Electrochemical deposition of highly porous reduced graphene oxide electrodes for Li-ion capacitors

Electrochimica Acta 337 (2020) 135861

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Electrochemical deposition of highly porous reduced graphene oxide electrodes for Li-ion capacitors Yi Zhan a, c, Eldho Edison a, William Manalastas a, Ming Rui Joel Tan a, c, Rohit Satish a, Andrea Buffa b, Srinivasan Madhavi a, c, **, Daniel Mandler b, c, * a b c

School of Materials Science and Engineering, Nanyang Technological University, 639798, Singapore Institute of Chemistry, The Hebrew University of Jerusalem, Jerusalem, 9190401, Israel Singapore-HUJ Alliance for Research and Enterprise (SHARE), Campus for Research Excellence and Technological Enterprise (CREATE), 138602, Singapore

a r t i c l e i n f o

a b s t r a c t

Article history: Received 4 August 2019 Received in revised form 27 December 2019 Accepted 4 February 2020 Available online 5 February 2020

Electrochemical deposition (ECD) is a promising and efficient technique for film assembly and electrode fabrication in energy storage and conversion devices. Herein, reduced graphene oxide (ErGO) with high porosity was driven by high-voltage ECD to form a binder-free capacitor electrode for lithium ion capacitors (LICs). The high voltage was used to drive the continuous and constant reduction of graphene oxide as well as for generation hydrogen, which was responsible for the porous structure. The latter not only prevented the severe stacking problem of reduced graphene oxide (rGO) but also facilitated the mass transfer of the electrolyte for the capacitive adsorption/desorption. One of the advantages of ECD is that it allows controlling very well the thickness and weight of the electrochemically deposited layer. Therefore, the effect of film thickness on the capacitive performance was also investigated. We found that increasing the film thickness did not linearly increase the areal capacitance, which was attributed to the resistive electrolyte diffusion through internal pores. Furthermore, a good capacitance as high as 168 F g
1 at 0.1 A g
1 was obtained by combining ErGO with V2O5 nanoparticles, benefiting from the integration of the high surface area of ErGO and the redox activity of V2O5. © 2020 Published by Elsevier Ltd.

Keywords: Supercapacitors Electrochemical deposition Graphene Graphene oxide Li-ion capacitor

1. Introduction Energy storage systems (ESSs) such as lithium-ion batteries (LIBs) and supercapacitors (SCs) can find numerous applications in consumer electronics, (hybrid) electric vehicles etc. [1e7] Nowadays, it is the Holy Grail to develop ESSs with high energy and power density. While LIBs can deliver high energy density as the most commercially successful battery in the market, they suffer from low power density and poor cycling performance [1e4]. The situation is opposite for SCs: high power density and good cyclability but low energy density [5,7,8]. The differences between LIBs and SCs result from their different charge storage mechanisms: LIBs rely on the faradaic lithiation/delithiation process limited by the

* Corresponding author. Institute of Chemistry, The Hebrew University of Jerusalem, Jerusalem, 9190401, Israel. ** Corresponding author. School of Materials Science and Engineering, Nanyang Technological University, 639798, Singapore. E-mail addresses: [email protected] (S. Madhavi), [email protected]. ac.il (D. Mandler). https://doi.org/10.1016/j.electacta.2020.135861 0013-4686/© 2020 Published by Elsevier Ltd.

sluggish solid-state ion and electron transport while SCs depend on highly reversible electric double-layer capacitive (EDLC) process, which involves non-faradic ion adsorption/desorption at the electrolyte/electrode interface [9e11]. This has been the motivation to developing lithium-ion capacitors (LICs) to deliver both high energy and power density without sacrificing cycling stability. LICs could bridge the gap between LIBs and SCs by integrating one battery electrode as anode with one supercapacitor electrode as cathode in the electrolyte dissolving Li salt [12e14]. Organic electrolyte is commonly used due to the larger potential window than that of aqueous electrolyte, thus providing a higher energy density. Since the capacity of the capacitor electrode is much lower than that of the battery electrode, the cathode is the key to improve the energy performance of LIC, according to the capacity equation [15].

