Fuel 355 (2024) 129530
Available online 16 August 2023
0016-2361/© 2023 Elsevier Ltd. All rights reserved.
New insights into microbial electrolysis cells (MEC) and microbial fuel cells (MFC) for simultaneous wastewater treatment and green fuel
(hydrogen) generation
Jayaseelan Arun
a, PanneerSelvam SundarRajan
b, Kirubanandam Grace Pavithra
c,
Packiyadoss Priyadharsini
a, Sivaprasad Shyam
d, Rangarajan Goutham
e, Quynh Hoang Le
f,g, Arivalagan Pugazhendhi
f,g,*aCentre for Waste Management, International Research Centre, Sathyabama Institute of Science and Technology, Jeppiaar Nagar (OMR), Chennai 600119, Tamil Nadu, India
bDepartment of Chemical Engineering, Saveetha Engineering College, Thandalam, Chennai, Tamil Nadu, 602105, India
cDepartment of Infrastructural Engineering, Saveetha School of Engineering, Chennai 602105, India
dEnvironmental Engineer, Stantec, FL, USA
eDepartment of Chemical Engineering & Applied Chemistry, University of Toronto, 200 College Street, Toronto, ON M5S 3E5, Canada
fSchool of Medicine and Pharmacy, Duy Tan University, Da Nang, Vietnam
gInstitute of Research and Development, Duy Tan University, Da Nang, Vietnam
A R T I C L E I N F O Keywords:
Microbial fuel cell Microbial electrolysis cell Wastewater
Bio-hydrogen Bio-electricity Catalyst
A B S T R A C T
Hydrogen is the green fuel with higher eco-friendly and efficient vehicular fuel. In past hydrogen gas is derived from non-renewable resources. In recent years renewable resources are utilized for generation of bio-hydrogen gas. Hydrogen gas is preferably produced via electrochemical, photochemical, thermochemical and biochemical processes. Microbial fuel cell (MFC) and microbial electrolysis cell (MEC) are the notable systems used for hydrogen production from organic wastes and wastewater. This review mainly focuses on bio-hydrogen pro- duction from both MFC and MEC from wastewater. The mechanism, factors influencing the process, different types of MFCs, energy harvesting ways are signposted in this review. This review addresses on the difficulties in harvesting the energy as well as the economic aspect of harvested energy in detail. In MEC, microbial activity happens at anode and in electrode the hydrogen evolution occurs. Upscaling of the process is mainly influenced by the reactor design and microbiology. Performance of various catalysts on hydrogen evolution are discussed in detail. The scalability of integrating MFC and MEC needs to be decoded in near future for the better of hydrogen production and achieving sustainable development goals (SDGs).
1. Introduction
In recent decades, there has been a significant spike in global energy consumption. The necessity for alternative energy resource develop- ment has been prompted by the depletion of fossil fuel reserves, signif- icant cost fluctuations, and associated environmental consequences.
Green fuels are regarded as a promising sustainable energy source because of their high efficiency, ease of manufacture, and pollution-free operation [1,2]. In recent times, wastewater is recognized as a vital source of energy, materials (cellulose, bioplastics, fibers, alginate, and metals) and nutrients (phosphorus and nitrogen) [3]. Microbial
electrochemical technologies (MET) based on microbial fuel cells (MFC) or microbial electrolysis cells (MEC) provide the possibility to biologi- cally treat wastewater while also recovering energy. Exploring energy and hydrogen cogeneration in microbial electrolysis cells (MECs), might become an alternate technique for decentralized hydrogen production coupled with a cheap and effective wastewater treatment processing strategy. This is also an innovative way to lower the expenses associated with the production of electricity and therefore make the entire system viable [4,5]. In the MEC, the cathode is a vital part that produces hydrogen and other chemical molecules. For practical use in MECs for hydrogen production, several studies have focused on the development
* Corresponding author.
E-mail address: [email protected] (A. Pugazhendhi).
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Fuel
journal homepage: www.elsevier.com/locate/fuel
https://doi.org/10.1016/j.fuel.2023.129530
Received 21 March 2023; Received in revised form 6 August 2023; Accepted 14 August 2023
and implementation of a low-cost cathode or cathode catalyst [6]. In the recent years, several laboratory and pilot scale research have been conducted in microbial fuel cells (MFCs) and microbial electrolysis cells (MECs). Microorganisms with extracellular electron transfer (EET) ca- pabilities transport electrons produced during oxidation from waste- water to the anode in MFCs or MECs.
