Fe–N–Si tri-doped carbon nanofibers for efficient oxygen reduction reaction in alkaline and acidic media

The most ideal substitute for Pt/C to catalyze the oxygen reduction reaction (ORR) is the transition metal and nitrogen co-doped carbon-based material (TM-N-C). However, large particles with low catalytic activity are formed easily for the transition metals during high-temperature carbonization. Herein, PAN nanofibers uniformly distributed with FeCl₃ were coated with SiO₂ and then carbonized to obtain Fe–N–Si tri-doped carbon nanofibers catalyst (Fe–N–Si-CNFs). The SiO₂ can further anchor the Fe atoms, thus preventing agglomeration during the carbonization process. Meanwhile, Si atoms have been doped in CNFs during this process, which is conducive to the further improvement of catalytic performance. The Fe–N–Si-CNFs catalyst has a 3D network structure and a large specific surface area (809.3 m² g⁻¹), which contributes to catalyzing the ORR. In alkaline media, Fe–N–Si-CNFs exhibits superior catalytic performance (E_1/2 = 0.86 V vs. RHE) and higher stability (9.6% activity attenuation after 20000s) than Pt/C catalyst (20 wt%).     

Highly efficient PtCo nanoparticles on Co–N–C nanorods with hierarchical pore structure for oxygen reduction reaction

Developing platinum-based nanoparticles on carbon catalysts with high activity and stability for oxygen reduction reaction (ORR) is of great significance for the practical application of fuel cells. Herein, a synchronous strategy of preparing nano-sized PtCo supported on atomic Co and N co-doped carbon nanorods (PtCo/Co–N–C NR) was developed to replace the conventional method of impregnating Pt sources into ready-made carbon materials, in which metal-organic frameworks (MOFs) with Co and Zn ions of rhombic dodecahedron were first prepared using 2-methylimidazole as building block and then their morphology was transformed into porous nanorods via the reduction of Co ions to Co–B–O complex in the MOFs by NaBH₄; subsequently, Pt was deposited on the Co–Zn MOF nanorods through the displacement reaction of PtCl₆^2- and metallic Co and coordination between MOF and PtCl₆^2-; after pyrolysis and acid-leaching process, highly dispersed PtCo/Co–N–C NR was obtained. Attributed to its unique characteristics of hierarchical pore structure, uniform PtCo alloy nanoparticles with the average size of 7.0 nm and strong supporting interaction effect, the catalyst exhibits high ORR activity and stability with the mass activity of 577.0 mA mg^?1_Pt and specific activity of 1.4 mA cm^?2 at 0.9 V vs RHE in 0.1 M HClO₄, which is about 3.6 times and 3.5 times high than that of commercial Pt/C catalyst respectively. This strategy would provide a flexible route to develop highly active and stable ORR electrocatalysts with various morphologies for optimizing the exposure of active sites.     

The synthesis of Fe/N–C@CNFs and its electrochemical performance toward oxygen reduction reaction

The one-dimensional filamentous carbon nanofibers hold great promise to substitute noble catalysts ascribing to the excellent physicochemical properties, environmentally friendly, easy to prepare, etc. Synthesizing non-noble catalysts with outstanding electro-catalytic activity for oxygen reduction reaction and excellent durability and application in the field of commercialization still exist lots of challenges. Herein, we report a facile synthesis of carbon nanofibers coated with iron doping nitrogen-carbon (Fe/N–C@CNFs) derived from carbon nanofibers coated with polyaniline (PANI@CNFs) via chemical vapor deposition and heat-treatment, which exhibit an outstanding catalytic activity toward oxygen reduction reaction. In detail, the Fe/N–C@CNFs exhibit onset potential of 0.99 V (vs RHE) and half-wave potential of 0.80 V (vs RHE) in 0.1 M KOH solution, indicating superior electrochemical activity for oxygen reduction reaction (ORR). Meantime, the transferred electron number of oxygen reduction reaction was 3.77, suggesting a nearly 4e⁻ transferring process with little intermediate product (H₂O₂). Moreover, the relative current value of carbon nanofibers coated with nitrogen-carbon film (N–C@CNFs) and Fe/N–C@CNFs maintain 89.6% and 88.2% respectively after 40,000 s, exhibiting good stability and durability. This facile and easy method could provide inspiration for synthesizing carbon nanofibers-based (CNFs-based) oxygen reduction reaction catalysts with excellent catalytic activity and good stability.     

