lable at ScienceDirect

Journal of Power Sources 258 (2014) 238e245

Contents lists avai

Journal of Power Sources

journal homepage: www.elsevier .com/locate/ jpowsour

Fabrication of gas diffusion electrodes via electrophoretic depositionfor high temperature polymer electrolyte membrane fuel cells

Cecil Felix*, Ting-Chu Jao, Sivakumar Pasupathi, Vladimir M. Linkov, Bruno G. PolletHySA Systems Competence Centre, South African Institute for Advanced Materials Chemistry (SAIAMC), University of the Western Cape,Robert Sobukwe Road, Bellville 7535, Cape Town, South Africa

h i g h l i g h t s

� EPD was adapted for the fabrication of GDEs for high temperature PEMFCs operating at 160 �C.� EPD GDEs showed higher porosity and more uniform particle size distribution compared to ultrasonically sprayed GDEs.� Heat treatment of GDEs negatively affected MEA performance.� EPD MEA showed w12% peak power increase over ultrasonically sprayed MEA at w4% lower Pt loading.� PTFE was shown to be more suitable than Nafion� ionomer for MEAs in HT-PEMFCs.

a r t i c l e i n f o

Article history:Received 2 October 2013Received in revised form6 January 2014Accepted 8 February 2014Available online 20 February 2014

Keywords:Electrophoretic depositionHigh temperature polymer electrolytemembrane fuel cellGas diffusion electrodeMembrane electrode assemblyZeta potential

* Corresponding author.E-mail address: cecilfelix09@gmail.com (C. Felix).

http://dx.doi.org/10.1016/j.jpowsour.2014.02.0500378-7753/� 2014 Elsevier B.V. All rights reserved.

a b s t r a c t

The Electrophoretic Deposition (EPD) method was adapted to fabricate Gas Diffusion Electrodes (GDEs)for Membrane Electrode Assemblies (MEA) for High Temperature Polymer Electrolyte Membrane FuelCells (HT-PEMFC) operating at 160 �C. Suspensions containing the Pt/C catalyst, polytetrafluoroethylene(PTFE) and NaCl were studied. Stable catalyst suspensions were observed when the NaCl concentrationswere �0.1 mM. Mercury intrusion porosity analysis showed that the GDEs obtained via the EPD methodhad higher porosity (30.5 m2 g�1) than the GDEs fabricated by the ultrasonic spray method (25.2 m2 g�1).Compared to the ultrasonically sprayed MEA, the EPD MEA showed w12% increase in peak power at aslightly lower (w4 wt %) Pt loading. Electrochemical Impedance Spectroscopy (EIS) analysis showed alower charge transfer resistance for the EPD MEA compared to the ultrasonically sprayed MEA whileCyclic Voltammetry (CV) analysis showed w16% higher Electrochemical Surface Area (ECSA) for the EPDMEA compared to the ultrasonically sprayed MEA. These observations were attributed to the higherporosity and better catalyst particle size distribution of the EPD GDEs. A comparison between PTFE andNafion� ionomer in the Catalyst Layers (CL) of two EPD MEAs revealed that PTFE yielded MEAs withbetter performance and is therefore more suitable in HT-PEMFCs.

� 2014 Elsevier B.V. All rights reserved.

1. Introduction

Electrophoretic Deposition (EPD) is an electrochemical methodattracting increasing interest as a material processing technique.The EPD method is highly efficient and is most commonlyemployed in the processing of ceramics, coatings and compositematerials from colloidal suspensions [1]. During EPD, chargedparticles dispersed in a suitable liquid are deposited onto a targetsubstrate under the force of an externally applied electric field.EPD has already been demonstrated for the deposition of Catalyst

Layers (CL) in Membrane Electrode Assemblies (MEA) wheredeposition was achieved on electron-conducting substrates (likeGas Diffusion Layers, GDLs) which simultaneously served as one ofthe electrodes as well as ion-conducting substrates (like protonexchange membranes) where the membrane was placed betweentwo external electrodes [2e6]. Munakata et al. [4] used the EPDmethod to deposit the catalyst particles directly onto a Nafion�

membrane to form a catalyst coated membrane. They observedthat the CLs attached well onto the membrane and their EPD MEAsshowed better performance than the hot-pressed MEAs which wasmainly attributed to improved Pt utilisation. Morikawa et al. [6]showed that the EPD process has selectivity for particle sizesince they only observed fine carbon particles in the deposited CL.Selectivity for particle size would produce deposits of high

