Methanol Oxidation Reaction And Oxygen Reduction Reaction Mechanisms On Pt/CNTs Catalysts And Enhancement Effect By Ni/Co Phosphides

Direct methanol fuel cells(DMFCs), as one kind of portable mobile power supply with the characteristics of high specific energy, environmental friendliness and quick start, is one of the most important new energy technology in the 21 st century. So far, the catalyst for proton membrane fuel cells, is mainly based on Pt-based noble metal due to its best performance for practical application. However, the high price, scarcity of Pt and the low tolerance ability for poisoning block severely the commercialization of DMFCs. In order to cut down the Pt loading and improve its utilization, it is necessary to clarify the electrochemical reaction mechanism in anodic methanol oxidation reaction(MOR) and cathodic oxygen reduction reaction(ORR) for Pt catalyst with different Pt loadings and different Pt particle sizes. It is a scientific subject worthy of intensive study, and is also one of the main keys to solve the problems that the present Pt catalysts encounter for the large-scale commercial application.In this work, the effects of Pt loadings and particle sizes of Pt/CNTs catalyst on MOR and ORR performances were studied systematically. The corresponding mechanism about MOR and ORR was proposed. Trans-metal phosphide was adopted to modify the Pt/CNTs catalyst to impove its MOR activity and stability..The main content of this paper was listed in the following.(1) A series of Pt/CNTs with different Pt loadings were prepared and their micro-structures were characterized. The change rules of ORR and MOR activity along with the change of Pt loading amount were analyzed. Results show that for Pt/CNTs catalyst, mass transfer is the determing step for ORR. So the higher the Pt loading, the lower the mass specific activity and active surface activity of ORR became. While different from the ORR, proton transfer is the determing step for MOR. With the increase of Pt loading, the distance between Pt particles becomes closer, which enhances the proton transfer. So, Pt/CNTs catalyst shows no MOR activity at low Pt loading and better mass specific activity and active surface activity at higher Pt loading..(2) Pt catalysts with different particle sizes were prepared by hydrogen reduction in gas phase and by ethylene glycol in liquid phase, and the effects of the particle size of Pt catalysts on MOR and ORR performance were studied. Results indicated that Pt/CNTs-g catalyst with large Pt particle size prepared by hydrogen reduction shows poorer ORR performance than Pt/CNTs catalyst with small Pt particle size obtained by ethylene glycol method. But the MOR activity of Pt/CNTs-g catalyst with large Pt particle size is much better than Pt/CNTs catalyst with small Pt particle size. The mass specific activity and active surface activity of Pt/CNTs-g catalyst are 2 and 1.5 times that of the Pt/CNTs, respectively, at the same Pt content of 20%. Moreover, Pt/CNTs-g catalyst with large Pt particle size exhibitsbetter stability.(3) The mechanism that MOR is determined by proton transfer was proposed. When the Pt particle sizes are small, H+ mainly transports among Pt particles due to the high surface adsorption energy that makes H+ hard to desorb. When the Pt particle is large, H+ may desorb into the solution directly due to the decreased surface adsorption energy, leading to the increased proton transfer ability and thus improved intrinsic activity. The proton desorption needs group effect caused by a number of Pt nanoparticles. The group effect is associated with the distance of Pt nanoparticles. It is enhanced with the decrease of the distance between Pt nanoparticles. The direct desorption of H+ on large Pt particles can be regarded as the group effect caused by a number of small particles aggregated extremely.(4) Pt /CNTs was modified with Ni2 P to obtain Pt/Ni2P/CNTs. The effect of loading sequence of Ni2 P and Pt to the performance of Pt/CNTs and the effect of Ni2 P loading on morphology, structure and electrochemical activity of Pt/Ni2P/CNTs was studied. The result shows that the best MOR activity was found on the catalyst when Ni2 P was loaded first and then Pt was loaded. Ni2 P can change the size and electronic structure of Pt particles greatly. Pt/Ni2P/CNTs with 6% Ni2 P loading shows the highest MOR activity and improved durabitiywith the If of 1400 m Amg-1Pt that is more than 2 times that of Pt/C and Pt/CNTs-g with the same Pt loading.(5) Pt/Co P/CNTs catalyst modified with Co P was prepared. The influence of different Co P loadings on the morphology, structure and electrochemical activity of Pt/Co P/CNTs catalystwere investigated. Results indicate that P modification can improve the MOR activity of the catalyst. But the MOR activity of the catalyst modified with Co was greatly reduced. The MOR activity of the catalyst modified by Co P was greatly enhanced. The Pt/Co P/CNTs catalyst with 4% Co P loading shows the highest MOR activity with the If of 1600 m Amg-1Pt and improved durability, indicating that it is a potential MOR anode catalyst for application.