Flow-by Mode Research Articles

Electroreduction of CO2 to HCOOH on 3D-Ag nanosheet array supported on carbon membrane

Regulating local CO2 concentration and pH near the catalytic active sites could induce effective reaction pathways for electroreduction of CO2 to chemicals. Here, a membrane nanofluid reactor for efficient electroreduction of CO2 to formic acid was prepared by constructing high-density active sites of 3D-Ag nanosheet array on the surface and within the pores of carbon membrane (3D-Ag NA/CM). The local microenvironment of CO2 and H+ concentration was controlled by adjusting the pore size, CO2 flow rate, and mass transfer mode. In the flow-through mode, the CO2 was electro-reduced to formic acid with Faraday efficiency of 94% and the partial current density of 35.8 mA/cm2. Both the selectivity and activity were higher than those in flow-by mode. The low local concentration of CO2 in the channel was the key to the high selectivity of formic acid. When CO2 flowed through the membrane nanopores, the high collision frequency of CO2 with catalyst on the pore walls facilitated the electroreduction of CO2. It found that the presence of OV promoted the adsorption of CO2 and reduced the energy barrier for its activation into ∗CHOO. It lays a foundation for the research on optimizing the performance of CO2 electroreduction with nanofluid reactors.

Electrofiltration-Enabled Nitrate Reduction Reactivity of Carbon Nanotube Electrified Membrane for Drinking Water Treatment

Electrocatalytic nitrate reduction could enable nitrate removal in drinking water without generating concentrated waste streams. However, current processes heavily rely upon metal catalysts with high activity to achieve sufficient nitrate removal, which increases treatment costs and can inevitably lead to leaching of metals into treated water. In this study, we elucidate how electrified filtration enables sufficient nitrate conversion to meet drinking water standards in metal-free defective CNTs by tunable matching of mass transport and reaction timescales. The metal-free carbon nanotube (CNT)-based electrified membrane (EM) was employed as a porous flow-through cathode to decrease the diffusion boundary layer. Computational fluid dynamics simulations revealed that the flow-through mode mitigates diffusion limitations to enhance overall reaction activity, as compared to the conventional flow-by mode at the same flow rate. Additionally, the nitrate reduction rate could be tuned by controlling the nitrate advection rate using permeate flux (0 to 60 L h-1 m-2). A maximum removal efficiency of 86.9% was reached when the mass transport rate matched the reaction rate across a range of applied potentials. Furthermore, defects in CNTs were identified as the catalytic active sites. We employed density functional theory and molecular dynamics simulations to gain insight into CNT defect-catalyzed nitrate reduction under electrofiltration. Finally, the long-term stability, tolerance of environmental interferences, and sufficient nitrate removal and N2 selectivity to meet drinking water standards were demonstrated in synthetic surface water, suggesting that the CNT-EM and flow-through reactor may provide a promising solution for decentralized nitrate destruction in drinking water.

Investigation of the impact of the flow mode in all-vanadium-redox-flow battery using macroporous and mesoporous carbon electrodes

Energy storage in vanadium redox flow batteries (VRFBs) is significantly impacted by both the cell design and the kinetics of electron transfer at the electrode/electrolyte interfaces. In this work, a novel VRFB flow field was designed and evaluated, allowing a portion of the flow to go through the porous electrodes in the direction of current flow. It was found that, when using standard carbon paper electrodes with moderate hydrophilicity, as compared to flow-by or only flow-through, concurrent 50 % flow-through and 50 % flow-by mode decreased the voltage loss by 46 % and increased the voltage efficiency from 74 % to 80 % compared to flow-by only (flow rate of 60 ml s−1). The voltage efficiency and energy efficiency were also enhanced to 82 % and 68 %, respectively, in the flow-through mode. This was likely due to enhanced mass transfer and increased utilization of the internal electrode surface area, in turn lowering the overpotential during both charging and discharging. To further determine the benefits of flow-through conditions, the effect of a binder-free, hydrophilic, and mesoporous carbon scaffold electrode (NCS85) was also investigated under flow-through mode and compared with the standard carbon paper electrodes with moderate hydrophilicity. NCS 85 has monodisperse, 3-D interconnected 85 nm pore diameters and thus a much higher surface area, resulting in a further increase in voltage and energy efficiency to 86 and 65 %, respectively. Heat treatment of the NCS85 to graphitize it and thus lower its resistance showed further improvements when using the flow-through mode, giving a voltage efficiency of 94 % and energy efficiency of 72 %. These improvements show the benefits that can be realized by optimizing both flow mode and carbon electrode porosity and permeability on VRFB performance.

