Electrolyte Flow Rate Research Articles

A Comprehensive Flow–Mass–Thermal–Electrochemical Coupling Model for a VRFB Stack and Its Application in a Stack Temperature Control Strategy

In this work, a comprehensive multi-physics electrochemical hybrid stack model is developed for a vanadium redox flow battery (VRFB) stack considering electrolyte flow, mass transport, electrochemical reactions, shunt currents, and as heat generation and transfer simultaneously. Compared with other VRFB stack models, this model is more comprehensive in considering the influence of multiple factors. Based on the established model, the electrolyte flow rate distribution across cells in the stack is investigated. The distribution and variation in shunt currents, single-cell current and single-cell voltage are analyzed. The distribution and variation in temperature and heat generation and heat transfer are also researched. It can be found that the VRFB stack temperature will exceed 40 °C when operating at 60 A and 100 mA cm−2 at an ambient temperature of 30 °C, which will lead to electrolyte ion precipitation, affecting the performance and safety of the battery. To control the stack temperature below 40 °C, a new tank cooling control strategy is proposed, and the suitable starting cooling point and the controlled temperature are specified. Compared with the common room cooling strategy, the new tank cooling strategy reduces energy consumption by 27.18% during 20 charge–discharge cycles.

Simulation of continuous thermally regenerative electrochemical flow cycle and coupled with photovoltaic/thermal system

Thermally regenerative electrochemical cycle (TREC) is an efficient, clean and feasible low-temperature thermoelectric conversion technology. Coupling it with flow battery (FB) can not only achieve continuous operation, but also can be used to realize the heat energy recovery of the photovoltaic/thermal (PV/T) system. In this paper, a continuous TREC-FB system including two charging-free cells is proposed for outputting power at both high and low temperatures. An electrochemical-thermodynamic coupling algebraic model is established to explore the effects of electrolyte flow rate, temperature difference and design parameters of heat exchanger on the power density (PTREC) and thermoelectric efficiency (ηele,TREC) of the TREC-FB. Experimental study on TREC-FB is carried out for model validation. The results show that PTREC is determined by the temperature difference and the electrolyte flow rate, which contribute to enhanced voltage and mass transfer, respectively. For specific heat source scenarios, the ηele,TREC is mainly affected by the flow rate, followed by the heat transfer area AH, and finally is the flow channel height and the thermal conductivity of the heat exchanger. Moreover, for the continuous TREC-FB-PV/T coupled system, the energy conversion efficiency reaches an average of 11 % within one day by optimizing the designed temperature difference.

Research on jet electrochemical machining with coaxial megasonic assistance

In order to address the problem of poor localization in electrochemical machining (ECM), a coaxial megasonic assisted jet ECM method was proposed. Based on theoretical analysis, experiments were conducted to compare the effects of various electrolyte flow rates, electrolytic voltage and megasonic power levels on pit ECM. The results indicate that, in the range of experimental parameters, the increase of electrolyte flow rate and megasonic power can increase the machining depth, so as to improve the depth-diameter ratio of ECM pits. The use of coaxial megasonic-assisted jet ECM can enhance the depth-diameter ratio of etched pits compared to the without megasonic one. When applying a megasonic power of 22 W, the dimensions of the ECM pit were measured as 0.81 mm in depth and 5.73 mm in diameter, resulting in an depth-diameter ratio of 0.140. Under the same conditions, without megasonic assistance, the pit diameter is reduced to 0.65 mm while the pit depth increases to 6.36 mm, resulting in a depth-diameter ratio of 0.102. Additionally, The results also demonstrate that, the increase of electrolytic voltage makes the depth to diameter ratio of pit further increase on the original basis. With an electrolyte flow rate of 0.9 L/min and a megasonic power of 22 W, the use of electrolysis voltage of 50 V increased the depth-diameter ratio of etched pits to 0.173. Using the above preferred parameters, electrolytic milling of the wide groove is carried out. The depth-diameter ratio of the wide groove is increased from 0.039 to 0.059 by appending coaxial megasonic. This further verified the effectiveness of the coaxial megasonic-assisted jet ECM method.

