Alkaline Fuel Cell |Advantages |Limitations |Applications

Introduction

The first alkaline fuel cell was built by Francis Thomas Bacon in 1939. He used potassium hydroxide for the electrolyte and porous gas diffusion electrodes instead of acid electrodes and solid electrodes. Alkaline fuel cells were one the first fuel cell technology developed in the USA and widely used in US space program to produce electrical energy and water on board spacecraft.

Alkaline fuel cells ( AFCs )

• These fuel cells use solution of potassium hydroxide in water as the electrolyte and variety of non-precious metals as catalyst at the anode and cathode. 
• Recently polymer membrane as the electrolyte have been developed for AFCs. 
• These fuel cells are similar to that of PEM fuel cells except that they use alkaline membrane instead of acid membrane. 
• It produces carbon dioxide ( CO₂ ) which is key challenge. Even small percentage of carbon dioxide in the air greatly affect cell performance and durability due to formation of carbonate.  
• The alkaline cells with liquid electrolytes can be run in recirculating mode which helps to reduce the effects of carbonate formation in the electrolyte. 
• However, the recirculate mode introduces shunt current. 
• The recirculate electrolyte may suffer increased corrosion and difficult to handling different pressure. 
• The alkaline membrane fuel cells produce lower carbon monoxides as compared to that liquid electrolyte AFCs. 
• The applications of AFCs are limited to watt to kilo watts.

Alkaline Fuel Cells: Electrolyte layer

• When the fuel in the fuel cell travels to catalyst layer, the fuel molecules broken in to electrons and protons ( H+ ). 
• The electrons travel to external circuit and hydrogen protons travel through electrolyte until it reaches the cathode to combine with oxygen to form water. 
• In the AFC, the reaction is similar, but it occurs in the opposite direction with O₂ producing the hydroxide and reacting with hydrogen at the anode. 
• The electrolyte in the alkaline cell is aqueous solution of alkaline potassium hydroxide.

Alkaline Fuel Cells: Chemical reaction

• Anode : 2H₂ + 4OH^-  → 4H₂O + 4e^-
• Cathode : O₂ + 2H₂O + 4e^- → 4OH^-
• Overall reaction : 2H₂ + O₂ →  H₂O
• At cathode, O₂ is reduced in the presence of H₂O to produce OH^- which transfers to the anode and reacts with H₂ to produce H₂O.

Alkaline Fuel Cells: Property of electrolyte

• High ionic conductivity
• Be chemically and mechanically stable
• Low electronic conductivity
• Ease of availability
• Low cost

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Advantages

• Polymer electrolyte completely eliminates problem of electrolyte leakage
• Lower carbonate formation as compared to liquid AFCs
• Low operating temperature
• Quick start up

Limitations

• The key challenges are carbon dioxide, higher temperature operation, water management membrane conductivity, durability, power density and anode electrolysis.
• CO₂ as by product which affect cell performance and durability
• Recirculate electrolyte operation reduce effect of carbonate formation on the electrolyte but increased corrosion and difficult to handle different pressure.

Applications

• The AFC is well known for role in NASA apollo mission to provide electricity and water to crew members. 
• The AFCs use hydrogen as a fuel source, they are highly sensitive and can fail when exposed to carbon dioxide that why they are primarily used in under water applications and controlled aerospace.
• Military
• Space
• Back-up power
• Transportation

Stack size

1 – 100 kW