Identifying the Role of the Cationic Geometric Configuration in Spinel Catalysts for Polysulfide Conversion in Sodium-Sulfur Batteries

An AB2X4 spinel structure, with tetrahedral A and octahedral B sites, is a paradigmatic class of catalysts with several possible geometric configurations and numerous applications, including polysulfide conversion in metal-sulfur batteries. Nonetheless, the influence of the geometric configuration and composition on the mechanisms of catalysis and the precise manner in which spinel catalysts facilitate the conversion of polysulfides remain unknown. To enable controlled exposure of single active configurations, herein, Cotd2+ and Cooh3+ in Co3O4 catalysts for sodium polysulfide conversion are in large part replaced by Fetd2+ and Feoh3+, respectively, generating FeCo2O4 and CoFe2O4. Through an examination of electrochemical activation energies, the characterization of symmetric cells, and theoretical calculations, we determine that Cooh3+ serves as the active site for the breaking of S-S bonds, while Cotd2+ functions as the active site for the formation of S-Na bonds. The current study underlines the subtle relationship between activity and geometric configurations of spinel catalysts, providing unique insights for the rational development of improved catalysts by optimizing their atomic geometric configuration.This work was supported by the Innovation fund for small and medium-sized Enterprises in Gansu Province (No. 22CX3JA006), Lanzhou Talent Innovation and Entrepreneurship Project (No. 2022-2-81), National Natural Science Foundation of China (Grant Nos. 61801200), and partially by the Fundamental Research Funds for the Central Universities (Grant No.: lzujbky-2021-it33). J.S. Li acknowledges financial support from the Natural Science Foundation of Sichuan province (2022NSFSC1229). ICN2 acknowledges funding from Generalitat de Catalunya 2021SGR00457. This study is part of the Advanced Materials programme and was supported by MCIN with funding from European Union NextGenerationEU (PRTR-C17.I1) and by Generalitat de Catalunya. The authors thank the support from the project NANOGEN (PID2020-116093RB-C43), funded by MCIN/AEI/10.13039/501100011033/ and by “ERDF A way of making Europe”, by the “European Union”. ICN2 is supported by the Severo Ochoa program from Spanish MCIN/AEI (Grant No.: CEX2021-001214-S) and is funded by the CERCA Programme/Generalitat de Catalunya. AGM has received funding from Grant RYC2021-033479-I funded by MCIN/AEI/10.13039/501100011033 and by European Union NextGenerationEU/PRTR. L.B. thanks the Ministry of Science and Innovation of Spain through the OXISHOT project (PID2021-128410OB-I00). ICN2 acknowledges funding from Project IU16-014206 (METCAM-FIB) from Generalitat de Catalunya and by “ERDF A way of making Europe”, by the European Union.With funding from the Spanish government through the ‘Severo Ochoa Centre of Excellence’ accreditation (CEX2019-000917-S).Peer reviewe

Mechanical behaviour of carbon nanotube fibers spun from vertically aligned arrays

