Critical Nuclei Size, Rate, and Activation Energy of H2 Gas Nucleation

The smallest electrochemical bubbles

Many of the relevant electrochemical processes in the context of catalysis or energy conversion and storage, entail the production of gases. This often implicates the nucleation of bubbles at the interface, with the concomitant blockage of the electroactive area leading to overpotentials and Ohmic drop. Nanoelectrodes have been envisioned as assets to revert this effect, by inhibiting bubble formation. Experiments show, however, that nanobubbles nucleate and attach to nanoscale electrodes, imposing a limit to the current, which turns out to be independent of size and applied potential in a wide range from 3 nm to tenths of microns. Here we investigate the potential-current response for disk electrodes of diameters down to a single-atom, employing molecular simulations including electrochemical generation of gas. Our analysis reveals that nanoelectrodes of 1 nm can offer twice as much current as that delivered by electrodes with areas four orders of magnitude larger at the same bias. This boost in the extracted current is a consequence of the destabilization of the gas phase. The grand potential of surface nanobubbles shows they can not reach a thermodynamically stable state on supports below 2 nm. As a result, the electroactive area becomes accessible to the solution and the current turns out to be sensitive to the electrode radius. In this way, our simulations establish that there is an optimal size for the nanoelectrodes, in between the single-atom and ∼3 nm, that optimizes the gas production.

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.

A Systematic Study on Long-acting Nanobubbles: Current Advancement and Prospects on Theranostic Properties

Delivery of diagnostic drugs via nanobubbles (NBs) has shown to be an emerging field of study. Due to their small size, NBs may more easily travel through constricted blood vessels and precisely target certain bodily parts. NB is considered the major treatment for cancer treatment and other diseases which are difficult to diagnose. The field of NBs is dynamic and continues to grow as researchers discover new properties and seek practical applications in various fields. The predominant usage of NBs in novel drug delivery is to enhance the bioavailability, and controlled drug release along with imaging properties NBs are important because they may change interfacial characteristics including surface force, lubrication, and absorption. The quick diffusion of gas into the water was caused by a hypothetical film that was stimulated and punctured by a strong acting force at the gas/water contact of the bubble. In this article, various prominent aspects of NBs have been discussed, along with the long-acting nature, and the theranostical aspect which elucidates the potential marketed drugs along with clinical trial products. The article also covers quality by design aspects, different production techniques that enable method-specific therapeutic applications, increasing the floating time of the bubble, and refining its properties to enhance the prepared NB's quality. NB containing both analysis and curing properties makes it special from other nano-carriers. This work includes all the possible methods of preparing NB, its application, all marketed drugs, and products in clinical trials.

Hydrodynamics-Controlled Single-Particle Electrocatalysis

Electrocatalysis is considered promising in renewable energy conversion and storage, yet numerous efforts rely on catalyst design to advance catalytic activity. Herein, a hydrodynamic single-particle electrocatalysis methodology is developed by integrating collision electrochemistry and microfluidics to improve the activity of an electrocatalysis system. As a proof-of-concept, hydrogen evolution reaction (HER) is electrocatalyzed by individual palladium nanoparticles (Pd NPs), with the development of microchannel-based ultramicroelectrodes. The controlled laminar flow enables the precise delivery of Pd NPs to the electrode-electrolyte interface one by one. Compared to the diffusion condition, hydrodynamic collision improves the number of active sites on a given electrode by 2 orders of magnitude. Furthermore, forced convection enables the enhancement of proton mass transport, thereby increasing the electrocatalytic activity of each single Pd NP. It turns out that the improvement in mass transport increases the reaction rate of HER at individual Pd NPs, thus a phase transition without requiring a high overpotential. This study provides new avenues for enhancing electrocatalytic activity by altering operating conditions, beyond material design limitations.

Performance Enhancement of Electrocatalytic Hydrogen Evolution through Coalescence-Induced Bubble Dynamics

The evolution of electrogenerated gas bubbles during water electrolysis can significantly hamper the overall process efficiency. Promoting the departure of electrochemically generated bubbles during (water) electrolysis is therefore beneficial. For a single bubble, a departure from the electrode surface occurs when buoyancy wins over the downward-acting forces (e.g., contact, Marangoni, and electric forces). In this work, the dynamics of a pair of H2 bubbles produced during the hydrogen evolution reaction in 0.5 M H2SO4 using a dual platinum microelectrode system is systematically studied by varying the electrode distance and the cathodic potential. By combining high-speed imaging and electrochemical analysis, we demonstrate the importance of bubble-bubble interactions in the departure process. We show that bubble coalescence may lead to substantially earlier bubble departure as compared to buoyancy effects alone, resulting in considerably higher reaction rates at a constant potential. However, due to continued mass input and conservation of momentum, repeated coalescence events with bubbles close to the electrode may drive departed bubbles back to the surface beyond a critical current, which increases with the electrode spacing. The latter leads to the resumption of bubble growth near the electrode surface, followed by buoyancy-driven departure. While less favorable at small electrode spacing, this configuration proves to be very beneficial at larger separations, increasing the mean current up to 2.4 times compared to a single electrode under the conditions explored in this study.

