Appropriate Acid Etching to Obtain Defect‐Rich and Porous Zeolitic‐Imidazolate‐Framework‐Derived Undercoordinated Fe‐NC Catalysts Toward Boosted Oxygen Reduction Reaction

Synthesis of nitrogen-rich porous carbon nanotubes coated Co nanomaterials as efficient ORR electrocatalysts via MOFs as precursor

Graphdiyne Electrocatalyst

Boosting oxygen reduction reaction with Fe and Se dual-atom sites supported by nitrogen-doped porous carbon

One major limitation for polymer electrolyte membrane fuel cells is the sluggish cathode kinetics. Development of efficient noble‐free catalysts is the key resolution to the problem of the oxygen reduction reaction (ORR) in both acid and alkaline solutions. Herein, we report a new type of efficient non‐precious‐metal catalyst for the ORR through the direct pyrolysis of poly[2,2′‐(1,1′‐cobaltocenium)‐5,5′‐dibenzimidazole]. The cobalt oxides were produced after pyrolysis at 900 °C (Cp2‐Co+‐PBI‐900, where PBI is polybenzimidazole). The obtained catalysts exhibit higher electrocatalytic activity and stability for the ORR under both alkaline and acidic conditions. Structural characterization manifested that Cp2‐Co+‐PBI‐800 had the highest graphitic N content and Cp2‐Co+‐PBI‐900 was also produced. In alkaline media, Cp2‐Co+‐PBI‐900 showed the highest ORR activity with onset potential of 998 mV (vs. RHE), which was only 22 mV higher than that of Pt/C under identical conditions. Besides, in acidic media, Cp2‐Co+‐PBI‐800 exhibited excellent ORR activity with an onset potential of 847 mV (vs. RHE) after leaching in 6 M HCl solution for 12 h. Both optimal catalysts displayed high durability, especially in acidic media. The half‐wave potential was also improved by 11 mV after 5000 CV scanning cycles in N2. The catalysts possessed diverse active sites in different working conditions. In acid conditions, cobalt acted as the promotor, whereas, in alkaline conditions, CoO was the activity site. Moreover, graphite N and pyridine N were the main activity sites in acid and alkaline conditions, respectively. PBI has a long‐chain and π‐conjugated system, indicating that the PBI precursor can be used as a non‐precious‐metal catalyst.

Synergistic effect between M-NX sites and MXOY particles in hierarchical porous M–N-C catalysts holds great promises in boosting oxygen reduction reaction (ORR). In this work, 1,4-dicyanobenzene was utilized as a molecular template to prepare the hierarchical porous Fe–N-C catalysts with size-tunable Fe3O4 particles for enhanced ORR in Zn-air battery. The as-prepared Fe3O4#Fe–N/CDB0.1 owned a half-potential of 0.90 V vs RHE, exceeding that of commercial 20%Pt/C (E1/2 = 0.82 V vs RHE), showing a maximum power density of 321 mW cm−2 in a homemade Zn-air battery. Density functional theory (DFT) calculations indicate that the electronic interaction between Fe3O4 and Fe-N4 sites enhances the adsorption energy of *OOH, effectively optimizing the energy barrier for *O formation, significantly reducing the limiting energy barrier. Such superior ORR activity in Fe3O4#Fe–N/C originated from the optimized hierarchical pores and synergistic effect between Fe-NX sites and Fe3O4 particles. This work provides a new and facile template strategy for engineering hierarchical porous carbon-based materials to achieve highly efficient catalytic reactions.

Nonprecious metal (NPM) catalysts are considered as the most promising candidate to replace Pt‐based electrocatalysts for oxygen reduction reaction (ORR). However, in comparison with the commercial Pt catalyst, the development of high efficiency and low cost NPM catalysts for ORR still remains a big challenge. Here, a simple but efficient way to fabricate porous N‐doped graphene immobilized molybdenum nitride (MoN) nanoparticles is reported, and simultaneously, the introduction of H2O2 plays a key role in modulating the particle size of MoN and the microstructure of the composite to achieve different configuration. As results, it is shown that the as‐prepared material owns outstanding ORR activity and excellent stability in an alkaline medium. To the best knowledge, this catalyst possesses the best performance among the same class catalysts as reported. It is believed that the H2O2‐assisted strategy can provide new insights in synthesis of high efficient metal nitride/carbon hybrid materials toward advanced energy conversion and storage.

