reduced graphene oxide nanocomposite as an efficient oxygen reduction reaction catalyst

Journal of Electroanalytical Chemistry 851 (2019) 113480

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Mn-doped ceria/reduced graphene oxide nanocomposite as an efficient oxygen reduction reaction catalyst I. Hota a, S. Soren a, B.D. Mohapatra a, A.K. Debnath b, K.P. Muthe b, K.S.K. Varadwaj a, P. Parhi a, * a b

Department of Chemistry, Ravenshaw University, Cuttack, Odisha 753003, India Technical Physics Division, Bhabha Atomic Research Center, Mumbai 400085, India

A R T I C L E I N F O

A B S T R A C T

Keywords: CeO2 Microwave solvothermal synthesis Oxygen reduction reaction Linear sweep voltammetry

The platinum based catalyst exhibits good “oxygen reduction reaction” (ORR) activity while its lower methanol tolerance and high cost obstructs its commercialization. In order to replace this precious metal containing ORR catalysts, transition metal oxides and their composites have been extensively studied. Among the various transition metal oxides, oxides of manganese have attracted massive interest because of their high electrochemical stability, availability, low cost, variable oxidation states and effective catalytic properties. Among the various rare earth oxides, CeO2 is extensively studied. To facilitate the catalytic activity of CeO2, either doping with transition metal and/or integration with a conducting framework is required. In this paper, we have reported the synthesis of manganese doped CeO2/reduced graphene oxide (Mn-CeO2/rGO) nanocomposite with varying manganese concentration using microwave mediated solvothermal method. The ORR activity of synthesized nanocomposites has been studied in basic medium. An optimized 5% Mn doped CeO2/rGO leads to highest ORR activity among the studied catalysts. The synthesized nanocomposite surpasses Pt/C in terms of methanol tolerance and stability.

1. Introduction Among the major types of fuel cells, direct methanol fuel cell (DMFC) has been widely studied due to its advantages that includes low operating temperature, high energy density, and ease of handling [1,2]. Efficiency of DMFC is decided by studying Oxygen Reduction Reaction (ORR) at the cathodic site of the cell [3]. ORR process is generally slow in aqueous electrolyte due to sluggish electrochemical reaction kinetics [4–7]. Reduction of oxygen during ORR follows two pathways, one is 4 electron pathway with formation of water and another is 2 electron pathway with formation of unwanted by-product hydrogen peroxide [8–10]. Still today development of oxygen electrode catalysts for a 4 electron ORR process is a challenge for the researchers [11]. Though there are few reports available for the successful synthesis of cathode materials but the commercialization of these materials in devices is still a challenge [12]. In spite of high cost, low stability and low methanol tolerance, Pt based catalysts are most widely used as cathode materials because of their high activity [13–16]. Among various synthetic cathode materials, Transition Metal Oxides (TMOs) are widely studied due to their high abundance [17]. The different oxidation states of metal cations in TMOs are responsible for their improved electrocatalytic activity. In recent years, TMOs such as manganese oxide, iron oxide, copper oxide, cobalt oxide

and nickel oxide have attracted tremendous attention as ORR catalysts [18–21]. However, due to low electrical conductivity and poor stability, TMOs are often combined with conductive support (carbon based materials) to accelerate the ORR process [22–27]. It is found that, the electronic interactions between the metal nanoparticle and conducting carbon substrates also play a vital role in determining ORR activity [28,29]. Further doping in TMOs could effectively enhance the catalytic activity towards ORR. Mathur et al. observed enhanced ORR activity of Fe doped MnO2 nanorods [30]. Similarly, Song et al. demonstrated enhanced ORR and Oxygen Evolution Reaction (OER) activity of Ni and Mn incorporated mesoporous Co3O4 [31]. However, in all these studies, the reason behind enhancement in electrocatalytic activity isn’t clearly understood. It is assumed that the enhancement in activity could be due to factors, such as; (a) electronic interaction between the hetero-metal atom dopant and metal oxide or (b) presence of multiple metal centres as active site or (c) simultaneous presence of both factors (a) and (b). Recently, rare earth metal oxides have been investigated for ORR activity due to their unique electronic properties and availability of catalytically active sites [32,33]. Particularly, Ceria (CeO2) is catalytically active because the oxidation state of cerium cation can change between the þ3 and þ4 states and therefore acts as an oxygen buffer. In fuel cell applications, CeO2 is also used as a co-catalyst [34–36].

* Corresponding author. E-mail address: [email protected] (P. Parhi). https://doi.org/10.1016/j.jelechem.2019.113480 Received 25 June 2019; Received in revised form 28 August 2019; Accepted 9 September 2019 Available online 10 September 2019 1572-6657/© 2019 Published by Elsevier B.V.

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Journal of Electroanalytical Chemistry 851 (2019) 113480

Table 1 Precursor’s concentration for microwave solvothermal synthesis of different cerium oxide based system at 180  C for 20 min and their d-spacing and lattice parameters. Sample CeO2 5% Mn-CeO2 CeO2/rGO 1% Mn-CeO2/rGO 3% Mn-CeO2/rGO 5% Mn-CeO2/rGO 7% Mn-CeO2/rGO

Ammonium cerium (IV) nitrate 0.45 mM 0.45 mM 0.45 mM 0.45 mM 0.45 mM 0.45 mM 0.45 mM

(250 mg) (250 mg) (250 mg) (250 mg) (250 mg) (250 mg) (250 mg)

MnCl2.7H2O – – – 0.0045 mM 0.0135 mM 0.0225 mM 0.0315 mM

d spacing (A )

Solvent

(~1 mg) (~3 mg) (~5 mg) (7 mg)

1,4-Butanediol 1,4-Butanediol 1,4-Butanediol 1,4-Butanediol 1,4-Butanediol 1,4-Butanediol 1,4-Butanediol

(15 mL) (15 mL) (15 mL) (15 mL) (15 mL) (15 mL) (15 mL)

3.130 3.145 3.120 3.143 3.141 3.140 3.144



A A A A A A A

Particle size (nm) 7.7 nm 7.5 nm 7.4 nm 12.07 nm 8.40 nm 6.61 nm 14.14 nm

CeO2 precursor and GO ratio constant. In order to prepare 1% Mn-CeO2/rGO, about 50 mg GO was ultrasonically dispersed in 5 ml DD H2O and then it is transferred to a teflon vessel. Thereafter, 100:1 M ratio of ammonium cerium (IV) nitrate (0.45 mM (250 mg)) and MnCl2.4H2O (0.0045 mM (~ 1 mg)) were added to the above solution. 15 mL of 1,4-Butanediol is added as solvent. The teflon vessel was treated in a microwave (MDS 6, Sineo, China) for 20 min at 180  C. After completion of reaction the Teflon vessel was cooled to room temperature and the product was collected by centrifugation. The product was washed with DD H2O for twice and finally once with ethanol. Finally the product was dried in an oven at 80  C. In order to prepare 3, 5 and 7% Mn-CeO2/rGO, about 0.0135 mM, 0.0225 mM and 0.0315 mM of MnCl2.4H2O was used respectively. The other reaction conditions such as precursors, reaction parameters and washing procedures are same as discussed for 1% Mn-CeO2/rGO. To study the effect of rGO and Mn properly Mn doped CeO2 and CeO2/rGO were prepared in the similar manner as discussed above. The detail of the synthesis condition is summarized in Table 1.

However, poor conductivity of CeO2 limits its catalytic activity. We have reported the ORR activity of Ceria (CeO2) by integrating it with nitrogen doped graphene (N-graphene). CeO2/N-graphene nanocomposite showed much higher ORR activity as compared to its individual components i.e. N-graphene and CeO2 [37]. Recently, we have reported the enhanced ORR activity of CeO2/gC3N4 composite as compared to its individual counterpart [38]. Parwaiz et al. demonstrated improved ORR activity of Co-doped CeO2/rGO nanocomposite as compared to bare CeO2 [39]. Tan et al. have reported the synthesis of Ni doped CeO2 nanoparticle with enhanced catalytic activity for ethanol elecrooxidation [40]. Recently, Sun et al. reported the enhanced ORR activity of Ag doped CeO2 with Vulcan XC-72 as conducting framework [41]. Liu et al. have reported the enhancement of ORR activity by mixing with CeO2 nanoparticles with Co3O4/KB [42]. For the first time, Chen et al. demonstrated the enhancement in ORR activity on MnOx-CeO2 loaded ketjenblack (KB) [43] and they proposed that CeO2 nanoparticles which are very close to MnOx, help in oxygen transfer to MnOx nanoparticles resulting in much higher ORR activity. Based on these observations by different researchers, it can be concluded that integration of TMO and RMO or doping of transition metal in RMO with conducting support could be a path to achieve higher electrocatalytic ORR activity. In this paper we have reported the catalytic activity of Mn doped CeO2-reduced graphene oxide (rGO) nanocomposites (Mn-CeO2/rGO) synthesized by microwave mediated solvothermal method. Manganese doping percentage was varied to study its effect on ORR activity. Mn-CeO2/rGO nanocomposites showed better stability and methanol tolerance ability as compared to Pt/C describing its importance as promising electrode material.

