Fuel cells


Content – Energy generation


A fuel cell is a reactor converting chemical energy from a fuel (hydrogen or hydrogen rich), directly into electricity through a chemical reaction between the fuel and an oxidizing agent.

In a fuel cell, the fuel is supplied to the anode, while air (or oxygen) is supplied to the cathode.

The anode and cathode contain catalysts that cause the fuel to undergo oxidation that generate positive hydrogen ions and electrons. The hydrogen ions are drawn through the electrolyte after the reaction. At the same time, electrons are drawn from the anode to the cathode through an external circuit, producing direct current electricity. At the cathode, hydrogen ions, electrons, and oxygen react to form water (H2O).

A Hydrogen-powered fuel cell will not emit CO2 or NOx.

Fuel cells that are fuelled by natural gas (ex.methane) will have CO2 emissions. Since fuel cells provide higher energy yield than diesel engines and gas turbines, the emission of CO2 is considered less per energy unit.

Both batteries and fuel cells generate direct current electricy through chemical reactions.

Fuel cells are different from batteries in that they require a continuous source of fuel and oxygen or air to sustain the chemical reaction, whereas in a battery the chemicals present in the battery react with each other to generate electricity. Batteries either has to be replaced or recharged when discharged. Recharging would be by reversing the chemical reactions such that it reverts to it´s original chemical composition that can generate electricity when connected to an external circuit.

As the main difference among fuel cell types is the electrolyte, fuel cells are classified by the type of electrolyte they use and by the difference in startup time ranging from 1 second for proton exchange membrane fuel cells (PEM fuel cells, or PEMFC) to 10 minutes for solid oxide fuel cells (SOFC).

Individual fuel cells produce relatively small electrical potentials, about 0.7 volts, so cells are normally “stacked”, or placed in series, to create sufficient voltage to meet the requirements. In addition to electricity, fuel cells produce water, heat and, depending on the fuel source, small amounts of nitrogen dioxide and other emissions.

When hydrogen (anode) and oxygen (cathode) pass over each of the electrodes electricity and heat is produced by means of a chemical reaction.

The chemical reaction split the hydrogen into one electron and one proton.

The electrons will by means of a membrane be prevented from flowing through the electrolyte to the cathode but rather through an external circuit. This flow of electrons produce the useable electric current.

The by product of the fuel cell is water (H2O)

Types of fuel cells

AFC: Alkaline Fuel Cell

Alkaline fuel cells differ from other types of fuel cells in the chemical reaction and the operating temperature.

AFC uses an aqueous solution of potassium hydroxide (held in a matrix material, if a static electrolyte type fuel cell) electrolyte, and operates at temperatures of 90-100°C. The alkaline electrolyte provides fast cathode reactions, which enables high performance. The AFC requires pure hydrogen,

The chemical reaction that occurs at the anode is: 2H2 + 4OH− → 4H2O + 4e. The reaction at the cathode occurs when the electrons pass around an external circuit and react to form hydroxide ions, OH, as shown:

O2 + 4e + 2H2O → 4OH

This is a technology that has been used by NASA for many years

PAFC: Phosphoric Acid Fuel Cell

PAFC uses a matrix soaked with liquid phosphoric acid electrolyte and operates at temperatures of 175-200° C. The PAFC offers high fuel efficiency since the generation of electrical energy often is combined with a system for recovering the heat energy. The PAFC can use impure hydrogen as fuel.

This technology is used for electrical utility generation and for transportation.

MCFC: Molten Carbonate Fuel Cell

MCFC uses a liquid solution of lithium, sodium, and/or potassium carbonates soaked in a matrix. It operates at temeratures of 600-1000° C. The MCFC offers high fuel efficiency.

Note that there are two moles of electrons and one mole of CO2 transfered from the cathode to the anode. This is given by:
H2 + ½ O2 + CO2 (cathode) → H2O + CO2 (anode)

The Nernst reversible potential for a molten carbonate fuel cell can be described as:

E= E^0 + \dfrac{RT}{2F}ln(\dfrac{P_{H^2}P^{\dfrac{1}{2}}_{O_2}}{P_{H_2O}}) + \dfrac{RT}{2F}ln(\dfrac{P_{CO_{2a}}}{P_{CO_{2C}}})

Where “a” and “c” correspond to the anode and cathode gas supplies respectively.

The CO2 produced at the anode is commonly recycled and used by the cathode.

This allows the reactant air to be preheated while burning unused fuel and the waste heat can be used for alternate purposes as necessary.

This configuration also allows the CO2 to be supplied externally from a pure CO2 source.

SOFC: Solid Oxide Fuel Cell

SOFC uses a solid zirconium oxide to which a small amount of yttria is added. It operates at temperatures of 600-1000° C. The SOFC offers high fuel efficiency, fuel flexibility and the ability to use a variety of catalysts.

SPFC: Solid Polymeric Fuel Cell

SPFC fuel cells often go under the name of PEMFC or PEM, where PEM stands for Proton Exchange Membrane. PEM cells supplied directly with methanol are deemed DMFC (Direct Methanol Fuel Cell).

