2, Obafemi Awolowo University, Ile Ife, Ile-Ife, , Nigeria
3, African University of Science and Technology (AUST), Abuja, , Nigeria
In this study, activated carbon was derived from polypyrole (PPY) using a K2CO3 activating agent with varying mass ratios of the activating agent to PPY polymer (AA:PP), for the optimization of the hierarchical pore structure necessary for improved electrochemical performance. The textural study of the as-synthesized samples (AC-PPY) displayed an increase in the specific surface area (SSA) and pore volume with increase in the amount of the activating agent up to a threshold for AA:PP of 6:1. The increase in the SSA was due to the presence of hierarchical pores in the material structure for efficient ion penetration. Initial half-cell electrochemical tests performed on the different activated carbon samples with varying SSA revealed superior charge storage capability for the 6:1 sample in both negative and positive operating potentials. The highest current response value was obtained from the signatory EDLC-type cyclic voltammogram, along with the longest discharge time from the chronopotentiometry plot as a result of the lowest ion-diffusion length for successful fast ion transport reported from the impedance spectroscopy analysis.
A full symmetric device (AC-PPY-6) assembled from the best material using KNO3 neutral electrolyte yielded a specific capacitance of 140 F g-1, 12.4 Wh kg-1 energy density at 0.5 A g-1 gravimetric current. An energy density of 7.12 Wh kg-1 was still maintained at a specific current of 2 A g-1. Interestingly, after the ageing test to ascertain device stability, the device energy density increased back to 12.2 Wh kg-1 as a result of the creation additional active pores within the nanostructured material for charge storage via voltage holding tests which also led to the enhancement in specific capacitance to 137.5 F g-1 at 2 A g-1. A 99.0% capacitance retention was recorded even after 10000 cycles at a moderate specific current of 2 A g-1. A substantial approach was used to elucidate the degradation phenomena from the device self-discharge profile which showcased the device retaining up to 70% of its operating potential after 80 h (> 3 days) on open circuit. The results obtained demonstrate the potential of adopting the AC-PPY material in potential device for energy storage purposes.