Integrated platinum carbon nanotube based electrocatalyst for high efficiency proton exchange membrane fuel cells 2

In addition, a series of characterization tests are also used to evaluate the physical and electrochemical properties of the synthesized Pt/CNT-based electrodes, including ex situ tools

Chapter 2 Experimental Methodology

2.1 Introduction

This chapter introduces the experimental methods used in this work to provide a thorough understanding on the electrocatalytic performance of the integrated Pt/CNT-based electrodes for PEMFCs These experimental methods include both fabrication and characterization methods for the Pt/CNT-based electrodes The fabrication processes basically consist of electrode and MEA preparation based on Pt/CNT and Pt/VXC72R-based catalysts In addition, a series of characterization tests are also used to evaluate the physical and electrochemical properties of the synthesized Pt/CNT-based electrodes, including ex situ tools such as scanning electron microscopy (SEM), Brunauer-Emmett-Teller measurement (BET), transmission electron microscopy (TEM), Raman spectroscopy and X-ray photoelectron spectroscopy (XPS), as well as in situ techniques like polarization curve measurement, electrochemical impedance spectroscopy (EIS), cyclic voltammetry (CV) and accelerated durability test (ADT)

2.2 Fabrication Process of Membrane Electrode Assembly (MEA)

In this study, the electrochemical performance of the Pt/CNT-based electrodes was characterized mainly based on single-cell membrane electrode assemblies (MEAs), comparing with conventional carbon black-based (Vulcan XC-72R, Carbot) electrodes as reference This section depicts the detailed fabrication processes of Pt/CNT and Pt/VXC72R-based electrodes used in this study, as well as the standard procedures to assemble the electrodes into PEMFC MEAs

2.2.1 Fabrication Process of Pt/CNT-based Electrode

As described in Section 1.3.3, previously Pt/CNT-based electrocatalysts were mostly prepared by chemical reduction of Pt precursors onto free-standing CNT supports The synthesis process usually consisted of CNT growth, CNT surface oxidation and Pt deposition on CNTs Sometimes the as-deposited Pt catalysts required a further reduction process under H2 due to incomplete chemical reduction [1] Although significant progress has been achieved in recent years on improving the

Pt dispersion on CNT supports via this wet-chemical preparation process, real development yet has not been made in terms of fabrication-efficiency for the Pt/CNT catalysts In this study, an integrated Pt/CNT catalyst was prepared for PEMFC electrodes by in situ growth of a dense CNT layer on carbon paper using a thermal CVD technique followed by direct sputter-deposition of Pt nanoparticles onto the CNT layer This combined fabrication method contains only two main steps and it also simplifies the fabrication process of Pt/CNT-based MEAs The experimental details for preparing Pt/CNT-based MEAs are presented in the following sections

In situ Growth of CNTs on Carbon Paper

In this study, CNTs were directly grown on carbon paper via a thermal chemical vapor deposition (CVD) process The CVD technique was chosen due to its ease of being scaled-up and relatively low growth temperature [2] In a typical CVD process for CNT growth, carbon precursors are catalytically decomposed on the surface of transition metal particles where carbon atomization takes place to form graphitic structure One of the key advantages of the CVD technique is that CNT growth with controlled structure and morphology can be achieved via a variety of experimental parameters such as growth temperature, catalytic materials, gas flow rates and so forth

Currently a number of forms of CVD techniques are in wide use for CNT growth differing in their activation and process conditions, such as general thermal CVD, floating catalyst CVD, aerosol-assisted CVD and plasma-enhanced CVD (PECVD)

Figure 2.1 shows the schematic diagram of the thermal CVD system used to grow CNTs on carbon paper Prior to CNT growth, a thin layer of a transition metal such as Fe, Ni or Co was firstly sputter-deposited onto a commercial carbon paper TGPH090 (Toray Inc.) as growth catalyst As illustrated in Fig 2.1, during growth process the metal-layer catalyzed carbon paper was placed at the center of a tube furnace and it was heated up to the growth temperature under a gas mixture of Ar + 5 vol% H2 Carbon feedstock gas C2H4 was then introduced into the tube when the growth temperature was reached After CNT growth for a certain period, C2H4 was cut off and the furnace system was then cooled down to room temperature The carrier gas Ar + 5 vol.% H2 was maintained at 100 sccm (cm3/min) throughout the whole process The optimization of the CNT growth process was conducted by varying a series of process parameters based on the structure and morphology of the as-grown CNTs, including catalyst type, catalyst loading, catalyst reduction, growth

