Ionic liquids as precursors for Fe-N doped carbon nanotube electrocatalysts for the oxygen reduction reaction

Cellulose-based carbon nanotubes array with lawn-like 3D architecture for oxygen reduction reaction

The conversion of biomass into high-performance carbon-based materials provides an opportunity to valorize biomass for advanced applications. Achieving this necessitates requires dedicated efforts and innovations in biocarbon synthesis, design, and applications. This study proposes the controllable conversion of biomass-derived cellulose into well-distributed carbon nanotubes (CNTs) by tuning the precipitation of cellulose pyrolysis generated vapors with in-situ formed ferric metal nanoparticles. The obtained CNTs exhibited lawn-like 3D architecture with similar length, uniform alignment, and dense distribution. The combined use of ferric chloride and dicyandiamide as the reagents with a mass ration of 0.162:1.05, demonstrated optimal performance in controlling the morphology of CNTs, enhancing the graphitization, and increasing the content of graphitic-N and pyridine-N. This multi-dimensional modification enhanced the electrocatalytic performance of the obtained CNTs, achieving an onset potential of 0.875 V vs. relative hydrogen electrode (RHE), a half-wave potential of 0.703 V vs. RHE, and a current density of -4.95 mA cm-2 during the oxygen reduction reaction. Following microbial fuel cells (MFCs) tests achieved an output voltage of 0.537 V and an output power density of 412.85 mW m-2, comparable to MFC with Pt/C as the cathode catalyst. This biomass-derived catalyst is recommended as a high-quality, non-noble metal alternative to traditional noble-metal catalysts.

Faraday cage-type self-powered immunosensor based on hybrid enzymatic biofuel cell

Self-powered immunosensors (SPIs) based on enzymatic biofuel cell (EBFC) have low sensitivity and poor stability due to the high impedance of the immune sandwich and the vulnerability of enzymes to environmental factors. Here, we applied the Faraday cage-type sensing mode on a hybrid biofuel cell (HBFC)-based SPI for the first time, which exhibited high sensitivity and stability. Cytokeratin 19 fragment (CYFRA 21-1) was used as a model analyte. Au nanoparticle-reduced graphene oxide (Au-rGO) composite was used as the supporting matrix for immunoprobe immobilized with detection antibody and glucose dehydrogenase (GDH), also the builder for Faraday cage structure on the bioanode in the presence of antigen. After the combination of immunoprobe, antigen, and the antibody on the bioanode, the Faraday cage was constructed in case the AuNP-rGO was applied as a conductive cage for electron transfer from GDH to the bioanode without passing through the poorly conductive protein. With the assistance of the Faraday cage structure, the impedance of the bioanode decreased significantly from 4000 to 300 Ω, representing a decline of over 90%. The sensitivity of the SPI, defined as the changes of open circuit voltage (OCV) per unit concentration of the CYFRA 21-1, was 68 mV [log (ng mL-1)]-1. In addition, Fe-N-C was used as an inorganic cathode material to replace enzyme for oxygen reduction reaction (ORR), which endowed the sensor with 4-week long-term stability. This work demonstrates a novel sensing platform with high sensitivity and stability, bringing the concept of hybrid biofuel cell-based self-powered sensor.

Contracted Fe-N5-C11 Sites in Single-Atom Catalysts Boosting Catalytic Performance for Oxygen Reduction Reaction

Promoting the catalyst performance for oxygen reduction reaction (ORR) in energy conversion devices through controlled manipulation of the structure of catalytic active sites has been a major challenge. In this work, we prepared Fe-N-C single-atom catalysts (SACs) with Fe-N₅ active sites and found that the catalytic activity of the catalyst with shrinkable Fe-N₅-C₁₁ sites for ORR was significantly improved compared with the catalyst bearing normal Fe-N₅-C₁₂ sites. The catalyst C@PVI-(TPC)Fe-800, prepared by pyrolyzing an axial-imidazole-coordinated iron corrole precursor, exhibited positive shifted half-wave potential (E_1/2 = 0.89 V vs RHE) and higher peak power density (P_max = 129 mW/cm²) than the iron porphyrin-derived counterpart C@PVI-(TPP)Fe-800 (E_1/2 = 0.81 V, P_max = 110 mW/cm²) in 0.1 M KOH electrolyte and Zn-air batteries, respectively. X-ray absorption spectroscopy (XAS) analysis of C@PVI-(TPC)Fe-800 revealed a contracted Fe-N₅-C₁₁ structure with iron in a higher oxidation state than the porphyrin-derived Fe-N₅-C₁₂ counterpart. Density functional theory (DFT) calculations demonstrated that C@PVI-(TPC)Fe-800 possesses a higher HOMO energy level than C@PVI-(TPP)Fe-800, which can increase its electron-donating ability and thus help achieve enhanced O₂ adsorption as well as O-O bond activation. This work provides a new approach to tune the active site structure of SACs with unique contracted Fe-N₅-C₁₁ sites that remarkably promote the catalyst performance, suggesting significant implications for catalyst design in energy conversion devices.

