Nanostructured catalysts for fuel cells with proton-exchanged membranes
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The relevance of research in the field of elucidation of the relationship between the manufacture methods, structure and electrocatalytic activity of platinum-carbon materials is largely due to the need to improve low-temperature hydrogen/air and air/methanol fuel cells (FC). Nanoparticles of platinum or its alloys deposited on microparticles of carbon materials with a developed surface are used as electrocatalysts in commercially produced FC. Other systems based on platinum-free catalysts or other types of carriers are today significantly inferior to platinum-carbon ones and, at best, are considered as materials of the “tomorrow” day. Commercial production of low-temperature fuel cells involves the extension of their durability and a decrease in the content of precious metals in the catalytic layer. The solution of the above problems of hydrogen energy is impossible without clarifying the fundamental issues regarding the relationship of the catalytic activity of platinum-carbon materials with their composition and structure, as well as the dependence of the latter on the synthesis conditions of electrocatalysts. We also note that scientific research in the field of synthesis and study of the properties of new platinum-containing catalytically active materials stimulates the development of modern technologies, deepens the fundamental aspects of electrocatalysis and the chemistry of heterogeneous solid-phase reactions.
Platinum-carbon catalysts are self-organized nanostructured systems consisting of many particles of platinum or its alloy, located on the surface and in the pores of the carbon carrier. Such nanoparticles are characterized by a certain dispersion in size (size distribution) and a more or less equally probable surface distribution (spatial distribution). According to the latest literature data, metal nanoparticles can have a more or less pronounced crystalline structure, whereby the metal component of the catalyst includes X-ray amorphous and crystalline regions. The specific catalytic activity of the surface of “amorphized” and “crystallized” particles, and in the case of platinum alloys also their composition, can vary. It is assumed that well-crystallized particles, ceteris paribus, exhibit a higher specific catalytic activity in electrode reactions. However, methods for assessing the degree of amorphization of the metal component of platinum-carbon materials have not been developed.
Optimization of the structure of the electrocatalyst involves the formation of systems with a small size of metal nanoparticles and a small dispersion of their size distribution. Considering that a decrease in the size of nanoparticles increases the surface area of the metal, but decreases its specific catalytic activity, we can talk about the existence of an optimal size of nanoparticles for these operating conditions. Therefore, the use of the average nanoparticle size as a parameter characterizing the potential activity of the catalyst in the corresponding electrochemical reaction is not always correct.
The fact of a decrease in the specific catalytic activity of platinum nanoparticles with a decrease in their size in a number of publications is associated with a disordering of the crystal lattice of the metal under conditions of the predominance of the surface over the volume. Nevertheless, a priori it cannot be argued that the degree of such disordering is associated only with the size of platinum nanoparticles. Our experience shows that Pt/C materials can be obtained with a similar average size of platinum nanoparticles, but with significantly different catalytic activity. Perhaps this is due to the different proportion of well crystallized particles and/or the degree of disordering of the platinum crystal lattice in these materials. Accordingly, finding physicochemical parameters sensitive to such a difference may be the key to selecting the most promising catalytic systems and developing methods for their preparation.
The shape of nanocrystals can have a large effect on the activity of platinum nanoparticles and Pt/C catalyst. The fact is that different types of faces, edges and vertices of crystals are characterized by different activities in current-determining reactions. Therefore, controlling the shape of nanoparticles in the synthesis of catalysts is also of significant interest.
For low-temperature fuel cell catalysts, the slowdown of the cathodic oxygen reduction reaction is a big problem. A noticeable increase in the catalytic activity of platinum in this reaction can be achieved by doping it with some d-metals. It is shown that materials based on alloys of platinum with nickel, cobalt, chromium, iron, copper, vanadium, tungsten, etc. can be more active electrocatalysts than just Pt/C Figure 2. At the same time, the practical use of bimetallic catalysts is difficult due to the selective dissolution of the alloying component, differences in the composition and structure of de-alloyed nanoparticles formed during operation from the initial ones.
To a large extent, the factors listed in Figure 2 are related. Apparently, each of them, depending on operating conditions, to one degree or another affects the catalytic properties of binary catalysts. Finding the physicochemical parameters characterizing the structural features of Pt-M/C materials that are “positive” in terms of catalytic activity is an even more complicated task than that for Pt/C systems.
An important characteristic of the functional behavior of electrocatalysts along with their activity is also durability. The fact is that during operation, the rate of conversion of the reactants in the catalytic layers decreases, and the catalytic layers (and catalysts) themselves degrade. The stability of the catalysts depends both on their initial composition and structure, and on operating conditions. Knowledge of the degradation mechanisms of electrocatalysts, understanding the nature of the influence of various factors on the speed of this process allows to find compositions and structures that optimally combine activity and stability.