Upconversion emission of Yb3+/Er3+ co-doped BaF2/CeO2 composites and their role in enhancing the capacitance of graphene-based supercapacitors

Yb 3+ /Er 3+ co-doped BaF 2 /CeO 2 (BaF 2 /CeO 2 :Yb 3+ ,Er 3+ ) composite powders were synthesized via the combustion method at 520 °C, and their upconversion (UC)/electrochemical properties were evaluated as a function of their calcination temperature. The uncalcined composite (BC520 sample) exhibited BaF 2 as the unique crystalline phase. In contrast, the calcination of the powders at 620 °C (BC620 sample) and at 800 °C (BC800 sample) induced additional crystallization of CeO 2 . The BC520 composite produced no detectable UC emission under excitation at 980 nm. Conversely, the calcined composites exhibited UC emission, displaying characteristic green (524/545 nm) and red (656/676 nm) emission bands associated with Er 3+ transitions. Notably, increasing the calcination temperature resulted in an enhancement of UC intensity by factors of 7.4 and 4.7 for the green and red emissions, respectively, accompanied by a color shift from yellowish-orange to yellow-green. Furthermore, the BC800 composite displayed the strongest emission, attributed to its higher crystallinity and reduced content of oxygen vacancies in comparison with the other composites. On the other hand, flexible graphene-based supercapacitors (SCs) were fabricated using BC520, BC620, and BC800 composites as redox-active materials, which were deposited onto the graphene electrodes. The SCs incorporating these composites showed capacitance enhancements of 62.5–203 % compared to the reference device (Gr-SC) made exclusively with graphene electrodes. In particular, the maximum capacitance of 827.8 F g −1 and energy density of 73.6 W·h kg −1 were obtained in the SC made with the BC620 composite (Gr/620-SC device). This higher performance is attributed to reversible redox reactions, which were facilitated by the presence of CeO 2 . Spectroscopic studies (UV-Vis, Raman, and XPS) revealed that oxygen vacancies, along with Ce 3+ /Ce 4+ species, act as redox centers for charge storage. Additionally, the Gr/620-SC device demonstrated high electrochemical stability under stressing conditions because it retained 87.2 % of its initial capacitance after 1000 charge/discharge cycles and above 78 % after 1000 bending cycles. Therefore, our findings confirm that the Yb 3+ /Er 3+ co-doped BaF 2 /CeO 2 composite is a versatile multifunctional material for applications in bioimaging and energy storage. • BaF 2 /CeO 2 :Yb 3+ ,Er 3+ composites were synthesized via combustion and calcined at 620 °C and 800 °C. • Calcination enhanced upconversion emission by increasing crystallinity and reducing defects. • SCs made with composite calcined at 620 °C produced a capacitance of 827.8 F g −1 and energy density of 73.6 W·h kg −1 . • Ce 3+ /Ce 4+ species and oxygen vacancies defects were the main redox centers for the charge storage.