Hydrogen Evolution Catalysis

Most semiconductor surfaces, including that of silicon, are poor catalysts for the hydrogen evolution reaction (HER). Semiconductor water-splitting devices, then, generally require a chemically distinct catalyst species in order to store energy as H-H bonds. Platinum has often been used in the past toward this end, since it catalyzes the HER very efficiently. But due to its low abundance and concomitant high cost, platinum is not desirable for use in a low-cost system. We are pursuing two approaches to developing cheaper catalysts for our water-splitting membrane – one homogeneous and the other heterogeneous.

Homogeneous Catalysis

Along with a number of our CCI collaborators, we are pursuing the development of homogeneous compounds capable of efficient hydrogen evolution. Due to the prohibitive cost of catalytic noble metals, we are exploring the development of homogeneous H2-generating catalysts derived from so-called “earth abundant” materials. We are primarily focused on transition metals such as Fe, Co and Ni. Although such elements are not great HER catalysts in their metallic state, complexes of Co(I/II) and Ni(0/II) derived from organic ligands have shown great promise in recent years. Such transition metal complexes have access to a wide variety of oxidation states that can both assist in electron transfer from any electrode surface and participate mechanistically in the catalytic cycle of H2 production. Thus, the synthesis of both known and new organic ligands is an important focus, and we are working to isolate and electrochemically characterize complexes bearing the low oxidation states of Co and Ni.

Another challenge to the development of abundant silicon-coupled HER catalysts is attachment of the catalyst to the Si surface. In this regime, the semiconductor is capable of generating excited-state electrons that can participate in reduction of the metal-center, thereby initiating the catalytic cycle of H2 generation under sunlight. Thus we are working closely with our surface chemistry group on techniques to functionalize the photoactive Si surfaces with well-defined molecular catalysts. As a result, we are pursuing novel syntheses of promising organic ligands with pendant functional “tethers” for coupling chemistry.

Heterogeneous Catalysis

We are also currently examining mixtures of inexpensive transition metals as heterogeneous HER catalysts. This work draws on the extensive literature of pure water electrolysis, from which have emerged a number of promising candidates. We are in the process of developing techniques for depositing known “dark” HER catalysts onto silicon surfaces and understanding the requirements for their efficient function.

Lewis Research Group, Division of Chemistry and Chemical Engineering
Caltech 127-72, 1200 East California Boulevard, Pasadena, California 91125