Interface optimization mechanism and quantitative analysis of hybrid graphite anode for fast-charging lithium-ion batteries.

Due to the inherent characteristics of traditional graphite anode material, its lithium diffusion kinetic is significantly constrained, easily leading to a noticeable capacity degradation during rapid charge/discharge cycling. Although modifying the graphite by mixing the hard carbon can effectively enhance its fast-charging performance, yet the underlying mechanism of improvement effect and structure design of interface are still needed to further investigate. To address this research gap, hard carbon-coated graphite (HCCG) material has been designed and synthesized through simple interface engineering, which is aimed to explore and elucidate the optimization mechanisms on fast-charging performance from the graphite interface perspective. According to the electrochemical calculations, the HCCG anode exhibits significant enhancements. Specially, its reversible lithium content is increased by approximately 8 % at various states of charge, its exchange current density is tripled, and its Tafel slope is reduced to one-quarter of the original graphite. Therefore, the HCCG maintains an impressive 86.89 % capacity retention and a high capacity of 202.3 mAh g-1 after 1450 cycles at ultrahigh rate of 5C. These improvements indicate a substantial reduction in electrode polarization during fast charging, which is ascribed to the abundant lithium intercalation pathways and accommodation space provided by the intimate hard carbon coating layer. Moreover, as a "buffer layer," hard carbon coating can accommodate considerable amount of lithium deposited on the graphite surface, effectively mitigating the capacity loss caused by lithium deposition and maintaining effective electrochemical contact without delamination. This comprehensive analysis of hard carbon coating illustrates the improvement mechanism of fast-charging performance, which can offer valuable insights into the dynamic and structural optimization of graphite anode interfaces.

Ternary nanostructure of CdS-rGO-Ag for enhanced photoelectrochemical application.

In the present study, we have synthesized the ternary nanostructure of CdS-rGO-Ag by using the solvothermal method for the enhanced photocatalytic as well as electrocatalytic activity of the material. The optical properties of the prepared samples were characterized by using UV-visible spectroscopy and photoluminescence (PL) spectroscopy. A decline in the energy gap of CdS-rGO-Ag nanostructure to 1.90 ± 0.05 from 2.40 ± 0.08 eV (CdS) is observed and may be attributed due to the rGO-Ag-induced creation of trap states in between the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) levels of CdS. The observed significant PL quenching also confirms the inclusion of trap states in the prepared nanostructure CdS-rGO-Ag, suggesting efficient photoactivity. The enhanced photocatalytic activity of CdS-rGO-Ag for methylene blue (MB) dye degradation supports the photocatalytic activity of prepared nanostructures. Furthermore, the photoelectrochemical catalytic activity of the CdS-rGO-Ag catalyst investigated by using cyclic voltammetry (CV), linear sweep voltammetry (LSV), and electrochemical impedance spectroscopy (EIS) depicts the improved current density, lowering of overpotential. The decrement in charge transfer resistance and enhancement in photocurrent density under light illumination indicates the better photoelectrochemical performance of CdS-rGO-Ag.

Dual-Site Bridging Mechanism for Bimetallic Electrochemical Oxygen Evolution.

Heterogeneous dual-site electrocatalysts are emerging cutting-edge materials for efficient electrochemical water splitting. However, the corresponding oxygen evolution reaction (OER) mechanism on these materials is still unclear. Herein, based on a series of in situ spectroscopy experiments and density function theory (DFT) calculations, a new heterogeneous dual-site O-O bridging mechanism (DSBM) is proposed. This mechanism is to elucidate the sequential appearance of dual active sites through in situ construction (hybrid ions undergo reconstruction initially), determine the crucial role of hybrid dual sites in this mechanism (with Ni sites preferentially adsorbing hydroxyls for catalysis followed by proton removal at Fe sites), assess the impact of O-O bond formation on the activation state of water (inducing orderliness of activated water), and investigate the universality (with Co doping in Ni(P4O11)). Under the guidance of this mechanism, with Fe-Ni(P4O11) as pre-catalyst, the in situ formed Fe-Ni(OH)2 electrocatalyst has reached a record-low overpotential of 156.4 mV at current density of 18.0 mA cm-2. Successfully constructed Fe-Ni(P4O11)/Ti uplifting the overall efficacy of the phosphate from moderate to superior, positioning it as an innovative and highly proficient electrocatalyst for OER.

