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Development of multiwalled carbon nanotubes platinum nanocomposite as efficient PEM fuel cell catalyst

Development of multiwalled carbon nanotubes platinum nanocomposite as efficient PEM fuel cell... Mater Renew Sustain Energy (2016) 5:2 DOI 10.1007/s40243-015-0066-5 ORIGINAL PAPER Development of multiwalled carbon nanotubes platinum nanocomposite as efficient PEM fuel cell catalyst 1,2 1,2 1 • • Chanchal Gupta Priyanka H. Maheshwari Sanjay R. Dhakate Received: 11 June 2015 / Accepted: 20 December 2015 / Published online: 14 January 2016 The Author(s) 2016. This article is published with open access at Springerlink.com Abstract Multiwalled carbon nanotubes platinum Introduction nanocomposite has been prepared via chemical route by reduction of Pt salt on MWCNTs in ethylene glycol solu- Among the various types of fuel cells, Polymer Electrolyte tion while refluxing in Argon atmosphere. The effect of Membrane (PEM) fuel cells have attracted most of the different pH media during the reduction process on their researchers because of their advantages like low emissions, physical and electrochemical properties, as well as on their low operating temperature and high power density and use performance in unit PEM fuel cell has been studied and of all solid state components, which make them suitable for further compared with the reaction carried out while portable applications [1–4]. Despite various advantages, refluxing in air as already demonstrated by the authors. The the sluggish kinetics of ORR (oxygen reduction reaction) is I-V performance of unit PEM fuel cell shows a peak Power the limiting factor for energy conversion in PEM fuel cells. -2 density of 156 mW cm with catalyst prepared in alkaline This necessitates the use of an efficient electro catalyst, medium, an increase of [ 110 % as compared to wherein Pt or Pt-based alloy catalyst supported on high -2 72 mW cm obtained while employing catalyst prepared surface area carbon black is generally used. Due to the in acidic medium and tested under similar conditions. This amorphous nature of carbon black, it tends to agglomerate is attributed not only to the small particle size of reduced Pt and corrode with repeated fuel cell operation. This not only NPs, but also to its uniform distribution when reduction is increases the cost of the catalyst, but also leads to insuf- carried out in alkaline medium. This has been further ficient durability [5–9]. Therefore, development of effi- explained by detail reaction mechanism under alkaline cient, low cost and highly durable catalyst is the major conditions. concern towards the commercialization of fuel cells. In this context, carbon materials like ordered mesoporous carbons Keywords Carbon nanotubes  Catalyst  Fuel cell  Mid- (OMCs) [10], carbon aerogels [11], carbon nanotubes wave potential  Polarization (CNTs) [5, 12–15], carbon nano-horns (CNHs) [16], car- bon nano-coils (CNCs) [17] and carbon nano fibers (CNFs) [18] have attracted much interest as electro catalyst sup- port. Amongst them CNTs have been considered as the most attractive support material due to its high crys- tallinity, hydrophobicity, high conductivity, chemical inertness and high surface area [19–24]. These properties not only make them suitable for catalyst support, but they & Priyanka H. Maheshwari are increasingly being used in other cell components like hedap@mail.nplindia.ernet.in GDL [25–28], carbon paper [29] and bipolar plates [30]. However, chemically inert nature of CNTs lowers the Physics and Engineering of Carbon, CSIR-National Physical Laboratory, New Delhi, India effective reaction sites for attachment of metal nanoparti- cles. To overcome this, many efforts have been done to Academy of Scientific and Innovative Research (AcSIR), modify the surface of CNTs by chemical functionalization CSIR-NPL Campus, New Delhi, India 123 2 Page 2 of 11 Mater Renew Sustain Energy (2016) 5:2 [31–33]. The chemical oxidation method introduces the refluxing in an inert atmosphere, will control the defect acid functional groups on the surface of CNTs; neverthe- formation and lead to a smooth surface, thereby providing less it introduces irreversibly structural defects, which identical surface sites, resulting in an enhanced catalytic reduces the electrical conductivity and the durability of activity of the nanocomposite formed. CNTs [34, 35].Various treatments including sono-chemical This study gives a detail account of how the varying [36], electrochemical [37], microwave heating [38] and conditions of pH during the reduction process affect the reflux heating [39, 40] have been proposed to incorporate properties of the synthesized nanocomposites and their Pt nano particles onto the CNT surface. corresponding behavior as catalyst for PEM fuel cell. This In the present study we use pristine CNTs to avoid the in turn is supported by characterizations like TGA, TEM, surface oxidation process and we demonstrate the devel- XRD, Raman spectroscopy, XPS and electrochemical opment of Pt/CNT by reduction of chloroplatinic acid via techniques like cyclic voltammetry (CV) and linear sweep refluxing in varying pH media and in argon environment. voltammetry (LSV) and finally the samples have been The above decision was driven by the thought that tested for their performance in unit PEM fuel cell. Materials and methods Development of Pt/CNT nanocomposite Commercially available Nanocyl 7000 MWCNTs with diameter in the range of 20–30 nm and aspect ratio [1000 were used for the preparation of Pt/CNTs. Ethylene glycol and Hexachloroplatic acid (H PtCl .6H O) were procured 2 6 2 from Merc Ltd. and Acros Organics, respectively. Ar gas with 99.9 % purity and double deionized water is used wherever required. Pristine MWCNTs were uniformly dispersed in ethylene glycol by ultra-sonication for *3 h. Solution of 0.01 M H PtCl (chloroplatinic acid) in IPA was added to the 2 6 above drop by drop under constant magnetic stirring such that the ratio of Pt:CNT is 1:4. The pH of the above Fig. 1 X-ray diffraction curves of pristine