Efficient oxygen reduction to alkaline peroxide at current densities up to 500 mA cm−2 on a gas diffusion electrode with hydrophobic carbon microporous layer: Influence of fluid dynamics and reactor operation on scale-up

The two-electron electrochemical O2 reduction reaction (2e-ORR) on a gas diffusion electrode (GDE) using clean electricity is an attractive process for generating the green oxidant hydrogen peroxide (H2O2). Electrolyte flooding through the porous catalyst and diffusion layers is one of the major factors inhibiting the operational stability and longevity under industrially relevant current densities (>100 mA cm−2). This study reports for the first time that optimizing the two-phase flow and the mode of reactor operation brings about a remarkable improvement in the high current density operation (up to 500 mA cm−2) on a scalable (20 cm2 geometric area) and commercially available Sigracet 39BB GDE with the microporous carbon layer (MPL) acting as the catalyst layer. The interacting effects of current density, gas and liquid flow rates and the flooding threshold limit for the differential liquid/gas pressure were determined. While high peroxide Faradaic efficiency (FE > 95 %) was attained in a gas flow-by mode during short-term experiments, long-term stability was only established when the gas was switched to the flow-through or quasi-flow-through mode of operation. Under the latter conditions, at 500 mA cm−2 operational stability of more than 20 h was demonstrated with a peak FE of 97 % in 1.0 M KOH, at 293 K, 1 atm. It was found that optimizing the reactor operation has a profound effect during scale-up, and developing novel catalytic materials for 2e-ORR to alkaline peroxide is not warranted since a low-cost microporous carbon-based GDE is sufficient for stable and efficient operation.

A continuous flow reactor for tubular gas diffusion electrodes

Tubular gas diffusion electrodes (GDEs) have been mainly characterized in H-cell environments without consideration of changing flow conditions and scale-up possibilities. In this study, a continuous flow reactor for tubular GDEs is presented, which allows for different electrode configurations and manipulation of flow conditions to control mass transfer. 3D printed copper GDEs are employed for CO2 electroreduction while membrane electrode assemblies (MEAs) from stainless steel electrodes are used on the anode side. We demonstrate the impact of the gas supply in either flow-through or flow-by mode on catalyst performance. Flow-through mode yields a variety of typical products, such as CO, HCOOH, CH4 and C2H4. Flow-by mode results in mainly CH4, which we link to a higher surface coverage of reaction intermediates with diffusion as the governing transport mechanism in flow-by operation. Additionally, a hybrid porosity GDE is introduced that combines different functional domains in one electrode for improved CO2 mass transfer. Microcomputed tomography confirms the successful realization of these domains in close proximity to each other. With the presented approach, we aim to highlight the importance of the reaction environment for an effective scale-up of tubular concepts in CO2 electroreduction and many other electrochemical applications.

Boosting nitrate electroreduction reactivity with a 3D Cu-Pd bimetallic flow-through electrode