A long-cycle-life reversible Li-CO2 battery enabled by engineering the active sites of graphene with Pd nanoparticles.

Li-CO2 batteries have been recognized as an emerging technology for energy storage systems owing to their high theoretical specific energy and environmentally friendly CO2 fixation ability. However, their development for applications requires a high energy efficiency and long cycle-life, this is currently limited to the formation of wide-bandgap insulator Li2CO3 during discharge. Here, nanoparticle Pd supported on reduced graphene oxide (rGO) is utilized as cathodes for Li-CO2 batteries, Pd nanoparticles as active centers significantly enhance CO2RR/CO2ER reaction activity, which can support the fast formation and decomposition of Li2CO3 in organic electrolytes and achieve a high discharge capacity of 7500 mAh g-1. It also performs remarkably high cycling stability of over 500 cycles with a long cycle-life of 5000 hours. The observed super electrochemical performance is attributable to the thick electrode design and uniform distribution of ultrafine catalyst nanoparticle Pd. When Li2CO3 is adsorbed on Pd particle, the Li-O bond in Li2CO3 will be elongated due to the interactions of two nucleophilic O atoms with Pd, resulting in a weakening of the Li-O bond and activation of Li2CO3. Our work suggests a way to design catalysts with high activity that can be used to activate the performance of Li-CO2 batteries.

Corrosion behavior of Ti-Pt-coated stainless steel for bipolar plates in polymer electrolyte membranes under water electrolysis conditions.

In this study, the corrosion behavior and degradation mechanism of Ti-Pt-coated stainless steel bipolar plates were investigated through electrochemical tests and surface analysis in a polymer electrolyte membrane water electrolysis (PEMWE) operating environment. The coated bipolar plate has a corrosion current density of only 1.68 × 10-8 A/cm2, which is an order of magnitude lower than that of the bare SS316L substrate (1.94 × 10-7 A/cm2), indicating that its corrosion resistance is superior to that of bare SS316L substrate. However, in the PEMWE operating environment, the protection efficiency of the coating and the corrosion resistance of the coated bipolar plate decreased. The degradation of the coated bipolar plate can be attributed to electrolyte penetration into the blistering areas of the coating layer with micro voids. Defects in the coating layer occur because of the pressure of oxygen gas generated within the coating layer under high-potential conditions, thereby exposing the substrate to the electrolyte and corrosion.

α-Bi2Mo3O12: A Dual-Functional material for electrocatalytic water splitting and supercapacitor applications.

This study explores the functionality of α-Bi2Mo3O12 (BMO) as an electrocatalyst for water splitting and its suitability for supercapacitor applications. BMO was synthesized by the solvothermal method and characterized in pre-calcination [BMO (BC)], post-calcination [BMO (AC)], and base-etched forms [BMO (BE)]. Structural analysis confirmed the formation of α-Bi2Mo3O12 with well-defined crystallographic planes. Electrochemical analysis revealed that BMO (AC) exhibited the lowest overpotential for hydrogen evolution reactions (HER) and BMO (BC) exhibited the lowest overpotential for oxygen evolution reactions (OER), indicating its superior electrocatalytic activity. The Tafel slope and electrochemical impedance spectroscopy results confirmed the superior kinetics and charge transfer properties of BMO material. Furthermore, BMO samples demonstrated excellent stability during prolonged chronoamperometry (CA) testing for 12 h. For supercapacitor performances, the BMO (BE) exhibits a superior specific capacitance value of 398 F/g at 2.0 A/g. Thus, the BMO material delivers prominent electrocatalytic activity as well as supercapacitor performance. Overall, this study demonstrates the potentiality of α-Bi2Mo3O12 in different forms as a dual-functional material for efficient energy storage and conversion.

