C as an ORR catalyst in air-cathode microbial fuel cell

Accepted Manuscript Low-cost nanowired α-MnO2/C as an ORR catalyst in air-cathode microbial fuel cell

Mir Reza Majidi, Fatemeh Shahbazi Farahani, Mirghasem Hosseini, Iraj Ahadzadeh PII: DOI: Reference:

S1567-5394(18)30231-7 doi:10.1016/j.bioelechem.2018.09.004 BIOJEC 7206

To appear in:

Bioelectrochemistry

Received date: Revised date: Accepted date:

17 June 2018 5 September 2018 6 September 2018

Please cite this article as: Mir Reza Majidi, Fatemeh Shahbazi Farahani, Mirghasem Hosseini, Iraj Ahadzadeh , Low-cost nanowired α-MnO2/C as an ORR catalyst in aircathode microbial fuel cell. Biojec (2018), doi:10.1016/j.bioelechem.2018.09.004

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ACCEPTED MANUSCRIPT Low-cost Nanowired α-MnO2/C as an ORR catalyst in air-cathode microbial fuel cell Reza

Majidi

1,*

[email protected],

Fatemeh

Farahani

1,*

3

1

CR

IP

2 [email protected], Mirghasem Hosseini , Iraj Ahadzadeh

Shahbazi

T

Mir

Deptartment of Analytical Chemistry, Faculty of Chemistry, University of Tabriz, 51664

2

US

Tabriz, Iran

Electrochemistry Research Laboratory, Department of Physical Chemistry, Tabriz University,

Research Laboratory for Electrochemical Instrumentation and Energy Systems, Department of

M

3

AN

Tabriz, Iran

Corresponding authors:

Abstract

AC

CE

*

PT

ED

Physical Chemistry, Faculty of Chemistry, University of Tabriz, Tabriz, Iran

In this work, low cost α-MnO2 nanowires and α-MnO 2 nanowires supported on carbon Vulcan (α-MnO2 /C) have been synthesized via a simple and facile hydrothermal method for application in microbial fuel cells. The prepared samples have been characterized by X-ray diffraction (XRD), Raman spectroscopy and field emission scanning electron microscopy (FESEM). Electrocatalytic activities of the samples have been evaluated by means of cyclic 1

ACCEPTED MANUSCRIPT voltammetry

(CV),

linear

sweep

voltammetry

(LSV)

and

electrochemical

impedance

spectroscopy (EIS) in a neutral phosphate buffer solution. EIS was performed at different potentials to gain further insight into the kinetic properties of α-MnO2 /C. Both catalysts were used in air cathode microbial fuel cells to achieve power densities of 180 and 111 mWm-2 for α-

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MnO2 /C and pristine α-MnO2 nanowires, respectively. α-MnO2 /C functions as a good and

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economical alternative for Pt free catalysts in practical MFC applications, as shown by the

US

CR

findings of stability test and voltage generation cycles in long-term operation of MFC.

AC

1 Introduction

CE

PT

ED

M

AN

Keywords: α-MnO2 nanowire, α-MnO 2 /C, ORR, Microbial fuel cell

Microbial fuel cells (MFC) are novel sources of energy which convert organic substrates into electrical energy via biological and electrochemical reactions and are used for wastewater treatment. This application has attracted many researchers’ increasing attention during the last decades to achieve promising sustainable energy (1, 2). However, real applications of MFCs are limited because of their low small scale power yield and high cost of materials, in particular catalysts used to enhance the oxygen reduction reaction (ORR) at the cathode side (3-6). As a

2

ACCEPTED MANUSCRIPT matter of fact, oxygen reduction takes place by two possible pathways: oxygen reduction to water or peroxide by direct 4-electron or 2-electron pathways, respectively (7-9). The performance of ORR with 4-electron pathway is usually enhanced by platinum (Pt) based catalysts due to their high efficiency. Platinum catalyst has been extensively used for application

and low stability in polluted media

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which makes commercialization of MFCs uneconomical,

