Three-dimensional nitrogen-doped carbon nanotubes/carbon nanofragments complexes for efficient metal-free electrocatalyst towards oxygen reduction reaction

Heteroatom-doped carbon materials as one of the most promising oxygen reduction reaction (ORR) catalysts have attracted much attention. Rational design and exploration of suitable heteroatom-doped carbon materials greatly affects their ORR performance. Herein, we successfully prepared nitrogen-doped carbon nanotubes/carbon nanofragments (NCNT/CNF) complexes by a pyrolysis process using oxidized open-ended carbon nanotubes (OCNT)/oxidized carbon nanofragments (OCNF) hybrids as carbon precursors. The effect of carbon precursors on the synthesis of the corresponding nitrogen-doped carbon products was systematically investigated. The result showed the OCNT retained good conductivity, while the OCNF offered adequate structure defects for efficient post-doping. Benefiting from the co-merits of sole constitute, the obtained NCNT/CNF_1-15 (1–15 refers to the mass ratio) complexes possessed a typical three-dimensional architecture and much increased specific surface area, which facilitated reactant/electrolyte infiltration and ion/electron transfer. More importantly, they built the most optimized balance on ORR catalytic sites and conductivity. Thus, the NCNT/CNF_1-15 complexes showed much enhanced ORR performance. Clearly, our work provides a good guidance on the design of advanced heteroatom-doped carbon-based ORR catalysts.     

Nitrogen-doped carbon coated ZrO2 as a support for Pt nanoparticles in the oxygen reduction reaction

A new nitrogen-doped carbon (CN_x) support for Pt electrocatalysts was prepared by carbonizing polypyrrole on the surface of ZrO₂ (ZrO₂@CN_x) at high temperature. Well-dispersed Pt nanoparticles were easily formed on the ZrO₂@CN_x. The electrocatalyst was characterized by FT-IR, XRD, TEM, XPS. The electrochemical performances indicate that the presence of ZrO₂ modified the electro-structure of Pt on the catalyst surface and that ZrO₂@CN_x had superior oxygen reduction activity compared to a nitrogen-doped carbon coated carbon (C@CN_x).     

Highly stable and effective Pt/carbon nitride (CNx) modified SiO2 electrocatalyst for oxygen reduction reaction

We report a durable and active electrocatalyst, Pt/carbon nitride (CN_x) modified silicon dioxide (SiO₂) composite (donated as CN_x/SiO₂), for oxygen reduction reaction (ORR). CN_x/SiO₂ composite is synthesized by calcination of polypyrrole coated SiO₂ (Ppy/SiO₂) at 800 °C. The structure and composition are assessed using Fourier transform infra-red spectroscopy, X-ray diffraction, transmission electron microscope and energy dispersive spectroscopy. Voltammetry is used to study the activities of Pt immobilized on Vulcan XC-72R and CN_x/SiO₂, respectively. The electrochemical data indicate that Pt supported on CN_x/SiO₂ possesses higher electrocatalytic activity and durability for ORR compared with those of Vulcan XC-72R. All these demonstrate that CN_x/SiO₂ is a promising ORR electrocatalyst support for low temperature fuel cells.     

Hybrid dual-template induced nitrogen-doped hierarchically porous carbon as highly efficient oxygen reduction electrocatalyst

Fuel cell, as a promising future energy device, is one of the potential electric generators maintaining the sustainable development of our society and alleviating the major problems related to the energy shortage and environment pollution. However, the high cost and the limited resource of Pt remain the key obstacles for the commercialization of fuel cells. A hybrid dual-template strategy is developed to synthesize the nitrogen-doped ordered hierarchically porous carbon (NHMC) via a surfactant-templating organic resol self-assembly with F127 as soft template and SiO₂ nanosphere as the hard-template. The NHMC catalyst presents three-dimensional hierarchically porous structure, composed of small ordered mesopore (∼3.8 nm), large 3-D interconnected mesopore (∼12 nm) as well as micropore. The catalyst exhibits tailored pore structures and a well-tuned surface chemical environment. It displays high onset potential of 0.91 V and half-wave potential of 0.76 V as well as high limiting current via four-electron pathway for oxygen reduction reaction (ORR). The NHMC also shows high stability and excellent methanol tolerance. This work brings inspiration for the synthesis of low-cos Pt-free catalyst with high activity, stability as well as high methanol tolerance.     