Fuel transport mechanisms and power generation enhancement in air-breathing microfluidic fuel cells with discrete-hole anode

Current challenges in air-breathing microfluidic fuel cells (AMFCs) technology include insufficient fuel transfer and unsatisfactory power density. To address these issues, this study develops a three-dimensional computational model to optimize the performance of AMFC with a discrete-hole anode. This model reveals that the "petal-shaped" non-uniform fuel distribution is due to the shearing and dilution effects of the electrolyte. Additionally, discrete-hole structures such as size have a slight influence on fuel transport and current production, with performance deviations under 1.6%. Significant improvements are achieved by reducing the electrolyte flow rate and electrode length. Transitioning from a single cell with a long anode to a series/parallel stack of cells with short electrodes significantly boosts net power output and fuel utilization. Under optimal conditions, a maximum power density of 349.9 mW cm−3 and an output of 19.2 mW are obtained. This research provides valuable insights into fuel transport mechanisms and power enhancement strategies in the AMFCs, guiding the future design and optimization of microfluidic reactors for energy conversion.

Electrolyte Gradients and Capacity Drop in Large Flow Battery Tanks: From Experiments to Fluid-Dynamic Investigations

A study of the effects of electrolyte flow inside the tanks on electrical performance of an industrial-size Vanadium Redox Flow Battery (VRFB) is presented. The concentration of electrolytes feeding the stacks of VRFBs is crucial in determining the device voltage and overall power performance. Usually, the assumption of perfect mixing in the tanks is assumed, but the hydraulic behavior of large VRFB can deviate significantly from this assumption depending on the electrolyte flow rate, stochiometric factor and geometry of the tanks. In this study, the State of Charge (SoC) measurements have been analyzed, starting from the signal produced by an Open Circuit Voltage (OCV) cell. An experimental campaign has been conducted during charge/discharge roundtrip cycles at different current levels and constant stoichiometric factor. Building on the delay in the voltage response of the battery, a fluid-dynamic model was developed to study how the hydraulic behavior of electrolytes inside tanks influences the electrolyte concentration and, consequently, the SoC of the electrolyte that feeds the stack and the electrical time response of the battery. The model indicated that a stratification of charged electrolyte is responsible for this delay resulting in an initial voltage plateau. A different hydraulic behavior was found in each tank and in each charge/discharge phase. These results highlight the tank role in the industrial-scale VRFB performance. References Sanchez-Diez, E. Ventosa, M. Guarnieri, A. Trovò, C. Flox, R. Marcilla, F. Soavi, P. Mazur, E. Aranzabe, R. Ferret, “Redox flow batteries: status and perspective towards sustainable stationary energy storage”, J. Power Sources, 481, (2021) 228804.Kurilovich, A. Trovò, M. Pugach, K. J. Stevenson, M. Guarnieri. Prospect of Modeling Industrial Scale Flow Batteries – from Experimental Data to Accurate Overpotential Identification, Renew. Sust. Energ. Rev. 167, (2022) 112559.Trovò, V. Di Noto, J. E. Mengou, C. Gambaro, M. Guarnieri. Fast Response of kW-Class Vanadium Redox Flow Batteries, IEEE Trans. Sustain. Energy 12, (2021) 2413–2422.Trovò, M. Rugna, N. Poli, M. Guarnieri. Prospects for industrial vanadium flow batteries, Ceram. Int., 49 (14), (2023) 24487-24498.Maggiolo, A. Trovò, M. Guarnieri, “Reactant Flow in Flow Batteries”, in L. Cabeza (ed.), Encyclopedia of Energy Storage, vol. 4, pp. 231-242. Oxford: Elsevier. March 2022.A. Prieto-Díaz, S. E. Ibáñez, M. Vera. Fluid dynamics of mixing in the tanks of small vanadium redox flow batteries: Insights from order-of-magnitude estimates and transient two-dimensional simulations, Int. J. Heat Mass Transf. 216, (2023) 124567.Trovò, N. Poli, M. Guarnieri. New strategies for the evaluation of Vanadium Flow Batteries: testing prototypes, Cur. Opin. Chem. Eng. 37, (2022) 100853. Figure 1

