Kilowatt-scale alkali-cation-free CO_2 electrolysis via accelerating mass transfer

Domain: physics
Published: 2026-02-12
Editorial outcome: major_revision

Abstract
Electrocatalytic CO2 reduction (ECO2R) presents a sustainable pathway for industrial decarbonization by converting CO2 into carbon-neutral fuels and chemicals. Despite progress in catalyst design, industrial scalability is hindered by slow mass-transfer kinetics. Here, we introduce a high-diffusion-flux gas diffusion electrode (HDF-GDE) that overcomes this limitation in alkali-cation-free systems, achieving CO2 conversion rates at industrial current densities. Kinetic analysis demonstrates that conversion is governed by mass transfer efficiency rather than flow rate. By optimizing the GDE structure to maximize CO2 diffusion and GDE utilization, we realize a kW-scale ECO2R system with stability (>1000 hours), producing CO or C2H4 depending on the catalyst. Operating with a 3 L/min CO2 flow rate, the system delivers 144 kg of CO (1.29 kW) or 17 kg of C2H4 (1.95 kW) over 1000 h. The alkali-cation-free ECO2R system, equipped with HDF-GDEs, demonstrates economic viability for large-scale ECO2R-to-CO/C2H4 production. Our findings bridge the gap between lab innovation and real-world deployment, advancing carbon-neutral chemical manufacturing.

Introduction
The electrocatalytic CO2 reduction (ECO2R) technology, as a vital technical link between fossil fuel and sustainable energy, is considered one of the most promising options for rapidly cutting CO2 emissions at the industrial level 1–3. In order to improve its practical feasibility, the ECO2R technology has advanced from the H cell to flow and membrane-electrode-assembly (MEA) cells utilizing the gas diffusion electrode (GDE) (Fig. 1) (refs. 4,5). However, their stabilities were still problematic (e.g., carbonate formation and crossover) (refs. 6–8). More recently, MEA system stability has been improved by approximately one order of magnitude (>1000 h for ECO2R-to-C2H4) using an alkali-cation-free (pure-H2O-fed) MEA system9. Still, the low current density (only ~120 mA/cm2 towards C2H4) resulted in limited product yield (low CO2 conversion: ~1% for ECO2R-to-C2H4), fatally hindering the industrialization of the ECO2R technology.

The ECOR MEA cell with GDEs was derived from the fuel cell community (Fig. 1a–c and Supplementary Fig. 2) (refs. 10–13). However, the ECOR system is more complicated than the fuel cell one (see Supplementary Note 1) (refs. 1,14). Taking the carbon paper GDE (CP-GDE) as an example, for ECOR, at the cathode, the H2 from the competing hydrogen evolution reaction (HER), multiple products, and unreacted CO2 make the mass transfer in the system complicated (Fig. 1b). This results in irreconcilable contradictions between the high ECOR product selectivity (Faradaic efficiency: FE) and the high CO2 conversion (see Supplementary Note 1, and Supplementary Fig. 3 and Table 7 for more information). That is, lowering the reactant CO2 inlet flow rate could increase CO2 conversion, but this could considerably decelerate CO2 mass transfer kinetics in CP-GDE and decrease product selectivity.

To achieve both high CO2 conversion and high product selectivity, we proposed the high-diffusion-flux GDE (HDF-GDE) that consists only of the catalyst (Fig. 1d). HDF-GDE can allow CO2 to diffuse directly into the catalyst layer without having to pass through the inert CP-GDE substrate (Fig. 1e). This could not only significantly increase the overall flux of species both to and from the HDF-GDE but also greatly enhance the space utilization of GDE. From a chemical kinetics perspective, HDF-GDE could accelerate the system’s mass transfer kinetics, increasing CO2 and key reaction intermediate coverage (such as *CO) on the catalyst surface and decreasing the coverage of *H from water transfer15. This could improve ECO2R activity due to first-/second-order dependence on CO2/*CO coverage1,15.

In this study, we aimed to achieve a high operating current density (i.e., high CO2 conversion) in a high-stability alkali-cation-free ECO2R system while maintaining high product selectivity by accelerating mass transfer kinetics. Here, the alkali-cation-free condition contains trace amounts of sodium ions originating from the water source9. To achieve this, we developed an HDF-GDE comprising the catalyst and a stainless-steel mesh skeleton in the catalyst’s middle layer. A high-performance stepped-surface Ag (SS-Ag) catalyst was used as a model catalyst to study the system’s mass-transfer kinetics, given its single-product selectivity (i.e., CO). As a result, by improving the mass transfer efficiency, the current density of the alkali-cation-free ECO2R-to-CO system was increased from 150 to 400 mA/cm2 while maintaining high product FE (~93%). To further improve CO2 conversion and advance the industrialization of ECO2R technology, we designed and customized a kW-scale alkali-cation-free MEA cell stack assembled with HDF-GDEs to evaluate the entire system’s stability. At a power of ~1.29 kW (60 A) and 3 L/min CO2 inlet flow rate, CO2 conversion broke 81% with a CO FE of ~90%. Furthermore, the HDF-GDE strategy was also successfully applied to ECO2R-to-C2H4. At ~1.95 kW (75 A) and 3 L/min CO2 inlet flow rate, CO2 conversion for ECO2R-to-C2H4 was increased to ~15 times. Their system stability was over 1000 hours, yielding ~144 kg CO and ~17 kg C2H4, respectively. Real-time analysis of mass transfer kinetics could revolutionize the design of the ECO2R system and accelerate its industrial deployment.

Results
To create high-energy surfaces in the catalyst, thus improving the ECO2R performance, we prepared the model catalyst SS-Ag nanoparticles (∼20−30 nm) with abundant stacking faults (SFs) and grain boundaries (GBs) (Fig. 2a and Supplementary Figs. 4–6). Aberration-corrected four-dimensional scanning TEM (STEM) (4D-STEM) revealed that abundant SFs and GBs in SS-Ag intersect with each other and result in the stepped surface formation (Fig. 2a–c and Supplementary Fig. 6). SFs and GBs finally lead to a decrease in the coordination number (CN) of Ag and an increase in lattice tensile strain (see Supplementary Figs. 7–11, Supplementary Table 8, and Supplementary Note 2). As a result, for ECO2R, SS-Ag delivered that FE towards CO reached ~99% under a wide potential window (−0.32 V to −0.65 V versus a reversible hydrogen electrode (RHE)) in a flow cell with the conventional electrolyte (1 M KOH) (Supplementary Figs. 12 and 13). We also found that the low CN and high tensile strain boosted ECO2R performance (see Supplementary Figs. 12–18 and Supplementary Note 3 for more information).

To suppress carbonate formation and crossover during ECO2R and improve the system stability, we adopted an alkali-cation-free MEA architecture with the anion- and proton-exchange membranes (AEM and PEM assembly: APMA) to evaluate the ECO2R performance of SS-Ag CP-GDE (Supplementary Fig. 19) (ref. 9). The reaction temperature was optimized at 60 °C to decrease the reaction overpotential of the pure-H2O system (Supplementary Fig. 20; the following ECO2R tests were conducted at 60 °C, unless otherwise noted).

In the alkali-cation-free APMA MEA system, at 150 mA/cm2, SS-Ag CP-GDE achieved CO and H2 FEs of ~93% and 6%, respectively (Fig. 2d, e). The cell voltage was ~3.88 V (without iR compensation for all tests in the MEA system throughout the text). In-situ Raman and isotope experiments proved that this alkali-cation-free APMA MEA system could effectively suppress carbonate formation (see Supplementary Figs. 21, 22 and Supplementary Note 4). However, the low current density (only 150 mA/cm2) resulted in a low CO2 conversion (~3.23%), drastically limiting its viability.

To increase CO2 conversion, we first decreased the CO2 inlet flow rate from 30 to 2 Standard Cubic Centimeters per Minute (sccm) (Fig. 2f). As expected, CO2 conversion gradually increased from ~3% to 43% (Fig. 2g). However, the CO FE and partial current density gradually decreased as reducing CO2 inlet flow rate (FECO: from ~93% to 82%; jCO: from ~139 to 123 mA/cm2) and those of H2 gradually increased (FEH2: from ~6% to 17%; jH2: from ~9.7 to 25.9 mA/cm2) (Fig. 2h and Supplementary Fig. 23). That is, in this CO2 mass transport limited regime (i.e., low CO2 flow rate), the side product (H2) FE increased at the expense of that of the product (CO). The cell voltage also slightly changed from ~3.88 to 3.49 V (Fig. 2f, g). These showed that, for high CO2 conversion, a very low CO2 inlet flow rate could decrease CO2 mass transfer efficiency, leading to lower CO2/*CO coverage on the catalyst surface and thus suppressing ECO2R and increasing HER15,16. In other words, based on CP-GDE, a high CO2 inlet flow rate (i.e., fast CO2 mass transfer kinetics) could favor a higher formation rate of ECO2R product but lower the fraction of CO2 converted into product; thus, a lower CO2 conversion efficiency was observed (see Supplementary Fig. 24 and Supplementary Note 5) (refs. 17,18).

To achieve both high product selectivity and high CO2 conversion in the alkali-cation-free system, the mass transfer in the ECO2R system must be revolutionized, especially in the GDE design. Since the coverage of CO2 and the key intermediate *CO on the catalyst surface is determined by the overall flux of species both to and from the GDE surface, we re-designed and assembled an HDF-GDE by screen printing (Supplementary Fig. 25). HDF-GDE could increase the coverage of CO2 and the key intermediate *CO and overcome the mass transfer and diffusion restrictions in traditional GDEs (such as carbon paper and porous polytetrafluoroethylene (PTFE)) and ultimately increase the operation current density (i.e., CO2 conversion) of the alkali-cation-free system without lowering the CO2 flow rate.

To enhance the mechanical strength of HDF-GDE, a stainless-steel mesh (SSM) was used as the supporting skeleton for the catalyst sheet (Fig. 3a and Supplementary Figs. 26, 27). The porous SS-Ag HDF-GDE, whose thickness was controlled to ~300 μm by the mechanical mold, exhibited hydrophobicity due to the incorporated PTFE in the catalyst layer (Fig. 3a and Supplementary Figs. 28, 29). And HDF-GDE’s contact angle was up to ~143.9° (Supplementary Fig. 30). SS-Ag HDF-GDE only contains the catalyst, PTFE and an SSM skeleton located in the middle layer of the catalyst, allowing CO2 to directly contact the catalyst without passing through the inert substrate (such as CP and PTFE GDEs). This could essentially increase the overall flux of CO2/*CO to and from the HDF-GDE surface, accelerating the system’s mass transfer kinetics.

Upon acquiring the expected SS-Ag HDF-GDE, we evaluated its ECO2R performance in the alkali-cation-free MEA cell (Fig. 3b). The SS-Ag HDF-GDE still maintained high CO FE (~90%) at a cell voltage of ~3.19 V (Fig. 3c). More importantly, compared to SS-Ag CP-GDE, the peak current density based on SS-Ag HDF-GDE increased from 150 to 400 mA/cm2 with a negligible decline in CO FE (from ~93% to ~90%) and a slight increase in cell voltage (from ~3.88 to ~3.91 V) (Fig. 3d, f). It is worth noting that this is a pure-H2O-fed system, with a current density of 400 mA/cm2, which is even better than that of alkali-metal electrolyte systems. Additionally, HER was effectively suppressed, especially at the high current density (from ~70% to ~14% at 500 mA/cm2) (Fig. 3e). These results showed that more CO2/*CO could occupy active sites in SS-Ag while decreasing *H coverage on the catalyst surface, which improved the ECO2R activity and suppressed HER19,20. As a result, CO2 conversion efficiency on SS-Ag HDF-GDE was significantly increased to ~2.6 times without decreasing the CO2 inlet flow rate (Fig. 3g), compared with SS-Ag CP-GDE.

To verify that the improved overall flux of species to and from the HDF-GDE surface caused the increase in CO2 conversion efficiency, we investigated the CO2 mass transfer kinetics in SS-Ag HDF-GDE. On a macroscopic level, SSM possesses visible holes that are more conducive to gas transfer and diffusion than the CP-GDE substrate (Supplementary Fig. 31). On a microscopic level, although the front microporous carbon powder layer of CP-GDE can facilitate the uniform diffusion of the gas to some extent (Supplementary Fig. 32), it can also inhibit access of the gas transfer and diffusion and cause the high pressure gradient21,22, compared with SSM (Supplementary Fig. 33). Thus, when SS-Ag was assembled on different GDEs (Supplementary Fig. 34), SS-Ag CP-GDE could further limit the gas transfer and diffusion, compared to SS-Ag HDF-GDE. Scanning electron microscopy (SEM) images showed that SS-Ag was evenly distributed on different GDEs (Supplementary Fig. 35). The pore size of HDF-GDE was much larger than that of SS-Ag CP-GDE, which was in the micron and nano scales, respectively (Supplementary Fig. 35a–f), indicating that HDF-GDE could benefit CO2 mass transfer and diffusion, compared to that of CP-GDE.

