1.4.1 Background
CO2 conversion technology refers a chemical or an electrochemical process to utilize CO2, a greenhouse gas, to produce useful carbon compounds. Since the Kyoto Protocol and the Paris Agreement have been issued, many countries and organizations have much endeavored to reduce carbon footprint enormously emitted from transportations, industries, shipping business, power generations, et cetera.8,10,11 In this regards, the chemical conversion methods to utilize CO2 to produce carbon compounds such as methanol, ethanol, syngas, ethylene, gasoline, diesel, plastics, etc., using reducing agents such as hydrogen at a high temperature or pressure have been extensively studied.51 However, the chemical conversion is still inadequate as an efficient CO2 abatement technology because the process is energy-intensive and sluggish from the considerably stable nature of CO2
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1.4.2 Electrochemical conversion
Electrochemical conversion of CO2 is considered as a more effective conversion technology than the chemical conversion technology since the electrochemical system are easy to scale-up and requires less complicated processes by using electrocatalysts and electrical energy. Mainly, the electrochemical conversion processes can be categorized into a spontaneous system and a non- spontaneous system.
The spontaneous electrochemical CO2 conversion generally utilizes the spontaneous oxidation of fuels (e.g., lithium, sodium, aluminum, zinc, etc.) and the spontaneous reduction of CO2 onto the cathode surface while generating electrical energy. Usually, these systems are also known as metal- CO2 batteries and present following electrochemical mechanisms (Equation 1.23-1.26):52–56
4Li(s) + 3CO2 ⇌ 2Li2CO3(s) + C Eo = 2.80 V vs. Li/Li+ (1.23) 4Na(s) + 3CO2 ⇌ 2Na2CO3(s) + C Eo = 2.35 V vs. Na/Na+ (1.24) 4Al(s) + 9CO2 ⇌ 2Al2(CO3)3(s) + 3C Eo = 1.30 V vs. Al/Al3+ (1.25) Zn(s) + CO2 + 2OH- + 2H+ ⇌ ZnO + HCOOH + H2O Eo = 0.96 V vs. Zn/Zn2+ (1.26)
Because the electrochemical nature of these aprotic metal-CO2 cells, the products of metal-carbonates are formed onto the surface of catalysts during a discharge reaction. Thus, these spontaneous CO2
conversion systems are closer to CO2 utilization systems rather than CO2 conversion technology since the consumed CO2 can be regenerated during the charging process.
The non-spontaneous electrochemical CO2 conversion is mainly focused on the electrocatalytic studies because the electrochemical reduction of CO2 can generate various products such as methanol,
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ethanol, CO, C2H4, etc., depending on the intrinsic properties of catalysts.57,58 This CO2 conversion system is generally configured in three-electrode system using a catalyst-loaded gas diffusion layer as a working electrode, an appropriate counter electrode such as platinum wire or graphite rod, and a reference electrode such as Ag/AgCl. Since the conversion process is usually proceeded under an aqueous KHCO3 solution, a hydrogen evolution reaction could be inevitably involved as a competitive reaction. Thus, the development of active electrocatalysts for CO2 reduction with non- active toward hydrogen evolution is an important criterion.
1.4.3 Electrocatalysts
As briefly introduced in Chapter 1.4.2, the electrocatalysts for CO2 conversion by use of electrical energy require selective electrochemical activity toward CO2 reduction reaction, not HER.
Thus, the Faradaic efficiency usually used to determine the electrochemical efficiency of the catalysts.
The Faradaic efficiency can be calculated to measure the amount of generated carbon compounds during reduction reaction via quantitative analysis methods such as a gas chromatography. By dividing the consumed electrical energy for the desirable electrochemical reaction with the energy input for the electrolysis, the Faradaic efficiency can be calculated. Generally, the Faradaic efficiency for electrochemical conversion is less than 30 % from complex electrochemical mechanisms involving undesirable side reactions such as a formation of CO, H2, etc.
