Chapter 2: Fuel Cells Flashcards
What is a ‘Fuel Cell’?
A Fuel cell is an electrochemical energy converter. It
converts chemical energy of fuel (typically H2) directly
into electrical energy. As such, it must obey the laws of
thermodynamics.
What is a ‘Galvanic Cell’?
A galvanic cell consists of at least two half cells, a reduction
cell and an oxidation cell Chemical reactions in the two half
cells provide the energy for the galvanic cell operations
Each half cell consists of an electrode and an electrolyte
solution. Usually the solution contains ions derived from the
electrode by oxidation or reduction reactions
Galvanic cell = Voltaic cell
The spontaneous reactions provide the electric energy or current.
Explain the ‘Daniell cell’
Anode (oxidation) : Zn (s) ==> Zn^(2+) (aq)+ 2e^-
Cathode (reduction): Cu^(2+) (aq) + 2e^- ==> Cu (s)
Overall Reaction: Zn (s) + Cu^(2+) (aq) ==> Zn^(2+) (aq) + Cu (s)
What is ‘EMF’ ?
Electromotive force, ΔE
The maximum potential difference between two electrodes of a galvanic (or voltaic) cell when no current is flowing.
ΔE= (E _2) - (E_1)
The SI unit of E is the volt, symbol V.
This quantity is related to the tendency for an element, compound or an ion to acquire (i.e gain) or release (loss) electrons.
Standard Hydrogen Electrode, SHE
a reference electrode defined as having zero electrode potential.
It consists of a platinum electrode in contact with gaseous H2 (p = 1atm) and aqueous hydronium (H3O^+) (a = 100)
Magnitude of a redox reaction
The magnitude of a redox potential is a measure of the relative strength of an oxidant or reductant.
The more positive the redox the stronger the the oxidant and the weaker the reductant (and vice versa)
Proton Exchange Membrane Fuel Cell (Reactions)
Anode: 2H2 ==> (4H^+) + (4e^-), E^0 = 0 V
Cathode: O2 + (4H^+) + (4e^-)==> 2H2O, E^0 = 1.23 V
Overall reaction:
2H2 + O2 ==> 2H2O
Anode Catalyst: Pt (fast)
Cathode catalyst: Ni, Pt and PtNi alloys except Pt3Ni (slow)
Why does voltage decrease as current increases?
- Activation losses
- Ohmic losses
- Mass transport losses
A typical fuel cell produces a voltage from 0.6 V to 0.7 V at full rated load
What is ‘ V (i)’
= Net output voltage of a fuel cell at a certain current density ,
= (V_rev) - (V_irrev)
V_rev = E_r = maximum (reversible) voltage of the fuel cell
V_irrev = Irriversible voltage loss “Overpotential”
V_irrev = v_act + v_ohmic + v_conc
v_act = activation overpotential
v_ohmic = Ohmic overpotential
v_coc = concentration overpotential
What is ‘OCP’?
“Open circuit potential”
When the fuel cell is supplied with reactant gases but the electrical circuit is not closed.
The cell potential is not close to the theoretical cell potential, OCP is significantly lower (< 1V) Losses even when no external current is generated
What happens when the electrical circuit is closed with a load?
- The potential drops even further
- Voltage losses
- Kinetics of electrochemical reactions
- internal electrical and ionic resistance
- Difficulties in getting the reactants to reaction sites
- Internal (stray) currents
- Crossover of reactants
What is the polarization of a fuel cell?
Definition: The drop from open circuit voltage as a result of drawing electric current
Activation polarization:
- Sluggish electrode kinetics
- The higher the exchange current density ==> The lower the activation polarization losses
O2 reduction requires much higher overpotentials ==> Much slower reaction than H2 oxidation
Activation losses are represented by:
ΔV = a + b*log(i) (Tafel equation)
a = (-2.3) * RT/αF * log(i_0)
b = 2.3 (RT/αF)
i = current density
α = charge transfer coefficient
i_0 = reaction exchange current density (= reaction rate)
Ohmic (resistive) losses:
- These losses occur because of resistance to the flow of ions and electrons in the electrolyte and through the electrically conductive fuel cells components, respectively
ΔV_ohm = i * R_i
R_i = R_i,i + R_i,e + R_i,c
R_i,e = negligible
R_i,i, R_i,c = same order of magnitude
i = current density , A/cm^2
R_i = Total cell internal (ionic, electronic, and contact) resistance, ohms cm^2
R_i = 0.1 - 0.2 ohm cm^2
Concentration polarization
These losses occur because when a reactant is rapidly consumed at the electrode by the electrochemical reaction so that the concentrations gradients are established.
