Download Lecture #8 Fuel Cells and Hydrogen File

PHYS-C6370 - Fundamentals
of New Energy Sources
Fuel cells and hydrogen
Janne Halme
University Lecturer, Department of applied physics
Main reference: Ryan O’Hayre et al: Fuel Cell Fundamentals. Wiley, 2006.
Slides: Peter Lund and Janne Halme
9.11.2016
Why H2 - relevance?
Hydrogen is not an energy source but:
• a clean energy carrier
• produced from all primary
energy sources
• can be converted to all final
energy forms
• important industry raw
material
”Hydrogen economy”?
Why H2 – abundance ?
• Hydrogen is abundantly available in
compounds, but not in free form; e.g. water,
hydrocarbons, alcohols, chemicals
― Covalent bond: H2O, CH4
― Hydrogen bond: H2O molecules, DNA,
proteins
― Hydrid compounds with electropositive
elements (H-): MeHx
― 1 m3 of: water= 111 kg H2 , methanol = 100
kg, LH2 =71 kg
• Fuels with H2:
― Hydrocarbons: light HCs are gases and heavy
5-12 C-atoms liquids
― Alcohols: OH connects to HCs
Hydrogen storage - general
• Storage properties of hydrogen
– Density 0.08 kg/m3, heating value 120 MJ/kg (33.3 kWh/kg)
– Volumetric energy density
• STP 2.7 kWh/m3
• If 200 bar 500 kWh/m3; 1000 bar 2700 kWh/m3
• For comparison: gasoline 8,000 kWh/m3, 42 MJ/kg
• Ways to store hydrogen
– Compressed gas (high p< 1000 bar)
– LH2 (cryogenic temperature -252.8°C)
– Material based storage (on material surfaces or inside materials)
What is a fuel cell ?
• A fuel cell is an electrochemical device that converts chemical energy from
a fuel into electrical energy without any moving parts
• Fuel cells are operationally equivalent to a battery, but the reactants or
fuel in a fuel cell can be replaced unlike a standard disposable or
rechargeable battery
Fuel cell advantages and
disadvantages
• Advantages:
– More efficient than combustion engines
– Power and capacity can easily be scaled
– No moving parts, silent, no emissions
• Disadvantages
– Costs
– Volumetric power density poor,
gravimetric power density better
– Fuel (e.g. Hydrogen)
– Several operational issues
Examples of applications
• Mobile, stationary and portable power applications
• Power range from mWs to few hundred kWs
Chemical energy release –
reconfiguring of bonds
•
•
•
•
Atoms are connected through bonds that lower their total energy
Bond is formed  energy is released
Bond is broken  energy is absorbed
Net release of energy : energy released > energy absorbed
Simple combustion reaction
• Basic combustion equation: H2 + ½ O2
H2O + heat
• Collision of molecules  O2 and H2 bonds break  New H2O bonds formed
 Energy of new configuration lower  Heat released
• Reconfiguration of bonds involves fast electron transfer;
Q: how can we slow the e- transfer from fuel species to oxidant species ?
A: separate reactants so that electron reconfiguration is much slower
Physical principle of a fuel cell
• In a fuel cell, electrons are forced to move through
an external circuit before completing the reaction
(i.e. reconfiguring the bonds)
• How ? An electrolyte is employed to allow ions (e.g.
H+, O2-) but not electrons (e-) to flow
• Electrolyte = ionic conductor
• A simple fuel cells has two electrodes (for both half
reactions) and an electrolyte
• An ionically permeable membrane may be used to
keep the gases separate
Gas
separation
Basic operation of FC
• Reaction area determines the current (electricity)
production
 large areas lead to large current
 maximize surface-to-volume
 thin and porous structures
• Anode = oxidation reaction (electrons liberated)
• Cathode = reduction reaction (electrons consumed)
• Good gas access necessary; oxidant (air) and reactant
(fuel) separated by the electrolyte
Major steps in a fuel cell
1.
2.
3.
4.
Flow field plates (channels, groves) distribute the reactants over the
electrodes
Fast electrochemical reactions result in high current; catalysts
needed; kinetics is a limiting factor
Charge balance requires ion transport (by hopping), slow and losses
 thin electrolyte preferred
Product removal. Similar to 1)
Fuel cell types
• Fuel cells are distinguished based on the electrolyte used
• All have same underlying operation principle, but operate at different
temperatures, use different materials, differ in performance, etc.
• Most important fuel cells are: Polymer electrolyte membrane fuel cell (PEMFC)
and solid oxide fuel cell (SOFC).
SOFC
Fuel cell general characteristics
Peter Lund 2013
PEM Fuel Cell
Anode:
Cathode:
Overall:
• The most popular fuel cell type used in mobile,
stationary and portable applications (1 mW-100
kW); low temperature operation (<<100 oC)
• Solid electrolyte (polymer) that requires water to
make H+ conductive; slow electrochemistry on the
cathode (air) requiring a Pt-catalyst
• Mass and heat flow management important
Peter Lund 2013
Fuel cell components (example: PEMFC)
System
Stack
Cell
Fuel cell components (example: PEMFC)
Membrane electrode assembly (MEA)
Reaction sites at the
electrodes (threephase boundary)
MEA
SOFC Fuel Cell
Anode:
Cathode:
Overall:
• SOFC= high temperature fuel cells (>700 oC);
ceramic electrolyte with T-dependent ion
conductivity
• High T  flexible to fuels, no catalysts
• High T  material problems, slow response
• Well suitable for co-generation
Fuel cell performance
• Fuel cell performance is described by the current-voltage (I-V) curve
• Normalized : current density mA/cm2, power density W/cm2 (kW/L, W/kg))
• Ideal thermodynamical voltage versus real voltage with loss mechanisms
– Activation losses (electrochemical reactions, kinetics)
– Ohmic losses (ionic and electronic conduction)
– Concentration losses ( mass transport)
Thermodynamic open circuit voltage
Linking Gibbs energy to electric work
• A change in Gibbs free energy is
– G= U – TS + pV  dG = dU – TdS – SdT + p dV + V dp
– dU = TdS – dW = T dS – p dV - dWelec ; insert above
 dG=-S dT + V dp – dWelec
– Assume constant pressure and temperature  dG= – dWelec
 Welec = - ∆g rxn
(per mole basis, kJ/mol)
(maximum amount of electric work that can be extracted from a
chemical reaction/fuel at constant conditions)
Linking Gibbs energy and work to voltage
• What is electric work Welec ?