Microbial fuel cells (MFCs) have received high expectations in recent decades as a promising candidate to address the contradiction between effluent treatment and energy conservation [7]. As stated, the energy in wastewater is 10 times more than what is needed to meet discharge standards. Several studies have been conducted on a variety of MFC- related topics, including electrode materials, membrane types, cell de- signs, and electron transport methods [89]. In any case, there are two fundamental obstacles that must be addressed: high cost and low overall performance. The typical capital investment in municipal wastewater treatment plants using MFC technology is 30 times greater than the expenditure required by the conventional activated sludge approach. Up to 80% of the entire fund intake is consumed by the high cost of the membrane and electrode, which includes the abiotic catalyst.
Laboratory-level MFC systems achieve power densities of just 1–3 W/
m2, compared to the theoretical maximum of 17–19 W/m2 [9,10].
2. Wastewater and MFC/MEC
There are a variety of causes for water pollution, including resi- dences, industry, mining, and infiltration, but one of the most significant is the widespread use of water by industry on a massive scale [11].
Wastewater is often classified into following categories: Rainfall (runoff from impermeable surfaces), residential wastewater, industrial waste- water and agricultural wastewater. The industrial wastewater includes cooling water, washing effluent (which varies in composition), and manufacturing or process water (which is biodegradable and/or possibly harmful) [12]. In essential manner, wastewaters differ greatly from drinking water sources (often rivers, lakes, or reservoirs): When compared to contamination levels in wastewaters resulting from industrial-type operations, the level of contaminants are higher than drinking water sources [13]. As a result of dumping wastewater from a variety of sources (domestic and industrial), the ecosystem is harmed.
These effects deplete water sources by causing hypertrophication, hyp- oxia, and algal growth [14]. In general, their toxicity varies with composition, which depends on the origin. A wide range of pollutants are present in wastewater from numerous sources, so the problems that wastewater treatment industry faces are complex in nature. As a result, there are several kinds of effluents to deal with, each with its own set of characteristics that necessitate unique treatment methods [12].
Chemical, physical and biological procedures and operations are commonly used in traditional wastewater treatment to remove solids, such as colloids and organic debris, as well as nutrients and soluble pollutants (such as heavy metals and organics) [12]. Each treatment has its own set of restrictions, including cost, feasibility, dependability, ef- ficiency, environmental effect, operation, sludge generation, pre- treatment needs, and the development of potentially harmful by- products.
The existing wastewater treatment methods are expensive to implement and require a large quantity of electrical power and also the systems in use do not generate revenue or add value addition to the wastewater being treated. Sustainable wastewater treatment is an effective idea that may potentially address the issues of energy scarcity, resource depletion, and pollution. A sustainable treatment process is commonly agreed to aim for minimal resource consumption, neutral- energy operation, steady treatment performance, high effluent quality to fulfill water reclamation and reuse requirements, balanced invest- ment and economic output, strong social equity, and a low environ- mental impact [14].
In general, contaminants are removed from effluents using physi- cochemical and/or biological methods, with research focusing on less
expensive, effective combinations of systems or novel alternatives. MFC and MEC are the techniques which found to be less expensive as it uti- lizes microorganisms as biocatalyst and the biocatalyst involves in the oxidation of organic matter and in the transfer of electrons to the anodic surface for the production of bioelectricity. This review focuses on the detailed description of MFC and MEC, its types, mechanism and the influencing factors. The practical difficulties in energy harvesting as well as the economic assessment of MFC and MEC towards wastewater treatment [4].
3. Microbial electrolysis system
Due to its minimal energy consumption and significant production efficiency, the microbial electrolysis cell (MEC) is a viable and sub- stantial technology for biohydrogen production [15]. The biohydrogen production in microbial electrolysis cells has resulted in technical terms such as “electrohydrogenesis [16], microbial electrochemical cells [17], electrochemically assisted hydrogen production [18], bio- electrochemical systems [15], bio-catalyzed electrolysis [19] and mi- crobial electrochemical systems (MES)” [16]. So far, “microbial elec- trolysis cell” has been the most widespread and specific term. Two chambers are employed in a conventional MEC. Anaerobic conditions are used in the anode and cathode chambers. Both chambers are divided using a cation exchange membrane (CEM) or an anion exchange mem- brane (AEM). The bacteria that grow on the anode surface are electro- chemically active, oxidising organic materials while producing electrons and protons that also acidify the anolyte solution. When protons are absorbed at the cathode in the presence of a strong voltage, hydrogen gas is produced, which results in the alkalization of the catholyte, which is a chemical reaction [20]. Different anode and cathode catalysts used for hydrogen generation is tabulated in Table 1.
3.1. Anode catalysts
The majority of MEC studies conduct electrogenic reactions directly on the anode of MFCs [21]. Carbon-based materials (activated carbon, graphite and carbon felt) have probably been the most prevalent and have frequently resulted in very effective microbial anodes. Carbon- based electrodes are very efficient and homogeneous. They are charac- terized by cross-linked structures, which contribute to the formation of a strong binding between the particles and anodes. There are several non- carbon electrodes with excellent electrocatalytic activity, such as plat- inum and stainless steel. Carbon brushing was employed as an anode [22], with three distinct systems being utilized: MEC with power supply, MEC linked hydraulically, and the other non-hydraulically connected to TEG (thermoelectric generators) [1].
