Oxygen is sourced through the air. Hydrogen is sourced from either:

• a number of conventional hydrocarbon fuels such as  natural gas, methanol/methane, gasoline, etc.
• or from renewable energy sources such as solar, wind, electrolysis of water, biomass, etc.

A fuel cell system including a "fuel reformer" to extract hydrogen from a hydrocarbon produces minimal NOx and less CO 2 than conventional combustion technology due to increased efficiency.
If hydrogen from renewable sources is used then there is no carbon in the chain and harmful emissions are practically zero.
The only exhaust of a fuel cell is water - pure water. The US National Aeronautics Space Administration (NASA) has for years used hydrogen fuel cells to power its systems onboard and to provide drinking water for the crew! more >>