Sediment microbial fuel cells as a new source of renewable and sustainable energy: present status and future prospects

Abstract

Active biocatalysts such as microorganisms or enzymes liberate electrons while electron donors are consumed in biological fuel cells. Biological fuel cells are a novel technology producing bio-electrochemical power using various materials such as complex organic waste or natural organic matter under anaerobic anode conditions. Recently, great attention has been paid to biological fuel cells due to their mild operating conditions and use of a variety of biodegradable substrates as fuel. Sediment microbial fuel cells (SMFCs) are a kind of microbial fuel cell (MFC) that can produce electrical current from the organic matter content of sediment using bacterial metabolism. SMFCs have been developed in the past decade to provide a renewable power source and organic matter removal. SMFCs differ from other MFCs due to their essentially complete anoxic conditions at the anode and their membrane-less structure. To further improve SMFC technology, this paper focuses on the limitations and challenges of SMFCs and collects the latest surveys in this field.

---

Atieh Zabihallahpoor

Atieh Zabihallahpoor is a Ph.D student at the Biofuel & Renewable Energy Research Center in the Faculty of Chemical Engineering in Babol Noshirvani University of Technology under the supervision of Dr Mostafa Rahimnejad. She received her masters degree in biotechnology for her work on biological ferrous iron oxidation for gas desulfurization with Dr Parisa Hejazi at Tehran University of Science and Technology in 2012. Her research interest is in the field of electrochemical sciences.

Mostafa Rahimnejad

Mostafa Rahimnejad obtained his Ph.D in biochemical engineering from the University of Mazandran, Iran in 2011. He is now a member of the scientific mission at Babol Noshirvani University of Technology, Babol, Iran. He is head of Biofuel & Renewable Energy Research Center at Faculty of Chemical Engineering in Babol Noshirvani University of Technology. His interests are in the field of experimental biochemical engineering and fermentation technology especially biological fuel cells. He is the author of numerous scientific publications on chemical and biochemical engineering.

Farid Talebnia

Farid Talebnia received his Ph.D in Biochemical Engineering from the Chalmers University of Technology, Sweden, in 2008. From 2008 to 2010, he was a postdoctoral fellow at the Technical University of Denmark (DTU), working on an EU-funded project related to biorefinery for conversion of whole oilseed biomass. In 2010, he joined the Biotechnology group at Babol Noshirvani University of Technology, Iran, and since then has worked as a senior researcher/lecturer at the Biotechnology and Biofuels Research Center.

---

1. Introduction

Energy is needed to preserve life. Different kinds of energy are formed and used in different countries. This energy is dissipated into the atmosphere and disrupts normal atmospheric circulation patterns, which cause changes in the Earth’s atmosphere, temperature, greenhouse effect etc.¹ Fossil fuel energy is the basic engine for growth in many economies. Energy consumption is related to economic growth.² So in recent years, energy consumption has grown exponentially in developing economies.³ As can be seen in Fig. 1, most of the energy consumed in the world in 2012 was non-renewable energy. For example according to the U.S. Energy Information Administration, only 8% of energy consumption in 2010 in the USA came from renewable energy sources versus 9% from nuclear energy and others from non-renewable energy sources. Although energy sources have shifted from fossil fuels to renewable energy sources, oil and gas are still the major primary energy sources powering the world’s industries. So the demand for new renewable energy sources still remains.

Fig. 1 World consumption of primary energy, 2012.

Energy needs are increasing around the world and traditional sources of energy such as fossil fuels have several disadvantages. Alternative sources of energy are required. Most of the energy sources used around the world are non-renewable energy sources. This kind of energy sources is running out and their utilization causes problems like emission of greenhouse pollutants, including SO_x, CO_x, NO_x, C_xH_y, soot, ash, droplets of tar, and other organic compounds, which are released into the atmosphere as a result of their combustion. Also, fossil fuels are inefficient sources for preparing energy requirements due to pollution and finite supplies.^3–5 So, the researchers round the world are working to find new energy platforms to provide sufficient energy without CO₂ emission and greenhouse gas problems.^5,6 Many countries all over the world have made an effort to solve the energy crisis by turning their attention to renewable energy sources such as solar energy or energy derived from wind or water. One renewable alternative energy source is the fuel cell, which has attracted lots of attention for generating energy. Fuel cells have several advantages such as no emission of environment polluting gases (such as SO_x, NO_x, CO₂, CO), higher efficiency, no mobile parts and, as a result, and a lack of sonic pollution, etc.^7,8 In contrast, high costs as well as mass generation are the only disadvantages of these new energy sources.^7,9 There are several kinds of fuel cells. One of the most interesting fuel cells is the microbial fuel cell (MFC). MFCs were discovered by Potter. In 1911, Potter observed electrical current generated by bacteria. Until the early 1990s, few studies were made in this field.¹⁰ Over the last 20 years and especially in this last decade, we have observed a rapid development in MFC technologies.^11,12

MFC technologies represent a novel energy harvesting technology and energy transducer and comprise an anaerobic anode, an aerobic cathode and typically a cation exchange membrane. The two electrodes are connected via a conductive wire.^11,13–15 Fig. 2 shows the essential concept of an MFC. Microorganisms or active biocatalysts break down organic matter in their surrounding environment.^6,16,17 Some MFCs need artificial electron mediators for transfer of electrons produced by biocatalysts from the substrate to the anode electrode.^17–20 H⁺ or other cations pass through the anodic chamber to the cathode chamber by a proton exchange membrane. The resultant electrons from the organic matter degradation are transferred through the cathode via an external circuit and reduce oxygen according to reaction (1):^11,13,16,21

O₂ + 4e⁻ + 4H⁺ → 2H₂O (1)

Fig. 2 Schematic diagram of a microbial fuel cell.