A metal-organic framework-derived Fe–N–C electrocatalyst with highly dispersed Fe–Nx towards oxygen reduction reaction

Metal and nitrogen co-doped catalysts have been promising alternatives to platinum group metal (PGM) catalysts for oxygen reduction reaction (ORR) over the past few decades. Herein, we have synthesized an efficient Fe–N–C catalyst by the co-calcination of NH₂-MIL-101@PDA and melamine. The best Fe–N–C shows a positive half-wave potential of 0.844 V, which is 14 mV higher than that of Pt/C catalyst, as well as superior methanol resistance and long-term durability in alkaline electrolyte. In addition, Fe–N–C also exhibits pronounced catalytic activity and a direct 4e⁻ reaction pathway in acid electrolyte. We ascribed the excellent ORR performance of Fe–N–C to its crumpled structure, large specific surface area (584.6 m² g⁻¹) and high content of Fe-Nx sites (1.22 at. %). This study provides a simple way for the fabrication of excellent PGM-free ORR catalysts.     

Effect of synthesis method on the oxygen reduction performance of Co–N–C catalyst

In recent years, Co, N co-doped carbon (Co–N–C) materials as oxygen reduction reaction (ORR) catalysts have attracted great attention because of their good ORR stability as well as decent activity. Co-doped zeolitic imidazolate framework-8 (Co@ZIF-8) as the precursor for synthesizing Co–N–C has attracted great interest recently. Co@ZIF-8 synthesis method may affect the properties of the as-synthesized Co@ZIF-8 precursors, which will surely affect the properties and ORR performance of Co@ZIF-8-derived Co–N–C catalysts. Herein, three methods, viz. room-temperature stirring method, reflux method, and hydrothermal method, were used to synthesize Co@ZIF-8 precursors. Physical characterization shows that the synthesis method has a great influence on the textural properties, composition, and graphitization degree of the as-synthesized Co–N–C catalysts. Electrochemical characterization shows that Co–N–C-R synthesized with reflux method exhibits an onset potential (E_onset) of 0.905 V, a half-wave potential (E_1/2) of 0.792 V and a limiting current density (J_L) of 5.85 mA cm^?2 in acidic media, which are higher than those of Co–N–C–S (E_onset = 0.870 V, E_1/2 = 0.770 V, J_L = 4.71 mA cm^?2) and Co–N–C–H (E_onset = 0.892 V, E_1/2 = 0.785 V, J_L = 4.68 mA cm^?2) synthesized with room-temperature stirring method and hydrothermal method, respectively. The better ORR activity observed on Co–N–C-R can be attributed to its larger graphitization degree and larger mesopore volume. Catalytic stability test shows that Co–N–C-R has negligible activity loss after 5000 potential cycles. This work demonstrates that reflux method is a more suitable method for synthesizing Co–N–C catalysts for ORR.     

One-step prepared prussian blue/porous carbon composite derives highly efficient Fe–N–C catalyst for oxygen reduction

Preparation of high-efficiency oxygen reduction reaction (ORR) catalysts with abundant and inexpensive biomass materials have been a hot research topic. We use nitrogen-rich lentinus edodes and potassium ferrate (K₂FeO₄) to simultaneously activate the carbon material and prepare prussian blue (PB), and a porous carbon composite (PB/C) containing PB is synthesized. Finally, using ammonium chloride (NH₄Cl) as a nitrogen source to further synthesize a Fe–N–C catalyst (PB/CN₁T₈₀₀) containing a trace amount of Fe for ORR. Results show that the prepared PB/CN₁T₈₀₀ catalyst forms a coral-like structure, which mainly contains mesopores and possesses a large specific surface area of approximately 1582 m² g⁻¹. Moreover, the onset potential of PB/CN₁T₈₀₀ is 0.95 V, and the half-wave potential is 0.83 V, which are consistent with those of commercial Pt/C. Thus the PB/CN₁T₈₀₀ material is an ORR catalyst with excellent performance. This work provides a basis for simple and efficient conversion of rich biomass into PB/porous carbon composites to prepare highly efficient catalysts.     

