Photoelectrochemical Hydrogen Production

At the University of Hawaii (UH), the approach to developing high-efficiency, low-cost photoelectrochemical (PEC) processes for the direct production of hydrogen has included the use of integrated electrochemical/optical models to design photoelectrodes based on multijunction thin-film technology; materials research to identify critical issues on photoelectrode efficiency and stability; and the fabrication and testing of photoelectrodes for optimization and life-testing. In recent years, we designed photoelectrodes using monolithically stacked triple-junctions of amorphous silicon (a-Si) and mechanically interconnected side-by-side copper-indium-galliumdiselenide (CIGS). This year, significant progress was made in optimizing constituent films for these photoelectrode designs. Photoelectrodes using both a-Si and CIGS were fabricated and tested. Both types of photoelectrodes were fabricated using optimized NiMo, Fe:NiOx, ITO (indium-tin-oxide), and polymer-encapsulation films deposited at the University of Hawaii. The a-Si solar cells used in these studies were deposited by the University of Toledo using a process that yielded electrical efficiencies as high as 12.7%. The 13% efficient CIGS cells were provided under subcontract by the University of Delaware. Based on the electrical efficiency of the cells and prior experiments using these catalytic coatings, solar-to-hydrogen efficiencies of 6% to 8% were expected for the a-Si based photoelectrodes. However, peak efficiencies of only 2.5% were measured in outdoor tests. The discrepancy has been attributed to handling-induced degradation in the a-Si triple-junction performance. Severe degradation also occurred in CIGS diodes when cut for photoelectrode fabrication. Open-circuit voltage in the CIGS triple-stacks was reduced from 1.8 V to below 1 V, making water-splitting impossible, despite predicted solar-to-hydrogen efficiencies in excess of 10%. Planned future work includes development of improved handling techniques to demonstrate the true hydrogen production potential in the photoelectrodes, with greater emphasis on the higher-efficiency CIGS devices. Plans also include continued development of the hybrid solid-state/PEC photoelectrode described at the FY 2001 Annual Review Meeting. This design combines a double-junction amorphous silicon cell with a dye-sensitized TiO2 or WO3 photoelectrochemical junction. Introduction Under the sponsorship of the U.S. Department of Energy, research at the Hawaii Natural Energy Institute of the University of Hawaii has aimed at developing high-efficiency, potentially lowcost, photoelectrochemical (PEC) systems to produce hydrogen directly from water using sunlight as the energy source. The main thrust of the work has been the development of integrated multijunction photoelectrodes, comprising semiconductor, catalytic, and protective thin-films deposited on low-cost substrates (such as stainless steel), for solar hydrogen production (Rocheleau et al. 1998). In the illustration of a generic hydrogen photoelectrode shown in Figure 1, sunlight shining on photoactive regions of the electrode produces electric current to drive the hydrogen and oxygen evolution reactions (HER, OER) at opposite surfaces. Hydrogen photoelectrode operation represents a complex interaction of photovoltaic, optical, and electrochemical effects, and an important part of the UH research has been the development of integrated models combining these effects (Rocheleau and Vierthaler 1994). Figure 2 shows a schematic of the model used in the analysis of a triple-junction photoelectrode. Figure 1. Photoelectrochemical hydrogen production In order to meet DOE goals, a PEC system must be low-cost, operate at solar-to-chemical conversion efficiencies greater than 10%, and have long operating lifetimes. Numerous approaches involving a variety of semiconductors have been explored since the early 1980s, but none have successfully satisfied both the efficiency and stability criteria. The high voltage required to dissociate water and the corrosiveness of the aqueous electrolytes have been major hurdles. Based on results from numerous modeling and proof-of-concept experiments conducted at UH over the course of the PEC research, our approach has been to develop photoelectrodes incorporating multijunction thin-film photoconvertors (for high voltage) and thin-film catalyst and protective layers (for stability). electrolyte sunlight substrate/photoconverter catalyzed surfaces