Flow-through Porous Electrodes Research Articles

Removal of Chromium ions (Cr6+) and Nickel ions (Ni2+) from Simulated Industrial Wastewater Using Flow-by-Porous Electrode

The quality of water is significantly impacted by the presence of Cr6+ and Ni2+ ions. This study investigates the effectiveness of a flow-by porous graphite electrode cell in removing these contaminants from simulated industrial wastewater. We explore the impact of various factors on the removal process, demonstrating the method's potential for efficient removal. The initial concentration of nickel and chromium ions (20 to 80 mg/l and 20 to 100 mg/l, respectively), the feed flow rate (0.28 to 1.11 ml/s), current density (0.2 to 2.25 mA/cm2) and pH all influence the removal rate and efficiency. A higher feed flow rate negatively affects the removal efficiency of both Ni2+ and Cr6+ ions. Nickel removal efficiency decreased by 34.9% at 20 ppm and 26% at 80 ppm, representing the highest and lowest reductions in efficiency, respectively. Chromium removal efficiency decreased by 19% at 100 ppm and 6.5% at 50 ppm, indicating the highest and lowest reductions in efficiency, respectively, under the same flow rate change. Under optimal conditions, the removal efficiency for Ni2+ was 99.47% after 15 min of operation at a current density of 1.96 mA/cm2, a flow rate of 0.28 ml/s, and a pH of 8 and the removal efficiency for Cr6+ was 99.97% after 10 min of operation at a current density of 2.25 mA/cm2, a flow rate of 0.28 ml/s, and a pH of 2. The flow-through porous electrode system achieves efficient heavy metal removal with operating costs of 0.24 USD/m3 for nickel and 0.38 USD/m3 for chromium at optimal conditions.

Flow-through porous electrode for enhanced oxygen evolution reaction by promoting gas bubble release from water splitting

Several CoS2/Ni flow-through porous electrodes are fabricated to promote gas bubble release for enhanced oxygen evolution reaction. Structural characterization shows that CoS2 catalyst is immobilized on the porous Ni substrate. The bubble average diameter can be decreased from 29.21 ± 17.9 μm to 13.38 ± 5.05 μm, and the desorption time is decreased from 1.615 s to 0.067 s, with the electrolyte velocity increasing from 0 cm/s to 2.89 cm/s at the current density of 100 mA/cm2. A mathematical model is proposed to describe gas bubble growth, which is found that higher current density is not bring much bigger gas bubble. The overpotential of the CoS2/Ni electrode for OER is 498 mV at 100 mA/cm2. The overpotential of CoS2/Ni electrode can be decreased from 754 mV to 661 mV with the electrolyte velocity increase from 0 cm/s to 9.42 cm/s at 200 mA/cm2 with the energy required for hydrogen production reduced by 5.01 kWh/kmol.

Alkalinity Enhancement during Reject Brine Electrolysis: Role of Electrocatalyst Placement on the Outer Surfaces of Porous Flow-Through Electrodes

Alkalinity Enhancement during Reject Brine Electrolysis: Role of Electrocatalyst Placement on the Outer Surfaces of Porous Flow-Through Electrodes

(Invited) Electrifying Nitrogen Transformations: Decarbonizing Ammonia Using Plasma Catalysis and Electrochemical Reduction of Nitrate in Wastewaters