Flow-Engineered 3-D Printed Electrodes for Alkaline Water Electrolysis

Initially, industrial alkaline electrolyzers had 2-D bipolar plates as electrodes in a configuration in which there is an important physical gap between the electrodes (e.g., 2 mm). This setup presented a high ohmic resistance and is commonly limited to a current density in the order of 0.5 A·cm-2 due to gas screening, i.e., the produced gases form a non-conductive layer over the entire electrode surface, limiting current density. The more advanced designs of today include the use of a zero-gap cell where the interelectrode distance is limited to the thickness of the separator (hundreds of micrometers for diaphragms and tens of micrometers for membranes). Commonly, 2-D electrodes such as perforated plates or an expanded metal mesh are used. As the electrode is pressed against the separator, the gases are forced to be evacuated at the backside of the electrode, avoiding the screening effect. As a consequence, much higher current densities can be achieved. More recently, there has been an increasing interest in the integration of 3-D electrodes in the zero-gap configuration, as they present higher surface areas than their counterpart 2-D electrodes. Nevertheless, bubbles can get trapped inside the intricate 3-D electrode structures. Several strategies are used to mitigate bubble entrapment, including the use of a forced electrolyte flow.The current study aims to investigate the possible electrochemical performance improvement of a single alkaline water electrolysis cell by using 3-D printed electrodes with graded structures, engineered to enhance bubble removal under the effect of a forced electrolyte flow. As the majority of the current is concentrated near the separator, the tailored 3-D printed electrodes present low strut distances and, consequently, a high surface area in this region. Further away from the separator, the distance between the struts gradually increases, decreasing flow-related energy dissipation and avoiding bubble entrapment. This tailored laterally graded structure promotes a difference in pressure along the direction perpendicular to the separator, allowing to extract bubbles from the region near the separator and to evacuate them towards the region with higher strut distances more efficiently. Furthermore, our flow cell was designed to promote a homogeneous upstream flow at the entrance of the electrode. Based on previous findings [1], a Schwarz triply periodic minimum surface was chosen as electrode geometry, as this geometry can channel the flow and reduce bubble-induced ohmic resistance. This geometry is defined by the following mathematical equation: t = sin(2·π·z/L)·sin(2·π·x/L) - 0.4·sin(2.4·π·y/L)·cos(2·π·z/L)·cos(2·π·x/L)with L the periodicity (e.g., an L = 1 mm means that the structure repeats itself in the space at each 1 mm), and t relates to void fraction.Different electrodes with a Schwarz geometry were analyzed in a combined experimental and simulation approach. Electrodes were 3-D printed using selective laser melting, and a strut in the order of 500 μm was achieved. Explicit computational fluid dynamics simulations were performed to estimate bubble residence time and path. Experimental analyses were conducted in a flow-through setup operating under industrial standard conditions of 30 wt% KOH electrolyte at 70°C to elucidate the performance disparities between these structures. Our findings indicate that such tailored 3-D printed electrodes are capable of significantly outperforming previously reported performances in the literature using more stochastic 3-D electrodes like foams. Furthermore, results show that the printed structures are more sensitive to electrolyte flow, presenting an important overvoltage reduction with increase in electrolyte flow rate. We also identified the most critical structural parameters controlling bubble evacuation, thereby significantly reducing the cell's ohmic resistance. Ultimately, this study also emphasizes the critical role of efficient gas evacuation for high-rate alkaline electrolysers using a zero-gap design.[1] Rocha F, Delmelle R, Georgiadis C, Proost J. Electrochemical Performance Enhancement of 3D Printed Electrodes Tailored for Enhanced Gas Evacuation during Alkaline Water Electrolysis. Advanced Energy Materials 2022;13:2203087.

Phase-Field Modeling of Zinc Dendrite Growth in Aqueous Zinc Batteries

The growth of zinc dendrites poses a major challenge in developing stable aqueous zinc batteries. It is crucial to understand the mechanism underlying the dendrite growth and the influences of various factors on the process of dendrite growth. In this study, a phase field model is constructed to simulate the growth of zinc dendrite and investigate the temporal and spatial distributions of ions and electric potential in both static and flowing electrolytes. The influences of various critical factors, including overpotential, anisotropic strength and electrolyte flow rate, on the morphology evolution of electrodeposit are investigated. Results reveal that the surface morphology evolution is intimately dependent on the competition between ion transport and the electrochemical reaction, and it can be modulated by various parameters. In particular, flowing electrolyte not only enhances the convective ion transport but also suppresses the dendrite bifurcation on the surface, resulting in a uniform distribution of the concentration gradient and a compact deposit. However, nonuniformity induced by the flowing electrolyte from the inlet to outlet may impede the stable cycling of zinc anodes in the flow batteries. Therefore, further design and optimization of the flow field are desired for zinc-based flow batteries. This work provides insights into the growth process of zinc dendrites and the influences of several critical parameters, which could inform the design of stable aqueous zinc batteries. Figure 1

