Effects of Supporting Electrolyte and pH on Charge Transport within Electropolymerized Poly(o-phenylenediamine) Films Deposited on Graphite Electrodes

The homogeneous charge-transport process within electropolymerized electroactive poly(o-phenylenediamine) (PPD) films on graphite electrodes in various supporting electrolytic solutions (i.e., NaCl, NaClO4, Na2SO4, LiCl, CF3COONa, tetraethylammonium chloride (TEACl) and sodium p-toluenesulfonate (NaPTS)) at various pH’s was examined by potential-step chronoamperometry and chronocoulometry. The overall charge-transport process was found to obey Fick’s diffusion laws and thus the apparent diffusion coefficients (Dapp) for the diffusion-like charge-transport process were estimated. The Dapp values for the oxidation and reduction processes (i.e., Dappa and Dappc) were almost the same at pH 1.0 irrespective of the thickness of PPD films and were actually independent of the supporting electrolytes except for NaPTS. As pH was increased from 1 to 7, the Dappa values decreased slightly from ca. 5.6×10−8 to 1.6×10−8 cm2 s−1 and the Dappc values did more largely from ca. 5.6×10−8 to 2.2×10−9 cm2 s−1. This behavior was essentially common to all the supporting electrolytes examined. Such pH dependences of Dappa and Dappc are discussed on the basis of the fact that the charge-transport process within PPD films involves the intrinsic electron-transfer process between adjacent electroactive sites as well as the proton addition–elimination process, the charge-compensating counterion motion, the motion of solvent and the segmental motion of the polymeric chain. Especially, it is demonstrated that in this system the proton addition–elimination process must necessarily occur in order for the charge-transport process to proceed.