Shengli Zhai

Transforming Pristine Carbon Fiber Tows into High Performance Solid‐State Fiber Supercapacitors

A facile activation strategy can transform pristine carbon fiber tows into high-performance fiber electrodes with a specific capacitance of 14.2 F cm(-3) . The knottable fiber supercapacitor shows an energy density of 0.35 mW h cm(-3) , an ultrahigh power density of 3000 mW cm(-3) , and a remarkable capacitance retention of 68%, when the scan rate increases from 10 to 1000 mV s(-1) .

All-carbon solid-state yarn supercapacitors from activated carbon and carbon fibers for smart textiles

Smart textiles are intelligent devices that can sense and respond to environmental stimuli. They require integrated energy storage to power their functions. An emerging approach is to build integratable fiber-/yarn-based energy storage devices. Here, we demonstrate all-carbon solid-state yarn supercapacitors using commercially available activated carbon and carbon fiber yarns for smart textiles. Conductive carbon fibers concurrently act as current collectors in yarn supercapacitors and as substrates for depositing large surface area activated carbon particles. Two hybrid carbon yarn electrodes were twisted together in polyvinyl alcohol/H3PO4 polymer gel, which is used as both an electrolyte and a separator. A 10 cm long yarn supercapacitor, with the optimum composition of 2.2 mg cm−1 activated carbon and 1 mg cm−1 carbon fiber, shows a specific length capacitance of 45.2 mF cm−1 at 2 mV s−1, an energy density of 6.5 μW h cm−1, and a power density of 27.5 μW cm−1. Since the yarn supercapacitor has low equivalent series resistance at 4.9 Ω cm−1, longer yarn supercapacitors up to 50 cm in length were demonstrated, yielding a high total capacitance of up to 1164 mF. The all-carbon solid-state yarn supercapacitors also exhibit excellent mechanical flexibility with minor capacitance decreases upon bending or being crumpled. Utilizing three long yarn supercapacitors, a wearable wristband was knitted; this wristband is capable of lighting up an LED indicator, demonstrating strong potential for smart textile applications.

Space-confined assembly of all-carbon hybrid fibers for capacitive energy storage: realizing a built-to-order concept for micro-supercapacitors

Miniaturized portable and wearable electronics have diverse power requirements, ranging from one microwatt to several milliwatts. Fiber-based micro-supercapacitors are promising energy storage devices that can address these manifold power requirements. Here, we demonstrate a hydrothermal assembly method using space confinement fillers to control the formation of nitrogen doped reduced graphene oxide and multi-walled carbon nanotube hybrid fibers. Consequently, the all-carbon hybrid fibers have tunable geometries, while maintaining good electrical conductivity, high ion-accessible surface area and mechanical strength; this allows us to address two important issues in micro-supercapacitor research. First, we found a clear correlation between the geometry of the hybrid fibers and their capacitive energy storage properties. Thinner fibers (30 μm in diameter) have higher specific volumetric capacitance (281 F cm−3), superior rate capability, and better length dependent performance. In contrast, larger-diameter hybrid fibers (236 μm in diameter) can achieve much higher specific length capacitance (42 mF cm−1). Second, we realized the first built-to-order concept for micro-supercapacitors by using all-carbon hybrid fibers with diversified geometry as electrodes. The device energy can cover two orders of magnitude, from <0.1 μW h to nearly 10 μW h, and the device power can be tuned in four orders of magnitude, from 0.2 μW to 2000 μW. Furthermore, multiple mechanically flexible fiber-based micro-supercapacitors can be integrated into complex energy storage units with wider operation voltage windows, demonstrating broad application potentials in flexible devices.

Amorphous Bimetallic Oxide–Graphene Hybrids as Bifunctional Oxygen Electrocatalysts for Rechargeable Zn–Air Batteries

