[ecs 214th ecs meeting - honolulu, hi (october 12 - october 17, 2008)] ecs transactions - the use of...

The Use of Ultrasound (20 kHz) as a Novel Method for Preparing Proton Exchange Membrane Fuel Cell Electrodes

B. G. Polleta

a Fuel Cells Group, School of Chemical Engineering, The University of Birmingham,

Edgbaston Road, Birmingham, B15 2TT, England, U.K.

A novel Sonoelectrochemical method for preparing catalyst layers for Proton Exchange Membrane Fuel Cell (PEMFC) is described. In order to optimize the system, initial studies focused on potentiostatic platinum electrodeposition on glassy carbon (GC) disc electrodes in the absence and presence of ultrasound (20 kHz, up to 29 W.cm-2) and at (313 ± 1) K. It was found that ultrasound affects significantly the electrodeposition process of Pt on GC electrodes, i.e. increased limiting currents were observed compared to those obtained under ‘silent’ conditions (no agitation). Platinum loaded on Nafion-bonded carbon anodes were prepared in K2PtCl4 aqueous solutions by galvanostatic pulse electrodeposition in the absence and presence of low-frequency high-power ultrasound; and MEAs were fabricated by conventional method. It was found that electrodes prepared sonoelectrochemically showed better performance compared to those prepared by (i) galvanostatic pulse method only (i.e. in the absence of ultrasound) and (ii) conventional method.

Introduction

The main objective for the development and successful market deployment of Proton Exchange Membrane Fuel Cells (PEMFC) is to reduce the platinum catalyst loading of the electrodes (both anode and cathode) and the associated cost without decreasing the fuel cell performance [1,2]. In order to achieve this challenging goal, it is necessary to increase the effective surface area of the Pt catalyst, in other words, the surface contact between the electrode catalyst layers (CL), the carbonaceous electronic conductor (gas diffusion layer, GDL), the polymer electrolyte membrane (Nafion®, PEM) and the reactants (hydrogen and oxygen). Since the electrochemical reactions occur in this active part of the electrodes (also known as the ‘three-phase reaction zone’), the fuel cell performance depend greatly on the kinetics of interfacial phenomena [1,2]. Electrodes for PEMFCs are usually constituted of carbon black powder acting as a catalyst support mixed with solid polymer electrolyte e.g. Nafion® [1]. In this case, to increase the performance of the electrodes (in other words, the ‘true’ catalyst surface area), either (i) an increase in CL thickness, for a given Pt catalyst loading or (ii) an increase in the amount of Pt catalyst in the CL is required. However, increasing the thickness of the catalyst layer leads to a decrease in reactants diffusion rate towards Pt catalytic sites, whereas increasing the electrocatalyst loading generally leads to an increase in particle size, thus a decrease in fuel cell efficiency [1,2].

ECS Transactions, 16 (2) 2031-2041 (2008)10.1149/1.2982043 © The Electrochemical Society

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There are a variety of well-documented methods describing the application of electrocatalyst to the substrate (GDL) and the membrane electrolyte (PEM). For example, Litster and McLean [1] and Wee et al. [2] give an excellent overview of PEMFC electrode fabrication methods. The most common method for the fabrication of electrocatalyst layers (CL) is to mix carbon supported on Pt agglomerates (Pt/C) by the colloidal route, with a solubilised polymer electrolyte membrane (e.g. Nafion ionomer) and apply the ‘paste’ onto the support either by decal, blade process, screen-printing or spray method. The main disadvantage of these methods is that (i) heating i.e. oxidative treatment is required in order to ‘clean’ the electrocatalyst particles from preparative chemical contamination, (ii) these treatments can greatly affect the surface structure/morphology of the electrocatalyst particles and (iii) the presence of inactive platinum sites for electrochemical reactions at the ‘three-phase reaction zone’ is observed [1,2].

