Herein, we report energy storage devices, which are based on densely packed, vertically aligned MoS2 (VA-MoS2) or planar oriented MoS2 (PO-MoS2) and compare their electrochemical performances. The VA-MoS2 films have been processed by chemical vapor deposition (CVD) to reach unprecedented micron-scale thick films while maintaining the vertical alignment for the whole thickness. The VA-MoS2 and the PO-MoS2 films form a high-performance Li-ion electrode, reaching the theoretical limits of reversible capacity for this material (800 mAh/g; twice the specific capacity of graphite). The vertical alignment allows faster charge-discharge rates while maintaining a high specific capacity (C-rate measurements). Noteworthy, the reversible cycling of the Li-ion electrode also benefits from the vertical alignment. In this article, we present the full synthesis, structural and electrochemical characterization of VA-MoS2 along with the properties of PO-MoS2 to deconvolute the intrinsic properties of MoS2 from the influence of the layers’ orientation.
Platinum is the most used electrocatalyst in proton exchange
membrane
fuel cells (PEMFCs). Nonetheless, it suffers from various types of
degradation. Identical location electron microscopy has previously
been used to observe local catalyst changes under accelerated stress
tests (ASTs), giving insight into how individual catalyst particles
degrade. However, it is important that such studies are carried out
under relevant reaction conditions, as these can differ substantially
between liquid half-cells and real PEMFC conditions. In this work,
a single cell PEMFC was used to study the degradation of a commercial
Pt-catalyzed membrane electrode assembly by performing square wave
voltage ASTs in a potential range of 0.6 to 1.0 V. Identical location
scanning electron microscopy (IL-SEM) was used to follow the degradation
of the cathodic catalytic layer (CL) throughout 14,000 AST cycles.
From the IL-SEM, we can conclude that the Pt nanoparticles degrade
via Ostwald ripening, crystal migration, and coalescence. Small Pt
nanoparticles agglomerate to larger particles or dissolve and redeposit
to more stable particles, increasing the average particle size during
the ASTs. In addition, cross-sectional TEM images show thinning of
the ionomer layer during the AST procedure. The IL-SEM technique facilitates
observation of local degradation of the CL in real PEMFCs, which will
help to understand different degradation mechanisms, allowing for
better solutions to be designed.
The efficient formulation of silicon based, highloading electrode with good capacity retention and cycling stability remains challenging. To gain a better understanding of the ongoing processes and failure mechanisms occurring during battery performance, operando micro-Raman spectroscopy is helpful to map the active silicon sites. Herein, we present the investigation of the electrochemical performance of anodes composed of plasmonic metal (Ag and Au) decorated silicon, designed for enhancing Raman signal. Following the discovery that only a partial amount of the electroactive material undergoes lithiation in the first cycle, we show that the plasmonic metal tips can enhance the connectivity of the Si particles. The micro-Raman mapping of electroactive silicon material reveals how the plasmonic metals influence the distribution of silicon active sites during battery cycling. The ratio of electroactive Si is found to increase from Si to Si/Au and Si/Ag electrodes, and the results are explained in terms of interconnectivity of the particles.
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