Ultra-low casting of Pt based nano-ink for electrooxidation of glycerol and ethylene glycol fuels in alkaline medium

Fuel 158 (2015) 659–663

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Ultra-low casting of Pt based nano-ink for electrooxidation of glycerol and ethylene glycol fuels in alkaline medium R.S. Sai Siddhardha a, M.S. Brahma Teja a, P.J. Tejkiran a, Susana Addo Ntim b, P. Sai Siva Kumar a, V. Lakshminarayanan c, Somenath Mitra b, Sai Sathish Ramamurthy a,⇑ a b c

Department of Chemistry, Sri Sathya Sai Institute of Higher Learning, Prashanthi Nilayam, 515134, India Department of Chemistry and Environmental Science, New Jersey Institute of Technology, Newark, NJ 07102, USA Raman Research Institute, C.V. Raman Avenue, Bangalore 560080, India

h i g h l i g h t s

g r a p h i c a l a b s t r a c t

 Ultra-low, binder free casting of Pt

based nano-ink for fuel electrooxidation.  Electrooxidation of industrial by-product such as glycerol and ethylene glycol.  Catalyst has shown 33 fold increased current density than commercial Pt on carbon.  Low onset potentials and high current densities for a facile electrooxidation.  Synergy between platinum nanoparticles & carbon nanotubes during electrooxidation.

a r t i c l e

i n f o

Article history: Received 27 April 2015 Received in revised form 3 June 2015 Accepted 4 June 2015 Available online 13 June 2015 Keywords: Glycerol Ethylene glycol Carbon nanotubes Fuel cells Catalyst Carbon paste electrode

a b s t r a c t For the first time we present an ultra-low casting of Pt based nano-ink on carbon paste electrodes (CPEs) for electrooxidation of glycerol and ethylene glycol fuels in alkaline medium. The platinum loaded carbon nanotube (Pt-CNT) synthesized by a microwave induced reaction has been extensively characterized by microscopic, spectroscopic and thermal gravimetric techniques. Pt-CNT has shown 6 and 33 fold improved activity for the Glycerol electrooxidation reaction (GOR) and ethylene glycol electrooxidation reaction (EGOR) vis-à-vis the commercial Johnson Matthey Pt-C. The low onset potentials and high current densities achieved using this novel nano ink is indicative of its plausible role in fuel cell applications. Ó 2015 Elsevier Ltd. All rights reserved.

1. Introduction Fuel cells have drawn significant attention owing to their portable, mobile and stationary applications [1]. Recently, substantial ⇑ Corresponding author. Tel.: +91 8790314405. E-mail address: [email protected] (S.S. Ramamurthy). http://dx.doi.org/10.1016/j.fuel.2015.06.017 0016-2361/Ó 2015 Elsevier Ltd. All rights reserved.

interest in alcohols as fuel has presented a competing alternative to hydrogen gas [2,3]. Among various alcohols, methanol is a toxic chemical, however, ethanol is non-toxic. The energy density of ethanol furthermore, is greater than methanol (8.01 kW h kg 1 vs. 6.09 kW h kg 1) [4]. However the caveat in the advance of direct ethanol fuel cell (DEFC) is the difficulty in the breaking of the CAC bond for a complete oxidation process at low

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temperatures [5]. The application of DEFC is also limited to countries where there is a huge supply chain. The recent alternatives to these small molecules are the polyols such as glycerol and ethylene glycol (EG). These polyols are non-toxic, with high water solubility, low volatility and low flammability [6]. In terms of their global production, there is an oversupply of glycerol (0.3 USD kg 1 & 2.4 million tons per year) as a byproduct from bio-diesel production [7], whereas EG is mainly produced from heterogeneous hydrogenation of cellulose derivatives [8]. The growing demand for methyl esters as fuel additives has further boosted glycerol production coupled with its plummeting cost [9]. The theoretical energy density of glycerol & EG are also quite high than that of methanol. Interestingly, the catalytic electrooxidation of glycerol & EG is advantageous to be carried out in alkaline medium than in acidic medium due to the less corrosive environment and faster kinetics of the former [10].

Usually, carbon paste electrodes (CPEs) are widely utilized in electrochemistry applications on account of its low cost, ease of fabrication, renewable surface and corrosion resistance than metallic electrodes [11]. These electrodes as such have no catalytic activity, hence significant research has been carried out to modify their surface with suitable nanoscale catalysts. Nanoscale catalysts have drawn wide attention due to their high surface to volume ratio. Multi-walled carbon nanotubes (MwCNTs) in this context are effective supports that are chemically inert and provide large surface area for immobilization of nanoparticles resulting in composites with interesting applications from bio-sensors, plasmonics to catalysis [12–14]. Numerous synthetic routes have been devised to reduce the metal ions on functionalized CNTs (MwCNT-COOH) surface, but many of these methods involve elaborate and expensive procedures [15]. Several other challenges posed by these techniques involve uniform dispersiblity of the nanoparticles over the

Fig. 1. FESEM images of (a) CNT, (b) Pt-CNT, inset: EDAX spectrum, (c) TEM image of Pt-CNT; XPS surface scan spectra of (d)(i) MwCNT, (d)(ii) MwCNT-COOH, (e)(iii) Pt-CNT, (e)(iv) PtCl2.

