Zhimi Hu

Scalable salt-templated synthesis of two-dimensional transition metal oxides.

Two-dimensional atomic crystals, such as two-dimensional oxides, have attracted much attention in energy storage because nearly all of the atoms can be exposed to the electrolyte and involved in redox reactions. However, current strategies are largely limited to intrinsically layered compounds. Here we report a general strategy that uses the surfaces of water-soluble salt crystals as growth templates and is applicable to not only layered compounds but also various transition metal oxides, such as hexagonal-MoO3, MoO2, MnO and hexagonal-WO3. The planar growth is hypothesized to occur via a match between the crystal lattices of the salt and the growing oxide. Restacked two-dimensional hexagonal-MoO3 exhibits high pseudocapacitive performances (for example, 300 F cm−3 in an Al2(SO4)3 electrolyte). The synthesis of various two-dimensional transition metal oxides and the demonstration of high capacitance are expected to enable fundamental studies of dimensionality effects on their properties and facilitate their use in energy storage and other applications.

Salt-Templated Synthesis of 2D Metallic MoN and Other Nitrides

Two-dimensional (2D) transition-metal nitrides just recently entered the research arena, but already offer a potential for high-rate energy storage, which is needed for portable/wearable electronics and many other applications. However, a lack of efficient and high-yield synthesis methods for 2D metal nitrides has been a major bottleneck for the manufacturing of those potentially very important materials, and only MoN, Ti4N3, and GaN have been reported so far. Here we report a scalable method that uses reduction of 2D hexagonal oxides in ammonia to produce 2D nitrides, such as MoN. MoN nanosheets with subnanometer thickness have been studied in depth. Both theoretical calculation and experiments demonstrate the metallic nature of 2D MoN. The hydrophilic restacked 2D MoN film exhibits a very high volumetric capacitance of 928 F cm-3 in sulfuric acid electrolyte with an excellent rate performance. We expect that the synthesis of metallic 2D MoN and two other nitrides (W2N and V2N) demonstrated here will provide an efficient way to expand the family of 2D materials and add many members with attractive properties.

Rapid mass production of two-dimensional metal oxides and hydroxides via the molten salts method

Because of their exotic electronic properties and abundant active sites, two-dimensional (2D) materials have potential in various fields. Pursuing a general synthesis methodology of 2D materials and advancing it from the laboratory to industry is of great importance. This type of method should be low cost, rapid and highly efficient. Here, we report the high-yield synthesis of 2D metal oxides and hydroxides via a molten salts method. We obtained a high-yield of 2D ion-intercalated metal oxides and hydroxides, such as cation-intercalated manganese oxides (Na0.55Mn2O4·1.5H2O and K0.27MnO2·0.54H2O), cation-intercalated tungsten oxides (Li2WO4 and Na2W4O13), and anion-intercalated metal hydroxides (Zn5(OH)8(NO3)2·2H2O and Cu2(OH)3NO3), with a large lateral size and nanometre thickness in a short time. Using 2D Na2W4O13 as an electrode, a high performance electrochemical supercapacitor is achieved. We anticipate that our method will enable new path to the high-yield synthesis of 2D materials for applications in energy-related fields and beyond.

Activated carbon derived from melaleuca barks for outstanding high-rate supercapacitors.

Activated carbon (AC) was prepared via carbonizing melaleuca bark in an argon atmosphere at 600 °C followed with KOH activation for high-rate supercapacitors. This AC electrode has a high capacitance of 233 F g(-1) at a scan rate of 2 mV s(-1) and an excellent rate capability of ∼80% when increasing the sweep rate from 2 to 500 mV s(-1). The symmetric supercapacitor assembled by the above electrode can deliver a high energy density of 4.2 Wh kg(-1) with a power density of 1500 W kg(-1) when operated in the voltage range of 0-1 V in 1 M H2SO4 aqueous electrolyte while maintaining great cycling stability (less than 5% capacitance loss after 10 000 cycles at sweep rate of 100 mV s(-1)). All the outstanding electrochemical performances make this AC electrode a promising candidate for potential energy storage application.