Bubble Transport in Porous Media for Electrochemical Applications

Bubbles and their flow dynamics strongly impact the performance of gas-evolving electrochemical systems with porous electrode architectures. The presence of bubbles on the electrode surface reduces the number of active catalytic sites that are accessible for the liquid electrolyte, leading to limitations in mass transport and/or reaction kinetics. Additionally, bubbles can obstruct the flow of liquid electrolyte that is able to pass through the porous structure, leading to pressure buildup. Therefore, to mitigate bubble blockage, optimal electrolyzer performance requires understanding bubble dynamics within the porous electrode in the presence of an applied flow; an accurate and complete physical model of this setup is not yet available. Here, we present volume-of-fluid simulation results and a theoretical framework to describe the bubble/electrolyte/electrode interaction, i.e. the triple phase boundary, ultimately with an aim to predict whether the bubble does or does not break through the porous media. Informed by the numerical results, the constructed theory describes the interaction of buoyancy, surface tension, and pressure drop for the range of different porosities, bubble sizes, and electrolyte flow rates. Comparison of these results with experimental findings are discussed. This work serves as a first step in building the framework for the design and optimization of relevant porous media systems that aim towards the improved control of bubble dynamics.This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344. LLNL release number: LLNL-ABS-857509.

H2Bro – a Novel Decoupled Electrocatalytic-Catalytic Water Splitting Process for Green Hydrogen Production

Green hydrogen produced by water splitting using renewable electricity is essential to achieve net-zero carbon emissions. Present water electrolysis technologies are uncompetitive with low-cost grey hydrogen produced from fossil fuels, limiting their scale-up potential. Disruptive processes that decouple the hydrogen and oxygen evolution reactions and produce them in separate cells or different stages emerge as a prospective route to reduce system cost by enabling operation without expensive membranes and sealing components.An elegant and innovative electrocatalytic-catalytic process was suggested for continuous high-current H2 production via membraneless decoupled water splitting (Schematic 1). H2 as the desired product is evolved on the cathode of the electrolytic compartment in near-neutral conditions, Br- is electrochemically oxidized on the anode side to Br2, which reacts in the bulk of the aqueous electrolyte to form bromate anions (with hypobromous acid and hypobromite anions as the intermediate products), whereas a catalytic transformation of the soluble redox couple (BrO3 -/Br-) mediates O2 evolution in the catalytic compartment.Bromate-mediated decoupled water splitting system offers decoupling of the H2 and O2 gases as a safety measure, a membraneless architecture as a cell design simplification, continuous operation mode at low temperature of 60 °C and less corrosive environment along with the potential use of abundant (electro)catalytic materials. The use of the redox mediator also allows bypassing the energetically unfavorable oxygen evolution reaction and, by doing so, decreasing the cell voltage.The concept of the new electrocatalytic-catalytic system was verified experimentally in batch-to-batch mode, resulting in long-term Faradaic efficiency of 98±2% and voltage efficiency of 62% at 1 A cm-2 already at its infancy stage [1]. Nevertheless, the merging of the electrolytic and catalytic parts into an integrated continuous flow system is still in process.In this work, a variety of possible system part designs were suggested and discussed, with an emphasis on integrating the electrolytic and catalytic parts into a continuous flow system. The first iteration of the laboratory scale experimental setup was designed in a continuous stirred reactor mode, using Pt as the cathode material for HER, DSA as the anode material for BER, and RuO2 Adams powder as the catalyst for bromate decomposition, and aiming for a high bromide utilization (BrO3 -/Br- ratio) level at the cell voltage of 1.6 V and the current density values above 0.2 A cm-2. The system was designed to automatically control the electrochemical parameters of the electrolytic process, to adjust the electrolyte flow rate through the electrolytic cell and the bromate/bromide mixture residence time in the catalytic cell, to control the H2 evolution rate, and to detect undesired O2 evolution (if any) in the electrolytic cell, and to optimize and maintain the temperature and pH values. A set of the key parameters affecting Faradaic efficiency, cell voltage and voltage efficiency are analyzed in detail.Some light was also shed on the development of a new type of solid-buffer PGM-free electrocatalysts for hydrogen evolution reaction (HER) in near-neutral conditions, as well as on deciphering the mechanism of bromate decomposition process and the development of novel PGM-free catalyst materials as more affordable alternatives for RuO2 in the catalytic cell.References Slobodkin, E. Davydova, M. Sananis, A. Breytus, A. Rothschild “Electrochemical and chemical cycle for high-efficiency decoupled water splitting in a near-neutral electrolyte", accepted for publication in Nature Materials Funded by the European Union (ERC), Project #101097966 “Continuous electrolytic-catalytic decoupled water electrolysis for green hydrogen production” (H2Bro). Views and opinions expressed are however those of the author(s) only and do not necessarily reflect those of the European Union or the European Research Council Executive Agency. Neither the European Union nor the granting authority can be held responsible for them.” Figure 1

