Droplet microfluidic electrochemical reactor for two-phase systems

Abstract

The application belongs to the technical field of electrochemical engineering, and discloses a droplet microfluidic electrochemical reactor for a two-phase system, wherein the surface of the flow channel plate of the working electrode chamber is engraved with a cross-shaped microchannel, four ports of the cross-shaped microchannel are respectively two continuous phase inlets, one dispersed phase inlet and one liquid outlet, the surface of the cross-shaped microchannel is covered with an inorganic hydrophilic film layer, the aqueous phase fluid is taken as the continuous phase and is injected from the two continuous phase inlets, and the organic phase fluid is taken as the dispersed phase and is injected from the dispersed phase inlet; due to the extremely hydrophilic state of the surface of the cross-shaped microchannel, the aqueous phase continuous phase is preferentially wetted and constructs a physically isolated aqueous phase liquid film, the organic dispersed phase is subjected to the symmetrical shearing of the aqueous phase continuous phase, an oil-in-water (O / W) discrete droplet flow type is generated, and therefore the contact area of the two phases can be increased at the microscale, the current path can be effectively shortened to reduce the ohmic loss, and the low-voltage stable operation of the system can be ensured.

Description

Technical Field

[0001] This invention belongs to the field of electrochemical engineering technology, specifically, it relates to a droplet microfluidic electrochemical reactor for two-phase systems. Background Technology

[0002] In recent years, with the rapid development of green chemistry and sustainable manufacturing, electrochemical synthesis technology, which utilizes renewable electricity to drive chemical production, has received widespread attention. Compared with traditional thermochemical reactions, electrochemical synthesis can be carried out at ambient temperature and pressure, and the reaction selectivity can be controlled by adjusting the potential, making it an important pathway to achieve the green upgrading of high-value-added chemicals. In practical applications, many key electrochemical reactions often involve immiscible liquid-liquid two-phase systems, such as highly conductive aqueous electrolytes reacting with nonpolar or weakly polar organic phases. How to achieve efficient and stable electrochemical conversion in two-phase systems is a significant challenge currently facing the chemical industry (Electrochemical Hydrogenation and Oxidation of Organic Species Involving Water. Nat. Rev. Chem. (2024)).

[0003] Currently, the commonly used equipment for two-phase electrochemical reactions in industry and laboratories mainly includes traditional batch reactors and conventional parallel-plate continuous flow electrochemical reactors. However, when dealing with immiscible liquid-liquid two-phase systems, these existing devices reveal significant technical bottlenecks. In traditional batch reactors, the mixing of the two phases mainly relies on mechanical stirring. This macroscopic emulsification not only produces uneven droplet size distribution but also results in a limited overall phase interface area, leading to significant mass transfer resistance for the reactants and severely limiting reaction rates and conversion rates. On the other hand, while conventional continuous flow reactors shorten the distance between the electrodes, when handling two-phase flows, the flow channel dimensions are often on the order of millimeters to centimeters. The fluid is easily stratified or forms uncontrollable slug flows due to gravity and surface tension, resulting in uneven wetting of the electrode surfaces. Furthermore, many organic phases have extremely low conductivity. In the case of two-phase stratification or uneven mixing, the current encounters extremely high resistance when passing through the organic phase. This not only significantly increases the ohmic voltage drop and operating energy consumption of the reactor but may also trigger local overheating and side reactions, significantly reducing Faraday efficiency. Although microchannel reactors can improve the aforementioned mass transfer problems, the throughput of a single channel is extremely low, and simply increasing the channel size will lose the original mass transfer advantages of the microscale, making it difficult to achieve both high mass transfer and high throughput. Summary of the Invention

[0004] To address the technical problems in existing two-phase electrochemical systems, where the intrinsic hydrophobicity of polymer microfluidic substrates easily leads to organic phase adhesion to the electrodes, resulting in phase-to-phase mass transfer blockage, severe electrode passivation, and a sharp increase in polarization resistance, this invention provides a droplet microfluidic electrochemical reactor for two-phase systems. By setting up cross-shaped microchannels and constructing an inorganic hydrophilic thin film layer on the surface of these microchannels, the system stably generates highly uniform oil-in-water (O / W) droplet flow patterns. This increases the contact area between the two phases at the microscale, while effectively shortening the current path to reduce ohmic losses and ensuring stable low-voltage operation of the system.

