Revealing the Size and Potential Dependent D2O Microkinetics on Pt Nanoparticles Using Grand Canonical Ensemble Modeling

Revealing the potential and nanoparticle size effect is significant for understanding the electrochemical microkinetic behaviors under real reaction conditions. Herein, an efficient strategy of combining the robust fully converged constant potential (FCP) algorithm and the size dependent site distribution rule assumption was proposed. A simple reaction of isotopic D2O/H2O adsorption and dissociation on Pt nanoparticles was set as the model reaction. The results show that the cathodic negative potential and the anodic positive potential would result in the D2O orientation of the D-down/O-down physisorption configuration. Microkinetic simulations by this strategy obtained electrochemical widows for D2O/H2O dissociation, and the optimal Pt nanoparticle diameter was predicted to be 1.8 nm, which agrees well with the experimental observation of ∼2 nm threshold. The kinetic isotope effect (KIE) rate constant ratio at the optimal potential of -0.80 V vs SHE was calculated to be ∼1.83. This work provides a guideline in studying electrochemical electrode-electrolyte interactions on nanoparticles.

Cations Determine the Mechanism and Selectivity of Alkaline Oxygen Reduction Reaction on Pt(111)

The proton-coupled electron transfer (PCET) mechanism of the oxygen reduction reaction (ORR) is a long-standing enigma in electrocatalysis. Despite decades of research, the factors determining the microscopic mechanism of ORR-PCET as a function of pH, electrolyte, and electrode potential remain unresolved, even on the prototypical Pt(111) surface. Herein, we integrate advanced experiments, simulations, and theory to uncover the mechanism of the cation effects on alkaline ORR on well-defined Pt(111). We unveil a dual-cation effect where cations simultaneously determine i) the active electrode surface by controlling the formation of Pt-O and Pt-OH overlayers and ii) the competition between inner- and outer-sphere PCET steps. The cation-dependent transition from Pt-O to Pt-OH determines the ORR mechanism, activity, and selectivity. These findings provide direct evidence that the electrolyte affects the ORR mechanism and performance, with important consequences for the practical design of electrochemical systems and computational catalyst screening studies. Our work highlights the importance of complementary insight from experiments and simulations to understand how different components of the electrochemical interface contribute to electrocatalytic processes.

Inverse kinetic isotope effects in the oxygen reduction reaction at platinum single crystals

Although the oxygen reduction reaction (ORR) involves multiple proton-coupled electron transfer processes, early studies reported the absence of kinetic isotope effects (KIEs) on polycrystalline platinum, probably due to the use of unpurified D₂O. Here we developed a methodology to prepare ultra-pure D₂O, which is indispensable for reliably investigating extremely surface-sensitive platinum single crystals. We find that Pt(111) exhibits much higher ORR activity in D₂O than in H₂O, with potential-dependent inverse KIEs of ~0.5, whereas Pt(100) and Pt(110) exhibit potential-independent inverse KIEs of ~0.8. Such inverse KIEs are closely correlated to the lower *OD coverage and weakened *OD binding strength relative to *OH, which, based on theoretical calculations, are attributed to the differences in their zero-point energies. This study suggests that the competing adsorption between *OH/*OD and *O₂ probably plays an important role in the ORR rate-determining steps that involve a chemical step preceding an electrochemical step (CE mechanism).

Accelerated water activation and stabilized metal-organic framework via constructing triangular active-regions for ampere-level current density hydrogen production

Two-dimensional metal-organic frameworks (MOFs) have been explored as effective electrocatalysts for hydrogen evolution reaction (HER). However, the sluggish water activation kinetics and structural instability under ultrahigh-current density hinder their large-scale industrial applications. Herein, we develop a universal ligand regulation strategy to build well-aligned Ni-benzenedicarboxylic acid (BDC)-based MOF nanosheet arrays with S introducing (S-NiBDC). Benefiting from the closer p-band center to the Fermi level with strong electron transferability, S-NiBDC array exhibits a low overpotential of 310 mV to attain 1.0 A cm^-2 with high stability in alkaline electrolyte. We speculate the newly-constructed triangular "Ni₂-S₁" motif as the improved HER active region based on detailed mechanism analysis and structural characterization, and the enhanced covalency of Ni-O bonds by S introducing stabilizes S-NiBDC structure. Experimental observations and theoretical calculations elucidate that such Ni sites in "Ni₂-S₁" center distinctly accelerate the water activation kinetics, while the S site readily captures the H atom as the optimal HER active site, boosting the whole HER activity.

