Influence of Edge- and Basal-Plane Sites on the Vanadium Redox

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Influence of Edge- and Basal-plane Sites on the Vanadium Redox Kinetics for Flow Batteries Nir Pour, David Gator Kwabi, Thomas J. Carney, Robert M Darling, Mike L. Perry, and Yang Shao-Horn J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp5116806 • Publication Date (Web): 16 Feb 2015 Downloaded from http://pubs.acs.org on February 18, 2015

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The Journal of Physical Chemistry

Influence of Edge- and Basal-Plane Sites on the Vanadium Redox Kinetics for Flow Batteries Nir Pour1,*, David G. Kwabi1, Thomas Carney2, Robert M. Darling3, Michael L. Perry3, Yang Shao-Horn1,2,* 1

Department of Mechanical Engineering,

2

Department of Materials Science & Engineering, Massachusetts Institute of Technology,

Cambridge, MA 02139, USA 3

United Technologies Research Center, East Hartford, CT 06118

Abstract:

The reaction kinetics of VII/VIII and VIVO2+/VVO2+ redox on carbon electrodes in sulfuric acid limit the development vanadium redox flow batteries (VRFB) with high power and efficiency characteristics. Cyclic voltammetry and symmetric flow cell measurements on selectively masked graphite-foil and highly oriented pyrolytic graphite electrodes revealed that edge carbon sites provide faster kinetics for VII/VIII and VIVO2+/VVO2+ redox than basal carbon, especially at low vanadium concentrations. The understanding was used to explain the marked enhanced kinetics of carbon paper electrodes with heat-treatments in air relative to without, which was supported by X-ray photoelectron spectroscopy measurements that showed much higher amounts

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of surface functional groups on the heat-treated carbon upon exposure to the VV species in the electrolyte. Of particular significance to note is that markedly enhanced kinetics for the VII/VIII redox for the heat-treated carbon were found at both low and high Vanadium concentrations while similar enhancement was for the VIVO2+/VVO2+ redox for low Vanadium concentrations but much smaller increased kinetics were noted for high Vanadium concentrations required for practical flow batteries. This result was further confirmed by symmetric flow cell measurements that show much higher currents for the VII/VIII electrolyte using heat-treated carbon in comparison to the as-received while comparable currents were found for

VIVO2+/VVO2+

electrolyte, indicating that the redox kinetics of VII/VIII can be limiting for VRFBs using asreceived carbon (low edge carbon and oxygen functional groups). These findings provide new insights and strategies for carbon electrode designs for high-power VRFBs.

KEYWORDS: Vanadium, Flow-Batteries, Edge-plane carbon, Basal-plane carbon, electrode kinetics, electrochemistry and energy storage.

Introduction Redox Flow Battery (RFB) is promising for large-scale storage of electrical energy,1 which stores energy using anolyte and catholyte separated by a membrane. The membrane is typically Nafion-based, used in several fuel cell applications.2–5 The Vanadium Redox Flow Battery (VRFB)6–14

is among the most developed flow-battery technologies,15,16 which have been

demonstrated up to MWs in power and MWHs in energy-storage capacity.17–24 The VRFB is comprised of the VIVO2+/VVO2+ redox couple for the positive electrode and the VII/VIII redox


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couple for the negative electrode, where the reactants are typically dissolved in sulfuric acid. The reactions at the positive and negative electrodes can be described as:

Positive half-cell: Charge

VIVO2+ + H2O

VVO2+ + 2H+ + e- E0 = 1 V vs. RHE

[1]

Discharge

Negative half-cell: Charge III

-

VII E0 = -0.26 V vs. RHE

V +e

[2]

Discharge

The concentration ratio of the catholyte (VIVO2+/VVO2+) and the anolyte (VII/VIII) governs the energy storage density (see Supporting Information). In a fully charged cell, the catholyte consists solely of VV while the anolyte consists of only VII. The most common VRFB electrodes are based on carbon such as graphite felts, carbon-fiber papers, glassy carbon25 and carbon nanotubes,26,27 which are stable in strong acids (typically 2.0 M H2SO4) and a wide electrochemical-stability window16. The kinetics of VII/VIII and VIVO2+/VVO2+ redox reactions on carbon are not well understood, which limits the development of electrodes with faster reaction kinetics and VRFBs with high power and efficient characteristics. Previous studies have shown that surface oxygen functional groups (e.g. C-O) on carbon surface can greatly promote the kinetics of the redox reactions of VII/VIII and VIVO2+/VVO2+ 11–14,28–31,26,27 and increase Coulombic, voltage and energy efficiencies of the full flow battery cell.28,32,33 The enhanced kinetics of VII/VIII and VIVO2+/VVO2+ redox reactions has been attributed to binding of vanadium ions to a surface oxygen group on the electrode (proposed to be found on edge sites12), followed by an electron transfer and the


