Hanoi 6-6-2012

Seminar: Fuel cells

Class: Materials Science EngineeringTeacher : Phạm Ngọc Diệu QuỳnhStudent: Hoàng Văn Tiến

What is fuel cell ?

-A fuel cell is a device that converts the chemical energy from a fuel into electricity through a chemical reaction with oxygen or another oxidizing agent.

-Fuels: +Hydrogen ( the most common fuel.)

+Hydrocarbons : natural gas,alcohols ..…..

-Air pollution emissions almost “equal zero”.

design

A block diagram of a fuel cell

clasification

Based on types of electrolyte: polymer electrolyte membrand fuel

cell alkaline fuel cell phosphoric acid fuel cell molten carbonat fuel cell solid oxides fuel cell

Case study: polymer electrolite membran fuel cell (PEMFC)

reactions-Anode : H2 --- 2H+ +2e-

*E =0 VSHE (standard hidrogen electrode)

-Cathode: 1/2O2 +2H+ +2e- --->H2O

*E0=1.229 VSHE

Overall reaction: H2 +1/2O2 -- H2O

*E0=1.229 VSHE

Electrochemical Aspects

-The standard free energy change of the fuel cell reaction is indicated by the equation :

∆G = –nFE-

The value of ∆G corresponding :

*∆G= −229 kJ/mol,

*n = 2,

*F = 96500 C/g.mole electron, E = 1.229 V.

The enthalpy change ∆H for a fuel cell reaction:

∆H = –nFEt

-Nernst equation :

E = E0+ (RT/2F) ln [PH2/PH2O] + (RT/2F) ln [PO2 1/2]

-The maximum electrical work obtainable in a fuel cell operating at constant temperature and pressure is given by the change in the Gibbs free energy of the electrochemical reaction:

W = ∆G = –nFE

-The difference between ∆G and ∆H is proportional to the change in entropy ∆S:

∆G = ∆H – T∆S

Thermodynamic Principles

The effect of temperature and pressure on the cell potential:

− ∆V: change in volume,

∆S : entropy change,

E :cell poten-tial,

T:temperature,

P :reactant gas pressure,

n :the number of electrons transferred,

F: Faraday’s constant.

Fuel Cell Efficiency

the efficiency :

The ideal efficiency of a fuel cell operating irreversibly:

The thermal efficiency of an ideal fuel cell operating reversibly on pure hydrogen and oxygen at standard conditions:

The thermal efficiency of the fuel cell ( in terms of the actual cell voltage):

Based on the higher heating value of hydrogen:

Advantages and disadvantages

Applicaions-Power:

Type 212 submarine with fuel cell propulsion of the German Navy in dry dock

Other applications

Providing power for base stations or cell sites Distributed generation An uninterrupted power supply (UPS) Base load power plants Fuel cell APU for Refuse Collection Vehicle Hybrid vehicles, pairing the fuel cell with either an ICE or

a battery. Notebook computers for applications where AC charging

may not be readily available. Portable charging docks for small electronics (e.g. a belt

clip that charges your cell phones or PDA). Smartphones, laptops and tablets. Small heating appliances.

References

www.wikipedia.org Nice, Karim and

Strickland,Jonathan. "How Fuel Cells Work: Polymer Exchange Membrane Fuel Cells". How Stuff Works, accessed August 4, 2011

www.fuelcells.org www.fuelcellenergy.com onlinelibrary.wiley.com