Final answer:
In a fuel cell, hydrogen and oxygen produce electrical energy through an electrochemical reaction, with high efficiency and low pollution. Direct combustion of hydrogen and oxygen, on the other hand, yields thermal energy with significant energy loss due to thermodynamic limitations.
Step-by-step explanation:
The reaction of hydrogen and oxygen to produce energy in a fuel cell differs substantially from their interaction during the direct combustion of hydrogen and oxygen. In a fuel cell, hydrogen is oxidized at the anode, and oxygen is reduced at the cathode, which results in water as a byproduct and the production of electrical energy directly via an electrochemical reaction. This process is efficient, producing electricity with up to 50%-75% energy efficiency.
In contrast, the direct combustion of hydrogen and oxygen is a much less efficient process, as it generates energy in the form of heat through a combustion reaction. The energy released in direct combustion, when used to power a heat engine, succumbs to second-law thermodynamics constraints, resulting in at least half of the energy being "wasted" to the surroundings with only about 20%-25% energy efficiency.
Moreover, fuel cells are nonpolluting, significantly reduce greenhouse gas emissions, and contribute to less dependence on fossil fuels. However, challenges like economic viability and dependence on natural gas for hydrogen production still hinder the wide adoption of fuel cell technology for applications like fuel-cell vehicles (FCVs).