Abstract

Objective

Introduction

The sun is setting on a brisk fall evening, and all of a sudden your TV turns off right in the middle of your favorite show! Without the sun, the solar cells on your roof cannot generate electricity to run your appliances. That is not a good scenario! Many forms of renewable energy, such as solar energy and wind energy, are not available at all times of the day and night. These renewable energies are intermittent sources of energy. We, as a society, however, need energy to be available at all times. For this reason, renewable energy sources like solar energy pose particular challenges to engineers if they are to be used by power plants that generate electrical energy for your house, or for your car in the place of gasoline. Currently, power plants that supply energy to your house run on coal, natural gas, or gasoline as a fuel source and can do so 24 hours a day, 7 days a week. However, these fuels are carbon-based, meaning that they can generate pollutants. Additionally, fossil fuels are unsustainable, meaning that they will eventually run out. Using clean, renewable energy would be a solution to these problems.

Sunlight is a form of renewable energy that is virtually limitless: more solar energy strikes the earth each hour than the entire world uses in a year! However, for solar energy to become practical, we need a cheap and efficient method of storing the solar energy for when the sun does not shine. One method to achieve efficient storage of solar energy is in chemical bonds. Specifically, what scientists and engineers seek to do is use the energy from the sun to rearrange low energy bonds to form high energy bonds. The high energy bonds can be used later to deliver the energy back to us when we need it. This concept is comparable to when you charge a reusable battery. You get electricity from the wall socket, store it in the battery, and when you need to run your phone or computer, the energy is available.

In fact, society is already familiar with this concept of energy storage in chemical bonds in the form of fossil fuels (like gasoline and natural gas). By burning these fuels, energy contained within those high-energy bonds is released (and used by humans) along with carbon dioxide (CO₂) (which is not used by humans). The carbon dioxide molecules have lower energy bonds than the bonds in the fossil fuels. In this instance, the process is irreversible—once the carbon dioxide is created, it is released and cannot be easily converted back to fuel.

A primary goal for renewable energy research is to develop fuel storage methods that are scalable, sustainable, and do not used carbon-based fuels. The ideal energy cycle would be a fully renewable one where the fuels can be utilized (burned to extract the energy from the high-energy bonds) and the waste products (low energy bonds) can be captured and reused to form the same fuel again.

One attractive approach for renewable energy storage is to use solar energy to drive the rearrangement of water (low-energy bonds) into molecular hydrogen gas and oxygen gas (high-energy bonds). This transformation, often called water splitting, provides attractive alternatives to hydrocarbon fuels because combustion of hydrogen fuel (which is really the "burning" of hydrogen fuel, H₂, by letting it react with oxygen, O₂) releases water instead of carbon-based molecules, such as carbon dioxide and carbon monoxide. The water that is formed upon combustion of the hydrogen can be collected and "split" again to remake the fuel. In this way, the cycle is sustainable.

Achieving efficient "splitting" of water requires a catalyst that assists oxygen-oxygen bond formation between two oxygen atoms derived from water. A catalyst is a material or molecule that increases the rate of a reaction between other starting materials, but is not used up in the reaction. Since the catalyst increases the reaction rate, less energy input is required to produce the product. This means that with the same amount of energy available, the product can be produced at a faster rate—it is more energy efficient.

Consider the reaction in Equation 1 above. With a catalyst present, we do not need as much energy to make C as we would have needed if the catalyst was absent, and with a catalyst present, we can make C much more quickly. One example to help explain this is to consider that A is a taxi driver and B is a person at the airport. The goal of the "reaction" is to get A and B together so that the taxi driver can take the passenger to a desired location called C. Without a catalyst, both the taxi driver and the person would wander around the airport, requiring a large amount of energy (through walking or driving) and time to bump into each other. However, if there is a taxi stand, acting as the catalyst, the taxi driver and the person would be able to find each faster with less energy wasted.

One fuel-forming reaction that interests scientists and engineers is the splitting, or breaking up, of water to form molecular hydrogen and oxygen using light from the sun. This chemical reaction is shown below in Equation 2. The molecular hydrogen and oxygen can then be stored separately, and later brought together as fuel in what is called a fuel cell.

Sun-light does not directly act on water to cause it to split into these elemental components. Hence catalysts are needed to effect the overall transformation. In Nature, the water-splitting reaction is accomplished by the process of photosynthesis inside of the leaf. Outside of the leaf, solar fuels other than hydrogen may be produced with the protons and electrons extracted from water, including the reduction of carbon dioxide to methanol. However, all water-splitting schemes require oxygen (O₂) production. This turns out to be a very inefficient step and a barrier to using artificial photosynthesis (the splitting of water into H₂ and O₂) as a renewable energy. Thus the focus of many research programs involves the design and development of catalysts that assist water oxidation to molecular oxygen (O₂).

