Electrolyte Flow Research Articles

A gradient electrospinning electrode structure both in the in/through-plane directions for non-aqueous iron-vanadium redox flow battery

To boost the performance of non-aqueous redox flow batteries (RFBs), it is important to synergistically improve the flow/mass transfer efficiencies and the uniformity of overpotential distribution into the porous electrode. In this work, electrostatic spinning technology is developed to propose a novel porous electrode with gradient pore distribution both in the in-plane and through-plane directions, and applied in deep eutectic solvent (DES) electrolyte-based iron-vanadium RFB. On the one hand, the new in-plane gradient design modifies the distribution of reactive species of electrode near the membrane side, resulting in the decreasing polarization loss. On the other hand, the increasing porosity of electrodes from the flow field side to the membrane side attains a trade-off between the charge transfer and the electrolyte flow resistances. According to the experimental results, compared to the graphite felt electrode, the energy efficiency of this RFB with three-dimensional gradient electrode improves by 74.2 % at a current density of 10 mA·cm−2. Moreover, the numerical simulation reveals the reactive transfer behaviors of three-dimensional gradient porous electrode. The results show that the proposed three-dimensional gradient design can enhance the uniformity of overpotential distribution and achieve the decrease of polarization resistance, thus improving the performance of DES electrolyte-based iron-vanadium RFB effectively.

Analysis of fluid-structure interaction in diaphragm plug valves for filling electrolyte in lithium-ion battery cells

In this paper, the stress and the local liquid residue of the diaphragm plug valve were simulated. Simulation of the diaphragm plug valve opening process was performed by using a bidirectional fluid-structure interaction based on dynamic mesh technique. The pressure and velocity distribution of the internal flow were obtained with boundary conditions for the fluid–structure interaction during the opening process. The finite element method was used to compare the equivalent stress with FSI method. The results indicate that mechanical tensile stress plays a prominent role in determining the equivalent stress of the diaphragm, with the highest stress concentration occurring in the root region. The electrolyte flow in the valve tends to be stable when reaching an opening degree of 56.6 %. Crystallization accumulation will occur due to the existence of vortices in the flow field around the diaphragm which affects the fluidity of the fluid, this area needs to be focused on.

Jet Electroforming of High-Aspect-Ratio Microcomponents by Periodically Lifting a Necked-Entrance Through-Mask.

High-aspect-ratio micro- and mesoscale metallic components (HAR-MMMCs) can play some unique roles in quite a few application fields, but their cost-efficient fabrication is significantly difficult to accomplish. To address this issue, this study proposes a necked-entrance through-mask (NTM) periodically lifting electroforming technology with an impinging jet electrolyte supply. The effects of the size of the necked entrance of the through-mask and the jet speed of the electrolyte on electrodeposition behaviors, including the thickness distribution of the growing top surface, deposition defect formation, geometrical accuracy, and electrodeposition rate, are investigated numerically and experimentally. Ensuring an appropriate size of the necked entrance can effectively improve the uniformity of deposition thickness, while higher electrolyte flow velocities help enhance the density of the components under higher current densities, reducing the formation of deposition defects. It was shown that several precision HAR-MMMCs with an AR of 3.65 and a surface roughness (Ra) of down to 36 nm can be achieved simultaneously with a relatively high deposition rate of 3.6 μm/min and thickness variation as low as 1.4%. Due to the high current density and excellent mass transfer effects in the electroforming conditions, the successful electroforming of components with a Vickers microhardness of up to 520.5 HV was achieved. Mesoscale precision columns with circular and Y-shaped cross-sections were fabricated by using this modified through-mask movable electroforming process. The proposed NTM periodic lifting electroforming method is promisingly advantageous in fabricating precision HAR-MMMCs cost-efficiently.

