Capacitive Mixing Research Articles

Electrochemical Conversion of Salinity Gradient Energy by Using Acid Surface Modified Activated Carbon Electrode

Salinity gradient energy is a sustainable, clean, and renewable ocean energy source. In this work, we used citric acid, phosphoric acid, and nitric acid to modify the surface of activated carbon. Based on the electrochemical capacitive mixing technology, a salinity gradient energy extraction device was constructed using acid-modified activated carbon as cathode and MoS2/MWCNTs as anode. Surface modification by different acids improves the performance of activated carbon electrodes for salt gradient energy conversion in the salt difference battery. The modification effect of nitric acid was the most pronounced, the specific capacitance of the modified electrode reaches 156 F g−1, and the concentration response voltage reaches 170 mV, with the assembled energy extraction system obtaining an energy density of 10.65 J m−2, an enhancement of 53.0% compared to the unmodified activated carbon electrode.

Polyelectrolyte-coated zeolite-templated carbon electrodes for capacitive deionization and energy generation by salinity exchange

As global demands for freshwater and renewable energy intensify, capacitive deionization (CDI) emerges as a promising technique for water purification, desalination, and ionic separation. Reciprocally, energy harvesting has been made possible from exchanging solutions with different salinity, using the so-called capacitive mixing (CapMix) methods, now taking their first steps towards a wider range of application. In all the techniques mentioned, porous electrodes are used in order to maximize the stored charge, making it essential to properly select the material of which the electrode is composed. This work focuses on exploring the performance of zeolite-templated carbon (ZTC) as a highly promising electrode material. ZTC offers ordered pore distribution and high electrical conductivity, making it in principle ideal for energy harvesting and water purification applications. In order to optimize its performance, the surface of the ZTC is for the first time modified by application of polyelectrolyte coatings, resulting in so-called soft electrodes or SEs. This combination of electrode material and functionalization gives rise to a highly efficient and low energy-consuming strategy to fully realize the potential of this carbon material in CDI and CapMix techniques for tackling global freshwater and energy challenges. Energy and power generation values up to 25 mJ and 7.5 mW m−2 have been obtained (with measured potential rises of 131 mV by only exchanging salinities of the bathing solution), which overcome values such as 4.3 mW m−2 previously found in our laboratory under similar conditions.

Asymmetric CapMix Device Exploiting Functionalized Graphene Oxide and SPEEK Coatings for Improved Energy Harvesting From Salinity Gradients

AbstractThis work investigated the possibility of using functionalized graphene oxide and sulfonated poly(ether ether ketone) coatings on activated carbons to obtain an asymmetric device suitable for Capacitive Mixing (CapMix) application. Herein the synthesis and fabrication procedures of both materials and electrodes are reported, together with a comprehensive list of electrochemical characterization techniques. The results from the electrochemical characterization are used to design the optimal operational parameters for CapMix experiments involving the materials being studied. A homemade cell is designed and fabricated for CapMix application by polymer micromachining. Electrochemical tests are conducted to optimize the process parameters and the results of this optimization are reported, proving that both the materials and the process parameters play a key role in CapMix. The final device can generate a net power output of 12 mW m−2 by mixing simulated seawater and freshwater. This result paved the way for further application of graphene oxide in the field of renewable energy.

Improved performance of flow-electrode capacitive mixing through N-doping of activated carbon

Flow electrodes, which are integral to flow-electrode capacitive mixing (F-CapMix), primarily consist of activated carbon (AC), conductive additive and an aqueous electrolyte with a salt. For the better performance of F-CapMix, the AC must have a high surface area and excellent electrical conductivity to facilitate efficient charge percolation and electron transport between the AC particles within the flow electrode. In this study, nitrogen-doped activated carbon (N-doped AC) was synthesized through a straightforward process involving the heating of a mixture of raw AC and melamine in a covered container. The resulting N-doped AC powder exhibited an average of 20 % higher electrical conductivity than raw AC powder without sacrificing the surface area (AC: 2284.7 m2/g vs N-doped AC: 2125.6 m2/g) owing to the incorporation of graphitic nitrogen and oxidized nitrogen into the raw AC surface. The surface characteristics of N-doped AC were found to be similar to those of raw AC. Electrochemical impedance spectroscopy analysis of F-CapMix utilizing N-doped AC in the flow electrode revealed reduced resistance and increased capacitance, leading to an enhancement in power density from 0.706 to 0.828 W/m2. Consequently, integrating N-doped AC into the flow electrode can contribute to the improved performance of F-CapMix.

