A cost-effective alkaline polysulfide-air redox flow battery enabled by a dual-membrane cell architecture

Electrocatalysts and Membranes for Aqueous Polysulfide Redox Flow Batteries

Redox flow batteries have demonstrated attractive attributes in large-scale stationary energy storage, but practical applications are impeded by high capital cost. Polysulfides are exceedingly cost-effective candidates of redox-active materials for achieving cost reduction, and a recent revival has been witnessed. But the slow conversion kinetics and irreversible crossover loss of polysulfides are daunting challenges that have caused severe technoeconomic stress and even system failure. Solutions to these issues capitalize on the innovations of powerful electrocatalysts and permselective membranes. To inspire viable development strategies and further advance polysulfide redox, this Review presents a critical overview of the state of the art of electrocatalysts and membranes, highlighting their working mechanisms, design protocols, and performance metrics. We briefly describe the complicated processes of the polysulfide reaction and the major spectroscopic methods for polysulfide speciation. Next, we point out the specific characteristics of polysulfide redox and summarize the metallic, metal sulfide, and molecular electrocatalysts to elucidate the fundamental requirements for imparting strong catalytic effects. We then discuss the possible origins of polysulfide crossover and outline the major families of membrane chemistries targeting polysulfide retention. Finally, the remaining challenges and the future perspectives for potential considerations are provided, aiming to realize efficient, durable polysulfide flow batteries.

Aqueous-S vs Organic-S Battery: Volmer-Step Involved Sulfur Reaction

Aqueous-S batteries (ASBs) are emerging as promising energy storage technologies due to their high safety, low cost, and high theoretical energy density. However, the present understanding of sulfur evolution in water relies on experience derived from conventional organic electrolyte-based sulfur batteries (OSBs). The gap between ASB and OSB has impeded progress in advancing the rational design of sulfur catalysts in the aqueous phase. Herein, we reveal the unique interaction between H2O and S species, which is fundamentally distinguishable from the organic counterparts. A series of spectroscopy analyses discloses that elemental sulfur is initially reduced to polysulfides (mainly S42-), which subsequently react with H2O to generate HS-, involving both polysulfide conversion and the Volmer step of water dissociation. Combined electrochemical and computational analysis further proposes an aqueous-S catalyst selection metric based on simultaneous polysulfide adsorption and Volmer-step catalysis. As a proof of concept, we have successfully prioritized the Mo2C-catalyzed ASBs with a superior rate capability of 1040 mAh g-1 than the Fe3C (693 mAh g-1) and pure C (510 mAh g-1) at a high current density of 5 A g-1. This work provides insights into the aqueous-S charge storage mechanism and establishes a foundational catalyst research paradigm for advancing the following ASBs.

Automated Microfluidics for Efficient Characterization of Cyclohexanol Electrooxidation for Sustainable Chemical Production

The electrochemical conversion of biomass-derived compounds into value-added chemicals using renewable electricity has attracted attention as a promising pathway for sustainable chemical production, with the electrooxidation of cyclohexanol being a typical example. However, optimizing and upscaling these processes have been hindered due to a limited understanding of the underlying mechanisms and limiting factors. To address this, there is a critical need for experimental tools that enable more efficient and reproducible measurements of these complex processes. In this work, we develop an automated microfluidic platform and use it to conduct controlled and efficient measurements of cyclohexanol electrooxidation on nickel electrodes under various electrolyte compositions and flow rates. The platform features microchannel networks integrated with multiple analytical instruments such as pumps, an electrochemical workstation, and a digital microscope to perform laboratory functions including electrolyte preparation, reaction control, microscopy, and electrochemical characterization, all streamlined through automation. Cyclohexanol electrooxidation on nickel is found to follow Fleischmann's mechanism, with an irreversible heterogeneous reaction as the rate-determining step. The effects of ionic and nonionic surfactant additives are screened, both demonstrating the ability to enhance current densities through different mechanisms. The developed platform is readily transferable for measuring other power-to-chemical processes and is believed to be a powerful tool for accelerating the understanding and development of sustainable electrosynthesis.

Thiosulfate-Mediated Polysulfide Redox for Energetic Aqueous Battery

Sulfur-based aqueous batteries (SABs) are regarded as promising candidates for safe, low-cost, and high-energy storage. However, the sluggish redox kinetics of polysulfides pose a significant challenge to the practical performance of SABs. Herein, we report a unique redox regulation strategy that leverages thiosulfate-mediated ligand-chain interaction to accelerate the polysulfide redox process (S0/S2-). The S2O3 2- species in the electrolyte can induce the rapid reduction of polysulfide through a spontaneous chemical reaction with sulfur species, while facilitating the reversible oxidation of short-chain sulfides. Moreover, the thiosulfate redox pair (S2O3 2-/S4O6 2-) within the K2S2O3 electrolyte contributes additional capacity at higher potential (E0 >0 V vs SHE). Consequently, the elaborate SAB delivers an unprecedented K+ storage capacity of 2470 mAh gs -1, coupled with a long cycling life exceeding 1000 cycles. Remarkably, thiosulfate-mediated SAB achieves an energy density of 616 Wh kgS+Zn -1, surpassing both organic K-S batteries and conventional aqueous battery systems. This work elucidates the mechanism underlying the thiosulfate-mediated polysulfide redox process, thereby opening a pathway for the development of high-energy aqueous batteries.

