Friday, 13 July 2007

Electrochemistry (1)

I just had my electrochemistry exam. So I thought it's a good idea to give a quick summary over modern batteries and fuel cells. Both are systems that use spontaneous chemical reactions to generate electricity. Fuel cells work continuously, batteries discontinuously.

For every electrochemical process there are four components that have to be considered: anode, cathode, electrolyte, electron conductor. "Red cat" tells you that reduction occurs at the cathode.

Probably the most important secondary (i.e. rechargeable) cell ist the lithium ion battery. You find it in pretty much every cellphone or portable computer. If charged the anode consists of anionic carbon (LiC6) and the cathode of lithium-cobalt(III/IV)-oxide (LiCo2O4). Both of those don't sound very stable. In other words we have a high voltage (about 3.5V). With high voltage and low density components the lithium ion battery has a very high energy density.

There is no way to use water in such a system (it decomposes at 1.2V). Instead ethylene-carbonate with LiPF6 is taken (large PF6- means low lattice energy and therefore good solubilty even in an organic solvent).

The two half-cell reactions are:

anode: LiC6 -> C6 + Li+ + e-
cathode: LiCo2O4 + Li+ + e- -> Li2Co2O4

Lithium goes through the electrolyte, the electron goes through the conductor (and your cell phone). They meet at the cathode and reduce Co(IV) to Co(III). The nice thing is that Li+ just intercalates back and forth without altering the electrodes. This "rocking chair" mechanism makes recharging possible. Compare this to the various types of lithium batteries containing lithium electrodes. It is not possible to make the electrode appear in its original shape through recharging. Dendrites will lead to short circuits.

Another way to get around dendrite growth is having the electrodes in their molten state. A typical example is the Zebra battery which works at 300°C and gives 2.6V. It has liquid sodium as an anode and NiCl2 dispersed in liquid NaAlCl4 as a cathode. The electrolyte is a solid sodium conducting ceramic, e.g. NaAl11O17. Zebra batteries are used in electric vehicles.

A more down to earth method is the nickel-methal-hydride battery (1.2V). We have an alloy hydride (MH) anode, a NiOOH cathode and a KOH electrolyte. Half cell reactions are (the proton is transported via the OH-):

MH + OH- -> M + H2O + e-
NiOOH + H2O + e- -> Ni(OH)2 + OH-

Fuel cells maybe tomorrow. Have a happy summer!


mevans said...

Just as an aside, lithium ion batteries are being exploited by electric car companies like Tesla Motors...Tesla is still in "get popular with the rich and famous by only selling one model for $95,000" mode, but they seem to want to market to the general public someday.

Felix said...

but who pays 95000 dollars for a car that doesn't make noise

Anonymous said...

Actually, even the ZEBRA battery can develop Na dendrites through the Na-beta"-alumina at high enough current densities.

Felix said...

ok, thanks for the clarification