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Investigation of Charge Transfer Kinetics at Carbon/Hydroquinone Interfaces for Redox-Active-Electrolyte Supercapacitors Jinwoo Park, Vipin Kumar, Xu Wang, Pooi-See Lee, and Woong Kim ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b06863 • Publication Date (Web): 12 Sep 2017 Downloaded from http://pubs.acs.org on September 15, 2017

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ACS Applied Materials & Interfaces

Investigation of Charge Transfer Kinetics at Carbon/Hydroquinone Interfaces for RedoxActive-Electrolyte Supercapacitors Jinwoo Park,† Vipin Kumar,‡ Xu Wang,‡ Pooi See Lee,‡,* and Woong Kim †,** †

Department of Materials Science and Engineering, Korea University, Seoul 02841,

Republic of Korea ‡

School of Materials Science and Engineering, Nanyang Technological University, 50

Nanyang Avenue, 639798, Singapore

KEYWORDS carbon nanotube, reduced graphene oxide, hydroquinone, scanning electrochemical microscopy, charge transfer kinetics, redox-active electrolyte, supercapacitor

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ABSTRACT

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The redox-active electrolyte supercapacitor (RAES) is a relatively new type

of energy storage device. Simple addition of selected redox species in the electrolyte can greatly enhance the energy density of supercapacitors relative to traditional electric double layer capacitors (EDLCs) owing to redox reactions. Studies on the kinetics at the interface of the electrode and redox mediator are important when developing RAESs. In this work, we employ highly accurate scanning electrochemical microscopy (SECM) to extract the kinetic constants at carbon/hydroquinone interfaces. The charge transfer rate constants are 1.2 × 10-2 and 1.3 × 10-2 cm s-1 for the carbon nanotube/hydroquinone and reduced graphene oxide/hydroquinone interfaces, respectively. These values are higher than those obtained by the conventional cyclic voltammetry method, approximately by an order of magnitude. The evaluation of heterogeneous rate constants with SECM would be the cornerstone for understanding and developing high performance RAESs.

INTRODUCTION Recent studies on redox-active-electrolyte supercapacitors (RAESs) have attracted much attention due to their huge electrode capacitance.1-7 For example, electrode capacitances of 5017 and 4700 F/g were respectively achieved when hydroquinone and cupric chloride were used as redox mediators in aqueous electrolyte.3,4 More recently, it was demonstrated that the same principle can be applied to supercapacitor with organic electrolytes in which the addition of a redox mediator not only increased the capacitance but also extended the operational voltage window.5 The reported capacitance values from RAES are comparable to those of state-of-the-art pseudocapacitors (e.g. 1525 F/g for Co3O4 and 4173 F/g for Ni(OH)2).8,9 Furthermore, the fabrication process of the RAESs is extremely simple and only requires the insertion of a redox mediator into the electrolyte. In the RAES, a large amount of 2 ACS Paragon Plus Environment

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redox mediators in the electrolyte can participate in the faradaic reaction at the electrode/electrolyte interface.1-7 Contrarily, the redox reactions of the pseudocapacitors are practically restricted to the near-surface region of the electrode materials and hence the oxide- or hydroxide-based pseudocapacitors often need sophisticated electrode structures to fully exploit the redox reaction of the electrode materials.10 Thus, developments of RAESs is considered an important, new, and simple way to attain high capacitance and hence high energy density of supercapacitors. Quantitative studies of charge transfer kinetics at electrode/electrolyte interfaces are important for developing high performance RAESs. The capacitances of RAESs are much higher than those of electric double layer capacitors (EDLCs) owing to their faradaic charge storage mechanism. RAESs operate based on faradaic chemical reaction derived from the redox mediator added to the electrolyte, whereas the operation of the EDLCs is based on the non-faradaic process of ion adsorption and desorption at the electrode surface. However, the high capacitances of RAESs are not fully accessible at high rate conditions due to the relatively slow ion diffusion to and from the electrode and/or kinetic charge transfer rates at the electrode/electrolyte interface.1-3,5 While proper design of electrode structure may facilitate mass transfer by shortening the diffusion pathway of electrolyte ions, the charge transfer dynamics at the electrode/electrolyte interface is greatly dependent on the materials of the redox active electrolytes and electrodes themselves.11-13 Therefore, accurate kinetic studies will provide invaluable information at the electrode/electrolyte interface, which would allow proper comparison of different interfaces between the electrode and electrolyte and even to rationally suggest new interfaces for the best performance. Previously, Nicholson’s method has widely been adopted to extract the heterogeneous rate constant owing to its simplicity.14,15 The only information required in this method is the 3 ACS Paragon Plus Environment