A Room-Temperature All-Solid-State Na-Ag Battery with a Long Cycle Life and Low Overpotential.

Aqueous Zn-Ag batteries have been developed and commercialized for nearly a century, offering stable discharge and high specific energies. Sodium, with its lower redox potential, smaller charge-to-mass ratio, and abundant resources, presents a promising alternative to zinc. In this study, we successfully developed an all-solid-state Na-Ag battery system. This battery demonstrates stable discharge and charge voltages, low overpotential (0.27 V), high energy efficiency (>91%), and long cycle life under moderate humidity at room temperature. The reaction mechanism was elucidated through combined analyses using differential electrochemical mass spectrometry (DEMS), X-ray diffraction (XRD), Raman spectroscopy, and X-ray photoelectron spectroscopy (XPS). Our findings indicate that metallic Ag in the cathode materials acts as an effective catalyst for the oxygen reduction reaction during the initial discharge process, forming NaOH as the discharge product. Ag is then oxidized during the charging process and recovered during discharge, serving as an active reactant in the Na-Ag battery. This work demonstrates superior performance of all-solid-state Na-Ag battery over aqueous Zn-Ag battery. Na-Ag battery may be of interest in applications with stringent requirements on stable discharge voltage and high specific energy.

Potential of V-Si72, V-C72, and V-Al36N36 as catalysts for oxygen reduction reaction.

CONTEXT: The capacities of V-Si72, V-C72, and V-Al36N36 nanocages to catalyze the ORR processes have been investigated. The acceptable pathways of ORR processes on V-Si72, V-C72, and V-Al36N36 nanocages have been examined by DSD-PBEPBE-D3/aug-cc-pVDZ, PW91PW91/aug-cc-pVDZ, and COSMO model in the gas phase and water. The ΔGreaction values of reaction steps of ORR pathways on V-Si72, V-C72, and V-Al36N36 nanocages are calculated. The Eadoption and Eformation of V-Si72, V-C72, and V-Al36N36 nanocages are negative values and these nanostructures are stable materials. The H2O has the lowest Eadsorption on V-Si72, V-C72, and V-Al36N36 nanocages. The *OH formation, creation of *OH-OH*, and formation of O* are rate-determining steps of ORR mechanisms. The overpotential values of ORR processes on V-Si72, V-C72, and V-Al36N36 nanocages are 0.41, 0.37, and 0.30 V, respectively. The V-Al36N36 nanocage have lower overpotential for ORR processes than V-Si72 and V-C72 nanocages by DSD-PBEPBE-D3/aug-cc-pVDZ, PW91PW91/aug-cc-pVDZ, and COSMO model in the gas phase and water. The V-Al36N36 nanocage have more negative ∆Greaction for reaction steps of ORR than V-Si72 and V-C72 nanocages. The V-Al36N36 nanocage with lower overpotential is proposed as an effective catalyst for ORR processes via studied pathways. METHODS: The DSD-PBEPBE-D3/aug-cc-pVDZ method has been used to optimize and calculate the frequencies of V-Si72, V-C72, and V-Al36N36 nanocages in GAMESS software. The complexes of O, OH, OOH, and H2O with V-Si72, V-C72, and V-Al36N36 nanocages are optimized and frequencies are determined by the DSD-PBEPBE-D3/aug-cc-pVDZ method. The Gactivation and ∆Greaction of ORR pathways on V-Si72, V-C72, and V-Al36N36 nanocages are calculated by DSD-PBEPBE-D3/aug-cc-pVDZ, PW91PW91/aug-cc-pVDZ, and COSMO model in the gas phase and water.

Influence of the Bis-Carbene Ligand on Manganese Catalysts for CO2 Electroreduction.

