*3.1. Biosupercapacitor*

Biosupercapacitors are self-powered energy storage devices using bioelectrocatalysis to charge capacitors [22]. In the biosupercapacitor, electric power is generated by a biocathode and/or a bioanode. The fundamental difference between biosupercapacitors and biofuel cells is the separation of the output of the current and bioelectrocatalytic reaction. Expressed as an equivalent circuit, a biosupercapacitor is a biofuel cell and a capacitor connected in parallel. Regardless of the connection to the external circuit, the bioelectrocatalytic reaction injects the electrons into the supercapacitor. When the external circuit requires the current, the biosupercapacitor provides the current. An advantage of biosupercapacitors is that a large current can flow even if the kinetics of the bioelectrocatalysis is inadequate. Additionally, biosupercapacitors are compatible with the self-powered biosensors [118,119].

The supercapacitor is classified into two types: the electrical double layer capacitor (EDLC) and the electrochemical pseudocapacitor (EPC). EDLC is the electrode with a large surface area, the electric charge is accumulated in the electrical double layer at the interface between the electrode and the electrolyte solution. EPC is constructed with the electrode and the reversible electrode-active redox species. The externally applied voltage shifts the electrode potential at the anode and a cathode in the negative and positive directions, respectively, as expressed in the Nernst equation.

Generally, redox-active species are immobilized at the electrode surface. Since an electrochemical capacitor contains two electrodes and an electrolyte solution, electrochemical capacitors are also classified into three types: EDLC-EDLC, EPC-EPC, and EDLC-EPC [120]. Therefore, biosupercapacitors are basically classified into the three types. The EDLC is compatible with DET-type bioelectrocatalysis and MET-type bioelectrocatalysis is adapted to the EPC. In order to avoid the mixing of mediators, EPCs in the biosupercapacitor are frequently employed redox polymers as mediators to immobilize at the electrode surface.

Electrochemically inactive porous electrodes in an electrolyte solution work as the EDLC. Additionally, DET-type bioelectrocatalytic activity is required for the EDLC-type biosupercapacitor. The reported materials suitable for the EDLC and the bioelectrode are carbon nanotubes [121–123], porous gold [124], gold nanoparticles [125], and indium tin oxide nanoparticles [126]. Porous electrodes often show high activity for DET-type bioelectrocatalysis. Therefore, it is considered that the investigation of bioelectrodes for DET-type biofuel cells will be directly useful for the biosupercapacitor.

The separation of oxidant and reductant is important in EPCs, since the mixing of the two species causes the cross-reaction discharge. Therefore, in the case of biosupercapacitors using the MET-type bioelectrocatalysis, careful attention is required to head off the outflow of mediators. As mentioned above, redox polymers are employed as mediators in biosupercapacitors [42,127–129]. The immobilization of redox-active proteins at the electrode surface has been investigated for the improvement of the capacitance of EPC-type biosupercapacitors [130,131].
