walker electrochemical paper

Electrochimica Acta 182 (2015) 1008–1018

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Electrochimica Acta

jo ur na l ho me p a ge: w ww.elsev i e r .c o m/ loca te/e lectacta

Electrodeposited MnO2 For Pseudocapacitive Deionization: Relating Deposition Condition and Electrode Structure to Performance

P.J. Walkera, M.S. Mauterb,c, J.F. Whitacrea,b,*a Department of Materials Science and Engineering, Carnegie Mellon University, 5000 Forbes Ave, Pittsburgh, PA 15217, USAb Department of Engineering and Public Policy, Carnegie Mellon University, 5000 Forbes Ave, Pittsburgh, PA 15217, USAc Department of Civil and Environmental Engineering, Carnegie Mellon University, 5000 Forbes Ave, Pittsburgh, PA 15217, USA

A R T I C L E I N F O

Article history:Received 25 June 2015Received in revised form 21 September 2015 Accepted 22 September 2015Available online 9 October 2015

A B S T R A C T

Electrodeposited MnO2 was screened as a possible pseudocapacitive material for use in a capacitive deionization (CDI) environment. Two simple electrochemical deposition conditions (galvanostatic and cyclic voltammetric) were used to produce MnO2 coatings on porous carbon substrates with contrasting surface areas and morphologies. These samples were then tested in static half-cell test fixtures with the intention of probing how these types of substrates and coatings might work in a CDI flow cell. The results show that the best sodium ion uptake performance on per-mass basis was not necessarily the most compelling electrode-level solution for practical application, and that a high surface area nanoporous substrate displayed evidence of a self-limited deposition wherein the internal electrode volume was not able to participate in the ion removal reactions of interest.

ã 2015 Elsevier Ltd. All rights reserved.

1. Introduction

There is an increasing demand for water for agricultural purposes around the world, particularly in areas that have been suffering more prolonged and frequent periods of drought. Capacitive deionization (CDI) has been put forth as a scalable low-energy technique to desalinate brackish water (on the order of 1000 mg/L total dissolved salt) than can be used alone or in conjunction with other water purification technologies [1]. Many designs for CDI devices already exist, generally relying on carbon to electrosorb ions at the electrode surface [2,3]. In particular, carbon electrodes with high surface area and appropriate porosities have been investigated, including graphenes, activated carbon, ordered mesoporous carbon and carbon aerogels [4–10]. Additional research has explored oxidizing or reducing the surface of carbon electrodes to increase ion-removal ability [11,12]. Several groups have also looked into enhancing ion-uptake through coating carbon substrates with materials such as silica or TiO2 [13,14]

Similarly, there has been a great deal of research into MnO2 for

use in aqueous electrolyte supercapacitors, however there has been far less work on the application of this materials system for the CDI environment [15–20]. In several cases MnO2/carbon

* Corresponding author at: Department of Engineering and Public Policy, Carnegie Mellon University, 5000 Forbes Ave, Pittsburgh, PA 15217, USA.

E-mail address: [email protected] (J.F. Whitacre).

http://dx.doi.org/10.1016/j.electacta.2015.09.1260013-4686/ ã 2015 Elsevier Ltd. All rights

reserved.

composites electrodes have been produced that achieved reason- able performance in Na-containing aqueous electrolytes [21,22]. In one case, a redox reaction to simply precipitate MnO2 on a silica substrate and the latter through anodic electrodeposition. An analysis of the relative performance of these materials in terms of mass loading and electrode area was not presented. This is a crucial metric for CDI systems, since it is the overall dimensions and volume of the CDI flow cell environment that will determine the economics and applicability of the technology, and it is entirely possible that optimizing for mass loading of MnO2 will result in sub-optimal performance at the electrode level [23–25]. To this end, we explore here the possibility of using electrodeposited MnO2 coating on porous carbon substrate for improving ion- removal ability for CDI systems, with a focus on understanding the trade-offs associated with mass loading, surface area, and electrode-level function.

