energy conversion devices,such as zinc-air batteries?ZABs?and proton exchange membrane fuel cells?PEMFCs?,use clean energy?hydrogen,wind,water and solar?as fuels.They are considered to be an important green technologies to solve energy waste and environmental pollution problems due to its high energy conversion efficiency,safety and non pollution.During the energy conversion process in these batteries,oxygen reduction reaction?ORR?occurs on the cathode.ORR requires an efficient catalyst to improve the energy conversion efficiency.Platinum?Pt?-based materials are recognized as highly efficient catalysts to promote ORR performance.However,Pt-based catalysts are prone to aggregate,resulting in poor stability and methanol tolerance.Furthermore,the scarcity and high cost of the Pt-based catalysts significantly restrict their widespread usage.Therefore,exploring alternative earth-abundant and high-efficiency ORR catalysts is imperative and urgent.Zeolitic imidazolate frameworks?ZIFs?have three-dimensional network structures which are formed by imidazole organic ligands and metal ions through coordination bonds.Due to the N atoms in organic ligands,nitrogen atoms are doped into the carbon skeleton during the pyrolysis treatment.Thus ZIFs as the precursor are benefit to generate the metal-nitrogen-carbon?M-N-C?active materials.At the same time,ZIFs have large specific surface area and rich porosity after the pyrolysis,which can effectively improve the transmission efficiency of reactants and products,improving the ORR performance.In this paper,we used Co/Zn-ZIF-67 as the precursor to obtain the CoN₃-NPs or cobalt-zinc bimetallic materials by adjusting the Co/Zn molar ratio in the precursor or the pyrolysis condition.Furthermore,we investigated their ORR performance.The specific research contents are as follows:?1?CoN₃ nanoparticles?NPs?were controllably synthesized by precisely adjusting the molar ratio of Co/Zn species in the Co/Zn-ZIF-67 precursor.The precursors were subsequently heated at 1000°C under Ar atmosphere.During the pyrolysis progress,the precursors were first converted into Co-NPs embedded in N-doped porous carbon?Co@NC?,accompanied by the release of NH₃ from decomposition of the ZIF structure.The abundant micropores and large surface area can be formed by the evaporation of Zn.When the molar ratio of Co/Zn is in the range of 5/95?9/91,the large surface area and enough micro-pores can facilitate full contact between the Co species and the NH₃molecules to generate CoN₃-NPs.When the Co/Zn molar ratio is 7/93,the prepared catalyst?CoN₃@NC-7-1000?delivers an outstanding ORR activity in 0.5 M H₂SO₄(half-wave potential E_1/2=0.72 V?vs.RHE?)and 0.1 M KOH electrolyte(E_1/2=0.825 V?vs.RHE?).This is mainly ascribed to the large specific surface area and porous structure of the catalyst.They are beneficial to expose more CoN₃ active sites and effectively shorten the diffusion distance between the active sites and the reactants,accelerating the transmission of reactants and products.?2?The Co/Zn-ZIF-67 as the precursor?Co/Zn molar ratio is 7/93?was pyrolyzed at750°C under argon atmosphere for 4 h.In this stage,a part of Zn and Co can be stably present in the framework with Zn-N-C and Co-N-C structures.And then the obtained material was pyrolyzed at 900°C under argon atmosphere.Zn atoms exist in the form of Zn-Nx while Co atoms aggregate to form Co-NPs.In the sample,some inactive NPs can be washed away by HCl treatment.The porous carbon material with bimetallic active sites??Zn,Co?/NC?was synthesized.In acidic medium?0.5 M H₂SO₄?,the material shows excellent ORR catalytic performance(E_1/2=0.72 V?vs.RHE?)and stability.In addition,it has excellent half-wave potential?0.864 V?vs.RHE??and current density in 0.1 M KOH medium.