Flow Reactor for Sustainable Electrosynthesis Fabricated via Cost‐Effective Electroplating and Adhesive Bonding

Abstract

The use of precious metal electrodes, although capable of improving reaction efficiency, inevitably increases the fabrication cost of electrochemical flow reactors. Additionally, the commonly employed mechanical fastening method consistently poses a risk of leakage. Herein, a low‐cost method for fabricating leak‐proof electrochemical flow reactors that can withstand high internal pressures is reported using electroplating and chemical bonding methods. By selectively electroplating Pt only on the electrode portion where the electrochemical reaction occurs, the Pt usage on the electrode is reduced by up to 99.4% (selective electroplating of 5‐μm‐thick Pt = 0.077 g vs. 0.2‐mm‐thick sheet of Pt = 13.8 g). The reactor shows no liquid leakage, even at a high pressure of 320 psi, and is composed of materials with excellent chemical resistance; therefore, various organic solvents and compounds can be used without restriction. Additionally, the reactor performance is verified by confirming a high yield of up to 88% in the C—H/S—H cross‐coupling reaction. Compared with that of conventional mechanical fastening methods, the fabrication cost is reduced by 97.4%. In addition, this process enables easy fabrication of a reactor of any shape. These findings may increase interest in electrochemical flow synthesis and lead to the industrial application of this approach.

A low‐cost method for fabricating leak‐proof electrochemical flow reactors that can withstand high internal pressures is developed using selective Pt electroplating (5 μm thick) and chemical bonding methods. These findings may increase interest in electrochemical flow synthesis and lead to the industrial application of this approach.

1. Introduction

Electrosynthesis is an environmentally friendly process that significantly reduces waste and improves energy efficiency by using electrical energy instead of oxidizing and reducing agents to induce reactions and is receiving great attention from researchers.^{[ 1} , ^{2 ]} Electrochemical batch reactors have been continuously developed to increase the robustness and versatility of electrosynthesis, and recently, efforts have been made to standardize reactors to reduce the variations in the reaction results depending on the reactor geometry and to obtain predictable results.^{[ 3 ]} In batch reactions, the current gradient increases as the distance between the electrodes increases because of the low conductivity of the organic solvent, which causes an uneven distribution of electrical energy within the reaction solution.^{[ 4 ]} These problems can be overcome to some extent by increasing the concentration of the supporting electrolyte; however, large amounts of the solvent and energy must be consumed to remove the supporting electrolyte after the reaction.^{[ 5 ]} In contrast, in an electrochemical flow reaction, the current gradient can be minimized by selectively controlling the distance between the electrodes, and the diffusion layers of the electrodes can overlap or combine to reduce side reactions and byproduct formation. In addition, electrochemical flow reactions have recently attracted much attention because they have the advantage of reducing or minimizing the use of supporting electrolytes, and research results have indicated that electrochemical flow reactions can be utilized in industrial processes.^{[ 4} , ⁶ , ⁷ , ⁸ , ^{9 ]}

In continuous‐flow processes, new fabrication methods that consider the reaction characteristics are continuously being reported because cost reduction and process efficiency improvement are possible through adjustments to the reactor design and fabrication methods. In particular, research is being actively conducted on new commercial production processes based on low‐cost fabrication technologies for continuous‐flow reactors.^{[ 10} , ^{11 ]} In recent studies on continuous‐flow reactor fabrication, new approaches have been proposed that aim for “rapid prototyping” and “low‐cost mass fabrication,” and many studies on quantitative cost estimation have also been reported.^{[ 12} , ¹³ , ^{14 ]} In the field of flow electrosynthesis, design optimization has been demonstrated to improve the materials, structure, and reaction efficiency of flow reactors, and tools have been developed that are advantageous for scale‐up by converting known batch reactions into flow processes while maintaining environmentally friendly characteristics.^{[ 15} , ¹⁶ , ¹⁷ , ^{18 ]} However, compared with the steady increase in research on batch‐based electrosynthesis over the past several decades, research on flow‐process‐based electrosynthesis is still in its infancy (Figure S1, Supporting Information). The main reason for this situation is that, just as the standardization of batch electrosynthesis took time to achieve predictable reaction outcomes, the development of flow electrosynthesis not only requires an understanding of new technologies, but also increases the complexity of reactor utilization.^{[ 4 ]} In order to advance flow‐type electrosynthesis research, reactors suitable for each reaction must be provided at low cost and must be made easier to utilize. Today, a wide range of laboratory‐scale reactors are available, from reactors costing a few hundred dollars with simple structures and inexpensive graphite electrodes to reactors costing tens of thousands of dollars with precious metal electrodes. However, some reactors are not sufficiently durable for some reactions, or they have fixed internal structures that are often unsuitable for others; thus, researchers must purchase suitable reactors or custom‐build them for certain reactions.^{[ 19} , ²⁰ , ^{21 ]} Because most electrochemists are familiar with batch reactors, for flow reactors to become attractive tools, extensive research must be conducted on the fabrication of reactors so that the optimal reactor for each reaction can be purchased separately at a low cost, or the fabrication technology must be disseminated so that researchers can easily fabricate the reactors they need.^{[ 4 ]} Creating this research environment is expected to contribute significantly to the process of scaling‐up laboratory‐level research toward commercial production.^{[ 11 ]}

