D. Tsiplakides

Electrode Work Function and Absolute Potential Scale in Solid-State Electrochemistry

A two-Kelvin-probe arrangement was used to measure, for the first time in situ, the work functions, Φ, of the gas-exposed surfaces of porous Pt, Au, and Ag working and reference electrodes exposed to O 2 -He, H 2 -He and O 2 -H 2 mixtures, and deposited on 8% Y 2 O 3 -stabilized ZrO 2 IYSZ) in a three-electrode solid electrolyte cell. It was found that at temperatures above 600 K the potential difference, U WR , between the working (W) and reference (R) electrode reflects the difference in the actual, spillover, and adsorption-modified work functions, Φ W and Φ R of the two electrodes cU WR = Φ W - Φ R [1] This equation, typically valid over 0.8-1 V wide U WR ranges, was found to hold for any combination of the Pt, Au, and Ag electrodes. It is consistent with the previously reported equation eΔU WR = ΔΦ W [2] which is also confirmed here, and allows for the definition of a natural absolute electrode potential U O (abs) in solid-state electrochemistry from U O2 (abs) = Φ/e [3] where Φ is the work function of the gas-exposed electrode surface of the metal (any metal) electrode in contact with the YSZ solid electrolyte. It expresses the energy of solvation of an electron from vacuum to the Fermi level of the solid electrolyte. The value U O2 0 (abs) = 5.14 ± 0.05 V was determined as the standard U O2 (abs) value at p O , = I bar and T = 673 K.

Activation of Catalyst for Gas‐Phase Combustion by Electrochemical Pretreatment

The catalytic activity of IrO2 catalyst used as an electrode on a YSZ solid electrolyte cell for the gas phase combustion of ethylene can be increased by electrochem. pre-treatment. Thus, the polarization of the IrO2 electrode for 90 min at 300 mA, relative to a Au electrode, both deposited on YSZ, increases the activity of the IrO2 catalyst after current interruption by a factor of 3. In situ catalyst work function measurements showed that after the electrochem. pre-treatment the IrO2 catalyst obtains higher work function. The activation of the catalyst is explained through the formation of a higher oxide, IrO2+d. [on SciFinder (R)]

Work Function and Catalytic Activity Measurements of an IrO2 Film Deposited on YSZ Subjected to In Situ Electrochemical Promotion

In order to investigate the origin of the effect of non faradaic electrochem. modification of catalytic activity (NEMCA), a Kelvin probe was used to measure in situ the changes induced in the work function of an IrO2 catalyst film deposited on yttria-stabilized zirconia upon electrochem. supply of O2- to the catalyst under reaction conditions. Ethylene oxidn. was chosen as a model reaction system. For this purpose an electrochem. reactor of novel design was used in which work function measurements could be carried out in situ during kinetic measurements. It was found that the changes in catalyst work function equal to changes in catalyst ohmic-drop-free potential and that the reaction rate depends exponentially on catalyst work function at low applied potentials. [on SciFinder (R)]

Scanning tunneling microscopy observation of the origin of electrochemical promotion and metal–support interactions

Abstract Scanning tunneling microscopy (STM) was used to investigate the surface of Pt single crystal catalyst surfaces interfaced with O2− conducting catalyst supports under conditions simulating electrochemical promotion and metal–support interactions. In both cases STM has clearly shown the reversible migration on the catalyst surface of promoting O2− species which are entirely distinct from normally chemisorbed oxygen originating from the gas phase. These observations provide useful information for the mechanism of electrochemical promotion and metal–support interactions, reveal the existence and fast migration of O2−, a most effective anionic promoter, on metal surfaces and underline its role in inducing the phenomena of electrochemical promotion and of metal–support interactions.

