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Biochemical Engineering Journal 54 (2011) 10–15

Contents lists available at ScienceDirect

Biochemical Engineering Journal

journa l homepage: www.e lsev ier .com/ locate /be j

lectricity generation in continuous flow microbial fuel cells (MFCs) withanganese dioxide (MnO2) cathodes

iang Lia, Boxun Hub, Steven Suibb,c,d, Yu Leid, Baikun Lia,∗

Department of Civil and Environmental Engineering, University of Connecticut, Storrs, CT 06269, United StatesInstitute of Materials Science, University of Connecticut, Storrs, CT 06269, United StatesDepartment of Chemistry, University of Connecticut, Storrs, CT 06269, United StatesDepartment of Chemical and Biomolecular Engineering, University of Connecticut, Storrs, CT 06269, United States

r t i c l e i n f o

rticle history:eceived 8 September 2010eceived in revised form 7 December 2010ccepted 11 January 2011vailable online 19 January 2011

eywords:icrobial fuel cells

a b s t r a c t

A novel cost-effective cathode catalyst, manganese dioxide (MnO2) featured with a cryptomelane-typeoctahedral molecular sieve (OMS-2) structure, was examined in continuous flow microbial fuel cells(MFCs). Power generation and organic substrate removal efficiency of two metal ions doped OMS-2cathodes (cobalt (Co)-OMS-2 and copper (Cu)-OMS-2) were compared with platinum (Pt) cathodes underdifferent hydraulic retention times (HRTs) and chemical oxygen demands (CODs). The 600-h continuousflow tests showed that Cu-OMS-2 MFCs and Co-OMS-2 MFCs achieved the stable power generation of200 ± 8 mV and 190 ± 5 mV, and were 50–60 mV higher than that of Pt MFCs. The COD removal efficiencies

ontinuous flowanganese dioxidesctahedral molecular sievesperation condition effect

of Cu-OMS-2 MFCs and Co-OMS-2 MFCs were 83–87%, which were 15–19% higher than that of Pt MFCs.The power generation and COD removal efficiency increased with longer HRTs. The Cu-OMS-2 exhibitedthe highest power density (201 mW/m2) at the COD of 1000 mg/L. However, Co-OMS-2 cathodes hadthe better performance than Cu-OMs-2 at high COD concentrations of 2000–4000 mg/L, with the powerdensity of 897 mW/m2 and COD removal efficiency of 46%. The continuous flow MFC tests demonstratedthat the fast reaction rate of OMS-2 cathodes enhanced power generation and COD removal efficiency,

to be

and had a great potential

. Introduction

Microbial fuel cells (MFCs) are capable of producing energyhrough wastewater treatment and hold a great potential forenewable bioresource generation. In the anode compartment,lectrons are generated by anaerobic electrogenic microorgan-sms through degrading organic substances (i.e., acetate, glucose),

hile in the cathode compartment, electrons are acceptedy electron acceptors (i.e., oxygen, ferricyanide (K3[Fe(CN)6]))1–3]. Due to the ability of simultaneous organics removalnd power generation, MFCs attract substantial interest inastewater treatment [4–7]. New configurations such as tubu-

ar MFCs, up-flow MFCs, and novel electrode materials suchs brush anodes, stainless steel cathodes have been developed8–11]. Enhancing power generation and developing low-cost

lectrode materials for large-scale applications become criti-al for MFC technology. In recent studies, oxygen reductionnd electron acceptance in the cathode chamber have beenound as limiting factors for electricity production, due to

∗ Corresponding author. Tel.: +1 860 486 2339; fax: +1 860 486 2298.E-mail address: baikun@engr.uconn.edu (B. Li).

369-703X/$ – see front matter. Published by Elsevier B.V.oi:10.1016/j.bej.2011.01.001

applied in real-world wastewater treatment processes.Published by Elsevier B.V.

the slow reaction kinetics of oxygen reduction rates (ORRs)[12,13].

