Electrochemical Separation Process Research Articles

Anion-Exchange Membrane Oxygen Separator.

Anion-exchange membranes (AEMs), known for enabling the high conductivity of hydroxide anions through dense polymeric structures, are pivotal components in fuel cells, electrolyzers, and other important electrochemical systems. This paper unveils an unprecedented utilization of AEMs in an electrochemical oxygen separation process, a new technology able to generate enriched oxygen from an O2/N2 mixture using a small voltage input. We demonstrate a first-of-its-kind AEM-based electrochemical device that operates under mild conditions, is free of liquid electrolytes or sweep gases, and produces oxygen of over 96% purity. Additionally, we develop and apply a one-dimensional time-dependent and isothermal model, which accurately captures the unique operational dynamics of our device, demonstrates good agreement with the experimental data, and allows us to explore the device's potential capabilities. This novel technology has far-reaching applications in many industrial processes, medical oxygen therapy, and other diverse fields while reducing operational complexity and environmental impact, thereby paving the way for sustainable on-site oxygen generation.

Separation of Lithium over Magnesium, Sodium, and Calcium By Electrodialysis Using Commercial Membranes

Lithium is considered a critical raw material worldwide due to the growing market for electric vehicles. Due to the risk of supply interruption in the short and medium term, alternative sources of lithium are being explored for battery supply. Among these, salt-lake brines have become a key supply because their abundance represents about 59% of lithium resources 1. A secondary source in the medium term is the recycling of spent Li-ion batteries by hydrometallurgical processes 2,3. Together, recovering lithium from brine and recycling can supply the demand for transport electrification. Conventional lithium extraction techniques from both brine and battery recycling face the challenge of selective separation over contaminants and low extraction efficiencies 4,5.As an alternative, electrodialysis (ED) is an important selective separation technique with high selectivity, productivity, and less consumption of chemical reagents under energy consumption. This electrochemical membrane separation process uses an electric field inducing anion and cation transport across alternating anion and cation exchange membranes. While standard electrodialysis can be used to desalinate water and generate more concentrated brine), selective electrodialysis processes that up-concentrate a specific ion from a mixed salt solution are less developed. Our goal with this work is to evaluate the effect of common ions on lithium separation by ED using three different commercial membranes. The experiments were carried out in a 4-chamber reactor where the feed solutions consisted of equimolar chloride salts of lithium and either nickel (representing battery recycling), magnesium, sodium, or calcium (representing brine). Anolyte, receiving, and catholyte chambers (Figure 1) contained 1.0 M H2SO4.Here, we present the results in terms of cation permselectivity, productivity, and energy consumption using three different commercial membranes for the cation exchange membrane in the ED setup: a standard cation exchange membrane (CXM-200), a monovalent cation exchange membrane (CXP-S) and a nanofiltration membrane (NF-90). The performance of each membrane was tested under varying applied current densities to assess the impact of operating conditions on ion transport. Results have demonstrated monovalent cationic membranes are strongly selective for lithium over magnesium and nickel, achieving selectivity of 43 and 57, respectively. The increase in current density from 2.8mA/cm² to 11.5mA/cm² also increased the fluxes of lithium and co-ions by the monovalent membrane, consequently decreasing the selectivity from 56.7 to 2.4. In addition, nickel precipitates in the feed chamber as the current density increases from 2.8mA/cm² to 5.7mA/cm². We then translate these results into a process model for upscaling the ED reactor, considering the species transport rate through the reactor chambers and selectivity for lithium. Keywords: battery recycling; brine; electrochemical membrane separation; critical raw materials.Figure 1: ED reactor used for lithium selective separation by commercial membranes References (1) Fuentealba, D.; Flores-Fernández, C.; Troncoso, E.; Estay, H. Technological Tendencies for Lithium Production from Salt Lake Brines: Progress and Research Gaps to Move towards More Sustainable Processes. Resources Policy 2023, 83 (January). https://doi.org/10.1016/j.resourpol.2023.103572.(2) Guimarães, L. F.; Botelho Junior, A. B.; Espinosa, D. C. R. Sulfuric Acid Leaching of Metals from Waste Li-Ion Batteries without Using Reducing Agent. Miner Eng 2022, 183 (May), 107597. https://doi.org/10.1016/j.mineng.2022.107597.(3) Martins, L. S.; Rovani, S.; Botelho Junior, A. B.; Romano Espinosa, D. C. Sustainable Approach for Critical Metals Recovery through Hydrometallurgical Processing of Spent Batteries Using Organic Acids. Ind Eng Chem Res 2023. https://doi.org/10.1021/acs.iecr.3c03048.(4) Asadi Dalini, E.; Karimi, G.; Zandevakili, S. Treatment of Valuable Metals from Leaching Solution of Spent Lithium-Ion Batteries. Miner Eng 2021, 173 (September), 107226. https://doi.org/10.1016/j.mineng.2021.107226.(5) Xu, S.; Song, J.; Bi, Q.; Chen, Q.; Zhang, W. M.; Qian, Z.; Zhang, L.; Xu, S.; Tang, N.; He, T. Extraction of Lithium from Chinese Salt-Lake Brines by Membranes: Design and Practice. J Memb Sci 2021, 635 (May), 119441. https://doi.org/10.1016/j.memsci.2021.119441. Figure 1

