In a typical fuel cell, the reactants (R) and products (P) are :-

1. $R=H_{2\left(g\right)}, O_{2\left(g\right)}; P=H_2{O}_{2\left(l\right)}$

2. $R=H_{2\left(g\right)}, O_{2\left(g\right)}; P=H_2{O}_{\left(l\right)}$

3. $R=H_{2\left(g\right)}, O_{2\left(g\right)}, C{l}_2\left(g\right); P=HCl{O}_{4\left(aq\right)}$

4. $R=H_{2\left(g\right)}, N_{2\left(g\right)}; P=N{H}_{3\left(aq\right)}$

A device that converts energy of combustion of fuels, like hydrogen and methane, directly into electrical energy is known as :