Self-supported Ru-Fe-Ox nanospheres as efficient electrocatalyst to boost overall water-splitting in acid and alkaline media.

Developing electrocatalysts with high activity and durability for oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) in both acidic and alkaline electrolytes remains challenging. In this study, we synthesize a self-supported ruthenium-iron oxide on carbon cloth (Ru-Fe-Ox/CC) using solvothermal methods followed by air calcination. The morphology of the nanoparticle exposes numerous active sites vital for electrocatalysis. Additionally, the strong electronic interaction between Ru and Fe enhances electrocatalytic kinetics optimization. The porous structure of the carbon cloth matrix facilitates mass transport, improving electrolyte penetration and bubble release. Consequently, Ru-Fe-Ox/CC demonstrates excellent catalytic performance, achieving low overpotentials of 32 mV and 28 mV for HER and 216 mV and 228 mV for OER in acidic and alkaline electrolytes, respectively. Notably, only 1.48 V and 1.46 V are required to reach 10 mA cm-2 for efficient water-splitting in both mediums, exhibiting remarkable stability. This research offers insights into designing versatile, highly efficient catalysts suitable for varied pH conditions.

Regulating Mo-based alloy-oxide active interfaces for efficient alkaline hydrogen evolution assisted by hydrazine oxidation.

Improving the efficiency of overall water splitting (OWS) is crucial due to the slow four-electron transfer process in the oxygen evolution reaction (OER). The coupling of the thermodynamically favorable hydrazine oxidation reaction (HzOR) with the hydrogen evolution reaction (HER) significantly boosts hydrogen production. A Ru-decorated MoNi/MoO2 micropillar (Ru-MoNi/MoO2) has been synthesized using a hydrothermal followed by reduction annealing. Benefiting from Ru moderating the active interface of Mo-based alloys/oxides and the unique one-dimensional micropillar morphology. The synthesized Ru-MoNi/MoO2 exhibits outstanding bifunctional activity for HER and HzOR, achieving 10 mA cm-2 at merely -13 mV and -34 mV in 1 M KOH and 1 M KOH + 0.5 M N2H4, respectively. Notably, with Ru-MoNi/MoO2 in a dual-electrode setup, only 0.57 V is needed to achieve 50 mA cm-2, demonstrating good stability and facilitating hydrazine-assisted water splitting (OHzS). This work offers insights into the modulation of alloy/metal oxide active interfaces, contributing to the development of efficient bifunctional catalysts for HER and HzOR.

Synergistic Effects of Pyrrolic N/Pyridinic N on Ultrafast Microwave Synthesized Porous CoP/Ni2P to Boost Electrocatalytic Hydrogen Generation.

Transition metal phosphides are ideal inexpensive electrocatalysts for water-splitting, but the catalytic activity still falls behind that of noble metal catalysts. Therefore, developing valid strategies to boost the electrocatalytic activity is urgent to promote large-scale applications. Herein, a microwave combustion strategy (20 s) is applied to synthesize N-doped CoP/Ni2P heterojunctions (N-CoP/Ni2P) with porous structure. The porous structure expands the specific surface area and accelerates the mass transport efficiency. Importantly, the pyrrolic N/pyridinic N content is adjusted by changing the amount of urea during the synthesis process and then optimizing the adsorption/desorption capacity for H*/OH* to enhance the catalyst activity. Then, the synthesized N-CoP/Ni2P exhibits small overpotentials of 111 and 133 mV for HER in acidic and alkaline electrolytes and 290 mV for OER in alkaline electrolytes. This work provides an original and efficient approach to the synthesis of porous metal phosphides.

Recent progress in the synthesis of transition metal nitride catalysts and their applications in electrocatalysis.