Nitrogen fixation in fertilizers forms the basis of modern agriculture and mediates global food insecurity. Energy-dense nitrogen-based compounds also represent promising carbon-free alternative fuels when synthesized using sustainable energy and materials. However, conventional thermally-activated nitrogen conversion processes consume substantial amounts of fossil fuels as materials and energy inputs, leading to an unsustainable energy and carbon footprint. Therefore, electron-driven approaches are needed to establish fossil-free nitrogen interconversions. Electrochemical and plasma-activated reactions operate under mild conditions, enabling facile coupling to intermittent renewable energy sources to reduce CO2 emissions and reactive nitrogen pollution, facilitating storage and transportation of renewable energy in nitrogen-based fuels.Non-thermal plasma has been used to synthesize ammonia under mild conditions, but the dearth of fundamental understanding of plasma catalytic reactions handicaps the development of efficient plasma catalytic N2 conversion. Therefore, an in situ FTIR reactor was employed in combination with steady-state flow reactor experiments and plasma kinetic modeling to elucidate the surface reaction mechanisms and plasma-catalyst interactions. Ammonia yield can be influenced by plasma-derived intermediates and their interactions with catalyst surfaces, which lead to different reaction pathways. Furthermore, techno-economic analysis was employed to reveal the threshold efficiency required for a plasma process to become environmentally and economically competitive.Beyond electrification of N2 fixation for ammonia synthesis, existing waste streams containing fixed nitrogen as nitrate may be mined for fertilizer or carbon-free renewable energy storage while avoiding the need for H2 as a reagent, via selective nitrate electroreduction to ammonium. Nitrate electrochemical reduction can eliminate the production of concentrated waste streams while avoiding the addition of reductant or hole scavenger chemicals. However, major challenges for nitrate removal from water via electrochemical conversion involve reducing the use of expensive precious metal electrocatalysts while also improving the reaction activity and selectivity, stability, and mass transport of nitrate to electrocatalyst active sites. Therefore, electrochemical membranes were employed as porous flow-through electrodes to address these challenges based on improved mass transport and altered kinetics under flow conditions within membrane pores.Conductive electrified membranes, fabricated using inexpensive materials and green synthesis methods, showed significantly higher nitrate removal efficiency during electrified filtration compared to flow-by mode. The small pore sizes in the membrane reduced the diffusional boundary layer by several orders of magnitude with respect to flow-by mode, overcoming diffusional mass transport limitations. During electro-filtration, the nitrate conversion activity was easily controlled by tuning the permeate flow rate, regardless of applied potential. These multifunctional electrified membranes for reduction of nitrate in wastewaters have the potential to partially displace carbon-intensive industrially synthesized ammonia while simultaneously accomplishing water decontamination.

ESTIMATION OF THE OPTIMUM BED THICKNESS OF A FLOWTHROUGH POROUS ELECTRODE (FTPE) WORKING UNDER MASS TRANSFER CONTROL

In this paper, a theoretical analysis of optimum bed thickness operates under mass transfer control for realizing a high efficiency and reaction conversion of an electrochemical reactor has been made based on flowthrough porous electrode (FTPE) configuration. Many models have been used to represent the optimum bed thickness by taking a look into previous works concerned and collecting all related information, data, and models. The parameters that affect the optimum bed thickness have been visualized and reviewed, and almost all of them have been examined by experimental data from different sources and based on the various models. It has been found that the increase in electrolyte flow rate, concentration, limiting current density, and specific surface area reduce the optimum bed thickness, and the increase in electrolyte conductivity, void fraction, and overpotential range increases optimum bed thickness. The most important design parameter that has a great effect on optimum bed thickness is found to be the electrolyte flow rate for any certain operation. It has been concluded that the most appropriate two models to represent the optimum bed thickness of FTPE electrochemical reactor operating under mass transfer control based on the results are those predicted theoretically and stated by Kreysa in (1978) and Doherty et al. in (1996).

The optimal electrode pore size and channel width in electrochemical flow cells

Microfluidic fuel cells, electrolyzers, and redox flow batteries utilize laminar flow channels to provide reactants, remove products and avoid their crossover. These devices often also employ porous flow-through electrodes as they offer a high surface area for the reaction and excellent mass transfer. The geometrical features of these electrodes and flow channels strongly influence energy efficiency. We derive explicit analytical relations for the optimal flow channel width and porous electrode volumetric surface area from the perspective of energy efficiency. These expressions are verified using a two-dimensional tertiary current distribution and porous electrode flow model in COMSOL and are shown to be able to predict optimal parameters in commonly used flow-through and interdigitated flow fields. The obtained analytical models can dramatically shorten modelling time and expedite the industrial design process. The optimal channel width and pore sizes we obtain, in the order of 100 microns and 1 micron respectively, are much smaller than those often used. This shows that there is a significant room for improvement of energy efficiency in flow cells that can sustain the resulting pressure drop.

