Iron chelates as low-cost and effective electrocatalyst for oxygen reduction reaction in microbial fuel cells

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Iron chelates as low-cost and effective electrocatalyst for oxygen reduction reaction in microbial fuel cells Minh-Toan Nguyen a, Barbara Mecheri a, Alessandra D’Epifanio a,*, Tommy Pepe` Sciarria a,b, Fabrizio Adani b, Silvia Licoccia a a

Department of Chemical Science and Technology & NAST Center, University of Rome Tor Vergata, Via della Ricerca Scientifica 1, 00133 Rome, Italy b RICICLA GROUP, Dipartimento di Scienze Agrarie e Ambientali: Produzione, Territorio, Agroenergia, Via Celoria 2, 20133 Milan, Italy

article info

abstract

Article history:

Iron-chelated electrocatalysts for the oxygen reduction reaction (ORR) in a microbial

Received 10 August 2013

fuel

Received in revised form

hydroxyphenylacetic acid) (FeE), sodium ferric diethylene triamine pentaacetic acid (FeD)

10 February 2014

supported on carbon Vulcan XC-72R carbon black and multi-walled carbon nanotubes

Accepted 12 February 2014

(CNTs). Catalyst morphology was investigated by TEM; and the total surfaces areas as well

Available online 14 March 2014

as the pore volumes of catalysts were examined by nitrogen physisorption characteriza-

cell

(MFC)

were

prepared

from

sodium

ferric

ethylenediamine-N,N0 -bis(2-

tion. The catalytic activity of the iron based catalysts towards ORR was studied by cyclic Keywords:

voltammetry, showing the higher electrochemical activity of FeE in comparison with FeD

Iron-based catalyst

and the superior performance of catalysts supported on CNT rather than on Vulcan XC-72R

Iron chelates

carbon black. FeE/CNT was used as cathodic catalyst in a microbial fuel cell (MFC) using

Oxygen reduction reaction (ORR)

domestic wastewater as fuel. The maximum current density and power density recorded

Microbial fuel cell (MFC)

are 110 (mA m2) and 127  0.9 (mW m2), respectively. These values are comparable with those obtained using platinum on carbon Vulcan (0.13 mA m2 and 226  0.2 mW m2), demonstrating that these catalysts can be used as substitutes for commercial Pt/C. Copyright ª 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.

1.

Introduction

Among the numerous systems under investigation to develop sustainable energy production devices, microbial fuel cells (MFCs) have the added value to contribute to wastewater treatment while producing energy. These bio-electrochemical

systems in fact, convert the chemical energy stored in the biodegradable organic matter present in wastewater into electrical energy through the catalytic action of microorganisms [1e3]. Several limiting factors regulate MFC performance, most important being reactor configuration, internal resistance, and electrode reactions. The power density of MFC is greatly affected by the cathode side [4], where oxygen is the

* Corresponding author. E-mail address: [email protected] (A. D’Epifanio). http://dx.doi.org/10.1016/j.ijhydene.2014.02.064 0360-3199/Copyright ª 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 9 ( 2 0 1 4 ) 6 4 6 2 e6 4 6 9

