Emerging Flexible Sensor Technologies for In-Situ Monitoring in PEMFCs: A Comprehensive Review

Proton exchange membrane fuel cells (PEMFCs) play a crucial role in the transition toward hydrogen-based clean energy systems. However, their performance and durability are highly sensitive to internal environmental conditions, which remain difficult to monitor in real time. Traditional monitoring methods, whether intrusive or non-intrusive, often suffer from limitations such as low spatial and temporal resolution, structural complexity, or incompatibility with stack integration. This comprehensive review highlights the paradigm shift toward emerging flexible sensor technologies as a promising approach for in-situ, multi-parameter monitoring in PEMFCs. We systematically categorize and evaluate both non-intrusive and intrusive sensing strategies, detailing their operating principles, advantages, and limitations. Special emphasis is placed on flexible microsensors, which enable real-time monitoring of temperature, humidity, pressure, gas composition, voltage, and current with minimal structural disruption. We further summarize recent advances of flexible sensors for in-situ monitoring in PEMFCs according to application scenarios. The transition from single-parameter sensing to multi-parameter integration, as well as from single-cell investigations to stack-level applications, is also discussed. Finally, we examine the trade-offs between sensor integration and fuel cell performance, and propose future research directions aimed at improving sensor durability, spatial resolution, and systemlevel integration for intelligent PEMFC management.