Identifying and developing new renewable energy sources is a top priority for humankind. The sun is the only available source capable of providing the immense amounts of carbon-free energy necessary to keep CO2 levels in the atmosphere below dangerous levels. However, to be useful, sunlight must generally be converted to fuel, that is, a substance that can be burned to produce heat or power, since more than 70% of current global energy use requires fuel.
Hydrogen fuel can be obtained from water by splitting water molecules into hydrogen and oxygen in a photoelectrolysis system. However, no known material can efficiently and cheaply do this while remaining stable under illumination in an electrolyte solution for the many years necessary to justify the expense of building such a system. A material capable of this is the Holy Grail of photoelectrochemistry. We believe a nanostructured oxide semiconductor, in which the features are smaller than the distance the photogenerated charge carriers can travel, has the best chance of being stable, cheap, and efficient. But which oxide? There are about 60 metals in the periodic table, and a complex multi-component oxide material would be needed to obtain the desired combination of properties, including stability, light absorption over a large portion of the solar spectrum, and efficient evolution of hydrogen and oxygen. Given the large number of possible metal oxides containing three or four suitable components, a combinatorial search is required. This would enable many candidate oxide compositions to be quickly produced and screened for photoelectrolysis activity.
Printing and screening a four-metals-three-at-a-time pattern, with a compositional zoom for an iron-cesium-neodymium-copper (Fe-Cs-Nd-Cu) system. (A) False color template showing the positions and gradients used for printing the four-metal precursor solutions. (B) Photograph of the printed and fired film. Note the triangular internal standards of iron oxide (α-Fe2
) and copper oxide (upper right and left, respectively) with thickness gradients (bottom to top) that are used as internal standards. (C) False color photocurrent image of the film shown in B. (D) Photocurrent scan of a triangular composition zoomed in on the brightest area of the Fe-Cs-Nd triangle shown in C, which has a photocurrent of about twice that of the α-Fe2
internal standard (smaller triangle to the lower right). (Reproduced with permission.1
We devised a method for rapid production and screening of metal oxides for photoelectrolysis activity that uses inkjet printers to print metal oxide precursors in overlapping compositional gradient patterns onto conductive glass substrates. Subsequent firing of the substrates at ∼500°C produces patterns of mixed metal oxides. The substrate with the metal oxide film is then immersed in an electrochemical cell containing an electrolyte solution, and the pattern is scanned with a visible light laser. Measuring the photocurrent as a function of the laser position makes it possible to obtain an image that reveals which areas of the metal oxide ‘library’ exhibit photoelectrolysis activity. These areas in the image can be associated with the corresponding metal oxide composition (see Figure 1).1
It is young people whose future prosperity is at stake if energy and climate change problems are not addressed. Thus, we have been recruiting high school and undergraduate students into the Solar Hydrogen Activity Research Kit (SHArK) Project to help produce and screen the millions of possible multi-element oxides for photoelectrolysis activity. We are developing simple, inexpensive, and flexible kits to facilitate the involvement of a large number of students. In one version of the kit, a laser scanner that uses an inexpensive laser pointer is constructed from a Lego Mindstorms® set. Metal oxide libraries can be produced by printing or pipetting metal nitrate solutions on the provided conductive glass substrates, which are then fired at 500°C to produce thin films of metal oxides. A USB-powered electronics box, laser goggles, and a simple electrochemical cell are also provided in the kit. A website2 allows the students to download their results and communicate problems and successes.
We have had a tremendous response to the SHArK Project from researchers, educators, students, businesses, and even parents. There are now over 40 SHArK sites in the US, Canada, and Germany. We have had more demand for kits than we are presently able to accommodate. Consequently, we are in the process of scaling up the project. We feel that this enthusiasm is due to the unique approach of the SHArK Project, which engages the hands and minds of young people in learning about and participating in actual research. They have a real opportunity to discover a material that can make a difference in the future of energy production and storage and help solve a global problem.
I would like to thank Hewlett-Packard for initial help with inkjet printing, the Dreyfus Foundation for the seed grant, and the US National Science Foundation for funding the Powering the Planet Center for Chemical Innovation based at the California Institute of Technology. I also thank the students at Colorado State University and faculty members and students at the SHArK sites, with a special thanks to Maggie Geselbracht for her enthusiasm and for coming up with the SHArK acronym. Additional thanks go to Robert Herrick for software and website development, Aaron Wolfe for help with early development of the Lego concept, and Harry Gray for his enthusiastic support. Jennifer Schuttlefield, who currently serves as SHArK national coordinator, and Craig Markum kept the project going at the University of Wyoming.
Department of Chemistry and School of Energy Resources
University of Wyoming
Bruce Parkinson is a professor of chemistry.
1. M. Woodhouse, G. S. Herman, B. A. Parkinson, A combinatorial approach to identification of catalysts for the photoelectrolysis of water, Chem. Mater. 17, pp. 4318-4324, 2005.