Gregory Jerkiewicz

Relation of energies and coverages of underpotential and overpotential deposited H at Pt and other metals to the 'volcano curve' for cathodic H2 evolution kinetics

Abstract Previous literature shows that electrocatalysis in the cathodic H 2 evolution reaction (HER) can be comparatively characterized at various metals by a ‘volcano curve’ relating the log of the exchange current-density to the bond energy of H chemisorbed to the metal or the standard Gibbs energy of chemisorption of H at the metal. The platinum metals lie at or near the apex of such a curve and this apex arises in the theoretical rationalization of the volcano plot when the standard Gibbs energy of dissociative chemisorption of H to the metal from H 2 is zero [R. Parsons, Trans. Faraday Soc. 34 (1958) 1053], whereas the corresponding bond energy to H from the gas-phase, or in underpotential deposition of H, is experimentally≃250 kJ mol −1 , or equivalently, from 1/2H 2 , −35 kJ mol −1 , calculated as the enthalpy of chemisorption. However, at the platinum metals, uniquely, cathodic H 2 evolution takes place on a surface almost filled by underpotentially deposited H already at the H 2 reversible potential, so the binding energy of the H intermediate in the HER is not that to a free Pt surface. The consequences of this situation, in relation to the significance of the volcano curve for the HER, are discussed in detail with regard to differences between underpotential and overpotential deposited states of H at Pt.

Theoretical investigations of the oxygen reduction reaction on Pt(111).

Computational modeling can provide important insights into chemical reactions in both applied and fundamental fields of research. One of the most critical processes needed in practical renewable energy sources is the oxygen reduction reaction (ORR). Besides being the key process in combustion and corrosion, the ORR has an elusive mechanism that may proceed in a number of complicated reaction steps in electrochemical fuel cells. Indeed, the mechanism of the ORR on highly studied Pt(111) electrodes has been the subject of interest and debate for decades. Herein, we first outline the theory behind these types of simulations and then show how to use these quantum mechanical approaches and approximations to create a realistic model. After reviewing the performance of these methods in studying the binding of molecular oxygen to Pt(111), we then outline our own results in elucidating the ORR and its dependence on environmental parameters, such as solvent, thermodynamic energies, and the presence of an external electrode potential. This approach can, in principle, be applied to other equally complicated investigations of other surfaces or electrochemical reactions.

Limit to extent of formation of the quasi-two-dimensional oxide state on Au electrodes

Potentiostatic polarization of Au electrodes in 0.5 M aqueous H2SO4 and KOH solutions at polarization potentials Ep between 2.00 and 2.43 V (RHE) for polarization times tp up to 104 s leads to the formation of thick oxide films which comprise up to four oxide states, designated OC1, OC2, OC3 and OC4, as resolved from linear sweep voltammetry oxide reduction profiles. The oxide growth proceeds by inverse logarithmic kinetics with two distinguishable kinetic regions leading to linear relations between 1/qox and log tp. The first 1/qox vs. log tp linear region corresponds to development of the quasi-2D state (OC1) and the very initial growth of the OC2 state, and the oxide growth in this tp domain is slow. Further growth of OC2 and formation of the OC3 and OC4 states is significantly faster. The oxide growth kinetics change when 1/qox reaches a value of ca. 0.10 mC−1 cm2. In situ ellipsometry measurements indicate the presence of two distinct layers within the oxide films, the inner layer designated α and the outer one designated β. The α film corresponds to the OC1 state whereas the β film corresponds to the OC2, OC3 and OC4 states. The OC1 state reaches a limiting thickness of 3 equiv. monolayers of AuO or Au(OH)2 in aqueous H2SO4 and 1 monolayer of AuO or Au(OH)2 in aqueous KOH. The β state which resides on top of the α one grows without reaching any limiting thickness and up to 100 equiv. monolayers of Au2O3 or Au(OH)3, depending on the electrolyte composition, can be formed under the conditions described in the present paper.

