Electrochemical Supercapacitors from Diamond - The Journal of

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The Journal of Physical Chemistry

Electrochemical Supercapacitors from Diamond Siyu Yu†, Nianjun Yang†,*, Hao Zhuang†, Jan Meyer†, Soumen Mandal‡, Oliver A. Williams‡, Inga Lilge,§ Holger Schönherr,§ Xin Jiang†,* †

Institute of Materials Engineering, University of Siegen, 57076 Siegen, Germany ‡

School of Physics and Astronomy, Cardiff University, Cardiff CF24 3AA, UK §

Physical Chemistry I, Department of Chemistry and Biology, University of Siegen, 57076 Siegen, Germany

E-mail address: [email protected]; [email protected] Tel: 0049-271 760 2531 (N.Y.); 0049-761 7602966 (X. J.)

Boron doped diamond has been utilized as an electrode material to construct an electric double layer capacitor (EDLC) as well as an electrode support to form a pseudocapacitor. In a 1.0 M NaSO4 solution, the capacitance of diamond EDLC is in the range of 3.6-7.0 µF cm-2, comparable with those of EDLCs based on other carbon materials. During a charge/discharge process for 1000 cycles at a scan rate of 100 mV s-1, the capacitance only decreases 5%, indicating high stability and a long life-time of such an EDLC. To improve the capacitance of diamond EDLCs, diamond was coated with a MnO2 film to construct a pseudosupercapacitor. The MnO2 films were electrodeposited at a constant potential of 0.9 V vs. Ag/AgCl in 0.2 M MnSO4 solution. The mass of MnO2 deposited per unit area, so-called the area density, calculated from the deposition charge, was controlled via the deposition time. The MnO2 films were characterized using various techniques like SEM, XPS, and Raman spectroscopy, etc. In a 1.0 M NaSO4 solution, the capacitance of the MnO2/diamond based pseudosupercapacitor rises with an increase of the mass of MnO2 on diamond. Its maximum capacitance was found to be reached at a MnO2 area density of 24 µg cm2

. The capacitance obtained from voltammetry is 384 µF, or 326 F g-1 at a scan rate of 10 mV s-1,

which is comparable with the value of 406 µF, or 349 F g-1, obtained from charge/discharge process at a current density of 3 A g-1 in the potential range 0 to 0.8 V. The capacitance was reduced by 34% after 1000 subsequent charge/discharge cycles carried out at a scan range of 100 mV s-1. The comparison of the performance of the MnO2/diamond pseudosupercapacitor with that of those pseudosupercapacitors based on MnO2 and other carbon materials indicates that diamond could be suitable for electrochemical supercapacitor applications. 1 ACS Paragon Plus Environment


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1 Introduction Supercapacitors (SC), high-capacity electrochemical devices to storage energy, have received increasing attention and have thus been extensively researched in recent years.1-2 In comparison to rechargeable batteries, SCs have features of higher power density, longer cycle life, faster charge and discharge rate and wider operating temperature range.1-2 SCs either store electrostatically the charges via ion adsorption/accumulation at electrode/electrolyte interfaces without chemical reactions, or are based on fast and reversible faradaic reactions of redox media in which electron transfer occurs. The first case refers to so-called electrical double layer capacitors (EDLC), where two electrodes and an ion permeable separate membrane are needed. In the latter case, the electrodes are coated with redox species (e.g. metal oxide, polymer), and thus the devices, so-named pseudocapacitors, possess higher capacitance than the EDLC.3-4 To attain high capacitance of a SC, the key is to use highly conductive and stable electrodes as well as one electrode with a large specific surface area. In this context, carbon materials (e.g. various dimensionalities from 0 to 3D, a series of shapes, forms and sizes distribution) are extremely attractive and have been extensively utilized for the construction of EDLCs and pseudocapacitors.5-6 Among them, nanocarbons, such as graphene, nanotubes, onions, fibers and nanohorns, etc. have been widely applied.7 Boron doped diamond (BDD) is a promising 3D carbon material as the electrode material to construct EDLCs. Beside its general favorable properties, such as high mechanical hardness, high thermal conductivity, chemical inertness, excellent stability etc., it shows unique advantages as an electrochemical electrode, including a wide working potential window in aqueous or nonaqueous electrolytes, which probably leads to high energy and power density of SCs.8 For example, Fujishima and his coworkers have applied as early as on 2000 for the first time honeycomb structured BDD to construct EDLCs in aqueous and organic solutions. The working potential window of their BDD was shown to be ca. 2.5 V in aqueous electrolytes and 7.3 V in organic electrolytes, which is much wider than that of activated carbon, glassy carbon or highly ordered pyrolytic graphite.9-11 However, the capacitance, power and energy density of these EDLCs were comparatively low. To improve the capacitance of BDD based EDLCs it was suggested to enlarge the surface area.9-14 Various top-down and bottom-up approaches have been investigated. For instance, honeycomb structured BDD based EDLCs produced using the top-down etching of diamond with an Al2O3 mask by Fujishima et al. achieved a high capacitance of 3910 µF cm-2 2 ACS Paragon Plus Environment

