UCLA

The present study investigates the complex multi-scale coupled transport and electrochemical phenomena involved in charge storage in pseudocapacitors. It presents rigorously developed models for simulating pseudocapacitive electrodes in both hybrid pseudocapacitors and in three-electrode systems under cyclic voltammetry. The models account for (i) charge transport in the electrodes and electrolyte, (ii) formation and dissolution of the electric double layer (EDL) at the electrode/electrolyte interface, (iii) steric repulsions due to finite ion size, (iv) redox reactions at the pseudocapacitive electrode/electrolyte interface, and (v) insertion and deinsertion of the reaction product in the pseudocapacitive electrode. They were used to study the behavior of electrochemical pseudocapacitors and provide physical interpretation of experimentally obtained measurements.

First, this study determined the respective contributions of faradaic reactions and EDL formation to charge storage in hybrid pseudocapacitors. It demonstrated the existence of two regimes for hybrid pseudocapacitors. First, a faradaic regime dominated by redox reaction and limited by the diffusion of Li in the pseudocapacitive electrode. Second, a capacitive regime dominated by the formation and dissolution of the EDL. A b-value of unity was shown to be associated with both regimes. The dip in b-value often observed experimentally was attributed to the transition between the two regimes.

Second, this study presented an extensive parametric study for the design of the pseudocapacitive electrode in a hybrid pseudocapacitor. Increasing the fraction of the potential window dominated by faradaic current was found to increase the performance of the device. Indeed, the faradaic reactions resulted in a significantly larger current magnitude than the EDL charge storage. The effect of the electrode thickness and Li diffusion coefficient on this fraction were investigated for different scan rates. To study the interplay between these parameters, a scaling analysis was performed to identify the relevant dimensionless similarity parameters governing Li transport and intercalation in the pseudocapacitive electrode. A dimensionless parameters was derived accounting for the respective contributions of the thickness, diffusion coefficient and scan rate. Furthermore, above a critical value, the device was found to operate under a diffusion-independent regime.

Finally, this study presented a model for simulating individual pseudocapacitive electrodes in three-electrode experiments. Notably, this model accounted for the variation of the redox reaction equilibrium potential with the oxidation state of the electrode. It was found to agree qualitatively with experimental measurements well and was used to provide physical interpretation to experimental measurements for Nb2O5 electrodes.

These models and results could help in the design of pseudocapacitive electrodes to achieve maximum energy and power density.