TD and Kinetics by rogerholland

VIEWS: 17 PAGES: 66

									                    Chem 360   Monday
Fuel Cells                     November 24




             RLG 2008-3                 1
  Global Warming                         Monday
                                         November
over Past Millenium                        24

 • anthropocene climate regime
 • 20th century: human population quadrupled,
   energy consumption increased sixteenfold!
 • global warming from fossil fuel use became
   dominant factor in climate change
 • global mean surface temperature is higher today
   than it’s been for at least a millennium


                       RLG 2008-3               2
                               RLG 2008-3   3
Slide from Marty Hoffert NYU
RLG 2008-3   4
                                           Monday
   Efficiencies                            November
                                             24

• How much energy of the fuel is actually utilized to
  power a car?



     ~8%




                        RLG 2008-3                5
                                            Monday
   Efficiencies                             November
                                              24

• There is nothing like an energy crisis…
• BUT: efficiency or entropy “crisis”?

• This also is not really a crisis

• Scientific perspective:
        it’s just an implication of the 2nd Law of TD!


                          RLG 2008-3               6
  Fuel Cells and                           Monday
                                           November
Energy Conversion                            24

• Current energy use:
• largely based on fossil fuels
• dwindling resources (3-fold increase in oil price
  in recent years)
• adverse environmental impact (air pollution,
  global warming)
• dependence on instable geographic regions


                       RLG 2008-3                 7
  Fuel Cells and                                Monday
                                                November
Energy Conversion                                 24

• Need for a new energy infrastructure –
  humanity’s No. 1 problem!
  – has to be solved within ~ 50 years
• sustainable infrastructure: decentralized,
  renewable, clean
• Hydrogen as a fuel?
  – fuel cells are a key enabling technology of a
    hydrogen economy


                          RLG 2008-3                  8
  Fuel Cells and                            Monday
                                            November
Energy Conversion                             24

2 aspects:

1. The fuel and fuelling infrastructure –
   hydrogen???

2. The energy conversion device – fuel cells (factor
  2.5 in efficiency)


                        RLG 2008-3                9
                                   Monday
Hydrogen as a Fuel?                November
                                     24




                      RLG 2008-3         10
                                            Monday
Hydrogen as a Fuel?                         November
                                              24

 • The Hindenburg disaster (1937)
 • – a problem of hydrogen? – maybe, but…
 • paint (dark iron oxide and reflective aluminum)
   caught fire (electrical discharge, thunderstorm)
 • hydrogen burned quickly and safely
 • all deaths caused by jumping/falling of people



                         RLG 2008-3                   11
                                          Monday
Hydrogen as a Fuel?                       November
                                            24

 • hydrogen is less flammable than gasoline
 • highly volatile, burns upward, quickly
 • non-toxic




                        RLG 2008-3              12
The Fuel Cell Effect -                        Monday
                                              November
      History                                   24

               • Christian Schönbein (1799-1868):
                 Prof. of Phys. and Chem. in Basel,
                 first observation of fuel cell effect
                 (Dec. 1838, Phil. Mag.)




                          RLG 2008-3                 13
The Fuel Cell Effect -                       Monday
                                             November
      History                                  24

                 • William Robert Grove (1811 –
                   1896): Welsh lawyer turned
                   scientist, demonstration of fuel
                   cell (Jan. 1839, Phil. Mag.)




                         RLG 2008-3                   14
The Fuel Cell Effect -                        Monday
                                              November
      History                                   24
                    • Friedrich Wilhelm Ostwald
                      (1853-1932, Nobel prize 1909):
                      founder of the field of physical
                      chemistry, provided much of the
                      theoretical understanding of
                      how fuel cells operate
                    • “I do not know whether all of us
                      realise fully what an imperfect
                      thing is the most essential source
                      of power which we are using in
                      our highly developed
                      engineering – the steam engine”

                         RLG 2008-3                  15
The Fuel Cell Effect -                   Monday
                                         November
      History                              24