[0005] To solve the above-mentioned technical problems, the present invention is achieved through the following technical solution:

[0006] This invention provides a droplet microfluidic electrochemical reactor for a two-phase system, comprising a counter electrode chamber flow channel plate, a counter electrode, an ion exchange membrane, a working electrode, and a working electrode chamber flow channel plate; the surface of the working electrode chamber flow channel plate is provided with cross-shaped microchannels, the four ports of which are two continuous phase inlets, one dispersed phase inlet, and one outlet; wherein the two continuous phase inlets are arranged opposite to each other, and the dispersed phase inlet and the outlet are arranged opposite to each other; the surface of the cross-shaped microchannels is covered with an inorganic hydrophilic thin film layer;

[0007] The two continuous phase inlets are used to inject an aqueous fluid as the continuous phase, the dispersed phase inlet is used to inject an organic fluid as the dispersed phase, and the outlet is used to discharge the two-phase fluid. During the electrochemical reaction: due to the extremely hydrophilic state of the surface of the cross-shaped microchannel, the aqueous continuous phase preferentially wets the entire surface of the cross-shaped microchannel and the surface of the working electrode, and constructs a physically isolated aqueous liquid film; due to the cross-shaped structure of the cross-shaped microchannel, the organic dispersed phase is subjected to symmetrical shearing by the aqueous continuous phase, generating an oil-in-water (O / W) discrete droplet flow pattern to block the direct contact between the organic dispersed phase and the working electrode.

[0008] Furthermore, gaskets are respectively provided between the counter electrode chamber flow channel plate and the counter electrode, and between the working electrode and the working electrode chamber flow channel plate. The gaskets are made of PTFE material and are corrosion resistant.

[0009] Preferably, the branch between the continuous phase inlet and the cross intersection, and the branch between the dispersed phase inlet and the cross intersection, are all of the same length and shorter than the length of the branch between the liquid outlet and the cross intersection.

[0010] Preferably, in the cross-shaped microchannel: the branch between the continuous phase inlet and the cross intersection, and the branch between the dispersed phase inlet and the cross intersection, have a width of 0.2-1.0 mm; the branch between the liquid outlet and the cross intersection has a width of 0.2-1.6 mm.

[0011] Preferably, the depth of the cross-shaped microchannel is 0.2-1.6 mm.

[0012] Furthermore, the surface water contact angle of the cross-shaped microchannel after surface modification with an inorganic hydrophilic film layer is less than 50°.

[0013] Furthermore, both the counter electrode and the working electrode are made of conductive material with flat surfaces, and are led outward through metal current collectors to achieve external power supply.

[0014] Furthermore, both the counter electrode chamber flow channel plate and the working electrode chamber flow channel plate are made of highly transparent polymers, preferably at least one of polymethyl methacrylate or polycarbonate.

[0015] Preferably, the inorganic hydrophilic film layer is a transparent amorphous silicon dioxide film.

[0016] Preferably, the inorganic hydrophilic thin film layer is prepared by physical vapor deposition using magnetron sputtering.

[0017] More preferably, the preparation method using magnetron sputtering physical vapor deposition includes: placing the working electrode chamber flow channel plate having the cross-shaped microchannels in the magnetron sputtering deposition chamber, using 99.999% pure silicon dioxide as the sputtering target, and evacuating the base vacuum of the magnetron sputtering deposition chamber to 6 × 10⁻⁶. -4 Under conditions of Pa below 30-50 sccm, high-purity argon gas flow rate stable at 30-50 sccm, and working pressure maintained at 1.0-4.0 Pa, sputter continuously for 5-30 min with 50-200 W RF power.