NO3− pollution in water and wastewater is a pressing global environmental issue, leading to the deterioration of water quality and an increased risk to human health. Herein, we present a three-dimensional (3D) flow-through electrode composed of electrodeposited Cu-Pd bimetallic catalysts on a nickel foam. The Cu-Pd bimetallic particles exhibited cluster-like shapes on the surface of the nickel foam. This electrode design enhanced the electrochemical activity of the NF/Cu63Pd37 electrode for NO3− reduction, as evidenced by linear sweep voltammetry (LSV) measurements. By tailoring the Cu-Pd ratio (Cu:Pd 0.63:0.37), the NF/Cu63Pd37 electrode showed the highest reactivity of NO3− reduction and selectivity (43.1 %) toward N2. Importantly, the NF/Cu63Pd37 electrode not only achieved over 99 % NO3− removal but also exhibited a higher kinetic rate constant of 1.44 h−1 in the flow-through mode. In contrast, in the flow-by mode, NO3− removal was 67.5 % with a lower reaction rate constant of 0.30 h−1. Furthermore, the NF/Cu63Pd37 electrode exhibited a reduced energy consumption (EEO) of 6.86 kWh m−3 order−1 in the flow-through mode, contrasting with 13.66 kWh m−3 order−1 in the flow-by mode. Moreover, the results also demonstrated successful NO3− remediation across various initial pH values (pH 3, 6, 9, and 11) and NO3− concentrations (100, 200, and 300 mg L−1). The results of this study highlight that the 3D Cu-Pd bimetallic flow-through electrode, with its high reactivity in reducing NO3−, marks a significant advancement in efficiently treating nitrate-contaminated water.

Effective attenuation of extracellular antibiotic resistance gene risks in wastewater by capacitive deionization

Despite the effective inactivation on antibiotic resistant bacteria, current water and wastewater treatment technologies are often ineffective in removing antibiotic resistance genes (ARGs), which remain active outside cells, known as extracellular ARGs (eARGs). eARGs can persist for a long time and facilitate bacterial transformation in receiving environments, thereby, posing a threat to public health. This study applied capacitive deionization (CDI), as a promising desalination technology, for eARGs removal from wastewater. Results showed that CDI using carbon electrodes was significantly more effective than activated carbon adsorption in removing eARGs, with the removal efficiency and rate constant that are 2.1–11.2 times (p < 0.05) and 2.5–13 times (p < 0.05) higher. In the flow-by mode, CDI achieved the highest removal of eARGs (3.5 log) in just 15 min. Furthermore, CDI treatment significantly decreased the frequency of horizontal gene transfer (HGT) and increased potential for natural degradation, thereby reducing the risk of eARGs propagation. The high electric field (>1000 V/m) applied in CDI induces DNA conformational damage and DNA sequence mutations, which are the main mechanisms for removing eARG. This study will provide technical guidance and theoretical support for the removal of eARG in wastewater.

Efficient multi-stage electrochemical flow-through system for refractory organic pollutant treatment: Kinetics, mass transfer, and thermodynamic analysis

Improving the kinetics rate and mass transfer is essential for expanding the potential of electrochemical technologies in wastewater treatment. The electrochemical flow-through configuration promises a high oxidation efficiency and low energy consumption. We aimed to provide a thorough understanding of the enhanced kinetics, mass transfer, and thermodynamic parameters during the degradation of amoxicillin (AMX) in a multi-stage flow-through (MSFT) system using porous Ti-ENTA/SnO2–Sb anodes. All operating conditions strongly influenced the kinetics of AMX degradation and followed pseudo-first-order rate kinetic model (R2 > 0.85), with the highest kobs of 0.228 min−1 at high temperature (318 K).In comparison to the flow-by mode, the AMX removal rate in the three-stage flow-through mode was greatly enhanced by 70%, exhibiting the superior capacity of a porous anode. This system exhibited outstanding performance regarding the high kinetics rate and mass transfer rate (km), which increased by factors of 3.46 and 10.74, respectively, obtained in the flow-by mode. It also revealed that •OH generation was 5.64 times higher, and the EE/O was 19.89-fold lower than those in flow-by mode. Temperature plays a vital role in the reaction process, and thermodynamic features found the positive enthalpy (ΔHo) of +27.06 kJ mol−1, signifying the process was endothermic. A Hatta number (Ha) of >0.02 at all temperatures proved this finding, confirming an undeniable role in mass transfer. Finally, these findings reveal the system's performance and offer the possibility of establishing a multi-stage flow-through for wastewater treatment.

