Taking a tip from the plants: Mimicking photosynthesis to produce fuels

By Lakshmi Chandrasekaran

Nathan La Porte dreams of a field full of solar panels leading to a pump in the corner fueling station. The sun shines down on the panels collecting enough energy to power a process that produces a liquid fuel to fill up the gas tank of a car.

If you think this falls in the realms of science fiction, think again. La Porte, a post-doctoral scientist with Michael Wasielewski’s chemistry lab at Northwestern University, is working to convert the concept into reality. And the inspiration for this comes from the billion-year-old phenomenon perfected by plants to produce their food – photosynthesis.

Wasielewski, director of the Institute of Sustainability and Energy at Northwestern is developing artificial photosynthesis as a way to harness solar energy and produce different types of fuels while tapping atmospheric carbon dioxide as a resource for the process. Carbon dioxide is the driving force of climate change and a key ingredient of photosynthesis.

“We are running the most dangerous experiment in history right now, which is to see how much carbon dioxide the atmosphere can handle before there is an environmental catastrophe,” said Elon Musk, CEO of Tesla Motors, in an interview with USA Today.

Artificial photosynthesis for solar liquid fuels of the future. (Animation scripted and reported by Lakshmi Chandrasekaran/MEDILL; produced by Next Media Animations)

Each year, we are spewing into the atmosphere 36 billion tons of carbon dioxide, a greenhouse gas that turns up the thermostat on global warming. A recent study estimates that just to meet the 2-degree global warming curb set by the Paris Climate Accord, we must soak up 1.8 billion tons of carbon dioxide every year until 2100.

“The important point is that carbon capture and storage that is happening at a rate of tens of millions of tons currently, needs to be ramped up by hundreds of times,” said associate engineering professor Eric Masanet at Northwestern University’s recent climate change symposium.

The answer may be to take a cue from nature and put all that carbon dioxide to good use. .

Mimicking a natural plant phenomenon

Plants absorb sunlight, water and carbon dioxide for  photosynthesis, using solar energy to split the water molecule into hydrogen and oxygen and  power a chemical reaction that assimilates carbon dioxide, producing carbohydrates used to fuel the plants’ survival. Splitting of water releases breathable oxygen. Scientists like Wasielewski are trying to replicate this phenomenon in their labs by using artificial light to split water as well as convert carbon dioxide into useful products such as a liquid fuels.

However, scientists are not just trying to mimic photosynthesis in a lab, but scale it up and make it more sustainable. “Natural photosynthesis is fragile and biological systems break down easily. If we build a system to do something similar we have to build in a way that it is going to have less of a tendency to break down,” said Wasielewski.

Artificial photosynthesis as a field of research surfaced almost 40 years ago but it actually took off only 10 years ago. At that point, scientists started grappling with how to successfully couple photo-driven systems (akin to sunlight in nature) and catalysts – an important part of the photosynthetic system helping to break carbon dioxide down to incorporate into carbohydrates.

Carbon dioxide and water “are very stable molecules. To convert them without a catalyst to the products that we want, namely oxygen and fuel, would require wasting a lot of energy,” said La Porte. Although there are several catalysts to reduce carbon dioxide, only a select few may work in artificial photosynthesis.

“Frequently you can take two really good systems – one very good catalyst and one good photosensitizer, put them together and they don’t work at all,”  Wasielewski said.

Getting at a working connection involves searching for the right combination of catalyst and photosensitizer. Easier said than done. This multi-step research is  steeped heavily in fundamental chemistry that looks straightforward in theory but proves to be challenging when tested in the lab.

Creating an effective barrier

The Wasielewski lab is working on coupling a light-absorbing molecule, the chromophore to a catalyst that will channel the light energy to reduce carbon dioxide to carbon monoxide. The carbon monoxide is then converted into methanol that can be used as a fuel. What makes this similar to natural photosynthesis is that this process uses light energy to produce the electrons that the catalyst needs, just as plants do, rather than generating electricity as an intermediate step. Another group at the Joint Center for Artificial Photosynthesis at Berkeley have built a device that powers photosynthesis using solar electricity from silicon-based solar cells.

