Direct electrosynthesis of 52% concentrated CO on silver’s twin boundary

The gaseous product concentration in direct electrochemical CO2 reduction is usually hurdled by the electrode’s Faradaic efficiency, current density, and inevitable mixing with the unreacted CO2. A concentrated gaseous product with high purity will greatly lower the barrier for large-scale CO2 fixation and follow-up industrial usage. Here, we developed a pneumatic trough setup to collect the CO2 reduction product from a precisely engineered nanotwinned electrocatalyst, without using ion-exchange membrane. The silver catalyst’s twin boundary density can be tuned from 0.3 to 1.5 × 104 cm−1. With the lengthy and winding twin boundaries, this catalyst exhibits a Faradaic efficiency up to 92% at −1.0 V and a turnover frequency of 127 s−1 in converting CO2 to CO. Through a tandem electrochemical-CVD system, we successfully produced CO with a volume percentage of up to 52%, and further transformed it into single layer graphene film.

Cdl is calculated from the corresponding slope. The double-layered capacitance of a flat metal electrode is ~ 20 μF cm -2 . The roughness factor can be calculated by equation (4):

Intrinsic Activity Calculation
The intrinsic activity calculation is based on three assumptions: i) The intrinsic activity is a constant value at a given potential and unvaried with TB density changing; ii) Mass transfer is efficient and doesn't control the reaction; iii) TB is the only active site besides facet atoms. The jCO therefore linearly increases with TB density rising, and the slope in jCO-TB density plot (kTB) represents the jCO per centimeter of TB. When TB density is zero, there is only plane atoms, indicating the interception is the intrinsic jCO of plane (jplane). This relationship can be expressed by the following equation: Assuming the area of TBs in electrode surface is TB density × silver's diameter (DAg, ~350 pm), the intrinsic jCO of TB (jTB) is as following: j TB (mA cm -2 )=k TB (mA cm -1 )÷D Ag (cm) (4)

Turnover Frequency (TOF) Calculation
The TOF values of TB and plane atoms are based on their intrinsic jCO. For TB atoms, the TOF of TB (TOFTB) is calculated with equation (5): Where    24 The electrode is connected with a waterproof wire and then inserted into pneumatic trough. Before the electrochemical reaction occurs, air in pneumatic trough is pumped out and electrolyte thereby fills in the pneumatic trough. CO2 is bubbled from inlet, partially dissolves in the electrolyte and diffuses to the electrode surface to produce CO.
The undissolved CO2 is vented in the form of bubble outside the pneumatic trough.
Therefore, the products we collected is relatively pure with some CO2 diffusing from electrolyte. The CO product with high concentration is then pumped out for graphene growth.
Although the maximum volume fraction of CO can theoretically reach its FECO, the products can be practically diluted by CO2 escaping from electrolyte. Therefore, the CO concentration is not only relative to the FECO but also controlled by CO2 escape. In this system, there are two factors dominating CO2 escape: temperature and surface roughness of device. Temperature affects partition coefficient of CO2 between electrolyte and gas phase. The higher the temperature is, the larger the escape rate will be. In our experiment, the initial CO concentration can only reach up to ~33% at room temperature, but ~52% at 0 °C. A rough surface of device has more nucleate center for CO2, which can accelerate CO2 escape from electrolyte. In the bicarbonate-CO2 electrolyte, the proton produced from OER quickly reacts with bicarbonate anion to generate CO2 and water. The depletion of bicarbonate in the anolyte makes the potassium ions redundant, driving the K + ions to migrate across the membrane to the catholyte side. Meanwhile, the OHproduced from CO2RR reacts with CO2 to form more bicarbonate in the catholyte, which cannot pass through the protonexchange membrane, and balance with the influent K + ions in the catholyte. The overall effects give rise to a continuous depletion of KHCO3 in the anolyte and a continuous accumulation of KHCO3 in the catholyte.
To prove that, we monitored the pH value and electric conductivity (κ) changes in both anolyte and catholyte. The initial pH in both anolyte and catholyte is ~7.5, and the corresponding conductivity is ~ 73 mS cm -1 . After a 12-hour reaction, the pH and 27 conductivity in anolyte decreased to 4.7 and 10 mS cm -1 , respectively. Whereas, the pH and conductivity in catholyte rose to 7.8 and 119 mS cm -1 , respectively. This indicates both a KHCO3 depletion in anolyte and a KHCO3 accumulation in catholyte.
We think the "migration" of KHCO3 is caused by the asymmetrical ion transfer in proton-exchange membrane, which can only admit the pass of cations. Without this membrane, we observed that H-type cell can maintain a stable cell voltage for much longer time (Fig. 3b), for the bicarbonate can diffuse freely and keep a constant concentration in both sides. We also measured the conductivity in membrane-free system, which shows little changes after running for 24 h. The energy conversion efficiency can be calculated by the following equation: Energy Efficiency= (∆G m θ (CO)-∆G m θ (CO 2 )) 2UF ×FE CO (7) Where ∆G m θ (CO) and ∆G m θ (CO 2 ) stand for the molar Gibbs free energy of CO and CO2, equaling to -137.168 kJ mol -1 and -394.359 kJ mol -1 , respectively. U is the absolute value of cell voltage and F is Faradaic constant.