The adsorption of bisulfate and sulfate anions over a Pt(111) electrode: A first principle study of adsorption configurations, vibrational frequencies and linear sweep voltammogram simulations

pKa as a Predictive Descriptor for Electrochemical Anion Adsorption

The adsorption of anions onto metal surfaces is important in many applications including effective (electro)catalyst design, metal surface modification, and contaminant removal in wastewater treatment. In electrocatalysis, anions can be both reactive intermediates or site-blocking spectators, where their adsorption strength therefore dictates the rate of reaction. In this work, we have measured the adsorption energy of a series of carboxylic acids on a Pt (111) single-crystal electrode surface from aqueous solution. We find that the adsorption strength of the carboxylate anion is linearly correlated with its acid-dissociation constant (pKa) and therefore the heterolytic O-H bond dissociation strength in solution. Using density functional theory modeling, we split the anion adsorption energy into a sum of the adsorption energy and electron affinity of a neutral (carboxyl) radical. Surprisingly, the adsorption energy of the carboxyl radicals are similar and therefore the large difference in electron affinity is what dictates anion adsorption strength; the greater the cost in energy to remove the electron from the anion upon adsorption, the weaker its binding. Therefore, at least within a class of anions with similar structure and surface binding atoms, both electron affinity and acidity are predictive descriptors of adsorption strength.

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.

Acid anion electrolyte effects on platinum for oxygen and hydrogen electrocatalysis

Platinum is an important material with applications in oxygen and hydrogen electrocatalysis. To better understand how its activity can be modulated through electrolyte effects in the double layer microenvironment, herein we investigate the effects of different acid anions on platinum for the oxygen reduction/evolution reaction (ORR/OER) and hydrogen evolution/oxidation reaction (HER/HOR) in pH 1 electrolytes. Experimentally, we see the ORR activity trend of HClO4 > HNO3 > H2SO4, and the OER activity trend of HClO4 [Formula: see text] HNO3 ∼ H2SO4. HER/HOR performance is similar across all three electrolytes. Notably, we demonstrate that ORR performance can be improved 4-fold in nitric acid compared to in sulfuric acid. Assessing the potential-dependent role of relative anion competitive adsorption with density functional theory, we calculate unfavorable adsorption on Pt(111) for all the anions at HER/HOR conditions while under ORR/OER conditions [Formula: see text] binds the weakest followed by [Formula: see text] and [Formula: see text]. Our combined experimental-theoretical work highlights the importance of understanding the role of anions across a large potential range and reveals nitrate-like electrolyte microenvironments as interesting possible sulfonate alternatives to mitigate the catalyst poisoning effects of polymer membranes/ionomers in electrochemical systems. These findings help inform rational design approaches to further enhance catalyst activity via microenvironment engineering.

Influence of an electrified interface on the entropy and energy of solvation of methanol oxidation intermediates on platinum(111) under explicit solvation

Liquid water and electric fields play significant roles in phenomena occurring at catalytic and electrocatalytic interfaces; however, how their interplay influences interfacial energetics remains uncertain. Electric fields control the orientations of water molecules, which we hypothesized would influence the solvation thermodynamics of surface species. To explore this hypothesis, we used multiscale simulations involving density functional theory and classical molecular dynamics. We computed the energies and entropies of solvation of surface species on Pt(111), specifically, adsorbed CH₃OH, COH, and CO, which are intermediates in the pathway of methanol oxidation, in the presence of electric fields spanning -0.5 to +0.5 V Å^-1. We found that both the energy and entropy of solvation depend on the strength and direction of the field, with the entropy of solvation being significantly impacted. Both the energy and entropy dependence on the field can be ascribed to water molecule orientations. Specifically, more positive fields orient water molecules so that they can more effectively hydrogen bond with surface species, which strengthens the energies of solvation. However, at more negative fields, competition with the surface species causes interfacial water molecules to reorient, which leads to disorder in the water structure and hence increased entropy.

A simple method to calculate solution-phase free energies of charged species in computational electrocatalysis

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 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.

Role of C-Defective Sites in CO Adsorption over ϵ-Fe2 C and η-Fe2 C Fischer-Tropsch Catalysts

Herein, we report the crucial importance of C-defective sites on the CO adsorption over ϵ-Fe₂ C and η-Fe₂ C Fischer-Tropsch catalysts via systematic DFT calculations. The simulated XRD and Wulff construction show the significant differences in their equilibrium shapes and most exposed surfaces. It is observed that the ϵ-Fe₂ C exposes a high proportion (89 %) of facets (1 2 ‾ 1) with similar structure to that of η-Fe₂ C (011) which has been proved to be the active surface of CO activation.

Sulfate, Bisulfate, and Hydrogen Co-adsorption on Pt(111) and Au(111) in an Electrochemical Environment

The co-adsorption of sulfate, bisulfate and hydrogen on Pt(111) and Au(111) electrodes was studied based on periodic density functional calculations with the aqueous electrolyte represented by both explicit and implicit solvent models. The influence of the electrochemical control parameters such as the electrode potential and pH was taken into account in a grand-canonical approach. Thus, phase diagrams of the stable coadsorption phases as a function of the electrochemical potential and Pourbaix diagrams have been derived which well reproduce experimental findings. We demonstrate that it is necessary to include explicit water molecules in order to determine the stable adsorbate phases as the (bi)sulfate adsorbates rows become significantly stabilized by bridging water molecules.

