Understanding Ionic Transport in Polypyrrole/Nanocellulose Composite Energy Storage Devices

In this work, we aim to resolve different diffusion processes in polypyrrole/cellulose composites using a combination of impedance spectroscopy and finite element simulations. The computational model involves a coupled system of Ohm's law and Fickian diffusion to model electrode kinetics, non-linear boundary interactions at the electrode interfaces and ion transport inside the porous electrodes, thereby generating the impedance response. Composite electrodes are prepared via chemical polymerization of pyrrole on the surface of a nanocellulose fiber matrix, and the electrochemical properties are investigated experimentally using cyclic voltammetry, impedance spectroscopy and galvanostatic cycling. It is demonstrated that the onset frequency of the capacitive (or pseudocapacitive) process depends on the counter ion electrolyte diffusion, which is modulated by adding different amounts of sucrose to the aqueous electrolyte solution. Consequently, the electrochemical properties can be controlled by diffusion processes occurring at different length scales, and the critical diffusion processes can be resolved. It is shown that under normal operating conditions, the limiting process for charge transport in the device is diffusion within the electrolyte filled pores of the composite electrode.