Matteo Zago

Vanadium redox flow battery (VRFB) is a promising technology for stationary energy storage because of the independent energy to power ratio and the long cycle life. However, the competitiveness of the technology is hindered by some technological issues, among which the severe capacity decay induced by vanadium cross-over, that results in the adoption of relatively thick membranes.

Redox flow batteries (RFBs) are commonly addressed as a promising post-lithium technology for grid energy storage. Among RFBs, Vanadium Redox Flow Batteries (VRFBs) are the most consolidated since they exploit vanadium-based electrolyte in both the half-cells. The keys advantages of this technology are the independent energy to power ratio and its intrinsic safety and durability. However, VRFB is characterized by technological issues such as the transport of vanadium-ions and water through the membrane. These phenomena cause the imbalance of the electrolytes which result in a dramatic capacity loss. If on the one hand through periodic rebalancing procedures it is possible to mitigate this performance loss, on the other hand these procedures introduce high operation and maintenance costs. In literature, Luo et al. observed as while vanadium concentrations almost stabilize after the initial capacity decay, water transport continuously causes electrolyte volume variation in the two tanks [1]. Moreover, through the coupling of experimental mitigation and modelling analysis Toja et al. [2] and Shin et al. [3] have evidenced how osmosis is responsible for the volume variation during the first cycles and how through the modification of the electrolytes it is possible to mitigate water transport. However, what literature still lacks is a complete description of water transport during long term cycling. In this work, combining experimental and modelling activities, vanadium and water transport is completely quantified and analyzed during more than 400 charge and discharge cycles with fixed cut-off voltages (1-1.65 V) and constant current (0.1 A cm ^-2 ). The 25 cm ² VRFB cell employs an Aquivion ^® membrane (E98-05) and carbon felt electrodes (Sigracell ^® GFD 2.5 EA thermal activated). The vanadium content of the 50 ml of electrolyte used (initial composition of 1.6 M vanadium dissolved in 2 M of sulphuric acid) has been monitored throughout the duration of the experimental test thanks to Inductively Coupled Plasma-Optical Emission Spectroscopy (ICP-EOS). Moreover, the positive and negative electrolyte volumes have been recorded daily. The coupling of these measurements has allowed to experimentally quantify the observed electrolyte evolution. In order to provide a critical analysis of water transfer, a previously developed 1D model [4] has been integrated with an innovative detailed description of water transport mechanisms. The main water transport mechanisms involved in the model are three: osmosis (OSM), Electro-Osmotic Drag (EOD) and vanadium ion drag (DRAG). The first refers to the spontaneous movement of solvent molecules (water in this case) through a selectively permeable membrane from a diluted solution to a concentrated one, the other two mechanisms refer to the water molecules that, being electrostatically bonded to the ion dissolved in the electrolyte in the so-called Hydration Shell (HS), are dragged by the movement of the ion itself. In particular, new equations based on the physics of electrolyte solutions have been implemented. As a matter of fact, a distinction between bonded water molecules (typical of hydration shells) and free water molecules is proposed. After calibration, the model has demonstrated its robustness since has been able to reproduce the experimental trend of the vanadium moles variation in the two tanks, the volume variation experienced by the positive and the negative electrolyte tanks along with the battery discharged capacity, battery energy efficiency and cell voltage. The osmotic flux has resulted in the dominant effect since after almost 300 hours of test it causes a variation of 17 ml which increases the positive volume and decreases the negative one. The model has confirmed the results present in literature: what occurs in the first charge and discharge cycle is influenced by osmosis which causes the initial increase of 4 ml of the positive volume and the resulting loss of 4 ml for the negative volume. Finally, the model suggests that in order to mitigate water transport and the resulting electrolyte volume variation it is necessary to limit the osmotic mechanism. This is possible by a modification of the osmotic driving force (positive and negative electrolyte free water molar fraction difference). References [1] Luo et al. ChemSusChem 6 (2013) 286-274. [2] Toja et al. Applied Energy 354 (2024) 122262. [3] Shin et al. Journal of Power Sources 526 (2022) 231144. [4] Cecchetti et al. Journal of Energy Storage 68 (2023) 107846. Figure 1