CNTs and sample Ar1, solution was measured and found to be less than 2. To Ar2, and Ar3 Table 1 Miller indices, d-spacing and crystallite size for the given diffraction angle for samples Ar1, Ar2, and Ar3 Sample Miller Peak position (in d-spacing FWHM (in Crystallite size Average crystallite size of Pt name indices degrees) (nm) degrees) (nm) (nm) Ar1 C(002) 25.67 3.46 4.04 2.01 Pt(111) 39.99 2.25 2.45 3.44 3.62 Pt(200) 46.06 1.96 3.56 2.43 Pt(220) 67.88 1.38 2.28 4.22 Pt(311) 81.70 1.17 2.39 4.39 Ar2 C(002) 25.42 3.51 6.19 1.31 Pt(111) 39.85 2.26 3.43 2.46 2.67 Pt(200) 45.96 1.97 4.15 2.07 Pt(220) 67.57 1.38 3.47 2.75 Pt(311) 81.21 1.18 3.06 3.41 Ar3 C(002) 25.54 3.48 4.66 1.74 Pt(111) 39.82 2.26 3.98 2.12 2.29 Pt(200) 46.24 1.96 4.32 1.99 Pt(220) 67.67 1.38 4.23 2.25 Pt(311) 81.92 3.74 1.18 2.81 123 Mater Renew Sustain Energy (2016) 5:2 Page 3 of 11 2 make the samples of different pH (i.e., 2, 7 and 11) 0.1 M The filtrate was further dried to obtain powdered catalyst. NaOH was added with continuous stirring. This was further Dilute solution of NaBH was added to residue to detect refluxed at 140 C for 3 h in argon. This process of the presence of unreacted platinum. The samples prepared reduction, immobilizes Pt on CNTs forming Pt/CNTs by refluxing the solution in acidic, neutral, and alkaline nanocomposites. The solution was filtered followed by mediums (pH 2, 7, and 11) in argon have been designated washing with copious amount of double de-ionized water. as Ar1, Ar2, and Ar3, respectively. Physical characterization The X-ray diffraction (examination of the samples was performed on Rikagu powder X-ray diffractometer model: XRG 2KW using Cu Ka radiation. The mean crystallite size and lattice parameters were calculated from line broadening and d-spacing measurements using the (002) and (100) reflections. Thermal gravimetric analysis of the electrode samples was carried out on TGA/DSC 1600 by Mettler Todedo. The experiments were carried out in air at the rate of 10 C/min. The structural details of the MWCNT samples were studied with the help of trans- mission electron microscopy (TEM) using Tecnai G2 F30 S-Twin instrument. Raman spectroscopy was carried out using a Renishaw InVia Reflex Micro Raman Spectrometer equipped with the CCD detector at room temperature and Fig. 2 TGA curves of pristine CNTs and sample Ar1, Ar2, and Ar3 Fig. 3 a–c TEM images of Ar3 sample and d particle size distribution curve for Ar3 123 2 Page 4 of 11 Mater Renew Sustain Energy (2016) 5:2 determined experimentally using cyclic voltammetry in N purged 0.5 M HClO solution. Kinetic activity for ORR is measured ex situ with Linear scan voltammetry (LSV) in rotating disk electrode (RDE) apparatus with 1600 rpm -1 rotating rate and 5 mVs sweep rate in O purged 0.5 M HClO solution. For fuel cell performance Toray carbon paper samples of size 25 cm were teflonized and gas diffusion layer -2 (GDL) was prepared by coating 1.5 mg cm of carbon black (Vulcan XC-500) by brush coating technique, fol- lowed by sintering at 350 C for 30 min. The catalyst ink was prepared by mixing the synthesized catalyst with nafion (7 wt% of catalyst). The ink was then brush coated onto the GDL such the amount of catalyst loading is -2 0.3 mg cm Pt. Nafion Membrane—1135 (DuPont) was sandwiched in between the two electrodes (as prepared above) and hot pressed at 120–130 C for 3 min. to make the membrane electrode assembly (MEA). The cells were tested at 60 C and 100 % humidified conditions with the flow rate of hydrogen and oxygen gasses maintained at 200 -1 and 300 mL min , respectively, at atmospheric pressure. Measurements of cell potential with varying current den- sities were conducted galvanostatically using MODEL- LCN4-25-24/LCN 50-24 procured from Bitrode Instru- ments (US). Results and discussion The XRD patterns for Pt/CNTs are shown in Fig. 1. Diffraction peak at * 26 is attributed to graphite crys- Fig. 4 a Raman spectra of pristine CNTs and sample Ar1, Ar2, and tallographic planes (002) of CNT. Pt/CNTs show the Ar3; b HRTEM image of pristine multiwalled carbon nanotube presence diffraction peaks at nearly 40,46, 67.5, and in air. Green laser (excitation line 514 nm) was used to 81 corresponding to (111), (200), (220) and (311) planes excite the samples. One scan per sample was recorded of Platinum, respectively. These are analogous to the face- wherein the samples were exposed to the laser power of centered cubic structure of the noble metal. For all 25mW for 10 s. diffraction peaks the average crystallite size has been cal- culated from FWHM (full width at half maximum) values Electrochemical characterization using Debye–Scherrer equation [41]. Table 1 gives a detailed account of the observed diffraction peaks along The electrochemical measurements were performed using with their corresponding d-value and FWHM. The crys- tallite size of platinum has been calculated as an average of Biologic instrument (VSP model and EC-Lab software) at 25 C. Conventional three electrode system was used for that obtained from different peaks. The average crystallite CV measurements saturated calomel electrode (SCE), Pt size decreases with increase in the pH of the refluxing foil and glassy carbon (GC) as reference electrode, counter medium. This can be explained as follows. At higher pH - - electrode and working electrode, respectively. The catalyst OH ions are available to replace Cl in the hexachloro ink was prepared by dispersing 4.26 mg of catalyst in 1 ml platinic acid since OH ligand has higher field strength ethanol. A drop of 5 % nafion solution was then added to than Cl in the spectro-chemical series. Further, the the above solution and sonicated for 30 min. The working Extended X-ray adsorption fine structure studies (EXAFS) electrode was prepared by drop casting the catalyst ink on confirmed that Pt-OH bond is smaller than Pt–Cl bond [42, the glassy carbon electrode such the platinum loading is 43]. Hence, due to steric contraction effect the complexes 60 lg/cm . Four electrodes per sample were prepared and formed in basic medium is smaller than the complexes