1) Cobalt Nitrate
2) 9V Battery (X4)
3) 22-gauge electrical wire
4) Alligator clips (X12)
5) Wire cutters and strippers
6) Breadboard, about 3" x 2"
7) 10K Ohm resistor
8) Voltmeter/Multimeter (must be able to read 10 millivolts)
9) Cola; any brand, such as Coke, Pepsi, or a generic brand, will work
10) Cup or jar that the nickel metal strips can completely fit inside (1); must be taller than 5 inches
11) 250 mL beaker
12) Nickel metal strips (2 of them); strips should be approximately 5 inches tall and ¾ inch wide
13) Small Styrofoam block
15) 0.1 M phosphate buffer solution, pH 7.0 (500ml)
This buffer should contain 2.63 g monopotassium phosphate (KH₂PO₄) (FW 136.09 g/mol) and 4.35 g sodium phosphate (Na₂HPO₄) (FW 141.96 g/mol) (to lower the pH to 7.0) and brought to 500 mL using deionized (DI) water for a total phosphate concentration of 0.1 M.
16) Magnetic stir plate and stir bar (1)
17) Pair of disposable gloves
18) Metal scoop for chemicals
1)Build a circuit on the breadboard consisting of the batteries, resistor, and voltmeter/multimeter, as shown in Figure 1 below.
2)Using Figure 1 as a visual reference, connect the four 9V batteries in series using some wire and 6 alligator clips.
- Cut each piece of wire to the desired length with the wire cutters.
- 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.
- Note: For some alligator clips, such as the ones in the Science Buddies kit, the stripped wire will need to be coiled around the alligator clips and then screwed in place. This is shown in Figure 2 below.
- Connect the batteries so that the negative end of one battery is connected to the positive end of the next battery in the series
Figure 2. If you are using alligator clips similar to these ones, you will need to coil the stripped wire around the alligator clip and then screw the wire in place. Note: This type of alligator clip is included in the Science Buddies kit.
- Using a piece of wire, connect the positive end of the series of batteries to the breadboard power bus
- In Figure 1, for the type of breadboard shown on the left this is the far left "+" column, and for the type of breadboard shown on the right this is the unmarked far left column.
- Connect the 10K Ohm resistor as shown in Figure 1. (Note: orientation does not matter.)
- For the breadboard shown on the left, one end of the resistor is connected to the same column as the positive end of the batteries, and the other end of the resistor is connected to position 1a.
- For the breadboard shown on the right, one end of the resistor is connected to the same row as the positive end of the batteries, and the other end of the resistor is connected to another position in the same column.
- Connect the positive (red) lead from the voltmeter/multimeter to the breadboard as shown in Figure 1.
- For both types of breadboards, the voltmeter/multimeter's positive lead is connected to a position in the same row as the end of the resistor (in the breadboard shown on the left, this is position 1c).
- Connect the negative (black) lead from the voltmeter/multimeter to the breadboard as shown in Figure 1.
- For both types of breadboards, the voltmeter/multimeter's negative lead is connected to a position on the right half of the breadboard. (In the breadboard on the left in Figure 1, position 7h is used.)
- If you are using the type of breadboard shown on the left in Figure 1, connect a small piece of wire between a position in the same row as the voltmeter/multimeter's negative lead (7j is used in Figure 1) and the ground bus (the far right "-" column).
- If you are using the type of breadboard shown on the right, skip this step.
- Using a piece of wire, attach the negative (-) end of the series of batteries to the ground bus (far right column) of the breadboard as shown in Figure 1.
- If you are using the type of breadboard shown on the right, make sure this is in the same row as the voltmeter/multimeter's negative lead.
- The circuit is now complete and should look like the circuit in Figure 1. Using the voltmeter/multimeter, make sure the circuit reads >30V.
- Use the nickel metal strips as electrodes. The nickel electrodes will serve as the scaffold for formation or electroplating of the cobalt catalyst.
- 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.
- If the jar is too small to immerse the electrodes, do the procedure once then flip the electrodes over (putting the end that was not previous immersed in the Cola) and repeat.
- Note: If you are using the Science Buddies kit for this project idea, use the 500 mL jar for this step.
- Construct a method to secure electrodes within a small beaker or jar that leaves 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 3 below using a piece of Styrofoam to hold the electrodes securely in place.
