Platinum has been established as the most powerful electrocatalyst for the 4-electron reduction of oxygen to water in acid medium in a conventional fuel cell. The reaction becomes less efficient and its efficiency suffers from the formation of the hydrogen peroxide intermediate at lower Pt loadings or at higher current densities. Recent progress in the area concerns fabrication of electrocatalysts in a form of carbon-supported platinum nanoparticles assembled within organized multilayers or inks deposited on a proton-conducting membrane. Important issues include the stabilization of Pt nanoparticles (to prevent their agglomeration causing decrease of the active surface area), the enhancement of their reactivity (e.g. by modification of their surfaces or by formation of novel bimetallic systems), the interfacial protection of nanostructured Pt from poisoning (e.g. by the development of methanol or CO tolerant systems using Ru-Se nanoparticles), and the activation of metal nanoparticles through the immobilization within reactive matrices (bifunctional mechanism). The above studies are paralleled by more or less successful attempts to develop the noble metal free electrocatalysts (e.g. metal phtalocyanines, porphyrins and their including pyrolyzed structures) of potential utility to fuel cells operating in acid media.

Biofuel cells can be viewed as analogues of conventional fuel cells in which biocatalysts (enzymes), biofules, and neutral or slightly acidic electrolytes are used. The interest in biofuel cells has been raised by biomedical applications and environmental concerns. Since most of conventional electrocatalysts for oxygen reduction are largely useless in neutral media, the present research centers on development of the methods of immobilization of enzymes (highly specific but slow electron-transfer biocatalysts) within redox mediators at electrode surfaces. Among present limitations is not only the long-term stability and reactivity of enzymes but also the durability and ability of polymeric redox mediators to propagate charge (electron transfers, displacement of ions) to the redox centers. When referring to the theory and methodology of electrocatalysis at polymer modified electrodes, care has to exercised to produce highly conducting (electronically, ionically) three-dimensional redox polymer films containing possibly the highest concentration of catalytic (enzymatic) centers. Recent examples of redox polymers, the choice of enzymes, and the possibility of use of carbon nanostructures will be discussed. The concept of bifunctional electrocatalysis with use of both biological and inorganic redox centers will be addressed too.

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