Proton Exchange Membrane (PEM) Fuel Cells are a promising technology for the clean energy production, especially in the automotive field. Actually, the main commercial catalysts employed in this system are based on Pt Nanoparticles supported on high surface area Carbon. The main issues associated to PEM Fuel Cells deal with the sluggish kinetic of oxygen reduction (ORR) at Platinum based electrode, with the low stability of both the carbon support and the metal phase, that tend respectively to oxidize and dissolve or diffuse and with the high cost due to rare and expensive Pt. In fact, nowadays high costs and low durability are the two factors that make PEM fuel cells still not competitive with internal combustion engine. For these reasons, research now focuses on obtaining more stable material with higher performances toward ORR. Two strategies are possible to improve catalyst for oxygen reduction. The first one deals with the enhancing of Pt activity modifying its electronic properties by alloying Pt with other transition metal (ligand effect) or by reducing the Pt-Pt distance (geometric effect). In both cases a Pt d-band shift occurs, which is responsible for the modification of adsorption and desorption energies of all species involved in ORR and has as a direct consequence a modulation in the electrochemical activity. The second strategy deals with the utilization of supports more stable respect to corrosion, like graphene, carbon nanotubes or mesoporous carbons. Furthermore, doping of carbon support with heteroatoms like N or S, can help to stabilize the metal nanoparticles. In fact, doping creates homogeneous and narrow dispersion of small metallic nanoparticles, strongly bound to the surface of carbon support and with a higher resistance to agglomeration. Furthermore, doping has as well an influence on the electronic structure of the Pt catalyst, resulting in a modulation of its electrochemical activity. Doping is not beneficial only in noble-metal catalyst but may also modify properties of the carbon support in which heteroatoms are present. Wettability, electrical conductivity and electrochemical activity are generally boosted when heteroatoms are inserted in carbonaceous substrates such as carbon blacks (CBs). The topics of this PhD thesis are Platinum NPs on doped carbon and Platinum-Yttrium alloy NPs on carbon. The goal consists in the understanding how the different synthesis parameters can influence the Pt-Y alloy formation and can modify the NPs growing. An increment of interact means increasing the electrochemical performance vs. the Oxygen Reduction Reaction (ORR). The Platinum deposition investigation is conducted via solid state reduction of several Platinum and Yttrium salts, in order to find the best conditions which, allow to have a good Pt NPs distribution over all surface of the carbon support. The synthetized PtxY@C catalysts are characterized by TEM, SEM, ICP, XRD, XPS and TGA techniques. Cyclic Voltammetry in steady conditions and with Rotating Disk Electrode are employed for the determination of electrochemical surface area (ECSA) and catalytic activity toward ORR, respectively, and compared to a commercial Pt/C catalyst. The catalytic activity of pure platinum can be increase by interaction with heteroatoms which permits to modify the absorption energy of oxygen and increase the Oxygen Reduction Reaction rate. The typical heteroatoms which interact very strongly with the platinum are sulphur and nitrogen. Platinum on nitrogen doped carbon was synthetized via solid state synthesis using particular platinum complex which contain nitrogen ligand. The goal consists in the synthesis of catalysts with a nitrogen surface distribution very close to the platinum NPs for increasing the Pt-N interaction and so for increasing the electrochemical performance. The metal-support interactions (MSI) between sulphur doped carbon and Pt nanoparticles (NPs) were investigated, for understanding how sulphur functional groups can improve the electrocatalytic activity of Pt NPs towards the oxygen reduction reaction (ORR). Sulphur doped carbons were synthetized by hard template method, tailoring the density of sulphur functional groups, and Pt NPs were deposited by thermal reduction of Pt(acac)2. The metal-support interaction was evaluated and proved by X ray photoelectron spectroscopy and X ray diffraction, the analysis revealed a strong electronic interaction between Pt and S proportional to the density of sulphur group. The combination between the micro-strain and the electronic effects resulted in a high catalytic activity of Pt NPs vs. ORR, showing a correlation of the electrochemical activity with the sulphur content in the carbon support. Sulphur affords a clear metal support interaction between Pt NPs and the doped carbon support; the NPs dimension and distribution are influence by heteroatom concentration in the support but especially by the morphology (in terms of surface area, pore dimension and pose distribution) of the carbon matrix. The surface area of sulphur doped carbon was modify by steam treatment. The carbon matrixes were completely physic-chemical characterized with TEM, Raman BET, AE. The platinum NPs were deposited by high temperature solid state synthesis with Pt(acac)2 using a temperature of 300 °C for 3 h and 8% H2. XPS, XRD and N2 Adsorption/Desorption analysis show a double correlation between the electrochemical activity and the sulphur concentration and the carbon morphology.

Synthesis and characterization of materials for PEM-FC, based on Pt alloyed nanoparticles supported on next generation mesoporous carbon / Brandiele, Riccardo. - (2019).

Synthesis and characterization of materials for PEM-FC, based on Pt alloyed nanoparticles supported on next generation mesoporous carbon.