One of the most promising methods to increase Pt active site and effective Pt utilisation of the electrocatalyst at the ‘three-phase reaction zone’ is to electrodeposit Pt from plating baths. Several workers showed that Pt electrodeposited either potentiostatically or galvanosatically in continuous DC or pulse modes is a viable and attractive method for the fabrication of PEMFC electrodes [3-13]. Recently, Kim et al. [10,11,13] showed that low Pt loading PEMFC electrodes can be prepared by galvanostatic pulse electrodeposition on a Nafion-bonded carbon layer. They demonstrated that using this method, it is possible to significantly reduce the thickness of the CL as well as the amount of Pt loading and hence increasing Pt utilisation efficiency. However, in views of increasing the Pt utilisation, Cheh et al. [3] and Coutanceau et al. [9] showed that the rate determining step of Pt electrodeposition is mainly controlled by mass transport. One of the many approaches to increase mass transport in such processes is to employ efficient stirring or forced convection in the form of ultrasound. The use of ultrasound on electrochemical systems or Sonoelectrochemistry was first observed by Moriguchi as early as 1934 [14] and continues to be an active and exciting research area [15]. Extensive work has been carried out in which low-frequency high power ultrasound (20kHz-2MHz) was applied to various electrochemical processes leading to several industrial applications and many publications over a wide range of subject areas including: electrodeposition, electroplating, electrochemical dissolution and corrosion testing [15]. It has been shown that the use of high intensity ultrasonic irradiation on electrochemical processes leads to both chemical and physical effects, for example, mass-transport enhancement, surface cleaning and radical formation. Early work from Coury et al. [16,17] , Compton et al. [18] and Pollet et al. [19] have shown that sonication lead to a decrease in the diffusion layer thickness (δ) thereby giving a substantial increase in limiting current (Ilim), which can be attributed to effects of cavitation and/or micro and macro-streaming. It has also been shown that ultrasonic irradiation is more effective than traditional hydrodynamic methods e.g. rotating disc electrode (RDE) in reducing the diffusion layer thickness, δ, and thus both cavitational and acoustic streaming effects contribute significantly to the increase in observed experimental currents [15-19]. The experimental decrease in the diffusion layer thickness is also thought to be due to asymmetrical collapse of cavitation bubbles at the electrode surface leading to the formation of high velocity jets of liquid being directed toward its

ECS Transactions, 16 (2) 2031-2041 (2008)



surface. This jetting, together with acoustic streaming, can lead to random puncture and disruption of the mass transfer boundary layer at the electrode surface.

In this paper, it is reported for the first time the use of ultrasound for the fabrication of PEMFC electrodes by galvanostatic pulse electrodeposition of Pt at room temperature, atmospheric pressure and at 20 kHz. In this study, MEA performance is compared to PEMFC electrodes prepared in the absence and presence of ultrasound (20 kHz), at a fixed ultrasonic intensity.

Experimental Potentiostatic Studies of the Electrodeposition of Pt on Glassy Carbon in the Absence and Presence of Ultrasound (20 kHz)

Potentiostatic experiments were carried out using an Autolab PGSTAT302N/FRA2 potentiostat connected to a PC for data acquisition and control. Electrochemical experiments were performed using a 50 cm3 jacketed cooling cell (Figure 1). A glass cooling coil was also placed in the cell to provide better control and regulation of the electro-analyte temperature. Both the jacketed cooling cell and the glass cooling coil were linked to two Grant thermostatted baths operating at preset temperatures. The temperature of the electro-analyte was measured with a Fluke 51 digital thermometer fitted to a K-type thermocouple. The electrochemical cell was placed in a Faraday cage. The working electrode (WE) was a rotating disc (RD) glassy carbon (GC) electrode (∅ = 0.2 cm, area = 0.0314 cm2).

Figure 1. Sonoelectrochemical (20 kHz) set-up employed for potentiostatic studies.

