The quasi-liquid layer of ice revisited : the role of temperature gradients and tip chemistry in AFM studies

In this work, we present new results of atomic force microscopy (AFM) force curves over pure ice at different temperatures, performed with two different environmental chambers and different kinds of AFM tips. Our results provide insight to resolve the controversy on the interpretation of experimental AFM curves on the ice\u2013air interface for determining the thickness of the quasi-liquid layer (QLL). The use of a Mini Environmental Chamber (mEC) that provides an accurate control of the temperature and humidity of the gases in contact with the sample allowed us for the first time to get force curves over the ice\u2013air interface without jump-in (jump of the tip onto the ice surface, widely observed in previous studies). These results suggest a QLL thickness below 1 nm within the explored temperature range ( 127 to 122\u25e6C). This upper bound is significantly lower than most of the previous AFM results, which suggests that previous authors overestimate the equilibrium QLL thickness, due to temperature gradients, or indentation of ice during the jump-in. Additionally, we proved that the hydrophobicity of AFM tips affects significantly the results of the experiments. Over-all, this work shows that, if one chooses the experimental conditions properly, the QLL thicknesses obtained by AFM lie over the lower bound of the highly disperse results re-ported in the literature. This allows estimating upper boundaries for the QLL thicknesses, which is relevant to validate QLL theories and to improve multi phase atmospheric chemistry models

Increased performance of an all-organic redox flow battery model: Via nitration of the [4]helicenium DMQA ion electrolyte

Redox flow batteries (RFBs), through their scalable design and virtually unlimited capacity, are promising candidates for large-scale energy storage. While recent advances in the development of redox-active bipolar organic molecules satisfy the prerequisites for the pioneering emergence of symmetrical all-organic redox flow batteries (SORFBs), problems of low durability or low energy density remain a bottleneck for their wide-spread application. The present work reports that nitration of the [4]helicenium dimethoxyquinacridinium (DMQA+) ion core (NO2C+) results in a significantly enhanced electrochemical performance of DMQA+ as the electrolyte for SORFBs. The physical and kinetic properties of NO2C+ were evaluated by cyclic voltammetry (CV) and UV-visible spectroscopy in acetonitrile and compared to those of its precursor (HC+). The ability for electron storage of NO2C+ was investigated in three different types of static H-cell experiments. In the first experiment, NO2C+ provided an open circuit voltage (OCV) of 2.24 V resulting in demonstrated good stability, as well as high coulombic (>98%) efficiencies, over more than 200 charge/discharge cycles. In the second experiment, a charge-discharge cycling over the entire redox window of NO2C+ (OCV > 3 V) resulted in 80 cycles at a potential energy density above 12 W h L-1. During the last experiment, a bipolarization stress-test was performed in which NO2C+ demonstrated a remarkable durability of 90 cycles at 100% load with a perfect retention of capacity and coulombic efficiency. The enhanced electrochemical performance of this redox material highlights that DMQA+ ions are robust and versatile materials for the emergence of SORFBs. © 2021 The Royal Society of Chemistry.Open access journalThis item from the UA Faculty Publications collection is made available by the University of Arizona with support from the University of Arizona Libraries. If you have questions, please contact us at [email protected]

Environmental chamber with controlled temperature and relative humidity for ice crystallization kinetic measurements by atomic force microscopy

The present work describes the development of an environmental chamber (EC), with temperature and humidity control, for measuring ice growth kinetics over a substrate with an atomic force microscope (AFM). The main component of the EC is an AFM fluid glass cell. The relative humidity (RH) inside the EC is set by the flow of a controlled ratio of dry and humid nitrogen gases. The sample temperature is fixed with an AFM commercial accessory, while the temperature of the nitrogen gas inside the EC is controlled by circulating cold nitrogen vapor through a copper cooler, specially designed for this purpose. With this setup, we could study the growth rate of ice crystallization over a mica substrate by measuring the force exerted between the tip and the sample when they approach each other as a function of time. This experimental development represents a significant improvement with respect to previous experimental determinations of ice growth rates, where RH and temperature of the air above the sample were determined far away from the ice crystallization regions, in opposition to the present work