Facile Synthesis and Characterization of Novel Carbon Nanofiber-Doped MWO4 (M-Mn, Co, Ni, Mn-Co, Mn-Ni, Ni-Co)-Based Nanostructured Electrode Materials for Application in Electrochemical Supercapacitors and Photoelectrochemical Water Splitting

More efficient, clean, and renewable energy sources must be developed to replace fossil fuels and protect the environment from their harmful effects. Electrochemical energy storage devices and energy conversion (hydrogen production) sources are two effective approaches for replacing fossil fuels. Herein, single metal tungstates, such as MnWO4 flakes, NiWO4 nanoparticles, and CoWO4 nanoparticles, as well as bimetallic tungstates, such as MnNiWO4, MnCoWO4, and NiCoWO4, were synthesized using a simple hydrothermal method. Furthermore, carbon nanofibers (CNFs) were adopted to modify bimetallic tungstate to improve the electron transfer and extraction of electron-hole pairs. To modify the CNFs with bimetallic tungstate as a composite electrode, a simple and convenient process called the wet impregnation method was employed. The resulting composite materials exhibited better performances in supercapacitor and photoelectrochemical water-splitting studies than those of single and bimetallic tungstates. The hybrid composite, MnNiWO4/CNF, showed a high specific capacity of 1374 F g(-1) at a current density of 0.5 A g(-1) in a three-electrode configuration, owing to its nonfaradaic and faradaic processes. This performance was 4.2, 10.3, and 3 orders of magnitude higher than those of MnWO4, NiWO4, and MnNiWO4 electrodes, respectively. In photoelectrochemical water-splitting studies, the development of heterostructures decreases electron-hole recombination and improves interfacial charge transfer in composite materials. In this study, the MnNiWO4/CNF nanocomposite material exhibited a maximum applied bias photon-to-current efficiency (ABPE) of 3.47%, approximately 6, 6, and 2 orders of magnitude higher than those of bare MnWO4, NiWO4, and MnNiWO4, respectively, under illumination. The crystallinities, morphologies, absorptions, and chemical compositions of the synthesized materials were investigated using electrochemical and spectroscopic techniques. The results indicate that the synthesized hybrid materials could be promising candidates as electrode materials for remarkable supercapacitor and photoelectrochemical water-splitting applications.