A planar microfluidic membraneless flow cell. The design eliminates the need for a mechanical membrane, such as a polyelectrolyte membrane (PEM) in a fuel cell, by providing a flow channel in which laminar flow regimes exist in two fluids flowing in mutual contact to form a "virtual interface" in the flow channel. In the flow cell, diffusion at the interface is the only mode of mass transport between the two fluids. In a fuel cell embodiment, a planar design provides to large contact areas between the two streams, which are fuel and oxidant streams, and between each stream and a respective electrode. In some embodiments, silicon microchannels, of fixed length and variable width and height, have been used to generate power using formic acid as fuel and oxygen as oxidant. Power densities on the order of 180 .mu.W/cm.sup.2 have been obtained using this planar design.