Pencil shape pores in porous silicon membrane toward improved efficiencies in reverse electrodialysis energy harvesting system
(2023) Journées SCOPe 2023 — Location: Louvain-la-neuve (9.November.2023)
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- Reverse electrodialysis (RED) principle convert saline gradient of electrolyte solutions into electrical power thanks to ion-exchange membranes. Although, the energy densities currently reached (0.6 Wh/L) are still low compared to the common lithium battery (200-500 Wh/L), RED could meet the energy needs of low-power Internet-of-Things nodes (0.5 mW – 10 mW) and be directly integrated in silicon technologies [1]. In this view, inorganic nanostructured materials have attracted an increasing interest as ion-exchange membranes [2] due to their potentially higher ionic selectivity for an improved efficiency. This higher selectivity results from the overlapping of the electrical double layer (EDL) in the nanostructure of the membrane and RED processes using such membranes are called nano-fluidic reverse electrodialysis (nRED). Porous silicon (PSi) is a good candidate for nRED membranes as its structural attributes are controllable during the multiple fabrication processes, i.e., dry and electrochemical etching, and it is one common material in the integrated semiconductors industry [3]. Currently, research have only studied membranes with cylindrical-shape pore and have shown that, on one hand, narrow pores show good ionic selectivity but higher resistivity and the other hand, larger pores will present lower selectivity and resistivity, resulting in both cases in lower power density. PSi with such pore shape has already been studied in a previous study (0.21 mW/m2 in 10 mM vs. 1 mM NaCl solutions) but the advantages of PSi are yet to be investigated [4]. As an alternative, conically shaped pores have been demonstrated to give higher efficiencies due to the trade-off between the good ionic selectivity from small pores and the low ionic resistance of the large pore and have been demonstrated to give higher efficiencies [5,6], but have only been done on a single-pore polymer membranes and not silicon. In this work, we have fabricated PSi membranes with various pore shapes, such as conical and pencil shapes, by monitoring the current density applied during the electrochemical etching steps on heavily p-doped silicon substrates. The SEM images indicate a reduction of the pore size along the thickness of the membrane. Because the EDL overlap depends on the pore size and the ionic concentration, and so the strength of the selective property, better performances are seen when the small end of the pore is exposed to the lowest ionic concentration. This asymmetry is experimentally observed by I-V tests (cfr. Figure 3), while it is absent for cylindrical pores. Furthermore, a comparison with the latter membrane indicates a greater power density of 1.32 mW/m2 for the pencil shape PSi membrane. [1] Ramato Ashu Tufa, et. Al., Progress and prospects in reverse electrodialysis for salinity gradient energy conversion and storage. Applied Energy, Volume 225, 1 September 2018, Pages 290-331. [2] Azadeh Nazif, et. Al. Recent progress in membrane development, affecting parameters, and applications of reverse electrodialysis: A review. Journal of Water Process Engineering, Volume 47, June 2022, 102706 [3] Roselien Vercauteren, et. Al., Porous silicon membranes and their applications: Recent advances. Sensors and Actuators A: Physical, Volume 318, 1 February 2021, 112486 [4] Hanus R. (2020) Harvesting blue energy using porous silicon. Master’s thesis, UCLouvain [5] Hung-Chun Yeh, et. Al., Reverse electrodialysis in conical-shaped nanopores: salinity gradient-driven power generation. RSC advances, Issue 6, 2014. [6] Liuxuan Cao, et. al. Towards understanding the nanofluidic reverse electrodialysis system: well matched charge selectivity and ionic composition. Energy & Environmental Science, Issue 6, 2011. [7] P Apel. Track etching technique in membrane technology. Radiation Measurements, Volume 34, Issues 1–6, June 2001, Pages 559-566.
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Hanus, R., Alicandro, S., Francis, L., & et al. (2023). Pencil shape pores in porous silicon membrane toward improved efficiencies in reverse electrodialysis energy harvesting system. Journées SCOPe 2023, Louvain-la-neuve. https://hdl.handle.net/2078.5/233241