Quantum Tunnelling Driven H2 Formation on Graphene

It is commonly believed that it is unfavorable for adsorbed H atoms on carbonaceous surfaces to form H₂ without the help of incident H atoms. Using ring-polymer instanton theory to describe multidimensional tunnelling effects, combined with ab initio electronic structure calculations, we find that these quantum-mechanical simulations reveal a qualitatively different picture. Recombination of adsorbed H atoms, which was believed to be irrelevant at low temperature due to high barriers, is enabled by deep tunnelling, with reaction rates enhanced by tens of orders of magnitude. Furthermore, we identify a new path for H recombination that proceeds via multidimensional tunnelling but would have been predicted to be unfeasible by a simple one-dimensional description of the reaction. The results suggest that hydrogen molecule formation at low temperatures are rather fast processes that should not be ignored in experimental settings and natural environments with graphene, graphite, and other planar carbon segments.

Models of Electron Transfer at Different Electrode Materials

Electron transfer is the most important electrochemical process. In this review, we present elements of various aspects of electron transfer theory from the early work of Marcus and Hush to recent developments. The emphasis is on the role of the electronic, and to a lesser extent the geometrical, properties of the electrode. A variety of experimental works are discussed in light of these theoretical concepts. Because the field of electron transfer is so vast, this review is far from comprehensive; rather, we focus on systems that offer a special interest and illuminate aspects of the theory.

Theoretical Modeling of Electrochemical Proton-Coupled Electron Transfer

Proton-coupled electron transfer (PCET) plays an essential role in a wide range of electrocatalytic processes. A vast array of theoretical and computational methods have been developed to study electrochemical PCET. These methods can be used to calculate redox potentials and pK_a values for molecular electrocatalysts, proton-coupled redox potentials and bond dissociation free energies for PCET at metal and semiconductor interfaces, and reorganization energies associated with electrochemical PCET. Periodic density functional theory can also be used to compute PCET activation energies and perform molecular dynamics simulations of electrochemical interfaces. Various approaches for maintaining a constant electrode potential in electronic structure calculations and modeling complex interactions in the electric double layer (EDL) have been developed. Theoretical formulations for both homogeneous and heterogeneous electrochemical PCET spanning the adiabatic, nonadiabatic, and solvent-controlled regimes have been developed and provide analytical expressions for the rate constants and current densities as functions of applied potential. The quantum mechanical treatment of the proton and inclusion of excited vibronic states have been shown to be critical for describing experimental data, such as Tafel slopes and potential-dependent kinetic isotope effects. The calculated rate constants can be used as input to microkinetic models and voltammogram simulations to elucidate complex electrocatalytic processes.

Electrocatalysis in Alkaline Media and Alkaline Membrane-Based Energy Technologies

Hydrogen energy-based electrochemical energy conversion technologies offer the promise of enabling a transition of the global energy landscape from fossil fuels to renewable energy. Here, we present a comprehensive review of the fundamentals of electrocatalysis in alkaline media and applications in alkaline-based energy technologies, particularly alkaline fuel cells and water electrolyzers. Anion exchange (alkaline) membrane fuel cells (AEMFCs) enable the use of nonprecious electrocatalysts for the sluggish oxygen reduction reaction (ORR), relative to proton exchange membrane fuel cells (PEMFCs), which require Pt-based electrocatalysts. However, the hydrogen oxidation reaction (HOR) kinetics is significantly slower in alkaline media than in acidic media. Understanding these phenomena requires applying theoretical and experimental methods to unravel molecular-level thermodynamics and kinetics of hydrogen and oxygen electrocatalysis and, particularly, the proton-coupled electron transfer (PCET) process that takes place in a proton-deficient alkaline media. Extensive electrochemical and spectroscopic studies, on single-crystal Pt and metal oxides, have contributed to the development of activity descriptors, as well as the identification of the nature of active sites, and the rate-determining steps of the HOR and ORR. Among these, the structure and reactivity of interfacial water serve as key potential and pH-dependent kinetic factors that are helping elucidate the origins of the HOR and ORR activity differences in acids and bases. Additionally, deliberately modulating and controlling catalyst-support interactions have provided valuable insights for enhancing catalyst accessibility and durability during operation. The design and synthesis of highly conductive and durable alkaline membranes/ionomers have enabled AEMFCs to reach initial performance metrics equal to or higher than those of PEMFCs. We emphasize the importance of using membrane electrode assemblies (MEAs) to integrate the often separately pursued/optimized electrocatalyst/support and membranes/ionomer components. Operando/in situ methods, at multiscales, and ab initio simulations provide a mechanistic understanding of electron, ion, and mass transport at catalyst/ionomer/membrane interfaces and the necessary guidance to achieve fuel cell operation in air over thousands of hours. We hope that this Review will serve as a roadmap for advancing the scientific understanding of the fundamental factors governing electrochemical energy conversion in alkaline media with the ultimate goal of achieving ultralow Pt or precious-metal-free high-performance and durable alkaline fuel cells and related technologies.

