Gas-evolving Electrodes Research Articles

Self-similar solution for laminar bubbly flow evolving from a vertical plate

The development of a bubble plume from a vertical gas-evolving electrode is driven by buoyancy and hydrodynamic bubble dispersion. This canonical fluid mechanics problem is relevant for both thermal and electrochemical processes. We adopt a mixture model formulation for the two-phase flow, considering variable density (beyond Boussinesq), viscosity and hydrodynamic bubble dispersion. Introducing a new change of coordinates, inspired by the Lees–Dorodnitsyn transformation, we obtain a new self-similar solution for the laminar boundary layer equations. The results predict a wall gas fraction and gas plume thickness that increase with height to the power of 1/5 before asymptotically reaching unity and scaling with height to the power 2/5, respectively. The vertical velocity scales with height to the power of 3/5. Our analysis shows that self-similarity is only possible if gas conservation is entirely formulated in terms of the gas specific volume instead of the gas fraction.

Gas Evolution in Water Electrolysis.

Gas bubbles generated by the hydrogen evolution reaction and oxygen evolution reaction during water electrolysis influence the energy conversion efficiency of hydrogen production. Here, we survey what is known about the interaction of gas bubbles and electrode surfaces and the influence of gas evolution on practicable devices used for water electrolysis. We outline the physical processes occurring during the life cycle of a bubble, summarize techniques used to characterize gas evolution phenomena in situ and in practical device environments, and discuss ways that electrodes can be tailored to facilitate gas removal at high current densities. Lastly, we review efforts to model the behavior of individual gas bubbles and multiphase flows produced at gas-evolving electrodes. We conclude our review with a short summary of outstanding questions that could be answered by future efforts to characterize gas evolution in electrochemical device environments or by improved simulations of multiphase flows.

Exploring transition metal hydroxides performance in membrane-free electrolyzer based decoupled water splitting for step-wise production of hydrogen and oxygen

Alkaline decoupled electrolysis is an emerging safest approach for efficient green hydrogen production. Bifunctional electrocatalysts subjected to reverse polarity can be effectively used as the gas-evolving electrodes (GEE) in decoupled water electrolysis for generating both hydrogen and oxygen at different rates and at different times. Designing a bifunctional electrocatalyst with excellent activity and durability will successfully pave the way towards non-precious metal-based decoupled electrolysis systems. Here, for the first time, ternary NiFeMn layered hydroxide is presented as the bifunctional electrocatalyst associated with the β-Ni(OH)2 solid-state redox mediator (SRM) for independently generating hydrogen and oxygen in an alkaline solution without membranes. This electrocatalyst had shown low overpotentials in HER (230 mV at 10 mA/cm2) and OER (270 mV at 50 mA/cm2). In this work, a chemically synthesized hexagonal β-Ni(OH)2 nanosheet was employed as the solid-state redox mediator for fabricating the membrane-free electrolysis setup. Accompanying a bifunctional electrocatalyst in decoupled electrolysis, this device displayed a 1.91 V cell voltage at 10 mA/cm2. When considered together, the complexity and expense of the electrolyzer were decreased by using NiFeMn LH/NF as a bifunctional electrocatalyst. This approach can be considered for constructing a cost-effective and step-wise green hydrogen harvesting system of membrane-free water electrolyzer.

Roles of Wettability and Wickability on Enhanced Hydrogen Evolution Reactions.

