A review on semiconductor photocathode in bioelectrochemical systems: Mechanism, limitation, and environmental application

Bio-electrochemical system (BES), a promising green treatment and resource recovery technique, suffers mainly from the cathodic limitation toward practical application. An emerging way to avoid this limitation is to apply novel or semiconductor catalysts on the cathode. Semiconductor photocatalysts are very promising that can combine the photoexcited and bio-generated electrons in BES. The possibilities of photocatalysts (bio and abiotic) for BES anode and cathode were jointly discussed in several review articles. However, comprehensive discussion on the semiconductor photocatalysts for BES cathodic application is limited. Therefore, this review aimed to explore the semiconductor photocatalysts for cathodic improvement in BES. An in-depth examination on their selection criteria, technical factors for catalyst layer fabrication, and photocathodic reactions under visible light spectrum are highlighted. The review found that photocathodic reactions in BES were promisingly applied for (i) oxygen evolution and reduction, (ii) hydrogen evolution, and (iii) pollutant removal. However, there is a lack of studies on the recently developed visible light heterojunctions/composites for the cathodic enhancement in BES. Tunable bandgap with highly efficient semiconductors should be considered for Type II and z-scheme heterojunctions preparation. Graphitic carbon nitride, bismuth oxi-halides, and earth-abundant metal-based oxides are the new generation photocatalyst choices for BES cathode. Bio-electrochemical desalination using semiconductor photocatalyst cathodes is still an unexplored field. Emphasis should be given more on efficient cathode catalyst layer preparation for better interactions among the electrolyte, light, and electron acceptors. Highly efficient photocatalytic air cathode can be a promising step toward a self-sustaining BES for a future solution.