The structure-activity relationship of Pd–Co/C electrocatalysts for oxygen reduction reaction

The Pd–Co/C alloy catalysts with an atomic ratio of 3:1 were deposited at various pH values and reduced at different temperatures for oxygen reduction reaction (ORR). The structure-activity relationship of the prepared catalysts has been elucidated. The pH values and reduction temperatures during the preparation process affect the deposition and reduction rates of Pd and Co ions significantly, and thus the degrees of alloying, surface species, and ORR activities of the Pd–Co/C catalysts are also influenced. Due to the enhancement of Co surface segregation and the formation of Co oxide on the surface, a deterioration of ORR activity for the catalysts reduced at high temperatures and high pH values is observed. The catalysts deposited at pH value of 9 and reduced at a very low temperature of 390 K have well-formed Pd–Co alloy structure, Pd-rich surface, and excellent ORR activity.     

Efficient electrocatalyst of Pt–Fe/CNFs for oxygen reduction reaction in alkaline media

Transition metal iron-based catalysts are promising electrocatalysts for oxygen reduction reaction (ORR), and they have the potential to replace noble metal catalysts. The one-dimensional of carbon nanofibers with tubular structure can effectively promote the electrocatalytic activity, which facilitates electron transport. Herein, the Pt–Fe/CNFs were synthesized by electrospinning and subsequent calcination. Benefiting from the advantages of one-dimensional structure, Pt–Fe/CNFs-900 with fast electrochemical kinetics and excellent stability for ORR with excellent onset of 0.99 V, a low Tafel slope of 62 mV dec⁻¹ and high limiting current density of 6.00 mA cm⁻². Long-term ORR testing indicated that the durability of this catalyst was superior to that of commercial Pt/C in alkaline electrolyte. According to RRDE test, the ORR reaction process of Pt–Fe/CNFs-900 was close to four-electron transfer routes.     

Exploration of unalloyed bimetallic Au–Pt/C nanoparticles for oxygen reduction reaction

The synthesis of carbon-supported unalloyed Au–Pt bimetallic nanoparticles using polyol method at a temperature as low as 85 °C is reported. Various compositions of Au–Pt/C bimetallic nanoparticles are characterized using transmission electron microscopy (TEM), X-ray florescence (XRF), X-ray diffraction and cyclic voltammetry. Electron microscopy shows that the particles have a near-narrow size distribution that peaks at an average size of ∼5 to 6 nm. The electrocatalytic activity of Au–Pt/C nanoparticles towards the oxygen reduction reaction (ORR) is studied by linear sweep polarization measurements obtained using a rotating disc electrode (RDE). The results reveal that a four-electron transfer pathway is mainly operative for ORR and the half-wave potential for ORR on bimetallic Au–Pt/C (20%:20%) is ∼100 mV less negative when compared with that of Pt/C (home-made and E-Tek). Studies of the methanol oxidation reaction (MOR) on these catalysts show that the MOR activity is significantly lowered with increasing content of Au in Au–Pt/C.     

Eu2O3–Cu/NC nanocomposite catalyst with improved oxygen reduction reaction activity for Zn-air batteries

Rare earth oxide promoted transition metal composite catalyst Eu₂O₃–Cu/NC with outstanding oxygen reduction reaction (ORR) performance, is constructed by hydrothermal and subsequent high-temperature calcination, considering replacing Pt/C. This synthesis method yields Eu₂O₃–Cu nanoparticles with uniform distribution, improved oxygen vacancies and increased content of N-doping. And the strong synergistic effect was created between promoter Eu₂O₃ and chief Cu. In addition, the accommodate adsorption and transfer of O species endow Eu₂O₃–Cu/NC the improved ORR activity than Eu₂O₃/NC and Cu/NC. Meanwhile, the stability of Eu₂O₃–Cu/NC is also strengthened compared to Cu/NC on account of the interaction of active sites, and the H₂O₂ yield of Eu₂O₃–Cu/NC is very low. For practical application, a rechargeable Zn-air battery with an air cathode of Eu₂O₃–Cu/NC displays a larger power density, excellent charge-discharge cycle stability and good rate capability. The designed composite shows potential application prospects in the fields of energy conversion.     

