Bernoulli’s principle-mediated Cl_2 electrosynthesis

Domain: chemistry
Published: 2026-01-24
Editorial outcome: major_revision

Abstract
Existing technologies for chlorine (Cl2) synthesis are generally suffered from low productivity or high production cost. Guided by Bernoulli’s principle, here we report an efficient yet cost-effective electrochemical system for Cl2 electrosynthesis, which is composed of anodic chlorine evolution reaction (CER) connected to gas chamber by triple-phase gas diffusion layer. The key is to modulate gas diffusion layer by Bernoulli’s principle, wherein the pressure difference at triple-phase boundary drives oriented Cl2 migration directly into gas chamber, thus preventing the crossover of anodic/cathodic products. By further joining with a pH-tolerant catalyst, a standalone prototype device is built for high-rate Cl2 production, operating at the Faradaic efficiencies of 96.3%~87.6% in the current density range of 0.1~1.14 A cm−2, having superior Cl2 synthesis performance. Further technical-economic evaluations of our synthetic scheme demonstrate reduced Cl2 production cost, saving 6.75% (1.17 million dollar per year) as comparison to conventional chlor-alkali design. We expect these findings offer broader opportunities to develop industrially production processes for other chemical commodities.

Introduction
Chlorine (Cl2) is one of the 100 most important chemical compounds in the world with diverse applications in disinfection goods production, pharmaceutical manufacture and wastewater treatment1. Up to 2024, the global production of Cl2 was more than 90 million tons per year, its market value being over US $20 billion. However, an efficient yet cost-effective technology for Cl2 production is still lacking.

The chemical oxidation method for Cl2 synthesis was developed centuries ago by Berthollet (1789)2 and Deacon (1868)3, but is limited by low productivity. After 1900s, membrane-based chlor-alkali processes have become the dominant routes for industrial Cl2 production, which are composed of anodic chlorine evolution reaction (CER)4, cathodic hydrogen evolution reaction (HER)5 in addition to membranes (like asbestos fleeces6 or fluorine-containing ion exchange membranes7) for separating Cl2 production at the anode, hydrogen (H2) production at the cathode and hydroxyl ions (OH−) in the electrolyte. In spite of their improved productivities, a major problem associated with these systems includes high operation costs arising from limited lifetime and low tolerance to contaminant ions of membranes8,9. Further, these membranes also increase system maintenance cost because of complicated configuration, as gas pressures in an anodic and cathodic chambers must remain in balance, otherwise aggravating membrane degradation10.

By contrast, a membrane-free chlor-alkali design would allow for simplified system configuration and durable operation, thus potentially delivering production cost savings (Fig. 1a, b). Yet, realizing such a design has met substantial challenges because of anodic/cathodic product crossover11:

Anodic CER: 2Cl− → Cl2 + 2e−, Eo = 1.36 V vs. RHE (1)

Cathodic HER: 2H2O + 2e− → H2 + 2OH−, Eo = 0 V vs. RHE (2)

Side OER: 2H2O → O2 + 4H+ + 4e−, Eo = 1.23 V vs. RHE (3)

Without a membrane, gaseous products of anodic Cl2 and cathodic H2 directly mix to form an H2/Cl2 mixture, especially at high current densities coupled with high reaction rates, which tends to explode upon ignition or light irradiation (H2/Cl2 explosion limit: 5%~87.5%)12. Further, hydroxyl ions (OH−) from cathodic HER migrants to the anodic CER chamber that elevates pH (pH > 7), and according to Pourbaix diagram (Fig. 1c)13, resulting in parasitic oxygen evolution reaction (OER) that consumes electrons otherwise for producing Cl27. In spite of several preliminary studies separating anodic/cathodic products by redox mediators (like Hg/NaHg14 and Na0.44MnO216), their Cl2 productivities are prohibitively low, with the maximum electricity efficiency of 50% by considering a whole operating cycle. Further, problems associated with these systems involve production costs, complex configuration and limited lifetimes of redox mediators15–17.

In this work, we report a strategy to address the stumbling problem of anodic/cathodic product crossover, which is achieved by taking advantage of “Bernoulli’s principle”, an energy conservation model firstly reported in 173818,19. The local pressure of a streaming gas can be leveraged by tuning its speed, with the pressure being lower the higher the speed. Yet, the conventional Bernoulli’s model only describes binary-phase systems. For gas-involving electrochemistry (like CER7 and other redox reactions of O220/CO221/N222), this model needs be revised to include triple-phase interfaces, as high-rate reactions occurs at the ternary boundaries of gas/aqueous electrolytes/catalysts. Guided by the updated model, here we have conducted numerical simulations for the local pressure of Cl2 at triple-phase boundary (Please see Figs. 1b, 5c, d and Supplementary Video 1). By manipulating the flow rates of liquids (0~8.4 mL min−1) and carrier gases (0~80 mL min−1), a substantial pressure difference can be easily achieved (0~116.3×10−3 Pa). This offers the opportunity to promote oriented migration of Cl2 away from reaction interfaces (i.e., H2/OH− at cathode/anode), which cannot only prevent H2/Cl2 crossover but also circumvent the Pourbaix diagram problems, leading to enhanced activities. Finally, our strategy has been demonstrated for solving key challenging problems in other chemical/catalytic reactions like water splitting (2H2O → 2H2 + O2, Supplementary Fig. 79).

Results
Limitation of Pourbaix diagram
To illustrate typical limitations of Pourbaix diagram in CER (Fig. 1b), the benchmark dimensionally stable anode (DSA, Supplementary Fig. 1), as developed half a century ago23, has been integrated into a flow cell to evaluate its activities. Firstly, the LSV of DSA in 5 M NaCl (pH = 1) were measured, showing a current density of 10 mA cm−2 at an overpotential of 80 mV (Supplementary Fig. 2), which is consistent to the literature5. Nevertheless, by elevating the pH from 1 to 13 with the DSA catalyst, Cl2 yield rates and Faradaic efficiencies (FE) change significantly. At the potential of 1.8 V, the Cl2 gas Faradaic efficiencies are 46.6%, 4.3% and 0% at pH = 1, 7 and 13, respectively. This illustrates the above-discussed pH effects on catalysts. Analogously, the Cl2 gas yield rates are 3, 0.21, and 0 mmol h−1 cm−2 at pH = 1, 7, and 13, respectively. The above phenomenon is consistent to the Pourbaix diagram applied to CER (Fig. 1c), which shows the Cl−/Cl2 couple in the pH < 4.3 range, HOCl/Cl− couple in the range of 4.3~7.5 and OCl−/Cl− couple at pH > 7.5. Therefore, Cl2 prefers to be produced in an acidic environment, while it easily hydrolyzes to HOCl and OCl− in neutral and alkaline conditions. Accordingly, it requires to develop pH-tolerant strategies and catalysts to circumvent the limitation of the Pourbaix diagram24.

pH-universal Cl2 electrosynthesis
We have prepared a titanium-based metal-organic framework (Ti-MOF) catalyst by a facile two-step procedure, starting by the coordinate assembly of Ti nodes and 2,6-naphthoic ligand, followed by low-temperature calcination at 450 oC to improve its crystallinity. Morphological characterizations show Ti-MOF comprised of nanosheet structures with Ti, O, C as the main components, whereas a small portion of sodium originates from the organic ligand precursor (please see the experiment sections). Close examination of a Ti-MOF nanosheet reveals its well-defined crystallinity with the adjacent lattice spacing of 0.21 nm (Fig. 2a and Supplementary Fig. 3), which agrees well with X-ray diffraction (XRD) containing a series of characteristic peaks corresponding to metal-organic framework architectures with dominant (210) crystal face (Fig. 2b and Supplementary Table 1)25. Further, the successful synthesis of Ti-MOF has been demonstrated by the Fourier transform infrared spectra (FTIR, Fig. 2c), showing characteristic vibrations of 1400–1630 cm−1 of naphthalene ring from the organic ligand (Supplementary Figs. 4−6) and the vibrational peak at 785 cm−1 from Ti-O bonding26,27. Notably, Ti-MOF can survive in pH-universal conditions, owing to the robust Ti-O bonding within the framework28–30.

