P. Sridhar

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.

A new mixed-matrix membrane for DMFCs

A new mixed-matrix membrane based on stabilized phosphotungstic acid (PTA) incorporated to chitosan (CS)-hydroxy ethyl cellulose (HEC) for application in direct methanol fuel cells (DMFCs) is reported. Membranes are characterised using Fourier Transform Spectroscopy (FTIR), Thermo-Gravimetric Analysis (TGA), Scanning Electron Microscopy (SEM) and their mechanical properties are evaluated. The PTA content in the CS-HEC blend and its influence on proton conductivity, water/methanol sorption, and methanol cross-over in the DMFC is studied. The DMFC with 3 wt. % stabilized PTA-CS-HEC mixed-matrix membrane delivers peak power-density of 58 mW/cm2 at a load current-density of 210 mA/cm2 with a lower methanol cross-over than that observed for a DMFC operating with a Nafion membrane electrolyte.

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.

Sodium-alginate-based proton-exchange membranes as electrolytes for DMFCs

Novel mixed-matrix membranes prepared by blending sodium alginate (NaAlg) with polyvinyl alcohol (PVA) and certain heteropolyacids (HPAs), such as phosphomolybdic acid (PMoA), phosphotungstic acid (PWA) and silicotungstic acid (SWA), followed by ex-situ cross-linking with glutaraldehyde (GA) to achieve the desired mechanical and chemical stability, are reported for use as electrolytes in direct methanol fuel cells (DMFCs). NaAlg-PVA-HPA mixed matrices possess a polymeric network with micro-domains that restrict methanol cross-over. The mixed-matrix membranes are characterised for their mechanical and thermal properties. Methanol cross-over rates across NaAlg-PVA and NaAlg-PVA-HPA mixed-matrix membranes are studied by measuring the mass balance of methanol using a density meter. The DMFC using NaAlg-PVA-SWA exhibits a peak power-density of 68 mW cm(-2) at a load current-density of 225 mA cm(-2), while operating at 343 K. The rheological properties of NaAlg and NaAlg-PVA-SWA viscous solutions are studied and their behaviour validated by a non-Newtonian power-law.

A DMFC with Methanol-Tolerant-Carbon-Supported-Pt-Pd-Alloy Cathode

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, XPS, FESEM, EDAX and TEM techniques. The optimum atomic weight ratio for Pt to Pd in the carbon-supported alloy catalyst as established by linear-sweep voltammetry (LSV) and cell polarization studies is found to be 2:1. A direct methanol fuel cell (DMFC) employing carbon-supported Pt-Pd (2:1) alloy (Pt-Pd/C) catalyst as the cathode catalyst delivers a peak-power density of 115 mW/cm(2) at 70 degrees C as compared to peak-power density of 60 mW/cm(2) obtained with the DMFC employing carbon-supported Pt (Pt/C) catalyst operating under similar conditions. In the literature, DMFCs operating with Pt-TiO2 (2:1)/C and Pt-Au (2:1)/C methanol-tolerant cathodes are reported to exhibit maximum ORR activity among the group of these methanol-tolerant cathodes with varying catalysts compositions. Accordingly, the present study also provides an effective route to design methanol-tolerant-oxygen-reduction catalysts for DMFCs

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

Mesoporous Carbon and Poly(3,4-ethylenedioxythiophene) Composite as Catalyst Support for Polymer Electrolyte Fuel Cells

In situ polymerization of 3,4-ethylenedioxythiophene with sol-gel-derived mesoporous carbon (MC) leading to a new composite and its subsequent impregnation with Pt nanoparticles for application in polymer electrolyte fuel cells (PEFCs) is reported. The composite exhibits good dispersion and utilization of platinum nanoparticles akin to other commonly used microporous carbon materials, such as carbon black. Pt-supported MC-poly(3,4-ethylenedioxythiophene) (PEDOT) composite also exhibits promising electrocatalytic activity toward oxygen reduction reaction, which is central to PEFCs. The PEFC with Pt-loaded MC-PEDOT support exhibits 75% of enhancement in its power density in relation to the PEFC with Pt-loaded pristine MC support while operating under identical conditions. It is conjectured that Pt-supported MC-PEDOT composite ameliorates PEFC performance/durability on repetitive potential cycling

