Reduced Graphene Oxide Nanocomposite as an

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Cobalt-Doped Ceria/Reduced Graphene Oxide Nanocomposite as an Efficient Oxygen Reduction Reaction Catalyst and Supercapacitor Material Shaikh Parwaiz, Kousik Bhunia, Ashok Kumar Das, Mohammad Mansoob Khan, and Debabrata Pradhan J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b06846 • Publication Date (Web): 24 Aug 2017 Downloaded from http://pubs.acs.org on August 25, 2017

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The Journal of Physical Chemistry

Cobalt-Doped Ceria/Reduced Graphene Oxide Nanocomposite as an Efficient Oxygen Reduction Reaction Catalyst and Supercapacitor Material Shaikh Parwaiz,1 Kousik Bhunia,1 Ashok Kumar Das,1 Mohammad Mansoob Khan,2 Debabrata Pradhan1,* 1

Materials Science Centre, Indian Institute of Technology, Kharagpur 721 302, W. B., India

2

Chemical Sciences, Faculty of Science, Universiti Brunei Darussalam, JalanTungku, BE1410,

Brunei Darussalam

Abstract Design and development of highly active and durable oxygen reduction reaction (ORR) catalyst to replace Pt- and Pt-based materials are the present challenges in fuel cell research including direct methanol fuel cells (DMFC). The methanol crossover and its subsequent oxidation at cathode is another unwanted issue that reduces the efficiency of DMFC. Herein we report cobaltdoped ceria (Co-CeO2) as a promising electrocatalyst with competent ORR kinetics mainly through a four electron reduction pathway and it surpasses Pt/C by a great margin in terms of stability and methanol tolerance. The Co-CeO2 nanoparticles of diameter 4−7 nm were uniformly grown on reduced graphene oxide (rGO) by a facile single-step hydrothermal process. The assynthesized Co-CeO2 nanoparticles/rGO nanocomposites are further demonstrated as active energy storage materials in supercapacitor underlying the importance of the studied material in renewable energy industries.

1 ACS Paragon Plus Environment


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INTRODUCTION Proton exchange membrane fuel cell (PEMFC) has been emerging as one of the potential energy conversion technology from the energy stored in small molecules such as hydrogen, methanol, and ethanol. Minimal pollutant emission with efficient energy conversion of PEMFC draws attention of researchers for its commercialization by solving the challenges associated with it.1 To establish PEMFC for practical applications, it is necessary to reduce the cost of cell fabrication and increase the cell performance. The PEMFC performance strongly depends on the oxygen reduction kinetics of the cathode half-cell reaction.2 Reduction of oxygen at cathode halfcell encounters either a 4-electron or a 2-electron pathway with former forming stable byproduct water whereas latter results in the formation of unwanted harmful byproduct hydrogen peroxide. The slow oxygen reduction reaction (ORR) kinetics and formation of corrosive hydrogen peroxide are considered as major limiting factors for the power generation through PEMFC.3 Pt and Pt-based alloy catalysts are considered as the best catalysts for the ORR. However, the high cost, poor abundance, and poor stability along with the catalyst poisoning restrict its wider practical applications.3 Alternative non-precious transition metals,4 metal oxides,5 metal complexes,6 heteroatom doped carbons,7 and conducting polymers8 have shown potential competition towards ORR compared to expensive Pt-based catalysts. Outstanding results obtained from transition metal oxide composites with graphene9,10 encourage researchers to explore the catalytic behavior of rare-earth metal oxides and their composites.11 So, a greater significance relies on the development of efficient non-precious metal oxide electrocatalysts for ORR. Among the various rare earth metal oxides, CeO2 and doped CeO2 have shown promising electrocatalytic activity due to their unique structural properties. Ce4+ ion is prone to get reduced


