Bubble Detachment Research Articles

Mechanical Analysis of the Critical Conditions for Trapping and Detachment of Microscale Air Bubbles on the Pure Water Freezing Front.

Icing is a widespread phase change phenomenon with implications for daily life and industrial production. Air bubbles form on the freezing front of pure water with dissolved air during the icing, which may affect the physical properties of ice. Controlling the behavior of air bubbles will be one method to change the physical properties of ice. To analyze the critical conditions for trapping and detachment of microscale air bubbles on a pure water freezing front, a mathematical model describing the forces on air bubble is developed on the basis of the principle of force equilibrium. Results show that the average accuracy of the present model in predicting the average air bubble detachment radius is about 62%, which is 30% higher than the model with the best prediction accuracy in the literature. Buoyant, temperature gradients, and hydrodynamic forces push air bubbles to detach from the freezing fronts, while adhesion force and gravity impede their detachment. Temperature gradient and adhesion forces are the main factors affecting the detachment of air bubbles from freezing fronts. The temperature gradient has the greatest effect on the air bubble detachment radius, while the tilt angle and liquid density have a lesser effect. When the temperature gradient is increased from 1000 to 10 000 K/m, the air bubble detachment radius decreases by 37.78%. Studying the forces acting on the air bubbles on the pure water freezing front is an important reference for the production of special ice bodies, phase change cold storage, and de-icing technology.

A multi-year study of acoustic propagation and ambient sound in a Thalassia testudinum seagrass meadow in a shallow sub-tropical lagoona).

Seagrasses provide a multitude of ecosystem services and act as important carbon sinks. However, seagrass habitats are declining globally, and they are among the most threatened ecosystems on earth. For these reasons, long-term and continuous measurements of seagrass parameters are of primary importance for ecosystem health assessment and sustainable management. This paper presents results from both active and passive acoustical methods for ecosystem monitoring in seagrass meadows. From a propagation perspective, gas bodies contained within the seagrass tissue as well as photosynthetic-driven bubble production result in attenuation, dispersion, and scattering of sound that produce increased transmission loss. For the passive approach, the detachment of gas bubbles from the plants is an important component of the ambient soundscape. Examples of both techniques will be presented based on data collected as part of a two-year continuous deployment of an acoustical measurement system operating in a moderately dense seagrass bed dominated by Thalassia testudinum (turtle grass) in Corpus Christi Bay, Texas. The data show annual trends related to the seasonal growth pattern of Thalassia as well as diurnal trends correlated with photosynthetically active radiation.

Experimental study of bubble growth and detachment behavior under different constraint strengths in a von Kármán swirling flow

Experimental study of bubble growth and detachment behavior under different constraint strengths in a von Kármán swirling flow

Science and Engineering of Superaerophobic Surfaces for Electrochemical Gas—Evolving Reactions: A Review of Recent Advances and Perspective

AbstractIn energy conversion processes and various industries, gas evolution reactions (GERs) play an important role. To achieve a future without fossil fuels, the development of high‐efficiency electrocatalysts is necessary, as they directly affect the catalytic performance and overall efficiency of reactions. In addition to the discovery of highly active catalysts, the rapid removal of gaseous products on the electrode surface is equally important for GERs. The adherence of bubbles to the electrode surface introduces substantial resistance, significantly diminishing the system's efficiency. One promising solution to reduce the adhesion of bubbles is the development of electrocatalysts with superaerophobic levels. These surface structures, such as nanotubes, nanosheets, and nanowires, prevent gas bubbles from adhering and promote their rapid removal from the electrode. The aim of this review is first to obtain a deep understanding of mechanisms related to the creation of superaerophobic surfaces, including their characteristics, methods of creation, and bubble detachment behavior. Furthermore, recent advances in the application of these surfaces in various gas‐evolving reactions to enhance electrocatalytic properties are discussed. By taking this innovative approach, valuable insights can be gained into advancing the field of electrocatalysis and driving progress toward sustainable energy solutions.

