Interaction of vanadium species with a functionalized graphite electrode: A combined theoretical and experimental study for flow battery applications

Journal of Power Sources 420 (2019) 134–142

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Interaction of vanadium species with a functionalized graphite electrode: A combined theoretical and experimental study for flow battery applications

T

Mohadeseh Meskinfam Langroudia, Christian Silvio Pomellib,∗, Romano Gigliolic, Cinzia Chiappeb, Maida Aysla Costa de Oliveirad, Barbara Mecherid, Silvia Licocciad, Alessandra D'Epifaniod a

Dept. Industrial Engineering, University of Florence, Via Santa Marta 3, 50139, Florence, Italy Dept. of Pharmacy, University of Pisa, Via Bonanno 33, 56125, Pisa, Italy c DESTEC, University of Pisa, Largo Lazzarino, 56122, Pisa, Italy d Dept. Chemical Science and Technologies, University of Rome, Tor Vergata, Rome, Italy b

HIGHLIGHTS

study of model systems of graphite electrode/V /V redox couple system. • DFT behavior of doped carbon-based electrodes for redox flow battery. • Electrochemical is to provide more active sites for adsorption of V /V redox couple. • Aim • Beneficial effect of electrode doping with N and O. 2+

2+

3+

3+

ARTICLE INFO

ABSTRACT

Keywords: Vanadium redox flow battery V3+/V2+ redox couple Electrode functionalization Density functional theory Electrochemistry

Vanadium redox flow battery (VRFB) is one of the most promising large-scale energy storage system; however, a widespread VRFB development is still limited by the poor electrochemical activity of graphite electrodes and a poor understanding of redox reactions occurring at electrode/electrolyte interface. In this work, DFT was performed to study the first solvation shell structure of all vanadium ions and to investigate the reactivity of modified graphite electrodes toward the V2+/V3+ redox species. The results suggest that the presence of oxygen and nitrogen functionalities at the electrode edges provides more active sites for adsorption of the V2+/V3+ redox couple, and therefore improve electron transfer kinetics. These results have been experimentally validated by means of Cyclic Voltammetry and Electrochemical Impedance Spectroscopy with carbon black electrode having different density of oxygen and nitrogen-containing surface groups.

1. Introduction Nowadays, the requirement of renewable energy resources is vigorously rising due to the reduction of reservoirs of fossil fuels, environmental issues, and increasing demand for production of energy. One of the big challenges for the commercialization of renewable sources is their intermittent natural disposition, which prevents them to provide stable and continuous power supply. One approach to solve this problem is the implementation of the electrical energy storage systems which have the capability to accumulate energy and make it accessible at the time when is needed [1]. One of the promising candidates, which have received considerable ∗

attention for large-scale electrical energy storage systems, is a Redox flow battery (RFB) system. Some of its advantages with respect to conventional batteries are their tolerance to overcharge/over-discharge, long lifespan, and their capability to frequently convert electrical energy to chemical energy and contrariwise [2,3]. Another unique advantage of RFBs is decoupled power and energy, because volume of electrolyte determines energy storage capacity, while power depends on the electrode surface area and on the number of cell stacks [4]. Among to the various types of RFBs that are under investigation by the scientific community, all-vanadium RFBs (VRFBs) have received significant attention due to their many advantages, including rapid

Corresponding author. E-mail addresses: [email protected] (C.S. Pomelli), [email protected] (A. D'Epifanio).

https://doi.org/10.1016/j.jpowsour.2019.02.083 Received 25 October 2018; Received in revised form 28 January 2019; Accepted 23 February 2019 0378-7753/ © 2019 Elsevier B.V. All rights reserved.

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response time, long lifetime, reversibility, low-cross contamination, design flexibility, deep discharge capability, and low retention cost [5,6]. The VRFBs system is made up of VO2+(IV)/VO2+(V) redox couple in the positive half-cell and V2+/V3+ in the negative half-cell as active species dissolved in an acidic electrolyte and separated by an ionexchange membrane. VRFB system has a standard voltage of 1.26 V, which is generated by the following reactions:

Cathode: VO2 + + H2 O

e

Charge Discharge

simulate their interaction with vanadium complexes in reducing environment and enhancing the electron transfer kinetics. The results obtained from theoretical calculations have been validated by cyclic voltammetry and electrochemical impedance spectroscopy at the interface of carbon black electrodes having different densities of oxygen and nitrogen-containing surface groups in vanadiumion-containing electrolyte solutions.

