Improvement of Electrochemical Activity of Pt / MWCNT Catalyst for Proton Exchange Membrane Fuel Cell

In last years, the carbon nanotubes have been studied as an advanced metal catalyst support for proton exchange membrane fuel cell. This study focuses on the sonochemical treatment of multi walled carbon nanotubes (MWCNTs) as a platinum supporting material for proton exchange membrane fuel cell (PEMFC) by mixture of sulfuric acid and nitric acid and mixture of sulfuric acid and hydrogen peroxide. X-ray diffraction (XRD) and Infrared (IR) spectroscopy were used to characterize the surface of sonochemically treated MWCNT and nanostructured electrocatalyst Pt/MWCNT. According to the experimental results of this work, the surface of MWCNT can be more successfully functionalized with hydroxyl and carboxyl groups after sonochemical treatment by mixture of sulfuric acid and nitric acid. The particle size of prepared Pt electrocatalyst on MWCNT was determined 3.4 nm by XRD.


Introduction
arbon nanotubes (CNT) have attracted much attention due to their extraordinary electrical, mechanical and structural properties [1].In recent years, there has been increasing interest in carbon nanotubes as heterogeneous catalyst support.Studies have shown that metal nanoparticles supported on carbon nanotubes may provide much improved catalytic activity [2].
Recently, carbon nanotubes have been proposed as promising support materials for fuel cell catalyst due to their unique characteristics, including high aspect ratio, high electron conductivity, and enhanced mass transport capability [3].The catalytic activity of the Pt based catalysts is strongly dependent on the composition, structure, morphology, particle size, alloying degree [4,5] and catalyst supports [6,7].
In many of these applications, CNTs have to be surface functionalized [8].Among various surface functionalization techniques, oxidation is probably the most widely studied.Oxidation of CNTs has been used to remove amorphous carbon for purification purposes [9] and to open CNT ends for metal nanoparticle insertion [10].Early treatment techniques have involved gas-phase oxidation in air and oxidative plasmas but these techniques have led to an over-oxidation of CNTs, often severely damaging the CNTs and removing the amorphous carbon in addition.
Liquid-phase oxidation involves acidic etching with nitric and/or sulfuric acids.Compared to gas-phase oxidation, this C 20 p 20-23 method is mild and slow, and can provide a high yield of oxidized CNTs.For an amorphous carbon removal and end opening, oxidation damage to the surface of the CNTs is not desired [11].
Noble metals, such as Pt, Ru or Pt-Ru alloys, supported on carbon materials with high surface area are important electrochemical catalysts for PEM fuel cells.It is well known that the catalytic activities highly depend on the size and dispersion of the supported metal nanoparticles as well as the particle interactions with the support substrates [12].
Carbon black, the most common catalytic support, has good electronic conductivity, high surface area, promissing electrochemical performance and stability in acidic environments [13].However, the main reason for using CNTs as support is to reduce Pt loading through increasing the catalyst utilization and improving the catalyst activity.

Experimental
For the experiment MWCNT was taken and its surface was sonochemicaly treated with sulfuric and nitric acid.After the surface treatment, the MWCNT was washed with ethanol a few times.Finally, the slurry was dried in a drying oven at 70 0 C during overnight.After drying it was mixed with100 ml ethylene glycol and the carbon suspension was placed in a three-necked flask.H 2 PtCl 6 •6H 2 O dissolved in ethylene glycol was dropped gradually to the suspension of MWCNTs.The mixture was stirred for 12 hours in additionally.After that, the mixed solution was adjusted with 1M NaOH until the pH reached 12, and stirred again for 1 hour.Before the mixture was heated, a few drop of 5% nafion117 (Sigma-Aldrich) solution was added to adjusted mixture.The mixture was heated at 160 0 C for 3h.Prepared catalyst was washed with ethanol and then dried overnight at the temperature of 80 0 C. The treated catalyst support was characterized by IR method and prepared catalyst was characterized by XRD method.

Results and Discussion
Whether the functional groups were formed or not formed on the surface of multi walled carbon nanotube before and after chemical treatment was determined by infrared spectroscopy.It is shown in Figure 2, IR spectrum of multi walled carbon nanotube after sonochemical treatment with sulfuric acid and hydrogen peroxide mixture.The formation of hydroxyl group was here not so strong.
The sonochemical treatment of MWCNT with sulfuric and nitric acid was shown in figure 3.
The peaks coming in range of 1700-1500cm -1 show that obviously after sonochemical treatment hydroxyl, carbonyl and carboxylat groups were formed.
The bride peak of 3500-3000 cm -1 shows obviously after sonochemical treatment hydroxyl groups were formed and the treatment with sulfuric and nitric acid is more effective than the treatment with mixture of sulfuric acid and hydrogen peroxide.This can be also confirmed by XRD result.In figure 4 it is shown that after treatment with sulfuric and nitric acid the peaks of amorphous carbons are disappeared which occur in range of 2θ=64 0 and 2θ=77.5 0 .After the treatment with sulfuric acid and hydrogen peroxide the peaks of amorphous carbon not completely disappeared so that the treatment with sulfuric and nitric acid gives better result than the treatment with sulfuric acid and hydrogen peroxide.For further experiment which is the preparation of electrocatalyst of Pt/MWCNT sulfuric and nitric acid treated MWCNT was used as a catalyst support.The prepared electrocatalyst of Pt/MWCNT was characterized by XRD.The XRD patterns in Figure 5 show the Pt loading of the catalyst.

Conclusions
Multi walled carbon nanotube was used as a support material for fuel cell catalysts.Its surface modification was carried out by sonochemical treatment with sulfuric and nitric acid also sulfuric acid and hydrogen peroxide.The surface was characterized by IR-spectroscopy and XRD.According to the experimental results of this work, the surface of MWCNT can be more successfully functionalized with hydroxyl and carboxyl groups after sonochemical treatment by mixture of sulfuric acid and nitric acid.Synthesized catalyst of Pt/MWCNT has the Pt particle size of 3.4 nm determined by XRD which is a relatively good result for electrocatalyst of PEMFC.

Fig 1 .
Fig 1. IR spectrum of MWCNT before the chemical treatment In figure 1 it is shown IR spectrum of untreated MWCNT.Before chemical treatment the lines of 3223.05 and 3305.99 in IR spectrum are for NH 2 and OH groups.C-O bonds appeared in range of 1000-1250 sm -1 .Below 1000 aromatic compounds are coming.The carbon nanotube has many aromatic rings with -OH, -NH 2 groups that is shown a multiple structure of compounds.

Figure 2 .Figure 3 .
Figure 2.After treatment of Multi walled carbon nanotube with sulfuric acid and hydrogen peroxide by IR spectroscopy

Fig 5 .
Fig 5. X-ray diffraction patterns of Pt/MWCNT catalysts The patterns clearly show the five characteristic peaks of face centered cubic crystalline Pt namely the 2θ=39.7 0 [111], 46.2 0 [200], 67.4 0 [220], 81.2 0 [311] and [222].The first broad peak is associated with the multi-walled carbon nanotube as support material and on the XRD is not demonstrated the amorphous carbon.The average particle size of Pt was calculated by following equation (1).