How do hydrogen fuel cells actually work?

Hydrogen fuel cells have been hailed as the ‘fuel of the future’, because of their efficient and environmentally friendly potential. They can be up to 3 times more efficient than the combustion engines usually seen in cars, and only produce water and heat as waste products.

KPMG’s global automotive executive survey found that the automotive industry is thought to be entering a ‘restructuring phase’, with the rise of fuel cell electric vehicles being the number one trend in 2050.

Aside from being efficient, hydrogen fuel cells are very reliable, safe and are scalable, so can be combined into larger systems which can be very useful.

Surprisingly, the science behind this ‘magic’ fuel and how it makes electricity is very simple!

The Basics

Electricity is generated through the flow of electrons, so essentially, the aim of a fuel cell is to somehow produce electrons. Hydrogen atoms make up the fuel source for these cells and consist of protons which carry a positive charge and electrons which carry a negative charge.

The fuel cell is made up of an anode (positively charged), a cathode (negatively charged) and a ‘proton exchange membrane’ between them. The proton exchange membrane is an electrolyte, which only lets certain chemicals travel across it.

When hydrogen is fed into the cell, the positively charged anode reacts with the hydrogen to split it into protons and electrons. The electrons are then fed into a circuit from anode to cathode to generate electricity. Meanwhile, the protons travel across the proton exchange membrane towards the cathode. Here, oxygen is fed in, to combine with the protons, and electrons from the circuit to form water molecules (H2O) which are the waste product.

 In some models of these cells, the process is slightly different, and instead oxygen ‘picks up’ electrons at the cathode, then travels through the proton exchange membrane to the anode where it combines with the protons to form water there instead.

Diagram source: Enzymatic Glucose Biofuel Cell and its Application

There are a number of ways to get the hydrogen needed as the fuel source:

  1. Thermolysis –Heating steam to 2000oC, to decompose the water into hydrogen and oxygen.
  2. Electrolysis – Passing a current through water, to split it up into hydrogen and oxygen ions, then passing the hydrogen ions over a cathode, to form hydrogen gas.
  3. Natural Gas Reforming – Reacting steam with methane to form carbon monoxide and hydrogen. The carbon monoxide then reacts with steam again to form more hydrogen.

Why are they not widely available?

Whilst the technology for many types fuel cells has been developed, there are none that are efficient enough to be widely marketed. They are all currently too expensive to be a viable replacement for current combustion technologies. Fuel cells which use liquid electrolytes are at risk from leaking, and those using solid ones are at risk from showing cracks.

Different types of fuel cell:

Type Proton exchange membrane Catalyst Properties
 Solid Oxide Fuel Cell  Hard ceramic  compound of metal  oxides  Non- platinum Operate at very high temperatures (about 1000oC)
 Direct Methanol Fuel Cell  Polymer membrane  Platinum Use liquid methanol as a fuel, run at low temperatures
 Alkaline Fuel Cell  Potassium hydroxide solution in water  Platinum  Require pure hydrogen fuel, operate at about 150-200oC
 Phosphoric Acid Fuel Cell  Phosphoric acid and ceramic electrolyte  Platinum Can use various fuel sources. Operate at about 150-220oC
 Proton Exchange Membrane Fuel  Cell  Thin sheet of polymer electrolyte  Platinum Operate at low temperatures, and need purified fuels
 Molten Carbonate Fuel Cell  Salt carbonates  Nickel Carbon dioxide needs to be periodically added. Operates at about 650oC

 

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