PEMFC fuel cell uses a solid organic polymer polyperfluorsulfonic acid electrolyte membrane. It operates at temperatures of 60-100° C. The PEMFC provides quick start-up.

The chemical reaction between hydrogen and oxygen that powers the fuel cell is the same as when hydrogen is burned:
2H2 +O2 ➞2H2O

The main difference between burning hydrogen and fueling a fuel cell by hydrogen is the efficiency. When hydrogen is burned in an internal combustion engine the reaction is hard to control and only converts around 20% of the available energy into useable kinetic enrgy.

The rest of the energy would be wasted as heat and spent overcoming the friction between all the moving parts of the engine. In a fuel cell the reaction between hydrogen and oxygen is very controlled and happends at a slower rate.

The main parts of a fuel cell are: a membrane (the electrolyte) (Nafion membrane), an anode catalyst, and a cathode catalyst. In a hydrogen fuel cell, hydrogen is fed into the cell and flows over the anode catalyst. When hydrogen molecules hit the anode catalyst, the H2 molecules separates into two hydrogen ions (two protons) and two electrons by the following chemical reaction:
H2➞2H +2e

The electrons flow from the anode to the cathode through the external circuit. The hydrogen ions flows from the anode through the membrane to the cathode where oxygen is separated into oxygen atoms, which have their negative charge, increased from the electrons arriving at the cathode such that it picks up the positively charged hydrogen ions:
O2 (g) → 2O(adsorbed)
O(adsorbed) + e + H+ → OH(adsorbed)

When hydrogen ions reach the cathode catalyst, they react with the oxide ions to form water molecules:
OH (adsorbed) + e + H+ → H2O

The electrons flowing through the external circuit represents the electric current generated.

Illustration of typical PEM fuel cell .

1 – The platinum catalyst at the anode makes the hydrogen split into protons (positive hydrogen ions) and electrons.
2 – Only positively charged ions can pass through the polymer electrolyte membrane to the cathode.The negatively charged electrons travels through the external circuit and an electrical current is created.
3 – Electrones and protons (positively charged hydrogen ions) are combined with oxygen at the cathode to form water which is the by product from the fuel cell.

Fuel cell efficiency

The energy efficiency of a fuel cell is measured by the amount of energy generated by the system compared to the energy stored in the fuel. We segregate between the total energy efficiency, which includes both electrical and thermal (heat) energy and the electrical energy efficiency. The total efficiency ratio is used if the fuel cell system can utilise the heat energy generated.

Factors to take into consideration when comparing the efficiency of different technologies, fuels and configuration:

  • What is measured theoretical or actual generation of useful (for work) energy.
  • Energy needed to produce, store and transport the fuel.
  • Energy needed to manufacture the system.

Fuel cells are currently assumed to provide an efficiency of 35% – 60% for simple systems and close to 80% for systems with heat recovery.

Thermal efficiency of a fuel cell

The thermal efficiency is basically defined as the amount of useful energy produced compared to the chemical energy of the fuel consumed. For a fuel cell that would be

the energy released when the fuel (hydrogen) is reacted with an oxidant (the oxygen of air)

\eta_e= \dfrac{useful-output-energy}{\Delta H}

This may be expressed as the Gibbs function change (measures the electrical work) to the Enthalpy change (measures the heating value of the fuel) in the cell reaction.

\eta_e= \dfrac{\Delta G}{\Delta H}

The efficiency of a fuel cell is expressed based on the change in the free energy for the cell reaction:

H_2 + \dfrac{1}{2}O_2 + H_2O

The produced water is in liquid form.

The chemical energy in the hydrogen – oxygen reaction is of 285.8kJ/mole (68,317 cal/g mole of H2) and the free energy available for useful work is 237.1 kJ/mole (or 56,690 cal/g mole of H2).

The thermal efficiency of an ideal (theoretical) fuel cell operating reversibly on pure hydrogen and oxygen at standard conditions would be:

\eta_e= \dfrac{237,1}{285,8}\approx0,83

The efficiency of an actual fuel cell can be expressed as the “voltage efficiency” which is the ratio of the operating cell voltage to the ideal (theoretical) cell voltage.

The actual cell voltage ould be less than the ideal cell voltage due to internal losses.

The ideal voltage of a fuel cell operating reversibly with pure hydrogen and oxygen in standard conditions is 1.229 V.

The termal efficiency based on the higher heating value for hydrogen is then:

\eta_e= \dfrac{0,83V_{cell}}{V_{ideal}}=\dfrac{0,83V_{cell}}{1,229}=0,675V_{cell}

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Fuel Cell Type Efficiency Applications
Alkaline (AFC) 60–70% electric • Military
• Space
Phosphoric Acid (PAFC) 80–85% overall with combined heat and power (CHP)
(36–42% electric)
• Distributed generation
Polymer Electrolyte Membrane or Proton Exchange Membrane (PEM)* 50–60% electric • Back-up power
• Portable power
• Small distributed generation • Transportation
Molten Carbonate (MCFC) 85% overall with CHP
(60% electric)
• Electric utility
• Large distributed generation
Solid Oxide (SOFC) 85% overall with CHP (60% electric) • Auxiliary power
• Electric utility
• Large distributed generation