1 1 1 ¼ þ CLIC Ccathode Canode

(1)

Activated carbon (AC) is a commonly used cathode material for LIC due to its high specific surface area (SSA), good electrical

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conductivity and low cost [16e20]. However, AC possesses relatively low specific capacity [21,22]. Hence, to develop LICs as a competitive technology, it is important to develop alternative cathode materials with high specific capacity and good rate capability. Graphene has attracted much attention because of its extraordinary properties, such as high theoretical surface area (2630 m2 g
1), good electrical conductivity (108 S m
1) and good mechanical strength (an intrinsic tensile strength of 130.5 GPa and a Young’s modulus of 1 TPa) making it an ideal candidate as cathode material delivering high energy and power density [7,9,23,24]. To fabricate advanced graphene SC materials, it is crucial to increase SSA, prevent graphene layer stacking and control the pore size distribution. For instance, it has been shown that self-assembled graphene hydrogel consisting of 3D porous frameworks could provide multidimensional electron transport pathway and facilitate electrolyte access by minimizing the transport distance [21,22,25]. Modification of graphene material by a strong alkali such as KOH at high temperature is another effective, yet complicated way to improve SSA and also create micropores on graphene sheet [26]. Furthermore, a class of hybrids called internal parallel hybrids has been explored as the combination of battery materials with AC electrodes in order to improve the energy storage behavior of such materials, including Li3V1$95Ni0$05(PO4)3 [27], LiFePO4 [28] and LiMn2O4 [29]. However, these battery materials are dominant components in the hybrids and contribute negligible pseudocapacitive storage. Electrodeposition, including electrochemical deposition (ECD) and electrophoretic deposition (EPD), is a widely used industrial process where particles or ions suspended in a liquid medium migrate under the influence of an electric field and are deposited onto an electrode. Possessing the advantages of high efficiency, controllability, scalability and cost-effectiveness, electrodeposition is an efficient method to fabricate electrode and assembly film in energy storage and conversion devices [30e32]. Traditional electrode preparations such as blade-coating show limitations at thickness control and film assembly efficiency, where active material is physically mixed with conductive agent and binder, to make a slurry paste and then coated on a current collector. The nonactive materials not only decrease the energy density and the tap density of the electrode, but also increase the interfacial resistance [32]. Therefore, one cannot neglect the lure of ECD to fabricate binder-free electrodes with high effectiveness and controllability of film thickness. However, the application of ECD and its detailed study are still in void for LIC. Herein, we developed electrodeposited rGO (ErGO) with porous structure as LIC cathode and evaluated its performance in both Li half-cell and symmetric capacitor and further improved the capacitance by compositing with V2O5 nanoparticles. The formation mechanism of porous structure of ErGO, which involves highvoltage ECD, and the effect of film thickness on the capacitive performance are studied in detailed in this work. To the ErGO, V2O5 was added, which is a very attractive pseudocapacitive additive for the cathode due to it redox peaks located at high potential window and low-cost. ErGO/V2O5 exhibited good capacitance as high as 168 F g
1 at 0.1 A g
1, resulting from the integration of the high surface area of ErGO and the redox activity of V2O5. 2. Experimental 2.1. Chemicals All chemicals were used as received. Potassium permanganate (KMnO4, 99%), vanadium pentoxide (V2O5, 98%) hydrogen peroxide solution (H2O2, 30%), oxalic acid (H2C2O4, 98%), isopropanol

