Understanding the electrochemical double layer at the hematite/water interface: A first principles molecular dynamics study

On the origin of multihole oxygen evolution in haematite photoanodes

The oxygen evolution reaction (OER) plays a crucial role in (photo)electrochemical devices that use renewable energy to produce synthetic fuels. Recent measurements on semiconducting oxides have found a power law dependence of the OER rate on surface hole density, suggesting a multihole mechanism. In this study, using transient photocurrent measurements, density functional theory simulations and microkinetic modelling, we have uncovered the origin of this behaviour in haematite. We show here that the OER rate has a third-order dependence on the surface hole density. We propose a mechanism wherein the reaction proceeds by accumulating oxidizing equivalents through a sequence of one-electron oxidations of surface hydroxy groups. The key O–O bond formation step occurs by the dissociative chemisorption of a hydroxide ion involving three oxyl sites. At variance with the case of metallic oxides, the activation energy of this step is weakly dependent on the surface hole coverage, leading to the observed power law.

Nanosecond solvation dynamics of the hematite/liquid water interface at hybrid DFT accuracy using committee neural network potentials

Metal oxide/water interfaces play an important role in biology, catalysis, energy storage and photocatalytic water splitting. The atomistic structure at these interfaces is often difficult to characterize by experimental techniques, whilst results from ab initio molecular dynamics simulations tend to be uncertain due to the limited length and time scales accessible. In this work, we train a committee neural network potential to simulate the hematite/water interface at the hybrid DFT level of theory to reach the nanosecond timescale and systems containing more than 3000 atoms. The NNP enables us to converge dynamical properties, not possible with brute-force ab initio molecular dynamics. Our simulations uncover a rich solvation dynamics at the hematite/water interface spanning three different time scales: picosecond H-bond dynamics between surface hydroxyls and the first water layer, in-plane/out-of-plane tilt motion of surface hydroxyls on the 10 ps time scale, and diffusion of water molecules from the oxide surface characterized by a mean residence lifetime of about 60 ps. Calculation of vibrational spectra confirm that H-bonds between surface hydroxyls and first layer water molecules are stronger than H-bonds in bulk water. Our study showcases how state of the art machine learning approaches can routinely be utilized to explore the structural dynamics at transition metal oxide interfaces with complex electronic structure. It foreshadows that c-NNPs are a promising tool to tackle the sampling problem in ab initio electrochemistry with explicit solvent molecules.

Ultrathin space charge layer in hematite photoelectrodes: A theoretical investigation

The space charge layer in hematite photoelectrodes has been analyzed by means of Poisson-Boltzmann equations, the Stern model, and density functional theory, in view of its application for photoelectrochemical water oxidation. The width of the space charge layer can be smaller than ∼10 Å under experimental conditions. In this regime, a substantial part of the potential drop takes place in the Helmholtz layer, leading to important corrections to the Mott-Schottky behavior of the space charge layer capacitance. These results shed light on an unexpected regime of high photoelectrocatalytic efficiency, different from the classical picture of the electrochemical interface of a semiconducting photocatalyst, which is also amenable to direct study by quantum-mechanical atomistic simulations. Density functional theory has been used to calculate the band bending (BB) in the space charge layer in atomistic models of pristine stoichiometric and hydroxylated surfaces. These surface terminations display BBs of 0.14 and 0.49 eV, respectively, with an increasing width of the space charge layer, however still in the sub-nanometer regime. This work shows that, at high doping, the width of the space charge layer of a hematite photoelectrode can become comparable with interatomic distances.

Thermodynamic Cyclic Voltammograms Based on Ab Initio Calculations: Ag(111) in Halide-Containing Solutions

Cyclic voltammograms (CVs) are a central experimental tool for assessing the structure and activity of electrochemical interfaces. Based on a mean-field ansatz for the interface energetics under applied potential conditions, we here derive an ab initio thermodynamics approach to efficiently simulate thermodynamic CVs. All unknown parameters are determined from density functional theory (DFT) calculations coupled to an implicit solvent model. For the showcased CVs of Ag(111) electrodes in halide-anion-containing solutions, these simulations demonstrate the relevance of double-layer contributions to explain experimentally observed differences in peak shapes over the halide series. Only the appropriate account of interfacial charging allows us to capture the differences in equilibrium coverage and total electronic surface charge that cause the varying peak shapes. As a case in point, this analysis demonstrates that prominent features in CVs do not only derive from changes in adsorbate structure or coverage but can also be related to variations of the electrosorption valency. Such double-layer effects are proportional to adsorbate-induced changes in the work function and/or interfacial capacitance. They are thus especially pronounced for electronegative halides and other adsorbates that affect these interface properties. In addition, the analysis allows us to draw conclusions on how the possible inaccuracy of implicit solvation models can indirectly affect the accuracy of other predicted quantities such as CVs.

Improved description of hematite surfaces by the SCAN functional

Controversies on the surface termination of α-Fe₂O₃ (0001) focus on its surface stoichiometry dependence on the oxygen chemical potential. Density functional theory (DFT) calculations applying the commonly accepted Perdew-Burke-Ernzerhof (PBE) exchange-correlation functional to a strongly correlated system predict the best matching surface termination, but would produce a delocalization error, resulting in an inappropriate bandgap, and thus are not applicable for comprehensive hematite system studies. Besides, the widely applied PBE+U scheme cannot provide evidence for existence of some of the successfully synthesized stoichiometric α-Fe₂O₃ (0001) surfaces. Hence, a better scheme is needed for hematite DFT studies. This work investigates whether the strongly constrained and appropriately normed (SCAN) approximation reported by Perdew et al. could provide an improved result for the as-mentioned problem, and whether SCAN can be applied to hematite systems. By comparing the results calculated with the PBE, SCAN, PBE+U, and SCAN+U schemes, we find that SCAN and SCAN+U improves the description of the electronic structure of different stoichiometric α-Fe₂O₃ (0001) surfaces with respect to the PBE results, and that they give a consistent prediction of the surface terminations. Besides, the bulk lattice constants and the bulk density of states are also improved with the SCAN functional. This study provides a general characterization of the α-Fe₂O₃ (0001) surfaces and rationalizes how the SCAN approximation improves the results of hematite surface calculations.

Special Topic on Interfacial Electrochemistry and Photo(electro)catalysis

Interfacial electrochemistry and photo(electro)catalysis are key processes that convert the energy of photons or electrons to chemical bonds in many energy conversion and storage technologies. Achieving a molecular level understanding of the fundamental interfacial structure, energetics, dynamics, and reaction mechanisms that govern these processes represents a broad frontier for chemical physics and physical chemistry. This Special Topic contains a collection of articles that range from the development of new experimental and computational techniques to the novel application of those techniques for mechanistic studies, as the principal investigators seek a fundamental molecular understanding of both electrode/electrolyte interfaces and the relevant electrocatalytic, photocatalytic, and photoelectrochemical reactions taking place thereabout. Altogether, this collection of articles captures the current state of this very active, frontier research field and highlights the current and remaining key scientific challenges and opportunities.