MFCs are able to harvest electricity from different substrates.^6,19,22,23 In addition to liquid phase substrates, solid phase substrates such as sediments, sludge and contaminated soil can be fuel for solid state microbial fuel cells.²⁴

Sediments in aquatic environments are a potentially long-term source of water contamination. Soil and sediments are derived from plant and animal detritus, settlement of dead bacteria and plankton, fecal matter and anthropogenic organic materials.²⁵ Sediments’ organic carbon content generally ranges from 0.4 to 2.2 wt%.²⁶ Thus, sediments’ organic carbon content may be seen as a sufficient energy resource in some locations. Many high cost and energy consumption physicochemical methods have been practised for sediment remediation.²⁷ But this organic carbon content can be consumed by exoelectrogens directly transporting electrons outside of the cell. Sediment bioremediation by sediment microbial fuel cells (SMFCs) was recently developed due to its cost-effectiveness and environmental benignity.²⁷ An SMFC is a simple configuration of an MFC, which generates electricity from the aquatic environment.

SMFCs are a bioelectricity production technique for low-power applications. SMFCs enhance organic matter removal (oxidation) from submerged soil at the anode along with energy production. In fact, SMFCs are membrane-free MFCs and their unique property of removing organic compounds from sediment has attracted significant attention recently.

SMFCs consist of an anode electrode embedded in an anaerobic sediment connected through an electrical circuit to a cathode electrode suspended in overlying water^28,29 (Fig. 3). Unlike conventional MFCs, SMFCs do not require protons to be transferred via a dissolved oxygen gradient along the water depth or a membrane.^30–32 SMFCs differ from other MFCs in that the anode is essentially under completely anoxic conditions.¹⁰ Inspired by the experiments of Reimers and Tender, the first functional SMFCs were created about 10 years ago.^25,33,34

Fig. 3 Schematic set up of a sediment microbial fuel cell.

Marine environments, rivers, fresh water, rice paddies and other aquatic sediments rich in organic matter have been used in various surveys.^35,36 Marine microbial fuel cells (MMFCs) and benthic microbial fuel cell (BMFCs) are also known as SMFCs.

SMFCs are a special kind of MFC that generate bioelectricity by using an active microorganism from the sediment. Although the electrical current produced by bacteria was observed by Potter in 1911,³⁷ limited feasible results were obtained in this subject in the next 50 years.³⁸ At the beginning of the year 1990, the attraction of fuel cells became stronger and the field of MFCs and SMFCs began to improve.³⁹ The steep slope of MFC progress began in 1999 when it was discovered that a mediator was not a required component.^40,41 But interest in SMFCs has increased in recent years. Until now, limited research has been done on SMFC scale-up. In general, the most significant investigations on sediment microbial fuel cells have been conducted by Tian-Shun Song,^{24,29,42–44} Chun-Chong Fu^45–48 and Seok Won Hong.^25,49–51 Recently, there has been a rise in the number of published articles on this subject with the United States of America being the major source of such publications. The number of articles in the field of SMFCs from 2006 to 2014 is presented in Fig. 4 having used the key words “sediment microbial fuel cell” in a Scopus search.

Fig. 4 Published articles on SMFCs in recent years.

Fig. 5 Published articles on SMFCs throughout the world.

Fig. 5 presents the different countries that are working on SMFCs. From Fig. 5, you can understand that all areas in the world have an interest in working in this renewable research area.

2. Mechanism

Microorganisms and substrates play important roles in SMFC performance. Fig. 6, illustrates the role of microorganisms in SMFCs with a number of main reactions which are determined to occur at the anode and cathode. The anode biofilm is enriched by two types of sedimentary microorganisms: the Geobacteracea family (most similar to Desulfuromonas acetoxidans) and the Desulfobulbus or Desulfucapsa genera. Geobacteracea oxidizes acetate in the sediment directly reducing the anode, while the Desulfobulbus or Desulfucapsa genera oxidize anode generated S⁰ to SO₄²⁻.^52–54 Acetate is provided by organic matter fermentation by other anaerobic microorganisms in sediment (e.g., Clostridium). Another reaction occurring at the anode is the oxidation of S²⁻ to S⁰. When organic matter is oxidized, O₂, MnO₂, Fe₂O₃ and SO₄²⁻ reduce in order between the sediment surface layer and anode. With increasing sediment depth, each layer accumulates more and more potent reductants.^52–55 So, as illustrated in Fig. 6, electrons produced by an active biocatalyst can be delivered to the anode from (1) microbes enriched on the anode surface, or (2) from the dissolved and solid-phase forms of reduced ions contained in the sediment (e.g., sulfides in marine sediment).⁵⁰ Some MFCs need electron mediators to transfer the produced electrons to the anode surface^17,18,20,56 and also a proton exchange membrane to transfer generated protons to the cathode chamber.^57,58 But SMFCs do not need proton exchange membranes nor electron mediators.

Fig. 6 Microorganism roles and main reactions in SMFCs.^59,60

3. Advantages

SMFCs have a number of functional advantages compared to other energy sources. These include direct conversion of organic matter into current at a high efficiency, working under a wide range of environmental conditions including low operating temperatures,^34,35 low cost,⁶¹ less frequent maintenance requirements (e.g. periodic replacement),⁵³ simple construction, wide varieties of and cheap fuel sources,³⁴ and they can be easily placed in remote locations⁶² and have no generation of toxic components.⁶³ On the other hand, SMFCs have some limitations such as nonlinear scaling up, low operating voltages, low cell potentials^53,62,64 and a failure to provide continuous power.⁶⁵

4. Applications

Two broad applications are discussed for sediment microbial fuel cells within the literature: (1) providing renewable power sources for instruments deployed in marine environments, rivers, lakes, freshwater, oceans etc. for long-term monitoring; and (2) removal of organic matter from sediments. In the following, each of these applications is explained separately.

