Bilen Akuzum, Ph.D.

In this work, Ti3C2Tx MXene was investigated as electrocatalyst material for the anodic V2+/V3+ reaction in vanadium redox flow batteries (VRFBs). A simple drop coating process was established using additive-free, aqueous MXene dispersions to fabricate MXene-coated carbon paper electrodes. The performance of Ti3C2Tx as an anodic electrocatalyst was studied using cyclic voltammetry and electrochemical impedance spectroscopy in a three-electrode cell. Furthermore, flow battery testing was… Show more

In this work, Ti3C2Tx MXene was investigated as electrocatalyst material for the anodic V2+/V3+ reaction in vanadium redox flow batteries (VRFBs). A simple drop coating process was established using additive-free, aqueous MXene dispersions to fabricate MXene-coated carbon paper electrodes. The performance of Ti3C2Tx as an anodic electrocatalyst was studied using cyclic voltammetry and electrochemical impedance spectroscopy in a three-electrode cell. Furthermore, flow battery testing was performed to determine the performance of the modified electrodes. At a current density of 50 mA cm−2, the electrode with Ti3C2Tx loading of 0.2 mg cm−2 enabled a 7% higher energy efficiency and 22% higher electrolyte utilization rate than the pristine electrode. At a higher current density (100 mA cm−2), the energy efficiency and electrolyte utilization were increased by 17% and 46%, respectively. At 50% SOC, the coated electrode was able to reach a limiting current density of 220 mA cm−2 while maintaining a voltaic efficiency above 80%, whereas the pristine electrode could only reach up to 160 mA cm−2 at the same voltaic efficiency. The improved performance was mainly attributed to the enhanced electrode kinetics, increased electrochemically active surface area, and improved wetting properties due to the addition of Ti3C2Tx nanoflakes. Show less

The development of high capacitance materials with high packing density and low viscosity in suspension electrodes is critical for progressing towards high-efficiency, low-footprint electrochemical flow capacitors (EFCs). Here, we report on the first electrochemical and rheological characterization of MXene-based suspension electrodes, using multilayer Ti3C2Tx as the active material and carbon black (CB) as the conductive additive in symmetric and asymmetric EFC devices. In the case of… Show more

The development of high capacitance materials with high packing density and low viscosity in suspension electrodes is critical for progressing towards high-efficiency, low-footprint electrochemical flow capacitors (EFCs). Here, we report on the first electrochemical and rheological characterization of MXene-based suspension electrodes, using multilayer Ti3C2Tx as the active material and carbon black (CB) as the conductive additive in symmetric and asymmetric EFC devices. In the case of symmetric Ti3C2Tx devices, the Ti3C2Tx concentration is fixed to 22 vol.% in the slurry and the CB concentration is varied from 0.5 to 2.0 vol.%. The symmetric device arrangement offers a high capacitance of 240 F ml−1 (2 mV s−1) and volumetric energy density of 2.65 Wh l−1 @ power density of 47.82 W l−1. Additionally, to extend the potential window, an asymmetric device assembly of activated carbon and Ti3C2Tx is investigated. This arrangement allows a stable operating potential window of 1 V with an energy density of 4.12 Wh l−1 and power density of 31.73 W l−1. Overall, multilayer Ti3C2Tx seems to be excellent candidate for flowable electrode applications, offering high capacitance, energy density and low viscosity due to its high electrochemical activity, excellent electrical conductivity, and versatile surface chemistry. Show less

The flow configuration selected for a capacitive deionization (CDI) system can impact the desalination performance due to drastic changes to the ion transport. Herein, a zero-gap CDI cell fixture with various flow configurations was utilized to investigate the effects of flow directionality on the CDI performance of activated carbon cloth (ACC) electrodes. Salt adsorption capacities and salt adsorption rates were determined for three commonly studied flow field designs (parallel (PFF)… Show more

The flow configuration selected for a capacitive deionization (CDI) system can impact the desalination performance due to drastic changes to the ion transport. Herein, a zero-gap CDI cell fixture with various flow configurations was utilized to investigate the effects of flow directionality on the CDI performance of activated carbon cloth (ACC) electrodes. Salt adsorption capacities and salt adsorption rates were determined for three commonly studied flow field designs (parallel (PFF), interdigitated (IDFF), and serpentine (SFF)) at various flow rates (2–128 mL/min). Increasing the flow rate was found to result in decreasing CDI performance for SFF and IDFF designs. On the other hand, the peak performance was observed for the parallel flow field at 32 mL/min flow rate. Additionally, the pressure drop values for different flow configurations were measured, and the energy consumptions were calculated. Overall, the findings showed that the performance of CDI systems strongly depends on the selected flow field geometry. Among the tested flow fields, the parallel configuration offered the best balance between CDI performance and energy efficiency. However, the designs that exert high hydrodynamic forces on the electrode plane showed poor performance due to rip-off of ions from the double layer causing a significant capacity loss for ACC electrodes. Show less

