A fuel cell, as used in road vehicles, is a device which converts hydrogen into electricity through an electro-chemical process. Fuel cells, in combination with electric motors, are more efficient than internal combustion engines. When fuelled by hydrogen, the only by-products of the fuel cell process are heat and water.
Fuel cells are used in a large number of different applications: portable power generation (for example as a small device to recharge your phone), stationary power generation (for example to increase the efficiency of your heater/boiler, using heat as a by-product), and power for transport (with hydrogen as a fuel in your vehicle).
Read more about fuel cells here
Fuel cell buses are a kind of electric bus; indeed, they share many components with battery electric buses and use electric motors rather than traditional engines for propulsion.
However, rather than storing energy in large batteries, fuel cell buses use compressed hydrogen as fuel and use on board fuel cell(s) to generate electricity which powers the electric motor. This device uses hydrogen and oxygen to generate electricity through an electro-chemical process producing only water and heat as by-products.
Hydrogen offers much higher energy density compared to electrical storage systems (e.g. batteries, super-capacitors), this leads to a substantial range for the buses (more than sufficient for a day’s operation).
Modern fuel cell electric buses generally additionally have a small battery or super capacitors. These devices improve the performance of the fuel cell and overall energy efficiency of the vehicle, for example they boost the acceleration and allow recuperation of braking energy. Apart from that, the bus structure and the other non-electric components of the bus are the same as conventional buses. See in the pictures on the right side the main components of an EvoBus, a Van Hool and a Wrightbus fuel cell bus.
You can find here a video explaning the assembly of The Van Hool Fuel Cell Bus deployed in Oslo