Solutal Marangoni force controls lateral motion of electrolytic gas bubbles

Electrochemical gas-evolving reactions have been widely used for industrial energy conversion and storage processes. Gas bubbles form frequently at the electrode surface due to a small gas solubility, thereby reducing the effective reaction area and increasing the over-potential and ohmic resistance. However, the growth and motion mechanisms for tiny gas bubbles on the electrode remains elusive. Combining molecular dynamics (MD) and fluid dynamics simulations (CFD), we show that there exists a lateral solutal Marangoni force originating from an asymmetric distribution of dissolved gas near the bubble. Both MD and CFD simulations deliver a similar magnitude of the Marangoni force of ∼0.01 nN acting on the bubble. We demonstrate that this force may lead to lateral bubble oscillations and analyze the phenomenon of dynamic self-pinning of bubbles at the electrode boundary.

Revealing the Heterogeneous Bubble Nucleation at Individual Silica Nanoparticles

Deep understanding of the bubble nucleation process is universally important in systems, from chemical engineering to materials. However, due to its nanoscale and transient nature, effective probing of nucleation behavior with a high spatiotemporal resolution is prohibitively challenging. We previously reported the measurement of a single nanobubble nucleation at a nanoparticle using scanning electrochemical cell microscopy, where the bubble nucleation and formation were inferred from the voltammetric responses. Here, we continue the study of heterogeneous bubble nucleation at interfaces by regulating the local nanostructures using silica nanoparticles with a distinct surface morphology. It is demonstrated that, compared to the smooth spherical silica nanoparticles, the raspberry-like nanoparticles can further significantly reduce the nucleation energy barrier, with a critical peak current about 23% of the bare carbon surfaces. This study advances our understanding of how surface nanostructures direct the heterogeneous nucleation process and may offer a new strategy for surface engineering in gas involved energy conversion systems.

Molecular Mechanisms of the Generation and Accumulation of Gas at the Interface

Gas-evolving reactions are widespread in chemical and energy fields. However, the generated gas will accumulate at the interface, which reduces the rate of gas generation. Understanding the microscopic processes of the generation and accumulation of gas at the interface is crucial for improving the efficiency of gas generation. Here, we develop an algorithm to reproduce the process of catalytic gas generation at the molecular scale based on the all-atom molecular dynamics simulations and obtain the quantitative evolution of the gas generation, which agrees well with the experimental results. In addition, we demonstrate that under an external electric field, the generated gas molecules do not accumulate at the electrode surface, which implies that the electric field can significantly increase the rate of the gas generation. The results suggest that the external electric field changes the structure of the water molecules near the electrode surface, making it difficult for gas molecules to accumulate on the electrode surface. Furthermore, it is found that gas desorption from the electrode surface is an entropy-driven process, and its accumulation at the electrode surface depends mainly on the competition between the entropy and the enthalpy of the water molecules under the influence of the electric field. These results provide deep insight into gas generation and inhibition of gas accumulation.

Nucleation of surface nanobubbles in electrochemistry: Analysis with nucleation theorem

The formation of single bubbles at nanoelectrodes during electrochemical reactions allows to accurately identify the critical nucleus for bubble formation. As demonstrated before, combining nanoelectrode experiments and an analysis approach based on classical nucleation theory (CNT) delivers useful insight into bubble nucleation. In this work we propose an alternative approach to analyze the critical nuclei by applying the nucleation theorem (NT), which is able to overcome the inherent shortcomings of CNT. The size of the critical nucleus can be calculated more accurately by fitting experimental data in a simple form of the NT. Simulating the local gas concentration using a finite element approach, and considering the effect of gas oversaturation on the interfacial tension and the real gas compressibility, we obtain a more realistic estimation of the critical nuclei morphology. With the NT-based analysis presented, we re-analyze the nucleation data reported before. The properties of the critical nuclei obtained here are roughly consistent with those obtained from the CNT-based approach. In addition, we confirm that the critical nucleus for bubble formation in high gas oversaturation is featured with a contact angle much larger than Young's contact angle.

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.

Electrifying Carbon Capture by Developing Nanomaterials at the Interface of Molecular and Process Engineering

ConspectusCarbon capture is an indispensable step toward closing the anthropogenic carbon cycle. However, the large-scale implementation of conventional thermochemical carbon capture technologies is hindered by their low energy efficiency, limited sorbent stability, and complexity in infrastructure integration. A mechanistically different alternative, commonly known as electrochemically mediated carbon capture (EMCC), has garnered increasing research traction over the past few years and relies on electrochemical stimuli instead of thermal or pressure swings for the capture and release of carbon dioxide (CO2). Compared to conventional methods, EMCC can be operated under mild conditions driven by intermittent renewable energy sources and has a flexible design to meet the multiscale demands of carbon capture, offering a potentially sustainable, energy-efficient, and cost-effective solution to CO2 concentration from dilute mixtures or the ambient environment.Nanomaterials have played a crucial role in carbon capture research. For instance, nanoporous materials can provide increased free volumes, surface areas, and active sites for carbon capture through physical or chemical adsorption from the gaseous phase. In contrast, EMCC relies on chemical absorption via acid-base interactions using solubilized CO2 in electrolytes. Therefore, most EMCC sorbents and mediators explored so far have been developed as molecules rather than nanomaterials. In recent years, our team has been focusing on electrifying the carbon capture processes at the molecular, materials, and process levels. We seek to address the most pressing issues associated with EMCC, either in fixed-bed or flow systems, that prevent their practical use. These issues include parasitic reactions with molecular oxygen, insufficient electrode capacity utilization, sorbent crossover, etc. To address these problems, there is an urgent need to develop rationally designed nanomaterials at the interface of molecular electrochemistry and device engineering. This Account provides an overview of recent progress on developing new chemistries and engineering batch/continuous processes for EMCC. We discuss the limitations of current EMCC technology and emphasize why nanomaterials are critical for electrifying carbon capture. First, we introduce the design principles for EMCC sorbents based on redox-active organic CO2 carriers and discuss metrics for their performance evaluation. Second, we showcase how molecular design can tackle problems of sorbent solubility, oxygen stability, and electrolyte compatibility in EMCC. Third, we discuss the early results of nanomaterials as solid sorbents in fixed-bed systems, nonswelling membranes for flow systems, and high-surface-area gas-liquid contactors. Finally, building on the foundation we established through our prior work, we offer perspectives on future directions for nanomaterials to help address the challenges in EMCC.

