Electrochemical & other Processes within a Fuel Cell

Question 1

Hydrogen Oxidation Rection

Answer 1

• proceeds very rapidly
• typically not considered a limitation
• happens at anode
• H2 -> 2H+ + 2e-
• has lossesHOR

Question 2

Oxygen Reduction Reaction

Answer 2

• “sluggish” -> O2 is comparatively stable
• very active catalyst needed to enhance reaction rate; tricky bc
  
  • 4 electrons need to be transferred
  • several adsorbed intermediate species involved -> adsorption sites can’t be perfect match for all
• voltage losses due to ORR very significant in low-temperature FCs
• High temperatures enhance the reaction kinetics
• at cathode
• 0.5O2 + 2H+ + 2e- -> H2O
• lots of efforts focusing on ORR catalyst research

Question 3

Mass Transport Mechanisms

Answer 3

• when high currents are drawn hydrogen at anode & oxygen at cathode is used -> needs to be continuously transported there
• Triple Phase boundary: electrolyte, catalyst/electrode, gaseous reactants
• product water needs to be removed -> can worsen reactant transport otherwise

Question 4

mass transport losses

Answer 4

• affected by anode, cathode, structural thickness; both rise
• ineffective transport of reactants to the catalytically active centers
• ineffective product removal
• experimentally determined
• simple model: m * exp(ni) (m&n experimentally determined)

Question 5

Ohmic losses in a fuel cell

Answer 5

• largely influenced by electrolyte thickness; both rise
• “inner resistance”
• resistances to charge transfer (proton/electron transport) -> follows Ohm’s law ΔUohm =iR =f(T)
  
  • ion transport through the electrolyte
  • electron flow through electronic conductors
• can become dominant in ceramic cells

Question 6

Activation losses

Answer 6

• reactions are kinetically hindered
• main contributor:
  - ORR as t is complex, multistep process
  - Electrode Kinetics
  - Catalyst Activity
  - Temperature
  - Electrode Surface Area
  - Overpotential
  - Electrolyte and Proton Conduction
• = ± a * log (i/i0)
  
  • a = Tafel slope
  • i0 exchange current density

Question 7

“internal current”

Answer 7

• gas crossover (mainly H2) -> mixed cathode potential
• radical formation
• remaining electronic conductivity of electrolyte
• relevant & hardly avoidable in low-temperature cells
• negligible in ceramic electrolyte cells

Question 8

Influence of losses on the FC voltage

Answer 8

• initially theoretical reversible cell voltage Urev
• Urev - internal current = open circuit voltage OCV
• OCV - activation losses - ohmic losses - mass transport losses = measured performance
• depending on the Fuel Cell the influence of each loss differs

Question 9

Operating point impacts

Answer 9

• fuel efficiency
• waste heat
• degradation behaviour
• important bc in aeronautical application high power density & specific power are necessary

Question 10

Effects of temperature changes on reversible potential & losses

Answer 10

• Urev: fuel cell reactions are exothermal -> T↑Urev↓
• 𝜂activation: temperature helps overcome activation barrier -> T↑𝜂act↓
• 𝜂ohmic: T↑ ionic conductivity↑, T↑ electronic conductivity↓; as ohmic losses typically dominated by losses arising fro ion transport -> T↑𝜂↓
• 𝜂masstransport: Differens effects depending on the dominant means of transport

Question 11

Effects of pressure changes on individual contributors

Answer 11

• Urev: Concentration change -> effects can be determined with Nernst equation
• 𝜂act: Different effects due to multistep reactions behaviour experimentally determined
• 𝜂ohmic: p↑𝜂ohmic roughly constant
• 𝜂mt: Gas concentration is increased with pressure -> p↑𝜂mt↓

Question 12

Take away messages

Answer 12

• measured voltage in a FC is combination of the reversible potential at the operating condition minus losses (overpotentials) arising from sluggish kinetics, limited mass transport & ohmic resistance
• depending on type of FC & operating point, relative share of these contributors differ
• ORR dominates kinetic losses
• polarisation curve is key for fuel cell performance assessment