Capacitive deionization (CDI) — wherein salt ions are removed from solution and stored in electric double layers — is an emerging electrochemical separations technology for desalination of low salinity water sources such as brackish aquifers and reclaimed wastewater. However, the viability of CDI systems is limited by the low charge efficiency and specific salt adsorption of carbon electrodes — large amounts of activated carbon are necessary to store the salt removed during desalination. The most common method of increasing charge efficiency in CDI systems is to add ion exchange membranes (IEMs) over the electrodes in membrane capacitive deionization (MCDI). However, the high cost of IEMs is a significant barrier to cost-effective desalination. One means of overcoming the limitations of both CDI and MCDI is the introduction of pseudocapacitive materials, which store charge and absorb ions through reversible redox reactions on the surface of the electrode. In this study, pseudocapacitive MnO2 was deposited onto carbon aerogels via cyclic voltammetry (CV) and electroless deposition (ED) at varying deposition times. Under CV deposition, the precursor electrode is suspended in a Mn2+ electrolyte, and an applied potential is linearly swept between two vertices to electrodeposit MnO2. ED utilizes no externally applied electric current or potential; instead, precursor electrodes are placed in a permanganate solution and chemically oxidized to form MnO2. Deposition techniques and conditions were evaluated according salt absorption capacity, maximum salt adsorption rate, and charge efficiency. MnO2 cathodes where then paired with a carbon slurry flow-anode to allow for operation of MnO2 electrodes without counter electrode capacity limitations. ED MnO2 was found to absorb approximately 15 times more salt than bare carbon aerogels, whereas CV MnO2 displayed only fourfold increase in capacity. Similarly, observed charge efficiency increased from 0.2 for bare carbon electrodes to 0.25-0.8 for ED MnO2, depending on deposition time and operating current. SEM imaging and XRD analysis of the different deposition conditions reveal uniform, homogeneous deposition of MnO2 under ED, while CV deposition produced localized dendritic growth leading to less complete aerogel coverage. Our results suggest that the incorporation of pseudocapacitive materials into CDI systems can greatly increase desired characteristics for desalination of brackish water without IEMs.

Pseudocapacitive deionization: Desalination of water using high capacity MnO2 electrodes

253rd National Meeting (2017)

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.