Применение эволюционных и роевых методов для оптимизации многопараметрического управления процессом нанесения гальванического покрытия

One of the key parameters of the quality of ceramic coatings is its thickness, which must meet technical requirements and provide the necessary degree of corrosion protection, wear resistance and appearance of the coating. Ensuring the uniformity of the galvanic coating is one of the most important and complex tasks of high-tech machine-building production, for which a number of methods have been proposed, such as controlling current modes, the location of electrodes in the bath, and the electrolyte flow rate. However, in most cases, these methods require solving the problem of optimal multiparametric control. Prompt and accurate optimization when changing the conditions of electrolysis (electrode potentials, composition and properties of the electrolyte), as well as, if necessary, taking into account the multi-extreme nature of the dependence of the uniformity coefficient on the process parameters (current density, interelectrode distance, electrolyte flow rate) is a rather complex and ambiguous multifactorial task that limits the use of classical methods for- the claim of a global extreme. The article explores the possibility and expediency of using intelligent heuristic methods, such as evolutionary and swarm methods, to solve this problem. Objective. The objective of this study is to determine the effectiveness of using genetic algorithms and the particle swarm method to solve the problem of optimizing the multiparametric control of the electroplating process. Materials and methods. Methods of mathematical modeling, methods and software environment of numerical modeling and optimization were used to conduct the study. Results. The article investigates the effectiveness of the application of evolutionary and swarm optimization methods in relation to the process of applying chrome plating in a galvanic bath with many anodes with multiparametric control of current density, interelectrode distance and electrolyte flow rate. The best approximation to the extreme of the uniformity coefficient can be achieved by genetic algorithms using the mutation operation and the particle swarm method, while achieving the extreme using the particle swarm method is achieved in fewer iterations. Conclusion. The results of the study substantiate the expediency of using evolutionary and swarm methods to solve the problem of optimizing the multiparametric control of the electroplating process, while it is possible to increase the efficiency of these methods due to additional tuning.

Methodology for fast testing of carbon-based nanostructured 3D electrodes in vanadium redox flow battery

Progress in material chemistry is manifested daily by the variety of prepared functional materials, often with nanodimensional structuring. The electrodes for vanadium redox flow batteries have been shown to benefit from incorporating nanostructured materials such as carbon nanotubes. However, the methods of such incorporation are far from optimal, relying mainly on physical deposition or insertion into a binder. Here, we describe a technique for integrating carbon-based rod-like nanomaterials into a vanadium redox flow battery and a methodology for fast nanomaterial performance testing. The technique is based on creating a fixed nanomaterial bed sandwiched between two graphite felt electrodes, forming a 3D flow-through electrode in the battery. Performing various positive and negative control experiments, we show the beneficial effect of a nanostructured bed on the primary battery characteristics obtained from short-term electrochemical experiments. We characterize carbon nanotubes exhibiting promising electrochemical behavior in vanadium electrolytes, as observed in our previous study. The load curves obtained from charge-discharge steps at various current densities and electrolyte flow rates revealed considerable differences in the performance of the tested materials, with few-walled carbon nanotubes reaching unsurpassable characteristics. At room temperature, with 50 %-SOC-working solutions and the highest tested linear velocity of 14.6 cm/min, the evaluated power density for this material reached values above 500 mW/cm2. For comparison, thermally treated graphite felt, used as a benchmark material, provided a power density of around 300 mW/cm2 under identical conditions. Although developed for vanadium redox flow batteries, the method enables testing tube-like and rod-like (nano-)materials for flow electrochemical systems.

