Architecturing Highly-efficient Carbon-based Electrocatalysts Based On Microstructural Regulation And Metal-support Interaction

To improve the energy conversion efficiency and increase the output power in energy conversion and storage technologies such as fuel cells,metal-air batteries and water electrolysis,it is necessary to reduce the overpotential of electrode reaction.Thus,highly-efficient electrocatalysts are needed to reduce the energy barrier of the reactions.The catalysts for these electrode reactions are mainly precious metal or composites with high precious metal content.However,the precious metals have limited reserves on earth,and being expensive,which hinders its large-scale commercial application.Therefore,it is imperative to develop materials with available rich raw materials resources and low cost to replace the precious metal catalyst.Carbon materials with abundant resource and various species,have multiple excellent physical and mechanical properties,being widely used in electrocatalysis.With doping and modification,carbon materials can serve as electrocatalyst.It can also act as support materials to increase the specific surface area and enhance the conductivity.Based on microstructural regulation and metal-support interaction,combined with different materials preparation methods and unlocked the full potential of carbon materials themselves,this thesis developed several low cost and high efficient carbon based electrocatalysts and studied the relation between the microstructure and electrocatalysis performance.The contents of specific research are as follows:1.Nitrogen-doped amorphous carbon（a-C）film was prepared by magnetron sputtering,and then used as electrocatalyst for oxygen reduction reaction（ORR）.Though regulating the partial pressure of nitrogen and argon,a-C films with different nitrogen content were prepared.After annealing at 700°C,the ORR activity of the films is gradually improved with the increase of nitrogen content.This indicates that the ORR activity has positive correlation with total nitrogen content.Moreover,once the partial pressure of nitrogen and methane is changed,a-C films with different nitrogen content were also prepared.However,the ORR activity of these a-C films has no correlation with nitrogen content.Because methane not only is the sputtering gas,but also exists as reaction gas.This indicates that the nitrogen content has no direct dependence on ORR activity when the changes occurred in reaction gas.Moreover,before annealing,the film has no catalytic activity.After mild annealing,the film structure remains amorphous,with a transformation tendency from amorphous to nanocrystalline feature.The films show ORR activity at this point.The results indicate that the occurrence of ORR activity is mainly due to that other types of nitrogen transform into more stable pyridinic N and the increase of sp²C clusters in the a-C films.According to the analysis of ORR activity,pyridinic N benefits a 4e^-pathway for ORR,and the output limiting current is improved.The increase of sp²C clusters has a positive effect on ORR onset potential.The inner stress of nitrogen-doped a-C film will be gradually release,inducing some microcracks in the films.These microcracks increase the contact area between the film and solution,thus improving the ORR activity.However,these newly exposed amorphous surface is active,and being easy passivated in air.Thus,the ORR activity decreases gradually after a long time stay in air.2.After annealing at high temperature in reducing atmosphere,graphene derived from Hummers method was loaded with a small amount of Pd nanoparticles and used for ORR electrocatalysis.The loading of Pd particles induces great changes for the graphene structure,such as the significantly increased degree of disorder,the vibration mode change of the functional groups like C-O and C-H.This is the specific manifestation of the interaction between Pd and carbon support.Moreover,the Pd-C interaction makes part of Pd nanoparticles encapsulated by carbon,leading to the decrease of the electrochemical active surface area.The disorder degree of graphene increases with boron（B）-doping.However,the disorder degree decreases significantly after Pd loaded on B-doped graphene,indicating that B-doping suppress the Pd-C interaction.Combined with other analysis,it was found that B-doping not only suppress the interaction between Pd and carbon in graphene,but transfer this interaction and recover the structural disorder induced by B-doping and Pd loading.Moreover,it also avoids the Pd nanoparticles encapsulated by carbon and affords improved ORR activity.In particular,the methanol and carbon monoxide resistance of the catalyst were improved.Theoretical calculations indicate that B-doping improves the electron transfer between Pd and support.Besides,Pd has a tendency that adsorbs on the support at the adjacence of doping sites.This type of catalyst reduces the activation energy of O-O bond,and benefiting for a 4e^-ORR pathway.Moreover,B-doping suppresses the interaction between Pd and carbon and relieves the Pd from carbon,leading to an easily formed passivation film on Pd in electrolyte.Thus,it shows a negative differential resistance phenomenon in electrochemical impedance spectra,which has a negative influence on its ORR activity.3.Ni/CeO₂ prepared by sol-gel methods was first used for catalyzing methane decomposition.The obtained solid product Ni/CeO₂/carbon nanotubes（CNT）were further used as electrocatalyst for hydrogen evolution reaction（HER）.Ni/CeO₂shows high activity for catalyzing methane decomposition in comparison with the catalyst without CeO₂.This is mainly due to that the existence of CeO₂ prevents the Ni nanoparticles encapsulated by carbon layers in the heat treatment of sol-gel precursor.Moreover,because of the metal-support interaction between Ni and CeO₂,the Ni particles are dispersive in CeO₂,which contributes to the excellent catalytic activity for methane decomposition,and produce CNT support for themselves.The presence of CNT improves the electron conductivity as well as dispersing the agglomerated Ni/CeO₂.Thus,more catalytic active site was exposed,and the HER electrocatalytic activity of Ni/CeO₂ was thereof improved.Moreover,the CNT skeletons provide sufficient H₂ diffusion paths and bypass the catalyst peeling off from the electrode.Moreover,along with the methane decomposition and the growth of CNT,Ni and CeO₂ are gradually separated,leading to the reduced interface between them.The HER activity decreases for the Ni/CeO₂/CNT derived from a long time methane decomposition.This indicates that the metal-support interface between Ni and CeO₂ plays an important role in HER.In particular,methane decomposition is a hydrogen production process which solid product then acts as catalyst for HER for hydrogen generation.This two steps hydrogen production improves the overall hydrogen yield.4.Little transition metal decorated carbon nanofiber（CNF）derived from electrospunning with polyacrylonitrile（PAN）and metal nitrate as precursor,was used as a bifunctional catalyst for ORR and HER.The ORR activity of pure CNF is very poor,and has hardly any HER activity.After decorated by transition metal Fe,Co and Ni,the CNF show HER activity and improved ORR activity.This indicates that the decorated transition metals are the vital component of the active sites.Then,the CNF-Fe/Co/Ni samples were etched by hydrochloric acid,and reheat-treatment by previous temperature for structural recovering.The catalytic activity of the samples after etching decreases significantly.However,their activity was improved obviously and the functional groups containing hydrogen and oxygen disappeared after structural recovering.This indicates that the functional group with hydrogen and oxygen may occupy the electrocatalytic active sites on the surface,and lead to the reduced activity for ORR and HER.Moreover,the activity of partial samples after structural recovering,reached to or even super to the activity prior to etching.In particular,the surface defects increase after structural recovering relative to ones before etching.This indicates that the surface defects may be one of the catalytic activity origins for the carbon-based electrocatalyst.