2.4. Characterization The crystal lattice structure of as synthesized products was analyzed using powder X-ray diffraction using Rigaku Ultima IV X-ray Diffractometer. Thermo Fisher Scientific Nicolet iS5 FT-IR spectrometer with KBr pellets was used to record FT-IR spectra in the range of 4000–400 cm1. The morphological structure of the as synthesized materials was obtained from the Scanning Electron Microscope (SEM) (Zeiss ultra 55). Transmission Electron Microscope (TEM), Model FEI Technai G2S-Twin was used to evaluate the particle growth of synthesized nanoparticles. X-ray photo electron spectra (XPS) was recorded by UHV analysis system (SPECS, Germany) with an Al-Kα monochromatized Xray source (E ¼ 1486.6). Raman spectroscopy was done for better insight into graphitic framework. Raman analysis of the samples were carried out using Horiba HR 800UV confocal Raman spectrophotometer with λ ¼ 632.8 nm and He–Ne laser as excitation source.

2. Experimental details 2.1. Materials Ammonium Cerium (IV) nitrate ((NH4)2[Ce(NO3)6]), Manganese(II) chloride tetrahydrate (MnCl2.4H2O), 1,4 Butane diol, Graphite powder were purchased from Sigma Aldrich. All chemicals were used as received.

2.5. Electrochemical ORR measurements 2.2. Synthesis of graphene oxide The electrochemical measurements were performed in a three electrode system by using a Metrohm Autolab 204 RRDE electrochemical workstation (Metrohm Autolab B.V. Netherland). For ORR measurements, modified glassy carbon (GC), Pt wire and Ag/AgCl (3 M KCl) were used as working electrode, counter electrode and reference electrode respectively. Synthesized electrocatalyst material was loaded on a precleaned GC electrode during preparation of working electrode (S ¼ 0.196 cm2). For the preparation of catalytic ink 5 mg of the as synthesized electrocatalysts were ultrasonically dispersed in a mixture of 2 ml isopropanol, 3 ml double distilled water and 25 μL (0.5%) of Nafion. About 33.3 μL (0.017 mg/cm2) of slurry was dropcasted on a GC electrode and allowed to dry in a decicator. The Cyclic Voltammogram (CV) was recorded in N2 or O2 saturated 0.1 M KOH solution in potential range of 0.8 to 0.2 V vs. Ag/AgCl (scan rate 10 mV/s). Linear Sweep voltammetry (LSV) was performed in O2 saturated 0.1 M KOH solution using rotating disk electrode (RDE) and rotating ring disk electrode (RRDE) (scan rate 10 mV/s). For both RDE and RRDE studies the GC disk is

Graphene oxide was prepared by modified Hummer’s Method. About 1 g of graphite flakes was grinded with 20 g of NaCl and then washed with 250 mL of double distilled water (DD H2O) and then dried at 80  C for 1 h. The flakes were then mixed with 23 mL of H2SO4 and 1 g NaNO3 in a 250 mL round bottom flask and kept for 24 h stirring in an ice bath. 3 g of KMnO4 was added slowly to the above solution. After 1 h of stirring 100 ml of DDH2O and 2 mL of H2O2 was mixed and the temperature was elevated to 90  C–95  C. A thick paste was formed. The product was washed with water and ethanol. The product was dried at room temperature for 24 h. 2.3. Synthesis of Mn doped CeO2/rGO by varying Mn % A facile microwave mediated solvothermal synthesis was employed for the synthesis of Mn-doped CeO2/rGO nanocomposites. To investigate the effect of doping amount of manganese precursor was varied keeping 2

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Journal of Electroanalytical Chemistry 851 (2019) 113480

Fig. 1. (a) XRD pattern of rGO, Mn doped CeO2/rGO nano composites and pure CeO2 (b) Shifting of peak on doping.

