# Research Progresses and Application of Biofuel Cells Based on Immobilized Enzymes
## 2 A Variety of Carrier Materials for Enzyme Immobilization in EBFCs
### Some Nanomaterials and Nanocomposites for Enzyme Immobilization in EBFCs
With an external charge pump, the cell successfully powered a light-emitting diode (LED), where the power could be scaled up from \(\upmu\)W to mW, showing the potential to power electronic devices [19]. [PERSON] and [PERSON] et al. developed a glucose-fueled photobiofuel cell based on two-dimensional graphite-phase carbon nitride (g-C\({}_{3}\)N\({}_{4}\)) nanosheets, which are widely used in photocatalysis [20]. They first prepared different ruthenium complexes and then incorporated the ruthenium complexes on a substrate composed of MWCNTs and g-C\({}_{3}\)N\({}_{4}\) to obtain MWCNTs/g-C\({}_{3}\)N\({}_{4}\)@Ru anode. The anode was loaded with GDH as a photobionode. BOD was used at the cathode. The resulting cell produced a maximum power density of 26.7 \(\pm\) 0.4 \(\upmu\)W cm\({}^{-2}\) under normal conditions (no light) and 28.5 \(\pm\) 0.10 \(\upmu\)W cm\({}^{-2}\) under light. The current density increases by 6.2% under light conditions, probably due to the use of ruthenium complexes that promoted anode electron transfer and thus increase the current density [20]. [PERSON] and [PERSON] et al. successfully immobilized GDH by electropolymerizing toluidine blue (Poly (toluidine blue), PTB) on rGO to create a GDH/PTB/rGO bioanode [21]. In addition, they prepared a GOx@HRP/MWCNTs bio-cathode by cross-linking GOx and HRP on MWCNTs. The resulting cell had an open-circuit voltage of 0.65 V in 40 mM glucose solution and a maximum power density of 31.3 \(\upmu\)W cm\({}^{-2}\) at 0.3 V [21]. Similarly, EBFCs with excellent performance using peroxidase elimination of by-products in the anode region were also reported [22; 23; 24]. [PERSON] and [PERSON] et al. demonstrated the synthesis of ZnO nanoparticles by mixing neem leaf extracts with zinc nitrate solution [16]. Then, PIn was used to functionalize MWCNTs for preparing ZnO/PIn@MWCNTs/Frt/GOx electrode. The maximum current density of this electrode was 4.9 mA cm\({}^{-2}\)[16]. [PERSON] and [PERSON] et al. put SWCNTs in a mixed acid to obtain hydrophilic HOOC-SWCNTs [25]. Then, a surface defective two-dimensional nanomaterial NiMoSe\({}_{2}\) was prepared by hydrothermal synthesis. HOOC-SWCNTs were then mixed with NiMoSe\({}_{2}\) in a ferric chloride solution, thereby the pyrrole monomer was polymerized onto the mixture to obtain HOOC-SWCNTs@Ppy@NiMoSe\({}_{2}\)Similarly, the resulting HOOC-SWCNTs@Ppy@NiMoSe\({}_{2}\)/Frt/GOx electrode was obtained. The open-circuit potential of the resulting cell was 0.35 V and the maximum current density of 9.01 mA cm\({}^{-2}\) was obtained in 50 mM glucose solution [25]. [PERSON] et al. demonstrated a method for preparing a 3D carbon composite of PANI1600@CNTs by in situ polymerizing aniline monomer in MWCNTs and carbonizing it at 1600 \({}^{\circ}\)C. The resulting composite exhibited a rhizobia-like structure that interconnected adjacent carbon tubes, leading to a high-performance bioenzyme electrode when used for GOx and Lac [26]. The assembled cells had an open-circuit voltage of 0.78 V and a maximum power density of 1.12 mW cm\({}^{-2}\) at 0.45 V. The three prepared cells were connected in series to successfully light up a yellow light-emitting diode with a voltage requirement of about 1.8 V. The power density was still maintained at 82.85% after two weeks of smooth operation. Composites of conductive polymers with nanomaterials are efficient loading materials for improving material conductivity for the immobilization of enzymes in the field of EBFCs or biosensors [15; 16; 17; 25; 27]. [PERSON] and [PERSON] et al. have made modifications to the reduced graphene oxide (rGO) structure to increase enzyme immobilization sites, leading to improved performance of EBFCs [28]. They used rGO with three-dimensional porous structure, prepared by spray freeze-drying method, as the substrate material for immobilized GOx. Based on this material, the bioenzyme electrode made with glutaraldehyde as a cross-linking agent had twice the loading capacity of GOx than when GO was used.