Effect of Meso- and Micro-Porosity in Carbon Electrodes on Atomic Layer Deposition of Pseudocapacitive V2O5 for High Performance Supercapacitors

Electrochemical capacitors can deliver high power density and long cyclability, but it is necessary to increase their energy density to promote their widespread use.^1,2 The charge storage capacity of supercapacitors can be increased by adding a pseudocapacitance material to create a Faradaic charge storage layer. High surface area activated carbon electrodes can give good power performance, but adding a conformal pseudocapacitance layer could further improve charge storage. The pseudocapacitive properties of V₂O₅ have been well studied and V₂O₅ has been incorporated into V₂O₅/carbon nanotube composites, but it has not been studied with activated carbon.^3,4 In this work, we show that atomic layer deposition (ALD) of V₂O₅ is a viable means to add pseudocapacitance to supercapacitors constructed from activated carbon powders.

Commercially available activated carbon powders with varying microporous and mesoporous surface areas were characterized before and after ALD of V₂O₅ using nitrogen adsorption, FTIR, mass uptake, SEM and TEM analysis. An acid treatment of the activated carbon before ALD aided in V₂O₅ deposition. For the ALD process, we used vanadium tri-isopropoxide and water to grow thin films of amorphous V₂O₅ on the activated carbon electrodes. The V₂O₅ ALD on the high surface-area microporous carbon with pore diameters less than 11 Å decreased the net microporous surface area, thereby decreasing the available area for charge storage in the electrochemical double layer and the capacitance of the supercapacitor. The mesoporous carbon with slightly less starting surface area, however, showed increased charge storage capacity, consistent with more conformal coatings in the larger mesopores.

For the mesoporous electrodes, we used cyclic voltammetry, galvanostatic charging/discharging, and electrochemical impedance spectroscopy (EIS) to evaluate detailed changes in supercapacitor performance as a function of pseudocapacitive V₂O₅ ALD coating thickness. 1.5 nm thick films of V₂O₅ deposited on mesoporous carbon increased the capacitance by 46%. The V₂O₅ exhibited a maximum pseudocapacitance of 540 F/g(V₂O₅) from a 0.5 nm thick V₂O₅ film. Electrodes with up to 4.5 nm thick V₂O₅ films maintained high capacitive current (91-93% of total current), similar to uncoated electrodes (89%), and minimal diffusion limited current. Electrodes coated with 33 nm of V₂O₅ reported no net change in capacitance compared to uncoated electrodes due to decreased surface area (70% decrease) and reduced capacitive current (67% of total current). EIS analysis reported low equivalent series resistance for all of the electrodes, and similar capacitance and diffusion trends as seen with cyclic voltammetry. Electrodes coated with 4.5 nm of V₂O₅ maintained 89% capacitance after 10,000 charge/discharge cycles, compared to 88% for the uncoated electrodes.

This work demonstrates the basic challenges for ALD on highly porous electrodes, and points to further analysis of the V₂O₅ ALD process that could further improve supercapacitor performance.

References

1. P. Simon and Y. Gogotsi, Nat. Mater., 7, 845–854 (2008).

2. F. Béguin, V. Presser, A. Balducci, and E. Frackowiak, Adv. Mater., 26, 2219–2251, 2283 (2014).

3. S. Boukhalfa, K. Evanoff, and G. Yushin, Energy Environ. Sci., 5, 6872–6879 (2012).

4. Z. Chen, V. Augustyn, J. Wen, Y. Zhang, M. Shen, B. Dunn, and Y. Lu, Adv. Mater., 23, 791–5 (2011).