Nanobubble Stability and Formation on Solid-Liquid Interfaces in Open Environments

Are surface nanobubbles transient or thermodynamically stable structures? This question remained controversial until recently, when the stability of gas nanobubbles at solid-liquid interfaces was demonstrated from thermodynamic arguments in closed systems, establishing that bubbles with radii of hundreds of nanometers can be stable at modest supersaturations if the gas amount is finite. Here we develop a grand-canonical description of bubble formation that predicts that nanobubbles can nucleate and remain thermodynamically stable in open boundaries at high supersaturations when pinned to hydrophobic supports as small as a few nanometers. While larger bubbles can also be stable at lower supersaturations, the corresponding barriers are orders of magnitude above kT, meaning that their formation cannot proceed via heterogeneous nucleation on a uniform solid interface but must follow some alternative path. Moreover, we conclude that a source of growth-limiting mechanism, such as pinning or gas availability, is necessary for the thermodynamic stabilization of surface bubbles.

Nanoelectrochemistry in electrochemical phase transition reactions

Electrochemical phase transition is important in a range of processes, including gas generation in fuel cells and electrolyzers, as well as in electrodeposition in battery and metal production. Nucleation is the first step in these phase transition reactions. A deep understanding of the kinetics, and mechanism of the nucleation and the structure of the nuclei and nucleation sites is fundamentally important. In this perspective, theories and methods for studying electrochemical nucleation are briefly reviewed, with an emphasis on nanoelectrochemistry and single-entity electrochemistry approaches. Perspectives on open questions and potential future approaches are also discussed.

A Single-Entity Method for Actively Controlled Nucleation and High-Quality Protein Crystal Synthesis

Lack of controls and understanding in nucleation, which proceeds crystal growth and other phase transitions, has been a bottleneck challenge in chemistry, materials, biology, and other fields. The exemplary needs for better methods for biomacromolecule crystallization include (1) synthesizing crystals for high-resolution structure determinations in fundamental research and (2) tuning the crystal habit and thus the corresponding properties in materials and pharmaceutical applications. Herein, a deterministic method is established capable of sustaining the nucleation and growth of a single crystal using the protein lysozyme as a prototype. The supersaturation is localized at the interface between a sample and a precipitant solution, spatially confined by the tip of a single nanopipette. The exchange of matter between the two solutions determines the supersaturation, which is controlled by electrokinetic ion transport driven by an external potential waveform. Nucleation and subsequent crystal growth disrupt the ionic current limited by the nanotip and are detected. The nucleation and growth of individual single crystals are measured in real time. Electroanalytical and optical signatures are elucidated as feedbacks with which active controls in crystal quality and method consistency are achieved: five out of five crystals diffract at a true atomic resolution of up to 1.2 Å. As controls, those synthesized under less optimized conditions diffract poorly. The crystal habits during the growth process are tuned successfully by adjusting the flux. The universal mechanism of nano-transport kinetics, together with the correlations of the diffraction quality and crystal habit with the crystallization control parameters, lay the foundation for the generalization to other materials systems.

Dynamic Electrodeposition on Bubbles: An Effective Strategy toward Porous Electrocatalysts for Green Hydrogen Cycling

ConspectusClosed-loop cycling of green hydrogen is a promising alternative to the current hydrocarbon economy for mitigating the energy crisis and environmental pollution. It stores energy from renewable energy sources like solar, wind, and hydropower into the chemical bond of dihydrogen (H₂) via (photo)electrochemical water splitting, and then the stored energy can be released on demand through the reverse reactions in H₂-O₂ fuel cells. The sluggish kinetics of the involved half-reactions like hydrogen evolution reaction (HER), oxygen evolution reaction (OER), hydrogen oxidation reaction (HOR), and oxygen reduction reaction (ORR) limit its realization. Moreover, considering the local gas-liquid-solid triphase microenvironments during H₂ generation and utilization, rapid mass transport and gas diffusion are critical as well. Accordingly, developing cost-effective and active electrocatalysts featuring three-dimensional hierarchically porous structures are highly desirable to promote the energy conversion efficiency. Traditionally, the synthetic approaches of porous materials include soft/hard templating, sol-gel, 3D printing, dealloying, and freeze-drying, which often need tedious procedures, high temperature, expensive equipment, and/or harsh physiochemical conditions. In contrast, dynamic electrodeposition on bubbles using the in situ formed bubbles as templates can be conducted at ambient conditions with an electrochemical workstation. Moreover, the whole preparation process can be finished within minutes/hours, and the resulting porous materials can be employed as catalytic electrodes directly, avoiding the use of polymeric binders like Nafion and the consequent issues like limited catalyst loading, reduced conductivity, and inhibited mass transport.In this Account, we summarize our contributions to the dynamic electrodeposition on bubbles toward advanced porous electrocatalysts for green hydrogen cycling. These dynamic electrosynthesis strategies include potentiodynamic electrodeposition that linearly scans the applied potentials, galvanostatic electrodeposition that fixes the applied currents, and electroshock which quickly switches the applied potentials. The resulting porous electrocatalysts range from transition metals to alloys, nitrides, sulfides, phosphides, and their hybrids. We mainly focus on the 3D porosity design of the electrocatalysts by tuning the electrosynthesis parameters to tailor the behaviors of bubble co-generation and thus the reaction interface. Then, their electrocatalytic applications for HER, OER, overall water splitting (OWS), biomass oxidation (to replace OER), and HOR are introduced, with a special emphasis on the porosity-promoted activity. Finally, the remaining challenges and future perspective are also discussed. We hope this Account will encourage more efforts into this attractive research field of dynamic electrodeposition on bubbles for various energy catalytic reactions like carbon dioxide/monoxide reduction, nitrate reduction, methane oxidation, chlorine evolution, and others.

