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Recharging with added vitamins
Materials Today Volume 19, Number 9 November 2016
NEWS
In two new studies, Stanford scientist Yi Cui and colleagues have developed technologies that could overcome a couple of the major energy challenges of the 21st century, that of cleaner fuel for transportation using hydrogen fuel from water as well as improved large-scale energy storage from zinc batteries. Hydrogen fuel has been intensively researched as a more environmentally friendly alternative to gasoline, and hydrogen-powered cars – which are emission-free – are already being produced for the mass market, although sales remain limited. However, producing hydrogen fuel for the cars is not emission-free, as it tends to involve natural gas in a process that releases carbon dioxide. To resolve this, the team looked at photovoltaic water splitting, which uses a solar-powered electrode immersed in water. On sunlight hitting the electrode, it generates an electric current that separate the water into its constituent parts of hydrogen and oxygen. To produce clean hydrogen from water is not easy, as silicon solar electrodes corrode when exposed to oxygen as a byproduct of the water splitting. In this study, published in the journal Science
Advances [Qiu, et al., Sci. Adv. (2016), doi:10.1126/sciadv.1501764], the researchers used bismuth vanadate as a coating, as it is cheap, absorbs sunlight and produces modest amounts of electricity. However, the material is not an effective conductor of electricity – to carry a current, solar cells made from bismuth vanadate have to be sliced so thinly that it is nearly transparent. This allows visible light that can be used to generate electricity to pass through the cell. To capture the sunlight, microscopic arrays containing thousands of silicon nanocones were developed. The arrays were deposited on a film of bismuth vanadate, and both layers were then positioned on a solar cell made of the photovoltaic material perovskite. On being submerged, the device immediately started to split water at a useful solar-to-hydrogen conversion efficiency. As Cui revealed, ‘‘The tandem solar cell continued generating hydrogen for more than 10 hours, an indication of good stability’’. In a second paper, which appeared in Nature Communications [Higashi, et al., Nat. Commun. (2016), doi:10.1038/ ncomms11801], Yi Cui, Shougo Higashi
from Toyota Central R&D Labs and colleagues showed a new battery design with electrodes made of zinc and nickel. Although there is already a range of zinc metal batteries available, not many are rechargeable due to the dendrites that form on the zinc electrode during charging. The dendrites can keep growing until they reach the nickel electrode, resulting in the battery short-circuiting. They overcame this challenge by redesigning the battery so that, instead of the zinc and nickel electrodes facing each other as in a traditional battery, they were separated by a plastic insulator and a carbon insulator was wrapped around the edges of the zinc electrode. Zinc ions are reduced and deposited on the exposed back surface of the zinc electrode during charging so that if zinc dendrites do form, they will grow away from the nickel electrode and not short the battery. They showed the stability of the battery by successfully charging and discharging it over 800 times without shorting. The design is also straightforward and could be applied to a wide range of metal batteries. Laurie Donaldson
flexible, and versatile power sources to accommodate future device energy requirements. Unfortunately, current batteries use transition metal-based cathodes that need energy-intensive processing and extraction methods, all of which is less than environmentally benign. Moreover, about one third of the cost of such batteries, whether powering a smart phone or an electric smart car, is due to the metal oxide or phosphate cathode material. The researchers explain that a lithium-ion battery built with their bio-derived polymer has a capacity of 125 milliamp hours per gram and an operational voltage of about 2.5 volts. Charge transport within the battery can be improved by forming hierarchical structures of the polymer with carbon black. The team also adds that preliminary experiments have offered new insights into the mechanisms that underlie electrode degradation and should help inform the design of polymer electrodes in general.
Redox active organic molecules have a high theoretical capacity, are low density, but strong materials, with tunable electronic properties. The polymers derived from a redox active molecule seem to be even more suited to a role in lithium-ion batteries than the small molecule systems. As such, ‘‘Our proposed new concept of using biologically derived polymers to store energy is an attractive strategy to address these issues,’’ Seferos told Materials Today. ‘‘We have identified a very similar polymer that is stable at high capacity and is able to avoid degradation,’’ Seferos adds. ‘‘We plan to make flexible batteries with this polymer. We hope to also extend this methodology to other redox-active bio-molecules in order to build a library of bio-derived electrode materials.’’ He adds that the next step will be to extend this work towards flexible devices that can conform to the form factors required in many different applications’’. David Bradley
Recharging with added vitamins The first bio-derived pendant polymer cathode for lithium-ion batteries has been developed by researchers in Canada (Schon, et al., Adv. Funct. Mater. (2016), doi:10. 1002/adfm.201602114). The team has used a flavin molecule derived from vitamin B2, also known as riboflavin, as the redox-active energy storage unit and suggests that it could represent a sustainable way to make high-performance rechargeable batteries for a wide range of applications. The team’s semi-synthetic route to the requisite pendant polymer in which two flavin units are coupled to a poly(norbornene) backbone allows for a high capacity and high voltage system to be built with a minimal number of synthetic steps. According to Tyler Schon, Andrew Tilley, Colin Bridges, Mark Miltenburg, and Dwight Seferos of the University of Toronto, the growth in portable electronic gadgets and the emergence of the Internet of Things will increasingly require inexpensive,
