Rufan Zhang

Superlubricity in centimetres-long double-walled carbon nanotubes under ambient conditions

Friction and wear are two main causes of mechanical energy dissipation and component failure, especially in micro/nanomechanical systems with large surface-to-volume ratios. In the past decade there has been an increasing level of research interest regarding superlubricity, a phenomenon, also called structural superlubricity, in which friction almost vanishes between two incommensurate solid surfaces. However, all experimental structural superlubricity has been obtained on the microscale or nanoscale, and predominantly under high vacuum. Here, we show that superlubricity can be realized in centimetres-long double-walled carbon nanotubes (DWCNTs) under ambient conditions. Centimetres-long inner shells can be pulled out continuously from such nanotubes, with an intershell friction lower than 1 nN that is independent of nanotube length. The shear strength of the DWCNTs is only several pascals, four orders of magnitude lower than the lowest reported value in CNTs and graphite. The perfect structure of the ultralong DWCNTs used in our experiments is essential for macroscale superlubricity.

Growth of Half-Meter Long Carbon Nanotubes Based on Schulz–Flory Distribution

The Schulz-Flory distribution is a mathematical function that describes the relative ratios of polymers of different length after a polymerization process, based on their relative probabilities of occurrence. Carbon nanotubes (CNTs) are big carbon molecules which have a very high length-to-diameter ratio, somewhat similar to polymer molecules. Large amounts of ultralong CNTs have not been obtained although they are highly desired. Here, we report that the Schulz-Flory distribution can be applied to describe the relative ratios of CNTs of different lengths produced with a floating chemical vapor deposition process, based on catalyst activity/deactivation probability. With the optimized processing parameters, we successfully synthesized 550-mm-long CNTs, for which the catalyst deactivation probability of a single growth step was ultralow. Our finding bridges the Schulz-Flory distribution and the synthesis of one-dimensional nanomaterials for the first time, and sheds new light on the rational design of process toward controlled production of nanotubes/nanowires.

Catalytic oxidation of Li2S on the surface of metal sulfides for Li-S batteries

Significance A series of metal sulfides were systematically investigated as polar hosts to reveal the key parameters correlated to the energy barriers and polysulfide adsorption capability in Li−S batteries. The investigation demonstrates that the catalyzing oxidation capability of metal sulfides is critical in reducing the energy barrier and contributing to the remarkably improved battery performance. Density functional theory simulation allows us to identify the mechanism for how binding energy and polysulfides trapping dominate the Li2S decomposition process and overall battery performance. The understanding can serve as a general guiding principle for the rational design and screening of advanced materials for high-energy Li−S batteries. Polysulfide binding and trapping to prevent dissolution into the electrolyte by a variety of materials has been well studied in Li−S batteries. Here we discover that some of those materials can play an important role as an activation catalyst to facilitate oxidation of the discharge product, Li2S, back to the charge product, sulfur. Combining theoretical calculations and experimental design, we select a series of metal sulfides as a model system to identify the key parameters in determining the energy barrier for Li2S oxidation and polysulfide adsorption. We demonstrate that the Li2S decomposition energy barrier is associated with the binding between isolated Li ions and the sulfur in sulfides; this is the main reason that sulfide materials can induce lower overpotential compared with commonly used carbon materials. Fundamental understanding of this reaction process is a crucial step toward rational design and screening of materials to achieve high reversible capacity and long cycle life in Li−S batteries.

