doi.org/10.26434/chemrxiv.7125107.v1
Flow Battery Electroanalysis 2: Influence of Surface Pretreatment onFe(III/II) Redox Chemistry at Carbon ElectrodesTejal Sawant, James McKone
Submitted date: 24/09/2018 • Posted date: 25/09/2018Licence: CC BY-NC-ND 4.0Citation information: Sawant, Tejal; McKone, James (2018): Flow Battery Electroanalysis 2: Influence ofSurface Pretreatment on Fe(III/II) Redox Chemistry at Carbon Electrodes. ChemRxiv. Preprint.
Redox flow batteries are attractive for large-scale electrochemical energy storage, but sluggish electrontransfer kinetics often limit their overall energy conversion efficiencies. In an effort to improve ourunderstanding of these kinetic limitations in transition metal based flow batteries, we used rotating-diskelectrode voltammetry to characterize the electron-transfer rates of the Fe3+/2+ redox couple at glassy carbonelectrodes whose surfaces were modified using several pre-treatment protocols. We found that surfaceactivation by electrochemical cycling in H2SO4(aq) electrolyte resulted in the fastest electron-transfer kinetics:j0 = 0:90 mA/cm2 in an electrolyte containing 10 mM total Fe. By contrast, electrodes that were chemicallytreated to either remove or promote surface oxidation yielded rates that were at least an order of magnitudeslower: j0 = 0:07 and 0:08 mA/cm2, respectively. By correlating these findings with X-ray photoelectronspectroscopy data, we conclude that Fe3+/2+ redox chemistry is catalyzed by carbonyl groups whose surfaceconcentrations are increased by electrochemical activation.
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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 slug-
gish electron transfer kinetics often 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 cy-
cling in H2SO4(aq) electrolyte resulted in the fastest electron-transfer kinetics: j0 = 0.9±0.07
mA/cm2 in an electrolyte containing 10 mM total Fe. By contrast, electrodes that were chem-
ically 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. By cor-
relating these findings with X-ray photoelectron spectroscopy data, we conclude that Fe3+/2+
redox chemistry is catalyzed by carbonyl groups whose surface concentrations are increased
by electrochemical activation.
1
Keywords
Redox flow battery, electroanalysis, kinetics, glassy carbon, rotating disk electrode voltammetry,
surface treatment, cleaning
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.
RFBs have been under investigation for decades, but academic and commercial interest
has grown substantially over the last several years.6–15 Early work focused on aqueous transition-
metal compounds—particular Fe and Cr aquo complexes—as the active components of liquid-
phase battery electrolytes.16–18 The aqueous all-vanadium flow battery has also been studied ex-
tensively 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 chem-
ically 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,
2
foams, or felts to produce a porous electrode with high surface area over which redox reactions
can occur.39–42 These materials are broadly similar in chemical composition to glassy carbon (GC)
electrodes, which are commonly employed for electroanalytical studies.43–47
In the context of electroanalysis using carbon electrodes, a variety of preparation proto-
cols are commonly used to clean and/or activate their surfaces toward electrochemistry.48 These
procedures often include polishing steps using aqueous alumina slurries, followed by additional
treatments intended to remove impurities or modify the chemical functionality of the carbon sur-
face.49,50 For example, freshly polished carbon electrodes are often activated by electrochemi-
cal cycling in aqueous acidic electrolytes over a potential range that slightly exceeds the elec-
trode/solvent stability window.44,51 These treatments are understood to help remove impurities
and generate a partially oxidized carbon surface. Chemical oxidants can also be used to gener-
ate 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 been found to improve the appar-
ent rates of electron transfer to aqueous transition metal RFB redox couples, but the mechanistic
basis of these improvements remains unclear. Several groups have argued that catalytic enhance-
ment 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 re-
active 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 Thus, ambiguity in the kinetics of carbon electrodes in RFB applica-
tions closely mirrors controversies in the electroanalytical literature regarding the catalytic proper-
ties of nominally well-defined model carbon surfaces.67–70 Resolving these ambiguities associated
3
with carbon surface chemistry and electron-transfer rates will greatly improve our understanding
of carbon-based 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 flow battery electrolytes. The present study extends on prior results in which we character-
ized the kinetics of aqueous Fe3+/2+ as a prototypical RFB positive electrolyte at Pt and Au surfaces
using 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
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 H2O2, respectively. These results were surprising because
the electrochemical and H2O2 treatments both increased the extent of carbon surface oxidation, but
only electrochemical activation improved the electron-transfer kinetics. Moreover, X-ray photo-
electron spectroscopy (XPS) and Raman analysis showed only modest differences in the apparent
extent of 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 sur-
face chemical functionality rather than defects or oxidized sites 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
4
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
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 elec-
trochemical 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 comprised of 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 ES-
CALAB 250Xi. Each electrode was tested at 3 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 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 (285 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
5
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 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 working electrode GC
was prepared first by polishing with 5, 1, and 0.05 µm alumina slurries. Then the GC surfaces
were further treated using one of three different procedures. The first carbon treatment comprised
of 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 sonicating in
water for 30 seconds. The electrode was then introduced into the cell before it dried. The second
carbon treatment involved first completing the AC/IPA cleaning step, followed by electrochemical
activation. The electrode was cycled in 0.5 M H2SO4(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
by hydrogen peroxide. After solvent cleaning, 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 have reported previ-
ously.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 exchange current
density was found by fitting the transport-free polarization data to a version of the Butler-Volmer
equation in which the symmetry factors, αox and αred, were allowed to vary arbitrarily between 0
6
and 1.