2. Experimental

We examined the performance of MnO2 coatings in simple static half-cell test environments on two contrasting porous substrates to probe the importance of structure and surface area as it relates to ion removal efficacy on both a per mass and per electrode area basis. We explored different deposition techniques, specifically galvanostatic and a cyclical sweeping voltage approach. We then characterized the resulting materials, and examined subsequent electrode performance in terms of ion

3 P.J. Walker et al. / Electrochimica Acta 182 (2015) 1008–

removal ability. The finished electrodes' capacities were determined in a static half-cell, and were physically characterized using scanning electron microscopy, energy dispersive spectroscopy, x- ray diffraction, and Brunauer-Emmett-Teller (BET) analysis. Care was taken to compare the performance of the electrodes in terms of specific capacity, electrode areal capacity, and BET area capacity, and we suggest that the best result for practical use may be different than that which maximizes MnO2 utilization, since the degree of cation uptake per unit electrode area would lead to the most efficient use of an electrode assembly in a true CDI system.

2.1. Description/characterization of carbon substrates

Substrates were chosen that had approximately two orders of magnitude difference in surface area and vastly different morphologies and structures. These included a high surface-area carbon nanofoam (Market Tech “grade 1”) and low surface-area carbon fiber paper produced by Toray. The listed specific surface area of the Market Tech material was 400 m2/g while the Toray,paper, which consisted of a nonwoven sheet of rv10 mm thick fibershad an unspecified specific surface area, but has been reported to be on the order of no more than 10 m2/g [26,27]. Under SEM observation the Toray carbon paper was seen to be a very open structure composed of overlapping thin carbonaceous fibers. Under SEM observation the carbon nanofoam was seen to be a relatively flat surface containing large numbers of micropores and a few macropores.

2.2. Materials Synthesis

MnO2 coatings were deposited on these different substrates using a three-electrode half-cell set up. Platinum foil with sufficient effective surface area (typically at least 3 times that of the working electrode) was used as the counter electrode and the reference electrode consisted of either the Hg/HgSO4 or Ag/Ag2SO4. The electrolyte used was a mixture of 0.1 M Na2SO4

and 0.1 M Mn (CH3COO)24H2O in deionized water. Stainless steel contacts were used to submerge the carbon substrates in the solution. Prior to deposition, care was taken to completely infiltrate the electrodes by exposing them to vacuum while in electrolyte. A Biologic SA Model VMP3 S/N Potentiostat was used to supply and control current. After deposition, the electrodes were then dried in an open-air oven overnight at approximately 85 o C. The geometric area of the coated electrode was assumed to be a rectangle and estimated using a ruler.

Either a galvanostatic, or a cyclic voltammetric deposition

2.3. X-ray diffraction

X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy-dispersive X-ray spectroscopy (EDAX) were used to characterize the MnO2 coatings present on the carbon substrates. The XRD characterization was carried out using an X'Pert Pro MPD X-ray diffractometer. The X-ray response was measured over the range of 10o -80o at a scan rate of 0.0003o /s with a detection 2theta angle of 3.77.

2.4. Scanning Electron Microscopy

The samples were also evaluated using Phillips XL30 SEM at either 15 kV or 20 kV after a 4 nm coating of platinum. Both flat and angled specimen stands were used for the SEM in order to allow both surface and interior images of the electrode.

2.5. BET

The Bennet-Emmett-Teller method was used to characterize the surface area of select samples. The measurements were done with liquid nitrogen at 77 K using a Micromeritics Gemini VII instrument. The samples done on nanofoam were outgassed for approximately twenty-four hours at 160 o C while the Toray paper was outgassed for approximately forty-eight hours at 160 o C.