Bubble dynamics significantly impact mass transfer and energy conversion in electrochemical gas evolution reactions. Micro-/nanostructured surfaces with extreme wettability have been employed as gas-evolving electrodes to promote bubble departure and decrease the bubble-induced overpotential. However, effects of the electrodes' wickability on the electrochemical reaction performances remain elusive. In this work, hydrogen evolution reaction (HER) performances are experimentally investigated using micropillar array electrodes with varying interpillar spacings, and effects of the electrodes' wettability, wickability as well as bubble adhesion are discussed. A deep learning-based object detection model was used to obtain bubble counts and bubble departure size distributions. We show that microstructures on the electrode have little effect on the total bubble counts and bubble size distribution characteristics at low current densities. At high current densities, however, micropillar array electrodes have much higher total bubble counts and smaller bubble departure sizes compared with the flat electrode. We also demonstrate that surface wettability is a critical factor influencing HER performances under low current densities, where bubbles exist in an isolated regime. Under high current densities, where bubbles are in an interacting regime, the wickability of the micropillar array electrodes emerges as a determining factor. This work elucidates the roles of surface wettability and wickability on enhancing electrochemical performances, providing guidelines for the optimal design of micro-/nanostructured electrodes in various gas evolution reactions.

Threshold current density for diffusion-controlled stability of electrolytic surface nanobubbles

Understanding the stability mechanism of surface micro/nanobubbles adhered to gas-evolving electrodes is essential for improving the efficiency of water electrolysis, which is known to be hindered by the bubble coverage on electrodes. Using molecular simulations, the diffusion-controlled evolution of single electrolytic nanobubbles on wettability-patterned nanoelectrodes is investigated. These nanoelectrodes feature hydrophobic islands as preferential nucleation sites and allow the growth of nanobubbles in the pinning mode. In these simulations, a threshold current density distinguishing stable nanobubbles from unstable nanobubbles is found. When the current density remains below the threshold value, nucleated nanobubbles grow to their equilibrium states, maintaining their nanoscopic size. However, for the current density above the threshold value, nanobubbles undergo unlimited growth and can eventually detach due to buoyancy. Increasing the pinning length of nanobubbles increases the degree of nanobubble instability. By connecting the current density with the local gas oversaturation, an extension of the stability theory for surface nanobubbles [Lohse and Zhang, Phys. Rev. E 91, 031003(R) (2015)] accurately predicts the nanobubble behavior found in molecular simulations, including equilibrium contact angles and the threshold current density. For larger systems that are not accessible to molecular simulations, continuum numerical simulations with the finite difference method combined with the immersed boundary method are performed, again demonstrating good agreement between numerics and theories.

Mass transport at gas-evolving electrodes

Direct numerical simulations are utilised to investigate mass-transfer processes at gas-evolving electrodes that experience successive formation and detachment of bubbles. The gas–liquid interface is modelled employing an immersed boundary method. We simulate the growth phase of the bubbles followed by their departure from the electrode surface in order to study the mixing induced by these processes. We find that the growth of the bubbles switches from a diffusion-limited mode at low to moderate fractional bubble coverages of the electrode to a reaction-limited growth dynamics at high coverages. Furthermore, our results indicate that the net transport within the system is governed by the effective buoyancy driving induced by the rising bubbles and that mechanisms commonly subsumed under the term ‘microconvection’ do not significantly affect the mass transport. Consequently, the resulting gas transport for different bubble sizes, current densities and electrode coverages can be collapsed onto one single curve and only depends on an effective Grashof number. The same holds for the mixing of the electrolyte when additionally taking the effect of surface blockage by attached bubbles into account. For the gas transport to the bubble, we find that the relevant Sherwood numbers also collapse onto a single curve when accounting for the driving force of bubble growth, incorporated in an effective Jakob number. Finally, linking the hydrogen transfer rates at the electrode and the bubble interface, an approximate correlation for the gas-evolution efficiency has been established. Taken together, these findings enable us to deduce parametrisations for all response parameters of the systems.

Self-healable, Tolerant Superaerophobic Coating for Improving Electrochemical Hydrogen Production.