To further unravel the coordination environment inside Ti-MOF, we have conducted analyses of X-ray absorption near-side structure (XANES) and extended X-ray absorption fine structure (EXAFS) spectroscopy (Fig. 2d, e, Supplementary Figs. 7, 8). The Ti-MOF has presented XANES absorption edge energy between Ti-foil and TiO2 (4962.95 vs. 4961.22 and 4963.46 eV), indicating its valence state of Ti in the range of 0~+4. Different from Ti foil bearing only Ti-Ti coordination (distance: 2.89 Å), the Fourier transform EXAFS spectrum of Ti-MOF shows featured shells of Ti-Ti (3.12 Å) and Ti-O (1.92 Å) coordination. The Ti-Ti distance (3.12 Å) corresponds to interlayer interaction between Ti atoms in the form of multi-nuclear titanium clusters31. The Ti-O distance (1.92 Å) is significantly shorter as comparison to the Ti-C bond distance (2.14 Å, Supplementary Figs, 9, 10). Further, the coordination number of Ti foil is quantified as 6 (for Ti-Ti shell), while that of Ti-MOF is 4.05 for Ti-Ti shell (Supplementary Table 2). This result agrees well with XPS results (Supplementary Figs. 11, 12) as well as the wavelet transform (WT) diagrams (Fig. 2f). It is known that WT analysis can serve as a qualitative supplement to EXAFS fitting, which can resolve overlapping coordination shells and validate scattering paths. Consequently, quantitative structural parameters of WT analysis have been derived from r-space fitting. As comparison to its precursor without calcination, the Ti-O coordination number of Ti-MOF only shows slight decline from 3.98 to 3.36, indicating calcinationstreatment does only weakly affect at least the local metal-organic framework architecture. Ti atom can theoretically coordinate with six other atoms in the form of a hexacoordinated architecture, which represents the global symmetry constraints for Ti atoms. On the other hand, the coordination number of Ti-O in Ti-MOF has been experimentally determined by XAS (Fig. 2d, e and Supplementary Figs. 7, 8), where the average number is 3.98 (before low-temperature calcination) or 3.36 (final structure after calcination). This phenomenon indicates the evaporation of some organic ligands during the calcination process, which can expose more Ti active sites for electrochemical reaction.

Subsequently, we start the electrochemical performance evaluation for different samples. The gas diffusion electrode (GDE) substrate has shown low Cl2 Faradaic efficiencies (Supplementary Fig. 13). The activities of Ti-MOF have been evaluated by tuning the pressure difference according to Bernoulli’s principle (Fig. 2g, h and Supplementary Figs. 14−29, 30a–d). On one side of the GDE is the microporous layer loaded with Ti-MOF sample and the electrolyte circulated through the liquid chamber with a fixed rate of 8.4 mL min−1, while on the otherside is the gas chamber with air flow rate of 80 mL min−1, which generates a pressure difference of 116.3×10−3 Pa at the three-phase boundary according to the Bernoulli’s equation (Fig. 2g, Supplementary Video 2 and Supplementary Fig. 30e). Without pressure difference, Ti-MOF has demonstrated diminished activities with elevated pH values, i.e., Faradaic efficiency of 72.8% (yield rate of 2.28 mmol cm−2 h−1) at pH = 1, 5.8% (yield rate of 0.21 mmol cm−2 h−1) at pH = 7 and 0% (yield rate of 0 mmol cm−2 h−1) at pH = 13. With pressure difference, the activities of Ti-MOF shows seldom fluctuations in the pH range of 1~13 at applied potential of 1.8~2.4 V. Specifically at the potential of 1.8 V with the Ti-MOF catalyst, the Cl2 Faradaic efficiencies are 93.3, 94, and 88.8% at the pH of 1, 7, and 13, respectively.

For reliable compare with benchmark DSA, we have further loaded the main components (RuO2 and TiO2) of DSA on the gas diffusion electrode (GDE, Supplementary Fig. 31). By applying interfacial pressure difference, the activities increased significantly, i.e., Faradaic efficiency of 86.3% (yield rate of 3.42 mmol cm−2 h−1) at pH = 1, 88% (yield rate of 3.3 mmol cm−2 h−1) at pH = 7 and 81.4% (yield rate of 3.3 mmol cm−2 h−1) at pH = 13. To facilitate comparison across diverse pH conditions, all of the potentials have been converted to RHE (Supplementary Fig. 32 and Supplementary Table 3). Furthermore, to investigate the dissolution of Cl2 under different pH conditions, Supplementary Fig. 33 shows that when the pH becomes alkaline, More Cl2 will dissolve in the electrolyte. Based on above data, we consider the pH is not a highly influential parameter anymore. In other words, we can move out of the limits of the Pourbaix diagram to produce Cl2 efficiently across a wider range of pH conditions.

Notably, the CER activity of Ti-MOF is further validated by other experimental data. For example, we have investigated the effect of calcination temperatures of Ti-MOF on CER performances (150, 250, 350, 450, and 550 oC; Supplementary Figs. 34−37). The Cl2 Faradaic efficiencies of Ti-MOF increase with calcination temperatures, starting from 38.6% at 150 oC to reach the maximum value (88.8%) at 450 oC, and finally decrease to 77.7% at 550 oC. Analogously, the Cl2 Faradaic efficiencies are 38.6%, 66.6%, 64.7%, 88.8% and 77.7% at the temperatures of 150, 250, 350, 450, and 550 oC, respectively. Importantly, the Ti-MOF also exhibits strong stability in universal pH conditions (Fig. 2h). Over the period of 50 hrs at pH = 1, 7, 13, the Ti-MOF electrode has shown little morphology and structure decay after stability test (Supplementary Figs. 38−40).

Standalone prototype device
Next, a standalone prototype device has demonstrated for membrane-free chlor-alkali process (Supplementary Video 3 and Supplementary Figs. 41–43). Figure 3a shows the schematic view of the prototype device, which is composed of a liquid chamber for cathodic HER/anodic CER, a gas chamber for carrier gas and external pumps driving NaCl electrolyte and gas flow circulating through the system. The prototype device is present in the form of a cuboid in the size of 24×22×12 cm3 (Fig. 3b and Supplementary Video 3). Manipulated by triple-phase Bernoulli’s principle, the pressure difference has formed that takes as-produced Cl2 into gas chamber, as evidenced by KI titration solutions with brown I2 formed during the operation (the top right corner of Fig. 3b). In addition, gas chromatography demonstrates the successful separation of Cl2 and H2 in the membrane-free prototype device (the bottom right corner of Fig. 3b and Supplementary Figs. 44, 45). Finally, The H2 generated at the cathode is collected and measured by gas chromatography, gas-phase results (Supplementary Fig. 46) demonstrate that H2 is discharged from the reaction system along with the electrolyte.

Next, we have evaluated the performances of the prototype device with fixed NaCl electrolyte rate (8.4 mL min−1) and tunable gas flow rates in the range of 0~80 mL min−1. It shows the Cl2 Faradaic efficiencies closely affiliated with flow rates (Fig. 3c). For Ti-MOF, the Cl2 Faradaic efficiencies increase by elevating flow rates, for example, 17.6% at 0 mL min−1 (yield rate: 0.228 mmol h−1 cm−2), 32.25% at 10 mL min−1 (yield rate: 0.48 mmol h−1 cm−2), 45.12% at 20 mL min−1 (yield rate: 0.6 mmol h−1 cm−2) and 75.57% at 30 mL min−1 (yield rate: 0.978 mmol h−1 cm−2). This phenomenon can be explained by the triple-phase Bernoulli equation: low rates of gas flow only result in the small pressure difference (0~7.26×10−3 Pa), making it difficult to take away the as-generated Cl2 at the three-phase boundary. While beyond 30 mL min−1, the Cl2 Faradaic efficiencies have reached the maximum value (75.8~82.6% and yield rate: 1.02~1.08 mmol h−1 cm−2) in the range of 40~80 mL min−1 with moderate pressure difference (16.3~116.3×10−3 Pa). Notably, other than air flow, controlled experiments have been conducted in the same condition by using CO2 flow and a vacuum condition. As expected, the Cl2 Faradaic efficiencies are comparable to the air counterpart, thus demonstrating the generality for our system (Fig. 3c, Supplementary Fig. 47 and Supplementary Table 4, 5).

