Chaohe Xu

Targeted Synthesis of Unique Nickel Sulfide (NiS, NiS2) Microarchitectures and the Applications for the Enhanced Water Splitting System

Water splitting is one of the ideal technologies to meet the ever increasing demands of energy. Many materials have aroused great attention in this field. The family of nickel-based sulfides is one of the examples that possesses interesting properties in water-splitting fields. In this paper, a controllable and simple strategy to synthesize nickel sulfides was proposed. First, we fabricated NiS2 hollow microspheres via a hydrothermal process. After a precise heat control in a specific atmosphere, NiS porous hollow microspheres were prepared. NiS2 was applied in hydrogen evolution reaction (HER) and shows a marvelous performance both in acid medium (an overpotential of 174 mV to achieve a current density of 10 mA/cm2 and the Tafel slope is only 63 mV/dec) and in alkaline medium (an overpotential of 148 mV to afford a current density of 10 mA/cm2 and the Tafel slope is 79 mV/dec). NiS was used in oxygen evolution reaction (OER) showing a low overpotential of 320 mV to deliver a current density of 10 mA/cm2, which is meritorious. These results enlighten us to make an efficient water-splitting system, including NiS2 as HER catalyst in a cathode and NiS as OER catalyst in an anode. The system shows high activity and good stabilization. Specifically, it displays a stable current density of 10 mA/cm2 with the applying voltage of 1.58 V, which is a considerable electrolyzer for water splitting.

Hydrothermal synthesis of nanostructured graphene/polyaniline composites as high-capacitance electrode materials for supercapacitors

As known to all, hydrothermal synthesis is a powerful technique for preparing inorganic and organic materials or composites with different architectures. In this reports, by controlling hydrothermal conditions, nanostructured polyaniline (PANi) in different morphologies were composited with graphene sheets (GNS) and used as electrode materials of supercapacitors. Specifically, ultrathin PANi layers with total thickness of 10–20 nm are uniformly composited with GNS by a two-step hydrothermal-assistant chemical oxidation polymerization process; while PANi nanofibers with diameter of 50~100 nm are obtained by a one-step direct hydrothermal process. Benefitting from the ultrathin layer and porous structure, the sheet-like GNS/PANi composites can deliver specific capacitances of 532.3 to 304.9 F/g at scan rates of 2 to 50 mV/s. And also, this active material showed very good stability with capacitance retention as high as ~99.6% at scan rate of 50 mV/s, indicating a great potential for using in supercapacitors. Furthermore, the effects of hydrothermal temperatures on the electrochemical performances were systematically studied and discussed.

A hybrid polymer/oxide/ionic-liquid solid electrolyte for Na-metal batteries

The development of solid electrolytes with superior electrical and electrochemical performances for the room-temperature operation of sodium (Na)-based batteries is at the infant stage and still remains a challenge. Herein, we, for the first time, report hybrid solid electrolytes consisting of PEO20–NaClO4–5% SiO2–x% Emim FSI (x = 50, 70) designed for solid-state Na-metal batteries. The hybrid design yields a solid electrolyte featuring a high room-temperature ionic conductivity of 1.3 × 10−3 S cm−1, suitable mechanical property, a wide voltage stability window of 4.2 V and a high Na+ transference number of 0.61. A prototypical Na-metal battery using this hybrid solid electrolyte demonstrates promising long-term cycling performances at room temperature and at an elevated temperature of 60 °C for 100 cycles. The finding implies that the hybrid solid electrolyte is promising for Na-metal batteries operating at room temperature.

The interfacial mechanical properties of functionalized graphene–polymer nanocomposites

The interfacial mechanical properties between graphene (GR) and a polymer matrix play a key role in load transfer capability for GR/polymer nanocomposites. Grafting of polymer molecular chains on GR can improve the dispersion of the GR in a polymer matrix and change the interfacial mechanical properties between the GR and the polymer matrix. In this work, we investigated the interfacial mechanical properties between GR functionalized with polymer molecular chains and a polyethylene (PE) matrix using molecular dynamics simulations. The influences of grafting density and chain length on the interfacial mechanical properties were analyzed. The results show that grafting of short PE molecular chains on GR can significantly improve the interfacial shear strength and interfacial Mode-II fracture toughness in functionalized GR/PE nanocomposites.

Na-rich layered Na 2 Ti 1−x Cr x O 3−x/2 (x = 0, 0.06): Na-ion battery cathode materials with high capacity and long cycle life

Rechargeable lithium batteries have been well-known and indispensable for portable electronic devices, and have the potential to be used in electric vehicles and smart grids. However, the growing concerns about the availability of lithium resources for large-scale applications have revived interest in sodium ion batteries. Of many obstacles to commercialization of Na-ion batteries, achieving simultaneously a large reversible capacity and good cycling capability of electrode materials remains a major challenge. Here, we report a new cathode material, Na-rich layered oxide Na2Ti0.94Cr0.06O2.97, that delivers high reversible capacity of 336 mAh g−1 at current density of 18.9 mA g−1 along with promising cycling capability of 74% capacity retention over 1000 cycles at current of 378 mA g−1. The high capacity is associated to the redox reaction of oxygen, which is confirmed here by a combined experimental and theoretical study. The present work therefore shows that materials beyond mainstream layered oxides and polyanion compounds should be considered as candidate high-performance cathodes for Na-ion batteries.

