Q.Explain why hydrogen is regarded as a promising fuel of the future, and distinguish between burning hydrogen directly and using it in a hydrogen fuel cell.

Hydrogen is considered a strong candidate for a clean fuel of the future for two combined reasons: its energy density and the cleanliness of its combustion product.

Energy density and clean combustion. Weight for weight, burning hydrogen releases substantially more energy than any conventional fossil fuel, following the simple combustion equation

and because the only product is water, hydrogen combustion generates none of the carbon dioxide, soot, or sulfur/nitrogen oxide pollutants that fossil-fuel combustion does -- an important advantage both for local air quality and for reducing greenhouse-gas emissions.

Direct combustion vs. the fuel cell. Hydrogen can be used as a fuel in two quite different ways. It can be burnt directly, exactly like a conventional fuel, in an internal combustion engine adapted for hydrogen gas, or in rocket engines that burn liquid hydrogen with liquid oxygen; here the chemical energy is released as heat, and how much of that heat can be turned into useful mechanical work is limited by the ordinary thermodynamic efficiency of a heat engine. …

State the combustion equation and its two key advantages (high energy density, clean product), then explicitly contrast the direct-burning route (heat, heat-engine-limited) with the …

Describing a fuel cell as simply 'burning hydrogen more efficiently' rather than explaining it is an electrochemical (non-combustion) proces …