Atomically dispersed Co atoms in nitrogen-doped carbon aerogel for efficient and durable oxygen reduction reaction

Developing non-noble-metal-based electrocatalysts as alternatives to replace Pt-based catalysts for oxygen reduction reaction (ORR) is crucial for large scale industrial application of fuel cells. Herein, we report a facile method to synthesize atomically dispersed Co atoms anchored on nitrogen-doped carbon aerogels with a 3D hierarchically porous network structure via F127-assisted pyrolysis of a phenolic resin/Co²⁺ composite and subsequent HCl etching treatment. HRTEM, AC-STEM, XRD, XPS, and Raman spectroscopy measurements demonstrate that Co atoms are homogeneously atomically dispersed on nitrogen-doped carbon aerogels within the porous structure by coordination with pyridinic-N. Among a series of samples, the Co-NCA@F127-1: 0.56 catalyst exhibits an enhanced ORR activity with onset potential (E_onset) of 0.935 V vs. RHE, the high diffusion limiting current density of 5.96 mA cm⁻² at 0.45 V, as well as an excellent resistance to methanol poisoning and good long-term stability in alkaline medium, comparable to the state-of-the-art Pt/C catalyst. This work may provide a novel and ingenious thought in the design and engineering of efficient and robust electrocatalysts based on single transition-metal atoms supported by nitrogen-doped carbon materials.     

Nanosized-Fe3PtN supported on nitrogen-doped carbon as electro-catalyst for oxygen reduction reaction

Novel nano-crystalline Fe₃PtN supported on nitrogen-doped carbon materials are synthesised via double pyrolysis, under Ar and NH₃ with two sets of temperatures namely, 800 and 900 °C. An improved catalytic activity has been observed in terms of higher values of onset and half-wave potentials, with larger kinetic currents and low hydrogen peroxide yields. The activity upon comparing with Pt/C with simultaneous experiments shows impressive results. Within the double annealed samples a comparative study has been done on the basis of active sites available for the oxygen reduction reactions. We have revealed the origin of its activity by intensively investigating the composition and the structure of the catalyst and their correlations with the electrochemical performance.     

Pt nanoparticles sputter-deposited on TiO2/MWCNT composites prepared by atomic layer deposition: Improved electrocatalytic activity towards the oxygen reduction reaction and durability in acid media

Acid-treated multi-walled carbon nanotubes (MWCNTs) were decorated with TiO₂ using the atomic layer deposition (ALD) technique followed by uniform distribution of platinum nanoparticles (PtNPs) through magnetron sputtering. Surface analyses were performed by scanning electron microscopy (SEM), scanning transmission electron microscopy (STEM) and X-ray photoelectron spectroscopy (XPS). Electrochemical decontamination and characterization of the Pt-TiO₂/MWCNT electrodes were carried out by CO stripping followed by cyclic voltammetry in acid media. The oxygen reduction reaction (ORR) was studied in O₂-saturated 0.05 M H₂SO₄ solution using the rotating disk electrode (RDE) method. Durability of the prepared catalysts was examined by repetitive potential cycling. Electrochemical data obtained was analyzed and compared to that of the commercial Pt/C catalyst. It was revealed that the Pt-TiO₂/MWCNT catalysts possess higher ORR activity and better durability as compared to that of the commercial Pt/C.     

