Energy%20Materials%20Fuel%20cell%202nd%20print

Energy%20Materials%20Fuel%20cell%202nd%20print - Last class...

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1 Last class - Introduction of fuel cells This class - Thermodynamics of fuel cells
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2 Review last class! 1. Fuel cells a) (open or closed) system b) Energy (storage or conversion) device c) Active materials are (solids or gases) d) they produce electricity, heat and water 2. What is this? a) A substance that participates in a reaction, increasing its rate, but is not consumed in the reaction. catalyst b) Individual fuel cells connected in series within a generating assembly. stack 3. Hydrogen can be stored in many ways. - liquid hydrogen in a special tank, chemical hydrides, metal hydrides
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3 4. Merits of Fuel cells : Environmentally benign- No pollution, Decreased dependence of fossil fuels, high efficiency 5. Challenges of Fuel cells : Hydrogen safety and storage and cost 6. In a fuel cell, hydrogen is oxidized at the anode and oxygen is reduced at the cathode. : H 2 2H + + 2e - at anode : 1/2O 2 + 2H + + 2e - H 2 O at cathode
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4 Internal energy (U) of H 2 : No movement in macroscopic view, but microscopic movement exist. • Kinetic energy – Molecular • Chemical (potential) energy – Bonding between atoms Thermodynamics of fuel cells A fuel cell converts energy stored within a fuel into electrical energy. The total intrinsic energy of a fuel is quantified by internal energy. Internal energy is the energy associated with microscopic movement and interaction of particles on the atomic scale.
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5 Laws of Thermodynamics First law: conservation of energy d(Energy) univ = d(Energy) system + d(Energy) surroundings = 0 d(Energy) system = -d(Energy) surroundings Q: heat, W: work, U: internal energy dU = dQ –dW Means that the change in the internal energy of a closed system (dU) is equal to the heat transferred to the system (dQ) minus the work done by the system (dW) Assumed that only mechanical work is done by a system, (dW) mech = pdV dU= dQ –pdV
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6 Second law: entropy : an expression of the universal principle of entropy, stating that the entropy of an isolated system which is not in equilibrium will tend to increase over time, approaching a maximum value at equilibrium Entropy of a system increases (dS univ 0) Microscopic: S = k log Ω (k: boltzmann’s constant Ω : No. of possible microstates accessible to the system) :Macroscopic: dS=dQ rev /T means that the entropy change dS of a system undergoing any infinitesimal reversible process is given by δ q / T, where δ q is the heat supplied to the system and T is the absolute temperature of the system.
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7 Entropy Example A perfect crystal of 100 atoms: only 1 microstate (configuration) S = k log Ω = k log 1 = 0 A crystal of 97 atoms with 100 lattice space Ω = 100 C 3 =1.7 * 10 5 S = 7.22 * 10 -23 J/K
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8 Thermodynamic potentials : a scalar function used to represent the thermodynamic state of a system. : rules about how energy can be transferred from one to
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Energy%20Materials%20Fuel%20cell%202nd%20print - Last class...

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