We have found the catalyst of Ti-MOF an important contributor in the membrane-free chlor-alkali process, which even outperforms benchmark DSA for Cl2 electrosynthesis. As showing in Fig. 3d, e, the Cl2 gas Faradaic efficiencies for Ti-MOF is 94% at the potential of 1.8 V, which is 18 times the one of DSA under the same test conditions and set-up. Analogously, the Cl2 gas yield rate of Ti-MOF is 3.4 times the one of DSA at a potential of 1.8 V. Further with Ti-MOF, the performances with and without membranes have been compared, which shows minor alternations in Cl2 Faradaic efficiencies (94% vs. 92%) and yield rates (0.93 vs. 0.9 mmol h−1 cm−2 Fig. 3d, e and Supplementary Fig. 48). We have also compared the activities in terms of other indicators (like electricity consumption, overpotential and Tafel plot), all of which underpinning the excellent performances of Ti-MOF (Fig. 3f).

Finally, durability is a noteworthy merit for large-scale applications. As shown in Fig. 3g, the blue dots in the figure represent the potentials driven the long-term stability test of the prototype device, which shows little fluctuation for 200 h at a current density of 100 mA cm−2. Analogously, the bar chart represents the overall Cl2 Faradaic efficiencies, which can be divided into gaseous Cl2 (blue) and the dissolved Cl2 species (pompadour). The overall Cl2 Faradaic efficiencies maintain stable over the whole 200-hr period (Supplementary Fig. 49), with little dissolved Cl2 detected (pompadour, Faradaic efficiencies of 1.7%~7.5%).

Technical-economic evaluations
To further evaluate the performances under the operating conditions of the literature and industrial chlor-alkali electrolysis, the prototype device has worked under industrial operation conditions of 70 °C by collecting both gaseous Cl2 and residue chlorine (denoted as active chlorine or AC)7. In spite of the “activity-selectivity trade-off” relationship from competitive OER (Supplementary Fig. 50), the productivity of 18.675 mmol h−1 cm−2 has been achieved with a Faradaic efficiency of 87.6% at a current density of 1.14 A cm−2, which is competitive among all the chlorine synthesis technologies described in the literature. (Fig. 4a, b and Supplementary Table 6). In spite of the membrane-involving system achieving comparable activities under the same testing conditions, the separation membrane turned black under elevated temperature at high current densities (Supplementary Figs. 51, 52). Finally, the stability of the prototype device was tested under 1.14 A cm−2 (Supplementary Fig. 53), which demonstrates durability for 40 hrs.

Consequently, we have analyzed the capital production cost over a 20-year production by techno-economics simulations, where the cost breakdown is presented as a function of current densities by normalizing both capital investment and operating cost (Fig. 4c, Supplementary Figs. 54−56 and Supplementary Tables 7−10)32. Overall, the capital cost of both membrane-free/based systems generally decreases with the elevated current densities from 0.18 to 1.14 A cm−2, while our membrane-free device has presented substantial cost saving as comparison to the membrane-based counterpart over the whole current density range. This result is consistent to single variable sensitivity analysis performed to examine the main parameters contributing to determining production cost (Fig. 4d, e). In both membrane-free/based systems, the capital cost is most susceptible to the variation in raw materials, followed by separation, electricity and production cost (labor and equipment), while the use of membrane has accounted for additional production cost of 6.75% (1.17 million dollar per year) in the membrane-based systems. Consequently, the profit profile has been calculated from the difference between sales and cost after 25% tax deduction recorded as financial net present value (FNPV)20:

FNPV = Σn (t=0) (Cl−CO) × (1+i) (−t) (4)

where Cl is the present value of future cash flow (product revenue), CO is the present value of the original investment (capital and operating costs and 25% tax), i is the discount rates and t is the duration. The profits difference with/without the membrane has been intuitively described in Fig. 4g, h, where the red dotted line divides the boundary between loss and profit. Obviously, more sections of FNPV profit have been demonstrated for the Ti-MOF-membrane-free system as compared to the membrane-based counterpart, indicating the elimination of membrane can lead to economic savings. The above result is consistent to the payback duration profiles (Fig. 4i). In spite of both membrane-free/-based systems show similar payback duration within four years, the profit gap becomes significant over 20-year operation (60.44 vs 49.48 million dollars) originated from the removal of membrane in the system.

Finally, we have made a corresponding comparison with DSA system with membranes, which is a benchmark process for current industrial chlor-alkaline process (Supplementary Fig. 57 and Supplementary Table 11). It shows that the incorporation of DSA significantly elevates NaOH/Cl2 production costs from the industrial chlor-alkaline process, which underperform Ti-MOF-based membrane-free system in the range of 1~10 year’s operation. After 11 years, the traditional DSA-membrane system begins to gain an advantage due to its lower power consumption. Overall, the production cost per kg of Cl2/NaOH are comparable for the DSA-based membrane system and our Ti-MOF-based membrane-free system ($0.36768 vs $0.36069).

Mechanism study
To unravel the mechanism for good performances, operando Raman spectra has been conducted to capture the short-lifetime intermediate species (i.e., *OCl) in the CER process. The Raman measurements have been performed in a home-made flow-type cell equipped with a quartz window that can detect the Raman signal from GDE at a 532 nm excitation wavelength. The applied potentials have been polarized from 1.0 to 2.0 V in 200 mV steps (Fig. 5a and Supplementary Fig 58). The operando Raman spectra has displayed a subset of vibrational data: the band at 1601 cm−1 is assigned to the C=C stretching vibration of the benzene ring, while the band at 1374 cm−1 corresponds to the symmetric stretching vibration of the carboxylate group (-COO−) in the MOF substrate.; the band at 3400 cm−1 from H2O; the potential-dependent band at 1093 cm−1 from the Cl-O stretching mode of *OCl during CER process33.

At the pressure difference of 116.3×10−3 Pa, the operando spectra begin to show the stretching modes of *OCl at ~1.0 V. The peak intensity of this band increases continuously until reaching the maximum value at 2.0 V, indicating the generation and rapid accumulation of *OCl on the surface of the catalyst. Quantitatively, Fig. 5b and Supplementary Fig. 58b show the relative peak intensities of *OCl without/with pressure difference at different potentials, which agrees well with the fact of Cl2 produced at the electrode/electrolyte interfaces immediately taken away by gas flow following triple-phase Bernoulli’s principle, leading to decreased *OCl peak signal. To further verified above result, this presence of residue Cl2 has been examined by local pH analyses performed in a RRDE system (Supplementary Fig. 59), which shows the increase of protons at electrode/electrolyte interfaces owing to the dissolution of Cl2 in aqueous electrolytes (Cl2 + H2O → 2H+ + Cl− + ClO−)20. Therefore, it provides additional evidence of Bernoulli’s principle contributing to separating *OCl species at elevated potentials, and according to Le Chatelier principle, promoting the rate-determining step (RDS) of *OCl-Cl2 on the catalyst surfaces34.

In order to understand more clearly the oriented Cl2 migration following triple-phase Bernoulli’s principle, we have conducted finite element simulations for the operating system (Figs. 5c, d and Supplementary Video 1). Figure 5c shows the pressure distribution map with pressure difference of 29×10−3 Pa (Please see other pressure difference diagrams in Supplementary Figs 60−63), which show the NaCl solution flowing at a constant rate of 8.4 mL min−1 in the anode/cathode chamber (light blue area), and the carrier gas flowing at high rates (80 mL min−1) but narrow areas from the inlet to outlet (warm color area). We can propose the as-generated Cl2 at electrode/electrolyte interface would be driven by the negative pressure into gas chamber, which is then taken out by carrier gas away from the system. Notably, Fig. 5d presents the negative pressure generated by carrier gas at different flow rates and positions in the gas chamber. When the flow rate reaches 40 mL min−1, the negative pressure generated by the flow rate begins to distinct change, which agree well with the experiment results (Fig. 3c) that Cl2 selectivity begins to reach the plateau at the carrier gas flow rate exceeding 40 mL min−1. So, we consider the interfacial pressure difference is the key to achieve pH-universal Cl2 electrosynthesis.

