Stability characterization and modeling of robust ...

Bioresource Technology 144 (2013) 477–484

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Stability characterization and modeling of robust distributed benthic microbial fuel cell (DBMFC) system Udayarka Karra a, Guoxian Huang b, Ridvan Umaz b, Christopher Tenaglier a, Lei Wang b, Baikun Li a,⇑ a b

Department of Civil and Environmental Engineering, University of Connecticut, Storrs, CT 06269, USA Department of Electrical and Computer Engineering, University of Connecticut, Storrs, CT 06269, USA

h i g h l i g h t s Distributed benthic MFC (DBMFC) with multi-anode/cathode. DBMFCs had higher stability than traditional BMFCs. Anode is the significant limiting factor for BMFC. Computational models confirmed the high stability of DBMFC.

a r t i c l e

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Article history: Received 9 May 2013 Received in revised form 23 June 2013 Accepted 25 June 2013 Available online 2 July 2013 Keywords: Benthic microbial fuel cell (BMFC) Stability Multi anode/cathode arrays Bioturbation Computational model

a b s t r a c t A novel and robust distributed benthic microbial fuel cell (DBMFC) was developed to address the energy supply issues for oceanographic sensor network applications, especially under scouring and bioturbation by aquatic life. Multi-anode/cathode configuration was employed in the DBMFC system for enhanced robustness and stability in the harsh ocean environment. The results showed that the DBMFC system achieved peak power and current densities of 190 mW/m2 and 125 mA/m2 respectively. Stability characterization tests indicated the DBMFC with multiple anodes achieved higher power generation over the systems with single anode. A computational model that integrated physical, electrochemical and biological factors of MFCs was developed to validate the overall performance of the DBMFC system. The model simulation well corresponded with the experimental results, and confirmed the hypothesis that using a multi anode/cathode MFC configuration results in reliable and robust power generation. Published by Elsevier Ltd.

1. Introduction Abbreviations: BMFC, benthic microbial fuel cell; COD, chemical oxygen demand; DBMFC, distributed benthic microbial fuel cell; DO, dissolved oxygen; Icell, current of mfcs; LSV, linear sweep voltammetry; MFC, microbial fuel cell; NMOS, Negative-channel metal-oxide semiconductor; OCP, open circuit potential; ORR, oxygen reduction rate; PTFE, polytetrafluoroethylene; Rext, external resistance; Rin, internal resistance; V, voltage; gohm, ohmic overpotential; gact, activation overpotential; gcon, concentration overpotential; gA,act, activation overpotential at anode; gA,con, concentration overpotential at anode; gC,act, activation overpotential at cathode; gC,con, concentration overpotential at cathode; k10, rate constant of the anode reaction at standard conditions; KAC, half velocity rate constant for acetate; a, charge transfer coefficient at the anode; b, charge transfer coefficient at the cathode; c, exchange current coefficient; rA, reaction rate occurring at the anode; rI, non-ideal current loss; F, Faraday constant; R, gas constant; T, operation temperature; Vopen, open circuit potential; X, concentrations of biomass at the anode surface; CAC, concentrations of acetate at the anode surface; CO2, concentration of the dissolved oxygen at the cathode surface; daq, distance of the electrodes in the solution; ds, distance of the electrodes in the sediment; qaq, resistivity of the solution; qs, resistivity of the sediment; Aaq, cross-section area of the solution; As, cross-section area of the sediment; Pa/c, power output of one anode/cathode pair; Ptotal, total harvested power; Itotal, overall output current. ⇑ Corresponding author. Tel.: +1 860 486 2339 E-mail address: [email protected] (B. Li). 0960-8524/$ - see front matter Published by Elsevier Ltd. http://dx.doi.org/10.1016/j.biortech.2013.06.104

Aquatic exploration has garnered enormous attention in the last decade for marine environment monitoring, pollution detection, surveillance for undersea infrastructure, ocean disaster deterrence, and coastal security surveillance. This rapid development has posed a critical need for autonomous and distributed underwater sensor networks (Proakis et al., 2011; Heidemann et al., 2006), which require reliable, stable and scalable power supply. Currently, battery power is the most used source, but is expensive and unsustainable (Heidemann et al., 2006). Their short service life hinders the long-term operation of underwater sensor networks. Even though some alternate energy sources like solar, wind, wave, and salinity gradients have been studied (Paradiso and Starner, 2005; Yen and Lang, 2006), they are neither practical nor adequate to meet the energy capacity. Henceforth, there is a critical need to explore a more sustainable energy source appropriate for underwater sensor networks. As such, there is an abundant supply of nutrients (e.g. organics remnants of marine microfossils), diverse