Design of a flow channel with vortex generators for performance improvement of flow-electrode capacitive mixing under low pressure drop conditions

Salinity gradient power generation is a novel renewable energy technology that generates electricity by exploiting the disparity in salt concentration between bodies of water with high and low salt content, using techniques such as capacitive mixing with flow electrodes (F-CapMix). In this research, we conducted a comprehensive numerical analysis to examine the fluid dynamics and particle behavior of saline water and activated carbon within a flow electrode system equipped with vortex generators on the inner walls of the flow channels. The maximum and net power densities of the F-CapMix for a flow channel with vortex generators were evaluated experimentally. On average, the introduction of vortex generators into the flow channel resulted in a 35.6 % increase in the maximum power density of the F-CapMix compared to a plain channel. Furthermore, when a vortex generator was incorporated, the net power density, considering the pumping power, showed an average enhancement of 26.3 % at flow rates of 15 ml/min or less. Therefore, we anticipate that the flow channel with integrated vortex generators can be effectively utilized in the F-CapMix electrode system with a negligible impact on the pressure drop.

Comparative study on the performance of a two-cell system of Flow Electrode Capacitive Mixing (F-CapMix) for continuous energy production

In recent years, Capacitive Mixing (CapMix) has garnered growing interest as a novel method for harnessing energy from the salinity gradient between seawater and freshwater. However, the challenge of extracting energy in a continuous way remains to be solved in traditional CapMix system. In this study, we demonstrate the feasibility of achieving continuous energy extraction through the use of a two-cell flow electrode Capacitive Mixing (F-CapMix) system. The performance of the F-CapMix system is evaluated under various experimental conditions including the activated carbon loading, carbon black additives, velocity of the flow electrode and feed water and external resistance in the circuit. The results suggest that the power density of the system can be significantly increased by approximately 800 % or 400 % with an increase in the carbon loading or the addition of carbon black additives, respectively. Meanwhile, reducing the flow rate of the flow electrode and feedwater from 20 mL/s to 5 mL/s was found to improve the system's performance. In addition, it is crucial that the external resistance is matched to the internal resistance of the cell for achieving a maximum power density. These results highlight the potential of F-CapMix and provide guidance for its further optimization.

Optimization criteria of flow-electrode and feed water flow rate to achieve positive net power of flow-electrode capacitive mixing

Flow-electrode capacitive mixing (F-CapMix) produces continuous electricity due to infinite ion storage capacity of the flow-electrode. Researchers have predicted that achieving positive net power from F-CapMix is difficult due to the high pumping energy consumption of flow-electrodes. In this work, the power densities and net power densities of F-CapMix were evaluated under different flow rate condition of feed water (FW) and flow-electrode (FE) using low- and high-viscosity FEs. The power densities increased with a FE flow rate increment and a flow rate reduction of the FW. The power density difference under low and high flow-rate ratios between the FE and FW was small. As the power densities tended to become saturated with the flow rate ratio increase, indicating that a fast flow of the FE in F-CapMix is not necessary. Positive net power densities were achieved at low flow rates. The difference in the net power densities between high- and low-viscosity FEs was smaller than the power density difference between the two FEs, due to the high pumping energy consumption of highly viscous FE. Based on these results, the criteria for flow-rate optimization for the F-CapMix positive net power were suggested.

Electrochemical Conversion of Salinity Gradient Energy via Molybdenum Disulfide Electrode

The energy generated when sea water meets river water is called salinity gradient energy. At present, the main methods to extract salinity gradient energy are pressure-retarded osmosis, reverse electrodialysis and capacitive mixing technology. The selection of electrode materials has always been the focus of capacitive mixing technology. Here we report a device assembled based on capacitive mixing technology with molybdenum disulfide as anode and activated carbon as cathode. The energy density of the device is 6.12 J g−1, which shows that it has application prospects in the development of renewable energy.

