An All-Soluble Fe/Mn-Based Alkaline Redox Flow Battery System

Redox flow batteries (RFBs) are membrane-separated rechargeable flow cells with redox electrolytes, offering the potential for large-scale energy storage and supporting renewable energy grids. Yet, creating a cost-effective, high-performance RFB system is challenging. In this work, we investigate an Fe/Mn RFB alkaline system based on the [(TEA)Fe-O-Fe(TEA)]3-/4- and MnO4-/2- redox couples with a theoretical cell voltage of ∼1.43 V. This combination has not been systematically studied previously, but it can lead to a very low-cost and sustainable materials for high energy storage. Constant current cycling tests were performed at ±41 mA cm-2 between 20% and 80% SOC over 800 h (400 cycles) with an apparent Coulombic efficiency (CE) approaching 100%, while the voltage efficiency (VE) gradually decreased from ∼75.3% to ∼61.4% due to increasing internal resistances. The voltage efficiency loss can be mitigated through a periodic acid treatment to remove MnO2 deposits from the separator.

Vanadate-Based Fibrous Electrode Materials for High Performance Aqueous Zinc Ion Batteries

Aqueous zinc-ion batteries (AZIBs) are considered as attractive energy storage systems with great promise owing to their low cost, environmental friendliness and high safety. Nevertheless, cathode materials with stable structure and rapid diffusion of zinc ions are in great demand for AZIBs. In this work, a new kind of potassium vanadate compound (KV3 O8 ) is synthesized with fibrous morphology as an excellent cathode material for AZIBs, which shows outstanding electrochemical performance. KV3 O8 exhibits a high discharge capacity of 556.4 mAh g-1 at 0.8 A g-1 , and the capacity retention is 81.3% at 6 A g-1 even after a long cycle life of 5000 cycles. The excellent performance of the KV3 O8 cathode is benefited from the structural stability, sufficient active sites, and high conductivity, which is revealed by in situ X-ray diffraction and various other characterizations. This work offers a new design strategy into fabricating high efficiency cathode materials for AZIBs and beyond.

An All-Climate Nonaqueous Hydrogen Gas-Proton Battery

Rechargeable hydrogen gas batteries, driven by hydrogen evolution and oxidation reactions (HER/HOR), are emerging grid-scale energy storage technologies owing to their low cost and superb cycle life. However, compared with aqueous electrolytes, the HER/HOR activities in nonaqueous electrolytes have rarely been studied. Here, for the first time, we develop a nonaqueous proton electrolyte (NAPE) for a high-performance hydrogen gas-proton battery for all-climate energy storage applications. The advanced nonaqueous hydrogen gas-proton battery (NAHPB) assembled with a representative V2(PO4)3 cathode and H2 anode in a NAPE exhibits a high discharge capacity of 165 mAh g-1 at 1 C at room temperature. It also efficiently operates under all-climate conditions (from -30 to +70 °C) with an excellent electrochemical performance. Our findings offer a new direction for designing nonaqueous proton batteries in a wide temperature range.

Advanced Electrolyte Formula for Robust Operation of Vanadium Redox Flow Batteries at Elevated Temperatures

Insufficient thermal stability of vanadium redox flow battery (VRFB) electrolytes at elevated temperatures (>40 °C) remains a challenge in the development and commercialization of this technology, which otherwise presents a broad range of technological advantages for the long-term storage of intermittent renewable energy. Herein, a new concept of combined additives is presented, which significantly increases thermal stability of the battery, enabling safe operation to the highest temperature (50 °C) tested to date. This is achieved by combining two chemically distinct additives-inorganic ammonium phosphate and polyvinylpyrrolidone (PVP) surfactant, which collectively decelerate both protonation and agglomeration of the oxo-vanadium species in solution and thereby significantly suppress detrimental formation of precipitates. Specifically, the precipitation rate is reduced by nearly 75% under static conditions at 50° C. This improvement is reflected in the robust operation of a complete VRFB device for over 300 h of continuous operation at 50 °C, achieving an impressive 83% voltage efficiency at 100 mA cm-2 current density, with no precipitation detected in either the electrode/flow-frame or electrolyte tank.

