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Journal of Power Sources 272 (2014) 320e327

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Journal of Power Sources

journal homepage: www.elsevier .com/locate/ jpowsour

Hybrid binuclear-cobalt-phthalocyanine as oxygen reduction reactioncatalyst in single chamber microbial fuel cells

Baitao Li a, *, Xiuxiu Zhou a, Xiujun Wang a, Bingchuan Liu b, Baikun Li b

a Key Laboratory of Fuel Cell Technology of Guangdong Province, School of Chemistry and Chemical Engineering, South China University of Technology,Guangzhou 510640, Chinab Department of Civil and Environmental Engineering, University of Connecticut, Storrs, CT 06269, USA

h i g h l i g h t s

� Bi-CoPc/C had better performance than normal CoPc/C.� Bi-CoPc/C hybrided with NiO and CoO substantially enhanced ORR in SCMFCs.� Integration of NiO and CoO to Bi-CoPc increased the O and N functional groups.� Power density of SCMFC with Bi-CoPceNiO/C cathode was 400 mW m�2, close to Pt/C.

a r t i c l e i n f o

Article history:Received 7 May 2014Received in revised form25 July 2014Accepted 14 August 2014Available online 28 August 2014

Keywords:Binuclear-cobalt-phthalocyanineMetal oxideOxygen reduction reactionMicrobial fuel cellX-ray photoelectron spectroscopy

* Corresponding author. School of Chemistry andChina University of Technology, Guangzhou 5106487112943.

E-mail address: btli@scut.edu.cn (B. Li).

http://dx.doi.org/10.1016/j.jpowsour.2014.08.0580378-7753/© 2014 Elsevier B.V. All rights reserved.

a b s t r a c t

A novel hybrid binuclear-cobalt-phthalocyanine (Bi-CoPc) is developed as the cathode catalyst to replacethe costly platinum (Pt) in single chamber microbial fuel cells (SCMFCs). Bi-CoPc/C is integrated withmetal oxides (NiO and CoO) to form macrocyclic complex for enhanced oxygen reduction rate (ORR). Thecharacteristics of hybrid catalysts (Bi-CoPc/CeCoO and Bi-CoPc/CeNiO) are compared with Co-containedcatalysts (CoPc/C and Bi-CoPc/C) and metal oxide catalysts (NiO and CoO). The increase in O and Nfunctional groups indicates the benefits of NiO and CoO to the cathode catalysts. The cyclic voltammetry(CV) shows the reduction peak for Bi-CoPc/CeNiO and Bi-CoPc/CeCoO at �0.12 V and �0.22 V,respectively. The power densities (368 mW m�2 and 400 mW m�2) of SCMFCs with Bi-CoPc/CeCoO andBi-CoPc-NiO/C are the highest among the cathodes tested, and close to that of Pt (450 mW m�2). Thisstudy demonstrates that hybrid Bi-CoPc/C with metal oxides has a great potential as a cost-effectivecatalyst in MFCs.

© 2014 Elsevier B.V. All rights reserved.

1. Introduction

Microbial fuel cell (MFC), a novel electrobiochemical systemcapable of converting organic contaminants to electricity, hasdrawn intensive attention in the past decade [1,2] Electrogenicbacteria growing on the anode surface generate electrons fromdegrading organic contaminants, and electrons are then transferredto the cathode and reacted with electron acceptors [3,4]. Oxygen inair has beenwidely used as the electron acceptor in single chamberMFCs (SCMFCs), but the oxygen reduction reaction (ORR) on cath-odes requires efficient catalysts to overcome the high overpotential.

Chemical Engineering, South0, China. Tel./fax: þ86 20

Platinum (Pt) has been widely used as the cathodic catalyst due toits high ORR catalyzing capability [5,6]. However, Pt can easily getpoisoned by a variety of chemicals (e.g. HS�, Cl�, CO) [7], and thehigh cost and scarcity limits its application inMFCs. Therefore, cost-effective catalysts have been developed (e.g. MnO2, PbO2, Co) toreplace Pt in MFCs [8e11].

Transition metal macrocycles have showed good performance,especially, Cobalt phthalocyanine (CoPc) exhibits unique ORRcapability [12] due to their large conjugated molecular structurewith strong pep interactions between aromatic rings. The ORRcatalyzed by nitrogen (N)-chelates of transition metals is mainlyinvolved with a modified ‘redox catalysis’, in which oxygen is firstadsorbed on the metal center to form an oxygen-catalyst adduct,electrons are then transferred from the metal center to bound O2,and the reduced N-chelates are later regenerated [13]. Comparedwith common cobalt oxide (CoO), CoPc exhibits better electron-donating properties, since nitrogen is an n-type dopant and the

