Towards membraneless alkaline water electrolysis by improved bubble management

Green hydrogen produced by water electrolysis using renewable energies has become a central technology for the transition towards a carbon-neutral industry, thereby replacing fossil fuels with renewable energy sources. Alkaline water electrolysis represents a promising and mature technology, with the potential to produce high-purity green hydrogen on a large scale. Nevertheless, compared to conventional methods such as steam methane reforming, alkaline water electrolysis is currently not economically viable. This is due to the low efficiency of alkaline water electrolysis, which is attributable to several factors, including losses at the gas evolving electrodes where the hydrogen and oxygen evolution reactions take place. The evolving gas bubbles block the active catalytic centers on the electrode surface and increase the electrolyte resistance, resulting in increased activation, concentration, and ohmic overpotentials. Thus, the thesis begins by investigating the influence of electrode surfaces on bubble dynamics. The aim is to create surfaces that can efficiently remove bubbles from catalytic sites, thereby improving electrode performance by reducing the bubble-related overpotentials. A novel three-electrode cell is introduced to run parametric studies of hydrogen and oxygen evolution in an alkaline electrolyte on various electrodes. Electrochemical methods are combined with a high-speed optical measurement system to simultaneously characterize the electrode performance and the bubble dynamics. This thesis focuses on utilizing readily available materials and scalable manufacturing techniques. For this reason, nickel is chosen as the electrode material because it is widely used in alkaline electrolyzers. First, a proof-of-concept study is presented to investigate the influence of a forced flow through porous electrodes. Three different expanded metals out of nickel are investigated at applied current densities of j = –10 to –200 mA cm–2. The applied current density is found to significantly influence both the size of detached bubbles and the surface coverage of the electrodes. The forced flow through the electrodes strongly reduces the bubble size up to current densities of about –100 mA cm–2. Second, the electrode surface is optimized in terms of bubble management using laser-structuring techniques to increase the overall efficiency of alkaline water electrolysis. Here, two structures on nickel electrodes are investigated: linear- and pillar-patterned surfaces. The former is produced using Direct Laser Interference Patterning and a systematic experimental study is performed to analyze the influence of the spatial period and structure depth on the electrode performance. Using a statistical design of experiments, it is found that the spatial distance between the line-patterned laser structures is the decisive processing parameter for lowering the electrode overpotentials. The electrochemically active surface area could be increased by a factor of 12 compared to a non-structured electrode. For the oxygen evolution reaction, a significantly lower onset potential and overpotential (≈ –164 mV at 100 mA cm–2) is found compared to an unstructured electrode. This is explained by the superhydrophilic behavior of the laser-induced grooves of the line-patterned surfaces and their influence on bubble growth. Consequently, there are fewer active nucleation sites and larger detached bubbles. Together with the fully wetted electrode surface, this results in reduced electrode blocking and lower ohmic resistance. The subsequent section introduces dual-wetting, pillar-patterned surfaces produced using Direct Laser Writing to further control bubble nucleation and growth. These surfaces combine hydrophilic grooves with hydrophobic ripples on top of each pillar. The laser-induced grooves still show superspreading properties due to the capillary forces within them, which enables a fully wetted surface. However, the hydrophobic pillars now serve as initial nucleation sites where the bubbles remain pinned during their growth. This results in a larger detachment size of the bubbles. In combination with an increase in the electrochemically active surface area by a factor of 9, a reduction in the overpotential for hydrogen evolution reaction of ≈ 24 % is found at –100 mA cm–2 compared to a non-structured electrode. Third, exploring new cell designs can lower the capital expenditures for alkaline water electrolyzers. Therefore, a novel Y-shaped membraneless flow-through electrolyzer is introduced to achieve a homogeneous electrochemical reaction across the entire electrode in a cost-efficient cell design with effective product separation. Numerical simulations of the electrolyte flow and electrical current within the already known I- and T-shaped cells motivate the newly proposed Y-shape cell. Furthermore, a new design criterion is developed based on the balance between bubble removal and gas generation. The experimental results using the Y-shaped electrolyzer serve as proof-of-concept, demonstrating homogeneous gas distributions across the electrode and efficient product separation via electrolyte flow. The Y-shaped membraneless alkaline electrolyzer is then studied in greater detail regarding its process limits. It is found that the electrolyte flow is essential to detach the bubbles at an early stage and therefore, guarantee a small bubble size. This is needed to enable safe operation without any blockage of the electrode gap. Furthermore, by changing the electrode porosity, the minimum electrolyte flow rate needed to ensure a bubble-free electrode gap is decreasing with increasing porosity. By mounting laser-structured electrodes into the membraneless cell, the cell voltage can be significantly lowered. Compared to a Y-shaped electrolyzer with non-structured electrodes, this results in an improvement of the overall cell voltage of 7.7 %. Finally, another application of laser-structured electrodes is presented. To study the influence of bubbles at elevated absolute pressures, different bubble sizes are needed to determine how they affect the electrode potential. Thus, different sized pillar-patterned electrodes were mounted in a pressurized alkaline electrolyzer to allow for the formation of hydrogen bubbles of various sizes. At a low current density of –25 mA cm–2, the agreement between the experimental results and the theoretical thermodynamic voltage losses as a function of the absolute pressure is good for all studied electrodes. Hence, the cathodic electrode potential rises with increasing pressure. A higher current density of –100 mA cm–2 results in more dominant bubble-induced losses. Due to the smaller bubble sizes at elevated pressures, the electrode potential and thus the efficiency improve with increasing pressure. The dissertation concludes with a summary of the key findings and provides an outlook on future work that builds on the insights gained here.