Metallic anodes offer an attractive alternative. Stainless steel has demonstrated a potential ability to generate microbial anodes, particu- larly when in the form of foam [23]. The use of copper does not appear to be particularly appropriate, despite the fact that copper has demon- strated an outstanding capacity to form microbial anodes [24]. This is because, with a normal potential dissolution of 0.34 V/SHE (0.1 V/SCE), the anode potential will have to be regulated quite precisely, and any unintended drift towards high values might result in the release of Cu2+ ions into solution. These ions are extremely harmful to microbial cells.
Copper-polymer hybridized complexes could be a promising alternative to conventional copper compounds [25]. Due to the weak conductivity of titanium’s passivation layer, it is widely recognized that it is unsuit- able for use as an anode metal. By covering titanium with metal oxides such as ruthenium and tantalum, the issue has been resolved, allowing for the development of so-called dimensionally stable anodes (DSA) [26].
A significant issue with MECs (mainly membrane less MECs) is the simultaneous synthesis of methane and hydrogen caused by the presence of methanogens [27]. This methane production is mostly related to the use of CO2 and electrochemically produced H2. There are other types of
bacteria that make hydrogen less available, such as homoacetogens that turn CO2 and H2 into acetate [28]. These are more challenging than electrogene and methanogens because they have faster growth rates and the ability to expand chemolithotrophically by using CO2 and H2. As a result, it is important to stop the growth of non-electrogenic bacteria to
obtain a considerable yield of hydrogen out of MECs. There have been reports of different chemical-based inhibitors, including 2-bromometha- nesulfonate (BES) and lumazine. The use of such chemicals raises the total cost of the process; as a result, low-cost alternatives to chemical methanogen suppression are required [29].
3.2. Cathode catalysts
The cathode’s base material as well as the catalyst would have a major impact on the performance of MECs. The cathode’s base material might be graphite, titanium, or any other conductive substance that can conduct electricity. The hydrogen evolution reaction (HER) on simple carbon electrodes takes a long time and needs a high overpotential to produce hydrogen and to control the excess potential a catalyst was utilized on the cathode [6]. Platinum catalysts were widely used in the earlier, but it’s not advised because of high costs, lack of sustainability, and sulphide poisoning [17].
Due to their low cost and availability, nickel and stainless steel are the best materials for making MEC cathodes. The use of nickel catalysts and the direct application of metallic nickel have both been proven effective. A stainless-steel combination of iron, carbon, chromium, and nickel has also been widely utilised in potential implementation. It is possible that stainless steel will be favoured as MEC cathodes since some of its alloys have comparatively low corrosion weight losses when compared to certain other alloys and metals [30]. Other materials that have been explored as MEC cathode catalysts include molybdenum disulphide, which is similar to platinum in terms of the volcano plot for the H2 evolution process and has been used as a cathode catalyst in a number of different experiments. Because of its poor conductivity and lack of active sites, molybdenum disulphide containing MEC only pro- duced H2 at a rate that was 61% of that obtained by nickel and only 49%
of that obtained by platinum [24]. As stated in the same paper, the adoption of an alloy containing molybdenum and nickel, which has a rate of reaction similar to that of platinum catalysts, might be a viable alternative to platinum catalysts [30].
3.3. Mechanism and mode of operation
Generally, MEC consists of an anode, a cathode, and an ion exchange membrane which is represented in Fig. 1 [1]. Microorganisms oxidize organic materials in the anolyte and on the anode surface. Protons are released into the medium as a result of this oxidation process and pro- ceed to the cathode via a separator, in which they are converted to molecular hydrogen by the electron (using an external circuit that are transferred from anode to cathode). It is feasible to collect H2 bubbles because they detach from the cathode, elevate and dissociate from the liquid phase, which makes hydrogen easier to collect. The Table 1
Biohydrogen production with different anode and cathode materials.
