(8d) Electrochemical Aging Effects of Pt Thin Film Electrodes Under Microfluidic Device Conditions

Many of the functions in a microfluidic device, such as sensing, transport and separation are based on electrodes. Noble metals are very commonly used as electrodes in microfluidic applications, and in the microfluidic electrokinetic realm, they are widely regarded as inert materials. However, they are known to undergo various chemical reactions and morphological changes and to be subject to fouling. In microfluidic devices, subtle changes in electrode behavior and appearance have been observed. Surface changes of Pt electrodes were previously studied whereby Pt electrodes utilized in common buffer compositions yielded surface chemical changes including Pt oxidation, Pt dissolution, Pt redeposition, and fouling by solute species. Chemical changes were observed with SEM-EDS and XPS while morphological changes were observed via SEM and optical light microscopy. However, the effect of these changes on electrode performance was not investigated. 

In the present work, the electrochemical behavior of thin film Pt electrodes was evaluated through cyclic voltammetry of a 1 mM K₃Fe(CN)₆, 0.5 M KCl solution. The cyclic voltammograms illustrate the redox reactions of the model compound K₃Fe(CN)₆ in unused Pt electrodes and were compared to the voltammograms obtained with the same devices after aging in various conditions. The conditions comprised a matrix of phosphate buffer solution conductivities (0.01 and 0.10 S/m) and different applied DC current levels, as well as an applied 5 V_pp, 1 kHz AC signal. Different devices of identical design were aged under each set of buffer and electric signal conditions. SEM-EDS was performed on the unused and aged electrodes to evaluate surface chemical and morphological changes. Similar tests of Pt wire electrodes showed that after 6 hrs under a 100 μA current in a 0.10 S/m phosphate buffer solution, K₃Fe(CN)₆ redox peak heights were significantly decreased. Further, the cyclic voltammograms were different than the control CV experiments in 0.5 M KCl solution and water electrolysis peaks were persistent. These results suggest that aging weakens the electrochemical response of Pt electrodes, but it does not completely passivate Pt surfaces even after long usage times.