VO+2 Eo = 1. 00V

2. Experimental

+ 2H+ Anode: V 3+ + e

Charge Discharge

2.1. Materials

V 2 + Eo =

Overall: VO2 + + H2 O + V 3 +

Charge Discharge

0. 26V

Black pearls 2000 (BP) were acquired from Cabot Corporation and partly used without further treatments (BP). In addition, BPs were modified by a two-step treatment; the first step consisted in heating BPs in concentrated HNO3 (65 wt%) under reflux at 90 °C for 16 h, followed by filtration and thorough washing with distilled water until reaching to the neutral pH. The powder was dried in an oven at 70 °C overnight, and consecutively grounded in an agate mortar, obtaining a sample labeled as BP (O)-modified. Afterwards, BP(O)-modified was annealed in a tubular oven at T = 400 °C (heating rate 5 °C min−1) under a flow of anhydrous ammonia for 4 h, and the final product was labeled as BP (N). Table 1 reports the elemental analysis results of BP-unmodified, BP (O)-modified, and BP (N) modified electrodes, as previously reported [34].

VO+2 Eo = 1. 26V

+ 2H+ + V 2 + One of the main issues, which limit their energy storage efficiency, is their slow rate of electron transfer kinetics at the electrode-electrolyte interface, which hinders their widespread commercialization. The crucial role of an electrode is providing active sites for charge transfer of vanadium couples. Therefore, to improve the electron transfer kinetics of redox reactions and to enhance energy and power densities of VRFB, the investigation of the atomic details in the interaction of the electrode surface and vanadium ions in the electrolyte solution is required [7]. There are various experimental and theoretical investigations on vanadium complexes [8–16], but the structure of hydrated vanadium ions at their different states of oxidation are still not well defined. Therefore, in the first step, we performed the first principle based electronic structure calculations for understanding the atomic structure of vanadium species in the aqueous electrolyte, their coordination number, hydration structures, and charge distribution of all four vanadium cations in bulk water. In the recent past, some experimental studies have been done to better understand the kinetics and redox transformation of V2+/V3+ and VO2+/VO2+ couples at carbon electrode [6–21]. The carbon-based electrode such as graphite, graphene and carbon nanotubes have been widely used as VRFB electrodes, due to their large operating potential range, low cost, high conductivity, and electrochemical resistance in concentrated acidic environments [22–27]. Despite their advantages, carbon-based electrodes have low electrochemical activity and poor kinetic reversibility. To overcome these problems, a general understanding of structure and properties of the electrolyte/electrode interface and an investigation of reactivity of carbon-based electrodes towards vanadium redox couples at microscale are needed. Until now, for improving the electrochemical activity of carbon-based electrodes different modification methods have been reported, including the acid treatment [28], electrochemical oxidation [29], heat treatment [30], and surface treatment by introducing the functional groups. Some experimental studies have confirmed that the presence of oxygen defects such as C-O, C=O, OH, and COOH groups [19–21] at the edge site of graphite electrodes allow enhancing the reaction kinetics for both redox couples. Also, nitrogen-functionalized carbon nanostructures have recently attracted the attention of several research groups [31–33], but the overall effect of nitrogenfunctionalities on the electron transfer kinetics of V2+//V3+ redox couple is far to be fully understood. In this work, we performed some DFT calculations in order to investigate the reactivity of two different edge surfaces of modified models of the graphite-like surface by means of two minimal models consisting of three condensed benzene rings: anthracene (zigzag) and phenanthrene (armchair) toward the V2+/V3+ redox species. This work was mainly focused on the enhancement of electrochemical activity of graphite electrode toward the V2+/V3+ couple by introducing oxygen and nitrogen containing groups at the surface of nanostructured carbon. We performed some calculations using the naive framework of frontier orbitals: an electron is added to the models of graphite in order to

