Hydrogen Underpotential Deposition Research Articles

Assemblies of polyvinylpyrrolidone-capped tetrahedral and spherical Pt nanoparticles in polyelectrolytes: hydrogen underpotential deposition and electrochemical characterization

Polyvinylpyrrolidone (PVP)-capped Pt nanoparticles (NPs) were synthesized in mostly tetrahedral (TH-Pt, [edge] = 4.3 ± 0.7 nm) or spherical (S-Pt, [d] = 3.4 ± 0.8 nm) shapes and assembled layer-by-layer in poly(diallyldimethylammonium) chloride on electrodes driven by electrostatic and hydrophobic interactions. The nanostructured Pt electrodes were characterized using hydrogen underpotential deposition (H(upd)) in 1 M H2SO4. The H(upd) charge increased linearly with the PDDA-Pt NP adsorption cycle measured up to 10 cycles revealing a linear incorporation of Pt NPs per cycle, indicative of reproducible surface charge reversal despite the submonolayer NP coverage imaged by TEM on a PDDA layer, and showing the feasibility of charge and mass transport in the thickness of the films. H(upd) at both PVP-TH-Pt and PVP-S-Pt occurred in two states, a major weak-adsorption H(W) peak, and a minor strong-adsorption state H(S) appearing as a shoulder. H(upd) features and other electrochemical processes at assemblies of PVP-Pt NP in PDDA were compared to assemblies of 2.5 nm polyacrylate-capped Pt NPs in PDDA and to polycrystalline Pt. Results indicated that H(W) adsorption likely occurs on a PVP-modified Pt NP surface without being accompanied by PVP desorption, while H(S) occurs on free (100) sites. The PVP-Pt NPs were resistant to surface oxidation and were stable against usual surface restructuring when scanned into the Pt-oxide potential region as they remained modified with PVP. O2 evolution was also suppressed by PVP-capping compared to PAC-Pt NPs and polycryst-Pt, but the assemblies were electrocatalytic for hydrogen evolution, hydrogen oxidation, and oxygen reduction. Increasing anodic polarization increased the H(W) charge but without causing a potential shift, indicating absence of PVP decapping or Pt surface restructuring, but possibly some structural polymer rearrangement increasing the accessibility of buried sites for H-adsorption.

Hydrogen oxidation on ordered intermetallic phases of platinum and tin – A combined experimental and theoretical study

Hydrogen oxidation on the surfaces of the intermetallic compounds Pt3Sn, PtSn and PtSn2 has been studied by the rotating disc electrode technique. Pt3Sn and PtSn were found to be good catalysts, about as good as Pt, while PtSn2 was inactive over the investigated range of potentials. Underpotential deposition of hydrogen is observed only on Pt3Sn. These results are explained by theoretical calculations based on a theory developed within our own group, and by density functional theory.

Kinetics of hydrogen underpotential deposition at polycrystalline platinum in acidic solutions

Kinetics of the hydrogen underpotential deposition (HUPD) reaction at smooth polycrystalline platinum has been studied in 0.1M H2SO4 and HClO4 using cyclic voltammetry and electrochemical impedance spectroscopy (EIS). Impedance was measured at Pt electrodes (i) at lower frequencies (LF) up to 10kHz in a classical set up with the potentiostat and the frequency response analyzer at a single Pt disk electrode in a hanging meniscus configuration or Pt wire and in a large frequency range up to 1MHz with the FRA in a stand-alone mode and two arrangements of the electrodes, (ii) at small twin Pt electrodes, and (iii) at one hanging meniscus electrode and another of similar geometric size but of large surface area. Real surface area of the Pt electrodes was determined by the hydrogen adsorption method. All impedance spectra exhibited a CPE behavior. High frequency measurements permitted determination of the double layer capacitance, hydrogen adsorption pseudocapacitance and charge transfer resistance of the HUPD reaction. Deviations from the ideal capacitive response were larger at high than at low frequencies. The possible origins of this dispersion and the electrical equivalent circuits have been discussed.

Diffusion of adsorbed CO on platinum (100) and (111) oriented nanosurfaces

Platinum nanocubes exhibiting oriented surface domains were used to study the electrooxidation of chemisorbed CO (COchem.). The amount of oriented surface nanodomains was characterized by hydrogen underpotential deposition (Hupd), bismuth and germanium spontaneous adsorption/oxidation. The surface diffusion of COchem toward low coordination sites, where oxidation is expected to take place, is more rapid on (111) than on (100) nanodomains and no diffusion of CO from (100) domains toward (111) nanodomains seems to occur.

