A hydrodynamic and mass transfer perspective of structured electrodes for electrochemical flow reactors

Electrochemical flow reactors play a crucial role in applications such as electrosynthesis, energy storage, and wastewater treatment, where performance is strongly influenced by mass transfer and fluid flow near the electrodes. Traditional planar or mesh electrodes are limited by low surface area and inefficient flow, while conventional porous electrodes can suffer from high pressure drops and uneven flow distribution. To address these limitations, this work investigates the design, fabrication, and evaluation of structured electrodes, specifically pillar-array and Triply Periodic Minimal Surface (TPMS) geometries. These designs aim to improve mass transfer by optimising flow distribution while balancing surface area and hydraulic resistance using a newly introduced metric: the Hydrodynamic Electrode Performance Factor (HEPF). In the first part of this work, the HEPF metric is introduced to evaluate and compare structured electrodes. HEPF combines mass transfer coefficient (mass transfer limiting region) and pumping losses into a single quantitative figure of merit. To validate this metric, structured electrodes were fabricated from graphite and 316L stainless steel using precision milling for pillar arrays and additive manufacturing for TPMS structures. Their performance was analysed through electrochemical experiments and computational fluid dynamics (CFD) simulations, providing insight into mass transfer coefficients and hydrodynamic behaviour such as wake formation. HEPF proved effective in identifying optimal electrode geometries under different operating conditions. The second part focuses on pillar-array electrodes and examines how their geometry influences hydrodynamic and electrochemical performance. Mass transfer was evaluated using the ferri-/ferrocyanide redox couple with chronoamperometry, while pressure drop was measured using differential pressure sensors. CFD-simulations revealed wake formation even at relatively low Reynolds numbers. The study highlights the importance of analysing pressure drop across structured electrodes. Among the tested configurations, electrodes with small pillar radii (1.5 mm) and small interpillar distances (1.5 mm) showed the highest HEPF values, demonstrating an effective balance between surface area and favourable flow behaviour. The third part evaluates five 3D-printed TPMS electrodes alongside a flat reference electrode. These mathematical 3D structures provide high surface area and controlled porosity, enabling improved flow paths and reduced pressure losses. The Schwarz-D structure exhibited the highest electrochemical performance and HEPF, outperforming other TPMS types such as Fisher-Koch-S and Lidinoid. A key finding is that strong mixing and favourable flow distribution can outweigh surface area in determining performance. Additionally, post-fabrication characterisation of surface area and effective porosity proved essential, as deviations from CAD designs significantly influence flow parameters such as interstitial velocity and Reynolds number. Overall, this work introduces HEPF as a valuable metric for evaluating structured electrodes in electrochemical flow reactors. By linking electrochemical performance with hydrodynamic behaviou, it enables optimisation of electrode geometries. The results provide a tool and modelling approaches for developing energy-efficient, high-performance electrode structures for electrochemical applications.