modified with 0.017 mg/cm2 Mn-doped CeO2/rGO nanocomposite. The potential of Pt ring in RRDE study was set at 0.6 V vs. Ag/AgCl to oxidise the H2O2; if generated. The collection efficiency of Pt ring was calculated to be N ¼ 0.25. The electro-catalytic stability of nanocomposites towards ORR was tested via current–time (i-t) chronoamperometric technique at 0.35 V vs. Ag/AgCl with RDE electrode rotating at 1000 rpm. 2.6. EIS impedance study The Charge transfer behaviour of electrode electrolyte interface was investigated via EIS using CHI660E electrochemical analyser. EIS was measured in the frequency range of 100 kHz to 50 Hz at open circuit potential at 0.210 V vs Ag/AgCl with an AC voltage of 5 mV. 3. Result and discussion 3.1. Composition and structure characterization of Mn-CeO2/rGO Fig. 1 shows the XRD Patterns of 1%, 3%, 5%, 7% Mn-doped CeO2/ rGO composite. All the composites display characteristic diffraction patterns for CeO2 (JCPDS: 34–0394). However, it is observed that for all composites the most intense diffraction peak for CeO2 is shifted to higher 2θ as compared to pristine CeO2 which indicates incorporation of Mn in the CeO2 crystal lattice. Again, the shift towards higher 2θ values suggests that Mn has replaced some of Cerium atoms from its crystal structure [44]. The crystallite size of CeO2 was calculated using Debye-Scherer formula by taking the FWHM (Full Width at Half Maxima) of the most intense (111) diffraction peak. The crystallite size was measured to be 7.7 nm, 12.07 nm, 8.40 nm, 6.61 nm and 14.14 nm, for pure CeO2 nanoparticles, 1% Mn-CeO2/rGO, 3% Mn-CeO2/rGO, 5% Mn-CeO2/rGO, and 7% Mn-CeO2/rGO nanocomposites, respectively. Loche et al. reported that with increase in La-doping the crystallite domains size of ceria decreases [45]. Bezkrovnyi et al observed downsizing effect upon doping europium in ceria nanotubes [46]. Retardation of the particle growth was due to segregation of dopant ions (Eu3þ) at the surface of ceria nanocrystals. Pichestapong et al. observed that the crystallite size initially decrease and later on it increases with increase in samarium doping [47]. Our result also shows similar trends which needs further investigation. As Mn loading increases, the intensity of the diffraction peaks increases gradually, which represent the enhancement of degree of crystallinity in the samples (Fig. 1a). On enlarging the (111) diffraction peak in the X-ray diffraction pattern (Fig. 1b) the shifting of peak on doping can be better visualised. The shifting of XRD peak towards higher 2θ, on doping indicates larger d-spacing on incorporation of Mn in CeO2 crystal lattice (Fig. 1b).

Fig. 2. FTIR of rGO, Mn doped CeO2/rGO nano composites and pure CeO2.

FT-IR spectra (Fig. 2) of CeO2 sample showed broad and intense band centred at ~3427 cm1 attributing to OH stretching vibration [ν(OH)] of physisorbed water molecules linked to CeO2 nanoparticles. The corresponding bending vibration [δ(OH)] is observed at 1627 cm1. The band at ~523.34 cm1 is attributed to stretching frequency of Ce–O bond. Anti-symmetric and symmetric vibration of adsorbed nitrate group (present in the starting precursor) also found at 1551 cm1 and 1412 cm1 [48]. In addition to Ce–O bond at ~523.34 cm1 FT-IR peaks – O stretching vibration), 1631.31 cm1 (C– –C at 1736.36 cm1 (C– stretching vibrations in epoxide), 1389 cm1 (-OH deformation vibrations) and 1023 cm1 (C-OH stretching vibration in carboxyl group) observed in all composites [39] (Fig. 2). 3.2. ORR activity study The ORR activity was explored in an aqueous 0.1 M KOH solution saturated with N2 or O2 in a potential range of 0.8 V to 0.2 V at a fixed scan rate of 10 mV/ s. The CV studies of Mn doped CeO2/rGO nanocomposites in N2 saturated electrolyte showed redox inactive behaviour (dotted lines) in the potential range 0.8 V to 0.2 V (Fig. 3a). However, in O2 saturated electrolyte, all the nanocomposites as well as CeO2 showed distinct cathodic peak, which can be attributed to reduction of oxygen. 3

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Fig. 3. CV Comparison of (a) Mn- CeO2/rGO nano composites, (b) 5% Mn-CeO2/rGO with CeO2 and 5% Mn-CeO2.