C. Felix et al. / Journal of Power Sources 258 (2014) 238e245 239

uniformity and increase Pt utilisation. The same researchersapplied the EPD method to fabricate MEAs for Low TemperaturePolymer Electrolyte Membrane Fuel Cell (LT-PEMFC) applications.Due to the limitations associated with LT-PEMFCs, such as a needfor high purity hydrogen and a sophisticated water managementsystem, many researchers have given attention to HT-PEMFCs thatoperate at temperatures between 100 and 200 �C. At these tem-peratures important advantages over low temperature operationexist i.e. faster electrode kinetics, higher CO tolerance of the Ptcatalyst, use of lower purity hydrogen, no requirements for hu-midification, and a simpler cooling system [7,8]. In LT-PEMFCs,Nafion� ionomer is incorporated into both anode and cathodeCLs to increase the formation of the triple phase boundaries and toimprove proton conduction. The addition of the Nafion� ionomerto the catalyst inks also produces stable suspensions due to thesteric repulsive forces generated by the negatively charged sul-fonic acid end groups. In our previous work [9], we demonstratedthat the Pt/C particles coated with Nafion� ionomer, readilydeposited when subjected to an externally applied electric field.The fabricated EPD MEAs showed a significant improvement(w73%) in peak power performance in comparison to the con-ventional hand-sprayed MEAs and were stable at 160 �C HT-PEMFC operation conditions. However the Nafion� ionomer isnot commonly incorporated into the CLs for HT-PEMFC because itoperates at temperatures above the glass transition temperature(Tg) of the Nafion� ionomer. Above the glass transition tempera-ture of the Nafion� ionomer, decomposition of Nafion� occurswhere sulphur oxide from the sulfonic acid side chains is lost [10].The decomposition of the Nafion� ionomer could have a negativeimpact on the overall MEA performance. Polytetrafluoroethylene(PTFE) is a polymer that is hydrophobic and is commonly incor-porated in the micro porous layer of the Gas Diffusion Electrode(GDE) to maintain the integrity of carbon particles and to improvewater management [11]. PTFE forms porous catalyst structures[12] and allow both reactant gases and liquid acid to access theactive catalyst sites [12,13]. PTFE also facilitates the formation oftriple phase boundaries [13]. For these reasons, PTFE arecommonly incorporated into the CL structure of HT-PEMFC elec-trodes [8,14]. The main drawback of HT-PEMFCs is that it requiressignificantly higher Pt loadings (0.6e1.2 mg cm�2 Pt on each side;total w2.4 mg cm�2 Pt) than LT-PEMFCs [13,15]. Therefore, alter-native MEA fabrication methods are needed to improve thePolymer Electrolyte Membrane Fuel Cell (PEMFC) efficiency andreduce the Pt loading, hence the investigation into the EPDmethod for the fabrication of GDEs in HT-PEMFCs. An ultrasonicspray method was previously demonstrated [16,17] and provedvery useful for the fabrication of ultra-low Pt loadings in MEAs forLT-PEMFCs. Thus MEAs were fabricated via the ultrasonic spraymethod and used as a comparison in this study. A comparisonbetween EPD MEAs containing PTFE and Nafion� ionomer in theCL structure is also presented.

2. Experimental section

2.1. Materials

HiSpec 4000, 40 wt% Pt/C (Johnson Matthey, United Kingdom)was used as received as catalyst material for all experiments. PTFEsolution 60 wt% (Electrochem, USA) was diluted with ultra purewater (R ¼ 18.3 MU) to obtain a 20 wt% solution. Nafion� solution,5 wt% (Johnson Matthey, United Kingdom) was used as received.Isopropyl alcohol (Kimix, South Africa) was used as the dispersingmedium. NaCl (Kimix, South Africa) was added to adjust the con-ductivity of the catalyst suspension. A commercially available GDL(Freudenberg H2315 CX196, Germany) was used as received. A

commercially available poly-(2,5-benzimidazole) also known asABPBI (Fumatech, Germany) was doped in phosphoric acid (PA)(Kimix, South Africa) for 24 h at 130 �C prior to use.

