However, the optimised system exhibited deterioration of current densities over the hour. The decrease can be attributed to the build-up of hydrogen gas in the pores and on the electrode as demonstrated by the rejuvenation of the activity after the bubbles were removed from the electrode (Figure ). Whilst the simulation does account for the mass transport effects of bubble formation (Supporting Information, Solution Domain), it does not account for bubble blocking or other effects. For example, H2 bubble formation may cause increased film loss, through enzyme displacement, re-orientation and degradation. H2 also affects the thermodynamic potential according to the Nernst equation, an effect even more pronounced at the low overpotentials applied here. Though usually H2 is a key inhibitor of H2ases, DvH