Electrode Potentials and Fuel Cells Flashcards
What is the oxidation state of uncombined elements?
Zero.
For a monoatomic ion, what is the oxidation state of the element?
Its the same charge of the ion.
What are the oxidation states of oxygen?
The oxygen state in all compounds is -2 but in hydrogen peroxide (H2O2), when it is -1, or in OF2 when it’s +2.
What is the sum of all oxidation states in a neutral compound?
Zero.
What must the sum of all the oxidation states in a compound equal?
The overall charge.
What is oxidation?
The loss of electrons.
What is reduction?
The gain of electrons.
What is a reducing agent?
A reactant that causes another reactant to be reduced in a reaction (itself, usually, being oxidised).
What is an oxidising agent?
A reactant that causes another reactant to be oxidised (itself usually being reduced).
What do fuel cells do?
They generate electricity from an electrochemical reaction in which oxygen (from air) and a fuel (e.g. hydrogen) combine to form water. The electricity produced can be use to power cars, buses etc. The by-product, heat, can also be used.
Describe the structures of fuel cells.
Fuel cells consist of 2 electrodes, a negative anode and a positive cathode.
The electrodes are separated by a by a solid or liquid electrolyte.
Catalysts (e.g. Pt) are often used to speed up reaction.
Electricity is generated when oxygen and hydrogen combine to form water.
In a Proton Exchange Membrane Fuel Cell (PEMFC), what is the fuel and what is the catalyst?
Fuel is Hydrogen, catalyst is Platinum.
In a Proton Exchange Membrane Fuel Cell (PEMFC), describe the operation.
- Hydrogen is oxidised to protons at the anode
- Protons move through the electrolyte
- Electrons pass through the external circuit
- Oxygen is produced at the cathode
- Water is produced
- A catalyst accelerates the reaction at the electrodes
In a Proton Exchange Membrane FUEL CELL (PEMFC), What happens at the anode?
2H2(g) -> 4H+(aq) + 4e-
Eo = 0.00V
In a Proton Exchange Membrane FUEL CELL (PEMFC), what happens at the cathode?
O2(g) + 4H+(aq) + 4e- -> 2H2O(l)
E0 = +1.23V
In a Fuel Cell (PEMFC), what is the overall reaction?
2H2(g) + O2(g) -> 2H2O(l)
Eo = +1.23V
What happens if other substances travel though an electrolyte?
They can disrupt the chemical reaction.
Why use fuel cells?
Need alternative to fossil fuels which are non-renewnable and prices and resources are dwindling. The atmosphere is becoming more polluted.
What are the limitations of using hydrogen-oxygen fuel cells?
- storage of hydrogen and transportation
- feasibility of liquefied hydrogen under pressure
- limited life cycle of ad/bsorber and cell
- high production cost and use toxic chem. in cells.
How is hydrogen manufactured?
Ideally from non-polluting and renewable resources; (solar, wind or hydropower), from hydrocarbon fuels by reforming, from methane of ethanol, electrolysis of water.
How is hydrogen made from methane?
Reforming
CH4 + H2O –> CO + 3H2
How do you store hydrogen?
- liquid stored under pressure or
- adsorbed on the surface of a solid or
- absorbed within a solid
What are the advantages of having fuelled cell vehicles (FCV’s)?
- Produce less pollution from exhaust gases (no NOx, CO, unburnt hydrocarbons)
- produce less CO2
- are more efficient
What are the pros for using fuel cells?
- Water is the waste product (no poluuting waste products)
- It is more efficient that conventional cells
What are the cons of using fuel cells?
- They’re expensive to make (Pt and membrane is $$).
- Production of fuel cells involve the use of toxic chemicals, which need to be disposed of afterwards ($$$).
- Have a limited life span so need to be replaced which means new ones need to be made and old ones need to be disposed of (which is $$$).
Describe the standard hydrogen electrode.
Reaction: 2H+(aq) + 2e- H2(g)
Pt electrode, 1M of H+ solution, H2 gas at 1atm, potential is 0.00V.
What is standard electrode potential?
The potential difference of a cell when the the electrode is connected to the standard hydrogen electrode under standard conditions.
What is the standard elecrode potential value affected by?
- temperature
- pressure of any gas
- solution concentration
The ultimate reference for SEP is the standard hydrogen electrode. What are its conditions?
298K, 1m= mol dm-3 with respect to H+ and hydrogen at 1atm, Pt catalyst.
Why use platinum as a catalyst?
You need an inert metal to get the electrons in/out. They’re inert and aren’t involved in the reaction.
What needs to be overcome before a hydrogen economy can be established?
- Ppl concerned over safety and reliability
- Infrastructure for HF supplies is $$ to set up
- If H is going to be the future, it needs to be cheap or cheaper than existing fuels
- Hydrogen is energy carrier so if you want to make clean fuel you need to make it using a clean energy source (e.g. wind) which is v $$.
What is meant by the term ‘hydrogen is an energy carrier’?
It needs an energy source to make it.
