2025 Volume 93 Issue 2 Pages 027002
The use of streaming potential in chemical production with microfluidic devices offers a promising approach to reducing energy consumption. This study explored H2 generation through the combined use of applied voltage and streaming potential in a microfluidic system. The microfluidic system with two platinum electrodes embedded in a microchannel was designed to explore the hydrogen evolution reaction (HER) and quantitative analysis was performed using the device with a liquid/gas loop system. The prepared microfluidic device demonstrated that the reduction current increased with flow rate when using an HCl aqueous solution. The results showed an increase in H2 production efficiency by up to 23 % compared to a static state, which can be attributed to the streaming potential. Enhancing chemical production efficiency through streaming potentials could contribute to establishing an innovative chemical production process using microfluidic devices.
Various liquids are widely used as solvents and substrates for reactions in industrial chemical production. The development of novel methods in industrial chemical manufacturing that employ these liquids could advance a sustainable society.1–4 Microfluidic devices have been reported to improve efficiencies in numerous chemical processes owing to their reduced volume,5–7 making them promising candidate for innovative chemical production methods. While their chemical production capacity is lower than that of large conventional chemical plants, microfluidic devices offer several advantages within a novel chemical production format.4,8 This new format, centered on local production for local consumption,9 can drastically reduce energy consumption by producing the required amount of chemicals directly at the point of demand. Unlike large-scale transitional chemical plants, which require the storage and transport of large volumes of chemicals globally at a significant energy cost,10–13 the microfluidic devices in localized production can minimize the energy demand associated with the handling of bulk chemicals. To replace large conventional chemical plants with microfluidic devices, energy efficiency is a crucial evaluation criterion.9,14,15 Various strategies are necessary to enhance the energy efficiency of microfluidic devices. One approach involves minimizing the energy losses associated with the liquid flow dynamics, as energy is essential for liquid movement within a microfluidic system. The use of a streaming potential and current induced by liquid flow is an inevitable occurrence in microfluidic devices, which may improve the energy efficiency of chemical production.
Potential differences arise between the upstream and downstream positions in a microchannel due to flow of ion-containing liquids along a charged wall surface. This phenomenon is referred to as the streaming potential and current.16–18 The mechanisms behind streaming potential and current are attributed to the polarization caused by the migration of counter-ions in the flow direction relative to the fixed ions in the electrical double layer. Numerous studies have reported applications of streaming potential and current in sensor devices,19–22 wearable electronic devices,23 and techniques to convert liquid low to electric energy.24–27 In contrast, research on the streaming potential in electrochemical reactions remains limited.28,29 If the potential gradient is sufficiently large, oxidation and reduction reactions can occur on the electrodes within the microchannel without applying an external voltage.28,29
Combining the streaming potential with an external voltage application is also an approach to reducing energy consumption. This concept involves using the streaming potential to supply part of the energy required to generate the electric potential for electrochemical reactions, leading to overall energy savings. However, studies on this combination of streaming potential and application of external voltage for chemical production are limited.30 Hydrogen (H2) generation through water electrolysis using streaming potential can enhance the energy efficiency of chemical production in microfluidic devices, as H2 serves as an energy source. Water electrolysis requires a voltage of 1.23 V based on the standard redox potential.31 When water is the liquid in the microfluidic device, H2 production is anticipated through water electrolysis. However, achieving H2 production solely from the streaming potential in microfluidic devices is a challenge. The voltage needed for H2 production can be supplemented by the energy deficit from the streaming potential, combined with the externally applied voltage, resulting in a decrease in the electrolytic voltage required for water electrolysis. This combination of streaming potential and applied voltage enhances H2 production efficiency by the amount of streaming potential, compared to the use of only applied voltage. The energy efficiency of chemical conversion systems in microfluidic devices is improved because the generated H2 can be utilized as the energy source to drive these devices.
In this study, we explored the feasibility of H2 generation by combining applied voltage and the streaming potential using a microfluidic system. To achieve this, we prepared a microfluidic device incorporating two Pt electrodes within a microchannel for H2 generation. Quantitative analysis of H2 production was performed using the prepared microfluidic device along with a liquid/gas loop system. Although channel shape and size, electrodes, and solvent require optimization for industrial applications, this study primarily focused on proof-of-concept. The results obtained are expected to serve as a stepping stone toward efficient hydrogen production using microfluidic devices and to improve energy efficiency by using the generated H2 as an energy source for driving microfluidic devices.