2.2. Zeta potential and particle size

The zeta potential and size of the Pt/C particles in isopropylalcohol were obtained using the Zetasizer Nano ZS (Malvern In-strument Ltd., United Kingdom). The catalyst inks were obtainedvia ultrasonic mixing (20 kHz) of the Pt/C, PTFE solution and iso-propyl alcohol for 5 min using a Biologics 3000 ultrasonic ho-mogenizer (Biologics, Inc., USA). Catalyst suspensions with NaClconcentrations between 0 and 0.5 mM were studied. A syringewas used to fill a semi-disposable capillary cell with the samplewhich was then immersed into a temperature controlled blockholder to avoid thermal gradients in the absence of the appliedelectric field [18]. Electrophoretic mobility was measured byapplying a fixed voltage of 100 V. The instrument calculated thezeta potential from electrophoretic mobility using the Smo-luchowski equation (Eq. (1)) [9].

u ¼ 330zEh

(1)

where z is the zeta potential, E is the electric field strength and330 and h are the dielectric constant and the viscosity of dispersionmedium, respectively. All measurements were performed at 25 �C.

2.3. Fabrication of MEAs by EPD

Catalyst inks were obtained via ultrasonic mixing of the Pt/C,PTFE or Nafion� ionomer solution and isopropyl alcohol for30 min. The ionic strength of the suspension was adjusted byadding NaCl. An in-house EPD cell [9] was used to fabricate theGDEs. A microelectrophoresis power supply (Consort, Belgium)was used to deposit the catalyst particles onto the GDLs. The ob-tained GDEs were placed in a vacuum oven (Binder GmbH, Ger-many) at room temperature and heated to 50 �C (w1.5 �C min�1)to dry the CL. Heat treatment of GDEs were carried out at 340 �Cunder Ar atmosphere to remove the surfactant from the PTFEbinder and to increase the porosity of the GDE. GDEs and MEAsfabricated via the EPD method are termed EPD GDEs and EPDsMEA respectively. Heat treatment of the GDEs was performed byheating the furnace to 340 �C within w1 h followed by main-taining 340 �C for 30 min and then cooling the furnace to roomtemperature without artificial cooling. GDE/MEAs that receivedheat treatment are termed “340 �C HT” while GDE/MEAs thatreceived no heat treatment is termed “no HT”. The MEAs wereobtained by sandwiching the anode and cathode GDEs and theacid doped membrane together inside a high temperature cell. Noprior hot pressing step was performed on the MEAs.

2.4. Fabrication of MEAs by Sonotek

Catalyst inks were obtained via ultrasonic mixing of Pt/C, PTFEsolution, ultra pure water and ethanol for 30 min. The well-mixedcatalyst ink was loaded into the Sonotek ultrasonic sprayer(Sono-tek Corporation, USA) then sprayed onto the GDLs. The ul-trasonic nozzle operated at 120 kHz and an ultrasonic power of 3W.The heat treatment procedures of the GDEs were the same as forthe EPD GDEs. MEAs were obtained by the same procedures as forthe EPD MEAs. GDEs and MEAs fabricated via the ultrasonic spraymethod are termed Sonotek GDEs and Sonotek MEAs respectively.

Fig. 1. (a) Zeta potential and (b) average particle size of Pt/C in isopropyl alcohol withvarious PTFE compositions and NaCl concentrations.

C. Felix et al. / Journal of Power Sources 258 (2014) 238e245240

2.5. Electrochemical characterisation of MEAs

An in-house HT-PEMFC test bench was used to study the elec-trochemical properties of the MEAs. The in-house HT-PEMFC testbench consisted of a PC loaded with Labview software to controlthe electronic load (Höcherl&Hackl GmbH, Germany) and massflow controllers (Bronkhorst, The Netherlands). A cell compressionunit (Pragma Industries, France) controlled the cell pressure andtemperature. All measurements were carried out at 160 �C and acell compression pressure of 20 bar while supplying dry air (1 slpm)at the cathode and dry hydrogen (0.5 slpm) at the anode. MEAswere activated for 3 h at þ0.55 V followed by measuring thepolarisation curve between Open Circuit Voltage (OCV) and þ0.2 V.The Autolab PGSTAT302N (The Netherlands) was used for EISanalysis at a cell voltage of 0.6 V in a frequency range of 0.1 Hze50,000 Hz and an amplitude of 5mV. The Autolab PGSTAT302Nwasalso used for CV analysis by cycling the potential between 20 mVand 800 mV at a scan rate of 50 mV s�1.

