Constructing Straight Polyionic Liquid Microchannels for Fast Anhydrous Proton Transport

A Critical Review of Electrolytes for Advanced Low- and High-Temperature Polymer Electrolyte Membrane Fuel Cells

In the 21st century, proton exchange membrane fuel cells (PEMFCs) represent a promising source of power generation due to their high efficiency compared with coal combustion engines and eco-friendly design. Proton exchange membranes (PEMs), being the critical component of PEMFCs, determine their overall performance. Perfluorosulfonic acid (PFSA) based Nafion and nonfluorinated-based polybenzimidazole (PBI) membranes are commonly used for low- and high-temperature PEMFCs, respectively. However, these membranes have some drawbacks such as high cost, fuel crossover, and reduction in proton conductivity at high temperatures for commercialization. Here, we report the requirements of functional properties of PEMs for PEMFCs, the proton conduction mechanism, and the challenges which hinder their commercial adaptation. Recent research efforts have been focused on the modifications of PEMs by composite materials to overcome their drawbacks such as stability and proton conductivity. We discuss some current developments in membranes for PEMFCs with special emphasis on hybrid membranes based on Nafion, PBI, and other nonfluorinated proton conducting membranes prepared through the incorporation of different inorganic, organic, and hybrid fillers.

Polybenzimidazole Confined in Semi-Interpenetrating Networks of Crosslinked Poly (Arylene Ether Ketone) for High Temperature Proton Exchange Membrane

As a traditional high-temperature proton exchange membrane (HT-PEM), phosphoric acid (PA)-doped polybenzimidazole (PBI) is often subject to severe mechanical strength deterioration owing to the “plasticizing effect” of a large amount of PA. In order to address this issue, we fabricated the HT-PEMs with a crosslinked network of poly (arylene ether ketone) to confine polybenzimidazole in semi-interpenetration network using self-synthesized amino-terminated PBI (PBI-4NH₂) as a crosslinker. Compared with the pristine linear poly [2,2′-(p-oxdiphenylene)-5,5′-benzimidazole] (OPBI) membrane, the designed HT-PEMs (semi-IPN/xPBI), in the semi-IPN means that the membranes with a semi-interpenetration structure and x represent the combined weight percentage of PBI-4NH₂ and OPBI. In addition, they also demonstrate an enhanced anti-oxidative stability and superior mechanical properties without the sacrifice of conductivity. The semi-IPN/70PBI exhibits a higher proton conductivity than OPBI at temperatures ranging from 80 to 180 °C. The HT-PEMFC with semi-IPN/70PBI exhibits excellent H₂/O₂ single cell performance with a power density of 660 mW cm⁻² at 160 °C with flow rates of 250 and 500 mL min⁻¹ for dry H₂ and O₂ at a backpressure of 0.03 MPa, which is 18% higher than that of OPBI (561 mW cm⁻²) under the same test conditions. The results indicate that the introduction of PBI containing crosslinked networks is a promising approach to improve the comprehensive performance of HT-PEMs.

A Critical Review on the Use of Ionic Liquids in Proton Exchange Membrane Fuel Cells

This work provides a comprehensive review on the incorporation of ionic liquid (ILs) into polymer blends and their utilization as proton exchanges membranes (PEM). Various conventional polymers that incorporate ILs are discussed, such as Nafion, poly (vinylidene fluoride), polybenzimidazole, sulfonated poly (ether ether ketone), and sulfonated polyimide. The methods of synthesis of IL/polymer composite membranes are summarized and the role of ionic liquids as electrolytes and structure directing agents in PEM fuel cells (PEMFCs) is presented. In addition, the obstacles that are reported to impede the development of commercial polymerized IL membranes are highlighted in this work. The paper concludes that the presence of certain ILs can increase the conductivity of the PEM, and consequently, enhance the performance of PEMFCs. Nevertheless, the leakage of ILs from composite membranes as well as the limited long-term thermal and mechanical stability are considered as the main challenges that limit the employment of IL/polymer composite membranes in PEMFCs, especially for high-temperature applications.

Ionic Liquid in Phosphoric Acid-Doped Polybenzimidazole (PA-PBI) as Electrolyte Membranes for PEM Fuel Cells: A Review

Increasing world energy demand and the rapid depletion of fossil fuels has initiated explorations for sustainable and green energy sources. High-temperature polymer electrolyte membrane fuel cells (HT-PEMFCs) are viewed as promising materials in fuel cell technology due to several advantages, namely improved kinetic of both electrodes, higher tolerance for carbon monoxide (CO) and low crossover and wastage. Recent technology developments showed phosphoric acid-doped polybenzimidazole (PA-PBI) membranes most suitable for the production of polymer electrolyte membrane fuel cells (PEMFCs). However, drawbacks caused by leaching and condensation on the phosphate groups hindered the application of the PA-PBI membranes. By phosphate anion adsorption on Pt catalyst layers, a higher volume of liquid phosphoric acid on the electrolyte-electrode interface and within the electrodes inhibits or even stops gas movement and impedes electron reactions as the phosphoric acid level grows. Therefore, doping techniques have been extensively explored, and recently ionic liquids (ILs) were introduced as new doping materials to prepare the PA-PBI membranes. Hence, this paper provides a review on the use of ionic liquid material in PA-PBI membranes for HT-PEMFC applications. The effect of the ionic liquid preparation technique on PA-PBI membranes will be highlighted and discussed on the basis of its characterization and performance in HT-PEMFC applications.

