Role of ion-selective membranes in the carbon balance for CO2 electroreduction via gas diffusion electrode reactor designs†

In this work, the effect of ion-selective membranes on the detailed carbon balance was systematically analyzed for high-rate CO2 reduction in GDE-type flow electrolyzers. By using different ion-selective membranes, we show nearly identical catalytic selectivity for CO2 reduction, which is primarily due to a similar local reaction environment created at the cathode/electrolyte interface via the introduction of a catholyte layer. In addition, based on a systematic exploration of gases released from electrolytes and the dynamic change of electrolyte speciation, we demonstrate the explicit discrepancy in carbon balance paths for the captured CO2 at the cathode/catholyte interface via reaction with OH− when using different ion-selective membranes: (i) the captured CO2 could be transported through an anion exchange membrane in the form of CO32−, subsequently releasing CO2 along with O2 in the anolyte, and (ii) with a cation exchange membrane, the captured CO2 would be accumulated in the catholyte in the form of CO32−, while (iii) with the use of a bipolar membrane, the captured CO2 could be released at the catholyte/membrane interface in the form of gaseous CO2. The unique carbon balance path for each type of membrane is linked to ion species transported through the membranes.


CO 2 capture via high local pH
During CO 2 reduction in flow electrolyzers using 1 M KHCO 3 , CO 2 from the gas chamber of the electrolyzers reacts with OHions produced at the cathode/electrolyte interface accoding to the below reactions of CO 2 and OH -: * This is at a CO 2 partial pressure of 1 bar in 1 M HCO 3 -. 5

CO 2 reduction performance
The electrocatalytic reduction of CO 2 was peformed in a three-chamber flow electrolyzer made from Teflon at ambient temperature and pressure. At the reactor, an ion-selective membrane was used to sperate catholyte and anolyte flow compartments. Catholyte and anolyte bottles were filled with 50 ml 1 M KHCO 3 , respectively ( Figure S2). In addition, a fixed geometric surface area (2 cm 2 ) of Cu layer was used for all the tests in this study.
CO 2 was purged into gas compartment at a constant flowrate of 45 ml/min, and then a fraction of gaseous CO 2 diffuses to the surface of the catalyst in electrolyte for CO 2 conversion. In addtion, CO 2 also can be captured to form carbonate (equation S8 and S9) via the reaction of CO 2 with OHgenerated at the cathode/electrolye inteface. 5 Thus, the high CO 2 conversion rate to gas (C 2 ) and liquid products as well as high local pH can lead to a substantial CO 2 consumption at high current densities, correspondingly varying the gas flow (gas mixture) out of the reactor. The volumetric flowrate of gas outlet (gas mixture) after reactor was monitored by flow meter in CO 2 reduction ( Figure S2), and then faradaic efficiencies of gas products were evaluated under the consideration of gas flow variation between inlet and outlet. It should be noted that the average catalytic selevitity of gas products during 2.5 h CO 2 reduction electrolysis was used in this work. Figure S2. Schematic illustration of flow cell setup for electrocatalytic CO 2 reduction.

Gas and liquid product analysis
Gas products mixing with unreacted CO 2 flowed out of the electrolyzers, directly injecting into the gas-sampling loop of a gas chromatography (PerkinElmer, Clarus ® 590). Ar was used as a carrier gas with a contant flowrate of 10 sccm. The gas chromatography was equipped with a packed Molecular sieve 13x column and a packed Hayesep Q column to separate the gas products. Thus, H 2 , CO, CH 4 and C 2 H 4 could be identified at different reaction times using a thermal conductivity detector.
In addition, the peak area of each gas product was compared to standards (calibration gases) to determine the corresponding concentration of gaseous products. Thus, we can get the faradaic efficiency of a certain gas product as follows: where n is the number of electrons required for producing one molecule of the related gas product, and C gas product is the concentration of gas product measured by GC (here, C gas product is S6 the mole fraction of gas product in the total gas outlet mixture). and is the gas flowrate out ∅ of the electrolyzers and the electrolysis time, respectively. is the ambient pressure, is the ideal gas constant, T is the absolute temperature, F is Faraday constant, and I is the applied current.
The liquid-phase products are analyzed after the electrolysis using a high performance liquid chromatography (HPLC, Agilent 1200 series). Liquid-phase products were separated by an Aminex HPX-87H column (Bio-Rad) that was maintained at 50 °C for the duration of the detection. The HPLC was equipped with a diode array detector (DAD) and a refractive index detector (RID), and the signal response of the DAD and RID was calibrated by known concentration solutions. Thus, we can get the concentration of the detected liquid-phase product. The faradaic efficiency of liquid products can be calculated by equation: where n is the number of electrons required for producing one molecule of the related liquid product, and C product is the molar concentration of gas product measured by HPLC. V is the volume of the electrolyte. To obtain accurate selectivity of liquid products, we measured the volume of catholyte and anolyte after electrolysis, respectively.