              • Grove‘s fuel cell,1839




                         RLG 2008-3            16
The Fuel Cell Effect -                Monday
                                      November
      History                           24

 • his “gas chain”
   from 1842




                         RLG 2008-3         17
    Friedrich Wilhelm                                          Monday
   Ostwald – visionary                                         November
           ideas                                                 24
  energy conversion in combustion engines limited by Carnot
    efficiency
  unacceptable levels of atmospheric pollution
                               vs.
             energy conversion in galvanic elements
                 direct generation of electricity
                         highly efficient
                          no pollution

  Transition would be technical revolution
  Prediction: practical realization could take a long time

                                                  RLG 2008-3         18
Source: Z. Elektrochemie, Vol. 1, p. 122-125, 1894.
                                           Monday
What is a fuel cell?                       November
                                             24
• Grove's fuel cell was an alkaline type. Fuel cells
  are typed by the kind of electrolyte they use.
• Fuel cells are of two general types:
     mobile (portable, as in cars, flashlights, boom
  boxes, lap tops, and cell phones)
      stationary (fixed, structural - like a power
  plant).
• Fuel cells produce electrical energy from
  hydrogen without any moving parts.

                        RLG 2008-3                19
                                             Monday
What is a fuel cell?                         November
                                               24
• Alkaline fuel cells - the first kind - used in Apollo
  space vehicles.
• High-temperature AFCs operate at temperatures
  between 100°C and 250°C. Newer AFC designs
  operate at lower temperatures of roughly 23°C to
  70°C.
• These fuel cells use a solution of potassium
  hydroxide in water as the electrolyte and can use
  a variety of non-precious metals as a catalyst at
  the anode and cathode.
                         RLG 2008-3                 20
                                           Monday
Alkaline fuel cells                        November
                                             24
• The anode and cathode are
  made of porous materials. In
  such materials, like a sponge,
  the two gasses soak into the
  materials.
• While the electrolyte is not a
  good conductor, it produces
  OH- or hydroxyl ions, which
  facilitate the oxidation
  reaction and produce
  electron flow, which
  generates an electrical
  current.


                              RLG 2008-3         21
                                                   Monday
Alkaline fuel cells                                November
                                                     24
• Hydroxyl ions meet with hydrogen coming in through the
  porous anode, to produce water. After the water
  formation, the electrons are released and flow through the
  anode. The chemical equation is as follows:
  Anode + 2H2 +4OH-  Anode + 4e- + 4H2O (oxidation)
  Cathode + O2 + 4e- +2H2O  Cathode + 4OH- (reduction)
   AFCs' high performance is due to the rate at which
  chemical reactions take place in the cell. They have also
  demonstrated efficiencies near 60 percent in space
  applications .

                            RLG 2008-3                        22
                                                  Monday
Alkaline fuel cells                               November
                                                    24

• The disadvantage of this fuel cell type is that it is
  easily poisoned by carbon dioxide (CO2 ).
   – even the small amount of CO2 in the air can affect this
     cell's operation, making it necessary to purify both the
     hydrogen and oxygen used in the cell.
   – This purification process is costly.
   – Susceptibility to poisoning also affects the cell's
     lifetime (the amount of time before it must be
     replaced), further adding to cost.

                           RLG 2008-3                    23
                                                        Monday
Alkaline fuel cells                                     November
                                                          24
• The problem is the reaction:

                  2KOH + CO2  K 2CO3 + H2O


• Potassium hydroxide is changed to potassium carbonate
  and water.
   – air with carbon dioxide cannot be used.
   – must use pure hydrogen.
   – need a process to get rid of the carbon dioxide and the water.



                               RLG 2008-3                        24
                                                      Monday
Alkaline fuel cells                                   November
                                                        24
• One solution to the water removal is to circulate
  hydrogen.
   – But then, what if the gases get mixed (i.e., hydrogen on
     the cathode and oxygen migrating to the anode)?
   – This generates a short circuit, because the electrons will
     not flow through the anode to the cathode, but flow
     directly to the cathode through the electrolyte.
• The next solution is to make the electrolyte solid (to
  avoid the pumping of the liquid electrolyte with its
  related problems).



