Simultaneous Regulation of Solvation Shell and Oriented Deposition toward a Highly Reversible Fe Anode for All-Iron Flow Batteries

Enhanced Reversibility of Iron Metal Anode with a Solid Electrolyte Interphase in Concentrated Chloride Electrolytes

Iron is a promising candidate for a cost-effective anode for large-scale energy storage systems due to its natural abundance and well-established mass production. Recently, Fe-ion batteries (FeIBs) that use ferrous ions as the charge carrier have emerged as a potential storage solution. The electrolytes in FeIBs are necessarily acidic to render the ferrous ions more anodically stable, allowing a wide operation voltage window. However, the iron anode suffers severe hydrogen evolution reaction with a low Coulombic efficiency (CE) in an acidic environment, shortening the battery cycle life. Herein, a hybrid aqueous electrolyte that forms a solid-electrolyte interphase (SEI) layer on the Fe anode surface is introduced. The electrolyte mainly comprises FeCl2 and ZnCl2 as cosalts, where the Zn-Cl anionic complex species of the concentrated ZnCl2 allows dimethyl carbonate (DMC) to be miscible with the aqueous ferrous electrolyte. SEI derived from DMC's decomposition passivates the iron surface, which leads to an average CE of 98.3% and much-improved cycling stability. This advancement shows the promise of efficient and durable FeIBs.

Research Progress and Prospect of All-Iron Redox Flow Battery Anolyte

The all-iron redox flow battery (AIRFB) has garnered significant attention in the field of energy storage due to its advantages of cost, aqueous chemistry, safety, and sustainability. The traditional deposited AIRFB has a limited depth of discharge and is prone to metal dendrite growth and hydrogen evolution side reactions during operation. In contrast, the all-soluble AIRFB, where the active substances are all fully soluble, offers flexible operation that is more conducive to industrial development of AIRFB. However, the all soluble AIRFB electrolyte still requires improvement of its solubility, long-term stability, reactivity, and electrode potential. This review introduces the concepts for modification of the AIRFB electrolyte and presents the main ideas and methods for electrolyte improvement, as well as the challenges faced and possible research directions for the future.

Conductive Polymer Coatings Control Reaction Selectivity in All-Iron Redox Flow Batteries

Aqueous all-iron redox flow batteries are an attractive and economic technology for grid-scale energy storage owing to their use of abundant and environmentally benign iron as the redox active material and water as solvent. However, the battery operation is challenged by the plating/stripping reactions of iron and the competing hydrogen evolution reaction at the negative electrode, which hinder performance and durability. Here, the reaction selectivity of the negative electrode is tailored by introducing conductive polymer coatings onto porous carbonaceous electrodes. Two conductive polymers, poly(3,4-ethylenedioxythiophene) (PEDOT) and poly(pyrrole) (PPy) are conformally coated with the dopant poly(4-styrenesulfonate) (PSS) and the resulting electrochemistry is studied on model electroanalytical platforms and redox flow batteries. Both polymers decrease the hydrogen evolution current on rotating disc electrodes, with PPy/PSS strongly inhibiting the reaction at high overpotentials. In full all-iron redox flow cells, PPy/PSS coating extends cyclability and significantly reduces hydrogen evolution, while PEDOT/PSS coating improves the round-trip efficiency, possibly acting as a redox shuttle for the iron stripping reaction. These findings motivate broader investigation and implementation of conductive polymers to engineer reaction selectivity for flow batteries and other electrochemical technologies.

High-Stable All-Iron Redox Flow Battery with Innovative Anolyte based on Steric Hindrance Regulation

All-soluble all-iron redox flow batteries (AIRFBs) are an innovative energy storage technology that offer significant financial benefits. Stable and affordable redox-active materials are essential for the commercialization of AIRFBs, yet the battery stability must be significantly improved to achieve practical value. Herein, ferrous complexes combined with the triisopropanolamine (TIPA) ligand are identified as promising anolytes to extend battery life by reducing cross-contamination due to a pronounced steric hindrance effect. The coordination structure and failure mechanism of our Fe-TIPA complexes were determined by molecular dynamics simulation and spectroscopic experiments. By coupling with [Fe(CN)6]4-/3-, Fe-TIPA/Fe-CN AIRFBs retained excellent stability exceeding 1831 cycles at 80 mA ⋅ cm-2, yielding an energy efficiency of ~80 % and maintaining a steady discharge capacity. Moreover, the all-soluble electrolyte was tested in an industrial-scale Fe-TIPA/Fe-CN AIRFB prototype energy storage system, where an energy efficiency of 81.3 % was attained. Given the abundance of iron resources, we model the TIPA AIRFB electrolyte cost to be as low as 32.37 $/kWh, which is significantly cheaper than the current commercial level. This work demonstrates that steric hindrance is an effective measure to extended battery life, facilitating the commercial development of affordable flow batteries.

A Double-ligand Chelating Strategy to Iron Complex Anolytes with Ultrahigh Cyclability for Aqueous Iron Flow Batteries

Aqueous all-iron flow batteries (AIFBs) are attractive for large-scale and long-term energy storage due to their extremely low cost and safety features. To accelerate commercial application, a long cyclable and reversible iron anolyte is expected to address the critical barriers, namely iron dendrite growth and hydrogen evolution reaction (HER). Herein, we report a robust iron complex with triethanolamine (TEA) and 2-methylimidazole (MM) double ligands. By introducing two ligands into one iron center, the binding energy of the complex increases, making it more stable in the charge-discharge reactions. The Fe(TEA)MM complex achieves reversible and stable redox between Fe3+ and Fe2+ , without metallic iron growth and HER. AIFBs based on this anolyte perform a high energy efficiency of 80.5 % at 80 mA cm-2 and exhibit a record durability among reported AIFBs. The efficiency and capacity retain nearly 100 % after 1,400 cycles. The capital cost of this AIFB is $ 33.2 kWh-1 (e.g., 20 h duration), cheaper than Li-ion battery and vanadium flow battery. This double-ligand chelating strategy not only solves the current problems faced by AIFBs, but also provides an insight for further improving the cycling stability of other flow batteries.

A Universal Additive Design Strategy to Modulate Solvation Structure and Hydrogen Bond Network toward Highly Reversible Fe Anode for Low-Temperature All-Iron Flow Batteries

Aqueous all-iron redox flow batteries (RFBs) are promising competitors for next-generation grid-scale energy storage applications. However, the high-performance operation of all-iron RFBs in a wider temperature range is greatly hindered by inferior iron plating/stripping reaction and low solid-liquid transition temperature at Fe anode. Herein, a universal electrolyte additive design strategy for all-iron RFBs is reported, which realizes a highly reversible and dendrite-free Fe anode at low temperatures. Quantum chemistry calculations first screen several organic molecules with oxygen-containing functional groups and identify N,N-Dimethylacetmide (DMAc) as a potential candidate with low cost, high solubility, and strong interactions with Fe2+ and H2 O. Combined experimental characterizations and theoretical calculations subsequently demonstrate that adding DMAc into the FeCl2 solution effectively reshapes the primary solvation shell of Fe2+ via the Fe2+ -O (DMAc) bond and breaks hydrogen-bonding network of water through intensified H-bond interaction between DMAc and H2 O, thereby affording the Fe anode with enhanced Fe/Fe2+ reversibility and lower freezing point. Consequently, the assembled all-iron RFB achieves an excellent combination of high power density (25 mW cm-2 ), long charge-discharge cycling stability (95.59% capacity retention in 103 h), and preeminent battery efficiency at -20 °C (95% coulombic efficiency), which promise a future for wider temperature range operation of all-iron RFBs.