Advancing sustainable hydrogen production relies on the creation of effective electrocatalysts for the hydrogen evolution reaction (HER). Ruthenium (Ru) is a potential alternative to platinum due to its lower cost and high activity, yet its performance in alkaline HER remains limited by sluggish reaction kinetics. Herein, we propose a strategy to modulate electronic asymmetry within Ru clusters by anchoring Ru nanoclusters on N-doped graphene (Ru-NC), optimizing the charge distribution between high-valent Run+ and metallic Ru0 sites. The Ru-NC catalyst, having a Ruⁿ+/Ru0 ratio of 57.1%, demonstrates superior HER activity with an overpotential of merely 29 mV at 10 mA·cm−2 and a Tafel slope of 46 mV·dec−1 in 1.0 M KOH, surpassing the performance of Pt/C catalyst. Experimental and structural analyses reveal that the asymmetric electronic configuration facilitates H2O dissociation at Run+ sites and efficient H adsorption/desorption at Ru0 sites, synergistically lowering energy barriers for HER intermediates. Furthermore, Ru-NC demonstrates remarkable stability, retaining its activity after 5000 cycles and operating continuously for 80 h at 100 mA·cm−2 with minimal degradation. This work highlights the pivotal role of electronic asymmetry in enhancing HER kinetics and provides a rational design principle for high-performance Ru-based electrocatalysts.
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RuO2 is a powerful alternative to IrO2 catalyst for acidic oxygen evolution reaction (OER), but its widespread application is hampered by its susceptibility to degradation in acidic environments. This instability is primarily due to the detrimental involvement of lattice oxygen, culminating in the formation of the labile RuO4 species and large amount of unstable oxygen vacancies. In this context, the electronic configuration and the local coordination environment of RuO2 are precise tailored by Sn doping. The resulting asymmetric Ru–O–Sn structure accelerates proton transfer and facilitates the formation of high oxidation state Ru centers. The resulting Sn-doped RuO2 electrocatalyst has demonstrated remarkable OER performance in 0.5 M H2SO4, with a minimum overpotential of 197 mV at a current density of 10 mA·cm−2 and impressive durability. The proposed strategy involves the incorporation of Sn into the RuO2 lattice, which reduces the Ru–O covalency, inhibits over-oxidation, and reduces the adsorption energy of reaction intermediates, resulting in a significant improvement in catalyst activity and stability.
Sodium-ion batteries with their abundant resource availability and low cost, show a tremendous potential in energy storage. Sodium-ion batteries emerge as a more economical alternative to lithium-ion batteries, making it highly attractive for large-scale grid energy storage applications. To enhance the competitiveness of sodium-ion batteries, the development of high-energy cathode materials is crucial. As a result, recent efforts are made to explore sodium-ion cathode materials, particularly sodium layered oxides and polyanion compounds. The most extensively studied polyanion sodium-ion cathode materials are vanadium phosphates (i.e., Na3V2(PO4)3 or NVP) and fluorophosphates (i.e., Na3V2(PO4)2F3 or NVPF). NVP with its sodium fast ion conductor (Na-superionic conductor, NASICON) structure is considered as a high-performance sodium-storage cathode material. The related experimental results indicate that the actual specific capacity of pure-phase NVP is lower than theoretical expectations, and its cycling stability is generally poor. Adjusting the composition and structure of NVP through elemental doping is an effective strategy to enhance its cycling reversibility, increase reversible capacity, and improve sodium ion diffusion kinetics. However, some challenges exist in accurately measuring the properties of sodium-ion cathode materials during experiments coupled with high trial-and-error costs and the lack of scientific theoretical guidance. This study was to utilize first-principles calculations to investigate the electronic structure characteristics of mixed polyanions doped with different transition metals, aiming to elucidate the conductivity mechanisms within polyanion systems. In addition, the microscopic processes of sodium ion insertion/extraction during the charge and discharge cycles of polyanion cathode materials were also analyzed via simulating all possible sodium ion migration pathways and calculating the corresponding energy barriers and voltage platforms in an ideally conductive polyanion system.