Fig. 2. Cyclic voltammograms of (a) Pt-CNT and (b) Pt-C in 0.1 N HClO4; scan rate 50 m V s

1

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MwCNTs and high loading of the metal. Over the last decade, we have extensively worked upon the functionalization of MwCNTs and their subsequent metal loading using a microwave induced reaction [16]. In the present study we have synthesized Platinum loaded carbon nanotube (Pt-CNT) by a microwave induced reaction. For the first time we have studied the electrocatalytic oxidation of glycerol & EG on a modified Pt-CNT CPE in an alkaline medium. The study draws a comparison between Pt-CNT as an electrocatalyst vis-à-vis the commercially available Johnson Mathey Pt on carbon (Pt-C). The Pt loading on both the catalysts is cross-linked to their catalytic activity. 2. Materials and method All the used chemicals in this study were of analytical grade. Highly purified water (Millipore) was used throughout the study. The catalyst synthesis, electrode fabrication and procedure for electrocatalytic experimentation were briefed in Supplementary information. 3. Results and discussion The nanotubes are coiled and tubular in structure as presented in scanning electron microscope (SEM) analysis Fig. 1a. The loading of Pt nanoparticles on the MwCNTs has not altered its morphology as seen in Fig. 1b. The energy dispersive X-ray spectrum (EDAX) elemental composition for this composite material has been presented as an inset in Fig. 1b. The nanoparticles are globular and roughly around 10–20 nm in size as inferred from the transmission electron microscopy (TEM) images as demonstrated in Fig. 1c. The SEM and TEM images also reveal the uniform distribution of nanoparticles over the MwCNTs surface. X-ray photoelectron spectroscopy (XPS) has been used to further investigate the surface

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elemental pattern of MwCNTs, MwCNT-COOH and the Pt-CNT. The characteristic carbon 1s component is seen at 284.0 eV for both MwCNT and MwCNT-COOH as presented in Fig. 1d. The Cl (2p3/2) peak at 203.3 eV has been observed only in the PtCl2 salt but not in the Pt-CNT spectrum. This reveals that there is a complete decomposition of the salt during the composite synthesis. In this regard XPS data corroborates the analysis obtained through EDAX spectrum. The possible oxidation states of the Pt are Pt(0), Pt(II) and Pt(IV) with their binding energies 71.1–74.4, 72.8–76.1 and 74.3–77.6 eV corresponding to Pt (4f) binding energies. PtCl2 has exhibited a characteristic Pt (II) peak around 76.0 eV and this has shifted to a lower energy in the case of Pt-CNT suggesting the reduction of the metal ion in the nanocomposite. The thermo gravimetric (TGA) analysis reveals a Pt loading of around 40% on the MwCNT surface as shown in Fig. S1. The Fourier transform infra-red (FT-IR) spectra clearly indicate the functionalization of MwCNTs and their subsequent homogeneous metal loading as shown in Fig. S2. The FT-IR peak at 1576 cm 1 reflects the C@C stretching of the carbon grid. Functionalization of the MwCNTs has resulted in the appearance of a new carbonyl stretching peak at 1716 cm 1 as observed in Fig. S1b. The position of the 1576 cm 1 peak has shifted slightly to a higher wavelength upon metal loading and a new peak at 993 cm 1 confirms the presence of the metals as shown in Fig. S1c. The electrochemical active surface area (ECSA) has been calculated from the charge obtained under the Pt-H desorption curves in the negative potential region as presented in Fig. 2. Generally the charge related to the monolayer adsorption of hydrogen is 210 lC cm 2 [17]. The charge measured under the hydrogen desorption peak around 0.3 V to 0 V in Fig. 2a and b has been used to calculate the ECSA for Pt-CNT and Pt-C. The ECSA for Pt-CNT and Pt-C has been found to be 0.173 & 2.04 respectively for an electrode geometric area of 0.071 cm2. Henceforth, all the electrochemical data has been normalized with respect to their ECSA to

Fig. 3. Scan rate study using Pt-CNT for (a) GOR, (b) EGOR and of Pt-C (c) GOR (d) EGOR.

