Methanol-Tolerant Platinum Electrocatalyst For Oxygen Reduction By Encapsulating Platinum Nanoparticles Inside Carbon-Based Nanocages

Direct methanol fuel cells (DMFCs) are expected to play an important role in addressing the growing energy demand and environmental crisis for their high efficiency, safety, easy storage and transport of methanol. Compared with the alcohol oxidation reaction (AOR) at anode, the oxygen reduction reaction (ORR) at cathode in acidic media is a crucial factor limiting the performance of DAFCs due to its sluggish kinetics. Nowadays, precious Pt-based electrocatalysts, though expensive and scarce, are the most popular due to the much better performances than those of the non-Pt catalysts, especially in acidic media. To improve Pt utilization and performance, usually Pt-based nanoparticles are highly dispersed on the carbon supports with high specific surface area, good chemical stability and conductivity. One of the main challenges for Pt-based ORR catalysts is the performance degradation owing to the alcohol crossover effect. To date, little progress has been achieved in developing anti-alcohol permeable membrane. Thus, it is of great significance to explore the alcohol-tolerant Pt-based ORR electrocatalysts and has attracted broad interests. Pt alloying with some other metals could enhance the methanol tolerance but only to a certain extent. By embedding Pt nanoparticles into the nanochannels of mesoporous carbon with the help of the ordered mesoporous silica (SBA-15) or metal-organic frameworks, the methanol tolerance could be obviously improved. Very recently, it was reported that a special complex containing single-atom Pt exhibits a high methanol tolerance with good ORR performance. In these approaches, a multi-step chemical process is usually involved. Recently, we have developed a novel 3D hierarchical carbon-based nanocages featured with the high specific surface area, good conductivity and coexisting micro-meso-macro-pores, which exhibit excellent performances as energy materials. Interestingly, there are numerous micropores of ca.0.6 nm in size in the graphitized shells, which can serve as mass-transfer channels between the inside and outside of the nanocages. This dissertation is focusing on the design, synthesis and alcohol-tolerant ORR performance of encapsulating Pt inside carbon-based nanocages. The main contents are summarized as follows:1. Construction of Pt@NCNC and Pt/NCNC catalysts with Pt nanoparticles inside or outside hollow nanocages:Mesostructured carbon nanocages (CNC) and nitrogen-doped CNC (NCNC) with high electrical conductivity and large specific surface area were first prepared using benzene and pyridine as precursor and in situ MgO as template. Pt@CNC and Pt@NCNC, hereinafter referred to as Pt@(N)CNC, with Pt nanoparticles of ca.1.3 nm in size inside hollow nanocages were constructed by facile vacuum-filling process. Similarly, Pt/(N)CNC with Pt nanoparticles of ca.2.6 nm in size outside the surface of nanocages were constructed by microwave-assisted ethylene glycol (EG) reduction.2. Excellent ORR activity, durability and alcohol tolerance of Pt@(N)CNC catalysts:Pt@(N)CNC with inside Pt present an excellent ORR activity in acidic media as usual for Pt-based electrocatalysts, e.g., the onset potential of Pt@NCNC is 650 mV (vs. Ag/AgCl), which is slightly smaller than ca.680 mV of Pt/NCNC and Pt/C. They demonstrate the excellent alcohol immunity and durability much better than the cases for the counterpart Pt/(N)CNC with outside Pt and the commercial Pt/C catalyst. The superb alcohol-tolerant ORR performance of Pt@(N)CNC results from the molecule-sieving effect of the micropores penetrating through the shells of nanocages, which admit the small-sized oxygen and ions but block the large-sized alcohols into the nanocages, as confirmed by examining the size dependence of ORR and AOR activities by regulating the micropore sizes and quantities. The excellent stability benefits from the much retarded erosion of Pt species due to its encapsulation inside hollow nanocages. The results in this study suggest a promising strategy to develop the superior alcohol-tolerant Pt-based ORR electrocatalyst in acidic media by making use of molecule-sieving effect, or even beyond.3. In situ synthesis and ORR activity of Pt@NCNC:Pt@NCNC were in situ synthesized by the facile method. Briefly, Pt nanoparticles were first supported on the surface of MgO by microwave-assisted EG reduction, dipped with magnesium acetate, then pyrolyzed to form new MgO layer, deposited CNx layer using pyridine as carbon source, finally removed the MgO template with the diluted HC1. The as-prepared Pt@NCNC demonstrates the completely alcohol-tolerant performance in acidic media although the ORR activity is not good enough. Of course, it only is very preliminary result. Thus, there should be huge space to develop the superb alcohol-tolerant Pt-based ORR electrocatalyst in acidic media by in situ encapsulating Pt nanoparticles inside the cavity of carbon nanomaterials with numerous micropores, which admit the small-sized oxygen and ions but block the large-sized alcohols through the shell.