B. Li et al. / Journal of Power Sources 272 (2014) 320e327 321

CoPc plane contains an extra electron that is easily donated [14].Several non-noble transition metals (e.g. Co, Fe and Mn) have beenstudied in the forms of FePc [15], MnPc [16], CoPc [17], and cobalttetramethoxyphenylporphyrin (CoTMPP) [18]. Iron phthalocyanine(FePc) and nitrogen-doped graphene composite produced the po-wer density similar to Pt electrodes [19]. Polyaniline or aminofunctionalized carbon nanotube supported FePc exhibited animproved ORR catalytic activity as a result of the synergy of FePcand the supporting materials [20]. Cobalt[tetrakis(o-aminophenyl)porphyrin] (CoTAPP) anchored onto graphene surfaces also showeda high electro-catalytic ORR capability [21]. Unfortunately, all thesecatalysts only had good ORR performance in the initial stage, andslowly declined over time. The stability of these transition metalmacrocycles needs further improvement.

Binuclear metal phthalocyanine (PcM(II)ePcM(II) or Bi-MPc)has recently attracted interests due to its symmetric disk shapeand highly conjugated structure [22,23], in which two phthalocy-anine units share a common benzene ring (Fig. 1). Unlike thecommonMPc (e.g. CoPc, and FePc) containing only one metal ion atone valence, Bi-MPc contains two metallic ions and the entiremolecule has a big p-electronic conjugated system, so that twometallic ions are able to achieve the valence transformation (e.g.PcM(II)ePcM(II) to PcM(I)ePcM(III)) during the catalytic reactions[24], and O2 can obtain electrons easily from the highest occupiedmolecular orbital (HOMO) of the binuclear phthalocyanine [25],which is expected to accelerate ORR in MFCs. Bi-MPc has beendeveloped as an efficient catalyst in lithium/thionyl chloride bat-tery [25] and atrazine dechlorination [26]. Until now, Bi-MPc hasnot been examined inMFCs. In addition, iron group transitionmetaloxide (i.e., CoO and NiO) can catalyze the disproportionation re-action of hydrogen peroxide (an undesirable intermediate in ORR)[27e29], and prolong the catalyst lifetime in MFCs [30,31]. Bi-MPccould be integrated with these transition metal oxides to form anew catalyst with high ORR capacity and long stability.

With the Cobalt as the fundamental element, this study aimed atdeveloping a hybrid Bi-CoPc/C catalyst by integrating Bi-CoPc withthe carbon supported nickel oxide (CeNiO) and cobalt oxide(CeCoO). These novel hybrid catalysts had two-fold significance forMFCs. First, Bi-CoPc/C itself could have a higher ORR activity thanCoPc. Second, the hybrid Bi-CoPc/C with metal oxide could furtherenhance the catalytic activity. In this study, two types of hybridcomposites, Bi-CoPc/CeNiO and Bi-CoPc/CeCoO were compre-hensively compared in terms of characteristics, ORR capability, andpower generation. The surface and chemical characterization of thedeveloped catalysts were conducted using X-ray photoelectronspectrometer (XPS) and transmission electron microscope (TEM).The ORR activities of these two catalysts were evaluated using the

Fig. 1. Optimized structure of carbon supported Bi-CoPc.

cyclic voltammetry (CV). The power generation of the hybrid cat-alysts was examined in SCMFCs and compared with Pt cathodes (asthe positive control), two Co-based catalysts (Bi-CoPc/C and CoPc/C), two transition metal oxide catalysts (NiO/C and CoO/C), andplain carbon cathode (as the negative control). Finally, the physi-cochemical characterization and the electrochemical performanceof hybrid catalysts were correlated.

2. Materials and methods

2.1. Synthesis of carbon supported Bi-CoPcemetal oxide composites

The chemicals used in this study had the analytical reagentgrade quality without the need of further purification. To preparemetal oxide cathodes (NiO/C and CoO/C), 198 mg nickel nitratehexahydrate (Ni(NO3)2$6H2O) crystal was powdered and mixedwith 200 mg carbon support (SBET ¼ 1042 m2 g�1, Hesen ElectricCo., Ltd. China) in a mortar. The mass ratio of Ni to C was 1:5. Themixed solid was dried at 110 �C for 1 h and subsequently calcined ina muffle furnace at 400 �C for 3 h in order to decomposeNi(NO3)2$6H2O to NiO. This solid product was denoted as NiO/C.CoO/C was prepared with the same procedure except that Co(N-O3)2$6H2O was used instead of Ni(NO3)2$6H2O. To prepare Co-based cathodes (Bi-CoPc/C and CoPc/C), 100 mg Bi-CoPc (NENUIndustry of Fine Chem. Co) was dissolved in 100 mL deionizedwater, and 200 mg carbon support was then added to the solution.The mixture was sonicated at room temperature for 4 h. The pastewas dried at 70 �C for 12 h to obtain Bi-CoPc/C. CoPc/Cwas preparedwith the similar steps as Bi-CoPc/C by using 20mg CoPc and 100mgcarbon support. To prepare the hybrid composites, 200mg Bi-CoPc/C was immersed in 60 mL chloroform by sonication. 200 mg NiO/Cwas dispersed to Bi-CoPc/C suspension and the mixture was stirredand purged with nitrogen gas for 5 h. The product was filtered,washed with chloroform, and dried at 30 �C. The catalyst wasdenoted as Bi-CoPc/CeNiO. Bi-CoPc/CeCoO was prepared bythe same method as Bi-CoPc/CeNiO except using CoO/C instead ofNiO/C.

