*10.1.2 Synthetic anode materials*

It is quite evident that 3-dimensional carbon fiber (non-woven) can achieve a maximal current density of up to 31 A/m<sup>2</sup> which is prepared by electrospinning and blowing the solution. The performance and efficiency of MFCs also depends on the system architecture, based on these 3D materials [78]. Double-sided air cathode reduces the boundaries of mass transfer. The stainless steel frame was

used for this design as a current assimilator and a carbon fiber support in the 3D matrix. In another study [79], it has been shown that an upgraded adaptation of the carbon-based multi-brush anode achieved admirable power generation. The power generated is similar to that obtained with a carbon anode with a single brush design. Because of cathodic limitations, the MFC system [80] gave a comprehensive comparison of carbon-based material for anodes, like graphite, carbon fiber veil, polycrystalline carbon rod, glossy carbon rod, graphite foil. The maximal current density attainable was calculated using a standardized biofilm grown in domestic wastewater. At 30°C, graphite, and polycrystalline carbon-based rods, both reached catalytic currents peaks of around 501 μA cm−2. By comparison, carbon fiber veil or paper-based material delivered a 40.1% higher current than graphite anode due to its large, microbial rich surface area [80]. In comparison with steady-state reactor, the rotational motion of carbon brush anodes in the tubular microbial fuel cell resulted in a 2.6 times rise in performance. The rotation was adequately mixing the nutrient and minimizing the limitation of mass transport. In general, several studies have shown that the existence and electrode content affected the kinetics of the biocatalyst. It has also been shown that the internal resistance is a major aspect affecting the overall performance. The use of 3-dimensional anode models, like carbon nanotubes (CNTs), nanofibers (CNF), gold/poly (e-caprolactone) microfibers (GPM), and gold/poly (e-caprolactone), to reduce the internal resistance increasingly preferred in microbial fuel cells. 3-dimensional anode material has less internal resistance than two-dimensional anodes. Such anode materials serve to increase the efficiency of nutrients, H<sup>+</sup> , and O2 transfer via biofilm as compared to macroscopic carbon-based paper and planar gold-based anodes. Chemical assisted surface alteration of the CNT/CNF-based anodes has been demonstrated to reduce kinetic losses and cellular toxicity. Ren et al. [81] investigated vertically aligned CNT, randomly aligned CNT, and spin-spray layered CNT. The studied nanotube-materials have a 4000 m−1 very large surface area to volume ratio which is very huge. The results showed that CNT-based anodes attracted more electrogenic microbes than bare gold, resulting in a thicker and more stable formation of biofilms. Using CNTs in a miniature MFC device, a maximal power density of 3321 W/m3 was achieved [81]. This was 8.5 times greater than that attained with the 2D-electrode systems.

#### **10.2 Composite anodes**

Composite anodes have intrigued extensive interest recently. These materials were utilized to attain synergistic effects with two or more materials to alter original content, resulting in increased anodic kinetics efficiency.

#### *10.2.1 Graphite-polymer composites*

Tang, Yuan, Liu, & Zhou prepared a nano-structured capacitive layer of modified 3D anode consisting of core-shell nanoparticles derived from titanium dioxide (TiO2) and egg albumin (EWP). This was built into a loofah sponge carbon (LSC) to achieve an efficient 3-dimensional electrode. The LSC's coating with TiO2 and heat treatment caused tiny particles to cover its entire surface. The resulting altered anode supplied greater power than a graphite anode. The increased power was associated with the increased electrochemical capacity of 3-dimensional anodes and to the synergistic effects of carbon derived TiO2 and EMP with good characteristics like more surface area, improved biocompatibility, and favorable surface functionality for easier extracellular electron transport [82]. The anodes of opencelled carbon scaffold (CS) and carbon scaffold graphite (CS – GR) were created by

**111**

*Treatment of Dairy Wastewaters: Evaluating Microbial Fuel Cell Tools and Mechanism*

fibers achieved a maximal current density at a peak of 35.8 A/m<sup>2</sup>

whereas the power density improved by 1.49 times.

*10.2.3 Multi-walled carbon nanotubes composite*

*10.2.2 Carbon nanotubes composite*

significant attention lately.