487
NEWS
Improving hydrogen fuel and rechargeable batteries
NEWS
In two new studies, Stanford scientist Yi Cui and colleagues have developed technologies that could overcome a couple of the major energy challenges of the 21st century, that of cleaner fuel for transportation using hydrogen fuel from water as well as improved large-scale energy storage from zinc batteries. Hydrogen fuel has been intensively researched as a more environmentally friendly alternative to gasoline, and hydrogen-powered cars – which are emission-free – are already being produced for the mass market, although sales remain limited. However, producing hydrogen fuel for the cars is not emission-free, as it tends to involve natural gas in a process that releases carbon dioxide. To resolve this, the team looked at photovoltaic water splitting, which uses a solar-powered electrode immersed in water. On sunlight hitting the electrode, it generates an electric current that separate the water into its constituent parts of hydrogen and oxygen. To produce clean hydrogen from water is not easy, as silicon solar electrodes corrode when exposed to oxygen as a byproduct of the water splitting. In this study, published in the journal Science
Advances [Qiu, et al., Sci. Adv. (2016), doi:10.1126/sciadv.1501764], the researchers used bismuth vanadate as a coating, as it is cheap, absorbs sunlight and produces modest amounts of electricity. However, the material is not an effective conductor of electricity – to carry a current, solar cells made from bismuth vanadate have to be sliced so thinly that it is nearly transparent. This allows visible light that can be used to generate electricity to pass through the cell. To capture the sunlight, microscopic arrays containing thousands of silicon nanocones were developed. The arrays were deposited on a film of bismuth vanadate, and both layers were then positioned on a solar cell made of the photovoltaic material perovskite. On being submerged, the device immediately started to split water at a useful solar-to-hydrogen conversion efficiency. As Cui revealed, ‘‘The tandem solar cell continued generating hydrogen for more than 10 hours, an indication of good stability’’. In a second paper, which appeared in Nature Communications [Higashi, et al., Nat. Commun. (2016), doi:10.1038/ ncomms11801], Yi Cui, Shougo Higashi
from Toyota Central R&D Labs and colleagues showed a new battery design with electrodes made of zinc and nickel. Although there is already a range of zinc metal batteries available, not many are rechargeable due to the dendrites that form on the zinc electrode during charging. The dendrites can keep growing until they reach the nickel electrode, resulting in the battery short-circuiting. They overcame this challenge by redesigning the battery so that, instead of the zinc and nickel electrodes facing each other as in a traditional battery, they were separated by a plastic insulator and a carbon insulator was wrapped around the edges of the zinc electrode. Zinc ions are reduced and deposited on the exposed back surface of the zinc electrode during charging so that if zinc dendrites do form, they will grow away from the nickel electrode and not short the battery. They showed the stability of the battery by successfully charging and discharging it over 800 times without shorting. The design is also straightforward and could be applied to a wide range of metal batteries. Laurie Donaldson
flexible, and versatile power sources to accommodate future device energy requirements. Unfortunately, current batteries use transition metal-based cathodes that need energy-intensive processing and extraction methods, all of which is less than environmentally benign. Moreover, about one third of the cost of such batteries, whether powering a smart phone or an electric smart car, is due to the metal oxide or phosphate cathode material. The researchers explain that a lithium-ion battery built with their bio-derived polymer has a capacity of 125 milliamp hours per gram and an operational voltage of about 2.5 volts. Charge transport within the battery can be improved by forming hierarchical structures of the polymer with carbon black. The team also adds that preliminary experiments have offered new insights into the mechanisms that underlie electrode degradation and should help inform the design of polymer electrodes in general.
Redox active organic molecules have a high theoretical capacity, are low density, but strong materials, with tunable electronic properties. The polymers derived from a redox active molecule seem to be even more suited to a role in lithium-ion batteries than the small molecule systems. As such, ‘‘Our proposed new concept of using biologically derived polymers to store energy is an attractive strategy to address these issues,’’ Seferos told Materials Today. ‘‘We have identified a very similar polymer that is stable at high capacity and is able to avoid degradation,’’ Seferos adds. ‘‘We plan to make flexible batteries with this polymer. We hope to also extend this methodology to other redox-active bio-molecules in order to build a library of bio-derived electrode materials.’’ He adds that the next step will be to extend this work towards flexible devices that can conform to the form factors required in many different applications’’. David Bradley
Recharging with added vitamins The first bio-derived pendant polymer cathode for lithium-ion batteries has been developed by researchers in Canada (Schon, et al., Adv. Funct. Mater. (2016), doi:10. 1002/adfm.201602114). The team has used a flavin molecule derived from vitamin B2, also known as riboflavin, as the redox-active energy storage unit and suggests that it could represent a sustainable way to make high-performance rechargeable batteries for a wide range of applications. The team’s semi-synthetic route to the requisite pendant polymer in which two flavin units are coupled to a poly(norbornene) backbone allows for a high capacity and high voltage system to be built with a minimal number of synthetic steps. According to Tyler Schon, Andrew Tilley, Colin Bridges, Mark Miltenburg, and Dwight Seferos of the University of Toronto, the growth in portable electronic gadgets and the emergence of the Internet of Things will increasingly require inexpensive,
487
NEWS
Improving hydrogen fuel and rechargeable batteries