Binary Solvent Induced Stable Interphase Layer for Ultra-Long Life Sodium Metal Batteries.

Sodium foil, promising for high-energy-density batteries, faces reversibility challenges due to its inherent reactivity and unstable solid electrolyte interphase (SEI) layer. In this study, a stable sodium metal battery (SMB) is achieved by tuning the electrolyte solvation structure through the addition of co-solvent 2-methyl tetrahydrofuran (MTHF) to diglyme (Dig). The introduction of cyclic ether-based MTHF results in increased anion incorporation in the solvation structure, even at lower salt concentrations. Specifically, the anion stabilization capabilities of the environmentally sustainable MTHF co-solvent lead to a contact-ion pair-based solvation structure. Time-of-flight mass spectroscopy analysis reveals that a shift toward an anion-dominated solvation structure promotes the formation of a thin and uniform SEI layer. Consequently, employing a NaPF6-based electrolyte with a Dig:MTHF ratio of 50% (v/v) binary solvent yields an average Coulombic efficiency of 99.72% for 300 cycles in Cu||Na cell cycling. Remarkably, at a C/2 cycling rate, Na||Na symmetric cell cycling demonstrates ultra-long-term stability exceeding 7000 h, and full cells with Na0.44MnO2 as a cathode retain 80% of their capacity after 500 cycles. This study systematically examines solvation structure, SEI layer composition, and electrochemical cycling, emphasizing the significance of MTHF-based binary solvent mixtures for high-performance SMBs.

A Review of Transition Metal Boride, Carbide, Pnictide, and Chalcogenide Water Oxidation Electrocatalysts.

Transition metal borides, carbides, pnictides, and chalcogenides (X-ides) have emerged as a class of materials for the oxygen evolution reaction (OER). Because of their high earth abundance, electrical conductivity, and OER performance, these electrocatalysts have the potential to enable the practical application of green energy conversion and storage. Under OER potentials, X-ide electrocatalysts demonstrate various degrees of oxidation resistance due to their differences in chemical composition, crystal structure, and morphology. Depending on their resistance to oxidation, these catalysts will fall into one of three post-OER electrocatalyst categories: fully oxidized oxide/(oxy)hydroxide material, partially oxidized core@shell structure, and unoxidized material. In the past ten years (from 2013 to 2022), over 890 peer-reviewed research papers have focused on X-ide OER electrocatalysts. Previous review papers have provided limited conclusions and have omitted the significance of "catalytically active sites/species/phases" in X-ide OER electrocatalysts. In this review, a comprehensive summary of (i) experimental parameters (e.g., substrates, electrocatalyst loading amounts, geometric overpotentials, Tafel slopes, etc.) and (ii) electrochemical stability tests and post-analyses in X-ide OER electrocatalyst publications from 2013 to 2022 is provided. Both mono and polyanion X-ides are discussed and classified with respect to their material transformation during the OER. Special analytical techniques employed to study X-ide reconstruction are also evaluated. Additionally, future challenges and questions yet to be answered are provided in each section. This review aims to provide researchers with a toolkit to approach X-ide OER electrocatalyst research and to showcase necessary avenues for future investigation.

Tailoring 3D-Printed Electrodes for Enhanced Water Splitting.

Alkaline water electrolysis, a promising technology for clean energy storage, is constrained by extrinsic factors in addition to intrinsic electrocatalytic activity. To begin to compare between catalytic materials for electrolysis applications, these extrinsic factors must first be understood and controlled. Here, we modify extrinsic electrode properties and study the effects of bubble release to examine how the electrode and surface design impact the performance of water electrolysis. We fabricate robust and cost-effective electrodes through a sequential three-dimensional (3D) printing and metal deposition procedure. Through a systematic assessment of the deposition procedure, we confirm the close relationship between extrinsic electrode properties (i.e., wettability, surface roughness, and electrochemically active surface area) and electrochemical performance. Modifying the electrode geometry, size, and electrolyte flow rate results in an overpotential decrease and different bubble diameters and lifetimes for the hydrogen (HER) and oxygen evolution reactions (OER). Hence, we demonstrate the essential role of the electrode architecture and forced electrolyte convection on bubble release. Additionally, we confirm the suitability of ordered, Ni-coated 3D porous structures by evaluating the HER/OER performance, bubble dissipation, and long-term stability. Finally, we utilize the 3D porous electrode as a support for studying a benchmark NiFe electrocatalyst, confirming the robustness and effectiveness of 3D-printed electrodes for testing electrocatalytic materials while extrinsic properties are precisely controlled. Overall, we demonstrate that tailoring electrode architectures and surface properties result in precise tuning of extrinsic electrode properties, providing more reproducible and comparable conditions for testing the efficiency of electrode materials for water electrolysis.