Gas-evolving electrodes often suffer from the blocking of catalytic active sites-due to unwanted and unavoidable adhesion of generated gas bubbles, which elevates the overpotential for the electrochemical hydrogen evolution reaction (HER)- by raising the resistance of the electrode. Here, a catalyst-free and self-healable superaerophobic coating having ultra-low bubble adhesion is introduced for achieving significantly depleted overpotentials of 209 and 506mV at both low (50mAcm-2) and high (500mAcm-2) current densities, respectively, compared to a bare nickel-foam electrode. The optimized coating ensured an early detachment of the generated tiny (0.8±0.1mm) gas bubble-and thus, prevented the undesired rise in resistance of the coated electrode. The systematic association of physical (i.e., ionic interactions, H-bonding, etc.) cross-linkage, β-amino ester type covalent cross-linkage and reinforced halloysite nano clay enables the design of such functional material embedded with essential characteristics-including improved mechanical (toughness of 63.7kJm-3, and tensile modulus of 26kPa) property and chemical (extremes of pH (1 and 14), salinity, etc.) stability, rapid (<10min) self-healing ability (even at alkaline condition) and desired bubble-wettability (bubble contact angle of 158.2±0.2°) with ultralow force (4.2±0.4µN) of bubble adhesion.

Machine learning-guided discovery of gas evolving electrode bubble inactivation.

The adverse effects of electrochemical bubbles on the performance of gas-evolving electrodes are well known, but studies on the degree of adhered bubble-caused inactivation, and how inactivation changes during bubble evolution are limited. We study electrode inactivation caused by oxygen evolution while using surface engineering to control bubble formation. We find that the inactivation of the entire projected area, as is currently believed, is a poor approximation which leads to non-physical results. Using a machine learning-based image-based bubble detection method to analyze large quantities of experimental data, we show that bubble impacts are small for surface engineered electrodes which promote high bubble projected areas while maintaining low direct bubble contact. We thus propose a simple methodology for more accurately estimating the true extent of bubble inactivation, which is closer to the area which is directly in contact with the bubbles.

Raising the curtain: Bubble size measurement inside parallel plate electrolyzers

Alongside vertical gas-evolving electrodes, gas bubbles nucleate, grow, coalesce, detach, and ultimately form a bubble curtain, affecting the fluid dynamic, electrochemical reaction, mass transport, and ohmic drop. Due to the small inter-electrode gap in parallel plate electrolyzers, the size determination of detached bubbles inside the bubble curtain close to the electrodes is challenging. In particular, the size determination of detached oxygen bubbles needs to be addressed. We present an optical setup to record images from detached oxygen bubbles inside the bubble curtain of a membrane-separated, parallel plate electrolyzer. The images are postprocessed using an adopted region-based convolutional neuronal network to quantify the bubble sizes. Next, the electrolyte flow velocity, concentration, current density, and catalysts are varied, and the effect on the oxygen bubble size distribution is studied. The median size ranges from 140 to ▪, doubling values from literature measured in artificial electrolyzers.

An analytical model for the velocity and gas fraction profiles near gas-evolving electrodes

Understanding multiphase flow close to the electrode surface is crucial to the design of electrolyzers, such as alkaline water electrolyzers for the production of green hydrogen. Vertical electrodes develop a narrow gas plume near their surface. We apply the integral method to the mixture model. Considering both exponentially varying and step-function gas fraction profiles, we derive analytical relations for plume thickness, velocity profile, and gas fraction near the electrode as a function of height and current density. We verify these analytical relations with the numerical solutions obtained using two-dimensional mixture model simulations. We find that for low gas fractions, the plume thickness decreases with an increase in current density for an exponentially varying gas fraction profile. In contrast, the plume thickness increases with increasing current density at high gas fractions for an approximately step-function-shaped gas fraction profile, in agreement with experiments from the literature.

