microbial fuel cell experiment

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

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

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:

• MudWatt Classic , MudWatt Science Fair Pack , or MudWatt Classroom Pack
• Any container (if you're using a different vessel)

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: http://fuelcellstore.com/fuel-cell-components/gas...

Titanium Wire: http://fuelcellstore.com/fuel-cell-components/gas...

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!

Setting Up a Microbial Fuel Cell: An Absolute Beginner's Guide

By a beginner, for beginners

Setting Up a Microbial Fuel Cell: An Absolute Beginner’s Guide

So, you want to set up your first microbial fuel cell, but you have no idea where to start? That makes two of us - or at least it would, if you were talking to me from 2 months ago. I have a fair amount of experience in fundamental electrochemistry, but it was only upon starting my fellowship as a Marie Skłodowska Curie at the Université de Rennes 1 that I actually got around to assembling an actual fuel cell. This short blog will serve as a quick guide on how to go about this. I’m not claiming that this method is perfect (feel free to chip in with improved tips or suggestions if you are reading this as an expert) but it should be enough to get started. Also, while my focus is on microbial fuel cells, the basic assembly described here should also be suitable for a variety of similar devices.

A Preview of What We Will be Making

Below is a preview of the end product - a fully assembled ‘H type’ fuel cell. The anode and cathode compartments are separated by a proton exchange membrane in the centre.

Probably the most important part of the whole process is deciding on the type of glassware you are going to use. Above is a photo of one half of the H cell, which is formed based on a 250 mL borosilicate glass bottle. After a trip to the local glassblower, 3 ports were added to the bottle in addition to the bridge at the bottom for the H-cell assembly. I wanted 3 ports to have one space reserved for a reference electrode, one space reserved for introducing inert gas, and one final port to use for pumping out the cell contents. Looking back now, it might have been preferable to have this 3rd port placed lower on the glass bottle for ease of pumping - try to think about what your particular application will need (pH meter? thermometer? microelectrode probe?…) and come up with a design that suits.

Joining the Glassware

I don’t know the proper name for the above piece of kit that ‘joins’ the 2 cells together, but it is essentially like the blue twist caps pictured on the bottles, except with 2 sides and a hole in the centre. The idea is to simply twist both cells on to this ‘joiner’ via the lower port, forming the ‘H-cell’ shape. In our lab, we have also had some success with an alternative design which uses a flat cylindrical glass port on each bottle and a clamp to hold the cells together. I will focus on the screw cap design in this post, though, as I am more aware of its pitfalls!

Before using the joiner, inspect the interior threads for any debris or defects. In particular if you are re-using an old joiner, rinse it well with millipore water (or at least distilled water) to remove any lingering salts remaining from buffer solutions. Anything blocking the threads will make it difficult to screw the glassware together and will cause headaches later.

In order to use the ‘double screw’ connection type, you will need to form a seal between the glass bottles and the proton exchange membrane, so that the solution inside does not leak out of the fuel cell or cross over from one side to the other. We accomplish this using the O-ring pictured below, but you can also try with a more typical black vycor o-ring of the same diameter.

Screw it on Straight!

Start by screwing the joiner on to the port of one of the glass bottles. Take care to ensure that it is screwed on perfectly straight - with these screw assemblies, it is very easy to ‘tighten’ them without properly orienting the glass threads into the corresponding grooves on the cap. If that happens you are liable to either have a leaking fuel cell, or to break the glass assembly when trying to tighten it due to misalignment.

Once you have screwed on the joiner, check that you can easily insert the o ring in the middle. It should fit snugly inside the joiner on top of the glass, as pictured above. If there are any noticeable gaps or evidence of a poor seal (e.g. if you are re-using an old o-ring or joiner) save yourself time later by replacing them now. If the cell doesn’t seal properly, you will have to disassemble everything later anyway.

The Membrane

Below is an example of a proton exchange membrane that we use in our H-cells. We also have transparent Nafion membranes, which seem to be more common in the literature, but for illustration purposes I will stick with this one as it is easier to photograph!

I recommend using the outer diameter of the o-ring as a template for cutting out the membrane. Try to cut out a circle which is slightly larger than this diameter, and then trim the edges.

