Metal-glass interactions during the hydrogen oxidation reaction sensed by scanning electrochemical microscopy on Pt microelectrodes

MARIELA A. BRITES HELÚ; JOSE LUIS FERNANDEZ

Buenos Aires

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

International Society of Electrochemistry

The hydrogen oxidation reaction (hor) proceeds on Pt and other noble metals over potential ranges where atomic hydrogen is electroadsorbed (Had) from proton or water, either as a reaction intermediate or as an spectator (i.e. the under-potential deposited HUPD). The behaviors of these adsorbed species (adsorption energy, surface mobility, among others) have strong influence on the hor reaction rate. Moreover, the coverage of Had is a key factor that governs the current-potential dependence for the hor. Thus, modification of the Had coverage by cooperative effects from other catalyst components that surround the active material may be an interesting strategy to increase the hor current. In particular, oxides are able to adsorb H+ through their surface acid-base equilibria, and may act as additional H sources for the hor when they are in contact with a catalyst. Scanning electrochemical microscopy (SECM) has the capability to sense the adsorption and surface diffusion of Had on metals and of H+ad on oxides when operating in the feedback mode using the H+/H2 couple as mediator, through a variant usually known as scanning electrochemical induced desorption. The local variation of the mediator concentration underneath the tip causes a potential-dependent gradient of the Had surface coverage at the substrate over the tip-affected region, which drives the Had surface diffusion toward this area and the consequent positive feedback of mediator, reaching a steady state. The method was successfully applied to detect H electroadsorption and surface diffusion on Pt and Au, and the effects of these processes on the SECM responses. The purpose of this work is to apply this SECM-based method on an heterogeneous material composed by an active metal (i.e. Pt) and an inert and non-conductive source of Had (i.e. glass) with the goal to sense not only the adsorption/surface diffusion of Had on these two materials but also to detect the possible transfer of Had from/to the oxide to/from the metal. This study was carried out on Pt disk microelectrodes sealed in borosilicate glass by setting the SECMID configuration, in deareated solutions of 0.02 M HClO4 - 0.1 M LiClO4. Once the tip was located right over the Pt disk by screening the substrate activity, steady-state tip current (IT) vs. substrate potential (ES) curves were measured at different tip-substrate distances (L), 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. The IT(ES) curves present a sharp peak at ca. 0.1 V vs. RHE overlapped to the hor response. This increment on feedback current is attributed to the diffusion of H+ adsorbed on glass being transferred to the Pt-disk surface through the Pt/glass interface. At ES values where Had is consumed on Pt, a Had coverage gradient is generated in the Pt/glass interface causing a flux of Had from the glass surface. According to numerical simulations and to previous works, such surface process leads to a peak in the IT(ES) dependence, whose intensity increases when L decreases. Such effect was not observed in SECM experiments performed on glass-free Pt disks.