Surfactant Improved Interface Morphology and Mass Transfer for Electrochemical Oxygen-Evolving Reaction

The surface microstructure of a catalyst coating layer directly affects the active area, hydrophilicity and hydrophobicity, and the high porosity is desirable especially for solid–liquid–gas three-phase catalytic reactions. However, it remains challenging to customize catalyst distribution during the coating process. Here, we report a simple strategy for achieving ultrafine nanocatalyst deposition in a porous structure via introducing the surfactant into coating inks. For a proof-of-concept demonstration, we spin-coated the nanoscale IrO2 sol with a surfactant of sodium dodecyl sulfate (SDS) onto the glassy carbon (GC) electrode for oxygen evolution reaction (OER). Due to the surfactant action, the deposited IrO2 nanocatalyst is evenly distributed and interconnected into a highly porous overlayer, which facilitates electrolyte permeation, gas bubble elimination and active-site accessibility, thus affording high-performance OER in alkaline media. Particularly, the SDS-modified electrodes enable the industrial-level high-current-density performance via enhanced mass transfer kinetics. Such manipulation is effective to improve the coating electrodes’ catalytic activity and stability, and scalable for practical applications and suggestive for other gas-evolving electrodes.

Ionomer Optimization for Hydroxide-Exchange-Membrane Water Electrolyzers Operated with Distilled Water: A Modeling Study

The hydroxide-exchange-membrane water electrolyzer (HEMWE) is a promising means to store intermittent renewable energy in the form of hydrogen chemical energy. The hydroxide-exchange ionomer (HEI) in the gas-evolving electrodes and the hydroxide-exchange membrane (HEM) are key components of HEMWE. In this work, we simulate the cell and examine explicitly the impact of HEI and HEM properties with a focus on improving HEMWE performance when operated with distilled water (i.e., no supporting electrolyte). The tradeoff between the ionic conductivity gain and electrochemically active surface area (ECSA) loss is studied. For a constant catalyst loading, distributing more catalyst next to the HEM or making thinner but denser catalyst layer is beneficial for HEMWE performance. The results demonstrate that a higher water diffusion coefficient is desired for HEM to supply reactant water to the cathode. In contrast, a lower water diffusion coefficient is preferred for the cathode HEI to retain the water in the regions with high reaction rates. Overall, the findings provide important insights to optimizing HEI/HEM materials for improved HEMWE performance.

Impact of Bubbles on Electrochemically Active Surface Area of Microtextured Gas-Evolving Electrodes.

The adverse effects of electrochemical bubbles on the performance of gas-evolving electrodes have been extensively studied. However, the ways in which bubbles dynamically alter the electrochemically active surface area during bubble evolution are not well understood. Here, we study hydrogen evolution at industrially relevant current densities by using controlled microtexture to examine this fundamental relationship. Surprisingly, the most densely microtextured electrodes have the lowest performance on an active surface area basis. Using high-speed imaging, we show that the benefits of microtexture to release smaller bubbles more consistently are outweighed by the inactivation induced by bubbles growing within the denser microtexture, causing these performance limitations. Additionally, we show that the area beneath adhered bubbles is electrochemically active, contrary to currently held assumptions. Our study therefore has broad implications for electrode design to avoid ineffective use of precious catalyst materials, which is especially critical for porous electrodes and three-dimensional structures with high specific surface areas.

How to minimise destabilising effect of gas bubbles on water splitting electrocatalysts?

Development of efficient electrocatalytic gas-evolving electrodes is one of the essential prerequisites for the deployment of hydrogen-based electrochemical energy conversion and storage. Gas bubbles generated by electrolysis at electrocatalytic interfaces manifest into undesirable increase in overpotential that simultaneously compromises stability of the electrocatalytic materials. A key research question is how to use theory and advanced experimental tools to holistically understand the mechanism of gas-evolution phenomena and finally arrive at principles of electrode design that will assure facile gas evolution. The analysis given in this work offers an optimistic framework how to significantly reduce overpotential and enhance electrode stability during water electrolysis.

