Understanding the properties and behavior of carbon materials is of paramount importance in the pursuit of sustainable energy solutions and technological advancements. As versatile and abundant resources, carbon materials play a central role in various energy conversion and storage applications, making them essential components in the transition toward a greener and more efficient future. This study explores the impact of diammonium hydrogen phosphate (DAHP) impregnation on activated carbon fibers (ACFs) for efficient energy storage and conversion applications. The viscose fibers were impregnated with varying DAHP concentrations, followed by carbonization and activation processes. The capacitance measurements were conducted in 6 mol dm−3 KOH, 0.5 mol dm−3 H2SO4, and 2 mol dm−3 KNO3 solutions, while the oxygen reduction reaction (ORR) measurements were performed in O2-saturated 0.1 mol dm−3 KOH solution. We find that the presented materials display specific capacitances up to 160 F g−1 when the DAHP concentration is in the range of 1.0 to 2.5%. Moreover, for the samples with lower DAHP concentrations, highly selective O2 reduction to peroxide was achieved while maintaining low ORR onset potentials. Thus, by impregnating viscose fibers with DAHP, it is possible to tune their electrochemical properties while increasing the yield, enabling the more sustainable and energy-efficient synthesis of advanced materials for energy conversion applications.