©2023. This manuscript version is made available under the CC-BY-NC-ND 4.0 license https://creativecommons.org/licenses/by-nc-nd/4.0/, This study presents a vanadium redox flow battery model that considers the most important variables that have a crucial role in the performance of the system. A complete model divided in an electrochemical, thermal, hydraulic and voltage submodels is presented. The analytic analysis of the model is carried out to reduce the system order according to some conservation laws. Based on this analysis, a subsequent calibration of the model parameters is developed using real experimental data. The validation is performed comparing the real measured voltage and the one estimated with the model. To calibrate the model an algorithm based on the implementation of a particle swarm optimizer is used. Results obtained in both short and long-term operation are presented, in order to compare and validate if the model can be used for both state of charge and state of health estimation., This research has been developed within the CSIC Interdisciplinary
Thematic Platform (PTI+) Transición Energética Sostenible+ (PTI-
TRANSENER+) as part of the CSIC program for the Spanish Recovery,
Transformation and Resilience Plan funded by the Recovery and Re-
silience Facility of the European Union, established by the Regulation
(EU) 2020/2094, the Spanish Ministry of Science and Innovation
under projects MAFALDA (PID2021-126001OB-C31 and C32 funded by
MCIN/AEI/10.13039/501100011033/ERDF, EU), and MASHED, Spain
(TED2021-129927B-I00), and by the Spanish Ministry of Universities
funded by the European Union - NextGenerationEU (2022UPC-MSC-
93823)., Peer reviewed

9 figues, 5 tables., In this research, the flow behavior through a commercial electrode used in Redox Flow Batteries (RFB) was characterized by means of Planar Laser Induced Fluoresce imaging (PLIF) for different cell configurations, without channels or with channels machined either in the back plate or in the electrode. Measurements of the pressure drop were obtained for each case, and the evolution of the electrode soaking process was quantified defining an electrode surface wetting rate. Using the pressure drop measurements, equivalent Darcy and intrinsic permeability coefficients were calculated for the different configurations. It was observed that the tight fitting of the electrode within its housing appears to be crucial for an adequate distribution of the electrolyte within the porous media. In general, when channels are available, the electrolyte tends to circulate along them, without crossing through the electrode felt, and lower pressure losses are accompanied by lower wetting rates. The flow field with serpentine channels in the back plate could be considered the most suitable configuration in terms of energy consumption, nearly 87% less than in a case without any channels, but this same percentage of electrolyte will exit the cell without traversing the electrode, and thus not contributing to the electrochemical reactions., This research has been developed within the CSIC Interdisciplinary Thematic Platform (PTI+) Transición Energética Sostenible+ (PTI-TRANSENER+) as part of the CSIC program for the Spanish Recovery, Transformation and Resilience Plan funded by the Recovery and Resilience Facility of the European Union, established by the Regulation (EU) 2020/2094. Partial funding from the Spanish Ministry of Science and Innovation under project MAFALDA (PID2021–126001OB-C32), and support of the Regional Government of Aragon to the Fuel Conversion Research Group (T06_23R) are also acknowledged., Peer reviewed

12 figures, 10 tables.-- Part of the special issue VIII Symposium on Hydrogen, Fuel Cells and Advanced Batteries - HYCELTEC 2022, VIII Symposium on Hydrogen, Fuel Cells and Advanced Batteries Buenos Aires, July 10-13, 2022.-- © 2023. This manuscript version is made available under the CC-BY-NC-ND 4.0 license https://creativecommons.org/licenses/by-nc-nd/4.0/, This paper proposes a methodology for characterizing electrochemical parameters in non-isothermal three-dimensional (3D) simulation models of fuel cell stacks. The proposed methodology involves utilizing only easily measurable construction and non-invasive operational data. In order to achieve a reasonable computational cost, an iterative method developed on various 3D computational domains is combined with a genetic algorithm optimization technique. The effectiveness of the methodology is demonstrated through its application on a real scale 40-cell high temperature PEM fuel cell (HTPEMFC) stack, with the results indicating good agreement between the model and experimental data. This approach has the potential to significantly reduce the computational cost of optimizing fuel cell designs while still maintaining accuracy., This research is part of the CSIC program for the Spanish Recovery, Transformation and Resilience Plan funded by the Recovery and Resilience Facility of the European Union, established by the Regulation (EU) 2020/2094. Support of the CSIC Interdisciplinary Thematic Platform (PTI+) Transición Energética Sostenible+ (PTI-TRANSENER+) is acknowledged. Funding provided by the Spanish Ministry of Science and Innovation under project MAFALDA (PID2021-126001OB-C32, MCIN/AEI/10.13039/501100011033), as well as the support of the Regional Government of Aragon to GCC are also acknowledged., Peer reviewed