Due to their unique structure, carbon nanotubes (CNTs) have excellent properties including high thermal conductivity, high current capacity, and superior mechanical and chemical properties, which make them excellent one-dimensional materials with potentially wide applications. In order to utilize these properties, nano-scale CNTs must be prepared into macro-scale assemblies. CNT fibers, notably the fibers spun from vertically aligned CNT arrays, have therefore attracted great attention in recent years. Although these fibers have the inherent potential of leveraging on the excellence of CNTs, the individual fiber’s properties strongly depend on their structures and morphologies. The strength of the best fibers is still much lower than the theoretical and experimental results from individual CNTs or small CNT bundles. Moreover, up to now, no attention has been paid to the influence of test condition, specifically strain rate effects on mechanical performances of CNT fibers. The goal of this research is to investigate the structural properties of CNT fibers, and study the strain rate dependent failure mechanisms and its relation to the fibers’ structure and morphology of failure. CNT fibers were directly spun from vertically aligned CNT arrays. The mechanical behaviors of CNT fibers were investigated for a wide range of tensile strain rates employing several characterization techniques, including static/dynamic tensile tests, polarized Raman spectroscopy, and fiber fracture surface observations. From these systematic studies, the failure mechanism of CNT fibers was analyzed and their relation to fibers’ structures was deduced. The results could be divided into the following sections: Firstly, the general properties of CNT fibers that were spun from CNT arrays were provided. The mechanical strength and electrical conductivity of CNT fibers were measured statistically. The structural properties of CNT fibers were characterized by Raman spectroscopy and non-uniform twisting was found inside the fiber. It was found that once the spinning process was fixed, the uniformity of fibers’ mechanical strength and electrical conductivity was fixed. Based on these systematic experiments, a fixed fiber spinning process was determined for the follow-on studies. Then, CNT fibers were tensile tested in a wide range of strain rates. It was found that the mechanical response of CNT fibers exhibited a strain rate strengthening effect, and two different failure mechanisms dominated at high and low tensile strain rates, respectively. The key factors, inter-tube slippage and CNT alignment, that limited the mechanical properties of current CNT fibers were then discussed and possible failure mechanisms were proposed based on fibers’ mechanical behavior and the observations of fracture surfaces by scanning electron microscopy (SEM). Mechanical behavior and reliability of CNT fibers were then studied in detail under low strain rates. Static mechanical test and electrical measurement, combined with an in situ Raman spectroscopy, were used to monitor the load transfer and failure process. The morphology of CNTs in the fibers was characterized by transmission electron microscopy (TEM). The results further confirmed that the performance of CNT fibers at low strain rates was determined by slippage and breakage between nanotubes or small tube bundles. An improvement approach was proposed and studied by introducing a second element with a 3-D network structured polymer to improve the load transfer efficiency between CNTs (or small CNT bundles) inside the fibers. It was demonstrated that the CNTs in the fiber could be effectively constrained, resulting in more effective load transfer. Comparatively, at high strain rates, the structure dependent mechanical performance of the CNT fibers was investigated. A facile approach was applied to improve the alignment of CNTs by re-wetting, swelling, and re-drying the CNT fibers in a dilute HCl solution. The alignment of CNTs and its influence on fibers were examined. The mechanical behaviors of CNT fibers before and after post-treatment were compared as well. It was found that the alignment of CNTs showed significant influence on fibers’ mechanical strength. Finally, statistical methods employing modified Weibull models was developed to analyze the distribution of mechanical strength and failure mechanisms of fibril materials, was introduced to examine the breaking mechanisms under different strain rates. Variation of fibers’ strength and its dependence on fibers’ diameters and strain rates were investigated. Based on statistical analysis of our experimental data, further mathematical evidence could be deduced to support the failure mechanisms proposed for low and high strain rates.DOCTOR OF PHILOSOPHY (MAE

Microfiber devices based on carbon materials

Microfiber devices are able to extend the micro/nano functionalities of materials or devices to macroscopic scale with excellent flexibility and weavability, promising a variety of unique applications and, sometimes, also improved performance as compared with bulk counterparts. The fiber electrodes in these devices are often made of carbon materials (e.g., carbon nanotubes and graphene) because of their exceptional electrical, mechanical, and structural properties. Covering the latest developments and aiming to stimulate more exciting applications, we comprehensively review the preparation and applications of carbon-microfiber devices on energy conversion and storage, electronics, sensors and actuators.ASTAR (Agency for Sci., Tech. and Research, S’pore)Published versio

Fabrication of Microscale Carbon Nanotube Fibers

Carbon nanotubes (CNTs) have excellent mechanical, chemical, and electronic properties, but realizing these excellences in practical applications needs to assemble individual CNTs into larger-scale products. Recently, CNT fibers demonstrate the potential of retaining CNT's superior properties at macroscale level. High-performance CNT fibers have been widely obtained by several fabrication approaches. Here in this paper, we review several key spinning techniques including surfactant-based coagulation spinning, liquid-crystal-based solution spinning, spinning from vertical-aligned CNT arrays, and spinning from CNT aerogel. The method, principle, limitations, and recent progress of each technique have been addressed, and the fiber properties and their dependences on spinning parameters are also discussed

Design of manganese dioxide for supercapacitors and zinc-ion batteries: similarities and differences

