Mediator-Free SECM for Probing the Diffusion Layer pH with Functionalized Gold Ultramicroelectrodes.

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.

Enhancement of Oxygen Evolution Activity of Nickel Oxyhydroxide by Electrolyte Alkali Cations.

Herein, the effect of the alkali cation (Li+ , Na+ , K+ , and Cs+ ) in alkaline electrolytes with and without Fe impurities is investigated for enhancing the activity of nickel oxyhydroxide (NiOOH) for the oxygen evolution reaction (OER). Cyclic voltammograms show that Fe impurities have a significant catalytic effect on OER activity; however, both 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. In situ surface enhanced Raman spectroscopy (SERS), shows this cation dependence is related to the formation of superoxo OER intermediate (NiOO- ). 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 Ni-OO- species leading to the formation of NiOO- -M+ species that is stabilized better by bigger cations (Cs+ ). This species would then act as the precursor to O2 evolution, explaining the higher activity.

Complex Electron Transfer Pathway at a Microelectrode Captured by in Situ Nanospectroscopy.

In the quest for analytical tools which enable the characterization of materials at the nanoscale and under the condition of their operation (in situ, operando), the emerging tip-enhanced Raman spectroscopy (TERS) now enters the spotlight. We demonstrate in this work that a TERS tip can be functionalized and partially insulated to be used as a microelectrode enabling electrochemical substrate enhanced Raman sectroscopy (EC-SERS) at a single hotspot. This "SERS at a tip" experiment enables one to capture the electrochemical transformation of a molecular layer self-assembled on a tapered gold microelectrode. The proposed setup and protocol open new perspectives in the characterization and development of complex redox architectures for molecular devices.