T

in MFCs. However, the application of platinum has recently been stopped given its high cost,

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containing anions such as chloride as well as methanol and sulfur (10). The high cost and limited accessibility of Pt, regretfully, restrict the further scaling-up MFCs. Consequently, non-precious

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metal catalysts have been extensively investigated as Pt alternatives in fuel cells (11-16). The

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application of platinum group metal (PGM) free catalysts is a feasible alternative approach for the reduction of costs and simultaneously increasing ORR. PGM free catalysts, prepared by

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supporting transition metals such as Fe, Co, Zr and Ni on different supports, have been

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reportedly used as cathodes in MFCs to yield electrochemical performance comparable to Pt (14,

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17-19).

Electrode materials based on manganese dioxide are promising candidates in green

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energy conversion/storage systems extending from fuel cells to metal/air batteries on account of

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their low cost, abundance, high energy density and being ecologically well-disposed (20-23). This ceramic compound is considered as an effective catalyst for ORR in MFCs as well (24-26). The ORR activity of MnO 2 is influenced by different factors such as size, morphology (common nanoparticles, 1D or 2D nanostructures), crystal structures (α, β and γ) and surface area (27, 28). α and β-MnO2 nanoflowers/nanorods/nanotubes can be prepared by hydrothermal synthesis using Mn+2 and MnO 4 - as manganese sources. The following Eq. (1) is the overall reaction for KMnO 4 and MnSO 4 staring materials (29): 3

ACCEPTED MANUSCRIPT 2 KMnO 4 + 3 MnSO 4 + 2 H2 O → 5 MnO 2 + K2 SO4 + 2H2 SO4 Among different kinds of

(1)

MnO 2 nanoparticles, nanotubes and nanowires (1D

nanostructures), MnO 2 nanowires are considered the

best due to their large surface area,

transmission of flexibility and stability in the form of thin films (30). However, MnO 2

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T

nanostructures individually are not effective enough to give the desirable reduction kinetics toward oxygen molecules because of their lower electrical conductivity and specific capacitance,

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which encourages the use of highly conductive materials such as graphene, porous carbon,

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carbon nanotubes and activated carbons as MnO 2 supports (31, 32). Electricity and power generation are consistent with publications, which report on the performance of MFCs assembled

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with MnO x or platinum free catalyst based cathodes, considering various cell configurations and

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operation conditions (13, 19, 33-35).

ED

In this work, α-MnO2 nanowires and α-MnO2 /C have been synthesized by a facile hydrothermal method. MnO2 nanostructures synthesized hydrothermally are frequently applied

PT

in batteries and pseudocapacitors (36, 37). Nevertheless, the application of MnO 2 nanostructures in MFCs and more extensively comparative analysis of their performance have hardly been

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reported. To the best of the authors’ knowledge, the catalytic characteristics of hydrothermally

AC

synthesized α-MnO2 nanowires supported on carbon Vulcan as air cathode catalysts in microbial fuel cells have been studied for the first time in this work. 2 Experimental 2-1 Materials Preparations In order to synthesize manganese oxide nanowires supported on carbon, 2.1 mg/mL of XC-72R Carbon Vulcan (C), supplied by Cabot Corporation, was dispersed in 105 mL of DI

4

ACCEPTED MANUSCRIPT water under sonication. 0.65 mmol of MnSO 4 (Aldrich) and 1.22 mmol of KMnO 4 (Aldrich) with molar ratio of 1.87:1 were then added to the suspension and the mixture obtained was stirred 30 minutes at 80 °C (38). Afterwards, the mixture was subjected to sonication for about 90 minutes. Eventually, the suspension was transferred to a 200 mL Teflon lined stainless steel

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autoclave and heated at 180 °C for 12 hours in a furnace. The product was finally washed with