Spin-polarized DFT calculations were used within the Vienna ab initio simulation package. The projector-augmented wave (PAW) method was applied to solve the ion-electron interactions in a periodic system. The generalized gradient approximation (GGA) with Perdew−Burke−Ernzerhof (PBE) functionals was used to take into consideration of the exchange-correlation interactions in the Kohn−Sham equations. A plane-wave basis set with a cutoff energy of 400 eV was used to expand the Kohn–Sham valence states. A k-point mesh of dimensions of 3 × 3 × 1 was employed for the Brillouin zone integration. For calculating the electronic structure, a Monkhorst-Pack grid of 5 × 5 × 1 was sampled. The self-consistent field convergence criteria were set at 1 ×10–6 eV. All the structures were sufficiently relaxed until the force at each atom was less than 0.02 eV∙Å–1. The climbing-image nudged elastic band (CI-NEB) was used to calculate the migration energy barrier corresponding to the migration path of Na ions. This is an effective way to find the saddle points of the ion diffusion energy. Five intermediate images were constructed between the initial and final states along the sodium ion diffusion path. The calculation formula of charge and discharge voltage is: V = (E(NaxMP) + (y–x)E(Na)–E(NayMP))/(y–x), where E(NayMP), E(NaxMP) and E(Na) indicate the discharge completion state, charging completion state and the energy of Na atoms, respectively.
The results show that Na3TiMn(PO4)3 and Na3TiFe(PO4)3 have more abundant electronic state near the Fermi level, compared to the original Na3V2(PO4)3. The conductivity of Na3TiMn(PO4)3 and Na3TiFe(PO4)3 is better than that of the original Na3V2(PO4)3, accelerating the kinetic process of electron transfer and ion extraction. The electronic structure characteristics of mixed polanion based on iron and manganese are analyzed, and the conductive mechanism of polyanionic system is proved via the first-principles calculations, which is helpful to screen the polanion cathode materials with an ideal conductivity.
The CI-NEB method is used to calculate the three diffusion paths and energy barriers in polanionic cathode materials (i.e., Na3TiMn(PO4)3, Na3TiFe(PO4)3 and Na3V2(PO4)3). The energy barriers for diffusion paths 1 and 3 of the primitive Na3V2(PO4)3 are both 0.12 eV. The energy barrier of diffusion path 2 is 0.11 eV, which is better than that of diffusion path 1 and 3. For Na3TiMn(PO4)3 and Na3TiFe(PO4)3, the energy difference between the initial and final state structures is too large (i.e., 0.6 eV) although the energy barrier of the diffusion path 1 is extremely low (i.e., only 0.04 eV), which is not conducive to the cyclic diffusion of sodium ions in the cathode material from the thermodynamic perspective. Sodium at Na1 is not conducive to prolapse, and sodium at Na2 is easier to prolapse during diffusion. The structural characteristics of Na3MM'(PO4)3 and experimental studies both thus indicate that sodium prolapsed at different positions has different effects on the structure. In terms of diffusion paths 2 and 3, the energy difference between the initial and final structures is close to 0, and the corresponding diffusion energy barrier is between 0.12 eV and 0.18 eV, favoring the diffusive transport of sodium ions both thermodynamically and kinetically.
As the charging process proceeds, the voltage gradually increases, while desodium and sodium value decreases. The Na2 position is the first sodium ion effluent, which is consistent with that of the experimental studies. The theoretical calculation shows that the voltage platform of Na3V2(PO4)3 changes, the structure of Na3V2(PO4)3 changes in the process of sodium extraction, and the voltage platform of Na3TiMn(PO4)3 and Na3TiFe(PO4)3 is more stable than the voltage platform of original Na3V2(PO4)3.
