Paper-Based Lactate Biofuel Cell Using Poly(glycidyl methacrylate)-Grafted MgO-Templated Carbon
Authors: Satake Mizuki , Yoshihara Yuko , Hoshi Yoshinao , Watanabe Hikari , Shitanda Isao , Itagaki Masayuki , Tsujimura Seiya 
Published: December 2020

Enzymatic lactate/O2 biofuel cells (BFCs) are energy conversion systems in which enzymes oxidize lactate at a bioanode and reduce oxygen at a biocathode. These enzymes are highly safe toward humans, and highly selective for substrates. In addition, lactate is contained abundant in a human sweat. Thus, the lactate/O2 BFCs have been attracting attention as a next-generation wearable power source1).
We have produced a paper-based lactate biofuel cell in which lactate oxidase (LOx) and a mediator are physically absorbed on MgO-templated carbon (MgOC) having mesopores suitable for enzyme2).
By using MgOC for electrode, long-term stability may be improved because the elution of the enzyme is suppressed by entrapping the enzyme in the mesopore3). On the other hand, covalent immobilization of mediators and enzymes seems to be effective for further improvement of stability of the electrodes. We recently reported that poly(glycidyl methacrylate) (poly(GMA)) bearing pendant glycidyl groups, grafted on the surface of MgO-templated carbon (GMgOC), was useful for forming strong multipoint covalent bonds with amino functional groups on the surface of flavin adenine dinucleotide-dependent glucose dehydrogenase (FAD-GDH) molecules4). The stability of FAD-GDH-immobilized GMgOC electrode showed good stability with a simple immobilization scheme by suppressing elution of FAD-GDH from the electrode surface.
In this study, we newly fabricated a paper-based lactate biofuel cell using a GMgOC electrode formed by screen printing. A lactate biofuel cell anode, in which Azure A, as a mediator, was covalently immobilized on the GMgOC. LOx was also immobilized in the GMgOC. We evaluated the output current and the stability of the electrode. The synthesis reaction of the GMgOC is shown in Fig. 1(a). MgOC was irradiated with an electron beem to generate radicals, and then glycidyl methacrylate was introduced to form GMgOC on the electrode surface. Polyvinylidene difluoride(PVdF) as a binder and N-methyl-2-pyrrolidinone(NMP) as a solvent were added to GMgOC as a carbon ink.
The paper-based carbon electrodes were fabricated by screen printing. A carbon leads and MgOC ink was printed on a water-repellent paper substrate, successively. The AzureA as a mediator and LOx as an enzyme were dropped on the porous carbon electrode surface. Chronoamperometry was performed using the prepared bioanode. The measurement was performed by the three-electrode method using a platinum wire as the counter electrode and a saturated KCl/Ag/AgCl electrode as the reference electrode, respectively. The chlonoamperometry was performed in 1 mol dm-3 phosphate buffer containing 0.1 mol dm-3 lactate. The potential was set at 0.1 V vs. Ag/AgCl.
Figure 1(b) shows the cyclic voltammetry of the GMgOC electode. A catalytic oxidation current was observed at a nobler potential than -0.1 V. The peak current value of the present enzyme electrode was larger than that of an enzyme electrode in which MgOC was used as the electrode material. The results indicated that the use of GMgOC will obtain a larger peak value because of immobilizing mediator and enzymes makes electrons to transport more efficiently to the electrode.
A chronoamperograms using GMgOC and MgOC electrodes were measured at 0.1 V. Figure 1(c) shows that the current value of MgOC was 0.09 mA cm-2 after 30000 s, and that of GMgOC was 1.7 mA cm-2. It was found to exhibit that the current density was about twice as high as MgOC. It’s because, mediators and enzymes are immobilized by physical adsorption on the carbon surface when using MgOC, while elution of LOx could be suppressed by using GMgOC because the amino group of the enzyme shell and the epoxy group on the carbon surface are bonded successfully.
Reference
1) E. Katz et al., J. Electroanal. Chem., 479 (1999) 64.
2) I. Shitanda et al., J. Electrochem. Soc, 166(12) (2019) B1063-B1068.
3) S. Tsujimura et al., Electrochemistry
83 (2015) 372.
4) I. Shitanda, T. Kato et al., Bull. Chem. Soc. Jpn., 93 (2020) 32.
Acknowledgments
This work was partially supported by JST-ASTEP Grant Number JPMJTS1513, JSPS Grant Number 17H02162 and Private University Research Branding Project (2017-2019) from Ministry of Education, Culture, Sports, Science and Technology, and Tokyo University of Science Grant for President's Research Promotion．

Figure 1