CVD Grown CNTs-Modified Electrodes for Vanadium Redox Flow Batteries.

Vanadium redox flow batteries (VRFBs) are of considerable importance in large-scale energy storage systems due to their high efficiency, long cycle life and easy scalability. In this work, chemical vapor deposition (CVD) grown carbon nanotubes (CNTs)-modified electrodes and Nafion 117 membrane are utilised for formulating a vanadium redox flow battery (VRFB). In a CVD chamber, the growth of CNTs is carried out on an acid-treated graphite felt surface. Cyclic voltammetry of CNT-modified electrode and acid-treated electrode revealed that CNTs presence improve the reaction kinetics of V3+/V2+ and VO2+/VO2+ redox pairs. Battery performance is recorded for analysing, the effect of modified electrodes, varying electrolyte flow rates, varying current densities and effect of removing the current collector plates. CNTs presence enhance the battery performance and offered 96.30% of Coulombic efficiency, 79.33% of voltage efficiency and 76.39% of energy efficiency. In comparison with pristine electrodes, a battery consisting CNTs grown electrodes shows a 14% and 15% increase in voltage efficiency and energy efficiency, respectively. Battery configured without current collector plates performs better as compared to with current collector plates which is possibly due to decrease in battery resistance.

Three-Dimensional Modeling of Anion Exchange Membrane Electrolysis: A Two-Phase Flow Approach

Anion exchange membrane water electrolyzers (AEMWEs) are attracting growing interest as a green hydrogen production technology. Unlike proton exchange membrane (PEM) systems, AEMWEs operate in an alkaline environment, allowing one to use less expensive, non-noble materials as catalysts for the reactions and non-fluorinated anion exchange polymer membranes. However, the performance and stability of AEMWEs strongly depend on the alkaline electrolyte concentration. In this work, a three-dimensional multi-physics model considering two-phase flow effects is applied to understand the impact of KOH electrolyte concentration and its flow rate on AEMWE performance, as well as on the current and gas volume fraction distributions. The numerical results were compared to experimental data published in the literature. For current densities above 1 A/cm2, a strongly non-uniform H2 and O2 gas volume distribution could be evidenced by the 3D simulations. Increasing the KOH electrolyte flow rate from 10 to 100 mL/min noticeably improves cell performance for current densities above 1 A/cm2. These results show the importance of accounting for the three-dimensional geometry of an AEMWE and two-phase flow effects to accurately describe its operation and performance.

Bioinspired flow Fields: A numerical investigation into Nature-Mimicking designs for boosting vanadium redox flow batteries

Efficient flow field structures are crucial for improving the performance of all-vanadium redox flow batteries (VRFBs). Considering the large pressure drop and pump losses in traditional serpentine flow fields (SFF), this paper proposes a novel biomimetic flow field structure (BFF) for VRFB. A three-dimensional multiphysics model comparing SFF and BFF designs for VRFB is developed, the assessment was further enhanced by considering the impact of electrolyte flow rates on multiple performance indicators. The results show that BFF’s unique design substantially reduces pressure drop, especially at higher flow rates. For both SFF and BFF designs, at a low flow rate of 60 ml/min, the SFF has a slightly higher pump-based voltage efficiency, but as the flow rate increases to 120 ml/min and 180 ml/min, the BFF outperforms the SFF, demonstrating advantages of 2.057 % and 6.888 %, respectively. This comprehensive VRFB modeling analysis provides valuable insights into optimizing future flow battery designs.