Transition metal nitrides (TMNs) have become excellent substitutes for precious metals such as Pt and Ir in the field of electrocatalysis because of their excellent electrocatalytic performance, high conductivity, good corrosion resistance and stability. As we all know, the commonly utilized carbon-based materials corrode easily during electrocatalysis, which will lead to catalyst falling off and agglomeration. Compared with carbon-based materials, TMNs have stronger corrosion resistance and higher stability. In the metal nitrides, a variety of chemical bonds (metal bond, ionic bond and covalent bond) coexist, among which the ionic bond between metal atoms and nitrogen atoms can make the d-band shrink and narrow, which leads to TMNs having characteristics similar to precious metals in the electrocatalytic process; thus, they can be used as a substitute for precious metal catalysts. In this paper, the synthesis method and catalytic principle of transition metal nitrides and their applications in the fields of hydrogen evolution reaction (HER), oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) are discussed, and the shortcomings of TMNs as a catalyst, the challenges faced in catalyst research and the developments and prospects for the future are pointed out.

Controllable Crystallization of Two-Dimensional Bi Nanocrystals with Morphology-Boosted CO2 Electroreduction in Wide pH Environments.

Two-dimensional low-melting-point (LMP) metal nanocrystals are attracting increasing attention with broad and irreplaceable applications due to their unique surface and topological structures. However, the chemical synthesis, especially the fine control over the nucleation (reduction) and growth (crystallization), of such LMP metal nanocrystals remains elusive as limited by the challenges of low standard redox potential, low melting point, poor crystalline symmetry, etc. Here, a controllable reduction-melting-crystallization (RMC) protocol to synthesize free-standing and surfactant-free bismuth nanocrystals with tunable dimensions, morphologies, and surface structures is presented. Especially, ultrathin bismuth nanosheets with flat or jagged surfaces/edges can be prepared with high selectivity. The jagged bismuth nanosheets, with abundant surface steps and defects, exhibit boosted electrocatalytic CO2 reduction performances in acidic, neutral, and alkaline aqueous solutions, achieving the maximum selectivity of near unity at the current density of 210 mA cm-2 for formate evolution under ambient conditions. This work creates the RMC pathway for the synthesis of free-standing two-dimensional LMP metal nanomaterials and may find broader applicability in more interdisciplinary applications.

Ultrafast Synthesis of Metal-Layered Hydroxides in a Dozen Seconds for High-Performance Aqueous Zn (Micro-) Battery.

Efficient synthesis of transition metal hydroxides on conductive substrate is essential for enhancing their merits in industrialization of energy storage field. However, most of the synthetic routes at present mainly rely on traditional bottom-up method, which involves tedious steps, time-consuming treatments, or additional alkaline media, and is unfavorable for high-efficiency production. Herein, we present a facile, ultrafast and general avenue to synthesize transition metal hydroxides on carbon substrate within 13 s by Joule-heating method. With high reaction kinetics caused by the instantaneous high temperature, seven kinds of transition metal-layered hydroxides (TM-LDHs) are formed on carbon cloth. Therein, the fastest synthesis rate reaches ~ 0.46 cm2 s-1. Density functional theory calculations further demonstrate the nucleation energy barriers and potential mechanism for the formation of metal-based hydroxides on carbon substrates. This efficient approach avoids the use of extra agents, multiple steps, and long production time and endows the LDHs@carbon cloth with outstanding flexibility and machinability, showing practical advantages in both common and micro-zinc ion-based energy storage devices. To prove its utility, as a cathode in rechargeable aqueous alkaline Zn (micro-) battery, the NiCo LDH@carbon cloth exhibits a high energy density, superior to most transition metal LDH materials reported so far.

Fast constructing polarity-switchable zinc-bromine microbatteries with high areal energy density.

Microbatteries (MBs) are promising candidates to provide power for various miniaturized electronic devices, yet they generally suffer from complicated fabrication procedures and low areal energy density. Besides, all cathodes of current MBs are solid state, and the trade-off between areal capacity and reaction kinetics restricts their wide applications. Here, we propose a dual-plating strategy to facilely prepare zinc-bromine MBs (Zn-Br2 MBs) with a liquid cathode to achieve both high areal energy density and fast kinetics simultaneously. The Zn-Br2 MBs deliver a record high areal energy density of 3.6 mWh cm-2, almost an order of magnitude higher than available planar MBs. Meanwhile, they show a polarity-switchable feature to tolerate confusion of cathode and anode. This strategy could also be extended to other battery systems, such as Zn-I2 and Zn-MnO2 MBs. This work not only proposes an effective construction method for MBs but also enriches categories of microscale energy storage devices.