To further verify that the fast CO2 mass transfer kinetics in HDF-GDE resulted in higher operation current density (i.e., higher CO2 conversion), rather than the higher catalyst loading, we prepared SS-Ag CP-GDE with the same catalyst loading as SS-Ag HDF-GDE (~10 mg/cm2). SS-Ag CP-GDE showed a significant decline in CO selectivity at 400 mA/cm2 with a CO FE of ~63% (Supplementary Fig. 36), compared with SS-Ag HDF-GDE (~90% CO FE). For SS-Ag CP-GDE, a high catalyst loading could inhibit CO2 access to the catalyst surface and thus suppress ECOR (i.e., low space utilization). We also investigated the effect of SS-Ag loading in HDF-GDE on ECOR performance (Supplementary Fig. 37). Catalyst loadings that are too low or too high can also significantly suppress ECOR. A low catalyst loading cannot provide enough active sites; too high a catalyst loading can also inhibit CO2 access to the catalyst surface due to the dense catalyst layer (Supplementary Fig. 38) (refs. 10,22). In addition, we employed the flow cell with 1 M KOH as the electrolyte to evaluate SS-Ag HDF-GDE ECOR performance (Supplementary Fig. 39). FE towards CO remained ~99% over a wide potential window. Still, the peak total current density increased from ~360 mA/cm2 to ~1700 mA/cm2 (Supplementary Fig. 40), compared to SS-Ag CP-GDE. These emphasized that HDF-GDE accelerated CO2 mass transfer kinetics, and that the fast CO2 mass transfer was a primary factor determining ECOR performance for SS-Ag HDF-GDE.

Further, we carried out in-situ Raman experiments to study ECO2R product (CO/*CO) coverage on different GDEs (Fig. 3h). Due to the effective CO2 mass transfer and diffusion, CO2 coverage on the HDF-GDE surface should be higher than that on the CP-GDE surface, which can cause a higher CO/*CO coverage during ECO2R9,23,24. For SS-Ag HDF-GDE, the intensity of CO/*CO located at ~230-360 cm-1 is significantly higher than that based on the SS-Ag CP-GDE, directly proving the overall flux of species both to and from HDF-GDE was increased (i.e., fast mass transfer kinetics)23,24. The high-efficiency CO2 mass transfer and diffusion can also suppress the accumulation of *H by transferring H2O on the catalyst surface, thereby improving the peak current density of ECO2R and increasing CO2 conversion without decreasing the CO2 inlet flow rate.

To quantitatively analyze mass transfer kinetics in GDEs, we simulated CO2 mass transfer. Although the models have limitations, the trend of mass transfer under the same conditions can reflect the mass transfer efficiency of different GDEs. The traditional CP-GDE contains carbon fiber and carbon powder layers with a total thickness of ~200-500 μm (Supplementary Fig. 41a) (ref. 4). Generally, the catalyst loading on CP-GDE is ~1 mg/cm2 and its thickness is ~2 μm (Supplementary Fig. 41b, c) (ref. 23). Under the real reaction environment, CO2 and products can present a concentration gradient distribution in the catalyst layer (Fig. 1a). Taking the vertical and parallel directions (dVertical and dParallel) of the catalyst layer as examples (Supplementary Fig. 41de), the closer to the cell outlet, the lower the concentration of CO2, and the opposite for products.

Under unreacted steady-state conditions, modeling of CO2 mass transfer kinetics in different GDEs showed that HDF-GDE could slightly accelerate CO2 mass transfer kinetics in the catalyst layer, compared with that of CP-GDE in both the vertical and parallel directions (Supplementary Fig. 42, see multiphysics simulation section in Supplementary information for more information). The effects of the thickness and porosity of HDF-GDE and CO2 flow rate on CO2 mass transfer were also investigated, showing that HDF-GDE’s thickness and CO2 flow rate were the two main factors influencing CO2 mass transfer efficiency under unreacted steady-state conditions (Supplementary Figs. 43–48). The GDE’s thickness was negatively correlated with CO2 mass transfer, and the CO2 inlet flow rate was positively correlated with CO2 mass transfer. Furthermore, in the vertical direction to the catalyst layer, there was almost no significant difference in CO2 mass transfer in HDF-GDE and CP-GDE (Supplementary Figs. 49–51).

When the current density of ECO2R was 150 mA/cm2, the CO2 mass transfer kinetics in GDEs were also simulated (to simplify the model, HER was not considered). At the same CO2 inlet flow rate, at the inlet, the efficiency of CO2 mass transfer to the catalyst surface in HDF-GDE was significantly higher than that in the CP-GDE in the dVertical and dParallel directions (Fig. 4a, b and Supplementary Fig. 52), indicating that CO2 coverage on the catalyst surface in HDF-GDE was higher than that in the CP-GDE at the inlet. As the reaction progressed, the CO2 coverage decreased gradually for both GDEs along the flow channel direction (Fig. 4c–e). Near the outlet, the CO2 coverage on the catalyst surface in HDF-GDE is less than in the CP-GDE. This is because, in the catalyst layer, the product (CO/*CO) coverage in HDF-GDE was higher than in the CP-GDE, which was almost a factor of magnitude difference (Fig. 4f–h and Supplementary Fig. 53). One CO2 molecule can generate one CO molecule, thus, the overall system’s mass transfer needed to consider CO2 and CO (i.e., the overall carbon concentration). Obviously, the system’s mass transfer efficiency of HDF-GDE is much higher than that of CP-GDE. Furthermore, the high carbon coverage on the catalyst surface in HDF-GDE could occupy more reaction sites, thereby decreasing the *H coverage, suppressing HER, and enhancing ECO2R-to-CO activity. In addition, the polarization curves of SS-Ag CP-GDE and HDF-GDE under N2 showed that SS-Ag HDF could indeed be capable of suppressing HER (Supplementary Fig. 54). These synergistically improved the peak operation current density for ECO2R and, thus, increased CO2 conversion.

After demonstrating ECO2R-to-CO in the alkali-cation-free MEA single cell with an SS-Ag HDF-GDE area of 1×1 cm2, we designed and customized a stack containing six MEA cells (each MEA cell with an area of 15×10 cm2) to validate the practical application possibilities of HDF-GDE combined with the alkali-cation-free MEA system (Figs. 5a–c). The scale-up demonstration and stability testing can demonstrate its practical viability, further advancing the ECO2R technology toward industrial deployment.

During scaling up, various engineering problems may arise. For example, under high power, the temperature at the MEA core can increase significantly due to resistive heating (Supplementary Table 9), and a uniform voltage distribution throughout the stack is also necessary25–28. As the power of the alkali-cation-free MEA cell stack assembled with SS-Ag HDF-GDE was scaled up to a kilowatt level, the initial temperature of pure H2O anolyte was lowered to 30 °C to balance the temperature at the MEA core and the CO2 inlet flow rate was increased to 3000 sccm.

To examine the stack’s durability, the scaled-up alkali-cation-free MEA cell stack assembled with SS-Ag HDF-GDEs was operated for 1000 h at a total current of 60 A (Fig. 5d and Supplementary Fig. 55). The full-cell-stack voltage remained remarkably stable at 21-22 V, with an operating power of ~1.29 kW. Six SS-Ag HDF-GDEs gave a relatively stable CO FE of ~89-80% and yielded ~144 kg CO throughout the stability test (Fig. 5d). The slight voltage increase and CO FE decrease could be attributed to the mild GDE flooding under the high power. We also monitored each cell’s operation voltage, which fluctuated slightly but showed nearly the same overall voltage throughout the 1000 h measurement (Supplementary Fig. 56). The CO2 conversion in this kW-scale cell stack was up to ~81% at a CO2 inlet flow rate of 3000 sccm, which was ~25 times more than that of SS-Ag CP-GDE (~3.2%) in this single alkali-cation-free MEA cell (Fig. 3g, f, and Supplementary Table 7). Furthermore, the peak current density of HDF-GDE in the alkali-cation-free system is comparable to or even better than that of CP-GDE operated in alkali metal electrolytes (Supplementary Table 7). The stack system with HDF-GDEs achieved high CO2 conversions from ECO2R to CO at the high CO2 flow rate (Fig. 5f). We also investigated SS-Ag stability with SFs and GBs. HAADF-STEM images showed that the SS-Ag retained its structural integrity after 1000 h of ECO2R (Supplementary Fig. 57). After 1000 h of ECO2R, CP-GDE’s contact angle significantly dropped from ~137.5° to ~90.1°, while that of HDF-GDE only slightly decreased from ~143.9° to ~134.1° (Supplementary Fig. 30). The durable hydrophobicity of HDF-GDE might be due to the direct addition of PTFE to the catalyst layer, rather than just on the substrate, compared to that of CP-GDE. The stable SS-Ag catalyst, durable HDF-GDE, and alkali-cation-free MEA system, along with fast mass transfer kinetics in HDF-GDE, synergistically boosted ECO2R performance.

We sought to selectively convert CO2 to C2H4 using a Cu electrocatalyst, given the high value and broad applications of C2H4 in the chemical industry. Based on our previously reported SS-Cu9, we fabricated SS-Cu HDF-GDE. We evaluated its ECO2R-to-C2H4 performance in the alkali-cation-free MEA cell (Supplementary Fig. 58). The peak C2H4 FE reached ~45% with a cell voltage of ~4.36 V, comparable to SS-Cu CP-GDE (FE: ~43%; cell voltage: ~4.30 V)9, but the total current density was increased from 300 to 500 mA/cm2 (ref. 9), indicating that the HDF-GDE strategy was also suitable for alkali-cation-free ECO2R-to-C2H4/C2+. At a total current of 75 A, the stability of the kW-scale (~1.95 kW) alkali-cation-free APMA MEA cell stack assembled with 6 SS-Cu HDF-GDEs was also over 1000 h, yielding ~14 kg C2H4 (Fig. 5e and Supplementary Figs. 59, 60). And ECO2R-to-C2H4 conversion in this alkali-cation-free APMA MEA cell stack increased to approximately ~15 times, compared to the SS-Cu CP-GDE in the initial single APMA MEA cell9. For both SS-Ag and SS-Cu (ECO2R-to-CO/C2H4), HDF-GDEs improved CO2 conversion efficiency by increasing ECO2R current density rather than decreasing the CO2 inlet flow rate, while maintaining high product selectivity, effectively alleviating the trade-off between high product selectivity and high CO2 conversion. Furthermore, the operating current density of the pure-H2O-fed (alkali-cation-free) system was effectively exceeded.

The rapid acceleration toward commercially viable ECO2R-to-CO/C2H4 necessitates techno-economic analysis (TEA) to identify key areas for improvement. Thus, an industrial-level TEA based on the performance of our durable alkali-cation-free kW-scale ECO2R-to-CO/C2H4 system was provided (see the Techno-economic analysis section in Supplementary information for more information). TEA includes additional costs for gaseous product purification, assuming unreacted CO2 is separated and recycled as fresh feedstock. The kW-scale alkali-cation-free ECO2R-to-CO is economically viable with a levelized CO cost of ~0.48 USD/kg (Supplementary Fig. 61a), which is below the market price of ~0.6 USD/kg (ref. 3). Our alkali-cation-free kW-scale ECO2R-to-CO system is competitive with the systems with CsHCO3 as the electrolyte3. The most significant advantage of the alkali-cation-free system is its high stability. With a longer lifetime (5-year MEA replacement period) for the alkali-cation-free APMA system assembled with HDF-GDEs, the levelized CO cost can be reduced by an additional 13% (~0.42 USD/kg). The MEA lifetime is negatively correlated with product cost (Supplementary Fig. 62). Compared with SS-Ag CP-GDE, the HDF-GDE strategy reduced the cost of ECO2R-to-CO by 35% (Supplementary Fig. 61a). Electricity consumption is the most significant contributor to the levelized cost of CO production. Sensitivity analysis shows that a 20% increase in electricity price leads to an over 10% rise in the levelized CO cost (Supplementary Fig. 61b). Other economic parameters, such as the MEA replacement period and CO2 price, affect the final TEA result by less than 5% with a 20% fluctuation.

The choice of electricity sources can also impact the economic viability of the ECO2R-to-CO system. Electricity from coal combustion with carbon capture and storage is over 300% more expensive than the lowest-cost option, making it economically unacceptable. The significant uncertainty in renewable energy sources, such as wind power, leads to varied outcomes regarding the economic viability of the alkali-cation-free ECO2R system (see Supplementary Fig. 63).

For ECO2R-to-C2H4, although the alkali-cation-free system assembled with HDF-GDEs was competitive with the reference study, which used KHCO3 as an electrolyte28,29, the levelized C2H4 cost reached ~5.63 USD/kg, exceeding the market price of ~1.0 USD/kg (refs. 4,28), which is due to the low C2H4 selectivity in the alkali-cation-free system. With an assumed extended 5-year lifetime for the MEA, the levelized C2H4 cost can be further reduced by ~10% (Supplementary Fig. 61c). Taking into account the market value of the byproducts (H2 and CO), the C2H4 cost can be lowered to ~1.66 USD/kg. The economic roadmap proposes that implementing a carbon tax of 60 USD/ton and reducing electricity prices (based on China’s green electricity at 0.2 CNY/kWh) can render alkali-cation-free ECO2R-to-C2H4 economically viable at ~0.48 USD/kg. Together, from TEA, the alkali-cation-free ECO2R system assembled with HDF-GDEs can achieve economic viability at scale for ECO2R-to-CO/C2H4.