Li-Zn Overlayer to Facilitate Uniform Lithium Deposition for Lithium Metal Batteries.

The highly reactive nature and rough surface of Li foil can lead to the uncontrollable formation of Li dendrites when employed as an anode in a lithium metal battery. Thus, it could be of great practical utility to create uniform, electrochemically stable, and "lithiophilic" surfaces to realize homogeneous deposition of Li. Herein, a LiZn alloy layer is deposited on the surface of Li foil by e-beam evaporation. The idea is to introduce a uniform alloy surface to increase the active area and make use of the Zn sites to induce homogeneous nucleation of Li. The results show that the alloy film protected the Li metal anode, allowing for a longer cycling life with a lower deposition overpotential over a pure-Li metal anode in symmetric Li cells. Furthermore, full cells pairing the modified lithium anode with a LiFePO4 cathode showed an incremental increase in Coulombic efficiency compared with pure-Li. The concept of using only an alloy modifying layer by an in-situ e-beam deposition synthesis method offers a potential method for enabling lithium metal anodes for next-generation lithium batteries.

Anodized Nickel Foam for Oxygen Evolution Reaction in Fe-Free and Unpurified Alkaline Electrolytes at High Current Densities.

To achieve practically high electrocatalytic performance for the oxygen evolution reaction (OER), the active surface area should be maximized without severely compromising electron and mass transport throughout the catalyst electrode. Though the importance of electron and mass transport has been studied using low surface area catalysts under low current densities (∼tens of mA/cm2), the transport properties of large surface area catalysts under high operating current densities (∼500 mA/cm2) for practical OER catalysis have rarely been explored. Herein, three-dimensional (3D) hierarchically porous anodized nickel foams (ANFs) with large and variable surface areas were synthesized via electrochemical anodization of 3D nickel foam and applied as OER electrocatalysts in Fe-free and unpurified KOH electrolytes. Using Fe-free and in situ Fe-doped ANF that were prepared in Fe-free and unpurified electrolytes, respectively, we investigated the interdependent effects of active surface area and transport properties on OER activity under practically high current densities. While activity increased linearly with active surface area for Fe-free ANF, the activity of Fe-doped ANF showed a nonlinear increase with active surface area due to lower electrocatalytic activity enhancement. Detailed investigations on the possible factors (Fe incorporation, mass transport, and electron transport) identified that electron transport limitations played the major role in restricting the activity enhancement with increasing active surface area for Fe-doped ANF, although Fe-doped ANF has electron transport properties better than those of Fe-free ANF. This study exemplifies the growing significance of electron transport properties in large surface area catalysts, especially those with superb intrinsic catalytic activity and high operating current density.

Modulating Charge Transfer Efficiency of Hematite Photoanode with Hybrid Dual-Metal-Organic Frameworks for Boosting Photoelectrochemical Water Oxidation.