The electrode materials used in electrosynthesis include glassy carbon, Pt, graphite, Ni, Au, and Pd.^{[ 22 ]} C‐based electrodes such as graphite are inexpensive but have low electrical conductivity, are not durable, and tend to crumble easily.^{[ 23 ]} By contrast, electrodes composed of precious metals such as Pt, Au, and Pd have high electrochemical stability and excellent electrical conductivity, which can lower the overvoltage and improve the reaction efficiency. In addition, because precious metal electrodes have inactive surfaces, they suppress unnecessary side reactions, such as radical generation and redox reactions, and are therefore utilized in various electrosyntheses.^{[ 2} , ²⁴ , ²⁵ , ^{26 ]} However, the continuous flow method requires a larger electrode area than batch reactions, and the reactor fabrication cost inevitably increases when precious metal materials are used as electrodes, which is disadvantageous for commercial production in terms of economic feasibility.^{[ 27} , ^{28 ]} In this study, we attempted to reduce the electrode cost by using inexpensive metals as substrates and plating them with a precious metal.

Until now, the mechanical fastening method has been generally applied to the fabrication of electrochemical flow reactors. The holder is made of heavy and strong metals with excellent chemical resistance and durability, such as Al or stainless steel, and is fixed with fasteners, making the reactor easy to assemble and disassemble. The fastening method has the advantage of solving problems such as impurities adsorbing on the electrode surface or clogging the inside of the flow path. Additionally, the fluid path, reaction volume, and distance between electrodes can be controlled to some extent by replacing the flow channel.^{[ 4 ]} A structure that arranges two electrode plates in parallel, such as a parallel plate flow reactor, is suitable for applying a mechanical fastening method and is the most widely used configuration to date.^{[ 29} , ^{30 ]} Cantillo and coworkers have studied specialized reactors, such as a rotating cylinder electrode reactor that uses rotating electrodes to address issues such as channel clogging or bubble retention in slurry or gas‐producing reactions.^{[ 31 ]} In addition, Reek, Noël, and coworkers have developed special‐purpose reactors optimized for specific reaction conditions, such as an electrophotochemical reactor that uses electricity and light simultaneously to increase reaction selectivity.^{[ 32 ]} Recently, Stahl, Merck, and coworkers have actively attempted to scale up from the laboratory to the industrial scale by enlarging the reactor cell itself, and Xu and coworkers have employed a numbering‐up strategy by connecting 20 parallel plate cells in parallel.^{[ 33} , ^{34 ]}