Methods
Chemicals and materials
Iridium(IV) oxide (IrO2, ≥ 99.9%, 206237), NafionTM solution (5 wt.%, 274704), 3-(Trimethylsilyl)propionic-2,2,3,3-d4 acid sodium salt (TSP, ≥ 98.0% (NMR), 269913), Polytetrafluoroethylene preparation (PTFE solution, 60% in H2O, 665800), Deuterium oxide (D2O, 99.9 at.% D, 151882), Silver nitrate (AgNO3, ≥ 99.0%, 209139), Water-18O (H218O, ≥ 10 at.% 18O, 332089), α−terpineol (90%, 432628), 2-(2-butoxyethoxy)ethanol (≥ 99%, 579963), Oleylamine (70%, O7805), acetone (≥ 99.5%, 179124), Copper(I) chloride (CuCl, 97%, 212946), n-hexane (C6H14, 99%, HX0293), Octadecylamine (≥ 99%, 305391), Ethyl 2-cyanoacrylate (E1505, 7085-85-0), Trioctylphosphine (90%, 117854) and squalane (96%, 234311) were purchased from Sigma Aldrich. The alkaline ionomer solution (5% in ethanol, Sustainion XA-9) and anion exchange membrane (Sustainion X37-50) were purchased from Dioxide Materials. Potassium hydroxide (KOH, ≥ 85.0%), stainless-steel mesh (SSM, 0.10 mm thickness, 500×500 mesh; warp, ~25 μm) and Titanium fiber felt (0.25 mm thickness, 99.9%) were purchased from Sinopharm Chemical Reagent Co., Ltd. (China). Acrylic resin (E0545) was purchased from Shenzhen JITIAN Chemical Co., LTD. Isopropanol (C3H8O, IPA, ≥ 99.5%, 3776) was purchased from Fisher Scientific. The anion exchange membrane (Fumasep FAA-3-PK-75), gas diffusion layer (carbon paper, GDE, Sigracet 39BC), and Nafion® 117 membrane (591239) were purchased from Fuel Cell Store.

Catalyst synthesis
In a typical synthesis, 0.07 g of AgNO3 and 0.02 g of octadecylamine were dissolved in 0.8 mL of squalane, and the solution was heated for 0.5 h at 80 °C under an inert gas (Ar) atmosphere to form an Ag-based stock solution. Meanwhile, a mixed solution of 10 mL oleylamine and 0.04 mL trioctylphosphine was heated to 170 °C under an Ar atmosphere with vigorous magnetic agitation. Subsequently, the Ag-based stock solution was injected into the above mixture and heated to 170 °C for 5 h. After natural cooling, the resulting sample was collected by centrifugation and washed several times with n-hexane. Finally, the sample was dried at 80 °C in a vacuum before use. The sample was denoted as SS-Ag.

The SS-Ag samples were annealed at various temperatures (250, 350, and 450 °C) in the tube furnace for 2 h under a mixed gas (H2/Ar: 5 v/v%; 200 sccm), denoted as Ag-250, Ag-350, and Ag-450, respectively.

The Cu sample (SS-Cu) was also prepared by the above method, using a Cu-based stock solution (CuCl) instead of the Ag-based stock solution, and the reaction temperature was increased to 200 °C (see ref. 9 for more detailed information).

GDEs fabrication
For conventional/traditional CP-GDEs in the flow cell, carbon paper (Sigracet 39BC) served as the gas diffusion layer. The cathode catalyst was dispersed in a mixed solution of H2O and IPA (1:4 v/v) and a NafionTM solution (~3 wt.% vs. catalyst) by sonication for 1 h to form the catalyst ink (1 mg/mL). Then, the catalyst ink was sprayed onto the microporous carbon layer of the carbon paper at a catalyst loading of 1–3 mg/cm2, followed by drying at 120 °C under vacuum for 1 h before testing. The anode GDE using the commercial IrO2 as the anode catalyst was also prepared by the above method.

For conventional/traditional CP-GDEs used in the APMA MEA cell with pure H2O as the anolyte, the NafionTM solution in the cathode catalyst ink was replaced by an alkaline ionomer solution (Sustainion XA-9), and nothing else changed. In addition, Ir-supported Ti fiber felt (Ir/Ti) was used as the anode electrode. Ir was sputtered on the Ti fiber felt using a pure Ir target in an Ar environment (5×10-3 Torr) in a magnetron sputtering system. Then, Ir/Ti was pressed onto the carbon paper to form the anode GDE.

For high-diffusion-flux (HDF) GDEs, the cathode catalyst (SS-Ag/SS-Cu) was printed onto the SSM skeleton by screen printing, followed by vacuum drying overnight. After that, the SSM-skeleton-supported catalyst was pressed at 1000 pounds of pressure for 2 h and the thickness of the HDF-GDE was controlled to ~300 μm. Finally, it was calcined at 350 °C for 30 min. The printing ink was prepared by mixing the catalyst and PTFE solution with an organic solution consisting of α-terpineol, 2-(2-butoxyethoxy)ethanol, and acrylic resin, with an approximate weight ratio of 65:15:20. The catalyst loading in HDF-GDE was controlled to about 10 mg/cm2.

Electrocatalytic CO2 reduction measurements
Electrocatalytic CO2 reduction (ECO2R) in the flow cell was to employ an electrochemical workstation (CHI 660E) connected to a current booster (CHI 680C). The CO2 flow rate (30 sccm, unless otherwise noted) was controlled by a mass flow controller (MFC, Alicat Scientific MC). The electrolyte (1 M KOH, pH = ~14) flow rate was ~10 mL/min, controlled by a peristaltic pump. The cathode area in the flow cell was 1 cm×1 cm. The flow cell consists of two titanium gas flow channels, two PTFE buffer layers, a membrane (Fumasep FAA-3-PK-75) between the two buffer layers, two titanium current collectors, and two aluminum end plates. The titanium gas flow channel is engraved with a serpentine flow channel to guide the CO2 stream and conduct the electricity. The serpentine flow channel is featured with a width of 1 mm and an interval of 1 mm. Both cathode and anode chambers have two 0.5-cm-thick PTFE buffer layers, through which the 1 M KOH electrolyte stream flows. All ECO2R measurements in the flow cell were carried out at room temperature and the Hg/Hg2Cl2 (SCE, saturated KCl) was used as the reference electrode, and all cathode potentials (vs. Hg/Hg2Cl2) were converted to the RHE scale via the following Eq. 1:

E(RHE) = E(Hg/Hg2Cl2) + 0.241 + 0.0591 × pH + iR (1)

where R is the resistance between the cathode and reference electrodes in the flow cell measured by electrochemical impedance spectroscopy (EIS) with a frequency range from 105 Hz to 0.01 Hz at the working potential.

For ECO2R in the single MEA cell, pure H2O (pH = ~7) was used as the anolyte. The cathode area in the flow cell was 1 cm×1 cm. The MEA cell consists of two titanium gas flow channels and a membrane between the cathode and the anode. The titanium gas flow channel is engraved with a serpentine flow channel, which has a width of 1 mm and an interval of 1 mm. The cathode and anode GDEs were separated by an AEM (Sustainion X37-50) and a PEM (Nafion 117) assembly (APMA architecture). The areas of both AEM and PEM are 2 cm×2 cm. AEM with a dry thickness of ~50 μm is soaked in 1 M KOH solution for storage and is washed with water before use. For the PEM with a dry thickness of ~183 μm, it was soaked in ~3% hydrogen peroxide for 1 h at 80 °C, deionized water for 1 h at 80 °C, and 0.5 M H2SO4 for another 1 h at 80 °C. And the resulting PEM was cleaned and preserved in deionized water at room temperature. Therein, the cathode GDE and the anode GDE were in contact with the AEM and the PEM, respectively. Cell voltages were directly presented without iR compensation for all MEA measurements. The reaction temperature was set at 60 °C. Unless otherwise noted, other reaction conditions were the same as those tested in the flow cell.

For scaled-up APMA-MEA stack measurements, an integrated circuit based on the Arduino UNO R3 (A000066) was used as an aid for monitoring, connected to the CoolTerm serial terminal application. The scaled-up APMA-MEA stack consists of two titanium gas flow channels and membranes between the cathode and the anode. The titanium gas flow channel is engraved with a serpentine flow channel, which has a width of 1 mm and an interval of 1 mm. All ECO2R measurements in the APMA-MEA stack were performed using the customized Variable DC power supply (10 kW). The cathode area was 150 cm2. The flow rates of the anolyte and CO2 were 20 mL/min and 3000 sccm, respectively. The initial temperature of the pure H2O anolyte was set to 30 °C rather than 60 °C due to the higher heat generation from the stack at high power. AEM (Sustainion X37-50) was replaced by one (MTCP-50) for the high stability30. Full-cell-stack voltages were directly presented without iR compensation. Unless otherwise noted, other reaction conditions were the same as those tested in the single MEA cell.

Products analysis
For ECO2R, the gas and liquid products were quantified by gas chromatography (GC, GC-2030, Shimadzu) and nuclear magnetic resonance (NMR, ECZ500R, 500 MHz, JEOL), respectively. GC was equipped with two thermal conductivity detectors (TCDs) for H2, O2, N2, He, CO, and CO2 signals, and a flame ionization detector (FID) for CH4, C2H4, and C2H6 signals. GC was composed of packed columns of two Porapak-N, a Molecular sieve-13X, a Molecular sieve-5A, a Porapak-Q and an HP-PLOT AL/S, and employed He (99.999%) and N2 (99.999%) as the carrier gases. Ar was used as the internal standard to calibrate the CO2 flow rate at the cell outlet (fCO2), which was fed at 2 sccm and mixed with the cell outlet gas stream before injection into GC23. The FEs of gas products were calculated by the following Eq. 2:

FE(%) = N_x × F × m_x × f_CO2 / j_Total × 100% (2)

where N_x is the number of electrons transferred for the specific product (x), F is the Faradaic constant, m_x is the molar fraction of the specific product (x) determined by GC, f_CO2 is the molar flow rate of the CO2, and j_Total is the total current density.

The liquid products were analyzed by 500 MHz 1H NMR spectroscopy (ECZ500R, JEOL) with water suppression. TSP and D2O were used as the reference standard and lock solvent, respectively. The FEs of liquid products were calculated by the following Eq. 3:

FE(%) = N_x × F × C_x × V_x / Q_Total × 100% (3)

where N_x is the number of electrons transferred for the specific liquid product (x), F is the Faradaic constant, C_x is the concentration of the specific liquid product (x) determined by 1H NMR, V_x is the volume of the electrolyte, and Q_Total is the total charge.

CO2 conversion was calculated using the following Eqs. 4–6:

f_x = F_x / (N_total × t × x_A) (4)

For SS-Ag catalyst:

CO2 Conversion (%) = f_CO2 / f_CO2_inlet × 100% (5)

For SS-Cu catalyst:

CO2-to-C2H4 Conversion (%) = 2 × f_C2H4 / f_CO2_inlet × 100% (6)

where f_x is the molar rate of the product (x) formation, t is the electrolysis reaction time, and A is the geometric area of the electrode.

Multiphysics simulation
The simulation was performed using COMSOL Multiphysics v. 5.6. The ECO2R process at the cathode was simulated under a fixed inlet flow rate, steady-state conditions, and involves two half-reactions: ECO2R-to-CO and HER (Eqs. 7 and 8). Although HER can affect CO2 diffusion through convection between H2 and CO2, the resulting diffusion trends for CO2 and H2 are in agreement. Thus, to simplify the calculation, Eq. 8 was not considered during the simulation.

CO2 + H2O + 2e− → CO + 2OH− (7)

2H2O + 2e− → H2 + 2OH− (8)

The momentum conservation in the serpentine channel, porous gas diffusion electrode and catalyst layer is expressed by the compressible Navier-Stokes equation31 and Darcy’s law32, respectively (Eqs. 9–11):

ρ∇·u = 0 (9)

ρ(u·∇)u = ∇·[−pI + μ(∇u + (∇u)T)] (10)

μ/K u + ∇p = 0 (11)

where ρ, u, p and μ represent density, flow velocity, gauge pressure and dynamic viscosity, respectively. K represents the permeability of the porous structure, including the GDE and catalyst layer, which can be expressed as (Eq. 12):

K = d_f^2 × ε^3 / K_CK(1−ε)^2 (12)

where ε is the porosity, d_f is the mean fiber diameter or the size of the particle, and K_CK is the Carman-Kozeny constant. The specific surface area (a) (ref. 33) of the porous structure can be expressed as (Eq. 13):

a = 4(1−ε) / d_f (13)

The mass transfer of gas in the porous structure and the flow channel involves both convection and diffusion. Therefore, the mass transfer process of all participating particles can be expressed by the Maxwell-Stefan equation (Eqs. 14–18) (ref. 34):

∇·j_i + ρ(u·∇)ω_i = R_i (14)

j_i = −(ρω_i Σ D_ik ∇x_k + D_i^T ∇T / T) (15)

d_k = ∇x_k + [(x_k − ω_k) ∇p / p] (16)

x_k = ω_k / M_k / Σ (ω_i / M_i) (17)

ω_i = c_i M_i / ρ (18)

where the subscript i represents species type. j_i is the relative mass flux vector, ω_i is the mass fraction, D_ik is the Maxwell-Stefan diffusion coefficient, d_k is the diffusional driving force, D_i^T is the thermal diffusion coefficient, x_k is the molar fraction, M_i is the reciprocal of the molar mass, and p is the absolute pressure.

