Easy-to-Lay Poly-N Heterocyclic Additives Enable Long-Term Stabilization of Zinc-Ion Capacitor Anodes under Deep Plating/Stripping.

Addition of organic compounds containing O/N heteroatoms to aqueous electrolytes such as ZnSO4 (ZS) solutions is one of the effective strategies to inhibit Zn anode dendrites and side reactions. However, addressing the stability of Zn plating/stripping at high current densities and areal capacities by this method is still a challenge, especially in capacitors known for high power and long life. Herein, an organic heterocyclic compound of 1, 4, 7, 10-tetraazacyclododecane (TC) containing four symmetrically distributed N atoms is employed as ZS additive, expanding the life of Zn anodes from ≈ 30 h to 1000 and 240 h at deep plating/stripping conditions of 10 and 20 mA cm-2/mAh cm-2, respectively; the cumulative capacity is as high as 5.0 Ah cm-2 with 99% Coulombic efficiency, far exceeding reported additives. TC with higher binding energies than H2O for Zn species tends to adsorb to Zn (002) in a lying manner and participate in the solvation shell of Zn2+, thus avoiding Zn dendrites and side-reaction damage, especially at high current densities. The TC-endowed Zn anode's stability under such extreme conditions is verified in Zn-ion capacitors (i.e., > 94.6% capacity retention after 28 000 cycles), providing new insights into the development of high-power Zn-based energy storage devices.

Competition-Induced Macroscopic Superlubricity of Ionic Liquid Analogues by Hydroxyl Ligands Revealed by in Situ Raman.

High load-bearing capacity is one of the crucial indicators for liquid superlubricants to move toward practicality. However, some of the current emerging systems not only have low contact pressures but also are highly susceptible to further degradation due to water adsorption and even superlubricity failure. Herein, a novel choline chloride-based ionic liquid analogues (ILAs) of a superlubricant with triethanolamine (TEOA) as the H-bond donor is reported for the first time; it obtains an ultralow coefficient of friction (0.005) and high load-bearing capacity (360 MPa, more than 2 times that of similar systems) due to adsorption of a small amount of water (<5 wt %) from the air. In situ Raman combined with 1H NMR and FTIR techniques reveals that adsorbed water competes with the hydroxyl group of TEOA for coordination with Cl-, leading to the conversion of some strong H-bonds to weak H-bonds in ILAs; the localized strong H-bonds and weak H-bonds endow the ILAs with high load-bearing capacity and the formation of ultralow shear-resistance sliding interfaces, respectively, under the shear motion. This study proposes a strategy to modulate the interactions between liquid species using adsorbed water from air as a competing ligand, which provides new insights into the design of ILA-based macroscopic liquid superlubricants with a high load-bearing capacity.

Highly Concentrated Electrolyte Superlubricants Enhanced by Interfacial Water Competition Around Chemically Active MgO Additives.

The low concentration of water-based lubricants and the high chemical inertness of the additives involved are often regarded as basic norms in the design of liquid lubricants. Herein, a novel liquid superlubricant of an aqueous solution containing a relatively high concentration of salt, lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), is reported for the first time, and the superlubricity stability and load-bearing capacity of the optimized system (MgO0.10/LiTFSI10) are effectively strengthened by the addition of only trace (0.10 wt %) water-chemically active MgO additives. It demonstrates higher applicable loads, lower COF (∼0.004), and stability relative to the base solution. Only a trace amount of MgO additive is needed for the superlubricity, which makes up for the cost and environmental deficiencies of LiTFSI10. The weak interaction region between free water and the outer-layer water of Li+ hydration shells becomes a possible ultralow shear resistance sliding interface; the Mg(OH)2 layer, generated by the reaction of MgO with water, further creates additional weakly interacting interfaces, leading to the formation of an asymmetric contact between the clusters/particles within the hydrodynamic film by moderating the competition between interfacial water and free water, thus achieving high load-bearing macroscopic superlubricity. This study deepens the contribution of electrolyte concentration to ionic hydration and superlubricity due to the low shear slip layer formed by interfacial water competition with water-activated solid additives, providing new insights into the next generation of high load-bearing water-based liquid superlubricity systems.

In Situ Raman Study of Voltage Tolerance Up to 2.2 V of Ionic Liquid Analogue Supercapacitor Electrolytes Immune to Water Adsorption Conferred by Amphoteric Imidazole Additives.

