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

Chapter 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/CNTbased 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. 39 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. 40 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). C2H4 Furnace Metal catalyst Quartz tube Ar + 5% H2 Carbon paper Exhaust Fig. 2.1 Schematic diagram of a thermal tube-furnace-type CVD system. 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 + 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 + 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 41 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]. 42 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 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. 43 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 °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 ml mixture of DI water and ethanol (1:2 vol. ratio) and the PTFE was stirred in 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 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. 44 Catalyst Layer Preparation In a conventional CB-based electrode, the catalyst layer is prepared via an inkspread 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 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 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 to improve the dispersion of Nafion ionomers. 2.2.3 MEA Assembly Process In order for electrochemical characterization of the Pt/CNT and Pt/VXC72Rbased 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. 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 45 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 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 hour and finally they were rinsed in boiling DI water for 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 wellknown 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 46 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/CNTbased 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 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. 47 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 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 ASAP2000 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 48 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. 49 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 unltrasonicating 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. 50 2.3.5 X-ray Photoelectron Spectroscopy (XPS) X-ray photoelectron spectroscopy (XPS) is a quantitative spectroscopic technique commonly used for surface characterization of elemental composition, empirical formula, chemical state and electronic state of the elements in a material. XPS spectra are usually obtained by irradiating a material with a beam of X-rays while simultaneously detecting the kinetic energy and number of photoelectrons excited from the top few layers of atoms at the sample surface. In previous PEMFC research, XPS spectra of Pt nanoparticles synthesized via chemical reduction processes were usually investigated to verify the chemical state of as-deposited Pt nanoparticles on CB and CNT supports [1, 7-9]. A number of studies showed that the deposited Pt catalyst may contain a considerable amount of Pt oxides and the Pt oxide content significantly depends on the reducing agent used [1, 7-9]. XPS spectra of Pt nanoparticles can quantify the Pt element at different chemical states by spectra deconvolution using XPS analysis software. In order to determine the chemical state of the sputter-deposited Pt catalysts on in situ grown CNTs, XPS characterization was performed using an Axis Ultra DLD spectrometer equipped with a hemispherical analyzer and an Al Kα anode (1486.6 eV). The pass energy was 20 eV and the energy increment was 0.05 eV for highresolution scans of the Pt 4f level. Spectra deconvolution was performed for Pt chemical state determination using software XPSPEAK v.4.1. Narrow-scan spectra of carbon were also performed to provide the reference binding energy of C 1s level. 51 2.4 In situ Characterization Methods In addition to ex situ characterization methods, a series of in situ characterization methods, such as polarization curve measurement, electrochemical impedance spectroscopy (EIS), in situ cyclic voltammetry (CV) and accelerated durability test (ADT), were performed in this study to examine the electrochemical activity and stability of the Pt/CNT-based electrodes toward PEMFC reactions. These in situ characterization methods can give the most accurate and realistic information of the Pt/CNT-based electrocatalyst as a potential substitute for conventional Pt/VXC72Rbased electrocatalysts used for high-efficiency PEMFC applications. The experimental details of each method are elaborated in the following subsections, where the characterization methods are ordered in the experimental sequence. 