Optimization of Vertically-Aligned CNT Carpets’ Macro- and Nano-properties for Supercapacitor and Capacitive Deionization

icon Optimization of Vertically-Aligned CNT Carpets’ Macro- and Nano-properties for Supercapacitor and Capacitive Deionization

M. Hashempour, H. Mutha, E. Wang, C. V. Thompson
Sponsorship: MIT Energy Initiative (MITEI), Progetto Roberto Rocca

The science and technology of water desalination is receiving increasingly intensive attention due to human population growth and subsequent growing demands in domestic, industrial and agricultural sectors. Currently, desalination technology relies mainly on energy intensive techniques such as multi-stage flash (MSF), multi-effect distillation (MED), and reverse osmosis (RO). Capacitive deionization (CDI) is an emerging high-efficiency charge-based desalination technique with the ability to remove a wide range of ionic contaminants with high recovery rates. In CDI, an electrical potential difference is maintained between high-surface-area electrodes as a salt water stream passes through them, resulting in adsorption of anions and cations at the surface of positively and negatively polarized electrodes, respectively. Since CDI removes salt ions from water rather than treating the bulk water molecules, it requires much less energy than competing techniques. One of the main research directions in the field of CDI is development of electrode materials with high electrical conductivity, electrochemical stability, and morphologies allowing for a maximized surface area along with a minimized distance for ionic migration. In this project, we investigated the

 

optimization of vertically aligned carbon nanotube (CNT) carpets for this application. CNTs were synthesized via chemical vapor deposition (CVD) and over a wide range of growth variables. We realized that the flow rate or partial pressure of the carbon precursor gas (C2H4 in our work) has the most fundamental effect on a carpet’s macro-properties such as areal and volumetric density due to the modification of the number of CNT walls as well as their number density (Figure 1 a-d). Electrochemical studies of these CNTs in a supercapacitor set-up (Figure 2 a-b) confirm the the highest gravimetric capacities (up to 25% compared to other CNTs) are found for CNTs grown at the lowest C2H4 flow rates, which provided the highest specific surface area and lowest number of CNT walls. The highest volumetric capacitance (by a factor of 2 compared to other CNTs) was, in contrast, found for CNTs grown at higher C2H4 flow rates, which provided the highest number density and lowest inter-CNT spacing. Preliminary CDI experiments were carried out, and successful capacitive desalination via these electrodes was demonstrated (Figure 2 c). More detailed parametric studies of the CDI performance along with development of an ion transport model are underway.

 

CNT carpets

CNTs

FURTHER READING
  • M. A. Shannon, P. W. Bohn, M. Elimelech, J. G. Georgiadis, B. J. Mariñas, and A. M. Mayes, “Science and technology for water purification in the coming decades,” Nature, vol. 452, pp. 301–310, 2008. doi:10.1038/nature06599.
  • M. E. Suss, S. Porada, X. Sun, P. M. Biesheuvel, J. Yoon, and V. Presser, “Water desalination via capacitive deionization: what is it and what can we expect from it?” Energy Env. Sci. vol. 8, pp. 2296–2319 2015. doi:10.1039/C5EE00519A.
Last Updated on Thursday, 21 July 2016 20:22