Alkaline polymer electrolyte fuel cells completely free from noble metal catalysts
Proceedings of the National Academy of Sciences of the United States of America
In recent decades, fuel cell technology has been undergoing revolutionary developments, with fundamental progress being the replacement of electrolyte solutions with polymer electrolytes, making the device more compact in size and higher in power density. Nowadays, acidic polymer electrolytes, typically Nafion, are widely used. Despite great success, fuel cells based on acidic polyelectrolyte still depend heavily on noble metal catalysts, predominantly platinum (Pt), thus increasing the cost
... reasing the cost and hampering the widespread application of fuel cells. Here, we report a type of polymer electrolyte fuel cells (PEFC) employing a hydroxide ionconductive polymer, quaternary ammonium polysulphone, as alkaline electrolyte and nonprecious metals, chromium-decorated nickel and silver, as the catalyst for the negative and positive electrodes, respectively. In addition to the development of a high-performance alkaline polymer electrolyte particularly suitable for fuel cells, key progress has been achieved in catalyst tailoring: The surface electronic structure of nickel has been tuned to suppress selectively the surface oxidative passivation with retained activity toward hydrogen oxidation. This report of a H2-O2 PEFC completely free from noble metal catalysts in both the positive and negative electrodes represents an important advancement in the research and development of fuel cells. nonprecious metals ͉ hydrogen oxidation ͉ oxygen reduction F uel cells have been recognized as an alternative powergeneration technique for the future in both mobile and stationary uses (1, 2). After decades of evolution, fuel cells of various types have been developed (2), such as alkaline fuel cell (AFC), phosphoric acid fuel cell (PAFC), molten carbonate fuel cell (MCFC), solid oxide fuel cell (SOFC), and polymer electrolyte fuel cell (PEFC). Among them, PEFC has been the most developed one in the past 2 decades (3), featuring rapid startup and high power density particularly suitable for vehicle applications (1-3). Compared with the aqueous electrolytes traditionally used in low-temperature fuel cells, polymer electrolytes completely eliminate the problems caused by electrolyte leakage and can effectively separate the fuels (such as hydrogen) and the oxidant (oxygen) with a thin film of a few tens of microns in thickness. For decades, the commonly used polymer electrolytes have been limited to proton exchange membranes, typically Nafion . Nowadays, many Nafion-based fuel cell systems of different sizes are being demonstrated or tested on a variety of applications across the world. Although they are promising, the Nafion-based fuel cells still face a number of obstacles to commercialization, one of which has been the severe dependence of catalysts on platinum (Pt), an expensive and scarce resource in the earth. Such dependence stems from the strong acidic nature of the protonexchange membrane; and thermodynamically, only noble metals can be relatively stable in this corrosive environment. Despite tremendous efforts devoted to the search for non-Pt and nonprecious metal catalysts with a few interesting preliminary results reported (4-8), there has hitherto been no demonstration of PEFC completely free from noble metal catalysts in both the positive and negative electrodes. To fundamentally get rid of the dependence on noble metal catalysts, alkaline electrolyte should be used. However, fuel cells based on aqueous alkaline electrolytes, namely AFC, have been suffering from the carbonation issue when air is used as oxidant, a fatal result of which is that the waterproof character of the gas-diffusion electrode would be broken by the K 2 CO 3 precipitate, thus causing failure of the entire fuel cell. A solution to this issue is to create a polymer version of alkaline electrolyte, because in such an alkaline polymer electrolyte (APE), the cation, usually quaternary ammonium, is attached on the polymer chain, and there is no dissociated cation in the liquid phase to form K 2 CO 3 precipitate. The working mechanism of an APE fuel cell (APEFC) is similar but not identical to that of a Nafion-based PEFC. As illustrated in Fig. 1 , at the cathode side of an APEFC, O 2 is reduced in the presence of H 2 O to produce OH Ϫ , which transfers through the hydroxide ion-conductive polymer to the anode side and reacts with H 2 to produce H 2 O. Although the potential advantages of APEFC have long been recognized (9-12), progress has not been satisfactory. Up to now, prototypes of APEFC are still using Pt catalysts (especially in the anode) (13-19); the long-expected totally precious-metalfree APEFC has not yet appeared. This situation has been caused by the difficulties arising from both the polymer electrolyte and the catalyst aspects. The APE suitable for fuel-cell applications is still not readily available, and, in most cases, fuel-cell researchers have to develop APE by themselves. An ideal APE should be Nafion-like, namely, the polymer electrolyte should not only serve as a highly conductive and mechanically stable membrane separator but also be soluble in certain solvents so that the catalyst layer of fuel cell electrodes can be easily impregnated with the polymer electrolyte. Such a Nafionlike APE is still rare. In some reports, the membrane had to be wetted with aqueous alkaline solution to remedy the poor ionic conductivity (20-23), so the fuel cells thus made were not genuine APEFC per se. On the other hand, transferring the non-Pt catalysts from AFC to APEFC is not as straightforward as expected. Although Ag has been found to be still applicable as the cathode catalyst for oxygen reduction in APEFC (18, 19) , attempts to use Ni as the anode catalyst for hydrogen oxidation failed unexpectedly and unexceptionally until now (9).