Local reactivity of hydrogen oxidation reaction on heteroatomic junctions investigated by scanning electrochemical microscopy

FERNÁNDEZ, JOSÉ L.; BRITES HELÚ, MARIELA A.

Conferencia; VI San Luis Conference; 2019

Scanning electrochemical microscopy (SECM) is an electroanalytical scanning probe technique capable of imaging substrate topography and local reactivity with high resolution. It allows to restrict the kinetic analysis over confined submicrometer-sized domains of a macroelectrode and to achieve high mass transport rates. These properties make SECM an exceptional tool to analyze the effects of the surface state on reactions electrocatalyzed by heterogeneous electrode surfaces where intermediate coverage could be affected by cooperative effects from other catalyst components that surround the active material. The aim of this work is to apply the SECM to analyze the complete mechanism of the hydrogen oxidation reaction (H2 2H+ + 2e-) on heterogonous substrates composed by an active metal (i.e. Pt) and an inert metal (i.e. Au) with the goal to sense the local reactivity at its junctions and detect changes from mono-metallic substrates. This study was carried out on highly ordered arrays of Pt micro-hexagons (19 µm) supported on Au in deareated solutions of 5 mM HClO4 - 0.1 M LiClO4. The electrodes were fabricated by mask assisted electron beam lithography using a benchtop SEM. A Au substrate was spin-coated with Poly(methyl methacrylate) (PMMA) and baked in an oven for 30 min. A TEM grid were placed onto the surface and used as a mask. After the complete exposure, the irradiated zones were developed in methyl isobutyl ketona (MIBK) ? isopropyl alcohol (IPA) (1:3) for 2 min under gently agitation, rinsed in pure IPA for 1 min and ultrapure water, and finally dried under hot air stream. Then Pt was electrodeposited into hexagonal patches in 25 mM H2PtCl6 - 0.5 M H2SO4 solution by applying -0.05 V vs. Ag/AgCl for 2 min. Finally, for obtaining the bi-metallic arrays the PMMA layer was removed by sonicating the substrates in pure MIBK for 30 min.In order to increase the resolution, nanometric SECM probes were developed. Conical sharped SECM tips fabricated by a method described elsewhere1 were dip-coated with PMMA and baked for 30 minutes. This process covers all the surface of the metallic cone but the apex exposing a disk of a few nanometers in diameter. SECM experiments were carried out in the feedback mode using the H+/H2 couple as mediator. Once the tip was located right over the junction by screening the substrate activity, steady-state tip current (IT) vs. substrate potential (ES) curves were measured at different tip-substrate distances (L) across the junction cross section, keeping the tip potential at -0.65 V vs. RHE and sweeping the substrate potential from 0.5 V to -0.25 V vs. RHE. It was possible to detect changes in the hor kinetic parameters across the interface. On one hand, the characteristic shoulder of transition between mechanistic routes (Tafel-Volmer and Heyrovsky-Volmer routes) appears at lower overpotentials than Pt. On the other hand, from the correlations of IT(ES) curves, the Heyrovsky equilibrium rate increases sharply in the junction region.