CHEM 3310 Fuel Cell Change in Gibbs Free

CHEM 3310 Fuel Cell

Change in Gibbs Free Energy, G G is the free energy that is available to do useful work such as electrochemical work. G = - n. FE G < 0 Go = - n. FEo An electrochemical cell is able do work if its cell potential, E, is positive. where n is the number of moles of electrons that flow in the cell, F is Faraday’s constant, 96500 coulombs / mole of electrons, Eo is the standard voltage measured of the electrochemical cell. CHEM 3310 2

Change in Gibbs Free Energy, G G is the free energy that is available to do useful work such as electrochemical work. Go < 0 An electrochemical cell is able do work if its cell potential, Eo, is positive. Go = - n. FEo where n is the number of moles of electrons that flow in the cell, F is Faraday’s constant, 96500 coulombs / mole of electrons, Eo is the standard voltage measured of the electrochemical cell. CHEM 3310 3

Example: Hydrogen fuel cell Use solar energy to break apart water Solar panel CHEM 3310 Electrolyzer to generate H 2 and O 2 as fuel for the fuel cell. Hydroge n Fuel cell Load box 4

Example: Hydrogen fuel cell Electrolyzer Solar panel Hydroge n Fuel cell Load box Eo=1. 23 V Anode: 2 H 2 (g) 4 H+ (g) + 4 e- Cathode: O 2 (g) + 4 H+ (g) + 4 e- 2 H 2 O (l) CHEM 3310 2 H 2 (g) + O 2 (g) 2 H 2 O (l) Eo = 1. 23 V 5

Part A: The Electrolyzer Theoretical decomposition voltage of water 2 H 2 O (l) O 2 (g) + 2 H 2 (g) O 2 (g) E° = - 1. 23 V H 2 (g) In practice, the external voltage applied to split water always exceeds 1. 23 V. The difference between theoretical decomposition voltage and the actual decomposition voltage is called overpotential or overvoltage. CHEM 3310 6

Part A: The Electrolyzer 2 H 2 O (l) O 2 (g) + 2 H 2 (g) E° = - 1. 23 V Adjust the power supply to obtain, roughly, the following current readings: 0. 01, 0. 02, 0. 03, 0. 04, 0. 05, 0. 06, 0. 08, 0. 10, 0. 15, 0. 20, 0. 25, 0. 30, 0. 35, 0. 40, 0. 45 A CHEM 3310 7

Part A: The Electrolyzer 2 H 2 O (l) O 2 (g) + 2 H 2 (g) E = - 1. 23 V NOTE: Do NOT let the current Exceed 0. 5 Amp! Will damage the electrolyzer Overpotential = V – 1. 23 CHEM 3310 8

Part B: The Faraday Efficiency of the Electrolyzer Measure the Volume of H 2 (experimental) produced in approximately 180 s. CHEM 3310 9

Part B: The Faraday Efficiency, ŋ, of the Electrolyzer Cathode: 4 H+ (g) + 4 e- 2 H 2 (g) Anode: 2 H 2 O (l) O 2 (g) + 4 H+ (g) + 4 e 2 H 2 O (l) O 2 (g) + 2 H 2 (g) 2 moles of electrons per mole of H 2 O E = - 1. 23 V . • One mole of electrons has a charge equal to 96500 coulombs. • At 20 o. C, the molar volume of H 2 (g) is 24000 m. L. Determine ŋ for the each of the two currents. CHEM 3310 In commercial electrolyzers, the Faraday efficiency must be close to 1 (i. e. 100%). 10

Part C - The Characteristic Curve of the Hydrogen Fuel Cell 2 H 2 (g) 4 H+ (g) + 4 e- O 2 (g) + 4 H+ (g) + 4 e- 2 H 2 O (l) Nafion. TM PEM Two platinum coated carbon electrodes bonded to a Proton Exchange Membrane (PEM) CHEM 3310 11

Part C - The Characteristic Curve of the Hydrogen Fuel Cell 2 H 2 (g) 4 H+ (g) + 4 e- O 2 (g) + 4 H+ (g) + 4 e- 2 H 2 O (l) CHEM 3310 12

Part C - The Characteristic Curve of the Hydrogen Fuel Cell • Record the voltage and current of the cell when the cell is subjected to following loads: 200 , 100 , 50 , 10 , 5 , 3 , and 1 ; Lamp; Motor CHEM 3310 13

Part C - The Characteristic Curve of the Hydrogen Fuel Cell • Record the voltage and current of the cell when the cell is subjected to following loads: 200 , 100 , 50 , 10 , 5 , 3 , and 1 ; Lamp; Motor CHEM 3310 14

Part C - The Characteristic Curve of the Hydrogen Fuel Cell Power = Voltage Current CHEM 3310 15

Other fuel cell: Methanol fuel cell Anode: CH 3 OH(l) + H 2 O(l) CO 2(g) + 6 H+ (g) + 6 e. Cathode: O 2(g) + 4 H+(g) + 4 e- 2 H 2 O(l) Overall cell reaction: CH 3 OH(l) + 1. 5 O 2(g) CO 2(g) + 2 H 2 O(l) E° = 1. 21 V At the anode methanol is supplied. At the cathode, oxygen from air is fed in. CHEM 3310 16

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