The absolute potential scale in solid state electrochemistry

Abstract The absolute electrode potential in solid state electrochemistry is discussed, defined and measured. The term “absolute” potential denotes an electrode potential not based on another reference electrode system but to a given reference electronic energy taken as zero. This is important because then the energy scale of electrochemical systems can be directly compared with that of solid/gas or solid/vacuum interfaces. A two-Kelvin probe arrangement was used to measure for the first time in situ the work functions, Φ, of the gas-exposed surfaces of porous Pt, Au and Ag working and reference electrodes, exposed to O2–He, H2–He and O2–H2 mixtures, and deposited on 8% Y2O3-stabilized-ZrO2 (YSZ) in a three-electrode solid electrolyte cell. It was found that at temperatures above 600 K, the potential difference, UWR, between the working (W) and reference (R) electrode reflects the difference in the actual, spillover and adsorption-modified, work functions, ΦW and ΦR of the two electrodes: (1) e U WR =Φ W −Φ R This equation, typically valid over 0.8–1 V wide UWR ranges, was found to hold for any combination of the Pt, Au and Ag electrodes. It is consistent with the previously reported equation: (2) eΔ U WR = Δ Φ W also confirmed here. This is due to the creation via ion spillover of an effective electrochemical double layer on the gas-exposed electrode surfaces in solid electrolyte cells, which is similar to the double layer of emersed electrodes in aqueous electrochemistry. Eq. (1) allows the definition of a natural absolute electrode potential UO2(abs) in solid state electrochemistry from: (3) U O 2 ( abs )=Φ/ e where Φ is the work function of the gas-exposed electrode surface of the metal (any metal) electrode in contact with the YSZ solid electrolyte. It expresses the energy of “solvation” of an electron from vacuum to the Fermi level of the solid electrolyte. The value UO2o(abs)=5.14(±0.05) V was determined as the standard UO2(abs) value for YSZ at pO2=1 bar and T=673 K.

Electrochemical promotion of a classically promoted Rh catalyst for the reduction of NO

The reduction of NO by CO in presence of O2, a reaction of great technological importance, was investigated on porous polycrystalline Rh catalyst-electrodes deposited on YSZ (Y2O3-stabilized-ZrO2 )a n O 2 conductor. It was found that application of current or potential between the Rh catalyst electrode and a Au counter-electrode enhances the rate of NO reduction and CO2 formation by up to a factor of 20. These rate increases are strongly non-Faradaic with apparent Faradaic efficiencies, L, up to 20, manifesting the effect of Electrochemical Promotion or Non-faradaic Electrochemical Modification of Catalytic Activity (NEMCA). The Rh catalyst electrodes were subsequently promoted in a classical way, via dry impregnation with NaOH, followed by drying and calcination. The thus Na-promoted Rh films were found, as expected, to exhibit much higher catalytic activity than the unpromoted films, with a pronounced decrease in their light-off temperature from 440 to 320°C. The effect of Electrochemical Promotion was then studied on these, already Na-promoted Rh catalyst. Positive ( 1 V) potentials were found to further increase the rate of NO reduction by up to a factor of 4 with a Faradaic efficiency up to 20 and concomitant reduction in light-off temperature down to 260°C. This is the first demonstration of electrochemical promotion on an already promoted catalyst surface.

Investigation of electrochemical promotion using temperature-programmed desorption and work function measurements

Abstract The origin of electrochemical promotion was investigated via temperature-programmed desorption (TPD) of oxygen from polycrystalline Pt and Ag films deposited on YSZ under high-vacuum conditions and via measuring the work function of a Pt film deposited on YSZ under catalytic reaction conditions at atmospheric pressure. It was found that electrochemical O 2− pumping to Pt and Ag catalysts in the presence of pre-adsorbed oxygen causes backspillover of large amounts of oxygen on the catalyst surface and leads to the formation of two oxygen adsorption states, i.e. strongly bonded anionic oxygen along with weakly bound atomic oxygen. Furthermore, the desorption activation energy of oxygen adsorbed on Pt and Ag catalyst films was found to decrease linearly with increasing catalyst potential. Finally, by increasing the Pt-catalyst potential from −0.9 to +1.3 V during C 2 H 4 oxidation on Pt, i.e. under electrochemical promotion conditions, a 1.42 eV increase in catalyst work function was observed. These results provide a straightforward explanation of the effect of electrochemical promotion on Pt and Ag deposited on O 2− conducting solid electrolytes.

