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Introduction Flashcards

1
Q

Define Fuel cells.

A

Fuel cells are electrochemical cells, which are continuously supplied with a fuel and an oxidant*. This process is ideally isothermal and produces electrical energy and heat. Electrodes and electrolyte do not undergo any inherent changes **.

  • An oxidant is a reactant that oxidizes or removes electrons from other reactants during a redox reaction, e.g. oxygen, hydrogen peroxide, …
    ** Degradation occurs though
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2
Q

Sources for Hydrogen

A
  • Wind power
  • Solar power: PV, Solar thermal
  • Hydropower
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3
Q

Use cases for Hydrogen

A

• Transportation → high revenues (allowable cost about 20 ct/kWh)
- Competion from battery drives
- Tranport modes w/ higher energy requirement
- Focus on heavier passenger vehicles, light and heavy duty trucks, trams, trains, ships, airplanes
• Industry
- As a substitute to gas and coal
- H2 as an energy source esp. for high grade process energy
- Chemical industry (allowable cost about 10 to a few cts/kWh depending on product)
- Steel industry allowable cost < 2 cts/kWh (compares to cost of coal energetically)
- Glass and cement industry
- Power to chem: expected energy equivalent for DE 650 TWh annually
• Households
- Tbd: maybe in large cities esp. for high rises, maybe for buildings under historic preservation

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4
Q

Hydrogen Storage methods

A

• Gaseous
- Containers
- gas holders, tube storage larger storage purposes
- Composite tanks 350/700 bar mainly for transportation applications;
- Rock salt caverns as bulk storage
- Exploited oil and gas fields (sluggish dynamics, heavy contamination w/ subsequent clean-up reqiurement)
- Pipelines will NOT provide storage option: 1st. low volumetric energy density; 2nd low allowable ∆p
• Liquid
- As a compound; e.g. methanol, ammonia (poisonous substances; esp. viable as chemical precursors)
- As a hydrate; → niche markets, often too heavy and temperature levels for dehydration do not fit
- Liquid organic hydrogen carriers (LOHC); (energy requirement for dehydration about 30%, aromatic carrier)
- Liquifaction of H2 losses will come down to about 21% for large modern plants

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5
Q

Methods for transport of Hydrogen

A

• Pipelines; conversion of natural gas pipeline to hydrogen bears great opportunity for the energy transition
• Trucks (and trains): gaseous up to 500 bar; liquid; potentially LOHC
• Ships (technology derived from LNG ca. -163°C; H2 ca. -250°C ); potentially LOHC

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6
Q

Electrochemical cells can be classified into what groups?

A
  • Electricity storage
  • Electrochemical converters
  • Sensors
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7
Q

Why are fuel cell types classified according to the electrolyte?

A
  • They determine the working temperature and necessary process engineering environment
  • Influence other important variables, such as humidification, and thus indirectly the necessary operating pressure
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8
Q

What are characteristic features of electrolytes?

A
  • Operating temperature
  • Polymer vs ceramic vs liquid vs molten salt
  • Precious metal group (PGM) catalysts vs non PGM catalysts
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9
Q

Some applications of fuel cells?

A
  • Fuels (e.g. hydrogen, methanol, ethanol, etc.)
  • Reforming (technology of converting [mostly conventional] fuels into hydrogen rich synthetic gas)
  • Pressure level
  • Cell size or stack size
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10
Q

Energy flow in fuel cells vs batteries

A

Fuel cells:
- fuel and oxidant in
- electrical current, heat dissipation and exhaust gas out

Batteries:
- electrical current in
- electrical current and heat dissipation out

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11
Q

Operation in fuel cells vs batteries

A

Continuous operation possible vs only discontinuous operation

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12
Q

Reactants in fuel cells vs batteries

A

Supplied continuously vs supplied once at production

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13
Q

Energy storage in fuel cells vs batteries

A

No (in a fuel tank yes) vs yes

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14
Q

Electrolyte-Electrode-System in fuel cells vs batteries

A

No inherent change vs change of composition according to SOC

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15
Q

Periphery in fuel cells vs batteries

A

FC:
- Fuel cell control system,
- gas supply / gas storage
- gas treatment
- cleaning (particles, S, CO, etc.) reforming w/ carbonaceous fuels

Batteries:
Battery control system

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16
Q

SOFC full form

A

Solid Oxide Fuel Cell

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17
Q

Electrolyte used in SOFC

A

(YSZ) Zirconia (ZrO2) doped with Yttrium oxide (Y2O3)

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18
Q

SOFC operating temperature

A

750C-1000C with attempts to make it 600C

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19
Q

What does an Interconnector (IC) do in a SOFC?

A

Interconnector (IC) is a critical component of solid oxide fuel cell (SOFC) stack for current collection and gas distribution.