2.2. Methods: electrochemical characterization The electrochemical behavior of the three samples (BP-unmodified, BP(O)modified, BP(N)-modified) toward in term of electrochemistry of V2+/V3+ and VO2+/VO2+ couples was studied by Cyclic Voltammetry (CV) and Electrochemical Impedance Spectroscopy (EIS). A conventional three-electrode cell was used with a reference electrode (saturated calomel-SCE, Amel 805/SCG/12), a counter electrode (platinum wire, Amel 805/SPG/12) and a working electrode consisting of a glassy carbon disk (GC, 0.196 cm2 area), modified with a BP-based ink at 0.3 mgcm−2 powder loading. The catalyst ink was prepared by dispersing 4 mg of BP-unmodified, BP(O)-modified, and BP(N)-modified in 455 μL of a solution containing water, ethanol and 5.0 wt % of Nafion (Sigma Aldrich). 7 μL of the ink were deposited on GC and dried at room temperature. The CV measurements were performed in the potential windows of +2.0 ÷ −1.3 (vs. SCE) with a scan rate of 25 mV s−1. The electrodes were immersed in a N2-saturated solution of 1 mol L−1 VOSO4 and 3 M H2SO4 at room temperature, and connected to a VMP3 (BioLogic Science Instruments) Potentiostat controlled by a computer through EC-Lab V10.18 software. EIS was recorded in the same conditions for BP-unmodified, BP(O)modified and BP(N)-modified electrodes, under different potentials with scan rate of 0 mVs−1 to −650 mVs-1, and a frequency from 200 KHz to 100 mHz at 10 mV sinusoidal amplitude. 2.3. Computational details Calculations have been performed using the Gaussian 16 package at the DFT theory level with the B3LYP functional. A universal solvation model with water as a solvent (implemented in Gaussian program as Table 1 Elemental analysis results and BET surface area of BP electrodes before and after treatments [36].

135

Sample

C wt. %

O wt. %

N wt. %

BET SA m2g−1

BP-unmodified BP(O)-modified BP(N)-modified

99.15 86.77 88.98

0.74 12.60 3.32

0.11 0.63 7.70

1359 1381 1317

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Fig. 1. Optimized structures of (a) [V(H2O)6]2+ and [V(H2O)6]3+, (b) [VO(H2O)5]2+, (c) VO2 (H2O)4+, and (d) VO2(H2O)3+ complexes, and their first solvation shell water molecules.

IEFPCM model) were applied to all the structures. The two basis sets of 6-31G (d, p) and 6-311 + G (d, p) have been used for studying the all hydrated structures of vanadium cations. While, 6-311 + G (d, p) has been used for understanding the reactivity of pristine and modified graphite models toward the V2+/V3+ redox couple. All the geometries have been optimized in accord with the level of the theory and characterized as minima with analytical frequencies calculation.

Table 2 The octahedral geometries of V2+ and V3+ (RV-Oa) (all in Å).

V2+ V3+

Computational

Experimental

2.21b, 2.19c, 2.21d, 2.20e 2.07b, 2.02c, 2.09h, 2.06j

2.15f, 2.13g 2.00j

a Average value of the bond distance between vanadium and oxygen atom of water molecules in Å. b Calculated in this work with B3LYP/6-31G(d, p)/IEFPCM in Gaussian. c Calculated in this work with B3LYP/6-311 + G(d, p)/IEFPCM in Gaussian. d [39]. e [40]. f [37]. g [38]. h [40]. j [38].

3. Results and discussion 3.1. Theoretical results: DFT calculations 3.1.1. Hydration shell geometry According to the literature analysis [35,36] and ab initio studies [37,38], for both V2+ and V3+ ions, the first hydration shell presents a fair regular octahedral geometry. Fig. 1(a) represents the first hydration shell structure around the vanadium center for both V2+and V3+ ions. While, Table 2 reports the average value of RV-O calculated in this work with some other available literature data. The trivalent vanadium 136

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Table 3 The average structural parameters for [VO2 (H2O)4]+ and [VO2(H2O)3]+ complexes(all in Å). Computational RV-O 2+

a b c g d e f h

RV-O

ax b

[VO2 (H2O)5] [VO2(H2O)4]+ [VO2 (H2O)3]+

Experimental

c

d

e

1.55 , 1.58 , 1.57 , 1.57 1.58b, 1.59c, 1.61g 1.58b, 1.58c, 1.61g

ax b

w (a)

RV-O c

d

e

b

2.22 , 2.23 2.17 , 2.32 2.22b, 2.34c, 2.28g 2.13b, 2.13c, 2.10g

c

d

2.05 , 2.06 , 2.03 , 2.12 2.06b, 2.08c, 2.13g 2.08b, 2.08c, 2.13g

e

ax f

1.59 1.63h 1.64g

RV-O

ax

w (a)

f

RV-O

ax f

2.22 2.23h 2.13g

2.04 2.01h 2.10g

Average distance of vanadium with Oaxw which indicate the axial oxygen atom of water molecules in Å. Calculated in this work with B3LYP/6-31G (d, p)/IEFPCM in Gaussian. Calculated in this work with B3LYP/6-311 + G (d, p)/IEFPCM in Gaussian. [43]. [44]. [45]. [46]. [41].