[0018] The beneficial effects of this invention are:

[0019] (i) Based on the cross-shaped structure of the cross-shaped microchannel and the inorganic hydrophilic film layer on its surface, the present invention can force the aqueous continuous phase to preferentially spread and wet the entire cross-shaped microchannel and the surface of the working electrode. The organic dispersed phase is subjected to highly symmetrical hydrodynamic shearing action of the aqueous continuous phase in the cross-shaped microchannel, stably generating uniformly sized oil-in-water (O / W) discrete droplets. Thus, the aqueous continuous phase spread on the surface of the working electrode forms a physically isolated aqueous liquid film, blocking the direct contact and disordered adhesion between the insulating organic phase droplets and the electrochemical reaction interface, eliminating the resistance change and electrode passivation caused by the organic insulating layer, and ensuring the long-term stability of the reactor tank pressure.

[0020] Due to the cross-shaped structure of the cross-shaped microchannel, the organic dispersed phase is subjected to symmetrical shearing by the aqueous continuous phase, generating oil-in-water (O / W) discrete droplets to block direct contact between the organic dispersed phase and the working electrode.

[0021] (ii) This invention achieves the transformation of the surface of the cross-shaped microchannel from an intrinsically hydrophobic and oleophilic state to an extremely hydrophobic and oleophobic state, and the water contact angle of the surface can be sharply reduced from more than 90° in the untreated state to less than 50°. Therefore, while achieving the reversal of surface properties, the precise hydrodynamic dimensions and structure of the original microfluidic channel design are preserved; and the hydrophilic film has extremely strong substrate adhesion and can resist the mechanical shearing of two-phase fluids for a long time. Attached Figure Description

[0022] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0023] Figure 1 This is a schematic diagram of the overall structure of the droplet microfluidic electrochemical reactor provided by the present invention.

[0024] Figure 2 This is a plan view of the flow channel plate in the working electrode chamber of the droplet microfluidic electrochemical reactor provided by the present invention.

[0025] Figure 3 This is a schematic diagram illustrating the working principle of the radio frequency / direct current (RF / DC) magnetron sputtering system used in the embodiments of the present invention.

[0026] Figure 4 The images shown are Raman (a) and Fourier transform infrared (b) spectra of the inorganic SiO2 hydrophilic thin film layer deposited in this embodiment of the invention.

[0027] Figure 5This is a comparison of the water contact angles of the working electrode chamber flow channel plate substrate surface before (a) and after (b) hydrophilic modification in an embodiment of the present invention.

[0028] Figure 6 This is a schematic diagram of the operation of the droplet microfluidic electrochemical reactor in an embodiment of the present invention.

[0029] Figure 7 This is a comparison of optical photographs of the droplet flow state inside the microfluidic electrochemical reactor before (W / O flow pattern) and after (O / W flow pattern) in the embodiment of the present invention (blue ink was added to the aqueous phase to distinguish the two phases).

[0030] Figure 8 This is a comparative graph showing the effect of different flow patterns (unmodified oil phase adhesion and modified O / W droplet flow pattern) on the operating pressure stability of a droplet microfluidic electrochemical reactor under constant current drive (black line represents pure aqueous phase system; blue line represents oil-in-water droplet system; green line represents disordered water-oil stratification system).

[0031] The accompanying figure is labeled as follows:

[0032] 1: Counter electrode chamber flow channel plate; 2: Counter electrode; 3: Gasket; 4: Ion exchange membrane; 5: Working electrode; 6: Working electrode chamber flow channel plate; 7: Cross-shaped microchannel; 701: Dispersed phase inlet; 702: Continuous phase inlet; 703: Liquid outlet; 704: Cross-shaped intersection. Detailed Implementation

[0033] To further understand the content, features, and effects of this invention, the following embodiments are provided, and detailed descriptions are given below in conjunction with the accompanying drawings:

[0034] like Figure 1 As shown, a droplet microfluidic electrochemical reactor for a two-phase system includes a counter electrode chamber flow channel plate 1, a counter electrode 2, a gasket 3, an ion exchange membrane 4, a working electrode 5, and a working electrode chamber flow channel plate 6.

