Energetics and Kinetics of Hydrogen Electrosorption on a Graphene-Covered Pt(111) Electrode.

The Angstrom-scale space between graphene and its substrate provides an attractive playground for scientific exploration and can lead to breakthrough applications. Here, we report the energetics and kinetics of hydrogen electrosorption on a graphene-covered Pt(111) electrode using electrochemical experiments, in situ spectroscopy, and density functional theory calculations. The graphene overlayer influences the hydrogen adsorption on Pt(111) by shielding the ions from the interface and weakening the Pt-H bond energy. Analysis of the proton permeation resistance with controlled graphene defect density proves that the domain boundary defects and point defects are the pathways for proton permeation in the graphene layer, in agreement with density functional theory (DFT) calculations of the lowest energy proton permeation pathways. Although graphene blocks the interaction of anions with the Pt(111) surfaces, anions do adsorb near the defects: the rate constant for hydrogen permeation is sensitively dependent on anion identity and concentration.

Morphological Stability of Copper Surfaces under Reducing Conditions.

Though copper is a capable electrocatalyst for the CO2 reduction reaction (CO2RR), it rapidly deactivates to produce mostly hydrogen. A current hypothesis as to why this occurs is that potential-induced morphological restructuring takes place, leading to a redistribution of the facets at the interface resulting in a shift in the catalytic activity to favor the hydrogen evolution reaction over CO2RR. Here, we investigate the veracity of this hypothesis by studying the changes in the voltammetry of various copper surfaces, specifically the three principal orientations and a polycrystalline surface, after being subjected to strongly cathodic conditions. The basal planes were chosen as model catalysts, while polycrystalline copper was included as a means of investigating the overall behavior of defect-rich facets with many low coordination steps and kink sites. We found that all surfaces exhibited (perhaps surprisingly) high stability when subjected to strongly cathodic potentials in a concentrated alkaline electrolyte (10 M NaOH). Proof for morphological stability under CO2RR-representative conditions (60 min at -0.75 V in 0.5 M KHCO3) was obtained from identical location scanning electron microscopy, where the mesoscopic morphology for a nanoparticle-covered copper surface was found unchanged to within the instrument accuracy. Observed changes in voltammetry under such conditions, we found, were not indicative of a redistribution of surface sites but of electrode fouling. Besides impurities, we show that (brief) exposure to oxygen or oxidizing conditions (i.e., 1 min) leads to copper exhibiting changing morphology upon cathodic treatment which, we posit, is ultimately the reason why many groups report the evolution of copper morphology during CO2RR: accidental oxidation/reduction cycles.

Nanoscale morphological evolution of monocrystalline Pt surfaces during cathodic corrosion.

This paper studies the cathodic corrosion of a spherical single crystal of platinum in an aqueous alkaline electrolyte, to map out the detailed facet dependence of the corrosion structures forming during this still largely unexplored electrochemical phenomenon. We find that anisotropic corrosion of the platinum electrode takes place in different stages. Initially, corrosion etch pits are formed, which reflect the local symmetry of the surface: square pits on (100) facets, triangular pits on (111) facets, and rectangular pits on (110) facets. We hypothesize that these etch pits are formed through a ternary metal hydride corrosion intermediate. In contrast to anodic corrosion, the (111) facet corrodes the fastest, and the (110) facet corrodes the slowest. For cathodic corrosion on the (100) facet and on higher-index surfaces close to the (100) plane, the etch pit destabilizes in a second growth stage, by etching faster in the (111) direction, leading to arms in the etch pit, yielding a concave octagon-shaped pit. In a third growth stage, these arms develop side arms, leading to a structure that strongly resembles a self-similar diffusion-limited growth pattern, with strongly preferred growth directions.

Double Layer at the Pt(111)-Aqueous Electrolyte Interface: Potential of Zero Charge and Anomalous Gouy-Chapman Screening.

We report, for the first time, the observation of a Gouy-Chapman capacitance minimum at the potential of zero charge of the Pt(111)-aqueous perchlorate electrolyte interface. The potential of zero charge of 0.3 V vs. NHE agrees very well with earlier values obtained by different methods. The observation of the potential of zero charge of this interface requires a specific pH (pH 4) and anomalously low electrolyte concentrations (<10-3 m). By comparison to gold and mercury double-layer data, we conclude that the diffuse double layer structure at the Pt(111)-electrolyte interface deviates significantly from the Gouy-Chapman theory in the sense that the electrostatic screening is much better than predicted by purely electrostatic mean-field Poisson-Boltzmann theory.