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desorption of the vanadium species12,32,33. Moreover, Gattrell et al.34,35 have suggested that the bound vanadium species can act as an active site, which can also transfer electrons from the electrode to another vanadium species either in the electrolyte or bound to the electrode surface.34 In this study, we examine the role of edge and basal carbon on the redox kinetics of VII/VIII and VIVO2+/VVO2+ through cyclic voltammetry and pulse voltammetry techniques, on graphite electrodes. These electrodes serve as a model system for measuring the difference in redox kinetics between edge and basal sites. Edge sites and basal sites are known to exhibit different responses for faradaic and non-faradaic processes36,37. For example, carbon edge sites can have markedly higher specific double layer capacitance (~60 µF/cm2) relative to basal carbon (~2 µF/cm2) and an increase rate for the redox reaction of Fe(CN)6(3-/4-) having rate constant (k0) many orders of magnitude higher for the edge sites (0.06 cm/sec) than that for basal sites (< 1x10-7 cm/sec).37 More recently, the edge of single graphene layer has been reported to have 4 orders of magnitude in specific capacitance than basal carbon.38 Here, using unmasked and masked graphite-foil and highly oriented pyrolytic graphite (HOPG) electrodes as a model system to study the kinetic activity of edge and basal sites, we show that increasing edge carbon greatly enhances the kinetics of VII/VIII and VIVO2+/VVO2+ Such findings from these model electrodes can be used to explain the much enhanced kinetics of VII/VIII and VIVO2+/VVO2+ found on carbon paper heat-treated in air relative to as-received carbon paper. With XPS results of pristine, soaked and potentiostatically polarized electrodes, we propose that increasing edge carbon and oxygen functional groups largely associated with edge carbon can greatly enhance the kinetics and can be responsible for the greater kinetics of heat-treated carbon.


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Experimental section: Electrodes – Edge-plane and basal-exposed electrodes were prepared by sealing the unwanted plane of the graphite-foil (Alfa Aesar 99.9% 0.13 mm thick, B.E.T surface area ~15 m2/g, 0.0137 g/cm2geo). The surface areas of graphite-foil electrodes were obtained by single-point Brunauer, Emmet, Teller (BET) analysis using a QuantaChrome ChemBET Pulsar. Graphite-foil electrodes were cleaned with ethanol and D.I water before use. In addition, edge-plane and basal-exposed electrodes were prepared from highly oriented pyrolytic graphite (HOPG –Alfa Aesar) electrode with paraffin wax (Figure S1). Moreover, carbon paper electrodes (0.189 mm thick, as-received B.E.T surface area ~2 m2/g, 0.006 g/cm2geo, heat-treated B.E.T surface area ~90 m2/g, 0.006 g/cm2geo) were obtained commercially and used typically for United Technologies Research Center (UTRC) high power density VRB cells. The carbon paper heat treatment was performed at 400 oC for 30 h under Air (the active surface area of the electrode was 23 cm2). Electrolyte synthesis - VIV solutions were prepared by dissolving VOSO4 nH2O (Alfa Aesar, 99.9%) in 2 M H2SO4 (Sigma, 99.999%).

VII, VIII and VV solutions, were obtained by

electrolysis of VOSO4-H2SO4 solutions in a two-compartment cell separated by a cationexchange membrane (Nafion® 117), where graphite foil (Alfa Aesar, 0.13mm, 99.8%) electrodes were used for both anode and cathode. The negative side of the two-compartment cell was kept under Ar atmosphere during the synthesis to prevent reaction with atmospheric oxygen. Electrochemistry - Electrochemical measurements were performed in a three-electrode cell using graphite foil (GF) as counter electrode and saturated-calomel electrode (SCE) as reference electrode (Pine Instrument). The potential was controlled using a Biologic SP- 300 potentiostat. The cyclic voltammetry for the VII electrolyte was performed under Ar atmosphere to prevent reaction with ambient oxygen. All the CVs started at the specific OCV of the electrolyte (~-0.6 V


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for VII, -0.4 V for VIII, 0.6 V for VIV and ~0.9 V for VV all vs. SCE). The experiments in the VII and VIII electrolyte was scanned first to the low voltage (-1.1 V vs. SCE) and back up to 1.45 V vs. SCE), and vice versa for VIV and VV. All CVs presented in this manuscript are steady-state CV, which were obtained after one full CV scan cycle with voltages described above. Graphite foil was used as a counter electrode for the CVs, and the size was roughly 20 times larger than the working electrode (~2 cm2). The electrical double-layer currents were measured for unmasked graphite-foil, edge-exposed graphite-foil, basal-exposed graphite foil, heat-treated carbon paper and as-received carbon paper electrodes (Figure S2) between -1 V and 1.5 V vs. SCE in 2 M H2SO4. Taking positive currents between 0.56 V to 0.87 V vs SCE, the double-layer capacitance was found to be 91, 460, 103, 378, 16 mF/cm2geo., respectively. The heat-treated carbon had an electrochemical capacitance ~25 times greater than the as-received carbon while the unmasked graphite-foil electrodes have ~5 times greater than the as-received carbon. Normal pulse voltammetry was performed by scanning from OCV to the end of the linear region, found by the CVs (0.65 V vs SCE for reduction and 0.95 V vs SCE for oxidation) with a pulse height of 0.5 mV, pulse width of 150 ms and step time of 1000 ms. Symmetric flow-battery testing - As the transference of different vanadium ions through the membrane of full-cell VRFBs can change the state of charge, which complicates redox kinetics studies, UTRC39 has used symmetric cells that circulate a single electrolyte (either the catholyte or the anolyte) to study the influence of electrode-electrolyte combinations on redox kinetics under steady-state conditions, which eliminates reactant crossover that occurs when different electrolytes are placed on either side of the cell (Figure S3). Stock solution of different oxidation states was prepared electrochemically from a starting solution of 1.5 M VOSO4 in 2.6 M H2SO4.


