整體而言，本研究證實g-C3N4奈米管之氫氣吸附能力可透過煅燒條件最佳化與酸鹼表面改質有效提升。H3PO4處理有助於引入極性表面物種，而KOH處理則可促進結構剝離、缺陷生成與表面活化，兩者皆顯示表面化學調控在g-C3N4儲氫行為中扮演關鍵角色。La0.6Ce0.4Ni5/g-C3N4複合結果則進一步說明，g-C3N4在合金系統中主要作為促進活化、改善顆粒分散、增加界面接觸與輔助氫氣傳輸之功能性添加物，而非主要儲氫相。本研究建立了表面改質、結構缺陷、表面電性與g-C3N4近室溫儲氫性能之關聯，可作為後續非金屬固態儲氫材料與複合儲氫系統設計之參考依據。;In this study， graphitic carbon nitride (g-C3N4) nanotubes were investigated as non-metallic hydrogen storage materials， with emphasis on the effects of morphology control and surface modification on hydrogen adsorption behavior. g-C3N4 possesses a nitrogen-rich conjugated framework， good chemical stability， and tunable surface chemistry. Its surface nitrogen sites， defect structures and functional groups may influence the interaction between hydrogen molecules and the material surface. However， pristine g-C3N4 is limited by dense layered stacking and weak physisorption at room temperature， resulting in limited near-room-temperature hydrogen storage performance. Therefore， this study focused on morphology optimization and surface modification to establish the relationship between material structure， surface chemistry and hydrogen adsorption performance.
First， g-C3N4 nanotubes were synthesized using a one-dimensional fibrous self-assembly method and the effects of calcination conditions on their structure and hydrogen storage performance was investigated. The results showed that the sample calcined at 550 °C for 4 h， denoted as 550-4， exhibited a more complete hollow tubular morphology， better thermal stability， and improved pore structure. Therefore， 550-4 was selected as the base material for subsequent surface modification. The hydrogen storage capacity of 550-4 was 0.095 wt.% at room temperature. When the temperature was decreased to 0 °C， the capacity increased to 0.192 wt.%. Under a high pressure of 10 MPa， the hydrogen storage capacity further increased to 0.344 wt.%. These results indicate that the hydrogen adsorption behavior of g-C3N4 nanotubes shows obvious temperature and pressure dependence and is mainly governed by a physisorption mechanism.
For surface modification， different acid and base solutions were used to treat the 550-4 sample in order to tune its surface structure and chemical environment. The results confirmed that the modified samples still retained the main carbon-nitrogen framework of g-C3N4， indicating that the chemical treatments mainly affected the surface and edge regions rather than destroying the main structure. Acid treatment improved surface accessibility and polarity through mild etching and the introduction of functional groups. Among the acid-treated samples， g-AH3PO4-5 showed an improved hydrogen storage capacity of 0.199 wt.%. In comparison， KOH treatment caused more significant structural exfoliation and surface activation. The g-BKOH-10 sample reached a hydrogen storage capacity of 0.200 wt.%. At 0 °C， the hydrogen storage capacities of g-AH3PO4-5 and g-BKOH-10 further increased to 0.269 wt.% and 0.301 wt.%， respectively. These results suggest that surface modification can effectively enhance the hydrogen adsorption ability of g-C3N4 and the improvement is related not only to pore structure but also to surface defects， functional groups and surface charge.
In addition to the surface modification of g-C3N4 nanotubes， the materials were further composited with La0.6Ce0.4Ni5 alloy as an extended application and synergistic effect evaluation. La0.6Ce0.4Ni5 is a typical AB5-type hydrogen storage alloy with reversible hydrogen absorption and desorption capability， but its initial hydrogen absorption kinetics still require improvement. The results showed that pure La0.6Ce0.4Ni5 alloy had a relatively high room-temperature hydrogen storage capacity， but its hydrogen absorption rate was slow. After adding an appropriate amount of g-C3N4， the hydrogen absorption time was significantly shortened. Among the composite samples， 78LCN maintained a hydrogen storage capacity of 1.256 wt.% and reduced the t90 value from 575 s for the pure alloy to 211 s， demonstrating a better balance between capacity and absorption rate. In addition， because 78LCN reduces the usage of rare-earth and Ni-based AB5 alloy while maintaining acceptable hydrogen storage performance， it also shows potential for reducing material cost.
Overall， this study demonstrates that the hydrogen storage ability of g-C3N4 nanotubes can be effectively enhanced through calcination optimization and surface modification. H3PO4 treatment helps introduce polar surface species， while KOH treatment promotes structural exfoliation， defect formation， and surface activation. Both treatments indicate that surface chemical regulation plays an important role in the hydrogen storage behavior of g-C3N4. The La0.6Ce0.4Ni5/g-C3N4 composite results further indicate that g-C3N4 mainly acts as a functional additive in the alloy system by promoting activation， improving particle dispersion， increasing interfacial contact and assisting hydrogen transport， rather than serving as the primary hydrogen storage phase. This study establishes the relationship among surface modification， structural defects， surface charge， and the near-ambient temperature hydrogen storage performance of g-C3N4， providing a reference for the future design of non-metallic solid-state hydrogen storage materials and composite hydrogen storage systems.