MEC Configuration Anode Cathode Hydrogen
production References Dual
chambered Graphite Nickel (Ni) foam-based cathode
Overall recovery 9.99%, Cathodic recovery 16.88%
[31]
Dual
chambered Carbon brush Carbon cloth electrode modified with carbon/iron- based nanorod catalysts
Overall recovery 30.3%, Cathodic recovery 79.5%
[32]
Single
chambered Wet carbon
cloth Nickel
molybdenum- based cathode
Production rate 2.1 m3/ m3 day, Cathodic recovery 90%
[33]
Dual
chambered Platinum (Pt)- deposited carbon cathode
Plasma-treated carbon felt anode
Yield 50–76%, Production rate 4.3 L/L day
[34]
Dual
chambered Pt-coated wet proof carbon cloth
Non-treated dry
carbon cloth Yield 23.3 mmol/mol- acetate
[35]
Single
chambered Graphite fiber
cloth Carbon cloth
coated with Pt/
C
Yield 4.66 mg H2/g VSS
[36]
Double
chambered Carbon cloth anode E-TEK W1400L
Carbon cloth cathode adhered with MoS2-EF catalyst
Production rate 0.133 m3/m3 day
[37]
Single
chambered Nitrogen treated Carbon cloth
Stainless steel
mesh cathode Production rate 1.31 m3/m3 day
[38]
Dual
chambered Heat-treated
graphite brush Pt-coated stainless steel mesh cathode
Production rate 2.2 m3/ m3 day
[39]
Dual
chambered Graphite Ni powder blended activated carbon cathode
Production rate 0.38 L/
L day
[40]
Single
chambered Isomolded
graphite plates Electroformed Ni mesh cathode
Overall recovery 89.3%, Production rate 4.18 m3/m3 day
[41]
Dual
chambered Graphite brush Graphite fiber cloth coated with platinum
2.6 L/L day [42]
Dual
chambered Carbon brush Carbon cloth Production rate 61.8 L/
m3 day
[43]
Single
chambered Carbon fiber
brush Stainless steel fiber felt cathode
Recovery 76.37%, Production rate 3.66 m3/m3 day
[44]
Single
chambered Plasma-treated carbon cloth with immobilized Geobacter sulfurreducens
Pt-coated
carbon cloth Production rate 0.56 m3/m3 day
[45]
Fig. 1.Production of hydrogen energy from microbial electrolysis cells (MEC) upon wastewater treatment.
implementation of MEC technology, like other biological wastewater treatment methods, entails several side reactions, which are caused by the complicated nature of the wastewater and the range of microbe- s found in the mixed culture. The complete process is demonstrated using acetate, which is the most often utilized carbon source for BES bioanodes in laboratory investigations [16]. The reactions occur in anode, cathode is shown as per Eq. (1)–(3).
Anode:
CH3COO−(ac)+4H2O(l)→2HCO−3(ac)+9H+(ac)+8e− (1) Cathode:
8 (
H+(ac)+e−→1 2H2(g)
)
(2) Total reaction:
CH3COO−(ac)+4H2O(l)→2HCO−3(ac)+H(ac)+ +4H2(g) (3) 4. Microbial fuel cell
4.1. Types of MFC
MFC’s are bio-electrochemical devices that deploys micro-organisms as catalysts for converting the chemical energy stored in organic and inorganic substrates to generate electric current [46]. Different anode, cathode and the proton exchange membranes used in MFC’s are used for a wide range of applications such as wastewater treatment, electricity generation, bio-hydrogen production and bio-sensors [47]. There are different types and generations of MFC’s, classified based on historical development and mechanisms of electron transfer. Ieropoulos et al. [48]
categorized MFC’s into three generations viz Gen- 1, Gen-2 and Gen-3.
Gen-1 involved the use of gram-negative E. coli or other micro- organisms in conjunction with synthetic redox mediators. Gen-2 uti- lized natural media like sulphate/sulphide with sulfate reducers like Desulfovibrio desulfuricans. Gen-3 MFC’s did not involve the use of sol- uble mediators and were based on anodophilic species like Geobacter sulfurreducens. [47] classified MFC’s into different types based on the type of design and method of construction viz double chamber, single chamber, upflow and stacked.
4.2. Mechanism and mode of operation
The mechanism and mode of operation of single-chamber, double (dual) chamber, upflow and stacked MFC’s are discussed.
4.2.1. Single-chamber MFC
Single-chamber MFC as the name suggests consists of only one chamber that contains both anode and cathode placed on the opposite ends [49]. One face of the cathode is in directly contact with the elec- trolyte while the opposite face is in direct contact with air. The internal ohmic resistance is directly proportional to the interelectrode spacing and internal ohmic resistance can be diminished by avoiding the use of catholyte for connecting the chambers, thereby increasing the power density. Single-chamber MFC is simple in design and economical in construction but there are certain disadvantages such as biofouling, adulteration of microbes and reversal of oxygen flow from cathode to anode [50].
4.2.2. Double-chamber MFC
Double-chamber or dual-chamber MFC consists of two reaction chambers or units, where one chamber acts as anode and the other chamber acts as cathode separated by a proton exchange membrane.
The major advantage of dual-chamber MFC is its configuration which enable to keep the liquid components of cathodic and anodic chamber separately. Fig. 2 shows the microbial fuel cell with two component system (anode and cathode) separated by cation exchange membrane.