A reductive strategy with ultrahigh selectivity for sequential recovery of uranium from radioactive wastewater

Uranium is valuable metal, and it is of great significance to develop a simultaneous separation process of uranium and cesium from radioactive wastewater. Herein, a process was designed to separate sequentially uranium and cesium from wastewater. By comparing the limiting ability of OH− and CO32− to Co2+, Cd2+, and Sr2+ ions, it was found that CO32− could effectively limit the above ions to the solid phase. The result of X-ray photoelectron spectroscopy (XPS) and species analysis revealed that the restriction was achieved through complexation. Through this property, more than 99.9 % of cobalt and cadmium were limited in the solid phase, and U-Cs co-separation was realized. Furthermore, the results of cyclic voltammetry and XPS indicated that U(VI) could be reduced in the liquid phase, but Cs(I) could not, which was the basis for uranium and cesium separation. And more than 99.5 % of uranium was separated from cesium by the electrochemical separation process. Simultaneously, effective separation of uranium and cesium under different pHs has been achieved via selective uranium leaching using carbonate. The co-separation combined with electrochemical separation realized the recovery of pure uranium products, indicating a large potential for realizing uranium resources in wastewater.

Highly conductive grain boundaries in cold-sintered barium zirconate-based proton conductors

A novel processing route for barium zirconate based protonic ceramics using cold sintering enables decreased sintering temperatures as well as high grain boundary conductivity.

(Invited) Recent Progress in Redox Flow Deionization

Recently, redox flow deionization (RFD) has gained great interest in water desalination as a promising electrochemical ion separation process. The RFD system advances the redox flow battery by adapting intervening channels for the water stream. As a unique cell feature, the system is comprised of multi-channels allowing the independent control of the flow streams (e.g., water stream and redox flows). This leads to several advantages compared to the conventional electrochemical ion separation processes such as capacitive deionization and electrodialysis: (1) sustainable redox reaction enables low operational cell voltage and high diversity in the employment of electrode materials, (2) the mild reaction environments allow less byproduct production and less variation in pH as this system facilitating downstream processes, and (3) the stored energy at redox couples provides energy recovery process with a simultaneous ion separation leading to high energy efficiency in water desalination.In line with the development of RFD, this presentation aims to introduce the basic principle of the RFD system and recent progress, including cell architecture, electrodes, operations, and scale-up strategy. In addition, various applications are addressed such as resource recovery and pollutant control. Finally, we briefly propose the practical challenges and opportunities for future research direction.

