Engineering Sulfur Defects, Atomic Thickness, and Porous Structures into Cobalt Sulfide Nanosheets for Efficient Electrocatalytic Alkaline Hydrogen Evolution

The development of nonprecious metal-based electrocatalysts with high mass activity and efficient atom utilization for alkali hydrogen evolution reaction (HER) is of great importance for the preparation of hydrogen resource. The combination of ultrathin and porous structure, especially with the assistance of vacancy, is expected to be beneficial for achievement of high mass activity, but the development of a facile, robust, and generalized strategy to engineer ultrathin, porous, and vacancy-rich structure into nonlayer structured transition metal-based electrocatalysts is highly challenging. Here, we propose a plasma-induced dry exfoliation method to prepare nonlayer structured Co₃S₄ ultrathin porous nanosheets with abundant sulfur vacancies (Co₃S₄ PNS_vac), which show an onset overpotential of only 18 mV and an extremely large mass activity of 1056.6 A g^–1 at an overpotential of 200 mV. Experimental results and theoretical calculations confirm that the efficient alkaline HER performance could be attributed to the abundant active sites, good intrinsic activity, and accelerated electron/mass transfer. Additionally, the plasma-assisted conversion method can also be extended to fabricate CoSe₂ and NiSe₂ ultrathin porous nanosheets with selenium vacancies

Spatial Progression of Polysulfide Reactivity with Lithium Nitrate in Li–Sulfur Batteries

LiNO3 is a common electrolyte additive in Li–S batteries, but its stabilizing effect is not well-understood due to the complex electrolyte chemistry. This complexity often hampers the clear characterization and interpretation of data. Herein, we explore the LiNO3 reactivity with polysulfide through in operando sulfur K-edge spectroscopy, using a sulfur-free electrolyte with LiNO3 as the sole salt. We reveal a spatially progressing chemical reaction influenced by the polysulfide concentration gradient. Polysulfides are electrochemically generated near the sulfur cathode, leading to a high local concentration. As a result, they are incompletely oxidized by LiNO3 to sulfites, which are gradually further oxidized into sulfonates and sulfates. Conversely, polysulfides near the anode side have a lower local concentration as they are diffused from the cathode side, thus leading to more highly oxidized species like sulfonates and sulfates. These reaction products are stable during electrochemical cycling, suggesting their capabilities to passivate the electrodes and contribute to the cycling stabilities of Li–S batteries

Identification and Catalysis of the Potential-Limiting Step in Lithium-Sulfur Batteries

The Li-S chemistry is thermodynamically promising for high-density energy storage but kinetically challenging. Over the past few years, many catalyst materials have been developed to improve the performance of Li-S batteries and their catalytic role has been increasingly accepted. However, the classic catalytic behavior, i.e., reduction of reaction barrier, has not been clearly observed. Crucial mechanistic questions, including what specific step is limiting the reaction rate, whether/how it can be catalyzed, and how the catalysis is sustained after the catalyst surface is covered by solid products, remain unanswered. Herein, we report the first identification of the potential-limiting step of Li-S batteries operating under lean electrolyte conditions and its catalysis that conforms to classic catalysis principles, where the catalyst lowers the kinetic barrier of the potential-limiting step and accelerates the reaction without affecting the product composition. After carefully examining the electrochemistry under lean electrolyte conditions, we update the pathway of the Li-S battery reaction: S8 solid is first reduced to Li2S8 and Li2S4 molecular species sequentially; the following reduction of Li2S4 to a Li2S2–Li2S solid with an almost constant ratio of 1:4 is the potential-limiting step; the previously believed Li2S2-to-Li2S solid–solid conversion does not occur; and the recharging reaction is relatively fast. We further demonstrate that supported cobalt phthalocyanine molecules can effectively catalyze the potential-limiting step. After Li2S2/Li2S buries the active sites, it can self-catalyze the reaction and continue driving the discharging process

<i>In Situ</i> Raman Spectroscopy of Copper and Copper Oxide Surfaces during Electrochemical Oxygen Evolution Reaction: Identification of Cu^III Oxides as Catalytically Active Species