Highly conductive and flexible molybdenum oxide nanopaper for high volumetric supercapacitor electrode

Paper-like electrodes with high conductivity and flexibility hold great potential for assembling high-performance flexible electronic devices. In this study, a flexible conductive film was fabricated via vacuum filtration using highly conductive MoO3−x ultralong nanobelts. This film has low sheet resistance of 5.1 Ω sq−1 and exhibits a stable three-dimensional structure even under 1000 times of bending test. Significantly, this free-standing film has a high volumetric capacitance of 652 F cm−3. Moreover, a symmetric device based on this electrode demonstrates good cycling stability with a capacitance retention of 85.7% after 25000 cycles. We anticipate that this strategy can also be applied to other three-dimensional flexible porous films based on one-dimensional conductive nanostructure, which could open up new opportunities for energy storage and conversion.

HxMoO3−y nanobelts with sea water as electrolyte for high-performance pseudocapacitors and desalination devices

Pseudocapacitors, which store charge through fast reversible redox reactions occuring at the surface of the electrode, could offer higher specific capacitance than electrochemical double-layer capacitors (EDLCs). However, the prime pseudocapacitive phase, transition metal oxides usually have poor electronic conductivity, limiting their practical application. Here, we fabricated highly conductive HxMoO3−y nanobelts as electrode materials for pseudocapacitors. This electrode has demonstrated great electrochemical activity in various aqueous cation solutions with the highest volumetric capacitance of over 350 F cm−3. Significantly, this electrode has high specific capacitance as well as excellent cycling stability in sea water. Moreover, this hybrid composite also exhibits remarkable ion electrosorptive capacity (4 mg g−1) as a desalination electrode.

Band gap engineering of MnO2 through in situ Al-doping for applicable pseudocapacitors

Band gap engineering was achieved by in situ doping method for high electrical conductivity and chemical activity of MnO2. By in situ releasing and adsorption during electrodeposition, Al3+ with close ion radius to Mn4+ could replace the position of Mn4+ in MnO2. The in situ doping process brings impurity level in MnO2 and changes the energy band structure. The narrower band gap of MnO2 after Al doping with higher electron concentration on conduction band could improve the conductivity of MnO2. The specific capacitance of Al-doped MnO2 achieves 430.6 F g−1 which is almost 2.5 times of the capacitance of initial MnO2 (177 F g−1), shedding light on its practical applications.

2D vanadium doped manganese dioxides nanosheets for pseudocapacitive energy storage

Ultrathin two-dimensional (2D) crystals have been predicted to have high electrochemical activity because nearly all active atoms are exposed to the electrolytes, which offers great potential for energy storage. However, to construct layered structure metal oxides, simplifying the synthetic methods and improving the electronic conductivity remain a challenge. Herein, we synthesized 2D vanadium doped manganese oxides through a facile hydrothermal method. Vanadium dopant is also used as a template agent for the formation of nanosheet-shaped MnO2, further leading to high specific surface area as well as significant enhancement of the electronic conductivity, as confirmed by the first-principle calculations and four-point probe method. For the sake of a shortened ion transport distance and enhanced electronic conductivity, V-doped MnO2 nanosheets display an excellent electrochemical performance as a supercapacitor electrode.

Natural Materials Assembled, Biodegradable, and Transparent Paper-Based Electret Nanogenerator

Developing eco-friendly and low-cost electronics is an effective strategy to address the electronic waste issue. In this study, transparent cellulose nanopaper (T-paper) and polylactic acid (PLA) electret were used to construct a biodegradable and transparent paper-based electret nanogenerator. The nanogenerator could be assembled with paper products to form a self-powered smart packaging system without impairing the appearance, due to the high transparency and desirable output performance. Furthermore, the self-degradation property in the natural soil of the nanogenerator is demonstrated, indicating that the nanogenerator is recycled and will not pollute the environment. We anticipate that this study will provide new insights to develop eco-friendly power source and paper-based electronics.

Energy Harvest from Organics Degradation by Two-Dimensional K+-Intercalated Manganese Oxide

Pollution treatment, a problem our world is being deeply puzzled by, has required large amounts of energy during enrichment and degradation. However, some pollutants, for instance organics in wastewater, could offer an energy supply but instead are wasted. Here we report an energy harvesting galvanic cell, built by using a Pt foil as an anode and 2D K+-intercalated MnO2 as a cathode, which combines both dye degradation in wastewater and energy harvesting during the degradation process. Owing to the galvanic effect, this cell could accelerate the degradation rate and indicate the progress of degradation. Different kinds of organics could be degraded and produce energy in this cell with a stable open-circuit voltage (0.45 V). Magnification and imitation of this strategy offer a new chance to harvest waste energy in other exothermic reactions.