formed in acidic medium [44]. tested under similar conditions. The value of ECSA is 123 Mater Renew Sustain Energy (2016) 5:2 Page 5 of 11 2 Fig. 5 CV curves of 4 different electrodes prepared for samples a Ar1, b Ar2, and c Ar3 in N purged 0.5 M HClO 2 4 Figure 2 shows the TGA and DTG curves of the catalyst samples along with that for the pristine CNTs. Fig. 6 CV curves of sample a Ar1, b Ar2, and c Ar3 for 500 cycles TGA for Nanocyl MWCNT without Platinum addition shows a residue about 8.5 % which may be due to the the platinum (i.e., 20 %) was reduced on the CNT presence of catalyst (metal/oxide) used during the CNT surface. As shown by the DTG curves, the thermal stability of synthesis process. For the Pt/CNTs sample prepared in argon atmosphere with different pH medium shows the the nanocomposites reduces as compared to pristine CNTs. This is because, (1) the metal nanoparticles acts as defects residue of *29 %, which indicates that nearly all of 123 2 Page 6 of 11 Mater Renew Sustain Energy (2016) 5:2 and catalyze the oxidation process; and (2) there is a (as shown by the TGA curve); (3) imperfect CNT walls as probability that defects are also introduced in the nanotube shown by the high-resolution TEM image (Fig. 4b). structure during the refluxing process. These results have After incorporation of platinum the I /I value increases D G been further confirmed by Raman spectroscopy. which indicated the stresses induce on the hexagonal TEM micrograph for Pt/CNT nanocomposite in basic structure of carbon. A slight increase in the values of I /I G G medium (Ar3) is shown in Fig. 3 reveals that platinum indicated that Pt incorporation also influences the coupling particles are attached to the outer walls of CNT. Fig- of few CNT layers. ure 3(a) inset shows the presence of closed ends of CNT The CV curves of catalyst samples (Ar1, Ar2, and Ar3) this means during synthesis process the CNTs remains of the four electrodes prepared are shown in Fig. 5a, b, c, highly intact and deposition of platinum particles only respectively, while the curves uptill 500 cycles for one of occurs at the surface of CNTs. The particle size distribution the electrodes are shown in Fig. 6a, b, c, respectively. The (PSD) curve has been plotted for Ar3 for average of 500 curves exhibit typical characteristic of crystalline Pt elec- particles, shows that the mean crystallite size is nearly trodes in the hydrogen and oxygen adsorption–desorption 2.37 nm which is in close agreement with the values regions which illustrates the presence of platinum particles measured from XRD. on the carbon surface. The ECSA of the hydrogen des- Figure 4a shows the Raman spectra for catalysts sam- orption peak calculated from the CV curves gives a mea- ples. The D (defect), G (graphitic) and G (second-order sure of the HOR. ECSA has been calculated for all the harmonic to D) bands are clearly visible. D band is related electrodes prepared for Ar1, Ar2, and Ar3 (as shown in to the defects presents on CNTs, whereas the G band Table 2) and the standard deviation for the four curves of originates from the presence of sp -hybridized carbon sites each sample has been calculated. The average value was present in the sample and hence represents the frequency of found to be maximum for sample Ar3. The increase in carbon–carbon double bond stretching vibration [45]. The ECSA is mainly attributed towards the decrease in crys- G band is the second-order harmonic to D and represents tallite size of the Pt with increase in pH of the refluxing defect in the stacking sequence. The intensities of the medium. Importantly, there is no decrease in the current bands were determined by the area under the spectral with successive cycling (as shown in Fig. 6), which in turn curve. The intensity of the G band (I ) has been used as a is the measure of the durability of the sample, as reported reference in determining the relative intensities of the D elsewhere [46–48]. band (I ) and G band (I ). However, for fuel cells the ORR at cathode side is more D G For pristine CNTs I /I ratio is 1.15 which is quite high critical as it is much slower than the HOR at anode. The D G and indicates the presence of defects. The major contrib- LSV curves for the four different electrodes prepared for utors to the defects are, (1) the large number of pentagons the Pt/CNT nanocomposites are shown in Fig. 7a, b, c. The that are present at the closed ends of the tubes; (2) presence curves show a low value of limiting current for all the of metal oxides used as catalyst for initiating the growth of catalyst samples, probably because of the defects present CNTs which accounts to nearly 8.7 % of the CNT weight on the CNTs [as is clear from the high value of I /I D G Table 2 ECSA, diffusion limiting current density, onset potential, and mid-wave potential calculated from the CV curves for all the electrodes prepared for samples Ar1, Ar2, and Ar3 2 2 Sample ECSA (m /g) DLCD (mA/cm ) Onset potential (V) Mid-wave potential (V) Ar1 17 0.3866 0.7498 0.3224 21 0.4143 0.7784 0.3374 19 0.4338 0.7794 0.2772 20 0.3702 0.7783 0.2575 Ar2 20 0.4163 0.7601 0.4673 20.9 0.4935 0.7748 0.3093 20.5 0.4257 0.7744 0.2856 18.5 0.4850 0.7749 0.2686 Ar3 24 0.2714 0.7696 0.5383 25 0.1860 0.7773 0.4633 25.2 0.3192 0.7821 0.3983 23 0.3659 0.7813 0.3852 123 Mater Renew Sustain Energy (2016) 5:2 Page 7 of 11 2 (Raman data)]which may give rise to unwanted side reac- From the LSV curves the onset potential and mid-wave tions, thus lowering the net useful current. Further, these potentials were measured. The onset potential gives a defects may act as traps for the O and inhibit the diffusion potential at which the ORR initiates and was found to be of the reactant and product species thus lowering the cat- highest for sample Ar3. High activity in terms of onset alytic activity. potential can be attributed to the increase in the electro conductivity of the catalyst [49]; and the increase in the density of the electrochemical active sites due to the enhancement in the surface atoms with a decrease in the particle size and therefore a rise in