ΔV = (RT/nF) * ln(C_B/C_S)
C_B = bulk conc. of reactant, mol/cm^3
C_S = conc. of reactant at the surface of catalyst, com/cm^3
- Based on ‘Ficks law’ the flux of reactant is proportional to concentration gradient
N = (D* (C_b - C_S))/ δ ) * A
N = flux of reactants, mol/s
D = diffusion coefficient of reacting species,
cm^2 /s
A = electrode active area, cm^2
δ = diffusion distance, cm
In steady state, the rate at which the reactant species is consumed in the electrochemical reaction is equal to the diffusion flux
N = i/nF from i = nFj
nF= charge transferred, Coulombs/mol
j= flux of reactant per unit area, mol/s cm^2
The reactant conc. at the catalyst thus depends on current density: the higher the current density, the lower the surface concentration
The surface conc reaches zero when the rate of cosumption exceeds the diffusion rate, i.e the reactant is consumed faster than it can reach the surface. The current density at which this happens is called ‘ limiting current density’
Therefore, for C_s =0, i = i_L and the limiting current density is
i_L = nFD*C_B / δ
And combing previous equations
ΔV_ conc = RT/nF * ln( i_L / i_L -i )
However, the limiting current is almost never experienced in fuel cells because of non - uniform conditions over the pourous electrode surface.
The polymer membrane is …
- not electrically conductive (only ions pass through)
- Impermeable to reactant gases
Even though the rates of hydrogen permeation and electron crossover are several orders of magnitude lower than hydrogen consumption rate, these losses have dramatic effect when the fuel cell is at OCP or operates at very low current densities
Fuel cell combined in series:
- Yields higher voltages
Fuel cell combined in parallel:
- Allows higher currents to be supplied
Fuel cell stack;
Fuel cells combined in series and parallel
The cell surface area can be increased, to allow higher current from each cell
The main components of ‘ Proton Exchange Membrane Fuel Cells’ PEMFCs
- Electrolyte (Usually defines the type of fuel cell)
- Fuel being utilised (H2 in this case on slides)
- Catalysts at the anode (usually noble metal such as Pt powder)
- Catalysts at the cathode (Usually Ni powder)
- Proton conductive polymer membrane
Electrode characteristics
Must be Pourous: Reactant gases are fed from the back and they must reach the electrode/membrane interface
Reactant gases reach the ‘ Catalyst layers’
‘Catalyst layers’ = part of porous electrode or part of the membrane (depending on manufacturing process)
what does ‘MEA’ stand for and what is it?
Multilayer Electrode Assembly = multilayer assembly of the membrane sandwiched between two electrodes
“MEA” = assembly stack of (i) proton conductive membrane + (ii) catalyst electrodes + (iii) gas diffusion layers
MEA is sandwiched between collector/separator plates
“collector” = collect and conduct electrical current
“separator” = they separate gases in the adjacent cells (in multicell configuration)
In a multicell configuration they connect (physically and electrically) the cathode of one cell with the anode of an adjacent cell ==> Bipolar Plates
What are “Bipolar plates”
Bipolar plates are conductive plates in a fuel cell stack that act as an anode for one cell and as a cathode for the next cell
Bipolar plates can be made of metal, carbon or conductive composite polymer (usually incorporating carbon). Polymer (plastic plates are also in development)
It must evenly distribute reactant gases over the surface of the anode, and O2 / air over the cathode
Bipolar plates may also need to carry a cooling fluid, and in addition, need to keep all these gases and cooling fluids separate.
Properties of bipolar plates
- The must be electrically conductive so they can connect cells electrically in series
- They must be impermeable to gases so they can separate gases in adjacent cells
- They must have adequate strength but must also be light weight so they can provide structural support for the stack
- They must be thermally conductive so they can conduct heat from active cells to the cooling cells
- They must be conformable so they can house the flow field channel
They must be corrosion resistant, cheap, and suitable for mass production
The electrical contacts should be as large as possible
The plate should be thin to minimize resistance
Gas needs to flow easily across the plate
They must be able to provide the pathways for the flow of reactant bases (‘flow fields’) but also cell structural rigidity
What are ‘GLD’ Gas diffusion layers?