The evolution of the global energy scenario highlights the necessity of reliable, scalable and efficient energy storage systems due to the expanding share of energy production from not programmable and intermittent energy sources. Among the different energy storage technologies, Vanadium Redox Flow Batteries (VRFBs) attracted increasing interest due to their unique features, such as decoupled energy and power and long cycle life [1]. Indeed, the ability of storing energy in external tanks containing liquid electrolytes of dissolved vanadium ions permits for flexible designs and low safety risks. However, due to the high and volatile cost of vanadium electrolyte and the intrinsically low energy density, the competitiveness of VRFB is hindered for energy storage time higher than 12 hours, while in the next future the increasing penetration of renewables will inevitably require higher storage times. This work presents the development of a dual redox flow battery based on the adoption of a hybrid hydrogen-vanadium flow battery [2] coupled with a vanadium-oxygen cell, permitting to increase the storage time thanks to the possibility to produce additional hydrogen. A simplified schematic is reported in Figure 1. The concept of dual redox flow batteries based on the adoption of vanadium chemistries has been already demonstrated in [3], in which an all-vanadium flow battery was coupled with a catalytic reactor placed at the negative side of a VRFB and a vanadium-oxygen cell at the positive side [3]. In this work the catalytic reactor for hydrogen production was removed and the all-vanadium flow battery was replaced by a hybrid hydrogen-vanadium flow battery. During charge, the hydrogen-vanadium battery produces hydrogen at the negative electrode while at the positive electrode VO ²⁺ oxidation occurs. With a conventional hydrogen-vanadium flow battery the stored energy is defined by the amount of vanadium electrolyte and when it is completely converted to VO ₂ ⁺ the battery is fully charged. However, the presented dual flow configuration permits to store additional energy with further hydrogen production, that is possible with the utilization of the vanadium-oxygen cell, leading to VO ₂ ⁺ reduction. In this work the hydrogen-vanadium flow battery was composed by a Pt/C negative electrode with a Pt loading of 0.5 mg cm ^-2 , a Nafion ^® 211 membrane and a Freudenberg ^® H23 as positive electrode. During charge, the average voltage values were 1.11 V to 1.2 V at an operating current density of 50 mA cm ^-2 and 250 mA cm ^-2 , respectively. Instead, the vanadium-oxygen cell was composed by a DSA mesh positive electrode, a Nafion ^® 117 membrane and a thermically activated carbon felt as negative electrode. This cell can operate with vanadium electrolyte at high state of charge and presented an average voltage value of 0.6 V and 1.05 V at an operating current density of 20 and 100 mA cm ^-2 , respectively. In the dual redox flow battery OER and HER can be decoupled in time, thus permitting to design different nominal operating conditions between the hydrogen-vanadium flow battery and the vanadium-oxygen cell. Due to the higher overpotentials of the latter, this cell was operated at lower current densities. Considering the preliminary results of the presented proof of concept, the efficiency of hydrogen production can be estimated between 72% and 55%. These results prove the potentialities of the developed dual flow battery system, that can increase the storage time with limited volumes of vanadium electrolyte. Further improvements in system performance are expected working on the optimization of the vanadium-oxygen cell. Figure 1 – Schematic of the developed dual redox flow battery. Acknowledgement This work has been performed within the MIAMI (Materiali Innovativi per Sistemi di Accumulo Ibrido) project, funded by Fondo per la Ricerca di Sistema Elettrico – CSEA, reference number CSEAA_00014. References [1] M. Cecchetti et al., Electrochimica Acta 517 (2025) 145750. [2] V. Yufit et al., Journal of The Electrochemical Society 160 (2013) A856. [3] J. Piwek et al., Journal of Power Sources 439 (2019) 227075. Figure 1