Ionic liquid analogues (ILAs) are promising electrolytes for supercapacitors due to their low cost and considerable voltage (>2.0 V). However, the voltage is <1.1 V for water-adsorbed ILAs. Herein for the first time, an amphoteric imidazole (IMZ) additive is reported to address this concern by reconfiguring the solvent shell of ILAs. Addition of only 2 wt % IMZ increases the voltage from 1.1 to 2.2 V, with an increase in capacitance from 178 to 211 F g-1 and an increase in energy density from 6.8 to 32.6 Wh kg-1. In situ Raman reveals that the strong H-bonds formed by IMZ with completive ligands 1,3-propanediol and water induce a reversal of the polarity of the solvent shells, suppressing absorbed water electrochemical activity and thus increasing the voltage. This study solves the problem of low voltage for water-adsorbed ILAs and reduces the equipment cost of ILA-based supercapacitor assembly (e.g., assembly in air without a glovebox).

Unravelling High-Load Superlubricity of Ionic Liquid Analogues by In Situ Raman: Incomplete Hydration Induced by Competitive Exchange of External Water with Crystalline Water.

A high load-carrying capacity is the key to the practicality of liquid superlubricity, but it is difficult to achieve high load and low friction simultaneously by relying solely on a liquid film. Herein, a choline chloride-based ionic liquid analogue (ILA) macroscale superlubricant is first reported by tuning down strong hydrogen bonding in the ILA via introducing 2-10 wt % water, with a high load of 160 MPa and a low coefficient of friction of 0.006-0.008. In situ Raman reveals that competitive exchange between external water and crystalline water induces weak H-bond-dominated incomplete hydration, conferring a low-shear interface and considerable load-carrying capacity inside the lubricant. It is a hydrodynamic lubrication film rather than a tribochemical/physical adsorption film, allowing it to be applied to friction pairs of various materials. This study unveils the principle of water mediation of high-viscosity ILAs and also provides new insights into the design of practical ILA-based superlubrication materials with high load-carrying capacity.

Synthesis and Study of Photothermal Properties of a Mixed-Valence Nanocluster CuI/CuII with Strong near-Infrared Optical Absorption Supported by 4-tert-Butylcalix[4]arene Ligand.

The first mixed-valence nanocluster CuI/CuII with the highest percentage of CuII ions was synthesized by using 4-tert-butylcalix[4]arene (Calix4), with the formula DMF2⊂[(CO3)2-@CuII6CuI3(Calix4)3Cl2(DMF)5(H3O)]•DMF (1), as a photothermal nanocluster. Its structure was characterized using single-crystal X-ray diffraction, Fourier-transform infrared spectroscopy, and powder X-ray diffraction. In addition, the charge state and chemical composition of the nanocluster were determined using electrospray ionization spectrometry and X-ray photoelectron spectroscopy (XPS) spectrum. The results of the XPS and X-ray crystallography revealed that there are two independent CuII and CuI centers in nanocluster 1 with the relative abundances of 66.6 and 33.3% for CuII and CuI, respectively. The nanocluster contains three four-coordinated CuI ions with a square-planar geometry and six five-coordinated CuII ions with a square pyramid geometry. The nanocluster shows strong near-infrared optical absorption in the solid state and excellent photothermal conversion ability (the equilibrium temperature ∼78.2 °C) with the light absorption centers in 286-917 nm over previous reported pentanucleus CuI4CuII clusters and CuII compounds.

Ferroelectric benzimidazole additive-induced interfacial water confinement for stable 2.2 V supercapacitor electrolytes exposed to air.

Deep eutectic solvents (DESs) are known as low-cost and environmentally friendly electrolytes for supercapacitors. However, because DESs are particularly vulnerable to moisture adsorption in the air, the voltage window (<1.2 V) is significantly lower than expected. Herein, we report a new ferroelectric benzimidazole (BI) additive that, by restricting water electrochemical activity at the DES/carbon electrode interface, allows air-exposed DESs to reach a high voltage of 2.2 V. The optimized DES with 0.5 wt% BI addition not only increases the voltage but also the capacitance and energy density while maintaining excellent cycling stability. This study addresses the voltage drop of DESs in air, providing insights into the design of additives that inhibit interfacial water splitting.

Hydrogen Bond Interaction in the Trade-Off Between Electrolyte Voltage Window and Supercapacitor Low-Temperature Performances.

Liquid electrolyte determines the voltage window and extreme working temperature of supercapacitors. However, the effect of weak interaction between electrolyte species on voltage window and low-temperature capacitive performance is unclear. Herein, an electrolyte model system with increasing H-bond interaction was constructed to clarify this concern. The results indicated that strong H-bond interaction was positively correlated with the number of hydroxyls, which was beneficial to expand voltage window, but deteriorated rate performance; weak H-bond improved low-temperature performance. Supercapacitors with an optimized electrolyte presented high voltage and good low-temperature performance; even at -40 °C, the maximum energy density could be maintained at 7.0 Wh kg-1 (>80 % retention relative to at -20 °C). This study revealed the mechanism of the influence of the H-bonds on electrolyte voltage window and anti-freezing capability and provided a new insight for the design of electrolytes with both high working voltage and low-temperature performance.

