Integrated platinum carbon nanotube based electrocatalyst for high efficiency proton exchange membrane fuel cells 4

Chapter 4 Integrated Pt/CNT-based Electrocatalyst for PEMFCs 4.1 Introduction Following our optimization studies on the in situ growth of CNTs on carbon paper shown in Chapter 3, the a

Chapter 4 Integrated Pt/CNT-based Electrocatalyst for PEMFCs

4.1 Introduction

Following our optimization studies on the in situ growth of CNTs on carbon paper shown in Chapter 3, the application of these in situ grown CNTs is presented in this chapter where fabrication and characterization of an integrated Pt/CNT-based electrocatalyst are intensively introduced The integrated Pt/CNT-based electrocatalyst was fabricated for PEMFC electrodes via direct Pt sputter-deposition and their electrocatalytic performance on PEMFC reactions was characterized in a real PEMFC system The aim of this work was to optimize the fabrication method for the integrated Pt/CNT-based electrode as well as to evaluate the effectiveness of this electrode as an integrated PEMFC component for high efficiency PEMFCs

It has been reported in a large number of studies that Pt/CNT-based electrocatalysts demonstrated higher electrochemical activity and stability than those

of the conventional Pt/VXC72R electrocatalyst [1-8] To exploit such improvement of Pt/CNT-based electrocatalysts, several research groups developed their Pt/CNT-based electrocatalysts based on in situ grown CNTs on carbon paper as the catalyst support [9-12] The synthesis processes of the in situ grown CNTs in their studies were described in Section 3.1 and this section mainly introduces their synthesis methods for

Pt deposition onto the in situ grown CNTs In earlier study by Wang and coworkers [9], Pt catalysts were electrodeposited onto the in situ grown multi-walled carbon

H2SO4 aqueous solution The deposition process was carried out at a potential of 0 V

vs SCE and the Pt loading was controlled by the total charge applied It was reported

that a total Pt loading of 0.2 mg cm-2 was obtained by this electrodeposition method and the average Pt particle size was around 25 nm However, the large Pt particles were found to be the major handicaps that caused a lower polarization performance compared to that of the conventional ink-process prepared electrode, where the Pt particle size was in the range of 2–4 nm To reduce the Pt particle size, they used a chemical reduction method in their subsequent work to synthesis Pt/CNT-based electrocatalysts [10] Prior to Pt deposition, the in situ grown CNTs were first surface

for 3 h To deposit Pt catalysts, a certain amount of Pt precursor solution was sprayed onto the CNT-grown carbon paper under a 60−80 °C heating condition, and then followed by reduction in 20% H2 in N2 at 150 °C for 2 h The Pt precursor solution

the in situ CNTs as determined the weight difference of the CNT-grown carbon paper According to the TEM micrograph of the deposited Pt catalysts, the Pt particle size was reduced to 4 nm via this chemical reduction method and enhanced Pt utilization was attained to give higher polarization performance However, it was found that the polarization performance of the Pt/CNT-based electrode remained very low without brushing an additional gas diffusion layer on the backside of the carbon paper

Based on Wang’s exploration on Pt catalysts supported on in situ grown CNTs,

in 2006 Villers et al also prepared a Pt/CNT-based electrocatalyst for PEMFC electrodes [11] In their method, the in situ grown MWNTs were first immersed in a

precursor were then air-dried and chemically reduced in H2 at 500 °C for 15 min After reduction, they found that the deposited Pt catalysts were nanosized particles with an average particle size of 3−5 nm However, chemical state analysis of the Pt nanoparticles by XPS revealed that there were still some Pt(II) (22%) and Pt(IV) (4%) present after Pt deposition Moreover, it was found that one deposition process corresponded to a Pt loading of 0.16 mg cm-2 and increasing Pt loading was achieved

