FU Yubin, LU Zhikai, ZAI Xuerong, and WANG Jian

1)Institute of Materials Science and Engineering,Ocean University of China,Qingdao266100,P.R. China

2)China Lucky Group Corporation,Baoding071000,P.R. China

3)College of Chemistry and Chemical Engineering,Ocean University of China,Qingdao266100,P.R. China

Carbon Foam Anode Modified by Urea and Its Higher Electrochemical Performance in Marine Benthic Microbial Fuel Cell

FU Yubin1),*, LU Zhikai1),2), ZAI Xuerong3), and WANG Jian1)

1)Institute of Materials Science and Engineering,Ocean University of China,Qingdao266100,P.R. China

2)China Lucky Group Corporation,Baoding071000,P.R. China

3)College of Chemistry and Chemical Engineering,Ocean University of China,Qingdao266100,P.R. China

Electrode materials have an important effect on the property of microbial fuel cell (MFC). Carbon foam is utilized as an anode and further modified by urea to improve its performance in marine benthic microbial fuel cell (BMFC) with higher voltage and output power. The electrochemical properties of plain carbon foam (PC) and urea-modified carbon foam (UC) are measured respectively. Results show that the UC obtains better wettability after its modification and higher anti-polarization ability than the PC. A novel phenomenon has been found that the electrical potential of the modified UC anode is nearly 100 mV lower than that of the PC, reaching -570 ±10 mV (vs. SCE), and that it also has a much higher electron transfer kinetic activity, reaching 9399.4 mW m-2, which is 566.2-fold higher than that from plain graphite anode (PG). The fuel cell containing the UC anode has the maximum power density (256.0 mW m-2) among the three different BMFCs. Urea would enhance the bacteria biofilm formation with a more diverse microbial community and maintain more electrons, leading to a lower anodic redox potential and higher power output. The paper primarily analyzes why the electrical potential of the modified anode becomes much lower than that of others after urea modification. These results can be utilized to construct a novel BMFC with higher output power and to design the conditioner of voltage booster with a higher conversion ratio. Finally, the carbon foam with a bigger pore size would be a potential anodic material in conventional MFC.

marine benthic microbial fuel cell; carbon foam anode; urea modification; low anode potential; high kinetic activity; high output voltage

1 Introduction

Microbial fuel cell (MFC) is a device that uses bacteria to catalyze the conversion of organic matter into electricity (Ashley and Kelly, 2010). Substrate is oxidized by bacteria generating electrons and protons at the anode. Electrons are transferred through an external circuit while the protons diffuse through the solution to the cathode, where electrons combine with protons and oxygen to form water (Rabaey and Verstraete, 2005). Marine benthic microbial fuel cell (BMFC) is a field-deployable and uniquely configured microbial fuel cell that relies on the natural redox processes in aqueous sediments (Reimerset al., 2006). Its anode is imbedded in the sediment and cathode is planted in overlying water. Meanwhile, the anode is connected by an external circuit to the cathode. The cells are under development as long-term power sources for autonomous sensors and acoustic communication devices deployed in shallow or deep sea environments (Reimers, 2001; Alberte, 2005). However, it pro-duces relatively low levels of power density, prohibiting its widespread application. Researchers have used various methods, such as selection of high efficient anode material or its modification to increase power density (Wei, 2011; Zhou, 2011).

A variety of anodic materials have been used in the MFCs, including plain graphite, carbon paper, carbon cloth and carbon felt. It appears to be a general trend that the higher surface area anodic material, the higher power output (Chaudhuri and Lovley, 2003). Carbon foam has an exceptionally high void volume (97%), a large pore size (≥ 2 mm), and low diffusion resistance. Thus, it is an ideal anode material in the BMFC. On the other hand, the anode modification is one of the effective ways to improve the output power. For example, ammonia gas has been used to modify carbon mesh and make the cell power density reach 1015 mW m-2due to the introduction of N-containing groups; carbon mesh has a high N/C ratio to improve electron transfer (Wanget al., 2009). Urea molecule has a higher content of the element N. Further increasing the amount of N-containing group can yield higher power output. Therefore, urea-modified anode is fabricated in the present paper.