Collection of liquid from electrolyte
It should be noted that the ion species carried with water molecules (hydrated ion) transports via membrane, which means the volume of catholyte and anolyte was vaired after electrolysis.
For AEM, a decrease in catholyte volume was observed with increased anolyte volume after several hours of CO 2 reduction electrolysis, because of the transportion of the anion species hydrated with water molecules from catholyte to anolyte via AEM as charge carriers. In contract, the use of CEM experienced an inceased catholyte volume with correspondingly decreased anolyte over the course of electrolysis, due to that the cation species hydrated water molecules transported from anolyte to catholyte via AEM as charge carriers. Notably, no obvious variation in both cathlyte and anolyte when BPM was used, which is due to that water supplied almost equally from both catholye and anolyte was disociated into H + and OH -, transporting to catholyte and anolyte, respectively.

S7
Based on the aforementioned discussion, in order to obtain accurate selectivity of liquid products, volume of catholyte and anolyte was also measured for each test after electrolysis, respectively.

Collection of liquid products evaporated from GDEs
Some liquid products can be evaporated from the gas diffusion layer of GDE and then flow out of the gas compartment of the reactor with unreacted CO 2 and gas products. To collect the evaporated liquid products from GDEs (i.e. gas chamber), gas outlet flow after the reactor was directly purged into a sealed bottle filled with 30 ml de-ionized water (the outlet flow tube was immersed into de-ionized water), as shown in Figure S3. After completion of CO 2 reduction, the liquid products diluted with de-ionized water in that sealed bottle were analysed via highperformance liquid chromatography (HPLC). Figure 2b presents the faradaic efficiencies of liquid products evaporated from GDEs when using distinct ion-selective membranes, indicating that only alcohols products such as ethanol and propanol evaporate and escape from the cathode/electrolyte interface irrespective of membrane types, which is due to their high volatility.
In addition, both catholyte and anolyte in the given reservoirs were collected for quantification of liquid products, owing to liquid products crossover from catholyte to anolyte via membranes. 5 Thus, the total amount of one certain liquid product formed on cathode GEDs can be written as: where and are the amount of one certain liquid product ℎ detected in anolyte and catholyte, respectively. is the amount of one certain liquid product evaporated from GDEs. Here, the evaporation ratio of one certain liquid product formed on cathode GDEs can be calculated based on the below equation: Thus, the equation (S13) was used to calculate a ratio between the amount of one certain liquid product evaporated from GDEs and the total amount of corresponding liquid product formed on the cathode, as shown in Figure S4.  Figure S4. Evaporation ratio of related liquid products escaped from GDEs (i.e. gas chamber) at 200 mA/cm 2 when using AEM, CEM and BPM, respectively.

Analysis of gas released from the anolyte
When the electrocatalytic CO 2 reduction occurs on the surface of the cathode, water oxidation reaction (i.e. O 2 evolution) takes place on the anode surface. By the water oxidation reaction, a large amount of H + can be created at the anode/electrolyte interface, which leads to a decrease of pH locally near the anode. Subsequently, H + produced at the anode/electrolyte interface can be neutralized with HCO 3 -, CO 3 2-or OHin anolyte. The H + neutralization with HCO 3or CO 3 2-forms CO 2 , leading to CO 2 degassing from anolyte with the stream of O 2 . 5 For analysing the gases released from anolyte over the course of CO 2 reduction, the flow electrolyser setup in Figure S5 was utilized. In that setup, N 2 at a constant flowrate was used as a carrier, thus gases released from anolyte were diluted with N 2 , directly venting into the gas sampling-loop of the GC for periodical quantification. The volumetric gas flow released from anolyte was also monitored by in situ flow meter over the electrolysis, as shown in Figure S5. S10 Figure S5. The schematic illustration of flow cell setup for detection of gases released from the anolyte over the course of CO 2 reduction.