by repetitive deposition, which indicated that this Pt deposition process could not provide efficient control of Pt catalyst loading Later Saha and coworkers [12] claimed that a high Pt loading on in situ grown CNTs could be obtained using glacial acetic acid as the reducing agent In their study, the MWNT-grown carbon paper was first chemically oxidized in a 5 M HNO3 aqueous solution for 5 h before Pt deposition Afterward, the MWNT-grown carbon paper was washed with deionized water and dried in vacuum at 90 °C for another 5 h During the Pt deposition process, the MWNT-grown carbon paper was immersed into a mixture dispersion of Pt acetylacetonate (Pt(acac)2) and 25 ml glacial acetic acid and ultrasonicated for 2 min

at room temperature Then it was heated up to 110 °C for Pt reduction and held for 5 h with constant stirring At last, the Pt-deposited MWNT-grown carbon paper was rinsed with deionized water and dried at 90 °C overnight in a vacuum oven The Pt loading obtained via this process was around 0.42 mg cm-2 determined by inductively coupled plasma-optical emission spectroscopy (ICP-OES) Although the deposited Pt nanoparticles showed high density and small size distribution range (2−4 nm) on the

in situ MWNTs, it should be noted that this deposition process was rather tedious and time-consuming In addition, the wet-chemical deposition methods described above all revealed difficulties in control of Pt loading and Pt distribution, considering that simultaneous Pt deposition may occur on the carbon fibers in carbon paper, making

these Pt catalysts inaccessible to reactant gases Furthermore, despite the enhanced polarization performance observed from the Pt/CNT-based electrocatalysts prepared

by the above wet-chemical methods, it is worth noting that an additional based gas diffusion layer was always needed on the backside of the carbon paper that adds further complexity to electrode preparation In view of all these limitations, the effectiveness of the in situ grown CNTs as catalyst support is greatly undermined by the Pt deposition process

To resolve the above mentioned difficulties in Pt deposition onto in situ grown CNTs, controllable Pt deposition was conducted via direct sputter-deposition in this study Sputter-deposition technique has the advantages of being able to directly deposit Pt catalyst with excellent control of deposition rate, as demonstrated in a number of studies [4, 13-17] Moreover, as it is a surface deposition technique, it allows us to disperse the Pt catalyst highly localized at the surface of the CNT-grown carbon paper, leading to higher Pt utilization [4] Last but not least, sputter-deposition technique was chosen for Pt deposition owing to its ability to directly deposit Pt catalysts onto the dense in situ grown CNT layer, thus additional CNT surface oxidation process can be eliminated In the following sections, the fabrication process

of the Pt/CNT-based electrocatalyst via sputter-deposition will be introduced and the overall effectiveness of this Pt/CNT-based electrocatalyst will be evaluated by a series

of ex situ and in situ characterization studies This chapter mainly concentrates on the electrochemical activity and mass transport properties of the Pt/CNT-based electrocatalyst for PEMFC electrodes The electrochemical stability of this Pt/CNT-based electrocatalyst will be demonstrated and elaborated in the subsequent chapter

4.2 Fabrication and Characterization of Integrated Pt/CNT-based Electrocatalyst

This section presents the fabrication process of the sputter-deposited Pt catalysts,

as well as the structural and compositional characterization towards them The structural and compositional analysis is of much importance in evaluating the integrated Pt/CNT-based electrocatalyst in that it can provide valuable insights with regard to understanding the electrocatalytic performance of the catalyst in addition to the electrochemical characterization

4.2.1 Fabrication of Integrated Pt/CNT-based Electrocatalyst

In this study, direct sputter-deposition of Pt catalysts onto the in situ grown CNTs was carried out using a R.F magnetron sputtering system (Denton Discovery-18) In contrast to previous studies where wet-chemical deposition processes were used [9-12], several pieces of CNT-grown carbon papers were directly transferred into the sputtering chamber after CNT growth without surface oxidation During the sputtering process, the Ar gas pressure was maintained at 10 mTorr while the specific deposition rate of Pt catalysts was varied by controlling the output sputter-power The specific deposition rate of the Pt catalysts at a given output power was determined by weight difference of the CNT-grown carbon paper before and after Pt deposition Controllable Pt deposition was then realized by varying the sputtering time based the calculated specific deposition rate Initially a Pt loading of 0.04 mg cm-2 obtained at