Membrane-free electrochemical CO2 conversion using serially connected porous flow-through electrodes

<h2>Summary</h2> Bicarbonate electrolysis offers a promising approach to conducting electrochemical carbon dioxide (CO₂) conversion using aqueous carbon capture solutions as the electrolyte. However, conventional membrane-based electrolyzers face risks associated with membrane degradation as well as challenges to scale up that arise from concentration gradients that develop laterally along electrodes as the electrolyte flows parallel to their surfaces. Herein, we present a membrane-free electrolyzer concept based on porous flow-through electrodes for which the aqueous bicarbonate electrolyte flows sequentially through alternating cathodes and anodes. As shown by colorimetric imaging recorded during electrolysis, this configuration facilitates rapid re-balancing of pH between adjacent electrodes while promoting <i>in situ</i> generation of CO₂ immediately upstream of cathodes. Results also show that CO₂ utilization increases monotonically with the number of cells incorporated into a multi-cell device, and modeling predicts that CO₂ utilization rates >80% are possible for optimized multi-cell devices.

Design Rules for Porous Flow-through Electrodes Used in Membrane-Free Electrolyzers for Treatment of Desalination Reject Brine

The world’s growing demand for fresh water is creating increasing stress on existing fresh water sources and environmental hazards related to the generation of high salinity reject brine streams that are produced during seawater desalination. Electrochemical separations technologies offer attractive sources to treat brines to reduce brine salinity and/or harvest valuable chemicals or materials from dissolved species present in the brine. In this work, our team has explored the use of membrane-free electrochemical electrolyzers for treatment and separation processes through their ability to create large pH differentials between anode and cathode effluent streams. Because these devices do not contain membranes, they are attractive for operating in the harsh brine environment, which contains many impurities that can result in membrane scaling or fouling. In the first part of this talk, I will describe the basic operating principles of these electrolyzers, which rely on the use of flow-induced separation of anode and cathode effluent streams to create large pH gradients. During operation, it is generally desirable to maximize pH differentials by choosing flow conditions that limit cross-over of product species, thus ensuring high current utilization, while avoiding unnecessary dilution of H+ and OH- within the product streams. This trade-off between cross-over and dilution was studied for porous flow-through electrodes, where we systematically explore the combined influences of volumetric flow rates, catalyst placement, and operating current densities on the pH differential generated by electrolyzers fed with pH neutral brine solutions. Aided by the use of in situ high speed videography and colorimetric imaging of pH differentials, these experiments show that the Damköhler number is an excellent descriptor that can be used to identify the optimal operating conditions that maximize pH differential. Further, we show that the in-plane distribution of active catalyst loading on the porous electrode support has a large impact on the maximum pH differential that can be achieved. Finally, this presentation will also describe the tolerance of electrode durability to the presence of dissolved alkaline earth metal ions such as Mg2+, which can deposit on cathodes and result in scaling if not properly managed.

Electrochemical CO2 Conversion with Packed Bed Membraneless Electrolyzers

(Bi)carbonate electrolysis offers an attractive opportunity for integrated carbon dioxide (CO2) capture and conversion whereby a carbonate-laden aqueous capture solution can be directly fed to an electrolyzer before being recycled to the CO2 capture unit(s). Among the previous studies that have demonstrated the viability of (bi)carbonate electrolysis, bipolar membranes are commonly used to deliver protons to the cathode where they convert (bi)carbonate into CO2 that is subsequently reduced at the cathode. However, these membranes can be susceptible to fouling or degradation, which may lead to device failure. Here, we present a scalable and potentially low-cost packed bed membraneless electrolyzer (PBME) concept for the conversion of bicarbonate into CO based on train of porous flow-through electrodes. At the anode, hydrogen oxidation reaction is used to produce protons, which rapidly react with bicarbonate to generate CO2 for electrochemical CO2 reduction at the downstream cathode in this membrane-free electrolyzer design. This study highlights the ability of the PBME design to minimize the magnitude of pH swings within the electrolyzer and enable high CO2 utilization rates. Tests of multi-cell PBMEs show enhanced performance compared to single-cell PBMEs and demonstrate the scalability of this PBME design.

