Lead‐Free, Ultrastable, Tungsten‐Based Ruddlesden–Popper and Hybrid Perovskite Frameworks for Aqueous Pseudocapacitive Charge Storage

The instability of hybrid organic–inorganic perovskites (HOIPs) in several electrolytes and the toxicity of heavy metals such as lead hinder their application in many electrochemical devices. Herein, an already existing Ruddlesden–Popper (R–P) structure of tungstic acid variants as a generic framework to achieve ultrastable HOIPs, serving as stable and safer alternatives to lead‐based HOIPs in aqueous electrochemical devices, is introduced. An enormous improvement (of the tungsten‐based framework) in electrochemical performance is achieved by converting electrochemically sluggish H 2 W 2 O 7 to oxygen‐deficient H 2 W 2 O 7 − δ to leverage a facile and reversible W 6+ → W 5+ transition along with local defect‐mediated H + insertion/extraction. This local structural modification results in a remarkable pseudocapacitive performance (specific capacitance of ≈622 F g −1 or specific capacity 155.5 mAh g −1 at 64 C) with no observable capacity fade (≈100% specific capacity retention after thousands of cycles) in 0.5 m H 2 SO 4 aqueous solution. To extend the scope of utilization of this R–P phase in aqueous electrochemical energy storage devices, OA 2 W 2 O 7 − δ (OA = octylammonium), a HOIP, which similarly displays impressive EES performance is synthesized. Most importantly, when used as an electrode material, this HOIP exhibits remarkably high stability in aqueous acidic electrolyte.