The new catalyst, cryptomelane-type manganese dioxide(MnO2), has been recently found to replace costly platinum asthe cathodic catalyst. The cryptomelane-type MnO2 is an octahe-dral molecular sieve consisting 2 × 2 edge-shared MnO6 octahedralchains (OMS-2), which are corner shared to form one-dimensionaltunnels (Fig. 1) [14]. Mn4+ and Mn3+ ions are located in the octahe-dral sites of cryptomelane. The catalytic activity of OMS-2 can beenhanced by doping metal ions, since the substitution of frameworkMn4+ with the doped metals (e.g., Cu2+ and Co3+) can create moreoxygen vacancies to maintain an overall charge. Consequently, oxy-gen transfer and reduction take place at these sites [15]. It had beenfound that the power density of the MFC with carbon-supportedMnO2 particles can reach 161 mW/m2 compared to 19 mW/m for abenchmark Pt/C at room temperature [16]. Nano-structured MnOx

was also used as the MFC cathode catalyst and achieved a peakpower density of 772.8 mW/m3 [17].

The MnO2 featured with OMS-2 structure has high ORR and goodcatalytic properties [18]. The cost of MnO2 cathodes is only 5% of thetraditional platinum (Pt) cathodes. The doping of copper (Cu-OMS-2) and cobalt (Co-OMS-2) ions were capable of enhancing powergeneration and organic removal rates [19]. In lab-scale batch mode

X. Li et al. / Biochemical Engineeri

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Fig. 1. The structure of OMS-2.

FC tests, the Cu-OMS-2 and Co-OMS-2 exhibited good perfor-ance with power densities of 165 and 180 mW/m2, which were

lose to that of Pt cathodes (198 mW/m2). Moreover, the batch-ode tests showed that the OMS-2 cathodes shortened the cycle

uration to less than half that of Pt cathodes, which was the resultf high catalytic ability and faster reaction rates of OMS-2 cathodes.his means that the MFCs equipped with OMS-2 cathodes can uti-ize organic substances more efficiently and treat high volume of

astewater at short retention time [20].However, there has been no study on OMS-2 cathodes under

ontinuous flow conditions, which are commonly applied ineal-world operations and could be affected by several criticalperational parameters including organic loading rates (OLRs) andydraulic retention time (HRT). Only a few studies were conducted

or the continuous flow MFCs treating of wastewater. By usingingle-chamber MFCs (SCMFCs), Nam et al. found that the powerensity increased from 1884 mW/m3 to 2981 mW/m3 with OLRs

ncreasing from 1.92 g/L d to 3.84 g/L d [21]. By using continuousow SCMFCs with graphite anodes and Pt cathodes, Liu et al. foundhat the peak power density was 26 mW/m2, and the power out-ut increased with HRTs (3–33 h) and wastewater contaminantoncentrations (50–220 mg/L) [22]. Until now, almost all of MFCests employed Pt cathodes. Considering the low cost and highower generation of the OMS-2 cathodes, continuous-flow MFCests should be conducted for future real-world operations.

There were two main objectives in this study. First, the Co-OMS-and Cu-OMS-2 cathodes were compared with the Pt cathodes in

ontinuous-flow SCMFCs in order to determine the improvementn electricity production. Second, the effects of HRT and chemicalxygen demand (COD) concentration on the power generation ofontinuous-flow SCMFCs were investigated. The MFC performanceas evaluated based on electricity generation capabilities, internal

esistance (Rin), open circuit potential (OCP), coulombic efficiencyCE) and COD removal efficiencies.

. Methods

.1. MFC set-up and continuous flow operation

The SCMFCs consisted of granular activated carbon (GAC)nodes and OMS-2 cathodes were used in this study [23]. The vol-me of the SCMFC was 0.5 L. About 0.35 L of GAC particles (GC× 30, General Carbon, Paterson, NJ) was packed into the SCMFCs.he OMS-2 catalyst was synthesized using hydrothermal methodsnd pasted on 30% water-proofed carbon cloth (Fuel Cell Earth,toneham, MA) [20]. The OMS-2 catalyst coated carbon cloth was

hen screwed on top of the SCMFCs with the catalyst layer facinghe water and the polytetrafluoroethylene (PTFE) coating layer fac-ng air. Oxygen in air acted as the electron acceptor. The SCMFCsonsisting of GAC anodes and platinum cathodes were used as theontrol at all COD and HRT tests. The platinum cathode was made

ng Journal 54 (2011) 10–15 11

following the method used in a previous study [24]. The catalystloading on cathode was 0.5 mg/cm2 in all tests. The external resis-tance (Rext) was 100 � unless otherwise stated. The voltage overRext was recorded with a Keithley 2700 data logging system at 2 hintervals. All experiments were conducted in a 30 ◦C incubator.

