Redox titration of gold and platinum surface oxides at porous microelectrodes

Abstract

The extension of the surface interrogation mode of scanning electrochemical microscopy (SI-SECM) is demonstrated for porous electrode materials. These materials are often high surface area powders which are very important electrocatalysts for instance in fuel cells or water electrolyzers. The powdered electrocatalyst material is filled into a cavity-microelectrode which is then operated as the sample electrode in SECM. After a surface oxide generation step, the oxides on the porous sample are reduced by [Ru(NH)] formed at the microelectrode probe of the SECM while the sample is at open circuit potential. Such porous electrodes pose the difficulty to cope with unavoidable variations in the filling of the cavity and to access the entire surface by the mediator. The electrochemically active surface area is used to compensate the variation in filling. It can also be used for calculating coverages of surface oxides for a better comparison between different electrodes. We found a complete and fast accessibility for all investigated porous electrodes which is based on electron transfer. Therefore, we propose a "vertical feedback" mechanism analogous to SECM feedback experiments on extended flat samples at open circuit potential. Moreover, the current transients indicate that distinctive oxide species with different kinetics are present. Taken together, these measures ensure consistent determination of oxide coverages for nanoporous gold and carbon-supported platinum nanoparticles.

Oxygen Reduction Reaction Activity of Mesostructured Cobalt‐Based Metal Oxides Studied with the Cavity‐Microelectrode Technique

“…One possibility is the calibration of the cavity volume by the usage of a powder of known properties . This approach was also used by some of us to estimate the amount of material loaded into the cavity by measuring the charge of gold oxide reduction peaks or of H UPD on carbon‐supported Pt catalyst . In addition, the application of electrochemical impedance spectroscopy and the post‐mortem dissolution of the studied material was suggested .…”

“…CMEs were prepared by potentiostatic electrochemical etching in hydrochloride solution of Au microelectrodes with a diameter of 50 μm (Figure 1c). [43,44] A smooth cavity bottom indicates a uniform etching process ( Figure 1d) and the cavity depth h was obtained from topographic reconstruction of a stack of confocal laser scanning microscopy (CLSM) images (Supporting Information, SI-2). The CMEs were filled by tapping into a small amount of the powder sample.…”

“…[46] This approach was also used by some of us to estimate the amount of material loaded into the cavity by measuring the charge of gold oxide reduction peaks or of H UPD on carbon-supported Pt catalyst. [44] In addition, the application of electrochemical impedance spectroscopy [43] and the post-mortem dissolution of the studied material was suggested. [47] Here, the linear increase of cavity depth and the amount of material loaded into the cavity is demonstrated for CuCo 2 O 4 by determining the area of metal oxides for CME with various depths (Figure 2c).…”

“…CMEs were prepared by a procedure described elsewhere . First, gold‐microelectrodes are prepared by sealing a gold wire with a nominal radius r of 25 μm inside a borosilicate glass capillary.…”

“…CMEs were prepared by a procedure described elsewhere. [43,44] First, gold-microelectrodes are prepared by sealing a gold wire with a nominal radius r of 25 μm inside a borosilicate glass capillary. The electrode was polished with several grades of abrasive paper down to 0.01 μm grain size and connected to a copper wire with silverepoxy glue (EPO-TEK®, John P. Kummer GmbH).…”