The transport of current in the electrocatalysis process occurs via electron transport at the electrodes. Therefore, they can be calculated through Ohm’s law (Eq. 19):

i_s = −σ_eff^s ∇φ_s (19)

where i_s and φ_s are the current density and potential, respectively. And the relationship between effective conductivity (σ_eff^s) and electrode material conductivity (σ_s) can be expressed by the Bruggeman correlation (Eq. 20) (ref. 35):

σ_eff^s = (1−ε)^1.5 σ_s (20)

The relationship of electrochemical reaction kinetics in the catalyst layer is described by the Butler-Volmer equation (Eq. 21) (ref. 36):

i_cs = i_0 [ (c_CO2 / c_CO2_ref) exp(α_c F η_c / RT) − (c_CO / c_CO_ref) exp(α_a F η_c / RT) ] (21)

where the i_cs stands for the transfer current density of the cathode. c_i and c_i_ref are the surface concentration and bulk concentration of the porous structure, respectively. α_c and η_c are the cathode charge transfer coefficient and the cathode overpotential, respectively. Here, i_0 represents the exchange current density of the cathode, which can be given as follows (Eq. 22) (ref. 36):

i_0 = F · k_c · (c_CO2)^α_c · (c_CO)^α_a (22)

where the k_c is the electrochemical reaction rate constant.

The overpotential can be expressed as follows (Eq. 23):

η_c = φ_s − φ_l − E_eq (23)

where subscripts s and l represent the solid phase and liquid phase, respectively. E_eq stands for the equilibrium potential. The Nernst equation can be used to describe the equilibrium potential as follows (Eq. 24):

E_eq = E^0 + RT / nF ln(c_CO2 / c_CO) (24)

where E^0 represents the standard electrode potential of the cathode versus the standard hydrogen electrode at 298 K.

Boundary conditions for the cathode half-reaction in the alkali-cation-free APMA system are as follows. The galvanostatic condition was used during the electrocatalytic process; hence, a constant current density was utilized on the current collector. In addition, a fixed mass flux (Q) and species mass fraction of CO2 were set at the inlet boundary and other boundary conditions were listed in Supplementary Table 1. The relevant transport parameters in the present simulation are listed in Supplementary Table 2. To ensure the numerical model accurately simulates the electrochemical reaction process, the electrochemical reaction validation was carried out under fixed operating conditions.

It can be seen that, across different CO2 inlet flow rates, the simulated CO/CO2 outlet flow rate is in good agreement with the experimental data (Supplementary Fig. 1). The slight deviation maybe attributed to the model’s omission of a side reaction. Therefore, the model could be used to simulate the electrochemical reaction process. To select the appropriate number of grids, the model calculates the corresponding pressure drop and CO2 concentration for different numbers of grids, and the results are listed in Supplementary Table 3. The data show that the differences in pressure drop and CO2 concentration across different grid numbers are small (about 0.11%). As the number of grids increases from 55751 to 75673, the pressure drops, and the CO2 concentration is grid-independent. Therefore, given the time and computational resource requirements, it can be concluded that the grid-dependent effect can be avoided when 55751 meshes are used to simulate the cathode ECO2R at the macroscopic scale.

Techno-economic analysis
The TEA of the CO2 electrolysis system is based on published models37,38, with slight adjustments to account for the recycling of unreacted CO2, thereby offsetting the cost of purchasing CO2. An industrial-scale production capacity of 50,000 kg/day is assumed. The actual single-pass conversion rate of CO2, derived from experimental studies, was utilized in the calculations. A pressure swing adsorption (PSA) unit was virtually designed to purify gaseous products from both the cathode and anode. The capital investment for the PSA unit was estimated based on reference costs, and its operating expenditure was calculated at 0.23 kWh per m3/h of gas mixture39. For typical electrodes, the membrane replacement cost was simplified to be 1180 $/m2 for all cases39. The parameters and assumptions used for TEA are summarized in Supplementary Table 4 and the TEA results are listed in Supplementary Tables 5 and 6.

The levelized cost of carbon monoxide production (LCOC) is employed as the techno-economic indicator, as defined in Eq. 25.

LCOC = Σ (TAC + OPEX_r)t / Σ (PD + D_r)t (25)

where TAC and WCP are the total annual cost and working capital, respectively. BAL is the balance of the plant. PD is the annual amount of product generated, which is 5000 kg/day for all case studies. r is the nominal interest rate and n is the plant lifetime.

Total annual cost is composed of the operating expenditure OPEX, and the depreciation cost DEP of the capital assets, as defined in Eq. 26.

TAC = OPEX + DEP (26)

where the calculation of OPEX includes the purchase cost of electricity and water, the replacement cost of membrane and stack, the purchase cost of carbon dioxide, maintenance cost, and operating expenses of the PSA unit. The depreciation rate of capital assets is assumed to average 10% annually until reaching 20% of the end-of-life salvage value of the facility37.

In-situ ECO2R Raman measurements
In-situ Raman measurements were carried out by a customized spectro-electrochemical flow cell fabricated with a sapphire window (thickness: 0.15 ± 0.02 mm; area: 1.4 cm×1.4 cm) in front of the cathode GDE. The Ni felt was used as a counter electrode. The overall system was operated in a two-electrode setup. The electrolyte was pumped through a sapphire window at a constant flow rate of 5 mL/min using a peristaltic pump over the cathode GDE, and the electrolyte layer thickness on the cathode surface was 1.5 mm. CO2 was supplied to the back of the cathode GDE through the serpentine flow channel to guide the CO2 at a flow rate of 30 sccm controlled by an MFC (Alicate Scientific MC). Raman spectra were collected under an accumulation time of 4 s and an accumulation number of 10 times using a WITEC Confocal Raman microscope with an objective (50×) and a 633 nm laser. The cell voltage was applied in potentiostatic mode and recorded without iR compensation.

In-situ heating TEM measurements. In-situ heating TEM measurements were performed on the JEOL Model JEM-2100F at 200 kV with a Fusion Select holder (Protochips) and a holey carbon-coated microelectromechanical system E-chip.

Materials characterizations
TEM images were collected on a JEOL JEM-2100F at 200 kV. SEM images were obtained on the field-emission Tescan MAIA3. Aberration-corrected HAADF-STEM and 4D-STEM images were collected on a TFS Spectra 300 at 300 kV. The XRD patterns were recorded on a Rigaku SmartLab 9 kW-Advance diffractometer with Cu Kα radiation (λ = 1.5418 Å). The hard X-ray absorption spectroscopy measurements were conducted at the beamline BL01C of the Synchrotron Radiation Research Center in Hsinchu (Taiwan).