Brandiele, Riccardo
2019

Abstract

Proton Exchange Membrane (PEM) Fuel Cells are a promising technology for the clean energy production, especially in the automotive field. Actually, the main commercial catalysts employed in this system are based on Pt Nanoparticles supported on high surface area Carbon. The main issues associated to PEM Fuel Cells deal with the sluggish kinetic of oxygen reduction (ORR) at Platinum based electrode, with the low stability of both the carbon support and the metal phase, that tend respectively to oxidize and dissolve or diffuse and with the high cost due to rare and expensive Pt. In fact, nowadays high costs and low durability are the two factors that make PEM fuel cells still not competitive with internal combustion engine. For these reasons, research now focuses on obtaining more stable material with higher performances toward ORR. Two strategies are possible to improve catalyst for oxygen reduction. The first one deals with the enhancing of Pt activity modifying its electronic properties by alloying Pt with other transition metal (ligand effect) or by reducing the Pt-Pt distance (geometric effect). In both cases a Pt d-band shift occurs, which is responsible for the modification of adsorption and desorption energies of all species involved in ORR and has as a direct consequence a modulation in the electrochemical activity. The second strategy deals with the utilization of supports more stable respect to corrosion, like graphene, carbon nanotubes or mesoporous carbons. Furthermore, doping of carbon support with heteroatoms like N or S, can help to stabilize the metal nanoparticles. In fact, doping creates homogeneous and narrow dispersion of small metallic nanoparticles, strongly bound to the surface of carbon support and with a higher resistance to agglomeration. Furthermore, doping has as well an influence on the electronic structure of the Pt catalyst, resulting in a modulation of its electrochemical activity. Doping is not beneficial only in noble-metal catalyst but may also modify properties of the carbon support in which heteroatoms are present. Wettability, electrical conductivity and electrochemical activity are generally boosted when heteroatoms are inserted in carbonaceous substrates such as carbon blacks (CBs). The topics of this PhD thesis are Platinum NPs on doped carbon and Platinum-Yttrium alloy NPs on carbon. The goal consists in the understanding how the different synthesis parameters can influence the Pt-Y alloy formation and can modify the NPs growing. An increment of interact means increasing the electrochemical performance vs. the Oxygen Reduction Reaction (ORR). The Platinum deposition investigation is conducted via solid state reduction of several Platinum and Yttrium salts, in order to find the best conditions which, allow to have a good Pt NPs distribution over all surface of the carbon support. The synthetized PtxY@C catalysts are characterized by TEM, SEM, ICP, XRD, XPS and TGA techniques. Cyclic Voltammetry in steady conditions and with Rotating Disk Electrode are employed for the determination of electrochemical surface area (ECSA) and catalytic activity toward ORR, respectively, and compared to a commercial Pt/C catalyst. The catalytic activity of pure platinum can be increase by interaction with heteroatoms which permits to modify the absorption energy of oxygen and increase the Oxygen Reduction Reaction rate. The typical heteroatoms which interact very strongly with the platinum are sulphur and nitrogen. Platinum on nitrogen doped carbon was synthetized via solid state synthesis using particular platinum complex which contain nitrogen ligand. The goal consists in the synthesis of catalysts with a nitrogen surface distribution very close to the platinum NPs for increasing the Pt-N interaction and so for increasing the electrochemical performance. The metal-support interactions (MSI) between sulphur doped carbon and Pt nanoparticles (NPs) were investigated, for understanding how sulphur functional groups can improve the electrocatalytic activity of Pt NPs towards the oxygen reduction reaction (ORR). Sulphur doped carbons were synthetized by hard template method, tailoring the density of sulphur functional groups, and Pt NPs were deposited by thermal reduction of Pt(acac)2. The metal-support interaction was evaluated and proved by X ray photoelectron spectroscopy and X ray diffraction, the analysis revealed a strong electronic interaction between Pt and S proportional to the density of sulphur group. The combination between the micro-strain and the electronic effects resulted in a high catalytic activity of Pt NPs vs. ORR, showing a correlation of the electrochemical activity with the sulphur content in the carbon support. Sulphur affords a clear metal support interaction between Pt NPs and the doped carbon support; the NPs dimension and distribution are influence by heteroatom concentration in the support but especially by the morphology (in terms of surface area, pore dimension and pose distribution) of the carbon matrix. The surface area of sulphur doped carbon was modify by steam treatment. The carbon matrixes were completely physic-chemical characterized with TEM, Raman BET, AE. The platinum NPs were deposited by high temperature solid state synthesis with Pt(acac)2 using a temperature of 300 °C for 3 h and 8% H2. XPS, XRD and N2 Adsorption/Desorption analysis show a double correlation between the electrochemical activity and the sulphur concentration and the carbon morphology.
2019
Fuel Cell Platinum Catalyst ORR Pltinum Alloy Doped mesoporous carbon
Synthesis and characterization of materials for PEM-FC, based on Pt alloyed nanoparticles supported on next generation mesoporous carbon / Brandiele, Riccardo. - (2019).
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Utilizza questo identificativo per citare o creare un link a questo documento: https://hdl.handle.net/11577/3422707
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