# Analysis of Technologies for Carbon Dioxide Capture from the Air
## 2 DAC Technologies
### Electrochemical Method
It works as an electro-swing adsorption (ESA) process; a schematic representation is shown in Figure 9. The cell is composed of two cathode electrode substrates coated with a CO\({}_{2}\)-binding quinone-carbon nanotube (Q-CNT) composite, sandwiching an anode electrode substrate coated with a ferrocene-carbon nanotube (Fc-CNT) composite, with separator membranes between the electrodes. The first one can capture CO\({}_{2}\) through a redox reaction via carboxylation of quinone, while it can release CO\({}_{2}\), reversing the polarity. On the other hand, the electrode with ferrocene acts as an electron source and sink for quinone reduction and oxidation, respectively, to regulate the uptake and release of CO\({}_{2}\). A room-temperature ionic liquid on the electrode substrates allows the passage of ionic currents through the electrolyte for the activation and deactivation of the electrodes and the diffusion of CO\({}_{2}\) into the electrolyte-wetted cathode during the capture. Any reactions between quinone and oxygen (causing the oxidation of reduced quinones) are avoided by tuning the electron density of the quinone polymer. Figure 8: Fuel cell concentrator scheme. Reproduced under the techconnect license [23]. During the battery charging, when electricity runs into the battery, an electrochemical reaction occurs at the surface of each electrode, which, with high natural affinity for CO\({}_{2}\), capture this gas. During the battery discharging, CO\({}_{2}\) is released. The particularity of this system is the binary nature of the electrode affinity to CO\({}_{2}\), depending on the charging or discharging mode. In fact, electrodes have a high affinity and no affinity to CO\({}_{2}\) if these are in the charging or discharging phase, respectively. The suggested system eliminates parasitic energy losses by using redox carriers, ensuring a Faradaic efficiency higher than 90% and with an energy consumption of 40-90 kJ/molCO\({}_{2}\) for CO\({}_{2}\) molar concentrations between 0.6% and 10%. The authors plan to develop a pilot-scale plant within a few years and estimate a cost between USD 50 and USD 100 per ton of CO\({}_{2}\) captured. Among electrochemical methods, an interesting suggestion has been proposed by [PERSON] et al. [121], demonstrating experimentally simultaneous alkaline solution regeneration and CO\({}_{2}\) desorption using an H\({}_{2}\) recycling electrochemical cell. As shown in Figure 10, the process is based on an H\({}_{2}\)-recycling electrochemical system (HRES) connected to a membrane contactor. The HRES is composed of three compartments: an anode, an \"acidifying\" and a cathode compartment. During the operation, protons (H\({}^{+}\)) are obtained through the H\({}_{2}\) oxidation at the anode and are transported towards the \"acidifying\" compartment that is also fed by the spent solution coming from the air contactor, decreasing the pH and converting (bi)carbonate ions into carbonic acid. Once the solution is saturated with carbonic acid, a further pH decrease allows the desorption of CO\({}_{2}\), for which gas-liquid separation occurs in the membrane contactor. On the other hand, the alkaline solution is regenerated at the cathode side due to the production of OH\({}^{-}\) and is sent again to the air contactor. The authors find that the experimental energy consumption was 374 kJ/molCO\({}_{2}\), while 164 kJ/molCO\({}_{2}\) was obtained in a simulation of the system. Figure 9: Schematic of a single electro-swing adsorption electrochemical. Reproduced under the Creative Commons attribution noncommercial 3.0 unported licence [120]. Nevertheless, the electrochemical approach requires more studies and development.