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "The developed developed an all-metal-based GDE, fabricated from a stainless steel skeleton, Ag or Cu nanocatalysts, and a binder, with by pressing and sintering processes. This electrode, along with a forward BPM and an anode, constituted the MEA for CO2-pure water co-electrolysis to produce CO or C2 products. Compared to traditional carbon-based GDEs, this all-metal GDE is thinner, which facilitates CO2 gas diffusion and maintains good Faradaic efficiency and CO2 conversion at high power densities. The study presents a high-performance CO2 reduction electrolysis system, though further experimental validation is needed to clarify the origin of its superior performance. Major revison is necessary before being published on Nature Communications.\n1.Figure 3a shows that the HDF-GDE exhibits superhydrophobicity. What is the primary source of this superhydrophobicity —is it the surfactants used in synthesizing the Ag and Cu nanoparticles, or is it the PTFE? The contact angles of the HDF-GDE and CP-GDE should be measured and compared, along with changes in contact angle before and after electrolysis. Additionally, cross-sectional SEM and EDS mapping images of the electrode should be provided to examine the distribution of each component.\n2. In the Raman spectrum of Figure 3h, the peak at 237 cm⁻¹ is assigned to the *CO species. Which vibrational mode of *CO does this peak correspond to? Multiple species may exhibit Raman peaks near this wavenumber—are there experimental results confirming that this peak indeed belongs to *CO? A more characteristic CO stretching vibration peak of *CO species should appear near 2100 cm⁻¹. Did the authors observe such a peak?\n3. The finite element analysis simulation results shown in Figures 4c-e reveal that while there are some differences in CO2 concentration distribution between the two electrode surfaces, the actual variation in CO₂ concentration is confined to a very narrow range (only from 35.4 mM to 35.8 mM). Such a minor difference does not appear to be significant. What meaningful conclusion can actually be drawn from this result? Whether this small variation still leads to observable performance differences in CO₂ reduction reactions?", "reviewer_2": "The manuscript of She et al reports on the CO2 conversion to CO and C2H4. They use a new GDE configuration, and scale up to a stack, demonstrating CO2 electrolysis at kW-scale.\nI would say that the achievement of high FE and reasonable stability are credible, but the explanations and mechanisms are substandard for Nat.Comm. I can specify that in the following issues:\n- The use of a GDE that only consists of catalyst (i.e., carbon-free diffusion electrodes) is not new as such. The work of Turek already demonstrated Ag-based GDEs, and similar carbon-free GDE material is also commercially produced by Covestro.\n- Also, removing the fiber substrate completely is not straightforward. If you would just press a CCM to the flow field, assuming a flow-by mode, a substantial part of the catalytically active area would be made inaccessible due to the flow field ribs. There needs to be some layer to provide the CO2 to the catalytic layer. The authors are not clear how that is for their SS-based GDE. The info in Fig. S24-S25 is too little to reproduce or judge whether this innovation makes sense. There needs to be at minimum a thickness of the layers, porosity, warp size and loading.\n- It is also unclear how the hydrophobicity is obtained. Using stainless steel and silver is not directly making the layer hydrophobic (in fact, I would expect this to get hydrophilic). How is this hydrophobicity obtained, and how persistent is the hydrophobicity after operation?\n- The section around figure 4, discussing the mass transport, is immature. The authors take a new approach (modelling some cases and varying hypothetical thickness and porosity), and it’s not easy to read. But more important: the results show basically very little sensitivity to the configurations, because the CO2 concentration is not varying much. That is even less when looking to the SI: Figure S38-S48 show very small differences in CO2 concentration, and that is not convincing that you now solved the mass transfer problem. Why not compare the mass transfer coefficients? Or limiting current densities? I would say it’s much easier and more clarifying to look at these metrics.\n- The comparison in Figure 5c, which is tabulated in table S7, is a bit out of context. It seems that all literature values in table S7 are for C2+ products, and the authors compare that to both their Cu-based C2+ cases and the Ag-based CO-case. For the latter, there have been much better performances. I think the work of Janaky and co-workers, and Sargent and co-workers, are a good starting points for the CO-case.\n- The techno-economic analysis has quite some assumptions. For example, the electricity prices (3.4 $ct per kWh and 0.2 CNY/kWh) are substantially lower than what other works typically use, and it’s unclear what is included in the cost analysis and what not (grid connection? Separating products to what purity? Production costs of GDEs? Etc.). I understand that the TEA cannot be very elaborate on this topic, because it’s only for the outlook, but then it might be better to skip from the manuscript text. Although the TEA gives a good indication, it dilutes the message and needs more nuance."}, "author_response": "A point-by-point response to Reviewers’ comments\nWe thank the Reviewers for their professional and constructive comments, which significantly improved the quality of our manuscript. We have revised the manuscript in accordance with the reviewers' comments. To highlight the changes made to the manuscript, the color of the changed text has been changed to Blue.\nThe developed developed an all-metal-based GDE, fabricated from a stainless steel skeleton, Ag or Cu nanocatalysts, and a binder, with by pressing and sintering processes. This electrode, along with a forward BPM and an anode, constituted the MEA for CO -pure water co-electrolysis to produce CO or C 2 2 products. Compared to traditional carbon-based GDEs, this all-metal GDE is thinner, which facilitates CO gas diffusion and maintains good Faradaic 2 efficiency and CO conversion at high power densities. The study presents a 2 high-performance CO reduction electrolysis system, though further 2 experimental validation is needed to clarify the origin of its superior performance. Major revison is necessary before being published on Nature Communications.\nResponse: We thank you for your meticulous review of our manuscript and for providing constructive comments. Also, we appreciate your positive feedback on this manuscript. In the following point-by-point response, we will address your comments and concerns.\nQuestion 1: Figure 3a shows that the HDF-GDE exhibits superhydrophobicity. What is the primary source of this superhydrophobicity—is it the surfactants used in synthesizing the Ag and Cu nanoparticles, or is it the PTFE? The contact angles of the HDF-GDE and CP-GDE should be measured and compared, along with changes in contact angle before and after electrolysis. Additionally, cross-sectional SEM and EDS mapping images of the electrode should be provided to examine the distribution of each component.\nResponse: We regret that we overlooked the issue of hydrophobicity and did not elaborate on the details in the main text sufficiently. We only provided a brief explanation in the experimental section. And the Reviewer #2 also noticed this problem (question 3 from Reviewer #2). We greatly appreciate the Reviewers for raising this critical point.\nActually, the HDF-GDE hydrophobicity originates from PTFE. When preparing the printing ink, we added superhydrophobic PTFE. The printing ink was prepared by mixing the prepared catalyst and PTFE solution in an organic solution consisting of α-terpineol, 2-(2-butoxyethoxy)ethanol and acrylic resin with an approximate weight ratio of 65:15:20. Thereinto, the ratio of the catalyst and PTFE is about 100:2. Thus, the HDF-GDE exhibits superhydrophobicity.\nFurthermore, following your comments and suggestions, we have performed contact angle experiments, as well as cross-sectional SEM and EDS mapping images of HDF-GDE, as shown in Figures R1 and R2. Before ECO R, both CP- and 2 HDF-GDEs exhibited hydrophobicity, with contact angles of up to ~137.5° and 2 ~143.9°, respectively (Figure R1a and R1c). After 1000 h ECO R, CP-GDE’s contact 2 angle significantly dropped to 90.1°, while that of HDF-GDE only slightly decreased to 134.1° (Figure R1b and R1d). The improved hydrophobicity of HDF-GDE may be attributed to the direct addition of PTFE to the catalyst layer, rather than just to the substrate (CP-GDE). From cross-sectional SEM and EDS mapping images of HDF-GDE, it can be known that SSM effectively supports the catalyst layer (Figure R2a and R2b). And PTFE (i.e., element F) is evenly dispersed in the catalyst layer (Figure R2c-R2e).\nThus, the corresponding explanations and clarifications have been included in the revised manuscript, as shown below.\n“To strengthen the mechanical strength of HDF-GDE, the stainless-steel mesh (SSM) was used as the supporting skeleton of the catalyst sheet (Fig. 3a and Supplementary Figs. 24 and 25). The porous SS-Ag HDF-GDE, whose thickness was controlled to ~300 µm by the mechanical mold, showed hydrophobicity due to the introduced PTFE in the catalyst layer (Fig. 3a and 3b and Supplementary Fig. 26). And HDF-GDE’s contact angle was up to ~143.9° (Supplementary Fig. 27). SS-Ag HDF-GDE only contains the catalyst and an SSM skeleton located in the middle layer of the catalyst, allowing CO to directly contact the catalyst without passing through 2 the inert substrate (such as CP and PTFE GDEs). This could essentially increase the overall flux of CO /*CO to and from the HDF-GDE surface, accelerating the system’s 2 mass transfer kinetics.”\nFigure R1. The contact angle measurements of different SS-Ag GDEs before and after 1000 h ECO R. a,b, CP-GDE. c,d, HDF-GDE. 2\nQuestion 2: In the Raman spectrum of Figure 3h, the peak at 237 cm⁻¹ is assigned to the *CO species. Which vibrational mode of *CO does this peak correspond to? Multiple species may exhibit Raman peaks near this wavenumber—are there experimental results confirming that this peak indeed belongs to *CO? A more characteristic CO stretching vibration peak of *CO species should appear near 2100 cm⁻¹. Did the authors observe such a peak?\nResponse: We appreciate the Reviewer for raising this critical point. This is a very professional question. For Figure 3h, we aim to utilize in-situ Raman experiments to investigate the ECO2R product (CO/*CO) coverage on various GDEs. Due to the effective CO mass transfer and diffusion, CO coverage on the HDF-GDE surface 2 2 should be higher than that on the CP-GDE surface, which can cause a higher CO/*CO coverage during ECO R. 2\nAs the Reviewer pointed out, multiple ECO R species may exhibit Raman peaks near 2 this wavenumber. According to reports, for ECO R, the peaks located in the range of 2 230-360 cm-1 are attributed to the frustrated rotation of Ag-CO and the Ag-CO stretch. [R1-R4] The peaks at ~2100 cm-1 can be ascribed to the C≡O stretch of the surface-absorbed CO, including atop-bound CO and bridge-bound CO. To present the CO coverage on different GDEs more clearly, we chose the Ag-CO peak rather than C≡O because the CO product (i.e., free CO) can influence the concentration of CO on GDE.\nAccording to the Reviewer’s comment (a more characteristic CO stretching vibration 4 peak of *CO species should appear near 2100 cm-1), we re-carried out in-situ Raman experiments in an in-situ flow cell for detecting C≡O stretch of the CO. From Figure R3, it can be found that the stretching peaks of Ag-CO and C≡O both appear simultaneously, located at the range of 230-360 cm-1 and ~2100 cm-1.\n[R1] Shang, W. Y. et al. In situ surface-enhanced Raman spectroscopic evidence on the origin of selectivity in CO electrocatalytic reduction. ACS Nano, 2020, 14, 2 11363-11372.\n[R2] Liu, Q. et al. Converting CO to ethanol on Ag nanowires with high selectivity 2 investigated by operando Raman spectroscopy. Sci. China Chem., 2023, 66, 259-265.\n[R3] Chernyshovaa, I. V., Somasundaran P., Ponnurangam, S. On the origin of the elusive first intermediate of CO electroreduction. PNAS, 2018, 115, 2 E9261-E9270.\n[R4] Wang, X. et al. Efficient electrically powered CO -to-ethanol via suppression of 2 deoxygenation. Nat. Energy, 2020, 5, 478-486.\nFigure R3. In-situ Raman measurements on SS-Ag for ECO R in an 2 alkali-cation-free system at different cell voltages.\nThus, we supplemented Supplementary Fig. 20 (Figure R3) in the supplementary materials and revised the explanation in the main text, as shown below.\n“Further, we carried out in-situ Raman experiments to study ECO R product 2 (CO/*CO) coverage on different GDEs (Fig. 3h). Due to the effective CO mass 2 5 transfer and diffusion, CO coverage on the HDF-GDE surface should be higher than 2 that on the CP-GDE surface, which can cause a higher CO/*CO coverage during ECO R9,23,24. For SS-Ag HDF-GDE, the intensity of CO/*CO located at ~230-360 cm-1 is significantly higher than that based on the SS-Ag CP-GDE, directly proving the overall flux of species both to and from HDF-GDE was increased (i.e., fast mass transfer kinetics)23,24. The high-efficiency CO mass transfer and diffusion could also 2 suppress the accumulation of *H through the transfer of H O on the catalyst surface, 2 improving the peak current density of ECO R and increasing CO conversion without 2 2 decreasing the CO inlet flow rate.” 2\nQuestion 3: The finite element analysis simulation results shown in Figures 4c-e reveal that while there are some differences in CO concentration distribution 2 between the two electrode surfaces, the actual variation in CO concentration is 2 confined to a very narrow range (only from 35.4 mM to 35.8 mM). Such a minor difference does not appear to be significant. What meaningful conclusion can actually be drawn from this result? Whether this small variation still leads to observable performance differences in CO reduction reactions? 2\nResponse: We highly agree with the Reviewer that a small difference in CO 2 concentration alone may not lead to such performance.\nRegarding this question, first of all, we must admit that we made a significant mistake in the layout of the pictures. The system mass transfer is not only related to CO but 2 also to the product CO; however, we have separated the simulation results of CO 2 and CO. Although the variation in CO concentration is confined to a very narrow 2 range, that of the product CO is much larger (Figure R4). That is to say, the back pressure of the system is different because faster mass transfer kinetics can result in faster reaction kinetics. From Figure R4f-R4h, the product CO concentration on SS-Ag HDF-GED is significantly higher than that on CP-GDE. And the difference in CO concentration is almost an order of magnitude. Noted that one CO molecule can 2 generate one CO molecule; thus, the overall system’s mass transfer of HDF-GDE needs to consider CO 2 and CO (i.e., the overall carbon concentration). Obviously, the variation in carbon concentration is very large for HDF-GDE and CP-GDE. The effective mass transfer in HDF-GDE caused accelerated reaction kinetics (i.e., higher current density), but it balanced the CO2 concentration on GDE, leading to a higher CO concentration.\nThus, we re-prepared Fig. 4 in the main text (i.e., Figure R4) and some discussions in the manuscript, as shown below.