2.6. Performance Assessment: Cyclic Voltammetry

The cyclic voltammetric profiles of the samples were evaluated using a Biologic SA Model VMP3 S/N Potentiostat. The MnO2/ carbon electrode was the working electrode, a platinum counter electrode, and either an Hg/HgSO4 or an Ag/Ag2SO4 reference electrode. The electrolyte was 0.1 M Na2SO4 in deionized water. The voltage window was 0.25 V to -0.3 V vs. Hg/HgSO4 and the scan rates were 0.25, 0.5, and 1.0 mV/s performed in that order. The electrodes were cycled six times for each scan rate and the final cycle of the 0.25 mV/s was used as the representative cycle. The mass was calculated simply by taking the difference in the uncoated electrode's mass and the coated electrodes mass using a Mettler Toledo AG245 scale. The specific capacity also known as the mass-normalized capacity, C (in mAh/g), was calculated using the potentiostat's software via the summation of the product of the instantaneous applied currents (Ii) and the duration of time steps(Dti) over which they were applied divided by deposited mass ofthe functional material (m). I i * D t i

conditions were used, as briefly discussed in [28]. The galvanostatic technique delivered a current density of 1.3 +/- 0.1 mA/cm2 (electrode area basis), and was voltage-limited to 1.2 V vs. the

C ¼ Si mð1Þ

reference electrode. The 7 hour 20 minute and 14 hour depositions on carbon paper terminated early since they reached the 1.2 V limit. Cyclic voltammetry depositions were performed over the range of -0.11 V to 0.19 V vs. the mercury/mercury sulfate electrodes and the scan rate was 0.25 mV/s. Table 1 contains details the full range of experiments performed.

3. Results

3.1. Electrodeposition

Characteristic deposition curves for the cyclic voltammetric (CV), and galvanostatic techniques are shown in Fig. 1. The CV

Table 1Table of MnO2 deposition conditions for galvanostatic, potentiostatic, and cyclic voltammetric techniques.

Deposition Technique Nanofoam Paper

Cyclic Voltammetry 3:20 7:20(CV/7/NF) 14:00 3:20 7:20 14:00+(-0.11 V to 0.19 V at (CV/3/NF) (CV/14/NF) (CV/3/P) (CV/7/P) (CV/14/P)0.25 mV/s) (cycles)

Galvanostatic (1 mA)(h:min) (Ref. ElectrodeAg/AgSO4) 3:20 7:20 14:00 3:20 7:20*(GS/7/P) 14*

P.J. Walker et al. / Electrochimica Acta 182 (2015) 1008–1018

1009(GS/3/NF) (GS/7/NF) (GS/14/NF) (GS/3/P) (GS/14/P)

*terminated earlier (3.9 h) when voltage limit was reached. + Deposited over -0.148 V to 0.152 V vs. Ag2/AgSO4.


Fig. 1. Deposition profiles of cyclic voltammetric, and galvanostatic techniques on carbon nanofoam: (a) The cyclic voltammetric profile over a voltage range of -0.11 to0.19 V vs. Hg/Hg2SO4 using a scan rate of 0.25 mV/s over five cycles or approximately 200 minutes. (b) The galvanostatic deposition at a current of 0.8 mA/cm2 with alimiting voltage of 1.2 VL (Ag/Ag2SO4) over 200 minutes.

deposition curves shown in Fig. 1(a) shows that the current responses fell progressively over subsequent cycles until reaching equilibrium by the fifth cycle. The extended “tails” on the left side of Fig. 1 a are due to the first cycle starting at the inherent potential of the electrode and then returning to that voltage upon the reverse part of the cycle. A characteristic deposition curve of the galvanostatic deposition curve is shown in Fig. 1(b). The voltage response shows an initially sharp increase before settling into a less dramatic long-term trend.

