A solid-state Li-air battery: computational studies of interfaces and relevance to discharge mechanism.

There is much interest in developing new energy storage systems to replace currently available ones that mainly work based on Li-ion intercalations. One attractive area is the Li-air battery for which most of the research has involved liquid electrolytes. There have been few studies on the use of a solid electrolyte in a Li-air battery. Recently, we reported the successful use of a solid-state electrolyte in a Li-air battery resulting in a Li2O product and potentially much higher energy density than in a Li-air battery based on either a Li2O2 or LiO2 product (Science, 2023, 379, 499). In this paper we discuss how the discharge mechanism involved in this solid-state Li-air battery differs from that of a Li-air battery with a liquid electrolyte. The solid-state mechanism is further explored with density functional studies of various interfaces involving the discharge product. We discuss the relevance of the results to the discharge mechanism in the solid-state Li-air battery.

A room temperature rechargeable Li2O-based lithium-air battery enabled by a solid electrolyte.

A lithium-air battery based on lithium oxide (Li2O) formation can theoretically deliver an energy density that is comparable to that of gasoline. Lithium oxide formation involves a four-electron reaction that is more difficult to achieve than the one- and two-electron reaction processes that result in lithium superoxide (LiO2) and lithium peroxide (Li2O2), respectively. By using a composite polymer electrolyte based on Li10GeP2S12 nanoparticles embedded in a modified polyethylene oxide polymer matrix, we found that Li2O is the main product in a room temperature solid-state lithium-air battery. The battery is rechargeable for 1000 cycles with a low polarization gap and can operate at high rates. The four-electron reaction is enabled by a mixed ion-electron-conducting discharge product and its interface with air.

Selective Hydration of Rutile TiO2 as a Strategy for Site-Selective Atomic Layer Deposition.

The feasibility of a site-selective hydration strategy that enables site-selective atomic layer deposition (ALD) is investigated among four rutile TiO2 facets [(110), (100), (101) and (001)] and their most prevalent step edges. First-principles simulations of asymmetric slab models were utilized to create accurate representations of pristine terrace and step edge sites. The adsorption free energies for molecular and dissociative adsorption of H2O were calculated to evaluate this strategy as a viable route to step edge selectivity. We predict that selective hydroxylation is possible on the 110 and 001 step edges and further computationally evaluate three metalorganic ALD precursors for their compatibility with the selective hydration strategy. Experimental evidence for delayed nucleation of ALD on rutile (001), (110), and (100) TiO2 single crystals corroborates predictions of the dehydration of the surface and suggests the possibility of site-selective ALD.

Nanostructured Conductive Metal Organic Frameworks for Sustainable Low Charge Overpotentials in Li-Air Batteries.

Lithium-oxygen batteries are among the most attractive alternatives for future electrified transportation. However, their practical application is hindered by many obstacles. Due to the insulating nature of Li2 O2 product and the slow kinetics of reactions, attaining sustainable low charge overpotentials at high rates becomes a challenge resulting in the battery's early failure and low round trip efficiency. Herein, outstanding characteristics are discovered of a conductive metal organic framework (c-MOF) that promotes the growth of nanocrystalline Li2 O2 with amorphous regions. This provides a platform for the continuous growth of Li2 O2 units away from framework, enabling a fast discharge at high current rates. Moreover, the Li2 O2 structure works in synergy with the redox mediator (RM). The conductivity of the amorphous regions of the Li2 O2 allows the RM to act directly on the Li2 O2 surface instead of catalyst edges and then transport through the electrolyte to the Li2 O2 surface. This direct charge transfer enables a small charge potential of <3.7 V under high current densities (1-2 A g-1 ) sustained for a long cycle life (100-300 cycles) for large capacities (1000-2000 mAh g-1 ). These results open a new direction for utilizing c-MOFs towards advanced energy storage systems.

Activation of N2 on Manganese Nitride-Supported Ni3 and Fe3 Clusters and Relevance to Ammonia Formation.

Dual-site models were constructed to represent manganese nitride (Mn4N)-supported Ni3 and Fe3 clusters for NH3 synthesis. Density functional theory calculations produced an energy barrier of approximately 0.55 eV for N-N bond activation at the interfacial nitrogen vacancy sites (Nv); also, the hydrogenation and removal of interfacial N is promoted by earth-abundant Ni and Fe metals. Steady-state microkinetic modeling revealed that the turnover frequencies of NH3 production follow an order of Fe3@Mn4N ≈ Ni3@Mn4N > Mn4N > Fe ≫ Ni. Moreover, we present clear evidence that, before NH3 formation, NH migrates from Nv onto the metallic sites. Using N binding energy (BEN) and the transition-state energy of N2 activation (ETS) as descriptors, we concluded that the beneficial effects owing to interfacial Nv sites are the most pronounced when BEN is either too strong or too weak while ETS is high; otherwise, excessive Nv sites may hinder catalyst performance.

Theoretical evidence of water serving as a promoter for lithium superoxide disproportionation in Li-O2 batteries.

