SOFCs: Advanced Materials and History

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Questions and Answers

What is one of the challenges facing SOFCs in portable applications?

  • Low thermal cycling capability
  • High weight
  • Short startup time (correct)
  • High operating temperature

What characteristic allows SOFCs to achieve higher power densities in planar designs?

  • Reduced thermal cycling
  • Use of high-temperature seals
  • Higher cell voltage per stack
  • Simpler manufacturing processes (correct)

Which type of SOFC design does not require high-temperature seals to isolate oxidant from the fuel?

  • Planar
  • Tubular (correct)
  • Electrolyte-supported
  • Stacked

What is a common misconception about the voltage produced by a typical single SOFC cell?

<p>It can generate voltages of less than 1 V (A)</p> Signup and view all the answers

What issue is a critical area to address for the commercialization of planar SOFCs?

<p>Uniform distribution of temperature (A)</p> Signup and view all the answers

What does the open circuit voltage (OCV) of a hydrogen fuel cell depend on?

<p>The number of electrons transferred (B), The calorific value of the fuel (C)</p> Signup and view all the answers

Which factor is NOT a cause of voltage drop in a fuel cell?

<p>Differences in temperature (B)</p> Signup and view all the answers

How is fuel cell efficiency generally defined?

<p>The ratio of actual voltage to theoretical voltage (D)</p> Signup and view all the answers

Which of the following accurately describes a reason for efficiency losses in a fuel cell?

<p>The speed of chemical reactions on the electrodes (D)</p> Signup and view all the answers

In a typical hydrogen fuel cell, which of the following is a consequence of fuel crossover?

<p>Reduction in overall voltage (C)</p> Signup and view all the answers

What is the primary challenge of maintaining high voltage in operational fuel cells?

<p>Dealing with activation losses (B)</p> Signup and view all the answers

Which property of fuel cells allows them to be applied to reactions beyond hydrogen?

<p>Versatile electrochemical processes (D)</p> Signup and view all the answers

What defines the electromotive force (EMF) of a fuel cell?

<p>The maximum voltage available from the cell (A)</p> Signup and view all the answers

What are some advantages of solid oxide fuel cells (SOFC)?

<p>High electrical efficiency and fuel flexibility (B), Limited pollution emission (D)</p> Signup and view all the answers

What does the Open Circuit Voltage (OCV) of a fuel cell represent?

<p>The maximum voltage the fuel cell can produce without any load (A)</p> Signup and view all the answers

Which of the following best describes the efficiency of a fuel cell?

<p>Efficiency is difficult to define due to variable energy input-output ratios. (D)</p> Signup and view all the answers

What is a common cause of voltage drop in solid oxide fuel cells?

<p>Inherent resistance in the cell components (B)</p> Signup and view all the answers

Which historical figure is associated with the first reports on fuel cells?

<p>Sir William Grove (A)</p> Signup and view all the answers

How does the efficiency of solid oxide fuel cells compare to that of reciprocating engines?

<p>SOFCs have higher efficiency than reciprocating engines. (A)</p> Signup and view all the answers

What role does the power coefficient (Cp) play in wind-driven generators?

<p>It relates to the efficiency of converting wind kinetic power to mechanical power. (A)</p> Signup and view all the answers

In a hydrogen fuel cell, how many electrons are passed for each water molecule produced?

<p>Two electrons per water molecule (A)</p> Signup and view all the answers

Flashcards

Faraday constant

The charge carried by one mole of electrons, equal to 96485 Coulombs.

Open circuit voltage (OCV)

The maximum voltage a fuel cell can produce under ideal conditions when no current is being drawn.

Fuel cell efficiency

The ratio of the actual voltage produced by a fuel cell to its maximum theoretical voltage (OCV).

Activation losses

Voltage drop caused by slow reactions at the fuel cell electrodes.

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Fuel crossover

Fuel leaking through the electrolyte, reducing fuel efficiency in a fuel cell.

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Ohmic losses

Voltage drop due to resistance in the fuel cell components (electrodes, wires, electrolyte).

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Mass transport losses

Voltage drop caused by difficulty in moving reactants to or products away from the electrodes, related to concentration changes.

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Theoretical OCV

The maximum potential voltage that a fuel cell could yield if the whole energy from fuel was completely transferred to electrical energy.

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SOFC for Portable Applications

Solid Oxide Fuel Cells (SOFCs) face challenges in portable applications, including needing to be lightweight, having a quick start-up time, and maintaining thermal stability.

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SOFC Advantages for Portable Applications

SOFCs offer the advantage of superior fuel flexibility in portable applications, allowing them to operate on various fuels.

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SOFC Design: Planar

Planar SOFC design involves stacking individual cells, typically with electrolyte, electrode, or metal support. It offers advantages like simpler manufacturing and higher power density.

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Challenges of Planar SOFCs

Planar SOFCs face difficulties with gastight seals between components, managing gas flow, and ensuring uniform temperature and current distribution to prevent damage.

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SOFC Design: Tubular

Tubular SOFC designs offer a cylindrical arrangement with advantages like eliminating high-temperature seals and providing stable long-term performance.

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What is SOFC?

A solid oxide fuel cell (SOFC) is a device that converts chemical energy of a fuel, like hydrogen or hydrocarbons, into electricity through electrochemical reactions. No combustion is involved.