2.6. Physical characterization of GDEs

Morphology of the GDEs were characterised by High ResolutionScanning ElectronMicroscopy (HR-SEM) using the Carl Zeiss AurigaHRFEG SEMworking at 5 kV. Porosity of the GDEswere obtained viamercury intrusion porosimetry analysis using the Autopore IV 9510(Micromeritics, USA) mercury porosimeter applying pressures be-tween 0.0145 and 4136.85 bar.

3. Results and discussion

3.1. Catalyst ink characterisation

The first step in EPD is to prepare a stable suspension of inde-pendent particles [19]. Suspension stability is characterised by theparticle settling rate and the tendency for particles to undergo oravoid flocculation [20,21]. To prepare stable suspensions suitablefor EPD the following four conditions should be met: (i) the parti-cles in solution need to be charged otherwise the particles will notmigrate with the externally applied electric field, (ii) the particlesshould be well dispersed and stable over the EPD duration to formhomogeneous deposits, (iii) ions other than the charged particlesshould be kept to a minimum as these lower the transport numberof the particles. Compression of the electrical double layer occurs athigh ionic concentration which reduces the stability of the sus-pension, and (iv) the particles should strongly adhere to the targetsubstrate which may be improved by the addition of polymerbinders. Measuring the particle zeta potential is useful to under-stand the stability of the suspension. For stable suspensions, zetapotential values more positive than þ30 mV or more negativethan �30 mV, depending on the sign is recommended. For somesystems, stable suspensions with particles having high zeta po-tentials are obtained by simply dispersing the particles in the so-lution. However in general, particles can be positively or negativelycharged by adding the appropriate amount of acid, base or poly-electrolytes having functional groups such as carboxylic acidgroups or amino groups on the side chains, to the suspension.Particle surfaces modified by polyelectrolytes have charges corre-sponding to the functional groups. The addition of excess acid orbase is disadvantageous as the un-adsorbed ions compress theelectrical double layer and affects the suspension stability.Furthermore, free ions become the majority of charge carriersduring EPD and the transport number of the charged particlesdrops which affects the deposition rate of the particles resulting inthin deposited layers [22]. In this study, we added a charger salt(NaCl) to adjust the ionic concentration and conductivity of the

catalyst suspension. For the case of Carbon Nanotubes (CNT), it wasobserved that the addition of charger salts can increase the sus-pension stability by associating a charge with the CNT surface in thesolvent and improve the adhesion and deposition of the CNTs to thetarget substrate [1]. Fig. 1a and b shows the zeta potential andaverage sizes of the Pt/C particles mixed with various PTFE contentsin solutions with various NaCl concentrations, respectively. Fig. 1ashows that the catalyst particles were negatively charged in allsuspensions and would therefore deposit onto the positivelycharged electrode. When no NaCl is added to the solution, thecatalyst particles show the highest zeta potentials rangingbetween �44 mV and �52 mV with average particle sizes rangingbetween w248 nm and w263 nm. These suspensions were stablesince no significant change in particle size was observed which isindicative of very slow coagulation. These types of suspensions arecommonly prepared when using other coating techniques such asspraying where the conductivity of the suspension is not crucial.For these suspensions, the highest zeta potential (�52 mV) andsmallest particle size (w248 nm) were observed when 20 wt% PTFEwas added while the lowest zeta potential (�44 mV) and largestparticle size (w263 nm) were observed when no PTFE was added.These suspensions however are not suitable for EPD as there isinsufficient conductivity due to the low dielectric constant

Fig. 2. HR-SEM images of (a) EPD GDE (no HT) at 1,000� magnification, (b) Sonotek GDE (no HT) at 1,000� magnification, (c) EPD GDE (no HT) at 50,000� magnification, (d)Sonotek GDE (no HT) at 50,000� magnification, (e) EPD GDE (340 �C HT) at 50,000� magnification and (f) Sonotek GDE (340 �C HT) at 50,000� magnification.

