Zero-Gap Porous Silicon Membrane Electrodes for Alkaline Electrolysis

Abstract

Despite only accounting for a fraction of the annual global hydrogen production, commercial application of water electrolysis is expected to grow significantly in line with global climate action goals1. Furthermore, water electrolysis driven by renewable energy is expected to aid in nullifying the intermittency of renewable energy sources such as wind and solar photovoltaics2.Two main types of water electrolysis have found widespread commercial application – alkaline water electrolysis (AWEs) and polymer electrolyte membrane electrolysis (PEMEs). While PEMEs operate at higher current densities and higher efficiencies, they require expensive precious metal catalysts such as Ir-based cathodes and Pt-based anodes3. In addition, PEMEs generally have a shorter lifespan than AWEs4. Therefore, AWEs are better suited for gigawatt-scale deployment5. Furthermore, it has been shown that AWEs with Ni-based catalysts can compete with Ir and Pt-based PEMEs if a dramatic reduction in the thickness of the separator can be achieved6.A key development in AWEs is the adoption of a zero-gap electrode (ZGE) configuration in which the electrodes are pressed directly on the surface of the separator. It is possible to leverage silicon-based lithographic techniques to produce separators with well-defined pore sizes and porosities, which are significantly thinner than commercially available AWE separators. This has been previously demonstrated for photoelectrochemical devices7. Such porous silicon devices typically suffer from the high gas crossover. One method to limit gas crossover is infilling the pores with ionomers. However, due to volumetric expansion in electrolytes, ionomers – such as the commonly used PEME membrane material, Nafion – are not suitable for microfabricated devices8.In this work, we report the fabrication of stable porous silicon separators with metal electrodes in a zero-gap configuration. These porous silicon separators exhibit ionic resistance comparable to Zirfon (the most commonly used AWE separator9) but at an order of magnitude lower porosity. The ordered and well-defined pore structure of the microfabricated separators allows us to study the influence of separator properties such as pore size and porosity on ionic conductivity and gas crossover in cylindrical pores with no interconnections. References International Energy Agency, The Future of Hydrogen, IEA, IEA, Paris, (2021), p. 203 https://www.iea.org/reports/hydrogen.L. M. Pierpoint, Energy Policy, 96, 751–757 (2016).A. S. Gago et al., ECS Trans., 85, 3 (2018).M. David, C. Ocampo-Martínez, and R. Sánchez-Peña, Journal of Energy Storage, 23, 392–403 (2019).M. Schalenbach, Aleksandar R. Zeradjanin, Olga Kasian, Serhiy Cherevko, and Karl J.J. Mayrhofer, Int. J. Electrochem. Sci., 1173–1226 (2018).M. Schalenbach et al., J. Electrochem. Soc., 163, F3197 (2016).W. J. C. Vijselaar et al., Advanced Energy Materials, 9, 1803548 (2019).T. Pichonat and B. Gauthier-Manuel, Fuel Cells, 6, 323–325 (2006).R. Phillips and C. W. Dunnill, RSC Adv., 6, 100643–100651 (2016).