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and 666 µF cm-2 (geometric area) in aqueous and organic solution, respectively.9-11 Van Wyk’s team provided sintered mass chemical vapor deposition (CVD) BDD as the electrode for EDLCs through a process of crushing under very high pressure and sintering. The surface area was greatly enhanced, resulting in an increase of capacitance, as well as the achievement of specific energy of the EDLC in the range from 1.3 to 1.8 W h kg-1.12 Nebel and his coworkers fabricated diamond foam using a bottom-up overgrowth of silicon oxide particles followed by etching of silicon oxide with HF. Diamond foam based EDLCs attained specific capacitances of 598 and 436 µF cm-2 (geometric area) in aqueous and organic solutions, respectively, as well as high power density of 807 W cm-3, which touched the best power performance of electrolytic capacitors.13 Later, the same group proposed another concept to enlarge the surface area of BDD via bottom-up overgrowth of silicon nanowires with a 240 nm thin BDD film. The capacitance of such an electrode based EDLC was 105 µF cm-2 in ionic liquid. A high energy density of 84 µJ cm-2 and power density of 0.94 mW cm-2 and good stability (retention stability of 93.3% after 10,000 cycles at a scan rate of 5 V s-1) were demonstrated.14 Although diamond based EDLCs appear to be quite promising, the detailed demonstration (e.g. advantages, disadvantages) of macro and flat (or unstructured) diamond electrode for EDLCs is surprisingly rare in literature. As another alternative approach to achieve higher capacitance of diamond-based supercapacitors, diamond has been applied as an electrode support to fabricate pseudocapacitors. The combination of BDD and transition metal oxides (e.g. RuO2) and conducting polymers (e.g. Polyaniline) has been reported. Spataru et al. deposited electrochemically RuOx on the BDD film as well as on diamond powders. The specific capacitance obtained in 0.5 M H2SO4 was 132 and 324 F g-1 for the film and powders, respectively.15 However, the cost for using RuO2 is relatively high and RuO2 is toxic, inhibiting the possibility to commercialize those pseudocapacitors. As a promising alternative material, MnO2 has attracted much attention, because it has the features of low cost, environmental friendliness, natural abundance, high theoretical capacitance (up to 1370 F g-1),1617

and ease of producing with various methods (e.g. sol-gel process18-19, hydrothermal method20-21

and electrodeposition22-23). Electrodeposition is the most effective process in that high-quality and morphology controlled MnO2 can be growth.24 For example, Yu et al. electrodeposited nanostructured MnO2 on graphene-coated textiles and achieved a specific capacitance of 315 F g1

at a scan rate of 2 mV s-1 in 0.5 M Na2SO4 aqueous solution.25 Liu et al. electrodeposited MnO2

on graphite under supergravity field and a specific capacitance of 367.77 F g-1 at current density of 0.5 A g-1 was realized in 6 M KOH aqueous solution.26 Liu et al. electrodeposited MnO2 thin 3 ACS Paragon Plus Environment


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films on a vertically aligned carbon nanofiber array and showed a maximum specific capacitance value of 365 F g-1 at the charge-discharge current density of 0.05 mA cm-2 (i.e., ∼0.385 A g-1) in 0.1 M Na2SO4 aqueous solution.27 Surprisingly, according to our best knowledge, MnO2 coated diamond pseudocapacitors have not been reported up to now. Here we report on the performance of diamond EDLC in aqueous solutions, as investigated using cyclic voltammetry (CV) and the charge-discharge technique, and compare the data to the performance of EDLCs based on other carbon materials. To make the comparison of those EDLCs have more sense, quite similar potential windows with those on other carbon materials were applied on diamond in this study. To address the shortcoming identified, electrochemically deposited MnO2 films on diamond are then introduced for pseudocapacitor applications. Following a thorough characterization and in particular optimization of the MnO2 area density, the performance of the MnO2/ diamond pseudosupercapacitor in 1.0 M NaSO4 solution was optimized. The performance is finally benchmarked against the data taken form the literature.

