Highly Localized C–N2 Sites for Efficient Oxygen Reduction

Leveraging Curvature on N-Doped Carbon Materials for Hydrogen Storage

Carbon sorbent materials have shown great promise for solid-state hydrogen (H2) storage. Modification of these materials with nitrogen (N) dopants has been undertaken to develop materials that can store H2 at ambient temperatures. In this work density functional theory (DFT) calculations are used to systematically probe the influence of curvature on the stability and activity of undoped and N-doped carbon materials toward H binding. Specifically, four models of carbon materials are used: graphene, [5,5] carbon nanotube, [5,5] D5d-C120, and C60, to extract and correlate the thermodynamic properties of active sites with varying degrees of sp2 hybridization (curvature). From the calculations and analysis, it is found that graphitic N-doping is thermodynamically favored on more pyramidal sites with increased curvature. In contrast, it is found that the hydrogen binding energy is weakly affected by curvature and is dominated by electronic effects induced by N-doping. These findings highlight the importance of modulating the heteroatom doping configuration and the lattice topology when developing materials for H2 storage.

Molecular Design of Porous Organic Polymer-Derived Carbonaceous Electrocatalysts for Pinpointing Active Sites in Oxygen Reduction Reaction

The widespread application of fuel cells is hampered by the sluggish kinetics of the oxygen reduction reaction (ORR), which traditionally necessitates the use of high-cost platinum group metal catalysts. The indispensability of these metal catalysts stems from their ability to overcome kinetic barriers, but their high cost and scarcity necessitate alternative strategies. In this context, porous organic polymers (POPs), which are built up from the molecular level, are emerging as promising precursors to produce carbonaceous catalysts owning to their cost-effectiveness, high electrical conductivity, abundant active sites and extensive surface area accessibility. To enhance the intrinsic ORR activity and optimize the performance of these electrocatalysts, recognizing, designing, and increasing the density of active sites are identified as three crucial steps. These steps, which form the core of our review, serve to elucidate the link between the material structure design and ORR performance evaluation, thereby providing valuable insights for ongoing research in the field. Leveraging the precision of polymer skeletons based on molecular units, POP-derived carbonaceous catalysts provide an excellent platform for in-depth exploration of the role and working mechanism for the specific active site during the ORR process. In this review, the recent advances pertaining to the synthesis techniques and electrochemical functions of various types of active sites, pinpointed from POPs, are systematically summarized, including heteroatoms, surficial substituents and edge/defects. Notably, the structure-property relationship, between these active sites and ORR performance, are discussed and emphasized, which creates guidelines to shed light on the design of high-performance ORR electrocatalysts.

Efficient hydrogen generation of a cobalt porphyrin-bridged covalent triazine polymer

Hydrogen production obtained by electrocatalytic water splitting exhibits great promise in addressing both energy shortage and environmental contamination. Herein, we prepared a novel cobalt porphyrin (CoTAPP)-bridged covalent triazine polymer (CoTAPPCC) by covalently linking CoTAPP with cyanuric chloride (CC) for catalytic hydrogen evolution reaction (HER). Both experimental techniques and density functional theory (DFT) calculations were used to evaluate the correlation of HER activity with molecular structures. Benefiting from the strong electronic interactions between the CC unit and the CoTAPP moiety, a standard current density at 10 mA cm^-2 is obtained for CoTAPPCC with a low overpotential of 150 mV in acid, which is comparable to or better than the best records reported previously. Additionally, a competitive HER activity in basic medium is obtained for CoTAPPCC. The strategy reported herein is valuable for designing and developing porphyrin-based efficient HER electrocatalysts.

Isometric Covalent Triazine Framework-Derived Porous Carbons as Metal-Free Electrocatalysts for the Oxygen Reduction Reaction

Covalent triazine frameworks (CTFs) and their derivative N-doped carbons have attracted much attention for application in energy conversion and storage. However, previous studies have mainly focused on developing new building blocks and optimizing synthetic conditions. The use of isometric building blocks to control the porous structure and to fundamentally understand structure-property relationships have rarely been reported. In this work, two isometric building blocks are used to produce isometric CTFs with controllable pore geometries. The as-prepared CTF with nonplanar hexagonal rings demonstrates higher surface area, larger pore volume, and richer N content than the planar CTF. After pyrolysis, nonplanar porous CTF-derived N-doped carbons exhibit admirable catalytic activity for oxygen reduction in alkaline media (half-wave potential: 0.86 V; Tafel slope: 65 mV dec^-1 ), owing to their larger pore volume and the abundance of pyridinic and graphitic N species. When assembled into a zinc-air battery, the as-made electrocatalysts show high capacities of up to 651 mAh g^-1 and excellent durability.

