Flow Battery Electroanalysis 2: Influence of Surface Pretreatment on

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C: Energy Conversion and Storage; Energy and Charge Transport

Flow Battery Electroanalysis 2: Influence of Surface Pretreatment on Fe(III/II) Redox Chemistry at Carbon Electrodes Tejal V. Sawant, and James R. McKone J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b09607 • Publication Date (Web): 18 Dec 2018 Downloaded from http://pubs.acs.org on December 20, 2018

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The Journal of Physical Chemistry

Flow Battery Electroanalysis 2: Influence of Surface Pretreatment on Fe(III/II) Redox Chemistry at Carbon Electrodes Tejal V. Sawant and James R. McKone∗ Department of Chemical and Petroleum Engineering, Swanson School of Engineering, University of Pittsburgh, Pittsburgh, PA 15261, USA E-mail: [email protected]

Abstract Redox flow batteries are attractive for large-scale electrochemical energy storage, but sluggish electron transfer kinetics can limit their overall energy conversion efficiencies. In an effort to improve our understanding of these kinetic limitations in transition metal based flow batteries, we used rotating-disk electrode voltammetry to characterize the electron-transfer rates of the Fe3+/2+ redox couple at glassy carbon electrodes whose surfaces were modified using several pre-treatment protocols. We found that surface activation by electrochemical cycling in H2 SO4 (aq) resulted in the fastest electron-transfer kinetics: j0 = 0.90 ± 0.07 mA/cm2 in an electrolyte containing 10 mM total Fe. By contrast, electrodes that were chemically treated to either remove or promote surface oxidation yielded rates that were at least an order of magnitude slower: j0 = 0.07 ± 0.01 and 0.08 ± 0.04 mA/cm2 , respectively. Correlating these results with X-ray photoelectron spectroscopy data suggests that surface carbonyl groups catalyze Fe3+/2+ redox chemistry in water.

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Introduction Renewables like solar and wind power are highly desirable energy sources for their environmental sustainability but problematic because they are spatially and temporally intermittent. As a result, there is a clear need for cost effective technologies that can efficiently store large quantities of renewable electricity. 1–4 Although conventional (solid phase) secondary batteries can be deployed for this type of energy storage, they are considerably more developed for mobile applications in which high energy and power density are paramount. 5 By contrast, redox flow batteries (RFBs) are less technologically mature, but they have garnered significant attention for their potential to store electricity on a large scale. 6 RFBs have been under investigation for decades, but academic and commercial interest has grown substantially over the last several years. 7–15 Early work focused on aqueous transitionmetal compounds—particular Fe and Cr aquo complexes—as the active components of liquidphase battery electrolytes. 16–18 The aqueous all-vanadium flow battery has also been studied extensively and remains popular due to its use of a single set of interconvertible vanadium aquo complexes as both the negative and positive electrolytes. 19–28 More recently, there has emerged significant interest in non-aqueous RFB chemistries, many of which are also based on transition metal complexes as the redox-active components of the electrolytes. 29–34 Because RFBs are anticipated to be useful in large-scale installations, they require stack components—electrodes, separators, bipolar plates, cell housing, and plumbing—that are chemically robust and low in cost. To this end, carbon is the material of choice for the positive and negative electrodes in practical RFBs. 35–38 Generally graphitic carbons are processed into cloths, foams, or felts to produce a porous electrode with high surface area over which redox reactions can occur. 39–42 These materials are similar in chemical composition to glassy carbon (GC) or pyrolytic graphite electrodes, which are commonly employed for electroanalytical studies. 43–47 In the context of electroanalysis, a variety of preparation protocols are commonly used to clean and/or activate carbon electrodes. 48 These procedures often include polishing steps us2


