Electrocatalyst specificity factors

One factor that may be important, but not systematically investigated, is the influence of the Pt electrocatalyst-support interactions on the electrocatalytic activity for O2 reduction. In Figure 14, an attempt to incorporate the pHzpc as a qualitative measure of the importance of carbon surface chemistry and metal-support interaction on the electrocatalytic activity of Pt is reported. The trend of the data in Figure 14 suggests that the specific activity for oxygen reduction increases as the pHzpc of the surface becomes more basic this effect may be related to the parallel increase of the particle size with the pHzpc of the catalyst. At this stage, one... 

For molecular electrocatalysts otherwise, and especially transition metal macrocycles, the electrocatalytic activity is often modified by subtle structural and electronic factors spanning the entire mechanistic spectrum, that is, from strict four-electron reduction, as for the much publicized cofacial di-cobalt porphyrin, in which the distance between the Co centers was set at about 4 A [12], to strict two-electron reduction, as in the monomeric (single ring) Co(II) 4,4, 4",4" -tetrasulfophthalo-cyanine (CoTsPc) [20] and Co(II) 5,10,15,20-tetraphenyl porphyrin (CoTPP) [21]. Not surprisingly, nature has evolved highly specific enzymes for oxygen transport, oxygen reduction to water, superoxide dismutation and peroxide decomposition. 

What factors determine the activity of electrocatalysts and are likely to introduce specificity into an electrode reaction At the present time, a complete answer to this question is not possible and we have certainly not reached the stage where it is possible to design electrocatalysts from theoretical considerations. On the other hand a number of general principles can be set out. Thus while metals, alloys, semiconductors (particularly oxides) and complexes have been shown to exhibit catalytic properties, invariably catalysts are based on transition metals and it seems that the design of a catalyst requires the placing of transition-metal ions or atoms in a matrix which serves to optimize their electronic configuration and position with respect to each other. 

On bulk Pt-M alloys, where M is a 3d-transition metal, the specific activity for the ORR is enhanced by two to four times with respect to pure Pt [121,136-143]. Interestingly, the enhancement factor is maintained on nanometer size carbon-supported electrocatalysts [121,140,141,144-146]. Nevertheless, the long-term stability of Pt-alloy electrocatalysts remains questionable in the harsh operating conditions of a PEMFC. Dubau et al. [147-149] showed that PtsCo/C electrocatalysts suffer compositional changes at the nanoscale in real PEMFC... 

As mentioned previously, real surface area (RSA) of a catalyst is one of the most important parameters when it comes to its evaluation. Part of RSA which participates in electrochemical reaction is denoted as electrochemically active surface area (ESA or EASA). However, it should be noted that ESA is usually smaller than RSA (determined by some non-electrochemical method such as gas physisorption analysis, particle size measurement etc.) due to the possibility that entire surface of the electrocatalyst is not available to electrolyte. Hence, the ratio between ESA and RSA gives catalyst utilization. The ration between ESA and geometrical cross section of an electrode gives roughness factor (Rf). There are number of different approaches to determine RSA, both electrochemical and non-electrochemical, however one should note that when electrochemical method is used it is ESA what is determined. These methods are summarized and critically overviewed by Trasatti and Petrii [13], while following section will focus on specific electrochemical methods based on voltammetry. 

In the equation 25 n is the number of electrons and F, k and C(02) are Faraday constant, rate constant and bulk O2 concentration, respectively. In addition, P and y are symmetry factors, while E is the electrode potential. The term 0ad relates to total surface coverage by OHads and adsorbed anions. The effect of surface oxides on the metal electrode surface was clearly demonstrated for series of Pt-based electrocatalysts [53, 54], leading to a general recipe for design of electrocatalysts with improved ORR activity. In specific, if oxide formation is hindered onset potential for ORR is shifted to higher anodic potentials. Underlying principles of this route have been set by combining electrochemical measurement... 

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