Titanium mesh as the anode of electrochemically active biofilm sensor for improved sensitivity in water toxicity real-time early-warning

As serious water ecological pollution caused by toxicant leakage occurs frequently, early-warning for toxicity presented in water environment attracts increasing attentions as it saves time to retain water safety and human health. Electrochemically active biofilm (EAB) sensor is a promising device for in situ real-time water toxicity early-warning. To improve the sensitivity of EAB sensor particularly for low-concentration toxicity warning, this study employed titanium mesh (TiM) as the anode to construct an EAB sensor. Compared to traditional EAB sensor with carbon felt (CF) anode, the sensitivity of the TiM sensor was increased up to 37.4 times. The effects of mesh size (TiM50, TiM100, TiM150) and operation mode (flow-by and flow-through) on the sensitivity of TiM sensors were further investigated. Results showed the sensor with TiM100 anode had the highest inhibition rate (IR) in flow-by mode, attributed to low charge transfer resistance (Rct) and fast mass transfer. Flow-through operation could further enhance TiM100 sensor's sensitivity from flow-by operation and succeeded to signal as low as 0.0025% formaldehyde, the lowest so far tested in EAB sensor with sensing anode. Multiple toxicity shocks on flow-through TiM100 sensor revealed its good recoverability towards all tested formaldehyde concentration from 0.01% to 0.0025%, during which electrochemical activity degradation and biomass accumulation partially impaired the repeatability. This work highlights the great improvement of EAB sensors by utilizing titanium mesh as EAB carrier and provides a reference for the practical application of metallic materials for EAB sensors.

(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.

Electrochemical abatement of aquatic metoprolol by porous foam-titanium based metal oxide anodic membranes

Electrochemical membrane filtration (EMF) is a promising advanced oxidation technology for eliminating aquatic refractory pharmaceuticals. In this work, three porous foam-titanium based metal oxide (MOx) anodic membranes were prepared for comparison of their effectiveness against the model pharmaceutical pollutant metoprolol. The electrooxidation efficiencies of metoprolol by all flow-by operated anodic membranes were found to be elevated with increasing charging voltage and decreasing electrolyte solution pH, in which the PbO2 membrane exhibited more efficient metoprolol degradation efficiency over SnO2 and RuO2 membranes. At the chosen charging voltage of 2 V and electrolyte solution pH 7, compared to the flow-by mode, the higher total organic carbon (TOC) removal and less electrical energy consumption was achieved by the flow-through operated PbO2 membrane, ascribed to the enhanced mass-transfer of metoprolol and its intermediate by-products toward the anode surface. The physisorbed and free HO• produced by PbO2 membrane were found to be the key oxidants accounting for metoprolol degradation, which induced the transformation of metoprolol by attacking its benzene ring and side chains. Apart from the capacity for detoxifying metoprolol-containing influent, PbO2 membrane also exhibited favorable stability in flow-through mode. The study provides an effective strategy for the electrochemical remediation of pharmaceutical-polluted water and wastewater.

Electrochemical removal and recovery of phosphorus from wastewater using cathodic membrane filtration reactor

Removal and recovery of phosphorus (P) from wastewater is of great importance to addressing the challenges of eutrophication and phosphorus shortage. The P removal and recovery performance of conventional electrochemical precipitation approach was constrained by the limited mass transfer rate. Herein, a cathodic membrane filtration (CMF) reactor was developed using Ti/SnO2-Sb anode and titanium mesh cathodic membrane module to achieve efficient removal and recovery of P in wastewater. Compared with the flow-by mode, the CMF system in the flow-through mode exhibited excellent P removal performance due to the markedly enhanced mass transfer. At the current density of 4 A/m2, membrane flux of 16.6 L m−2 h−1, and Ca/P molar ratio of 1.67, the removal efficiency of P was 96.2% and the energy consumption was only 45.7 kWh/kg P. The local high pH of cathode surface played a vital role in P removal, which substantially accelerated the nucleation of calcium phosphate (CaP). Based on the crystalline and morphological characterization of the precipitates, the hydroxyapatite was the most stable crystalline phase of CaP, which was transformed from intermediate phases (such as dicalcium phosphate and amorphous calcium phosphate). This study paves the way for applying electrochemical membrane filtration system for P removal and recovery from wastewater.