- It is important to ensure that the separation between the electrodes remains the same throughout the experiment. When securing the electrodes make sure to:
- Position the electrodes 1-2 centimeters (cm) apart.
- Make sure the electrodes are securely in place and not dangling freely or touching the sides of the beaker.
- Position the electrodes so that they will, later, once the buffer has been added to the beaker, only be immersed half way in the buffer. Note: It is critical that the top of the electrodes do not touch the buffer.
- Note: If you are using the Science Buddies kit for this project idea, use the blue Styrofoam block and 250 mL beaker in the kit for this step.
Figure 3. The photos above show one possible method for suspending the nickel electrodes in the beaker to make the electrochemical cell.
- Add 0.1 M phosphate buffer solution, pH 7.0, to the beaker with electrodes so that the nickel electrodes are submerged half way in the buffer solution.
- Note: If you only have 500 mL of phosphate buffer, the most you should fill the beaker with is 250 mL because in a later step you will need to use this same amount of phosphate buffer again.
- Place the stir bar in the bottom of the jar.
- Make sure that the electrodes are not so low that they will be bumped by the stir bar. If they are, raise them up until they are not.
- Connect the nickel electrodes to the rest of the circuit using copper wire and alligator clips as shown below in Figure 4.
- For both types of breadboards shown, one wire is connected to a position in the same row as the voltmeter/multimeter's positive lead, and the other wire is connected to a position in the same row as the voltmeter/multimeter's negative lead (on the other half of the breadboard).
- For example, for the breadboard shown on the left, one wire is connected to position 1e, and the other wire is connected to position 7f.
Figure 4. The completed galvanostatic electrochemical cell, using two different types of breadboards, is pictured in the top two pictures. One type of breadboard is shown in the left pictures, and a second type is shown in the right pictures. In the top left picture, the leads going up are connected to the nickel electrodes (not pictured). The bottom left picture shows a close-up of the breadboard shown in the top left picture. The bottom right image is a schematic of the completed galvanostatic electrochemical cell pictured in the top right.
Adding the Cobalt Catalyst and Measuring Its Effects
- With the electrodes securely in place inside the small beaker, place the beaker on the magnetic stir plate. Turn on the stir plate and get the stir bar moving at a constant rate.
- Make sure that the stir bar does not bump the electrodes. Adjust the electrodes if needed, but then keep them in the same position throughout the rest of the experiment.
- Monitor the voltage readout on the voltmeter/multimeter. It should range between 1.9-2.4v 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.
- Using Equation 4, below, and the information in Technical Note #2, calculate the baseline efficiency of the water splitting reaction in your electrochemical cell.
- Now it is time to add the reactant necessary to form the cobalt-based catalyst. Put on a pair of disposable gloves and, using the metal scoop, add a pinch of the cobalt nitrate to the jar with the 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.
- 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 5, below, for a visual reference of how much cobalt nitrate to add at a time.
|Figure 5. 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, after several minutes, 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.
- As the reaction takes place, you will see tiny bubbles forming on the nickel electrodes, similar to those in Figure 6 below.
Figure 6. In the beginning, as shown in the photo on the top, 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 bottom.
- 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 asmall amount of cobalt nitrate at a time.
- Record in your lab notebook how long it takes the voltage to stabilize again and what that final voltage reading is.
- 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.
- This may take a total of four or five additions of small amounts of cobalt nitrate, and with each addition it may take around 5 to 20 (or more) minutes for the voltage to stabilize.
- As you add more cobalt nitrate, how does the brown film on the anode change? Record any observations in your lab notebook.
- Fill a second beaker with the same amount of phosphate buffer solution that you put in the first beaker.
- Carefully remove the nickel electrodes from the phosphate buffer and put them into the second beaker that you just filled with fresh phosphate buffer.
- The beaker that the electrodes were in will still contain some un-reacted cobalt nitrate, so the electrodes should be transferred to a beaker with fresh phosphate buffer.
- Note: Be careful not to jostle the electrodes when transferring them to the new beaker. It is important that they remain in the same position relative to each other and stay the same distance apart or it could give you inaccurate results.
