# Mini-Review: Recent Technologies of Electrode and System in the Enzymatic Biofuel Cell (EBFC)
## 2 The Enzyme, Support, and Substrate in EBFC
Kraton has high ion mobility, film-forming ability, and high proton conductivity that can produce a current as high as 1.14 mA/cm\({}^{2}\) at a glucose concentration of 60 mM. [PERSON] et al. [39] developed a rectangular carbon tube with a concave surface to immobilize the GOx and laccase (LAC) from rectangular polypyrrole (RPPy). RPPy is carbonized at high temperatures so that the resulting surface structure has defects at a high degree of disorder and low degree of intralayer binding of the amorphous carbon. In addition, [PERSON] et al. [2] used pyrene carboxylic acid with MWCNT and reduced graphene oxide-ceria for the electrostatic immobilization of enzymes via calcium ions. The calcium ion is produced on the electrode surface by dipping the electrode in a calcium chloride solution for two hours. The electrode consists of a substrate and support material. The support material is used to support the enzyme and will be on the surface of the substrate. The commonly used substrate is carbon cloth [44], carbon paper [45], indium tin oxide (ITO) [22], glassy carbon electrode [46], and buckypaper [47], because these carbon-based materials provide advantages such as high compatibility, being user-friendly, and being environmentally friendly and made of reusable material [40], through physical adsorption or chemical bonding to form the carbon composites [39]. Substrates are rarely studied in EBFCs, but [PERSON] et al. [48], using graphene paper [49], used the reduction method followed by crosslinking to strengthen the mechanical stability of the material and used it for the anode and cathode in the EBFC, as shown in Figure 3. In the EBFC system developed, authors use pyrroloquinoline-dependent glucose dehydrogenation (PGG-GDH) and bilirubin oxidase (BOx) as the enzyme anode and cathode. [PERSON] et al. [50] used carbon paper to solve the  
Figure 1: Enzymes immobilized on an electrode surface via (**a**) physical adsorption to a polymer, (**b**) covalent attachment to a polymer (as shown by the black and white tethers), or (**c**) encapsulation in [35]. Copyright 2012 Elsevier. problem of the low solubility of the gas molecule in the aqueous solution with polytetrafluoroethylene to increase its hydrophobicity. Low power due to low oxygen solubility was increased by producing power of 53.0 uW/cm\({}^{2}\) at 0.45 V. [PERSON] et al. [51] applied the buckypaper as a bioelectrode for both GOx and lactose at the bioanode and biocathode without a redox co-factor, which offers various advantages, such as highly efficient electron transfer, scalable production, high electrical conductivity, and large specific areas [52], respectively. [PERSON] et al. [44] modified the carbon cloth with MgO-template porous carbon to further increase the electrochemically active surface area on the electrode surfaces. Both modified electrodes use an anodic and cathode binder, namely poly(vinyldienedifluoride) and polytetrafluoroethylene. The authors modified the carbon cloth using an MgO template to form mesoporous carbon and were able to control the pore size distribution to immobilize the enzyme. The enzyme will attach to the surface of the support material, where the oxidation and decomposition reactions take place to produce current or power. However, it should be noted that enzymes will oxidize and reduce, such as in Equations (2) and (3), and the resulting electrons need to flow to the electrode. There are two ways of electron transfer in EBFC, namely mediated electron transfer (MET) and direct electron transfer (DET) [22]. The redox mediator is used in MET as an electron acceptor for electron transfer. These mediators are encouraged to attach at the electrode [28] to increase electron transfer by covalently attaching to a polymer backbone or directly adsorbing on the electrode surface by weak noncovalent immobilization or physical adsorption by drop-casting; both methods still carry the problem of mediator leaching. [PERSON] et al.