Electrocatalysts carbon

An alternative to using commercially available carbon for electrocatalyst carbon substrates is to build a specific carbon structure having controlled properties. Thus, carbons have been prepared by the controlled pyrolysis of polyacrylonitrile (PAN) and contain surface nitrogen groups that act as peroxide decomposing agents.62... 

In a carbon-supported metal electrocatalyst, the electronic interaction between metal and carbon support has a significant effect on its electrochemical performance [4], For carbon-supported Pt electrocatalyst, carbon could accelerate the electron transfer at the electrode-electrolyte interface, leading to an accelerated electrode process. Typically, the electrons are transferred from platinum clusters to the oxygen species on the surfece of a carbon support material and the chemical bond formation or the charge transfer process occurs at the contacting phase, which is considered to be beneficial to the enhancement of the catalytic properties in terms of activity and stability of the electrocatalysts. Experimentally, the investigation into the electron interaction between metal catalyst and support materials could be realized by various physical, spectroscopic, and electrochemical approaches. The electron donation behavior of Pt to carbon support materials has been demonstrated by the electron spin resonance (ESR) X-ray photoelectron spectroscopy (XPS) studies, with the conclusion that the electron interaction between Pt and carbon support depends on their Fermi level of electrons. It is considered that the electronic structure change of Pt on carbon support induced by the electron interaction has positive effect toward the enhancement of the catalytic properties and the improvement of the stability of the electrocatalyst system. However, the exact quantitative relationship between electronic interaction of carbon-supported catalyst and its electrocatalytic performance is still not yet fully established [4]. 

Of practical importance is the contribution that is made by carbonaceous materials as an additive to enhance the electronic conductivity of the positive and negative electrodes. In other electrode applications, carbon serves as the electrocatalyst for electrochemical reactions and/or the substrate on which an electrocatalyst is located. In... 

In acid electrolytes, carbon is a poor electrocatalyst for oxygen evolution at potentials where carbon corrosion occurs. However, in alkaline electrolytes carbon is sufficiently electrocatalytically active for oxygen evolution to occur simultaneously with carbon corrosion at potentials corresponding to charge conditions for a bifunctional air electrode in metal/air batteries. In this situation, oxygen evolution is the dominant anodic reaction, thus complicating the measurement of carbon corrosion. Ross and co-workers [30] developed experimental techniques to overcome this difficulty. Their results with acetylene black in 30 wt% KOH showed that substantial amounts of CO in addition to C02 (carbonate species) and 02, are... 

Other experiments by Ross and co-workers [30] clearly indicate that the common metal (Co, Ni, Fe, Cr, Ru) oxides that are used for oxygen electrocatalysts also catalyze the oxidation of carbon in alkaline electrolytes. 

Carbon shows reasonable electrocatalytic activity for oxygen reduction in alkaline electrolytes, but it is a relatively poor oxygen electrocatalyst in acid electrolytes. A detailed discussion on the mechanism of... 

The overpotentials for oxygen reduction and evolution on carbon-based bifunctional air electrodes for rechargeable Zn/air batteries are reduced by utilizing metal oxide electrocatalysts. Besides enhancing the electrochemical kinetics of the oxygen reactions, the electrocatalysts serve to reduce the overpotential to minimize... 

In redox flow batteries such as Zn/Cl2 and Zn/Br2, carbon plays a major role in the positive electrode where reactions involving Cl2 and Br2 occur. In these types of batteries, graphite is used as the bipolar separator, and a thin layer of high-surface-area carbon serves as an electrocatalyst. Two potential problems with carbon in redox flow batteries are (i) slow oxidation of carbon and (ii) intercalation of halogen molecules, particularly Br2 in graphite electrodes. The reversible redox potentials for the Cl2 and Br2 reactions [Eq. (8) and... 

In their earlier work, Modes et al. [53] described low polarization electrodes composed of Teflon-bonded high surface area carbon, loaded with different electrocatalysts (Pt, platinized Pt, Co, Ni). The best results were obtained by using cobalt... 

We have already referred to the Mo/Ru/S Chevrel phases and related catalysts which have long been under investigation for their oxygen reduction properties. Reeve et al. [19] evaluated the methanol tolerance, along with oxygen reduction activity, of a range of transition metal sulfide electrocatalysts, in a liquid-feed solid-polymer-electrolyte DMFC. The catalysts were prepared in high surface area by direct synthesis onto various surface-functionalized carbon blacks. The intrinsic... 

Recently, rhodium and ruthenium-based carbon-supported sulfide electrocatalysts were synthesized by different established methods and evaluated as ODP cathodic catalysts in a chlorine-saturated hydrochloric acid environment with respect to both economic and industrial considerations [46]. In particular, patented E-TEK methods as well as a non-aqueous method were used to produce binary RhjcSy and Ru Sy in addition, some of the more popular Mo, Co, Rh, and Redoped RuxSy catalysts for acid electrolyte fuel cell ORR applications were also prepared. The roles of both crystallinity and morphology of the electrocatalysts were investigated. Their activity for ORR was compared to state-of-the-art Pt/C and Rh/C systems. The Rh Sy/C, CojcRuyS /C, and Ru Sy/C materials synthesized by the E-TEK methods exhibited appreciable stability and activity for ORR under these conditions. The Ru-based materials showed good depolarizing behavior. Considering that ruthenium is about seven times less expensive than rhodium, these Ru-based electrocatalysts may prove to be a viable low-cost alternative to Rh Sy systems for the ODC HCl electrolysis industry. 

