2012-11-27 | Editor : elsiefu 8901 pageviews

Artificial Photosynthesis to Drive Solar Energy Technology Forward

Dr Dino R Ponnampalam

Solar energy technology is based on the principle that energy from the Sun can be absorbed and then converted to generate electricity, a form that is useful to society.  This usable form is one significant part of the transition away from what has been the status quo for some time: burning finite carbon-based fossil fuels. 

The earth absorbs roughly 89 petawatts (PW; 1 petawatt = 1 x 1015watt) of energy from the Sun per year, while the world consumes about 0.016 petawatts of energy per year.  As such, solar energy offers the prospect of a clean and reliable energy source with the prospect of meeting the energy demands of the world quite easily.  However, harnessing this powerful source has been uneven over the past six decades.  Thankfully, there is a potential solution to making a quantum leap forward and this is replicating a process found in Nature.

While solar energy technology has improved immensely over the decades, two issues prevent the technology from truly reaching its potential.  The two main sticking points concerning the progress of this technology are manufacturing costs and the electrical energy conversion rate.  By mimicking the biological mechanism common to certain organisms and integrating this process in solar cell technology, the possibility of an extremely efficient method of converting sunlight, photosynthesis, is within reach. 

Assimilating Photosynthesis in Developing Solar Energy Technology

For millions of years, chlorophyll-containing plants and organisms have efficiently harnessed sunlight in order to catalyze a reaction that produces an organic compound and oxygen.  This reaction is what sustains life on earth, a reaction of paramount importance, and yet this key conversion process shares a couple of similarities with solar energy technology. 

For example, both photosynthesis and solar energy technology focus on absorbing sunlight.  Over the course of millions of years, nature installed unique photosensitive receptors allowing for this astonishingly efficient conversion to take place.  These evolved photosensitive receptors, called chloroplasts, contain the pigment chlorophyll.  This pigment allows for the efficient absorption of sunlight.  In solar cell technology, sunlight is absorbed through the use of a semiconducting material with the most conventional semiconducting material being silicon. 

Another similarity concerns the conversion of sunlight (effectively viewed as a considerable beam of photons).  Both photosynthesis and solar energy technology utilize sunlight to manufacture something usable.  In the process of photosynthesis, sunlight helps to produce organic compounds (sugars) and this reaction also releases oxygen; for solar energy technology, the photons kick-start a reaction where the movement of electrons is captured and expressed as energy in the form of electricity.

While there are fundamental differences between the principles of photosynthesis and the operations involved in solar energy technology, the fact that photosynthesis is an amazingly efficient process cannot be overlooked.  Thus, investigating into how a process millions of years in the making can be between 75% and 95% efficient will help in pushing the efficiency rates of current solar energy technology higher. 

Laboratory-based solar cells can reach efficiency ratings of around 40%, but commercially deployed solar modules peak at around 20%.  Both are woefully poor efficiency ratings when compared to the process of photosynthesis.  However, this deficiency is in itself a cause for hope.  By understanding the process of photosynthesis and by attempting to replicate the process, further development of solar energy technology is guaranteed.    

Research Advancements in Artificial Photosynthesis

Using this natural process as a template, the world of academia has recently tried and succeeded in replicating vital mechanisms of photosynthesis.  This allows for the hope that more efficient solar cells, and better energy storage devices, could be produced.  For example, Professor Daniel G Nocera, the Henry Dreyfus Professor of Energy and Professor of Chemistry at the Massachusetts Institute of Technology (MIT) in the USA is one such researcher who has been working and making progress in this field. 

Professor Licheng Sun of the Department of Chemistry at the Royal Institute of Technology (KTH) in Stockholm (Sweden) has also been working in this field but from a different angle.  Professor Sun’s research concerns molecular catalyzers, and how to integrate molecular catalyzers into solar energy technology to improve the efficiency. 

 [i] Developing a Cheaper, Efficient Artificial Leaf 

Professor Nocera’s work has been covered before(Energytrend June 2011; Quantum Effects in Advancing the Solar Industry) and since 2011, a lot of progress has been made.  Professor Nocera’s research concerns creating an artificial leaf for energy storage.  By efficiently capturing sunlight, a chain reaction began in Nocera’s card-sized artificial leaf prototype that resulted in water molecules being split into hydrogen atoms and oxygen atoms.  By placing the card-sized prototype into a container of water, the hydrogen gas was released and this could be used for generating electricity.

While the prototype released last year was indeed a significant breakthrough, Professor Nocera aimed to improve upon his groundbreaking work.  The prototype version, which was ten times more efficient than a chlorophyll-containing green leaf at photosynthesis, mostly used inexpensive raw materials that are relatively abundant in nature.  However, the artificial leaf also used one considerably expensive raw material: platinum.  Furthermore, due to the use of platinum, the manufacturing process was not cost-effective.

