Recent Trends in Electricity Generation from Lignocellulosic Biomass-Fueled Microbial Fuel Cells

Abstract

Microbial fuel cells (MFC) have gained immense popularity in recent years as a promising sustainable technology to meet ever-growing energy demand. The MFC technology is based on the exploitation of catabolic activities of microorganisms for the conversion of organic waste to sustainable electricity. Many different substrates have been investigated for use as feedstock for MFC. Among the main substrates that have been used, the prominent one is lignocellulosic biomass (LCB). The LCB as a feedstock is a viable option due to its abundance and renewability. However, it cannot be directly utilized by the microorganisms for energy conversion due to its recalcitrant nature. There are several other practical limitations of MFCs in fulfilling high energy demands. The performance of the MFC depends not only on the substrates utilized but also on the microorganisms used, electrode material, the reactor design, and the operating parameters such as type of substrate, temperature, pH, oxygen flow rate electrode material and construction, and choice of catalyst. The article provides a complete understanding of Microbial fuel cells including the pretreatment methods to disintegrate the recalcitrance of LCB, the diversity and role of microorganisms used in biomass-fueled MFC, various aspects impacting their performance, power output, advantages, various challenges, and prospects.

Introduction

The prodigious demand for energy across the globe has led to the exhaustion of non-renewable fossil fuel reserves. To bring down the dependence of the growing population on fossil energy sources, the utilization of sustainable resources has picked up momentum [1]. Over the past years, Bioelectrochemical systems (BESs) have been extensively reviewed as a new technology not only for sustainable electricity generation, but is used for waste remediation, and resource recovery [2]. The BESs are unique systems that encompass a broad category of bio-based catalysts or microorganisms with diverse functions primarily to achieve energy conservation [3]. Microbial fuel cells (MFCs) are a major type of BESs that can generate electrical energy from complex LCB through the catabolic activity of microbes. The MFC technology has several benefits such as its environmental-friendly nature, remarkable efficiency, and requirement of mild operational conditions without the production of toxic by-products like SOX, CO₂, NOX, and CO [4–6]. Therefore, recent years have witnessed MFCs emerging as a potent technology to produce bioelectricity. The application of MFCs is not only limited to the production of electric power but can also be exploited for bioremediation purposes such as the treatment of wastewater, hydrogen generation, seawater desalination, and biosensors and remote powerhouse [7–10]. MFC uses electrodes and chemicals during electrolysis reactions for the generation of electric current while the biochemical reaction involved in the current generation depends on the potential of microorganisms. MFC relies on unique kinds of microorganisms called electroactive microorganisms (EAMs) or electrogenic bacteria (EABs) [7]. These bacteria produce electricity by degrading inorganic or organic substrate using diverse metabolic pathways. The electrons generated by microorganism(s) during oxidation of substrate are gathered at an appropriate electrode present in a properly designed MFC.

Several factors influence the output of MFC including substrate and microorganisms used, electrode material, electron acceptor, and the configuration of bioreactor [11]. An extensive exploration of a variety of cathodes has been carried out over the past decade. These electrodes can be divided into three primary categories based on the presence or absence of catalysts such as air–cathode, aqueous air–cathode, and bio-cathodes [12]. A diverse array of substrates ranging from pure chemicals to organic wastewater to LCB have been explored [13]. The utilization of organic wastewater and LCB offers a promising solution for waste valorization and simultaneous energy production [14]. The metabolic processes in the MFC are dependent on various operation conditions including pH, temperature, and organic loading rate [15]. Previous studies including Leropoulos and Greenman [16], Muritala and Adewole [17], Prasad [18], Wang et al. [15], Moradian et al. [14], Shrivastava and Sharma [19], Pandit et al. [20], Antolini [21], Pandey et al. [22], Zhao et al. [23] have reviewed generation of electricity by MFC using LCB. However, the studies on factors affecting MFC’s performance using LCB and future direction of the technology using LCB as a substate are scant. Therefore, the study fills the knowledge gaps regarding these aforementioned areas and can help academicians, researchers, engineers, and technicians working in the waste-to-energy discipline. Given this, the central objective of this review article is to investigate the potential of MFCs in electricity production to achieve sustainable development and environmental protection. Specifically, the following will be discussed: (i) LCB as substrate and the various pretreatment strategies to break its crystalline structure. (ii) the configuration and working mechanism of different MFC (iii) the role of diverse EAMs used in MFC (iv) parameters affecting the performances of MFC and optimizations for performance improvement in MFCs

Lignocellulosic Biomass: A Global Concern

LCB is a plant-based renewable, abundantly available raw material produced by the process of photosynthesis using water, solar energy, and carbon dioxide present in the atmosphere. It comes under the category of second-generation biomass i.e., non-food crops such as wood chips, miscanthus, switchgrass, wheatgrass, etc., or non-edible part of food crops including rice straw, wheat straw, bagasse, corn stover, etc. [24–26]. Second-generation biomass is preferred as it counteracts the major drawbacks concerning first-generation biomass (edible food crops). The worldwide annual production of LCB is roughly 181.5 billion tonnes, which is often burned in situ to reduce transportation and storage expenditure [2, 27]. The huge amounts of biomass generated per annum can be exploited to produce various value-added materials with a focus on an integrated biorefinery approach. The smooth running of biorefinery operations employing lignocellulosic feedstock requires a cost of $50 to $97 per tonne [28]. LCB possesses a recalcitrant structure due to the presence of lignin in the outermost region which makes it difficult for enzymes to reach the cellulosic and hemicellulose portion of biomass [29]. The 80% loss of residual sugars is due to the presence of lignin which hinders the process of enzyme action [24]. Therefore, for effective removal of sugars and enzymatic saccharification of the biomass, the LCB must be pre-treated to open its intricate structure by disrupting the molecular bonds between cellulose, hemicellulose, and lignin.

