Physical Interpretation of Cyclic Voltammetry for Hybrid Pseudocapacitors

This study aims to elucidate the respective contributions of faradaic reactions and electric double layer formation to charge storage in hybrid pseudocapacitors. It also aims to provide physical interpretation of experimental cyclic voltammetry (CV) measurements. First, a physicochemical transport model was derived froth first-principles for simulating coupled interfacial, transport, and electrochemical phenomena in hybrid pseudocapacitors. The model simultaneously accounted for (i) charge transport in both electrodes and electrolyte, (ii) the dynamics of the electric double layer, steric repulsion due to finite ion sizes, (iv) redox reactions, and (v) intercalation. Then, CV curves were simulated for different electrode thicknesses and Li diffusion coefficients in the planar pseudocapacitive electrode. Particular attention was paid to the so-called b-value characterizing the power law evolution of the total current with respect to scan rate for a given potential. Overall, trends observed in numerically generated CV curves showed good agreement with experimental measurements. In addition, the results indicated that a b-value of unity across the potential window can be associated with purely faradaic charge storage with fast ion intercalation in the thin-film pseudocapacitive electrode. The study also demonstrates that under diffusion-limited conditions of Li intercalation in the pseudocapacitive electrode, the CV curves exhibited two distinct regimes: a faradaic regime dominated by faradaic reactions and a capacitive regime dominated by electric double layer formation. The b-value was near 1.0 in both regimes. However, a dip in the b-value, often observed experimentally, was also obtained and attributed to the transition between the capacitive and the faradaic regimes.