2 Experimental Materials The BDD films were grown on silicon wafers using microwave plasma assisted chemical vapor deposition (MWCVD) using conditions previously described.28 Before the growth the silicon wafer (2 inch) was seeded with hydrogen-terminated diamond nanoparticles (5-10 nm) in an ultrasonic bath.29 The density of diamond nanoparticles was in excess of 1011 cm-2.30 The growth was carried out at 875 oC. The reaction gases were CH4 (1%) and H2 (99%). The boron source was trimethyl boron (TMB) and the B/C ratio was 6500 ppm. The growth rate was 0.075 µm h-1. The boron-concentration was estimated to be 5×1020 cm-3 from precious calibrations.31 Prior to experiments, the BDD films were cleaned in a piranha solution (the mixture of H2SO4 and H2O2 with a volume ratio of 3:1) (Caution: dangerous and highly corrosive solution. Please handle it with utmost care), then using ultrasound in deionized water for three times and finally dried in a N2 atmosphere. The electrodeposition of MnO2 films on BDD was conducted using a CHI600E Potentiostat/Galvanostat (Shanghai Chenhua Inc., China) in a standard three electrode cell, which consists of BDD as the working electrode, Ag/AgCl (3M KCl) as the reference electrode and Pt as the counter electrode. The effective region of the working electrode exposed to the electrolyte 4 ACS Paragon Plus Environment

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was a circular area with a diameter of 2.5 mm or an area of 0.05 cm2. Electrodeposition of MnO2 films was carried out in an aqueous solution of 0.2 M MnSO4 in a potentiostatic mode at a constant potential of 0.9 V vs. Ag/AgCl. By altering the electrodeposition time, namely the charges transferred, various area densities of MnO2 on BDD were obtained.

Characterization The surface morphology, microstructure and the elemental composition of the materials prepared (BDD and MnO2) were investigated with a Field Emission Scanning Electron Microscopy (FESEM, Zeiss ultra55, Germany). The Raman spectra were taken in the range of 300 cm-1 to 2000 cm-1 on a home-made micro Raman configuration with a 532 nm laser. Atomic force microscope (AFM) images of BDD were recorded on AFM PSIA (XE-100, South Korea) in a noncontact mode. X-ray photoelectron spectroscopy (XPS, Surface Science Instruments, SSX-100 Sprobe photoelectron spectrometer, USA) was applied to characterize the chemical states of Mn in the MnO2 film on the BDD surface using an Al Kα radiation of 200W. The XPS spectra were analyzed using CasaXPS processing software version 2.3.16 PR 1.6. The binding energies (BE) were calibrated by reference to the C 1s (285.0 eV) signal.

Performance of supercapacitors The performance of the diamond EDLC and MnO2 coated diamond pseudocapacitor were examined in 1.0 M Na2SO4 aqueous solution using CV and a galvanostatic charge/discharge method. The CV measurements were performed in a potential window between 0 - 0.8 V vs. Ag/AgCl at scan rates ranging from 10 - 100 mV s-1. The charge/discharge cycles were tested at current densities in ranges of 0.05 - 1 µA cm-2 for diamond EDLC and 3 - 30 A g-1 for MnO2/ diamond pseudocapacitor, respectively.

3 Results and Discussion Performance of diamond EDLCs Figure 1 shows the SEM and AFM images of BDD samples used. The surface morphology (FESEM) and topography (AFM) of the BDD samples are clearly seen. They have grain sizes of about 0.3 to 1.0 µm (Figure 1a) and their average thickness is about 1.5 µm as measured from a SEM cross-section in a side view (Figure 1b). The root-mean-square roughness value measured 5 ACS Paragon Plus Environment


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by non-contact AFM at a scan size of 5×5 µm2 is 61 nm. The rough surface will be promising to enhance surface area of the BDD electrode as well as to grow nanomaterials on its surface.

Figure 1. (a, b) SEM and (c) three-dimensional AFM images of BDD samples (a, c) in a top view and (b) in a side view.

CV was applied first to evaluate the capacitance of the BDD film. Figure 2a shows the cyclic voltammograms of BDD in 1.0 M aqueous Na2SO4 solution at scan rates from 10 to 100 mV s-1. At all scan rates, the shape of cyclic voltammograms is symmetrical rectangular, indicating the ideal capacitive behavior of the BDD electrode used. As expect the capacitive current is enhanced with the scan rate. The specific capacitance Cs (F cm-2) was then calculated by integrating the charges during anodic and cathodic cycles. The specific capacitance was calculated to be 3.6 µF cm-2 at the scan rate of 10 mV s-1. This value alters in the range of 3.6 – 7.0 µF cm-2 for different BDD samples, which fits in the range of those published in literature.32 The very small variation of the capacitance of BDD samples is probably due to the difference of their surface states resulting from their surface morphology. The capacitance of BDD was evaluated as well using the galvanostatic charge/discharge method. Figure 2b shows those charge/discharge curves at current densities in the range of 0.05 – 1 µA cm-2. The symmetric curves are linear in both charging and discharging cycles, demonstrating the excellent capacitive characteristic and high reversibility of the capacitor. Longer times are required for one charge cycle when a lower charging current density is applied. For example, when a charge current density of 0.05 µA cm-2 was applied, a charge time of 55.9 s time was required, which was 2.6 s when a charge density of 1 µA cm-2 was applied. Through the integration of the charges in charge/discharge cycles during the galvanostatic process, Cs was calculated to be 3.6 µF cm-2 at the current density of 0.05 µA cm-2, which agrees well with that obtained from CV.