3.2. Materials Characterization: X-ray diffraction.

The phase of the coatings were determined using XRD. The data, plotted in Fig. 2, show mixed phases on both the cyclic voltammetrically coated nanofoam over three hours and twenty minutes and the galvanostatically-coated carbon paper over fourteen hours were observed. In the nanofoam sample, the dominate species were the ramsdelite MnO2 and manganite MnOOH phases (Fig. 2(a)). The broad X-ray diffraction peaks observed also suggest that the sample was nano-crystalline or amorphous in nature, as observed in similar types of coatings produced when MnO2 species are electrodeposited. The XRD results from the galvanostatically deposited MnO2 on carbon paper are shown in Fig. 2(b) and

Fig. 2. X-ray diffraction data collected from: (a) An electrode created using a cyclic voltammetric technique over about three hours and twenty minutes on carbon nanofoam. The peaks marked M indicate the phase manganite (MnOOH), the peaks marked R indicated ramsdelite (MnO2), and peaks with both M and R indicate a peak that contains both phases. Additionally, some of the width of the peak for M [210] may be due to the presence of the carbon substrate. (b) An electrode created using a galvanostatic technique over fourteen hours on carbon nanofoam. The peaks marked B are Birnessite, A for Akhtenskite, O for Manganese Oxide MnO1.937.

indicate birnessite, akhtenskite, and manganese oxide MnO1.9. This sample also had a lower signal to noise ratio than the cyclic voltammetric as seen in Fig. 2(a).

3.3. Scanning Electron Microscopy.

SEM images showing the morphologies of the deposited films are in Figs. 3–7 , with the uncoated carbon paper shown in Fig. 3 (with (a) being the uncoated sample) and the carbon nanofoam shown in Fig. 4 (with (a) being uncoated). CV (0.25 mV/s for 5 cycles (3 h 20 min))was used to produced the samples shown in Fig. 3(b), (c), (d), (e) and Fig. 4 (b), (c), (d), and (e). The high surface area nanofoam was coated in relatively even sheets; the deposition following the same sort of geometry as the uncoated nanofoam Fig. 4(b). The structural difference between the nanofoam and the fiber paper gave rise to the massive difference in BET surface area, as discussed below. The high contrast material seen at the joints of the low surface area paper was thought to be residual polymer binder from when the electrode was created (Fig. 3(a)).

Fig. 3c shows how the CV-deposited encrusted individual carbon strands of the paper, growing as rosettes that eventually

P.J. Walker et al. / Electrochimica Acta 182 (2015) 1008– 1

Fig. 3. SEM micrographs of uncoated and cyclic voltammetrically coated carbon paper electrodes: (a) the uncoated carbon paper substrate. (b), (c), and (d) are micrographs that of the cyclic voltammetrically coated carbon paper over approximately 200 minutes that illustrate the spheres of platelets that coat the individual strands of carbon. (e) Is a higher magnification micrographs of the same electrode that shows the surface of the platelets (note: images are at different magnifications).

form a uniform but rough coating. The surface of this coating exhibits a structure of platelets, as shown in Fig. 3(e). Fig. 3(d), however, shows that this coating does not extend deeply into the substrate, as evidenced by the lack of coating in the bottom left of the figure as well as on the fibers in the background. Both of these images show a similar structure to the ones in Fig. 4(d).

Both electrodes exhibited smaller rosette like structures, which are shown in detail in Fig. 3(c) and Fig. 4(d). The EDS data for Fig. 3(b) electrode revealed 34% Mn, O 55%, and C 11% by atomic percent for the coated low surface area paper. The Fig. 4(b) micrograph on the more heavily coated side (all white) gave 30% Mn, 57% O, and 13% C by atomic percent, while the lesser-coated side exhibited 3.59% Mn, 27% O, and 70% C. The combined SEM and EDS data suggested that MnO2 was built up on the carbon slowly, potentially changing chemical ratio as the deposition progresses, although the EDS data was inconclusive due to the large error. Figs. 3 and 4e both reveal the spikes of the platelet layers of the respective coatings.