A Two-Dimensional Transient Model to Investigate the Influence of Flow Field Design on Zinc Deposition and Performance in a Zinc-Iodide Flow Battery

Zinc-based flow batteries are attractive because they have high aqueous solubility and hence high energy density, as well as rapid kinetics [1,2]. Among the aqueous redox flow batteries (RFBs), the zinc-iodide flow battery has been reported to have very high discharge energy density [1]. However, aqueous zinc batteries suffer from dendrite formation resulting in capacity fading and a decreased battery performance. In our previous study [3], we evaluated a novel flow field design for the zinc-iodide flow battery. When compared to a conventional flow-by (FB) mode, significantly higher charge and energy efficiencies were obtained with the novel arrangement. In the novel flow design, part of the was drawn through the porous electrode towards the current feeder, and this was found to enhance the effective surface area and decrease the ohmic loss. This design, called flow-by-through (FBT), with a component of flow in the direction of the current feeder, can prevent the blockage of the mass/ion transport pathway and enhance the flow distribution in hybrid flow batteries [4]. Based on X-ray computed tomography and SEM characterization, a denser zinc deposit, with more zinc deposition inside the felt were observed with the FBT mode, which may explain the increased discharge capacity and cycle life of the battery [3].A mathematical model of the zinc-iodide RFB can help us better understand the battery performance in the different flow modes. Models of RFBs have been used to investigate the impact of the flow field on performance since 1980 [5]. In this study, we have used a two-dimensional transient model with the COMSOL multi-physics software package to study the galvanostatic charge/discharge behavior of the zinc-iodide RFB in FB and FBT modes. The model considers the electrochemical kinetics, mass transport and the current and potential distribution in a single cell zinc-iodide flow battery. This is the first reported modeling study of the influence of the FBT design on the performance of an RFB.The galvanostatic charge-discharge of a zinc-iodide battery was modelled at a current density of 20 mA cm-2, and a charge duration of 20000 s, corresponding to a state of charge of 25.7%. These conditions were chosen to enable comparison and validation with experimental data are shown in Figure 1. The rate of zinc ion consumption and the amount of zinc deposition on the graphite felt electrode for both FB and FBT arrangements were evaluated using the model. The distribution of zinc deposition, potential and current, will be presented, which provide new insights into the impact of the flow arrangement on the battery performance. The model will be applied to predict cell voltage and efficiency, maximum discharge capacity and maximum discharge power density of the zinc-iodide RFB for FB and FBT arrangements.[1] Li, B., Nie, Z., Vijayakumar, M., Li, G., Liu, J., Sprenkle, V., & Wang, W. (2015). Ambipolar zinc-polyiodide electrolyte for a high-energy density aqueous redox flow battery. Nature communications, 6(1), 1-8.[2] Higashi, S., Lee, S. W., Lee, J. S., Takechi, K., & Cui, Y. (2016). Avoiding short circuits from zinc metal dendrites in anode by backside-plating configuration. Nature communications, 7(1), 1-6.[3] ShakeriHosseinabad, F., Daemi, S. R., Momodu, D., Brett, D. J., Shearing, P. R., & Roberts, E. P. (2021). Influence of Flow Field Design on Zinc Deposition and Performance in a Zinc-Iodide Flow Battery. ACS Applied Materials & Interfaces, 13(35), 41563-41572.[4] Zhou, X., Lin, L., Lv, Y., Zhang, X., Fan, L., & Wu, Q. (2020). Elucidating effects of component materials and flow fields on Sn–Fe hybrid flow battery performance. Journal of Power Sources, 450, 227613.[5] Trainham, J. A., & Newman, J. (1981). A comparison between flow-through and flow-by porous electrodes for redox energy storage. Electrochimica Acta, 26(4), 455-469. Figure 1

An efficient and robust flow-through electrochemical Ti4O7 membrane system for simultaneous Cr(VI) reduction and Cr immobilization with membrane cleaning by a periodic polarity reversal strategy

