First, the IO electrode IO-TiO2|H2ase25 was run with the same loading (25 pmol) and at the same applied potential (-0.6 V vs. SHE) with 240 mM MES and a bulk pH of 4.24 to demonstrate the increased mass transport of the macroporous structure (Figure ). As the local pH increased less than in the confined mesoITO structure, the local pH remained below the optimum and therefore the current density for IO-TiO2 was significantly lower (-2.2 mA cm -2 ) than for mesoITO (-4.8 mA cm -2 ). If the local proton consumption is then increased beyond the mass transport limit by doubling the enzyme loading and applying a more reductive potential (IO-TiO2|H2ase50), the local pH again increases as in the mesoporous electrode.