Advances in proton exchange membranes in polyelectrolytefuel cells

Abstract

<p indent="0mm">Polyelectrolyte fuel cells (PEFCs) are promising devices for clean power generation in automotive, stationary, and portable applications. As a transition, the proton exchange membrane (PEM) is one of the important components in PEFCs, always promotes H⁺ ion transport, isolates the reaction gas (H₂ and O₂), and prevents electronic transmission between cathodes and anodes. Therefore, the performance of PEMs will directly affect the efficiency and stability of PEFCs, and the PEM with excellent performance is of great significance to PEFCs. Generally, PEMs should have extremely low gas permeability, high ion conductivity, fast water transmission and electrical insulation. To achieve this goal, PEM materials usually consist of three parts: Main chains, fixed groups, and ion exchange groups. Ion exchange groups provide faster ion transmission, while main chains and fixed groups endow PEMs with excellent chemical and mechanical stability for more stable mass transport in proton exchange membrane fuel cells (PEMFCs). Nowadays, due to better microphase separation (ordered arrangement between hydrophobic backbones and hydrophilic side chains), perfluorosulfonic acid (PFSA) membranes are the most widely used and commercialized PEMs. Moreover, to reduce the cost of PEMs, fluorine-containing PEMs and non-fluorine PEMs have been well developed. Specifically, the fluorine-containing PEMs with better microphase separation, fabricated by Suzuki-Miyaura coupling reactions or superacid catalysis methods, can provide high H⁺ conductivity and low cost. Surprisingly, some research has broken through the original H⁺ ion transport mechanism (Vehicular and Grotthuss mechanisms) and effectively improved the H⁺ ion transport rate by the modification of ion channels, such as polyrotaxane prepared via host-guest interactions, COFs-PEMs and PIMs-PEMs. This review mainly summarizes the research advances in PEMs in the past two decades. It starts from the principle of PEFCs and the important role of PEMs in PEFCs. The structural characteristics and mass transport characteristics of PEMs are then investigated while emphasizing the influence of molecular structures on the performance of PEFCs, mainly including PFSAPEMs, fluorine-containing PEMs and non-fluorine PEMs, as well as the recently developed new generation PEMs (polyrotaxane, COFs-PEMs and PIMs-PEMs). Meanwhile, the development and challenges of high-temperature proton exchange membranes (HT-PEMs) are introduced. HT-PEMs can effectively extend the operating temperature range of PEMFCs. HT-PEMFCs have attracted more attention due to several advantages, including improved electrode reaction kinetics, enhanced tolerance to fuel/air impurities, simple plate design, and better heat/water management. These advantages may mean the potential for using low Pt metal loading or even Pt metal free catalysts. Finally, according to the study of PEMs in recent years, this review looks forward to and predicts the direction of low-cost, high-performance PEMs in future. Up to now, although PEMs have been well developed, some performances are still not enough for the harsh environment during fuel cell operation, such as poor resistance to free radical, trade-off effects between cost and performance, and so on. Therefore, the development of PEMs should not only improve the H⁺ ionic conductivity, but also enhance the synergy between PEMs and other components in PEMFCs. Future research should focus on the performance of PEMs in the actual operating environment of PEMFCs.