Numerical study of flow boiling on microcavity surface under the action of an electric field using lattice Boltzmann method

The microcavity setting promotes bubble nucleation, and the presence of an electric field enables bubble detachment. In this study, the synergistic effect of electric field and microcavity in improved flow boiling heat transfer was analyzed based on the pseudopotential lattice Boltzmann method. The investigation was focused on the influence of wall superheat, Reynolds (Re) number, electric field intensity, liquid subcooling, and gravitational acceleration on the flow boiling performance under two forms of electric field distributions for uniform conducting microcavity surface (MC–UCS) and conducting–insulating microcavity surface (MC–CIS). Compared with the MC–UCS, the MC–CIS maintained a stable wetting state in the microcavity through inhibition of sliding and merging of bubbles. The increase in electric field intensity and Re number improved convective heat transfer in the microcavity but at the expense of a larger frictional pressure drop. The boiling phenomenon on the microcavity surface weakened with the increase in liquid subcooling, which caused difficulty for the electric field to improve the heat transfer performance by improving bubble behavior. Electrical force can replace buoyancy force to perturb the vapor–liquid interface under low gravitational acceleration. This condition allows the fluid shear force to drive bubbles away from the microcavity surface quickly. The results can provide theoretical and technical guidance for the realization of the enhanced synergistic heat transfer between electric fields and microstructures.

Effects of ultrasound on bubble dynamic behavior of flow boiling in microchannel

Bubble dynamics is paramount in comprehending the heat transfer mechanisms of flow boiling in the microchannel within ultrasonic field, which is regarded as a promising method to confront challenges of thermal management posed by microelectronic devices. Nevertheless, the impact of ultrasound on bubble behaviors and its underlying mechanisms remain largely unexplored. This study first delves into the effect of ultrasonic parameters on bubble dynamic behaviors and associated mechanisms, subsequently further analyzing the forces acting on bubbles through the constructed force model. The findings suggest that although growth force serves as the significant resistance, the primary Bjerknes force dominates the rapid detachment of bubbles. The secondary Bjerknes force results in the bubble only sliding along the bottom wall rather than lifting off. Furthermore, the elevated ultrasonic pressure amplitude resulting from augmenting ultrasonic power induces a substantial increase in the critical detachment diameter and growth rate by 55.49% and 59.42%, respectively. The enhanced primary Bjerknes force, attributed to the rise in ultrasonic frequency, leads to a 71.42% increase in sliding velocity and a 46.45% reduction in growth time. The positive impacts arising from ultrasonic power and frequency are anticipated to notably enhance the thermal performance of microchannels. Besides, surface tension acts as the resistance and diminishes slightly with an augmentation of the boiling number, resulting in a moderate variation in sliding velocity and growth time.

Numerical investigation on the influence of porous media structural parameters on pool boiling heat transfer performance

This work investigates pool boiling heat transfer (BHT) and bubble dynamics from a porous medium. The influence of the porous media structural parameters, such as porosity, pore density, porous medium height, thermal conductivity, and wettability, are mainly investigated. The findings indicate that the presence of porous media can increase the critical heat flux (CHF) by an average of 3.75 times and the BHT coefficient by an average of 3.84 times when porosity varies between 57.5% and 98.0% as compared to the plain surface. It is also found that both the CHF and BHT coefficient increase as the porosity decreases if porosity ε≥71.4%. However, they drop with the porosity decreases if porosity ε≤71.4%. On the other hand, the number of nucleation sites, heat transfer area, and bubble escape resistance increase as pore density increases. In addition, increasing the porous media height may enhance BHT performance, but too high a porous media increases the bubble escape resistance and restricts the separation of bubbles. Moreover, the CHF value and the maximum BHT coefficient increase with the thermal conductivity of porous media linearly. Finally, the stronger the wettability, the faster the bubble detachment, and the stronger the BHT performance.