Vanadium redox flow batteries (VRFBs) represent a promising technology for the integration of stationary energy storage with renewable energy sources, particularly solar and wind. These sources are characterized by significant fluctuations and intermittency, which necessitate reliable energy storage solutions. A distinctive advantage of VRFBs lies in their ability to decouple energy storage capacity—determined by the volume of the electrolyte—from power output, which depends on the size of the electrochemical device. Despite their potential, the widespread commercialization of VRFBs is still hindered by several technological challenges that emerge during system scale-up. Among these are the issues of cross-contamination between the positive and negative electrolytes, as well as the difficulty of ensuring effective and uniform distribution of the electrolyte throughout the porous electrode structure. In this study, several design and process improvements previously reported in the literature [1–2] are further developed and adapted for large-area cells within the framework of the Italian project Multilayered Urban Sustainability Action (MUSA) , which promotes social and environmental sustainability in urban contexts. The impact of these improvements on performance and durability is critically assessed with a view toward scaling up to full-stack and system-level applications. This work has been partially performed within the MUSA – Multilayered Urban Sustainability Action – project, funded by the European Union – NextGenerationEU, under the National Recovery and Resilience Plan (PNRR) Mission 4 Component 2 Investment Line 1.5: Strenghtening of research structures and creation of R&D “innovation ecosystems”, set up of “territorial leaders in R&D. CUP: ECS MUSA, D43C22001410007”.

Vanadium Redox Flow Battery (VRFB) is a promising technology for stationary energy storage because of its long cycle life and independent energy to power ratio. However, the competitiveness of the technology is hindered by some technological issues, among which vanadium cross-over, that is the undesired transport of vanadium ions trough the ion-exchange membrane, inducing capacity loss and electrolytes imbalance. For this reason, VRFB usually employs thick separators, leading to higher ohmic losses and increased system cost. Moreover, also the high cost of both positive and negative vanadium electrolytes further limits the potential widespread of the technology. This work presents the development of a high-performance hydrogen vanadium redox flow battery [1,2] operating with a thin separator. In particular, the influence of membrane thickness and equivalent weight (EW) has been analysed in a 25 cm² single cell during charge-discharge cycles at constant current densities, imposing 0.8 V and 1.3 V as lower and upper voltage limits. The positive electrode was a Freudenberg H23, while the negative one a commercial Pt/C gas diffusion electrode with 0.6 mg cm^-2 of Pt loading. The vanadium electrolyte was a commercial solution of 1.6 M vanadium in 2 M of sulfuric acid. The cell hardware was also equipped with local reference hydrogen electrodes [3], permitting to monitor the evolution of electrodes potential during operation. Thicker membranes and higher EW resulted in worse battery performance, in particular during the charge phase of the battery at low state of charge. This unexpected behaviour has been further analysed with the support of a 1D physics-based VRFB model [4]. Model simulations evidenced the importance of charge balance in the membrane for an efficient battery operation. Higher membrane resistance, associated with an increased thickness or a high EW, hinders the back diffusion flux of V³⁺, that is formed at the negative electrode due to vanadium cross-over and limits its operation. The best configuration turned out to be the one with a membrane thickness of 20 mm and EW-720, permitting to reach 84% energy efficiency and 60% electrolyte utilization at a power density in discharge of 250 mW cm^-2. At 150 mW cm^-2both energy efficiency and electrolyte utilization increased to 90% and 80%, respectively. This configuration was then tested for hundreds of cycles, highlighting also a reduced capacity decay with respect to the conventional all-vanadium technology. These results prove the potentialities of the developed hydrogen-vanadium flow battery and in future works the obtained performance will be further improved optimizing positive and negative electrode properties. Acknowledgement This work was funded by A2A S.p.A. in the framework of the research project entitled: “Development of hybrid hydrogen-vanadium redox flow batteries”. References [1] V. Yufit et al., Journal of The Electrochemical Society 160 (2013) A856-A861. [2] J. Rubio-Garcia et al., Energy Storage Materials 31 (2020) 1–10. [3] M. Cecchetti et al., Journal of Power Sources 400 (2018) 218–224. [4] M. Cecchetti et al., Journal of Energy Storage 68 (2023) 107846.

