Neutron Depolarization Under Electrochemical Polarization: Investigating Phase Change Reactions in All-Iron Redox Flow Batteries

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Large-scale and low-cost energy storage systems are necessary to integrate renewable energy technologies into the electrical grid. Among the options, all-iron redox flow batteries (AIRFBs) are a promising candidate as they can strike a good balance between flexibility and modularity [1]. Additionally, they employ sustainable and abundant raw materials in both half cells, thus decreasing their capital costs and their need for maintenance, which contribute to lower levelized cost of storage [2]. However, AIRFBs still suffer from certain operational challenges, such as the unoptimized and segregated electrodeposition of the active species and the competitive hydrogen evolution reaction [3]. Conventional characterization techniques cannot accurately determine the spatial distribution of both reactions within the electrochemical cell, and thus the operational losses they generate still remain unclear.In recent years, several groups have explored non-invasive operando characterization of RFBs by use of nuclear magnetic resonance[4] or fluorescence microscopy[5], among others. Recently, our group developed a neutron radiography-based method to visualize concentrations in solution of redox or charge carrier species, enabling visualization of local concentration effects [6]. Nevertheless, this approach relies solely on species in solution providing high attenuation, such as H-, B-, or Li- containing electrolytes. Therefore, this attenuation mechanism severely restricts the types of species or phases that can be probed, and the regions of interest where the metal electroplating occurs would remain difficult to quantify with conventional neutron imaging [7].In this work, we expand the workspace of neutron radiography in electrochemical cells to systems employing substances of lower neutron cross section by combining the use of in-plane transmission and polarized neutron imaging. With this dual characterization approach, we are able to visualize for the first time the distribution of iron plated species and hydrogen evolution during electrochemical operation in an all-iron redox flow battery. First, we validate the proposed methodology by performing ex-situ measurements of the depolarization response caused by the magnetic momentum of the various iron-containing species that could form during the charging process of the AIRFB system. We find that only purely ferromagnetic substances (e.g. microscale alpha-iron particles) produce a strong and depolarizing response, with a linear dependence between the depolarization coefficient and the weight fraction of the magnetic phase, thus enabling the detection of subtle changes in plated iron content. On the contrary, weak ferri- or paramagnetic compounds, such as Fe2O3, Fe3O4 or FeOOH, do not cause interference in the measured signal.Subsequently, we run an all-iron redox flow battery at the BOA neutron beamline and performed simultaneous neutron imaging under two modes. By means of the polarization contrast neutron imaging, we tracked the electrodeposition of iron during charge and its removal during discharge within the negative electrode, correlating operating conditions with the distribution of the plated species in the porous carbon electrodes. Simultaneously, the neutron transmission mode and subtractive imaging enabled visualization of hydrogen gas evolution and its dynamics within the cell. These results provide insights into the break-in behavior of phase change reactions and the distribution of the simultaneous reactive fronts of the multiphase, hybrid type of flow cell. In turn, we were able to obtain fundamental understanding of the effects of operational and design parameters, such as electrolyte composition, flow field design and flow distribution geometry, and their impact on the mass transfer characteristics and efficiency of all-iron flow batteries. Acknowledgements IGG gratefully acknowledges financial support by “la Caixa Foundation” (ID 100010434) under the fellowship number LCF/BQ/EU20/11810076. AFC gratefully acknowledges funding by the European Union (ERC, FAIR-RFB, ERC-2021-STG 101042844). Views and opinions expressed are however those of the author(s) only and do not necessarily reflect those of the European Union or the European Research Council. Neither the European Union nor the granting authority can be held responsible for them. The results of this project are based on experiments performed at the Swiss Spallation Neutron Source, SINQ, Paul Scherrer Institute (Switzerland), with proposal number 20222727 References [1] Dunn, B., Kamath, H., & Tarascon, J. M. (2011). Science, 334(6058), 928-935.[2] Li, B. & Liu, J. (2017). National Science Review, 4(1), 91-105.[3] Zhang, H., & Sun, C. (2021). Journal of Power Sources, 493, 229445.[4] Zhao, A. et al. (2020) Nature¸ 579, 224[5] Wong, A.A., Rubinstein, S.M. & Aziz, M.J. (2021) Cell Reports Physical Science 2(4), 100388[6] Jacquemond, R.R. et al. (2023) ChemRXiv , ID 10.26434/chemrxiv-2023-8xjv5[7] Kardjilov, N. et al. (2011) Materials. Today, 14(6) 248-256