Tapered, interdigitated channels for uniform, low-pressure flow through porous electrodes for desalination and beyond

Abstract

Interdigitated flow fields (IDFFs) stand out among flow fields used in electrochemical energy storage and conversion for their modest pressure drop through electrodes. While recent efforts have sought to optimize flow-field topologies and channel shapes, we present a bottom-up strategy to taper channel cross-sections in IDFFs to maximize flow uniformity, eliminating the dead zones that plague straight channels. A linear variation in channel hydraulic conductance is shown to produce constant inter-channel velocity with minimal pressure drop, and mapping between conductance and cross-section geometry with appropriate Poiseuille flow relations enables their implementation. Using a micro-engraving process, we design and manufacture tapered channels having piecewise-constant cross-sections chosen from a library with different nominal widths and depths. The spacing between channels is then optimized to simultaneously minimize material removal and maximize apparent hydraulic permeability. Such tapered-channel IDFFs are embedded in porous, cation-intercalating electrodes for use in desalination by symmetric Faradaic deionization (FDI), where an increase in hydraulic permeability of tapered channels greater than two-fold compared to straight channels is shown to reduce pumping energy by 62% when desalinating seawater-salinity feeds. Energy efficiency doubles at 50% salt removal as a result, motivating hybridization of FDI with conventional desalination processes. Total energy consumption levels of 7.3 kWh/m³ and 0.69 kWh/m³ to produce freshwater respectively from seawater-salinity and brackish feeds is lower than small-scale reverse osmosis and thermal distillation. Low-pressure, high-efficiency operation enabled by IDFFs designed with optimally tapered channels motivates their broad use in flow-based electrochemical separations, energy storage, and energy conversion.