ZIF-8 derived bimetallic Fe–Ni-Nanoporous carbon for enhanced oxygen reduction reaction

The development of highly efficient nοn-nοble meta? catalysts for (ОRR) in PEMFCs is at the heart of the research, yet their performance is not satisfactory. The Fe–N active sites enclosed in carbon matrix are generally agreed to be the most promising active sites for ORR. In view of this, further effort is made to increase the Fe–N active sites. Here we present the fabrication of novel FeNi bimetallic electrоcatalyst obtained from ZIF, which consists of FeNi nanоallоys incorporated in N-doped carbon (FeNi-NC) featuring carbon nanotubes and porous carbon demonstrates outstanding results for ОRR. The Fe–N and Ni–N active sites synergistically enhance the ORR activity of FeNi-NC catalyst. The FeNi-NC showed remarkable performance in KОH with the half-wave and onset potential of 0.89 V and 0.99 V vs RHE, respectively. This catalyst shows exceptional stability in methanol equivalent to Pt/C commercial. The FeNi-NC retained 71%, while Pt/C commercial retained only 59% of its original current density. The exceptional stability and activity might be associated with the interplay among FeNi active sites and N-doped carbon, the distinct nanо-structure made up of porous carbon and carbon nanotubes with a high graphitization degree.     

PtFe and Fe3C nanoparticles encapsulated in Fe–N-doped carbon bowl toward the oxygen reduction reaction

The high-temperature calcination strategy facilitates the formation of alloy atoms but inevitably results in the aggregation and deactivation of the metal particles for the oxygen reduction reaction (ORR) electrocatalysts. Herein, we report the successful encapsulation of Platinum–Iron (PtFe) nanoparticles (∼4.7 nm) in the N-doped hollow carbon hemisphere matrix (NCB) containing Fe–N and Fe₃C without employing high-temperature pyrolysis, which effectively facilitates the well-dispersed Pt nanoparticles and the formation of PtFe nanoalloys. The hollow carbon hemisphere structure contributes to the expansion of the specific surface area and exposure of active sites of the catalyst, meanwhile, the modification of the surface of the carbon nano-bowl from a predominantly Fe to a functional electrocatalyst with a primarily PtFe alloy can boost the ORR catalytic activity and stability. It is found that the Pt3Fe/Fe₃C-NCB catalyst exhibits the optimum ORR performance with a mass activity (0.97 A mg⁻¹_Pt), 5.10 times higher than the commercial Pt/C (0.19 A mg⁻¹_Pt). Pt3Fe/Fe₃C-NCB also displays excellent durability in comparison to the commercial Pt/C after 20,000 potential cycles. Combined with the Physical characterization and the electrochemical test results, Fe₃C-NCB plays a strong metal-support role for the encapsulated PtFe nanoparticles structure, thereby preventing nanoparticle migration and corrosion. Experimental characterization and theoretical calculations show that the appropriate PtFe alloy composition and the strain effect induced by Fe–N/Fe₃C active sites are sufficient to accelerate the detachment of oxygenated species from the alloy surface, resulting in a catalyst with excellent ORR performance.     

Mechanisms insight into oxygen reduction reaction on sulfur-doped Fe–N2 graphene electrocatalysts

The density functional theory (DFT) calculations were performed to investigate the stability of the S-doped Fe–N₂G electrocatalysts, as well as ORR mechanism and activity. The most stable configuration is the Fe–N₂S1G because of forming a strong bond structure of Fe–S. In addition, the structures of Fe–N₂S3G and Fe–N₂S4G also exhibit the higher stability compared to the undoped Fe–N₂G. According to the distinct charge difference on the surface, the O-contained intermediates would like to adsorb on the active sites of Fe–N₂ complex active sites. The binding strength of OH on these different catalysts follows the increasing order of Fe–N₂S4G < Fe–N₂S3G < FeN₂G < Fe–N₂S1G < Fe–N₂S5G < Fe–N₂S2G < Fe–N₂S6G < Fe–N₂S7G, implying the opposite order of the catalytic activity. The calculations of the free energy diagrams show that all elementary reaction steps on Fe–N₂S4G, Fe–N₂S3G, FeN₂G and Fe–N₂S1G are downhill. Besides, the rate determining step (RDS) for these catalysts (excluded Fe–N₂S4G) is the fourth reduction step (OH*+H⁺+e⁻→H₂O+*). The study of the reaction mechanisms predicted that the direct 4-electron reduction process is the favorable ORR pathway, and the alternative reaction pathways containing the formation of OH* + OH* co-adsorbate also process without the formation of H₂O₂ for these catalysts. Particularly, Fe–N₂S4G also exhibits the outstanding performance for H₂O₂ reduction. In general, since the higher stability and working potential for ORR, Fe–N₂S4G is predicted to be the prior candidate site for ORR among S-doped FeN₂G catalysts.     

