Hydrogen-Induced Step-Edge Roughening of Platinum Electrode Surfaces

Electrode surfaces may change their surface structure as a result of the adsorption of chemical species, impacting their catalytic activity. Using density functional theory, we find that the strong adsorption of hydrogen at low electrode potentials promotes the thermodynamics and kinetics of a unique type of roughening of 110-type Pt step edges. This change in surface structure causes the appearance of the so-called "third hydrogen peak" in voltammograms measured on Pt electrodes, an observation that has eluded explanation for over 50 years. Understanding this roughening process is important for structure-sensitive (electro)catalysis and the development of active and stable catalysts.Article / Letter to editorLIC/ES/Catalysis and Surface Chemistr

Thermodynamics of the formation of surface PtO2 stripes on Pt(111) in the absence of subsurface oxygen

This paper examines the thermodynamics of PtO2 stripes formed as intermediates of Pt(111) surface oxidation as a function of the degree of dilation parallel to the stripes, using density functional theory and atomistic thermodynamics. Internal energy calculations predict 7/8 and 8/9 stripe structures to dominate at standard temperature and pressure, which contain 7 or 8 elevated PtO2 units per 8 or 9 supporting surface Pt atoms, respectively. Moreover, we found a thermodynamic optimum with respect to mean in-stripe Pt-Pt spacing close to that of alpha-PtO2. Vibrational zero point energies, including bulk layer contributions, make a small but significant contribution to the stripe free energies, leading to the 6/7 stripe being most stable, although the 7/8 structure is still close in free energy. These findings correspond closely to experimental observations, providing insight into the driving force for oxide stripe formation and structure as the initial intermediate of platinum surface oxidation, and aiding our understanding of platinum catalysts and surface roughening under oxidative conditions.Catalysis and Surface Chemistr

Nucleation and growth of dendritic islands during platinum oxidation-reduction cycling

Platinum is the model catalyst in fuel cells because of its high activity toward oxygen reduction and hydrogen oxidation. However, its applicability is limited due to the degradation of the catalyst under operating conditions. This degradation process has been extensively studied by repeatedly oxidizing and reducing the electrode, which leads to the roughening of the surface due to the nucleation and growth of platinum nano-islands. Although the general picture of this surface roughening is well known, the atomic details concerning the nucleation and early growth of the islands are still under debate. In this work, we use Density Functional Theory (DFT) to calculate formation energies and diffusion barriers of an adatom, in both the unoxidized and the oxidized state, with the aim to provide further insight into the nucleation phenomena. Moreover, we analyze from STM images obtained experimentally the shape of the nano-islands during the first stages of growth. Our results show not only that the islands form during the reduction of the surface, but also that they grow with a dendritic island shape, similarly to the platinum islands formed in vacuum by Molecular Beam Epitaxy (MBE).Catalysis and Surface Chemistr

How palladium inhibits CO poisoning during electrocatalytic formic acid oxidation and carbon dioxide reduction

Catalysis and Surface Chemistr

Effect of Step Density and Orientation on the Apparent pH Dependence of Hydrogen and Hydroxide Adsorption on Stepped Platinum Surfaces

The effect of the alkali-metal cation (Li+, Na+, K+, and Cs+) on the non-Nernstian pH shift of the Pt(554) and Pt(533) step-associated voltammetric peak is elucidated over a wide pH window (1-13), through computation and experiment. In conjunction with our previously reported study on Pt(553), the non-Nernstian pH shift of the step-induced peak is found to be independent of the step density and the step orientation. In our prior work, we explained the sharp peak as due to the exchange between adsorbed hydrogen and hydroxyl along the step and the non-Nernstian shift as a result of the adsorption of an alkali-metal cation and its subsequent weakening of hydroxyl adsorption. Our density functional theory results support this same mechanism on Pt(533) and capture the effect of alkali-metal cation identity and alkali cation coverage well, where increasing electrolyte pH and cation concentration leads to increased cation coverage and a greater weakening effect on hydroxide adsorption. This work paints a consistent picture for the mechanism of these effects, expanding our fundamental understanding of the electrode/electrolyte interface and practical ability to control hydrogen and hydroxyl adsorption thermodynamics via the electrolyte composition, important for improving fuel cell and electrolyzer performance.Catalysis and Surface Chemistr

Alkali Metal Cation Effects in Structuring Pt, Rh, and Au Surfaces through Cathodic Corrosion

Catalysis and Surface Chemistr

Electrochemical oxidation of Pt(111) beyond the place-exchange model

Catalysis and Surface Chemistr

The role of adsorbed hydroxide in hydrogen evolution reaction kinetics on modified platinum

The bifunctional mechanism that involves adsorbed hydroxide in the alkaline hydrogen oxidation and evolution reactions, important in hydrogen fuel cells and water electrolysers, is hotly debated. Hydroxide binding has been suggested to impact activity, but the exact role of adsorbed hydroxide in the reaction mechanism is unknown. Here, by selectively decorating steps on a Pt single crystal with other metal atoms, we show that the rate of alkaline hydrogen evolution exhibits a volcano-type relationship with the hydroxide binding strength. We find that Pt decorated with Ru at the step edge is 65 times more active for the hydrogen evolution reaction (HER) than is the bare Pt step. Simulations of electrochemical water dissociation show that the activation energy correlates with the OH* adsorption strength, even when the adsorbed hydroxide is not a product, which leads to a simulated volcano curve that matches the experimental curve. This work not only illustrates the alkaline HER mechanism but also provides a goal for catalyst design in targeting an optimum hydroxide binding strength to yield the highest rate for the alkaline HER. A three-dimensional (H and OH adsorbed species) HER activity volcano and the implications for hydrogen oxidation are discussed.The appropriate descriptors for a catalyst's hydrogen evolution activity in alkaline electrolyte are debated. Combining simulations and single-crystal studies of metal-decorated Pt surfaces, McCrum and Koper show that activity exhibits a volcano-type relationship with the hydroxide binding strength of the catalyst, providing a target for catalyst design.Catalysis and Surface Chemistr

Hydrogen-Induced Step-Edge Roughening of Platinum Electrode Surfaces

Electrode surfaces may change their surface structure as a result of the adsorption of chemical species, impacting their catalytic activity. Using density functional theory, we find that the strong adsorption of hydrogen at low electrode potentials promotes the thermodynamics and kinetics of a unique type of roughening of 110-type Pt step edges. This change in surface structure causes the appearance of the so-called "third hydrogen peak" in voltammograms measured on Pt electrodes, an observation that has eluded explanation for over 50 years. Understanding this roughening process is important for structure-sensitive (electro)catalysis and the development of active and stable catalysts.Catalysis and Surface Chemistr

On the presence of surface bound hydroxyl species on polycrystalline Pt electrodes in the "hydrogen potential region" (0-0.4 V-RHE)

Catalysis and Surface Chemistr