The oxygen reduction reaction (ORR) limits the efficiency of fuel cells, and thus it is of great interest to manufacturers to discover nanocatalysts with an ORR activity superior to that of platinum (Pt). Panasonic Corporation (Osaka, JP) garnered U.S. Patent 7,695,850 for nano-scale cobalt catalysts for use in electrodes that are applicable to air cells, fuel cells, sugar biofuel cells, electrochemical sensors and like electrochemical devices.
According to inventors Tadashi Sotomura, Mitsuru Hashimoto and Yuka Yamada, derivatives of cobalt phthalocyanine and cobalt tetrapyrazinoporphyrazine can used as catalysts which provide very stable oxygen-reducing electrodes. The catalysts achieve electrochemical reduction of oxygen that is equal to platinum but are more resistant to poisoning from carbon dioxide and carbon monoxide.
The oxygen-reducing electrode contains CoPyrz(CF3)8 as a catalytic component. The catalyst is supported on a conductive substrate of (CF-AuNano) comprising carbon felt (CF) and gold nanoparticles (AuNano). The cobalt catalyst can also be supported on carbon nanotubes (CNT). Examples of preferable conductive substrates include carbon fibers (CFibre), carbon papers (CP), carbon felts (CF), carbon sponges (CSponge), carbon nanotubes (CNT), gold nanoparticles (AuNano), etc. Such conductive substrates may be used in a combined manner.
There are no limitations on how the oxygen-reducing electrode containing CoPyrz(CF3)8 can be handled and can be treated in the same manner as conventional oxygen-reducing catalysts. For example, it is possible to support CoPyrz(CF3)8 directly on a glassy carbon (GC) electrode or gold (Au) electrode. It is also possible to support CoPyrz(CF3)8 on a separately prepared carrier (conductive substrate) and then dispose the carrier so as to be in contact with a GC electrode or an Au electrode.
Cobalt phthalocyanine is a stable oxygen-reducing catalyst. It is known that cobalt phthalocyanine can obtain excellent oxygen-reducing characteristics without using a high valency metal element.
There is a demand for chemically stable oxygen-reducing catalysts by which an oxygen-reducing electrode exhibiting as high an oxygen reduction potential as that of platinum catalysts. Platinum achieves high performance in this respect, but platinum deposits are limited and therefore cannot meet the demand on an industrial scale.
There is a demand for chemically stable oxygen-reducing catalysts by which an oxygen-reducing electrode exhibiting as high an oxygen reduction potential as that of platinum catalysts. Platinum achieves high performance in this respect, but platinum deposits are limited and therefore cannot meet the demand on an industrial scale.
However, compared with cobalt phthalocyanines, platinum catalysts have greater oxygen reduction potentials. Among cobalt phthalocyanines, CoPc(CN)8 and CoOBuPc have excellent oxygen reduction potentials, but are still smaller than that of platinum by 0.1 to 0.15 V.
The Panasonic oxygen-reducing electrode uses a high-performance oxygen-reducing catalysts which can attain as high an oxygen reduction potential as that of a platinum catalyst.
The Panasonic inventors conducted extensive research and found that the platinum's oxygen reduction ability can be equaled by using a specific nanoscale cobalt tetrapyrazinoporphyrazine derivative as an oxygen-reducing catalyst when supported on carbon nanotubes, or nanofibers with nano-gold.
Furthermore, because cobalt does not exhibit a high valence (monovalence or bivalence), deterioration due to oxidation of constituent components of an electrochemical device (e.g., electrolyte, electrodereed, current collector, etc.) can be prevented. The catalysts and carriers (including electrode pellets) used in each oxygen-reducing electrode are shown in Table 1 below. Silver/silver chloride is used as the reference electrode, and an aqueous 0.1 mol/l potassium hydroxide solution at pH 13 is used as the electrolyte.
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