In Fig. 4, the MnO2 coating formed a continuous rosette pattern over the surface of the carbon paper, the platelets of which altered the BET surface area (Fig. 9). The CV deposited material on the carbon paper showed a similar structure to that of the carbon nanofoam, a small compact sphere that built up into a larger

structure Fig. 3(b) and Fig. 4(b) and (d). Within the cyclic voltammetrically coated nanofoam images, the (c) image in Fig. 4 shows the type of structures found at the far right of image 4(b), and exhibits larger and more connected spikes than those in Fig. 4(d). Fig. 4(d) shows the type of structure found towards the center of 4(b), before it becomes fused together as in the left of Fig. 4(b).

Figs. 5(a) and Fig. 6(a) show the results of the galvanostatic deposition; a much heavier coating was apparent. In Fig. 5(a) the coating again tends to cover the individual strands of carbon, in fact to a degree that causes the films to split. The Fig. 5(b) shows that the morphology of the film was not nearly as consistent as that of the previous carbon paper sample in Fig. 3(e). The galvanostatically coated carbon nanofoam electrodes in Fig. 6(b) and (c) also exhibited a less regular structure than the previous CV deposition (Fig. 4(c) and (d)). These electrodes are also appeared more porous than the CV deposited electrodes.

This CV coating on carbon nanofoam was further characterized with SEM on an angled stand to allow both surface and internal micrographs as seen in Fig. 7. The electrode that was examined was put in liquid nitrogen and then cut with an equally cooled razor in order to produce a clean fracture. Fig. 7(a) shows that the top surface of the electrode was coated in rosette-like structures that


Fig. 4. SEM micrographs of uncoated and cyclically voltammetrically coated carbon nanofoam electrodes: (a) the uncoated carbon nanofoam substrate. (b) Shows a carbon nanofoam electrode coated with a cyclic voltammetric technique over approximately 200 minutes. Both a more discrete platelet structure as well as a more open spaced structure seen on the right side of the micrograph, (c) shows is a close up of the latter while (d) is a close up of the former. (e) is a higher magnification micrographs that shows the surface of the platelets.

formed a continuous network. Fig. 7(b), an image of the internal surface, however showed no such structures, only the microporous internal carbon of the substrate.

3.4. Energy Dispersive Spectroscopy

EDS mapping was performed on the freeze-fractured electrode at an angle that allowed examination of the interior, as shown in Fig. 8. Fig. 8(b) and (c) show that primarily carbon was detectedfrom the interior of the electrode a thin layer, less than 0.5 mm, ofMnO2 was observed near the top and bottom of the samples. Small amounts of sodium and sulfur were also detected, along with the external coating of platinum. Fig. 8c also shows that the external surface of the electrode received a heavy coating of MnO2 obscuring the underlying carbon.

3.5. Post-deposition BET

Several coated electrodes were characterized using BET analysis, as shown in Fig. 9. The results confirmed the high initial surface area of the carbon nanofoam relative to the carbon paper.

Additionally, the data showed an increase in surface area with MnO2 coatings, with the exception of the cyclic voltammetric coating over five cycles. The surface areas that our BET characterization returned for the uncoated carbon nanofoam and uncoated carbon paper were 220 m2/g and 1.5 m2/g, respectively. The BET returned 120 m2/g for the cyclic voltammetrically coated material, carbon nanofoam and 16 m2/g for the carbon paper coated using the same technique. The BET also returned 210 m2/g for the galvanostatically coated carbon nanofoam and 36 m2/g for the carbon paper coated using the same technique.

3.6. Electrode performance in a static half-cell environment

The electrodes were characterized through cyclic voltammetry by testing in a static half-cell environment. The uncoated carbon paper had a capacity of 0.0058 mAh/g and the uncoated carbon nanofoam a capacity of 0.39 mAh/g, or approximately 2.2 x 10-7

Moles and 1.5 x 10-5 Moles of monovalent cations respectively, asnormalized to the total mass of the carbon.


Fig. 5. SEM micrographs galvanostatically coated carbon paper electrodes over approximately fourteen hours: (a) illustrates the thick coating of MnO2 around the fibers and subsequent cracking. (b) Shows the irregular features found on this electrode. (c) Illustrates the more uniform surfaces of thick MnO2 coatings present on said electrodes. (Note: images are at different magnifications).

