(Invited) Construction of an Enzymatic Biofuel Cell Which Produces Electric Energy and a Platform Chemical Simultaneously
Authors: Nakamura Nobuhumi 
Published: February 2020

Introduction

Enzymes can catalyze selective and environmentally friendly production of platform chemicals from biomass. When a platform chemical is produced through one or more oxidation reactions, the electrons can obtain from the substrate. The electrons are usually just dumped to electron accepters. If a biofuel cell anode is used as an electron acceptor for the process, we can obtain the electric energy.
Organic acids have especially attracted attention among the bio-derived chemical raw materials. The US Department of Energy (DOE) selected 12 chemicals from more than 300 biomass-derived chemicals on the basis of cooperative research with industry and academia, to develop the production methods preferentially [1]. More than half of the 12 chemicals are organic acids. The organic acids obtained by the oxidation of monosaccharides and oligosaccharides are referred to as sugar acids. The aldose oxidation at its aldehyde group forming into the carboxyl group produces aldonic acid while corresponding oxidation at its terminal hydroxyl group gives another mono-carboxylic acid, uronic acid. Aldaric acid is a di-carboxylic acid produced by the oxidation of both groups. Because various application studies of aldaric acids have been conducted, the development of their production methods are eagerly expected. Hence, we set a goal of producing aldaric acids and electric energy simultaneously from biomass with enzymatic biofuel cells. We struggle with the production of aldaric acids from cellulose and pectin so far.
It is desirable to use a flow-cell so that the fuel and product can be provided and removed efficiently. To construct a flow-cell type biofuel cell, enzymes and electron mediators should be fixed on electrodes so that they do not flow out. Here, the construction of a flow-cell type enzymatic biofuel cell for the D-glucose oxidation are mainly reported (Fig. 1).

Methods

-Examination of poly(methylene green) as an electron mediator for enzymatic anode-
Methylene green was polymerized on glassy carbon (GC) electrodes by cyclic potential sweeping method as previously reported [2]. Onto the poly(methylene green) (PMG) modified electrode, FAD-dependent glucose dehydrogenase (FAD-GDH) was cast. The electrode was evaluated by cyclic voltammetry (CV) measurement in 100 mM glucose / 90 mM potassium phosphate buffer solution.
-Flow-cell type biofuel cell construction-
A carbon fiber (CF) electrode was modified by PMG and immersed in FAD-GDH solution to use as the anode. Another CF electrode was immersed in bilirubin oxidase solution to use as an O2-reducing cathode. A flow-cell type biofuel cell consists of the anode and the cathode was constructed as Fig. 1. Evaluation of the cell was conducted by linear sweep voltammometry measurement with a flow of 100 mM glucose / 100 mM potassium phosphate buffer solution at a rate of 11 μL/min.

Results and discussion

Voltammograms of cyclic potential sweeping in methylene green solution by GC electrodes had the same features as previously reported one. This result shows that a film of PMG was formed on the surface of the electrode. When the PMG modified electrode was washed and examined in potassium phosphate buffer, the cyclic voltammogram showed a couple of peak currents around the redox potential previously reported [2]. This also confirmed adsorption of PMG on the electrode. The CV measurement of the FAD-GDH/PMG modified GC electrode exhibited a catalytic current in glucose solution. This demonstrated electron transfer mediation by PMG from FAD-GDH to the GC electrode. To maximize the catalytic current, the number of potential sweeping for PMG synthesis was optimized. The maximum catalytic current by the FAD-GDH/PMG/GC electrode was obtained when the PMG was synthesized by 20-time potential sweeping. Because cyclic voltammograms of the FAD-GDH/PMG/GC electrode displayed catalytic current in glucose solution, a flow-cell type biofuel cell was constructed.
Evaluation of the flow-cell type biofuel cell demonstrated that the open-circuit voltage was 549 mV, maximum current density was 143 μA cm-2, and maximum power density was 23.6 μW cm-2. The current density and power density were lower than what was expected in advance taking the CV measurements of the anode and the cathode respectively. It can be considered a cause that less bioelectrocatalysis reactions proceeded due to outflow of enzymes on electrodes. Further studies to improve this issue are under way.

References

[1] T. Werpy and G. Petersen, “Top Value Added Chemicals from Biomass: Volume 1—Results of Screening for Potential Candidates from Sugars and Synthesis Gas,” DOE/GO-102004-1992, 2004.
[2] D.M. Zhou, H.Q. Fang, H.Y. Chen, H.X. Ju, and Y. Wang, “The electrochemical polymerization of methylene green and its electrocatalysis for the oxidation of NADH,” vol.329, Issues 1-2, pp.41-48, 1996.

Figure 1