A major challenge for development of better electrocatalysts for fuel cell and other applications is improving the understanding of surface reaction mechanisms at electrochemical interfaces. Whereas major strides have been made in describing surface reactivity at a metal-vacuum interface, catalytic reactions on metal electrodes are typically characterized by a polarized metal-electrolyte interface. In part because of the complexity of this interface, little study has been devoted to modeling the influence of the electric double layer (EDL) environment on surface reactivity. In this work, the effects of two important facets of the EDL on surface processes will be studied in detail. First, the role of solvent-adsorbate interactions in altering surface reaction pathways will be characterized. Second, the influence of near-surface electric fields on those same reactions will be investigated to determine how interactions of adsorbate dipoles with interfacial electric fields can affect reaction mechanisms.
The proposed studies will be carried out using a so-called non-situ approach, in which effects of the EDL will be modeled on single crystals in ultrahigh vacuum, where powerful spectroscopic techniques may be employed to characterize surface intermediates in detail. This approach is not designed to provide a highly accurate simulation of a working electrocatalyst. Instead, the goal is to identify the fundamental mechanisms by which the EDL environment affects surface reactions. Temperature programmed desorption (TPD), high resolution electron energy loss spectroscopy (HREELS), and work function measurements, among other methods, will be employed to characterize the surfaces under study, and density functional methods will be used for the required theoretical studies. To investigate the effects of solvation, controlled amounts of water and other solvents will be added above an adsorbate-covered surface and subjected to various thermal treatments. The interface polarization studies will largely be conducted by adsorbing strong electron donors, especially alkali metals, together with surface adsorbates. The effects of solvation and polarization will be probed for several key processes, such as oxygen adsorption and dissociation, proton formation, and proton transfer. Because this research deals with a fundamental study of surface processes essential for many electrocatalytic processes, its potential for broad impact across disciplines is great. For example, the ability to account for trends in solvation environment and electrode potential is important for first-principles development of improved electrocatalysts. The proposed work is partly motivated by preliminary theoretical studies, and an additional objective of this work is to provide experimental spectroscopic information to help validate ongoing and future theoretical modeling efforts. The proposed studies will also provide insights into mechanisms important for reactions conducted in acidic media, including those associated with biorefinery conversions, and surface processes conducted in the presence of electric fields. The PI will work with the National Society of Black Engineers (NSBE) and the Society of Hispanic Professional Engineers (SHPE) to recruit undergraduate researcher assistants to promote diversity efforts in energy education. Outreach efforts to a local area high school focused on communicating the ability of students to impact renewable energy research are also planned.
The purpose of this research was to investigate physical factors that control the ability to isolate protons at metal surfaces under controlled conditions. Formation and reaction of such protons are critical steps in a variety of electrocatalytic reactions, including those that occur in hydrogen fuel cells. By isolating and studying the behavior of protons under well-defined ultrahigh vacuum conditions, it is possible in principle to better understand the mechanisms that control key electrocatalytic reactions, and therefore design improved catalysts. Our research employed both computational and experimental methods to study proton formation on surfaces of metals such as platinum and palladium. Using computation, we found that the favorability of proton formation on metal surfaces was related to both the electronic properties of the surface (in particular, the work function) and the organization of water (which serves as the stabilizing solvent for the protons) near the surface. The presence of electron-withdrawing surface species such as oxygen was predicted to enhance the ability to form protons near surfaces, while the presence of certain other species (e.g. hydrocarbon-derived species) was found to decrease the favorability of protons at surfaces. This latter finding may be an important consideration in efforts to design catalysts for reduction of carbon dioxide, since such reactions require the electrochemical consumption of protons. Experimentally, it has been found that protons were isolable on an oxygen-covered Pt surface, in agreement with computation. However, the observed failure to isolate protons on Pd surfaces in the presence of electron-withdrawing coadsorbates signals the need for more work in developing interfacial models.