Superstrong Ultralong Carbon Nanotubes for Mechanical Energy Storage

Energy storage in a proper form is essential for a good grid strategy. The systems developed so far mostly use batteries or capacitors in which energy is stored electrochemically or electrostatically. Mechanical energy storage is also one of the most important ways for energy conversion. In fact, water reservoirs on high mountains store mechanical energy using the gravitational potential on the earth, and the surplus energy can be mechanically stored in water pumped to a higher elevation using pumped storage methods. Other systems for mechanical energy storage were realized, such as flstoring mechanical energies by the use of a rapidly rotating mass and steel springs storing mechanical energies by their elasticity. However, such mechanical energy storage usually is operated on a macroscopic scale, and the energy density is not very high. With the fast development of nano- and micro-electromechanical systems (N/MEMS) and actuators, nanoscale mechanical energy storage is highly required. Developing a robust nonmaterial with good mechanical performance and stable supply is the fi rst step. Ultralong carbon nanotubes (CNTs) with the properties of 1‐2 TPa modulus and 100‐200 GPa strength, [ 1‐4 ] the strongest material ever known, have shown promising potential for the storage of mechanical energy, either by their deformation in the composite materials, [ 5‐7 ] or by their elastic deformation produced by stretching or compressing the pristine tubes or tube arrays. [ 8 ] Theoretical calculation suggested that the energy storage capacity, in the latter case, can be at least three orders higher than that of steel spring and several times that of the fl ywheels and lithium ion batteries. [ 9 , 10 ] The mechanical energy storage capacity of CNTs depends on their mechanical properties, while which directly depend on their molecular structures. Besides, CNTs that simultaneously have theoretically high strength (100‐200 GPa), high tensile modulus (1‐2 TPa) and high breaking strain ( > 15%) are not yet experimentally available on the macroscale. [ 2 , 11‐20 ] This is mainly due to the existence of defects in the fabricated CNTs. Even for CNTs with little defects, the highest reported breaking strain is 13.7% ± 0.3%, [ 21 ] which is still lower than the theoretical value. [ 22 , 23 ] Here we experimentally demonstrate that the as-grown defect-free CNTs with length over 10 cm, have breaking strain up to 17.5%, tensile strength up to 200 GPa and Young’s modulus up to 1.34 TPa. They could endure a continuously repeated mechanical strain-release test for over 1.8 × 10 8 times. The extraordinary mechanical performance qualifi es them with high capacity for the storage of mechanical energy. The CNTs can store mechanical energy with a density as high as 1125 Wh kg − 1 and a power density as high as 144 MW kg − 1 , indicating the CNTs can be a promising medium for the storage of mechanical energy.

Self-healing SEI enables full-cell cycling of a silicon-majority anode with a coulombic efficiency exceeding 99.9%

Despite active developments, full-cell cycling of Li-battery anodes with >50 wt% Si (a Si-majority anode, SiMA) is rare. The main challenge lies in the solid electrolyte interphase (SEI), which when formed naturally (nSEI), is fragile and cannot tolerate the large volume changes of Si during lithiation/delithiation. An artificial SEI (aSEI) with a specific set of mechanical characteristics is henceforth designed; we enclose Si within a TiO2 shell thinner than 15 nm, which may or may not be completely hermetic at the beginning. In situ TEM experiments show that the TiO2 shell exhibits 5× greater strength than an amorphous carbon shell. Void-padded compartmentalization of Si can survive the huge volume changes and electrolyte ingression, with a self-healing aSEI + nSEI. The half-cell capacity exceeds 990 mA h g−1 after 1500 cycles. To improve the volumetric capacity, we further compress SiMA 3-fold from its tap density (0.4 g cm−3) to 1.4 g cm−3, and then run the full-cell battery tests against a 3 mA h cm−2 LiCoO2 cathode. Despite some TiO2 enclosures being inevitably broken, 2× the volumetric capacity (1100 mA h cm−3) and 2× the gravimetric capacity (762 mA h g−1) of commercial graphite anode is achieved in stable full-cell battery cycling, with a stabilized areal capacity of 1.6 mA h cm−2 at the 100th cycle. The initial lithium loss, characterized by the coulombic inefficiency (CI), is carefully tallied on a logarithmic scale and compared with the actual full-cell capacity loss. It is shown that a strong, non-adherent aSEI, even if partially cracked, facilitates an adaptive self-repair mechanism that enables full-cell cycling of a SiMA, leading to a stabilized coulombic efficiency exceeding 99.9%.