To measure the voltage difference between two Pt electrodes, a one-way flow device was prepared, as shown in Fig. 1a. The microchannel in the flow device was made of quartz glass other than the platinum electrodes. The setup included a reservoir, a flow pump (TACMINA, Q1-5-KR-UP-S) and a flow meter (Sensirion, SLI-1000) connected to the prepared microfluidic device using PTFE tubes (GL Sciences, ID: 0.75 mm, OD: 1/16 inch). The actual flow rate was monitored by the flow meter, as it often differed from the set value of the flow pump due to factors such as pulsation. Although it is possible to install an air chamber to suppress the effect of pulsation, we decided to monitor the actual flow rate with the flow meter in this study. Two Pt electrodes of the same size and shape (diameter = 7 mm) were placed in the flow microchannel, designated as the upstream electrode (Ptup), serving as the counter electrode, and the downstream electrode (Ptdown), which acts as the working electrode. The center locations of Ptup and Ptdown were 18 mm and 26 mm from the inlet port of the flow channel, respectively, as shown in Fig. 1b. Voltage differences were measured in aqueous hydrochloric acid (HCl, FUJIFILM Wako Pure Chemical, 36 wt%) solution diluted to concentrations ranging from 1 mol L−1 (M) to 0.001 M using a digital multimeter (KEITHLEY, DMM6500). Prior to measurement, the flow channel was flushed with each concentration of HCl solution, allowing sufficient time for replacement. The potential was measured for 240 s, with the HCl solution flowed at 100 µL/min after a 120 s period of no flow.
(a) Schematic of the one-way flow device with two Pt electrodes embedded in the microchannel. (b) Photograph of the prepared microfluidic device and detailed view of the microchannel. (c) Conceptual potential diagram of H2 generation reactions by combining streaming potential and external applied potential.
The current was measured at different flow rates using chronoamperometry with a potentiostat (MEIDEN HOKUTO, HSV-110). The one-way flow device (Fig. 1a) was utilized for current measurements, and the two Pt electrodes were connected as the working electrode and counter electrode, similar to the setup for measurement of voltage difference. Current measurements were performed for 360 s, with flow rates changed every 60 s in the following order: 100, 400, 800, 400, 100, and 50 µL/min, corresponding to the set values of the flow pump. The applied voltages were set at −1.2 V and −1.5 V.
2.3 Quantitative analysis of H2 production in microfluidic deviceFor the quantitative analysis of H2 production, the prepared microfluidic device was integrated with a liquid/gas loop system. Before electrochemical measurements, the gas loop path, including the gas bag (GL Sciences, SMART BAG PA CCK-1), was flushed with argon (Ar) gas (Uno Sanso, 99.999 %), and approximately 500 mL of Ar gas was introduced into the gas loop system. The introduced Ar gas and generated H2 gas were circulated using a gas pump at a flow rate of 100 mL/min. Chronoamperometry measurement were performed at −1.5 V for 30 min with varying flow rates of 50 and 800 µL/min. For H2 concentration analysis, a 1 mL sampled of the looped gas was taken and analyzed every 5 min for 30 min using gas chromatography (NISSHA, SGHA-P3-A). After the chronoamperometry measurements, the gas volume in the gas loop path, including the sampling bag, was accurately measured. The amount of H2 production was calculated based on the gas volume and the results from gas chromatography.
The one-way flow device (Fig. 1a) was prepared to evaluate the streaming potential, considering the effects of solution conditions, flow rate, and the externally applied voltage. To accurately determine the relationship between flow rate and streaming potential, the flow rate was monitored using a flow meter positioned before the microchannel. The microchannel dimensions were 52 mm in length, 7 mm in width, and 50 µm in thickness (volume = 18.2 µL). Two rounded Pt electrodes (diameter = 7 mm) were placed within the microchannel. For clarity, the Pt electrodes positioned in the upstream and downstream locations along the liquid flow path were designated as Ptup and Ptdown, respectively. The center positions of Ptup and Ptdown were located 18 mm and 26 mm from the inlet port of the microchannel, with Ptup and Ptdown serving the anode and cathode, respectively.
The streaming potential can be expressed by the following equation:32
| \begin{equation*} E_{\text{str}} = \frac{\epsilon_{0}\epsilon_{\text{r}}\zeta}{\eta K_{\text{L}}}\Delta P. \end{equation*} |
Here, $\epsilon_{0}$, $\epsilon_{\text{r}}$, ζ, ΔP, η, and KL denote electrical permittivity of vacuum, relative permittivity of the solution, zeta-potential of the channel walls, pressure drop across the microchannel, solution viscosity, and solution conductivity, respectively. The pressure drop resulting from liquid flow causes the streaming potential to increase in proportion to the pressure drop, meaning that the streaming potential rised with an increased flow rate.33,34 The potential difference between two electrodes can be harnessed by positioning them within the microchannel.28 Figure 1c illustrates a schematic of the combined effects of externally applied voltage and streaming potential.
Even if the applied voltage alone is insufficient to initiate electrochemical reactions, these reactions can proceed if the total electrode potential, boosted by the streaming potential generated from solution flow, is high enough (Fig. 1c). This effect can reduce energy consumption for electrochemical reactions within the microchannel. In most existing chemical production processes, flow energies is typically wasted. However, if this unused flow energy is efficiently converted into streaming potentials for chemical production, it can pave the way for innovative processing methods. To evaluate this concept, we investigated the H2 generation reaction by combining streaming potential and applied voltage. Here, the roles of cathode and anode are determined by the charge on the microchannel wall.35,36 The anticipated reactions on the Ptup (anode) electrode are Cl2 and O2 generation, while on Ptdown (cathode) electrode is H2 generation.
3.2 Optimization of HCl concentration for H2 generationThe potential difference between two Pt electrodes is complex, as it is influenced by various factors, including electrolyte concentration,37 pH,38 and the presence of redox species.39 The streaming potential generally decreases as the electrolyte conductivity increases.32,40 Furthermore, it is highly dependent on the pH of the solution, given that the surface charge in the microchannel varies with pH.41,42 Given these dependencies, we investigated the optimal concentration of HCl that would maximize the voltage difference between the electrodes. The voltage difference between the two Pt electrodes was evaluated using HCl solutions at different concentrations, as shown in Fig. 2a. The voltage was defined as E = Edown − Eup. The left and right halves of Fig. 2a illustrate the relative voltage without and with flow, respectively. Before the measurements, the liquid in the microfluidic device was replaced with each concentration of HCl aqueous solution. The liquid flow for the replacement of the solution in the device was stopped at t = 0 s, and then the solution flow was restarted with a flow rate of 100 µL/min from t = 120 s when a steady state was almost reached.
(a) Relative voltage difference between two Pt electrodes at various HCl concentrations, with relative voltages set to zero based on measurements taken immediately before flow initiation at 120 s. (b) Voltage change at 240 s with HCl solutions of varying concentrations. (c) Flow rate dependence of voltage changes with 0.1 M HCl aqueous solution.
To determine relative voltages, the voltages measured immediately before the flow started at 120 s were set to zero. The results suggested that the peak relative voltage difference increased with higher HCl concentrations. Moreover, the rate of voltage increase immediately after the flow initiation also increased with higher HCl concentration, indicating that a lower pH and higher electrolyte concentration can lead to a quicker stabilization of the electrode potential. The potential changes (streaming potential) in Fig. 2a showed negative shifts. According to the streaming potential equation, the sign of the potential change is governed by the ζ potential, suggesting that the observed negative potential changes correspond to a negative ζ potential in this system. The quartz glass forming the microchannel may exhibit neutral or positive ζ potential as a result of a decrease in pH.43,44 Therefore, these results suggest that Pt electrodes significantly contribute to the observed behavior in the device.
For chemical production applications involving streaming potential, the long-term stability of voltage differences is critical. In the 1 M HCl solution, a clear voltage decrease was observed after the peak, posing a potential limitation for sustained chemical production. Although the exact cause of the decrease in relative voltage with 1 M HCl is not fully understood, it may involve proton adsorption and oxidation on the Pt electrode surface, resulting in time-dependent changes in electrode potential. In contrast, the voltage results at lower HCl concentrations showed smaller fluctuations over time. Figure 2b shows the voltage changes at 240 s in various concentrations of HCl, indicating that the voltage change increased with HCl concentration up to a maximum at 0.1 M. Based on these findings, 0.1 M was selected as the optimal concentration of HCl for sustained electrochemical reactions, and subsequent experiments were performed at this concentration. The flow rate dependence of streaming potential with 0.1 M HCl aqueous solution was evaluated (Fig. 2c). The results show that the streaming potential increases with increasing flow rate, which is consistent with the streaming potential equation, because ΔP increases with increasing flow rate. In contrast, the magnitude of the increase gradually decreases above 200 µL/min, indicating that the flow rate and ΔP are not proportional.
The increase in HCl concentration leads to a greater magnitude of potential change (Figs. 2a and 2b), which cannot be fully explained by the streaming potential equation alone. Although the dependence of potential changes on HCl concentrations may be due to the production of H2, Cl2 and O2, the detailed mechanism remains unclear. Further experiments and considerations are required to reveal the dependency mechanism of HCl concentrations.
3.3 Relationship between current and flow rate under a combination of streaming potential and external applied voltageA minimum applied voltage of 1.23 V is necessary for proton reduction at the working electrode and water oxidation at the counter electrode, where the applied voltage values are set to −1.2 and −1.5 V. Figure 3a presents a chronoamperogram at an applied voltage of −1.5 V, where the flow rate was adjusted stepwise approximately every 60 s. The flow rate values, measured with a flow meter, as shown in Fig. 3a, show oscillatory behavior due to pump pulsation, with oscillation periods dependent on the flow rate settings. To account for these fluctuations, we calculated averaged values over 60 s, given the oscillatory nature of both current and flow rate caused by pump pulsation.
(a) Chronoamperogram at −1.5 V in 0.1 M HCl, showing the measured flow rate. The pump flow rate was set sequentially to 100, 400, 800, 400, 100, and 50 µL/min. (b) Averaged current over 60 s at −1.2 V and −1.5 V for each flow rate. The zero-flow rate point is estimated by extrapolating the linear fit function. (c) Dynamic responses of streaming potential and current analyzed by fitting with the exponential function. The same data in Fig. 2a (0.1 M HCl) and Fig. 3a (current) were used to this analysis.
Figure 3b plots the relationship between averaged current and flow rate at applied voltages of −1.2 and −1.5 V. The reductive currents show a proportional increase with rising average flow in both cases. The zero-flow current (at average flow rate = 0) extrapolated from the first-order fitting represents the current in the absence of flow effects. Assuming that the flow rate and streaming potential are proportional relation and that the current increases in accordance with the Tafel law, the increase in the reduction current should follow an exponential function. However, as shown in Fig. 2c, the negative increase in streaming potential with an increasing flow rate becomes less in larger flow rate values. Therefore, the increase in the reduction current in Fig. 3b appears to be linear, and we concluded that fitting it with a linear function is the most appropriate. The area of current influenced by flow demonstrates the energy efficiency enhancement due to liquid flow. Calculated percentage increases in current at a flow rate of 500 µL/min yield values of 14 % and 23 % for −1.2 V and −1.5 V, respectively. These findings suggest that both applied voltage and flow rate significantly impact energy efficiency improvements via streaming potential. Although the current flow rate was limited to a set value of 800 µL/min due to the pressure resistance of the microfluidic device, increasing the flow rate could further enhance electrochemical reaction efficiency within the microchannel by optimizing the microfluidic system. The liquid flow may affect diffusions of reactants and products of electrochemical reactions, inducing current changes. At present, it is difficult to understand the effects of streaming potential and diffusion separately. In this study, the dynamic response of streaming potential and current was analyzed with time constants τ obtained by fitting with the exponential function (Fig. 3c). The time constants τ of the streaming potential and the current were 0.82 and 0.76 s, respectively, indicating almost the same time constants. The results suggested that the current change by the liquid flow originated from the streaming potential.
3.4 H2 generation by combining streaming potential and applied voltageTo investigate H2 generation through the combined effect of streaming potential and applied voltage, a loop-type device was prepared, as illustrated in Fig. 4a. The microchannel configuration and Pt electrodes are identical to those in the one-way device (Fig. 1a). The generated H2 gas was collected in the gas bag, gas pump, and tubing comprising the gas-loop system. The outlet port of the gas pump was submerged in the reservoir solution to capture dissolved H2. H2 generation experiments were performed with an applied voltage of −1.5 V. Flow rates of 50 and 800 µL/min were used to assess the streaming potential’s effect. The 800 µL/min flow rate represents the maximum rate permissible considering the pressure resistance of the microfluidic channel. For the control experiment, a zero-flow rate could not be achieved, as H2 transport to the reservoir requires some liquid flow. Therefore, the control was conducted at a low flow rate of 50 µL/min.
(a) Schematic of the loop-type device. (b) Molar amount of H2 produced at an applied voltage of −1.5 V. The average flow rates over 30 min were 460 µL/min and 24 µL/min, with pump set values indicated in brackets.
Figure 4b presents the molar amount of H2 generated with the applied voltage of −1.5 V and flow rates of 50 and 800 µL/min (set values). H2 production increased over time for both flow rates, with significantly higher production at 800 µL/min. These results indicate that the enhancement in reductive current by the streaming potential contributes directly to increased H2 production. During these experiments, the applied voltage remained fixed at −1.5 V, suggesting that the potential of both Pt electrodes shifted in a negative direction owing to the streaming potential created by liquid flow. The slopes of the H2 production in Fig. 4b leveled out after approximately 20 min, indicating a steady state. The faradaic efficiency for H2 production at 800 µL/min after 30 min was approximately 60 %. The possible reason for the decrease in faradic efficiency of H2 production on Ptdown was reactions of Cl2 and O2 molecules produced on Ptup. The use of the liquid/gas loop system that re-dissolves all the gas products into the solution may have caused in a further decrease in the faradic efficiency of H2 production. To improve the efficiency of H2 production by addressing the crossover between two electrodes, an optimization of the device design that separates the products generated at the anode and cathode is required.
This study demonstrated that the efficiency of electrochemical production can be enhanced by streaming potential in microfluidic devices. Using hydrogen evolution reaction (HER) as a model, we investigated the boosting effect of streaming potential on H2 production at the Pt electrodes placed in the flow microchannel. The reduction currents measured with the prepared microfluidic device increased with the flow rate of the HCl aqueous solution. Our results indicated that the reduction current at −1.5 V was boosted by as much as 23 % owing to streaming potential, which contributes to the decreased energy consumption required for HER. Although the flow rate was limited in this study by the mechanical durability of the microfluidic device, a higher flow rate could further enhance energy efficiency beyond 23 %. We focused on the proof of concept for highly efficient electrochemical production by combining applied voltage and streaming potential using a microfluidic system. The boosting effect of streaming potential should be improved by optimizing various parameters, such as flow rate, microchannel shape and size, electrode positioning, and electrolyte composition. Our results indicate the potential for achieving highly efficient material production through various electrochemical reactions, including substance conversion, synthesis, and decomposition using microfluidic devices. Further advancements in electrochemical production with microfluidic devices can lead to the establishment of innovative processes that may replace conventional chemical plants in the future.
This work was supported by JSPS KAKENHI Grant Number JP24K01509, and Kanazawa University SAKIGAKE Projects 2022 and 2024.
Shinji Nagamatsu: Conceptualization (Lead), Data curation (Lead), Investigation (Lead), Writing – original draft (Lead)
Masayuki Morimoto: Investigation (Lead), Methodology (Equal), Writing – original draft (Equal)
Ryosuke Izumi: Investigation (Equal), Methodology (Supporting)
Masanori Hamanaka: Investigation (Supporting), Methodology (Lead)
Akio Ohta: Methodology (Supporting), Project administration (Equal)
Hitoshi Asakawa: Conceptualization (Lead), Methodology (Equal), Project administration (Lead), Writing – review & editing (Lead)
Shinji Nagamatsu was an employee of Daicel Corporation but conducted this study as a doctoral student at Kanazawa University.
Japan Society for the Promotion of Science: JP24K01509
Kanazawa University: SAKIGAKE Project 2022
Kanazawa University: SAKIGAKE Project 2024
M. Morimoto and H. Asakawa: ECSJ Active Members
