Santoshkumar D. Bhat

Pervaporation Separation Using Sodium Alginate and Its Modified Membranes—A Review

Abstract Sodium alginate (NaAlg) and its modified forms have been widely used as membranes in pervaporation (PV) separation of aqueous‐organic solutions because of the hydrophilic nature and versatility to modify/tune their structures to achieve the desired separation. A survey of the literature indicates that in recent years, NaAlg‐based membranes have been reported to perform outstanding separation characteristics in dehydrating the aqueous‐organic mixtures. In view of the availability of extensive data on the PV performance of NaAlg and its modified membranes, particularly in separating water‐organic mixtures, we thought of compiling the available literature PV data on NaAlg membranes with an aim to assess their performances over other membranes. The present review addresses the application of PV technique using NaAlg membranes for dehydration of organic liquids of different kinds. Literature data on NaAlg‐based membranes such as grafted NaAlg, blends of NaAlg, hybrid composites of NaAlg and mixed matrix membranes of NaAlg are widely scattered. Hence, this review compiles and critically evaluates such data. Literature data on modified NaAlg including blends, grafts, filled matrices, composites, etc., are discussed with respect to a variety of aqueous‐organic mixtures. Factors such as effect of feed composition, membrane swelling, flux and selectivity are assessed with reference to solvents like ethanol, isopropanol, tetra‐hydrofuran, 1,4‐dioxane, acetic acid, and acetonitrile.

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.

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.

Impact on the ionic channels of sulfonated poly(ether ether ketone) due to the incorporation of polyphosphazene: a case study in direct methanol fuel cells

Blend membranes are fabricated from sulfonated poly(ether ether ketone) (SPEEK) and poly[bis(phenoxy)phosphazene] (POP). The effect of POP content on the distribution of ionic channels is investigated by atomic force microscopy (AFM). The water uptake and methanol permeability for the blend membranes are also investigated. The blend membranes are characterized in terms of their thermal and mechanical properties in conjunction with their ionic conductivity. The proton conductivity of the blend membranes slightly decreased with increasing POP content in comparison with the pristine SPEEK membrane. The hydrophobic nature of POP blocks the ionic channels in the SPEEK matrix, subsequently decreasing its water uptake and methanol permeability. The blend membranes showed higher power density compared to a pristine SPEEK membrane in direct methanol fuel cells (DMFCs).

Nitrogen functionalized graphite nanofibers/Ir nanoparticles for enhanced oxygen reduction reaction in polymer electrolyte fuel cells (PEFCs)

Nitrogen functionalization of graphite nanofibers (N-GNF) was performed using hexa methyl tetra amine (HMTA) as the nitrogen source and used as a support material for metal nanoparticle deposition. The successful incorporation of nitrogen was confirmed using X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy analysis. Iridium (Ir) nanoparticles with a particle size of ∼2.2 nm were deposited onto N-GNF by a simple ethanol reduction method. The oxygen reduction reaction (ORR) activity of N-GNF and the ameliorating effect of ORR on Ir deposited N-GNF (Ir/N-GNF) were studied by various physicochemical and electrochemical methods. The enhancement of ORR activity for Ir/N-GNF was evidenced by high onset potentials and mass activities. The presence of nitrogen in the Ir/N-GNF catalyst facilitates quick desorption of the –OH species from the Ir surface and accelerates the electrochemical reaction of Ir particles which in turn enhances the ORR activity. The electrochemical stability of the Ir/N-GNF was investigated by repeated potential cycling up to 2500 cycles and was found to have excellent stability for ORR activity. The PEFC with Ir/N-GNF catalyst delivers a peak power density of 450 mW cm−2 at a load current density of 1577 mA cm−2, while the PEFC with Ir/GNF catalyst delivers a peak power density of only 259 mW cm−2 at a load current density of 1040 mA cm−2 under identical operation conditions.

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

Dendrimer confined Pt nanoparticles: electro-catalytic activity towards the oxygen reduction reaction and its application in polymer electrolyte membrane fuel cells

Dendrimers have shown as a promising candidate in controlling the size, shape and avoid agglomeration of the metal nanoparticles. Platinum nanoparticles are stabilized by fourth generation amine terminated poly(amidoamine) (G4-PAMAM) dendrimers and anchored successively onto carbon by two methods, namely ester functionalization (Pt-DENs/Cest) and active anhydride functionalization (Pt-DENs/Canh) as the cathode catalyst in polymer electrolyte membrane fuel cells (PEMFCs). The effects of pH and reaction time on the complexation reaction are examined by the UV-vis spectroscopic technique. The electronic and structural features of nanoparticles are studied by X-ray photoelectron spectroscopy (XPS), the X-ray diffraction technique (XRD) and Transmission Electron Microscopy (TEM). Electrochemical behaviour and catalytic activity of the catalysts are studied by cyclic voltammograms (CV) and linear sweep voltammograms (LSV) in N2 and O2 saturated 0.1 M aqueous HClO4. Effect of functionalization is studied and a significant result has been observed. The PEMFC performance of the catalysts synthesized using the dendrimer was compared with the catalyst without the dendrimer. Pt-DENs/Canh show significant performance over Pt/C prepared in the absence of the dendrimer.

