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... Show morePlatinum 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). Show less
Chen, X.; Granda Marulanda, L.P.; McCrum, I.T.; Koper, M.T.M. 2022
The ubiquity of aqueous solutions in contact with charged surfaces and the realization that the molecular-level details of water-surface interactions often determine interfacial functions and... Show moreThe ubiquity of aqueous solutions in contact with charged surfaces and the realization that the molecular-level details of water-surface interactions often determine interfacial functions and properties relevant in many natural processes have led to intensive research. Even so, many open questions remain regarding the molecular picture of the interfacial organization and preferential alignment of water molecules, as well as the structure of water molecules and ion distributions at different charged interfaces. While water, solutes and charge are present in each of these systems, the substrate can range from living tissues to metals. This diversity in substrates has led to different communities considering each of these types of aqueous interface. In this Review, by considering water in contact with metals, oxides and biomembranes, we show the essential similarity of these disparate systems. While in each case the classical mean-field theories can explain many macroscopic and mesoscopic observations, it soon becomes apparent that such theories fail to explain phenomena for which molecular properties are relevant, such as interfacial chemical conversion. We highlight the current knowledge and limitations in our understanding and end with a view towards future opportunities in the field.What do a rock in a river, a red blood cell in our body and the electrodes inside a car battery have in common? Charged surfaces in contact with water. Although a unified approach to study such a variety of systems is not available yet, the current understanding - even with its limitations - paves the road to the development of new concepts and techniques. Show less
Granda Marulanda, L.P.; McCrum, I.T.; Koper, M.T.M. 2021
Determining the adsorption potential of adsorbed ions in the field of computational electrocatalysis is of great interest to study their interaction with the electrode material and the solvent, and... Show moreDetermining the adsorption potential of adsorbed ions in the field of computational electrocatalysis is of great interest to study their interaction with the electrode material and the solvent, and to map out surface phase diagrams and reaction pathways. Calculating the adsorption potentials of ions with density functional theory and comparing across various ions requires an accurate reference energy of the ion in solution and electrons at the same electrochemical scale. Here we highlight a previously used method for determining the reference free energy of solution phase ions using a simple electrochemical thermodynamic cycle, which allows this free energy to be calculated from that of a neutral gas-phase or solid species and an experimentally measured equilibrium potential, avoiding the need to model solvent around the solution phase ion in the electronic structure calculations. While this method is not new, we describe its use and utility in detail and show that this same method can be used to find the free energy of any ion from any reaction, as long as the half-cell equilibrium potential is known, even for reactions that do not transfer the same number of protons and electrons. To illustrate its usability, we compare the adsorption potentials obtained with DFT of I*, Br*, Cl*, and SO4 * on Pt(111) and Au(111) and OH* and Ag* on Pt(111) with those measured experimentally and find that this simple and computationally affordable method reproduces the experimental trends. Show less
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.... Show moreThe 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. Show less
Chen, X.; Granda Marulanda, L.P.; McCrum, I.T.; Koper, M.T.M. 2020
Specific adsorption of anions is an important aspect in surface electrochemistry for its influence on reaction kinetics in either a promoted or inhibited fashion. Perchloric acid is typically... Show moreSpecific adsorption of anions is an important aspect in surface electrochemistry for its influence on reaction kinetics in either a promoted or inhibited fashion. Perchloric acid is typically considered as an ideal electrolyte for investigating electrocatalytic reactions due to the lack of specific adsorption of the perchlorate anion on several metal electrodes. In this work, cyclic voltammetry and computational methods are combined to investigate the interfacial processes on a Pd monolayer deposited on Pt(111) single crystal electrode in perchloric acid solution. The "hydrogen region" of this PdMLPt(111) surface exhibits two voltammetric peaks: the first "hydrogen peak" at 0.246 V-RHE actually involves the replacement of hydrogen by hydroxyl, and the second "hydrogen peak" H-II at 0.306 V-RHE appears to be the replacement of adsorbed hydroxyl by specific perchlorate adsorption. The two peaks merge into a single peak when a more strongly adsorbed anion, such as sulfate, is involved. Our density functional theory calculations qualitatively support the peak assignment and show that anions generally bind more strongly to the PdMLPt(111) surface than to Pt(111). Show less
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... Show moreElectrode 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. Show less
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... Show moreElectrode 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. Show less
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),... Show moreThe 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. Show less
