Herein, the effect of the alkali cation (Li + ,Na + ,K + , and Cs + )inalkaline electrolytes with and without Fe impurities is investigated for enhancing the activity of nickel oxyhydroxide (NiOOH) for the oxygen evolution reaction (OER). Cyclic voltammograms showthat Fe impurities have asignificant catalytic effect on OER activity;h owever,b oth under purified and unpurified conditions,the trend in OER activity is Cs + > Na + > K + > Li + ,suggesting an intrinsic cation effect of the OER activity on Fe-free Ni oxyhydroxide.I nsitu surface enhanced Raman spectroscopy( SERS), shows this cation dependence is related to the formation of superoxo OER intermediate (NiOO À ). The electrochemically active surface area, evaluated by electrochemical impedance spectroscopy (EIS), is not influenced significantly by the cation. We postulate that the cations interact with the NiÀOO À species leading to the formation of NiOO À ÀM + species that is stabilized better by bigger cations (Cs + ). This species would then act as the precursor to O 2 evolution, explaining the higher activity.Supportinginformation and the ORCID identification number(s) for the author(s) of this article can be found under: https://doi.
Probing
pH gradients during electrochemical reactions is important
to better understand reaction mechanisms and to separate the influence
of pH and pH gradients from intrinsic electrolyte effects. Here, we
develop a pH sensor to measure pH changes in the diffusion layer during
hydrogen evolution. The probe was synthesized by functionalizing a
gold ultramicroelectrode with a self-assembled monolayer of 4-nitrothiophenol
(4-NTP) and further converting it to form a hydroxylaminothiophenol
(4-HATP)/4-nitrosothiophenol (4-NSTP) redox couple. The pH sensing
is realized by recording the tip cyclic voltammetry and monitoring
the Nernstian shift of the midpeak potential. We employ a capacitive
approach technique in our home-built Scanning Electrochemical Microscope
(SECM) setup in which an AC potential is applied to the sample and
the capacitive current generated at the tip is recorded as a function
of distance. This method allows for an approach of the tip to the
electrode that is electrolyte-free and consequently also mediator-free.
Hydrogen evolution on gold in a neutral electrolyte was studied as
a model system. The pH was measured with the probe at a constant distance
from the electrode (ca. 75 μm), while the electrode potential
was varied in time. In the nonbuffered electrolyte used (0.1 M Li2SO4), even at relatively low current densities,
a pH difference of three units is measured between the location of
the probe and the bulk electrolyte. The time scale of the diffusion
layer transient is captured, due to the high time resolution that
can be achieved with this probe. The sensor has high sensitivity,
measuring differences of more than 8 pH units with a resolution better
than 0.1 pH unit.
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