First row transition metal complexes have attracted attention as abundant and affordable electrocatalysts for CO2 reduction. Manganese complexes bearing bis-N-heterocyclic carbene ligands defining 6-membered ring metallacycles have proven to reduce CO2 to CO selectively at very high rates. Herein, we report the synthesis of manganese carbonyl complexes supported by a rigid ortho-phenylene bridged bis-N-heterocyclic carbene ligand (ortho-phenylene-bis(N-methylimidazol-2-ylidene), Ph-bis-mim), which defines a 7-membered ring metallacycle. We performed a comparative study with the analogues complexes bearing an ethylene-bis(N-methylimidazol-2-ylidene) ligand (C2H4-bis-mim) and a methylene-bis(N-methylimidazol-2-ylidene) ligand (CH2-bis-mim), and found that catalysts comprising a seven-membered metallacycle retain similar selectivity and activity as those with six-membered metallacycles, while reducing the overpotential by 120-190 mV. Our findings reveal general design principles for manganese bis-N-heterocyclic carbene electrocatalysts, which can guide further designs of affordable, fast and low overpotential catalysts for CO2 electroreduction.

Surface-Modified Ruthenium Nanorods for an Ampere-Level Bifunctional Hydrogen Evolution Reaction/Oxygen Evolution Reaction Electrocatalyst.

The practical applications of bifunctional ruthenium-based electrocatalysts with two active sites of Ru nanoparticles covered with RuO2 skins are limited. One reason is the presence of multiple equally distributed facets, some of which are inactive. In contrast, ruthenium nanorods with a high aspect ratio have multiple unequally distributed facets containing the dominance of active faces for efficient electrocatalysis. However, the synthesis of ruthenium nanorods has not been achieved due to difficulties in controlling the growth. Additionally, it is known that the adsorption capacity of intermediates can be impacted by the surface of the catalyst. Inspired by these backgrounds, the surface-modified (SM) ruthenium nanorods having a dominant active facet of hcp (100) through chemisorbed oxygen and OH groups (SMRu-NRs@NF) are rationally synthesized through the surfactant coordination method. SMRu-NRs@NF exhibits excellent hydrogen evolution in acid and alkaline solutions with an ultralow overpotential of 215 and 185 mV reaching 1000 mA cm-2, respectively. Moreover, it has also shown brilliant oxygen evolution electrocatalysis in alkaline solution with a low potential of 1.58 V to reach 1000 mA cm-2. It also exhibits high durability over 143 h for the evolution of oxygen and hydrogen at 1000 mA cm-2. Density functional theory studies confirmed that surface modification of a ruthenium nanorod with chemisorbed oxygen and OH groups can optimize the reaction energy barriers of hydrogen and oxygen intermediates. The surface-modified ruthenium nanorod strategy paves a path to develop the practical water splitting electrocatalyst.

Sequentially Regulating Potential-Determining Step for Lowering CO2 Electroreduction Overpotential over Te-Doped Bi Nanotips.

Electrocatalytic conversion of CO2 into formate is recognized an economically-viable route to upgrade CO2, but requires high overpotential to realize the high selectivity owing to high energy barrier for driving the involved proton-coupled electron transfer (PCET) processes and serious ignorance of the second PCET. Herein, we surmount the challenge through sequential regulation of the potential-determining step (PDS) over Te-doped Bi (TeBi) nanotips. Computational studies unravel the incorporation of Te heteroatoms alters the PDS from the first PCET to the second one by substantially lowering the formation barrier for *OCHO intermediate, and the high-curvature nanotips induce enhanced electric field that can steer the formation of asymmetric *HCOOH. In this scenario, the thermodynamic barrier for *OCHO and *HCOOH can be sequentially decreased, thus enabling a high formate selectivity at low overpotential. Experimentally, distinct TeBi nanostructures are obtained via controlling Te content in the precursor and TeBi nanotips achieve >90 % of Faradaic efficiency for formate production over a comparatively positive potential window (-0.57 V to -1.08 V). The strong Bi-Te covalent bonds also afford a robust stability. In an optimized membrane electrode assembly device, the formate production rate at 3.2 V reaches 10.1 mmol h-1 cm-2, demonstrating great potential for practical application.

A photothermal assisted zinc-air battery cathode based on pyroelectric and photocatalytic effect.

