# Research Progresses and Application of Biofuel Cells Based on Immobilized Enzymes
## 2 A Variety of Carrier Materials for Enzyme Immobilization in EBFCs
### Some Paper-Based and Other Flexible Materials for Enzyme Immobilization in EBFCs
proposed a hybrid biofuel cell (HBFC) where the anode used metal to catalyze oxidation of glucose and oxygen was reduced at the cathode with immobilized BOD [44]. Platinum nanoparticles were loaded on braided gold wire at the anode, and then the BOD was cross-linked on BP at the cathode. The resulting cell had an open-circuit voltage of 0.735 V and a power density of 46.31 \(\mu\)W cm\({}^{-2}\) in 3 mM of glucose. This cell can supply continuous and stable power to small electronic devices [44]. [PERSON] et al. developed a MWCNTs paper-based enzyme cell with glutaraldehyde cross-linked electron transfer mediators Tetrathiofulvalene (TTF) and GOx at the anode and Lac at the cathode [45]. The resulting single-chamber MWCNT-based enzymatic biofuel cells with four Y-shaped paper microchannels cascaded in series had a maximum stable open-circuit potential of 1.65 V and a maximum power density of 46.4 \(\mu\)W cm\({}^{-2}\) was obtained at 0.8 V (Figure 3). Comparing to cross-shaped paper microchannels, Y-shaped paper microchannels could allow more sustainable and stable microfluidics of biofuel and oxidants for various series-parallel arrangements of enzymatic anodes and cathodes in a shelf-stack configuration with maximum and stable power output [45]. The output power generated was sufficient to power small portable devices [45]. [PERSON] et al. developed a disposable paper-based membrane-free microfluidic EBPCs by preparing a composite slurry of cellulose ionic liquids and MWCNTs as carriers for GOX at the anode and Lac at the cathode [46]. The resulting cell achieved a better power density of 141.2 \(\pm\) 3.35 \(\mu\)W cm\({}^{-2}\) and current density of 615.6 \(\pm\) 3.14 \(\mu\)A cm\({}^{-2}\) in a Y-type paper substrate structure and it was more stable than the cross-type structure. They also  
Figure 3: EBFC based on shelf-stacked Y-shaped paper microchannels [45]. discussed the effect of glucose concentration with the best cell performance at 100 mM for the Y-type and 200 mM for the cross-type. The better cell performance was obtained when the electrode was prepared with multi-walled carbon nanotubes instead of single-walled carbon nanotubes, and the enzyme was immobilized on the electrode instead of dissolved in the electrolyte [46]. [PERSON] et al. prepared bioelectrodes to construct EBFCs by immobilizing GOx and BOD, respectively, on screen-printed paper and successfully obtained energy from glucose [47]. The LEDs were successfully lit without the use of a boost circuit, and the output voltage of the series battery pack could be adjusted as needed. In addition, they developed another battery pack by matrix arrangement to acquire 0.97 \(\pm\) 0.02 mW at 1.4 V in a 4-series/4-parallel arrangement toward wearable devices [48]. Moreover, [PERSON] and [PERSON] et al. successfully expanded the application of CC for EBFCs by modifying the CC surface to immobilize GOx and supply fuel to the battery in a self-pump manner [49, 50]. These paper-based or sheet-based or flexible electrode materials provide an effective way for miniaturization and mass production of EBFCs including those wearable devices with simple methods and mechanical flexibility in the process of electrode and cell fabrication. Some more carbon-based flexible materials applied in EBFCs were listed in Table 2.