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Lithium–sulfur (Li–S) batteries are considered as one of the most promising next-generation energy storage devices because of their ultrahigh theoretical energy density beyond lithium-ion batteries. The cycling stability of Li metal anode largely determines the prospect of practical applications of Li–S batteries. This review systematically summarizes the current advances of Li anode protection in Li–S batteries regarding both fundamental understanding and regulation methodology. First, the main challenges of Li metal anode instability are introduced with emphasis on the influence from lithium polysulfides. Then, a timeline with 4 stages is presented to afford an overview of the developing history of this field. Following that, 3 Li anode protection strategies are discussed in detail in aspects of guiding uniform Li plating/stripping, reducing polysulfide concentration in anolyte, and reducing polysulfide reaction activity with Li metal. Finally, 3 viewpoints are proposed to inspire future research and development of advanced Li metal anode for practical Li–S batteries.

Electrochemical water splitting is one of the most sustainable approaches for generating hydrogen. Because of the inherent constraints associated with the architecture and materials, the conventional alkaline water electrolyzer and the emerging proton exchange membrane electrolyzer are suffering from low efficiency and high materials/operation costs, respectively. Herein, we design a membrane-free flow electrolyzer, featuring a sandwich-like architecture and a cyclic operation mode, for decoupled overall water splitting. Comprised of two physically-separated compartments with flowing H 2 -rich catholyte and O 2 -rich anolyte, the cell delivers H 2 with a purity >99.1%. Its low internal ohmic resistance, highly active yet affordable bifunctional catalysts and efficient mass transport enable the water splitting at current density of 750 mA cm −2 biased at 2.1 V. The eletrolyzer works equally well both in deionized water and in regular tap water. This work demonstrates the opportunity of combining the advantages of different electrolyzer concepts for water splitting via cell architecture and materials design, opening pathways for sustainable hydrogen generation. Seawater electrolysis is promising for grid-scale H 2 production without freshwater reliance, but high energy costs and detrimental Cl chemistry reduce its practical potential. Here, authors developed an energy-saving hybrid seawater electrolyzer for chlorine-free H 2 production and N 2 H 4 degradation.

Membrane electrode assemblies enable CO2 electrolysis at industrially relevant rates, yet their operational stability is often limited by formation of solid precipitates in the cathode pores, triggered by cation crossover from the anolyte due to imperfect ion exclusion by anion exchange membranes. Here we show that anolyte concentration affects the degree of cation movement through the membranes, and this substantially influences the behaviors of copper catalysts in catholyte-free CO2 electrolysers. Systematic variation of the anolyte (KOH or KHCO3) ionic strength produced a distinct switch in selectivity between either predominantly CO or C2+ products (mainly C2H4) which closely correlated with the quantity of alkali metal cation (K+) crossover, suggesting cations play a key role in C-C coupling reaction pathways even in cells without discrete liquid catholytes. Operando X-ray absorption and quasi in situ X-ray photoelectron spectroscopy revealed that the Cu surface speciation showed a strong dependence on the anolyte concentration, wherein dilute anolytes resulted in a mixture of Cu+ and Cu0 surface species, while concentrated anolytes led to exclusively Cu0 under similar testing conditions. These results show that even in catholyte-free cells, cation effects (including unintentional ones) significantly influence reaction pathways, important to consider in future development of catalysts and devices.

Challenges in CO 2 capture, CO 2 crossover, product separation, and electrolyte recovery hinder electrocatalytic CO 2 reduction (CO 2 R). Here, we present an integrated electrochemical recovery and separation system (ERSS) with an ion separation module (ISM) between the anode and cathode of a water electrolysis system. During ERSS operation, protons from the anolyte flow through the anodic cation exchange membrane (CEM) into the ISM, acidifying the CO 2 R effluent electrolyte. Cations like K + in the ISM flow through the cathodic CEM into the catholyte to balance the OH − ions from hydrogen evolution. ERSS recycles electrolyte-adsorbed CO 2 , recovers KOH with a 94.0% K + yield, and achieves an 86.2% separation efficiency for CO 2 R products. The recovered KOH can capture CO 2 from air or flue gas or be utilized as a CO 2 R electrolyte, closing the CO 2 capture, conversion, and utilization loop. Compared to the conventional acid-base neutralization process, ERSS saves $119.76 per ton of KOH recovered and is applicable to other aqueous alkaline electrosynthesis reactions. CO 2 capture, product separation, and electrolyte recovery are critical challenges in CO 2 electroreduction. Here, the authors present an electrochemical recovery-separation system that addresses these challenges and facilitates the application of CO 2 reduction technology.