The electrolysis-graded leaching process for rapid resource regeneration of Th(IV)/U(VI) from wastewater

Uranium and thorium are strategic metals, and the mining and utilization of uranium and thorium produce a large amount of radioactive wastewater. In this paper, a new process combining electrolysis and leaching is proposed to achieve resource regeneration of uranium and thorium from radioactive wastewater. The results revealed that carbonate was the optimal separation system. The uranium-thorium separation was based on the formation of soluble UO2(CO3)34− and insoluble ThO2. And the separation efficiency was determined to reach a maximum value of 4.8 × 104 at pH 10.5. Surprisingly, UO2(CO3)34− could be effectively electrochemically recovered, and more than 90% of uranium could be recovered within 3 h, which indicated that uranium could be efficiently recovered. In addition, U–Th leaching studies were performed, and the results showed that selective leaching of uranium could be achieved at 0.01 M, while effective leaching of Th could be achieved at 0.3 M, thus establishing a graded-leaching process for U–Th. To reduce the impact of Ba2+ on the uranium electrochemical separation process, the SO42--CO32- combined system was developed, and the concentration of Ba2+ was reduced by two orders of magnitude. The final result revealed that the purity had been roughly tripled via this process. This study provides an effective strategy to reduce the waste of non-renewable uranium-thorium resources and process wastewater, achieving the recycling of non-renewable resources and the sustainable development of nuclear energy.

Exploring Performance and Cost Trade-Offs in Electrochemical CO2 Separation Processes

Increasing global energy demand coupled with a continued reliance on carbon-intensive fuels has contributed to the rise in anthropogenic greenhouse gas emissions that are adversely impacting the global climate.1,2 While the expansion of low-cost renewable electricity (often coupled with energy storage) offsets fossil-fuel-based generation, complete mitigation of carbon emissions will likely require the capture, utilization, and/or storage of carbon dioxide (CO2) at scale. Today’s state-of-the-art separation technologies rely on thermochemical processes to extract CO2 from industrial waste streams, post-combustion flue gases, or directly from the atmosphere. Such approaches are energetically intensive and typically rely on fossil-fuel-derived heat to release CO2 and regenerate the sorbent, ultimately limiting process effectiveness.3 Electrochemical approaches to CO2 capture and concentration are increasingly seen as a viable pathway to reducing carbon emissions, as these processes enable several advantages including direct integration with renewables, modular deployment, operation near ambient conditions, and high energetic efficiencies.4 Despite this promise, these concepts are at an early-stage and the relationships between cost and performance are not well-understood at the molecular-, cell-, or system-level.In this presentation, we will discuss how techno-economic modeling can be used to inform design criteria for cost-competitive electrochemical CO2 separation systems. We will specifically focus on a 4-stage system with soluble redox-active species for direct binding / debinding with CO2. First, we describe and integrate all process units (i.e., electrochemical reactor, absorber, desorber) via thermodynamic, chemical, and electrochemical relationships and then estimate capital, fixed, and variable costs via a levelized cost of capture model. Second, we investigate key scaling correlations and cost sensitivities for the 4-stage system, including the role of resistive losses in the electrochemical reactor and binding affinity in the absorption column. Third and finally, we connect system-level performance and cost targets to molecular / system properties and operating envelopes. While our focus is on a particular system, the framework is generalizable and can be extended to different electrochemical and thermochemical approaches to CO2 separation, potentially enabling technology comparisons on a common basis. Acknowledgements This work was supported by the Alfred P. Sloan Foundation. K.M.R. gratefully acknowledges the support of the National Defense Science and Engineering Graduate (NDSEG) Fellowship. References Ritchie, H., Roser, M. & Rosado, P. CO2 and Greenhouse Gas Emissions. Our World in Data (2020). Annual Energy Outlook 2021. U.S. Energy Information Administration (2021).Khan, F. M., Krishnamoorthi, V. & Mahmud, T. Modelling reactive absorption of CO2 in packed columns for post-combustion carbon capture applications. Chemical Engineering Research and Design 89, 9, 1600–1608 (2011).Renfrew, S. E., Starr, D. E. & Strasser, P. Electrochemical Approaches toward CO2 Capture and Concentration. ACS Catal. 10, 21, 13058–13074 (2020).

