Nature has developed the process of photosynthesis in which ambient energy (sunlight) is converted into chemical energy, which is then used by the organism for sustaining life. Sunlight is one of the few sources of energy that is constant and "renewable". The reaction pathway involved in photosynthesis is very complex but boils down to two basic processes. These processes are the light dependent reaction and "dark" reaction. [1] As the name implies, the light dependent reaction requires the energy of ambient sunlight to convert freely available chemicals into fuel:
The light-independent reaction uses the products of the light-dependent reaction as well as atmospheric CO2 to produce glucose, a fuel that can be used by the organism for a variety of purposes. [1] In total, the efficiency of photosynthesis is about 6%, that is, 6% of the energy contained in the photons absorbed during photosynthesis is available in the products of this series of reactions. [2]
Although statistically photosynthesis appears to be a somewhat inefficient process, the chemical conditions under which these reactions proceed need to be tolerable to the organism in which the process is occurring. In the case of photosynthesis, the conditions are mild and use plentiful earth elements. This system may not be the most efficient but consists of "cheap", "abundant" resources and has thrived for millions of years. To use the popular cliché, photosynthesis could be described as the definition of "green" chemistry.
In 1954 Bell labs invented the first photovoltaic solar cells in which sunlight was directly converted to a Direct Current (DC) of electricity, a form of energy that can be used by modern society. [3] Solar cells have continued to be developed since their invention to increase efficiency to over 40% for some systems. [4] Mass produced solar panels have an efficiency of about 6-20%. Two major drawbacks of traditional solar power are the storage of electricity as well as the toxicity and scarcity of the chemical components that compose the most efficient solar cells. Most solar cells store energy in traditional batteries, which contain toxic rare elements and degrade over time. This inefficiency of storing energy severely limits the usefulness of current solar power technology. [5] The rare metals used in some of the most efficient systems prohibit mass production and implementation of solar energy as a viable replacement for fossil fuels.
In a 2008 Science paper, Daniel Nocera and Matthew Kanan (then at MIT) demonstrated a solar cell that draws its inspiration directly from nature. [6] They developed a system of catalysts that function to split water into O2 and H2 (the artificial equivalent of NADPH), analogous to the foundation of photosynthesis. [6] This system is remarkable in that it functions under very mild conditions, neutral pH at 1 ATM and uses abundant earth elements. At the anode, O2 is generated and at the Cathode, H2 is generated. These byproducts can be stored and recombined to produce electricity in an efficient process common in fuel cells. Batteries degrade over time and in this system, the catalyst responsible for O2 generation precipitates from solution. This phenomenon results in the eventual loss of catalytic current and fuel generation. [6] However, Nocera has been able to implement a self-healing mechanism into the system where catalyst levels essentially remain constant over a period of days. [7] This advancement has greatly increased the lifetime of the system.
At the March 27, 2011 meeting of the American Chemical Society, Nocera unveiled a prototype artificial leaf that he reports to function at 10 times the photosynthetic efficiency of natural leafs, with no drop in efficiency after 45 hours of operation. Although this technology is in its infancy (the detailed scientific paper is not yet available) the mild conditions under which it is reported to operate, inspiration from natural photosynthesis, self-healing characteristics and fuel-cell like efficiency make this system very attractive for mass implementation. It can be foreseen that such a system could eventually be used to turn homes into their own power plants.
The first steps of enacting artificial photosynthetic technology on a super-laboratory scale have been taken, albeit very recently. In his announcement at the March, 27 2011 ACS meeting Nocera opined the deployment of this system to third world countries that don't consume huge amounts of electricity as the modern American home does. This strategy reflects the environment in which photosynthesis evolved (plants being energetically efficient) and could empower the impoverished. Although this system is unproven to the general scientific community, if it behaves as described, it could give rise to a new energy economy that begins to distance itself from the fossil fuels so demonized by many politicians and scientists alike. The results are yet to be demonstrated on any scale that would provide grounds to predict future impact. However, the initial scientific findings appear very promising.
© Timothy Ray Blake. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
[1] J. M. Berg, J. L. Tymoczko and L. Stryer, Biochemistry (W. H. Freeman, 2007), Ch. 19.
[2] R. E. Blankenship et al., "Comparing Photosynthetic and Photovoltaic Efficiencies and Recognizing the Potential for Improvement," Science 332, 805 (2011).
[3] K. A. Tsokos, Physics for the IB Diploma, 5th ed." (Cambridge U. Press, Cambridge, 2006).
[4] M. A. Green et al., "Solar Cell Efficiency Tables (Version 35)," Prog. Photovolt. Res. Appl. 18, 144 (2010).
[5] M. L. Wald, "New Ways to Store Solar Energy for Nighttime and Cloudy Days," New York Times, 15 Apr 08.
[6] M. W. Kanan and D. G. Nocera, "In Situ Formation of an Oxygen-Evolving Catalyst in Neutral Water Containing Phosphate and Co2+," Science 321, 1072 (2008).
[7] D. A. Lutterman, Y. Surendranth and D. G. Nocera, A Self-Healing Oxygen-Evolving Catalyst". J. Am. Chem. Soc. 131, 3838 (2009).