Preparation and investigation of block polybenzimidazole membranes with high battery performance and low phosphoric acid doping for use in high-temperature fuel cells

Analysis of proton exchange membranes for fuel cells based on statistical theory and data mining

Fuel cells (FCs) have attracted widespread attention as a highly efficient, clean, and renewable energy conversion technology. Proton exchange membrane (PEM), as one of the core components of FCs, plays a crucial role, and a comprehensive summary of its development is essential for promoting rapid progress in the field of sustainable energy. This article provides a comprehensive review of the development status and research trends of PEMs over the past twenty-eight years, based on statistical analysis and data mining techniques. Price, sustainability, stability, and compatibility issues are the main challenges faced by current PEMs used in FCs research. The current research focuses mainly on the characterization, performance optimization, enhancement mechanisms, and applications of PEMs in FCs. This review provides a systematic summary of PEM materials, serving as a valuable reference for the development, application, and promotion of new PEM materials in FCs.

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.

Metal-coordinated polybenzimidazole membranes with preferential K+ transport

Membranes with fast and selective ion transport are essential for separations and electrochemical energy conversion and storage devices. Metal-coordinated polymers are promising for fabricating ion-conducting membranes with molecular channels, however, the structures and ion transport channels remain poorly understood. Here, we reported mechanistic insights into the structures of metal-ion coordinated polybenzimidazole membranes and the preferential K⁺ transport. Molecular dynamics simulations suggested that coordination between metal ions and polybenzimidazole expanded the free volume, forming subnanometre molecular channels. The combined physical confinement in nanosized channels and electrostatic interactions of membranes resulted in a high K⁺ transference number up to 0.9 even in concentrated salt and alkaline solutions. The zinc-coordinated polybenzimidazole membrane enabled fast transport of charge carriers as well as suppressed water migration in an alkaline zinc-iron flow battery, enabling the battery to operate stably for over 340 hours. This study provided an alternative strategy to regulate the ion transport properties of polymer membranes by tuning polymer chain architectures via metal ion coordination.

Advancements of Polyvinylpyrrolidone-Based Polymer Electrolyte Membranes for Electrochemical Energy Conversion and Storage Devices

Polymer electrolyte membranes (PEMs) play vital roles in electrochemical energy conversion and storage devices, such as polymer electrolyte membrane fuel cell (PEMFC), redox flow battery, and water electrolysis. As the crucial component of these devices, PEMs need to possess high ion conductivity and electronic insulation, remarkable mechanical and chemical stability, and outstanding isolation function for the materials on both sides of the cathode and anode. Polyvinylpyrrolidone has received widespread attention in the research of PEMs owing to its tertiary amine basic groups and exceptional hydrophilic properties. This review focuses on the application status of polyvinylpyrrolidone-based PEMs in PEMFC, vanadium redox flow battery, and alkaline water electrolysis, and describes in detail the key scientific problems in these fields, providing constructive suggestions and guidance for the application of polyvinylpyrrolidone-based PEMs in electrochemical energy conversion and storage devices.

Customized Structure Design and Functional Mechanism Analysis of Carbon Spheres for Advanced Lithium-Sulfur Batteries

Lithium-sulfur batteries (LSBs) are attracting much attention due to their high theoretical energy density and are considered to be the predominant competitors for next-generation energy storage systems. The practical commercial application of LSBs is mainly hindered by the severe "shuttle effect" of the lithium polysulfides (LiPSs) and the serious damage of lithium dendrites. Various carbon materials with different characteristics have played an important role in overcoming the above-mentioned problems. Carbon spheres (CSs) are extensively explored to enhance the performance of LSBs owing to their superior structures. The review presents the state-of-the-art advances of CSs for advanced high-energy LSBs, including their preparation strategies and applications in inhibiting the "shuttle effect" of the LiPSs and protecting lithium anodes. The unique restriction effect of CSs on LiPSs is explained from three working mechanisms: physical confinement, chemical interaction, and catalytic conversion. From the perspective of interfacial engineering and 3D structure designing, the protective effect of CSs on the lithium anode is also analyzed. Not only does this review article contain a summary of CSs in LSBs, but also future directions and prospects are discussed. The systematic discussions and suggested directions can enlighten thoughts in the reasonable design of CSs for LSBs in near future.