You can see in the below image that the circle is not perfect - in particular there is a jagged corner on the bottom that must be trimmed off with the scissors.

After you cut the membrane to size, insert it into the joiner on top of the o-ring. It should look like the photo below. If you have difficulty getting the membrane to stay flush with the o-ring, you need to trim the edges a little more. In particular if the membrane is ‘bending’ against the sides of the joiner, you won’t get a good seal in the final assembly and it will more than likely leak. Trim the edges until it fits comfortably. Don’t trim them too much, however, as this will leave enough space for a leak across the cells (and compromise your experiment). It is OK if you can see part of the o-ring beneath the membrane (as I can in the bottom right of the joiner in the photo below) but any more than this will result in a leak, at least in my experience.

The Hardest Part: Screwing the Full Assembly

Once you have screwed on the joiner, placed the o-ring and inserted the membrane, the hardest step is to screw on the second half of the cell. I recommend holding the glassware together from one side and ‘turning’ the joiner like screwing on a bottle cap. If you feel some resistance, verify that the whole assembly is perfectly straight before applying pressure. If you aren’t careful you can force the glassware together at an angle. Then not only will it not seal properly, but you also will have a hard time unscrewing it again for a second try without breaking it. Basically, you need to screw it on straight, relatively tight, without disturbing the o-ring and membrane inside. I find that once it is close to tight enough, you can finish off tightening by turning the bottles themselves around once. Don’t push it! If it feels like you need to join a gym and lift weights to tighten it up, stop! You will more than likely just break the glass and have to start over again.

Once you have assembled the cell, it’s time to test your work. First, fill both bottles up to the same level with pure water and carefully inspect the join for leaks. If the seal is very poor (for reasons outlined above) it will more than likely leak immediately, and you can disassemble the cell and try again. However even if the assembly doesn’t leak right away, a slow leak could still develop over time. For this reason I recommend placing the cell inside a basin on top of some tissue and leave it for at least 24 hours. You can mark the level of the water inside both bottles with a marker so that it will be obvious if the solution level drops below.

Regardless of the type of fuel cell you will run, I recommend making use of ferricyanide solution at this stage as a means of checking the cell’s integrity. Ferricyanide is often used as an electron acceptor in microbial fuel cells, but even if you intend to run your cell based on oxygen or to instead run it as an electrolysis cell, ferricyanide can be invaluable in diagnosing leaks or issues with the membrane before starting a long experiment.

Choose one compartment as your cathode and label it as such. Fill it with ferricyanide solution (50 mM in buffer should be enough) and fill the anode compartment with the same buffer (minus ferricyanide) to avoid osmotic pressure issues. The assembly should look as below. The yellow color of aqueous ferricyanide is invaluable here, as it will be very obvious if there is a leak either out of the cell or from cathode to anode. After at least 24 hours (ideally slightly longer), use a 3 electrode setup with the working and reference electrode in the anode compartment and counter electrode in the cathode compartment. If there are no ferricyanide peaks found in the anode (about 0.2 V if using an Ag/AgCl reference) then your seal is probably acceptable and you can start your experiment.

Some Finer Details

At this point the ‘hard’ part is over, but there are still many things to think about for your fuel cell setup. In my case, we need the anodic compartment to be anaerobic, so the ports of the cell need to be tightly sealed with stoppers or septa like the one pictured below. These allow for inoculation of the solution with syringes or sparging with inert gas and (if new) keep a good seal after repeated use.

For sealing off the top of the cell, I prefer to swap the typical screw cap with a disk shaped septum like the one below.

This can be combined with an open top screw cap (below) or simply a regular screw cap with some holes drilled to allow for your electrode to fit inside.

The septum + cap should look as below when assembled:

You can be as creative as you want for the insertion of electrodes here. A simple solution is to simply pierce the septum with the electrode, as shown for the graphite rod below. However this may not allow for a good enough seal in the case of strictly anaerobic microbial fuel cells, or if trying to grow pure cultures. In that case waterproof contacts attached to wires may be preferable, or the septum can be replaced for another material (such as blue butyl stoppers used often in anaerobic culturing).

For the reference electrode, a home made Ag/AgCl like the one below works well and can be left in the assembly for some time to monitor a long experiment reliably. It will periodically need to be reconditioned or replaced, but this can easily be evaluated using a standard probe such as ferricyanide.