A platinum flag was used as the counter electrode (CE, area = 1.0 cm2). For the 20 kHz sonoelectrochemical experiments, the distance between the ultrasonic probe (20 kHz, area = 1.33 cm2) and the working electrode was d = (2 ± 0.1) mm (Figure 1). All Pt electrodes were electrochemically cleaned by cycling in sulphuric acid (1.0 mol dm-3) for 10 minutes prior to the experiments. They were then washed with high quality MilliQ

Ultrasonic probe (20 kHz)

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water. RD GC electrodes were used in preliminary platinum electrodeposition investigations, since GC allows the preparation of a readily reproducible and low surface area carbon support. These GC electrodes (2 mm diameter, embedded in a PTFE cylinder) were polished with a 0.05 µm Al2O3 suspension (BAS Inc.) and cleaned ultrasonically (Langford 40 kHz ultrasonic bath) in Milli-Q for 5 min at room temperature before each experiment. Baseline cyclic voltammograms of the polished GC electrodes were then recorded in 1 M H2SO4 to ensure a smooth, reproducible surface (i.e. unchanged over ten cyclic voltammograms at 50 mV.s−1) without any evidence of platinum or surface oxide functionalities. These electrodes were electrochemically scanned extensively in the range −0.23 to +1.2 V vs. sce and repolished, when required. They were then repolished and cleaned ultrasonically before checking via cyclic voltammetry. At this final stage only ten baseline CV scans were performed as to avoid oxidation of the GC surface. All chemical reagents were of AnalaR grade or equivalent. Aqueous solutions of K2PtCl4 (Aldrich, AR, 10 mmol.dm-3) in sodium chloride (Aldrich, AR, 0.5 mol.dm-3, employed as background electrolyte) and H2SO4 (Aldrich, AR, 1.0 mol dm-3) were prepared in (high quality MilliQ water, resistivity = 12 MΩ.cm-1). A saturated calomel electrode (SCE) was employed as reference electrode. All aqueous solutions were degassed by bubbling nitrogen for 20 minutes prior to experiments. Ultrasound was provided by a 20 kHz Vibra-Cell VC750 ultrasonic probe (maximum power of 155 W). Ultrasonic powers were determined calorimetrically using the Margulis’ method [21] and ultrasonic intensities (ψ) are quoted as W.cm-2 or otherwise stated. Preparation of Nafion-bonded Carbon Substrates

The preparation of Nafion-bonded carbon substrates was performed by using the method suggested by Kim et al. [13]. Vulcan XC-72 carbon powder (E-Tek) and Nafion solution (Aldrich, wt. 5 % in water/aliphatic alcohols, 1100 EW) were mixed with isopropyl alcohol (IPA, Aldrich) and sonicated using a Langford 40 kHz ultrasonic bath. Glycerol (Aldrich) and equivalent amount of aqueous solution of sodium hydroxide (Aldrich) were added to the mixture to form a hydrophilic layer and to convert H-form of Nafion to sodium-sulfonated Nafion (Nafion-Na+) respectively. The mixture was then sonicated and was applied onto the hydrophobic carbon gas diffusion layer (E-Tek, LT 1200-W, 10 cm2 geometric surface area) by spraying. The hydrophilic carbon layer was loaded with 0.3 mg.cm−2 of carbon. Galvanostatic Pulse Electrodeposition of Pt on Nafion-bonded Carbon Substrate in the Absence and Presence of Ultrasound (20 kHz) Pulse electrodeposition of Pt was performed galvanostatically using a Radiometer Voltalab 80 potentiostat (with current pulsing capabilities) connected to a PC for data acquisition and control. Galvanostatic experiments were performed using a 50 cm3 jacketed cooling cell (Figure 2). The electrodeposition of Pt occurred on the Nafion-bonded carbon blank electrode in a Pt plating bath made of K2PtCl4 (Aldrich, AR, 10 mmol.dm-3) in sodium chloride (Aldrich, AR, 0.5 mol.dm-3, employed as background electrolyte) in the absence and presence of ultrasound (20 kHz) and at (313 ± 1) K. For the sonoelectrochemical experiments, an ultrasonic intensity of 18 W.cm-2 was used.