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into Ce3+ ion, which introduces a mixed ion electronic conductivity in the CeO2 nanostructures.12 However, the electronic conductivity of pure CeO2 is not enough to achieve desired electrocatalytic activity. Thus CeO2 is combined and/or doped with various transition metals such as Ni,13 Sm,14 and Sn15 which not only alters the electronic properties but also enhances the electrocatalytic activity. Cobalt oxide is reported as one of the promising ORR catalysts in recent years.16,17,18 However, cobalt-doped CeO2 (Co-CeO2) has not been studied for ORR and it is expected to overcome some of the obstacles associated with the PEMFC cathode. Due to poor electronic conductivity of pure CeO2, suitable conductive support materials such as CNT16 and reduced graphene oxide (rGO)19 can be used for further improvisation in its electrocatalytic activity. The residual functional groups present in the rGO have the ability to anchor the oxide nanoparticles and thereby can restrict agglomeration of nanoparticles. On the other hand, high electrical conductivity, thermal stability, chemical resistance, and large surface area make rGO a favorable support material.20 In the present work, single-step hydrothermal synthesis of Codoped CeO2 nanoparticles/rGO (Co-CeO2/rGO) composite and its ORR activity is reported. Energy storage systems are equally essential as energy conversion technologies. In the recent years, apart from Li-ion batteries, supercapacitors have established their potential as emerging candidates for energy storage because of their unique features such as higher power density, better charge-discharge ability, and excellent cyclic stability over the available counterparts (conventional capacitors and batteries).21 On the basis of charge storage mechanisms, supercapacitors are subdivided into two groups: Electric double-layer capacitors (EDLCs) and pseudocapacitors. Pseudocapacitive materials are said to possess higher specific capacitance as well as energy density than the EDLC type materials.22 In this regard, RuO2 is believed to be superior to all other existing pseudocapacitor materials.23 But, the high cost, poor



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abundance, and toxicity restrict the commercial utilization of RuO2 in a large scale. So, efforts have been made to achieve high specific capacitance using the low-cost electrode materials. In particular, extensive researches have been performed on the metal oxide/hydroxides such as MnO2, NiO/Ni(OH)2, Co3O4/Co(OH)2, Fe2O3, Fe3O4, and binary oxides.24 In addition to metal oxides, rare earth metal oxides are also studied as pseudocapacitive materials. The tendency of rapid transformation from Ce(IV) to Ce(III) is believed to make CeO2 as a good electrode material for pseudocapacitors.25 However, pure CeO2 exhibits lower specific capacitance ( 1% Co-CeO2/rGO (57.07 Ω) > Pt/C (42.44 Ω) > 7% Co-CeO2/rGO (39.74 Ω) > 5% Co-CeO2/rGO (37.63 Ω) > 3% Co-CeO2/rGO (34.81 Ω). The lowest Rct obtained for 3% Co-CeO2/rGO indicates its superior charge transport property among the as-synthesized composites. The observed electrochemical results clearly indicate that the incorporation of Co in the CeO2 nanostructures increases the ORR activity significantly. An optimum 3% Co-doped composite produced maximum defects as observed in the Raman study is ascribed to create maximum active sites thereby increasing the ORR activity. Furthermore, the rGO sheets provide active sites for oxygen adsorption which reduces the activation energy and in turn, boosts the charge transfer kinetics of ORR. In particular, the onset potential (−0.223 V) and half wave potential (−0.295 V) of 3% Co-CeO2/rGO was found to be closer to Pt/C implying importance of Co-doped CeO2 as an ORR electrocatalyst.


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Figure 4. (a) CV profiles of the as-synthesized electrocatalysts along with Pt/C at 10 mV s−1, (b) LSV plots of 3% Co-CeO2/rGO nanocomposite at different RDE rotation speeds, (c) K-L plots of 3% Co-CeO2/rGO nanocomposite. (d) EIS spectra of as-synthesized electrocatalysts along with Pt/C and inset shows equivalent circuit diagram to fit the EIS spectra in order to measure the Rct values. In panel (a) and (d), the electrocatalysts includes (i) CeO2/rGO, (ii) 1% CoCeO2/rGO, (iii) 3%Co-CeO2/rGO, (iv) 5% Co-CeO2/rGO, and (v) 7% Co-CeO2/rGO nanocomposites, and (vi) Pt/C. The CVs, LSVs, and EIS were performed in O2 saturated 1 M KOH except the case of (vii) 3% Co-CeO2/rGO in N2 saturated 1 M KOH as shown in panel (a).


