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Solvent free CO2 capture using Membrane Capacitive DeIonization (MCDI) Louis Legrand, Olivier Schaetzle, Robert de Kler, and Hubertus V.M. Hamelers Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.8b00980 • Publication Date (Web): 11 Jul 2018 Downloaded from http://pubs.acs.org on July 16, 2018
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Solvent Free CO2 Capture using Membrane Capacitive Deionization (MCDI).
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L. Legrand, †‡ O. Schaetzle, † R.C.F. de Kler, † H.V.M. Hamelers †,* †
Wetsus, European Centre of Excellence for Sustainable Water Technology, Oostergoweg 7, 8911 MA Leeuwarden, The Netherlands. ‡ Department of Environmental Technology, Wageningen University, Bornse Weilanden 9, 6708 WG Wageningen, The Netherlands. *
Corresponding author:
[email protected]. Phone: +31-58-2543000.
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Abstract
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Capture of CO2, both from fossil origin like coal combustion and from renewable origin
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like biogas appears to be one of the greatest technological challenges of this century. In this
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study, we show that Membrane Capacitive Deionization (MCDI) can be used to capture CO2
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as bicarbonate and carbonate ions produced from the reaction of CO2 with water. This novel
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approach allows capturing CO2 without chemicals usage, at room temperature and
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atmospheric pressure. In this process, the adsorption and desorption of bicarbonate ions
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from the deionized water solution drives the CO2(g) absorption/desorption from a gas phase.
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In this work, the effect of the current density and the CO2 partial pressure were studied. We
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found that between 55-75% of the electrical charge of the capacitive electrodes can be
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directly used to absorb CO2 gas. The energy requirement of such a system was found ≈40 kJ
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mol-1 at 15% CO2 and could be further improved by reducing the ohmic and non-ohmic
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energy losses of the MCDI cell.
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Keywords: MCDI, CO2 capture, Adsorption, activated carbon
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Introduction
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Projections, made by the International Energy Agency predict that achieving a net zero
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CO2 emission by the mid-century (≈2050) is critical to limit the temperature increase to 2°C1.
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While renewable energy sources like sun and wind already replaced fossil fuels for power
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production in part, production of fuels and chemicals from renewable sources is slow. It
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seems that some form of CO2 capture from different sources such as flue gas (coal, gas or
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biomass power plants), biogas or even from ambient air is required to achieve net zero
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emissions in 2050. The captured CO2 can be either stored or utilized as a chemical building
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block by electrochemical CO2 reduction2-4 or thermocatalytic CO2 conversion5,6.
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To capture CO2 from a gas mixture, various concepts have been developed including
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adsorption7-9,
absorption10-12,
separation13-14,
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electrochemical methods16-18 and biochemical methods19,20. Among these concepts, amine
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scrubbing (chemical absorption) is the most developed and applied technology. This
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technology is based on the chemical interaction between CO2 and an amine group, which
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drives the absorption process of CO2 into the amine solvent. Despite its wide usage, this
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process shows several disadvantages, such as high amount of heat energy needed to
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regenerate the solvent21,22 and solvent degradation, which leads to high costs and some
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other toxic emissions24, solvent loss25 and corrosion effects26. Thus, more energy-efficient
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and environmentally friendly CO2 capture methods are still of importance to investigate.
membrane
cryogenic
separation15,
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Alternative approaches based on electrochemistry attract more and more attention.
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This approach uses fewer chemicals and can potentially minimize the energy consumption.
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Moreover, electrochemical systems, based on electrical power, are more suitable for CO2
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removal from emission points where insufficient waste heat is available for solvent
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regeneration. Various concepts have been explored such as Molten carbonate fuel cell27,28,
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pH swing with ion-exchange membranes16,29, electrochemical generation of nucleophile30-33
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and supercapacitive swing adsorption34. In this study, we propose an alternative concept to
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capture CO2 based on Membrane Capacitive Deionization (MCDI)35-37.
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MCDI cells are composed of activated carbon electrodes and ion-exchange membranes,
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and are mainly used to desalinate water. By applying a current through the MCDI cell, ions
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are removed from the electrolyte into the pores of the electrodes, and are stored in the
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electrical double layer (EDL). During this step, energy is temporarily stored in the electrodes
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due to its capacitive behavior. By reversing the current, the ions are desorbed from the pores
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of the electrodes to the electrolyte, and the energy previously stored is released. One
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electrode is covered by an anion-exchange membrane, and the other electrode is covered by
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a cation-exchange membrane. Due to the selectivity of ion-exchange membranes, MCDI
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shows higher ions adsorption capacity38 for monovalent salt (NaCl, KCl) compared to CDI.
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In this study, we show that MCDI technology can be used to capture CO2(g) in the form of
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HCO3- and CO32-. These ions are produced by the reaction between CO2(g) and deionized
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water, producing ions as described in eqs 1-3. Here, H2CO3* stands for the combined
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concentrations of CO2(aq) and H2CO3. These two species are usually added up as they are
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difficult to distinguish. The value for Hcc (defined as aqueous concentration over gaseous
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concentration) was calculated from ref39,40, and the K1 and K2 values were taken from ref41.
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∗
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∗ +
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+
with Hcc=0.83
(1)
with K1=10-6.35 M
(2)
with K2=10-10.33 M
(3)
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Depending on the CO2 content in the gas, the reactions between CO2(g) and deionized
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water lead to an equilibrium composition. Figure 1b shows the concentration of CO2(g) and
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H2CO3* as well as the pH, based on the eqs 1-3 as a function of HCO3- concentration in
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deionized water. The concentration of CO32- can be neglected as it is lower than 10-11 mM,
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due to the low pH of the electrolyte (pH