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The cells had identical carbon paper electrodes with active surface area of 23 cm2 and were separated by Nafion® 212 membrane. The reservoir contained 90 mL of electrolyte and was purge under nitrogen. A peristaltic pump circulated the electrolyte at 120 mL/min.39

XPS – Both heat-treated and as-received electrodes were analyzed after undergoing different treatments: pristine electrodes, electrodes soaked in 0.5 M V(V) / 2 M H2SO4, and electrodes that were polarized in 0.5 M V(V) / 2 M H2SO4 at 1.1 V vs. SCE. All electrodes were dried in a vacuum oven at 70 °C for 8 hours prior to XPS measurement. For identifying the surface chemistry of the carbon electrodes, a Physical Electronics Versaprobe II X-ray Photoelectron Spectrometer was used. Survey spectra were used to quantify atomic concentrations of C, O and S in each sample. A small percentage (< 2%) of V was detected in samples that had been soaked/polarized in the electrolyte. Curve fitting of the photoemission spectra was performed following a Shirley-type background subtraction. The C 1s peak from sp2 hybridized carbons centered at 284.5 eV was used as the reference. All other peaks were fitted with a Gaussian– Lorentzian function. The relative sensitivity factors used to scale the peak areas of the C 1s and O 1s, S 2p and V 2p regions were 58.8, 137.4, 153.0 and 298.7 respectively.

Results and Discussion Redox kinetics of VII/VIII and VIVO2+/VVO2+ on model graphite-foil electrodes: In order to examine the role of edge and basal sites of carbon on the kinetics of VII/VIII and VIVO2+/VVO2+ redox reactions, CV measurements were performed on unmasked graphite-foil electrodes, which consisted of stacked graphene sheets (Figure S4), having carbon in the basal plane as the majority and carbon on the edge sites as the minority. Steady-state CV features for


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VII/VIII and VIVO2+/VVO2+ redox observed in the VII, VIII, VIV and VV electrolytes are shown in Figure 1. The oxidation and reduction peaks with small peak separation centered at -0.5 V vs. SCE can be attributable to the VII/VIII redox,8 which was similar regardless the oxidation state of vanadium in the electrolyte. The currents between ~0.0 and ~0.4 V vs. SCE were changed from positive in the VII electrolyte to negative in the VV electrolyte while VIV and VIII had a negligible currents, indicating the lack of oxidizable (VII) or reducible (VV) species in this range. Moreover, two oxidation waves centered at ~0.9 and ~1.1 V vs. SCE were found for the VII, VIII and VIV electrolytes while one oxidation wave centered at ~0.9 V vs. SCE was observed for the VV electrolyte, which is accompanied with one reduction wave at ~0.75 V vs. SCE.

It is

hypothesized that the oxidation and reduction peaks centered at ~0.9 V vs. SCE can be attributed to the faster redox kinetics of VIVO2+/VVO2+ on the edge sites of carbon, and the oxidation peak occurred at the higher voltage of 1.1 V. vs. SCE (with accompanying reduction at ~0.9 V vs. SCE) can be attributed to the slower oxidation kinetics of VIVO2+ to VVO2+ on the basal sites of carbon.12,40


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Figure 1. Steady-state CVs of graphite-foil electrode (0.1 cm2 and 0.014 g/cm2geo.) measured in a 0.5 M VII (purple), VIII (turquoise), VIV (blue) and VV (yellow) electrolytes all with 2 M H2SO4 at a scan rate of 20 mV/s. Graphite foil and SCE were used as the counter and reference electrode, respectively.

