Study On CO₂ Electrochemical Reduction Properties Of Diamond-based Thin Film Materials

Nowadays,environmental protection and sustainable development have triggered a great interest in the research on conversion of greenhouse gas CO₂ into value-added substrates.As one of promising solution to CO₂ conversion,electrocatalytic reduction of CO₂ has been extensively studied and got more and more attentions,in which electrode materials are one of the most important key technology factors.Comparing with widely used metal-based electrodes,diamond-based thin film materials is a powerful candidate because of its wide potential window in aqueous solution which can effectively suppress hydrogen evolution reaction,a competitive side effect for CO₂ conversion,as well as possessing excellent comprehensive physical and chemical properties like high chemical inertness and mechanical durability.However,the electrocatalytic reaction on the diamond film electrode is a very complicated process,numerous factors may influence the electrochemical behavior such as the dopant type,concentration of dopants,non-diamond carbon impurities,surface termination state and grain orientation,etc.Although many studies on diamond utilizing as CO₂electrochemical reducing have been accomplished,there are still few attention on the influence of the intrinsic characteristics of diamond film on its CO₂ electrochemical reducing performances.Therefore,in this thesis systematically studies were focused on the relationship between intrinsic characteristics of diamond film and their CO₂electrochemical performances.Establish theoretical support for obtaining high-efficiency and high-quality diamond-based CO₂ electrocatalytic reduction film electrode materials.The research contents are as follows:（1）A series of Boron Doped Diamond（BDD）films were prepared by hot filament chemical vapor deposition method by changing the amount of Boron in the reaction gas source during deposition process.The morphology and quality of the films were characterized by SEM,Raman and XRD.The electrochemical properties of the films were analyzed by LSV and CO₂ reduction experiments.The influence of boron content on the composition and CO₂electrochemical reduction performance of the films was investigated.The results show that the increase of boron introduce can get rise to more secondary nucleation,and also can increase their growth rate when the B/C ratio chosen within a suitable range（2000-17000 ppm）,along with to enhance their electrocatalytic activity and CO₂ electrochemical performance.When B/C in the reaction gases too high（=40,000 ppm）,however,the incorporation of boron will seriously damage the diamond structure,reduce the quality of the diamond film,and increase the occurrence of hydrogen evolution side reactions.At the same time,it is also found that the increase of boron doping will increase the adsorption of CO₂ reduction intermediate(CO₂^?﹣)on the surface of the diamond film,thereby result in CO produced.（2）To further make clear the relationship between intrinsic characteristics of BDD materials with their CO₂reduction performance,the boron-doped diamond films also have been prepared by only change the BDD growth time using Heat Filament chemical vapour deposition technology under two kinds of B/C（7000 and 17000 ppm）.SEM observation predicted that with the longer deposition time,the thickness,surface grain size of the films increased synchronously,and the crystallinity would improve correspondingly while the grain orientation,the boron total content and the surface termination of the films maintaining approximately the same observed by XRD,Raman and XPS analysis.Moreover,the abundances of B-C relative to B-B bonds of the films also would increased when growing more time,which was demonstrated by the results of XPS and Hall effect test.Using the obtained BDD films as cathode to electrochemical reducing for CO₂,an increased formic acid yield,Faradaic efficiency and positive shifted potential emerged for the longer depositing time BDD electrodes,which can be attributed to their bigger superficial area,better crystallinity and more available B doping.So for this kind of potential CO₂ electrochemical reducing electrode material,a simple and convenient path to effectively improve the performance of BDD is to lengthen their growing duration to get more favourable material features for electrocatalytic reduction performance.（3）Two novel nitrogen-doped diamond films were prepared by MPCVD through changing deposition temperatures（750℃and 850℃）.SEM and XRD analysis showed that the average grain size of the film grown at 750℃is less than 30 nm,the proportion of grain boundary is very large and as-film only contained diamond phase.Matched with the widen G peak and D peak in the Raman spectroscopy,it could be proved that this film is a typical N-doped Ultra-nano Crystalline Diamond film（N-UNCD）.For the film prepared at 850℃,the composed phases include diamond and graphite along with the extremely thin graphite sheets are vertically cross arranged to forming a honeycomb shape structure,in which distributed many extremely small diamond grains.All these composition and structure characteristics indicated this is a Nitrogen-doped Diamond/Multilayer Graphene Nano-hybrid composite film（N-DMGN）.Electrochemical test results indicated that the two novel nitrogen-doped diamond films both have wide potential window（～3 V）,which can effectively inhibit the competitive reaction of hydrogen evolution during CO₂.At the same time,both electrodes could effectively electrocatalytically reduce CO₂ to generate formic acid and CO,which maximum Faraday efficiencies could achieve 38.37%and 15.32%,respectively.But comparatively,the N-DMGN film grown at 850°C has better electrochemical activity,higher CO Faraday efficiency and can produce multi-electron transfer product-methane.The reason could attribute to its own"honeycomb-like"structure that increases the surface area.Moreover,the suitable sp³-C/sp²-C ratio can effectively combine with the intermediate of CO₂ reduction,and further increase the diversity of products.So for nitrogen-doped diamond film materials,this composite film with abundant pore structure is a more potential electrode material using in CO₂ electrochemical reduction.