Electrochemical Polishing Method for Titanium Alloys with a Microgroove Structure

TI–6AL–4V alloys are widely used in various fields owing to their excellent corrosion resistance, high-temperature resistance, and low-temperature toughness. Herein, a microgroove fixture was used to simulate the microgrooves in a titanium alloy with different aspect ratios to study the influence of the electrolyte flow rate on the polishing effect. The optimization of the electrochemical polishing parameters was conducted using experiments and simulations. The effects of process parameters, such as the concentration of sodium chloride (NaCl) and zinc chloride (ZnCl2), polishing time, and processing voltage, on the quality of the post-polished surface were studied. Experiments were conducted on microgrooves with different aspect ratios under the optimized polishing process parameters. Changes in the surface elements of the microgrooves after polishing were detected. The experimental results indicated that the optimal electrochemical polishing solution flow rate, NaCl concentration, ZnCl2 concentration, polishing time, and processing voltage were 0.2 m/s, 4.0 wt.%, 0.4 wt.%, 8 min, and 90 V, respectively. After 8 min of electrochemical polishing, a TiO2 passivation film was formed on the surface of the microgroove. The surface roughness of the notch and bottom of the microgroove decreased from 250 nm to below 40 nm, with a minimum of 24.5 nm.

Correlating Electrolyte Infiltration with Accessible Surface Area in Macroporous Electrodes using Neutron Radiography

Carbon-based porous electrodes are commonly employed in electrochemical technologies as they provide a high surface area for reactions, an open structure for fluid transport, and enable compact reactor architectures. In electrochemical cells that sustain liquid electrolytes (e.g., redox flow batteries, CO2 electrolyzers, capacitive deionization), the nature of the interaction between the three phases - solid, liquid and gas - determines the accessible surface area for reactions, which fundamentally determines device performance. Thus, it is critical to understand the correlation between the electrolyte infiltration in the porous electrode and the resulting accessible surface area in realistic reactor architectures. To tackle this question, here we simultaneously perform neutron radiography with electrochemical measurements to correlate macroscopic electrode saturation/wetting with accessible surface area. We find that for untreated electrodes featuring neutral wettability with water, the electrode area remains underutilized even at elevated flow rates, both for interdigitated and parallel flow fields. Conversely, increasing the electrode hydrophilicity results in an order-of-magnitude increase in accessible surface area at comparable electrode saturation, and is less influenced by the electrolyte flow rate. Ultimately, we reveal useful correlations between reactor architectures and electrode utilization and provide a method that is broadly applicable to flow electrochemical reactors.

Catalyst-less efficient electrochemical production of hydrogen peroxide

Hydrogen peroxide is produced efficiently in a catalyst-less electrochemical cell manufactured at lab scale (4 cm2 of electrode size) by resin and titanium 3-D printing. The electrolyte inlet and outlet of the cell are designed to promote a very turbulent flow pattern that provides good mixing of the liquid and gas phases and minimizes the mass transport limitations. The mechanization of the titanium contributes to an efficient gas diffusion in the cathode which was successfully adapted to the 3-D printed cell resulting in a very efficient gas–liquid reactive absorbent unit. The effect of the electrolyte, operation mode, gas and liquid flow rates were evaluated on the electrochemical production of hydrogen peroxide and the results obtained were successfully fitted to a simple phenomenological model that helped to obtain a deep understanding of the process. The proposed electrochemical gas–liquid absorber achieved coulombic efficiencies as high as 69.72 % and energy consumptions close to 9 mg H2O2 (Wh)−1 when the system was operated in continuous mode with the most efficient conditions (no cell compartmentation and feeding pure oxygen with electrolytes containing sulphate salts at pH near 3). Those values of efficiencies are in the upper range of the results found in the literature for the electrochemical production of hydrogen peroxide, even though no catalytic coating was used to promote this cathodic reaction. An unexpected effect of the salt contained in the electrolyte was observed indicating the existence of other production mechanisms that require further characterization.

Universal Approach to Direct Spatiotemporal Dynamic In Situ Optical Visualization of On-Catalyst Water Splitting Electrochemical Processes.