4.1. Renewable power sources

One of the main applications of SMFCs is to provide a power source for wireless equipment used for environmental monitoring, oceanographic studies, and military tactical surveillance where real-time data acquisition from remote locations is required^35,65 (Table 1). These instruments have no cable connection with the surface, such that they need a kind of power supply such as a battery. However, batteries are associated with limited calendar lifetimes due to their requiring high cost periodic replacement, especially in deep water.³⁴ These challenges can be overcome by applying SMFCs as a power source. SMFCs can empower various wireless sensors including those identifying temperature, salinity, tidal patterns, the presence of algae and other life forms, migration patterns of fish and other marine wildlife, organic contamination from oil production, metallic compounds from other industrial processes,⁵⁵ pH, humidity, aquatic life, invasive species,³⁰ and also biological oxygen demand (BOD) biosensors, and dissolved oxygen (DO) sensors.⁶⁶

Table 1 SMFCs used as power supplies in different practical applications

Instrument	Power requirement (mW)	Reference
Meteorological buoy	18	53
Acoustic modem	3	34
Tele-communication system	300	62
Temperature sensor	2.2	30
Remote sensor	2500	67
Submersible ultrasonic receiver	15	64
Turbidity meter	42	68
Acoustic receiver	28	68
Wireless temperature probe	49.5	68

---

4.2. Organic matter removal

Organic-rich sediments, as an important component of aquatic environments, can be considered as an abundant potential source of renewable energy. But drainage of industrial wastewater and municipal sewage has infected the surface layer of sediments with pollutants such as organic matter, nitrogen, and phosphors,²⁹ resulting in water-quality issues and even methane emission. Furthermore, these compounds are toxic for organisms due to their carcinogenic and mutagenic potential.⁶⁹ One way to remove these compounds is to reduce them as a fuel in SMFCs. There is a linear relationship between the generated current and the removal efficiency of organic matter from sediments.^43,51 A comparison of the removal efficiency of organic matter and power density of SMFCs for different types of carbon sources is shown in Table 2.

Table 2 Removal efficiency of organic matter and power density of SMFCs for different types of carbon

Type of fuel	Removal efficiency (%)		Power density (mW m^−2)	References
Aquaculture water	Max COD (g m^−2 d^−1)	3.99	4.52	70
	Max TN (g m^−2 d^−1)	0.21
Fresh water sediments	Phenanthrene	99.47 ± 0.15	—	69
	Pyrene	94.79 ± 0.63
Waterlogged soil	Phenol	90.01	29.45	71
Tidal river sludge	Carbon removal	9.6 ± 1.1	7.5 ± 0.3	72
Fresh water sediment	Carbon removal	29 ± 1	—	25
Hydrocarbon contaminated sediments	Carbon removal	24 ± 4	6.3 ± 0.2	73
Aquaculture pond water	COD	84.4	0.241	74
Lake sediment	Nitrate	62	42	75
	Nitrite	77
Fresh water lake	COD	95.5	86.7	76
Lake sediment	COD	76.2	72	76
River sediment	Organic matter remove	29	1000	36
Fresh water sediment	COD	28.3 ± 1.9	3.15 ± 0.07	43

---

5. Evaluation of SMFC operation

The operation of electrical sources can be evaluated by their provided current, voltage, power density and current density. The current can be calculated using Ohm’s law (I = V/R_ext), where I represents the current in amperes, V represents the potential difference between two electrodes in volts, and R represents the external resistance measured in ohms.¹⁰ Voltage can be calculated using the open circulate voltage (OCV) and the internal resistance (R_int, thus V = OCV − IR_int). Internal resistance is associated with ohmic losses (R_o), activation losses (R_a), and mass transfer losses (R_mt); giving eqn (2):⁷⁷

R_int = R₀ + R_a + R_mt (2)

Power generation is our other major purpose for SMFCs. The power output is calculated via:

It is common to normalize power production by the surface area of the anode, A, or total reactor volume, v, so that the normalized power density produced is:¹⁰

or

6. Challenges in SMFCs

A significant portion of investigations performed on SMFCs has been concerned with improving their electricity generation capacity over a longer timeframe. As shown in Table 3, the investigations mostly focused on the anode, cathode, sediments, overlying water, equipment configuration and operational conditions, each of which is discussed in the following.

Table 3 Classification of all investigations of SMFCs

Overall classification	Partial classification
Anode	Material
	Geometry
	Surface modification
Cathode	Material
	Geometry
	Bio-cathode
Sediment	Sediment Organic Matter (SOM) availability
	Pre-treatment
	Conductivity
	pH
	Mass transfer
	Conductivity
	TDS
Overlying water	Dissolved Oxygen (DO)
	pH
	Flow pattern
	Temperature
	Nature
	Substrate
Equipment configuration	Electrode spacing
	Electrode surface area ratio
	Water depth
	Anode chamber
	Depth of embedded anode
	Electrode configuration
	External resistance
	Parallel SMFCs
Operational condition	Temperature

---

Moreover, SMFCs produce bioelectricity and remediate contaminated sediments simultaneously. In spite of low electricity generation from SMFCs, it has been demonstrated that this system can be used successfully to power low-power electronic devices in an aquatic ecosystem. Nevertheless, SMFC technology is facing many challenges to become a reliable renewable energy source and research in this field of fuel cells must be continued to improve SMFC performances.

6.1. Anode

Microorganisms metabolize available organic matter in sediments and release electrons (e⁻) through electron acceptors known as anodes.⁷⁸ Anode reduction is done by microorganisms colonized on the anode surface.⁵² The anode material, geometry and surface modifications are the key parameters in optimizing electricity harvesting from sediments. These parameters affect microbial adhesion to the anode surface, electron transfer and substrate oxidation.³⁶

The anode material requires high conductivity, environmental stability and good redox reversibility.⁴⁸ Current and power densities for different anode materials and geometries are shown in Table 4. Graphite, stainless steel and carbon are common materials used in SMFCs. The maximum power and current densities for the graphite tank are 100 mW m⁻² and 3500 mA m⁻², respectively.⁷⁴

Table 4 Current and power densities for different anode materials and geometries

Anode material	Anode geometry	Current density (mA m^−2)	Power density (mW m^−2)	References
Graphite	Plate	3	—	51
	Felt	10	—	51
	Felt		33.5 ± 1.5	42
	Felt	—	45	80
	Tank	3500	100	81
	Rode	23.72	19.57 ± 0.35	35
	Disk	5.39	8.72 ± 1.39	35
Stainless steel	Grid	8200	—	50
	—	100	10	81
	Plate	140	23	82
Activated carbon	Fiber felt		10.6	42
Carbon	Sponge	100	55	55
	Cloth	50	19–27.5	55
	Fiber	5.0	4.5	55
	Reticulated vitreous (RVC)	0.8	0.2	55

---

Surface modification alters the surface contact angle and wettability of the anode by introducing several hydrophilic groups onto the electrode surface,⁴²which results in higher and better adhesion of bacteria.^46,79 Several investigated anode modifications are shown in Table 5. As can be observed, the power density is significantly increased with Fe/ferric oxide in some cases such as electrolytic deposition of graphite.