10 figures, 3 tables., In the present research, the performance of three commercial graphite felts (a 6 mm thick Rayon-based Sigracell®, a 4.6 mm thick PAN-based Sigracell®, and a 6 mm thick PAN-based AvCarb®) used as electrodes in vanadium redox flow batteries (VRFBs) is analyzed before and after thermal activation. The thermal treatment of the electrodes at 500 °C for 1 h in a self-designed industrial furnace under a synthetic air atmosphere. XPS confirms that thermal activation provides with different C=O/Csingle bondO and sp2/sp3 ratios to the graphite electrodes depending on their carbon precursors, providing different catalitic behavior. T-GFD4.6-EA felt electrode was also oxidized by cycling in H2SO4 and in 0.4 M VOSO4 + 2 M H2SO4 solution. In the first case, the graphite electrode increased the current density 24.27 mA cm−2 for the VO2+ electrooxidation, however the cathodic current density (VO2+ reduction reaction) was decreased 36 mA cm−2. In the second case, the increase was of 21.27 mA cm−2, whereas the current density for the VO2+ reduction hardly changed. Thermally treated GFD4.6-EA graphite felt chemical composition also changed differently when exposed to different laboratory experiments which mimic the electrode behavior in a VRFB. XPS analysis confirmed that the chemical modification of the graphite surfaces can either improve or decrease the electrocatalytic activity of the electrodes depending on their carbon precursors., This research is part of the CSIC program for the Spanish Recovery, Transformation and Resilience Plan funded by the Recovery and Resilience Facility of the European Union, established by the Regulation (EU) 2020/2094. Support of the CSIC Interdisciplinary Thematic Platform (PTI+) Transición Energética Sostenible+ (PTI-TRANSENER+). Funding provided by the Spanish Ministry of Science and Innovation under project MAFALDA (PID2021–126001OB-C32), as well as the support of the Regional Government of Aragon to the Fuel Conversion Research Group (T06_23R)., Peer reviewed

Under a Creative Commons license by-nc-nd 4.0, Figure S1: Thermostated electrochemical reactor with a typical three-electrode configuration. Figure S2: Deconvoluted spectra of C1s peak for a) T-GFD4.6 (100 cycles in 2 M H2SO4), b) T-GFD4.6 (immersion 2 M H2SO4 0.4 M VOSO4) and c) T-GFD4.6 (100 cycles 2 M H2SO4 0.4 M VOSO4). Figure S3: Deconvoluted spectra of O1s peak for a) T-GFD4.6 (100 cycles in 2 M H2SO4), b) T-GFD4.6 (immersion 2 M H2SO4 0.4 M VOSO4) and c) T-GFD4.6 (100 cycles 2 M H2SO4 0.4 M VOSO4). Figure S4: Deconvoluted spectra of N1s peak for a) T-GFD4.6 electrode after being immersed 5 days in 2 M H2SO4 + 0.4 M VOSO4 electrolyte b) the T-GFD4.6 activated after 100 CV cycles in 2 M H2SO4. Figure S5: Tafel plots of the a) GFD4.6-EA, GFA6 and Avcarb6 and b) T-GFD4.6-EA, T-GFA6 and T-Avcarb6 obtained from the Linear Sweep Voltammetry curves. Figure S6: Linear relationship between the logarithm of the redox peak current and the logarithm of the scan rate for the thermal pre-treated electrodes; a)T-GFD4.6, b) T-AvCarb6 and c) T-GFA6. Table S1. Table S2. Table S3., This research is part of the CSIC program for the Spanish Recovery, Transformation and Resilience Plan funded by the Recovery and Resilience Facility of the European Union, established by the Regulation (EU) 2020/2094. Support of the CSIC Interdisciplinary Thematic Platform (PTI+) Transición Energética Sostenible+ (PTI-TRANSENER+). Funding provided by the Spanish Ministry of Science and Innovation under project MAFALDA (PID2021–126001OB-C32), as well as the support of the Regional Government of Aragon to the Fuel Conversion Research Group (T06_23R)., Peer reviewed