Experimental evidence has demonstrated that the presence of water in non-aqueous electrolytes significantly affects Li-O2 electrochemistry. Understanding the reaction mechanism for Li2O2 formation in the presence of water impurities is important to understand Li-O2 battery performance. A recent experiment has found that very small amounts of water (as low as 40 ppm) can significantly affect the product formation in Li-O2 batteries as opposed to essentially no water (1 ppm). Although experimental as well as theoretical work has proposed mechanisms of Li2O2 formation in the presence of much larger amounts of water, none of the mechanisms provide an explanation for the observations for very small amounts of water. In this work, density functional theory (DFT) was utilized to obtain a mechanistic understanding of the Li-O2 discharge chemistry in a dimethoxyethane (DME) electrolyte containing an isolated water and no water. The reaction pathways for Li2O2 formation from LiO2 on a model system were carefully evaluated with different level of theories, i.e. PBE (PW), B3LYP/6-31G(2df,p), B3LYP/6-311++G(2df,p) and G4MP2. The results indicate that the LiO2 disproportionation reaction to Li2O2 can be promoted by the water in DME electrolyte, which explains why there is a significant difference compared to when no water is present in the experimentally observed discharge product distributions. Ab initio molecular dynamics calculations were also used to investigate the disproportionation of LiO2 dimer in explicit DME. This work adds to the fundamental understanding of the discharge chemistry of a Li-O2 battery.

2D Copper Tetrahydroxyquinone Conductive Metal-Organic Framework for Selective CO2 Electrocatalysis at Low Overpotentials.

Metal-organic frameworks (MOFs) are promising materials for electrocatalysis; however, lack of electrical conductivity in the majority of existing MOFs limits their effective utilization in the field. Herein, an excellent catalytic activity of a 2D copper (Cu)-based conductive MOF, copper tetrahydroxyquinone (CuTHQ), is reported for aqueous CO2 reduction reaction (CO2 RR) at low overpotentials. It is revealed that CuTHQ nanoflakes (NFs) with an average lateral size of 140 nm exhibit a negligible overpotential of 16 mV for the activation of this reaction, a high current density of ≈173 mA cm-2 at -0.45 V versus RHE, an average Faradaic efficiency (F.E.) of ≈91% toward CO production, and a remarkable turnover frequency as high as ≈20.82 s-1 . In the low overpotential range, the obtained CO formation current density is more than 35 and 25 times higher compared to state-of-the-art MOF and MOF-derived catalysts, respectively. The operando Cu K-edge X-ray absorption near edge spectroscopy and density functional theory calculations reveal the existence of reduced Cu (Cu+ ) during CO2 RR which reversibly returns to Cu2+ after the reaction. The outstanding CO2 catalytic functionality of conductive MOFs (c-MOFs) can open a way toward high-energy-density electrochemical systems.

3D ordered macro-/mesoporous carbon supported Ag nanoparticles for efficient electrocatalytic oxygen reduction reaction.

Three-dimensionally ordered macro-/mesoporous carbon (OMMC)-supported Ag nanoparticles (Ag/OMMC) with homogeneously dispersed Ag particles are prepared and investigated as effective electrocatalysts for oxygen reduction reaction (ORR) in alkaline aqueous system. The obtained Ag/OMMC catalyst displays smaller Ag particle size, higher Ag dispersion, and enhanced catalytic activity and durability compared with the carbon black Vulcan XC-72R supported Ag (Ag/XC-72R). The sizes of Ag particles supported on the OMMC and XC-72R are 4.3 and 6.5 nm, respectively. The prepared Ag/OMMC catalyst shows a positive half-wave potential of 0.79 V vs. RHE and a large diffusion-limited current of 5.6 mA cm-2 at 0.4 V, superior to Ag/XC-72R catalyst. The better ORR performance of the Ag/OMMC is probably ascribed to the unique 3D ordered interconnected macro-/mesoporous structure, which contributes to facilitating the mass/charge transport, improving the Ag particle dispersion, and preventing the Ag particle growth and aggregation.

Elucidating Molecular Interactions in Glycerol Adsorption at the Metal-Water Interface with Density Functional Theory.

Glycerol is an extremely versatile platform molecule for chemical and fuel production, as evidenced by successful demonstrations in electrochemical and thermochemical processes, where key catalytic chemistries occur at the solid-liquid interface. Despite the remarkable progress made in enriching the first-principles-based computational tool set to reveal and characterize solvent structures in the past decade, techniques for realistic and efficient molecular-level modeling to study aqueous-phase glycerol chemistry are still far from mature. Many aqueous-phase catalytic systems are deemed too complex for routine modeling because of their highly correlated structures at the heterogeneous solid-liquid interface. This invited feature article merges recent developments in quantum mechanical solvation models and oxygenated hydrocarbon conversion chemistry by revisiting the molecular interactions of adsorbed glycerol and its dehydrogenation intermediates at the water-metal interface. Explicit participation of water through the establishment of water-adsorbate, water-water, and water-metal interactions on Pt(111) was investigated using density functional theory. In periodic models, the adsorption favors networklike structures with adsorbates as nodal points linked by coadsorbed water molecules. We also showed that these adsorption patterns actually preserve the original bond-order-based scaling relationship framework established without the consideration of solvent. This behavior can be exploited to improve computational efficiency for future analysis of catalytic polyol conversions in the aqueous-phase environment.