Preparation Of Silicon-Doped Carbon Cathode Electrocatalysts For Fuel Cells

Fuel cells as new power generating devices are the most promising alternatives to the traditional energy in the future. However, traditional noble metal Pt-based cathode catalysts used in fuel cells have a lot of disadvantages, such as high cost, poor stability and weak methonal tolercence, which has been a bottleneck restricting the large-scale application of fuel cells. In recent years, as one of the most promising alternatives to the Pt-based catalysts, the novel metal-free, heteroatom-doped carbon catalysts with low cost, excellent electrocatalytic activity and methonal tolercence are becoming one of the hottest research topics in the field of fuel cells. In order to develop more excellent metal-free cathode carbon catalysts, we tried to introduce a new doping atom of silicon into the carbon framework and evaluated oxygen reduction reaction(ORR) activity, stability and methanol tolercence. The results are as follows:Silicon doped carbon nanospheres(Si-CNSs) were synthesized by chemical vaporous deposition and measuraed their ORR activities by electrochemical tests. Although the atomic radius of silicon is much larger than that of carbon atom, test results showed that the doping of silicon into the carbon framework could improve the ORR activity of carbon catalyst. The ORR activity of Si-CNSs in alkaline medinum was 2.4 times that of undoped CNSs. The calculated average value of n for the Si-CNSs(2.8) was higher than CNSs(2.0). Furthermore, the stability and methanol tolercence of Si-CNSs all exceeded the commercial Pt-C catalyst.Given the larger specific area of carbon nanotubes(CNTs) than that of CNSs, its electrocatalytic activity of CNTs frequently surpassed that of CNSs. Thus, with Fe-Mo/Al2O3 as catalyst, we tried to synthesize a series of Si-doped CNTs with different silicon contents. Electrochemical tests showed that the electrocatalytic activities of the synthesized Si-doped carbon nanotubes towards the ORR all higher than that of pure carbon nanotubes. Their ORR activities did not enhance proportionally with the increase of silicon doping content in alkaline medinum. After silicon doping, the energy gap(ΔE) of SiCNTs decreased from 2.75 to 2.01 eV, the absorption style of oxygen changed, the average value of n for the Si-doped CNTs increased from 2.17 to 3.06. These results strongly suggested that the electrons in Si-doped CNTs are much more easier to be excited to O2 than those in pure CNTs.Meanwhile, in order to further improve the ORR activity of silicon doped carbon catalysts, we tried to prepare Si, N-codoped carbon nanotubes(SiN-CNTs) as well. The effect of thermolysis temperature on the electrochemical performance was also investigated. Test results showed that the best thermolysis temperature(SiN-CNTs700) was 700 oC. The diffusion current densities of Si N-CNTs700 below-0.1 V was about 1.4 to 2.6 times those of N-CNTs700. Moreover, the current densities of SiN-CNTs700 below-0.45 V even exceeded those of the commercial Pt-C catalyst. The calculated average value of n for the SiN-CNTs700(3.3) was obviously considerably higher than that(2.0) of the pure CNTs, indicating that the the proportion of the four-electron ORR process would increase after the doping of Si and N into the CNTs. Density functional therory calculations were carried out for Si,N-codoped graphene sheet. The results showed that ΔE(2.52 eV) of the pure graphene was much higher than that(1.53 eV) of Si,N co-doped graphene sheet, indicating that the electrons in Si, N-doped grapheme could be more easily transferred to O2. Moreover, SiN-CNTs700 had more excellent stability and methanol tolerance than the commercial Pt-C catalyst.It is worth noting that, rare studies on the fabrication and ORR activity of Si-doped carbon previously.