\n“When the current density of ECO R was 150 mA/cm2, the CO mass transfer kinetics 2 2 6 in GDEs were also simulated (to simplify the model, HER was not considered). At the same CO inlet flow rate, at the inlet, the efficiency of CO mass transfer to the 2 2 catalyst surface in HDF-GDE was significantly higher than that in the CP-GDE in the d and d directions (Fig. 4a, 4b, and Supplementary Fig. 49), indicating that Vertical Parallel CO coverage on the catalyst surface in HDF-GDE was higher than that in the 2 CP-GDE at the inlet. As the reaction progressed, the CO 2 coverage decreased gradually for both GDEs along the flow channel direction (Fig. 4c-4e). Near the outlet, the CO coverage on the catalyst surface in HDF-GDE is less than in the CP-GDE. 2 This is because, in the catalyst layer, the product (CO/*CO) coverage in HDF-GDE was higher than in the CP-GDE, which was almost a factor of magnitude difference (Fig. 4f-4h and Supplementary Fig. 50). One CO molecule can generate one CO 2 molecule, thus, the overall system’s mass transfer needed to consider CO and CO 2 (i.e., the overall carbon concentration). Obviously, the system’s mass transfer efficiency of HDF-GDE is much higher than that of CP-GDE. Furthermore, the high carbon coverage on the catalyst surface in HDF-GDE could occupy more reaction sites, thereby decreasing the *H coverage, suppressing HER, and enhancing ECO R-to-CO activity. These improved the peak operation current density for ECO R 2 2 and, thus, increased CO conversion.” 2\nThe manuscript of She et al reports on the CO conversion to CO and C H . 2 2 4 They use a new GDE configuration, and scale up to a stack, demonstrating CO 2 electrolysis at kW-scale.\nI would say that the achievement of high FE and reasonable stability are credible, but the explanations and mechanisms are substandard for Nat.Comm. I can specify that in the following issues:\nResponse: We highly appreciate your meticulous efforts in reviewing our manuscript and providing constructive comments. Also, we appreciate your positive feedback on the ECO R performance test results. 2\nQuestion 1: The use of a GDE that only consists of catalyst (i.e., carbon-free diffusion electrodes) is not new as such. The work of Turek already demonstrated Ag-based GDEs, and similar carbon-free GDE material is also commercially produced by Covestro.\nResponse: We greatly appreciate the Reviewer's information. We have carefully read the recent work on carbon-free Ag GDEs from Prof. Turek. We strongly agree with Prof. Turek that thinner GDEs with a shorter transport distance for the liquid electrolyte enhance Faradaic efficiency and carbon efficiency for ECO2R, providing better access to fresh electrolyte. Their presented method identifies fundamental transport phenomena and design-performance relationships, which are qualitatively transferable across a broad range of GDE architectures, as well as further operation and electrode parameters, the analysis of non-steady states, long-term durability, and higher current densities in future research, thereby enabling a deeper understanding of the underlying processes. And in the revised manuscript, we have cited these papers.\nIn this work, we used the pure-H O-fed APMA system to perform ECO R, resulting 2 2 in the low current density (i.e., slow reaction kinetics). On account of this, we prepare carbon-free HDF-GDEs to accelerate the system’s mass transfer kinetics, in turn, to improve the current density. Thus, the focus of our work and Prof. Turek’s work on carbon-free GDEs differs. But these two works reveal the future development trend of GDE, especially for carbon-free GDE.\nWe are incredibly excited that the Covestro company mentioned by the Reviewer is already producing the carbon-free GDE material. It at least indicates that our research, along with Prof. Turek’s, has broad prospects for practical application. We will contact the Covestro company to actively seek cooperation for scaling up our HDF-GDE and further optimizing the carbon-free GDE’s mass transfer efficiency.\nWe thank the Reviewer for his/her unreserved introduction of carbon-free GDE again, which leads us to think more deeply about the structure of carbon-free GDE.\nResponse: There is no doubt that this is a professional and excellent question. We highly agree with the Reviewer that if only SSM is pressed into the flow field (i.e., flow-by mode), a substantial part of the catalytically active area would be made inaccessible due to the flow field ribs.\nFor HDF-GDE, we pre-pressed the HDF-GDE onto the membrane (Figure R5). On the one hand, HDF-GDE will come into close contact with the membrane, thereby reducing the resistance. On the other hand, this will also prevent the flow channel from exerting pressure on SSM, thereby avoiding deformation of SSM (Figure R5). When preparing HDF-GDE, the thickness of the HDF-GDE was controlled to ~300 μm by the mold. The thickness of SSM was about 100 μm, the warp size of SSM was ~25 μm, and the mesh count of SSM was 500 × 500 (Figure R6).\nFigure R5. Schematic of HDF-GDE. Left: Pre-pressing process. Right: the structure of MEA assembled with HDF-GDE.\nAccording to your suggestions, we added the cross-sectional SEM and EDS mapping images (Figure R6) and provided corresponding information, including layer thickness, warp size, and loading, as shown below.\n“For high-diffusion-flux (HDF) GDEs, the cathode catalyst (SS-Ag/SS-Cu) was printed on the SSM skeleton by screen printing, followed by drying in a vacuum overnight. After that, the SSM-skeleton-supported catalyst was pressed at 1000 pounds of pressure for 2 h and the thickness of the HDF-GDE was controlled to ~300 10 µm. Finally, it was calcined at 350 °C for 30 min. The printing ink was prepared by mixing the prepared catalyst and PTFE solution in an organic solution consisting of α-terpineol, 2-(2-butoxyethoxy)ethanol and acrylic resin with an approximate weight ratio of 65:15:20. The loading of the catalyst in HDF-GDE was controlled to about 10 mg/cm2.”\n“Potassium hydroxide (KOH, ≥ 85.0%), stainless-steel mesh (SSM, 0.10 mm thickness, 500 × 500 mesh; warp, ~25 µm) and Titanium fiber felt (0.25 mm thickness, 99.9%) were purchased from Sinopharm Chemical Reagent Co., Ltd. (China).”\n“To further verify that the fast CO mass transfer kinetics in HDF-GDE resulted in 2 higher operation current density (i.e., higher CO conversion), rather than the higher 2 catalyst loading, we prepared SS-Ag CP-GDE with the same catalyst loading as SS-Ag HDF-GDE (~10 mg/cm2). SS-Ag CP-GDE showed a significant decline in CO selectivity at 400 mA/cm2 with a CO FE of ~63% (Supplementary Fig. 33), compared with SS-Ag HDF-GDE (~90% CO FE).”\nFigure R6. Cross-sectional SEM and the EDS mapping images of HDF-GDE. a,b, Cross-sectional SEM images. c-e, and EDS elemental mapping images. (d) and (e) correspond to (c), and they represent the elements Ag and F, respectively.\nResponse: We apologize for not presenting the detailed information on HDF-GDE clearly in the manuscript previously. The Reviewer #1 also raised this question (Question 1 from Reviewer #1). Thank you for your friendly suggestions again.\nActually, the HDF-GDE hydrophobicity originates from PTFE. When preparing the printing ink, we added superhydrophobic PTFE. The printing ink was prepared by mixing the prepared catalyst and PTFE solution in an organic solution consisting of α-terpineol, 2-(2-butoxyethoxy)ethanol and acrylic resin with an approximate weight ratio of 65:15:20. Thereinto, the ratio of the catalyst and PTFE is about 100:2. Thus, the HDF-GDE exhibits superhydrophobicity.\nFurthermore, we have performed the contact angle experiments (Figure R7). Before ECO R, both CP- and HDF-GDEs exhibited hydrophobicity, with contact angles of 2 up to ~137.5° and ~143.9° (Figure R1a and R1c), respectively. After 1000 h ECO R, 2 CP-GDE’s contact angle significantly dropped to 90.1°, while that of HDF-GDE only slightly decreased to 134.1° (Figure R1b and R1d). The better hydrophobicity of HDF-GDE might be due to the direct addition of PTFE to the catalyst layer, rather than just on the substrate (just like CP-GDE).\nFigure R7. The contact angle measurements of different SS-Ag GDEs before and after 1000 h ECO R. a,b, CP-GDE. c,d, HDF-GDE. 2\nThus, the corresponding explanations and clarifications have been included in the revised manuscript, as shown below.\n“To strengthen the mechanical strength of HDF-GDE, the stainless-steel mesh (SSM) 12 was used as the supporting skeleton of the catalyst sheet (Fig. 3a and Supplementary Figs. 24 and 25). The porous SS-Ag HDF-GDE, whose thickness was controlled to ~300 µm by the mechanical mold, showed hydrophobicity due to the introduced PTFE in the catalyst layer (Fig. 3a and 3b and Supplementary Fig. 26). And HDF-GDE’s contact angle was up to ~143.9° (Supplementary Fig. 27). SS-Ag HDF-GDE only contains the catalyst and an SSM skeleton located in the middle layer of the catalyst, allowing CO to directly contact the catalyst without passing through 2 the inert substrate (such as CP and PTFE GDEs). This could essentially increase the overall flux of CO /*CO to and from the HDF-GDE surface, accelerating the system’s 2 mass transfer kinetics.”\nQuestion 4: The section around figure 4, discussing the mass transport, is immature. The authors take a new approach (modelling some cases and varying hypothetical thickness and porosity), and it’s not easy to read. But more important: the results show basically very little sensitivity to the configurations, because the CO concentration is not varying much. That is even less when 2 looking to the SI: Figure S38-S48 show very small differences in CO 2 concentration, and that is not convincing that you now solved the mass transfer problem. Why not compare the mass transfer coefficients? Or limiting current densities? I would say it’s much easier and more clarifying to look at these metrics.\nResponse: This is yet another particularly well-posed and highly specialized question. We thank the Reviewer for your meticulous effort in reviewing our manuscript and providing constructive comments. As the Reviewer pointed out, we take a new approach to model the mass transfer of the system. In this work, we considered two scenarios of mass transfer (Figure R8). One is under unreacted steady-state conditions (with a current density of 0), and the other is at a current density of 150 mA/cm². Because, at the cathode, under unreacted steady-state conditions, the system’s mass transfer only includes CO (Figure R8a), which makes the model 2 relatively simple. But during the ECO R, at the cathode, the system’s mass transfer 2 behavior becomes very complex because ECO R products from the catalyst layer and 2 the fresh CO can form opposite flows (Figure R8b), which can increase the mass 2 transfer resistance of the reactant CO to the catalyst layer and result in low CO 2 2 coverage on the catalyst surface, especially at a high current density. The low CO 2 coverage in the catalyst layer can suppress ECO R and concurrently increase HER, 2 limiting the current density of ECO R and resulting in low CO conversion. 2 2\nAdditionally, the catalyst loading on CP-GDE is limited to a very low value, approximately 1 mg/cm², creating a thin catalyst layer that facilitates CO mass 2 13 transfer and diffusion from the carbon paper to the catalyst layer. This results in extremely low space utilization for CP-GDE. The thin catalyst layer is also easily flooded due to the rapid water transfer at high current densities.\nFigure R8. Schematic illustrating the mass transfer in CP-GDE. a, Under the unreacted steady-state conditions, CO mass transfer. b, Under the reaction conditions, 2 the system’s mass transfer.\nGenerally, increasing the reactant CO flow rate can accelerate the CO mass transfer 2 2 kinetics in CP-GDE; however, this can considerably lower the CO conversion. 2\nCurrently, for high CO conversion, the CO flow rate is often excessively lowered 2 2 (from ~100 to 0.06 sccm). The low flow rate, however, can slow down the CO mass 2 transfer and diffusion, which can suppress ECO R and increase HER. Thus, the high 2 ECO R product selectivity (Faradaic efficiency: FE) and the high CO conversion are 2 2 regarded as irreconcilable contradictions that pose a considerable challenge in practical industrial applications.\nUnder the unreacted steady-state conditions, the simulated results show basically very little sensitivity to the configurations for CO concentration. This is because the 2 thickness of both CP-GDE and HDF-GDE is minimal (325 µm - 150 µm), thus the change in CO volume concentration is also very small. 2\nHowever, under the reaction conditions (current density: 150 mA/cm2), the system’s mass transfer is not only related to CO but also to the product CO. Although the 2 variation in CO concentration is confined to a very narrow range, that of the product 2 CO is much larger (Figure R9). That is to say, the back pressure of the system is different because faster mass transfer kinetics can result in faster reaction kinetics. From Figure R9f-R9h, the product CO concentration on SS-Ag HDF-GED is significantly higher than that on CP-GDE. And the difference in CO concentration is almost an order of magnitude. Noted that one CO molecule can generate one CO 2 molecule; thus, the overall system’s mass transfer of HDF-GDE needs to consider CO 2 and CO (i.e., the overall carbon concentration). Obviously, the variation in carbon concentration is substantial for HDF-GDE and CP-GDE. The effective mass transfer in HDF-GDE caused accelerated reaction kinetics (i.e., higher current density), but it balanced the CO concentration on GDE, leading to a higher CO 2 concentration.\n“For the fuel cell, the mass transfer of the reactant (e.g., H ) at the anode is 2 unrestricted because the unreacted fuel H can be cycled in the system 2 (Supplementary Fig. 2) (ref. S16). The mass transfer of the reactant O at the cathode 2 can be improved by increasing its flow rate, without considering its conversion efficiency, as it is derived from the air.S17 However, for ECO R, at the cathode, the H 2 2 from the competing hydrogen evolution reaction (HER), multiple products, and unreacted CO make the mass transfer in the system extremely complicated (Fig. 2 1b).”\n“Taking the commonly used carbon-paper GDE (CP-GDE) as an example (Fig. 1c), the reactant CO needs to pass through the carbon fiber and carbon powder layers to 2 the catalyst layerS18-20. Thus, ECO R products from the catalyst layer and the fresh 2 CO can form opposite flows (Fig. 1b), which can increase the mass 2 transfer resistance of the reactant CO to the catalyst layer and result in low CO coverage on 2 2 the catalyst surfaceS21-23, especially at a high current density (i.e., high potential). The low CO coverage in the catalyst layer can suppress ECO R and concurrently increase 2 2 HER, limiting the current density of ECO R and resulting in low CO 2 2 conversionS24,25,26. Additionally, the catalyst loading on CP-GDE is limited to be very low, at around 1 mg/cm2, creating a thin catalyst layer that allows for CO mass 2 transfer and diffusion from the carbon paper to the catalyst layerS21,22. This results in extremely low space utilization for CP-GDE. The thin catalyst layer is also easily flooded due to the rapid water transfer at the high current densityS21,23,27-29. ”\n“Generally, increasing the reactant CO flow rate could accelerate the CO mass 2 2 transfer kinetics in CP-GDES30,31, but this could considerably lower the CO 2 conversion (Supplementary Fig. 3 and Supplementary Table 4). Currently, for high CO conversion, the CO flow rate is often excessively lowered (from ~100 to 0.06 2 2 sccm). The low flow rate, however, can slow down the CO mass transfer and 2 diffusion, which can suppress ECO R and increase HERS21,26,32. Thus, the high 2 ECO R product selectivity (Faradaic efficiency: FE) and the high CO conversion are 2 2 regarded as irreconcilable contradictions that pose a considerable challenge in practical industrial applications. ”\nResponse: Thanks for these kind and sincere suggestions. For Figure 5c and Table S7, our primary intention is to emphasize two key points: first, within a certain range, decreasing the CO inlet flow rate can increase CO conversion; and second, 2 2 excessively decreasing the CO inlet flow rate can significantly decrease product selectivity. Thus, to improve CO conversion, we cannot simply reduce the CO inlet 2 2 flow rate. In this work, we aim to accelerate the system’s mass transfer kinetics, thereby achieving both high CO conversion and high product selectivity. Thus, we 2 proposed and assembled the high-diffusion-flux GDE (HDF-GDE). Additionally, the reported CO conversion primarily involved the ECO R-to-C H /C + process. 