Pre-treatment of LCB is a cost-intensive step, responsible for deciding the fate of the biomass valorization process, ignoring which can significantly decrease the sugar yields and increase the total cost. A promising pre-treatment technique is characterized by several conditions such as low catalyst cost or inexpensive methods for recycling catalysts, ability to process the bulk amount of biomass, efficient biomass conversion to assimilable sugars with the limited formation of inhibitors, mild operational conditions, and low capital investment [30, 31]. The pre-treatment of biomass leads to complete or partial solubilization of lignin and hemicellulosic fractions in the liquid medium and cellulose fraction in the leftover solid residue. Eventually, each fraction of biomass can be further utilized for the synthesis of respective chemicals or materials with a focus on LCB-based biorefinery to make the overall process economically viable. The pre-treatment differs significantly depending on the type of biomass as some methods solubilize the lignin, others efficiently hydrolyze the hemicellulosic fraction, while others help in reducing the crystallinity of cellulose for effective enzymatic hydrolysis [32]. Thus, the selection of the pre-treatment methods depends on the type of biomass and its lignocellulosic composition.

Disintegration of Lignocellulosic Biomass

A total of 30%–80% of biomass weight is made of lignin, hemicellulose, and cellulose, which constitute a major part of plant cell walls. For instance, the total lignocellulose content of Zea mays Linn stalk is 69% [33, 34]. Before converting biomass to energy, the intricate structure is broken down into simple monomeric units such as glucose and xylose as the complex macromolecules inhibit microbial metabolism [35].

Physical Pre-Treatment

The fermentation media containing solid biomass as substrate yields low power density due to the slow hydrolysis process, which indicates the importance of particle size for efficient production of bioenergy. The physical pretreatment of LCB involves milling, grinding, shredding, chipping, ozonolysis, and pyrolysis to increase the surface area and porosity and break down the crystallinity of cellulose fibers. A maximum power density (PD) of 102 mW/m² at 600 W ultrasonic power was obtained when the dairy manure was given ultrasonic pretreatment, which was estimated to be 241% higher in comparison to the untreated substrate [36]. Oil palm frond fiber was pretreated using the ball milling method yielding 87% glucose and approximately 82% xylose from the pretreated biomass [37]. LCB can be pretreated using the freezing method involving a change in the volume of water due to transformation from liquid to solid at low temperatures. The increase in the volume of water during freezing leads to the breakdown of the biomass cell wall. A high glucose yield was obtained from LCB using four cycles of freezing and thawing at − 18 and 22 °C, respectively [38]. Similarly, microwave-assisted pretreatment techniques have also been found effective for the pretreatment of switchgrass, miscanthus, rice straw, and Agave lechugialla [39–41].

Chemical Pre-Treatment

Acidic pretreatment is preferably utilized for LCB pretreatment due to its cost-effectiveness, high reaction rates, and high xylan to the xylose conversion efficiency of approximately 70–95% [42]. The acidic treatment breaks down the glucosidic linkages between the hemicellulosic and cellulosic portion of LCB resulting in maximum disruption of hemicellulose and partial disruption of cellulose [43]. Acid hydrolysis of LCB can enhance the amenability of enzymes and energy conversion efficiency for MFCs. The acids typically employed for pre-treatment include H₂SO₄, HCl, and HNO₃. The pretreatment process is performed at higher temperatures (> 180 °C) and shorter residence time (1–5 min), or at low temperatures (≤ 121 °C) and longer time durations (30–90 min) with low or high concentrations of acid, respectively [30]. As per the report of Borole et al. [44] corn stover was pre-treated with sulphuric acid at 158 °C at 5 min residence time to produce bioenergy via MFC yielding 1180 mW/m² PD. Wang et al. [45] reported a maximum PD of 17.2 ± 0.3 mW/ m² by using dilute sulphuric acid pretreated corn cob treated with Shewanella oneidensis MR-1. Other chemicals such as KOH, NaOH, Ca(OH)_2, and anhydrous NH₃ are used in the alkaline pretreatment of LCB [46]. Rice straw was pretreated using 2% NaOH at 80 °C for 2.5 h followed by microwave and microbial pre-treatment with Xanthomonas translucens ICBB 9762. An exoelectrogenic Staphylococcus saprophyticus ICBB 9554 yielded 337.90 mV electrical voltage; 0.39 mA electrical current; and 26.20 mW/m² power density from microwave assisted-alkali and microbe pretreated rice straw [47]. Another study showed a 3.6 X higher PD of 140 mW/m² from 5% NaOH rice in comparison to untreated solid-phase microbial fuel cells [48]. The use of alkaline pretreatment (concentrated sodium hydroxide) for sludge-fueled MFC showed a PD generation of 46.82–55.88 mW/m² [49]. Kim et al. [50] treated barley straw at 90–190 °C, with sulfuric acid (0.02–2.3%) allowing a reaction time between 1.5–18.4 min. Redox enzymes were immobilized on each electrode using graphite, cobalt, and chitosan as electron transfer mediators. The hydrolysate sample’s highest power outputs were measured to be 552, 828, and 1,254 μWcm⁻² at current densities of 1,500, 2,300, and 3,000 mAcm⁻², respectively. The maximum current density dropped as the hydrolysate dilution rose. The hydrolysates of 2.0, 1.0, and 0.5% glucose were found to have peak currents of about 1.2, 0.5, and 0.15 mA cm⁻², respectively. The rate constant (k) of the hydrolysis-based enzymatic fuel cell (EFC) system was determined to be 14.83 s⁻¹. The results indicated the effectiveness of the electron generation process using the redox reaction hydrolysate regulated by fungal redox enzymes. Although physical pretreatments are extensively employed, the downside of these procedures is stringent pre-treatment condition requirements and more expensive equipment costs. Chemical reagents may affect succeeding fermentation negatively [51–53].