12 figures, 2 tables., This study presents an online algorithm capable to simultaneously estimate the state of charge and state of health of a vanadium redox flow battery. Starting from a general electrochemical model, some order reductions are carried out considering different conservation laws. Based on these low-order models, the observer is designed considering the terminal voltage of the battery. This observer is firstly analyzed using numerical tools. Secondly, an experimental validation is carried out with real data provided by a vanadium redox flow battery stack, consisting on current and voltage measurements., This research has been developed within the CSIC Interdisciplinary Thematic Platform (PTI+) Transición Energética Sostenible+ (PTI-TRANSENER+) as part of the CSIC program for the Spanish Recovery, Transformation and Resilience Plan funded by the Recovery and Resilience Facility of the European Union, established by the Regulation (EU) 2020/2094, the Spanish Ministry of Science and Innovation under projects MAFALDA (PID2021-126001OB-C31 and C32 funded by MCIN/AEI/ 10.13039/501100011033/ERDF, EU) and MASHED (TED2021-129927B-I00), the European Union’s H2020 research and innovation programme under grant agreement number 963652 Project HYBRIS and by the Spanish Ministry of Universities funded by the European Union - NextGenerationEU (2022UPC-MSC-93823)., Peer reviewed

12 figures, 7 tables., This paper presents a comprehensive numerical investigation into the degradation mechanisms of High-Temperature Proton Exchange Membrane Fuel Cells (HT-PEMFCs) with a particular focus on catalyst layer degradation. The study integrates advanced models to accurately describe the electrochemistry of platinum and carbon oxidations, crucial processes influencing the cell’s performance over time. Additionally, a heuristic model is employed to account for the loss of proton conductivity in the membrane attributed to the depletion of phosphoric acid. The degradation processes are seamlessly integrated into a 3D computational code. By utilizing optimized values for the electrochemical parameters, the numerical simulations demonstrate excellent agreement with experimental degradation observed in an HT-PEMFC operating under steady-state conditions. In addition, the numerical approach offers insights that are not easy to achieve by experimental means, such as the equilibrium of the different oxidation states of platinum and carbon., The authors would like to acknowledge the financial support provided by the Spanish Ministry of Science and Innovation under projects DOVELAR (ref.: RTI2018-096001-B-C31) and MAFALDA (ref.: PID2021-126001OB-C32), and to the Aragon Government under the projects LMP246_18 and T06_23R. This research has been developed within the CSIC Interdisciplinary Thematic Platform (PTI+) Transición Energética Sostenible+ (PTI-TRANSENER+) as part of the CSIC program for the Spanish Recovery, Transformation and Resilience Plan funded by the Recovery and Resilience Facility of the European Union , established by the Regulation (EU) 2020/2024., Peer reviewed

9 figures, 10 tables., In this work an all-vanadium redox flow battery 3D model is developed to study the crossover phenomena causing electrolyte imbalance in an perpendicularly assembled battery. Fluid flow is fully modeled including transition from porous media to non-porous zones coupling the Navier–Stokes equations with the Brinkman corrections. General conservation laws are applied to model the electrochemistry in the electrolyte and electrochemical equilibrium is used to model Donnan effect at the membrane-electrolyte interface. Exclusion of the co-ions in the membrane is carefully modeled leading to a novel treatment of the sulfuric acid dissociated species in this domain. The impact of different proton and vanadium concentrations in the battery half-cells is studied in detail showing a potential way to reduce electrolyte imbalance., This research has been developed within the CSIC Interdisciplinary Thematic Platform (PTI＋) Transición Energética Sostenible＋ (PTI-TRANSENER＋) as part of the CSIC program for the Spanish Recovery, Transformation and Resilience Plan funded by the Recovery and Resilience Facility of the European Union, Spain, established by the Regulation (EU) 2020/2094. The authors would like also to acknowledge the financial support provided by CSIC, Spain under the Special Intramural Project (ref.: PIE-201980E101), to the Spanish Ministry of Science and Innovation, Spain under projects DOVELAR (ref.: RTI2018-096001-B-C31) and MAFALDA (ref.: PID2021-126001OB-C32), and to the Aragon Government, Spain under the project LMP246_18., Peer reviewed