the probability of early as well as simultaneous reactions, as also stated elsewhere [50, 51]. High mid-wave potential for Ar3 further reflects low binding energy of the catalyst surface with the reac- tants which in turn implies fast catalytic activity as the catalyst surface is quickly available for reducing more O . However, some deviation in the CV and LSV curves and the related parameters have been observed which has been explained later in the text. Fig. 8 Polarization curves for catalyst samples Ar1, Ar2, and Ar3 Fig. 9 IR corrected cell polarization curves for the catalyst samples Ar1, Ar2, and Ar3. Inset shows the tafel plots for activation Fig. 7 LSV curves of 4 different electrodes prepared for samples polarization region sample a Ar1, b Ar2, and c Ar3 in O purged 0.5 M HClO 2 4 123 2 Page 8 of 11 Mater Renew Sustain Energy (2016) 5:2 The actual activity of the catalyst is best judged from its Improved performance of sample Ar3 as compared to fuel cell performance curves. The MEA for catalyst sam- Ar1 and Ar2 is probably because of smaller particle size ples was fabricated and tested in single cell. MEA per- and homogeneous distribution of platinum nano particles. formance is reflected in its polarization curve, a plot of cell To calculate the electrode kinetic parameters, the IR voltage versus current density. Figure 8 shows the com- corrected cell polarization curves were taken into account parative fuel cell performance with the different catalyst as shown in Fig. 9. This helps us to remove the IR con- samples. The peak power density achieved while employ- tribution coming from the membrane and/or electrolyte so ing Ar1, Ar2, and Ar3 as catalyst was found to be 73, 92, that pure catalyst performance can be evaluated. The tafel -2 and 156 mWcm , respectively. plots are shown in the inset while the data are summarized Table 3 Electrochemical kinetic parameters for the unit PEM fuel cells using catalyst samples Ar1, Ar2, and Ar3 Sample E (V) j (A/cm ) b (V/decade) a 0 0.9 Ar1 0.906 1.21 0.1220 ± 0.0121 0.1052 Ar2 1.151 60.61 0.1150 ± 0.0106 0.1116 Ar3 1.068 68.15 0.1110 ± 0.0077 0.1156 Fig. 10 Detailed reaction mechanism for reaction occurring in basic medium 123 Mater Renew Sustain Energy (2016) 5:2 Page 9 of 11 2 in Table 3.The valueof j represents the kinetic cur- Conclusions 0.9 rent density (at the cell potential of 0.9 V), while a the charge transfer coefficient is calculated from the slope of MWCNTs platinum nanocomposites have been synthe- the tafel plots. The values of a and j increases for sized as PEM fuel cell catalyst by reflux heating in argon 0.9 samples Ar1 to Ar3 whereas the tafel slopes of the atmosphere under different pH conditions. High viscosity polarization curves decreases, indicating comparative of ethylene glycol is effective in stabilizing the diffusion of feasibility of the reaction with sample Ar3. The kinetics Pt nanoparticles. The catalytic activity of CNT supported of the reaction in alkaline medium can be explained by Pt nanoparticles greatly depends on the particle size and the following probable reaction mechanism. Addition of distribution and related parameters, which in turn is largely OH ions (increasing basicity) along with EG can cat- affected by the pH of the synthesizing medium and the alyze the entire reaction as shown in Fig. 10.Thisis refluxing environment. The I-V performance of unit PEM because OH ions have the tendency to release protons fuel cell shows a peak power density of 156 mW/cm with from EG. catalyst prepared in alkaline medium, which is nearly The OH–(CH ) –O ion thus produced will bind on the 110 % more as compared to that obtained while employing 2 2 CNT surface for stabilizing its charge. These additional catalyst prepared in acidic medium and tested under similar OH ions will also be available to reduce Pt which can conditions. bind with EG (forming square planar complexes) which is Acknowledgments The authors are grateful to Director, NPL, New already on CNT surface. Thus, the reaction will become Delhi for his support, encouragement and permission to publish the much more feasible as compared to when it is carried out in results. One of the authors CG wants to thank CSIR-SRF for financial acidic medium (where it will be difficult to release proton). support. Evaluation of the fuel cell performance and technical dis- Further coordinate bonds will be stronger leading to sta- cussions by the scientists of Central Electrochemical Research Insti- tute (CSIR), Chennai, are gratefully acknowledged. Thanks are also bility of the catalyst formed. due to Dr. Vidyanand and Singh, Mr. R. K. Seth and Mr. Naval K. However, when we compare the above results (kinetic Upadhyay for carrying out the TEM, TGA and XRD studies. The parameters) with our previous study when the refluxing studies have been carried out under the CSIR, project entitled was carried out in air [15], a decrease in the catalyst ‘‘Development of New Generation Energy Efficient Devices (D- NEED)’’. performance is noted in the present case, i.e., when the reaction was carried out in argon. Since refluxing in air Open Access This article is distributed under the terms of the has a greater probability of increasing the defects (since Creative Commons Attribution 4.0 International License (http:// air has a number of components along with oxygen) as creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give compared to that in an inert atmosphere like argon, it was appropriate credit to the original author(s) and the source, provide a assumed that argon refluxing would reduce the formation link to the Creative Commons license, and indicate if changes were of defects leading to a smooth and uniform surface, made. resultinginanenhancedcatalytic activity of the synthe- sized catalyst. However, our thought was not actualized as there are a References number of possibilities while a substance is in the reacting stage. It is important to note that the catalyst performance 1. 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Appl Catal B: Environ. 88(1),1–24 (2009) Sung, Y.-E.: Effect of platinum amount in carbon supported http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Materials for Renewable and Sustainable Energy Springer Journals