C. Felix et al. / Journal of Power Sources 258 (2014) 238e245 241

( 3r z 18.23) of isopropyl alcohol and the particles would not de-posit under the force of the externally applied electric field. For thesuspensions containing 0.1 mM NaCl concentration, the zeta po-tentials ranged between �33 mV and �35 mV. The zeta potentialswere noticeably smaller in magnitude however the particle sizeswere not affected. The particle sizes ranged betweenw246 nm andw255 nm. For suspensions containing 0.2 mM NaCl concentration,the zeta potentials ranged between �30 mV and �34 mV. Forsuspensions containing 0.5 mM NaCl concentration, the particlezeta potentials became less negative than �30 mV and the particlesizes was significantly affected. The zeta potentials decreasedfrom �25 mV for 0 wt% PTFE tow �17 mV for 40 and 50 wt% PTFE.

The average particle sizes ranged fromw472 nm for 0 wt% PTFE, tow1600 nm for 40 and 50 wt% PTFE. This signified significantcoagulation and unstable suspensions. At high ionic concentrationswhere electrical double layer compression occurs, PTFE had a sig-nificant influence on the stability of the suspension. From the Fig. 1it was clear that to obtain stable catalyst suspensions for EPD, theNaCl concentration should be �0.1 mM.

3.2. GDE fabrication and physical characterisation

A NaCl concentration of 0.01 mM was selected for all catalystsuspensions to minimise the NaCl concentration deposited in the

Fig. 4. Mercury intrusion porosity analysis of (a) EPD and Sonotek GDEs with PTFE inCLs and (b) EPD GDEs with PTFE and Nafion� ionomer in CLs.

C. Felix et al. / Journal of Power Sources 258 (2014) 238e245242

CLs and to avoid the formation of unstable suspensions. Ali et al.[23] observed that the chloride impurity can drastically decreasethe performance and durability of the PBI based MEAs. It wasproposed that Pt chloro-complexes such as hexachloroplatinateðPtCl2�6 Þ or tetrachloroplatinate ðPtCl2�6 Þ were formed in the pres-ence of chloride impurities. A NaCl concentration of 0.01 mMyielded a stable electric field during EPD whereas at lower NaClconcentrations the electric field was notmaintained and depositionceased. At higher NaCl concentrations, the excess free ions becamethe major charge carriers resulting in high currents and jouleheating significantly affecting the morphology of the deposited CL.The EPD GDEs had a 0.48 mg cm�2 Pt loading while the SonotekGDEs had a 0.5 mg cm�2 Pt loading. The PTFE content was 40 wt%for all GDEs. Fig. 2a and b shows the HR-SEM images of the EPD GDE(no HT) and the Sonotek GDE (no HT) at 1,000� magnification,respectively. The EPD GDEwas covered in cracks which is the resultof drying of a relatively thick (w25 mm) CL in a single step. TheSonotek GDE has relatively smooth and uniform morphology. TheSonotek ultrasonic sprayer is capable of spraying a very fine mist ofthe catalyst particles which is continuously dried via a hot plateoperating at 80 �C during the spraying process. Fig. 2c and d showsthe HR-SEM images of the EPD GDE (no HT) and the Sonotek GDE(no HT) at 50,000�magnification, respectively. The EPD GDE formsa uniform network of porous structures while the Sonotek GDEmorphology is notably different. The Sonotek GDE morphologyexhibits catalyst particles that are smaller than that of the EPD GDEbut also contains larger particles (>300 nm). Energy Dispersive X-Ray (EDX) analysis showed that these larger particles had higher(w60%) PTFE contents than the smaller catalyst particles. The PTFEresulted in the agglomeration of the catalyst particles in the CL ofthe Sonotek GDE. Fig. 2e and f shows the HR-SEM images of the EPDGDE (340 �C HT) and the Sonotek GDE (340 �C HT) at 50,000�magnification, respectively. The EPD GDE showed no notablechange in morphology after the heat treatment process. TheSonotek GDE, however, showed significant agglomeration of thecatalyst particles due to the melting of PTFE whereas no agglom-eration of the catalyst particles was noted for the EPD GDE. Thissuggested that for EPD, the Pt/C particles are uniformly coated withPTFE probably due to the formation of stable suspensions.Furthermore it suggested that only fine catalyst particles depositduring EPD which is consistent to that reported in the literature[4,6]. Fig. 3a and b shows the HR-SEM images of the EPD GDE (noHT) with PTFE in the CL and the EPD GDE (no HT) with Nafion�

ionomer in the CL at 50,000� magnification, respectively. The EPDGDE containing the Nafion� ionomer in the CL shows slightly largercatalyst particle structures and larger pore sizes. Fig. 4a shows the

Fig. 3. HR-SEM images of (a) EPD GDE (no HT) with PTFE in CL at 50,000� magnification and (b) EPD GDE (no HT) with Nafion� ionomer at 50,000� magnification.