100 W output sputter-power was used in this study, which is one tenth of the Pt loading in a commercial Pt/VXC72R-based electrode [18]

4.2.2 Characterization of Integrated Pt/CNT-based Electrocatalyst

In order to reveal the microstructure and morphology of the sputter-deposited Pt catalysts on the in situ grown CNTs, their TEM micrographs were examined using a high-resolution JEM-2010 FETEM system Figure 4.1 (a) shows the TEM micrograph

of the CNT grown on carbon paper at a C2H4 flow rate of 20 sccm It can be seen that the CNTs grown on carbon paper by the thermal CVD process were multi-walled carbon nanotubes (MWNTs) with a typical outer diameter of about 30 nm and an inner diameter of around 10 nm The Fe catalysts can also be observed as small particles buried inside the tube, which agrees well with previous studies [19, 20] Moreover, it is noteworthy that the in situ grown MWNTs showed very coarse and incontinuous external graphitic walls, suggesting that various defects were generated

on the CNT skin during growth This result is in excellent agreement with the Raman spectra study of the in situ grown CNTs shown in Section 3.3.2 Nevertheless, the defects on the CNT surface may be a favorable feature for Pt deposition due to the presence of numerous anchoring sites for Pt particles, in contrast to the inertness of a perfect CNT skin that was reported unable to be wetted by liquids with a surface

oxidation Figure 4.1 (b) and (c) illustrate the TEM micrographs of the as-deposited Pt catalysts on in situ grown MWNTs Contrary to previous studies where Pt catalysts prepared by various chemical reduction methods usually showed poor dispersion on untreated CNTs [10, 21], numerous nanosized Pt particles were observed to be densely dispersed on the CNT support via direct sputter-deposition as shown in Fig 4.1 (a) Instead of forming a Pt thin film, as was seen on the smooth Si substrate, the sputter-deposited Pt catalyst showed a scaly structure with a homogeneous

distribution on the CNT layer due to its high surface roughness and porosity The average particle size of the Pt nanoparticles was approximately 2−3 nm The Pt loading of 0.04 mg cm-2 corresponds to a Pt thin film of thickness about 20 nm on a smooth Si substrate Given the extremely high roughness and porosity of the in situ grown MWNT layer, it is understandable that the Pt layer on MWNTs was of much lower thickness and thus Pt nanoparticles were formed and uniformly dispersed on the MWNT surface According to Fig 4.1 (c), the grain size distribution of the Pt nanoparticles on a single CNT shows a relatively small range mostly from 1−5 nm TEM investigation demonstrates that well-dispersed Pt nanoparticles have been successfully deposited onto the in situ grown CNTs by direct sputter-deposition, which provides significant advances over the wet-chemical processes that Pt particle distribution is greatly enhanced and CNT surface oxidation is eliminated

(a)

(b)

0 1 2 3 4 5 6 0

Fig 4.1 TEM micrographs of the in situ grown CNTs (a) before and (b) after

Pt sputter-deposition, (c) Pt nanoparticles on a CNT support Pt loading: 0.04

mg cm-2(c)

Fig 4.2 Histogram of Pt particle size distribution based on Fig 4.1 (c)

mass activity of Pt catalyst has a strong correlation to the Pt particle size and the maximum mass activity corresponds to a grain size around 3 nm Therefore the sputter-deposited Pt catalysts may give rise to a high mass activity for PEMFC reactions based on the particle size distribution demonstrated in Fig 4.2

Pt polycrystalline MWNT

Graphite layers

(b) (a)

Fig 4.3 HRTEM micrographs of (a) an as-grown CNT tip and (b) a CNT tip with sputter-deposited Pt catalysts

