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  (Source: Meyer Group)
Future dye-based electrochemical cells may reach efficiencies of up to 15 percent, team says

The top 1 percent gets it all.  No we're not talking about America's income gap, we're talking about a tantalizing, but still rather inefficient method of solar power grid storage -- hydrogen.
 
I. Solar Storage -- A Big Problem
 
One of the toughest problems of the alternative energy industry is the question of variability.  While utilities can control when to burn fossil fuels, to directly satisfy demand, we can't control when the sun is shining or the wind is blowing.  A possible solution is to store the energy, either chemically or mechanically.
 
Much of the early work has focused on grid storage using spent electric vehicle batteries.  But given the low levels of EV sales and distribution difficulties, this method can't fully satisfy the power demands of millions of Americans at present.  Compounding that method’s flaws is the fact that these aren't very good storage devices anymore in the first place -- hence why they've been declared "spent."

Solar storm
Solar is a tantalizing, but inconsistent power source. [Image Source: NASA]

Many have suggested that we break water into diatomic oxygen molecules and hydrogen; plants do this as a precursor to sugar production during photosynthesis.  One way to do this is to harvest the solar energy as electricity and then perform on-demand hydrolysis.  Energy losses on the PV end are already bad and since most solar cells degrade or short in water, they become worse as you have to transfer electricity out of the solar cell and into an electrolysis device.  In short, the compounding losses mean it takes a lot of electricity to produce a little hydrogen.
 
An alternative method is to use water-soluble catalysts  -- chemicals that speed up chemical reactions by reducing the energy barrier that a series of reactants must overcome to transform into a series of products (in this case pure hydrogen and oxygen fuel).  To do this you would generally use a chromophore-catalyst -- a catalyst activated by light -- to drive the reaction.
DSSC
Catalysts can rip electrons away from water to produce storable hydrogen fuel.
[Image Source: Meyer Group]

Chromophore catalysts operate by ripping electrons away from a compound such as water, which creates a flow of current that can be used to drive electrochemical reactions (e.g. water splitting).  As this process is all localized, theoretically you could minimize losses much more than in an approach relying on standard cells.  Better still, its heavy use of abundant materials like carbon and nitrogen, could  -- in the long run -- reduce costs.

Artificial photosynthesis
Nanoparticle-based systems can create artificial photosynthesis.
[Image Source: The Meyer Group]

Bacteria, protists, and plants have been doing this for billions of years using the chromophore-catalyst chlorophyll to produce electronics, which in turn split water, which in turn is used to produced sugar, the "battery molecule" of the organic world.  But chlorophyll isn't necessarily the easiest molecule to produce; so human researchers have been examining novel chromophores (light absorbing compounds, aka pigments) to alternatively use.

artifical solar
[Image Source: The Meyer Group]

Aside from tuning the pigment, there's a host of other issues including mounting of the pigment to a nanoparticle.  In plants this is done by nanostructures called "thylakoids", which are stacked inside chloroplasts.  Thylakoids vary in diameter, but tend to be around 15 nm thin in higher plants.  A crucial question is how plants "glue" their chlorophyll molecules in place inside the thylakoids; in manmade chromophore-loaded nanoparticles, the chromophores tend to break away, decreasing efficiency and slowly killing the conversion cell.
 
Another key challenge is how to shuttle away electrons to prevent a "traffic jam", electrically.  So long as the charge is being transferred elsewhere, the chromophore-catalyst is free to keep ripping electrons away from water.  But once the chromophore becomes charged, it has to wait for that charge to dissipate before continuing the process.  Again, understanding of how the charge transfer occurs in plants is a topic of ongoing research.
 
But in terms of synthetic analogs to the plant's stacked thylakoid energy storage system, researchers are making significant advances to overcome these problems, using common nanomaterials.
 
II. The First Fully Manmade Photosynthesis Device
 
A team of researchers led by the University of North Carolina - Chapel Hill campus (UNC-Chapel Hill) Professor Tom Meyer and North Carolina State University (NCSU) Professor Gregory Parsons, have used a complex nanostructured material to capture 1 percent of the energy in the sunlight received.  That's far worse than the average last year for conventional panels -- 15 to 16 percent (according to estimates by Forbes).  With some premium cells hitting efficiencies of 21+ percent, 1 percent just isn't cutting it.

Professor Thomas Meyer
UNC Professor Thomas Meyer [Image Source: Auburn.edu]

On the other hand the result is far better than past chromophore catalyst results, and the faculty members are convinced they can achieve even better.

Professor Meyer comments:

When you talk about powering a planet with energy stored in batteries, it’s just not practical.  It turns out that the most energy dense way to store energy is in the chemical bonds of molecules. And that’s what we did – we found an answer through chemistry.

