December 1-6, 2013 | Boston
Meeting Chairs: Charles Black, Elisabetta Comini, Gitti Frey, Kristi Kiick, Loucas Tsakalakos
Nanowires, with their unique capability to bridge the nanoscopic and macroscopic worlds, have already been demonstrated as important materials for different energy conversion. One emerging and exciting direction is their application for solar to fuel conversion. The generation of fuels by the direct conversion of solar energy in a fully integrated system is an attractive goal, but no such system has been demonstrated that shows the required efficiency, is sufficiently durable, or can be manufactured at reasonable cost. One of the most critical issues in solar water splitting is the development of suitable photoelectrodes with high efficiency and long-term durability in an aqueous environment. Semiconductor nanowires represent an important class of nanostructure building block for direct solar-to-fuel application because of their high surface area, tunable bandgap and efficient charge transport and collection. Nanowires can be readily designed and synthesized to deterministically incorporate heterojunctions with improved light absorption, charge separation and vectorial transport. Meanwhile, it is also possible to selectively decorate different oxidation or reduction catalysts onto specific segments of the nanowires to mimic the compartmentalized reactions in natural photosynthesis. Recently, We have developed a fully integrated system of nanoscale photoelectrodes assembled from inorganic nanowires for direct solar water splitting. Similar to the photosynthetic system in a chloroplast, the artificial photosynthetic system comprises two semiconductor light absorbers with large surface area, an interfacial layer for charge transport, and spatially separated cocatalysts to facilitate the water reduction and oxidation. Under simulated sunlight, a 0.12% solar-to-fuel conversion efficiency is achieved, which is comparable to that of natural photosynthesis. The result demonstrates the possibility of integrating material components into a functional system that mimics the nanoscopic integration in chloroplasts. It also provides a conceptual blueprint of modular design that allows incorporation of newly discovered components for improved performance.
The push for the development of inexpensive, earth-abundant catalysts for photoelectrochemical water-splitting has demonstrated significant advancements. For example, Nocera&’s cobalt-phosphate and nickel borate catalysts have been successfully used for the oxidation of water with moderate overpotentials. However, these catalysts require near-neutral and weakly basic pH electrolytes in order to maintain their performance and usability. In order to successfully collect the generated fuels in photoelectrochemical cells (PECs), the use of electrolytic membranes and porous separators is necessity. In this study, we focus on identifying and measuring the series resistance losses that could arise in aqueous buffered-membrane electrolytes in PECs. Potentiometric and pH measurements were used to study the contribution of solution resistance, membrane resistance and pH gradient formation at 25 mA cm-2. A comparison between different combinations of membranes and buffered electrolytes was done, in which we identified membrane pH gradient formation as the greatest contributor to voltage losses on the PECs.
CIGS (CuInxGa1-xSe2) is a well-known solar cell material, and should thus be an interesting material for solar water splitting applications. Despite this almost no work has been directed towards utilizing CIGS for renewable hydrogen production. Here we demonstrate that CISG is a highly interesting material for solar hydrogen applications, with the potential of deliver photocurrents of technological importance. CIGS in itself has a suitable conduction band position for the hydrogen producing half-reaction, and photocurrents of 6 mA/cm2 for the photo reduction are demonstrated. The stability in water under illumination is however a problem as for most other efficient materials. We demonstrate how the problem can be solved by spatially separating the charge carrier generation process from the catalysis step by increasing the distance of charge t