Highly Enhanced Gas Sorption Capacities of N‑Doped Porous Carbon Spheres by Hot NH₃ and CO₂ Treatments

Highly enhanced CO₂ and H₂ adsorption properties were achieved with a series of phenolic resin-based carbon spheres (resorcinol–formaldehyde carbon (RFC) and phenol–formaldehyde carbon (PFC)) by carbonization of RF and PF polymer (RFP and PFP) spheres synthesized via a sol–gel reaction and subsequent activation with hot CO₂ or NH₃ treatment. Monodisperse and size-tunable (100–600 nm) RFC and PFC spheres had intrinsic nitrogen contents (ca. 1.5 wt %), which are attributed to the synthesis conditions that utilized NH₃ as a basic catalyst as well as nitrogen precursor. A series of CO₂-activated and N-doped RFC and PFC spheres showed almost perfect correlation (<i>R</i>² = 0.99) between CO₂ adsorption capacities and accumulated pore volumes of fine micropores (ultramicropore <1 nm) obtained using the nonlocal density functional theory (NLDFT) model. Interestingly, NH₃ activation served not only as an effective method for heteroatom doping (i.e., nitrogen) into the carbon framework but also as an excellent activation process to fine-tune the surface area and pore size distribution (PSD). Increased nitrogen doping levels up to ca. 2.8 wt % for NH₃-activated RFC spheres showed superior CO₂ adsorption capacities of 4.54 (1 bar) and 7.14 mmol g^–1 (1 bar) at 298 and 273 K, respectively. Compared to CO₂-activated RFC spheres with similar ultramicropore volume presenting CO₂ uptakes of 4.41 (1 bar) and 6.86 mmol g^–1 (1 bar) at 298 and 273 K, respectively, NH₃-activated nitrogen-enriched RFC was found to have elevated chemisorption ability. Moreover, prolonged activation of RFC and PFC spheres provided ultrahigh surface areas, one of which reached 4079 m²g^–1 with an unprecedented superb H₂ uptake capacity of 3.26 wt % at 77 K (1 bar), representing one of the best H₂ storage media among carbonaceous materials and metal–organic frameworks (MOFs)

Variation in Crystalline Phases: Controlling the Selectivity between Silicon and Silicon Carbide via Magnesiothermic Reduction using Silica/Carbon Composites

Magnesiothermic reduction of various types of silica/carbon (SiO₂/C) composites has been frequently used to synthesize silicon/carbon (Si/C) composites and silicon carbide (SiC) materials, which are of great interest in the research areas of lithium-ion batteries (LIBs) and nonmetal oxide ceramics, respectively. Up to now, however, it has not been comprehensively understood how totally different crystal phases of Si or SiC can result from the compositionally identical parent materials (SiO₂/C) via magnesiothermic reduction. In this article, we propose a formation mechanism of Si and SiC by magnesiothermic reduction of SiO₂/C; SiC is formed at the interface between SiO₂ and carbon when silicon intermediates, mainly <i>in situ</i>-formed Mg₂Si, encounter carbon through diffusion. Otherwise, Si is formed, which is supported by an <i>ex situ</i> reaction between Mg₂Si and carbon nanosphere that results in SiC. In addition, the resultant crystalline phase ratio between Si and SiC can be controlled by manipulating the synthesis parameters such as the contact areas between silica and carbon of parent materials, reaction temperatures, heating rates, and amount of the reactant mixtures used. The reasons for the dependence on these synthesis parameters could be attributed to the modulated chance of an encounter between silicon intermediates and carbon, which determines the destination of silicon intermediates, namely, either thermodynamically preferred SiC or kinetic product of Si as a final product. Such a finding was applied to design and synthesize the hollow mesoporous shell (ca. 3–4 nm pore) SiC, which is particularly of interest as a catalyst support under harsh environments

Bioinspired Synthesis of Melaninlike Nanoparticles for Highly N‑Doped Carbons Utilized as Enhanced CO₂ Adsorbents and Efficient Oxygen Reduction Catalysts

Highly N-doped nanoporous carbons have been of great interest as a high uptake CO₂ adsorbent and as an efficient metal-free oxygen reduction reaction (ORR) catalyst. Therefore, it is essential to produce porosity-tunable and highly N-doped carbons through cost-effective means. Herein, we introduce the bioinspired synthesis of a monodisperse and N-enriched melaninlike polymer (MP) resembling the sepia biopolymer (SP) from oceanic cuttlefish. These polymers were subsequently utilized for highly N-doped synthetic carbon (MC) and biomass carbon (SC) spheres. An adequate CO₂ activation process fine-tunes the ultramicroporosity (<1 nm) of N-doped MC and SC spheres, those with maximum ultramicroporosities of which show remarkable CO₂ adsorption capacities. In addition, N-doped MC and SC with ultrahigh surface areas of 2677 and 2506 m²/g, respectively, showed excellent ORR activities with a favored four electron reduction pathway, long-term durability, and better methanol tolerance, comparable to a commercial Pt-based catalyst

Elucidating Relationships between Structural Properties of Nanoporous Carbonaceous Shells and Electrochemical Performances of Si@Carbon Anodes for Lithium-Ion Batteries

The encapsulation of silicon in hollow carbonaceous shells (Si@C) is known to be a successful solution for silicon anodes in Li-ion batteries, resulting in many efforts to manipulate the structural properties of carbonaceous materials to improve their electrochemical performance. In this regard, we demonstrate in this work how both the shell thickness and pore size of nanoporous carbonaceous materials containing silicon anodes influence the electrochemical performance. Structurally well-defined Si@C materials with varying carbon-shell thicknesses and pore sizes were synthesized by a nanocasting method that manipulated the carbon shell and by a subsequent magnesiothermic reduction that converted the amorphous silica cores into silicon nanocrystals. When these materials were employed as anodes, it was verified that two opposite effects occur with respect to the thickness of carbon shell: The weight ratio of silicon and the electrical conductivity are simultaneously affected, so that the best electrochemical performance is not obtained from either the thickest or the thinnest carbon shell. Such countervailing effects were carefully confirmed through a series of electrochemical performance tests and the use of electrochemical impedance spectroscopy. In addition, the effect of pore size was elucidated by comparing Si@C samples with different pore sizes, revealing that larger pores can further improve the electrochemical performance as a result of enhanced Li-ion diffusion

Dynamic Nuclear Polarization of Selectively ²⁹Si-Enriched Core@shell Silica Nanoparticles

29Si silica nanoparticles (SiO2 NPs) are promising magnetic resonance imaging (MRI) probes that possess advantageous properties for in vivo applications, including suitable biocompatibility, tailorable properties, and high water dispersibility. Dynamic nuclear polarization (DNP) is used to enhance 29Si MR signals via enhanced nuclear spin alignment; to date, there has been limited success employing DNP for SiO2 NPs due to the lack of endogenous electronic defects that are required for the process. To create opportunities for SiO2-based 29Si MRI probes, we synthesized variously featured SiO2 NPs with selective 29Si isotope enrichment on homogeneous and core@shell structures (shell thickness: 10 nm, core size: 40 nm), and identified the critical factors for optimal DNP signal enhancement as well as the effective hyperpolarization depth when using an exogenous radical. Based on the synthetic design, this critical factor is the proportion of 29Si in the shell layer regardless of core enrichment. Furthermore, the effective depth of hyperpolarization is less than 10 nm between the surface and core, which demonstrates an approximately 40% elongated diffusion length for the shell-enriched NPs compared to the natural abundance NPs. This improved regulation of surface properties facilitates the development of isotopically enriched SiO2 NPs as hyperpolarized contrast agents for in vivo MRI