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ing water-alumina slurries, followed by additional treatments intended to remove impurities or modify the chemical functionality of the carbon surface. 49,50 For example, freshly polished carbon electrodes are often activated by electrochemical cycling in aqueous acidic electrolytes over a potential range that slightly exceeds the electrode/solvent stability window. 44,51 These treatments are understood to help remove impurities and generate a partially oxidized, and therefore more chemically reactive, carbon surface. Chemical oxidants can also be used to generate surface oxygen species. 25,48,52,53 By contrast, thermal treatments in vacuum or under reducing atmospheres have been used to remove surface oxygen functional groups. 24,54–57 McCreery, et al. reported a straightforward treatment involving incubation of carbon electrodes in organic solvents that had been pre-purified with activated carbon. 45,58,59 This method was also found to decrease the amount of oxidized carbon on the electrode surface (albeit less effectively than heat treatments), which was attributed to the removal of residual impurities that remain even after polishing. A variety of carbon surface preparation protocols have also been found to improve the apparent rates of electron transfer to RFB redox couples, but the mechanistic basis of these improvements remains unclear. Several groups have argued that catalytic enhancement results from the presence of surface oxygen functionalities broadly. 44,60,61 By contrast, some have suggested that the primary reason for enhanced kinetics is the presence of a greater ratio of reactive edge sites to unreactive basal planes. 62,63 Recent work on vanadium electrolytes in particular has suggested that both oxidative and reductive surface treatments can have a significant effect on electron-transfer rates. 64–66 This ambiguity in the kinetics of carbon electrodes in RFB applications closely mirrors controversies in the electroanalytical literature regarding the catalytic properties of model carbon surfaces. 67–70 Resolving these ambiguities would greatly improve our understanding of carbonbased electrocatalysis and help drive improved performance in functional RFBs. We are working to understand, and ultimately eliminate, kinetic limitations in RFBs by leveraging insights and methods from electroanalytical chemistry to investigate electron-transfer in model battery electrolytes. The present study extends on prior results in which we characterized the kinetics of aqueous Fe3+/2+ as a prototypical RFB positive electrolyte at Pt and Au surfaces using 3



rotating disk electrode (RDE) voltammetry. 71 We have now executed a series of experiments examining the electron transfer kinetics of the Fe3+/2+ redox couple at GC electrodes as a function of surface preparation. Using an electrolyte containing 10 mM total Fe in HCl(aq) solution, we found geometric exchange current densities of 0.07 ±0.01 mA/cm2 , 0.9 ±0.07 mA/cm2 , and 0.08 ±0.04 mA/cm2 at GC electrodes that were pretreated by incubation in pre-purified isopropanol, electrochemical activation, and chemical oxidation with H2 O2 , respectively. These results were surprising because the electrochemical and H2 O2 treatments both increased the extent of carbon surface oxidation, but only electrochemical activation improved the electron-transfer kinetics. Moreover, X-ray photoelectron spectroscopy (XPS) and Raman analysis showed only modest differences in surface oxidation between all three treatments. These results lead us to conclude that the catalytic activity of carbon toward aqueous Fe3+/2+ redox chemistry depends on specific surface chemical functionality rather than defects or oxidized sites more generally. Based on additional scrutiny of XPS results, we conclude that the relatively higher proportion of carbonyl groups on electrochemically treated GC surfaces are responsible for catalyzing Fe3+/2+ electrochemistry.

Experimental FeCl2 (tetrahydrate salt, 98%), FeCl3 (hexahydrate salt, 97%), hydrochloric acid (Certified ACS plus), sulfuric acid (trace metal grade), isopropanol (Certified ACS grade), hydrogen peroxide (30 wt%, stabilized with sodium stannate) and Ag/AgCl (gel-type reference electrodes with 3 N NaCl fill solution) were obtained from Fisher scientific and were used as obtained. Deionized water with resistivity of ≥18.2 MΩ-cm was obtained using a Millipore Milli-Q Advantage A10 system and used for preparation of all solutions. Activated carbon (425–850 µm particle size) was obtained from Alfa Aesar. Two water-based detergents, Citranox and Alconox, were obtained from W.W. Grainger Inc. 5, 1 and 0.05 µm alumina powders and 8 inch polishing micropads were obtained from Pace Technologies. Graphite electrodes were obtained from Electron Microscopy Sciences; they were of spectroscopic grade with a porosity of 16.5 % and a diameter of 1/4 in. Zero grade 4