Low-cost Zinc-Iron Flow Batteries for Long-Term and Large-Scale Energy Storage

Aqueous flow batteries are considered very suitable for large-scale energy storage due to their high safety, long cycle life, and independent design of power and capacity. Especially, zinc-iron flow batteries have significant advantages such as low price, non-toxicity, and stability compared with other aqueous flow batteries. Significant technological progress has been made in zinc-iron flow batteries in recent years. Numerous energy storage power stations have been built worldwide using zinc-iron flow battery technology. This review first introduces the developing history. Then, we summarize the critical problems and the recent development of zinc-iron flow batteries from electrode materials and structures, membranes manufacture, electrolyte modification, and stack and system application. Finally, we forecast the development direction of the zinc-iron flow battery technology for large-scale energy storage.

Solid-Liquid-Gas Management for Low-Cost Hydrogen Gas Batteries

Aqueous nickel-hydrogen gas (Ni-H₂) batteries with excellent durability (>10,000 cycles) are important candidates for grid-scale energy storage but are hampered by the high-cost Pt electrode with limited performance. Herein, we report a low-cost nickel-molybdenum (NiMo) alloy as an efficient bifunctional hydrogen evolution and oxidation reaction (HER/HOR) catalyst for Ni-H₂ batteries in alkaline electrolytes. The NiMo alloy demonstrates a high HOR mass-specific kinetic current of 28.8 mA mg^-1 at 50 mV as well as a low HER overpotential of 45 mV at a current density of 10 mA cm^-2, which is better than most nonprecious metal catalysts. Furthermore, we apply a solid-liquid-gas management strategy to constitute a conductive, hydrophobic network of NiMo using multiwalled carbon nanotubes (NiMo-hydrophobic MWCNT) in the electrode to accelerate HER/HOR activities for much improved Ni-H₂ battery performance. As a result, Ni-H₂ cells based on the NiMo-hydrophobic MWCNT electrode show a high energy density of 118 Wh kg^-1 and a low cost of only 67.5 $ kWh^-1. With the low cost, high energy density, excellent durability, and improved energy efficiency, the Ni-H₂ cells show great potential for practical grid-scale energy storage.

Aqueous Zinc-Chlorine Battery Modulated by a MnO2 Redox Adsorbent

Aqueous zinc-chlorine battery with high discharge voltage and attractive theoretical energy density is expected to become an important technology for large-scale energy storage. However, the practical application of Zn-Cl₂ batteries has been restricted due to the Cl₂ cathode with sluggish kinetics and low Coulombic efficiency (CE). Here, an aqueous Zn-Cl₂ battery using an inexpensive and effective MnO₂ redox adsorbent (referred to Zn-Cl₂ @MnO₂ battery) to modulate the electrochemical performance of the Cl₂ cathode is developed. Density functional theory calculations reveal that the existence of the intermediate state Cl_ads free radical catalyzed by MnO₂ on the Cl₂ cathode contributes to the charge storage capacity, which is the key to modulate the electrode and improve the electrochemical performance. Further analysis of the Cl₂ cathode kinetics discloses the adsorption and catalytic roles of the MnO₂ redox adsorbent. The Zn-Cl₂ @MnO₂ battery displays an enhanced discharge voltage of 2.0 V at a current density of 2.5 mA cm^-2 , and stable 1000 cycles with an average CE of 91.6%, much superior to the conventional Zn-Cl₂ battery with an average CE of only 66.8%. The regulation strategy to the Cl₂ cathode provides opportunities for the future development of aqueous Zn-Cl₂ batteries.

