The simulation of electrochemical reaction dynamics from first principles remains challenging, since over the course of an elementary step, an electron is either consumed or produced by the electrode. For example, the hydrogen evolution reaction begins with a simple proton discharge to a metal surface, but with conventional electronic structure methods, the simulated potential, which is manifested as the metal’s workfunction, varies over the course of the simulation as the electron is consumed in the new metal–hydrogen bond. Here, we present a simple approach to allow the direct control of the electrochemical potential via charging of the electrode surface. This is achieved by changing the total number of electrons in the self-consistent cycle, while enforcing charge neutrality through the introduction of a jellium counter charge dispersed in an implicit solvent region above the slab. We observe that the excess electrons localize selectively at the metal’s reactive surface and that the metal workfunction responds nearly linearly to the variation in electronic count. This linear response allows for control of the potential in simulations with a minimal computational penalty compared to standard electronic structure calculations. This scheme can be straightforwardly implemented with common electronic structure calculators (density functional theory in the present work), and we find this method to be compatible with the commonly used computational hydrogen electrode model, which we expect will make it useful in the construction of potential-dependent free-energy diagrams in electrochemistry. We apply this approach to the proton-deposition (Volmer) step on both Au(111) and Pt(111) surfaces and show that we can reliably control the simulated electrode potential and thus assess the potential dependence of the initial, transition, and final states. Our method allows us to directly assess the location along the reaction pathway with the greatest amount of charge transfer, which we find to correspond well with the reaction barrier, indicating this reaction is a concerted proton–electron transfer. Interestingly, we show that the Pt electrode has not only a more favorable equilibrium energy with adsorbed hydrogen but also a lower intrinsic barrier under thermoneutral conditions.

Platinum is a nearly perfect catalyst for the hydrogen evolution reaction, and its high activity has conventionally been explained by its close-to-thermoneutral hydrogen binding energy (G∼0). However, many candidate non-precious metal catalysts bind hydrogen with similar strengths, but exhibit orders-of-magnitude lower activity for this reaction. In this study, we employ electronic structure methods that allow fully potential-dependent reaction barriers to be calculated, in order to develop a complete working picture of hydrogen evolution on platinum. Through the resulting ab initio microkinetic models, we assess the mechanistic origins of Pt's high activity. Surprisingly, we find that the G∼0 hydrogen atoms are kinetically inert, and that the kinetically active hydrogen atoms have ∆G's much weaker, similar to that of gold. These on-top hydrogens have particularly low barriers, which we compare to those of gold, explaining the high reaction rates, and the exponential variations in coverages can uniquely explain Pt's strong kinetic response to the applied potential. This explains the unique reactivity of Pt that is missed by conventional Sabatier analyses, and suggests true design criteria for non-precious alternatives.

Charge transport through single molecules can be influenced by the charge and spin states of redox-active metal centres placed in the transport pathway. These molecular intrinsic properties are usually addressed by varying the molecule's electrochemical and magnetic environment, a procedure that requires complex setups with multiple terminals. Here we show that oxidation and reduction of organometallic compounds containing either Fe, Ru or Mo centres can solely be triggered by the electric field applied to a two-terminal molecular junction. Whereas all compounds exhibit bias-dependent hysteresis, the Mo-containing compound additionally shows an abrupt voltage-induced conductance switching, yielding high-to-low current ratios exceeding 1000 at voltage stimuli of less than 1.0 V. DFT calculations identify a localized, redox-active molecular orbital that is weakly coupled to the electrodes and closely aligned with the Fermi energy of the leads because of the spin-polarised ground state unique to the Mo centre. This situation opens an additional slow and incoherent hopping channel for transport, triggering a transient charging effect of the entire molecule and a strong hysteresis with unprecedented high low-to-high current ratios.

Nanostructured Cu catalysts have increased the selectivities and geometric activities for high value C-C coupled (C2) products (ethylene, acetate, and ethanol) in the electrochemical CO(2) reduction reaction (CO(2)RR). The selectivity...

Electrochemical conversion of CO(2) into hydrocarbons and oxygenates is envisioned as a promising path toward closing the carbon cycle in modern technology. To date, however, the reaction mechanisms toward the plethora of products are disputed, complicating the search for alternative catalyst materials. To conclusively identify the rate-limiting steps in CO reduction on Cu, we analyzed the mechanisms on the basis of constant-potential density functional theory (DFT) kinetics and experiments at a wide range of pH values (3–13). We find that *CO dimerization is energetically favored as the rate-limiting step toward multicarbon products. This finding is consistent with our experiments, where the reaction rate is nearly unchanged on a standard hydrogen electrode (SHE) potential scale, even under acidic conditions. For methane, both theory and experiments indicate a change in the rate-limiting step with electrolyte pH from the first protonation step under acidic/neutral conditions to a later one under alkaline conditions. We also show, through a detailed analysis of the microkinetics, that a surface combination of *CO and *H is inconsistent with the measured current densities and Tafel slopes. Finally, we discuss the implications of our understanding for future mechanistic studies and catalyst design.

