Liquid water microjets have been successfully employed for both electrical power generation and gaseous hydrogen production, but the demonstrated efficiencies have been low. Here, we employ a combination of a modified Poisson-Boltzmann description, continuum hydrodynamic equations, and microjet electrokinetic experiments to gain detailed insight into the origin of the streaming currents produced in pure water. We identify the contributions to the streaming current from specific ion adsorption at the solid/liquid interface and from long-ranged electrostatic interactions, finding that the portion originating from the latter dominate at charged surfaces. The detailed understanding afforded by theory and the close agreement with experimental results elucidates design principles for optimizing hydrogen production and power generation. Changing the sign of the surface charge density through targeted use of surface coatings via silanization switches the primary charge carrier between hydronium and hydroxide and therefore switches the corresponding production of molecular hydrogen to oxygen at the target electrode. Moreover, hydrophobic surface coatings reduce dissipation due to fluid/solid friction, thereby increasing the conversion efficiency.
