Combining enzymatic biofuel cells with supercapacitors to self-charging hybrid devices

Enzymatic biofuel cells are energy conversion devices utilizing biocatalysts to directly convert chemical energy to electricity. Due to their biocompatible, sustainable and maintenance-free properties, they hold the promise as attractive energy sources for powering next generation medical electronics for personalized healthcare. Low current and power output are main bottlenecks of enzymatic biofuel cells to hinder their practical applications. Supercapacitors are able to harness ambitious energy and deliver high-power pulses. Combining enzymatic biofuel cells with supercapacitors to establish self-charging energy-conversion/energy-storage hybrid systems are considered as an effective strategy to improve the current and power output. This design enables the hybrid electric devices to scavenge ambient energy and simultaneously store it and thus increases the efficiency and facilitates the miniaturization for practical application. In this review, we first discuss various structural configurations of these self-charging hybrid systems, and then focus on explaining their charge storage mechanisms, including electrochemical double-layer capacitance, pseudocapacitance and hybrids. Several proof-of-concept applications as implantable and wearable power sources are enumerated. Finally, we provide an overview of challenges and opportunities for research and development of self-charging hybrid devices.