Pencil shape pores in porous silicon membrane toward improved efficiencies in reverse electrodialysis energy harvesting system-PSST conference

Hanus, Romain;Alicandro, Sophie;Francis, Laurent
(2024) PSST-Porous Semiconductor Science and Technology 2024 — Location: Brno-Czech republic (28.April.2024)

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1. INTRODUCTION Reverse electrodialysis (RED) principle converts the saline gradient of electrolyte solutions into electrical power thanks to ion-exchange membranes[1]. 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 with 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 with smaller scale RED systems. This higher se-lectivity 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]. Up to now, investigations were mainly focused on membranes with cylindrical-shape pores and have shown that, on one hand, narrow pores show good ionic selectivity but higher resistivity and, on 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 fully investigated [4]. As an alternative, conically shaped pores (cfr. Schema 1(b)) have been demonstrated to give higher effi-ciencies 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 single-pore polymer mem-branes and not with Silicon. In this work, to demonstrate the feasibility of conical shaped pores in Silicon membranes, we have fabricated pSi membranes with various pore shapes, such as conical and pencil shapes, and tested their ionic selec-tive properties as well as the maximal power possible between each of the pore shapes for different gradient concentra-tions in a reverse electrodialysis cell. 2. EXPERIMENTAL RESULTS AND DISCUSSIONS PSi were prepared by electrochemical etching of monocrystalline p-type Silicon from the front side of the wafer with a polySilicon mask. Pore shapes were obtained by monitoring the current density applied during the electrochemical etch-ing. Membranes of 1 mm2 area were then obtained by deep reactive ion etching (DRIE) released from the back with an aluminum hard mask. The substrates were highly Boron-doped (0.6-0.8mOmh.cm) chosen in order to obtain small pores while keeping a high porosity. The former point is a requirement for a good EDL overlap while the latter allows for more pores for the ionic flow, hence giving a small ionic resistance. Finally, the pSi was oxidized in an ambient air environment at 250°C for 1h to be more chemically stable over time in salted waters. Three different shapes were tested, namely cylindrical, conical and pencil (70%-30%). Top view and cross sections SEM images confirmed the reduction of the pore size along the thickness of the membrane for the conical and pencil shapes. Furthermore, since the EDL overlap depends on the pore size and the ionic concentration, and so is the strength of the selective property. An asymmetric selectivity was thus well observed for the conical and pencil pore shapes when switching the concentration gradient, while cylindrical pores were symmetrical under the same circumstances. First, I-V curves were measured to assess the asymmetric ionic selectivity. Second, comparison of the maximal power between each membrane with a given pore shape were computed from the experimental data. Better performances were seen when the small end of the pore is exposed to the lowest ionic concentration. The asym-metry is experimentally observed by I-V tests, while it is absent for cylindrical pores. Furthermore, as shown in Table 1, comparison of the maximal power between each membrane indicates a power density of 1.32 mW/m2 for the pencil shape pSi membrane, the highest between the three pore shapes. Even by considering the thickness difference between the membranes, the ionic resistance of cylindrical would be two times lower. This would only give a maximal power of 0.84W/m2 for the cylindrical pores. Although The energy conversion efficiency is still better with cylin-drical because better selectivity (t+) is reached with long narrow pores, the higher ionic resistance associated with those pores reduces the maximal power achievable. 3. CONCLUSIONS We have successfully demonstrated the potential of pSi membranes with conical and pencil pore shape for improved per-formances as IEM in a RED energy harvesting system. pSi pore shapes were obtained by monitoring the current density during the anodization. SEM images confirmed the obtained membranes had pore size varying along their thickness and I-V test confirmed the asymmetric ionic selectivity. Finally, higher maximal power (1.32mW/m^2) could be seen for the pen-cil shapes due to a better tradeoff between a high ionic selectivity and low ionic resistance compare to the cylindrical pore shape. REFERENCES [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 electrodialy-sis: 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, Université catholique de Louvain-la-neuve [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. (2024). Pencil shape pores in porous silicon membrane toward improved efficiencies in reverse electrodialysis energy harvesting system-PSST conference. PSST-Porous Semiconductor Science and Technology 2024, Brno-Czech republic. https://hdl.handle.net/2078.5/213337