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water and ethanol several times and dried in an oven at 40 °C. The final product was labeled as

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α-MnO2 /C. α-MnO2 nanowires were synthesized by the same procedure, except for the addition

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of carbon Vulcan. 2-2 Materials Characterizations

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The morphology of α-MnO2 nanowires and α-MnO2 /C was studied by field emission

M

electron microscopy (FE-SEM, MIRA3FEG-SEM, Tescan). The crystallographic structure of manganese oxide were performed by a Philips PW1730 X-ray diffractometer with Cu K α

ED

radiation (λ=1.5406 Å). Raman spectroscopy was carried out using a Bruker Instruments, model

measurements

including

cyclic

voltammetry

and

linear

sweep

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Electrochemical

PT

SENTERRA (2009) Germany spectrophotometer with laser number fixed at 532 nm.

voltammetry were carried out by a Potentiostat/Galvanostat Autolab PGSTAT 30 (Eco Chemie

AC

B. V., The Netherlands) instrument. EIS tests were performed using an EG&G Princeton applied Research PARSTAT 2263 advance electrochemical system (USA). Fitting and analysis of EIS data were done by Zview software. The electrolyte for electrochemical experiments was a 100 mM phosphate buffer solution saturated by bubbling nitrogen and oxygen. Electrochemical tests were carried out in a three electrode system consisting of modified glassy carbon, saturated calomel and Pt wire as working, reference and counter electrodes, respectively. All electrodes were purchased from Azar electrode (Iran). To prepare modified electrode with loading of 0.31 5

ACCEPTED MANUSCRIPT mg/cm2 , 4 mg of the catalyst and Pt were dispersed by ultrasonication in 455 μL of ethanol, water and 5.0 wt. % Nafion (Aldrich) with volume ratio of 5.4:2.7:1. 7 µL of the catalyst ink were then deposited on the surface of glassy carbon electrode with surface area of 0.196 cm2 . Cyclic voltammetry experiments were carried out in a potential window of +1.1V  -0.8 V (vs.

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SCE) at a scan rate of 10 mV/s. Linear sweep voltammetry with rotating disc electrode was

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performed in a potential window of +0.25 V  -1 V (vs. SCE) at 10 mV/s and different rotation

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speeds of 200 to 2500 rpm. Impedance spectra were registered at different potentials of E = 0.00,

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-0.25, -0.5, -0.75 V (vs. SCE) over a frequency range of 10 mHz to 100 kHz for α-MnO2 /C catalyst, as the representative catalyst.

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2-3 MFC operation

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2-3-1 Cathode preparation

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Cathodes for microbial fuel cell were prepared by brushing α-MnO2 nanowires and αMnO2 /C catalysts on GDL carbon cloths (AvCarb P75). The diffusion layer was faced to air and

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the catalyst layer was exposed to the solution. 5 mg of the catalyst were dispersed in 0.83 µL of DI water, 6.67 µL of Nafion solution and 3.33 µL of 2-propanol, followed by brushing on the

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surface of carbon cloth and air drying overnight at room temperature (18, 39). The amount of

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catalysts and Pt loading were 0.5 mg/cm2 . 2-3-2 MFC operation MFC were designed in single chamber air cathode with a volume of 28.84 mL and cathode with a diameter of 3.5 cm and graphite brush as the anode. The fiber brush electrodes were heated to 450°C for half an hour in ambient air prior to use (40). MFCs were first acclimated using a mixture of domestic wastewater (Tabriz sludge) with mixed culture and 100 mM phosphate buffer solution. Feeding solution was composed of 9.15 mg/mL NaHPO 4 , 4.9 6

ACCEPTED MANUSCRIPT mg/mL NaH2 PO4 , 0.62 mg/mL NH4 Cl, 0.26 mg/mL KCl and 1 g/L sodium acetate (41). Two air cathode single chamber MFCs, which were equipped with same anodes for each catalyst, were assembled for comparison of the performance of the cathodes prepared. The cells were operated in parallel and batch modes. MFC polarizations were measured by successively lowering the

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value of a homemade precision resistor box externally connected to the anode and cathode via

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some wire and two crocodile clips and simultaneously recording the potential across the load

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resistor (42). The resistance of box variation was between 10000 and 10 Ω and the potential was registered every 30 minute at room temperature (23 ± 1 °C). Moreover, voltage generation cycles

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were acquired under external resistance of 1000 Ω and plotted after 48 hours.

