Promoting Iron Redox for Rechargeable Aqueous Batteries

Stationary battery systems are vital to implementing renewable energy infrastructures for a sustainable decarbonized economy by storing wind and solar power in the electricity grid to align the fluctuating demand and supply of electricity. Utilizing aqueous electrolytes and earth-abundant elements is an instrumental step forward to next-generation technologies to mitigate the supply chain risks of commercial batteries for large-scale implementation. 

Iron metal-based aqueous batteries hold great potential for extensive grid-scale energy solutions due to iron’s high theoretical specific capacity, ultralow cost, and abundance. However, rechargeable aqueous iron batteries are inadequately employed since iron redox suffers from parasitic side reactions, resulting in poor efficiency and requiring a better understanding of foundational aspects in their materials sciences. Our investigation using multimodal synchrotron techniques such as operando X-ray scattering, transmission X-ray microscopy, ambient pressure X-ray photoelectron spectroscopy, X-ray absorption spectroscopy, and X-ray fluorescence micro-imaging is beneficial for investigating the redox chemistry of iron electrodes and the structural chemistry of aqueous electrolytes. Such intricate information about the operations of earth-abundant iron for batteries using eco-friendly aqueous electrolytes promises their immense impact on the industrialization of long-duration grid storage technologies. 

My previous research work (during PhD) was on the Fe2+ --> Fe3+ conversion as a sustainable redox. However, the Fe(OH)2 layered double hydroxide (LDH) undergoes two parallel oxidation processes to electrochemically passive spinel (Fe3O4) and reversible FeOOH phase. However, my challenge is overcoming spinel phase formation by various methods, including but not limited to anion-intercalation into the Fe-O interlayers. 

During the discharge (oxidation process), the Fe(OH)2 accommodates anions from the electrolyte in between their interlayers to form an intermediate phase called green rust with a chemical formula [Fe2+1-xFe3+x(HO-)2]X+ (An-x/n)X-, where An- are the anions such as SO42-, Cl-, etc., My former colleague Dr. Fenghua Guo and I worked on the SO4-intercalation, which proved to concede the spinel phase and thus promote FeOOH formation. We published the electrochemical and molecular dynamics results elucidated by the operando synchrotron X-ray diffraction (XRD) in the Journal of American Chemical Society. Even though the divalent nature of SO4 has excellent electrostatic attraction to the positively charged Fe-LDH, their sizeable tetrahedral geometry did not facilitate spontaneous adsorption onto the Fe-O framework as much as the spherical monovalent Cl- ions. We found that the stable adsorption by Cl- ions in between the Fe-LDH resulted in a higher Fe(OH)2/FeOOH conversion (64.7%) and a better cycle life (400 cycles) than the SO4-intercalated conversion (19%). The excellent performance of the chloride insertion report was reported in the Chemistry of Materials and mentioned in the 'Highlights from 2023 and Chemistry of Materials’ 35th Year'.

Another exciting way to promote the Fe(OH)2 --> FeOOH conversion is to modify the water activity in the electrolyte by using additives. During the charging (reduction process), the Fe(OH)2 partially converts to Fe, triggering the HER. Polysilicates (Na2SiO3), in their extremely low concentration (parts per million) in the alkaline electrolyte, have two-fold effectiveness on Fe-redox: (i) on charging (reduction), it strengthens the hydrogen bond networks of the water to passivate the Fe(OH)2 --> Fe conversion and block HER on Fe surface; (ii) the suppressed Fe(OH)2 --> Fe conversion allows a lower reduction potential on charging to improve Fe(OH)2 formation successively benefiting Fe(OH)2 --> FeOOH conversion on discharging. The discharge capacity was improved six times with Na2SiO3 additives compared to the electrolyte without them. The detailed study has been published in the ChemSusChem journal. My Ph.D. advisor and I have also applied for two US patents reporting this mechanism (US Patent App. 63/445,386 and 63/632,588).

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