Suparna Das

Utilization of Conducting Polymers in Fabricating Polymer Electrolyte Membranes for Application in Direct Methanol Fuel Cells

Search for suitable materials for fabricating polymer electrolyte membranes (PEMs) for application in polymer electrolyte membrane fuel cells, and particularly in direct methanol fuel cells (DMFCs), has been an important field of research for the last several decades. Notable candidates that have emerged from this extensive and seemingly exhaustive research are Nafion®, poly(vinylidenefluoride), sulfonated poly(etheretherketone), poly(benzimidazole), Dow XUS®, Flemion® R, 3P-energy, Aciplex-S®, Gore-Tex®, Gore-Select®, and their different blends, copolymers and interpenetrating networks with compounds, such as poly(hexafluoropropylene), poly(acrylonitrile), poly(styrenesulfonate), and poly(methylmethacrylate). Nevertheless, the objective of achieving a much reduced methanol crossover, while maintaining a substantial level of proton conductivity, has remained by and large elusive. However, a ray of hope has been provided by conducting polymers (CPs), and this has led to a considerable number of researchers to plunge into the exploitation of this possibility. This review focuses on the application of CPs, mainly polyaniline and polypyrrole, as PEM constituents. Detailed comparisons between their functioning, and their respective utility in terms of achieving this objective have been provided. We have also discussed the following critical points: first, the effect of CPs on methanol crossover, proton conductivity, and gas diffusion, and second, thermal stability of CPs in the temperature range within which DMFC operates.

Enhancements of Catalyst Distribution and Functioning Upon Utilization of Conducting Polymers as Supporting Matrices in DMFCs: A Review

Fuel cells (FCs) have evolved as a potential alternative energy harnessing device, with direct methanol fuel cell (DMFC) as one of the front-runners. Although it has achieved significant progress, and is currently getting available in the commercial market; however, from a broader perspective, DMFCs, like most other devices, suffer from certain critical drawbacks. This, in turn, demands considerable progress to be made in order to realize ultimate commercialization, i.e., cheap, reliable, durable, and portable DMFCs with easily accessible fuel. In this respect, one important area of real concern is the DMFC electrodes, consisting of catalysts and catalyst-supporting matrices. Sluggish reaction rates and use of highly expensive and scarce catalysts are two critical drawbacks. Conducting polymers (CPs) have found extensive use in the fabrication of these matrices and have resulted in better dispersion, distribution and anchoring of catalysts, which is important to enhance their reaction efficiencies. This review attempts to summarize the potential contributions of CPs, their critical roles, and possible future trends toward fabricating catalyst-supporting matrices in DMFCs.

Nickel nanocatalysts supported on sulfonated polyaniline: potential toward methanol oxidation and as anode materials for DMFCs

The development of potential anode catalysts and catalyst supporting matrices for application in direct methanol fuel cells (DMFCs) has been an active area of research for the last couple of decades. The conventionally used Pt catalyst suffers from (a) high cost, (b) limited abundance and (c) the catalyst poisoning effect induced by the in situ generated carbon monoxide. In this work, a comparatively less expensive and more abundant Ni metal catalyst [supported on Vulcan carbon, polyaniline (PAni) and partially sulfonated PAni (SPAni)] has been utilized as a potential alternative to the Pt metal catalyst for the oxidation of methanol. SPAni emerged as the best matrix for the deposited Ni catalyst nanoparticles. This combination generated a peak current density of 306 μA cm−2 at +0.57 V. In addition, the Ni/SPAni catalyst produced a higher IF/IB ratio compared to the commercial Pt–Ru/C catalyst. Furthermore, a current density of 135 mA cm−2 (at +0.2 V potential) and a maximum power density of 27 mW cm−2 were obtained at 60 °C upon utilizing Ni/SPAni as the anode catalyst for DMFCs. The results, thus obtained, were better than those obtained for the commercial Pt–Ru/C, as well as, the Ni/C and Ni/PAni catalysts.

Synthesis, Preparation, and Performance of Blends and Composites of π-Conjugated Polymers and their Copolymers in DMFCs

π-conjugated aromatic polymers (π-CPs) are a class of high performance materials used in fabrication of a number of devices. Their use in direct methanol fuel cells (DMFCs) has resulted in improving the performance of this prospective alternative energy harnessing device. This review focuses on these aspects of π-CPs, from fabrication techniques to structure-property relationships and finally performance evaluation and comparison with other potential candidates. Along with an overview of the step-by-step progress that has been made in the last two decades, this review consists of a separate section dedicated to the advancements made in the last five years. This period has seen considerable progress in terms of preparation techniques, such as fabrication of layer-by-layer assembly and core-shell assembly; materials used, such as variety of dopants for π-CPs and sulfonated polymers in case of PEMs and π-CP composites with carbon nanotubes and graphenes in case of catalyst supporting matrices; and surface morphology, such as use of nanofibers, nanotubes, and nanowires. In addition, different polymerization strategies and solubility aspects of π-CPs have also been discussed. All these modifications have resulted in yielding high power and current densities, mass specific activity, stability, durability, and judicious utilization of costly materials, like Pt and Nafion.

