Impact of Surface Carbonyl- and Hydroxyl-Group Concentrations on

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C: Energy Conversion and Storage; Energy and Charge Transport

Impact of Surface Carbonyl- and Hydroxyl- Group Concentration on Electrode Kinetics in an All-Vanadium Redox Flow Battery Yue Li, Javier Parrondo, Shrihari Sankarasubramanian, and Vijay K. Ramani J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b11874 • Publication Date (Web): 25 Feb 2019 Downloaded from http://pubs.acs.org on March 1, 2019

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The Journal of Physical Chemistry

Impact of Surface Carbonyl- and Hydroxyl- Group Concentration on Electrode Kinetics in an All-Vanadium Redox Flow Battery Yue Li1,2, Javier Parrondo1,3, Shrihari Sankarasubramanian1, Vijay Ramani* Center for Solar Energy and Energy Storage and Department of Energy, Environmental and Chemical Engineering, Washington University in St. Louis, One Brookings Dr., St. Louis, MO 63130, USA. * Corresponding Author’s email: [email protected] Abstract This study investigates the effect of thermal activation of all-vanadium redox flow battery (RFB) carbon-felt electrodes on their electrode kinetics. Using x-ray photoelectron spectroscopy (XPS), thermal activation is shown to increase the content of the C-OH group, decrease the content of the C=O group and not affect the O-C=O group, with all these surface moieties already being present in the non-activated carbon-felt. Rotating disk electrode (RDE) studies were performed using custom electrodes fabricated using the carbon-felt to investigate the kinetics of the V2+/V3+ and VO2+/VO2+ redox couples in H2SO4 and to deconvolute the impact of the thermal activation on electrode kinetics. We demonstrate that V2+/V3+ kinetics is sluggish compared to VO2+/VO2+ kinetics (equilibrium rate constant (𝑘0) = 4.98x10-8 m.s-1 vs. 8.81x10-8 m.s-1) and that thermal activation enhanced V2+/V3+ kinetics while inhibiting VO2+/VO2+ kinetics. The enhancement in V2+/V3+ kinetics was attributed to the oxygen-containing groups -C-OH added during thermal activation. Using thermally-activated carbon-felt V2+/V3+ electrodes yielded an overall increase in energy efficiency from 75±3.7% to 90±4.5% and voltage efficiency from 76±4% to 92±4.6%

. On the other hand, using thermally-activated carbon-felt VO2+/VO2+

electrodes lowered energy efficiency from 75±4% to 73±3.6% and voltage efficiency from

These authors contributed equally to this study Present address: Redox Power Systems, College Park, MD 20742, United states 3 Present address: Nissan Technical Center North America, Farmington Hills, MI 48331, United States 1 2

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76±4% to 74±4% . The optimal combination of thermally-activated carbon-felt V2+/V3+ electrodes and untreated carbon-felt VO2+/VO2+ electrodes resulted in the most efficient RFB configuration. 1. Introduction The accelerating, wide-spread adoption of inherently intermittent renewable electricity generation platforms such as wind turbines and solar PV modules necessitates the concurrent deployment of large-scale energy storage solutions to avoid demand-supply gaps and disruptions.

1-4

Redox flow batteries (RFBs) are a leading contender for this

application due to their myriad advantages in terms of cost, flexible siting requirements and their ability to be scaled-up sub-linearly due to their inherent decoupling of power and energy 4. These advantages have led to an extensive research effort encompassing a variety of RFB chemistries including all-vanadium, iron-chromium, polysulfide-bromine, soluble metalbromine and others 4. The all-vanadium redox flow battery (VRFB), first introduced by Skyllas-Kazacos is furthest along in terms of commercial development. This system employs a V2+/V3+ anolyte and a VO2+/VO2+ catholyte typically supported by mineral acids such as H2SO4 and/or HCl

5-6.

The standard half-cell reactions and their corresponding

reduction potentials against the standard hydrogen electrode are as follows: V3+ + 1e- ↔ V2+ (E0 = -0.26V)

(1)

VO2+ + 1e- ↔ VO2+ (E0 = 1.00V)

(2)


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Extensive research has been conducted on components such as the electrode 7-8, electrolyte 9-10,

and membrane

11-13

with a view towards improving the overall performance of the

VRFB. In terms of electrodes, carbon-based materials including carbon-felts, carbon paper and carbon fibers have been investigated given their wide operational potential range and acceptable stability in acidic environments 3. Furthermore, improvement and optimization of these electrodes has been attempted by seeking to understand the kinetics and mechanisms of the electrochemical reactions taking place at these carbon-based electrodes in a VRFB 14-22. Gattrell et al. studied the reduction mechanism of VO2+ to VO2+ in acidic media at high overpotentials 22. By using glassy carbon (GC) and graphite disc electrodes in a rotating disk electrode (RDE) cell, they proposed a multistep chemicalelectrochemical-chemical (CEC) mechanism at low overpotentials which changed to a multistep electrochemical-chemical-chemical (ECC) mechanism at higher overpotentials 21.