Rocking-chair capacitive deionization for phosphate recovery via rejection mode using ion-exchange membranes

Rocking-chair capacitive deionization (CDI), in combination with commercially available ion-exchange membranes, was evaluated for the feasibility of recovering phosphate. Here, contrary to the typical ion-selective adsorption methods in CDI, we aim to isolate and recover phosphate via a rejection mode. We have successfully removed 80 % of chloride from a binary solution containing equimolar concentrations of chloride and monovalent phosphate, while retaining virtually 100 % of phosphate. Further studies have revealed that ≥90 % of chloride can be removed while sacrificing as little as 15–20 % of the original phosphate concentration. To optimize the electrochemical ion separation process, we have studied several different working potentials, flowrates, and cycle times in these experiments. Moreover, we have observed similar selectivity results for both carbon-based and intercalation electrodes (Prussian blue analogues, PBAs) in a direct comparison. Within the constraints of the testing parameters, both systems exhibit no pronounced differences, with PBAs marginally outperforming carbon-based electrodes in terms of total chloride removed (94 % vs. 92 %). However, during the early stages of a desalination run, the carbon-based system revealed a minor edge on the ion separation performance compared to the PBA system.

Impact of the Donnan electrolytes on selectivity of cation exchange membranes evaluated via the ionic membrane conductivity

The selectivity of an ion exchange membrane (IEM) is an essential property in related electro-chemical separation and clean production processes. The impact of the Donnan electrolytes on the counter-ion selectivity of three representative cation exchange membranes (CEMs) evaluated via the ionic membrane conductivity is systematically studied in the mixed MgSO4/Na2SO4 and H2SO4/Na2SO4 electrolytes, over an external total sulfate concentration range of 0.1 ∼ 1.0 M. The counter-ions selectivity is decoupled into the partitioning selectivity coefficient (Kji) and the mobility ratio (uim/ujm), both of which are related to the CEM conductivity. If the Donnan electrolytes conductivity is excluded, for the heterogeneous CEM (3361BW), the selectivity calculated from the ion-exchanged conductivity (κE) can be perfectly matched with the electrodialysis (ED) results under a lower external sulfate concentration (0.1 M). The Mg2+/Na+ selectivity values of two homogeneous CEMs (Nafion117 and CSE) obtained from the nonlinear correlation method derived from the Kohlrausch's law (σ) are closer to the ED values, while the linear correlation approaches are more suitable at the presence of H2SO4. By analyzing the variation of Kji and uim/ujm with the κE separately, the main reason for the variation of the Mg2+/Na+ selectivity for these three CEMs, when the external electrolytes concentration increases, is the different equilibrium partitioning selectivity coefficients (KNa+Mg2+) between the membrane and the solution. On the contrary, the H+ ions transport much faster than the Na+ ions within the CEMs, and it is the change of this mobility ratio that causes the different H+/Na+ selectivity results. This work not only expands the application of the proposed conductivity method, but also provides a new perspective for studying the sorption and transport mechanism of ions in electro-chemical processes based on IEMs.

Geochemical carbon dioxide removal potential of Spain

Many countries have made pledges to reduce CO2 emissions over the upcoming decades to meet the Paris Agreement targets of limiting warming to no >1.5 °C, aiming for net zero by mid-century. To achieve national reduction targets, there is a further need for CO2 removal (CDR) approaches on a scale of millions of tonnes, necessitating a better understanding of feasible methods. One approach that is gaining attention is geochemical CDR, encompassing (1) in-situ injection of CO2-rich gases into Ca and Mg-rich rocks for geological storage by mineral carbonation, (2) ex-situ ocean alkalinity enhancement, enhanced weathering and mineral carbonation of alkaline-rich materials, and (3) electrochemical separation processes. In this context, Spain may host a notionally high geochemical CDR capacity thanks to its varied geological setting, including extensive mafic-ultramafic and carbonate rocks. However, pilot schemes and large-scale strategies for CDR implementation are presently absent in-country, partly due to gaps in current knowledge and lack of attention paid by regulatory bodies. Here, we identify possible materials, localities and avenues for future geochemical CDR research and implementation strategies within Spain. This study highlights the kilotonne to million tonne scale CDR options for Spain over the rest of the century, with attention paid to chemically and mineralogically appropriate materials, suitable implementation sites and potential strategies that could be followed. Mafic, ultramafic and carbonate rocks, mine tailings, fly ashes, slag by-products, desalination brines and ceramic wastes hosted and produced in Spain are of key interest, with industrial, agricultural and coastal areas providing opportunities to launch pilot schemes. Though there are obstacles to reaching the maximum CDR potential, this study helps to identify focused targets that will facilitate overcoming such barriers. The CDR potential of Spain warrants dedicated investigations to achieve the highest possible CDR to make valuable contributions to national reduction targets.

