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Graphene Origami

09 November 2015 - Making It Move XVII

graphene origami.PNGA new contestant in our continuous coverage of self-propelling, swimming or walking chemical entities, be it at the molecular level or the macroscopic. At long as it MOVES. See previous episode here. This week Jiuke Mu et al. treat us to graphene origami! (DOI).

Through a masking technique a strip of graphene oxide (GO) is selectively coated on one side with dopamine forming polydopamine (PDA, a melamine-like material) line patterns. This layer makes it resistant to oxidation with HI. PDA-rich regions of the strip then remain hydrophilic but the regions without PDA are reduced to hydrophobic rGO (reduced GO). When exposed to water, the PDA-rich sections tend to swell and as the PDA-poor regions do not, local bending of the strip is observed. By light exposure, water evaporates and the process is reversed. The novelty has thus far resulted in GO walkers and GO opening- and closing boxes. The more serious application the researchers are going for are artificial muscles.

Lithium to air battery redesign

05 November 2015 - Electrochemistry

LiOH gray 2015.PNGIn the news this week: a new development in battery design that allows you to drive from London to Edinburgh on a single car battery (DOI), at least according to several news outlets (BBC,FT, The Independent). Now, in The Netherlands we have a lot of electrical hybrid cars but only for tax evasion purposes (range only 50 Km). On the other hand the latest Tesla model S in sale here boasts a range of 528 Km and that is almost that distance (664 Km). So who is making this claim of driving between the two capital cities? Not the PR department as you might expect but the lead of the science team, Professor Gray.

The battery redesign concerns the classic lithium-air battery which is made up of a solid lithium anode and a porous carbon cathode. During discharge lithium is oxidized at the anode and oxygen is reduced at the cathode. The equations are Li -> Li+ + e- for the anode and Li+ + e- + O2 -> LiO2 and LiO2 + Li+ + e- -> Li2O2 for the cathode. Lithium peroxide is an insoluble compound and will occupy the available cathode cavities. In an ideal battery the pores are filled to maximum capacity, at one extreme a single crystal forms, and during recharge the pores are completely emptied by the reverse chemical process. Main attraction for this type of battery is the energy density. The theoretical value is 12 kW·h/kg which is close to that of gasoline at 13 kW·h/kg. The value for the Tesla model S is 0.14 kW·h/kg ? (link). Another factor to take into account is overpotential, the higher the overpotential, the more energy it requires to recharge a battery beyond what you would expect based on thermodynamics alone.

So what did the redesign involve? First of all the cathode was replaced with reduced graphene oxide (GO). Secondly as electrolyte the lithium salt of bistriflimide in dimethoxyethane solvent was added into the equation with additional lithium iodine thrown in. The amounts were small but the consequences were profound. The battery capacity increased a lot and the required overpotential at charging was also smaller. The reason is that all of a sudden it is no longer lithium peroxide that is formed during discharge but lithium hydroxide. The lithium iodine is able to make a difference by acting as a so-called redox mediator and by facilitating lithium ion diffusion as well as oxygen diffusion. Significantly LiOH is much more efficient at filling the GO pores. The essential reactions for discharging are now: 4Li+ + 4e- + 4O2 -> 4LiO2 and 4LiO2 + H2O -> 4LiOH + 3O2. The charge reactions are 6I- -> 2I3- +4e- and 4LiOH + 2I3- -> 4Li+ + 6I- + 2H2O + O2 The proton required for this reaction to happen is likely to originate from stray water. In fact the battery is quite resistant to water and LiOH crystal growth even benefits. Nice to know if you are heading to Edinburgh anyway.