Hydrogen & Fuel Cells | Reactions | Chemistry | FuseSchool
Channel: FuseSchool - Global Education
Date: 2017-07-13

Hydrogen & Fuel Cells | Reactions | Chemistry | FuseSchool

In this video, you will learn about how we are trying to design a way to power vehicles of the future on hydrogen, oxygen and sunlight. 

In order to obtain energy for the vehicle from hydrogen (that is stored as a fuel cell for motor vehicles), we need to rejoin the hydrogen with oxygen. This is done most efficiently in what is called a fuel cell. 

In an ordinary electric cell, a metal, say zinc, ionizes at one electrode, the anode, giving off two electrons. The electrons are pushed around the circuit carrying energy, to, for example, a motor and absorbed by metal ions of a less reactive metal, say copper. The circuit is completed by the movement of metal ions through the solution.

In a fuel cell, the reactants are gases instead of metals. Hydrogen gives its electrons. The electrons flow round, driving the motor, and arrive at the cathode where they are recombined with hydrogen in the presence of the reactive gas oxygen, which provides the driving energy to form water again. The electrodes can be made of porous carbon coated with a catalyst, such as platinum or nickel. 

The advantage of combining a fuel and oxygen in a cell is that you can in theory convert most of the chemical energy to electricity whereas burning them, as happens in the internal combustion engine of a car, has a maximum efficiency of about 50% and in practice only about 25% of the chemical energy does useful work in driving the engine - the rest comes out as waste heat. 

There are two major problems to using hydrogen fuel cells in vehicles. 

Firstly, where do you get the hydrogen? Currently most industrial hydrogen is derived from methane and the carbon is rejected as carbon dioxide, thus adding to the greenhouse effect. The hope is that we can learn to mimic photosynthesis by using sunlight to split water molecules apart, giving us a clean and simple source of hydrogen. Currently this is achieved by using photovoltaic cells to generate electricity, which then electrolyses water, forming hydrogen and oxygen. To take the analogy further, the hydrogen is transported to the fuel cell, like biomass passing along a food chain, and the oxygen, which we tend to take for granted, is vented to the atmosphere. The fuel cell then gets its oxygen from the atmosphere, just like in respiration. The energy is stored whilst the hydrogen and oxygen are kept apart.

The other problem is how to store and transport the hydrogen gas once you've got it. It's extremely difficult to liquefy and rather dangerous if kept as a gas under pressure, particularly if the vehicle crashes. Research is therefore focusing on hydrides, compounds of elements with hydrogen, which are solid or liquid at room temperature and which give out their hydrogen reversibly and without too much energy input. This allows hydrogen to be pumped in, reacting to form the hydride and, then, on the journey, the hydrogen is given off to be used in the fuel cell to drive the vehicle. For example, ammonia borane, a solid at room temperature with the same structure of ethane, gives up its hydrogen on heating. 

However, it will probably be more energy efficient to run the cars on batteries which are charged from green electricity.

Fuel cells generate electricity from the reaction between a fuel and oxygen. Using them on vehicles means that we need to develop a way of storing hydrogen, which is difficult. It is more likely that vehicles of the future will be powered by batteries.


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