Thankfully, improvements have been made that has led to these two issues - choice of the platinum catalyst and the manufacturing issue - becoming less important and less limiting.  Figure 1 shows a photograph of the artificial leaf prototype.

  

Figure 1: Photograph showing the prototype card-sized artificial leaf placed on a leaf.  Credit: Dominic Reuter

First, Professor Nocera replaced the platinum catalyst (which is used for hydrogen gas production) with a catalyst made up with a trinity of elements: nickel, zinc, and molybdenum.  This nickel-molybdenum-zinc catalyst is less expensive than the original catalyst choice of platinum, and equally important, the new catalyst is easier to deal with from a manufacturing point-of-view.  This new ease will result in lower manufacturing costs, which in turn will help to lower costs overall for this new version of the artificial leaf.  The parallels for the solar energy industry, especially for solar cell manufacturers, are striking.    

While Professor Nocera’s artificial leaf opens up the prospect of generating electricity from sunlight in remote and off-grid environments as and when required, the true potential lies in energy storage.  By developing mechanisms in which to store the hydrogen gas (for use when needed), this artificial leaf will truly revolutionize the solar industry and the energy industry and turn this device into a 24-hour standalone energy device.   

 [ii] Boosting the Efficiency of Solar Energy Technology Through A Catalyst

By oxidizing water into oxygen, especially at rapid speeds, researchers are hoping to replicate one of the key processes during photosynthesis and improve the concept of the artificial leaf.  And by reproducing this key process, the efficiency of solar cell technology will also dramatically rise.  As such, a lot of work has been invested in the area of molecular catalyzers. 

Professor Sun research group, at Stockholm’s Royal Institute of Technology (KTH) in Sweden, designed a molecular catalysts that is able to compete with a green leaf on converting molecules of water into molecules of oxygen.  In a conventional setting, a green leaf is able to initiate a turnover rate of conversion that ranges between 100 conversions per second to 400 conversions per second.  The molecular catalyzer developed by Professor Sun is capable of achieving 300 conversions per second. 

The new development, at 300 conversions per second and with direct benefits for progress in the area of the artificial leaf and for solar technology as well, is based on the use of a specifically-designed ruthenium compound as the catalyst.  This catalyst involves a ruthenium ion bound to a bipyridine-dicarboxylic structure as well as to an isoquinoline compound.  In a solution of water, this catalyst was found to significantly increase the rate of conversion of water molecules.  If a complementary hydrogen storage technology were to be fitted, then this molecular catalyzer could very well be what ushers in the hydrogen economy.     

By speeding up the rate of conversion, Professor Sun and collaborators have shown the benefit of such a catalyzed reaction on modern solar cell technology.  By increasing the rate of reaction, more electrons will be produced which will positively impact the efficiency of the solar cell.  Such improvements make feasible the thought of solar cells and solar modules having efficiency ratings that exceed the Shockley–Queisser limit, which is the theoretical efficiency limit of conventional first-generation solar cells.

However, there are some drawbacks to this exceptionally clever and well-thought-out catalyst.  Ruthenium is one of the rarest elements in the world and requires expensive manufacturing processes to extract and refine.  Therefore, the prohibitive costs associated with using ruthenium would prevent this metal from being used commercially on a large scale.  Accordingly, Professor Sun and co-workers have begun the process of finding suitable alternatives to ruthenium that offer exceptionally high conversion rates, allowing for developments to take place for both solar energy technology and for the artificial leaf to be cost-comparable to coal and other fossil fuels.

A Photosynthetic Mechanism for the Energy Future

Photosynthesis can be viewed as the very reason why Earth began to support life.  By efficiently using sunlight, a reaction took place that offered the ideal conditions for all organisms to thrive.  That reaction took place around two and a half billion years ago.  Now, in this modern energy-intensive world where fossil fuels are being consumed at an environmentally unacceptable price, it is to nature that we turn to in order to accelerate the transition to a world run on clean energy. 

By turning to nature, a solution to the problem of using renewable forms of energy on a widespread basis might just have been found that includes the prospect of developing a hydrogen economy.  For solar cell technology entities, nature has indicated how improvements could be made to the efficiency rating as well as reducing manufacturing costs.

There is an elegant, and ‘green’, symmetry to pushing the envelope on solar energy technology based on the principles and mechanisms of photosynthesis.  The notion of meeting the energy demands of the modern world by generating clean sources of energy, such as solar, has no better source of inspiration than nature.  After all, this is where clean energy generation began. 

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