The optimum degree of sulfonation (DS) for sulfonated poly(ether ether ketone) (SPEEK) membranes is determined by comprehensive characterization results, including proton conductivity, swelling ratio, water uptake, chemical stability, thermal stability, mechanical indicators, and proton exchange membrane fuel cell (PEMFC) performance. The PEMFC with a membrane electrode assembly containing a SPEEK-62 (DS = 62%) membrane realizes the power density of 482.08 mW/cm², surpassing that of commercial Nafion-212 under identical conditions. In the crucial Fenton test for durability, the SPEEK-51 membrane demonstrated outstanding dimensional and chemical stability, with a decomposition time of up to 137 min, far surpassing the durability of SPEEK-62 or other membranes with a higher DS. The results indicate that in comparison to the SPEEK-67 membrane as reported in the literature, SPEEK membranes with a DS = 51~62% hold great potential for future applications in PEMFC, and further modifications of these membranes can be a promising approach to enhance the conductivity while maintaining good chemical stability.

As a type of electrochemical energy storage, the vanadium redox flow battery system (VRFB) is currently one of the most promising large-scale energy storage methods. Nevertheless, the ability to accurately estimate the state of charge (SOC) is one of the critical factors restricting the commercialization of VRFBs. This review summarizes the estimation methods for the SOCs of VRFBs used by scholars in the past 10 years, comprehensively discusses the main factors affecting the accuracy of SOC estimation, and discusses the direct measurement methods, combined with modeling filter estimation methods and data-driven SOC estimation approaches currently investigated by mainstream scholars. Although several recent literature reviews describe the current modeling and estimation methods for VRFBs, there has been relatively little attention paid to the more common equivalent circuit modeling methods and parameter identification approaches. This review mainly focuses on common equivalent circuit model (ECM) modeling methods and filter estimation algorithms using modeling, and it summarizes their advantages and disadvantages. Finally, a description of potential research directions for VRFB modeling and SOC estimation in the future is presented.