## 3 Results and Discussion
### MFC Performance Analysis  
With the aim to evaluate the real efficiency of using natural wolframite as a MFC cathode photocatalyst, two sets of MFCs equipped with both a natural wolframite-coated graphite cathode and a normal graphite cathode were operated simultaneously. Their performances under dark/light conditions were quantified and compared via polarization curves and power density curves (Figure 5). The open circuit voltages (OCV) for the graphite cathode and the wolframite-coated cathode MFC were 489.2 and 520.2 mV, but their values increased to 496.3 and 575.3 mV under light irradiation, as summarized in Table 1. Notably, the system resistance remarkably decreased from 1134.7 to 583.6 \(\Omega\), suggesting that the semiconducting wolframite successfully played the role of cathode photocatalyst in the MFC. Figure 4: UV-Vis of natural wolframite (**a**) and linear sweep voltammetry (LSV) curves under dark/light conditions in O\({}_{2}\)/N\({}_{2}\) (**b**). As far as the maximum power density is concerned, the highest value of 135.57 mW\(\cdot\)m\({}^{-2}\) was realized with a photo-wolframite cathode, while it was only 95.51 mW\(\cdot\)m\({}^{-2}\) without light irradiation (Figure 5b). However, the maximum power density of MFC with a bare graphite cathode was only 41.47 and 57.74 mW\(\cdot\)m\({}^{-2}\) under dark and light conditions, respectively. Compared with the photo-wolframite cathode and photo-graphite cathode, a nearly 2.4-fold increment of the efficiency was realized (Table 1). In an sterile control experiment, only a slight current was observed, which was even lower than 0.1 \(\mu\)A\(\cdot\)cm\({}^{-2}\) with 100 \(\Omega\) resistance, suggesting that effective electron transfer could not establish between the anode culture medium and natural wolframite. The results demonstrated that semiconducting wolframite was a suitable candidate for cathode photocatalysts in MFC. To gain a better understanding of the performance of wolframite photocathodes MFC, their power density was compared with MFCs in recent studies with different types of catalysts. For example, the maximum power density of a natural hematite-coated cathode MFC was 144.4 \(\pm\) 7.5 mW\(\cdot\)m\({}^{-2}\)[19], which was 45.4 mW\(\cdot\)m\({}^{-2}\) in a natural pyrrhotite-coated cathode MFC [20]. Moreover, manganese oxides nanoparticles and lead oxides were characterized as the cathode materials for MFCs, and the maximum power density of MnO\({}_{x}\)/C-MFC was 161 mW\(\cdot\)m\({}^{-2}\)[16], and 22.1, 97.8, 82.6, and 78 mW\(\cdot\)m\({}^{-2}\) for \(\alpha\)-MnO\({}_{2}\), \(\beta\)-MnO\({}_{2}\), \(\gamma\)-MnO\({}_{2}\), and PbO\({}_{2}\)-coated air cathode microbial fuel cells, respectively [15; 18]. The maximum power density was much higher than 200 mW\(\cdot\)m\({}^{-2}\) when using a complex catalyst, such as nickel nanoparticle/C, phthalocyanine/C, copper-phthalocyanine/C, activated carbon, polytetrafluoroethylene, alumina/nickel nanoparticles-dispersed hierarchical structure of activated carbon microfibers or carbon nanofibers (CNFs) [35; 36; 37]. Notably, the value of maximum power density in our study was higher than that of the light-Cu\({}_{2}\)O cathode MFC (77 mW\(\cdot\)m\({}^{-2}\)) and rutile-coated cathode (120.3 mW\(\cdot\)m\({}^{-2}\)). In addition, no dissolved Mn/Fe was detected in the cathode solution after ten days, suggesting the wolframite was stable. These comparison results demonstrated that wolframite should be considerable as a stable and available substitute for cathode photocatalysts, that is inexpensive, abundant, and without complex composites compared with other existing expensive artificial cathode materials.