In this video, chemists Dr. Nocera and Dr. Kanan explain how they discovered the Co-Pi catalyst, and what it might mean for the future of renewable energy.

A catalyst formed from cobalt ions (Co) in a phosphate (Pi) buffered solution (Co-Pi) has recently been discovered, in Dr. Nocera's lab at MIT, that is capable of water oxidation to O₂ at low overpotentials and high rates. The catalyst is comprised of inexpensive, earth-abundant elements, and its formation is robust under a host of conditions. See the video below for more details about the discovery of the Co-Pi catalyst.

In this science project, you will construct an experimental set-up that will allow you to synthesize this cutting-edge cobalt-based catalyst. You will perform measurements to determine how much the catalyst reduces the energy needed for the water-splitting reaction to form oxygen. Does it sound too challenging? If you watch the video below you’ll see that setting up the experiment really isn’t that hard. Especially if you follow all the steps in the Experimental Procedure! Once you get the hang of it, you can use your set-up to try to discover your own new and improved catalysts! Maybe you'll find the one that we'll all be using someday to power the planet.

Watch this video from MIT researchers Yogesh Surendranath, Thomas Teets, and Dr. Elizabeth R. Young to see how you can perform water splitting experiments with just a few simple materials.

Terms, Concepts, and Questions to Start Background Research

• Renewable energy
• Fossil fuels
• Solar energy
• Low energy bonds
• High energy bonds
• Water splitting
• Fuel cell
• Potential energy
• Overpotential
• Electroplating
• Electrochemical cell
• Baseline

Questions

• What is the key problem with using water splitting as a way of storing energy?
• How much energy does it take to split water? How much energy is retrieved by recombining the molecular hydrogen and oxygen during hydrogen combustion?
• There are many reactions that store about the same amount of energy as water splitting. For example, hydrochloric acid (HCl) can be split into hydrogen (H₂) and chlorine(Cl₂) to store solar energy. What are some of the advantages that water splitting has over alternative energy-storing reactions like HCl splitting?
• Why was cobalt chosen as a possible water splitting catalyst? Based on this information, can you suggest other elements that might also work as a catalyst for this reaction?

Bibliography

This resource will give you more information about catalysts:

• Clark, Jim. (2002). The Effect of Catalysts on Reaction Rates. Retrieved February 18, 2011 from http://www.chemguide.co.uk/physical/basicrates/catalyst.html#top

More information about renewable energy can be found at these websites:

• Culverco, LLC. (2002). Pacific Gas and Electricity: Alternative Energy Sources. Retrieved February 18, 2011 from http://www.pge.com/microsite/pge_dgz/more/alternative.html
• National Renewable Energy Laboratory. (2009, August 20). Learning About Renewable Energy. Retrieved February 18, 2011 from http://www.nrel.gov/learning/

More information about water splitting and the original discovery of the Co-Pi catalyst can be found using these resources:

• Witcombe, C. and Hwang, S. (n.d.) The Hydrogen and Oxygen of Water. Retrieved February 18, 2011 from http://witcombe.sbc.edu/water/chemistryelectrolysis.html
• Chemical Explorers. (n.d.) Tiny Bubbles. Retrieved February 18, 2011 from http://techtv.mit.edu/videos/273-tiny-bubbles
• Kanan, M.E., Surendranath, Y., and Nocera, D.G. "Cobalt-phosphate oxygen-evolving compound" Chemical Society Review, 2009, 38, 109-114.

For more information about setting up a circuit on a breadboard, or electronics terms, use this primer:

• Science Buddies Staff. (n.d.) Electronics Primer: Introduction Retrieved February 18, 2011 from http://sciencebuddies.org/science-fair-projects/project_ideas/Elec_primer-intro.shtml

Materials and Equipment

• Breadboard, about 3"x2", for example Radio Shack part # 276-003
• 22-gauge electrical wire, for example Radio Shack part # 278-1218
• Voltmeter/Multimeter (must be able to read 10 millivolts), for example this one at Amazon.com
• 10K Ohm resistor, for example Radio Shack part # 271-1335
• 9V batteries (4)
• Alligator clips (12), for example Radio Shack part # 270-380
• Small jars or beakers (2); should be taller than 5 inches
• Magnetic stir plate and stir bar, try borrowing these from school. If that doesn't work, they can be purchased online.
• Nickel metal strips (2), purchase these from Unitednuclear.com
• Cobalt Nitrate (1 container), purchase this from Unitednuclear.com
• Phosphate buffer solution, pH 7 (500ml), available from Amazon.com
• Cola; any brand (Coke, Pepsi, or a generic) will work
• Lab notebook