Comparison of Fluid Dynamics of Different Flow Geometries in an Iron Flow Battery Prototype

Introduction: Renewable energy sources demand is increasing in the world due to their less environmental effect. Solar and wind energy, which requires largescale electrical energy storage, are major contributors to this growth. The iron flow battery is one of the Redox flow battery technologies, which is encouraging for electrical energy storage because of their long lifetime, flexibility to increase storage, and minimum chemical hazard compared to conventional batteries Method: In this research, the engineering design and fabrication of a locally made Iron flow battery prototype is described. Then, electrolyte flow simulation was carried out through COMSOL Multiphysics to compare the fluid velocity and pressure profiles in three different flow geometries such as plain, parallel, and serpentine, to select the most compatible flow pattern for the Iron flow battery. Result: The resulting velocity profiles indicated that plain flow had stagnated points, the parallel flow had uneven velocity distribution, and serpentine flow had a uniform and high-velocity profile. The simulation results of pressure profiles showed the serpentine, parallel, and plain flows had inlet pressures of 34.38 Pa, 10.01 Pa, and 0.254 Pa, respectively. Conclusion: Given that pumping power is directly proportional to the pressure gradient, the power requirement of electrolyte pumps from highest to lowest is as serpentine flow, parallel flow, and plain flow.

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.

Modeling of nanochannels in synthetic membranes

The behavior of a diluted electrolyte in a system of joint microchannel and nanochannel with charged dielectric walls under the action of external potential difference and external pressure is investigated numerically. The surface charge on the nanochannel walls prevents the ions of corresponding polarity from passing through it. Consequently, the system in question acquires ion-selective properties and can, under certain assumptions, be viewed as a fragment of an ion-selective membrane, including one synthesized by creating nanopores in a dielectric material. Such systems are used in experiments to control the movement of charged particles through concentration polarization. The objective of the work is to investigate the influence of a single pore on electrolyte flow and the possibilities to control that flow by changing the geometric and physical properties of the pore. The investigation relies on the specially developed simplified models based on cross-section-averaged Nernst-Planck, Poisson and Stokes equations that are subsequently reduced to a single nonlinear differential equation. The simplified models allow identifying the impact of different physical mechanisms of electrolyte movement: pressure-based (generated by the external mechanical action) and electroosmotic (generated by the electric field). A finite-difference method with semi-implicit time integration is used for the numerical solution of equations. It has been found that the behavior of the system qualitatively matches the behavior of a cell based on a non-ideally-selective ion-exchange membrane. In particular, the model correctly predicts the underlimiting and limiting electric current regimes, as well as vortex formation near the nanochannel inlet due to concurrency between electrolyte movement mechanisms. The proposed models can be extended to describe a channel with arbitrary geometry and an electrolyte with arbitrary number of charged species.

Facile Preparation of Stable NiII‐ and CoII‐tetraaminophthalocyanine Electropolymers for Highly Efficient Heterogeneous Carbon Dioxide Reduction

This study focused on preparation of stable polymer films of NiII‐ and CoII‐tetraaminophthalocyanines, p‐NiTAPc and p‐CoTAPc, respectively, for highly efficient heterogeneous electrochemical carbon dioxide (CO2) reduction in a flow electrolysis cell. Major development represented in this work was fabrication of p‐NiTAPc and p‐CoTAPc films via electropolymerization of their corresponding monomers on carbon‐based substrates without using binder or conducting additive materials to obtain efficient gas diffusion electrodes (GDEs) for scalable, productive and selective CO2‐to‐CO conversion. The target polymers were characterized by UV‐visible spectrophotometry, attenuated total reflection‐Fourier transform infrared spectroscopy, scanning electron microscopy, energy dispersive X‐ray spectroscopy and cyclic voltammetry. According to controlled potential electrolysis and gas chromatography, p‐NiTAPc‐catalyzed CO2 reduction at –0.99V vs. reversible hydrogen electrode (RHE) gave 953 mL of CO in a period of 16 hours with current density and Faradaic efficiency (FE) of 109±1 mA·cm‐2 and 99±2%, respectively. A p‐CoTAPc‐modified GDE exhibited superior catalytic performance to the case of p‐NiTAPc in terms of catalyst stability and CO productivity by performing the continuous CO2 reduction at the potential of –1.10 V vs. RHE for up to 41 hours and affording almost 3 times higher amount of CO with the current density of 161±5 mA/cm2 and 95±2% FE.