2.2. SCMFC configuration and operation

The SCMFC (volume: 28 mL) was made from a plexiglass cy-lindrical bottle (depth: 4 cm, diameter: 3 cm) consisting an anodechamber and a cathode extension. Anode was immersed into theanode media solution, with one side of cathode facing the anodesolution and the other side facing air. The distance between theanode and cathode electrodes was 2.0 cm. Both anode and cathodeelectrodes had a projected area of 7.1 cm2 (diameter: 3 cm). Theanode was made of non-wet-proofed carbon cloth, and the cathodewas made of a wet-proofed carbon cloth and a catalytic layer [3,32].The catalyst loading was 1.0 mg cm�2 unless otherwise stated. In atypical preparation process of cathode, 1 mg of the synthesizedcatalyst (hybrid catalysts, or Co-based catalysts, or metal oxidecatalysts) was successively mixed with 0.83 mL water, 6.67 mL 5%nafion solution, and 3.33 mL isopropyl alcohol (IPA) to form a slurry,which was then applied onto one side of the 30% wet-proofedcarbon cloth. To compare the effects of catalyst loadings on thecathode, the Bi-CoPc/C contents were varied in the range of0.5e2.5 mg cm�2 by changing the mass of Bi-CoPc/C catalyst in theslurry. The Pt cathode containing 20% of Pt/C (Hesen Electric Co.,Ltd. China) was prepared as the same steps and used as the positivecontrol sample. The Pt loading of 0.5 mg cm�2 was used [3,16],which was the half of the hybrid catalyst loading applied in thisstudy, since this loading has been widely used in most MFC studies[3]. The plain carbon electrode (HCP330, Hesen Electric Co., Ltd.China) was used as the negative control sample. All electrodes were

B. Li et al. / Journal of Power Sources 272 (2014) 320e327322

dried overnight at ambient temperature before being installed inSCMFCs.

The inoculants of SCMFCs were taken from a wastewatertreatment plant (Guangzhou, China). Sodium acetate (1.0 g L�1) andphosphate buffer solution (50 mM) containing mineral(12.5 mL L�1) and vitamin (5 mL L�1) were added to stimulate theanaerobic electrogenic bacterial growth [33]. The SCMFCs wereoperated in batch modes at 30 ± 1 �C in an incubator, and eachcatalyst was examined in duplicate cycles. The external resistancewas 1000 U. The voltage across the external resistor was recordedby a 32-channel USB data collection (AD8233h) at 0.5 h intervals.The medium was replaced once the voltage below 100 mV.

2.3. Surface chemistry characterization

The surface chemistry of the catalysts was characterized usingX-ray photoelectron spectrometer (XPS) (Axis Ultra DLD, KRATOS)equipped with a monochromatic Al Ka excitation source(hn ¼ 1486.6 eV) at 12 kV and a 10 mA current. The pressure of theanalysis chamber was 5 � 10�9 torr. The binding energy was cali-brated using C 1s photoelectron peak at 284.6 eV as a reference. Thesurface morphological properties and dispersion of the catalystswere examined using a transmission electron microscope (TEM)(JEM-2100HR, JEOL) equipped with a LaB6 electron gun at anaccelerating voltage of 200 kV. Before the measurement, the sam-ples were dispersed ultrasonically in ethanol and then deposited ona copper grid.

Fig. 2. TEM images for three binuclear-cobalt-phthalocyanine (Bi-CoPc) catalysts. (A)Bi-CoPc/C, (B) hybrid Bi-CoPc/CeNiO, (C) hybrid Bi-CoPc/CeCoO.

2.4. Electrochemical characterization

The catalytic activity of the electrodes was examined using thecyclic voltammetry (CV) and linear sweep voltammetry (LSV). Anelectrochemical workstation (CHI660D, Shanghai CH instrumentCo., Ltd. China) with a three-electrode system was used, in whichthe coated glassy carbon (GC) electrode acted as the workingelectrode, Ag/AgCl as the reference electrode, and Pt wire as thecounter electrode. The working electrode was prepared as follows:the GC electrodes (3 mm diameter) were polished with finealumina powder, and 5 mg catalyst (hybrid catalyst or metal oxidecatalyst) was dispersed in 2 mL ethanol and 0.1 mL of 5% nafionsolution, followed by ultrasonically blending for 10 min. The 5 mLsolutionwas then carefully deposited onto the GC electrode surfaceby a micropipette. The catalyst-coated GC electrodes were dried at60 �C for 20 min to remove the solvent. The electrochemical testwas taken from �0.5 V to þ0.5 V in 50 mM phosphate bufferelectrolyte (pH ¼ 7.0) at a scan rate of 10 mV s�1.

