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Lecture 6. Mass Transport Diffusive Transport • Reaction drives transport • Limiting current density • Concentration affects Nernst voltage • Concentration affects reaction rate • Concentration loss explained on j-V curve
Fuel Cell Performance Curve Reversible Voltage (Chapter 2) Activation Loss (Lecture 4) Ohmic Loss (Lecture 5) Cell voltage(V) Cell voltage(V) Cell voltage(V) Current Density (A/cm2) Current Density (A/cm2) Current Density (A/cm2) Concentration Loss (Lecture 6) Net Fuel Cell Performance Cell voltage(V) Cell voltage(V) Current Density (A/cm2) Current Density (A/cm2)
Convection and Diffusion a) Convection b) Diffusion
Flow Channel GDL H2 H2 H+ c0H2 DiffusionLayer Concentration c*H2 Flow Channel Electrode Distance Both Convection and Diffusion Are Important in FCs H2 O2 Anode Electrolyte Cathode
Reaction Drives Diffusion Flow Channel GDL Catalyst Electrolyte Reactants (R) In JR jrxn JP Products (P) Out Reaction in catalyst layer consumes R, generates P c0R JR c*P Concentration JP c0P c*R d
c*P c*R Reaction Drives Diffusion CatalystLayer Flow Channel Anode Electrode c0R Concentration c0P d Distance
Limiting Current Density • High • Large • Small
Concentration Affects Nernst Voltage when j << jL, is negligible; when j → jL, is increase sharply.
Concentration Affects Reaction Rate j We only need to concentrate on the high-current density region where the concentration effects become pronounced. j =
Concentration Affects Reaction Rate = From concentration effects On Nernst voltage we have: + Combine both effects: = c
1.2 Cell voltage(V) Reactant depletion (C*R < C0R) yields concentration loss (ηconc) Theoretical EMF or Ideal voltage Concentration Loss jL = 1.0A jL = 1.5A jL = 2.0A 0.5 1.0 2.0 Current (A)
Recap • Convection vs. Diffusion • Convection dominates in the flow channels • Diffusion dominates in the electrode/catalyst (GDL) • Reactant depletion leads to jL = limiting current density • jL = max operating j for fuel cell; determined when CR* = 0 • jL = nFDeff(CR0/d) • High jL = GOOD • hconc = Loss due to CR* < CR0 • hconc = cln[jL/(jL-j)] • Decrease hconc by increasing jL