Next, the reaction pathways of CER over Ti-MOF have been examined by theoretical calculations using density functional theory (DFT). According to XRD and XAFS results (Fig. 2b–f), the theoretical structure of Ti-MOF has been built on the basis of cluster model composed of Ti coordinated with 2,6-naphthalene (Supplementary Figs. 64−65, the structural information before and after optimization is presented in Supplementary Data 1). The reaction pathways of CER are proposed for Ti-MOF by using Gibbs free energy as a descriptor with potential correction of U = 1.36 V (Fig. 5e)35. Generally, one electron-transfer CER is composed of two cascade steps of the catalysts adsorbing chlorine-containing species, followed by the dissociation to Cl2 for recovering the active centers9. Particularly in the formation of chlorine-containing species, two intermediates might appear according to their chlorine sources (Fig. 5f): the local modeling diagram for *O → *OCl pathway and the lower part of * → *Cl pathway36,37. As showing in Fig. 5e, the energy for *O → *OCl is 0.36 eV, while * → *Cl is −0.59 eV. However, the required energy for dissociation to Cl2 is −0.36 eV for *OCl → *O + Cl2 while 0.59 eV for *Cl → * + Cl2. Consequently, the Ti-MOF prefers the *OCl reaction pathway (*O → *OCl → *O + Cl2) with an energy barrier of 0.36 eV than the *Cl pathway of 0.59 eV (* → *Cl → * + Cl2) under the same computation condition.

To understand the above results from an electronic perspective, further theoretical calculations have been performed for the density of states (DOS, Fig. 5g and Supplementary Figs. 66−67) and differential charge density (Supplementary Figs. 68−71) for Ti-MOF adsorbed by *OCl and *Cl intermediates. In the DOS profiles in Fig. 5g, Ti-MOF-*OCl has demonstrated higher and sharper peak intensity near the Fermi energy level as comparison to Ti-MOF-*Cl, which reflecting the stronger adsorption of *OCl on the Ti-MOF surface than *Cl counterpart36. This result is consistent with the larger overlap in the binding states of 0.85~3.59 eV between O p orbital and Cl p orbitals for Ti-MOF-*OCl than *Cl counterpart, which explains the large energy barrier for the formation of *OCl as comparison to *Cl (0.36 vs. −0.59 eV)9,38. On the other hand, the subsequent intermediate step of dissociation to Cl2 has been examined by the three-dimensional schematic illustration of charge density electron/proton transfer occurs between Ti-MOF-*OCl in comparison to that of the *Cl counterpart36. Quantitative analyses shows the charge transfer of 0.2 eV for Cl→O inside Ti-MOF-*OCl while 0.48 eV for Cl→Ti inside Ti-MOF-*Cl. As a consequence, it requires small energy to break Cl-O binding of Ti-MOF-*OCl to generate Cl2 as comparison to Cl-Ti of Ti-MOF-*Cl (−0.36 vs. 0.59 eV) (Phonon spectrum information showed in Supplementary Tables 12−13).

The strong-resistant property of Ti-MOF for pH-universal CER has been explained (Fig. 5h), whose surface forming energy (SFE) representing the durability in different applied potentials and pH values. Without applied potentials (or negative SPF), the Ti-MOF material situates at the dark blue area characteristic of good stability in the pH range from 1 to 13. Next, the Ti-MOF has been polarized positively, which generate *OH at the local environment owing to competitive oxygen evolution (OER). The Ti-MOF can still maintain its structure in pH = 1~13 (Please see the blue area). Further, the deprotonation of *OH to *O occurs at 1.8−2.4 V (vs. Ag/AgCl) that initialize the *OCl reaction pathway (*O → *OCl → *O + Cl2), and the Ti-MOF can still maintain its durability in all pH ranges (as shown in the pompadour area)39. Ti-MOF catalyst prefers to promote CER to side OER, as evidenced by its low energy barrier of 0.36 as comparison to 1.14 eV for OER under the same condition (Fig. 5i, Supplementary Figs. 72, 73−78, intermediate model and properties and Phonon spectrum information showed in Supplementary Tables 14−16)40.

Finally, we have demonstrated the generality of our configuration for water splitting (Supplementary Fig. 79a). In a similar electrolysis design, the system produces H2 in 1 M KOH electrolyte, which is taken away by carrier gas (i.e., CO2) from GDEs according to Bernoulli’s principle. The gas chromatography data shows the characteristic peak of H2 product during the test (Supplementary Fig. 79b). The H2 Faradaic efficiency increases with flow rates and pressure difference (Supplementary Fig. 79c), and comparable for both membrane-based and -free configurations at different potentials (Supplementary Fi. 79d).

Methods
Materials synthesis
Synthesis of pH-tolerant Ti-MOF catalyst. 2,6 naphthoic acid (5 g) and sodium hydroxide (1.85 g) were mixed in a water/ethanol solution (1/1 vol) and dried overnight. Next, a portion of above sample (2.916 g) was dispersed in a mixed solution of N,N-Dimethylformamide (28 mL) and methanol (3 mL), where tetraisopropyl titanate (1.153 g) was added with vigorous stirring, and heated at 150 °C for 24 hrs. The as-obtained product was finally calcined at 450 oC for 3 hrs at a heating rate of 5 oC min−1 under argon atmosphere.

Materials characterization
Transmission electron microscopy (TEM) was conducted on a JEOL JEM-F200 TEM. X-ray diffraction (XRD) was conducted on a SmartLab SE with Cu Kα radiation operating at 40 kV and 30 mA. X-ray photoelectron spectroscopy (XPS) was collected on a Thermo Scientific ESCALAB 250Xi with Al Kα X-ray source. Fourier transform infrared (FT-IR) was conducted on a Nicolet iS20 FT-IR spectrometer. Ti K-edge X-ray absorption near edge structure (XANES) and extend X-ray absorption fine structure (EXAFS) experiments was conducted at Shanghai Synchrotron Radiation Facility. Gas was detected in the PANNA A91 Plus gas chromatography.

Electrochemical analyses
Electrode fabrication. The dimensionally stable anode (DSA) was purchased commercially and used without further treatment (electrode area: 1×1 cm2). The Ti-MOF electrode was prepared by a spray-coating procedure: firstly, the catalyst ink was prepared by mixing Ti-MOF (5 mg), acetylene black (1 mg) and nafion (50 μL) in isopropyl alcohol (950 μL) with mild ultrasonication. Next, the ink (300 μL) was filled into a pipette gun and sprayed onto a gas diffusion electrode (1×1 cm2) with the catalyst loading of 1.5 mg cm−2. More details of materials, electrodes and reactor unit are shown in Supplementary Figs. 30a–d.

Electrochemical testing setup. As shown in Supplementary Fig. 30e, our test was conducted in a flow-type reaction device. The anode and cathode have faced each other, and the reference electrode is placed between the anode and cathode. The middle chamber is connected by the electrolyte, while the rightmost chamber allows the carrier gas to pass through. Both the electrolyte and the carrier gas enter through the lower liquid (gas) inlet and exit through the upper liquid (gas) outlet. All the tests were conducted in an air environment, and the room temperature was maintained at around 20 oC except for the heating test.

We have evaluated the performances of the prototype device with a fixed NaCl electrolyte rate (8.4 mL min−1) and tunable gas flow rates (Supplementary Video 2). The reaction chamber can hold an electrolyte volume of 1×2×2 cm3. Since DSA is manufactured by coating an electrocatalytic metal oxide layer (such as RuO2, IrO2) on a titanium substrate, its structure is a solid titanium plate, so it is impossible to apply pressure difference treatment to the back of the electrode. Therefore, the effective components (RuO2 and TiO2) were used as a surrogate for DSA. In details, the RuO2 and TiO2 mixture with a mass ratio of 1:1 was prepared as ink and sprayed onto GDE under the same mass loading.