The GDL is a pourous material composed of a dense array of carbon fibres, which also provides an electrically conductive for current collection
Role of the GDL
- Provide a pathway for reactant gases from the flow field to the catalyst layer ==> Gases need to access the entire active area, not just those adjacent to the channels
- Provide a pathway for the product water from the catalyst layer to the flow field channels :
Electrically connecting the catalyst layer to the bipolar plate, allowing electrons to complete the circuit
- Conducting heat generated in the electrochemical reactions in the catalyst layer to the bipolar plate, allowing heat removal:
Provide mechanical support to MEA, preventing it from sagging into the flow field channels
Properties of GDL
Must be sufficently porous ==> Flow of reactant gases and water (these fluxes are in opposite direction) Design of the flow is important, especially diffusion in both through-plane and in-plane
Must be electrically and thermally conductive ==> Interfacial (contact) resistance (in both through-plane and in-plane) is important
The catalyst is made of discrete small particles ==> The pores of GDL facing the catalyst must not be too big
Must be suffiecently rigid to support the MEA but having some flexibility to maintain good electrical contacts ==> carbon fibre based materials
The GDL must provide a pathway for excess water removal
excess water ==> flooding ==> fuel starvation
GDL is commonly made hydrophobic to enhance water removal, in practice, flooding is still a major cause of PEMFC performance degradation
The fuel cell is assembled under compressive loads to prevent gas leakage and to minimize contact resistance, however compression leads to changes in surface property and material characteristics of the porous layers of the fuel cell
Forms of GLD
- Carbon cloth
- Carbon fibre
The GDL is a core component of a fuel cell, enabling transport of gases, liquids, electricity within the cell
The quality of the GDL plays an important role in the overall performance and cost.
Inside a fuel cell…
- Gas flows through the channels ==> Convective flows
Gas flows through porous media
Electrochemical reactions
Proton transport through proton conducting polymer membranes
Electron conduction through electrically conductive cell components
Water transport through polymer membrane ==> electrochemical drag and back diffusion
Water transport (vapor + liquid) through the porous catalyst layer and GDL
Two phase flow of unused gas carrying water droplets
Heat transfer ==> conduction through solid components of cell, convection to reactant gases and cooling medium
First fuel cell law
Changing one parameter in a fuel cell causes the change of at least two other parameters, and atleast one of them has an opposite effect of the one expected to be seen
Proton exchange fuel cell
- each individual cell produces about 0.7 V (EMF) when operating in air
in order to produce a useful voltage, the electrons of many cells must be linked in series
In addition to connecting the cells, we must ensure that reactant gases can still reach the electrodes and that the resistance of the electrodes has a minimal effect
The membranes purpose
- to separate the anode and cathode
- to prevent mixing of the fuel and the oxidant
- to provide a conductive path way for protons
Membranes require characteristics
- High ionic conductivity ( and zero electronic conductivity) under cell operating conditions
- Long term chemical and mechanical stability at elevated temperatures in oxidizing and reducing environments
Good mechanical strength, with resistance to swelling
- Low oxidant and low fuel cross-over
- Interfacial compatibility with catalyst layers
- Low cost
Advantages and Disadvantages of ‘Nafion’
Advantages:
- Stable material
- Selective ion permeability
- Compatible with current fuel technology
- High proton connductivity under aqueous conditions ~ 0.1 S/cm
Disadvantages:
- Low conductivity at low water content
-Peremableat MeOH (this is relevant for the direct methanol fuel cell)
-Poor mechanical strength at high temps
-Processibility and fabrication issues, Ex: Nafion dehydrates at T > 80 C and RH < 100%
-Nafion is expensive : 1000 $ / m^2
Water uptake
