Study sheet: Fuel Cell Technology

📋 Course Outline

1. Fuel Cell Fundamentals
2. Operating Principle and Stack
3. Fuel Cell Classification
4. Phosphoric Acid and Alkaline Cells
5. Polymer and Methanol Cells
6. High-Temperature Fuel Cells
7. Fuel Cell Characteristics
8. Applications and Comparisons

📖 1. Fuel Cell Fundamentals

🔑 Key Concepts & Definitions

• Fuel cell : A device that converts the chemical energy of a fuel into electricity through a chemical reaction of positively charged hydrogen ions with oxygen or another oxidizing agent.

★ Must-know

📌 Fuel cells require a continuous source of fuel and oxygen or air, whereas batteries generate electromotive force from chemicals already present inside the battery.

Further detail

• Fuel cells can produce electricity continuously for as long as fuel and oxygen or air are supplied, while a battery supplies electricity only while its internal chemical reaction persists.

💡 Memory Hook

Fuel cells need continuous fuel and oxygen, whereas batteries consume chemicals stored inside them.

📖 2. Operating Principle and Stack

★ Must-know

• A fuel cell uses an anode, an electrolyte, and a cathode to separate ion transport from electron transport and generate electricity.

• The electrolyte is designed to allow ions to pass through it while preventing electrons from passing through it.

• A typical fuel cell produces 0.6 V to 0.7 V at full rated load, and its voltage decreases as current increases.

📌 Fuel cells are connected in series to obtain higher voltage and in parallel to supply higher current; this arrangement is called a fuel cell stack.

Further detail

• William Grove developed the first fuel cell in 1838 using sheet iron, copper, porcelain plates, copper sulfate solution, and dilute acid.

💡 Memory Hook

Anode → electrolyte → wire → cathode

📖 3. Fuel Cell Classification

★ Must-know

• Classification by electrolyte includes:

  • Alkaline fuel cell
  • Phosphoric acid fuel cell
  • Polymer electrolyte membrane or solid polymer fuel cell
  • Molten carbonate fuel cell
  • Solid oxide fuel cell

• The operating-temperature classes are:

  • Low temperature: below 150°C
  • Medium temperature: 150°C to 250°C
  • High temperature: 250°C to 800°C
  • Very high temperature: 800°C to 1100°C

Further detail

• Classification by fuel and oxidant includes:

  • Hydrogen–oxygen
  • Hydrogen-rich gas–air
  • Hydrazine–oxygen or hydrogen peroxide
  • Ammonia–air
  • Synthesis gas–air
  • Hydrocarbon gas–air

• Fuel cells are also classified by application as space, vehicle propulsion, submarine, defense, and commercial fuel cells, and by electrolyte chemistry as acidic, alkaline, or neutral.

📖 4. Phosphoric Acid and Alkaline Cells

🔑 Key Concepts & Definitions

• Phosphoric acid fuel cell : A fuel-cell type that uses liquid phosphoric acid as its electrolyte and was the first fuel-cell type to be commercialized.
• Alkaline fuel cell : A fuel cell that consumes hydrogen and pure oxygen and produces potable water, heat, and electricity through a redox reaction.

★ Must-know

📐 Formula — The PAFC reactions are: anode $2H_2 \rightarrow 4H^+ + 4e^-$, cathode $O_2 + 4H^+ + 4e^- \rightarrow 2H_2O$, and overall $2H_2 + O_2 \rightarrow 2H_2O$.

• PAFCs operate at about 150°C to 210°C, achieve 37% to 42% electrical efficiency alone and 85% efficiency in electricity-and-heat cogeneration, and are typically used for stationary power generation.

📐 Formula — In an AFC, the anode reaction is $2H_2 + 4OH^- \rightarrow 4H_2O + 4e^-$, the cathode reaction is $O_2 + 2H_2O + 4e^- \rightarrow 4OH^-$, and the net reaction is $2H_2 + O_2 \rightarrow 2H_2O$.

Further detail

• PAFC electrolyte is highly concentrated or pure phosphoric acid saturated in a silicon carbide matrix, and its carbon-paper electrodes are coated with a finely dispersed platinum catalyst.

• AFCs can reach 60% efficiency in space applications but are easily poisoned by carbon dioxide, which can combine with KOH to form potassium carbonate and increase resistance.

💡 Memory Hook

PAFC uses acidic phosphoric acid; AFC uses alkaline hydroxide ions.