Table 5 Different anode modifications proposed in the literature

Material	Modification	Power density (mW m^−2)	References
Graphite	Anthraquinone-1,6-disulfonic acid (AQDS)	98	52
	1,4-Naphthoquinone (NQ)	—	52
	Hydroxyl and carboxyl groups	358.1	83
	Electrolytic deposition with Fe/ferric oxide	740	46
	Sulfide oxidizing Sb(V)	—	59
	Oxidize	—	59
Ceramic–graphite composite	Mn^2+ and Ni^2+	105	52
Graphite paste	Fe3O4 and Ni^2+	—	52
	Fe3O4	—	52
	Sulfonated polyaniline powder with a PTFE solution	129.1	48
	Sulfonated polyaniline/vanadate composite powder with a PTFE solution	187.1	48
Glassy carbon	Anthraquinone-1,6-disulfonic acid (AQDS)	—	59
	Sulfide oxidizing Sb(V)	—	59
	Oxidize	—	59

---

6.2. Cathode

In SMFCs, electrons (e⁻) harvested from sediments flow from anode to cathode through an external circuit, reducing oxygen in the overlying water. Similar to the anode, the cathode material and geometry are important factors in SMFC operation. Electron transfer efficiency and oxygen reduction rate of the cathode material are important factors.⁸⁴ Current and power densities for different cathode materials and geometries are shown in Table 6 where maximum power and current density are obtained from polyaniline graphene nano-sheets.

Table 6 Current and power densities for different cathode materials and geometries

Cathode material	Cathode geometry	Current density (mA m^−2)	Power density (mW m^−2)	References
a Algae-assisted cathode.b Tetramethoxyphenyl porphyrin.c Polyaniline graphene nanosheets.
Graphite	Felt	—	23.6	42
	Disk		7	85
	Thick felt (porous)	45.4	2.00 ± 0.11	49
	Thin felt (porous)	37.6	2.00 ± 0.11	49
	Plate (non-porous)	13.9	1.25 ± 0.15	49
Stainless steel	Plate	140	23	82
	Round	—	1.0	44
Activated carbon	Granule	—	3.5	44
Carbon	Cloth	80	55	85
	Paper		0.2	85
	Reticulated vitreous (RVC)		12	85
	Sponge		38	85
	Nanotube^a		38	86
Co-TMPP^b	—		32	85
Fe–Co TMPP	—	160	62	85
Platinised carbon (Pt/C)	—		8	85
PANI–GNS^c	Nano sheet	479.8	99	84

---

Microorganisms can catalyze the reduction of oxygen at the cathode by growing a biofilm on that electrode. Biological oxygen reducing cathodes are called bio-cathodes. The advantages of this system are its low cost, self-replenishment, better sustainability and having no mediator involved.^11,36,87 Mixed culture microorganisms,⁸⁷ oxygenic phototrophs¹¹ and iron oxidizing bacteria⁸⁸ are several biofilms used as bio-cathodes.

6.3. Sediment

As mentioned before, the electrons in SMFCs are provided by bacterial degradation of organic matter, so SMFC power output can be improved by using a higher organic matter content.²⁶ In this regard, several researchers have increased sediment organic matter (SOM) by addition of substrates such as glucose, cellulose, chitin,^26,44,89 acetate,^35,90 biomass,²⁴ and milk.⁸¹

Plant microbial fuel cells (PMFCs) (first reported in 2008) can also provide organic matter.⁹¹ In PMFCs, plants grow within sediments, producing carbohydrates via the photosynthetic process (Fig. 7-B). Some percentage of the produced carbohydrates is absorbed by the root structure.⁹² Microorganisms in the rhizosphere break down these carbohydrates and produce electrons. A scheme of the plant-MFC is shown in Fig. 7-B. Growing plants in sediments can also solve the mass transfer limitations for electron donors to reach the anode in SMFCs.⁴⁴ Several plants used in PMFCs are presented in Table 7.

Fig. 7 (A) Sediment-type photo-MFC; (B) plant microbial fuel cell.⁹⁷

Table 7 Plants used in PMFCs

Plant used	Power density (mW m^−2)	Current density (mA m^−2)	References
Rice paddy field	6	—	93
Spartina anglica at roof top	88	—	91
Duckweed (Lemna minuta)	380 ± 19	1620 ± 100	94
Duckweed (Lemna valdiviana)	—	226 ± 11	92
Rice paddy rhizosphere	1.3	—	88
Glyceria maxima	12	—	95
Grass species Spartina anglica	679	2080	87
Spartina anglica	21	31	96
Arundinella anomala	10	39	96
Arundo donax	No stable electricity production	—	96

---

Microalgae and some bacteria such as Cyanobacteria⁹⁰ contribute to increased SOM via photosynthesis⁹⁷ (Fig. 7-A). This technology is called sediment-type photoMFCs, where electricity generation exhibits an inverse relationship with illumination, because of the negative effect of the oxygen accumulation via photosynthesis.^97,98 This problem can be overcome by using color filters or increasing the thickness of the sediment.⁹⁸

Sediment bed conductivity is the other important factor affecting the efficiency of power generation in SMFCs. Adding conductive materials to sediment may improve the conductivity of the sediment by serving as electron conduits between the bacterial cells and anodes.^32,99 Graphite flakes³² and colloidal iron oxyhydroxide⁹⁹ are such conductive materials that can be used for enhancing the performance of SMFCs.

The operating pH in the sediment bed⁷⁰ and different pretreatment methods for sediments²⁹ also affect the efficiency of power generation.

6.4. Overlying water

The nature, origin, flow conditions, characteristics, functional activities, total dissolved solids, pH and temperature of water bodies play crucial roles in power generation.^51,78 For example stagnant water bodies have shown higher power generation compared with running water bodies.⁷⁸ But dissolved oxygen (DO) is the most significant factor. Oxygen is utilized as an electron acceptor.⁶³ Therefore DO is another important factor for SMFCs.^50,86 Researchers’ findings indicate that different methods can be used for increasing DO in water. These methods are summarized in Table 8.