The surface structure, stability, and catalytic performances toward O2 reduction of CoP and FeCoP2

The systematic atomistic level investigation of low-index surface structures, stabilities, and catalytic performances of CoP and FeCoP₂ towards the O₂ reduction reaction (ORR) is vital for their applications. Employing first-principles calculations, it is revealed that CoP and FeCoP₂ present the same surface stability in the order of (101) ≈ (011) > (111) > (001) > (110) > (010) > (100). They also possess a similar Wulff equilibrium crystal shape with (101) and (011) exposing the largest surface area. From the electronic view, FeCoP₂ presents improved electronic conductivity compared with CoP. From the energy view, whether FeCoP₂ delivers improved electrocatalytic activity toward the ORR with respect to CoP depends on the reactive surfaces and sites. Among the 4 surfaces considered, only CoP(101), FeCoP₂(101) and FeCoP₂(011) delivered ORR performances theoretically when the bridge metal-metal site acts as the reactive center, which makes CoP(011) the only exception. CoP(101)-b_Co-Co and FeCoP₂(011)-b_Fe-Co exhibit a larger thermodynamic limiting potential than FeCoP₂(101)-b_Co-Co, suggesting their higher performances toward the ORR. The last step of HO* desorption as the rate-limiting step accounts for 3/4. The third step of transformation from O* to HO* as the most sluggish step accounts for 1/4. The work function, d-band center, Bader charge, and electronic localization function calculations are performed to reveal the HO adsorption nature. The present work provides fundamental insight into the effect of Fe doping into CoP, the determination of the catalyst surface and the key species adsorption nature to guide the rational design of high-performance materials.

Design Strategies for Single-Atom Iron Electrocatalysts toward Efficient Oxygen Reduction

The oxygen reduction reaction (ORR) is a pivotal half-reaction for full cells and metal-air batteries. However, the intrinsic sluggish kinetics of the ORR inhibits the practical applications of these environmentally friendly energy-conversion devices. Therefore, highly efficient electrocatalysts with low cost are required to promote the ORR process. Carbon materials with single-atom Fe coordinated with N and C (Fe-N-C) stand out from various non-precious electrocatalysts, and great progress of both catalysts design and mechanism understanding has been achieved in the past. In this Perspective, we start with the recent advance in design strategies of active sites in Fe-N-C and emphasize the importance of spatial configuration and electron distribution. We discuss diverse Fe-N-C species as well as their corresponding properties. At last, we give our outlook for the future development of advanced Fe-N-C electrocatalysts.

Activity origin of boron doped carbon cluster for thermal catalytic oxidation: Coupling effects of dopants and edges

Catalytic oxidation plays important roles in energy conversion and environment protection. Boron-doped crystalline carbocatalyst has been demonstrated effective; however, the application potential of boron-doped amorphous carbocatalyst remains to be explored. For amorphous carbon material, finite-sized carbon clusters are the basic structural units, which exhibit unique activity due to edge and size effect. Herein, using sulfur dioxide (SO₂) and carbon monoxide (CO) oxidation as probe thermal-catalysis reactions, we found the distribution and reactivity of active sites in boron-doped carbon clusters are simultaneously determined by dopants and edges. According to comparisons of oxygen (O₂) chemisorption energy at different sites of symmetric and non-symmetric carbon cluster, the most active site is found to be the edge carbon atom with high electron donation ability, which can be accurately identified by electrophilic Fukui function. More importantly, the reactivity of boron-doped cluster is simultaneously influenced by doping configuration and the type of edge, based on which -O-B-O- configuration embedded into K-region edge (isolated carbon-carbon double bonds that do not belong to Clar sextet) is predicted to exhibit the highest reactivity among various boron doping configurations. This work clarifies unique activity origin of heteroatom-doped amorphous carbon materials, providing new insights into designing high-performance carbocatalysts.

Conjugated Porous Polydiaminophenylsulfone-Triazine Polymer-A High-Performance Anode for Li-Ion Batteries

Organic compounds are promising electrode materials because of their resource sustainability, environmental friendliness, and highly tailorable properties. The porous conjugated polymer shows great potential as an electrode material for its tunable redox nature, conjugated skeleton, and porous structure. Herein, a novel conjugated porous polymer, polydiaminophenylsulfone-triazine, was synthesized by a simple nucleophilic substitution reaction. The conjugated structure and triazine ring can improve the conductivity, charge-transfer efficiency, and physicochemical stability. Also, the porous polymeric framework shows a large specific surface area and high porosity, providing a large contact area with electrolytes and reducing diffusion distance. The polymer demonstrates highly stable cycling performance and good rate capability as an anode for lithium-ion batteries, suggesting a promising strategy to design a competitive electrode material.