Lignin-derived sulfonate base metal-free N, S co-doped carbon microspheres doped with different nitrogen sources as catalysts for oxygen reduction reactions.

The oxygen reduction reaction (ORR) is an important step in the widespread application of metal-air batteries, so it is necessary to study and develop low-cost and efficient metal-free carbon-based catalysts to catalyze the ORR reaction. Heteroatomic doping, especially N and S co-doped carbon materials, has received much focus as a promising ORR catalyst. Meanwhile, the lignin material has high carbon content, wide source, and low price, and has wide application prospects for the preparation of carbon material catalysts. Here we report a hydrothermal­carbonation preparation method for the synthesis of carbon microspheres by utilizing lignin derivatives as carbon precursors. And a variety of N, S co-doped carbon microsphere materials were prepared by adding different nitrogen sources (urea, melamine, NH4Cl) to the microspheres. The N, S co-doped carbon microspheres (NSCMS-MLSN) catalysts achieved with NH4Cl as the nitrogen source displayed superior RR catalytic activity with high half-wave potential (E1/2 = 0.83 V vs. RHE) and current density (JL = 4.78 mA cm-2). This work provides some references on the method of preparing carbon materials co-doped with N and S and the choice of nitrogen sources.

Magnesium lignosulfonate-derived N, S co-doped 3D flower-like hierarchically porous carbon as an advanced metal-free electrocatalyst towards oxygen reduction reaction.

The development of metal-free electrocatalytic materials that are economical, friendly to the environment, and efficiency towards the oxygen reduction reaction (ORR) is of significant interest. Hence, this paper synthesizes nitrogen and sulfur co-doped three-dimensional magnesium lignosulfonate (MLS-derived) flower-like hierarchical porous carbon (NSLPC) materials by a simple and green method. The synthesized NSLPC uses magnesium lignosulfonate as the sulfur source and carbon precursor, melamine as nitrogen source, MgO as hard template, and ZnCl2 as the activator. We also investigated the effect of the ratio of MgO to ZnCl2 on the catalyst performance. When the ratio of MgO to ZnCl2 is 10:0.5, NSLPC-1005 possesses the highest ORR activity with an enormous surface area (1752.54 m2 g-1), abundant active sites, and a hierarchical porous network structure. In alkaline media, NSLPC-1005 has an initial potential of 0.97 V, as well as an excellent half-potential of 0.86 V (vs. Hg/HgO), and an ultimate current density of 5.35 mA cm-2. It exhibits attractive ORR performance as well as outstanding cyclic stability that are comparable to commercial Pt/C electrocatalysts. This research developed an effective approach to synthesize metal-free carbon materials with high activity and long-term durability as electrocatalysts, which have a promising application in sustainable energy conversion technology.

Route to stable dispersion-managed mode-locked Yb-doped fiber lasers with near-zero net cavity dispersion.

We numerically investigate the stability of a dispersion-managed mode-locked Yb-doped fiber laser of near-zero net cavity dispersion. The instability is primarily due to the filtering effect of the chirped fiber Bragg grating. The size of the unstable region is dependent on the modulation depth of the saturable absorbers. At modulation depth higher than 30%, stable mode-locking can operate throughout the dispersion region. Based on the simulation results, stable mode-locking around zero cavity dispersion is experimentally viable by a SESAM with a 34% modulation depth. The fiber laser can generate laser pulses with a 17-nm spectral bandwidth and a 139-fs dechirped pulse duration.