In the initial batch-mode inoculation period, wastewater influ-ent collected from the University of Connecticut WastewaterTreatment Plant was used as the inoculums. The initial COD ofwastewater was 300 mg/L and pH was 7.2. Sodium acetate wasadded as an extra carbon source to accelerate the inoculation ofanaerobic electrogenic bacteria in MFCs. Once the MFCs achieved astable power output (180–220 mV), they were changed to continu-ous flow mode and fed with artificial wastewater (AW) containingsupplemental sodium acetate to achieve the designated COD con-centrations (500–4000 mg/L). The HRTs (10–40 h) in MFCs wereadjusted with a multi-channel cassette pump (CARTER 12/6 Cas-sette Pump System, 115VAC, Barrington, IL). All measurementswere performed after five HRT cycles for each MFC. The experi-mental data presented in this study were the average of triplicatemeasurements.

2.2. Analysis

The internal resistance (Rin) consumes the power generatedby the MFCs and lowers the power generation efficiency. The Rinof SCMFCs at different HRTs and COD concentrations was deter-mined using the polarization curve plotted by changing the externalresistors (Rext) from 30 to 1500 �. The voltage over each Rext

was recorded with a multimeter. The Rin was calculated at themaximum power density point (where Rin equals to Rext) on thepolarization curve [2]. Power densities (W/m2) and current density(A/m2) were calculated based on

Power density = V2

RextA(1)

and

Current density = V

RextA(2)

V is the cell voltage (V), Rext is the external resistor (�), and A is thearea of the cathode (m2).

The open circuit potentials (OCPs) of anodes and cathodes inMFCs were measured using a potentiostat (Gamry Reference 600),with the target electrode (anode or cathode) as the working elec-trode, and an Ag/AgCl reference electrode as the counter electrodeand the reference electrode.

Coulombic efficiency (CE, �c (%)) was defined as the ratio of theactual charge generated to the theoretical charge generated if thesubstrate is completely converted to electricity. CE was calculatedusing Eq. (3), where M = 32 is the molecular weight of oxygen, I isthe current, F is the Faraday constant (96,485), b = 4 is the num-ber of electrons exchanged per mole of oxygen, q is the volumetricinfluent flow rate, and �COD is the change of COD concentrationsbetween the influent and effluent of SCMFCs.

�c = M I

F b q �COD(3)

3. Results and discussion

3.1. The comparison of electricity generation of OMS-2 cathodesand Pt cathodes in continuous-flow SCMFCs

The OMS-2 SCMFCs produced higher power than Pt SCMFCsin continuous-flow tests. The stable voltage output of Cu-OMS-2 was 200 ± 8 mV (Fig. 2) over a 600-h operation period (at thecondition of COD = 1000 mg/L and HRT = 25 h), while the voltage of

12 X. Li et al. / Biochemical Engineering Journal 54 (2011) 10–15

Fig. 2. Continuous voltage generation with Cu-OMS-2 SCMFCs and Pt SCMFCs(COD = 1000 mg/L, HRT = 25 h).

Table 1COD removal efficiency (%) of the OMS-2 and Pt SCMFCs at different HRTs and CODconcentrations.