Generally, Cu-based electrocatalysts are reported to be active toward CO2 reduction reaction because of their favorable carbon dioxide fixation energy.57,59–61 To initiate electrochemical reactions, the electrochemical adsorption is the important step of the overall procedures. Because of complex electrochemical mechanisms and various carbon compounds, the products from the CO2 reduction could be so various as methanol, ethanol, ethylene, carbon monoxide, oxalate, methane, formic acid, etc. The development of efficient CO2 reduction catalysts with understanding of insights into detailed CO2 reduction steps would be an important criterion.
In case of electrocatalysts adopted in aprotic metal-CO2 batteries discussed in previous Chapter 1.4.2, the mechanism of the CO2 conversion is different because CO2 is directly reacting with metal ions (e.g., Li+, Na+, and Al3+) at the surface of catalysts.52–56 Thus, low-dimension carbon catalysts, such as N-doped graphene, carbon nanotubes, graphite, etc, are usually adopted than Cu- base electrocatalysts. However, from the electrochemical nature, the formation of solid products (e.g., Li2CO3, Na2CO3, and Al2(CO3)3) accumulating on the surface of catalysts is inevitable and the cell capacity is limited. Therefore, the finding of new CO2 conversion system with an efficient chemistry is important because these metal-CO2 cells are closer to CO2 utilization systems rather than CO2
conversion systems.
12 1.4.4 Next generation CO2 conversion devices
Since conventional electrochemical CO2 conversion technologies utilize a direct reduction of CO2 molecule, the reactions present low Faradaic efficiency, sluggish kinetics, and are energy intensive. Desirable CO2 conversion technologies should meet some conditions as follows: 1) High selectivity toward CO2 conversion without side reactions (high Faradaic efficiency). 2) Facile conversion rates (operation at a high current density). 3) Economic feasibility.
Recently, the indirect conversion technologies of CO2 utilizing the spontaneous dissolution of CO2 in aqueous electrolytes have been suggested.8,62 Since CO2 is easily dissolve in an aqueous solution and provides the acidity by a formation of carbonic acid (H2CO3), the continuous consuming of the acidity (H+) could induce the continuous dissolution of CO2 in the aqueous electrolyte (Equation 1.27 and 1.28).
CO2(aq) + H2O(l) ⇌ H2CO3(aq) Kh = 1.70 × 10-3 (1.27) H2CO3(aq) ⇌ HCO3-(aq) + H+(aq) pKa1 = 6.3 (1.28)
The consumption of the acidity (H+) can be easily realized by adopting kinetically fast hydrogen evolution reaction (HER) as a reduction reaction of cathodes (Equation 1.29). As briefly introduced in Chapter 1.4.3, the electrocatalysts for direct CO2 conversion struggle from the competitive reaction of HER since HER is thermodynamically much favored than CO2 reduction reactions. If this chemistry is adopted for the CO2 conversion purpose, the cathodic reaction utilizing the acidity of CO2 can be defined as Equation 1.30 derived by Equation 1.27-1.29.
2H+ + 2e- ⇌ H2(g) Eo = 0.000 V vs. SHE (1.29) 2CO2 + 2H2O + 2e- ⇌ H2(g) + 2HCO3-(aq) Eo = 0.000 V vs. SHE (1.30)
By combining appropriate anodic reactions with suggested new cathodic reaction (Equation 1.30), new electrochemical cells producing electrical energy and hydrogen gas by consuming CO2 can be suggested. If sodium oxidation reaction (Equation 1.31) is adopted as an anodic reaction, then cell mechanism is provided as follows (Equation 1.32):
2Na(s) ⇌ 2Na+ + 2e- Eo = -2.71 V vs. SHE (1.31)
2CO2 + 2H2O + 2e- ⇌ H2(g) + 2HCO3-(aq) Eo = 0.000 V vs. SHE (1.30) 2Na(s) + 2CO2 + 2H2O ⇌ H2(g) + 2NaHCO3(aq) Eo = 2.71 V vs. Na/Na+ (1.32)
Finally, CO2 is changed into a form of NaHCO3 well-known as a baking soda. Thus, these indirect
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CO2 conversion electrochemical cells adopting spontaneous CO2 dissolution as a reaction mechanism could potentially serve as a new CO2 utilization technologies. Detailed researches could be found at Chapter 6 and Chapter 7.