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3 Results and discussion

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3-1 Catalysts characterization

ED

To observe the crystalline phase of the prepared samples, XRD was used. Fig. 1a shows the diffraction peak and pattern related to MnO 2 nanowire. All the diffraction peaks can be

PT

indexed to the tetragonal phase of α-MnO 2 (JCPDS 44-0141) with no other characteristic peaks related to other phases or impurities according to the literature (43, 44). Additionally, Fig. 1b

CE

shows the Raman spectra as a further indication of the presence of manganese oxides and

AC

carbonic features of its support. There is a distinctive peak at 620.92 cm-1 corresponding to the stretching vibration of Mn-O bonds (45). Raman spectra of C and α-MnO2 /C show doublet peaks at (1342.18, 1582.81 cm-1 ) and (1346.13, 1594.51 cm-1 ), associated with D-band or defect and Gband or vibration of sp2 carbons in carbonic structures (46). Fig. 2 shows the morphology of the prepared samples characterized by scanning electron microscopy (SEM). Fig. 2 (a to d) shows the SEM images at different magnitude resolutions. It is clearly observed that α-MnO2 nanowires are properly spread (Fig. 2a,b) over the carbon 7

ACCEPTED MANUSCRIPT Vulcan particles (Fig. 2c,d) so that a porous surface has been formed. In general, the diameter and length of MnO 2 nanowires prepared by hydrothermal method were in the range of 5-50 nm and several tens of micrometers, respectively (47, 48).In this study the average diameter of αMnO2 nanowires with and without carbon support is estimated as 43.12 ± 5.06 and 40.54 ± 7.50,

IP

T

respectively, which are distributed from 30 to 50 nm and consistent with literatures (48, 49). 3-2 Electrochemical characterizations

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For further characterization of the prepared samples, cyclic voltammetry was applied.

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Fig. 3 shows the electrochemical response of α-MnO2 and α-MnO2 /C toward ORR in N 2 and O 2 saturated PBS. A reduction peak in N 2 saturated solution was shown by the CV curve

AN

corresponding to α-MnO 2 supported with carbon. This can be justified as a result of carbon

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Vulcan ORR activity, which is unavoidable for carbon based material interfaces and oxygen functionalities. In fact introduction of H2 SO4 in the synthesis procedure can lead to oxygen

ED

functionalization (such as hydroxyl, carboxyl, carbonyl and others) on the surface of carbon

PT

Vulcan during hydrothermal (17, 26, 50). It is clearly observed that both α-MnO 2 /C and α-MnO2 are active toward oxygen reduction because of the appearance of such an intense peak in the

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presence of oxygen inside the solution. The reduction potential for α-MnO2 /C is more positive

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(E= -0.27 V vs. SCE) than that of α-MnO2 (E= -0.57 V vs. SCE) and the current density for αMnO2 is much lower (J= -0.16 mA/cm2 ) than that of α-MnO2 (J= -0.55 mA/cm2 ). In general, CV results indicate that α-MnO2 /C is more catalytically active toward oxygen reduction. In order to further evaluate the α-MnO 2 ORR kinetics on prepared electrocatalysts, linear sweep voltammetry was performed in the presence of O 2 at rotation rates of 200 to 2500 rpm (Fig. 4a). The faster the rotating rates, the easier will be the O 2 flux to the electrode surface followed by increasing current densities. The same experiments were performed on the electrode 8