Results and Discussion
As an initial indication of the relative rates of electron transfer, Figure 1(a) presents cyclic voltam-
metry (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 com-
pared to electrodes with AC/IPA (420mV) or H2O2 (510mV) treatments. Thus, it is immediately
apparent that the kinetics of Fe3+/2+ were considerably faster at electrochemically activated GC
as 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 elec-
trocatalysis.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 H2O2 treated GC. Complete details are included in the Supporting Information.
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
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 densi-
ties 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 H2O2 treated electrodes both exhib-
ited 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 H2O2 treated electrodes; in fact,
7
Figure 1: (a) Current density versus potential data representing peak to peak separations at a scanrate of 200mV/s at 0 rpm for AC/IPA treated GC, electrochemically treated GC and H2O2 treatedGC, (b) Peak to peak separation as a function of scan rate over the range from 10-3000 mV/s in5mM FeCl2 and 5 mM FeCl3 in 0.5 M HCl at 0 rpm.
Figure 2: RDE current density versus potential data for (a)AC/IPA treated GC, (b)Electrochemically treated GC and (c) H2O2 treated GC in 5mM FeCl2 and 5 mM FeCl3 in 0.5 MHCl.
8
steady-state limiting currents were not observed even over a 1.2 V scan range for H2O2 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, partic-
ularly at relatively low 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 measure-
ment. We attribute this variability to changes in electrode surface composition during electrochem-
ical measurements, which are qualitatively similar to the changes that occur during electrochemical
activation.
Figure 4 presents the natural log of kinetic current density versus overpotential (η) along
with the associated Butler-Volmer fits. The results from these fits were found to be in good agree-
ment with the experimental data if the α values were constrained between 0 and 1 but were not
required to sum to 1. Table 1 summarizes the transport and kinetics properties of the Fe3+/2+ redox
couple (10 mM total Fe concentration) for each carbon surface preparation. 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 H2O2 treated GC. All α values were
found to be between 0.25 and 0.45 and were also indistinguishable within two standard deviations.
Our initial expectation was that any surface treatment that oxidized GC surfaces would
result in increased electron-transfer rates.60,61,77,78 We did observe substantial increases in interfa-
cial capacitance, along with the emergence of pseudocapactive redox features, in GC electrodes
9
Figure 3: Koutecky-Levich analysis representing inverse current versus inverse square root ofrotation 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 treatedGC, (e) iron oxidation at H2O2 treated GC and (f) iron reduction at H2O2 treated GC in 5mM FeCl2and 5 mM FeCl3
10
Figure 4: Butler-Volmer fit for iron oxidation and reduction at (a) AC/IPA treated GC, (b) Electro-chemically treated GC and (c) H2O2 treated GC in 5mM FeCl2 and 5 mM FeCl3 in 0.5 M HCl
Table 1: Transport and kinetics properties of iron oxidation and reduction at GC electrodes. Stan-dard deviations are reported based on n=5 set of experiments.
