PhD Defence Akash Raman | Bubbles and Membranes: Electrolysis at the mesoscale
Akash Raman is a PhD student in the department Mesoscale Chemical Systems. Promotors are prof.dr.ir. D. Fernandez Rivas and prof.dr. J.G.E. Gardeniers from the faculty of Science and Technology .
Heavily polluting processes such as coal gasification and steam methane reforming (SMR) continue to be the dominant sources of hydrogen. On the other hand, water electrolysis offers a clean method to produce hydrogen when connected to renewable energy sources such as wind and solar power. As a result, water electrolysis is expected to play a critical role in reaching global climate change mitigation goals. However, further technological innovations are required for water electrolysis to reach cost parity with fossil fuel-based pathways, and for large-scale adoption.
This thesis focuses on two key sources of inefficiency in electrolyzers – bubbles, and membranes. The unifying strategy employed in the study of bubbles and membranes in this thesis is the use of simplified, well-controlled model systems to elucidate complex puzzles. A major focus throughout this thesis is on understanding the mass transfer phenomena - both at the electrode-electrolyte-gas bubble interface, and across membranes. The design and fabrication of these simplified systems is made possible through silicon photolithography which enables the production of highly precise geometries.
In chapters 1 and 2 uses a ring-pit electrode system wherein a ring-shaped electrode surrounds a super-hydrophobic pit at its center. This hydrophobic pit acts as a preferential nucleation site for electrolytic bubbles thus achieving the spatial decoupling of the site of electrolysis and the site of bubble nucleation. This system simplifies bubble evolution and makes it possible to study successions of electrolytic bubbles one at a time, in the absence of electrode coverage due to bubbles. Experimental data in combination with numerical models showed that the bubbles lower the concentration of dissolved hydrogen near the electrode - leading to a decrease in the concentration overpotential. Despite this, the Ohmic overpotential was seen to lead to a net increase in total overpotential as the bubbles grew larger. In addition to this, direct numerical simulations showed that the bubbles in this study outgrew the concentration boundary layer leading to bubble dissolution at the top.
Chapter 3 adopts a novel electrode architecture with multiple preferential bubble nucleation sites. This controlled system makes it possible to study electrolytic bubbles in the presence of coalescence – as expected on commercially used electrodes. The spacing between the bubble nucleation sites is found to be a key determinant of the bubble departure radius. This proves that the dominant mechanism of bubble departure is through coalescence with adjacent bubbles and opens the possibility for passive bubble control using optimally spaced nucleation sites. Analysis of the growth rates of bubbles driven by different electrolysis currents indicated the presence of a physical phenomenon that enhanced bubble growth rates with increasing current.
Chapter 4 describes the fabrication and characterization of a novel, porous silicon-based, zero-gap electrode for alkaline electrolysis. The porous silicon separator consists of an ordered array of cylindrical pores. As with the distance between the pits in Chapter 3, the distance between the pores on this electrode determines the separator resistance and gas crossover. The ionic resistance is found to increase with decreasing porosity, while the gas crossover decreases with decreasing porosity.