Guild AF, Gancs L, Allen RJ, Mukerjee S (2007) Carbon-supported low-loading rhodium sulfide electrocatalysts for oxygen depolarized cathode applications. Appl Catal A 326 227-235... 

Of special Interest as O2 reduction electrocatalysts are the transition metal macrocycles In the form of layers adsorptlvely attached, chemically bonded or simply physically deposited on an electrode substrate Some of these complexes catalyze the 4-electron reduction of O2 to H2O or 0H while others catalyze principally the 2-electron reduction to the peroxide and/or the peroxide elimination reactions. Various situ spectroscopic techniques have been used to examine the state of these transition metal macrocycle layers on carbon, graphite and metal substrates under various electrochemical conditions. These techniques have Included (a) visible reflectance spectroscopy (b) laser Raman spectroscopy, utilizing surface enhanced Raman scattering and resonant Raman and (c) Mossbauer spectroscopy. This paper will focus on principally the cobalt and Iron phthalocyanlnes and porphyrins. 

Quantitative analysis can be carried out by chromatography (in gas or liquid phase) during prolonged electrolysis of methanol. The main product is carbon dioxide,which is the only desirable oxidation product in the DMFC. However, small amounts of formic acid and formaldehyde have been detected, mainly on pure platinum electrodes. The concentrations of partially oxidized products can be lowered by using platinum-based alloy electrocatalysts for instance, the concentration of carbon dioxide increases significantly with R-Ru and Pt-Ru-Sn electrodes, which thus shows a more complete reaction with alloy electrocatalysts. 

The effects of dispersion of the electrocatalyst and of particle size on the kinetics of electrooxidation of methanol have been the subject of numerous studies because of the utilization of carbon support in DMFC anodes. The main objective is to determine the optimum size of the platinum anode particles in order to increase the effectiveness factor of platinum. Such a size effect, which is widely recognized in the case of the reduction of oxygen, is still a subject of discussion for the oxidation of methanol. According to some investigators, an optimum of 2 nm for the platinum particle size exists, but studying particle sizes up to 1.4 nm, other authors observed no size effect. According to a recent study, the rate of oxidation of methanol remains constant for particles greater than 4.5 nm, but decreases with size for smaller particles (up to 2.2 nm). 

Specific Activity (SA) and Mass Activity (MA) of Pt Electrocatalysts Supported on Different Carbon Powders Characterized by Specific Surface Area (S) and Particle Size (d)... 

Finally, a simple method for a rapid evaluation of the activity of high surface area electrocatalysts is to observe the electrocatalytic response of a dispersion of carbon-supported catalyst in a thin layer of a recast proton exchange membrane.This type of electrode can be easily obtained from a solution of Nafion. As an example. Fig. 11 gives the comparative... 

Similarly, Pd, Ag, and Pd-Ag nanoclusters on alumina have been prepared by the polyol method [230]. Dend-rimer encapsulated metal nanoclusters can be obtained by the thermal degradation of the organic dendrimers [368]. If salts of different metals are reduced one after the other in the presence of a support, core-shell type metallic particles are produced. In this case the presence of the support is vital for the success of the preparation. For example, the stepwise reduction of Cu and Pt salts in the presence of a conductive carbon support (Vulcan XC 72) generates copper nanoparticles (6-8 nm) that are coated with smaller particles of Pt (1-2 nm). This system has been found to be a powerful electrocatalyst which exhibits improved CO tolerance combined with high electrocatalytic efficiency. For details see Section 3.7 [53,369]. 

Establishment of carbon-supported Pt catalysts as a means to achieve higher and more stable dispersion of the precious metal electrocatalyst on an electronically conducting support [Petrow and Allen, 1977]. 

Zhang J, Mo Y, Vukmirovic MB, Klie R, Sasaki K, Adzic RR. 2004. Platinum monolayer electrocatalysts for O2 reduction Pt monolayer on Pd(lll) and on carbon-supported Pd nanoparticles. J Phys Chem B 108 10955-10964. 

Arenz M, Stamenkovic V, Blizanac BB, Mayrhofer KJJ, Markovic NM, Ross PN. 2005. Carbon-supported Pt-Sn electrocatalysts for the anodic oxidation of H2, CO, and H2/CO mixtures. Part 11 The structure-activity relationship. J Catal 232 402-410. 

Wang W, Zheng D, Du C, Zou Z, Zhang X, Xia B, Yang H, Akins DL. 2007. Carbon-supported Pd-Co bimetallic nanoparticles as electrocatalysts for the oxygen reduction reaction. J Power Sources 167 243-249. 

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