Energies 2015, 8, 2082-2096; doi:10.3390/en8032082 energies
ISSN 1996-1073 www.mdpi.com/journal/energies
Fabrication and Test of an Air-Breathing Microfluidic Fuel Cell
Jin-Cherng Shyu 1,*, Po-Yan Wang 1, Chien-Liang Lee 2,†, Sung-Chun Chang 3,†,
Tsung-Sheng Sheu 4,†, Chun-Hsien Kuo 5,†, Kun-Lung Huang 2,† and Zi-Yi Yang 5,† 1 Department of Mechanical Engineering, National Kaohsiung University of Applied Sciences,
Kaohsiung 80778, Taiwan; E-Mail: firstname.lastname@example.org 2 Department of Chemical and Materials Engineering,
National Kaohsiung University of Applied Sciences, Kaohsiung 80778, Taiwan;
E-Mails: email@example.com (C.-L.L.); firstname.lastname@example.org (K.-L.H.) 3 Material and Chemical Research Laboratories, Industrial Technology Research Institute,
Hsinchu 31040, Taiwan; E-Mail: email@example.com 4 Department of Mechanical Engineering, R.O.C. Military Academy, Kaohsiung 83059, Taiwan;
E-Mail: firstname.lastname@example.org 5 Department of Mold and Die Engineering, National Kaohsiung University of Applied Sciences,
Kaohsiung 80778, Taiwan; E-Mails: email@example.com (C.-H.K.); firstname.lastname@example.org (Z.-Y.Y.) † These authors contributed equally to this work. * Author to whom correspondence should be addressed; E-Mail: email@example.com;
Tel.: +886-7-381-4526 (ext. 5343); Fax: +886-7-383-1373.
Academic Editor: Andrés G. Muñoz
Received: 18 January 2015 / Accepted: 5 March 2015 / Published: 16 March 2015
Abstract: An air-breathing direct formic acid microfluidic fuel cell, which had a self-made anode electrode of 10 mg/cm2 Pd loading and 6 mg/cm2 Nafion content, was fabricated and tested. The microfluidic fuel cell was achieved by bonding a PDMS microchannel that was fabricated by a soft-lithography process and a PMMA sheet that was machined by a CO2 laser for obtaining 50 through holes of 0.5 mm in diameter. Formic acid of 0.3 M, 0.5 M, and 1.0 M, mixed with 0.5-M H2SO4, was supplied at a flow rate ranging from 0.1 to 0.7 mL/min as fuel. The maximum power density of the fuel cell fed with 0.5-M HCOOH was approximately 31, 32.16, and 31 mW/cm2 at 0.5, 0.6, and 0.7 mL/min, respectively. The simultaneous recording of the flow in the microchannel and the current density of the fuel cell at 0.2 V, within a 100-s duration, showed that the period and amplitude of each unsteady current
Energies 2015, 8 2083 oscillation were associated with the bubble resident time and bubble dimension, respectively.
The effect of bubble dimension included the longitudinal and transverse bubble dimension, and the distance between two in-line bubbles as well.
Keywords: air-breathing; fuel cell; formic acid; bubble resident time 1. Introduction
In the past few years, miniature fuel cells without a polymer electrolyte membrane, called membraneless fuel cells or microfluidic fuel cells, have been widely proposed and tested. Some of those microfluidic fuel cells operate a co-laminar flow in a microchannel to keep both the fuel and oxidant streams separated, such that both anode and cathode half-cell reactions are able to properly take place at relative catalyst-coated electrodes with slight mixing at the inter-diffusion zone. Various aqueous fuel/oxidant combinations for generating electricity from microfluidic fuel cells can be found.
Ferrigno et al.  and Kjeang et al.  tested the performance of microfluidic fuel cells using redox couples V(V)/V(IV) and V(III)/V(II) dissolved in sulfuric acid solution as aqueous oxidant and fuel, respectively. Accompanying a cathodic half-cell reaction using hydrogen peroxide or oxygen-saturated electrolyte as oxidant, methanol or formic acid is another common aqueous fuel [3–6] for microfluidic fuel cells. Zhu et al.  proposed an air-breathing direct formic acid microfluidic fuel cell using graphite cylinder arrays as the anode, in order to extend the reactive surface area and improve fuel utilization by the three-dimensional anode. Various effects including the spacer configuration, fuel and electrolyte concentration, reactant flow rate, and dynamic behavior of generated CO2 bubbles on the cell performance were investigated. With a formic acid concentration and electrolyte concentration of 0.5 M and 1.0 M, respectively, their microfluidic fuel cell yielded a peak power density of 21.5 mW/cm3 at a flow rate of 20 mL/h, and a maximum fuel utilization of 87.6% at the flow rate of 1 mL/h. Moreover, there is a particular electrochemical reaction for microfluidic fuel cells that uses hydrogen peroxide as both fuel and oxidant [8,9].
However, microfluidic fuel cells using an aqueous reactant suffer a low species diffusivity and low oxidant concentration, and thus hinder the cell from enhancing cell output. In order to address such problems, another kind of microfluidic fuel cell that reacts with air as a gaseous oxidant by breathing air through a porous gas diffusion electrode was proposed.
The first prototype of an air-breathing microfluidic fuel cell was proposed by Jayashree et al.  in 2005. Their fuel cell used formic acid containing 0.5-M sulfuric acid as fuel and breathed air as an oxidant through an air-exposed porous GDE. Results showed that both the fuel utilization and the power density for this microfluidic fuel cell were able to significantly improve, compared with the traditional microfluidic fuel cell using an aqueous oxidant. At lower flow rates (e.g., 0.1 mL/min), fuel utilization was 33% for a single pass. In addition, a power density in air-breathing laminar flow fuel cells of 26 mW/cm2 was obtained.
Because of the marked performance improvement of the air-breathing microfluidic fuel cells, several studies investigating the various effects on the performance of similar fuel cells reacting with either methanol or formic acid were carried out [11–16]. In order to have a notable microfluidic fuel cell
Energies 2015, 8 2084 performance, the anodic gas diffusion electrodes of those fuel cells were always heavily loaded with a catalyst, ranging from 7 mg/cm2 to 10 mg/cm2. Besides electricity, a gaseous product, CO2, is also produced in the anode based on the electrochemical reaction. Based on a careful examination of the related literature [10–16], it can be found that the anode catalyst loading of an air-breathing microfluidic fuel cell is usually so high (>8 mg/cm2) that the bubbles are most likely to be presented on the electrode due to the high cell current output.