Ayokunle Omosebi

Surface charge enhanced carbon electrodes for stable and efficient capacitive deionization using inverted adsorption–desorption behavior

Unsustainable and inefficient capacitive deionization (CDI) performance has been observed through CDI operation with carbon xerogel (CX) electrodes for 50 hours using a constant-voltage charging method. This behavior is primarily accounted for by changes in the surface chemistry for the studied material via oxidation of the carbon electrodes in an aqueous solution. In order to improve performance stability, we have developed a novel CDI system using an anode with net negative surface charges and a cathode with net positive surface charges. As a result, salt separation in this system is achieved in an opposing manner to the conventional CDI system, e.g., when the system is charged using a power source, cations and anions are desorbed at the anode and cathode, respectively. This system is named the inverted capacitive deionization (i-CDI) system. Most importantly, salt separation in the i-CDI system was maintained for over 600 hours, which is approximately an increase of 530% in lifetime compared to a CDI system operated under similar conditions. This enhanced performance stability is attributed to the use of oxidized anodes in the i-CDI system, which limits the possibility for loss in separation performance due to carbon oxidation in an aqueous solution.

Asymmetric Electrode Configuration for Enhanced Membrane Capacitive Deionization

Long-term performance of capacitive deionization (CDI) and membrane-assisted capacitive deionization (MCDI) single cells equipped with the same pristine carbon xerogel (CX) electrodes configured as the anode and cathode was investigated. Unlike CDI, which was subject to performance degradation in a short period of time, MCDI showed performance preservation during the 50 h of operation due to its ability to mitigate charge leakage from parasitic electrochemical reactions that result in carbon oxidation. Differential capacitance measurements of the used CDI and MCDI electrodes revealed shifting of the potential of zero charge (EPZC) of the CDI anode from -0.1 to 0.4 V but only to 0.1 V for the MCDI anode. CDI and MCDI cells tested with electrodes having EPZCs at -0.1 and 0.5 V showed strongly contrasting results depending on the anode-cathode EPZC configuration. The MCDI cell configured with a 0.5 V EPZC cathode and -0.1 V EPZC anode displayed the best performance of all the tested cells, benefiting from increased counterion excess within the potential window, and the membrane was in-place to reject expelled co-ions from accessing the bulk.

Enhanced Salt Removal in an Inverted Capacitive Deionization Cell Using Amine Modified Microporous Carbon Cathodes.

Microporous SpectraCarb carbon cloth was treated using nitric acid to enhance negative surface charges of COO(-) in a neutral solution. This acid-treated carbon was further modified by ethylenediamine to attach -NH2 surface functional groups, resulting in positive surface charges of -NH3(+) via pronation in a neutral solution. Through multiple characterizations, in comparison to pristine SpectraCarb carbon, amine-treated SpectraCarb carbon displays a decreased potential of zero charge but an increased point of zero charge, which is opposed to the effect obtained for acid-treated SpectraCarb carbon. An inverted capacitive deionization cell was constructed using amine-treated cathodes and acid-treated anodes, where the cathode is the negatively polarized electrode and the anode is the positively polarized electrode. Constant-voltage switching operation using NaCl solution showed that the salt removal capacity was approximately 5.3 mg g(-1) at a maximum working voltage of 1.1/0 V, which is an expansion in both the salt capacity and potential window from previous i-CDI results demonstrated for carbon xerogel materials. This improved performance is accounted for by the enlarged cathodic working voltage window through ethylenediamine-derived functional groups, and the enhanced microporosity of the SpectraCarb electrodes for salt adsorption. These results expand the use of i-CDI for efficient desalination applications.

Continuous operation of membrane capacitive deionization cells assembled with dissimilar potential of zero charge electrode pairs.

The performance of single stack membrane assisted capacitive deionization cells configured with pristine and nitric acid oxidized Zorflex (ZX) electrode pairs was evaluated. The potentials of zero charge for the pristine and oxidized electrodes were respectively -0.2V and 0.2V vs. SCE. Four cell combinations of the electrodes including a pristine anode-pristine cathode, oxidized anode-pristine cathode, pristine anode-oxidized cathode, and oxidized anode-oxidized cathode were investigated. When the PZC was located within the polarization window of the electrode, diminished performance was observed. The cells were operated at 1.2 V and based on potential distribution results, the effective working potentials were ∼0.9, 0.8, 1.2, and 1.1 V for the pristine anode-pristine cathode, oxidized anode-pristine cathode, pristine anode-oxidized cathode, and oxidized anode-oxidized cathode cells, respectively. The highest electrosorption capacity of 17 mg NaCl/g ZX was observed for the pristine anode-oxidized cathode cell, where both PZCs were outside of the polarization window.

Structured and Surface-Modified Carbon Xerogel Electrodes for Capacitive Deionization

The use of porous carbon materials for desalination and water treatment applications has been a heavily studied topic over the last few decades. In particular, the field of capacitive deionization (CDI) has become an increasingly popular method for the desalination of brackish water streams with carbon electrode design and porosity defined as key metrics in addition to operating parameters. In this chapter, carbon xerogel electrodes are reviewed and the effect porosity, surface charge, and the potential of zero charge are highlighted for their impact on the salt adsorption process. Ultimately, all of these parameters predict the desalination performance of a CDI cell, and assessments are made for future electrode needs in this field.

Correction to Asymmetric Electrode Configuration for Enhanced Membrane Capacitive Deionization.

ACS Appl. Mater. Interfaces 2014, 6 (15), 12640−12649. DOI: 10.1021/am5026209 P 12647. There is a slight mistake in the paper that may cause confusion on the effect of EPZC on electrosorption. Near Figure 9 in the paragraph before the “Asymmetric Electrodes in CDI and MCDI Operation” section, the sentence that reads “However, if an oxidized electrode is configured as the anode and a pristine electrode as the cathode...” should read “However, if a pristine electrode is configured as the anode and an oxidized electrode as the cathode...”. There is no impact on the conclusions of the work, but the authors needed to make this correction due to possible reader confusion. The authors apologize for any confusion caused by this error.