The defect and N-doping engineering are critical to developing the highly efficient metal-free electrocatalysts for oxygen reduction reaction (ORR), mainly because they can efficiently regulate the geometric/electronic structures and sur-/interface properties of the carbon matrix. Herein, we provide a facile and scalable strategy for the large-scale synthesis of N-doped porous carbon nanosheets (NPCNs) with hierarchical pore structure, only involving solvothermal and pyrolysis processes. Additionally, the turnover frequency of ORR (TOFORR) was calculated by taking into account the electron-transfer number (n). Benefiting from the trimodal pore structures, high specific surface area, a higher pore volume, high-ratio mesopores, massive vacancies/long-range structural defects, and high-content pyridinic-N (~2.1%), the NPCNs-1000 shows an excellent ORR activity (1600 rpm, js = ~5.99 mA cm−2), a selectivity to four-electron ORR (~100%) and a superior stability in both the three-electrode tests (CP test for 7500 s at 0.8 V, Δjs = ~0.58 mA cm−2) and Zn–Air battery (a negligible loss of 0.08 V within 265 h). Besides, the experimental results indicate that the enhancement of ORR activity mainly originates from the defects and pyridinic-N. More significantly, this work is expected to realize green and efficient energy storage and conversion along with the carbon peaking and carbon neutrality goals.

Advanced Hierarchically 2D and 3D Nanostructured Materials for Electrochemical Clean Energy Conversion

Fuel cells (FCs) have emerged as a promising alternative for efficient electricity production in the future [1]. However, their widespread commercialization is hindered by their prohibitive cost due to the large amount of platinum used in these electrochemical systems [2]. Platinum is used to accelerate the unfavorable kinetics of the cathodic oxygen reduction reaction (ORR) [3]. To achieve large-scale and cost-effective development of FCs, it is essential to find high-performance platinum-free catalysts. Two solutions are mainly reported in the literature: the development of non-precious metal-based carbon catalysts and metal-free carbon catalysts. However, the work reported on these two types of carbon catalysts is sometimes contradictory regarding the actual nature of the active sites and the mechanisms of reduction of oxygen molecules on the catalyst [4].Pyrolyzed metal-nitrogen-carbon (M-N-C) catalysts for the ORR, especially single-atom catalysts (SACs), have been widely studied because they exhibit excellent electrocatalytic performance, thanks to the high content of metal atoms anchored on a carbon support [5]. Recent research has indicated that the combination of single-atom active sites with metal nanoclusters can further improve the catalytic activity, but the underlying mechanism remains unclear [6]. Therefore, it is necessary to develop model catalysts based on both single-atoms and nanoclusters to better understand their catalytic activity.In this work, we prepared nitrogen-doped and iron-doped carbon catalysts (called Fe-NC below) with precise control of the type of iron in the material. First, the functionalization of an activated carbon was performed by carbonizing it under air in the presence of urea. Then, the sample was placed in an autoclave for hydrothermal treatment in the presence of FeCl3.6H2O to allow the incorporation of iron, and thermal post-treatment was applied to synthesize model catalysts (Fig. 1a). The precursor ratio and final pyrolysis temperature were adjusted and optimized to produce either iron-based single atoms (SA) (here referred to as FeSA-NC) or atomic clusters (AC) associated with single atoms (here referred to as FeSA+AC-NC). The results show that the iron-based catalysts exhibit outstanding catalytic activity (Fig. 1b) and durability (Fig. 1c) in alkaline medium for the ORR, even better than commercial Pt (20 wt.%)/C in the case of FeSA+AC-NC. The analysis provided insight into the development of highly efficient multifunctional electrocatalysts, where the presence of iron nanoclusters was found to be a crucial determinant of the catalytic activity for the ORR.[1] M.K. Debe, Electrocatalyst approaches and challenges for automotive fuel cells, Nature. 486 (2012) 43–51. https://doi.org/10.1038/nature11115.[2] C. Sealy, The problem with platinum, Mater. Today. 11 (2008) 65–68. https://doi.org/10.1016/S1369-7021(08)70254-2.[3] M. Borghei, J. Lehtonen, L. Liu, O.J. Rojas, Advanced Biomass-Derived Electrocatalysts for the Oxygen Reduction Reaction, Adv. Mater. 30 (2018) 1703691. https://doi.org/10.1002/adma.201703691.[4] A. Dessalle, J. Quílez-Bermejo, V. Fierro, F. Xu, A. Celzard, Recent progress in the development of efficient biomass-based ORR electrocatalysts, Carbon. 203 (2023) 237–260. https://doi.org/10.1016/j.carbon.2022.11.073.[5] K. Liu, J. Fu, Y. Lin, T. Luo, G. Ni, H. Li, et al., Insights into the activity of single-atom Fe-N-C catalysts for oxygen reduction reaction, Nat. Commun. 13 (2022) 2075. https://doi.org/10.1038/s41467-022-29797-1.[6] X. Wan, Q. Liu, J. Liu, S. Liu, X. Liu, L. Zheng, et al., Iron atom–cluster interactions increase activity and improve durability in Fe–N–C fuel cells, Nat. Commun. 13 (2022) 2963. https://doi.org/10.1038/s41467-022-30702-z. Figure 1

