By Jake Wilkinson
Inspired by the natural process of photosynthesis in plants, which converts energy from the sun into sugars, new solar cells have been developed that allow artificial photosynthesis. The cells convert carbon dioxide emissions into solar fuel and even function effectively if they are placed underwater.
As part of a revolutionary technology known as artificial photosynthesis, the engineers have shown how solar energy can be used to combine water and carbon dioxide (CO2) to produce non-toxic chemical products.
The newly developed solar cells can work under water. However, instead of feeding the power into the grid, the energy generated by these specialized solar cells can be used to stimulate chemical reactions. This behaviour can be exploited to convert captured greenhouse gases, such as CO2, into indutrial fuel sources such as natural gas.
Photosynthesis, a process which occurs in plants to convert solar energy into chemical energy by combining water and CO2 and into sugar. The sugar then acts as an essential energy source for plants and allows them to survive. Similarly, artificial photosynthesis will use the power generated by the newly developed solar cells to combine water and CO2 to create industrial fuels.
Artificial photosynthesis using conventional methods has two major disadvantages. Standard silicon solar cells corrode easily under water and also current solar cells that are resistant to corrosion cannot capture sufficient amounts of sunlight whilst under water to drive carbon capture reactions.
However, four years ago a research team at Stanford University, headed by Paul McIntyre, developed novel solar cells that do not corrode in water. This latest innovation has set a new record for solar energy output under water.
“The results reported in this paper are significant because they represent not only an advance in performance of silicon artificial photosynthesis cells, but also establish the design rules needed to achieve high performance for a wide array of different semiconductors, corrosion protection layers and catalysts.” Paul McIntyre – Stanford University
The specialized solar cells can become a part of a larger system to combat the climate change. The aim of the study is to direct greenhouse gases from the atmosphere into large, transparent chemical tanks. The solar cells within the chemical tanks can be used to drive chemical reactions, and therefore convert water and greenhouse gases into ‘solar fuels’.
The researchers dedicated many years of study to develop the novel solar cells.
Earlier in 2011, the issue of solar cell corrosion issue was resolved by McIntyre’s laboratory. The researchers used a thin and transparent titanium dioxide layer to coat the electrodes in the specialized solar cells. This ultra-thin coating can accommodate 25,000 layers which can be stacked up to the width of a single sheet of paper. However, despite this advancement, the corrosion-resistant solar cells failed to extract a sufficient amount of power from the sunlight after it had traveled through the water.
Eventually, they demonstrated that corrosion-proof solar cells can be made stronger by introducing a charged silicon layer between the silicon cell and the titanium oxide layers. The resulting device includes three layers which each have different electronic properties. The layer of active silicon, which uses photons from sunlight to activate electrons, is placed at the bottom. A silicon dioxide ‘booster’ layer is placed over the active silicon layer, which improves the voltage considerably. This booster is then covered with a transparent titanium dioxide layer which, whilst acting as a conductor, also closes the system and prevents corrosion.
Iridium is used to coat these layers. This element acts as a catalyst and allowing water and CO2 molecules to combine to create solar fuel. Electricity is conducted from velow the iridum layer which is used to break the chemical bonds in the molecules, a process known as electrolysis, which then form new bonds to give methane (natural gas) and pure oxygen.
The unique system developed for artificial photosynthesis operates just like a battery but in the opposite direction.
The research has been reported in the journal ‘Nature Materials’.