Coupled redox and ion-exchange dynamics in [Ru(bpy)3]-Intercalated layered MnO2

Abstract

Layered manganese dioxide (MnO₂) films intercalated with tris(2,2′-bipyridine)ruthenium(II) ([Ru(bpy)₃]²⁺) complexes were electrodeposited under potential control, enabling the selective formation of double-layered (D-type) and single-layered (S-type) intercalation structures. Upon immersion in water, D-type films underwent a controlled structural contraction to form an SD-type single-layer structure while partially retaining the Ru complexes. Comprehensive structural and spectroscopic analyses (XRD, Raman, FT-IR, UV–vis, EDS) revealed that Na⁺ ion exchange was strongly suppressed in the SD-type, whereas direct ion exchange occurred readily in the D-type. In contrast, S-type films exhibited intrinsic structural stability with minimal Ru leaching. Electrochemical measurements (CV, EIS) demonstrated that residual [Ru(bpy)₃] in the SD-type acted as a redox mediator, significantly lowering the charge-transfer resistance and enhancing pseudocapacitance, whereas S-type films promoted ion diffusion through their open layered framework. After 50 charge–discharge cycles, specific capacitances of 268 and 234 F g⁻¹ were obtained for SD- and S-type films, respectively, exceeding that of Na@MnO₂. This work provides direct evidence for redox–ion exchange coupling in layered MnO₂ and presents a rational strategy to tune interlayer structure and interfacial charge-transfer kinetics through controlled intercalation of redox-active metal complexes.