Structure And Electrocatalytic Performance Of Carbon Based Non-noble Metal Catalysts Derived From Metal-organic Framework For Cathode Of Fuel Cells

Hydrogen energy is a vital green energy technology that can contribute to the new wave of clean energy transition.Fuel cells are devices that convert the chemical energy of fuels into electrical energy through electrochemical reactions.They offer high fuel efficiency,low noise,and zero emissions,making them suitable for various applications.Among different types of fuel cells,proton exchange membrane fuel cells（PEMFC）are considered as one of the key solutions to address future environmental challenges.However,there are numerous drawbacks to PEMFC,including its expensive price,short lifespan,and low power density.The oxygen reduction reaction（ORR）at the cathode is the main factor that affects the overall performance of PEMFCs,as it involves a slow four-electron transfer process.Therefore,one of the major challenges for the development of PEMFC is to find a high-efficiency ORR catalyst that can operate in acidic media without using noble metals.To enhance the activity and stability of the cathode catalyst and thus improve the overall performance of PEMFCs,it is crucial to design and synthesize novel materials with rational structures and active sites.In this paper,we report a multistage porous catalyst derived from nitrogen-rich zeolitic imidazolate frameworks（ZIF）precursors,which exhibits a hierarchical pore structure of macro-,meso-and micropores and a uniform distribution of active sites.By combining experimental and computational methods,we investigate the catalytic mechanism of the diatomic catalysis on the ORR steps and reveal the structure-activity relationship of the material for facilitating mass transfer,suppressing hydrogen peroxide and its derivatives,and lowering the activation energy.The main contents of this paper are summarized as follows:1.A Co/Zn-ZIF structure supported by a Co and Zn mixture was designed to address the issue of low utilization of catalytic sites in traditional catalysts due to poor contact between the active site and the reactive gas.After high-temperature heat treatment,a porous structure with a high surface area and porosity was formed,which facilitated mass transfer for the sites inside the catalyst.Density functional theory calculations and experiments confirmed that Co and Zn alternately formed ZIF structures rather than separate ZIF-8 or ZIF-67 structures.This unique interwoven combination ensured abundant catalytic sites and structural stability.Moreover,the evaporation of Zn during heat treatment promoted the formation of a multilevel pore structure with more mesopores,which further enhanced the utilization of catalytic sites.The effects of different sizes of ZIF particles on pore volume and Co agglomeration,as well as the effects of different pyrolysis methods and temperatures on the electrocatalytic performance of the prepared catalysts were investigated.The results showed that the 200 nm Co/Zn-ZIF catalyst prepared by 950°C flash heating method has the highest catalytic activity.2.To address the issue of low utilization of catalytic sites in traditional catalysts due to agglomeration of transition metal-based active sites and poor contact with reactive gas,a Co/Zn-ZIF structure was grown in situ on both sides of GO by precisely adjusting the mole ratio of Co/Zn species in the Zn-Co-ZIF precursor.A stable Co-N-RGO catalyst was successfully prepared by high-temperature heat treatment.The ZIF precursor contained rich zinc species,which acted as a barrier to enlarge the distance between adjacent Co atoms and prevent them from forming zero-valent Co particles with no catalytic activity during heat treatment.This enabled more Co-N_x sites to be evenly dispersed in the catalyst,increasing the number of catalytic sites.The evaporation of Zn during heat treatment created a large number of conductive pores in the ZIF and established a hierarchical porous structure connected by uniformly dispersed Co-N_x sites.This exposed the Co-N_x active sites in the bulk phase and improved their utilization rate.It also allowed Nafion and O₂ to enter the pores and form a three-phase catalytic site with them,while facilitating the export of H₂O and avoiding micropore flooding.The reduced graphene oxide（RGO）enables the formation of a connectivity network among the derived carbon particles,which reduces the electron transfer impedance and increases the electron transfer efficiency,thus improving the overall power density and durability of the fuel cell.The peak power density of the Co-N-RGO-1.5 sample reached 334 m W cm^-2.This strategy avoided additional acid etching and secondary heat treatment steps that are traditionally required.3.The intrinsic activity of Co-N_x for ORR in acidic environment is low,which limits the catalyst half-wave potential and the fuel cell power density under high voltage.Selenium（Se）atom has a special electronegativity and a high electrical polarizability,which are expected to enhance the electron transport and the catalytic ORR activity of materials.In this work,we introduce Se element by ball milling strategy and anchor atomically dispersed Co and Se diatoms on nitrogen-doped carbon to construct Co/Se diatomic catalyst.The Co-Se-N-C catalyst exhibits significantly higher ORR activity in acidic electrolytes than both Co-N-C and Se-N-C,demonstrating the synergistic effect of its diatomic sites.In addition to Se-C serving as the ORR reaction’s active site,the introduction of Se atom can effectively modulate the charge redistribution and modifies the electronic state of the Co-N_x active site.As a result,the activation energy barrier of the rate-determining step was lowered from 0.52 e V to 0.38 e V,encouraging charge transfer and decreasing the membrane electrode’s charge transfer resistance from 0.507 to 0.326.Moreover,due to the low sublimation temperature of Se O₂,most of it volatilizes in the calcination process,leaving abundant pore structure on the catalyst that facilitates mass transfer.This study not only proves that the construction of Co/Se diatomic site catalysts can improve the ORR activity through multiple effects,but also provides a unique insight into the rational design of more efficient catalysts by introducing nonmetallic site atoms.It offers a new strategy to improve the electrochemical performance of metal-nonmetal diatomic catalysts.4.In view of the problem that Fe-N-C,as ORR catalyst,tends to degrade rapidly in acidic PEMFC environment,it is analyzed that one of the main reasons for this problem is that H₂O₂,the intermediate product produced by 2e^-step in the ORR reaction process,reacts with Fe to form free radicals.Oxidation corrodes the Fe-N-C catalyst itself and the proton exchange membrane,resulting in poor durability of PEMFC.Therefore,we design a dual catalytic site and introduce Mn O_x next to the Fe-N_x site,because Mn O_x can accelerate the degradation of the by-product H₂O₂ through disproportionation reaction,thus weakening Fenton reaction and reducing or even eliminating the stability problem.At the same time,after coating Mn O_x,a layer of ZIF structure is coated twice to provide N source,which contributes to the generation of Fe-N_x site,and effectively avoids the direct generation of Fe₃C with no catalytic activity in the carbonization process of Fe.The synthesized Fe-N_x/Mn O_x dual-site catalysts exhibited inhanced activity and stability in both electrochemical tests and fuel cell tests,and the current density was maintained at 209 m A cm^-2 after 36 h of endurance testing at 5.5 V,which was a significant improvement compared with 108 m A cm^-2 for the Fe-N-C sample.