Nonthermal atmospheric pressure plasmas generated using noble gases act as gaseous electrodes capable of promoting electrochemical reactions when in contact with aqueous media. Despite overwhelming evidence in favor of the interfacial region between the plasma and solution being a reducing environment, the plasma-liquid interface has not been evaluated from an electrochemical perspective. Herein, we present an electrochemical method to measure the overpotential at the plasma-liquid interface of solutions containing organic redox couples. Negative overpotentials indicate that the interfacial region is a reducing environment, which is consistent with the results of colorimetric assays using redox indicators. The reaction rates are related to the magnitude of the overpotential, as expected from electrochemical kinetic models. The ability to characterize the overpotential of plasma-liquid interfaces adds to the plasma electrochemistry toolkit and contributes to efforts being made to tune gaseous electrodes for the rational control of redox reactions.
Nonequilibrium plasma treatments of carbon fiber reinforced thermoplastic (CFRTP) composites can activate the robust, chemically inert material into a nonequilibrium state that can be used to fabricate bonded structural assemblies with high toughness. The nonequilibrium surface is unstable; thus, the development of nondestructive characterization techniques is of paramount importance to assess the material before use. Herein, a discovery is reported that plasma-activated CFRTP surfaces are characterized by a sharp, well-defined surface potential distribution, the width of which is correlated to the fracture toughness of bonded assemblies. The activated surface has a maximum lifetime on the order of days to weeks and is hypothesized to be composed of metastable radical-ion complexes. The hypothesis is consistent with several independent pieces of evidence, including Kelvin probe force microscopy, contact angle measurements, magnetic force microscopy, and radical probe experiments.
Nonthermal
atmospheric pressure plasma in contact with a liquid
yields a variety of energetic photons, ions, and electrons, which
can be transported into the plasma–liquid interface (PLI).
Similar to the electrochemical interface formed between a solid electrode
and electrolyte in conventional electrochemical systems, the charge-transfer
process across the PLI is able to promote reduction–oxidation
(redox) reactions. However, in the case of free plasma jets in contact
with liquids, the absence of solid electrodes obscures the spatial
locations of the electrochemical half-reactions. Herein, we present
a spatial electrochemical measurement technique used to characterize
an aqueous solution in contact with an atmospheric pressure plasma
jet. The technique is based on measuring the potential difference
between two identical Ag/AgCl electrochemical electrodes positioned
at different locations within the solution. More specifically, electrochemical
maps were made by measuring the potential of one electrochemical electrode
positioned at different locations near the PLI with respect to the
other electrochemical electrode positioned far away from the PLI in
the bulk solution. Regions in the map with negative and positive potential
differences between these electrochemical electrodes were used to
identify the electrodeless cathode and anode, respectively. Visualization
of the spatial distribution of molecular colorimetric redox indicators
by multispectral imaging revealed that reduction was occurring near
the plasma jet centerline while oxidation was occurring further away
in solution, which constitutes an independent confirmation of the
electrochemical maps.
Nonthermal plasmas in contact with liquids have been shown to generate a variety of reactive species capable of initiating reduction-oxidation (redox) reactions at the electrochemically active plasma-liquid interface. In conventional...
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