Membraneless fuel cell : Fluid physics and electrochemistry

The proton conducting membrane has been identified as a key component affecting the fuel cell performance. On the contrary of many efforts to understand and improve the properties of the membrane, a novel design that eliminates the need of using membrane has recently received increasing interests. The microfluidic fuel cell (MFC), built under microfluidics, can utilize the laminar regime to naturally separate the fuel and oxidant streams. The absence of membrane has many benefits, such as reduced cost, simplified assembling, minimized fuel crossover, and easy water management.The manipulation of fluids and electrode behavior of MFC, with characteristics length in microns and fluid volume in sub-micro-liter scale, has emerged as a distinct new field. This is because, partly, such transitional scale is in between microfluidics and macrofluidics. It not only shares physicochemical properties of both scales, but also has unique characteristics rarely reported in previous studies. On the other hand, in order to optimize the cell performance, one must minimize the mass transfer at the liquid-liquid interface to prevent the fuel crossover, while at the same time, mass transfer through the electrode boundary layer should be enhanced for higher current density and fuel utilization. These two conflicting requirements give enormous challenges to the manipulation of the local physicochemical interactions at such a small scale.In this presentation, a review of key physical and electrochemical phenomena involved in the MFC is presented, with a discussion on the fuel cell mechanisms. A list of dimensionless numbers are introduced to parameterize the relative importance of various physical and chemical phenomena. Specifically, this study explores the Péclet numbers to describe the interfacial mixing and parasitic current generation; the Graetz number and Damköhler number to account for the interactions between mass transfer and electrode reaction; the Sherwood number to address the mass transfer enhancement and consequently current density increase; etc. We attempt to use simple scaling arguments to describe the physics and electrochemistry behind the rich variety of micro-scale phenomena of MFC to gain intuitive understanding of this complex problem.