Feasibility Study of Coffee Husk Char-Derived Carbon Dots to Enhance Solar Photovoltaic-Thermal Applications

Coffee is one of the most traded commodities in the world. Coffee husk makes up the majority of the solid debris generated during the dry processing of coffee, which is between 30 to 50%. Pyrolysis is a reliable and feasible technology to valorise the coffee husk. The present study proposes and investigates a novel method for producing carbon dots from coffee husk char through pyrolysis. Nanomaterials are becoming more popular to improve the thermal and optical properties of working fluids used in solar photovoltaic and thermal conversion systems. The coffee husk-char derived carbon dot (CHC-CD) nanofluids are prepared with different char concentrations (1%, 2%, 3% and 4%). The chemical and optical properties of the nanofluid produced from coffee husk char (CHC-CD nanofluid) are investigated. The morphology, size and particle size distribution of the CHC-CD particles were studied using field emission scanning electron microscopy (FESEM) and high-resolution transmission electron microscopy (HRTEM). The particles were found to have an average size of 5.38 nm. The CHC-CD nanofluid had a basic pH and negative zeta potential, which was found to decrease with an increase in the char concentration and pH. The nanofluid prepared with 2% char was found to have a zeta potential of -61.4 mV. It was evident from the UV-vis transmittance spectra that the CHC-CD nanofluid prepared with a char concentration of 2% had very limited transmittance in the UV region and good transmittance above 735 nm. This transmittance range is found to be favourable for the smooth and efficient operation of silicon-based solar PV cells. The CHC-CD nanofluid also showed a maximum intensity emission wavelength of 522 nm at an excitation wavelength of 450 nm. Characterisation of the carbon phase, surface functional groups, and surface defects was also done using XRD, FTIR, and Raman spectroscopy, respectively. The optical and chemical properties indicate that the developed CHC-CD nanofluid can find application not only in solar photovoltaic and thermal systems but also in applications in biomedical and chemical sensor applications.