# Mini-Review: Recent Technologies of Electrode and System in the Enzymatic Biofuel Cell (EBFC)
## 3 Current Development on the Bioanode and Biocathode in EBFC
Another study used chitosan as the composite electrode, synthesized by  
Figure 5: Characterization of the RPPy and CPPy before and after being carbonized at 1600 \({}^{\circ}\)C. SEM images of (**A**) RPPy, (**B**) RPPy1600, (**C**) CPPy, and (**D**) CPPy1600; TEM images of (**A1**) RPPy, (**B1**) RPPy1600, (**C1**) CPPy, and (**D1**) CPPy1600 [39]. Copyright 2019 Elsevier. [PERSON] et al. [64] to increase porosity and surface area for highly optimized enzyme loading. The condition of glucose at 20 mM produced a current density of 3.5 mA/cm\({}^{2}\) using the combination of chitosan/reduced graphene oxide/polyaniline as the electrode. [PERSON] et al. [65] developed the carboxyl-functionalized mesoporous carbon electrode and used Nafion as an electron transfer medium. The carboxyl-functionalized mesoporous carbon electrode is optimized using oxidation time, concentration, and temperature as the main factors to increase enzyme activity and enzyme immobilization up to 140.72 and 242.74%, respectively. There are various electrodes with a high density of the hydroxyl and amino groups in chitosan, which have the advantage of biocompatibility suitable for use in EBFC, and [PERSON] et al. [66] combined them with cobalt metal and graphene oxide as a composite electrode to immobilize the enzyme. The authors claimed that improved electrochemical performance in EBFC affects rheological and morphological properties due to the acidity pH of the chitosan. [PERSON] et al. [15] produced electrode material without binder or adhesive using a combination of reduced graphene oxide (rGO), functionalized magnetic nanoparticles (f-Fe\({}_{3}\)O\({}_{4}\)), and polyaniline matrix. Biocompatible polymers with high conductivity, porosity, and surface area, together with the rGO and f-Fe\({}_{3}\)O\({}_{4}\) tested with appropriate enzyme loading, have increased electron transfer. The good electrical conductivity in graphene leads to the production of 3D graphene as a support for glucose oxidase with an extended enzyme lifetime and reduced leaching, developed by [PERSON] et al. [26]. The leaching is prevented by entrapping the enzyme at the 3D graphene surface and thus produces power at 164 \(\upmu\)W/cm\({}^{2}\). Three-dimensional graphene-related studies were also conducted by [PERSON] et al. [59] together with polyethylenimine (PEI) and ferrocene carboxylic acid (FcCOOH) using the drop-casting method. This study also tested the effect of Nafion solution on 3D graphene/PEI/FCCOOH on the carbon paper surface. The catalytic activity reduced by about 20% in the stability test, and the electrode can be stored for a week at a temperature of 4 \({}^{\circ}\)C. [PERSON] et al. [67] synthesized the doped nitrogen and sulfur on carbon nano-onion as an enzyme support, in which the carbon nano-onion has properties of the graphene layer. The nitrogen and doped sulfur change the carbon-nano-onion capacitance characteristics and are suitable for the H\({}_{2}\)O\({}_{2}\) sensor in EBFC. Reduced graphene oxide was dispersed on the 3D carbon paper electrode as a biocathode to minimize the aggregation problem of \(\pi\)-\(\pi\) stacking in reduced graphene oxide (rGO) by [PERSON] et al. [68]. This rGO/3D carbon paper combination provides enzyme orientation and confinement control to increase direct electron transfer efficiency with modification at the rGO surface through linker molecule 4-aminoobenzoic acid (4-ABA). The bilirubin oxidase enzyme used in this study increased the half-life by 5 hours. [PERSON] et al. [69] used a modified electrode using the graphene-coated carbon fiber to help solve insufficient oxygen dissolved in the fuel liquid. Using graphene-coated carbon fiber with high porosity and surface area allowed the permeability of the oxygen gas to reach the electrode surface. As stated earlier, the mediator is encouraged to be directly adsorbed on the electrode surface [28]. [PERSON] et al. [70] took the initiative to produce the immobilized mediator redox protein cytochrome c (Cyt c) and used it on both anode and cathode.