                             RLG 2008-3                       25
                                          Monday
Alkaline fuel cells                       November
                                            24
• A variation is to use hydrazine (H2NNH2) or
  methanol (CH3OH) as fuels. The reaction for
  methanol is:

         CH3OH + 6OH-  5H2O + CO2 + 6e-

• The production of nitrous oxide and carbon
  dioxide is not exactly a total "clean"
  environmental optimal situation.

                       RLG 2008-3               26
                                                  Monday
Alkaline fuel cells                               November
                                                    24

• Why aren’t these being used every where?
• Cost!!
  – is less of a factor for remote locations such as space or
    under the sea.
• To effectively compete in most mainstream
  commercial markets, these fuel cells will have to
  become more cost-effective.



                           RLG 2008-3                    27
Alkaline fuel cells

• AFC stacks have been shown to maintain sufficiently
  stable operation for more than 8,000 operating
  hours.
  – To be economically viable in large-scale utility
    applications, these fuel cells need to reach operating
    times exceeding 40,000 hours,
  – has not yet been achieved due to material durability
    issues.
  – possibly the most significant obstacle in commercializing
    this fuel cell technology.
• Today production and research is turning away from
  the alkaline fuel cell to the PEM.

                            RLG 2008-3                     28
Proton exchange
   membrane
• Proton exchange
  membrane - a solid
  polymer fuel cell using
  a perfluorosulfonic
  acid plastic, which
  allows hydrogen
  protons to pass
  through. PEMs operate
  at temperatures
  between 50°C and
  100°C.

                            RLG 2008-3   29
Proton exchange                             Monday
                                            November
   membrane                                   24
• deliver high power density
• advantages of low weight and volume, compared to
  other fuel cells.
• use a solid polymer as an electrolyte and porous
  carbon electrodes containing a platinum catalyst.
• need only hydrogen, oxygen from the air, and water
  to operate and do not require corrosive fluids like
  some fuel cells.
• typically fueled with pure hydrogen supplied from
  storage tanks or onboard reformers.

                        RLG 2008-3                 30
Proton exchange                                               Monday
                                                              November
   membrane                                                     24
• operate at relatively low temperatures, around 80°C (176°F).
   – allows them to start quickly (less warm-up time) and results in less
     wear on system components, resulting in better durability.
• requires that a noble-metal catalyst (typically platinum) be used
  to separate the hydrogen's electrons and protons,
   – Cost!!!
• The platinum catalyst is extremely sensitive to CO poisoning,
   – Must use an additional reactor to reduce CO in the fuel gas if the
     hydrogen is derived from an alcohol or hydrocarbon fuel.
   – Cost!!!
• Developers are currently exploring platinum/ruthenium
  catalysts that are more resistant to CO.



                                 RLG 2008-3                               31
Proton exchange                               Monday
                                              November
   membrane                                     24

• Sulfonation is a familiar process used in making
  detergent, where the chain repels water
  (hydrophobic) and clings to the dirt.
• This is an " acid" fuel cell.
  – The sulfonated fluoroethylene is called
    perfluorosulfonic acid.




                          RLG 2008-3                 32
Proton exchange
   membrane




Figure 3. Schematic presentation of proton conduction in perfluorosulfonic acid (left)
and acid-doped polybenzimidazole (right) membranes.
                                         RLG 2008-3                                      33
Proton exchange
   membrane
• Dupont has the patent on Nafion, and other
  fluorosulfonate ionomers.
  –   Similar to Teflon
  –   tough and chemically resistant.
  –   absorb large quantities of water.
  –   allow H+ ions to pass through without the electron passing
      through.
• They can be made in thin films.
  – reduces the distance between the anode and cathode,
    thus reducing the ohmic resistance, which causes voltage
    drops.