The discovery of liquid crystalline (LC) phases in dispersions of two-dimensional (2D) materials has enabled the development of macroscopically aligned three-dimensional (3D) macrostructures. Here, we report the first experimental observation of self-assembled LC phases in aqueous Ti3C2Tx MXene inks without using LC additives, binders, or stabilizing agents. We show that the transition concentration from the isotropic to nematic phase is influenced by the aspect ratio of MXene flakes. The… Show more

The discovery of liquid crystalline (LC) phases in dispersions of two-dimensional (2D) materials has enabled the development of macroscopically aligned three-dimensional (3D) macrostructures. Here, we report the first experimental observation of self-assembled LC phases in aqueous Ti3C2Tx MXene inks without using LC additives, binders, or stabilizing agents. We show that the transition concentration from the isotropic to nematic phase is influenced by the aspect ratio of MXene flakes. The formation of the nematic LC phase makes it possible to produce fibers from MXenes using a wet-spinning method. By changing the Ti3C2Tx flake size in the ink formulation, coagulation bath, and spinning parameters, we control the morphology of the MXene fibers. The wet-spun Ti3C2Tx fibers show a high electrical conductivity of ∼7750 S cm–1, surpassing existing nanomaterial-based fibers. A high volumetric capacitance of ∼1265 F cm–3 makes Ti3C2Tx fibers promising for fiber-shaped supercapacitor devices. We also show that Ti3C2Tx fibers can be used as heaters. Notably, the nematic LC phase can be achieved in other MXenes (Mo2Ti2C3Tx and Ti2CTx) and in various organic solvents, suggesting the widespread LC behavior of MXene inks. Show less

Understanding the percolation characteristics of multi-component conducting suspensions is critical for the development of flowable (semi-solid) electrochemical systems for energy storage and capacitive deionization with optimal electrochemical and rheological performance. Despite its significance, not much is known about the impact of the selected particle morphology on the agglomeration kinetics and the state of dispersion in flowable electrodes. In this study, impact of the conductive… Show more

Understanding the percolation characteristics of multi-component conducting suspensions is critical for the development of flowable (semi-solid) electrochemical systems for energy storage and capacitive deionization with optimal electrochemical and rheological performance. Despite its significance, not much is known about the impact of the selected particle morphology on the agglomeration kinetics and the state of dispersion in flowable electrodes. In this study, impact of the conductive additive morphology on the electrochemical and rheological response of capacitive flowable electrodes has been systematically investigated. Critical viscosity limits have been determined for common carbon additives that offer slurry formulations with improved electrochemical and rheological performance. For instance, at the same electrical conductivity of 60 mS cm-1, higher aspect ratio particles, such as graphene and carbon nanotubes, offered 4- and 2.4-times lower viscosity compared to carbon black due to the improved packing and conformity of the high-aspect-ratio particles. On the other hand, thixotropic measurements showed that the flowable electrodes with carbon black exhibit the fastest agglomeration kinetics, offering 25 % less time to recover from the applied shear due to spherical morphology and facile agglomeration kinetics. Overall, our findings show that the particle morphology has a significant impact on the electrochemical and rheological performance of flowable electrodes with up to 40 % difference in capacitance for similar viscosity electrodes. Furthermore, a direct correlation between the rheological and the electrochemical properties was established, offering morphology-independent practical guidelines for formulating slurries with optimal performance. In this manner, particles that can achieve the highest density of packing prior to the critical limit were found to offer the optimal balance between electrochemical and rheological performance. Show less

Introduction of new nanomaterials with conductivity, salt adsorption capacity (SAC) and rate (SAR) exceeding that of carbon electrodes may greatly improve capacitive deionization of water. However, those materials show a different electrochemical behavior, which must be studied and optimized for practical use. Here, effects of operating conditions on desalination performance of pre-conditioned Ti3C2Tx-MXene-based electrodes in a symmetric membrane capacitive deionization (MCDI) system were… Show more