MFC produces electricity in combination with two half-cell reactions (in anode substrate utilization and in cathode oxygen reduction). Anodic chamber is maintained under anaerobic conditions to promote the growth of microbes. A biofilm of exoelectrogenic microorganisms is formed at the anode when microorganisms adhere to the anodic surface and bio-electrochemical reaction is carried out to oxidize organic matter via metabolic reactions. This releases protons, electrons and CO2 to the solution. The protons get transferred from anodic chamber to cathodic chamber via proton exchange membrane. The cathodic chamber func- tions under aerobic conditions, as oxygen accepts electrons and results in the formation of water [51].
4.2.3. Upflow MFC
Upflow MFC is a relatively new type of design which is cylinder shaped. It consists of cathode chamber at the top and anode chamber at the bottom separated by a proton exchange membrane [52]. The sub- strate is usually pumped from the bottom of the chamber through the anode and flows upward through the cathode on top. A potential gradient is formed between the electrodes which enables the action of fuel cell. Upflow MFC’s have the potential to be easily scaled in com- parison to other MFC’s in the light of wastewater treatment rather than power applications. The setback for power application is the energy costs associated with pumping the substrate which is greater than the output power produced [8]. These MFC’s are prevalently studied in laboratory scale and studies reveal that high-internal resistance and electrode-based losses lowers the power density.
4.2.4. Stacked MFC
Stacked MFC’s are usually a combination of multiple MFC’s that are aligned in series or parallel to increase the power output [53]. The objective of this configuration is to enhance the power output produced by combining multiple MFC’s but the final voltage of combined MFC’s would not be equivalent to the sum of voltage of individual MFC’s which could be attributed to the losses in series or parallel connection [51]. It is reported that parallelly stacked MFC’s have higher bio-electrochemical reaction rate and produce more power output than series-connected MFC’s. In the light of wastewater treatment, a greater COD removal is attained for enhanced wastewater treatment using parallel connection or stacking of MFC’s [47]. The operational performance of different MFC’s in the light of wastewater treatment focusing on COD removal is summarized in Tables 2 and 3.
Fig. 2.Microbial fuel cell with two component system (anode and cathode) separated by cation exchange membrane. MFC produces electricity in combi- nation with two half-cell reactions (in anode substrate utilization and in cath- ode oxygen reduction).
4.2.5. Nature of microorganism and microbial metabolism
Many kinds of microbes are proven to be very useful in MFCs which includes aechea, bacteria and fungi. Mixed consortium is found to be synergistic community as each organism plays a role for building up nutrient cycle ecosystem [60]. The inoculation of MFC’s can be per- formed either using pure culture or mixed culture of bacteria. Few of the commonly used microorganisms in the anode are Escherichia coli, Pseu- domonas aeruginosa, Shewanella putrefaciens, Geobacter sulfurreducens, etc, [61]. The anodic potential plays a vital role in determining the cell potential of a MFC depends upon the nature of microorganism and their metabolism pathway. The catabolism of bacteria is the rate limiting step in MFC’s [62]. The mixed consortium such as Clostridium, Tetrathiobacter and Desulfovibrio were utilized as a biocatalyst in the sulfate-laden wastewater using MFC. The pH and salinity tests performed shows that the biofilm generated by this consortium prefers alkaline environ- ment which provides electricity production [63]. The consortium con- taining Thermoanaero bacterium and Paenibacillus were utilized in the MFC for the treatment of wood hydrothermal-treatment wastewater.
The power generation was remarkably increased from 70 to 360 mW/m2
after utilization of consortium from municipal wastewater [64]. Fig. 3 highlights the factors influencing the operation mechanism of MFC.
4.2.6. Electron transfer mechanism
Extracellular electron transfer (EET) involves the transfer of elec- trons between the microbes and anode. This could take place by two different mechanisms viz direct electron transfer and mediated electron transfer [65]. Direct electron transfer involves the establishment of physical contact between the cell membrane of microbe and an electrode to facilitate the process and does not involve the use of dissolved redox mediator. Direct electron transfer takes place via outer membrane cy- tochrome present on the external surface of the microbe. Geobacter and Shewanella are two of the widely studied species that contains intricate network of outer membrane cytochrome capable of performing direct electron transfer. There are also microbes that do not possess the inherent capability of facilitating electron transfer because they lack contact with electrode surface. A redox mediator acts as a carrier by gaining electrons from bacterial cells to transfer the electrons to anode, thereby getting oxidized in the process. The oxidized mediator can perform subsequent electron transfer. A good mediator should possess (1) positive redox potential to facilitate electron transfer, (2) good sol- ubility in anolyte, (3) easy ability to cross the microbe cell membrane, and (4) non-toxic towards bacteria [48,65].