Single-atom Rh/N-doped carbon electrocatalyst for formic acid oxidation

To meet the requirements of potential applications, it is of great importance to explore new catalysts for formic acid oxidation that have both ultra-high mass activity and CO resistance. Here, we successfully synthesize atomically dispersed Rh on N-doped carbon (SA-Rh/CN) and discover that SA-Rh/CN exhibits promising electrocatalytic properties for formic acid oxidation. The mass activity shows 28- and 67-fold enhancements compared with state-of-the-art Pd/C and Pt/C, respectively, despite the low activity of Rh/C. Interestingly, SA-Rh/CN exhibits greatly enhanced tolerance to CO poisoning, and Rh atoms in SA-Rh/CN resist sintering after long-term testing, resulting in excellent catalytic stability. Density functional theory calculations suggest that the formate route is more favourable on SA-Rh/CN. According to calculations, the high barrier to produce CO, together with the relatively unfavourable binding with CO, contribute to its CO tolerance.

Adsorption processes on a Pd monolayer-modified Pt(111) electrode

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 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 Pd_MLPt(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 Pd_MLPt(111) surface than to Pt(111).

Specific adsorption of anions is an important aspect in surface electrochemistry for its influence on reaction kinetics in either a promoted or inhibited fashion.

Resolving the chemical identity of H2SO4 derived anions on Pt(111) electrodes: they're sulfate

Understanding how electrolyte composition controls electrocatalytic reactions requires molecular-level insight into electrode/electrolyte interaction. Perhaps the most basic aspect of this interaction, the speciation of the interfacial ion, is often controversial for even relatively simple systems. For example, for Pt(111) in 0.5 M H₂SO₄ it has long been debated whether the adsorbed anion is SO₄^2-, HSO₄^- or an H₃O⁺SO₄^2- ion pair. Here we apply interface-specific vibrational sum frequency (VSF) spectroscopy and theory to this problem and perform an isotope exchange study: we collect VSF spectra of Pt(111) in H₂SO₄(H₂O) and D₂SO₄(D₂O) as a function of bias and show that at all potentials they are identical. This is the most direct spectroscopic evidence to date that SO₄^2- is the dominant adsorbate, despite the fact that at 0.5 M H₂SO₄ bulk solution is dominated by HSO₄^-. This approach is based on the unique selection rule of the VSF spectroscopy and thus offers a new way of accessing general electrode/electrolyte interaction in electrocatalysis.

Effects of Sulfate on the Oxygen Reduction Reaction Activity on Stabilized Pt Skin/PtCo Alloy Catalysts from 30 to 80 °C

The effects of the concentration of H₂SO₄ ([H₂SO₄]), which is the major decomposition product of polymer electrolyte membranes during the operation of fuel cells, on the performance of stabilized Pt skin/PtCo alloy nanocatalysts supported on high-surface-area carbon (Pt_xAL-PtCo/C) were investigated. Kinetically controlled activities for the oxygen reduction reaction (ORR) and the H₂O₂ yields ( P(H₂O₂)) on the Pt_xAL-PtCo/C were examined based on hydrodynamic voltammograms in O₂-saturated 0.1 M HClO₄ + X M H₂SO₄ ( X = 0 to 5 × 10^-2) by use of the channel flow double electrode method at temperatures between 30 and 80 °C. At X ≤ 10^-6 (1 μM) and all temperatures examined, the apparent ORR rate constants k_app@0.85 V (per unit electrochemically active surface area) on Pt_xAL-PtCo/C at 0.85 V vs the reversible hydrogen electrode (RHE) were nearly identical with those in sulfate-free 0.1 M HClO₄ and were at least twice as high as those on a commercial Pt/C catalyst (c-Pt/C). The values of k_app@0.85 V on both Pt_xAL-PtCo/C and c-Pt/C decreased linearly with log[H₂SO₄] in the concentration range 10^-6 < X ≤ 5 × 10^-2. The detrimental effect by H₂SO₄ was less pronounced on Pt_xAL-PtCo/C than on c-Pt/C at high temperatures; the k_app@0.85 V value at X = 5 × 10^-2 on the former at 80 °C was maintained as high as 87%, whereas that of the latter was 66% (34% loss). The values of peroxide production percentage P(H₂O₂) on Pt_xAL-PtCo/C at 80 °C were nearly constant (ca. 0.22% at 0.76 V vs RHE) up to X = 5 × 10^-2. These superior characteristics are ascribed to weakened adsorption of sulfate on the Pt skin surface, supported by DFT calculations, which provides the great advantage of robustness in the presence of impurities, maintaining active sites for the ORR during the PEFC operation.

