3D Porous Fe/N/C Spherical Nanostructures As High-Performance Electrocatalysts for Oxygen Reduction in Both Alkaline and Acidic Media

A High-Performance Hybrid Biofuel Cell with a Honeycomb-Like Ti3 C2 Tx /MWCNT/AuNP Bioanode and a ZnCo2 @NCNT Cathode for Self-Powered Biosensing

This work focusses on developing a hybrid enzyme biofuel cell-based self-powered biosensor with appreciable stability and durability using murine leukemia fusion gene fragments (tDNA) as a model analyte. The cell consists of a Ti₃ C₂ T_x /multiwalled carbon nanotube/gold nanoparticle/glucose oxidase bioanode and a Zn/Co-modified carbon nanotube cathode. The bioanode uniquely exhibits strong electron transfer ability and a high surface area for the loading of 1.14 × 10^-9  mol cm^-2 glucose oxidase to catalyze glucose oxidation. Meanwhile, the abiotic cathode with a high oxygen reduction reaction activity negates the use of conventional bioenzymes as catalysts, which aids in extending the stability and durability of the sensing system. The biosensor offers a 0.1 fm-1 nm linear range and a detection limit of 0.022 fm tDNA. Additionally, the biosensor demonstrates a reproducibility of ≈4.85% and retains ≈87.42% of the initial maximal power density after a 4-week storage at 4 °C, verifying a significantly improved long-term stability.

Synergistic Effect of Sn and Fe in Fe-Nx Site Formation and Activity in Fe-N-C Catalyst for ORR

Iron-nitrogen-carbon (Fe-N-C) materials emerged as one of the best non-platinum group material (non-PGM) alternatives to Pt/C catalysts for the electrochemical reduction of O₂ in fuel cells. Co-doping with a secondary metal center is a possible choice to further enhance the activity toward oxygen reduction reaction (ORR). Here, classical Fe-N-C materials were co-doped with Sn as a secondary metal center. Sn-N-C according to the literature shows excellent activity, in particular in the fuel cell setup; here, the same catalyst shows a non-negligible activity in 0.5 M H₂SO₄ electrolyte but not as high as expected, meaning the different and uncertain nature of active sites. On the other hand, in mixed Fe, Sn-N-C catalysts, the presence of Sn improves the catalytic activity that is linked to a higher Fe-N₄ site density, whereas the possible synergistic interaction of Fe-N₄ and Sn-N_x found no confirmation. The presence of Fe-N₄ and Sn-N_x was thoroughly determined by extended X-ray absorption fine structure and NO stripping technique; furthermore, besides the typical voltammetric technique, the catalytic activity of Fe-N-C catalyst was determined and also compared with that of the gas diffusion electrode (GDE), which allows a fast and reliable screening for possible implementation in a full cell. This paper therefore explores the effect of Sn on the formation, activity, and selectivity of Fe-N-C catalysts in both acid and alkaline media by tuning the Sn/Fe ratio in the synthetic procedure, with the ratio 1/2 showing the best activity, even higher than that of the iron-only containing sample (j_k = 2.11 vs 1.83 A g^-1). Pt-free materials are also tested for ORR in GDE setup in both performance and durability tests.

Valorization of spent double substituted Co-Ni-Zn-Fe LDH wastewater nanoadsorbent as methanol electro-oxidation catalyst

Finding suitable non-expensive electrocatalyst materials for methanol oxidation is a significant challenge. Waste valorization of spent wastewater nanoadsorbents is a promising route toward achieving circular economy guidelines. In this study, the residual of layered double hydroxide (LDH) can be used as an electrocatalyst in direct methanol fuel cells as a novel approach. The Co-Ni-Zn-Fe LDH was prepared by the co-precipitation method followed by the adsorption of methyl orange (MO). Moreover, the spent adsorbent was calcined at different temperatures (200, 400, and 600 °C) to be converted to the corresponding mixed metal oxides (MMO). The prepared samples were characterized using XRD, FTIR, HRTEM, zeta potential, and hydrodynamic size measurements. The spent adsorbent was tested as an electro-catalyst for direct methanol electro-oxidation. The spent LDH/MO adsorbent showed a maximum current density of 6.66 mA/cm² at a 50 mV/s scan rate and a 1 M methanol concentration. The spent MMO/MO adsorbent showed a maximum current density of 8.40 mA/cm² at a 200 °C calcination temperature, 50 mV/s scan rate, and a 3 M methanol concentration. Both samples show reasonable stability over time, as indicated by the chronoamperometric response. Further nanoengineering of used nanoadsorbents could be a promising path to repurposing these wastes as electro-oxidation catalysts.