4.2.7. Type of substrate
A wide range of substrates from simple molecules such as glucose, butyrate, acetate and glycerol to complex organic compound mixtures and wastewaters were studied [66]. Different wastewater sources such as dairy, livestock, distillery, agricultural and pharmaceuticals are a great potential substrate [66]. The concentration of substrate plays a Table 2
A summary of different types of MFC’s, wastewater and COD removal (%).
Type of MFC Type of wastewater COD Removal (%) Reference
Upflow Brewery >90 [54]
Single chamber Acetate rich wastewater >99 [55]
Single chamber Butyrate rich wastewater >98
Double chamber Petroleum refinery 64 [56]
Single chamber Starch processing 98 [57]
Dual chamber Winery 8.76 [58]
Single chamber Tannery 88 [59]
Table 3
Describes about the different types of wastewater utilized in mfcs and mecs.
Type of substrate MFC/MEC Anode Cathode Power/Current density Reference
Petroleum wastewater SMFC Ag Ag 286.7 mWm−2 [68]
Labanehy whey wastewater MFC-SC Graphite brush Plain carbon cloth 800 mV [69]
Petroleum wastewater MFC-DC Grapihte brush Platium coated carbon cloth 1089 Mw/m2 [70]
Petroleum refiner wastewater MFC-SC Graphite Graphite 222 mW/m2 [71]
Cr(VI) wastewater CW-MFC Stainless-steel mesh Stainless-steel mesh 0.462 W/m3 [72]
Domestic sewage MFC Graphite rod Graphite rod 22.5KW [73]
Food wastewater CSMEC Carbon-brush Pressed activated carbon 1125.35 mA/m2 [74]
Swine wastewater MEC Iron mesh twined on titanite wire Carbon cloth 57 L/m3.d (H2) [75]
Dairy industry wastewater SC-MEC Carbon cloth Graphite sheet 10.6 mWcm−2 [76]
Oil sands process wastewater MEC Titanium Stainless steel – [77]
Sugar industry wastewater MEC Carbon cloth Carbon cloth 3.6 Ml/l/h [78]
Fig. 3. Factors influencing the performance of MFC’s.
vital role in electricity generation and wastewater treatment applica- tions. For instance, [67] indicated that the bioelectricity generation and wastewater treatment efficacies in air–cathode microbial fuel cell coupled with constructed wetland systems increased with increase in substrate concentration. Similarly, [50] reported that increasing the sodium acetate (substrate) concentration from 100 to 850 ppm, increased the power output from 0.2 W m−3 to 1.2 W m−3. For further increase in substrate concentration above 1000 ppm, the power output levelled off.
4.2.8. Type of membrane
Proton exchange system in MFC’s consists of a proton exchange membrane (PEM). The membrane contains charged side walls with the presence of pores for facilitating the transfer or movement of protons from anode to cathode. This membrane plays a critical role in influ- encing the power output of MFC’s [79]. The presence of membrane in- creases the internal ohmic resistance in MFC’s and the diffusion of ions through the membrane leads to concentration polarization losses which directly lowers the power output of the MFC’s. Nafion™ is one of the widely used PEM’s in MFC technology that is selectively permeable to the protons [80]. It is also subjective to biofouling and the recovery of energy is reported to be lower than other porous and microfiltration membranes [81]. A membrane free MFC in the future could be envi- sioned for large scale practical applications. A membrane ceramic MFC and membrane-less self-stratifying MFC were compared were calibrated with same experimental conditions. The ceramic MFC took 50 days to obtain the full maturity and the energy conversion efficiency of ceramic MFC increased with longer HRT. The self-stratifying MFC reaches its maturity with in 25 days. The C-MFC and S-MFC observed differences in maturation and in output stability and this could be due to the presence and absence of membranes [82]. Three MFC’s with different electrode materials and membranes were compared. The MFC-A consist of elec- trode material carbon brush and proton exchange membrane. The MFC- B consist of graphite electrode material and proton exchange membrane and MFC-C consist of carbon brsh electrode material and cation ex- change membrane. These three MFC’s were operated stably for 300 days and there is no change in the removal efficiencies but the variations are seen in the production of electricity and in the nitrification, denitrifi- cation related to bioelectricity generation. The mechanical properties and the stability of PEM was found to be longer when compared to CEM [82].
4.2.9. Type of electrode
In general, the electrodes used for anode and cathode in MFC’s should be conductive, non-fouling, non-corrosive and cost-effective. A greater magnitude of electrode surface area is considered to be benefi- cial for MFC [83,84]. A wide range of electrode materials are used such as activated carbon acts as a good porous adsorbent material for physical processes, phosphorus and ferric salt/metal electrode for chemical processes and microbes for biological processes [85].