complex is more distorted than divalent, which can be due to its higher charge density. In addition, the average bond distance between the oxygen atom of the water molecule and V3+ is nearly 0.1A° shorter than V2+ which is in good agreement with value reported by Cotton et al. (0.14A°) [36]. Due to the higher charge density of V3+, the electrostatic attraction with water molecules is stronger and consequently the ligands get closer to the V3+ ion than to V2+. In the case of VO2+ (aq) cation, the first hydration shell has five water molecules with an octahedral geometry, which shows in Fig. 1(b). The average first-shell structural parameters of VO2+ and VO2+ ions along with some literature data are listed in Table 3. The average distance of equatorial water molecules is shorter than the axial water molecules from central vanadium ion of VO2+. Furthermore, due to the repulsion that is caused by oxo group atom of VO2+, the equatorial plane is distorted. The average bond distance of vanadium and oxygen of VO2+ cation in comparison to V3+ and V2+ ions and oxygen from first-shell water molecules is shorter about 0.5-0.4 Å and the geometry is a slightly more distorted octahedron. According to the analysis reported in the literature [40,41], the VO2+ (aq) cation have two stable hydration shell structures. Fig. 1(c) shows the structure of the [VO2 (H2O)4]+ complex with four water molecules which has octahedral geometry, while Fig. 1(d) represents the [VO2 (H2O)3]+ complex with three water molecule and the tri-hydrated bipyramidal geometry. The average value of RV-Oax and RV-Oeq are effectively the same in the case of both [VO2 (H2O)4]+ and [VO2 (H2O)3]+ complexes. The NBO atomic charges for all vanadium complexes are reported in Table 4. The atomic charge on V2+ is 1.54 and 1.25 with the inclusion of diffuse function, which is near to its formal charge. While for V3+ is 1.88 and 1.31 along with diffuse function. The atomic charge of vanadium for VO2+ and VO2+ complexes show to be smaller in comparison to the V2+ and V3+ ions. For both V2+ and V3+ ions, the atomic charges of water and oxygen are almost close to each other and the atomic charge of oxo-oxygen for VO2+ is slightly lower than VO2+ complex. For all the vanadium complexes, the binding energy of central vanadium cations to water molecules calculated and reported in Table 5. It is remarkable that the binding energy is significantly larger with the

Table 5 The binding energy of central vanadium cations with water molecules, which calculated at B3LYP/6-31G (d, p)/IEFPCM and B3LYP/6-311 + G (d, p)/ IEFPCM levels of theory. (all in Kcal/mol). Structure

2+

[V(H2O)6] [V(H2O)6]3+ [VO(H2O)5]2+ [VO2 (H2O)4]+ [VO2 (H2O)3]+

Structure

V

O

O(H2O)a

H(H2O)a

[V(H2O)6]2+ [V(H2O)6]3+ [VO(H2O)5]2+ [VO2(H2O)4]+ [VO2 (H2O)3]+

1.55(1.25) 1.31(1.88) 1.10(1.32) 0.88 (1.12) 1.00(1.19)

−0.23(-0.34) −0.36(-0.41) −0.36(-0.41)

−0.47(-0.57) −0.82(-0.46) −0.41(-0.92) −0.84(-0.92) −0.83(-0.90)

0.25(0.32) 0.55(0.28) 0.26(0.56) 0.52(0.54) 0.53(0.55)

ΔEa

ΔE + ZPEb

6-31G (d, p) 6-311 + G (d, p)

6-31G (d, p) 6-311 + G (d, p)

−299.46 −285.72 −140.54 −91.91 −61.09

−286.14 −273.33 −127.38 −80.94 −57.71

−106.87 −167.74 −100.66 −63.32 −56.47

−97.07 −151.85 −87.85 −52.71 −48.94

a binding energy which measured based on uncorrected total electronic energies. b binding energy which measured based on ZPE corrected total electronic energies.