A novel phenomenon is found that the modified anode potential is 570 ± 10 mV (vs. SCE), 100 mV lower than the plain (-440 ± 10 mV,vs. SCE) and the output voltage of the modified BMFC is 870 ± 15 mV. This result has a significant meaning for the device design of a voltage booster. At present, the main limitations of the BMFCs to drive small monitor or equipment to work for a long-term service on ocean floor are their low power and output voltage (Zhang and Angelidaki, 2012). So we need to design a voltage booster device to improve output voltage and satisfy the voltage requirement for driving marine electronic instruments. In the paper, higher cell output voltage will be beneficial to design the booster conditioner with a higher power conversion ratio.

2 Experimental Section

2.1 Materials

Carbon foam (pore diameter ＞ 2 mm) was purchased from Qingdao Gaotai New Material Co., Ltd, China (shown in Fig.1a). Urea (purity ＞ 99.0) from Tianjin Guangcheng Chemical Co., Ltd. Graphite powder (500 mesh) from Qingdao Tianhe Graphite Co., Ltd. Graphite block and plate from Qingdao Duratight Carbon Co., Ltd. The materials were all directly used without further treatment. Natural sediment and seawater were then collected from the Jiaozhou Bay in Qingdao, China (at 36°10.3′N, 120°18.1′E).

2.2 Urea-Modified Anode Preparation

Three types of anodes were employed in the BMFCs. All of the anodes only had one work surface (2 cm × 2 cm) and the other five surfaces were coated by epoxy resin. These three anodes were called plain graphite block (PG), plain carbon foam (PC) and urea-modified carbon foam (UC) respectively. The UC was prepared according to the following procedure： 30 g of urea and 15 g of graphite powder were mixed and heated for 5 min -10 min in the temperature range 120 -130℃, then PC was soaked in the melted urea mixture for 1 min and dried at the room temperature for 12 h. The modified anodes characterized by X-ray diffraction (XRD) and water contact angle, were accurately weighed by electronic analytical balance to calculate the mass fraction of urea.

Carbon foam is a porous material and it is necessary to design a joint point to protect the lead wire from the corrosion of seawater. We used a glue to adhere carbon foam to a graphite plate and then sintered them under a high temperature (more than 2000℃) in the N2atmosphere to form a unity material. After that, copper lead wire was put into a small hole in the graphite plate by using conductive glue to protect the lead wire. Finally, a carbon foam anode was formed (Fig.1a).

2.3 BMFC Design and Configuration

Three BMFCs were constructed with the designed anodes (UC, PC, PG) and cathode. Cathode was then made of a big graphite plate (length 25 cm, width 20 cm, thickness 1.2 cm) to ensure that it has a great larger area than the anode. The PG, PC and UC as working electrodes were respectively imbedded in sediment; meanwhile the graphite plate and a reference electrode (saturated calomel electrode SCE) were positioned in the overlying seawater. The depth of the marine sediment was 20 cm, seawater 15 cm and the distance between cathode and anode was 15 cm (Fig.1b). The cells constructed by the UC, PC and PG anode were called BMFC-UC, BMFCPC and BMFC-PG respectively. Three parallel experiments were conducted in an identical configuration to obtain average results.

Fig.1 Carbon foam anode (a-1), carbon plate (a-2) and the BMFC configuration (b).

2.4 Analytical Measurements

XRD patterns were obtained from Macscience-M18 XHF Instrument. Water contact angle was measured by a contact angle meter (KRUSS DSA100 Germany). Cathode potentials were directly measured by the SCE and the anode potentials were calculated by subtracting the cell voltage from cathode potentials (Song and Jiang, 2011).Voltage and current produced from the BMFCs during the experiments were recorded at 15 min intervals using digital multimeters (Shenzhen Huayi Instrument Co., Ltd). Power (P) was calculated according toP=IU, whereUis voltage andIcurrent. Current and power were normalized to the geometric surface area of the anode when calculating current and power densities (Ryckelyncket al., 2005). The maximum power density of the BMFC was established from polarization curve using a variable resistor box (from 10000 Ω to 0 Ω, ZX-21, Shanghai Precision & Scientific Instrument Co., LTD). Tafel plots (log [exchange current densityi0] versus overpotentialη) were recorded for each anode by sweeping voltage at 10 mV s-1fromη= 0 -200 mV, whereη= 0 is the OCP of the anodevs. SCE (Lowyet al., 2006). The exchange current density (i0) of each anode was determined from Tafel plots by extrapolation toη= 0 of a linear regression betweenη= 75 -150 mV. It must be chosen at a low overpotential range so that thei0values will depend on the charge-limited electrochemical process and the effect of mass transfer on anodic potential can be negligible. The potential sweep curves (sweeping voltage at 1 mV s-1from the OCP to 200 mV) of different anodes were obtained from a Model LK2005A Electrochemical Workstation (Tianjin Lanlike Co., Ltd). All of the measurements of electrochemical behavior were conducted in a three-electrode configuration： the anode imbedded in the sediment as a working electrode, the cathode as a counter and the SCE as a reference electrode.

