UC San Diego

Light assisted or driven fuel generation by carbon dioxide and proton reduction can be achieved by a p-type semiconductor/liquid junction. There are four different types of schemes which are typically used for carbon dioxide and proton reduction for fuel generation applications. In these systems, the semiconductor can serve the dual role of a catalyst and a light absorber. Specific electrocatalysts (heterogeneous and homogeneous) can be driven by p-type semiconductor where it works only as light absorber in order to achieve better selectivity and faster rates of catalysis. The p-type semiconductor/ molecular catalyst junction is primarily explored in this dissertation for CO₂ and proton photoelectrochemical reduction. A general principle for the operation of p-type semiconductor/molecular junctions is proposed and validated for several molecular catalysts in contact with p-Si photocathode. It is also shown that the light assisted homogeneous and heterogeneous catalysis can coexist. This principle is extended to achieve direct conversion of CO₂ to methanol on Platinum nanoparticles decorated p-Si in aqueous medium through pyridine/ pyridinium system for CO₂ reduction. An open circuit voltage higher than 600 mV is achieved for p-Si/Re(bipy- tBu)(CO)₃Cl [where bipy-tBu = 4,4'- tert-butyl-2,2'- bipyridine] (Re-catalyst) junction. The photoelectrochemical conversion of CO₂ to CO₃ using a p-Si /Re-catalyst junction is obtained at 100 % Faradaic efficiency. The homogeneous catalytic current density for CO₂ by p-Si/Re-catalyst junction under illumination scales linearly with illumination intensity (both polychromatic and monochromatic). This indicates that the homogeneous catalysis is light driven for the p-Si/Re-catalyst junction system up to light intensities approaching one sun. The photoelectrochemical reduction of other active members of Re(bipyridyl)(CO)₃Cl molecular catalyst family is also observed on illuminated p-Si photocathode. Effects of surface modification and nanowire morphology of the p- Si photocathode on the homogeneous catalytic reduction of CO₂ by using p-Si/Re-catalyst junction are also described in this dissertation. For phenyl ethyl modified p-Si photocathode, the rate of homogeneous catalysis for CO₂ reduction by Re-catalyst is three times greater than glassy carbon electrode and six times greater than the hexyl modified and the hydrogen terminated p-Si photocathodes. When hexyl modified p-Si nanowires are used as photocathode, the homogeneous catalytic current density increased by a factor of two compared to planar p-Si (both freshly etched and hexyl modified) photocathode. A successful light assisted generation of syngas (H₂:CO = 2: 1) from CO₂ and water is achieved by using p-Si/Re- catalyst. In this system, water is reduced heterogeneously on p-Si surface and CO₂ is reduced homogeneously by Re- catalyst. The same principle is extended to the homogeneous proton reduction by using p-Si/[FeFe] complex junction where [FeFe] complex [Fe₂([Mu]-bdt)(CO)₆] (bdt = benzene-1,2-dithiolate)] is a proton reduction molecular catalyst. A short circuit quantum efficiency of 79 % with 100 % Faradaic efficiency and 600 mV open circuit are achieved by using p-Si/[FeFe] complex for proton reduction with 300 mM perchloric acid as a proton source. Cobalt difluororyl-diglyoximate (Co-catalyst) is a proton reduction catalyst with only 200 mV of overpotential for the hydrogen evolution reaction (HRE). The Co-catalyst is photoelectrochemically reduced with a photovoltage of 470 mV on illuminated p-Si photocathode. For p-Si photocathodes, the overpotential for proton reduction is over 1 V. In principle, p-Si/Co-catalyst junction can reduce proton to hydrogen homogeneously at underpotential. In a concluding effort, a wireless monolithic dual face single photoelectrode (multi junction photovoltaic cell which can generate a voltage higher 1.7 V) based photochemical cell is proposed for direct conversion of solar energy into liquid fuel. In this device, the two faces of the multijunction photoelectrode are serve as an anode and a cathode for water oxidation and fuel generation, respectively, and are separated by proton exchange membrane