Photosynthetic water oxidation versus photovoltaic water electrolysis

13 May 2011

Professor Tom Moore, a leader of Subtask 1 (Total systems analysis, assembly and testing) in the Center, is a coauthor of the review paper “Comparing Photosynthetic and Photovoltaic Efficiencies and Recognizing the Potential for Improvement” published in May 13 issue of the Science magazine. In this paper, the multidisciplinary team of authors presented a consensus on comparison of solar energy conversion of photosynthesis and photovoltaics that outlines a road map for engineering of more efficient photosynthetic systems.

Search for efficient and cheap substitute for the oil is a real challenge for the 21st century. The world is dangerously dependent on the fossil fuels. Apparently, harnessing energy of sun is a solution to humankind energy problems because solar energy is inexhaustible. Annually, about 120,000 terawatts of solar energy hits the surface of the Earth. This is 8,000 times of the global energy production that would satisfy annual human requirements (~ 15 terawatts). This equals to the amount of solar energy that hits the surface of the Earth within one hour!

There is a need for a device that is capable to produce fuel from the energy of the sun based on either natural photosynthetic biomass/biofuel production or photovoltaic devices utilizing solar energy for electrical current production. Several years ago the Department of Energy has initiated discussions among scientists about the search for cost-effective technologies for using sunlight. These discussions have identified two major trends in solar-driven research, photosynthetic production of biomass/Biofuels and photovoltaic solar energy fuel production.

The paper in Science highlights the results of this discussion and defines the prospects of research in the fields of natural and artificial photosynthesis for decades ahead. Fifteen authors of the paper are renowned researchers in the field of natural photosynthesis and photovoltaics (see links in the end). These are scientists from different areas of research: biologists, physicists, engineers, researchers on algae, agriculture. Tom Moore says that the important conclusion of the article is that the underlying principles that govern photosynthesis and photovoltaics are the same and that photosynthesis can be reengineered to do as well as photovoltaic devices.

Basis for comparison of photosynthesis and photovoltaics

Photosynthesis and photovoltaics systems both use solar energy; however they operate in different ways and produce different products. Therefore, comparison of efficiencies in these systems is not trivial task. Photosynthesis is a natural “nanotechnology” selected during billions years of evolution of macromolecular machines that catalyze solar driven water splitting to feed carbon fixation apparatus for storage of energy in chemical bonds of sugars or other compounds. In natural photosynthesis bacteria, algae and plants use solar energy to produce biomass. However, in evolution selective pressure is directed towards reproductive success of the organisms, not for high biomass production. In photovoltaics systems the solar energy is used to produce electrical current. In semiconducting materials such as silicon, cadmium telluride, or copper indium gallium selenide/sulfide the solar radiation is converted into direct current electricity due to the photovoltaic effect, which is governed by light-dependent transfer of electrons from the valence to conduction band of the semiconductor.In photovoltaics system energy is stored as a potential driving the current between the electrodes. The authors have found a basis for a comparison proposing a process in which PV stores energy in chemical bonds as in photosynthesis. This is a system of photovoltaic driven electrolysis of water (Figure 1) as compared to a photosynthetic water oxidation (Figure 2).

In the photovoltaic-driven electrolysis of water decomposition of water for hydrogen and oxygen occurs due to imposing the electric current between the electrodes with the electrical energy supplied by the photovoltaic cell. The free energy (ΔG°) needed to split H2O into H2 and O2 is equal to 1.23 eV. Efficiency of such a system primarily depends on the photovoltaic cell. Standard silicon based PV efficiency is about 18% calculated as the percentage of the ratio of power converted from absorbed light to electrical energy to the stored energy. The following factors should be taken into account when calculating the efficiency of PV-driven water electrolyzer:

  • Due to changing solar zenith angle, the annual averaged PV efficiency is about 95% of the maximum.
  • Efficiency of modern commercial electrolyzers is ~ 80%.
  • Mismatch between the photovoltage generated by the PV array and the voltage required for electrolysis.

