Grantee Research Project Results
Final Report: Nanostructured Carbon Based Capacitive Desalination
EPA Contract Number: EPD17039Title: Nanostructured Carbon Based Capacitive Desalination
Investigators: Agarwal, Sandip
Small Business: Vuronyx Technologies
EPA Contact: Richards, April
Phase: I
Project Period: September 1, 2017 through February 28, 2018
Project Amount: $100,000
RFA: Small Business Innovation Research (SBIR) - Phase I (2017) RFA Text | Recipients Lists
Research Category: Small Business Innovation Research (SBIR) , SBIR - Water
Description:
Capacitive deionization (CDI) is a robust and energy efficient technology for water desalination. In a CDI process, saline water flows between two charged porous carbon electrodes, which removes the salt from the water. The energy efficiency of CDI for water desalination of low and moderate salt content brackish water is that CDI aims to remove the minority component in the water, i.e. the salts. This is an obvious improvement over other approaches, such as Reverse Osmosis, that aim to extract the majority phase, i.e. the water, from the salt solution.
In this Phase 1 SBIR project, Vuronyx Technologies investigated new materials for efficient desalination of water using CDI. A conventional CDI electrode consists for three components: activated carbon powder for salt adsorption, carbon black for electrical conductivity, and a binder such as PTFE or PVDF for mechanical integrity. We investigated the following changes to CDI electrode materials: (1) use of nanostructured high surface area activated carbon in place of conventional activated carbon, (2) use of carbon nanotubes to replace conductive carbon black to improve the conductivity and capacitance of electrodes, and (3) use of novel binders to improve coulombic efficiency and reduce the oxidation of carbon electrodes.
Summary/Accomplishments (Outputs/Outcomes):
In the first set of experiments, we investigated CDI performance with electrodes made with ultra high surface area carbon (USAC) material. We used polymer precursors to synthesize ultra-high surface area carbon which consisted of six membered ring structures, like in graphene. However, in contrast to graphene, these six membered pieces do not stack on each other. As such, BET surface area of our nanostructured carbon is ~3500 m2/g [graphene surface area is 2630 m2/g, commercial activated carbon is 600 m2/g for Norit DARCO, 1700 m2/g for YP50F, and 2200 m2/g for YEC-8A]. The use of USAC material increased the salt adsorption capacity to 9 mg/g, compared to 5 mg/g for YP-50F.
In the second set of experiments, we investigated replacing conductive carbon black with carbon nanotubes for improved conductivity and capacitance of the electrodes. When using vacuum filtration for fabricating electrodes, we were able to demonstrate >50% increase in capacitance of the electrodes. However, upon using doctor blading method to make electrodes, we did not see any improvement in CDI measurements, possibly due to insufficient dispersion of CNTs. Given the high cost of CNTs and vacuum filtration being a non-manufacturable process, we did not pursue use of CNTs further.
The third set of experiments relied on adding novel binders to improve coulombic efficiency and reduce oxidation of the electrodes. Commercial CDI equipment sold by Voltea uses ion exchange membrane, which adds a significant cost of CDI electrodes. Electrode oxidation with time is a known issue with CDI, especially at higher voltages. Electrodes made with our USAC material and PVDF binders showed oxidation beyond 0.5 V, which reduced desalination performance. We investigated the use of novel binders in our electrode formulations that can enable improved CDI performance at higher voltage.
Conclusions:
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The compositions of the formulations seem to ensure that mechanically robust yet conductive electrodes could be made that undergo no delamination in contact with NaCl solution.
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The desalination is highest for the electrode with the highest surface area carbon indicating that the capacitance values are directly translating to good CDI performance.
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The formulations with CNT as one of the constituents does not provide required capacitance
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With additional modifications, salt adsorption capacity with our USAC powder is 2.5-3x higher compared to specialty grade activated carbon available commercially.
The perspectives, information and conclusions conveyed in research project abstracts, progress reports, final reports, journal abstracts and journal publications convey the viewpoints of the principal investigator and may not represent the views and policies of ORD and EPA. Conclusions drawn by the principal investigators have not been reviewed by the Agency.