Pervaporation separation of water–isopropanol mixtures using silicotungstic acid loaded sulfonatedpoly(ether ether ketone) composite membranes

Composite membranes of sulfonatedpoly(ether ether ketone) (sPEEK) containing nascent and modified silicotungstic acid (STA) nanoparticles were tested for pervaporation (PV) separation of water–isopropanol mixtures. The performance of filler loaded composite membranes was better than that of nascent polymer membranes. The chemically modified STA containing membranes showed improved PV performance compared to those of nascent STA loaded membranes. This effect is attributed to leaching of STA during the PV cycles. The leaching effect was reduced after modification of STA with cesium carbonate. Among all the composite membranes prepared by the solution casting method, the membrane containing 7 wt% modified STA gave the highest PV separation performance compared to those containing 3, 5 and 10 wt% STA. The PV performance of the 7 wt% STA loaded membrane was further tested as a function of feed water composition from 10 to 25 wt% and a temperature range of 30–60 °C. Characterization of the membranes was done by universal testing machine (UTM), scanning electron microscopy (SEM), atomic force microscopy (AFM) and X-ray diffraction (XRD) techniques. The PV results have been explained in terms of sorption–diffusion principles and Arrhenius activation parameters.

Bio-functionalized hybrid nanocomposite membranes for direct methanol fuel cells

Bio-molecules are responsible for rapid proton conduction in biological systems. In view of this, hybrid nanocomposite membranes are prepared by incorporating amino acid functionalized titaninum dioxide (AA–TiO2) bio-hybrid nanoparticles into a polyvinylalcohol (PVA) matrix. Sequential binary cross-linking of PVA with sulfosuccinic acid (SSA) and glutaraldehyde (GA) provides adequate mechanical strength and thermal stability. TiO2 nanoparticles are functionalized with three different amino acids (AAs) namely glycine, phenyl alanine and lysine. AA functionalization to TiO2 improves the interaction and degree of adhesion between organic–inorganic phases leading to a homogenous dispersion when used as an additive in PVA. Interfacial interactions between AA–TiO2 and PVA are confirmed by Fourier transform infrared spectroscopy (FTIR), Scanning Electron Microscopy (SEM), thermogravimetry (TG) and UV-Visible spectral analyses. Elemental mapping of the PVA–LY–TiO2 hybrid nanocomposite membrane confirms the incorporation of AA–TiO2 bio-hybrid nanoparticles within the PVA matrix. The suitability of these membranes as electrolytes for direct methanol fuel cells (DMFCs) has been demonstrated through assessment of proton conductivity and methanol permeability. The zwitterion (dipolar ion) architecture created by the AA–TiO2 bio-hybrid nanoparticles substantially facilitates proton conduction through acid–base pairs analogous to biological systems and simultaneously eases methanol permeation. The optimized membrane PVA–lysine–TiO2 is used as an electrolyte in a DMFC which delivers nearly a two-fold higher peak power density of 58 mW cm−2, compared to the pristine PVA membrane.

A novel multi-walled carbon nanotube (MWNT)-based nanocomposite for PEFC electrodes

A novel nanocomposite comprising MWNTs and mixed-conducting polymeric components (electronic and ionic) is prepared, characterized and investigated as a support for platinum (Pt). Nanocomposite of MWNTs and poly (3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT–PSS) is prepared by in situ polymerization and characterized using Fourier–Transform infrared spectroscopy (FT–IR), thermogravimetric analysis (TGA) in conjunction with scanning electron microscopy (SEM). Atomic force microscopy (AFM) studies are also carried out to characterize the surface topography of MWNTs/PEDOT–PSS nanocomposite. X-ray diffraction (XRD) studies reveal that MWNTs/PEDOT–PSS nanocomposite provides better backbone for the improved dispersion of Pt as evidenced by the reduced Pt crystallite size over MWNTs/PEDOT–PSS nanocomposite compared to MWNTs. Electrochemical characterization studies performed with Pt/nanocomposite and Pt/MWNTs demonstrate the superior catalytic activity of Pt/nanocomposite under reduced Nafion loadings in relation to Pt/MWNTs. It is observed that mixed conducting nanoporous network of MWNTs/PEDOT–PSS composite structure promotes the catalytic activity of Pt by enhancing catalyst utilization.