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In order to utilize BDD for EDLC applications, the performance of diamond EDLC was examined. As a key factor, its capacitance retention, namely, cycling stability was tested. The test was conducted in 1.0 M Na2SO4 by CV at a scan rate of 100 mV s-1. Figure 2c shows the variation of the capacitance retention of diamond EDLC as a function of cycles applied. Only 5% reduction of its capacitance is observed even after 1000 charge/discharging cycles. By contrast, for other carbon electrodes based EDLCs33-34, the reduction of their capacitance retentions is higher than 5%. For example, on hollow mesoporous shell carbon capsules only 88% of initial capacitance was retained after 2000 cycles.35 Although the full cell tests using diamond EDLCs are required to obtain more detailed capacitor performance with respect to energy density, power density, etc.; the comparison of specific capacitance, IR drop, etc. with those obtained from impedance tests is needed, these facts illustrate that diamond EDLC has the highest cycling stability. It is well-known that diamond with boron doping levels in the range of 3.0×1020 -2.0×1021 cm-3 is an excellent electrode for electrochemical sensing applications in that it has a much smaller capacitance than metal electrodes.8, 32, 36 In order to clarify the advantages and disadvantages of such a small capacitance for EDLC applications, we list and compare in Table 1 the capacitances of diamond EDLCs in different media, together with those for other carbon materials (e.g. carbon nanotube (CNT), activated carbon (AC), carbon aerogel and graphene) based EDLCs. For CNT, AC, carbon aerogel and graphene, Cs was normalized for the caparison using the equation of Cs = Cg/SBET (Cg: gravimetric specific capacitance, SBET: the surface area measured using BET technique). In aqueous solutions, organic solvents, or ionic liquids, Cs for diamond EDLCs falls respectively in the same order of the magnitudes as those for other carbon materials based EDLCs. This is to be expected as all of these materials are carbon based. The very small changes of the capacitances in different media and for the different types of carbon materials are due to the varied surface electronic states, surface morphology, etc.37 For example, the density of electronic states for sp2 contained CNTs and graphene is relatively higher than sp3 carbon materials such as diamond. All these facts lead to the conclusion that BDD is quite promising for EDLC applications. To improve Cs of diamond EDLCs with respect to higher Cg, generating diamond electrodes with large surface areas is a promising and efficient approach. Surface nanostructuring of diamond using top-down technique9-11, overgrowth of other nanostructures using bottom-up approach13-14, and surface coating of diamond with other nanomaterials15 have been proved to be possible methodologies. Moreover, due to the wider working potential window of BDD (e.g. 2.5 V in aqueous 7 ACS Paragon Plus Environment


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electrolytes, 7.3 V in organic electrolytes) than other carbon materials, diamond based EDLCs will have much higher capacitances than other carbon materials under identified conditions. For example, diamond foam based EDLCs showed three magnitudes higher capacitance than that of conventional supercapacitors with a power performance of 807 W cm−3.13 In a room temperature ionic liquid, diamond coated silicon nanowires showed a capacitance of 105 µF cm-2.14 Recently, we have successfully produced diamond networks using a template-free approach by selective etching of β-SiC in diamond/β-SiC nanocomposite film with a mixture of HF and HNO3. Their porosity was achieved in a range from 15 to 68%, leading to hundreds of times enhancement of the surface areas than that of flat diamonds (e.g. 490-fold for a 3 µm thick diamond network).38 For example, in 0.1 M H2SO4, the double layer capacitance of one typical diamond network was calculated to be 13.7 F g-1 or 17.3 F cm-3 at a scan rate of 100 mV s-1. Further optimization and proper decoration of diamond networks with other metal oxides (i.e., MnO2, NiO, etc.) and/or conducting polymers, these capacitors will have high power densities together with long lifetimes. Those diamond networks might be the best diamond electrode material for EDLC applications. In future, diamond films without defects, scale-up sizes, and reduced costs are highly required from technical and practical viewpoints. These considerations may be the only disadvantages of diamond EDLCs.

