Hydrogen is considered a promising candidate for encountering energy necessity since it is a clean, nontoxic, economical, abundant, environment-friendly, and renewable energy source [[1], [2], [3]]. Numerous hydrogen storage mechanisms have been projected in the last decades via the physisorption and chemisorption processes to discover suitable material [[4], [5], [6]]. The U.S Department of Energy (DOE) system has fixed a target of 5.5 wt% gravimetric hydrogen storage capacity and 0.05 Kg hydrogen/L volumetric capacity for onboard light-duty vehicles, materials-handling equipment, and moveable power applications by the end of 2025 [7]. Nevertheless, developing materials that can accumulate hydrogen by that standard and function under ambient conditions is not easy.

The hydrogen bonding is either too feeble when interacting with carbon nanostructure or too robust as inorganic molecules and light metal hydrides [8,9]. So, continuous efforts are given to find the ideal storage system with binding energy intermediate between physisorption and chemisorption. Formerly, LiBH4 has been considered a suitable material due to its 18.4 wt% hydrogen storage capacity, but this hybrid assembly is comparatively unbalanced at high temperatures (400 °C) [10].

Recently, nanostructures based on carbon and graphene, such as 2D carbon nitrides, have been recommended for photocatalytic hydrogen evolution [11], dye degradation [12], toxic gas sensing [13,14] and hydrogen storage applications [15,16] due to their excellent surface-to-volume ratio, porous structure, lightweight, electronic and thermodynamic properties [17]. Zhang et al. have reported that graphitic carbon nitride (g-C3N4) (triazine-based) is an admirable material for the steady and sound-dispersed embellishment of Ti atoms with high hydrogen adsorption capacity and binding strength appropriate for mobile application [18]. M.D. Ganji et al. have studied the hydrogen storage capacity of Si-decorated graphene sheets using density functional theory (DFT) and found ∼15 wt% of hydrogen storage capacity by considering hydrogen on both sides of the Si-decorated graphene sheet [19]. Menghao Wu et al. have found that Li-functionalization on g-C3N4 has a large capacity of 10 wt% for hydrogen storage [20]. Also, Y. Wang et al. have shown that g-C3N4 nanotubes functionalized with Na and Li atoms have a storage capacity of 9.09 wt% at 0 K for hydrogen [21]. Nevertheless, the binding strength of Li on the g-C3N4 has been computed to be just 2.20 eV at 0 K, which would encourage the development of the Li-clusters at advanced process temperature and afterward reduce hydrogen storage capacity. Y. Liu et al. have found that up to 7.8 wt% of H2 molecules can be stored by Ti-decorated graphene [22]. N. Song et al. have stated that up to 7.6 wt% of H2 molecules can be stored with Ti-decorated boron-carbon-nitride [23].

Most theoretical studies on hydrogen storage are in triazine-based carbon nitride, even though the allotrope heptazine-based g-C3N4 is more stable and readily available for experimental studies. Besides this, heptazine-based C3N4 has more lone pair electrons than triazine due to the additional nitrogen atoms, which makes it more desirable in adsorption or sensing applications [14]. Wu et al. have found that Li-functionalized heptazine-based g-C3N4 is a hopeful material for H2 storage [20]. Hussain et al. have extended the work done by Wu et al. by using dispersion corrections and considering large supercells in the DFT calculations [15]. They find out the effect of different Li decorations on the storage capacity of heptazine-based g-C3N4 for H2 storage. Our recent work has demonstrated that P doping as distinct and organized with metal outcomes in sturdy delocalization of frontier molecular orbitals (MOs), which slows and opposes the charge recombination rate in the g-C3N4 [24]. The P doping in g-C3N4 changes the electronic structure of g-C3N4 with lesser bandgap energy and boosts electric conductivity and dye degradation capacity [25,26]. Thus, in this work, we have investigated the synergetic consequence of lithium and phosphorus decoration on the structural, electronic, and optical properties along with the hydrogen storage capacity of the heptazine-based g-C3N4 monolayer.