Investigation On Design, Fabrication And Electro-catalytic Properties Of Nano-structured Pt And Pd Catalysts

In this paper, in order to fabricate high-performance catalysts for direct methanol fuel cells (DMFCs), the Al-based precursor alloys containing Pt were fabricated by mechanical alloying or rapid solidification process, and then the precursor alloys were dealloyed in NaOH and HNO3solutions. Finally, the nanoporous Pt-based binary or ternary alloys can be obtained with high performance for methanol oxidation reaction (MOR) and oxygen reduction reaction (ORR) in DMFCs. On the other hand, Pd nanoparticles with superior electro-catalytic properties were spontaneously formed through the combination of anodization and cyclic voltammogram (CV) reduction processes.Through mechanical alloying and subsequent two-step dealloying method, ultrafine nanoporous Pt3Cu (np-Pt3Cu) and PtNi (np-PtNi) alloys have been successfully fabricated from Al2(Cu95Pts) and Al74Ni25Pt1precursor alloys, respectively, which were characterized by the X-ray diffraction (XRD) and electron microscopic characterization analysis. Electrochemical tests indicate that the obtained np-Pt3Cu alloy exhibits superior electro-catalytic activity and stability towards MOR, as well as enhanced ORR activities in acidic circumstance compared to the commercial PtC catalyst. Density functional theory (DFT) calculations reveal that the electronic structure of Pt has been modified with the shift of Pt d-band center due to alloying with Cu, which can decrease CO poisoning and enhance the MOR and ORR activities. Moreover, the nanoporous structure and connecting ligaments of the np-Pt3Cu alloy are much favorable for the mass and electron transport in three-dimension, which greatly accelerates the reaction kinetics on the nanoporous alloy modified electrode surfaces. Similarly, the np-PtNi alloy fabricated by mechanical alloying and two-step dealloying method also exhibits enhanced MOR activity compared to the commercial PtC catalyst.Through rapid solidification and subsequent two-step dealloying method, nanoporous PtM (M=Co, Cu, Ni)(np-PtM) alloys have been successfully fabricated. Electrochemical tests indicate that the obtained np-PtM alloys exhibit superior MOR and ORR activities and stability compared to the commercial PtC catalyst, and different elements additions have an obvious effect on the improvement degree of the electro-catalytic activities. It is thought that the electronic structure varies from that of pure Pt among these alloys, which is responsible for the improved electro-catalytic activities of np-PtM alloys.By mechanical alloying and subsequent different dealloying processes, nanoporous PtPdCu (np-PtPdCu) ternary alloys with different compositions (T1-T6alloys) have been obtained from the AlCuPtPd precursor, which was characterized by the XRD and electron microscopic characterization analysis. Electrochemical tests suggest that there are obvious difference of electro-catalytic MOR and ORR activities among these alloys (T1-T6alloys), which is mainly caused by the different compositions due to the different dealloying conditions. Finally, the as-dealloyed T3alloy (dealloying in2M NaOH and1M HNO3solutions) exhibits the best electro-catalytic activities for MOR and ORR.By mechanical alloying and subsequent dealloying process, nanoporous PtRuCu and PtRuNi ternary alloys have been fabricated from the AlCuPtRu and AlNiPtRu precursors, respectively. Electrochemical tests demonstrate that nanoporous PtRuCu exhibits better electro-catalytic activities for MOR than the commercial PtC and PtRuC catalysts, while its CO poisoning resistance ability is a little poor than the commercial PtRuC catalyst. Meanwhile, the nanoporous PtRuNi alloy shows no electro-catalytic activity for MOR and certain electro-catalytic activity for electro-oxidation towards formic acid.The anodization and subsequent CV reduction processes result in the formation of Pd nanostructures on the electrode surface. Compared to the bulk Pd, the anodization of Pd in H2SO4solutions leads to different CV behaviors including well separated adsorption/desorption peaks in the hydrogen region and relatively larger reduction peak areas. The improvement of electrochemically active surface areas (ECSAs) corresponding to the larger reduction peak areas of the anodized Pd samples results in better electro-catalytic activities towards methanol, ethanol and formic acid compared to the bulk Pd. Meanwhile, the electrolyte concentration, the applied potential and polarization time have a significant influence on the anodization process of Pd. The ECSAs of the anodized Pd obtained under the optimum polarization conditions (1.0M H2SO4,2.0V vs. MSE,90min) can reach as large as890times compared to its geometric area. The present findings provide a promising route to fabricate nanostructured Pd electrocatalysts with ultrahigh ECSAs.