most popular cathodic electron acceptor due to its charge-free nature and high redox potential characteristic. The use of platinum as catalyst for the oxygen reduction reaction (ORR) significantly contributes to device cost, thus representing one obstacle to the commercial development of MFCs. The cathodic materials account for 47% of the MFC total cost [5,6] which would be largely reduced by decreasing the platinum loading or replacing platinum with less expensive species. The platinum loading has been successful reduced from 0.5 to 0.1 mg cm2 with a slight decrease of air-cathode MFC power density [7]. Other studies have reported on the use of platinum alloys containing less expensive metals such as iron [8], nickel [9], cobalt [10]. These catalysts however, are still Ptcontaining and suffer from catalyst poisoning when the cell is operated in single-chamber configuration. Thus, several nonplatinum materials, i.e. cobalt [7,11], manganese [12], titanium [13], lead [14], copper [15], have been investigated. Among these metals, iron is particularly appealing because of its low cost and high abundance. At iron chelated cathodes of MFCs, the ORR can proceed through either a 4 electron pathway producing water, or through a 2 electron pathway producing hydrogen peroxide as an intermediate [16]. The peroxide pathway and the consequent possible production of other intermediate reactive oxygen species does not necessarily impair the final MFC performance by reducing microbial catalytic activity. In fact, the production of hydroxyl radicals could considered beneficial based on their ability to degrade wastewater industrial pollutants, such as 4-nitrophenol [17], or to treat the residual COD in the effluent stream. Different iron-based catalysts such as iron phthalocyanine [18], iron acetate, CleFeIII tetramethoxyphenylporphyrin [19], iron ethylenediaminetetraacetic acid [20e22] have been explored as cathodic catalysts for MFC applications demonstrating that Fe-containing species are promising candidates as ORR cathodic catalysts. The design of a catalyst implies depositing the metal species on a carbon support. Carbon nanotubes (CNTs) are characterized by high electronic conductivity, uniform pore size distribution, meso- and macro-pore structure, inert surface properties, and resistance to acidic and basic environment [23]. Some examples on the use of CNTs as support material for the cathodic catalyst in MFCs have been reported [24e26]. In this study, catalysts containing iron chelates of ethylenediamine-N,N0 -bis(2-hydroxyphenylacetic acid) and diethylene triamine pentaacetic acid supported on multiwalled carbon nanotubes (CNTs) or Vulcan XC-72R carbon black were prepared and characterized. The structure and catalytic activity of the prepared catalysts were investigated by TEM, BET, and cyclic voltammetry. Their performance as cathodic

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catalyst of an MFC fed with domestic wastewaters was tested and compared with that of reference Platinum.

2.

Experimental

2.1.

Catalyst preparation

Multi-walled carbon nanotubes (CNTs, >95% carbon content, diameter  length 6e9 nm  1 mm) were purchased from Aldrich and purified by heat-treatment at 300  C for 3 h to eliminate the amorphous carbon, and refluxing in HNO3 65 wt.% at 90  C for 16 h to remove metal impurities [27]. Purified CNTs were separated by filtration and washed with pure water until neutral pH, obtaining a carbon paste which was dried at 70  C overnight. Sodium ferric ethylenediamine-N,N0 -bis(2-hydroxyphenylacetic acid) (Fe EDDHA) and diethylene triamine pentaacetic acid pentasodium salt solution (DTPA) were purchased from Aldrich and used as received. Catalysts were prepared as follows: solutions of iron containing precursors were prepared either dissolving sodium Fe EDDHA (0.392 g) or a mixture of sodium DTPA (1.132 g) and FeCl3$6H2O (0.243 g) in water (100 mL). Vulcan XC-72R or purified CNTs (0.5 g) was added to the solution. The mixture was stirred for 0.5 h, filtered and the resulting wet solid dried at 70  C overnight. The powder was ground, placed in a ceramic vessel and heated for 1.5 h at 800  C under argon to obtain the catalysts which were labeled as indicated in Table 1. Commercial platinum 10 wt.% on Vulcan XC-72R carbon black (Pt/C) was obtained by Quintech and used as reference.

2.2.

Catalyst characterization

The morphology of the iron based catalysts was investigated by TEM images acquired with a Philips CM 12 instrument operating at 120 kV. Elemental analysis was carried out by energy dispersive X-ray (EDX) analysis using an Oxford INCA ver. 4.04 year 2003 system. To measure the total surface area and the pore volume of the catalysts, nitrogen sorption isotherms were measured at 196  C down to relative pressure P/Po 5  107 with a Quantachrome Instrument Autosorb-1. Before sorption measurements, each sample was heated to 120  C for 2 h under vacuum. Sorption data were analyzed using Autosorb Software from Quantachrome Instrument. Cyclic voltammetry (CV) experiments were carried out using a Ring-Disk Electrode Model 636A (Princeton Applied Research, Ametek) and a Potentiostat VMP3 (Bio Logic Science Instruments). The system was integrated with a personal