Power density (W m�2) was calculated from the cell voltage U(V), external resistor Rext (U), and area of the cathode S (m2) ac-cording to power density ¼ U2/RextS. Current density (A m�2) wascalculated according to current density ¼ U/RextS. Rext werechanged from 20 to 30000 U during the polarization curve mea-surement. Internal resistance (Rin) was calculated at the maximumpower density point on the polarization plot where Rin was equal toRext [34]. The anodic and cathodic open circuit potentials (OCPs)were measured with respect to the Ag/AgCl reference electrode.Open circuit voltages (OCVs) were calculated based on OCPs.

The chemical oxygen demand (COD) concentration of the mediasolution in SCMFCs was measured using a commercial COD de-tector (COD-571, Shanghai precision & scientific instrument Co.,Ltd. China). The Coulombic Efficiency (CE) was defined as the ratioof the actual charge transferred to the anode from the substrate, tothe maximum available electrons if all substrate were oxidized andconverted to electrical current [34]. CE was obtained by thefollowing equation:

MtbIdt

CE ¼

Z0

FbvAnDCOD

Where M ¼ 32, the molecular weight of oxygen; F is Faraday'sconstant; b ¼ 4, the number of electrons exchanged per mole ofoxygen; vAn is the volume of liquid in the anode compartment;DCOD is the change in COD over time tb.

3. Results and discussion

3.1. Surface structures of Bi-CoPc-containing catalysts

The morphologies of the catalysts showed that the Bi-CoPcparticles were homogeneously absorbed onto the carbon surface

Fig. 3. Wide scan of XPS spectra for Co-contained plain catalysts and hybrid catalysts.(A) CoPc/C, (B) Bi-CoPc/C, (C) Bi-CoPc/CeCoO, (D) Bi-CoPc/CeNiO.

B. Li et al. / Journal of Power Sources 272 (2014) 320e327 323

(Fig. 2A). The deep dark spots especially accumulated at the centerof TEM images for the Bi-CoPc/CeNiO and Bi-CoPc/CeCoO catalysts(Fig. 2B and C) indicated the incorporation of NiO and CoO into themacrocyclic complex of Bi-CoPc/C.

XPS results showed the presence of Co and Ni on the catalystsurface (Fig. 3AeD). Predominant C 1s at 284.6 eV, N 1s at 399.8 eV,O 1s at 532.1 eV and Co 2p at 781.1 eV peaks were observed overCoPc/C, Bi-CoPc/C, Bi-CoPc/CeCoO and Bi-CoPc/CeNiO catalysts.The binding energy of Ni 2p at 853.4 eV was clearly appeared in Bi-CoPc/CeNiO (Fig. 3D), which was the characteristic peak of NieO[35]. The NiO doping changed the natural structure of functionaloxygen groups in the Bi-CoPc/C catalyst. The O 1s peak for Bi-CoPc/CeNiO (Fig. 3D) was much higher than that for Bi-CoPc/C (Fig. 3B).A higher content of oxygen in the catalysts usually leads to astronger ability for oxygen adsorption, which was favorable for ORRin MFCs [36,37].

The high resolution N 1s spectra were curve-fitted to evaluatethe chemical changes of the hybrid Bi-CoPc/CeNiO and Bi-CoPc/CeCoO catalysts. The N 1s peaks clearly upshifted in these twohybrid composites. In the Bi-CoPc/C (Fig. 4A), N 1s signal showed amain peak at 398.9 eV that could be ascribed to two chemicallynon-equivalent N atoms, e.g. four central nitrogen atoms (the innernitrogens bonded with the cobalt in the pyrrole ring) and four aza-bridging nitrogen atoms (the outer ones bonded with two carbonatoms) [38]. For the metal containing-Pc molecules, these twotypes of nitrogen atoms had similar net charge density [39], andwere separated by a small binding energy difference (0.28 eV) [40].The latter shoulder peak at 400.1 eV could be attributed to a shake-up as an excitation channel of the N 1s peak [41]. In the spectrum ofthe Bi-CoPc/CeCoO composite (Fig. 4B), the N 1s peak shifted tohigher binding energy by 0.6 eV than that in pure Bi-CoPc/C (from398.9 eV to 399.5 eV). This positive shift became more substantialfor Bi-CoPc/CeNiO (Fig. 4C), which was 1.3 eV with respect to pureBi-CoPc/C (from 398.9 eV to 400.2 eV). High binding energy isassociated with low electronic density. The shift in the hybridcomposites (Bi-CoPc/CeNiO and Bi-CoPc/CeCoO) showed thelower electronic density around the nitrogen nucleus, which couldbe indicative of the electron transfer from Bi-CoPc/C to CoO or NiOin the composite catalysts. The addition of CoO and NiO to the Bi-CoPc/C decreased the electron density on bi-phthalocyanine mac-rocycle. This deficient electron density could effectively promoteOeO bond cleavage and hydrogen peroxide formation, thus facili-tates the ORR reaction [42]. Bi-CoPc/CeNiO exhibited a clear elec-tron deficiency and was expected a higher ORR activity.