Effect of the advancing contact angle on coal particle–bubble detachment at the mesoscopic scale

Improving particle–bubble adhesion stability and reducing particle–bubble detachment are crucial in froth flotation processes. In this study, a nonequilibrium molecular dynamics approach is applied to investigate the dynamic detachment process of low-rank coal particles adhering with/without a collector from bubbles under different pull forces at the mesoscopic scale. Results indicate that a collector oil film on the surface of a low-rank coal particle can considerably increase the advancing contact angle and counteract the contraction of the three-phase contact line, enabling the particle to withstand a greater detachment force. Conversely, the residual water clusters on the particle surface tend to cause contraction of the contact line. Force analysis of the liquid surrounding the particle and the contact line indicates that the maximum static capillary force between the particle and the bubble is equivalent to the capillary force when the bending angle of the liquid surface reaches the advancing contact angle. When the bending angle of the liquid surface exceeds the forward contact angle, the force on the liquid around the contact line becomes unbalanced, causing the contact line to move to decrease the bending angle of the liquid surface and restore balance to the surrounding liquid. Hence, on mesoscopic scales of ten to tens of nanometers, the advancing contact angle still determines particle–bubble detachment. The study of particle–bubble detachment mechanisms at the mesoscopic scale is expected to establish a relation between microscopic mechanisms at the molecular scale and macroscopic experimental studies at the micrometer to millimeter scale

Numerical Simulation and Experimental Research on Effect of Flow Regimes for Refrigeration Performance of Wave Rotor

Abstract Wave rotors exchange energy through the interaction of fluids with different pressures to produce a cooling effect without the application of mechanical parts, which makes them suitable for the complex flow field environments. But the flow situation inside the wave rotor is complex, and research on the impact of various flow regimes for the refrigeration efficiency is currently incomplete. This study deals with the numerical simulation and the experimental verification of a four-port fixed rotor gas wave refrigerator. Various typical flow regimes within the wave rotor were obtained through simulation to analyze their impact on the refrigeration efficiency. To further demonstrate the effectiveness of the simulation results, experimental verification was conducted. The simulation results indicate that separation bubbles and vortices will be respectively generated on the upper and lower walls of the wave rotor tube during the gradual opening stage. Vortex affects the airflow flow and causes detachment of separation bubbles as the airflow moves, which results in the loss of pressure energy and kinetic energy of the incident gas. As a result, the loss caused by the gradual opening process leads to a decrease in the self-expansion ability of high-pressure gas during the gradual closing stage, thereby affecting the refrigeration effect. Therefore, reducing the influence of vortices plays an important role in improving the cooling efficiency of wave rotors. Finally, this study analyzes the main factors that affect the cooling effect within the wave rotor through flow regime and provides new insights for improving refrigeration efficiency.

Two-phase closed thermosiphon with grooved the evaporator section applied for low-grade energy recovery: Thermo-hydrodynamic performance and potentials

High thermal conductivity and simple structure of two-phase closed thermosyphons (TPCTs) facilitate it extensively applied in low-grade thermal recovery, such like geothermal and solar energy exploitation. In present work, by incorporating threaded grooves into the evaporator section of the TPCTs, heat transfer performance could be significantly enhanced. Thermal transport mechanism of thermosyphons with various threaded groove structures was comprehensively investigated, and corresponding thermo-hydrodynamic performance was modelled through CFD/VOF simulations. Numerical procedures were validated well after comparison with experimental ones felled within 5.0 %. Under various heating power levels and filling ratios, heat transfer performance of thermosyphons with threaded grooves evidently surpassed that of smooth thermosyphons. Depending on delicate numerical results, critical point at 33 % filling ratio and 90 W heating power were confirmed that total thermal resistance of the grooved thermosyphon decreased almost close to 4.6 %. Furthermore, deviating from that critical point, higher heating power and lower filling ratio instead confined heat transfer performance of the threaded TPCT, which was primarily due to the rate of bubble generation and growth being significantly was slower than its detachment rate from the wall. In addition, heat and mass transfer performance of thermosyphons with threaded grooves of various pitches and apex angles were further analyzed under various conditions. Our results indicated that, when the groove apex angle turned to 90°, overall thermal transportation capability of the TPCT achieved optimal, with a decline in thermal resistance of up to 6.5 % compared to thermosyphons with smaller apex angles. Apex angle primarily affected the number of nucleation sites and the bubble detachment rate. Additionally, as the screw pitch decreased, boiling heat transfer enhancement, leading to more robust boiling heat transfer of TPCTs. Under high heating power, thermosyphons with larger screw pitch exhibited superior heat transfer performance. Overall, this innovative TPCTs with threaded grooves could be well applied in low-grade energy exploitation, particularly like as geothermal and solar energy fields.

