Electrocatalyst activity

There is an increasing interest in the development of electrochemical sensors and microsensors for detecting and monitoring NO or N02, due to their importance in clinical and environmental analysis. It has been suggested that transition metal electrocatalysts active for NO or N02 coordination and reduction could be exploited for the development of metal-complex film electrodes for N02 and NO sensing. However, most of the sensory devices reported so... 

In addition, one needs not to deal with impure surfaces so much because the reaction takes place in solution and surface layers (oxides for example) may dissolve under appropriate potential conditions. Several procedures for achieving activation are also known for electrocatalysts. Ion implantation may be a convenient method of activating and modifying an electrocatalyst. Activation takes place by the introduction of defects and modification by the production of surface-doped layers of varying composition. 

Besides the activation overpotential, mass transport losses is an important contributor to the overall overpotential loss, especially at high current density. By use of such high-surface-area electrocatalysts, activation overpotential is minimized. But since a three-dimensional reaction zone is essential for the consumption of the fuel-cell gaseous reactants, it is necessary to incorporate the supported electrocatalysts in the porous gas diffusion electrodes, with optimized structures, for aqueous electrolyte fuel-cell applications. The supported electrocatalysts and the structure and composition of the active layer play a significant role in minimizing the mass transport and ohmic limitations, particularly in respect to the former when air is the cathodic reactant. In general, mass transport limitations are predominant in the active layer of the electrode, while ohmic limitations are mainly due to resistance to ionic transport in the electrolyte. For the purposes of this chapter, the focus will be on the role of the supported electrocatalysts in inhibiting both mass transport and ohmic limitations within the porous gas diffusion electrodes, in acid electrolyte fuel cells. These may be summarized as follows ... 

Recently, Lamy and co-workers [4,5] described that PtSn/C electrocatalysts were more active than PtRu/C electrocatalysts for ethanol oxidation. For electrocatalysts prepared by co-impregnation-H2 reduction and Bonneman methods, they found that the optimum tin composition was in the range of 10-20 at.%. In these conditions, the electrode activity was enhanced and the CO-intermediates coming from ethanol dissociative chemisorption were reduced. Xin and co-workers [6-9] prepared PtRu/C and PtSn/C electrocatalysts by a polyol method and tested for ethanol oxidation. It was observed that the addition of some elements, like W, could improve the PtRu/C electrocatalyst activity. However, the activities of the PtRu/C electrocatalysts were inferior to those of PtSn/C electrocatalysts. It was also found that PtSn/C electrocatalysts with Pt Sn atomic ratios of 60 40 and 50 50 were more active than electrocatalysts with 75 25 and 80 20 atomic ratios. Thus, it seems that the performance of PtSn/C electrocatalysts depends greatly on their preparation procedure. 

The cathode must also display good electrocatalyst activity for the reduction of O2 and offer good electronic conductivity, since it must serve as the current collector. The cathode material most commonly used is porous or mesoporous perovskite manganite with the formula Lai xSr cMn03 (0.10cathode reaction is as follows ... 

Fig. 1.6 Time dependence of the PIml/Ru electrocatalysts activity at 0.69 V with two different compositions indicated in the graph and a commercial PtRu/C catalyst for methanol oxidation [77] (reproduced with permission from J. Electrochem. Soc. 155, B185 (2008). Copyright 2003, The Electrochemical Society)...

Metal deposition on the electrocatalysts active sites can take place as well. Although elements such as chromium are used to enhance the activity of cathode catalysts, it must be in the alloyed form for such enhancement, while in the case of corrosion of metal plates, it will form an adlayer on the catalytic surface, blocking platinum atoms becoming inactive for oxygen or hydrogen dissociation. 

Table 15.2 Targets for automotive PEMFC electrocatalyst activity and costs. Mass Activity 0.90 V, H2IO2, 80°C, Ph2. P02 = 100 kPa (based on multiple sources)...

Electrocatalyst Activity, conductivity. L, e, loading, particle size,... 

---