Correlation of electrical and physical properties of photoanode with hydrogen evolution in enzymatic photo-electrochemical cell

In this study, the electrical and physical properties, including the current density, open-circuit voltage, morphology and crystalline structure, of an anodized TiO₂ electrode on a titanium foil are correlated with the hydrogen production rate in an enzymatic photo-electrochemical system. The effect of light intensity at ca. 74 and ca. 146 mW cm^-2 on the properties is also examined. Anodizing (20 V; bath temperature 5 °C; anodizing time 45 min) and subsequent annealing (350-850 °C for 5 h) of the Ti foils in an O₂ atmosphere led to the formation of a tube-shaped, or a compact layered, TiO₂ film on the Ti substrate depending on the annealing temperature. The annealing temperature has a similar effect on the properties of the sample and the hydrogen evolution rate. The generated electrical value, the chronoamperometry (CA), is +13 to -229 and +13 to -247 μA for light intensities of ca. 74 and ca. 146 mW cm^-2, while the corresponding open-circuit voltage (OCV) is in the range of -41 to -687 and -144 to 738 mV, respectively. In the absence of light (dark), the CA is 13-29 μA and the OCV is +258 to -126 mW cm^-2. The trend in the electrical properties for the different samples is well matched with the rate of hydrogen evolution. The samples with higher activities (450, 550, and 650 °C) have similar X-ray diffraction (XRD) patterns, which clearly indicates that the samples showing the highest evolution rate are composed of both anatase and rutile, while those showing a lower evolution rate are made of either anatase or rutile. Increasing the intensity of the irradiated light causes a remarkable enhancement in the rate of hydrogen production from 71 to 153 μmol h^-1 cm^-2.