Hydrogen Underpotential Deposition Research Articles

Kinetics of hydrogen underpotential deposition at ruthenium in acidic solutions

Kinetics of the hydrogen underpotential deposition (H UPD) reaction at polycrystalline ruthenium in 0.1 M H 2SO 4 and HClO 4 solutions was studied using cyclic voltammetry and electrochemical impedance spectroscopy. Surface of Ru electrode was prepared by: (i) simple mechanical polishing and (ii) polishing followed by cleaning in concentrated HNO 3 and reduction at −0.05 V. The electrode real surface area was determined from the copper UPD charge and the specific double layer capacitance was estimated from impedance. The charge transfer resistance and double layer and adsorption capacitances were determined as functions of the electrode preparation method and anodic limit of the potential sweep in cyclic voltammetry. The kinetics is faster in sulfuric than in perchloric acid but cycling to positive potentials has smaller effect on the charge transfer resistance. It has been found that the kinetics of the H UPD at Ru is much slower than that at Pt and even slower than at Pd. The obtained results could give a new insight into electrosorption properties of the ruthenium catalyst.

Electrochemical studies of irreversibly adsorbed ethyl pyruvate on Pt{ h k l} and epitaxial Pd/Pt{ h k l} adlayers

Cyclic voltammetry (CV) has been used to investigate the irreversible adsorption of ethyl 2-oxopropanoate (ethyl pyruvate (EtPy)) on well-defined Pt and Pd electrodes. Following dosing of EtPy at open circuit on Pt and Pd surfaces, adsorption was found to be structure sensitive with extensive chemisorption being exhibited at {1 1 0} and {1 0 0} terraces together with {1 1 1} × {1 0 0} and {1 1 1} × {1 1 1} steps. However, only very limited adsorption of EtPy was observed at {1 1 1} terraces under identical dosing conditions. In particular, Pt{1 0 0} terraces were found to give rise to significant decarbonylation of molecularly adsorbed EtPy in the hydrogen underpotential deposition (H upd) potential region to generate chemisorbed CO. This finding concerning EtPy adsorption and decomposition agrees with previous investigations on polycrystalline Pt. In contrast, no electrochemical evidence for EtPy decomposition to form adsorbed CO on Pd was ever detected, irrespective of the surface crystallography of the electrode. EtPy decarbonylation was found to be suppressed on Pt{1 0 0} by pre-modification with (R)-[(4S, 5R, 7S)-5-ethenyl-1-azabicyclo[2.2.2]octan-7-yl]-quinolin-4-yl methanol (cinchonidine (CD)), often applied as a chiral modifier in the Pt-catalysed enantioselective hydrogenation of α-ketoesters. The degree of inhibition was found to be proportional to the amount of CD adsorbed. This surface sensitivity suggested that EtPy requires a very specific adsorption geometry or number of free Pt sites to undergo decarbonylation. For all surfaces studied, EtPy adsorption products could readily be removed either by electrochemical hydrogenation (leaving behind a clean metal surface) or, in the case of adsorbed CO, by performing potential excursions to 0.85 V (Pd/H) to facilitate electrooxidation of CO to CO 2. The results highlight the relative importance of ‘active’ {1 1 0} and {1 0 0} defect sites (relative to {1 1 1} terraces) for the chemisorption of EtPy on polycrystalline Pt catalysts used in the enantioselective hydrogenation of α-ketoesters and this finding may have wider implications for catalyst design methodologies.

Impedance spectroscopy of H and OH adsorption on stepped single-crystal platinum electrodes in alkaline and acidic media

The adsorption kinetics of hydrogen and hydroxyl on Pt(111) and stepped Pt[n(111) × (110)] electrodes with n = 29, 9, and 4 in acidic and alkaline electrolytes have been studied using impedance spectroscopy. We found a potential dependent charge transfer resistance, R(ct), (∼30 Ω cm(2) to ∼1 kΩ cm(2)) for hydrogen underpotential deposition in alkaline media (0.05 M NaOH), whereas in acidic media (0.025 M HClO(4)) R(ct) was too low to be determined accurately. Assuming simple (mean field) isotherms to fit our data, we obtain repulsive interactions between H(upd) at the (111) terrace and effective attractive interactions at the steps. The adsorption of OH on (111) terraces is fast in both acidic and alkaline media.

