A good day to dye: how to turn sunlight into fuel
Plants are brilliant at converting sunlight to energy — humans, not so much. Lasers are helping us get just a little better at it.
Through photosynthesis, plants are brilliant at converting the Sun’s energy into fuel — humans, not so much. A collaboration between Newcastle University and Sorbonne Université aims to rectify this by creating fuel through artificial photosynthesis. To do this they employed the ULTRA laser at the Central Laser Facility (CLF) to investigate a promising type of solar cell that uses a combination of dyes and nanoparticles to turn water into hydrogen fuel.
Copying nature to harness the Sun
One of the biggest problems facing our civilisation today is the production and storage of energy — everything we do, use, and need, relies on having a readily-available supply of energy — and we need to find that energy without further adding to the warming of our planet.
The Sun is a giant, hydrogen-burning, power plant that has the potential to supply us with an unlimited supply of energy — if only we can find a way to exploit it to its fullest. Although solar panels are becoming increasingly cheap to produce and efficient, their effectiveness is limited, particularly at night and during winter, compared to nature’s own solution: photosynthesis.
Plants have been perfecting the process of capturing photons and using their energy to power the conversion of water into the fuel they need to thrive over billions of years. Scientists have been attempting to replicate the process and create the machinery for artificial photosynthesis for over a century. The most common method for harnessing the Sun’s photonic bounty is through the use of silicon solar cells, but more recently, scientists have been turning to something called dye-sensitised solar cells, or dye cells.
A dye hard solution
Traditional solar cells use crystalline silicon as a semiconductor to absorb light, convert its energy into electrons and to transport those electrons. To achieve this requires the use of very pure materials and, if you want to create a global infrastructure, you need those pure materials in huge quantities. Another downside with silicon solar cells is that they require direct sunlight, so they work best only when the Sun is striking them at a direct angle, which (even when we are blessed with a cloudless sky) only happens for a few hours each day.
Dye cells work differently. Instead of asking one semiconducting material to do all of the heavy lifting, dye cells separate the process of light collection and charge transport. They use a dye to collect the photons and a separate semiconductor to handle the electron transport part of the operation.
Attaching a coloured dye molecule to a semiconducting nanoparticle allows light to be collected from many angles at the same time, which makes the best use of available light even when it is not direct. It also means that, unlike silicon wafers, they can be printed onto a range of materials, in different colours, making them much more versatile and attractive than conventional solar panels. Also unlike other proposed alternatives to silicon solar cells, dye cells do not require the use of toxic or rare metals, which means they could be deployed on a global scale without being limited by the availability of raw materials.
POM dyedly POM POM
Generating electricity is only one part of the equation though — what if you could fully realise the potential of artificial photosynthesis and, like a plant, use the Sun’s rays to create fuel? To do this, you need to able use the energy collected to split water (H2O) into oxygen and hydrogen, which can be used in hydrogen fuel cells to generate power. For this to happen, you need a nanoparticle that is capable of accumulating electric charge, and that can also act as a catalyst to drive the chemical reaction that separates hydrogen fuel from water. The collaboration between Newcastle University and Sorbonne Université investigated one potential candidate for this: Polyoxometalates (POMs).
POMs are metal oxide catalysts abundantly available in the natural environment that are emerging as alternative to rare metals as catalysts in artificial photosynthetic systems. Most POMs, however, do not absorb sunlight well, so POM nanoparticles have to be paired with dye molecules to act as light-gathering antennae.
The collaboration, led by Dr. Elizabeth Gibson from Newcastle University, used the ULTRA laser at CLF to investigate how POMs perform when paired with dye light-absorbing antennae. ULTRA allowed the team to monitor and record the POMs’ light-absorption and charge-separation characteristics as the reaction unfolded. The experiments revealed that the POM/dye-antenna hybrid was able to rapidly absorb light, convert it to a charge and pass the charge (as an electron) to the POM, which was then able to hold the charge for a long period.
To complete the system to be able to generate hydrogen in a fuel-forming solar cell, requires the materials to be printed onto an electrode, so that an electrical current can flow between the electrode making oxygen and the electrode making hydrogen. This enables the two gasses to be separated so there is no explosive mixture.
As the next phase of their research, the team plan on anchoring the POM hybrids to the surface of electrodes and then using ULTRA to monitor the process as it happens in a working device.