A promising metal-organic complex, iron (Fe)-NTMPA 2 , consisting of Fe(III) chloride and nitrilotri-(methylphosphonic acid) (NTMPA), is designed for use in aqueous iron redox flow batteries. A full-cell testing, where a concentrated Fe-NTMPA 2 anolyte (0.67 M) is paired with a Fe-CN catholyte, demonstrates exceptional cycling stability over 1000 charge/discharge cycles, and noteworthy performances, including 96% capacity utilization, a minimal capacity fade rate of 0.0013% per cycle (1.3% over 1,000 cycles), high Coulombic efficiency and energy efficiency near 100% and 87%, respectively, all achieved under a current density of 20 mA·cm - ². Furthermore, density functional theory unveils two potential coordination structures for Fe-NTMPA 2 complexes, improving the understanding between the ligand coordination environment and electron transfer kinetics. When paired with a high redox potential Fe-Dcbpy/CN catholyte, 2,2′-bipyridine-4,4′-dicarboxylic (Dcbpy) acid and cyanide (CN) ligands, Fe-NTMPA 2 demonstrates a notably elevated cell voltage of 1 V, enabling a practical energy density of up to 9 Wh/L. Here, authors report an iron flow battery, using earth-abundant materials like iron, ammonia, and phosphorous acid. This work offers a solution to reduce materials cost and extend cycle life in energy storage applications for grid decarbonization.

Aqueous soluble organic (ASO) redox-active materials have recently attracted significant attention as alternatives to traditional transition metal ions in redox flow batteries (RFB). However, reported reversible capacities of ASO are often substantially lower than their theoretical values based on the reported maximum solubilities. Here, we describe a phenazine-based ASO compound with an exceptionally high reversible capacity that exceeds 90% of its theoretical value. By strategically modifying the phenazine molecular structure, we demonstrate an increased solubility from near-zero with pristine phenazine to as much as 1.8 M while also shifting its redox potential by more than 400 mV. An RFB based on a phenazine derivative (7,8-dihydroxyphenazine-2-sulfonic acid) at its near-saturation concentration exhibits an operating voltage of 1.4 V with a reversible anolyte capacity of 67 Ah l −1 and a capacity retention of 99.98% per cycle over 500 cycles. Redox flow batteries (RFBs) based on organic redox-active molecules are attractive, but the solubility of those molecules, and consequently the capacity, is generally low. Here, the authors develop a phenazine derivative with high solubility as an energy-dense anolyte for RFBs.

Aqueous organic redox flow batteries (AORFBs) are a promising technology for large-scale electricity energy storage to realize efficient utilization of intermittent renewable energy. In particular, organic molecules are a class of metal-free compounds that consist of earth-abundant elements with good synthetic tunability, electrochemical reversibility and reaction rates. However, the short cycle lifetime and low capacity of AORFBs act as stumbling blocks for their practical deployment. To circumvent these issues, here, we report molecular engineered dihydroxyanthraquinone (DHAQ)-based alkaline electrolytes. Via computational studies and operando measurements, we initially demonstrate the presence of a hydrogen bond-mediated degradation mechanism of DHAQ molecules during electrochemical reactions. Afterwards, we apply a molecular engineering strategy based on redox-active polymers to develop capacity-boosting composite electrolytes. Indeed, by coupling a 1,5-DHAQ/poly(anthraquinonyl sulfide)/carbon black anolyte and a [Fe(CN) 6 ] 3−/4− alkaline catholyte, we report an AORFB capable of delivering a stable cell discharge capacity of about 573 mAh at 20 mA/cm 2 after 1100 h of cycling and an average cell discharge voltage of about 0.89 V at the same current density. Aqueous organic redox flow batteries are affected by short cycle life and low capacity. Here, the authors develop composite dihydroxyanthraquinone/polymer anolytes capable of improving the cycling stability and discharge capacity of aqueous organic redox flow batteries.