CV measurements from only top-exposed graphite-foil surface (more basal) or side-exposed surfaces (more edge sites) exposed to the electrolyte (Figure S1) were compared with unmasked graphite-foil electrodes. Edge-exposed-electrodes (blue) had more symmetric oxidation and reduction peaks, with small voltage separation between oxidation and reduction peaks, while


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basal-exposed electrodes (red) exhibited greater peak separation and more asymmetric oxidation and reduction currents for both VII/VIII and VIVO2+/VVO2+ redox in comparison to unmasked graphite-foil electrodes, as shown in Figure 2a. The onset currents of oxidation peaks were shifted to lower voltages while those of reduction peaks were shifted to high voltages for the edge-exposed electrodes in comparison to unmasked and basal-exposed electrodes. The unmasked electrodes were found to have combined responses from top-exposed and edgeexposed electrodes. These observations can be explained by the following hypothesis that the edge sites are attributable to faster kinetics of VII/VIII and VIVO2+/VVO2+ redox than basal sites. This hypothesis is supported by CV measurements of VIVO2+/VVO2+ redox on basal- and edgeexposed HOPG electrodes (Figure S5), which shows much greater kinetics of VIVO2+/VVO2+ redox on HOPG edges. Further evidence came from CV measurements with decreasing vanadium concentration in the electrolyte, as shown in Figure 3. With decreasing vanadium ions in the electrolyte, the redox peaks assigned to the edge sites became more pronounced, and the oxidation or reduction peak attributable to the basal sites reduced significantly, revealing much greater kinetics on the edge sites at low vanadium concentrations. Lastly, while the onset currents of redox peaks in the CV are governed by the electrode kinetics, all redox peak intensities scaled linearly with respect to the square root of the scan rate and with the concentration of vanadium in the electrolyte (Figure S6 and S7), which suggests that the diffusion of vanadium ions in the electrolyte governs the CV peak intensities.


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Figure 2. Steady-state CVs measured in 0.5 M VIV in 2 M H2SO4 electrolyte. (a) graphite-foil (GF) electrodes: unmasked (dashed-black line), edge- (blue line) and basal-exposed (red line) GF electrodes with schematics and scanning He-Ion microscope images in the insets, and (b) uncovered GF (dashed-black 0.1 cm2 and 0.014 g/cm2geo.), as-received carbon-paper (blue line) and heat-treated carbon-paper (red line) electrodes (0.1 cm2 and 0.006 g/cm2geo). All CV were collected from a 3-electrode cell setup, with a scan rate of 20 mV/s, with graphite foil and SCE used as the counter and reference electrode, respectively.


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Figure 3. Steady-state CVs measured with unmasked graphite-foil electrode (0.1 cm2 and 0.014 g/cm2geo.) in 0.5 M, 0.2 M, 0.1 M and 0.05 M (a) VII, (b) VIII, (c) VIV and (d) VV in 2 M H2SO4 electrolytes with a scan rate of 20 mV/s. These measurements were collected from a threeelectrode cell setup, where graphite-foil and SCE were used as the counter and reference electrode, respectively.

Redox kinetics of VII/VIII and VIVO2+/VVO2+on as-received and heat-treated carbon-paper electrodes: Steady-state CV measurements of the as-received carbon paper electrode in VII, VIII, VIV and VV electrolytes showed universally large separation for reduction and oxidation peaks for the


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VII/VIII and VIVO2+/VVO2+ redox (Figure 4), which exhibit much poorer kinetics than graphitefoil electrodes discussed earlier. Similarly poor kinetics were found for as-received carbon with mixed electrolytes such as 50 at.% of VIVO2+ and VVO2+ (Figure S8). In contrast, the heattreated carbon had faster kinetics than the as-received carbon and graphite-foil electrodes (Figure 4 and Figure 2b), as evidenced by more symmetric redox currents and smaller voltage separation for the VII/VIII and VIVO2+/VVO2+ redox found universally in VII, VIII, VIV and VV electrolytes. Unlike the graphite-foil electrodes, CV data of the VII/VIII and VIVO2+/VVO2+ redox reactions did not change significantly with decreasing vanadium ion concentration for both as-received and heat-treated carbon (Figure S9). Only at very high vanadium ion concentrations in the electrolyte, CV data of as-received and heat-treated carbon for the VIVO2+/VVO2+ redox became very similar (Figures S9g and S9h). On the other hand, even at very high vanadium ion concentrations (e.g. 2 M), the heat-treated carbon still exhibited higher redox peak currents and small peak separation for the VII/VIII redox (Figures S9a and S9b) than as-received carbon. These observations suggest that at high vanadium ion concentrations, the reaction kinetics of VII/VIII are more sensitive to carbon surface area and chemistry than those of VIVO2+/VVO2+. Lastly, similar to the graphite-foil electrodes, all redox peak intensities obtained from both as-received and heat-treated carbon scaled linearly with the square root of the scan rate (Figure S10), and with the concentration of vanadium in the electrolyte (Figure S11) suggesting that peak currents were governed by diffusion of the vanadium ions to the electrode surface.


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Figure 4. Steady-state CVs obtained with as-received and heat-treated carbon-paper electrodes (0.1 cm2 and 0.006 g/cm2geo) in different 0.5 M vanadium-ion solutions including VII (purple), VIII (turquoise), VIV (blue) and VV (yellow) in 2M H2SO4 with a scan rate 20 mV/s. Measurements were conducted in a three-electrode cell setup, where graphite foil and SCE were used as the counter and reference electrode, respectively. Dashed lines represent zero current for each electrolyte solution and optical photographs of the corresponding solutions are shown in the inset.

