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