[0035] High-transparency polymethyl methacrylate (PMMA) was selected as the substrate for the counter electrode chamber flow channel plate 1 and the working electrode chamber flow channel plate 6. A conventional straight-line microchannel was engraved on the counter electrode chamber flow channel plate 1 using precision CNC machining; a cross-shaped microchannel 7 was engraved at the corresponding position on the working electrode chamber flow channel plate 6.

[0036] like Figure 2 As shown, the four ports of the cross-shaped microchannel 7 are a dispersed phase inlet 701, two continuous phase inlets 702, and a liquid outlet 703. The two continuous phase inlets 702 are arranged opposite each other, and the dispersed phase inlet 701 and the liquid outlet 703 are arranged opposite each other.

[0037] In the cross-shaped microchannel 7 of this embodiment, the branch length between a dispersed phase inlet 701 and the cross intersection 704, and the branch length between two continuous phase inlets 702 and the cross intersection 704 are the same and less than the branch length between an outlet 703 and the cross intersection 704.

[0038] In this embodiment, to ensure a more precise distribution of shear and flow focusing forces of the two-phase fluid in the cross-shaped microchannel 7, the branch width between the dispersed phase inlet 701 and the cross-shaped intersection 704 is set to 1.0 mm, the branch width between the two continuous phase inlets 702 and the cross-shaped intersection 704 is set to 1.0 mm, and the branch width between the outlet 703 and the cross-shaped intersection 704 is set to 1.6 mm. Furthermore, the uniform depth of the four branches of the cross-shaped microchannel 7 is controlled at 1.6 mm, and the dimensional tolerances of the width and depth are strictly controlled within the range of ±10 to 20 μm.

[0039] like Figure 3 As shown, the PMMA substrate with the working electrode chamber flow channel plate 6 engraved with cross-shaped microchannels 7 is ultrasonically cleaned and dried, and then placed in the radio frequency (RF) magnetron sputtering deposition chamber. Pure silica is used as the target material for magnetron sputtering physical vapor deposition. The specific process parameters are precisely controlled as follows: Before starting deposition, the background vacuum of the deposition chamber is evacuated to 6 × 10⁻⁶. -4 Below Pa; then high-purity argon (Ar) is introduced as the working discharge gas, with the flow rate stabilized at 30 sccm; the working gas pressure inside the deposition chamber is locked at 1 Pa; the output power of the RF DC power supply is set to 100 W; the continuous sputtering time is 20 min.

[0040] After the above magnetron sputtering treatment, a silicon dioxide thin film is formed on the surface of the cross-shaped microchannel 7, and its spectroscopic characterization yields... Figure 4 The Raman spectrum in (a) and the Fourier transform infrared spectrum in (b) are shown. Figure 4 As shown, the Raman spectrum at 490 cm⁻¹ -1 The typical symmetric stretching vibration peak of silicon-oxygen tetrahedra was clearly detected; the FTIR spectrum also showed the characteristic absorption band of Si-O-Si bonds.

[0041] like Figure 5 As shown, the droplet contact angle precision tester shows that after this sputtering treatment, the water contact angle of the surface of the cross-shaped microchannel 7 is sharply reduced from 95.6° (hydrophobic state) of the substrate polymer to 13.4° (extremely hydrophilic state), successfully reversing the hydrophilicity of the surface of the cross-shaped microchannel 7.

[0042] All components are assembled in the following order: counter electrode chamber flow channel plate 1, counter electrode 2, gasket 3, ion exchange membrane 4, gasket 3, working electrode, and hydrophilically treated working electrode chamber flow channel plate 6. During assembly, the electrode assembly sandwiched between the counter electrode chamber flow channel plate 1 and the working electrode chamber flow channel plate 6 uses flat-surfaced hydrophilic carbon paper as the counter electrode 2 and working electrode 5. This flatness ensures a tight fit between the working electrode 5 and the edge of the cross-shaped microchannel 7, avoiding microscopic leakage or localized flow field disturbances caused by surface undulations. To achieve low-impedance external potential connection, a connection scheme is adopted where copper tape is attached to the edges of the counter electrode 2 and working electrode 5.

