The Lifetime of Hydroxyl Radical in Realistic Fuel Cell Catalyst Layer.

The lifetime of hydroxyl radicals (⋅OH) in the fuel cell catalyst layer remains uncertain, which hampers the comprehension of radical-induced degradation mechanisms and the development of longevity strategies for proton-exchange membrane fuel cells (PEMFCs). In this study, we have precisely determined that the lifetime of ⋅OH radicals can extend up to several seconds in realistic fuel cell catalyst layers. This finding reveals that ⋅OH radicals are capable of carrying out long-range attacks spanning at least a few centimeters during PEMFCs operation. Such insights hold great potential for enhancing our understanding of radical-mediated fuel cell degradation processes and promoting the development of durable fuel cell devices.

Revealing a Double-Volcano-Like Structure-Activity Relationship for Substitution-Functionalized Metal-Phthalocyanine Catalysts toward Electrochemical CO2 Reduction.

Electron-donating/-withdrawing groups (EDGs/EWGs) substitution is widely used to regulate the catalytic performance of transition-metal phthalocyanine (MPc) toward electrochemical CO2 reduction, but the corresponding structure-activity relationships and regulation mechanisms are still ambiguous. Herein, by investigating a series of substitution-functionalized MPc (MPc-X), this work reveals a double-volcano-like relationship between the electron-donating/-withdrawing abilities of the substituents and the catalytic activities of MPc-X. The weak-EDG/-EWG substitution enhances whereas the strong-EDG/-EWG substitution mostly lowers the CO selectivity of MPc. Experimental and calculation results demonstrate that the electronic properties of the substituents influence the symmetry and energy of the highest occupied molecular orbitals of MPc-X, which in turn determine the CO2 adsorption/activation and lead to diverse CO2 reduction pathways on the EWG or EDG substituted MPc via different CO2 adsorption modes. This work provides mechanism insights that could be guidance for the design and regulation of molecular catalysts.

Mechanism of Particle-Mediated Inhibition of Demetalation for Single-Atom Catalytic Sites in Acidic Electrochemical Environments.

Demetalation, caused by the electrochemical dissolution of metal atoms, poses a significant challenge to the practical application of single-atom catalytic sites (SACSs) in proton exchange membrane-based energy technologies. One promising approach to inhibit SACS demetalation is the use of metallic particles to interact with SACSs. However, the mechanism underlying this stabilization remains unclear. In this study, we propose and validate a unified mechanism by which metal particles can inhibit the demetalation of Fe SACSs. Metal particles act as electron donors, decreasing the Fe oxidation state by increasing the electron density at the FeN4 position, thereby strengthening the Fe-N bond, and inhibiting electrochemical Fe dissolution. Different types, forms, and contents of metal particles increase the Fe-N bond strength to varying extents. A linear correlation between the Fe oxidation state, Fe-N bond strength, and electrochemical Fe dissolution amount supports this mechanism. Our screening of a particle-assisted Fe SACS led to a 78% reduction in Fe dissolution, enabling continuous operation for up to 430 h in a fuel cell. These findings contribute to the development of stable SACSs for energy applications.

Singlet Oxygen Induced Site-Specific Etching Boosts Nitrogen-Carbon Sites for High-Efficiency Oxygen Reduction.

Targeted construction of carbon defects near the N dopants is an intriguing but challenging way to boost the electrocatalytic activity of N-doped carbon toward oxygen reduction reaction (ORR). Here, we report a novel site-specific etching strategy that features targeted anchoring of singlet oxygen (1 O2 ) on the N-adjacent atoms to directionally construct topological carbon defects neighboring the N dopants in N-doped carbon (1 O2 -N/C). This 1 O2 -N/C exhibits the highest ORR half-wave potential of 0.915 VRHE among all the reported metal-free carbon catalysts. Pyridinic-N bonded with a carbon pentagon of the neighboring topological carbon defects is identified as the primary active configuration, rendering enhanced adsorption of O2 , optimized adsorption energy of the ORR intermediates, and a significantly decreased total energy barrier for ORR. This 1 O2 -induced site-specific etching strategy is also applicable to different precursors, showing a tremendous potential for targeted construction of high-efficiency active sites in carbon-based materials.

Ampere-level current density ammonia electrochemical synthesis using CuCo nanosheets simulating nitrite reductase bifunctional nature.

The development of electrocatalysts capable of efficient reduction of nitrate (NO3-) to ammonia (NH3) is drawing increasing interest for the sake of low carbon emission and environmental protection. Herein, we present a CuCo bimetallic catalyst able to imitate the bifunctional nature of copper-type nitrite reductase, which could easily remove NO2- via the collaboration of two active centers. Indeed, Co acts as an electron/proton donating center, while Cu facilitates NOx- adsorption/association. The bio-inspired CuCo nanosheet electrocatalyst delivers a 100 ± 1% Faradaic efficiency at an ampere-level current density of 1035 mA cm-2 at -0.2 V vs. Reversible Hydrogen Electrode. The NH3 production rate reaches a high activity of 4.8 mmol cm-2 h-1 (960 mmol gcat-1 h-1). A mechanistic study, using electrochemical in situ Fourier transform infrared spectroscopy and shell-isolated nanoparticle enhanced Raman spectroscopy, reveals a strong synergy between Cu and Co, with Co sites promoting the hydrogenation of NO3- to NH3 via adsorbed *H species. The well-modulated coverage of adsorbed *H and *NO3 led simultaneously to high NH3 selectivity and yield.

Two-dimensional metal-organic frameworks and their derivatives for electrochemical energy storage and electrocatalysis.