((CH3)2CHOH), tetrabutylammonium tetrafluoroborate ((CH3CH2CH2CH2)4N(BF4), 99%) and lithium perchlorate (LiClO4, 99%) were purchased from Sigma-Aldrich. Graphite powder (~200 mesh, 99.9995%) was obtained from Alfa-Aesar. Sulfuric acid (H2SO4, 98%), nitric acid (HNO3, 70%) and hydrochloric acid (HCl, 37%) from Merck were also used in this study. 2.2. Synthesis 2.2.1. Preparation of graphene oxide (GO) GO was synthesized by the modified Hummers’ method [33]. In a typical preparation, 1 g graphite powder was added to 40 mL 98% H2SO4. 6 g KMnO4 was then added slowly to the mixture under stirring. After stirring for a further 2 h, the mixture was placed in an ice bath. 50 mL deionized water (DW) was added, followed by 50 mL DW and 10 mL 30% H2O2 15 min later. The reaction product (graphite oxide) was washed with 5% HCl solution and isopropanol (IPA), centrifuged several times and finally redispersed in 200 mL IPA. The graphite oxide suspension prepared as such was exfoliated to graphene oxide (GO) nanosheets after 20 min of ultrasonication on a 750 W SONICS Vibra-Cell. 2.2.2. Electrodeposition of GO The high-voltage ECD driven of graphene-based films was carried out in a two-electrode compartment at ambient conditions with a constant voltage applied by a power supply (MINI PRO 300 V POWER SUPPLY, MAJOR SCIENCE). Carbon paper (CP, Toray Paper 090, 1  1.5 cm2) was used as the working electrode by immersing the area of 1  1 cm2 in the solvent and Pt foil (1.5  1.5 cm2) was used as the counter electrode. 1 mg mL
1 GO dispersion in IPA with the addition of 5 mM HNO3 was used as the deposition solution with CP as cathode and Pt as the anode. The applied voltage was 30e70 V and the time was 1e5 min. The resultant ErGO was washed with DW by using a tissue to adsorb solvent from the backside of CP and expedite the solvent exchange. ErGO was then freeze-dried (ScanVac CS110-4) at
110  C overnight and finally calcined at 350  C for 1 h in air. 2.2.3. Preparation of ErGO/V2O5 VOC2O4 precursor was prepared by mixing V2O5 (2 mmol) with H2C2O4 (10 mmol) in 20 mL of DW under vigorous stirring. ErGO/ V2O5 was fabricated by a similar procedure of ErGO, namely, ErGO was electrochemically deposited followed by immersing into an aqueous solution with different VOC2O4 concentrations (0.01, 0.02, 0.05, 0.1e0.2 M) which caused VOC2O4 to adsorb onto the ErGO. The samples were free-dried and thermally treated as above. 2.3. Morphology, structure and electrochemical measurement Field-emission scanning electron microscopy (SEM) was performed on a Zeiss Supra 55 microscope operating at 5 kV accelerating voltage. X-ray diffraction (XRD) patterns were obtained by a Bruker GADDS XRD powder diffractometer using a Cu Ka source (l ¼ 1.5418 Å) at 40 kV and 30 mA. Fourier-transform infrared spectroscopy (FTIR) was carried out on a PerkinElmer Frontier infrared spectrometer. Raman spectroscopy was performed on a Confocal Raman Spectrometer (Alpha300 SR, WITec) using laser excitation at l ¼ 488 nm. Half-cell configurations were assembled using the electrodeposited film as cathode and Li-metal foil (Kyokuto Metal Co., Japan; 0.59 mm thick) as the anode in CR2016 type coin-cells. The two electrodes were separated by a microporous glass fiber separator (Whatman, Cat. no. 1825047, UK). A symmetric capacitor was assembled by using two identical ErGO electrodes. The electrolyte was 1 M LiPF6 in EC/DEC ¼ 1:1 (v:v, purchased from Sigma-Aldrich).

Y. Zhan et al. / Electrochimica Acta 337 (2020) 135861

Electrochemical characterizations such as cyclic voltammetry (CV) measurements and galvanostatic charge-discharge profiles were carried out using Solartron 1470 E and the charge-discharge cycles were performed on a Neware battery tester at ambient conditions. Specific capacitance derived from the CV curve was calculated according to the following equation:

Cspecific ¼

1 v$ðVf
Vi Þ

V ðf

idV

(2)

Vi

Specific capacitance could be also calculated from the discharging profile by the following equation:

Cspecific ¼

I  Dt Vf
Vi

(3)

The specific energy density E and power density P were calculated by the following equations, respectively:

P ¼ 1000  I  DV

(4)

Vf þ Vi 2

(5)

DV ¼

And

E¼

Pt 3600

(6)

where Cspecific is the specific capacitance (F g
1), v is the scan rate (V s
1), Vf and Vi is the upper limit and lower limit of the operating potential (V), i is the voltammetric current density (A g
1), P is power density (W kg
1), I is the current density (A g
1), E is the energy density (Wh kg
1) and t is the discharging time (s).