Directed Synthesis of MC-PEDOT Composite Catalyst-Supports for Durable PEFCs

Mesoporous carbon (MC)-Poly (3,4-ethylenedioxythiophene) (PEDOT) composites are synthesized using structure-directing agents and explored as catalyst supports for polymer electrolyte fuel cell (PEFC) electrodes. To this end, platinum nanoparticles are deposited onto the composite supports as also on Vulcan XC-72 carbon black from platinum salts by formaldehyde reduction. The morphologies and crystallinity of Pt/Vulcan XC-72 and various Pt/PEDOT-MC are characterized using powder X-ray diffraction and transmission electron microscopy, which suggest Pt nanoparticles to be uniformly dispersed onto the supports. The durability of MC-PEDOT-supported catalysts in PEFCs is attributed to enhanced corrosion-resistance of MC. Indeed, the non-destructive functionalization of MC with conducting polymer makes them promising catalyst-supports for PEFCs.

Modified-Pore-Filled-PVDF-Membrane Electrolytes for Direct Methanol Fuel Cells

The polyvinylidene fluoride (PVDF) membrane is modified by the chemical etchant-route employing a sodium naphthalene charge-transfer complex followed by impregnation with Nafion ionomer or polyvinyl alcohol (PVA)-polystyrene sulfonic acid (PSSA) polymeric blend solutions by a dip-coating technique to form pore-filled-membrane electrolytes for application in direct methanol fuel cells (DMFCs). The number of coatings on the surface-modified PVDF membrane is varied between 5 and 15 and is found to be optimum at 10 layers both for Nafion and PVA-PSSA impregnations for effective DMFC performance. Hydrophilicity of the modified-membrane electrolytes is studied by determining average contact angle and surface-wetting energy. Morphology of the membranes is analyzed by a cross-sectional scanning electron microscope. The modified PVDF membrane electrolytes are characterized for their water-methanol sorption in conjunction with their mechanical properties, proton conductivity, and DMFC performance. Air permeability for the modified membranes is studied by a capillary-flow porometer. Methanol crossover flux across modified-PVDF-membrane electrolytes is studied by measuring the mass balance of methanol using a density meter. DMFCs employing membrane electrode assemblies with the modified PVDF membranes exhibit a peak power-density of 83 mW/cm(2) with Nafion impregnation and 59 mW/cm(2) for PVA-PSSA impregnation, respectively. Among the membranes studied here, stabilities of modified-pore-filled PVDF-Nafion and PVDF-PVA-PSSA membranes with 10-layers coat are promising for application in DMFCs

Carbon supported Pt–Sn/SnO2 anode catalyst for direct ethanol fuel cells

Binary Pt–Sn/SnO2–C electro-catalysts comprising Pt and Sn in varying weight ratio, namely 31 : 9, 33 : 7 and 35 : 5, were synthesized by an alcohol-reduction process using ethylene glycol as solvent and reducing agent. The electro-catalysts were characterized by XRD, XPS, TEM, SEM-EDAX, ICP-OES, Cyclic Voltammetry (CV), chronoamperometry and CO stripping techniques. XRD spectra reveal shifting of Pt diffraction peaks to lower angles with the addition of Sn compared with Pt–C and also the presence of SnO2. XPS results also confirm the presence of Sn in the form of PtSn alloy and in the form of SnO2 phase in the catalyst. The effect of composition towards electro-oxidation of ethanol has been studied by the CV technique. The direct ethanol fuel cells (DEFCs) with Pt–Sn/SnO2–C anode catalyst with reduced Pt loading exhibits an enhanced peak power density of 27.0 mW cm−2 while a peak power-density of only 2.2 mW cm−2 is obtained for the DEFC employing Pt–C at 90 °C.