Tailoring Pore Size and Catalytic Activity in Cobalt Iron Layered Double Hydroxides and Spinels by Microemulsion-Assisted pH-Controlled Co-Precipitation

Cobalt iron containing layered double hydroxides (LDHs) and spinels are promising catalysts for the electrochemical oxygen evolution reaction (OER). Towards development of better performing catalysts, the precise tuning of mesostructural features such as pore size is desirable, but often hard to achieve. Herein, a computer-controlled microemulsion-assisted co-precipitation (MACP) method at constant pH is established and compared to conventional co-precipitation. With MACP, the particle growth is limited and through variation of the constant pH during synthesis the pore size of the as-prepared catalysts is controlled, generating materials for the systematic investigation of confinement effects during OER. At a threshold pore size, overpotential increased significantly. Electrochemical impedance spectroscopy (EIS) indicated a change in OER mechanism, involving the oxygen release step. It is assumed that in smaller pores the critical radius for gas bubble formation is not met and therefore a smaller charge-transfer resistance is observed for medium frequencies.

Quantitative Description of Bubble Formation in Response to Electrolyte Engineering

The green hydrogen economy is expected to play a crucial role in carbon neutrality, but industrial-scale water electrolysis requires improvements in efficiency, operation costs, and capital costs before broad deployment. Electrolysis operates at a high current density and involves the substantial formation of gaseous products from the electrode surfaces to the electrolyte, which may lead to additional resistance and a resulting loss of efficiency. A detailed clarification of the bubble departure phenomena against the electrode surface and the surrounding electrolytes is needed to further control bubbles in a water electrolyzer. This study clarifies how electrolyte properties affect the measured bubble detachment sizes from the comparisons with analytical expressions and dynamic simulations. Bubble behavior in various electrolyte solutions and operating conditions was described using various physical parameters. A quantitative relationship was then established to connect electrolyte properties and bubble departure diameters, which can help regulate the bubble management through electrolyte engineering.

Toward Precision Deposition of Conductive Charge-Transfer Complex Crystals Using Nanoelectrochemistry

The lack of understanding for precise synthesis and assembly of nano-entities remains a major challenge for nanofabrication. Electrocrystallization of a charge-transfer complex (CTC), tetrathiafulvalene bromide (TTF)Br, is studied on micro/nanoelectrodes for precision deposition of functional materials. The study reveals new insights into the entire CTC electrocrystallization process from the initial nanocluster nucleation to the final elongated crystals with hollow ends grown from the working electrode to the neighboring receiving electrode. On microelectrodes, the number of nucleation sites is reduced to one by lowering the applied overpotential or precursor concentration. Certain current-time transients exhibit significant induction periods prior to stable nucleus growth. The induction regime contains small fluctuating current spikes consistent with stochastic formation of precritical nanoclusters with lifetimes of 0.1-30 s and sizes of 20-160 nm. Electrochemical analyses further reveal rate, size distribution, and formation/dissipation dynamics of the nanoclusters. Crystal growth of (TTF)Br is further studied on triangular nanoelectrode patterns with thickness of 5-500 nm, which shows a mass-transfer-controlled process applicable for precision deposition of functional (TTF)Br crystals. This study, for the first time, establishes CTC nanoelectrochemistry as a platform technology for precise deposition of conductive crystal assemblies spanning the source and drain electrode for sensing applications.

Confined Electrochemical Behaviors of Single Platinum Nanoparticles Revealing Ultrahigh Density of Gas Molecules inside a Nanobubble

Understanding the basic physicochemical properties of gas molecules confined within nanobubbles is of fundamental importance for chemical and biological processes. Here, we successfully monitored the nanobubble-confined electrochemical behaviors of single platinum nanoparticles (PtNPs) at a carbon fiber ultramicroelectrode in HClO₄ and H₂O₂ solution. Due to the catalytic decomposition of H₂O₂, a single oxygen nanobubble was formed on individual PtNPs to block the active surface of particles for proton reduction and to suppress their stochastic motion, resulting in significantly distinguished current traces. Furthermore, the combination of theoretical calculations and high-resolution electrochemical measurements allowed the nanobubble size and the oxygen gas density inside a single nanobubble to be quantified. Moreover, the ultrahigh oxygen density inside (1046 kg/m³) was revealed, indicating that gas molecules in a nanosized space existed with a high state of aggregation. Our approach sheds light on the gas aggregation behaviors of nanoscale bubbles using single-entity electrochemical measurements.

