*10.2.2 Carbon nanotubes composite*

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

#### *10.2.3 Multi-walled carbon nanotubes composite*

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 is used [85] producing maximal power densities of 1422 mW/m2 and 457 mW/m<sup>2</sup> , 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 on the reaction site and its redox reactions (i.e. anode and biofilm) [87].

#### *10.2.4 Graphene anodes*

Graphene is an allotrope of 2D crystalline carbon with unusual characteristics such as large surface area (up to 2600 m2 /g), exceptionally high electrical conductivity (7200 S/m), and exceptional tensile strength up to 35 GPa [88]. Graphene-modified

*Environmental Issues and Sustainable Development*

to increase the efficiency of nutrients, H<sup>+</sup>

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 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

was achieved [81]. This was 8.5 times greater than that attained with the

Composite anodes have intrigued extensive interest recently. These materials were utilized to attain synergistic effects with two or more materials to alter original

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

content, resulting in increased anodic kinetics efficiency.

, and O2 transfer via biofilm as compared

**110**

3321 W/m3

2D-electrode systems.

**10.2 Composite anodes**

*10.2.1 Graphite-polymer composites*

stainless steel mesh (GMS) power density was recorded to be 18 times higher than that of a stainless steel mesh anode (SSM) and 17 times higher than that of polytetrafluoroethylene modified SSM (PMS) [68]. The significant improvement was recognized due to increased surface area of the electrodes, improved adhesion of bacterial biofilms, and efficient extracellular electron transfer. The current stainless steel collector (SS) boosts electrical conductivity for electrode, and the overall efficiency of the system is enhanced by the current SS assimilator which reduces internal resistance. Chen et al. [69] used an ice template as an anode to create a versatile macroporous 3D graphene sponge. The microporous 3D graphene allowed the random propagation of bacteria and resulted in a high biofilm span and increased performance [69]. From another study, tin oxide (SnO2) nanomaterials were utilized on the reduced graphene oxide surface (R-GO-SnO2) able to generate electricity that was approximately 5 times higher than the use of an unaltered graphene oxide (reduced). Collegial effects among SnO2 and graphene and strong biocompatibility were liable for the much stable formation of bacterial biofilms and the efficiency of charges transfer [86]. Reduced graphene oxide/carbon nanofibers (R-GO-CNTs sponges) melamine sponges based on dip-coating technique tend to cater to a huge electrically conductive surface area for *Escherichia coli* growth as well as electron transport in MFC [65]. Four R-GO-CNT sponges were tested with varied thicknesses and configurations, but the thinnest one (with a thickness of 1.5 mm) displayed prime efficiency, generating a maximal current density of 336 A/m3 [65]. The usage of a redesigned anode built from graphenepolyaniline nanocomposite was also found to produce power three times greater than carbon cloth [70]. Often used as an anode for MFC was a 3-dimensional reduced graphene oxide-nickel foam (R-GO-Ni) by accurate deposition of R-GO sheets to the nickel foam substratum. The R-GO thickness may be modified in comparison to the surface region of the electrode by initiation cycles. This macro-porous scaffolding design not only offers a 3-dimensional surface for microbial growth but also promotes the mobility of substrates inside the culture medium. The efficiency was extensively better than with the usage of nickel foam and various graphite materials dependent on anodes [63, 64]. The formation in MFC of highly crystalline graphene or nickel electrode with Shewanella putrefaciens provided the power density of typical MFC carbon cloth anode 13 times greater. Because of the minimal cost of hollow Ni and the low weight percent of graphene (5% w), this composite electrode provides good potential in the development of efficient MFCs for greater power generation [71].