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The initial GO amount was also varied to find its optimum quantity in the composites keeping the cerium and cobalt precursor concentration fixed, i.e., 8 mL of 50 mM CeCl3·7H2O and 240 µL of 50 mM CoCl2·6H2O. The electrochemical ORR activity (Figure S7, SI) such as onset

potential, half-wave potential, and electron transfer number of the nanocomposites synthesized with different GO concentration is presented in Table S3 (SI). The best electrochemical performance was observed for the nanocomposite synthesized by taking 60 mg of initial GO. Thermogravimetric analysis (Figure S8, SI) of these samples synthesized with different initial GO concentration was carried out in air atmosphere from room temperature to 800 °C at a heating rate of 10°C/min to find any correlation with the experimental findings on electrochemical behavior. Initial weight loss of around 7% within 200°C was due to the removal of adsorbed moisture and oxygen containing functional moieties attached to GO matrix.50 A maximum weight loss of around 37% was observed in the temperature range of 200−400°C for 3% Co-CeO2/rGO nanocomposite with 60 mg GO (3-CCG-60) suggesting larger quantities of functional groups in it which gets removed in the heat treatment.51,52 Presence of maximum number of functional groups on the GO matrix in the case of 3-CCG-60 nanocomposite is thus ascribed to its superior electrochemical behavior among the composites synthesized in the present work. Methanol crossover effect and durability study. The methanol crossover is a major problem in direct methanol oxidation fuel cell (DMFC) in which methanol crosses to the cathode chamber where it gets oxidized along with ORR and thus decreasing the performance of DMFC significantly. An ORR electrocatalyst is thus desirable which would preferentially reduce oxygen but ineffective towards methanol oxidation. To study the crossover phenomena and stability of ORR electrocatalyst, CeO2/rGO, 3% Co-CeO2/rGO, and Pt/C modified electrodes were



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investigated further. The crossover effect was studied in presence of methanol using amperometric current-time response at −0.3 V (Figure 5a). Not only a sharp increase in current was found for Pt/C electrode after addition of 1.0 M methanol but also the cathodic current was shifted into the anodic region just after the methanol addition indicating methanol oxidation.53 On the other hand, the 3% Co-CeO2/rGO modified electrode displays negligible change in current upon methanol addition suggesting higher methanol oxidation tolerance. The observed results indicate that 3% Co-CeO2/rGO nanocomposite has a better fuel selectivity towards ORR than that of state-of-the-art catalyst Pt/C. The noncompetitive ORR and methanol oxidation reactions are ascribed to the higher methanol tolerance on 3% Co-CeO2/rGO surface. On the other hand, Pt exhibits ORR activity in the same potential range as methanol oxidation reaction.54 Thus, 3% Co-CeO2/rGO encounters far less active sites for methanol oxidation than that of Pt/C at the studied potential. Additionally, Co facilitates the higher methanol tolerance as that reported for Pd-Co electrocatalyst.54 The comparative durability study of the as-synthesized 3% Co-CeO2/rGO electrocatalyst with commercial Pt/C was carried out in 1 M KOH for 10000 second at −0.3 V (Figure 5b). After 10000 second, around 70% and 12% reduction of initial current was found for Pt/C and 3% Co-CeO2/rGO nanocomposite, respectively. In addition, the current response was found to be noisy for Pt/C which could be due to surface poisoning of Pt. Thus it can be explicitly stated that the 3% Co-CeO2/rGO nanocomposite not only has much higher methanol tolerance but also more stable than that of commercial Pt/C electrocatalyst.


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Supercapacitor Study. The as-synthesized Co-doped CeO2/rGO nanocomposites were further employed for electrochemical energy storage (supercapacitor) application. The energy storage performance of the as-synthesized nanocomposites was first studied in a three electrode system (active materials modified GC as working, Pt as counter, and SCE as reference electrodes) in a neutral aqueous medium of 0.5 M Na2SO4. The CV profiles (Figure S9) were collected at a constant potential window of 0.0 to 0.8 V (vs. SCE) for pure CeO2 nanoparticles and all the composites by varying the scan rates from 2 mV s−1 to 80 mV s−1. Figure 6a shows typical CV curves with 3% Co-CeO2/rGO nanocomposite at different scan rates. The CV curves were almost symmetrical to zero current and such symmetric I-V response along with the quasi-rectangular shape neatly defines the pseudocapacitive nature of the as-synthesized materials.25,27 It is to be noted that the pseudocapacitor materials show such quasi-rectangular CV plot in Na2SO4 electrolyte.55 Moreover, the same material shows much higher current and clear redox peaks in



