Active sites of the functionalized coals and carbons for oxygen reduction reaction in a fuel cell

The active sites of the coals and carbons functionalized with added nitrogen, oxygen and iron were studied for the oxygen reduction reaction (ORR) in a fuel cell. The catalysts were characterized based on the XPS, Raman, TEM, XRD and N₂ adsorption measurements. The ORR activity was promoted by the addition of iron and aluminum as the inorganic components of the ash to the ash-free brown coal. The ORR activity of the ash-components added to the ash-free brown coal was correlated to the I _D/I _G ratio (deficient carbon degree) and the pyridinic nitrogen based on the Raman and XPS analyses, respectively. The active sites of the brown coal were formed at the pyridinic nitrogen on parts of the defective carbons associated with iron on the alumina. On the other hand, for the nitrogen-doped carbons without iron, the ORR activity was related to the pyrrolic-NH, pyridinic nitrogen species and the defective carbon degree. Based on these results, the active sites of the iron-added and nitrogen-doped coals and carbons were the iron sites coordinated with the pyridinic nitrogen, while the active sites of the iron-free and nitrogen-doped carbons without iron were the pyrrolic-NH and pyridinic-NH⁺ sites of parts of the defective carbons. The difference between the active sites of the nitrogen-doped coals and carbons in the presence of iron and those in the absence of iron was discussed. These results suggested that the pyridinic N as a base site transformed into pyridinic-NH⁺ as an acid site by attack of the proton from the anode.     

Doping effect of boron and phosphorus on nitrogen-based mesoporous carbons as electrocatalysts for oxygen reduction reaction in acid media

Doped mesoporous carbons comprising nitrogen, boron, and phosphorus (N, B, and P, respectively) were prepared as non-Pt catalysts for oxygen reduction reaction (ORR) in an acidic solution. The N-doped carbons were varied to increase their catalytic activity through by additionally doping of B and P. All the mesoporous carbons were synthesized by carbonizing polyaniline at 900 °C for the N species, while the B and P species were inserted into the carbon structure at the carbon growth step. The linear sweep voltammogram recorded in the acidic solution showed that the ORR activity of the N-doped carbon catalysts increased significantly after the addition of B. An approximately 19 % increase in the pyridinic N content at the carbon surface was observed, along with B-N-C moieties with a binding energy of 399.5 eV. The non-precious metal ORR catalysts were prepared via pyrolysis, with the insertion of an additional transition metal (iron, Fe). The deconvoluted X-ray photoelectron spectroscopy (XPS) results showed that the Fe-N peak was generated after the pyrolysis. The peak intensity of the quaternary N also increased compared with the pyridic and pyrrolic N, which indicates that Fe serves to catalyze the modification of N species. The numerical examinations showed that N- and B-doped mesoporous carbon (NBC) 1.5 % Fe had the highest limited current (4.94 mA/cm²), with the B-doped carbon still the most active mesoporous carbon catalyst for ORR. As a result, it can be said that Fe positively contributes to the formation of graphitic N, which is known to be an active site for ORR. The cyclic voltammetry results showed that the peak area of the NBC 1.5 % Fe catalyst was larger than that of the N-doped mesoporous carbon (NC) 1.5 % Fe catalyst. It was concluded that B doping enhances the ORR activity and the stability of carbon materials even after 1000 cycles under acidic conditions.     

Self-sacrificial template synthesis of Fe,N co-doped porous carbon as efficient oxygen reduction electrocatalysts towards Zn-air battery application

Designing highly efficient non-precious based electrocatalysts for oxygen reduction reaction（ORR） is of significance for the rapid development of metal-air batteries.Herein,a hydrothermal-pyrolysis method is employed to fabricate Fe,N co-doped porous carbon materials as effective ORR electrocatalyst through adopting graphitic carbon nitride（g-C₃ N₄） as both the self-sacrificial templates and N sources.The gC₃ N₄ provides a high concentration of unsatur...     

