Prof Nathan Lewis, talks to AZoM about his research into a nanoscience-based system and the development of an artificial photosynthetic system.
Please can you give an introduction to your research and what you presented at Pittcon 2016?
I'm working on how to make a nanoscience-based system do what a plant does: take sunlight, water and/or carbon dioxide, and directly make a fuel that we could use in our infrastructure.
At the Pittcon presentation I discussed the latest results of how we try to implement this vision into something that could be manufactured in principle, and could be scaled to produce a lot of energy from this source.
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How and why are you developing an artificial photosynthetic system?
We're developing an artificial photosynthetic system because the intermittency of renewable solar and wind is a major technology barrier for implementing them at a large scale.
The sun goes out locally every single night, and there are days and days at a time when the wind doesn't blow. To build a full energy system, you have to store the energy somehow. The densest form of storing energy, other than the nucleus of an atom, is in chemical bonds.
The best battery available has an energy density of 200 watt-hours in a kilogram, but gasoline has an energy density of 12,000 watt-hours in a kilogram. So it is clear that the best way to take an intermittent resource like solar energy and to store it so that it can be supplied on demand is to store it in chemical fuel, just like nature did to make all of the fossil energy from photosynthesis over millennia to many, many hundreds of thousands of years.
We want to do it more efficiently and over a short enough time scale, so that it can be sustained instead of only used once, as we're doing with fossil fuels.
What are the potential applications?
A potential application of artificial photosynthesis would be creating a so-called drop-in fuel - a fuel that would be totally transparent as to its source to a consumer, you wouldn't know whether the gasoline, diesel, fertilizer, or the plastics came from a renewable source, the sun, or from fossil energy.
They would produce the same products, just in a carbon-neutral, scalable, sustainable way, which is secure for our energy and environmental future at the same time.
Please can you outline your modular, parallel development approach?
Our approach involves the use of two photo systems, just like photosynthesis. A solar cell has one photo system and one light absorber, but to make fuel, you need two photo systems to get enough voltage, like two batteries connected in series to provide enough voltage to power a laser pointer.
We have two such light absorbers: one absorbs in the blue and passes the red, the second one absorbs in the red. This is an advantage compared to natural photosynthesis, because both chlorophylls in photo system one and photo system two are the same color, so they fight for photons.
Instead a blue and a red absorber would complement each other, and work together to yield a higher efficiency and absorb more photons from the solar spectrum.
In addition to the two absorbers that we nanostructure, we need two catalysts: one catalyst to evolve oxygen from water, the other one to reduce water and/or carbon dioxide to make a fuel.
So the system is: two absorbers, two catalysts and a membrane to hold the system together, but also to separate the products. Otherwise, if you make an explosive mixture of hydrogen and oxygen over an active catalyst for recombination, that is simply unsafe. We have to have a membrane to ensure safety, as well as to separate the products for efficiency.
That's the blueprint: two absorbers, two catalysts, and a membrane. Then we have to put all of the pieces together and make sure they work under the same conditions, that they're efficient, and that they function hand in hand with all of the parts of the solar spectrum that we can harvest, in order to get the energy we need to make and break chemical bonds.
What are your design principles?
Our design principles are mapped out from the inspiration of natural photosynthesis, but they're adopted in the same way that although birds fly with feathers, airplanes aren't built out of feathers to fly faster and further.
We do have the idea of two photo systems, just like in natural photosynthesis, but we don't use chlorophylls because we use inorganic materials that are more stable and don't fight with each other for photons. In addition, we want a very long axis to absorb the light, but like aspen trees, have a very short axis to move the carrier sideways.
This way we can use much cheaper materials, because the excited states don't have to go all of the way back up the same way they came down - they can go sideways a short distance.
The nano and micro rods bridged by the membrane lets us use very cheap materials, and place our catalyst wherever we want in this internal structure, in order to get the optimum absorption and electrochemical transport to match the whole system synergistically integrated, and have the whole function dictated by more than just the individual parts or their sum.
What will the photoanode and photocathode consist of and why?
The photoanode and the photocathode are the two different materials that we still have to work on to get them to function under mutually compatible conditions, where the membrane and where the catalysts work.
We can work in an alkaline media or in an acidic media, and get a safe system so that we can neutralize the pH gradient, by protons going with the electrons or hydroxides going against the electrons. They are the only two choices we have. In order to do that, we need anodes and cathodes that are stable under the same conditions under which the membrane allows us to work.
For silicon, we have demonstrated that we can reduce water to hydrogen with catalyzed silicon photocathodes, but silicon is not stable in alkaline media: it etches. We can work in acid, but we don't yet have a photoanode that is stable in acid. On the other hand, we do have a protected photoanode that is stable in alkaline media, and we can protect silicon in alkaline media.
