Harness the power of rust! Metal-Air batteries are a way to capture the power of metal oxidation in a salt solution. In this instructable, I'll be building an iron-air battery, turning iron into rust using salt and atmospheric oxygen.

For a quick background, take a look at the energy that can be released from various metals:

While aluminum would release more energy, My goal here is to create a power source that is cheap to refuel (using rebar or scrap as the fuel source), and could be made from easliy obtainable materials in a third world environment. additionally, iron oxide is easily smelted back into raw iron using charcoal, and is non toxic - unlike aluminum, which is nasty stuff.

A metal-air cell is just: ANODE of metal (in this case iron), CATHODE of carbon/air, and a salt electrolyte. So far, my experiments have yielded Epsom Salts are a pretty darn good electrolyte.

What DOESN'T WORK? Seems like urea-based sidewalk salt is a huge failure, avoid everything containing urea.

COVER PICTURE: This is a carbon-block water filter. It was a little difficult to build the cell, as there is no inside liner. The newer filters (in the rest of the instructable, you will notice a white liner).

UPDATED: 3/10/14 - Added tables and additional data on most pages. Added diagram of how it works to main page.

UPDATED: 3/11/14 - added positive/negative to diagram and added Polarity step. Updated Filter page to indicate preferred filter (block) and style.

UPDATED: 3/12/14 - Polyethylene Glycol (PEG)- otherwise known as Miralax (the laxative).. immediate boost in power under load. 10Ohm resistor 0.5v fell under 0.01.. after PEG, jumped to 0.113 and was climbing when I left the basement... will let it run a few hours to see what happens.

UPDATED: 3/14/14 - Updated results with current output from a two cell in the final step, a few other edits in the mix as well.

UPDATED: 3/17/14 - 2 cells running with PEG since 3/12, 0.88v, 10Ohm resistor since 2/12 = 10.560 Ah * 0.88v = 9.2928 Watts expended. I think it's time to scale this up to 12v.

UPDATED: 3/17/14 - removed all iron from the outside of the cells, voltage went from ~0.4 to 0.55 immediately. Added two more cells to scale up to an initial voltage of 2.0v .. once it stabilizes I'll add the 10 Ohm resistor for a 200mA load .. and we'll see how it goes.

UPDATED: 3/18/14 - I attempted a recharged on the 4-cell at 3v for about an hour, which made the cells hold a nice 2.34 volts .. the two fresh cells seem to have a higher internal resistance, and don't show the surface crystals the older ones do. I think building up this lattice of epsom, salt, and PEG has something to do with efficiency... keeps the unit wet (from atmospheric water) without dripping.

UPDATED: 5/6/2014 - realized Maalox is incorrect, the product is Miralax, composed of only Polyethylene glycol.

Step 1: Water Filters

Here's my latest experimental filters.

They're cheap, but the carbon isn't a block like my current best cell. The carbon seems to be a roll of paper. I'll update on the progress with this filter, it *might* work - we'll see.

The idea is to create a damp, spongy CATHODE, so that oxygen can meet the electrolyte and iron ions. But you don't want it TOO loose, or you won't keep the elctrolyte damp. Additionally, having some kind of lining inside will prevent accidental short circuits, iwhen you fill the chamber with metal or activated carbon.

The second image shows how I cut off the white plastic netting, so I could wrap my CATHODE electrode around the body of the cell.

The bottom hole has to be plugged with a rubber stopper. You may have to trim the filter's hole to get the stopper to fit. This will prevent your electrolyte form just pouring through.

Finally, once prepared I filled the elctrodes with distilled water to ensure everything was moistened and ready to roll. Finally, I just let them sit in the water to soak up what they could.

Carbon-block water filters: :) - will make you happy.

You're more likely to be successful with these. The carbon is brittle, but not difficult to handle. Becuase the carbon is so thick, you're more likely to have a dry connection for your wire, and less likely to have the "reverse current" problem of the carbon paper. I was able to find $8/pair carbon block filters in my local hardware store's water filtration section.

"Carbon Paper" filters (like the one's in the picture): :( - will make you sad.

Seem to work well enough, but I think they might not be as conductive. Additionally, because they stay completely wet, you are at risk of having your metal electrodes form a secondary air-battery, sapping all your power. I'm experimenting with making carbon-paste electrodes (glue and activated charcoal) or some other similar method to extract power without the part you clip to being wet.

<p>This is really interesting stuff! Thanks for sharing all the details of what has worked so far, and what hasn't. The PEG as a wetting agent was a great idea. </p><p>For your current &quot;best&quot; battery, how does the voltage/current hold up over time?</p>
Well.. looks like the big thing is having a &quot;dry&quot; outer carbon electrode ... I'm able to get the voltage up to 0.7-0.8/cell, and can get some reasonable power - but It seems like the carbon-block electrodes were the ones that gave me the best/most-stable power. I think the carbon-paper has too high of an internal resistance to really be useful.<br><br>I had the original two carbon block cells running an LED for more than a week... and then had a 10Ohm resistor in there - my final calculation was close to 9 or 10 watts over that time.<br><br>So I'm going to ditch the paper filters, and go all carbon-block for my &quot;lessons learned&quot;/refined post. I think I'll just make a new instructable at that point. I've designed a PVC case as well that I'm going to build for a 12v system.
<p>try putting them in a pvc capped tube to hold the liquids in?</p>
<p>Unfortunately - the carbon has to be in contact with air. I suppose fresh water around the carbon, bubbled with air might work, if you used Tyvek or a similar vapor-permeable membrane.... or just floated the carbon on top of a tank of electrolyte.</p>
I am very interested in how this turns out keep us informed I can't wait for your next installment.
<p>Should I make a part 2, or just amend this one? </p>
amend this one, easier to find later
<p>Updated, tried to lay out things a little better, and add some more information.</p>
<p>Will do.</p>
Cool idea. This would be a great thing to work on if you live on a boat. You could recycle your water filter to generate power. I wonder how well evaporated sea water would work.
<p>try using the sanding wool. way better results me think an cheaper.</p>
wow! i will read it with great care as it sounds excellent idea!! well done!!!

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