Superhydrophilic and Underwater Superaerophobic Dual-Function Peony-Shaped Selenide Micro-nano Array Self-Supported Electrodes for High-Efficiency Overall Water Splitting Driven by Renewable Energy.

With the intensification of global environmental pollution and resource scarcity, hydrogen has garnered significant attention as an ideal alternative to fossil fuels due to its high energy density and nonpolluting nature. Consequently, the urgent development of electrocatalytic water-splitting electrodes for hydrogen production is imperative. In this study, a superwetting selenide catalytic electrode with a peony-flower-shaped micronano array (MoS2/Co0.8Fe0.2Se2/NixSey/nickel foam (NF)) was synthesized on NF via a two-step hydrothermal method. The optimal catalytic activity of cobalt-iron selenide was achieved by adjusting the Co/Fe ratio. The intrinsic catalytic activity of the electrodes was enhanced by incorporating transition metal selenides, which then served as a precursor for the subsequent loading of MoS2 nanoflowers on the surface to fully expose the active sites. Furthermore, the superwetting properties of the electrode accelerated electrolyte penetration and electron/mass transfer, while also facilitating bubble detachment from the electrode surface, thereby preventing "bubble shielding effect". This resulted in superior oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) performance, as well as overall water splitting capabilities. In a 1.0 M KOH solution, the electrode required only 166 and 195 mV overpotential to achieve a current density of 10 mA cm-2 for OER and HER, respectively. When functioning as a bifunctional catalytic electrode, only 1.60 V of voltage was necessary to drive the electrolyzer to reach a current density of 10 mA cm-2. Moreover, laboratory simulations of wind and solar energy-driven water splitting validated the feasibility of establishing a sustainable energy-to-hydrogen production chain. This work provides new insights into the preparation of low-overpotential, high-catalytic-activity superhydrophilic and underwater superaerophobic catalytic electrodes by rationally adjusting elemental ratios and exploring changes in electrode surface wettability.

Development of multi-view 3D reconstruction system for bubble flow measurement

This work introduces the development of bubble measurement method utilizing a three-dimensional (3D) reconstruction technique from multi-view images. Multiple synchronized cameras were positioned around a water container, and calibration was performed to obtain external and internal camera parameters. Images of bubbles emerging from a nozzle were captured, and a machine learning technique was used to extract bubble silhouettes as foreground probability distributions, enabling the extraction of bubbles from images obtained with a simple lighting setup. These distributions were then projected onto a 3D voxel space using the visual hull method. It was confirmed that the method can successfully capture bubble generation, detachment, and rise behaviors, offering insights into understanding bubble dynamics and potential applications in 3D computational fluid dynamics validation.

Bionic Janus microfluidic hydrogen production with high gas–liquid separation efficiency

Water electrolysis offers the advantages of emission-free and highly efficient energy conversion in terms of sustainable energy development and environmental pollution. However, water electrolysis is strongly affected by the detachment of bubbles from the electrodes commonly enabled by buoyancy, thereby reducing the performance of the electrolytic cells in harsh environments like in space. Herein, a bionic Janus microchannels with active gas–liquid separation for water electrolysis is proposed. There is a Janus membrane with superaerophilic top surface and inner micropores over the microchannels, and such a unique type of microchannels can capture and unidirectionally manipulate the hydrogen (H2) bubbles generated on the surface of the electrode during the water electrolysis reaction at high speed with long-term stability. It is worth noting that the gas–liquid separation through the Janus membrane does not hinder the flow of electrolyte in the microchannel due to its hydrophilic inside surfaces, which can maintain the great stability of the electrolysis. Moreover, mimicked leaves and trees for water catalysis are further demonstrated with ultrahigh electrolysis efficiency based on the active detachment of H2 bubbles enabled by Janus micropores. The present bionic Janus microchannels for high-efficient water electrolysis to produce H2 is intended to provide a new power supply solution for human space living and exploration.