Fast-Decoding Algorithm for Electrode Processes at Electrified Interfaces by Mean-Field Kinetic Model and Bayesian Data Assimilation: An Active-Data-Mining Approach for the Efficient Search and Discovery of Electrocatalysts

The microscopic origins of the activity and selectivity of electrocatalysts has been a long-lasting enigma since the 19th century. By applying an active-data-mining approach, employing a mean-field kinetic model and a statistical approach of Bayesian data assimilation, we demonstrate here a fast decoding to extract key properties in the kinetics of complicated electrode processes from current-potential profiles in experimental and literary data. As the proof-of-concept, kinetic parameters on the four-electron oxygen reduction reaction in the 0.1 M HClO4 solution (ORR: O2 + 4e- + 4H+ → 2H2O) of various platinum-based single-crystal electrocatalysts are extracted from our own experiments and third-party literature to investigate the microscopic electrode processes. Furthermore, data assimilation of the mean-field ORR model and experimental data is performed based on Bayesian inference for the inductive estimation of kinetic parameters, which sheds light on the dynamic behavior of kinetic parameters with respect to overpotential. This work shows that a fast-decoding algorithm based on a mean-field kinetic model and Bayesian data assimilation is a promising data-driven approach to extract key microscopic features of complicated electrode processes and therefore will be an important method toward building up advanced human-machine collaborations for the efficient search and discovery of high-performance electrochemical materials.

Emergent electrochemical functions and future opportunities of hierarchically constructed metal-organic frameworks and covalent organic frameworks

Designing spatial and architectural features across from the molecular to bulk scale is one of the most important topics in materials science which has received a lot of attention in recent years. Looking back to the past research, findings on the influences of spatial features denoted as porous structures on the applications related to mass transport phenomena have been widely studied in traditional inorganic materials, such as ceramics over the past two decades. However, due to the difficulties in precise control of the porous structures at the molecular level in this class of materials, the mechanistic understanding of the effects of spatial and architectural features across from the molecular level to meso-/macroscopic scale is still lacking, especially in electrochemical reactions. Further understanding of fundamental electrochemical functions in well-defined architectures is indispensable for the further advancement of key next-generation energy devices. Furthermore, creating periodic porosity in reticular structures is starting to be recognized as an emerging approach to control the electronic structure of materials. In this review, we focus on the investigations on preparing well-defined molecular-level crystalline porous materials known as metal-organic frameworks (MOFs) and covalent organic frameworks (COFs) into hierarchically constructed architectures from molecular structures lower than the reticular frameworks to meso-/macroscopic scale structures. By connecting well-defined nanosized porous structures in MOFs/COFs and additional length-scale space or shapes, emergent electrochemical functions towards emerging devices, such as beyond Li-ion batteries including all-solid-state rechargeable batteries, are expected to be obtained. By summarizing recent advancements in synthetic strategies of hierarchically constructed MOF/COF based materials and fundamental investigation of their structural effect in a wide spectrum of electrochemical applications, we highlight the importance and future direction of this developing field of hierarchically constructed MOFs/COFs, while emphasizing the required chemical stability of the MOFs/COFs which meet the use in the game-changing electrochemical devices.

Biological concepts for catalysis and reactivity: empowering bioinspiration

Biological systems provide attractive reactivity blueprints for the design of challenging chemical transformations. Emulating the operating mode of natural systems may however not be so easy and direct translation of structural observations does not always afford the anticipated efficiency. Metalloenzymes rely on earth-abundant metals to perform an incredibly wide range of chemical transformations. To do so, enzymes in general have evolved tools and tricks to enable control of such reactivity. The underlying concepts related to these tools are usually well-known to enzymologists and bio(inorganic) chemists but may be a little less familiar to organometallic chemists. So far, the field of bioinspired catalysis has greatly focused on the coordination sphere and electronic effects for the design of functional enzyme models but might benefit from a paradigm shift related to recent findings in biological systems. The goal of this review is to bring these fields closer together as this could likely result in the development of a new generation of highly efficient bioinspired systems. This contribution covers the fields of redox-active ligands, entatic state reactivity, energy conservation through electron bifurcation, and quantum tunneling for C-H activation.

Quantum electrocatalysts: theoretical picture, electrochemical kinetic isotope effect analysis, and conjecture to understand microscopic mechanisms

The fundamental aspects of quantum electrocatalysts are discussed together with the newly developed electrochemical kinetic isotope effect (EC-KIE) approach.

Advances and challenges for experiment and theory for multi-electron multi-proton transfer at electrified solid–liquid interfaces

Understanding microscopic mechanism of multi-electron multi-proton transfer reactions at complexed systems is important for advancing electrochemistry-oriented science in the 21st century.