ELECTROCHEMICAL APPLICATIONS OF THE BENDING BEAM METHOD .1. MASS-TRANSPORT AND VOLUME CHANGES IN POLYPYRROLE DURING REDOX

We introduce the bending beam method to studies of the electrochemistry of polypyrrole (PPy) electrodes, where a bipolymer strip made of a PPy layer and a polyethylene layer bound together responds, by bending, to volume changes in the PPy layer during its electrochemical doping and undoping. A correlation between the local linear strain and the curvature change of the strip is developed. Upon reduction (undoping) in an aqueous solution of LiClO4, a tosylate-doped PPy undergoes initial rapid swelling and subsequent rapid shrinking and continuous slow shrinking. The volume changes are in the range 0.65-3.4%. Cation insertion from the solution for charge compensation, which controls the net rate of the undoping, salt (sodium tosylate) draining out of the reduced neutral PPy0 at zero current, and phase relaxation in the PPy0 after salt draining are proposed and confirmed by cyclic voltammetry. The cation insertion is fast with D(Li) around 10(-7) cm2 s-1, and salt draining is slow with D(s) around 10(-9) cm2 s-1, depending on the thickness of the PPy layer. Mathematical models of the cation insertion, salt draining, and bending behavior are given and compared with the experimental results.