Electrochemical Behavior of Silver Halogenocomplexes in an Amide-Type Ionic Liquid

The electrode reactions of haloargentates were investigated in an aprotic and hydrophobic amide-type ionic liquid, 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)amide (BMPTFSA) in the presence of the halide ions. Silver halides, AgX (X- = Cl-, Br-, and I-) were found to be soluble in BMPTFSA containing 0.5 M BMPX to form halogenocomplex, [AgX3]2-. The cathodic reduction of [AgX3]2- to metallic Ag was observed within the electrochemical potential window of the ionic liquid, while the reduction potential was lower than that in the ionic liquid in the absence of X-. The equilibrium potentials of [AgX3]2-/Ag were in the order of [AgCl3]2- > [AgBr3]2- > [AgI3]2-, probably reflecting the thermodynamic stability of the complexes. The diffusion coefficients of [AgCl3]2-, [AgBr3]2-, and [AgI3]2- were 2.5, 2.0, and 1.6 × 10-7 cm2 s-1, respectively. The morphology of deposits strongly depended on the reduction potential. The nucleation and growth mechanism of Ag deposition was considered to be progressive rather than instantaneous. Formation of Ag nanoparticles dispersed in the ionic liquids was confirmed after potentiostatic cathodic reduction at -2.5 V vs Ag|Ag(I) using a transmission electron microscope.