Modification of Electrode Substrates Using Nanocomposites Including Carbon Nanomaterials and Multimetallic Nanoparticles for Electrocatalysis of some Fuel Cells Reactions

Rezaee, Sharifeh | 2020

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  1. Type of Document: Ph.D. Dissertation
  2. Language: Farsi
  3. Document No: 53312 (03)
  4. University: Sharif University of Technology
  5. Department: Chemistry
  6. Advisor(s): Shahrokhian, Saeed
  7. Abstract:
  8. In this research, nanocomposite containing reduced graphene oxide nanosheets (RGO) and trimetallic three-dimensional (3D) Pt-Pd-Co porous nanostructures was fabricated by galvanic replacement technique. First, GO suspension was drop-casted on the electrode surface, then GO film reduction was carried out by cycling the potential in negative direction to form the RGO on GCE (RGO/GCE). Then, electrodeposition of the cobalt nanoparticles (CoNPs) as sacrificial seeds was performed onto the RGO/GCE by using cyclic voltammetry. Afterward, Pt-Pd-Co 3D porous nanostructures fabrication occurs through galvanic replacement method based on a spontaneous redox process between CoNPs and solution containing PtCl2, PdCl2. The performance of the prepared modified electrode toward ethylene glycol electrooxidation was investigated by various electrochemical methods. Also, for comparison other electrodes such as Pt-Pd-Co/GCE, Pd-Co/RGO/GCE, Pd-Co/GCE, Pt-Co/RGO/GCE and Pt-Co/GCE prepared. The study on electrocatalytic performances revealed that Pt-Pd-Co/RGO/GCE exhibit a lower onset potential, higher peak current density, high stability for the ethylene glycol electrooxidation. The excellent performances are attributed to the RGO as catalysts support and resulting synergistic effects of the trimetallic and appropriate characteristics of the resulted 3D porous nanostructures.In the second part of the thesis, a new nanocomposite containing PtPd nanoflowers and Cu2O nanosheets supported on RGO (PtPd-NFs/Cu2O-NSs/RGO) prepared. SEM images showed that vertical-standing nanosheets arrays of Cu2O with an edge length up to 1 µm and thickness of about 20 nm are electrodeposited on the surface of RGO film. Also, PtPd needle-like NFs with visible and clear pricks totally covered the Cu2O-NSs/RGO surface. Also for comparison, Cu2O nanoparticles and PtPd nanoparticles were prepared by the electrodeposition method at the surface of RGO (PtPd-NPs/Cu2ONPs/RGO). The catalytic activities of the prepared nanocomposites toward methanol oxidation are studied through different electrochemical tests. PtPd-NFs/Cu2O-NSs/RGO exhibited an outstanding electrocatalytic activity, lower onset potential and high poisoning tolerance toward methanol oxidation. The obtained result can be ascribed to the synergetic effect between bimetallic PtPd, Cu2O, and RGO. Also, Cu2O nanostructure can be appeared as a catalytic mediator, facilitating the charge transfer and enhance the CO poisoning oxidation through spillover of OH to PtPd surface. the unique shape and morphology of the Cu2O-NSs and PtPd-NFs have a significant influence on the catalytic behavior of the nanocomposite. So, according to results, PtPd-NFs/Cu2O-NSs/RGO can be considered as a promising anode catalyst in direct methanol fuel cells.In the third part of the thesis, a nanocomposite containing manganese dioxide (MnO2) and bimetallic palladium-nickel nanoparticle (Pd-Ni) supported on RGO is prepared by electrodeposition method. The catalytic activities of different electrodes based on Pd/GCE, Pd/C/GCE, Pd/RGO/GCE, Pd-Ni/RGO/GCE, and Pd-Ni/MnO2/RGO/GCE for ethanol oxidation are compared using through different electrochemical tests. The obtained results revealed significantly higher electroactive surface area (ECSA), higher catalytic activity, and better stability of Pd-Ni-MnO2/RGO/GCE toward the electrooxidation of ethanol compared to the other electrodes. The overall results corroborate the role of MnO2, Ni, and RGO important constituents that significantly improve the electrocatalytic behavior, stability, and CO poisoning tolerance of Pd during the electrooxidation process. Thus, Pd-Ni-MnO2/RGO/GCE catalyst can be considered as a promising anode catalyst for alkaline direct ethanol fuel cells.In the fourth part of the thesis, 3D ultrathin petal-like NiCo/NiO-CoO/nanoporous carbon composite (NiCo/NiO-CoO/NPCC) prepared based on bimetallic MOF. This nanocomposite is synthesized by a two-steps procedure involving preparation of bimetallic MOFs by partially substituting Ni2+ in the Ni-MOF structure with Co2+ (Ni-Co/BDC) and direct carbonization. The prepared nanocomposite was used directly as a non-precious electrocatalyst for methanol oxidation reaction. The results indicated that, in comparison to the Ni/NiO/NPCC and Co/CoO/NPCC, NiCo/NiO-CoO/NPCC exhibits excellent performance toward the methanol electrooxidation. The unique porous petal-like structure with free pores and the enlarged specific surface area provides fast ion/electron transfer, leading to faster kinetics, lower over-potential, and higher electro-catalytic reactivity. Also, the excellent conductivity of carbon frame, as well as metals/metal oxides synergistic effects provides favorable catalytic activity for the electro-oxidation of methanol.In the fifth part of the thesis, an MOF-carbon based composite is developed to synthesize a transition metal phosphide catalyst for the electrocatalytic oxidation of urea. Hence, in this work, first poly (pyrrole-co-aniline, PPCA) hollow nanospheres are fabricated through in-situ emulsion polymerization of a mixture of aniline and pyrrole in the presence of Triton X-100. Then, simple carbonization treatment of PPCA hollow spheres leads to carbonized hollow carbon nanospheres with ultrahigh surface areas and nano-morphologies. After that, bimetallic MM'/MOFs (M/M'= Ni, Co) uniformly are grown around HCNs by a simple hydrothermal reaction (NiCO/MOF@HCNs). Also, during the synthesis process by adjusting Ni/Co ratios, MOFs morphology can be engineered, so that by reducing Ni/Co ratio flower-like structures change to urchin-like structures. Finally, this NiCO/MOF@HCNs precursor with different Ni/Co ratio during in-situ carbonization/phosphorization, chemically converted into Ni-Co mixed metal phosphides (NixCo2-xP/C@HCNs). Finally, the electrocatalytic activity of the prepared catalysts is tested for urea oxidation reaction and hydrogen evolution reaction. Obtained results indicate that, NixCo2-xP/C@HCNs as a bifunctional electrocatalyst can be used for H2 production and simultaneous urea oxidation in cathode and anode, in the two-electrode electrolyzer cell
  9. Keywords:
  10. Methanol Electro-Oxidation ; Direct Methanal Fuel Cell ; Reduced Graphene Oxide ; Hydrogen Evolution Reaction ; Urea Electrooxidation ; Ethanol Electrooxidation ; Ethylene-Glycol Electrooxidation

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