Table 8 Different methods proposed to increase DO in water

DO increasing method	References
Rotating cathode	31 and 100
Floating cathode	36 and 61
Air cathode	66, 73, 101 and 102
Cold seep sediments	54 and 103
Algae assisted cathode	86
Plant rhizosphere	88
Bio cathode	8 and 104

---

In rotating cathode technology, rotation of the cathode disks also increases the dissolved oxygen in the overlying water (Fig. 8-C). No external energy input is required when disk drives are rotated by natural water currents.³¹ In a floating SMFC (Fig. 8-B), the cathode is floated on the water surface, so that a part of its surface is exposed to air. This technology overlaps with the air–cathode method in which the cathode is placed at the air–water interface (Fig. 8-A). On the other hand, algae such as Chlorella vulgarisis fix CO₂ during photosynthesis, releasing oxygen as a byproduct⁸⁶ and saturating water with oxygen.

Fig. 8 Schematic of (A) an air-cathode; (B) a floating cathode; (C) a rotating cathode technology; and (D) a biocathode.

Part of the produced oxygen in plant-MFCs is released into the rhizosphere by the roots. Hence rhizospheres can act as an oxygen source provided that one cathode is placed in the sediment part of the SMFC.⁸⁸

DO is also a function of temperature such that it increases with decreasing temperature, leading to an increased power generation efficiency in the cold seep ocean.

6.5. Equipment configuration

Power generation is affected by several additional factors related to SMFC configuration. These factors include electrode spacing, water depth, depth of the embedded anode, the anode chamber, cathode arrangement etc.

When the electrode spacing is increased, ohmic losses increase, decreasing the amount of current generated from the SMFC.⁵⁰ To minimize these ohmic losses, a new type of floating SMFC with a constant inter-electrode spacing can be used.⁵¹ Increasing the water and embedded anode depths also contributes to an increase in the electrode spacing,^105,106 while increasing the anode depth enhances the internal resistances of SMFCs.¹⁰⁷ On the other hand, at different depths of sediment, certain substrates and microorganisms are active, enhancing anode performance at greater depths.¹⁰⁷ Therefore, anode embedded depth should be determined locally.

A major challenge in SMFCs is anode passivation.⁷⁷ Passivation is the inhibition of the dissolution reaction caused by the formation of non-dissolving films. Anode passivation results in lost production capacity, increased power costs, and decreased cathode quality. In 2007, Guzman et al. avoided this problem by using a chambered SMFC design in which the anode is placed in a semi-enclosed chamber that rests securely on the seafloor (Fig. 8).¹⁶ By using a one-way check valve, water solely outflows from the underlying sediment into the chamber and not in the opposite direction. This water is nutrient-rich and depleted of oxygen due to oxygen consumption by microbes present within the sediment.¹⁶

According to Ohm’s law, an increase in current generation should be observed as decreasing external resistance.⁷⁸ External resistance may also affect power density, such that the power density increases with external resistance. Therefore, the highest power density and produced current may not be achieved at the same external resistance; so, our ultimate goal is to determine an optimum value of external resistance.

The effects of electrode surface area ratio⁵⁰ and various electrode arrangements^30,44,78,108 have also been investigated in the literature.

7. Scale-up

Until now, apart from storing the energy and using it intermittently, there has been no MFC (including SMFCs) capable of producing Watt-level power.¹⁰⁹ Power management systems (PMSs) can be used to overcome low power generation issues by cyclic charging and discharging of a capacitor that converts a low potential into a high one.⁶⁴ A major component of a PMS is shown in Fig. 9. In the recent literature, different PMSs have been developed for SMFCs.^{30,62,64,65,67,109}

Fig. 9 The major necessary components of a power management system.

It is obvious that a scalable technology SMFC that enables Watt-level power generation will be more useful. Two approaches to scale SMFCs up are: (1) connecting multiple MFCs in series; and (2) increasing the surface area of the electrodes equivalent to connecting multiple MFCs in parallel.^110–112 The first alternative is impossible because all the electrodes are immersed in the same solution. On the other hand, it is impossible to scale an SMFC up by increasing the surface area of the electrodes due to the resultant sharp decline in current density.¹⁰⁹ Indeed, the surface area needs to be increased by almost 100-fold to merely double the power output; this is clearly problematic.¹¹⁰ In 2014, Ewing et al. made it possible to scale-up SMFCs by using smaller-sized individually operated SMFCs connected to a power management system. In this system, the electrodes were electrically isolated (Fig. 10).

Fig. 10 Scaled-up SMFC for practical applications.

8. Summary and future development

In this article, sediment type microbial fuel cells were overviewed including issues such as new materials for use in anodes and cathodes, sediment and overlying water properties, types of equipment configuration etc.

It seems that fossil fuels may not supply the increasing energy demand of the future, so it is essential to find sustainable and renewable sources of energy. The results of recent studies suggest that SMFCs will be of practical use in bioenergy production and waste removal from sediments in the future. SMFCs need to provide a higher power output at a lower cost to be considered practically and commercially affordable. In this respect, nano-materials are examples of current research lines. However, to move this technology from laboratory trials to field applications all the challenges discussed in this review need to be further considered before wide application of SMFCs can be realized. In order to further develop SMFC technology, it is suggested to evaluate energy collecting methods, develop PMS and scale-up technologies, and to use cost-effective materials, process monitoring and control, etc.

Until recently, most developed SMFCs were not designed for sediment remediation. For remediation applications, power output is not the major goal. For this issue, simulating sediment bioremediation, in situ bioremediation processes, and environmental, ecological and social considerations require further attention and using a moderate input of external energy (from renewable energy sources) to improve bioremediation must be studied in future. Also, most recent studies have been done on non-complex materials in sediment/soil. So, bioremediation of complex materials in sediments should be considered in the future.