Ternary Pt-Ru-Ni catalytic layers for methanol electrooxidation prepared by electrodeposition and galvanic replacement.

Ternary Pt-Ru-Ni deposits on glassy carbon substrates, Pt-Ru(Ni)/GC, have been formed by initial electrodeposition of Ni layers onto glassy carbon electrodes, followed by their partial exchange for Pt and Ru, upon their immersion into equimolar solutions containing complex ions of the precious metals. The overall morphology and composition of the deposits has been studied by SEM microscopy and EDS spectroscopy. Continuous but nodular films have been confirmed, with a Pt ÷ Ru ÷ Ni % bulk atomic composition ratio of 37 ÷ 12 ÷ 51 (and for binary Pt-Ni control systems of 47 ÷ 53). Fine topographical details as well as film thickness have been directly recorded using AFM microscopy. The composition of the outer layers as well as the interactions of the three metals present have been studied by XPS spectroscopy and a Pt ÷ Ru ÷ Ni % surface atomic composition ratio of 61 ÷ 12 ÷ 27 (and for binary Pt-Ni control systems of 85 ÷ 15) has been found, indicating the enrichment of the outer layers in Pt; a shift of the Pt binding energy peaks to higher values was only observed in the presence of Ru and points to an electronic effect of Ru on Pt. The surface electrochemistry of the thus prepared Pt-Ru(Ni)/GC and Pt(Ni)/GC electrodes in deaerated acid solutions (studied by cyclic voltammetry) proves the existence of a shell consisting exclusively of Pt-Ru or Pt. The activity of the Pt-Ru(Ni) deposits toward methanol oxidation (studied by slow potential sweep voltammetry) is higher from that of the Pt(Ni) deposit and of pure Pt; this enhancement is attributed both to the well-known Ru synergistic effect due to the presence of its oxides but also (based on the XPS findings) to a modification effect of Pt electronic properties.

Insights into the Surface Reactivity of Cermet and Perovskite Electrodes in Oxidizing, Reducing, and Humid Environments

Understanding the surface chemistry of electrode materials under gas environments is important in order to control their performance during electrochemical and catalytic applications. This work compares the surface reactivity of Ni/YSZ and La0.75Sr0.25Cr0.9Fe0.1O3, which are commonly used types of electrodes in solid oxide electrochemical devices. In situ synchrotron-based near-ambient pressure photoemission and absorption spectroscopy experiments, assisted by theoretical spectral simulations and combined with microscopy and electrochemical measurements, are used to monitor the effect of the gas atmosphere on the chemical state, the morphology, and the electrical conductivity of the electrodes. It is shown that the surface of both electrode types readjusts fast to the reactive gas atmosphere and their surface composition is notably modified. In the case of Ni/YSZ, this is followed by evident changes in the oxidation state of nickel, while for La0.75Sr0.25Cr0.9Fe0.1O3, a fine adjustment of the Cr valence and strong Sr segregation is observed. An important difference between the two electrodes is their capacity to maintain adsorbed hydroxyl groups on their surface, which is expected to be critical for the electrocatalytic properties of the materials. The insight gained from the surface analysis may serve as a paradigm for understanding the effect of the gas environment on the electrochemical performance and the electrical conductivity of the electrodes.

Investigation of the state of the electrochemically generated adsorbed O species on Au films interfaced with Y2O3-doped-ZrO2

Adsorbed O species on Au interfaced with Y2O3-doped-ZrO2 are generated by electrochemical O2− supply. It was found that two oxygen chemisorbed states are formed, which desorb at 420 °C (state α) and 550 °C (state β) with activation energies of desorption ranging between 115–145 kJ/mol and 235–270 kJ/mol, respectively. The strong interaction of the β-state O species with the Au surface causes an over 600 mV increase in Au surface potential and work function while the α-state O species is formed at even more positive catalyst-electrode potential. State α is attributed to normally adsorbed atomic O while the more ionic state β is only created electro-chemically and is mainly responsible for the work function increase of the Au catalyst-electrode surface. Their desorption activation energies of both states decrease linearly with increasing catalyst-electrode potential with slopes of the order of four.