MoO2 nanocrystals down to 5 nm as Pt electrocatalyst promoter for stable oxygen reduction reaction

The single molybdenum oxide (MoO₂) crystals down to 5 nm in diameter on carbon (denoted as C-MoO₂) are synthesized based on ion-exchange principle for the first time. The structures, morphologies, chemical and electrocatalytic performances of as-synthesized nanomaterials are characterized by physical, chemical and electrochemical methods. The results indicate that electrocatalysts made with Pt nanoparticles supporting on C-MoO₂ (denoted as Pt/C-MoO₂) are highly active and stable for oxygen reduction reaction (ORR) in fuel cells. A mass activity of 187.4 mA mg⁻¹_Pt at 0.9 V is obtained for ORR, which is much higher than that on commercial Pt/C (TKK) electrocatalyst (98.4 mA mg⁻¹_Pt). Furthermore, the electrochemical stability of Pt/C-MoO₂ is more excellent than that of Pt/C (TKK). The origin of the improvement in catalytic activity can be attributed to the synergistic or promotion effect of MoO₂ on Pt. The improvement in electrochemical stability is due to the strong interaction force between Pt and MoO₂.     

Silver cluster supported on nitrogen-doped graphene as an electrocatalyst with high activity and stability for oxygen reduction reaction

An Ag₈ cluster deposited on three different types of nitrogen (N)-doped graphene was studied using density functional theory calculations with empirical pair potentials (DFT-D). Among the different kinds of N-doped graphene, the pyridinic-N₃ (P-N₃) type can act as the best anchor position to stabilize Ag₈. In addition, it is found that supported Ag₈ clusters show higher activity in oxygen reduction reaction compared to unsupported clusters due to significant decrease in O₂ adsorption energy and higher charge transfer to O₂. Electron transfer from Ag to O₂ leads to the elongation of the OO bond, which facilitates the breaking of this bond in the oxygen reduction reaction. All results suggest that N-doped graphene support can play a significant role in the chemical reactivity of a Ag₈ cluster in oxygen reduction reaction.     

Synergy between isolated-Fe3O4 nanoparticles and CNx layers derived from lysine to improve the catalytic activity for oxygen reduction reaction

In order to seek heterogeneous electrocatalyst with efficient catalytic activity for oxygen reduction reaction (ORR), Fe₃O₄-CN_x composite reported in our previous work was studied as electrocatalyst for ORR and showed poor catalytic activity. To improve the catalytic activity, Fe₃O₄-CN_x composite is modified by the CN_x layers derived from lysine through pyrolysis. The physical characterization show that the coral-shaped morphology of the resultant composite (Fe₃O₄-CN_x-Lys) is still retained, while the degree of its graphitic crystalline increases. Besides, Fe₃O₄-CN_x-Lys has 364.7 m² g⁻¹ of surface area with hierarchical porous structure. Electrochemical tests show that the catalytic activity Fe₃O₄-CN_x-Lys for ORR is not only higher than those Fe₃O₄-CN_x, XC-72-Lys derived from lysine and XC-72 Vulcan carbon but also comparable to that of commercial Pt/C (20 wt%).     

Cobalt selenide electrocatalyst supported by nitrogen-doped carbon and its stable activity toward oxygen reduction reaction

We have prepared durable catalysts of CoSe₂/N-carbon using low-cost raw materials, measured their activities, peroxide yields, stabilities in reducing molecular oxygen, and characterized their crystalline phases and morphology. CoSe₂/N-carbon is featured with an active support, N-carbon, which by itself shows high stability as evidenced in its small activity decay. After 1000 CV cycles, the half-wave potential (E_1/2) of N-carbon decreases from 0.667 V to 0.636 V in 0.5 M H₂SO₄. Loading of CoSe₂ enhances the activity of N-carbon, when the samples were synthesized above 385 °C and formulated with the Se/Co ratio higher than 10. The higher activity is attributed to the pyrite phase of CoSe₂. But the stability of pyrite CoSe₂ is less than that of N-carbon. Corrosion during the stability test exposes the active sites of underlying N-carbon, which sustains the catalyst activity. Consequently the E_1/2 value of the active CoSe₂/N-carbon decreases moderately, from 0.711 V to 0.644 V after 1000 CV cycles. In contrast, the E_1/2 value of CoSe₂/C descends much more, from 0.681 V to 0.475 V.     