As for the Cr(VI) reduction and subsequent Cr immobilization from the neutral and basic water, acidification and subsequent alkalization are necessitated, which usually suffers from the cumbersome operation, high chemical cost and increased water salinity. In this study, a porous Ti4O7 reactive electrochemical membrane (REM) was utilized as both the flow-through cathode and microfiltration membrane for the reduction of Cr(VI), and precipitation and interception of Cr(OH)3 particles synchronously, where the electrolysis cell was divided by a polytetrafluoroethylene (PTFE) membrane. As compared with the flow-by mode, the flow-through mode exhibited a 3-fold increase in the mass transfer rate constant and therefore nearly 100% of Cr(VI) was reduced when passing through the Ti4O7 REM cathode. The divided electrolysis cell was operated to generate a high alkaline environment (pH ∼ 11.0) throughout the whole volume of the cathodic chamber regardless of the initial pH changing from 5.0 to 10.0, which favored the formation of Cr(OH)3 particles and its interception by Ti4O7 REM. Thus, increasing initial pH from 5.0 to 10.0 exhibited negligible effects on the Cr(VI) removal efficiency and Cr(total) immobilization efficiency. Under the optimum condition (initial Cr(VI) concentration 500 ppb, current density 1 mA cm−2, flux 287 L m-2 h−1), the Cr(total) removal efficiency could achieve at as high as 95% with residual Cr(total) concentration of 27 μg L-1. As the reaction progressed, Cr(OH)3 passivated the membrane gradually, decreasing the Cr(VI) reduction efficiency to 84% at 30 h. Interestingly, the fouled Ti4O7 REM could be recovered completely after 30 min’s membrane cleaning operation just by periodically reversing the polarity at current density of 5 mA cm−2. Generally, this process can be considered as a promising and suitable treatment method for Cr(VI) detoxification from distributed water with a very low specific energy consumption of 0.109 kWh m−3.

Fabrication of SnO2-Sb reactive membrane electrodes for high-efficiency electrochemical inactivation of bacteria and viruses in water

Exposure to waterborne pathogens is still a widespread threat to public health, and efficient and reliable disinfection technologies are urgently needed for decentralized applications. Herein, we developed a novel freestanding and flow-through SnO2-Sb reactive membrane electrode (RME) for electrochemical wastewater disinfection. When water was forced to flow through the porous RME, large amounts of reactive ·OH and H+ were generated within the interior pores of the anode, creating a strongly acidic local environment enriched with ·OH for the rapid inactivation of bacteria and viruses. At an applied voltage of 3.5 V, 100% removal of Escherichia coli (>6.3-log inactivation) was achieved by the flow-through RME at a high flux, which was significantly more effective than the system operated in the conventional flow-by mode (2.7-log inactivation). Using the bacteriophage MS2 strain as a model virus, a disinfection efficiency of 100% (>8.3-log inactivation) was achieved by the RME at 3.5 V. The transmission electron microscopy analysis revealed that E. coli and MS2 in the disinfected water suffered from serious destruction and disintegration, reflecting the strong electrocatalytic oxidation capacity of the charged RME. The SnO2-Sb RME-based electrochemical system performed very well in disinfecting complex municipal wastewater, demonstrating its potential for practical applications.

Electro-activation of peroxymonosulfate by a graphene oxide/iron oxide nanoparticle-doped Ti4O7 ceramic membrane: mechanism of singlet oxygen generation in the removal of 1,4-dioxane

Electro-activation of peroxymonosulfate (PMS) has been widely investigated for the degradation of organic pollutants. Herein, we employ graphene oxide (GO)/Fe3O4 nanoparticles (NPs) doped into a Ti4O7 reactive electrochemical membrane through strong chemical bonding as the cathode to activate PMS for the degradation of 1,4-dioxane (1,4-D). The strong chemical interaction between GO, Fe3O4-NPs, and Ti4O7 via Fe–O---GO---O–Ti bonds enhances the electron-transfer efficiency and provides catalytically active sites that boost the electro-activation of PMS. As a result, the 1,4-D oxidation rate of the GO/Fe3O4-NPs@Ti4O7 REM cathode is ~3 times higher (7.21 × 10-3 min-1) than those of other Ti4O7 ceramic membranes, and 1O2 plays a key role (59.9%) in the degradation of 1,4-D. The 1O2 generation mechanism in the electro-activation process of PMS was systematically investigated, and we claimed that 1O2 is mainly generated from the precursors H2O2 and O2•–/HO2• rather than by O2 or •OH, as has been reported in previous studies. A flow-through mode test in the PMS electro-activation system is firstly reported, and the 1,4-D decay efficiency is 7.1 times higher than that obtained by a flow-by mode, showing that an improved PMS mass transfer efficiency enhances the conversion to reactive oxygen species.

