When discussing future power sources for ships, fuel cells are generally considered to be the most likely candidates and are described as being clean with zero emissions other than water and heat. While that may be true of the stack which is the main component of a fuel cell, there are more aspects that to the technology that need to be taken into account.
The usual description of a fuel cell is a device for combining hydrogen with oxygen in an electrochemical reaction with electrical energy being produced for power utilisation and water and heat as by products. What is often omitted is that there are many more components needed to reform the fuel if it is not pure hydrogen, heaters, pumps and more that equate to the fuel conditioning and delivery systems, starting air compressors and exhausts of a conventional diesel engine. These ancillary systems require their own power source which depending upon the type of fuel cell may produce undesirable by products and pollutants.
In its simplest form as a proton exchange membrane fuel cell the only requirements are a constant feed of hydrogen to the anode and oxygen via air to the cathode. The hydrogen is fed to the anode of the fuel cell where the electrons in the hydrogen are separated to pass as an electric current to the motor or other system. The electrons continue on to the cathode to meet the hydrogen ions (protons) which have passed across the membrane and are combined with oxygen from the air to form water.
To function, the membrane must conduct hydrogen ions but not electrons as this would in effect "short circuit" the fuel cell. The membrane must also not allow either gas to pass to the other side of the cell, a problem known as gas crossover. Finally, the membrane must be resistant to the reducing environment at the cathode as well as the harsh oxidative environment at the anode. Splitting of the hydrogen molecule is relatively easy by using a platinum catalyst.
Using hydrogen as the fuel presents problems for ships that may be unsurmountable for large vessels making long voyages. Hydrogen in gaseous form is probably the least energy dense fuel possible so must be refrigerated and kept under pressure as a liquid for a PEM fuel cell to be viable for marine use.
Although it is the hydrogen that is important for a fuel cell to function, it is not necessary for it to be kept as pure hydrogen. Any fuel that contains hydrogen including diesel, LNG, methanol and many more can be used instead. However, except with a very few fuel types, the fuel has to be reformed to extract the hydrogen before it can be used in the fuel.
As an example methane as LNG (CH4) can be reformed by using steam (H20) to produce hydrogen and carbon monoxide. The hydrogen is then used to power the fuel cell while the carbon monoxide reacts further with some of the oxygen making the waste products of the fuel cell water and carbon dioxide. In another type of fuel cell, methanol (CH3OH) can be used directly as a liquid alone or mixed with water with all of the hydrogen atom electrons producing the current and again water and CO2 are the waste products.
Although it is possible for oil fuels to be used, any sulphur and metals present will contaminate the fuel cell and rapidly degrade it unless great care is taken to remove the contaminants before the hydrogen is fed to the fuel cell.