ALKALINE FUEL CELLSNallapaneni Manoj KumarM.Tech-Renewable EnergyKarunya University

What is Fuel Cell???• A fuel cell is a device that converts the chemical energy from a fuel into electricity through a chemical reaction with oxygen or another oxidizing agent.

• Hydrogen is the most commonly used fuel, but hydrocarbon such as natural gas and alcohols like methanol are sometimes used.

• Fuel cells are different from batteries in that they require a constant source of fuel and oxygen/air to sustain the chemical reaction; however, fuel cells can produce electricity continuously for as long as these inputs are supplied.

History of Fuel Cell• The first fuel cells were invented in 1838. Barrister William Grove wrote about their development of first crude fuel cells in The London and Edinburgh Philosophical Magazine and Journal of Science.

• German physicist Christian Friedrich Schonbein discussed the current generation from hydrogen and oxygen.

• In 1939, British engineer Francis Thomas Bacon successfully developed a 5 kW stationary fuel cell.

• UTC Power was the first company to manufacture and commercialize a large, stationary fuel cell systems for NASA space programs to generate power for probes, satellites and space capsules.

 William Grove's 1839 Fuel Cell

• Solid electrodes are used

• Acid electrolytes• A combination of sheet iron, copper and porcelain plates, and a solution of sulphate of copper and dilute acid are used in the fuel cell design.

Bacon's 1939 Fuel Cell 

• Gas-diffusion electrodes.• Alkali electrolytes like potassium hydroxide (KOH).• Electrodes increased the surface area in which the reaction between the electrode, the electrolyte and the fuel occurs.

• Bacon used pressurized gases to keep the electrolyte from "flooding" the tiny pores in the electrodes.

• One of the first demonstrations : In 1959 Allis-Chalmers farm tractor powered by a stack of 1,008 cells, with 15,000 watts of power, the tractor generated enough power to pull a weight of about 3,000 pounds.

Types of Fuel Cells Fuel cells are classified primarily by the kind of electrolyte they employ.

• Polymer Electrolyte Membrane (PEM) Fuel Cells• Direct Methanol Fuel Cells• Alkaline Fuel Cells(AFC)• Phosphoric Acid Fuel Cells(PAFC)• Molten Carbonate Fuel Cells(MCFC)• Solid Oxide Fuel Cells(SOFC)• Regenerative Fuel Cells

Alkaline Fuel Cells(AFC)• The alkaline fuel cell (AFC), also known as the Bacon fuel cell.

• It is one of the most developed fuel cell technologies, one among the most efficient fuel cells, having the potential to reach 70%.

• NASA has used alkaline fuel cells since the mid-1960s, in Apollo-series missions and on the Space Shuttle.

• AFCs consume hydrogen and pure oxygen producing potable water, heat, and electricity.

• High-temperature AFCs operate at temperatures between 100°C and 250°C . However, newer AFC designs operate at lower temperatures of roughly 23°C to 70°C .

AFC Design• Potassium hydroxide (KOH) as electrolyte.

• Gas-diffusion electrodes.

• Non precious metals are used as catalyst at anode and cathode.

• Designs: Static electrolyte Flowing electrolyte

Chemistry of AFC• The fuel cell produces power through a redox reaction between hydrogen and oxygen.

• At the anode, hydrogen is oxidized.

• The electrons flow through an external circuit and return to the cathode, oxygen is reduced.

• The net reaction consumes one oxygen atom and two hydrogen atoms in the production of one water molecule. Electricity and heat are formed as by-products of this reaction.

  CO2 Poisoning Effect in AFC

• Aqueous alkaline solutions do not reject carbon dioxide (CO2) so the fuel cell can become "poisoned" through the conversion of KOH to potassium carbonate (K2CO3).

• In this case blocking of cathode occurs through K2CO3

• The main problem in AFC is CO2 concentration and this is limited to some extent by making some changes in the design (Electrolyte).

• Due to this the performance and the life lime of the AFC reduces.

Methods to Reduce CO2 Concentration In AFC• Absorption• Adsorption • Electrochemical Process • Electrolyte Circulation• Use of Liquid Hydrogen • Use of Solid Anionic Exchange Membranes• Use of Pure Hydrogen as a fuel &• Use of Pure Oxygen

Lifetime of alkaline fuel cells• Another major drawbacks confronting AFC is its lifetime. The main factors limiting AFC lifetime are CO2 poisoning, corrosion, and electrode weeping

Comparison between AFC and PEMFC

Applications• Alkali cells also provide drinking water for the astronauts

• Military• In Space Satellite applications

Research ScopeSolid Anionic Exchange Membranes for Removal of Carbon dioxide