Chapter 6: Fuel cells as a source of energy

What are the advantages of fuel cells?

+ Hydrogen fuel cells produce water and heat as by-products. No greenhouse gases, like carbon dioxide, are released. + Fuel cells can use a variety of fuels. + Electricity can be generated on-site and users are not reliant on connection to an electricity grid.

What are the disadvantages of fuel cells?

+ They are expensive. Fuel cell technology is still developing. + Some types of fuel cell use expensive electrolytes and catalysts, leading to high purchase price and maintenance costs.. + At present, hydrogen for fuel cells is mainly sourced from fossil fuels.

SPECIFIC Applications of fuel cells?

+ Transport vehicles +Small portable electronic devices + Electricity (and sometimes heat) in homes and hospitals that doesn't require connection to the main grid

What is the definition of fuel cells?

+A fuel cell is a type of galvanic cell that convert the chemical energy of a fuel into electricity. +Requires ongoing supply of fuel and oxygen (reactants)

What is the definition of Galvanic cells?

+A primary or secondary electrochemical cell that converts chemical energy in a spontaneous reaction to electrical energy. +Definite supply of reactants - once used, it has to be recharged or discarded

Properties of hydrogen

+Can adsorb onto metal hydrides as either molecules or atoms +can be absorbed into lattice structure +can react reversibly with a range of of different chemicals.

Combustion of Fuel cells SAFETY:

+Difficulty with storing and transporting hydrogen as it burns readily and can result in a combustion reaction. +Low density however means that if a leak occurs, the gas can disperse quickly in well ventilated areas.

What are features of a Galvanic cell?

+Reactants can be electrodes or ions found in the electrolyte +Electrodes can be kept in separate vessels +Electrodes can take part in the reaction or be inert +Products build up in the cell (primary) +Wet cell - contains salt bridge, dry cell does not +Oxidation occurs at negative charged anode (vice versa)

What is hydrogen economy?

+Scientists predict that in the future, hydrocarbons will be replaced by hydrogen as the principle source of energy for transport and other purpose. +A fuel cell using hydrogen produces electricity, water, heat and very small amounts of nitrogen dioxide.

What are features of a fuel cells?

+Uses fuels eg. hydrogen, methane, methanol +two separate compartments, one for inflow of fuel and the other for oxygen/ air. +Electrodes are separated but contained within the same vessel +Electrodes are porous and usually contain catalysts to increase cell efficiency +Products are removed +No salt bridge

What is the overall equation for the reaction?

2H2 (g) + O2 (g) => 2H2O (l)

What is the efficiency of fuel cells?

40-60% as 85% of heat produced by the cell is used to generate electricity.

What is the efficiency of Galvanic cells?

60-90% (high in comparison to hydrocarbon cars)

What is steam reforming?

95% of hydrogen is produced through steam reforming in coal, oil and natural gas. In this process steam reacts with the fossil fuel at high temperature in the presence of a nickel catalyst.

What is a feature of Hydrogen production?

A fuel cell using hydrogen can be described as a zero-emission device because water is almost the only product apart from heat and electricity.

What is a fuel cell?

A type of electrochemical cell in which the reactants are supplied continuously, allowing continuous production of of electrical energy from chemical energy.

Fuel cell common design features ELECTRODES

Are porous which allows reactants (H2 and O2) to diffuse through and react with ions in electrolyte. Constant production of electrical energy due to constant supply of reactants (H2 and O2) O2 is always the oxidant

How are fuel cells applied in the real world?

Are used in submarines and some transport vehicles. Not yet read for large-scale commercial use due to cost

Combustion of Fuel cells ENERGY EFFICIENCY:

Direct conversion from chemical to electrical energy

Combustion of Fuel cells ENVIRONMENTAL IMPACT:

Doesn't contribute greenhouse gases

Combustion of Fuels APPLICATIONS:

Electricity (large scale), transportation

Efficiency of a Fuel cell?

Have a much higher efficiency then thermal power stations because chemical energy is directly converted into electrical energy.

Compressed hydrogen:

Hydrogen can be stored in high pressured tanks. However these need to be large in order to store the same amount of energy as a liquid hydrogen tank. Would need to be larger then a car boot to replace the effectivness of a hydrocarbon fuel.

Combustion of Fuel cells FUEL SUPPLY:

Hydrogen, methane, methanol etc.

Applications of fuel cells

Most major vehicle manufacturers are investigating the use of fuel cells as an alternative to the internal combustion engine as fuel cells have better fuel efficiency and lower emissions of greenhouse gases. Likewise fuel cells do not rely on oil

Fuel cell common design features CATHODE

O2 (g) + 4H(aq)+ 4e => 2H2O (l) Where O2 is being reduced.

Fuel cell common design features ELECTROLYTE:

Only allows the appropriate ions (H) to pass, not electrons which pass from one electrode to another.

Combustion of Fuel cells APPLICATIONS:

Portable and stationary energy storage E.G transport, portable devices, homes

Combustion of Fuels FUEL SUPPLY:

Predominately fossil fuels & biofuels (non-renewable)

How are Galvanic cells applied in the real world?

Primarily batteries for portable devices whilst secondary cells are used in transport.

Combustion of Fuels ENVIRONMENTAL IMPACT:

Production of greenhouse gases - CO2, CO, SO2 etc

Combustion of Fuels ENERGY EFFICIENCY:

Requires multiple energy transformations (As a result, often energy can be wasted along the process and therefore reduces efficiency)

liquid hydrogen:

The energy available per litre of liquid hydrogen is four times less than for a liquid hydrocarbon fuel such as petrol so a vehicle using liquid hydrogen requires a larger tank.

Fuel cell common design features ANODE

The hydrogen gas (the fuel) is oxidised by reacting with hydroxide ions from the electrolyte. H2 (g) + 2OH (aq) => 2H2O (l)+ 2e

What is the difference between a fuel cells and a secondary/ primary cell?

The reactants are not stored in a fuel cell, they must be continuously supplied form an external source.

Combustion of Fuels SAFETY:

Valves used to transport fuels, extraction processes

Hydrogen storage:

Widespread adoption of hydrogen as a primary fuel would require a massive expenditure on infrastructure include changes to pipelines and and filling methods.

What is the function of fuel cells?

to produce electrical energy, particularly useful as a continuous source of high electric current for portable/ fixed applications.

What is the function of Galvanic cells?

to produce electrical energy, particularly useful as a source of portable energy.