Core/shell design of efficient electrocatalysts based on NiCo2O4 nanowires and NiMn LDH nanosheets for rechargeable zinc–air batteries

Rechargeable zinc–air (Zn–air) batteries having high theoretical energy density are the most attractive energy technologies for future electric vehicles and flexible/wearable electronics. However, the serious lack of highly efficient and cost-effective oxygen electrocatalysts is one of the major obstacles for their future commercialization. Herein, we presented a core/shell design based on porous NiCo2O4 nanowires and ultrathin NiMn LDH nanosheets as an efficient method for the synthesis of electrocatalysts for rechargeable Zn–air batteries. Due to the large active surface area, rapid mass/charge transport, and high electron conductivity as well as unique structures, the core/shell NiCo2O4@NiMn LDH materials could deliver a rather low OER overpotential of 255 mV at 10.0 mA cm−2 while maintaining good stability in alkaline media. When these materials were further employed as air-cathode materials for Zn–air batteries, they exhibited an ultrahigh energy density (866 W h kg−1), superior reversibility (initial round-trip efficiency of 63.5%) and excellent stability (voltage gap increased by only about 20 mV after 500 cycles), which were much better than those of commercial Ir/C catalyst. Furthermore, the as-prepared flexible solid Zn–air battery also displayed very good mechanical properties, long cycle life and outstanding round-trip efficiency (70–74%).

Conductive PVDF-HFP/CNT composites for strain sensing

A strain sensor based on the composites of poly (vinylidene fluoride-hexafluoropropylene) (PVDF-HFP) filled by multi-walled carbon nanotube (MWNT) was prepared using a proposed fabrication process. Three kinds of MWNT loadings, i.e., 1.0wt.%, 2.0wt.% and 3.0wt.% were employed. Due to good dispersion state of MWNT in PVDF-HFP matrix, which was characterized by scanning electron microscope (SEM), this sensor was found to be of high sensitivity and stable performance. The sensor’s piezoresistivity varied in a weak nonlinear pattern, which was probably caused by the tunneling effect among neighboring MWNTs. The gauge factor of the sensor of 1.0wt.% MWNT loading was identified to be the highest, i.e., 33. This sensor gauge factor decreased gradually with the increase of addition amount of MWNT, which was 5 for the sensor of 3.0wt.% MWNT loading. This gauge factor was still higher than that of conventional metal-foil strain sensors. The electrical conductivity of PVDF-HFP/MWNT composites was also studied. It was found that with the increase of the addition amount of MWNT, the electrical conductivity of the PVDF-HFP/MWNT composites varied in a perfect percolation pattern with a very low percolation threshold, i.e., 0.77 vol.%, further indicating the very good dispersion of MWNT in the PVDF-HFP matrix.

Highly sensitive humidity sensors made from composites of HEC filled by carbon nanofillers

Abstract Resistive type humidity sensors made from the composite films of HEC (hydroxyethyl cellulose) and MWCNTs (multiwalled carbon nanotubes) were developed. Three kinds of HEC/MWCNT composite films were fabricated, which contained 7.0, 8.0 and 9.0 wt-% MWCNTs. Rapid response capability and high sensitivities of the composite sensors were confirmed. For the convenience of sensor’s calibration, the non-linear resistance change ratios can be linearised by performing natural logarithm operation on them. A new sensitivity factor was defined based on this linear relationship between the logarithm of the resistance change ratio and relative humidity. For comparison, a reference humidity sensor was also made from the composite film of HEC filled by 50.0 wt-% CBs (carbon blacks). Compared with the HEC/MWCNT sensors, the HEC/CB sensor was of much higher sensitivity, but lower repeatability and stability under cyclic humidity changes. Possible sensing mechanisms were discussed in detail.

Quasi-parallel arrays with a 2D-on-2D structure for electrochemical supercapacitors

Construction of two-dimensional nanostructured arrays is important to improve the specific capacitance and rate performance of supercapacitors. In this work, we demonstrate a novel type of pseudocapacitive NiMn oxide@MnO2 quasi-parallel array assembled via 2D-on-2D structures. The quasi-parallel arrays markedly reduce the “dead area” caused by the bending stack of traditional 2D assembled structures, which obviously increases the number of active sites of the MnO2 electrode. The small aspect ratio of the quasi-parallel arrays ensures mechanical adhesion and electrical connection between the active material and the current collector. Therefore, the NiMn oxide@MnO2 arrays exhibit outstanding pseudocapacitive performance, including high specific capacitance (801 F g−1 at 1 A g−1, capacity: 151.3 mAh g−1), long-term cycling stability, and outstanding rate capability (79% capacitance retention rate at 40 A g−1). The constructed asymmetric supercapacitor based on a NiMn oxide@MnO2 cathode and an activated graphene anode exhibits extraordinary cyclic stability (96% capacitance retention after 10 000 cycles). This new type of quasi-parallel array could become a versatile electrode platform, which would open up a wide range of applications in supercapacitors, batteries and electrocatalysis.