XPS results showed the differences in elemental compositionbetween Bi-CoPc/C and CoPc/C (Table 1) with the biggest differencein the oxidation extents. Compared with CoPc/C, Bi-CoPc/C catalystshowed a slight increase in oxygen and nitrogen contents. For Bi-CoPc/CeCoO, a continuous increase in N content was observed,while for Bi-CoPc/CeNiO, a substantial increase in O (from 2.55 to10.06%) and slight increase in N (from 2.78 to 4.04%) were detected.This XPS result showed that the integration of Bi-CoPc/C with CoOor NiO changed the surface chemistry. Bi-CoPc/C significantlyincreased the oxidation extents compared to CoPc, and the NiOaddition produced even higher O and N contents than Bi-CoPc/C.

3.2. The performance of SCMFCs with different Bi-CoPc/C loadingson the cathodes

The Bi-CoPc/C catalyst loadings on the cathodes greatly affectedthe SCMFC performances (Fig. 5). The SCMFCs with the Bi-CoPc/Cloadings (1e2 mg cm�2) exhibited higher power generation thanthat with low Bi-CoPc/C loading (0.5 mg cm�2). The current densityincreasedwith higher Bi-CoPc/C loadings, with 2.4 Am�2 for the Bi-CoPc/C loading of 0.5 mg cm�2, 3.1 A m�2 for 1 mg cm�2, and

3.2 Am�2 for 2 mg cm�2. However, further increasing the Bi-CoPc/Cloading (2.5 mg cm�2) in the cathode did not lead to a higher powerdensity, which could be caused by the low catalyst dispersion onthe cathode surfaces and the interference of the Bi-CoPc/C catalyticefficiency.

Fig. 4. High-resolution N 1s XPS spectra. (A) Bi-CoPc/C, (B) hybrid Bi-CoPc/CeCoO, (C)hybrid Bi-CoPc/CeNiO. (The red line through the data points is the resulting fit). (Forinterpretation of the references to color in this figure legend, the reader is referred tothe web version of this article.)

B. Li et al. / Journal of Power Sources 272 (2014) 320e327324

The power generation was not clearly correlated with thecatalyst loading. The SCMFCs with the Bi-CoPc/C loading of2 mg cm�2 obtained the highest power density of 370 mW m�2

(Fig. 5A), the loading of 1 mg cm�2 came the next (350 mW m�2),and the loading of 0.5 mg cm�2 was 50% lower (240 mWm�2). Thisindicated that the decrease in the Bi-CoPc/C loading from 2 to1 mg cm�2 only lowered 5% of power generation. Based on thepower density (Fig. 5A) and the potential values (Fig. 5B),1 mg cm�2 Bi-CoPc/C-loaded cathode was selected to balance thecost and the performance of SCMFCs.

3.3. The electrocatalytic activity of Bi-CoPc/CeNiO and Bi-CoPc/CeCoO

The oxygen reduction activity of Co-based catalysts (CoPc/C andBi-CoPc/C) and hybrid composites (Bi-CoPc/CeNiO and Bi-CoPc/

CeCoO) were evaluated using CV (Fig. 6A) and LSV (Fig. 6B) andcompared with metal oxides (NiO/C and CoO/C). The oxygenreduction peak at �0.24 V was observed for Bi-CoPc/C, which wasmore positive than that of CoPc/C (�0.3 V). This result indicatedthat the ORR activity of Bi-CoPc was better than CoPc/C, possiblydue to the extended p conjugated plane of Bi-CoPc/C that isfavorable for the catalytic activity [43]. In terms of metal oxides, nocatalytic activity for oxygen was observed on CoO/C and NiO/C,indicating that these two nanoparticles alone were unable tocatalyze electrochemical reduction of oxygen into water. However,as CoO/C and NiO/C were hybridized with Bi-CoPc/C, positivereduction peaks with substantial increase in current wereobserved, which were �0.12 V for Bi-CoPc/CeNiO and �0.22 V forBi-CoPc/CeCoO. This indicated a significant electro-catalytic effectand a good electrochemical response of the composite catalysts.The same trend was observed on LSV curves (Fig. 6B). These resultsdemonstrated that the integration of NiO or CoO with Bi-CoPc/Cimproved the ORR catalytic activity, and the NiO addition pro-duced higher ORR activity than CoO addition.