5. Operating conditions
pH, temperature, salinity, and ionic strength are the critical factors that influences the growth of bacteria. [86] studied the effects of tem- perature, pH and salinity on a dual chamber MFC in the light of wastewater treatment. The effect of salinity was studied in the range from 0 to 6.7 g KCl L-1, pH was varied in the range of 6–9, and temper- ature was varied from 24 to 35 ◦C. It was observed that the maximum power generation of 66 mW m−2 and a COD removal efficiency of 70%
were maintained steady until a salinity level of 4.1 g/L. Further increase in salinity from 4.1 to 6.7 g/L decreased the power generation and COD removal efficiencies by 92% and 25.3%. The maximum power density increased by 37% to 50.6 mW m−2 as the pH of the anolyte was increased from 6 to 9. Similarly, the maximum power density increased by 64% to 59 mW m−2 as the temperature was increased from 24 to
35 ◦C. The COD removal efficiency remained constant within 75–80% at all temperatures and pH.
6. Practical energy harvesting systems using MFC and MEC The energy harvesting systems efficiencies can be increased by reducing the components and their losses. Proper design of energy harvesting systems which plays a major role in energy consumption and self-sustainability within the system must be done to obtain net positive production of power and this is represented in diagrammatic format Fig. 4. Limitations are seen in MFC’s and in MEC’s due to its less voltage output that is between 0.2 V and 0.6 V, but 3 V is considered as a working voltage when it comes to practical applications. In research, MFCs are connected in series for increasing the MFCs working voltage but these kind of MFCs shows voltage reversal which results in loss of voltage [87].
To overcome these type of voltage problem MFC’s and MEC’s are combined with energy harvesting strategies to increase the attention.
The combination of MFC-energy harvesting includes (i) energy man- agement system development for harvesting energy for transmitters and remote sensors, (ii) optimization of external resistance for achieving maximum power in a given system (iii) increasing the power capacity and voltage output. The important aspect in energy harvesting tech- nique is that living microorganisms act as a power source and the quantity of energy as well as the frequency at which the energy har- vesting system has to perform need to be noted [88].
6.1. Capacitor-MFC systems
In a system which consist of capacitor and MFC, capacitors behave like variable resistors and the MFC testers are developed for testing the performance of MFCs power source. The whole system is controlled by analog-to-digital converter board (ADC). The usage of ADC and MFCT determines the capacitor frequency of the MFC system.
In this system the current-limiting factor is electrode as well as the size of the capacitor. Power management system (PMS) techniques are utilized for the enhancement of MFCs output voltage, and this system consist of rectifiers, boost converters, capacitors, regulators and charge pumps. The work of PMS is to harvest energy from MFCs and convert it to a energy form which can be utilized directly. The harvesting tech- nique is different from the conventional technique where the current passing through the resistors is not stored in the reusable form of energy instead the energy is dissipated [89,90]. Fig. 5 shows the diagrammatic representation of capacitor MFC systems.
6.2. Charge pump -MFC and boost converter-MFC systems
It is a simple circuit with inductor-less DC/DC converter, capacitor and the charge pump. Here, in this system the capacitor is utilized for storing energy and to transfer the stored energy either to lower or higher voltages. The capacitor-transformer-converter type power management system can utilize low voltage outputs and has higher power efficiency.
The other type of power convertor which are normally utilized is a boost converter due to its simple architecture and its ability towards output voltage increase in MFCs [91]. The Fig. 6 showed the diagrammatic representation of charge-pump-MFC systems.
6.3. Maximum power point tracking (MPPT)
The MPPT consist of a transformer for transferring and amplifying energy, an inductor for storing energy in magnetic fields and a diode. In general, in MFCs the electric load is obtained from DC/DC convertor but in case of MPPT, it controls the operating point of MFC for the optimi- zation of power harvesting level at any load [92]. For the effective operating conditions, resistors which are unnecessary will be eliminated and exploration of energy tracking and harvesting has to be performed.
Due to microbial activity variations, fluctuations in output were seen which makes the MFC maintenance with MPP difficult. To overcome this type of fluctuations, potentiometer with digital control can be utilized for harvesting hysteresis controller-based energy. This technique will track MPP as well as maintain the performance of energy harvesting.
The digital controlled potentiometers control the hysteresis controller system and eliminates the external resistance [88].
6.4. Economic assessment of MFC and MEC
Commercialization of MEC as well as MFC technique results in the
expansion in the industry and also enhances the research for the advancement. Some of the factors such as feedstock, reactor volume, cathode and anode material and system architecture. Both the commercialization and the system performance depend upon the fac- tors. In future the construction of MEC as well as MFC are to be in such as manner as it should cut down the anode costs, increase in organic loading rates and electrode life [93].
