Platinum arguably is the most important electrocatalyst material, not only because it is the best single element catalyst in a variety of important electrocatalytic reactions but also due to its relatively high stability. However, in the corrosive environment of real electrocatalysis systems, such as fuel cells, even platinum can structurally degrade. Moreover, the presence of strongly adsorbing species, in particular carbon monoxide (CO), can substantially increase these degradation effects.
A team led by Prof. CHEN Yanxia from University of Science and Technology of China (USTC) of CAS, in cooperation with Prof. Olaf Magnuseen, reported in situ video-STM observations of additional point defects in the presence of this dynamic CO adlayer. The STM observations presented in this work provide direct insights into their dynamic behavior and formation mechanisms. The research results were published in Chemical Communications on June 17th.
Adsorbed CO is known to interact with Pt electrodes, causing a relaxation of Pt surface atoms and a weakening of the Pt–Pt bond. In situ scanning tunneling microscopy (STM) studies of Pt(111) in the presence of dissolved CO found that initially disordered were transformed into perfectly straight (111) steps by potential cycling into the CO oxidation regime. CO can increase Pt surface mobility during CO oxidation, which can reduce the amount of available low coordinated sites, leading to a restructuring of the Pt steps.
In the previous work by the team, they studied the atomic-scale structural dynamics of CO adlayers on Pt(111) electrodes in CO-saturated 0.1 M H2SO4 using in situ video-rate scanning tunneling microscopy (STM) and density functional theory (DFT).
In the recent work, their video-STM results of Pt(111) in CO-saturated 0.1 M H2SO4 revealed specific defects within the apparent (1 × 1)-CO adlayer, which we assign to vacancies in the topmost Pt layer. The presence of these Pt vacancies as well as the observed fluctuations at the Pt steps show that even under very benign conditions, i.e., for the particularly stable Pt(111) electrode surface in the CO pre-oxidation regime, the presence of CO can induce some structural degradation.
Thus, CO may affect the stability of Pt electrocatalysts already at potentials as low as 0.30 VAg/AgCl, which for example may be relevant in direct methanol fuel cells. The atomic scale data obtained by video-STM provide fundamental insights into the structure–activity and structure–stability relationships, which may contribute to the knowledge-based design of better Pt electrocatalysts.
Sequence of in situ video-STM images obtained at 0.30 V near a step of the Pt(111) electrode, showing step fluctuations and defect formation/annihilation at steps. (Image by WEI Jie et al.)
In addition, their results show that for suitable systems the dynamics of individual electrode point defects, such as vacancies, can be directly investigated in electrochemical environment.
The observed dynamic behavior suggests a complex interaction between adsorbed CO and surface vacancies which also should affect the electrochemical reactivity of CO and needs to be explored in future experimental and theoretical studies.
（Written by LI Xiaoxi, USTC News Center）