Evidence for high performances of low Pt loading electrodes based on capped platinum electrocatalyst and carbon nanotubes in fuel cell devices

Recently we reported the preparation and electrochemical behaviour of porous electrodes based on the controlled combination of carbon nanotubes and capped platinum nanoparticles towards oxygen reduction. Due to the organic crown of the nanoparticles, the electrodes exhibited low hydrogen underpotential deposition (H upd) electroactive surface areas but significant activity towards oxygen reduction was recorded down to very low platinum loadings of few μg/cm 2. While the presence of organic stabilizing material, at the surface of the electrocatalyst synthesized by wet chemistry, may be considered as a potential drawback in fuel cell community, we present in this paper results showing that our capped electrocatalyst associated with carbon nanotubes can be used without any pre-treatment and exhibit high performances in fuel cell devices, in spite of low platinum loadings. Beyond the practical interest of such capped nanoparticles in fuel cell technology demonstrated here, fundamental question related to the high performances of the capped electrocatalyst are still opened and are currently under investigation.

Impedance Analysis for Hydrogen Adsorption Pseudocapacitance and Electrochemically Active Surface Area of Pt Electrode

Electrochemically active surface area (ECA) of a polycrystalline Pt electrode is measured from the pseudocapacitance (Cp) values that are associated with hydrogen underpotential deposition. The potential-dependent Cp values are extracted from the raw impedance data by removing the interferences coming from the double-layer charging and hydrogen evolution. Three different approaches have been made: (i) by using the proportionality between the capacitance and area of the capacitive peak on imaginary capacitance plots, (ii) by complex nonlinear least-squares (CNLS) fitting on both the imaginary and real part of complex capacitance with appropriate equivalent circuits, and (iii) by using the modified Kramers-Kronig (K-K) relation. The first approach is the simplest one for the Cp measurement but cannot be used in the hydrogen evolution region (<0.05 V vs RHE), whereas the measurement can be extended down to -0.01 V with the second method. The isotherm fitting on the Cp(E) profile shows that the saturation of adsorbed hydrogen is reached at -0.1 V vs RHE. Faster data acquisition is possible with the third approach since the data analysis can be made without the time-consuming low frequency data (<100 Hz). The roughness factor and ECA of the Pt electrode are calculated from the electric charge that is obtained by integrating the potential-dependent Cp values; the roughness factor (1.4-1.5) lies within the normal range for planar electrodes.

High-performance core–shell PdPt@Pt/C catalysts via decorating PdPt alloy cores with Pt

A core–shell structured low-Pt catalyst, PdPt@Pt/C, with high performance towards both methanol anodic oxidation and oxygen cathodic reduction, as well as in a single hydrogen/air fuel cell, is prepared by a novel two-step colloidal approach. For the anodic oxidation of methanol, the catalyst shows three times higher activity than commercial Tanaka 50 wt% Pt/C catalyst; furthermore, the ratio of forward current I f to backward current I b is high up to 1.04, whereas for general platinum catalysts the ratio is only ca. 0.70, indicating that this PdPt@Pt/C catalyst has high activity towards methanol anodic oxidation and good tolerance to the intermediates of methanol oxidation. The catalyst is characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy (XPS). The core–shell structure of the catalyst is revealed by XRD and TEM, and is also supported by underpotential deposition of hydrogen (UPDH). The high performance of the PdPt@Pt/C catalyst may make it a promising and competitive low-Pt catalyst for hydrogen fueled polymer electrolyte membrane fuel cell (PEMFC) or direct methanol fuel cell (DMFC) applications.

PEMFC electrode preparation by electrospray: Optimization of catalyst load and ionomer content

Electrodes for proton exchange membrane fuel cell (PEMFC) have been prepared by means of electrospray deposition, using different ionomer contents and catalyst loads. The electrodes have been mounted in single PEMFC, and tested as cathode. The electroactive platinum area of electrosprayed electrodes has been measured by the hydrogen underpotential deposition method. Polarization curves have been measured to determine the optimal concentration for the platinum catalyst (Pt/C, 20 wt.%) and the ionomer (Nafion ®). Best performance is observed with 15% ionomer content in the electrosprayed cathode due to minimal cell internal resistance. This optimal concentration is lower than that found for electrodes prepared by other standard methods, like airbrushing or impregnation, which is attributed to the improved ionomer distribution within the electrospray deposited catalyst layer. On the other hand, the optimum value for catalyst load is similar to that encountered for electrodes prepared by the other methods, which reflects that electrospray has little or no effect neither on the catalyst nor on the electronic resistance of the catalyst layer.