The glorious charge transfer efficiency of photoanode is an important factor for efficient photoelectrochemical (PEC) water oxidation. However, it is often limited by slow kinetics of oxygen evolution reaction. Herein, a dual transition metal-based metal-organic frameworks (MOF) cocatalyst, Fe@Ni-MOF, is introduced into a titanium-doped hematite (Fe2O3:Ti) photoanode. The combination of Ni and Fe can optimize the filling of 3d orbitals. Moreover, the introduction of Fe donates electrons to Ni in the MOF structure, thus, suppressing the irreversible (long-life-time) oxidation of Ni2+ into Ni3+. The resulting Fe@Ni-MOF/Fe2O3:Ti photoanode exhibits ∼threefold enhancement in the photocurrent density at 1.23 V versus the reversible hydrogen electrode. Kinetic analysis of the PEC water oxidation processes indicates that this performance improvement is primarily due to modulating the charge transfer efficiency of hematite photoanode. Further results show that a single transition metal-based MOF cocatalyst, Ni-MOF, exhibits slow charge transfer in spite of a reduction in surface charge recombination, resulting in a smaller charge transfer efficiency. These findings provide new insights for the development of photoelectrodes decorated with MOFs.

Simultaneous Sulfite Electrolysis and Hydrogen Production Using Ni Foam-Based Three-Dimensional Electrodes.

The electrochemical oxidation of sulfite ions offers encouraging advantages for large-scale hydrogen production, while sulfur dioxide emissions can be effectively used to obtain value-added byproducts. Herein, the performance and stability during sulfite electrolysis under alkaline conditions are evaluated. Nickel foam (NF) substrates were functionalized as the anode and cathode through electrochemical deposition of palladium and chemical oxidation to carry out the sulfite electro-oxidation and hydrogen evolution reactions, respectively. A combined analytical approach in which a robust electrochemical flow cell was coupled to different in situ and ex situ measurements was successfully implemented to monitor the activity and stability during electrolysis. Overall, satisfactory sulfite conversion and hydrogen production efficiencies (>90%) at 10 mA·cm-2 were mainly attributed to the use of NF in three-dimensional electrodes with a large surface area and enhanced mass transfer. Furthermore, stabilization processes associated with electrochemical dissolution and sulfur crossover through the membrane induced specific changes in the chemical and physical properties of the electrodes after electrolysis. This study demonstrates that NF-based electrocatalysts can be incorporated in an efficient electrochemical flow cell system for sulfite electrolysis and hydrogen production, with potential applications at a large scale.

Lithium Fluoride Coated Silicon Nanocolumns as Anodes for Lithium Ion Batteries.

Silicon (Si) films are promising anode materials in thin-film lithium batteries due to their high capacity of 3578 mAh g-1, but the huge volume expansion of lithiated Li15Si4 and the unstable solid electrolyte interphase (SEI) preclude their practical application. Here lithium fluoride (LiF) coated Si nanocolumns are fabricated by glancing angle evaporation to address the obstacle. The LiF coating can elevate the lithium ion diffusion coefficient (LDC) of Si electrodes upon the alloying reaction and reduce the LDC upon the SEI formation. The composition evolution of the outer SEI layer in the LiF/Si electrodes is studied by ex situ X-ray photoelectron spectroscopy. The modified surface and mitigated volume expansion enable the LiF/Si nanocolumns to exhibit superior rate capability and higher cycling stability compared with the pristine Si nanocolumns. This work demonstrates the positive effect of LiF coating for reducing the polarization and forming a robust SEI film on Si anodes.

Cu4SnS4-Rich Nanomaterials for Thin-Film Lithium Batteries with Enhanced Conversion Reaction.

Through a simple gelation-solvothermal method with graphene oxide as the additive, a Cu4SnS4-rich composite of nanoparticles and nanotubes is synthesized and applied for thin and flexible Li-metal batteries. Unlike the Cu2SnS3-rich electrode, the Cu4SnS4-rich electrode cycles stably with an enhanced conversion capacity of ∼416 mAh g-1 (∼52% of total capacity) after 200 cycles. The lithiation/delithiation mechanisms of Cu-Sn-S electrodes and the voltage ranges of conversion and alloying reactions are informed by in situ X-ray diffraction tests. The conversion process of three Cu-Sn-S compounds is compared by density functional theory (DFT) calculations based on three algorithms, elucidating the enhanced conversion stability and superior diffusion kinetics of Cu4SnS4 electrodes. The reaction pathway of Cu-Sn-S electrodes and the root cause for the unstable capacity are revealed by in situ/ex situ characterizations, DFT calculations, and various electrochemical tests. This work provides insight into developing energy materials and power devices based on multiple lithiation mechanisms.