A dual-site bifunctional mechanism had been proposed for co-balt based ORR catalysts (i.e. cobaltepolypyrrole/C), where oxygenwas reduced to an intermediate reaction product (HO2

�) and furtherreduced to OH� over cobalt oxide nanoparticles in alkaline media[44]. However, the nanoparticles (CoO/C or NiO/C) used in thisstudy were not electrochemically active towards ORR. N4eCocenter was the only catalytic active site in Bi-CoPc/CeNiO and Bi-CoPc/CeCoO, and no other chemical center was involved in theelectrocatalytic process [45]. Consequently, the reason for theenhancement of the hybrid catalyst activity might be the lowerelectron density via the addition of NiO or CoO (as shown in the XPSN 1s result) and the chemisorptions of the oxygenmolecules on thecatalyst surface.

3.4. The performance of SCMFCs with different cathodes

The cathodes with different catalysts were installed in theSCMFCs and operated in the batch mode for 380 h (Fig. 7). Becausethese SCMFCs had the same volume, inoculum, anode materials,and operational conditions, the variation of power generation wasmainly caused by the cathode catalysts (metal oxide: CoO/C, NiO/C;Co-based: CoPc/C and Bi-CoPc/C; hybrid composite: Bi-CoPc/CeCoO and Bi-CoPc/CeNiO). In the 1st cycle (the first 50 h), thevoltage production were low in all SCMFCs, which was caused bythe lag phase of biomass growth after inoculation started. Thevoltage output rose rapidly in the 2nd cycle, but was still unstable.The voltage increased fast in the SCMFCs with Bi-CoPc/CeCoO andBi-CoPc/C (0.35 V after 60 h), followed by Bi-CoPc/CeNiO (0.33 Vafter 85 h) and CoPc/C (0.31 V after 75 h). The two lowest outputvoltages were NiO/C and CoO/C (0.23 V after 90 h). The voltageoutput of all SCMFCs became stable after the 3rd cycle. Three Bi-CoPc-contained cathodes had higher voltage output than that ofCo-Pc/C, indicating the superior catalytic activity for binuclear-cobalt-phthalocyanine. Pure Bi-CoPc/C (340e350 mV) withoutmetal doping had lower catalytic property than the hybrid Bi-CoPc/CeNiO and Bi-CoPc/CeCoO materials. In addition, Bi-CoPc/CeNiOexhibited a higher voltage generation (400e419 mV) than Bi-CoPc/CeCoO (350e363 mV) at the steady state. This result suggested thedoping of metal oxide enhanced the catalytic activity of Bi-CoPc/Cin SCMFCs and the voltage increase was dependent on the type ofmetal oxides doped (NiO vs. CoO).

The Bi-CoPc/C-containing cathodes exhibited higher reactionrate than CoPc-C cathodes. The cycle duration of the SCMFCs withBi-CoPc/CeNiO, Bi-CoPc/CeCoO, and Bi-CoPc/C was shorter thanthat of CoPc/C, NiO/C, and CoO/C cathodes (cycle 3rde6th) (Fig. 7).Specifically, hybrid Bi-CoPc/CeNiO (39e42 h) and Bi-CoPc/CeCoO

Table 1XPS analysis of elemental compositions (%) for Co-contained plain catalysts (CoPc/Cand Bi-CoPc/C) and hybrid catalysts (Bi-CoPc/CeCoO and Bi-CoPc/CeNiO).

Element CoPc/C Bi-CoPc/C Bi-CoPc/CeCoO Bi-CoPc/CeNiO

C 1s 94.10 92.50 88.00 85.20N 1s 2.78 2.85 3.33 4.04O 1s 2.55 4.23 8.43 10.06Co 2p 0.16 0.15 0.69 0.14Ni 2p e e e 0.56

B. Li et al. / Journal of Power Sources 272 (2014) 320e327 325

(41e49 h) had shorter cycle time than pure Bi-CoPc/C (43e50 h). Ashorter cycle duration indicates a faster reaction rate [8], and themetal oxide doping in the cathodes enhanced the ORR in MFCs.

Power densities were measured at different external resistancesand were plotted as a function of current density (Fig. 8A). Thetrend of the power density was the same as the voltage generation(Fig. 7). After 200 h of operation, all SCMFCs reached stable volt-ages. The power density of the SCMFCs with Bi-CoPc (324mWm�2)was higher than that of CoPc/C (256 mW m�2), indicating thatmacrocyclic complex significantly enhanced the ORR activity [46],probably due to a stronger bond with the cathode electrode byextended organic complex [47]. The power density of Bi-CoPc/Cwas greatly improved when doped with metal oxides (Bi-CoPc/CeNiO: 400 mW m�2 and Bi-CoPc/CeCoO: 368 mW m�2), whilethe metal oxide alone (CoO/C: 150 mW m�2, NiO/C: 121 mW m�2)had much lower power density. The results reinforced that theintegration of NiO and CoO with Bi-CoPc/C was feasible to improvethe catalytic ORR activity of Bi-CoPc/C cathodes in SCMFCs.

