Ytterbium-doping effect on the electrochemical and piezoelectric properties of poly(vinylidene fluoride) films

The optimal molar ratio of poly(vinylidene fluoride) (PVDF): dimethyl sulfoxide (DMSO) was investigated that promoted the beta-phase formation, used during the PVDF film synthesis. The solution cast method was used for doping the PVDF films with ytterbium (III) nitrate salt hexahydrate. In general, Yb-doping in PVDF films improves thermal resistance, promotes beta-phase formation, modifies the monomer conformation, reduces reversibility to electrochemical processes, and increases the d33 coefficient of piezoelectricity. The beta-phase formation caused by Yb-doping alters the internal structures of the trans-gauche-trans-gauche' (TGTG') and all-trans (TTTT) chains inside the polymeric network. The coexistence between the trans-gauche and all-trans structures provides in both films the alpha-phase and beta-phase coexistence. The presence of self-polarized CH2-CF2 dipoles, where the concentration of fluorine remains the same and the CH2 drastically increases after Yb3+ incorporation. The quasi-reversible shape shown in both voltammetry cyclic curves is related to irreversible oxidation and reduction reactions. Yb-PVDF exhibits specific pseudocapacitance values that are 30% higher than those of the PVDF films. Finally, the piezoelectric properties were improved with ytterbium incorporation.