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Interview: Fueling your car with sun and water
Nanotechnologist Jin Zhang on the elusive dream of hydrogen.

Jin Zhang of U.C. Santa Cruz’s Department of Chemistry and Biochemistry will receive $535,000 in grants from the U.S. Department of Energy for his part in two research projects aimed at developing new technologies for the production and storage of hydrogen fuel using nanostructured materials. Zhang talked about his research in July of 2005 with Earth & Sky’s Jorge Salazar.

Salazar: What are some of the issues involved with making hydrogen a viable source of energy?

Zhang: Hydrogen is potentially a very efficient and clean fuel. Just imagine using hydrogen as a gasoline. The only by-product from this process is water. Of course, water is very useful and clean. But there are a number of issues. How do you generate hydrogen efficiently in large amounts? Currently, the hydrogen that we buy from chemical suppliers is made from fossil fuels Ñ so that defeats the purpose.

The second issue is, how do you store hydrogen? Hydrogen is a gas. So when you buy a tank of hydrogen, the density is very, very low.

When you have a very small amount of hydrogen in a big tank, it’s not going to be very easy to use for automobiles of the future because you don’t want to carry a big tank in a car all of the time. The solution of how you store hydrogen may be in a solid material with a very high density. The next step is, once you have hydrogen, you need to have fuel cells, where you use a fuel cell to convert hydrogen at a reasonably low temperature, and ambient conditions, into electricity. Once you have the electricity, you can imagine using it to power cars or anything that you want.

Salazar: Could you describe what’s involved in generating hydrogen?

Zhang: We propose to use solar energy and water to generate hydrogen. This is essentially a holy grail in science and technology. People have been pursuing this for many, many years, since at least the 70s. The idea is very simple, but it’s not easy to do. The idea is to take water, which is two parts of hydrogen and one part of oxygen (in terms of molar ratios), so it’s H20. If you can extract the hydrogen from water, in a cheap and efficient manner, that would be ideal, because water itself is reasonably abundant and cheap.

Generally though, you have to put in energy to extract the hydrogen from water. The question is how do you do that? You can put in electricity, or light, or maybe other means, but no matter how you do it, it will cost you energy. Let’s say we use electricity to break down water to generate hydrogen. That’s called electrolysis. But there you are using electricity, which is from fossil fuels or nuclear power. What we, and others in the past, propose to do is to use solar energy, which is free, essentially. If we can use solar energy, and water, and break down the water to generate hydrogen, that will be the most ideal way to generate hydrogen, in my mind. We propose to use a solar cell to generate electricity to power a photoelectrochemical cell, in which hydrogen is generated, using electricity and light. But the electricity needed is from the solar cell. At the very end, the idea is, if we make a device at the end of the project, all we need to do is take the device outside, leave it in the sun, fill it with water, and we should be able to generate hydrogen from it.

Of course, in conjunction with that, a separate proposal, is that we would like to develop new nanomaterials, in this case nanowires, so they can be used to store hydrogen at high density.

We propose to generate and to store the hydrogen at high-density ideally, higher than we can do currently. Part of the idea for the proposal is that nanowires offer some advantages that bulk materials, or conventional materials cannot offer. For example, nanowire, or nanomaterials in general, have huge surface areas, with respect to their volume. In other words, if you have a big chunk of material and you break it down into small pieces, the smaller the pieces become, the larger the surface area for the same amount of material. That’s called a surface-to-volume ratio. So that’s one big advantage.

If you think about hydrogen being stored on the surface of the nanomaterial, by going down from one centimeter cubed, down to a 10 nanometer particle, we gain a factor of one million in surface area. You can imagine increasing the density of storage significantly.

Currently, solar cells are based on bulk silicon materials, or polycrystalline silicon materials. And the material we proposed is silicon nanowires, or nanosprings Ñ basically very, very small in size nanomaterials of silicon. Again, we have a larger surface area, and we have quantum confinement effects that can be used to control the properties of the materials. So we can use the same materials, silicon, but we can design them by changing the structure to get different properties, and therefore functionality, out of just silicon. In bulk, you have silicon, that’s it. But if you dope the materials, you get different properties. But there’s a very limited degree of freedom that we can play with. By using nanomaterials, we can really change the properties in a dramatic way, the physical and chemical properties.

Salazar: How do these new materials you’re testing compare to the solar cells today?

Zhang: To give you some rough numbers, the solar cells that people currently use, the commercial solar cells Ñ can reach 30% quite easily.

A lot of people are doing photovoltaic, or solar cell research right now. But we would like to design materials that can potentially have better efficiency, or if the efficiency is not better, maybe cheaper, so the cost for commercial applications is not just efficiency Ñ it’s also the cost. Let me give you an example. Silicon is relatively expensive Ñ that’s why we don’t have solar cells on our roofs nowadays for most families Ñ it’s still cheaper to use electricity from conventional sources.

An area that a lot of people are working on is the use of titanium dioxide, metal oxides that are nanoparticles and relatively cheap, when compared to silicon. Currently the best efficiency for solar cells is based on titanium dioxide (TIO2) nanoparticles. Titanium dioxide by itself is white, it’s a powder and it cannot absorb solar light. In this case we put either a polymer or a dye molecule on the surface of these nanoparticles so that they can absorb sunlight. These molecules absorb sunlight and then inject an electron into TIO2, completing a circuit where we can produce electricity. This is already being done. The efficiency is about 10%. It’s not as efficient as silicon, that’s true, and while right now it’s more expensive because it’s not in commercial production, but let’s just for the purpose of argument that if we can produce these cells at 1/10 the price of silicon cells, even if my efficiency is a factor of three lower it’s still 10% rather than 30% efficiency, but I gain in price, the price is lower by a factor of ten. That’s still commercially viable for producing such solar cells. We’re not really striving to achieve the best efficiency, but we’re hoping to produce cells that have the best combination of efficiency and cost. These are two factors that we need to consider together, and also, making them environmentally friendly. To produce silicon, it requires a lot of chemical processes that use energy, chemicals, and by-products. TIO2, which you might have seen in some of the paint materials that you use, is non-toxic and very abundant. It’s pretty cheap. “We propose to use solar energy and water to generate hydrogen. This is essentially a holy grail in science and technology. . .”