Biological Pre-Treatment

Lignin-degrading microorganisms such as bacteria or fungi as whole cells, or the ligninolytic enzymes isolated from these microorganisms are used to pretreat LCB for the removal of lignin. Ligninolytic or cellulolytic microorganisms are generally used for the pretreatment process, bacterial species producing cellulase enzyme include Clostridium, Cellulomonas, Bacteroides, Bacillus, Ruminococcus Streptomyces, Microbispora, and Thermomonospora. Amongst the fungi, White rot fungi are extensively used for biological pretreatment of LCB [54]. Biological pretreatment has many advantages including mild operational parameters, low energy input, and environment-friendly nature with least or no formation of inhibitory compounds [55]. Nevertheless, it also comes with several disadvantages such as longer reaction times (hours to several days), strict processing conditions, requirement of ample space for experimentation, and at times some part of the fermentable sugars generated as a result of treatment is utilized by the microorganism as the carbon source. Because of these disadvantages, the biological pretreatment method is not considered applicable for commercial use [56]. Xanthomonas translucens ICBB 9762 is reported for rice straw pretreatment for bioenergy generation via MFC [47]. Wheat straw was pretreated with Phlebia floridensis and Phlebia brevispora to produce catholyte for its use in the generation of electricity in MFC [57]. A novel strategy was applied using three-chambered MFCs for simultaneous pretreatment of sugarcane bagasse and corncob using co-culture of Oscillatoria and Gluconobacter sp. to produce bioelectricity with PD of 8.78 W/ m³ and 6.73 W/m³, respectively [58] (Fig. 1).

Fig. 1.

Composition of lignocellulosic biomass, pretreatment strategies, and disintegrated structure

Combined Pretreatments

This strategy involves combining physical, chemical, and/or enzymatic hydrolysis. The cellulosic component is made accessible to enzymes before hydrolysis through physical or chemical modification. Enzymatic hydrolysis has the advantages of gentle operation conditions and selectivity thus producing significant quantities of reducing sugars [58, 59]. For instance, the technology of hydrothermal hydrolysis involves cooking biomass in water at a temperature of more than 100 °C. Water enters the cell structure of the biomass during this process, hydrating cellulose, solubilizing hemicellulose, and to a lesser extent stripping lignin. The method is also effective in increasing cellulose degradability to enzymes by exposing the susceptible surface area of cellulose [60]. The degradation of hemicellulose predominantly yields xylose, mannose, galactose, and acetic acid while cellulose degradation primarily yields glucose. Xylose degrades further into furfural at high temperature and pressure conditions. Similarly, 5–hydroxymethyl furfural (HMF) is produced through the breakdown of hexose while furfural and HMF break down to release formic acid. Levulinic acid is produced through the decomposition of HMF. Phenolic chemicals are produced by the partial breakdown of lignin, and they have also been claimed to be produced by the breakdown of carbohydrates [59]. These chemical byproducts, including furans and phenols, do not appear to impair MFCs’ ability to generate energy [60]. Microwave treatment is simple to execute and has a high heating efficiency. It disintegrates the complex structure of lignin rendering the sugars accessible for enzymatic attack. Jablonska et al. [61] employed microwave processing of rapeseed straw for the generation of energy. Adekunle et al. [62] utilized sonication of 20 kHz for 10 min for the fermentation of cassava peel. It was discovered that the pretreatments increased the system’s internal resistance. Conditioning at a higher temperature, long residence time, and lowering the pH of the extract facilitated the acclimatization and colonization of the anode by the microorganism flora, resulting in a faster and more consistent action [62]. Kim et al. [63] pre-treated rice straw with a 10% (w/v) FeCl₃ solution for one hour before heating. This resulted in maximum degradation of hemicellulose leaving approximately 80% glucan intact. Following rice straw pre-treatment, almost 70 percent of ferric iron was converted to ferrous iron. The electrons that were transferred to ferric iron originated from xylose or lignin. The functioning of the ferrous fuel cell revealed that the maximal open circuit potential rose linearly with increasing pH and ferrous iron concentration (0.001–0.008 mol/L). At pH 7, the maximum PD was 1290 mW/m². The power density achieved with FeCl₃ was much lower than with pure FeCl₂, probably due to the presence of other chemicals in the hydrolysate. A high-pressure steam of 433–533 K is applied to treat the LCB for a required duration of time with subsequent release of steam to depressurize the vessel to atmospheric pressure. The combination of high temperature and sudden explosive decompression causes the disintegration of hemicellulose, which is liberated in the water-soluble fraction called hydrolysate. Further, a large portion of the original cellulose component of biomass is preserved, and only a little disintegration occurs at mild reaction conditions [64]. Zuo et al. [65] explored the conversion of hemicelluloses present in corn stover to soluble sugars by subjecting the biomass to steam explosion or acid-incorporated steam-explosion processes. Maximum PD of 371 mW/m² in neutral hydrolysate and 367 mW/m² in acid hydrolysate were obtained [66] (Table 1).

Table 1.

