Development of electrochemical system for carbon dioxide absorption and mineralization

Abstract

As an effort to capture and utilize the ever-problematic greenhouse gas, carbon dioxide ($CO_2$), an electrochemical system, facilitating $CO_2$ absorption and enabling its mineralization, has been developed. The electrochemical cell consists of three compartments with anion and cation exchange membranes in-between. When the potential is applied, electrolytic reactions occurred as follows

first, hydroxyl ions (OH-), which rapidly turn $CO_2$ into bicarbonate/carbonate ions, are produced by water electrolysis in the cathode compartment, and second, cations which are electro-migrated from the middle to the cathode compartment result in forming bicarbonate/carbonate minerals. As a proof of concept, the electrochemical experiment for $CO_2$ capture and mineralization was carried out first, and the average current density of 25.0 mA $cm^{-2}$ was stably obtained under the condition of 4 V and 1 M NaCl. This value corresponds to the required operation hour of 2.43×107 h $cm^{-2}$ so as to absorb 1 ton of $CO_2$, which demands 2437 kWh of energy. By using the Taguchi L16 orthogonal method, the optimal conditions corresponding with the lowest cell voltage of 4.2 V was obtained: 0.5 M of anolyte concentration, 1.5 M of brine concentration, 0.84 cm $s^{-1}$ of brine feeding velocity, $65^\circ C$ of brine temperature, and 31 A $cm^{-2}$ of constant current density. In fact, this electrochemical cell had one fatal problem: proton crossover through the anion exchange membrane (AEM). This phenomenon critically lowers the pH of the brine compartment, and neutralizes OH- which participates in $CO_2$ absorption. In the subsequent research, therefore, the systematic approach to mitigate proton crossover was proposed as a workable solution. Since proton crossover through the quaternary amine-based AEM is an inherent phenomenon that is not entirely excluded, studies have been focused on the reduction of proton transport from the electrolyte to the AEM itself by changing the -OH dipoles of cations. Based on theoretical speculation, along with analytical measurement, aluminum chloride was found to reduce the phenomenon to a great extent and therefore selected as the anodic electrolyte. The proton crossover problem, conversely, was able to be solved by means of adopting a sacrificial electrode which oxidizes itself instead of generating water oxidation at the anode. For achieving the higher current efficiency further as well as preventing proton production, the metallic sacrificial anode of nickel (Ni) and zinc (Zn) was applied to the electrochemical $CO_2$ mineralization system. As using a nickel sacrificial anode, the specific time efficiency for 1 ton of CO2 absorption reached to 1.08×107 h $cm^{-2}$. However, since Ni regenerating process was additionally necessary, 4569 kWh of energy was extra demanded for mineralizing 1 ton of $CO_2$. In addition, the specific Ni regenerating condition like temperature needs to be maintained for reusing Ni as a sacrificial anode, so another sacrificial anode like Zn was suggested as the next sequence. Fortunately, the regeneration conditions of Zn are at the atmospheric level and can be oxidized / reduced in one composition, so there is no need to install additional Zn regenerating process but only to change the charge of electrode anode to cathode conversely. By doing so, the required operating time for absorbing 1 ton of CO2 became 7.73×106 h $cm^{-2}$. Though it had higher $CO_2$ absorbing capacity, the extra energy of 6101 kWh was demanded for Zn regeneration for absorbing 1 ton of $CO_2$. To overcome this excessive energy demands, the oxygen ($O_2$), which came out from the Zn regeneration process, was utilized as cathodic fuel in $CO_2$ mineralization. As a result, the Zn-$CO_2$ mineralization system was turned to the energy generating fuel cell, producing 1.179 V of potential. In fact, it is similar to the concept of redox flow battery that is actively studied as a way of storing intermittent renewable energy. Consequently, the energy demand for Zn regeneration was decreased to 4044 kWh, as well as the energy consumption to 1715 kWh for $CO_2$ mineralization. It was found that the electrochemical approach has many advantages, such as simple operation, good scalability, and proven safety, together with valuable by-products. For minimizing the scale of the system and enhancing the specific $CO_2$ mineralizing capacity, there are several steps for development: 1) improving OH- productivity via development of cell configurations like the catalytic electrodes and the distance of electrodes, 2) searching other alternatives to prevent the unexpected cation transport through AEM, 3) increasing nickel retrieving efficiency for lowering energy consumption in the nickel regeneration, and finally 4) applying artificial combustion gas considering with interfering compounds and operating temperature. Moreover, high energy cost is still a huge issue in the electrochemical system, and the carbon emission and absorbing ratio cannot be promised yet. Therefore, the combination of the electrochemical $CO_2$ mineralization system with the renewable energies must be concerned with carbon neutral for actualizing this technology in industry.