Symmetric sponge-like porous polybenzimidazole membrane for high temperature proton exchange membrane fuel cells

Semi-interpenetrating polybenzimidazole membrane containing polymeric ionic liquid with high power density and enhanced proton conductivity for fuel cells

In phosphoric acid (PA)-doped polybenzimidazole (PBI) membranes designed for high-temperature proton exchange membranes (HT-PEMs), increasing the PA doping is essential. Yet, excessive PA doping causes a decline in mechanical strength, which in turn affects the cell performance. We utilize a strategy that integrates elevated PA absorption, increased mechanical strength, and enhanced PA retention. An azide-type ionic liquid (IL) containing double bonds was synthesized and crosslinked with PBI via free radical polymerization reaction. In addition, the IL can also self-polymerize to form long-chain polymeric ionic liquid (PIL). Together, the two structures together form a semi-interpenetrating polymer network (sIPN) system, which has good mechanical properties. The synthesized alkaline ionic liquid can absorb and retain a large amount of PA through acid-base interactions and inter-ionic interactions. Consequently, the proton conductivity of the amino-type polybenzimidazole (AmPBI)-polymeric ionic liquid (PIL)-30 (where 30 stands for the wt% of IL) membrane in an anhydrous environment at 180 °C reached 138.2 mS cm-1. After PA retention test at 160 °C/0 % relative humidity (RH) for 240 h, the proton conductivity reached 99.4 mS cm-1 at 180 °C. The AmPBI-PIL-10 membrane exhibited a significant power density of 635.4 mW cm-2 at 160 °C. The AmPBI-PIL-X composite membranes exhibited exceptional performance.

Autohomogenization of Polybenzimidazole Composites with Enhanced Mechanical Performance by Air Incorporation

Polybenzimidazoles are one of the most thermally and chemically stable polymers due to their rigid chemical structure with π-π stacking and conjugated bonding. Poly(2,5-benzimidazole) (ABPBI), the simplest structure of polybenzimidazole, was synthesized, but the cast film was not homogeneous and featured thick brown areas, which limited their further application. Silica nanospheres were adapted as porogen to generate nanopores in the ABPBI film by successive etching with hydrofluoric acid. As a result of air-composite formation, the ABPBI film became homogeneous and its surface roughness was reduced from 10.0 to 2.5 nm. The obtained air-composite ABPBI film had more favorable mechanical properties than the original film. An air-composite film prepared with 50 wt % silica content had a tensile strength of 128 MPa and an elongation at break of 23%, both of which values were approximately twice as high as the corresponding values of the original film.

Strategies for Mitigating Phosphoric Acid Leaching in High-Temperature Proton Exchange Membrane Fuel Cells

High-temperature proton exchange membrane fuel cells (HT-PEMFCs) have become one of the important development directions of PEMFCs because of their outstanding features, including fast reaction kinetics, high tolerance against impurities in fuel, and easy heat and water management. The proton exchange membrane (PEM), as the core component of HT-PEMFCs, plays the most critical role in the performance of fuel cells. Phosphoric acid (PA)-doped membranes have showed satisfied proton conductivity at high-temperature and anhydrous conditions, and significant advancements have been achieved in the design and development of HT-PEMFCs based on PA-doped membranes. However, the persistent issue of HT-PEMFCs caused by PA leaching remains a challenge that cannot be ignored. This paper provides a concise overview of the proton conduction mechanism in HT-PEMs and the underlying causes of PA leaching in HT-PEMFCs and highlights the strategies aimed at mitigating PA leaching, such as designing crosslinked structures, incorporation of hygroscopic nanoparticles, improving the alkalinity of polymers, covalently linking acidic groups, preparation of multilayer membranes, constructing microporous structures, and formation of micro-phase separation. This review will offer a guidance for further research and development of HT-PEMFCs with high performance and longevity.

Grafting of Amine End-Functionalized Side-Chain Polybenzimidazole Acid-Base Membrane with Enhanced Phosphoric Acid Retention Ability for High-Temperature Proton Exchange Membrane Fuel Cells

A high phosphoric acid uptake and retention capacity are crucial for the high performance and stable operation of phosphoric acid/polybenzimidazole (PA/PBI)-based high-temperature proton exchange membranes. In this work, amine end-functionalized side-chain grafted PBI (AGPBI) with different grafting degrees are synthesized to enhance both the phosphoric acid uptake and the acid retention ability of the accordingly formed membranes. The optimized acid-base membrane exhibits a PA uptake of 374.4% and an anhydrous proton conductivity of 0.067 S cm-1 at 160 °C, with the remaining proton conductivity percentages of 91.0% after a 100 h stability test. The accordingly fabricated membrane electrode assembly deliver peak power densities of 0.407 and 0.638 W cm-2 under backpressure of 0 and 200 kPa, which are significantly higher than 0.305 and 0.477 W cm-2 for the phosphoric acid-doped unmodified PBI membrane under the same conditions.