C. Felix et al. / Journal of Power Sources 258 (2014) 238e245 243

mercury intrusion porosity analysis of the EPD and Sonotek GDEswith PTFE in the CLs, heat treated and non heat treated. The EPDGDE (no HT) shows a higher porosity (30.5 m2 g�1) than theSonotek GDE (no HT) which had a total pore area of 25.2 m2 g�1.Heat treatment of the GDEs yielded an increase in porosity for bothGDE types which were expected due to the presence of the PTFEbinder [24]. However the increase in porosity was mainly a result ofa significant increase in macro pores. Fig. 4b shows the mercuryintrusion analysis of the EPD GDEs containing PTFE and Nafion�

ionomer in the CLs. The EPD GDE with Nafion� ionomer in the CLhad a slightly lower total pore area of 27.4 m2 g�1. The figure showssignificantly more macro pores for the EPD GDE with Nafion�

ionomer in the CL which is consistent to that observed from HR-SEM (Fig. 3b).

3.3. Single cell test

Fig. 5 shows the polarisation and power density curves of theEPD and Sonotek MEAs. The EPD MEA (no HT) shows the highestMEA performance with w12% increase in peak power compared tothe Sonotek MEA (no HT). Heat treatment of the GDEs resulted in aslight lowering in performance of the EPD MEA while significantlylowering the performance of the Sonotek MEA. CV analysis of theEPD and Sonotek MEAs are shown in Fig. 6. The ECSA of each MEAwas calculated using Eq. (2) [25].

ECSA ¼ 105 � Ad

C �m� v(2)

where Ad is the integral area of the hydrogen adsorption peak (AV),C is the coefficient of hydrogen adsorbed by Pt (0.21 mC cm�2),m isthe mass of Pt at the cathode (mg cm�2) and v is the potential scanrate (mV s�1). Fig. 6a shows that the ECSAs decreased by w3% andw51% after heat treatment for EPD and Sonotek MEAs, respectively.The reduction in ECSA implies that the available surface Pt sites arereduced which affects MEA performance. The increase in porosityof the GDEs usually results in improved MEA performance due toimproved access of both the reactant gases and the liquid acid tothe active catalyst sites [12,13]. On the contrary however, the in-crease in porosity (due tomacro pores)may allow easier covering ofthe active Pt sites by PTFE and PA. The PO4�-ions in PA strongly

Fig. 5. Polarisation and power density curves of EPD and Sonotek MEAs with PTFE inCLs.

adsorbs onto the active Pt sites causing it to be inaccessible forelectrochemical reactions. The significant lowering in the perfor-mance of the Sonotek MEA (340 �C HT) can be explained using theHR-SEM image (Fig. 2f) where the melting of PTFE clearly resultedin the agglomeration of the catalyst particles and covering of theactive Pt sites. Fig. 7 shows the EIS analysis of the EPD and SonotekMEAs and revealed highest charge transfer resistance (observedfrom the larger diameter of the arc) for the Sonotek MEA (340 �CHT). This is an indication that the Sonotek GDE (340 �C HT) had lessactive Pt sites which was the result of being covered by PTFE andbecoming inaccessible to the reactant gases for electrochemicalreactions. The EPD MEA (no HT) showed the lowest charge transferresistance indicative of better electrode kinetics which resulted inbetter MEA performance. Fig. 7 shows differences in the membraneresistances of the MEAs therefore IR free polarisation curves, whereinternal cell resistances were corrected based on the high fre-quency resistances of the EIS analysis, are provided in Fig. 8. TheEPD MEA (no HT) clearly exhibits better electrode kinetics whichresulted in the better MEA performance. The IR free curves showsthat the Sonotek (no HT) and EPD (340 �C) had comparable

Fig. 6. Cyclic voltammetry analysis of (a) EPD and Sonotek MEAs with PTFE in CLs and(b) EPD MEAs with PTFE and Nafion� ionomer in CLs.

Fig. 7. EIS analysis of EPD and Sonotek MEAs with PTFE in CLs at 0.6 V.Fig. 9. Polarisation and power density curves of EPD MEAs with PTFE and Nafion�

ionomer in CLs.

