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Current Applied Physics 18 (2018) 728e736

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Current Applied Physics

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Electrospray-assisted fabrication of porous platinum-carboncomposite thin layers for enhancing the electrochemical performanceof proton-exchange membrane fuel cells

Nian Shan a, Hun Jung a, Ji Young Ahn b, Ji Hoon Kim a, Soo Hyung Kim a, b, c, *

a Department of Nano Fusion Technology, Pusan National University, 30 Jangjeon-dong, Geumjung-gu, 609-735 Busan, Republic of Koreab Research Center for Energy Convergence Technology, Pusan National University, 30 Jangjeon-dong, Geumjung-gu, 609-735 Busan, Republic of Koreac Department of Nano Energy Engineering, Pusan National University, 30 Jangjeon-dong, Geumjung-gu, 609-735 Busan, Republic of Korea

a r t i c l e i n f o

Article history:Received 30 August 2017Received in revised form6 December 2017Accepted 30 January 2018Available online 1 February 2018

Keywords:Platinum-carbon composite thin layerElectrosprayMicro- and nano-porous structuresProton-exchange membrane fuel cellsSurfactant templates

* Corresponding author. Department of Nano FusioUniversity, 30 Jangjeon-dong, Geumjung-gu, 609-735

E-mail address: sookim@pusan.ac.kr (S.H. Kim).

https://doi.org/10.1016/j.cap.2018.01.0201567-1739/© 2018 Korean Physical Society. Published

a b s t r a c t

Membrane electrode assembly (MEA) in proton-exchange membrane fuel cells (PEMFCs) have beenfabricated using electrospray-assisted deposition of platinum-carbon composites on carbon-fiber-basedpaper substrate, because the technique is versatile, operated in atmospheric pressure, and easy to scaleup for commercialization. In this study, we investigate the effects of electrospray-assisted platinumloadings from 0.1 to 0.5mg cm�2 on the electrochemical performance of PEMFCs. The PEMFCs withplatinum loading of 0.3 mg cm�2 generate the highest power density, which is ~35% higher than that ofPEMFCs fabricated by traditional brush-deposited catalyst layers. Relatively high platinum loading(>0.3mg cm�2) enhances the pressure drop in MEA; therefore, the resulting power density is decreaseddue to low-reacting gas permeability. We also examine the effect of porous structures on the electro-chemical performance of PEMFCs. Brij 58-based surfactant templates create micro- and nano-porousstructures in the platinum-carbon composite thin layers via thermal removal. These porous structuresin the platinum-carbon composite thin layers increase the reacting gas permeability and simultaneouslylower the cell resistance, significantly enhancing the electrochemical performance of PEMFCs withporous structures.

© 2018 Korean Physical Society. Published by Elsevier B.V. All rights reserved.

1. Introduction

Hydrogen fuel cells have been extensively developed as highlyefficient, cost-effective, and less environmentally harmful energyconversion devices. Proton-exchange membrane fuel cells(PEMFCs) constitute one of versatile and useful fuel cell typesbecause they are operated at relatively low temperatures and areconstructed with relatively inexpensive materials. Therefore,PEMFCs are considered potentially important parts that can meetthe stringent cost requirements in automobile and stationary po-wer generation markets [1e5].

Generally, catalyst layers in PEMFCs are composed of relativelyexpensive platinum (Pt) or its alloys. Reducing the manufacturingcost and improving the utilization of catalytic materials are major

n Technology, Pusan NationalBusan, Republic of Korea.

by Elsevier B.V. All rights reserve

issues for their applications in the energy industry. Various effec-tive methods to fabricate optimized catalyst layers have been re-ported, which significantly improve the electrochemicalperformance of the PEMFCs [6e10]. To improve the utilization ofcatalyst layers, various catalyst deposition methods have also beendeveloped, including cast [11,12], spray [13,14], and impregnation[15,16], which are known to be effective, but unable to provide ahighly uniform catalyst distribution. Chemical vapor deposition[17,18] and sputter [19,20] are known as uniform catalyst deposi-tion methods; however, they require the precisely controlled vac-uum conditions and pressure controls, which are expensive,complex, and not easily adaptable to commercial scale-up.