Development of multiwalled carbon nanotubes platinum nanocomposite as efficient PEM fuel cell catalyst

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Abstract

Mater Renew Sustain Energy (2016) 5:2 DOI 10.1007/s40243-015-0066-5 ORIGINAL PAPER Development of multiwalled carbon nanotubes platinum nanocomposite as efficient PEM fuel cell catalyst 1,2 1,2 1 • • Chanchal Gupta Priyanka H. Maheshwari Sanjay R. Dhakate Received: 11 June 2015 / Accepted: 20 December 2015 / Published online: 14 January 2016 The Author(s) 2016. This article is published with open access at Springerlink.com Abstract Multiwalled carbon nanotubes platinum Introduction nanocomposite has been prepared via chemical route by reduction of Pt salt on MWCNTs in ethylene glycol solu- Among the various types of fuel cells, Polymer Electrolyte tion while refluxing in Argon atmosphere. The effect of Membrane (PEM) fuel cells have attracted most of the different pH media during the reduction process on their researchers because of their advantages like low emissions, physical and electrochemical properties, as well as on their low operating temperature and high power density and use performance in unit PEM fuel cell has been studied and of all solid state components, which make them suitable for further compared with the reaction carried out while portable applications [1–4]. Despite various advantages, refluxing in air as already demonstrated by the authors. The the sluggish kinetics of ORR (oxygen reduction reaction) is I-V performance of unit PEM fuel cell shows a peak Power the limiting factor for energy conversion in PEM fuel cells. -2 density of 156 mW cm with catalyst prepared in alkaline This necessitates the use of an efficient electro catalyst, medium, an increase of [ 110 % as compared to wherein Pt or Pt-based alloy catalyst supported on high -2 72 mW cm obtained while employing catalyst prepared surface area carbon black is generally used. Due to the in acidic medium and tested under similar conditions. This amorphous nature of carbon black, it tends to agglomerate is attributed not only to the small particle size of reduced Pt and corrode with repeated fuel cell operation. This not only NPs, but also to its uniform distribution when reduction is increases the cost of the catalyst, but also leads to insuf- carried out in alkaline medium. This has been further ficient durability [5–9]. Therefore, development of effi- explained by detail reaction mechanism under alkaline cient, low cost and highly durable catalyst is the major conditions. concern towards the commercialization of fuel cells. In this context, carbon materials like ordered mesoporous carbons Keywords Carbon nanotubes  Catalyst  Fuel cell  Mid- (OMCs) [10], carbon aerogels [11], carbon nanotubes wave potential  Polarization (CNTs) [5, 12–15], carbon nano-horns (CNHs) [16], car- bon nano-coils (CNCs) [17] and carbon nano fibers (CNFs) [18] have attracted much interest as electro catalyst sup- port. Amongst them CNTs have been considered as the most attractive support material due to its high crys- tallinity, hydrophobicity, high conductivity, chemical inertness and high surface area [19–24]. These properties not only make them suitable for catalyst support, but they & Priyanka H. Maheshwari are increasingly being used in other cell components like hedap@mail.nplindia.ernet.in GDL [25–28], carbon paper [29] and bipolar plates [30]. However, chemically inert nature of CNTs lowers the Physics and Engineering of Carbon, CSIR-National Physical Laboratory, New Delhi, India effective reaction sites for attachment of metal nanoparti- cles. To overcome this, many efforts have been done to Academy of Scientific and Innovative Research (AcSIR), modify the surface of CNTs by chemical functionalization CSIR-NPL Campus, New Delhi, India 123 2 Page 2 of 11 Mater Renew Sustain Energy (2016) 5:2 [31–33]. The chemical oxidation method introduces the refluxing in an inert atmosphere, will control the defect acid functional groups on the surface of CNTs; neverthe- formation and lead to a smooth surface, thereby providing less it introduces irreversibly structural defects, which identical surface sites, resulting in an enhanced catalytic reduces the electrical conductivity and the durability of activity of the nanocomposite formed. CNTs [34, 35].Various treatments including sono-chemical This study gives a detail account of how the varying [36], electrochemical [37], microwave heating [38] and conditions of pH during the reduction process affect the reflux heating [39, 40] have been proposed to incorporate properties of the synthesized nanocomposites and their Pt nano particles onto the CNT surface. corresponding behavior as catalyst for PEM fuel cell. This In the present study we use pristine CNTs to avoid the in turn is supported by characterizations like TGA, TEM, surface oxidation process and we demonstrate the devel- XRD, Raman spectroscopy, XPS and electrochemical opment of Pt/CNT by reduction of chloroplatinic acid via techniques like cyclic voltammetry (CV) and linear sweep refluxing in varying pH media and in argon environment. voltammetry (LSV) and finally the samples have been The above decision was driven by the thought that tested for their performance in unit PEM fuel cell. Materials and methods Development of Pt/CNT nanocomposite Commercially available Nanocyl 7000 MWCNTs with diameter in the range of 20–30 nm and aspect ratio [1000 were used for the preparation of Pt/CNTs. Ethylene glycol and Hexachloroplatic acid (H PtCl .6H O) were procured 2 6 2 from Merc Ltd. and Acros Organics, respectively. Ar gas with 99.9 % purity and double deionized water is used wherever required. Pristine MWCNTs were uniformly dispersed in ethylene glycol by ultra-sonication for *3 h. Solution of 0.01 M H PtCl (chloroplatinic acid) in IPA was added to the 2 6 above drop by drop under constant magnetic stirring such that the ratio of Pt:CNT is 