Electrochemical reactions are the unrivaled backbone of next-generation energy storage, energy conversion, and healthcare devices. However, the real-time visualization of electrochemical reactions remains the bottleneck for fully exploiting their intrinsic potential. Herein, for the first time, a universal approach to direct spatiotemporal-dynamic in situ optical visualization of pH-based as well as specific byproduct-based electrochemical reactions is performed. As a highly relevant and impactful example, in-operando optical visualization of on-catalyst water splitting processes is performed in neutral water/seawater. HPTS (8-hydroxypyrene-1,3,6-trisulfonicacid), known for its exceptional optical capability of detecting even the tiniest pH changes allows the unprecedented "spatiotemporal" real-time visualization at the electrodes. As a result, it is unprecedentedly revealed that at a critical cathode-to-anode distance, the bulk-electrolyte "self-neutralization" phenomenon can be achieved during the water splitting process, leading to the practical realization of enhanced additive-free neutral water splitting. Furthermore, it is experimentally unveiled that at increasing electrolyte flow rates, a swift and severe inhibition of the concomitantly forming acidic and basic 'fronts', developed at anode and cathode compartments are observed, thus acting as a "buffering" mechanism. To demonstrate the universal applicability of this elegant strategy which is not limited to pH changes, the technique is extended to visualization of hypochlorite/ chlorine at the anode during electrolysis of sea water using N-(4-butanoic acid) dansylsulfonamide (BADS). Thus, a unique experimental tool that allows real-time spatiotemporal visualization and simultaneous mechanistic investigation of complex electrochemical processes is developed that can be universally extended to various fields of research.

Stable Operation of Paired CO2 Reduction/Glycerol Oxidation at High Current Density.

Despite the considerable efforts made by the community, the high operation cell voltage of CO2 electrolyzers is still to be decreased to move toward commercialization. This is mostly due to the high energy need of the oxygen evolution reaction (OER), which is the most often used anodic pair for CO2 reduction. In this study, OER was replaced by the electrocatalytic oxidation of glycerol using carbon-supported Pt nanoparticles as an anode catalyst. In parallel, the reduction of CO2 to CO was performed at the Ag cathode catalyst using a membraneless microfluidic flow electrolyzer cell. Several parameters were optimized, starting from the catalyst layer composition (ionomer quality and quantity), through operating conditions (glycerol concentration, applied electrolyte flow rate, etc.), to the applied electrochemical protocol. By identifying the optimal conditions, a 75-85% Faradaic efficiency (FE) toward glycerol oxidation products (oxalate, glycerate, tartronate, lactate, glycolate, and formate) was achieved at 200 mA cm-2 total current density while the cathodic CO formation proceeded with close to 100% FE. With static protocols (potentio- or galvanostatic), a rapid loss of glycerol oxidation activity was observed during the long-term measurements. The anode catalyst was reactivated by applying a dynamic potential step protocol. This allowed the periodic reduction, hence, refreshing of Pt, ensuring stable, continuous operation for 5 h.

Role of Mass Transport in Electrochemical CO2 Reduction to Methanol Using Immobilized Cobalt Phthalocyanine.

Electrochemical CO2 reduction (CO2R) using heterogenized molecular catalysts usually yields 2-electron reduction products (CO, formate). Recently, it has been reported that certain preparations of immobilized cobalt phthalocyanine (CoPc) produce methanol (MeOH), a 6-electron reduction product. Here, we demonstrate the significant role of intermediate mass transport in CoPc selectivity to methanol. We first developed a simple, physically mixed, polymer (and polyfluoroalkyl, PFAS)-free preparation of CoPc on multiwalled carbon nanotubes (MWCNTs) which can be integrated onto Au electrodes using a poly(3,4-ethylenedioxythiophene) polystyrenesulfonate (PEDOT:PSS) adhesion layer. After optimization of catalyst preparation and loading, methanol Faradaic efficiencies and partial current densities of 36% (±3%) and 3.8 (±0.5) mA cm-2, respectively, are achieved in the CO2-saturated aqueous electrolyte. The electrolyte flow rate has a large effect. A linear flow velocity of 8.5 cm/min produces the highest MeOH selectivity, with higher flow rates increasing CO selectivity and lower flow rates increasing the hydrogen evolution reaction, suggesting that CO is an unbound intermediate. Using a continuum multiphysics model assuming CO is the intermediate, we show qualitative agreement with the optimal inlet flow rate. Polymer binders were not required to achieve a high Faradaic efficiency for methanol using CoPc and MWCNTs. We also investigated the role of formaldehyde as an intermediate and the role of strain, but definitive conclusions could not be established.