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microorganisms (e.g. anaerobic bacteria, metal oxidizing/reducing bacteria), and dissolved oxygen in ocean waters, which can be used for underwater energy harvesting (Logan and Regan, 2006a; Logan et al., 2006b; Rabaey and Verstaete, 2005). Microbial fuel cells (MFCs) are a novel biotechnology to generate electrical energy through bacterial metabolic activities and organic substrate degradation. They have been extensively studied in wastewater treatment and for hydrogen production from biomass (Du et al., 2007; Rabaey et al., 2007). In recent years, MFCs have been explored for energy harvesting in aquatic systems, known as benthic MFCs (BMFCs) (Reimers et al., 2001), which could supply steady power for remote oceanographic devices by converting the chemical energy stored in the benthic sediments into electrical power. Microbially mediated redox reactions (redox potential of 0.7–0.8 V) (Girguis et al., 2010) and electron transfer process, accompanying organic matter degradation are the underlying principles of BMFC (Rezaei et al., 2007). In a BMFC, microorganisms in the sediments act as anode and oxygen in ocean water as cathode (Fig. 1). The electrons generated through the bacterial degradation of organic substances are collected at the anode buried in the sediments, while protons migrate to the cathode via diffusion through the ocean water. The electrons pass through an external circuit (e.g. sensor electronics) to the floating cathode and react with dissolved oxygen, through which electricity is generated. BMFCs have dual attributes of supplying power for underwater sensors and understanding microbially mediated biogeochemical processes. Numerous factors influence the proper functioning and stable power generation in BMFCs, and foremost of all is the issue of bioturbation by diverse aquatic organisms (Girguis et al., 2010). The anodes in BMFCs are buried in sediments and are operated under anaerobic conditions insulated from oxygen. However, bioturbation by aquatic organisms at the anode could seep oxygen resulting in the electron depletion. This will eliminate the net potential difference across the BMFC electrodes, which is the essential driving force. In order to enhance the stability and robustness of BMFCs, bioturbation problem need to resolved. Earlier developed BMFC prototypes used a simple configuration of buried-anode and floating cathode, with the purpose of examining differences in power production among underwater locations (Tender et al., 2002). Later advancements like the Benthic Unattended Generator (B.U.G) (Tender et al., 2008) and chambered BMFC (floating anode system) (Nielsen et al., 2008) enhanced power densities and prolonged the power generation duration period. However, the use of large electrodes with huge footprint for the purpose of improving power generation have raised concerns for their extensive field application, as such making them vulnerable to bioturbation. A new configuration of MFC, termed Distributed Benthic Microbial Fuel Cell (DBMFC) employing an array of distributed single cathode associated with multiple anodes was developed in this study to investigate the robustness and stability of scale-up BMFCs. The distinct advantage of this configuration is that the performance of BMFCs can be greatly improved by using multiple anodes (as shown in Fig. 1) in conjunction with a cathode, and using numerous arrays of these, potential bioturbation from aquatic organisms in the natural environment can be addressed. In the real world sce-

WATER H

+

CATHODE

1

2

n

MULTI-STACK

ANODE

LOAD e

-

SEDIMENTS

Fig. 1. Basic principle of DBMFCs with different electrode arrays.

nario of a conventional BMFC system with only one anode deployed, any bioturbation to the electrodes can result in reverse reactions, i.e., electrochemical redox reactions arise not only at the anode as expected but also at the cathodes (Rapoport et al., 2012). This will eliminate the net potential difference across the electrodes and reduce power supply. Whereas, in a DBMFC system with multiple (2, 3 or n) anodes in the sediment associated with one cathode, if one anode is impaired, the problem will only develop in the local area of this specific anode without disturbing other anode pairs. A majority of electrode arrays can still provide stable power supply for underwater sensor networks, which substantially enhances the stability and reliability of the DBMFCs in harsh habitats. Furthermore, the distributed facet of the DBMFC utilizes multiple arrays of anode/cathode pairs that can be placed in individual casings and stacked along the ocean sediment floor, hence providing for a compact configuration with minimal footprint. Theoretical models are necessary to understand the mechanisms of MFCs through evaluating the various parameters of the system and provide the design guidelines to improve the energy harvesting performance. Some theoretical models were developed for chemical fuel cells, but could not be applied to BMFCs, since the biochemical attributes of BMFCs were not considered in these models. Besides, existing simple models for traditional underwater energy harvesting systems (Shantaram et al., 2005; Nielsen et al., 2007; Picioreanu et al., 2007; Zeng et al., 2010; Dai et al., 2011) need to consider the distributed structure to simulate the attributes developed to minimize bioturbation. The objectives of this study were to characterize the robustness and stability of the DBMFC for long term operations. Detailed analysis of system performance in terms of voltage, power, and current production were conducted using multiple arrays of anodes/cathodes. In order to facilitate the design of DBMFC for real applications, a computational model was developed to quantify the unique multi-electrode configuration of the DBMFC system, and allow for systematic quantification of various key parameters to understand their overall relationship with the operation of the DBMFC system.

