Defined Carbon Porosity for Sustainable Capacitive Charging

Abstract

Disclosed are activated carbon electrodes fabricated according to a pore mouth diameter mixture profile that is optimized for a given electrochemical application. In a given pore mouth diameter mixture profile, the pore mouth diameter and conductivity of activated carbon are tightly controlled and provide unexpected long-term charging/discharging (aka cycling) performance. A given pore mouth diameter mixture profile optimizes a mixture of pore mouth diameters for a given electrochemical application, such as energy storage, desalination, deionization, hydrolysis, and dialysis, inter alia.

Claims

1. A carbon electrode used in an electrochemical system, wherein an average pore mouth diameter of the carbon is in the range of 2.5 to 10 nm achieved with a pore mouth diameter profile from 0% to 30% microporous activated carbon and from 70% to 100% mesoporous activated carbon.

2. A carbon electrode used in an electrochemical system, wherein an average pore mouth diameter of the carbon is in the range of 2.5 to 10 nm achieved with a pore mouth diameter profile from 80% to 100% mesoporous activated carbon and from 0% to 20% macroporous activated carbon.

3. The electrode of claim 1, wherein the electrochemical system is selected from the group consisting of energy storage, batteries, supercapacitors, CDI desalination, i-CDI desalination, deionization, hydrolysis, dialysis, electrodialysis reversal, and fuel cells.

4. The electrode of claim 2, wherein the electrochemical system is selected from the group consisting of energy storage, batteries, supercapacitors, CDI desalination, i-CDI desalination, deionization, hydrolysis, dialysis, electrodialysis reversal, and fuel cells.

5. The electrode of claim 1, wherein the electrochemical system is selected from the group consisting of energy storage, batteries, supercapacitors, CDI desalination, i-CDI desalination, deionization, hydrolysis, dialysis, electrodialysis reversal, and fuel cells, and wherein the carbon has an average pore mouth diameter in the range of 3 to 5 nm achieved with a pore mouth diameter profile from 0% to 30% microporous activated carbon and from 70% to 100% mesoporous activated carbon.

6. The electrode of claim 2, wherein the electrochemical system is selected from the group consisting of energy storage, batteries, supercapacitors, CDI desalination, i-CDI desalination, deionization, hydrolysis, dialysis, electrodialysis reversal, and fuel cells, and wherein the carbon has an average pore mouth diameter in the range of 3 to 5 nm achieved with a pore mouth diameter profile from 80% to 100% mesoporous activated carbon and from 0% to 20% macroporous activated carbon.

7. A method of selecting a pore mouth diameter profile in fabricating electrodes for an electrochemical application by: excluding activated carbon with a pore mouth diameter of less than 2.5 nm, excluding volume percentages of microporous carbon of more than 30%, and maximizing the volume percentage of mesoporous activated carbon without a drop in the specific charge passed/mCg.sup.1 of more than 30% based on at least 100 cycles of charging/discharging in a selected electrochemical system.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0055] FIG. 1. Charge passed with commercially available microporous (SC and KN), a predominately mesoporous (CG), and mesoporous (CX, aka generic CX) carbon electrodes in a flow-through capacitive deionization cell. The order of the carbon types in the legend is also the order of the starting traces.

[0056] FIG. 2. Charge passed with KN and CG carbon electrodes in a flow-by capacitive deionization cell. The order of the carbon types in the legend is also the order of the starting traces.

[0057] FIGS. 3A and 3B. Schematic for two pairs of electrodes used for desalination for flow-through (FIG. 3A) and flow-by (FIG. 3B) cell architectures. The components present in FIG. 3A are the same for FIG. 3B, only the flow path changes. In flow-through the stream flows perpendicular to the electrodes, while inflow-by the stream flows parallel. 1anode (carbon), 2current collector, 3separator, 4cathode (carbon), 5power supply, arrowdirection of water flow.