Thermodynamic and electrode kinetic factors in cathodic hydrogen sorption into metals and its relationship to hydrogen adsorption and poisoning

Abstract In the present paper, we examine how H sorption should depend on the cathodic H2 evolution mechanism, through the potential dependence of the fractional coverage by adsorbed H. The states of adsorbed and absorbed H are treated in terms of site-fraction statistics in order to determine the chemical potentials of the adsorbed and absorbed H. Then, the relationships to the overpotential in the H2 evolution reaction are derived and the efficiency of H sorption in relation to the coevolution of H2 (important for metal hydride battery electrode charging) can be examined in terms of reaction mechanisms. Difficulties arising with application of the Nernst equation to H sorption when appreciable overpotentials for H2 evolution are involved, i.e. at appreciable cathodic current densities, are examined. Finally, the interesting effects of ‘catalyst poisons’, which promote H sorption into host metals, are examined in terms of the effects of competitive adsorption between the poison and H at the surface. Conventional ideas about these effects are inconsistent with information on the diminution of H coverage caused by competitive adsorption of poisons; alternative mechanisms are discussed and a new surface-thermodynamic basis for the enhancement of H sorption by adsorbed poisons is proposed.

Electro-oxidation of CO(chem) on Pt nanosurfaces: solution of the peak multiplicity puzzle.

An understanding of the oxidation of chemisorbed CO (CO(chem)) on Pt nanoparticle surfaces is of major importance to fuel cell technology. Here, we report on the relation between Pt nanoparticle surface structure and CO(chem) oxidative stripping behavior. Oxidative stripping voltammograms are obtained for CO(chem) preadsorbed on cubic, octahedral, and cuboctahedral Pt nanoparticles that possess preferentially oriented and atomically flat domains. They are compared to those obtained for etched and thermally treated Pt(poly) electrodes that possess atomically flat, ordered surface domains separated by grain boundaries as well as those obtained for spherical Pt nanoparticles. A detailed analysis of the results reveals for the first time the presence of up to four voltammetric features in CO(chem) oxidative stripping transients, a prepeak and three peaks, that are assigned to the presence of surface domains that are either preferentially oriented or disordered. The interpretation reported in this article allows one to explain all features within the voltammograms for CO(chem) oxidative stripping unambiguously.

Surface Science and Electrochemical Analysis of Nickel Foams

Open-pore nickel (Ni) foams are characterized using surface science and electrochemical techniques. A scanning electron microscopy analysis reveals interconnected Ni struts that generate small and large pores of ca. 50 and 500 μm in size, respectively. An X-ray photoelectron spectroscopy (XPS) analysis of the surface-chemical composition of the Ni foams shows that there are oxidized and metallic sections within their surfaces despite being prepared by sintering in an oxidizing atmosphere at a high temperature and being stored in moist air. The ratio of the areas of oxidized and metallic sections is evaluated using XPS data. Chemical etching of the Ni foams results in removal of the native surface oxide/hydroxide without altering the three-dimensional structure; it also increases the roughness (R) of the surfaces of Ni struts giving rise to an increase in the electrochemically active surface area (Aecsa). Thermal treatment of Ni foams in an H2(g) atmosphere at 500 °C reduces the native surface oxide/hydroxide but does not increase R or Aecsa. Electrochemical behavior of the Ni foams is examined in 0.5 M aqueous KOH solution using cyclic-voltammetry (CV) and electrochemical impedance spectroscopy (EIS). As-received, chemically etched, thermally reduced and electro-oxidized Ni foams generate distinct CV profiles; their features are assigned to oxidized and metallic surface states. The observations made on the basis of XPS measurements are corroborated by the results of CV analyses. The application of CV and XPS or EIS allows in situ determination of Aecsa and the specific surface area (As) of the chemically etched and thermally reduced Ni foams. The values of As determined on the basis of joint CV and XPS measurements are 227 ± 74 and 149 ± 48 cm(2) g(-1) for the etched and reduced Ni foams, respectively. The values of As determined on the basis of CV, XPS and EIS measurements are 241 ± 80 and 160 ± 23 cm g(-1) for the etched and reduced Ni foams, respectively.