📖 5. Polymer and Methanol Cells

🔑 Key Concepts & Definitions

• Polymer electrolyte membrane fuel cell : A fuel cell that uses a polymeric membrane electrolyte and porous carbon electrodes containing a platinum catalyst.
• Direct methanol fuel cell : A PEM-like fuel cell that uses a polymer membrane and a platinum–ruthenium anode catalyst to extract hydrogen directly from liquid methanol.

★ Must-know

• In PEM fuel cells, protons move through the electrolyte to the cathode, where they combine with oxygen and electrons to produce water and heat.

• PEM fuel cells operate at about 80°C, start quickly, have high power density, and offer low weight and volume.

• In a DMFC, methanol is oxidized at the anode, water is consumed at the anode, protons cross the membrane, oxygen reacts at the cathode to produce water, and electrons pass through the external circuit.

Further detail

• PEM fuel cells require hydrogen, oxygen from air, and water, and they are suitable candidates for cars, buildings, and smaller applications.

• DMFCs operate from 60°C to 130°C and are targeted mainly at portable applications because their efficiency is quite low but methanol has high energy density and is easy to transport and store.

💡 Memory Hook

PEMFC uses hydrogen; DMFC extracts hydrogen directly from liquid methanol.

📖 6. High-Temperature Fuel Cells

🔑 Key Concepts & Definitions

• Molten carbonate fuel cell : A fuel cell that uses a molten carbonate salt as its electrolyte and can be fueled by coal-derived gases, methane, or natural gas.
• Solid oxide fuel cell : An electrochemical conversion device with a solid oxide or ceramic electrolyte that produces electricity directly from fuel oxidation.

★ Must-know

• In MCFCs, negative carbonate ions travel through the electrolyte to the anode, where they combine with hydrogen to generate water and electrons.

• MCFCs operate at 650°C and above, can reach 60% efficiency, use non-precious catalysts, and do not require an external reformer.

• SOFCs operate at temperatures up to about 1000°C, conduct negatively charged oxygen ions through the electrolyte, and can internally reform methane, propane, and butane.

Further detail

📐 Formula — During internal reforming in an MCFC, methane reacts with water according to $CH_4 + H_2O \rightarrow 3H_2 + CO$.

• SOFC generating efficiency can reach about 60%, and high-temperature operation removes the need for precious-metal catalysts and permits carbon monoxide to be used as fuel.

💡 Memory Hook

High temperature → internal reforming and cheaper non-precious catalysts.

📖 7. Fuel Cell Characteristics

★ Must-know

• Fuel-cell voltage regulation is poor at very small and very large output currents, so practical operation is fixed in the BC region where regulation is best and output voltage is about 0.6 V to 0.8 V.

• As a fuel cell is loaded, its terminal voltage and efficiency decrease from the theoretical open-circuit value because of activation, resistance, and concentration polarisation.

• Concentration polarisation includes electrolyte polarisation from slow diffusion in the electrolyte and gas-side polarisation from slow reactant or product diffusion through porous electrodes.

Further detail

• Activation polarisation is significant at low current density and can be reduced using an effective electrochemical catalyst or increasing operating temperature.

• Resistance polarisation results from electrolyte resistance and electrode–electrolyte contact resistance and can be reduced with a higher-conductivity electrolyte, higher operating temperature, and suitable electrolyte shape and spacing.

💡 Memory Hook

Activation → resistance → concentration losses

📖 8. Applications and Comparisons

★ Must-know

• The main charge carriers are H+ in PEMFC and PAFC, OH− in AFC, CO3^2− in MCFC, and O^2− in SOFC.

• Electrical-efficiency ranges are 35% for PEMFC, 40% to 60% for AFC, 40% to 50% for PAFC, 50% to 60% for MCFC, and 50% to 65% for SOFC.

Further detail

• Documented fuel-cell applications include:

  • Space systems
  • Vehicle propulsion
  • Submarines
  • Defense systems
  • Commercial power generation
  • Portable devices
  • Buildings
  • Stationary power generation

• The German Navy used fuel-cell propulsion in a Type 212 submarine, Toyota introduced the Mirai fuel-cell vehicle in Japan in 2014, and Toyota displayed the FCHV-BUS at Expo 2005.

📊 Synthesis Tables

Fuel Cell Type Comparison

Type	Electrolyte and charge carrier	Operating temperature	Electrical efficiency
PEMFC	Solid polymer membrane; H+	80°C	35%
AFC	KOH in water; OH−	120°C–150°C	40%–60%
PAFC	Phosphoric acid; H+	200°C	40%–50%
MCFC	Molten lithium–potassium carbonate; CO3^2−	650°C	50%–60%
SOFC	Yttrium-stabilized zirconia; O^2−	800°C–1000°C	50%–65%