MFCs HRT (h) (COD: 1000 mg/L) COD (mg/L) (HRT: 25 h)

10 25 40 500 1000 2000 4000

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Cu-OMS-2 59 87 95 97 87 58 40Co-OMS-2 52 83 92 92 83 60 46Pt 27 68 87 86 68 39 29

t SCMFCs was only 140 ± 6 mV. The main reason for the higherower generation of OMS-2 SCMFCs was that OMS-2 cathodes pos-essed high oxygen reduction rates (ORRs) [20]. In our previousatch-mode test, the voltage generation in OMS-2 SCMFCs wasnly 10 mV higher than Pt SCMFCs, but the cycle duration of OMS-SCMFCs (90 h) was less than half of that of Pt SCMFCs (202 h),hich means that the OMS-2 cathodes possessed faster ORR than

t cathodes. The advantage of fast reaction rates of OMS-2 cath-des was amplified in continuous flow tests. Because the retentionime (25 h) in continuous flow tests was much shorter than theycle duration (90–202 h) in batch mode tests, the contact periodf anodic biofilms and organic substances became less sufficient.hereby, a fast reaction rate became critical for degrading organicubstances and reacting with electrons, protons, and oxygen onhe cathode. The fast reaction rate of OMS-2 cathodes led to thefficient degradation of organic substances and high power gener-tion. About 198 mW/m−2 and 180 mW/m−2 power densities werebtained in Cu-OMS-2 and Co-OMS-2 SCMFCs, while the maximumower density of Pt SCMFCs was only 105 mW/m−2 (Fig. 3).

The Cu-OMS-2 SCMFCs had a better performance than Co-OMS-SCMFCs since the former had lower Rin and higher OCP than the

atter. The Rin was affected by the electrode and solution conductiv-ty and electrolyte resistance [2]. The conductivity of the Cu-OMS-2lm measured by the four-probe method [25] is 0.15 S/cm, which isigher than that of the Co-OMS-2 film (0.09 S/cm). Under moderate

rganic loading (COD = 1000 mg/L, HRT = 25 h), the conductivity ofhe solution (0.01 S/cm) was low and the cathode conductivity con-ributed more to the Rin, which explained that the Rin of Cu-OMS-2115 ± 4 �) was slightly lower than that of Co-OMS-2 (118 ± 3 �)Table 2). The OCP was measured to evaluate the cathode catalyst

able 2nternal resistance (Rin, �) of the OMS-2 and Pt SCMFCs at different HRTs and CODoncentrations.

MFCs HRT (h) (COD: 1000 mg/L) COD (mg/L) (HRT: 25 h)

10 25 40 500 1000 2000 4000

Cu-OMS-2 118 ± 3 115 ± 4 116 ± 5 150 ± 5 115 ± 4 45 ± 2 35 ± 3Co-OMS-2 120 ± 3 118 ± 3 116 ± 5 148 ± 7 118 ± 3 43 ± 3 30 ± 2Pt 120 ± 5 121 ± 5 120 ± 3 151 ± 5 121 ± 5 80 ± 5 50 ± 3

Fig. 3. The polarization curve of the OMS-2 and Pt SCMFCs at different HRTs (COD:1000 mg/L): (A) HRT = 10 h, (B) HRT = 25 h, and (C) HRT = 40 h.

activity. The OCP of Cu-OMS-2 was higher than that of Co-OMS-2,which illustrated that the Cu-OMS-2 had a higher catalytic activ-ity (Table 3). The OCP values of OMS-2 cathodes were much higherthan Pt cathodes, which attributed to the high power generation ofOMS-2 cathodes.

Table 3OCP (vs Ag/AgCl, mV) of the OMS-2 and Pt SCMFCs at different HRTs and CODconcentrations.

MFCs HRT (h) (COD: 1000 mg/L) COD (mg/L) (HRT: 25 h)

10 25 40 500 1000 2000 4000

Cu-OMS-2 136 137 140 125 137 146 150Co-OMS-2 121 124 123 120 124 159 161Pt 115 118 119 116 118 127 132

X. Li et al. / Biochemical Engineeri

Table 4CE (%) of the OMS-2 and Pt SCMFCs at different HRTs and COD concentrations.