However, the method of using fasteners by drilling holes in the parts for mechanical fastening requires a heavy and thick frame, which increases the weight and volume of the reactor and causes stress concentration around the fixed area; thus, other areas are not sufficiently compressed, often resulting in solution leakage between layers.^{[ 35 ]} In particular, when attempting to build large‐area reactors, connections are often concentrated at the edges of the reactor, causing bulging between the holes for fastening (Figure S2, Supporting Information).^{[ 4} , ^{36 ]} These limitations make utilizing flow reactors difficult for electrochemists accustomed to batch reactors and have been major factors in reducing industry interest in flow methods. In this study, we applied an adhesive bonding method that can evenly distribute stress over a wide area and modularized the reactor to solve the leakage problem.^{[ 37 ]} Adhesive bonding may be less suitable than mechanical fastening for cleaning contaminated electrode surfaces or troubleshooting problems such as clogging within the flow path. However, if the electrode cost, which accounts for the largest proportion of the fabrication cost, can be reduced by using a selectively plated electrode, such critical issues can be avoided. Moreover, instead of using a general‐purpose reactor for various reactions, a reactor optimized for each application can be fabricated by modifying the internal structure of the reactor.

2. Results and Discussion

2.1. Pt Electroplating on Ti Surface

The first attempt to fabricate Pt‐electroplated electrodes involved directly plating Pt onto a Ti substrate.^{[ 38 ]} However, this method had several limitations. Pt plating did not proceed uniformly on the oxidized Ti surface, and the Pt layer was severely delaminated (Figure S3, Supporting Information).^{[ 39 ]} To address these issues, we then attempted to polish the surface of the oxidized Ti substrate using sandpaper or chemical etching.^{[ 40} , ^{41 ]} Mechanical surface treatment of titanium was performed by sequentially polishing the titanium substrates (10 mm × 30 mm) using 600–3000 grit sandpaper. The polished substrates were ultrasonically cleaned with acetone to remove the surface oxides and contaminants. For chemical etching, substrates of the same size were treated with NaOH, HCl, and oxalic acid solutions. The etched substrates were thoroughly rinsed with distilled water and dried with nitrogen gas. Subsequently, electroplating was performed on both types of substrates under the same conditions (Supporting Information, S3.2). However, during this preliminary work, an oxide films rapidly reformed on the Ti surface, and satisfactory plating quality was not attained (Figure S4 and S5, Supporting Information). To mitigate the peeling of the Pt layer, multistage physical surface treatment processes were performed. The results of each process were visually inspected, and the elemental distribution on the surface was analyzed using scanning electron microscopy–energy dispersive X‐ray spectrometry (SEM–EDS) (SEM: Merlin Compact, Carl Zeiss, Germany; EDS: Ultim Max 65, Oxford Instruments, UK). First, we attempted to change the morphology of the Ti surface via sandblasting. Sandblasting uses abrasives to remove surface contaminants and to roughen smooth surfaces, thereby increasing the surface area and improving the bonding strength of coatings or metal plating.^{[ 42 ]} In this process, a sandblaster (Conveyor type (medium), ORIGIN, Korea) and 90–100 grit aluminum oxide oxide as abrasives were used to remove the oxidized layer on the Ti surface and to transform the smooth surface into a rough surface (Figure  1a,b). Next, a physical vapor deposition method, namely, electron‐beam (e‐beam) evaporation, was introduced to form a uniform, stable Pt layer on a Ti substrate. This process consisted of two steps in which pure Ti with a thickness of 200 nm was first deposited on a sandblasted Ti substrate (deposition rate 1.5 Å/s), followed by a thin (200 nm) layer of Pt (deposition rate of 1.0 Å/s) while maintaining a vacuum. Through this process, the oxidized layer on the Ti substrate could be minimized, and a Pt layer could be stably deposited on the Ti surface.^{[ 43 ]} Deposition was performed using an e‐beam evaporator (KVE‐E2000L, KOREA VACUUM Technology, Korea), and the deposition conditions were 5.0 × 10^{− 7} Torr (deposition pressure) and room temperature (deposition temperature). The SEM analysis results show that the surface deposited after sandblasting maintained a surface structure similar to that of the Ti surface and was brighter in color than the Ti surface before deposition, confirming that Pt was deposited uniformly (Figure 1c). However, the EDS analysis results revealed that Ti was still present on the surface even after the deposition process (Ti: 22.64 at%, Pt: 77.36 at%). Thus, the deposited Pt did not completely cover the entire electrode, and Ti is exposed in some areas. Because exposed Ti can negatively affect the electrochemical synthesis; thus, a more uniform Pt layer was formed by electroplating the Pt‐coated Ti substrate. The electroplating process was carried out in an undivided cell using a plating solution prepared by mixing 200 mg of hexachloroplatinic acid (H₂PtCl₆), 4 mL of 37% hydrochloric acid (HCl), and 16 mL of DI water. A Pt‐coated electrode was used as the counter electrode, and the process was conducted under ambient air at 70 °C by applying a constant voltage of 1.2 V for 1 h.