On these grounds, an electrospray method can be considered asone of potential candidates for fabricating uniform catalyst layers inPEMFCs because of its various advantages of relatively simpleexperimental set-up, operability under atmospheric conditions,and easy commercialization by scale-up processes [21e23]. Inaddition, the amount of catalyst loading can be controlled moreaccurately by preparing the homogeneously dispersed catalyst

d.

Fig. 1. (a) Photographs of (left) the electrospray device and (right) the Pt-C compositelayer formed on the surface of carbon-fiber-based paper substrate using the electro-spray device operated at 8.5 kV (Here, the distance between syringe nozzle tip andcarbon fiber paper was ~2.5 cm and the temperature of heater was kept at ~80 �C). (b)The schematic and (c) photograph of various components of the membrane electrodeassembly (MEA) used in PEMFCs.

Fig. 2. Photographs of various electrospray modes operated at different applied voltages o12.5 kV (Here, the flow rate of the Pt-C suspension was fixed at ~80 mLmin�1 and the dista

N. Shan et al. / Current Applied Physics 18 (2018) 728e736 729

suspension. Several research groups have attempted to employ theelectrospray method to fabricate PEMFC electrodes, in which ultra-low Pt loadings (0.0125e0.1mg cm�2) were used. A linear rela-tionship was commonly observed between the Pt loading amountand the obtained power density. However, Pt loadings much higherthan 0.1mg cm�2 fabricated by electrospray deposition have notyet been explored to optimize the electrochemical performance ofPEMFCs, which can be strongly affected by the stacking structures(e.g., catalyst layer thickness, catalyst amounts, etc.) of catalystlayers on carbon-fiber-based paper substrate [24e27].

In this study, we systematically investigated the effects ofelectrospray-assisted deposition of catalyst layers on the electro-chemical performance of PEMFCs. Briefly, we fabricate PEMFCswith catalyst layers deposited on carbon-fiber-based paper sub-strates using electrospray method. The effect of various Pt loadings(0.1e0.5mg cm�2) on the electrochemical performance of PEMFCswas examined in terms of cell performance and resistance analyses.A traditional brush method was also used to fabricate catalystlayers for comparison with the electrospray method. We alsoexamined the effects of porous structures in the Pt-carbon (C)composite layers on the electrochemical performance of PEMFCs.Briefly, surfactant templates (e.g., Brij 58) were added to the Pt-Ccomposite thin layers followed by thermal removal, so that mi-cro- and nano-sized pores were successfully created to improve thereacting gas permeability through the Pt-C composite thin layers.

2. Experimental

2.1. Preparation of catalyst suspension

The catalyst suspension for the electrospray process was pre-pared with Pt-C nanopowders (20wt%, E-TEK), Nafion® solution(5wt% in lower aliphatic alcohols) as the ionomer, and isopropylalcohol (IPA) as the solvent. The aforementioned precursor-dispersed solution was sonicated in an ultrasonic bath with200W and 40 kHz for 1 h. For the catalyst suspension with sur-factant templates, ~4.5wt% non-ionic Brij 58 surfactant (poly-ethylene glycol hexadecyl ether, HO(CH2CH2O)20C16H33, Sigma-Aldrich) was added to the total mass of Pt-C and Nafion®.

f (a) 2.5 kV, (b) 4.5 kV, (c) 6.5 kV, (d) 8.5 kV, (e) 9.5 kV, (f) 10.5 kV, (g) 11.5 kV, and (h)nce between the nozzle tip and substrate surface was fixed at ~2.5 cm)

Fig. 3. Top-view and cross-sectional SEM images of brush-deposited Pt-C composite thin layer on carbon-fiber-based paper (a and b, respectively), and of electrospray-deposited Pt-C composite layer formed on carbon-fiber-based paper (c and d, respectively). (Pt loading was fixed at ~0.3mg cm�2).

