The future of planet Earth will rely on renewable energy. Without a complete remaking of humanity’s energy generation systems, there is no hope of staying under a 2°C warming due to climate change. The higher above 2°C that our atmosphere rises, the lower the chances we are to be able to counteract the irreparable damage that will ensue. Renewable energy sources are being sought more than ever before due mainly to rising greenhouse gas emissions resulting from the burning of fossil fuels. All renewable energy sources, from geothermal, wind, and solar energy, to biomass fuels and hydrogen combustion, will play a role in replacing fossil fuels. However, in a world run on fossil fuel combustion, it is extremely unlikely that society will transition away from oil, gas, and coal immediately. Instead, policies that encourage the development and use of emerging technologies such as artificial photosynthesis will bridge our present with the future and reduce humanity’s carbon footprint. While artificial photosynthesis is still in its developmental stages, many are yearning to know if it could be a feasible method of carbon dioxide reduction in years to come. With continuing research into this progressive technology, it is now understood that the development of artificial photosynthetic systems was not intended to reduce the amount of carbon dioxide in the atmosphere, nor can it currently be done effectively via this method. To find out what artificial photosynthesis is capable of doing, the technology must be understood, misconceptions must be addressed, and the challenges and sustainability of the process assessed.
Artificial photosynthesis is a chemical process which mimics the natural action of photosynthesis. A system like this is said to be “biomimetic,” or, mimicking biological processes, as it imitates the models, systems, and elements of nature for the purpose of solving the intricate issues of humanity (Vincent 12). In this system, it is clear that artificial photosynthesis mimics the natural process of plant photosynthesis. Natural photosynthesis converts sunlight, water, and carbon dioxide into glucose and oxygen, while artificial photosynthesis is a method of capturing the energy from sunlight and storing it in the chemical bonds of a solar fuel (DOE/Brookhaven National Laboratory, Molecular System). A solar fuel is a synthetic, chemical fuel created from solar energy through photochemical, thermochemical, and electrochemical reactions, such as the reactions that occur during artificial photosynthesis (Heeger 7). In these reactions, the sun’s rays provide the energy source which is transduced into chemical energy, often by reducing protons to hydrogen, in a liquid or gaseous form.
To fully comprehend this artificial process, one must first understand the function of the natural process which artificial photosynthesis mimics. In natural photosynthesis, water molecules are photo-oxidized, or, degraded as facilitated by solar energy, into oxygen and protons (Nocera 767). In the second half of plant photosynthesis, a light-independent reaction known as the Calvin Cycle occurs, where carbon dioxide is converted into glucose, a carbohydrate. When respiration later takes place within living plant cells, some glucose and oxygen molecules are used to produce energy that is necessary for life. The remaining glucose is then stored as a starch for later conversion back into glucose, which would then be used to produce energy through respiration (Khan 2:01). Today, scientists are researching and developing photocatalysts, or, substances that will increase the rate of a chemical reaction, for both the light dependent reactions as well as the Calvin Cycle (Yarris, Turning Sunlight…). Further research on artificial photosynthesis encompasses designing the machines that will be able to directly produce solar fuels, while simultaneously creating enzymes that will also produce biofuels or bio hydrogen from solar energy (Crabtree 4). In natural photosynthesis, the sun releases photons which plants harness in order to create glucose molecules. In artificial photosynthesis, protons that are released from the splitting of water molecules through photocatalytic reactions may be used for the production of hydrogen, with the only byproduct being oxygen (Thrust 2, JCAP). In current technologies, hydrogen and oxygen are often separated by running an electric current through water (Dockrill, Artificial Photosynthesis…). In summary, the natural and artificial both use solar energy to produce a solar fuel, one to be used by the plant, and one to be used by the people, respectively.
