## 1 Introduction  
Microbial fuel cells (MFCs) are a famous device and have received considerable attention in recent years because of their great potential in alternative energy generation, wastewater treatment, and environmental monitoring [1; 2; 3; 4; 5]. The performance of MFCs is influenced by many factors, including electrode material, electricgens, equipment configuration, and so on. Because the most widely used cathode electron acceptor is oxygen, the cathodic oxygen reduction reaction is always a key limiting factor for MFCs [3; 4; 5]. So far, a variety of electrocatalysts and photocatalysts have been synthetized and utilized in MFCs to improve cathodic reactivity. Platinum has high catalytic activity and is the most commonly used catalyst, but its scarcity and high cost poses an obstacle for application in MFCs [6]. Thus, metal-free carbon materials and non-precious metal-based catalysts have been extensively investigated, and are often chosen as affordable alternatives to Pt-based catalysts [7; 8; 9; 10; 11]. For example, nitrogen-doped carbon nanotubes/graphene, Co-napthalocyanine (CoNPc), pyrolyzed iron ethylenediaminetetraacetic acid-modified activated carbon, and a Co porphyrinylene-ethynylene framework have been studied [7; 8; 9; 10; 11]. Although the performance of MFCs could be remarkably enhanced, these catalysts still cannot be used widely owing to the complicated synthesis conditions and extra cost for high temperature and special inert atmosphere [10; 11]. Moreover, the utilization of biocathodes is known as an alternative to noble/non-noble metal cathode catalysts in recently studies [12; 13]. Unfortunately, the bacterial electron-transfer process in the cathode was unstable and their mechanisms have not been fully understood, and more significant research is required for their economic development and practical application. It is commonly known that MFCs are developed on the basis of the extracellular electron transfer (EET) process of microorganisms. Iron and manganese oxide minerals serve as the most common natural electron acceptors for EET in natural environments [4; 5; 14]. Inspired by nature, some simple metal oxides have been used for cathode catalysts, including MnO\({}_{2}\) (\(\alpha\), \(\beta\), and \(\gamma\)), TiO\({}_{2}\), and PbO\({}_{2}\)[15; 16; 17; 18]. After using these simple metal oxides, the performance of MFCs was increased, but these materials still waste some chemical reagents during their synthesis processes. It is worthwhile mentioning that [PERSON] et al. first investigated the utilization of natural hematite as a cathode catalyst in a traditional MFC, which indicated that the maximum power density in a hematite-coated cathode MFC was 2.2 times higher than that in a graphite cathode MFC, and a current density of 330.66 mA\(\cdot\)m\({}^{-2}\) was stabilized over 10 days [19]. [PERSON] et al. studied the performance and Cr(VI) removal efficiencies by natural pyrrhotite-coated cathode MFCs and demonstrated that the natural pyrrhotite not only played the role of catalyst, but also acted as a reactive site for Cr(VI) reduction [20]. It must also be mentioned that [PERSON] et al. used the semiconducting mineral natural rutile as a novel photocathode catalyst for MFCs, which produced a maximal power density of 7.64 and 12.03 W\(\cdot\)m\({}^{-3}\) under dark and light irradiation, and provided a cost-effective alternative photocatalyst from nature [21]. With the help of natural semiconducting rutile, photocatalytic technology was successfully combined with MFC technology. As we all know, sunlight sustains the processes of all life, and is the most important energy source on Earth. This renewable and clean solar energy can be utilized by semiconductor photocatalytic technology, which has broad prospects in treating environmental pollution and energy-saving [22].