Synthesis and characterization of nanofiber-type hydrophobic organic materials as electrodes for improved performance of PVDF-based piezoelectric nanogenerators

Poly(3,4-ethylenedioxythiophene) (PEDOT) derivatives are synthesized by oxidative polymerization using sodium dodecyl sulfate (SDS) as an anionic surfactant dopant. The resulting polymeric materials featuring nanofiber-type one-dimensional (1D) structures are identified as poly(2-butyl-2,3-dihydrothieno[3,4-b][1,4] dioxine: dodecyl sulfate (PEDOT-C-4:DS) and poly(2-hexyl-2,3-dihydrothieno[3,4-b][1,4] dioxine: dodecyl sulfate (PEDOT-C-6:DS). The ratio of the DS anion doped into PEDOT-C-4:DS and PEDOT-C-6:DS is 0.16 and 0.23, respectively. The contact angle of water on the PEDOT-C-4:DS and PEDOT-C-6:DS films is 76.6 degrees and 87.7 degrees, respectively, showing hydrophobic properties similar to that with water on PVDF. It facilitated the fully uniform film formation due to excellent surface matching. Peeling force of PEDOT-C-4:DS and PEDOT-C-6:DS is stronger than the one of PEDOT:PSS-CNT composite. GIWAX analysis showed that PEDOT-C-4:DS formed the highly ordered edge-on structure and PEDOT-C-6:DS formed the bimodal orientation consisting of edge-on structure mainly and face-on structure slightly. The electrical conductivity (sigma(PEDOT-C4:DS) = 50.0 S cm(-1)) of PEDOT-C-4:DS is 41.7 times higher than that of PEDOT:PSS (sigma(PEDOT:PSS) = 1.2 S cm(-1)). The output signals (maximum voltages/currents) of piezoelectric nanogenerators (PNGs, electrode/PVDF/electrode) using these materials as electrodes are PNG-1 (PEDOT:PSS-CNT composite) 1.25 V/128.5 nA, PNG-2 (PEDOT-C-4:DS) 1.54 V/166.0 nA, and PNG-3 (PEDOT-C-6:DS) 1.49 V/159.0 nA. Of these, PNG-2 & PNG-3 show maximum piezoelectric output power of 63.0 nW and 59.9 nW at 9 M Omega compared to PNG-1 (41.0 nW at 10 M Omega). They are enhanced up to 53.7%. The excellent surface matching between a piezoelectric active material and an electrode material leads to high output power.