Table 1 e Catalysts labeling, total surface area, total pore volume, potential (Ep), and current (I) peak. Sample Purified MWCNTs Carbon Vulcan NaFe/EDDHA/CNTs NaFe/EDDHA/C NaFe/DTPA/CNTs NaFe/DTPA/C

Label

Total surface area (m2 g1)

Total pore volume (cc g1)

Ep (V versus SCE)

Ip (mA cm2)

CNT C FeE/CNT FeE/C FeD/CNT FeD/C

275 212 171 116 164 117

1.59 1.09 1.03 0.58 0.87 0.57

0.244 0.329 0.017 0.114 0.22 0.288

0.388 0.775 0.702 0.587 0.396 0.396

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catalyst with 135 mL of water, 270 mL of ethanol, and 217 mL of Nafion in potassium salt form (made by mixing 1 mL of a 3 M KCl aqueous solution and 10 mL of Nafion solution 5 wt.% in alcohol) [17]. Then, an amount of 7 mL of catalyst ink was deposited on the glassy carbon electrode and dried at 70  C for 10 min before starting CV experiments. All cyclic voltammograms were measured at room temperature in 50 mM phosphate buffer solution (pH ¼ 7), saturated with either nitrogen or oxygen gas, with a scan rate of 10 mV s1.

2.3.

Fig. 1 e The molecular structure of a) sodium ferric ethylenediamine-N,N0 -bis (2-hydroxyphenylacetic acid) (NaFeEDDHA, NaFeE), and b) sodium ferric diethylene triamine pentaacetic acid (NaFeDTPA, NaFeD).

computer and controlled by EC-Lab V10.18 software. A conventional three-electrode cell was employed, the counter electrode was a platinum wire and the reference electrode a saturated calomel electrode (SCE). The catalyst deposited on a 5 mm diameter glassy carbon disk was used as working electrode. The catalyst ink was prepared by blending 10 mg of

Cathode preparation

The cathode electrode was prepared according to a previously published procedure [7,28]: one side of a carbon cloth with 30 wt.% PTFE wet-proofing (Quintech) was covered with the diffusion layer, the other side was painted with the catalyst layer. The diffusion layer was prepared as follows: the carbon layer was made by brushing a suspension containing Vulcan XC-72R and PTFE 40 wt.% in water with carbon loading of 1.56 mg cm2 and 12 mL of PTFE per 1 mg of carbon, respectively. Subsequently, the electrode was dried at 70  C for 1 h before annealing at 370  C for 30 min. A PTFE layer was then deposited on the carbon layer by brushing a PTFE 60 wt.% suspension in water (Aldrich) followed by pyrolysis at 370  C for 15 min. The PTFE deposition was repeated 3 times. For the catalyst layer, a suspension prepared from iron-based catalyst with DI water, Nafion 5 wt.% solution in alcohol (Aldrich), and iso-propanol was brushed with a uniform distribution on the side opposite to the diffusion layer. The loading of iron was 0.5 mg cm2 and the total volume of DI water, Nafion solution, and iso-propanol were 0.83 mL, 6.67 and 3.33 mL per mg of catalyst, respectively. The same procedure was carried out with PtC 10 wt.% with platinum loading of 0.5 mg cm2 for comparison. All electrodes were dried at room temperature for 24 h before use.

2.4.

Air-cathode MFC configuration and operation

2.4.1.

MFC reactor

Two single-chamber air-cathode MFCs were assembled as previously reported [29,30]. The main MFC body made of Plexiglas in cylindrical chamber has a total inner volume of 28 mL. The anode consists of graphite fiber brush with

Fig. 2 e a) TEM image of FeE/CNT and b) EDS spectrum.

