Evaluation of a Lactate/Oxygen Biofuel Cells Using a Flow through Air-Breathing Microfluidic Design
Authors: Escalona-Villalpando Ricardo Antonio , Reid Russ C. , Milton Ross , Minteer Shelley D. , Arriaga Luis G. , Ledesma-García Janet 
Published: February 2020

A microfluidic biofuel cell (µBFC) can be defined as a BFC where the delivery of fuel and removal of waste takes place via a microfluidic channel incorporating electrode structures. These devices typically do not require a physical barrier to separate the anodic and cathodic compartments. One type of µBFC is a flow-through device that consists of porous electrodes where the fuel and oxidant flow-through the electrodes. The implementation of flow-through µBFC technologies allow for higher enzyme loadings and high surface area to volume ratios to promote the surface-based electrochemical reactions catalyzed by the immobilized enzymes. Although µBFC have been evaluated for glucose and ethanol mainly, they have never evaluated the use of lactate as fuel. Lactate/O2 BFC can have high theoretical energy densities of up to 507 Wh/L using a sinlge enzyme and the complete oxidation of lactate could theoretically result in a maximum energy density of 3041 Wh/L. For this reason, lactate is an interesting fuel source for alternative energy generation. In this work, lactate oxidase (LOx) was coupled with a ferrocene-based redox polymer (dimethylferrocene-modified linear polyethylenimine, FcMe2-LPEI) as the bioanode and laccase (Lc) was compared connected to pyrene-anthracene and anthracene-modified carbon nanotubes (PyrAn-MWCNT and An-MWCNT) to facilitate the direct electron transfer at the biocathode and TBAB-modified Nafion was added and it was deposited onto a TCP electrode using a brush and allowed to dry for 2 h. Both electrodes were evaluated in two µBFC configurations using different concentrations of lactate, in the range found in sweat (0, 2, 10, 20 and 40 mM). The µBFC with an air-breathing cathode was fabricated using two plates made of PMMA [poly-(methyl methacrylate)] with a silicon elastomer film (Silastic®) compressed between the plates used to seal the device and define the channel pattern. In the top plate, an air-intake window was constructed with a dimension of 0.22 cm2 enabling oxygen delivery from the air. The FcMe2-LPEI/LOx bioanode was characterized by oxidation and reduction peak potentials of 0.28 and 0.16 V (vs. SCE). In the presence of 70 mM lactate, the limiting current density was 213 mA cm-2, corresponding to the mediated oxidation of lactate by LOx. The catalytic kinetics of the LOx bioanode were studied by amperometry similar current density of 204.1 mAcm-2 using 70 mM lactate and an apparent Michaelis constant of 8.9 ± 1.1 mM calculated by double-reciprocal evaluation of the Lineaweaver-Burk equation. On other hand, two different modifications of An-MWCNT and PyrAn-MWCNT were performed by cyclic voltammograms in 0.1 M PBS at pH 5.6 and. An-MWCNT/Lc electrodes resulted in a current density of 68 mA cm-2 without stirring, while PyrAn-MWCNT/Lc electrodes yielded a current density of 111.3 mA cm-2. Therefore, we utilized the PyrAn-MWCNT/Lc electrodes for µBFC experiments. The performance of the µBFC using 0.05 M PBS and a pH of 5.6 as anolyte and catholyte achievement a maximum OCP of 0.67 V using 20 and 40 mM of lactate, 1700 ± 77 mA cm-2 and 305 ± 20 mW cm-2 using 40 mM lactate, respectively. In order to take advantage of the µBFC design that allows the use of individual electrolyte inlets for the anode and cathode, the pH of the anolyte was changed to pH 7.4 (0.05 M), as appropriate for use in physiological conditions, and the pH of the catholytewas kept at 5.6 (0.05 M). With these change, the OCP increased to 0.73 V and remained constant at concentrations ranging from 2 to 40 mM of lactate, achieving maximum current and power densities of 2087 ± 84 mA cm-2 and 402 ± 23 mW cm-2 using 10 mM of lactate. This is a 19% and 24% enhancement in performance compared to employing the same pH. Also, the effect of flow rate was evaluated using a concentration of 10 mM lactate in 0.05 M at pH 7.4 for the anolyte and pH 5.6 for the catholyte. The OCP of the µBFC was 0.73 V. The results show that a flow rate of 3mL h-1 allowed the OCP to reach a maximum in less time, 6.9 min, compared to a flow rate of 9 mL h-1 which resulted in an improved current density of 1875 mA cm-2 although the average stabilization time was 11.2 min. Interestingly, the current density begins to decrease at a flow rate of 12 mL h-1, perhaps due to flow-induced mechanical instabilities in the CNT immobilization. This is the highest performing lactate/oxygen biofuel cell utilizing a single enzyme system. Future work will focus on incorporating multi-enzyme cascades to further improve performance.