# 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
The assembled cell has an open-circuit voltage of 0.85 V, and the maximum power density is 380 \(\mu\)W cm\({}^{-2}\), which is 1.4 times higher than that when using GO. Similarly, [PERSON] and [PERSON] et al. prepared three-dimensional coralloid nitrogen-doped hierarchical micro- and mesoporous carbons aerogels (3D-NHCAs) as the electrode substrate for immobilization of AOx and BOD for wearable EBFCs [29]. [PERSON] et al. developed a 3D foam material through vapor deposition using nickel foam and ethanol as the carbon source [30]. Then, the SWCNTs were directly deposited on this foam to obtain CNT@Gr material, with which GOx and Lac were immobilized, respectively. The open-circuit voltage of this cell reached about 1.2 V with a power density of 2.27 \(\pm\) 0.11 mW cm\({}^{-2}\)[30]. [PERSON] and [PERSON] et al. designed a spider nest-shaped 3D substrate composed of rGO and nickel foam (Ni foam) with excellent electrical conductivity and large electroactive surface area [31]. GOx and Lac were encapsulated thereby into bioanode and biocathode, respectively (Figure 2). The resulting cell had an open-circuit voltage of 0.7 V and a maximum power density of 7.05 \(\pm\) 0.05 mW cm\({}^{-2}\). After 60 days of cyclic storage experiments, the performance of the cell remained 84.2% [31]. [PERSON] and [PERSON] et al. synthesized a bamboo-shaped hollow composite of N-doped carbon nanotubes (N-CNTs) and g-C\({}_{3}\)N\({}_{4}\) nanosheets (N-CNTs@g-C\({}_{3}\)N\({}_{4}\)) by a simple synthetic method [32]. The combination of one-dimensional N-CNTs with an open structure and two-dimensional g-C\({}_{3}\)N\({}_{4}\) nanosheets formed a three-dimensional cross-linked network on which GOx was well immobilized to produce a high open-circuit voltage of 0.93 V with a maximum power density of 0.57 mW cm\({}^{-2}\) and the ability to harvest energy directly from various soft drinks. This composite also demonstrated the ability to harvest energy directly from various soft drinks, suggesting the promising application of g-C\({}_{3}\)N\({}_{4}\)-based nanocomposites as electrode materials [32]. These results demonstrate that the use of 3D substrates with large surface areas, special morphology and good electrical conductivity can significantly improve the performance and stability of bioelectrochemical cells. Table 1 provides an overview of several other nanomaterials and nanocomposites used in EBFCs. The use of some carbon-based nanomaterials as electrode substrates have solved more or less some bottlenecks of EBFCs: (1) the high specific surface area of nanomaterials and nanocomposites increases the enzyme attachment sites and ensures efficient consumption of biofuels; (2) the stability and biocompatibility of EBFCs can be enhanced with incorporation of carbon-based nanomaterials; (3) the unique 3D structure can shorten the distance between the enzyme active site and the electrode, which increases the electron transfer efficiency, high conductivity and thus enhances the performance  
of the cell; and (4) the conductive polymers can help improve material conductivity for enzymatic electrodes.