Due to the attractive combination of a relatively high specific heat of combustion with a large specific energy capacity, molecular hydrogen (H 2) is being investigated for use as an alternative to fossil fuels. Energy-efficient H 2 production and safe storage remain key technical obstacles to implementation of an H 2 based economy, however. ZnO has been investigated for use as an alternative photocatalytic electrode to TiO 2 for solarpowered photo-electro-chemical (PEC) electrolysis, in which H 2 is generated by direct water splitting in a cell with a metal cathode and a semiconducting anode. In this investigation, ZnO NR grown on Si (100) substrates by pulsed laser deposition were investigated for use as electrodes in the Hydrogen Evolution Reaction (HER). The electrochemical potential and Fermi energy of the ZnO NR were estimated from the electrochemical current density in acid and alkaline solutions via phenomenological thermodynamic analysis. As well as acting as an effective electrocalytic cathode, the ZnO NR appear to operate as a hydrogen reservoir. These results indicate that the ZnO NR have excellent potential for the storage of evolved H 2.