Influence of Pore Architecture and Chemical Structure on the Sodium Storage in Nitrogen-Doped Hard Carbons.

Hard carbon is the material of choice for sodium ion battery anodes. Capacities comparable to those of lithium/graphite can be reached, but the understanding of the underlying sodium storage mechanisms remains fragmentary. A two-step process is commonly observed, where sodium first adsorbs to polar sites of the carbon ("sloping region") and subsequently fills small voids in the material ("plateau region"). To study the impact of nitrogen functionalities and pore geometry on sodium storage, a systematic series of nitrogen-doped hard carbons is synthesized. The nitrogen content is found to contribute to sloping capacity by binding sodium ions at edges and defects, whereas higher plateau capacities are found for materials with less nitrogen content and more extensive graphene layers, suggesting the formation of 2D sodium structures stabilized by graphene-like pore walls. In fact, up to 84% of the plateau capacity is measured at potentials less than 0 V versus metallic Na, that is, quasimetallic sodium can be stabilized in such structure motifs. Finally, gas physisorption measurements are related to charge-discharge data to identify the energy storage relevant pore architectures. Interestingly, these are pores inaccessible to probe gases and electrolytes, suggesting a new view on such "closed pores" required for efficient sodium storage.

Fundamental Insights into the Reductive Covalent Cross-Linking of Single-Walled Carbon Nanotubes.

Single-walled carbon nanotubes (SWCNT) have been covalently cross-linked via a reductive functionalization pathway, utilizing negatively charged carbon nanotubides (KC4). We have compared the use of difunctional linkers acting as molecular pillars between the nanotubes, namely, p-diiodobenzene, p-diiodobiphenyl, benzene-4,4'-bis(diazonium), and 1,1'-biphenyl-4,4'-bis(diazonium) salts as electrophiles. We have employed statistical Raman spectroscopy (SRS), a forefront characterization tool consisting of thermogravimetric analysis coupled with gas chromatography and mass spectrometry (TG-GC-MS) and aberration-corrected high-resolution transmission electron microscopy imaging series at 80 kV to unambiguously demonstrate the covalent binding of the molecular linkers. The present study shows that the SWCNT functionalization using iodide derivatives leads to the best results in terms of bulk functionalization homogeneity ( Hbulk) and degree of addition. Phenylene linkers yield the highest degree of functionalization, whereas biphenylene units induce a higher surface area with an increase in the thermal stability and an improved electrochemical performance in the oxygen reduction reaction (ORR). This work illustrates the importance of molecular engineering in the design of novel functional materials and provides important insights into the understanding of basic principles of reductive cross-linking of carbon nanotubes.

Nitro Lignin-Derived Nitrogen-Doped Carbon as an Efficient and Sustainable Electrocatalyst for Oxygen Reduction.

The use of lignin as a precursor for the synthesis of materials is nowadays considered very interesting from a sustainability standpoint. Here we illustrate the synthesis of a micro-, meso-, and macroporous nitrogen-doped carbon (NDC) using lignin extracted from beech wood via alkaline hydrothermal treatment and successively functionalized via aromatic nitration. The so obtained material is thus carbonized in the eutectic salt melt KCl/ZnCl2. The final NDC shows an excellent activity as electrocatalyst for the oxygen reduction reaction.