Note how the reference below has been inserted into a septum and held in place with parafilm - this will allow the reference electrode to seal tightly onto the port and avoid it detaching from the septum and falling into the solution…not that that ever happened to me…

Checking the Resistance

At this point it is a good idea to check the resistance across the H-cell in your buffer using a voltmeter. Tens to hundreds of ohms in resistance is ideal, but even a few kOhm is acceptable. If you are reading high kOhms or MOhms in a reasonable buffer (such as PBS), you have a problem with your membrane or there is an air bubble in the joint. Try to agitate the cell (gently) and retest the resistance. If it doesn’t drop, you will need to disassemble the cell and change the membrane. It is also a good idea to monitor this resistance periodically during your experiment - if it is a microbial fuel cell, colonisation of the membrane could increase the resistance across the cell, or a trapped bubble of gas could effectively close the circuit. Monitoring isn’t too time consuming and can save time in the long run by catching these problems.

Closing the Circuit

For microbial fuel cells, a typical means of completing the cell circuit is with an external resistor like the one above. You can match your resistor to the internal resistance of the cell measured with the voltmeter above, or just use a common value (like ~500-1000 ohms). Of course if you intend to control the anode potential with a potentiostat or otherwise apply a potential, this is unnecessary.

Here’s One we Prepared Earlier…

Below is a microbial fuel cell running with an anode colonised by exoelectrogenic bacteria fed with acetate running against a ferricyanide cathode. Both electrodes use a 10 mM phosphate buffer at pH close to 7. You can see a fairly typical open circuit potential of ~760 mv as measured with a voltmeter. The cell is kept at about 30 degrees in a water bath and protected from light with Al foil. A cell like this can be used for a variety of experiments for a long time, with only periodic monitoring of important parameters (like the internal resistance or the buffer pH) required to keep the system going. Also note that the cell is being degassed using Ar with an inlet and exhaust syringe - take care when sparging not to use too high of an overpressure of gas and to be sure you have an outlet before turning the gas on. Otherwise you might have septa flying across the lab…

I hope that this simple guide was helpful to anyone just starting out assembling fuel cells. In the future, I may follow up with similar content about some of the points I glossed over here, or try to cover some of the fundamentals for a less technical or specialist audience. Stay tuned!

James A. Behan

Chargé de recherche/assistant professor cnrs.

I am a Chargé de Recherche (Assistant Professor) based at the Institut des Sciences Chimiques de Rennes, France. My research focuses on the areas of nanomaterial electrochemistry, bioelectrochemistry and interfacial science for energy applications. I previously worked as a Marie Curie fellow (Université de Rennes) and postdoctoral researcher at the Centre for BioNano Interactions (CBNI) in the School of Chemistry, University College Dublin and as a postdoc and lecturer in Trinity College Dublin where I completed my PhD in physical chemistry with a focus on electrochemical and spectroscopic characterisation of nitrogenated carbon materials. I completed my Bachelor’s Degree in Chemistry at Trinity in 2014, when I graduated at the top of my class.

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Projects from Make: Magazine

Generate electricity from bacteria in mud.

• Time Required!: 1–3 Hours
• Difficulty: Easy
• Price: $0-$50
• Print this Project

By Jendai E. Robinson

Jendai e. robinson.

Jendai E. Robinson works in the Center for Nanotechnology at NASA. She is currently a NASA Harriet G. Jenkins Fellow.

With some mud, salt, and water, you can create a closed circuit that generates a current. This is called a microbial fuel cell, a device that uses bacteria to create electrical power by oxidizing simple compounds like glucose or organic matter in wastewater. Given the finite supply of fossil fuels, this biofuel cell is a promising approach for generating power in a renewable, carbon-neutral way. Check out these examples:

The fuel cell works when bacteria attach to the electrode in an anode chamber of a cell that is oxygen-free. Since the bacteria do not have oxygen, they must transfer their electrons somewhere else. The cathode however is exposed to oxygen; thus, the two electrodes are at different potentials and create a bio-barrier or a “fuel-cell.”

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Project Steps

1. make the salt bridge.