[0043] To address the extremely high internal pressure drop caused by continuous two-phase flow in the microfluidic system, corrosion-resistant PTFE gaskets 3 with a thickness of 0.2 mm were embedded between the counter electrode chamber flow channel plate 1 and the counter electrode 2, and between the working electrode chamber flow channel plate 6 and the working electrode 5. An assembly torque of 1.2 N·m was uniformly applied using a torque wrench through a network of M5 mechanical fastening bolts distributed around the droplet microfluidic electrochemical reactor. Extreme fluid dynamics tests showed that this encapsulation system could withstand pressure drops up to 0.2 mL / min. -1 At a total injection flow rate, it can smoothly withstand an internal pressure drop of 0.4 MPa without any leakage.

[0044] like Figure 6 As shown, this embodiment verifies the key role of droplet microfluidic electrochemical reactor in reducing system polarization resistance and maintaining long-term stable operation through two-phase fluid dynamics and electrochemical coupling tests. First, fluid transport tests were conducted in the electrochemical reactor under ambient temperature and open-circuit (no electric field polarization) conditions using the untreated and hydrophilically treated working electrode chamber flow channel plate 6 respectively: the aqueous phase fluid (continuous phase, containing KOH electrolyte) was injected at an equal volumetric flow rate from the two continuous phase inlets 702 of the cross-shaped microchannel 7 by a dual-channel micro-injection pump; the organic phase fluid (dispersed phase, 2-MeTHF solvent) was injected at a flow rate controlled at 0.05-0.4 mL / min by a single-channel micro-injection pump from the dispersed phase inlet 701 of the cross-shaped microchannel 7.

[0045] like Figure 7As shown, the real-time monitoring comparison of the optical window of the working electrode chamber flow channel plate 6 under the above two states shows that in the intrinsic PMMA channel without inorganic hydrophilic film layer modification, the organic phase fluid spontaneously wets the hydrophobic wall of the PMMA substrate, and forms a water-in-oil (W / O) flow pattern after shearing in the cross-shaped region, resulting in the organic phase fluid directly adhering to the surface of the working electrode 5. In contrast, in the droplet microfluidic electrochemical reactor modified with the SiO2 inorganic hydrophilic film layer of this invention, the aqueous phase fluid preferentially spreads and completely wets the surface of the cross-shaped microchannel 7 and the working electrode 5, forming a continuous phase; the organic dispersed phase is subjected to hydrodynamic focusing and symmetrical shearing of the continuous aqueous phase on both sides in the cross-shaped region, stably generating water-in-oil (O / W) discrete droplets with uniform size and height.

[0046] Subsequently, 5-hydroxymethylfurfural (HMF) was added as a model molecule to the organic phase, and the mixture was subjected to a constant current (10 mA / cm²). 2 In-situ tank pressure monitoring is performed under the drive. For example... Figure 8 As shown, the operating tank pressure of the O / W droplet microfluidic electrochemical reactor of the present invention remains highly stable, and its electrochemical impedance performance is highly consistent with that of a system using only a single aqueous phase medium. Conversely, if no inorganic hydrophilic film layer modification is performed, i.e., the system transforms into a W / O flow pattern, the insulating passivation effect of the oil phase causes a sharp increase in polarization resistance, leading to a significant rise in the system tank pressure. This result demonstrates that in the O / W droplet microfluidic system, the continuous aqueous phase spread on the surface of the working electrode 5 constructs a stable physically isolating liquid film, effectively blocking the disordered adhesion and coverage of the electrochemically active interface by the insulating organic phase droplets. The above comparative experiments fully demonstrate the decisive role of the surface engineering and fluid control strategies proposed in this invention in ensuring the continuous and efficient operation of the droplet microfluidic electrochemical reactor.

[0047] Although the preferred embodiments of the present invention have been described above in conjunction with the accompanying drawings, the present invention is not limited to the specific embodiments described above. The specific embodiments described above are merely illustrative and not restrictive. Those skilled in the art can make many specific modifications under the guidance of the present invention without departing from the spirit of the invention and the scope of protection of the claims, and these modifications all fall within the scope of protection of the present invention.