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N2 (g) (99.998%) and Ultra-high purity H2 (g) (99.999%) were obtained from Matheson. The electrochemical apparatus was a Pine MSR electrode rotator, equipped with a 5mm Change-Disk electrode assembly. A Gamry Interface 1000E potentiostat was used for all electrochemical measurements. Sonication was performed in a Branson M1800 ultrasonic cleaner, in which electrodes were first placed in 20 mL vials filled with deionized water that were in turn placed in the sonicator bath. The electrochemical cell was a 100 mL glass chamber with a tight Teflon cap bearing holes to introduce the electrodes and a gas purge tube. Raman microscopy was perfomed on a Renishaw InVia Raman microscope. Spectra were collected using 633 nm laser light with 1800 l/mm grating through a 20× objective lens at 50% laser power. Three accumulations of the Raman spectra were taken over the range 1000–1800 cm−1 for 20 seconds each, and each electrode was tested at 2 different locations on the sample. X-ray photoelectron spectroscopy (XPS) was performed on a Thermo Scientific ESCALAB 250Xi. Each electrode was tested at 5 different points on the sample. Survey spectra were first collected to identify atomic concentrations of surface carbon and oxygen. The C 1s and O 1s peaks were found at 284.6 eV and 532 eV and the relative sensitivity factors used were 1 and 2.93, respectively. High resolution scans of the C 1s region were then collected and peak deconvolution was performed to extract ratios of functional groups present in the sample. 72–74 For this deconvolution, XPS data were fit to graphitic groups (284.6 eV), alcoholic groups (286 eV), carbonyl groups (287 eV) and carboxylic groups (288.6 eV) after background subtraction using a built-in protocol in the instrument software. The peak positions were allowed to vary over ±0.1 eV and the full width half max (FWHM) of the peaks was constrained between 1.3-1.9 eV. For electrochemistry measurements, we followed the same protocol that we recently reported for Pt and Au RDE voltammetry of Fe-based RFB electrolytes. 71 Briefly, all glassware was cleaned by boiling in aqueous detergents (Citranox followed by Alconox), and the cell was assembled with an electrolyte consisting of 5mM FeCl2 and 5mM FeCl3 in 0.5 M HCl. The GC working electrode was prepared first by course grinding with silicon carbide lapping film

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down to 1200 grit, followed by polishing with 5, 1, and 0.05 µm alumina slurries. Then the GC surfaces were further treated using one of the three different procedures. The first carbon treatment comprised solvent cleaning using isopropanol that had been pre-purified with activated carbon (hereafter referred to as AC/IPA). 45 A 1:3 vol/vol of activated carbon and isopropanol was first prepared by stirring for 5 minutes followed by sonicating for 5 minutes. This slurry was then allowed to stand for at least 30 minutes prior to use. The active surface of a freshly polished glassy carbon electrode was submersed in this mixture for 10 minutes followed by a copious water rinse and sonication in water for 30 seconds. The electrode was then introduced into the cell before it dried. The second carbon treatment involved again completing the AC/IPA cleaning step, followed by electrochemical activation. The electrode was cycled in 0.5 M H2 SO4 (aq) solution from -0.25 to 1.5 V versus Ag/AgCl for a total of 100 cycles at 200 mV/s and then from -0.25 to 1.7 V versus Ag/AgCl for 20 cycles at 200 mV/s. It was then rinsed and introduced into the Fe3+/2+ electrolyte before it dried. The third carbon treatment involved again cleaning with AC/IPA, followed by chemical oxidation with hydrogen peroxide wherein the electrode surface was introduced into a vial containing 5 ml of 30% hydrogen peroxide and left for 10 minutes. It was then thoroughly rinsed with water and introduced into the test cell before it dried. RDE measurements and analysis also followed the methods we reported previously. 71 While we found that it was essential to clean the surface of Pt and Au electrodes between data collection at each rotation rate, this was not found to be necessary for GC electrodes. From the RDE data, diffusivity was extracted using Koutecky-Levich (KL) analysis and geometric exchange current densities were found by fitting the transport-free polarization data to a version of the ButlerVolmer equation in which the symmetry factors, αox and αred , were allowed to vary arbitrarily between 0 and 1.