                             RLG 2008-3                     34
Proton exchange                                       Monday
                                                      November
   membrane                                             24

 The basic structure of Nafion 117 is:


 CF2=CFOCF2CFOCF2CF2SO3H
            |
            CF 3


 Notice in acid fuel cells like this, the acid will corrode the
 less precious metals. This is a challenge: how to develop
 electrodes that use cheaper metals.

                             RLG 2008-3                       35
Proton exchange                             Monday
                                            November
   membrane                                   24

• PEM fuel cells are used primarily for
  transportation applications and some stationary
  applications.
• Due to their fast startup time, low sensitivity to
  orientation, and favorable power-to-weight ratio,
  PEM fuel cells are particularly suitable for use in
  passenger vehicles, such as cars and buses.



                        RLG 2008-3                 36
Proton exchange                                              Monday
                                                             November
   membrane                                                    24
• A significant barrier to using these fuel cells in vehicles is
  hydrogen storage.
   – must store the hydrogen onboard as a compressed gas in pressurized
     tanks.
   – Hydrogen is a low density fuel
   – difficult to store enough hydrogen onboard to allow vehicles to travel
     the same distance as gasoline-powered vehicles
• Higher-density liquid fuels such as methanol, ethanol, natural
  gas, liquefied petroleum gas, and gasoline can be used for fuel,
   – vehicles must have an onboard fuel processor to reform the
     methanol to hydrogen.
   – increases costs and maintenance requirements.
   – reformer releases carbon dioxide, but less than that emitted from
     current gasoline-powered engines.


                                 RLG 2008-3                            37
Proton exchange                Monday
                               November
   membrane                      24




                  RLG 2008-3         38
RLG 2008-3   39
Phosphoric acid fuel                               Monday
                                                   November
      cells                                          24
 • earliest commercial cells developed
    – worked best in handling the carbon produced in producing
      the hydrogen from reforming carbon sources like fossil
      fuel or methane gas.
 • PAFC CHP systems operate at around 220°C.
 • can generate 200,000 watts of electrical power.
 • have been used for stationary applications with a
   combined heat and power efficiency of about 80%,
 • continue to dominate the on-site stationary fuel cell
   market.

                            RLG 2008-3                    40
Phosphoric acid fuel
      cells

  • "first generation" of
  modern fuel cells.
  • one of the most mature
  cell types
  • the first to be used
  commercially,
       -over 200 units currently
       in use.
  • typically used for
  stationary power generation
  • May be used to power
  large vehicles such as city
  buses.
                                   RLG 2008-3   41
Phosphoric acid fuel                                 Monday
                                                     November
      cells                                            24
 • The electrodes are made of carbon paper coated
   with a finely-dispersed platinum catalyst, which make
   them expensive to manufacture.
    – not affected by carbon monoxide impurities in the
      hydrogen stream.
 • Phosphoric acid solidifies at a temperature of 40 °C,
    – makes startup difficult and restrains PAFCs to continuous
      operation.
 • at an operating range of 150 to 200 °C, the expelled
   water can be converted to steam for air and water
   heating  bonus!
                             RLG 2008-3                      42
Phosphoric acid fuel                                    Monday
                                                        November
      cells                                               24
 • are 85 percent efficient when used for the co-
   generation of electricity and heat, but less efficient at
   generating electricity alone (37 to 42 percent).
    – only slightly more efficient than combustion-based power
      plants, which typically operate at 33 to 35 percent
      efficiency.
 • PAFCs are also less powerful than other fuel cells,
   given the same weight and volume.
    – As a result, these fuel cells are typically large and heavy.
 • PAFCs are also expensive.
    – require an expensive platinum catalyst
    – costs between $4,000 and $4,500 per kilowatt to operate.

                               RLG 2008-3                        43
Direct Methanol                                   Monday
                                                  November
   Fuel Cells                                       24
• DMFCs are powered by pure methanol, which is
  mixed with steam and fed directly to the fuel cell
  anode.
• do not have many of the fuel storage problems
   – methanol has a higher energy density than hydrogen (less
     than gasoline or diesel)
   – easier to transport and supply to the public using our
     current infrastructure since it is a liquid
• technology is relatively new
   – research and development are roughly 3-4 years behind
     that for other fuel cell types.
                           RLG 2008-3                     44
Molten carbonate                   Monday
                                   November
    fuel cells                       24

• MCFC systems operate
  at 650°C.
• They generate power
  in the megawatt range.




