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##### Lecture 6. Mass Transport

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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