Bioelectricity generation using varied substrates and MFC designs

MFC Type	Design	Modifications made	Lignocellulosic source	Treatments given	References
SAIDKOCC MFC (SAIDKOCC10091720, Fujian, China)	Anode: made up of copper, 40 cm2 area. Cathode: made up of zinc, 62.5 cm2 area	NR	Tangerine residue	Tangerine waste was passed through an extractor to obtain juice	[67]
NA	NA	Ranges of slurry concentration, temperature, pH, slurry pretreatment, and resistance	Banana peel slurry	NA	[68]
Dual chambered MFC	Anode: Rectangular made of stainless steel modified with binder-free activated charcoal. Cathode: cylindrical graphite rods of 10 cm length and 0.5 cm radius	At anode activated charcoal was used because of its large surface area-to-volume ratio	Banana peel	The peels were dried at 105 °C, ground, and sieved through 44 mesh-sized sieves. For slurry preparation, the peels were washed and ground in a mixer grinder	[69]
Dual chambered MFC	Anode: rectangular stainless-steel mesh. Cathode: cylindrical graphite rod	NA	Pretreated sweet lime peel slurry	NA	[70]
Two-compartment MFC	Graphite felt discs-based anode and cathode (Φ = 3 cm)	NR	Corn stover	NR	[71]
Dual chambered MFC	Cylindrical anode and cathode chambers with acrylic sheet material. Total electrode surface area: 50 cm2	NR	Unsterilized clarified cashew apple juice	Cashew apple juice was subjected to mechanical pressing, centrifugation, and filtration	[72]
Single-chamber air cathode	Anode: graphite brushes Cathode: custom 5-layer membrane electrode assembly with a 0.5 mg cm − 2 60% PtC Gas diffusion layer	A sampling port was added and the airtightness of the anodic chamber was increased	Cassava peel extracts	I. Sonication at 20 kHz for 10 min II. Anaerobic fermentation was carried out in a bioreactor at 50 °C for 48 h III. Fermentation was allowed to continue in a fraction of the fermented peel pulp-whyme mixture at 23 °C for a further 168 h	[62]
Dual-chamber MFCs	Electrode material: Carbon cloth Anode geometric area: 2.5 cm × 2.5 cm Cathode geometric area: 2.5 cm × 3 cm	NR	NR	NR	[73]
Single-chamber air cathode	Cathode: Carbon cloth–based cathode (10 cm × 10 cm) Anode: A ring of graphite bushes (surface area: 9,600 m2)	NR	Cassava peels	Cassava peels were removed, and ground up to a particle size of 1 mm	[61]
Two-chamber microbial fuel cell	Cathodic: Height: Dia: Length (in.): 15:9:- Anodic: H:D:L: 15:9:NR Salt bridge: NR:4:4	NR	Carbo manure	NR	[61]
Dual-chamber (H type), two-electrode fuel cell	Anode: Graphite (projected surface area of 0.5 dm2) Cathode: Pt/C coated graphite	NR	Raw rapeseed straw	1. Hydrothermal treatment:120 °C 1.2 bar for 2 h. 2. Microwave treatment: (2.45 GHz, 600 W for 2 h, 3. Enzymatic treatment: cellulase, xylanase, and cellobiase	[61]
Enzymatic fuel cell	NR	Gold electrode	Barley straw	Dilute acid pretreatment and hydrolysate preparation: Reaction temperatures: 90–190 °C, sulfuric acid concentration: 0.02–2.3%, and reaction time: 1.5–18.4 min	[50]
Direct lignocellulosic biomass fuel cell	Anode: carbon cloth Cathode: Pt/C coated carbon cloth	PMo12 with an active electrode area of 40 cm2	(1) Commercial Sodium Lignosulfonate (2) a Kraft lignin (KL) (3) Enzymatic hydrolysis lignin residue obtained after SPORL pretreatment pretreated poplar (4) Two spent liquors from SPORL pretreated lodgepole pine	Pre-oxidation with the help of lignin. Treatment with Conc.H3PO4 (85 wt%)	[74]
Enzyme-based two-compartment biofuel cell	Glassy carbon electrode (GCE, diameter of 3 mm) with 0.05 wt% Nafion solution and Multi-walled carbon nanotube; Cathode: Composed of 10 mg/ml laccase and 1 wt% glutaraldehyde	Laccase/MWNTs/GCE	Corn cob, Cornstalk	1. Enzymatic pretreatment 2. Physico-chemical pretreatment	[75]
Single chamber new solid-substrate MFC	Trial 1: two graphite rods with a total surface area of 121.6 cm2 as the primary anodes. Trial 2: carbon-fiber brushes with titanium stems (14 cm (O.D.) × 33 cm (L) as anode. Cathode: Platinum coated (0.45 mg Pt/cm2) carbon cloth	Bulking material: Trial 1: Graphite granules combined with corncob pellets (4:5 (v/v)) Trial 2: Plastic packing material (Jaeger Tri Packs, 1.25" diameter with corncob pellets (1:1 (v/v))	Commercially available corncob pellets	Corncob pellets were sieved to a particle diameter of 2.36–4.76 mm and an effective porosity of 0.3	[76]
Ferrous-based fuel cell	Cathode: Pt/C (10 wt. % Pt/C, 0.5 mg/cm2) coated carbon cloth. Anode: Carbon cloth Projected surface area:15 cm2	NR	Rice straw	1. FeCl3 pre-treatment: rice straw pretreated with 0.1 mol/L FeCl3 solution at 10% (w/v) solid loading. 2. Enzymatic hydrolysis: a mixture of cellulase from Trichoderma reesei and β-glucosidase from Aspergillus niger	[63]
Air–cathode MFC	Cathode: Pt-coated carbon paper (2 × 2 cm2, 2 mg cm−2), surface area: 1m2/m3 Anode: A plain carbon paper (3 × 3 cm2, nonwet-proofed), surface area: 2.25m2/m3	NR	Canna indica	Air-drying and milling	[77]

Microbial Fuel Cells

The two predominant categories of LCB-powered fuel cells are indirect biomass fuel cells and direct biomass fuel cells. The indirect biomass fuel cell requires pre-conversion of biomass to readily assimilable sugars such as glucose and xylose. These are classified based on their temperature requirement; solid oxide fuel cells and direct carbon fuel cells (high temperatures), and microbial fuel cells (low temperatures). The direct biomass fuel cells, in contrast, bypass the pre-processing and pre-conversion stages and directly produce electricity [78]. The microbial fuel cell is an emergent technology that focuses on a greener approach to produce bioelectricity by oxidizing both the organic as well as inorganic compounds of the biomass using a biocatalyst. It follows a similar working principle as that of traditional fuel cells of producing electrical energy from chemical energy but with the help of microbial activity [79].

MFC Reactor Types and Configuration

As appropriately designed MFCs are crucial for their performance, researchers over the years have developed many MFC designs with better performance. The widely used types of MFC belong to single and dual-chambered categories. A single-chambered MFC has a simple and more affordable design because of the absence of a cathode chamber, catholyte, and proton exchange membrane. In this type, the cathode is exposed to the air and water, usually in the atmosphere (outside) and the anolyte (inside), respectively [80] (Fig. 2a). A dual-chambered MFC comprises two divisions called the anode (contains anolyte) and cathode chamber (contains catholyte) separated by a proton exchange membrane (Fig. 2b). The microorganisms oxidize the substrate in the anaerobic chamber where the anode is located and release CO_2, protons, and electrons to the anode. The protons generated through the proton exchange membrane are passed over to the cathode which further reacts to form H₂O in the presence of oxygen [70]. The electron transfer at the cathode occurs through an external circuit leading to electricity generation. The electron transfer mechanism in an anode chamber can be of various types including direct (through cytochromes, periplasmic extensions, nanowire structures, or conductive pili), and indirect with the help of redox-active mediating molecules (flavins, quinones, and polyphenols) [81]. These mediators act as electron carriers and transport electrons by linking the redox reactions occurring inside and outside of the microbial cells. Platinum is a commonly reported catalyst for lignocellulosic substrates [61, 63, 76, 77]. The electrons extracted by the bacterium from the substrate must have access to the electrode. Either chemical mediators, such as phenazine derivatives, conductive appendages, or coupled intercellular circuits can transport the electrons to the electrode. Clarified cashew apple juice obtained through mechanical pressing, centrifugation, and filtration was used by Priya et al. [72] to generate energy in a dual-chambered MFC. The current system was able to generate 31.58 mW/m² PD, 350 mA/m² current, and a maximum open circuit voltage (OCV) of 0.4 V. In addition to these two popular designs, various modifications have been made to the construction and design of MFCs. Fan et al. [75] created a unique two-compartment laccase/ multiwalled carbon nanotubes/ glassy carbon electrode MFC to exploit the catalytic activity of laccase. The specially designed enzyme-based fuel cell displayed high steadiness and potential. The power density of the cell reached 45.1 and 53 µWcm⁻² when fueled by corn cob and corn stalk, respectively. Zhao and Zhu [78] created novel polyoxometalates (POMs)-mediated direct biomass fuel cells that directly converted lignin to electricity at low temperatures with a high-power output and Faraday efficiency. It offered several advantages, such as suitability for mild operating conditions, elimination of the need for expensive noble metal catalysts for anode, and higher degrees of chemical and thermal stability.

