This problem has several parts: a. If a portable electronic device draws 1 A at

Question

This problem has several parts:

a. If a portable electronic device draws 1 A at a voltage of 2.5 V, what is the power requirement for the device?

b. You have designed a fuel cell that delivers 1 A at 0.5 V. How many of your fuel cells are required to supply the above portable electronic device with its necessary voltage and current requirements?

c. You want the electronic device to have a operating lifetime of 100 h. Assuming 100% fuel utilization, what is the minimum amount of H2 fuel required (in grams)?

d. If this H2 fuel is stored as a compressed gas at 500 atm, what volume would it occupy (assume ideal gas, room temperature)? If it is stored as a metal hydride at 5 wt% hydrogen, what volume would it occupy? Assume the metal hydride has a density of 10 g cm3 .

Explanation / Answer

a) Power = Voltage X Current = 2.5V X 1 amp = 2.5 Watt

b) The power obtained from each cell = 0.5 X 1 amp = 0.5 Watt

The required power = 2.5 watt

So number of cells required = .5 / 0.5 = 5

c) In stacking the cells in series the current will remain the same

Now for 2.5 watt of power , we have to supply 2.5J/s of hydrogen

The amount of hydrogen per cell = NH2 = i /nF

Charge needed = Current X time = 1 amp x 100 X 60 X 60 seconds = 3600,00 C

Charge in Faraday = 3600,00 / 96500 = 3.731 Faraday

For each moles of Hydrogen we need = 2F of charge

so moles of hydrogen converted = 3.731 / 2 = 1.866

Mass of hydrogen = Moles x molecular weight = 1.866 x 2 = 3.731 grams

Now each fuel cell will use = 3.731 grams

So for stack of five = 5 X 3.731 = 18.655 grams

d) Pressure = 500atm

Temperature = 25C = 298 K

So for ideal gas

PV =nRT

V = nRT / P = 1.866 X 5 X 0.0821 (L atm / mol K ) X 29K / 500 atm = 0.457 L

If it is stored as a metal hydride at 5 wt% hydrogen, the hydrogen density = 10g /cm^3 X 5 / 100 = 0.5g/ cm^3

So for 18.655 grams the volume needed = Mass / Density = 18.655 / 0.5 = 37.31 cm^3

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