Fig. 10. EIS analysis of EPD MEAs with PTFE and Nafion� ionomer in CLs at 0.6 V.

C. Felix et al. / Journal of Power Sources 258 (2014) 238e245244

performances however the Sonotek (no HT) showed slightly betterperformance under high current conditions.

Fig. 9 shows the polarisation and power density curves of theEPDMEAs (no HT) containing PTFE and Nafion� ionomer in the CLs.The EPD MEA containing PTFE in the CLs exhibited better MEAperformance than the EPD MEA containing the Nafion� ionomer inthe CLs. The tests were performed at 160 �Cwhich is above the glasstransition temperature of the Nafion� ionomer which may nega-tively affect the CL structure. Above the glass transition tempera-ture, the polymer chain may rearrange which can lead to structuralchanges. The structural changes may have a negative effect on thestability and performance of the Nafion� ionomer [26]. Fig. 6bshows the CV analysis of the EPD MEAs (no HT) with PTFE and theNafion� ionomer in the CLs. The EPD MEA (no HT) with PTFE in theCL shows w53% higher ECSA than the EPD MEA (no HT) with theNafion� ionomer in the CL. The HR-SEM image (Fig. 3b) revealedlarger catalyst structures when the Nafion� ionomer was incorpo-rated into the CLs which are associated with a reduction in theECSA. Both GDEs showed porous morphologies however the CLscontaining the Nafion� ionomer consisted of significantly moremacro pores which may lead to easier poisoning of the Pt sites by

Fig. 8. IR free polarisation curves of EPD and Sonotek MEAs with PTFE in CLs. Fig. 11. IR free polarisation curves of EPD MEAs with PTFE and Nafion� ionomer in CLs.

Fig. 12. Stability of EPD MEAs with PTFE and Nafion� ionomer in the CLs at 0.55 V.

C. Felix et al. / Journal of Power Sources 258 (2014) 238e245 245

PA. Fig. 10 shows the EIS analysis of the EPD MEAs (no HT)containing PTFE and Nafion� ionomer in the CLs. Significantlyhigher charge transfer resistance is noted when the Nafion� ion-omer was present in the CLs. Fig. 11 shows the IR free polarisationcurves of the EPD MEAs containing PTFE and Nafion� ionomer inthe CLs and also revealed that PTFE resulted in higher MEA per-formance under high temperature (160 �C) operation. Fig. 12 showsthe stability behaviour of the EPD MEAs with PTFE and the Nafion�

ionomer in the CLs. The tests were performed at 160 �C at aconstant voltage of þ0.55 V. Both MEAs showed stable behaviourover the duration of the test (i.e. 180 h). The EPD MEAwith PTFE inthe CL showed double the current density (i.e. w340 mA cm�2)compared to the EPD MEA with the Nafion� ionomer in the CL(i.e. w170 mA cm�2).

4. Conclusions

MEAs for HT-PEMFCs were fabricated via the EPD method.Suspension conditions were studied and suspensions having lowNaCl concentrations (<0.1 mM) should be considered for thefabrication of the GDEs. For comparison purposes, MEAs fabricatedusing the Sonotek ultrasonic sprayer were tested and characterisedunder similar conditions. HR-SEM images revealed cracked butuniform morphologies for the EPD GDEs while the Sonotek GDEsexhibited morphology consisting of small, fine particle structuresand larger particle structures that contained higher PTFE contents.Heat treatment of both GDE types resulted in higher porosity but alowering in MEA performance. For the Sonotek GDE, the melting ofthe PTFE resulted in the covering of the active Pt sites whichsignificantly affected the electrochemical performance of the MEA.Under the test conditions the EPD MEA (no HT) performed betterthan the EPDMEA (340 �C HT) and SonotekMEAs as revealed by thepolarisation measurements. EPD MEAs containing PTFE in the CLsyielded better MEA performance compared to the EPD MEA con-taining the Nafion� ionomer in the CLs under high temperature(160 �C) operation. Future work will focus on the long term dura-bility of the fabricated EPD MEAs.

Acknowledgements

This work was supported by the Hydrogen and Fuel Cell Tech-nologies RDI Programme (HySA) funded by the Department ofScience and Technology (DST) in South Africa (Project KP1-S01).

References

[1] A.R. Boccaccini, J. Cho, J.A. Roether, B.J.C. Thomas, E. Jane Minay, M.S.P. Shaffer,Carbon 44 (2006) 3149e3160.