Introduction of new nanomaterials with conductivity, salt adsorption capacity (SAC) and rate (SAR) exceeding that of carbon electrodes may greatly improve capacitive deionization of water. However, those materials show a different electrochemical behavior, which must be studied and optimized for practical use. Here, effects of operating conditions on desalination performance of pre-conditioned Ti3C2Tx-MXene-based electrodes in a symmetric membrane capacitive deionization (MCDI) system were investigated. Specifically, influences of discharge potential, half-cycle length (HCL), and flow rate were systematically studied. Results showed different degrees of performance dependence on operating conditions. For instance, lower discharge potentials increased SAC and SAR by 152%. However, longer HCL increased SAC by 32% while decreasing SAR by 54%. Finally, faster flow rates decreased both SAC and SAR by 20%. Desalination performances of symmetric pre-conditioned MXene and activated carbon cloth (ACC) electrodes were gravimetrically and volumetrically compared in MCDI system. Pre-conditioned MXene electrodes gravimetrically performed 30% lower than ACC due to their notably higher density. Yet, pre-conditioned MXene electrodes volumetrically outperformed ACC by 162%. Results suggest that although MXenes offer high electrochemical activity and hydrophilicity, making them promising candidates for CDI applications, the strong dependence of desalination performance of MXenes on operating conditions requires in-depth understanding and warrants further research. Show less

The projected decline in fresh water reservoirs over the next century attracted researchers towards the development of novel technologies that can offer low-cost and robust treatment of brackish and saline waters. Among many proposed technologies, flow-electrode capacitive deionization (F-CDI) is a promising capacitive desalination technology that can mitigate the majority of the shortcomings associated with conventional CDI systems and offer continuous and scalable treatment of brackish and… Show more

The projected decline in fresh water reservoirs over the next century attracted researchers towards the development of novel technologies that can offer low-cost and robust treatment of brackish and saline waters. Among many proposed technologies, flow-electrode capacitive deionization (F-CDI) is a promising capacitive desalination technology that can mitigate the majority of the shortcomings associated with conventional CDI systems and offer continuous and scalable treatment of brackish and saline water resources. In this study, we have developed an integrated diagnostics tool for testing flowable water treatment technologies. Show less

In this study, we have investigated the effects of varying flow channel depths and addition of various channel obstructions on the electrochemical performance and pumping power requirements of vanadium redox flow batteries (VRFBs). Specifically, 3D-printed ramps and prismatic obstructions were inserted into the channels of interdigitated flow field (IDFF) and parallel flow field (PFF) designs to observe the effect of non-uniform channel depth on the mass transport properties of open- and… Show more

In this study, we have investigated the effects of varying flow channel depths and addition of various channel obstructions on the electrochemical performance and pumping power requirements of vanadium redox flow batteries (VRFBs). Specifically, 3D-printed ramps and prismatic obstructions were inserted into the channels of interdigitated flow field (IDFF) and parallel flow field (PFF) designs to observe the effect of non-uniform channel depth on the mass transport properties of open- and closed-ended flow channels. Results were compared with conventional flow field geometries. Integration of ramps into the closed-ended (i.e., IDFF) flow channels resulted in 15% improvement in peak power density (PPD) at a flow rate of 50 mL min−1. Addition of ramps to IDFF has also resulted in a significant 40% drop in required pumping pressure due to guided and gradual delivery of the electrolyte to the electrode plane. In addition, the effects of varying channel depths in open-ended (i.e., PFF) channels were found to be much more drastic with improvements in PPD up to 150%. Overall, findings of this study highlight the significance of varying channel depths on improving the mass transport characteristics of VRFBs and offer an alternative approach for design of high-performance flow cells. Show less

The objective of this study is to understand the effects of operating conditions and cathode parameters on the salt removal performance of hybrid capacitive deionization systems (HCDI). Hence, the effects of half cycle length, flow rate, cathode thickness, and conductive additive loading in the cathode are systematically investigated. Hydrothermally synthesized α-MnO2 was selected as the active material in the cathode. Desalination results indicate notable dependence of HCDI performance on the… Show more

The objective of this study is to understand the effects of operating conditions and cathode parameters on the salt removal performance of hybrid capacitive deionization systems (HCDI). Hence, the effects of half cycle length, flow rate, cathode thickness, and conductive additive loading in the cathode are systematically investigated. Hydrothermally synthesized α-MnO2 was selected as the active material in the cathode. Desalination results indicate notable dependence of HCDI performance on the investigated parameters. For instance, increasing half cycle length increases the salt adsorption capacity (SAC) by ~58% but decreases the peak salt adsorption rate (PSAR) by ~28%. On the other hand, increasing the flow rate leads to an increase of the SAC and PSAR by ~25% and ~115%, respectively. Increase in the cathode thickness also showed a notable decay in performance with 43% drop in SAC. The amount of conductive additive in the cathode was also investigated to observe the impact of electrical conductivity on the CDI performance. Salt adsorption capacity and rate of HCDI systems containing identical active materials show strong dependence on the operation conditions and cathode parameters, which suggests a necessity of developing an understanding of the impact of these conditions on the system performance. Show less