Claims

1. A droplet microfluidic electrochemical reactor for a two-phase system comprising a counter electrode chamber flow channel plate, a counter electrode, an ion exchange membrane, a working electrode, and a working electrode chamber flow channel plate; characterized in that, The surface of the working electrode chamber flow channel plate is provided with a cross-shaped microchannel. The four ports of the cross-shaped microchannel are two continuous phase inlets, one dispersed phase inlet, and one liquid outlet. The two continuous phase inlets are arranged opposite to each other, and the dispersed phase inlet and the liquid outlet are arranged opposite to each other. The surface of the cross-shaped microchannel is covered with an inorganic hydrophilic film layer. The two continuous phase inlets are used to inject an aqueous fluid as the continuous phase, the dispersed phase inlet is used to inject an organic fluid as the dispersed phase, and the outlet is used to discharge the two-phase fluid. During the electrochemical reaction: due to the extremely hydrophilic state of the surface of the cross-shaped microchannel, the aqueous continuous phase preferentially wets the entire surface of the cross-shaped microchannel and the surface of the working electrode, and constructs a physically isolated aqueous liquid film; due to the cross-shaped structure of the cross-shaped microchannel, the organic dispersed phase is subjected to symmetrical shearing by the aqueous continuous phase, generating an oil-in-water (O / W) discrete droplet flow pattern to block the direct contact between the organic dispersed phase and the working electrode.

2. A droplet microfluidic electrochemical reactor for two-phase systems according to claim 1, characterized in that, Gaskets are provided between the counter electrode chamber flow channel plate and the counter electrode, and between the working electrode and the working electrode chamber flow channel plate. The gaskets are made of PTFE anti-corrosion gaskets.

3. The droplet microfluidic electrochemical reactor for two-phase systems according to claim 1, characterized in that, The branch between the continuous phase inlet and the crossroads, and the branch between the dispersed phase inlet and the crossroads, are all of the same length and shorter than the length of the branch between the outlet and the crossroads.

4. The droplet microfluidic electrochemical reactor for two-phase systems according to claim 1, characterized in that, In the cross-shaped microchannel: the branch between the continuous phase inlet and the cross intersection, and the branch between the dispersed phase inlet and the cross intersection, have a width of 0.2-1.0 mm; the branch between the liquid outlet and the cross intersection has a width of 0.2-1.6 mm; and the depth of the cross-shaped microchannel is 0.2-1.6 mm.

5. The droplet microfluidic electrochemical reactor for two-phase systems according to claim 1, characterized in that, The surface water contact angle of the cross-shaped microchannel after surface modification with an inorganic hydrophilic thin film layer is less than 50°.

6. The droplet microfluidic electrochemical reactor for two-phase systems according to claim 1, characterized in that, Both the counter electrode and the working electrode are made of conductive materials with flat surfaces, and are led outward through metal current collectors to achieve external power supply.

7. A droplet microfluidic electrochemical reactor for a two-phase system according to claim 1, characterized in that, Both the counter electrode chamber flow channel plate and the working electrode chamber flow channel plate are high-transparency polymers, and are made of at least one of polymethyl methacrylate or polycarbonate.

8. A droplet microfluidic electrochemical reactor for a two-phase system according to claim 1, characterized in that, The inorganic hydrophilic film layer is a transparent amorphous silicon dioxide film.

9. The droplet microfluidic electrochemical reactor for two-phase systems according to claim 1, characterized in that, The inorganic hydrophilic thin film layer was prepared by physical vapor deposition using magnetron sputtering.

10. A droplet microfluidic electrochemical reactor for two-phase systems according to claim 9, characterized in that, The physical vapor deposition method using magnetron sputtering includes: placing the working electrode chamber flow channel plate with the cross-shaped microchannels into the magnetron sputtering deposition chamber; using 99.999% pure silicon dioxide as the sputtering target; and evacuating the base vacuum of the magnetron sputtering deposition chamber to 6 × 10⁻⁶. -4 Under conditions of Pa below 30-50 sccm, high-purity argon gas flow rate stable at 30-50 sccm, and working pressure maintained at 1.0-4.0 Pa, sputter continuously for 5-30 min with 50-200 W RF power.

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