Electrolysis-Driven and Pressure-Controlled Diffusive Growth of Successive Bubbles on Microstructured Surfaces

Life beyond Fritz: On the Detachment of Electrolytic Bubbles

We present an experimental study on detachment characteristics of hydrogen bubbles during electrolysis. Using a transparent (Pt or Ni) electrode enables us to directly observe the bubble contact line and bubble size. Based on these quantities we determine other parameters such as the contact angle and volume through solutions of the Young-Laplace equation. We observe bubbles without ("pinned bubbles") and with ("spreading bubbles") contact line spreading and find that the latter mode becomes more prevalent if the concentration of HClO4 is ≥0.1 M. The departure radius for spreading bubbles is found to drastically exceed the value predicted by the well-known formula of W. Fritz [Phys. Z. 1935, 36, 379-384] for this case. We show that this is related to the contact line hysteresis, which leads to pinning of the contact line after an initial spreading phase at the receding contact angle. The departure mode is then similar to a pinned bubble and occurs once the contact angle reaches the advancing contact angle of the surface. A prediction for the departure radius based on these findings is found to be consistent with the experimental data.

Bridging Innovations of Phase Change Heat Transfer to Electrochemical Gas Evolution Reactions

Bubbles play a ubiquitous role in electrochemical gas evolution reactions. However, a mechanistic understanding of how bubbles affect the energy efficiency of electrochemical processes remains limited to date, impeding effective approaches to further boost the performance of gas evolution systems. From a perspective of the analogy between heat and mass transfer, bubbles in electrochemical gas evolution reactions exhibit highly similar dynamic behaviors to them in the liquid-vapor phase change. Recent developments of liquid-vapor phase change systems have substantially advanced the fundamental knowledge of bubbles, leading to unprecedented enhancement of heat transfer performance. In this Review, we aim to elucidate a promising opportunity of understanding bubble dynamics in electrochemical gas evolution reactions through a lens of phase change heat transfer. We first provide a background about key parallels between electrochemical gas evolution reactions and phase change heat transfer. Then, we discuss bubble dynamics in gas evolution systems across multiple length scales, with an emphasis on exciting research problems inspired by new insights gained from liquid-vapor phase change systems. Lastly, we review advances in engineered surfaces for manipulating bubbles to enhance heat and mass transfer, providing an outlook on the design of high-performance gas evolving electrodes.

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.

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.

Solutal Marangoni effect determines bubble dynamics during electrocatalytic hydrogen evolution

Understanding and manipulating gas bubble evolution during electrochemical water splitting is a crucial strategy for optimizing the electrode/electrolyte/gas bubble interface. Here gas bubble dynamics are investigated during the hydrogen evolution reaction on a well-defined platinum microelectrode by varying the electrolyte composition. We find that the microbubble coalescence efficiency follows the Hofmeister series of anions in the electrolyte. This dependency yields very different types of H2 gas bubble evolution in different electrolytes, ranging from periodic detachment of a single H2 gas bubble in sulfuric acid to aperiodic detachment of small H2 gas bubbles in perchloric acid. Our results indicate that the solutal Marangoni convection, induced by the anion concentration gradient developing during the reaction, plays a critical role at practical current density conditions. The resulting Marangoni force on the H2 gas bubble and the bubble departure diameter therefore depend on how surface tension varies with concentration for different electrolytes. This insight provides new avenues for controlling bubble dynamics during electrochemical gas bubble formation.

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.

Arrays of Microscale Linear Ridges with Self-Cleaning Functionality for the Oxygen Evolution Reaction

