What is a fuel cell?

A fuel cell uses the chemical energy of hydrogen, or other fuels to produce clean and efficient electricity. Compared to combustion engine technology, they are more energy-efficient and environmentally friendly and they produce water as a by-product instead of harmful gases.

Although the manufacturers claim hydrogen fuel cells are eco-friendly, hydrogen itself has many drawbacks. It is not freely available like crude oil or lithium. Electrolysis produces hydrogen in power plants. It is then stored and transferred to the manufacturer’s stack of fuel cells. This brings down the efficiency to approximately 40% as compared to 20% efficiency in combustion engines. The operating temperature of the hydrogen fuel cell engines is around 400°C to 1000°C.

Working of a fuel cell

Although there are various types of fuel cells, the working mechanism is more or less the same. A hydrogen atom goes into the anode. A catalyst like platinum splits the atom into electron and proton, the electron then flows to make electricity. At the cathode end, the proton and electrons of hydrogen and oxygen bond to form water. Thus, a fuel cell requires hydrogen and oxygen atoms to operate.

As fuel cells generate electricity using an electrochemical process instead of combustion, they are not limited by thermodynamic laws which is the case with conventional power plants. Because of this reason, it significantly reduced the losses with fuel cell technologies. On top of good efficiency, they do zero-emission of toxic gases like sulfur dioxide or carbon dioxide. This produces DC output, which can be converted into AC using an inverter.

Electrolytes in fuel cell technologies

The illustration of fuel cell technology looks easy to understand, but it is more difficult to construct an inexpensive and efficient fuel cell. The imperative components in fuel cells are the electrolytes which decide the price and efficiency of the cell. Nowadays, some of the widely used electrolytes are molten carbonate, phosphoric acid, polymer electrolyte membrane, and solid oxide. The operating temperature of the cell depends on the electrolyte. For example, molten carbonate fuel cells run hot, just as the name implies.

Types of fuel cells

There are several types of fuel cells available in the market and are currently undergoing development to improve the price to output ratio. Some of them are:

  • Polymer electrolyte membrane fuel cells (PEMFC)
  • Phosphoric acid fuel cells (PAFC)
  • Solid oxide fuel cells (SOFC)
  • Molten carbonate fuel cells (MCFC)
  • Direct methanol fuel cells (DMFC)

Polymer electrolyte membrane fuel cells

The PEM fuel cell technology uses acidic electrolyte and platinum-based electrodes. These cells work at low operating temperatures (80-100°C). Due to low operating temperature and usage of precious metal electrodes, they must use pure hydrogen. Scientists are working to decrease the price of these fuel cells. As platinum is a core component, this makes the fuel cell technology more expensive as compared to its counterparts. Recently, PGM-free (platinum group metals) catalysts have been developed and research is eliminating the platinum-based electrodes to reduce the power cost to $30/kWh as per the United States Department of Energy.

Electrochemical process inside a PEMFC
Image Credits: https://commons.wikimedia.org | Albris

Phosphoric acid fuel cells

The electrolyte in these fuel cells is phosphoric acid. The acid is in a silicon carbide matrix and carbon electrodes containing platinum as a catalyst. These were one of the first commercially used fuel cells and are suitable for stationary power generation. PAFCs are more tolerant of impurities like carbon monoxide as compared to PEMFCs. Although their solo electricity production efficiency is not very good, which lies between 36%-41%, but still better than coal-fired power plants whose efficiency is around 32%. But, co-generation can lead to efficiency as high as 85%.

Solid oxide fuel cells

These are made of hard ceramic electrolytes and produce around 60% efficiency. Unlike pure hydrogen fuel cells, they produce carbon monoxide which can be used for other tasks. Applications that can use waste heat (co-generation) can increase the overall fuel efficiency to 85%. They are also known as solid oxide electrolyzer cells.

In chilly places, the SOFCs can produce electrical energy, and the waste heat energy generated can warm rooms. This is co-generation and also applies to other fuel cells.

Molten carbonate fuel cells

MCFCs are fuel cells that operate at around 650°C and use electrolytes made of molten carbonate and lithium aluminum oxide matrix. Since they operate at such high temperatures, inexpensive elements can be used at anode and cathode instead of platinum.