The type of wastewater and the degree of contamination in the water determines the type and the duration of the treatment, energy quantity needed and reactor size. The wastewater having high strength will take longer time period, therefore the hydraulic retention time (HRT) will be increased. Longer HRT rates are disadvantageous in the industrial level as it reduces the volume of waste and demands large reactors. The high- strength wastewater offers more energy, and they are economically viable in the phase of energy production but in the case of low strength wastewater, the HRT is low but it’s difficult to justify the type of treat- ment as it links with lower organic content. When the water strength increases the treatment process which is economically feasible [93].
Research is in process regarding the microbial population interactions with different substrates. Due to its availability and the chemical composition of wastewater, the wastewater was found to be a cost- effective substitute for pure chemicals in MECs and in MFCs [94].
The performance of anode is determined by the bio-electrochemical systems which is based upon the electrochemical reactions at the anode.
In cathode, either methane or hydrogen are produced as per the requirement of the manufacturer. In general, graphite as well as carbon materials are utilized as electrode materials due to its affordability, conductivity and abundance [95]. Carbon nanotubes (CNTs) have enormous amount of mechanical, thermal, biological and electrical properties and in recent studies, CNTs are utilized as electrode material despite of its issues in biotoxicity, biodegradability and biosafety [96].
Molybdenum and stainless-steel materials are utilized as anode where the molybdenum showed excellent durability and non-corrosivity and the stainless-steel electrodes has high amount of nickel concentrations.
In a research, stainless-steel brushes were utilized as cathode and hydrogen production attained was at the rate of 1.7 m3.m−3.day−1 and the efficiency at the cathode was maintained as 84% and this system. Th Fig. 4. Energy harvesting system for low output power amplifications.
Fig. 5. Capacitor-transformer-converter type.
Fig. 6. Charge pump-capacitor converter type.
increased rate of hydrogen production is due to higher nickel content and the larger specific area of the cathode [97].
7. Future research needs on energy harvesting process
The limitation of MFC is the molecular oxygen reduction at the cathode. Reduction of oxygen on the surface of electrode increases the potential. So far, many catalysts are utilized at the cathode side, but the oxygen reduction is considered as a limiting factor when dealing with MFC. This limitation can be rectified by using the increased ration of cathode when compared to cathode and the utilization of biological catalyst. The industrial suitability of MFC becomes crucial when it comes to energy production and wastewater treatment and this is due to the instability of electrode material utilized in MFCs [98]. Further research is needed in coupling of energy-harvesting system with MFCs which has a potential to store and increase the output voltage at the time of wastewater treatment. Research in the area of cost reduction, improved harvesting efficiency and design optimization are needed. The sustainable energy source of MFC can be designed by utilizing the fraction of power which is harvested form the MFC to make self-powered energy harvesting system [99].
MEC is oxidation–reduction reaction dependent and influenced by many operating conditions. For the optimization of the system, meta- bolic behaviour of microbial extracellular electron transfer has to be performed. The electrode and microbe interactions need to be studied to understand the exo-electrogenic activity. The electrodes which are of biocompatible and low-cost need to be identified to achieve effective electron kinetics [100]. The limitation in MEC is the microbial anodes acidification and it can be rectified by polarity reversing where anode can be worked as cathode in which the protons in the biofilm can be consumed [101,102].
8. Conclusion
Microbial fuel cell (MFC) and microbial electrolysis cell (MEC) will serve for future towards green fuel (hydrogen) generation. Apart from the MFC types, its mechanism, operation and factors influencing, the manuscript focuses on the practical difficulties in harvesting the hydrogen from MFC and MEC and the economic aspect of MFC and MEC as well which covers the different reader. Some of the points in which upgradation in the upcoming research is needed is discussed further and they are simultaneous wastewater treatment and ecological hydrogen production helps in integrated sustainability achievement. Performance of numerous catalysts, anodes, cathodes, substrates and microbial community on hydrogen production need to be studied simultaneously.
MEC and MFC helps in economical hydrogen production when compared to conventional processes. Routes of hydrogen storage and transport needs to be identified soon for easy commercialization of the process. Hydrogen storage container material and size be optimised.
Both environmental and ecological facts of hydrogen upon usage, stor- age and transport needs to be addressed for day to day usage. Scalability of the process should consider the operational cost, economical fabri- cation and efficiency.
CRediT authorship contribution statement
Jayaseelan Arun: Conceptualization, Data curation, Formal anal- ysis, Writing – original draft. PanneerSelvam SundarRajan: Method- ology, Writing – original draft. Kirubanandam Grace Pavithra:
Visualization, Writing – original draft. Packiyadoss Priyadharsini:
Writing – review & editing. Sivaprasad Shyam: Conceptualization, Writing – review & editing. Rangarajan Goutham: Writing – review &
editing. Quynh Hoang Le: Writing – review & editing. Arivalagan Pugazhendhi: Supervision, Conceptualization, Writing – review &
editing.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Data availability
Data will be made available on request.
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