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Experimental Procedure

Creating the Galvanostatic Electrochemical Cell

1. To start this science fair project, you should assemble the necessary chemicals, electrodes, components, and instruments you need, as specified in the Materials and Equipment list above. Make sure you are familiar with all of the items in the Terms, Concepts, and Questions section above. If you get stuck, or need more detailed explanations, watch the video in the Introduction by MIT researchers Yogesh Surendranath, Thomas Teets, and Dr. Elizabeth R. Young. It takes you through the project step-by-step.
2. You will build a circuit on the breadboard consisting of the batteries, resistor, and voltmeter/multimeter. If you don't have experience building a circuit on a breadboard, use the Science Buddies Breadboard Guide as a reference. Figure 1 below shows you what the completed circuit should look like.
   1. Using Figure 1 as a visual reference, connect the four 9V batteries in series using some wire and 6 alligator clips.
   1. Cut each piece of wire to the desired length with the wire cutters.
   2. When using wire to attach components in a circuit, the ends of each piece of wire need to be stripped, with the wire stripper, before creating the connection.
   2. Using a piece of wire, connect the positive end of the series of batteries to the breadboard power bus (first column, see the Science Buddies Breadboard Guide if you are confused).
   3. Connect the 10K Ohm resistor as shown below in Figure 1a. (Note: orientation does not matter.)
   4. Connect the positive (red) lead from the voltmeter/multimeter to the breadboard as shown below in Figure 1b.
   5. Connect the negative (black) lead from the voltmeter/multimeter to the breadboard as shown below in Figure 1b.
   6. Using a piece of wire, attach the negative (-) end of the series of batteries to the ground bus (last column) of the breadboard.
3. The circuit is now complete and should look like the circuit in Figure 1b. Using the voltmeter/multimeter, make sure the circuit reads >30V. If you need help using a multimeter, consult the Science Buddies Guide to Using a Multimeter, as well as the instruction manual that came with your voltmeter/multimeter.

Figure 1. The photo on the left (1a) shows how the various components in the circuit should be wired onto the breadboard. The completed circuit, including breadboard and multimeter leads, is shown in the photo on the right (1b).

4. Use the nickel metal strips as electrodes. The nickel electrodes will serve as the scaffold for formation or electroplating of the cobalt catalyst.
   1. To clean the electrodes (nickel metal strips), pour some cola into a cup or jar. Put both electrodes in the cola. Make sure the nickel is entirely immersed. Cola contains phosphoric acid. This acid will do a great job of cleaning the surface of the electrodes. After a few minutes, remove the nickel electrodes, wash them off with plain water, and dry them.
   2. Construct a method to secure electrodes within a small jar leaving the top of the electrodes readily available to make an electrical connection to the rest of the circuit you started preparing above. An example of a method to secure the electrodes is shown in Figure 2 below using a piece of Styrofoam to hold the electrodes securely in place. Note: It is important to ensure that the separation between the electrodes remains the same throughout the experiment. When securing the electrodes make sure to:
      1. Position the electrodes 1-2 cm apart.
      2. Make sure the electrodes are securely in place and not dangling freely or touching the sides of the jar.
      3. Position the electrodes so that they will, later, once the buffer has been added to the jar, only be immersed half way in the buffer. Note: It is critical that the top of the electrodes do not touch the buffer.

Figure 2. The photos above show one possible method for suspending the nickel electrodes in the jar to make the electrochemical cell.

5. Add pH 7 phosphate buffer to the jar with electrodes and fill so that the nickel electrodes are submerged half way in the buffer solution.
6. Place the stir bar in the bottom of the jar.
7. Connect the nickel electrodes to the rest of the circuit using copper wire and alligator clips as shown below in Figure 3.

Figure 3. The completed galvanostatic electrochemical cell is pictured above on the top (3a) and represented as a schematic on the bottom (3b).