The Nafion-bonded carbon substrate (acting as the cathode) was placed on a PTFE holder facing the ultrasonic probe and the counter electrode (Pt mesh, current collector, anode) (Figure 2) in the chloroplatinic acid bath. For this experiment, one face of the Nafion-bonded carbon substrate was electroactive and the other face was covered with a non-conducting material. A saturated calomel electrode (SCE) was employed as reference electrode. The distance between the ultrasonic probe and the Nafion-bonded carbon substrate electrode was d’ = (10 ± 0.1) mm (Fig. 2). The time programme for Pt electrodeposition in the absence and presence of ultrasound was 10 ms (ton) and 100 ms (toff) at a galvanostatic current density of 300 mA.cm-2. After Pt electrodeposition, the electrochemically catalysed electrode was rinsed thoroughly with ultrapure water to remove any residue of the Pt precursor. The electrodes were heated at 523 K in H2 (10%)/N2 (90%) gas for 30 mins and then immersed in 0.1 mol.dm-3 H2SO4 solution for 30 mins at 353 K. The electrodes were then thoroughly rinsed with ultrapure water.

Figure 2. Sonoelectrochemical (20 kHz) set-up employed for galvanostatic studies. MEA Fabrication and Fuel Cell Testing The MEA fabrication was carried out by using the method suggested by Kim et al. [13]. A cathode catalyst slurry was prepared by thoroughly mixing the supported catalyst (Pt/C, 20 wt.% E-TEK), Nafion solution (5 wt.%), and an appropriate amount of IPA ultrasonically (Langford 40 kHz ultrasonic bath). The ratio of the supported catalyst to Nafion was typically 2:1 by weight. The ‘sonicated’ slurry was sprayed on a GDL (E-TEK, LT 1200-W) with 0.3 mg Pt cm−2. The electrodeposited anode and the sprayed electrode were placed on either side of a Nafion 112 ® membrane (Dupont). The MEAs were prepared by hot-pressing (408 K, 180 s, 100 kgf.cm-2). For comparison purposes, conventional MEAs were prepared using identical GDLs, spraying method with a Pt loading of 0.3 mg.cm-2 (anode and cathode). The fuel cell tests were performed using a Bio-logic PEM Fuel Cell FCT-50S test stand. All measurements were made at 70oC (343 K) and pressures of 1 atm. on the anode and cathode side in H2 and O2 (1.5/2 stoics) respectively.

Ultrasonic probe (20 kHz)

Pt mesh (CE)

SCE (REF)

Water-jacketed electrochemical cell

Thermocouple

Nafion-bonded GDL placed on a PFTE holder (WE)

N2 bubbler

d’

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Results and Discussion

Potentiostatic Studies of Pt electrodeposition on Glassy Carbon in the Absence and Presence of Ultrasound (20 kHz) Silent conditions. A cyclic voltammogram (CV) of a freshly polished and cleaned glassy carbon (GC) electrode in a solution of 10mM PtCl4

2- in 0.5 M NaCl in the potential range of +0.4 V vs. sce; –0.8 V vs. Sce, at 10mV.s-1 and at (313 ± 1) K in the absence of ultrasound is shown in Figure 3. The CV was recorded after the electrode was held at +0.8 V vs. sce for 5 mins. On a freshly polished GC electrode, the reduction of PtCl4

2- occurs in the whole potential region during the negative scan and the resulting cathodic current appears to commence at around +100 mV vs. sce (see inset of Fig. 3) and increases continuously to reach its maximum at -328 mV vs. sce (Epc(I)). It was observed (not show here) that at potentials more negative than -950 mV vs. sce, the cathodic current was mainly due to the evolution of hydrogen occurring on the platinum clusters already formed.

Figure 3. Cyclic voltammogram of 10mM PtCl42- in 0.5 mol. dm-3 NaCl on a freshly

polished and cleaned glassy carbon (GC) electrode at 10 mV.s-1 and at (313 ± 1) K in the potential range [+0.4 V vs. sce; –0.8 V vs. sce] in the absence of ultrasound. The inset figure shows a linear sweep voltammogram (LSV) of 10 mM PtCl4

2- in 0.5 mol.dm-3 NaCl on a freshly polished and cleaned glassy carbon (GC) electrode at 10 mV.s-1 in the range of [+0.4 V vs. sce; –0.8 V vs. sce] in the absence of ultrasound and at (313 ± 1) K. It is now well-accepted in the literature, that this cathodic peak (Epc(I)) corresponds to the reduction of Pt(II) to Pt according to Equation (1) [13,22]:

PtCl42- + 2e- Pt(S) + 4Cl- Eo = 0.73 V vs. she [22] [1]

In the reverse scan, i.e. the anodic scan, Pt (II) ions are further reduced to a maximum of ca. -400 mV vs. sce (Epc(II)) and continues up to +100 mV vs. sce suggesting that platinum is further deposited on the nuclei already formed. These findings suggest that: (i) a

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substantial overpotential is necessary to drive the formation of Pt nuclei on the glassy carbon surface and (ii) are in excellent agreement with literature [13,22]. During the course of this investigation, it is assumed that the contribution from parasitic processes, such as the double layer charging and the hydrogen evolution during the electrodeposition process, are negligible under the given experimental conditions. Ultrasonic conditions. Figure 4 shows a series of linear sweep voltammograms (LSV) of a freshly polished and cleaned glassy carbon (GC) electrode in a solution of 10mM PtCl4

2- in 0.5 M NaCl in the potential range of [+0.2 V vs. sce; –0.7 V vs. sce] recorded at 10 mV.s-1 and at (313 ± 1) K in the absence and presence of ultrasound at various ultrasonic intensities. The LSVs were recorded after the electrode was held at +0.8 V vs. sce for 5 mins.

Figure 4. Linear sweep voltammograms (LSV) of 10mM PtCl42- in 0.5 mol.dm-3 NaCl on

a freshly polished and cleaned glassy carbon (GC) electrode at 10 mV.s-1 and at (313 ± 1) K in the potential range [+0.2 V vs. sce; –0.7 V vs. sce] in the absence of (silent) and presence of ultrasound (20 kHz, 18 and 29 W.cm-2). Figure 4 clearly shows that ultrasound affects the electro-reduction of Pt(II). For example, at the two ultrasonic intensities employed, limiting current densities, ilim, were obtained and a 2-fold increase in ilim was observed compared to silent conditions. This finding is an excellent agreement with previous studies of the electrodeposition of silver and copper on Pt electrodes in the presence of ultrasound (20 kHz) [23,24]. It is also interesting to note that the deposition of Pt(II) on GC electrode starts earlier compared to silent conditions. For example, discharge potentials (Ed) were found to be approximately -0.06 V vs. sce at 18 W.cm-2 and -0.03 V vs. sce at 29 W.cm-2 i.e. an anodic potential shift, ∆Ed, of ca. + 30 mV was observed. Again, this observation is in very good agreement with previous findings [23,24]. It was shown that this finding could be due to possible contributions [23] such as: (a) an increase in mass-transport caused by high stirring induced by sonication, b) an increase in local temperature, (c) a decrease in overpotential, (d) a decrease in concentration overpotential, (e) the cleaning effect of ultrasound or (f) hydroxyl radicals produced by sonication [23].

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MEA Performance for PEMFC electrodes prepared in the absence and presence of ultrasound Many workers have reported that the Pt catalyst prepared by electrodeposition is greatly influenced by the experimental conditions such as: the chloroplatinic precursor concentration, background electrolyte composition and concentration, carbonaceous substrate composition, total charge density, galvanostatic current density, duty cycle and temperature [1-13]. In our conditions, the electrodeposition from 10 mM PtCl4

2- in 0.5 M NaCl on Nafion-bonded carbon substrate in the absence and presence of ultrasound was performed at a fixed duty cycle of 10 ms (ton) and 100 ms (toff) and at various galvanostatic current densities ranging from 100 to 600 mA.cm−2. However, in views of reducing the electrodeposition time and controlling the amount of loaded Pt on the Nafion-bonded carbon substrate, a fixed total charge density of 2 C.cm-2 was employed. Silent Conditions. Figure 5 shows the polarization curves for MEAs prepared by using various galvanostatic current densities at a fixed total charge density of 2 C.cm-2 and a duty cycle of 10 ms (ton)/100 ms (toff) in the absence of ultrasound. The figure shows that electrodes prepared at a current density of 300 mA.cm-2 exhibit better performances than those deposited at lower current densities e.g. 100 and 200 mA.cm-2. However, above 300 mA.cm-2, the figure also shows that electrodes prepared using current densities up to 600 mA.cm-2 exhibit the lowest performance. These findings are in excellent agreement with those found by Kim et al. [13]. They showed that the optimum galvanostatic current density for the electrodeposition of 10 mM PtCl4

