An air-breathing direct formic acid membraneless microfluidic fuel cell using graphite cylinder arrays as the anode is proposed. The three dimensional anode volumetrically extends the reactive surface area and improves fuel utilization. The effects of spacer configuration, fuel and electrolyte concentration as well as reactant flow rate on the species transport and cell performance are investigated. The dynamic behavior of generated CO2 bubbles is visualized and its effect on current generation is discussed. The results show that the absence of two spacers adjacent to the cathode surface improves the cell performance by reducing the proton transfer resistance. The CO2 gas bubbles are constrained within the anode array and expelled by the fluid flow periodically. Proper reactant concentration and flow rate are crucial for cell operation. At optimum conditions, a maximum current density of 118.3 mA cm(-3) and a peak power density of 21.5 mW cm(-3) are obtained. In addition, benefit from the volumetrically stacked anodes and enhanced fuel transfer, the maximum single pass fuel utilization rate reaches up to 87.6% at the flow rate of 1 mL h(-1). (C) 2013 Elsevier B.V. All rights reserved.