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voltage difference between the anodic and cathodic peaks in the cyclic voltammogram (CV). Although this CV measurement with a macroelectrode is convenient, the rate constants obtained are often not very precise because of inherent error sources such as large ohmic potential drop and charging current. Further, this method is conducted in a transient state, which might impede high accuracy and reproducibility of results. These error sources can be minimized by using a rotating-ring disk electrode (RRDE). The RRDE encourages high flux of analytes to the electrode and suppresses mass-transfer-related errors, which allows the precise evaluation of kinetic parameters. However, there are difficulties in the theoretical treatment of hydrodynamic problems such as the establishment of the solution flow velocity profiles with respect to rotation rate, viscosity, and density and the fabrication of electrodes and cells that can provide reproducible mass transfer conditions.15,16 On the other hand, scanning electrochemical microscopy (SECM) is a powerful technique to study heterogeneous electron transfer kinetics.17-20 SECM employs a micro-sized tip electrode, which can eliminate the effects of large ohmic drop in solution and double layer charging. The tip electrode also provides a steady-state in milliseconds or seconds and allows the localized kinetic property, which permits understanding the electrochemical process. Moreover, SECM can calculate the kinetic constant with a quantitative theory derived from steady-state diffusion equations.17-20 Additionally, sample preparation is less restricted than in RRDE, allowing a variety of sample morphologies (such as spots and thin/thick films, etc.) owing to the micro-sized tip electrode capable of probing localized interfacial kinetics under substantially high diffusive mass fluxes.21,22 In this work, we investigated the heterogeneous electron transfer kinetics of carbon electrodes/hydroquinone using SECM. The hydroquinone/benzoquinone (HQ/BQ) redox couple is one of the most widely investigated organic redox materials and has been 4 ACS Paragon Plus Environment

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successfully incorporated in RAESs.2,3,23 Single-walled carbon nanotubes (SWNTs) and reduced graphene oxides (RGOs) were employed as electrode materials due to their outstanding material properties for electrochemical applications.24,25 SECM measurement under steady state conditions provides accurate kinetic constants of charge transfer at the electrode/electrolyte interfaces. On the other hand, the conventional Nicholson’s method significantly underestimated the values, indicating that caution is required when using transient methods. We believe that our demonstration greatly contributes to improved understanding and development of high performance RAESs.

EXPERIMENTAL SECTION Preparation of electrodes and electrolyte. Carbon electrode materials were employed as received: SWNT (P3-SWNT, > 90%, average diameter 1.4 nm, bundle length ~1.0 µm, bundle diameter 4-5 nm, Carbon solutions) and RGO (TNRGO, > 99 wt%, thickness 0.553.74 nm, size 0.5-3 µm, specific surface area 500-1000 m2 g-1, Times Nano). The carbon electrodes were prepared by the drop-casting method. The carbon inks were prepared by dispersing SWNTs or RGOs in ethanol (1 g L-1) via bar-sonication for 1h. The carbon solution was pasted on a piece of non-conducting petri dish (polystyrene, Biomedia) and a graphite paper (carbon > 99.9%, density 1.2-1.3 g cm-3, X2 Labwares) for each SECM and Nicholson’s measurement and dried on a hotplate (80 °C) overnight. The mass of carbon materials was approximately 0.3 mg on an area of 1 cm2. To prepare the electrolyte, hydroquinone (≥ 99.5 %, Sigma-Aldrich) was dissolved in deionized water (Milli-Q, Millipore Corp.) with H2SO4 (95.0-98.0 %, Fisher Scientific). The concentrations of HQ and H2SO4 were 1 and 100 mM, respectively. An ultramicroelectrode (UME) was fabricated by sealing a Pt wire (25 µm diameter, Goodfellow, Bad Nauheim, Germany) in a borosilicate 5 ACS Paragon Plus Environment