Multi-electrode scale-up strategy and parametric investigation of redox-flow desalination systems

Redox flow desalination (RFD) has emerged as a promising energy-efficient and sustainable electrochemical ion separation process for water desalination. Despite the effectiveness of RFD systems for desalination, prior studies have generally focused on fundamental aspects and lab-scale studies; therefore, the practical feasibility and potential for deployment in industry remain unclear. Here, a strategy for scaling-up RFD systems was developed, and the efficacy of RFD for desalination on a larger scale and its practical feasibility for industrial applications were assessed. A new multielectrode stacking strategy employing a carbon electrode and titanium current collector enabled enhanced charge transfer and improved desalination performance without the need for additional channels. This facile strategy of scaling up the RFD remarkably reduces the size of the system compared with unit cell stacking in the conventional electrochemical system. The larger scale RFD system exhibited a salt removal rate of 852 mmol/m2/h and specific energy consumption of 120 kJ/mol (≅0.1 kWh/m3). Multi-parametric optimization was carried out via interactive data visualization by varying the cell voltage, flow rate, number of carbon electrode stacks, and membrane arrangement. The RFD system achieved an energy consumption of 60–80 kJ/mol and removal rate of 200–300 mmol/m2/h. Overall, the feasibility of RFD for water desalination was confirmed through a rational scale-up strategy, comprehensive understanding of the desalination parameters, and techno-economic analysis. Through continued development and further scale-up investigations, we envision the deployment of RFD systems for energy-efficient desalination on the industrial scale as an alternative to conventional electrochemical desalination processes (i.e., electrodialysis and capacitive deionization).

Mechanism of Tungsten Recovery from Spent Cemented Carbide by Molten Salt Electrodeposition

The accumulation of spent carbide (YG8), not only pollutes the environment but also causes waste of tungsten, cobalt and other rare metal resources. To better address this issue, we proposed a combined electrochemical separation process of low-temperature aqueous solution and high-temperature molten salt for tungsten and cobalt. H<sub>2</sub>WO<sub>4</sub> was obtained from spent carbide in an aqueous solution, and we calcined it to obtain WO<sub>3</sub>, which was used as a raw material to obtain tungsten by using molten salt electrodeposition. The influence of the current efficiency and the electrochemical behavior of the discharge precipitation of W(VI) were also studied. The calcination results showed that the morphology of WO<sub>3</sub> was regular and there were no other impurities. The maximum current efficiency of 82.91% was achieved in a series of electrodeposition experiments. According to XRD and SEM analysis, the recovered product was high purity tungsten, which belongs to the simple cubic crystal system. In the W(VI) reduction mechanism experiments, the electrochemical process of W(VI) in NaCl-Na<sub>2</sub>WO<sub>4</sub>-WO<sub>3</sub> molten salt was investigated using linear scanning voltammetry (LSV) and chronoamperometry in a three-electrode system. The LSV showed that W(VI) was reduced at the cathode in two steps and the electrode reaction was controlled by diffusion. The fitting results of chronoamperometry showed that the nucleation mechanism of W(VI) was an instantaneous nucleation mode, and the diffusion coefficient was 7.379×10<sup>−10</sup> cm<sup>2</sup>·s<sup>−1</sup>.

