Boron Doped Diamond Films Prepared By Hot Filament Chemical Vapor Deposition As Catalysts For Oxygen Reduction Reaction

Fuel cell is a widely concerned power generation technology with clean,environmentally friendly,and low emissions.However,it has not yet been widely used in industry.Main reasons are a slow oxygen reduction reaction in the fuel cell cathodic and a highly irreversible in the dynamic.It means that oxygen reduction reaction needs catalysts to enhance reaction.Borondoped diamond is one kind of metal-free promising catalyst which possesses a certain catalytic performance,meanwhile maintains the excellent mechanical and thermal properties of diamond.Up to now,the research on the mechanism of oxygen reduction and the influence of borondoped diamond as catalyst are not clear.In this paper,the effect of doped boron and graphite content of boron-doped diamond on the oxygen reduction reaction was studied.The main works consist of the selection of substrates,the preparation of boron-doped diamond catalysts,the control of boron and graphite content.The morphology,microstructure,composition,and electrochemical properties were investigated.Diamond films were deposited on graphite,titanium,silicon and silicon powder respectively by hot filament chemical vapor deposition.SEM,XRD and Raman spectroscopy analysis showed that the diamond grain on graphite substrate is complete.It is very difficult to distinguish between the graphite from the diamond films and from the substrate by XRD pattern and Raman spectrum.Although the diamond grains on the silicon substrate are also complete and the XRD pattern and the Raman spectra show that the diamond peaks are obvious,the silicon substrate has poor conductivity.Diamond on silicon powder has poor crystallinity.These substrates are not suitable for the subsequent analysis.The diamonds on titanium substrate have good quality and XRD pattern and Raman spectra show that the diamond peaks are obvious.Titanium substrate also has good conductivity.It demonstrates that the Titanium substrate is suitable for the subsequent test and analysis.Boron doped diamond films with different boron contents were prepared on titanium substrate by solid boron-doped method.The boron content is controlled by changing the diameter of the boron source container so as to control the boron concentration in the reaction atmosphere.The analysis results show that diamond has been boron-doped in the atomic level.With the increase of boron concentration,boron doped diamond grain size decreased,boron content increased gradually,diamond(220)crystal surface decreased and(111)crystal surface increased.The potential of oxygen reduction reaction did not change obviously,which was maintained near-0.77 V.The reaction current density increased first and then decreased,which was the result of the increase of the defects in boron doped diamond and the decrease of the step atom density.At the same time,the boron doped diamond has a good durability and methanol resistance.By changing the ratio of carbon source concentration(CH4/H2)in the deposition atmosphere to adjust the content of graphite in the boron-doped diamond under constant other parameters.The results show that with the increase of carbon source concentration,the grain size of boron-doped diamond decreases,the grain boundary increases,and the graphite content increases.The current density of oxygen reduction reaction does not change obviously,and the reaction potential moves forward with the increase of graphite content.Comprehensive analysis shows that the most suitable substrate for this experiment is titanium,boron content in boron-doped diamond has a great influence on the current density and graphite content has a great influence on the potential for oxygen reduction reaction.Borondoped diamond has good durability,stable performance in the reaction for a long time,and good resistance to methanol.