Hydrogen generation in additively manufactured membraneless microfluidic electrolysis cell: Performance evaluation and accelerated stress testing

Reducing the cost of sustainable energy technologies is key to enabling the global decarbonization movement. Implementing microfluidics for H2 production reduces the assortment of components required for traditional water electrolyzer, as laminar flow is prevalent in microchannels, eliminating the need for a membrane. Here we demonstrate microfluidic electrolysis cells (microECs) produced by additive manufacturing, which enables rapid and reproducible prototyping. The microEC consisted of three parallel microchannels encompassing a pair of porous flow-through electrodes. The design allows the H2 and O2 gases to be transported through the porous electrodes and collected in outer channels with negligible cross-over between gases and high H2 yield. The space–time yield of H2 increased significantly in the microEC owing to rapid mass transfer in the porous electrodes and microchannel. The electrolyte velocity is optimized (0.16 – 0.34 m s−1) to attain minimum charge and mass transfer resistances while averting gas product intermixing in membraneless operation. The microEC exhibited a current density of 747 mA cm−2 at 2.5 V and 0.34 m s−1 electrolyte velocity. The H2 and O2 volumetric collection efficiency is 99.88 % and 98.35 %, respectively, with 99.35 % energy conversion efficiency based on 1 mm interelectrode distance. Accelerated stress testing on the microEC was performed by imposing a dynamic current stressor with periodic shutdown to mimic power supply from renewable energy sources. The degradation mechanism was identified from the analysis of electrochemical and physicochemical characterizations.

Fabrication of a Porous Flow-Through Electrode for CO2 Reduction to CO in Organic Electrolyte with HCl Oxidized to Cl2 on Anode

Carbon dioxide (CO2) can be electrochemically reduced to value-added fuels and chemicals, which offers a promising way for CO2 recycling utilization. In the present work, we have designed a membrane electrolyzer for CO2 reduction to carbon monoxide (CO) in organic electrolyte, with hydrochloric acid (HCl) oxidized to chlorine (Cl2) on the anode. The obtained CO and Cl2 can be used as feedstock to produce phosgene. Using this method, CO2 can be converted into high-value products, and Cl2 can be recovered from industrial off-gases, which contain HCl. To substantially increase the current density of CO2 electroreduction, we have proposed a novel noncyanide electroplating method to deposit Au on Cu foam to fabricate a porous flow-through electrode. By enlarging the specific surface area of the cathode, and providing a large amount of Au nanoparticles for CO2 electroreduction, the cathodic current density has been increased to 106 mA·cm–2, with faradic efficiency of CO formation stabilized at 90%. On the anode, the faradic efficiency of Cl2 generation is stabled at 92%, with the anodic current density archived to 42 mA·cm–2. Such designed electrolysis cell has a broad application prospect in the chemical industry.

Boosting cell performance and optimizing gas distribution of flow-through microfluidic fuel cells by adjusting flow configuration

Microfluidic fuel cells (MFCs) with flow-through porous electrodes produce gas bubbles under acidic condition, which undoubtedly cover the catalyst surface, hinder the fuel transport and increase the ionic transfer resistance between electrodes. Herein, an effective approach is proposed to optimize the gas bubble distribution and reduce their influence on cell performance. Different from the flow configuration in the conventional flow-through MFCs, the reactant solutions flow from the microchannel between the anode and the cathode to the external channel through porous electrodes. After adjusting the flow configuration, the nucleation and distribution of gas bubbles are changed from the microchannel between anode and cathode to the external one, leading to a decreased ionic transfer resistance between the anode and the cathode. As a result, compared with the MFC with the conventional flow configuration, the maximum power density of the MFC with the new configuration (MFC-N) is increased by 52.9%, and a higher fuel utilization rate is achieved. Moreover, the MFC-N reaches a more stable voltage even at a high current density of 80 mA cm−2. The proposed flow configuration is a good and feasible method for gas management and performance enhancement in the flow-through MFCs.

