Introduction

Question 1

Define Fuel cells.

Answer 1

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

Question 2

Sources for Hydrogen

Answer 2

• Wind power
• Solar power: PV, Solar thermal
• Hydropower

Question 3

Use cases for Hydrogen

Answer 3

• 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

Question 4

Hydrogen Storage methods

Answer 4

• 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

Question 5

Methods for transport of Hydrogen

Answer 5

• 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

Question 6

Electrochemical cells can be classified into what groups?

Answer 6

• Electricity storage
• Electrochemical converters
• Sensors

Question 7

Why are fuel cell types classified according to the electrolyte?

Answer 7

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

Question 8

What are characteristic features of electrolytes?

Answer 8

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

Question 9

Some applications of fuel cells?

Answer 9

• 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

Question 10

Energy flow in fuel cells vs batteries

Answer 10

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

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

Question 11

Operation in fuel cells vs batteries

Answer 11

Continuous operation possible vs only discontinuous operation

Question 12

Reactants in fuel cells vs batteries

Answer 12

Supplied continuously vs supplied once at production

Question 13

Energy storage in fuel cells vs batteries

Answer 13

No (in a fuel tank yes) vs yes

Question 14

Electrolyte-Electrode-System in fuel cells vs batteries

Answer 14

No inherent change vs change of composition according to SOC

Question 15

Periphery in fuel cells vs batteries

Answer 15

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

Batteries:
Battery control system

Question 16

SOFC full form

Answer 16

Solid Oxide Fuel Cell

Question 17

Electrolyte used in SOFC

Answer 17

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

Question 18

SOFC operating temperature

Answer 18

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

Question 19

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

Answer 19

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

Question 20

SOFC Advantages

Answer 20

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

Question 21

Disadvantages of SOFC

Answer 21

• 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)

Question 22

MCFC operating temperature

Answer 22

650C

Question 23

MCFC Electrolyte

Answer 23

LixK2-xCO3 in porous LiAlO2 matrix

Question 24

MCFC Advantages

Answer 24

Large cell area
Simple system
Internal reforming
CO as fuel

Question 25

MCFC disadvantages

Answer 25

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

Question 26

PAFC operating temperature

Answer 26

220C-160C

Question 27

PAFC Electrolyte

Answer 27

H3PO4; PBI + H3PO4

Question 28

PAFC Advantages

Answer 28

Simple cooling No water management

Question 29

PAFC disadvantages

Answer 29

Advanced PBI: high catholic overpotential

Question 30

Usual catalysts for high temperature FC

Answer 30

Nickel catalysts

Question 31

Usual catalysts for low temperature FC

Answer 31

PGM catalysts

Question 32

PEFC operating temperature

Answer 32

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

Question 33

PEFC and DMFC electrolyte

Answer 33

Sulfonamide polymer

Question 34

PEFC advantages

Answer 34

Industrialised Robust with pure H2

Question 35

PEFC disadvantages

Answer 35

Intricate water management Needs clean H2

Question 36

DMFC advantages

Answer 36

Robust Liquid energy carrier

Question 37

DMFC disadvantages

Answer 37

Methanol permeation Efficiency limited to < 30%

Question 38

Which fuel cells are ideally operated with H2?

Answer 38

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

Question 39

What is the functional principle of a fuel cell?

Answer 39

- 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.

Question 40

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

Answer 40

Oxidation occurs at anode and reduction at cathode

Question 41

Explain the principle working of a PEFC

Answer 41

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.

Question 42

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

Answer 42

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).

Question 43

What is the working principle of SOFC?

Answer 43

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.

Question 44

What is the working principle of MCFC?

Answer 44

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.

Question 45

What are the components of a FC stack?

Answer 45

Electrolyte Cathode Anode IC (bipolar plate)

Question 46

What is the function of electrolyte in FC stack?

Answer 46

Ionic transport Separation of fuel and oxidant

Question 47

What is the function of cathode in FC stack?

Answer 47

Reduction reaction Gas transport to/from electrolyte interface Electronic conductivity

Question 48

What is the function of anode in FC stack?

Answer 48

Oxidation reaction Gas transport to/from electrolyte interface Electronic conductivity

Question 49

What is the function of IC in FC stack?

Answer 49

Electronic conductivity Separation of fuel and oxidant

Question 50

What is the requirement for electrolyte in FC stack?

Answer 50

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

Question 51

What is the requirement for cathode in FC stack?

Answer 51

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

Question 52

What is the requirement for Anode in FC stack?

Answer 52

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

Question 53

What is the requirement for IC in FC stack?

Answer 53

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