Ion-conducting membranes are essential components in many electrochemical devices, but they often add substantial cost, limit performance, and are susceptible to degradation. This work investigates membraneless electrochemical flow cells for hydrogen production from water electrolysis that are based on angled mesh flow-through electrodes. These devices can be fabricated with as few as three parts (anode, cathode, and cell body), reflecting their simplicity and potential for low-cost manufacture. 3D printing was used to fabricate prototype electrolyzers that were demonstrated to be electrolyte agnostic, modular, and capable of operating with minimal product crossover. Prototype electrolyzers operating in acidic and alkaline solutions achieved electrolysis efficiencies of 61.9% and 72.5%, respectively, (based on the higher heating value of H-2) when operated at 100 mA cm(-2). Product crossover was investigated using in situ electrochemical sensors, in situ imaging, and by gas chromatography (GC). GC analysis found that 2.8% of the H-2 crossed over from the cathode to the anode stream under electrolysis at 100 mA cm(-2) and fluid velocity of 26.5 cm s(-1). Additionally, modularity was demonstrated with a three-cell stack, and high-speed video measurements tracking bubble evolution from electrode surfaces provide valuable insight for the further optimization of electrolyzer design and performance. (C) The Author(s) 2016. Published by ECS. All rights reserved.