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Fig. 4. Comparative CV plot between various controls and Pt-CNT for GOR (a) & (b) and for EGOR (c) & (d).

yield current densities. Glycerol electrooxidation reaction (GOR) and ethylene glycol electrooxidation reaction (EGOR) onset potentials and peak potentials have not varied significantly for Pt-CNT as presented in Fig. 3a and b. The scan rate study for Pt-CNT has also followed the Randle–Sevcik equation indicating that the reaction is diffusion controlled. The commercial catalyst Pt-C, however, has not shown an incremental current density at various scan rates for EGOR. Although an extremely small quantity of Pt-CNT (5 lg) has been drop-cast on the electrode (geometric area 0.071 cm2), the catalyst has shown excellent current densities towards GOR and EGOR than any other experimental controls as presented in Fig. 4. The onset potential for electrooxidation of Pt-CNT has been around 0.44 V than Pt-C’s 0.32 V, on account of this, the reaction has been more facile in the former as illustrated in Fig. 4b. The other controls of the experiment such as bare CPE and MwCNT-COOH have negligible catalytic effect in both GOR and EGOR as shown in Fig. 4c and d. Likewise, the onset potential for Pt-CNT for EGOR has been 0.49 V that is more negative than 0.23 V of Pt-C. For a detailed note on the catalytic efficiencies, a comparison between Pt-CNT and Pt-C for GOR and EGOR has been tabulated in Table 1. At a potential of 0.1 V for GOR, Pt-CNT has shown 6 folds higher current density than Pt-C. Likewise for EGOR at the same potential, the current density values are even higher, resulting in 33 fold enhancement over the commercial catalyst. This highlights that for an application at the same potential, the Pt-CNT catalyst has greater ability to oxidize the fuel (glycerol and EG) than the commercial catalyst. To realize this increased performance, we combine several enhancing elements: (i) functionalized MwCNT with high surface area and superior electronic conductivity; (ii) Pt nanoparticles for catalytic turnover of glycerol and EG molecules; (iii) Pt-CNT composite for strong electronic relay with enhanced reaction rates. The synergistic catalyst frame work with an extraordinary aqueous dispersiblity of the nano ink has been achieved through microwave

Table 1 Comparative potentials and current densities: Pt-CNT vis-à-vis Pt-C (j in mA cm Reaction

Catalyst

GOR GOR EGOR EGOR

Pt-CNT Pt-C Pt-CNT Pt-C

Onset potential (V) 0.44 0.32 0.49 0.23

Peak potentials (V) 0.08 0.04 0.15 0.07

2

j-0.4 V

j-0.3 V

j-0.2 V

j-0.1 V

0.20 0.03 1.57 0.02

0.40 0.06 4.88 0.08

0.80 0.13 9.63 0.18

1.23 0.20 9.50 0.29

).

induced reaction that ensures uniform distribution of Pt nanoparticles on the MwCNT surface. An infinitesimal quantity, 5 lg of the Pt-CNT was drop-cast on the electrode surface when compared to 35 mg used in the case of the commercial Pt-C catalyst. This clearly indicates the enhanced catalytic turn over achieved using Pt-CNT in comparison with the commercial Pt-C that requires a large amount (700 times) of catalyst loading. 4. Conclusions In summary, the study has shown an efficient use of industrial by-products such as glycerol and EG. In-situ homogeneous decoration of Pt nanoparticles on functionalized MwCNT was achieved through microwave based synthetic design. Binder free, ultra-low castings of Pt-CNT on corrosion free CPEs exhibited superior catalytic performance in the case of GOR and EGOR. This novel catalytic material can be specifically tuned to wide spectrum applications in the energy sector. Acknowledgements SSR and RSSS acknowledge the funding from DBT-Ramalingaswami fellowship (Sanction Order No. 102/IFD/SAN/1118/2014-15) and

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UGC JRF fellowship (award letter number 2121110003), Govt. of India respectively. Part of this work has been funded by the National Institute of Environmental Health Sciences (NIESH) – United States (Grant number RC2 ES018810). The opinions, findings and conclusions expressed in this manuscript are those of the authors(s) but do not reflect the views of NIESH. Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.fuel.2015.06.017. References [1] Selvaraj V, Vinoba M, Alagar M. Electrocatalytic oxidation of ethylene glycol on Pt and Pt–Ru nanoparticles modified multi-walled carbon nanotubes. J Colloid Interface Sci 2008;322:537–44. [2] Badwal SPS, Giddey S, Kulkarni A, Goel J, Basu S. Direct ethanol fuel cells for transport and stationary applications – a comprehensive review. Appl Energy 2015;145:80–103. [3] Chen Y, Bellini M, Bevilacqua M, et al. Direct alcohol fuel cells: toward the power densities of hydrogen-fed proton exchange membrane fuel cells. ChemSusChem 2015;8:524–33. [4] Habibi E, Razmi H. Glycerol electrooxidation on Pd, Pt and Au nanoparticles supported on carbon ceramic electrode in alkaline media. Int J Hydrogen Energy 2012;37:16800–9. [5] Artem LM, Santos DM, De Andrade AR, Kokoh KB, Ribeiro J. Development of ternary and quaternary catalysts for the electrooxidation of glycerol. Sci World J 2012;2012:6.

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