Cobalt phosphide nanoparticles encapsulated in manganese, nitrogen co-doped porous carbon nanosheets with rich nanoholes for high-efficiency oxygen reduction reaction

It is a challenging task to research oxygen reduction electrocatalysts with cost-effectiveness, high-performance and ultra-stability to replace traditional noble metal catalysts in renewable energy conversion/storage devices. Herein, cobalt phosphide (Co₂P) nanoparticles encapsulated in Mn, N co-doped porous carbon nanosheets with abundant nanoholes (Co₂P/Mn,N-PCNS) were prepared by a alizarin complexone coordination regulated pyrolysis at 800 °C. In the controlled experiments, the pyrolysis temperature and metal types were investigated in details. The resultant catalyst exhibited rapid mass/charge transfer and superior oxygen reduction reaction (ORR) performance (E_onset = 0.96 V; E_1/2 = 0.86 V vs RHE), surpassing commercial Pt/C. This work presents some constructive guidelines for synthesis of appealing ORR electrocatalysts in renewable energy technology.

Sulfur Doping versus Hierarchical Pore Structure: The Dominating Effect on the Fe-N-C Site Density, Activity, and Selectivity in Oxygen Reduction Reaction Electrocatalysis

Nitrogen doping has been always regarded as one of the major factors responsible for the increased catalytic activity of Fe-N-C catalysts in the oxygen reduction reaction, and recently, sulfur has emerged as a co-doping element capable of increasing the catalytic activity even more because of electronic effects, which modify the d-band center of the Fe-N-C catalysts or because of its capability to increase the Fe-N_x site density (SD). Herein, we investigate in detail the effect of sulfur doping of carbon support on the Fe-N_x site formation and on the textural properties (micro- and mesopore surface area and volume) in the resulting Fe-N-C catalysts. The Fe-N-C catalysts were prepared from mesoporous carbon with tunable sulfur doping (0-16 wt %), which was achieved by the modulation of the relative amount of sucrose/dibenzothiophene precursors. The carbon with the highest sulfur content was also activated through steam treatment at 800 °C for different durations, which allowed us to modulate the carbon pore volume and surface area (1296-1726 m² g^-1). The resulting catalysts were tested in O₂-saturated 0.5 M H₂SO₄ electrolyte, and the site density (SD) was determined using the NO-stripping technique. Here, we demonstrate that sulfur doping has a porogenic effect increasing the microporosity of the carbon support, and it also facilitates the nitrogen fixation on the carbon support as well as the formation of Fe-N_x sites. It was found that the Fe-N-C catalytic activity [E_1/2 ranges between 0.609 and 0.731 V vs reversible hydrogen electrode (RHE)] does not directly depend on sulfur content, but rather on the microporous surface and therefore any electronic effect appears not to be determinant as confirmed by X-ray photoemission spectroscopy (XPS). The graph reporting Fe-N_x SD versus sulfur content assumes a volcano-like shape, where the maximum value is obtained for a sulfur/iron ratio close to 18, i.e., a too high or too low sulfur doping has a detrimental effect on Fe-N_x formation. However, it was highlighted that the increase of Fe-N_x SD is a necessary but not sufficient condition for increasing the catalytic activity of the material, unless the textural properties are also optimized, i.e., there must be an optimized hierarchical porosity that facilitates the mass transport to the active sites.

Efficient oxygen reduction electrocatalyst derived from facile Fe,N-surface treatment of carbon black

The development of nonprecious metal-based electrocatalysts for oxygen reduction reaction (ORR) is a central task in renewable electrochemical energy conversion and storage technologies. Iron-nitrogen doped carbon-based (Fe-N/C) materials are promising alternatives to Pt-based ORR electrocatalysts. Owing to large specific surface area and outstanding electrical conductivity, carbon black is an inborn support for electrocatalysts. Unfortunately, the direct incorporation of Fe-N_x moieties onto the surface of carbon black has not been realized to date. Herein, Fe-N_x moieties are directly incorporated onto the surface of carbon black through surface modification and the following Fe and N co-doping. The obtained Fe and N co-doped carbon back (Fe-N/CB) catalyst has very large specific surface area and abundant accessible Fe-N_x moieties. As a result, Fe-N/CB electrocatalyst exhibits a more positive half-wave potential (0.86 V) than Pt/C. The Fe-N/CB catalyst also displays better stability and methanol resistance than Pt/C. The Zn-air battery with Fe-N/CB as cathodic catalyst shows a maximum power density of 68 mW cm^-2 and a specific capacity of 676 mAh g_Zn^-1. Our finding provides a convenient and low-cost approach to fabricating efficient M-N/C-based catalysts and will be helpful to the development of renewable electrochemical energy conversion and storage technologies.