ACCEPTED MANUSCRIPT loaded with Pt/C as a reference and the results were compared with those of α-MnO2 /C and αMnO2 at a rotation rate of 1600 rpm (Fig. 4b). It can be observed in Fig. 4b that the onset potential of α-MnO2 occurs at less positive potentials (Eonset = -0.46 V vs. SCE). Moreover, αMnO2 /C possesses a more positive onset potential (Eonset = -0.2 V vs. SCE) and higher limiting

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current density (JL= -4.00 mA/cm2 ) compared to α-MnO 2 (JL= -2.07 mAcm-2 ) and is roughly

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comparable with Pt/C (Eonset = 0.18 V vs. SCE, JL= -4.46 mA/cm2 ). This means that α-MnO2

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supported on carbon has an increased ORR activity as a result of higher surface area compared

US

with pristine α-MnO2 . These results are consistent with CV results as well. Koutecky-Levich (KL) theory was used to analyze the data by measuring the current

AN

using Eq. (2):

M

(2)

ED

in which JK and JD are the kinetic current density and limiting current density, respectively. JD is as following equation (Eq. (3)): ⁄

⁄

PT

⁄

(3)

CE

in which n is the number of exchanged electrons, F is Faraday’s constant (C/mol), C is the concentration of oxygen (mol/L), D is oxygen diffusivity (cm2 /s),  is the electrolyte

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kinematic viscosity (cm2 /s) and ω is the electrode rotation rate (rad/s) (12). The Koutechy-Levich (J-1 vs. ω-1/2 ) corresponding to α-MnO2 /C as the representative catalyst at different potentials were plotted and showed good linearity and parallelism, indicating that ORR kinetics is first order (51). This means that the slopes at different potentials from -0.5V to -0.8V remains stable and therefore the number of electrons exchanged during the ORR can be approximately constant in this potential range (Fig. 4c).

9

ACCEPTED MANUSCRIPT Furthermore, the kinetic parameters for α-MnO2, α-MnO2 /C as the representative catalyst and Pt/C as reference based on plotting the inverse measured current density vs. inverse square root of the rotation rate, KL plots obtained at E= -0.6 V (vs. SCE) were analyzed and plotted as Fig. 4d and listed in Table 2 (52). The number of electrons calculated for α-MnO2 /C was 3.51 ±

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0.01, which is an electron transfer route involving almost four electrons and shows excellent

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ORR activity.

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To evaluate kinetics properties and having a comparison of Carbon Vulcan and α-

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MnO2 /C, EIS was used for both in N 2 -and O 2 -saturated PBS solution. To have further insight into the reaction kinetics of α-MnO2 /C surface as a representative catalyst, EIS was performed at

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different potentials to study O 2 reduction process. Fig. 5a,b demonstrate the Nyquist plots and

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Fig. 5c shows the equivalent circuit model fitting well for all plots. Rs, Rct and CPE are the solution resistance, charge transfer resistance and constant phase element corresponding to

ED

double layer capacitance in non-homogeneous surface, respectively. According to the plots, the

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charge transfer related to the redox reaction on the surface of the electrode is the main process throughout the range of due frequencies. Additionally, the values of C DL were calculated

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according to the following Eq. (4) (52, 53): 1

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( R  CPE )   ct Rct

C DL

(4)

in which α is the deviation from ideality and roughness of the surface. All parameters extracted from EIS fitting results at different potentials and the values of relevant CDL are presented in Table 1 and Table 2. According to Fig. 5a and related data in Table 1, less amount of Rct and higher amount of C DL for α-MnO 2 /C in both N 2 - and O 2 -saturated PBS 10

ACCEPTED MANUSCRIPT solution can be seen which is concluded that by exposing α-MnO2 nanowires to carbon Vulcan, charge transfer occurs with higher rate and double layer capacitance increases. This fact can be caused by synergistic effect of Carbon and α-MnO2 which is compatible with CV and LSV results as well. A significant difference in diameter of the semicircle arc can be seen in N 2 - and

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O2 -saturated solution which obviously indicates that the electron transfer occurred between

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oxygen molecules and electrode surface modified with α-MnO2 /C catalyst. This can be an

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evident that the charge transfer semicircle diameter is related to both presence of oxygen and rate of oxygen reduction. In other word, α-MnO2 nanowires intensified the capacitive properties of

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capacitance and rate of oxygen reduction reaction.