Dynamic impedance study of ethanol and acetaldehyde oxidation at platinum in acid solutions

Ethanol and acetaldehyde oxidation at polycrystalline platinum electrode in 0.5M sulfuric acid were studied using cyclic voltammetry and dynamic electrochemical impedance spectroscopy and different concentrations of electroactive species. It was found that sharp peaks on cyclic voltammograms of ethanol observed by various authors were related to the appearance of negative resistances. The simplest electrical equivalent circuits explaining the impedance were found. Double layer capacitance measurements revealed that hydrogen underpotential deposition exists at the least positive potentials. In the potential range 0.3–0.7V low capacitance of ∼10μFcm−2 was observed due to strong adsorption of organic fragments and increased upon platinum oxidation. These results were reproducible upon cycling. In the potential zone of the formation of the first anodic peak the logarithmic slopes of dE/dRct were larger for the positive sweep than for the negative sweep. The zones of the negative resistances were identified.

Colloidal Syntheses of Shape- and Size-Controlled Pt Nanoparticles for Electrocatalysis

Different colloidal synthesis methods of platinum nanoparticles with controlled sizes and shapes that are relevant for electrocatalysis studies are reviewed. Four main methods, i.e., water in oil microemulsion (w/o) method, polyacrylate (PA) method, tetradecyltrimethylammonium bromide (TTAB) method, and ethylene glycol method, are used to synthesize platinum nanoparticles. The PA method allowed us to synthesize reproducibly nanocubes, nanooctahedrons, or nanocuboctahedrons/truncated nanooctahedrons with size between 8 and 10 nm, the TTAB method led to the synthesis of nanocubes of about 10 nm, and the w/o method allowed the synthesis of spherical particles of about 3 nm. All these samples could be cleaned for further electrochemical characterization of their surface structure by hydrogen underpotential deposition and by spontaneous deposition and oxidation of bismuth and germanium, leading to a quantitative determination of the (100) and (111) surface domains. The samples prepared by the ethylene glycol method, in the presence or not of polyvinylpyrrolydone as surfactant, were size or shape controlled, but did not allow the electrochemical characterization of their surface due to remaining of strongly adsorbed organic species even after cleaning steps.

Electrochemical behavior of Pt–Co(111), (100) and (110) alloy single-crystal electrodes in 0.1 M HClO4 and 0.05 M H2SO4 solution as a function of Co content

We investigated the electrochemical behavior of Pt–Co(111), (100) and (110) alloy single-crystal electrodes in 0.1M HClO4 and 0.05M H2SO4 solutions as a function of Co composition. Successive changes of cyclic voltammograms at the Pt–Co single-crystal electrodes with increasing Co content revealed alloying effects on the underpotential deposition of hydrogen and the surface oxidation process, with a strong dependence on crystal plane and electrolyte.

In depth analysis of complex interfacial processes: in situ electrochemical characterization of deposition of atomic layers of Cu, Pb and Te on Pd electrodes

A combination of cyclic voltammetry, electrogravimetry, and electrochemical impedance spectroscopy has been used to characterize, in situ, the underpotential deposition (UPD) of atomic layers of Cu, Pb and Te on Pd electrode surfaces. This approach provides co-adsorption and competitive adsorption of anions to be measured and quantified during the UPD processes, highlighting the complex competitive processes that can e.g. hinder the design of new catalysts. The formed Cu, Pb and Te atomic layers on the Pd electrode showed no evidence of anion co-adsorption or surface alloying effects, which indicates that these systems, when formed in a perchlorate medium, could act as building blocks for catalysts. The mode of deposition was found to vary greatly for each overlayer. Cu was found to form a compact monolayer on the Pd surface, while Te formed a bilayer structure on the Pd surface, of which ∼1/4 of a monolayer was found to be irreversibly adsorbed. The formation of Pb overlayers was complicated by background UPD of hydrogen and its absorption to the underlying Pd substrate. While perchloric acid media are suitable for the formation of the overlayer, catalytic application of the formed Pb-layers would require a higher pH to negate such processes.

