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Molecular catalysis in a fuel cell.

Molecular hydrogen is considered to be the most promising of chemical fuels in terms of removing our dependence on fossil fuels, but the development of cheap, efficient, fuel-cell systems has not yet been realized. Currently, fuel cells are based on the heterogeneous, homolytic splitting of H2 on a platinum surface, but these fuel cells have the obvious problem that Pt is both scarce and expensive. Furthermore, few improvements in efficiency have been achieved in over a hundred years, so a new model for fuel-cell catalysis is required to generate a fuel-cell-based economy. Though a few related systems have been reported, the successful construction of a new type of fuel cell has not been achieved to date. Fuel-cell development can be taken in an entirely new direction by the introduction of molecular catalysts capable of working in homogeneous solutions. Molecular catalysts have the advantage of being highly variable in terms of design, and solution-phase catalysis is important because it enables us to directly observe the details of the mechanism (Figure 1). By combining these two features, we can view the path to greater efficiency. This laboratory has previously reported just such a catalyst and, as a result of its solution-phase behavior, we were able to determine the action of this catalyst in precise detail. The catalyst is based on a biologically inspired Ni Ru aqua complex, [NiLRu(H2O)(h -C6Me6)](NO3)2 ([1](NO3)2, L = N,N’-dimethyl-N,N’-bis(2-mercaptoethyl)1,3-propanediamine; Scheme 1) and is a functional model of natural [NiFe]hydrogenases. It is soluble in water, and aqueous-phase reactions showed that it was able to catalyze the oxidation of H2 to protons. Importantly, the catalyst functioned by means of a mechanism new to chemistry, involving two molecules of H2 in one cycle and a remarkable, low-valent, Ni Ru active center. This cycle also proceeded via an unusually stable hydride complex [Ni(H2O)L(mH)Ru(h-C6Me6)](NO3) ([2](NO3)), which was so stable that it was more convenient to use 2 as the starting point for experiments, and hence the experimental descriptions below refer to 2 rather than 1. In a major development for this chemistry, we have now been able to apply 2 to the catalysis of the complementary reduction, that is, O2 to H2O. In other words, 2 is capable of catalyzing both the oxidation of H2 and the reduction of O2 in the formation of H2O. This ability to promote both sides of the process has not been observed for any kind of molecular catalyst to date. We are thus able to report the performance of a functioning molecular fuel cell with Ni Ru hydride complex 2 as the catalyst for both the anode and the cathode. Furthermore, 2 functions in both the solid and solution phases, which provides both the handling convenience of the solid phase and the analytical clarity of the solution phase. The fuel cell was monitored for performance with combinations of 1 or 2 in either or both electrodes and with either catalyst in the solution or solid phase (Table 1). In solution-phase experiments, the electrodes bore conduits that contained solutions of the catalyst, and streams of watersaturated H2 or O2 gas were bubbled through. In solid-phase experiments, the NO3 counteranion was replaced by CF3SO3 (OTf ) to yield the corresponding water-insoluble catalyst. This insoluble catalyst was immobilized on carbon cloth and sandwiched between the electrodes and the protonconducting polymer electrolyte while the saturated gases Figure 1. Direct observation of catalysts in a fuel cell—an abstract depiction of a fuel cell. Water-saturated H2 and O2 gases flow through conduits in the electrodes (red and blue). In solution-phase experiments, water-soluble catalysts with the NO3 counteranion are dissolved in the gas conduits. In solid-phase experiments, water-insoluble catalysts with the CF3SO3 counteranion are immobilized on carbon cloth sandwiched between the electrodes and the proton-conducting polymer electrolyte. Protons pass through the polymer electrolyte and electrons flow through an electrical circuit to the cathode.