Table 1. Specific capacitances of carbon materials based EDLCs Electrode Materials

Electrolyte

Cg / F g-1

Cs / µF cm-2

Ref.

Carbon nanotube

aqueous

20-200

9-40

7

(CNT)

organic

50-100

6-10

7

ionic liquid

30-450

30-150

7, 39-40

Activated carbon

aqueous

30-330

3-15

7, 41

(AC)

organic

20-240

1-8

7, 41

ionic liquid

60-200

4-10

7

aqueous

40-200

4-20

7

organic

100-130

7-8

7

aqueous

120-280

10-30

7

organic

100-200

4-14

7, 42-43

ionic liquid

75-200

5-8

7

Carbon aerogel

Graphene


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BDD

aqueous

-

3.6-7.0

This work

organic

-

14-20

44

ionic liquid

-

11-15

45

100 mV s-1-1 50 mV s -1 20 mV s -1 10 mV s

(a)

Current (µA)

0.02

0

-0.02 0 0.2 0.4 0.6 0.8 Potential (V vs. Ag/AgCl) (b)

(c)

Capacitance Retention(%) Potential (V vs. Ag/AgCl)

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0.8

1.0 µA cm-2 0.5 µA cm-2 0.2 µA cm-2 -2 0.1 µA cm -2 0.05µA cm

0.6 0.4 0.2 0

0

30

60 90 Time (s)

120

0

300 600 Cycle Number

900

100 80 60 40



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Figure 2. Electrochemical performance of diamond EDLCs in 1.0 M Na2SO4: (a) cyclic voltammograms at scan rates from 10 - 100 mV s-1, (b) charge/discharge curves at the current densities of 0.05 - 1.0 µA cm-2 in a potential range of 0 - 0.8 V, (c) capacitance retention of a diamond EDLC as a function of charge/discharge cycles tested at a scan rate of 100 mV s-1.

Performance of MnO2/diamond pseudocapacitors As alterative promising and efficient approach to improve the capacitance of diamond based capacitors, MnO2 films were utilized to coat diamond electrode for the construction of diamond pseudocapacitors. In our work, the electrodeposition of MnO2 on BDD was carried out in a 0.2 M MnSO4 solution by applying a constant operational potential (e.g. 0.9 V vs. Ag/AgCl). During such a process, the Mn2+ species were oxidized on the BDD surface via a two-electron transfer process46: Mn + 2H O → MnO + 4H + 2e

(1)

Assuming all MnO2 produced was coated on the BDD surface, the amount of deposited MnO2 can be calculated by transferred charge using the following equation: 1 1 × × × 2 =

(2)

=

(3)

where m is the mass of MnO2, Q is the charge transferred during deposition and integrated from the i-t deposition curve, e (=1.6×10-19 C) is the electron charge, NA (=6.022×1023 mol-1) is the Avogadro constant, M (= 86.94 g mol-1) is the molar mass of MnO2 and A (=0.05 cm2) is the electrode area. For further analysis area normalized mass of MnO2, so-called area density (ms), was used. As an example, i-t deposition curve for a deposition time of 270 s is shown in Supporting Information. The deposited mass of MnO2 was controlled by adjusting the total transferred charge, namely the electrodeposition time. Figure 3 shows three example SEM images at different magnifications where ms is 2.0 (a, d), 24 (b, e), and 26 (c, f) µg cm-2, respectively. In the case of 2.0 µg cm-2 the crystal boundaries of BDD are still clear, while the boundaries turn blurry when ms increases to 24 µg cm-2. As ms rises to 26 µg cm-2, cracks occur in the oxide film, owning to increased internal stress in the thicker MnO2 film. Similar phenomena have been observed on other electrode surface.47 Moreover, both the low magnification SEM images (Figure 3a-c) and the corresponding 10 ACS Paragon Plus Environment

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images acquired at high magnification (Figure 3d-f) reveal that MnO2 films on the diamond surface possess nanostructures with fibrous features and distribute uniformly on BDD. These nanostructures are shown on Pt48 and Au-coated (100 nm thick) glass49 electrodes as well. With increasing transferred charge, the amount and density of nanostructures as well as the thickness of MnO2 film increased gradually. For ms of 2.0 µg cm-2, some bare diamond surface area is still observed. With a rise of ms, the density of nanostructures as well as the thickness of MnO2 film increases. With ms = 24 µg cm-2 the BDD surface is fully covered by MnO2 nanostructures. As ms further increases to 26 µg cm-2, the film is broken at some areas. Therefore, 24 µg cm-2 is probably the optimal ms for MnO2/diamond pseudocapacitor applications.