2 2 2 4 2\nActually, for our pure-H O-fed ECO R-to-C H system, its performance is not 2 2 2 4 particularly outstanding (Figure R10). And CO -to-C H conversion was only ~16%. 2 2 4 This is mainly because our ECO R system is a pure-H O-fed system, which resulted 2 2 in a low C H FE. Thus, in the next step, we will primarily focus on enhancing the 2 4 product selectivity in the pure-H O-fed ECO R APMA system. 2 2\nAdditionally, we are also very familiar with the works from Janaky’s and Sargent’s groups. However, to improve system stability, we chose the pure-H O-fed APMA 2 system, thereby sacrificing a portion of the current density. Thus, when conducting the performance comparison, we listed the flow- and MEA-cell (Figure R10). Additionally, to conduct a more objective and fair comparison of the performance, we extensively cited the research works of Janaky and Sargent.\nFigure R10. CO conversions of ECO R on different catalysts from literature and this 2 2 work at different CO inlet flow rates. 2\nResponse: We sincerely thank the Reviewer for his/her understanding. During the review process of other works, we encountered cases where some individuals strongly requested that we add TEA. We strongly agree with the Reviewer that TEA is for the outlook and can provide a good indication, but it dilutes the message. Once again, we sincerely appreciate your understanding.\nBased on your kind and constructive comments, we have incorporated the TEA section into SI. Thus, we revised Figure 5 (i.e., Figure R11) and revised the manuscript, as shown below.\nFigure R11. Scaled-up kW-scale alkali-cation-free APMA MEA cell stack 18 assembled with HDF-GDEs for ECO R. a-c, Digital photographs of the flow 2 channel plate in the cell stack system (a), the assembled cell stack in side view (b), and the cell stack in front view (c). d,e, The system stability performance of ECO R 2 on the SS-Ag HDF-GDE at a current of 60 A (d) and SS-Cu HDF-GDE at a current of 75 A (e). f, CO conversions of ECO R on different catalysts from literature and this 2 2 work at different CO inlet flow rates. 2\nParameter Value Origin\nProduction Capacity 5000 kg/day S9\nPlant Lifetime 20 years S9\nOperating Time 350 days/year S9\nElectrolyzer Cost 550 $/kW S9\nElectrolysis Stack Cost 61% of the electrolysis cost S8\nWorking Capital 5% of the capital cost S9\nDepreciation Rate 10% S8\nEnd-of-Life Salvage Value of the 20% of the capital cost S8\nFacility\nNominal Interest Rate 10% S11\nBalance of Plant 39% of the electrolysis cost S9\n2.5% of the electrolyzer\nMaintenance Cost S9\ncost\nWorking Capital 5% of the capital cost S9\nMembrane Cost 5% of the electrolyzer cost S9\nMembrane Replacement Cost 1180 $/m2 S9\n15% of the electrolyzer\nCell Compartment Replacement Cost S9\ncost\nCapital Investment in Pressure Swing 61% of the total capital cost S10\nAdsorption\nElectricity Consumption of Pressure 0.23 kWh/m3 S10\nSwing Adsorption\nFaraday Constant 96485 C/mol S9\nWater Price 0.0054 $/gal S9\nElectricity Price (long-term-operation 0.034 $/kWh S11\nnuclear)\nChina Green Electricity 0.2 CNY/kWh S12\nCO Price 40 $/ton S9\nEthylene Price 1.0 $/kg S14\nHydrogen Price 6 $/kg S15\n20"}, {"round": 2, "reviewer_comments": {"reviewer_1": "My comments have been addressed in this revised manuscript. I recommend this manuscript to be accepted.", "reviewer_2": "The revised manuscript of She et al demonstrates a good progress in CO2 electrolysis at higher current density with good Faradaic efficiency and reasonable stability at larger scale. The revisions have improved the manuscript. The points regarding hydrophobicity and the layer structure are clear now. I have to say that still the use of carbon-free GDE, or GDEs with higher mass transfer in general, are not unique (as elaborated earlier), but I think the advances in the GDE combined with CO2 utilization and scale-up make the manuscript valuable. Nevertheless, some points need to be addressed.\nSpecifically:\n- One of the main developments in this paper is the new GDE structure, but it’s hard to understand which configuration the authors are using for what figure. For example, this information in Fig. S12 is a ‘SS-Ag’ structure, but SS-Ag was also used later in the figure 2 and in Figure 3, so this is not descriptive. It would help to make a scheme of the different layer structures (including where’s the gas channel and membrane, so maybe full cell structures) every time the authors change to discussing another configuration. That could be a sub-figure in Fig.2 and Fig. 3 (as a suggestion, the figure 2h is superfluous, as 2g is already presenting the same information, and figure 3b is not very informative, which makes some space for a schematic layer structure).\n- Also in the text, above Figure 2, the step towards alkali-cation-free and AEM+PEM assembly is not straightforward and goes too quickly. The AEM+PEM structure is basically a bipolar membrane. Is this used for the other results as well? And was a catalyst involved between the AEM+PEM layer (like others do for BPMs)?\n- The FE for the case with 1 M KOH (early results, and Fig. S12) is extremely high. Generally, some formic acid is formed as well, decreasing the FE towards CO. Did the authors check for formic acid?\n- The authors mention the use of alkali-cation-free conditions, but that depends on the pre-treatment of the membrane. If the membrane is pre-treated in a solution with high concentration of alkali-cations, the cations are still in the membrane and have no way out. That doesn’t have to be a bad thing, but please consider that feeding pure water does not per se imply alkali-cation-free.\n- The modelling part, with the local concentration of CO2 and CO, is still a weak part. I see the point that the CO concentration changes relatively much more than the CO2 concentration, but I have 2 issues with this:\n1) The local CO concentration does not pose a limiting current (such as CO2 can do), because it doesn't deplete. Therefore, basically, when it changes a factor 10, it would give a Nernst potential, but that’s it. A change of 59 mV, which Nernst would predict by a factor 10 change in concentration, would not make a big difference, I would say. The authors need to better underpin why this causes so much impact. Maybe the HER is very sensitive, but then I think they then need to compare this with the same setup in HER mode (feeding N2, for example).\n2) The real mass transfer improvement is perpendicular to the catalyst layer, while the authors plot the xy concentrations. This does not really show the mass transfer benefits of the different GDE structures.\n- For the larger scale experiments, the authors started with 30 deg C. Did the authors observe heating? Literature has recently picked up effects of temperature, and would be good to compare these new results to literature.\n- The paper is rather long. Not sure what the editor thinks about the length, but if needed, I think the authors can write their thoughts on the C2H4 case more dense, because it’s not really needed for the core of the paper.\nSome minor comments:\n- The authors do not use a correct number of significant digits. The FE is sometimes expressed in 82.26% or 42.97%, while I assume that their precision is not up to 2 digits. Please reduce the number of digits to the estimated precision of the measurements.\n- Text above Fig. 3 says ‘(…) traditional GDEs (just like carbon paper (…)’. Should that be ‘just like’ -> ‘such as’ ?\n- While the PTFE is now mentioned in the manuscript, another line still says ‘SS-Ag HDF-GDE only contains the catalyst and an SSM skeleton (…)’, while I think there needs to be PTFE added then?\n- The manuscript could use some language/grammar editing. The authors often omit an article (the/an) and could use an overall language check."}, "author_response": "A point-by-point response to the Reviewers’\ncomments\nWe highly thank the Reviewers for their constructive comments, which have greatly improved our manuscript’s quality. We have added further analysis and experiments to address the Reviewer’s concerns and suggestions fully. Below, we address the points raised by the Reviewers. To highlight the changes made to the manuscript, the changed text has been colored Blue.\nResponse to Reviewer #1\nMy comments have been addressed in this revised manuscript. I recommend this manuscript to be accepted.\nResponse: We highly appreciate the reviewer’s support of our work for publication and the constructive suggestions that substantially improved the manuscript’s quality.\nResponse to Reviewer #2\nThe revised manuscript of She et al demonstrates a good progress in CO\n2\nelectrolysis at higher current density with good Faradaic efficiency and\nreasonable stability at larger scale. The revisions have improved the manuscript.\nThe points regarding hydrophobicity and the layer structure are clear now. I\nhave to say that still the use of carbon-free GDE, or GDEs with higher mass\ntransfer in general, are not unique (as elaborated earlier), but I think the\nadvances in the GDE combined with CO utilization and scale-up make the\n2\nmanuscript valuable. Nevertheless, some points need to be addressed.\nResponse: We greatly appreciate the reviewer’s positive response to our response.\nYour professional questions have prompted us to reconsider the carbon-free GDE and\nto improve the quality of our manuscript. Thank Reviewer #2, again for your\nrecognition of our response. We have carefully addressed your concerns.\nSpecifically:\nQuestion 1: One of the main developments in this paper is the new GDE\nstructure, but it’s hard to understand which configuration the authors are using\nfor what figure. For example, this information in Fig. S12 is a ‘SS-Ag’ structure,\nbut SS-Ag was also used later in the figure 2 and in Figure 3, so this is not\ndescriptive. It would help to make a scheme of the different layer structures\n(including where’s the gas channel and membrane, so maybe full cell structures)\nevery time the authors change to discussing another configuration. That could be\na sub-figure in Fig.2 and Fig. 3 (as a suggestion, the figure 2h is superfluous, as\n2g is already presenting the same information, and figure 3b is not very\ninformative, which makes some space for a schematic layer structure).\nResponse: We highly appreciate the reviewer for this sincere and friendly suggestion.\nFollowing your suggestions, we have included the relevant content from the GDE and\ncell configurations to clarify the different system configurations, as shown below.\nSupplementary Fig. 12 | Schematic of the flow cell used in this work.\nSupplementary Fig. 18 | Schematic of the alkali-cation-free APMA MEA cell for\nECO\nFig. 2 | Characterizations of SS-Ag and the ECO R performance in the\n2\nalkali-cation-free APMA MEA cell. a, HAADF-STEM image of SS-Ag. b,\nFalse-color orientation map from four-dimensional STEM (4D-STEM),\ncorresponding to (a). The color map shows grain orientations classified based on the\nangular position of 200 diffraction spots per probe position. c, Atomic-resolution\nHAADF-STEM image of SS-Ag showing stepped surfaces induced by a stacking\nfault and a twin boundary, both along {111} planes (dashed white lines). d, FEs\ntoward ECO R products on SS-Ag under a range of applied current densities and the\n2\ncorresponding cell voltages. e, Schematic of the APMA ECO R system based on\n2\nalkali-cation-free APMA MEA cell at 150 mA/cm2 and different CO inlet flow rates.\n2\ng, The CO conversions and cell voltage for ECO R-to-CO on SS-Ag under different\n2 2\nCO inlet flow rates. h, FEs towards products under different CO inlet flow rates for\n2 2\nECO R. Values are means and error bars indicate SD (n = 3 replicates).\nb, Schematic of the APMA ECO R system based on SS-Ag HDF-GDE. c, The FEs\n2\ntowards ECO R products on SS-Ag HDF-GDE under a range of applied current\n2\ndensities and the corresponding cell voltages. d-f, Comparisons of CO FEs (d), H\nmA/cm2.\nQuestion 2: Also in the text, above Figure 2, the step towards alkali-cation-free\nand AEM+PEM assembly is not straightforward and goes too quickly. The\nAEM+PEM structure is basically a bipolar membrane. Is this used for the other\nresults as well? And was a catalyst involved between the AEM+PEM layer (like\nothers do for BPMs)?\nResponse: Thank you for raising this important point. This is a very important and\nprofessional question. As you said, the AEM+PEM structure is basically a bipolar\nmembrane. The AEM+PEM architecture can also be used for other results, such as\nthe ECO R-to-C H reaction using the Cu catalyst.\n2 2 4\nAs the reviewer noted, under reverse-bias mode (Figure R1a), a water dissociation\ncatalyst (e.g., TiO , Al(OH) , or IrO ) is typically required in the BPM middle layer to\n2 3 2\nlower the overpotential for water dissociation. H+ and OH- from water dissociation\ncan be transferred to the cathode and anode to take part in ECO R and OER,\n2\nrespectively.\nFigure R1. a, The BPM system under reverse bias mode. b, The APMA system under\nforward bias mode.\nUnder forward bias mode (Figure R1b), the water can directly take part in ECO R\n2\nand OER at the cathode and anode, respectively, and OH- from the cathode and H+\nfrom the anode can spontaneously form water in the BPM middle layer (standard\nGibbs free energy change, < 0). For our AEM+PEM architecture, it was\noperated in forward bias mode;𝜃𝜃 thus, there is no catalyst between the AEM+PEM\n𝛥𝛥𝛥𝛥𝐻𝐻2𝑂𝑂\nlayers.\nTo make the alkali-cation-free AEM+PEM assembly clearer and easier to understand,\nwe have supplemented additional explanations to make the process smoother, as\nshown below.\n“To suppress carbonate formation and crossover during ECO R and improve the\n2\nsystem stability, we adopted an alkali-cation-free MEA architecture with the anion-\nand proton-exchange membranes (AEM and PEM assembly: APMA) to evaluate the\nECO R performance of SS-Ag CP-GDE (Supplementary Fig. 19) (ref. 9). The\n2\nreaction temperature was optimized at 60 °C to decrease the reaction overpotential of\nthe pure-H O system (Supplementary Fig. 20; the following ECO R tests were\n2 2\nconducted at 60 °C, unless otherwise noted).”\nSupplementary Fig. 19 | Schematic of the alkali-cation-free APMA MEA system\narchitecture for ECO R. In the APMA system, the water dissociation occurs at the\n2\ncathode and anode, and the produced OH- and H+ can spontaneously form water at the\ninterface ( < 0).\n𝜃𝜃\n∆𝛥𝛥𝐻𝐻2𝑂𝑂\nQuestion 3: The FE for the case with 1 M KOH (early results, and Fig. S12) is\nextremely high. Generally, some formic acid is formed as well, decreasing the FE\ntowards CO. Did the authors check for formic acid?\nResponse: According to your suggestion, we re-check the liquid product of ECO R on\n2\n(Figure R2a). The catholyte did not contain the liquid product. Actually, we found\nthat trace amounts of CH could form (Figure R2b), especially at high potentials.