Pseudocapacitive Contributions to Electrochemical Energy Storage in TiO₂ (Anatase) Nanoparticles

TLDR

Nanostructured materials improve electrochemical energy storage mainly by shortening ion transport paths. This study investigates the role of capacitive effects in TiO₂ nanoparticles as particle size decreases. The authors analyzed voltammetric sweeps of nanocrystalline TiO₂ to separate capacitive from lithium intercalation charge contributions. Below 10 nm, TiO₂ nanoparticles exhibit pseudocapacitive behavior with capacitance >100 μF cm⁻², increasing total charge storage and enabling faster charge/discharge rates, thus improving both power and energy density.

Abstract

The advantages in using nanostructured materials for electrochemical energy storage have largely focused on the benefits associated with short path lengths. In this paper, we consider another contribution, that of the capacitive effects, which become increasingly important at nanoscale dimensions. Nanocrystalline TiO2 (anatase) was studied over a dimensional regime where both capacitive and lithium intercalation processes contribute to the total stored charge. An analysis of the voltammetric sweep data was used to distinguish between the amount of charge stored by these two processes. At particle sizes below 10 nm, capacitive contributions became increasingly important, leading to greater amounts of total stored charge (gravimetrically normalized) with decreasing TiO2 particle size. The area normalized capacitance was determined to be well above 100 μF/cm2, confirming that the capacitive contribution was pseudocapacitive in nature. Moreover, reducing the particle size to the nanoscale regime led to faster charge/discharge rates because the diffusion-controlled lithium ion intercalation process was replaced by faradaic reactions which occur at the surface of the material. The charge storage and kinetics benefits derived from using nanoscale metal oxides provide an interesting direction for the design of materials that offer both power density and energy density.