Synthesis and Characterization of Pyridine-Based Benzoxazines and Their Carbons

Novel benzoxazines were synthesized to prepare nitrogen-doped carbons with high nitrogen content. Monomers (2pd, 3pd) were successfully synthesized from aminopyridine, orth-hydroxybenzaldehyde and paraformaldehyde. Their chemical structures were confirmed via Fourier transform-infrared spectroscopy (FTIR), ¹H-NMR and ¹³C-NMR. The thermogravimetric analysis and elemental analysis results demonstrated that the cured benzoxazines had high char yield and their carbides had high nitrogen content. The nitrogen configurations of the carbons obtained from pyridine-based benzoxazines were pyridinic N, pyrrolic N and graphitic N, which were the active centers in nitrogen-doped carbons. We believe that these pyridine-based benzoxazines will be applied as the precursors for functional carbon materials in many fields such as hydrogen storage, fuel cell and supercapacitor.     

On the mechanism of enhanced oxygen reduction reaction in nitrogen-doped graphene nanoribbons

Nitrogen (N)-doped carbon materials were shown in recent studies to have promising catalytic activity for oxygen reduction reaction (ORR) as a metal-free alternative to platinum, but the underlying molecular mechanism or even the active sites for high catalytic efficiency are still missing or controversial both experimentally and theoretically. We report here the results of periodic density functional theory (DFT) calculations about the ORR at the edge of a graphene nanoribbon (GNR). The edge structure and doped-N near the edge are shown to enhance the oxygen adsorption, the first electron transfer, and also the selectivity toward the four-electron, rather than the two-electron, reduction pathway. We find that the outermost graphitic nitrogen site in particular gives the most desirable characteristics for improved ORR activity, and hence the active site. However, the latter graphitic nitrogen becomes pyridinic-like in the next electron and proton transfer reaction via the ring-opening of a cyclic C-N bond. This inter-conversion between the graphitic and pyridinic sites within a catalytic cycle may reconcile the controversy whether the pyridinic, graphitic, or both nitrogens are active sites.     

Fe/N/C氧还原催化剂的热稳定性及活性位结构

研制高活性的Fe/N/C氧还原催化剂对于降低燃料电池成本、实现商业化应用有重要意义. 为了实现Fe/N/C催化剂的理性设计,需要深入研究其活性位结构. 本文我们发展一种研究活性位结构的新策略,即以预先合成好的聚间苯二胺基Fe/N/C催化剂(PmPDA-FeNx/C)为起始物,对其在1000~1500 ^oC高温下再次进行热处理并使其失活,通过关联催化剂热处理前后的结构变化与氧还原催化性能来揭示活性位结构. 实验结果表明,随着热处理温度升高,活性中心结构被破坏,铁原子析出团聚并形成纳米颗粒,氮元素挥发损失,导致催化剂失活. XPS分析显示,低结合能含氮物种的含量与催化剂的ORR活性呈良好的正相关性,表明活性中心很可能是由吡啶N和Fe-N物种构成的.     

Dual‐Template Pore Engineering of Whey Powder‐Derived Carbon as an Efficient Oxygen Reduction Reaction Electrocatalyst for Primary Zinc‐Air Battery

Cost‐effective and high‐performance electrocatalysts for oxygen reduction reactions (ORR) are needed for many energy storage and conversion devices. Here, we demonstrate that whey powder, a major by‐product in the dairy industry, can be used as a sustainable precursor to produce heteroatom doped carbon electrocatalysts for ORR. Rich N and S compounds in whey powders can generate abundant catalytic active sites. However, these sites are not easily accessible by reactants of ORR. A dual‐template method was used to create a hierarchically and interconnected porous structure with micropores created by ZnCl₂ and large mesopores generated by fumed SiO₂ particles. At the optimum mass ratio of whey power: ZnCl₂ : SiO₂ at 1 : 3 : 0.8, the resulting carbon material has a large specific surface area close to 2000 m² g^?1, containing 4.6 at.% of N with 39.7% as pyridinic N. This carbon material shows superior electrocatalytic activity for ORR, with an electron transfer number of 3.88 and a large kinetic limiting current density of 45.40 mA cm^?2. They were employed as ORR catalysts to assemble primary zinc‐air batteries, which deliver a power density of 84.1 mW cm^?2 and a specific capacity of 779.5 mAh g^?1, outperforming batteries constructed using a commercial Pt/C catalyst. Our findings open new opportunities to use an abundant biomaterial, whey powder, to create high‐value‐added carbon electrocatalysts for emerging energy applications.     

