The demand for potable water from an increasing population has led to the investigation of methods for improving performance of electrodes in purifying water, such as in capacitive deionization technology, which removes salt ions from water by storing the ions in capacitive electrodes. Applications of porous electrodes also extends into fields such as batteries and supercapacitors, all of which are searching for means of improving performance. Within the structure of a porous electrode, one factor limiting the rate at which ions transport to electric double layers is known the “tortuosity.” Tortuosity quantifies the irregular path length the ions must take through the pores and around solid particles, increasing the effective distance they must travel regardless of the dimensions of the electrode cross-section. By introducing straight macroscopic pores, oriented normal to the separator between the electrodes, the apparent tortuosity can be decreased, allowing a greater utilization of the electrode’s material by giving ions a more direct path to active particles. [1] Current micromachining technology makes the manufacture of such patterns physically feasible by milling straight channels into the surface of cast electrodes, studied here in the case of activated carbon electrodes. As a result of this patterning method, electrodes have been fabricated which are capable of achieving higher specific and areal capacitance at increasing sweep rates over their unpatterned counterparts. The performance improves with increasing sweep rate and thickness, in the best cases reaching over double the performance of unpatterned electrodes during experiments.

Cyclic voltammetry experiments were performed on YP-50 activated carbon electrodes, composed of 85% active material, 5% carbon black and 10% PVDF binder, of thicknesses between 50 and 250 μm to study the effect of patterning. [2] The results of these experiments showed that the patterned, or “bi-tortuous” electrodes of 25% macro-pore coverage, showed a slower decline with respect to sweep rate than unpatterned ones. Electrochemical impedance spectroscopy was used to characterize the electronic and ionic resistance by applying a transmission line model. These findings confirmed a ~40% decrease in tortuosity and roughly twice the effective ionic conductivity compared with unpatterned electrodes of similar dimensions. [3][4] Further experiments varying the concentration of the aqueous NaCl electrolyte between 10 and 100 mM shows that the measured tortuosities for both patterned and unpatterned electrodes remained constant within 12%.

From these results we conclude that reduced tortuosity results in a higher effective ionic conductivity in the direction of the current, increasing the rate of charging in the electric double-layer. Despite the patterning process removal of some of the electrode’s mass, and thereby decreasing theoretical areal capacitance, increased effective ionic conductivity compensates for the reduced mass at increasing sweep rates due to more efficient utilization of electric double-layers. The resulting bi-tortuous electrodes, while possessing the same composition and microporous structure as their unpatterned counterparts, demonstrate as high as a 100% increase in overall capacitance at high sweep rates due to their macroscopic features. Application of similar macroscopic pores could reduce ionic resistance in systems using porous electrodes, and improve performance of technologies in fields such as energy storage and desalination.

Figure 1: A) A comparison of the cross-sections of unpatterned electrode and a patterned electrode cast onto a graphite foil current collector with a 100 μm wide macropore milled into it, including expected paths for the current to flow. B) and C) show the areal capacitance values found from cyclic voltammetry curves for the unpatterned and patterned electrodes, respectively. Data is from experiments on 214 μm-thick unpatterned and patterned electrodes, tested at sweep rates ranging from 1-30 mV/s.

This work was funded by the US National Science Foundation Award No. 1605290.

References:

[1] V. P. Nemani, S. J. Harris, and K. C. Smith, “Design of Bi-Tortuous, Anisotropic Graphite Anodes for Fast Ion-Transport in Li-Ion Batteries,” J. Electrochem. Soc., vol. 162, no. 8, pp. A1415–A1423, 2015.

[2] S. Porada, L. Weinstein, R. Dash, der W. van A., M. Bryjak, Y. Gogotsi, and P. M. Biesheuvel, “Water Desalination Using Capacitive Deionization with Microporous Carbon Electrodes.,” ACS Appl. Mater. Interfaces, vol. 4, no. 3, pp. 1194–1199, 2012.

[3] J. Landesfeind, J. Hattendorff, A. Ehrl, W. A. Wall, and H. A. Gasteiger, “Tortuosity Determination of Battery Electrodes and Separators by Impedance Spectroscopy,” J. Electrochem. Soc., vol. 163, no. 7, pp. A1373–A1387, 2016.

[4] S. Malifarge, B. Delobel, and C. Delacourt, “Determination of Tortuosity Using Impedance Spectra Analysis of Symmetric Cell,” J. Electrochem. Soc., vol. 164, no. 11, pp. 3329–3334, 2017.