Exchange current density | EPFL Graph Search

In electrochemistry, exchange current density is a parameter used in the Tafel equation, Butler–Volmer equation and other electrochemical kinetics expressions. The Tafel equation describes the dependence of current for an electrolytic process to overpotential. The exchange current density is the current in the absence of net electrolysis and at zero overpotential. The exchange current can be thought of as a background current to which the net current observed at various overpotentials is normalized. For a redox reaction written as a reduction at the equilibrium potential, electron transfer processes continue at electrode/solution interface in both directions. The cathodic current is balanced by the anodic current. This ongoing current in both directions is called the exchange current density. When the potential is set more negative than the formal potential, the cathodic current is greater than the anodic current. Written as a reduction, cathodic current is positive. The net current

Chemical and electrochemical promotion of supported rhodium catalyst

Olena Baranova

The chemical and electrochemical promotion of highly dispersed nanofilm Rh catalysts (dispersion: about 10 %, film thickness: 40 nm) has been investigated for the first time. To this end Rh metal was sputter-deposited, either on a purely ionic conductor (8 % Y2O3-stabilized ZrO2) or on a mixed ionic-electronic conductor (TiO2), the latter being a highly dispersed layer of TiO2 (4 µm) deposited on YSZ. These catalysts are designated as Rh/YSZ and Rh/TiO2/YSZ, respectively. It was established analytically that both in the Rh/YSZ and in the Rh/TiO2/YSZ system, the catalyst films have a nanoparticle-size grain structure. The Rh supported on titania is rather porous, exhibiting a higher dispersion and surface area than Rh on YSZ. Both after reduction (H2, T=400 °C) and after oxidation (O2, T=400 °C), Rh supported on TiO2 was found to be in a more highly reduced state than Rh on YSZ. After reducing treatment, the Rh/TiO2/YSZ samples contain a larger amount of weakly bonded oxygen, which can be attributed to oxygen "backspillover" from the TiO2. A major feature of this research was the electrochemical characterization of the oxygen/Rh/solid electrolyte three-phase boundary by steady-state polarization measurements and by impedance spectroscopy. These are powerful techniques for extracting experimental trends and details that are useful for an understanding of the electrochemical promotion principles. It was shown that the exchange current densities at the Rh/solid electrolyte interface are lower on account of the TiO2 layer. The exchange current densities are more than twice lower at Rh/TiO2(4 µm)/YSZ than at Rh/YSZ, demonstrating that the former interface is much more polarizable. The mechanism of oxygen exchange occurring close to equilibrium (O2/O2- couple) was investigated for the first time at Rh catalyst electrodes interfaced with solid electrolyte. The processes of cathodic oxygen reduction and anodic oxygen evolution are not symmetric, but they are similar in the two systems, Rh/YSZ and Rh/TiO2(4 µm)/YSZ. The cathodic process consists of three steps: dissociative adsorption of oxygen at the gas-exposed Rh surface, atomic oxygen diffusion to the electrochemical reaction sites (ERS), and a two-electron transfer to this oxygen on the ERS. The cathodic process is limited by interfacial diffusion of oxygen atoms from the gas-exposed metal surface to the ERS. The anodic process includes two steps: two-electron transfer reaction, which is the rate-determining step, and oxygen desorption to the gas phase. With data obtained from impedance spectroscopy at the equilibrium potential, it was possible to confirm the reaction scheme proposed. It was demonstrated that Rh nanofilm catalysts interfaced with YSZ or TiO2/YSZ can be electrochemically promoted for the reaction of ethylene oxidation. Small anodic currents cause periodic oscillations in catalytic rate and potential of the Rh/YSZ catalyst, while at Rh/TiO2/YSZ they give rise to a stable and reversible rate enhancement by up to a factor 80. The increase in ethylene oxidation rate is up to 2000 times larger than the electrochemical rate, I/2F, of O2- oxidation. The pronounced electrochemical promotion behavior that has been observed is due to the anodically controlled migration of O2- species from the electrolyte to the Rh/gas interface. At the Rh surface, these species destabilize the formation of rhodium surface oxide (Rh2O3). The existence of backspillover oxygen species has been confirmed by impedance measurements under positive applied potential. Another significant result for heterogeneous catalysis is the finding that thick as well as thin films of Rh/TiO2/YSZ catalyst are open to chemical promotion of ethylene oxidation and partial methane oxidation, ie, they offer a higher catalytic activity and stability than the Rh/YSZ catalysts. The modification of catalytic activity observed for Rh/TiO2/YSZ was attributed to either a "long-range" electronic-type SMSI mechanism or to a self-driven electrochemical promotion mechanism. In both cases, the ultimate cause of promotion are different work functions of catalyst and support. Equilibration of the work functions of two solids in contact induces surface charging, a migration of O2- ions to the catalyst/gas interface, and a weakening of the Rh-O chemisorptive bonds. It facilitates reduction of oxidized surface sites. Another important achievement of the present work was that of exploring and confirming the possibilities of current-assisted activation of Rh/TiO2/YSZ catalysts. In partial methane oxidation using close to stoichiometric gas compositions (CH4 : O2 = 2 : 1) at 550 °C, the inactive (oxidized) Rh/TiO2/YSZ catalyst was successfully activated by applied currents, either positive or negative. This phenomenon is an example of "permanent" electrochemical promotion furnishing a permanent rate enhancement ratio of γ = 11. The activation by negative currents is explained in terms of an electrochemical reduction of rhodium surface oxide, while the activation by positive currents can be explained by the mechanism of electrochemical promotion.

