Abstract
Objective:Although photoelectrochemical (PEC) water splitting heralds the emergence of the hydrogen economy, the need for external bias and low efficiency stymies the widespread application of this technology. By coupling water splitting (in a PEC cell) to a microbial fuel cell (MFC) using Escherichia coli as the biocatalyst, this work aims to successfully demonstrate a sustainable hybrid PEC–MFC platform functioning solely by biocatalysis and solar energy, at zero bias. Through further chemical modification of the photo-anode (in the PEC cell) and biofilm (in the MFC), the performance of the hybrid system is expected to improve in terms of the photocurrent generated and hydrogen evolved.Methods:The hybrid system constitutes the interconnected PEC cell with the MFC. Both PEC cell and MFC are typical two-chambered systems housing the anode and cathode. Au-TiO2 hollow spheres and conjugated oligoelectrolytes were synthesised chemically and introduced to the PEC cell and MFC, respectively. Hydrogen evolution measurements were performed in triplicates.Results:The hybrid PEC–MFC platform generated a photocurrent density of 0.35 mA/cm2 (~70× enhancement) as compared with the stand-alone P25 standard PEC cell (0.005 mA/cm2) under one-sun illumination (100 mW/cm2) at zero bias (0 V vs. Pt). This increase in photocurrent density was accompanied by continuous H2 production. No H2 was observed in the P25 standard PEC cell whereas H2 evolution rate was ~3.4 μmol/h in the hybrid system. The remarkable performance is attributed to the chemical modification of E. coli through the incorporation of novel conjugated oligoelectrolytes in the MFC as well as the lower recombination rate and higher photoabsorption capabilities in the Au-TiO2 hollow spheres electrode.Conclusions:The combined strategy of photo-anode modification in PEC cells and chemically modified MFCs shows great promise for future exploitation of such synergistic effects between MFCs and semiconductor-based PEC water splitting.
Highlights
Achieving a hydrogen economy can alleviate the universal fossil fuel crunch and rampant pollution driven by the insatiable demand for energy
The P25 standard electrode in the hybrid system produced an efficiency of 0.19% at zero bias (0 V vs. Pt), while the PEC–microbial fuel cell (MFC) hybrid system using Au-TiO2 hollow spheres photoanode/DSSN+-incorporated E. coli achieved an improved efficiency of 0.44% at the same potential
Hydrogen concentration was quantified in the modified PEC–MFC hybrid system after 36 h to elucidate the effects of the modifications on hydrogen evolution (Figure 6)
Summary
Achieving a hydrogen economy can alleviate the universal fossil fuel crunch and rampant pollution driven by the insatiable demand for energy. Solar-illuminated semiconductor-based photoelectrochemical (PEC) water splitting is an attractive strategy to generate hydrogen, which has witnessed significant breakthroughs recently.[1,2,3,4] Titanium dioxide (TiO2) and iron oxide (Fe2O3) have been extensively studied as common materials for application as photo-anodes in PEC cells owing to their favourable optoelectronic properties.[1,4] there is much to improve on, such as enhancing visible light absorption capabilities to maximise the full solar spectrum.[5,6,7] In particular, TiO2 and Fe2O3 can only absorb in certain regions of the solar spectrum This can be addressed through the synthesis of novel nano-structured hybrid materials, which can be tailored to manipulate material composition, shape, size and geometrical configurations.[8] To fully exploit the complete solar spectrum, gold nanoparticles (AuNPs) can be used to functionalize TiO2 to impart strong visible light photoabsorption capabilities, at ~ 550 nm.[5] This phenomenon can be attributed to localised surface plasmon resonance, which is characteristic of AuNPs.[9,10] Further, hollow spheres morphology can be fabricated to increase the surface area for photocatalytic reactions. AuNPs entrapped within the hollow sphere architecture can facilitate direct charge transfer from metal to TiO2 and function as traps to minimise electron and energy back flow in the form of surface charge recombination.[11,12]
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