Why the future of fuel lies in artificial photosynthesis

By Carol Clark

Most people, especially technical experts, may agree that we have an energy crisis, but it’s much harder to come to a consensus on how to solve it.

Fossil fuels, wind power, biofuels, geothermal power, nuclear energy and solar power are all pieces in the puzzle for how to keep Earth’s burgeoning civilization running, says Emory inorganic chemist Craig Hill.

He adds, however, that an energy source that will be essential to manage the crisis in the coming decades is the least developed: Artificial photosynthesis.

Hill and other top experts in the nascent field of artificial photosynthesis co-wrote an opinion piece on the topic published in the journal Energy and Environmental Science.

“Humanity is on the threshold of a technological revolution that will allow all human structures across the earth to undertake photosynthesis more efficiently than plants,” the authors write.

The 18 authors on the opinion piece, from leading research universities and national laboratories in the United States, Europe and Australia, represent the broad range of expertise, from chemistry to biology to engineering, working on the problem.

The aim of artificial photosynthesis is to use solar energy to split water, to generate hydrogen as a cheap and abundant source of carbon-free fuel.

“The development and global deployment of such artificial photosynthesis (AP) technology,” the authors write, “addresses three of humanity’s most urgent public policy challenges: to reduce anthropogenic carbon dioxide emissions, to increase fuel security and to provide a sustainable global economy and ecosystem. Yet, despite the considerable research being undertaken in this field … AP remains largely unknown in energy and climate change public policy debates.”

“Globally, our energy requirements our expected to double in the next 30 to 40 years, maybe less,” Hill says. “It’s a staggering problem that puts everything else in perspective. Everything derives from energy. If we don’t have enough energy, we’re not going to have enough food and water.”

Fracking has opened up new sources of fossil fuels in the United States, but ultimately fossil fuels are going to run out. Fossil fuel use is also coming at a rapidly escalating environmental cost, including rising global temperatures and acidification of the oceans.

The only energy source that can come close to sustainably powering our long-term needs is terrestrial sunlight, Hill says.

The solar power industry, which converts sunlight into electricity, continues to grow, but it has severe limitations, Hill says. A great deal of space is required for solar panels to collect the sun’s energy, and that energy must be stored in batteries.

“We’re at the point now where we have solar powered buildings and electric cars, but we are never going to be able to run airplanes and ships and most other forms of transportation on electricity,” Hill says. “That’s why we ultimately need artificial photosynthesis, which is just another way of saying solar fuel.”

The goal of artificial photosynthesis is to do what plants do, only better.

“Plants use sunlight, water and carbon dioxide to make fuel in the form of carbohydrates,” Hill explains. “The process, however, is incredibly inefficient. It works for plants because they don’t have to worry about finances.”

Scientists currently know how to mimic plant photosynthesis, but not in ways that are powerful and efficient enough for practical application. Breakthroughs are needed in both fundamental science and materials engineering, says Hill, who is working on perfecting a key aspect of the problem, a water oxidation catalyst. Hill’s lab has developed the fastest homogeneous water oxidation catalyst to date.

“Artificial photosynthesis is a tremendous challenge,” Hill says, “but it’s also tremendously exciting.”

Hill foresees that we will eventually make the necessary breakthroughs to generate solar fuel. We simply have no other choice, he adds, as the human population approaches 10 billion by 2050.

Meanwhile, Hill and the co-authors of the Energy and Environmental Science opinion piece are calling for a globalized approach to artificial photosynthesis, to help raise the field’s public policy profile, remove logistical and governmental hurdles to its development, and strengthen an international commitment to clean, sustainable energy.

They envision scenarios like a network of light capture facilities situated in coastal cities where seawater would be catalytically converted to hydrogen and oxygen.

“Photosynthesis is the great invention of life,” they write. “Like biodiversity, the atmosphere, the moon, outer-space, the human genome and the world’s cultural and natural heritage, it could be treated as subject to common heritage requirements under international law, perhaps through a specific UN or UNESCO declaration. Common heritage of humanity status putatively limits private or public appropriation; requires representatives from all nations to manage such resources on behalf of all, actively share the benefits, restrain from their militarization and preserve them for the benefit of future generations.”

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