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3. Results and discussion Although aqueous electrochemical deposition (ECD) is more common in the field, organic media can avoid the electrolysis of water at the electrodes when high voltages are applied [30,34]. Therefore, IPA was employed as a solvent in this study. Graphene oxide (GO) nanosheets are negatively charged due to different functional groups on the surface, e.g. hydroxyl and carboxylate, and therefore, tend to migrate to the anode under the influence of the applied voltage. This would cause the electrophoretic deposition (EPD) of GO. However, such migration is largely suppressed by the presence of an electrolyte. Hence, instead of being attracted by the anode, GO can be reduced at the cathode and lose partially the functional groups in protons coupled processes, resulting in the decrease of the surface charge. The electrochemically reduced GO (ErGO) thus, deposits on the cathode (Fig. 1a) because of its low dispersibility in the solvent caused by the low zeta potential. Interestingly, ErGO has macroporous structure (Fig. 1b), suggesting the formation of ErGO aerogel by a facile one-step ECD. The porous structure should be beneficial to the mass transfer in the course of ion adsorption-desorption process, which should improve the rate capability. To maintain the porous structure after drying, freezedrying is found to be a proper process to dry ErGO. However, the freezing point of IPA is
89  C, which is too low to freeze dry under safe conditions using a standard lyophilizer. Therefore, a solventexchange operation was necessary to exchange IPA in the fresh ErGO with water because of the much higher freezing point of water. ErGO without solvent-exchange was less porous than the solvent-exchanged one, suggesting the necessity of the solventexchange operation (Fig. S1a). Porous ErGO has also been reported by Shi’s group, where ErGO electrodes were prepared by electrolyzing 3 mg mL
1 GO aqueous suspension containing 0.1 M LiClO4 [35]. However, the underlying mechanism for the formation of porous structure has never been studied. In Shi’s study, both Liþ and Hþ were present in the electrolyte during ECD so that it was difficult to differentiate, which ion

Fig. 1. Photo image of ErGO deposited on CP (a); SEM images of ErGO (b) and ErGO-TBA(c); and the XRD patterns of ErGO and GO (d).

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Y. Zhan et al. / Electrochimica Acta 337 (2020) 135861

played the major role in the porous structure formation. In this study, proton is the only cation involved in the ECD, suggesting that protons should be the key role in the formation of porous ErGO by evolving H2 gas. To further validate this hypothesis, tetrabutylammonium tetrafluoroborate (TBATFB) or LiClO4 were used as the electrolyte instead of HNO3, where ErGO is denoted as ErGO-TBA or ErGO-Li, respectively. The non-porous ErGO-TBA was expected regardless of solvent-exchange since TBATFB is a stable and inert electrolyte and no gases were evolved to cause porosity in ErGOTBA (Figs. 1c and S1b). In the presence of Liþ, GO was not only electrochemically reduced but also further lithiated [36]. Highly reactive ErGO-Li should react with water to evolve H2 gas during solvent-exchange operation (bubbles were also observed by naked eye). Therefore, it also showed porous structure for ErGO-Li undergoing solvent-exchanged, while it was non-porous without solvent-exchange (Figs. S1c and S1d). X-ray diffraction (XRD) pattern of ErGO shows a diffraction peak centered at 2q ¼ 25.1 which can be attributed to the diffraction of (002) plane with a lattice spacing of 0.36 nm (Fig. 1d). The lattice spacing was slightly larger than that of graphite due to the presence of residual oxygen functionalities on the surface. GO showed a typical sharp peak located at 2q ¼ 10.3 from the diffraction of (001) plane. The peak at 2q ¼ 10.3 disappeared for ErGO due to the removal of the intercalated water and functional groups. Fouriertransform infrared spectroscopy (FTIR) of GO showed the characteristic peaks of hydrate (OeH), aldehyde (C]O), alkene (C]C), carbohydroxylic (CeOH) and carboxylic (CeO) groups centered at 3203, 1726, 1620, 1412 and 1045 cm
1, respectively (Fig. S2a). Both peaks of hydrate and carbohydroxylic disappeared for ErGO while other peaks can still be observed suggesting the presence of