Besides active, functional molecular building blocks such as diodes or switches, passive components as, e.g., molecular wires, are required to realize molecular-scale electronics. Incorporating metal centers in the molecular backbone enables the molecular energy levels to be tuned in respect to the Fermi energy of the electrodes. Furthermore, by using more than one metal center and sp-bridging ligands, a strongly delocalized electron system is formed between these metallic "dopants", facilitating transport along the molecular backbone. Here, we study the influence of molecule-metal coupling on charge transport of dinuclear X(PP) 2 FeC 4 Fe(PP) 2 X molecular wires (PP = Et 2 PCH 2 CH 2 PEt 2 ); X = CN (1), NCS (2), NCSe (3), C 4 SnMe 3 (4) and C 2 SnMe 3 (5)) under ultra-high vacuum and variable temperature conditions. In contrast to 1 which showed unstable junctions at very low conductance (8.1 · 10 −7 G 0 ), 4 formed a Au-C 4 FeC 4 FeC 4 -Au junction 4 after SnMe 3 extrusion which revealed a conductance of 8.9 · 10 −3 G 0 , three orders of magnitude higher than for 2 (7.9 · 10 −6 G 0 ) and two orders of magnitude higher than for 3 (3.8 · 10 −4 G 0 ). Density functional theory (DFT) confirmed the experimental trend in the conductance for the various anchoring motifs. The strong hybridization of molecular and metal states found in the C-Au coupling case enables the delocalized electronic system of the organometallic Fe 2 backbone to be extended over the molecule-metal interfaces to the metal electrodes to establish high-conductive molecular wires.KEYWORDS: Molecular wire, Single-molecule junctions, electronic transport, break-junctions, organometallic compounds, density functional theory Molecular electronics aims at employing single molecules as functional building blocks in electronic circuits. Besides such active components which provide, e.g., current rectifying or switching properties, also passive components such as molecular wires are required for the realization of molecular-scale electronics. Generally, an ideal wire has lowest resistance with almost linear (ohmic) and length-independent (ballistic) transport properties. For molecular wires, the required high conductance can in principle be achieved if low injection barriers for charge-carriers are present at the molecule-metal interfaces, if molecular orbitals (MOs) are available close to the Fermi energy of the electrodes, and if a large degree of electronic conjugation across the backbone is present. Already the first task seems to be difficult to achieve as the most frequently used thiol anchoring 1,2 suffers from an electronically weak molecule-metal coupling. Additionally, multiple bonding sites available on the Au surface for the thiol bond give rise to alter- † IBM Research -Zurich ‡ University of Vienna ¶ University of Zurich nating energy barriers for charge-carrier injection and result in large fluctuations in the transport properties. Therefore other anchoring schemes such as nitriles, 3 isocyanides, 4 amines, 5 and pyridines 6 were ...

Azulene (Az) is a non-alternating, aromatic hydrocarbon composed of a five-membered, electron-rich and a sevenmembered, electron-poor ring; an electron distribution that provides intrinsic redox activity. By varying the attachment points of the two electrode-bridging substituents to the Az centre, the influence of the redox functionality on charge transport is evaluated. The conductance of the 1,3 Az derivative is at least one order of magnitude lower than those of the 2,6 Az and 4,7 Az derivatives, in agreement with density functional theory (DFT) calculations. In addition, only 1,3 Az exhibits pronounced nonlinear current-voltage characteristics with hysteresis, indicating a bias-dependent conductance switching. DFT identifies the LUMO to be nearest to the Fermi energy of the electrodes, but to be an active transport channel only in the case of the 2,6 and the 4,7 Az derivatives, whereas the 1,3 Az derivative uses the HOMO at low and the LUMO+1 at high bias. In return, the localized, weakly coupled LUMO of 1,3 Az creates a slow electron-hopping channel responsible for the voltage-induced switching due to the occupation of a single MO.

Many electrochemical processes are governed by the transfer of protons to the surface, which can be coupled with electron transfer; this electron transfer is in general non-integer and unknown a priori, but is required to hold the potential constant. In this study, we employ a combination of surface spectroscopic techniques and grandcanonical electronic-structure calculations in order to rigorously understand the thermodynamics of this process. Specifically, we explore the protonation/deprotonation of 4-mercaptobenzoic acid as a function of the applied potential. Using grand-canonical electronic-structure calculations, we directly infer the coupled electron transfer, which we find to be on the order of 0.1 electron per proton; experimentally, we also access this quantity via the potential-dependence of the pK a . We show a striking agreement between the potential-dependence of the measured pK a and that calculated with electronic-structure calculations. We further employ a simple electrostatics-based model to show that this slope can equivalently be interpreted to provide information on the degree of coupled electron transfer or the potential change at the point of the charged species.