Metal oxides of earth-abundant elements are promising electrocatalysts to overcome the sluggish oxygen evolution and oxygen reduction reaction (OER/ORR) in many electrochemical energy-conversion devices. However, it is difficult to control their catalytic activity precisely. Here, a general three-stage synthesis strategy is described to produce a family of hybrid materials comprising amorphous bimetallic oxide nanoparticles anchored on N-doped reduced graphene oxide with simultaneous control of nanoparticle elemental composition, size, and crystallinity. Amorphous Fe0.5 Co0.5 Ox is obtained from Prussian blue analog nanocrystals, showing excellent OER activity with a Tafel slope of 30.1 mV dec-1 and an overpotential of 257 mV for 10 mA cm-2 and superior ORR activity with a large limiting current density of -5.25 mA cm-2 at 0.6 V. A fabricated Zn-air battery delivers a specific capacity of 756 mA h gZn-1 (corresponding to an energy density of 904 W h kgZn-1 ), a peak power density of 86 mW cm-2 and can be cycled over 120 h at 10 mA cm-2 . Other two amorphous bimetallic, Ni0.4 Fe0.6 Ox and Ni0.33 Co0.67 Ox , are also produced to demonstrate the general applicability of this method for synthesizing binary metal oxides with controllable structures as electrocatalysts for energy conversion.

Sandwich-Architectured Poly(lactic acid)–Graphene Composite Food Packaging Films

Biodegradable food packaging promises a more sustainable future. Among the many different biopolymers used, poly(lactic acid) (PLA) possesses the good mechanical property and cost-effectiveness necessary of a biodegradable food packaging. However, PLA food packaging suffers from poor water vapor and oxygen barrier properties compared to many petroleum-derived ones. A key challenge is, therefore, to simultaneously enhance both the water vapor and oxygen barrier properties of the PLA food packaging. To address this issue, we design a sandwich-architectured PLA-graphene composite film, which utilizes an impermeable reduced graphene oxide (rGO) as the core barrier and commercial PLA films as the outer protective encapsulation. The synergy between the barrier and the protective encapsulation results in a significant 87.6% reduction in the water vapor permeability. At the same time, the oxygen permeability is reduced by two orders of magnitude when evaluated under both dry and humid conditions. The excellent barrier properties can be attributed to the compact lamellar microstructure and the hydrophobicity of the rGO core barrier. Mechanistic analysis shows that the large rGO lateral dimension and the small interlayer spacing between the rGO sheets have created an extensive and tortuous diffusion pathway, which is up to 1450-times the thickness of the rGO barrier. In addition, the sandwiched architecture has imbued the PLA-rGO composite film with good processability, which increases the manageability of the film and its competency to be tailored. Simulations using the PLA-rGO composite food packaging film for edible oil and potato chips also exhibit at least eight-fold extension in the shelf life of these oxygen and moisture sensitive food products. Overall, these qualities have demonstrated the high potential of a sandwich-architectured PLA-graphene composite film for food packaging applications.

Nickel hydroxide–carbon nanotube nanocomposites as supercapacitor electrodes: crystallinity dependent performances

Nickel hydroxide (Ni(OH)2) is a promising pseudocapacitive material to increase the energy storage capacity of supercapacitors. Ni(OH)2 has three common crystalline structures: amorphous (amor-), α-, and β-Ni(OH)2. There is a lack of good understanding on their pros and cons as supercapacitor electrodes. In this work, we synthesized three nanocomposites with thin layers (10-15 nm) of amor-, α-, and β-Ni(OH)2 deposited on conductive multi-walled carbon nanotubes (MWCNTs). The mass loading of Ni(OH)2 is analogous in these nanocomposites, ranging from 49.1-52.2 wt% with a comparable narrow-pore size distribution centered around 4-5 nm. They were fabricated into supercapacitor electrodes at a mass loading of 6 mg cm(-2) with a thickness of ∼250 μm, similar to the electrodes used in commercial supercapacitors. Our results show that MWCNT/amor-Ni(OH)2 has the highest specific capacitance (1495 or 2984 F g(-1), based on the mass of total active materials or Ni(OH)2 only at the scan rate of 5 mV s(-1) in 1 M KOH electrolyte). It also has the best rate capability among the three nanocomposites. Better performances can be attributed to its disordered structure, which increases its effective surface area and reduces diffusion resistance for redox reactions. However, superior performances gradually deteriorate to the same level as that of MWCNT/β-Ni(OH)2 over 3000 charge/discharge cycles, because amor- and α-Ni(OH)2 transform slowly to more ordered β-Ni(OH)2. Our results highlight that the electrochemical performances of MWCNT/Ni(OH)2 nanocomposites depend on the crystallinity of Ni(OH)2, and the performances of electrodes change upon the crystalline structure transformation of Ni(OH)2 under repeated redox reactions. Future research should focus on improving the structure stability of amor-Ni(OH)2.