Pulsed dynamic electrolysis enhanced PEMWE hydrogen production: Revealing the effects of pulsed electric fields on protons mass transport and hydrogen bubble escape

The transition of hydrogen sourcing from carbon-intensive to water-based methodologies is underway, with renewable energy-powered proton exchange membrane water electrolysis (PEMWE) emerging as the preeminent pathway for hydrogen production. Despite remarkable advancements in this field, confronting the sluggish electrochemical kinetics and inherent high-energy consumption arising from deteriorated mass transport within PEMWE systems remains a formidable obstacle. This impediment stems primarily from the hindered protons mass transfer and the untimely hydrogen bubbles detachment. To address these challenges, we harness the inherent variability of electrical energy and introduce an innovative pulsed dynamic water electrolysis system. Compared to constant voltage electrolysis (hydrogen production rate: 51.6 mL h−1, energy consumption: 5.37 kWh Nm−3 H2), this strategy (hydrogen production rate: 66 mL h−1, energy consumption: 3.83 kWh Nm−3 H2) increases the hydrogen production rate by approximately 27% and reduces the energy consumption by about 28%. Furthermore, we demonstrate the practicality of this system by integrating it with an off-grid photovoltaic (PV) system designed for outdoor operation, successfully driving a hydrogen production current of up to 500 mA under an average voltage of approximately 2 V. The combined results of in-situ characterization and finite element analysis reveal the performance enhancement mechanism: pulsed dynamic electrolysis (PDE) dramatically accelerates the enrichment of protons at the electrode/solution interface and facilitates the release of bubbles on the electrode surface. As such, PDE-enhanced PEMWE represents a synergistic advancement, concurrently enhancing both the hydrogen generation reaction and associated transport processes. This promising technology not only redefines the landscape of electrolysis-based hydrogen production but also holds immense potential for broadening its application across a diverse spectrum of electrocatalytic endeavors.

Tip-intensified engineering of interfacial microenvironment toward pH-universal hydrogen evolution reaction

Engineering a robust non-platinum electrode toward hydrogen evolution reaction (HER) is important for water electrolysis. Here we develop a nanotip-structured electrode through a solution phase etching procedure. Based on the experimental and finite element simulation, the results show that the nanotip-structured design of electrodes cannot only lower the adhesion force of bubbles, but also induce a local-concentrated electric field that can promote the formation of Marangoni effect, and K+ concentrated and local acid-like microenvironment, consequently facilitating the detachment of bubbles and the HER kinetics. The electrochemical measurements and density function theory (DFT) calculation indicate that the constructed electrode exhibits a low H* binding energy and an outstanding HER performance that outperforms the commercial Pt/C in pH-universal medium. Overall, this work provides a feasible strategy for electrode design with a pH-universal feasibility by precisely constructing electrode interfaces.

Superaerophobic Ni3N/Ni@W2N3 multi-heterointerfacial nanoarrays for efficient alkaline electrocatalytic hydrogen evolution reaction

The hydrogen evolution performances of electrocatalysts are greatly restricted by their unfavorable hydrogen evolution kinetics and the undesirable accumulation of as-evolved gas bubbles on the electrocatalysts surface. Herein, we present the fabrication of self-supporting multi-heterointerfacial nanoarrays electrode toward addressing these challenges simultaneously. A monolith Ni3N/Ni@W2N3 electrode is prepared, which exhibits a multi-heterojunction interface between the different components. The construction of this multi-heterojunction interface allows for the redistribution of the electrons, thus alleviating the strong Ni-H bond and optimizing the hydrogen adsorption free energy toward more efficient HER catalysis. Moreover, by virtue of the distinct nanoarray structure, the Ni3N/Ni@W2N3 nanoarrays exhibit improved hydrophilicity and superaerophobicity compared with Ni/Ni3N, facilitating water contact and enabling more efficient detachment of the as-evolved H2 bubbles from the surface. Benefiting from these attractive features, the Ni3N/Ni@W2N3 nanoarrays deliver an attractive alkaline HER catalytic activity with a low overpotential of 66 mV to achieve a current density of 10 mA cm−2, which is comparable with that of the benchmark Pt/C electrode. Moreover, the electrode exhibits remarkable stability during long-term electrolysis. The simultaneous interface and nanostructure engineering provide a feasible pathway for obtaining high-performance electrodes and beyond for various electrochemical reactions.