Pd–O2 interaction and singlet oxygen formation in a novel reactive electrochemical membrane for ultrafast sulfamethoxazole oxidation

In this study, a novel reactive electrochemical membrane (REM) made of Pd-loaded porous Ti was prepared to achieve ultrafast and efficient oxidation of trace antibiotics sulfamethoxazole (SMX) under anodic polarization. At a current density of 0.5 mA/cm2, the percentage removal of SMX by the REM was 96.3% in flow-through mode, with the energy cost (i.e., the electrical energy per order of removal, 1.34 Wh/m3) being much lower than the flow-by mode (125.76 Wh/m3) under the same operating condition. Quenching experiments and electron spin resonance spectra demonstrated that singlet oxygen (1O2) was the main reactive oxygen species in the REM system, playing a vital role in SMX degradation. Results of this study indicated that 1O2 was mainly generated on the anode via Pd–O2 interaction that directly activated oxygen, rather than the evolution of 1O2 precursors (e.g., O2− and H2O2). The constantly high electrochemical removal of SMX (88.5–91.9%) from actual surface water demonstrated that the matrix effect on the process performance was minimal in realistic applications, highlighting this novel REM to be a viable way to remove trace organic contaminants from surface water.

Electrochemical oxidation of reverse osmosis concentrates using macroporous Ti-ENTA/SnO2-Sb flow-through anode: Degradation performance, energy efficiency and toxicity assessment

Due to poor mass transfer performance and high energy consumption of the traditional electrochemical flow-by mode, this study developed a high-efficiency electrochemical oxidation system in flow-through mode based on three-dimensional macroporous enhanced TiO2 nanotube array/SnO2-Sb (MP-Ti-ENTA/SnO2-Sb) anode. The effects of initial pH, current density and flow rate on the COD degradation of reverse osmosis concentrates (ROCs) from reclaimed wastewater plant were investigated. Besides, the energy efficiency, biodegradability and acute biotoxicity were studied during electrochemical flow-through process. Compared with the flow-by mode, the flow-through mode based on the MP-Ti-ENTA/SnO2-Sb anode had a COD removal rate of 0.38 mg min−1 (current density: 5 mA cm-2) and an electrical efficiency per order (EE/O) of 5.3 kW h m-3. The three-dimensional fluorescence spectrum showed that the fulvic acids, humic acids and soluble microbial metabolites of ROCs could be effectively removed by the flow-through anode. In addition, the luminescence inhibition rate of the effluent was 22.4 %, indicating that the acute biotoxicity was reduced by more than 40 %. The electrochemical flow-through process of ROCs treatment required relatively low energy consumption without extra chemical agent addition, showing a broader application prospect.

Combined Simulation and Experimental Study of Electrolysis Flow Cell for Continuous CO2 Conversion