Figure 3. SEM images of MnO2 films coated on diamond with a deposited masses of (a, d) 2.0, (b, e) 24 and (c, f) 26 µg cm-2 at (a, b, c) low and (d, e, f) high magnifications.

Manganese is known to possess several oxidation states that show correspondingly different redox activity, and hence different capacitance.50-52 The oxidation states can be estimated from the splitting widths of the Mn 3s and Mn 2p peaks, as well as the binding energy (BE) of the Mn 2p3/2 peak in the XPS spectra of MnO2 films, as shown in literature.53-55 The high resolution element scans of the Mn 3s, Mn 2p and O 1s core levels of a typical MnO2 film with ms of 24 µg cm-2 are presented in Figure 4a, b and c, respectively. The solid lines represent the experimental data, which are overlaid with the corresponding fits (broken lines). According to the Mn 3s spec11 ACS Paragon Plus Environment


trum (Figure 4a), the energy separation between two peaks is 4.97 eV, which is close to the value for MnO2 reported in the literature (4.78 eV).53 Two peaks at BEs of 654.6 eV and 642.9 eV are observed in the Mn 2p spectrum (Figure 4b), which are assigned to Mn 2p1/2 and 2p3/2 states. The BE of the Mn 2p3/2 signal agrees favorably with that reported for Mn4+ states.54 Furthermore, the peak separation between the Mn 2p1/2 and 2p3/2 peaks is 11.7 eV, in good accordance with the literature value for MnO2 (11.6 eV).55 Therefore, Mn4+ ions are dominant in the electrochemically deposited film. The O 1s core level spectrum shown in Figure 4c is de-convoluted into three components, which are associated with O comprising Mn-O-Mn bonds (530.5 eV), Mn-OH bonds (532.2 eV) and H-O-H bonds (533.7 eV).53 According to the integrated areas for the Mn 2p and O 1s peaks, the atomic ratio between Mn and O is calculated to be about 1:2(±0.1). The assignment of the oxidation states and bonding situation of Mn were further confirmed with a clear peak at 649 nm in the Raman spectra of such a film (Supporting Information)56-58. Subsequently, MnO2 is the main compound on the BDD surface.

Intensity (a.u.)

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(a) Mn 3s

4.97 eV

(b) Mn 2p

Mn 2p3/2

(c) O 1s

Mn-O-Mn

Mn 2p1/2 Mn-OH H-O-H

95

90

85

80

656

648

640 540

534

528

Binding energy (eV) Figure 4. XPS spectra of (a) Mn 3s, (b) Mn 2p and (c) O 1s core levels of MnO2 film.

Notably, the porous characteristic of MnO2, namely nanostructured MnO2, is expected to improve the active surface area and produce high specific capacitance of the electrode underneath. As shown in Figure 3, a rise of transferred charge leads to increases in the amount and density of nanostructures as well as the thickness of MnO2 film. The capacitance of MnO2 coated BDD with different ms were then evaluated. CV in a potential range of 0 - 0.8V was conducted in 1.0 M Na2SO4 solution at a scan rate of 10 mV s-1. Figure 5a illustrates cyclic voltammograms with different ms of 2.0, 12, 24, 26 µg cm-2. All curves represent rectangular and symmetrical characteristics. This is typically the pseudocapacitive behavior of MnO2.48, 59 The pair of broad waves at 12 ACS Paragon Plus Environment

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around 0.45 V result from the redox activity of MnO2 in a solution with such electrolyte cations (C+ = H+, Na+, etc.) in the following form24, 46, 60: MnO + C + e ↔ MnOOC

(4)

Furthermore, the capacitive currents of those MnO2/diamond pseudocapacitors are enhanced with ms. Their capacitances were further calculated to be 0.94, 4.3, 7.82, and 7.9 mF cm-2, respectively, for ms of 2.0, 12, 24, and 26 µg cm-2. The capacitance of the MnO2/diamond pseudocapacitor is about 150-2200 times higher than that (3.6-7.0 µg cm-2) of diamond EDLC in previous session. The capacitances of MnO2/diamond pseudocapacitors with various ms were then estimated using CV at a scan rate of 10 mV s-1. All obtained cyclic voltammograms exhibit the properties of rectangular shape and mirror image. Figure 5b shows the calculated Cs as a function of ms. Cs increases approximately linear with ms up to 24 µg cm-2, remains almost constant (7.9 mF cm-2) with ms in the range of 26 µg cm-2 to 28 µg cm-2, and then decreases with an increase of ms. The decrease of Cs was noticed on MnO2/graphene23 as well as MnO2/carbon cloth47 pseudocapacitors. This is probably attributed to the poor electrical conductivity (10-5 - 10-6 S cm-1) of MnO261 when higher ms is applied. In our case cracks are noticed as well in some areas of the MnO2 film (e.g. ms = 26 µg cm-2), leading to the reduction of active surface area and eventually the deterioration of its capacitance. Hence, ms of 24 µg cm-2 is the optimal amount to achieve maximal capacitance of MnO2/diamond pseudocapacitor.



