Explicating charge transfer dynamics in anodic TiO2/ZnO/Zn photocatalytic fuel cell for ameliorated palm oil mill effluent treatment and synchronized energy generation

Abstract

A sustainable green device known as photocatalytic fuel cell (PFC) uses organics found in wastewater as fuel for the energy conversion. Hence, the effective PFC constructing from anodic TiO2/ZnO/Zn and cathodic TiO2/CuO/Cu was proposed for refractory palm oil mill effluent (POME) removal as well as energy recovery. Among the as-prepared photoanodes, anodic ZnO/Zn coated with 2 layers of TiO2 (TZ2) showed high crystallinity, low band gap, appropriate hydroxyl group and interfacial charge carrier separation that enable its superior photoactivity, which presented an enhancement of 3.67 times and 4.64 times in photocatalytic and energy performance over pristine ZnO/Zn. Among several examined influential parameters, the electrolyte concentration was the most influential factor due to its provision of OH− ions that can capture the photogenerated holes to enhance the electron-hole pair separation. The optimized system reached 90 % of COD removal efficiency of POME. Furthermore, an open circuit voltage (Voc), short circuit current density (Jsc) and power density (Pmax) of 1173 mV, 0.2652 mA cm-2 and 0.0734 mW cm-2 was achieved, respectively. The photoanode maintained high recyclability performance after repeated experimental cycles. In-depth mechanism investigations confirmed that the marked cell performance was attributed to not only the accelerated charge carrier separation due to deposition of TiO2 onto ZnO/Zn as verified by the photoelectrochemical tests, but also the dominant active species, including hydroxyl (OH) radicals and hole (h+) found in the radical trapping study. The assessed electrical energy efficiency of PFC using anodic TZ2 manifested reduced energy consumption of 1.77 times as compared to PFC using pristine anodic ZnO. The toxicity experiments via wild algae indicated reduction in toxicity. This work advances the practical application of promising PFC for environmental protection and energy recovery.