Design, Preparation And Performance Of Electrocatalysts For Fuel Cells Based On Nitrogen- And Boron-doped Carbon Nanotubes

Fuel cells, especially direct methanol fuel cells (DMFCs) and alkaline fuel cells (AFC), are attracting considerable interest as a future power source due to their high energy conversion efficiency, ambient operating conditions and environmental benefits. The exploration of the novel electrocatalysts with low cost, high performance and good stability is the main issue for the commercialization of fuel cells. Based on the advancement of carbon-based nanotubes and the research progress in our group, this dissertation will concentrate on the application of nitrogen or boron doped carbon nanotbues as supports for Pt-based electrocatalysts or non-metal electrocatalysts for oxygen reduction reaction in DMFCs and AFCs. The main contents of this dissertation are summarized as follows:1. Taking advantage of the inherent chemical activity of nitrogen doped carbon nanotubes (NCNTs) that arises from the nitrogen incorporation, we have developed a facile strategy for the construction of monometallic Pt, Ru, Ni or Co/NCNTs and binary alloyed Pt-M/NCNTs (M=Ni,Ru,Co) electrocatalysts. Pt-based alloyed nanoparticles with an average size of about 2-4 nm, have been highly and homogeneously dispersed on NCNTs without pre-modification. The composition of Pt and M could be modulated by changing the ratio of the corresponding precursors. High dispersion of Pt-based nanoparticles, the modulated metal composition and the good conductivity of NCNTs provide us a platform for the optimization of the electrocatalytic performance of these catalysts.2. The electrochemical properties of the Pt-M/NCNTs electrocatalysts for methanol oxidation and oxygen reduction reaction have been systematically evaluated. Binary Pt-M alloyed composites with moderate molar ratio of (Pt:M) present enhanced electrocatalytic activities, improved tolerance to CO poisoning and competitive stability with the commercial catalyst or the monometallic Pt/NCNTs catalysts in acid electrolyte. The good performance of the catalysts mainly arises from i) the high dispersion of Pt-based species, ii) the alloying effect of Pt-M, and iii) the high conductivity of NCNTs. These results imply that the so-constructed composite catalysts have the potential for applications in direct methanol fuel cells.3. A novel metal-free electrocatalyst, i.e., boron-doped carbon nanotubes (BCNTs), has been developed for oxygen reduction reaction (ORR). In alkaline medium, BCNT catalyst demonstrates a low overpotential, negligible methanol crossover effect and strong resistance to CO poisoning, which is essential for fuel cells. The electrocatalytic activity of BCNT increases with increasing boron dopant, and the BCNT catalyst with 2.24 at% B dopant presents an excellent performance, even better than the commercial Pt/C catalyst with a 40.0 wt% Pt loading. The promising progress in this study suggests the intensive exploration of the metal-free electrocatalysts in the heteroatom-doped CNTs with mono-or multi-dopants such as B, N and P, which might be a hopeful strategy for developing the advanced practical electrocatalysts for fuel cells.4. To further understand this superior catalytic activity, a natural bond orbital analysis was performed for BCNT before and after O2 adsorption. Theoretical calculations show that B dopant plays the crucial role in collecting electrons from%conjugation and localizing some π* electrons in conjugated carbon system. Furthermore, B dopant also creates a tunnel between conjugated electrons and chemisorbed O2 and promotes obvious charge transfer to O2, which effectively weakens the 0=0 bond and facilitates ORR. The rational understanding of the superior performance of BCNT for ORR provides us a basis for the exploration of heteroatom-doped CNTs as advanced non-metal ORR catalysts.