1:4. The pH of the above Fig. 1 X-ray diffraction curves of pristine CNTs and sample Ar1, solution was measured and found to be less than 2. To Ar2, and Ar3 Table 1 Miller indices, d-spacing and crystallite size for the given diffraction angle for samples Ar1, Ar2, and Ar3 Sample Miller Peak position (in d-spacing FWHM (in Crystallite size Average crystallite size of Pt name indices degrees) (nm) degrees) (nm) (nm) Ar1 C(002) 25.67 3.46 4.04 2.01 Pt(111) 39.99 2.25 2.45 3.44 3.62 Pt(200) 46.06 1.96 3.56 2.43 Pt(220) 67.88 1.38 2.28 4.22 Pt(311) 81.70 1.17 2.39 4.39 Ar2 C(002) 25.42 3.51 6.19 1.31 Pt(111) 39.85 2.26 3.43 2.46 2.67 Pt(200) 45.96 1.97 4.15 2.07 Pt(220) 67.57 1.38 3.47 2.75 Pt(311) 81.21 1.18 3.06 3.41 Ar3 C(002) 25.54 3.48 4.66 1.74 Pt(111) 39.82 2.26 3.98 2.12 2.29 Pt(200) 46.24 1.96 4.32 1.99 Pt(220) 67.67 1.38 4.23 2.25 Pt(311) 81.92 3.74 1.18 2.81 123 Mater Renew Sustain Energy (2016) 5:2 Page 3 of 11 2 make the samples of different pH (i.e., 2, 7 and 11) 0.1 M The filtrate was further dried to obtain powdered catalyst. NaOH was added with continuous stirring. This was further Dilute solution of NaBH was added to residue to detect refluxed at 140 C for 3 h in argon. This process of the presence of unreacted platinum. The samples prepared reduction, immobilizes Pt on CNTs forming Pt/CNTs by refluxing the solution in acidic, neutral, and alkaline nanocomposites. The solution was filtered followed by mediums (pH 2, 7, and 11) in argon have been designated washing with copious amount of double de-ionized water. as Ar1, Ar2, and Ar3, respectively. Physical characterization The X-ray diffraction (examination of the samples was performed on Rikagu powder X-ray diffractometer model: XRG 2KW using Cu Ka radiation. The mean crystallite size and lattice parameters were calculated from line broadening and d-spacing measurements using the (002) and (100) reflections. Thermal gravimetric analysis of the electrode samples was carried out on TGA/DSC 1600 by Mettler Todedo. The experiments were carried out in air at the rate of 10 C/min. The structural details of the MWCNT samples were studied with the help of trans- mission electron microscopy (TEM) using Tecnai G2 F30 S-Twin instrument. Raman spectroscopy was carried out using a Renishaw InVia Reflex Micro Raman Spectrometer equipped with the CCD detector at room temperature and Fig. 2 TGA curves of pristine CNTs and sample Ar1, Ar2, and Ar3 Fig. 3 a–c TEM images of Ar3 sample and d particle size distribution curve for Ar3 123 2 Page 4 of 11 Mater Renew Sustain Energy (2016) 5:2 determined experimentally using cyclic voltammetry in N purged 0.5 M HClO solution. Kinetic activity for ORR is measured ex situ with Linear scan voltammetry (LSV) in rotating disk electrode (RDE) apparatus with 1600 rpm -1 rotating rate and 5 mVs sweep rate in O purged 0.5 M HClO solution. For fuel cell performance Toray carbon paper samples of size 25 cm were teflonized and gas diffusion layer -2 (GDL) was prepared by coating 1.5 mg cm of carbon black (Vulcan XC-500) by brush coating technique, fol- lowed by sintering at 350 C for 30 min. The catalyst ink was prepared by mixing the synthesized catalyst with nafion (7 wt% of catalyst). The ink was then brush coated onto the GDL such the amount of catalyst loading is -2 0.3 mg cm Pt. Nafion Membrane—1135 (DuPont) was sandwiched in between the two electrodes (as prepared above) and hot pressed at 120–130 C for 3 min. to make the membrane electrode assembly (MEA). The cells were tested at 60 C and 100 % humidified conditions with the flow rate of hydrogen and oxygen gasses maintained at 200 -1 and 300 mL min , respectively, at atmospheric pressure. Measurements of cell potential with varying current den- sities were conducted galvanostatically using MODEL- LCN4-25-24/LCN 50-24 procured from Bitrode Instru- ments (US). Results and discussion The XRD patterns for Pt/CNTs are shown in Fig. 1. Diffraction peak at * 26 is attributed to graphite crys- Fig. 4 a Raman spectra of pristine CNTs and sample Ar1, Ar2, and tallographic planes (002) of CNT. Pt/CNTs show the Ar3; b HRTEM image of pristine multiwalled carbon nanotube presence diffraction peaks at nearly 40,46, 67.5, and in air. Green laser (excitation line 514 nm) was used to 81 corresponding to (111), (200), (220) and (311) planes excite the samples. One scan per sample was recorded of Platinum, respectively. These are analogous to the face- wherein the samples were exposed to the laser power of centered cubic structure of the noble metal. For all 25mW for 10 s. diffraction peaks the average crystallite size has been cal- culated from FWHM (full width at half maximum) values Electrochemical characterization using Debye–Scherrer equation [41]. Table 1 gives a detailed account of the observed diffraction peaks along The electrochemical measurements were performed using with their corresponding d-value and FWHM. The crys- tallite size of platinum has been calculated as an average of Biologic instrument (VSP model and EC-Lab software) at 25 C. Conventional three electrode system was used for that obtained from different peaks. The average crystallite CV measurements saturated calomel electrode (SCE), Pt size decreases with increase in the pH of the refluxing foil and glassy carbon (GC) as reference electrode, counter medium. This can be explained as follows. At higher pH - - electrode and working electrode, respectively. The catalyst OH ions are available to replace Cl in the hexachloro ink was prepared by dispersing 4.26 mg of catalyst in 1 ml platinic acid since OH ligand has higher field strength ethanol. A drop of 5 % nafion solution was then added to than Cl in the spectro-chemical series. Further, the the above solution and sonicated for 30 min. The working Extended X-ray adsorption fine structure studies (EXAFS) electrode was prepared by drop casting the catalyst ink on confirmed that Pt-OH bond is smaller than Pt–Cl bond [42, the glassy carbon electrode such the platinum loading is 43]. Hence, due to steric contraction effect the complexes 60 lg/cm . Four electrodes per sample were