Understanding the Activity of Co‐N4−xCx in Atomic Metal Catalysts for Oxygen Reduction Catalysis

Atomic metal catalysis (AMC) provides an effective way to enhance activity for the oxygen reduction reaction (ORR). Cobalt anchored on nitrogen‐doped carbon materials have been extensively reported. The carbon‐hosted Co‐N₄ structure was widely considered as the active site; however, it is very rare to investigate the activity of Co partially coordinated with N, for example, Co‐N_4?xC_x. Herein, the activity of Co‐N_4?xC_x with tunable coordination environment is investigated as the active sites for ORR catalysis. The defect (di‐vacancies) on carbon is essential for the formation of Co‐N_4?xC_x. N species play two important roles in promoting the intrinsic activity of atomic metal catalyst: N coordinated with Co to manipulate the reactivity by modification of electronic distribution and N helped to trap more Co to increase the number of active sites.     

Metal‐Free Electrocatalyst for Oxygen Reduction: Synthesis‐Controlled Density of Catalytic Centers and Impact on ORR

This work demonstrates a rapid and scalable route for the preparation of N‐doped carbon spheres of 80–120 nm via pyrolysis of polypyrrole as the only carbon and nitrogen source. The resulting porous catalyst has a nitrogen doping level of 6–8 at%. Electrochemical studies show that N‐doped C is very active toward oxygen reduction in alkaline electrolyte and the mechanism of ORR process is controlled by the surface concentration of catalytic active sites that promote either a direct four‐electron or two‐electron process. An interesting observation is that we can generate precursors for the N‐doped carbon with desirable particle size, shape and with the preferential structure (linear polypyrrole from the α? α coupling during slow polymerization or cross‐linked polypyrrole from α? β coupling during fast polymerization) that promotes the formation of favorable catalytic sites for O₂ reduction. The XPS analysis in conjunction with RDE voltammetry highlights the effect of polymer precursor synthesis on the chemical structure and a resulting electrochemical activity of the N‐doped carbon materials.     

B,N Co-Doped Three-Dimensional Carbon Aerogels with Excellent Electrochemical Performance for the Oxygen Reduction Reaction

Herein, an ordinary and mass-production approach is reported to synthesize boron (B) and nitrogen (N) co-doped three-dimensional (3D) carbon aerogels (CA) by using glucose and borax as the raw materials by a simple hydrothermal method and then carbonization in NH₃ atmosphere. The porous material (BN-CA-900) possesses a large specific surface area (1032 m² g⁻¹) and high contents of doped pyridinic N and graphitic N. The onset potential (0.91 V vs. reversible hydrogen electrode, RHE), half-wave potential (0.77 V vs. RHE), and current density (5.70 mA cm⁻² at 0.2 V vs. RHE) of BN-CA-900 for ORR are similar to those of commercial Pt/C, indicating that BN-CA-900 has a comparable catalytic activity with Pt/C in alkaline media. The number of electron transfer is 3.86–3.99 and the yield of hydrogen peroxide is less than 6.8 %. BN-CA-900 also presents decent catalytic performance in acidic medium. Moreover, the stability and methanol tolerance of BN-CA-900 are superior to commercial Pt/C in both alkaline and acidic media. The prepared BN-CA-900 is a promising candidate that may be applied in other areas, such as the adsorption of pollution, porous conductive electrodes, and lithium-ion batteries.     

One-Step Preparation of Fe/N/C Single-Atom Catalysts Containing Fe−N4 Sites from an Iron Complex Precursor with 5,6,7,8-Tetraphenyl-1,12-Diazatriphenylene Ligands

Fe/N/C single-atom catalysts containing Fe−N_x sites prepared by pyrolysis are promising cathode materials for fuel cells and metal-air batteries due to their high oxygen reduction reaction (ORR) activities. We have developed iron complexes containing N2- or N3-chelating coordination structures with preorganized aromatic rings in a 1,12-diazatriphenylene framework tethering bromo substituents as precursors to precisely construct Fe−N₄ sites in an Fe/N/C catalyst. One-step pyrolysis of the iron complex with carbon black forms atomically dispersed Fe−N₄ sites without iron aggregates. X-ray absorption spectroscopy (XAS) and electrochemical measurements revealed that the iron complex with N3-coordination is more effectively converted to Fe−N₄ sites catalyzing ORR with a TOF value of 0.21 e site⁻¹ s⁻¹ at 0.8 V vs. RHE. This indicates that the formation of Fe−N₄ sites is controlled by precise tuning of the chemical structure of the iron complex precursor.     

