Alkaline fuel cells (AFCs) are an electrochemical energy conversion technology that produces electricity from the chemical reaction between an oxidizing agent like oxygen or air and a fuel like hydrogen. Similar to other fuel cell types, AFCs consist of two electrodes - an anode and a cathode - bonded on both sides of an electrolyte. At the anode, hydrogen atoms split into positive hydrogen ions (protons) and negatively charged electrons. The electrolyte allows only the positively charged hydrogen ions to pass through it but not the electrons. These electrons are routed in an external circuit to provide electric power. At the cathode, oxygen from the air reacts with the hydrogen ions and electrons to produce water.
Principle and Mechanism of AFCs
An Alkaline Fuel Cells uses a highly concentrated aqueous solution of potassium hydroxide (KOH) as its electrolyte. The electrolyte does not conduct protons, only hydroxide ions (OH-). At the anode, hydrogen gas is catalytically split into protons and electrons. The protons react with the hydroxide ions in the electrolyte to form water. The electrons travel through the external circuit to provide electricity. At the cathode, oxygen reacts with the water, electrons and hydroxide ions to form more water. The overall reaction is:
Anode: 2H2 → 4H+ + 4e-
Cathode: O2 + 2H2O + 4e- → 4OH-
Overall: 2H2 + O2 → 2H2O
Working of an AFC
The AFC functions best at temperatures between 60-220°C. As the electrolyte is an aqueous solution of potassium hydroxide, corrosion resistant nickel alloys are commonly used as the electrodes and separator plates in AFCs. At the anode, the hydrogen gas is oxidized into protons which migrate through the electrolyte. The electrons are conducted through an external circuit to provide DC power. At the cathode, oxygen molecules are reduced by gaining electrons from the external circuit and hydroxide ions from the electrolyte to form hydroxyl ions and water. Efficient catalysts like silver, palladium or platinum are necessary at both electrodes to facilitate the electrode reactions and prevent overpotentials.
Advantages of AFCs
Some of the key advantages of AFCs include:
- High efficiency: AFCs have very high theoretical efficiencies of around 85%. Practical efficiencies average around 60-70%.
- Carbon-neutral: The only byproducts of an AFC are water and heat, making it an environmentally benign power source.
- High power density: AFCs can deliver up to 250-300 mW/cm2 which is relatively high for fuel cells.
- Fuel flexibility: Besides pure hydrogen, AFCs can also operate on hydrogen-rich fuels like methanol with minor redesigns.
- Cold start capability: Unlike PEMFCs, AFCs can start up smoothly even in sub-zero temperatures.
- Long-term stability: AFCs show good stability and lesser degradation rates if run continuously for long periods.
Challenges of AFCs
Some challenges currently hindering the widespread commercialization of AFCs include:
- Electrolyte management: The electrolyte needs to be managed carefully and maintained highly concentrated to prevent water formation.
- Catalyst poisoning: Impurities in fuel/air can poison the anode/cathode catalysts like palladium, decreasing performance and stability over time.
- Materials issues: The alkaline electrolyte is corrosive and requires expensive nickel alloys as structural materials increasing costs.
- Pressure/thermal management: Temperature and pressure need tight control for optimal performance.
- Durability: Repeated start/stop cycles and improper shut down/start up procedures can shorten durability.
- Cost: Use of precious metal catalysts and alloy components makes initial capital costs higher than other fuel cell types currently.
Applications of AFCs
Despite the challenges, AFCs have found niche commercial and industrial applications leveraging their benefits:
- Spacecraft/Satellite power: NASA has successfully flown over 140 AFC power systems in satellites for their reliable cold start capability and high efficiency.
- Military equipment: U.S. military has deployed AFC powered generators, batteries and submarines to take advantage of their cold temperature operation.
- Customized zero-emission vehicles: Small companies offer fuel cell vehicles, forklifts and backup power based on bespoke AFC systems.
- Stationary power: Backup power for telecom towers, homes, remote area electrification projects use AFC based microgrids and generators.
- Portable power: AFC based portable chargers, lanterns and batteries offer off-grid electric power for outdoor recreation and emergencies.
- Specialty submarines: Some militaries employ AFC based propulsion systems for underwater vehicles requiring endurance and silence.
Ongoing Research and Development
Research continues focused on overcoming the existing challenges to enable more widespread commercial use of AFCs. Key areas of ongoing development include:
- Improved electrolyte management techniques
- Non-precious and abundantly available catalysts
- Novel low-cost metal alloy compositions
- Thermal and water management protocols
- Membrane and electrode redesigns for higher power/efficiency
- Cost-effective fabrication methods
- Expanding applications to stationary, microgrid and transportation sectors
- Developing refueling/recharging infrastructure
- Demonstrations in actual field conditions
alkaline fuel cells offer an environmentally friendly pathway for distributed electric power generation using hydrogen as the fuel. With ongoing research aimed at reducing costs and improving performance/lifetimes, AFCs have the potential to become a serious contender for carbon-neutral stationary and portable power in both developed and developing world contexts.
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About Author:
Money Singh is a seasoned content writer with over four years of experience in the market research sector. Her expertise spans various industries, including food and beverages, biotechnology, chemical and materials, defense and aerospace, consumer goods, etc. (https://www.linkedin.com/in/money-singh-590844163)
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