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Catalysts 2020, 10, 1287 4 of 25 electrolytes. Additionally, the very small differences between the thermodynamic potentials for CO2 reduction products bring about a challenge to selectively produce the desirable carbon compounds. Actually, the required potentials to drive CO2 reductions are more negative than the equilibrium ones, leading to the overpotentials [21]. The presence of overpotential originates from the difficult rearrangement of the linear CO2 molecule to a bent radical anion, which requires enormous energy to occur [22]. Therefore, reducing or even eliminating the overpotentials will facilitate the electroreduction of CO2. Table 1. Half-reactions and the corresponding thermodynamic redox potentials of CO2 reduction to the main products in aqueous solutions [20]. Half-Reactions CO2 +2H+ +2e− =CO+H2O CO2 + 2H+ + 2e− = HCOOH CO2 +4H+ +4e− =HCHO+H2O CO2 +6H+ +6e− =CH3OH+H2O CO2 +8H+ +8e− =CH4 +2H2O 2CO2 + 8H+ + 8e− = CH3COOH + H2O 2CO2 + 12H+ + 12e− = C2H5OH + H2O 2H+ +2e− =H2 E0 (V vs. NHE) −0.53 −0.61 −0.48 −0.38 −0.24 −0.29 −0.33 −0.41 Recently, a kind of flow cell using the gas-diffusion electrode (GDE) and flowing catholyte has been developed to accelerate the technology of CO2 electroreduction toward its envisioned application of neutralizing CO2 emission on a global scale [23,24]. In this system, the electrolysis cell should consist of three compartments: cathode chamber, anode chamber and gas chamber (Scheme 2B). An ion exchange membrane between cathode chamber and anode chamber is necessary to separate the catholyte and the anolyte. The catalyst is incorporated into electrolyzers on a GDE commonly by drop-casting, spray-coating and in situ growing (self-supporting GDE) [25]. The coated catalyst side of GDE directly contacts the catholyte. Gaseous CO2 with a certain flow rate passes through the gas chamber at the back side of GDE, and reacts at the catalyst/electrolyte interface, constituting a three-phase boundary (Scheme 2C). This configuration could overcome the mass transfer limitations of two-phase interface and the low solubility of CO2 in aqueous solution (Scheme 2D). Thus, it enables rapid delivery of CO2 to the catalyst, resulting in higher or even commercially relevant current densities. Furthermore, this GDE configuration allows the use of more alkaline electrolytes such as KOH, which results in an increased local pH, favoring HER suppression and C2 (ethanol) production. 2.2. CO2 Photoreduction A CO2 photoreduction system commonly consists of a light source, a photocatalyst, a sacrificial electron donor and a photoreactor that is used as the light absorber [12]. Generally, solar energy can be applied as the energy source for CO2 conversion [9]. However, CO2 is optically inert at visible and UV radiation in the wavelengths of 200–900 nm [26]. Thus, a photocatalyst with a suitable band structure is required to promote the CO2 photoreduction so that the electrons can be excited by sunlight and transfer to CO2 species adsorbed on the surface of catalysts. The band structure constitutes a conduction band (CB), a valence band (VB) and a bandgap between them with no electron configuration [27]. Accordingly, the CB bottom of the photocatalysts must be more negative than the reduction potentials of CO2 and its reduced products, while the top of the VB must be more positive than the oxidation potential of sacrificial reagents [4,28]. Thus, the electrons would transfer from the CB of photocatalysts to surface-adsorbed CO2 species and convert it to solar fuels. As a sacrificial reagent, H2O is an ideal electron donor and a hydrogen source for the CO2 photoreduction [29]. However, it can compete with CO2 photoreduction for the electrons to generate H2, as shown in HER in Table 1. Therefore, over the semiconductor photocatalysts, H2O can be not only oxidized to O2 by trapping the photogenerated holes in the VB, but also reduced to H2 by catching the photogeneratedPDF Image | Advances in Clean Fuel Ethanol Production from CO2 Reduction
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