In this study, we report on an approach to flow cell design that can enable a significantly improved power output (∼10x) for electrochemical flow capacitors (EFCs), even at large flow channel gaps. Reticulated vitreous carbon (RVC) electrodes of various average pore sizes (0.43–2 mm) were integrated into EFC flow cell fixtures with channel gaps of 5 mm. Electrochemical testing under flow conditions showed a 10-fold improvement in the power density with the RVC integration (290 W/m2, 580 W/kg)… Show more

In this study, we report on an approach to flow cell design that can enable a significantly improved power output (∼10x) for electrochemical flow capacitors (EFCs), even at large flow channel gaps. Reticulated vitreous carbon (RVC) electrodes of various average pore sizes (0.43–2 mm) were integrated into EFC flow cell fixtures with channel gaps of 5 mm. Electrochemical testing under flow conditions showed a 10-fold improvement in the power density with the RVC integration (290 W/m2, 580 W/kg) for the same slurry composition. This improvement was mostly attributed to the presence of a 3D porous electrode insert (i.e., RVC) that shortened the travel distance of the electrons to the current collectors. Pressure drop in the RVC containing cells was also investigated and found to increase up to 30% depending on the pore size. However, this increase was found to be offset by the increase in the channel depth, yielding almost no change in pressure as compared to conventional narrow-gap (>0.75 mm) flow channels. RVCs having an average pore size of 0.55 mm showed the best performance out of all studied cases with improved coulombic efficiency and good specific capacity (85 F/g) under flowing conditions. Show less

In this study, effects of thermal treatment conditions on the capacitive deionization performance (CDI) of activated carbon cloth (ACC) electrodes have been investigated. A total of 8 different treatment conditions has been studied by systematically changing the type of gas (Ar, CO2, N2) and the treatment temperature (700, 800, 850 °C). Treated electrodes were subjected to electrochemical testing and morphological analysis in order to assess the changes in the CDI performance. Results indicated… Show more

In this study, effects of thermal treatment conditions on the capacitive deionization performance (CDI) of activated carbon cloth (ACC) electrodes have been investigated. A total of 8 different treatment conditions has been studied by systematically changing the type of gas (Ar, CO2, N2) and the treatment temperature (700, 800, 850 °C). Treated electrodes were subjected to electrochemical testing and morphological analysis in order to assess the changes in the CDI performance. Results indicated a major discrepancy between the electrochemical and the CDI performance of the treated electrodes depending on the treatment condition. For instance, electrochemical testing showed 15% improvement in charge storage for N2-treated electrodes, while CDI performance was found to decrease by 20%. On the other hand, improvements in both electrochemical (25%) and CDI performances (60%) were observed for Ar and CO2 treated electrodes. These findings indicate that different treatment conditions promote distinct charge compensation mechanisms at the electrode surface; some of which are not beneficial for salt adsorption. Moreover, results highlight the significance of selecting a suitable thermal treatment condition for achieving enhanced performance in CDI systems utilizing ACC electrodes. Show less

Understanding the rheological properties of two-dimensional (2D) materials in suspension is critical for the development of various solution processing and manufacturing techniques. 2D carbides and nitrides (MXenes) constitute one of the largest families of 2D materials with >20 synthesized compositions and applications already ranging from energy storage to medicine to optoelectronics. However, in spite of a report on clay-like behavior, not much is known about their rheological response… Show more

Understanding the rheological properties of two-dimensional (2D) materials in suspension is critical for the development of various solution processing and manufacturing techniques. 2D carbides and nitrides (MXenes) constitute one of the largest families of 2D materials with >20 synthesized compositions and applications already ranging from energy storage to medicine to optoelectronics. However, in spite of a report on clay-like behavior, not much is known about their rheological response. In this study, rheological behavior of single- and multilayer Ti3C2Tx in aqueous dispersions was investigated. Viscous and viscoelastic properties of MXene dispersions were studied over a variety of concentrations from colloidal dispersions to high loading slurries, showing that a multilayer MXene suspension with up to 70 wt % can exhibit flowability. Processing guidelines for the fabrication of MXene films, coatings, and fibers have been established based on the rheological properties. Surprisingly, high viscosity was observed at very low concentrations for solutions of single-layer MXene flakes. Single-layer colloidal solutions were found to exhibit partial elasticity even at the lowest tested concentrations (<0.20 mg/mL) due to the presence of strong surface charge and excellent hydrophilicity of MXene, making them amenable to fabrication at dilute concentrations. Overall, the findings of this study provide fundamental insights into the rheological response of this quickly growing 2D family of materials in aqueous environments as well as offer guidelines for processing of MXenes. Show less