Scanning electron microscopy, X-ray diffraction, cyclic voltammetry, chronoamperometry, <i>in situ</i> Raman spectroscopy, and X-ray absorption near-edge structure spectroscopy (XANES) were used to investigate the electrochemical oxygen evolution reaction (OER) on Cu, Cu₂O, Cu­(OH)₂, and CuO catalysts. Aqueous 0.1 M KOH was used as the electrolyte. All four catalysts were oxidized or converted to CuO and Cu­(OH)₂ during a slow anodic sweep of cyclic voltammetry and exhibited similar activities for the OER. A Raman peak at 603 cm^–1 appeared for all the four samples at OER-relevant potentials, ≥1.62 V vs RHE. This peak was identified as the Cu–O stretching vibration band of a Cu^III oxide, a metastable species whose existence is dependent on the applied potential. Since this frequency matches well with that from a NaCu^IIIO₂ standard, we suggest that the chemical composition of the Cu^III oxide is CuO₂^–-like. The four catalysts, in stark contrast, did not oxidize the same way during direct chronoamperometry measurements at 1.7 V vs RHE. Cu^III oxide was observed only on the CuO and Cu­(OH)₂ electrodes. Interestingly, these two electrodes catalyzed the OER ∼10 times more efficiently than the Cu and Cu₂O catalysts. By correlating the intensity of the Raman band of Cu^III oxide and the extent of the OER activity, we propose that Cu^III species provides catalytically active sites for the electrochemical water oxidation. The formation of Cu^III oxides on CuO films during OER was also corroborated by <i>in situ</i> XANES measurements of the Cu K-edge. The catalytic role of Cu^III oxide in the O₂ evolution reaction is proposed and discussed

Spatial Imaging and Speciation of Lead in the Accumulator Plant <i>Sedum alfredii</i> by Microscopically Focused Synchrotron X-ray Investigation

Sedum alfredii (Crassulaceae), a species native to China, has been characterized as a Zn/Cd cohyperaccumulator and Pb accumulator though the mechanisms of metal tolerance and accumulation are largely unknown. Here, the spatial distribution and speciation of Pb in tissues of the accumulator plant was investigated using synchrotron-based X-ray microfluorescence and powder Extended X-ray absorption fine structure (EXAFS) spectroscopy. Lead was predominantly restricted to the vascular bundles of both leaf and stem of the accumulator. Micro-XRF analysis revealed that Pb distributed predominantly within the areas of vascular bundles, and a positive correlation between the distribution patterns of S and Pb was observed. The dominant chemical form of Pb (>60%) in tissues of both accumulating (AE) and nonaccumulating ecotype (NAE) S. alfredii was similar to prepared Pb-cell wall compounds. However, the percentage of the Pb-cell wall complex is lower in the stem and leaf of AE, and a small amount of Pb appeared to be associated with SH-compounds. These results suggested a very low mobility of Pb out of vascular bundles, and that the metal is largely retained in the cell walls during transportation in plants of S. alfredii

Single-Atomic Cu with Multiple Oxygen Vacancies on Ceria for Electrocatalytic CO₂ Reduction to CH₄

The electrocatalytic reduction of CO₂ into value-added chemicals such as hydrocarbons has the potential for supplying fuel energy and reducing environmental hazards, while the accurate tuning of electrocatalysts at the ultimate single-atomic level remains extremely challenging. In this work, we demonstrate an atomic design of multiple oxygen vacancy-bound, single-atomic Cu-substituted CeO₂ to optimize the CO₂ electrocatalytic reduction to CH₄. We carried out theoretical calculations to predict that the single-atomic Cu substitution in CeO₂(110) surface can stably enrich up to three oxygen vacancies around each Cu site, yielding a highly effective catalytic center for CO₂ adsorption and activation. This theoretical prediction is consistent with our controlled synthesis of the Cu-doped, mesoporous CeO₂ nanorods. Structural characterizations indicate that the low concentration (<5%) Cu species in CeO₂ nanorods are highly dispersed at single-atomic level with an unconventionally low coordination number ∼5, suggesting the direct association of 3 oxygen vacancies with each Cu ion on surfaces. This multiple oxygen vacancy-bound, single atomic Cu-substituted CeO₂ enables an excellent electrocatalytic selectivity in reducing CO₂ to methane with a faradaic efficiency as high as 58%, suggesting strong capabilities of rational design of electrocatalyst active centers for boosting activity and selectivity

One-Pot Synthesis of Fe(III)–Polydopamine Complex Nanospheres: Morphological Evolution, Mechanism, and Application of the Carbonized Hybrid Nanospheres in Catalysis and Zn–Air Battery

