Recent advances in the application of scanning electrochemical microscopy for studying the hydrogen electrode reaction mechanism

JOSE LUIS FERNANDEZ; MARIELA A. BRITES HELÚ

Buenos Aires

Congreso; 20th Topical Meeting of the International Society of Electrochemistry; 2017

International Society of Electrochemistry

The hydrogen electrode reaction (HER) proceeds through a mechanism involving the Volmer (V), Heyrovsky (H), and Tafel (T) steps, and one adsorbed intermediate (Had), so it can proceed via two parallel and independent routes (for instance, VT and VH routes). The study of this mechanism requires high mass-transport rates to visualize the contributions of all the elementary steps to the global reaction rate over a large-enough potential range before the reaction becomes fully limited by the mass transport of reactant. Precisely, scanning electrochemical microscopy (SECM) is a technique that allows to establish very high mass-transport rates by approaching a microelectrode tip toward another surface (substrate) down to sub-micrometer distances, defining a pseudo thin-layer cell (TLC) between these two electrodes. On that sense, even though the HER was previously studied by SECM in the feedback mode, theoretical formalisms with very simplified kinetic models were employed to process the data, which impeded to obtain complete mechanistic information. This work describes an approach that allows to use the full capacity of SECM to analyze the HER reaction mechanism, which can be applied to the study of the HER cathodic branch (the hydrogen evolution reaction - her), and of the HER anodic branch (the hydrogen oxidation reaction - hor). The method proposes to analyze the complete experimental dependence of the normalized tip current (IT) on the substrate or tip potential (ES or ET, respectively) and on the normalized tip-substrate distance (L) by modeling it through a TLC. This equation involves the TLC current (ITLC) which must be solved either for the her or for the hor operating through the VHT mechanism, and correction factors caused by the additional contributions to the SECM response. This work shows the SECM analysis of the her on a Pt microelectrode tip under very high mass transport rates, and the SECM analysis of the hor on Pt films deposited on glassy carbon (GC), and on Pt nanoparticle (NP) ensembles deposited on HOPG.