EPFL2005

Self and induced stabilized nanoparticles for electrocatalysis

Mériadec Limat

Since few decades the search of new catalysts formulations for fuel cell applications has motivated numerous projects especially due to the future lack of fossil fuel, environmental and economical reasons. Consequently, Direct Alcohol Fuel Cell (DAFC) or Direct Formic Acid Fuel Cell domains have received great attention. In spite of the promising results reported recently, these types of fuel cell still exhibit some problems. In fact, platinum, bulk or nanoparticles, undergoes surface poisoning involved by intermediates (as acetaldehyde, acetic acid, carbon monoxide) which are strongly bonded to the surface. These poisoning species block the active sites at the surface electrode and drastically decrease the catalytic activity and also the life-time of the cell. Probable solutions are suggested essentially in Pt-based alloys catalysts or nanocatalysts in order to avoid this phenomenon. Therefore, multicomponent catalysts deposited at carbon electrodes (Boron-Doped Diamond and Highly Oriented Pyrolytic Graphite) has been investigated during this thesis. These two substrates have been chosen due to their outstanding properties. In the first part of this study, self stabilized nanoparticles at BDD substrate have been studied. Ex-situ ternary Pt-Ru-Sn microemulsion-synthesized nanoparticles deposited at BDD substrate were investigated towards both methanol and ethanol electrooxidation. Such type of composite electrode seemed to possess a greater efficiency towards methanol oxidation than observed for ethanol, leading essentially for the latter to C2 oxidation products. The inability of this catalyst to activate the C-C bond scission can explain this observation. However, the major enhancement was observed for the methanol electrooxidation. Recent studies have shown that gold nanoparticles exhibited surprising catalytic activity, very different from the bulk, making them a privileged candidate to replace or be added to platinum. Gold nanoparticles prepared with a twofold technique (sputtering followed by a heat treatment in air) have been characterized with both outer and inner-sphere redox reactions. It has been observed that the resulting electrochemical behavior of such type of composite electrode was similar to a gold microelectrodes array, due to the high difference between the two material electrochemical rate constants. In a morphological point of view, the processes of nucleation and growth engaged in the stabilized and well-dispersed gold nanoparticles formation have especially attracted a great attention. At BDD surface, the gold nanoparticles were located at specific sites (grain boundaries, defects, impurities,…). This phenomenon has motivated further investigations at HOPG substrate, known to possess a quite smooth and perfect surface. In the second part of this thesis, induced stabilized nanoparticles at HOPG surface were studied. In a first time, HOPG was morphologically and electrochemically characterized. The surface state has appeared to be a key parameter, essentially with the presence of the two surface species, the edge plane and the basal plane. High modifications in the electrochemical behavior of HOPG electrode were observed after an anodic pretreatment in acidic media, due to surface concentration increase of the edge plane species, which are highly reactive. Based on these results a tailoring of the HOPG has been performed in order to create nanocontainers for a further gold deposition. HOPG surface has been bombarded by gold clusters and then oxidized (etching step) under well-defined experimental conditions, leading in a tailored pattern of nanopits with high reliability and reproducibility. Gold deposition was performed by evaporation. The gold nanoparticles were located in the created nanocontainers and were found to be stable under cyclic voltammetric measurements in acidic media. A new synthesis method for a bimetallic or alloy system formed by Au and Pt nanoparticles stabilized at tailored HOPG surface was developed. Pt electrodeposited at Au/HOPG electrode surface have been observed to preferentially deposit on Au. In fact, gold nanoparticles were used as nucleation sites for the platinum particles. The resulting Pt-Au/HOPG electrodes were then heated at 400 °C in vacuum conditions to improve interactions between both metals. The appearance of an alloy was observed and characterized by a negative shift in the peak potential of Pt oxides reduction. Thanks to these results, Pt-Au/HOPG composite electrodes were applied for electrocatalytic oxidation. At a surface composition of P55-Au45 alloy, Pt-Au/HOPG after the heat treatment has demonstrated a higher electrocatalytic activity than before the heat treatment towards formic acid electrooxidation. It seemed that electronic and geometrical effect was responsible of such enhancement. Finally, bismuth adsorption at both P55-Au45/HOPG and polycrystalline platinum was investigated towards formic acid electrooxidation. The effect of Bi adatoms has been essentially ascribed to geometrical restrictions for CO to adsorb at platinum active sites. Drastic enhancement in current density at a fixed potential and dramatic decrease of the onset potential of formic acid oxidation were observed. Thus, it seemed that avoiding the CO poisoning can be the most judicious development for new electrocatalysts in fuel cell applications.

EPFL2009