Heterogeneous cobalt polysulfide leaf-like array/carbon nanofiber composites derived from zeolite imidazole framework for advanced asymmetric supercapacitors.

Developing new electrode materials is one of the keys to improving the energy density of supercapacitors. In this article, a novel cobalt polysulfide/carbon nanofibers (C,N-CoxSy/CNF) film derived from zeolitic imidazolate framework is first prepared by a facile strategy. The composite material with two-dimensional leaf-shaped nanoarray neatly grown on the surface of carbon nanofibers is composed of CoS, CoS2, Co9S8, N-doped carbon nanosheets, and carbon nanofibers. It is found that the composite can not only increase the contact area with the electrolyte but also provide abundant redox-active sites and a Faraday capacitance for the entire electrode. The C,N-CoxSy/CNF composite exhibits excellent electrochemical properties, including a high capacity of up to 1080F g -1 at 1 A g -1 and a good rate capability (80.4 % from 1 A g -1 to 10 A g -1). A C,N-CoxSy/CNF//AC asymmetric supercapacitor device is assembled using C,N-CoxSy/CNF as the positive electrode and activated carbon as the negative electrode, showing high energy density (37.29 Wh kg -1@813.6 W kg -1) and good cycle stability (90.5% of initial specific capacitance at 10 g-1 after 5000 cycles). This C,N-CoxSy/CNF composite material may also be used as a potential electrode for future lithium-ion batteries, zinc-ion batteries, lithium-sulfur batteries, etc.

Electrocatalysis of S-doped carbon with weak polysulfide adsorption enhances lithium-sulfur battery performance.

Heteroatom-doped nanocarbons are beneficial for the performance improvement of lithium-sulfur batteries, and the reason is usually attributed to their strong adsorption to the soluble polysulfides. Herein, we found that, despite the weak polysulfide adsorption on hierarchical S-doped carbon nanocages (hSCNCs), the hSCNC-encapsulated sulfur cathode still exhibited better performance than the counterpart using undoped carbon nanocages, showing a high capacity of 579 mA h g-1 at 2 A g-1 after 400 cycles, and a high areal capacity of 4.7 mA h cm-2 with a high sulfur loading of 4.5 mg cm-2. The electrocatalysis-promoted mechanism of S-doped carbon was demonstrated, which facilitated polysulfide conversion and suppressed the polarization effect, thereby leading to superior performance.

The simplest construction of single-site catalysts by the synergism of micropore trapping and nitrogen anchoring.

Single-site catalysts feature high catalytic activity but their facile construction and durable utilization are highly challenging. Herein, we report a simple impregnation-adsorption method to construct platinum single-site catalysts by synergic micropore trapping and nitrogen anchoring on hierarchical nitrogen-doped carbon nanocages. The optimal catalyst exhibits a record-high electrocatalytic hydrogen evolution performance with low overpotential, high mass activity and long stability, much superior to the platinum-based catalysts to date. Theoretical simulations and experiments reveal that the micropores with edge-nitrogen-dopants favor the formation of isolated platinum atoms by the micropore trapping and nitrogen anchoring of [PtCl6]2-, followed by the spontaneous dechlorination. The platinum-nitrogen bonds are more stable than the platinum-carbon ones in the presence of adsorbed hydrogen atoms, leading to the superior hydrogen evolution stability of platinum single-atoms on nitrogen-doped carbon. This method has been successfully applied to construct the single-site catalysts of other precious metals such as palladium, gold and iridium.

Strong Graphene 3D Assemblies with High Elastic Recovery and Hardness.

The rational design and construction of 3D graphene assemblies is a crucial step to extend the graphene properties for practical applications. Here, a novel interfacially reactive self-assembling process is reported to prepare well-organized 3D honeycomb-like graphene assemblies with unique polygonal nanopores interconnected by silicon-oxygen chemical bonds. The newly developed silicate-bridged graphene assembly (SGA) exhibits an exceptionally high hardness of 13.09 GPa, outperforming all existing 3D graphene materials, while maintains high Young's modulus (162.96 GPa), elastic recovery (75.27%), and superb thermal stability (600 °C in air). The observed unusual merits are resulted from unique pore structure combining the mechanical stability of the trihedral-nanopore structure and the deformability of the other polygonal nanopores. As a filling material, a merely 0.05% (w/w) addition of SGA could double the impact resistance of unsaturated resins (e.g., polyester). While SGA is attractive for various applications, including body armors, wearable electronics, space elevators, and multifunctional reinforcement fibers for automobiles, and aerospace vehicles, the novel liquid sodium-water interfacial reactive self-assembling developed in this study could open avenues for further development of various well-defined 3D assemblies from graphene and many other materials.

Compressing Carbon Nanocages by Capillarity for Optimizing Porous Structures toward Ultrahigh-Volumetric-Performance Supercapacitors.

