Electrochemistry at 3D-Printed Electrodes: Revealing How Nanoscale Features Contribute to Macroscopic Properties

Résumé fourni par la source

Abstract 3D-printed carbon composite electrodes are growing in popularity, but characterization studies are typically limited to voltammetric measurements, with peak-to-peak separation (for fast electron transfer redox couples) being used as a key metric of performance and properties. To understand voltammetric responses, investigation of the nano to microscale origins is critical. In this study, carbon black in poly-lactic acid electrodes, printed using fused deposition modeling and mechanically polished using silicon carbide abrasive paper, are investigated using cyclic voltammetry (CV), conducting atomic force microscopy (C-AFM), and scanning electrochemical cell microscopy (SECCM). The CV response for ferrocenylmethyl trimethylammonium (FcTMA+) oxidation in 0.1 M potassium nitrate shows peak-shaped responses which increase in magnitude with scan rate, up to 5 V s–1 where the current response becomes more sigmoidal. C-AFM reveals an RMS surface roughness of 79 ± 13 nm (n = 5, 3 on the same electrode, 2 on different electrodes) after polishing and localized conducting regions covering 11–15% of the surface (n = 5). These sites vary in size from the 10’s of nm to the ∼μm scale. The active sites lie mostly in small recesses in the electrode surface. Voltammetric SECCM maps of the surface, using a double-channel tip ∼580 nm outer diameter, support the C-AFM data, showing a significant fraction of the surface has no electrochemical activity. For those sites with electrochemically active regions, different CV characteristics are observed. Capacitance values obtained from the foot of each CV are used as a proxy for estimating active surface area in the SECCM footprint (not otherwise visible), and the limiting current density provides a measure of the mass transport rate to the site. At 0.5 V s–1, SECCM CV responses range from slightly peak-shaped, associated with the largest active sites, to steady-state responses with different limiting currents and interquartile potentials. These data are analyzed to reveal a greater than two orders of magnitude variation in the standard rate constant, ko, at carbon black sites. Moreover, the SECCM data help explain the macroscopic scan rate voltammetric data where widely spaced active sites become increasingly diffusionally isolated, and the wide distribution of local kinetics becomes more apparent as scan rate increases. Both factors are reflected in the observed change in CV waveshape. Finally, the data highlight that the Randles–Ševčík and Nicholson equations for CV analysis should be applied with caution. The printed electrodes used in this work show a peak current response (at 0.1 V s–1) ∼ half that predicted using the modified Randles–Ševčík equation for quasi-reversible electron transfer.

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