MFCs HRT (h) (COD: 1000 mg/L) COD (mg/L) (HRT: 25 h)

10 25 40 500 1000 2000 4000

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Cu-OMS-2 2.8 9.5 14.7 18.1 9.5 6.2 5.0Co-OMS-2 2.9 9.5 14.4 20.3 9.5 6.0 5.2Pt 4.3 8.5 10.7 13.7 8.5 4.8 4.5

Besides good power generation, the OMS-2 SCMFCs also exhib-ted high COD removal efficiency. About 87% and 83% of organicubstances in the influent were degraded in the Cu-OMS-2 and Co-MS-2 SCMFCs, respectively, while only 68% organic substancesere degraded in the Pt SCMFCs (Table 1). This could be explained

y the high ORR and high OCP of the OMS-2 cathodes. The OCP ofu-OMS-2 and Co-OMS-2 cathodes were 137 mV and 124 mV, buthe OCP of Pt cathode was 118 mV (Table 3). This indicates that theeaction rate order was Cu-OMS-2 > Co-OMS-2 > Pt, which was con-istent with the COD removal rates order (Table 1). Considering thathe cost of OMS-2 cathode was only 5% of the Pt cathode, the OMS-cathodes can indeed enhance the continuous power generation

nd organic removal rate economically and effectively.

.2. The effect of HRT on continuous-flow OMS-2 SCMFCs

The effect of HRT on SCMFC performances was studied at theRT range of 10–40 h with a COD concentration of 1000 mg/L.he reason for keeping COD concentration at 1000 mg/L was thaticroorganisms in MFCs have sufficient substrates for power pro-

uction at this level of COD concentration and too high CODoncentrations might hinder the microbial activities. Both powereneration and COD removal rates increased with HRTs (Fig. 3 andable 1). At HRT of 10 h, the power densities of Cu-OMS-2 and Co-MS-2 SCMFCs were only 79 and 73 mW/m2 (Fig. 3A). The COD

emoval efficiencies were only 59% and 52% and the CE valuesere 2.8% and 2.9%, respectively, which demonstrated that sig-ificant amounts of organic substances were not used effectively

n MFCs at this short HRT (Tables 1 and 4). The degradation ofrganic substances required sufficient HRT for the contact withnodic biofilms in the MFCs. When HRT increased to 25 h, the powereneration values of Cu-OMS-2 and Co-OMS-2 were 198 mW/m2

nd 180 mW/m2, which were 94 mW/m2 and 75 mW/m2 higherhan that of Pt SCMFCs (Fig. 3B). The COD removal efficiencies of theu-OMS-2 and Co-OMS-2 SCMFCs were 87% and 83%, respectively,hich were 19% and 15% higher than that of the Pt SCMFCs (Table 1).hen HRT was prolonged to 40 h, the power densities and COD

emoval efficiencies of the OMS-2 cathodes slightly increased com-ared to those obtained at an HRT of 25 h (Fig. 3C and Table 1). About5% and 92% of COD were removed in the Cu-OMS-2 and Co-OMS-SCMFCs, respectively, and the power densities were 201 mW/m2

nd 187 mW/m2. Long HRTs were beneficial for metabolic activitynd biomass growth of anaerobic microorganisms, which improvedhe COD removal efficiency. However, even at HRT of 40 h, the PtCMFCs still had lower COD removal efficiency and lower powerensity than OMS-2 SCMFCs. This indicated that the Pt SCMFCseeded longer HRT to reach the same power recovery efficiency andOD removal efficiency as OMS-2 cathodes (Fig. 3C and Table 1).

Although HRT had clear effects on COD removal and power gen-ration, HRT only had minor impact on Rin and OCP (Tables 2 and 3).hen the HRT increased from 10 h to 40 h, the Rin of the Cu-OMS-and Co-OMS-2 SCMFCs slightly decreased by 2–3 �, while the

CP values of these two SCMFCs slightly increased by 2–3 mV.he Rin was determined by the electrode and solution conductiv-ty and electrolyte resistance, while the OCP was determined byhe electrode electrochemical properties such as surface area andatalytic ability [26]. Because these properties associated with Rin

ng Journal 54 (2011) 10–15 13

and OCP remained almost the same in SCMFCs under the same CODconcentration at different HRTs, the Rin and OCP did not exhibit dis-tinct changes in HRT tests. However, the CE values of MFCs clearlyincreased with longer HRTs (Table 4). As shown in Eq. (3), althoughthe COD removal (�COD) slightly increased at long HRTs, the flowrate (q) decreased substantially, which led to an overall increase inthe CE values.

