Approaching the Self-Consistency Challenge of Electrocatalysis with Theory and Computation

This opinion piece centers around challenges involved in developing first- principles electrochemical methods. In recent years, theory and computation have become quintessential tools to navigate the parameter space that controls the activity and stability of electrocatalytic materials and electrochemical devices. Viable methods process as input details on materials structure, composition and reaction conditions. Their output includes metrics for stability and activity, phase diagrams, as well as mechanistic insights on reaction mechanisms and pathways. The core challenge, connecting input to output, is a self-consistency problem that couples the electrode potential to variables for the electronic structure of the solid electrode, solvent properties and ion distributions in the electrolyte as well as specific properties of a boundary region in between. We will discuss a theoretical framework and computational approaches that strive to accomplish this feat

Unifying Theoretical Framework for Deciphering the Oxygen Reduction Reaction on Platinum

Rapid conversion of oxygen into water is crucial to the operation of polymer electrolyte fuel cells and other emerging electrochemical energy technologies. Chemisorbed oxygen species play double-edged roles in this reaction, acting as vital intermediates on one hand and site-blockers on the other. Any attempt to decipher the oxygen reduction reaction (ORR) must first relate the formation of oxygen intermediates to basic electronic and electrostatic properties of the catalytic surface, and then link it to parameters of catalyst activity. An approach that accomplishes this feat will be of great utility for catalyst materials development and predictive model formulation of electrode operation. Here, we present a theoretical framework for the multiple interrelated surface phenomena and processes involved, particularly, by incorporating the double-layer effects. It sheds light on the roles of oxygen intermediates and gives out the Tafel slope and exchange current density as continuous functions of electrode potential. Moreover, it develops the concept of a rate determining term which should replace the concept of a rate determining step for multi-electron reactions, and offers a new perspective on the volcano relation of the ORR. &nbsp

Physical Modeling of Materials for PEFCs: A Balancing Act of Water and Complex Morphologies

Peer reviewed: YesNRC publication: Ye

Interface Properties of the Partially Oxidized Pt(111) Surface Using Hybrid DFT–Solvation Models

This article reports a theoretical–computational effort to model the interface between an oxidized platinum surface and aqueous electrolyte. It strives to account for the impact of the electrode potential, formation of surface-bound oxygen species, orientational ordering of near-surface solvent molecules, and metal surface charging on the potential profile along the normal direction. The computational scheme is based on the DFT/ESM-RISM method to simulate the charged Pt(111) surface with varying number of oxygen adatoms in acidic solution. This hybrid solvation method is known to qualitatively reproduce bulk metal properties like the work function. However, the presented calculations reveal that vital interface properties such as the electrostatic potential at the outer Helmholtz plane are highly sensitive to the position of the metal surface slab relative to the DFT-RISM boundary region. Shifting the relative position of the slab also affects the free energy of the system. It follows that there is an optimal distance for the first solvent layer within the ESM-RISM framework, which could be found by optimizing the position of the frozen Pt(111) slab. As it stands, manual sampling of the position of the slab is impractical and betrays the self-consistency of the method. Based on this understanding, we propose the implementation of a free energy optimization scheme of the relative position of the slab in the DFT-RISM boundary region. This optimization scheme could considerably increase the applicability of the hybrid method

Oxygen desorption – Critical step for the oxygen evolution reaction

The oxygen evolution reaction (OER) has been widely investigated in computational electrocatalysis. Recent studies suggest that the final oxygen desorption step could be rate-limiting, or even inhibiting, for the classical OER mechanism on the benchmark IrO2 electrocatalyst, and a novel reaction mechanism has been proposed circumventing this bottleneck. In this review, we provide an overview of recent progress in OER electrocatalysis with a concise focus on computational studies that explicitly accounted for the elementary step of O2 desorption. We highlight the computational and methodological intricacies that led to not considering this step as crucial by earlier OER studies. Key suggestions are provided for future studies to open new directions in OER electrocatalysis

Properties of the Pt(111)/electrolyte electrochemical interface studied with a hybrid DFT–solvation approach

Self-consistent modeling of the interface between solid metal electrode and liquid electrolyte is a crucial challenge in computational electrochemistry. In this contribution, we adopt the effective screening medium reference interaction site method (ESM–RISM) to study the charged interface between a Pt(111) surface that is partially covered with chemisorbed oxygen and an aqueous acidic electrolyte. This method proves to be well suited to describe the chemisorption and charging state of the interface at controlled electrode potential. We present an in-depth assessment of the ESM–RISM parameterization and of the importance of computing near-surface water molecules explicitly at the quantum mechanical level. We found that ESM–RISM is able to reproduce some key interface properties, including the peculiar, non-monotonic charging relation of the Pt(111)/electrolyte interface. The comparison with independent theoretical models and explicit simulations of the interface reveals strengths and limitations of ESM–RISM for modeling electrochemical interfaces

Electronic Structure and Conformational Properties of Polybenzimidazole-Based Ionenes—A Density Functional Theory Investigation

Polybenzimidazole-based ionenes are explored for use in both alkaline anion-exchange membrane fuel cells and alkaline polymer electrolyzers. Poly-(hexamethyl-p-terphenylbenzimidazolium) (HMT-PMBI), the material of interest in this article, is exceptionally hydroxide-stable and water-insoluble. The impact of the degree of methylation on conformations and electronic structure properties of HMT-PMBI oligomers, from the monomer to the pentamer, is studied with density functional theory calculations. Optimization studies are presented for both the gas phase and in the presence of implicit water. In addition, time-dependent density functional theory is employed to generate the UV–vis absorption spectra of the studied systems. Results are insightful for experimentalists and theorists investigating the impact of synthetic and environmental conditions on the conformation and electronic properties of polybenzimidazole-based membranes