Bridging the Gap: Unraveling the Mechanisms of Nanobubble Detachment from Electrode Surfaces

Gas evolution reactions play a fundamental role in the functionality of various electrochemical devices. Recent experimental studies have provided valuable insights into the behavior of gas bubble formation on nanoelectrodes; however, the experiments have limited spatial resolution to fully understand the underlying mechanisms governing the formation of nanobubbles and their stability on the surface as well as the effects of nanoscopic surface defects on their behavior. A comprehensive understanding of how bubbles behave on electrode surfaces and in the electrolyte is essential for optimizing and designing electrochemical processes. In this work, we employ all-atom molecular simulations to explore the effects of surface chemistry and defect geometry on detachment behavior of hydrogen gas bubbles on nickel electrodes at molecular level. Analyzing the dynamics of bubble detachment on the surfaces with different chemistries reveals that the rate of bubble detachment is higher on the strongly hydrophilic surface compared to the surface exhibiting weak hydrophilicity. In addition, our findings indicate that the defect geometry has significant effects on the detachment of bubbles from the surface. Specifically, the width and depth of canonical-shaped defects are demonstrated to be critical parameters affecting the behavior of the hydrogen bubbles on the nickel surface. Overall, our results show there is a complex balance between the degree of surface wettability and surface defects, determining the stability of nanobubble formation on the electrode surfaces. The results of this study could enhance our understanding of bubble detachment dynamics on the electrode surfaces, providing valuable insights for the future design of practical applications.This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC5207NA27344. Release: LLNL-ABS-858292

Nanocone-Modified Surface Facilitates Gas Bubble Detachment for High-Rate Alkaline Water Splitting

Alkaline water splitting (AWS) is one of the most attractive technologies for green hydrogen production. In comparison to the acidic proton exchange membrane (PEM), AWS offers greater flexibility in using non-platinum group metal catalysts and could generate higher-purity hydrogen. However, commercial AWS systems are normally operated at a lower current density (~400 mA cm-2) than PEM electrolyzers. To rapidly generate hydrogen at an industrial scale, it is critical to boost the operating current density of AWS to a level of ~1000 mA cm-2. Nevertheless, the significant amount of gas bubbles generated during high-rate AWS could be detrimental to the process. Bubbles can block the catalysts’ active site, hinder electrolyte diffusion, and increase Ohmic resistance, thus deteriorating the reaction rate and restricting the cell voltage efficiency. The detachment of gas bubbles may also damage the catalyst layer. In this study, we demonstrate a general strategy for facilitating bubble detachment by modifying the nickel electrode surface with nickel nanocone nanostructures, which turns the surface into underwater superaerophobic. The simulation and experimental data show that bubbles take a considerably shorter time to detach from the nanocone-modified nickel foil than the unmodified foil. As a result, these bubbles also have a smaller detachment size and less chance for bubble coalescence. The nanocone-modified electrodes, including nickel foil, nickel foam, and 3D-printed nickel lattice, all show substantially reduced overpotentials at 1000 mA cm-2 compared to their pristine counterpart. The electrolyzer assembled with two nanocone-modified nickel lattice electrodes retains >95% of the performance after testing at ~900 mA cm-2 for 100 hours and the surface NC structure is also well preserved, demonstrating its excellent electrochemical and mechanical stability at high current density. Our findings offer an exciting and simple strategy for enhancing the bubble detachment and, thus, the electrode activity for high-rate AWS.

Exploiting Non-Classical Nucleation for Improved Gas Bubble Mitigation on Nickel Electrocatalysts for the Oxygen Evolution Reaction