                      RLG 2008-3         45
Molten carbonate                              Monday
                                              November
    fuel cells                                  24
• currently being developed for natural gas and coal-
  based power plants for electrical utility, industrial,
  and military applications.
• use an electrolyte composed of a molten carbonate
  salt mixture suspended in a porous, chemically inert
  ceramic lithium aluminum oxide (LiAlO2) matrix.
• Since they operate at extremely high temperatures of
  650°C and above, non-precious metals can be used
  as catalysts at the anode and cathode
  – reduces costs.

                         RLG 2008-3                  46
Molten carbonate                                   Monday
                                                   November
    fuel cells                                       24
• Improved efficiency over PAFCs.
  – MCFCs can reach efficiencies approaching 60 percent,
    considerably higher than the 37-42 percent efficiencies of
    a PAFC.
  – When the waste heat is captured and used, overall fuel
    efficiencies can be as high as 85 percent.
• don't require an external reformer to convert more
  energy-dense fuels to hydrogen.
  – Due to the high operating temperatures, these fuels are
    converted to hydrogen within the fuel cell itself by a
    process called internal reforming
  – reduces cost!!

                           RLG 2008-3                      47
Molten carbonate                                     Monday
                                                     November
    fuel cells                                         24

• not prone to carbon monoxide or carbon dioxide
  "poisoning"
   – can even use carbon oxides as fuel
   – Could use gases made from coal.
• more resistant to impurities than other fuel cell types
   – could even be capable of internal reforming of coal,
     assuming they can be made resistant to impurities such as
     sulfur and particulates that result from converting coal, a
     dirtier fossil fuel source than many others, into hydrogen.


                             RLG 2008-3                      48
Molten carbonate                                Monday
                                                November
    fuel cells                                    24

• The primary disadvantage of current MCFC
  technology is durability.
  – The high temperatures and the corrosive electrolyte
    used accelerate component breakdown and corrosion,
    decreasing cell life.
  – Scientists are currently exploring corrosion-resistant
    materials for components as well as fuel cell designs
    that increase cell life without decreasing performance.



                          RLG 2008-3                   49
                                      Monday
Solid oxide fuel cells                November
                                        24

 • operate between
   500°C and 1000°C.
 • SOFCs generate
   power from 2,000
   watts to several
   megawatts.




                         RLG 2008-3         50
                                                  Monday
Solid oxide fuel cells                            November
                                                    24
 • SOFCs use a hot, hard, non-porous ceramic
   compound as the electrolyte.
 • Since the electrolyte is a solid, the cells do not
   have to be constructed in the plate-like
   configuration typical of other fuel cell types.
 • SOFCs are expected to be around 50-60 percent
   efficient at converting fuel to electricity.
    – In applications designed to capture and utilize the
      system's waste heat (co-generation), overall fuel use
      efficiencies could top 80-85 percent.

                            RLG 2008-3                   51
                                                 Monday
Solid oxide fuel cells                           November
                                                   24

 • High temperature operation removes the need
   for precious-metal catalyst,
    – reducing cost.
 • It also allows SOFCs to reform fuels internally,
    – enables the use of a variety of fuels
    – reduces the cost associated with adding a reformer to
      the system.
 • The high temperature makes reforming methane easier
   for producing the hydrogen needed for the process.

                           RLG 2008-3                   52
                                                  Monday
Solid oxide fuel cells                            November
                                                    24

 • the most sulfur-resistant fuel cell type;
    – can tolerate several orders of magnitude more sulfur
      than other cell types.
 • not poisoned by carbon monoxide (CO), which
   can even be used as fuel.
    – allows SOFCs to use gases made from coal.