Distinguishing the effects of altered morphology and size on the visible light-induced water oxidation activity and photoelectrochemical performance of BaTaO2N crystal structures.

Factors, including crystallinity, morphology, size, preferential orientation, growth, composition, porosity, surface area, etc., can directly influence the optical, charge-separation, charge-transfer and water oxidation and reduction properties of particle-based photocatalysts. Therefore, these factors must be considered when designing high-performance particle-based photocatalysts for solar water splitting. Here, a flux growth method was applied to alter the morphology and size of Ba5Ta4O15 precursor oxide crystals using BaCl2, KCl, RbCl, CsCl, KCl + BaCl2 and K2SO4 at different solute concentrations, and the impact of nitridation with and without KCl flux was studied. Specifically, the effects of altered morphology and size on the visible light-induced water oxidation activity and photoelectrochemical performance of the BaTaO2N crystal structures were investigated. Upon nitridation, the samples became porous due to the lattice shrinkage caused by the replacement of 3 O2- with 2 N3- in the anionic network. The BaTaO2N crystal structures obtained by nitridation without KCl flux show higher surface areas than do their counterparts prepared by nitridation with KCl flux because of the formation of porous networks. All of the samples exhibited a high anodic photocurrent upon nitridation without KCl flux compared with those of the samples obtained by nitridation with KCl flux. These findings demonstrate that it is important to specifically engineer photocatalytic crystals to reach their maximum potential in solar water splitting.

Interface Engineering and its Effect on WO3-Based Photoanode and Tandem Cell.

During photoelectrochemical (PEC) water splitting, the reactions occur on the surface of the photoelectrode. Therefore, the properties of the interfaces between the various components of the electrode (semiconductor/semiconductor, semiconductor/catalyst, or photoelectrode/electrolyte) affect the PEC performance of the composite material. Notably, surface trap states may hinder charge transfer and transport properties, and also cause Fermi pinning, affecting the quasi-Fermi level and onset potential under illumination, which may in turn influence the PEC performance of the corresponding tandem cells. In this study, plate-like WO3 array films prepared by an aqueous chemical growth method were employed to highlight the effect of interfacial properties on the performance of a WO3-based photoanode. The Mott-Schottky and linear sweep voltammetry experiments prove the existence of surface trap states and Fermi pinning for pristine WO3, which are alleviated after an "etching" treatment and disappeared after surface passivation by a Ga2O3 layer. Both etching and passivation increase the oxygen evolution activity and the Faradaic efficiency for the oxygen evolution reaction (OER). After loading a permeable catalyst (FeOOH), the photocurrent is further increased, and there is a synergistic effect between loading of the electrocatalyst with etching or passivation. The onset potentials of the samples follow the trends: etch-WO3/FeOOH < WO3/FeOOH ≤ WO3/Ga2O3/FeOOH < etch-WO3 < WO3 < WO3/Ga2O3, indicating that the interfacial properties have a significant effect on the PEC performance. Meanwhile, the modified WO3-based electrode was combined with a dye-sensitized solar cell to fabricate tandem cell, which showed 2.42-fold photocurrent density compared with the pristine WO3-based tandem cell.

Self-Assembled Cu-Sn-S Nanotubes with High (De)Lithiation Performance.

Through a gelation-solvothermal method without heteroadditives, Cu-Sn-S composites self-assemble to form nanotubes, sub-nanotubes, and nanoparticles. The nanotubes with a Cu3-4SnS4 core and Cu2SnS3 shell can tolerate long cycles of expansion/contraction upon lithiation/delithiation, retaining a charge capacity of 774 mAh g-1 after 200 cycles with a high initial Coulombic efficiency of 82.5%. The importance of the Cu component for mitigation of the volume expansion and structural evolution upon lithiation is informed by density functional theory calculations. The self-generated template and calculated results can inspire the design of analogous Cu-M-S (M = metal) nanotubes for lithium batteries or other energy storage systems.