6 research outputs found
Elucidating Gas Evolution of Prussian White Cathodes for Sodium‐ion Battery Application: The Effect of Electrolyte and Moisture
As global energy storage demand increases, sodium-ion batteries are often considered as an alternative to lithium-ion batteries. Hexacyanoferrate cathodes, commonly referred to as Prussian blue analogues (PBAs), are of particular interest due their low-cost synthesis and promising electrochemical response. However, because they consist of ~50 wt% cyanide anions, a possible release of highly toxic cyanide gases poses a significant safety risk. Previously, we observed the evolution of (CN)2 during cycling via differential electrochemical mass spectrometry (DEMS), but were unable to determine a root cause or mechanism. In this work, we present a systematical investigation of the gas evolution of Prussian white (PW) with different water content via DEMS. While H2 is the main gas detected, especially in hydrated PW and during overcharge (4.6 V vs. Na+/Na), the evolution of CO2 and (CN)2 depends on the electrolyte conductive salt. The use of oxidative NaClO4 instead of NaPF6 is the leading cause for the formation of (CN)2. Mass spectrometric evidence of trace amounts of HCN is also found, but to a much lower extent than (CN)2, which is the dominant safety risk when using NaClO4-containing electrolyte, which despite being a good model salt, is not a viable option for commercial applications
Electrochemical oxidation of dihydronicotinamide adenine dinucleotide (NADH) : Comparison of highly oriented pyrolytic graphite (HOPG) and polycrystalline boron-doped diamond (pBDD) electrodes
The electro-oxidation of nicotinamide adenine dinucleotide (NADH) is studied at bare surfaces of highly oriented pyrolytic graphite (HOPG) and semi-metallic polycrystalline boron-doped diamond (pBDD). A comparison of these two carbon electrode materials is interesting because they possess broadly similar densities of electronic states that are much lower than most metal electrodes, but graphite has carbon sp2-hybridization, while in diamond the carbon is sp3-hybridised, with resulting major differences in bulk structure and surface termination. Using cyclic voltammetry (CV), it is shown that NADH oxidation is facile at HOPG surfaces but the reaction products tend to strongly adsorb, which causes rapid deactivation of the electrode activity. This is an important factor that needs to be taken into account when assessing HOPG and its intrinsic activity. It is also shown that NADH itself adsorbs at HOPG, a fact that has not been recognized previously, but has implications for understanding the mechanism of the electro-oxidation process. Although pBDD was found to be less susceptible to surface fouling, pBDD is not immune to deterioration of the electrode response, and the reaction showed more sluggish kinetics on this electrode. Scanning electrochemical cell microscopy (SECCM) highlights a significant voltammetric variation in electroactivity between different crystal surface facets that are presented to solution with a pBDD electrode. The electroactivity of different grains correlates with the local dopant level, as visualized by field emission-scanning electron microscopy. SECCM measurements further prove that the basal plane of HOPG has high activity towards NADH electro-oxidation. These new insights on NADH voltammetry are useful for the design of optimal carbon-based electrodes for NADH electroanalysis
Electrochemical oxidation of dihydronicotinamide adenine dinucleotide (NADH): comparison of highly oriented pyrolytic graphite (HOPG) and polycrystalline boron-doped diamond (pBDD) electrodes
Nanoscale Surface Charge Visualization of Human Hair
The
surface charge and topography of human hair are visualized
synchronously at the nanoscale using scanning ion conductance microscopy
(SICM), a scanning nanopipette probe technique that uses local ion
conductance currents to image the physicochemical properties of interfaces.
By combining SICM data with finite element method (FEM) simulations
that solve for ion transport at the nanopipette under bias, one is
able to quantitatively correlate colocated surface charge and topography.
The hair samples studied herein, from a 25-year-old Caucasian male
with light hair (as an exemplar), reveal that untreated hair, in areas
ca. 1 cm from the root, has a fairly uniform negative charge density
of ca. −15 mC/cm–2 (in pH 6.8 aqueous solution),
with some higher magnitude negative values localized near the boundaries
between hair cuticles. Common chemical treatments result in varying
degrees of charge heterogeneity. A bleach treatment produces some
highly negatively charged localized regions (−80 to −100
mC/cm–2 at pH 6.8), due to hair damage, while a
chemical conditioner treatment causes an overall increase in the homogeneity
of the surface charge, together with a shift in the surface charge
to positive values. Bleached surfaces are temporarily repaired to
some extent through the use of a conditioner, as judged by the surface
charge values. Finally, SICM is able to detect differences in the
surface charge density of hair at different distances from the root
(equivalent to hair age). Presently, the assessment of hair surface
charge mainly relies on zeta-potential measurements which lack spatial
resolution, among other drawbacks. In contrast, SICM enables quantitative
surface charge mapping that should be beneficial in deepening understanding
of the physicochemical properties of hair and lead to the rational
development of new treatments and the assessment of their efficacy
at the nanoscale. Given the widespread interest in the surface charge
properties of interfaces, this work further demonstrates that SICM
should generally become an important characterization tool for surface
analytical chemists
Ageing analysis and asymmetric stress considerations for small format cylindrical cells for wearable electronic devices
Performance assessments on miniature cylindrical cells used in Fitbit Flex 2 fitness trackers have been performed to understand the dominant ageing modes and small format implications. We utilise electrochemical testing, x-ray photoelectron Spectroscopy (XPS), x-ray computed tomography (XCT) and scanning electron microscopy (SEM), to reveal device and component structural features and changes. The cell maintains 82% cell capacity retention after 500 continuous charging and discharging cycles at 3.0–4.35 V, 0.75C rate at 20 °C. The anode shows severe delamination due to high bending stress exerted on the cell components, however this seemingly has minimum impact on the electrochemical performance if the coating is sufficiently compressed in the jelly roll with a good electrical contact. After ageing, the surface layers continue to grow, with more LiF found on the cathode and anode. The formation of LiF is discussed and we suggest the main ageing mechanism of the Fitbit cell is related to increasing charge transfer resistance due to the transportation of Li+ ions being inhibited by the thicker surface layer, which contains LiF. That preferential delamination on the inner sides of the electrode coatings was observed consistently opens up an interesting avenues for advances in cylindrical cell manufacturing at large
Dissolution of Bicalutamide Single Crystals in Aqueous Solution: Significance of Evolving Topography in Accelerating Face-Specific Kinetics
The
dissolution kinetics of individual microscale bicalutamide
(BIC) form-I crystals are tracked over time using in situ atomic force
microscopy (AFM), with the evolution of crystal morphology used to
obtain quantitative data on dissolution kinetics via finite element
method (FEM) modeling of the dissolution reaction-diffusion problem.
Dissolution is found to involve pit formation and roughening on all
dissolving surfaces of the BIC crystal, and this has a strong influence
on the overall dissolution process and kinetics. While all of the
exposed faces (100), {051}, and {1̅02} show dissolution kinetics
that are largely surface-kinetic controlled, each face has an intrinsic
dissolution characteristic that depends on the degree of hydrogen
bonding with aqueous solution, with hydrogen bonding promoting faster
dissolution. Moreover, as dissolution proceeds with pitting and roughening,
the rate accelerates considerably, so that there is an increasing
diffusion contribution. Such insight is important in understanding
the oral administration of poorly soluble active pharmaceutical ingredients
(APIs) in crystal form. Evidently, surface roughening and defects
greatly enhance dissolution kinetics, but the evolving crystal topography
during dissolution leads to complex time-dependent kinetics that are
important for modeling and understanding API release rates