Electrocatalytic activity, modification

In order to enhance the reversibility of the electrode reaction and the electrocatalytic activity, modification of the electrode materials is necessary by... 

The activation observed in titania-supported Au electrocatalysts is unlikely to arise from electronic effects in monolayer or bilayer Au [Valden et al., 1998 Chen and Goodman, 2004], since the electrocatalytic activity was correlated with the size of three-dimensional titania-supported Au particles [Guerin et al., 2006b Hayden et al., 2007a, c]. The possibility that titania-induced electronic modification of three-dimensional particles below 6.5 nm is responsible for the induced activity, however, could not be excluded. It was pointed out, though, that such electronic effects should dominate for the smaller particle regime (<3 nm), where deactivation of the Au is observed on all supports. 

The electrocatalytic activities data indicate that there is a major enhancement in comparison with that of Au/C catalysts in terms of the peak potential (by -500 mV) and the peak current (by 20 x). The presence of a small fraction of Pt in the Au-based bimetallic nanoparticles significantly modified the catalytic properties. While a detailed characterization is needed, it is clear that the bimetallic AuPt composition played a significant role in the observed modification of the catalytic properties. 

In a recent communication [91], Ni surface alloyed with Cu, Ti, or Cu + Ti by ion implantation was examined for its redox and electrocatalytic activities by cyclic voltammetry. The surface was characterized by XPS, x-ray, and electron diffraction, as well as by electron and atomic force microscopies. This type of material exhibited a unique voltammetric response of Ni and was shown to stabilize the / -modification of the Ni oxide/hydroxide. It was demonstrated that the morphology and microstructure differ from those of bulk materials. 

Bulk Pt alloys for the electrooxidation of formic acid have been less frequently studied compared to underpotential deposition (upd) modified Pt surfaces. The Pt50Ru5o surface was again found to be one of the most active Pt-Ru surfaces. Underpotentially deposited metals, such as Bi, Se, Sb, were studied as reaction modifiers for Pt surfaces and provided significant electrocatalytic activity increases. Electronic factors (ligand effects) rather than bifunctional effects were held responsible for these activity modifications, because the metal coverages that caused the activity gains were extremely small. 

Mechanical strain of Pd and Pt has been observed [278] to give rise to overpotential variations. Such an effect has been studied to gain insight into the factors responsible for the relationship between surface modifications and electrocatalytic activity. The analysis has shown that, of the various factors scrutinized including the possible variation in the interatomic distance, the most probable one is a change in ionic specific adsorption. 

While attempting to use platinum in fuel cells, it has been demonstrated that its surface exhibits important electrocatalytic activities toward the oxidation of organic compounds. However, this effect can sometimes be enhanced by the use of bimetallic surfaces [1-10]. The physical mixture and the electronic interaction of the alloy components lead to a modification in the interaction between the adsorbate and the substrate in an electrocatalytic reaction. As a consequence of the structural changes at the single crystal surfaces during the electrochemical activation (examined with in situ STM) [11], it has been demonstrated that most of the catalysts are constituted by randomly oriented islands [12-14]. 

A thin-film electrode is relatively dense, as the metallic film does not have the electrocatalytic properties that a porous electrode has. Therefore, in many instances, the surface of the thin film is chemically or electrochemically modified to enhance its electrocatalytic activity. For instance, thin platinum film electrodes can be platinized electrochemically forming a porous platinum black layer. This platinum black layer is electrocatalytically more active than the thin platinum film. Thin-film processes are more capital and labor intensive and the process is more complicated than thick-film processes. Thin-film deposition is also a batch process which may produce sensors of limited numbers of silicon substrates. This is very desirable in prototype development, for it allows modification on prototypes with minimum cost. 