                           RLG 2008-3                   53
                                                  Monday
Solid oxide fuel cells                            November
                                                    24

 • High-temperature operation has disadvantages.
    – results in a slow startup
    – requires significant thermal shielding to retain heat
      and protect personnel
    – acceptable for utility applications but not for
      transportation and small portable applications.
    – place stringent durability requirements on materials.
    – development of low-cost materials with high
      durability at cell operating temperatures is the key
      technical challenge facing this technology.
                            RLG 2008-3                   54
                                             Monday
Solid oxide fuel cells                       November
                                               24

 • Scientists are currently exploring the potential for
   developing lower-temperature SOFCs operating
   at or below 800°C that have fewer durability
   problems and cost less.
 • Lower-temperature SOFCs produce less electrical
   power, however, and stack materials that will
   function in this lower temperature range have not
   been identified.


                         RLG 2008-3                 55
Regenerative fuel                          Monday
                                           November
     cells                                   24

• Regenerative fuel cells produce electricity from
  hydrogen and oxygen and generate heat and
  water as byproducts, just like other fuel cells.
• However, regenerative fuel cell systems can also
  use electricity from solar power or some other
  source to divide the excess water into oxygen and
  hydrogen fuel by electrolysis.
• This is a comparatively young fuel cell technology
  being developed by NASA and others.
                       RLG 2008-3                56
RLG 2008-3   57
RLG 2008-3   58
RLG 2008-3   59
Problems with fuel                                Monday
                                                  November
      cells                                         24

The closed circuit in the fuel cell measures the
  kinetic energy, rather than the potential energy.
Voltage drops in the fuel cells are caused by:
• Polarization
   – as protons move from the anode to the cathode,
     charge builds up.
   – positive charge repels further proton movement, until
     the proton is united with oxygen and electrons
     arriving at the cathode to neutralize this polar charge.

                           RLG 2008-3                     60
Problems with fuel
      cells

Voltage drops in the fuel cells are caused by:
• Fuel crossing over to the other electrode.
   – This happens when oxygen shows up at the anode
     and hydrogen gas shows up at the cathode.
   – the electron does not flow from anode to the
     cathode, but ends up flowing from cathode to
     cathode.
   – This is called a short circuit. No work is done and all of
     the energy is entropy.

                            RLG 2008-3                      61
Problems with fuel                                 Monday
                                                   November
      cells                                          24

• Voltage drops in the fuel cells are caused by:
• Transport holes
   – when fuel is consumed at the electrode, until more
     fuel arrives at the site, a reaction "hole" exists where
     no electrons split off from protons on the anode.
   – causes a drop in the potential and kinetic energy.
   – Pressure and concentration of pure gas relieve this
     problem.


                            RLG 2008-3                     62
Problems with fuel
      cells

• Voltage drops in the fuel cells are caused by:
• Ohmic losses
   – This is normal resistance to electron flow.
   – The more electrons one tries to ram through the
     circuit, the lower the voltage drops.
   – Voltage = current * resistance




                          RLG 2008-3                   63
Problems with fuel                          Monday
                                            November
      cells                                   24
• Needless to say, the entropy results in production
  of heat. If you were to try to pass a lot of current
  through a resisting circuit at high voltage, you
  would get the "flash bulb" effect. The circuit
  would overload, smoke, and ultimately burn.

  The solution in fuel cells is to use metals with
  good conductivity and low resistance. The
  electrolyte provides resistance, so make it thin
  and reduce the distance of anode and cathode.

                        RLG 2008-3                   64
Problems with fuel                                  Monday
                                                    November
      cells                                           24
• The problems mentioned above with voltage drop
  are solved by making the cell parts smaller. This is
  a good thing, according the Robert G. Hockaday,
  founder of Energy Related Devices, Inc., because
  fuel cell production can use the similar
  production schemes used in computer chip
  manufacture.
   – Can you see the day when Intel makes fuel cells?
• He blew up his mother's stove in the 1970's
  baking his fuel cell electrodes.
  http://www.energyrelatedevices.com/ourtechs.htm
                                     RLG 2008-3           65
             Monday
             November
               24




RLG 2008-3         66

								
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