We have pieces of each but currently they don’t all work together at once, which is the next step in our development program, in order to demonstrate that we can build a whole system, not just individual parts, and that they can work all together.
How have you minimized costs whilst sacrificing energy conversion efficiency?
The beauty of a system like this is that it's designed from the inception, bottom-up, to be something that could be scaled, manufactured, and be low-cost. We could make fuel right now from a photovoltaic installation, and connect it through a power supply to an electrolysis unit, both of which are commercialized and comprise a very expensive system of fuel production.
Relative to that, think about a multi-layer fabric that you could just unroll, which could wick up water vapor from the air and absorb sunlight, and wick out your product in a very simple, seamless way, much like installing AstroTurf rather than installing solar panels to electrolyzers.
That is the form factor we started with, to have a plastic membrane that was bridged by our multi-layer fabrics, which are these nanofibers or microfibers that can wick up water vapor from the air and wick out our product down below. That is how we address cost, and we do it with scalable manufacturing methods that make materials that would otherwise be inefficient, except in this architecture. We can combine scalability, low cost, and efficiency by integrating them into this form factor.
Is current technology limiting your research in any way?
The limits on our research right now are still in what we call "materials by design". We know the functions we want: we want an anode; we want a cathode; we want the catalysts to not be scarce, and to be scalable and inexpensive; we want the membrane to not be high-performance or very expensive in large areas.
We know the functions that we want but we don't have the materials that can provide those functions, not only individually but together into a system that will function seamlessly.
We are limited by our creativity and ability to invent and discover these materials that we want to design, and so we use theory, we use directed experiment, we use high throughput experiment. We use all of the tools at our disposal as materials chemists in order to try and develop the compatible set of systems that will meet our performance specifications.
Image Credits: Greg Brave/shutterstock.com
What do you think the future holds for this field?
The future of this field clearly shows that we can, to keep the analogy going: “Be the Wright Brothers and get this plane to fly”.
We want to make an artificial photosynthetic system in a scalable, sustainable way, with an efficiency 10 times greater than the fastest growing crop, and makes a fuel that we can directly use in our infrastructure from the sun. If we do that, then history shows that humans are very good at innovating and developing next generations of technology.
Making it faster, better, cheaper, replacing the catalyst and optimizing the designs, and turning the Wright Brothers into 787s by evolution of systems that needs invention also, but it is that first leap that is our goal, to try to show that this is really possible and not just a scheme in a presentation.
Then we can form the basis for thinking about new generations of materials and new performance attributes, to enhance the system so it could be commercialized at scale.
Which talks at Pittcon have you found particularly interesting and relevant to your research?
At the session I was in, nanomaterials was, of course, very interesting, and relevant to my research, both specifically and in general. I heard about new methods of in situ characterization, operando characterization of electrocatalysts, and electrochemical interfaces that could be broadly applicable.
We heard about new designs for membranes and redox flow batteries that could be specifically applicable to help charge and discharge our fuel-forming reactions in new ways. We heard about many other cross-cutting aspects.
We're very interested in putting catalysts in the right places in our three-dimensional network. Should we have little pointed structures at the tips to evolve bubbles, or should the catalyst be further into the structure so that we could look at the electrochemical transport of a bubble refracting light into the rods that we're designing?
These concepts, the three-dimensional nature of electrochemistry, ion transport, mass transport, and some of the new tools that are offered by the vendors at the exhibition, are very relevant to help us think about a path forward in our research.
What are the key benefits you believe people gain from attending Pittcon? What does Pittcon mean to you?
Pittcon is unique for people like me, because of the combination of the expertise in electrochemistry and analytical chemistry, as well as the ability to go through the exhibition and to see the latest tools that you can use to advance your research in a way that is difficult to find in that combination at a society meeting that is more specialized, or would have more of a scientific focus as opposed to a blend between the exhibition, as well as the science. That is where Pittcon really stands out.
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About Prof Nathan Lewis
Dr. Nathan S. Lewis is the George L. Argyros Professor of Chemistry at the California Institute of Technology.
Professor Lewis is Principal Investigator of the Beckman Institute Molecular Materials Resource Center. His research interests include artificial photosynthesis and electronic noses. Nate continues to study ways to harness sunlight and generate chemical fuel by splitting water to generate hydrogen.
He is developing the electronic nose, which consists of chemically sensitive conducting polymer film capable of detecting and quantifying a broad variety of analytes. Technical details focus on light-induced electron transfer reactions, both at surfaces and in transition metal complexes, surface chemistry and photochemistry of semiconductor/liquid interfaces, novel uses of conducting organic polymers and polymer/conductor composites, and development of sensor arrays that use pattern recognition algorithms to identify odorants, mimicking the mammalian olfaction process.
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