Proton-Trapping Agent for Mitigating Hydrogen Evolution Corrosion of Zn for an Electrolytic MnO2/Zn Battery

A rechargeable aqueous electrolytic MnO₂/Zn battery (EMZB) based on a reversible Mn²⁺/MnO₂ two-electron redox reaction in an acidic electrolyte is very attractive for large-scale energy storage due to its high output voltage, large gravimetric capacity, and low cost. However, severe hydrogen evolution corrosion (HEC) of the Zn anode in an acidic electrolyte limits its application. Here, a proton-trapping agent (PTA) is introduced in the electrolyte to improve the electrochemical performance of the EMZB. Experimental results and theoretical calculations demonstrate that HEC of the Zn electrode can be effectively mitigated through high binding energy between the protons and PTA. The optimized EMZB regulated by a PTA of acetate (EMZB-20% Ac) delivers a high discharge voltage of 1.91 V and over 400 stable cycles at 1 C, which is more than 5 times the cycle life of the battery without PTA. EMZB-20% Ac also shows a Coulombic efficiency of 90.7% at a high areal capacity of 8 mAh cm^-2 and an energy retention of 83.6% after 1000 cycles at 5 C with an areal capacity of 1 mAh cm^-2. This work provides a promising electrolyte regulation strategy for the design and application of a high-performance EMZB and other energy storage systems.

Descriptor-Driven Computational Design of Bifunctional Double-Atom Hydrogen Evolution and Oxidation Reaction Electrocatalysts for Rechargeable Hydrogen Gas Batteries

Rechargeable hydrogen gas batteries (RHGBs) have been attracting much attention as promising all-climate large-scale energy storage devices, which calls for low-cost and high-activity hydrogen evolution/oxidation reaction (HER/HOR) bifunctional electrocatalysts to replace the costly platinum-based catalysts. Based on density functional theory (DFT) computations, herein we report an effective descriptor-driven design principle to govern the HER/HOR electrocatalytic activity of double-atom catalysts (DACs) for RHGBs. We systematically investigate the d-band center variation of DACs and their correlations with HER/HOR free energies. We construct activity maps with the d-band center of DACs as a descriptor, which demonstrate that high HER/HOR electrocatalytic activity can be achieved with an appropriate d-band center of DACs. This work not only broadens the applicability of d-band center theory to the prediction of bifunctional HER/HOR electrocatalysts but also paves the way to fast screening and design of efficient and low-cost DACs to promote practical applications of RHGBs.

Theory-Driven Design of a Cationic Accelerator for High-Performance Electrolytic MnO2 -Zn Batteries

Aqueous electrolytic MnO₂ -Zn batteries are considered as one of the most promising energy-storage devices for their cost effectiveness, high output voltage, and safety, but their electrochemical performance is limited by the sluggish kinetics of cathodic MnO₂ /Mn²⁺ and anodic Zn/Zn²⁺ reactions. To overcome this critical challenge, herein, a cationic accelerator (CA) strategy is proposed based on the prediction of first-principles calculations. Poly(vinylpyrrolidone) is utilized as a model to testify the rational design of the CA strategy. It manifests that the CA effectively facilitates rapid cations migration in electrolyte and adequate charge transfer at electrode-electrolyte interface, benefiting the deposition/dissolution processes of both Mn²⁺ and Zn²⁺ cations to simultaneously improve kinetics of cathodic MnO₂ /Mn²⁺ and anodic Zn/Zn²⁺ reactions. The resulting MnO₂ -Zn battery regulated by CA exhibits large reversible capacities of 455 mAh g^-1 and 3.64 mAh cm^-2 at 20 C, as well as a long lifespan of 2000 cycles with energy density retention of 90%, achieving one of the best overall performances in the electrolytic MnO₂ -Zn batteries. This comprehensive work integrating theoretical prediction with experimental studies provides opportunities to the development of high-performance energy-storage devices.

Tungsten Nanoparticles Accelerate Polysulfides Conversion: A Viable Route toward Stable Room-Temperature Sodium-Sulfur Batteries

Room-temperature sodium-sulfur (RT Na-S) batteries are arousing great interest in recent years. Their practical applications, however, are hindered by several intrinsic problems, such as the sluggish kinetic, shuttle effect, and the incomplete conversion of sodium polysulfides (NaPSs). Here a sulfur host material that is based on tungsten nanoparticles embedded in nitrogen-doped graphene is reported. The incorporation of tungsten nanoparticles significantly accelerates the polysulfides conversion (especially the reduction of Na₂ S₄ to Na₂ S, which contributes to 75% of the full capacity) and completely suppresses the shuttle effect, en route to a fully reversible reaction of NaPSs. With a host weight ratio of only 9.1% (about 3-6 times lower than that in recent reports), the cathode shows unprecedented electrochemical performances even at high sulfur mass loadings. The experimental findings, which are corroborated by the first-principles calculations, highlight the so far unexplored role of tungsten nanoparticles in sulfur hosts, thus pointing to a viable route toward stable Na-S batteries at room temperatures.