Fig. 4. LSV curves of (a) different % Mn doped CeO2/rGO nano composite, (b) 5% Mn doped CeO2 with Pt/C, 5% Mn doped CeO2, CeO2/rGO and CeO2 at scan rate 10 mV/s and rotation 1600 RPM.

The decreasing order of ORR onset potential values for Mn-doped composites is as follows 1% Mn-CeO2/rGO (0.20 V) < 3% Mn-CeO2/rGO (0.169 V) < 7% Mn-CeO2/rGO (0.162 V) < 5% Mn-CeO2/rGO (0.136 V) (Fig. 3a). Among all the prepared nanocomposites 5% MnCeO2/rGO system showed the best result in term of onset potential. 5% Mn-CeO2/rGO system has a positive shift in ORR onset potential as compared to undoped CeO2/rGO as well as bare CeO2 and 5% Mn doped CeO2 (Fig. 3b). From the comparison CV plot, it was observed that doping of Mn in CeO2 can effectively enhance ORR activity in presence of rGO. With this we can conclude that Mn doping (0.827 V vs. RHE) has enhanced ORR catalytic activity than Co-doping (~0.739 V vs. RHE) in CeO2 in presence of rGO (as reported in the literature) [39]. This may be due to the variable oxidation states of manganese that can heal the oxygen vacancy more perfectly as compared to cobalt. The ORR activity of the catalyst was further study by RDE at different rotation rates. Fig. 4a shows the polarisation curves for all composites at 1600 rpm at a scan rate 10 mV/s. Again, the more positive onset potential and higher current density at 0.9 V (0.152 V vs Ag/AgCl, 4.83 mA/ cm2) of 5% Mn doped CeO2/rGO confirmed its improved catalytic activity for ORR as compared to other Mn-CeO2/rGO composites. This 5% Mn-CeO2/rGO showed better catalytic activity than 5% Mn-CeO2 as well as CeO2/rGO which indicates the synergistic interaction of rGO and Mn doping for enhancement of ORR activity of CeO2 (Fig. 4b). Further, the limiting current density for 5% Mn-CeO2/rGO at 0.9 V Vs Ag/AgCl is comparable to that of Pt/C (JL ¼ 5.71 mA/cm2) (Fig. 4b). The half wave

Table 2 Half wave potential, onset potential, limiting current density of synthesized electrocatalysts obtained from LSV at 1600 rpm. Sample

Half wave potential (V)

Onset potential (V)

Limiting current density (mA/cm2)

CeO2 CeO2/rGO 1% Mn-CeO2/ rGO 3% Mn-CeO2/ rGO 5% Mn-CeO2/ rGO 7% Mn-CeO2/ rGO 5% Mn-CeO2 Pt/C

0.352 0.340 0.392

0.23 0.20 0.20

1.71 2.23 4.02

0.382

0.169

3.83

0.336

0.136

4.83

0.372

0.162

4.02

0.350 0. 09

0.23 0.05

2.51 5.17

potential (E1/2), onset potential (Eonset) and limiting current density (JL) are the important parameter to describe ORR catalytic activity, which is summarized for synthesized catalysts in Table 2. To gain more insight into the ORR mechanism of composites, RRDE technique was employed to directly measure the rate of peroxide formation and electron transfer number (n). Fig. 5 shows the polarisation curve (Id) and the corresponding ring current (Ir) for 5% Mn-CeO2/rGO nanocomposites, 5% Mn-CeO2 and CeO2 in O2 saturated 0.1 M KOH at 4

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n¼

4ID   ID þ INR

(1)

here, the value of ‘N’ is 0.25, denoting the collection efficiency which is a design parameter provided by the RRDE manufacturer. Further the fraction of peroxide intermediate produced in the reaction can be estimated by the following equation

%HO2 

  200 INR   ¼ ID þ INR

(2)