Electrochemical carbon dioxide reduction reaction (CO2RR) is a promising strategy to sequester CO2 while synthesizing valuable chemicals and utilizing intermittent renewable energy supply from solar and wind energy.1 Electrolysis is often studied in H-cells that are composed of planar electrodes immersed in an aqueous electrolyte, which have severely limited mass transport across the electrolyte and hydrodynamic boundary layer.2-3 To avoid these limitations alkaline flow cells with a gas diffusion electrode (GDE) operated in a flow-by mode are sometimes used to achieve more realistic conditions. Although they provide higher current densities (CD) and energy efficiencies (EE), they suffer from carbonate salt precipitation in the stagnant pores of the GDE, moreover in KOH electrolyte CO2 is parasitically converted to bicarbonate. To remedy the latter problem neutral electrolytes, such as K2SO4 or KHCO3, can replace alkaline electrolytes, but these have so far demonstrated low EE due to high ohmic resistance and overpotentials in the GDE.In this work we present a flow-through compact membrane electrode assemble (MEA) electrolysis cell for continuous CO2RR, which has following advantages. Firstly, the neutral electrolytes flowed through the porous electrode with carbon in the form of dissolved CO2 and HCO3 —.4 Electrolysis was carried out to produce CO gas and formate ions, which only need to pass through a thin boundary layer with minimized mass transport resistance. The porous electrode was pressed onto the membrane to ensure good ionic conductivity at the electrode−electrolyte interface. Secondly, flowing electrolyte eliminated degradation related to electrolyte flooding and carbonate precipitation. Finally, the overpotential was lowered through catalyst tuning and localized alkaline environment,5 contributing to cost competitive electroreduction of CO2 to CO, which exhibited partial current density (PCDCO) exceeding 150 mA cm−2 at cell overpotentials (|ηcell|) less than 2 V.Reference 1. De Luna, P.; Hahn, C.; Higgins, D.; Jaffer, S. A.; Jaramillo, T. F.; Sargent, E. H., What would it take for renewably powered electrosynthesis to displace petrochemical processes? Science 2019, 364 (6438).2. Liu, M.; Pang, Y.; Zhang, B.; De Luna, P.; Voznyy, O.; Xu, J.; Zheng, X.; Dinh, C. T.; Fan, F.; Cao, C.; de Arquer, F. P. G.; Safaei, T. S.; Mepham, A.; Klinkova, A.; Kumacheva, E.; Filleter, T.; Sinton, D.; Kelley, S. O.; Sargent, E. H., Enhanced electrocatalytic CO2 reduction via field-induced reagent concentration. Nature 2016, 537 (7620), 382-386.3. Wen, G.; Lee, D. U.; Ren, B.; Hassan, F. M.; Jiang, G.; Cano, Z. P.; Gostick, J.; Croiset, E.; Bai, Z.; Yang, L.; Chen, Z., Orbital Interactions in Bi-Sn Bimetallic Electrocatalysts for Highly Selective Electrochemical CO2 Reduction toward Formate Production. Adv. Energy Mater. 2018, 8 (31), 1802427.4. Weng, L. C.; Bell, A. T.; Weber, A. Z., Modeling gas-diffusion electrodes for CO2 reduction. Phys Chem Chem Phys 2018, 20 (25), 16973-16984.5. Verma, S.; Hamasaki, Y.; Kim, C.; Huang, W.; Lu, S.; Jhong, H.-R. M.; Gewirth, A. A.; Fujigaya, T.; Nakashima, N.; Kenis, P. J. A., Insights into the Low Overpotential Electroreduction of CO2 to CO on a Supported Gold Catalyst in an Alkaline Flow Electrolyzer. ACS Energy Letters 2018, 3 (1), 193-198.

Preferential removal of 2,4-dichlorophenoxyacetic acid from contaminated waters using an electrocatalytic ceramic membrane filtration system: Mechanisms and implications

A selective electrocatalytic ceramic membrane filtration (ECMF) system was developed using molecular imprinting (MI) TiO2@SnO2-Sb (MI-TiO2@SnO2-Sb) anode for selectively eliminating 2,4-dichlorophenoxyacetic acid (2,4-D). The introduction of MI sites enhanced the removal of 2,4-D compared to the control. At a charging voltage of 3 V under flow-by mode, the 2,4-D removal rate of MI-TiO2@SnO2-Sb ECMF system was 1.91 times that of the control (without MI), attributed to the specific recognition sites (leading to selectivity) and the enhanced reactive oxygen species production in the presence of MI sites. In the coexistence of interferent phenoxyacetic acid herbicides, MI-TiO2@SnO2-Sb ECMF system also showed higher selective coefficient than the control (without MI). More importantly, flow-through mode could enhance the selectivity since the membrane filtration enhanced the mass transfer of target micropollutant towards MI sites. The imparted selectivity was mainly attributed to the size matching and special interaction between the target molecules and imprinted cavities.