Single atom catalysts exhibit particularly high catalytic activities in contrast to regular nanomaterial-based catalysts. Until recently, research has been mostly focused on single atom catalysts, and it remains a great challenge to synthesize bimetallic dimer structures. Herein, we successfully prepare high-quality one-to-one A-B bimetallic dimer structures (Pt-Ru dimers) through an atomic layer deposition (ALD) process. The Pt-Ru dimers show much higher hydrogen evolution activity (more than 50 times) and excellent stability compared to commercial Pt/C catalysts. X-ray absorption spectroscopy indicates that the Pt-Ru dimers structure model contains one Pt-Ru bonding configuration. First principle calculations reveal that the Pt-Ru dimer generates a synergy effect by modulating the electronic structure, which results in the enhanced hydrogen evolution activity. This work paves the way for the rational design of bimetallic dimers with good activity and stability, which have a great potential to be applied in various catalytic reactions.
Many literature reports show that layered Li-Ni-Mn-Co oxides (NMC) have a surface reconstruction to a rocksalt (Fm3m) structure which is claimed to be responsible for the increase in cell impedance during high voltage cycling. It is important to determine if appropriate electrolyte additives can suppress the surface reconstructions of NMC materials. LiNi 0.8 Mn 0.1 Co 0.1 O 2 (NMC811)/Graphite pouch cells with different electrolyte additives and different upper cutoff potentials were charge-discharge cycled and the electrodes were recovered for z-contrast scanning transmission electron microscope (STEM) studies. It was found that there was no significant surface layer growth for cells cycled between 2.8 and 4.1 V. For cells with an upper cutoff voltage of 4.3 V, the electrodes from cells with control electrolyte (no additives) showed the thickest surface layer. The electrolyte additives vinylene carbonate (VC) and prop-1-ene-1,3-sultone (PES) were found to suppress the growth of the surface layer. However, cells with PES showed a more rapid capacity fade than control cells or cells with 2% VC showing that, at least for NMC811/graphite cells with PES or VC additives, failure cannot only be solely ascribed to a growing rocksalt surface layer. Other processes, for example associated with electrolyte oxidation, are believed to be responsible for failure. High energy density lithium-ion batteries that are cheaper, safer and with longer lifetimes need to be developed in order to meet the increasing demand for applications such as electric vehicles. LiNi 0.8 Mn 0.1 Co 0.1 O 2 (NMC811) can deliver a high capacity of ∼200 mAh/g with an average discharge voltage of ∼3.8 V (vs. Li+/Li), making it a promising positive electrode material for high energy density lithium-ion batteries.1 However, electrochemical tests of NMC811 from half cells and full cells show poor cycling performance when charged to voltages above 4.2 V.2 In-situ and ex-situ X-ray diffraction showed that there are no significant irreversible structural changes in the bulk of the material during charge-discharge cycling. Instead, the parasitic reactions between the electrolyte and the surface of the positive electrode particles at high voltages were suggested to be the cause of the failure of cells cycled above 4.2 V. 2Layered NMC materials have a hexagonal layered structure (α-NaFeO 2 -type structure described in the Rm space group), where Li and transition metal atoms form alternating layers between oxygen layers and Li atoms have a 2-D diffusion path. [3][4][5] Lin et al. 6 showed that the surface of LiNi 0.42 Mn 0.42 Co 0.16 O 2 (NMC442) went through a structural reconstruction from layered (Rm) to rocksalt (Fm3m). In that transition, transition metal ions migrated to the lithium layers with a possible loss of Li and O from the surface of the structure. This was cited as one of the causes of a significant increase in cell impedance under high voltage cycling conditions. This surface reconstruction phenomenon was also observed in many other reports abou...
Pt single-atom catalysts are receiving more and more attention due to their different properties compared with nanostructures. As one typical kind of single-atom catalysts, Pt-based single-atom alloys (SAAs) have generated significant interest due to their application in several heterogeneous catalytic reactions. However, almost all of the reported Pt-based SAAs are on Cu surface. In addition, it is still great challenge to apply Pt single-atom alloys in electrocatalytic reactions. Herein, we demonstrated a fabrication of Pt/Pd SAA catalysts on nitrogen-doped carbon nanotubes by atomic layer deposition. The asprepared octahedral Pt/Pd SAA catalysts exhibited greatly improved activity compared to commercial Pt/C catalysts for electrochemical catalytic reactions. According to the X-ray adsorption spectrum, the Pt atoms in Pt/Pd SAA catalysts exhibited higher unoccupied 5d character density of states and a lower Pt−Pt coordination number, compared to those in core−shell structures. In addition, we used density functional theory calculation results to explain the enhanced mechanism of Pt/Pd SAA catalysts for electrochemical reactions. This study opens up an avenue of developing different types of Pt-based catalysts for electrocatalytic reactions and brings insight understanding about catalytic performances of SAA catalysts.
A novel two-step surface modification method that includes atomic layer deposition (ALD) of TiO followed by post-annealing treatment on spinel LiNi Mn O (LNMO) cathode material is developed to optimize the performance. The performance improvement can be attributed to the formation of a TiMn O (TMO)-like spinel phase resulting from the reaction of TiO with the surface LNMO. The Ti incorporation into the tetrahedral sites helps to combat the impedance growth that stems from continuous irreversible structural transition. The TMO-like spinel phase also alleviates the electrolyte decomposition during electrochemical cycling. 25 ALD cycles of TiO growth are found to be the optimized parameter toward capacity, Coulombic efficiency, stability, and rate capability enhancement. A detailed understanding of this surface modification mechanism has been demonstrated. This work provides a new insight into the atomic-scale surface structural modification using ALD and post-treatment, which is of great importance for the future design of cathode materials.
Configuring metal single‐atom catalysts (SACs) with high electrocatalytic activity and stability is one efficient strategy in achieving the cost‐competitive catalyst for fuel cells’ applications. Herein, the atomic layer deposition (ALD) strategy for synthesis of Pt SACs on the metal–organic framework (MOF)‐derived N‐doped carbon (NC) is proposed. Through adjusting the ALD exposure time of the Pt precursor, the size‐controlled Pt catalysts, from Pt single atoms to subclusters and nanoparticles, are prepared on MOF‐NC support. X‐ray absorption fine structure spectra determine the increased electron vacancy in Pt SACs and indicate the Pt–N coordination in the as‐prepared Pt SACs. Benefiting from the low‐coordination environment and anchoring interaction between Pt atoms and nitrogen‐doping sites from MOF‐NC support, the Pt SACs deliver an enhanced activity and stability with 6.5 times higher mass activity than that of Pt nanoparticle catalysts in boosting the oxygen reduction reaction (ORR). Density functional theory calculations indicate that Pt single atoms prefer to be anchored by the pyridinic N‐doped carbon sites. Importantly, it is revealed that the electronic structure of Pt SAs can be adjusted by adsorption of hydroxyl and oxygen, which greatly lowers free energy change for the rate‐determining step and enhances the activity of Pt SACs toward the ORR.
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