Efficient solar-driven water splitting by nanocone BiVO4-perovskite tandem cells

Efficient solar water splitting is achieved by a nanocone BiVO4 photoelectrochemical cell in tandem with a perovskite solar cell. Bismuth vanadate (BiVO4) has been widely regarded as a promising photoanode material for photoelectrochemical (PEC) water splitting because of its low cost, its high stability against photocorrosion, and its relatively narrow band gap of 2.4 eV. However, the achieved performance of the BiVO4 photoanode remains unsatisfactory to date because its short carrier diffusion length restricts the total thickness of the BiVO4 film required for sufficient light absorption. We addressed the issue by deposition of nanoporous Mo-doped BiVO4 (Mo:BiVO4) on an engineered cone-shaped nanostructure, in which the Mo:BiVO4 layer with a larger effective thickness maintains highly efficient charge separation and high light absorption capability, which can be further enhanced by multiple light scattering in the nanocone structure. As a result, the nanocone/Mo:BiVO4/Fe(Ni)OOH photoanode exhibits a high water-splitting photocurrent of 5.82 ± 0.36 mA cm−2 at 1.23 V versus the reversible hydrogen electrode under 1-sun illumination. We also demonstrate that the PEC cell in tandem with a single perovskite solar cell exhibits unassisted water splitting with a solar-to-hydrogen conversion efficiency of up to 6.2%.

Nanofiber Air Filters with High-Temperature Stability for Efficient PM2.5 Removal from the Pollution Sources

Here, we developed high-efficiency (>99.5%) polyimide-nanofiber air filters for the high temperature PM2.5 removal. The polyimide nanofibers exhibited high thermal stability, and the PM2.5 removal efficiency was kept unchanged when temperature ranged from 25-370 °C. These filters had high air flux with very low pressure drop. They could continuously work for >120 h for PM2.5 index >300. A field-test showed that they could effectively remove >99.5% PM particles from car exhaust at high temperature.

Roll-to-Roll Transfer of Electrospun Nanofiber Film for High-Efficiency Transparent Air Filter

Particulate matter (PM) pollution in air has become a serious environmental issue calling for new type of filter technologies. Recently, we have demonstrated a highly efficient air filter by direct electrospinning of polymer fibers onto supporting mesh although its throughput is limited. Here, we demonstrate a high throughput method based on fast transfer of electrospun nanofiber film from roughed metal foil to a receiving mesh substrate. Compared with the direct electrospinning method, the transfer method is 10 times faster and has better filtration performance at the same transmittance, owing to the uniformity of transferred nanofiber film (>99.97% removal of PM2.5 at ∼73% of transmittance). With these advantages, large area freestanding nanofiber film and roll-to-roll production of air filter are demonstrated.

In situ fabrication of depth-type hierarchical CNT/quartz fiber filters for high efficiency filtration of sub-micron aerosols and high water repellency

We fabricated depth-type hierarchical CNT/quartz fiber (QF) filters through in situ growth of CNTs upon quartz fiber (QF) filters using a floating catalyst chemical vapor deposition (CVD) method. The filter specific area of the CNT/QF filters is more than 12 times higher than that of the pristine QF filters. As a result, the penetration of sub-micron aerosols for CNT/QF filters is reduced by two orders of magnitude, which reaches the standard of high-efficiency particulate air (HEPA) filters. Simultaneously, due to the fluffy brush-like hierarchical structure of CNTs on QFs, the pore size of the hybrid filters only has a small increment. The pressure drop across the CNT/QF filters only increases about 50% with respect to that of the pristine QF filters, leading to an obvious increased quality factor of the CNT/QF filters. Scanning electron microscope images reveal that CNTs are very efficient in capturing sub-micron aerosols. Moreover, the CNT/QF filters show high water repellency, implying their superiority for applications in humid conditions.

Sulfiphilic Nickel Phosphosulfide Enabled Li2S Impregnation in 3D Graphene Cages for Li–S Batteries

A 3D graphene cage with a thin layer of electrodeposited nickel phosphosulfide for Li2S impregnation, using ternary nickel phosphosulphide as a highly conductive coating layer for stabilized polysulfide chemistry, is accomplished by the combination of theoretical and experimental studies. The 3D interconnected graphene cage structure leads to high capacity, good rate capability and excellent cycling stability in a Li2S cathode.