- Proton conductivity of a polymer membrane is strongly dependent on membrane structure and water content
Water content = g of H2O / g of polymer dry weight = nr. of H2O molecules / nr. of sulfonic acid groups = λ
Maximum amount of water ==> state of the water used to be equilibrate the membrane
Swelling of nafion
Membrane swelling is a very significant factor in a fuel cell design and assembly
The dimensional changes are in the order of 10% depending on water content ==> Reinforcement using composite materials
Proton conductivity
- it is the most important function of the polymer membrane used in fuel cells
- Nafion (EW 1100)
==> - Charge carrier density is similar to that in 1M aqueous H2SO4
==>
In a fully hydrated Nafion membrane the proton mobility is only one order of magnitude lower than that in aqueous H2SO4
Conductivity = f(water content, T)
Water management
- Water is produced at the cathode as a result of the electrochemical reaction
- water is dragged from the anode to the cathode by protons moving through the electrolyte (electroosmotic drag)
- Water generation and electroosmotic drag create a large concentration gradient ==> water diffuses back to the anode
General considerations
- Nafion will dehydrate (thus losing proton conductivity) when T>80 C
- This is limitation strongly affects the design of fuel cells, because higher T are desirable for a better efficiency and CO tolerance of the platinum catalyst
- Silica and zirconium phosphate can be incorporated into nafions water channels through in situ chemical reactions to increase the working temperature to above 100 C
Gas permeation
- Membrane has a porous structure, water content and because of solubility of H2, O2 in water, some gases do permeate through the membrane
Permeability = Pm = D x S
D = diffusivity (cm^2 s^-1)
S = solubility ( mol cm^-3 Pa ^-1)
1 Barrer = 10^-10 cm^3 s^-1 cmHg^-1
Modifications to Nafion
- addition of water-retaining (hydrophilic) additives
- non-conducting inorganic particles (SiO2, TiO2)
- Proton conducting inorganic salts and heteropolyacids, HPA (e.g zirconium phosphate)
- Substitution of water with a high boiling proton solvent: Ionic fluids
- If water boiled at 200 C most of the problems related to PEM fuel cell operation at T > 100 C would be resolved
Alternatives to sulfonated polymer membranes
- Fluoropolymers
- Polysiloxanes
- Aromatic polymers
Lowering material costs
Improving the operating temperature
Improving chemical and thermal stability
Improving the proton conduction properties
Preferential transport characteristics
Fluoropolymers
Sulfonated polystyrenes - first generation polymer electrolytes for fuel cells
- suffers from a short lifetime - mechanical/chemical stability
Partially fluorinated polymer: FEP and PVDF
Prepared by grafting and then sulfonating the styrene groups
High water uptake & high proton conductivity
Polysiloxanes
- Organic modified silicate electrolyte (ORMOLYTE): arylsulfonic anions or alkylsulfonic anions grafted to the benzyl group
Proton conductivity of 10^-2 Scm^-1 at RT
Chemically and thermally stable up to 200 C
Water uptake alteration are possible
Aromatic polymers
- Cost effective and avalibilty
Material suitable for high temperature range (> 100 C ) applications
Different materials
Polyether etherketone (PEEK)
polysulfones (PSF) or Polyethersulfone (PES)
Polybenzimidazoles (PBI)
polyimides (PI)
polyphenylenes (PP)
poly(4 phenoxybenzoyl 1,4 phenylene) (PPBP)
Drawbacks (?)
- no dignificant improvement in proton conductivity at low humidty ( water retention is not as good as expected and water is not where it is needed)
- Imidazolium groups are not as good as water in solvating membrane acid groups
Future trends for high T, H2 / air, fuel cell membranes
- eliminate the need for water in PEMS
basic polymer with added excess acid (PBI doped with concentrated phosphoric acid)
Acidic polymer with added base (sulfonated polymer with absorbed imidazole, benzimazole or some other proton acceptor)
Inorganic solid acids ( bound in a polymeric matrix)
- Hold water in PEMS
Developing membranes having highly uniform nanopore structure
Utilize capillary condensation to hold water in membrane pores at T > 100 C and Rh < 100%
Use rigid cross-linked polymers
What is ‘ORR’ ?