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KOH electrolyte.55 The specific capacitance (Cs) value (in F g−1) from the CV plot was calculated using following equation (equation 5) and shown in Figure 6b.56 Cs = ∫ I(v)dV / (V2−V1) ν m

(5)

Where, ∫ I(v)dV is the area under the CV curve, V2−V1 is the potential window, ν is the scan rate in V s−1, and m is the mass (g) of the active material on the electrode surface. Specific capacitance of CeO2 was found to be increased by 1.86 times (at 2 mV s−1) by incorporation of rGO suggesting its importance. Moreover, the maximum capacitance observed for 3% CoCeO2/rGO nanocomposite at 2 mV s−1 is found to be 3.97 and 2.13 times higher than that of pure CeO2 and CeO2/rGO nanocomposite, respectively. The specific capacitance of 3% Co-CeO2/rGO nanocomposite was decreased from 594.3 F g−1 to 229.16 F g−1 with the increase in scan rate from 2 to 80 mV s-1. At higher scan rate, the insertion of the electrolyte into the electrode material is less effective resulting decrease of active surface sites.57 Upon increasing the scan rate from 2 mV s−1 to 80 mV s-1, 82.67 %, 75.72% and 61.44% decrease in specific capacitance was observed for pure CeO2, CeO2/rGO, and 3% Co-CeO2/rGO, respectively. The superior charge retention with 3% Co-CeO2/rGO than other materials synthesized here suggest optimum cobalt doping not only improves but also retains the capacitive behavior for a long range of sweep rate. The supercapacitor behavior of the as-synthesized materials was further investigated using galvanostatic charge/discharge (GCD) technique. The GCD of all the materials was performed with a potential window of 0.0−0.8 V at current densities of 0.25 A g−1 to 8 A g−1 in 0.5 M Na2SO4 (Figure S10, SI). The GCD plots show nearly symmetric triangular shape with a negligible internal resistance (IR) drop. The CV and GCD curves suggest reversible Faradaic reaction between Na+/H+ and CeO2 during the charge/discharge process.55 A representative GCD


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plot of 3% Co-CeO2/rGO is shown in Figure 6c. The specific capacitance (Figure 6d) for all the composites at different applied current densities was also calculated from GCD plots using equation 6.58 Cs= I∆t / m∆v

(6)

Where I is the applied current, ∆t is the discharge time, m is the mass of the active material, and ∆v is the potential window. The specific capacitance from GCD plots follow the order of CeO2 < CeO2/rGO < 1% Co-CeO2/rGO < 5% Co-CeO2/rGO < 7% Co-CeO2/rGO < 3% Co-CeO2/rGO. This order is found to be same as that measured from CV profiles. The specific capacitance was found to vary inversely with current density (Figure 6d). As ion migration is controlled by the diffusion process, at low current densities, it gets enough time to access the active electrode surface. In contrary, at high current densities, the interaction of the ions with the electrode surface is reduced because of shorter accessible time of the migrating ions than that at low current densities.59 The measured specific capacitance of 298 F g−1 with 3% Co-CeO2/rGO at an applied current density of 1 A g−1 is found to be higher than reported for CeO2-based materials such as MnO2/CeO2 (274.3 F g−1 at 0.5 A g−1),60 CeO2 nanowire@MnO2 nanostructures (255.2 F g−1 at 0.25 A g−1),61 RGO/CNT/CeO2 (215 F g−1 at 0.1 A g−1)62 and also higher than other oxides/rGO composites such as MnO2/graphene hybrid (197.0 F g−1 at 1 A g−1),63 ternary rGO/Fe3O4/PANI composite (283.4 F g−1 at 1 A g−1),64 Co3O4/graphene composite (157.7 F g−1 at 0.1 A g−1).52 At a current density of 8 A g−1, 3% Co-CeO2/rGO nanocomposite retains 47.11% of its specific capacitance measured at 0.25 A g−1 whereas, 33.16 and 28.74% retention was found in the same conditions for CeO2/rGO and CeO2 systems, respectively. This confirms that the high capacitive retention over a wide range of current densities can be achieved by the incorporation of rGO and Co-doping the CeO2 matrix. Uniform distribution of CeO2



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nanoparticles over highly conductive rGO matrix accounts for an increase in specific capacitance for CeO2/rGO system than pure CeO2. The specific capacitance was further increased by Codoping and an optimum of 3% Co-doping in the CeO2/rGO system delivers maximum capacitance and retention. The reduction in specific capacitance at higher Co-doping is believed to be due to the self-agglomeration of Co atom in the CeO2/rGO matrix.