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glass capillary (Hilgenberg GmbH, Malsfeld, Germany) followed by grinding the end of the electrode.26 Characterization of carbon electrodes. The surface morphologies of SWNTs and RGOs were investigated using field-emission SEM (S-4800, Hitachi). Chemical and elemental analysis on the carbon materials were carried out using X-ray photoelectron spectroscopy (Xtool, ULVAC-PHI) and Raman spectroscopy (LabRAM ARAMIS IR2, Horiba Jobin Yvon) equipped with a 532 nm laser source. Electrochemical measurements. SECM measurements were carried out with a home-built instrument. A tip and a glass radius (rT and rglass) of a UME were 12.5 and 87.5 µm, respectively. The UME attached to an xyz-positioning system was used as a working electrode. A carbon electrode (SWNT or RGO) was floating. A Pt wire and an Ag/AgCl electrode were used as a counter and a reference electrode, respectively. The four electrodes were exposed to electrolyte (1 mM HQ & 0.1 M H2SO4) and connected to a bipotentiostat (PGSTAT 30, Autolab). The CV curves were measured with the UME voltage scanned from -0.1 to 1.0 V vs. Ag/AgCl (scan rate = 5 mV s-1) at the distances of ~200 µm and ~ 5 µm from the carbon electrodes. Feedback mode approach curves were obtained by monitoring how UME tip current changed as the UME (fixed at 1.0 V vs. Ag/AgCl) approached the carbon electrode. During the approach, the distance between UME and carbon electrode was changed from ~200 to ~5 µm at a translation rate of 2 µm s-1. The obtained approach curves were normalized and fitted to the theoretical curves proposed by Cornut and Lefrou27 to extract the normalized rate constant, κ. As a result, the heterogeneous kinetic constant (keff) was acquired from κ = keff rT/D, where rT is the radius of the UME and D is the diffusion coefficient of the redox mediator, HQ. Detailed information on quantitative determination of keff from the SECM approach curves is provided in the Supporting Information. CV 6 ACS Paragon Plus Environment

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measurements for Nicholson’s method were performed with an electrochemical analyzer (VSP-300, Biologic) in a three-electrode configuration: SWNTs or RGOs on graphite paper, Pt gauze, and Ag/AgCl (3 M) were used as a working, a counter, and a reference electrode, respectively. In Nicholson’s method, the keff was determined by averaging five keff values obtained from the CVs measured at five different scan rates.

RESULTS AND DISCUSSION Morphologies of two types of carbon electrode materials (SWNTs and RGOs) were characterized using scanning electron microscopy (SEM). Entangled SWNT bundles and crumpled RGO platelets were observed (Figures 1a and b). Randomly intertwined nanotubes and closely stacked multi-layer structures introduce the various-sized pores and high specific surface area, which can accommodate electrolyte ions and redox mediators. Due to these unique surface morphologies, both carbon materials frequently are electrodes of choice in RAESs.28-30 Raman spectra of SWNTs and RGOs show three representative peaks (D, G, and 2D bands) of carbon materials (Figures 1c and d). The D band peaks at ~1342 cm-1 (SWNT) and 1347 cm-1 (RGO) originate from the defect sites of a graphite structure or a graphene edge. The G bands at ~1597 cm-1 (SWNT) and 1588 cm-1 (RGO) arise from the stretching of C-C bonds in graphitic materials. The 2D band, which is an overtone mode of the D band, arises from a two-phonon, inter-valley, second-order Raman scattering process (2674 cm-1 and 2676 cm-1 for SWNT and RGO, respectively).31-33 The 2D band of RGO is broader and right-shifted, which confirms the presence of a multilayer stacked graphene structure in the RGO sample.33,34 The radial breathing mode (RBM) peak (~171 cm-1) of SWNTs indicates the tubular structure. The RBM frequency value agrees with the measured diameter of SWNTs (~1.4 nm).35 7 ACS Paragon Plus Environment


