11.19.2016

The transition metal battery

In the past I have made various types of batteries from aluminum air to nobel metal batteries.  But now it is time for something more practical.

Introducing the transition metal battery.  This battery uses one oxidation resistant cathode such as copper or carbon or preferably iridium, and one transition metal anode.  This anode can be any transition metal but most preferably would be manganese or vanadium.  Chromium, zinc, and iron will also work but give diminishing voltage (but may be able to be recharged quicker).  Any combination of these metals or other transition metals work in a transition metal battery.

There are many reasons why these metals are good anodes.  First of all we need a metal that has a negative redox potential that has a lower absolute value than chlorine's redox potential 1.358v.  This means that if you try to charge the battery cell higher than 1.358v then the chloride ions in the salt or acid solution will turn into chlorine gas and not charge your battery.  For manganese it needs 1.185v to charge and for vanadium it requires 1.175v to charge so these metals are safe.  These voltages are also what the battery will be able to produce which is similar to normal batteries.  1 volt is the standard voltage of a battery cell.  Using an iridium cathode instead of copper can get up to roughly 1.4 volts.  The amperage is pretty good, roughly a square inch of copper cathode gives 0.01A in salt water and that same piece of copper in HCL gives around 0.1 A.  The amperage is determined by how much oxidation happens on the cathode.  So increasing the cathode surface area or increasing the aeration at the cathode (or adding an oxidizer) can improve the amperage.  All of these can be done to the transition metal battery.

So for electrolytes preferably we would use a chloride based one.  Fluoride based works for even higher power batteries like aluminum, magnesium, lithium, etc.  But we don't need fluoride for transition metals.  Chloride (salt) works which is completely safe and environmentally friendly.  HCL would be able to deliver the most amperage as an electrolyte, but unlike nobel metal anodes that have positive redox potentials and can handle the acid, transition metals self discharge in acid.  This is because acid has a 0 redox potential so any metal with a negative redox potential like manganese and vanadium will be dissolved by acid.  To be able to still get the chloride ions without the acid the HCL has to be neutralized with a base like sodium or potassium hydroxide.  When HCL is neutralized with sodium hydroxide we have what is known as table salt.  So table salt or any other chloride salt would be a great electrolyte for this battery.  Calcium chloride may be the best cheap and safe option.  
http://chempaths.chemeddl.org/services/chempaths/?q=book/General%20Chemistry%20Textbook/Reactions%20in%20Aqueous%20Solutions/1426/ions-solution-electrolytes
This can be a water based electrolyte, glycerine based, or any other solvent or ionic liquid suspension.

The main thing that sets this battery apart from conventional batteries is the anode and cathode chemistry don't change throughout the charge/discharge cycle.  What happens is manganese ions will be dissolved during discharge and they will electro-deposit during recharge.  Care must be taken in design to prevent dendrites of manganese to connect between the anode and cathode.  Also care must be taken to not too fully discharge the cell so there is no longer an anode left to recharge.

In conclusion we have a rechargeable ~1.2 volt battery with only 3 basic ingredients; copper, manganese, and salt water.  Higher nobel metals like iridium can be used as the cathode instead of (or plated on) copper to improve performance, carbon can be added to or used as the cathode to increase surface area and improve performance, air or another oxidizer can be added to the cathode to improve performance, a reducing agent could be added to the anode side (including sunlight) to increase performance, and a different neutral chloride based electrolyte can be used to increase performance as well.

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