We report one-pot synthesis of Fe­(III)–polydopamine (PDA) complex nanospheres, their structures, morphology evolution, and underlying mechanism. The complex nanospheres were synthesized by introducing ferric ions into the reaction mixture used for polymerization of dopamine. It is verified that both the oxidative polymerization of dopamine and Fe­(III)–PDA complexation contribute to the “polymerization” process, in which the ferric ions form coordination bonds with both oxygen and nitrogen, as indicated by X-ray absorption fine-structure spectroscopy. In the “polymerization” process, the morphology of the complex nanostructures is gradually transformed from sheetlike to spherical at the feed Fe­(III)/dopamine molar ratio of 1/3. The final size of the complex spheres is much smaller than its neat PDA counterpart. At higher feed Fe­(III)/dopamine molar ratios, the final morphology of the “polymerization” products is sheetlike. The results suggest that the formation of spherical morphology is likely to be driven by covalent polymerization-induced decrease of hydrophilic functional groups, which causes reself-assembly of the PDA oligomers to reduce surface area. We also demonstrate that this one-pot synthesis route for hybrid nanospheres enables the facile construction of carbonized PDA (C-PDA) nanospheres uniformly embedded with Fe₃O₄ nanoparticles of only 3–5 nm in size. The C-PDA/Fe₃O₄ nanospheres exhibit catalytic activity toward oxygen reduction reaction and deliver a stable discharge voltage for over 200 h when utilized as the cathode in a primary Zn–air battery and are also good recyclable catalyst supports

β‑FeOOH: An Earth-Abundant High-Capacity Negative Electrode Material for Sodium-Ion Batteries

Thanks to the great earth abundance and excellent energy density of sodium, sodium-ion batteries are promising alternative energy storage devices for large-scale applications. Developing cheap, safe, and high-capacity sodium-ion battery anode materials is one of the critical challenges in this field. Here, we show that β-FeOOH is a very promising low-cost anode material, with a high reversible capacity (>500 mAh g^–1 during initial cycles). The fundamental characteristics associated with the discharge/charge processes, in terms of the redox reactions, formation/deformation of the solid electrolyte interface (SEI) layers, and structural and morphological changes, are comprehensively investigated. In addition, a comparison study shows that the smaller-sized FeOOH has more serious kinetic restrictions, and thus lower capacities, while it shows better cyclability than the bigger one. Origins of the large overpotential are discussed, and it is suggested that the overpotential should be mainly due to the features of the surface-concentration-dependent potential and the slow diffusion of Na⁺; in addition, the presence of the SEI layers may also contribute to the overpotential

Degree of Geometric Tilting Determines the Activity of FeO₆ Octahedra for Water Oxidation

Fe oxides and (oxy)­hydroxides are promising cost-effective catalysts for scalable water electrolysis. For an improvement in the understanding of the structural factors required by the most active Fe sites, the role of geometric tilting in determining the activity of the FeO₆ octahedron for water oxidation was investigated. The catalytic performance of the FeO₆ octahedron in a series of crystalline structures, i.e., perovskites AFeO₃, spinel ZnFe₂O₄, and β-FeOOH, was found to be negatively correlated with their octahedral tilting degree. This correlation was rationalized through the Fe–O covalency, which is reflected by the O 2p band center as well as the charge-transfer energy obtained from ab initio calculations. Thus, it was disclosed that FeO₆ octahedral tilting alters the activity for water oxidation through changing the covalency degree of Fe–O bonds

Degradation of Lithium Iron Phosphate Sulfide Solid-State Batteries by Conductive Interfaces

The superionic solid-state argyrodite electrolyte Li6PS5Br can improve lithium and lithium-ion batteries’ safety and energy density. Despite many reports validating the conductivity of this electrolyte, it still suffers from passivating electrode degradation mechanisms. At first analysis, lithium iron phosphate (LFP) should be more thermodynamically stable in contact with sulfide electrolytes. However, without substantial improvements to interfacial engineering, we find that LFP is not inherently stable against Li6PS5Br. We hypothesize argyrodite oxidation favorably competes with LFP’s delithiation, insulating the electrolyte–electrode interface and causing large overpotential losses. We show that compared to LiNixMnyCozO2, LFP has no actual electrochemical stability advantage despite operating at a lower voltage. We utilize tender energy XAS and XPS to show that chemical reactions occur between LFP and the Li6PS5Br solid electrolyte and these reactions are exacerbated by cycling. We also show that electrochemical degradation occurs at the interface between the solid electrolyte ion conductor and any electron conductor, namely, the active material and carbon additives. We further demonstrate that LiNbO3 cathode coatings on LFP can delay electrochemical degradation by electronically insulating the LFP–sulfide electrolyte interface but not prevent its occurrence at the carbon–electrolyte interface