Zinc-air battery as one of the new generations of battery system, its theoretical specific energy is as high as 1086 Wh kg-1, specific capacity up to 820 mAh/g, and zinc has the advantages of environmental friendliness, resource abundance, low cost and good safety, so it has attracted much attention. However, due to its slow reaction kinetic process, zinc-air battery will produce a large charging overpotential usually up to 2 V, it is far beyond the theoretical voltage of 1.65 V, so reducing the overpotential of zinc-air batteries is extremely necessary, and the most common way to solve this problem is to use excellent catalyst cathode to improve the oxygen reduction and oxygen evolution kinetics of zinc-air batteries. So we developed a new photothermal assisted zinc-air battery system with Hollow carbon nanosphere@poly (vinylidene fluoride-trifluoroethylene-chlorofluoroethylene)@CdS(HCN@PVTC@CdS) photocathode, the pyroelectric and photocatalysis effect can effectively promote the reaction kinetics and reduce the reaction overpotential. With the pyroelectric and photocatalysis synergistic effect, the zinc-air has displayed a high discharge potential of 1.33 V and a low charging potential of 1.5 V with good cycle stability. This multi-assist technology with built-in electric and light fields paves the way for the development of high-performance zinc-air batteries and other energy storage systems.

A High-Entropy Single-Atom Catalyst Toward Oxygen Reduction Reaction in Acidic and Alkaline Conditions.

The design of high-entropy single-atom catalysts (HESAC) with 5.2 times higher entropy compared to single-atom catalysts (SAC) is proposed, by using four different metals (FeCoNiRu-HESAC) for oxygen reduction reaction (ORR). Fe active sites with intermetallic distances of 6.1 Å exhibit a low ORR overpotential of 0.44 V, which originates from weakening the adsorption of OH intermediates. Based on density functional theory (DFT) findings, the FeCoNiRu-HESAC with a nitrogen-doped sample were synthesized. The atomic structures are confirmed with X-ray photoelectron spectroscopy (XPS), X-ray absorption (XAS), and scanning transmission electron microscopy (STEM). The predicted high catalytic activity is experimentally verified, finding that FeCoNiRu-HESAC has overpotentials of 0.41 and 0.37 V with Tafel slopes of 101 and 210 mVdec-1 at the current density of 1 mA cm-2 and the kinetic current densities of 8.2 and 5.3 mA cm-2, respectively, in acidic and alkaline electrolytes. These results are comparable with Pt/C. The FeCoNiRu-HESAC is used for Zinc-air battery applications with an open circuit potential of 1.39 V and power density of 0.16 W cm-2. Therefore, a strategy guided by DFT is provided for the rational design of HESAC which can be replaced with high-cost Pt catalysts toward ORR and beyond.

Constructing Metal(II)-Sulfate Site Catalysts toward Low Overpotential Carbon Dioxide Electroreduction to Fuel Chemicals.

Precise regulation of the active site structure is an important means to enhance the activity and selectivity of catalysts in CO2 electroreduction. Here, we creatively introduce anionic groups, which can not only stabilize metal sites with strong coordination ability but also have rich interactions with protons at active sites to modify the electronic structure and proton transfer process of catalysts. This strategy helps to convert CO2 into fuel chemicals at low overpotentials. As a typical example, a composite catalyst, CuO/Cu-NSO4/CN, with highly dispersed Cu(II)-SO4 sites has been reported, in which CO2 electroreduction to formate occurs at a low overpotential with a high Faradaic efficiency (-0.5 V vs. RHE, FEformate=87.4 %). Pure HCOOH is produced with an energy conversion efficiency of 44.3 % at a cell voltage of 2.8 V. Theoretical modeling demonstrates that sulfate promotes CO2 transformation into a carboxyl intermediate followed by HCOOH generation, whose mechanism is significantly different from that of the traditional process via a formate intermediate for HCOOH production.

Identification of Suitable Mesh Size of Commercial Stainless-Steel for Electrochemical Oxygen Evolution Reaction.