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(a)

SWNT

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(b)

RGO

200 nm

(c)

(d) G

RBM

500 nm

D

SWNT

D

G

2D

RGO

2D

Figure 1. SEM images and Raman spectra of SWNT (a,c) and RGO (b,d). X-ray photoelectron spectroscopy (XPS) shows similar chemical properties for SWNTs and RGOs (Figure 2). Figures 2a and b show the survey scan XPS spectra of the carbon materials including the C 1s, O 1s, C-Auger, and O-Auger peaks.36 The atomic ratios of carbon to oxygen (C:O) in SWNT and RGO were 87:13 and 89:11, respectively. Deconvolution of the strong C 1s peaks (Figures 2c and d) provided more detailed information on the carbon. The sp2 (C=C) hybridized carbon peak originating from the graphitic structure and the sp3 (C-C) peak attributed to defects on the carbon surface were located at approximately 284.5 and 285.4 eV, respectively. Oxygen-containing functional groups such as hydroxyl, carbonyl, carboxyl, and carbonate were identified at approximately 8 ACS Paragon Plus Environment

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(a)

(b) RGO

SWNT

C 1s

C 1s

O 1s

C-auger

O 1s

C-auger

O-auger

O-auger

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(d) sp2

SWNT

sp3

RGO

sp2 sp3

C-O C=O O-C=O carbonate π-π*

C-O C=O O-C=O carbonate π-π*

Figure 2. XPS survey scan and C 1s scan spectra of SWNT (a,c) and RGO (b,d). 286.2, 287.2, 288.5, and 289.8 eV, respectively. The π-π* shakeup satellite peak at ~291.5 eV confirmed the presence of conjugated sp2 carbons.36 The integrated areas of sp2 and sp3 peaks indicate that sp2/sp3 carbon ratios of SWNTs and RGOs were 4.4 and 7.1, respectively. SECM was carried out using a UME to study the charge transfer kinetics between SWNTs or RGOs and HQ (Figure 3). A schematic of the home-built SECM cell is described in Figure 3a. A four-electrode system including a UME and a carbon electrode, a Pt wire counter electrode, and an Ag/AgCl reference electrode was assembled with a custom-made 9 ACS Paragon Plus Environment


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(b)

Figure 3. Schematics of (a) SECM cell configuration and (b) charge transfer process at the UME/carbon electrode interface. Teflon cell. All electrodes were placed in the cylindrical space (diameter ~1 cm) centered on the upper Teflon cell and submerged together in HQ-containing electrolyte (1 mM HQ in 0.1 M H2SO4). To prevent the electrolyte from leaking out of the cylindrical space, the joints were properly sealed with rubber o-rings and the Teflon cell was fixed with four securing screws. The charge transfer processes among UME, HQ, and carbon electrode were represented in detail (Figure 3b). At the tip of UME with sufficiently positive potential, HQs are oxidized to BQs. The BQs diffuse onto the surface of the carbon electrode due to a concentration gradient. Charges are transferred from the carbon electrode to BQs, reducing the BQs back to HQs. The regenerated HQs at the carbon electrode diffuse to the tip and additionally contribute to the tip current. A CV measured with a UME confirms that the SECM measurements were carried out in a steady state condition. The tip potential (ET) was scanned over the range from -0.1 to 1.0 V with respect to Ag/AgCl while the UME recorded the steady-state tip current (iT) resulting from the redox reaction at a specific tip-substrate distance. As expected, the CVs had 10 ACS Paragon Plus Environment