prepared and formed in basic medium is smaller than the complexes formed in acidic medium [44]. tested under similar conditions. The value of ECSA is 123 Mater Renew Sustain Energy (2016) 5:2 Page 5 of 11 2 Fig. 5 CV curves of 4 different electrodes prepared for samples a Ar1, b Ar2, and c Ar3 in N purged 0.5 M HClO 2 4 Figure 2 shows the TGA and DTG curves of the catalyst samples along with that for the pristine CNTs. Fig. 6 CV curves of sample a Ar1, b Ar2, and c Ar3 for 500 cycles TGA for Nanocyl MWCNT without Platinum addition shows a residue about 8.5 % which may be due to the the platinum (i.e., 20 %) was reduced on the CNT presence of catalyst (metal/oxide) used during the CNT surface. As shown by the DTG curves, the thermal stability of synthesis process. For the Pt/CNTs sample prepared in argon atmosphere with different pH medium shows the the nanocomposites reduces as compared to pristine CNTs. This is because, (1) the metal nanoparticles acts as defects residue of *29 %, which indicates that nearly all of 123 2 Page 6 of 11 Mater Renew Sustain Energy (2016) 5:2 and catalyze the oxidation process; and (2) there is a (as shown by the TGA curve); (3) imperfect CNT walls as probability that defects are also introduced in the nanotube shown by the high-resolution TEM image (Fig. 4b). structure during the refluxing process. These results have After incorporation of platinum the I /I value increases D G been further confirmed by Raman spectroscopy. which indicated the stresses induce on the hexagonal TEM micrograph for Pt/CNT nanocomposite in basic structure of carbon. A slight increase in the values of I /I G G medium (Ar3) is shown in Fig. 3 reveals that platinum indicated that Pt incorporation also influences the coupling particles are attached to the outer walls of CNT. Fig- of few CNT layers. ure 3(a) inset shows the presence of closed ends of CNT The CV curves of catalyst samples (Ar1, Ar2, and Ar3) this means during synthesis process the CNTs remains of the four electrodes prepared are shown in Fig. 5a, b, c, highly intact and deposition of platinum particles only respectively, while the curves uptill 500 cycles for one of occurs at the surface of CNTs. The particle size distribution the electrodes are shown in Fig. 6a, b, c, respectively. The (PSD) curve has been plotted for Ar3 for average of 500 curves exhibit typical characteristic of crystalline Pt elec- particles, shows that the mean crystallite size is nearly trodes in the hydrogen and oxygen adsorption–desorption 2.37 nm which is in close agreement with the values regions which illustrates the presence of platinum particles measured from XRD. on the carbon surface. The ECSA of the hydrogen des- Figure 4a shows the Raman spectra for catalysts sam- orption peak calculated from the CV curves gives a mea- ples. The D (defect), G (graphitic) and G (second-order sure of the HOR. ECSA has been calculated for all the harmonic to D) bands are clearly visible. D band is related electrodes prepared for Ar1, Ar2, and Ar3 (as shown in to the defects presents on CNTs, whereas the G band Table 2) and the standard deviation for the four curves of originates from the presence of sp -hybridized carbon sites each sample has been calculated. The average value was present in the sample and hence represents the frequency of found to be maximum for sample Ar3. The increase in carbon–carbon double bond stretching vibration [45]. The ECSA is mainly attributed towards the decrease in crys- G band is the second-order harmonic to D and represents tallite size of the Pt with increase in pH of the refluxing defect in the stacking sequence. The intensities of the medium. Importantly, there is no decrease in the current bands were determined by the area under the spectral with successive cycling (as shown in Fig. 6), which in turn curve. The intensity of the G band (I ) has been used as a is the measure of the durability of the sample, as reported reference in determining the relative intensities of the D elsewhere [46–48]. band (I ) and G band (I ). However, for fuel cells the ORR at cathode side is more D G For pristine CNTs I /I ratio is 1.15 which is quite high critical as it is much slower than the HOR at anode. The D G and indicates the presence of defects. The major contrib- LSV curves for the four different electrodes prepared for utors to the defects are, (1) the large number of pentagons the Pt/CNT nanocomposites are shown in Fig. 7a, b, c. The that are present at the closed ends of the tubes; (2) presence curves show a low value of limiting current for all the of metal oxides used as catalyst for initiating the growth of catalyst samples, probably because of the defects present CNTs which accounts to nearly 8.7 % of the CNT weight on the CNTs [as is clear from the high value of I /I D G Table 2 ECSA, diffusion limiting current density, onset potential, and mid-wave potential calculated from the CV curves for all the electrodes prepared for samples Ar1, Ar2, and Ar3 2 2 Sample ECSA (m /g) DLCD (mA/cm ) Onset potential (V) Mid-wave potential (V) Ar1 17 0.3866 0.7498 0.3224 21 0.4143 0.7784 0.3374 19 0.4338 0.7794 0.2772 20 0.3702 0.7783 0.2575 Ar2 20 0.4163 0.7601 0.4673 20.9 0.4935 0.7748 0.3093 20.5 0.4257 0.7744 0.2856 18.5 0.4850 0.7749 0.2686 Ar3 24 0.2714 0.7696 0.5383 25 0.1860 0.7773 0.4633 25.2 0.3192 0.7821 0.3983 23 0.3659 0.7813 0.3852 123 Mater Renew Sustain Energy (2016) 5:2 Page 7 of 11 2 (Raman data)]which may give rise to unwanted side reac- From the LSV curves the onset potential and mid-wave tions, thus lowering the net useful current. Further, these potentials were measured. The onset potential gives a defects may act as traps for the O and inhibit the diffusion potential at which the ORR initiates and was found to be of the reactant and product species thus lowering the cat- highest for sample Ar3. High activity in terms of onset alytic activity. potential can be attributed to the increase in the electro conductivity of the catalyst [49]; and the increase in the density of the electrochemical active sites due to the enhancement in the surface