Quantitative SNIFTIRS studies of (bi)sulfate adsorption at the Pt(111) electrode surface

CoCrFeNi High-Entropy Alloy as an Enhanced Hydrogen Evolution Catalyst in an Acidic Solution

High-entropy alloys (HEAs) have intriguing material properties, but their potential as catalysts has not been widely explored. Based on a concise theoretical model, we predict that the surface of a quaternary HEA of base metals, CoCrFeNi, should go from being nearly fully oxidized except for pure Ni sites when exposed to O2 to being partially oxidized in an acidic solution under cathodic bias, and that such a partially oxidized surface should be more active for the electrochemical hydrogen evolution reaction (HER) in acidic solutions than all the component metals. These predictions are confirmed by electrochemical and surface science experiments: the Ni in the HEA is found to be most resistant to oxidation, and when deployed in 0.5 M H2SO4, the HEA exhibits an overpotential of only 60 mV relative to Pt for the HER at a current density of 1 mA/cm2.

CuS nanoparticles in humid environments: adsorbed water enhances the transformation of CuS to CuSO4

Covellite copper sulfide nanoparticles (CuS NPs) have attracted immense research interest due to their widespread use in a range of biological and energy applications. As such, it is crucial to understand the transformations of these nanomaterials and how these transformations influence the behavior of these nanoparticles in environmental and biological systems. This study specifically focuses on understanding the role of water vapor and adsorbed water in the transformation of CuS NP surfaces to CuSO4 in humid environments. Surface sulfide ions are oxidized to sulfate by oxygen in the presence of water vapor, as detected by atomic force microscopy based photothermal infrared spectroscopy (AFM-PTIR) and in situ attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy. These results show that the transformation of CuS to CuSO4 is highly dependent on relative humidity (RH). While sulfide to sulfate conversion is not observed to a great extent at low RH (<20%), there is significant conversion at higher RH (>80%). X-ray photoelectron spectroscopy (XPS) analysis confirms that sulfide is irreversibly oxidized to sulfate. Furthermore, it shows that initially, the Cu ions possess the original oxidation state similar to the original covellite, i.e. Cu+, but they are oxidized to Cu2+ at higher RH. The formation of CuSO4 has also been confirmed by HRTEM. These analyses show that adsorbed water on the NP surfaces enhances the conversion of sulfide to sulfate and the oxidation of Cu+ to Cu2+ in the presence of molecular oxygen.

Recognizing conductive islands in polymeric redox surfaces using electrochemical-coupled vibrational spectromicroscopy

We introduce a set up by coupling multiplex FTIR microscopy to electrochemistry through a home-made spectroelectrochemical cell to observe real time changes in the electronic states of polymeric islands by monitoring the oxidation states of polyaniline (PANI). The resultant technique, called electrochemical-coupled vibrational spectromicroscopy (EVSM), enables the measurement of structural changes in the conductive islands of PANI with the spatial resolution as high as 2.5 μm. Unique 2D and 3D chemical maps obtained by the integration of the spectral bands in the subtractively normalized interfacial infrared (SNIFTIR) spectra reveal electrochemical heterogeneity, showing promising topological properties control for conducting polymer-based electronic devices.

New Approach to Simultaneous In Situ Measurements of the Air/Liquid/Solid Interface Using PM-IRRAS

