Where Most Frameworks Degrade: Flexible Bimetallic Phosphonate Crystals as pH‐Universal Supercapacitor Electrodes

ABSTRACT Herein, we report the charge storage and plastic properties of the redox‐active, bimetallic metal phosphonate framework of [Cu(2,2′‐bpy)VO(O 3 PC 6 H 5 ) 2 ]. The flexible crystals of [Cu(2,2′‐bpy)VO(O 3 PC 6 H 5 ) 2 ] combine high energy storage with mechanical flexibility on the same platform, which is an unusual and significant property that is not observed in traditional rigid layered electrode materials. In contrast to RuO 2 , graphene, or MXenes, which prefer concentrated acidic or basic electrolytes to operate effectively as electrodes, [Cu(2,2′‐bpy)VO(O 3 PC 6 H 5 ) 2 ] operates between pH values of 4 and 10 while reaching a specific capacitance of about 140 F/g in H 3 PO 4 at pH 4 and in NaOH at pH 10 at 1 mV/s. It also demonstrates high chemical and electrochemical stability between pH 2 and 12 and in lithium hexafluorophosphate for extended periods. The use of [Cu(2,2′‐bpy)VO(O 3 PC 6 H 5 ) 2 ] as electrodes eliminates the need for harsh chemical environments, generating more sustainable and environmentally friendly energy storage solutions, and [Cu(2,2′‐bpy)VO(O 3 PC 6 H 5 ) 2 ] can be synthesized in water at mild temperatures. The combination of chemical stability, mechanical flexibility of [Cu(2,2′‐bpy)VO(O 3 PC 6 H 5 ) 2 ], and compatibility with mild electrolytes makes [Cu(2,2′‐bpy)VO(O 3 PC 6 H 5 ) 2 ] a more sustainable alternative to conventional metal oxides, MXenes, and carbon‐based electrodes in next‐generation supercapacitors and battery technologies.