Fig. 10 shows the specific current vs. voltage profiles for both the best mass specific performer and the best area specific performer electrodes. Fig. 10(a) depicts a more rectangular profile characteristic of the CV coating over five cycles on carbon nanofoam, suggesting that the coating was acting like a capacitor. Fig. 10(b) however shows that the galvanostatically deposited coating over fourteen hours on carbon nanofoam has a significantly more sloped profile, indicative of a more resistive coating.

As depicted in Figs. 11 and 12, the electrochemical calculated capacity screening technique the electrode with the greatest mass-

Fig. 6. SEM micrographs galvanostatically coated carbon nanofoam electrodes over approximately fourteen hours: (a) illustrates the thick coating of MnO2

on the surfaces of the carbon nanofoam. (b) Shows the irregular features found on this electrode. (c) Illustrates caterpillar like structures found on this electrode. (Note: images are at different magnifications).

specific capacity was found to be cyclic voltammetrically coated nanofoam (Fig. 11), while the results were different if the performance was normalized to electrode area (Fig. 12). These materials were found to be stable over multiple samples over many hours of testing. However, when considering the capacity normalized to geometric surface area the best electrode was found to be the galvanostatically coated carbon nanofoam. Fig. 11


Fig. 7. SEM micrographs of the exterior and interior of an electrode created via a cyclic voltammetric technique: (a) shows the rosette-like MnO2 structures coating the external planes of the sample (b) shows the internal structure, microporous carbon rather than MnO2.

also compares the relationships between areal specific capacity, BET surface area specific capacity, and mass specific capacity.

4. Discussion

The data from the samples coated onto the Toray fiber paper revealed the importance of the coating morphology; although the carbon paper starts with approximately 1.5 m2/g the surface area can be increased through deposition, and the surface functionality of the deposited MnO2 greatly enhanced functionality. Further- more the surface area continued to increase the longer the deposition. The use of the nanofoam substrate resulted in a more complicated dependence on surface area; the initial, uncoated substrate had high surface area, which was cut in half by a three- hour cyclic voltammetry coating. In general, coatings deposited over longer durations showed higher surface area, again suggesting that surface area can be built up by the deposition of MnO2 that continued to roughen with thickness.

The XRD results for the cyclic voltammetrically produced compound reveal the expected MnO2 compound in the ramsdellite phase, indicating that we were successful in producing the compound in the correct oxidation state. The large presence of MnOOH was also to be expected considering the technique that was used to deposit the MnO2 was cyclic voltammetry and would likely resulted in previously deposited MnO2 being reduced by the continued cycling in the presence of water [28]. This in turn indicates that the material should also be able to store other cations [29–31].

The galvanostatically-coated carbon paper shown in Fig. 2(b) did not have high phase purity relative to the material deposited

Fig. 8. Energy dispersive spectroscopy mapping of a freeze-fractured electrode (a) shows the electron image of the electrode (b) shows the manganese map, highlight in red and green, while (c) is the carbon map, highlighted in blue.


Fig. 9. A comparison of the specific surface area of the uncoated substrates and those coated using either a cyclic voltammetric technique over five cycles (three hours, twenty minutes) electrodes or a galvanostatic technique over fourteen hours: Normalized to the mass of the total electrode, the nanofoam electrodes exhibit significantly higher specific surface area to the paper ones.

via cyclic voltammetry onto carbon nanofoam. The birnessite phase accounted in part for the three most prominent peaks with some contributions coming from Akhtenskite (and a small amount of MnO1.9), however the majority of the peaks shown are not of comparable intensity to those in Fig. 2(a), in particular the peaks at 37 and 66 degrees. We believe that this was due to the fact that the voltage of the surface of the electrode changed considerably over the course of the deposition as the film becomes thicker, which in turn leads to a variety of deposited materials that are not uniform through the thickness of the film.