Hydrodynamic behavior of bubbles at gas-evolving electrode in ultrasonic field during water electrolysis

In electrochemical processes, gas bubbles on the electrode can cause an increase in both overpotential and ohmic voltage drop which leads to higher energy consumption. Applying power ultrasound during water electrolysis can help to reduce the overpotential, enhance mass transfer, and save energy. In this study, we investigated the effect of ultrasound (20 kHz) on the hydrogen evolution reaction (HER) on a stainless steel plate with varying concentrations of NaOH solutions at 298 K, using linear sweep voltammetry (LSV). We especially focused on understanding the bubble behavior on the stainless steel plate during HER using high-speed imaging in ultrasonic field. When ultrasound was applied to solutions with NaOH concentrations of 0.1, 0.5, 1 M, the current density increased by about 9.0, 5.9, 2.8 %, respectively. As the ultrasound irradiation began, the bubbles tended to hover around on the electrode surface, coalescing with other bubbles, rather than rising. When the size of the coalesced bubbles became too large to stay on the surface of the electrode, they were expelled from the ultrasonic field. The repeated collapse and coalescence of these bubbles was observed while they were rising. The velocity increased about 2 times when ultrasound irradiation began, and increased by more than 6 times in the ultrasonic field. More nucleation of bubbles was observed on the electrode in the ultrasonic field. Using ultrasound reduced the critical diameter of bubbles which detached from the electrode, from 58.0 to 15.9 μm, and the residence time of the bubbles, from 533 to 118 ms. Further, when the ultrasound was applied, the mean diameter of bubbles decreased from 71.8 to 17 μm. Hence, bubble coverage on the electrode surface decreased from 8.3 to 1 % despite an increase in the total number of bubbles. As a result, ultrasound was found to be effective for hydrogen production during water electrolysis, increasing current by the faster removal of gas from the stainless steel plate.

Efficient Mesh Interface Engineering: Insights from Bubble Dynamics in Electrocatalysis.

Electrochemical catalysis offers great potential in energy and mass conversion in academy and industry. However, bubble dynamics and its influence on gas-evolving electrode systems remain ambiguous. Detailed information on the local transport process between different phases and the underlying mechanism are required for the full understanding of two-phase flow evolution and distribution. Here, we construct a three-electrode water splitting reaction system to study the bubble dynamics and system efficiency of titanium electrodes with different morphologies. The dynamics of a gas bubble at an electrode with a plate and 100-mesh, 150-mesh, and 300-mesh structures is systematically investigated with respect to applied voltage conditions. Parameters and underlying mechanisms that influence the two-phase flow evolution and electrochemical reaction performance are carefully discussed. Finally, the underlying dynamic force balance on the gas bubble is analyzed to illustrate the mechanism and experimental observations. Our study provides insights in gas-evolving electrocatalysis and offers opportunities for the design and fabrication of high-performance electrocatalytic reactors.

Hydrodynamic Behavior of Bubbles on a Gas-Evolving Electrode in an Ultrasonic Field During Water Electrolysis

In electrochemical processes, gas bubbles on the electrode can cause an increase in both overpotential and ohmic voltage drop which leads to higher energy consumption. Applying power ultrasound during water electrolysis can help to reduce the overpotential, enhance mass transfer, and save energy. In this study, we investigated the effect of ultrasound (20 kHz) on the hydrogen evolution reaction (HER) on a stainless steel plate with varying concentrations of NaOH solutions at 298 K, using linear sweep voltammetry (LSV). We especially focused on understanding the bubble behavior on the stainless steel plate during HER using high-speed imaging when an ultrasonic field was applied. When ultrasound was applied to solutions with NaOH concentrations of 0.1, 0.5, 1 M, the current density increased by about 9.0, 5.9, 2.8%, respectively. As the ultrasound irradiation began, the bubbles tended to hover around on the electrode surface, coalescing with other bubbles, rather than rising. When the size of the coalesced bubbles became too large to stay on the surface of the electrode, they were expelled from the ultrasonic field. The repeated collapse and coalescence of these bubbles was observed while they were rising. The velocity increased about 2 times when ultrasound irradiation began, and increased by more than 6 times in the ultrasonic field. More nucleation of bubbles was observed on the electrode in the ultrasonic field. Using ultrasound reduced the critical diameter of bubbles which detached from the electrode, from 58.0 to 15.9 μm, and the residence time of the bubbles, from 533 to 118 ms. Further, when the ultrasound was applied, the mean diameter of bubbles decreased from 71.8 to 17 μm. Hence, bubble coverage on the electrode surface decreased from 8.3 to 1% despite an increase in the total number of bubbles. As a result, ultrasound was found to be effective for hydrogen production during water electrolysis, increasing current by the faster removal of gas from the stainless steel plate.

