energy storage

1. Electrolysis
“What does this have to do with fuel cells?”
By providing
energy from a
battery, water
(H2O) can be
dissociated into
the diatomic
molecules of
hydrogen (H2)
and oxygen
(O2).
Figure 1

2. Fuel Cell Basics
“Put electrolysis in reverse.”
Fuel cell H2O
O2
H2
heat
work
The familiar process of electrolysis requires work to proceed, if the
process is put in reverse, it should be able to do work for us
spontaneously.
The most basic “black box” representation of a fuel cell in action is
shown below:

4. How do they work?
•Fuel (H2) is first transported
to the anode of the cell
•Fuel undergoes the anode
reaction
•Anode reaction splits the fuel
into H+ (a proton) and e-
•Protons pass through the
electrolyte to the cathode
•Electrons can not pass
through the electrolyte, and
must travel through an
external circuit which creates
a usable electric current
•Protons and electrons reach
the cathode, and undergo the
cathode reaction

5. Hydrogen is oxidized on the anode and oxygen is reduced on the
cathode. In nature, molecules cannot stay in an ionic state, therefore
they immediately recombine with other molecules in order to return
to the neutral state. Hydrogen protons in fuel cells stay in the ionic
state by traveling from molecule to molecule through the use of
special materials. The protons travel through a polymer membrane
made of persulfonic acid groups with a Teflon backbone. The
electrons are attracted to conductive materials and travel to the load
when needed. On the cathode, oxygen reacts with protons and
electrons, forming water and producing heat. Both the anode and
cathode contain a catalyst to speed up the electrochemical
processes. Reactants are transported by diffusion and/or convection
to the catalyzed electrode surfaces where the electrochemical
reactions take place. The water and waste heat generated by the fuel
cell must be continuously removed.

6. Fuel Cells Basics
• Fuel cells convert chemical energy directly into electrical energy.
• Difference with batteries: fuel cells require a fuel to flow in order to
produce electricity.
• Heat is produced from chemical reaction and not from combustion.
• Types of fuel cells:
• Proton exchange membrane (PEMFC)
• Direct Methanol fuel cell (DMFC)
• Alkaline fuel cell (AFC)
• Phosphoric acid fuel cell (PAFC) (*)
• Molten-carbonate fuel cell (MCFC) (*)
• Solid-oxide fuel cell (SOFC) (*)
(*) Suitable for micro-grids.

7. Fuel cells operation
• Example: PEMFC
• The hydrogen atom’s electron and proton are separated at the anode.
• Only the protons can go through the membrane (thus, the name proton
exchange membrane fuel cell).
Hydrogen
Oxygen
Water
Heat
2 2
1/2 2 2 1
O H e H O
 
  
Membrane
(Nafion)
Catalyst (Pt)
Anode (-)
Catalyst (Pt)
Cathode (+)
dc current
2 2 2
2 2 ( 1.23 )
r
O H H O E V
  
2 2 2
H H e
 
 

8. Advantages & Disadvantages of fuel cell systems
Some advantages of fuel cell systems are as follows:
• Fuel cells have the potential for a high operating efficiency.
• There are many types of fuel sources, and methods of supplying
fuel to a fuel cell.
• Fuel cells have a highly scalable design.
• Fuel cells produce no pollutants.
• Fuel cells are low maintenance because they have no moving
parts.
• Fuel cells do not need to be recharged, and they provide power
instantly when supplied with fuel.

9. Some limitations common to all fuel cell systems include the
following:
• Fuel cells are currently costly due to the need for materials with
specific properties. There is an issue with finding low-cost
replacements. This includes the need for platinum and Nafion
material.
• Fuel reformation technology can be costly and heavy and needs
power in order to run.
• If another fuel besides hydrogen is fed into the fuel cell, the
performance gradually decreases over time due to catalyst
degradation and electrolyte poisoning.