Energy storage devices, e.g., supercapacitors (SCs) and zinc-ion batteries (ZIBs), based on aqueous electrolytes, have the advantages of rapid ion diffusion, environmental benignness, high safety and low cost. Generally, SCs provide excellent power density with the capability of fast charge/discharge, while ZIBs offer high energy density by storing more charge per unit weight/volume. Although the charge storage mechanisms are considered different, manganese dioxide (MnO2) has proven to be an appropriate electrode material for both SCs and ZIBs because of its unique characteristics, including polymorphic forms, tunable structures and designable morphologies. Herein, the design of MnO2-based materials for SCs and ZIBs is comprehensively reviewed. In particular, we compare the similarities and differences in utilizing MnO2-based materials as active materials for SCs and ZIBs by highlighting their corresponding charge storage mechanisms. We then introduce a few commonly adopted strategies for tuning the physicochemical properties of MnO2 and their specific merits. Finally, we discuss the future perspectives of MnO2 for SC and ZIB applications regarding the investigation of charge storage mechanisms, materials design and the enhancement of electrochemical performance

Failure mechanisms of carbon nanotube fibers under different strain rates

Strong and uniform carbon nanotube (CNT) fibers with tensile strength around 1.2 GPa were prepared from vertically aligned CNT arrays, and their mechanical properties were studied using a wide range of tensile strain rates. The cyclic load/unload process, polarized Raman measurements, and fiber fracture surfaces were also used to study the failure mechanism of the CNT fibers. It is found that the fibers exhibit a strain-rate strengthening effect, and have different failure mechanisms at high and low strain rates. The key factors that limit the mechanical properties of the CNT fibers were then investigated based on a failure mechanism analysis: inter-tube slippage happens at low strain-rates, and “cascade-like” breaking dominates at high strain-rates. The maximum strength of the fibers appears at high strain rates, and is mainly determined by the CNT alignment

Self-powered, visible-light photodetector based on thermally reduced graphene oxide–ZnO (rGO–ZnO) hybrid nanostructure

Here we report a new type of self-powered, visible-light photodetector fabricated from thermally reduced rGO–ZnO hybrid nanostructure. The photocurrent generation of the photodetectors under zero-bias enables hybrid rGO–ZnO devices to work like photovoltaic cells, which could power themselves without electrical power input. The thermal treatment at elevated temperature not only reduces graphene oxide (GO) into reduced graphene oxide (rGO), but also dopes the ZnO nanoparticles with carbon atoms, enabling their visible-light photoresponse capability. The pronounced and fast photocurrent generation was attributed to the efficient charge transfer between the rGO and carbon-doped ZnO nanoparticles, which were in intimate contact. The efficient charge transfer of the rGO–ZnO hybrid nanostructures also indicates that there could be applications in other light energy harvesting devices, including solar cells, sensors and visible-light photocatalysis

Probing structure and strain transfer in dry-spun carbon nanotube fibers by depth-profiled Raman spectroscopy

The structural properties of dry-spun carbon nanotube (CNT) fibers were characterized by depth-profiled polarized Raman spectroscopy. Results showed that the twisting cannot be fully transferred through the whole fiber and the CNTs within fibers possess non-uniform alignments in radial direction. Effective twisting depth was determined from the residue strain distribution within fibers. Larger surface twisting angles can result in higher residue strain, better alignment degree, and deeper twisting depth. This research suggests a balance should be built between the enhancement of CNT interactions and the increase of defect density to obtain high-performance fibers.Published versio

Electrochemical chlorine sensor with multi-walled carbon nanotubes as electrocatalysts

It has been reported for the first time that an electrochemical gas sensor modified with multi-walled carbon nanotubes (MWNTs) film as electrocatalyst was fabricated for the determination of chlorine (Cl2).Here, MWNTs and graphite were compared with each other in terms of their electrochemical properties using cyclic voltammetry. Cl2 gas was allowed through the cathode surface of the sensor and the resulting galvanic effects were monitored. Results indicated that both of the MWNTs and graphite have the electrocatalytic activity for the reduction of Cl2 while the MWNTs-modified electrode exhibited a higher accessible surface area in electrochemical reactions, excellent sensitivity, stable response, reproducibility and recovery for the determination of Cl2. Keywords: Carbon nanotubes, Sensor, Chlorine, Electrocatalyst, Cyclic voltammetr