*3.4. The CO2 Conversion Performance Test*

The electrochemical reduction of CO2 was performed in an H-type dual-chamber reactor. The Nafion N-117 ion-exchange membrane was used to separate the cathode chamber and the anode chamber. A silver plate (1cm × 1cm × 0.3mm) and a graphite rod were used as the working electrode and counter electrode, respectively. ILs/PC mixture solution (0.1 M) and H2SO4 aqueous solution (0.1 M) were used as the cathodic and anodic electrolytes, respectively.

In a typical procedure, the reactor was connected to a gas circulation and online sampling system (Labsolar-III AG, Beijing Perfectlight Technology Co., Ltd. (Beijing, China), and the details are stated in our previous work [46,72]). Subsequently, the air solubilized in the electrolyte solution, and the air in the circulation channel was evacuated for 30 min. Then, CO2 was introduced into the electrolyte solution from the bottom of the cathodic cell for one hour to reach the solubility equilibrium of CO2. Then two hours of the reaction was carried out to make sure that the concentration of products was high enough to exceed the detection limit of the detectors for reducing the analytic errors.

#### *3.5. The Products Analysis and Calculation of Faradaic E*ffi*ciency*

The products of H2 and CO were online sampled and in-situ quantified by gas chromatography (GC 9790II, Zhejiang Fu Li Analytical Instrument Co. Ltd., Zhejiang, China). The separation of H2, CO, and CO2 feed gas was realized by a TDX-01 column and the separated gas was subsequently brought into two paths. In one path, the products of H2 and CO were guided into a Molsieve5 A column, and the H2 was then measured using a thermal conductivity detector (TCD), while the gas CO was passed through a mechanizer to be transformed into methane by the nickel catalyst at 380 ◦C, and was ultimately detected by a flame ionization detector (FID). In another path, CO2 and other possible hydrocarbons that have longer retention time than H2 and CO in the TDX-01 column were introduced

into a Porapak N column. Finally, the CO2 gas was vented out, and the gas of hydrocarbons were measured by FID.

The FE is calculated from the product analysis by the equation:

$$\text{FE} = \frac{n\_i z\_i}{n\_e}$$

where *ni* is the mole number of a specific product (mol), *zi* is the number of electrons transferred for this product *i* (*zi* = 2 for CO and H2), and *ne* is the total mole number of electrons passed through the circuit (mol). Furthermore, the *ne* (mol) can be determined by the equation:

$$m\_{\ell} = \frac{Q}{F}$$

where *Q* is the passed charge (C) that can be obtained from the integration of the recorded chronoamperometric (*i*-*t*) curve, and *F* is the Faradaic constant (96500 C/mol).
