Lithium-ion batteries

When you walk around having a chat to your friends on your mobile phone, or watch a show on your tablet, or do your homework on your laptop in a cosy cafe, have you ever stopped to wonder about the amazing revolution in chemistry that allows you to do these "every day" things?

Batteries that are small, that can store enough electrical energy so that they can be used continuously for hours, can be quickly recharged, and can be discharged and recharged many, many times, are a very recent development. Without these batteries your life-style would be a lot less mobile!

What makes these batteries so special?

Find out in the December 2019 issue of AUS-e-NEWS

https://www.ausetute.com.au/ausenews.html

New Electrochemistry Resources

AUS-e-TUTE has been updating its electrochemistry section.
The latest additions have been tutorials, games, tests, exams and drills on the following topics:

• Strength of Electrolytes
• Galvanic (Voltaic) Cell Diagrams
• Electroplating Concepts

Oxygen Evolution Reactions

Oxygen Evolution Reactions (OER) produce molecular oxygen via chemical reactions. These reactions are being studied because of their importance to the development of energy-storage systems.

During photosynthesis, molecular oxygen is produced from water as shown in the equation below:
2H₂O → 4e + 4H⁺ + O_2(g)so photosynthesis is an Oxygen Evolution Reaction.

The electrolysis of water, is another Oxygen Evolution Reaction. Oxygen gas is produced according to the equation shown below:
2H₂O_(l) → O_2(g) + 2H_2(g)
In either case, the chemical reaction to split water in order to produce oxygen gas is not spontaneous, it requires the addition of energy. In the case of the electrolysis of water in the laboratory, electrical energy is supplied from a battery or a power pack. In the case of photosynthesis, sunlight provides the energy for the reaction in the form of photons.
A catalyst can be used to speed up the reaction. In the case of photosynthesis, chlorophyll is the catalyst present in green plants. Scientists are continually working to find a good catalyst that will work just like chlorophyll. TiO₂, SrTiO₃, and BaTiO₃ have all been investigated as possible catalysts.

A team of MIT researchers have just found one of the most effective catalysts ever discovered for an Oxygen Evolution Reaction. The new catalyst is composed of cobalt, iron, oxygen and some other metals.

Reference
J. Suntivich, K. J. May, H. A. Gasteiger, J. B. Goodenough, Y. Shao-Horn. A Perovskite Oxide Optimized for Oxygen Evolution Catalysis from Molecular Orbital Principles. Science, 2011; DOI: 10.1126/science.1212858

Further Reading
Carbon Cycle
Electrolysis
Mass-Mole Calculations
Mole Calculations
Gas Tests

Suggested Study Questions

1. Assuming 18g of water is to be split to provide oxygen gas. Calculate the maximum mass of oxygen gas that could be obtained.
2. Assuming 1L of water is to be electrolyzed at 25^oC and 1 atmosphere pressure. What is the maximum volume of oxygen gas that could be produced?
3. During the electrolysis of water, what is produced at the
   • cathode
   • anode
4. During the electrolysis of water, what is the charge on the
   • cathode
   • anode
5. During electrolysis name the electrode at which each of the processes below occurs:
   • oxidation
   • reduction
6. Write balanced equations to represent the reaction that occurs at the
   • cathode
   • anode
7. Calculate the minimum voltage that would be required in order for the electrolysis of water to produce both hydrogen gas and oxygen gas.
8. Describe a test that you could conduct in the laboratory that would allow you to say which of the gases evolved during the electrolysis of water is oxygen and which is hydrogen.
   
   
   
   
   

Hydrogen from Water Splitting

The production of hydrogen as an alternative fuel to current fossil fuels relies on the creation of a suitably cheap and efficient way to split water using the power of sunlight. Monash University scientists in Australia, working with UC Davis scientists in the USA, have found that a manganese mineral known as birnessite can be used as a catalyst to speed up the splitting of water into hydrogen and oxygen gases.

Birnessite, a soft, black mineral formed from precipitation reactions in lakes, oceans and groundwater, is predominantly an oxide of manganese, but calcium, potassium and sodium are also present in smaller amounts.
The formula for birnessite is (Na_0.3Ca_0.1K_0.1)(Mn⁴⁺,Mn³⁺)₂O₄ · 1.5 H₂O
As a catalyst for the water splitting reaction, the manganese in the birnessite cycles between oxidation states. First, when a voltage is applied manganese (II) is oxidized to manganese (IV). Then in sunlight, birnessite goes back to the manganese (II) state.

The water splitting reaction has two steps:

1. Two molecules of water are oxidized to form one molecule of oxygen gas, four protons and four electrons.
   
2. The protons and electrons combine to form two molecules of hydrogen gas

Reference:
Rosalie K. Hocking, Robin Brimblecombe, Lan-Yun Chang, Archana Singh, Mun Hon Cheah, Chris Glover, William H. Casey, Leone Spiccia. Water-oxidation catalysis by manganese in a geochemical-like cycle. Nature Chemistry, 2011; DOI: 10.1038/nchem.1049

Further Reading
Oxidation States (Numbers)
Oxidation and Reduction
Balancing Half Equations
Electrolysis - Electrolytic Cells
Percentage Composition

Study Questions:

1. What is meant by the term oxidation state (or oxidation number)?
2. What is the oxidation state (or oxidation number) for each of the following:
   
   • Mn³⁺
   • Mn⁴⁺
   • manganese (II)
   • manganese (IV)
     
3. Write equations to represent each of the following:
   
   • The oxidation of manganese (II) to manganese (IV)
   • The reduction of manganese (IV) to manganese (II)
4. For each reaction in question 3 above, identify:
   • the oxidant
   • the reductant
     
5. Write an equation to represent the first step in the water splitting reaction.
6. Write an equation to represent the second step in the water splitting reaction.
7. Use the equations in question 5 and 6 above to write an overall reaction for the water splitting reaction.
8. For each equation in questions 5 and 6,
   • label the reaction as an oxidation or reduction reaction
   • identify the oxidizing agent(s)
   • identify the reducing agent(s)
9. In the formula of birnessite, (Na_0.3Ca_0.1K_0.1)(Mn⁴⁺,Mn³⁺)₂O₄ · 1.5 H₂O, what does the 1.5 H₂O mean?
   
10. Calculate the percentage composition of birnessite.