Gas management during electrocatalytic water splitting is vital for improving the efficiency of clean hydrogen production. The accumulation of gas bubbles on electrode surfaces prevents electrolyte access and passivates the electrochemically active surface area. Electrode morphologies are sought to assist in the removal of gas from surfaces to achieve higher reaction rates at operational voltages. Herein, regular arrays of linear ridges with specific microscale separations were systematically studied and correlated to the performance of the oxygen evolution reaction (OER). The dimensions of the linear ridges were proportional to the size of the oxygen bubbles, and the mass transfer processes associated with gas evolution at these ridges were monitored using a high-speed camera. Characterization of the adhered bubbles prior to detachment enabled the use of empirical methods to determine the volumetric flux of product gas and the bubble residence times. The linear ridges promoted a self-cleaning effect as one bubble would induce neighboring bubbles to simultaneously release from the electrode surfaces. The linear ridges also provided preferential bubble growth sites, which expedited the detachment of bubbles with similar diameters and shorter residence times. The linear ridges enhanced the OER in comparison to planar electrodes prepared by electrodeposition from the same high-purity nickel (Ni). Linear ridges with a separation distance of 200 μm achieved nearly a 2-fold increase in current density relative to the planar electrode at an operating voltage of 1.8 V (vs Hg/HgO). The electrodes with linear ridges having a separation distance of 200 μm also had the highest sustained current densities over a range of operating conditions for the OER. Self-cleaning surface morphologies could benefit a variety of electrocatalytic gas evolving reactions by improving the efficiency of these processes.

Harnessing bubble behaviors for developing new analytical strategies

Gas bubbles are easily accessible and offer many unique characteristic properties of a gas/liquid two-phase system for developing new analytical methods. In this minireview, we discuss the newly developed analytical strategies that harness the behaviors of bubbles. Recent advancements include the utilization of the gas/liquid interfacial activity of bubbles for detection and preconcentration of surface-active compounds; the employment of the gas phase properties of bubbles for acoustic imaging and detection, microfluidic analysis, electrochemical sensing, and emission spectroscopy; and the application of the mass transport behaviors at the gas/liquid interface in gas sensing, biosensing, and nanofluidics. These studies have demonstrated the versatility of gas bubbles as a platform for developing new analytical strategies.

Concentration profile of dissolved gas during hydrogen gas evolution: an optical approach

We develop an optical image tracking technique for the simultaneous observation of a wide area in proximity to the electrode, and study the growth of bubbles during hydrogen gas evolution in alkaline water electrolysis. Using a diffusion model we can successfully extract the concentration profile of dissolved hydrogen gas as a function of distance from the electrode. The obtained concentrations agree well with the values by the electrochemical method.

Nucleation and Growth of a Nanobubble on Rough Surfaces

We study the nucleation and growth of a nanobubble on rough surfaces using molecular dynamics simulations. A nanobubble nucleates and grows by virtue of a heterogeneous surface reaction which results in the production of gas molecules near the surface. We study the role of surface roughness in the nucleation and growth behavior of a nanobubble. We perform simulations at various reaction rates and surface morphology and quantified the growth dynamics of a nanobubble. Our simulations show that after the onset of nucleation, the nanobubble grows rapidly with radius following t^1/3 behavior followed by a diffusive growth regime which is marked by t^1/2 growth behavior. This growth behavior remains independent of surface roughness and reaction rates over the range considered in this study. We also quantified the oversaturation of gas required for nucleation of a nanobubble and demonstrated its dependence on the surface morphology.

Adsorption and Desorption of a Phospholipid from Single Microbubbles under Pulsed Ultrasound Irradiation for Ultrasound-Triggered Drug Delivery

Microbubbles have potential for applications as drug and gene delivery systems, in which the release of a substance is triggered by an ultrasonic pulse. In this paper, we discuss the adsorption and desorption of a film of phospholipid on the surface of a single microbubble under ultrasound irradiation. Our optical observation system consisted of a high-speed camera, a laser Doppler vibrometer, and an ultrasound cell; 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) was used as the surfactant. The adsorption of the DMPC molecules onto the surface of the bubble was evaluated by measuring the contact angle between the bubble and a glass plate. A decrease of the contact angle of the bubble indicates desorption of the DMPC molecules from the bubble surface into the surrounding aqueous solution. The amount of DMPC molecules adsorbed on the bubble's surface is shown to decrease over time after bubble generation. The type and intensity of the pulsed ultrasound waves were varied so as to mimic ultrasound-triggered drug release. Increasing the number of cycles and the amplitude of the sound pressure of the pulsed ultrasound yielded a greater increase of the contact angle. We also measured the radial vibrations of the microbubbles in the ultrasound field. The vibrational characteristics of the microbubbles and the desorption characteristics of the DMPC molecules showed the same variation; namely, a greater sound pressure amplitude induced greater vibrational displacement and a larger amount of molecular desorption under resonance conditions. These results support the possibility of controlling drug release with pulsed ultrasound in a microbubble-based drug delivery system.