fuel cell fundamentals

Why do we use current density rather than current

Normalizes the effects of system size

Voltage loss caused by kinetic limitations

Activation loss

What relationship describes the connection between over voltage and current density

Butler volmer eqn

What does exchange current density measure

Equilibrium rate at which reactant and product are exchanged when no over-voltage is present

Four ways to increase exchange current density

1. Increase rxn concentration

2. Increase rxn temp

3. Decrease Ea by Add a catalyst

4. Increase # of rxn sites

Main requirements for an effective fuel cell catalyst

1. Activity

2. Conductivity

3. Stability

What does increasing the number of reaction sites per unit area mean

Maximizing triple phase boundary conditions where the elyte, rxt and catalytically active electrode phases meet

Why is the rate (current) proportional to surface area

Bc electrochemical reactions happen on the electrode surface

What is charge transport in fuel cells driven by

Voltage gradient

Charge transport aka conduction

Ohmic overvoltage

Voltage expended to drive conductive charge transport

Why are Area-specific resistances (ASRs) used for fuel cells design

Because resistance scales with area

Magnitude comparison in good electrolytes vs metals

In good electrolytes, ion conductivity is 4 to 8x lower than in metals

Requirements for a good electrolyte

1. High ionic conductivity

2. 
   

Stable in highly reducing and oxidizing regions

3 Major electrolyte classes in fuel cells

1. Liquid

2. Polymer

3. Ceramic

What is mobility/conductivity determined by in aqueous electrolytes

The balance between Ion acceleration and frictional drag (viscosity)

Smaller ion —> Greater charge —> great mobility

What determines the conductivity in nafion (polymer elyte)

Water content

What determines conductivity in a ceramic elyte

Defects in the crystal lattice

Introduce more defects via doping

Mixed ionic and electronic conductors (MEICs)

Conduct ions and electrons

Used for SOFCs

Improve reactivity by making 3phase boundaries into 2phase rxn zones