(Invited) Electrochemical Gas Separation

Electrochemical gas separators are electrically powered systems that selectively remove a targeted gas species from a mixture of gases by virtue of electrochemical reactions. Electrochemical separation is an attractive option owing to its higher efficiency and lower cost compared to incumbent technologies like pressure swing adsorption, cryogenic processes, and selective permeation. This work focuses on two specific electrochemical separation processes – (1) separation of hydrogen from a mixture of gases to help develop the hydrogen distribution infrastructure, and (2) nitrogen separation from air for aircraft fuel tank inerting.Reductions in CO2 emission since the beginning of this century by the US electric power sector are mainly attributed to the replacement of coal by natural gas in power plants. Nevertheless, the combustion of natural gas still contributes heavily to global warming and climate change. It is imperative to find alternatives to combustion-based energy technologies and nurture the growth of renewable energy systems. In this scenario, hydrogen is a leading candidate as a carbon-free fuel with high energy density and is expected to play a key role in future energy systems. However, hydrogen faces serious obstacles in its distribution due to the lack of a nationwide hydrogen pipeline network. Developing a dedicated hydrogen pipeline network will be quite expensive, therefore, it is worthwhile to examine whether existing natural gas pipelines could be effectively deployed for hydrogen distribution. This would be accomplished by directly injecting a prescribed amount of hydrogen at the point of production into a natural gas pipeline. Such a mixture of hydrogen and methane is labeled as hythane. While this enables the convenient transport of hydrogen across large distances, the process can only be completed by separating hydrogen from methane at the destination point. Electrochemical hydrogen separation (ECHS) systems built around proton-selective polymer electrolyte membranes (PEMs) represent an effective platform to separate and simultaneously compress hydrogen in a continuous operation. Furthermore, ECHS ensures that the resulting gas is not contaminated by lubrication oil as observed in conventional systems. In ECHS, the hythane mixture enters the anode compartment wherein the hydrogen is selectively dissociated to protons and electrons. The protons are then driven across the PEM by an externally applied voltage to recover hydrogen at the cathode.The first part of this study demonstrates hydrogen purification using low-temperature PEM-based ECHS from various gas mixtures including methane/hydrogen, carbon dioxide/hydrogen, water gas shift effluent, and hythane. ECHS performance is first investigated for pure hydrogen as a function of membrane thickness, cell temperature, and relative humidity of the anode stream. In the second set of experiments, various ratios of methane/hydrogen and carbon dioxide/hydrogen are introduced to examine the effect of hydrogen concentration in the feed gas mixture on ECHS performance. Finally, experiments are performed for hydrogen purification from a water gas shift (WGS) effluent mixture as well as a practical hythane gas feed. ECHS performance for all gas mixtures was benchmarked against the pure hydrogen case. The purity of the separated hydrogen gas was measured to confirm the effectiveness of the method. The results show that ECHS represents a good solution to separate hydrogen from the hythane mixture at the downstream end of the pipeline.Pertinent to the second electrochemical separation process examined here, after the TWA flight 800 disaster due to a fuel tank explosion in 1996, inerting of aircraft fuel tanks became a priority. During fuel tank inerting, an inert gas like nitrogen is supplied to the tank to reduce its flammability. An electrochemical gas separation and inerting system (EGSIS) is a device that generates nitrogen enriched air (NEA) from ambient air by the application of electrical power. EGSIS combines a PEM electrolyzer anode wherein water is dissociated to release oxygen, and a PEM fuel cell cathode where atmospheric air is converted to NEA. Aircraft tank inerting requires varying NEA flowrates (low during takeoff and ascent, and high during descent). In conventional hollow fiber membrane air separation modules typically used in current aircraft, the total membrane surface area is determined by the maximum required NEA flow rate which results in large and expensive modules. On the other hand, the NEA flow rate can be easily controlled in EGSIS by simply adjusting the applied voltage. This portion of the study focuses on results for a single EGSIS cell and its optimization. Various EGSIS stack configurations are also described in order to develop a practical system. Finally, a techno-economic analysis of EGSIS is presented to show that EGSIS can compete favorably with incumbent technologies in terms of fuel usage and cost.