Observations in the H-Pd{111} System via Stress and EQCM Measurements

The under-potential deposition (UPD) & absorption of hydrogen in thin film palladium (45–75 nm) was investigated electrochemically utilizing both EQCM and thin film stress measurement techniques. The total frequency change during H absorption (desorption) as measured by EQCM is attributed to both an increase (decrease) in mass, Δfm, as well as the associated thin film stress evolution, Δfs. In order to deconvolute these two contributions, the changes in the Pd{111} thin-film stresses were measured directly by a high-resolution cantilever curvature-based stress monitoring technique. Mass incorporation and stress generation due to hydrogen absorption were recorded during cyclic voltammetric scans of increasingly cathodic potentials as well as during chronoamperometric holds in the hydrogen UPD region. The combination of stress measurements and mass uptake of hydrogen in the purely elastic regime provided a measure of a biaxially constrained partial molar volume of hydrogen in the Pd thin films, = 0.44 cm3/mol. Additionally, the occurrence of hysteresis and dramatic changes in slope of stress vs. H concentration during loading and unloading of thin films is explained in the context of elastic-plastic transitions that occur in the film.

The Active Surface Area of Nanoporous Metals during Oxygen Reduction

Nanoporous metals formed by electrochemical dealloying are seeing increased attention as catalysts for fuel cells because they combine the positive attributes of high geometric surface area, facile processing, and an active core/shell composition profile. Analysis of oxygen reduction in such materials is complicated by the problem of the coupled reaction/diffusion kinetics of oxygen in the small (<5 nm) pores of the material. In porous electrocatalytic media, we show that such kinetics lead to a potential-dependent active surface area that has little to do with the surface area measured by hydrogen underpotential deposition (UPD) - at high overpotential, the effective area participating in oxygen reduction is approximately equal to the geometric area, and this effective area grows when approaching lower overpotentials for oxygen reduction where there is a decreased reaction rate.

Study of CO Oxidation on Polycrystalline Pt Electrodes in Acidic Solution by ATR-SEIRAS

Adsorption and electro-oxidation of CO on a polycrystalline Pt electrode in acidic solutions were systematically revisited by in situ attenuated-total-reflection surface-enhanced infrared absorption spectroscopy (ATR-SEIRAS) in conjunction with related Gram–Schmidt response analysis. CO was either adsorbed in the double-layer region, i.e., 0.45 V (RHE) (denoted as CO@DL) or in the hydrogen underpotential deposition region, i.e., 0.1 V (RHE) (denoted as CO@UPD). The results indicate that the CO@UPD and H2Ofree coexisted structure (or simply costructure) forms only at a sufficiently high global CO coverage (H2Ofree denotes hydrogen-bonding-broken water); In contrast, the CO@DL and H2Ofree costructure forms in an earlier adsorption phase, less dependent on the global CO coverage. The partial oxidation of CO from solution and weakly adsorbed COL at the active sites is suggested to yield a prepeak that occurs with the relaxation of the COad-H2Ofree costructure and the disorganization of the outer water net layers. In the main oxidation process, the oxidation of CO@UPD tends to proceed via the “mean-field approximation” kinetics due to the high COad mobility resulting from the oxidation prepeak. The oxidation process of CO@DL is, however, likely via the “nucleation and growth” kinetics due to the good stability of the local CO@DL and H2Ofree costructured islands. The H2Ofree can be better assigned to the “probe” of the local COad coverage rather than the main oxygenated species for COad oxidation according to the spectral results for both CO@UPD and CO@DL.

Unprecedented Structural Sensitivity toward Average Terrace Width: Nafion Adsorption at Pt{hkl} Electrodes

Ultrathin Nafion overlayers ranging from 0 to 3.3 nm in thickness have been deposited on platinum single crystal electrodes in order to study their voltammetric behavior as a function of both Nafion surface coverage and platinum surface step density. Unusual structural sensitivity of Nafion-induced voltammetric peaks ascribable to Nafion interactions with step sites is observed as a function of average terrace width. For stepped surfaces containing both {111} and {100} terraces, average terrace widths of 2–3 atoms give rise to the most intense Nafion-step electrosorption peaks. Rapid quenching of the Nafion-step voltammetric peaks is observed as the average terrace width is gradually increased until for >5 atom wide terraces all such electrochemical features are completely attenuated. This rapid diminution of the step-Nafion voltammetric peak, within just three atoms of terrace width, is unprecedented, and a parallel with the “splitting” of hydrogen underpotential deposition (HUPD) step peaks in perchloric acid aqueous electrolyte for closely spaced steps is noted. On the basis of these measurements, it is suggested that Nafion is a molecular probe of adsorbed OH species on Pt electrodes.