Analysis of onset currents away from the voltammetry current peaks for the VII/VIII and VIVO2+/VVO2+ redox further supports faster kinetics for the heat-treated than as-received carbon,


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especially at low vanadium concentrations. The heat-treated carbon had much faster kinetics than as-received carbon for oxidation from VII to VIII, with a different Tafel slope of ~100 mV/dec (vs. ~150 mV/dec) and ~40 times greater geometric-surface-area-normalized current than asreceived carbon for the 0.05 and 0.5 M electrolytes (Figure S12a ). Similarly, both heat-treated and as-received electrodes were found to have similar Tafel slopes of ~150 mV/dec for the reduction from VVO2+ to VIVO2+ (Figure S12b). While geometric-surface-area-normalized currents were found to be ~30 times greater for heat-treated carbon than as-received at low vanadium concentrations (e.g. 0.05 M), increasing vanadium concentration resulted in smaller differences (~2 times greater at 0.5 M). Similar observation were obtained from the double-layer currents measured from CV in 2M H2SO4 (experimental section) were the heat-treated electrode displayed ~24 times greater geometric-surface-area-normalized current than as-received carbon. These observations are further supported by pulse voltammetry measurements of VIVO2+/VVO2+ redox at low vanadium concentrations (Fig. 5), where the geometric-surface-area-normalized currents were ~40 times greater for the heat-treated carbon and the Tafel slopes were much higher (~150 mV/dec for oxidation and ~120 mV/dec for reduction) compare to the AR (~100 mV/dec for oxidation and ~150 mV/dec for reduction). These Tafel slops suggest that there is a low apparent symmetry factor for the vanadium redox, which was reported previously for different carbon electrodes.34 However, the differences between the reduction and the oxidation Tafel slops are not well understood, which requires further studies.


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Figure 5. Normal pulse voltammetry data obtained with as-received (red) and heat-treated (blue) carbon-paper electrodes (0.1 cm2 and 0.006 g/cm2geo.) in 0.05 M VIV/VV electrolyte in 2 M H2SO4. Measurements were conducted in a three-electrode cell setup, where graphite foil and SCE were used as the counter and reference electrode, respectively.

It is proposed that the enhanced kinetics of VII/VIII and VIVO2+/VVO2+ redox of the heat-treated carbon in comparison to as-received carbon, most evident at low vanadium concentrations, can be attributed to a greater density of surface defects such as oxygenated edge sites introduced by the heat-treatment in air in comparison to as-received carbon, which is in accordance with previous studies.26–28,30,31 This hypothesis is supported by further experimental observations. The kinetics of VII/VIII redox on the as-received carbon can be greatly enhanced induced by exposure to the redox of VIVO2+/VVO2+, as shown in Figure 6. The as-received carbon exhibited poor kinetics for VII/VIII redox in the first cycle (Figure 6a), having small reduction and negligible oxidation currents. The oxidation peak of VIVO2+ to VVO2+ appeared at ~1 V vs. SCE while the oxidation of the as-received carbon began at ~1.2 V vs. SCE, with some current instability, where the evolution of molecular oxygen was not observed. Interestingly, the VII/VIII redox


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kinetics on the as-received carbon were markedly increased in the second cycle (greater reduction and oxidation currents than those in the first cycle), where the enhanced redox kinetics were only observed after scanning to high positive potentials (Figure S13). In contrast, no significant increase in the VII/VIII redox kinetics was found for heat-treated carbon (Figure 6b) in the second cycle relative to the first. It should be noted, however, that much faster kinetics of the VII/VIII and VIVO2+/VVO2+ redox were found for heat-treated carbon (Figure 6b) relative to asreceived carbon in the second cycle (Figure 6a) and in the steady state (Figure 4). We also note that this study does not distinguish between the relative effects of the hybridization of the edge site vs the presence of the oxygenated functional groups on vanadium redox kinetics.

Figure 6. Cyclic voltammograms of (a) as-received and (b) heat-treated carbon paper (0.1 cm2 and 0.0063 g/cm2geo.) in a 0.5 M VIII with 2 M H2SO4 electrolyte at a scan rate of 20 mV/s. The CV started at OCV (~-0.4 V vs. SCE), scanning down to -1.1 V (vs. SCE) and back up to 1.45 V (vs. SCE) The first and second cycles were collected from a three-electrode cell, having graphite


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foil and SCE as the counter and reference electrode, respectively. Scanning He-Ion microscope images of the corresponding electrodes are shown in the insets.

