G. Selvarani

A Sol-Gel Modified Alternative Nafion-Silica Composite Membrane for Polymer Electrolyte Fuel Cells

Nafion-silica composite membranes are fabricated by embedding silica particles as inorganic fillers in perfluorosulfonic acid ionomer by a novel water hydrolysis process. The process precludes the use of an added acid but exploits the acidic characteristic of Nafion facilitating an in situ polymerization reaction through a sol-gel route. The use of Nafion as acid helps in forming silica/siloxane polymer within the membrane. The inorganic filler materials have high affinity to water and assist proton transport across the electrolyte membrane of the polymer electrolyte fuel cell (PEFC) even under low relative humidity (RH) conditions. In the present study, composite membranes have been tested in hydrogen/oxygen PEFCs at varying RH between 100 and 18% at elevated temperatures. Attenuated total reflectance-Fourier transform infrared spectroscopy and scanning electron microscopy studies suggest an evenly distributed siloxane polymer with Si-OH and Si-O-Si network structures in the composite membrane. At the operational cell voltage of 0.4 V, the PEFC with an optimized silica-Nafion composite membrane delivers a peak power density value five times higher than that achievable with a PEFC with conventional Nafion-1135 membrane electrolyte while operating at a RH of 18% at atmospheric pressures.

PVA-PSSA membrane with interpenetrating networks and its methanol crossover mitigating effect in DMFCs

A membrane with interpenetrating networks between poly�vinyl alcohol� �PVA� and poly�styrene sulfonic acid� �PSSA� coupled with a high proton conductivity is realized and evaluated as a proton exchange membrane electrolyte for a direct methanol fuel cell �DMFC�. Its reduced methanol permeability and improved performance in DMFCs suggest the new blend as an alternative membrane to Nafion membranes. The membrane has been characterized by powder X-ray diffraction, scanning electron microscopy, time-modulated differential scanning calorimetry, and thermogravimetric analysis in conjunction with its mechanical strength. The maximum proton conductivity of 3.3 � 10−2 S/cm for the PVA–PSSA blend membrane is observed at 373 K. From nuclear magnetic resonance imaging and volume localized spectroscopy experiments, the PVA–PSSA membrane has been found to exhibit a promising methanol impermeability, in DMFCs. On evaluating its utility in a DMFC, it has been found that a peak power density of 90 mW/cm2 at a load current density of 320 mA/cm2 is achieved with the PVA–PSSA membrane compared to a peak power density of 75 mW/cm2 at a load current density of 250 mA/cm2 achievable for a DMFC employing Nafion membrane electrolyte while operating under identical conditions; this is attributed primarily to the methanol crossover mitigating property of the PVA–PSSA membrane.

Carbon-Supported Pt – TiO2 as a Methanol-Tolerant Oxygen-Reduction Catalyst for DMFCs

Carbon-supported Pt–TiO2 Pt–TiO2/C catalysts with varying at. wt ratios of Pt to Ti, namely, 1:1, 2:1, and 3:1, are prepared by the sol–gel method. The electrocatalytic activity of the catalysts toward oxygen reduction reaction ORR, both in the presence and absence of methanol, is evaluated for application in direct methanol fuel cells DMFCs. The optimum at. wt ratio of Pt to Ti in Pt–TiO2/C is established by fuel cell polarization, linear sweep voltammetry, and cyclic voltammetry studies. Pt–TiO2/C heattreated at 750°C with Pt and Ti in an at. wt ratio of 2:1 shows enhanced methanol tolerance, while maintaining high catalytic activity toward ORR. The DMFC with a Pt–TiO2/C cathode catalyst exhibits an enhanced peak power density of 180 mW/cm2 in contrast to the 80 mW/cm2 achieved from the DMFC with carbon-supported Pt catalyst while operating under identical conditions. Complementary data on the influence of TiO2 on the crystallinity of Pt, surface morphology, and particle size, surface oxidation states of individual constituents, and bulk and surface compositions are also obtained by powder X-ray diffraction, scanning and transmission electron microscopy, X-ray photoelectron spectroscopy, energy dispersive analysis by X-ray, and inductively coupled plasma optical emission spectrometry.

