A cell in which @R05039@ 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, @R05039@ provides a Gibbs energy to drive a reaction such as \(\text{H}_{2}\text{O}\rightarrow \text{H}_{2}+\frac{1}{2}\text{O}_{2}\) , 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 \(\Delta G < 0\) so there is no net storage of chemical energy, but the @R05039@ speeds up a slow reaction.