Wasielewski’s student Jose Martinez designs and synthesizes light-absorbing molecules (chromophores) that become excited when exposed to sunlight. In their high-energy state, an electron transfer takes place from the chromophore to the carbon dioxide reducing catalyst bound to the chromophore – a step known as charge separation. Now that the chromophore is in a low-energy state, it attracts the electron back from the catalyst – a step referred to as charge recombination. It is imperative for the electron to remain bound to the catalyst to enable the reduction of carbon dioxide to carbon monoxide. So, the question is focused on how to avoid the electron transfer back to the chromophore. Can it be prevented or at least delayed long enough to allow the carbon monoxide conversion?

Enter spectroscopist La Porte, who grapples with these uncertainties by exploring the “photophysics of the system,” Martinez said. Sitting in a state-of-the-art climate and humidity controlled spectroscopy lab at Northwestern, La Porte hits the molecule under study (in this case, the chromophore) with short discrete laser light pulses that are less than a trillionth of a second (a picosecond) in duration and observes how the electrons move in very short time spurts. These experiments allow La Porte to determine how fast the electron leaves the chromophore or jumps back onto it. Understanding the “lifetimes” of the electron transfer steps allow La Porte to create what is known as a “barrier” – an intermediate step between the electron transfer from the chromophore to the catalyst.

“We would put in this extra molecule, the barrier that the electron could easily hop onto,” he said.

Martinez and La Porte have shown that without the barrier, the electron transfer took place in 800 picoseconds. However, adding the barrier delayed it by 22 microseconds – which means, it takes more than 20,000 times longer for the electron to go back to the chromophore, a way of maintaining the all-important bond to the catalyst to allow for carbon dioxide reduction to occur.

The team is now expanding this strategy – such as introducing a second barrier to provide an electron source long-lived enough to reduce carbon dioxide efficiently. This is a challenge mainly because carbon dioxide is “an extremely stable molecule that does not react readily with the sorts of molecules we could use to reduce it. Reducing CO2 is a multi-electron, multi-proton process in which the electrons and protons have to be delivered in the correct order at the correct time if catalysis is going to proceed efficiently,” La Porte said.

The upcoming decade

 “There are big challenges and opportunities at the same time in the next few years such as selectivity – being able to get the exact product you want using earth-abundant catalysts that are driven by renewable energy,” said Wasielewski, who has overseen several sustainability projects in his role as director of the Institute for Sustainability and Energy at Northwestern.

One of the recent projects includes the Ubben Climate and Carbon Science program to understand how and why the climate is changing, with a focus on reducing fossil fuel burdens that are driving warming temperatures. To this end, ISEN’s recently concluded climate change symposium at Northwestern fostered impactful dialogue between science, business and policy. As part of the university’s outreach efforts, the packed conference, also sponsored by the Department of Earth and Planetary Sciences, welcomed the public.

So when will La Porte’s dream be turned into reality? If we are to see a commercial photosynthesis product on the market, Wasielewski suggests maintaining the current or accelerated levels of activity in solar energy research. “Also, it needs government policy to not get in the way. If those two things happen, a 10-year time span is not unrealistic. If things slow down a little bit, 15-20 years maybe,” an optimistic Wasielewski said.

Investing in renewable energy is among the top 10 solutions recommended in the New York Times bestseller book – Project Drawdown, published earlier this year by environmentalist Paul Hawken. And indeed, researchers have been looking at efficient and cost-effective means to harness energy from solar power.

Compared to five years ago, the cost of installing solar panels has gone down considerably, making it a competitive energy alternative to fossil fuels. In fact, in September the U.S. Department of Energy released a new report showing the utility-scale solar cost target was met ahead of schedule, beating the government’s 2020 target by three years!

But while conventional solar cells have been making inroads into the energy sector, researchers are working hard to expand the reach of solar cells to not only increase their efficiencies but developing various applications such as artificial photosynthesis to harness solar energy.

Photo at top: Mike Wasielewski, director of the Institute of Sustainability and Energy at Northwestern, runs the research to  make of artificial photosynthesis a reality that can produce liquid fuels to power our cars, in future. (Lakshmi Chandrasekaran/MEDILL)