Prepare an agar solution according to the instructions on the packaging. Add ½ cup of salt to the water.

Cover one end of the PVC pipe with plastic wrap to contain the prepared agar solution. Place the pipe vertically in a dish, then pour in the solution and allow it to cool (Figure A).

2. OBTAIN A MUD SAMPLE

The mud must come from the benthic zone — anything associated with or occurring on the bottom of a body of water. This is where you will find the electrochemically active anaerobic bacteria. If the sample is being collected from the bottom of a creek, pond, or lake, it should be black in color. Topsoil mixed with distilled water can also work if it contains enough anaerobic bacteria. Place mud in a container and cover.

3. BUILD THE HOUSING

Using a permanent marker, outline a hole on the side of one of your plastic containers large enough to fit a PVC fitting. With a ruler, measure the location of the mark and make a mark at exactly the same location on the side of the second plastic container. Ensure that your marked outlines are exactly opposite and facing each other, then cut out the holes.

Mark the center of each of the two lids for the containers. Drill a small hole in each lid for the copper wire to run through, and an optional hole on one of the containers for the placement of an air pump. Place the PVC fitting into the holes and glue into place. Allow glue to dry (Figure B).

CAUTION:  Some adhesives can be irritating to the skin. I wear gloves while handling glue.

4. PREPARE THE ELECTRODES

Strip off each end of the red and black copper wires with a wire stripper and fold one of the ends around a sheet of aluminum mesh. Bind it with the mesh (Figure C) or, optionally, with a paper clip.

Insert the other ends into the pre-drilled holes on the containers and seal with glue.

5. ASSEMBLE THE FUEL CELL

Insert the PVC nipple into the FHT PVC fittings on the containers and hand tighten.

Insert the air pump tube into the pre-drilled hole on the cathode container and seal with glue (optional).

Insert each end of the salt bridge into the pipe connectors and tighten until it is securely in place. Be sure to create a watertight seal (Figure D).

6. FILL THE CONTAINERS

Using gloves, fill the first container halfway with your sludge. Take one of the electrodes and bury it. Remove any air bubbles and continue filling the container. This will be your anode.

Next, fill the second container with distilled water. Add ½ cup of salt and stir. This will be your cathode.

Place your second electrode and seal both containers with their lids. Attach alligator clips to the ends of the protruding copper wires.

Optionally, turn on your aquarium pump to aerate the cathode solution.

Begin to measure voltage output on the multimeter by attaching the respective ends of the alligator clips to the multimeter.

The performance of your biofuel cell can be evaluated by determining its voltage output. Turn on the multimeter and measure the voltage between the anode and cathode. Next, try attaching a resistor and determine the performance of the biofuel cell using its power output, or put an LED light onto the ends of the anode and cathode wire to see if there’s enough energy to illuminate it. Additionally, applying a small voltage to the bacteria produced at the anode can modify the cell. By not using oxygen at the cathode, you should be able to produce pure hydrogen gas. This modified process is known as a microbial electrolysis cell and is based on the idea that fuel cells produce electricity whereas electrolysis produces hydrogen.

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Student Projects

Examples of mfc projects by students and other researchers (universities and public schools).

It is great to see all the different types of reactors that students have built over the years for science and engineering competitions. This is a great idea for a project as the reactors can be quite inexpensive to build, and yet scientifically they cross so many disciplines of biology, materials, and chemistry, with room for art (make them beautiful), science and engineering.

The photos are temporarily missing as we work to transfer materials from the old website to this new website.

Have you built one?  Send me your photos, website, or other information (email: [email protected] ).

Eric A. Zielke (Humboldt State University)

Design of a Single Chamber Microbial Fuel Cell (1.40 mb),

Probabilistic Analysis of a Monod-type equation by use of a single chamber Microbial Fuel Cell (374kb),

Application of Microbial Fuel Cell technology for a Waste Water Treatment Alternative – (1021kb)

Thermodynamic Analysis of s single chamber Microbial Fuel Cell (707kb)

Numerical Analysis of a one dimensional Diffusion Equation for a single chamber Microbial Fuel Cell using a Linked Simulation Optimization (LSO) technique (924kb)

Poster presented at the American Institute of Chemical Engineers (AIChE) 2006 Conference (945kB)