Analysis of gas released from the catholyte using BPMs
No any gas evolution in catholtye was observed when AEM or CEM was used. However, under the use of bipolar membrane in flow electrolyzers, we found that gas bubbles released from the catholyte, which is unique in comparison with the other two membranes. To analyze the gas released from catholyte over the course of CO 2 reduction using BPM, a test setup in Figure S6 was utilized. Similar to gas analysis from anolyte, a constant N 2 flow was also used as a carrier gas, which mixed with gases released from catholyte, venting into the gas sampling-loop of the GC for periodical quantification, followed with an in situ volumetric flow meter ( Figure S6).
We found CO 2 gas releasd from the cathlyte, along with only trace amount of H 2 in Figure 4. It should be noted that the mole ratio of CO 2 /H 2 released from the catholyte is 5000, which means that the purity of released CO 2 is about 99.98%. S11 Figure S6. The schematic illustration of flow cell setup for detection of gases released from the catholyte over the course of CO 2 reduction under the use of BPM. 1 M KHCO 3 was used as initial catholyte (50 ml) and anolyte (50 ml).

Applied potentials on the cathode
Potentiostatic electrochemical impedance spectroscopy (PEIS) was conducted on Cu deposited GDE in the flow electrolyzer to determine the solution resistance (Rs). A detailed procedure was described in a previous work. 5 It should be noted that the distance between reference and cathode was less than 2 mm in order to reduce Rs in this work. Table S1 shows the solution resistance for the different ion-selective membranes. However, the cathodic reactions at high current densities can lead to a significant change of ion species and related concentration in the vicinity of the cathode, which indicates a difference in solution resistance near the cathode at high current densities compared to that of PEIS which was performed at relative low current densities. Thus, this difference in the solution resistance is closely correlated with the accuracy in IR-corrected potentials at a high current.

Theoretical estimation of O 2 and CO 2 flowrate generated from electrolyte
Assuming that all charge passed through the anode is just employed for oxidation of water into O 2 , thus theoretical O 2 flowrate released from anolyte can be expressed as 5 : where n and Q tot are the number (here is 4) of electrons lost from 2 H 2 O for forming one O 2 molecule and totoal charge passed through the anode, respectively. F is the faradaic constant, is ideal gas constant, T is absolute temperature, and is ambient pressure.
From our prevous work, the ratio of CO 2 and O 2 released from the anolyte will be 4, 2 and 0 if the only anion species for neutralization reaction with H + is HCO 3 -, CO 3 2or OH -. 5 Thus, after S13 getting the O 2 flowrate at 200 mA/cm 2 (the cathode with 2 cm 2 geometric active area was used for all the tests) based on the equation S14, we can easily get the related flow of CO 2 , as shown in Table S2.

Electrolyte pH and conductivity measurements
pH of the catholyte and the anolyte was monitored by a pH meter (pH 110, VWR) during the electrolysis. In addition, the pH meter was also equipped with a temperature sensor for the temperature-compensation. The pH meter was calibrated by a standard pH 7 buffer and a standard pH 10 buffer before the measurement.
The conductivity of the catholyte and the anolyte was monitored by a conductivity meter (PCE-PHD 1-PH, PCE Instruments) during CO 2 reduction electrolysis. Before the measurement, the conductivity meter was calibrated via conductivity standard of 1413 µS / cm (25 °C; 0.01 M KCl) and 111.8 mS / cm (25 °C; 1 M KCl) purchased from VWR. It should be noted that both of the calibration and the measurement were temperature-compensated due to that the solution conductivity is also temperature-dependent at a fixed solution concentration.