Splitting water is extremely difficult to do.  You need to take four electrons away from two water molecules, transfer them somewhere else, and make hydrogen, and, once you have done that, keep the hydrogen and oxygen separated. How to design molecules capable of doing that is a really big challenge that we’ve begun to overcome.

So called 'solar fuels' like hydrogen offer a solution to how to store energy for nighttime use by taking a cue from natural photosynthesis.  Our new findings may provide a last major piece of a puzzle for a new way to store the sun’s energy – it could be a tipping point for a solar energy future.

To attain the current results Professor Meyers began by hunting for manmade chromophores that could absorb visible light.
 
He found a perfect candidate in a molecule with a number of aromatic nitrogen-carbon rings (bipyridine) mounted to a ruthenium metallic center.  The organic compounds act as the "antenna", while the ruthenium acts as the "signal processor", converting the solar energy into electron flow.  Professor Meyers named the compound "the blue dimer".  Published in the peer-reviewed ACS journal Inorganic Chemistry in 2012, this was a crucial work as it was the first time man had found a synthetic chromophore-catalyst for water splitting.
 
Prior to that work some researchers had managed to achieve similar results -- but not from manmade compounds (they tricked viruses into acting as organic equivalents to the manmade blue dimer).  Other solar cells were merely made to mimic the look of the leaf, for efficiency reasons, but chemically were still traditional thin-film photovoltaic cells.


blue dimerruthenium
[Image Source: The Meyer Group]

Then in 2012 through late last year, Professor Meyers put the new catalysts and its derivatives to use, mounting them to nanoparticles of indium tin oxide (nanoITO).  The group's latest work, for example, uses (PO3H2)2bpy)2Ru(4-Mebpy-4-bimpy)Rub(tpy)(OH2)]4+ catalyst molecules mounted to the nanoparticle.
 
The result was the first "Dye Sensitized Photoelectrosynthesis Cell (DSPEC)".  Unlike the similar "Dye Sensitized Solar Cell" (DSSC), which produces current and potential (electricity), the DSPEC directly produces stored energy in the form of chemical fuel (hydrogen).
Ruthenium chromophoreRuthenium chromophore
[Image Source: The Meyer Group]

But as huge of an advance as this was, the previously mentioned problems rendered it essentially a theoretical curiosity; the cells produced little hydrogen.  The problem was two-fold; the blue dimer loaded nanoparticles needed a better glue to keep the pigments from floating away and they needed a way to enhance transport of electrons out of the pigment and into the nanoparticle.

III. From Curiosity to Contender

Professor Parsons proposed a solution to both issues -- "spraying" the nanoparticle with a thin film of metal oxide.  Films of so-called "transport conducting oxides" (TCOs) TiO2, ZrO2, and SnO2 were evaluated.  His lab found that the best results in terms of electron transport and pigment adhesion came from the titanium oxide films, which also happened to be a relatively cheap ingredient to use (TiO2 nanoparticles are widely used in sunscreens).
artificial water splitting
Then fresh layer of metal-oxide increases the electron transfer from the chromophore "antenna". [Image Source: Meyer Group]

With the gathered electrons, further electrolysis is possible by funneling the electrons to a platinum electrode.  This dual-approach strategy
 
The approach improved the efficiency of the cell to about 1 percent of the incidents solar energy striking the cell, or about 4.4 percent (at maximum) of the absorbed solar energy.  Crucially, it also greatly enhanced the cell longevity.

scaffold
An illustration of the new coated nanoparticle [Image Source: Parsons Group]

Professor Meyer and his colleagues (including Professor Parsons) are currently working in a variety of directions to try to refine the organic electrochemical reaction.  They're examining better substrates (they're currently using poly(methyl methacrylate) (PMMA) and SiO2 sol-gels), on which to adhere the pigment-loaded nanoparticles.  They're examining more complex chromophore compounds, such as protein-like polymers with pigment side chains.  They're also looking at more novel coatings.

ruthenium peptide
Mounting the chromophores to peptide backbones could improve efficiencies.
[Image Source: Meyer Group]

The collaborators feel confident they can boost their efficiencies to around 15 percent.
 
That's a pretty ambitious goal, but if they can achieve it and find a way to produce their catalyst compounds inexpensively (perhaps with genetically engineered helper-microbes) it would be a game changer for solar.  No longer would solar be plagued by variability or use of expensive rare earth metals storage devices; solar would be on more of an even footing with other non-fossil fuel energy solutions such as algal biofuels and clean nuclear power.

artificial solar
An artist's rendering of the catalyst mounted to the coated nanoparticle [Image Source: Meyer Group]

The pair's latest work can be found in a pair of papers -- one published in the Oct. 2013 edition of the prestigious peer-reviewed journal PNAS, and another published in the Nov. 2013 edition of the journal.  The work was funded, in part, by grants from the U.S. Department of Energy (DOE).

Sources: UNC, NSCU





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