The efficient transport of gas poses a significant challenge in the design of high-performance gas evolving electrocatalysts. Inefficiencies due to mass transport limitations become more pronounced at the high current densities required in industrial electrolyzers, largely as a result of gas bubbles that obstruct a portion of the electrochemically active surface area of the anode and/or cathode, while also hindering electrolyte transport to the electrode surfaces. One promising approach to mitigate these sources of inefficiency is to engineer micro-to-nanoscale surface textures that promote the favorable growth and release of these bubbles. A range of structures have been shown to improve electrocatalyst performance under both stagnant and induced convection conditions, particularly in applications for hydrogen production via electrocatalytic water splitting in alkaline water electrolyzers.1 Trends between surface texture and electrocatalyst performance have been observed for a range of features, such as pillars,2,3 ridges,4 dimples,5 “cracked” structures,6 and electrodeposited crystalline structures,7 however substantial work is still needed to optimize the designs of these materials. These textured surfaces can influence bubble evolution in various ways, such as by altering the wetting properties of a surface, or by providing preferential nucleation sites. Previous studies have noted that surface structures that provide good control over bubble nucleation can often be adversely affected by increased bubble retention.3–5 The present study investigates a method of fabricating patterned electrocatalysts that provides a fine, tunable control over the distribution of bubble nucleation sites on the surfaces of planar Ni electrocatalysts for the oxygen evolution reaction (OER). These textured surfaces are designed to take advantage of so-called “non-classical nucleation” or “type IV nucleation”, a scenario in which gas is trapped at specific surface sites with an effective radius larger than the critical radius for classical nucleation, thereby allowing spontaneous bubble growth without being required to overcome an activation energy barrier.8 The separation between these sites on the surface of the OER active material (i.e., Ni) is then varied to obtain the most favorable distribution of these non-classical nucleation sites. Chronoamperometry is used to compare the current densities achieved by these patterned Ni electrodes to that of a planar Ni electrode. High speed videography is used to directly measure how these textures influence processes such as bubble growth and detachment, and to determine how these processes influence performance. A better understanding of these gas evolution processes can help lower the energy requirements for electrolyzers and may have more general implications for the design of energy efficient gas evolving electrocatalysts relevant to a variety of industrially important reactions.

Numerical Investigation on Two-Phase Flow-Electrochemical Reaction Coupling in PEM Water Electrolyzers Porous Transport Layer Under High Operating Pressures

Proton exchange membrane water electrolyzers (PEMWEs) operated at high pressures have advantages such as lower energy consumption, reduced noise, and absence of component slippage. They have a more robust potential for commercial applications than other electrolyzers. The transport characteristics of two-phase flow within the porous transport layer (PTL) and channels are not well understood, especially under high-pressure conditions. The gas bubbles generated in the catalyst layer (CL) pass through the PTL and reach the flow channel. However, oxygen evolution reaction in the anode CL may be limited if these bubbles block the pathway of liquid water, causing reactant starvation. The interaction between electrochemical reactions and gas bubbles thus directly impacts a PEMWE's efficiency. This study aims to investigate the coupling between the two-phase flow and electrochemical reactions under high-pressure conditions.A two-dimensional (2D) CFD model was developed to simulate the bubbly flow through a reconstructed PTL. The volume of fluid (VOF) method was employed to capture bubble growth, movement, and detachment, as illustrated in Figure 1. The coupling of bubble transport and electrochemical reactions was achieved through the application of a weighted averaging method. A cross-section of a stochastically reconstructed titanium felt model was generated as the computational domain; see Fig. 1(a). The computation domain consists of a liquid water channel, a PTL, and a simplified CL interface at the bottom. The CL's reaction rate is modeled as a function of local bubble coverage on the CL interface. Transient simulations of the two-phase flow of the PEMWE anode side were carried out for pressures ranging from 1 bar to 200 bar. The simulation results elucidate the flow characteristics of liquid water in the PTL under high-pressure conditions and quantitatively reveal the coupling between two-phase flow and electrochemical reactions. The behavior of the two-phase flow in PEMWE is found to be sensitive to operating pressure. As the operating pressure increases, the size of bubbles generated on the CL interface decreases, resulting in a higher permeability of liquid water than that under low pressures. When the flow rate of liquid water in the channel is sufficiently high, bubbles do not fully develop and are rapidly removed from the domain. This phenomenon significantly shifts the two-phase flow regime within the channels,. Furthermore, the activation overpotential is substantially reduced under high-pressure conditions. The present study demonstrates a numerical simulation tool for high-pressure PEMWE. A future study of a full 3D PTL model is currently underway. Figure 1