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A rechargeable aluminum-ion battery based on chloroaluminate electrolytes has received intense attention due to the high abundance and chemical stability of aluminum. However, the fundamental intercalation processes and dynamics in these battery systems remain unresolved. Here, the energetics and dynamics of chloroaluminate ion intercalation in atomically thin single crystal graphite are investigated by fabricating mesoscopic devices for charge transport and operando optical microscopy. These mesoscopic measurements are compared to the high-performance rechargeable Al-based battery consisting of a few-layer graphene-multiwall carbon nanotube composite cathode. These composites exhibit a 60% capacity enhancement over pyrolytic graphite, while an ∼3-fold improvement in overall ion diffusivity is also obtained exhibiting ∼1% of those in atomically thin single crystals. Our results thus establish the distinction between intrinsic and ensemble electrochemical behavior in Al-based batteries and show that engineering ion transport in these devices can yet lead to vast improvements in battery performance.

Isostructural [M 2 (DOBDC)(EG) 2 ] (M = Mg, Co, Ni) frameworks are first synthesized by controlling the pH* in the reaction medium. Coordinated ethylene glycols form a hexagonal OH cluster, which works as a template to grow single crystals with high crystallinity. After the liberation of solvated molecules, [M 2 (DOBDC)] shows notably higher surface areas than the reported values and completely different CO 2 and CO separation properties depending on the kinds of unsaturated metal. Therefore, breakthrough experiments using a CO 2 /CO mixed gas show that Mg-MOF has a longer breakthrough time for CO 2 than for CO, whereas Co/Ni-MOFs have longer breakthrough times for CO than for CO 2 . Apart from CO 2 and CO, other gases such as CH 4 , H 2 , and N 2 were almost not adsorbed at all in these materials at 298 K. To reveal the role of unsaturated metal sites, CO 2 and CO adsorption sites are unequivocally determined by single-crystal X-ray diffraction analysis. One of very interesting discoveries is that there are two CO 2 and CO adsorption positions (sites A and B) in the hexagonal channels. Site A is the unsaturated metal center working as Lewis acidic sites, and site B is the secondary adsorption site located between two A sites. A close inspection of crystal structures reveals that unsaturated Co(II) and Ni(II) sites adsorb both CO 2 and CO, whereas the unsaturated Mg(II) sites strongly capture only CO 2 , not CO. Density functional theory calculations elucidate the discrepancy in CO affinity: Co(II) and Ni(II) form strong π-back-donating bonds with CO via electron transfer from the d orbitals of the transition metals to the antibonding molecular orbitals of CO, whereas Mg(II) does not participate in electron transfer or orbital overlap with CO. This observation provides new insight into the synthesis of novel functional materials with high CO 2 /CO separation performance.

The catalytic activity for the hydrogen evolution reaction (HER) at the anion vacancy of 40 2D transition-metal dichalcogenides (TMDs) is investigated using the hydrogen adsorption free energy (Δ G H ) as the activity descriptor. While vacancy-free basal planes are mostly inactive, anion vacancy makes the hydrogen bonding stronger than clean basal planes, promoting the HER performance of many TMDs. We find that ZrSe 2 and ZrTe 2 have similar Δ G H as Pt, the best HER catalyst, at low vacancy density. Δ G H depends significantly on the vacancy density, which could be exploited as a tuning parameter. At proper vacancy densities, MoS 2 , MoSe 2 , MoTe 2 , ReSe 2 , ReTe 2 , WSe 2 , IrTe 2 , and HfTe 2 are expected to show the optimal HER activity. The detailed analysis of electronic structure and the multiple linear regression results identifies the vacancy formation energy and band-edge positions as key parameters correlating with Δ G H at anion vacancy of TMDs.