2.4.1 Polarization Curve Measurement As described in Section 1.2.3, polarization curve is the most common diagnostic technique that shows straightforward current-voltage relationship of a PEMFC. PEMFC polarization curves are usually measured by recording corresponding potentials at given current densities [10]. This process is done by a potentiostat or galvanostat unit that gradually increases cell current density from zero till the maximum current is reached. In this study, an Electrochem 890B single cell test system (Electrochem Inc., Scribner Associate) was employed to measure the polarization curves for Pt/CNT-based electrodes. This test system consists of four main components: two gas distribution units (GDU), two gas humidifiers, a computer controlled electronic load system and a cm2 single cell test fixture (see Fig. 2.3). As illustrated in Fig. 2.3, the GDU has two units that are responsible for the control of inlet gas flow rate and gas pressure, respectively. The GDU is connected to two 52 humidifiers that are basically two water bottles with heaters and temperature controls, to manage the humidity of the reactant gases for anode (H2) and cathode (O2). The reactant gases flow through the humidifiers into the anode and cathode of the single cell fixture where a MEA is inserted and tested. The single cell fixture is electrically connected to the electronic load system that applies an external resistor on the cell to form a complete electrical circuit. During polarization curve measurement, the electronic load system alters the magnitude of its resistance to draw different current densities from the cell and correspondingly causes different amount of polarization of the cell output voltage. The polarization curve is recorded by a computer via the FuelCell@ test software (Scribner Associates). (a) (a) (b) (b) (c) (d) Fig. 2.3 Photograph of an Electrochem 890B single cell test system: (a) gas distribution units, (b) gas humidifiers, (c) electronic load system and (d) 5cm2 single cell fixture. It is well-known that operating conditions play a crucial role in cell performance, notwithstanding the properties of materials used in the electrode. The main operating 53 conditions that affect cell polarization performance include system warm-up, cell temperature, gas humidity, gas pressure, gas flow rate and compression force [10]. In this study the operating conditions were computer-controlled by the FuelCell@ test software. In order to evaluate the effectiveness of the integrated Pt/CNT-based electrode, the cell operating conditions were first optimized based on conventional ink-process prepared MEAs and they were fixed during polarization curve measurement for the Pt/CNT-based MEAs. The optimized conditions are presented as follows. System Warm-up A system warm-up process is usually performed prior to polarization curve measurement to ensure the system in a steady state. In this study, all MEAs underwent a warm-up process by operating at 0.5 A cm-2 for 30 to activate the Pt catalysts and warm up the electrolyte membrane. Cell Temperature The cell temperature was maintained at 80 °C during polarization curve measurement. It was found in our optimization study that reaction kinetics and mass transport are greatly limited at temperatures lower than 80 °C whereas the polymer electrolyte membrane may experience dry-out when the temperature is too high. Gas Humidity Gas humidification is a necessary measure for PEMFC operation in that the proton conductivity of Nafion electrolyte significantly depends on its water content. However, it has been reported that over-humidification of the reactant gases can cause 54 electrode flooding, especially at cathode where water is produced. Therefore gas humidification is an essential factor in water management of PEMFC operation. In this study, the relative humidity of reactant gases was managed by controlling the humidifier temperatures. The optimized humidifier temperatures were 85 °C for H2 and 80 °C for O2, respectively. As a result, the relative humidity of H2 and O2 were estimated to be 85% and 80%, respectively. Gas Pressure The pressure of inlet reactant gases is also a very important factor that affects reaction kinetics and mass transport of a PEMFC. High inlet gas pressure can improve cell performance; however, gas permeation through electrolyte is also elevated causing direct combustion of fuels. The gas pressure was fixed at 30 psi for both H2 and O2 throughout the polarization curve measurement of this study. Gas Flow Rate Gas flow rates of H2 and O2 were controlled by two gas flow meters embedded in the GDU. In order to minimize the possibilities of reactant insufficiency, high flow rates of both H2 and O2 were guaranteed by feeding a stoichiometry of for H2 (14 cc min-1 A-1) and O2 (7 cc min-1 A-1) based on an initial flow rate of 300 sccm. The high gas