Figure 1.

SEM images and EDS elemental mapping of Ti and Pt during the electrode fabrication process (Figure S6, Supporting Information). a) Ti surface before sandblasting (scale bar: 100 μm). b) Ti surface after sandblasting (scale bar: 50 μm). c) Surface after Pt/Ti deposition (scale bar: 50 μm). d) Electroplated surface (scale bar: 50 μm). e) Electroplated surface (scale bar: 10 μm). f) Cross‐section of Pt‐plated electrode (Pt layer: average thickness of 5 μm).

The final electroplated surface was analyzed using SEM, confirmed that the electroplated layer is formed uniformly over the entire surface (Figure 1d). The high‐magnification image in Figure 1e clearly shows Pt crystals on the surface. EDS did not detect Ti in the electroplated layer, confirming that Pt was electroplated over the entire electrode without exposing the Ti layer. To observe the cross‐section of the electroplated electrode via SEM, a 15 mm × 15 mm sample was cut using wire‐cut electrical discharge machining (EDM), AL600G, Sodick, Japan. The cross‐sectional SEM images show an electroplated Pt crystal layer with an average thickness of 5 μm on the Ti surface (Figure 1f and Figure S7, Supporting Information).

2.2. Fabrication of Reactors Through Adhesive Bonding

To design an electrochemical flow reactor to improve the structure and reaction efficiency and reduce the fabrication cost, we first attempted the commonly used mechanical fastening method based on research on the fabrication of conventional reactors (Figure  2a and Figure S8, Supporting Information). The reactor used Pt‐plated electrodes with an average Pt thickness of 5 μm. When fabricating a reactor, various reaction factors such as mass transfer, heat transfer, and residence time must be considered, and the essential characteristics of the reactor, such as pressure resistance due to gases and byproducts generated during the reaction, must also be satisfied. Therefore, as the number of factors considered while fabricating a reactor increases, the fabrication process inevitably becomes more complex.^{[ 44} , ⁴⁵ , ⁴⁶ , ^{47 ]} However, despite the electrode area (40 mm × 70 mm) of the reactor (40 × 70 × 60 mm) being 28.0 cm², liquid continued to leak between the fastened reactor components, making proceeding with the electrochemical reactions difficult. These problems made it challenging to improve the productivity using large‐area electrodes or multilayer structures (Figure 2b).^{[ 36 ]}

Figure 2.

Reactor fabricated using the mechanical fastening method. a) Overall configuration of the reactor (60 × 70 × 40 mm). ① and ⑧: reactor holders (Al) ② and ⑦: electrode holder (PEEK) ③: rubber gasket (fluorocarbon rubber (FKM)) ④: anode (Pt‐plated electrode, internal area: 40 mm × 70 mm) ⑤: flow path (PTFE) ⑥: cathode (Pt‐plated electrode, internal area: 40 mm × 70 mm) ⑨: flow port (PEEK). b) Disassembled mechanical fastening reactor, showing that solution leaks outside the flow path. c) Photograph of the mechanical fastening reactor assembled with alligator clips and flangeless nuts (PEEK).