[0058] FIGS. 4A and 4B. FIG. 4A shows a shaded band that was the target zone for charge passed per gram of the inventors' high-efficiency carbon electrode for inverted capacitive deionization (i-CDI) cells with 12 pairs of electrodes in 18.5 L of 4.3 mM NaCl using a charging voltage of 0.8 V (for desorption) and a discharging voltage of 0 V (for adsorption). Total cycle time was 3 h. Pristine Spectracarb (SC), Kynol (KN), Calgon (CG) or carbon xerogel (CX) was used for the cathodes and nitric acid oxidized SC, KN, CG, or CX was used to make the anodes used in the experiments. FIG. 4B shows the actual results (black ++++ data points) using the inventors' high-efficiency carbon electrode, which achieved the target charge passed per gram of electrode for i-CDI cells shown in FIG. 4A. The order of the carbon types in the legend is also the order of the starting CV traces.

[0059] FIGS. 5A, 5B, and 5C. FIG. 5A shows representative pore sizes of carbon electrode materials. FIG. 5B shows a schematic view of microporous carbon pores (pristine, and roofed until pore mouth collapse). FIG. 5C shows a schematic view of mesoporous carbon pores (pristine, and partially roofed without pore mouth collapse).

[0060] FIGS. 6A, 6B, 6C, 6D, and 6E. FIG. 6A through 6E show cyclic voltammograms (CV) at a scan rate of 0.5 mV/s in 4.3 mM NaCl with carbon as the cathode, Pt coated Ti as the anode, and a standard calomel electrode (SCE) as the reference. Accelerated oxidation studies were performed at 2.0 V for 6 hours. Pristine carbon is compared to electrochemically oxidized carbon at 3 and 6-hour time steps for a given carbon. FIG. 6A shows CVs for microporous SC. After 3 hours of electrochemical oxidation the E.sub.PZC has shifted to the right, indicative of the formation of oxygen surface groups on the carbon electrode. After 6 hours the E.sub.pzc is no longer present and there is a noticeable decrease in current, signifying pore collapse. FIG. 6B shows CVs for microporous KN. The same trend is observed as for SC in FIG. 6A. FIG. 6C shows CVs for CG, a primarily mesoporous carbon. After 3 and 6 hours of electrochemical oxidation the E.sub.PZC has shifted to the right, indicative of the formation of oxidized surface groups on the carbon electrode, but the current is maintained due to non-collapsed pores. FIG. 6D shows CVs for mesoporous CX and FIG. 6E shows CVs for mesoporous HE, both of which follow the same trend observed for CG in FIG. 6C: current is maintained due to non-collapsed pores.

[0061] FIGS. 7A and 7B. Schematic depicting the differences between a supercapacitor (FIG. 7A) and a capacitive deionization (CDI) cell (FIG. 7B). 1cathode (carbon), 2current collector, 3electrolyte, 4separator, 5anode (carbon), and 6power supply. A supercapacitor cell is a closed cell; a CDI cell has inlet and outlet streams.

[0062] FIGS. 8A and 8B. A schematic of two electrodes used to demonstrate the mass balancing technique. FIG. 8A shows a conventional electrode set-up where the cathode and anode are the same size. FIG. 8B shows a mass balanced electrode set-up where the anode is larger than the cathode to lower the distributed voltage to the anode. 1cathode (carbon), 2anode (carbon), and 3power supply.

[0063] FIG. 9. A schematic of an EDR cell with one pair of electrodes. Water flows through the cell producing desalinated streams, shown as product, and concentrated waste streams, shown as concentrate. 1current collector, 2cathode (carbon), 3cation membrane, 4anion membrane, 5anode (carbon), 6power supply, and arrowsdirection of water flow.

