[NiFeSe]-hydrogenase and [W]-formate dehydrogenase (denoted as H2ase and FDH thereafter) from Desulfovibrio vulgaris Hildenborough (DvH) were expressed, characterised and purified using previously published methods . Stock solutions of H2ase (20 µM in 20 mM TRIS-HCl, pH 7.6) and FDH (50 µM in 20 mM TRIS-HCl, 10% glycerol, 10 mM NaNO3, pH 7.6) were stored at -40 °C in an anaerobic glovebox (MBraun, N2 atmosphere, <0.1 ppm O2) and thawed immediately prior to use. While H2ase could be used directly for electrochemical experiments, FDH required an additional activation step via pre-incubation with DTT for 20 minutes . For experiments requiring 100 pmol loadings of H2ase and FDH each, 5 µL and 2 µL of the respective stock solutions were drop-cast onto the IO-TiO2 electrodes and left to incubate for 2-3 minutes before full immersion in the buffer solution. Proportional volumes were used for higher enzyme loadings of 250 and 500 pmol. Experiments with carbonic anhydrase (CA, 100 pmol) involved an additional but similar drop-casting step on IO-TiO2|H2ase or IO-TiO2|FDH electrodes before electrochemical characterisation.