Two-dimensional (2D) metal-organic frameworks (MOFs) and their derivatives with excellent dimension-related properties, e.g. high surface areas, abundantly accessible metal nodes, and tailorable structures, have attracted intensive attention as energy storage materials and electrocatalysts. A major challenge on the road toward the commercialization of 2D MOFs and their derivatives is to achieve the facile and controllable synthesis of 2D MOFs with high quality and at low cost. Significant developments have been made in the synthesis and applications of 2D MOFs and their derivatives in recent years. In this review, we first discuss the state-of-the-art synthetic strategies (including both top-down and bottom-up approaches) for 2D MOFs. Subsequently, we review the most recent application progress of 2D MOFs and their derivatives in the fields of electrochemical energy storage (e.g., batteries and supercapacitors) and electrocatalysis (of classical reactions such as the HER, OER, ORR, and CO2RR). Finally, the challenges and promising strategies for the synthesis and applications of 2D MOFs and their derivatives are addressed for future development.

Ultrasmall 2 D Cox Zn2-x (Benzimidazole)4 Metal-Organic Framework Nanosheets and their Derived Co Nanodots@Co,N-Codoped Graphene for Efficient Oxygen Reduction Reaction.

The development of nonprecious metal-nitrogen-carbon (M-N-C) materials with efficient metal utilization and abundant active sites for the oxygen reduction reaction (ORR) is of great significance for fuel cells and metal-air batteries. Ultrasmall 2 D Cox Zn2-x (benzimidazole)4 [Cox Zn2-x (bim)4 ] bimetallic metal-organic framework (MOF) nanosheets (≈2 nm thick) are synthesized by a novel bottom-up strategy and then thermally converted into a core-shell structure of sub-5 nm Co nanodots (NDs) wrapped with 2 to 5 layers of Co,N-codoped graphene (Co@FLG). The size of the Co NDs in Co@FLG could be precisely controlled by the Co/Zn ratio in the Cox Zn2-x (bim)4 nanosheet. As an ORR electrocatalyst, the optimized Co@FLG exhibits an excellent half-wave potential of 0.841 V (vs. RHE), a high limiting current density of 6.42 mA cm-2 , and great stability in alkaline electrolyte. Co@FLG also has great ORR performance in neutral electrolyte, as well as in Mg-air batteries. The experimental studies and DFT calculations reveal that the high performance of Co@FLG is mainly attributed to its great O2 absorptivity, which is endowed by the abundant Co-Nx and pyridinic-N in the FLG shell and the strong electron-donating ability from the Co ND core to the FLG shell. This elevates the eg orbital energy of CoII and lowers the activation energy for breaking the O=O/O-O bonds. This work sheds light on the design and fabrication of 2 D MOFs and MOF-derived M-N-C materials for energy storage and conversion applications.

Morphology-dependent electrochemical performance of Ni-1,3,5-benzenetricarboxylate metal-organic frameworks as an anode material for Li-ion batteries.

The performance of energy storage materials is substantially dependent on their nanostructures. Herein, Ni-1,3,5-benzenetricarboxylate metal-organic frameworks (Ni-BTC MOFs) with different morphologies are controllably synthesized using a facile solvothermal method by simply adjusting the solvent and their electrochemical performance as an anode material for lithium-ion batteries is thoroughly investigated. Among the synthesized Ni-BTC MOFs with different morphologies, a hierarchical mesoporous flower-like Ni-BTC MOF (Ni-BTCEtOH) assembled from two-dimensional nanosheets shows the best electrochemical properties including a high capacity of 1085 mA h g-1 at 100 mA g-1 (358 mA h g-1 at 5000 mA g-1), excellent cycling stability at 1000 mA g-1 for 1000 cycles, and great rate performance, which is superior to most of the reported MOF-based anode materials for lithium-ion batteries. The outstanding electrochemical performance of Ni-BTCEtOH is originated from its unique and stable hierarchical mesoporous morphology with a high specific surface area and improved electrical/ionic conductivity. Moreover, our study demonstrates that the charge-discharge mechanism of the Ni-BTCEtOH electrode involves the insertion/extraction of Li ions to/from the organic moieties in Ni-BTCEtOH during the charge-discharge process without the direct engagement of Ni2+. This work highlights that the nanostructure design is an effective strategy to obtain promising energy storage materials.

Plasma-Induced Oxygen Vacancies in Urchin-Like Anatase Titania Coated by Carbon for Excellent Sodium-Ion Battery Anodes.

The incorporation of oxygen vacancies in anatase TiO2 has been studied as a promising way to accelerate the transport of electrons and Na+ ions, which is important for achieving excellent electrochemical properties for anatase TiO2. However, wittingly introducing oxygen vacancies in anatase TiO2 for sodium-ion anodes by a facile and effective method is still a challenge. In this work, we report an innovative method to introduce oxygen vacancies into the urchin-like N-doped carbon coated anatase TiO2 (NC-DTO) by a facile plasma treatment. The superiorities of the oxygen vacancies combined with the conductive N-doped carbon coating enable the obtained NC-DTO of greatly improved sodium storage performance. When served as the anode for sodium-ion batteries, the NC-DTO electrode shows superior electrochemical performance (capacity: 272 mA h g-1 at 0.25 C, capacity retention: 98.8% after 5000 cycles at 10 C, as well as ultrahigh capacity: 150 mA h g-1 at 15 C). Density functional theory calculations combined with experimental results suggest that considerably improved sodium storage performance of NC-DTO is due to the enhanced electronic conductivity from the N-doped carbon layer as well as narrowed band gap and lowered sodiation energy barrier from the introduction of oxygen vacancies. This work highlights that introducing oxygen vacancies into TiO2 by plasma is a promising method to enhance the electrochemical property of TiO2, which also can be applied to different metal oxides for energy storage devices.