Direct methanol fuel cells

These cells are powered by pure methanol which is mixed with water before sending it to the anode. Methanol is easier to transport as it is available in liquid form like gasoline. We use this technology in portal fuel cell applications like cell phones or laptops.

Applications of fuel cells in vehicles

Automobile manufacturers have been trying to tap into the FCEV (Fuel cell electric vehicles) market. Some vehicles are:

  • Hyundai NEXO fuel cell SUV
  • Honda Clarity fuel cell
  • Hyundai XCIENT

The Honda Clarity fuel cell is a hydrogen-powered vehicle that uses a proton exchange membrane fuel cell with an energy output of 103 kW. As compared to IC engine cars, vehicles based on fuel cell technologies are quieter when moving because of a fewer number of moving parts. It is also available in two more variants, electric and plug-in hybrid. The price of a Honda clarity fuel cell is around $35,000, which is expensive compared to similar cars running on IC engines. The average price of hydrogen fuel is $17/kg. The capacity of a fuel cell vehicle is 5kg-6kg, traveling up to 400 miles in one fill-up. If we compare to conventional cars, then refueling an empty hydrogen fuel tank is a lot more expensive. To refuel, an empty hydrogen fuel cell will cost around $90, while it costs around $35 to fill an empty gasoline tank. Although the mileage is great provided its tank size and short refueling time, refueling stations are rare in the USA and most of them are in California.

A Honda Clarity fuel cell car charging at a fuel cell recharge station.
CC BY-SA 4.0 | Image Credits: https://commons.wikimedia.org | Mariordo (Mario Roberto Durán Ortiz)

Fuelcell energy

FuelCell Energy is a Danbury-based organization that uses its state-of-the-art fuel cell technologies to design, install and maintain power plants based on green energy.

Electric vehicles

A battery-electric vehicle solely relies on chemical energy stored in rechargeable battery packs. They use electric motors instead of IC engines or fuel cells. Recharging stations for battery-powered vehicles are increasing at an excellent rate, so, unlike FCEV, BEV owners can easily refuel their vehicles. Although charging time is drastically different in either case. BEVs take around 8 to 10 hours while FCEVs takes less than 5 minutes to full charge.

Context and Applications

The study of fuel cell technology and electric vehicles are included in various undergraduate and postgraduate degrees like-

  • Bachelors in Automobile Engineering
  • Bachelors in Science in Chemistry
  • Bachelors in Mechanical Engineering
  • Masters in Power Engineering

Practice Problems

1. Which of the following is a crucial element when producing electrical energy from fuel cells?

  1. Nitrogen
  2. Oxygen
  3. Carbon
  4. Sulfur

Correct option- b

Explanation: Oxygen combines with hydrogen to form water at the cathode end of the fuel cell hence making the process eco-friendly.

2. Which of the following batteries do electric vehicles use?

  1. Lead-acid batteries
  2. Sulfur batteries
  3. Aluminum batteries
  4. Nickel batteries

Correct option- a

Explanation: Electric vehicles use lead-acid batteries for electrical loads because of their low price and high capacity.

3. Which element is used as a catalyst in fuel cells?

  1. Silver
  2. Gold
  3. Carbon
  4. Platinum

Correct option- d

Explanation: We use Platinum as a catalyst in fuel cells because of its ability to split hydrogen atoms. The hydrogen electron is used for creating electricity and the proton is transferred to the cathode.

4. What is the meaning of an off-grid EV charging station?

  1. Facility to charge electric vehicles using hydropower energy.
  2. Facility to charge electric vehicles through wind energy.
  3. Facility to charge electric vehicles in remote areas where there is no grid supply.
  4. Facility to charge electric vehicles using geothermal energy.

Correct option- c

Explanation: Rural areas where there is no electricity supply, manufacturers build off-grid EV charging stations so that electric vehicles can recharge their batteries.

5. What does a hydrogen compressor do to hydrogen?

  1. Decreases volume and increases pressure
  2. Decreases pressure and increases volume
  3. Increases both pressure and volume
  4. Decreases both volume and pressure

Correct option- a

Explanation: A hydrogen compressor lowers the volume by the increasing pressure of hydrogen so that we can store it in smaller capacity storage. By compressing, we get compressed hydrogen or liquid hydrogen.

  • Electrochemical cell
  • Lithium-ion batteries
  • Solar off-grid system

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