The polarization curves of different catalysts were summarized(Fig. 8B). Voltage decreased as the current density increased inSCMFCs. The order of the voltage was the same as the powerdensity. The highest potential was obtained by Bi-CoPc/CeNiO,

Fig. 5. Power density versus current density (A) and polarization curves (B) for theSCMFCs with different Bi-CoPc/C loadings on the cathodes.

followed by Bi-CoPc/CeCoO. Pure Bi-CoPc/C always exhibitedhigher potential than CoPc/C. The metal oxides NiO/C and CoO/Chad the lowest potentials.

Due to lower internal resistance and higher OCP values (Table 2),the SCMFCs with the Pt/C, Bi-CoPc/CeNiO, and Bi-CoPc/CeCoOcathodes were expected to have a better cathodic reaction andhigher power generation than other cathodes. The higher powerdensities resulted from the higher working potentials of the cath-odes, since the anodic OCP values (�0.47 to �0.49 V) were rela-tively consistent in all SCMFCs tested. This implied that the anodicOCP values were not affected by cathodematerials, and the cathodeperformance was the main factor for power generation in MFCs.

3.5. COD removal and the coulombic efficiency

The coulombic efficiency (CE) values indicated the conversionefficiency of organic substrates to electricity in MFCs (Fig. 9). Thecontrol SCMFC with the plain carbon cathode exhibited the lowestCOD removal (60.5%) and the lowest CE efficiency (5.0%), meaningthat the plain carbon electrode had a low conductivity and lowelectrochemical reaction activity. The OCVs for the NiO and CoOelectrodes were ~0.60 V (Table 2) and COD removal increased to70e74%. The CoPc electrode further increased the COD removal to78% and CE to 12.4%, and decreased the Rin to 302 U, indicating theCoPc improved the electricity generation.

Compared with CoPc, the Bi-CoPc/C-based catalysts had highervoltage generation, shorter cycle duration (Fig. 7), higher OCV(0.70e0.75 V) (Table 2), and good COD removal efficiency (83e89%)(Fig. 9). Among all the non-precious metal catalysts tested, theSCMFCs with the hybrid Bi-CoPc/CeNiO cathode had the lowest Rin,the highest OCV, and the highest CE value, which could becontributed to its high electrochemical activity and ORR activity.

Fig. 6. Cyclic voltammetry curves (A) and linear sweep voltammetry (B) of the cata-lysts with a scan rate of 10 mV s�1 in oxygen saturated 50 mM buffer solution. (Eachcatalyst cathode acted as the working electrode with a Pt-wire as counter electrode(CE), and an Ag/AgCl as reference electrode).

Fig. 7. The voltage generation of the batch-mode SCMFCs with the cathodes tested. (A)Bi-CoPc/CeNiO, (B) Bi-CoPc/CeCoO, (C) Bi-CoPc/C, (D) CoPc/C, (E) CoO/C, (F) NiO/C.

Table 2Parameters comparison of cathode materials.

Cathodematerial

OCV (V) AnodicOCP (V)

CathodicOCP (V)

Rin (U) Pmax

(mW m�2)

NiO/C 0.592 �0.483 0.109 465 121CoO/C 0.605 �0.479 0.126 492 150CoPc/C 0.660 �0.480 0.180 302 256Bi-CoPc/C 0.701 �0.491 0.210 311 324Bi-CoPc/CeCoO 0.729 �0.488 0.241 297 368Bi-CoPc/CeNiO 0.752 �0.485 0.267 300 400Pt/C 0.764 �0.470 0.294 319 450

Fig. 9. The COD removal and CE of SCMFCs with different cathodes tested.

B. Li et al. / Journal of Power Sources 272 (2014) 320e327326

The Pt cathodes had the similar Rin as the Bi-CoPc/C-containingcatalysts, the highest OCV (0.764 V), the highest power density(450 mW m�2), and the highest CE (16.3%) among all the cathodestested, but slightly lower COD removal efficiency (78.3%), Thismeans that Pt is effective at transforming organic substrates toelectricity. The Bi-CoPc/CeNiO catalyst with the highest voltagegeneration had a lower CE value (15.5%) than Pt, which wasconsistent with previous studies that high COD removal wasassociatedwith low CE values [48]. This was probably caused by theconsumption of organic substrates by bacterial growth rather thanby electricity generation. The Bi-CoPc/CeNiO catalyst was slightlylower in catalytic activity than Pt, but more cost-effective thanexpensive noble Pt.