– If charge is q and potential difference is E, then qE is the work needed to
move the charge, i.e. Welec = qE = nFE , if the charge carries n electrons;
F=Faraday’s constant
– From Welec = - ∆g rxn follows that ∆g= - nFE
–  E = -∆g /(nF); n is the number of electrons involved in the reaction
• Example H2+ ½O2  H2O
– ∆g rxn = -237 kJ/mol (STP)
 E = -∆g /(nF) = 237 kJ/mol /( 2 mol e-/mol × 96400 C/mol) = 1.23 V
• Example CH4+ 4O2  2 H2O + CO2
– ∆g rxn = -763 kJ/mol (STP)
 E = -∆g /(nF) = 763 kJ/mol /( 8 mol e-/mol × 96400 C/mol) = 0.99 V
Efficiency of a fuel cell
• Heat engine limited by the Carnot efficiency
– ηI= energy produced/∆H
– ηII=1-T2/T1
– In practice η= 30-60%
(1st law)
(2nd law)
• In a fuel cell, ∆H turned into ∆G (=electricity)
– Max ηI = ∆G/∆H
(1st law)
– ∆G/∆H ≈ 80-95% (T=25 C)
• Example H2+ ½O2  H2O
– ∆g rxn = -237 kJ/mol (STP)
– ∆h rxn(HHV) = -286 kJ/mol (STP)
 ηthermo= -237/-286 = 83%
Enthalphy
= ∆H
Chemical
energy
Heat
Mechanical
energy
Electricity
Practical fuel cell efficiency
• Ideal Efficiency = Useful energy ÷ Total energy (=work/enthalpy)
• Ideal efficiency ηthermo = ∆g/∆h
• Main loss mechanisms: Voltage loss and fuel utilization losses
– ηvoltage = V/Ethermo
– ηfuel = (i/nF)/υfuel = 1/λ
λ= stochiometric factor (λ>1 overstochiometric)
υ=injection rate of fuel (mol/sec)
i = current generated (A)
• Practical efficiency:
– ηreal = ∆g/∆h × V/E × 1/λ
Activation losses
Activation losses are caused by
slow electrode kinetics
•
•
Electrochemical reactions (≠ chemical reactions) involves charge transfer
between an electrode and a chemical species;
Heterogeneous reactions
−
−
•
surface-limited reaction taking place at the electrode/electrolyte interface
E.g. electrons cannot exist inside the electrolyte, nor H+ inside electrodes
The rate of the electrochemical reactions determine the current i
Electrode kinetics are limited by
an activation barrier
Reaction rate is finite
•
•
Current is limited due to the activation barrier ∆G which is
due to the minimum energy path of hydrogen H2 2H++eProbability to cross the barrier depends exponentially on the
barrier height.
Net current across the interface is the sum
of forward and reverse reaction currents
• At equilibrium j1 = j2= j0;
• With η away from equil.
• Net current = j1-j2
Butler-Volmer equation
• Accounting for surface concentrations (rate limitations)
(R=reactancts, P=products)
No concentration limitations:
With concentration limitations:
Fuel cell kinetics depends on fuel
cell type (1)
• Hydrogen oxidation is often fast, oxygen reduction slow
Kinetics can be improved by
catalyst and electrode design
• Triple phase boundary (TPB): electrolyte, gas, catalyst in contact
• Pt most important catalyst (PEM), high-T SOFC nickel, ceramic cat.
• Electrodes 100-400 µm, catalyst layers 10-50 µm
Charge transport causes ohmic
losses
Effect of ohmic losses on fuel cell
performance
R= ∑ Ri
Mass transport constraints in a
fuel cell
Diffusion of the reactants
• Electrochemical reactions
consume reactants on the
electrode-electrolyte surface 
surface concentration of
reactants drops and reaction
products increase
– cR*< cR0 and cP*> cP0
(0=concentrations in flow
channels)
• Two consequences:
1) Nernstian losses
2) Reaction rate (activation) losses
(Butler-Volmer eq)
Flow channel design
• Flow channel patterns:
– Parellel flow
– Serpentine (series) flow
– Interdigitated flow
Fuel cell performance summary
Fuel cell microgeneration
• Distributed combined heat and
power plants (1-100 kW)
ALSTOM-Ballard P2B (212kW PEMFC ηe = 34 %)
• Buildings a good application
• Different fuels: biogas, bioalcohols,
natural gas, hydrogen
Siemens-Westinghouse (100kW SOFC ηe = 45 %)
(Lähde: Laurikko,VTT)
Residential Use PEMFC Cogeneration
System:
Matsushita Electric
(Collaboration with
OSAKA GAS)
Sanyo Electric
(JGA Project)
Fuel cells in transport