A cell in which radiant energy causes a net chemical conversion in the cell, e.g. so as to produce hydrogen as a useful fuel. These cells can be classified as photosynthetic or photocatalytic. In the former case, radiant energy provides a Gibbs energy to drive a reaction such as ${\text{H}}_2\text{O}\to {\text{H}}_2+\frac{1}{2}{\text{O}}_2$ , and electrical or thermal energy may be later recovered by allowing the reverse, spontaneous reaction to proceed. In a photocatalytic cell the photon absorption promotes a reaction with $\mathrm{\Delta }G<0$ so there is no net storage of chemical energy, but the radiant energy speeds up a slow reaction.