Characterization and single cell testing of Pt/C electrodes prepared by electrodeposition

Electrodes for proton exchange membrane fuel cells (PEMFC) have been prepared by the electrodeposition method. For this task, the electrodeposition of platinum is carried out on a carbon black substrate impregnated with an ionomer, proton conducting, medium. Before electrodeposition, the substrate is submitted to an activation process to increase the hydrophilic character of the surface to a few microns depth. Electrodeposition of platinum takes place inside the generated surface hydrophilic layer, resulting in a continuous phase covering totally or partially carbon substrate grains. Cross sectional images show a decay profile of platinum towards the interior of the substrate, reflecting a deposition process limited by diffusion of PtCl 6 2− through the porous substrate. Electrodes with different platinum loads have been prepared, and membrane electrode assemblies (MEA) have been mounted with the electrodeposited electrodes as cathode and other standard components (commercial anode and Nafion R 117 membrane). The electrochemically active surface area determined from hydrogen underpotential deposition charge, is lower on the electrodeposited electrodes than on standard electrodes. However, single cell testing shows higher mass specific activity on electrodeposited cathodes with low and intermediate Pt load (below 0.05 mg Pt cm −2).

Underpotential deposition of hydrogen on Pt(111): a combined direct molecular dynamics/density functional theory study

A combined direct molecular dynamics/density functional theoretical study of the electro-oxidation of molecular hydrogen at the Pt(111)/water interface has been carried out to provide insights into the adsorbed underpotential deposition (UPD) states of hydrogen at submonolayer coverages of bridging hydrogen atoms Hads(bridge). As in our previous study, H2 oxidation proceeds via the Heyrovsky process, forming a hydrated H+ ion and an UPD hydrogen atom, which is strongly adsorbed and is lying nearly flat in a bridging position on the Pt(111) surface. At submonolayer coverage, the UPD Hads(bridge) atoms spontaneously self-assemble to form a hexagonal 2D honeycomb network on the Pt(111) surface, a signature that could be considered to be characteristic of UPD states of hydrogen.

On the enhanced electrocatalytic activity of Pd overlayers on carbon-supported gold particles in hydrogen electrooxidation

Palladium-gold particles with varied composition were prepared by Pd electrochemical deposition on Au nanoparticles immobilized on model carbon support. Pd-Au/C catalysts were characterized ex situ by transmission electron microscopy, energy dispersive X-ray analysis and X-ray photoelectron spectroscopy, and in situ, by underpotential deposition of hydrogen and copper adatoms, and CO stripping. Hydrogen oxidation reaction on pristine and CO-poisoned Pd-Au/C particles was studied using rotating disk electrode (RDE) technique. It was found that the decrease of the effective Pd overlayer thickness below ca. two monolayers resulted in a two-fold increase of the exchange current density of the hydrogen oxidation reaction and in significant increase of CO tolerance.

Deposición de Hidrogeno a Subpotencial sobre Oro Policristalino y con Orientación Preferida

Hydrogen deposition at lower potentials than those corresponding to the hydrogen evolution reaction on polycrystalline gold whose surface has been preferentially oriented according to the three low index planes, has been studied. Several processes contribute to the anodic and cathodic currents at the double layer charging potential region: the specific anion adsorption and desorption, the lift of surface reconstruction, the hydrogen oxidation and the hydrogen underpotential deposition. The relative heights of different current contributions change with the surface preferential orientation. The hydrogen oxidation peaks grow continuously while those corresponding to the lift of surface reconstruction reach a limited height. These facts allow distinguishing the faradic current contributions from the capacitive current contributions at the double layer charging potentials in 0.5 M sulphuric acid solution.

Oxygen Reduction at Nanostructured Electrodes Assembled from Polyacrylate-Capped Pt Nanoparticles in Polyelectrolyte

