A 3D N-doped graphene foam with a 6.8 at% nitrogen content is prepared by annealing a freeze-dried graphene oxide foam in ammonia. It is used as an anode in sodium ion batteries to deliver a high initial reversible capacity of 852.6 mA h g(-1) at 1 C between 0.02 and 3 V with a long-term retention of 69.7% after 150 cycles.

Over the past decade, considerable progress has been made in the synthesis and applications of nanoporous carbon spheres ranging in size from nanometres to micrometres. This Review presents the primary techniques for preparing nanoporous carbon spheres and the seminal research that has inspired their development, presented potential applications and uncovered future challenges. First we provide an overview of the synthesis techniques, including the Stöber method and those based on templating, self-assembly, emulsion and hydrothermal carbonization, with special emphasis on the design and functionalization of nanoporous carbon spheres at the molecular level. Next, we cover the key applications of these spheres, including adsorption, catalysis, separation, energy storage and biomedicine — all of which might benefit from the regular geometry, good liquidity, tunable porosity and controllable particle-size distribution offered by nanoporous carbon spheres. Finally, we present the current challenges and opportunities in the development and commercial applications of nanoporous carbon spheres

A series of nitrogen-containing polymer and carbon spheres were obtained by the sol–gel method. In particular, the nitrogen-rich carbon spheres were prepared by one-pot hydrothermal synthesis in the presence of resorcinol/formaldehyde as carbon precursors and ethylenediamine (EDA) as both a base catalyst and nitrogen precursor, followed by carbonization in nitrogen and activation with CO2. The introduction of EDA to the sol–gel system resulted in structurally bonded nitrogen-containing carbon spheres. The nitrogen doping level and the particle size can be tuned by varying the EDA amount in the reaction mixture. The maximum nitrogen doping level of 7.2 wt % in carbon spheres could be achieved without sacrificing the spherical morphology. The diameter of these carbon spheres (CS) can be tuned in the rage of 50–1200 nm by varying the EDA amount. N2 adsorption analysis showed that the aforementioned activated carbon spheres exhibited high surface area reaching up to1224 m2/g. Ultra high CO2 adsorption capacities, 4.1 and 6.2 mmol/g, corresponding to an equilibrium pressure of 1 bar, were measured on nitrogen-containing activated carbon spheres at 25 and 0 °C, respectively. Electrochemical measurements performed on these carbon spheres for double layer capacitors showed very high capacitance up to ∼388 F/g at 1.0 A/g, outstanding rate capability (60% capacitance retention at 100 A/g), and unprecedented cycling stability (∼98% capacitance retention even after 8000 cycles) in 1 M H2SO4 electrolyte solution.

Phenolic resin-based carbon spheres obtained by a slightly modified Stöber method are shown to be superior CO 2 adsorbents. A direct KOH activation of polymeric spheres gave carbons with small micropores (<0.8 nm) and large specific surface area (2400 m 2 g À1 ), which are able to adsorb an unprecedented amount of CO 2 (up to 8.9 mmol g À1 ) at 0 C and ambient pressure.

A series of carbon spheres (CS) was prepared by carbonization of phenolic resin spheres obtained by the one-pot modified Stöber method. Activated CS (ACS), having diameters from 200 to 420 nm, high surface area (from 730 to 2930 m(2)/g), narrow micropores (<1 nm) and, importantly, high volume of these micropores (from 0.28 to 1.12 cm(3)/g), were obtained by CO2 activation of the aforementioned CS. The remarkably high CO2 adsorption capacities, 4.55 and 8.05 mmol/g, were measured on these AC spheres at 1 bar and two temperatures, 25 and 0 °C, respectively.