High volumetric energy density combined with high power density is highly desired for electrical double-layer capacitors. Usually the volumetric performance is improved by compressing carbon material to increase density but at the much expense of power density due to the deviation of the compressed porous structure from the ideal one. Herein the authors report an efficient approach to increase the density and optimize the porous structure by collapsing the carbon nanocages via capillarity. Three samples with decreasing sizes of meso- and macropores provide us an ideal model system to demonstrate the correlation of volumetric performance with porous structure. The results indicate that reducing the surplus macropores and, more importantly, the surplus mesopores is an efficient strategy to enhance the volumetric energy density while keeping the high power density. The optimized sample achieves a record-high stack volumetric energy density of 73 Wh L-1 in ionic liquid with superb power density and cycling stability.

Alcohol-Tolerant Platinum Electrocatalyst for Oxygen Reduction by Encapsulating Platinum Nanoparticles inside Nitrogen-Doped Carbon Nanocages.

Pt-based electrocatalysts are the most popular for direct alcohol fuel cells, but their performances easily deteriorate for the oxygen reduction reaction (ORR) at the cathode because of the alcohol crossover effect. Herein, we report the novel Pt electrocatalyst encapsulated inside nitrogen-doped carbon nanocages (Pt@NCNC), which presents excellent alcohol-tolerant ORR activity and durability in acidic media, far superior to the Pt counterpart immobilized outside the nanocages (Pt/NCNC). The superb performance is correlated with the molecule-sieving effect of the micropores penetrating through the shells of the nanocages, which admit the small-sized oxygen and ions but block the large-sized alcohols into the nanocages. This mechanism is confirmed by examining the size dependence of ORR and alcohol oxidation activities by regulating the micropores sizes. This study provides a promising strategy to develop the superior alcohol-tolerant Pt-based ORR electrocatalyst in acidic media.

Manganese oxide-induced strategy to high-performance iron/nitrogen/carbon electrocatalysts with highly exposed active sites.

Iron/nitrogen/carbon (Fe/N/C) catalyst is so far the most promising non-precious metal electrocatalyst for oxygen reduction reaction (ORR) in acidic medium, whose performance depends closely on the synthesis chemistry. Herein, we report a MnOx-induced strategy to construct the Fe/N/C with highly exposed Fe-Nx active sites, which involves the uniform spreading of polyaniline on hierarchical N-doped carbon nanocages by a reactive-template polymerization, followed by the successive iron incorporation and polyaniline pyrolysis. The resulting Fe/N/C demonstrates an excellent ORR performance, including an onset potential of 0.92 V (vs. RHE), four electron selectivity, superb stability and immunity to methanol crossover. The excellent performance is well correlated with the greatly enhanced surface active sites of the catalyst stemming from the unique MnOx-induced strategy. This study provides an efficient approach for exploring the advanced ORR electrocatalysts by increasing the exposed active sites.

Graphene oxide as efficient high-concentration formaldehyde scavenger and reutilization in supercapacitor.

Graphene oxide (GO) was investigated as a low-cost and high-efficient scavenger for high-concentration formaldehyde in alkali media. It showed very high removal capacity, 411 mg of formaldehyde per milligram of GO, and strong resistant to temperature changes. Additionally, the used GO can be easily renewed by a simple electrochemical method. By analyzing the componential and electrochemical characterizations of GO before and after use, the results showed that the degradation mechanism of formaldehyde is a collaborative process of chemical oxidation and physical adsorption, and the former dominates the degradation process. With the aid of oxygen-containing groups in GO, most formaldehyde can be easily oxidized by GO in alkaline media (this is equivalent to GO was reduced by formaldehyde). On the other hand, the used GO (reduced GO, noted as rGO) exhibits more ideal electronic double-layer capacitor (EDLC) feature than GO, along with higher rate capacitance (up to 136 F g(-1) at 50 A g(-1)). In short, GO is not only an efficient formaldehyde scavenger, but the used GO (rGO) can serve as promising electrical energy storage material. This study provides new insights for us to reutilize the discarded adsorbents generated from the environmental protection.

Graphene oxide film as solid lubricant.

As a layered material, graphene oxide (GO) film is a good candidate for improving friction and antiwear performance of silicon-based MEMS devices. Via a green electrophoretic deposition (EPD) approach, GO films with tunable thickness in nanoscale are fabricated onto silicon wafer in a water solution. The morphology, microstructure, and mechanical properties as well as the friction coefficient and wear resistance of the films were investigated. The results indicated that the friction coefficient of silicon wafer was reduced to 1/6 its value, and the wear volume was reduced to 1/24 when using GO film as solid lubricant. These distinguished tribology performances suggest that GO films are expected to be good solid lubricants for silicon-based MEMS/NEMS devices.