Relevance of Channel Shape in Air-Breathing Membraneless Direct Formic Acid Microfluidic Fuel Cell: Experimental and Numerical Investigations

This paper discusses a novel air-breathing membraneless microfluidic fuel cell (µFFC) U-shaped concept. A 3-D model is established by considering mass transport phenomena, electrochemical reactions, electrode porosity, air breathing at the cathode and other features such as electrodes properties, fluid density and fluid viscosity. The model is applied to µFFC V- and U-shaped microfluidic channels to predict experimental results. A very good agreement between the experimental and numerical results in terms of predicted polarization curves, power outputs and fuel utilization indicate that the model can be used as a platform to study a wide set of stream architectures under numerous functioning conditions for the optimization of the fuel cell. The power output performance and fuel efficiency utilization were improved by reducing the gap between the anode and the cathode and avoiding the fuel crossover. An optimum performance was achieved with a µFFC-U using 0.5 M HCOOH and flow rate of 100 μ L min -1 delivering maximum current density and maximum power output as high as 985 mA cm -2 and 165 mW cm -2 , respectively. A proof of concept is demonstrated by using a stack of three µFFC-Us (of 9.6 cm 2 footprint and a total volume of 10.6 cm 3 ) connected in serial mode for powering four green LEDs (each of requiring a 2.1-2.5 V and 4.2-5 mW power to function) during 20 hours with a constant low flow rate of 16.7 µL min -1 (~ 4.23 µL min -1 in each cell). These results represent an important step towards the construction of microenergy systems to be used as long-time electricity generators for low power electronics applications.