flow rate also provided another benefit that it could purge the excess water accumulated in the porous electrode. Compression Force In polarization curve measurement, a MEA is fixed between two graphite current collectors by clamping them with a compression force. Optimization of this 55 compression force is required as it can impose direct influences on cell performance. A low compression force would yield a higher ohmic loss of cell performance due to the high contact resistance between the cell components. On the other hand, a greater concentration loss is caused in an over-compressed cell where the porous structure of the gas diffusion electrode is diminished by the large compression force. In this study, an optimized compression force was applied on all MEAs by using a torque wrench to compact the two current collectors at a fixed torque of Nm. 2.4.2 Electrochemical Impedance Spectroscopy (EIS) Electrochemical impedance spectroscopy (EIS), also known as AC impedance spectroscopy, has become a primary diagnostic tool in PEMFC research utilized by an increasing number of researchers [11-14]. The EIS technique provides an exceptional diagnostic approach for evaluating cell performance in that it has the capability to distinguish the influences from various processes occurring simultaneously in PEMFCs. The EIS technique is conducted by measuring the frequency dependence of the impedance of a PEMFC by applying a small sinusoidal AC potential or current as a perturbation signal to the cell and measuring the current or voltage response along with the corresponding phase angle [15]. To analyze these data the real and imaginary impedances are calculated and plotted against each other for different perturbation frequencies whereby a Nyquist spectrum is obtained to present the impedance data of the cell. In practice, EIS measurements are usually performed on a working PEMFC by means of two-electrode connections between the EIS instrument and the PEMFC, to provide in situ information about the impedance data of the cell [16]. The twoelectrode method basically connects the PEMFC cathode to the working electrode whereas uses the anode as both the counter electrode and the reference electrode. This 56 method is commonly used as inserting a conventional reference electrode such as SCE or Ag/AgCl into MEA is difficult and the contribution of anode impedance is usually negligible compared to that of cathode impedance in a pure hydrogen PEMFC. Currently the majority of EIS studies on PEMFCs have been focused on investigating various transport processes contributing to cathode impedance, such as charge transfer process and oxygen diffusion process. In this study, the EIS measurements were performed subsequent to polarization curve measurement by using a Solartron SI1280B electrochemical workstation (AMETEK Inc.). The SI1280B electrochemical workstation is a combined system that integrates an electrochemical interface (ECI) and a frequency response analyzer (FRA) in one unit. The ECI can be used as a potentiostat or a galvanostat that applies DC polarization on any electrochemical cells. The FRA, on the other hand, is an AC signal generator and an analyzer for measuring cell impedance data. When measuring cell impedance data, the working electrode was connected to the cathode of the working cell while both the counter electrode and reference electrode were connected to the anode. Therefore the reference electrode was a dynamic hydrogen electrode (DHE). During EIS measurements, the amplitude of the AC signal was 10 mV and the frequency range was from 10 kHz to 0.1 Hz. Impedance spectra of the cell were obtained under cathodic potentiostatic conditions at values from open circuit voltage to 0.4 V vs. DHE. The impedance spectra were presented in Nyquist plots in our results and obtained by ZPlot software from Scribner Associates. 57 2.4.3 In situ Cyclic Voltammetry (CV) Cyclic voltammetry (CV) is a prevalent characterization method for the electrocatalyst study in PEMFC research to determine the electrochemical active surface area (ECSA) of PEMFC electrocatalysts. In cyclic voltammetry, the electrode potential ramps linearly vs. time in a triangular waveform. The potential between the working electrode and the reference electrode is measured, while the current response is measured between the working electrode and the counter electrode. This data is then plotted in a cyclic voltammogram in which current vs. potential diagram is presented. In cyclic voltammograms, current peaks in the curves usually contain information about the redox processes occurring in the cell [17]. PEMFC electrocatalysts may be examined for their ECSA by ex situ or in situ CV measurements. In ex situ CV experiment, the properties of the electrocatalyst are evaluated using a standard three-electrode cell with an aqueous solution of acid (typically 0.5 M