Relevance of channel shape in air-breathing membraneless direct formic acid microfluidic fuel cell: Experimental and numerical investigations

A 3-D model is established by considering mass transport phenomena, electrochemical reactions, electrode porosity, air breathing at the cathode, electrodes properties, fluid density and fluid viscosity. The model is applied to mu FFC V-and U-shaped microfluidic channels. The agreement between the experiments and simulation in terms of predicted polarization curves, power outputs and fuel utilization indicate that the model can provide a trustworthy platform to study a wide set of stream architectures under numerous operating conditions. The power output and fuel efficiency utilization were improved by reducing the gap between the anode and the cathode. An optimum performance was achieved with a mu FFC-U using 0.5 M HCOOH and flow rate of 100 mu L min(-1) delivering current density and power density output as high as 985 mA cm(-2) and 165 mW cm(-2), respectively. A proof of concept is demonstrated with a stack of three mu FFC-Us (9.6 cm(2) footprint and total volume of 10.6 cm(3)) connected in series powering four green LEDs (each of requiring a 2.1-2.5 V and 4.2-5 mW) for 20 h with low flow rate of 16.7 mu L min(-1). These results represent an important step towards the construction of microenergy systems for low power electronics applications.