10. Chemistry behind the technology
Oxidation
At the anode of the cell,
a catalyst (platinum
powder) is used to
separate the proton from
the electron in the
hydrogen fuel.
Anode half-reaction:
2H2  4H+ + 4e-
Eo = 0.00V
Reduction
At the cathode of the cell, a
second catalyst (nickel) is
used to recombine the
protons, electrons, and
oxygen atoms to form water.
Cathode half- reaction:
4H+ + O2 + 4e-  2H2O
Eo = 0.68V
In electrochemistry, the Eo
cell value (energy) of a fuel cell is equal to the
Eo of the cathode half-reaction minus the Eo of the anode half-reaction.
For a hydrogen fuel cell, the two half reactions are shown above. So to
calculate the energy of one fuel cell, we need to subtract the anode
energy from the cathode energy. For a HFC, the Eo
cell = 0.68V – 0.00V
which equals 0.68V

11. NTNU, 29 June 2007 11
Fuel Cell Stack
Ucell = 0,5 - 0.9 V
 Stacking N cells in series leads
to higher voltages.
 Larger cross sectional area A
leads to higher currents:
+
- +
-
-
-
-
+
+
+
UStack = N · Ucell
Stack by ZSW, Germany
Electrical Power: 1 kW
100 mm
100 mm
Stack
Single Cell
IStack = Icell = A · icell,av

12. Fuel Cell Basics-Components
Anode: Where the fuel reacts or "oxidizes", and releases electrons.
Cathode: Where oxygen (usually from the air) "reduction" occurs.
Electrolyte: A chemical compound that conducts ions from one
electrode to the other inside a fuel cell.
Catalyst: A substance that causes or speeds a chemical reaction
without itself being affected.
Cogeneration: The use of waste heat to generate electricity.
Harnessing otherwise wasted heat boosts the efficiency of power-
generating systems.
Reformer: A device that extracts pure hydrogen from hydrocarbons.
Direct Fuel Cell: A type of fuel cell in which a hydrocarbon fuel is
fed directly to the fuel cell stack, without requiring an external
"reformer" to generate hydrogen.

14. The primary functions of the electrodes are:
(a) Ability to transport reactants and products through the
porous structure.
(b) Capacity to adsorb the reactants and enable charge transfer
through electrolytic and electronic continuity throughout the
matrix (translates to low over-potentials for a particular
electrochemical reaction).
(c) Exhibit low energies of adsorption for products.
(d) Ability to selectively oxidize and reduce reactants.

15. Types of Fuel Cells
The five most common types:
• Alkali
• Molten Carbonate
• Phosphoric Acid
• Proton Exchange Membrane
• Solid Oxide

16. Alkali Fuel Cell
compressed hydrogen and
oxygen fuel
potassium hydroxide (KOH)
electrolyte
~70% efficiency
150˚C - 200˚C operating
temp.
300W to 5kW output
requires pure hydrogen fuel and platinum catylist → ($$)
liquid filled container → corrosive leaks

17. • The main advantage is that their cost is relatively low (when
considering the fuel cell stack only without “accessories”.
• Reactions:
• Anode
• Cathode
• Developed for the Apollo program.
• Very sensitive to CO2 poisoning. So these FCs can use impure
hydrogen but they require purifying air to utilize the oxygen.
• Issues:
• Cost (with purifier)
• Short life (8000 hours)
• Relatively low heat production
Alkaline Fuel Cells (AFCs)
2 2
2 2 2
H OH H O e
 
  
2 2
1/2 2 2 2
O H O e OH
 
  

18. Molten Carbonate Fuel Cell (MCFC)
carbonate salt electrolyte
60 – 80% efficiency
~650˚C operating temp.
cheap nickel electrode
catylist
up to 2 MW constructed, up
to 100 MW designs exist
The operating temperature is too hot for many applications.
carbonate ions are consumed in the reaction → inject CO2 to
compensate

19. • One of the main advantages is the variety of fuels and catalyst than
can be used.
• Reactions:
• Anode
• Cathode
• They operate at high temperature. On the plus side, this high
temperature implies a high quality heat production. On the minus
side, the high temperature creates reliability issues.
• They are not sensitive to CO poisoning.
• They have a relatively low cost.
• Issues:
• Extremely slow startup
• Very slow dynamic response
2
2 3 2 2 2
H CO H O CO e
 
   
2
2 2 3
1/2 2
O CO e CO
 
  
Molten Carbonate Fuel Cells (MCFCs)

