Axial heteroatom (P, S and Cl)-decorated Fe single-atom catalyst for the oxygen reduction reaction: a DFT study.

An FeN4 single-atom catalyst (SAC) embedded in a graphene matrix is considered an oxygen reduction reaction (ORR) catalyst for its good activity and durability, and decoration on the Fe active site can further modulate the performance of the FeN4 SAC. In this work, the axial heteroatom (L = P, S and Cl)-decorated FeN4 SAC (FeN4L) and pure FeN4 were comparatively studied using density functional theory (DFT) calculations. It was found that the rate-determining step (RDS) in the ORR on pure FeN4 is the reduction of OH to H2O in the last step with an overpotential of 0.58 V. However, the RDS of the ORR for the axial heteroatom-decorated FeN4L is the reduction of O2 to OOH in the first step. The axial P and S heteroatom-decorated FeN4P and FeN4S exhibit lower activity than pure FeN4 since the overpotentials of the ORR on FeN4P and FeN4S are 1.02 V and 1.09 V, respectively. Meanwhile, FeN4Cl exhibits the best activity towards the ORR since it possesses the lowest overpotential (0.51 V). The main reason is that the axial heteroatom decoration alleviates the adsorption of all the species in the whole ORR, thus modulating the free energy in every elementary reaction step. A volcano relationship between the d band center and the ORR activity can be determined among the axial heteroatom-decorated FeN4L SACs. The d band center of the Fe atom in various FeN4L SACs follows the order of FeN4 > FeN4Cl > FeN4S > FeN4P, whereas the overpotential of the ORR on various catalysts follows the order of FeN4Cl > FeN4 > FeN4S ≈ FeN4P. ΔG(*OH) is a simple descriptor for the prediction of the ORR activity of various axial heteroatom-decorated FeN4L, although the RDS in the ORR is either the first step or the last step. This paper provides a guide to the design and selection of the ORR over SACs with different axial heteroatom decorations, contributing to the rational design of more powerful ORR electrocatalysts and achieving advances in electrochemical conversion and storage devices.

Tungsten phosphide on nitrogen and phosphorus-doped carbon as a functional membrane coating enabling robust lithium-sulfur batteries.

Lithium-sulfur batteries (LSBs) hold great potential as future energy storage technology, but their widespread application is hampered by the slow polysulfide conversion kinetics and the sulfur loss during cycling. In this study, we detail a one-step approach to growing tungsten phosphide (WP) nanoparticles on the surface of nitrogen and phosphorus co-doped carbon nanosheets (WP@NPC). We further demonstrate that this material provides outstanding performance as a multifunctional separator in LSBs, enabling higher sulfur utilization and exceptional rate performance. These excellent properties are associated with the abundance of lithium polysulfide (LiPS) adsorption and catalytic conversion sites and rapid ion transport capabilities. Experimental data and density functional theory calculations demonstrate tungsten to have a sulfophilic character while nitrogen and phosphorus provide lithiophilic sites that prevent the loss of LiPSs. Furthermore, WP regulates the LiPS catalytic conversion, accelerating the Li-S redox kinetics. As a result, LSBs containing a polypropylene separator coated with a WP@NPC layer show capacities close to 1500 mAh/g at 0.1C and coulombic efficiencies above 99.5 % at 3C. Batteries with high sulfur loading, 4.9 mg cm-2, are further produced to validate their superior cycling stability. Overall, this work demonstrates the use of multifunctional separators as an effective strategy to promote LSB performance.

Electronic Spin Alignment within Homologous NiS2/NiSe2 Heterostructures to Promote Sulfur Redox Kinetics in Lithium-Sulfur Batteries.

The catalytic activation of the Li-S reaction is fundamental to maximize the capacity and stability of Li-S batteries (LSBs). Current research on Li-S catalysts mainly focuses on optimizing the energy levels to promote adsorption and catalytic conversion, while frequently overlooking the electronic spin state influence on charge transfer and orbital interactions. Here, hollow NiS2/NiSe2 heterostructures encapsulated in a nitrogen-doped carbon matrix (NiS2/NiSe2@NC) are synthesized and used as a catalytic additive in sulfur cathodes. The NiS2/NiSe2 heterostructure promotes the spin splitting of the 3d orbital, driving the Ni3+ transformation from low to high spin. This high spin configuration raises the electronic energy level and activates the electronic state. This accelerates the charge transfer and optimizes the adsorption energy, lowering the reaction energy barrier of the polysulfides conversion. Benefiting from these characteristics, LSBs based on NiS2/NiSe2@NC/S cathodes exhibit high initial capacity (1458 mAh·g⁻1 at 0.1C), excellent rate capability (572 mAh·g⁻1 at 5C), and stable cycling with an average capacity decay rate of only 0.025% per cycle at 1C during 500 cycles. Even at high sulfur loadings (6.2 mg·cm⁻2), high initial capacities of 1173 mAh·g⁻1 (7.27 mAh·cm⁻2) are measured at 0.1C, and 1058 mAh·g⁻1 is retained after 300 cycles.