more extended basis set (6-311 + G (d, p)). This is useful to tune the further calculation that will be presented in the article: in the second part of this study, we will use this basis set only. The stabilization energies trend is reasonable. Triple charged vanadium complex is more stable than the double charged one. This for two factors: the stronger ion-water bond and the larger solvation energy. The presence of a one less d electron does not invert this trend. The stabilization energies of the oxo ions are lower due to the reduced number of d electrons. Note the [V (H2O)6]2+ and [VO(H2O)5]2+ that have a similar structure and identical charge have similar stabilization energy. 3.1.2. Reactivity of modified graphite edge toward the V2+/V3+ redox reaction According to the hypothesis of some experimental literature [40–44], the nitrogen doping and the introduction of OH functionalities on the graphite felt surface could enhance the poor electrochemical activity of graphite-felt electrode toward the vanadium redox reactions. We considered how nitrogen doping and different adsorption sites for the different number of OH functional groups could improve the kinetics of electron transfer in electrode/electrolyte interface. In this paper, we compare the adsorption behavior of V2+/V3+ redox reaction in aqueous phase toward the minimal graphite models of modified zigzag and armchair edge surfaces (pristine, nitrogen-doped, and hydroxylated surface). We performed some calculations with an added electron to the models of graphite electrode, in order to simulate their interaction in the reducing environment, and study the possible enhancement of the redox kinetics. Fig. 2 shows the different modified graphite models, which have been considered in this work. Fig. 3 represents the results of the highest occupied molecular

Table 4 The atomic charges of all vanadium cations calculated at B3LYP/6-31G (d, p)/ IEFPCM (in parenthesis) and B3LYP/6-311 + G (d, p)/IEFPCM levels of theory.

a

RV-O

eq

Average value. 137

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process of detaching of the reduced species: after the electron transfer, they can easily leave their place for the reduction of further complexes. The HOMO, and LUMO energies and the gap between them for all the graphite models are reported in Table 7. The results suggest that, the presence of V-O bond, nitrogen doping, and addition of the electron to the graphite models improve the adsorption of vanadium ions toward the graphite electrode and enhance the electron transfer kinetics. According to the energies, which are obtained from the HOMO-LUMO gaps, the zigzag models show to have a smaller gap than armchair models, which shows the faster transfer of electron in electrode/electrolyte interface. This can be related to the different nature of the model aromatic systems. Note that in the real graphite both situations can occur and this difference cannot be related to experimental observations. 3.2. Experimental results: electrochemistry Fig. 4 shows CV curves of BP electrodes in N2-saturated 3 mol L−1 H2SO4 (dashed line) and 1 mol L−1 VOSO4 and 3 molL-1 H2SO4 (solid line). In the absence of vanadium ions in the electrolyte, all BP electrodes shows a well-defined cathodic peak at −0.3 V and one weak anodic peaks around 0.1 V. Those peaks can be ascribed to the selfredox activity of BPs in acidic environment, similarly to what previously observed for different carbon nanostructures [48]. The presence of VOSO4 in solution resulted in two couple of peaks at around 1.0 V and −0.5 V, related to the redox reactions of VO2+/VO2+ and V3+/ V2+ redox couples, respectively [49]. Although the shape of CV curves of BP(O)-modified and BP(N)-modified electrodes in VOSO4 containing electrolyte solution is similar to that of BP-unmodified electrode, main differences can be detected. Table 8 lists the electrochemical parameters extrapolated from CV analysis focused on the V3+/V2+ redox couple to compare the experimental results with the computational achievements. As shown in Table 8, peak potential values due to V3+/V2+ reduction (Epc) are less negative for BP(O)- and BP(N)-modified electrodes indicating a higher reaction rate for the functionalized electrodes as compared to unmodified BP. As a previous reported [44,45], this finding indicates that the presence of O and N functionalities at the electrode surface improved charge transfer for V2+/V3+ redox couple, which is in good agreement with the computational achievements. Peak current density (Jpc) of BP-unmodified and BP(O)-modified electrodes is quite similar, while the BP(N)-modified electrode displays lower current density as compared to BP-unmodified and BP(O)-modified samples. Lower current density values for BP(N)-modified electrode can be associated to a reduced density of active sites, as compared to BP and BP(O)-modified electrode. This arises from a partial collapse of BP porous structure induced by ammonia treatment, which reduces surface area as confirmed by surface area values shown in Table 1 [46–50]. Moreover, the separation between anodic and cathodic peak potential, ΔEp, is around 250 mV, which is much lower than typical values

Fig. 2. The graphite models of the pristine zigzag (1), and armchair (2), zigzag functionalized with one OH group (1a), and armchair (2a), zigzag functionalized with two OH groups (1b), armchair (2b), zigzag nitrogen-doped functionalized with OH (1c), and armchair (2c) and zigzag nitrogen-doped (1d), armchair (2d).

orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) of zigzag graphite model doped with nitrogen and functionalized with OH group. The results of molecular orbital for all graphite models except for the pristine models and the ones without the additional of an electron suggest that the LUMO is related to an antibonding orbital of the aromatic system while HOMO is according to an orbital largely of the d system of vanadium. Thus, the further electron is localized on the metal atom as expected. This can be seen also by analyzing the values of the Vanadium NBO charges for model systems interacting with complexes with and without a further electron, which reported in Table 6. The added electron charge, in almost is totality, is employed to reduce the charge of the metal atom. The added electron reduces also a considerable amount of the stabilization energy. This is favorable for the

Fig. 3. The LUMO (a) and HOMO (b) orbitals of N-doped graphite model functionalized with an hydroxyl group. 138

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Table 6 The geometric, electrostatic, and energetic quantities of model systems interacting with complexes with and without a further electron. Complex

Spin/charge 3+

[V(H2O)6] [V(H2O)6]3+ [V(H2O)6]3+ [V(H2O)6]3+ [V(H2O)6]3+ [V(H2O)6]3+ [V(H2O)6]3+ [V(H2O)6]3+ [V(H2O)5]3+ [V(H2O)5]3+ [V(H2O)5]3+ [V(H2O)5]3+ [V(H2O)4]3+ [V(H2O)6]3+ [V(H2O)5]3+ [V(H2O)6]3+ [V(H2O)6]3+ [V(H2O)6]3+

+1 +1+2 +2+ 1a + 1a+ 1b + 1b+1a +1b +2a +2b+2a +2b +1c +2c +1d +2d -

Triplet/3+ Doublet/2+ Triplet/3+ Doublet/2+ Triplet/3+ Doublet/2+ Triplet/3+ Doublet/2+ Doublet/2+ Doublet/2+ Doublet/2+ Doublet/2+ Doublet/2+ Doublet/2+ Doublet/2+ Doublet/2+ Doublet/2+ Doublet/2+

Charge V 2.237 1.203 1.946 1.203 2.310 1.204 2.236 1.201 1.213 1.221 1.206 1.263 1.211 1.207 1.205 1.205 1.196 1.147

Charge V complex

Charge graphite model

3.009 2.006 2.959 1.985 3.038 1.924 3.031 2.0003 1.887 1.904 1.864 1.863 1.882 1.901 1.783 1.897 1.899 1.897

3+

(1) + [V(H2O)6] (2) + [V(H2O)6]3+ (1a) + [V(H2O)5]3+ (2a) + [V(H2O)5]3+ (1b) + [V(H2O)4]3+ (2b) + [V(H2O)6]3+ (1c) + [V(H2O)5]3+ (2c) + [V(H2O)6]3+ (1d) + [V(H2O)6]3+ (2d) + [V(H2O)6]3+

V-C, N, O

(Å)

ΔE + ZPE (Kcal/mol)

(C)

−0.009 −0.006 0.041 0.015 −0.038 0.026 −0.031 −0.0003 0.113 0.096 0.136 0.137 0.118 0.099 0.207 0.103 0.101 0.103

Table 7 The energy of HOMO and LUMO orbitals and their gap in Kcal/mol.

R

5.14 5.37 (C) 4.51 (C) 4.55 (C) 4.03 (O) 3.80 (O) 4.55 (O) 5.24 (O) 2.18 (O) 2.30(O) 2.22(O) 2.62(O) 2.12 (O23) 2.15(O25) 4.01(O27) 4.34(O25) 2.17(O) 3.79(N) 3.84(O) 4.03(N) 4.14(N) 4.09(N)

−60.17 −9.95 −62.37 −13.04 −55.93 −3.88 −64.68 −2.40 −14.88 −21.30 −11.39 −15.15 −24.90 −3.52 −18.82 −56.47 −4.64 −6.19

Table 8 The electrochemical parameters extrapolated from.