[2] K.-T. Jeng, W.-M. Huang, N.-Y. Hsu, Mater. Chem. Phys. 113 (2009) 574e578.[3] R.-F. Louh, A.C.C. Chang, V. Chen, D. Wong, Int. J. Hydrogen Energy 33 (2008)

5199e5204.[4] H. Munakata, T. Ishida, K. Kanamura, J. Electrochem. Soc. 154 (2007) B1368e

B1372.[5] R.-F. Louh, H. Huang, F. Tsai, J. Fuel Cell Sci. Technol. 4 (2007) 72e78.[6] H. Morikawa, N. Tsuihiji, T. Mitsui, K. Kanamura, J. Electrochem. Soc. 151

(2004) A1733eA1737.[7] J.J. Linares, C. Sanches, V.A. Paganin, E.R. Gonzalez, Int. J. Hydrogen Energy 37

(2012) 7212e7220.[8] M.S. Kondratenko, M.O. Gallyamov, A.R. Khokhlov, Int. J. Hydrogen Energy 37

(2012) 2596e2602.[9] C. Felix, T.-C. Jao, S. Pasupathi, B.G. Pollet, J. Power Sources 243 (2013) 40e47.

[10] D. Chu, D. Gervasio, M. Razaq, E. Yeager, J. Appl. Electrochem. 20 (1990) 157e162.

[11] S. Park, J.-W. Lee, B.N. Popov, J. Power Sources 177 (2008) 457e463.[12] A. Li, M. Han, S.H. Chan, N.-T. Nguyen, Electrochim. Acta 55 (2010) 2706e

2711.[13] Q. Li, J.O. Jensen, R.F. Savinell, N.J. Bjerrum, Prog. Polym. Sci. 34 (2009) 449e

477.[14] G.-B. Jung, C.-C. Tseng, C.-C. Yeh, C.-Y. Lin, Int. J. Hydrogen Energy 37 (2012)

13645e13651.[15] C. Wannek, W. Lehnert, J. Mergel, J. Power Sources 192 (2009) 258e266.[16] B. Millington, V. Whipple, B.G. Pollet, J. Power Sources 196 (2011) 8500e8508.[17] T.-H. Huang, H.-L. Shen, T.-C. Jao, F.-B. Weng, A. Su, Int. J. Hydrogen Energy 37

(2012) 13872e13879.[18] J.C.W. Corbett, R.O. Jack, Colloids Surf. A 376 (2011) 31e41.[19] J.J. Van Tassel, C.A. Randall, Key Eng. Mater. 314 (2006) 167e174.[20] Y.S. Joung, C.R. Buie, Langmuir 27 (2011) 4156e4163.[21] L. Besra, M. Liu, Prog. Mater. Sci. 52 (2007) 1e61.[22] Y. Sakka, T. Uchikoshi, KONA Powder Part. J. 28 (2010) 74e90.[23] S.T. Ali, Q. Li, C. Pan, J.O. Jensen, L.P. Nielsen, P. Møller, Int. J. Hydrogen Energy

36 (2011) 1628e1636.[24] M. Li, K. Scott, Electrochim. Acta 55 (2010) 2123e2128.[25] T.-. Jao, G.-B. Jung, P.-H. Chi, S.-T. Ke, S.-H. Chan, J. Power Sources 196 (2011)

1818e1825.[26] C. Yang, J. Membr. Sci. 237 (2004) 145e161.

List of abbreviations

EPD: Electrophoretic DepositionGDE: Gas Diffusion ElectrodeGDL: Gas Diffusion LayerPEMFC: Polymer Electrolyte Membrane Fuel CellHT-PEMFC: High Temperature Polymer Electrolyte Membrane Fuel CellLT-PEMFC: Low Temperature Polymer Electrolyte Membrane Fuel CellMEA: Membrane Electrode AssemblyPTFE: PolytetrafluoroethyleneEIS: Electrochemical Impedance SpectroscopyCV: Cyclic VoltammetryECSA: Electrochemical Surface AreaCL: Catalyst Layer

Nomenclature

m: electrophoretic mobility3: permittivity30: permittivity of vacuumz: zeta potentialE: electric field strengthh: viscosityAd: integral area of the hydrogen adsorption peakC: hydrogen adsorption coefficientm: massv: potential scan rate