Facile Fabrication of Bifunctional Hydrogen Catalytic Electrodes for Long-Life Nickel-Hydrogen Gas Batteries

The renaissance of long-lasting nickel-hydrogen gas (Ni-H₂) battery by developing efficient, robust, and affordable hydrogen anode to replace Pt is particularly attractive for large-scale energy storage applications. Here, we demonstrate an extremely facile corrosion induced fabrication approach to achieve a self-supporting hydrogen evolution/oxidation reaction (HER/HOR) bifunctional nanosheet array electrode for Ni-H₂ battery. The electrode is constituted by ultrafine Ru nanoparticles on Ni(OH)₂ nanosheets grown on nickel foam. Experimental and theoretical calculation results reveal that the electrode with optimized geometric and electronic structures ensures the efficient and robust catalytic hydrogen activities. The fabricated Ni-H₂ battery using the Ru-Ni(OH)₂/NF anode with an industrial scale areal capacity of 16 mAh cm^-2 demonstrates a high energy density, good rate capability and excellent durability without capacity decay over 1800 h. This study casts light on the development of low manufacturing cost and high performance bifunctional hydrogen catalytic electrodes for future hydrogen energy applications.

High Performance Aqueous Li-Ion Flow Capacitor Realized Through Microstructure Design of Suspension Electrode

Suspension electrode is the core of flowable electrochemical energy storage systems, which are considered suitable for large-scale energy storage. Nevertheless, obtaining suspension electrodes with both low viscosity and high conductivity is still a big challenge. In present work, spinel LiMn₂O₄ was chosen as an example to make suspension with low viscosity and high conductivity through microstructure morphology control of solid particles and the contact mode between active materials and conductive additives in suspension electrode. By coating a thin layer of polyaniline on the surface of spherical spinel LiMn₂O₄, the resulting suspension showed much higher electronic conductivity (about 10 times) and lower viscosity (about 4.5 times) as compared to irregular and bare spinel LiMn₂O₄-based suspension counterpart. As a result, the Li-ion flow capacitor based on LiMn₂O₄ and activated carbon suspensions exhibited a record energy density of 27.4 W h L⁻¹ at a power density of 22.5 W L⁻¹ under static condition to date, and can be smoothly work under an intermittent-flow mode. The strategy reported in this work is an effective way for obtaining suspension electrodes with low viscosity and high electronic conductivity simultaneously. It can not only be used in the flow capacitors, but also can be extended to other flowable electrochemical energy storage systems.

Nickel-hydrogen batteries for large-scale energy storage

Large-scale energy storage is of significance to the integration of renewable energy into electric grid. Despite the dominance of pumped hydroelectricity in the market of grid energy storage, it is limited by the suitable site selection and footprint impact. Rechargeable batteries show increasing interests in the large-scale energy storage; however, the challenging requirement of low-cost materials with long cycle and calendar life restricts most battery chemistries for use in the grid storage. Recently we introduced a concept of manganese-hydrogen battery with Mn²⁺/MnO₂ redox cathode paired with H⁺/H₂ gas anode, which has a long life of 10,000 cycles and with potential for grid energy storage. Here we expand this concept by replacing Mn²⁺/MnO₂ redox with a nickel-based cathode, which enables ∼10× higher areal capacity loading, reaching ∼35 mAh cm^-2 We also replace high-cost Pt catalyst on the anode with a low-cost, bifunctional nickel-molybdenum-cobalt alloy, which could effectively catalyze hydrogen evolution and oxidation reactions in alkaline electrolyte. Such a nickel-hydrogen battery exhibits an energy density of ∼140 Wh kg^-1 (based on active materials) in aqueous electrolyte and excellent rechargeability with negligible capacity decay over 1,500 cycles. The estimated cost of the nickel-hydrogen battery based on active materials reaches as low as ∼$83 per kilowatt-hour, demonstrating attractive characteristics for large-scale energy storage.

