SiO2 stabilizes electrochemically active nitrogen in few-layer carbon electrodes of extraordinary capacitance

Pyrrolic and pyridinic N dopants can dramatically increase the electrochemical activities of carbon and conducting polymers.Although N-doped conducting polymers suffer from rapid degradation,their carbon counterpart of extraordinary capacitance has remarkable rate performance and cycling endurance thanks to carbon’s excellent electrical conductivity.But high nitrogen content and high electrical conductivity are difficult to achieve in a high-surface-area carbon,because the high chemical vapor deposition(CVD)temperature required for obtaining high conductivity also destabilizes under-coordinated pyrrolic and pyridinic nitrogen and tends to lower the surface area.Here we resolve this dilemma by using SiO2 as an effective N-fixation additive,which stabilizes the N-rich nano few-layer sp2-carbon construct in1000℃CVD.This enables a scalable sol-gel/CVD fabrication process for few-layer carbon electrodes of extraordinary capacitance(690 F g^-1).The electrodes have excellent rate performance and can maintain90%of their initial capacitance after 30,000 cycles,thus potentially suitable for practical applications.

氮掺杂黑色二氧化钛用于高性能超级电容器

对于储能系统,同时实现高能量密度和高功率密度仍是一个巨大的挑战.电化学超级电容器通过表面吸附或表面氧化还原反应实现储能,是解决上述问题的潜在方法之一.本论文报道了一种新型高氮掺杂(9.29 at.%)黑色二氧化钛(TiO2-x:N)超级电容器电极材料.该材料具有独特的微观结构,由高导电的非晶壳层和一个纳米晶核组成.在酸性电解液中,该材料可以通过氮参与的氧化还原反应(TiO2-xNy+z H++ze■-TiO2-xNyHz)可逆地与质子结合实现能量的高效快速储存,实现极高的比电容(2 mV s-1扫速下容量高达750 F g-1,1 A g-1电流密度下容量可达707 F g-1)、高倍率特性(极高电流密度20 A g-1时容量仍可达503 F g-1)和长时间循环下的高稳定性.作为一种新型超级电容器电极材料,氮掺杂黑色二氧化钛或将引领金属氧化物型超级电容器的复兴.

New progress in development of ferroelectric and piezoelectric nanoceramics

There has been great progress in the last decade in the synthesis of nanopowders with highly controlled size and size distribution.Meanwhile,the development of an unconventional pressureless two-step sintering strategy enabling densification without grain growth provides a novel technology suitable for commercial production of nanograin ceramics.The particular interest concerning bulk dense nanograin ceramics is the manifestation of ferroelectricity,which remains a fundamental issue to be understood and exploited.Combining the best powder synthesis and optimized two-step sintering,high-density barium titanate(BT)and related nanograin ceramics have been fabricated to allow for a detailed determination of the size effect on nanometer-scale ferroelectricity and piezoelectricity of fundamental and industrial interest.These include dense ceramics of undoped BT with an average grain size down to 5 nm,and of(1x)BiScO_(3)xPbTiO_(3)(BSPT)solid solutions with an average grain size down to 10 nm.Here we review the fabrication methods of high-density BT and BSPT nanoceramics and the major findings of the size effect on their microstructure,phase transition and electrical properties.Robust ferroelectricity is demonstrated for the first time in 5 nm BT nanoceramics,while strong local piezoelectricity is present in 10 nm BSPT nanoceramics.

Electrodes with Electrodeposited Water-excluding Polymer Coating Enable High-Voltage Aqueous Supercapacitors

Aqueous supercapacitors are powerful energy sources,but they are limited by energy density that is much lower than lithium-ion batteries.Since raising the voltage beyond the thermodynamic potential for water splitting(1.23 V)can boost the energy density,there has been much effort on water-stabilizing salvation additives such as Li_(2)SO_(4) that can provide an aqueous electrolyte capable of withstanding~1.8 V.Guided by the first-principles calculations that reveal water can promote hydrogen and oxygen evolution reactions,here,we pursue a new strategy of covering the electrode with a dense electroplated polymerized polyacrylic acid,which is an electron insulator but a proton conductor and proton reservoir.The combined effect of salvation and coating expands the electrochemical window throughout pH 3 to pH 10 to 2.4V for both fast and slow proton-mediated redox reactions.This allows activated carbon to quadruple the energy density,a kilogram of nitrogen-doped graphene to provide 127 Watt-hour,and both to have improved endurance because of suppression of water-mediated corrosion.Therefore,aqueous supercapacitors can now achieve energy densities quite comparable to that of a lithium-ion battery,but at 100 times the charging/discharging speed and cycle durability.