Fig. 4. (a) Current density-voltage (J-V) and (b) current density-power density (J-P)curves for PEMFCs composed of brush- and electrospray-assisted MEAs. (The Ptloading was fixed at 0.3mg cm�2).

N. Shan et al. / Current Applied Physics 18 (2018) 728e736730

2.2. Electrospray-assisted deposition of Pt-C composite layers

Using both electrospray and brush deposition methods, Pt-Ccomposite layers were coated on the surface of carbon-fiber-based paper substrates (area¼ 2.25 cm2) and were pretreatedwith polytetrafluoroethylene. Fig. 1a shows the electrospray deviceused for depositing Pt-C composite layer on the surface of carbon-fiber-based paper substrate.

The distance between the syringe nozzle and the substratesurfacewas fixed at ~2.5 cm, whichwas experimentally determinedto be appropriate for covering the entire substrate area with Pt-Ccomposite layers. The carbon-fiber-based paper substrate washeated at ~80 �C using a heater (MSH 20-D, Witeg, Germany),which enabled complete evaporation of any residual isopropanol.The injection flow rate of the catalyst suspension was fixed at~80 mLmin�1. The resulting Pt-C composite thin layers with variousPt loadings (0.1e0.5mg cm�2) were obtained by controlling thetotal inject time of the catalyst suspension. In order to fabricateporous structures, Brij 58-added Pt-C layers were additionallyheated at ~350 �C for 30min to thermally remove the Brij 58 sur-factant templates from the Pt-C composite thin layers.

2.3. Fabrication and characterization of MEAs

To fabricate MEAs, firstly, Nafion® 212 membrane was insertedbetween the anode and the cathode, and was then pressed using ahot-press machine operated at 200 bar and 135 �C for 10min. Theschematic and photograph of the components assembled forPEMFCs are presented in Fig. 1b and c. The morphological struc-tures of the Pt-C composite thin layers were examined usingscanning electron microscopy (SEM; S-4700, Hitachi, Tokyo, Japan).The current density-voltage (J-V) curve was obtained using fuel cellmonitor pro 3.0 (FCM; U 103, H-TEC, Germany) feeding dry H2/O2

gases at the electrodes under room temperature and atmosphericpressure conditions. The cyclic voltammetry (CV; Reference 3000,GAMRY Instruments, USA) measurement was performed at50mV s�1 under H2/N2 flow in the electrodes. Electrochemicalimpedance spectroscopy (EIS; Reference 3000, GAMRY

N. Shan et al. / Current Applied Physics 18 (2018) 728e736 731

Instruments, USA) was performed to analyze the internal resistanceof the PEMFCs.

3. Results and discussion

3.1. Fabrication and characterization of Pt-C composite layers

We employed an electrospray process to generate fine Pt-Caerosol particles. The electrospray system consisted of syringe,liquid pump, high-voltage power supply, and electrically groundedcollector. A high voltage was applied to a syringe nozzle tip, andthen a liquid droplet was formedwith a semi-spherical shape at thetip of the syringe. The positive charge carriers were moved from

Fig. 5. SEM images of electrospray-deposited Pt-C composite thin layers formed on carbon-and f) 0.3mg cm�2, (g and h) 0.4mg cm�2, and (i and j) 0.5mg cm�2.

electrode to droplet.When the applied voltagewas greater than thethreshold, the liquid droplet turned into a Taylor cone, whichresulted in forming small Pt-C-contained particles due toCoulombic explosion. Fig. 2 shows the various electrospray modesat different voltages at the syringe nozzle tip (2.5e12.5 kV) with afixed Pt-C solution flow rate of ~80 mLmin�1 and a fixed distance of2.5 cm between the syringe nozzle tip and the substrate surface. Atapplied voltages lower than 4.5 kV, large droplets were drippeddirectly on the substrate, while fine droplets were stably generatedat voltages above 6.5 kV. At ~8.5 kV, an inverse cone shape wasformed and very fine droplets were deposited homogeneously onthe entire substrate surface. Multiple jet mode was obtained atapplied voltages much higher than 9.5 kV, which resulted in non-

fiber-based paper with Pt loadings of (a and b) 0.1mg cm�2, (c and d) 0.2mg cm�2, (e

Fig. 6. (a) Polarization curves, (b) CV curves, and (c) Nyquist plots for PEMFCs withvarious Pt loadings (0.1e0.5mg cm�2).

