INTRODUCTION:

Fuel cell is the source of current and it is a spotless i.e, clean technology. A variety of fuel cells are in different stages of development. Among the different kinds of fuel cells, direct alcohol fuel cells (DAFCs) draw tremendous attention as power sources in several applications at low temperature[1]. Aliphatic alcohols have been promoted as promising fuels for direct alcohol fuel cells (DAFCs) in particular ethanol[2]. This is due to the better energy efficiency, easy handling during storing and transporting. Ethanol is non-toxic. It is an interesting alternative renewable fuel, which can be formed in large quantity by the fermentation process. Moreover, the penetration efficiency of ethanol (EtOH) through the nafion membrane is much lower than that of methanol.

Mechanism of alcohol electro-oxidation on to platinum surface has been studied widely for long time. Although a breakthrough in the understanding of the ethanol electro oxidation mechanism was obtained by techniques like in situ infrared reflectance spectroscopy, in situ IR reflection-absorption spectroscopy, in situ surface-enhanced IR absorption spectroscopy etc [3-4].

Nowadays, special interest is paid to support materials of the catalysts and innovations of the support materials are challenging. Support materials for platinum (Pt) deposition in fuel cell applications are the vital components. Among supports, carbons such as activated carbon, carbon black, graphite material are used. The use of expanded graphite as a catalyst support was also reported [5-7].

The main requirements for carbon supports used for electrochemical applications are: a large surface area, high porosity and good electric conductivity. Platinum catalysts deposited on carbon supports are widely used as electrode materials in proton exchange membrane fuel cells (PEMFC) [8]. Due to the fine dispersion of metallic particles a much larger number of catalytic sites is accessible for reagents than in a corresponding bulk metal.

In this article systematic studies of ethanol electrooxidations onto normal graphite(C) and potassium expanded graphite (K-C) decorated with Pt particles have been reported.

MATERIALS AND METHODS:

Materials

Sulfuric acid (Merck), H₂PtCl₆ (Arrora Matthey Limited), were used as supplied. Ethanol (Bengal Chemicals) were distilled before use.

Electrode preparation

At first, graphite electrode was rubbed with sand paper and then boiled in distilled water for 15 minutes. Before deposition, the graphite surface was electrochemically cleaned by holding at 0.9 V vs. SCE for 30 seconds and then cycled in between -0.2 V to 1.1 V vs. SCE at a scan rate 30 mV s^-1. Depositions were carried out galvanostatically (PAR VersaStat^TMII) with a current density of 5 mA.cm^-2 for 30 minutes from chloroplatinic acid solution (0.01M) in 0.5M H₂SO₄ deposition bath. Pt particles were deposited onto nonexpanded graphite the electrode hereafter referred as Pt/C electrode. The anode was a 2cm² graphite plate obtained from the RandD of BHEL, India. Graphite plates were expanded using Potassium vapour ^[6,9]. Pt particles were deposited onto the expanded graphite electrode under galvanostatic (PAR VersaStatTMII) condition using 5 mA.cm^-2 current for 30 minutes and the electrode hereafter referred as Pt/K-C.

Electrochemical measurement

The catalytic activity of all the deposited electrodes towards EtOH oxidations were investigated by cyclic voltammetry and ampeometry technique. A three electrode setup was constructed for this study, where graphite supported electrodes were the working electrodes, the counter electrode was the Pt foil (1cm²), while a saturated calomel electrode (SCE) served as reference electrode. The electrolytes were 0.5 M sulfuric acid solution (blank), and 0.5 M sulphuric acid containing EtOH (1.0M) solution. The electrooxidations were measured by cyclic voltammetry at different scan rates. Electrochemical measurements were performed using a Potentiostat-galvanostat (VersaStat^TM II, Princeton Applied Research). All the experiments were carried out at 25^oC.

RESULTS AND DISCUSSION:

Figure 1 presents the cyclic voltammograms of ethanol electro oxidations in H₂SO₄ medium at 30 mVs^-1 scan rate. Initial kinetics of these electrodes are much slower followed by a sharp rise in current. Cyclic voltammograms of ethanol solution shows that three oxidation peaks are observed at about 0.7V and at about 1.07V in the forward scan separated by a deep minimum and in the cathodic sweep only one anodic peak ( at about 0.4 V) is observed and this is attributed to renewed oxidation of the fuel. For the electro-oxidation of EtOH on Pt, the occurrence of two peaks on the anodic scan can be attributed to the oxidation of the fuel by two kinds of chemisorbed oxygen species. In the region of first peak, a surface layer of Pt-OH is first formed on Pt and this is subsequently transformed in to a Pt-O layer at higher electrode potential ^[2].