XPS analysis of as-received and heat-treated carbon electrodes: To examine surface defects and oxygen functional groups on as-received and heat-treated carbon before and after chemical or electrochemical treatments, XPS surface analysis was performed. XPS wide scan surveys (Figure S14) of all the electrodes showed that O 1s peaks increased relative to C 1s peaks upon heat-treatment, soaking and polarization at 1.1 V, indicating surface oxygen incorporation. The resulting O/C ratio was considerably higher in the heat-treated than as-received carbon (0.15 vs 0.03), which suggests that heat-treatment in air created surface oxygen-containing functional groups on carbon. This result is consistent with high-resolution C 1s XPS spectra (Figure S15), which showed that the higher O/C ratio of the heat-treated carbon is accompanied by greater intensities at 286 ± 0.1, 286.9 ± 0.2 and 288.9 ± 0.2 eV, which can be assigned to C-O, C=O and COOH surface functional groups, respectively41,42,43. The observation of more oxygenated functional groups on heat-treated carbon is in agreement with CV measurements in 2 M H2SO4, which clearly show the presence of pronounced redox peaks characteristic to quinone/hydroquinone redox44,45 for the heat-treated carbon but not for the as-received carbon (Figure S2). It should be mentioned that although considerably higher oxygen functional groups were found for the heat-treated carbon than asreceived carbon, there was no noticeable difference in the Raman signal of these two samples (Figure S16). Soaking the electrodes in the 0.5 M VV and 2M H2SO4 electrolyte and potentiostatically holding them at 1.1 V vs. SCE increased surface O/C ratios for both as-received and heat-treated


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carbon, with greater increases upon the potentiostatic measurement and for the heat-treated carbon. As sulfur was detected in the soaked and potentiostatically held samples, surface adsorbed SO4-2 species on carbon also contributes to surface oxygen detected. Moreover, small percentages of vanadium were detected on carbon using the XPS measurements (Figure S15), largely in the VV valence state (presumably V2O5-like). Both higher concentrations of surface sulfur and vanadium were found for heat-treated carbon than as-received carbon after soaking and potentiostatic holding. Therefore, atomic ratios of surface oxygen to carbon (Table 1) in the soaked and potentiostatically held samples were calculated by subtracting the contribution of O from SO42- ions in the electrolyte and V2O5 from the total oxygen detected. This analysis revealed that soaking and potentiostatic holding at 1.1 V vs. SCE resulted in considerable surface oxygen functional groups on carbon for both as-received and heat-treated carbon, with the highest ratio found for potentiostatically held heat-treated carbon. This result is in good agreement with the (non-sp2)/(total carbon) ratio, which was found to correlate with the O/C ratio (Table 1). The observation is also in agreement with the notion that the heat treatment in air increases the concentration of edge sites that can be oxidized to form C-O, C=O and COOH functional groups upon exposure to the VV electrolyte.29,36 An estimation of the ratio of surface O ions to VV ions in the electrolyte shows an increase from 0.03 in the as-received carbon to 6.5 in the heat-treated carbon, confirming that a sufficient number of active sites exist for catalytic activity by these functional groups (details in the Supporting Information). Moreover, XPS analysis of graphite-foil electrodes that are unmasked or edge-exposed showed noticeable increase in the surface oxygen on carbon after soaking or potentiostatic holding at 1.1 V vs. SCE (Table S1 and Figure S17). These findings suggest that exposure to the VV electrolyte can lead to oxidation of carbon and increase of graphitic edge sites, and that this process is enhanced by


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electrochemical oxidation of the electrode. This deduction is in agreement with the fact that the Fermi level of carbon materials is lower than the energy level of VIVO2+/VVO2+redox (Figure S18), which would lead to carbon oxidation when carbon is in contact with the VV solution.

Table 1 Atomic fractions of C, O, S and V, O/C and non-sp2/total carbon from XPS spectra of pristine, as-received (AR) and heat-treated (HT) carbon paper electrodes, and after soaking in 0.5 M V(V)/2M H2SO4 electrolyte for 10 min, or potentiostatic hold at 1.1 V (vs. SCE) for 10 min in 0.5 M V(V)/2M H2SO4. Fractions of sp2, sp3, C=O, C-O and COOH species were obtained from component fits of the C 1s region and surface O/C ratios were calculated by discounting contributions from SO4 ions and V2O5 species. Components

Atomic ratios AR

HT

HTsoaked

AR 1.1V

HT 1.1V

67.0

25.1

51.2

23.4

sp2

90.8

sp3

3.3

3.9

2.5

4.5

5.2

8.1

C=O

0.8

1.1

0.6

1.5

1.4

2.7

COOH

0.6

2.2

1.0

2.3

1.5

1.4

1.3

3.3

2.4

0.8

2.8

1.8

96.8

87.0

73.5

34.1

62.2

37.4

O 1s (%)

3.2

13.0

22.0

53.0

31.6

53.2

S 2p (%)

--

--

3.7

10.6

5.7

7.3

V 2p(%)

--

--

0.73

2.3

0.56

2.1

C (%)

