Sulfonated polyaniline nano
fiber as Pt-catalyst conducting support for
proton exchange membrane fuel cell
Rong-Hua Wu
a, Ming-Jer Tsai
a, Ko-Shan Ho
a,*, Ting-En Wei
a, Tar-Hwa Hsieh
a,
Yu-Kai Han
a, Chung-Wen Kuo
a, Po-Hao Tseng
b, Yen-Zen Wang
caDepartment of Chemical and Materials Engineering, National Kaohsiung University of Applied Sciences, 415, Chien-Kuo Road, Kaohsiung 80782, Taiwan bGraduate Institute of Electrical Engineering, National Taiwan University, No. 1, Sec. 4, Roosevelt Road, Taipei 10617, Taiwan
cDepartment of Chemical and Materials Engineering, National Yun-Lin University of Science and Technology, 640 Yun-Lin, Taiwan
a r t i c l e i n f o
Article history:
Received 15 November 2013 Received in revised form 24 January 2014
Accepted 25 February 2014 Available online 5 March 2014
Keywords: Polyaniline Carbonization Sulfonation
a b s t r a c t
Carbonization at high temperature can significantly increase the conductivity of polyaniline nanofibers (PANF) but the created carbonized surface is too hydrophobic for Pt-loading. Sulfonic acid groups are then grafted on the carbonized PANF by ultra-sonication in concentrated sulfuric acid to increase the surface hydrophilicity and Pt-loading. The obtained sulfonated carbonized PANF was found to own not just high conductivity but good hydrophilicity which can load more than 18% of Pt on the surface from a 25% H2PtCl6(aq) and demonstrate better electrochemical activity in the cyclic voltaic and ORR testing. The surface area of the loaded Pt per unit support can be increased from 85.66 to 276.61 cm2mg1after 24 h of sulfonation. The single cell performance demonstrates an increasing power and maximum current density with degree of sulfonation for MEA made of the sulfonated carbonized PANF.
Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction
Based on the idea that replacing fossil fuels with hydrogen fuel can substantially reduce greenhouse gas emissions and smog pollution[1e3], hydrogen fuel cell technologies have been widely studied in many areas[4].
Recent researches focus on hydrogen and oxygen system of proton exchange membrane fuel cells (PEMFCs) under the consid-erations of its friendly by-products, high power density, low noise, and low operating temperatures. However, one of the main prob-lems to commercialize PEMFCs is how to decrease the cost of Pt-catalysts by increasing its catalytic efficiency and durability under the harsh working conditions[5]. The performance of the PEMFCs also depends on the properties of the gasflow in the membrane electrode assembles (MEAs)[6]in which interfacial areas between the reactant, electrolyte and catalyst itself, the so-called triple-phase boundary, play important roles on the catalytic efficiency of Pt-catalyst. To increase the electricity productivity of a PEMFC, the catalyst supporting material in the MEAs need to own good con-ducting capability for both ion and electron except improving the Pt loading %. In other words, we need lots of continuous pathways for
the produced protons to pass through and then go into the elec-trolyte layer and also for the electrons to go into or out of the circuit. Consequently, continuous catalyst supports with high porosity and conductivity are needed to allow the produced electrons, protons, and water to move faster. We can prepare conducting catalyst supports with nanoscaled pores to accept and disperse the implanted Pt[7]. At present, catalyst material for MEA is prepared by loading nano-scale platinum (Pt) particles on the surface of conducting/nanostructure carbon black (CB) in PEMFC. However, adopting CB as the fuel cell catalyst support still has some disad-vantages. For example, the particulate CB can significantly interrupt the electrons moving to the outer circuit since some of the CB particles do not contact with each other. Meanwhile, carbon sup-port in the cathode is subjected to severe corrosion in the presence of water and produces carbon dioxide at the high performing temperature[8e10], which can deteriorate the performance of the catalysts and shortens the lifetime of PEMFC. Pt-catalyst supporting material which can resist water corrosion at the working temper-ature is needed to replace CB, and conducting metal oxides and conducting polymers are two of the candidates[11,12,7]. However, considering the continuous conducting pathway for electron transportation, conducting polymer is the only option.