atoms with a decrease in the particle size and therefore a rise in the probability of early as well as simultaneous reactions, as also stated elsewhere [50, 51]. High mid-wave potential for Ar3 further reflects low binding energy of the catalyst surface with the reac- tants which in turn implies fast catalytic activity as the catalyst surface is quickly available for reducing more O . However, some deviation in the CV and LSV curves and the related parameters have been observed which has been explained later in the text. Fig. 8 Polarization curves for catalyst samples Ar1, Ar2, and Ar3 Fig. 9 IR corrected cell polarization curves for the catalyst samples Ar1, Ar2, and Ar3. Inset shows the tafel plots for activation Fig. 7 LSV curves of 4 different electrodes prepared for samples polarization region sample a Ar1, b Ar2, and c Ar3 in O purged 0.5 M HClO 2 4 123 2 Page 8 of 11 Mater Renew Sustain Energy (2016) 5:2 The actual activity of the catalyst is best judged from its Improved performance of sample Ar3 as compared to fuel cell performance curves. The MEA for catalyst sam- Ar1 and Ar2 is probably because of smaller particle size ples was fabricated and tested in single cell. MEA per- and homogeneous distribution of platinum nano particles. formance is reflected in its polarization curve, a plot of cell To calculate the electrode kinetic parameters, the IR voltage versus current density. Figure 8 shows the com- corrected cell polarization curves were taken into account parative fuel cell performance with the different catalyst as shown in Fig. 9. This helps us to remove the IR con- samples. The peak power density achieved while employ- tribution coming from the membrane and/or electrolyte so ing Ar1, Ar2, and Ar3 as catalyst was found to be 73, 92, that pure catalyst performance can be evaluated. The tafel -2 and 156 mWcm , respectively. plots are shown in the inset while the data are summarized Table 3 Electrochemical kinetic parameters for the unit PEM fuel cells using catalyst samples Ar1, Ar2, and Ar3 Sample E (V) j (A/cm ) b (V/decade) a 0 0.9 Ar1 0.906 1.21 0.1220 ± 0.0121 0.1052 Ar2 1.151 60.61 0.1150 ± 0.0106 0.1116 Ar3 1.068 68.15 0.1110 ± 0.0077 0.1156 Fig. 10 Detailed reaction mechanism for reaction occurring in basic medium 123 Mater Renew Sustain Energy (2016) 5:2 Page 9 of 11 2 in Table 3.The valueof j represents the kinetic cur- Conclusions 0.9 rent density (at the cell potential of 0.9 V), while a the charge transfer coefficient is calculated from the slope of MWCNTs platinum nanocomposites have been synthe- the tafel plots. The values of a and j increases for sized as PEM fuel cell catalyst by reflux heating in argon 0.9 samples Ar1 to Ar3 whereas the tafel slopes of the atmosphere under different pH conditions. High viscosity polarization curves decreases, indicating comparative of ethylene glycol is effective in stabilizing the diffusion of feasibility of the reaction with sample Ar3. The kinetics Pt nanoparticles. The catalytic activity of CNT supported of the reaction in alkaline medium can be explained by Pt nanoparticles greatly depends on the particle size and the following probable reaction mechanism. Addition of distribution and related parameters, which in turn is largely OH ions (increasing basicity) along with EG can cat- affected by the pH of the synthesizing medium and the alyze the entire reaction as shown in Fig. 10.Thisis refluxing environment. The I-V performance of unit PEM because OH ions have the tendency to release protons fuel cell shows a peak power density of 156 mW/cm with from EG. catalyst prepared in alkaline medium, which is nearly The OH–(CH ) –O ion thus produced will bind on the 110 % more as compared to that obtained while employing 2 2 CNT surface for stabilizing its charge. These additional catalyst prepared in acidic medium and tested under similar OH ions will also be available to reduce Pt which can conditions. bind with EG (forming square planar complexes) which is Acknowledgments The authors are grateful to Director, NPL, New already on CNT surface. Thus, the reaction will become Delhi for his support, encouragement and permission to publish the much more feasible as compared to when it is carried out in results. One of the authors CG wants to thank CSIR-SRF for financial acidic medium (where it will be difficult to release proton). support. Evaluation of the fuel cell performance and technical dis- Further coordinate bonds will be stronger leading to sta- cussions by the scientists of Central Electrochemical Research Insti- tute (CSIR), Chennai, are gratefully acknowledged. Thanks are also bility of the catalyst formed. due to Dr. Vidyanand and Singh, Mr. R. K. Seth and Mr. Naval K. However, when we compare the above results (kinetic Upadhyay for carrying out the TEM, TGA and XRD studies. The parameters) with our previous study when the refluxing studies have been carried out under the CSIR, project entitled was carried out in air [15], a decrease in the catalyst ‘‘Development of New Generation Energy Efficient Devices (D- NEED)’’. performance is noted in the present case, i.e., when the reaction was carried out in argon. Since refluxing in air Open Access This article is distributed under the terms of the has a greater probability of increasing the defects (since Creative Commons Attribution 4.0 International License (http:// air has a number of components along with oxygen) as creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give compared to that in an inert atmosphere like argon, it was appropriate credit to the original author(s) and the source, provide a assumed that argon refluxing would reduce the formation link to the Creative Commons license, and indicate if changes were of defects leading to a smooth and uniform surface, made. resultinginanenhancedcatalytic activity of the synthe- sized catalyst. However, our thought was not actualized as there are a References number of possibilities while a substance is in the reacting stage. It is important to note that the catalyst performance 1. 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