Stacked titanium dioxide nanotubes photoanode facilitates unbiased hydrogen production in a solar-driven photoelectrochemical cell powered with a microbial fuel cell treating animal manure wastewater

Photoelectrochemical water splitting is a green, sustainable technology for harnessing solar energy to generate H-2; however, the energy demand needed to drive this non-spontaneous reaction limits the technology's competitiveness. In addition, the poor efficiency of photoanodes in photoelectrochemical cells (PECs) has been one of the factors governing the overall solar conversion efficiency of PECs. Here, a novel stacking approach of n-type titanium dioxide nanotubes (TiO2 NTs) photoanodes was proposed to improve their light-harvesting and charge transfer properties. Interestingly, the stacked photoanodes exhibited a much higher photocurrent of -0.79 mA/cm2 at 1 V (vs. SHE) than single sheet TiO2 NTs photoanode (i.e., -0.07 mA/cm(2)) at the same po-tential, implying that TiO2 NTs stacking approach resulted in effective light absorbance and management and much lower charge transfer resistance across the interface of photoanode and electrolyte. In addition, a self-biased, integrated solar-microbial system was developed, in which a microbial fuel cell (MFC) fed with real animal manure wastewater was used as a power source to drive sustainable H-2 production in a photo-electrochemical cell having stacked TiO2 NTs photoanodes. Without any external bias, the integrated system, which comprises an MFC and a three-sheet TiO2 NTs photoanode-based PEC, generated a photocurrent of 0.44 mA/cm(2) with an H-2 production rate of 0.45 +/- 0.03-m(3) H-2/m(3) day under 1 sun illumination (100 mW/cm(2)), which is -1.5-1.9-fold higher than other tested systems. This study demonstrates the synergetic effect between MFCs and PECs, in which electrons recovered from wastewater biodegradation in MFCs significantly increase H-2 generation in PECs without the need for an external power source.