To further investigate the microstructure of the Pt/CNT-based electrocatalyst, high-resolution TEM (HRTEM) micrographs were obtained at magnifications up to 500,000 Figure 4.3 (a) shows the multi-graphitic walls on the tip of a CNT that was grown on carbon paper The layered graphitic walls were clearly seen at the closed end with Fe catalysts buried inside, indicating a tip-growth mechanism for the in situ grown CNTs [19] Considerable defects were also observed at the CNT surface, corresponding to the Raman results shown in the precious chapter After Pt sputter-deposition, the TEM micrograph of a CNT tip with sputter-deposited Pt nanoparticles

is illustrated in Fig 4.3 (b) It is noticeable that the Pt nanoparticles exhibited a clear lattice fringe of crystallite structure, suggesting that the Pt catalyst produced via sputtering technique has a high crystallinity This implies that the sputter-deposited Pt catalysts are probably in a pure metallic state with little oxide content

To examine the chemical state of the sputter-deposited Pt catalysts, XPS analysis was performed on the Pt/CNT-based electrocatalyst as shown in Fig 4.4 Determination of Pt chemical state was carried out by means of spectrum deconvolution using a Gaussian/Lorentzian shape line modified by an asymmetric function As can be seen in Fig 4.4, the Pt 4f7/2 core level revealed a binding energy

of 71.1 eV based on the Pt/CNT composite catalyst, with reference to 284.4 eV as the binding energy of C 1s This result substantially supports the TEM results that the sputter-deposited Pt catalyst is mostly in the pure metallic state and has a fine crystallite structure However, it is likely that a minute amount of oxidized Pt is present due to the anchoring oxidized groups on the CNT surface By contrast, Villers found a noticeable amount of oxidized Pt catalysts on the in situ grown CNTs that were deposited by wet-chemical methods [11] Therefore, a reduction process for the

84 82 80 78 76 74 72 70 68 66 64 0

post-deposited Pt particles was indispensable in their study The vast composition difference of the Pt catalysts may probably be due to the intrinsic properties of the deposition methods In a typical chemical reduction process, the content of Pt oxides

in the synthesized Pt catalyst greatly depends on the reducing agent used for the based solvent solution [11, 12, 23] Pt oxides are likely to form during deposition when the reduction of Pt precursor is insufficient While in the sputtering process, Pt atoms are ejected from a pure Pt target and directly deposited onto CNT/carbon paper substrates in a high-vacuum environment This process particularly reduces the possibilities of Pt oxide formation thus showing a great potential for Pt deposition for PEMFC applications as demonstrated in previous studies [13-17]

Pt-Fig 4.4 XPS spectrum of Pt nanoparticles on CNT/carbon paper by

sputter-deposition X-ray source: Al Kα 1486.6 eV, pass energy: 20 eV

4.3 Integrated Pt/CNT-based Cathode for PEMFCs

As described previously in Chapter 1, the oxygen reduction reaction (ORR) occurring at PEMFC cathode is the rate determining step for PEMFC operation Under normal conditions the ORR kinetics is very slow thus Pt-based catalysts are commonly used at the electrocatalyst for this reaction However, the ORR kinetics is still rather sluggish compared with the hydrogen oxidation reaction (HOR) that most

of the activation overpotential derives from the ORR while the contribution of the HOR is usually negligible [24] As such, in this study we focus on the integrated Pt/CNT-based electrocatalyst for the ORR by examining their in situ electrochemical performance as the PEMFC cathode electrocatalyst

4.3.1 Optimization of Electrode Preparation

To actively verify the electrochemical performance of the integrated based for the ORR, electrode preparation process was first optimized in terms of a series of experimental parameters, including Nafion impregnation, sputtering power,

Pt/CNT-Pt catalyst loading and CNT layer morphology In a typical MEA used in this study the integrated Pt/CNT-based electrocatalyst was made into the cathode while the anode was a conventional VXC72R-based gas diffusion electrode with a commercial Hispec4000 catalyst (40 wt% Pt/VXC72R, Johnson-Matthey) The Pt loading at

conventional VXC72R-based electrode was described in Section 2.2.3 For the Pt/CNT-based cathode, 1 mg PTFE was brushed onto the back side of the CNT/carbon paper backing before Pt deposition, in order to provide electrode hydrophobicity The optimization of electrode preparation process for the Pt/CNT-