Modulating the electronic structure and O-intermediates' chemisorption behavior of Pd metallene with boosted oxygen reduction reaction (ORR) performance is critical to advance proton exchange membrane fuel cells (PEMFCs). Herein, Hf doping Pd metallene (Hf-Pd metallene) is developed for efficient ORR electrocatalysis. Multiple characterizations and theoretical simulations disclose that the Hf dopant located in the inner atomic layers of Hf-Pd metallene could modulate the electronic configuration of Pd, lower the binding energies of the Pd d-band centers toward O-related intermediates, deliver a much reduced overpotential during O* hydrogenation into OH*, and thus enhance the catalytic activity. Consequently, the Hf-Pd metallene delivers superior ORR electrocatalytic activity together with excellent stability, surpassing commercial Pt/C and various advanced Pd-based catalysts. Encouragingly, when utilized as the cathode in a PEMFC, the Hf-Pd metallene achieved the higher maximum power density (722.75 mW cm−2) as compared to Pt/C-based batteries, elucidating the practical application of Hf-Pd metallene in PEMFCs.

Developing efficient and durable iron-nitrogen-carbon (Fe-N@C) electrocatalysts with optimal pore architecture is crucial for advancing the oxygen reduction reaction (ORR) in fuel cells. In this study, we demonstrate how hard-templating with tailored silica scaffolds (SBA-15, KIT-6, and a dual SBA-15/KIT-6 template) can tune the pore structure of Fe-N@C materials. In these materials, the pore structure influences the formation and accessibility of active sites for the ORR. The mesoporous Fe-N@CMK-3 electrocatalyst, derived from SBA-15, exhibits the highest ORR activity (onset potential: 0.99 VRHE in alkaline media, and 0.82 VRHE in acid) due to its well-defined 2D hexagonal pores, which facilitate efficient oxygen diffusion. In contrast, the microporous Fe-N@CMK-8 (KIT-6-derived) exhibits lower ORR activity due to limited oxygen accessibility to the active sites. The dual-templated Fe-N@CMK-3/8 combines micro/mesoporosity to deliver balanced performance despite its lower surface area and pore volume resulting from the pore connectivity. All electrocatalysts initially follow a quasi-4e- ORR pathway, but their behavior changes during the long-term testing: Fe-N@CMK-8 shifts to the 2e- pathway despite its notably durable activity in acidic media; Fe-N@CMK-3 exhibits the best stability in terms of activity under alkaline conditions also with a slight shift to the 2e- pathway; Fe-N@CMK-3/8 excels in terms of selectivity sustaining a 4e- pathway along time with medium stability in the activity in both acid and alkaline media. These findings establish pore engineering as a powerful tool to tailor Fe-N@C electrocatalysts for specific operational environments, contributing to the development of high-performance non-precious metal catalysts for the ORR in proton exchange membrane and alkaline fuel cell applications.

Mechano-thermal milling synthesis of atomically dispersed platinum with spin polarization induced by cobalt atoms towards enhanced oxygen reduction reaction

Hollow mesoporous metal-nitrogen-carbon electrocatalysts with enhanced oxygen reduction activity for zinc-air batteries.

Regulating the coordination environment of atomically dispersed catalysts is vital for catalytic reaction but still remains a challenge. Herein, an ionic exchange strategy is developed to fabricate atomically dispersed copper (Cu) catalysts with controllable coordination structure. In this process, the adsorbed Cu ions exchange with Zn nodes in ZIF-8 under high temperature, resulting in the trapping of Cu atoms within the cavities of the metal-organic framework, and thus forming Cu single-atom catalysts. More importantly, altering pyrolysis temperature can effectively control the structure of active metal center at atomic level. Specifically, higher treatment temperature (900 °C) leads to unsaturated Cu-nitrogen architecture (CuN3 moieties) in atomically dispersed Cu catalysts. Electrochemical test indicates atomically dispersed Cu catalysts with CuN3 moieties possess superior oxygen reduction reaction performance than that with higher Cu-nitrogen coordination number (CuN4 moieties), with a higher half-wave potential of 180 mV and the 10 times turnover frequency than that of CuN4 . Density functional theory calculation analysis further shows that the low N coordination number of Cu single-atom catalysts (CuN3 ) is favorable for the formation of O2 * intermediate, and thus boosts the oxygen reduction reaction.

Theory-Driven Design of Electrocatalysts for the Two-Electron Oxygen Reduction Reaction Based on Dispersed Metal Phthalocyanines