2. Methods 2.1. Materials and configuration of DBMFC The DBMFC system consisted of several arrays, each array constituted multiple anodes associated to single cathode (Fig. 2; only one anode for each array is being represented for convenience). Each anode is connected to the cathode through the source of a negative-channel metal-oxide transistor (NMOS) switch with the gate grounded and drain connected to the load. When the anode is operational, it is negatively poised due to the anaerobic conditions, allowing the NMOS switch to remain closed and connect to the load. On the other hand, if the anode cannot produce the expected potential due to reverse reactions, the NMOS switch opens and prevents the corresponding anode from affecting the rest of the circuit. The total volume of the DBMFC system was about 0.10 m3, with dimensions-length: 32 cm, width: 32 cm, height: 65 cm. The system had a stacked column of three anode casings and an cylindrical expanded metal frame structure made of stainless steel (316 3/4 #16 Flattened) to guard against potential bioturbation. Each anode casing contained nine anodes, and one anode from each of the three casing were connected together in parallel to one cathode, which means that one cathode was connected with three anodes. The DBMFC system consisted of a total of twenty seven anodes buried (at an optimal distance of 10 cm from the surface) into the sediments, and nine cathodes floating about 5 cm above the sediment casings in the water.

479


MULTI-STACK

WATER

CATHODE O2

H2O H+

CO2 +H 2O

LOAD

SEDIMENTS ORGANICS

ANODES

MICROBES

ee-

e-

MULTI-STACK Fig. 2. The DBMFC system setup for underwater energy harvesting. Schematic diagram of the DBMFC and power management system (only one anode for each array is being represented for convenience).

The anode material used was carbon fiber brush (Mill Rose Inc., OH; fiber stack diameter: 3.8 cm; and fiber stack height: 6.25 cm), which had large specific surface area for bacterial growth and electron transfer (Shantaram et al., 2005; Logan and Regan, 2006a). The cathode used was flat carbon cloth (30% wet proofing, Fuel Cell Earth, MA; geometric area: 58 cm2), with the side facing the anode in the water doped with platinum (10% by weight on carbon black) (0.5 mg/cm2) and the opposite side treated with a layer of carbon black (Cabot Vulcan XC-72) and 30% polytetrafluoroethylene (PTFE) to the support the platinum layer. Anaerobic sediments with diverse microbial consortium and organic substrates taken from the Mirror Lake at the University of Connecticut campus were used as inoculum. Surface waters collected from the same lake were used to support the benthic environment. To complement the ample availability of dissolved oxygen in natural waters, a small air pump (Aqua Culture, 5–15 Gallon, Dual Outlet) with stone diffusers was used near the cathode setup. Sodium acetate was added as a source of supplemental nutrients to enhance the acclamation of anaerobic electrogenic bacteria. The DBMFC system was operated at temperature of 20 °C. The operational period of the DBMFC system was over 1000 h (60 days).

four hours. The electrode potential under the open circuit condition, termed open circuit potential (OCP), which indicates the maximum voltage under the operated thermodynamic limits, was measured using a potentiostat (Gamry Reference 600). The target electrode (anode or cathode) acts as the working pole, Ag/AgCl served as reference electrode and platinum spiral as counter electrode. The voltages over a series of variable Rext (15–2500 O) were recorded using a multimeter (RadioShack, TX) to derive the polarization curves power curves. During the variable Rext recording, the voltage over each resistor was not taken until a steady state was reached (about 10 min). The power and current data were normalized to the cathode surface area for density estimates and comparison. The internal resistance (Rin) was determined at the maximum power density point on the power curves. The polarization curves record the current as a function of voltage (Heidemann et al., 2006; Lowy et al., 2006). All the measurements were performed in duplicate every 2 weeks. The power and current densities were calculated according to Eqs. (1) and (2).