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carbon Vulcan electrode by providing higher surface area and porosities which led to higher total

Table 1- EIS parameters for carbon Vulcan and α-MnO2 /C from fitting of EIS spectra in N2 - and O2 -saturated PBS solution at potential of -0.25 V vs. SCE.

α

52660

7.83

0.81

8.37x10-5

AC

R ct (Ω)

0.38

O2 98.65

N2 101.80

R s (Ω)

CPE (Ss α) CDL (mF/cm2 )

α-MnO2 /C

CE

Parameters

0.118

Carbon Vulcan 0.76

N2 100.34

2401

2.11

0.29

0.81

0.92

8.78x10-5 0.060

0.70

O2 97.45

1.33

70546

4.00

3727

2.81

0.84

0.89

0.22

0.86

0.65

3.02

2.66x10-5

1.04

3.45x10-5

3.39

0.028

0.024

11

ACCEPTED MANUSCRIPT According to Fig. 5b and Table 2 also, as one proceeds from zero to -0.25 V vs. SCE, where the oxygen reduction reaction mainly takes place, the charge transfer resistance and C DL sharply decrease. In addition, on bypassing the reduction potential towards negative values, the Rct and CDL become slightly larger again. This further verifies the previous results about α-

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MnO2 /C catalyst. Moreover, according to theories, the double layer capacitance consists of

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charges, which do not participate in faradic reactions (54, 55). Evidently, at potentials involving

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electrochemical reactions, the amount of charges with no faradic reaction, which can build up double layer, may decrease and replace with redox ions. This is why the minimum amount of

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CDL is observed at potential of -0.25 V vs. SCE, in which the electrochemical redox reactions are

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predominant. By moving away from this potential, C DL increases as faradic reactions decrease. The same phenomena was observed in Table 1, when oxygen molecules were present around the

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α-MnO2 /C electrode, double layer capacitance decreased (C DL in O 2 : 0.060 mF/cm2 ) compared

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with absence of oxygen (C DL in N 2 : 0.118 mF/cm2 ). The capacitance of the MnO 2 electrodes results mostly from pseudocapacitance, which is associated with the reversible redox reaction

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between Mn4+ and Mn3+, as well as double layer charge storage mechanism. The proton (H+) or

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alkali metal cation insertion/deinsertion into the MnO 2 structure supports this transition process: (5)

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MnO 2 + C+ + e- → MnOOH(s)

in which C+ shows H+ and alkali cations (Li+, Na+ and K +) (56). Oxygen reduction in the presence of oxygen in the solution and at the potential range of oxygen reduction succeeds this pseudocapacitance MnO 2 behavior. The following mechanism may be used to explain the corresponding ORR on the α-MnO2 /C surface in a protic solution (26): MnO 2 + H+ + e- → MnOOH(s)

(6a)

12

ACCEPTED MANUSCRIPT 2MnOOH(s) + O 2 → 2MnOOH(O 2ad)

(6b)

MnOOH(O 2ad) + e- →OH- + MnO2

(6c)

In the beginning, MnO 2 placed and deposited on the cathode surface, predominantly act

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as electron and proton acceptor reduce to MnOOH, which then absorbs oxygen to form the

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intermediate. During this process, the electron for the oxygen molecule is provided by the

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intermediate, which also accelerates oxygen reduction by continuous electron supply. This process continuously recycled by MnO 2 . As a matter of fact, The higher rate of oxygen reduction

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(during faradaic reaction of 6a to 6c), the lower amount of ions have chance to be adsorbed on the surface of electrode as double layer capacitance (57, 58). This has resulted that we witness

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the decrease of C DL.