Fig. 7. TGA measurements of Brij 58 and Pt-C composites.

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uniform coating of Pt-C solution on the given substrate. Thus, in thisstudy, the applied voltage was fixed at 8.5 kV for stably generatingfine droplets on the substrate.

SEM analyses were performed to examine the morphologicalstructure of the Pt-C composite layers formed by brush and elec-trospray deposition methods. Here, Pt loading was fixed at~0.3mg cm�2. After simple brush deposition, the Pt-C compositeswere randomly located among the carbon fibers, and some carbonfibers were not fully coated with the Pt-C composites, as shown inFig. 3a. In addition, Pt-C composites remained on the shallow re-gion of carbon-fiber-based paper (Fig. 3b). On the other hand, thePt-C composite layer prepared by the electrospray deposition

method was uniformly distributed along the carbon fibers on theentire substrate surface as shown in Fig. 3c. This suggests that ionand electron transfers were easily made on the entire substratesurface, ensuring stable electrochemical reaction in the PEMFCs.Simultaneously, Pt-C composite layer was uniformly formed on thesurface and inside of the carbon fiber paper (Fig. 3d), suggestingthat the Pt-C composites present both surface and inside of thecarbon fiber paper can also participate in improving the electro-chemical reaction.

The current density-voltage (J-V) and current density-powerdensity (J-P) curves of the PEMFCs fabricated by brush and elec-trospray deposition methods are presented in Fig. 4.

The Pt loading for the MEAs was fixed at ~0.3mg cm�2. The J-Vcurves and J-P curves were obtained after reaching steady state. Forthe PEMFCs fabricated by brush deposition method, the open cellvoltage was 0.91 V, the short circuit current density was90mA cm�2, and the highest power density was ~20mWcm�2.However, for the PEMFCs fabricated by electrospray depositionmethod, the open cell voltage was 0.95 V, the short circuit currentdensity was 132mA cm�2, and the highest power density was~27mWcm�2. The highest power density for the PEMFCs fabri-cated by the electrospray deposition method was ~35% higher thanthat for the PEMFCs fabricated by the brush deposition method.Thus, the PEMFCs fabricated by the electrospray deposition methodshows much better electrochemical performance than that ofbrush-assisted MEA because uniform Pt catalyst thin layers areformed on the surface and inside the carbon fiber paper, whichmaximizes the utilization of Pt.

3.2. Effects of various Pt loadings on the electrochemicalperformance of PEMFCs

MEAs with various Pt loadings (0.1e0.5mg cm�2) were fabri-cated by the electrospray deposition method in this study. SEManalyses were performed to examine Pt-C composite structures forelectrospray-assisted Pt loadings of 0.1, 0.2, 0.3, 0.4, and0.5mg cm�2 on the carbon-fiber-based paper substrate (Fig. 5).When the Pt loading was 0.1mg cm�2, the Pt-C composites weremostly deposited along the carbon fibers, and pores were observedamong the carbon fibers (Fig. 5a and b). When the Pt loading wasincreased to 0.2mg cm�2, some pores among the carbon fiberswere filled with Pt-C composites (Fig. 5c and d). When Pt loadingwas increased to �0.3mg cm�2, compactly dense Pt-C compositeswere found to cover the carbon fiber paper (Fig. 5eej).

Fig. 8. (a) SEM image of Pt-C composite layers fabricated for Pt loading of 0.3mg cm�2 without Brij 58 templates. SEM images of Pt-C composite thin layers fabricated with Ptloadings of (b) 0.3mg cm�2, (c) 0.4mg cm�2, and (d) 0.5mg cm�2 after the thermal removal of the Brij 58 templates.