Progress in Developments of Inorganic Nanocatalysts for Application in Direct Methanol Fuel Cells

Significant progress has been made in the last few years toward synthesizing highly dispersible inorganic catalysts for application in the electrodes of direct methanol fuel cells. In addition, research toward achieving an efficient catalyst supporting matrix has also attracted much attention in recent years. Carbon black- (Vulcan XC-72) supported Platinum and Platinum-Ruthenium catalysts have for long served as the conventional choice as the cathode and the anode catalyst materials, respectively. Oxygen reduction reaction at the cathode and methanol oxidation reaction at the anode occur simultaneously during the operation of a direct methanol fuel cell. However, inefficiencies in these reactions result in a generation of mixed potential. This, in turn, gives rise to reduced cell voltage, increased oxygen stoichiometric ratio, and generation of additional water that is responsible for water flooding in the cathode chamber. In addition, the lack of long-term stability of Pt-Ru anode catalyst, coupled with the tendency of Ru to cross through the polymer electrolyte membrane and eventually get deposited on the cathode, is also a serious drawback. Another source of potential concern is the fact that the natural resource of Pt and the rare earth metal Ru is very limited, and has been predicted to become exhausted very soon. To overcome these problems, new catalyst systems with high methanol tolerance and higher catalytic activity than Pt need to be developed. In addition, the catalyst-supporting matrix is also witnessing a change from traditionally used carbon powder to transition metal carbides and other high-performance materials. This article surveys the recent literature based on the advancements made in the field of highly dispersible inorganic catalysts for application in direct methanol fuel cells, as well as the progress made in the area of catalyst-supporting matrices.

Pt–Ru/Al2O3–C nanocomposites as direct methanol fuel cell catalysts for electrooxidation of methanol in acidic medium

The development of potential low cost anode catalyst–supporting matrix combinations for application in direct methanol fuel cells (DMFCs) has been an effective area of research till date, primarily due to the slower kinetics of the methanol oxidation reaction and poisoning of the catalyst by carbonaceous species. In this work, we have employed different ratios of Al2O3 and Vulcan carbon (i.e. Al2O3:Vulcan carbon (w/w) = 3:1, 2:1, 1:1) as supporting matrices for a Pt–Ru catalyst. Pt and Ru nanoparticles were deposited from a fixed amount of precursor salts H2PtCl6 and RuCl3, respectively, by employing a NaBH4 reduction method at 80 °C. Pt–Ru/Al2O3–Vulcan carbon exhibited the highest catalytic activity when the weight ratio of Al2O3 to Vulcan carbon was maintained at 2:1. This combination generated a peak current density of 399.6 μA cm−2. Furthermore, the Pt–Ru/Al2O3–Vulcan carbon (2:1) catalyst system produced a current density of 148.92 mA cm−2 at +0.2 V and a maximum power density of 29.78 mW cm−2, using non-humidified air as the catholyte at 60 °C. However, when humidified air was used as the catholyte, a current density of 220.1 mA cm−2 at +0.2 V and a maximum power density of 44.02 mW cm−2 were obtained for the Pt–Ru/Al2O3–C (2:1) anode catalyst.

Polyaniline nanowhiskers induced low methanol permeability and high membrane selectivity in partially sulfonated PVdF-co-HFP membranes

Sulfonated poly(vinylidene fluoride-co-hexafluoro propylene) (SPVdF-co-HFP) and polyaniline (PAni) have shown promise as polymer electrolyte membrane (PEM) materials, especially in direct methanol fuel cells, by virtue of their significantly high ion-exchange capacity (IEC) and membrane selectivity ratio. It was intuited that utilization of PAni nanostructures, with high surface areas, in PEMs can result in further improvement of these attributes. With this objective, positively-charged PAni nanofibers (NFs) were synthesized. Interestingly, when these NFs were exposed to a negatively-charged amphiphile solution for different time extents and followed by washing with an organic solvent, PAni nanowhiskers (NWs) having lengths between 40–120 nm and diameters between 10–40 nm were formed. BET surface area exhibited an increase upon decrease in the aspect ratio of the NFs. Membranes were fabricated using SPVdF-co-HFP as the continuous phase and PAni granules, NFs or NWs as the dispersed phase. SPVdF-co-HFP/PAni NWs and SPVdF-co-HFP/PAni NFs membranes exhibited enhanced IECs, proton conductivities and selectivities compared to SPVdF-co-HFP/PAni granules membrane. However, comparisons between SPVdF-co-HFP/PAni NFs and SPVdF-co-HFP/PAni NWs membranes revealed that the extended conjugation available for the former versus the higher surface area of the latter played a crucial role in determining the uptake and transport properties of the membranes.

Electrocatalytic potential of sulfonated polypyrrole-supported Ni-Ag towards methanol oxidation in acidic medium

Bimetallic Ni-Ag alloy nanostructures with different ratios of Ni and Ag, prepared through a simple co-reduction process from the respective precursor metal salts, were employed as the electrocatalysts toward methanol oxidation reaction in acidic electrolyte. The previously reported superior catalytic performance exhibited by deposited Ni catalyst on sulfonated polypyrrole (SPPy) matrix is encouraging enough to further utilize SPPy as a support matrix and Ni as a primary metal catalyst. In this work, the activity of the synthesized Ni based catalyst was strongly dependent on the composition of the Ni-Ag/SPPy catalyst systems. The best performance was obtained upon utilizing a Ni:Ag ratio of 80:20. For example, this Ni-Ag (80:20)/SPPy catalyst system exhibited a higher area specific current density, a stable current density, a higher IF/IB ratio and a better single cell DMFC performance in acidic electrolyte, than that of the other prepared catalyst systems, and also, the state-of-the-art Pt-Ru on C.

An analysis of electron beam evaporation of SrTiO3 on Si substrates

Thin films have been prepared by electron beam evaporation of strontium titanate (SrTiO3) on bare (111) p-type silicon substrate held at room temperature. The as deposited films were annealed at 700 degrees C in flowing oxygen to compensate for any loss of O from the sample. The as deposited and the annealed samples were analysed by Auger electron spectroscopy (AES). The AES analysis shows that there is no trace of Si present in the bulk of the film and the Si/film interface is fairly sharp. The results are discussed in the light of the usefulness of the e-beam deposition of SrTiO3 for preparation of a buffer layer on an Si substrate for the deposition of high-Tc superconducting materials in thick- and thin-film form.