Adding the Cobalt Catalyst and Measuring Its Effects

1. With the electrodes securely in place inside the jar, place the jar on the magnetic stir plate. Turn on the stir plate and get the stir bar moving at a constant rate.
2. Monitor the voltage readout on the voltmeter/multimeter. It should range between 2.1-2.4v and will take at least five minutes to stabilize. After the voltage reading has stabilized, record this voltage in your lab notebook. This is the baseline voltage value for your electrochemical cell.
3. Using Equation 4, below, and the information in Technical Note #2, calculate the baseline efficiency of the water splitting reaction in your electrochemical cell.

4. Now it is time to add the reactant necessary to form the cobalt-based catalyst. Add a pinch of the cobalt nitrate to the jar with the pH 7 phosphate buffer and either start the stopwatch, or write down the time in your lab notebook. With the cobalt source and the energy provided by the batteries, the catalyst will start to form.
   1. Adding small amounts of cobalt nitrate each time is critical. The cobalt nitrate concentration must remain very low so the solution does not become cloudy. See Figure 4, below, for a visual reference of how much cobalt nitrate to add at a time.

Figure 4. As shown in the picture above, only a small amount of cobalt nitrate should be added to the buffer at a time.

5. The cobalt-based catalyst will begin to electroplate onto the anodic (connected to + side of the battery) nickel electrode. As the catalyst film grows, you will see a brown film growing on the anode, and the voltage readout on the voltmeter/multimeter will slowly drop. Eventually the voltage will settle to a stable reading. Record this voltage readout. Also record how long it took, using either the stopwatch or clock, to reach a stable voltage reading.
   1. As the reaction takes place, you will see tiny bubbles forming on the nickel electrodes, similar to those in Figure 5 below.

Figure 5. In the beginning, as shown in photo on the left, there are no bubbles on the nickel electrodes. As the reaction proceeds, the gases formed can be seen as tiny bubbles covering the electrodes as shown in the photo on the right.

6. Once the voltage readout stabilizes, you can add more cobalt nitrate to the solution to initiate formation of more cobalt-based catalyst. Again, add only a small amount of cobalt nitrate at a time.
   1. Record in your lab notebook how long it takes the voltage to stabilize and what that final voltage reading is.
7. Repeat step 6 until the voltage does not appear to change with the addition of more cobalt nitrate. In this instance, the cobalt-based catalyst will continue to work, but no additional catalyst material will form.
8. Remove the phosphate buffer solution from the jar with the nickel electrodes (it will still contain some un-reacted cobalt nitrate) and replace it with fresh phosphate buffer. At this point you have finished forming the cobalt-based catalyst on the nickel electrodes. Measure and record the voltage one last time. The voltage readout in pure phosphate buffer reflects the operating potential of the electrochemical cell.
9. Analyze your data.
   1. Using Equation 4 and the information in Technical Note #2, calculate the final efficiency of the electrochemical cell with the cobalt-catalyst.
   2. Compare the original efficiency of the cell calculated in step 3 to the final efficiency. How much does the cobalt-based catalyst increase the efficiency of the electrochemical cell?
   3. How quickly did the cobalt-catalyst form?
   4. Plot the rate of increase in efficiency for the number of times you repeated step 6. Was the rate of efficiency increase constant?
10. Clean up and disposal.
    1. The phosphate buffer solution can safely be disposed of down the drain.
    2. The nickel electrodes can be saved with the cobalt-based catalyst still on them. Or, the nickel electrodes can be recovered and used again by rinsing them in cola as in step 4a, above, of the Creating the Galvanostatic Electrochemical Cell section.

Variations

• Consider adding a different resistor to your galvanostatic cell. Based on Technical Note #1, calculate the current that would pass through your cell. Add that resistor. What is the voltage now read by the voltmeter? You should stay within a range of 3mA to 30 μA (i.e. you should only use larger resisters than the 10K Ohms). Using a range of different resistors, you can determine the voltage required to run the electrochemical cell at each current delivered by the galvanostatic circuit. More information about this is provided in Technical Note #3.
• Can the water oxidation catalyst be made from metals other than cobalt? How well do these other metals work? You can purchase different metal salt compounds such as ones containing nickel, iron or manganese and try to add those for the electroplating procedure. You can compare the efficiencies of the electrochemical cells with various metal-based catalysts. How do they compare to the cobalt-based catalyst?
• In the Experimental Procedure above, electricity from batteries is used to split water. How would you change the setup to get the energy from sunlight instead of batteries?

• For more science project ideas in this area of science, see Chemistry Project Ideas.

Credits

Elizabeth R. Young, Ph.D., MIT

Yogesh Surendranath, MIT

Thomas Teets, MIT

Edited by Sandra Slutz, Ph.D., Science Buddies

Last edit date: 2011-11-04 12:00:00