Construction of Stable Wide-Temperature-Range Proton Exchange Membranes by Incorporating a Carbonized Metal-Organic Frame into Polybenzimidazoles and Polyacrylamide Hydrogels

Proton exchange membrane fuel cells (PEMFCs) are promising devices for clean power generation in fuel cell electric vehicles applications. The further request of high-efficiency and cost competitive technology make high-temperature proton exchange membranes utilizing phosphoric acid-doped polybenzimidazole be favored because they can work well up to 180 °C without extra humidifier. However, they face quick loss of phosphoric acid below 120 °C and resulting in the limits of commercialization. Herein UiO-66 derived carbon (porous carbon-ZrO₂ ), comprising branched poly(4,4'-diphenylether-5,5'-bibenzimidazole) and polyacrylamide hydrogels self-assembly (BHC1-4) membranes for wide-temperature-range operation (80-160 °C) is presented. These two-phase membranes contained the hygroscopicity of polyacrylamide hydrogels improve the low-temperature proton conductivity, relatively enable the membrane to function at 80 °C. An excellent cell performance of BHC2 membrane with high peak power density of 265 and 656 mW cm^-2 at both 80 and 160 °C can be achieved. Furthermore, this membrane exhibits high stability of frequency cold start-ups (from room temperature to 80 °C) and long-term cell test at 160 °C. The improvement of cell performance and stability of BHC2 membrane indicate a progress of breaking operated temperature limit in existing PEMFCs systems.

Highly Conductive Polybenzimidazole Membranes at Low Phosphoric Acid Uptake with Excellent Fuel Cell Performances by Constructing Long-Range Continuous Proton Transport Channels Using a Metal-Organic Framework (UIO-66)

Phosphoric acid (PA)-doped polybenzimidazoles generally require high PA doping levels to achieve high conductivity as high-temperature proton exchange membranes. However, high PA doping levels result in a significant decrease in the mechanical properties of and PA leaching from the membranes. Herein, a Zr-based metal-organic framework material (UIO-66) was introduced into poly[2,2'-(p-oxydiphenylene)-5,5'-benzimidazole] (OPBI) membranes. The composite membranes exhibited long-range continuous proton transport channels when the mass ratio of UIO-66 to OPBI was ≥30 wt %. The long-range continuous proton transport channels endowed the composite membranes with high proton conductivity at low PA doping levels. When the doping of UIO-66 in the OPBI membrane reached 40 wt %, the membrane exhibited the highest proton conductivity (0.092 S cm^-1, at 160 °C) at a low PA uptake (73.25%), while the conductivity of the pristine OPBI membrane was 0.050 S cm^-1 with a high PA uptake (217.43%). Additionally, in the oxyhydrogen fuel cell test, 40%UIO-66@OPBI membranes exhibited excellent fuel cell performance with a peak power density of 583 mW cm^-2 at 160 °C, which is 50% higher than that of the pristine OPBI membrane (374 mW cm^-2). A single cell based on 40%UIO-66@OPBI also demonstrated good durability and could remain at about 600 mV after 500 h of operation under a constant load of 200 mA cm^-2.

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.

R, M. K, Y. WW, J. P. Recent Progress in the Development of Aromatic Polymer-Based Proton Exchange Membranes for Fuel Cell Applications

Proton exchange membranes (PEMs) play a pivotal role in fuel cells; conducting protons from the anode to the cathode within the cell's membrane electrode assembles (MEA) separates the reactant fuels and prevents electrons from passing through. High proton conductivity is the most important characteristic of the PEM, as this contributes to the performance and efficiency of the fuel cell. However, it is also important to take into account the membrane's durability to ensure that it canmaintain itsperformance under the actual fuel cell's operating conditions and serve a long lifetime. The current state-of-the-art Nafion membranes are limited due to their high cost, loss of conductivity at elevated temperatures due to dehydration, and fuel crossover. Alternatives to Nafion have become a well-researched topic in recent years. Aromatic-based membranes where the polymer chains are linked together by aromatic rings, alongside varying numbers of ether, ketone, or sulfone functionalities, imide, or benzimidazoles in their structures, are one of the alternatives that show great potential as PEMs due totheir electrochemical, mechanical, and thermal strengths. Membranes based on these polymers, such as poly(aryl ether ketones) (PAEKs) and polyimides (PIs), however, lack a sufficient level of proton conductivity and durability to be practical for use in fuel cells. Therefore, membrane modifications are necessary to overcome their drawbacks. This paper reviews the challenges associated with different types of aromatic-based PEMs, plus the recent approaches that have been adopted to enhance their properties and performance.