Electrochemical testing. All measurements were conducted with CHI660 Electrochemical Analyzer in a flow-type cell using Ti-MOF as the working electrode, iridium-plated titanium oxide as the counter electrode, Ag/AgCl as the reference electrode in 5.0 M NaCl aqueous electrolytes (with pH adjusted by HCl or NaOH). Place the electrode to be calibrated and the standard reference electrode in a saturated potassium chloride solution, and record the open-circuit potential (OCP). Ensure that the potential remains within the ±5 mV range. At this point, the reference electrode can be used. Due to the fact that the pH value of the electrolyte fluctuates with temperature, according to the test, the range of the pH value change of the electrolyte is less than ±0.5. All electrolytes should be placed in a well-ventilated and safe area, and the electrolyte should be used immediately after it is prepared. The polarization curve was collected using a potentiostatic method with a potential range from 1.8 V to 2.4 V (vs. Ag/AgCl) at a 100 mV s−1 scan rate. All CV, LSV curves were corrected with iR compensation (90%). All compensated potentials are marked, and the uncompensated potentials are written normally. The impedance test was conducted using parameters of 105 for high frequency and 0.01 for low frequency.

Electrochemical IR correct compensation and heating testing. All measurements were conducted with CHI660 Electrochemical Analyzer connecting the high current amplifier in a flow-type cell using Ti-MOF as the working electrode, iridium-plated titanium oxide as the counter electrode, Ag/AgCl as the reference electrode in 0.1 M NaOH + 5.0 M NaCl aqueous electrolytes. The IR correct compensation is performed by manually entering the ohmic value of the compensation (1 Ω) into the electrochemical workstation software. The heating experiment is to heat the electrolyte in a water bath and then pass it into an electrolytic cell.

Prototype device fabrication
The general structure of the membrane-free device is similar to a conventional liquid flow electrolyzer. However, the membrane-free device has made some adjustments in some details. As shown in Supplementary Fig. 42, after the removal of the proton membrane, there is no distinction between the anode chamber and the cathode chamber. However, due to the special structure of the flow cell, the chamber where the anode and cathode are located is referred to as the anode region and the cathode region. The electrolyte flows into the entire chamber from the bottom of the anode region at a rate of 8.4 mL min−1. The direction of the liquid flow is mainly towards the anode region for filling, but also towards the cathode region (the multiple black arrows in the Supplementary Fig. 43). Finally, the electrolyte flows out from the upper part of the cathode region. The purpose of this is to create a pressure potential towards the cathode region for the hydrogen generated in the chamber, which can reduce the hydrogen diffusion in the flow cell to some extent. In order to more effectively reduce the movement of hydrogen in the entire electrolyzer, we attached a tape to the upper part of the gasket window, covering an area of about 0.5 cm−2 (Supplementary Fig. 43a). Supplementary Fig. 43b uses a simplified schematic to explain the mechanism of covering this area. When H2 forms at the Anode, it will float towards the upper part of the chamber due to its lower density. Most of the H2 will be discharged with the liquid flow. However, there will be some H2 that cannot be discharged in time, causing it to agitate in the chamber, which increases the risk of explosion due to contact with the generated Cl2 and disturbing the electrode surface material. By making the above modifications, the membrane-free device has been assembled. Notably, to prevent electrolyte penetration into gas chamber, the counter electrode was engineered with the specialized design shown in Supplementary Fig. 80. The feasibility of this approach has been demonstrated through contact angle measurements and PTFE waterproof testing (Supplementary Fig. 81).

The determination of Cl2 product
Cl2 is determined by the traditional KI titration method, which reacts with Cl2 to change the colorless KI to brown I2. Next, the above mixture was mixed with starch solution (1 mL, 1 wt%), and titrated with 0.1 M Na2S2O3 solution. The end point of titration is determined by the observation of the blue-black color disappearing after the last drop of 0.1 M Na2S2O3 into the absorption solution with no longer color change within half a minute. The relevant reaction equations are listed as follows:

Cl2 + 2KI = I2 + 2KCl (5)

I2 + 2Na2S2O3 = 2NaI + Na2S4O6 (6)

The Cl2 yield (mol h−1 cm−2) and Faradaic efficiency (FE) for the Cl2 production were calculated according to the equations:

Yield = 0.1 V(Na2S2O3) / 2t (7)

FE (%) = 0.1 F V(Na2S2O3) / It (8)

Where V(Na2S2O3) is the Na2S2O3 consumed by titration of I2 in the total absorption solution (L), t is reaction duration, F is the Faraday constant (96485 C mol−1) and I is the applied steady current.

To ensure greater accuracy of the results, in addition to the KI titration method, the FAS-DPD titration method and the amperometric titration method were also employed. The specific methods are described in the Experimental method section of the Supplementary information. Moreover, the test results are shown in (Supplementary Fig. 82), indicating that the results obtained using the KI titration method in this study are reliable over a wider range. Therefore, in the lower part of Supplementary Fig. 82, we analyzed the error of Faraday efficiency. And in other parts, the data were selected to be repeatedly tested, and the more average values were presented.

Standardization of Na2S2O3
The accurate concentration of Na2S2O3 was calibrated using the potassium dichromate method:
Weigh a certain mass (accurate weighing) of dried K2Cr2O7, and dissolve it in a specific volume of deionized water.
Add a KI solution acidified with dilute sulfuric acid, shake well, then place the mixture in a sealed dark environment and let it stand for 3−5 min.
Titrate with the Na2S2O3 solution to be calibrated until the solution turns pale yellow, then add 2 mL of starch indicator (the solution will turn dark blue at this point). Continue titrating until the blue color of the solution fades and does not revert within 30 seconds-this is the endpoint. Record the volume of Na2S2O3 solution consumed. Repeat the operation 3 times in parallel and take the average concentration.
The accurate concentration of the Na2S2O3 solution can be calculated from the titration results:

c(Na2S2O3) = 6 × m(K2Cr2O7) × 1000 / M(K2Cr2O7) × V(Na2S2O3) (9)

Among them, 6 represents the reaction molar ratio, m(K2Cr2O7) is the mass of K2Cr2O7 weighed, M(K2Cr2O7) = 294.18 g mol−1, and V(Na2S2O3) is the volume of the Na2S2O3 solution used for the titration.

Calibration of gas chromatography
Five mixed standard gases containing different concentrations of CH4, C2H6, C2H4, C3H8, and CO (however, these gases are not involved in this work, so no analysis of carbon-containing gases will be conducted), as well as H2, O2, and N2 were used to calibrate the relationship between concentration and gas phase signal.

In the most recent calibration, we used standard gases with H2 contents of 1.98, 0.0044, 5.9167, 3.843, and 0.02 mol mol−1. The peak areas measured by the instrument were 1078.87, 2.38, 3223.48, 2105.32, and 12.23 μV*s, respectively. From these data, the relationship between H2 concentration and peak area in the gas phase was calculated as: c = 0.00183*s − 0.0024. When conducting the H2 measurement in the gas bag, since H2 is lighter, it is necessary to thoroughly shake the gas bag and maintain an intake time of more than 30 s. This will yield more accurate data.

Since we have conducted a quantitative analysis of H2, the calculation formula for the Faraday efficiency of H2 is as follows:

FE = Qg / Qtotal × 100% = nzF / Qtotal × 100% = (0.0018s − 0.0024) × v carriergas × T × zF / Qtotal × Vm (10)

In the formula: s represents the measured area of H2 in the gas phase (μVs), v carriergas is the gas flow rate during the reaction process (mL min−1), T is the reaction time (s), z is the number of transferred electrons, F is the Faraday constant, F = 96485 C mol−1, Vm is the molar volume of the gas, which is 24.5 L mol−1 at normal temperature and pressure.