Fig. 6a and b shows the variation of n value and % of HO 2 formation respectively in the studied potential range for 5% Mn-CeO2/rGO along with CeO2 and 5% Mn-CeO2. By analysing both CV and RRDE data the reaction kinetics can be explained clearly. The CV curves for all composites show two reduction peaks due to ORR. Hence it is expected that ORR catalytic reaction may occur in two pathways. The RRDE data supports the result obtained from CV. The RRDE data reveals the ring current first increases in the potential window (0.3 V vs. Ag/AgCl to 0.5 V vs. Ag/AgCl) and then decreases in the potential window (0.5 V vs. Ag/AgCl to 0.7 V vs. Ag/AgCl). The corresponding n value decreases from 3.6 (0.2 V) to 2.5 (0.4 V) and then increases to 3.6 (0.6 V). In low overpotential region, the number of electron count decreases to ~2 resulting production of H2O2 which further reduces to HO 2 (58.45%) as a result ring current increases. In high overpotential region (0.6 V) the

Fig. 5. RRDE comparison of 5% Mn doped CeO2/rGO nano composite with 5% Mn doped CeO2, CeO2/rGO and bare CeO2 at 1600 rpm.

1600 rpm. In RRDE, the GC disk acts as working electrode and the concentric Pt ring detects the peroxide production [49]. From the Id and Ir values in the curves n value can be calculated by the following equation:

Fig. 6. Comparison of (a) Electron transfer number, (b) % of H2O formation of 5% Mn doped CeO2/rGO nano composite with 5% Mn doped CeO2 and bare CeO2.

Fig. 7. (a) FESEM and (b) TEM of 5% Mn doped CeO2/rGO nano composite (inset SAED pattern). 5

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doped CeO2 is 2.6 and 3.2 respectively. The high n value and low HO 2 generation in 5% Mn-CeO2/rGO is due to the synergistic interaction of rGO and Mn doping, which is clearly demonstrated in Fig. 6. As the 5% Mn-CeO2/rGO proved to be better ORR catalyst among all composite its morphology, particle size distribution, chemical bonding and graphitic structure was analyzed via SEM, TEM, XPS and Raman spectroscopy. The Mn doped CeO2 nanoparticles anchored to rGO is clearly observed in FESEM image (Fig. 7a). The morphology of 5% Mn-CeO2/ rGO composite was characterized by TEM. Fig. 7b shows the typical TEM image of 5% Mn-CeO2/rGO. It is observed that large number of 5% Mn doped CeO2 were uniformly distributed on rGO surface, which indicates the presence of graphene sheet obstructs agglomeration of CeO2 nanoparticles. Raman spectroscopy is a non-destructive and useful tool for characterization of graphene based materials [37]. It can be seen that two remarkable Raman peaks of graphene at about 1300 cm1 and 1590 cm1 is observed for 5% Mn-CeO2/rGO nanocomposite, indicating the presence of graphene. The D band (~1300 cm1) is associated with A1g mode of structural defects in sp2 network and G band (~1590 cm1) is associated with E2g Raman mode of graphitic carbon in graphene. The peak intensity ratio of the D band and G band (ID/IG) is often used to measure the degree of disorder and average size of the sp2 domain in graphene based materials [37,39]. The observed ID/IG ratio is least for GO, i.e. 0.92 and the value increases to 1.16 for rGO and for 5% MnCeO2/rGO nanocomposite the value is ~1.0 suggesting formation of more number of defect sites in sp2 domain (Fig. 8a). Further, one strong Raman peak centred at about 460 cm1 dominates the spectrum. This originates from the F2g Raman active mode of CeO2 cube structure i.e. a symmetrical stretching mode of the Ce–O8 vibrational unit [39,50] which is very sensitive to any disorder in the oxygen sub-lattices. In 5% Mn-CeO2/rGO nanocomposites the peak appears more asymmetric and shifts to lower wave number i.e. to 415.05 cm1 (Fig. 8b) [51]. The peak near 263.45 cm1 may be due to disorder in the system [52], and the peak at 553.88 cm1 may be attributed to the presence of Ce3þ and

Fig. 8. (a) Raman spectra of GO, rGO, CeO2/rGO and 5% Mn doped CeO2/rGO, (b) Raman spectra of 5% Mn doped CeO2/rGO in full scan range (inset enlarged view of marked region).

number of electron count nearly equal to direct 4e process. At 0.6 V vs Ag/AgCl, 5% Mn-CeO2/rGO adopted dominant 4-electron pathway with high n value i.e. 3.5. The n value at 0.6 V for undoped CeO2 and 5% Mn

Fig. 9. Deconvoluated XPS spectrum of (a) XPS survey spectrum, (b) Ce 3d5/2 and Ce 3d3/2, (c) O-1 s, (d) C-1s and (e) Mn-2p3/2 and Mn-2p1/2. 6

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Fig. 11. Methanol tolerance of 5% Mn doped CeO2/rGO nano composite.