H2SO4) to simulate the electrolyte in PEMFCs [18]. This method is commonly used for its convenience and relatively fast operation. For in situ CV experiment, a two-electrode method is used with the same connection configuration as EIS measurement whereas the cathode is purged with N2 instead of O2 [19]. In this study, in situ CVs were performed with the Solartron SI1280B electrochemical workstation after EIS measurements. Before CV scans began, the cathode was purged with 300 sccm N2 flow to serve as the working electrode whereas the anode was fed with 50 sccm H2 to work as both the reference and the counter electrodes simultaneously. The low H2 flow rate was used for minimizing the fuel gas cross-over that results in current shift in forward sweep due to H2 oxidation. When the cathode potential dropped below 0.1 V vs. DHE, in situ CV scans were initiated at a 58 scan rate of 100 mV s-1 between 0.1 and 1.2 V. A total of 20 CV cycles were scanned with the control and data acquisition by CorrWare software (Scribner Associate). 2.4.4 Accelerated Degradation Test (ADT) Accelerated degradation test (ADT) has been developed in recent years to investigate the long-term durability of PEMFC operation [20-23]. A number of ADT protocols have been standardized by the U.S. DOE office exclusively for individual cell component, including electrocatalyst, electrocatalyst supports, MEA chemical and MEA mechanical [24]. These protocols allow implementing durability tests in a relatively rapid way comparing conventional long-term tests that could last up to 10,000 h or more. In the ADT protocol for electrocatalyst support, the cell fed with H2/N2 would undergo a 200 h cycle of being held at 1.2 V for 24 h and characterized by polarization curve and ECSA repetitively [24]. Although this method is much faster than conventional durability tests, it still requires a long operation period which is not time-efficient and cost-effective enough for materials studies. In this study, a series of in situ accelerated degradation tests (ADT) were performed by the Solartron SI1280B electrochemical workstation based on potential oxidation of the Pt/CNT-based electrode. Both static and dynamic potential oxidation processes were investigated on their effectiveness for evaluating the durability of the CNT support, including CV cycling between 0.1−1.2 V, potentiostatic oxidation at 1.5 V and potential cycling between 0.6 and 1.8 V vs. DHE. In the potential cycling ADT test, the electrode was held at 0.6 V for 40 s and at 1.8 V for 20 s for a total of 100 oxidation cycles. In situ CV curves were measured before ADT and after every 10 oxidation cycles to evaluate the electrochemical stability of the Pt/CNT catalyst. 59 2.4 Summary The main materials and chemicals used in this study are listed in Table 2.1 shown below. Name Formula/Code Supplier Description Made of graphitized carbon fiber, 0.26mm in Carbon powder, 30−40 nm in diameter Pt nanoparticles supported on VCXC72R Pt nanoparticles supported on VCXC72R Carbon paper TGPH090 Toray Carbon black (CB) VCXC72R Cabot Commercial Pt catalyst 40wt% Pt/VCXC72R Johnson Matthey Commercial Pt catalyst 20wt% Pt/VCXC72R E-TEK Nafion membrane Nafion 112 Dupont PEM, 80 µm in thickness Nafion solution wt% Nafion Dupont wt% perfluorinated sulphonate resin Polytetrafluoroethy PTFE lene Sigma Aldrich 60 wt% PTFE suspension Carrier gas Ar + vol.% H2 NOX Gas mixture Carbon feedstock gas C2H4 NOX Pure Hydrogen H2 NOX Pure Oxygen O2 NOX Pure Nitrogen N2 NOX Pure Table 2.1Main chemicals and materials used in this study and their properties. 60 The main instruments and facilities used in this study are shown in Table 2.2 below. Name Model Application Tube-furnace chemical vapor deposition system (CVD) Carbolite MTF 12/38/250 Grow CNTs on carbon paper Radio-frequency magnetron sputtering system Denton Discovery-18 Deposit Pt catalysts on CNTs Scanning electron microscope (SEM) JEOL JSM 6700F Investigate the structure and morphology of in situ grown CNTs Brunauer-Emmett-Teller surface area analyzer (BET) Micromerities ASAP2000 Examine the surface area of CNT-grown carbon paper Renishaw 2000 Probe the sp2 and sp3 hybridized carbon content of the in situ grown CNTs Raman Spectrometer Transmission electron microscopy (TEM) JOEL JEM-2010 X-ray photoelectron spectrometer (XPS) Axis Ultra DLD Fuel cell test system Electrochem 890B Electrochemical workstation Solartron SI1280B Investigate the microstructure and dispersion of Pt nanoparticles on CNT Determine the chemical state of the sputterdeposited Pt catalysts on CNT Examine the electrochemical activity and stability of the Pt/CNT catalyst Examine the electrochemical activity and stability of the Pt/CNT catalyst Table 2.2 Main instruments and facilities used in this study and their applications. 61 References [1] D. 