Preparation and Application of Fe-N Co-Doped GNR@CNT Cathode Oxygen Reduction Reaction Catalyst in Microbial Fuel Cells

Through one-step pyrolysis, non-noble-metal oxygen reduction reaction (ORR) electrocatalysts were constructed from ferric trichloride, melamine, and graphene nanoribbon@carbon nanotube (GNR@CNT), in which a portion of the multiwall carbon nanotube is unwrapped/unzipped radially, and thus graphene nanoribbon is exposed. In this study, Fe-N/GNR@CNT materials were used as an air-cathode electrocatalyst in microbial fuel cells (MFCs) for the first time. The Fe-N/C shows similar power generation ability to commercial Pt/C, and its electron transfer number is 3.57, indicating that the ORR process primarily occurs with 4-electron. Fe species, pyridinic-N, graphitic-N, and oxygen-containing groups existing in GNR@CNT frameworks are likely to endow the electrocatalysts with good ORR performance, suggesting that a GNR@CNT-based carbon supporter would be a good candidate for the non-precious metal catalyst to replace Pt-based precious metal.

Highly active electrocatalysts of iron phthalocyanine by MOFs for oxygen reduction reaction under alkaline solution

Metal-nitrogen-carbon materials (Fe-N/C) have been extensively studied as one of the most excellent electrocatalysts with good catalytic activities and cheap price towards the oxygen reduction reaction (ORR). The rational design of metal-organic framework (MOF) derived carbon materials with rapid mass transport ability and good stability is a great challenge to achieve. Herein, intensive research of Fe-N/C catalysts prepared from assembling MOFs with cheap iron phthalocyanine (FePc) for the ORR is innovatively carried out. A series of Fe-N/C nano-architectures are simply synthesized by a convenient assembling method under different temperatures (800 to 1000 °C). The assembly method at high temperatures tunes the number of FeN x active sites and intensifies the exposure of interior active sites. The highly dispersing Fe20-N/C electrocatalyst treated at 900 °C exhibits remarkable stability and excellent ORR activities with a half-wave potential of 0.866 V (vs. RHE) in alkaline solution, which is higher than that of commercial Pt/C (0.838 V vs. RHE) under the same test conditions. X-ray photoelectron spectroscopy results illustrate that incorporated MOFs interact with the active centre of FePc, tend to enhance the electron transition and to promote the kinetics of the ORR. Overall, highly dispersed Fe-N/C MOF-based materials are excellent non-precious metal electrocatalysts for energy and environmental applications.

A 3D nitrogen-doped graphene aerogel for enhanced visible-light photocatalytic pollutant degradation and hydrogen evolution

The synergistic effect of the 3D structure and N-doping explain the high surface area of 536 m² g⁻¹ and excellent photocatalytic activity.

A Lactate/Oxygen Biofuel Cell: The Coupled Lactate Oxidase Anode and PGM-Free Fe-N-C Cathode

The rapid development of both wearable and implantable biofuel cells has triggered more and more attention on the lactate biofuel cell. The novel lactate/oxygen biofuel cell (L/O-BFC) with the direct electron transfer (DET)-type lactate oxidase (LOx) anode and the platinum group metal (PGM)-free Fe-N-C cathode is designed and constructed in this paper. In such a reasonable design, the surface-controlled direct two-electron electrochemical reaction of the lactate oxidase was determined by cyclic voltammetry (CV) on the carbon nanotube (CNT) modified electrode with favorable high electrochemical active surface area and electronic conductivity. Additionally, the biosensor based on DET-type LOx modified electrode impressively presented linear response to lactate with different concentrations from 0.000 mM to 12.300 mM. In particular, the apparent Michealis-constant (K_M^app) calculated as 0.140 mM clearly indicates that LOx on CNT has strong affinity to the substrate lactate. Meanwhile, 4e^- transfer oxygen reduction reaction (ORR) was proven to take place on the Fe-N-C catalysts inthe 0.1 M PBS system, indicating the advantage by using the Fe-N-C catalysts at the cathode of L/O-BFC. Last but not least, the L/O-BFC with the direct electron transfer (DET)-type lactate oxidase(LOx) anode and the Fe-N-C cathode produced an superior open circuit potential (OCP) of 0.264 V and a maximum output power density (OPD) of 24.430 μW cm^-2 in O₂ saturated 95.020 mM lactate solution. The above results will not only bring about significant interest in developing a DET-type biofuel cell, but also offer guiding direction to explore novel catalyst materials for the biofuel cell. This work enriches the research content and may push developments of the implantable and wearable biofuel cell forward.