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sigmoidal shape with saturation current and without hysteresis indicating that steady state was established at this scan rate (Figures 4a and b).37 When the tip is far away from the carbon electrode (e.g. d ~200 µm, Bulk), the tip current in a bulk solution (iT,∞) depends solely on the concentration of redox mediator in the electrolyte and is not affected by the substrate.17-19 As the tip voltage increased beyond the oxidation potential, the tip current rose from 0 to ~10 nA. In contrast, when the tip is brought sufficiently near the surface of the carbon electrode (e.g. d ~5 µm, Near), the regenerated HQs at the carbon electrode diffuse back to the UME and increase tip current. The tip current was approximately 16~17 nA at the UME voltage of 1.0 V. This value is higher by 6~7 nA than the tip current measured at 200 µm distance. Next, SECM approach curves can be obtained against SWNT and RGO electrodes in feedback mode. In this mode, the UME tip potential (ET) was fixed at a sufficiently high voltage (= 1.0 V) to oxidize the HQ into BQ and the oxidative tip current, iT, was measured while the tip was approaching a SWNT or RGO substrate. When the UME tip was sufficiently far from the carbon substrate, steady-state tip current (iT,∞) was generated due to the oxidation reaction (HQ BQ) at the UME tip. When the tip was brought closer to the carbon substrate, the tip current increased (iT > iT,∞, positive feedback) due to the additional flux of HQ regenerated at the carbon substrate via the reduction reaction (BQ HQ). More specifically, as the distance between the tip and SWNT substrate (d) reduced from bulk to ~34.4 µm (L = 2.75) and ~4.4 µm (L = 0.35), the tip current (iT) increased by 10% and 50%, respectively (Figure 4c). If the substrate is insulating, the tip current decreases (iT < iT,∞, negative feedback) because the proximity of the tip and the substrates blocks the diffusion of HQ to the UME tip.



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(a)

(c)

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(b) Near

Near

Bulk

Bulk

SWNT

RGO

(d)

Figure 4. CV curves of micro electrodes when the UME is far from and near (a) SWNT and (b) RGO (v = 5 mV s-1). Approach curves to (c) SWNT and (d) RGO in the SECM feedback mode (UME tip voltage = 1 V vs. Ag/AgCl; carbon electrode is floating). Experimental data and theoretical calculation27 are expressed as open symbols and solid lines, respectively (iT: tip current, iT,∞: tip current when the tip is far from the carbon electrode (~9.5 nA), d: distance between the tip and carbon electrode, and rT: radius of the tip). Approach curves of conductor and insulator were also theoretically calculated27 assuming that the conductor (keff ∞) or insulator (keff 0) was used as electrode instead of carbon electrode, respectively (see Supporting Information). 12 ACS Paragon Plus Environment

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Finally, keff of the charge transfer at SWNTs or RGOs is derived from the approach curve by fitting the data to the analytical approximation of Cornut and Lefrou (Supporting Information).27 Approach curves show the normalized tip current (IT) as a function of normalized distance (L) (Figures 4c and d). IT is the normalized iT value at a particular distance divided by the diffusion-controlled iT,∞ (IT = iT/iT,∞) and L is the ratio of the distance between the tip and the substrate to the tip radius (L = d/rT). By fitting the approach curve data (IT vs. L) to the analytical approximate expression, the normalized rate constant (κ) was extracted (Supporting Information). Finally, the effective heterogeneous charge transfer rate constant (keff) was determined by using the equation (κ = keff rT/D)26,38-40 where rT is the radius of employed UME (= 12.5 µm), and D is the diffusion coefficient of HQ (= 0.73×10-5 cm2 s-1).15 As a result, we obtained keff values of 1.2 × 10-2 and 1.3 × 10-2 cm s-1 for SWNTs and RGOs, respectively. When using the conventional Nicholson’s method, it was noticed that the charge transfer rate constant (keff) was underestimated relative to the SECM method. The CV curves were obtained at scan rates of 2, 5, 10, 25, and 50 mV s-1 in the voltage window from -0.1 to 1.0 V vs. Ag/AgCl (Figures 5a and b). The CVs displayed HQ/BQ redox peaks centered approximately at 0.45 V. Figures 5c and d show the anodic and cathodic peak currents (ip) as a function of the square root of scan rate (υ1/2). The Randles-Sevcik plot (ip vs. υ1/2) exhibited a linear trend with both carbon electrodes, which implies that the charge transfer process of the HQ redox reaction was diffusion-controlled. Then, the keff values were calculated from the difference between the anodic and cathodic peaks (∆Ep = |Epa – Epc|) using the following equation under the assumption that the charge transfer coefficient α = 0.5 and the diffusion coefficients of the reduced and oxidized species are equal (D = DO = DR): keff = (πDnFυ/RT)1/2 ψ 13 ACS Paragon Plus Environment