Density function theory (DFT) computation methods
Structure optimization. First-principles calculations were performed by using MedeA-VASP based plane wave set approach41,42. The electron-ion interactions were described by the density function method, and the electron-electron exchange correlations were described by the Perdew-Burke-Ernzerhof (PBE) functional43,44. The Ti-MOF model framework has been built on the basis of experiment results and literature, including: XRD patterns (Fig. 2b)28, FTIR (Fig. 2c)27 and X-ray absorption spectroscopy (XAS, Fig. 2d, e and Supplementary Tables 1−2).

For structure optimization, the RMM-DIIS update algorithm was used. We used low precision for efficient computation and normal precision for accurate computation. The convergence is 0.06 and 0.05 for efficiency and accuracy computation. Both efficiency and accuracy computation use 400 eV plane wave cutoff. A Gaussian smearing with a width of 0.05 eV was also utilized. For the surface Brillouin zone integration, a 1×1×1 Monkhorst−Pack k point mesh was used. The convergence criteria for electronic self-consistent iteration and ionic relaxation were set to 10−4 eV and 10−5 eV. Hubbard-U correction method (LSDA+U) was carried out to improve the description of highly correlated Ti with the value of U = 2.5 eV set to standard LDA or GGA.

Computational details
Based on Bernoulli’s principle for fluid dynamics, the local pressure of gas flowing through a conduit decreases as its flow velocity increases45. Consequently, the gas pressure within the backside region of the electrode in the gas chamber diminishes with elevated flow rates. This hydrodynamic relationship is further operationalized in our simulations through Darcy law:

u = k / μ ∇p (11)

which provides a concrete framework for modeling such systems. Given that the gas-liquid flow within the chambers can be regarded as laminar, the Darcy law offers an appropriate theoretical foundation for these calculations.

Theoretically, Bernoulli principle a basic rule in fluid dynamics illustrating the relationship between the surface velocity and pressure. In the present work, two fluids have presented at the three-phase boundary, carrier gas flow and electrolyte flow, and according to Bernoulli principle that can be illustrated as:

P1 + 1/2 ρ1 V1^2 + ρ1 gh1 = P2 + 1/2 ρ2 V2^2 + ρ2 gh2 (12)

where 1 represents carrier gas flow and 2 represents electrolyte flow.
So, the interfacial pressure difference (∇p) can be illustrated as:

∇p = 1/2 ρ1 V1^2 + ρ1 gh1 − 1/2 ρ2 V2^2 − ρ2 gh2 (13)

Quantitively, the pressure difference has been calculated by software (Fig. 5c, d, Supplementary Video 1), showing the pressure difference in the range of 0~116.3×10−3 Pa with the carrier gas flow of 0~80 mL min−1 and electrolyte flow of 8.4 mL min−1. Therefore, when high-velocity carrier gas flow and low electrolyte flow have been applied to the three-phase boundary, it will generate a large localized pressure gradient. This can promote Cl2 bubbles migration away from electrolyte flow, and into carrier gas (Please see Supplementary Video 2).

Model description
Complete model with anode gas chamber, gas diffusion layer, catalytic layer and electrolyte chamber is setup at the steady state with two-dimensional (2D) finite element approaches. Free software (like OpenFOAM, FreeCAD and SALOME) is applied to solve the finite element analysis and simulation problem. In order to analyze the influence of anode carrier gas flow rate on the overall pressure and gas flow state of the system, which is shown in Supplementary Fig. 60, following sections, including flow equations and mass transfer in porous medium are applied.

Mass Transport in the porous media
Darcy law and mass transfer equations are used to describe the gas transport in CDL and CCL:

u = k / μ ∇p (23)

k = dp^2 ε^3 / 180(1−ε)^2 (24)

Where J is the flux of carried gas (kg/(m^2*s)), ρ is the density (kg/(m^3)), μ is Dynamic viscosity (Pa*s), Deff is the effective diffusion coefficient (m2/s), к is the permeability (m2) of the medium, dp is particle diameter (m) and ε is the porosity of porous medium.

Mass Transport in the anode gas chamber and anode electrolyte chamber
The CO2 in the gas chamber and the liquid in the electrolyte chamber can be regarded as laminar flow, so it can be described by the following equation:

ρ(μ∇)μ = ∇(−pI + K) − (μκ−1)μ + F (25)

ρ∇μ = 0 (26)

K = μ(∇u + (∇u)T) − 2/3 μ(∇u)I (27)

Where u is the speed (m/s) of gas or liquid.

Operando Raman spectroscopic characterizations
Operando Raman was conducted on CHI1140C electrochemical workstation equipped with a HORIBA LabRAM HR Evolution Raman spectrometer with 532 nm solid laser as an excitation source. A self-made flow-type cell with a quartz window was used to detect the Raman signal from the catalyst electrode (prepared by loading catalysts onto GDE). Pt foil and Ag/AgCl electrode were used as the counter and reference electrodes, respectively. The concentration of NaCl electrolyte was reduced to 1 M to reduce the interference of Cl− on the Raman signal.