N. Shan et al. / Current Applied Physics 18 (2018) 728e736 733

The J-V curves were obtained for PEMFCs with various Pt load-ings (Fig. 6a). The electrochemical performances of the PEMFCsincreased upon increasing the Pt loading to 0.3mg cm�2. However,at Pt loadings >0.3mg cm�2, electrochemical performances were

significantly decreased. Fig. 6b shows the CV analyses for MEAswith various Pt loadings. The electrochemically active surface area(EASA, in m2 g�1) was calculated as follows:

Fig. 9. Current density-voltage curves measurement for PEMFCs with various amountsof Brij 58 surfactant in the fixed amount of 0.3mg cm�2 of Pt.

Fig. 10. (a) Pressure drops in MEAs as a function of the reacting gas flow rate. (b)Polarization curves and (c) Nyquist plots for various PEMFCs with electrospray-deposited Pt-C composite thin layers with and without using Brij 58 surfactanttemplates.

N. Shan et al. / Current Applied Physics 18 (2018) 728e736734

EASA¼Q/(210M)

where Q is the real adsorption of a hydrogen monolayer on themembrane surface (mC cm�2) and M is the catalyst loading on theelectrode (g cm�2). Here, the adsorption of hydrogen monolayer onthe surface of polycrystalline Pt was normalized by a factor of210 mC cm�2 [28]. The EASA values for the MEAs were determinedto be 31.7, 34.9, 47.6, 26.5, and 25.4m2 g�1, for Pt loadings of 0.1, 0.2,0.3, 0.4, and 0.5mg cm�2, respectively. It is generally known thathigher Pt loadings in MEAs result in larger EASA values [26].However, for the electrospray-deposited catalyst layers fabricatedin this study, the maximum EASA of 47.6m2 g�1 is found for a Ptloading of 0.3mg cm�2, and then it is considerably decreased to25.4m2 g�1 for the Pt loading of 0.5mg cm�2. This suggests that theutilization of Pt is limited by the compactly dense Pt-C compositethin layer structures formed by the relatively high Pt loadings(>0.3mg cm�2), which restricts easy passage of the H2/O2 reactinggases, thus deteriorating the electrochemical performance of thePEMFCs.

Nyquist plots (Fig. 6c) present the internal resistances of thePEMFCs obtained by varying the Pt loadings. The first intersectionpoint with the Z0 axis and the size of semi-circle represent theohmic resistance and the charge transfer resistance, respectively[29]. Both are significantly decreased upon increasing Pt loadingamount up to 0.3mg cm�2, and are suddenly increased uponincreasing the amount of Pt loadings beyond 0.3mg cm�2. Theelectrochemical impedance is initially large at relatively low Ptloadings (<0.3mg cm�2) due to the poor catalytic kinetics resultingfrom insufficient Pt loadings. However, too much Pt loading(>0.3mg cm�2) increases the electrochemical resistance due toformation of compactly dense Pt-C composite structures, whichprevent easy passage of the reacting gases and simultaneouslyelongate the transfer paths of the generated ions and electronsthrough the Pt-C composite thin layer.

3.3. Effects of porosity of Pt-C composite layers on theelectrochemical performance of PEMFCs

TGA analysis was performed to determine the appropriatecalcination temperature for thermally removing the Brij 58 tem-plates in the Pt-C composite layers as shown in Fig. 7. Pt-C com-posite can be significantly oxidized at higher than 400 �C. Thereforewe precisely controlled the calcination temperature at ~350 �C to

remove Brij-58 for minimizing the loss of Pt-C composite.The presence of pores in the Pt-C composite layers can facilitate

passage of the H2/O2 reacting gases, thus lowering the ion andelectron transfer resistances [30e32]. In order to increase theporosity of the electrospray-deposited Pt-C composite layers, Brij58 surfactant was added as a template to the Pt-C composite layersfollowed by their thermal removal by heating at ~350 �C. Fig. 8shows the comparison of the microstructures of Pt-C compositethin layers before and after the thermal removal of Brij 58 tem-plates for a Pt loading of 0.3mg cm�2. The compactly dense Pt-Ccomposite structures with large primary particles are shown in

N. Shan et al. / Current Applied Physics 18 (2018) 728e736 735

Fig. 8a. However, after the thermal removal of the Brij 58 templates,the large primary particles disappeared and microstructured net-works with micro- and nano-sized open pores were clearlyobserved in the Pt-C composite thin layers (Fig. 8bed).