Adhesive bonding between two layers has been attempted as a suitable alternative to solve the leakage problem. This method is expected to have the disadvantage of difficulty in recycling the electrode or removing blockages caused by substances such as electrolytes in the flow path. However, the adhesive bonding is worth attempting because the need for electrode recycling is reduced by utilizing the plated electrodes described above. The first attempt was with epoxy resin; however, applying the adhesive evenly to the bonding surfaces was difficult and problems existed with excessive adhesive leakage or blockage of the passages (Figure S9, Supporting Information). Based on previous studies, we attempted a thermal bonding method using fluorinated ethylene propylene (FEP) nanopowder, an adhesive material that bonds with the Al holder and electrode and has excellent bonding strength with the fluorine‐based polytetrafluoroethylene (PTFE).^{[ 48 ]} However, evenly applying the FEP nanodispersion was very difficult, and in some areas, the adhesion was insufficient, causing the reactor parts to peel off. In addition, similar to the case of epoxy resin, the dispersion leaked out or penetrated the flow channel (Figure S10, Supporting Information). To address these issues and improve the adhesion and uniformity, we sought to identify an adhesive material in film form that would not block channels or cause structural damage. Among the fluorinated polymers, PTFE, perfluoroalkoxyalkane (PFA), polyvinylidene fluoride (PVDF), and FEP in film form (0.1 mm thick) were compared in terms of chemical resistance and solvent resistance (Table  1 ).

Table 1.

Adhesive properties of fluoropolymer films.

Entry	Material	M.P. [°C]	Bonding Temp. [°C]	Chemical Resistance	Bonding Strength	Applicability
1	PTFE	327	350	Excellent	Weak	Not suitablea)
2	PFA	305	350	Excellent	Weak	Not suitableb)
3	PFA	305	310	Excellent	Weak	Not suitableb)
4	PVDF	177	200	Poor	Moderate	Partially suitablec)
5	FEP	260	280	Excellent	Strong	Optimald)

^a)Weak bonding caused by insufficient melting.

^b)Channel shrinkage during bonding.

^c)Partially suitable but lacking solvent resistance.

^d)Good bonding performance.

First, we tested a PTFE film, a material similar to that of the flow channel. The melting point of the PTFE film was 327 °C, and the tested bonding temperature was 350 °C. After bonding with the PTFE film, the flow channel was deformed; thus, the PTFE film was not suitable (Entry 1). Bonding was then performed at 350 °C using a PFA film with a melting point of 305 °C, but deformation occurred in the flow channel, as with the PTFE film (Entry 2). The temperature was lowered to 310 °C, which was lower than the melting point of PTFE, and the experiment was performed again, but the bonding was not sufficient (Entry 3). As expected excellent adhesion was obtained by bonding with PVDF film (melting point: 177 °C) at 200 °C, but its relatively low chemical resistance and solvent resistance were expected to limit the choice of organic solvents or reactants during the electrosynthesis (Entry 4). FEP film, which has a lower melting point (≈260 °C) than PTFE and PFA films and excellent chemical resistance, exhibited good adhesion without channel deformation (Entry 5). Therefore, FEP film was finally selected as the bonding material.

The entire surface of the electrode shown in Figure 2 was plated with Pt with an average thickness of 5 μm. However, in this case, Pt was unnecessarily plated in areas where the solution did not flow. Therefore, to minimize the amount of Pt used, a process was performed to plate only the flow path area where the actual electrosynthesis would occur. First, two Ti sheets (40 × 80 × 1 mm) were prepared, a hole with a diameter of 1.5 mm was machined into the upper substrate to connect the inlet and the flow path, and another hole was machined into the lower substrate to connect the flow path and the outlet using a metal laser cutting machine (PSJ‐1325, PSJ, China). Next, sandblasting was performed on the Ti surface using the same method as previously described, and then Ti and Pt were deposited with an e‐beam (Figure  3a). Using Kapton tape (Polyimide TAPE, ST‐830, DAEHYUN ST, Korea), the areas that did not require plating were masked, leaving a 2‐mm‐wide area for a 1.5‐mm‐wide channel area, and only the areas where the liquid flowed were plated (Figure 3b). Selective Pt plating reduced the amount of Pt used by 82% compared to that employed in full plating (Figure 3c and Figure S11, Supporting Information). The boundary between the plating surface and the deposition surface was analyzed using SEM–EDS. The difference in crystal structure between the deposition surface and the plating surface is confirmed by the SEM image, and the EDS analysis results show that the e‐beam‐deposited surface had some Ti exposed, similar to that in Figure 1c, but the plated area was uniformly plated with Pt crystals, similar to that in Figure 1d (Figure 3d and Figure S12, Supporting Information).

Figure 3.

