The Proposed Hybrid Na-CO2 Cell and Its Reaction Mechanism
A schematic illustration of the proposed hybrid Na-CO2 cell is presented in Figure 1. The digital photographs of the system are also presented in Figure S1. This system could work continuously with Na metal and CO2 as fuel at the anode and feedstock gas at the cathode, respectively. Na is regarded as a promising candidate as a substitute for Li in terms of its electrochemically similar behavior along with low cost (30 times cheaper than Li) from natural abundance and environmental friendliness (Noorden, 2014, Kwak et al., 2015). The Na metal anode is kept in an organic electrolyte to prevent a direct corrosion from an aqueous electrolyte separating by Na super ionic conductor (NASICON) membrane. The overall reaction mechanisms are composed of a chemical reaction and an electrochemical reaction.
The chemical reaction of CO2 dissolution mechanism is as follows:
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When CO2 is purged into an aqueous solution (e.g., distilled water, seawater, NaOH solution), CO2 dissolution proceeds and carbonic acid (H2CO3(aq)) is formed through the hydration of CO2 (Equation 1). For a standard state condition in pure water, this spontaneous chemical equilibrium of CO2 hydration is determined by the hydration equilibrium constant (Kh = 1.70 × 10-3) (Housecroft and Sharpe, 2005). Then, the carbonic acid dissociates into HCO3- and H+ determined by the first acid dissociation constant (Ka1 = 4.46 × 10-7), shown in Equation 2 (Harris, 2010). Because carbonic acid is a polyprotic acid dissociating multiple steps, an in-depth understanding of CO2 dissolution requires that the second acid dissociation step, i.e., HCO3-(aq) ⇌ CO32-(aq) + H+(aq) (Ka2 = 4.69 × 10-11), be considered (Harris, 2010). However, the second acid dissociation constant is significantly smaller than the first (Ka1 ≫ Ka2), making it negligible in calculating the proton concentration. Thus, when CO2 dissolved in water, it acidifies the aqueous solution and HCO3-(aq) is predominant over CO32-(aq). The concentration of carbonate ions when CO2 dissolves in water at normal atmospheric pressure is provided at Table S1. The mole fractions of carbonate ions depending on the pH of solution is shown in Figure S2.
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The electrochemical reactions are composed of anodic reaction of sodium metal oxidation (Equation 3) and cathodic reaction of hydrogen evolution (Equation 4):
Then, the electrochemical net equation is simply given as the oxidation of Na metal and the spontaneous evolution of hydrogen (Equation 5). Because the potential of cathodic reaction is closely influenced by the pH of aqueous solution, the dissolution of CO2 renders a favorable electrochemical reaction environment by acidifying the aqueous solution.
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