Ar + 5% H 2

C 2 H 4

Carbon paper Quartz tube

Furnace

Exhaust Metal catalyst

Fig 2.1 Schematic diagram of a thermal tube-furnace-type CVD system

temperature, growth duration, and C2H4 flow rate Results will be demonstrated and discussed in the next chapter

Sputter-deposition of Pt Electrocatalysts

Sputter-deposition is a physical vapor deposition method for depositing thin films

by sputtering, which ejects material atoms or ions from a source so called target, and deposits them onto a substrate The sputtered atoms are usually bombarded by sputtering gas atoms and ballistically fly from the target in straight lines onto the substrate in a vacuum chamber The sputtering gas is often an inert gas such as Ar During a magnetron sputtering process, electrons near the target surface are trapped

by strong electric and magnetic fields and they experience ionizing collisions with neutral Ar atoms Ar ions are generated as a result of these collisions, leading to

Fig 2.2 Schematic diagram of a magnetron sputtering system for Pt deposition [3]

intensive bombardment on the source atoms It ensures that the plasma produced by sputtering can be sustained at a lower pressure while a high deposition rate is maintained

In this study, a radio-frequency (R.F.) magnetron sputtering system (Denton Discovery-18) was used for Pt deposition onto the in situ grown CNT support Figure 2.2 shows a schematic diagram of the sputtering system for Pt deposition To start, a pure Pt target (purity 99.99%) was mounted at the sputter cathode of the system Afterward several 5 cm2 pieces of CNT-grown carbon papers were placed on a 8-inch sample stage Before sputtering began, the sputtering chamber was pumped down to a high vacuum of about 1×10-6 Torr During the sputtering process, the deposition rate

of Pt catalysts was basically controlled by two parameters: the output sputter-power and the Ar gas pressure To determine the specific deposition rate of Pt catalysts at a given condition, the loading of the sputter-deposited Pt catalysts was determined by weight difference of the CNT-grown carbon paper before and after Pt deposition By varying deposition time, a series of Pt/CNT-based electrodes with different Pt catalyst loadings were fabricated for further electrochemical characterization

2.2.2 Fabrication Process of Pt/VXC72R-based Electrode

The Pt/VXC72R-based electrodes were mostly used as reference electrodes to compare with the Pt/CNT-based electrodes in this study They were fabricated via a conventional ink-spread process which is commonly used to prepare thin-film PEMFC electrodes [4] This process usually consists of a series of ink-spread processes including carbon paper Teflonization, GDL preparation and CL preparation

Carbon Paper Teflonization

To fabricate a conventional CB-based electrode prepared by ink-spread process, a 5cm2 carbon paper (TGPH090, Toray Inc.) was first Teflonized by brushing 60wt% PTFE (polytetrafluoroethylene, Aldrich Chemical Company Inc.) dispersion in water onto both sides of the carbon paper The carbon paper was then dried on a hotplate and weighed to determine the Teflon content on the carbon paper Teflon content on the carbon paper was controlled around 30 wt% Afterwards, the Teflonized carbon paper was transferred to an oven and heated up to 350oC at 5 °C min-1 to gradually remove the dispersant agent present in PTFE and to even the PTFE dispersion by heat treatment at 350 °C for 30 min

Gas Diffusion Layer Preparation

Generally, gas diffusion layers for conventional CB-based electrodes are spread onto the Teflonized carbon papers, usually consisting of a mixture of carbon black VXC72R and PTFE The weight ratio of VXC72R to PTFE was 7:3 for both anode and cathode The carbon black VXC72R was first treated in an ultrasonicator in a 3

ml mixture of DI water and ethanol (1:2 vol ratio) and the PTFE was stirred in 1 ml

of DI water The PTFE solution was added to the carbon black ink and stirred to make

a homogeneous dispersion The diffusion ink was applied to one side of the carbon paper by means of spraying with an air brush When a typical GDL loading around 2

mg cm-2 based on dry weight was reached, the gas diffusion electrode was transferred

to an oven and sintered at 350oC under the same conditions as the heat treatment for Teflonizing carbon papers