In the past decades, conducting polymers have been under wide researches for lots of applications in thefields of corrosion pro-tection, electrochemical displays, energy conversions, * Corresponding author. Tel.: þ886 7 3814526x5122; fax: þ886 7 3830674.
E-mail address:[email protected](K.-S. Ho).
Contents lists available atScienceDirect
Polymer
j o u rn a l h o m e p a g e : w w w . e l s e v ie r . c o m / l o c a t e / p o l y m e r
http://dx.doi.org/10.1016/j.polymer.2014.02.066
0032-3861/Ó 2014 Elsevier Ltd. All rights reserved.
microelectronics, and sensors [13e17]. Among the conducting polymers, polyaniline and its derivatives[18,19]are qualified can-didates because of their high surface area and porosity when they are directly electrochemically polymerized on the electrode in acidic media[20].
Another approach to obtain polyanilines with conducting and stable three-dimensional nanostructure is to prepare conducting polyaniline with nanofibrous morphology (PANF) as the support by emulsion polymerization. Michel et al. [7] introduced a functionalized Pt/PA-F composite and demonstrated high per-formance in a single cell testing. André Wolz et al.[21]reported using polyol as reducing agent to deposit Pt nanoparticles on PANF and single-walled carbon nanotubes (SWCNTs) by building up alternating layers of PA supported catalyst and a high power densities was achieved. C.W. Lin et al. [22] investigated the morphology-dependent electrochemical properties of catalyst supporter made of polyaniline micro/nanostructures in DMFC applications. Bo Qu et al.[23] prepared coreeshell polyaniline/ Vulcan Carbon composite structures by in situ chemical poly-merization, to improve the CO anti-poisoning ability and catalytic efficiency of the Pt catalyst.
Recently, S. Mentus et al. preparing carbonization of nano-structured nitrogen-containing conducting polymers has opened new perspectives in the preparation of nitrogen-containing conducting nanomaterial [24]. Gavrilov et al. [25] prepared the nitrogen-containing nanotube/nanosheet of carbonized pol-yaniline as a new carbonaceous support for Pt nanoparticles, which demonstrated significant ORR in both acidic and alkaline media.
In this study, we try to prepare the carbonized and sulfonated PANF as the catalyst support in the electrode of MEA. Carbon-ization and sulfonation will be demonstrated to improve the conductivity and Pt-loading % of the catalyst support. Various properties such as, conductivity, BET surface area of the catalyst support, and the performance of the MEA based on this material will be measured.
2. Material and methods 2.1. Materials
The aniline monomer (TOKYO KASEI KOGYO CO.) was distilled under vacuum before use. Ammonium persulfate (APS, SHOWA CHEMICALS INSTRUMENT CO.), para-phenolsulfonic acid hydrate (PSA, TOKYO CHEMICAL INDUSTRY), hydrogen hexachloroplatinate (IV) hexahydrate (H2PtCl6‧6H2O, ALDRICH), ethylene glycol(EG, J.T.
Baker), hydrochloric acid (HCl, Riedel-de Haën (RDH)), Sulfuric acid(H2SO4, Riedel-de Haën (RDH)) were used without further
purification.
2.2. Synthesis of polyaniline nanofibers (PANF)
PANF were prepared through an emulsion polymerization method, described in previous publications[26e29].
The only difference from the previous preparation of PANF was we replace para-phenolsulfonic acid hydrate with n-dode-cylbenzene sulfonic acid as the doping protonic acid.
