Acidification of Polyphthalocyanine Boosts Solid‐State Proton Battery Performance with High Specific Capacity and Robust Cycling Stability

Abstract Solid‐state proton batteries are regarded as one of the most promising energy storage technologies. However, the shortage of efficient protonic electrolytes significantly hinders their development. In this study, a simple, efficient, and scalable method for synthesizing metal‐free polyphthalocyanine (H 2 PPc) with high purity and crystallinity is presented, which is subsequently integrated with methanesulfonic acid (MeSA) to give a solid protonic electrolyte, MeSA@H 2 PPc, demonstrating great potential for application in proton batteries. Remarkably, MeSA@H 2 PPc shows superior proton conduction exceeding 10 ‒3 S cm ‒1 under ambient conditions, along with excellent long‐term stability and the widest electrochemical stability window (ESW) reported to date for solid protonic electrolytes. More impressively, when used in solid‐state proton batteries, MeSA@H 2 PPc enables excellent rate performance, robust cycling durability, and a record‐high specific capacity, surpassing previously reported solid‐state systems and outperforming many proton batteries based on conventional liquid electrolytes.