New scientific discoveries are moving society toward the era of “personalized solar energy,” in which the focus of electricity production shifts from huge central generating stations to individuals in their own homes and communities. That’s the topic of a report by an international expert on solar energy published in the ACS’ Inorganic Chemistry, a bi-weekly journal. It describes a long-awaited, inexpensive method for solar energy storage that could help power homes and plug-in cars in the future while helping keep the environment clean.
Daniel Nocera explains that the global energy need will double by mid-century and triple by 2100 due to rising standards of living world population growth. Personalized solar energy – the capture and storage of solar energy at the individual or home level – could meet that demand in a sustainable way, especially in poorer areas of the world.
The report describes development of a practical, inexpensive storage system for achieving personalized solar energy. At its heart is an innovative catalyst that splits water molecules into oxygen and hydrogen that become fuel for producing electricity in a fuel cell. The new oxygen-evolving catalyst works like photosynthesis, the method plants use to make energy, producing clean energy from sunlight and water. “Because energy use scales with wealth, point-of-use solar energy will put individuals, in the smallest village in the nonlegacy world and in the largest city of the legacy world, on a more level playing field,” the report states.
Producing hydrogen from water using electrolysis driven by the energy from solar cells or wind power provides a pathway for these intermittent renewable power sources to meet our continuous energy demand1, as well as providing a potential fuel for transportation. But using the Sun’s energy to split water into its component gases directly via ‘artificial photosynthesis’ would be both more efficient and cost-effective than the two-step process. We already have catalysts for the hydrogen-production step, but oxygen catalysts have proved to be a more complex problem. Now, Kanan and Nocera2 have produced an oxygen-evolving catalyst from earth-abundant materials, using a very simple synthetic process. The catalyst also operates in water under ambient conditions. This is an important step towards enabling the commercial use of artificial photosynthesis, so how far along the path does it take us?
… The Hydrogen Economy — obtaining our fuel from sunlight and water — is eminently doable; we can do it now using electrolysers powered with renewable energy. But the direct splitting of water — artificial photosynthesis — is a much better way. We know that it will work, nature has been doing this for a couple of billion years, but we have slightly different priorities from nature. For any future energy system based on sunlight, we must be concerned with efficiency and cost, and that means better light-harvesting systems and low-cost catalysts; the work of Kanan and Nocera takes us closer to this goal.
All this talk about solar power and the weird thing is, plants have been doing it since the beginning of, well, plants. For years now, scientists have been trying to duplicate and improve upon the process of photosynthesis (even Jimmy Stewart tried it once.) And now a research group led by Osamu Ishitani has created a new catalyst that could turn CO2 into fuel efficiently, with only the power of the sun.
The new catalyst uses ruthenium and rhenium, two elements not found in your average leaf. But they do allow for the same first step (CO2 to CO) that plants use. In fact, it’s considerably more efficient and simpler than the way plants do things.
CO is far more reactive than CO2, and so it’s fairly simple to do a little bit of old-school organic chemistry to turn CO into burnable hydrocarbons like ethanol.
The trick was using the Ru catalyst to absorb the light, which it does very efficiently in the visual light spectrum, but then using the Re catalyst to actually take the electron produced and knock one of the oxygens off of the CO2. The Re complex has a quantum efficiency of 0.62, which means it actually uses 62% of the electrons it gets from the Ru catalyst to reduce the CO2. This number is extremely high.
The Ru-Re combination also excels at selecting CO2 over H2O. One big problem with artificial photosynthesis in the past is that these photocatalysts would often reduce water to OH just as easily as they reduced CO2 to CO. That waste of energy forced people to look into ways of scrubbing the CO2 of water (not a simple task.) But with this new catalyst, water isn’t a problem.
Now, the only problem is to make sure the catalyst is stable and doesn’t degrade over time. If they can do that, then there won’t be much between this research and a CO2 to fuel manufacturing plant.