2.3. Carbonization and sulfonation on PANF electrocatalyst supports
A 2 g of PANF powder prepared in 2.2 was carbonized in an oven at 1100C in a highly pure nitrogen atmosphere (CPANF)[30]. Then 0.5 g of CPANF was ultra-sonicated in 200 ml, 5 M H2SO4(aq), at 0C
for 6e24 h. Finally, the different periods of sulfuric acid treated
polyanilines (CPANF-Sx; x¼ 6, 12, 24) were washed with de-ionized water until thefiltrate became neutral and the filter cakes were dried in an oven at 60C for 24 h.
2.4. Platinum deposited on various polyanilines
A 14 mg (Pt ion is about 14 (195/518) ¼ 5.3 mg) of hexa-chloroplatinate (H2PtCl6) acid solid powders was added to 20 mL of
ethylene glycol (EG) in the presence of 16 mg of electrocatalyst support polyanilines with various degree of sulfonation. Then the mixture was under vigorous stirring for 30 min. Some NaOH was introduced to adjust the pH of the EG solution to above 11. The solution was then heated to 170C and refluxing for 2 h. The ob-tained Pt composites named as Pt/PANF (or Pt/CPANF, Pt/CPANF-Sx) were isolated byfiltration and washed with isopropanol, dried at 60C overnight. Theoretically, 25 wt% (5.3/(5.3þ 16) ¼ 25 wt%) of Pt will be present in the obtained Pt/polyaniline composite if it is fully reduced by EG.
2.5. Characterization 2.5.1. Raman spectroscopy
The Raman spectra of neat and degraded samples were carried out by a Triax 550 spectroscopy with a green laser light source of 520 nm wavelength. The samples were pressed into tablets before exposing to Raman source.
2.5.2. SEM (scanning electronic microscopy)
Images of various PANF, prepared from strewn on carbonic tape and followed by posting on ferric stage, were taken in a Field Emission SEM, HRSEM (HITACHI S-4200: accelerating voltage of 15 kV).
2.5.3. BET (Brunauer-emmett-teller specific surface area analyzer) Nitrogen adsorption isotherms were measured with an auto-mated gas sorption system (Micromeritics; ASAP2101) kept at low temperature in liquid nitrogen. The specific surface-area was calculated using BrunauereEmmetteTeller (BET) approach. 2.5.4. FTIR spectroscopy
The functional groups of carbonized and sulfonated samples were characterized by FTIR spectroscopy. The FTIR spectra were recorded on an IFS3000 v/s Fourier-transform infrared spectrom-eter at room temperature.
2.5.5. WXRD (wide angle X-ray diffraction)
An copper target (Cu-K
a
) Rigaku x-ray source with a wavelength of 1.5402 Å was used for diffraction. The scanning angle (2q
) started from 5to 100with a voltage of 40 kV and a current of 30 mA at 1min1.2.5.6. TEM (transmission electronic microscopy)
Samples forfield emission transmission electron microscope, HR-AEM (HITACHI FE-2000) werefirst dispersed in acetone and put on carbonic-coated copper grids in drop wise before subjecting to the emission.
2.5.7. TGA (thermogravimetric analysis)
The thermal degradation behavior of various Pt/polyaniline composites was examined by TGA (TA SDT-2960) thermograms. The amount of Pt deposited on the on the surface of catalyst sup-ports were characterized by the residual weight at 800C with a heating rate of 10C min1under purging air.
R.-H. Wu et al. / Polymer 55 (2014) 2035e2043 2036
2.6. Electrochemical characterization
Cyclic voltammetry (CV) method was used to determine the active electrochemical surface area of the catalyst supports in the electrode. The performance of the electrocatalyst support was tested with a three-electrode system. The working electrode with a square area of 1.5 cm2was prepared as follows. Ag/AgCl and plat-inum wire were used as the reference and counter electrode, respectively. The electrochemical test was carried out in a poten-tiostat/galvanostat (Autolab-PGSTAT 30 Eco Chemie) in 1 M H2SO4
solution and various cyclic voltammograms (CV) were obtained with potential scanned between0.2 and 1.2 V at a sweeping rate of 50 mVs1. The catalyst ink was prepared by mixing 3 mg support powder in isopropanol and stirred until uniform. Subsequently, 5% Nafion solution was added into the mixture as binder and the mixture was ultrasonicated for 1 h, the obtained ink was uniformly cast on the carbon paper for CV test.
