California Institute of Technology
Engineering & Science
07.29.14

Random Walk

Postdoc Bryce Sadtler, grad student James McKone, and grad student Jillian Dempsey give a lunchtime presentation at John Muir High School in Pasadena.

The Solar Army is Recruiting

In the battle to save the world from global warming, the newest weapons are LEGO Mindstorms robotics kits, inexpensive lasers, and ink-jet printers that spit out metal salts instead of ink. The newest recruits are high school students, guided by generals from Caltech’s chemistry department. The objective is to locate a metal oxide that can use sunlight to split water into hydrogen, a storable fuel, and thus wean us from fossil fuels.

The project is the brainchild of Bruce Parkinson (PhD ’78), now a chemistry professor at the University of Wyoming, who got the idea while watching his own high-school-age children playing with LEGOs. Parkinson combined LEGOs with a laser pointer and other cheap, readily available components to create a $600 computer-controlled, gear-driven testing device that very precisely scans a laser beam across three-millimeter-diameter metal-oxide samples printed on CD-sized glass slides. As the metal oxide pixels get illuminated one by one, the electrical current generated by this artificial sunlight is measured for each one.

Parkinson and Caltech’s Powering the Planet Center for Chemical Innovation have parlayed this gizmo into a project called SHArK, for Solar Hydrogen Activity Research Kit. (The small “r” allows the acronym to be written as a sequence of chemical elements.) Initially funded with seed money from the Dreyfus Foundation and now supported by the National Science Foundation, SHArK has the potential to rally young people around the globe to collaborate on solving a real-life problem.

“We’ve been flooded with requests from students and parents who want to join the Solar Army,” says Harry Gray, the Beckman Professor of Chemistry and a member of the center. “There are millions of possible metal oxide combinations that might work. We need thousands of students to check them out. We don’t have a really good idea which combinations will be the winners.”

The winners will be dirt-cheap, non-toxic, readily available compounds that can mimic photosynthesis and split water into hydrogen (to run a fuel cell to produce electricity; unlike sunlight, hydrogen can be stored for use at night) and oxygen. Plants photosynthesize with organic molecules that they replace every 30 minutes or so because oxidation is a brutal process that degrades living tissue. But an inorganic, metal-oxide catalyst would be able to withstand oxidation, because it’s already oxidized.

Finding that catalyst is a little like locating a rusty needle in the periodic haystack of elements. The best candidate will probably consist of an oxide containing three or four metals working together in order to absorb as broad a spectrum of sunlight as possible. That’s the easy part. The hard part is that the metal-oxide combo must then spit out electrons that are energetic enough to split water.

Grad students Jillian Dempsey, Suzanne Golisz, James McKone, and Leslie O’Leary, undergrad Carolyn Valdez (BS ’10), and postdoc Bryce Sadtler are mentoring student volunteers at three local high schools—Blair High School, John Muir High School, and Polytechnic School. Each student group is given their own kit to assemble, and is shown how to print their own slides and how to use the scanner.

Carolyn Valdez (right) explains the setup to Chelsea Newbold (left), an 11th-grader at Blair High School. The slide to be scanned goes in the frosted-glass tank at the right. The laser beam is marched across the slide by a pair of angled mirrors (follow Carolyn’s fingers) facing each other and mounted on two perpendicular assemblies of gears and connecting rods driven by the Mindstorm motor units (the gray cylinders, one of which is obscured in this view); the laser pointer itself rests behind the one red LEGO brick in the cradle at the left of the apparatus.

The project was born in 2004 when Parkinson, then at Colorado State, started work on a method to retrofit ink-jet printers to handle metal nitrate salt solutions rather than ink. Heating the slides to 500°C then transforms the salts into metal oxides. Nate Lewis (BS ’77, MS ’77), Caltech’s Argyros Professor and professor of chemistry, who had overlapped with Parkinson at Caltech while a student, heard about the work at a meeting and dispatched grad student Jordan Katz (PhD ’08) and undergrad Todd Gingrich (BS ’08) to visit Parkinson’s lab. The Lewis lab then adapted the idea, improving the design so that it yielded more data per sample.

Each slide contains 16,200 unique metal oxide samples grouped in four adjoining triangles. Each triangle contains oxides of three different metals in a gradual continuum of concentrations, ranging from equal amounts of all three metals at the triangle’s center point to 100 percent of one metal at each tip of the triangle. Each slide also has two smaller triangular arrays of metal oxides with known photoelectric behavior, used for calibrating the measurements.

In a process that takes some six to eight hours per slide, a pulsed green laser slowly scans each pixel while the slide is suspended in an electrically conductive solution of sodium hydroxide. If a metal oxide splits water, current will flow through the solution to an electrode, registering a “hit.” Teams share their information on the infobahn, and if the hit is validated by another high school or college, the formulation will be further tested at Wyoming or Caltech. Eventually, of course, we’ll have to figure out how to produce enough of the winning stuff to support the power grid.

The project continues to expand, and Parkinson and Gray plan to extend their recruiting activities to more colleges and high schools. (See www.thesharkproject.org for more information.) “These are the kids who are going to make the difference as to whether we’re going to have solar energy,” says Gray. “This is the future of the planet. And, by the way, it’s your future.” —LD