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Figure 6. Electrochemical energy storage performance in a three-electrode system. (a) CV profile of 3% Co-CeO2/rGO nanocomposite at different scan rates (2, 5, 10, 20, 30, 40, 50, 60, 70, and 80 mV s−1), (b) specific capacitance of different active materials measured from respective CV curves, (c) GCD curves of 3% Co-CeO2/rGO nanocomposite at different applied current densities, and (d) specific capacitance of different active materials measured from respective GCD curves.

EIS study was carried out to further determine the capacitive behavior of the material along with the solution resistance which suggests flow of ions to the electrode surface. Figure 7a represents the typical Nyquist plots for undoped and Co-doped (3%) CeO2/rGO nanocomposite



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studied in 0.5 M Na2SO4 solution. A small semi-circle along with a straight line was observed for both the composites. A semi-circle at high-frequency region corresponds to the solution resistance and the straight line at low-frequency region shows the capacitive behavior of the material. The fitting of the impedance spectra was done with an equivalent circuit (inset) consisting of solution resistance (Rs), charge transfer resistance (Rct), Warburg impedance (W), pseudocapacitive element (Cp), and double layer capacitance (Cdl). The Rct and Rs values of the Co-doped nanocomposite was estimated to be 1.128 Ω and 4.507 Ω, respectively, which are lower than the Rct (1.431 Ω) and Rs (4.897 Ω) values of the undoped sample. Lower Rct value of 3% Co-CeO2/rGO reveals efficient charge transport across the electrode/electrolyte interface in this system which accounts for its better capacitive behavior than other synthesized ones which is in well accordance with the CV and GCD data. The cyclic stability of the 3% Co-CeO2/rGO nanocomposite was performed using GCD analysis at a current density of 1 A g−1 in a potential window of 0.0 to 0.8 V for 2000 cycles. The plot of specific capacitance vs. number of cycles (Figure 7b) clearly shows 85% retention of initial specific capacitance value even after 2000 cycles which demonstrate the long-term cycling stability of the hydrothermally grown 3% CoCeO2/rGO nanocomposite. The decrease in specific capacitance is mainly due to the continuous insertion and de-insertion of the electrolyte ions into the active material which causes aggregation of the active material on the electrode surface. As a result, dissolution of some active materials occurs which leads to the reduction of the initial specific capacitance value.65Another important parameter i.e. Columbic efficiency was calculated before and after the long-term cycling stability test from the following relation66 η = td/tc ×100%

(7)


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where η is the Columbic efficiency, td is the time required to discharge, and tc is the time required to charge. From the inset of Figure 7b, η value was found to be 97% for the GCD done after 2000 cycles. It indicates an excellent kinetic reversibility of the 3% Co-CeO2/rGO nanocomposite. CeO2/rGO 3% Co-CeO2/rGO

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The supercapacitor performance of the nanocomposites synthesized with different initial GO quantity was also measured. The representative CV plots of these nanocomposites along with their variation in specific capacitance values with scan rate are shown in Figure S11, SI. The highest value of specific capacitance as well as its maximum retention at higher scan rates was observed for the nanocomposite synthesized with an initial amount of 60 mg GO.