Further, to date most studies have been carried out using laboratory-scale SMFCs. In the near future, investigators will face the new challenge of transitioning SMFCs from the laboratory to aquatic environments; for example, SMFC setup installation in aquatic sediments, passivation of electrode materials by electrochemical deposition, the corrosion of electrode materials and connections, SMFC setup destruction by current flow and fish grazing etc. On the other hand, electrode materials (such as nanomaterials which are used increasingly) and electrochemical reaction effects on the surrounding environment and ecological system must be considered. Challenges in SMFCs will be overcome by the cooperation of different disciplines.

References

1. G. Akhmat, K. Zaman, T. Shukui and F. Sajjad, Renewable Sustainable Energy Rev., 2014, 36, 123–134.
2. A. Tardast, M. Rahimnejad, G. Najafpour, A. Ghoreyshi, G. C. Premier, G. Bakeri and S.-E. Oh, Fuel, 2014, 117, 697–703.
3. M. Rahimnejad, A. A. Ghoreyshi, G. Najafpour and T. Jafary, Appl. Energy, 2011, 88, 3999–4004.
4. A. E. Franks and K. P. Nevin, Energies, 2010, 3, 899–919.
5. W. R. W. Daud, G. Najafpour and M. Rahimnejad, Iran. J. Energy Environ., 2011, 2, 262–273.
6. M. Rahimnejad, M. Ghasemi, G. D. Najafpour, M. Ismail, A. W. Mohammad, A. A. Ghoreyshi and S. H. A. Hassan, Electrochim. Acta, 2012, 85, 700–706.
7. S. Peighambardoust, S. Rowshanzamir and M. Amjadi, Int. J. Hydrogen Energy, 2010, 35, 9349–9384.
8. P. Izadi, M. Rahimnejad and A. Ghoreyshi, Biotechnol. Appl. Biochem., 2015, 62, 483–488.
9. M. Rahimnejad, A. Ghoreyshi, G. Najafpour, H. Younesi and M. Shakeri, Int. J. Hydrogen Energy, 2012, 37(7), 5992–6000.
10. B. E. Logan, Microbial fuel cell, Wiley, 2008.
11. X. A. Walter, J. Greenman and I. A. Ieropoulos, Algal Res., 2013, 2, 183–187.
12. S. Fatemi, A. Ghoreyshi, G. Najafpour and M. Rahimnejad, Iran. J. Energy Environ., 2012, 3, 2079–2115.
13. Y. Yang, G. Sun and M. Xu, J. Chem. Technol. Biotechnol., 2011, 86, 625–632.
14. A. Tardast, M. Rahimnejad, G. Najafpour, A. Ghoreyshi, G. C. Premier, G. Bakeri and S.-E. Oh, Fuel, 2014, 117, 697–703.
15. M. Ghasemi, W. R. W. Daud, M. Rahimnejad, M. Rezayi, A. Fatemi, Y. Jafari, M. Somalu and A. Manzour, Int. J. Hydrogen Energy, 2013, 38, 9533–9540.
16. J. J. Guzman, K. G. Cooke, M. O. Gay, S. E. Radachowsky, P. R. Girguis and M. A. Chiu, Proc. SPIE, 2010, 7666, 68–73.
17. N. Mokhtarian, W. Wan Ramli, M. Rahimnejad and G. Najafpour, World Appl. Sci. J., 2012, 18, 559–567.
18. M. Rahimnejad, G. Najafpour, A. Ghoreyshi, M. Shakeri and H. Zare, Int. J. Hydrogen Energy, 2011, 36, 13335–13341.
19. M. Rahimnejad, G. Bakeri, G. Najafpour, M. Ghasemi and S.-E. Oh, Biofuel Res. J., 2014, 1, 34–38.
20. M. Rahimnejad, G. D. Najafpour, A. A. Ghoreyshi, F. Talebnia, G. C. Premier, G. Bakeri, J. R. Kim and S.-E. Oh, J. Microbiol., 2012, 50, 575–580.
21. M. Rahimnejad, T. Jafary, F. Haghparast, G. D. Najafpour and A. A. Ghoreyshi, Turk. J. Eng. Environ. Sci., 2011, 34, 289–292.
22. M. Rahimnejad, A. Adhami, S. Darvari, A. Zirepour and S.-E. Oh, Alexandria Eng. J., 2015, 54, 745–756.
23. M. Rahimnejad, M. Ghasemi, G. Najafpour, A. Ghoreyshi, G. Bakeri, S. K. Hassani Nejad and F. Talebnia, Am. J. Biochem. Biotechnol., 2012, 8, 304–310.
24. T.-S. Song, D.-B. Wang, S. Han, X.-y. Wu and C. C. Zhou, Int. J. Hydrogen Energy, 2014, 39, 1056–1062.
25. S. W. Hong, H. S. Kim and T. H. Chung, Environ. Pollut., 2010, 158, 185–191.
26. F. Rezaei, T. L. Richard, R. A. Brennan and B. E. Logan, Environ. Sci. Technol., 2007, 41, 4053–4058.
27. W.-W. Li and H.-Q. Yu, Biotechnol. Adv., 2015, 33, 1–12.
28. S.-E. Oh, J. Y. Yoon, A. Gurung and D.-J. Kim, Bioresour. Technol., 2014, 165, 21–26.
29. T.-S. Song and H.-L. Jiang, Bioresour. Technol., 2011, 102, 10465–10470.
30. F. Zhang, L. Tian and Z. He, J. Power Sources, 2011, 196, 9568–9573.
31. Z. He, H. Shao and L. T. Angenent, Biosens. Bioelectron., 2007, 22, 3252–3255.
32. M. Lenin Babu and S. Venkata Mohan, Bioresour. Technol., 2012, 110, 206–213.
33. C. E. Reimers, L. M. Tender, S. Fertig and W. Wang, Environ. Sci. Technol., 2000, 35, 192–195.