A Deep Insight into Different Acidic Additives as Doping Agents for Enhancing Proton Conductivity on Polybenzimidazole Membranes

The use of phosphoric acid doped polybenzimidazole (PBI) membranes for fuel cell applications has been extensively studied in the past decades. In this article, we present a systematic study of the physicochemical properties and proton conductivity of PBI membranes doped with the commonly used phosphoric acid at different concentrations (0.1, 1, and 14 M), and with other alternative acids such as phytic acid (0.075 M) and phosphotungstic acid (HPW, 0.1 M). The use of these three acids was reflected in the formation of channels in the polymeric network as observed by cross-section SEM images. The acid doping enhanced proton conductivity of PBI membranes and, after doping, these conducting materials maintained their mechanical properties and thermal stability for their application as proton exchange membrane fuel cells, capable of operating at intermediate or high temperatures. Under doping with similar acidic concentrations, membranes with phytic acid displayed a superior conducting behavior when compared to doping with phosphoric acid or phosphotungstic acid.

S. S. Polybenzimidazole co-polymers: their synthesis, morphology and high temperature fuel cell membrane properties

Polybenzimidazole (PBI) random co-polymers containing alicyclic and aromatic backbones were synthesized using two different dicarboxylic acids (viz., cyclohexane dicarboxylic acid and terephthalic acid) by varying their molar ratios.

Poly(ionic liquid) composites

This review highlights recent advances in the development of poly(ionic liquid)-based composites for diverse materials applications.

Exploring the Gas-Permeation Properties of Proton-Conducting Membranes Based on Protic Imidazolium Ionic Liquids: Application in Natural Gas Processing

This experimental study explores the potential of supported ionic liquid membranes (SILMs) based on protic imidazolium ionic liquids (ILs) and randomly nanoporous polybenzimidazole (PBI) supports for CH₄/N₂ separation. In particular, three classes of SILMs have been prepared by the infiltration of porous PBI membranes with different protic moieties: 1-H-3-methylimidazolium bis (trifluoromethane sulfonyl)imide; 1-H-3-vinylimidazolium bis(trifluoromethane sulfonyl)imide followed by in situ ultraviolet (UV) polymerization to poly[1-(3H-imidazolium)ethylene] bis(trifluoromethanesulfonyl)imide. The polymerization process has been monitored by Fourier transform infrared (FTIR) spectroscopy and the concentration of the protic entities in the SILMs has been evaluated by thermogravimetric analysis (TGA). Single gas permeability values of N₂ and CH₄ at 313 K, 333 K and 363 K were obtained from a series of experiments conducted in a batch gas permeance system. The results obtained were assessed in terms of the preferential cavity formation and favorable solvation of methane in the apolar domains of the protic ionic network. The most attractive behavior exhibited poly[1-(3H-imidazolium)ethylene]bis(trifluoromethanesulfonyl)imide polymeric ionic liquid (PIL) cross-linked with 1% divinylbenzene supported membranes, showing stable performance when increasing the upstream pressure. The CH₄/N₂ permselectivity value of 2.1 with CH₄ permeability of 156 Barrer at 363 K suggests that the transport mechanism of the as-prepared SILMs is solubility-dominated.

Anisotropic Dye Adsorption and Anhydrous Proton Conductivity in Smectic Liquid Crystal Networks: The Role of Cross-Link Density, Order, and Orientation

In this work, the decisive role of rigidity, orientation, and order in the smectic liquid crystalline network on the anisotropic proton and adsorbent properties is reported. The rigidity in the hydrogen-bonded polymer network has been altered by changing the cross-link density, the order by using different mesophases (smectic, nematic, and isotropic phases), whereas the orientation of the mesogens was controlled by alignment layers. Adding more cross-linkers improved the integrity of the polymer films. For the proton conduction, an optimum was found in the amount of cross-linker and the smectic organization results in the highest anhydrous proton conduction. The polymer films show anisotropic proton conductivity with a 54 times higher conductivity in the direction perpendicular to the molecular director. After a base treatment of the smectic liquid crystalline network, a nanoporous polymer film is obtained that also shows anisotropic adsorption of dye molecules and again straight smectic pores are favored over disordered pores in nematic and isotropic networks. The highly cross-linked films show size-selective adsorption of dyes. Low cross-linked materials do not show this difference due to swelling, which decreases the order and creates openings in the two-dimensional polymer layers. The latter is, however, beneficial for fast adsorption kinetics.

Hierarchical Porous Polybenzimidazole Microsieves: An Efficient Architecture for Anhydrous Proton Transport via Polyionic Liquids

Liquid-induced phase-separation micromolding (LIPSμM) has been successfully used for manufacturing hierarchical porous polybenzimidazole (HPBI) microsieves (42-46% porosity, 30-40 μm thick) with a specific pore architecture (pattern of macropores: ∼9 μm in size, perforated, dispersed in a porous matrix with a 50-100 nm pore size). Using these microsieves, proton-exchange membranes were fabricated by the infiltration of a 1H-3-vinylimidazolium bis(trifluoromethanesulfonyl)imide liquid and divinylbenzene (as a cross-linker), followed by in situ UV polymerization. Our approach relies on the separation of the ion conducting function from the structural support function. Thus, the polymeric ionic liquid (PIL) moiety plays the role of a proton conductor, whereas the HPBI microsieve ensures the mechanical resistance of the system. The influence of the porous support architecture on both proton transport performance and mechanical strength has been specifically investigated by means of comparison with straight macroporous (36% porosity) and randomly nanoporous (68% porosity) PBI counterparts. The most attractive results were obtained with the poly[1-(3H-imidazolium)ethylene]bis(trifluoromethanesulfonyl)imide PIL cross-linked with 1% divinylbenzene supported on HPBI membranes with a 21-μm-thick skin layer, achieving conductivity values up to 85 mS cm^-1 at 200 °C under anhydrous conditions and in the absence of mineral acids.