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Single-atom catalysts have expanded the design paradigm for oxygen reduction reaction (ORR) relying on nonplatinum group metals (non-PGM). Here, density functional theory calculations were performe...

This study reports the preparation of a set of hybrid materials consisting of molybdenum disulfide (MoS2) nanopatches on reduced graphene oxide (rGO) nanosheets by microwave specific heating of graphene oxide and molecular molybdenum precursors followed by thermal annealing in 3% H2 and 97% Ar. The microwave process converts graphene oxide to ordered rGO nanosheets that are sandwiched between uniform thin layers of amorphous molybdenum trisulfide (MoS3). The subsequent thermal annealing converts the intermediate layers into MoS2 nanopatches with two-dimensional layered structures whose defect density is tunable by controlling the annealing temperature at 250, 325 and 600 °C, respectively. All three MoS2/rGO samples and the MoS3/rGO intermediate after the microwave step show a high Li-ion intercalation capacity in the initial 10 cycles (over 519 mA h gMoSx−1, ∼3.1 Li+ ions per MoS2) which is attributed to the small MoS2 nanopatches in the MoS2/rGO hybrids while the effect of further S-rich defects is insignificant. In contrast, the Zn-ion storage properties strongly depend on the defects in the MoS2 nanopatches. The highly defective MoS2/rGO hybrid prepared by annealing at 250 °C shows the highest initial Zn-ion storage capacity (∼300 mA h gMoSx−1) and close to 100% coulombic efficiency, which is dominated by pseudocapacitive surface reactions at the edges or defects in the MoS2 nanopatches. The fast fading in the initial cycles can be mitigated by applying higher charge/discharge currents or extended cycles. This study validates that defect engineering is critical for improving Zn-ion storage.

Nitrogen doping in carbon materials can modify the employed carbon material’s electronic and structural properties, which helps in creating a stronger metal-support interaction. In this study, the role of nitrogen doping in improving the durability of Pt catalysts supported on a three-dimensional vertically aligned carbon nanofiber (VACNF) array towards oxygen reduction reaction (ORR) was explored. The nitrogen moieties present in the N-VACNF enhanced the metal-support interaction and contributed to a reduction in the Pt particle size from 3.1 nm to 2.3 nm. The Pt/N-VACNF catalyst showed better durability when compared to Pt/VACNF and Pt/C catalysts with similar Pt loading. DFT calculations validated the increase in the durability of the Pt NPs with an increase in pyridinic N and corroborated the molecular ORR pathway for Pt/N-VACNF. Moreover, the Pt/N-VACNF catalyst was found to have excellent tolerance towards methanol crossover.

This study reports an approach to achieving stable 2 and 3 Li+ insertion, respectively, into vanadium pentoxide (V2O5) as lithium-ion battery (LIB) cathode materials using a core-shell structure based on a self-standing carbon nanofiber (CNF) membrane fabricated by an electrospinning process. Uniform coaxial V2O5 shells are coated onto continuous CNF cores via a pulsed electrodeposition. The materials analyses confirm that the V2O5 shell after 4 h of thermal annealing at 300 °C forms a partially hydrated amorphous structure. SEM and TEM images indicate that the uniform 30–50 nm thick V2O5 shell forms an intimate interface with the CNF core. Lithium insertion capacities up to 291 and 429 mAh g−1 are achieved in the voltage ranges of 4.0–2.0 V and 4.0–1.5 V, respectively, which are in good agreement with the theoretical values of 294 mAh g−1 for 2 Li+/V2O5 insertion and 441 mAh g−1 for 3 Li+/V2O5 insertion into crystalline V2O5 materials. Moreover, after 100 cycles, remarkable retention rates of 97% and 70% are obtained for 2 Li+/V2O5 and 3 Li+/V2O5 insertion, respectively. These results reveal that it is potentially feasible to fabricate the core-shell structure with electrospinning and electrodeposition processes to break the intrinsic limits of V2O5 and enabling this high-capacity cathode materials for future LIBs.

For both proton exchange membrane and direct methanol fuel cells, oxygen reduction reaction (ORR) is a performance-determining step. While Pt has been established as a standard bearer in ORR catalyst development, practical long-term applications require continuing effort to pursue active, stable, as well as affordable materials as alternatives to precious metals. In this talk, carbon-based vertically aligned carbon nanofibers (VACNFs), essentially a stack of conical graphitic structures, will be discussed as highly promising ORR catalysts. In an alkaline medium, at -0.4 V applied potential, the current density measured on VACNF cathode is twice more than that measured with commercial Pt catalyst (Pt/C). Molecular insights have been gained through the use of Density functional theory (DFT) calculations to: (1) reveal interactions of ORR intermediate species at the catalytically active sites; and (2) elucidate the origin of catalytic activities of VACNF under reaction conditions. Specifically, semi-periodic fishbone-like stacked graphene sheets, bare or terminated with H and OH, have been employed to understand the impact on ORR mechanism and corresponding reactivities. Bindings of O2, O, and OH at various configurations obtained from DFT are then used as descriptors to assess catalyst performance against simple Pt (111) and Pt (211) surfaces. In addition, Pt nanocatalysts supported on VACNF (Pt/VACNF), which can push the ORR current density even higher, will be discussed as well. Again, with DFT, a model with Pt anchored at the VACNF edges was proposed to investigate the functionalities of Pt/VACNF for alkaline ORR reactions. In this talk, both associative and dissociative ORR pathways will be analyzed based on the established models. In particular, the overall computed ORR thermodynamics are shown to be favored toward Pt/VACNF than Pt (111), consistent with experimental results. Nevertheless, the unique interactions of ORR intermediates such as O*, OOH*, and OH* with catalytic active VACNF edge sites, revealed by DFT calculations, suggest intriguing but also complex electrochemistries.

This study reports a 3D freeze-printing method that integrates inkjet printing and freeze casting to control both the microstructure and macroporosity via formation of ice microcrystals during printing. A viscous aqueous ink consisting of a molecular MoS2 precursor (ammonium thiomolybdate) mixed with graphene oxide (GO) nanosheets is used in the printing process. Post-treatments by freeze-drying and reductive thermal annealing convert the printed intermediate mixture into a hybrid structure consisting of MoS2 nanoparticles anchored on the surface of 2D rGO nanosheets in a macroporous framework, which is fully characterized with FESEM, TEM, XRD, Raman spectroscopy and TGA. The resulting hybrid MoS2-rGO aerogels are studied as anodes for sodium ion batteries. They present a high initial specific capacity over 429 mAh/g at C/3.3 rate in the potential range of 2.5–0.10 V (vs Na+/Na). The process involves both reversible 2 Na+ insertion and slow irreversible conversion of MoS2 to metallic Mo. At higher rates, the conversion reaction is suppressed and the electrode is dominated by fast Na+ intercalation with good stability. This demonstrates that the 3D printing technology can be used as a processing technique to control the materials properties for energy storage. Keywords: 3D printing, Hybrid MoS2/graphene aerogel, Freeze-casting, Sodium ion battery, Porous electrode materials