Differential Impact of Surface Conduction and Electroosmotic Flow on Ion Transport Enhancement by Microscale Auxiliary Structures

Our research investigates the impact of auxiliary structures on ion transport in electrochemical systems such as batteries and microscale desalination units, whose importance for sustainable development has increased dramatically in recent decades. The electrochemical systems typically feature ion-selective surfaces, such as electrodes and ion exchange membranes, where ion depletion can cause performance issues, including metal dendrite formation and flow instability. Recent research has shown that auxiliary structures in these electrochemical systems can enhance ion transfer near ion-selective surfaces, thereby resolving the instability problem and improving the energy conversion efficiency of the system. Our study leverages recent advancements in nanoscale electrokinetics to model these auxiliary structures as pillar arrays near an ion exchange membrane in a microchannel. We examine how these structures enhance ion transports relative to the characteristic length scale of microchannel depth and pillars’ proximity to the ion-selective surface. Results show that the effect of pillars varies significantly with their placement. Specifically, in deeper microchannels, where electrokinetic convection is stronger, the closer the auxiliary structure is to the ion-selective membrane, the better the ion transfer. However, in the thinner microchannel, the proximity of the auxiliary structure to the ion selective membrane has less significant correlation with the ion transfer. Therefore, this finding highlights the importance of spatial arrangement of the auxiliary structures in improving the performance of electrochemical devices. Conclusively, this study can help to better understand energy conversion systems such as fuel cells, salinity gradient power generation systems and electrochemical desalination systems, where auxiliary structures can be used in the vicinity of ion-selective surfaces. Especially, our fundamental electrokinetic study provides an effective mean for designing the efficient electrochemical platforms utilizing micro/nanofluidics.

Figure 1. (a) Image of the fabricated device with pillar arrays installed in main-microchannel. Two groups of micropillar arrays were installed in the central region of the main microchannel and these arrays were symmetrically aligned from a nanoporous membrane with different interdistance (L) of 0 μm, 500 μm, 1000 μm, 1500 μm, 2000 μm and 2500 μm. (b) Schematic diagram of main-microchannel with the formation of ion depletion boundary. Ion depletion boundary was symmetrically formed from the nanojunction and the boundary started to propagate through the pillar arrays when external voltage was applied.

Figure 2. The time-evolving snapshots of the concentration profile in the EOF governed regime (d = 15 μm) with (a) L = 0 μm and (b) L = 2500 μm. Applied voltage was 6 V. The propagations of the ion depletion boundary were different in each case. (c) Experimentally measured I-V characteristics as a function of various interdistance. Each curve was obtained from the measurements conducted five times per one device for five devices.

Figure 3. The time-evolving snapshots of the concentration profile in the SC governed regime (d = 3 μm) with (a) L = 0 μm and (b) L = 2500 μm. Applied voltage was 6 V. The length of ion depletion boundaries in each case was almost identical in this regime. (c) Experimentally measured I-V characteristics as a function of various interdistance. Each curve was obtained from the measurements conducted five times per one device for five devices.

Figure 4. (a) OLC enhancement as a function of interdistance in each governed mechanisms. OLC was invariant in the SC governed regime, while OLC depended on L in the EOF governed regime. (b) The time-evolving observation of ion depletion boundary in two mechanisms. Stable boundary was observed in the SC governed regime, while the boundary slightly fluctuated in the EOF governed regime.