A Durable PEFC with Carbon-Supported Pt – TiO2 Cathode: A Cause and Effect Study

Durability is central to the commercialization of polymer electrolyte fuel cells (PEFCs). The incorporation of TiO2 with platinum (Pt) ameliorates both the stability and catalytic activity of cathodes in relation to pristine Pt cathodes currently being used in PEFCs. PEFC cathodes comprising carbon-supported Pt-TiO2 (Pt-TiO2/C) exhibit higher durability in relation to Pt/C cathodes as evidenced by cell polarization, impedance, and cyclic voltammetry data. The degradation in performance of the Pt-TiO2/C cathodes is 10% after 5000 test cycles as against 28% for Pt/C cathodes. These data are in conformity with the electrochemical surface area and impedance values. Pt-TiO2/C cathodes can withstand even 10,000 test cycles with nominal effect on their performance. X-ray diffraction, transmission electron microscope, and cross-sectional field-emission-scanning electron microscope studies on the catalytic electrodes reflect that incorporating TiO2 with Pt helps in mitigating the aggregation of Pt particles and protects the Nafion membrane against peroxide radicals formed during the cathodic reduction of oxygen

PEFC Electrode with Enhanced Three-Phase Contact and Built-In Supercapacitive Behavior

Hydrous ruthenium oxide, which exhibits both protonic and electronic conduction, is incorporated in the cathode electrocatalyst layer of the membrane electrode assembly for polymer electrolyte fuel cells (PEFCs). The supercapacitive behavior of ruthenium oxide helps realize a fuel cell–supercapacitor hybrid. Platinum (Pt) nanoparticles are deposited onto carbon-supported hydrous ruthenium oxide and the resulting electrocatalyst is subjected to both physical and electrochemical characterization. Powder X-ray diffraction and transmission electron microscopy reflect the hydrous ruthenium oxide to be amorphous and well-dispersed onto the catalyst. X-ray photoelectron spectroscopy data confirm that the oxidation state of ruthenium in Pt anchored on carbon-supported hydrous ruthenium oxide is Ru4+. Electrochemical studies, namely cyclic voltammetry, cell polarization, intrinsic proton conductivity, and impedance measurements, suggest that the proton-conducting nature of hydrous ruthenium oxide helps extend the three-phase boundary in the catalyst layer, which facilitates improvement in performance of the PEFC. The aforesaid PEFC operating with hydrogen fuel and oxygen as oxidant shows a higher power density (0.62 W/cm2 @ 0.6 V) in relation to the PEFC comprising carbon-supported Pt electrodes (0.4 W/cm2 @ 0.6 V). Potential square-wave voltammetry study corroborates that the supercapacitive behavior of hydrous ruthenium oxide helps ameliorate the pulse-power output of the fuel cell.

A PEFC With Pt–TiO2/C as Oxygen-Reduction Catalyst

Carbon-supported Pt-TiO2 (Pt-TiO2/C) catalyst with varying atomic ratio of Pt to Ti, namely, 1: 1, 2: 1, and 3: 1, is prepared by sol-gel method and its electrocatalytic activity toward oxygen-reduction reaction (ORR) is evaluated for the application in polymer electrolyte fuel cells (PEFCs). The optimum atomic ratio of Pt to Ti in Pt-TiO2/C and annealing temperature are established by cyclic voltammetry and fuel-cell-polarization studies. Pt-TiO2/C annealed at 750 degrees C with Pt and Ti in atomic ratio of 2: 1, namely, 750 Pt-TiO2/C (2: 1), shows enhanced electrocatalytic activity toward ORR. It is found that the incorporation of TiO2 with Pt ameliorates both electrocatalytic activity and stability of cathode in relation to pristine Pt cathode, currently being used in PEFCs. A power density of 0.75 W/cm(2) is achieved at 0.6 V for the PEFC with 750 Pt-TiO2/C (2: 1) as compared with 0.62 W/cm(2) at 0.6 V achieved with the PEFC comprising Pt/C as cathode catalyst while operating under identical conditions. Interestingly, carbon-supported Pt-TiO2 cathode exhibits only 6% loss in electrochemical surface area after 5000 potential cycles while it is as high as 25% for Pt/C. DOI: 10.1115/1.4002466]

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