Fig. 10. (Current–time) chronoamperometric responses for ORR on 5% Mn doped CeO2/rGO nanocomposite and commercial Pt/C at 0.35 V at a rotational rate of 1000 rpm.

oxygen vacancies [50] (inset Fig. 8b). The X-ray photoelectron spectroscopy (XPS) measurement was performed to understand the composition and chemical nature of Mn-CeO2/ rGO nanocomposites. The XPS survey spectra of 5% Mn-CeO2/rGO shows peaks at 285, 532, 881 and 897 eV corresponds to C 1 s, O 1 s, Ce 3d5/ 2and Ce 3d3/2 respectively (Fig. 9a). In Fig. 9b the peaks at 881.3, 884.2 and 886.4 eV can be attributed to 3d5/2 of Ce4þ(45.81%), Ce (33.09%) and Ce3þ(21.09%) core electrons respectively, whereas the peaks at 897.5 and 900.5, 904.0 eV corresponds to 3d3/2 of Ce3þ, Ce and Ce4þ respectively [37]. The deconvoluted O 1 s spectrum can be attributed to three peaks at 529.4 (CeO2/MnO2), 531.2 (COOH) and 532.4 eV (-C¼O), 533.7 (-C-OH) suggesting formation of CeO2 on rGO (Fig. 9c). The XPS spectra of the C 1s core level for 5% Mn-CeO2/rGO can be deconvoluated into 3 components i.e. 63.02 (atm.) % C–C (284.8 eV) (of total carbon content), C–OH (286.3 eV) and COO groups (288.6 eV), indicating the presence of sp3 carbon atom and carbon atom bonded O moieties (Fig. 9d). Fig. 9e represents the high resolution Mn 2p spectra of 5% MnCeO2/rGO, which is deconvoluted into 4 peaks. Two peaks at 640.6 and 642.8 eV is assigned to contribution from Mn2þ and Mn4þ for Mn-2p3/2 and another two peaks at 654.0 and 656.8 eV is assigned to contribution from Mn2þ and Mn4þfor Mn-2p5/2 [53–55]. The relative areas (%) of Mn4þ is 49.5% and Mn2þ is 50.4% showing that the Mn species on the surface existed in both forms. Combining the change of valence states of Ce and Mn species, it can be considered that the presence of Mn species promoted the oxidation of Ce species that leads to the oxygen vacancy [56], which in turn provide active sites in ORR catalysis.

Fig. 12. CV cycle of 5% Mn doped CeO2/rGO electrocatalysts in O 0.1 M KOH solution before and after 300 cycles.

2

-saturated

3.3. Stability of the catalyst and methanol tolerance test Stability of electrocatalyst is an important parameter to be considered in catalysis. Hence the stability of 5% Mn-CeO2/rGO was tested via chronoamporometry. Chronoamperometric studies showed that, after 12,500 s of ORR test, relative current value for Pt/C decreased by 40%, while a decrease of only 20% was observed for 5% Mn-CeO2/rGO. This shows the better ORR stability of 5% Mn-CeO2/rGO to that of commercial Pt/C catalyst, which could be ascribed to presence of highly stable active sites formed due to the doping of Mn to CeO2 crystal system, and stronger interaction between 5% Mn-CeO2 and rGO (Fig. 10). Methanol poisoning of the cathode impacts the ORR process. So it is very much essential to address another important factor of ORR catalyst i.e. methanol tolerant capability [57]. The methanol tolerance capability of 5% Mn-CeO2/rGO nanocomposite and 20 wt% Pt/C were investigated and compared in Fig. 11. For this study, about 10 v/v% of 3 M methanol

Fig. 13. Nyquist plots obtained from EIS measurements of CeO2/rGO, Pt/C and Mn-CeO2/rGO (inset: Fitted circuit). 7

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was added at 600 s of chronoamperometric measurement. This resulted in the decrease in relative current by 29% for 20 wt% Pt/C, and only13% for 5% Mn-CeO2/rGO. This result clearly demonstrates the better methanol tolerance capability of 5% Mn-CeO2/rGO over commercial Pt/C in alkaline medium. Fig. 12 shows the ORR stability of 5% Mn-CeO2/rGO in CV test. In this study 5% Mn-CeO2/rGO modified GC electrode is cycled 300 times in O2 saturated electrolyte. We observed slight decrease in ORR current after 300 cycles. However, the ORR onset potential value remains almost constant indicating enhanced ORR durability of 5% MnCeO2/rGO nanocomposite.