Critical Nucleus Size and Activation Energy of Ag Nucleation by Electrochemical Observation of Isolated Nucleation Events

The induction times for electrodeposition of individual Ag nanoparticles on Pt nanodisk electrodes in acetonitrile were used to determine the critical nucleus size and activation energy barrier associated with the formation of Ag nuclei. Induction times for the nucleation and growth of a single Ag nanoparticle were determined following the application of a potential step to reduce Ag+ at overpotentials, η, ranging from -130 to -70 mV. Sufficiently small Pt electrodes (5.1 × 10-10-2.6 × 10-11 cm2) were used to ensure that the detection of a single Ag nucleation event occurred during the experimental observation time (150 ms-1000 s). Multiple measurements of Ag nucleation induction times were recorded to determine nucleation rates as a function of η using cumulative probability theory. Both classical nucleation theory (CNT) and the atomistic theory of electrochemical nucleation were employed to analyze experimental nucleation rates, without a requisite knowledge of the nucleus geometry or surface free energy. Using the CNT, the number of atoms comprising the critical size nucleus, Nc, was estimated to be 1-9 atoms for η ranging from -130 to -70 mV, in good agreement with 1-5 atoms obtained using atomistic theory. The experimental nucleation rates were also used to determine the activation energy barriers for nucleation from the CNT, which varied from 3.31 ± 0.05 to 13 ± 1 kT over the same range of η.

Electrolysis in reduced gravitational environments: current research perspectives and future applications

Electrochemical energy conversion technologies play a crucial role in space missions, for example, in the Environmental Control and Life Support System (ECLSS) on the International Space Station (ISS). They are also vitally important for future long-term space travel for oxygen, fuel and chemical production, where a re-supply of resources from Earth is not possible. Here, we provide an overview of currently existing electrolytic energy conversion technologies for space applications such as proton exchange membrane (PEM) and alkaline electrolyzer systems. We discuss the governing interfacial processes in these devices influenced by reduced gravitation and provide an outlook on future applications of electrolysis systems in, e.g., in-situ resource utilization (ISRU) technologies. A perspective of computational modelling to predict the impact of the reduced gravitational environment on governing electrochemical processes is also discussed and experimental suggestions to better understand efficiency-impacting processes such as gas bubble formation and detachment in reduced gravitational environments are outlined.

Nucleation as a rate-determining step in catalytic gas generation reactions from liquid phase systems

The observable reaction rate of heterogeneously catalyzed reactions is known to be limited either by the intrinsic kinetics of the catalytic transformation or by the rate of pore and/or film diffusion. Here, we show that in gas generation reactions from liquid reactants, the nucleation of gas bubbles in the catalyst pore structure represents an additional important rate-limiting step. This is highlighted for the example of catalytic hydrogen release from the liquid organic hydrogen carrier compound perhydro-dibenzyltoluene. A nucleation-inhibited catalytic system produces only dissolved hydrogen with fast saturation of the fluid phase around the active site, while bubble formation enhances mass transfer by more than a factor of 50 in an oscillating reaction regime. Nucleation can be efficiently triggered not only by temperature changes and catalyst surface modification but also by a mechanical stimulus. Our work sheds new light on performance-limiting factors in reactions that are of highest relevance for the future green hydrogen economy.

Challenges and opportunities in continuous flow processes for electrochemically mediated carbon capture

Carbon capture from both stationary emitters and dilute sources is critically needed to mitigate climate change. Carbon dioxide separation methods driven by electrochemical stimuli show promise to sidestep the high-energy penalty and fossil-fuel dependency associated with the conventional pressure and temperature swings. Compared with a batch process, electrochemically mediated carbon capture (EMCC) operating in a continuous flow mode offers greater design flexibility. Therefore, this review introduces key advances in continuous flow EMCC for point source, air, and ocean carbon captures. Notably, the main challenges and future research opportunities for practical implementation of continuous flow EMCC processes are discussed from a multi-scale perspective, from molecules to electrochemical cells and finally to separation systems.

Elucidating Cathodic Corrosion Mechanisms with Operando Electrochemical Transmission Electron Microscopy

Cathodic corrosion represents an enigmatic electrochemical process in which metallic electrodes corrode under sufficiently reducing potentials. Although discovered by Fritz Haber in the 19th century, only recently has progress been made in beginning to understand the atomistic mechanisms of corroding bulk electrodes. The creation of nanoparticles as the end-product of the corrosion process suggests an additional length scale of complexity. Here, we studied the dynamic evolution of morphology, composition, and crystallographic structural information of nanocrystal corrosion products by analytical and four-dimensional electrochemical liquid-cell scanning transmission electron microscopy (EC-STEM). Our operando/in situ electron microscopy revealed, in real-time, at the nanometer scale, that cathodic corrosion yields significantly higher levels of structural degradation for heterogeneous nanocrystals than bulk electrodes. In particular, the cathodic corrosion of Au nanocubes on bulk Pt electrodes led to the unexpected formation of thermodynamically immiscible Au-Pt alloy nanoparticles. The highly kinetically driven corrosion process is evidenced by the successive anisotropic transition from stable Pt(111) bulk single-crystal surfaces evolving to energetically less-stable (100) and (110) steps. The motifs identified in this microscopy study of cathodic corrosion of nanocrystals are likely to underlie the structural evolution of nanoscale electrocatalysts during many electrochemical reactions under highly reducing potentials, such as CO₂ and N₂ reduction.