20. Phosphoric Acid Fuel Cell (PAFC)
phosphoric acid electrolyte
40 – 80% efficiency
150˚C - 200˚C operating temp
11 MW units have been tested
sulphur free gasoline can be
used as a fuel
The electrolyte is very corrosive
Platinum catalyst is very expensive

21. • One of their main advantages is their long life in the order of
40,000 hours.
•The phosphoric acid serves as the electrolyte.
• The reactions are the same in a PEMFC. Hence, the reversible
voltage is 1.23 V
• The most commercially successful FC: 200 kW units
• They produce a reasonable amount of heat
• They support CO poisoning better than PEMFC
• They have a relatively slow dynamic response
• Relative high cost is an important issue
Phosphoric Acid Fuel Cells (PAFCs)

22. Proton Exchange Membrane (PEM)
thin permeable polymer sheet
electrolyte
40 – 50% efficiency
50 – 250 kW
80˚C operating temperature
electrolyte will not leak or crack
temperature good for home or vehicle use
platinum catalyst on both sides of membrane → $$

23. PEMFC Technology and issues
• Expected life of PEMFC is very short (5,000 hours).
• The most commonly used catalyst (Pt) is very expensive.
• The most commonly used membrane (Nafion – a sulfonated
tetrafluorethylene copolymer is also very expensive).
• PEMFCs are very expensive.
• CO poisoning diminishes the efficiency. Carbon monoxide (CO) tends
to bind to Pt. Thus, if CO is mixed with hydrogen, then the CO will take
out catalyst space for the hydrogen.
• Hydrogen generation and storage is a significant problem.

24. Solid Oxide Fuel Cell (SOFC)
hard ceramic oxide
electrolyte
~60% efficient
~1000˚C operating
temperature
cells output up to 100 kW
high temp / catalyst can extract the hydrogen from the fuel at the
electrode
high temp allows for power generation using the heat, but limits use
SOFC units are very large
solid electrolyte won’t leak, but can crack

25. Solid Oxide Fuel Cells (SOFCs)
• One of the main advantages is the variety of fuels and catalyst than
can be used.
• Reactions:
• Anode
• Cathode
• They operate at high temperature with the same plus and minus
than in MCFCs.
• They are not sensitive to CO poisoning.
• They have a relatively low cost.
• They have a relatively high efficiency.
• They have a fast startup
• The electrolyte has a relatively high resistance.
2
2 2 2
H O H O e
 
  
2
2
1/2 2
O e O
 
 

26. Fuel Cell Basics-Thermodynamics
H2(g) + ½O2(g) H2O(l)
Other gases in the fuel and air inputs (such as N2 and CO2) may be
present, but as they are not involved in the electrochemical
reaction, they do not need to be considered in the energy
calculations.
69.91 J/mol·K
205.14 J/mol·K
130.68 J/mol·K
Entropy (S)
-285.83 kJ/mol
0
0
Enthalpy (H)
H2O (l)
O2
H2
Table 1 Thermodynamic properties at 1Atm and 298K
Enthalpy is defined as the energy of a system plus the work
needed to make room for it in an environment with constant
pressure.
Entropy can be considered as the measure of disorganization of a
system, or as a measure of the amount of energy that is
unavailable to do work.

27. Enthalpy of the chemical reaction using Hess’ Law:
ΔH = ΔHreaction = ΣHproducts – ΣHreactants
= (1mol)(-285.83 kJ/mol) – (0)
= -285.83 kJ
Entropy of chemical reaction:
ΔS = ΔSreaction = ΣSproducts – ΣSreactants
= [(1mol)(69.91 J/mol·K)] – [(1mol)(130.68 J/mol·K) + (½mol)(205.14 J/mol·K)]
= -163.34 J/K
Heat gained by the system:
ΔQ = TΔS
= (298K)(-163.34 J/K)
= -48.7 kJ

28. The Gibbs free energy is then calculated by:
ΔG = ΔH – TΔS
= (-285.83 kJ) – (-48.7 kJ)
= -237 kJ
The external work done on the reaction, assuming reversibility
and constant temp.
W = ΔG
The work done on the reaction by the environment is:
The heat transferred to the reaction by the environment is:
W = ΔG = -237 kJ
ΔQ = TΔS = -48.7 kJ
More simply stated:
The chemical reaction can do 237 kJ of work and produces 48.7 kJ
of heat to the environment.