Fig. 2.

Categories of MFCs a Single chambered, b Dual chambered

Microorganisms in MFCs

MFC generates electricity by using the catabolic activities of microbes. The microorganisms oxidize organic matter at the anode and electrons are further transferred to the cathode via an external circuit resulting in electricity generation [82–84]. The microorganisms used in MFC are defined as electrochemically active bacteria (EAM) (electrogens), electricigens, anode-respiring bacteria (ARB), and exoelectrogens because of their capability to transfer the electrons outside the cells [85–87]. The EAMs are present in the environment where abundant solid-state electron acceptors or donors are naturally present, posing selective pressure on the EAMs to dominate such type of ecological niche [88]. The microorganisms used in MFCs for bioelectricity production are generally found in soil, wastewater systems, activated sludge or fresh water, and marine sediments. The environment that has a predominant state of solid-state electron donors includes iron-rich materials, metallic iron, and carbonaceous materials which can transfer electrons between metabolically co-dependent microbes [89–93]. The isolation of EAMs is considered challenging because the use of non-selective media leads to the loss of electroactivity of microorganisms in the absence of electrochemical technologies. Various studies showed that electrochemical strategies could be employed to isolate the EAMs without any loss in their electroactivity. The electroactive microorganisms can be enriched in situ by using the electrodes resulting in new strains of EAMs [94]. For instance, for in situ enrichment of EAMs from sediments, naturally occurring electrodes in benthic or sediment MFC can be exploited. Sediment MFCs operate with the anode inserted in the anoxic sediment and the cathode in water. The organic matter present in the sediment and O₂ in the water act as the source and sink of electrons, respectively. Another approach for in situ colonization of electroactive organisms from groundwater included the passing of a customized electrochemical reactor equipped with two electrodes at both oxidizing and reducing potentials, in the groundwater of depth 1478 m [95]. Geobacter, Pseudomonas, Clostridium, Shewanella, and Desulfuromonas are some species that are often inoculated into MFCs for the generation of hydrogen and electricity. These microbes are also able to oxidize butyrate, ethanol, propionate, acetate, or lactate as substrates [96] Fungi have a high capacity for cellulose degradation during the pre-treatment phase [97, 98]. The white rot fungi produce a multienzyme complex comprised of ligninolytic enzymes [14]. Wheat straw cellulose was degraded by white rot fungus, resulting in 35% saccharification efficiency based on consolidated bioprocessing (CBP) in which cellulose hydrolysis and electricity production processes occurred concurrently [99].

Genetically Modified Microorganisms (GMOs)

To accomplish the aim of achieving a high concentration of electrons at the anode, microorganisms are either mutated or altered genetically to form high recombinant strains (Table 2). Various biomolecular tools are utilized to tailor the microorganisms in a way to optimize the functionality of desired reactions, manipulating the pathways required during the transfer of electrons, and thus facilitating the high yield of electricity [14]. So far, the microorganisms are engineered genetically through the directed evolution of redox enzymes, heterologous gene expression, and metabolic engineering [100, 101]. A consortium of bioengineered Klebsiella pneumonia and S. oneidensis was designed to efficiently harvest electricity from corn stalk hydrolysate [102]. The lactate output could be increased in K. pneumoniae via the deletion of phosphotransacetylase and alcohol dehydrogenase genes which eliminated the ethanol and acetate pathways. Further, for lactate transportation, two genes lac dehydrogenase and lac transporter were expressed in the strain. Consequently, the hydrolysate was fermented by engineered K. pneumonia to lactate which was further used as fuel by S. oneidensis for electricity generation. Moreover, the efficiency of extracellular electron transfer (EET) was improved in S. oneidensis by expressing a heterogenous flavins synthesis pathway gene isolated from Bacillus subtilis [102]. In S. oneidensis MR-1, two gene clusters for flavin biosynthesis and metal-reducing conduit biosynthesis were coexpressed which improved the EET capacity in MFCs, and an increase in current density of approximately 110% [103]. S. oneidensis MR-1, a model microorganism for bioelectricity generation is unable to utilize the carbon sources as substrate. A 3X increase in power output was achieved upon the overexpression of three endogenous and one exogenous gene [104] (Table 2).

Table 2.