Separation of antimony from lead-antimony alloy by molten salt electrolysis

Lead-acid batteries (LABs) are widely used in electric motorcycles, uninterruptible power systems, and stationary energy storage devices. However, spent LABs should be recycled to reuse lead (Pb) and thereby close the Pb cycle and avoid Pb contamination. In order to reuse the recovered Pb, the purity of recovered Pb should meet the required standard. Since antimony (Sb) is a common alloying element in Pb alloy grid such as the Pb-Sb alloy, separating Sb from the recovered Pb is necessary. In this paper, we proposed a molten salt electrolysis method to separate Sb from liquid Pb-Sb alloys in terms of forming a solid Ca-Sb intermetallic at the top of the liquid Pb-Sb cathode. The electrochemical separation process was performed in molten NaCl-CaCl2. The content of Sb (atomic percent) in the Pb-Sb alloy was reduced from 1% to less than 0.01% (atomic ratio). The recovery principle lies in the different alloying potentials between Ca-Sb and Ca-Pb. Hence, molten salt electrolysis is a promising separation method to recover pure Pb from spent Sb-containing Pb alloys.

Hybrid surface modification of an anion exchange membrane for selective separation of monovalent anions in the electrodialysis process

Electrodialysis is an electrochemical separation process which is used for the removal of ionic species from aqueous solutions. In this study selectivity of an anion exchange membrane was tuned towards formate (CHOO-) anions compared to oxalates (C2O42- ) based on the characteristics of formate anions which show lower hydration energy. In order to enhance the removal efficiency of monovalent anions in electrodialysis, a hybrid surface modification on a heterogeneous anion exchange membrane was carried out. Surface modification consisted of three steps: plasma treatment (activation of the membrane surface), functionalization (acidification) and coupling with Polyethylenimine and Glutaraldehyde. Treatment of the membrane was verified using FTIR, Scanning Electron Microscopy, and Atomic Force Microscopy. The electrochemical properties of the membrane were measured before and after the modification. The results showed that due to the presence of the amide groups on the surface of the membrane upon modification, the water content of the membrane decreased by 8% while no other significant changes were observed in the membrane properties. Finally, the membrane selectivity towards CHOO-/ C2O42- in electrodialysis process increased from 0.88 to 4.29 upon surface modification.

(Invited) Toward Understanding Work Inputs for pH Swing-Based Electrochemical CO2 Separation

Large-scale CO2 separation and concentration from point sources, seawater and the atmosphere can play key roles in mitigating climate change and promoting the conversion of CO2 into chemical fuels and high-value products. Electrochemical CO2 separation processes have the potential to be more cost-effective than incumbent thermal (e.g. amine-based) analogues if they use inexpensive active materials and are sufficiently energy-efficient. Understanding the various thermodynamic and kinetic factors that contribute to the net electrical work consumed per unit of concentrated CO2 is therefore a critical step in developing practical systems.We discuss a method for electrochemical CO2 separation based on a pH swing cycle driven by redox reactions involving species capable of proton-coupled electron transfer (PCET). Electrochemical reduction of these species results in the formation of alkaline solution, into which may be CO2 is absorbed; subsequent electrochemical oxidation of the reduced molecules results in acidification of the solution, triggering the release of pure CO2 gas. This method is attractive for its operational simplicity, and because it can be driven by a wide range of aqueous-soluble or electrode-conjugated PCET-active reactants. Thermodynamic analysis and an experimental implementation of this idea in an electrochemical flow cell demonstrate that the net electrical work input per unit of separated CO2 can be low (< 100 kJ/molCO2). However, this work input comprises coupled kinetic and exergy losses that are highly sensitive to the cell’s electrochemical properties and its manner of operation. We describe how zero-dimensional modeling can play a role in rationalizing the various contributions to the work input, and thus promoting the development of energy-efficient systems which are capable of high throughput CO2 separation.

