Introduction: How to Make a Microbial Fuel Cell (MFC) Using Mud

About: We make fun and educational STEM kits that let you play with microbes. Create electricity from the bacteria living in mud, or make bread while racing yeast!

The MudWatt microbial fuel cell (affectionately dubbed the "Dirt Battery") is a device that uses bacteria to convert the organic matter found in mud into electricity. This Instructable will walk you through making your own microbial fuel cell using any MudWatt Science Kit.

To make a MudWatt, you will need:

How the MudWatt Works: The MudWatt is a fun and educational science kit that uses the micro-organisms naturally found in soil to generate electricity. Although invisible to the naked eye, these microbes, with bodies one-tenth the thickness of a human hair, live throughout virtually all soil and sediment on the planet. Among these diverse communities of microbes are particular species that have the unique ability to release electrons outside their bodies as part of their respiration process.

The MudWatt harnesses this remarkable ability by providing these mud-based microbes with two conductive graphite discs, called an anode and cathode. The anode is placed within the mud where the electrogenic microbes can grow, while the cathode is placed on top exposing it to oxygen in the air (see MudWatt diagram below).

Step 1: Making Mud

Put on gloves and find 3-4 handfuls of soil or swamp goo--the smellier the better! Make sure your soil is saturated but not soupy by either adding or pouring off water. Optional: Add extra nutrients to your soil, such as MudWatt packaging, shredded paper products, or food from your fridge.

Key notes: Avoid using soils with little white balls (perlite) which aerate the soil. The bacteria that power the MudWatt are anaerobes that need an environment without oxygen to build healthy communities.

Step 2: Making Electrodes

Bend both wires 90° where the plastic sheath ends. Straighten out the bare end of the wire. The green wire will be used to make the anode, and the orange wire will be used to make the cathode. Insert the bare end of the anode (green) wire into the side of the thin felt disc while wearing the gloves provided. Try to keep the wire from exiting the felt. Repeat this step with the cathode (orange) wire and the thick felt disc.

Step 3: Assembling

Pack an even layer of mud into the bottom of your container, at least 1cm deep. Place the anode (green) you constructed in Step 3 on top of the mud, pressing it down firmly to squeeze out air bubbles. Fill your container with more mud, at least 5cm deep, pressing down firmly to squeeze out air bubbles. Let your mud rest for a few minutes and drain any excess liquid. Finally, place the cathode (orange) gently on top of the mud. Do not cover the cathode with mud.

Step 4: Lidding (for MudWatt Kits That Come With Vessels)

If your kit came with the MudWatt Vessel:

Remove your gloves and attach the Hacker Board into the indentation on the lid. Pass the electrode wires through the lid. Facing the semicircular indentation, the cathode (orange) should be on the left and the anode (green) on the right. Now press the lid down onto the jar to snap it into place.

Step 5: Closing the Circuit

1. Bend and connect the cathode wire (orange) to ‘+’ and anode wire (green) to ‘-’ on the Hacker Board.

2. Connect the long end of the blue (10μF) capacitor to pin 1 and its short end to pin 2. You may need to bend the wires so that they fit snuggly.

3. Connect the LED ‘s long end to pin 5 and its short end to pin 6.

That’s it! You should start seeing the LED blink after a few days, once your MudWatt has developed a healthy community of microbes!

What do these components do?

Hacker Board: The Hacker Board takes the low voltage and low current coming from the MudWatt and converts into short bursts of higher voltage and higher current.

Capacitor: The Capacitor is a little energy storage component. It is able to build up energy as power comes in from the MudWatt, and then discharge that energy in a quick burst to blink the LED.

LED: The Light Emitting Diode (LED) takes the electrons being discharged by the capacitor and converts those electrons’ energy into light energy.

Step 6: Measuring Microbes and Power Output

Download the MudWatt Explorer App on the App Store or Google Play. You’ll be using it to measure, record, and analyze your MudWatt data in the few next steps!

Step 1: Ready, Aim...Measure!

Once your MudWatt’s blinker is blinking, open the MudWatt Explorer App and select Measure from the main menu. Line up the blinker in the target on your screen and the App will automatically measure your power and your population of electric bacteria!

Step 2: Record & Analyze Multiple Measurements

Record several measurements by using the Record button on the Measurement screen, and go to the Analyze section of the app to see how your MudWatt functions over time!

Step 3: Discover a Hidden World

Use your power readings to unlock chapters of a fun and educational comic following Shewy, the Electric Microbe. Discover the magic of microbes as Shewy explores this complex, muddy world.

Step 7: A Closing Note and DIY Resources

Thanks so much to everyone that has commented on our post! When Instructables first invited us to post about the MudWatt, we weren’t sure if it was the right fit, since it is indeed a product, and we were worried our Instructable would end up sounding like an infomercial. But we decided to do it because the MudWatt is designed to be a DIY MFC kit, after all. While we are of course excited when people buy our products, we’re also very excited when we inspire people to pursue their own creation and experimentation. Many people have requested more info for creating their own MFC using off-the-shelf parts. This is completely possible to do, and we’ve provided resources for that below. However, we’ve found that making a true MFC with off-the-shelf components is significantly more expensive than purchasing our components. The MudWatt actually started out using off-the-shelf components, but we've managed to get the price down by ordering materials in very large quantities and processing them ourselves.

We've seen several posts about DIY MFCs in the past, but we've found that these projects include the use of metal meshes, metal brushes, copper wire and other materials that corrode in harsh soil environments. Using these corrosive materials means that you're making a corrosion battery instead of an MFC. That corrosion often occurs at the junction between the electrode material and the wire. This junction can be sealed with epoxy, but this is very difficult to do in practice, especially if you're using high-surface-area or porous electrodes, which you'll need if you want to produce any significant power. The funny thing is that this corrosion will actually produce some significant power, which can be very easily mistaken to be coming from microbial activity.

For making your own DIY MFC, here are some places you can find non-corrosive materials:

Carbon electrodes:

Titanium Wire:

Charge-pump chip (for power an LED/electronics): S-882Z24-M5T1G

We encourage you to use the MudWatt as a launching point for your own research, and we'd be thrilled if you're able to create a true MFC using off-the-shelf materials at a cheaper price. Happy experimenting!