- The best way to use the energy from hydrogen is fuel cells.
- The main advantage of using hydrogen in a fuel cell over using as a fuel is the increase in efficiency which is around 60% for solid-oxide fuel cell and around 50% for PEM fuel cell. (heat energy is a poor form of energy compared to electric energy)
- Before you read about fuel cells do read the section on batteries below.
Principle behind fuel cells
- Fuel cell is just electrolysis in reverse. Meaning in a fuel cell you pump hydrogen and oxygen which combines to form water, in the process you can derive electricity.
How does it work?
- You pump hydrogen on one side and separate it into its constituent ion and electron.
- Make the electrons to pass through a wire and you have electricity.
- In addition, we use an electrolyte which is simply a membrane that allows ions to pass through and not electrons to pass through.
- The separation of hydrogen into its constituent electron and hydrogen ion is brought about by a platinum catalyst. The challenge is its high cost.
- Finally, when ions and electrons recombine the resultant hydrogen is mixed with oxygen to produce water. This produces heat which is a deciding factor in choosing the fuel cell. (operating temperature of fuel cell)
Types of fuel cells
- Please note that the only fuel cells that are feasible for use in automobile are the ones which have manageable operating temperatures.
- Following are the fuels cells that are relevant for use in electric vehicles.
Proton-exchange membrane fuel cell
- The example we have taken to illustrate the working of fuel cell above are called proton-exchange fuel cell.
Solid oxide or Methane Fuel cells
- Alternately we can use methane or even ethanol out of which you can derive hydrogen inside the fuel cell itself.
- However operating temperatures are high upon formation of water (around 200 degree celsius).
- Due to high operating temperature, we can use solid electrolyte, typically a meta oxide. Thus, the name.
- Having solid electrolyte is important for use in consumer electronics.
- However, one disadvantage of such fuel cell is that the solid electrolyte used are typically rare earth elements. (eg: cerium, gadolinium, yttrium called YSZ)
- These are expensive and also environmentally harmful)
Batteries and Fuel Cells
- Batteries or fuel cells are devices that derive electricity out of a chemical reaction that happens.
- The chemical reaction is simply pulling out electrons from something and making it travel along a metallic wire producing electricity. The thing about electron is once you pull it out of something it needs a place to go.
- The terminals from where electrons are pulled out and sent to are called electrodes.
- The terminal from where electrons are pulled out are called anode and those where electrons are sent to are called cathode.
- The chemical reaction that leads to pulling out of electrons is called oxidation and that that leads to dumping of electrons is called reduction.
- So typically, oxidation happens at anode and reduction happens at cathode. In the process of electron going from anode to cathode, the energy is flow of electrons (electricity) is captured.
How does battery store energy?
- Battery is a device where energy is stored for use later.
- So, in order to store energy, you need an energy source.
- The process of storing energy in a battery is what you know as charging the battery. Similarly, the process of using the energy is discharging of battery.
- Charging and Discharging the battery
- Here you pump energy from an outside source to take electrons of out some chemical from where it does not want to come and send it to another chemical where it does not want to go. Once you do this the electrons want to run back to its original place.
- This is like sending electrons uphill(charging) and allowing the electrons roll back downhill by itself (discharging).
What chemicals to use?
- Everything depends on element’s affinity (likeability) for electrons. Simply elements which like electrons and those that do not like electrons.
- The likeability for electrons decides the electrochemical potential of that element.
- Electrochemical potential is simply how readily an element wants to lose or gain electrons.
- Higher the electrochemical potential the element wants to readily lose electron, lower the electrochemical potential it wants to gain electrons.
Working of a battery
- The trick is to take 2 different metals with different electro-potential so that one wants to gain, and one wants to lose electron.
- Eg: Zinc and Copper.
- As it is evident from the table, Zinc wants to give electron and Copper wants take it. (that’s why Zinc is anode and Copper is cathode)
- So, if you somehow give a path for these electrons moving from Zn to Cu, you will have derived electricity.
- When all the electrons have come to Cu, you can no longer derive electricity.
- So, you have to forcefully pull electrons out of Cu (cathode) and put it back at Zn (anode). This is charging.
- Once you have forcefully pulled out all electrons from Cu and have put it back at Zn, you can again rely on the natural flow of electrons from Zn to Cu to derive electricity. This is discharging.
- One last thing is about the electrolyte. An electrolyte is like a semi-permeable membrane for electrons and ions. It allows the ions to pass through them but not electrons.
- Putting all the pieces together
- In a battery a chemical reaction takes place where you are separating electrons and ions from the anode material.
- Once separated you make 2 different paths for these electrons and ions to move. For electrons you connect the anode to a metallic wire and for ions you dip the anode in electrolyte.
- Electrons will move towards cathode along the wire and ions move towards cathode along the semi-permeable electrolyte.
- Electrons movement through the wire gives you electricity.