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(a)

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(b) SWNT

(c)

RGO

(d) SWNT

RGO

Figure 5. Cyclic voltammetry curves of macro electrodes for (a) SWNT and (b) RGO (scan rate = 2, 5, 10, 25, and 50 mV s-1). Plots of ip vs. υ1/2 obtained from CV curves of (c) SWNT and (d) RGO. where D is the diffusion coefficient of redox mediator (cm2 sec-1), n is the number of electrons participating in the redox reaction, F is Faraday’s constant (C mol-1), υ is the scan rate of CV measurement (V s-1), R is the gas constant (J K-1 mol-1), T is the temperature (K) and ψ is the kinetic parameter deduced from the peak separation (Table S1 & Figure S2). The calculated keff values were 1.0 × 10-3 and 1.2 × 10-3 cm s-1 for SWNTs and RGOs, 14 ACS Paragon Plus Environment

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respectively. These values are very close to the previously reported value (1.1 × 10-3 cm s-1) measured using Nicholson’s method on the HQ/SWNT-bucky paper interface.41 The keff values obtained from Nicholson’s method (~10-3 cm s-1) are smaller than those from SECM (~10-2 cm s-1) by an order of magnitude. This underestimation would be ascribed to several error sources in macro-electrode CV measurement such as high ohmic drop, charging current, and unsteady state. Our results reveal that it is important and significant to study RAESs with the SECM method to elucidate and provide microscopic understanding of system kinetics with precision.

CONCLUSIONS SECM was performed to investigate charge transfer kinetics between SWNTs or RGOs and HQ at steady state. The power performance of RAES greatly relies on the electrode/electrolyte interfacial kinetics. The sigmoidal CV shape obtained when using UME confirms that steady state was established during the measurement. SECM approach curves were obtained against SWNT and RGO electrodes in feedback mode. The heterogeneous charge transfer rates were precisely determined by analyzing the approach curve and are 1.2 × 10-2 and 1.3 × 10-2 cm s-1 for SWNT/HQ and RGO/HQ, respectively. Nicholson’s method significantly underestimated the charge transfer rate constant compared to the accurate SECM method. Our demonstration will be an important basis for the development of high performance RAESs.

ASSOCIATED CONTENT



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Supporting Information. Description of the electrochemical system used to obtain approaching curve data. Approach curve fitting and determination of kinetic constant. ∆Ep × n variation with ψ (Table S1 and Figure S2). This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION

Corresponding Author *E-mail: [email protected] (P.S. Lee) **E-mail: [email protected] (W. Kim)

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was financially supported by the Brain Korea 21 Plus Project in 2014 and the National Research Foundation of Korea (NRF-2017R1A2B2006209). Part of the work was supported by grant no. MOE2015-T2-1-129 awarded by the Ministry of Education, Singapore.

REFERENCES

(1) Lota, G.; Frackowiak, E. Striking Capacitance of Carbon/Iodide Interface. Electrochem. Commun. 2009, 11, 87-90. (2) Roldan, S.; Blanco, C.; Granda, M.; Menendez, R.; Santamaria, R. Towards a Further Generation of High-Energy Carbon-Based Capacitors by Using Redox-Active Electrolytes. Angew. Chem., Int. Ed. 2011, 50, 1699-1701. (3) Roldán, S.; Granda, M.; Menéndez, R.; Santamaría, R.; Blanco, C. Mechanisms of Energy Storage in Carbon-Based Supercapacitors Modified with a Quinoid Redox-Active Electrolyte. J. Phys. Chem. C 2011, 115, 17606-17611. (4) Mai, L.-Q.; Minhas-Khan, A.; Tian, X.; Hercule, K. M.; Zhao, Y.-L.; Lin, X.; Xu, X. Synergistic Interaction between Redox-Active Electrolyte and Binder-Free Functionalized Carbon for Ultrahigh Supercapacitor Performance. Nat. Commun. 2013, 4, 2923. 16 ACS Paragon Plus Environment

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