The galvanostatically deposited material on the carbon nanofoam showed a less regular and rougher surface than the corresponding technique on carbon paper; see for example Fig. 6(a) in comparison to Fig. 5(c). The more magnified images showed evidence of two structures; an uneven, coral-like structure in Fig. 6(b) not unlike that found on the carbon paper, as well as a primarily linear structure with small rods emerging from its core as in Fig. 6(c). The differences seen here between the SEM micrographs, much like the XRD graphs, are thought to be caused by the differences of the voltage imparted to the growth surface during deposition; the cyclic voltammetry method generated a wide range of potentials throughout film growth while the galvanostatic method did not. Such differences have been noted previously by D.P. Dubal et. al.; they found that their potentiodynamic technique (similar to our cyclic voltammetry) had superior electrochemical qualities, and attributed this to the potentiodynamic technique being discontinuous (as opposed to the galvanostatic technique) which allowed for the creation of a more porous sample [32]. Additionally, SEM analysis of their galvanostatically deposited material revealed a similar worm-like structure to the ones we found, as shown in Fig. 6(c).

The data in Fig. 7 indicates both an absence of MnO2

structures on the internal surfaces of the Marketech electrode as seen on the surface while Fig. 8 shows a low Mn signal count from the internal surfaces of the electrode. This suggests that there was not sufficient Mn cation transport into the bulk of this electrode during the deposition. Despite multiple attempts, it seems that the technique employed did not make full use of the surface area of the initial substrate, and only very thin depositions would be able to do so, resulting in an overall less appealing performance. The BET analysis

on these films indicated that the surface area dropped off for the lighter coating and then increase for the heavier coatings, which also suggests that the initial coating atop the electrode may close off access to the internal surface area of the carbon substrate. However, longer deposition times can build up additional surface roughness due to the inherent surface area of the MnO2.

In general, the coatings greatly increased the capacitance of theelectrodes, with the carbon nanofoam substrate typically out- performing the Toray carbon paper in terms of both specific capacity and electrode specific areal capacity. The electrodes produced through a cyclic voltammetric technique over five cycles on carbon nanofoam had the highest specific capacity of 30mAh/g while the electrode produced via a galvanostatic coating over fourteen hours on carbon paper had the greatest areal capacity at0.32 mAh/cm2. The coating had a much higher capacity than non-coated substrate by a factor of rv75. Additionally, the former electrode, with 30 mAh/g capacity, has a capacitance of 210 F/g, a value comparable to the best performing carbon materials

Fig. 10. (a) Example of electrochemical performance testing on is CV electrodeposited MnO2, The sweep rates started with 0.25 mV/s for 6 cycles and then moved to the next sweep (0.5 mV/s) rate for an additional 6 cycles, and then again for the final sweep rate (1.0 mV/s). (b) Electrochemical performance data (same test conditions as disclosed in (a) for the galvanostatically deposited MnO2 on the carbon nanofoam (14 hour deposition) with same test conditions as described in (a). The asymmetry and slant of the profile is characteristic of the galvanostatic samples. The data shown here is from the final two cycles (the final complete cycle from -0.3 V to 0.25 V and back to -0.3 V).


Fig. 11. Electrochemical performance normalized to: (a) electrode areal capacity of the (b) BET surface area, and (c) active material mass the coating or.

(200-250 F/g), without needing extremely tortuous morphologies or costly processing steps [33,34]. Further work is required to determine both how to coat the interior of the electrode as well as how to make better use of the MnO2 coating itself. Furthermore, it needs to be determined whether the electrode with better mass specific capacity, cyclic voltammetric deposition over 3.33 hours on nanofoam, or the one with areal specific capacity, galvanostatic deposition over 14 hours on nanofoam, is better suited for use in CDI systems.

The difference in cation uptake performance between the cyclic voltammetrically coated and the galvanostatically coated nanofoam samples was stark and manifested in two ways: the CV- coated nanofoam material exhibited both higher specific capacity on a per mass basis as well as a more classical rectangular current vs. voltage response during testing. The galvanostatically deposited material's current vs. voltage response showed a sloped profile that was likely due to the higher electrical resistance of the thicker

MnO2 coating. The difference in specific capacity was significant and was approximately 20 mAh/g as seen in Fig. 11.