The HRT tests showed that 25-h was the optimal HRT for thecontinuous-flow OMS-2 SCMFCs. For instance, at the HRT of 25 h,the COD removal efficiency of the Cu-OMS-2 SCMFCs reached 87%,which means almost all of the organic substances in the influentwere consumed by bacteria in the anode. Further increasing HRTto 40 h in the Cu-OMS-2 SCMFCs (HRT increased 37.5%) only led tothe increases of 1.5% and 8% in power density and COD removal,respectively. On the other hand, HRT of 10 h was too short toachieve a desirable power output and COD removal, with 41% ofCOD remained in the effluent (Table 1), which means that therewere many organic substances not degraded in the MFCs.

3.3. The effect of COD concentration on continuous-flow OMS-2SCMFCs

The power generation of the OMS-2 and Pt SCMFCs increasedwith COD concentrations in the range of 500–4000 mg/L (HRT:25 h). The power densities of the Cu-OMS-2, Co-OMS-2 and Pt SCM-FCs at the COD of 4000 mg/L were 15, 20 and 8 times as high as thoseat COD 500 mg/L, respectively (Fig. 4A and C). This tendency wasprobably related to the higher availability of organic substancesat high COD concentrations. Moreover, the OMS-2 SCMFCs exhib-ited better power production than the Pt SCMFCs at each CODconcentration. For instance, at COD of 4000 mg/L, the power gen-erations of Cu-OMS-2 and Co-OMS-2 were 2.6 and 3 times higherthan that of the Pt SCMFCs (Fig. 4C). Because Pt cathodes had lowerORR and lower OCP than OMS-2 cathodes (Table 3), their electronacceptance capacity and catalytic activity were lower than thoseof OMS-2 cathodes with the presence of the same amount of oxy-gen in the cathode chamber. For OMS-2 cathodes, due to high ORRand OCP, more electrons generated in the anodes can be utilizedand reacted with oxygen in the cathode chamber, thus resulting inhigher power production and higher COD removal efficiency.

The electrochemical properties (e.g., Rin, OCP, and CE val-ues) of OMS-2 cathode varied with COD concentrations. The Rindecreased with the increase in COD concentrations. The Rin wasmainly determined by the electrode and solution conductivityand electrolyte resistance [2]. Because Cu-OMS-2 had higher con-ductivity (1.5 S/m) than Co-OMS-2 (0.9 S/m), the Rin of Cu-OMS-2(150 ± 5 and 115 ± 4 �) was slightly lower than that of Co-OMS-2(148 ± 7 and 118 ± 3 �) at low and moderate COD concentrations(COD = 500 and 1000 mg/L) (Table 2). However, with the solutionconductivity increasing at higher COD concentrations, the conduc-tivity defects of Co-OMS-2 were made up, and the Rin of Co-OMS-2(30 ± 2 �) became lower than that of Cu-OMS-2 (35 ± 3 �) at thehigh COD concentration (4000 mg/L) (Table 2). The Cu-OMS-2 hasbetter conductivity but lower catalytic activity than Co-OMS-2 [27].Due to the high catalytic ability, Co-OMS-2 cathodes had fasterORR at high COD concentrations, which would be a driving forceto transfer more electrons generated in anodes to cathodes andresulted in low Rin. In terms of OCP values, Co-OMS-2 cathodes hadlower OCP than Cu-OMS-2 cathodes at low COD concentrations(COD: 500 and 1000 mg/L), but had higher OCP than Cu-OMS-2cathodes at high COD concentrations (COD: 2000 and 4000 mg/L)

(Table 3). The significant increase in OCP on Co-OMS-2 cathodesat higher COD concentrations was that Co-OMS-2 had higher sur-face area (44 m2/g) than Cu-OMS-2 (40 m2/g) and higher catalyticactivity [27]. At high COD concentration, more organic substanceswere degraded on the anode so that a high amount of oxygen was