The limited availability of a high-performance catholyte has hindered the development of aqueous organic redox flow batteries (AORFB) for large-scale energy storage. Here we report a symmetry-breaking design of iron complexes with 2,2′-bipyridine-4,4′-dicarboxylic (Dcbpy) acid and cyanide ligands. By introducing two ligands to the metal centre, the complex compounds (M 4 [Fe II (Dcbpy) 2 (CN) 2 ], M = Na, K) exhibited up to a 4.2 times higher solubility (1.22 M) than that of M 4 [Fe II (Dcbpy) 3 ] and a 50% increase in potential compared with that of ferrocyanide. The AORFBs with 0.1 M Na 4 [Fe II (Dcbpy) 2 (CN) 2 ] as the catholyte were demonstrated for 6,000 cycles with a capacity fading rate of 0.00158% per cycle (0.217% per day). Even at a concentration near the solubility limit (1 M Na 4 [Fe II (Dcbpy) 2 (CN) 2 ]), the flow battery exhibited a capacity fading rate of 0.008% per cycle (0.25% per day) in the first 400 cycles. The AORFB cell with a nearly 1:1 catholyte:anolyte electron ratio achieved a cell voltage of 1.2 V and an energy density of 12.5 Wh l –1 . The development of aqueous organic redox flow batteries suffers from the limited availability of high-performance catholytes. Here the authors design a metal organic complex catholyte material with a tunable redox potential, which offers promise for high-energy long-lasting flow batteries.

Aqueous batteries have been considered as the most promising alternatives to the dominant lithium-based battery technologies because of their low cost, abundant resources and high safety. The output voltage of aqueous batteries is limited by the narrow stable voltage window of 1.23 V for water, which theoretically impedes further improvement of their energy density. However, the pH-decoupling electrolyte with an acidic catholyte and an alkaline anolyte has been verified to broaden the operating voltage window of the aqueous electrolyte to over 3 V, which goes beyond the voltage limitations of the aqueous batteries, making high-energy aqueous batteries possible. In this Review, we summarize the latest decoupled aqueous batteries based on pH-decoupling electrolytes from the perspective of ion-selective membranes, competitive redox couples and potential battery prototypes. The inherent defects and problems of these decoupled aqueous batteries are systematically analysed, and the critical scientific issues of this battery technology for future applications are discussed. Developing aqueous batteries with high voltages is possible with the use of pH-decoupling electrolytes with an acidic catholyte and an alkaline anolyte.

The practical application of electrochemical carbon dioxide reduction reaction (CO 2 RR) technology remains hindered by poor stability, primarily owing to bicarbonate salt formation at the cathode, which blocks reactant CO 2 mass flow. Here, using operando characterization tools, we tracked the salt formation process and quantified salt precipitation under varying device operational conditions, elucidating a potential mechanism and optimizing anolyte conditions for long-term (>1,000 h) operation CO 2 RR to CO under >100 mA cm –2 . Liquid droplets carrying cations and (bi)carbonate ions were observed to migrate from the catalyst/membrane interface towards the backside of the gas diffusion electrode, driven by interfacial gas evolution and CO 2 flow. These droplets eventually dried, forming bicarbonate salt precipitates that blocked the gas flow channels. On the basis of this observation, we applied a hydrophobic parylene coating to the cathode gas flow channel surface, facilitating the removal of the droplets and extending stability from ~100 h to over 500 h under 200 mA cm –2 . Devices for electrochemical CO 2 reduction can suffer from salt precipitation that blocks gas flow, leading to instability. Hao et al. explore possible salt formation mechanisms and provide a means to mitigate it via application of hydrophobic surface coatings.