Figure1. Cyclic Voltammograms of Ethanol solution onto (a) Pt deposited K-expanded graphite material and (b) Pt deposited graphite material, Scan rate 30 mv/s.

It is clearly seen from figure 1 that the onset potential for electro-oxidation of EtOH is shifted to lower potential value on the expanded graphite surface (Pt/K-C). Two peak potentials onto (Pt/K-C) surface remarkably shifted to lower potential value compared to Pt/C surface. The anodic peak current of EtOH electro-oxidation onto Pt/K-C surface is also higher than that of Pt/C. All these indicate significant enhancement of catalytic activity of Pt in presence of expanded support material. Thus it is very clear that, though patterns are the same, the overall kinetics in expanded catalyst surface is superior to normal graphite support for ethanol electro-oxidation. This improved catalytic activity may be due to some ion channel formation in the electrode and higher surface area. Actually chemical species (potassium) inserted into interlayer spacings of graphite. The forward peak current ( considering the first peak only) in the anodic sweep and the reversed peak current in the cathodic sweep are compared in table 2, and a significant increase in both the current densities are observed with the expanded supported catalyst compared with the nonexpanded support.

Table:1 Peak currents and tolerance power

Supported mateial	If	Ib	Tolerance power (If/Ib)
Pt/CK	0.0714	0.05	1.4
Pt/C	0.044	0.04	1.1

However peak current on Pt/C-K was 1.5 order of magnitude higher than those obtained incase of Pt/C, which may be due to the higher surface area . It is seen from the table that the tolerance power of expanded material is also higher.

Figure-2. Peak current versus square root of scan rate (a) Pt deposited graphite material and (b) Pt deposited K-expanded graphite material

The current Ip (considering the first peak) increases with the square root of scan rate. Plots of the anodic peak currents of ethanol oxidation (Ip) versus scan rates (V) ranging from30 to 50mVs^-1 gave straight lines (figure 2) and thus obeys the following relationship^[10-13].

Ip =2.985*105 n [(1-á)ná] ^-1/2AD ^1/2 CV ^1/2 (1)

Where Ipa is the anodic peak current (mA), n is the number of electrons involved in the oxidation, A is the area of electrode (cm²), V is the scan rate (Vs^-1), C is

the concentration of the electro active species in bulk solution (mol cm^-3).Thus ethanol electro-oxidation onto both electrodes show irreversibility.

The durability of the electrocatalysts in the reaction medium for alcohol oxidations is one of the major criteria in DAFCs. This can be roughly estimated by amperometric studies (i–t curves). Figure 3 shows Amperometry curves of (a) Pt deposited K-expanded graphite material and (b) Pt deposited graphite material for ethanol oxidation at 0.45 V. Expanded graphite based electrode shows better catalytic activity towards ethanol oxidation.

Figure-3: Amperometric i-t curves of EtOH solution at 0.45 volt onto (a) Pt deposited K-expanded graphite material and (b) Pt deposited graphite material.

CONCLUSION:

In this work, a comparative study has been carried out on the electro-oxidation of ethanol onto graphite supported Pt particles and expanded graphite supported Pt particles. For the electrooxidation of ethanol the appearance of two peaks on the anodic sweep can be ascribed to the oxidation of the fuel by two kinds of chemisorbed oxygen species Cyclic voltammogram (CV), onset potential (Es), peak potential (Ep), peak current (Ip) and amperometry studies have been used to compare the electrocatalytic activity of the electrodes towards alcohol electrooxidations. On the basis of these parameters and amperometry studies it is seen that Pt particles deposited onto potassium expanded graphite act as a better electrode. The improved catalytic effect of expanded electrode is due to e nhanced surface area and nanochannel formation. It is also observed from the studies that alcohol oxidations are irreversible in nature.

ACKNOWLEDGEMENT:

Author is very much thankful to Prof. I. Basumallick and Dr. S. Ghosh for providing Lab facility, chemicals and valuable suggestions.

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Received on 09.02.2017 Modified on 15.02.2017

Asian J. Research Chem. 2017; 10(2):145-148.

DOI: 10.5958/0974-4150.2017.00024.4