Fuel cells convert the chemical energy stored in fuels such as hydrogen into electricity. They do so by reacting the fuel with oxygen or another oxidizing agent that can strip electrons from the fuel. An electrolyte—commonly a polymer or ceramic—interposed between the fuel and oxidizer helps shuttle ions within the fuel cell.
Fuel cells are typically more efficient and environmentally friendly than heat engines, such as the internal combustion engines that usually power cars. However, fuel cells are often limited by how well their electrolytes can prevent electrons from leaking through them at the interface where the fuel and the oxidizing agent meet. Such electron conduction not only reduces fuel cell power output, but it can also lead to catastrophic fractures in the electrolyte.