Catalyst Layer Preparation

In a conventional CB-based electrode, the catalyst layer is prepared via an spread process similar to GDL The catalyst ink was typically a blend of a commercial VXC72R supported Pt catalyst and Nafion ionomers In this study, two commercial catalysts were used as reference catalysts with different Pt weight ratios: 20 wt% Pt/VXC72R (E-TEK Inc.) and 40 wt% Pt/VXC72R (Johnson Matthey Inc.) The Nafion ionomer was a commercial 5 wt% Nafion perfluorinated resin solution (Sigma Aldrich Inc.) To make the catalyst ink, the Pt/VXC72R and Nafion (dry weight) were mixed at a weight ratio of 2:1 The mixture was then dispersed in a solvent of DI water and ethanol (1:1 vol ratio) by ultrasonicating for 30 min The desired amount of

ink-Pt catalyst loading (0.2 mg cm-2) was applied on top of the gas diffusion layer with an air brush Lastly, the GDL-CL-spread electrode was subjected to heat treatment at

130oC for 30 min to improve the dispersion of Nafion ionomers

2.2.3 MEA Assembly Process

In order for electrochemical characterization of the Pt/CNT and based electrodes, typically a combination of two electrodes, i.e anode and cathode, were hot-pressed with a Nafion 112 (Dupont Inc.) membrane into a PEMFC MEA In this study a variety of MEA combinations were investigated on their electrochemical performance, which were prepared via a standard procedure of PEM purification and MEA assembly as described below

Pt/VXC72R-PEM Purification

Before hot-pressing two electrodes into a MEA, the polymer electrolyte membrane needs to undergo a purification process to remove various impurities and

increase H+ content Nafion 112 was used as electrolyte throughout this study In the purification process, several membranes were first boiled in a 3% H2O2 solution for 1 hour to clean them from organic impurities Then they were rinsed with DI water and boiled in DI water for another hour After rinsing off residual H2O2, the membranes were cation-exchanged to H+ form by boiling them in a 0.5M H2SO4 solution for 1 hour and finally they were rinsed in boiling DI water for 1 hour After this purification process, the membranes were stored in DI water and ready for MEA hot-press

MEA Assembly

To hot-press a MEA, a Nafion 112 membrane was first sandwiched between two electrodes and this unassembled MEA was enveloped with two Furon (fiberglass reinforced Teflon) sheets Then this whole assembly was placed between two stainless steel plates, which were held at the hot-press temperature of 140 °C When the temperature was stabilized at 140 °C, a compress force was added to the two plates at

15 kg cm-2 by a hydration press The hot-press compression was maintained for 90 s

at 140 °C After hot-press, the assembled MEA was removed from the plates and then transferred to the fuel cell test system for electrochemical characterization

2.3 Ex situ Characterization Methods

This section introduces the ex situ characterization methods that were used to investigate the physical properties of the integrated Pt/CNT-based catalyst It is well-known that the cell performance of a PEMFC is greatly dependant on the structural and compositional properties of its electrocatalysts; thus suitable analytical tools and methods play an important role in understanding the overall efficiency and effectiveness of the integrated Pt/CNT-based electrodes In order to obtain the

relevant information, a number of ex situ characterization methods were used to reveal the microstructure and morphology of the Pt/CNT-based electrocatalyst, including scanning electron microscopy (SEM), Brunauer-Emmett-Teller measurement (BET), Raman spectroscopy, transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS) These characterization methods can provide insightful information to understand the physical properties of the Pt/CNT-based electrocatalyst as well as to optimize the structure and morphology of the Pt/CNT-based electrocatalyst to yield a maximized cell performance from the optimized electrode fabrication method

2.3.1 Scanning Electron Microscopy (SEM)

Scanning electron microscopes (SEM) are widely used to obtain visualized information on materials surface A SEM images the sample surface by scanning it with a high-energy beam of electrons in a raster scan pattern In the state of the art SEM, high-energy electrons are usually emitted via field emission (FE) and they actively interact with the surface atoms of the sample, generating signals that contain particular information about the sample’s surface topography, composition and so forth Typically the most important signal of the produced signals is secondary electrons and they are collected by a low-energy (< 50 eV) secondary electron detector This secondary electron imaging can produce very high-resolution images

of the sample surface, revealing structure details less than 1 nm in size In addition, SEM images can also provide three-dimensional information due to the large depth of field yielded by the extremely narrow electron beam These characteristics particularly enable the scanning electron microscopy to reveal the microstructure of sample surface without any physical or chemical destruction