1s C-O

Total

76.4

ARsoaked


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O –S*4 -2.5*V/C

0.03

0.15

0.07

0.13

0.12

0.50

(non-sp2)/C

0.06

0.12

0.09

0.27

0.18

0.37

Symmetric flow battery kinetics of VII/VIII and VIVO2+/VVO2+ on carbon-paper electrodes: The enhanced kinetics of VII/VIII and VIVO2+/VVO2+ on heat-treated carbon were confirmed in symmetric flow battery testing (see experimental). Figure 6 compares the polarization curves of as-received and heat-treated carbon in the positive symmetric cell (90% VVO2+/10% VIVO2+) and in the negative symmetric cell (90% VII/10%VIII). All the current-voltage profiles were found to pass through the origin, and exhibit one slope for each electrode-electrolyte combination, which indicate that the current was independent of positive or negative bias as expected for the symmetric cell. In the positive symmetric cell (Figure 7a), as-received and heat-treated carbon were found to have comparably large currents, which was not changed by polarizing in the negative symmetric cell with 90% VII/10%VIII (diamonds). In contrast, in the negative symmetric cell, as-received carbon showed much lower currents than heat-treated carbon (circles in Figure 7b) and those found in the positive symmetric cell, and polarizing in the positive symmetric cell with 90% VII/10%VIII (diamonds) led to a large current increase for as-received carbon but not for heat-treated carbon (diamonds in Figure 7b). Therefore, these measurements indicate that the VIVO2+/VVO2+ redox kinetics are more facile and less dependent on the carbon surface chemistry than those of VII/VIII, which kinetics can be enhanced greatly by surface oxygen functional groups on carbon using the heat-treatment in air and polarizing in the positive symmetric cell with 90% VVO2+/10% VIVO2+.


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Figure 7. Potentiostatic measurements of as-received (red) and heat-treated (blue) carbon-paper electrodes with the cell active area of 23 cm2 (0.0063 g/cm2geo.). (a) 1.5 M 90% VVO2+/10% VIVO2+ in 2.6 M H2SO4, measured at the beginning-of-life39 (circles) and after-potentiostatic measurements in the 1.5 M 90% VII/10%VIII in 2.6 M H2SO4 electrolyte (diamonds), and b) 1.5 M 90% VII/10%VIII in 2.6 M H2SO4, measured at the beginning-of-life (circles) and afterpotentiostatic measurements in the 1.5 M 90% VVO2+/10% VIVO2+ in 2.6 M H2SO4 solution (diamonds). (For each applied voltage, 30 measurements were conducted). Conclusions We examine the kinetics of VII/VIII and VIVO2+/VVO2+ redox reactions on carbon electrodes, which currently limits the development VRFBs with high power and efficiency. Using basalexposed and edge-exposed graphite-foil electrodes, we show that the kinetics of VII/VIII and VIVO2+/VVO2+ redox reactions can be greatly enhanced by having more edge sites, especially at low vanadium concentrations. This observation is further supported by similar observations made on basal-exposed and edge-exposed HOPG electrodes. Furthermore, very recent work has reported that the improved activity of the vanadium redox lies in the presence of oxygen defects


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at the edge sites of edge functionalized graphene nanoplatelet.12 This understanding is applied to postulate the physical origin responsible for the enhanced kinetics of VII/VIII and VIVO2+/VVO2+ redox reactions found on heat-treated carbon paper in air relative to as-received carbon paper. While the enhancement for the redox kinetics of VII/VIII is comparable for large and small vanadium concentrations, significant enhancement is noted for VIVO2+/VVO2+ at low vanadium concentrations but not at high concentrations typically used for practical VRFBs. X-ray photoelectron spectroscopy measurements support that higher amounts of edge sites and surface oxygen functional groups on the heat-treated carbon relative to as-received carbon and upon exposure to the VV species in the electrolyte are responsible for enhancement of kinetics of VII/VIII and VIVO2+/VVO2+ redox reactions. The influence of carbon surface chemistry on the VII/VIII and VIVO2+/VVO2+ redox reactions is further confirmed by symmetric flow cell measurements that show much higher currents for the VII/VIII electrolyte using heat-treated carbon in comparison to the as-received while comparable currents were found for the VIV/VV electrolyte, which reveals that the redox kinetics of VII/VIII can limit the rate capability of VRFBs when carbon electrodes with low edge sites and oxygen functional groups were used.

ASSOCIATED CONTENT ACKNOWLEDGMENT This work was supported by United Technologies Research Center (UTRC), East Hartford, CT through the MITEI SEED project.