DETAILED DESCIPTION OF THE PREFERRED EMBODIMENTS

[0064] The inventors are the first to disclose a carbon electrode that addresses the effects of pore mouth roofing and pore volume decrease from oxidative reactions in electrochemical systems. Pore mouth roofing, and pore volume decrease, result in major performance deterioration and short life of prior art electrodes in energy storage and desalination applications. Shown in FIG. 1 is an example of the charge passed per gram of carbon for microporous (Spectracarb (SC) and Kynol (KN)) carbon, a predominately mesoporous carbon formulated according to the pore mouth diameter profile method (CG), and mesoporous (carbon xerogel CX) carbon electrodes over the course of >200 cycles with each cycle lasting 3 hours (>300 total hours of charging, >300 total hours of discharging). The inventors observed that charge passed for the microporous carbon electrodes initially plateaus before a sharp drop in performance after 100 charge/discharge cycles (cycles). Unlike the microporous carbon, the mesoporous carbon shows continued, generally uniform performance over hundreds of cycles. Ideally, a carbon material would have sustained higher charging capacities for >100 cycles.

[0065] The various embodiments of the invention are tailored mixtures of both microporous and mesoporous carbon. Mixtures of microporous and mesoporous carbon are abbreviated based on percentage content, in a microporous volume percentage/mesoporous volume percentage format, e.g., 11% microporous and 89% mesoporous carbon is called an 11/89 mixture. The CG carbon used in the experiments reported herein is an 11/89 formulation developed using the pore mouth diameter profile method disclosed herein, and custom fabricated for the inventors. An 11/89 mixture has improved performance over the microporous carbon with a slower decay in performance, but it is not as stable as the generic mesoporous carbon. FIG. 2 shows a similar trend for microporous vs. mesoporous carbons for a flow-by cell design.

[0066] CX predominately mesoporous carbon contains an average pore mouth diameter of 20-40 nm and specific surface areas between 100-300 m.sup.2/g. In contrast to mesoporous carbon electrodes, commercially available microporous carbon electrodes have an average pore mouth diameter of <2 nm with specific areas between 1500-2000 m.sup.2/g. CG microporous/mesoporous carbon contains an average pore mouth diameter of 0.5-10 nm and specific surface areas between 500-1000 m.sup.2/g. Generic mesoporous carbon electrodes (average pore mouth diameter of 10-45 nm and specific surface areas between 100-300 m.sup.2/g) are called herein, generic CX.

[0067] HE predominately mesoporous carbon contains an average pore mouth diameter of 2.5 to 4 nm, containing >98% mesoporous carbon with the balance being macroporous carbon. HE was formulated by the inventors using the pore mouth diameter profile method disclosed herein.

[0068] The inventors' research focused on trying to develop a carbon material with the continued, generally uniform capacitive performance of generic CX but with much higher adsorption efficiency. The inventors developed a much higher adsorption efficiency carbon electrode material that has smaller mesopore (3-5 nm) pore mouth diameter and specific surface areas between 300-500 m.sup.2/g. The inventors' high-efficiency (HE) mesoporous carbon (1) has much higher charge storage per gram of carbon compared to commercially available carbon electrodes, (2) has sustained performance for >500 hours of cycling, and (3) avoids the sharp drop in performance after 100 cycles characteristic of microporous carbon.

[0069] Multiple cell designs have been shown for capacitive deionization (CDI) in the past, and each cell design can have an impact on oxidation routes of the carbon anode. The two most common designs are flow-through and flow-by. In Cohen, I.; Avraham, E.; Bouhadana, Y.; Soffer, A.; Aurbach, D. Long term stability of capacitive de-ionization processes for water desalination: The challenge of positive electrodes corrosion Electrochim. Acta 2013, 106, 91-100 and in Suss, M. E.; Baumann, T. F.; Bourcier, W. L.; Spadaccini, C. M.; Rose, K. A.; Santiago, J. G.; Stadermann, M. Capacitive desalination with flow-through electrodes, Energy Environ. Sci. 2015, 5, 9511-9519, a flow-through architecture is used for desalination. The use of a flow-through cell design has the benefits of increased adsorption kinetics, but the pH changes and reaction products from the carbon cathode can directly impact the carbon anode and increase the rate of pore collapse of the anode (points 2 and 3 above). Flow-by cell designs will still be impacted by these processes, but to a lesser extent. Likewise, both cell designs will experience electrochemical carbon oxidation at the anode, regardless of flow regime, making the use of a high-efficiency carbon electrode preferable to delay or eliminate the effects of pore collapse on the separation/ion storage process.