Data availability
All relevant data that support the findings of this study are presented in the manuscript and supplementary information file. The experimental datasets associated with this work are provided in the form of Source Data files. Source data are provided in this paper.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "This manuscript by Nie et al. present a breakthrough in chlorine (Cl2) electrosynthesis by integrating Bernoulli’s principle with a pH-tolerant Ti-MOF catalyst. The novel triple-phase gas diffusion electrode design enables oriented Cl2 migration via pressure differences, eliminating membrane requirements and preventing H2/Cl2 crossover. The manuscript is well-organized, clearly presented, and the results are convincing. However, there are still some questions need to be illustrated clearly. Also, the conclusion needs to be further revised under solid evidence. Below are specific critiques to address these issues.\n1. The abstract states Faradaic efficiencies of 96.3%~87.6% within 0.1~1.14 A cm-2, yet fails to clarity why the efficiency decreases with increasing current density.\n2. The working potential of CER is 1.8 V-2.4 V in this manuscript, which is relatively high compared to other literatures. Is this system really energy-efficient?\n3. While Bernoulli's principle explains Cl2 migration via carrier gas, the separation mechanism for Cl2 and carrier gas is unaddressed. Does the mixture of carrier gas with Cl2 cause Cl2 to be impure?\n4. Grammar mistake: please revise recurrent misuse of “despite of” to “in spite of” or “despite”.\n5. Please correct the anodic CER equation in introduction: The electron transfer number should be 2 not 1.\n6. Replace “mPa” into “MPa” throughout the manuscript.\n7. Grammar mistake in introduction: subject-verb agreement error of “circumvents”→ “circumvent”.\n8. In figure 2a, lattice spacing of 0.2 nm is not clear, which should be redone to better prove the (210) crystal face.\n9. Why in Figure 2e there exist Ti-Ti and Ti-O, but in Figure 2f authors only mention Ti-O?\n10. Why the coordination number of Ti-O in Ti-MOF is 3.183, but authors speculate Ti node is octahedrally coordinated? What’s the relationship between the two?\n11. In Figure 3b, gas chromatography doesn’t detect H2. Where does the H2 go? Is H2 be collected?\n12. The plateau in Cl2 Faradic efficiency beyond 40 mL min-1 gas flow is not explained mechanistically in Figure 3c. Please clarify whether this stems from gas-liquid equilibrium constraints or kinetic bottlenecks.\n13. In Figure 3d-e, the control experiment of DSA-membrane should be added.\n14. In Fig. 3g, the “dissolved Cl2 species” (brown) color scheme is inconsistent with the legend. Please replot color.\n15. The techno-economic analysis assumes industrial operation at 70°C. Justify this temperature is along with actual chlor-alkali industry standards with literature citations.\n16. Why Cl2 is counted as “cost” in Figure 4c?\n17. In technical-economic evaluations section, the profits difference is described in Figures 4 g-h not Figures 5 g-h. The same goes for payback duration profiles.\n18. In Figure 5a, there seems no change in *OCl peak at all. Please reexamine experimental conditions.\n19. Fig. 5h exhibits mismatched sector colors between graphical representation and textual description.\n20. The authors utilized Bernoulli's principle to regulate gas and liquid flow rates, forming a pressure difference of 116.3 mPa to promote Cl2 outflow. However, it remains unclear whether this pressure difference remained stable during the reaction process. Furthermore, complex factors inherent to electrochemical reactions could significantly alter the interfacial pressure, such as gas product evolution and interfacial mass transfer. Critically, the authors determined this value solely through calculation using the Bernoulli equation, without providing experimental evidence to substantiate that this minute pressure difference was the key factor enabling efficient Cl2 synthesis.\n21. The authors proposed the *OCl pathway in the CER process, where the Cl- adsorbs on the Ti-O site. However, the outermost electrons shell of Cl- is already full, and it’s extremely difficult and unreasonable for Cl- to adsorb onto the Ti-O site. Please provide a reasonable explanation.\n22. In Figure 5a, the intensity of the Raman characteristic peaks corresponding to Ti-*OCl didn’t exhibit an obvious dependence on applied potentials. In addition, no significant correlation can be identified between the peak intensity in Figure 5a and the intensity analysis diagram in Figure 5b. Please optimize the experimental conditions to obtain more conclusive results.\n23. The experimental data and supplementary videos could only confirm the absence of H2 in the flowing gas, and cannot determine whether H2 and Cl2 are mixed within the reactor. Please design experiments to prove the successful separation of the generated H2 and Cl2.\n24. Please provide the stability experiments at the current density of 1.14 A cm-2.", "reviewer_2": "This study presents an innovative membrane-free electrochemical system for chlorine (Cl₂) production, leveraging Bernoulli’s principle to drive Cl₂ migration via a triple-phase gas diffusion layer. The work demonstrates impressive Faradaic efficiencies (up to 96.3%) and stability, with potential cost savings over conventional chlor-alkali processes. While the concept is compelling and the results are promising, several key issues require clarification to strengthen the manuscript’s rigor and impact.\n(1)Given the harsh electrochemical conditions (high current densities, extreme pH), surface reconstruction or phase changes of the MOFs are inevitable. The authors should address this critical aspect more thoroughly.\n(2)If the Ti-MOF catalyst undergoes surface reconstruction under operational conditions, the initial structure used for calculations may not reflect the true active phase.\n(3)The authors should include the k-space of the XAFS data in the supplementary information. In addition, an R-space range of 1 to 6 Å is sufficient to capture the main coordination shells of the materials. The EXAFS fitting results in Supplementary Table 1 should include standard deviations or confidence intervals for each fitted parameter.\n(4)While wavelet transform analysis can provide additional insights into the EXAFS data, its utility in this context is limited. The structural information obtained from WT may not be highly accurate. Given this, the authors should either provide a more detailed justification for including WT analysis or consider omitting it if it does not significantly contribute to the understanding of the catalyst's structure.\n(5)The authors should include photographs of the internal construction of the prototype device. In addition, a critical aspect that remains unclear is how this membrane-free device ensures water does not leak into the adjacent gas chamber.", "reviewer_3": "The authors have reported a chlorine evolution system based on Bernoulli principle, which promotes oriented migration of Cl2 away from reaction interfaces. This has not only prevented H2/Cl2 crossover but also circumvented the Pourbaix diagram problems, leading to significantly enhanced activities. By further joining with a pH-tolerant catalyst, a membrane-free prototype device has been built for high-rate Cl2 production. Further, the strategy has been demonstrated for solving key challenging problems in other chemical/catalytic reactions like water splitting (2H2O → 2H2 + O2). Overall, the strategy is interesting, and the manuscript is well organized. It can be published in Nature Communications after addressing the following concerns.\n1. In Figure 3c, the authors have tested Cl2 evolution in membrane-free prototype device using CO2 and air carrier gases. However, only the Faradic efficiency of air carrier has been provided. Please provide the relevant data for CO2 carrier gas.\n2. The author discussed the activities of Cl2 electrosynthesis on gas diffusion electrode-supported catalysts. However, the performances of gas diffusion electrode substrate have not been provided.\n3. In terms of mechanism (Figures 5), why did the authors choose *Cl and *OCl as reaction intermediates? What is the reason of choosing these two intermediates for DFT calculates?\n4. Some experimental details need to be provided, such as the optical picture of the synthetic and electrochemical testing processes.\n5. For DFT calculations (Figures 5f), the author should provide more details on building the slab models. Why they use the cluster model for theoretical calculations?\n6. The authors have claimed Cl2 migration to gas chamber by pressure difference according to Bernoulli's principle. However, the description is too simple. More in-depth discussion should be given, both from theory and experiments prospect.\n7. Some typos and errors have been noted, particularly in the Reference section and the fourth paragraph of Introduction section. A careful recheck of the whole manuscript is recommended.", "reviewer_4": "In this manuscript, the authors reported a membrane-free electrochemical system for Cl2 synthesis with a titanium-based metal-organic framework (Ti-MOF) anode. Under the guidance of Bernoulli's principle, Cl2 could permeate through the gas diffusion electrode and be directly separated from the anode by the carrier gas on the other side during the reaction. Although the Cl2 separation strategy is of significance, the manuscript lacks sufficient evidences to support the conclusions. Meanwhile, the comparison of the activity data was not scientific. Therefore, this manuscript is not recommended for publication in Nature Communications.\nSpecific comments are as follows.\n1. The author's identification of the signal at 1.92 Å as the Ti-O scattering path was unconvincing without reference samples with Ti-O and Ti-C scattering pathways (such as TiO2, TiC2, etc.). In addition, the authors are required to clarify why the Ti-Ti coordination in Ti-MOF (2.89 Å) was significantly smaller than that in the reference Ti foil (3.11 Å).\n2. Line 90, the authors mentioned the well-defined crystallinity of Ti-MOF nanosheet, which was inconsistent with an amorphous structure revealed by the TEM image in Figure 2a. Moreover, the authors are suggested to index the sharp diffraction peaks (25-30 degree) in the XRD pattern (Figure 2b).\n3. The FTIR identification of organic ligands in Figure 2c should be confirmed by reference samples.\n4. Line 108-113, Page 6, the final structure of Ti-MOF did not align with the provided evidence. The coordination number of Ti-O was 3.183, inconsistent with the metal-oxygen-layers of a hexacoordinated TiO6 units.\n5. The authors were required to provide the complete calculation process of the pressure difference at the triple-phase interface in the main text, as well as the revised triple-phase Bernoulli formula.