MXene boosted metal-organic framework-derived Fe–N–C as an efficient electrocatalyst for oxygen reduction reactions

Iron-nitrogen-carbon (Fe–N–C) electrocatalysts offer great promise to replace their noble metal-based counterparts for oxygen reduction reactions (ORR). However, the practical applications of this type of catalyst are hindered by insufficient accessible active sies, low electrical conductivity, and poor durability. Here, we report a Ti₃C₂ MXene supported metal-organic framework (MOF)-derived Fe–N–C (Fe-N_x/N/Ti₃C₂) catalyst to simultaneously address the issues. Owing to the negatively charged characteristics, NH₂-MIL-53(Fe) is firmly anchored on Ti₃C₂ MXene, which not only serves as a conductive substrate to alleviate the collapse and agglomeration of MOFs during the pyrolysis, but also modulates the electronic properties of active FeN_x sites to improve the electrocatalytic activity and stability. As a result, the as-prepared Fe-N_x/N/Ti₃C₂ catalyst exhibits superb ORR activity and long-term stability in both alkaline and acidic electrolytes.     

Effect of Pt–Pd/C coupled catalyst loading and polybenzimidazole ionomer binder on oxygen reduction reaction in high-temperature PEMFC

Polybenzimidazole (PBI) was studied as an ionomer binder at varying ratios (1–7) in a 20–40 wt% Pt–Pd/C cathode-coupled catalyst layer for the oxygen reduction reaction (ORR) in a high-temperature proton exchange membrane fuel cell (HT-PEMFC). Catalytic activity was examined by CV and LSV, while the properties of the catalysts were characterized by FESEM-EDX, N₂ adsorption–desorption, XRD and FTIR. The results showed that the distribution of metals on the carbon surface, carbon wall thickness and the interaction between ionomer and coupled catalysts affected the ORR performance. The fabricated membrane electrode assembly with 5:95 PBI: 30 wt% Pt–Pd/C catalyst ratio exhibited the best performance and highest durability for HT-PEMFC at 170 °C, yielding a power density of 1.30 Wcm⁻² with 0.02 mg_Pt/cm Pt loading. This performance of ultra-low metal loading of coupled Pt–Pd/C electrocatalyst with PBI binder was comparable to those reported by other studies, highlighting a promising catalyst for fuel cell application.     

Effect of pretreatment on Pt–Co/C cathode catalysts for the oxygen reduction reaction

Carbon supported Pt and Pt–Co electrocatalysts for the oxygen reduction reaction in low temperature fuel cells were prepared by the reduction of the metal salts with sodium borohydride and sodium formate. The effect of surface treatment with nitric acid on the carbon surface and Co on the surface of carbon prior to the deposition of Pt was studied. The catalysts where Pt was deposited on treated carbon the ORR reaction preceded more through the two electron pathway and favored peroxide production, while the fresh carbon catalysts proceeded more through the four electron pathway to complete the oxygen reduction reaction. NaCOOH reduced Pt/C catalysts showed higher activity that NaBH₄ reduced Pt/C catalysts. It was determined that the Co addition has a higher impact on catalyst activity and active surface area when used with NaBH₄ as reducing agent as compared to NaCOOH.     

