External quantum efficiency (EQE) spectra of the corresponding photoelectrodes at 0 V vs. RHE showed a plateau between 550 and 700 nm, consistent with that of the underlying OPV (Figs. S23, 24) . Integrating the EQE spectra yielded ideal Jsc values of 13 mA cm -2 (H2ase+CA) and 16 mA cm -2 (FDH+CA), which differ from the respective -8 mA cm -2 and -5 mA cm -2 by PFV measurements. This indicates that electrode kinetics play a more substantial role at higher photocurrent densities obtained under 1 sun irradiation, compared to lower light intensities of EQE measurements . H2 bubbles produced also adhere to the electrode surface, affecting the mass transport of dissolved species such as H + and CO2 into the porous IO-TiO2 matrix . Likewise, EQE measurements at 0.6 V vs. RHE yielded low Jsc values of ~ 5.5 mA cm -2 for both photoelectrodes, further complicated by increased bimolecular recombination within the OPV under forward bias .