(Invited) Electrified Membranes for Transformation of Nitrate in Wastewaters

The release of nitrate to the environment from wastewater effluent and agricultural runoff contributes to groundwater contamination, harmful algal blooms, and disruption of biogeochemical nitrogen flows. Typical treatment methods are based on nitrate separation, which produces waste streams that are often discharged to the environment. Alternatively, nitrate conversion via electrochemical reduction eliminates the production of concentrated waste streams while avoiding the addition of reductant or hole scavenger chemicals to accomplish the reaction. However, major challenges for nitrate removal from water via electrochemical conversion involve reducing the use of expensive precious metal electrocatalysts while also improving the reaction activity and selectivity, catalyst stability, and mass transport of nitrate to electrocatalyst active sites. The use of electrochemical membranes as multifunctional porous flow-through electrodes could potentially address these challenges based on improved mass transport and altered kinetics under flow conditions within membrane pores.Conductive membranes were fabricated using polymers combined with carbonaceous materials such as reduced graphene oxide (rGO) and carbon nanotubes. The rGO was functionalized with non-precious transition metal oxynitride electrocatalysts, where these catalysts showed higher nitrate conversion activity compared to the unsupported transition metal nitrides. The influence of catalyst materials, membrane fabrication process, and filtration conditions on nitrate reduction activity and selectivity were evaluated. In addition to the environmental impacts of closing the nitrogen loop by converting nitrate into innocuous N2, selective nitrate reduction to ammonia provides opportunities for recovery as fertilizer or carbon-free renewable energy storage. The prospects for reactive nitrogen recovery based on nitrate electrochemical conversion to ammonia were analyzed for various potential source waters.

Preparation of a Porous Flow-Through Electrode for CO2 Reduction to CO in a Multi-Chamber Electrolyzer in an Organic Electrolyte

Preparation of a Porous Flow-Through Electrode for CO₂ Reduction to CO in a Multi-Chamber Electrolyzer in an Organic Electrolyte

Reactive-Transport Modeling of Electrochemical Oxidation of Perfluoroalkyl Substances in Porous Flow-through Electrodes

Reactive-Transport Modeling of Electrochemical Oxidation of Perfluoroalkyl Substances in Porous Flow-through Electrodes

Computational design of microarchitected porous electrodes for redox flow batteries

Porous flow-through electrodes are used as the core reactive component across electrochemical technologies. Controlling the fluid flow, species transport, and reactive environment is critical to attaining high performance. However, conventional electrode materials like felts and papers provide few opportunities for precise engineering of the electrode and its microstructure. To address these limitations, architected electrodes composed of unit cells with spatially varying geometry determined via computational optimization are proposed. Resolved simulation is employed to develop a homogenized description of the constituent unit cells. These effective properties serve as inputs to a continuum model for the electrode when used in the negative half cell of a vanadium redox flow battery. Porosity distributions minimizing power loss are then determined via computational design optimization to generate architected porosity electrodes. The architected electrodes are compared to bulk, uniform porosity electrodes and found to lead to increased power efficiency across operating flow rates and currents. The design methodology is further used to generate a scaled-up electrode with comparable power efficiency to the bench-scale systems. The variable porosity architecture and computational design methodology presented here thus offers a novel pathway for automatically generating spatially engineered electrode structures with improved power performance.

Boost performance of porous electrode for microfluidic fuel cells: electrochemical modification or structure optimization?