\n6. The applied potentials for all the activity experiments in the three-electrode system were referenced against the Ag/AgCl and had not been converted to the potentials relative to the more commonly used reversible hydrogen electrode (RHE). Therefore, it was unreasonable to investigate the effect of pH variation on activity at the same potential (vs. Ag/AgCl), as the potential (vs. RHE) would change with pH under these conditions.\n7. The authors compared the Cl2 yield rate of Ti-MOF with that of the most recently reported catalysts. However, when achieving nearly similar Cl2 generation rates, Ti-MOF required a higher current density and exhibited a lower Faraday efficiency, which did not meet the claimed “record activities”. Note that the overpotential at 10 mA cm-2 or 100 mA cm-2 is a common descriptor for chlorine evolution activity. The authors should provide the activity data of the electrocatalysts in the text and conduct a systematic comparison with the state-of-the-art materials, especially under different pH conditions.\n8. The authors claimed to observe Raman signals of Ti-*OCl in the operando Raman spectra, but the signal-to-noise ratio of the provided Raman spectra is too low to substantiate their conclusions.\n9. The model constructed in DFT calculations demonstrated the atomically dispersed Ti atoms. The authors need to further clarify whether Ti-MOF was a single-atom catalyst, as there was no relevant evidence or description in the main text. Furthermore, the author constructed a cluster model terminated by sodium ions without considering periodic boundary conditions, which seems to be an unreasonable approach, as both the XRD and HRTEM results demonstrated the periodic crystalline structure of Ti-MOF.\n10. It was meaningless for the authors to merely compare whether the presence of a membrane affected the production costs of NaOH and Cl2 with Ti-MOF as the anode. The authors should compare the costs of the products in a membrane-free system with Ti-MOF as the anode versus a traditional membrane system with DSA as the anode.\n11. Is pH-universal Cl2 electrosynthesis attributed to the use of Ti-MOF or the strategy of chlorine gas separation?\n12. It is still quite confusing whether the pH tolerance is brought by the characteristic of Ti-MOF or the pressure difference (namely the three-phase set up). Obviously, the DSA exhibits very poor CER performance at pH=7 and 13 in a traditional three-electrode set up. However, when testing the CER activity of Ti-MOF, the authors tuned the pressure difference which is not performed for the scenario of DSA. The authors should provide the experimental conditions clearly in the Method Section/Figure Illustration. Furthermore, if Ti-MOF is the main contributor for the pH tolerance, what property brings this benefit?\n13. The authors are suggested to provide solid data source during technical-economic evaluations. Important parameters include the price of electricity, NaCl, water, membrane, etc. As the American manufacturing industry is mentioned (Page 7, Supporting Information), we hypothesize this Cl2 production factory to be set up in the U.S. Based on the data on Global-Petrol-Prices., the industrial electricity price in the U.S. is estimated at $0.15/kWh in 2024, much higher than the $0.03/kWh used in the authors' estimates.\n14. The oxygen evolution reaction (OER) chemical equation (in Line 43, Page 3) was incorrect. Generally, OER involves a four-electron transfer rather than a six-electron transfer.\n15. The term \"Cl2 yield rates\" in Line 128-130, Page 7 was incorrect and should be replaced with \"the Cl2 Faradaic efficiencies\".\n16. There are many incorrect expressions and figure numbers in the main text. Such as “...the yield rate of Ti-MOF is 2.6 times the one of DSA...” in Line 161, Page 8, which should be change to “...the Cl2 yield rate of Ti-MOF is 2.6 times the one of DSA...”.", "reviewer_5": "In this article, the authors designed and prepared an efficient yet cost-effective electrochemical system for Cl2 electrosynthesis. It modulates gas diffusion layer by Bernoulli's principle, wherein the pressure difference at triple-phase boundary drives oriented Cl2 migration directly into gas chamber, thus preventing the crossover of anodic/cathodic products and reaching a pH-tolerant CER. This article provides excellent and comprehensive guidance on design and development of electrocatalytic chlorine evolution devices and effectively reduces industrial costs. However, there are still many problems in terms of structural characterization and mechanism exploration. In a whole, this work cannot be accepted for becoming suitable for this journal.\nQ1: The author points out that “a Ti-MOF nanosheet reveals its well-defined crystallinity with the adjacent lattice spacing of 0.2 nm, which agrees well with XRD containing a series of characteristic peaks corresponding to metal-organic framework architectures with dominant (210) crystal face” (Line 90-92). However, in fact, the so-called lattice spacing were not observed in Figure. 2a, and XRD (Figure. 2b) can only correspond to the three peaks before 20°, but basically did not match peaks after 20°. Therefore, a Fourier-transform TEM image or SAED image should be exhibited to support this view. At the same time, XRD needed to be verified for correctness.\nQ2: The data from XAS in Figure. 2d-e and Supplementary Table 1 contradicts the author’s description of catalysts structure in the article. The author points out that “The Ti node is octahedrally coordinated” (Line 111-112). But the EXAFS-fitting indicated a CN of 3.183 for Ti-O (Line 103 in Supplementary Table 1). It’s necessary to clarify the error values towards each parameter in Supplementary Table 1 and explain this contradictory phenomenon.\nQ3: The author used operando Raman spectroscopy to verify the intermediates in the reaction process and observed potential-dependent *OCl signals, but in reality, no corresponding signal was observed at 1077 cm-1 in either Figure 5a or Supplementary Figure 41a-b.\nQ4: Could the authors clearly describe the correlation of Bernoulli principle in its reaction process? It seems that there is no connection between them, and the mechanism study part has not been well reflected."}, "author_response": "..."}, {"round": 2, "reviewer_comments": {"reviewer_1": "Most of my concerns have been well addressed in this revision. Still, there are two issues to be solved.\n1. The influence of pH on the solubility of Cl2 should be considered. An environment with low pH (acidic) favors the existence of Cl2 in its molecular form (where physical dissolution dominates), while a high pH (alkaline) environment promotes chemical reactions of Cl2, significantly increasing its apparent total solubility.\n2. Why is the difference between Ti-MOF-membrane and Ti-MOF-membrane-free in Figure 3d so negligible? The performance disparity between the membraned and membrane-free configurations appears marginal and requires further clarification.\nIn Fig. 3f, the colors corresponding to the four catalysts are indistinguishable. Please recreate this figure with clearer and more distinct visual differentiation.", "reviewer_2": "The authors have comprehensively addressed all of my previous concerns. The revisions are satisfactory, and the manuscript now presents a complete and valuable piece of work that meets the high standards of Nature Communications.", "reviewer_3": "The revised version can be accepted.", "reviewer_4": "The authors have addressed most of my concerns, but there are still some minor issues should be solved before publication.\n1. The authors attributed the lower coordination number of Ti-O (3.36) in Ti-MOF compared to that of the theoretical TiO6 units to the difference in contributions from O at various positions (i.e., internal O contributes 1, while surface O contributes 0.5, Figure R2). They should also consider another possibility that the lower coordination number may be related to the presence of abundant oxygen vacancies in the Ti-O framework. Considering the fact of the lower coordination number, the authors are suggested to revise the theoretical model to better reflect the actual situation.\n2. Were the performance tests of DSA and Ti-MOF in Figure 3d and e conducted in the same prototype device shown in Figure 3b? Was there a pressure difference in both cases? Why did DSA exhibit such poor chlorine production performance (with a Faraday efficiency for chlorine gas below 10%)?\n3. The authors should assign the vibrational peaks of H₂NDC at 1668 and 1415 cm⁻¹ and of 2,6 naphthoic 2Na at 786 cm⁻¹.\n4. The author needs to provide the formulas used for calculating the economic costs, rather than only presenting the final calculation results.\n5. Why the current densities used in the three tables, namely Supplementary Table 9-11: Ti-MOF-membrane -free for CER, Ti-MOF-membrane for CER, and DSA-membrane for CER, were different? Would the difference in current density affect the calculation of economic analysis?", "reviewer_5": "The data presentation in the manuscript has several shortcomings that need to be addressed before the results can be considered reliable:\n1) The description of Cl₂ measurement is not clear or sufficiently detailed. Critical information such as reagents details including companies, purity, and prepared concentration, full titration procedure, and standardization of the Na₂S₂O₃ solution is missing. Without these details, the reproducibility and reliability of the results cannot be assessed.\n2) The KI–thiosulfate titration method is known to introduce significant errors, especially due to interferences and endpoint subjectivity. The manuscript does not provide any indication of experimental errors, replicate measurements, or data variability. The use of the FAS-DPD titration method would have been more suitable for this study, as it is widely regarded as more accurate and specific for chlorine analysis.\n3) There are several well-established, high-accuracy techniques for chlorine quantification (e.g., amperometric titration, ion chromatography, or membrane-covered amperometric analyzers). The manuscript should validate the reported chlorine yields with at least one of these methods to strengthen confidence in the results.\n4) The description of GC analysis for H₂, O₂, and N₂ is incomplete. Essential details such as the instrument type, detector used, column specifications, calibration procedures, and standards are not reported. Furthermore, the manuscript does not include calculations or data for the molar amounts or yields of each gas, which are crucial for evaluating the performance of the system."}, "author_response": "..."}, {"round": 3, "reviewer_comments": {"reviewer_1": "All my concerns have been well addressed in this revision. Congratulations!", "reviewer_4": "The authors have addressed all my concerns. I recommend its publication at this version.", "reviewer_5": "The comments are addressed. Suggest to accept for publication"}, "author_response": ""}]

Decision letter