[0070] Flow-through cells have the electrolyte flow directly through (perpendicular to) the carbon electrodes during separation and regeneration while inflow-by cells, the electrolyte will flow parallel to the electrode surface. Separation performance and charge stability for each carbon, reported below, was compared using flow-through inverted capacitive deionization (i-CDI) cells with 12 pairs of electrodes in 18.5 L of 4.3 mM NaCl using a charging voltage of 0.8 V and a discharging voltage of 0 V, with a total cycle time of 3 h. Flow-by i-CDI cells with 14 pairs of electrodes in 18.5 L of synthetic tap water using a charging voltage of 0.4 V and a discharging voltage of 0.2 V, with a total cycle time of 20 min, 30 min, or 1 h, reported below, were used to compare the KN microporous and CG (primarily mesoporous) carbons. Schematics of both cell architectures are shown in FIG. 3.

[0071] It will take longer for pore mouth collapse to occur with a flow-by CDI design. Pore collapse, which leads to significant decreases in carbon pore volume and surface area, is exacerbated due to oxidation routes at the carbon anode. This oxidation is a function of (1) electrochemical carbon reactions with water, (2) increases in the pH from reactions at the cathode that leads to increased driving force for oxidation reactions at the cathode, and (3) reaction products from the cathode such as hydrogen peroxide that can oxidize the carbon anode.

[0072] Mesoporous electrodes for desalination typically have a pore mouth diameter of 2-50 nm, but the inventors have identified 5 nm as optimal. Oxidative roofing is a greater problem in desalination cells compared to energy storage cells because the electrolyte salt concentration is lower, and the system is open.

[0073] While supercapacitors and CDI cells have similar components, such as carbon anodes, carbon cathodes, an electrolyte, and an external voltage source used to control adsorption/desorption of ions on the carbon surface through the electrical double layer, some notable discrepancies can be found. Supercapacitors adsorb and desorb ions from a concentrated electrolyte (such as 1 M sodium sulfate) inside a closed or sealed cell. The electrolyte is tailored for maximizing the charge storage of the cell and no outside compounds impact the carbon electrodes. In a CDI cell, the carbon material is in contact with an electrolyte that is constantly changing as this system is used to separate ions from an incoming feed stream, not for charge storage. The carbon electrodes used in CDI cells come into contact with dissolved oxygen and a lower concentration of salt species.

[0074] The differences between supercapacitors and CDI leads to a notable difference in the degradation of the carbon anodes. The anode will oxidize due to electrochemical reactions with water in both systems resulting in pore mouth roofing and pore mouth collapse. However, pore mouth collapse is intensified for carbon anodes in CDI cells due to lower electrolyte concentrations (higher likelihood of Faradaic reactions such as carbon oxidation), and the presence of dissolved oxygen that reacts at the carbon cathode and produces perturbations in pH and hydrogen peroxide that can increase the oxidation rate at the anode. Although pore mouth collapse can take place in both systems, the effect will be more profound earlier in the device lifetime for a CDI cell. In embodiments of the invention, a more stable carbon anode (high efficiency carbon electrode) can be used either in conjunction with mass balancing or as a standalone technique. A more stable, oxidation resistant anode can substantially increase the usable lifetime of both a supercapacitor and a CDI cell by limiting oxidation reactions that lead to pore mouth collapse. Pore mouth collapse decreases the surface area for charge storage or ion separation.