Based on that an annual averaged efficiency for solar water splitting by PV-driven electrolysis is about 10 -11%. The authors have stated that the ongoing research is providing opportunities to construct PV devices with considerably higher efficiencies. Replacing single-junction devices with tandem cells optimized for water oxidation and hydrogen production and tuning the devices for better use of the regions of the solar spectrum could increase the efficiency of the photoelectrolysis cells to as high as ~ 24%.

Photosynthetic water oxidation

Oxygen-evolving photosynthetic organisms use two photosystems (Photosystem I and Photosystem II) that work in series. In reaction centers of both photosystems, the excitation of the chlorophyll molecule of the primary donor is followed by its photoxidation. Photosynthetic machinery operates at nearly 100%  quantum efficiency meaning that each quantum that hits the primary donor in the reaction center (Figure 2) induces a charge separation due to a transfer of one electron in a series of redox cofactors. In the overall reaction of photosynthesis, assimilation of one molecule of carbon dioxide into carbohydrate requires two molecules of NADPH based on a concerted action of two photosystems and absorption of eight photons resulting in photoxidation of water. The free energy change associated with photosynthesis CO2 + H2O → (CH2O) + O2 (where (CH2O) is shorthand for carbohydrate) is equal to 1.24 eV that is essentially the same as the free energy change of the PV-driven water electrolyzer (Figure 1).

The following factors should be taken into account when calculating the efficiency of the photosynthesis:

  • Photosynthetic organisms use only half of the incident solar energy since the working spectral region for photosynthesis is 400 – 700 nm.
  • Due to reflection and transmission only 90% of this energy reaches the pigments.
  • Higher energy excited states of the chlorophyll tend to rapidly relax to lower energies due to ultrafast internal conversion resulting in a 6.6% energy loss in a heat.
  • Energy losses due to carbohydrate biosynthesis and other metabolic processes

As a result of multiple losses, the remarkable efficiency of the photosynthesis on the molecular level downgrades to 1-4% on the level of the photosynthetic organism.

Ways to increase efficiency of photosynthesis:

Here are some directions of the research that the authors propose to improve the efficiency of photosynthesis:

  1. Tuning photosystem light absorption by genetic modification of the pigments able to absorb more of solar light.
  2. Engineering plants and algae for fuel production
  3. Removing biochemical inefficiencies by acclimation of plants to high CO2 levels, suppressing photorespiration, engineering organisms with alternative carbon-fixation pathways
  4. Reducing the size of the antenna system to avoid the excess energy damage, manipulating the chlorophyll content in the crop plants for biofuel production
  5. Synthetic biology approach towards engineering photosynthetic organisms with two photosystems (tandem architecture) that use non-overlapping spectral regions but efficient energy transfer from antennas towards the reaction centers.
  6. Suppressing the production of reduced carbon storage products and diverting the electrons towards reducing H+ to hydrogen.  

The paper shows that both systems have a threshold energy that defines attainable light absorption, conversion efficiency, and energy storage capabilities and thus it is a correct “apples-to-apples” comparison of solar energy conversion of photosynthesis and photovoltaics that outlines a road map for engineering of more efficient photosynthetic systems.  Furthermore, artificial photosynthesis is the essential platform to design, test and put into practice systems that eliminate such problems and systems that guide synthetic biology.

Related links:

  • Reference: Blankenship, R. E., Tiede, D. M., Barber, J., Brudvig, G. W., Fleming, G., Ghirardi, M., Gunner, M. R., Junge, W., Kramer, D. M., Melis, A., Moore, T. A., Moser, C. C., Nocera, D. G., Nozik, A. J., Ort, D. R., Parson, W. W., Prince, R. C., and Sayre, R. T. (2011) Comparing Photosynthetic and Photovoltaic Efficiencies and Recognizing the Potential for Improvement, Science, 332, 805-809
  • Link to the paper online
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