Ashok Kumar Shukla

Enhanced Methanol Electro-Oxidation on Pt-Ru Decorated Self-Assembled TiO2-Carbon Hybrid Nanostructure

Direct Methanol Fuel Cells (DMFCs) are attractive for portable power applications owing to the easy transportation, storage and refueling of methanol in conjunction with the reduced system-weight, size, highenergy-efficiency and low-temperature operation (1). However, to improve the commercial viability of DMFCs, there are several scientific issues, such as methanol crossover, sluggish electrode kinetics and durability that need to be addressed with concomitant improvements in performance characteristics. One of the problems with the DMFCs is the limited activities of the pure platinum anode catalysts as pure platinum is easily poisoned by carboxylic reaction-intermediates produced during the methanol oxidation reaction (MOR). The use of alloy catalysts such as Pt:Ru has helped mitigating the aforesaid problem substantially. But the CO tolerance of PtRu alloy catalyst is still unsatisfactory for practical DMFC applications (2). Accordingly, it is imperative to further the catalytic activity of Pt-Ru alloy catalyst. It has been reported that the addition of transition metal oxides, such as, CeO2, TiO2, WO3, MoO3, etc., to PtRu alloy catalyst can improve its CO tolerance and activity towards MOR due to the “spillover” effect. Among these metal oxides, TiO2 seems most promising due to its natural abundance, cost and stability in acidic environment. Homogeneously-dispersed composite of PtRu alloy catalyst with TiO2 could be realized by (a) intimate mixing of Pt, Ru, and TiO2 precursor solutions, (b) impregnation and colloidal methods using Pt-Ru/C or TiO2 particles, and (c) physical mixing of PtRu/C with TiO2 particles. Among these colloidal methods and solgel routes are more effective to achieve homogenous nanoscale mixing of the metal and metal oxide phases. However, these methods need pyrolysis at high temperatures that affects the performance of the catalyst. Accordingly, it is desirable to develop an effective synthetic route to realize a homogenous nanocomposite catalyst devoid of any post-heat treatment. In the present study, a porous titanium oxide-carbon hybrid nanocomposite is directly synthesized using a supramolecular self-assembly concept with in situ crystallization process. The microstructure of the catalyst including surface area, morphology and crystallinity are characterized by Brunauer–Emmett–Teller (BET), Transmission electron microscope (TEM), X-ray diffraction (XRD) and Raman spectroscopy. Pt-Ru on titanium oxide-carbon composite is prepared by treating with chloroplatinic acid and ruthenium chloride followed by reduction with NaBH4. The crystalline nature and alloy formation are confirmed by XRD studies, and the morphology and particle-size distribution are studied by TEM. Methanol electro-oxidation and Accelerated Durability Test (ADT) are performed using Cyclic Voltammetry (CV). The catalysts have also been performance tested in DMFCs at 65C using methanol and oxygen. Fig. 1 shows electro-catalytic activities for PtRu/C and Pt-Ru decorated Titanium oxide-carbon towards methanol oxidation reaction. It is clear that titanium oxide-carbon composite supported electrocatalyst exhibit enhanced catalytic activity in relation to Pt-Ru/C. Besides the peak potentials for methanol oxidation are 0.52V and 0.56V for Pt-Ru supported on titanium oxide-carbon composite and Pt-Ru/C, respectively, suggesting that methanol oxidation occurs at a lower potential on titanium oxide-carbon composite supported catalyst in relation to carbon supported catalyst.

A 12 V Substrate-Integrated PbO2-Activated Carbon Asymmetric Hybrid Ultracapacitor with Silica-Gel-Based Inorganic-Polymer Electrolyte

A 12 V Substrate-Integrated PbO2-Activated Carbon hybrid ultracapacitor (SI-PbO2-AC HUCs) with silica-gel sulfuric acid electrolyte is developed and performance tested. The performance of the silica-gel based hybrid ultracapacitor is compared with flooded and AGM-based HUCs. These HUCs comprise substrate-integrated PbO2 (SI-PbO2) as positive electrodes and high surface-area activated carbon with dense graphite-sheet substrate as negative electrodes. 12 V SI-PbO2-AC HUCs with flooded, AGM and gel electrolytes are found to have capacitance values of 308 F, 184 F, and 269 F at C-rate and can be pulse charged and discharged for 100,000 cycles with only a nominal decrease in their capacitance values. The best performance is exhibited by gel-electrolyte HUCs.

Influence of Surface Pre-treatment of MWNTs Support on PEFC Performance

The influence of surface characteristics of multi-walled carbon nnanotubes (MWNTs) support on the catalytic performance of nPEFC electrodes is investigated by using oxidized and nonoxidized nMWNTs as the supports for platinum. The defect-free nmorphology, high electrical conductivity and favorable pore-size ndistribution of non-oxidized MWNTs ameliorate catalytic activity nand electrochemical stability of platinum. Physico-chemical nproperties of oxidized and non-oxidized MWNTs and the nrespective catalysts are studied by BET surface-area, XRD, XPS nand TEM measurements. Electrochemical stability of MWNTssupported nplatinum as PEFC electrodes is assessed using potential ncycling and potentiostatic techniques. Owing to the higher ncorrosion-resistance, platinum on non-oxidized MWNTs show nlower loss in electrochemical surface area (ESA) and also exhibit n22% lower corrosion current than oxidized MWNTs.