residual functional groups on ErGO surface after the electrochemical reduction. The Raman spectra of ErGO and GO showed two typical peaks located at 1357 and 1580 cm
1, which are commonly assigned as D and G bands (Fig. S2b). The D band results from phonons around K zone boundary activated by double resonance, which is strongly dispersive with excitation energy due to the Kohn Anomaly at K; while the G band is attributed to the E2g phonon mode of the sp2-bonded carbon atoms [37,38]. The intensity ratio of the D to G peak (ID/IG) is related to the degree of defects in graphene-based materials. The ID/IG was 0.99 for GO and 1.00 for ErGO in the study, suggesting that the defect density is almost maintained after the ECD, i.e. the sp2 domains were maintained [39]. The high controllability of the ECD technique could be well demonstrated by the dependence of the deposited weight on the deposition time and the applied voltage. As can be seen, the ErGO weight was nearly linearly dependent on time from 1 to 5 min applying 50 V (Fig. 2a). Similar trend is also observed for the weight dependence on the applied voltage of 30e70 V for 3 min of deposition (Fig. 2b). The thickness of ErGO layer was also measured and it generally increases linearly with the weight, suggesting a near constant porosity (density) of ErGO along with time and applied voltage in the measured window. This behavior could be obtained only by the high-voltage ECD. It should be noticed that the mechanism of deposition requires that GO be electrochemically reduced, which means that electron transfer must take place at the ErGO/electrolyte interface to keep the film growing. The high porosity of the film requires, therefore, very high over potential to keep the rate of deposition constant and independent on film thickness.

Fig. 2. The weight and thickness of ErGO as a function of the deposition time at 50 V (a) and the applied voltage within 5 min (b); CV of ErGO film at different scan rates (c) and the derived capacitances (d).

Y. Zhan et al. / Electrochimica Acta 337 (2020) 135861

The as-fabricated ErGO was first evaluated in a Li half-cell in the potential window of 3.0e4.5 V (vs. Li/Liþ). Cyclic voltammetry (CV) performed at different scan rates shows quasi-rectangular shapes with the absence of redox waves (Fig. 2c), alluding to the EDLC behavior of ErGO with anion (PF
6 ) adsorption/desorption. The CVderived capacitance of ErGO slightly decreased with increasing the scan rates (Fig. 2d). The capacitance was ca. 79.8 F g
1 at 1 mV s
1 and still kept at 55.3 F g
1 at 40 mV s
1, indicating of good rate capability of ErGO. This can be attributed to the macroporous structure of ErGO facilitating the fast mass transfer at high scan rates. It was expected that ErGO-TBA would exhibit the worst capacitance together with the poorest rate capability because of the non-porous structure; and that ErGO-Li would show the performance similar to ErGO. Fig. 2d shows that this is indeed observed, which confirms the structure-performance correlation. The increase in film thickness was expected to result in a decrease in the capacitive performance due to the formation of a bulky film, which is less accessible by ions. Traditional electrode preparations commonly use non-active additives such as conductive agents and polymer binders, which affect the energy and power density of the electrode. Therefore, it was highly interesting to study the effect of film thickness on the capacitive performance by taking the advantage of ECD, which allows controlling the thickness very precisely. Hence, ErGO with different thickness ranging from 3 to 79 mm was tested in a symmetric capacitor. Fig. 3a shows the near rectangular CV curves of a 3 mm thick ErGO at different scan rates (2e50 mV s
1) clearly exhibiting EDLC behavior. Areal-normalized CV curves of ErGO with thickness ranging from 3 to 79 mm are compared in Fig. 3b. It is evident that the

5

current (and therefore the areal capacitance) increases with the increase in the thickness. However, such increase in the capacitance is not linear with the thickness but levels off at high thickness (Fig. 3c). In principle, if the charging mechanism had been thickness independent, we would have expected to obtain capacitances that are linear with the thickness. Clearly, this is not the case. The observed trend suggests that the increase in the ErGO film thickness results in a decrease in the utilization efficiency of ErGO. This is presumable to the elongated diffusion path required for the electrolyte to access the film bulk. In other words, the resistance increases for the electrolyte diffusion to internal pores along with the thickness increase. The energy density and power density could be calculated from the charge-discharge profile and are shown in the Ragone plot (Fig. 3d). The latter is usually used to evaluate the capacitive performance by correlating energy density as a function of power density. As can be seen, ErGO with 79 mm thickness has the best performance with 23.7 mWh m
2 at 1.793 W m
2 and 14.9 mWh m
2 at 179.3 W m
2, which falls within the typical range of supercapacitor [6,8,40]. The energy density increases along with the increase of the thickness but levels off at high thickness. This follows the same trend of the CV curves: the internal pores become less accessible along with increasing the thickness. To increase the capacitance of ErGO, V2O5 was selected as the redox nanomaterial to provide pseudocapacitance when combined with ErGO. The weight ratio of V2O5 in the composite was controlled by rinsing ErGO with different concentrations of VOC2O4 solution of 0.01, 0.02, 0.05, 0.1e0.2 M (denoted as ErGO/V2O5-0.01, 0.02, 0.05, 0.1 and 0.2, respectively). The vanadium species