Numerical prediction of pressure drop and flow characteristics of activated carbon and salt-water for flow-electrode capacitive mixing (F-CapMix)

In flow-electrode capacitive mixing (F-CapMix), prediction of the flow characteristics and pressure drop of the flow-electrode is important as it pertains to both efficient charge percolation and electron transport of activated carbons in the flow-electrode, as well as the net power of F-CapMix. In this study, the flow characteristics and pressure drop of activated carbon and salt-water within the flow-electrode of F-CapMix are predicted by applying numerical simulations, when the channel depth for the flow-electrode is changed from 0.5 mm to 2.5 mm. The numerical simulation was validated by comparing the experimental results of pressure drop measurements of the flow-electrode. To analyze the variables related to electron transfer between the activated carbon and the current collector, the particle wall normal pressure was numerically calculated. As the channel depth for the flow-electrode decreased by 0.5 mm, the pressure exerted by the activated carbon particle on the wall increased by 32.92 % on average. Consequently, the power density of F-CapMix increased to 14.03 % owing to the elevated pressure at the wall. Predicting the behavior of activated carbon and the pressure drop through numerical simulation is a key step for the designing of the flow channel for the F-CapMix.

Tuning Surface Molecular Design of Porous Carbon for Blue Energy Harvesting.

Capacitive mixing is a promising blue energy technology due to its membrane-free electricity generation and long electrode life cycle. However, because of limited performance, existing systems do not lend themselves to practical implementation. Although it is a crucial factor directly influencing electrode behavior, surface chemistry has largely been overlooked in capacitive mixing. Here, we show that manipulating surface functionalization alone can tune the responses of electrodes to produce a high voltage rise without altering the pore structure of the electrodes. Our findings reveal that the spontaneous electrode potential of a surface-modified carbon electrode shifts negatively proportional to the surface charge due to the surface groups, which explains why and how manipulating the surface chemistry can improve the power generation capacity. Using electrodes fabricated with identical activated carbon material but with different surface treatments, we have achieved a remarkably high power density of 166 mW/m2 delivered to an electrical load under a 0.6 M to 0.01 M salinity gradient, with the total power generated of 225 mW/m2. The corresponding volumetric power densities were 0.88 kW/m3 net and 1.17 kW/m3 total. The volumetric power density of our prototype is comparable to or better than those of prevailing membrane technologies, such as pressure retarded osmosis and reverse electrolysis, whose volumetric power density values are 1.1 kW/m3 and 0.16 kW/m3, respectively. In the seawater stage, the net power density reached 432 mW/m2 or 2.3 kW/m3. Such performance far exceeds existing membrane-free systems, with the highest reported power density of 65 mW/m2 under a 0.5 M to 0.02 M salinity gradient (121 mW/m2 in this work). The device demonstrated unparalleled durability, maintaining 90% of the maximum energy capacity after 54,000 charge-discharge cycles.

A charge-free and membrane-free hybrid capacitive mixing system for salinity gradient energy harvesting

A charge-free and membrane-free hybrid capacitive mixing (H-CapMix) system for salinity gradient energy harvesting with EDA-YP80F as a capacitive anode for Cl− absorption and FeHCF as a battery-type cathode for Na+ incorporation.

A single-cell system of flow electrode capacitive mixing (F-CapMix) with a cross chamber for continuous energy production

In this system, all the negative ions move toward to left and positive ions move toward to right. The ions movement can be converted into electricity.

Control Strategy for Power Generation from a Capacitive Mixing Cell and Grid Injection

In this work, the possibility of extracting energy from a capacitive mixing cell based on the potential that exists as a result of the salinity gradient between fresh and salt water passing through the electrodes of a deionization cell is studied. For this, a two-stage process is defined. The first step uses a dual buckboost converter for the positive and negative cycles of the input voltage. This topology acts as a current source and is chosen due to its ability to provide high power factor and to work with extremely low voltage levels. The second stage consists of an Hbridge inverter synchronized with the AC grid to feed power into the grid. A hysteresis control system has been designed for the current flowing through the coil. With this, it has been analyzed how the efficiency of the total system (cell + power converter) varies with the changes in the series resistance of the CapMix cell or with the discharge current selected in the power converter. Simulations are carried out to characterize the energy extraction and some experimental results confirm the possibility of using this new renewable energy source.