S-Block Potassium Single-atom Electrocatalyst with K−N4 Configuration Derived from K+/Polydopamine for Efficient Oxygen Reduction

Currently, single-atom catalysts (SACs) research mainly focuses on transition metal atoms as active centers. Due to their delocalized s/p-bands, the s-block main group metal elements are typically regarded as catalytically inert. Herein, an s-block potassium SAC (K−N−C) with K-N₄ configuration is reported for the first time, which exhibits excellent oxygen reduction reaction (ORR) activity and stability under alkaline conditions. Specifically, the half-wave potential (E_1/2) is up to 0.908 V, and negligible changes in E_1/2 are observed after 10,000 cycles. In addition, the K−N−C offers an exceptional power density of 158.1 mW cm⁻² and remarkable durability up to 420 h in a Zn-air battery. Density functional theory (DFT) simulations show that K−N−C has bifunctional active K and C sites, can optimize the free energy of ORR reaction intermediates, and adjust the rate-determining steps. The crystal orbital Hamilton population (COHP) results showed that the s orbitals of K played a major role in the adsorption of intermediates, which was different from the d orbitals in transition metals. This work significantly guides the rational design and catalytic mechanism research of s-block SACs with high ORR activity.     

Nitrogen-doped mesoporous carbon nanospheres loaded with cobalt nanoparticles for oxygen reduction and Zn–air batteries

Mesoporous carbon supported with transition metals nanoparticles performs desired activities for oxygen reduction reaction (ORR) and clean energy conversion devices such as Zn–air batteries. In this work, we synthesized N-doped mesoporous carbon loaded with cobalt nanoparticles (CoMCN) through self-assembly method. There are sufficient mesopores on the carbon substrate which stem from the pore-forming agent. These mesopores can provide enough accessible active sites and profitable charge/mass transport for ORR. The high content of pyridinic and graphitic N is beneficial for promoting O₂ adsorption and reduction. The smaller value of I_D/I_G indicates the higher degree of graphitization of CoMCN, providing better electronic conductivity. The half-wave potential of CoMCN is 0.865 V in basic solution, which is 24 mV more positive than that of the commercial Pt/C (0.841 V). In addition, CoMCN performs excellent methanol tolerance and stability under both basic and acidic conditions. The Zn–air battery assembled with CoMCN performs the larger power density and open-circuit voltage than the commercial Pt/C-based battery, indicating the potential application in energy conversion systems. This work provides thoughtful ideas for fabricating transition metal nanoparticles based porous carbon for electrocatalysis and metal–air batteries.     

Highly Efficient Porous Carbon Electrocatalyst with Controllable N-Species Content for Selective CO2 Reduction

We report a straightforward strategy to design efficient N doped porous carbon (NPC) electrocatalyst that has a high concentration of easily accessible active sites for the CO₂ reduction reaction (CO₂RR). The NPC with large amounts of active N (pyridinic and graphitic N) and highly porous structure is prepared by using an oxygen-rich metal–organic framework (Zn-MOF-74) precursor. The amount of active N species can be tuned by optimizing the calcination temperature and time. Owing to the large pore sizes, the active sites are well exposed to electrolyte for CO₂RR. The NPC exhibits superior CO₂RR activity with a small onset potential of −0.35 V and a high faradaic efficiency (FE) of 98.4 % towards CO at −0.55 V vs. RHE, one of the highest values among NPC-based CO₂RR electrocatalysts. This work advances an effective and facile way towards highly active and cost-effective alternatives to noble-metal CO₂RR electrocatalysts for practical applications.     

High‐Rate Oxygen Electroreduction over Graphitic‐N Species Exposed on 3D Hierarchically Porous Nitrogen‐Doped Carbons

Nitrogen‐doped species (NDs) are theoretically accepted as a determinant of the catalytic activity of metal‐free N‐doped carbon (NC) catalysts for oxygen reduction reaction (ORR). However, direct relationships between ND type and ORR activity have been difficult to extract because the complexity of carbon matrix impairs efforts to expose specific NDs. Herein, we demonstrate the fabrication of a 3D hierarchically porous NC catalyst with micro‐, meso‐, and macroporosity in one structure, in which sufficient exposure and availability of inner‐pore catalytic sites can be achieved due to its super‐high surface area (2191 cm² g^?1) and interconnected pore system. More importantly, in‐situ formation of graphitic‐N species (GNs) on the surface of NC stimulated by KOH activation enables us to experimentally reveal the catalytic nature of GNs for ORR, which is of great significance for the design and development of advanced metal‐free NC electrocatalysts.     

