Review

What would it take for renewably powered electrosynthesis to displace petrochemical processes?

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Science  26 Apr 2019:
Vol. 364, Issue 6438, eaav3506
DOI: 10.1126/science.aav3506
  • Electrochemical CO2 conversion.

    Reduction of CO2 using renewably sourced electricity could transform waste CO2 emissions into commodity chemical feedstocks or fuels.

  • Fig. 1 Pathways and selectivities for renewable chemical synthesis.

    (A) Possible renewable energy–powered routes to commodity chemicals driven by electrocatalysis from H2O (gray) and CO2 (purple, red) as feedstocks. (B) Highest reported Faradaic efficiencies for carbon monoxide (gray squares), formic acid (purple triangles), ethylene (blue diamonds), and ethanol (red circles) and corresponding current densities (green) over the past three decades (table S3).

  • Fig. 2 Production costs of electrosynthesized chemicals.

    The graphs show technoeconomic analyses of hydrogen, carbon monoxide, ethanol, and ethylene costs as a function of electrolyzer energy conversion efficiency and electricity costs. We assume a pure CO2 price of $30/tonne, Faradaic efficiency of 90%, current density of 500 mA/cm2, electrolyzer cost of $300/kW, and plant lifetime of 30 years. The area above the white dashed line in lighter color indicates profitable production costs based on average global prices. We note that regional differences in market prices exist because of the nature of fossil fuel feedstocks.

  • Fig. 3 The emissions impact of electrosynthesized chemicals.

    (A) Market size and total emissions reductions of ethylene (58), ethanol (102), carbon monoxide (103), and formic acid (104). (B to E) Carbon emissions assessment of (B) formic acid, (C) carbon monoxide, (D) ethylene, and (E) ethanol. We assume a plant capacity of 500 MW, global warming potential (GWP) of formic acid and carbon monoxide = 1 kg CO2/kg product, and GWP of ethylene and ethanol = 5.75 kg CO2/kg product. Emissions reductions are calculated as a product of global production and GWP.

  • Fig. 4 Bio+electrocatalytic pathways toward long-chain commodity chemicals.

    Today, CO2 may be converted to syngas at very high selectivity using silver- or gold-based catalysts (top left). Alternatively, CO2 can be converted into a wide range of hydrocarbon and oxygenate products using copper-, tin-, or palladium-based catalysts (bottom left). These products can then be used as inputs for genetically engineered enzymes and bacteria to convert to more complex commodity chemicals.

  • Table 1 Current state of CO2 electrolyzers in comparison with hydrogen electrolyzers and their figures of merit.
    CatalystElectrolyteProductCell voltage (V)Current
    density (mA/cm2)
    Faradaic
    efficiency (%)
    Energy conversion
    efficiency (%)
    Cu (59)7 M KOHEthylene2.41107034
    Au (105)2 M KOHCarbon monoxide2.0999864
    Ag (106)1 M KOHCarbon monoxide3.035010145
    Ag (107)0.5 M K2SO4: 1 M KHCO3Carbon monoxide2.91978750
    Ag (79)0.1 M K2SO4: 1.5 M KHCO3Carbon monoxide4.72337825
    Sn (108)0.5 M KClFormate4.01638432
    Pb (109)0.5 M H2SO4Formate2.8509549
    Sn (110)0.5 M KHCO3 + 2 M KClFormate3.11338333
    Pt (111)Polymer electrolyteHydrogen1.2 to 2.2600 to 200010057 to 74
    Pt (111)AlkalineHydrogen1.5 to 2.0200 to 40010052 to 69
  • Table 2 Comparison of production cost and carbon emissions across various catalytic processes.
    ProductTechnologyProduction cost ($/tonne)Carbon emissions
    (tonne CO2e/tonne produced)
    EthyleneElectrocatalytic1100−0.01
    Biocatalytic (37)1200 to 26002.5
    Fossil fuel–derived (112, 113)600 to 13006
    Carbon monoxideElectrocatalytic200−0.85
    Biocatalytic
    Fossil fuel–derived (39, 41)1500.05
    EthanolElectrocatalytic515−1.00
    Biocatalytic (114, 115)6702.1
    Fossil fuel–derived
    Formic acidElectrocatalytic108−1.63
    Biocatalytic
    Fossil fuel–derived (41, 116)5700.01

    *Electrocatalysis assumes Faradaic efficiencies of 90%, electricity costs of 4 cents/kWh, energy conversion efficiency of 70%, capacity factor of 0.9, and grid intensities of 0.35 kg CO2e/kWh.

    Supplementary Materials

    • What would it take for renewably powered electrosynthesis to displace petrochemical processes?

      Phil De Luna, Christopher Hahn, Drew Higgins, Shaffiq A. Jaffer, Thomas F. Jaramillo, Edward H. Sargent

      Materials/Methods, Supplementary Text, Tables, Figures, and/or References

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      • Supplementary Text
      • Figs. S1 and S2
      • Tables S1 to S3
      • References
      Data S1 and S2

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