based cathode were carried out mainly based on polarization curve characterization,

of which results are shown below in sequence

Nafion Impregnation

It is well-known that Nafion impregnation into catalyst layer can significantly improve cell performance in that the three-phase reaction zone is greatly increased by impregnating Pt catalysts with Nafion electrolyte Paganin et al [25] investigated the effect of Nafion impregnation on commercial Pt/VXC72R-based electrode and found that the cell performance notably improved when the Nafion loading was increased

maximum when the Nafion loading was equivalent to 33 wt% of the catalyst layer weight Later Qi et al [26] also observed this optimum Nafion loading that the weight ratio between Nafion and Pt/VXC72R was 1:2 for conventional ink-process prepared electrodes To find out the optimum Nafion loading for the Pt/CNT-based cathode in this study, the effect of Nafion impregnation was investigated by spraying 0.5 wt% Nafion solutions onto several Pt/CNT-based electrodes bearing different Nafion loadings The corresponding polarization curves are shown in Fig 4.5 below

As can be seen in Fig 4.5, the cell performance was visibly enhanced when 40

µl 0.5 wt% Nafion solutions were sprayed onto the Pt/CNT-based cathode, compared

to that of the MEA without Nafion impregnation With increasing Nafion impregnation, a maximum cell performance was achieved from the MEA with a total amount of 60 µl Nafion solution sprayed at cathode, corresponding to the optimum

density of 610 mW cm-2 based on 0.04 mg cm-2 Pt at cathode, owing to the increased

proton conductivity within the catalyst layer However, when the Nafion loading further increased, it was observed that the cell performance dropped dramatically at large current densities It can be speculated that the voltage drop at large current density region may be probably due to the wrapping of Pt catalysts by the excess Nafion ionomers, causing increased mass transport resistance

Sputter Output Power

The effect of sputter output power was also investigated as a parameter for the optimization of electrode preparation Initially the sputter-deposition of Pt catalysts was carried out at an output power of 100 W, which gives a specific Pt deposition rate

of 35 µg cm-2 min-1 In this study, a series of sputter output powers, including 50, 30 and 15 W, were used to deposit Pt catalysts and their effects on cell performance were evaluated based on their polarization curves The impregnated Nafion loading was 60

Pt loading: 0.04 mg cm  at cathode

Fig 4.5 Polarization curves of Pt/CNT-based cathodes

with various Nafion loadings

µl of 0.5 wt% Nafion solution for all the samples As shown in Fig 4.6, the based electrodes obtained at 50 and 30 W sputter powers demonstrated similar cell performance, which was noticeably higher than those of the electrodes prepared under

Pt/CNT-100 and 15 W sputter powers According to Huang et al [27], lowering sputter power could produce more refined Pt particles at lower sputtering rate However, they found

that lower sputter power also reduced the kinetic energy of the ejected Pt atoms, leading to a diminished penetration depth of the Pt particles Accordingly, it is very likely that the sputter-deposited Pt catalysts at 50 and 30 W had smaller grain sizes than at 100 W, as well as greater penetration depths into the CNT layer than at 15 W, owing to the higher kinetic energy of the sputtered Pt atoms The specific Pt

min-1, respectively Therefore 50 W was chosen as the optimum sputter power for the

Fig 4.6 Polarization curves of Pt/CNT-based electrodes with

sputter-deposited Pt catalysts prepared at different output powers

subsequent Pt deposition, whereby a typical Pt loading of 0.04 mg cm-2 can be obtained from a 4 min sputter-deposition process

Pt Catalyst Loading

A number of studies have shown that sputter-deposition technique has great potential for fabricating ultra-low Pt loading electrodes for PEMFC applications [13-17] The key advantage of this technique lies in that it is able to deposit Pt nanostructures directly onto the rough electrode surface, where the reactant gases, catalysts, and electrolyte form the three-phase zone Hence the Pt utilization is considerably enhanced by sputter-depositing Pt catalysts into the electrode-electrolyte interface with an ultra-low Pt loading Previously in this study the Pt/CNT-based electrodes were all sputter-deposited with 0.04 mg cm-2 Pt catalysts In order to probe the cell performance of Pt/CNT-based electrodes with different Pt loadings, a set of electrodes were investigated on their polarization curves, where the Pt loadings were

electrodes, carbon papers with in situ CNTs grown at 20 sccm C2H4 were used as the sputtering substrates The sputter output power was 50 W and the Ar gas pressure was