34. Y. Gong, S. E. Radachowsky, M. Wolf, M. E. Nielsen, P. R. Girguis and C. E. Reimers, Environ. Sci. Technol., 2011, 45, 5047–5053.
35. N. J. Sacco, E. L. M. Figuerola, G. Pataccini, M. C. Bonetto, L. Erijman and E. Cortón, Bioresour. Technol., 2012, 126, 328–335.
36. A. Wang, H. Cheng, N. Ren, D. Cui, N. Lin and W. Wu, Front. Environ. Sci. Eng., 2012, 6, 569–574.
37. M. C. Potter, Proc. R. Soc. London, Ser. B, 1911, 84, 260–276.
38. K. Lewis, Bacteriol. Rev., 1966, 30, 101.
39. R. M. Allen and H. P. Bennetto, Appl. Biochem. Biotechnol., 1993, 39, 27–40.
40. B. H. Kim, D. H. Park, P. K. Shin, I. S. Chang and H. J. Kim, U.S. Pat. 5976719, 1999.
41. H. Kim, M. Hyun, I. Chang and B. H. Kim, J. Microbiol. Biotechnol., 1999, 9, 365–367.
42. T.-S. Song, W.-M. Tan, X.-Y. Wu and C. C. Zhou, J. Chem. Technol. Biotechnol., 2012, 87, 1436–1440.
43. T.-S. Song, Z.-S. Yan, Z.-W. Zhao and H.-L. Jiang, J. Chem. Technol. Biotechnol., 2010, 85, 1489–1493.
44. T.-S. Song, Z.-S. Yan, Z.-W. Zhao and H.-L. Jiang, Bioprocess Biosyst. Eng., 2011, 34, 621–627.
45. C.-C. Fu, T.-C. Hung, W.-T. Wu, T.-C. Wen and C.-H. Su, Biochem. Eng. J., 2010, 52, 175–180.
46. Y. Fu, Q. Xu, X. Zai, Y. Liu and Z. Lu, Appl. Surf. Sci., 2014, 289, 472–477.
47. Y. Fu, J. Yu, Y. Zhang and Y. Meng, Appl. Surf. Sci., 2014, 317, 84–89.
48. Y. Fu, Z. Zhao, J. Liu, K. Li, Q. Xu and S. Zhang, Sci. China: Chem., 2011, 54, 844–849.
49. S. Hong, I. Chang, Y. Choi, B. Kim and T. Chung, Bioprocess Biosyst. Eng., 2009, 32, 389–395.
50. S. W. Hong, I. S. Chang, Y. S. Choi and T. H. Chung, Bioresour. Technol., 2009, 100, 3029–3035.
51. S.-W. Hong, H.-J. Kim, Y.-S. Choi and T.-H. Chung, Bull. Korean Chem. Soc., 2008, 29, 2189–2194.
52. D. A. Lowy, L. M. Tender, J. G. Zeikus, D. H. Park and D. R. Lovley, Biosens. Bioelectron., 2006, 21, 2058–2063.
53. L. M. Tender, S. A. Gray, E. Groveman, D. A. Lowy, P. Kauffman, J. Melhado, R. C. Tyce, D. Flynn, R. Petrecca and J. Dobarro, J. Power Sources, 2008, 179, 571–575.
54. M. E. Nielsen, C. E. Reimers, H. K. White, S. Sharma and P. R. Girguis, Energy Environ. Sci., 2008, 1, 584–593.
55. K. Scott, I. Cotlarciuc, D. Hall, J. B. Lakeman and D. Browning, J. Appl. Electrochem., 2008, 38, 1313–1319.
56. M. Rahimnejad, N. Mokhtarian, G. Najafpour, A. Ghoreyshi and W. Dahud, World Appl. Sci. J., 2009, 6, 1585–1588.
57. M. Ghasemi, W. R. W. Daud, S. H. Hassan, S.-E. Oh, M. Ismail, M. Rahimnejad and J. M. Jahim, J. Alloys Compd., 2013, 580, 245–255.
58. G. Najafpour, M. Rahimnejad, N. Mokhtarian, W. R. W. Daud and A. Ghoreyshi, World Appl. Sci. J., 2010, 8, 1–5.
59. D. A. Lowy and L. M. Tender, J. Power Sources, 2008, 185, 70–75.
60. D. R. Lovley, Nat. Rev. Microbiol., 2006, 4, 497–508.
61. Y. Huang, Z. He, J. Kan, A. K. Manohar, K. H. Nealson and F. Mansfeld, Bioresour. Technol., 2012, 114, 308–313.
62. Y. R. J. Thomas, M. Picot, A. Carer, O. Berder, O. Sentieys and F. Barrière, J. Power Sources, 2013, 241, 703–708.
63. R. Renslow, C. Donovan, M. Shim, J. Babauta, S. Nannapaneni, J. Schenk and H. Beyenal, Phys. Chem. Chem. Phys., 2011, 13, 21573–21584.
64. C. Donovan, A. Dewan, D. Heo, Z. Lewandowski and H. Beyenal, J. Power Sources, 2013, 233, 79–85.
65. C. Donovan, A. Dewan, D. Heo and H. Beyenal, Environ. Sci. Technol., 2008, 42, 8591–8596.
66. D. Majumder, J. P. Maity, C.-Y. Chen, C.-C. Chen, T.-C. Yang, Y.-F. Chang, D.-W. Hsu and H.-R. Chen, Int. J. Hydrogen Energy, 2014, 39, 21215–21222.
67. C. Donovan, A. Dewan, H. Peng, D. Heo and H. Beyenal, J. Power Sources, 2011, 196, 1171–1177.
68. M. E. Nielsen, Doctoral, Oregon state university, 2008.
69. Z. Yan, N. Song, H. Cai, J.-H. Tay and H. Jiang, J. Hazard. Mater., 2012, 199, 217–225.
70. T. K. Sajana, M. M. Ghangrekar and A. Mitra, Aquacultural Engineering, 2014, 61, 17–26.
71. D.-Y. Huang, S.-G. Zhou, Q. Chen, B. Zhao, Y. Yuan and L. Zhuang, Chem. Eng. J., 2011, 172, 647–653.
72. N. Touch, T. Hibino, Y. Nagatsu and K. Tachiuchi, Bioresour. Technol., 2014, 158, 225–230.
73. J. M. Morris and S. Jin, J. Hazard. Mater., 2012, 213–214, 474–477.