HOMO

LUMO

HOMO-LUMO Gap

Sample

JPc (mAcm−2)

EPc (V)

ΔEP(V)

−85.642 −99.535 −103.018 −95.896 −103.884 −102.277 −96.328 −107.868 −103.926 −107.089

−30.804 −42.187 −52.591 −33.854 −51.223 −36.119 −58.910 −42.381 −54.665 −41.695

54.838 57.348 60.831 62.042 52.661 66.158 37.418 65.487 49.260 65.394

BP BP(O) BP(N)

22.6 21.2 17.7

−0.66 −0.62 −0.64

0.29 0.24 0.25

CV analysis focused on the V3+/V2+ redox couple.

previously reported in literature for V2+/V3+ redox couple at glassy carbon electrodes (ΔEp > 500 mV) [49]. As ΔEp is smaller, the redox reaction is more reversible. This indicates that BPs have a beneficial effect at enhancing the reversibility of V2+/V3+ redox reaction. Among the three samples, ΔEp is smaller for the functionalized samples (BP(O) and BP(N)) indicating enhanced electrochemical reversibility as compared to BP sample. By increasing the reversibility of electrode

Fig. 4. Cyclic voltammetries of BP-unmodified, BP(O)-modified, and BP(N)-modified electrodes in a 3.0 mol L−1 H2SO4 (dashed line) and 1.0 mol L−1 VOSO4 + 3.0 mol L−1 H2SO4 (solid line) electrolyte solution (scan rate at 25 mVs−1). 139

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behavior can be attributed to the adsorption of V2+/V3+ species at the electrode surface through both oxygen and nitrogen groups that contribute to form inner sphere complexes exhibiting a faster adsorption/ desorption kinetics [53,54]. We can exclude any surface area effect considering that the treatments with nitric acid and gaseous ammonia led only to small changes in surface area of the BP-modifies samples. In fact, SA of BP(O)-modified is 1.6% higher than that of BP-unmodified, while SA of BP(N)-modified samples is 3.1% lower than that of BPunmodified (as shown in Table 1). Such a slight difference can be ascribed on the one hand to amorphous carbon removal and external exposition of inner cavities induced by oxidation with nitric acid (which causes an increase in SA. On the other hand, a slight decrease in surface area of BP after NH3 treatment can be associated to a partial collapse of BP porous structure, as previously reported [55]. In both theoretical and experimental studies, surface modification of carbon-based electrodes was found to improve their electrocatalytic activity towards V3+/V2+ redox couple. According to the results of HOMO-LUMO gaps, shown in Table 7, the zigzag graphite model functionalized with OH group and nitrogen functionalities is the one with smaller band gap (faster electron transfer kinetics). The comparison between the above-mentioned model and the two models with single functionalization shows that there is a cooperative effect between these two moieties. In presence of a reasonable amount of functional groups on the graphite edge this kind of cooperation can be important. The experimental electrochemical analysis confirmed that O- and Nfunctionalization improved electrocatalytic activity of BPs. In fact, CV results indicated that peak potential values for V3+ reduction are less negative for functionalized BP, indicating higher reaction rate. Moreover, the fitting of EIS spectra indicated that the charge transfer resistance of V2+/V3+ redox couple was much lower for the BP(N)modified electrode, which was decorated with both oxygen and nitrogen surface groups, as compared to BP(O)-modified and BP-unmodified (4.66, 19.5 and 22.4 Ωcm2, respectively). According to previous reports [56], the rate constant for the V2+/ V3+ redox reaction increases with increasing density of oxygen groups at the surface of nanostructured carbon via the formation of innersphere complexes between adsorbed vanadium ions and surface oxygen groups. In addition, the presence of graphitic and pyridinic nitrogen allows charge delocalization phenomena, promoting synergistic catalytic effects of oxygen and nitrogen on different electrochemical processes, including oxygen reduction reaction and V2+/V3+ kinetics [57,58]. Hence, both experimental and theoretical studies converge to indicate that the simultaneous presence of nitrogen and oxygen moieties on the surface of carbon-based electrodes enhances electron transfer kinetics of V2+/V3+ redox couple. The improved kinetics is expected to enhance energy and power densities of VRFBs. In fact, a less negative reduction potential (as indicated by CV results) and a decreased charge transfer resistance (as indicated by EIS results) are beneficial for improving the energy storage efficiency because they imply a lower over potential for the redox reaction and a lower charge voltage for the redox flow battery.

