# Research Progresses and Application of Biofuel Cells Based on Immobilized Enzymes
## 1 Introduction  
Enzymatic biofuel cells (EBFCs) are electrochemical devices that use natural enzymes as catalysts to convert chemical energy in some bio-sourced fuels into electrical energy under mild conditions. EBFCs have gained significant attentions due to their potential applications in various fields, including biomedical devices, environmental monitoring, consumer electronics, military and defense, and among others, providing a sustainable and environmentally friendly alternative to traditional power sources. This technology has opened a new way for the utilization of clean energy and renewable resources, as EBFCs generally do not require expensive precious metal catalysts [1; 2]. The working principle of biofuel cells is depicted in Figure 1. In the anode region, EBFCs utilize biological enzymes such as glucose oxidase (GOx), glucose dehydrogenase (GDH), fructose dehydrogenase (FDH), lactate oxidase (LOx), alcohol dehydrogenase (ADH), and alcohol oxidase (AOx), including ethanol oxidase (EOx), as catalysts to catalyze the oxidation of biofuels such as glucose, fructose, and ethanol to lose electrons [3]. The electrons then reach the cathode through the external circuit of the cell. Sometimes, metallic platinum is used as the cathode material to receive the electrons. Additionally, some biological enzymes, bilirubin oxidase (BOD), lactose (Lac) and maybe some other enzymes, are used to catalyze the reduction of oxygen [4], so that the chemical energy is converted into electrical energy. EBFCs are promising because of their fuel abundance, biocompatibility, mild reaction conditions (physiological pH, temperature, etc.), and simplicity of operation, and they can find a wide range of applications including powering biomedical and wearable devices [5; 6; 7].  
However, EBFCs also face some significant challenges such as low output power, low open-circuit voltage, poor power stability, electron transfer barriers, easy leakage of electron transfer mediators and immobilized enzymes, and poor stability of enzymes [8]. One of the prominent problems is the activity stability of the immobilized enzymes on the electrodes [9]. Therefore, researchers use different immobilization methods, differentimmobilization materials, and various protection mechanisms to maintain high enzyme activity [1; 2; 3; 4; 5; 6; 7]. Another prominent problem is that direct electron transfer between the enzyme and the electrode is difficult because the active sites of most enzymes are deeply buried within their protein shells [10]. The efficiency of EBFCs is often enhanced by selecting more suitable and conductive materials for enzyme immobilization and introducing electron transfer mediators to improve the electron transfer between the enzyme and the electrode.  
We will briefly review and discuss the research progresses of enzymatic biofuel cells and their application prospects in terms of the selection of enzyme immobilization methods and materials related to electrode modification, the improvement of electron transfer between enzymes and electrodes, and the optimization of cell construction methods [1; 2]. Finally, we will explore current and future practical applications of EBFCs, including implantable and wearable enzymatic biofuel cells, and the issues that still need to be addressed to ensure the technical and commercial viability and sustainability of EBFCs [5; 6; 7].