Study of the photocurrent in a photocatalytic fuel cell for wastewater treatment and the effects of TiO2 surface morphology to the apportionment of the photocurrent

Abstract

A photocatalytic fuel cell (PFC) with simultaneous degradation of organism pollution and generation of electricity has been a promising technology in environmental wastewater remediation. However, a perspective on its photocurrent in the PFC, especially its relationship with the performance of organics degradation, is still not explicitly defined. Here, we demonstrated the co-existence of the oxygen evolution reaction (OER) and organic substance (COD) oxidation currents theoretically and technically in the PFC using TiO2 for wastewater remediation because the low valence band position of the TiO2 allows simultaneous interface reactions of water to oxygen and organics to carbon dioxide. These two reactions could compete for the exited holes. The benchmark of the COD current has been further measured and calculated from the J-V curves, which reveals a complicated apportionment, from 5.9% to 52.8% of the COD current, of the overall photocurrent in the PFC. These values are strongly affected by the morphologies, the electrolyte, and also the pollutant concentrations. A huge enhancement, more than two-fold the COD current, of the organic oxidation has been observed on morphologically reshaped long TiO2 nanotube arrays (TNTAs) with μm scaled large pit holes compared to the traditional long TNTAs with a planar surface. This result indicates a significant morphologic benefit, which in detail, probably allows a more efficient organic migration onto the tube wall surface because of the large pit holes. The comparison of the COD current between normal long-, short- TNTAs, a sol-gel TiO2 film, and the newly developed pit-type TNTAs demonstrates that the pit-type TNTAs are most robust for COD oxidation with a much larger COD current over all these traditional TiO2 electrodes. This benchmark of the COD current, including the demonstration of the photocurrent sources in a PFC, could probably provides insight into the design of the photoanodes and gives a more fair evaluation of the photoanode performance, which could also support the development of more efficient photoanodes in the future.