Reynolds number driven void fraction and ionic resistivity in a modular membraneless flow-by electrolyzer

Abstract

A modular membraneless flow-by electrolyzer was experimentally evaluated to determine how the Reynolds number (Re) governs the void fraction and the resulting ionic resistivity of the electrolyte. The study addressed the challenge of bubble-induced ohmic losses, a key limitation in membraneless hydrogen generators. The objective was to quantify the coupling between flow regime, gas holdup, and resistive behavior under controlled operating conditions. The prototype cell (active volume: X cm³, electrode gap: Y mm) was operated with KOH 1M and tested at flow rates corresponding to Re = A–B. Void fraction was measured using [método empleado, Directo  Análisis de video (Visualización óptica / PIV simplificada), Indirecto  A partir de conductividad efectiva], and electrolyte resistivity was obtained from polarization data at current densities of C–D A·cm⁻². An increase in Re from R₁ to R₂ reduced the void fraction by V%, and the ionic resistivity decreased by W%, leading to an improvement of energy efficiency of E%. Hydrogen generation rate reached F mmol·h⁻¹·cm⁻² at the highest Re tested. These trends were consistent with CFD predictions, which showed thinning of the ionic boundary layer and enhanced convective renewal near the electrode surfaces. Overall, the results demonstrated that manipulating the flow regime is an effective mechanism to mitigate bubble accumulation and reduce ohmic penalties in membraneless systems. These findings support the development of low-cost, scalable hydrogen generators as an alternative to PEM devices, with potential ohmic losses reduced by up to G% relative to reference membrane-based benchmarks.