74. T. K. Sajana, M. M. Ghangrekar and A. Mitra, Aquacultural Engineering, 2013, 57, 101–107.
75. Y. Zhang and I. Angelidaki, Water Res., 2012, 46, 6445–6453.
76. T.-S. Song, H.-Y. Cai, Z.-S. Yan, Z.-W. Zhao and H.-L. Jiang, Bioresour. Technol., 2012, 108, 68–75.
77. M. E. Nielsen, C. E. Reimers and H. A. Stecher, Environ. Sci. Technol., 2007, 41, 7895–7900.
78. S. V. Mohan, S. Srikanth, S. V. Raghuvulu, G. Mohanakrishna, A. K. Kumar and P. N. Sarma, Bioresour. Technol., 2009, 100, 2240–2246.
79. F. Y.-B. L. Zhi-Kai, X. Qian, L. Yuan-Yuan and Z. Ye-Long, J. Inorg. Mater., 2013, 28, 278–282.
80. L. D. Schamphelaire, P. Boeckx and W. Verstraete, Appl. Microbiol. Biotechnol., 2010, 87, 1675–1687.
81. C. Dumas, A. Mollica, D. Féron, R. Basseguy, L. Etcheverry and A. Bergel, Bioresour. Technol., 2008, 99, 8887–8894.
82. C. Dumas, A. Mollica, D. Féron, R. Basséguy, L. Etcheverry and A. Bergel, Electrochim. Acta, 2007, 53, 468–473.
83. L. Zhi-Kai, F. Yu-Bin, X. Qian, L. Yuan-Yuan and Z. Ye-Long, J. Inorg. Mater., 2013, 28, 278–282.
84. Y. Ren, D. Pan, X. Li, F. Fu, Y. Zhao and X. Wang, J. Chem. Technol. Biotechnol., 2013, 88, 1946–1950.
85. K. Scott, I. Cotlarciuc, I. Head, K. P. Katuri, D. Hall, J. B. Lakeman and D. Browning, J. Chem. Technol. Biotechnol., 2008, 83, 1244–1254.
86. D.-B. Wang, T.-S. Song, T. Guo, Q. Zeng and J. Xie, Int. J. Hydrogen Energy, 2014, 39, 13224–13230.
87. K. Wetser, E. Sudirjo, C. J. N. Buisman and D. P. B. T. B. Strik, Appl. Energy, 2015, 137, 151–157.
88. Z. Chen, Y.-C. Huang, J.-H. Liang, F. Zhao and Y.-G. Zhu, Bioresour. Technol., 2012, 108, 55–59.
89. T. K. Sajana, M. M. Ghangrekar and A. Mitra, Bioresour. Technol., 2014, 155, 84–90.
90. J. Zhao, X.-F. Li, Y.-P. Ren, X.-H. Wang and C. Jian, J. Chem. Technol. Biotechnol., 2012, 87, 1567–1573.
91. M. Helder, D. P. B. T. B. Strik, R. A. Timmers, S. M. T. Raes, H. V. M. Hamelers and C. J. N. Buisman, Biomass Bioenergy, 2013, 51, 1–7.
92. Y. Hubenova and M. Mitov, Bioelectrochemistry, 2015, 106, 226–231.
93. N. Kaku, N. Yonezawa, Y. Kodama and K. Watanabe, Appl. Microbiol. Biotechnol., 2008, 79, 43–49.
94. Y. Hubenova and M. Mitov, Bioelectrochemistry, 2012, 87, 185–191.
95. R. A. Timmers, D. P. B. T. B. Strik, H. V. M. Hamelers and C. J. N. Buisman, Biomass Bioenergy, 2013, 51, 60–67.
96. M. Helder, D. P. B. T. B. Strik, H. V. M. Hamelers, A. J. Kuhn, C. Blok and C. J. N. Buisman, Bioresour. Technol., 2010, 101, 3541–3547.
97. M. Rosenbaum, Z. He and L. T. Angenent, Curr. Opin. Biotechnol., 2010, 21, 259–264.
98. Z. He, J. Kan, F. Mansfeld, L. T. Angenent and K. H. Nealson, Environ. Sci. Technol., 2009, 43, 1648–1654.
99. Y.-L. Zhou, Y. Yang, M. Chen, Z.-W. Zhao and H.-L. Jiang, Bioresour. Technol., 2014, 159, 232–239.
100. D. Suor, J. Ma, Z. Wang, Y. Li, J. Tang and Z. Wu, Chem. Eng. J., 2014, 248, 415–421.
101. Z. Chen, Y.-C. Huang, J.-H. Liang, F. Zhao and Y.-G. Zhu, Bioresour. Technol., 2012, 108, 55–59.
102. Y. Yuan, S. Zhou and L. Zhuang, J. Soils Sediments, 2010, 10, 1427–1433.
103. C. E. Reimers, P. Girguis, H. A. Stecher, L. M. Tender, N. Ryckelynck and P. Whaling, Geobiology, 2006, 4, 123–136.
104. J. M. Pisciotta, Z. Zaybak, D. F. Call, J.-Y. Nam and B. E. Logan, Appl. Environ. Microbiol., 2012, 78, 5212–5219.
105. H. Deng, Y. C. Wu, F. Zhang, Z. C. Huang, Z. Chen, H. J. Xu and F. Zhao, Pedosphere, 2014, 24, 330–338.
106. B. Erable, R. Lacroix, L. Etcheverry, D. Féron, M. L. Delia and A. Bergel, Bioresour. Technol., 2013, 142, 510–516.
107. J. An, B. Kim, J. Nam, H. Y. Ng and I. S. Chang, Bioresour. Technol., 2013, 127, 138–142.
108. U. Karra, G. Huang, R. Umaz, C. Tenaglier, L. Wang and B. Li, Bioresour. Technol., 2013, 144, 477–484.
109. T. Ewing, P. T. Ha, J. T. Babauta, N. T. Tang, D. Heo and H. Beyenal, J. Power Sources, 2014, 272, 311–319.
110. A. Dewan, H. Beyenal and Z. Lewandowski, Environ. Sci. Technol., 2008, 42, 7643–7648.
111. M. Rahimnejad, G. Nnajafpour and Z. Najafgholi, Journal of RenewableEnergy and Environment, 2015, 2, 49–55.
112. M. Rahimnejad and Z. Najafgholi, Korean J. Chem. Eng.,  DOI:10.1007/s11814-015-0123-x.

---