Various genetic modification techniques used to improve microbial performance for bioenergy generation in MFC

Microorganism	Genetic modification	Importance	MFC performance	References
Escherichia coli	Gold binding motifs were introduced in two scaffolds: eCPX and FimH	Engineered to have an increased affinity for gold	Power density up to 0.27 mW/ m2 i.e., a tenfold increase in comparison to wild strains	[105]
Shewanella oneidensis MR-1	Expression of adenylate cyclase (an exogenous gene) isolated from Beggiatoa sp.	Enhanced bidirectional extracellular electron transfer capacities	-	[106]
Escherichia coli	Peptide-mediated binding of gold nanoparticles	Enhanced the bioelectrochemical charge transfer	Increase in power density by 4 times	[107]
Shewanella oneidensis	Overexpression of SO_1522, a D-lactate transporter gene	Increased substrate transport. D-lactate was metabolized at a 61% faster rate	1.3 times increase in photocurrent production	[108]
Escherichia coli	Expression of CymA and MtrCAB isolated from S. oneidensis	Cytochrome maturation system Ccm was constitutively expressed in native E. coli	Four-fold increase in current output	[109]
Pseudomonas aeruginosa	Overexpression of rhla gene	Higher production of surfactants (rhamnolipids)	Power output increased by 2.5%	[110]
Shewanella oneidensis	Overexpression of diguanylate cyclase (YdeH)	Enhanced biofilm formation and c-di-GMP synthesis	Maximum power density increased 2.8 times and 3.4 times higher stable current density	[85]

Xylose Degrading Microbes

Xylose is one of the integral components produced during the disintegration of LCB. Thus, the xylose conversion to electric energy via MFC is considered an imperative step. However, due to its sluggish rate of utilization and ineffective metabolic pathways, xylose metabolization presents many challenges. The S. oneidensis MR-1 strain GS was genetically modified by Li et al. [73] to contain the Gxf1 gene, which codes for the xylose facilitator from Candida intermedi, as genes that code for the xylose oxidoreductase pathway from Scheffersomyces stipites. The gene xylT of Clostridium acetobutylicum that encodes a D–xylose–proton symporter was incorporated to make S. oneidensis competent to metabolize xylose. In a dual chamber MFC setup, the recombinant S. oneidensis strain GS produced an output voltage (OV) of ~ 73.4 ± 5.8 mV. Under aerobic conditions, the modified strain could produce a PD of 2.1 ± 0.1 mW/m² by consuming xylose at a rate of ~ 35.2 M/h. The strain GS was also able to consume xylose at a rate of about ~ 17.2 μM/h with fumarate (electron acceptor) and xylose (electron donor) under anaerobic conditions [73].

Mixed Microbial Community

MFC can also use a mixed microbial community composed of cellulolytic and exoelectrogenic microbes. Ren et al. [111] utilized C. cellulolyticum, a cellulose-degrading bacterium, and Geobacter sulfurreducens, an electrochemically active bacterium for power generation from sodium carboxymethyl cellulose. Rismani-Yazdi et al. [112] revealed that rumen bacteria can hydrolyze microcrystalline cellulose while producing electricity and methane simultaneously. Cheng et al. [113] generated electricity from pure cellulose using wastewater as inoculum. Rezaei et al. [114] discovered that Enterobacter cloacae could be employed for both cellulose degradation and power production. In addition to pure cellulose, LCBs such as cornstalk [66], corncob pellets [76], and rice straw [115] were also used. Zang et al. [77] treated Canna indica (canna), a lignocellulosic aquatic plant in an air–cathode MFC, with rumen microbes. Shrestha et al. [71] observed that Geobacillus sp. strain WSUCF1 cultures included endogenous hydrolyzing enzymes (cellulase, hemicellulases, and laccase), allowing it to treat unprocessed maize stover without any physicochemical preparation by lignocellulolytic enzymes, biofilm production, and transmembrane & extracellular electron transport channels [116]. The Table 3 summarizes the MFC performance from different lignocellulosic biomass and their pretreatment strategies.

Table 3.

Various pretreatment methods of lignocellulosic biomass used for bioenergy generation in MFC

Lignocellulosic biomass	Method of pretreatment	Microorganisms used for bioenergy generation	MFC performance	References
Corn stover	Physical pretreatment- size reduction	Lsc-8 strain	6.19 mW m − 2	[117]
Wheat straw	Biological pretreatment of wheat straw by P. floridensis, P. chrysosporium, and P. brevispora	Pichia fermentans	34.99 mW m−2	[118]
Sago hampas	Enzymatic pretreatment with dextrozyme	Clostridium beijerinckii SR1	56.5 mW/cm2	[119]
Corn straw	Physical and chemical pretreatment- size reduction, and acid hydrolysate	Shewanella oneidensis MR-1	660 mW/m2	[66]
Raw rapeseed straw	Physical and enzymatic pretreatment- Hydrothermal, microwave treatment, and enzymatic hydrolysis	Culture from post-fermentation sludge	54 mW/m2	[61]
Cornstalk	Hydrothermal liquefaction (HTL)	Culture from anaerobic digester of municipal wastewater	680 mW/m3	[85]
Rice straw	Physical pretreatment- size reduction	Mixed culture of cellulose-metabolizing bacteria	190 mW /m2	[120]

Key Parameters Affecting the Performances of MFC

The microbial metabolism in MFC depends on various parameters such as type of substrate, operational as well as electrochemical conditions. [121]. To achieve the commercialization of MFCs, several steps have been initiated to improve the power efficiency of MFCs such as by modification of membranes or electrodes, alteration in the microorganisms at the molecular level, changing the operational parameters, and development of new designs [122, 123]. The following section discusses the operational and electrochemical parameters affecting the performance of MFC, including substrate (Fig. 3).

Fig. 3.

Factors affecting performance of MFCs

Substrate The substrate, which acts as a source of nutrition for microorganisms is an imperative factor that affects the performance of MFC. It can impact the start-up time of MFC, the complex matrix of the biofilm formed at the anode, current efficiency, and power density [124]. Different substrates tested for MFC performance include low molecular weight carbohydrates, alcohols, amino acids, inorganic compounds (sulfate), organic acids, complex substrates (cellulose, chitin, molasses), and wastewater (food processing, domestic, recycled paper, etc.) [122]. The EAMs generate electricity mainly by utilizing glucose. More recently, studies have investigated the utilization of xylose sugar from the hemicellulosic component of LCB. Rojas-Flores et al. [125] succeeded in producing voltage and current maxima of 1.191 ± 0.035 V and 1.43973 ± 0.05568 mA, respectively from tangerine wastes employing MFC made up of zinc and copper. The high voltage was attributed to the high levels of polyphenols and carbohydrates present in the fruits, and the low electrical resistance of the substrate contributed to the gradual increase in the electrical conductivity. Verma and Mishra [69] achieved maximum OV of 56 ± 1.5 mV, 210 ± 0.5 mV, and 307 ± 1.5 mV, with dried banana peel with S. cerevisiae (BDY), banana slurry (BS), and banana slurry with S. cerevisiae (BSY), respectively. At an external resistance of 1000Ω, there was a spurt in the voltage and current after the substrate was refilled into MFC (0 h–96 h). Verma and Mishra [70] recorded a maximum OCV of 792.33 ± 1.53 mV and 481.33 ± 3.51 mV and corresponding PD of 210.66 ± 6.11 mA/m² and 640.0 ± 2.0 mA/m² with co-culture of isolated cellulolytic bacteria and S. cerevisiae in pretreated sweet lime peel slurry.