[0075] The target properties of the inventors' high-efficiency mesoporous carbon material (shaded band in FIG. 4A were: pore mouth diameter 3-10 nm; thickness<600 m; conductivity>5 S/cm; surface area>400 m.sup.2/g; and pore volume>1 cm.sup.3/g. One embodiment of the high-efficiency carbon of the invention has an average pore mouth diameter of 3.4 nm, a thickness of 580 m, a conductivity of 49 S/cm, a specific surface area of 378 m.sup.2/g, and a pore volume of 0.235 cm.sup.3/g, and achieves the desired increase in performance (FIG. 4B). Table 1 lists the pertinent material properties for all carbons discussed herein.

[0076] When a voltage is applied across the carbon electrodes in a cell, the anode is at a positive potential (facilitating oxidation) and the cathode at a negative potential (facilitating reduction). This causes the anode to oxidize and oxygen surface groups to form over time. The smaller the pore diameter (micropore), the more likely it will get blocked by these oxygen surface groups (in a process called, roofing) and prevent ion adsorption within the pore volume of the carbon electrode (FIG. 5B). With a larger pore mouth diameter (mesopores), ions can enter the pore volume regardless of oxygen groups present at the surface (FIG. 5C), providing sustained stable salt adsorption performance; however, mesoporous carbon has significantly less total surface area compared to microporous carbon. The inventors' experimental data show that there is linear decrease in charge efficiency until an average pore diameter of 3-5 nm, below which diameter charge efficiency drastically and nonlinearly decreases. For CG, the micropores would collapse first, leaving only the remaining mesopores available for ion adsorption. This phenomenon is reflected by the gradual decay in specific charge passed shown in FIG. 1.

[0077] A simple comparison of carbon electrode pore diameters, shown in FIG. 5A, does not suggest the unexpected properties of the inventors' high adsorption-efficiency activated carbon mixture. Accelerated oxidation studies and loss of surface area from capacitance measurements support the effect of pore collapse and roofing observed with microporous carbon. Pore roofing is when the pore appearance, or pore mouth, becomes blocked by surface oxide groups due to reaction between the carbon electrode and the aqueous electrolyte, preventing ion adsorption into the pore volume. Once the pore mouth is completely (in physical, stearic, or ionic terms) blocked or filled, the pore is said to have experienced pore collapse. FIGS. 5B and 5C show pore roofing and ultimate collapse for pores with varying pore diameters. To demonstrate pore roofing, using commercial and high efficiency synthesized carbon electrodes of the invention, carbon was electrochemically oxidized at 2.0 V in 1 hour increments for a total of 6 hours. As shown in FIGS. 6A to 6E, cyclic voltammograms (CV) were performed before and after each oxidation step (an oxidation step herein is 3 hours of cycling) to determine whether there was a change in current, overall shape of the CV curve, and potential of zero charge (potential at which there is no net charge on an electrode, or E.sub.pzc). Representative CVs for each carbon are shown in FIGS. 6A to 6E. Microporous carbons (SC and KN) were significantly oxidized after 3 hours and completely collapsed after 6 hours (FIGS. 6A and 6B). The E.sub.pzcs shifted more anodic, indicative of oxidation, and the area under the curve dramatically reduced over time. Whereas CG (FIG. 6C), and mesoporous carbons, CX and HE (FIGS. 6D and 6E), showed oxidation after 6 hours but maintained current. The E.sub.pzcs shifted more anodic, but the area under the curves was preserved. The electrochemical surface areas were calculated for the pristine and electrochemically oxidized carbons after 6 hours at 2.0 V (Table 2). THE percent loss in surface area from lowest to highest is as follows: HE, CX<CG<SC<KN (stated more generally, HE or mesoporous<CG<microporous). This emphasizes that mesoporous carbons can handle a large applied voltage while maintaining charge and avoiding the effect of pores collapsing and roofing, leading to a reliable, long lifetime.