The study examines the oxygen evolution reaction (OER) electrocatalytic efficiency of various stainless-steel mesh (SSM) sizes in electrolytic cells. Stainless steel is chosen due to its widespread availability and stability, making it an economically viable option. The primary objective of this investigation is to determine the optimal stainless-steel mesh size among those currently widely available on the market. The classification of stainless-steel mesh sizes as SS304 is confirmed by the minimal compositional variations observed across all mesh sizes through electron dispersive X-ray (EDX) spectra and X-ray fluorescence (XRF) analyses. Remarkably, CV experiments carried out at different scan rates indicate that SSM 200 has the maximum specific electrochemical surface area (ECSA). As a result, SSM 200 demonstrates superior performance in terms of current density response and shows the lowest overpotential in the alkaline medium compared to other stainless-steel mesh sizes. Furthermore, the SSM 200 exhibits a low overpotential of 337 mV at a current density of 10 mA/cm2 and a Tafel slope of 62.2 mV/decade, surpassing the performance of several previously reported electrodes for the OER. Stability tests conducted under constant voltage further confirm the remarkable stability of SSM 200, making it an ideal anode for electrolytic cell applications. These findings emphasize the cost-effectiveness and high stability of SSM 200, presenting intriguing possibilities for future research and advancements in this field.

Cobalt-Based Nanoscale Material: An Emerging Electrocatalyst for Hydrogen Production.

Modern civilization has been highly suffering from energy crisis and environmental pollutions. These two burning issues are directly and indirectly created from fossil fuel consumption and uncontrolled industrialization. The above critical issue can be solved through the proper utilization of green energy sources where no greenhouse gases will be generated upon burning of such materials. Hydrogen is the most eligible candidate for this purpose. Among various methods of hydrogen generation, electrocatalytic process is one of the most efficient methods because of easy handling and high efficiency. In these aspects Co-based nanomaterials are considered to be extremely significant as they can be utilized as efficient, recyclable and ideal catalytic system. In this article a series of Co-based nano-electrocatalysts has been discussed with proper structure-property relationship and their medium dependency. Therefore, such type of stimulating summary on recently reported electrocatalysts and their activity may be helpful for scientists of the corresponding field as well as for broader research communities. This can be inspiration for materials researchers to fabricate active catalysts for the production of hydrogen gas in room temperature.

Thermodynamics, Adhesion, and Wetting at Li/Cu(-Oxide) Interfaces: Relevance for Anode-Free Lithium-Metal Batteries.

A rechargeable battery that employs a Li metal anode requires that Li be plated in a uniform fashion during charging. In "anode-free" configurations, this plating will occur on the surface of the Cu current collector (CC) during the initial cycle and in any subsequent cycle where the capacity of the cell is fully accessed. Experimental measurements have shown that the plating of Li on Cu can be inhomogeneous, which can lower the efficiency of plating and foster the formation of Li dendrites. The present study employs a combination of first-principles calculations and sessile drop experiments to characterize the thermodynamics and adhesive (i.e., wetting) properties of interfaces involving Li and other phases present on or near the CC. Interfaces between Li and Cu, Cu2O, and Li2O are considered. The calculations predict that both Cu and Cu2O surfaces are lithiophilic. However, sessile drop measurements reveal that Li wetting occurs readily only on pristine Cu. This apparent discrepancy is explained by the occurrence of a spontaneous conversion reaction, 2 Li + Cu2O → Li2O + 2 Cu, that generates Li2O as one of its products. Calculations and sessile drop measurements show that Li does not wet (newly formed) Li2O. Hence, Li that is deposited on a Cu CC where surface oxide species are present will encounter a compositionally heterogeneous substrate comprising lithiophillic (Cu) and lithiophobic (Li2O) regions. These initial heterogeneities have the potential to influence the longer-term behavior of the anode under cycling. In sum, the present study provides insights into the early stage processes associated with Li plating in anode-free batteries and describes mechanisms that contribute to inefficiencies in their operation.

Self-Zincophilic Dual Protection Host of 3D ZnO/Zn⊂CF to Enhance Zn Anode Cyclability.