Fig. 3. CVs of 3 mm thick ErGO film at different scan rates (a); areal-normalized CV comparison of ErGO with different thickness at 50 mV s
1 (b); the CV-derived capacitance as a function of the thickness (c); and the Ragone plot of ErGO in different thickness (d).

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Y. Zhan et al. / Electrochimica Acta 337 (2020) 135861

adsorbed onto the ErGO and turned into nanoparticles by annealing. Energy-dispersive X-ray spectroscopy (EDX) measurements showed the increase of V2O5 weight ratio from 3.1, 4.7, 13.9, 16.9 to 23.5 wt%, respectively. XRD pattern of ErGO/V2O5-0.2 demonstrated the strong signals of carbon paper substrate while one residual peak located at 2q ¼ 17.89 could be assigned to the diffraction from (220) planes of V3O7 (JCPDS #71e1591), suggesting the hybrid oxide constituted of 68.6% V2O5 (Fig. S3a). SEM image of ErGO/V2O5-0.2 showed the maintained porous structure after the introduction of V2O5 nanoparticles (Fig. S3b). V2O5 nanoparticles in tens of nanometers were well integrated onto the ErGO sheets, which should shorten the ion diffusion path and be beneficial to the surface de/lithiation processes (Fig. 4). Apparently, the aggregation of nanoparticles increased along with the increase of the vanadate precursor concentration. The CV of the as-synthesized ErGO/V2O5-0.2 at 1 mV s
1 shows a major redox peak centered at 2.88 V accompanying with two minor redox peaks at 3.22 and 3.41 V, and very small peak potential difference (DEpk ¼ 80 mV) indicating of the high electrochemical reversibility of the composite (Fig. 5a). It is worth noticing that the redox peaks at 3.22 and 3.41 V, which are assigned as the first lithium de/lithiation in two steps, are much more prominent than the peak at 2.88 V in the literature [41e44]. The peak at 2.88 V is

only present in few studies [43e45] and Wang et al. attributed it as the pseudocapacitive surface de/lithiation process [44]. The prominence of the redox peak at 2.88 V in the study indicates the pseudocapacitive behavior as the dominant process for the ErGO/ V2O5 composite, compared with the faradaic de/lithiation process represented by the peaks at 3.22 and 3.41 V. Only the redox peak at 2.88 V is still present at high scan rate of 40 mV s
1 and the peak current decreases with the decrease in V2O5 weight ratio and finally diminishes for ErGO/V2O5-0.01 (Figs. S4a and b). Generally, the peak current follows a power-law relationship with the scan rate as follows [25,46].

i ¼ avb

(7)

where a and b are adjustable parameters. According to the b value, it is possible to figure out the current contribution from the capacitive process (b ¼ 1) and the diffusion-limited process (b ¼ 0.5). The b value could be obtained as the slope by plotting the peak current against the sweep rate both in logarithmic scale. The obtained slopes of the linear-fitted lines give b values of 0.95, 0.91, 0.89, 0.88 and 0.87 for ErGO/V2O5-0.01, 0.02, 0.05, 0.1 and 0.2, respectively, suggesting the dominance of surface pseudocapacitive process for the composites and the increase of bulk capacity contribution along with the increase of V2O5 percentage (Fig. 5b).

Fig. 4. SEM images of ErGO (a) and ErGO/V2O5-0.01 (b)
0.02 (c),
0.05 (d)
0.1 (e) and
0.2 (f) composites.