Fabrication of selectively electroplated Pt electrodes. a) Substrate with 200 nm Ti/200 nm Pt layers deposited using an e‐beam evaporator before electroplating. b) Substrate with the region outside the flow channel masked after cutting Kapton tape (Polyimide TAPE, ST‐830, DAEHYUN ST, Korea) along the flow channel shape. c) Substrate selectively platinized only in the flow channel area by electroplating. d) SEM image of the boundary between the nonflow area and the flow channel in the electroplated Pt electrode, EDS layered image of Ti (green) and Pt (red) using EDS, and elemental mapping of Ti.

A targeted reactor was fabricated using an FEP film and electrodes via selective Pt electroplating. First, a reactor holder with holes for connecting the tube connections for reactant injection and electrode connectors for supplying power to the electrodes was fabricated from Al (40 × 80 × 10 mm) using a computer numerical control machining center (DNM‐500, Doosan, Korea). The holder was designed with a 6‐mm‐deep M6 thread and a 4‐mm‐deep, 1‐mm‐diameter hole leading to the channel and electrode. After bonding, each machined hole was secured using a flangeless nut (P‐307x, PEEK M6 Flat bottom for 1/8 inch OD, Upchurch Scientific, USA). The power supply and electrode were connected with Cu wires through holes in contact with the electrodes (Figure  4a ①), and a tube through which fluid could flow was inserted into the hole connected to the channel (Figure 4a ②). An identical holder containing an electrode connector connection for powering the electrodes and a tubing connection for product injection was fabricated and placed on the opposite side of the reactor (Figure 4a ③ and ⑦ and Figure S13, Supporting Information). The flow channel was designed using 3D CAD software (Autodesk Fusion360) on a rectangular PTFE sheet (40 mm × 80 mm × 1 mm) with a width of 1.5 mm through which the fluid flowed and was machined via laser cutting (VLS3.60DT, Universal Laser Systems, USA) (Figure 4a ⑤ and Figure S14, Supporting Information). The selectively plated electrodes (Figure 3) were placed on either side of the channel so that the machined holes aligned with the flow paths to facilitate smooth fluid flow (Figure 4a ④ and ⑥). The FEP film (40 × 80 × 0.1 mm, FEP0100, Alphaflon, Korea) was cut into a shape suitable for the locations where adhesion was required so that excess film would not block the holes and channels inside the reactor when heated. FEP film was inserted between the holder, electrode, and channel to bond each component to a single piece (Figure 4a ⑧ and Figure S15, Supporting Information). After aligning and stacking each part, they were placed in a furnace maintaining a uniform pressure of 10 kPa and heated at 260 °C for 1 h to bond them. After bonding, they were slowly cooled at a rate of 30 °C/h to prevent deformation of some reactor components due to rapid temperature changes. After cooling to room temperature, the joint condition was verified, and a flangeless nut was fixed to complete the reactor (Figure 4b). In addition, by miniaturizing the inlet and outlet connections, the thickness of the main body could be reduced to 3 mm (Figure 4c and Figure S16 and S17, Supporting Information). One side of the electrode was insulated with a PTFE sheet, and electricity was supplied through alligator clips.

Figure 4.

Electrochemical flow reactor fabricated via electroplating and adhesive bonding. a) Design and configuration of the reactor. ①: Electrode connector. ②: Inlet, outlet ports. ③ and ⑦: Holders (Al). ④ and ⑥: Selectively Pt‐plated electrodes. ⑤: Flow channel (PTFE). ⑧: Adhesive film (FEP). b) Photograph of the completed reactor (40 × 80 × 23 mm). c) Reactor with 3 mm body thickness and detailed side view.

One of the most important factors to consider when building a reactor is maintaining complete containment, which ensures that the reactants can maintain a steady flow without being affected by the external environment or atmosphere. To confirm that the fluid flowed smoothly without blockage inside the channel and that the solution did not leak outside, water was injected at a rate of 5 mL min⁻¹ using an high‐performance liquid chromatography (HPLC pump, KP‐22, GL Sciences, USA) with the reactor outlet open, and the water flowed stably without leakage. Subsequently, a pressure maintenance test was conducted by injecting water while the outlet was closed, and the pressure change over time was measured. The pressure of the HPLC pump was monitored with five reactors. A fluid leak was detected at an average of 320 psi, and a joint completely ruptured at 435 psi, causing a drop in internal pressure (Figure  5 ).

