Abstract Redox flow batteries are an emerging technology for stationary, grid-scale energy storage. Membraneless batteries in particular are explored as a means to reduce battery cost and complexity. Here, a mathematical model is presented for a membraneless electrochemical cell employing a single laminar flow between electrodes, consisting of a continuous, reactant-poor aqueous phase and a dispersed, reactant-rich nonaqueous phase, and in the absence of gravitational effects. Analytical approximations and numerical solutions for the concentration profile and current-voltage relation are derived via boundary layer analysis. Regimes of slow and fast reactant transport between phases are investigated, and the theory is applied to a membraneless zinc-bromine single-flow battery with multiphase flow. The regime of fast interphase reactant (bromine) transport is characterized by the negligible effect of advection within the cathode boundary layer, leading to a thin boundary layer whose size is largely independent of position, and by relatively high battery current capability. Increasing the nonaqueous (polybromide) phase volume fraction is shown to significantly improve battery performance, as has been observed in recent experiments. For the case of spherical polybromide droplets, the contribution of bromine release from the polybromide phase on the limiting current density becomes negligible for diameters above a critical droplet diameter, when the system can be characterized as having a slow interphase bromine transport. Overall, we show that our analytical approximations agree well with numerical solutions, and thus establish a useful theoretical framework for single-flow batteries with multiphase flow.