Two-Electrode Asymmetric Supercapacitor. An asymmetric supercapacitor (ASC) device was further fabricated using 3% Co-CeO2/rGO and activated carbon (AC) as positive and negative electrode material, respectively. The CV profiles of AC and 3% Co-CeO2/rGO nanocomposite



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were collected at 10 mV s−1 scan rate in 0.5 M Na2SO4 (Figure S12, SI). The AC electrode shows an almost rectangular type response whereas pseudocapacitive behavior was observed CeO2 based materials. The specific capacitance values for AC and 3% Co-CeO2/rGO nanocomposite was found to be 278.5 F g-1 and 459.76 F g-1 respectively at a scan rate of 10 mV s−1. The mass ratio of positive (m+) to negative (m−) electrode was calculated using the charge balance theory.67 To achieve maximum working potential window, the mass ratio should follow:

=

× ∆ × ∆

(8)

By incorporating the values of specific capacitances and potential windows for positive and negative electrode, the mass ratio was found to be 0.75. By maintaining this mass ratio, a total active mass of 7 mg was used to fabricate the ASC device. Total cell voltage was obtained by adding both the potential windows of AC and 3% Co-CeO2/rGO nanocomposite. Figure 8a and 8b display the CV and GCD curves of the fabricated ASC device by varying potential windows at constant scan rate (10 mV s−1) and current density (0.25 A g−1), respectively. In the smaller potential window (0.0 to 0.6 V), EDLC type behavior was predominantly appeared whereas further increasing the potential window to 1.8 V, the change in the shape of the CV curve indicates pseudocapacitive nature of the active material. The CV of the ASC device was also performed at different scan rates (2 mV s−1 to 80 mV s−1) in the potential window of 0.0 to 1.8 V (Figure 8c). A specific capacitance of 69.8 F g−1 at 2 mV s−1 was calculated from the CV curve of the ASC device. Further the electrochemical performance of the ASC device was tested by GCD technique. Figure 8d shows the GCD plots of the ASC device at different applied current densities with a small IR drop. Figure 8e presents the specific capacitance of ASC device at different current densities. In particular, the specific capacitance of the ASC device from GCD analysis was measured to be 60.28 F g−1 at 0.25 A g−1. Another advantageous characteristic of


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the ASC device arises from the coupling of EDLC behavior with the pseudocapacitive character which augments its cyclic stability.68 The cyclic stability of the device was tested by GCD analysis for 2000 cycles at 1 A g−1 current density as shown in Figure 8f. A capacitance retention of >90% was observed after 2000 cycles which suggests the long-term stability of 3% CoCeO2/rGO//AC ASC device. Inset of Figure 8f shows the GCD plots of initial few cycles along with final few cycles. EIS was performed before and after the stability test (Figure S13, SI).The charge transfer resistance values before and after long term cycling test were found to be 4.647 Ω and 4.752 Ω by fitting the EIS spectra with the equivalent circuit shown in the inset of Figure S13 (SI). An almost identical Nyquist plots in both the cases along with a small change in charge transfer resistance reveal the excellent charge transport phenomenon as well as supercapacitive behavior of the fabricated ASC device even after continuous cycling.



-2.0

−1

10 mV s -4.0

0.0

0.5

1.0

1.5

1.5 1.0 0.5

2.0

0

−1

0.25 A g−1 0.5 A g−1 1 A g−1 2 A g−1 3 A g−1 4 A g−1 5 A g−1 10 A g−1

1.6 1.2 0.8 0.4 0.0 0

200 400 600 800 1000

Time (s)

10 5 0

60

-5

-20

200 400 600 800 1000

0.4

0.8

1.2

1.6

100

(e)

30 15

0

0.0

1

2

3

2.0


45

0

-1

2 mV s -1 5 mV s -1 10 mV s -1 20 mV s -1 30 mV s -1 40 mV s -1 50 mV s -1 60 mV s -1 70 mV s -1 80 mV s

-10

Time (s) Specific Capacitance (F g )

(d)

(c)

-15

0.0

Potential (V vs. SCE) 2.0

15

4

−1

5

Current Density (A g )

80 60 40 20

(f) Potential (V vs. SCE)

0.0

0.25 A g

Capacitance Retention (%)

2.0

20

−1

(b)

Current (mA)

2.0

(a)


Current (mA)

4.0


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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1.0 1 A g

-1

Before 2000 cycles After 2000 cycles

0.8 0.6 0.4 0.2 0.0 0

500

1000 1500 2000

Time (s)

0 0

500

1000

1500

2000

Cycle number

Figure 8. The two-electrode electrochemical characteristics of an ASC device fabricated with the 3% Co-CeO2/rGO nanocomposite as negative electrode and AC as positive electrode. (a) CV curves at 10 mV s−1 and (b) GCD curves at 0.25 A g−1 by varying the potential window range, (c) CV curves at different scan rates, and (d) GCD curves at different applied current densities. (e) The specific capacitance as a function of current densities and (f) capacitance retention as a function of cycle number of ASC device fabricated with 3% Co-CeO2-rGO//AC. The inset of (f) shows GCD curves obtained before and after 2000 cycles.