Maruyama et al. studied the mechanism of the reduction of VO2+ to VO2+ using GC

RDEs

20

and proposed that the reduction of VO2+ involved both C=O and C-OH surface

functional groups. Discussions regarding these mechanisms are ongoing. There have been attempts to catalyze the electrode reactions by chemical modification of the electrodes or by the addition of suitable catalysts. Most of the materials used as catalysts were metals and metal oxides 23-32. Some representative examples are highlighted below. Ejigu et al. investigated the electrocatalytic activity of a composite nitrogen-doped, reduced graphene oxide/Mn3O4 (N-rGO/Mn3O4) toward the VO2+/VO2+ redox couple 24. They showed that the metal-nitrogen-carbon interaction was key to the catalytic performance and that the optimum Mn3O4 loading was ~24%. It was also concluded in the work of Kim et al. that cells with carbon-felt electrodes with Mn3O4 nanoparticles 3 ACS Paragon Plus Environment


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dispersed on the surface yielded increased coulombic, voltage, and energy efficiencies compared to cells employing bare carbon-felts

27.

In the study of Pham et al., MoO2

nanocrystals on interconnected mesocellular carbon foam (MoO2/MSU-F-C) yielded higher peak currents and smaller potential separation between anodic and cathodic peaks in cyclic voltammetry (CV) for the VO2+/VO2+ redox reaction and yielded higher energy and voltage efficiencies in flow battery tests, compared to bare mesocellular carbon foam electrodes 28. Bismuth modified carbon-felt was found to enhance the reversibility of the V2+/V3+ redox reactions and the long-term cycling performance of the electrode were significantly enhanced 23. Other kinds of metals and metal oxides, including CeO2 32, WO3 31,

Pt 29, Ir 26, etc., have also been used to decorate carbon-based electrode materials.

Parallel attempts at electrode modification aimed to introduce a variety of functional groups on the surface to enhance the reaction kinetics and battery performance. Such functional groups typically included nitrogen-containing groups

33-43

and oxygen-

containing groups 44-56. In the work of Jin et al. 35, nitrogen-doped graphene sheets (NGS) were prepared and quaternary nitrogen was identified as the catalytic active center for the VO2+/VO2+ redox reaction. NGS positive electrodes increased discharge capacity by 211% over pristine graphite, and by 34% over thermally oxidized graphite 37. Oxygen containing surface groups have been produced by a variety of methods including thermal treatment 46, acidic treatment 8, electrochemical treatment 51-56 and plasma treatment 47-50. The effect of introducing these groups is an area of active investigation. In the early work of SkyllasKazacos and Sun 8, carbon-felt treated at 400℃ for 30 hours increased the VRFB energy efficiency from 78% to 88%. They claimed that the increase in energy efficiency was due to the surface-active functional groups C ― O and C = O added by the thermal treatment. 4 ACS Paragon Plus Environment

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They concluded that the increased battery performance was due to the catalytic effect of the C ― O functional group towards the VO2+/VO2+ redox reaction. In contrast, Dixon et al.

47

stated that the C ― O functional group added to the carbon-felt surface by using

oxygen plasma had a slightly negative effect on the VO2+/VO2+ redox reaction and the enhancement of battery performance was attributed to the catalytic effects of C ― O groups towards the V2+/V3+ redox reaction. Xi et. al.

53

claimed that C ― O surface groups

introduced on carbon-felt by electro-oxidation were active towards the VO2+/VO2+ redox reaction and that this enhancement in kinetics increased the overall battery energy efficiency from ~72% to ~76%. In light of these conflicting results, it is necessary to clarify the types and amounts of oxygen-containing groups that can be introduced by using different treatment methods, to deconvolute the effect of introducing these functional groups on the kinetics of VO2+/VO2+ and V2+/V3+ redox couples, and to pinpoint the impact of the changes in the kinetics of each individual redox reaction on the overall performance of the VRFB. To address this situation, Bourke et al. conducted a series of investigations 51

using electrochemically treated carbon materials. These studies indicated that the

kinetics of the V2+/V3+ redox couple was enhanced by anodic treatment and inhibited by cathodic treatment of the electrode. In contrast, the kinetics of the VO2+/VO2+ redox couple was inhibited by anodic treatment and enhanced by cathodic treatment of the electrode. In summary, there exist three groups of observations on the effects of introducing oxygen surface groups on the kinetics of the VRFB redox reactions (V2+/V3+ and VO2+/VO2+). These observations are listed in the following table. Table 1 Effects of oxidation treatment of carbon electrodes on redox kinetics V2+/V3+:

Observations enhanced; VO2+/VO2+: enhanced

References 8, 46, 56, 44, 45, 48. 5

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V2+/V3+: enhanced; VO2+/VO2+: inhibited V2+/V3+: no mention; VO2+/VO2+: enhanced

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7, 51, 47. 53, 55, 49.

The aim of this work was to individually clarify the effect of thermal activation on the kinetics of the V2+/V3+ and VO2+/VO2+ couples. This was done using RDE linear sweep voltammograms that were subsequently validated by VRFB tests with asymmetric combinations of activated and non-activated V2+/V3+ and VO2+/VO2+ electrodes. The RDE results were fitted using the Marcus-Hush kinetic model and the solvent reorganization energy (𝜆) was used to quantify the impact of the electrolyte on the charge transfer kinetics and to establish the nature of charge transfer (inner-sphere vs. outer-sphere electron transfer). This insight into the effect of solvation (and hence the supporting electrolyte) is particularly important as it provides a framework to understand the reports of chloride or sulfate-chloride mixed electrolytes significantly enhancing all-V RFB performance.57-58 The RDE tests utilized custom-made disk-electrodes which only contained the active material of interest and electrochemically inactive binder. Thus, these electrodes exactly mimic the surface conditions on the carbon-felt. We show that the kinetics of the V2+/V3+ redox couple was significantly improved by electrode thermal activation, while the kinetics of the VO2+/VO2+ redox couple deteriorated. The results of RDE tests and VRFB tests mutually supported each other. X-ray photoelectron spectroscopy (XPS) measurements were conducted to pinpoint the amounts and types of oxygen-containing groups before and after the thermal-activation of the carbon-felts. C-OH groups introduced on the surface following thermal-activation could explain the improvement in the kinetics of the V2+/V3+ redox couple and in the overall performance of the flow battery. 2. Experimental 6 ACS Paragon Plus Environment

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2.1 Thermal activation Carbon-felt (SigraCELL GFA6, SGL carbon) was thermally activated by heating at 400ᵒC in air for 30 hours in a muffle furnace (Thermo Scientific; Lindberg/Blue Box Furnace). 2.2 Materials characterization XPS experiments were performed to analyze the surface chemical composition and elemental electronic environment of non-activated and thermally-activated carbon-felts. The measurements were performed on a Physical Electronics 5000 VersaProbe II scanning electron spectroscopy for chemical analysis (ESCA) instrument employing an AlK (1486.6 eV) X-ray source. Spectra were analyzed and deconvoluted using the IgorPro software package (version 6.0) with the baseline correction performed using a Shirley background function. Scanning electron microscopy was carried out using a FEI Nova NanoSEM 230 scanning electron microscope (SEM) with an attached energy dispersive analysis of X-rays (EDAX) detector. X-ray diffraction (XRD) was carried out using a Rigaku DMaxB instrument with a step size of 0.06⁰ and a dwell time of 2s. 2.3 Carbon-felt disk electrode preparation The standard approach to investigating the kinetics of an electrocatalyst employs a threeelectrode system with a thin-film working electrode wherein a slurry of the active material is deposited onto a glassy carbon (GC) disk electrode

29.

The kinetics measured in this

manner tends to deviate from observation in the actual device since the GC substrate also contributes to the measured kinetic currents. This problem was overcome by fabricating disk electrodes from pristine and thermally-activated carbon-felt using an optimal amount of binder. The composition of the fabricated carbon-felt disk electrode was optimized to 7 ACS Paragon Plus Environment


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20 wt.% of polyvinylidene fluoride (PVDF) binder and either thermally-activated or nonactivated carbon-felt powders. This composition was arrived at following the observation that when the PVDF content exceeded 20 wt.%, the resistance of the resultant disks became significant. The use of carbon-felt disk electrodes for RDE tests enabled the measurement of the electrocatalytic activity directly on the active material of interest without interference from any substrate.