[0078] The surface of an HE carbon electrode can be modified by adding functional groups to improve device performance, e.g., to shift the Epzc of electrodes for use in an i-CDI system. Although shifting the Epzc does improve ionic separation, based on data collected to date, shifting the Epzc does slow down pore mouth roofing. The functional groups for desalination are selected to match the surface charge for the intended application, meaning anodes with negative surface charge (positive E.sub.PZC) and cathodes with positive surface charge (negative E.sub.PZC) for i-CDI and the reverse for CDI.

[0079] Pore mouth diameter profile and pore volume (and associated carbon electrode density) are optimized for a given electrochemical application using the pore mouth diameter profile method disclosed herein. High pore volume in cm.sup.3/g will yield high specific surface area in m.sup.2/g, but the mass of carbon per volume of device will be low, which means less total carbon available. One criterion in selecting a pore mouth diameter profile in fabricating electrodes for a given energy storage, desalination, deionization, hydrolysis, dialysis, or other electrochemical application is to avoid (i) too small a pore mouth diameter, and (ii) too high a percentage of microporous carbon. A second criterion is selecting the largest pore mouth diameter in the profile. A third criterion is selecting the volume percentage of each type of carbon, which is done experimentally for a given electrolyte. For desalination, the average pore mouth diameter should be 3-10 nm (small mesopores) to prevent pore collapse in aqueous electrolytes. For desalination, pore volumes>0.1 cm.sup.3/g will be needed to have substantial adsorption space. For a given application, exact values related to carbon density, average pore mouth diameter, and pore mouth diameter profile (i.e., the volumetric ratios among microporous, mesoporous, and macroporous carbon) are determined experimentally using CVs.

[0080] Electrodialysis reversal (EDR) is a water treatment process that uses an electric field, carbon or metallic electrodes, and ion exchange membranes to selectively remove ions from water streams. A schematic of an EDR cell is shown in FIG. 9. Selective movement of ions is accomplished through the incorporation of cation and anion exchange membranes in the cell. In EDR, the electric field or voltage is periodically reversed to help prevent fouling of the ion exchange membranes. When carbon electrodes are used in an EDR cell stack, pore mouth roofing can occur due to similar mechanisms described above for CDI and supercapacitor systems. However, the voltages are larger and the pore mouth roofing can occur more quickly due to the combination of dilute electrolyte and higher voltages. E.sub.pzcn-shifting may be made with the HE carbon of the invention, thereby combining the advantages of E.sub.pzc shifting and decreased pore mouth roofing.

[0081] Experimental results, such as those presented above, provide the following guidelines for working of the invention:

[0082] For desalination CDI, i-CDI, EDR, or other electrochemical systems, carbon electrodes of the invention have an average pore mouth diameter in the range of 2.5 to 10 nm achieved with a pore mouth diameter profile either (i) from 0% to 30% microporous (<2.5 nm) activated carbon and from 70% to 100% mesoporous (>2.5 nm and <50 nm) activated carbon or (ii) from 80% to 100% mesoporous activated carbon and from 0% to 20% macroporous (>50 nm) activated carbon.

[0083] For desalination, CDI, i-CDI, EDR, or other electrochemical systems, carbon electrodes of the invention have an average pore mouth diameter preferably in the range of 2.5 to 8 nm achieved with a pore mouth diameter profile either (i) from 0% to 30% microporous (<2.5 nm) activated carbon and from 70% to 100% mesoporous (>2.5 nm and <50 nm) activated carbon or (ii) from 80% to 100% mesoporous activated carbon and from 0% to 20% macroporous (>50 nm) activated carbon.

[0084] For desalination, CDI, i-CDI, EDR, or other electrochemical systems, carbon electrodes of the invention have an average pore mouth diameter more preferably in the range of 2.5 to 5 nm achieved with a pore mouth diameter profile either (i) from 0% to 30% microporous (<2.5 nm) activated carbon and from 70% to 100% mesoporous (>2.5 nm and <50 nm) activated carbon or (ii) from 80% to 100% mesoporous activated carbon and from 0% to 20% macroporous (>50 nm) activated carbon.