This CV deposition technique always resulted in coatings with a higher mass-specific capacity compared to those coatings deposited using the galvanostatic techniques. We believe that this was due to both the more uniform surface coating, as seen in the SEM micrographs, as well as a more functional phase, as seen in the XRD data.

An increase in coating time does not appear to have a significant

effect on the mass specific capacity of the cyclic voltammetrically coated carbon Toray paper. This suggests that, for the range of coating thicknesses investigated, a sufficiently porous structure was deposited, which then allowed for a similar amount of the MnO2 to contribute to electrochemical functionality with continued growth. This was in contrast to previous work which shows a decrease in capacitance (and hence capacity) when the duration of the deposition was extended, though it was presumed that if we


Fig. 12. Comparative ionic functionality in terms of mAh normalized to either loading mass or electrode area measured during the standard CV screening test: (a) is the loading specific capacity performance in terms of mAh/g of the electrodeposited active material while (b) is the electrode area specific performance of the full

electrode structures in terms of mAh/cm2. Black bars indicated uncoated electrodes, green bars indicated a CV coating technique, and the red bars indicate that a galvanostatic technique was used.

deposited for sufficient time, the coating would eventually become self-limiting [35].

Conversely, both the galvanostatic and the cyclic voltammetric technique showed a decrease in mass specific capacity with increased time of deposition when depositing on the Marketech nanofoam. It is thought that the reason for this decrease in the nanofoam's capacity was due to the very dense MnO2 coating quickly choking off the internal surface area of the electrode and subsequently did not provide a sufficiently open structure for the proliferation of new MnO2 surface sites with longer deposition times. The carbon paper, with its larger sized pores, did not suffer from this complication and so additional material can potentially increase the capacity without limiting access to previously deposited material.

Additionally, in general, the galvanostatic technique on the nanofoam structures also led to higher areal-specific capacity (where the area was the geometric area of the electrode) compared to the cyclic voltammetric technique, an effect likely due to greater loadings of MnO2 with an ever-roughening surface morphology.

4.1. Future Work

The results presented here are a first step to showing the complex relationships that exist between electrode composition, morphology, phase content, and deposition condition for this class of pseudocapacitive materials. As such, the findings thus far do not indicate what the optimal or high performance set of parameters might be. As such, a more focused multi-variant study is needed to identify the statistical relationships between the various parameters identified as important here. Examples of this type of study can be found in the literature [36,37], and work of this nature is currently under way in our laboratory and follow on publications are forthcoming.

4.2. Summary/Conclusions

MnO2 was electrochemically deposited using two different techniques on carbon nanofoam and carbon fiber paper. There were significant differences observed in phase, morphology, and electrochemical performance. The coating greatly increased the ionic functionality of the structures in all cases. Findings include:

(a) the MnO2 deposited using the CV technique had higher performance in terms of specific capacity (mAh/g) than films of comparable deposited onto the same substrate using the galvanostatic technique,

(b) typically, the galvanostatic deposited films had higher performance in terms of area-specific capacity (mAh/cm2), with the highest performance being documented for longer depositions where it was likely that the nanoporous carbon structure had been completely encrusted by the MnO2

deposition, and(c) the nanoporous carbonaceous substrates were apt to be filled

in during deposition such that the influence of their initial structure did not manifest during performance screening.

These results suggest that depositing sodium interactive materials such as the MnO2 species explored here onto three dimensional structures is a good approach to enhancing the CDI function, however both the choice of the carbon structure as well as the deposition characteristics can have a profound impact on electrochemical performance.

Acknowledgements

The Authors acknowledge the support of the US National Science Foundation under award number CBET1403826 and the


United States – Israel Binational Science Foundation under award number 2012142.

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