Electrochemical oxygen reduction reactions
Relevance to:
fuel cells
metal-air abtteries
Industrial electrolytic processes
ORR is a for electron process
O2 = 4H^+ + 4e^- ==> 2H2O
E^o = 1.229 V vs NHE
ORR reaction is kinetically slow
Mechanisms of oxygen reduction reactions
Pathway A: direct pathway, involves four electron reduction
O2 + 4H^+ + 4e^- ==> 2H2O
Pathway B - indirect pathway, involves two electron reduction followed by further two-electron reduction
O2 + 2H^+ + 2e^- ==> H2O2
(E^0 = 0.695 V)
H2O2 + 2H^+ + 2e^- ==> 2H2O (E^0 =1.77 V)
Dissociative mechanism
(low current density)
1/2 O2 + * ==> O*
O* + H^+ + e^- ==> OH*
OH* + H^+ + e ==> H2O + *
Associative mechanism
(high current density)
O2 + * ==> O2*
O2* + H^+ + e ==> HO2*
HO2* + H^+ + e ==> OH*
O* + H^+ + e ==> OH*
OH* + H^+ + e ==> H2O + *
Electrocatalysts for ORR
- Reversible
- High oxygen adsorption capacity
- Structural stability during oxygen adsorption and reduction
- stability in electrode medium and also in suitable potential window
- ability to decompose H2O2
- good Conductivity
- Cheap
Work function
is the minimum energy needed to remove an electron from a solid to a point immediately outside the solid surface (or energy needed to move an electron from the fermi level into vacuum)
Pt electrocatalyst (advantages vs disadvantages)
Advantages:
- high work function Φ, (4.6eV)
- Ability to catalyse the reduction of oxygen
- good resistance to corrosion and dissolution
- High exchange current density
Disadvantages:
- Slow ORR due to the formation of -OH species at +0/8 V
- Pt is very expensive and scarce
Pt and PtO have different reactivity towards ORR
Pt electrocatalyst
Two major pathways:
- The first route (OOH^- formation pathway) in which the O2 molecule is not dissociated directly but reacts with hydrogen to form OOH or H2O2 which then dissociates
- The second route (O2 ^- dissociation pathway) involves the dissociation of O2 as one of the first reaction steps to generate two oxygen atoms on the surface, which then can react with adsorbed H atoms to form water
(The real mechanism is not yet fully understood)
Pt based electrocatalysts are still the state of the art cathodes for the ORR
It is well established that the activity for the ORR is strongly influenced by the presence of strongly adsorbed oxygenated species on the Pt surface, which is inseparable from the adsorption of oxygen.
The rate limiting step:
- Splitting oxygen molecules into oxygen atoms and the subsequent formation of water currently the ‘rate-limiting step’
- This is a strong restriction for the overall power production
- Finding more efficient catalysts for ORR is of paramount importance
for making low temperature fuel cells commercially viable
Size effect
When the size of the particle is reduced, the relative amount of surface atoms in edge and corner positions
Edge and corner atoms are electronically more active than other surface atoms
Applicalble to < 10 nm NPs
Effect of crystallographic orientation
- Pt has faced centered cubic crystal structure
3 basal planes: (111), (100), (110)
- Catalytic activity for ORR scales as:
Pt (110) > Pt(100) > Pt (111)
Carbon - based materials
(Chemical properties)
- Good corrosion resistance
- Available in high purity
- Forms intercalation compounds
Carbon-based materials
(Electrical properties)
- Good conductivity
Carbon-based materials
(Mechanical properties)
- Dimensionally & mechanically stable -
- Low modulus of elasticity
- Light weight & adequate strength
- Availability in a variety of physical structures
- Easily fabricated into composite structures
Pt based alloys
- Alloying with a different metal may change Pt electronic structure
- Alloy catalysts are used at the anode to increase the catalysts CO-tolerance
==> Hydrogen gas feed usually contains small amounts of CO which adsorbs on the surface and poisons the catalyst
CO : must be «_space;ppm level
Example : PtRu anode catalysts
Pt-(CO)_ads + Ru-(OH)_ads ==> Ru + Pt + CO_2 +2H^+ + 2e^-
- Ruthenium increases the affinity for adsorption of oxygen containing species at less positive potentials
The rate determining step in CO oxidation is the reaction between OH and CO species ==> the amount of poisoned surface sites decreases
The catalyctic activity of Pt towards ORR strongly depends on…
its O2 adsorption energy, the dissociation energy of the O-O bond, and the binding energy of OH on the Pt surface