3 Results and Discussion

3.1 Modified Anode and Characterization

XRD patterns show that several sharp peaks exist after urea modification (see the attached information), indicating that urea is still in the crystal state. Weight difference calculation before and after modification shows that urea has the mass percentage of 43.6% in the anode.

Fig.2 shows contact angle images and their sizes in three parallel experiments are shown in Table 1. The value of the modified anode reduces by 49.1° versus the plain anode. The decrease in the contact angle is caused by the polarity functional groups in urea. Studies have shown that the contact angle stands for the wettability and it is closely related to the electrochemical reaction (Suskiet al., 1999). The lower the contact angle is, the better the wettability is. Good wettability can improve the adhesion of microorganism and accelerate electron production because many biochemical reactions undergo on the hydrophilic surface.

Fig.2 Images of the contact angle, PC (a) and UC (b).

Table 1 Contact angles of the plain and modified anodes

3.2 Electrochemical Behavior of Different Anodes

Anode polarization curves are shown in Fig.3a (each data point is an average value from three times of measurement, error ± 10 mV). There is a sharper drop slope of the PG than the PC and UC do. It indicates that the PC and UC have better anti-polarization ability. Bigger porosity and larger surface area provide more space for microorganism to reside in, which in turn produces more electrons for transfer (Scottet al., 2007). After modification, urea may serve as a new fuel substrate to satisfy the demand of attached bacteria, or form hydrogen bonding with peptide bonds in bacterial cytochromes, which can remove the barrier for electron transfer (Zhenget al., 2007). Therefore, the UC has the best anti-polarization ability among them.

Fig.3 Anode polarization curves (a) and potential sweep curves of different anodes (b).

Fig.3b shows the potential sweep curves of the different anodes. It is a useful method to evaluate the anodic electrochemical property. The UC has the highest value of the anodic current density and the PG the least value. The UC curve has a current peak 1100 mA m-2at -50 mV and then it decreases and the current finally keeps steady after50 mV. It could reasonably be speculated that the urea on the anode surface may be serve as a substrate to donate more electrons than other anodes do at -50 mV, but then the mass diffusion resistance on the anode surface limits the current to increase further. Finally it reaches a balance and maintains a higher current value than the PG and PC.

Anode kinetic activity stands for the electron transfer ratio on its surface and means the anodic exchange current density. It is a key factor to increase the current and power density of the BMFC. Tafel plots (Fig.4) show that all curves become linear after an initial steep current increasing. According to the Tafel equation (η=a+blog |i|), thei0values (exchange current density) obtained from these plots are 16.6 mA m-2(PG), 125.9 mA m-2(PC) and 9399.4 mA m-2(UC) respectively. Thei0value of UC is almost three orders of magnitude higher than the PG, indicating that the activation energy of the biochemical reactions on the UC reduces greatly.

Fig.4 Tafel plots of different anodes.

3.3 BMFC Performance with Different Anodes

The BMFCs with different anodes show the variations of output voltage in Fig.5a (each data point is an average value from three times of measurement). The voltage of BMFC-UC is 870 ± 15 mV, BMFC-PC 770 ± 15 mV and BMFC-PG 740 ± 15 mV. Though the UC curve has a mild slope, it is close to be linear, indicating that it has a steady internal resistance and its activation and mass-transfer losses do not increase after discharge. The steep decrease in voltage at low current density suggests that the BMFC-PG has a high activation loss (Martinset al., 2010). The maximum power densities of the three different BMFCs are 256.0 mA m-2for UC, 151.3 mA m-2PC, 19.6 mA m-2PG respectively. The UC power is 13-fold higher than that of PG, and nearly 1.7-fold higher than that of PC.

Compared with the literature (Wanget al., 2009), the UC power in the paper is much lower than the cell power (1015 mA m-2) obtained by Wanget al. (2009), where the anode was modified by ammonia gas. The large structure difference in microbial fuel cells is another important reason for the big difference in power output. The BMFC directly uses natural seawater and sediment (sea mud) to construct fuel cell, in which case it can not be stirred or air-pumped. However, in the conventional MFC, the single and high effective cultured bacterial species is generally utilized and cell structure can be artificially stirred or air-pumped to increase its power output.

