Efficient upgrading of CO to C3 fuel using asymmetric C-C coupling active sites

The electroreduction of C1 feedgas to high-energy-density fuels provides an attractive avenue to the storage of renewable electricity. Much progress has been made to improve selectivity to C1 and C2 products, however, the selectivity to desirable high-energy-density C3 products remains relatively low. We reason that C3 electrosynthesis relies on a higher-order reaction pathway that requires the formation of multiple carbon-carbon (C-C) bonds, and thus pursue a strategy explicitly designed to couple C2 with C1 intermediates. We develop an approach wherein neighboring copper atoms having distinct electronic structures interact with two adsorbates to catalyze an asymmetric reaction. We achieve a record n-propanol Faradaic efficiency (FE) of (33 ± 1)% with a conversion rate of (4.5 ± 0.1) mA cm−2, and a record n-propanol cathodic energy conversion efficiency (EEcathodic half-cell) of 21%. The FE and EEcathodic half-cell represent a 1.3× improvement relative to previously-published CO-to-n-propanol electroreduction reports.

According to the energetic span 1 , the total barriers of OC-OCCO ( 1 ) and OC-OCCOH ( 2 ) pathways are: where U is the applied potential vs. The total barrier of OC-OCCOH on Ag-doped Cu is 0.76 eV ( Supplementary Fig.19), lower than that on pure Cu, giving a maximum TOF of 0.87 s -1 at reaction applied potential (-1.286 VCHE).
Therefore, after considering the proton/electron transfer in C1-C2 coupling, the designed Ag-doped Cu also favors C1-C2 coupling reaction compared to Cu.  We started with CO adsorption on Cu(111) surface as shown in Supplementary Fig. 1a and d.

Chemicals and Materials.
Commercial Cu nanopowder (99%) and silver nitrate (AgNO3, 99%) were purchased from Sigma-Aldrich. Potassium hydroxide (KOH) and methanol were purchased from Caledon Laboratory Chemicals. Gas diffusion layer (GDL, Freudenberg H14C9), anion exchange membrane (Fumasep FAB-PK-130) were received from Fuel Cell Store. Ni foam (1.6 mm thickness) was purchased from MTI Corporation. All chemicals were used as received. All aqueous solutions were prepared using deionized water with a resistivity of 18.2 M  cm -1 .

Calculation for Equilibrium Potential.
Equilibrium potentials for the half reactions of CO to n-propanol and CO2 to n-propanol were calculated based on the values of the standard molar Gibbs energy of formation at 298.15 K (ref. 35). We assumed that gases are at 1 atm and liquids are in the pure form.

Calculation for Cathodic Energy Conversion Efficiency.
The OER in anode side is one of main contributors to the energy lost, but here we excluded the effect of the OER and analyzed the cathode performance using cathodic energy conversion efficiency (EEcathodic half-cell), where the overpotential of oxygen evolution is assumed to be 0.
The n-propanol EEcathodic half-cell can be calculated as follows 36 : where E is applied potential versus RHE, FEn-propanol is the measured Faradaic efficiency of npropanol in percentage, and En-propanol = 0.20 VRHE for CORR or En-propanol = 0.10 VRHE for CO2RR.
As shown in the equation above, n-propanol EEcathodic half-cell is governed by both FE and overpotential, which are the two important factors in CORR.
For example, at -0.416 VRHE after iR compensation, Ag-doped Cu GDE delivered a n-propanol FE of 33% in CORR. Then the n-propanol EEcathodic half-cell for n-propanol can be calculated as follows: