Transport Of Gas Bubbles Research Articles

Mitigation of Performance Degradation of PEM Water Electrolyzers by Power Distribution Control of Fluctuating Renewable Energy

Introduction Hydrogen production by water electrolysis using renewable energy such as wind and solar power generation (i.e., green hydrogen production) has attracted considerable attention as a key technology towards decarbonization. Proton exchange membrane (PEM) water electrolyzers are often used in combination with renewable energy source since they show fast response to input power fluctuation. However, lifetime of PEM electrolyzer needs to be improved for reducing hydrogen production cost. Recently we reported a durability analysis of PEM electrolyzers under wind power voltage fluctuations, where the reversible and irreversible voltage losses were analyzed in terms of gas bubble transport and catalyst degradation [1]. We also developed a novel control strategy called the power distribution control (PDC) that controls the power converters so that the input fluctuating power is decomposed to several fluctuating patterns that less degrade the electrolyzers than the input pattern [2]. The purpose of this study is to validate the effect of the PDC on mitigation of performance degradation of PEM electrolyzers under wind power fluctuations. Experimental method A membrane electrode assembly (MEA) with an active area of 1 cm2 was prepared using Nafion 117 membrane, IrO2 anode catalyst and Pt/KB cathode catalyst. The catalyst loading for anode and cathode was 0.5 mg IrO2/cm2 and 0.3 mg Pt/cm2, respectively. A PEM water electrolysis cell was prepared by assembling the MEA into a cell holder. The cell temperature was kept at 80°C during the durability test.Figure 1a) and 1b) show a schematic system diagram for performing the PDC and the two voltage fluctuation patterns (A and B) used for the durability test, respectively. As in our previous study [1], actual 24-h voltage fluctuation of wind power turbine was used as the primary data. The pattern A was prepared from the primary data by (i) scaling down the maximum voltage by 1/100 and setting the minimum voltage higher than the standard potential of water electrolysis and (ii) accelerating the time scale by 5 times. The pattern B was prepared by applying the PDC, that is, converting the pattern A into an alternative sequence of a high-operating rate pattern and a low-operating rate pattern as described in Ref. [2]. Here we set the average voltage of the high-operating rate pattern (~2.4 V) and the low-operating rate pattern (~1.7 V) as the same value with the voltage of the pattern A (~2.05 V) for each time, ensuring that we model the situation in which the input fluctuating power is distributed between two series of electrolyzers as shown in Fig. 1a). We performed 240-h durability tests and analyzed performance degradation of the PEM cell for the pattern A and B, respectively. Results and Discussion Figure 2 summarizes the performance degradation of the PEM cell during the 240-h durability test. The cell current density at the cell voltage of 2.1 V showed a steep decrease during the initial ~30 h for both pattern A and B. For 30 – 240 h, the current density continuously decreased for the pattern A (−0.68 mAcm-2/h), while no distinguishable decrease was observed for the pattern B (+0.06 mAcm-2/h). Therefore we successfully demonstrated that performance degradation of the PEM electrolyzers can be effectively suppressed by the PDC when using actual wind power voltage fluctuation as the input power.To further investigate the performance degradation behavior, the increase in overpotential components of the PEM cell was analyzed. Figure 2b) shows the comparison of overpotential components before and after the 240-h test. For the pattern A, the diffusion overpotential (E diff) showed a significant increase after the test for the current density higher than 1.0 mAcm-2. In contrast, for the pattern B, increase in E diff was suppressed and confirmed only for > 1.4 Acm-2. Increase in the activation overpotential (E act) was small compared to increase in E diff for both pattern A and B. Furthermore, we confirmed that the performance degradation confirmed in this study was reversible, that is, the current density was almost recovered within 24-h rest time after the test. These results are consistent with our previous study [1] in that the reversible degradation is related to gas bubble transport, that is, the increase in E diff. Therefore it was shown that the PDC suppresses reversible degradation, which is important for mitigating irreversible degradation during long-term operation. Our results demonstrate that PDC is a promising technique for improving lifetime of PEM electrolyzers operating under fluctuating renewable energy.

Enhancement of bubble transport in porous electrodes and catalysts.

We investigate the formation and transport of gas bubbles across a model porous electrode/catalyst using lattice Boltzmann simulations. This approach enables us to systematically examine the influence of a wide range of morphologies, flow velocities, and reaction rates on the efficiency of gas production. By exploring these parameters, we identify critical parameter combinations that significantly contribute to an enhanced yield of gas output. Our simulations reveal the existence of an optimal pore geometry for which the product output is maximized. Intriguingly, we also observe that lower flow velocities improve gas production by leveraging coalescence-induced bubble detachment from the electrode/catalyst.

Enhanced Transport in Electrochemical Reactors Using Advanced 3D Printed Electrodes

Water electrolyzers employ porous architectures to facilitate the transport of mass and charge. Ensuring effective transport can improve energy efficiency and unlock stable operation at higher current densities. In order to realize these improvements, however, research needs to show the relationship between pore structure and performance. Advances in 3D printing research have enabled the manufacture of functional, electrically conductive porous materials at ever finer length scales and larger build areas. Using electrodes manufactured via projection microstereolithography (PuSL), we first show how this structural control can lead to computationally optimized electrodes which minimize the total power losses of a model single phase reaction. The physics of gas bubble transport in water electrolyzers is more complicated, however. At higher operating current densities, entrained gas bubbles in the triple phase boundary can be a source of degradation by occupying active sites and impeding the flow of ions between electrodes. Using structured pores, controlled surface tension boundaries offer an unprecedented degree of control over gas bubble transport. Integrating these ordered structures into water electrolyzers has the potential to improve the stability of devices at high current densities by guiding bubbles away from the reaction zone and into designed outlets.

Microporous structure flowing integrated with electrolytic water splitting based on stacked mesh electrode for hydrogen production

In order to promote gas bubble transport during water splitting for hydrogen production, stacked mesh electrodes integrated microporous structure flowing and electrolytic water splitting have been fabricated by electrodepositing CoFe alloy. The average size of the bubble on the mesh electrode surface is 40 μm and the release time of trapped bubbles in the pores of the electrode is 2.11 s during the process of water splitting at 50 mA cm−2 and 1 M KOH. The electrodes show excellent performance in both hydrogen evolution reaction (HER, 247 mV at 1000 mA cm−2) and oxygen evolution reaction (OER, 350 mV at 1000 mA cm−2). The required cell voltage at 1000 mA cm−2 for flow-through electrolyzer assembled with the stacked mesh electrodes is only 2 V under the condition of 6 M KOH, and 343 K. The voltage caused by the bubble in the flow-through electrolyzer assembled with stacked mesh electrodes is just about 50 mV at the current density of 1000 mA cm−2. The energy required for hydrogen production is only 4.04 kWh Nm−3 and 4.78 kWh Nm−3 under the current density of 200 mA cm−2 and 1000 mA cm−2, respectively. Besides, the flow-through electrolyzer assembled with the stacked mesh electrodes shows excellent stability with only a 60 mV increase in cell voltage after continuing electrolytic water splitting for 100 h at 1000 mA cm−2.

Numerical study on oxygen transport pattern in porous transport layer of proton exchange membrane electrolysis cells

A transient, two-dimensional and two-phase numerical model is established to study the nature of oxygen transport in proton exchange membrane electrolysis cells (PEMEC). Randomly generated circle fibers are used to construct a patterned porous transport layer (PTL), and the volume of fluid (VOF) method is employed to capture accurate gas-liquid interfaces. The patterns of oxygen transport and the relationship between multiple transport paths are analyzed in detail. We propose for the first time the concept of ‘isolation belt’, which is composed of a series of narrow throats, hindering the spontaneous combination of different oxygen transport paths. The influence of PTL perforation and spatially-graded fiber diameters on oxygen transport are explored. The numerical results show that perforation can probabilistically destroy the ‘isolation belt’, forming a transport path with high priority, which helps to reduce oxygen saturation. Moreover, when the porosity of PTL remains unchanged, reducing the fiber diameter in the region near the catalyst layer (CL) can significantly reduce the local oxygen saturation. The pressure variation reveals its consistency with the maximum throat width of the oxygen transport path, which indicates the law of capillary pressure. From the perspective of mathematical probability theory, a novel functional expression of the profile of oxygen saturation with respect to the structural properties of PTL is proposed to make a concise explanation of gas bubble transport in the context of PEMEC or similar gas-liquid porous flow. The highlights in this paper include: • Comprehensive spatiotemporal profile for in the process of oxygen penetration. • The concept of isolation belt for spontaneous merging of oxygen pathways. • A mathematical explanation for probability of oxygen saturation distribution. • Influence of PTL perforation and graded fiber diameters on oxygen saturation.

Understanding the impact of catholyte flow compartment design on the efficiency of CO2 electrolyzers

Catholyte flow compartment design impacts the CO2RR product selectivity by influencing gas bubble transport and local pH. According to the hydrodynamic volcano model, an optimal catholyte fluid velocity enables the highest CO2 reduction selectivity.

Optical in Situ Analysis of Gas Bubble Evolution in Electrochemical Machining Based on Similarity Theory Using Particle Image Velocimetry

Numerical simulations are used to shorten the tool development process for precise electrochemical machining. However, these simulations are often based on simplified models that disregard bubble transport or have not been validated. Therefore, this paper presents optical in situ measurements of gas bubble transport during precise electrochemical machining. These measurements, by means of particle image velocimetry on an upscaled working gap in accordance with the principle of dynamic similarity, are then compared to simulation results. It was observed that due to the coalescence of the gas bubbles, the bubble size grows rapidly and obstructs the fluid flow.

Remarkable gas bubble transport driven by capillary pressure in 3D printing-enabled anisotropic structures for efficient hydrogen evolution electrocatalysts

Additive manufacturing technologies have been proved as a promising method to achieve electrocatalysts with periodic micro-size pores, while the nano-sized interspace of the material structures and their corresponding gas bubbles transfer process are not explored in detail. Herein, we employ the shear force alignment in additive manufacturing to design NiMo-based structures with anisotropic porous channels as electrocatalysts for hydrogen evolution reaction (HER) in seawater. Based on the complementary experimental and theoretical investigation, the unique anisotropic structure not only fully exposes the active sites in the electrolyte, but also facilitates the rapid electrolyte-hydrogen phase conversion during electrochemical reactions. In this case, the obtained 3D electrode exhibits superior electrocatalytic performance and excellent long-term operational stability with an extremely low overpotential of ∼150 mV at a current density of 500 mA/cm2 in 1 M KOH seawater. This work will provide a practical scenario for designing highly-efficient HER electrocatalysts.

Lattice Boltzmann simulations of gas bubble transport in proton exchange membrane water electrolysis cells

Gas/liquid two-phase transport is the key fundamental scientific issue in the proton exchange membrane (PEM) water electrolysis cell, which has important effects on the overall performance of water electrolysis and microstructure optimization. In this study, a combination of the lattice Boltzmann two-phase model and the QSGS numerical method was used to numerically model the porous diffusion layer and the micro-channel structure, and investigate the effects of porosity and wettability properties on the gas/liquid two-phase transport process. Simulation results show that the value of porosity will obviously affect the association and distribution pattern of solid particles inside the porous diffusion layer, and the reduction of porosity will cause the association part of solid particles to be narrower and longer, resulting in complex and variable pore channels. Some closed pore channels will appear inside the diffusion layer, which is not conducive to the smooth transmission of gas bubbles. In addition, the increase of contact angle will enhance the interaction force between the bubble and the solid wall, making it difficult for the bubble to fall off from the solid surface. Moreover, the increased interaction force will slow down the sliding speed of gas bubbles, which will result in the fusion of more gas bubbles and increase the risk of blocking the flow micro-channel. This study has initially grasped the mechanisms of micro/meso scale gas bubble transport in the PEM water electrolysis cell, which will provide theoretical basis for the optimized development of high-performance water electrolysis system for hydrogen production.

Stimuli-Responsive Liquid-Crystal-Infused Porous Surfaces for Manipulation of Underwater Gas Bubble Transport and Adhesion.

Biomimetic artificial surfaces that enable the manipulation of gas bubble mobility have been explored in a wide range of applications in nanomaterial synthesis, surface defouling, biomedical diagnostics, and therapeutics. Although many superhydrophobic surfaces and isotropic-lubricant-infused porous surfaces have been developed to manipulate gas bubbles, the simultaneous control over the adhesion and transport of gas bubbles underwater remains a challenge. Thermotropic liquid crystals (LCs), a class of structured fluids, provide an opportunity to tune the behavior of gas bubbles through LC mesophase transitions using a variety of external stimuli. Using this central idea, the design and synthesis of LC-infused porous surfaces (LCIPS) is reported and the effects of the LC mesophase on the transport and adhesion of gas bubbles on LCIPS immersed in water elucidated. LCIPS are demonstrated to be a promising class of surfaces with an unprecedented level of responsiveness and functionality, which enables the design of cyanobacteria-inspired object movement, smart catalysts, and bubble gating devices to sense and sort volatile organic compounds and control oxygen levels in biomimetic cell cultures.

Gas injection test of remolded saturated soil with consolidation

Investigation of gas migration in saturated soil is of great theoretical and practical significance in marine science and engineering. The transport of gas bubbles in soil and its effects on soil physical and mechanical properties can be further studied. Gas injection and permeability tests were conducted using a self-developed oedometer with gas injection and water infiltration. Before the gas injection test, the remolded saturated soil was consolidated in an oedometer device to an effective vertical stress of 100 kPa. Gas injection tests were then performed to investigate the transport mechanism of bubbles in saturated soil and the changes in water permeability after gas injection. The results show that it is difficult for gas to migrate through capillary invasion and fracturing from the pore scale owing to the high gas entry pressure in fine-grained sediments. Hence, initial gas migration mechanism in pore scale is elastic-plastic expansion. The scale of cavities expands from micron to millimeter during gas injection, and gas pressure for fracturing reduces with the expansion of cavities. The fracturing initiated in millimeter scale and gas invades further into the soil. Interconnections of cavities and cracks resulting in an increasing of drainage rate and coefficient of permeability.

Ladderlike Conical Micropillars Facilitating Underwater Gas-Bubble Manipulation in an Aqueous Environment.

Underwater gas-bubble manipulation in aqueous environments is of great importance in industry and academia. Although the underwater gas bubble has been proved to be directionally transportable by various structures, transporting gas bubbles in 3D space remains a challenge. In this research, two kinds of tapered pillars, that is, ladderlike and helical ladderlike, were proposed for manipulating gas bubbles. To fabricate such unique structures, an improved alternative coating and etching method was developed. To meet the requirements of underwater gas-bubble transport, a modified gas-bubble slippery technology was also developed to enhance the aerophilic ability. The dynamics of the gas bubble was analyzed using a high-speed camera. The Laplace force that resulted from the geometry gradient was found to play a significant role in tuning the gas-bubble velocity. Through adjustments on the wettability, tilt angle, and geometry of each section of the tapered pillar, tuning the transport velocity from 113.9 ± 10.3 to 309.1 ± 5.8 mm/s becomes possible. On the basis of these findings, the helical ladderlike tapered pillar was fabricated and demonstrated to be able to transport gas bubbles in 3D space. These results may provide a new and systematic way to design and fabricate materials and structures for directional gas-bubble transport in 3D space.

Smart Manipulation of Gas Bubbles in Harsh Environments Via a Fluorinert-Infused Shape-Gradient Slippery Surface

Fundamental research and practical applications have examined the manipulation of gas bubbles on open surfaces in low-surface-tension, high-pressure, and high-acidity, -alkalinity, or -salinity environments. However, to the best of our knowledge, efficient and general approaches to achieve the smart manipulation of gas bubbles in these harsh environments are limited. Herein, a Fluorinert-infused shape-gradient slippery surface (FSSS) that could effectively regulate the behavior of gas bubbles in harsh environments was successfully fabricated. The unique capability of FSSS was mainly attributed to the properties of Fluorinert, which include chemical inertness and incompressibility. The shape-gradient morphology of FSSS could induce asymmetric driving forces to move gas bubbles directionally on open surfaces. Factors influencing gas bubble transport on FSSS, such as the apex angle of the slippery surface and the surface tension of the aqueous environment, were carefully investigated, and large apex angles were found to result in large initial transport velocities and short transport distances. Lowering the surface tension of the aqueous environment is unfavorable to bubble transport. Nevertheless, FSSS could transport gas bubbles in aqueous environments with surface tensions as low as 28.5 ± 0.1 mN/m, which is lower than that of many organic solvents (e.g., formamide, ethylene glycol, and dimethylformamide). In addition, FSSS could also realize the facile manipulation of gas bubbles in various aqueous environments, e.g., high pressure (~ 6.8 atm), high acidity (1 mol/L H2SO4), high alkalinity (1 mol/L NaOH), and high salinity (1 mol/L NaCl). The current findings provide a source of knowledge and inspiration for studies on bubble-related interfacial phenomena and contribute to scientific and technological developments for controllable bubble manipulation in harsh environments.

Bioinspired Slippery Cone for Controllable Manipulation of Gas Bubbles in Low-Surface-Tension Environment.

Manipulating bubbles in surfactant solutions or oil mediums is of vital importance in daily life and industries concerned with cosmetics, food, fermentation, mineral flotation, etc. However, realizing controllable regulation of a bubble's behavior is quite challenging in a low-surface-tension aqueous environment, which is mainly attributed to the strong affinity of liquid molecules to a solid surface to prevent the efficient interaction of gas bubbles with the solid surface. To address these issues, herein, we have taken inspiration from cactus spines and pitcher plants to develop a slippery copper cone (SCC), which can facilely manipulate gas bubble in surfactant solutions (as low as ∼29.9 mN/m, 20 °C), e. g., directional and continuous transportation of gas bubbles. This intriguing capability mainly originates from the cooperation of the conical morphology engendering a Laplace pressure and the slippery surface with low friction force but high affinity to bubbles. In addition, the SCC also shows an elegant capability of transporting gas bubbles in various organic solvents, such as formamide (57.4 mN/m, 20 °C), glycol (46.5 mN/m, 20 °C), dibutyl phthalate (37.0 mN/m, 20 °C), and dimethylformamide (35.8 mN/m, 20 °C). Furthermore, the prepared SCC also demonstrated distinguished feasibility in antibuoyancy bubble delivery, efficient collection of acidic CO2 microbubbles, and the underwater reaction of hydrogen and oxygen, endowing it with promising applications in various complex low-surface-tension environments.

Femtosecond laser fabrication of shape-gradient platform: Underwater bubbles continuous self-driven and unidirectional transportation

The capture and directional transportation of gas bubbles in an aqueous environment are important aspects of modern science and technology including methane harvesting, CO2 capture/reduction and electrochemical H2 generation and collection. However, designing and fabricating a device that functions to both harvest air bubbles from the aqueous solution and to directionally transport those bubbles with no external force remains a challenge. In this work, we develop a facile method to fabricate micro/nanoscale hierarchical structures on a trapezoidal platform using femtosecond laser direct cutting and surface structuring technology. The geometry-gradient platform surface exhibits superhydrophobicity in air and superaerophilicity in water and allows air bubbles to self-transport from the minor side onto the large side but not in the opposite direction. Furthermore, the as-prepared platform exhibits robust stability in corrosion and friction tests. In addition, a superaerophilic trapezoidal platform with a cross-shaped structure is developed that spontaneously captures, unidirectionally transports, and collects bubbles in a supersaturated gas solution. This superaerophilic trapezoidal platform exhibits tremendous potential in applications for advanced self-driven transport devices and is a promising tool in solving energy and environment problems.

A hierarchical superaerophilic cone: Robust spontaneous and directional transport of gas bubbles

In this paper, we present a rapid and simple method to fabricate superaerophilic polytetrafluoroethylene cones via a two-step femtosecond laser direct writing technique, which enormously improved processing efficiency compared to the existing method. The laser-treated cones contained hierarchical microstructures and exhibited self-driven and directional transport of gas bubbles along the cones and away from the tip, even when the cones were horizontal. Furthermore, the laser-treated cones exhibited excellent chemical and long-term stability. This work may provide an effective and simple approach to obtain efficient manipulation of gas bubbles in practical applications.

Robust laser-structured asymmetrical PTFE mesh for underwater directional transportation and continuous collection of gas bubbles

We report a simple, efficient method to fabricate micro/nanoscale hierarchical structures on one side of polytetrafluoroethylene mesh surfaces, using one-step femtosecond laser direct writing technology. The laser-treated surface exhibits superhydrophobicity in air and superaerophilicity in water, resulting in the mesh possessing the hydrophobic/superhydrophobic asymmetrical property. Bubbles can pass through the mesh from the untreated side to the laser-treated side but cannot pass through the mesh in the opposite direction. The asymmetrical mesh can therefore be designed for the directional transportation and continuous collection of gas bubbles in aqueous environments. Furthermore, the asymmetrical mesh shows excellent stability during corrosion and abrasion tests. These findings may provide an efficient route for fabricating a durable asymmetrical mesh for the directional and continuous transport of gas bubbles.

Spontaneous and Directional Bubble Transport on Porous Copper Wires with Complex Shapes in Aqueous Media.

Manipulation of gas bubble behaviors is crucial for gas bubble-related applications. Generally, the manipulation of gas bubble behaviors generally takes advantage of their buoyancy force. It is very difficult to control the transportation of gas bubbles in a specific direction. Several approaches have been developed to collect and transport bubbles in aqueous media; however, most reliable and effective manipulation of gas bubbles in aqueous media occurs on the interfaces with simple shapes (i.e., cylinder and cone shapes). Reliable strategies for spontaneous and directional transport of gas bubbles on interfaces with complex shapes remain enormously challenging. Herein, a type of 3D gradient porous network was constructed on copper wire interfaces, with rectangle, wave, and helix shapes. The superhydrophobic copper wires were immersed in water, and continuous and stable gas films then formed on the interfaces. With the assistance of the Laplace pressure gradient between two bubbles, gas bubbles (including microscopic gas bubbles) in the aqueous media were subsequently transported, continuously and directionally, on the copper wires with complex shapes. The small gas bubbles always moved to the larger ones.

Investigation of Influence Factors of Surface Roughness at Corner Area in ECM Process

Surface quality of workpiece with sudden dimension change processed by electrochemical machining (ECM) is affected by many factors, such as the electrolyte flow pattern, gap between tool and workpiece electrodes, machining by-products such as gas bubbles, etc. In this paper, surface polishing process of a workpiece with sudden dimension change was considered. The electrolyte flow velocity distribution and vortex formed due to the sudden dimension change was discussed. Then, the transport of gas bubbles by the flow and its effect on the current density distribution in the electrolyte as well as the effect of current density distribution on the generation of bubbles were also investigated. Results show that current distribution on the electrode surface was greatly affected by the generation and accumulation of gas bubbles around the corner area due to the sudden dimension change of flow channel geometry, which leads to poor surface quality near the workpiece corner.

Directional and Continuous Transport of Gas Bubbles on Superaerophilic Geometry‐Gradient Surfaces in Aqueous Environments

AbstractDue to the direct and sufficient contacting with the aqueous environment, the directional and continuous transport of gas bubbles on open surface without energy input will advance a variety of applications in heat transfer, selective aeration, water electrolysis, etc. Unfortunately, the behaviors of gas bubbles in aqueous environment are mainly dominated by the buoyancy moving gas bubbles upward, resulting in their difficult manipulation. Therefore, realizing the directional and continuous transport of gas bubbles on open surface still remains a great challenge. Herein, a novel strategy integrating the superaerophilic wettability with geometry‐gradient structure is proposed, which can engender high driving force and low hysteresis resistance force acting on the gas bubbles. In experiment, these fabricated superaerophilic geometry‐gradient polyethylene surfaces demonstrate distinguished performance of directionally and continuously transporting gas bubbles on open surfaces without energy input. In addition, the antibuoyancy bubble transportation device and the underwater bubble microreactor are successfully prepared in this manuscript, both of which illustrate the feasibility in the applications of complex environment and gas‐related fields. It can be envisioned that this study will promote the understanding and development of underwater functional superwettability materials to achieve the directional and continuous transport of gas bubbles on the open surface.