The amount of H2 produced by H2ase with time was monitored using a gas chromatograph (Agilent 7890A). Aliquots of gas (50 µL) withdrawn from the headspace of electrochemical or PEC cells were injected manually at periodic intervals, and quantified based the 2% CH4 internal standard. Formate produced by FDH was quantified using a Metrohm 882 Compact IC Plus ion chromatograph equipped with a conductivity detector (eluent comprising of 3.2 mM Na2CO3 and 1 mM NaHCO3). Reaction samples were diluted 10-fold with ultrapure deionised water prior to IC injection. O2 evolution was monitored in a N2-filled glovebox (Belle Technology) using a NeoFox-GT fluorometer and Fospor-R fluorescence oxygen sensor probe (Ocean Optics). Henry's law was used to determine the amount of dissolved H2 and O2 in the aqueous buffer solution. The faradaic yield (FY) was calculated using the equation: FY (%) = nZF/Q × 100%, where n is the number of moles of product (H2, formate or O2) produced, Z is the number of electrons needed per molecule of product (Z = 2 for both H2 and formate production, Z = 4 for oxygen production), F is the Faraday constant (96485 C mol -1 ), and Q is the total charge passed. The total charge Q was determined by integrating the current trace over a defined period. TON and TOF values for H2 and formate production were calculated based on the amount of enzyme drop-cast onto the IO-TiO2 electrodes, representing the lower limit assuming that all enzymes are immobilised and electrochemically active. Solar-to-fuel efficiencies were determined using the equation: STF (%) = (J × ΔU × FY)/P × 100%, where J is the absolute value of the operating photocurrent density, FY is the faradaic yield for the solar fuel produced and P is the incident solar power. As all tandem PEC experiments were performed at zero applied bias, ΔU = 1.23 for both overall water splitting and CO2 reduction coupled to O2 evolution.