Power density ¼

V2 Rext k

Current density ¼ 2.2. Water quality characterization The organic substrate concentrations (chemical oxygen demand, COD) in the DBMFC system were periodically measured using the HACH COD measurement kits and DR 220 spectroscopy (HACH, Loverland, CO, USA) according to the HACH analysis protocol. The dissolved oxygen (DO) concentration in water and sediment was measured using an optical DO meter (YSI ProODO, 0–50 mg/L). The conductivity of water was measured using a conductivity meter (Thermo Scientific Orion 3). 2.3. Electrochemical characterization The voltage over an external resistance (Rext) of 1000 O was recorded using a data log system (Keithley Instruments Inc.) every

V Rext k

ð1Þ

ð2Þ

where V is the voltage across the external resistor, Rext is the external resistance, and k is the cathode surface area. 2.4. Stability tests and analysis of DBMFC system Two types of tests were conducted for stability characterization of the DBMFC system under adverse conditions (e.g. bioturbation), and assessed for variations in power and current generation. The first test was assessing the reliability of multi-anode/cathode setup, for a given array consisting of three anodes and one cathode. Three different scenarios were considered: (i) 1A-when only one of the three anodes was assumed to be functional and the other two anodes experience reverse reactions caused by bioturbation, which results in the opening of the corresponding NMOS switches

480


(worst case scenario); (ii) 2A- when two anodes were functional; and (iii) 3A- when all the three anodes were functional (best case scenario). The second test involved examining all nine arrays (1– 9) of the distributed BMFC system. For each of the nine arrays, nine different scenarios were tested. On one end of the spectrum, only one array (1) of the multi-anode/cathode setup was tested, indicative of only one working array (worst case scenario), while the remaining eight arrays (8) were presumed to be affected by reverse reactions. On the other end, all nine arrays (9) were tested, suggesting that all nine arrays were functional (best case scenario). Comparison of the performance between different multi-anode/ cathode arrays and different number of anodes within each array would elucidate the stability of the DBMFC system. The tests were conducted in tandem for each of the arrays, at multiple times during the course of the operation. 2.5. Linear sweep voltammetry The performance of anode and cathode electrodes in the DBMFC system was examined weekly using linear sweep voltammetry (LSV) to assess the potential limiting factors. The LSV was performed using a potentiostat (Gamry Reference 600) in a three-electrode electrochemical cell setup. The current density at the working electrode (anode/cathode) was measured, while the potential between the working electrode and reference electrode (Ag/AgCl) were linearly swept in time, and the platinum spiral served as the counter pole. Anodic LSV was performed at a scan rate of 0.2–0.1 mV/s and cathodic LSV was performed at a scan rate of 0.4–0.5 mV/s. All LSV measurements were conducted in duplicates under similar conditions of pH, temperature, and solutions conductivities.

where CO2 is the concentration of the dissolved oxygen at the cathode surface, b is the charge transfer coefficient of the cathode, and c is the coefficient related to the exchange current. Consequently, the MFC over potentials are determined by a composite effect of electrode reactions, mass transfer of chemical species to or from the electrodes, the resistance to the flow of ions in the electrolyte, and the resistance to the flow of electrons through electrode materials. In summary, the output voltage (VMFC) can be quantified as a function of the structural and operational parameters as follows (Eq. (5)):

V MFC ¼ V open gA gC gohm ¼ V open

The key parameters of DBMFCs should be well understood in order to facilitate their design and application. Presently, MFC output voltage is usually lower than the theoretical value due to various internal and external losses (Logan et al., 2006b). There are three major sources for these losses: activation overpotential (gact), concentration overpotential (gcon), and ohmic overpotential (gohm); of which gact and gcon occur at both anode (gA,act, gA,con) and cathode (gC,act, gC,con). The anaerobic reactions at the anode in a MFC are controlled by the electrochemical potential at the anode. The anode overpotential can be derived by employing both Monod-type and Bulter–Volmer equations (Bailey and Ollis, 1986; Newman and ThomasAlyea, 2004) as below (Eq. (3)):

gA ¼ gA;act ¼ gA;con ¼

RT r A K AC þ C AC ln aF k01 X C AC

ð3Þ

where k10 is the rate constant of the anode reaction at standard conditions, KAC is the half velocity rate constant for acetate, a is the charge transfer coefficient of the anodic reaction, rA is the reaction rate occurring at the anode, F is the Faraday constant, R is the gas constant, T is the operation temperature, and CAC and X are the concentrations of acetate and biomass at the anode surface, respectively. The MFC cathodic reactions are governed by oxygen reduction reaction (ORR), controlled by the corresponding electrochemical potential at the cathode. The overpotential at the cathode can be derived by employing the exponential form of the Bulter–Volmer equation shown below (Eq. (4)):