The structure and electronic characteristics of Na3V2(PO4)3, Ti, Cr, Mn, Fe and other transition group metals were investigated. The density of state electronic structure of Ti, Mn and Ti and Fe substituted Na3MM′(PO4)3 polyanionic sodium ion battery cathode material. The results showed that Na3TiMn(PO4)3 and Na3TiFe(PO4)3 had more abundant electronic states near the Fermi level, the conductivity of Na3TiMn(PO4)3 and Na3TiFe(PO4)3 was better than that of the original Na3V2(PO4)3, which could effectively enhance the conductivity and accelerate the kinetic process of electron transfer and ion extraction. We calculated three sodium ion diffusion paths, analyzed the three diffusion paths of sodium ion in polyanionic cathode materials (i.e., Na3TiMn(PO4)3, Na3TiFe(PO4)3 and Na3V2(PO4)3) and the corresponding energy barriers of 0.12–0.18 eV, and explained that the materials Na3TiMn(PO4)3 and Na3TiFe(PO4)3 were thermodynamically and kinetically beneficial. The theoretical calculation further studied the influence of voltage platforms on cathode materials. Based on the electronic structure and ion diffusion path results, Na3TiMn(PO4)3 and Na3TiFe(PO4)3 were selected as NASICON sodium ion cathode materials with a superior comprehensive performance.
The distance effect of the doped heteroatoms away from the catalytic centers has rarely been reported. In this work, we conducted density functional theory calculations to thoroughly investigate the influence of heteroatom (N, P, B, and S atoms) doping distance on the oxygen reduction reaction (ORR) activity of graphene-based FeN4 sites. We uncovered a Sabatier-like relationship between heteroatom doping distance and ORR activity of FeN4 sites. The nearest doping does not significantly improve and even block the ORR activity of FeN4 sites. Optimal ORR activity is achieved when the heteroatoms are 4–5 Å (N, P, and S atoms) or 6–7 Å (B atoms) away from the Fe atoms. Analysis of electronic structure indicates that distance effect can modulate the local chemical environment of Fe atoms, thereby account for the changes in ORR activity along with the doping distance and doping atoms. This study provides insights into the influence of heteroatom doping on the chemical environment of reaction active centers, and provides the theoretical guidance for controlling the doping distance of heteroatoms to achieve optimal catalytic activity and selectivity.
Atomically-dispersed iron-based electrocatalysts are promising substitutes for noble metal electrocatalysts because of excellent performance in oxygen reduction reaction (ORR). Rationally modulating the local coordination environment of the Fe site and optimizing the binding energy of oxygen reduction intermediates are effective strategies to optimize ORR activity. Herein, we report a new method in which Ni is introduced to construct NiFe dual single atoms and iron nanoclusters loaded on the nitrogen-doped carbon with a highly porous structure. This design plays a synergistic role of dual single atoms and clusters, optimizes the 3d orbital and Fermi level of Fe, breaks the symmetrical structure of Fe-N4, and effectively improves the adsorption/desorption behavior of the oxygen-containing intermediates. Electrochemical tests show FeNCs/NiFeSAs-NC has an excellent intrinsic activity. Theoretical calculations show the oxygen-containing species on the Ni active site will move to the middle of NiFe (bridge site connection) after optimization and that the key step is OH desorption, with a reaction energy of 0.27 eV. The electron exchange between NiFe-N6 and Fe-cluster is very strong, further indicating the introduction of Ni species and Fe clusters has a regulatory effect on the electronic structure of Fe-N4.
Developing efficient and stable catalysts for the electrocatalytic N2 reduction reaction (NRR) shows promise in nitrogen fixation. Here, we proposed active and stable single-atom catalysts (SACs) toward NRR, where transition metals are anchored on nitrogenated carbon nanotubes (NCNTs). Among the screened nine common transition metals (Ti, V, Cr, Mn, Fe, Mo, Ru, Rh, and Ag) on (4, 4) NCNTs, we found Mo-NCNT possesses the most excellent NRR catalytic activity and selectivity with a low overpotential of 0.29 V. Then, the NRR performance of Mo-NCNT was further engineered by controlling the nanotube diameter, where the lowest overpotential is 0.18 V at a diameter of 9.6 Å. In addition, we found a linear scaling relation between *NNH and *NH2 on the studied catalysts with the exception of (2, 2) and (3, 3) Mo-NCNTs, owing to their extremely unstable structures. We attribute the outstanding NRR performance of Mo-NCNT to the moderate adsorption of N2 due to the slightly low d-band center of Mo, and the charge donating and accepting capacity of NCNTs. This work has provided a deeper insight into designing high-efficiency and stable NRR SACs supported by NCNTs.
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