In this study SEM was extensively used to examine the surface morphology of the CNTs grown on carbon paper using a JEOL JSM 6700F FESEM system During sample imaging, a CNT-grown carbon paper was placed into the SEM chamber and exposed to a high-energy electron beam emitted by a strong electrical field of 5 kV The image magnifications were ranged from 3−30 k, giving a high resolution in the measure of nanometers

2.3.2 Brunauer-Emmett-Teller (BET) Surface Area Measurement

It has been commonly recognized that an effective catalyst support material should provide high surface area for metal particles As such, determination of surface area characteristics of support materials is of vital importance for PEMFC applications The Brunauer-Emmett-Teller (BET) surface area characterization can provide a relatively accurate way for ex situ surface area measurement In general, the BET theory is a useful analytical method for the physical adsorption of gas molecules

on a solid surface, based on which the specific surface area of a material can be determined This theory basically extends the Langmuir equation assuming that inert gases such as N2, Ar will form multilayer adsorbates on a solid surface instead of monolayer molecular adsorption The details of the BET theory can be found in ref [5]

In this study, the BET surface area measurement was performed on a series of CNT layers grown on carbon paper under different growth conditions An ASAP-

2000 BET characterization system was used and the absorbate gas was N2 In the BET measurement, a certain amount of integrated CNT/carbon paper samples were dried out and gas-evacuated in a small flask They were then chilled down to liquid N2

temperature before introducing N2 absorbate The surface area of the integrated CNT/carbon papers was calculated from the BET isotherm adsorption curves, which were obtained by decreasing N2 pressure in the flask

2.3.3 Raman Spectroscopy

Raman spectroscopy utilizes the principle of Raman scattering of monochromatic light from a laser in the visible range, in order to study vibrational, rotational and other low-frequency modes of a material The laser light interacts with phonons or other excitations within the material, resulting in energy shift of the laser photons up

or down This energy shift can provide information of the phonon modes of the material As Raman scattering shows responsive symmetric shift to carbon materials with different structure and sizes, Raman spectroscopy has received considerable attention in CNT research since carbon nanotubes were found in 1991 [6] It is commonly used to identify the vibrational and rotational state of CNTs, giving useful information about their microstructure and other properties

In this study, a Renishaw 2000 Raman spectrometer was used to probe the sp2(ordered) and sp3 (disordered) hybridized carbon Raman peaks of the in situ grown CNTs Raman spectra of VXC72R and carbon paper were also investigated for comparison The incident laser light was a green Ar ion laser with a wavelength of 514.4 nm and the intensity was set at about 100 mW The Raman spectra were recorded over a 20 s interval and iterated for 10 times till the characteristic Raman peaks showed little variation in peak shape and intensity In addition, all spectra were normalized in terms of background noise thus direct comparison of peak intensities was conducted for data analysis

2.3.4 Transmission Electron Microscopy (TEM)

Transmission Electron Microscope (TEM) is also an electron microscopy

technique; while unlike SEM, it emits a beam of electrons that transmit through an ultra thin specimen A TEM image is formed when the transmitted electrons interact with the specimen as they pass through Although TEM images cannot provide sample depth profiles as SEM due to the direct transmitted electron projection, they can give a much higher resolution, being able to show fine structure details at atomic levels It is well-known that the particle size and dispersion of electrocatalysts on support material are very important parameters in determining their electrocatalytic performance Therefore, TEM is a very commonly used technique in PEMFC research to directly reveal the particle size and dispersion of supported Pt nanoparticles

To examine the Pt nanoparticles sputter-deposited on the CNT surface, their TEM micrographs were obtained with a JEOL JEM-2010 FETEM system The operating voltage was 200 kV The characterized samples were prepared by unltra-sonicating the integrated Pt/CNT/carbon paper electrode in ethanol and subsequently placing one drop of the Pt/CNT catalyst dispersion onto a 300 mesh Cu TEM grid (Electron Microscopy Sciences), which was then allowed to dry prior to imaging For the histograms of Pt particle size distribution, a total of 500−550 nanoparticles were counted to ensure a statistically representative sampling Besides the integrated Pt/CNT catalyst, TEM characterization of the commercial Pt/VXC72R catalysts was also performed to compare their Pt particle size and dispersion on the CNT and VXC72R supports, respectively