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Supporting Information. Additional cyclic voltammetry, Tafel plot, He-Ion microscope figures, XPS analysis and Raman spectra. This material is available free of charge via the Internet at http://pubs.acs.org AUTHOR INFORMATION Corresponding Author * To whom correspondence should be addressed, [email protected], [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. REFERENCES (1)

(2) (3)

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(44) Ji, X.; Banks, C. E.; Silvester, D. S.; Wain, A. J.; Compton, R. G. Electrode Kinetic Studies of the Hydroquinone−Benzoquinone System and the Reaction between Hydroquinone and Ammonia in Propylene Carbonate: Application to the Indirect Electroanalytical Sensing of Ammonia. J. Phys. Chem. C 2007, 111, 1496–1504. (45) Byon, H. R.; Lee, S. W.; Chen, S.; Hammond, P. T.; Shao-Horn, Y. Thin Films of Carbon Nanotubes and Chemically Reduced Graphenes for Electrochemical Micro-Capacitors. Carbon 2011, 49, 457–467. Insert Table of Contents Graphic and Synopsis Here


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Figure 1. Steady-state CVs of graphite-foil electrode (0.1 cm2 and 0.014 g/cm2geo.) measured in a 0.5 M VII (purple), VIII (turquoise), VIV (blue) and VV (yellow) electrolytes all with 2 M H2SO4 at a scan rate of 20 mV/s. Graphite foil and SCE were used as the counter and reference electrode, respectively. 123x184mm (300 x 300 DPI)


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Figure 2. Steady-state CVs measured in 0.5 M VIV in 2 M H2SO4 electrolyte. (a) graphite-foil (GF) electrodes: unmasked (dashed-black line), edge- (blue line) and basal-exposed (red line) GF electrodes with schematics and scanning He-Ion microscope images in the insets, and (b) uncovered GF (dashed-black 0.1 cm2 and 0.014 g/cm2geo.), as-received carbon-paper (blue line) and heat-treated carbon-paper (red line) electrodes (0.1 cm2 and 0.006 g/cm2geo). All CV were collected from a 3-electrode cell setup, with a scan rate of 20 mV/s, with graphite foil and SCE used as the counter and reference electrode, respectively. 177x74mm (276 x 276 DPI)



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Figure 3. Steady-state CVs measured with unmasked graphite-foil electrode (0.1 cm2 and 0.014 g/cm2geo.) in 0.5 M, 0.2 M, 0.1 M and 0.05 M (a) VII, (b) VIII, (c) VIV and (d) VV in 2 M H2SO4 electrolytes with a scan rate of 20 mV/s. These measurements were collected from a three-electrode cell setup, where graphite-foil and SCE were used as the counter and reference electrode, respectively. 133x100mm (300 x 300 DPI)


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Figure 4. Steady-state CVs obtained with as-received and heat-treated carbon-paper electrodes (0.1 cm2 and 0.006 g/cm2geo) in different 0.5 M vanadium-ion solutions including VII (purple), VIII (turquoise), VIV (blue) and VV (yellow) in 2M H2SO4 with a scan rate 20 mV/s. Measurements were conducted in a threeelectrode cell setup, where graphite foil and SCE were used as the counter and reference electrode, respectively. Dashed lines represent zero current for each electrolyte solution and optical photographs of the corresponding solutions are shown in the inset. 123x184mm (300 x 300 DPI)



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Figure 5. Normal pulse voltammetry data obtained with as-received (red) and heat-treated (blue) carbonpaper electrodes (0.1 cm2 and 0.006 g/cm2geo.) in 0.05 M VIV/VV electrolyte in 2 M H2SO4. Measurements were conducted in a three-electrode cell setup, where graphite foil and SCE were used as the counter and reference electrode, respectively. 58x40mm (300 x 300 DPI)


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Figure 6. Cyclic voltammograms of (a) as-received and (b) heat-treated carbon paper (0.1 cm2 and 0.0063 g/cm2geo.) in a 0.5 M VIII with 2 M H2SO4 electrolyte at a scan rate of 20 mV/s. The CV started at OCV (~-0.4 V vs. SCE), scanning down to -1.1 V (vs. SCE) and back up to 1.45 V (vs. SCE) The first and second cycles were collected from a three-electrode cell, having graphite foil and SCE as the counter and reference electrode, respectively. Scanning He-Ion microscope images of the corresponding electrodes are shown in the insets. 177x82mm (300 x 300 DPI)



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Figure 7. Potentiostatic measurements of as-received (red) and heat-treated (blue) carbon-paper electrodes with the cell active area of 23 cm2 (0.0063 g/cm2geo.). (a) 1.5 M 90% VVO2+/10% VIVO2+ in 2.6 M H2SO4, measured at the beginning-of-life39 (circles) and after-potentiostatic measurements in the 1.5 M 90% II V /10%VIII in 2.6 M H2SO4 electrolyte (diamonds), and b) 1.5 M 90% VII/10%VIII in 2.6 M H2SO4, measured at the beginning-of-life (circles) and after-potentiostatic measurements in the 1.5 M 90% VVO2+/10% VIVO2+ in 2.6 M H2SO4 solution (diamonds). (For each applied voltage, 30 measurements were conducted). 177x78mm (300 x 300 DPI)


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Influence of Edge- and Basal-Plane Sites on the Vanadium Redox

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