Kinetics of hydrogen underpotential deposition at polycrystalline rhodium in acidic solutions

Kinetics of hydrogen underpotential deposition (H UPD) reaction at smooth polycrystalline rhodium wire has been studied in 0.1 M H 2SO 4 and HClO 4 using cyclic voltammetry and electrochemical impedance spectroscopy. Real surface area of the Rh electrodes was determined by hydrogen adsorption method. For all impedance spectra registered at potentials corresponding to the H UPD deviations from the ideal capacitive behavior was revealed and a constant phase element was used instead of capacitance for description of Rh impedance behavior. Double layer and adsorption capacitances and charge transfer resistances of the H UPD were determined as functions of the electrode potential. It has been found that the kinetics of the H UPD reaction at Rh is slower than that at polycrystalline Pt but similar to that found at polycrystalline Ru. Slightly higher values of R ct were determined in perchloric acid.

ChemInform Abstract: Electrochemical Hydrogen Adsorption and Absorption. Part 1: Underpotential Deposition of Hydrogen

AbstractReview: 134 refs.

Nanocomposite electrodes based on pre-synthesized organically capped platinum nanoparticles and carbon nanotubes. Part II: Determination of diffusion area for oxygen reduction reflects platinum accessibility

In this paper we report the determination of the diffusion area for oxygen reduction in porous electrode structure having a controlled platinum loading and based on capped platinum electrocatalysts and carbon nanotubes. Such a parameter is expected to be higher than the macroscopic geometrical area of the active porous layer. The oxygen diffusion area is determined by cyclic voltammetry after impregnation of the electrode structure by the electrolyte, and using the equations available for peak potential and peak current as a function of scan speed for irreversible redox couple. First it is found first that the oxygen diffusion area is dependent on the total amount of platinum in the electrode. Second, for a given platinum loading, the diffusion area is higher when the mass ratio of platinum to carbon nanotube decreases. This point indicates that the accessibility of platinum capped electrocatalyst is better in such cases. It is thus concluded that the oxygen diffusion area determination in porous electrode structures may be used to characterize the accessibility of the capped electrocatalysts for oxygen reduction. Even if this area is different in nature from the one calculated by Hydrogen Underpotential Deposition, we believe that its determination might be of interest for the characterization of porous electrodes structures in which the electrocatalyst is combined with a finely divided carbon support.

Carbon supported Pt-based ternary catalysts for oxygen reduction in PEM fuel cells

Carbon supported platinum based ternary catalysts, namely, Pt6CuFe/C, Pt6AgCu/C and Pt6AgFe/C were prepared by the wet impregnation–reduction method in order to be used at the cathode of the PEM fuel cell. The catalysts were characterized by various techniques including N2 physisorption, XRD, SEM, EDX and CV. N2 physisorption analysis showed that the catalysts have surface areas varying between 152 and 166 m2/g. Estimated platinum mean particle sizes were between 3 and 4 nm according to the XRD data. Electrochemically active surface areas were evaluated by the hydrogen underpotential deposition method and Pt6AgFe/C was found to have the highest EAS with 557 cm2/mgPt and the highest mass activity with 29.1 mA/mg at 0.7 V, which is comparable to the mass activity of Pt/C.

Fourier transform electrochemical impedance spectroscopic studies on platinum electrodes in an acidic medium

The platinum electrode/electrolyte interface has been characterized in 0.50 M H 2SO 4 by staircase cyclic voltammetry–Fourier transform electrochemical impedance spectroscopy combined experiments in a wide potential range. From analysis of a large body of data, we propose a single equivalent circuit for the description of the interface, which accommodates impedance responses from capacitance dispersions, hydrogen underpotential deposition (H-UPD), and Faradaic reactions in 0.50 M H 2SO 4 within the potential range of 0.05–1.5 V. Analysis of the impedance data revealed that the equivalent circuits reported in the literature in specific potential regions represent specific forms of the circuit proposed here. The capacitance dispersions are rather small in H-UPD and oxide-covered capacitive regions due to large double-layer capacitances in relation to pseudo-capacitances. Electrochemical oxidation of platinum and its oxide reduction are shown to proceed via two step one-electron-transfer reactions and their respective kinetic parameters are reported.