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Results and Discussion As an initial indication of the relative rates of electron transfer, Figure 1(a) presents cyclic voltammetry (CV) data for glassy carbon electrodes after each surface treatment at a rotation rate of 0 rpm and a scan rate of 200 mV/s. The peak-to-peak separation values, which vary inversely with reaction kinetics, were considerably smaller for electrochemically activated GC (140mV) as compared to electrodes with AC/IPA (420mV) or H2 O2 (510mV) treatments. Thus, it is immediately apparent that the kinetics of Fe3+/2+ were considerably faster at electrochemically activated GC compared to the other treatments. This result is in good agreement with the popular use of electrochemical cycling methods to activate carbon electrodes for electroanalysis and RFB electrocatalysis. 43,64 Figure 1(b) presents peak-to-peak separations for all GC electrodes over a range of scan rates from 10–3000 mV/s. We made further use of these data to estimate reaction kinetics using the method reported by Nicholson. 75 This analysis showed that the electron-transfer rate at electrochemically treated GC was ∼15 times faster than AC/IPA treated GC, and ∼30 times faster than H2 O2 treated GC. Complete details are included in the Supporting Information.

Figure 1: (a) Current density vs. potential data at a scan rate of 200 mV/s for AC/IPA treated GC, electrochemically treated GC and H2 O2 treated GC; (b) peak to peak separation as a function of scan rate over the range from 10-3000 mV/s in 5mM FeCl2 and 5 mM FeCl3 in 0.5 M HCl at 0 rpm. 7



Figure 2 shows RDE current density vs. potential (j–E) data for each electrode type over a range of rotation rates from 100 to 2500 rpm. The equilibrium potential was found to be

Figure 2: RDE current density vs. potential data for (a) AC/IPA treated GC, (b) electrochemically treated GC and (c) H2 O2 treated GC in 5mM FeCl2 and 5 mM FeCl3 in 0.5 M HCl. 0.45 V versus Ag/AgCl (0.68 V vs. NHE), which agrees reasonably well with the reported value of 0.7 V vs. NHE for 1 M HCl supporting electrolyte. 76 Oxidative and reductive current densities increased monotonically in magnitude with increasing rotation rate at the outer limits of the measured potential range. Electrochemically activated GC electrodes exhibited well-differentiated current densities at low overpotentials, whereas AC/IPA and H2 O2 treated electrodes both exhibited overlapping j–E response over a range of at least 100 mV about the equilibrium potential, which is indicative of predominantly kinetic limitations. Moreover, larger potential ranges were required to achieve transport-limited behavior for the AC/IPA and H2 O2 treated electrodes; in fact, steady-state limiting currents were not observed even over a 1.2 V scan range for H2 O2 treated GC. Figure 3 depicts KL data for each GC treatment, comprising plots of inverse current vs. inverse square root of rotation rate for overpotentials from 40 to 180 mV in the positive and negative directions. These data exhibit linear trends in which the intercepts decrease monotonically and the slopes of the best fit lines approach a nearly constant value as overpotential increases. Thus, the slopes of the lines above 100 mV overpotential were used to extract diffusivity values. The KL data for Fe2+ oxidation at AC/IPA treated GC exhibited greater variability, particularly at small overpotentials. Upon further examination, we found that the variation was consistent with a monotonic increase in the electron-transfer rate over the course of the measurement. We attribute 8


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Figure 3: Koutecky-Levich plots comprising inverse current vs. inverse square root of rotation rate for (a) iron oxidation at AC/IPA treated GC, (b) iron reduction at AC/IPA treated GC, (c) iron oxidation at electrochemically treated GC, (d) iron reduction at electrochemically treated GC, (e) iron oxidation at H2 O2 treated GC and (f) iron reduction at H2 O2 treated GC.