*10.2.4 Graphene anodes*

as large surface area (up to 2600 m2

carbonizing the microcellular polyacrylonitrile (PAN) and composite PAN/graphite (PAN – GR). The PAN-GR was created by utilizing supercritical carbon dioxide (Sc-CO2), as a practical foaming agent. The maximal current density achieved with a CS altered anode was 102% greater than that with carbon felt. Improved performance has been referred to as enhanced hydrophilicity and biocompatibility caused by carbonization. Carbon nanofibers with improved graphite fibers and reduced nanotube-coated graphene oxide/carbon scaffold promise new composite anode materials. Using carbon nanofibers as anodes for MFC modified graphite

coated scaffold anode device with reduced graphene oxide/carbon obtained a power

used in combination with granular graphite (MFC-GG) in a tubular setup to boost the power density 5.2 and 1.3 times greater than that obtained with MFC-GG and MFC-GFB. The improved efficiency of the system was referred to the thick biofilm of the system, and scant internal resistance [83]. Six types of micro or nano-structured anodes utilized in micro-sized MFCs have been compared. The anodes under consideration included carbon nanotubes (CNTs), carbon nanofibers (CNFs), gold or poly (e-caprolactone) microfibers (GPM), nanofibers (GPN), planar gold (PG), and traditional carbon paper (CP). All anode's effectiveness was tested with the use of small and micro-liter sized MFC. A homemade 3-dimensional anode coating has been developed using the iron net as the structural anchor and fastened to a carbon felt layer [82]. The combination of carbon powder and a solution mixture of 30% polytetrafluoroethylene (PTFE) have greatly affected power generation. The performance was assessed using an acetate-fed MFC and the anode coating which improved the power generation considerably. The internal resistance measured in the MFC system was decreased by 59.3% compared to the non-coated iron net,

Due to their special intrinsic properties, including high conductivity, rust tolerance, high surface area and electrochemical inertness, the usage of CNTs has drawn

Multi-walled carbon nanotubes (MWCNTs) with carboxyl functional groups were utilized for MFC air respiration. It demonstrated a 2-fold improvement in power density relative to the carbon cloth electrode [84]. In a recent report, multiwalled carbon nanotubes/SnO2 nanocomposite coated on the glass fiber electrode

respectively [86]. The use of graphite coated with manganese oxide/multiwalled carbon nanotubes composites has greatly elevated benthic microbial fuel cells in another study. The composite provided greater hydrophobicity, kinetic movement, and power density when opposed to the standard graphite electrode. The shift seen was attributed to the consolidated impact of the Mn ions electron transfer shuttle

Graphene is an allotrope of 2D crystalline carbon with unusual characteristics such

(7200 S/m), and exceptional tensile strength up to 35 GPa [88]. Graphene-modified

/g), exceptionally high electrical conductivity

is used [85] producing maximal power densities of 1422 mW/m2

on the reaction site and its redox reactions (i.e. anode and biofilm) [87].

. Composite graphite fiber brush anode (MFC-GFB) was

. The nanotube-

and 457 mW/m<sup>2</sup>

,

*DOI: http://dx.doi.org/10.5772/intechopen.93911*

density of 335 mW/m3

#### *Treatment of Dairy Wastewaters: Evaluating Microbial Fuel Cell Tools and Mechanism DOI: http://dx.doi.org/10.5772/intechopen.93911*

carbonizing the microcellular polyacrylonitrile (PAN) and composite PAN/graphite (PAN – GR). The PAN-GR was created by utilizing supercritical carbon dioxide (Sc-CO2), as a practical foaming agent. The maximal current density achieved with a CS altered anode was 102% greater than that with carbon felt. Improved performance has been referred to as enhanced hydrophilicity and biocompatibility caused by carbonization. Carbon nanofibers with improved graphite fibers and reduced nanotube-coated graphene oxide/carbon scaffold promise new composite anode materials. Using carbon nanofibers as anodes for MFC modified graphite fibers achieved a maximal current density at a peak of 35.8 A/m<sup>2</sup> . The nanotubecoated scaffold anode device with reduced graphene oxide/carbon obtained a power density of 335 mW/m3 . Composite graphite fiber brush anode (MFC-GFB) was used in combination with granular graphite (MFC-GG) in a tubular setup to boost the power density 5.2 and 1.3 times greater than that obtained with MFC-GG and MFC-GFB. The improved efficiency of the system was referred to the thick biofilm of the system, and scant internal resistance [83]. Six types of micro or nano-structured anodes utilized in micro-sized MFCs have been compared. The anodes under consideration included carbon nanotubes (CNTs), carbon nanofibers (CNFs), gold or poly (e-caprolactone) microfibers (GPM), nanofibers (GPN), planar gold (PG), and traditional carbon paper (CP). All anode's effectiveness was tested with the use of small and micro-liter sized MFC. A homemade 3-dimensional anode coating has been developed using the iron net as the structural anchor and fastened to a carbon felt layer [82]. The combination of carbon powder and a solution mixture of 30% polytetrafluoroethylene (PTFE) have greatly affected power generation. The performance was assessed using an acetate-fed MFC and the anode coating which improved the power generation considerably. The internal resistance measured in the MFC system was decreased by 59.3% compared to the non-coated iron net, whereas the power density improved by 1.49 times.
