Binder Engineering for Phosphoric Acid Distribution and Retention in High-Temperature Proton Exchange Membrane Fuel Cell Cathodes.

High-temperature proton exchange membrane fuel cells (HT-PEMFCs) offer compelling advantages for energy conversion, including simplified water and thermal management and high tolerance to fuel impurities. In these systems, phosphoric acid (PA) serves as the primary proton conductor, enabling anhydrous proton transport. However, cathode performance and durability remain constrained by sluggish oxygen reduction kinetics and the challenge of controlling PA distribution and retention. Uneven acid distribution, manifesting as local flooding that blocks gas pathways and covers active sites or local depletion that disrupts proton-conducting networks, causes severe mass-transport losses and performance decay. Concurrently, gradual PA migration and loss during operation progressively degrade proton conduction pathways, accelerating voltage decay. Recent studies have established cathode binder engineering as an effective strategy to regulate both PA distribution and retention. This review systematically examines the design of cathode binder systems-encompassing polymer binder optimization and functional additive integration-and the characterization and diagnostic frameworks for evaluating their efficacy in PA management. We further integrate advanced PA characterization techniques with a comprehensive electrochemical performance evaluation framework. By establishing explicit connections among binder design, PA distribution and retention behavior, and device-level performance assessment, this review provides an instructive guide for the rational development of high-performance, durable HT-PEMFC cathodes.