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2.4.3.

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Operation

MFCs were first acclimated by using the effluent from previously operated MFCs fed with a mixture of domestic wastewater (Canegrate Wastewater Treatment Plant) and buffer solution (Phosphate buffer solution 100 mM: Na2HPO4$H2O 9.152 g L1, NaH2PO4$H2O 4.904 g L1, NH4Cl 0.62 g L1, KCl 0.26 g L1) containing 0.2 g L1 sodium acetate. The initial pH of the solution was 7.2  0.1. Once the maximum voltage output was similar after three consecutive cycles (maximum peak voltage of 0.4  0.05 V), the buffer solution was gradually omitted until only DW and sodium acetate 0.2 g L1 were supplied to the cells. After acclimation period, two cells were operated simultaneously for each cathode and were fed with DW and sodium acetate (0.2 g L1). Feed solutions were replaced when the voltage dropped below 40 mV, forming one complete cycle of operation. All the tests were carried out at room temperature of 23  3  C. Anodes and cathodes were connected with copper wire, and the voltage across an external resistor of 1000 U every 900 s was monitored using a

Fig. 3 e Cyclic voltammograms of bare glassy carbon electrode (GC), GC modified with Vulcan XC-72R carbon black (C) and GC modified with carbon nanotubes (CNT) in 50 mM phosphate buffer solution (pH [ 7) in a) saturated nitrogen, and b) saturated oxygen atmosphere (scan rate: 10 mV sL1).

titanium wire as core (Panex 33 160 K, Zoltek), 2.5 cm in both outer diameter and length. The estimated surface area is 0.22 m2 or 18 200 m2/m3 brush-volume, the porosity being 95%. The fiber brush electrodes were heated to 450  C for 30 min under ambient air before use. No catalyst was applied to the fiber brush anode. The cathode (diameter 4 cm) was prepared as described in the Previous (2.3) Section.

2.4.2.

Wastewater

Domestic wastewater (DW) samples were collected from a wastewater treatment plant in Canegrate (Italy), and stored at 4  C. DW was characterized before and after MFC tests. Total chemical oxygen demand (TCOD) was analyzed following standard methods (APHA, 1998). pH and conductivity were measured using pH meter and conductivity meter (PC 2700, Eutech Instruments, Netherlands), respectively. For MFC tests, DW was used as both inoculum and substrate.

Fig. 4 e Cyclic voltammograms of FeE/CNT, FeE/C, FeD/ CNT, and FeD/C in 50 mM phosphate buffer solution (pH [ 7) in a) saturated nitrogen, and b) saturated oxygen atmosphere (scan rate: 10 mV sL1).

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multimeter (2700, Keithley, United States) connected to a personal computer. Polarization curves were obtained by varying the external resistance (10e10 000 U) every 30 min, and measuring the cell voltage for each different resistance.

2.4.4.

Calculations

Coulombic efficiency (CE), defined as the fraction of electrons recovered as current versus that in the beginning organic matter, was calculated as follows [31]: CE ¼

Cp F  V  b  DCOD=M

where Cp is the total Coulombs calculated by integrating the current over time, F is Faraday’s constant (96 485 C mol1), V is the liquid volume (28 mL), b is the number of mol of electrons per mol of substrate (4), DCOD (g L1) is the substrate concentration change over the cycle, and M is the molecular weight of the substrate (M ¼ 32 for oxygen). The calculation of power density (mW m2) and current density (mA m2) was based on the surface area of one side of the cathode of 7 cm2.

3.

Results and discussion

3.1.