High-Performance Aqueous Zinc-Ion Battery Based on Layered H2 V3 O8 Nanowire Cathode

Rechargeable aqueous zinc-ion batteries have offered an alternative for large-scale energy storage owing to their low cost and material abundance. However, developing suitable cathode materials with excellent performance remains great challenges, resulting from the high polarization of zinc ion. In this work, an aqueous zinc-ion battery is designed and constructed based on H₂ V₃ O₈ nanowire cathode, Zn(CF₃ SO₃ )₂ aqueous electrolyte, and zinc anode, which exhibits the capacity of 423.8 mA h g^-1 at 0.1 A g^-1 , and excellent cycling stability with a capacity retention of 94.3% over 1000 cycles. The remarkable electrochemical performance is attributed to the layered structure of H₂ V₃ O₈ with large interlayer spacing, which enables the intercalation/de-intercalation of zinc ions with a slight change of the structure. The results demonstrate that exploration of the materials with large interlayer spacing is an effective strategy for improving electrochemical stability of electrodes for aqueous Zn ion batteries.

High-Capacity Aqueous Potassium-Ion Batteries for Large-Scale Energy Storage

A potassium iron (II) hexacyanoferrate nanocube cathode material is reported, which operates with an aqueous electrolyte to deliver exceptionally high capacities (up to 120 mA h g^-1 ). The cathode material exhibits excellent structural integrity, leading to fast kinetics and highly reversible properties. All of the battery materials are safe, inexpensive, and provide superior high-rate, long-cycle-life electrochemical performance.

Environmentally-friendly aqueous Li (or Na)-ion battery with fast electrode kinetics and super-long life

Current rechargeable batteries generally display limited cycle life and slow electrode kinetics and contain environmentally unfriendly components. Furthermore, their operation depends on the redox reactions of metal elements. We present an original battery system that depends on the redox of I(-)/I3 (-) couple in liquid cathode and the reversible enolization in polyimide anode, accompanied by Li(+) (or Na(+)) diffusion between cathode and anode through a Li(+)/Na(+) exchange polymer membrane. There are no metal element-based redox reactions in this battery, and Li(+) (or Na(+)) is only used for charge transfer. Moreover, the components (electrolyte/electrode) of this system are environment-friendly. Both electrodes are demonstrated to have very fast kinetics, which gives the battery a supercapacitor-like high power. It can even be cycled 50,000 times when operated within the electrochemical window of 0 to 1.6 V. Such a system might shed light on the design of high-safety and low-cost batteries for grid-scale energy storage.

Magnetic Field-Controlled Lithium Polysulfide Semiliquid Battery with Ferrofluidic Properties

Large-scale energy storage systems are of critical importance for electric grids, especially with the rapid increasing deployment of intermittent renewable energy sources such as wind and solar. New cost-effective systems that can deliver high energy density and efficiency for such storage often involve the flow of redox molecules and particles. Enhancing the mass and electron transport is critical for efficient battery operation in these systems. Herein, we report the design and characterization of a novel proof-of-concept magnetic field-controlled flow battery using lithium metal-polysulfide semiliquid battery as an example. A biphasic magnetic solution containing lithium polysulfide and magnetic nanoparticles is used as catholyte, and lithium metal is used as anode. The catholyte is composed of two phases of polysulfide with different concentrations, in which most of the polysulfide molecules and the superparamagnetic iron oxide nanoparticles can be extracted together to form a high-concentration polysulfide phase, in close contact with the current collector under the influence of applied magnetic field. This unique feature can help to maximize the utilization of the polysulfide and minimize the polysulfide shuttle effect, contributing to enhanced energy density and Coulombic efficiency. Additionally, owing to the effect of the superparamagnetic nanoparticles, the concentrated polysulfide phase shows the behavior of a ferrofluid that is flowable with the control of magnetic field, which can be used for a hybrid flow battery without the employment of any pumps. Our innovative design provides new insight for a broad range of flow battery chemistries and systems.