10 mTorr during Pt sputter-deposition Prior to MEA assembly, the Pt/CNT-based electrodes were brushed with 1 mg PTFE at the backside and air-sprayed with 60 µl

of 0.5 wt% Nafion solution on the electrode surface

Figure 4.7 shows the polarization curves of the Pt/CNT-based electrodes with various Pt loadings at cathode It can be observed that the cell performance was improved with increasing Pt loading at cathode This is expectable that a larger amount of Pt catalysts provides more reaction sites for the ORR However, it was

noted that only slight improvement in cell performance was obtained when the

densities from 0.02, 0.03, 0.04 and 0.06 mg cm-2 Pt catalysts were found to be 605,

635, 670 and 680 mW cm-2, respectively

TEM micrographs of the in situ grown CNTs with different Pt loadings are shown in Fig 4.8 to reveal the microstructure of the Pt/CNT-based catalysts It can be clearly seen that the particle size and density of the sputter-deposited Pt catalysts grew

catalysts were sputter-deposited onto the in situ grown CNTs as shown in Fig 4.8 (a), small Pt nanoparticles were observed on the CNT surface exhibiting a low distribution density As the Pt loading increased to 0.04 mg cm-2 (see Fig 4.8 (c)), it is noticeable that the Pt nanoparticles formed numerous nanoscaled Pt islands and densely dispersed on the CNT surface As a result, the corresponding Pt surface area is greatly

Fig 4.7 Polarization curves of Pt/CNT-based electrodes with

different Pt loadings at cathode

enhanced by the increased Pt loading, leading to a visibly improved cell performance

as demonstrated in Fig 4.7 However, it was revealed that the Pt islands coalesced into large grains and fully covered the CNT surface as the Pt loading was further increased to 0.06 mg cm-2 This result superbly explains the limited improvement in cell performance from the Pt/CNT-based electrode with 0.06 mg cm-2 Pt catalysts that the Pt/CNT catalysts can only give slight increment in total Pt surface area when

Fig 4.8 TEM micrographs of in situ grown CNTs with (a) 0.02,

(b) 0.03, (c) 0.04, and (d) 0.06 mg cm-2 Pt catalysts

higher Pt loadings are sputter-deposited beyond 0.04 mg cm-2 As such, the optimum

Pt loading was determined as 0.04 mg cm-2 for our subsequent experiment

CNT Layer Morphology

As previously depicted in Chapter 3, the in situ grown CNT layer at different

tunable surface roughness and porosity of the in situ grown CNT layers for Pt deposition The effect of CNT layer morphology has not yet been reported in previous studies where CNTs were also grown on carbon paper to serve as Pt catalyst support [9-12] In this study, we investigated the cell performance of Pt/CNT-based electrodes based on different in situ grown CNT layers with varied surface morphologies The in situ grown CNT layers on carbon paper were obtained from the optimized CNT growth described in previous chapter at a series of C2H4 flow rates of 5, 10, 15, 20 and 25 sccm, respectively Their SEM images are illustrated in Fig 3.8 The electrode preparation parameters used, such as Nafion loading, Pt sputtering power and Pt loading, were based on aforementioned optimization studies

