A highly sustainable hydrothermal synthesized MnO₂ as cathodic catalyst in solar photocatalytic fuel cell

A unidirectional flow solar photocatalytic fuel cell (PFC) was successfully developed for the first time to offer alternative for electricity generation and simultaneous wastewater treatment. This study was focused on the synthesis of α-, δ- and β-MnO₂ by wet chemical hydrothermal method for application as the cathodic catalyst in PFC. The crystallographic evolution was performed by varying the ratios of KMnO₄ to MnSO₄. The mechanism of the PFC with the MnO₂/C as cathode was also discussed. Results showed that the catalytic activity of MnO₂/C cathode was mainly predominated by their crystallographic structures which included Mn-O bond strength and tunnel size, following order of α- > δ- > β-MnO₂/C. Interestingly, it was discovered that the specific surface areas (S_BET) of different crystal phases did not give an impact on the PFC performance. However, the P_max could be significantly influenced by the micropore surface area (S_micro) in the comparison among α-MnO₂. Furthermore, the morphological transformation carried out by altering the hydrothermal duration demonstrated that the nanowire α-M3(24 h)/C with 1:1 ratio of KMnO₄ and MnSO₄ yielded excellent PFC performance with a P_max of 2.8680 μW cm^-2 and the lowest R_int of 700 Ω.