Zn dendrite growth and side reactions restrict the practical use of Zn anode. Herein, the design of a novel 3D hierarchical structure is demonstrated with self-zincophilic dual-protection constructed by ZnO and Zn nanoparticles immobilized on carbon fibers (ZnO/Zn⊂CF) as a versatile host on the Zn surface. The unique 3D frameworks with abundant zinc nucleation storage sites can alleviate the structural stress during the plating/stripping process and overpower Zn dendrite growth by moderating Zn2+ flux. Moreover, given the dual protection design, it can reduce the contact area between active zinc and electrolyte, inhibiting hydrogen evolution reactions. Importantly, density functional theory calculations and experimental results confirm that the introduced O atoms in ZnO/Zn⊂CF enhance the interaction between Zn2+ and the host and reduce Zn nucleation overpotential. As expected, the ZnO/Zn⊂CF-Zn electrode exhibits stable Zn plating/stripping with low polarization for 4200 h at 0.2 mA cm-2 and 0.2 mAh cm-2. Furthermore, the symmetrical cell displays a significantly long cycling life of over 1800 h, even at 30 mA cm-2. The fabricated full cells also show impressive cycling performance when coupled with V2O3 cathodes.

Low-Overpotential Rechargeable Na-CO2 Batteries Enabled by an Oxygen-Vacancy-Rich Cobalt Oxide Catalyst.

Rechargeable sodium-carbon dioxide (Na-CO2) batteries have been proposed as a promising CO2 utilization technique, which could realize CO2 reduction and generate electricity at the same time. They suffer, however, from several daunting problems, including sluggish CO2 reduction and evolution kinetics, large polarization, and poor cycling stability. In this study, a rambutan-like Co3O4 hollow sphere catalyst with abundant oxygen vacancies was synthesized and employed as an air cathode for Na-CO2 batteries. Density functional theory calculations reveal that the abundant oxygen vacancies on Co3O4 possess superior CO2 binding capability, accelerating CO2 electroreduction, and thereby improving the discharge capacity. In addition, the oxygen vacancies also contribute to decrease the CO2 decomposition free energy barrier, which is beneficial for reducing the overpotential further and improving round-trip efficiency. Benefiting from the excellent catalytic ability of rambutan-like Co3O4 hollow spheres with abundant oxygen vacancies, the fabricated Na-CO2 batteries exhibit extraordinary electrochemical performance with a large discharge capacity of 8371.3 mA h g-1, a small overpotential of 1.53 V at a current density of 50 mA g-1, and good cycling stability over 85 cycles. These results provide new insights into the rational design of air cathode catalysts to accelerate practical applications of rechargeable Na-CO2 batteries and potentially Na-air batteries.

Triiodide Anion as a Magnesium-ion Transporter for Low Overpotential Battery Cycling in Iodine-Containing Mg(TFSI)2 Electrolyte.

Magnesium iodide (MgI2) solid-electrolyte interface (SEI) layers have previously been shown to protect Mg metal anodes from passivation through products formed during Mg(TFSI)2 electrolyte decomposition (TSFI = trifluorosulfonimide). MgI2 formed in situ from small quantities of I2 added to the electrolyte shows a drastic decrease in the overpotential for magnesium deposition and stripping. In this work, a MgI2 SEI layer was created in an ex situ fashion and then the electrochemical characteristics of this MgI2 SEI layer were probed both alone and with small quantities of I2 or Bu4NI3 additives to identify the electroactive species. Chronopotentiometry (CP) and cyclic voltammetry (CV) show that the MgI2 SEI alone is insufficient for low overpotential magnesium cycling. I(3d) XPS data show that I3- is formed within the SEI layer, which can serve as the electroactive species when ligated with Mg2+ for low overpotential (<50 mV at 0.1 mA cm-2 current density) cycling. Moreover, Raman shifts at 110 and 140 cm-1 are consistent with I3- formation, and these signatures are observed before and after CP experiments. The Mg0 deposition curves in the CV with additives are consistent with diffusive species. Finally, electrochemical impedance spectroscopy (EIS) shows that there is a large decrease in the charge-transfer resistance within the SEI when either I2 or Bu4NI3 additives are used, which supports a solvating effect that facilitates magnesium deposition and stripping.