Proton Exchange Membrane Fuel Cell (PEMFC) is an electrochemical device which generates a direct electrical current from the chemical energy of a redox reaction between oxygen and hydrogen. One of the main components of a PEMFC is the gas diffusion medium (GDM), which has to guarantee an efficient water management within the cell and is composed by a carbon cloth macroporous gas diffusion layer (GDL) and a microporous layer (MPL) deposited on it. Graphene is an emerging and promising material which is composed by one or several atomic carbon monolayers that can be arranged in different shapes; the most significant property of this material is the conductivity and that is why it has been employed in this field. The present work aimed to prepare graphene based and graphene/carbon black composite microporous layers by using blade coating technique; moreover, two ageing processes, namely accelerated stress tests, both chemical and mechanical, have been developed in order to assess samples change of properties over time, i.e. durability. In the characterization step, morphological changes on the surfaces, both as prepared and upon ageing tests, with different compositions were investigated . In details, SEM analysis was employed to evaluate both MPLs surfaces and thickness of the deposited layers. Thermogravimetric analyses, performed up to 1000 °C, were useful to get the temperature values at which decomposition of the microporous layer components occurs. The static contact angles of the gas diffusion media were measured, on the MPL side, to evaluate hydrophobicity of the PEM fuel cell samples. The mercury intrusion porosimetry (MIP) was performed for all samples in order to get pore size distribution and pores volume of the GDMs. Electrical and electrochemical experiments were carried out in a lab-scale fuel cell with a commercial catalyst coated membrane assembled with prepared and with stressed samples aiming to assess possible changes due to accelerated stress tests. Furthermore, the resulting polarization curves were used to calculate different types of voltage losses, namely charge transfer losses, mass transport losses, crossover losses and ohmic losses. The last chapter of this work concerns the possible future outlook about gas diffusion media, i.e. graphene based MPLs obtained by evaporation of the solvent directly on the GDL substrate aiming to lower time of the whole preparation process.
La celle a combustibile con membrane a scambio protonico sono dei dispositivi che generano corrente elettrica sfruttando l’ energia chimica fornita da reazioni di ossidoriduzione tra ossigeno e idrogeno. Uno dei principali componenti delle PEM fuel cells è il gas diffusion medium (GDM), che è composto da un gas diffusion layer, tipicamente un tessuto di carbonio, ed un microporous layer, depositato sopra di esso. L’idea del nostro lavoro è quella di creare dei microporous layers composti principalmente da grafene o da compositi grafene-polvere di carbone. Il grafene è un nuovo materiale molto promettente ed è composto da multistrati di atomi di carbonio arrangiati in diverse possibili strutture. Una delle principali proprietà del grafene è la conduttività. Lo scopo del nostro lavoro è investigare come le proprietà di un gas diffusion media varino dopo i trattamenti di invecchiamento chimico e meccanico rispetto al campione non trattato. La prima analisi condotta è il SEM, con l’obiettivo di studiare come la struttura del gas diffusion media cambi in base alle diverse composizioni dei reagenti per i tre trattamenti; inoltre, una stima del possibile spessore dei campioni può essere valutata attraverso le immagini a risoluzione laterale. I campioni sono stati anche analizzati con la tecnica di termogravimetria fino a 1000°C per stabilire a quali temperature avvenga la decomposizione di tutti i componenti presenti nel GDM; inoltre, i campioni sono stati soggetti ad angolo di contatto per decretare se siano sufficientemente idrofobici da essere provati in cella. Dopo ogni trattamento tutti i campioni sono stati inseriti in una PEM fuel cell, utilizzando un’adeguata membrana catalitica, per registrare la variazione del voltaggio in base alle diverse correnti imposte. Queste misure sono state effettuate utilizzando un carico elettronico chiamato TDI POWER, RBL 488 50-150-800. Dopo aver estrapolato le curve di polarizzazione per stabilire quale tipo di campione avesse le migliori performance, sono stati effettuati dei test elettrochimici per calcolare tutti i tipi di perdite, ovvero: perdite a trasferimento di carica, perdite a trasporto di massa, perdite di crossover e perdite ohmiche. L’analisi finale condotta sui campioni è la porosità con intrusione di mercurio per stabilire se i GDM siano porosi e per stimare l’eventuale diametro medio dei micropori. L’ultimo capitolo di questo lavoro riguarda un possibile sviluppo futuro di gas diffusion media, ossia un GDM composto da grafene ed ottenuto per evaporazione del solvente.
Development of innovative graphene based gas diffusion media for PEM fuel cell applications
MUSIO, FRANCESCO
2016/2017
Abstract
Proton Exchange Membrane Fuel Cell (PEMFC) is an electrochemical device which generates a direct electrical current from the chemical energy of a redox reaction between oxygen and hydrogen. One of the main components of a PEMFC is the gas diffusion medium (GDM), which has to guarantee an efficient water management within the cell and is composed by a carbon cloth macroporous gas diffusion layer (GDL) and a microporous layer (MPL) deposited on it. Graphene is an emerging and promising material which is composed by one or several atomic carbon monolayers that can be arranged in different shapes; the most significant property of this material is the conductivity and that is why it has been employed in this field. The present work aimed to prepare graphene based and graphene/carbon black composite microporous layers by using blade coating technique; moreover, two ageing processes, namely accelerated stress tests, both chemical and mechanical, have been developed in order to assess samples change of properties over time, i.e. durability. In the characterization step, morphological changes on the surfaces, both as prepared and upon ageing tests, with different compositions were investigated . In details, SEM analysis was employed to evaluate both MPLs surfaces and thickness of the deposited layers. Thermogravimetric analyses, performed up to 1000 °C, were useful to get the temperature values at which decomposition of the microporous layer components occurs. The static contact angles of the gas diffusion media were measured, on the MPL side, to evaluate hydrophobicity of the PEM fuel cell samples. The mercury intrusion porosimetry (MIP) was performed for all samples in order to get pore size distribution and pores volume of the GDMs. Electrical and electrochemical experiments were carried out in a lab-scale fuel cell with a commercial catalyst coated membrane assembled with prepared and with stressed samples aiming to assess possible changes due to accelerated stress tests. Furthermore, the resulting polarization curves were used to calculate different types of voltage losses, namely charge transfer losses, mass transport losses, crossover losses and ohmic losses. The last chapter of this work concerns the possible future outlook about gas diffusion media, i.e. graphene based MPLs obtained by evaporation of the solvent directly on the GDL substrate aiming to lower time of the whole preparation process.| File | Dimensione | Formato | |
|---|---|---|---|
|
TESI_837483.pdf
accessibile in internet solo dagli utenti autorizzati
Descrizione: TESI
Dimensione
73.6 MB
Formato
Adobe PDF
|
73.6 MB | Adobe PDF | Visualizza/Apri |
I documenti in POLITesi sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
https://hdl.handle.net/10589/135600