Comparative study on the performance of capacitive mixing under different operational modes

Capacitive mixing (CapMix) is a renewable method of extracting energy from the salinity difference between seawater and freshwater. In this study, we systematically investigate the system behavior and performance of the CapMix system under four operational modes namely, capacitive energy extraction based on double layer expansion (CDLE), capacitive energy extraction based on the Donnan potential (CDP), and CDP with additional charging of constant voltage (CDP-CV) and constant current (CDP-CC). The results indicate that the application of additional charging in the CDP technique can break the limits of the Donnan potential and significantly improve the system’s performance. Accordingly, in terms of energy production and average power density, CDP-CC and CDP-CV are the two superior operational modes, followed by CDP and CDLE. In addition, our results reveal that CDP-CC is determined by the accumulated charge and applied current. CDLE is dependent on the applied voltage, while CDP-CV is not sensitive to the applied voltage. Increasing the external load can considerably increase the energy production of both CDLE and CDP. In summary, the findings in this study provide practical information for the optimization and application of CapMix technologies.

Multilayered Graphene-Coated Metal Current Collectors with High Electrical Conductivity and Corrosion Resistivity for Flow-Electrode Capacitive Mixing

Flow-electrode capacitive mixing (F-CapMix) is a novel technology, harvesting electric energy from salinity gradient power. A continuously circulating flow-electrode enables constant electrical power generation without any intermittent step. Graphite is a widely used current collector because of its high electrical conductivity and corrosion resistivity to seawater for F-CapMix. However, insulating polyacrylate-based polymers should be employed for shaping the graphite body, resulting in reduced electrical conductivity, hindering efficient charge percolation in flow-electrodes. Metal current collectors are difficult to use because of their low corrosion resistance to seawater despite their good electrical conductivity. Here, we first report high electrical conductivity and corrosion-resistive multilayered graphene as a protective layer on metal current collectors in F-CapMix. The graphene is synthesized on metal current collectors through the chemical vapor deposition process. We study chemical and electrochemical aspects of protective layer-coated current collectors by controlling parameters for graphene growth on metal substrates. The multilayered graphene protective layer can not only prevent corrosion from seawater but also enable efficient charge percolation with high electrical conductivity. By employing multilayered graphene-coated Ni current collectors in the F-CapMix unit cell, an almost twice higher power density of 0.75 W·m–2 and current density of 22.3 A·m–2 could be achieved compared to those of graphite.

Continuous power production using flowable electrodes based on waste-heat assisted capacitive mixing

• A thermally assisted capacitive mixing system is proposed for harvesting low-grade heat. • The system can produce power continuously via adopting multiple electrode flow capacitors. • Various parameters are adjusted to optimize the performance of the system. • The thermal efficiency of the system is calculated based on the exergy analysis. • The temperature effect contributes to an increase in the power density of the system by a factor 1.5. The capacitive mixing method generates energy through spontaneous ionic mixing caused by the salinity gradient between sea and river waters. We introduce a continuous thermal regenerative system to combine an intermittent cycle with electrode flow capacitors using flowable slurries as electrodes to improve the efficiency of the existing capacitive mixing method by adding low-grade heat as another energy source. A continuous harvesting system was constructed to utilize the capacitive Donnan potential effect by switching the temperature and salt concentration with four separate reactors and two flow-electrode slurries. Experiments were conducted for continuous cycle operations by varying the active carbon and salt concentrations of the slurry as well as a single electrode flow capacitor test to evaluate the contributions of two different energy sources, i.e. , concentration and temperature differences. The continuous cycle operation was further optimized, resulting in a maximum power density of 3.84 mW/m 2 . Based on the exergy analysis, the theoretical thermal efficiency of the thermally–assisted continuous system was calculated at 10.8%. Waste-heat energy was successfully used for continuous power production using an electrode flow capacitor, and a continuous power generation cycle was implemented by combining the concentration and temperature differences without supplying external power.

