Biomass oxidation through photoelectrochemical (PEC) processes using semiconductor materials has been considered an attractive approach to produce fossil-fuel-alternative energy sources. We fabricate a photoanode to simultaneously enhance the photoreactivity and selectivity of glycerol to glyceric acid using a cobalt-doped zinc oxide (ZnO) nanoparticle array. The introduction of cobalt in the ZnO array improves the photocatalytic ability by enhancing the accessibility of glycerol to the surface and tuning the electron states of ZnO. The photocurrent density and charge transfer of the electrode obtained by this strategy are 2.4 and 2.2 times higher than those of a pristine ZnO photoanode. These results suggest that the use of the optimal structure of the cobalt-doped ZnO could be an advanced strategy to enhance the photocatalytic properties of ZnO and provide PEC glycerol oxidation.