Dependence of the reliability of electrochemical quartz-crystal nanobalance mass responses on the calibration constant, Cf : analysis of three procedures for its determination

Abstract The acquisition of accurate results in use of the electrochemical quartz-crystal nanobalance (EQCN) for surface-electrochemical studies depends on reliability of the value of the calibration constant relating changes of the crystal oscillator frequency to the specific surface mass change resulting from some chemisorption or electrosorption process. Three electrochemical methods are employed comparatively to determine the calibration constant for use of an EQCN. In the present paper, study of electrodeposition/electrodissolution of silver on Pt(poly) electrode surfaces provides a means of determining values of the calibration constant for the EQCN response using cyclic voltammetry, chronopotentiometry and chronoamperometry. The experimentally determined value of the calibration constant (Cf) is found to depend on the electrochemical methodology and technique used for its measurement as well as the thickness of the Ag deposit. Use of these three complementary procedures applied under well-defined conditions provides a basis for adoption of a preferred and reliable procedure for determination of the calibration constant; the resulting mean value is precise and very reproducible.

Elucidation of the effects of competitive adsorption of Cl−and Br− ions on the initial stages of Pt surface oxidation by means of electrochemical nanogravimetry

Abstract At anodically polarized Pt electrodes in aqueous H 2 SO 4 solution, Cl − and Br − ions are adsorbed competitively with the oxygen species that is deposited between 0.8 and 1.1 V, RHE, forming the initial stage of anodic oxide-film generation at Pt anodes. The Cl − adsorption leads to selective and progressive blocking of the first 50% of coverage by O species at the Pt surface with increasing Cl − concentration (commencing at 10 −7 M) while relatively little effect is seen on the place-exchanged Pt/O→O/Pt lattice that is formed beyond 1.10 V, except at Cl − concentrations greater than 10 −1 M, where evolution of Cl 2 commences and hence interferes. The states of co-adsorbed Cl and O-containing species, and their relative surface coverages, determine the nature of the electrocatalytic surface on which anodic Cl 2 evolution takes place. How this selective blocking behavior arises has not previously been well understood. In the present paper, complementary applications of the electrochemical quartz crystal nanobalance (EQCN) technique with cyclic voltammetry are brought to bear on this matter and provide new results at high levels of sensitivity and reproducibility which help to elucidate the competitive and selective chemisorption effects of Cl − at Pt anodes. The results obtained with Cl − ions exhibit some peculiarities which comparative experiments on Br − adsorption help to rationalize.

Hydrogen adsorption on Pt and Rh electrodes and blocking of adsorption sites by chemisorbed sulfur

Research on the under-potential deposition of H, UPD H, on Pt and Rh electrodes in aqueous H2SO4 solution at temperatures between 273 and 343 K by cyclic-voltammetry followed by theoretical treatment leads to determination of ΔGads°, ΔSads° and ΔHads° and the bond energy between the metal substrate, M, and HUPD, EM−HUPD. Knowledge of EM−HUPD results in elucidation of the surface adsorption site of HUPD based on thermodynamic deliberation. The UPD H on Pt is suppressed completely by a monolayer of chemisorbed S, Schem, or partially by a submonolayer of Schem having its surface coverage less than 0.33, θS<0.33. A submonolayer of Schem having θS=0.10 affects ΔGads°, ΔSads° and ΔHads°; the Pt–HUPD bond energy, EPt−HUPD, becomes weaker in presence of the Schem submonolayer. The lateral interactions between HUPD and Schem are brought about by local electron withdrawing effects that propagate through the underlying metal which acts as a mediator.

Comprehensive Structural, Surface-Chemical and Electrochemical Characterization of Nickel-Based Metallic Foams

Nickel-based metallic foams are commonly used in electrochemical energy storage devices (rechargeable batteries) as both current collectors and active mass support. These materials attract attention as tunable electrode materials because they are available in a range of chemical compositions, pore structures, pore sizes, and densities. This contribution presents structural, chemical, and electrochemical characterization of Ni-based metallic foams. Several materials and surface science techniques (transmission electron microscopy (TEM), scanning electron microscopy (SEM), energy dispersive spectrometer (EDS), focused ion beam (FIB), and X-ray photoelectron spectroscopy (XPS)) and electrochemical methods (cyclic voltammetry (CV)) are used to examine the micro-, meso-, and nanoscopic structural characteristics, surface morphology, and surface-chemical composition of these materials. XPS combined with Ar-ion etching is employed to analyze the surface and near-surface chemical composition of the foams. The specific and electrochemically active surface areas (As, Aecsa) are determined using CV. Though the foams exhibit structural robustness typical of bulk materials, they have large As, in the range of 200-600 cm(2) g(-1). In addition, they are dual-porosity materials and possess both macro- and mesopores.