In a hydrogen - oxygen fuel cell, combustion of hydrogen occurs to

In hydrogen-oxygen fuel cell following reactions take place to create

 potential difference between two electrodes

$\begin{array}{l} 2{\mathrm{H}}_{2\left(g\right)}+4{{\mathrm{OH}}^{-}}^{\left(aq\right)}\to 4{\mathrm{H}}_2{\mathrm{O}}_{\left(l\right)}+4e^{-}\\ \frac{{\mathrm{O}}_{2\left(g\right)}+2{\mathrm{H}}_2{\mathrm{O}}_{\left(l\right)}+4e^{-}\to 4{\mathrm{OH}}^{-}\left(\text{aq }\right)}{\text{ Overall reaction }=2{\mathrm{H}}_{2\left(g\right)}+{\mathrm{O}}_{2\left(g\right)}\to 2{\mathrm{H}}_2{\mathrm{O}}_{\left(l\right)}}\end{array}$

the net reaction is the same as burning (Combustion) of 

hydrogen to form water.