Direct measuring of single-heterogeneous bubble nucleation mediated by surface topology

Heterogeneous bubble nucleation is one of the most fundamental interfacial processes ranging from nature to technology. There is excellent evidence that surface topology is important in directing heterogeneous nucleation; however, deep understanding of the energetics by which nanoscale architectures promote nucleation is still challenging. Herein, we report a direct and quantitative measurement of single-bubble nucleation on a single silica nanoparticle within a microsized droplet using scanning electrochemical cell microscopy. Local gas concentration at nucleation is determined from finite element simulation at the corresponding faradaic current of the peak-featured voltammogram. It is demonstrated that the criteria gas concentration for nucleation first drops and then rises with increasing nanoparticle radius. An optimum nanoparticle radius around 10 nm prominently expedites the nucleation by facilitating the special topological nanoconfinements that consequently catalyze the nucleation. Moreover, the experimental result is corroborated by our theoretical calculations of free energy change based on the classic nucleation theory. This study offers insights into the impact of surface topology on heterogenous nucleation that have not been previously observed.

Single Particle Hopping as an Indicator for Evaluating Electrocatalysts

The design and screening of electrocatalysts for gas evolution reactions suffer from little understanding of multiphase processes at the electrode-electrolyte interface. Due to the complexity of the multiphase interface, it is still a great challenge to capture gas evolution dynamics under operando conditions to precisely portray the intrinsic catalytic performance of the interface. Here, we establish a single particle imaging method to real-time monitor a potential-dependent vertical motion or hopping of electrocatalysts induced by electrogenerated gas nanobubbles. The hopping feature of a single particle is closely correlated with intrinsic activities of electrocatalysts and thus is developed as an indicator to evaluate gas evolution performance of various electrocatalysts. This optical indicator diminishes interference from heterogeneous morphologies, non-Faradaic processes, and parasitic side reactions that are unavoidable in conventional electrochemical measurements, therefore enabling precise evaluation and high-throughput screening of catalysts for gas evolution systems.

Single-Entity Electrochemistry of Nano- and Microbubbles in Electrolytic Gas Evolution

Gas bubbles are found in diverse electrochemical processes, ranging from electrolytic water splitting to chlor-alkali electrolysis, as well as photoelectrochemical processes. Understanding the intricate influence of bubble evolution on the electrode processes and mass transport is key to the rational design of efficient devices for electrolytic energy conversion and thus requires precise measurement and analysis of individual gas bubbles. In this Perspective, we review the latest advances in single-entity measurement of gas bubbles on electrodes, covering the approaches of voltammetric and galvanostatic studies based on nanoelectrodes, probing bubble evolution using scanning probe electrochemistry with spatial information, and monitoring the transient nature of nanobubble formation and dynamics with opto-electrochemical imaging. We emphasize the intrinsic and quantitative physicochemical interpretation of single gas bubbles from electrochemical data, highlighting the fundamental understanding of the heterogeneous nucleation, dynamic state of the three-phase boundary, and the correlation between electrolytic bubble dynamics and nanocatalyst activities. In addition, a brief discussion of future perspectives is presented.

Electroprecipitation of Nanometer-Thick Films of Ln(OH)3 [Ln = La, Ce, and Lu] at Pt Microelectrodes and Their Effect on Electron-Transfer Reactions

We report investigations of the deposition of nanometer-thick Ln(OH)₃ films (Ln = La, Ce, and Lu) and their effect on outer-sphere and inner-sphere electron-transfer reactions. Insoluble Ln(OH)₃ films are deposited from aqueous solutions of LaCl₃ onto the surface of 12.5 μm radius Pt microdisk electrodes during water or oxygen reduction. Both reactions produce interfacial OH^-, which complexes with Ln³⁺, resulting in the precipitation of Ln(OH)₃. Surface analyses by scanning electron microscopy (SEM), SEM-energy-dispersive X-ray spectroscopy, and atomic force microscopy indicate the formation of a 1-2 nm thick uniform film. Outer-sphere electron-transfer reactions (Ru(NH₃)₆³⁺ reduction, FcMeOH oxidation, and Fe(CN)₆^4-/3- oxidation/reduction) were investigated at Ln(OH)₃-modified electrodes of different film thicknesses. The results demonstrate that the steady-state transport-limited current for these reactions decreases with an increase in the film thickness. Moreover, the degree of blockage depends upon the redox species, suggesting that the Ln(OH)₃ films are free from pinholes greater than the size of the redox molecules. This suggests that the films are either ionically conducting or that electron tunneling occurs across these thin layers. A similar blocking effect was observed for the inner-sphere reductions of H₂O and O₂. We further demonstrate that the thickness of La(OH)₃ films can be controlled by anodic dissolution. Additionally, we show that La³⁺ lowers the supersaturation of dissolved H₂ required to nucleate a stable nanobubble.

On the growth regimes of hydrogen bubbles at microelectrodes

Beside classical growth (regime I), depending on potential and concentration, new growth regimes of hydrogen bubbles were found. These differ with respect to the existence of a carpet of microbubbles underneath and of current oscillations.

Characterizing the Onset Potential Distribution of Pt/C Catalyst Deposition by a Total Internal Reflection Imaging Method

A catalytic electrode with extraordinary performances for hydrogen evolution reaction (HER) should achieve a low onset potential of the bulk electrode, as well as its uniform distribution. Herein, a total internal reflection imaging (TIRi) method to characterize the onset potential distribution of the catalytic electrode surface is presented. When the potential scans toward negative in a linear sweep voltammetry, the equivalent refractive index of the electrolyte on the electrode surface will decrease due to H₂ microbubbles generation, leading to the increase in optical intensity. Analysis of the relationship between the optical intensity and potential in each region results in the onset potential distribution. The TIRi method reveals poor uniformity and repeatability in the catalytic electrodes which are fabricated by depositing Pt/C catalysts on a porous carbon support with polymer binders (e.g., Nafion). Further electrochemical stability test also shows poor durability, whose HER onset potential deteriorates from the edge to the middle of these catalytic electrodes. The present TIRi method realizes direct visualization of the activity distribution on the bulk electrode surface, which provides a powerful tool for better fabrication and evaluation of large-area HER electrodes in industrial energy devices.