S14
Because the AEMs are positively charged, anions can pass, and the crossover of positively charged ions will be decreased drastically. 4 In other words, no ideal AEM can absolutely avoid the crossover of positively charge ions. In this work, for the use of the AEM, when the anion species served as the main charge carriers transporting from the catholyte to the anolyte via the membrane, a small amount of K + slowly crossed over from the anolyte to the catholyte via the AEM. Thus, the anolyte concentration gradually decreased, along with the correspondingly increased catholyte concentration over electrolysis, leading to the related variation in the anolyte and catholyte conductivity over time ( Figure S8a). S15 Figure S8. Conductivity of catholyte and anolyte as a function of time when using AEM (a), CEM (b) and BPM (c) over the course of CO 2 reduction electrolysis at 200 mA/cm 2 . 1 M KHCO 3 was used for both catholyte (50 ml) and anolyte (50 ml).

Calculation of the carbon balance
The residual unreacted CO 2 flowrate in the gas outlet (gas mixture) out of gas compartment of flow electrolzyers can be written as: where is the monitored gas flowrate out of the ractor during CO 2 reduction electrolysis ∅ using the setup shown in Figure 2S respectively. Based on the equation S1-3, each molecule of CO, CH 4 and C 2 H 4 formation requires 1, 1 and 2 CO 2 molecule, Thus, the consumed CO 2 flowrate that is converted into all gas products (CO, C 2 H 4 and CH 4 ) in CO 2 reduction can be expresed as below: Depending on the number of carbon atoms in liquid molecule produced in CO 2 reduction, the consumed CO 2 flowrate involved in all liquid products formation can be written as: where , , and are the consumed CO 2 flowrate for forming C 1 , C 2 and C 3 liquid ∅ 1 ∅ 2 ∅ 3 products, respectively. For high-rate CO 2 reduction, the inevitably caputred CO 2 in forms of carbonate via reaction with OHcould consume substantial CO 2 flow, significantly reducing the total gas flow out of the reator (Figure 1c). It is known that the carbon element from CO 2 inlet flowrate should be eventually balanced with those of residual unreacted CO 2 , all products and carbonate formed via reaction between OHand CO 2 . Thus, the consumed CO 2 flowrate via the reaction with OHgenerated on the cathode surface can be expressed as: where is CO 2 flowrate fed into the gas chamber of the reactor. In this work, a ∅ 2 constant CO 2 flow was used. It should be noted that mass flow controller used in this work was calibrated for CO 2 flow by volumetric flow meter before and after each CO 2 reduction test for high accuracy. Thus, we got the below carbon balance in Table S3. S17 : the consumed CO 2 flowrate for all liquid products in CO 2 ∅ 2 reduction (such as ethanol); : the consumed CO 2 flowrate via the reaction with OH -∅generated in cathodic reactions; : the residual unreacted CO 2 flowrate in the gas ∅ 2 outlet (gas mixture) out of gas compartment of flow electrolzyers) S18

Correlation between the carbonate/bicarbonate ratio and pH
The pH of a buffer solution can be estimated by the Henderson-Hasselbalch equation, as below: where pKa is the acid dissociation constant, and [CO 3 2- Here, the pKa is 10.3 at 25 o C based on the equation S9. The pH of the catholyte was measured by the pH meter for the different membranes, as shown in Figure 3d, e and f. Thus, the concentration ratio of carbonate/bicarbonate in the catholyte can be estimated according to the equation S20.  (Figure 3d), corresponding to [CO 3 2-]/[HCO 3 -] ratio of 10, which is consistent with the catholyte transformation from bicarbonate to carbonate over the electrolysis. For the BPM, the catholyte pH was maintained ˂ 9 over the entire electrolysis (Figure 3f), indicating that most of anion species was bicarbonate.

Cell voltage
Cell voltage as a function of time when using the three different membranes over the course of CO 2 reduction electrolysis is shown in Figure S9, which indicates that an additional potential of ~ 2 V was required for the BPM compared to those of the use of CEM and AEM. In addition, it should be noted that an apparent fluctuation in the applied potentials was observed due to the effect of CO 2 degassing at the BPM/catholyte interface. With the use of the CEM, the anolyte conductivity rapidly decreased from ~70 mS/cm to ~3 mS/cm after ~ 3 h ( Figure S8b), which leads to the increased cell voltages for maintaining the constant current density of 200 mA/cm 2 ( Figure S9b).