Changrong Zhu

Synthesis of Free‐Standing Metal Sulfide Nanoarrays via Anion Exchange Reaction and Their Electrochemical Energy Storage Application

Metal sulfides are an emerging class of high-performance electrode materials for solar cells and electrochemical energy storage devices. Here, a facile and powerful method based on anion exchange reactions is reported to achieve metal sulfide nanoarrays through a topotactical transformation from their metal oxide and hydroxide preforms. Demonstrations are made to CoS and NiS nanowires, nanowalls, and core-branch nanotrees on carbon cloth and nickel foam substrates. The sulfide nanoarrays exhibit superior redox reactivity for electrochemical energy storage. The self-supported CoS nanowire arrays are tested as the pseudo-capacitor cathode, which demonstrate enhanced high-rate specific capacities and better cycle life as compared to the powder counterparts. The outstanding electrochemical properties of the sulfide nanoarrays are a consequence of the preservation of the nanoarray architecture and rigid connection with the current collector after the anion exchange reactions.

Array of nanosheets render ultrafast and high-capacity Na-ion storage by tunable pseudocapacitance

Sodium-ion batteries are a potentially low-cost and safe alternative to the prevailing lithium-ion battery technology. However, it is a great challenge to achieve fast charging and high power density for most sodium-ion electrodes because of the sluggish sodiation kinetics. Here we demonstrate a high-capacity and high-rate sodium-ion anode based on ultrathin layered tin(II) sulfide nanostructures, in which a maximized extrinsic pseudocapacitance contribution is identified and verified by kinetics analysis. The graphene foam supported tin(II) sulfide nanoarray anode delivers a high reversible capacity of ∼1,100 mAh g−1 at 30 mA g−1 and ∼420 mAh g−1 at 30 A g−1, which even outperforms its lithium-ion storage performance. The surface-dominated redox reaction rendered by our tailored ultrathin tin(II) sulfide nanostructures may also work in other layered materials for high-performance sodium-ion storage.

All Metal Nitrides Solid‐State Asymmetric Supercapacitors

Two metal nitrides, TiN porous layers and Fe2 N nanoparticles, are grown uniformly with the assistance of atomic layer deposition on vertically aligned graphene nanosheets and used as the cathode and anode for solid-state supercapacitors, respectively. Full cells are constructed and show good flexibility, high-rate capability, and 98% capacitance retention after 20,000 cycles.

Nonaqueous Hybrid Lithium‐Ion and Sodium‐Ion Capacitors

Hybrid metal-ion capacitors (MICs) (M stands for Li or Na) are designed to deliver high energy density, rapid energy delivery, and long lifespan. The devices are composed of a battery anode and a supercapacitor cathode, and thus become a tradeoff between batteries and supercapacitors. In the past two decades, tremendous efforts have been put into the search for suitable electrode materials to overcome the kinetic imbalance between the battery-type anode and the capacitor-type cathode. Recently, some transition-metal compounds have been found to show pseudocapacitive characteristics in a nonaqueous electrolyte, which makes them interesting high-rate candidates for hybrid MIC anodes. Here, the material design strategies in Li-ion and Na-ion capacitors are summarized, with a focus on pseudocapacitive oxide anodes (Nb2 O5 , MoO3 , etc.), which provide a new opportunity to obtain a higher power density of the hybrid devices. The application of Mxene as an anode material of MICs is also discussed. A perspective to the future research of MICs toward practical applications is proposed to close.

Ultrafast‐Charging Supercapacitors Based on Corn‐Like Titanium Nitride Nanostructures

Ultrahigh rates realized by ALD‐made TiN. The symmetric full‐cell supercapacitors deliver a typical capacitance of 20.7 F cm−3 at a scan rate of 1 V s−1, and retain 4.3 F cm−3 at high rate of 100 V s−1. The devices can be charged and discharged for 20 000 cycles with negligible capacitance loss and with an ultralow self‐discharge current (≈1 μA).

Self-branched α-MnO2/δ-MnO2 heterojunction nanowires with enhanced pseudocapacitance

Despite the extensive research on MnO2 as a pseudocapacitor electrode material, there has been no report on heterostructures of multiple phase MnO2. Here we report the combination of two high-capacitance phases of MnO2, namely, α-MnO2 nanowires and δ-MnO2 ultrathin nanoflakes, to form a core-branch heterostructure nanoarray. This material and structure design not only increases the mass loading of active materials (from 1.86 to 3.37 mg cm2), but also results in evident pseudocapacitance enhancement (from 28 F g−1 for pure nanowires to 178 F g−1 for heterostructures at 5 mV s−1). The areal capacitance is up to 783 mF cm−2 at 1 mV s−1. Upon 20 000 cycles, the heterostructure array electrode still delivers a reversible capacitance above 100 F g−1 at 4.5 A g−1. Kinetic analysis reveals that capacitances due to both capacitive and diffusion controlled processes have been enlarged for the self-branched heterostructure array. This work presents a new route to improve the electrochemical performance of MnO2 as a binder-free supercapacitor electrode.

In Situ Grown Epitaxial Heterojunction Exhibits High-Performance Electrocatalytic Water Splitting.

Electrocatalytic performance can be enhanced by engineering a purposely designed nanoheterojunction and fine-tuning the interface electronic structure. Herein a new approach of developing atomic epitaxial in-growth in Co-Ni3 N nanowires array is devised, where a nanoconfinement effect is reinforced at the interface. The Co-Ni3 N heterostructure array is formed by thermal annealing NiCo2 O4 precursor nanowires under an optimized condition, during which the nanowire morphology is retained. The epitaxial in-growth structure of Co-Ni3 N at nanometer scale facilitates the electron transfer between the two different domains at the epitaxial interface, leading to a significant enhancement in catalytic activities for both hydrogen and oxygen evolution reactions (10 and 16 times higher in the respective turn-over frequency compared to Ni3 N-alone nanorods). The interface transfer effect is verified by electronic binding energy shift and density functional theory (DFT) calculations. This nanoconfinement effect occurring during in situ atomic epitaxial in-growth of the two compatible materials shows an effective pathway toward high-performance electrocatalysis and energy storages.