Figure 4.9 shows the polarization curves of Pt/CNT-based electrodes with Pt catalysts sputter-deposited on different in situ grown CNT layers As can be seen in Fig 4.9, a notable improvement in cell performance was achieved by using CNT support layers grown at increasing C2H4 flow rate from 5 to 20 sccm However, this tendency reversed as the C2H4 flow rate was further increased to 25 sccm According

to the surface morphology of the in situ grown CNT layers as revealed by their SEM images, the dramatic difference in cell performance can be probably attributed to the distinct surface morphologies of the CNT layers in view of their surface area and

porosity It has been demonstrated that the surface area and porosity of the in situ

during growth Such enhancement would provide a considerably enlarged surface area for the sputter-deposited Pt catalysts However, it should be also noted that the pore size of the CNT layers greatly decreased with increasing C2H4 flow rate, due to which the penetration depth of the sputtered Pt particles might be limited when the pore size became extremely small Consequently, it is likely that the Pt catalysts were sputter-deposited mainly onto the top surface of the CNT layer, resulting in a reduced Pt surface area for the ORR This may probably be the reason for the cell performance

optimum surface morphology where both high surface area and large penetration depth of Pt catalysts are guaranteed

Pt loading: 0.04 mg cm -2

at cathode

Fig 4.9 Polarization curves of Pt/CNT-based electrodes with different in situ grown CNT layers as catalyst support

4.3.2 Electrochemical Characterization of Integrated Pt/CNT-based Cathode

Subsequent to the optimization studies for electrode preparation, electrochemical characterization of the integrated Pt/CNT-based cathode was carried out by means of

a series of in situ electrochemical tests, including polarization curve measurement, in situ cyclic voltammetry (CV), and electrochemical impedance spectroscopy (EIS) To evaluate the electrochemical performance of the Pt/CNT-based electrodes, two reference electrodes were also prepared using commercial 20 wt% Pt on VXC72R (E-TEK) and 40 wt% Pt on VXC72R (Johnson Matthey) catalysts The details of electrode preparation process have been described in Section 2.2.3 The three electrodes with different catalysts were used as the cathode with an identical Pt

electrode for all of them with 0.2 mg cm-2 Pt prepared by ink-spread method Nafion

112 (Dupont Inc.) was used as the polymer electrolyte

Figure 4.10 shows the polarization curves of the three electrodes with the Pt/CNT catalyst and two commercial Pt/VXC72R catalysts, respectively It can be clearly seen that the Pt/CNT-based electrode showed a pronounced improvement in the entire voltage region while the 40 wt% Pt/VXC72R catalyst gave the lowest polarization performance among the three catalysts This implies that the Pt/CNT-based electrode may have enhanced charge transfer and mass transport properties compared to those of the commercial Pt/VXC72R catalysts, thus leading to smaller activation, ohmic and mass transport overpotentials as shown in Fig 4.10 It was

was 2.8 W per mg of Pt for the Pt/CNT-based MEA, compared to 2.5 and 2.1 W per

mg of Pt for the two Pt/VXC72R-based MEAs Furthermore, it should be noted that

0.0 0.5 1.0 1.5 2.0 2.5 0.0

20 wt% Pt/VXC72R catalyst (E-TEK) at cathode

40 wt% Pt/VXC72R catalyst (Johnson Matthey) at cathode

the polarization curves of the Pt/VXC72R-based electrodes showed gradual deterioration usually after 10 curve scans, particularly at large current regions This is probably due to water flooding occurring at the cathode By contrast, the Pt/CNT-based electrode showed increasing polarization performance in the first 10 ~ 15 scans derived from the activation of the sputter-deposited Pt catalysts, and thereafter its

polarization curves were rather stable within the test scope of 30 scans in this study It

is likely that the inherent hydrophobicity and highly porous structure of the CNT layer may provide superior water transport property of the electrode to prevent water accumulating in the catalyst layer Therefore, the Pt/CNT-based electrode revealed a greatly reduced mass transport overpotential compared with the Pt/VXC72R-based electrodes as shown in their polarization curves