Vibrational spectroscopy techniques have evolved to measure gases, liquids, and solids at surfaces and interfaces. In the field of surface-sensitive vibrational spectroscopy, infrared spectroscopy measures the adsorption on surfaces and changes from reactions. Previous polarized modulated-infrared reflection-absorption spectroscopy (PM-IRRAS) measurements at the gas/solid interface were developed to observe catalytic reactions near reaction conditions. Other PM-IRRAS measurements use liquid cells where the sample is submerged and compressed against a prism that has traditionally been used for electrochemical reactions. This article presents a new method that is used to observe in situ adsorption of molecules using PM-IRRAS at the gas/liquid/solid interface. We demonstrate the meniscus method by measuring the adsorption of octadecanethiol on gold surfaces. Characterization of self-assembled monolayers (SAMs), the "gold standard" for PM-IRRAS calibration measurements, was measured in ethanol solutions. The condensed-phase (air/liquid) interface in addition to the liquid/solid interface was measured simultaneously in solution. These are compared with liquid attenuated total reflectance (ATR)-Fourier transform infrared (FTIR) spectroscopy measurements to confirm the presence of the SAM and liquid ethanol. A model of the three-phase system is used to approximate the thickness of the liquid ethanol layer and correlate these values to signal attenuation using PM-IRRAS. This proof-of-concept study enables the measurement of reactions at the gas/liquid/solid interface that could be adapted for other reactions at the electrode and electrolyte interfaces with applications in environmental science and heterogeneous catalysis.

Structure effects on electrocatalysts. Oxygen reduction on Te-modified Pt(111) surfaces: Site-blocking vs electronic effects

In this work, the oxygen reduction reaction (ORR) on tellurium-modified Pt(111) surfaces has been studied. Adsorption of Te adatoms on Pt(111) progressively shifts toward less positive values of both the ORR reaction onset and the half-wave potential in 0.1M HClO₄ for 0 < θ_Te < 0.25. However, at θ_Te > 0.25, the ORR activity increases relative to the one at θ_Te < 0.25, but remains lower than that on clean Pt(111). Results were analyzed in light of simulations of kinetic currents as a function of θ_Te, calculated by employing a simple mean field model including both site blocking and electronic effects. Inside this framework, experimental data are best explained by considering that oxygenated Te species inhibit the ORR by either negatively modifying adsorption energies of reaction intermediates or combined site-blocking and electronic effects. A redox ORR catalysis due to redox properties of Te adatoms is discarded. Contrarily, in 0.05M H₂SO₄, a positive catalytic effect has been found, interpreted in terms of a competitive adsorption-desorption mechanism involving the replacement of adsorbed sulfate by Te adatoms. On the other hand, despite the strong site-blocking effect on H_ads and OH_ads adsorption by Te adatoms, it appears that the reduced Te-Pt(111) adlayer does not inhibit the reaction, suggesting different active sites for H_ads and OH_ads adsorption and for the rate-determining step of the ORR mechanism.

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.

Interfacial Study of Nickel-Modified Pt(111) Surfaces in Phosphate-Containing Solutions: Effect on the Hydrogen Evolution Reaction

The surface modification of electrodes attracts great interest in electrocatalysis. It has often been observed that deposition of foreign adatoms on the surface of an electrode can originate a significant enhancement in the catalytic activity. For example, it has been reported that nickel deposits on Pt surfaces improve the rate of the hydrogen evolution reaction (HER, Nature Energy 2017, 2, 17031). During the deposition process of such metal adlayers, the pH and the nature of the ions in the electrolyte play an important role. Phosphate species are typically used to prepare buffer solutions in a wide range of pH. Therefore, electrolytes containing phosphate species are used in a large number of applications. However, the effect of phosphate on platinum surface modification with nickel deposits has not been studied yet. In this work, new data about the interaction of phosphate with nickel adatoms deposited on Pt(111) at pH 5 is investigated using cyclic voltammetry and infrared spectroscopy. The results show that, when nickel is in solution, the phosphate ions are adsorbed at lower potentials than in the absence of nickel. In addition, Laser-Induced Temperature Jump Technique demonstrates that nickel facilitates the adsorption of phosphate because of a shift of the potential of zero charge (pzc) toward negative potentials. This increases the magnitude of the positive electric field on the electrode surface, at a given potential E>pzc, facilitating the adsorption of anions. CO displacement technique has been also employed to obtain additional information about co-adsorbed phosphate on nickel adlayers. Finally, the HER has been studied at pH 5 in the presence of nickel, with and without phosphate in the bulk solution.