Salazar: Can you talk about the process of how these nanomaterials are made?

Zhang: In general, there are two ways to make nanomaterials. In my lab we use chemistry methods to make nanomaterials, semiconductors, metals, or insulators. The second method is to use physical methods, either by vapor deposition, you evaporate a source of material and collect a substrate. So it’s like a deposition technique, either photochemical or chemical. Or just evaporate it from a source, and then condense them onto a substrate. So basically there’s chemical methods and physical methods of making nanomaterials. One technique we use is called GLAD, which stands for glancing angle deposition. It’s somewhat uniquein that it offers a lot of flexibility in the deposition process. It’s a physical method of depositing materials in a controlled structure. It can be made into a short nanorod. That means that it is very small in two dimensions, and long in the third dimension. When it’s very, very long we call them nanowires. Let’s say the cross section in the X-Y direction is 5-10 nanometers. In the Z direction it can be 50 to 100s of nanometers. So normally, we call that a nanorod. But if in the Z direction the length is microns, much, much longer, then they’re called nanowires. We can also make, using this technique, something we call nanosprings. So basically, by changing the control parameters of the instrument, you can make these nanowires twist so they appear like a spring. But they’re all well controlled. The different springs that we make all have the same diameter and the same length, same structure. This is very difficult to do with chemical synthesis, where you can make nanomaterials, but you’ll have a broad distribution in size and shape. The GLAD technique offers much better control over the shape, size, and dimensionally of the nanostructure produce.

Salazar: Let’s get back to our “gas tank” and talk about these new nanomaterials and how they store hydrogen.

Zhang: There are two typical ways that people can store hydrogen right now. One is just to store the hydrogen in our gas tank, like how we deliver oxygen, or other chemical gasses. It’s in a big cylinder and compressed. But the density is still very low. It’s still a gas. Another technique is more in the research arena right now is to use chemical materials, solid materials that interact favorably with hydrogen to produce compounds or complex.

I’ll give you an example. I can take a metal of nickel or magnesium. These metals can form a complex with magnesium or hydrogen. There are two mechanisms under this category. The hydrogen molecules may stay at a molecule. That’s one mechanism. Another mechanism is that hydrogen molecules actually break apart into hydrogen atoms once they bind, or they adsorb onto the surface of the solid material like a metal. Similarly, people are trying to use carbon nanotubes Ñ also a solid material Ñ to store hydrogen. What’s nice about these methods is that you don’t have to worry about high pressure, which can be potentially dangerous. And, potentially it could also be high density.

Now let’s imagine that the action takes place on the surface of the material. So back to the issue that we talked about earlier, when you have nanomaterials, the surface area is much larger than micron size, or millimeter-size bulk materials. So you can imagine, just from the viewpoint of the surface area, you would like to have the material as small as possible. Of course in reality you would have to worry about interactions between the particles, what surface areas are really available. Having the area does not mean that the chemical sites located there are active. So that’s the research, that’s what people are working on. And ideally we want to have as many sites on the surface of either a bulk material or nanomaterial to be available for hydrogen adsorption, or storage, as possible.

This is a very interesting scientific problem here. It’s two steps. Let’s say that I have a piece of metal, or nanomaterial, and I put it in a bottle, a cylinder let’s say, and fill it with hydrogen. I hope that the hydrogen will get onto the surface of this piece of solid material. Now I can carry this from let’s say a gas station to a car. The first step is that we need to have the hydrogen adsorbed onto the material as quickly, as efficiently, as possible, with the highest density possible. Once I bring the material to my car, I need this hydrogen. That’s my fuel, and I want to get the hydrogen off the material. That’s a desorption step. Now at this step, I want to get it off as quickly as possible, as efficiently as possible, with 100% desorption. Because if it doesn’t desorb, whatever doesn’t desorb from the material you cannot use. So once it desorbs, you have the hydrogen gas that you can use to drive a fuel cell, or burn it in some cases for the application of interest, like running an engine. So in the storage project, the three issues that we have to work are, 1) getting the hydrogen onto the material, 2) and then to get it off. Now both steps depend critically on the property of the solid material, the chemical and physical nature of the material. What we are proposing is to use nanowires of different metals, different nanostructures, nanosprings, to see which nanostructure will give us the best properties for this purpose, adsorption and desorption of hydrogen, the rate, density, and so on. And we already know that large surface area is desired.

Salazar: Is there anything else that you’d like to share with the listeners of Earth & Sky?

Zhang: Generally, I would emphasize that the long-term goal here is to produce energy that is clean and efficient when we use them, but also that the process of generating the fuel uses as little resources and as cheaply as possible. In other words, we want to use solar light and water, which is relatively cheap and abundant and environmentally friendly. So I think this is one of the most exciting possibilities.

But I do want to say also that this is long-term. The target date is 2020, when we hope to start using hydrogen (commonly) in automobiles. I’m not certain that we can really reach that goal by 2020, but I think that there’s hope in the long run that this will work, and it should work, with more research, more investment in these areas from the federal government, state, and private sectors. I think this is one of the most exciting areas we should push to have more investment, more research done.