Contributed by Lauren Winstel, ACS Green Chemistry Institute® Research Assistant
Photosynthesis is one of the most fundamental processes in nature, absorbing ever-present sunlight, water and carbon dioxide, and then converting it to glucose and oxygen. When combined with modern technology, this basic process can be applied to many different industries. The result is a “bionic leaf” that can mimic energy conversions, creating various commercially viable fuels as well as chemical feedstocks. Conventionally, hydrogen is produced through the oxidation of methane, or the steam reforming of fossil fuel waste gases. This technology has the potential to sustainably produce hydrogen gas using simple inputs from the air using catalysts that are non-toxic and made from abundant elements. Over the last few years, two research teams have been developing different versions of the “bionic leaf” technology, both with their own advantages that have the potential to go above and beyond the initial goal of hydrogen gas production.
The basic concept of artificial photosynthesis is focused on water splitting, where H2O molecules are broken down into their separate oxygen and hydrogen atoms. Hydrogen production in this manner has been studied by many different research groups over the years, including one of the very first artificial leaves developed by Nocera Lab that used indium/fluorine tin oxide membranes combined with phosphate and cobalt catalysts. Daniel Nocera was one of the first scientists to devote his studies to artificial photosynthesis, and he is often unofficially credited with the creation of the first “practical bionic leaf” in 2008. A major development in the technology was made in 2014, when Nate Lewis and his research team at the California Institute of Technology (CalTech) greatly increased its sustainability, efficiency and safety over all previous attempts at the time because its reaction was chemically stable, and it used earth-abundant materials instead of rare elements.
On top of a silicon semi-conductor base, a thin coating of nickel-based oxide created a membrane that isolated the hydrogen and oxygen from each other. Separation is essential with highly reactive mixtures such as this because they are explosive when exposed to heat or electricity. When in close proximity to the current created by the leaf’s photoelectrodes, the mixture could be very dangerous. However, the transparent film produced at CalTech eliminates this possibility by holding the split molecules completely separate. The result is an infinite supply of usable hydrogen and oxygen created by a system solely powered by sunlight.
The most recent version of the “bionic leaf” has been developed by Daniel Nocera and his group of researchers at Harvard University. Nocera has continuously improved his technology over the years, and the newest version builds on the concepts from Nocera’s own product from 2011 as well as Lewis’ technology. The 2017 edition swaps the nickel-molybdenum-zinc catalyst for a cobalt-phosphorus alloy. The purpose for this change was to make the separation membrane more bacteria-friendly, since the previous nickel-based catalyst coating was poisonous to microbes. This allowed Nocera to pair the water-splitting technology with Ralstonia Eutropha bacteria, which takes the entire process one step further. Nocera’s version not only isolates hydrogen for potential fuel use, but it also creates a bioplastic fertilizer that improves crop growth.
Nocera’s team has engineered a microbial organism that works in tandem with their “bionic leaf.” The microbe’s sole food source is hydrogen, and it also contains nitrogenase, an enzyme that absorbs nitrogen from the atmosphere. This microbe intakes carbon dioxide combined with the hydrogen produced by the “bionic leaf,” and then it has the potential to produce two different products. One option is that the organism could create a hydrogen-based fuel that it expels. The second option is the creation of polyhydroxybutyrate, a bioplastic that would be stored inside the organism as a fuel source for basic cellular activity. This second option was implemented and put into a feedback loop, and as a result, Nocera and his team created a self-sustaining organism.
In addition, this organism also uses the hydrogen from the “bionic leaf” in a third way. Due to the nitrogenase within the microbe, it can absorb nitrogen and then combine it with hydrogen to create ammonia. Because of a molecule block inserted into the microbe, none of the ammonia is absorbed into the cells. Instead, the ammonia builds up until it begins to be expelled from the organism at a constant rate. At this point, the microbe can be buried and produce an infinite amount of ammonia-rich soil. The improvements in crop yields can be clearly seen in Figure 1. Not only does this fertilizer increase the size of crops, but it also isolates the ammonia under the ground in the soil instead of being topically applied like many conventional fertilizers. This could greatly reduce agricultural runoff when it rains, which would reduce algal blooms that are often found in waterways downstream of croplands.
While Nocera’s and Lewis’ basic technologies are very similar, the final applications are very different. The minor change in membrane material allowing for microbial growth created an entirely new avenue of applications in agriculture. Bionic leaves and artificial photosynthesis play an important role in the future of the green revolution. What began as mimicking nature has become a method of improving upon it and creating artificial processes that surpass their natural counterparts in both efficiency and sustainability.
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