Fossil fuels have a high energy density, meaning they contain a significant amount of energy per unit of volume, making them efficient for energy production and transport. Biodiesel is especially becoming a fossil fuel alternative and a key part of renewable energy. Several types of waste from homes, markets, street vendors, and other industrial places were collected and transesterified with Ni-doped ZnO nanoparticles for this study. These included castor oil, coffee grounds, eggshells, vegetable oil, fruit peels, and soybean oil. The Ni-doped ZnO’s were then calcined at 800 °C. The maximum conversion rate found in converting fruit peel waste into biodiesel is about 87.6%, and it was 89.6% when the oil-to-methanal ratio was about 1:2 and the reaction time was 140 min. This is the maximum biodiesel production compared to other wastes. Moreover, using vegetable oil with nanocatalyst, the maximum biodiesel production rate of about 90.58% was recorded with 15% catalyst loading, which is the maximum biodiesel production compared with the other wastes with nanocatalyst. Furthermore, at 75 °C and a concentration of catalyst of about 15% the maximum biodiesel production obtained by using castor oil is about 92.8%. It has the highest biodiesel yield compared with the yield recorded from other waste. The catalyst also demonstrated great stability and reusability for the synthesis of biodiesel. Using waste fruit peels with Ni-doped ZnO helps to progress low-cost and ecologically friendly catalyst for sustainable biodiesel production.

Polyaniline–graphene quantum dots (PANI–GQDs) are considered as an important candidate for applications in photovoltaic cells. In this work, GQDs were prepared using sono-Fenton reagent from reduced graphene oxide (rGO). PANI–GQD hybrid was also synthesized using the chemical in situ polymerization method. The synthesized materials were characterized using UV–visible (UV–Vis) spectroscopy, photoluminescence (PL) spectroscopy, current–voltage (I–V) characteristic, thermal gravimetric analysis (TGA), Raman spectroscopy, and X-ray diffraction (XRD). Dynamic light scattering was also used to estimate the lateral size of GQDs. The enhanced visible-light absorbance in the hybrid was confirmed by UV–Vis analysis and the decrease in intensity around 3461 cm−1 in FT-IR spectra was due to the interaction between functional groups of PANI with GQDs. This led to improved thermal stability and conductivity as observed from TGA and I–V analysis, respectively. Moreover, the Raman spectrum for PANI–GQDs showed a decrease in the peak at ~ 1348 and ~ 1572 cm−1 as compared to PANI and GQDs. Similarly, from the XRD profile of PANI–GQDs, a shift in peak was observed due to an alteration in the microstructure. A sandwich device with cell structure glass/ITO/PANI–GQDs/Al was fabricated and its application was tested. Current density–voltage (J–V) curve of the device was measured with a Keithley SMU 2400 unit under an illumination intensity of 100 Wm−2 simulating the AM 1.5 solar spectrum. The hybrid exhibited photovoltaic properties, and 0.857% efficiency was observed in response to the applied voltage. This work suggests that PANI can be used as an alternative material for photovoltaic cells.