(Keynote) Enzymatic Biofuel Cell Based on Porous Carbon Materials
Authors: Tsujimura Seiya 
Published: February 2020

Enzymatic biofuel cells (EBFCs) are a type of fuel cells, which employ enzymes instead of conventional noble metal catalysts. The working principle is the same as in conventional polymer electrolyte membrane fuel cells, namely fuel is oxidized at the anode side and the electrons reach the cathode, where they combine with an oxygen to water. EBFCs are promising for sustainable green energy applications; however, they are still at an early stage of development, with many yet-to-be-resolved fundamental scientific and engineering problems. Two critical problems are short lifetime and poor power density, both of which are related to enzyme stability, electron transfer rate, and enzyme loading. To achieve the practical application of EBFCs, a promising approach is to use porous carbon materials. Strategies for the designing of hierarchically structured supports composed of mesoporous and macroporous are considered: the large surface area of mesoporous materials can increase the enzyme loading and electron transfer efficiency, and the macropores enable the efficient fuel transport.
Our recent paper demonstrated that Magnesium oxide (MgO)-templated porous carbon forms are promising candidates as electrode materials for the elaboration of efficient bioelectrochemical devices. MgO-templated mesoporous carbon (MgOC, mean pore diameter 10, 20, 40, 80 nm) were used to increase the effective specific surface area for specific enzyme immobilization. MgOC particles were deposited on a current collector by an electrophoretic deposition method to generate micro meter-scale macropores to improve the mass transfer of glucose and electrolyte (buffer) ions. To create a glucose bioanode, the porous-carbon-modified electrode was further coated with a biocatalytic hydrogel composed of a conductive redox polymer, flavin adenine dinucleotide-dependent glucose dehydrogenase (FAD-GDH), and a cross-linker. The electrode coated with FAD-GDH and a redox polymer showed a 33-fold increase in glucose oxidation current density (ca. 100 mA cm−2) compared to that of the flat electrode. MgOC-modified electrode can be applied for the biocathode using bilirubin oxidase (BOD) as an oxygen reduction reaction catalysts.
Our recent wearable EBFC based on MgOC will be presented. FAD-GDH was used as an anodic catalyst. 1,4-naphtoquinone was modified on the MgOC mesopore surface. Bilirubin oxidase and ABTS mediator were modified on the MgOC surface. Aiming at the application of EBFCs to the wearable power devices, we modified the MgOC on the surface of flexible carbon cloth. The cell operation was initiated after addition of 0.4 mL of 1 M phosphate buffer solution containing various concentrations of glucose, ranging from 0.1 – 3 M. The maximum output power density per geometric surface area was 2 mW cm-2, which was limited by the cathode performance, especially O2 supply from the air. The operation time, i.e. cell capacity, was quantitatively determined by the mole number of applied glucose evaluated by galvanostatic experiments at 0.25 – 4 mA cm-2 of constant current density, indicating that the capacity was determined by the consumption of β-d-glucose. After consumption of the fuel, the cells were able to restart by adding the glucose solution. The operation time linearly depended on the mole number of glucose added.
In this presentation, recent developments in EBFCs technology using porous carbon materials are highlighted. The essential properties of the resulting materials with respect to the EBFC application are also discussed. A combination of electron transfer technology and porous carbon material would be helpful in achieving a much higher and stable current output, thus contributing to a practical advance in the sustainable energy field.