The development of high-performance porous electrode is highly desired in microfluidic fuel cells (MFCs) to improve their output for real-world applications. Previous studies have mainly focused on the electrochemical modification of the porous electrodes to enhance their reaction kinetics. Yet, less attention was paid on the optimization of the electrode porous structure for improved mass transport. To identify proper optimization strategies, a computational model of flow-through type MFC with porous electrodes is established in this work and systematic numerical analyses are conducted to compare the effects of the kinetic and structural parameters of the anode and cathode on the cell performance. Simulation results show that the constraint from the anode is more severe and concentration polarization brought by the insufficient mass transfer in the porous electrodes dominates the cell performance. Thus, structure optimization of the porous electrode for enhanced mass transfer is more promising in the performance enhancement of MFCs in comparison with the electrochemical modification. The specific surface area of the anode is identified as the most influencing factor. After proper structure optimization, an increase of 62.2% can be obtained in the current density at 0.8 V, which is three times of that in the electrochemical modification case. Based on the results, useful guidance is provided for the future development of MFCs. Novelty Statement A computational model of MFC with flow-through porous electrodes is established. Simulation results show that structure optimization of the porous electrode for enhanced mass transfer is more promising in the performance enhancement of MFCs in comparison with the electrochemical modification. After proper structure optimization, an increase of 62.2% can be obtained in the current density at 0.8 V.

Inertially enhanced mass transport using 3D-printed porous flow-through electrodes with periodic lattice structures.

Electrochemical reactors utilizing flow-through electrodes (FTEs) provide an attractive path toward the efficient utilization of electrical energy, but their commercial viability and ultimate adoption hinge on attaining high currents to drive productivity and cost competitiveness. Conventional FTEs composed of random, porous media provide limited opportunity for architectural control and engineering of microscale transport. Alternatively, the design freedom engendered by additively manufacturing FTEs yields additional opportunities to further drive performance via flow engineering. Through experiment and validated continuum computation we analyze the mass transfer in three-dimensional (3D)-printed porous FTEs with periodic lattice structures and show that, in contrast to conventional electrodes, the mesoscopic length scales in 3D-printed electrodes lead to an increase in the mass correlation exponent as inertial flow effects dominate. The inertially enhanced mass transport yields mass transfer coefficients that exceed previously reported 3D-printed FTEs by 10 to 100 times, bringing 3D-printed FTE performance on par with conventional materials.

Current Distribution of Lead Ions Deposition Within Porous Flow-Through Electrodes Operating Under Simultaneous Hydrogen Gas Evolution

Current Distribution of Lead Ions Deposition Within Porous Flow-Through Electrodes Operating Under Simultaneous Hydrogen Gas Evolution

ESSENCE – A rapid, shear-enhanced, flow-through, capacitive electrochemical platform for rapid detection of biomolecules

The rapid, sensitive, and selective detection of target analytes using electrochemical sensors is challenging. ESSENCE, a new Electrochemical Sensor that uses a Shear-Enhanced, flow-through Nanoporous Capacitive Electrode, overcomes current electrochemical sensors' response limitations, selectivity, and sensitivity limitations. ESSENCE is a microfluidic channel packed with transducer material sandwiched by a top and bottom microelectrode. The room-temperature instrument less integration process allows the switch of the transducer materials to make up the porous electrode without modifying the electrode architecture or device protocol. ESSENCE can be used to detect both biomolecules and small molecules by simply changing the packed transducer material. Electron microscopy results confirm the high porosity. In conjunction with the non-planar interdigitated electrode, the packed transducer material results in a flow-through porous electrode. Electron microscopy results confirm the high porosity. The enhanced shear forces and increased convective fluxes disrupt the electric double layer's (EDL) diffusive process in ESSENCE. This disruption migrates the EDL to high MHz frequency allowing the capture signal to be measured at around 100 kHz, significantly improving device timing (rapid detection) with a low signal-to-noise ratio. The device's unique architecture allows us multiple configuration modes for measuring the impedance signal. This allows us to use highly conductive materials like carbon nanotubes. We show that by combining single-walled carbon nanotubes as transducer material with appropriate capture probes, NP-μIDE has high selectivity and sensitivity for DNA (fM sensitivity, selective against non-target DNA), breast cancer biomarker proteins (p53, pg/L sensitivity, selective against non-target HER2).