Recent Progress of Crystalline Porous Frameworks for Intermediate-Temperature Proton Conduction

Proton exchange membranes (PEMs), especially for work under intermediate temperatures (100-200 °C), have attracted great interest because of the high CO toleration and facial water management of the corresponding proton exchange membrane fuel cells (PEMFCs). Traditional polymer PEMs faced challenges of low stability and proton carrier leaking. Crystalline porous materials, such as metal-organic frameworks (MOFs) and covalent organic frameworks (COFs), are promising to overcome these issues contributed by nanometer-sized channels. Herein we summarized the recent development of MOF/COF-based intermediate-temperature proton conductors. The strategies of framework engineering and pore impregnation were introduced in detail for raising proton conductivity. The proton-conducting mechanism was described as well. This spotlight will provide new insight into the fabrication of MOF/COF proton conductors under intermediate-temperature and anhydrous conditions.

Bifunctional Metal-Organic Framework Functionalized by Dimethylamine Cations: Proton Conduction and Iodine Vapor Adsorption

A metal-organic framework, {Zn₃(BTB)₂(μ₃-OH)[(CH₃)₂NH₂](H₂O)}_n (1), was synthesized based on H₃BTB (1,3,5-tri(4-carboxyphenyl)benzene). An AC impedance test proves that 1 has a relatively high conductivity performance of 1.52 × 10^-3 S·cm^-1 at 338 K and 98% RH. The proton conductivity of the composite film 1@CS-9 (CS = chitosan) reaches 1.84 × 10^-1 S·cm^-1 at 328 K and 98% RH. In addition, 1 is discovered to have a good adsorption effect on iodine vapor, and the adsorption capacity reaches 726 mg·g^-1. The multifunctionality caused by dimethylamine cations was investigated for the first time, which has implications for multifunctionality generated by host-guest molecules.

Ultrathin and Super Strong UHMWPE Supported Composite Anion Exchange Membranes with Outstanding Fuel Cells Performance

For high-performance anion exchange membrane fuel cells (AEMFCs), the anion exchange membrane (AEMs) should be as thin as possible to reduce the ohmic resistance. However, the mechanical stability of ultrathin AEMs cannot be guaranteed, as well as a huge risk of gas (H₂ &O₂ ) permeation. In this work, composite AEMs based on ultrahigh molecular weight polyethylene (UHMWPE) are prepared by in situ bulk polymerization. The as-prepared composite membranes can be as thin as 4 µm, and possess super high strength beyond 150 MPa. It also shows extremely low hydrogen permeation, low water uptake, low dimensional swelling, high conductivity, and good alkaline stability. In addition, the fuel cell performance based on the ultrathin composite AEMs exhibits outstanding peak power density of 1014 and 534 mW cm^-2 for H₂ -O₂ and H₂ -Air (CO₂ -free) at 65 °C, respectively, as well as good short-term durability.

Reinforcement of ultrahigh thermoresistant polybenzimidazole films by hard craters

Ultrahigh thermoresistant polybenzimidazole films with uniform pores and hard craters on the surface were prepared by a silica template method. The pore and crater formation enhanced elongation and Young's modulus.

Modifications on Promoting the Proton Conductivity of Polybenzimidazole-Based Polymer Electrolyte Membranes in Fuel Cells

Hydrogen-air proton exchange membrane fuel cells (PEMFCs) and direct methanol fuel cells (DMFCs) are excellent fuel cells with high limits of energy density. However, the low carbon monoxide (CO) tolerance of the Pt electrode catalyst in hydrogen-air PEMFCs and methanol permanent in DMFCs greatly hindered their extensive use. Applying polybenzimidazole (PBI) membranes can avoid these problems. The high thermal stability allows PBI membranes to work at elevated temperatures when the CO tolerance can be significantly improved; the excellent methanol resistance also makes it suitable for DMFCs. However, the poor proton conductivity of pristine PBI makes it hard to be directly applied in fuel cells. In the past decades, researchers have made great efforts to promote the proton conductivity of PBI membranes, and various effective modification methods have been proposed. To provide engineers and researchers with a basis to further promote the properties of fuel cells with PBI membranes, this paper reviews critical researches on the modification of PBI membranes in both hydrogen-air PEMFCs and DMFCs aiming at promoting the proton conductivity. The modification methods have been classified and the obtained properties have been included. A guide for designing modifications on PBI membranes for high-performance fuel cells is provided.