Active Area Determination of Porous Pt Electrodes Used in Polymer Electrolyte Fuel Cells: Temperature and Humidity Effects

This paper discusses the proper measure of the electrochemically active area (ECA) of carbon supported Pt catalyst in polymer electrolyte fuel cells employing in situ cyclic voltammetry. The charges of the hydrogen underpotential deposition and CO stripping peaks obtained in situ are compared, and the influence of operation temperature and relative humidity (40%–90%) is discussed. The results show that the charges of the decrease with rising temperature, while the corresponding charges of the CO stripping peak are essentially independent of temperature, at least at high relative humidity. The unexpectedly small charges are explained by the significant overlap with the hydrogen evolution reaction in a fuel cell at elevated temperatures. According to our results, it is proposed that a more reliable value of Pt ECA is estimated from the CO stripping charge. However, with decreasing humidity the charges of both and CO stripping peaks decrease, which is probably an effect of increasing blockage of Pt active sites by hydrophobic domains in the electrode ionomer. Some implications of varying cell conditions on the estimated Pt ECA and its correlation with fuel cell activity are discussed in an example from a fuel cell degradation test.

Preparation and Electrochemical Behavior of PtRu(111) Alloy Single‐Crystal Surfaces

The electrochemical behavior of a PtRu(111) single crystal with 1:1 bulk atomic ratio is investigated for the first time by means of cyclic voltammetry and scanning tunneling microscopy (STM). The electrode surfaces are enriched with either Ru or Pt, depending on the cooling conditions after inductive heating. Analysis of the surfaces by STM shows a typical topography with smooth terraces separated by monoatomic high steps. The voltammetric characterization of PtRu(111) in acid media clearly reveals an altered electrochemical behavior of the Pt and Ru surfaces compared to Pt(111) and Ru(0001), respectively. Systematic changes are observed for hydrogen adsorption and underpotential deposition of copper as test reactions. Based on theoretical calculations in the literature, it is experimentally verified that the Pt-rich and the Ru-rich surfaces of the PtRu(111) single-crystal alloy bind adsorbates such as hydrogen significantly weaker and stronger than the pure single-crystal electrode surfaces. Such changes in surface reactivity can be crucial for electrocatalytic reactions.

Electrochemical Hydrogen Adsorption and Absorption. Part 1: Under-potential Deposition of Hydrogen

The review article presents an atomic-level discussion of the fundamental steps involved in hydrogen electro-adsorption on transition-metal electrodes. It also discusses the basic events involved in hydrogen physisorption and chemisorption under low-pressure gas-phase conditions in order to identify characteristics that are unique to the electrochemical environment. The electrochemical environment that comprises the solid electrode, the liquid electrolyte solution, and the electrostatic field creates favorable conditions for the electro-adsorption of two different hydrogen species, the under-potential and over-potential adsorbed hydrogen (HUPD and HOPD). The electro-adsorption of HUPD and HOPD occurs in two different potential ranges. The electro-adsorption of HUPD is characteristic of Pt group metals only, while the electro-adsorption of HOPD takes place at all electrode materials at which H2(g) can be electrolytically generated. The existence of HUPD and HOPD is supported by spectroscopic and thermodynamic results. A thermodynamic analysis of the electro-adsorption of HUPD results in the determination of thermodynamic state functions that govern the process, namely the standard Gibbs energy, enthalpy and entropy of electro-adsorption, $$ {\Delta_{{\rm{ec - ads}}}}G^\circ \left( {{{\hbox{H}}_{\rm{UPD}}}} \right) $$ , $$ {\Delta_{{\rm{ec - ads}}}}H^\circ \left( {{{\hbox{H}}_{\rm{UPD}}}} \right) $$ , and $$ {\Delta_{{\rm{ec - ads}}}}S^\circ \left( {{{\hbox{H}}_{\rm{UPD}}}} \right) $$ , the Gibbs energy of later interactions, $$ \omega \left( {{{\hbox{H}}_{\rm{UPD}}}} \right) $$ , and the M–HUPD surface bond energy, $$ E^\circ \left( {{\hbox{M}} - {{\hbox{H}}_{\rm{UPD}}}} \right) $$ . Thermodynamic data for the electro-adsorption of HUPD on polycrystalline electrodes (Pt(poly) and Rh(poly)) and on Pt(111) in several aqueous electrolyte (H2SO4, HClO4, and NaOH) solutions are reported. Their analysis and the analysis of $$ {\Delta_{{\rm{ec - ads}}}}H^\circ \left( {{{\hbox{H}}_{\rm{UPD}}}} \right) $$ and $$ E^\circ \left( {{\hbox{M}} - {{\hbox{H}}_{\rm{UPD}}}} \right) $$ allow us to compare on a thermodynamic basis HUPD to Hchem, and to identify surface adsorption sites possibly occupied by HUPD and HOPD adatoms.