The effect of Pt particle size on oxidation of methanol has been studied by several researchers. Machida et al. [67] introduced several platinum-cluster-attached graphite electrodes and reported enhanced electrocatalytic activity in anodic methanol oxidation with Pt clusters ranging in size from Pt9 to Ptis [67]. The catalytic activity of supported Pt clusters is significantly higher than that of conventional Pt electrodes. They used platinum carbonyl clusters of the type Pt3n,(CO)6n (u= 3,5) as well as HRu3(CO)n. Modification of the graphite surface... 

The investigation on the electrochemical epitaxial growth of palladium on gold and platinum singlemonolayer coverage of palladium on Pt(lll) electrode by the immersion technique for the first time [74]. Llorca and coworkers investigated the irreversibly adsorbed palladium on Pt(hiJ) in acidic solution [75] and reported that electrocatalytic activity for the oxidation of formic acid on the Pt(lOO) electrodes vras improved drastically by the palladium-adlayer modification, while that on the Pt(lll) electrode was not greatly affected by the Pd-modification [76]. However, it is difficult to prepare an ultrathin film of palladium with various thickness by the immersion technique, and STM observation of the atomic structure of these surfaces is not available yet. 

As mentioned above, the alcohol crossover from the anode to the cathode is a important problems to be overcome to improve the DAFC performance. This is due to the fact that the commonly used Pt-based cathode electrocatalysts are also active for the adsorption and oxidation of methanol [1]. So, in addition to the resulting mixed potential at the cathode, there is a decrease in the fuel utilization. Therefore, considering the above exposed reactions for the alcohol electrooxidation, and the features that govern the ORR electrocatalytic activity, as discussed in the Sect. 5.2, it is ready to conclude the importance of the modification of the active ORR electrocatalyst surfaces in order to inhibit the methanol or ethanol oxidative adsorption steps. In the next sections, some recent materials being developed to overcome the problems caused by the alcohol crossover will be presented. 

O2 to H2O at the Au nanoparticles was directly proved by a four-electrode configuration. Also, they observed that after a heat treatment (600 °C under H2 atmosphere) of the samples, the electrocatalytic activity improved, which was attributed to a modification of the particle shape and consequently with a modification of the crystallographic orientations exposed at the surface of the nanoparticles. 

While Pd is less expensive than Pt, the electrocatalytic activity of bulk polycrystalline Pd for ORR is at least five times lower than that of Pt, which prevents it from being used directly in fuel cells. Great efforts have been dedicated to improve the activity of Pd by surface modification and alloying. This chapter attempts to summarize the recent progress of electrocatalysts containing Pd for ORR. The development of Pd electrocatalysts for electrooxidation of hydrogen and small organic molecules is not discussed in this chapter but has been adequately covered in Chaps. 5 and 6 and recent reviews [5-7]. 

Generally, the ad-atoms cause positive catalytic effects with significant enhancement of the electrocatalytic activity of platinum in several cases. Several reviews have been published on this subject [26-28, 67]. Very recently Ross [68] and Jarvi and Stuve [29] have discussed the more recent advances in our understanding of the fundamentals of Ci electrocatalysis by ad-atoms. The different types of enhancement (third-body effect, bifunctional mechanism, poison destabilization, and electronic modification) are well documented. The new information obtained from the in situ spectroscopic studies about the nature of poisons and the dependence of their coverages on potential, as well as the use of single-crystal electrodes with defined surface structure and specific reactivity, enables a deeper insight in the electrocatalysis by ad-atoms. As a general rule, one can say that, except for methanol, the more susceptible... 

The co-catalytic role of Sb is somewhat similar to that of Bi. The redox behavior of Sb in conjunction with Had on Sb determines the electrocatalytic activity in a Pt structure-sensitive fashion [161, 162]. Sb modification had a beneficial effect on the HCOOH electrooxidation activation energies on Pt(lll) and Pt(331), while exercising an inhibitory role on Pt(100), Pt(llO), and Pt(320) [161]. But the same group presented cyclic voltammetry data showing increased HCOOH oxidation currents on Sb-modified Pt(llO) and Pt(320) [162]. The effect of Sbad was very dependent on its surface coverage and there was an interaction... 

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