Bubble nucleation in core-shell structured molten oxide-based membranes with combined diffusion-bubbling oxygen mass transfer: experiment and theory

Oxygen-selective membranes are likely to play a leading part in the future separation processes relevant to energy engineering. A newly developed molten copper and vanadium oxide-based diffusion-bubbling membrane with core-shell structure and fast combined oxygen mass transfer is a promising candidate for efficient oxygen separation. In this work, the oxygen bubble nucleation and transport properties of the diffusion-bubbling membrane were experimentally and theoretically studied. Bubble size distribution and cumulative oxygen flux have been plotted as functions of oxygen partial pressure. The relationship between the bubble density, oxygen partial pressure, and oxygen permeation flux was established. The oxygen flux and bubble density vary in the ranges of 3.2 × 10^-8-1.4 × 10^-7 mol cm^-2 s^-1 and 1.3 × 10¹³-5.8 × 10¹³ m^-3 at ΔP_O2 = 0.1-0.75 atm, respectively. The mechanisms of homogeneous, heterogeneous, pseudo-classical and non-classical nucleation are reviewed within the framework of the Cahn-Hilliard model. It is shown that the homogeneous nucleation mechanism is most likely in the membrane core. The estimated values of the interfacial tension, energy barrier, and rate nucleation are 0.02 J m^-2, 5 kT, and 4 × 10²⁹ m^-3 s^-1, respectively.

Unravelling the last milliseconds of an individual graphene nanoplatelet before impact with a Pt surface by bipolar electrochemistry

Contactless interactions of micro/nano-particles near electrochemically or chemically active interfaces are ubiquitous in chemistry and biochemistry. Forces arising from a convective field, an electric field or chemical gradients act on different scales ranging from few microns down to few nanometers making their study difficult. Here, we correlated optical microscopy and electrochemical measurements to track at the millisecond timescale the dynamics of individual two-dimensional particles, graphene nanoplatelets (GNPs), when approaching an electrified Pt micro-interface. Our original approach takes advantage of the bipolar feedback current recorded when a conducting particle approaches an electrified surface without electrical contact and numerical simulations to access the velocity of individual GNPs. We evidenced a strong deceleration of GNPs from few tens of μm s-1 down to few μm s-1 within the last μm above the surface. This observation reveals the existence of strongly non-uniform forces between tens of and a thousand nanometers from the surface.

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.

Electrochemical Visualization of Gas Bubbles on Superaerophobic Electrodes Using Scanning Electrochemical Cell Microscopy

Electrocatalytic gas evolution reactions, where gaseous molecules are electrogenerated by reduction or oxidation of a species, play a central role in many energy conversion systems. Superaerophobic electrodes, usually constructed by their surface microstructures, have demonstrated excellent performance for electrochemical gas evolution reactions due to their bubble-repellent properties. Understanding and quantification of the gas bubble behavior including nucleation and dynamics on such microstructured electrodes is an important but underexplored issue. In this study, we reported a scanning electrochemical cell microscopy (SECCM) investigation of individual gas bubble nucleation and dynamics on nanoscale electrodes. A classic Pt film and a nonconventional transition-metal dichalcogenide MoS₂ film with different surface topologies were employed as model substrates for both H₂ and N₂ bubble electrochemical studies. Interestingly, the nanostructured catalyst surface exhibit significantly less supersaturation for gas bubble nucleation and a notable increase of bubble detachment compared to its flat counterpart. Electrochemical mapping results reveal that there is no clear correlation between bubble nucleation and hydrogen evolution reaction (HER) activity, regardless of local electrode surface microstructures. Our results also indicate that while the hydrophobicity of the nanostructured MoS₂ surface promotes bubble nucleation, it has little effect on bubble dynamics. This work introduces a new method for nanobubble electrochemistry on broadly interesting catalysts and suggests that the deliberate microstructure on a catalyst surface is a promising strategy for improving electrocatalytic gas evolution both in terms of bubble nucleation and elimination.

Integrated Bundle Electrode with Wettability-Gradient Copper Cones Inducing Continuous Generation, Directional Transport, and Efficient Collection of H2 Bubbles

The hydrogen evolution reaction (HER), as an efficient process of converting various energies into high-purity hydrogen, has attracted much attention from both scientific research studies and industrial productions. However, its wide applications still confront considerable difficulties, for example, bubble coverage on the electrode and bubble dispersion in the electrolyte, which will disturb current distribution and isolate active sites from reaction ions resulting in a high reaction overpotential and large Ohmic voltage drop. Consequently, timely removing the generated gas bubbles from the electrode as well as avoiding their direct release into the electrolyte can be an effective approach to address these issues. In this work, we have developed an elegant electrode, that is, the integrated bundle electrode with wettability-gradient copper cones, which is endowed with the multifunctions of continuous generation, direct transport, and efficient collection of hydrogen bubbles. All processes are proceeding on the electrode, which not only remove the generated hydrogen bubbles efficiently but also prevent the hydrogen bubbles from releasing into the electrolyte, which should greatly advance the development of water electrolysis and offer inspirations for people to fabricate more efficient HER devices.

