Preparation Of Polyimide Foam Supported Pd,Au,Co₃O₄ Electrodes And Their Electrocatalytic Properties For Hydrogen Peroxide

The ever-increasing energy demands in global and excessive depletion of the fossil fuels facilitated the development of new energy conversion and storage devices such as fuel cells and metal-air batteries.H₂O₂ has been used as the oxidizer and fuel in fuel cell.As an electron acceptor and electron donor,redox reaction occurs between cathode and anode,and then electrons are released on the anode.The electrons are transmitted to the cathode through the external circuit and combine with the oxidant to form ions.Under the action of the electric field,ions migrate to the anode through the electrolyte and react with the fuel to form a circuit,following with a generation of current.This process could realize the energy conversion from chemical energy to electrical energy.The performance and application range of fuel cells depend largely on the preparation methods of catalyst and the selection of catalyst support.The structure and properties of catalyst support as substrate material play an important role in the growth rate,particle size,dispersion,and catalytic performance.Due to the unique and stable physicochemical properties,polymer materials have gradually been paid attention in the research of fuel cell catalyst support.Based on the application background of polyimide-based electrode support materials,the poor conductivity of traditional polyimide support materials and the research status of nanometal catalysts in fuel cell,the construction of the electrode surface microstructure and various preparation methods of catalytic electrodes were investigated in this thesis.PI foam was prepared by“two-step”method.3D reduced graphene oxide/polyimide foam(3D RGO@PI foam,3D RPF)was obtained by loading conductive reduced graphene oxide through dip-drying or hydrothermal method,which exhibited a three-dimensional open porous structure and could be propitious to the formation of continuous conductive networks,providing the effective path for the escape of gas during the electrocatalytic period.The as-prepared 3D RPF substrate has the characteristics of light-weight and good thermal stability.A series of novel electrodes were prepared with different morphology and size nanometal catalysts by using diverse methods,following with 3D RPF as catalyst support.The catalytic performance of hydrogen peroxide electroreduction and electrooxidation were explored in acid and alkaline systems,respectively.3D RPF catalyst supportPalladium-loaded reduced graphene oxide/polyimide foam electrode Pd/3D RGO@PI foam(PRP)was successfully prepared on the 3D RPF substrate by electrodeposition.The structure,morphology and properties of PRP electrode were characterized by means of XRD,XPS,SEM,TEM,LSV,CA and CV.The catalytic activity,stability,reaction activation energy,reproducibility and repeatability of PRP electrode for H₂O₂ electroreduction and electrooxidation in acidic and alkaline systems were investigated.The results showed that PRP electrode exhibited good catalytic performance for H₂O₂ electroreduction and electrooxidation in acidic and alkaline systems.The good three-dimensional open skeleton structure,the dispersibility of Pd nanoparticles and RGO nanosheets as well as the ideal specific surface area played an important role in the catalytic performance of PRP electrode in both acid and alkaline systems.After 1000 circles of CV under the same conditions,the reduction or oxidation current density on PRP electrode could reach more than 93%of the original reduction or oxidation current density.Two kinds of gold-loaded reduced graphene oxide/polyimide foam electrodes which denoted as Au/3D RGO@PI foam(E-ARP)and 3D Au/RGO@PI foam(3D ARP)were prepared on the 3D RPF substrate by electrodeposition and one-pot method,respectively.The structure,morphology and H₂O₂ electroreduction behavior in acid of the E-ARP and 3D ARP electrodes were investigated.It showed that both of the two electrodes have good three-dimensional open structure.However,the size,loading amount and dispersion of Au nanocatalyst on both two electrodes were different.E-ARP electrode was loaded with a large amount of micro/nano Au catalyst,some of which were agglomerative.The particle size of Au catalyst supported on 3D ARP electrode was only 4.26±2.08 nm,following with good dispersion.It demonstrated that the loading of Au catalyst on the 3D ARP electrode was only one tenth of that of E-ARP electrode in geometric area of 1 cm².Both electrodes have good stability,reproducibility and repeatability.Compared with E-ARP electrode,3D ARP electrode which prepared by"one-pot method"exhibited lower reaction activation energy and better catalytic performance than that of E-ARP electrode for H₂O₂ electrooxidation under the same conditions.It can be attributed to the small-sized Au nanoparticles catalyst on the 3D ARP electrode,which could exhibit a better nanosize effect and achieve effective utilization of catalyst in unit area.Two electrodes with Co₃O₄ and Pd-Co₃O₄ nanocomposites which denoted as Co₃O₄/3D RGO@PI foam(CRP)and Pd-Co₃O₄/3D RGO@PI foam(PCRP)were prepared on the 3D RPF substrate by in situ coprecipitation and stratified assembly method.The structure,morphology and H₂O₂ electroreduction behavior in alkaline of both electrodes were investigated.As a result,both electrodes exhibited a 3D open free-standing structure on which Co₃O₄ exhibited hollow rhombic dodecahedral nanocages.Each adjacent RGO nanosheet was sandwiched between Co₃O₄ nanocage layers to support each other,forming a sandwich-like free-standing structure.Experimental results revealed that porous Co₃O₄ nanocages had low catalytic activity towards H₂O₂ reduction in alkaline medium.After decoration with noble metal,the PCRP electrode which loaded with Pd/Co₃O₄ nanocomposite showed high catalytic performance and good stability.On the one hand,it may be related to the free-standing structure on the electrode surface formed by small-sized Pd nanoparticles(4.25±1.38 nm),Co₃O₄ hollow nanocages and RGO nanosheets;On the other hand,it was possible that the porous Co₃O₄ nanocages could provide a good deposition framework for noble metal,which enabled the multiple possibility electroreduction reactions of H₂O₂ on PCRP electrode.Compared with PRP,E-ARP and 3D ARP electrodes,PCRP electrode exhibited a better electrocatalytic performance for H₂O₂electroreduction under the condition of low noble metal loadings.