Understanding the Fuel Cell Technology and the chemistry behind it
The Fuel Cell Technology : Hydrogen and oxygen in, water out, perfectly clean fuel-cell technology, but this whole thing is not simple like this sentence. But, the hydrogen that fuels them, that has to come from somewhere and that somewhere might be a lot less green than you think it is. Here our focus is not on the source of hydrogen but on the technology for this complete fuel cell operation.
The fuel cell technology has the potential to power the automotive industry
Basically, a hydrogen car is an electric car having the same powertrain which drives any BEVs. The only difference is the energy is stored in the form of hydrogen instead of electricity in a battery. Hydrogen density is too low so it is filled up in the hydrogen tank under 700 bar pressure.
Functioning of a fuel cell system
A hydrogen fuel cell is a lot like a sandwich. There is an anode or negative electrode on one side, then there’s an electrolyte in the middle. The electrolyte has protons that can move between the two sides. It could be a liquid electrolyte, like sulfuric acid, or a solid electrolyte known as a proton exchange membrane. And then, there’s a cathode or positive electrode on the other. Hydrogen gas flows over the anode where a catalyst, usually something like platinum, splits it apart into protons and electrons. The protons flow right through the proton exchange membrane electrolyte to the cathode. The electrons are also drawn toward the cathode where they’ll re-combine with the protons. But, they can’t get through the PEM. Instead, the electrons leave the fuel cell and take a different path through a circuit powering an electric motor and the motor turns the vehicle wheels. The electrons finally travel onto the cathode. Air, which is about 21% oxygen, flows over the cathode. So when the proton and electron from the hydrogen reach the other side of the stack they can combine with oxygen to form water.
So, in short – Hydrogen and oxygen are in power and water out – electricity generates which powers the electric motor to drive the fuel cell vehicle.
History of Fuel cell technology
- The first fuel cells were invented and demonstrated by Sir William Grove in 1839.
- The very first commercial use of fuel cells came more than a century later following the invention of hydrogen-oxygen fuel cell by Francis Thomas Bacon in 1932.
- The alkaline fuel cell, also known as the Bacon fuel cell after its inventor, has been used in NASA space programs since the mid-1960s to generate power for satellites and space capsules. Since then, fuel cells have been used in many other applications.
- Fuel cells are used for primary and backup power for commercial, Industrial and residential buildings and in remote or inaccessible areas.
- They are also used to power fuel cell vehicles, including forklifts, automobiles, buses, boats, motorcycles and submarines.
Advantages and Disadvantage of Fuel Cell technology
Advantages
Direct transfer of chemical energy to electrical energy.
Not dependent on the Carnot-process and higher efficiency at low temperatures
No emissions of pollutants or noise
No moving parts.
Very low refilling time as compared to batteries.
Disadvantages
High manufacturing costs (due to low volumes).
Expensive materials (e.g. Pt).
Generation, distribution and storage of hydrogen is expensive.
Types of Fuel Cell
Basically there are two criteria two simplify our understanding about the fuel cell technology:
Temperature
Electrolyte
• <600°C: Low temperature fuel cell (LT)
• >600°C: High temperature fuel cell (HT)
• Alkaline fuel cell (AFC)
• Polymer electrolyte membrane (PEM) – fuel cell (PEMFC)
• Direct methanol fuel cell (DMFC)
• Low temperature – PEM fuel cell (LT-PEMFC)
• High temperature – PEM fuel cell (HT-PEMFC)
• Molten carbonate fuel cell (MCFC)
• Solid oxide fuel cell (SOFC)
• Phosphoric acid fuel cell (PAFC)
- Key Facts about AFC:
- 60 – 80°C (LT)
- aqueous potassium hydroxide solution as electrolyte
– Environmental hazardous (Safety)
– short lifespan (due to corrosive electrolytes) - Fuel: H2
- H2 + ½ O2 → H2O
- inexpensive catalysts (Nickel, Silver)
- CO2-tolerance: ≤ 1ppm
– pure oxygen as oxidizer - 10 – 100kW
- Space program (Apollo, Space Shuttle)
- Key Facts about DMFC:
- Ca. 80°C (LT)
- Proton conductive membrane as electrolyte (humidification with material flows or product water)
- Fuel : CH3OH (methanol)
- CH3OH + 3/2 O2 → 2 H2O + CO2
- Up to 5kW
- No additional humidifier necessary
- Diffusion of methanol through the membrane reduces efficiency
- Military field batteries, entertainment electronics e.g. laptops,
- Power supply (<1kW) if H2, or natural gas not available
- Key Facts about LT-PEMFC:
- 60 – 120°C (LT)
- Proton conductive membrane as electrolyte (humidification with material flows or product water)
- Fuel: H2
- H2 + ½ O2 → H2O
- Up to 500kW
- Humidification / water management necessary
Anode: Back diffusion of water
Cathode: Humidification of the suppled air - Good dynamic behavior
- CO-tolerance: ≤ 10ppm
- Mainstream!
