| Improvements of electrochemical energy storage devices rely on the understanding of a complex conglomeration of electronic transport, ionic transport, and interfacial interactions between disparate materials. In this work, the complexity of a heterogeneous nanostructured pseudocapacitor is reduced down to an individual single-walled carbon nanotube (SWNT) current collector coated with sub-picograms quantities of an electrochemically deposited pseudocapacitive material, MnO 2. These individual SWNT-MnO2 pseudocapacitors allow for a unique, well-defined system that enables novel, quantitative measurements of the various impedances and structural deformations.;To obtain the device impedances, an equivalent circuit model is used to fit cyclic voltammogram (CV) data for the device resistance R, and capacitance C, as functions of scan rate nu, MnO2 mass loading m, collector surface area A, and collector material. It is found, as expected, that C has a surface component CS that is proportional to m, and a bulk component CB that is proportional to mnu -1/2, and that C does not depend on the collector chemistry or surface area. R is composed of a diffusion controlled (Warburg) component ZW that is proportional to m-1nu-1/2 and a SWNT-MnO2 interfacial component Ri that is proportional to A-1/2. The primary new result of this research is that R i is not sensitive to the collector chemistry so that pristine graphitic carbon, defective graphitic carbon, and Pt all exhibit the same behavior. Furthermore, Ri has an A-1/2 impedance dependence rather than the typical A-1 dependence. In particular, the SWNT, MWNT, and Pt-microelectrode current collectors all exhibited interface resistances of Ri=(93 Ohm-m)A-1/2 for MnO2 charging and Ri=(70 Ohm-m)A-1/2 for discharging. In addition to a detailed study of the interface impedances, this research also investigated MnO2 structural changes using a liquid atomic force microscope coupled to a 3-terminal electrochemical cell. This combined apparatus enabled the investigation of in situ MnO2 volume expansion as a function of the CV voltage window dV , the accumulated charge dQ, and the scan rate nu. The MnO2 volume expansion is found to have a superlinear dependence on dV with a 1% expansion for dV =0.6 V and a 5% expansion for dV =1.4 V.;In addition, the scan rate dependence of the volume expansion follows the same trend as the capacitance and charge; i.e., the total expansion dH, can be broken into two components: dH = dHS + dHB, where dHS is independent of nu and dHB is proportional to nu -1/2. The primary new result of this work is a direct, quantitative measurement of the correlation between dH and dQ, which is shown to have a proportionality or expansion factor of 15 nm/nC. Furthermore, this expansion factor is independent of both dV and nu and it appears to have the same value for both surface and bulk processes. |
