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03/18/2009
Better LEDs and Fuel Cells with Nitrogen
Last week my wife was clearing out one of my many piles of accumulated papers and came across a clipping dating back to 1968. The clipping was a Chicago Sun-Times Dispatch headlined "Bell Is Developing Revolutionary ‘Lamp’" and described the work at Bell Labs by Ralph Logan, Harry White and yours truly on what we apparently called our "solid state lamp". Our "lamp" was a light-emitting diode (LED). At the time, we were working on LEDs made from gallium phosphide, a compound of gallium and phosphorus, doped with the elements zinc and oxygen to produce red light.
Nitrogen will play a key role in this column. As I’ve probably mentioned before in these columns, others at Bell Labs found that doping gallium phosphide with nitrogen produced LEDs that emitted green light. With red and green LEDs, all we needed were blue LEDs and we could make all colors, by combining the three LEDs of different colors. Blue was the Holy Grail, which we failed to find. However, we did light up Bell System phones with our diodes, which required no additional power other than that already available on the telephone lines.
Today’s super bright LEDs, found in traffic lights, huge billboard displays, auto stop lights, etc. are primarily the result of work by Nick Holonyak, credited with the invention of the LED, and others who, instead of gallium phosphide, employed compounds combining gallium, aluminum, indium, phosphorus, arsenic. But it was Shuji Nakamura in Japan who found the Holy Grail about a decade ago. Nakamura made blue LEDs from a compound of gallium and nitrogen known as gallium nitride, GaN.
Nakamura is now at the University of California-Santa Barbara (UCSB) and, in an article in the Materials Research Society’s February MRS Bulletin, he discusses the current status of GaN-based solid state lighting. I was amazed to read that there are two other professors and 40 graduate students working on GaN-based LEDs at UCSB. It turns out that by mixing GaN with varying amounts of the nitrides of aluminum and/or indium, you can make LEDs that emit light of any wavelength from the infrared to the ultraviolet and all colors in between.
Now the Holy Grail is white light and Nakamura has scored again. We could make white light by combining the light from LEDs of three colors. However, Nakamura finds it less costly and simpler to use blue LEDs coated with phosphors that convert blue light to white light. We’ve all been encouraged to replace our incandescent light bulbs with compact fluorescent bulbs. The fluorescent bulbs use less energy to get the same light output and last longer. A problem is that they contain some mercury, which means you can’t just toss them in the trash when they’re burned out. Gallium nitride LEDs are nontoxic and last much longer than incandescent and compact fluorescent bulbs.
We can compare light sources by measuring their light outputs in lumens, a standard measure of brightness. A 60-watt incandescent bulb emits roughly a thousand lumens. If you’ve touched one while it’s lit, you also know it produces a substantial amount of heat, which is wasted energy. You only need a 13-watt compact fluorescent bulb to supply the same amount of light, or lumens. A mere 6-watt white LED bulb will supply the same amount of light, and no wasteful heat. Be advised, however, that your LED bulb may contain a fair number of LEDs, not just one. An LED generally is a pretty small chunk of material, e.g, GaN, and can’t handle too much current without burning out. So, you have to have more LEDs to spread out the power.
According to Nakamura, here in the U.S. 22% of our total electricity consumption is for lighting in all its various forms. If all our lighting were white LEDs, our electricity consumption for lighting could be cut in half, saving over a hundred billion dollars by 1925 and eliminating the need to build well over a hundred new power plants. In various countries spanning the globe, legislation has been passed to phase out the use of incandescent lighting over periods of years. I can still remember in my youth visiting my relatives in rural Maryland and getting together in the evenings by the light of kerosene lamps. In some areas of the world where kerosene lamps are still being used for lighting, white LEDs have been introduced along with solar cells to charge batteries to power the LEDs. In Pakistan, one village schoolmaster is quoted as saying that the children’s improved performance in school since the introduction of white LEDs has been "remarkable".
In the February 6 issue of Science, I found an article on another use of nitrogen that could be a boon to the future of fuel cells to drive vehicles and contribute to a greener world. The article, by Kuanping Gong and his coworkers at various facilities in Dayton, Ohio, deals with a problem of long standing in the world of fuel cells, the catalyst. Platinum is the catalyst in most fuel cells. One downside to platinum, if you’ve ever purchased platinum jewelry, is obvious - cost. Another problem is that carbon monoxide, a product of the reactions taking place in various types of fuel cells, poisons platinum, rendering it ineffective as a catalyst. Decades have been spent by numerous researchers trying to find less costly and longer lasting substitutes for platinum.
Gong and his colleagues have looked at the catalytic properties of carbon nanotubes as catalysts for the so-called oxygen reduction reactions that take place at the cathode of . These are reactions typically catalyzed by platinum. We’ve discussed carbon nanotubes, in which carbon atoms are arranged in a structure resembling rolled up pieces of chicken wire. Nanotubes can be single-walled, only one atom thick, or multi-walled, like concentric stacks of chicken wire.
One trick used by Gong et al involves growing carbon nanotubes all lined up vertically like stacks of bamboo poles bunched together. The other trick is to introduce nitrogen. I won’t go into details except to note the remarkable results. The nitrogen incorporates into the carbon nanotube structure in a way that the nitrogen tends to suck up electrons making the surrounding carbon atoms positively charged. This is a good thing. A mat of vertically aligned nanotubes containing nitrogen performs even better than platinum in catalyzing the oxygen reduction reaction. Not only do the nitrogen-containing nanotubes act as a good catalyst, but they also are not poisoned by carbon monoxide, even gobs of it that would do in platinum in short order.
What’s the bottom line? First, unlike platinum, carbon is everywhere. The problem is that manufacturing carbon nanotubes is not cheap. However, as applications for carbon nanotubes continue to emerge, the manufacturing learning curve should follow the typical route, with luck resembling that followed by the silicon chip and Moore’s Law, with the cost of carbon nanotubes falling dramatically over time. Hopefully, the Science paper’s concluding sentence proves true, that is, "..... these nitrogen-containing carbon nanotubes are clearly of practical importance."
Next column on April 2 or before.
Allen F. Bortrum
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