Fixture-free omnidirectional prestretching fabrication and integration of crumpled in-plane micro-supercapacitors.

Multidimensional folded structures with elasticity could provide spatial charge storage capability and shape adaptability for micro-supercapacitors (MSCs). Here, highly crumpled in-plane MSCs with superior conformality are fabricated in situ and integrated by a fixture-free omnidirectional elastic contraction strategy. Using carbon nanotube microelectrodes, a single crumpled MSC holds an ultrahigh volumetric capacitance of 9.3 F cm-3, and its total areal capacitance is 45 times greater than the initial state. Experimental and theoretical simulation methods indicate that strain-induced improvements of adsorption energy and conductance for crumpled microelectrodes are responsible for the prominent enhancement of electrochemical performance. With outstanding morphological randomicity, the integrated devices can serve as smart coatings in moving robots, withstanding extreme mechanical deformations. Notably, integration on a spherical surface is possible by using a spherical mask, in which a small area of the microdevice array (3.9 cm2) can produce a high output voltage of 100 V.

Enabling fast-charging selenium-based aqueous batteries via conversion reaction with copper ions.

Selenium (Se) is an appealing alternative cathode material for secondary battery systems that recently attracted research interests in the electrochemical energy storage field due to its high theoretical specific capacity and good electronic conductivity. However, despite the relevant capacity contents reported in the literature, Se-based cathodes generally show poor rate capability behavior. To circumvent this issue, we propose a series of selenium@carbon (Se@C) composite positive electrode active materials capable of delivering a four-electron redox reaction when placed in contact with an aqueous copper-ion electrolyte solution (i.e., 0.5 M CuSO4) and copper or zinc foils as negative electrodes. The lab-scale Zn | |Se@C cell delivers a discharge voltage of about 1.2 V at 0.5 A g-1 and an initial discharge capacity of 1263 mAh gSe-1. Interestingly, when a specific charging current of 6 A g-1 is applied, the Zn | |Se@C cell delivers a stable discharge capacity of around 900 mAh gSe-1 independently from the discharge rate. Via physicochemical characterizations and first-principle calculations, we demonstrate that battery performance is strongly associated with the reversible structural changes occurring at the Se-based cathode.

Reunderstanding the Reaction Mechanism of Aqueous Zn-Mn Batteries with Sulfate Electrolytes: Role of the Zinc Sulfate Hydroxide.

Rechargeable aqueous Zn-Mn batteries have garnered extensive attention for next-generation high-safety energy storage. However, the charge-storage chemistry of Zn-Mn batteries remains controversial. Prevailing mechanisms include conversion reaction and cation (de)intercalation in mild acid or neutral electrolytes, and a MnO2 /Mn2+ dissolution-deposition reaction in strong acidic electrolytes. Herein, a Zn4 SO4 ·(OH)6 ·xH2 O (ZSH)-assisted deposition-dissolution model is proposed to elucidate the reaction mechanism and capacity origin in Zn-Mn batteries based on mild acidic sulfate electrolytes. In this new model, the reversible capacity originates from a reversible conversion reaction between ZSH and Znx MnO(OH)2 nanosheets in which the MnO2 initiates the formation of ZSH but contributes negligibly to the apparent capacity. The role of ZSH in this new model is confirmed by a series of operando characterizations and by constructing Zn batteries using other cathode materials (including ZSH, ZnO, MgO, and CaO). This research may refresh the understanding of the most promising Zn-Mn batteries and guide the design of high-capacity aqueous Zn batteries.

A Flexible Aqueous Zinc-Iodine Microbattery with Unprecedented Energy Density.