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20
Q

SOFC Advantages

A
  • High efficiency
  • Simple system
  • Internal reforming
  • longevity proven up to 100,000 hrs, robust
  • CO is a fuel
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21
Q

Disadvantages of SOFC

A
  • Brittleness of ceramics
  • Corrosion of metallic IC
  • high thermal load of peripheral components
  • limited number of thermal achievable (some hundred)
  • limited cell area achievable due to brittleness (upto 20x20 or 20x30 cm2)
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22
Q

MCFC operating temperature

A

650C

23
Q

MCFC Electrolyte

A

LixK2-xCO3 in porous LiAlO2 matrix

24
Q

MCFC Advantages

A

Large cell area
Simple system
Internal reforming
CO as fuel

25
Q

MCFC disadvantages

A

Corrosion by molten salt
Phase transition l↔ s
Low spec. power density
Limited number of cycles
High thermal loads on the periphery

26
Q

PAFC operating temperature

A

220C-160C

27
Q

PAFC Electrolyte

A

H3PO4; PBI + H3PO4

28
Q

PAFC Advantages

A

Simple cooling
No water management

29
Q

PAFC disadvantages

A

Advanced
PBI: high catholic overpotential

30
Q

Usual catalysts for high temperature FC

A

Nickel catalysts

31
Q

Usual catalysts for low temperature FC

A

PGM catalysts

32
Q

PEFC operating temperature

A

80C, but 100C-120C are sought by the automotive industry to facilitate the cooling of the systems

33
Q

PEFC and DMFC electrolyte

A

Sulfonamide polymer

34
Q

PEFC advantages

A

Industrialised
Robust with pure H2

35
Q

PEFC disadvantages

A

Intricate water management
Needs clean H2

36
Q

DMFC advantages

A

Robust
Liquid energy carrier

37
Q

DMFC disadvantages

A

Methanol permeation
Efficiency limited to < 30%

38
Q

Which fuel cells are ideally operated with H2?

A

All except DMFC (uses methanol) and MCFC (requires CO2 to maintain its electrolyte)

39
Q

What is the functional principle of a fuel cell?

A
  • In a chemical reaction, electrons are transferred and voluntary reactions release energy.
  • This energy - minus the losses that occur during the conversion - is directly converted into electrical energy. A membrane between the electrodes is used with allows only ions but not electrons to pass through. Due to the potential difference between the electrodes, the electrons balance the charge between the 2 electrodes by passing through an external load.
40
Q

What must be remembered about anode and cathode in an electrochemical cell?

A

Oxidation occurs at anode and reduction at cathode

41
Q

Explain the principle working of a PEFC

A

With the example of Nafion. It consists of a backbone of CF2 groups, i.e. a PTFE backbone which includes side groups called spacers which at the end carry an SO3H group. Later when the Polymer comes in contact with water, due to the backbone being hydrophobic and the side groups being hydrophilic, the side chains split off from the head attached to the hydrogen atom as a proton in the aqueous environment. The proton ca now diffuse in the aqueous channel in the polymer electrolyte.

42
Q

What is the principle of working for an anion conductor (e.g. SOFC)?

A

If the fuel gas is hydrogen, the oxidant is assumed to be air. At the cathode, the O^2- ions are formed at the interface between the electrolyte and the gas space. The ions travel through the electrolyte along the chemical potential gradient. If the fuel gas concentration increases, more oxygen ions migrate to the anode. Then due to the difference in electrical potentials between the electrodes, an electrical voltage is generated, where e- go from anode to cathode.
This also describes the measuring principle of the lambda sensor. If no external return transport load perceives the electrons involved in the reaction (lambda sensor), these electrons build up and electric field opposing the flow of the ions and stopping the flow of them. It then settles in an equilibrium state, which can be calculated with the Nernst equation and thus represents the method for oxygen partial pressure measurement. The voltage of an idling fuel cell is also the highest.
Fuel cells are stacked and connected in series like a battery. The electrolyte is coating on both sides with one electron layer. Several such MEAs (Membrane Electrode Assemblies) are stacked, each separated by interconnectors /bipolar / separator plates).

43
Q

What is the working principle of SOFC?

A

The working principle of the SOFC is based on the fact that the oxygen sub lattice in the zirconia crystal has defects, which are naturally formed by thermal activation. In order to enhance this effect and make it technically usable, the crystal is doped with trivalent ions instead of the tetravalent zirconium ions. Voids are formed in the oxygen sublattice since the crystal must be electrostatically neutral. The more ions, the higher the conductivity.

44
Q

What is the working principle of MCFC?

A

In MCFC a carbonate diffuses from cathode to anode. The carbonate groups in the electrolyte must be constantly reformed, which is why CO2 must be supplied on the cathode usually via exhaust gas recirculation. There CO2 and O2 for the CO3^2- group with the release of 2 e- into the electrode. The carbonate diffuses in the electric field through the molten electrolyte.

45
Q

What are the components of a FC stack?

A

Electrolyte
Cathode
Anode
IC (bipolar plate)

46
Q

What is the function of electrolyte in FC stack?

A

Ionic transport
Separation of fuel and oxidant

47
Q

What is the function of cathode in FC stack?

A

Reduction reaction
Gas transport to/from electrolyte interface
Electronic conductivity

48
Q

What is the function of anode in FC stack?

A

Oxidation reaction
Gas transport to/from electrolyte interface
Electronic conductivity

49
Q

What is the function of IC in FC stack?

A

Electronic conductivity
Separation of fuel and oxidant

50
Q

What is the requirement for electrolyte in FC stack?

A

Ionic conductivity > 10 S/m
Gas tightness
Stability in oxidising and reducing atmosphere

51
Q

What is the requirement for cathode in FC stack?

A

High electro-catalytic activity
Easy gas diffusion
Electronic conductivity > 1000 S/m
Long term stability in oxidising environment

52
Q

What is the requirement for Anode in FC stack?

A

High electro-catalytic activity
Easy gas diffusion
Electronic conductivity > 1000 S/m
Long term stability in reducing environment

53
Q

What is the requirement for IC in FC stack?

A

Electronic conductivity > 1000 S/m
Gas tightness
Stability in oxidising and reducing atmosphere

GTB (14 decks)

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