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Figure 5. (a) Cyclic voltammograms of MnO2 film coated BDD electrode with a deposited MnO2 mass of 2.0, 12, 24, and 26 µg cm-2, (b) surface normalized capacitance of the MnO2/diamond pseudocapacitor as a function of deposited MnO2 amount. The solid line is for eye guiding. The MnO2/diamond pseudocapacitor with ms of 24 µg cm-2 was then investigated in details. Figure 6a shows the cyclic voltammograms in the potential range of 0 - 0.8 V at different scan rate in 1.0 M NaSO4 solution. All curves retain symmetric rectangular shape and the currents increase with the scan rate, demonstrating ideal capacitor behavior of such a MnO2/diamond pseudocapacitor. In order to compare our results with those reported in literature, gravimetric specific capacitance of MnO2 Cg (F g-1) was calculated. Cg was estimated to be 326, 308, 281, and 267 F g-1 at a scan rate of 10, 20, 50, and 100 mV s-1, respectively. The variation of Cg with the scan rate for our MnO2/diamond pseudocapacitor is similar to most pseudocapacitors, caused by the different charge/discharging times.62-63 Figure 6b presents the galvanostatic charge/discharge curves of the MnO2/diamond pseudocapacitor at current densities ranging from 3 A g-1 to 30 A g-1. The charging curve is almost symmetric to the corresponding discharging section, illustrating high reversibility of the material. In Figure 6b the small IR drops within 50 mV are possibly due to the internal resistance of the MnO2 film, the same as those for MnO2/duplex stainless steel64 and MnO2/graphene65 pseudocapacitors. Cg was calculated to be 349, 331, 308, 299, and 282 F g-1 at the current density of 3, 5, 10, 15, and 30 A g-1, respectively. To obtain energy and power density of MnO2/diamond pseudocapacitor the full cell tests are required. From impedance tests the specific capacitance and IR drop will be evaluated and compared. The cycling stability of the MnO2/diamond pseudocapacitors was examined by CV at a scan rate of 100 mV s-1. The results are shown in Figure 6c. After 1000 cycles only 66% of the initial capacitance was retained. The degradation of capacitance is probably due to the relatively weak binding between MnO2 film and BDD substrate. The SEM image (Supporting Information) characterized the morphology of MnO2 film after 1000 cycles shows clearly the cracks in some areas of the film, which reduce the active area and lower the capacitance. Additionally the decrease of capacitance of the MnO2/diamond pseudocapacitors may be attributed to the following issues23, 66: (i) irreversible chemical reactions during charging and discharging process, leading to disappearance of the surface oxygenated functionalities and thus losing the pseudo-capacitive properties on the surface; (ii) deposition of impurity molecules on the MnO2/diamond surface, blocking the po14 ACS Paragon Plus Environment

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rous structure and therefore reduce the electrode active area; (iii) mechanical expansion of MnO2 due to the intercalation and de-intercalation of electrolyte cations, decreasing the structure stability of MnO2/diamond based electrode. Deep understanding the interaction of MnO2 with BDD surface having different surface terminations will be the key to solve above problems and eventually leads to the formation of the MnO2/diamond pseudocapacitors with higher capacitance retention.



100 mV -1 s-1 50 mV s 20 mV s-1 10 mV s-1

Current (µA)

(a)

40 20 0 -20 -40

(b)

(c)

Potential (V vs. Ag/AgCl)

0

Capacitance Retention(%)

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0.2 0.4 0.6 0.8 Potential (V vs. Ag/AgCl)

0.8

30 A g -1 15 A g -1 10 A g-1-1 5 A g -1 3Ag

0.6 0.4 0.2 0

0

50

100 150 Time (s)

200

100 80 60 40

0

300 600 Cycle Number

900

Figure 6. Electrochemical performance of the MnO2/diamond pseudocapacitor: (a) cyclic voltammograms at scan rates of 10 – 100 mV s-1, (b) charge/discharge curves at current densities from 3 – 30 A g-1, (c) capacitance retention at a scan rate of 100 mV s-1. The potential range was 0 - 0.8 V, the mass of deposited MnO2 was 24 µg cm-2 and the electrolyte was 1.0 M NaSO4 solution. 16 ACS Paragon Plus Environment