14 X. Li et al. / Biochemical Engineerin

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nology, Environ. Sci. Technol. 40 (2006) 5181–5192.

ig. 4. The polarization curve of the OMS-2 and Pt SCMFCs at different CODoncentrations (HRT = 25 h): (A) COD = 500 mg/L, (B) COD = 2000 mg/L, and (C)OD = 4000 mg/L.

equired as the electron acceptor on the cathode to accept thelectrons generated from the anode. The higher catalytic activitynd higher surface areas of Co-OMS-2 cathodes led to the highermounts of oxygen adsorbed on the catalyst/cathode surface, sohat more oxygen was available for utilizing on the cathode and ledo higher OCP values. The CE values of Cu-OMS-2 and Co-OMS-

dropped from 18.1% and 20.3% to 5.0% and 5.2% as the CODncreased from 500 mg/L to 4000 mg/L (Table 4). The lower CE val-es at high COD concentration indicated that the lower portion ofOD was degraded at higher COD concentrations than lower COD

oncentrations. In Eq. (3), the COD change (�COD) at high COD con-entration was much higher than that at low COD concentration,ven though the COD removal rate was lower than that of low CODoncentration (Table 4).

g Journal 54 (2011) 10–15

Besides higher power generation, the OMS-2 SCMFCs alsohad higher COD removal efficiencies than that of the Pt SCM-FCs (Table 1). At the COD of 500 mg/L, 97% and 92% of COD weredegraded in the Cu-OMS-2 and Co-OMS-2 SCMFCs, compared with86% of COD being removed in the Pt SCMFCs. At the COD of4000 mg/L, 40% and 46% of COD were degraded in the Cu-OMS-2and Co-OMS-2 SCMFCs, compared with 29% of COD being removedin the Pt SCMFCs. The Co-OMS-2 was the better cathode catalyst athigh COD concentration, due to the low Rin and high OCP at highCOD concentrations (COD: 2000 and 4000 mg/L) (Tables 2 and 3).On the other hand, the Cu-OMS-2 was the better catalyst at low CODconcentrations (COD: 500 and 1000 mg/L), due to low Rin and OCP.The main reasons for the opposite trends of these two OMS-2 cat-alysts are that the advantage of catalytic ability and surface area ofCo-OMS-2 was suppressed by the low conductivity at low COD con-centration, while the high conductivity at high COD concentrationdiminished the conductivity difference between Co-OMS-2 and Cu-OMS-2 and then the advantage of catalytic ability and surface areaof Co-OMS-2 became dominant at high COD concentration.

4. Conclusions

The newly developed OMS-2 cathodes were compared with Ptcathodes in continuous-flow SCMFCs to enhance power generationand organic removal. The performance of the OMS-2 SCMFCs and PtSCMFCs was comprehensively examined at different levels of HRTsand COD concentrations. Three major conclusions were drawn fromthis study.

1. The OMS-2 SCMFCs improved power generation and organicremoval efficiency. The advantages (e.g., fast oxygen reductionrate) of OMS-2 cathodes over Pt cathodes were amplified incontinuous-flow tests than in batch mode tests. The power den-sities of the Cu-OMS-2 and Co-OMS-2 SCMFCs were 2.6 and 3times higher than that of the Pt SCMFCs at COD 4000 mg/L andHRT 25 h.

2. The HRT of 25 h was found as the optimal HRT with highestpower density and organic removal efficiency. The Rin and OCPvalues remained relatively stable at different HRTs, even thoughthe CE values were slightly increased due to the lower flow ratesat longer HRTs.

3. Co-OMS-2 cathode is more suitable than Cu-OMS-2 cathodeat high COD concentrations (2000–400 mg/L), while Cu-OMS-2cathode is more suitable than Co-OMS-2 cathode at low CODconcentrations (500–1000 mg/L).

Acknowledgements

This study was supported by UCONN Provost OutstandingGraduate Research Fellowship and UCONN CESE (Center forEnvironmental Science and Engineering) Multidisciplinary Envi-ronmental Graduate Research Scholarship and the Department ofEnergy, Office of Basic Energy Sciences, Division of Chemical, Geo-logical, and Biological Sciences.

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