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Lithium-sulfur batteries represent an attractive option for energy storage applications. A deeper understanding of the multistep lithium-sulfur reactions and the electrocatalytic mechanisms are required to develop advanced, high-performance batteries. We have systematically investigated the lithium-sulfur redox processes catalyzed by a cobalt single-atom electrocatalyst (Co-SAs/NC) via operando confocal Raman microscopy and x-ray absorption spectroscopy (XAS). The real-time observations, based on potentiostatic measurements, indicate that Co-SAs/NC efficiently accelerates the lithium-sulfur reduction/oxidation reactions, which display zero-order kinetics. Under galvanostatic discharge conditions, the typical stepwise mechanism of long-chain and intermediate-chain polysulfides is transformed to a concurrent pathway under electrocatalysis. In addition, operando cobalt K-edge XAS studies elucidate the potential-dependent evolution of cobalt’s oxidation state and the formation of cobalt-sulfur bonds. Our work provides fundamental insights into the mechanisms of catalyzed lithium-sulfur reactions via operando methods, enabling a deeper understanding of electrocatalysis and interfacial dynamics in electrical energy storage systems. 

Abstract The complex interplay and only partial understanding of the multi-step phase transitions and reaction kinetics of redox processes in lithium–sulfur batteries are the main stumbling blocks that hinder the advancement and broad deployment of this electrochemical energy storage system. To better understand these aspects, here we report operando confocal Raman microscopy measurements to investigate the reaction kinetics of Li–S redox processes and provide mechanistic insights into polysulfide generation/evolution and sulfur deposition. Operando visualization and quantification of the reactants and intermediates enabled the characterization of potential-dependent rates during Li–S redox and the linking of the electronic conductivity of the sulfur-based electrode and concentrations of polysulfides to the cell performance. We also report the visualization of the interfacial evolution and diffusion processes of different polysulfides that demonstrate stepwise discharge and parallel recharge mechanisms during cell operation. These results provide fundamental insights into the mechanisms and kinetics of Li–S redox reactions. 

A systematic study explores transition metal nitrides for nonprecious oxygen reduction electrocatalysts in alkaline fuel cells. 

Metal oxide semiconductors have attracted much attention due to their versatility in different applications, ranging from biosensing to green energy-harvesting technologies. Among these metal oxides, oxide-based diluted magnetic semiconductors have also been proposed for fuel cell applications, especially for the oxygen reduction reaction (ORR) and the oxygen evolution reaction. However, the catalytic mechanism has been proposed to follow a two-electron pathway, forming hydrogen peroxide, instead of the four-electron pathway. Herein, we report cobalt-doped zinc oxide (CoxZn1–xO, 0 < x < 0.018) materials prepared using a co-precipitation method suitable for the electrocatalytic production of hydrogen peroxide. The electrocatalytic performance of CoxZn1–xO materials showed up to 60% hydrogen peroxide production with onset potentials near 649 mV, followed by the two-electron ORR mechanism. Ex situ X-ray absorption spectroscopy experiments at the Co K-edge demonstrated the presence of Co(II) ions at tetrahedral sites within the ZnO lattice. 

Li–S batteries have attracted great attention for their combined advantages of potentially high energy density and low cost. To tackle the capacity fade from polysulfide dissolution, we have developed a confinement approach by in situ encapsulating sulfur with a MOF-derived CoS 2 in a carbon framework (S/Z-CoS 2 ), which in turn was derived from a sulfur/ZIF-67 composite (S/ZIF-67) via heat treatment. The formation of CoS 2 was confirmed by X-ray absorption spectroscopy (XAS) and its microstructure and chemical composition were examined through cryogenic scanning/transmission electron microscopy (Cryo-S/TEM) imaging with energy dispersive spectroscopy (EDX). Quantitative EDX suggests that sulfur resides inside the cages, rather than externally. S/hollow ZIF-67-derived CoS 2 (S/H-CoS 2 ) was rationally designed to serve as a control material to explore the efficiency of such hollow structures. Cryo-STEM-EDX mapping indicates that the majority of sulfur in S/H-CoS 2 stays outside of the host, despite its high void volumetric fraction of ∼85%. The S/Z-CoS 2 composite exhibited highly improved battery performance, when compared to both S/ZIF-67 and S/H-CoS 2 , due to both the efficient physical confinement of sulfur inside the host and strong chemical interactions between CoS 2 and sulfur/polysulfides. Electrochemical kinetics investigations revealed that the CoS 2 could serve as an electrocatalyst to accelerate the redox reactions. The composite could provide an areal capacity of 2.2 mA h cm −2 after 150 cycles at 0.2C and 1.5 mA h cm −2 at 1C. This novel material provides valuable insights for further development of high-energy, high-rate and long-life Li–S batteries. 

Designing high-performance nonprecious electrocatalysts to replace Pt for the oxygen reduction reaction (ORR) has been a key challenge for advancing fuel cell technologies. Here, we report a systematic study of 15 different AB 2 O 4 /C spinel nanoparticles with well-controlled octahedral morphology. The 3 most active ORR electrocatalysts were MnCo 2 O 4 /C, CoMn 2 O 4 /C, and CoFe 2 O 4 /C. CoMn 2 O 4 /C exhibited a half-wave potential of 0.89 V in 1 M KOH, equal to the benchmark activity of Pt/C, which was ascribed to charge transfer between Co and Mn, as evidenced by X-ray absorption spectroscopy. Scanning transmission electron microscopy (STEM) provided atomic-scale, spatially resolved images, and high-energy-resolution electron-loss near-edge structure (ELNES) enabled fingerprinting the local chemical environment around the active sites. The most active MnCo 2 O 4 /C was shown to have a unique Co-Mn core–shell structure. ELNES spectra indicate that the Co in the core is predominantly Co 2.7+ while in the shell, it is mainly Co 2+ . Broader Mn ELNES spectra indicate less-ordered nearest oxygen neighbors. Co in the shell occupies mainly tetrahedral sites, which are likely candidates as the active sites for the ORR. Such microscopic-level investigation probes the heterogeneous electronic structure at the single-nanoparticle level, and may provide a more rational basis for the design of electrocatalysts for alkaline fuel cells.