Fig. 2. Three-electrode electrochemical data collected using unmodified GaP(100) (black), CoP–GaP (green), or FeP–GaP (dark red) working electrodes in phosphate buffer (pH 7) including (a) chronoamperograms using working electrodes polarized at a constant potential of 0 V vs. RHE and under 1-sun illumination (100 mW cm^–2), (b) linear sweep voltammograms recorded in the dark (dashed) or under 1-sun illumination (solid), (c) an image of a CoP–GaP photocathode under photoelectrochemical testing, and (d) gas chromatograms obtained using samples of headspace gas collected from sealed photoelectrochemical cells containing working electrodes polarized at a constant potential of 0 V vs. RHE and under 1-sun illumination. The amount of hydrogen produced in these experiments corresponds to a faradaic efficiency (FE) of 97% following 30 min of illumination using CoP–GaP and 45% following 6 min of illumination using FeP–GaP. (e) Cyclic voltammetry data recorded using butyronitrile solutions of model cobalt (green) or iron (dark red) porphyrin compounds are included for comparison.