Figure 5.

Durability test for internal pressure (fluid leak at 320 psi, and joint ruptured at 435 psi).

2.3. Electrochemical Flow Synthesis

To verify the performance of the fabricated reactor, a C—H/S—H cross‐coupling reaction, which efficiently forms C—S bonds via electrochemical oxidation without a transition metal catalyst and oxidant, was performed in a batch reactor and the fabricated flow reactor.^{[ 49 ]} First, a batch reaction was performed using 1‐methylindole (1b, 0.50 mmol) and p‐toluenethiol (2, 1.0 mmol) as reactants and an electrolyte containing LiClO₄ (2.0 mmol) dissolved in acetonitrile (CH₃CN, 8.0 mL). Both the anode and cathode were composed of electroplated Pt (10 mm × 30 mm), and the distance between them was maintained at 2 mm. The reaction was carried out for 4 h while maintaining a current density of 5.33 mA cm⁻² in a N₂ atmosphere, and product 3b was obtained in 75% NMR yield and 94.3% Faradaic efficiency (Supporting Information, S4(a)). Subsequently, the reaction was performed in the fabricated reactor. Indole (1, 0.50 mmol) derivative and p‐toluenethiol (2, 1.0 mmol) were used as reactants, and LiClO₄ (2.0 mmol) dissolved in CH₃CN (8.0 mL) was used as the electrolyte. A current of 5.33 mA cm⁻² was applied. The reaction was performed at room temperature under a N₂ atmosphere for 1.5 h. The reactants were circulated at a flow rate of 10 mL min⁻¹ using a peristaltic pump. The internal volume of the fabricated reactor is 0.437 mL, the internal current area is 437.1 mm², and the residence time is ≈2.6 s (Figure S4(b) and Figure S18, Supporting Information). All reported yields are based on the average of five replicates (Table S1, Supporting Information). As a result of the experiment, 3a, a cross‐linking product of indole and p‐toluenethiol, was obtained in 75% yield and 57.5% Faradaic efficiency. Indole derivative 3b with a methyl group substituted at the R₁ position showed a yield of 86% and 66.0% Faradaic efficiency. This resulted in a shorter reaction time and higher yield compared to those of 3b synthesized via the batch process. Meanwhile, indole derivative 3c, in which a Cl atom was substituted at the 5‐position, exhibited a yield of 64% and 49.1% Faradaic efficiency, indicating relatively moderate reactivity. Additionally, 3d, a derivative with Br substitution at position 5 and a methyl group at position R₁, showed a high yield of 88% and 67.5% Faradaic efficiency. These results demonstrate that the fabricated reactor with selectively plated electrodes and adhesive bonding can be efficiently utilized for the electrochemical flow of C—H/S—H cross‐coupling to form C—S bonds without transition metal catalysts or oxidants Scheme  1 .

Scheme 1.

Oxidant‐free dehydrogenative C—H/S—H cross‐coupling with p‐toluenethiol. ^[a]Flow conditions: electroplated Pt anode, electroplated Pt cathode, current density = 5.33 mA cm⁻², 1 (0.50 mmol), 2 (1.0 mmol), LiClO₄ (2.0 mmol), CH₃CN (8.0 mL), room temperature, N₂ atmosphere, circulate at a flow rate of 10 mL min⁻¹, 1.5 h, isolated yield. ^[b]Batch conditions: Pt‐electroplated anode, Pt‐electroplated cathode, current density = 5.33 mA cm⁻², 1b (0.22 mmol), 2 (0.44 mmol), LiClO₄ (0.88 mmol), CH₃CN (3.6 mL), room temperature, N₂ atmosphere, 4 h, NMR yield.