Tuning of electrocatalytic activity of Mn–O–Co composite for alkaline hydrogen evolution reaction

We report the enhancement in electrocatalytic activity of Mn–O–Co composite electrode developed through chemical reduction method. The Mn–O–Co composite electrode exhibits high catalytic activity with a low Tafel slope of 123 mV dec⁻¹ and a low overpotential of 117 mV at a current density of 10 mA cm⁻². The enhancement in electrocatalytic activity of Mn–O–Co composite electrode is due to the synergistic activity of MnO and CoO with the NiP matrix. The intermetallic interaction among the half-filled orbitals of manganese with the fully occupied orbitals of cobalt and nickel leads to an effective electron delocalization in the catalytic system which enhances the HER performance of the coating. The C_dl value of the composite electrode is in the order of 254 μF, which is approximately ten fold higher than the bare NiP coating, due to the enhancement in interaction between the Mn–O–Co composite electrode and the reactive species in the HER medium. The Mn–O–Co composite electrode shows promising characteristics as an electrocatalyst with long term stability and remarkable competency with the commercially available electrodes.     

Effect of surface composition of Pt–Fe nanoparticles for oxygen reduction reactions

The aim of this work is primarily to rationalize the effect of surface composition on electrocatalytic activity. To investigate this point, we compared two types of nanoparticles with a different surface composition, namely Fe-rich and Pt–Fe mixed surfaces. We synthesized highly dispersed carbon-supported Pt₁Fe_x (x = 1, 2, and 3) nanoparticles with the Fe-rich surface (∼2 nm), through a preferential interaction of a capping agent and the metal, i.e., Fe-OOC. The electronic structure and electrocatalytic properties of Pt₁Fe_x nanoparticles with the Fe-rich surface were found to be virtually independent from the Pt/Fe ratio. In contrast, nanoparticles with the Pt–Fe mixed surface, prepared by utilizing the difference of segregation energy, showed a clear dependence of the electronic and electrochemical characteristics on the amount of Pt and Fe, possibly because of the interaction between these two metals on the surface of the electrocatalysts. Compared to Pt, the Pt₁Fe₂ nanoparticles with the Pt–Fe mixed surface showed the highest enhancement in the activity of the oxygen reduction reaction. This resulted from the development of a more electrochemically stable structure of the Pt–Fe mixed surface. This study demonstrated that the electrocatalytic properties of the Pt–Fe nanoparticles can be tuned using the surface composition rather than the bulk composition.     

Fe,N codoped porous carbon nanosheets for efficient oxygen reduction reaction in alkaline and acidic media

Electrospinning typically employed to fabricate nanofibers was first used to prepare Fe and N doped porous carbon nanosheets (Fe–N/CNs) as oxygen reduction reaction (ORR) electrocatalysts. Polyacrylonitrile nanofibers containing a small amount of ferrocenes (Fer-PAN) were produced by electrospinning. When Fer-PAN was preoxidized at 300 °C in the air (Fer-PAN-300), nanosheets were formed and occupied the interspace between nanofibers. Fe–N/CNs was finally obtained using carbonized Fer-PAN-300 at 900 °C in N₂. The Fe–N/CNs incorporated the advantages of carbon nanofiber webs and porous nanocarbon materials, inclusive of comparatively high conductivity and large specific surface area. In both alkaline and acidic electrolyte, the Fe–N/CNs took on similar even better ORR catalytic activity than other catalysts reported elsewhere, and better stability than those of commercial Pt/C.     

A strategy for easy synthesis of carbon supported Co@Pt core–shell configuration as highly active catalyst for oxygen reduction reaction

An oxygen-mediated galvanic battery reaction strategy has been developed to one-step synthesize carbon-supported Co@Pt core–shell nanostructures. Relying on this strategy, a structural evolution of 3-D Pt-on-Co bimetallic nanodendrites into Co@Pt core–shell configuration is readily achieved in our study. These well-supported and low-Pt-content nanostructures show superior electrocatalytic activities to oxygen reduction reaction. Especially, the supported Co@Pt core–shell electrocatalyst for oxygen reduction reaction shows a high activity with the maximal Pt-mass activity of 465 mA mg⁻¹ Pt at 0.9 V (vs. RHE). The present investigation clearly demonstrates that the design and synthesis of the core–shell nanostructures is a viable route for building Pt-based electrocatalysts with optimized utilization efficiency and higher cost performance.