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WIREs Energy Environ.
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# Electrocatalytic conversion of carbon dioxide to fuels: a review on the interaction between CO2 and the liquid electrolyte

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The electrochemical reduction of carbon dioxide using renewable energy resources can potentially accomplish the carbon‐neutral energy cycle while synthesizing fuels simultaneously. CO2 electrocatalysis typically requires the presence of protons in a liquid electrolyte, which acts as a possible cocatalyst for CO2 conversion to fuels. However, the following fundamental questions remain: (1) How does the interaction between CO2 and liquid electrolyte influence CO2 electrocatalysis? (2) How does the selectivity toward products from CO2 reduction depend upon the choice of electrolyte? In this review article, we emphasize on the interaction between CO2 and the liquid electrolyte, in order to find answers to these questions. Specifically, the nature of interactions along with the structures of solvated molecules for CO2 in water, organic solvents and ionic liquids are discussed. Finally, relevant questions are proposed which need to be studied carefully in order to gain a deeper understanding of CO2 electrocatalysis. WIREs Energy Environ 2017, 6:e239. doi: 10.1002/wene.239 This article is categorized under: Fuel Cells and Hydrogen > Climate and Environment
The Keeling Curve, which shows carbon dioxide concentrations measured at the Mauna Loa Observatory since 1958 (Source: Scripps Institution of Oceanography).
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Reaction pathways for the electrochemical reduction of CO2 in the (a) absence and (b, c) presence of [emim][Tf2N] at a Pb electrode in acetonitrile.
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Raman spectra of [bmim][Pf6]–CO2 system at 313 K. From top to bottom: (1) [bmim] [PF6]–CO2 at 140 bar; (2) [bmim][PF6] without CO2; and (3) pure CO2. Arrows indicate bands of pure CO2.
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The spatial radial distribution function of CO2 (yellow) around (a) B(CN)4 and (b) emim.
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(a) Correlation between gas‐phase cation–anion interaction energy and experimental CO2 solubitlity at 1 bar and 298 K for four ionic liquids with the same [emim]cation but different anions. (b) Correlation between gas‐phase CO2 anion interaction energy and the experimental CO2 solubility at 1 bar and 298 K for four ionic liquids with the same [emim] cation but different anions.
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Structure of [emim] cation in an ionic liquid.
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A schematic of how the free energy of the system changes during the reaction CO2 + 2H++2e ⇌ CO + H2O in water or acetonitrile (solid line)or EMIM–BF4 (dashed line).
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Ramanspectra of acetaldehyde: (a) the carbonyl stretch region in He (black) and CO2 (red) and (b) the aldehydic C–H stretch in He (Black) and CO2 (blue). Insets show the intensity changes upon complexation.
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Optimzed structure of the acetaldehyde–CO2 complex.
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Optimzed structure of complex formed during CO2 and DME interaction.
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Nyquist plot for different electrolytes obtained via electrochemical impedance spectroscopy at a cell potential of −2.25 V.
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Schematic illustrations of processes in the double layer that play a role in the kinetics of CO2 to CO conversion on a Ag cathode when using (a) KOH or (b) KCl as the electrolyte.
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Gibbs free energy change with temperature for CO2–(H2O)n, n = 1, 2, …, 8.
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Optimized structure of (CO2)2–H2O trimer.
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Optimized structure of CO2–H2O dimer.
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Diffusion coefficients of CO2 in ultra‐pure DI water at different and pressures.
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Structures of CO2–H2O complex in a T‐ and H‐type configuration.
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Pair correlation function between water and carbon dioxide: (a) C atom in CO2 and (b) O atom in CO2. Solid and dashed lines refer to O and H sites in water, respectively.
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Calculated NEXAFS carbon K‐edge spectra for various species (adapted).
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An illustration of effect of temperature, pressure, and salinity (ionic strength) on the concentration of CO2, , and species. (Kl* = K1; K2* = K2).
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Variations in concentration of CO2, , and with pH of the aqueous medium. DIC = 2.1 mmol/kg; Salinity, S = 35 and T = 25°C. (Kl* = K1; K2* = K2).
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Performance of a full electrochemical cell (a) without and (b) with a buffer layer. Anode catalyst: Pt/C; Cathode catalyst: Sn.
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A schematic of the full electrochemical cell featuring a buffer layer circulating a liquid electrolyte.
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