Cost-effective Co3O4 nanospheres on nitrogen-doped graphene used as highly efficient catalyst for oxygen reduction reaction

Developing low-cost, efficient and stable catalyst for the oxygen reduction reaction is meaningful and necessary for the industrialization of fuel cells. In this work, we report the controllable synthesis of Co₃O₄ nanospheres uniformly anchored on N-doped reduced graphene oxide (N-rGO) sheets with significant catalytic activity for the oxygen reduction reaction via a simple two-step approach. Results show that there is an interaction between Co₃O₄ nanospheres and N-rGO after riveting on N-rGO, which is beneficial to the electron transfer between them. The Co₃O₄ NS/N-rGO hybrid has excellent catalytic activity, comparable to commercial Pt/C catalyst. The transferred electron number of the oxygen reduction reaction on Co₃O₄ NS/N-rGO is around 3.95. The hybrid has more excellent durability and methanol resistance than commercial Pt/C. The Co₃O₄ NS/N-rGO catalyst in our work provides a cost-effective the oxygen reduction reaction catalyst alternative to the precious metal Pt in fuel cell.     

Astragali Radix-derived nitrogen-doped porous carbon: An efficient electrocatalyst for the oxygen reduction reaction

The development of biomass-derived nitrogen-doped porous carbons (NPCs) for the oxygen reduction reaction (ORR) is important for sustainable energy systems. Herein, NPCs derived from Astragali Radix (AR) via a cost-effective strategy are reported for the first time. The as-prepared AR-950-5 catalyst shows a stacked layer-like structure and porosity. Notably, the optimized AR-950-5 delivers catalytic activity comparable to that of commercial Pt/C (C-Pt/C), with high onset potential, positive half-wave potential and large limiting current density. It also displays superior long-term stability and methanol tolerance for ORR. This work will pave the way for a new approach in the development of highly active and low-cost NPCs for fuel cells.     

Pt nanodendrites anchored on bamboo-shaped carbon nanofiber arrays as highly efficient electrocatalyst for oxygen reduction reaction

In order to improve the Pt utilization and enhance their catalytic performance in fuel cells, a novel composite electrode composed of single-crystalline Pt nanodendrites and support constructed by bamboo-shaped carbon nanofiber arrays (CNFAs) on carbon paper, is reported. This electrode is designed by growing vertically CNFAs on carbon paper via plasma enhanced chemical vapor deposition, followed by the direct synthesis of Pt nanodendrites using a simple surfactant-free aqueous solution method. Electron microscopy studies reveal that the Pt nanodendrites are uniformly high dispersed and anchored on the surface of CNFAs. Electrochemical measurements demonstrate that the resultant electrode exhibits higher electrocatalytic activity and stability for oxygen reduction reaction than commercial Pt/C catalyst, suggesting its potential application in fuel cells.     

Nitrogen doped amorphous carbon as metal free electrocatalyst for oxygen reduction reaction

A non metal catalyst for the oxygen reduction reaction is prepared by simply pyrolyzing ion exchange resin D113 in NH₃. The product is nitrogen doped amorphous carbon. The pyrolysis of D113 exchanged with iron ion results in nitrogen doped graphitic carbon. The amorphous carbon is easier to be doped by NH₃ with higher nitrogen content. The nitrogen doped amorphous carbon is more active than graphitized carbon, together with much improved stability. The higher activity is explained by the higher total nitrogen content and higher pyridinic/graphitic nitrogen percentage. The higher stability is because there is no loss or dissolution of the active sites. The results of this work prove metal element and graphitization of carbon are not necessary factors for nitrogen doped carbon as non noble metal catalyst for the oxygen reduction reaction.     