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 (¹O₂) on the N-adjacent atoms to directionally construct topological carbon defects neighboring the N dopants in N-doped carbon (¹O₂−N/C). This ¹O₂−N/C exhibits the highest ORR half-wave potential of 0.915 V_RHE 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 ¹O₂-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.     

基于羧甲基纤维素钠制备氮掺杂多孔炭及其电容性能研究

以羧甲基纤维素钠(NaCMC)为碳源, 利用直接炭化工艺(无需进一步活化)制备多孔炭材料; 然后, 以CO(NH₂)₂为氮源, 形成了氮掺杂多孔炭材料. 氮的存在形式包括吡啶N、石墨N和吡咯N. 实验结果表明, 羧甲基纤维素钠与CO(NH₂)₂之间的配比可以有效控制氮存在形式、含量、样品的比表面积及孔的结构等. 样品的电化学性能测试表明, 氮掺杂后多孔炭材料的超电容性能得到了显著提升. 以carbon-N-1:20为例, 其比表面积可达858 m²·g^-1, 远高于未经氮掺杂carbon-blank 的463 m²·g^-1, 其质量比电容则由94.0 F·g^-1提高到了156.7F·g^-1.     

Understanding the Activity of Co-N4−xCx in Atomic Metal Catalysts for Oxygen Reduction Catalysis

Atomic metal catalysis (AMC) provides an effective way to enhance activity for the oxygen reduction reaction (ORR). Cobalt anchored on nitrogen-doped carbon materials have been extensively reported. The carbon-hosted Co-N₄ structure was widely considered as the active site; however, it is very rare to investigate the activity of Co partially coordinated with N, for example, Co-N_4−xC_x. Herein, the activity of Co-N_4−xC_x with tunable coordination environment is investigated as the active sites for ORR catalysis. The defect (di-vacancies) on carbon is essential for the formation of Co-N_4−xC_x. N species play two important roles in promoting the intrinsic activity of atomic metal catalyst: N coordinated with Co to manipulate the reactivity by modification of electronic distribution and N helped to trap more Co to increase the number of active sites.     

热处理碳载Fe-三聚氰胺及Fe-g-C3N4催化剂的氧还原催化性能

分别以三聚氰胺和三聚氰胺的聚合物为配体, 采用浸渍法合成了两种氧还原反应(ORR)催化剂Fe-N/C(1)和Fe-N/C(2). 通过X射线衍射光谱(XRD)、X射线光电子能谱(XPS)、透射电子显微镜(TEM)和电化学测试对催化剂的成分、形貌和电催化性能进行了表征. 结果表明, 以三聚氰胺聚合物为配体制备的Fe-N/C具有更高的ORR催化活性. 在高温热处理过程中, 催化剂表面能形成更多的石墨N活性点, 是其ORR性能提高的重要原因.     

Highly Efficient Porous Carbon Electrocatalyst with Controllable N‐Species Content for Selective CO2 Reduction

We report a straightforward strategy to design efficient N doped porous carbon (NPC) electrocatalyst that has a high concentration of easily accessible active sites for the CO₂ reduction reaction (CO₂RR). The NPC with large amounts of active N (pyridinic and graphitic N) and highly porous structure is prepared by using an oxygen‐rich metal–organic framework (Zn‐MOF‐74) precursor. The amount of active N species can be tuned by optimizing the calcination temperature and time. Owing to the large pore sizes, the active sites are well exposed to electrolyte for CO₂RR. The NPC exhibits superior CO₂RR activity with a small onset potential of ?0.35 V and a high faradaic efficiency (FE) of 98.4 % towards CO at ?0.55 V vs. RHE, one of the highest values among NPC‐based CO₂RR electrocatalysts. This work advances an effective and facile way towards highly active and cost‐effective alternatives to noble‐metal CO₂RR electrocatalysts for practical applications.