The effect of different mixing ratio between Pt and Brij 58 on theperformance of PEMFCs was examined. The amount of Pt loadingwas fixed at 0.3mg cm�2, and then the amount of Brij-58 wasvaried at 0,1, 5, and 10wt%, which resulted in the porosity of 0, 0.14,0.71, and 1.42%, respectively. When the amount of Brij 58 wasincreased, the electrochemical performance of PEMFCs wasimproved as shown in Fig. 9. However, when the amount of Brij 58was greater than 5wt%, the electrochemical performance ofPEMFCs was considerably deteriorated. This is presumably becausethe excessive Brij 58 resulted in serious damages in the Pt-C com-posite layer after calcination process. Therefore, we maintained theamount of Brij 58 at the fixed amount of 5wt% throughout thisstudy.

We examined the effects of porosity in the Pt-C composite layeron the pressure drop in the MEAs as a function of the reacting gasflow rate for various Pt loadings (0.3e0.5mg cm�2) (Fig. 10a). Uponincreasing the Pt loading amount without Brij 58, the pressure dropin MEAs was significantly increased due to the formation ofcompactly dense Pt-C composite layers. However, after creatingporous structures in the Pt-C composite layers, the pressure dropsin MEAs were decreased at all flow rates compared with thepressure drops in MEAs without porous structures. The use of moreBrij 58 templates was accompanied by lower pressure drops inMEAs. This suggests that the porosities of the Pt-C composite layerswere successfully improved by thermally removing the Brij 58templates and H2/O2 reacting gases easily passed through the thus-obtained porous Pt-C composite layers. J-V curves were obtained forvarious PEMFCs with and without using Brij 58-templated porousstructures (Fig. 10b). The electrochemical performances of PEMFCswith porous Pt-C composite layers at Pt loadings of0.3e0.5mg cm�2 were enhanced compared to the PEMFCs pre-pared without using Brij 58 templates (Fig. 10b). Finally, Nyquistplots for PEMFCs with and without using Brij 58 are shown inFig. 10c. The size of the semi-circle for the PEMFCs decreased for Pt-C composite layers with Brij 58-templated porous structures. Thissuggests that the internal resistance of the PEMFCs was reduced bycreating porous structures in the Pt-C composite thin layers so thatthe reacting gases can easily permeate, and the generated ions andelectrons can rapidly transfer through the Pt-C composite thinlayers.

4. Conclusions

In this study, we have investigated the effects of electrospray-deposited Pt-C composite thin layers on the electrochemical per-formances of PEMFCs. Upon increasing the Pt loadings up to0.3mg cm�2, the electrochemical performances of PEMFCs wereimproved due to the formation of uniform Pt-C composite layers onthe carbon fiber papers. However, increasing Pt loadings beyond0.3mg cm�2 formed compactly dense Pt-C composite thin layersthat restrict passage of the reacting gases through the MEAs, andsimultaneously elongate the transfer paths for the generated ionsand electrons. In order to enhance the reacting gas permeability, weemployed Brij 58 surfactant templates to create porous structuresin the Pt-C composite thin layers. The Brij 58 templates were addedto the Pt-C composite thin layers, followed by thermal removal. Themicro- and nano-porous structures created using the Brij 58 tem-plates lowered the pressure drop in MEAs. As such, the reacting gaspermeability was improved, which enhanced the electrochemicalperformance of the PEMFCs. This suggests that the creation ofmicro- and nano-porous structures in Pt-C composite thin layers

play an important role in improving the electrochemical perfor-mance of PEMFCs in practical energy production applications.

Acknowledgements

This study was supported by a 2-Year Research Grant of PusanNational University, South Korea.

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