Anode stability is a significant parameter for the BMFC application. The long-term discharge of the BMFC-UC in Fig.5b shows that the initial 20 d are developing period and it becomes steady in the following time. This experiment has operated for six months. The UC anode potential can still be sustained at -570 ± 10 mV and the voltage at 870 ± 15 mV. This result illustrates that the UC has a better stability in sediment and maintains a higher output voltage than the PC and PG do.

Fig.5 Cell polarization plots and power density curves of three BMFCs (a), UC long-term steady performance (b).

The result also indicates that the modified anodes with different urea contents have the identical electrochemical behavior, but higher contents of urea can make the cell have a long-lasting discharging time with higher voltage.

3.4 Electro-Catalytic Activity Analysis of the Modified Anode

The results of the three types of anodes are summarized in Table 2. The anode electrical potentials are -440 ± 10 mV (PG), -470 ± 10 mV (PC) and -570 ± 10 mV (UC) respectively. The UC potential is lower by nearly 100 mV and its cell voltage reaches 870 ± 15 mV. Its kinetic activity is 566.2-fold higher than that of PG and 74.5-fold higher than that of PC. The anode potential is one of the determining elements for the collection of energy from the microorganisms and it can regulate both the activityand growth of bacteria to sustain enhanced current and power generation (Zhouet al., 2011). In a word, the UC electrical potential is reduced and its cell voltage increases after modification. This is a novel phenomenon reported for the first time.

BMFC as a power source for driving marine instrument needs to design voltage booster device from 0.5 V to 5 V, 6 V, 12 V,etc. However, lower cell voltage leads to lower power conversion ratio of the booster device. Thus, higher cell output voltage will improve the power conversion ratio of the booster device. This study provides a new way for constructing higher voltage BMFCs by designing novel voltage booster conditioner with higher ratio.

Table 2 Electro-catalytic activity of three different anodes

A possible mechanism of the BMFC with urea- modified anode is presented in Fig.6. Songet al. (2012) reported that the output voltage increases by 30 mV due to increase of Fe (III) in sediment. Zhanget al. (2011) demonstrated that different substrates lead to different microbial community enrichment. Thus, it could be reasonably speculated that besides the traditional microbial oxidation reaction, UC selects a new microbial community with lower redox potential that can catalyze oxidation of urea and generate electrons (new bacterial species on modified anode surface as seen in Fig.6). This process may be beneficial for nitrogen cycle in the environment; equations I and II particularly show the nitrogen cycle process (AQDS is a widely existing product of humus respiration in anaerobic sediment (Wuet al., 2009)). However, it needs further detailed research on the attached bacteria to explain why the electrical potential of the modified anode decreases.

In the reaction (I), the urea acts as a reduction agent, which may cause the anodic electrical potential to decrease.

Fig.6 Schematic representation of the mechanism of the BMFC with modified anode. Darker color in the modified anode stands for a lower potential and the rod and sphere on its surface means much more different bacteria than on the plain electrode due to the urea.

4 Conclusions

Carbon foam modified by urea has been used as a novel anode for increasing the N/C ratio to improve anodic performance in marine benthic microbial fuel cell. A novel characteristic of the modified anode is that it has a lower potential (-570 ± 10 mV) than that of the plain graphite and the fuel cell obtains a higher output voltage (870 ± 15 mV). The power densities of the three different fuel cells are 256.0 mW m-2(modified carbon foam), 151.3 mW m-2(carbon foam) and 19.60 mW m-2(plain graphite) respectively. The higher exchange current density and higher kinetic activity of the urea-modified carbon foam have also been achieved. Urea as a microbial nitrogen source can promote the bacteria biofilm formation with a diversity of communities and produce more electrons leading to a higher power. Finally, the novelurea-modified anode offers good prospects for application in the marine BMFCs or other MFCs.

Acknowledgements

This work is supported by the Key Project of Natural Science Fund of Shandong Province, China (ZR2011 BZ008), and the Special Fund of Marine Renewable Energy from State Ocean Bureau, China (GHME2011GD 04).

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XRD patterns of urea before and after melting. Urea peaks have a good consistence with its standard card.

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(Edited by Ji Dechun)

(Received February 12, 2014; revised February 6, 2015; accepted March 2, 2015)

? Ocean University of China, Science Press and Springer-Verlag Berlin Heidelberg 2015

* Corresponding author. E-mail： ffyybb@ouc.edu.cn