The electrochemical activities of the Pt-electrocatalysts with different PANF supports were measured using a rotating-disk electrode (RDE) operated at 1600 rpm in O2-saturated 0.5 M
H2SO4. The oxygen reduction reaction (ORR) currents at the
measured voltage range (0.5e1.0 V) for each electrocatalyst mate-rial were recorded.
2.7. MEA preparation
A NafionÒ212 sheet purchased from Ion Power Inc., New Castle,
DE, USA was used as the polymer exchange membranes. In order to remove the surface organic impurities and to convert the mem-branes into protonated (Hþ) form for use in the PEM, the Na fion-212 (4 4 cm2) membrane was treated at 70C in 5 wt% H
2O2
aqueous solution for 1 h, followed by submerged in 1 M H2SO4
solution for 1 h, respectively and subsequently the treated mem-branes were dipped in distilled water for 15 min then stored in de-ionized water. The catalyst inks were prepared by mixing 20 mg Pt/ PANF (or Pt/CPANF, Pt/CPANF-Sx) powders in an isopropanol so-lution and stirred mechanically until uniform and 5% Nafion solu-tion was added into the mixture followed by ultra-sonicasolu-tion for 1 h. Similar to doctor blade coating, the catalyst inks was layer by layer coated on both side of the treated Nafion sheet as anode and cathode electrodes (2 2 cm2), respectively and hot pressed at
140C with 70 kg cm2for 5 min into an MEA. 2.8. Single-cell performance testing
The MEA was installed in a fuel cell test station for testing using a single-cell test equipment (model FCED-P50; Asia Pacific Fuel Cell
Technologies, Ltd.). The active cell area was 2 2 cm2. The
tem-peratures of anode, cell, and cathode and humidifying gas were all controlled at around 70C. Theflow rates of anode input H2and the
cathode input O2fuels were set at 100 and 200 mL min1,
respec-tively. To test the performance of the various Pt/polyaniline com-posites catalyst in MEA, polarization curves (IeV) and output power were constructed and recorded, respectively.
3. Results and discussion
3.1. Carbonization and sulfonation effects on PANF
The particles of the conventional electrocatalyst support of carbon black can easily build up resistance for the produced elec-trons if they are not contacting with each other. A PANF prepared in the absence of organic solvent from a simple emulsified polymer-ization can prevent it with its long, continuous chain. However, the poor conductivity neat PANF are found inTable 1, which is also unfavorable for electron transportation and even for Pt-loading. When PANFs are carbonized at 1100 C in the nitrogen atmo-sphere, their surface areas which are the place for Pt-implantation significantly rise more than 4 times from 40 to 188 square meter per gram (m2 g1). And the surface area can be even increased further by sulfonation. It abruptly rises from 188 to 422 m2g1after 6 h of sulfonation in sulfuric acid according toTable 1. Eventually, the area is even increased to be 513 m2g1after 24 h of sulfonation. The increasing surface area after high temperature treatment comes from the breakage of the PANF during carbonization and sulfonation as described inScheme 1 [30].
Furthermore, the conductivity of CPANF significantly increases to 6.667 from 0.032 S/cm due to surface carbonization effect. Although the conductivity drops again to 0.04 S/cm after 6 h of sulfonation according toTable 1, it can be partly recovered to be become 0.667 S/cm when sulfonation time is increased to 24 h. The loss of conductivity by sulfonation is compensated by the significant increase of hydrophilicity of the grafted sulfonic groups with which more Pt ions can be associated. The degree of sulfo-nation on the CPANF is monitored by carrying out elementary analysis. The elementary analysis data listed inTable 1indicates that the alkyl sulfonic protonic acids used as the primary dopant during PANF polymerizing are entirely destroyed by carbonization at 1100C, leading to the entire loss of hydrophilicity of CPANF. That is why we cannot do the sulfonation before carbonization since all the formed sulfonic groups from sulfonation will be degraded during carbonization. Actually, sulfonic acid groups are decomposed around 250 C, which will be found in thermogra-vimetric analysis (TGA).