Vanadium redox flow battery (VRFB) is a promising technology for energy storage because of its peculiarity to decouple power and stored energy, its high efficiency and long charge-discharge cycle life [1]. However, VRFB widespread commercialization is still hindered by some technological issues, among which one of the most important is the effective and homogeneous electrolyte distribution over large areas porous electrodes. In fact, the flow pattern imposed by the flow field regulates the distribution of ions concentration throughout the active area, strongly affecting reactions overpotential at local level [2]. Moreover, flow field geometry defines electrolyte velocities and thus the corresponding pressure drops. In this work, a Computational Fluid Dynamics (CFD) model was adopted to provide an insight into the effects of different features of the flow field on local distribution of electrolyte velocities. This analysis was initially conducted on a cell active area of 100 cm², investigating the impact of channels number, channel length and rib length. This analysis led to the design of an innovative flow-field geometry, in which an optimized electrolyte distribution is reached without any pressure drop increase. The promising performance improvement was experimentally demonstrated in a segmented cell hardware, permitting to evaluate the local current densities distribution. The performances of the developed flow-field were measured during charge/discharge cycles with fixed cut-off voltages, adopting commercial electrolyte (1.6 M V in 2 M of H₂SO₄) and two different electrodes: Sigracet^® 39AA treated in air at 400° for 1h and Freudenberg H23 treated in air at 450°C for 12h. The applied current densities ranged from 50 mA cm^-2 to the maximum value allowed by the two different configurations. The innovative distributor led to a more homogeneous distribution of operating current densities, leading to increased voltage efficiencies and higher Electrolyte Utilization (EU): at 50 mA cm^-2 the innovative flow-field coupled with H23 showed an EU of 81%, about 8% higher than the one of the standard flow-field. This difference became more evident at 250 mA cm^-2, where the innovative distributor reached an EU of 26%, about 7.3% greater compared to the standard flow-field. Finally, the advantages given by the improved 100 cm² geometry was scaled-up to a cell active area of 300 cm², demonstrating the scalability of the designed geometry without any performance loss and pressure drop increase, confirming once again the importance of homogeneous electrolyte distribution throughout cell active area. References [1] M. Aneke et al., Applied Energy 179 (2016) 350. [2] M. Messaggi et al., Journal of Power Sources 526 (2022) 231155. Acknowledgement This work has been partially performed within the MUSA – Multilayered Urban Sustainability Action – project, funded by the European Union – NextGenerationEU, under the National Recovery and Resilience Plan (NRRP) Mission 4 Component 2 Investment Line 1.5: Strenghtening of research structures and creation of R&D “innovation ecosystems”, set up of “territorial leaders in R&D”.

Vanadium redox flow battery (VRFB) is a very promising solution for large‐scale energy storage, but some technical issues need to be addressed. Crossover, i.e., the undesired permeation of vanadium ions through the cell separator, causes capacity loss and self‐discharge. Low‐cost and highly selective separators are thus required to improve the competitiveness of this technology. This work investigates the use of silica nanoparticles in an innovative selective layer to improve membrane selectivity and reduce its thickness. 1.5 μm thick barrier layers composed of 1100EW Nafion ionomer with silica (≈3–13 nm diameter) and Vulcan XC‐72R (≈40 nm) nanoparticles in different proportions are directly deposited on 50 μm thick Nafion membranes. The barrier layer composed only of silica nanoparticles reduces the self‐discharge due to crossover by 5 times and increases the average efficiency of the battery. Finally, during more than 1000 h of operation, the barrier layer on a 25 μm Nafion membrane demonstrates excellent stability, working with a constant coulombic efficiency higher than 99% and a capacity decay rate comparable with a thicker Nafion membrane, thus enabling the use of thinner membranes in VRFB, allowing an estimated 8% stack costs reduction with respect to NR212.