Fig. 8. Power density versus current density (A) and polarization curves (B) forSCMFCs with different cathode catalysts.

The Bi-CoPc modified with CoO and NiO improved the powergeneration due to higher amount of N and O (Fig. 10). Bi-CoPc/C hadslightly higher amount of N (2.85) and O (4.23%) than CoPc/C (N:2.78, and O: 2.55%). After integrated with CoO and NiO, the com-positions of N and O increased to 3e4% and 8e10%, respectively.Correspondingly, the SCMFCs with Bi-CoPc/CeNiO cathode pro-duced a maximum power density of 400 mW m�2 (Table 2). Thedifference in the surface chemistry and the increase in O and Nfunctional groups well demonstrated that the introduction of NiOand CoO to the cathode materials substantially enhanced powergeneration and COD removal in MFCs.

3.6. Significance of the hybrid Bi-CoPc catalyst for SCMFCs

The 380-h batch-mode SCMFC tests clearly showed that thehybrid Bi-CoPc catalysts improved the catalytic activity for ORR,

Fig. 10. Correlation between the maximum power density and N/O compositions ofthe Co-based cathodes.

Table 3Cost of materials used to build cathodes for SCMFCs.

Catalyst Raw materials Quantity (g) Cost ($) Amount used(mg cm�2)

Cost($ cm�2)

Pmax

(mW m�2)

Bi-CoPc/CeNiO Bi-CoPc 1 61 0.17 0.0104 400Ni(NO3)6$6H2O 100 28 0.25 e

Bi-CoPc/CeCoO Bi-CoPc 1 61 0.17 0.0104 368Co(NO3)6$6H2O 100 79 0.25 e

Pt/C 20% Pt/C 1 69 2.5 0.17 450

B. Li et al. / Journal of Power Sources 272 (2014) 320e327 327

COD removal efficiency, and had the power generation close to thecostly Pt cathodes. By integrating metal oxide with Bi-CoPc, thehybrid catalysts (Bi-CoPc/CeNiO and Bi-CoPc/CeCoO) had muchhigher contents of N and O functional groups, which fundamentallychanged the surface chemistry of cathodes and enhanced the cat-alytic activity.

Developing cost-effective durable catalysts to replace costly Pt iscritical for real-world applications of MFCs. The hybrid Bi-CoPccomposites show a great potential, with only 6% cost of thecommercialized Pt/C based cathode catalyst and almost the samepower density as Pt/C (Table 3). The simplemetal precursors (e.g. Nior Co) are abundant and will not have the cost problems like Pt. Thelong-term stability of the hybrid Bi-CoPc was not evaluated in thisstudy, but the 380-hr batch operation already demonstrated thehigh stability of the hybrid Bi-CoPc. It should be noted that theloading of NiO or CoO in the hybrid catalyst was only 6% in thisstudy, thus higher loadings (e.g. 10e20%) could further enhance thehybrid composite performance.

4. Conclusions

A novel hybrid Co-based catalyst was developed by integratingBi-CoPc with metal oxide nanoparticles (CoO and NiO) to enhancethe catalytic activity of ORR and increase power generation inSCMFCs. The surface characteristics and electro-chemical perfor-mance of the hybrid catalyst were compared with Co-containedcatalysts (CoO/Pc and Bi-CoPc), metal oxide catalysts (CoO andNiO), Pt catalyst, and plain carbon cathode (C) over 380-h batchtests. The hybrid catalysts had better performance than pure Bi-CoPc with respect to voltage generation, biochemical reactionrate, electrochemical catalytic activity as well as surface func-tional group. Three major conclusions were drawn from thisstudy.

First, the surface chemical analysis showed that the integrationof NiO and CoO to Bi-CoPc increased the O and N functional groups.

Second, the CV and LSV confirmed the high electrochemicalcatalytic activity of the hybrid Bi-CoPc/C catalysts than the pure Bi-CoPc/C catalyst. The reduction peak for Bi-CoPc/CeNiO and Bi-CoPc/CeCoO was clearly observed at �0.12 V and �0.22 V,respectively.

Third, the integration of metal oxide to cathodes enhanced theORR and the overall biochemical reactions in SCMFCs. The SCMFCswith Bi-CoPc/CeNiO cathode produced a maximum power densityof 400mWm�2, which was slightly lower than Pt/C (450mWm�2).

Fourth, the SCMFCs with the hybrid Bi-CoPc/CeNiO had thehighest COD removal efficiency among all the cathodes tested. TheBi-CoPc/C integrated with NiO or CoO showed the great potential toreplace the costly Pt catalyst.

Acknowledgments

Financial supports from the National Natural Science Founda-tion of China (Grant No. 21173086 and 20975040) and GuangdongNatural Science Foundation (10351064101000000) are gratefullyacknowledged.

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