Cobalt-Doped CuO Photocathodes with Improved Surface Morphology for Effective Photoelectrochemical Hydrogen Production: Performance Assessment

In this paper, the Co-doped CuO photocathodes with nominal Co contents of 0, 2, 5, 10, and 15% were designed and fabricated on fluorine-doped tin oxide by spin coating to investigate composition-dependent photoelectrochemical (PEC) hydrogen production. The XRD, FE-SEM, EDS, and XPS characterization analyses revealed morphological changes and successful cobalt incorporation while retaining the predominant monoclinic CuO phase. The 10% Co/CuO film exhibited the strongest visible-light absorbance and an apparent direct optical gap of 1.56 eV. Its fitted CuO/FTO interfacial resistance contribution decreased from 13.43 to 5.91 kΩ relative to pristine CuO, while Mott-Schottky analysis indicated retained p-type behavior and a flat-band potential of +0.137 V vs. RHE. Under 100 mW cm -2 illumination in 0.5 M KOH, this composition reached an illuminated cathodic current density of approximately -500 μA cm -2 at +0.420 V vs. RHE. Volumetric measurements yielded approximately 9800 μmol g -1 of hydrogen over 120 min, corresponding to an average production rate of 4898 μmol g -1 h -1 , twice that of pristine CuO. The calculated energy, exergy, and volumetrically derived Faradaic efficiencies reached 0.500%, 0.518%, and 94.34%, respectively. An extended 10 h experiment reached approximately 13,900 μmol g -1 , with a declining production rate at longer times. Increasing the Co content to 15% reduced PEC performance despite further optical-gap narrowing. These findings identify 10% Co/CuO as the best-performing composition within the investigated series and associate its enhanced hydrogen production with concurrent optical, morphological, and interfacial changes.