Currently, reported aqueous microbatteries (MBs) only show unsatisfactory electrochemical performance (≤120 mWh cm-3 volumetric energy density and <1000 µWh cm-2 areal energy density) and it remains challenging to develop durable aqueous MBs that can simultaneously offer both high volumetric and areal energy density. Herein, an in situ electrodeposition strategy is adopted to construct a flexible aqueous zinc-iodine MB (ZIDMB). Notably, the fabrication process well avoids the use of common additives (such as binders, conductive agents, and toxic solvent) and also bypasses subsequent time-consuming procedures such as grinding, coating, drying, etc., thus greatly simplifying the manufacture of the ZIDMB. Meanwhile, owing to the suppression of the shuttle effect of triiodide ions and the high ionic conductivity of the polyelectrolyte, the ZIDMB can simultaneously deliver record-high volumetric and areal energy densities of 1647.3 mWh cm-3 and 2339.1 µWh cm-2 , thus achieving values at least 13.5- and 2.3-fold better than those of best available aqueous MBs, respectively. This work affords an innovative strategy to construct an ideal micro-power-source for future miniaturized and integrated electronics.

Metal-Triazolate-Framework-Derived FeN4 Cl1 Single-Atom Catalysts with Hierarchical Porosity for the Oxygen Reduction Reaction.

The construction of single-atom catalysts (SACs) with high single atom densities, favorable electronic structures and fast mass transfer is highly desired. We have utilized metal-triazolate (MET) frameworks, a subclass of metal-organic frameworks (MOFs) with high N content, as precursors since they can enhance the density and regulate the electronic structure of single-atom sites, as well as generate abundant mesopores simultaneously. Fe single atoms dispersed in a hierarchically porous N-doped carbon matrix with high metal content (2.78 wt %) and a FeN4 Cl1 configuration (FeN4 Cl1 /NC), as well as mesopores with a pore:volume ratio of 0.92, were obtained via the pyrolysis of a Zn/Fe-bimetallic MET modified with 4,5-dichloroimidazole. FeN4 Cl1 /NC exhibits excellent oxygen reduction reaction (ORR) activity in both alkaline and acidic electrolytes. Density functional theory calculations confirm that Cl can optimize the adsorption free energy of Fe sites to *OH, thereby promoting the ORR process. The catalyst demonstrates great potential in zinc-air batteries. This strategy selects, designs, and adjusts MOFs as precursors for high-performance SACs.

A Cascade Battery: Coupling Two Sequential Electrochemical Reactions in a Single Battery.

Currently, rechargeable electrochemical batteries generally operate on one reversible electrochemical reaction during discharging and charging cycles. Here, a cascade battery that couples two sequential electrochemical reactions in a single battery is proposed. Such a concept is demonstrated in an aqueous Zn-S hybrid battery, where solid sulfur serves as the cathode in the first discharge step and the generated Cu2 S catalyzes Cu2+ reduce to Cu/Cu2 O to provide the second discharge step. The cascade battery shows many merits compared to traditional batteries. First, it integrates two batteries internally, eliminating the use of additional inactive connecting materials required for external integration. Second, it can more fully utilize the inactive reaction chamber of the battery than traditional batteries. Third, cascade battery can bypass the challenges of thick solid electrode to access high areal capacity. An ultrahigh areal capacity of 48 mAh cm-2 is achieved even at a low solid cathode loading (9.6 mg cm-2 ). The cascade battery design breaks the stereotype of conventional battery configuration, providing a paradigm for constructing two-in-one batteries.

The Emerging of Aqueous Zinc-Based Dual Electrolytic Batteries.