In vanadium redox flow batteries, the flow field geometry plays a dramatic role on the distribution of the electrolyte and its design results from the trade-off between high battery performance and low pressure drops. In the literature, it was demonstrated that electrolyte permeation through the porous electrode is mainly regulated by pressure difference between adjacent channels, leading to the presence of under-the-rib fluxes. With the support of a 3D computational fluid dynamic model, this work presents two novel flow field geometries that are designed to tune the direction of the pressure gradients between channels in order to promote the under-the-rib fluxes mechanism. The first geometry is named Two Outlets and exploits the splitting of the electrolyte flow into two adjacent interdigitated layouts with the aim to give to the pressure gradient a more transverse direction with respect to the channels, raising the intensity of under-the-rib fluxes and making their distribution more uniform throughout the electrode area. The second geometry is named Four Inlets and presents four inlets located at the corners of the distributor, with an interdigitated-like layout radially oriented from each inlet to one single central outlet, with the concept of reducing the heterogeneity of the flow velocity within the electrode. Subsequently, flow fields performance is verified experimentally adopting a segmented hardware in symmetric cell configuration with positive electrolyte, which permits the measurement of local current distribution and local electrochemical impedance spectroscopy. Compared to a conventional interdigitated geometry, both the developed configurations permit a significant decrease in the pressure drops without any reduction in battery performance. In the Four Inlets flow field the pressure drop reduction is more evident (up to 50%) due to the lower electrolyte velocities in the feeding channels, while the Two Outlets configuration guarantees a more homogeneous current density distribution.

In recent years, we have seen an impressive progress in the deployment of renewable energy sources, and in particular of photovoltaics and wind turbines. However, renewable energies have some well-known drawbacks due to their intermittent nature and direct usage of the electricity generated would destabilize the grid. To overcome these problems large scale energy storage systems have drawn great attention in the last few years both from the academic and industrial world. Redox flow batteries (RFB) are emerging as one more promising electrochemical energy storage technologies. Notably, the Vanadium redox flow battery (VRFB) is currently the most widespread technology as it allows for the implementation of strategies to recover the capacity decay induced by ion cross-over. Despite the many advantages, VRFBs have long suffered from low power density, mainly caused by the usage of electrodes which are not optimized for this application. The use of heteroatom dopants, such as nitrogen, phosphor, and boron, is currently the most common method to enhance the performance of carbon electrodes. Nitrogen has been extensively studied as a catalyzer for Vanadium redox reactions, showing exceptional boosts in performance in several works. In a previous work we developed a two-step process based on a plasma enhanced nano-aerosol jet deposition source and a vacuum thermal treatment. Starting from a gas precursor we deposited a nanostructured carbonaceous material on a commercially available substrate, this process allows to increase the current density to 400 mAcm^-2 with an energy efficiency of 80%, that is, 4 times greater than the commercial reference. The two-steps process permits a fine tuning of the properties and structure of the material leading to an exceptionally high surface area and a defect rich structure. The advantage of this process is that nitrogen can be added easily during the deposition phase by using a N₂-C₂H₂ mixture, without the need for any other processing steps. In this work we aim to further increase the performance of our nanostructured electrode by adding nitrogen functionalities to the carbon nanostructure, thus combining the effects of an incredibly high surface area with the excellent catalytic properties of nitrogen. The morphology and chemistry of the synthesized material is analyzed by different techniques such as SEM, Raman spectroscopy, XPS, BET analysis. Nitrogen doping was confirmed by XPS analysis, in particular a high amount of graphitic nitrogen, which is considered to be the most active towards vanadium ions, is present. Moreover, BET analysis shows an incredibly high surface area, up to 756 m²/g. The properties of the material are then correlated to the electrochemical properties by testing the material in increasingly complex setups, starting from a three-electrode setup, then in symmetric cell and finally in a full-cell setup. The kinetic activity of the material is studied by cyclic voltammetry and electrochemical impedance spectroscopy. Cyclic voltammetry showed an impressive activity of the electrodes towards the active species, especially towards the V²⁺/V³⁺ couple where a peak-to-peak separation of 65 mV was obtained, which is among the lowest reported in literature. These results were confirmed by EIS, from which the reaction rate constants were calculated following the method proposed by Stimming et al. [1] The addition of nitrogen led to an order of magnitude increase of the reaction rate with respect to the un-doped material. The performance enhancement was then validated by the test in symmetric cell. Polarization curves show a significant decrease in the overpotential, especially in V²⁺/V³⁺. Thus, we propose a vertically integrated process, from the synthesis of the material to the final application, which is able to deliver a comprehensive overview on the material properties, its electrochemical performance and the correlation between the two. [1] Friedl, J., Bauer, C. M., Rinaldi, A., & Stimming, U. (2013). Electron transfer kinetics of the VO2+/VO2+–Reaction on multi-walled carbon nanotubes. Carbon, 63, 228-239.