Catalyst characterization

Fig. 4 shows the CV curves of the two families of ironchelated catalysts: FeE and FeD, deposited either on Vulcan XC-72R carbon black (C) or on multiwalled carbon nanotubes (CNT). In nitrogen-saturated solution (Fig. 4a), the catalyst showed a couple of redox peaks ranging from 0.4 V to 0.0 V, that may be assigned to the feature of iron-chelates combined with carbon substrate [18]. Under oxygen purging (Fig. 4b), the peak due to ORR can be clearly recognized, indicating that the iron-chelated catalysts are active with oxygen. The reduction peaks are shifted towards positive voltage with respect to the substrates. The potential and current density of the ORR peaks for the different samples are listed in Table 1. On the basis of its highest oxygen reduction voltage (Ep) the best ironchelated catalyst appears to be FeE/CNT (Epr ¼ 0.017 V). This value is even higher than ORR peak potential obtained in the case of the reference Pt/C catalyst (Fig. 5) which displayed oxygen reduction peak at þ0.10 V. Moreover, the observation of reproducible voltammograms during repetitive scanning demonstrates the stability of the electrochemical activity of FeE/CNT. The superior ORR catalytic activity of carbon nanotubesbased electrodes can be explained on the basis of their higher total surface area and porosity which ease the dispersion of iron chelates. Electrochemistry and nitrogen physisoprtion converge to identify FeE/CNT as the best performing catalyst for ORR.

3.2. Fig. 1 shows the structural formula of the Fe complexes used to prepare the different catalysts under investigation. Both species have been deposited onto Vulcan XC-72R carbon black and CNTs generating the series of samples listed in Table 1. Table 1 also reports the total surface area and total pore volume values as derived from BrunauereEmmetteTeller (BET) analysis of sorption data. All samples containing CNTs have surface area and pore volume higher than that of the corresponding carbon black-based samples. Fig. 2a shows TEM image of sample FeE/CNT, chosen as an example of the series, at different magnification. The sample consist of homogeneous CNTs with iron catalyst (dark spots) homogeneously deposited on the surface of carbon nanotubes walls. The presence of iron was confirmed by EDS analysis of the sample (Fig. 2b). The catalytic activity towards ORR of the different catalysts was investigated by cyclic voltammetry. CV curves were acquired in either N2 or O2 atmosphere and compared with those measured for bare glassy carbon (GC), and GC with Vulcan XC-72R or purified carbon nanotubes (CNT) electrodes. Modified Vulcan XC-72R [32], activated carbon [33], or activated carbon fibers [34e36] have been previously used as ORR catalysts for in air-breath MFCs although showing much lower activity than what observed with common noble metal catalysts. Fig. 3 shows the CV curves obtained with the three electrodes in 50 mM neutral phosphate buffer. While substantially negligible activity is observed when the measurements are carried out in N2 (Fig. 3a), all the cyclic voltammograms recorded in oxygen (Fig. 3b) show a reaction peak during anodic scan indicating electrocatalytic activity towards ORR. The activity increases in the order GC < Vulcan < CNTs.

MFC tests

To compare the performance of FeE/CNT with a reference Pt/C cathode, two air-cathode single-chamber MFCs equipped with the same anode were assembled. The cells were operated in parallel in batch mode feeding on domestic wastewater (DW) and acetate 0.2 g L1. Table 2 summarizes the data relative to the performance of the two MFCs and Fig. 6 shows the polarization and power density curves. Only a slight decrease in power density was observed when FeE/CNT is used to substitute the commercial Pt/C catalyst (127 mW m2 versus

Fig. 5 e Cyclic voltammogram of Pt/C in 50 mM phosphate buffer solution (pH [ 7) in saturated oxygen atmosphere (scan rate: 10 mV sL1).