Energy Extraction from Capacitive Mixing: Experimental and Computational Analysis of Chemical Aspects

The double-layer expansion at the interfaces of charged electrodes when a concentrated salt solution (molar concentration Csalt ) is replaced by a diluted solution (C fresh), produces an increase of the cell voltage. Experiments and simulations were performed in order to study the effect of several variables on the obtained transient voltage, V(t), and the rising of voltage at equilibrium, Δφ eq. Carbon electrodes were prepared and characterized by cyclic voltammetry, amperometry, and N2-adsorption isotherms. The experimental results and finite elements simulations show that the shape of V(t) is influenced by the diffusion coefficients of the ions. On the other hand, experiments and simulations indicate that Δφ eq present a Nernstian-like dependence with the dilution coefficient, Cdil = C salt /Cfresh , while is independent of the initial charge and charging time. Quantitative features are interpreted by proposing a partial blocking of the electrodes for the dilution process, associated to irreversible incorporation of solution into the porous, presenting a model to estimate the blocking degree. Finally, cycles of charging-dilution-work-concentration were performed, extracting energies in the order of 100 μJ cm−2, which are in agreement with the model equations as function of Csalt and Cfresh .

Importance of channel dimension for flow-electrode flowing in flow-electrode capacitive mixing (F-CapMix): Evaluation of net power density under high-pressure-drop conditions

Flow-electrode capacitive mixing (F-CapMix) utilizes ion adsorption/desorption on active materials in the flow-electrode to harvest salinity gradient power. In order to facilitate flow-electrode flow in the F-CapMix unit cell, the current collectors should include a meandering flow channel for the cell assembly and should facilitate efficient electron transport between them and the flow-electrode active materials. This study evaluates the F-CapMix performance variation and pumping power of the flow-electrode and feed water depending on flow channel depth. It was found that both parameters increased as the cross-sectional area of the flow channel narrowed. This result was due to the increasing linear velocity of the flow-electrode, which itself was associated with the narrowing cross-sectional area. The maximum gross power densities of the F-CapMix increased in a parabolic shape with respect to the flow-electrode linear velocity increase. (0.68 → 0.83 → 0.86 → 1.03 → 1.14 W/m2) On the contrary, pumping power densities of the flow-electrode increased linearly (2.22 → 2.98 → 3.51 → 5.93 → 19.67 W/m2) with linear velocity increment of the flow-electrode. Further, the F-CapMix net power density was negatively affected by the increasing linear velocity of the flow-electrode (net power density variation from −2.06 to −19.10 W/m2), likely owing to the higher pumping power density required due to the narrower flow channel. This study verifies the importance of securing an appropriate cross-sectional area of the meandering flow channel to maintain positive net power of the F-CapMix.

High-Performance Asymmetric Flow-Electrode Capacitive Mixing with MnO2-Coated Activated Carbon Flow-Electrode for Energy Harvesting from Salinity Gradient Power

Flow-electrode capacitive mixing (F-CapMix) is a promising energy harvesting system, utilizing the salinity gradient difference between seawater and river water. However, the conventional F-CapMix system has a low power density due to the limited charge storage of porous carbon materials through the electrical double layer. In this study, we first report manganese dioxide-coated activated carbon (MO@AC) as a flow-electrode material for the F-CapMix system. MnO2 nanoparticles could be easily decorated on the surface of activated carbon (AC) by the reduction of KMnO4 in an acidic medium. We systematically study electrochemical properties of AC and MO@AC-based flow-electrodes as positive and negative electrodes for both symmetric and asymmetric F-CapMix systems. The asymmetric F-CapMix system employing the MO@AC flow-electrode exhibits an outstanding power density of 2.22 W/m2 through the reversible redox reaction with Na+ ions. Our strategies can be extended to other materials that have redox reactions with Na+ and Cl– ions for the F-CapMix system.

Numerical Prediction of Pressure Drop and Flow Characteristics of Activated Carbon and Salt-Water for Flow-Electrode Capacitive Mixing (F-Capmix)

Numerical Prediction of Pressure Drop and Flow Characteristics of Activated Carbon and Salt-Water for Flow-Electrode Capacitive Mixing (F-Capmix)