On Performance Enhancement of the STED Process Using Modified Tool Electrode

Shaped tube electrolytic drilling (STED) is an electrochemical machining variant that employs a tubular tool electrode to produce holes with a high aspect ratio on hard-to-machine materials. The tubular electrodes with a diameter below 1 mm produce undesired residue (center-peak) at the machining surface that hampers the machining rate. Therefore, this study attempts to improve the electrolyte flow and enhance the electrochemical dissolution through tool modification. The performance enhancement of the STED process in terms of material removal rate and average diametral overcut has been explored. The issues related to the limitations in material removal in the STED process are brought forth with the technique to resolve those difficulties. The experiments were conducted with the modified tubular tool, and the length of slits on the tool was selected based on the simulation insights and pilot study. The effects of input process parameters (applied voltage, tool feed rate, electrolyte concentration, and tool slit length) on the output responses obtained from the STED process are elaborated. Holes with diameters in the range 0.89–0.97 mm and 12 mm depth were fabricated. Further optimization of the process parameters in the design space is also presented to obtain sustainable process performance.

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.

A Versatile Electrochemical Cell for Operando XAS

AbstractIn situ and operando X‐ray absorption spectroscopy (XAS) provides fundamental insight into the working principles of electrocatalysts and is an important tool for future catalyst development. However, the design of an operando XAS electrocatalytic cell is not facile, and researchers designing cells, whether new cells or modifications to previous cells, often spend many hours on cell design before obtaining high‐quality XAS data. Here, we describe the design, with engineering drawings, and operation of a versatile XAS cell with options for gas flow, electrolyte flow, pH monitoring, temperature monitoring, and the ability to handle many catalyst forms (any catalyst that can be deposited onto a conductive X‐ray transparent substrate). We benchmarked XAS spectra collected using the new experimental cell to a previous cell design showing its ability to produce quality XAS data. We demonstrate the viability of this cell by providing insight into electrocatalysts by studying cation effects and show the tetrabutylammonium cation prevents bulk oxidation of copper. We hope the availability of this cell allows researchers to convert time typically spent on cell design to time spent on breakthroughs in electrocatalysis.

Molecular tuning boosts asymmetric C-C coupling for CO conversion to acetate.

Electrochemical carbon dioxide/carbon monoxide reduction reaction offers a promising route to synthesize fuels and value-added chemicals, unfortunately their activities and selectivities remain unsatisfactory. Here, we present a general surface molecular tuning strategy by modifying Cu2O with a molecular pyridine-derivative. The surface modified Cu2O nanocubes by 4-mercaptopyridine display a high Faradaic efficiency of greater than 60% in electrochemical carbon monoxide reduction reaction to acetate with a current density as large as 380 mA/cm2 in a liquid electrolyte flow cell. In-situ attenuated total reflectance surface-enhanced infrared absorption spectroscopy reveals stronger *CO signal with bridge configuration and stronger *OCCHO signal over modified Cu2O nanocubes by 4-mercaptopyridine than unmodified Cu2O nanocubes during electrochemical CO reduction. Density function theory calculations disclose that local molecular tuning can effectively regulate the electronic structure of copper catalyst, enhancing *CO and *CHO intermediates adsorption by the stabilization effect through hydrogen bonding, which can greatly promote asymmetric *CO-*CHO coupling in electrochemical carbon monoxide reduction reaction.

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.

Efficient Electrosynthesis of Hydrogen Peroxide Using Oxygen-Doped Porous Carbon Catalysts at Industrial Current Densities.

Metal-free carbon catalysts (MFCCs) are one of the commonly used catalysts for electrocatalytic two-electron oxygen reduction (2e- ORR) synthesis of hydrogen peroxide (H2O2). Oxygen doping is an effective means to improve the performance of MFCCs, but the performance of oxygen-doped carbon catalysts is still not high enough, and the contribution of different oxygen functional groups (OFGs) to the catalytic performance is still inconclusive. In this paper, carbon-based catalysts with different oxygen contents and ratios of OFGs were prepared, and the high 2e- ORR activity of COOH + C-OH was demonstrated by combining the results of experiments and theoretical calculations. The prepared oxygen-doped carbon-based catalyst C-0.1M80 achieved an onset potential of 0.795 V (vs RHE), a selectivity of up to 98.2% (0.6 V vs RHE), and a H2O2 oxidation current of 1.33 mA cm-2 (0.5 V vs RHE) in a rotating ring-disk electrode test (0.1 M KOH solution), which was an outstanding performance in MFCCs. In a solid electrolyte flow cell, C-0.1M80 achieved a Faraday efficiency of 97.5% at 200 mA cm-2 with a corresponding H2O2 production rate of 123.7 mg cm-2 h-1. In addition, a flow cell stability test was performed at an industrial current density (100 mA cm-2) with an astounding 200 h of uninterrupted operation, also achieving an outstanding average Faradaic efficiency (95.8%).