gC ¼ gC;act þ gC;con ¼

RT Icell ln bF cC O2

ð4Þ

ð5Þ

where Rin represents the internal resistance of the MFC. The effects of multi-anode/cathode configuration on the performance of DBMFC were analyzed based on the above described theoretical model. From (Eq. (3)), most of the parameters are independent of the number of anodes employed. Hence, the anode overpotential remains the same regardless of the anode number as long as the surrounding environment for each anode is the same. For the cathode, its over potential will only be affected by the cell current as long as the surrounding environment of the cathode is identical. In addition, the difference between the sediment MFC and the two-chamber MFC (Scott et al., 2008) is that one of the two chambers is replaced by the sediment in the sediment MFC. This won’t affect the ohmic over potential in the design, which can be expressed similar to the two-chamber MFC as follows (Eq. (6)):

gohm ¼ ðqaq 2.6. Computational model development for DBMFC system

RT r A ðK AC þ C AC Þ RT IMFC ln ln Rin Icell C AC bF cC O2 aF k01 X

daq ds þ qs ÞIcell Aaq As

ð6Þ

where daq and ds are the distance of the electrodes in the water and sediment; qaq and qs are the resistivity of the water and sediment; and Aaq and As are the cross-section area of the water and sediment for the ion exchange, respectively. For multiple anodes (1A, 2A and 3A), the effective ohmic resistance is reduced due to the increase in the conductive area brought by the multi-anode design. As a result, relatively lower ohmic resistance is expected, and this will improve the performance of multi-anode/cathode configuration when more anodes work in parallel. In the DBMFC system, because all the anodes and cathodes had the same structures, the charge transfer coefficients a at the anode and b at the cathode had the same values as those of a single anode/cathode array. The ohmic overpotential gohm for different anode/cathode array is also the same, since the anode–cathode arrays in the DBMFC system had the same structure and operated independently. On the other hand, the exchange current coefficient c between the anodes and cathodes was expected to increase proportionally with the number of anode/cathode arrays n, considering the parallel anode connection. The structure and set up of multi-anode/cathode arrays of the DBMFC system would result in a constant output voltage. To characterize the electrochemical behaviors of the multi-electrode configuration, the overall cell current (Icell (n)) can be expressed as a function of the number of anode/cathode arrays n as below (Eq. (7)):

Icell ðnÞ ¼ rI ðn; Icell Þ n Icell

ð7Þ

where the value of parameter 0 < rI (n, I) < 1 represents the nonideal current loss in the multi-electrode configuration. Thus, the total harvested power (Ptotal) is derived as below (Eq. (8)):

Ptotal ¼ V cell Itotal ¼ rI ðn; Icell Þ n Pcell

ð8Þ

481


where Pcell is the power output of one anode/cathode pair. It should be noted that each anode/cathode array consisted of 3 anodes and 1 cathode in the DBMFC system developed in this study. Note that relatively simple but sufficient models were used in this paper to study the attributes of BMFCs in terms of voltage, current and power production under multiple arrays of anodes/cathodes. Other related processes in BMFCs, e.g., impact of oxygen concentration variations, transport of microorganism in water bodies, etc., should be studied using different models established from biological and/or biochemical areas. 3. Results and discussion 3.1. Voltage output of the DBMFC in long term operation The average voltage for all the electrode arrays recorded over (Rext) was 0.1–0.15 V in the first 50 h, steadily increased to 0.35 V after 200 h, and then stabilized at 0.3 V after 600 h (data not shown). There are three main reasons for the variation in output voltage. First, these were the average values of multiple arrays (9 in total) of electrode, which oscillated based on the individual array performance. Second, the changes in the environment, such as ambient temperature, water temperature, sediment nutrient properties, and diurnal patters, could affect the MFC performance. Third, the water was replaced every two to three days to simulate the real underwater conditions, which could have resulted in the variation observed. Previous lab-scale BMFC systems showed fluctuations, similar to the DBMFC system (Shantaram et al., 2005; Scott et al., 2008). However, the field studies (Wotawa-Bergen et al., 2010; Gong et al., 2011) reported relatively high voltages (0.40–0.45 V) in the initial stage, and then dropped over time. The possible reasons may well be the dynamic natural environmental conditions. In the initial stage of deployment, the BMFCs could obtain high voltage output. But over a long operational period, the nutrient depletion, mass transfer resistance and anode/ cathode fouling, could lower the system performance (Scott et al., 2008; Scott et al., 2008a). 3.2. Analysis of DBMFC system performance The anode and cathode arrays in the DBMFC system exhibited different bio-electrochemical characteristics (Table 1). The organic substrate concentration (COD) was close to 250 mg/L in the anode sediments, but was