Preparation And Performance Of MOF-based Low Platinum Fuel Cell Catalysts

Proton exchange membrane fuel cells（PEMFCs）,as an efficient and clean energy conversion device,have been widely used due to their high power density and low operating temperature.However,the slow oxygen reduction reaction（ORR）process at the cathode leads to the accumulation of electrons,resulting in a high overpotential,so catalysts are needed to accelerate the reaction to improve the performance of the fuel cell.At present,commercial platinum（Pt）-carbon catalysts have the problems of high price,insufficient activity and durability.Therefore,many researchers have optimized the crystal structure,morphological characteristics and electronic structure of the active components of Pt-metal.At the same time,in order to improve the corrosion resistance of the carrier,a large number of researchers have modified the physical structure and charge distribution of the carbon carrier.The ORR performance can be improved effectively by regulating the catalyst components.Based on these research bases,in this paper,by regulating the metal active components and modified carbon support,including limiting effect,alloy structure and N doping strategy,the carbon support has a strong interaction with the metal,so as to reduce the amount of Pt and improve the ORR activity and stability of the catalyst.The morphology,physical structure,electronic structure and electrochemical properties of catalyst and carrier were tested and analyzed,and the structure-activity relationship between carrier and metal active components was obtained.The main research contents are as follows:（1）In order to improve the utilization rate of Pt metal in the catalyst,the ZIF-derived carbon protective layer was grown around the metal nanoparticles and on the surface of the carbon carrier,and the Pt alloy catalyst 3-Co-PtCu/NC with small particle size and uniform dispersion was obtained through the limiting effect.By adjusting the number of layers and thickness of the ZIF protective layer,according to TEM and AFM,when the number of coating layers is three,the thickness of the ZIF protective layer is about 3 nm,and the coating effect is the best at this time,so that the metal nanoparticles are partially coated,but not completely covered by the carbon layer.This can better immobilize and protect the metal nanoparticles,so that they can still maintain small size during synthesis and reaction,and improve the utilization rate of Pt metal.In addition,the ZIF precursor was transformed into an N doped carbon structure after high temperature calcination,and Co and N elements were introduced.Some of the Co species form alloys with Pt and Cu,and the other part exists in the form of Co–N bonds with N.The alloy structure and the N doping effect can adjust the crystal state and electronic structure of Pt,respectively,thereby changing the bond spacing of Pt–Pt and enhancing the metal-support interaction.Under the combined action of these effects,3-Co-PtCu/NC showed a mass activity of 0.69 A mg _Pt^-1,which showed excellent stability after 10,000 potential cycles.（2）In order to improve the oxidation corrosion of the carrier under acidic medium and high potential,nitrogen doped porous carbon material was independently prepared,and the alloyed PtCu/Co-NPC-1 catalyst was obtained after supporting PtCu alloy.The nitrogen doped porous carbon carrier was prepared by in-situ doping using PS microspheres as pore forming template and ZIF-67 as carbon and nitrogen sources.By adjusting the size of the PS microspheres of the pore-forming template,it was concluded that when the styrene input was 1 g and the diameter of the PS microspheres was about 150nm,the carrier Co-NPC-1 obtained after segmental calcination had higher specific surface area and mesoporous content,which could expose more active sites and facilitate mass transfer,so the pore-forming effect of PS microspheres of this size was the best.In addition,after pyrolysis of the precursor ZIF-67,Co and N elements were introduced,in which the metal Co formed an alloy with PtCu,and part of the Co ions formed Co–N bonds with N as the active site.According to BET,the NPs of Pt alloy is mainly reduced in mesoporous,and the size effect of mesoporous structure can effectively inhibit the aggregation and growth of NPs at high potential,so that NPs can maintain a small size.In addition,N doping can optimize the electronic structure of carbon materials and Pt,thereby improving the stability of carbon materials and enhancing the metal-carrier interaction.Through the template method,the particle size is maintained at 2-4 nm,which can effectively improve the utilization rate of Pt,so PtCu/Co-NPC-1 exhibits a mass activity of 0.68 A mg _Pt^-1,and only 13%of the mass activity is lost after stability test.（3）In order to further improve the structural stability of the carrier and catalyst,dodecahedral nitrogen doped porous carbon carrier was independently prepared.Using ZIF as a self-sacrificing template and as both nitrogen and carbon source,nitrogen-doped porous carbon dodecahedron was obtained by pyrolysis of Zn Co-ZIF.The alloy type catalyst PtCu-Co_0.1/NC was obtained by supporting PtCu metal.After adjusting the addition ratio of Zn to Co in the Zn Co-ZIF precursor and calcination at high temperature,according to the morphology and physical characterization of the Zn Co-ZIF precursor,the prepared Co_0.1/NC had the largest specific surface area when Zn∶Co=10∶1.Due to the low boiling point of Zn metal,it creates pores for carbon carriers after heat evaporation,and at the same time causes spatial isolation to Co,which alleviates the aggregation phenomenon of Co under high temperature conditions.This structure has a large specific surface area,which can better disperse Pt alloy NPs,and facilitate the transport of electrolyte and reactant.In addition,the alloy structure and N doping effect reduce the oxygen adsorption energy of Pt and promote the ORR process.PtCu-Co_0.1/NC exhibited a mass activity of0.62 A mg _Pt^-1,and the morphology and structure of the support were stable after10,000 cycles of potential cycling,without collapse,and the mass activity of the catalyst decreased by only 10%.（4）In order to enhance the interaction between metal and carrier in the catalyst,a catalyst with high nitrogen content was prepared.Using CuCo-ZIF as nitrogen and carbon sources,the alloy catalyst PtCuCo/NC with high nitrogen content was prepared by supporting Pt metal on ZIF by in-situ doping.After doping the second metal Cu in the Co-based ZIF precursor,according to SEM and XRD,the addition of Cu has no effect on the morphology and physical structure of ZIF.After loading Pt by dip reduction method,the XPS test showed that PtCuCo/NC had a higher content of N element than Pt Co/NC,which was due to the fact that N species tended to form metal–N bonds with Cu,and more N elements were fixed.The increase of N doping in the catalyst can alleviate the loss of N species during high temperature pyrolysis,and at the same time,more N can interact with Pt,thus optimizing the electronic structure of Pt.The incorporation of N atoms into the carbon network can improve the charge distribution of carbon,resulting in strong interaction between the carrier and the metal,and enhance the anchoring of the carbon carrier to the NPs of the Pt alloy.The shrinkage of the Pt lattice caused by the alloy structure weakens the adsorption energy of Pt to oxygen-containing intermediates,and metal–N can be used as the active site to assist in reduction.PtCuCo/NC with high nitrogen content exhibited mass activity of 0.63 A mg _Pt^-1 and retained a high half-wave potential after stability test.In summary,this paper adopted the limiting effect,alloy structure and nitrogen doping effect to adjust the metal active components in the catalyst,and modified the physical structure and charge distribution of the carbon carrier.Through the combined analysis of the morphology,size,physical structure,charge distribution and electrochemical performance of the two,the structure-activity relationship between the metal and the carbon carrier was studied.Thus,the ORR activity and durability of platinum-based catalysts are improved,which provides an experimental and theoretical basis for the preparation of low-cost and good durability Pt alloy catalysts.