2- on Nafion-bonded carbon substrate using a duty cycle of 10/100ms and a total charge density of 2 C.cm-2 was in the region of ca. 300 mA.cm-2. Furthermore, they concluded that the increase in performance is contributed to both the increase in Pt active surface area and Pt loading in the catalyst layer [13]. They also showed that: (i) Pt loading increases with the total charge and concentration of PtCl4

2- as well as, the platinum loading on the substrate was directly proportional to the concentration of PtCl4

2-, (ii) by careful TEM and XRD analyses, as the concentration of PtCl4

2- increased (e.g. up to 100 mM), the specific surface area decreased due to Pt particles overlapping and a slight increase in particle size, and (iii) the process was limited by mass transport [13].

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Figure 5. Polarization curves of the MEA anodes prepared at various galvanostatic current densities. The electrodeposited anodes were prepared using 10 mM PtCl4

2- in 0.5 mol dm-3 NaCl with a fixed total charge density at 2 C.cm-2 and various current densities, 100 [ ], 200 [ ], 300 [ ], 400 [], 500 [∆] and [◊] 600 mA.cm-2 and at (313 ± 1) K in the absence of ultrasound. The current duty cycle was 10/100ms. The fuel cell testing parameters were H2/O2 (1.5/2 stoics), 70oC and 1 atm. Ultrasonic Conditions. Figure 6 shows MEA performance for anodes prepared by the galvanostatic pulse method in the absence of ultrasound, the sono-galvanostatic pulse method (20 kHz, 18 W.cm-2) and conventional method (0.30 mg Pt cm-2 electrodes). All fabricated anodes were prepared galvanostatically at a current density of 300 mA.cm-2, at a fixed total charge density of 2 C.cm-2 with a duty cycle of 10 ms (ton)/100 ms (toff) in the absence and presence of ultrasound. The figure shows that electrodes prepared sonoelectrochemically give better performance compared to those prepared by: (i) galvanostatic pulse method only (i.e. in the absence of ultrasound) and (ii) conventional method (electrodes prepared with 20 wt.% Pt/C from E-TEK; 0.3 mg Pt cm−2 on both anode and cathode). For example, a power density value of 98.5 mW.cm-2 was found for anodes prepared sonoelectrochemically compared with 91.5 mW.cm-2 (by galvanostatic pulse method alone) and 86 mW.cm-2 (by conventional method), in other words, an increase of ca. 12 mW.cm-2 was found using the sono-galvanostatic pulse method.

Figure 6. Comparison of MEA performance between anodes prepared by: a) the galvanostatic pulse method in the absence of ultrasound [ ], b) the sono-galvanostatic pulse method (20 kHz, 18 W.cm-2) [ ] and c) conventional method (0.30 mg Pt cm-2 electrodes) [ ]. The fuel cell testing parameters were H2/O2 (1.5/2 stoics), 70oC and 1 atm.

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Conclusions

This paper reports for the first time, the use of ultrasound for the fabrication of PEMFC electrodes. Potentiostatic investigations showed that the electro-reduction of Pt(II) as K2PtCl4 in aqueous sodium chloride on GC electrode is greatly affected by sonication (20kHz). Mass-transport limited currents were obtained and a positive anodic shift of potentials, in the order of +30mV, was observed under ultrasound. PEMFC electrodes prepared (i) at various galvanostatic currents showed optimum performance at 300 mA.cm-2 and (ii) sonoelectrochemically showed better performance compared to those prepared by (a) the galvanostatic pulse method only (i.e. in the absence of ultrasound) and (b) conventional method. Further investigations are currently being carried out to optimise the ultrasonic effect on the performance of MEAs.

Acknowledgments

The author would like to thank Advantage West Midlands (AWM) and EPSRC for

their kind financial support.

References

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