In this study, the effects of dispersion time and mixing methodology on electrochemical performance of flowable carbon electrodes have been investigated. Specifically, 20 different cases have been tested by systematically changing the mixing time (1.5, 3, 4.5, 8, 15 min), the mixing methodology (stir-bar vs. high-speed shear mixing), and the electrode composition (activated carbon (AC) only and AC with multi-walled carbon nanotube slurries). Each case was subjected to direct current… Show more

In this study, the effects of dispersion time and mixing methodology on electrochemical performance of flowable carbon electrodes have been investigated. Specifically, 20 different cases have been tested by systematically changing the mixing time (1.5, 3, 4.5, 8, 15 min), the mixing methodology (stir-bar vs. high-speed shear mixing), and the electrode composition (activated carbon (AC) only and AC with multi-walled carbon nanotube slurries). Each case was subjected to direct current conductivity and cyclic voltammetry measurements to identify the contribution of studied parameters on the dispersion and electrochemical performance. Results indicate that up to 60% difference in conductivity can be observed by changing mixing time and methodology for the same electrode composition. Additionally, capacity differences up to 90% within the same slurry composition have been noticed by slightly changing the electrode preparation protocol. Such major discrepancy in electrochemical performance with minimal changes in the electrode preparation highlights the significance of the degree of particle dispersion in these systems and the necessity for establishing an optimal slurry preparation protocol to achieve the best performance for a selected composition and application.

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The continuous development and further miniaturization of portable electronic devices and microelectromechanical systems have led to increasing demands for micro or nanoscale power sources and energy storage units. Supercapacitors, also called electrochemical capacitors, are energy storage devices with a long service life and high power densities that can be fully charged and discharged in seconds. Small-scale supercapacitors, or micro-supercapacitors (MSCs), can be
integrated with… Show more

The continuous development and further miniaturization of portable electronic devices and microelectromechanical systems have led to increasing demands for micro or nanoscale power sources and energy storage units. Supercapacitors, also called electrochemical capacitors, are energy storage devices with a long service life and high power densities that can be fully charged and discharged in seconds. Small-scale supercapacitors, or micro-supercapacitors (MSCs), can be
integrated with self-powered microscale devices and provide the required power for a long duration of time without maintenance, serving as ideal stand-alone power sources. The intrinsic properties of electrode materials play a crucially important role in the performance of MSCs. Here, a novel MSC is fabricated by employing a new material, two-dimensional titanium carbide (MXene). The MXene MSCs offer a long lifetime and higher areal and volumetric capacities compared to most of the previously reported devices. This work opens up a door for the design of on-chip devices with high energy storage capability by employing a large family (B20 members) of 2D MXenes and their heterostructures. Show less

In this study, we investigate the mass transport effects of various flow field designs paired with raw and laser perforated carbon paper electrodes in redox flow batteries (RFBs). Previously, we observed significant increases in peak power density and limiting current density when perforated electrodes were used in conjunction with the serpentine flow field. In this work, we expand on our earlier findings by investigating various flow field designs (e.g., serpentine, parallel, interdigitated… Show more

In this study, we investigate the mass transport effects of various flow field designs paired with raw and laser perforated carbon paper electrodes in redox flow batteries (RFBs). Previously, we observed significant increases in peak power density and limiting current density when perforated electrodes were used in conjunction with the serpentine flow field. In this work, we expand on our earlier findings by investigating various flow field designs (e.g., serpentine, parallel, interdigitated, and spiral), and continuously measuring pressure drop in each configuration. In all cases, these perforated electrodes are found to be associated with a reduction in pressure drop from 4% to 18%. Flow field designs with a continuous path from inlet to outlet (i.e., serpentine, parallel, spiral) are observed to exhibit improved performance (up to 31%) when paired with perforated electrodes, as a result of more facile reactant delivery and resulting greater utilization of the available surface area. Conversely, flow fields with discontinuous paths which force electrolyte to travel through the electrode (e.g. interdigitated), are adversely affected by the creation of perforations due to the high permeability ‘channels’ in the electrode. These results demonstrate that mass transport can significantly limit the performance of RFBs with carbon paper electrodes. Show less