Yaqi Ren

Large-scale synthesis of hybrid metal oxides through metal redox mechanism for high-performance pseudocapacitors

Electrochemical performance and production cost are the main concerns for the practical application of supercapacitors. Here we report a simple and universally applicable method to prepare hybrid metal oxides by metal redox reaction utilizing the inherent reducibility of metals and oxidbility of for the first time. As an example, Ni(OH)2/MnO2 hybrid nanosheets (NMNSs) are grown for supercapacitor application by self-reaction of Ni foam substrates in KMnO4 solution at room temperature. The obtained hybrid nanosheets exhibit high specific capacitance (2,937 F g−1). The assembled solid-state asymmetric pseudocapacitors possess ultrahigh energy density of 91.13 Wh kg−1 (at the power density of 750 W kg−1) and extraordinary cycling stability with 92.28% capacitance retention after 25,000 cycles. Co(OH)2/MnO2 and Fe2O3/MnO2 hybrid oxides are also synthesized through this metal redox mechanism. This green and low-cost method is capable of large-scale production and one-step preparation of the electrodes, holding promise for practical application of high-performance pseudocapacitors.

Activated carbon with micrometer-scale channels prepared from luffa sponge fibers and their application for supercapacitors

In this paper, we report the preparation of macrochanneled activated carbon (MCAC) with a novel structure combining micropores, mesopores, and macrochannels and its electrochemical properties for supercapacitor applications. The MCAC was prepared by carbonizing luffa sponge fibers and subsequent activation with KOH. The MCAC has densely packed and parallel channels of 4–10 μm in diameter and 0.3–1 μm in wall thickness, which are inherited from the natural structure of the luffa sponge fibers. Micro- and mesopores were produced on the inner surface of the channel walls, forming a hierarchically porous structure. The MCAC exhibits excellent electrochemical performance for application as electrode materials of supercapacitors in appropriate electrolytes. The specific capacitances of the MCAC at 1 A g−1 are 167, 196, and 249 F g−1 in Na2SO4, KOH, and H2SO4 solutions, respectively. The MCAC materials reported here may have broad applications such as for supercapacitors, catalysis, and templates for supporting various functional materials.

Cotton-based hollow carbon fibers with high specific surface area prepared by ammonia etching for supercapacitor application

In this paper, carbon fibers with a high specific surface area (SSA) have been prepared from cotton for supercapacitor application. The cotton-based carbon fibers (CCFs) are prepared by carbonizing the cotton fibers in ammonia (NH3) and nitrogen gases at different temperatures. The CCFs possess a hollow tubular structure with an outer diameter of about 7 μm and an inner diameter of about 3 μm. The hollow structure is inherited from the natural structure of the cotton fibers. The SSA and pore structure of the CCFs depend on the carbonizing temperature and atmosphere. The CCFs carbonized in NH3 have high SSA up to 778.6 m2 g−1 with higher mesopore ratio. Higher nitrogen concentration (3.3 at%) and more CO functional groups are present in the CCFs carbonized in NH3. The maximum specific capacitance of the CCFs carbonized in NH3 is measured to be 355 F g−1 at 1 A g−1, 245.3 F g−1 at 0.8 A g−1, and 181.3 F g−1 at 0.2 A g−1 in KOH, H2SO4, and Na2SO4 electrolytes, respectively. The tubular structure, high SSA, higher mesopore ratio, nitrogen doping, and the presence of the oxygen functional groups are responsible for the excellent electrochemical performance. Comparing with the conventional activation process using KOH as an etchant the present process by NH3 etching to prepare high SSA carbon materials has the advantages of simplicity, no contaminants, and higher mesopore ratio.

Aligned polyaniline nanowires grown on the internal surface of macroporous carbon for supercapacitors

High utilization efficiency of electrode materials is of great importance for achieving excellent electrochemical performance of supercapacitors. In this paper, we report the growth of aligned polyaniline (PANI) nanowires on the internal surface of macroporous carbon (MC) derived from luffa sponge fibers for increasing their utilization efficiency. The pores in the MC are densely packed, straight, and parallel with the diameter at the micrometer scale, which provide easy paths for reaction solution to penetrate and thus enable the growth of the PANI nanowires on the internal wall surface. Due to full exposure to the electrolyte, the PANI nanowires exhibit high utilization efficiency, leading to high specific capacitance up to 1500 F g−1 (1 A g−1). As the macropores allow easy penetration of the electrolyte, the PANI nanowires show high rate capability with the capacitance retention up to 70% with increasing current density from 1 to 10 A g−1. Symmetric supercapacitors assembled using the MC/PANI materials possess high energy density (19 W h kg−1 at 0.5 kW kg−1) and long cycle life (83% retention after 7000 cycles). Considering the abundance and green production of the luffa sponge the MC/PANI composites are promising for industrial application of supercapacitors.

Solvothermal synthesis of a dendritic TiNxOy nanostructure for oxygen reduction reaction electrocatalysis

A novel dendritic titanium oxynitride (TiNxOy) nanostructure composed of nanowires of 15–20 nm in diameter has been prepared. The dendritic TiNxOy nanostructure nitrided at 900 °C possesses high electrocatalytic activity for ORR with a four-electron process, high onset potential of −0.13 V, and superior durability compared with the commercial 10% Pt/C catalyst.

A novel route towards well-dispersed short nanofibers and nanoparticles via electrospinning

Good control of the size, shape, and dispersion is of great significance for the preparation and application of nanomaterials. In this paper, we report a novel electrospinning–calcination–grinding route to prepare well-dispersed inorganic nanoparticles and short nanofibers. Continuous inorganic nanofibers are prepared by calcining the electrospun precursor nanofibers, which are then transformed into nanoparticles or short nanofibers after mild grinding. Due to the continuous fiber shape of the precursor agglomeration is avoided during calcination, and due to the fragile nature the calcined continuous inorganic nanofibers can be made into nanoparticles or short nanofibers easily by short-time mild grinding. On account of the versatility of electrospinning, the present route is applicable to most of the commonly seen materials. Furthermore, short nanofibers which are difficult to prepare for some materials by other conventional methods can be easily obtained by this method. A series of inorganic nanoparticles and short nanofibers have been prepared, including WC, TiC, ATO, W, Ni, and Cu. The prepared suspensions of WC, TiC, and W show outstanding dispersion stability and no sedimentation or flocculation is observed even after several months, indicating high promise for many applications such as printed electronics and three-dimensional (3D) printing.