\ndepends on the pre-treatment of the membrane. If the membrane is pre-treated\nin a solution with high concentration of alkali-cations, the cations are still in the\nmembrane and have no way out. That doesn’t have to be a bad thing, but please\nconsider that feeding pure water does not per se imply alkali-cation-free.\nResponse: We strongly agree with the reviewer that feeding pure water does not, per\nse, imply alkali-cation-free conditions. The AEM used in this work was immersed in\nKOH before use. PEM’s pre-treatment/activation, we followed the NAFION™ User's\nGuide. PEM was soaked in ~3% hydrogen peroxide for 1 h at 80 °C, deionized water\nfor 1 h at 80 °C and 0.5 M H SO for another 1 h at 80 °C. And the resulting PEM\n2 4\nwas cleaned and preserved in deionized water at room temperature. During assembly\nof the AEM+PEM architecture, both AEM and PEM were re-cleaned with deionized\nwater.\nAs the reviewer noted, during assembly of AEM+PEM, although they were cleaned\nwith deionized water, Na+ at the ppb level was detected by inductively coupled\nplasma mass spectrometry (ICP-MS) (Figure R3), originating from the water source.\nThus, the trace amounts of Na+ and K+ may still be in the membrane and have no way\nout. As a result, we re-defined the alkali-cation-free APMA system in the revised\nmanuscript, as shown below.\nFigure R3. ICP-MS measurements of the pure-H O-fed ECO R on SS-Ag CP-GDE in\n2 2\nthe APMA MEA system at 150 mA/cm2. (Left) K+ standard solution reference (500\nppb). (Right) ICP-MS spectra of the electrolyte after 2 h ECO R.\n2\n“In this study, we aimed to achieve a high operation current density (i.e., high CO\n2\nconversion) in a high-stability alkali-cation-free ECO R system while maintaining\n2\nhigh product selectivity by accelerating the mass transfer kinetics of the\nalkali-cation-free system. Here, the alkali-cation-free condition contains trace\namounts of sodium ions originating from the water source9. To achieve this, we\ndeveloped an HDF-GDE comprising the catalyst and a stainless-steel mesh skeleton\nin the catalyst's middle layer. A high-performance stepped-surface Ag (SS-Ag) was\nused as a model catalyst to study the system’s mass transfer kinetics due to its single\nproduct selectivity (i.e., CO).”\nQuestion 5: The modelling part, with the local concentration of CO and CO, is\n2\nstill a weak part. I see the point that the CO concentration changes relatively\nmuch more than the CO concentration, but I have 2 issues with this:\n2\n(1) The local CO concentration does not pose a limiting current (such as CO\n2\ncan\ndo), because it doesn't deplete. Therefore, basically, when it changes a factor\n10, it would give a Nernst potential, but that’s it. A change of 59 mV, which\nNernst would predict by a factor 10 change in concentration, would not make\na big difference, I would say. The authors need to better underpin why this\ncauses so much impact. Maybe the HER is very sensitive, but then I think\nthey then need to compare this with the same setup in HER mode (feeding N ,\n2\nfor example).\nResponse: This is a very important and highly specialized question. We greatly\nappreciate the reviewer's meticulous effort in reviewing our manuscript and\nproviding constructive comments. We agree with the reviewer that HER may be\nhighly sensitive, and HER performance on CP-GDE and HDF-GDE should be\ncompared.\nFrom a thermodynamic perspective, a 10-fold change in concentration results in\nonly a 59 mV change (i.e., a Nernst potential). Per your suggestion, we added the\nLSV experiments for SS-Ag CP-GDE and SS-Ag HDF-GDE under N (Figure R4).\ncould indeed suppress HER over a specific range (~40 mV change at 10 mA/cm2),\ncompared with SS-Ag CP-GDE. Additionally, from a dynamic perspective, the\noverall carbon flux to and from the HDF-GDE surface increased, accelerating the\nsystem’s mass transfer kinetics.\nFigure R4. Polarization curves of different gas diffusion electrodes with ~1 cm2\nin 1 M KHCO , with a scan rate of 5 mV/s and without insulation resistance\n3\ncompensation.\nThus, based on your suggestions, we added LSV experiments under N and the\n2\ncorresponding discussion to the manuscript, as shown below.\n“When the current density of ECO R was 150 mA/cm2, the CO mass transfer\n2 2\nkinetics in GDEs were also simulated (to simplify the model, HER was not\nconsidered). At the same CO inlet flow rate, at the inlet, the efficiency of CO\n2 2\nmass transfer to the catalyst surface in HDF-GDE was significantly higher than\nthat in the CP-GDE in the d and d directions (Fig. 4a, 4b, and\nVertical Parallel\nSupplementary Fig. 52), indicating that CO coverage on the catalyst surface in\nprogressed, the CO coverage decreased gradually for both GDEs along the flow\n2\nchannel direction (Fig. 4c-4e). Near the outlet, the CO coverage on the catalyst\n2\nsurface in HDF-GDE is less than in the CP-GDE. This is because, in the catalyst\nlayer, the product (CO/*CO) coverage in HDF-GDE was higher than in the\nCP-GDE, which was almost a factor of magnitude difference (Fig. 4f-4h and\nSupplementary Fig. 53). One CO molecule can generate one CO molecule, thus,\n2\nthe overall system’s mass transfer needed to consider CO and CO (i.e., the\n2\noverall carbon concentration). Obviously, the system’s mass transfer efficiency of\nHDF-GDE is much higher than that of CP-GDE. Furthermore, the high carbon\ncoverage on the catalyst surface in HDF-GDE could occupy more reaction sites,\nthereby decreasing the *H coverage, suppressing HER, and enhancing\nECO R-to-CO activity. In addition, the polarization curves of SS-Ag CP-GDE and\n2\nsuppressing HER (Supplementary Fig. 54). These synergistically improved the\npeak operation current density for ECO R and, thus, increased CO conversion.”\n2 2\nWe thank the reviewer again for their constructive suggestions, which led us to\nthink more deeply about the gas diffusion electrode.\n(2) The real mass transfer improvement is perpendicular to the catalyst layer,\nwhile the authors plot the xy concentrations. This does not really show the\nmass transfer benefits of the different GDE structures.\nResponse: We appreciate the reviewer for raising this critical point. We highly\nagree with the reviewer that the mass transfer perpendicular to the catalyst layer\nis of great significance. According to the reviewer’s suggestions, we have added\nthe relevant information and discussion in the manuscript, as shown below\n(Figure 4e and 4h, and Supplementary Figs. 46 and 52).\n“Under unreacted steady-state conditions, modeling of CO mass transfer kinetics\n2\nin different GDEs showed that HDF-GDE could slightly accelerate CO mass\n2\ntransfer kinetics in the catalyst layer, compared with that of CP-GDE in both the\nvertical and parallel directions (Supplementary Fig. 42, see multiphysics\nsimulation section in Supplementary information for more information). The\neffects of the thickness and porosity of HDF-GDE and CO flow rate on CO\n2 2\nmass transfer were also investigated, showing that HDF-GDE’s thickness and CO\n2\nflow rate were the two main factors influencing CO mass transfer efficiency\n2\nunder unreacted steady-state conditions (Supplementary Fig. 43-48). The GDE’s\nthickness was negatively correlated with CO mass transfer, and the CO inlet\n2 2\nflow rate was positively correlated with CO mass transfer. Furthermore, in the\n2\nvertical direction to the catalyst layer, there was almost no significant difference in\nCO mass transfer in HDF-GDE and CP-GDE (Supplementary Figs. 49-51).”\n2\nat 150 mA/cm2. a,b, CO concentration in different GDEs at different CO inlet\n2 2\nflow rates and the parallel direction and with an applied current density of 150\nmA/cm2. c,d, CO concentration on the surface of the catalyst layer on CP-GDE\n2\n(c) and HDF-GDE (d) at a CO inlet flow rate of 2 sccm and with an applied\n2\ncurrent of 150 mA/cm2. e, CO concentration in the four selected areas of the\n2\ncatalyst layer on different GDEs, corresponding to the structures in (c) and (d). f,g,\nCO concentration on the surface of the catalyst layer on CP-GDE (f) and\nHDF-GDE (g) at a CO inlet flow rate of 2 sccm and with an applied current of\n2\n150 mA/cm2. h, COconcentration in the four selected areas of the catalyst layer\non different GDEs, corresponding to the structures in (f) and (g).\nSupplementary Fig. 46 | Modeling of the CO concentration in different\n2\nvertical direction and without the applied current. a-e, CO inlet flow rates: 30,\n2\n20, 10, 5 and 2 sccm, respectively.\nSupplementary Fig. 52 | Modeling of the CO concentration in different\n2\ndirection and with an applied current of 150 mA/cm2. a, Traditional CP-GDE.\nb, HDF-GDE.\nQuestion 6: For the larger scale experiments, the authors started with 30 deg C.\nDid the authors observe heating? Literature has recently picked up effects of\ntemperature, and would be good to compare these new results to literature.\nResponse: For the larger-scale experiments, we observed the heating effect. The\nscaled-up alkali-cation-free MEA cell stack assembled with SS-Ag HDF-GDEs was\noperated at a total current of 60 A, delivering a power of ~1.29 kW for ECO R-to-CO.\nAbout the effects of temperature on ECO R performance, we did some experiments.\n2 2\nincreasing the reaction temperature. And we found that low temperature could cause\nhigh overpotential, while too high a temperature would suppress ECO R and increase\n2\nthe optimized reaction temperature was 60 °C.\nFigure R5. The FEs of products and corresponding cell voltages of the\nalkali-cation-free APMA MEA system at a total current density of 100 mA/cm2 and\ndifferent reaction temperatures.\nBased on your suggestions, we have carefully reviewed recent works on the effects of\ntemperature. For example, Prof. Sievers found that increasing temperature directly\naffects the CO conversion route to C and C products, beyond simply controlling\n1 2+\nlocal CO availability, mass transport, and elementary reaction rates (ACS\ncompared the ECO R temperature, as shown below (Table R1).\nEffect of higher\nTemperature Cell Type Electrolyte Ref.\nT on product FE\n(T)\nIncrease: H\n2\na.25 - 80 °C MEA Cell Pure H 2 O This work\nDecrease: CO\nIncrease: H , C H , CH\n2 2 4 4\n25 - 80 °C MEA Cell Pure H O S53\nIncrease: H\n4\n0.1 M\n25 - 75 °C H-Cell C H OH S54\n2 5\nKHCO\nMaximum at 45 °C: C H\n2 4\nIncrease: H , CO\n1.0 M Decrease: C H , HCOOH,\n2 4\n25 - 80 °C or S55\nKHCO C H OH, n-C H OH,\n3 2 5 3 7\nMEA\nC H O\n2 4 2\nIncrease: H\n4\n1.0 M\n18 - 70 °C H-Cell Constant: CO S56\nKHCO\n2 4\n,\nC H OH\n2 5\n1.0 M Increase: H , CO\n2\n5 - 45 °C H-Cell S57\nKHCO Decrease: CH\n3 4\nIncrease: H , CO\n2\n1.0 M\n2 - 42 °C H-Cell Decrease: CH , HCOOH S58\n2 4\nIncrease: H\n2\n0.65 M\n0 - 15 °C H-Cell Decrease: CH S59\n0.5 M Increase: C H\n2 4\n0 - 48 °C H-Cell S60\nKHCO Decrease: CH\n3 4\n0.5 M Increase: C H\n2 4\n0 - 22 °C H-Cell S61\nKHCO Decrease: CH\n3 4\nIncrease: H , CO, C H\n2 2 4\n0.5 M\n0 - 40 °C H-Cell Decrease: CH S62\na. The catalyst was an Ag catalyst; others are Cu-based catalysts.\nQuestion 7: The paper is rather long. Not sure what the editor thinks about the\nlength, but if needed, I think the authors can write their thoughts on the C H\n2 4\ncase more dense, because it’s not really needed for the core of the paper.\nResponse: Thanks for these kind and sincere suggestions. We have shortened the\nmanuscript. Per your suggestion, we have added content on C H , as shown below.\n2 4\n“We sought to selectively steer CO toward C H using the Cu electrocatalyst, in light\n2 2 4\nof the high value and broad application of C H in the chemical industry. Based on\n2 4\nour previously reported SS-Cu9, we fabricated SS-Cu HDF-GDE. We evaluated its\nECO R-to-C H performance in the alkali-cation-free MEA cell (Supplementary Fig.\n2 2 4\n58). The peak C H FE reached ~45% with a cell voltage of ~4.36 V, comparable to\n2 4\nSS-Cu CP-GDE (FE : ~43%; cell voltage: ~4.30 V)9, but the total current density\nC2H4\nwas increased from 300 to 500 mA/cm2 (ref. 9), indicating that the HDF-GDE\nstrategy was also suitable for alkali-cation-free ECO R-to-C H /C . At a total current\n2 2 4 2+\nof 75 A, the stability of the kW-scale (~1.95 kW) alkali-cation-free APMA MEA cell\nstack assembled with 6 SS-Cu HDF-GDEs was also over 1000 h, yielding ~14 kg\nC H (Fig. 5e and Supplementary Figs. 59 and 60). And ECO R-to-C H conversion\n2 4 2 2 4\nin this alkali-cation-free APMA MEA cell stack increased to approximately ~15 times,\ncompared to the SS-Cu CP-GDE in the initial single APMA MEA cell9. For both\nSS-Ag and SS-Cu (ECO R-to-CO/C H ), HDF-GDEs improved CO conversion\n2 2 4 2\nefficiency by increasing ECO R current density rather than decreasing the CO inlet\n2 2\nflow rate, while maintaining high product selectivity, effectively alleviating the\ntrade-off between high product selectivity and high CO conversion. Furthermore, the\n2\noperation current density of the pure-H O-fed (alkali-cation-free) system was\n2\neffectively broken through.”\nSome minor comments:\n- The authors do not use a correct number of significant digits. The FE is\nsometimes expressed in 82.26% or 42.97%, while I assume that their precision is\nnot up to 2 digits. Please reduce the number of digits to the estimated precision of\nthe measurements.\nResponse: Thanks for these kind and sincere suggestions. According to your\nsuggestions, we have double-checked the manuscript and corrected the number of\nsignificant digits.\n- Text above Fig. 3 says ‘(…) traditional GDEs (just like carbon paper (…)’.\nShould that be ‘just like’ -> ‘such as’ ?\nResponse: Thanks so much. Followed your suggestions.\n- While the PTFE is now mentioned in the manuscript, another line still says\n‘SS-Ag HDF-GDE only contains the catalyst and an SSM skeleton (…)’, while I\nthink there needs to be PTFE added then?\nResponse: Thank you for your tolerance of our carelessness. PTFE information has\nbeen added in the relevant statement. Thank you so much.\n- The manuscript could use some language/grammar editing. The authors often\nomit an article (the/an) and could use an overall language check.\nResponse: We had revised the grammatical errors and double-checked the\nmanuscript.\nAgain, we highly appreciate Reviewer #2's meticulous effort in reviewing our\nmanuscript and providing constructive comments."}, {"round": 3, "reviewer_comments": {"reviewer_1": "The revision has addressed all the comments properly. No more questions from my side.", "reviewer_2": "The manuscript by She et al. has well implemented the revisions, and is now recommended for publication. The description is now more clear and the section around the modeling has well improved. Maybe the only thing is that the partial current density still has up to 5 significant digits (when discussing figure 2h), and that should be probably up to 3 or so, but that is a correction that can be made in 1 minute."}, "author_response": "Response to Reviewer #1\nThe revision has addressed all the comments properly. No more questions from\nmy side.\nResponse: We highly appreciate the reviewer’s support of our work for publication\nand the constructive suggestions that substantially improved the manuscript’s quality.\nResponse to Reviewer #2\nThe manuscript by She et al. has well implemented the revisions, and is now\nrecommended for publication. The description is now more clear and the section\naround the modeling has well improved. Maybe the only thing is that the partial\ncurrent density still has up to 5 significant digits (when discussing figure 2h),\nand that should be probably up to 3 or so, but that is a correction that can be made in\n1 minute.\nResponse: According to the Reviewer’s suggestion, the partial current densities have\nbeen modified to have three significant digits.\nAgain, we highly thank Reviewer #2 for his/her meticulous effort in reviewing our\nmanuscript and providing constructive comments."}]

Decision letter