Alejandro Concheso

Redox flow batteries are attractive systems for large‐scale energy storage due to their capability to uncouple energy and power but still need to make several improvements to reach full commercial scale. The need to search for better components, including electrode materials that allow the internal flow of electrolytes and have optimal electrochemical performance is a hot topic in the development of this kind of battery. The use of direct ink writing technology to engineer complex electrode materials both in the architecture and chemical composition opens a new field of research to optimize electrode performance. In this study, several formulations are prepared using graphite, multiwall carbon nanotubes, and two different Polyacrylonitrile (PAN)‐based short carbon fibers. Furthermore, a graphitizable binder is added to the formulation to help consolidate the printed object into a highly conductive (3000–8000 Sm ⁻¹ ) and mechanically resistant carbon electrode after a moderate heat treatment (800 °C). The 3D electrodes are successfully tested in an all vanadium redox flow cell showing a competitive performance when compared to benchmark electrodes (graphite felts).

Redox flow batteries (RFBs) are attractive electrochemical systems for large‐scale energy storage. Despite the most developed ones being those based on vanadium, the search for new chemistries is essential to overcome several problems associated with this metal identified as a critical raw material. All‐iron redox flow battery (A‐IRFB) is an interesting device due to iron abundance and worldwide distribution. However, the poor performance of its negative half‐cell, due to the sluggish plating/stripping processes related to the Fe²⁺/Fe⁰ redox pair, negatively impacts its energy efficiency and long‐term performance. Here, it is demonstrated that the addition of a low concentration of NaHSO₃ (10 mM), as a novel additive, to an electrolyte formulation based on 0.5 M FeCl₂, 3 M NaCl, and 10 mM citric acid (H₃Cit) remarkably improves the electrochemical behavior of the negative half‐cell. The enhanced performance can be explained as the additive guarantees a low oxygen solution content (reductant agent), promotes the plating/stripping reactions (improving the kinetics of the Fe⁰ deposit through the formation of a FeHSO₃⁺ complex), and diminishes the contribution of the competitive hydrogen evolution reaction. The use of this key additive opens up a promising scenario for the development of A‐IRFBs with significantly enhanced electrochemical performance, thus boosting their potential commercial development.

Graphite felts act as electrodes in VRFBs thanks to their properties such as chemical strength and electrical conductivity or 3D-structure. However, there are significant drawbacks to be overcome, such as their low wettability, sluggish kinetic reversibility and electroactivity towards faradaic processes related to vanadium electroactive species. As a consequence, it is key to alter the fibres to enhance their electrochemical performance during battery operation. Most of the previously reported modifications have been focused on incorporating surface oxygenated functional groups, even though the role of those groups on the electrocatalytic activity towards vanadium faradaic processes is still not clear. Aiming to gain knowledge on this issue, this work investigates the influence of electro-oxidation and electro-reduction treatments, performed in different acidic media (H₂SO₄ or HNO₃ solutions), on their subsequent electrochemical response towards VO²⁺/VO₂⁺ and V³⁺/V²⁺ faradaic processes. The chemical and electrochemical properties of the modified felts were analyzed to understand two key parameters that affect the vanadium reaction catalysis: the depth and oxidation degree of the fibres. A treatment with HNO₃, a strong oxidizing agent, leads to a deep oxidation of the fibre and the development of a high density of oxygenated functional groups, mainly C=O, which hinder the redox reactions of vanadium, especially for the faradaic reactions from the catholyte.

Slags collected from the basic oxygen furnaces of two Linz-Donawitz steel making plants were tested as adsorbents for H₂S removal at room temperature (298 K). Two different particle size fractions, namely 2S g^–1 of slag were attained, in spite of the very low specific surface area of the steel slags. As expected, humidity played a crucial role in the removal of H₂S. Particle size had also an important effect on the capacity of the adsorption beds. Analysis of the exhausted slags revealed considerable amounts of elemental sulfur on the surface of the particles. Sulfates were also found on the exhausted slags, especially on the 212–500 μm size fractions. The characterization of the slags prior and after the H₂S adsorption experiments allowed us to postulate plausible mechanisms to understand the outstanding capacity of these steel byproduct for H₂S adsorption.