pH The pH values are also among the most influential parameters influencing the reactions mediated by microorganisms at both anode and cathode. When a buffer was introduced to the cassava extract solution that had already been pre-fermented at 50 °C, the peak voltage, current density, and power increased by 36%, 114%, and 150%, respectively. It has been demonstrated that neutral conditions promote substrate breakdown more than alkaline or acidic ones. The pH changes made during fermentation increased the extract’s conductivity by as much as 168%. The improvements in the MFC performances could be attributed to the conductivity enhancement [126]. The build-up of protons results in liquid acidification at the anode and the electrolyte alkalization at the cathode [127]. Zhang et al. [128] reported faster removal of chemical oxygen demand under acidic conditions. It was also observed that at pH ≤ 5.0, anodic biofilm got damaged and the concentration of cells reduced dramatically while at pH ≤ 4.0 the MFC failed due to changes in the composition of the microbial community [4, 128, 129]. The pH and concentration of soluble organic material increases during the fermentation of cassava peel to certain levels which adversely affects the power generation capacity of MFC [130]. Igboamalu et al. [131] stated that all the MFCs have a specific saturation potential that depends on the pH.

Temperature Temperature has a considerable influence on several variables that can greatly affect MFC power production including substrate conductivity, diffusion coefficient, charge transference, and metabolic activities of EAMs [132, 133]. Mesophilic microorganisms (25–40 °C < 45 °C) are extensively used in the MFCs [134]. Microbial reactions at low temperatures slow down thus rendering MFCs non-operational [135]. MFCs containing psychrophile EAMs (10–15 °C < 20 °C) have been reported to achieve high Coulombic Efficiency (CE). Ren et al. [136] analyzed the effect of temperature on miniaturized MFC and observed 49–53 °C as the optimal temperature. An increase from 21 °C to 49 °C resulted in an improvement of current density by 282%. Li et al. [137] studied the influence of temperature on a two-chamber MFC separated by a proton exchange membrane (PEM) over a wide range of 10 °C to 55 °C. The results obtained were comparable to those of Moon et al. [138] who reported maximum PD at 37 °C. Verma et al. [68] observed that the essential features to attain high power density are temperature, resistance, and pretreatment. A maximum PD was achieved with banana peel slurry and baker’s yeast as inoculum at a slurry concentration of 41.47 ml/L, pH < 7.25, and resistance < 285 Ω.

Final electron acceptor In most of the MFC for the cathodic reactions, oxygen serves as the final electron acceptor. Dissolved oxygen (DO) level is a critical factor that decides the power output of MFC. The use of a soluble electron acceptor, such as ferricyanide, can boost electric current output. When ferricyanide was used in place of dissolved oxygen, maximum power output increased by 50–80% [139]. The measured PD reached 0.72 mW cm⁻², compared to less than 0.1 mW cm⁻² using DO possibly due to the high mass transfer of ferricyanide and lower activation energy for the cathodic reaction [140]. Other electron acceptors employed include H₂O₂ [141] and potassium permanganate [142]. In a two-compartment continuous-flow MFC, H₂O₂ significantly increased PD over aeration (oxygen) [141].

Electrode The performance and cost of MFC are greatly influenced by electrode materials. The chosen electrode material should have a broad surface area, conductivity, and biocompatibility to be able to provide a suitable platform for the microbial cells to adhere and grow. Surface properties including the porosity and roughness of an electrode can influence biofilm growth and consequently electric power generation [74]. The yield of power density is directly affected by the electrode surface area [61, 63, 73, 76, 78]. The preferred electrode materials are graphite, carbon [62, 69, 70, 78], acrylic sheet, and gold [63, 72]. Carbon-based materials like carbon papers and graphite rods are widely utilized for the construction of anodes. Nano-composites are widely used due to their good conduction, catalytic potential, and mechanical stability [143]. Platinum-coated carbon materials are usually used for an air cathode. The deposition of Platinum on an air–cathode increases the oxygen affinity and decreases the activation energy of the cathode thereby improving power output. As per the report of Razalli et al. [144], Semantan bamboo was acid hydrolyzed to create a polyaniline/crystalline nanocellulose electrode. The electrochemical impedance spectroscopy of the electrode showed a lower value of e-charge transfer resistance (RCT) i.e.,148 cm² compared to the bare (177 cm²) and polyaniline (156 cm²) electrodes, indicating that polyaniline and nanocellulose inclusion can impedes the charge transference rate. Cathode modification with metal ions such as Fe (III) or Mn (IV) can also significantly increase electricity output [145, 146]. Thermal modification of carbon felt (CF) electrodes with HNO₃ and H₂O₂ in different ratios was achieved by Zhao et al. [147, 148]. The modification achieved a reduction of the anodic charge transfer resistance to 785.2 mW/ m², which was 51.1% lower than the native electrodes.

Biocathode is increasingly used due to its low cost, easy-to-maintain power supply, and employability as self-regenerate catalysts [149]. Biocathodes are reported to perform better than abiotic cathodes as microorganisms present at the cathode have a major role to play in improving the performance of MFC [149, 150]. Ramanaiah et al. [151] evaluated the working of MFC with abiotic cathodes and biocathodes and observed that the biocathode-containing MFC had better power and current densities of 54 mW/m² and 122 mA/m², respectively. Yaqoob et al. [152] opined that the type of electrodes in terms of their size influences the internal resistance. Verma and Mishra [68] fabricated a single-layer activated charcoal-stainless steel (AC-SSM) composite anode employing a binder-free and drying method, improving the microbial adhesion on the surface of the metal, consequently improving current generation. The oxygen utilization capability of algae is being investigated for its potential application in MFC. Cui et al. [153] used Chlorella vulgaris as a biocatalyst and reported that maximum PD and CE of 1926 21.4 mW/m² and 6.3 0.2%, respectively at 2500 mg COD/L.