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Table 2 compares the gravimetric specific capacitance and capacitance retention of the MnO2/diamond pseudocapacitors (ms = 24 µg cm-2) with those of other MnO2/carbon materials based pseudocapacitors. The optimal specific capacitances of MnO2/diamond pseudocapacitors achieved in this work were 326 F g-1 at a scan rate of 10 mV s-1 and 349 F g-1 at a current density of 3 A g-1, which are higher than many of values that were published (e.g. MnO2/graphene pseudocapacitor with a specific capacitance of 218 F g-1 at a current density of 5 A g-1, MnO2/graphene oxide nanocomposite pseudocapacitor with a specific capacitance of 216 F g-1 at a current density of 150 mA g-1 and carbon@MnO2 core-shell hybrid nanosphere pseudocapacitor with a highest capacitance of 252 F g-1 at a scan rate of 2 mV s-1, etc.).65, 67-68 These are advantageous properties of the MnO2/diamond pseudocapacitors. However, the cycling stability of MnO2 film based diamond pseudocapacitors is relatively poor in comparison to other MnO2 coated carbon materials. The relatively weak binding between MnO2 film and BDD substrate was the main reason. To improve the binding ability of these two materials, one possible way is to fabricate micro- and nanostructures on the surface, for example to growth structures using bottom-up approach or to produce structures using top-down etching techniques. An alteration of the surface chemistry of diamond, for example, electrochemical grafting of a linker in between diamond and MnO2 or varying surface terminations of diamond to generate suitable interface for the immobilization of MnO2, is another possible and promising approach. Table 2. Gravimetric specific capacitance and capacitance retention of MnO2 and carbon electrodes based pseudocapacitors. Substrate

Specific Capacitance / F g-1

Retention / %

Ref.

Graphene

218 (5 A g-1)

94

65

Graphene oxide

216 (150 mA g-1)

84

67

Carbon nanospheres

252 (2 mV s-1)

74

68

-1

Carbon cloth

425 (10 mV s )

98.5

47

Carbon nanotube

201 (1 A g-1)

100

69

66

This work

BDD

326 (10 mV s-1) 349 (3 A g-1)



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4 Conclusion Boron doped diamond films has been applied as an electrode to construct electric double layer capacitor. Its specific capacitance is 3.6 – 7 µF cm-2, which is comparable to that of other carbon materials. The capacitance reduces by 5% after 1000 cycles. Therefore the advantage of diamond in such applications is its high stability. To improve the performance of diamond EDLC, enhancing diamond surface area is required. Diamond is utilized as the electrode support to fabricate the MnO2/diamond pseudosupercapacitor. With an optimum MnO2 mass of 24 µg cm-2, gravimetric specific capacitances of 326 F g-1 at a scan rate of 10 mV s-1 and 349 F g-1 at a current density of 3 A g-1 are achieved. The capacitance (7.82 mF cm-2) is three orders of magnitude larger than that of diamond EDLCs. Due to the relatively weak adhesion between BDD substrate and MnO2, the retention stability shows only 66% after 1000 cycles at a large scan rate of 100 mV s-1. However, the full cell tests (e.g. energy density, power density, etc.) and impedance tests (specific capacitance, IR drop, etc.) of diamond EDLCs and diamond pseudosupercapacitor must be conducted. Although in future making diamond electrodes with large surface area and improving the binding ability of metal oxide (e.g. MnO2) or polymer on BDD needs to be done, boron doped diamond is one of the most suitable materials for electrochemical supercapacitor applications, e.g. constructing either electric double layer capacitors or pseudosupercapacitors. As thin, defect-free, costeffective diamond films are possible to be produced using different techniques, diamond is promising and has the potential to be applied as the electrode material to construct electrochemical capacitors for industrial applications where require the capacitors with long life-time and high stabilities.

Acknowledgement The authors thank Dipl.-Ing. Gregor Schulte for his excellent contributions in setting up and running the XPS. HS gratefully acknowledges the financial support from the EU (ERC project ASMIDIAS, Grant no. 279202). SY gratefully acknowledges the financial support from China Scholarship Council (Chinese Government Scholarship, Award no. 201408080015).


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Supporting Information available The current-time curve for the electrochemical deposition of MnO2 on BDD, Raman spectra of BDD before and after being coated with the MnO2 film, and the SEM image of the surface morphology of a MnO2 film after charge/discharge process are shown in Supporting Information. This material is available free of charge via the Internet at http://pubs.acs.org.

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Electrochemical Supercapacitors from Diamond - The Journal of

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