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3.4. Charge transfer rate studied by electrochemical impedance spectroscopy (EIS) Electrochemical impedance spectroscopy (EIS) is an useful tool to investigate on Charge transfer behaviour [58]. For better understanding the effect of doping in CeO2 and 5%Mn-CeO2 modified electrodes were tested via EIS. The obtained Nyquist Plots (Fig. 13) were fitted to corresponding circuits (inset Fig. 13) which indicates lower charge transfer resistance of 5% Mn-CeO2/rGO (RCT ¼ 55.3 Ω, RS ¼ 12.92 Ω) than CeO2/ rGO (RCT ¼ 104.4 Ω, RS ¼ 5.321 Ω) and Pt/C (RCT ¼ 73.3 Ω, RS ¼ 3.42 Ω) which supports the enhanced electron transfer kinetics of 5%Mn-CeO2/ rGO [39]. 4. Conclusion In accordance with the study report a simplistic and single step sustained microwave assisted solvothermal synthesis is adopted for synthesis of Mn doped CeO2/rGO nanocomposites. The XRD, XPS and Raman results reveals successful doping of Mn in CeO2 crystal structure resulting in oxygen vacancies. The rGO in the composite increases electronic conductivity and promotes ORR catalytic activity on Mn doped CeO2. An optimized 5%Mn-CeO2/rGO with highest ORR activity proceeds via facile ~4 electron process with better methanol tolerance ability than commercial Pt/C. The low cost and facile synthesis procedure predicts the future utility of the 5%Mn-CeO2/rGO composite in energy conversion system to meet energy crises. Acknowledgements This work is funded and supported by DST SERB, India (Grant no. EMR/2016/006050). References [1] A.S. Arico, S. Srinivasan, V. Antonucci, DMFCs: from fundamental aspects to technology development, Fuel Cells 1 (2001) 133–161. [2] T. Tsujiguchi, M.A. Abdelkareem, T. Kudo, N. Nakagawa, T. Shimizu, M. Matsuda, Development of a passive direct methanol fuel cell stack for high methanol concentration, J. Power Sources 195 (2010) 5975–5979. [3] Z. Chen, M. Waje, W. Li, Y. Yan, Supportless Pt and Pt-Pd nanotubes as electrocatalysts for oxygen-reduction reactions, Angew. Chem. Int. Ed. 46 (2007) 4060–4063. [4] J. Zhang, D.P. He, H. Su, X. Chen, M. Pan, S.C. Mu, Porous polyanilinederived FeNxC/C catalysts with high activity and stability towards oxygen reduction reaction using ferric chloride both as an oxidant and iron source, J. Mater. Chem. A 2 (2014) 1242–1246. [5] C. Xiong, Z.D. Wei, B.S. Hu, S.G. Chen, L. Li, L. Guo, Nitrogendoped carbon nanotubes as catalysts for oxygen reduction reaction, J. Power Sources 215 (2012) 216–220. [6] Y. Nie, X.H. Xie, S.G. Chen, W. Ding, X.Q. Qi, Y. Wang, Towards effective utilization of nitrogen-containing active sites: nitrogen-doped carbon layers wrapped CNTs electrocatalysts for superior oxygen reduction, Electrochim. Acta 187 (2016) 153–160. [7] S. Hussain, H. Erikson, N. Kongi, M. Merisalu, P. Ritslaid, V. Sammelselg, Heattreatment effects on the ORR activity of Pt nanoparticles deposited on multi-walled carbon nanotubes using magnetron sputtering technique, Int. J. Hydrog. Energy 42 (2017) 5958–5970. [8] J. Stacy, Y.N. Regmi, B. Leonard, M. Fan, The recent progress and future of oxygen reduction reaction catalysis: a review, Renew. Sust. Energ. Rev. 69 (2017) 401–414.

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