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SOFC Advantages

SOFCs offer high electrical efficiency (≥40%), fuel flexibility, quiet operation, and reduced greenhouse gas emissions, compared to traditional power sources like reciprocating engines or photovoltaics.

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Wind-Driven Generator Efficiency

The efficiency of a wind-driven generator is calculated by multiplying the power coefficient (Cp) of the wind turbine by the efficiency of the electric machine.

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Gibbs Free Energy Change

The Gibbs free energy change represents the maximum amount of energy available from a chemical reaction that can be converted into useful work, such as electrical energy.

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Hydrogen/Oxygen Fuel Cell Equation

The hydrogen/oxygen fuel cell reaction is: H2 + 1/2 O2 -> H2O. The negative values for Gibbs free energy change indicate energy release.

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Electron Flow in Hydrogen/Oxygen Fuel Cell

For every water molecule produced in a hydrogen fuel cell, two electrons flow through the external circuit.

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Study Notes

Advanced Materials for Energy Conversion Solid Cells (SOFCs)

  • Solid oxide fuel cells (SOFCs) convert fuel (hydrogen or hydrocarbons) into electricity through electrochemical reactions, not combustion.
  • Fuel: CO, H₂
  • Reaction Products: CO₂, H₂O
  • Anode Reaction (examples): H₂ + O₂ → H₂O + 2e⁻, CH₄ + 3O₂ → CO₂ + 2H₂O + 6e⁻
  • Cathode Reaction: O₂ + 4e⁻ → 2O₂⁻
  • Operating Temperature: 1000°C
  • Electrolyte: YSZ (Yttria-Stabilized Zirconia)
  • Other components: NiZrO₂ cermet, LaMnO₃

History of SOFCs

  • Faraday's early investigations of conduction in ceramics (1830s)
  • Sir William Grove first reported fuel cells in 1839.
  • Walther Nernst observed increased conductivity of mixed oxides (1890s).
  • Conceptual development of ion conduction through lattice defects (Schottky and Frenkel) in the 1930s.
  • SOFC patent filed by Siemens and Halske.
  • NASA's Project Gemini (1961)
  • Large stationary fuel cells develop in the 1990s & commercial applications
  • Honda begins leasing of Fuel Cell Electric Vehicles (FCV) in 2008
  • Fuel cells sold commercially as APU and stationary backup power in 2007

Efficiency and Open Circuit Voltage (OCV)

  • Electrical efficiency of SOFCs: ≥40%
  • Reciprocating engine efficiency: ≈35%
  • Photovoltaic efficiency: 6-20%
  • Wind turbine efficiency: ≈25%
  • Other advantages of SOFCs: Fuel flexibility, noise-free operation, less pollution, generating excessive heat, effective reduction of greenhouse gas

Wind-Driven Generator

  • Input: Kinetic energy of moving air
  • Output: Electrical energy
  • Power coefficient (Cp): Ratio of wind kinetic power to mechanical power in rotor shaft
  • Overall wind turbine efficiency = (Cp) x (electric machine efficiency)

Efficiency of Fuel Cell

  • Electrical power and energy output are easily calculated: Power = VI and Energy = VIt
  • Using Gibbs free energy calculation ∆Gf = Gf of products – Gf of reactants
  • ∆g f = (gfH₂O) – (gfH₂) − ½ (gfO₂)

Hydrogen/Oxygen Fuel Cell

  • Basic reaction: 2H₂ + O₂ → 2H₂O
  • 'Product' is one mole H₂O
  • 'Reactants' are one mole of H₂ and ½ mole of O₂
  • Example values of ∆h f, As, and Agf for various temperatures
  • Examples of values at 100°C, 300°C, 500°C, 700°C, 900°C, & etc
  • Negative ΔGf values indicate energy release

Open Circuit Voltage (OCV)

  • OCV formula: E = -∆Gf / 2F
  • Example: Hydrogen fuel cell at 200 °C, E = 220,000 / (2 x 96,485) = 1.14 V
  • z is the number of electrons transferred per molecule of fuel.

Fuel Cell Efficiency

  • Efficiency calculation for different water product forms. Includes different temperatures (e.g. 25°C, 80°C, 100°C, 200°C, 400°C, 600°C, 800°C, & etc.)
  • Relationship between Maximum Efficiency (or Reversible Open Circuit Voltage), ∆gf, and ∆h f

Efficiency and Fuel Cell Voltage

  • Operating voltage of a fuel cell is easily related to its efficiency
  • If all hydrogen fuel energy is transformed into electrical energy, EMF= -∆Hf / 2F
  • Cell efficiency = (actual voltage) / E (EMF)
  • efficiency = μf Actual Voltage / E

Additional information:

  • OCV of hydrogen fuel cell, various losses result in operational voltage (e.g. Activation, Ohmic, and concentration losses)
  • Causes of voltage drop in Fuel Cells
  • SOFC Design types (Planar and Tubular)
  • SOFC Advantages (simpler manufacturing, Higher power densities, High temperature operation & etc) and disadvantages (manufacturing costs, sealing problems, and etc)
  • SOFC applications (Small SOFC Systems, Large SOFC Systems, Portable SOFC, and etc)
  • SOFC classification by Temperature levels, Cell Type, and Flow

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