Table 1
Properties and elementary analysis (weight %) of various PANF supports.
sulfonation (CPANF-S24) according to Fig. 8. It implies more Pt atoms are loaded on the surface of sulfonated samples.
3.7.3. Single cell performance analysis
The surface area of the loaded Pt per unit of catalyst supports is also estimated and listed in the right column ofTable 2. We can understand that the surface area of the implanted Pt become smaller after carbonization due to less Pt-loading on CPANF. And it abruptly increases to three times after sulfonation and the increased surface area of implanted Pt is believed to be the main reason why we have better single cell performance for MEAs after sulfonation.
MEAs based on different types of PANFs are assembled into single cell and their electrochemical performances are evaluated by measuring their current density, voltage and power density in
Fig. 9. The carbonization at 1100 C (CPANF) does increase the conductivity, however, the current and power density (maximum 126.7 mW cm2) shown inFig. 9are small due to less implanted Pt catalyst. Besides, the voltage decays rapidly with current, resulting from severe concentration polarization.
The sulfonation on the surface of CPANF (CPANF-S series) can effectively improve the Pt-loading % and decrease the effect of concentration polarization. Only 6 h (CPANF-S6) of sulfonation can promote the maximum current (Imax) and power density (Pmax) to
959.4 mA cm2and 310.0 mW cm2, respectively, which are much higher than that of Pt/CPANF seen inFig. 9. Both Imaxand Pmaxgo
high steadily with increasing sulfonation time and reach at 1393.7 mA cm2for Imaxand 414.0 mW cm2for Pmaxafter 24 h of
sulfonation. And the cell experiences only slight voltage decaying with increasing current density due to little concentration polari-zation that attributes to the N-containing, conducting feature of sulfonated carbonized PANF.
4. Conclusions
Based on previous studies and experiences on the preparation of nanofibrous conducting polyanilines, a 1D conductor with affluent amino groups are successfully prepared. Its conductivity is further improved and surface area is significantly increased via carbon-ization at 1100C, after which some nitrogen-containing functional groups and 1D nanofibers are preserved. However, high hydro-phobicity that comes with carbonization prevents the association of the Pt ions. Lots of sulfonic acid groups are grafted to its surface
after ultra-sonication with concentrated sulfuric acids, leading to higher dispersibility in the aqueous solution and gain a high degree of Pt-loading of 74.12% according to its TGA thermograms.
The CV cycle test proves more stable redox reaction for sulfo-nated carbonized PANF and the electrocatalyst made reveals a higher reducing current than carbonized PANF in the ORR (oxygen reduction reaction) testing.
The single cell performance testing illustrates an increasing maximum power of and maximum current density with sulfona-tion time for MEA made of sulfonated carbonized PANF. Besides, this MEA does not experience a serious power density loss at high current density.
The carbonization at high temperature and subsequent sulfo-nation in sulfuric acids for PANF are able to prepare an optimized electrocatalytic support for Pt catalyst for better PEMFC perfor-mance. The N-containing, conducting and anti-water corrosion features make sulfonated carbonized polyaniline nanofiber a more competing candidate as the conducting catalyst supporting material.
Acknowledgment
The authors would like to appreciate thefinancial support from National Science Council in Taiwan, ROC through the grants of NSC 101-2221-E-151-035 and NSC 102-2221-E-151-033.
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and O2flows are set at 0.1 L min1and 0.2 L min1, respectively.