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this variability to changes in electrode surface composition during electrochemical measurements, which are qualitatively similar to the changes that occur during electrochemical activation. Figure 4 shows representative Tafel data for all carbon treatments depicting natural log of kinetic current density versus overpotential (η) along with the associated Butler-Volmer fits for each carbon surface preparation. Table 1 summarizes the transport and kinetics properties

Figure 4: Tafel plots and corresponding fits for iron oxidation and reduction at (a) AC/IPA treated GC, (b) electrochemically treated GC and (c) H2 O2 treated GC in 5mM FeCl2 and 5 mM FeCl3 in 0.5 M HCl extracted from the RDE data. Uncertainty values are reported as 1 standard deviation from mean of 5 replicates. Diffusivity values ranged from 2–4 x 10−6 cm2 /s and were indistinguishable within two standard deviations. The exchange current densities were found to be 0.07 ±0.01 mA/cm2 for AC/IPA treated GC, 0.9 ±0.07 mA/cm2 for electrochemically treated GC, and 0.08 ±0.04 mA/cm2 for H2 O2 treated GC. All α values were found to be between 0.25 and 0.45 and were also indistinguishable within two standard deviations. Table 1: Transport and kinetics properties of Fe3+/2+ electron transfer at GC electrodes. Electrode AC/IPA treated GC (n=5) Electrochemically treated GC (n=5) H2 O2 treated GC (n=5)

Diffusivity Dox (cm2 /s) 2.2 x 10−6 (±0.4 x 10−6 ) 3.8 x 10−6 (±0.5 x 10−6 ) 2.0 x 10−6 (±0.6 x 10−6 )

Diffusivity Dred (cm2 /s) 3.7 x 10−6 (±0.4 x 10−6 ) 2 x 10−6 (±0.5 x 10−6 ) 2.5 x 10−6 (±1 x 10−6 )

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Exchange current density j0 (mA/cm2 ) 0.07 (±0.01) 0.90 (±0.07) 0.08 (±0.04)


αox

αred

0.33 (±0.09) 0.35 (±0.07) 0.26 (±0.06)

0.31 (±0.05) 0.44 (±0.09) 0.36 (±0.01)

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Our expectation was that the electrochemical and H2 O2 surface treatments would both oxidize GC surfaces and would therefore result in increased electron-transfer rates. 60,61,77,78 Instead, only electrochemical activation resulted in faster rates while the H2 O2 treatment gave electron transfer kinetics that were essentially indistinguishable from AC/IPA cleaning alone. These unexpected results motivated us to further explore electrode surface chemistry. Figure 5 presents Raman data collected immediately after completion of each surface preparation. Two clear peaks are resolved, which are attributable to the presence of graphitic carbon at ∼1590 cm−1 (G band) and structural defects at ∼1330 cm−1 (D band). 79–82 Some reports also describe an additional peak that appears as a shoulder on the G band at ∼1610 cm−1 , which also indicates defective carbon. 53 Rather than a shoulder, we observed a broad G peak with substantial intensity at 1610 cm−1 . The intensity ratio of D to G bands (ID /IG ) for the AC/IPA and H2 O2 treated GC is 2.1 while that for the electrochemically treated GC is 1.7. This value is known to increase when the graphite crystallite size decreases, and a larger ID /IG ratio has also been found to correlate with increase in edge plane density and enhanced electron transfer kinetics. 51,62,63 These data are all indicative of highly defective graphitic carbon, which is consistent with the fact that glassy carbon is comprised of randomly oriented graphitic or fullerene-like nanocrystallites. 83,84 Thus, the high density of crystallographic defects and edge planes in the structure of glassy carbon provides ample sites for surface oxidation. Nevertheless, these Raman data do not provide a clear indication of the relative extent of oxidation resulting from each electrode treatment. To better quantify and compare the extent of surface oxidation, we performed XPS measurements on carbon electrodes after each treatment. Figure 6 presents representative XPS survey scan data for each type of carbon electrode. Surface oxidation was preliminarily estimated based on O/C ratios (after accounting for relative intensity factors), which were 0.13 for AC/IPA cleaned GC, 0.15 for electrochemically treated GC, and 0.17 for hydrogen peroxide treated GC. These data show that indeed the electrochemical and H2 O2 treatments resulted in increased surface oxidation, although the difference between all three treatments was small. The O/C ratio also does not correlate with the relative rates of electron transfer for Fe3+/2+ . Thus, the overall degree of surface 11