Ionic Transport The rate of electrochemical reactions is dependent on the electrolyte’s ability to ensure effective ion transport [154]. The chance of ion transport being rate-limited grows as the electrolyte’s ionic conductivity decreases. Cations such as H⁺ being slow, result in anolyte acidification and considerably limit power generation. The anolyte acidification can be eliminated with alkaline catholyte via electrolyte recirculation alleviating the pH drop [155] besides the presence of inorganic ion buffers which stabilize the pH of the electrolyte [156].

Clogging Media depletion occurs because of extended cathode leakage despite considerable cathode leak-proofing and evaporation, which allows oxygen to rapidly enter the anode compartment. The agglomeration of corncob pellets resulted in clogging as reported by Gregoire & Becker [76]. Methanogenesis can also contribute to the blockage due to its tendency to form gas bubbles which impede aqueous flow. The problem of clogging can be prevented by mixing the media with porous components like corncob pellets. The oxygen exposure is successful at suppressing methanogens, which is consequently beneficial to decreasing the proportion of electrons redirected from anode respiration to methanogenesis, hence increasing the current and power output by 157%. As per the report of Shrestha et al. [71] The "Beads / No beads" assays (calcium alginate encapsulation/no encapsulation) showed that Geobacillus sp. strain WSUCF1 cells stuck to the anode surface more readily in the no-beads-MFC than in the beads-MFC and control. Compared to beads-MFC (37 mW/m²), the power density in no beads-MFC was 2.9 × greater (107 mW/m²). No-beads-MFC’s performance was unaffected by the substitution of the medium (42 and 46 mW/m² before and after, respectively), whereas beads-MFC’s power output sharply decreased (e.g., from 14 to 0.16 mW/m²). When compared to MFC without beads, the riboflavin concentration was 1–3 × higher in the former.

Oxygen flow rate The highest oxygen flow rate was necessary to provide a high yield of electricity. This was predicated on the idea that when more protons were accepted from the anodic chamber, power generation would grow along with the flow rate of oxygen. Using a two-chamber MFC, Jatoi et al. [157] revealed that the maximum power generation from cow dung and carbo manure was 229 mv/l and 270 mv/l, respectively.

Choice of catalyst Due to the complicated lignin structure, lignin oxidation proceeds slowly without the aid of catalysts. The polyoxometalate (POM) holds immense potential to generate energy from lignin. Zhao and Zhu [78] reported that the PD increased from 0.014 to 0.10 mW cm⁻² on increasing the PMo₁₂ concentration from 5 mm to 0.25 mm. Cations may contaminate the proton exchange membrane (PEM), decreasing the membrane’s ability to conduct H⁺ and increasing the cell’s apparent internal resistance. Other elements, such as the use of liquid catalysts and oxygen ventilation, were said to be important in enhancing MFC performance. It has been observed that PMo₁₂ works well as an electron transporter and catalyst to move electrons from lignin to oxygen. The exterior vessel’s temperature, H₃PO₄ concentration, and lignin concentration are all directly correlated with the cell’s power density.

Areas of loss The three areas of loss connected to MFCs are activation, ohmic, and concentration. The requirement to colonize the electrodes and help the microbes adapt to the environment is important as the ohmic and activation losses are prominent. Adekunle et al. [62] used cassava peels for energy production in a single-chamber air cathode. For about 4 days, the recorded voltage increased steadily until it hit a plateau, which was a sign of improvement in the microorganisms’ metabolic activity. The loss of the MFC’s accessible organic material was credited for the voltage drop. By the end of 26 days, the three zones of loss had become noticeable. Electricity could be generated directly from Canna indica (Canna), in an air–cathode MFC seeded with rumen microbes, with a maximum PD of 0.405 W/m³ with an open circuit value of roughly 0.6 V. This canna-fed MFC had a total CE of 400 C/g and 767 C/g in the first and second cycle, respectively. As the current increased, the voltage decreased linearly, indicating that ohmic loss is the result of resistance between the electrodes and the electrolyte. The internal resistance of 513 Ω was predicted from the linear range of the polarisation curve’s slope. Acetate was the predominant volatile fatty acids (VFA) species in the Canna-fed-MFC throughout the experiment [150].

Conclusion and Prospects

MFCs are emerging as a state-of-the-art technology for sustainable energy generation. These are relatively low-cost, but efficient tools for exploiting waste material like LCB which is renewable, carbon–neutral, and sustainable. The biomass recalcitrance to sugar release is one major drawback that can be mitigated by using an effective pretreatment technology. The EAMs play a pivotal role in energy conversion, therefore various types of EAMs including fungi, bacteria; aerobic and anaerobic, xylose-degrading microbes, mixed microbial communities, and GMOs are used. The downside of MFC technology is that it has not yet been established on an industrial scale. However, commercializing MFC is not impossible with optimization and modifications. Maximum productivity and efficiency of the MFC are strongly dependent on start-up and operation conditions, and taking these into account, an optimal microbial fuel cell can be produced. The promising approaches in this regard are omics and functional analysis, genome mining for the optimization process, and biofilm engineering. Consolidated bioprocessing with the development of artificial consortiums can help in the integration of pretreatment, saccharification, and bioelectricity generation. The other drawback is low power density which can be improved upon by the isolation of potential microorganisms which can efficiently transfer the electrons to an anode or engineering of microorganisms by recombinant DNA technology. In addition to the technical considerations, novel process design, scale-up studies, durability, and profitability are urgently required for practical applications. The construction design of membrane, anode, and cathode using inexpensive material can play an important role in boosting efficiency and economizing the design. The modification at the anode and cathode can alleviate the electron transfer rate and oxygen reduction efficiency, thus improving the electric output. Another option that holds promise is the integration of MFC with other renewable resources such as solar or wind for improved performance and reliability. Finally, the studies on the acclimation of MFCs under diverse environmental conditions under the factors that govern their performance and operation can pave the path for the future development of MFCs.

Declarations

Conflict of interest

All authors declare that they have no conflicts of interest.