2.4. Cost Analysis

The cost‐effectiveness of the fabricated reactor was compared with that of a bonded reactor assuming the use of 0.1‐ or 0.2‐mm‐thick Pt sheets in a conventional mechanical fastening reactor.^{[ 50} , ^{51 ]} The costs were compared by categorizing them into material costs, labor costs, and operating costs (Supporting Information, S5). First, the material costs of the three reactors were compared, and the cost differences among the materials used for the holder, electrode substrate, and channel were found to be insignificant. The largest difference in material costs was related to the amount of Pt used. Pt is a precious metal, and its price can fluctuate significantly depending on market price fluctuations and has risen significantly in recent years. The material costs of the mechanical fastening reactor with 0.1‐ and 0.2‐mm‐thick Pt plates were 4,306.9 USD and 6,986.2 USD, respectively, which are incomparable to the fabrication cost of the reactor fabricated in this study, which was 183.1 USD. Additionally, selective plating could minimize unnecessary Pt use (Table S2, Supporting Information). The fabrication process of the conventional reactor could be completed within a short fabrication time, but in the case of the reactor reported here, processes such as deposition, electroplating, and bonding are added, making the overall fabrication process somewhat longer (Table S3, Supporting Information). Additional processes may increase the operating and labor costs. In addition, if the equipment is not directly owned, additional rental costs for equipment may be added (Table S4, Supporting Information). For the fabricated reactor, the combined operating and labor costs were approximately seven times higher than those of the conventionally fastened reactor (23.1 USD: 23.1 USD: 164.2 USD), but their increased cost could be offset by the large difference in material costs (Figure  6 and Table S5, Supporting Information). Finally, the total fabrication cost of the reactor was 183.1 USD, representing a 97.4% reduction in total cost compared with that of a mechanical fastening reactor with 0.2‐mm‐thick Pt sheet electrodes (6,986.2 USD), and the reactor could be fabricated at 1/100 of the price of commercial reactors using Pt electrodes that cost tens of thousands of dollars. In addition, in the case of scaling up to increase the reactor size to increase the reaction volume of the laboratory‐scale reactor, or by increasing the reaction volume by stacking multiple reactors, the amount of electrode material used when fabricating the reactor in a conventional manner also increases proportionally, significantly increasing the overall fabrication cost. Nonetheless, the electrode presented in this study can significantly reduce the amount of Pt required and thus is anticipated to be a more economical and efficient alternative for fabricating large‐scale reactors or multilayer reactors. In particular, as the production scale increases, cost reduction effects through automation and process optimization can be expected.

Figure 6.

Detailed analysis of fabrication cost according to fabrication method.

3. Conclusions

The use of precious metal electrodes, while ensuring high electrical conductivity and chemical stability, inevitably increases the fabrication cost of electrochemical flow reactors. Furthermore, the commonly utilized mechanical fastening method consistently presents a risk of leakage. In this study, we developed a new fabrication method to solve the leakage problem and cost disadvantages of conventional electrochemical flow reactors. First, to maximize the economic feasibility of the reactor fabrication, we introduced a process of selectively electroplating Pt only on the electrode surface where the reaction occurs. Analysis of the surface and cross‐section using SEM confirmed that a Pt layer with an average thickness of 5 μm was uniformly formed along the flow channel. The reactor was bonded by heat‐fusing the holder, PTFE channel, and electrode with FEP film at 260 °C for 1 h. In the durability test for the internal pressure, no solution leakage from the reactor occurred up to 320 psi. An electrochemical C—H/S—H cross‐coupling reaction was performed using the fabricated reactor, and the reaction of indole and p‐toluenethiol resulted in a maximum yield of 88%. By drastically reducing the use of Pt, which is an expensive precious metal, the fabrication cost can be decreased by up to 97.4% with respect those of the conventional mechanical fastening methods, and the cost‐saving effect is expected to be maximized as the size and number of reactors increase. The fabricated reactor is anticipated to be attractive to most electrochemists familiar with batch reactors because it includes chemical and solvent resistance, durability under high‐pressure conditions, and the ability to change the reactor structure appropriately depending on each reaction. Furthermore, this study may increase the potential for flow electrosynthesis to become a commercial process, as it can be easily configured in stacks or in parallel using large‐area electrodes.

Conflict of Interest

The authors declare no conflict of interest.

Supporting information

Supplementary Material

Data Availability Statement

The data that support the findings of this study are available from the corresponding author upon reasonable request.