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Mitsushima, S. Kawahara, K. I. Ota, and N. Kamiya, J. Electrochem. Soc. 154, B153 (2007). [23]D. A. Stevens, M. T. Hicks, G. M. Haugen, and J. R. Dahn, J. Electrochem. Soc. 152, A2309 (2005). [24] U.S. DOE, Cell Component Accelerated Stress Test Protocols for PEM Fuel Cells, 2007. 62 [...]... Electrochem Solid-State Lett., 8, A 42 (20 05) [20 ] X Wang, W Li, Z Chen, M Waje, and Y Yan, J Power Sources, 158, 154 (20 06) [21 ] Y Y Shao, G P Yin, Y Z Gao, and P F Shi, J Electrochem Soc 153, A1093 (20 06) [22 ] S Mitsushima, S Kawahara, K I Ota, and N Kamiya, J Electrochem Soc 154, B153 (20 07) [23 ]D A Stevens, M T Hicks, G M Haugen, and J R Dahn, J Electrochem Soc 1 52, A2309 (20 05) [24 ] U.S DOE, Cell Component... combustion of fuels The gas pressure was fixed at 30 psi for both H2 and O2 throughout the polarization curve measurement of this study Gas Flow Rate Gas flow rates of H2 and O2 were controlled by two gas flow meters embedded in the GDU In order to minimize the possibilities of reactant insufficiency, high flow rates of both H2 and O2 were guaranteed by feeding a stoichiometry of 2 for H2 (14 cc min-1... vol.% H2 NOX Gas mixture Carbon feedstock gas C2H4 NOX Pure Hydrogen H2 NOX Pure Oxygen O2 NOX Pure Nitrogen N2 NOX Pure Table 2. 1Main chemicals and materials used in this study and their properties 60 The main instruments and facilities used in this study are shown in Table 2. 2 below Name Model Application Tube-furnace chemical vapor deposition system (CVD) Carbolite MTF 12/ 38 /25 0 Grow CNTs on carbon. .. realistic information of the Pt/CNT -based electrocatalyst as a potential substitute for conventional Pt/VXC72Rbased electrocatalysts used for high- efficiency PEMFC applications The experimental details of each method are elaborated in the following subsections, where the characterization methods are ordered in the experimental sequence 2. 4.1 Polarization Curve Measurement As described in Section 1 .2. 3, polarization... V for 40 s and at 1.8 V for 20 s for a total of 100 oxidation cycles In situ CV curves were measured before ADT and after every 10 oxidation cycles to evaluate the electrochemical stability of the Pt/CNT catalyst 59 2. 4 Summary The main materials and chemicals used in this study are listed in Table 2. 1 shown below Name Formula/Code Supplier Description Made of graphitized carbon fiber, 0 .26 mm in Carbon. .. or more In the ADT protocol for electrocatalyst support, the cell fed with H2/N2 would undergo a 20 0 h cycle of being held at 1 .2 V for 24 h and characterized by polarization curve and ECSA repetitively [24 ] Although this method is much faster than conventional durability tests, it still requires a long operation period which is not time-efficient and cost-effective enough for materials studies In this... fiber, 0 .26 mm in Carbon powder, 30−40 nm in diameter Pt nanoparticles supported on VCXC72R Pt nanoparticles supported on VCXC72R Carbon paper TGPH090 Toray Carbon black (CB) VCXC72R Cabot Commercial Pt catalyst 40wt% Pt/VCXC72R Johnson Matthey Commercial Pt catalyst 20 wt% Pt/VCXC72R E-TEK Nafion membrane Nafion 1 12 Dupont PEM, 80 µm in thickness Nafion solution 5 wt% Nafion Dupont 5 wt% perfluorinated... Table 2. 2 Main instruments and facilities used in this study and their applications 61 References [1] D Villers, S H Sun, A M Serventi, J P Dodelet, and S Dsilets, J Phys Chem B, 110 (51), 25 916 (20 06) [2] W Z Li, D Z Wang, S X Yang, J G Wen, and Z F Ren, Chem Phys Lett., 335, 141 (20 01) [3] S Litster, and G McLean, J Power Sources, 130, 61 (20 04) [4] M Wilson, and S Gottesfeld, J Appl Electrochem., 22 ,... optimized humidifier temperatures were 85 °C for H2 and 80 °C for O2, respectively As a result, the relative humidity of H2 and O2 were estimated to be 85% and 80%, respectively Gas Pressure The pressure of inlet reactant gases is also a very important factor that affects reaction kinetics and mass transport of a PEMFC High inlet gas pressure can improve cell performance; however, gas permeation through... analyzer (BET) Micromerities ASAP2000 Examine the surface area of CNT-grown carbon paper Renishaw 20 00 Probe the sp2 and sp3 hybridized carbon content of the in situ grown CNTs Raman Spectrometer Transmission electron microscopy (TEM) JOEL JEM -20 10 X-ray photoelectron spectrometer (XPS) Axis Ultra DLD Fuel cell test system Electrochem 890B Electrochemical workstation Solartron SI 128 0B Investigate the microstructure . most accurate and realistic information of the Pt/CNT -based electrocatalyst as a potential substitute for conventional Pt/VXC72R- based electrocatalysts used for high- efficiency PEMFC applications boiled in DI water for another hour. After rinsing off residual H 2 O 2 , the membranes were cation-exchanged to H + form by boiling them in a 0.5M H 2 SO 4 solution for 1 hour and finally. 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