Electrode DiffusivityDox (cm2/s)
DiffusivityDred (cm2/s)
Exchange current densityj0 (mA/cm2) αox αred
AC/IPAtreated GC (n=5)
2.2 x 10−6
(±0.4 x 10−6)3.7 x 10−6
(±0.4 x 10−6)0.07
(±0.01)0.33
(±0.09)0.31
(±0.05)Electrochemicallytreated GC (n=5)
3.8 x 10−6
(±0.5 x 10−6)2 x 10−6
(±0.5 x 10−6)0.90
(±0.07)0.35
(±0.07)0.44
(±0.09)H2O2
treated GC (n=5)2.0 x 10−6
(±0.6 x 10−6)2.5 x 10−6
(±1 x 10−6)0.08
(±0.04)0.26
(±0.06)0.36
(±0.01)
11
after the electrochemical and H2O2 treatments (see Supporting Information). These observations
are consistent with substantial surface oxidation and/or increased electrode surface area. Thus,
we anticipated enhanced electron-transfer kinetics in both cases relative to the AC/IPA treatment.
Instead, the electrochemical treatment resulted in faster rates while the H2O2 treatment gave elec-
tron transfer kinetics that were essentially indistinguishable from AC/IPA cleaning alone. These
unexpected results motivated us to further explore electrode surface chemistry using Raman spec-
troscopy and XPS.
Figure 5 presents Raman microscopy 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 is also indicative of 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 H2O2 treated GC was ≈ 2.1 while that for the electrochemically treated GC was
1.7. This intensity ratio of D to G band has been reported 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
essentially amorphous, randomly oriented graphite nanocrystallites. 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 mea-
surements on carbon electrodes after each treatment. Figure 6 presents a region of the XPS survey
scan data for carbon electrodes based on their surface treatments. Surface oxidation was 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.
12
Figure 5: Raman data depicting D band at 1330 cm−1 and G band at 1590 cm−1 for AC/IPA treated,electrochemically treated and H2O2 treated GC electrodes. The intensities are normalized to themaxmimum intensity of G band for each electrode type.
These data show, then, that indeed the electrochemical and H2O2 treatments resulted in increased
surface oxidation, although the difference between all three treatments was small. More impor-
tantly, it is clear that the degree of oxidation of carbon alone does not correlate well with the rel-
ative kinetics of electron transfer for Fe3+/2+. Instead, we conclude that specific surface functional
groups are responsible for catalysis in this system.
Figure 6: XPS data presenting intensity as a function of binding energy(eV) for AC/IPA treated,Electrochemically treated and H2O2 treated GC electrodes representing carbon and oxygen peaks.
13
We collected high-resolution XPS data to elucidate the relative amounts of various types
of oxidized surface species, under the hypothesis that one or more functionality is responsible for
catalyzing Fe3+/2+ redox chemistry. Figure 7(a) presents a representative XPS peak deconvolution
performed on the C 1s spectrum for H2O2 treated GC. These data were fit to find the relative ratios
of graphitic groups (285 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 com-
prised 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 ox-
idation. 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 electrochemical treatment
was found to increase the relative proportion of carbonyl and carboxylic groups, while decreasing
the proportion of alcohol groups (likely via further oxidation of alcohols to carbonyls or carboxly-
ates). By contrast, the H2O2 treated electrodes slightly increased the fraction of carboxylic groups,
greatly increased the fraction of alcoholic groups, and did not significantly alter the fraction of
carbonyl groups.
The XPS data clearly indicate that carbonyls are the only oxidized surface functionality
whose coverage increases in electrochemically activated GC, but not in H2O2 treated GC. Thus,
we conclude that 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 also found to
correlate with carbonyl coverage.45,46,57 One possible mechanistic rationale for this enhancement
involves the formation of bridging complexes mediated by carbonyls. This picture is attractive for
its similarity to the comparatively well-established catalysis of transition metal redox chemistry by
forming bridging complexes with adsorbed anions at noble metal electrodes.83–86
Based on further analogies to solution-phase coordination chemistry, a single carbonyl
group is less likely to bind Fe centers than multiple adjacent carbonyl species resembling, e.g.,
14
Figure 7: (a) Representative XPS deconvolution data for H2O2 treated GC fitted after backgroundsubtraction to graphitic groups (285 eV), alcoholic groups (286 eV), carbonyl groups (287 eV) andcarboxylic groups (288.6 eV). The residual curve at the top indicates error on peak fitting to thehigh resolution carbon peak, (b) Bar chart of various functional groups present in AC/IPA treated,electrochemically treated and H2O2 treated GC electrodes obtained from carbon peak deconvolu-tion using XPS.
para-quinones (or hydroquinones) or 1,3-diones. This may help explain why electron-transfer rates
are faster in electrochemically activated GC relative to AC/IPA or H2O2 treated GC, even though
each electrode surface contains carbonyl groups. As total carbonyl surface coverage increases, the
statistical likelihood of near-adjacent carbonyl groups also increases. We speculate that the cat-
alytic mechanism may therefore involve self-catalysis by Fe species that are weakly coordinated
to the carbon surface by two or more carbonyl groups. Surface coordination could then facili-
tate 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 sur-
faces through pi stacking.59 It also 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.87,88 Further improvements in our ability to control the spe-
cific surface chemistry of model carbon electrodes would be advantageous for probing this and
other mechanistic hypothesis, and would in turn support the design of functionalization strategies
15
that could be used to minimize overpotential losses in functional RFBs.