Understanding how artificial photosynthesis works at a molecular level is crucial to assessing the potential efficiency, sustainability, and disadvantages of this system, as well as deciphering if this process may reduce carbon dioxide in the atmosphere, or if it simply generates green energy. Devens Gust, the Director of Energy Frontier Research Center for Bio-Inspired Solar Fuel Production, argues that artificial photosynthesis encompasses three main steps. First, “reaction complexes” take in sunlight and convert this solar energy into electrochemical energy. Next, a “water oxidation complex” catalyzes the reduction of water into hydrogen ions and oxygen molecules. Finally, another catalytic system uses these hydrogen ions to make fuels such as carbohydrates, lipids, or hydrogen gas (Gust 1890). Heinz Frei, a chemist who researched with University of California Berkeley Lab’s Physical Biosciences Division, states that “effective photooxidation requires a catalyst that is both efficient in its use of solar photons and fast enough to keep up with solar flux in order to avoid wasting those photons.” It was later found that clusters of cobalt oxide nanocrystals serve well with efficiency and speed, are abundant, and also have long enough lifetimes to “perfectly fit the bill” as they are able to carry out the photosynthetic reaction by splitting apart water molecules (Yarris, Turning Sunlight…). This process may operate in water at room temperature, and has to potential to become self-healing if technological advancement persists; as plants are able to fix malfunctions within themselves, someday so may this system. (Nocera 169). Moreover, there are other molecules being explored that may also function as effective catalysts for these photosynthetic reactions. For example, a review completed by Wee-Jun Ong, a chemical engineer at Monash University, is expected to open a new research doorway to a generation of graphitic carbon nitride, g-C3N4, based photocatalysts. These catalysts are expected to have higher performances than those in current use by “harnessing the outstanding structural, electronic, and optical properties” for the possibility of a more sustainable future, free from environmental harm (Ong 7159). Although researchers can effectively retrieve hydrogen fuel seemingly out of thin air, what science still synthesize are molecules that will actively reduce carbon dioxide in the air at atmospheric levels.
In society today, there is this stigma that human technology will advance at a fast enough rate to keep up with humanities massive deposits of greenhouse gases into the atmosphere. It is becoming increasingly clear that this is no longer possible, as humans are releasing carbon dioxide at a faster rate than ever before, and our current industrial systems and electrical grids are not fully compatible with renewable energy electricity generation (Neff, Renewable Energy). For this idea to become a reality, scientists need to continue developing green technologies, such as artificial photosynthesis, and integrate them into our current fossil fuel based systems in order to make the steady transition to a fully renewable energy system. Artificial photosynthesis is a perfect example of this integration, as it produces a fuel that can be used with our current infrastructure, including cars, buses, and other transportation methodologies.
Due to its emerging existence, there are many misconceptions about artificial photosynthesis. Artificial photosynthesis is not simply a way of taking carbon dioxide out of the atmosphere, undergoing a chemical process, and releasing fresh oxygen, nor is it a direct method of generating electricity. Artificial photosynthesis is a way of producing a biofuel through the process of undergoing a chemical reaction by means of solar energy. The purpose of undergoing these reactions is to produce a fuel that can be stored and used conveniently when solar energy, or sunlight, is not available, making it a competitive alternative to fossil fuels. At first glance, this may seem quite similar to photovoltaics, however, photovoltaics generate electricity directly from solar energy, while artificial photosynthesis may first create a fuel, such as hydrogen, which can later generate energy or electricity through another reaction, which may then be used to carry out some form of work (Sosnowski, The Difference Between Photosynthesis…). To clarify though, artificial photosynthesis systems may, in fact, use photovoltaic cells within their systems to generate the electricity which is used to split water into hydrogen, the fuel, and oxygen, the byproduct (Biello, Plants versus Photovoltaics). Hydrogen generated from these processes may someday be used as a fuel for transportation, replacing gasoline and diesel fuel, or may simply be used to generate electricity in fuel cells (Bard 141). Nevertheless, this fact is significant as hydrogen will not produce any pollutants, or greenhouse gases, through its combustion.