Fig 4.10 Polarization curves of Pt/CNT and Pt/VXC72R-based

electrodes with 0.04 mg cm-2 Pt at cathode

Figure 4.11 shows the in situ CVs of Pt/CNT and Pt/VXC72R-based electrodes

at a scan rate of 100 mV s-1 The typical characteristics of the cyclic voltammogram for Pt metal observed for both catalysts are a pair of hydrogen adsorption/desorption peaks located below 0.3 V and a pair of Pt oxidation/reduction peaks at around 0.8 V [28] It is noteworthy that the Pt oxidation peak of the Pt/CNT catalyst showed a positive shift to a higher potential compared with the Pt/VXC72R catalyst, which implies that the sputter-deposited Pt catalyst on the CNT support may have better electrochemical stability over the commercial catalyst In addition, the cyclic voltammogram for the Pt/CNT catalyst revealed a notably smaller double-layer charging region than the commercial catalysts, which is attributed to the high electronic conductivity of the CNT support as proposed by Xing [29] The in situ electrochemical active surface area (ECSA) was obtained for the three electrodes by

40 wt% Pt/VXC72R catalyst (Johnson Matthey)

Fig 4.11 In situ CVs of Pt/CNT and Pt/VXC72R-based electrodes with 0.04 mg cm-2 Pt at cathode

calculating the hydrogen adsorption/desorption peak areas excluding the double-layer capacitance region The average of the cathodic and anodic peak areas was used as the

ECSA can be calculated from S EC = Q H /(Q ref × (Pt loading)), where Q ref = 210 µC

hydrogen [11] The calculated in situ ECSA for the Pt/CNT-based electrode was 17.2

m2 g-1, in comparison to 29.1 m2 g-1 for the 20wt% Pt/VXC72R-based electrode and

from the 20wt% Pt/VXC72R-based electrode may account for its well-dispersed Nafion ionomers throughout the catalyst layer The Pt/CNT-based electrode also showed relatively high ECSA owing to its distinct morphological property that all the

Pt nanoparticles are highly localized at the electrode-electrolyte interface by direct sputter-deposition On the other hand, the Pt/VXC72R catalysts were reported to have

a rather dense structure, where the three-phase reaction zone was greatly impaired by the compact carbon particles Therefore, it is likely that the Pt/CNT-based electrode should give higher Pt utilization than the Pt/VXC72R-based electrodes, whereas further investigation on these three electrodes in addition to the ECSA evaluation is necessary to provide more insights into the notable cell performance improvement of the Pt/CNT-based electrode

Subsequently, a more in-depth investigation on the Pt/CNT-based electrode was conducted using a.c impedance spectroscopy, to further understand the underlying mechanisms for the performance improvement from this Pt/CNT layer Electrochemical impedance spectroscopy (EIS) was used in that it is a reliable diagnostic tool for evaluating the transport properties in fuel cells owing to its ability

to separate the impedance responses of the various transport processes occurring simultaneously in PEMFCs [30] A number of EIS simulation studies on working PEMFCs have suggested several in situ characteristics of a PEMFC MEA present in its impedance spectra [31-34]: (i) the cathode impedance predominates in the spectrum while the anode impedance is relatively negligible at low overpotential; (ii) the charge transfer resistance (Rct) of the cathode due to the oxygen reduction reaction

structure of the electrode; (iii) the ohmic resistance (Rohm) distributed in the electrode mainly caused by the electrolyte membrane; (iv) the oxygen diffusion resistance (Rod) derived from the diffusion limitation of oxygen in the cathode, especially when air is used as oxidant; (v) the water transport resistance (Rwt) as a result of water transport limitation in the membrane Generally, the high frequency region of the impedance spectrum reflects the charge transfer properties of the catalyst layer, whereas the low frequency region represents the mass transport properties in the electrode Consequently, EIS enables us to evaluate them separately in order to determine their individual influence on the overall fuel cell performance

In order to distinguish the contributions of different processes that take place in the Pt/CNT and Pt/VXC72R-based electrodes, in situ EIS tests were performed to measure the frequency dependence of the impedance of each electrode at different overpotential regions The EIS results of the three electrodes are presented in Nyquist plot as shown in Fig 4.12 At 0.8 V, which is near open circuit voltage, the rate determining step is controlled by the kinetics of ORR and thus the major impedance

capacitance Cdl in the cathode catalyst layer observed as a semicircular arc in Fig