Oxygen reduction and related processes are studied at nanostructured Pt electrodes assembled from polyacrylate-capped Pt nanoparticles (〈d〉 = 2.5 ± 0.6 nm) in poly(diallyldimethylammonium)chloride (PDDA) on indium tin oxide or glassy carbon with varying nanoparticle surface coverage. The nanoparticle density was varied laterally by varying the dipping time of PDDA-modified electrodes in the nanoparticles solution, or vertically with the number of nanoparticle/polyelectrolyte (bi)layers following a layer-by-layer assembly. TEM images revealed submonolayer coverage in one bilayer at 60 min dipping with a fractal distribution, and a significant surface coverage at four bilayers with evidence of multilayer assembly. Cyclic voltammetry in oxygen-containing electrolytes showed the assemblies to be electroactive for oxygen and hydrogen peroxide reduction, with a pH-dependent oxygen reduction peak shifting by −50 mV/pH unit. OH adsorption was found to be less favored occurring at more positive potential at the nanostructured electrode compared to polycrystalline Pt, while the oxide reduction peak was negatively shifted at the former electrode, in agreement with reports of increased oxophilicity with decreased particle size. The oxygen reduction peak potential shifted positively upon increasing Pt nanoparticles coverage, consistent with the catalytic activity of Pt for oxygen reduction. The active surface area of Pt nanoparticles was measured electrochemically from the charge of hydrogen underpotential deposition at the assemblies in H2SO4, and the diffusion-limited peak current for oxygen reduction measured per real Pt surface area is reported to decrease with increasing catalyst loading, as a result of reaching a limiting effective diffusion field.

Effect of inert gas flow on hydrogen underpotential deposition measurements in polymer electrolyte fuel cells

The effect of inert gas flow rate on hydrogen underpotential deposition ( H upd) measurements in polymer electrolyte fuel cells (PEFCs) was investigated using a novel experimental technique. The method combines local voltammetric measurements in PEFCs with the use of sectioned electrodes. The results give experimental proof that the high inert gas flow rate usually employed in voltammetric measurements in PEFCs at the working electrode results in high hydrogen reduction currents in both the cathodic and the anodic sweep, which hampers an accurate determination of the electrochemically active surface area (ECA). Strong spatial inhomogeneities occur at low potentials as a consequence of formation and accumulation of molecular hydrogen along the flow field. The results show that the flow of inert gas should be minimized or even stopped during a measurement to allow molecular hydrogen to accumulate at the working electrode and to provide uniform conditions along the flow field.

Direct molecular dynamics and density-functional theoretical study of the electrochemical hydrogen oxidation reaction and underpotential deposition of H on Pt(111)

Abstract A combined direct molecular dynamics/density-functional theoretical study of the electro-oxidation of hydrogen at the Pt(1 1 1)/water interface has been carried out to provide insights into the reaction mechanism. The H2 oxidation proceeds via the Heyrovsky process – a heterolytic bond breaking brought about by the impinging hydrogen molecule striking the metal surface. The products are a hydrated H+ ion and a strongly adsorbed hydrogen atom lying nearly flat in a bridging position on the Pt(1 1 1) surface. The bridging H interacts only weakly with the aqueous phase of the metal/solution interface and imparts unique electronic and vibrational properties characteristic of underpotentially deposited H. Potential-dependent activation energies for the hydrogen redox reactions were evaluated.

Electrochemical Behaviour of a Composite Rh/TiO&lt;sub&gt;2&lt;/sub&gt; Layer Formed Potentiodynamically on Titanium Surface

Potentiodynamic polarization of a mechanically polished titanium electrode in a diluted solution of Rhodium(III) chloride in 0.1 M perchloric acid was performed, resulting in simultaneous formation of both Rh and TiO2 films. The morphology of obtained Rh/TiO2 composite film followed the morphology of titanium support, as evidenced by SEM technique. This composite surface was examined by cyclic voltammetry in both acidic and alkaline solutions, in the potential region of both hydrogen and oxygen underpotential deposition. The charge related to hydrogen underpotential deposition corresponded to a surface roughness of 43. As a consequence of high surface roughness, the diffusion current of oxygen reduction in an oxygen saturated 0.1 M NaOH solution, measured by voltammetry on rotating disc electrode, was found to be comparable to the current of hydrogen underpotential deposition.

Electrocatalytic Activity of Nano-Sized Ebonex/Pt for Underpotential Deposition of Hydrogen

The underpotential deposition of hydrogen was studied in 0.5 mol dm-3 HClO4 solution on an electrode based on Ebonex-supported platinum electrocatalyst spread on rotation Au disk electrode (Ebonex/Pt). Pt catalyst was prepared by the impregnation method from 2-propanol solution of Pt(NH3)2(NO2)2 and Ebonex powder. Ebonex support (nonstoichiometric mixture of titanium oxides) was characterized by: X-ray diffraction (XRD), energy dispersive X-ray spectroscopy (EDX) and BET techniques. The synthesized catalyst was analyzed by TEM technique. Voltammetric profiles at the Ebonex/Pt catalyst surface in 0.5 mol dm-3 HClO4 aqueous solution obtained at different temperatures with the evaluation of the temperature effect on the reversible adsorption of the Hupd state are presented and the thermodynamic state functions for H adatom adsorption process are calculated. The chemisorptive energy strength of the Ebonex/Pt-H state is estimated in order to establish the relationship between the surface structure and the electrocatalytic activity of Ebonex/Pt electrode and compare it to the one for smooth polycrystalline Pt.

