Research ArticleGEOCHEMISTRY

Geoelectrochemical CO production: Implications for the autotrophic origin of life

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Science Advances  04 Apr 2018:
Vol. 4, no. 4, eaao7265
DOI: 10.1126/sciadv.aao7265
  • Fig. 1 Electrochemical CO2 reduction on various metal sulfides in 0.1 M NaCl saturated with 1 atm CO2 at room temperature (~25°C).

    (A and B) FEs for CO2 reduction to (A) CO (CO2 + 2H+ + 2e → CO + H2O) and (B) HCOO (CO2 + 2H+ + 2e → HCOO) at –0.8, –1.0, and –1.2 V versus the SHE. (C) CO production efficiencies on CdS and Ag2S as a function of electric potential. Error bars correspond to the overall reproducibility of the experimental data that was evaluated from three independent experiments.

  • Fig. 2 Thermodynamically predicted electric potentials of fluids from deep-sea hydrothermal systems.

    Redox potentials of (A) H2S (1 mmol kg–1), (B) H2S (10 mmol kg–1), (C) H2 (1 mmol kg–1), and (D) H2 (10 mmol kg–1) as a function of pH at 25°, 100°, 200°, and 300°C calculated assuming the half reactions of S + 2H+ (or H+) + 2e → H2S (or HS) [for (A) and (B)] and 2H+ + 2e → H2 [for (C) and (D)]. S (solid sulfur) was chosen as the H2S (or HS) oxidation product because the H2S/S redox couple provides a major potential control on the sulfide-rich hydrothermal vent environments (19). See the Supplementary Materials for the calculation procedure.

  • Fig. 3 Abiotic carbon fixation in the primitive hydrothermal system.

    (A) The hot and active mantle convection in the primitive Earth induced a lot of massive upwelling of komatiite/basaltic melts beneath the ocean floor (30), (B) where interactions between the solidified (ultra)mafic rocks and the CO2-rich seawater with different water/rock ratios led to a variety of end-member fluid chemistry, including the H2-rich, high-temperature (T), and alkaline type and the metal-rich, high-temperature, and acidic (or neutral) ones (31, 32). (C) On the ocean floor, mixing of the hydrothermal fluids and seawater generated sulfide-rich chimneys (22), and the potential gradient across the chimney drove a continuous electron flow. The electric potential at the chimney-seawater interface could reach less than –1 V (versus SHE) in alkaline hydrothermal vent environments. The low potential, in the presence of sulfides rich in Cd2+ and Ag+, allowed the electrochemical CO2 reduction to CO with the FE as high as dozens of percent, together with H2 evolution. The produced CO served as a driving force for the subsequent abiotic organic synthesis that preceded the origin of life as schematically indicated in the figure.

Supplementary Materials

  • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/4/4/eaao7265/DC1

    fig. S1. A schematic of the electrochemical cell.

    fig. S2. Abiotic organic synthesis driven by the electrochemically generated reductive gas on CdS at –1.0 V (versus SHE) in 100 mM NaCl saturated with 1 atm of CO2.

    fig. S3. X-ray diffraction patterns of metal sulfides.

    fig. S4. 1H NMR spectra of the CO2-saturated 0.1 M NaCl after applying –1.2 V (versus SHE) for 24 hours in the presence of metal sulfides.

    table S1. Summary of total current densities and FEs for CO2 reduction on metal sulfides.

    table S2. Peak list of x-ray diffraction patterns of metal sulfides.

  • Supplementary Materials

    This PDF file includes:

    • fig. S1. A schematic of the electrochemical cell.
    • fig. S2. Abiotic organic synthesis driven by the electrochemically generated reductive gas on CdS at –1.0 V (versus SHE) in 100 mM NaCl saturated with 1 atm of CO2.
    • fig. S3. X-ray diffraction patterns of metal sulfides.
    • fig. S4. 1H NMR spectra of the CO2-saturated 0.1 M NaCl after applying –1.2 V (versus SHE) for 24 hours in the presence of metal sulfides.
    • table S1. Summary of total current densities and FEs for CO2 reduction on metal sulfides.
    • table S2. Peak list of x-ray diffraction patterns of metal sulfides.

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