Dual-Function MnBi2O4: Ni for Flexible Supercapacitors and Sunlight-Driven Degradation of Organic Pollutants

We report Ni-doped MnBi 2 O 4 (MnNiBi) as a defect-engineered material that works as efficient photocatalyst and as redox material for flexible supercapacitors (SCs). Ni incorporation induces lattice distortions that create oxygen vacancies, which act as active sites for charge storage or for the degradation of contaminants. Firstly, SCs were fabricated with graphene (G) electrodes printed on flexible polyethylene and those electrodes were coated with MnBi or MnNiBi powders. The G/MnNiBi-SC device (made with electrodes of G+MnNiBi) showed a capacitance of 810 F g − 1 , high energy density of 165 Wh kg − 1 , and high capacitance retention (94%) after 1000 charging-discharging cycles. Also, a device was made with MnBi powder (without Ni dopant), but the capacitance and energy density decreased to 155 F g − 1 and 48 Wh kg − 1 , respectively. Thus, the presence of Ni in the redox powder increased the capacitance by 422%. Moreover, MnNiBi powder produced high photocatalytic degradation of methylene blue (MB, 93%) and glyphosate (75%) under direct sunlight, which was superior to the degradation percentages obtained with undoped MnBi (57–70%). Interestingly, the degradation % increased to 98% and to 92% for MB and glyphosate, respectively, under UV-light. Moreover, total organic carbon measurements indicated mineralization efficiencies of 75% for MB and 58% for glyphosate. XPS studies showed that oxygen vacancies defects increased ⁓225% after the use of the MnNiBi powder for photocatalysis, which accelerated the degradation of contaminants. These defects were also crucial for supercapacitors, because they worked as redox centers to store charge. Thus, multifunction MnNiBi powders were used for charge-storage/water-cleaning.