Electrocatalytic generation and tuning of ultra-stable and ultra-dense nanometre bubbles: an in situ molecular dynamics study

Electrocatalytic generation of nanometre gas bubbles (nanobubbles) and their tuning are important for many energy and chemical processes. Studies have sought to use indirect or ex situ methods to investigate the dynamics and properties of nanobubbles, which are of fundamental interest. Alternatively, we present a molecular dynamics simulation method, which features in situ and high spatial resolution, to directly address these fundamentals. Particularly, our simulations can quantitatively reproduce the generation of ultra-stable and ultra-dense nanobubbles observed in electrochemical experiments. More importantly, our results demonstrate that the classical nucleation theory is still valid even for the scale down to several nanometres, to predict the dynamics and properties of nanobubbles. This provides general guidelines to design efficient nanocatalysts and nanoelectrodes. In our specific case, nanoelectrodes with wetting angles below 71° can suppress the generation of surface nanobubbles.

Effect of Surfactant on Electrochemically Generated Surface Nanobubbles

Surfactants, mimics of contamination, play an important role in nanobubble nucleation, stability, and growth at the electrode surface. Herein, we utilize single-molecule fluorescence microscopy as a sensitive imaging tool to monitor nanobubble dynamics in the presence of a surfactant. Our results show that the presence of anionic and nonionic surfactants increase the rate of nanobubble nucleation at all potentials in a voltage scan. The fluorescence and electrochemical responses indicate the successful lowering of the critical gas concentration needed for nanobubble nucleation across all voltages. Furthermore, we demonstrate that the accumulation of surfactants at the gas-liquid interface changes the interaction of fluorophores with the nanobubble surface. Specifically, differences in fluorophore intensity and residence lifetime at the nanobubble surface suggest that the labeling of nanobubbles is affected by the nature of the nanobubble (size, shape, etc.) and the structure of the gas-liquid interface (surfactant charge, hydrophobicity, etc.).

Dynamic Equilibrium Model for Surface Nanobubbles in Electrochemistry

Gas bubbles are ubiquitous in electrochemical processes, particularly in water electrolysis. Due to the development of gas-evolving electrocatalysis and energy conversion technology, a deep understanding of gas bubble behaviors at the electrode surface is highly desirable. In this work, by combining theoretical analysis and molecular simulations, we study the behaviors of a single nanobubble electrogenerated at a nanoelectrode. With the dynamic equilibrium model, the stability criteria for stationary surface nanobubbles are established. We show theoretically that a slight change in either the gas solubility or solute concentration results in various nanobubble dynamic states at a nanoelectrode: contact line pinning in aqueous and ethylene glycol solutions, oscillation of pinning states in dimethyl sulfoxide, and mobile nanobubbles in methanol. The above complex nanobubble behavior at the electrode/electrolyte interface is explained by the competition between gas influx into the nanobubble and outflux from the nanobubble.

Visualization and Quantification of Electrochemical H2 Bubble Nucleation at Pt, Au, and MoS2 Substrates

Electrolytic gas evolution is a significant phenomenon in many electrochemical technologies from water splitting, chloralkali process to fuel cells. Although it is known that gas evolution may substantially affect the ohmic resistance and mass transfer, studies focusing on the electrochemistry of individual bubbles are critical but also challenging. Here, we report an approach using scanning electrochemical cell microscopy (SECCM) with a single channel pipet to quantitatively study individual gas bubble nucleation on different electrode substrates, including conventional polycrystalline Pt and Au films, as well as the most interesting two-dimensional semiconductor MoS₂. Due to the confinement effect of the pipet, well-defined peak-shaped voltammetric features associated with single bubble nucleation and growth are consistently observed. From stochastic bubble nucleation measurement and finite element simulation, the surface H₂ concentration corresponding to bubble nucleation is estimated to be ∼218, 137, and 157 mM, with critical nuclei contact angles of ∼156°, ∼161°, and ∼160° at polycrystalline Pt, Au, and MoS₂ substrates, respectively. We further demonstrated the surface faceting at polycrystalline Pt is not specifically correlated with the bubble nucleation behavior.

A Bubble-STORM Approach for Super-Resolved Imaging of Nucleation Sites in Hydrogen Evolution Reactions

Stochastic optical reconstruction microscopy (STORM) is a powerful strategy to achieve super-resolved imaging of biological structures by virtue of the stochastic photoactivation of fluorophores and superlocalization algorithm. Herein, we report a fluorophore-free bubble-STORM approach for super-resolved imaging of nucleation sites in hydrogen evolution reactions (HER). When applying an appropriate pulse potential to the electrode, rapid electro-reduction of protons created a local oversaturation of hydrogen molecules and thus the nucleation of sparsely distributed hydrogen nanobubbles. A surface plasmon resonance microscopy was employed to monitor the process and report the localization of each nanobubble via superlocalization fitting. The withdrawal of electrode potential, or the microconvection, led to the immediate disappearance of nanobubbles and recovered the electrode surface before the next pulse. By repeating the procedures for thousands of cycles, one was able to reconstruct a map of nucleation sites with a spatial resolution beyond the optical diffraction limit. This approach does not require a model fluorogenic reaction or fluorescent labeling to the nanobubbles, thus revealing the intrinsic nucleation sites in the natural states. Our results further indicated the fast growth, coalescence, and detachment behaviors of nanobubbles on a time scale of sub-milliseconds, underscoring the significance of high temporal resolution for studying nanobubble nucleation.