Study Of Potassium-intercalated Fullerene Derivatives As Possible Hydrogen Storage Materials

Hydrogen energy is a clean and renewable energy source.The bottleneck restricting its large-scale applications is hydrogen storage.In this thesis,we study K-doped PCBM as a new type of solid-state hydrogen storage material using density functional theory(DFT)calculations.Here PCBM is the collective name of four materials,including PC61BM,PC71BM,bisPCBM[60]and bisPCBM[70].Except PC61BM,the other three PCBMs all contain multiple isomers.For PC71BM and bisPCBM[70],we selected the most representative or experimentally separated isomer to study.For bisPCBM[60],we arbitrarily selected one of the 19 isomers that have been separated experimentally to study.The highest chemical ratio of K-PCBM compound is the prerequisite for hydrogen storage study.The specific K-PCBM compounds studied in this thesis are K7PC61BM,K9PC71BM,K10bisPCBM[70]and K8bisPCBM[60].The chemical ratios of the first two compounds are from experimental reports in the literature.For bisPCBM[70]and bisPCBM[60],which have not been reported on chemical ratios,we use photoelectron spectroscopy(PES)experiments and DFT calculations to determine the highest chemical ratios(K10bisPCBM[70]and KsbisPCBM[60]).The research goal is to achieve the hydrogen storage capacity(4.5 wt%for 2020 and 5.5 wt%for 2025)and adsorption temperature(above-40℃)targets established by the U.S.Department of Energy(DOE)for on-board hydrogen storage materials.The hydrogen storage capacity at room temperature(RT)is paid more attention.All calculations are performed on isolated K-PCBM complex molecules,but fully consider the limited number and limited size of interstitial sites available for H2 molecules in actual solid-phase materials.The research results show that the hydrogen capacity at RT of K7PC61BM and K8bisPCBM[60]are 5.07 and 5.34 wt%,respectively,which are higher than the DOE 2020 target;the hydrogen capacity at RT of K9PC71BM is 5.57 wt%,which is higher than the DOE 2025 target.The hydrogen capacity at RT of K10bisPCBM[70]is 4.40 wt%,which is slightly lower than the DOE 2020 target,but still much higher than the reported hydrogen capacity at RT for the current adsorbent hydrogen storage materials(less than or slightly higher than 1.0 wt%).Therefore,this thesis reveals that at least three K-doped PCBMs(PC61BM,PC71BM and bisPCBM[60])are expected to become practical hydrogen storage materials for vehicles.In terms of hy drogen storage mechanism,this thesis has also achieved rich results.It is revealed that the polarization of the charged carbon cage to H2 has a non-negligible effect on hydrogen storage.The synergistic effect of this polarization and the polarization of K ions on H2 makes a compact 6H2 ring adsorption structure formed.It also reveals that the van der Waals force contributes a lot to the adsorption energy of H2,exceeding 40%.