Y. Zhan et al. / Electrochimica Acta 337 (2020) 135861

7

Fig. 5. CV curve of ErGO/V2O5-0.2 at 1 mV s
1 (a); the anodic peak currents of ErGO/V2O5-0.2 plotting against the scan rate in logarithm (b); the discharge-charge profiles of ErGO/ V2O5-0.2 at different current densities (c); and the derived capacitances of different ErGO/V2O5 composites (d).

The capacitance derived from the CV curves was improved after introducing V2O5 and the capacitance was as high as 298 F g
1 at 1 mV s
1 and 143 F g
1 at 40 mV s
1 for ErGO/V2O5-0.2 (Fig. S4c). The decrease in capacitance retention along with the increase of V2O5 weight ratio was expected because of the involvement of the V2O5 redox species. The charge-discharge profiles of ErGO/V2O5-0.2, for example, are not that linear with some bumps, and the absence of clear plateaus at different current densities from 0.1 A g
1 to 5 A g
1, suggests a mixed behavior of capacitive adsorption/desorption, pseudocapacitive surface lithiation/delithiation and also the possible mass-diffusion controlled bulk reaction on the electrode (Fig. 5c). The profile shows that the discharge time is 3354 s at 0.1 A g
1 and it still can be discharged for 30 s at 5 A g
1, corresponding to a capacitance as high as 168 F g
1 (93.2 mA h g
1) and 90 F g
1 (50 mA h g
1), respectively (Fig. 5d). The capacitance competes with the state-of-the-art capacitor electrodes in the literature, suggesting the promising application of ErGO/V2O5-0.2 (Table S1). The stability test shows a capacitance retention of 89% after 5000 cycles for ErGO/V2O5-0.01 at 1 A g
1 while ErGO/V2O50.2 is less stable than ErGO/V2O5-0.01 (Fig. S4d). Although V2O5 could provide pseudocapacitance via the fast surface de/lithiation reaction, the large volume expansion/contraction during Liþ intercalation and deintercalation results in self-aggregation or pulverization, leading to a limited lifespan compared to ErGO that relies on the highly reversible physical adsorption-desorption process. Therefore, ErGO/V2O5-0.2 possessing the higher V2O5 weight ratio in the composite showed a less stability than that of ErGO/V2O5-0.01.

4. Conclusions Electrochemical deposition (ECD) applying high potentials is used as a means of controlling the build-up of an especially porous graphene based supercapacitor. The high voltage drives the continuous and constant reduction of graphene oxide and at the same time evolves hydrogen, which is necessary to generate the highly porous structure of the electrochemically reduced graphene oxide (ErGO). This binder-free layer was used as the cathode in a LIC. The ErGO weight and thickness could be easily controlled by the applied voltage and time, which made it possible to study the effect of the thickness on the performance of the electrode. We found that the ErGO utilization efficiency decreased along with the increase in film thickness. This was attributed to the increase in the resistance of the electrolyte diffusion to internal pores along with the thickness increase while near-surface pores were still accessible for the adsorption/desorption. An ErGO/V2O5 composite could deliver a capacitance as high as 168 F g
1 at 0.1 A g
1 benefiting from the pseudocapacitive contribution from V2O5; and compete with the state-of-the-art capacitor electrodes, demonstrating its promising application. We believe that ECD provides substantial benefits as compared with the conventional methods for assembling high-performance energy storage devices. CRediT authorship contribution statement Yi Zhan: Conceptualization, Methodology, Validation, Writing original draft. Eldho Edison: Validation, Writing - review & editing. William Manalastas: Validation. Ming Rui Joel Tan: Validation.

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Y. Zhan et al. / Electrochimica Acta 337 (2020) 135861

Rohit Satish: Validation. Andrea Buffa: Validation. Srinivasan Madhavi: Supervision, Project administration, Funding acquisition. Daniel Mandler: Supervision, Project administration, Writing review & editing.

[21]

[22]

Acknowledgement [23]

This work was conducted by Nanomaterials for Energy and Energy-Water Nexus (NEW) Programme under Singapore-HUJ Alliance for Research and Enterprise (SHARE) in the Campus for Research Excellence and Technological Enterprise (CREATE) that is supported by the National Research Foundation, Prime Minister’s Office, Singapore. Appendix A. Supplementary data

[24] [25]

[26]

[27]

Supplementary data to this article can be found online at https://doi.org/10.1016/j.electacta.2020.135861.

[28]

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