Ultrafine Fe3C nanoparticles embedded in N-doped graphitic carbon sheets for simultaneous determination of ascorbic acid, dopamine, uric acid and xanthine

A pyrolytic method is described for preparation of ultrafine Fe₃C nanoparticles incorporated into N-doped graphitic carbon nanosheets (Fe₃C@NGCSs). Iron phthalocyanine and graphitic carbon nitride (g-C₃N₄) are used as starting materials. The hybrid nanocomposite was placed on a glassy carbon electrode (GCE) and then applied to simultaneous determination of ascorbic acid (AA), dopamine (DA), uric acid (UA) and xanthine (XA). Figures of merits are as follows: for AA, the linear response range covers the 54.0-5491.0 μM range, the lower detection limit is 16.7 μM, and the best working voltage (vs. the saturated calomel electrode (SCE)) is 0.05 V. The respective data for DA are 1.2-120.8 μM, 0.34 μM and 0.19 V (vs. SCE). For UA, the respective data are 4.8-263.0 μM, 1.4 μM and 0.32 V (vs. SCE), and for XA the data are 4.8-361.0 μM, 1.5 μM and 0.71 V (vs. SCE). The method was successfully applied to their simultaneous determination in spiked serum samples. Graphical abstract Ultrafine Fe₃C nanoparticles embedded in N-doped graphitic carbon sheets for simultaneous determination of ascorbic acid, dopamine, uric acid and xanthine.

Metal and Nonmetal Codoped 3D Nanoporous Graphene for Efficient Bifunctional Electrocatalysis and Rechargeable Zn-Air Batteries

Developing bifunctional electrocatalysts with high activities and long durability for the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) is crucial toward the practical implementation of rechargeable metal-air batteries. Here, a 3D nanoporous graphene (np-graphene) doped with both N and Ni single atoms/clusters is reported. The predoping of N by chemical vapor deposition (CVD) dramatically increases the Ni doping amount and stability. The resulting N and Ni codoped np-graphene has excellent electrocatalytic activities for both the ORR and the OER in alkaline aqueous solutions. The synergetic effects of N and Ni dopants are revealed by density functional theory calculations. The free-standing Ni,N codoped 3D np-graphene shows great potential as an economical catalyst/electrode for metal-air batteries.

Balancing the Micro-Mesoporosity for Activity Maximization of N-Doped Carbonaceous Electrocatalysts for the Oxygen Reduction Reaction

Carbonaceous porous structures have instigated global research interest as promising low-cost electrocatalysts for numerous energy technologies. However, the rational design principle of pore structures for activity maximization is still unclear. In this work, a series of N-doped carbon (N-C) catalysts with exclusively different micro-mesoporosity are investigated for the oxygen reduction reaction (ORR). By combining the experiment results and a pioneering mathematical model, it was observed that the best catalytic activity can only be attained by balancing the micro-mesoporosity. These findings offer a definite criterion for pore structure optimization in carbon-based ORR catalysts, which is of great importance for various energy technologies.

Recent Advance on Polyaniline or Polypyrrole-Derived Electrocatalysts for Oxygen Reduction Reaction

The fuel cell, as one of the most promising electrochemical devices, is sustainable, clean, and environmentally benign. The sluggish oxygen reduction reaction (ORR) is an important fuel cell cathodic reaction that decides the efficiency of the overall energy conversion. In order to improve ORR efficiency, many efficient catalysts have been developed, in which the N-doped material is most popular. Polyaniline and polypyrrole as common aromatic polymers containing nitrogen were widely applied in the N-doped material. The shape-controlled N-doped carbon material can be prepared from the pyrolysis of the polyaniline or polypyrrole, which is effective to catalyze the ORR. This review is focused on the recent advance of polyaniline or polypyrrole-based ORR electrocatalysts.

Hierarchically porous Fe–N–C nanospindles derived from a porphyrinic coordination network for oxygen reduction reaction

NaCl-assisted pyrolysis of an Fe porphyrinic coordination network combined with concentrated-sulfuric-acid post-activation results in a hierarchically porous Fe–N–C nanospindle catalyst.

Reduced graphene oxide intercalated ZnS nanoparticles as an efficient and durable electrocatalyst for the oxygen reduction reaction

The oxygen reduction reaction (ORR) is a key reaction for fuel cells and metal–air batteries.