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 pre-
treatments. We found that electrochemical activation and chemical treatments with concentrated
H2O2 both increased the degree of carbon surface oxidation, but only the former resulted in en-
hanced kinetics. XPS data showed that electrochemical activation increased the proportion of
carbonyl groups on the surface, whereas H2O2 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 kinetics of flow bat-
teries, even for well-studied coordination complexes involving only a single electron transfer. Fur-
ther work is warranted to elucidate the precise surface chemistry responsible for catalysis in Fe-
based electrolytes and to determine whether the associated mechanism is general to all transition
metal aquo compounds of interest for RFB applications. These insights will help motivate im-
proved 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.
16
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Supporting Information
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]
Tabulated Fe3+/2+ kinetics data from literature
Table S1 present reported kinetics data for Fe3+/2+ redox chemistry at carbon electrodes. The rate
constants, all of which were collected using RDE voltammetry, varied over nearly three orders of
magnitude. Reported α values are close to 0.5 in most cases, while some reports did not explicitly
include α values.
Peak current vs. scan rate data
We performed cyclic voltammetry over a series of scan rates ranging from 10 to 200 mV/s (Fig-
ure S1) and collected values of anodic and cathodic peak currents. Figure S2 plots peak current
versus square root of scan rate, which show linear behavior as expected from a diffusional process.
S1
Table S1: Kinetics of Fe3+/2+ as reported in literature
Electrode Rate constantk0 (cm/s) α
Measuringtechnique
Supportingelectrolyte [Fe] Ref.
GC 4.4 x 10−3 0.5 RDE 0.1 M HCl 1
GC 1 x 10−3 0.5 RDE 0.3 M HCl 1
GC 2.9 x 10−3 0.5 RDE 0.1 M HNO31
GC >10−2 0.57 RDE 0.1-1 M HClO41
GC 1.3 x 10−3 0.5 RDE 0.1 M H2SO41
Carbon paste 5.4 x 10−5 0.63 RDE 1 M H2SO42
Carbon paste 8.5 x 10−5 RDE 0.1 M HCl 2
GC 2.3 x 10−3 RDE 0.2 M HClO4 5 mM 3
GC 2.3 x 10−4 RDE 1 M NaCl 0.1 M 4
Pyrolytic graphite 5.2 x 10−4 RDE 4 M HCl 0.1 M 5
Figure S1: Cyclic voltammograms of (a) AC/IPA treated GC, (b) electrochemically treated GC and(c) H2O2 treated GC electrodes depicting scan rate dependence in an electrolyte containing 5mMFeCl2 and 5mM FeCl3 in 0.5 M HCl(aq).
Figure S2: Peak current density vs. square root of scan rate for (a) AC/IPA treated GC, (b) electro-chemically treated GC and (c) H2O2 treated GC electrodes in an electrolyte containing 5mM FeCl2and 5mM FeCl3in 0.5 M HCl(aq).
S2
Kinetics estimates from CV Data
To estimate electron-transfer kinetics directly from CV data, we followed the method reported by
Nicholson, who showed that the relationship between ∆Ep and scan rate can be used to extract
k0.6 We first extracted the following mathematical relationship between ∆Ep and an empirical
parameter Ψ from Nicholson’s report:
∆Ep · n = 0.051 + 0.0345ψ−0.671 (1)
Nicholson showed that Ψ can be related to the apparent electron-transfer rate constant k0,app via
the following equation:
ψ = ν−12k0,app
[πnFD
RT
]− 12
(2)
such that a plot of Ψ vs ν−0.5 should give a line with y intercept that is near zero and a slope that is
proportional to k0. Figure S3 shows the corresponding plots for Fe3+/2+ voltammetric data at carbon
electrodes prepared by the three different methods over an extended range of scan rates from 10–
3000 mV/s. The data for AC/IPA and electrochemically treated carbon both gave excellent linear
Figure S3: Plots of ψ versus inverse square root of scan rate along with least-squares fits over arange of scan rates from 10–3000 mV/s for (a) AC/IPA treated GC, (b) Electrochemically treatedGC and (c) H2O2 treated GC
fits, whereas the H2O2 treated carbon gave a considerably poorer fit, which is consistent with the
greater relative uncertainty we found in the RDE results for these surfaces.