Figure 1. Procedure for preparing custom-made carbon-felt disk electrodes. As shown in Figure 1, the carbon-felt disk electrodes were prepared by ball-milling and mixing 0.03g PVDF and 1mL acetone in a strengthened glass bottle with two zirconia balls, agitated for 2 hours in a vortex mixer. In parallel, thermally-activated or non-activated carbon-felt was ground into a powder using a mortar for 20 mins. Following 2 hours of agitation, PVDF completely dissolved in acetone and 0.12g of carbon-felt powder was added to the bottle and further agitated for 10 hours to obtain a uniformly dispersed carbonfelt-PVDF slurry. The slurry was dried, further ground and then dried again at 60ᵒC for 30


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mins. 0.1g of the dry carbon-felt-PVDF composite was compressed for 2 hours in a custom stainless-steel die in a press under a pressure of 750MPa. The resultant carbon-felt disk was polished on a nylon cloth with 5μm alumina suspension to smoothen it and ensure fit in the RDE tip. This procedure was followed for preparing disk electrodes with both thermally-activated and non-activated carbon-felts. 2.4 Electrochemical measurements All electrochemical measurements reported here were carried out in a water jacketed, 5neck cell with suitable openings for the working, counter and reference electrodes, the gas purge line inlet and a gas outlet (Pine instruments, AKCELL3). The water jacket was used in conjunction with a recirculating bath to carry out electrochemical measurements at different temperatures. An RDE assembly was used to mount the custom-made carbon-felt disk electrode. The counter electrode consisted of a Pt spiral attached to a Pt wire in a fritted glass tube filled with the electrolyte. A saturated calomel reference electrode was employed (+0.2444V vs. SHE at 25ᵒC). Linear sweep voltammetry (LSV) measurements were performed on the custom-made, carbon-felt disk electrodes for both V2+/V3+ and VO2+/VO2+ redox couples. LSVs for VO2+/VO2+ using disks made of thermally-activated and non-activated carbon-felt powders were measured at the rate of 5mV/s from 0.6V to 1.2V vs. SCE in a mixed solution containing 75mM VO2+, 75mM VO2+, and 0.1M H2SO4. LSVs for the V2+/V3+ redox couple were conducted at 5mV/s in a mixed solution composed of 75mM V2+, 75mM V3+, and 0.1M H2SO4. The potential window was limited from 0.56V to -0.36V vs. SCE, to avoid hydrogen evolution and oxidation of V3+ to V4+. All measurements were carried out under a N2 blanket. 2.5 Vanadium redox flow battery tests 9 ACS Paragon Plus Environment


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Single cell redox flow battery testing was carried out in a 10cm2 acid-resistant hardware. The single cell was assembled by sandwiching a Nafion® 212 membrane between two carbon-felt electrodes that were either non-activated or thermally activated. VRFB testing was performed using 1.5M V2+/V3+ and 1.5M VO2+/VO2+ as the negative and positive electrolytes, respectively, in 3M H2SO4. The electrolytes were recirculated using two peristaltic pumps. Charge-discharge tests were conducted at a constant current density of 25 mA/cm2 at 25ᵒC. The cut-off voltages for the charge and discharge cycles were 1.65V and 0.65V respectively. The maximum cut-off voltage (1.65 V) used during charging was selected based on the maximum potential the cell could hold without excessive oxygen evolution. The lower cut-off cell voltage (0.65 V) during discharging was selected on the basis of allowing complete discharge of the flow battery. The metrics employed to evaluate performance were: a) Coulombic efficiency (CE), defined as the discharge capacity divided by the charge capacity, b) Energy efficiency (EE), defined as the discharge energy divided by the charge energy, and c) Voltage efficiency (VE), calculated as VE = EE/CE. To verify the results obtained from previous kinetic studies, we investigated the effect of thermal activation on the VO2+/VO2+ and V2+/V3+ redox couples separately. There were four different arrangements of positive / negative electrodes that we used in flow battery tests – 1) thermally-activated / non-activated, 2) non-activated / thermally-activated, 3) thermally-activated / thermally-activated and 4) and non-activated / non-activated. 3. Results and discussion 3.1 Characterization of the carbon-felt disk electrodes


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The custom-made carbon-felt disk electrodes were characterized by SEM, XRD and XPS. The disks were found to be largely non-porous as seen from the SEM images depicted in Figure 2. Through XRD spectra presented in ESI section S1, the disks were found to exhibit short range order with graphite, graphite oxide and reduced graphite oxide peaks being observed but the broadening of the peaks indicated a lack of long-range order while the noisy baseline indicated interspersion of amorphous phases between the ordered domains. The ball-milling process produced uniformly small particles of carbon in both the thermally-activated and non-activated cases with the particle size being

Impact of Surface Carbonyl- and Hydroxyl-Group Concentrations on

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