Capacitive processes for carbon capture and energy recovery from CO2 emissions : Shaping a new technology going from water to gas applications

Abstract

Nonetheless, how can we convert a water technology into a gas technology? The first step is to identify a strategy to produce ions from CO2 gas. As CO2 gas is not an ion, the CDI and CAPMIX cells are not directly usable with gas streams. Nevertheless, ions can be generated from CO2 by sparging CO2 gas in water, a process in which bicarbonate ions are formed (see Box 1). 1.3.2. Electrochemical pH swing CO2 capture can be achieved by generating a pH swing between two chambers in an electrochemical cell. The electrochemical pH swing has been mostly investigated in electrodialysis cell 62,63 and bipolar electrodialysis cell 49,53-55. The CO2 absorption takes place in the alkaline compartment (high pH) through the reaction of CO2 with hydroxide (CO + OH → HCO), whereas the CO2 desorption takes place in the acidic compartment (HCO + H → CO). Fig. 1.6 shows an example of CO2 capture with bipolar electrodialysis. Supercapacitive swing adsorption (SAA) is an electrical double-layer capacitor (EDLC) for CO2 capture 58-61. The concept consists of adsorbing CO2 on the cathode in a gas channel while charging the EDLC cell in a highly concentrated NaCl solution. Fig. 1.8 shows an illustration of the SAA concept. Three mechanisms have been suggested for the CO2 gas adsorption in the electrodes, i.e. (a) adsorption of gas molecules at gas-solid interface, (b) adsorption of gas molecules at the gas-liquid interface, and (c) adsorption of ionized gas molecule 61. Adsorption at the gas-solid interface would occur when CO2 gas is adsorbed in non-filled electrode pores stimulated by a change of affinity between the pore wall and CO2 during the charging step of the cell. Adsorption at the gas-liquid interface would occur due to an increased CO2 solubility in the electrical double layer (EDL) by charging the capacitive cell. Finally, ionization adsorption occurs through the adsorption where ϵ. is the dielectric constant, ϵ 0 is the permittivity of free space, R the gas constant, T the temperature, F the Faraday constant and c 4 the ion concentration in the bulk solution. Finally, the Stern layer represents the layer of solution separating the diffuse layer, and the carbon matrix surface. In theory, ions are surrounded by a hydration shell, and therefore, the charged ions are not infinitely close to the electrode surface. IEMs are ionomer, i.e., charged polymer, which primarily act as selective charge barriers. Ideally, only anions can be transported through the AEM, and only cations can be transported through the CEM (counter-ion transport). IEMs are mainly used in CDI to improve the charge efficiency (in Membrane CDI or MCDI) 70,72 and are required in CAPMIX to generate an electrical potential.