Carbon-Supported Pt-Pd Alloy as a Methanol-Tolerant-Oxygen-Reduction Electro-Catalyst for DMFCs

Methanol-tolerant Pt-Pd alloy catalysts supported on to carbon with varying Pt:Pd atomic ratios of 1:1, 2:1 and 3:1 are prepared by a novel wet-chemical method and characterized using powder XRD, FESEM and EDAX techniques. The optimum atomic weight ratio of Pt to Pd in the carbon-supported alloy catalyst as established by linear-sweep voltammetry (LSV) and cellpolarization studies is found to be 2:1. A direct methanol fuel cell (DMFC) employing the carbon-supported Pt-Pd(2:1) alloy catalyst as the cathode catalyst delivers a peak-power density of 115 mW/cm at 70°C as compared to the peak-power density value of 60 mW/cm obtained with the DMFC employing carbon-supported Pt (Pt/C) catalyst operating under similar conditions.

MWNTs-Poly(3,4-ethylenedioxy Thiophene) and Polystyrene Sulphonic Acid Nanocomposite as a Catalyst Support for PEFCs

A nanocomposite of MWNTs and poly(3,4-ethylenedioxythiophene)-poly(styrenesulphonate) is synthesized and characterized using Fourier-Transform Infra-Red spectroscopy (FTIR),Thermogravimetric analysis (TGA) and Scanning electron microscopy (SEM) in conjunction with Atomic force microscopy (AFM). X-ray diffraction (XRD) and electrochemical characterization performed on Pt/nanocomposite and Pt/MWNT reveal superior catalytic activity for Pt/nanocomposite under reduced platinum and Nafion loadings. It is found that mixed conducting nanoporous and inter-connected network of MWNTs and poly (3, 4-ethelendioxythiophene)-poly (styrenesulfonate) promotes the catalytic efficiency of platinum. The study suggests improved dispersion and consequent better catalyst utilization on nanocomposite support comprising MWNT and mixed (electronic and ionic) polymeric conductive components.

Anodic Oxidation of Hydrogen in PEFCs at Varying Platinum Loadings

An optimum Pt loading of 0.05 mg/cm 2 on the anode of PEFC is established through cell polarization and hydrogen-pump experiments. Hydrogen oxidation reaction (HOR) is an important reaction in PEFCs. In relation to oxygen reduction reaction (ORR) occurring on the cathode of the PEFCs, the HOR that takes place on the anode is relatively less investigated. This is because the anode contributes very little to the activation polarization under typical fuel cell operating conditions as HOR has a relatively much larger value of the exchangecurrent density than the ORR [1]. In most of the experimental and theoretical studies on PEFCs, the polarization of the anode has been considered negligible unless the operating current density is high enough to reflect mass-transport polarization effect on the anode. Platinum loading as low as 0.05 mg/cm 2 at the anode of the PEFC stacks is desired for automotive applications [2]. Hence, it is necessary to quantify the performance losses that may arise on reducing anode Pt loading from the present level of about 0.5 mg/cm 2 to 0.05 mg/cm 2 . Furthermore, knowledge of HOR in PEFCs is desired to understand quantitative electrode degradation arising due to local H2 starvation, start-up / shutdown, and cell reversal. The present study mainly focuses on hydrogen oxidation reaction for fuel cell anode with low Pt loading. In order to establish the optimum loading, the polarization curve (Figure 1) is obtained for H2/O2 PEFCs at varying Pt loading at the anode, namely 0.5, 0.25, 0.05 and 0.025mg/cm 2 keeping the cathode Pt loading constant at 0.5 mg/cm 2 . It is observed that PEFCs with Pt catalyst loading of 0.5, 0.25 and 0.05 mg/cm 2 at the anode exhibit almost similar performance. However, the PEFC with anode Pt loading of 0.025 mg/cm 2 shows lower performance, especially in higher current density region due to reduced availability of Pt active sites for HOR. Hydrogen-pump experiments are conducted to determine the performance losses during reduced Pt loading arising due to HOR and hydrogen evolution reaction (HER). From HOR and HER over-potential measurements for varying platinum loading of 0.5, 0.25 0.05 and 0.025 mg/cm 2 , it is clear that 0.05 mg/cm 2 Pt loading is the optimum anode loading. It is established that the performance loss is hardly 30mV at the anode for an operating current density of 1.5 A / cm 2 on reduction of Pt loading from 0.5 to 0.05 mg/cm 2 . These experiments corroborate the polarization data. Though the low-loading of Pt (0.05 mg/cm 2 ) on the anode shows almost identical performance with respect to highly-loaded anode containing 0.5 mg/cm 2 , the long-term operation (durability test) is an important issue for commercialization of PEFCs. Therefore, durability study on the PEFC is conducted with optimum platinum loading of 0.05mg/cm 2 on the anode. Figure 1: Polarization curves (voltage vs. current density) for PEFCs employing anode with varying platinum loading of 0.5, 0.25, 0.05, and 0.025 mg/cm 2 . 1. J. O’ M. Bockris and S. Srinivasan, Fuel cells: Their Electrochemistry, McGraw-Hill (1969). 2. K. C. Neyerlin, Wenbin Gu, Jacob Jorne and Hubert A. Gasteiger, J. Electrochem. Soc., 154 (2007) B631-B635. Acknowledgments Financial support from CSIR, New Delhi through a supra-institutional project during the XI Five Year Plan is gratefully acknowledged. G. Selvarani is grateful to CSIR, New Delhi for a Senior Research Fellowship. 0 50