Qiangfeng Xiao

Nitrogen-doped activated carbon for a high energy hybrid supercapacitor

Nitrogen-doped activated carbons (NACs) were prepared through a one-step process. The obtained NACs show high surface areas of up to 2900 m2 g−1 with a moderate N content of up to 4 wt%. Electrochemical evaluation of the NACs shows a high specific capacity of 129 mA h g−1 (185 F g−1) in an organic electrolyte at a current density of 0.4 A g−1, as well as excellent rate capability and cycling stability. The hybrid-type supercapacitor assembled using the NACs and a Si/C electrode exhibits a high material level energy density of 230 W h kg−1 at 1747 W kg−1. The hybrid device achieved 76.3% capacity retention after 8000 cycles tested at 1.6 A g−1.

Inward lithium-ion breathing of hierarchically porous silicon anodes

Silicon has been identified as a highly promising anode for next-generation lithium-ion batteries (LIBs). The key challenge for Si anodes is large volume change during the lithiation/delithiation cycle that results in chemomechanical degradation and subsequent rapid capacity fading. Here we report a novel fabrication method for hierarchically porous Si nanospheres (hp-SiNSs), which consist of a porous shell and a hollow core. On charge/discharge cycling, the hp-SiNSs accommodate the volume change through reversible inward Li breathing with negligible particle-level outward expansion. Our mechanics analysis revealed that such inward expansion is enabled by the much stiffer lithiated layer than the unlithiated porous layer. LIBs assembled with the hp-SiNSs exhibit high capacity, high power and long cycle life, which is superior to the current commercial Si-based anode materials. The low-cost synthesis approach provides a new avenue for the rational design of hierarchically porous structures with unique materials properties.

Regenerative Polysulfide-Scavenging Layers Enabling Lithium–Sulfur Batteries with High Energy Density and Prolonged Cycling Life

Lithium-sulfur batteries, notable for high theoretical energy density, environmental benignity, and low cost, hold great potential for next-generation energy storage. Polysulfides, the intermediates generated during cycling, may shuttle between electrodes, compromising the energy density and cycling life. We report herein a class of regenerative polysulfide-scavenging layers (RSL), which effectively immobilize and regenerate polysulfides, especially for electrodes with high sulfur loadings (e.g., 6 mg cm-2). The resulting cells exhibit high gravimetric energy density of 365 Wh kg-1, initial areal capacity of 7.94 mAh cm-2, low self-discharge rate of 2.45% after resting for 3 days, and dramatically prolonged cycling life. Such blocking effects have been thoroughly investigated and correlated with the work functions of the oxides as well as their bond energies with polysulfides. This work offers not only a class of RSL to mitigate shuttling effect but also a quantified design framework for advanced lithium-sulfur batteries.

Recent advances in Pt-based octahedral nanocrystals as high performance fuel cell catalysts

Cost, electrochemical activity and durability of the catalysts remain the key issues affecting the commercialization of fuel cells. An answer to these issues may be Pt-based octahedral nanocrystal catalysts because (111) facets promise high electrochemical activity and relatively long durability. This paper reviews the relative mechanisms, the preparation methods and characterization techniques for fuel cell Pt-based octahedral nanocrystal catalysts. First, we summarize the formation mechanisms and the activity enhancement mechanisms of Pt-based octahedral nanocrystal catalysts. Second, we present the preparation methods for the octahedral nanocrystal catalysts. Methods include the capping agent, surfactant-free organosol, microwave and solid phase reduction methods. Generally, the capping agent method was used most widely because it can easily control the structure and size of particles. In recent years, the surfactant-free organosol method, microwave method and solid phase reduction method have been developed rapidly. Catalysts prepared by the surfactant-free organosol method are of particular interest because their clean crystal surfaces result in extremely high electrochemical activity. In the second half of this review, the structure, composition and electrochemical characterization techniques associated with the octahedral nanocrystal catalysts will be detailed. We will conclude with a summary of problems facing this field and suggest prospective research directions.

One-Step Synthesis of Microporous Carbon Monoliths Derived from Biomass with High Nitrogen Doping Content for Highly Selective CO 2 Capture

The one-step synthesis method of nitrogen doped microporous carbon monoliths derived from biomass with high-efficiency is developed using a novel ammonia (NH3)-assisted activation process, where NH3 serves as both activating agent and nitrogen source. Both pore forming and nitrogen doping simultaneously proceed during the process, obviously superior to conventional chemical activation. The as-prepared nitrogen-doped active carbons exhibit rich micropores with high surface area and high nitrogen content. Synergetic effects of its high surface area, microporous structure and high nitrogen content, especially rich nitrogen-containing groups for effective CO2 capture (i.e., phenyl amine and pyridine-nitrogen) lead to superior CO2/N2 selectivity up to 82, which is the highest among known nanoporous carbons. In addition, the resulting nitrogen-doped active carbons can be easily regenerated under mild conditions. Considering the outstanding CO2 capture performance, low production cost, simple synthesis procedure and easy scalability, the resulting nitrogen-doped microporous carbon monoliths are promising candidates for selective capture of CO2 in industrial applications.

Robust lithium-ion anodes based on nanocomposites of iron oxide–carbon–silicate

Robust composite particles containing Fe3O4 cores and porous conductive carbon–silicate layers were synthesized using an aerosol-assisted process followed by vapor coating using organosilanol as the precursor. Such unique synthesis enables the composites with high capacity and good cycle performance, and can be extended towards other oxide composites for energy storage.

Creating Lithium‐Ion Electrolytes with Biomimetic Ionic Channels in Metal–Organic Frameworks

Solid-state electrolytes are the key to the development of lithium-based batteries with dramatically improved energy density and safety. Inspired by ionic channels in biological systems, a novel class of pseudo solid-state electrolytes with biomimetic ionic channels is reported herein. This is achieved by complexing the anions of an electrolyte to the open metal sites of metal-organic frameworks (MOFs), which transforms the MOF scaffolds into ionic-channel analogs with lithium-ion conduction and low activation energy. This work suggests the emergence of a new class of pseudo solid-state lithium-ion conducting electrolytes.

High performance octahedral PtNi/C catalysts investigated from rotating disk electrode to membrane electrode assembly

Octahedral PtNi/C catalysts have demonstrated superior catalytic performance in oxygen reduction reaction (ORR) over commercial Pt/C with rotating disk electrode (RDE). However, it is not trivial to translate such promising results to real-world membrane-electrode assembly (MEA). In this work, we have synthesized octahedral PtNi/C catalysts using poly(diallyldimethylammonium chloride) (PDDA) as a capping agent and investigated their performance from RDE to MEA. In RDE, mass activity and specific activity of the optimized octahedral PtNi/C catalyst for oxygen reduction reaction (ORR) are nearly 19 and 28 times high of the state-of-the-art commercial Pt/C, respectively. At MEA level, the octahedral PtNi/C catalyst exhibits excellent power generation performance and durability paired with commercial Pt/C anode. Its cell voltage at 1,000 mA·cm−2 reaches 0.712 V, and maximum power density is 881.6 mW·cm−2 and its performance attenuation is also less, around 11.8% and 7% under galvanostatic condition of 1,000 mA·cm−2 for 100 h. Such results are investiaged by thermodynamic analysis and fundametal performance modeling, which indicate the single cell performance can be further improved by reducing the size of PtNi/C catalyst agglomerates. Such encouraging results have demonstrated the feasibility to convey the superior performance of octahedral PtNi/C from RDE to MEA.