Two most useful parameters that determine the efficiency of a supercapacitor are energy density (ED) and power density (PD). The typical Ragone plot consisting of these two parameters at different current densities was plotted as shown in Figure 9. At a power density of 225 W kg−1, the maximum energy density of 27.13 Wh kg−1 was obtained for the fabricated ASC device with 3% Co-CeO2/rGO as an anode and AC as cathode. The energy density of 3% CoCeO2/rGO//AC ASC is found to be higher than the ASC available in literatures with active


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materials such as α-Co(OH)2/Co3O4//AC (22.4 Wh kg−1),69 Co(OH)2-nanowires//NTAC (13.1 Wh kg−1),70 activated carbon//MnO2 (15 Wh kg−1),71 aqueous AC//NaMnO2 (19.5 Wh kg−1),72 NiO//rGO (23.5 Wh kg−1),73 C-G/Ammonium ferric citrate//phenolic aldehyde porous carbon (18.3 Wh kg−1),74 graphene-MnO2//graphene (10.03 Wh kg−1),75 CuS//AC (15.06 Wh kg−1),76 and MOXC-700//NPC (17.496 Wh kg−1).77 2

R ef

.7

3

10

2

This work

R

4

ef .7

f. 6

9

1

R ef . Re 71 f Re . 7 0 f. 75

.7

Ref. 77 Ref. 76

R ef

10

Re

−1

Energy Density (Wh kg )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0

10 2 10

3

10

10

4

−1

Power Density (W kg ) Figure 9. Ragone plot of the 3% Co-CeO2/rGO//AC asymmetric supercapacitor along with previously reported data for comparison.

CONCLUSIONS Here, a facile and single-step hydrothermal synthesis technique is demonstrated to synthesize Co-doped CeO2/rGO nanocomposites. The Co-doped CeO2 nanoparticles of diameter 4−7 nm were anchored uniformly on the rGO sheets forming composite that exhibits enhanced ORR activity and supercapacitor performance. An optimized 3% Co-doping leads to the highest ORR activity and energy storage performance than not only to their individual counterpart but also other composites. The improved performance with 3% Co-CeO2/rGO is ascribed to the



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synergistic role of Co-doping that creates more active sites through formation of defects and rGO that increases the electronic conductivity and O2 adsorption. The Co-doped Co-CeO2/rGO also exhibits higher methanol tolerance activity and stability than that of state-of-art Pt/C, which are highly desirable for ORR catalyst to be practically useful for DMFCs. Bi-functional characteristics of 3% Co-CeO2/rGO nanocomposite in energy storage and conversion system along with its cost effectiveness and facile synthesis envisioned the importance of the studied materials.

ASSOCIATED CONTENT Supporting Information FTIR spectra GO, CeO2, and Co-CeO2/rGO nanocomposites; FESEM images; LSV and corresponding K-L plots; TGA profiles; CV, GCD and EIS plots

AUTHOR INFORMATION Corresponding Author * [email protected] Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENT This work was supported by Science and Engineering Research Board (SERB), Department of Science and Technology, New Delhi, India through the grant SB/S1/IC-15/2013.


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TOC Graphic Cobalt-Doped Ceria/Reduced Graphene Oxide Nanocomposite as an Efficient Oxygen Reduction Reaction Catalyst and Supercapacitor Material Shaikh Parwaiz,1 Kousik Bhunia,1 Ashok Kumar Das,1 Mohammad Mansoob Khan,2 Debabrata Pradhan1,*

1

Materials Science Centre, Indian Institute of Technology, Kharagpur 721 302, W. B., India

2

Chemical Sciences, Faculty of Science, Universiti Brunei Darussalam, JalanTungku, BE1410,

Brunei Darussalam


Reduced Graphene Oxide Nanocomposite as an

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