Artificial photosynthesis
Photosynthesis is the conversion of sunlight, carbon dioxide, and water into usable fuel and it is typically discussed in relation to plants where the fuel is carbohydrates, proteins, and fats. Using only 3 percent of the sunlight that reaches the planet, plants collectively perform massive energy conversions, converting just over 1,100 billion tons of CO2 into food sources for animals every year.
This harnessing of the sun represents a virtually untapped potential for generating energy for human use at a time when efforts to commercialize photovoltaic–cell technology are underway. Using a semiconductor–based system, photovoltaic technology converts sunlight to electricity, but in an expensive and somewhat inefficient manner with notable shortcomings related to energy storage and the dynamics of weather and available sunlight. However, recent advances have been made with artificial photosynthesis, which, if perfected, could provide unlimited, relatively inexpensive, and clean electricity, with a storage capability.
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[edit] Artificial Photosynthesis – A Developing Concept
To gain a clearer understanding of artificial photosynthesis, we need to delve deeper into the process that plants have perfected and then relate these concepts to commercial energy applications. Two things occur as plants convert sunlight into energy:
Sunlight is harvested using chlorophyll and a collection of proteins and enzymes, and Water molecules are split into hydrogen, electrons, and oxygen. These electrons and oxygen then turn the CO2 into carbohydrates, after which oxygen is expelled.
Rather than release only oxygen at the end of this reaction, an artificial process designed to produce energy for human use will need to release liquid hydrogen or methanol, which will in turn be used as liquid fuel or channeled into a fuel cell. The processes of producing hydrogen and capturing sunlight are not a problem. The challenge lies in developing a catalyst to split the water molecules and get the electrons that start the chemical process to produce the hydrogen.
There are a number of promising catalysts available, that, once perfected, could have a profound impact on how we address the energy supply challenge:
- Manganese directly mimics the biology found in plants.
- Titanium Dioxide is used in dye-sensitized cell.
- Cobalt Oxide is very abundant, stable and efficient as a catalyst
[edit] Artificial Photosynthesis Applications
- There is general agreement that fossil fuels will eventually disappear from the range of energy sources. While they are providing for the majority of our energy needs today, they significantly impact pollution and climate change. Most of the more traditional sources of renewable energy pose other challenges as well.
- Wind turbines have a negative impact on picturesque landscapes.
- Corn requires an extremely large amount of land to produce energy in an adequate supply, and
- Traditional solar technology is quite expensive, still inefficient, and limited in that it depends upon the presence of sunlight.
Artificial Photosynthesis could be a solution:
- It produces stored fuel
- It can produce more than one type of fuel: liquid hydrogen to be used like gasoline in hydrogen-powered engines, incorporated into a fuel-cell-setup and create electricity by combining hydrogen and oxygen into water, or methanol as either a gasoline additive or primary fuel source.
- It does not require mining, growing, or drilling. Since artificial photosynthesis involves water and carbon dioxide – neither of which is in short supply – it is virtually limitless, potentially less expensive, and not only does it not emit greenhouse gases but it removes large amounts of CO2 from the environment in the process of producing fuel. However, there are a number of obstacles that thus far preclude this process from being used on a large scale.
[edit] Obstacles and Challenges
Theoretical solutions applied in a lab environment have met with some success. However, artificial photosynthesis is not yet ready for full implementation, primarily because replicating what took plants billions of years to perfect takes a lot of trial and error. Some of the specific challenges include:
- The catalyst for plants, manganese, is unstable, has a relatively short lifespan, and will not dissolve in water, leading to an inefficient and impractical approach.
- Like the human body, the molecular makeup of plants is both complex and exact, making it difficult to replicate the overall process at the required level of intricacy.
- Stability remains an overriding issue as organic catalysts will degree or cause other reactions that can actually cause cell damage. Inorganic catalysts offer a potential solution, but the speed at which they have to work to make efficient use of the chemical reactions can be an issue.
- In the case of state-of-the-art dye sensitized cells, the issue is not with the catalyst (Titanium Dioxide) but rather with the electrolyte solution that is made of solvents that can erode other parts of the system.
- Progress has been made in the last few years with the further development of Cobalt Oxide as a stable, fast and abundant catalyst and even with the dye-sensitized cells, solutions have been developed that are less corrosive. Current estimates are that we are 10 years away from full scale implementation, if at all. The development of a comprehensive energy policy will require many dimensions and that the path to success is going to require pursuing a number of possibilities, expecting only a percentage of them to pass the test of time. Artificial photosynthesis should be one of the candidates.
[edit] Artificial Photosynthesis Approaches
To recreate the photosynthesis that plants have perfected, an energy conversion system has to be able to do two crucial things (probably inside of some type of nanotube that acts as the structural "leaf"): harvest sunlight and split water molecules.
Plants accomplish these tasks using chlorophyll, which captures sunlight, and a collection of proteins and enzymes that use that sunlight to break down H2O molecules into hydrogen, electrons and oxygen (protons). The electrons and hydrogen are then used to turn CO2 into carbohydrates, and the oxygen is expelled.
For an artificial system to work for human needs, the output has to change. Instead of releasing only oxygen at the end of the reaction, it would have to release liquid hydrogen (or perhaps methanol) as well. That hydrogen could be used directly as liquid fuel or channeled into a fuel cell. Getting the process to produce hydrogen is not a problem, since it's already there in the water molecules. And capturing sunlight is not a problem -- current solar-power systems do that.
The hard part is splitting the water molecules to get the electrons necessary to facilitate the chemical process that produces the hydrogen. Splitting water requires an energy input of about 2.5 volts [source: Hunter]. This means the process requires a catalyst -- something to get the whole thing moving. The catalyst reacts with the sun's photons to initiate a chemical reaction.
There have been important advances in this area in the last five or 10 years. A few of the more successful catalysts include:
Manganese: Manganese is the catalyst found in the photosynthetic core of plants. A single atom of manganese triggers the natural process that uses sunlight to split water. Using manganese in an artificial system is a biomimetric approach -- it directly mimics the biology found in plants.
Dye-sensitized titanium dioxide: Titanium dioxide (TiO2) is a stable metal that can act as an efficient catalyst. It's used in a dye-sensitized solar cell, also known as a Graetzel cell, which has been around since the 1990s. In a Graetzel cell, the TiO2 is suspended in a layer of dye particles that capture the sunlight and then expose it to the TiO2 to start the reaction.
Cobalt oxide: One of the more recently discovered catalysts, clusters of nano-sized cobalt-oxide molecules (CoO) have been found to be stable and highly efficient triggers in an artificial photosynthesis system. Cobalt oxide is also a very abundant molecule -- it's currently a popular industrial catalyst.