Parameters

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Table 2- EIS parameters for α-MnO2 /C from fitting of EIS spectra at different potentials in O2 -saturated PBS.

Different potenials (vs. SCE)-O2 E = -0.25 Errors E = -0.50 Errors V % V % 98.65 0.76 97.40 0.55

Errors % 0.36

42296

7.14

2401

2.11

6074

α

0.83

0.29

0.81

0.84

CPE (Ss α)

8.57x10-5

0.90

8.78x10-5

3.02

R s (Ω) R ct (Ω)

CDL (mF/cm2 )

AC

E = 0.00 V 103

0.111

0.060

E = -0.75 V 101.10

Errors % 0.37

2.16

16443

2.92

0.82

0.49

0.84

0.30

8.03x10-5

1.68

7.97x10-5

0.97

0.068

0.083

13

ACCEPTED MANUSCRIPT 3-3 MFC test To confirm the previous findings and investigate the performance of the prepared catalysts, α-MnO 2 /C and α-MnO 2 and Pt/C (as reference) were acquired in microbial fuel cells for having cathode polarization and power density curves. The relative parameters were indexed

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in Table 2. As shown in Fig. 7, the open circuit voltage for α-MnO2 /C is 0.46 ± 0.01 V, which is

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higher than that of α-MnO2 , with a voltage of 0.38 ± 0.01 V. On the other hand, there is an

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improvement of maximum power density for α-MnO2 /C (PD= 180 ± 4.65 mWm-2 ) with current density of 836.6 mAm-2 , which is higher than that of pristine α-MnO2 as a synergistic effect (111

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± 3.25 mWm-2 for power density and 496.1 mAm-2 for current density).

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Table 3- Electrochemical parameters extracted from KL Plot at potential of -0.6 V, polarization and power density curves

n @ -0.6 V 2.15 ± 0.01 3.51 ± 0.01 4.03 ± 0.02

OCV V 0.38 ± 0.01 0.46 ± 0.01 0.70 ± 0.01

PD mWm-2 111 ± 3.25 180 ± 4.65 256 ± 4.87

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α-MnO2 α-MnO2 /C Pt/C

JK / mAcm-2 @ -0.6 V 2.13 11.14 13.51

CE

Sample

In spite of higher performance of Pt/C in maximum power density (256 ± 4.87 mWm-2 ) and OCV (0.70 ± 0.01 V) than that of prepared catalysts, long-term voltage generation cycles in Fig. 6 indicates that α-MnO2 /C and Pt/C have achieved dual-purpose of constant and reproducible voltage even with slightly higher values for former one.

14

ACCEPTED MANUSCRIPT The performance of bio-related devices is exactly associated with a large number of internal and external factors due to the complex medium and its contamination, which can control the microorganism growth and activity or affect the formation of cathode biofilm. Nevertheless, many factors have not yet been studied. In general, compounds based on platinum

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are prone to cathodic poisoning by contaminants (e.g. methanol, chloride and sulfide) in

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wastewater. The cathode catalytic sites for ORR in MFCs can be severely deactivated by these

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compounds. On the contrary, inorganic catalysts based on inexpensive transition metals such as Mn are highly stable, biocompatible with enhanced bacteria attachment and biocathode

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formation and cheap, making them promising and efficient catalysts for application in MFCs.

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Catalyst poisoning of Pt-based cathodes are essential problems, which cause considerable kinetic losses in ORR; especially in the long time operations (10). In spite of higher power density of Pt

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in the polarization curve, poisoning of Pt cathode may occur by exposure to wastewater in long

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time operations of MFCs lead to the lower potential generation in Fig. 7.