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Table 2 e Performance of commercial platinum and FeE/CNTs as catalyst in a microbial fuel cell pH, conductivity (r), voltage at 1 kU (E), maximum current density (I), maximum power density (PD), total COD (TCOD) removed and coulombic efficiency (CE). Catalyst Pt/C FeE/CNTs

pH in/out 7.30 6.84 7.19 6.87

 0.2  0.2  0.1  0.1

r (mS) in/out

E (mV)

corrige I (mA m2)

corrige PD (mW m2)

TCOD removed (%)

C.E (%)

   

373

corrige 130

corrige 226  0.2

52.1%

9.5

379

corrige 110

127  0.9

43.2%

1493 942 1342 1011

149 67 120 130

31

226 mW m2). No substantial difference was observed in the current density generated by the two MFCs, the measured value being 1100 mA m2 for the cell containing the iron-based cathode and 1300 mA m2 for the one assembled with the reference Pt/C catalyst. In addition, the efficiency of wastewater treatment was evaluated as total chemical oxygen demand (TCOD) removed, calculated as the difference between the influent TCOD (2877 mg L1) and the effluent TCOD after each MFC cycle. TCOD removed was measured for both iron and platinum MFCs, being 43.21 and 52.09%, respectively (Table 2). These findings indicate that FeE/CNT catalyst has a good ability to remove organic matter from DW, at the same time generating electricity, similar to that observed in the case of Pt/C catalyst. Moreover, the calculated coulombic efficiency (CE) of the MFC equipped with FeE/CNT was 31%, more than 3 times higher than that of the MFC equipped with Pt/C. The stability of the FeE/CNT cathode was evaluated in continuous long-term operation. Fig. 7 shows the voltage cycles recorded for the FeE/CNT and Pt/C MFCs. For the cell equipped with the iron-based cathode, only a slight decrease in the maximum voltage (0.38 V) was observed after 6 cycles (1400 h). The commercial Pt/C catalyst showed a good repetitive performance with a maximum voltage after each inoculum around 0.4 V. However, the reduction of costs associated with the elimination of Pt is paramount to evaluate if the iron-chelate containing catalyst might represent a significant improvement in the development of MFCs. Assuming to use a metal loading (Fe or Pt) of 0.5 mg cm2, the cost of the commercial Pt/

C catalyst is 25.00 V/g while that of the catalyst investigated in this work, FeE/CNT results to be 5.30 V/g. This latter figure was calculated on the basis of a lab-scale procedure and could be significantly reduced in scaling-up the process. Joint analysis of performance and cost (reduced by ca. 80%) than that of Pt/C indicated that FeE/CNT is a potential catalyst for replacement of Platinum in the MFCs cathode.

Fig. 6 e Polarization (EeI) and power density (PD) curves of MFCs fed with domestic wastewater and equipped with Pt/ C and FeE/CNT cathodes.

Fig. 7 e Voltage generation in a single chamber MFC (1000 U) fed with domestic wastewater and acetate using Pt/C or FeE/CNT as the cathode electrodes.

4.

Conclusions

This study presents the development of iron-chelated catalyst with low-cost and high catalytic activity towards oxygen reduction reaction for microbial fuel cell applications. The electrocatalysts were prepared by annealing iron salts with carbon substrate in argon gas at high temperature, using either EDDHA or DTPA as chelating agent. Vulcan XC-72R carbon black and multiwalled carbon nanotubes were used as catalyst support. Cyclic voltammetry experiments indicated that, among the two families (EDHHA- and DTPA-based) of prepared catalysts, the most active iron-chelated catalyst was FeEDDHA supported on CNTs (FeE/CNT), due to the better activity of iron precursor of EDDHA and the superior characteristics of CNTs compared to those of Vulcan XC-72R carbon black. FeE/CNT catalyst was assembled in an MFC fed with domestic wastewater, which performance in terms of efficiency

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to wastewater treatment and electricity generation was tested. The results obtained demonstrated that FeE/CNT was active as cathodic catalyst and its performance was just slightly lower than that of Pt/C catalyst. In terms of cost, the expense of lab-scale catalyst was much lower than that of state of the art platinum, thus FeE/CNT could be a promising candidate for non-platinum MFCs.

Acknowledgments The authors thank Ms C. D’Ottavi for her valuable technical support. The financial support of the Italian Ministry for Environment (MATTM, Project MECH2) and of the Ager Consortium (Project AGER, grant n 2011-0283) is gratefully acknowledged.

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