A new analysis of two phase flow on hydrogen production from water electrolysis

It is clear that the entire world have to research, develop, demonstrate and plan for alternative energy systems for shorter term and also longer term. As a clean energy carrier, hydrogen has become increasingly important. It owes its prestige to the increase within the energy costs as a result of the equivocalness in the future availability. Two phase flow and hydrogen gas flow dynamics effect on performance of water electrolysis. Hydrogen bubbles are recognized to influence energy and mass transfer in gas-evolving electrodes. The movement of hydrogen bubbles on the electrodes in alkaline electrolysis is known to affect the reaction efficiency. Within the scope of this research, a physical modeling for the alkaline electrolysis is determined and the studies about the two-phase flow model are carried out for this model. Internal and external forces acting on the resulting bubbles are also determined. In this research, the analytical solution of two-phase flow analysis of hydrogen in the electrolysis is analyzed.

Mitigating Bubble Traffic in Gas-Evolving Electrodes via Spinodally Derived Architectures

Porous electrodes are widely used in the industry because of their high surface area to volume ratio. However, the stochastic morphology of most commercially available porous electrodes results in poor electrical connections in the solid phase and inefficient mass transport through the pore phase. This can be especially detrimental for gas-evolving processes such as water electrolysis for hydrogen and oxygen generation. Bicontinuous interfacially jammed emulsion gels (bijels) offer templates from which to create porous electrodes with robust solid-state interconnectivity and a uniform pore structure that facilitate improved electron and mass transport. In this study, gas release rates and electrochemical experiments are utilized to study the effects of powder- and bijel-derived microstructures on hydrogen generation by water electrolysis. The bijel-derived electrodes are shown to expel product gas faster and require up to 25% less overpotential to drive water electrolysis over the range of current densities tested (-5 to -40 mA/cm2) than their powder-derived analogs. Our findings suggest that the uniform and bicontinuous domains of bijel-derived porous electrodes can mitigate the limited current distribution and deleterious bubble effect found in stochastic electrodes, in turn improving the overall performance of electrolytic processes requiring transport of gaseous species.

Transport-Based Modeling of Bubble Nucleation on Gas Evolving Electrodes.

Bubble nucleation is ubiquitous in gas evolving reactions that are instrumental for a variety of electrochemical systems. Fundamental understanding of the nucleation process, which is critical to system optimization, remains limited as prior works generally focused on the thermodynamics and have not considered the coupling between surface geometries and different forms of transport in the electrolytes. Here, we establish a comprehensive transport-based model framework to identify the underlying mechanism for bubble nucleation on gas evolving electrodes. We account for the complex effects on the electrical field, ion migration, ion diffusion, and gas diffusion arising from surface heterogeneities and gas pockets initiated from surface crevices. As a result, we show that neglecting these effects leads to significant underprediction of the energy needed for nucleation. Our model provides a non-monotonic relationship between the surface cavity size and the overpotential required for nucleation, which is physically more consistent than the monotonic relationship suggested by a traditional thermodynamics-based model. We also identify the significance of the gas diffuse layer thickness, a parameter controlled by external flow fields and overall electrode geometries, which has been largely overlooked in previous models. Our model framework offers guidelines for practical electrochemical systems whereby, without changing the surface chemistry, nucleation on electrodes can be tuned by engineering the cavity size and the gas diffuse layer thickness.