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.

Early Investigations on Electrolyte Mixing Issues in Large Flow Battery Tanks

Most investigations on flow batteries (FBs) make the assumption of perfectly mixed electrolytes inside the tanks without estimating their likelihood, while specific analyses are missing in the literature. This paper presents a pioneering investigation of the electrolyte flow dynamics inside FB tanks. This study considers the Open Circuit Voltage (OCV) measured at the stack of a 9 kW/27 kWh Vanadium FB with 500 L tanks. Order-of-magnitude estimates of the measured dynamics suggest that differences in densities and viscosities of the active species drive gradients of concentrations with different patterns in the positive and negative tanks and in charge and discharge, affected by current and flow rate, which result in significant deviation from homogeneity, affecting the State of Charge (SoC) of the electrolytes flowed into the stack and thus the FB performance. In particular, stratifications of the inlet electrolytes may appear which are responsible for delays in reaching the outlets, with initial plateau and following step (s) in the SoC at the stack. These events can have a major impact in the performance of industrial FBs with large tanks and suggest that specific tank designs may improve the overall dynamics, calling for further analysis.

Filter paper as electrolyte flow transport using vaporized methanol as fuel in a microfluidic fuel cell: Experimental and numerical simulation

In this work, we present a design of a paper-based microfluidic fuel cell (μFC), which employs the spontaneous capillary flow of reactant solutions in a filter paper to accomplish passive conveyance of the fuel and oxidant. This self-pumping device uses methanol vapor as a fuel. The gas phase in the microfluidic fuel cell increases the fuel supply to the anode due to a higher diffusion coefficient of 1.5 ​× ​10−5 ​m2 ​s−1 compared with 5 ​× ​10−9 ​m2 ​s−1 for liquid phase. An air-breathing cathode is incorporated to paper-based μFC through which atmospheric oxygen is continuously supplied. The paper-based μFC performance is studied by polarization curves and chronoamperometry to determinate the power output and stability. Peak power of 1.49 ​mW and a stable current of 1.35 ​mA at 0.35 ​V for 28 ​h can be achieved with this prototype under room temperature. To interpret the device performance a numerical model is developed and validated with the experimental polarization curve. The fuel and oxidant concentration profiles in the electrodes from the model demonstrates a constant species availability at the cathode and anode and explains the stable current obtained in the experimental measurements. Subsequently, a stack of four MμFCFP was developed and evaluated in both series and parallel connections. In the parallel configuration, a maximum open circuit potential (OCP) of 0.69 ​V with a maximum current and power output of 34.53 ​mA and 4.14 ​mW are delivered, respectively. Conversely, in the series connection, a total current of 7.35 ​mA, an OCP of 2.39 ​V and a maximum power of 3.57 ​mW are reached. As a proof of concept, the stack successfully operates a 3 green LEDs array, each requiring a 2.1–2.5 ​V and 4.2–5 ​mW power to function, for a continuous duration of 3 ​h.

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.

Numerical Simulation of Cathode Nodule Local Effects

As one of the main factors decreasing current efficiency and product quality, the growth of nodules deserves attention in the copper electrorefining process. Three-dimensional (3D) Finite Element Method models combining tertiary current distribution and fluid flow were established in this study to investigate the details of the growth of columnar nodules, including the electrolyte flow around the nodule and its effects. Compared with an inert nodule, a significant impact of the electrochemical reaction of an active nodule has been observed on the fluid flow, which may be one of the reasons for the formation of small nodule clusters on the cathode. Furthermore, the local current density is not even on the nodule surface under the comprehensive influence of local electrolyte flow, local overvoltage, and the angle with the anode surface. Thus, the head of an active nodule grows faster than the root, and the upper parts grow faster than the lower parts, leading to asymmetric growth of the nodules.

Electrolytic bubble-based flow sensing using electrochemical resistance measurement in a microchannel

Electrolytic bubble-based flow sensing using electrochemical resistance measurement in a microchannel

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.