Simulation of the electrolyte imbalance in vanadium redox flow batteries

The stack is the core component of large-scale flow battery system. Based on the leakage circuit, mass and energy conservation, electrochemicals reaction in porous electrode, and also the effect of electric field on vanadium ion cross permeation in membrane, a model of kilowatt vanadium flow battery stack was established. The electro chemical reaction parameters, ion concentration and temperature of each single cell in the stack were calculated respectively. The imbalance of vanadium ion concentration and the effects of current density and electrolyte temperature on the electrolyte imbalance in the stack were studied.

Uniform Nanoscale Ion-Selective Membrane Prepared by Precision Control of Solution Spreading and Evaporation

Ion-selective membrane has broad application in various fields, while the present solution-processed techniques can only prepare uniform membrane with microscale thickness. Herein, a high-quality polymer membrane with nanoscale thickness and uniformity is precisely prepared by controlling solution spreading and solvent evaporation stability/rate. With the arrayed capillaries, the stable spreading of polymer solution with volume of microliter induces the formation of solution film with micrometers thickness. Moreover, the fast increase of solution dynamic viscosity during solvent evaporation inhibits nonuniform Marangoni flow and capillary flow in solution film. Consequently, the uniform Nafion-Li membranes with ∼200 nm thickness are prepared, while their Li+ conductivity is 2 orders of magnitude higher than that of commercially Nafion-117 membrane. Taking lithium-sulfur battery as a model device, the cells (capacities of 8-10 mAh cm-2) can stably operate for 150 cycles at a S loading of 12 mg cm-2 and an electrolyte/sulfur ratio of ∼7.

Development of Membranes and Separators to Inhibit Cross-Shuttling of Sulfur in Polysulfide-Based Redox Flow Batteries: A Review

The global rapid transition from fossil fuels to renewable energy resources necessitates the implementation of long-duration energy storage technologies owing to the intermittent nature of renewable energy sources. Therefore, the deployment of grid-scale energy storage systems is inevitable. Sulfur-based batteries can be exploited as excellent energy storage devices owing to their intrinsic safety, low cost of raw materials, low risk of environmental hazards, and highest theoretical capacities (gravimetric: 2600 Wh/kg and volumetric: 2800 Wh/L). However, sulfur-based batteries exhibit certain scientific limitations, such as polysulfide crossover, which causes rapid capacity decay and low Coulombic efficiency, thereby hindering their implementation at a commercial scale. In this review article, we focus on the latest research developments between 2012-2023 to improve the separators/membranes and overcome the shuttle effect associated with them. Various categories of ion exchange membranes (IEMs) used in redox batteries, particularly polysulfide redox flow batteries and lithium-sulfur batteries, are discussed in detail. Furthermore, advances in IEM constituents are summarized to gain insights into different fundamental strategies for attaining targeted characteristics, and a critical analysis is proposed to highlight their efficiency in mitigating sulfur cross-shuttling issues. Finally, future prospects and recommendations are suggested for future research toward the fabrication of more effective membranes with desired properties.

Hierarchical Nano-Electrocatalytic Reactor for High Performance Polysulfides Redox Flow Batteries

The aqueous polysulfides is an important Earth-abundant and multielectron redox couple to construct high capacity density and low-cost aqueous redox flow batteries (RFB) ; nevertheless, the sluggish conversion and kinetic behavior of S2-/Sx2- result in a low power density output and poor active material utilizations. Herein, we present nanoconfined self-assembled ordered hierarchical porous Co and N codoped carbon (OHP-Co/NC) as an electrocatalytic reactor to enhance the mass transfer and redox activity of aqueous polysulfides. Finite element method simulation proves that the OHP-Co/NC with interconnected macropores and mesopores exhibits an enhanced mass transfer and delivers a larger redox electrolyte utilization of 50.1% compared to 23.3% of conventional Co/NC. Notably, the OHP-Co/NC obtained at 850 °C delivers the smallest redox peak potential difference (ΔE = 99 mV). Comparison studies of in operando Raman for aqueous polysulfides in the redox electrolyte and in situ electrochemical Raman on the single OHP-Co/NC particle for the adsorbed polysulfides were carried out. And it confirms that the OHP-Co/NC-850 catalyst has a strong adsorption of S42- and can retard the strong disproportionation and hydrolysis behavior of polysulfides on the electrocatalyst interface. Therefore, the polysulfide/ferrocyanide RFB with an OHP-Co/NC-850 based membrane-electrode assembly (MEA) exhibited a high power density of 110 mW cm-2, as well as a steady capacity retention over 99.7% in 300 cycles.

Benchmarking organic active materials for aqueous redox flow batteries in terms of lifetime and cost

Flow batteries are one option for future, low-cost stationary energy storage. We present a perspective overview of the potential cost of organic active materials for aqueous flow batteries based on a comprehensive mathematical model. The battery capital costs for 38 different organic active materials, as well as the state-of-the-art vanadium system are elucidated. We reveal that only a small number of organic molecules would result in costs close to the vanadium reference system. We identify the most promising candidate as the phenazine 3,3'-(phenazine-1,6-diylbis(azanediyl))dipropionic acid) [1,6-DPAP], suggesting costs even below that of the vanadium reference. Additional cost-saving potential can be expected by mass production of these active materials; major benefits lie in the reduced electrolyte costs as well as power costs, although plant maintenance is a major challenge when applying organic materials. Moreover, this work is designed to be expandable. The developed calculation tool (ReFlowLab) accompanying this publication is open for updates with new data.