Deep learning for pH prediction in water desalination using membrane capacitive deionization

The pH of a solution has a large influence on the ion removal efficiency of the membrane capacitive deionization (MCDI) process, an electrochemical ion separation process. We developed a convolutional neural network linked with a long short-term memory (CNN-LSTM) model based on an artificial intelligence algorithm to predict the effluent pH of MCDI, as effluent pH is difficult to predict using conventional numerical modeling. The model accurately predicted effluent pH (R2≥0.998) based on the analysis of five input variables (current, voltage, influent conductivity and pH, and effluent conductivity) under standard operating conditions of MCDI using either constant-current or constant-voltage conditions. The developed model predicted effluent pH using only limited input variables, current and voltage, with high accuracy (R2≥0.997). Thus, the CNN-LSTM model can be used in practical applications as only the current and voltage of MCDI cells are often monitored in field applications.

Electrochemically driven adsorptive separation techniques: From ions to proteins and cells in liquid streams

• Electrochemical separation processes have a great potential beyond ion separation. • Both, faradaic and capacitive current, can be used to separate macromolecules. • The change in surface wetting upon applying electric stimuli is essential for the separation of macromolecules. • Surface plasmon resonance and atomic force spectroscopy techniques will be essential techniques to further investigate electrochemical separation approaches for macromolecules. Separation processes are of utmost importance for most industrial processes in the chemical, food, and pharma sectors. Electrochemical separation technologies gain more and more attention especially for the desalination of water, as they potentially lead to a reduction in the environmental footprint of the process. For salt, many different electrochemical desalination architectures and electrode designs have been proposed. Also other target molecules have been investigated, such as carboxylates, amino acids, proteins and whole cells. We note that the efforts are still scattered; therefore in this review we bundle and summarize the technological state of the art, and the application possibilities for capacitive as well as faradaic separation technologies for various targets. From this it is clear that there are still a lot of options that have been underused so far, and that electrochemical separation processes are key for the separation processes needed for a sustainable future.

Determination of impurities in copper metal using total reflection X‐ray fluorescence spectrometry after matrix separation: Method validation and uncertainty assessment

TXRF spectrometry has been utilized for the determination of impurities in copper metal. Multielement standard solution has been used for the calibration of TXRF spectrometer. To avoid the matrix effects, Cu was separated by electrochemical separation method under optimized condition followed by determination of impurities using TXRF. K, Ca, Fe, Ni, and Zn are the impurities found to be present in copper metal. Bottom up approach was employed to get the measure of the combined uncertainty in the results. The uncertainty associated with the recovery of electrochemical separation process is the highest. The TXRF results were validated by analyzing the same sample using ICP‐OES. Student's t‐test was performed to establish the statistical indistinguishability of the results obtained from TXRF and ICP‐OES.

(Invited) Investigations into Molecular Layer Deposition of Conjugated Amine Polymers

Conjugated amine polymers (CAPs) such as polyaniline and polypyrrole have high electrical conductivities (>500 S/cm), high specific capacities (>500 F/g), and are one of few known material classes containing redox-active sites for binding negatively-charged anions. These properties make CAPs appealing for a range of electrochemical applications including semiconductor devices, energy storage devices, sensors, and electrochemical separation processes. Unfortunately, existing synthesis routes for CAPs rely on rapid chemical or electrochemical oxidation reactions that provide limited control over the molecular structure and thickness of CAPs. This restricts the ability to use CAPs to address technological needs. In this report, we describe ongoing research in our group to deposit conformal CAP films with molecular-scale control of thickness and composition using sequential and self-limiting gas-phase surface reactions. Our approach is in line with established molecular layer deposition (MLD) techniques, and uses an oxidant as one of the MLD precursors in so-called oxidative molecular layer deposition (oMLD). Using select examples, we reveal new insights into oMLD growth behavior, characterize the molecular structures of the oMLD CAP films, and connect structural differences under varying deposition conditions with electrical and electrochemical properties of the films. The insights we report enable the deposition of electrically conductive and redox-active CAP films conformally onto high-surface area samples, opening new avenues to enable advanced electrochemical technologies based on ultrathin and conformal CAP films.