Table S2 collects apparent k0 values (k0,app) extracted from the slopes of these data along
S3
with the corresponding j0 values for a 5 mM reactant concentration under the assumption that
the diffusivity was 4 x 10−6 cm2/s and the relationship between k0,app and j0 obeys the following
equation:
j0 = nFCk0,app (3)
These j0 values agree reasonably well with those reported from RDE results in the main text.
However, because these results relied on a mathematical fit to Nicholson’s empirical relation, we
recommend that they be treated only as an order-of-magnitude estimate of reaction kinetics.
Table S2: j0 and k0,app obtained from analysis of voltammetry data
Electrodek0,app(cm/s)
j0(mA/cm2)
AC/IPA treated GC 2.4 x 10−4 0.12Electrochemically treated GC 3.7 x 10−3 1.8
H2O2 treated GC 1.2 x 10−4 0.06
Results of conventional Butler-Volmer fits with contrained α values
Figure S4 reports conventional Butler-Volmer fits of kinetics data obtained by constraining the fit
to αox + αred = 1. These results show a clear deviation in the fits for all 3 electrode surface
treatments, which were resolved by allowing the symmetry factors to vary arbitrarily between 0
and 1 as shown in the main text. Such a deviation is consistent with a reaction mechanism involving
both electrochemical and chemical elementary steps.
Residual surface roughness
As a part of our experimental procedure, we polished the surface of GC electrodes before em-
ploying various activation protocols. Although polishing is an important step in removing adventi-
tious species from GC surfaces, we were unable to consistently eliminate polishing damage in the
form of sub-micron scratches, as shown in Figure S5. This residual surface structure complicates
S4
Figure S4: Transport free polarization data for (a) AC/IPA treated GC, (b)Electrochemically treatedGC and (c) H2O2 treated GC electrodes in RFB electrolyte containing 5mM FeCl2 and 5mM FeCl3in 0.5 M HCl(aq). The black lines correspond to fits using the Butler-Volmer equation where thesymmetry factors were constrained to αox + αred = 1.
quantitative analysis of interfacial electron transfer rates due to the increase in electrochemically
accessible surface area per unit geometric area. However, even GC electrodes with completely
smooth surfaces are likely to exhibit considerable site heterogeneity arising from the high density
of crystallographic defects, which makes the determination of the “intrinsic” kinetics of these sur-
faces difficult. In fact, we argue that this roughness may in fact be advantageous for the study of
RFB electrokinetics, in that it is likely to expose a diversity of interfacial active sites as would be
expected to exist in structured carbon materials that are used in functional RFBs.
Figure S5: Scanning electron micrographs of (a) AC/IPA treated GC, (b) Electrochemically treatedGC and (c) H2O2 treated GC
S5
Effect of surface treatment on interfacial capacitance
Figure S6 overlays CV data of GC electrodes in 0.5 M H2SO4 solution over the potential range
from -0.2 to 0.8 V vs Ag/AgCl. In the absence of a redox couple, the data in this range are repre-
Figure S6: Current density versus potential data for AC/IPA treated, electrochemically treated, andH2O2 treated GC electrodes in 0.5 M H2SO4(Aq) solution.
sentative of the interfacial capacitance and/or surface redox chemistry of the electrodes. Whereas
prior work involving similar solvent treatments resulted in interfacial capacitance values on the
order of 50–100 µF/cm2,7 we found AC/IPA treated electrodes gave values on the order of 400
µF/cm2. We attribute this difference to residual surface roughness from alumina polishing, as dis-
cussed in the previous section. The apparent interfacial capacitance also increased markedly with
electrochemical and H2O2 treatments (by factors of 3 and 5, respectively). Moreover, reversible
voltammetric features emerged between 0.2 and 0.5 V vs Ag/AgCl, which can be attributed to
pseudocapacitive redox chemistry of oxidized surface species. These data collectively agree with
XPS results showing that electrochemical and H2O2 treatments both oxidize GC surfaces, where
the latter treatment introduces somewhat more surface oxygen functionality in spite of the lack of
increase in Fe3+/2+ electron transfer kinetics.
S6
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