The potential of artificial photosynthesis is often denounced due to lack of technological capability, minimal energy storing techniques, as well as due to its high expense and low efficiency, among other factors. Andrew Bocarsly of Princeton University argues that “we’ve been studying carbon dioxide chemistry for a long time, more than 100 years, and there’s very little evidence that we could do what a leaf does.” For example, plants have the innate ability to repair themselves, while human technology is not yet to the point where a system may effectively heal itself (Biello, Plants versus Photovoltaics). In homogeneous artificial photosynthesis systems, that is, a system in which hydrogen and oxygen are produced within the same compartment, the two molecules together may generate an explosive mixture. This poses a threat to the safety of the users of the system as well as to the system itself. The other current option, a heterogeneous system, consists of two separate electrodes, easily allowing for the separation of oxygen and hydrogen. However, a drawback of this safer system is that it is more complex, making it much more expensive to design and build. In comparison, photovoltaic cells appear to be more stable over longer periods of time since artificial photosynthesis systems often contain components that will corrode in water, not to mention that many hydrogen catalysts are oxygen sensitive and will degrade in its presence over time. Furthermore, photodamage may occur to artificial photosynthetic systems over time as well. Technology aside, at current prices, products such as hydrogen produced via artificial photosynthesis are not yet commercially viable to compete with fossil fuels. Ken Caldeira, an atmospheric scientist at Stanford University, says he is “skeptical that this is really going to be the economic path forward” admitting that, “in order to become commonplace, [artificial photosynthesis] would have to become a lot cheaper and a lot more efficient” (Ronson, Can Artificial Photosynthesis…). Oftentimes, the efficiencies are so low an external electrical input is required to catalyze the reactions, meaning that artificial photosynthetic systems are not even a fully renewable form of energy generation; this additional electricity is often derived from a non renewable source. It is clear that artificial photosynthesis is not yet ready for use outside of a laboratory setting (Gust 1891). Even if these systems could rely solely on solar energy for production, sunlight is intermittent (Neff, Renewable Energy). There are gaps in solar energy throughout the day due to clouds, and every night there is no solar energy to be harnessed. Research shows that man made solar electricity generation systems are already more efficient than the natural plant processes which they rival. A study performed at Michigan State University revealed that photovoltaic cells are already more efficient at converting solar photons to energy than even real plants are (Biello, Plants versus Photovoltaics). Furthermore, simple solar cells absorb more energy in sunlight than photosynthetic systems can because solar cells capture light across the electromagnetic spectrum, ranging from infrared to ultraviolet, whereas chlorophyll can only absorb only visual light (Biello, Plants versus Photovoltaics). While photovoltaics are used in artificial photosynthetic systems, these facts may be crucial to informing policy makers whether or not to continue to pursue biofuels by mimicking natural systems, or to simply stick with solar electricity. Regardless, even if investments were made in advancing research in fields such as artificial photosynthesis, hydrogen fuel-cell cars are still extremely expensive. Expenses like these may reduce the demand for hydrogen fuels produced via artificial photosynthesis. In this light, artificial photosynthesis appears not to be a feasible source for fuel production, or carbon dioxide reduction.
Despite a multitude of arguments against artificial photosynthesis, there are many potential advantages as well. The Global Artificial Photosynthesis conference concluded that, if artificial photosynthesis were to be implemented globally, it could serve as a development aid for ‘off-grid’ energy, and could “replace policy models of corporate globalization and ever-increasing economic growth predicated on preparation for war and use of non-renewable and polluting energy sources” (Faunce, GAP). Furthermore, it has been argued that artificial photosynthesis, if fully developed, will serve as a more efficient renewable energy source than similar renewable methods such as photovoltaics. In solar energy generation via photovoltaics, energy is transferred directly into electricity, but if it is not used right away, the energy discharges and is not available for human consumption. This is due to our lack of battery storage technology for electrical energy. Unlike photovoltaic energy generation, artificial photosynthesis can generate a solar fuel which can be stored for later use. This gives hope to fuel the next generation of transportation vehicles, free of carbon dioxide emissions. Production of solar fuels even has the potential to be cheaper than gasoline (Artificial Photosynthesis, Education). Chemist Nathan Lewis, director of the U.S. Department of Energy’s Joint Center for Artificial Photosynthesis states that “chemical fuels [hydrocarbons, like those in oil] would be the game-changer if you could directly make them efficiently from sunlight” because “it pairs the biggest source of energy and the biggest storage” (Biello, Plants versus Photovoltaics). While artificial photosynthesis is still not efficient enough for widespread use, “the development of artificial photosynthetic systems for water splitting and CO2 reduction on a large scale for practical applications is the ultimate goal towards worldwide sustainability” (Zhou 9870). In addition, according to Leone Spiccia, lead author of artificial photosynthesis research at Monash University in Melbourne, “electrochemical splitting of water could provide a cheap, clean and renewable source of hydrogen as the ultimately sustainable fuel,” arguing that her “latest breakthrough is significant in that it takes us one step further towards this becoming a reality” (Dockrill, Artificial Photosynthesis…). The Monash team was able to use nickel as a catalyst in artificial photosynthetic reactions, reaching a 22.4 percent efficiency compared to the typical 10 percent. This is the highest ever recorded efficiency for artificial photosynthetic systems, and is an extremely significant finding because nickel is a cheaper, and much more accessible resource compared to rarer metals that had been previously used. However, what artificial systems still cannot perform is the act of using atmospheric concentrations of carbon dioxide to undergo photosynthetic reactions.
In essence, the answer is no, artificial photosynthesis is not a feasible method of reducing atmospheric carbon dioxide. However, it may play a role in counteracting climate change through its emission free creation of hydrogen, a clean, green energy source. While the technology is not yet advanced enough yet to use outside of a laboratory setting, it is possible that in the near future artificial photosynthesis will provide our society with a renewable energy system that causes little environmental impact.
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