Advancing All-Iron Redox Flow Batteries: electrolytes, electrodes and diagnostics

Renewable energy technologies have experienced rapid development and cost reductions, thus becoming increasingly cost-competitive with conventional fossil-based generation. Despite these advances, a fundamental challenge remains: the inherent variability and intermittency of renewable resources are still poorly aligned with the continuous and reliable supply demanded by modern power grids. Without reliable large-scale energy storage solutions capable of buffering temporal fluctuations in supply and demand, the full integration of renewables into global electricity networks cannot be achieved. Electrochemical storage systems represent one of the most promising pathways to bridge this gap, with redox flow batteries (RFBs) standing out due to their flexible scaling capabilities, broad operation conditions and discharge durations. Furthermore, the inherent versatility of RFBs allows for the deployment of different redox chemistries, thereby providing opportunities to balance cost, efficiency, and sustainability considerations. Within this broad design space, all-iron redox flow batteries (AIRFBs) are particularly attractive, owing to the abundance, low cost, and environmental compatibility of iron. However, their widespread implementation has been limited by a number of persistent challenges, including inefficient electrodeposition control, the parasitic hydrogen evolution reaction, suboptimal electrode architectures, and an incomplete understanding of internal transport and degradation mechanisms. The central objective of this dissertation is to address these bottlenecks through a multifaceted approach that combines electrolyte engineering, the design and optimization of electrode materials and architectures, and the application of advanced operando diagnostic tools. Through these strategies, this work seeks to enhance the performance, efficiency, and long-term durability of AIRFBs, advancing their potential as a viable platform for grid-scale energy storage. In Chapter 1, a general introduction establishing the current energy paradigm is presented. The main alternatives for mid- to large-scale energy storage solutions are described, with a brief overview of their characteristics. Next, the potential of electrochemical energy storage is described as an option to aid in the expansion of renewable energy sources and reach carbon neutrality, before providing a more comprehensive analysis of a promising alternative for scalable energy storage: the redox flow battery, which is the focus of this dissertation. The different components and chemistries of redox flow batteries are explained, in addition to describing the main challenges for widespread implementation. The all-iron redox flow battery is then presented as a promising candidate, as it is reliant on sustainable, safe, abundant and cost-effective raw materials. The working principle of all-iron redox flow batteries is described and finally, the main goals of this dissertation are highlighted. In Chapter 2, the fundamentals of the iron electrodeposition were investigated within the context of all-iron rechargeable batteries. The process efficiency and material and system lifetimes are notably limited by the poorly understood deposition process and the parasitic hydrogen evolution reaction when operating in aqueous media. To bridge this knowledge gap, a methodology was introduced to reliably investigate the formation, growth and stripping of metallic iron and extract relevant metrics about the plating kinetics and spatial distribution of the iron plating centers over a range of potentials practical to battery operation. Subsequently, a number of molecules of different nature (e.g. organic, inorganic acids of increasing protic strength) were screened, studying their effects in regulating the electroplating process and/or suppressing side reactions. It was found that certain additives form highly stable iron-complexes that produced plated layers of superior quality due to a balance between nucleation enhancement and epitaxial electrodeposition with hydrogen evolution suppression, which ultimately resulted in increased cyclability. In Chapter 3, the use of metal-based electrode alternatives was explored, with the aim of overcoming the challenges of the conventional carbon fiber electrodes. More precisely, steel materials were investigated as due to their chemical and mechanical stability and lower production costs and compared to baseline materials such as pure iron and carbon fibers. First, different flat metal substrates, such as ferritic, martensitic or austenitic materials, were tested in a flow-by configuration, resulting in different electrochemical performance due to their microstructural properties and metallurgical phases. Advanced, three-dimensional structures were successively tested in a flow-through configuration, screening various fiber arrangements in order to elucidate the structural effects on mass transfer and their electrochemical polarization and cycling performance. In Chapter 4, the overall performance of an all-iron redox flow battery was investigated by employing a range of carbon materials. More specifically, the influence of their surface chemistry and electrode architecture was investigated. Firstly, a range of commercially available materials (e.g. carbon felt, paper, cloth) conventionally used in electrochemical flow cells, and based on carbon fiber mats, is investigated. To expand the design workspace of electrodes in hybrid, phase-changing systems, a novel electrode manufacturing technique based on non-solvent induced phase separation is explored. With this new synthetic route, a variety of electrode alternatives, with interconnected microstructures and a range of pore sizes and pore size distributions, can be obtained. The influence of some of the main critical synthesis parameters were tested, namely the casting thickness, the phase inversion step and the carbonization conditions. Through these, electrodes with different electrode-surface-area to volume ratios, porosity profiles and heteroatom contents were obtained. Finally, the impact of the microstructural and surface chemistry properties of the diverse manufactured electrodes were then methodically assessed, with the resulting electrochemical performance and fluid dynamics in the all-iron redox flow system. In Chapter 5, a coupled diagnostics method is developed, by combining electrochemical techniques with a high-sensitivity dual neutron imaging. This dual characterization mode enabled the correlation between cell-level macroscopic information with local transport and reaction phenomena within the electrochemical reactor. With this dual neutron radiography approach, the two main operational challenges of all-iron redox flow batteries were addressed in operando conditions. Firstly, neutron transmission mode and subtractive imaging enabled the visualization of hydrogen gas evolution and its dynamics within the cell. Secondly, by means of polarization contrast neutron imaging, the time- and space-resolved electrodeposition and stripping of iron during battery operation was tracked, due to the strong ferromagnetic properties of the deposited metal. A deeper insight into iron plating dynamics was obtained, which proved crucial for battery performance optimization as the observed non-uniform current distribution in the electrode leads to irregular iron plating and stripping, and thus electrolyte degradation and decreased system lifetime. More precisely, the dynamic plating patterns were mapped as a function of cycle number, in addition to studying the effects of the applied of current density on the distribution of iron plated on the porous electrode. Furthermore, the role of pH on the battery operation, reaction distribution and capacity decay were investigated. Finally, the effect of the flow field architecture on the iron plating and the hydrogen evolution distribution was studied, coupled with the influence of the flow rate on the mass transport limitations of the system. Through this systematic approach, insights into performance-limiting processes were obtained, which relate to inhomogeneous iron plating, formation of non-accessible iron deposits and hydrogen gas accumulation in the reactor and guidelines for improved operating conditions and reactor design are provided. In Chapter 6, the main findings of the present work are summarized. Here, an outlook is included, where research directions are proposed to accelerate the improvement of the performance of all-iron redox flow batteries, extensible to other hybrid flow cell systems, with the final aim to aid the widespread implementation of flow batteries for grid-scale energy storage. Collectively, the studies presented in this dissertation contribute towards advancing a holistic understanding of the factors governing performance, reversibility, and durability in all-iron redox flow batteries. By integrating electrolyte engineering, electrode design, and operando diagnostics, this work demonstrates how electrolyte composition, electrode architecture, and flow dynamics interact to control iron deposition behavior and parasitic reactions. The systematic investigation of these interrelated processes revealed that further developments of the all-iron redox flow battery system require a coupled approach, based on co-optimization of chemical stability, microstructural characteristics, and hydrodynamic control rather than on their independent treatment. Beyond incremental performance improvements, the methodologies developed in Chapters 2-5, establish a foundation for mechanistic and predictive studies of complex multiphase electrochemical systems. Together, these contributions provide a pathway for the rational design and implementation of efficient, durable, and sustainable AIRFBs, supporting their evolution from a promising concept toward practical grid-scale energy storage solutions.