Asymmetric Behavior of Positive and Negative Electrodes in Carbon/Carbon Supercapacitors and Its Underlying Mechanism

The existing asymmetric behavior of positive and negative electrodes has been early observed experimentally in carbon/carbon supercapacitors, however, the understanding of its working mechanism is still lacking. In this paper, experiment and molecular dynamics (MD) simulation were integrated to investigate this phenomenon and its underlying origins. Two different electrolytes, tetraethylammonium tetrafluoroborate (TEABF4)/propylene carbonate (PC) and lithium hexafluorophosphate (LiPF6)/PC, were employed in the electrochemical measurements. Regardless of whether anions are smaller than cations or not, the positive electrode where anions adsorbed possesses a larger capacitance than the negative electrode. This asymmetric behavior in electrolyte LiPF6/PC is more distinct than in TEABF4/PC. MD simulations were carried out to render a full-length understanding that the ion motion and desolvation as well as the size of solvated ions are the main factors accounting for the asymmetric behavior. As unbalanced capacitances of two identical electrodes can impose restrictions on the operating potential window of the device and consequently its energy density, this work would shed light on asymmetric behavior of electrodes in carbon/carbon supercapacitors and have the potential to give a new strategy to balance the capacitance of two electrodes in symmetric supercapacitors.