- At this point you have finished forming the cobalt-based catalyst on the nickel electrodes. Measure and record the stabilized voltage one last time. The voltage readout in pure phosphate buffer reflects the operating potential of the electrochemical cell.
- Analyze your data.
- Using Equation 4 and the information in Technical Note #2, calculate the final efficiency of the electrochemical cell with the cobalt-catalyst.
- 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?
- How quickly did the cobalt-catalyst form?
- Plot the rate of increase in efficiency for the number of times you repeated step 6. Was the rate of efficiency increase constant?
- Clean up and disposal.
- The phosphate buffer solution can safely be disposed of down the drain.
- 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.
By following steps 1-7 above you have constructed a simplified galvanostat. This means that the electrochemical cell passes the same current through the cell at all times, and the voltage read-out varies based on the efficiency or property of the electrodes.
The four 9-volt batteries generate a maximum of 36 (9v per battery x 4 batteries in series =36v). You typically will get less than 36 volts depending on how fresh the batteries are. The circuit is completed by two other resistors attached in series. One resistor is the electrochemical cell itself (the nickel electrodes in the phosphate buffer), and the other resistor is a 10,000 Ohm resistor you specifically place in series. This 10,000 Ohm resistor is critical to stabilize the electrochemical cell and ensure that a constant current is passed at all times. The reason for this is that most of the voltage (~30v) drops across the 10,000 Ohm resistor, and approximately 1.5-3v are dropped across the electrochemical cell. It is important to drop most of the voltage through the resistor because this will set the current that passes through the rest of circuit. Small variations in the electrochemical cell will have little effect because the 10,000 Ohm resistor is the dominant factor. Using Equation 3 below, and assuming that approximately 30 volts are dropped over the 10,000 Ohm resistor, the current can be calculated to be 3mA (30v / 10,000 Ohms = 0.003 A = 3mA). This calculation indicates there are 3mA of current flowing through the electrochemical cell.
The voltage readout you measure in step 2, above, is the voltage required by the electrochemical cell to maintain a constant current of 3mA (see Technical Note #1 to learn how the current was calculated). This voltage is the sum of the energy required to drive the water-splitting reaction 1.23v + overpotential) and any resistive losses in the cell. If there were no resistive losses, and the water splitting reaction was completely efficient, the necessary voltage to maintain the 3mA current would be 1.23v. From the Introduction, you already know that the water-splitting reaction is not completely efficient and instead has a significant overpotential. In the steps below, you are watching the overpotential drop as the cobalt catalyst is electroplated onto the nickel electrode (specifically, the anode). This drop in overpotential is reflected in a drop in the voltage you measure across the circuit. As the overpotential required by the cell decreases (through addition of the catalyst), the voltage necessary to maintain the 3mA current also decreases. This indicates that you are running the water splitting reaction closer to the absolute, ideal limit. The efficiency of the reaction can be calculated using Equation 4 below.
If the reaction was 100% efficient, it would require only 1.23v to maintain the 3mA of current passing through the cell. The voltmeter/multimeter would thus read 1.23v. As an example, let us imagine that the initial voltmeter/multimeter reading (in step 2 above) for your electrochemical cell was 2.46v. According to Equation 4, this would mean that without the catalyst, the reaction was 50% efficient (1.23v / 2.46v = 0.50 = 50%). If voltage readings after the addition of the catalyst dropped to 2.12v, then the efficiency would rise to 58% (1.23v / 2.12v = 0.58 = 58%).
Above, we saw that the efficiency of the reaction is determined by the voltage drop across the electrochemical cell. Higher voltages lead to lower efficiency. But what about the speed at which we produce hydrogen and oxygen? Above, we were running our cell at 3mA, and this current is directly proportional to the rate of hydrogen and oxygen production. What if we slow this rate to 1mA, 0.1mA, or 0.01mA? How would that affect the voltage? Knowing the relationship between the current (rate) and the voltage (energy input) is essential to designing water splitting systems that are practical. As described in Variation 1, change the resistor so that the current is varied between 3mA and 30mA. Plot the cell voltage as a function of the log of the current passed in the cell. Do you get a straight line? What is the slope? Repeat this experiment with catalysts formed for different metal salts (see Variation 2). Plot all of your results on the same graph to determine which catalyst performs the best. Remember that lower voltages and higher currents equal better catalysts.