Water electrolysis, hydrogen, bubble behaviour, membrane-free water electrolyser

Abstract

The global challenge of rapid population growth and environmental degradation has spurred the search for sustainable, clean, and low-carbon energy solutions. Hydrogen has been widely used as a feedstock for chemical processes and is recognised as a clean alternative to traditional fuels. Currently, a significant portion of H2 production relies on fossil fuel-based processes, contributing to global warming and toxic gas emissions. Consequently, there is an increasing interest in investigating different approaches for H2 production from non-fossil sources. Among these, water electrolysis has emerged as a leading method for generating clean H2. The membrane-free water electrolyser (MFE) is a novel cell design that eliminates the membrane in a conventional water electrolyser. This innovative approach not only reduces both capital and maintenance costs but also facilitates the increase of operating temperature and pressure, resulting in improved cell efficiency. Moreover, it reduces the overall cost of H2 considering the pressurisation or liquefaction of H2 for transportation. However, the membrane-free design faces challenges related to gas crossover and scalability, and these factors are closely related to bubble behaviour in the electrolyser channel. The research in this thesis is motivated by the need to address these challenges. The research aims to minimise the gas crossover in MFEs by developing the technique to better control bubble distribution in the electrolyser channel. The thesis first demonstrates the feasibility of using MFEs for industrial-scale liquid H2 (LH2) production. While MFE offers lower ohmic resistance compared to conventional ones, they face challenges in producing high-purity H2 due to the lack of a physical barrier to separate H2 and O2. To address this issue, a cryogenic cooling system is employed to purify H2 gas by removing O2. The specific energy consumption for LH2 production using MFEs and conventional alkaline water electrolysers under different operating conditions has been calculated. Findings indicate that MFEs could potentially provide more cost-effective operations in the production of LH2, since the electric power saved from their higher efficiency outweighs the cost of oxygen removal. Furthermore, improving the product H2 purity further reduces cooling load requirements, leading to reduced operating costs. This emphasises the significance of effectively managing bubble distribution in an MFE. To facilitate the investigation of bubble behaviour in MFEs, it is necessary to develop a technique that can generate bubbles in an electrolyser channel with controllable size and frequency. Consequently, the research focuses on the study of H2 bubble formation and departure from a microelectrode in an MFE. Systematic studies were carried out to examine the impact of applied current and the geometry of microelectrodes on the size and frequency of bubbles. Additionally, the process of single and continuous bubble formation is investigated. Building upon these findings, an electrolytic microbubble generator is developed, capable of producing single bubbles with diameters ranging from 0.3 mm to 1.4 mm, which encompasses the control target – the large bubbles with diameters greater than 0.3 mm, formed through coalescence in MFEs. Employing the technique of generating single microbubbles, this research includes an experimental study focused on the rising trajectories of H2 bubbles in an electrolyser channel. The lift and drag forces on bubbles are assessed based on their rising trajectories, and a new correlation is proposed for estimating the shear-induced lift forces on bubbles with diameters ranging from 0.3 mm to 1.0 mm. The study highlights the critical role of the electrolyte's velocity field in governing bubble distribution within an MFE. It is shown that maintaining a parabolic velocity profile in the electrolyte allows H2 and O2 bubbles to rise close to the electrode where they evolve. However, as the void fraction increases, bubbles near the electrode induce a high local velocity, disrupting the velocity field and leading to gas crossover. This highlights that depending solely on applying a parabolic electrolyte flow at the inlet of the electrolyser channel may not be effective in eliminating gas crossover in MFE. The final part of the research introduces a novel technique to suppress the crossover of H2 and O2 within the MFE channel. The approach involves the implementation of flow controllers to manipulate the velocity field in the electrolyser channel, generating a lift force that prevents H2 and O2 bubbles from mixing. This significantly alleviates gas crossover within the electrolyser. While this technique does not completely prevent gas crossover, it provides valuable insights into the potential of active flow control methods for enhancing the performance of MFE. The thesis makes significant contributions by developing a new understanding of bubble behaviour in an MFE, along with the development of techniques to control bubble distribution within such systems. The research demonstrates the viability of MFEs for LH2 production, positioning this technology as an appealing option for clean H2 generation. The creation of the microbubble generator introduces a straightforward yet highly effective tool for conducting fundamental studies on bubble behaviour. Additionally, the study on forces acting on rising bubbles highlights the critical role of the velocity field of the electrolyte in governing bubble distribution, a key factor for managing gas crossover. Moreover, the innovative flow controllers effectively address gas crossover issues and enable the scaling-up of MFEs. These findings provide valuable insights into the MFE technology, propelling the prospects of efficient and sustainable H2 production. The research outcomes contribute significantly to the clean energy sector, fostering the implementation of membrane-free water electrolysers as a promising pathway towards a greener future.