Fuel cells are efficient and clean energy sources that produce electricity by combining hydrogen and oxygen. They have a much higher efficiency and lower emissions compared to traditional combustion engines. The most common type is the proton exchange membrane fuel cell (PEMFC), ideal for vehicles and portable devices. Other types include phosphoric acid fuel cells (PAFC), solid-state fuel cells (SOFC), and molten carbonate fuel cells (MCFC). These cells offer different advantages, such as using natural gas or CO2 as fuel.