Protein film voltammetry (PFV) and controlled potential electrolysis (CPE) traces were recorded on an Ivium CompactStat potentiostat in a three-electrode configuration, consisting of an IO-TiO2 working electrode, a Ag/AgCl reference electrode (in saturated NaCl, BASi MW-2030) and a Platinum mesh counter electrode. The experiments were performed using a gas-tight two-compartment H-cell separated by a Selemion or Nafion ion exchange membrane, and in a pH 6.45 buffer solution containing 50 mM NaHCO3 and 50 mM KCl. The buffer solutions were purged with CO2 (containing 2% CH4 as an internal standard) prior to each experiment. PFV scans were collected at a scan rate of 5 mV s -1 within the range -0.4 to +0.2 V vs. RHE and in 3 successive cycles to verify the stability of the enzyme-IO-TiO2 interface. Potentials measured against Ag/AgCl were converted to the RHE scale using the equation: E (V vs. RHE) = E (V vs. Ag/AgCl) + 0.059 × pH + 0.197 V (at 298 K). CPE experiments conducted over a 10 h duration were performed at -0.4 V vs. RHE under continuous stirring. All reported current densities (in mA cm -2 ) are based on the geometrical surface area (0.19 cm 2 ) of the IO-TiO2 electrodes. TON and TOF values were calculated based on total H2ase or FDH drop-cast onto the IO-TiO2 scaffold.