As high performance and safety alternatives to the batteries with organic electrolytes, aqueous zinc-based batteries are still far from satisfactory in practical use because of the limitation of the intercalation reaction mechanism and the strict requirements for the cathodes. Very recently, zinc-based dual electrolytic batteries (DEBs), where the cathode and anode are both based on reversible electrolytic reactions, are emerging. It features with electrode-free configuration, thus avoiding the preliminary active materials or electrode fabrication procedures. Meanwhile, the new battery chemistry typically possesses a high specific capacity, output voltage, faster reaction rates, and long cycling life. Herein, the advances of the development of various zinc-based DEBs, including Zn-MnO2 , Zn-Br2 , and Zn-I2 DEBs, are systematically summarized. This review will focus on the working mechanisms of these batteries and how the decoupling catholyte and anolyte affect their output voltages. The perspectives of the opportunities and challenges are also suggested in the aspects of protecting zinc anode, enhancing volumetric energy density, suppressing fast self-discharge, and developing multifunctional integrated zinc-based DEBs.

A seamlessly integrated device of micro-supercapacitor and wireless charging with ultrahigh energy density and capacitance.

Microdevice integrating energy storage with wireless charging could create opportunities for electronics design, such as moveable charging. Herein, we report seamlessly integrated wireless charging micro-supercapacitors by taking advantage of a designed highly consistent material system that both wireless coils and electrodes are of the graphite paper. The transferring power efficiency of the wireless charging is 52.8%. Benefitting from unique circuit structure, the intact device displays low resistance and excellent voltage tolerability with a capacitance of 454.1 mF cm-2, superior to state-of-the-art conventional planar micro-supercapacitors. Besides, a record high energy density of 463.1 µWh cm-2 exceeds the existing metal ion hybrid micro-supercapacitors and even commercial thin film battery (350 µWh cm-2). After charging for 6 min, the integrated device reaches up to a power output of 45.9 mW, which can drive an electrical toy car immediately. This work brings an insight for contactless micro-electronics and flexible micro-robotics.

Retarding Ostwald Ripening to Directly Cast 3D Porous Graphene Oxide Bulks at Open Ambient Conditions.

Graphene aerogels (GAs) with attractive properties have shown tremendous potentials in energy- and environment-related applications. Unfortunately, current assembly methods for GAs such as sol-gel and freeze-casting processes must be conducted in enclosed spaces with unconventional conditions, thus being literally inoperative for in situ and continuous productions. Herein, a direct slurry-casting method at open ambient conditions is established to arbitrarily prepare three-dimensional (3D) porous graphene oxide (GO) bulks without macroscopic dimension limits on a wide range of solid surfaces by retarding Ostwald ripening of 3D liquid GO foams when being dried in air. A subsequent fast thermal reduction (FTR) of GO foams leads to the formation of graphene aerogels (denoted as FTR-GAs) with hierarchical closed-cellular graphene structures. The FTR-GAs show outstanding high-temperature thermal insulation (70% decrease for 400 °C), as well as superelasticity (>1000 compression-recovery cycles at 50% strain), ultralow density (10-28 mg cm-3), large specific surface area (BET, 206.8 m2 g-1), and high conductivity (ca. 100 S m-1). This work provides a viable method to achieve in situ preparations of high-performance GAs as multifunctional structural materials in aircrafts, high-speed trains, or even buildings for the targets of energy efficiency, comfort, and safety.

Hybrid Energy Storage Device: Combination of Zinc-Ion Supercapacitor and Zinc-Air Battery in Mild Electrolyte.

In this work, a new type of hybrid energy storage device is constructed by combining the zinc-ion supercapacitor and zinc-air battery in mild electrolyte. Reduced graphene oxide with rich defects, large surface area, and abundant oxygen-containing functional groups is used as active material, which exhibits two kinds of charge storage mechanisms of capacitor and battery simultaneously. Apart from the physical adsorption/desorption of anions on the surface of graphene, the zinc ions in electrolyte will be electrochemically adsorbed/desorbed onto the oxygen-containing groups of graphene during the charge/discharge process, contributing extra capacitance to the device. Moreover, the defects in graphene will further improve the electrochemical performance of the energy storage device via catalyzing the oxygen reduction reaction with exposure to air. Consequently, the synergistic effect leads to a record high capacitance of 370.8 F g-1 at a current density of 0.1 A g-1, which is higher than that of zinc-ion supercapacitors reported previously. Furthermore, the hybrid device exhibits a superior cycling stability with 94.5% capacitance retention even after 10000 charge/discharge cycles at a high current density of 5 A g-1. Interestingly, the developed hybrid device can be self-charging automatically after the power is exhausted in the ambient atmosphere. Other electrode materials, such as carbon nanotube paper, are also used to build a hybrid device to verify the feasibility of this strategy. This facile, green, and convenient strategy provides new insight for developing a high performance storage device, showing great application prospect in other hybrid energy storage devices in mild electrolyte.