Figure 5: Normalized Raman data for AC/IPA treated, electrochemically treated and H2 O2 treated GC electrodes in the region corresponding to the characteristic features of graphitic carbon: D band at 1330 cm−1 and G band at 1590 cm−1 . The intensities are normalized to the maximum intensity of the G band for each electrode type. oxidation does not offer an adequate explanation for the observed differences in reaction kinetics. Another possible explanation is increased surface area attributable to roughening of the GC surface during the electrochemical treatment. However, electron microscopy and atomic force microscopy measurements (see Supporting Information) showed that surface roughness was indistiguishable across all three surface treatments. We collected high-resolution XPS data to elucidate the relative amounts of oxidized surface species, under the hypothesis that one or more chemical functionalities is responsible for catalyzing Fe3+/2+ redox chemistry. Figure 7(a) presents a representative XPS peak deconvolution performed on the C 1s spectrum for H2 O2 treated GC. These data were fit to find the relative ratios of graphitic groups (284.6 eV), alcoholic groups (286 eV), carbonyl groups (287 eV) and carboxylic groups (288.6 eV). Figure 7(b) collects the corresponding results from all three surface treatments as a bar chart depicting fractional ratios of each type of functional group. Graphitic carbon comprised the primary component in each case, which is unsurprising in view of the fact that XPS measurements probe several nm of sample depth, whereas only surface sites are susceptible to oxidation. Indeed, we estimate that complete oxidation of a smooth GC surface (i.e., every surface carbon atom bound to oxygen) would only give rise to a ∼9 % fraction of oxidized 12


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Figure 6: XPS survey scan data for AC/IPA treated, electrochemically treated and H2 O2 treated GC electrodes. Peaks corresponding to the O1s and C1s regions are noted along with the associated O/C ratios. carbon by XPS (see Supporting Information). Interestingly, the AC/IPA carbon surface already exhibits nearly this level of oxidation, which we attribute to several factors: (a) solvent treatments do not entirely remove oxidized surface functionalities; (b) our carbon surfaces were not completely smooth; (c) there may be oxygen in the sub-surface of our GC electrodes; and (d) some adventitious oxygen-bearing impurities may be present in our XPS samples. If we treat the distribution of functional groups in the AC/IPA treated carbon as a baseline corresponding to a “clean” and relatively inactive carbon surface, the observed increases in oxidation for the electrochemical and H2 O2 treatments correspond to tens of percent of a monolayer equivalent. Moreover, the relative ratios of oxidized surface functionalities was quite different between the two. The electrochemical treatment was found to increase the proportion of all three types of oxidized surface groups. By contrast, the H2 O2 treatment only increased the relative amounts of carboxylic and alcoholic groups. The XPS data indicate that carbonyl is the only surface functionality whose coverage increases in electrochemically activated GC, but not in H2 O2 treated GC. Thus, we conclude that

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Figure 7: (a) Representative high-resolution XPS data in the C1s region for H2 O2 treated GC fit after background subtraction to the relative fraction of graphitic groups (284.6 eV), alcoholic groups (286 eV), carbonyl groups (287 eV) and carboxylic groups (288.6 eV). (b) Bar chart of various functional groups present in AC/IPA treated, electrochemically treated, and H2 O2 treated GC electrodes obtained from C1s peak deconvolution using XPS. Reported data are the mean of 5 replicates. carbonyl groups are primarily responsible for catalyzing electron transfer from carbon surfaces to the Fe3+/2+ redox couple. This agrees with several prior studies in which the catalytic activity of carbon electrodes toward transition metal aquo complexes was found to correlate with carbonyl coverage. 45,46,57 One possible mechanistic rationale for this enhancement involves the formation of Fe-based surface adsorbates mediated by carbonyls. This picture is attractive for its similarity to the established catalytic effect of adsorbing anions at noble metal surfaces, whereby bridging complexes including the anions are thought to accelerate electron transfer. 85–88 Notably, our measurements were all made in HCl electrolyte, so the sluggish electron transfer observed after the AC/IPA and H2 O2 treatments strongly suggests that chloride ions do not play the same catalytic role at carbon electrodes. Based on further analogies to solution-phase coordination chemistry, we speculate that the predominant mechanism may involve rapid electron transfer from the electrode to surface-bound Fe, followed by a rate-determining self-exchange type electron-transfer between surface-bound and solution phase Fe species. This mechanism is analogous to prior observations of self-catalysis by quinones bound to carbon surfaces through pi stacking. 59 It also

