Diffuse Layer Effect on Electron-Transfer Kinetics Measured by Scanning Electrochemical Microscopy (SECM)

Recent theoretical and experimental studies revealed strong effects of the electrical double layer (EDL) on mass transfer at nanometer-sized electrodes and in electrochemical nanogaps. Although the EDL effect is much stronger in weakly supported media, it can significantly influence the kinetics of electron-transfer processes involving multicharged ionic redox species, even at high concentrations of supporting electrolyte. We measured the kinetics of Fe­(CN)₆^4– oxidation in 1 M KCl solution at the Pt nanoelectrode used as a tip in the scanning electrochemical microscope. The apparent standard rate constant values extracted from tip voltammograms without double-layer correction increased markedly with the decreasing separation distance between the tip and substrate electrodes. The same steady-state voltammograms were fitted to the theory including the EDL effect and yielded the rate constant essentially independent of the separation distance

Scanning Electrochemical Microscopy Study of Electron-Transfer Kinetics and Catalysis at Nanoporous Electrodes

The complicated electrochemical properties of nanoporous electrodes arising from their geometry remain poorly understood because their complex structure defies easy interpretation of experimental results. The large surface area of a porous electrode results in a higher double-layer charging current and faster apparent heterogeneous rate constants, which are difficult to measure by voltammetry and other transient electrochemical techniques. In this article, we used a scanning electrochemical microscope equipped with a nanometer- or micrometer-sized tip to measure the rates of the same electron-transfer process at the flat and nanoporous Au electrodes. The origins and magnitude of the rate constant enhancement at the nanoporous surface (after the roughness factor correction) are discussed. Using the substrate generation/tip collection mode of the scanning electrochemical microscope operation, the higher catalytic activity of nanoporous Au for the oxygen reduction reaction was found from both substrate and tip voltammograms that can also be used for analyzing the reaction products

Dissolution of Pt during Oxygen Reduction Reaction Produces Pt Nanoparticles

The loss of Pt during the oxygen reduction reaction (ORR) affects the performance and economic viability of fuel cells and sensors. Our group previously observed the dissolution of Pt nanoelectrodes at moderately negative potentials during the ORR. Here we report a more detailed study of this process and identify its product. The nanoporous Pt surface formed during the ORR was visualized by AFM and high-resolution SEM, which also showed ∼5 nm sized Pt particles on the glass surface surrounding the electrode. The release of these nanoparticles into the solution was confirmed by monitoring their catalytically amplified collisions with a Hg-coated microelectrode used as the tip in the scanning electrochemical microscope (SECM)

In-Channel Electrochemical Detection in the Middle of Microchannel under High Electric Field

We propose a new method for performing in-channel electrochemical detection under a high electric field using a polyelectrolytic gel salt bridge (PGSB) integrated in the middle of the electrophoretic separation channel. The finely tuned placement of a gold working electrode and the PGSB on an equipotential surface in the microchannel provided highly sensitive electrochemical detection without any deterioration in the separation efficiency or interference of the applied electric field. To assess the working principle, the open circuit potentials between gold working electrodes and the reference electrode at varying distances were measured in the microchannel under electrophoretic fields using an electrically isolated potentiostat. In addition, “in-channel” cyclic voltammetry confirmed the feasibility of electrochemical detection under various strengths of electric fields (∼400 V/cm). Effective separation on a microchip equipped with a PGSB under high electric fields was demonstrated for the electrochemical detection of biological compounds such as dopamine and catechol. The proposed “in-channel” electrochemical detection under a high electric field enables wider electrochemical detection applications in microchip electrophoresis

Nonfaradaic Nanoporous Electrochemistry for Conductometry at High Electrolyte Concentration

Nanoporous electrified surfaces create a unique nonfaradaic electrochemical behavior that is sensitively influenced by pore size, morphology, ionic strength, and electric field modulation. Here, we report the contributions of ion concentration and applied ac frequency to the electrode impedance through an electrical double layer overlap and ion transport along the nanopores. Nanoporous Pt with uniform pore size and geometry (L₂-ePt) responded more sensitively to conductivity changes in aqueous solutions than Pt black with poor uniformity despite similar real surface areas and enabled the previously difficult quantitative conductometry measurements at high electrolyte concentrations. The nanopores of L₂-ePt were more effective in reducing the electrode impedance and exhibited superior linear responses to not only flat Pt but also Pt black, leading to successful conductometric detection in ion chromatography without ion suppressors and at high ionic strengths