Hydrogen evolution reaction activity of nickel phosphide is highly sensitive to electrolyte pH

The nickel phosphide (Ni2P) family of materials have become a hot subject in hydrogen evolution reaction (HER) electrocatalyst research. Various studies have reported their high activity, high stability, and high faradaic efficiency. To date, there have been no systematic studies regarding the influence of pH on the HER performance of Ni2P. Here we show that the pH of electrolytes can strongly influence the HER activity of Ni2P electrocatalysts. Tests in 19 electrolytes with pH ranging from 0.52 to 13.53 show that Ni2P is much more active in strongly acidic and basic electrolytes. With the increase of pH, the lower H+ concentration reduces the formation of adsorbed H atoms in the Volmer reaction, resulting in poorer activities. However, the high activity observed in the strongly basic electrolytes is not the intrinsic property of Ni2P. We found that Ni oxides/hydroxides are formed in strongly basic electrolytes under applied potentials, resulting in improved activities. Furthermore, the specific activity based on the electrochemically active surface area of recently reported Ni2P catalysts is not high and requires significant improvements for practical applications.

Metal-free bifunctional carbon electrocatalysts derived from zeolitic imidazolate frameworks for efficient water splitting

Metal-free carbon catalysts have attracted great interest because of their high electrical conductivity, tailorable porosity and surface area, affordability, and sustainability. In particular, their bifunctional activity for hydrogen and oxygen evolution reactions (HER and OER) is attractive for electrochemical splitting of water. However, pristine carbon materials have low activities for HER/OER. Here, a high-performance carbon electrocatalyst is demonstrated by first pyrolyzing a metal–organic framework (MOF), i.e., zeolitic imidazolate framework-8 (ZIF-8), followed by optimized cathodic polarization treatment (CPT). Pyrolyzing ZIF-8 produces a highly N-doped (8.4 at%) carbon material having a large specific surface area of 1017 m2 g−1 with micro and mesopores. CPT in 0.5 M H2SO4 for up to 8 hours modulates the composition of N- and O-containing surface functional groups of the pyrolyzed ZIF-8 without sacrificing its large surface area and pore size distribution. After the 6-hour CPT, this material shows an excellent HER activity in 0.5 M H2SO4 electrolyte with an overpotential of 155 mV, a Tafel slope of 54.7 mV dec−1, and an exchange current density of 0.063 mA cm−2. And the 4-hour CPT results in excellent OER activity in 0.1 M KOH electrolyte with an overpotential of 476 mV and a Tafel slope of 78.5 mV dec−1. In a demonstration, these two carbon electrocatalysts steadily run a two-electrode water electrolyzer at a current density of 10 mA cm−2 over 8 hours under a potential of 1.82 V with a Faradaic efficiency of 98.0–99.1% in 0.1 M KOH electrolyte. The superior activity of the designed carbon electrocatalysts can be attributed to the functional group composition modulation achieved by CPT. High-performance metal-free carbon electrocatalysts derived from MOFs show excellent potentials for energy and environmental applications.

Nano-RuO2-Decorated Holey Graphene Composite Fibers for Micro-Supercapacitors with Ultrahigh Energy Density

Compactness and versatility of fiber-based micro-supercapacitors (FMSCs) make them promising for emerging wearable electronic devices as energy storage solutions. But, increasing the energy storage capacity of microscale fiber electrodes, while retaining their high power density, remains a significant challenge. Here, this issue is addressed by incorporating ultrahigh mass loading of ruthenium oxide (RuO2 ) nanoparticles (up to 42.5 wt%) uniformly on nanocarbon-based microfibers composed largely of holey reduced graphene oxide (HrGO) with a lower amount of single-walled carbon nanotubes as nanospacers. This facile approach involes (1) space-confined hydrothermal assembly of highly porous but 3D interconnected carbon structure, (2) impregnating wet carbon structures with aqueous Ru3+ ions, and (3) anchoring RuO2 nanoparticles on HrGO surfaces. Solid-state FMSCs assembled using those fibers demonstrate a specific volumetric capacitance of 199 F cm-3 at 2 mV s-1 . Fabricated FMSCs also deliver an ultrahigh energy density of 27.3 mWh cm-3 , the highest among those reported for FMSCs to date. Furthermore, integrating 20 pieces of FMSCs with two commercial flexible solar cells as a self-powering energy system, a light-emitting diode panel can be lit up stably. The current work highlights the excellent potential of nano-RuO2 -decorated HrGO composite fibers for constructing micro-supercapacitors with high energy density for wearable electronic devices.