- Key Facts about HT-PEMFC:
- 120 – 200°C
- Polybenzimidazole – Membrane (PBI) as electrolyte doped with phosphoric acid
– Water not necessary for conductivity (no costly water management)
– High operating temperatures possible (small heat exchanger bottles, but higher temperatures in overall system) - Fuel: H2
- H2 + ½ O2 → H2O
- Up to 500kW
- CO-tolerance: ≤ 500ppm up to 1%
- Condensation of product water must be avoided
- High technological potential, but still in development phase
- Key Facts about MCFC:
- Ca. 650°C (HT)
- Carbonate salts – molten bath as electrolyte
- Fuel: H2, CO (through internal reformation of methane energy source)
- Oxidation gases: O2, CO2
- H2 + ½ O2 + CO2 → H2O + CO2
- Up to 2MW (small power plant)
- No expensive noble metal catalysts necessary
- High temperature materials needed
- Overall efficiency ca. 90% with heat utilization
- Key Facts about PAFC:
- 160 – 200°C (LT)
- Concentrated phosphoric acid (90-100%!) as electrolyte,
- Fuel: H2
- H2 + ½ O2 → H2O
- CO-tolerance: ≤ 1%
- 50kW – 11MW
- Small power plan
- High costs
- Key Facts about SOFC:
- Ca. 600-800°C (HT).
- Yttrium stabilized zirconium dioxide as electrolyte.
- Fuel: H2, CO (internal reforming).
- Oxidation gas: O2, CO2
- H2 + ½ O2 → H2O & CO + ½ O2 → CO2
- Up to 100MW
- External heater necessary to start fuel cell
- Simple, robust, no liquid management necessary.
- APU, REX, CHP, SOEC, Co-SOEC.
Fuel Cell application in Automotive Industry
The Fuel Cell & Battery Electric Vehicle topology is most comparable to the ICE EREV topology, but HV Battery sizing for mainstream FCEV compares to ICE Hybrid. Along with the car this technology is main future driver for the heavy commercial vehicle like – Fuel Cell Truck, Buses or also for commercial passenger vehicles.
Packaging of complete drive unit along with FC system
Whether we talk about the BEVs or FCEVs there is always a constant challenge which is packaging the whole system underneath the limited space available in a car. Fuel cell vehicles have fuel tanks similar to an ICE car but other components are very much similar to battery electric cars. Some of the fuel cell vehicles are also having one small battery pack to support quick and high power delivery on demand, also to store the energy generated through the regeneration process. This whole thing makes challenging for an engineer to package.
Below you see some of the actual fuel cell vehicle system packaging-
There are several question marks on the full existence of fuel cell vehicles in the market because the OEMs themselves look in dilemma. Where the all automotive manufacturers are focusing on BEVs but same time Hyundai and Toyota still pushing their fuel cell cars. BMW also testing its several sedan and SUVs on hydrogen-powered technology. Below you see the images of some of the BMW proto vehicles. Few of them are expected to launch soon.
There are several development programs related to fuel cell electric vehicles going on in India. From electric two-wheeler to passenger buses. That we will cover in upcoming articles.
Note: Source for categorization of fuel cell :
H. Eichlseder und M. Klell, Wasserstoff in der Fahrzeugtechnik; Erzeugung, Speicherung, Anwendung, Wiesbaden: Springer Vieweg, 2012
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