On-Chip in Situ Monitoring of Competitive Interfacial Anionic Chemisorption as a Descriptor for Oxygen Reduction Kinetics

The development of future sustainable energy technologies relies critically on our understanding of electrocatalytic reactions occurring at the electrode-electrolyte interfaces, and the identification of key reaction promoters and inhibitors. Here we present a systematic in situ nanoelectronic measurement of anionic surface adsorptions (sulfates, halides, and cyanides) on ultrathin platinum nanowires during active electrochemical processes, probing their competitive adsorption behavior with oxygenated species and correlating them to the electrokinetics of the oxygen reduction reaction (ORR). The competitive anionic adsorption features obtained from our studies provide fundamental insight into the surface poisoning of Pt-catalyzed ORR kinetics by various anionic species. Particularly, the unique nanoelectronic approach enables highly sensitive characterization of anionic adsorption and opens an efficient pathway to address the practical poisoning issue (at trace level contaminations) from a fundamental perspective. Through the identified nanoelectronic indicators, we further demonstrate that rationally designed competitive anionic adsorption may provide improved poisoning resistance, leading to performance (activity and lifetime) enhancement of energy conversion devices.

Speciation of Adsorbed Phosphate at Gold Electrodes: A Combined Surface-Enhanced Infrared Absorption Spectroscopy and DFT Study

Despite the significance of phosphate buffer solutions in (bio)electrochemistry, detailed adsorption properties of phosphate anions at metal surfaces remain poorly understood. Herein, phosphate adsorption at quasi-Au(111) surfaces prepared by a chemical deposition technique has been systematically investigated over a wide range of pH by surface-enhanced infrared absorption spectroscopy in the ATR configuration (ATR-SEIRAS). Two different pH-dependent states of adsorbed phosphate are spectroscopically detected. Together with DFT calculations, the present study reveals that pKa for adsorbed phosphate species at the interface is much lower than that for phosphate species in the bulk solution; the dominant phosphate anion, H2PO4(-) at 2 < pH < 7 or HPO4(2-) at 7 < pH < 12, undergoes deprotonation upon adsorption and transforms into the adsorbed HPO4 or PO4, respectively. This study leads to a conclusion different than earlier spectroscopic studies have reached, highlighting the capability of the ATR-SEIRAS technique at electrified metal-solution interfaces.

Quantitative Subtractively Normalized Interfacial Fourier Transform Infrared Reflection Spectroscopy Study of the Adsorption of Adenine on Au(111) Electrodes

Quantitative subtractively normalized interfacial Fourier transform infrared reflection spectroscopy (SNIFTIRS) was used to determine the molecular orientation and identify the metal-molecular interactions responsible for the adsorption of adenine from the bulk electrolyte solution onto the surface of the Au(111) electrode. The recorded p-polarized IR spectra of the adsorbed species were subtracted from the collected s-polarized IR spectra to remove the IR contributions of the vibrational bands of the desorbed molecules that are located within the thin layer cavity of the spectroelectrochemical cell. The intense IR band around 1640 cm(-1), which is assigned to the pyrimidine ring stretching vibrations of the C5-C6 and C6-N10 bonds, and the IR band at 1380 cm(-1), which results from a combination of the ring stretching vibration of the C5-C7 bond and the in-plane CH bending vibration, were selected for the quantitative analysis measurements. The transition dipoles of these bands were evaluated by DFT calculations. Their orientations differed by 85 ± 5°. The tilt angles of adsorbed adenine molecules were calculated from the intensity of these two vibrations at different potentials. The results indicate that the molecular plane is tilted at an angle of 40° with respect to the surface normal of the electrode and rotates by 16° around its normal axis with increasing electrode potential. This orientation results from the chemical interaction between the N10 and gold atoms coupled with the π-π parallel stacking interactions between the adjacent adsorbed molecules. Furthermore, the changes in the molecular plane rotation with the electric field suggests that the N1 atom of adenine must also participate in the interaction between the molecule and metal.

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.