Hydrocarbon adsorption in an aqueous environment: A computational study of alkyls on Cu(111)

Hydrocarbon chains are important intermediates in various aqueous-phase surface processes, such as CO2 electroreduction, aqueous Fischer-Tropsch synthesis, and aqueous phase reforming of biomass-derived molecules. Further, the interaction between water and adsorbed hydrocarbons represents a difficult case for modern computational methods. Here, we explore various methods for calculating the energetics of this interaction within the framework of density functional theory and explore trade-offs between the use of low water coverages, molecular dynamics approaches, and minima hopping for identification of low energy structures. An effective methodology for simulating low temperature processes is provided by using a unit cell in which the vacuum space is filled with water, employing the minima hopping algorithm to search for low-lying minima, and including dispersion (van der Waals) interactions. Using this methodology, we show that a high coverage of adsorbed alkyls is destabilized by the presence of water, while a low coverage of alkyls is stabilized. Solvation has a small effect on the energetics of hydrocarbon chain growth, generally decreasing its favorability at low temperatures. We studied higher temperatures by running molecular dynamics simulations starting at the minima found by the minima hopping algorithm and found that increased temperatures facilitate chain growth. The self-consistent continuum solvation method effectively describes the alkyl-water interaction and is in general agreement with the explicit solvation results in most cases, but care should be taken at high alkyl coverage.

Signal Enhanced FTIR Analysis of Alignment in NAFION Thin Films at SiO2 and Au Interfaces

Spin-cast NAFION samples were prepared on silicon native oxide and gold substrates with film thicknesses ranging from 5 to 250 nm. The influence of NAFION film thickness on the infrared spectrum of the polymer was investigated in substrate overlayer attenuated total reflection (SO-ATR) geometry at incident angles between 60° and 65°. In the grazing angle SO-ATR geometry, the thickness of the film significantly affected the position and absorbance of characteristic peaks in the FTIR spectrum of NAFION. Two major peaks in the NAFION spectrum at 1220 cm^-1 (predominantly v_as(CF₂) and v_as(SO₃^-)) and 1150 cm^-1 (predominantly v_as(CF₂)) appeared to systematically blueshift to 1256 and 1170 cm^-1, respectively, as the thickness of the film decreased from 250 to 5 nm. The changes in the NAFION thin film FTIR spectrum can be attributed to two factors; (1) ordering of NAFION at the interface during spin coating and film formation and (2) the increase in the p-polarization character of the infrared evanescent wave as the polymer film became thinner between the internal reflection element and the film substrate overlayer. The increase in p-polarization resulted in an increase in characteristic peak absorbances of dipoles aligned normal to the substrate due to the overlayer enhancement of the electric field with NAFION films on Si or Au film substrates. These results show that the specific thin film sampling geometry, especially in internal reflection experiments, must be considered to rationally quantify changes in NAFION thin film infrared spectra.

Enhanced hydrogen evolution properties obtained by electrochemical modification of carbon-supported platinum–copper bimetallic nanocatalysts and structural characterization

The three-dimensional surface with high specific surface area after being electrochemically modified tremendously enhances Pt–Cu/C hydrogen evolution properties.

Non-covalent interactions at electrochemical interfaces: one model fits all?

The shift with increasing concentration of alkali-metal cations of the potentials of both the spike and the hump observed in the cyclic voltammograms of Pt(111) electrodes in sulfuric acid solutions is shown to obey the simple model recently developed by us to explain the effect of non-covalent interactions at the electrical double layer. The results suggest that the model, originally developed to describe the effect of alkali-metal cations on the cyclic voltammogram of cyanide-modified Pt(111) electrodes, is of general applicability and can explain quantitatively the effect of cations on the properties of the electrical double layer.

Density Functional Theory Methods for Electrocatalysis

Electrocatalysis involves catalytic reactions occurring in electrochemical systems, where bond breaking and forming on the catalyst surface are coupled with electron and ion transfer. Electrocatalytic reactions occur in fuel cells, with examples such as hydrogen oxidation, methanol oxidation, and oxygen reduction as well as in electrolysis cells, with examples such as hydrogen evolution, water splitting, and carbon dioxide reduction. Density functional theory (DFT) can be used in a similar manner to its application to non-electrochemical catalytic reactions however, additional complexities arise owing to the electrochemical nature of the catalytic interface. As in typical heterogeneous catalysis, the electrocatalyst is generally a supported nanoparticle, and all of the same challenges in developing appropriate and computationally tractable model systems (use of low-index plane surfaces or small particles as models, for example) apply to electrocatalytic systems.

Electrochemical reactivity of urea at Pt(100) surface in 0.5 M H2SO4 by AC impedance spectroscopy

The present paper reports an alternate current impedance spectroscopic study on adsorption of urea (U) at Pt(100) single-crystal surface, examined in 0.5 M H₂SO₄ supporting electrolyte. The resulted information provided confirmation of the role of electrosorption of urea on the Pt(100) plane through evaluation of the associated charge transfer resistance and capacitance parameters. Obtained impedance results were compared to those previously recorded for guanidinium cation (G⁺) under analogous experimental conditions, especially with respect to the so-called ion pairing mechanism, as originally proposed for the G⁺ ion and bi(sulfate)/OH species, based on the voltammetric and in situ Fourier transform infrared spectroscopy results.