[0085] For desalination, CDI, i-CDI, EDR, or other electrochemical systems, carbon electrodes of the invention have an average pore mouth diameter more preferably in the range of 2.5 to 4 nm achieved with a pore mouth diameter profile either (i) from 0% to 30% microporous (<2.5 nm) activated carbon and from 70% to 100% mesoporous (>2.5 nm and <50 nm) activated carbon or (ii) from 80% to 100% mesoporous activated carbon and from 0% to 20% macroporous (>50 nm) activated carbon.

[0086] Other electrochemical systems comprise energy storage, batteries, supercapacitors, deionization, hydrolysis, dialysis, and fuel cells.

[0087] The preferred method of selecting a pore mouth diameter profile (aka, the pore mouth diameter profile method) in fabricating electrodes of the invention for an electrochemical application is by:

[0088] excluding activated carbon with a pore mouth diameter of less than 2.5 nm,

[0089] excluding volume percentages of microporous carbon of more than 30%, and

[0090] maximizing the volume percentage of mesoporous activated carbon without a drop in the specific charge passed/mCg.sup.1 of more than 30% based on at least 100 cycles of charging/discharging in a selected electrochemical system.

TABLE-US-00001 TABLE 1 Carbon Properties Resis- Pore Surface Thick- tivity.sup.b Conduc- Mouth Area.sup.a ness (Ohm .Math. tivity Diameter Material (m.sup.2/g) (mm) cm) (S/cm) (nm) Spectracarb (SC) 1951 0.54 0.69 1.45 <1 Kynol (KM) 1822 0.46 0.45 2.24 2.2 Calgon (CG) 718 0.66 0.47 2.14 0.5-10 Carbon Xerogel 250 0.32 0.013 78.09 10-45 (CX) (25 ave.) High-Efficiency 378 0.58 0.020 49.32 3.4 (HE) .sup.ameasured by Brunauer-Emmett-Teller (BET) theory .sup.baverage of three measurements taken with a four point probe

TABLE-US-00002 TABLE 2 Percent surface area lost due to electrochemical oxidation for 6 hours at 2.0 V in 4.3 mM NaCl as calculated from specific capacitance measurements. Cyclic voltammograms were run in 1 M NaSO.sub.4 at scan rates of 1, 5, 10, 20, 40, 60, 80, and 100 mV/s. The total current at 100 mV was plotted vs. the scan rate to obtain the geometric capacitance. The specific capacitance was obtained by dividing the geometric capacitance by the mass of carbon and the electrochemical surface area was then calculated from Eq. 1. Surface Area Surface Area from BET from Capacitance (m.sup.2/g) Material (m.sup.2/g) Pristine Cycled 6 h @ 2.0 V % Loss Spectracarb 1951 3712 1195 68 (SC) Kynol (KN) 1822 4758 1179 75 Calgon (CG) 718 3419 3074 10 Carbon Xerogel 250 434 2723 N/A (CX) High-Efficiency 378 970 1812 N/A (HE) Eq. 1: Electrochemical surface area (m.sup.2g.sup.1) = Specific capacitance(Fg.sup.g) [00001]$\frac{{\mathrm{cm}}^2}{{10}^{-5}.Math.F}\frac{1.Math..Math.m^2}{100.Math..Math.{\mathrm{cm}}^2}$

Example 1

[0091] 18.5 L of 4.3 mM NaCl solution was treated by a small PowerTech Water device (PowerTech LLC, Lexington, Ky.) where the anode had been oxidized using nitric acid and the cathode was a pristine carbon electrode. Between 10-15 grams of carbon was used in a flow-through inverted capacitive deionization cell (aka i-CDI, disclosed by the inventors in USPUB 20160167984) and operated using a cell charging potential of 0.8 V and a discharge potential of 0 V. The NaCl solution was sent directly through the capacitive deionization cell at 20 ml/min.