0.4 Hydrogen and fuel cells  (Page 2/4)

The chemistry of a fuel cell

A fuel cell is a galvanic cell in which electricity is generated by a combustion reaction. The fuel cell consists of two electrodes between which electrical contact is established by means of an electrolyte. Oxygen or just plain atmospheric air is fed continuously to the cathode and the fuel is fed continuously to the anode.

The fuel could be any of a vast number of combustible materials, e.g. methane, ethane or ethanol (all organic fuels) hydrogen, hydrazine or sodium borohydride (inorganic fuels). With the hydrogen burning cell as an example we can describe the chemistry of the cell by the following reactions:

Anode – at which oxidation of the fuel takes place:

H2 + 2OH- ->2H2O + 2e-

Cathode – at which reduction of oxygen takes places

½ O2 + H2O + 2e- ->2OH-

The next reaction for the cell:

H2 + ½ O2 ->H2O

With ethanol as the fuel the matter becomes somewhat more complicated, since ethanol is oxidized in steps to ethanal, ethanoic acid and carbon dioxide respectively. In an ideally working fuel cell we assume that ethanal and ethanoic acid are further oxidized so that the only carbon compound of the overall process is carbon dioxide. We have not succeed ( by simple chemical tests) to detect either ethanol or ethanoic acid (or rather ethanoate due to the strongly basic electrolyte solution) as intermediate products in our own cells. However we still suggest a three-step oxidation of ethanol(and at the same time admitting that the last step is dubious):

Anode:

Step 1: CH3CH2OH + 2 OH- ->CH3CHO + 2 H2O + 2e-

Step 2: CH3HO + 2 OH- ->CH3COOH + H2O + 2 e-

Step 3: CH3COOH +8 OH- ->2 CO2 + 6 H2O + 8 e-

Sum: CH3CH2OH + 12OH- ->2 CO2 + 9 H2O + 12 e-

Cathode:

3O2 + 6 H2O + 12 e- ->12OH-

Overall reaction:

CH3CH2OH + 3O2 ->2CO2 + 3H2O

Sodium borohydride can power a cell in either a direct or indirect manner. Indirectly sodium boroydride will decompose in water to produce NaBO2 (borax) and hydrogen

NaBH4 + 2H2O ->NaBO2 + 4H2

This hydrogen will then fuel the cell as shown above. However, sodium borohydride can directly power a cell with higher energy yields.

Anode:

NaBH4 + 8OH- ->NaBO2 + 6H2O + 8e-

Cathode:

2O2 + 4H2O + 8e- ->8OH-

While sodium borohydride costs ~$50 per kilogram, it has projected that mass production and borax recycling could reduce that price to as low as $1 per kilogram.

Experimental

Caution!!! plastic can burn.

To get good results, very careful measurements are required. Be sure to wear suitable eye protection.

Materials:

• 2X 50-60mL disposable hypodermic syringes without needles and pistons.
• 3X pieces of nickel net (2 cut to cover the flanges of the syringe cylinders approximately 2cm X 10cm + 1 extra piece) The net should be a very fine mesh.
• 2X machine screws with nuts and waters (all brass)
• 2X 20cm pieces of insulated 1mm copper wire with ~1.5cm insulation removed from each end
• Heating plate
• Aluminum plate 4-6mm thick with 7-8mm hole drilled through center
• Baking paper
• Screwdriver, drill, spanner, flat bit, scissors, wooden board and small saw
• tape
• Lab stand with clamps
• 600mL beakers
• 1.5V electric motor
• Red LED
• digital mulitimeter
• balloons
• electrical leads with alligator clips
• 1M sodium hydroxide solution
• 4M nitric acid
• ethanol
• methanol
• Palladium chloride solution (very expensive and should be recycled)
• NaBH4
• Oxygen gas
• Hydrogen gas