Curvature effects on the bifunctional oxygen catalytic performance of single atom metal-N-C

Understanding the fundamental relationship between the structural information of electrocatalysts and their catalytic activities plays a key role in controlling many important electrochemical processes. Recently, single-atom catalysts (SACs) with the so-called MN₄ structure, consisting of a central transition metal quadruply bound to four pyridine nitrogen atoms all situated in an extended carbon-based matrix, have attracted intensive scientific attention owing to their exceptional catalytic performance. In this work, we perform the first-principles density functional theory (DFT) calculations to explore the curvature effects of the carbon matrix surfaces on the catalytic activities for two fundamental electrochemical processes, namely, the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER). Our DFT results suggest that the curved surface structure can weaken the interaction between the metal atom and the N-doped carbon matrix, modify the electronic structure of the metal atom, and thus increase the adsorption strength of the reaction intermediates, resulting in enhanced OER and ORR catalytic activities of MN₄ catalysts. More importantly, a prediction model is developed to evaluate the bifunctional catalytic activities of such catalysts based on their directly obtained parameters including the surface curvature of the catalysts, the number of d electrons of the metal element, and the electronegativity of the metal atom and its coordination atoms in MN₄ catalysts. This prediction model not only provides some candidates, for example, FeN₄, CoN₄ and OsN₄ for the ORR; CoN₄, NiN₄, RuN₄, RhN₄ and IrN₄ for the OER; and CoN₄, RuN₄, IrN₄ and OsN₄ for the bifunctional ORR and OER, but also reasonably links the structure of catalysts with their catalytic performance, providing new possibilities for the quick design of high-performance catalysts.

Coordinated single-molecule micelles: a self-template approach for preparing mesoporous doped carbons

Porous carbons prepared using a self-template approach inherit the pore features of template, but they exhibit almost no evenly dispersed mesopores, which is significant for diffusion-limited applications. Herein, N-doped hierarchically porous carbons (NHPCs) with uniform mesopores are prepared using a self-template method. The spherical single-molecule micelle of polystyrene-b-poly(4-vinyl pyridine) (PS-b-P4VP) is turned into a Zn²⁺-coordinated PS-b-P4VP micelle (CPM) by coordination of Zn²⁺ with the P4VP shell. Then, the self-template of the CPM is carbonized into a hollow carbon nanosphere. During carbonization, the PS core is decomposed to generate the central mesopore, whereas the Zn²⁺-coordinated P4VP shell is transformed into a carbonaceous shell. These even hollow carbon nanospheres aggregate to form uniformly mesoporous carbon lumps. Simultaneously, the coordinated Zn²⁺ of the CPM is reduced to metal zinc at high temperatures and then it is evaporated, thus creating numerous micropores in the carbonaceous shell. These NHPCs with uniform mesopores display a high specific surface area. As a demonstration in diffusion-limited applications, their catalytic performances for the oxygen reduction reaction (ORR) are investigated. Strikingly, NHPCs exhibit outstanding catalytic performances for the ORR. This self-template method paves a facile approach for preparing mesoporous carbons with high performances.

Host-guest interactions promoted formation of Fe-N4 active site toward efficient oxygen reduction reaction catalysis

FeNC is the most promising material to replace the noble metal catalyst for cathodic oxygen reduction reaction in proton exchange membrane fuel cells (PEMFCs). However, the practical performance of FeNC catalyst is significantly limited by its low active site (Fe-N₄) density. Herein, we propose to promote the formation of Fe-N₄ active sites in FeNC catalyst by strengthening the interaction of N precursors and Fe precursors during the carbonization synthesis. In our approach, ionic liquid (IL, [EMIM][NTf₂]) with high nitrogen content and good thermal stability is caged in the pores of Fe-ZIF-8 through the host-guest interactions. These interactions are critical for the preservation of Fe and N species and formation of active sites during the synthesis. The optimal catalyst developed with this approach (Fe_0.05NC/10) has a high density of accessible Fe-N₄ sites (1.88*10¹⁹ sites g^-1). Therefore, in both acidic and alkaline media, Fe_0.05NC/10 showed excellent ORR activity comparable to commercial Pt/C catalyst. Moreover, PEMFC performance with a peak power density of 300 mW cm^-2 was demonstrated with Fe_0.05NC/10 under H₂/O₂ conditions. The synthetic approach reported herein may be used for tailoring of advanced catalyst with high intrinsic activity.