Manipulating irreversible phase transition of NaCrO2 towards an effective sodium compensation additive for superior sodium-ion full cells.

During the first charge process of full cells, a solid electrolyte interphase (SEI) film is formed when the active ion from the cathode is consumed, resulting in irreversible capacity loss. This phenomenon has shown to be more serious in sodium-ion full cells than in lithium-ion full cells. Although many strategies have been employed to alleviate the loss of sodium ions, such as presodiation and construction of an artificial solid electrolyte interface, they are both cumbersome and time-consuming. For the first time, NaCrO2 was used as an effective self-sacrificing sodium compensation additive in sodium-ion full cells due to the irreversible phase transition of NaCrO2 in a high voltage region can deliver an irreversible capacity of up to 230 mAh g-1. Based on this design, sodium-ion full cells coupled with hard carbon as the anode exhibited higher capacity, less polarization, greater energy density, and superior cycle stability than those of a pristine electrode. This is mainly attributed to the removal of sodium ions from NaCrO2, which compensates for the loss of sodium ions consumed during the formation of the SEI film on the anode surface during the first charge process. Overall, this work opens up a new avenue for exploring sodium compensation strategy and contributing to practical application of sodium-ion full cells.

Nitrogen-Doped Carbon as a Host for Tellurium for High-Rate Li-Te and Na-Te Batteries.

A nitrogen-doped hierarchical porous carbon sponge, used as a matrix for tellurium accommodation, was designed and prepared in this work. The porosity of the matrix played an important role in enhancing the electrochemical performance of Li/Na-Te batteries. Specifically, the mesopores could accommodate active materials whereas the macropores provided sufficient space for partial Te accommodation and volume expansion in discharge. In addition, N heteroatoms in carbon species could enhance the electrical conductivity and widen its application in lithium/sodium storage. The monolithic and flaky architecture of the nitrogen-doped hierarchical porous carbon sponge/tellurium composite offered a highly conductive network for fast electron transportation. As a result, the nitrogen-doped hierarchical porous carbon sponge/tellurium composite achieved a superior rate performance for Li-Te and Na-Te batteries.

Self-Supported FeCo2S4 Nanotube Arrays as Binder-Free Cathodes for Lithium-Sulfur Batteries.

Inhibiting the shuttle effect, buffering the volume expansion, and improving the utilization of sulfur have been the three strategic points for developing a high-performance lithium-sulfur (Li-S) battery. Driven by this background, a flexible sulfur host material composed of FeCo2S4 nanotube arrays grown on the surface of carbon cloth is designed for a binder-free cathode of the Li-S battery through two-step hydrothermal method. Among the rest, the interconnected carbon fiber skeleton of the composite electrode ensures the basic electrical conductivity, whereas the FeCo2S4 nanotube arrays not only boost the electron and electrolyte transfer but also inhibit the dissolution of polysulfides because of their strong chemical adsorption. Meanwhile, the hollow structures of these arrays can provide a large inner space to accommodate the volume expansion of sulfur. More significantly, the developed composite electrode also reveals a catalytic action for accelerating the reaction kinetic of the Li-S battery. As a result, the FeCo2S4/CC@S electrode delivers a high discharge capacity of 1384 mA h g-1 at the current density of 0.1 C and simultaneously exhibits a stable Coulombic efficiency of about 98%.