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bears resemblance to more recent observations of redox-silent but catalytically competent coordination complexes that were covalently bound to graphite surfaces through well-defined molecular linkers. 89,90 In the context of RFB device engineering, it is apparent that transition metal aquo complexes do not behave as outer-sphere electron transfer reagents but instead exhibit remarkably complex interfacial redox chemistry. Our results suggest that only certain types of oxidized carbon surfaces competently catalyze electron transfer to/from Fe3+/2+ (aq), and the catalytic requirements of carbon substantially diverge from those of noble metals. 71,85 Analogous work on vanadium redox chemistry has also shown widely variable results depending on the specific method that is used to modify the surface chemistry of carbon electrodes. For example, Bourke et al. found that vanadium RFB electron transfer can either be enhanced or suppressed by electrochemical surface modifications depending on whether the treatment is oxidizing or reducing and whether the electrolyte of interest is V3+/2+ or V5+/4+ . 64,65 Zeng et al. reported that oxygen-containing functional groups that were generated using an alkaline hydrothermal treatment catalyze the V3+/2+ redox reaction but have no significant influence on the kinetics of V5+/4+ . 91 Taylor et al. argued that structural disorder, and not surface oxidation, was primarily responsible for enhancing the kinetics of V5+/4 redox chemistry at glassy carbon electrodes. 92 These remarkably diverse outcomes suggest that there would be considerable value in developing mechanistically-grounded design rules for carbon surface modifications, but these rules may in fact be different for each redox couple of interest. Improvements in our ability to control the precise surface chemistry of carbon would be advantageous for testing hypotheses regarding RFB electrocatalysis. Moreover, the microstructures of model materials like glassy carbon or pyrolytic graphite are considerably different from the graphitized carbon fibers that are used as RFB electrodes. Thus, there is also a need for additional work to determine whether chemical modifications developed on model electrodes can be directly translated to technologically relevant materials.

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Conclusions We have used quiescent and hydrodynamic voltammetry to assess the rate of electron-transfer between glassy carbon and RFB-mimicking Fe3+/2+ electrolytes after three different surface treatments. We found that electrochemical activation and chemical treatment with concentrated H2 O2 both increased the degree of carbon surface oxidation, but only the former resulted in enhanced kinetics. XPS data showed that electrochemical activation slightly increased the proportion of carbonyl groups on the surface, whereas H2 O2 did not, leading us to conclude that carbonyls are primarily responsible for catalyzing Fe3+/2+ redox chemistry on carbon. Nonetheless, the specific molecular structure and areal density of active catalytic sites remain unclear. These results underscore the remarkable complexity underlying the electrochemical kinetics of flow batteries, even for well-studied coordination complexes involving only a single electron transfer. Further work is warranted to elucidate the precise carbon surface chemistry responsible for catalysis in Fe-based electrolytes and to determine whether the associated mechanism is applicable to other transition metal aquo compounds of interest for RFB applications. These insights will help motivate improved surface functionalization strategies that can be applied to technologically relevant carbon electrodes in the interest of minimizing energy efficiency losses attributable to kinetics in practical flow batteries.

Acknowledgements We gratefully acknowledge the Swanson School of Engineering at the University of Pittsburgh for support of this work via startup funds for the McKone Laboratory. Thank you to Dr. Susan Fullerton and Jierui Liang for assistance with AFM measurements.

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Supporting Information Available Tabulated Fe3+/2+ kinetics. Peak current vs. scan rate. Kinetics estimates from CV data. Results of conventional Butler-Volmer fits with constrained α values. Detailed consideration of surface roughness effects. Exchange current density normalized to carbonyl content. Additional Discussion of XPS surface oxidation measurements.

This material is available free of charge via the

Internet at http://pubs.acs.org/.

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Graphical TOC Entry C-OH

C=O

C-OOH

e-

Fe3+ (aq)

Fe2+ (aq)

purified solvent treatment

420 mV electrochemical activation

140 mV conc. H2O2 treatment

510 mV 0

5% 10% surface carbon [by XPS]

15%

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Flow Battery Electroanalysis 2: Influence of Surface Pretreatment on

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