Fig. 5. Nyquist plots of the BP-unmodified, BP(O)-modified, and BP(N)-modified electrodes acquired at open circuit potential in N2-saturated 1.0 mol L−1 VOSO4 + 3.0 mol L−1 H2SO4 electrolyte solution. Solid lines refer to experimental data and the dashed lines to fitting results. Table 9 EIS fitting parameters of BP-unmodified, BP(O)-modified and BP(N)-modified electrodes. R values were normalized with respect to the geometric area of the electrode (0.196 cm2). Parameters

R1 (Ω cm2) Q1 (Ssn) R2 (Ω cm2) Q2 (Ssn) R3 (Ω cm2)

Samples BP-unmodified

BP(O)-modified

BP(N)-modified

0.84 5.8 × 10−5 19.47 1.0 × 10−3 90.3

0.72 6.2 × 10−5 22.38 3.9 × 10−4 182.9

0.70 3.3 × 10−5 4.66 5.8 × 10−4 101.4

materials, the energy conversion for VRFBs will be facilitated. To get deeper insights on the effect of surface modification on the electron transfer kinetics of the V2+/V3+ redox couple at the surface of BP-based electrode, electrochemical impedance spectroscopy (EIS) was used to further characterize the samples. The Nyquist plots in Fig. 5 shows a semicircle at high frequency range and a linear part in the low frequency range, for all samples. The first characteristic represents the charge transfer process, and the second one represents the diffusionlimited process involved in the V2+/V3+ couple redox reaction [45,51]. EIS spectra were modeled by using a Randles-type circuit depicted as an inset of Fig. 5. This circuit is suitable for modeling finite linear diffusion in heterogeneous materials [52,53]. The elements in the equivalent circuit consist of an ohmic resistance (R1), a charge transfer resistance activation polarization (R2), a mass transfer resistance - concentration polarization (R3), a constant phase element (Q1) associated to the double layer capacitance and Q2 as a further constant phase element associated to the catalyst ink at the electrode. The results of fitting are shown in Table 9. As expected, R1 values are similar for all the three electrode, indicating that ohmic resistance is not affected by the functionalization of BP surface; in fact, ohmic resistance is mainly represented by the electrolyte solution. On the other hand, R3 is similar for BP-unmodified and BP(N)-modified, while it is much higher for BP (O)-modified. This can be ascribed to aggregation phenomena induced by the presence of oxygen surface groups in BP(O), as previously reported for similar carbon nanostructures [53]. Interestingly, R2 is similar for BP(O)- and BP-unmodified and much lower for BP(N)-modified, indicating that the charge transfer process is faster on BP(N). This

4. Conclusion In conclusion, our theoretical and experimental results reveal that the surface treatments of carbon-based electrodes by introducing oxygen functional groups (BP (O)) and dual oxygen and nitrogen groups (BP (N)) at the surface of BP samples synergistically enhanced electron transfer kinetics of the V3+/V2+ redox couple. The surface modification allows increasing active sites for adsorption of V3+/V2+ redox couple and facilitate electron transfer thanks to charge delocalization phenomena induced by the presence of nitrogen and the formation of innersphere complexes between adsorbed vanadium ions and surface oxygen groups. 140

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At the molecular level, this interaction has been studied trough the ab-initio study of the interaction between minimalistic-doped graphite models. The introduction of heteroatoms that have Lewis basic behavior facilitates the direct interaction between the electron reservoirs i.e. the V2+ ion and the electrode. This through the direct coordination of the metallic center to one or more of these heteroatoms whose orbitals (unbonding for both OH and N, π for nitrogen) are directly involved in the graphite delocalized π system. Other effects of this interaction are: 1) the reduction of the HOMO-LUMO gap value that is a critical value for any electron transfer/excitation phenomena; 2) an increase of the time of permanence of the V2+ ion near the electrode. This last effect can be explained considering that surface functionalization with oxygen and nitrogen facilitates the adsorption and coordination of V2+ on the catalytically active site, through the formation of reactant-support complexes. From the experimental point of view, cyclic voltammetry results indicated an enhanced redox reaction rate and reversibility of V2+/V3+ couple at the surface of BP-modified electrodes, as pointed out by less negative peak potential value of V2+/V3+ reduction and lower anodic and cathodic peak separation (ΔEp), as compare to BP-unmodified electrodes. Furthermore, EIS results demonstrated that a dual oxygen and nitrogen functionalization (BP(N)) synergistically increased electron transfer kinetics, in good agreement with the outcome of calculations. Finally, experimental and computational results agree on the importance of graphite functionalization in order to enhance vanadium ions/electrode interaction and, hence, to improve electrochemical performance of VRFB. Further studies are necessary to investigate the effect of graphite functionalization on the kinetics of VO2+/VO2+ electron transfer, providing a correlation between the computational point of view and more realistic graphite models.

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