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4 Conclusion

In this paper, pristine α-MnO2 nanowires and α-MnO2 supported on carbon Vulcan were

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synthesized via a facile hydrothermal method. The electrochemical study of ORR of the

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synthesized catalysts surface was carried out using a three electrode configuration in neutral media. According to cyclic voltammetry and RDE results, α-MnO2 /C was selected as representative for further investigations. EIS results at different potentials revealed that the predominant reaction on the surface of electrode is the charge transfer for redox reactions. Finally, the microbial fuel cell was tested in terms of polarization and power density by applying pristine α-MnO2 nanowires and α-MnO2 /C as air cathodes. Eventually, α-MnO 2 /C was found to be more efficient for MFCs because of surface structure and higher surface area of carbonic 15

ACCEPTED MANUSCRIPT support, which provide rather active sites for redox reactions compared with pristine α-MnO2 nanowires. Also it is shows good stability and voltage generation respect to commercial and expensive Pt/C catalyst. Based on the performance and cost analysis as well as the possibility of further cost reduction associated with scaling-up of the synthesis process, α-MnO2 /C is

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considered a promising catalyst for application in MFCs.

Acknowledgements

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This work was financially supported by post graduate office of the University of Tabriz.

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Furthermore, we are deeply grateful to Materials and Devices for Energy at University of Tor

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M

Vergata for their valuable technical support and guidance.

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References

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56. Wei W, Cui X, Chen W, Ivey DG. Manganese oxide-based materials as electrochemical supercapacitor electrodes. Chemical society reviews. 2011;40(3):1697-721. 57. Aoki KJ, Chen J, Zeng X, Wang Z. Decrease in the double layer capacitance by faradaic current. RSC Advances. 2017;7(36):22501-9. 58. Bao J, Wang Z, Liu W, Xu L, Lei F, Xie J, et al. ZnCo 2 O 4 ultrathin nanosheets towards the high performance of flexible supercapacitors and bifunctional electrocatalysis. Journal of Alloys and Compounds. 2018;764:565-73.

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Fig. 1- a) Identification of XRD peaks of α-MnO2 nanowires according to JCPDS, b) Raman

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spectra of carbon Vulcan and α-MnO2 /C Fig. 2- FE-SEM images of α-MnO2 nanowires (a,b), α-MnO 2 /C (c,d) Fig. 3- Cyclic voltammograms of α-MnO2 nanowires and α-MnO2 /C in N 2 (dashed lines) and O2 saturated (solid lines) 100 mM neutral PBS solution. Scan rate = 10 mV/s. Fig. 4- a) LSV curves at different electrode rotation rates of α-MnO2 /C; b) LSV curves at 1600 rpm Pt/C, α-MnO 2 /C and α-MnO2 nanowires, c) KL plots of α-MnO2 /C at different

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ACCEPTED MANUSCRIPT potentials (Inset: Number of exchanged electrons calculated at different potentials); d) Comparison of KL plots of α-MnO2 /C and Pt/C at ‒0.6 V (Inset: Jk and n @ -0.6 V) Fig. 5- a) Nyquist plots for α-MnO2 /C and Carbon Vulcan in N 2 - and O 2 -saturated neutral PBS at potential of -0.25 V vs. SCE, b) Nyquist plots for α-MnO2 /C in O 2 -saturated neutral PBS

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at different potentials, c) Equivalent circuit related to EIS plots.

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Fig. 6- Polarization and power density (PD) curves of MFCs assembled with α-MnO2 nanowires and α-MnO2 /C cathodes fed with 1 mg/L sodium acetate in neutral phosphate buffer.

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Fig. 7- Voltage cycles with 1 kV external resistance versus time.

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Facile and low cost electrocatalyst preparation used in microbial fuel cell system Electrocatalysts based on manganese dioxide and carbon Vulcan α-MnO2 /C proceed ORR through first order kinetics mechanism Performance improvement of α-MnO2 /C respect to pristine α-MnO2

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