Comparative Potentiodynamic Study of Nickel and Hydrogen Underpotential Deposition at Polycrystalline Platinum Electrode in Weak Acid and Neutral Solutions

Underpotential deposition of nickel and hydrogen on polycrystalline platinum in weak acid and neutral solutions (3.5 ≤ pH ≤ 7.0), with and without Ni2+-ions, has been examined using cyclic voltammetric technique in the range of temperature from 274 to 313 K. The nickel and hydrogen ad-atom surface coverages were calculated from the voltammetric adsorption and desorption charges. The Temkin isotherm was fitted for both underpotential depositions, and thermodynamic adsorption parameters were calculated for both atoms. The value of the bonding energy between hydrogen and surface in the presence of underpotentially deposited nickel was calculated and found to amount to 287 kJ mol-1, which is 40 kJ mol-1 stronger bond than the same of pure platinum. The presence of nickel on the surface facilitates interfacial transfer of hydrogen into the metal bulk and consequent hydride formation significant in design of hydrogen based fuel cells.

Ta 2O 5 templated growth of droplet-like platinum particles by potentiodynamic polarization of tantalum in aqueous solution of hexachloroplatinic acid

By potentiodynamic polarization of mechanically polished tantalum in a diluted aqueous solution of hexachloroplatinic acid, droplet-like platinum microparticles were electrodeposited, embedded into the simultaneously formed Ta 2O 5 film. The roughness factor of platinum of 31 was achieved. Within the potential region of both hydrogen and oxygen underpotential deposition, in both acidic and alkaline solutions, the composite Pt/Ta 2O 5 electrode displayed an excellent electrochemical response characteristic of smooth polycrystalline platinum. The preparation method applied in this work presents an easy way to obtain an electrode surface combining the behaviour of smooth polycrystalline platinum with the behaviour of microdisc arrays. Its electrocatalytic effectiveness was demonstrated for an oxygen reduction reaction in alkaline solutions.

Comparison of Voltammetric Responses over the Cathodic Region in LiPF6 and LiBETI with and without HF

This study reports the effects of HF on the voltammetric responses below (vs. ) in solutions being widely used in rechargeable lithium-ion batteries (LIBs). During the first cathodic scan on a Pt electrode, two reduction peaks are observed between 3.0 and in solutions irrespective of the solvents used. The reduction reactions are absent in LiBETI solutions but present after the addition of trace amount of HF. Thus, the reduction reactions in solutions are due to the HF, an inevitable impurity of solutions. Based on the results of the rotating disk electrode and the rotating ring disk electrode experiments, two reduction peaks are assigned to the hydrogen underpotential deposition and the hydrogen evolution reaction, respectively. HF reduction reactions forms a surface layer of which the main constituent is LiF. The surface layer suppresses the other reduction reactions otherwise occurring below . For example, the electrodeposition of metallic Mn, which takes place below in LiBETI solutions, is severely hampered in solutions. The passivity by HF reduction is observed not only for a Pt electrode but also for a glassy carbon electrode.

Electrochemical response of a composite Pt/TiO 2 layer formed potentiodynamically on titanium surfaces

By potentiodynamic polarization of mechanically polished titanium in a diluted aqueous solution of hexachloroplatinic acid, nearly spherical Pt microparticles were grown, embedded into the simultaneously formed TiO 2 film. The real surface of platinum may be easily controlled by the number of potentiodynamic cycles, and roughness factors exceeding 100 were obtained. Within the potential region of both hydrogen and oxygen underpotential deposition, in both acidic and alkaline solutions, the electrode displayed an excellent electrochemical response characteristic of smooth polycrystalline platinum, unlike to highly dispersed platinum electrodes produced by other methods. The preparation method applied in this work presents an easy way to obtain an electrode surface combining the behaviour of smooth polycrystalline platinum with the behaviour of microdisc arrays. Its high electrocatalytic effectiveness was demonstrated for oxygen reduction and bromide ion adsorption/oxidation.