Pore-rich iron-nitrogen-doped carbon nanofoam as an efficient catalyst towards the oxygen reduction reaction

Developing cost-effective electrocatalysts is crucial for oxygen reduction reactions. In this work, a highly active iron-nitrogen co-doped carbon nanofoam is proposed and developed via a facile salt-assisted pyrolysis process of chitooligosaccharides. i) Chitooligosaccharides, a nitrogen-rich polymer with high water solubility and high thermostability, induce a coordination reaction between iron ions and amino/hydroxyl groups, forming large active sites. ii) FeCl₃/KCl salt, acting as a template, etching agent and catalyst for graphitization, facilitates the formation of pore-rich carbon, leading to enhanced mass transfer and electronic conduction. It has been found that this iron-nitrogen co-doped carbon nanofoam presents competitive activity over commercial Pt/C, as follows: i) a high onset potential approximately 0.965 V, ii) strong durability with a current retention rate of 88% after 30000 s, and iii) excellent fuel tolerance to methanol, ethanol and formate. This work provides a simple, low-cost and environmentally friendly method to prepare iron-nitrogen co-doped carbon materials that has great potential to be extended to other catalysts for energy-related applications.     

Flame synthesis of nitrogen,boron co-doped carbon as efficient electrocatalyst for oxygen reduction reaction

The flame synthesis provides a simple low-cost method to produce novel carbon materials. In this study, N, B co-doped carbon (NBC) materials have been prepared by flame synthesis. Among many as-prepared samples, the NBC catalyst which prepared under carbonization temperature of 1000 °C for 3 h with acetonitrile/acetone precursor of 1:1 exhibits the best catalytic activity and stability, as well as good resistance to methanol interference for oxygen reduction reaction (ORR), with half-wave potential being almost nearly to Pt/C, and a quasi-four-electron transfer process. This study would provide an economic, environmental feasible and scalable approach for fabricating novel heteroatom co-doped carbon materials for ORR applications.     

Soybean milk derived carbon intercalated with reduced graphene oxide as high efficient electrocatalysts for oxygen reduction reaction

Exploring high-performance and low-cost metal-free oxygen reduction reaction (ORR) catalysts from biomass-derived materials is vital to the development of novel energy conversion devices such as fuel cells, etc. Herein, nitrogen-enriched soybean milk derived carbon (BDC/rGO-HT-NH₃) intercalated with reduced graphene oxide (rGO) electrocatalyst is prepared via one-pot hydrothermal synthesis method followed with nitridation by NH₃. The resultant catalyst with high surface area, good conductivity and high content of N (9.4 at.%) shows high electrocatalytic activity towards ORR in alkaline medium, which mainly happens through the direct 4-electron pathway. The onset potential of BDC/rGO-HT-NH₃ catalyzed ORR is 0.96 V vs RHE, which is only 0.11 V lower than that of the commercial Pt/C (20 wt%) catalyst. In addition, the BDC/rGO-HT-NH₃ catalyst shows superior long-term running durability. The desirable catalytic performances enable the facile synthesis approach of BDC/rGO-HT-NH₃ to be a promising methodology for transforming other biomass materials to N-enriched carbon based materials as low-cost and environmental friendly catalysts for ORR.     

Pyrolyzed CoN4-chelate as an electrocatalyst for oxygen reduction reaction in acid media

A novel platinum-free electrocatalyst CoTETA/C for oxygen reduction reaction (ORR) was prepared from pyrolysis of carbon-supported cobalt triethylenetetramine chelate under an inert atmosphere. X-ray diffraction (XRD) measurement showed that nanometallic face-centered cubic (fcc) crystalline α-Co phase embedded in graphitic carbon was present on the pore surface of this catalyst. Cyclic voltammogram experiment showed that the ORR peak potential appears at 710 mV (vs. NHE) in oxygen-saturated acidic media (0.5 M H₂SO₄). The Koutecky–Levich analysis indicated that the ORR follows the first-order kinetic reaction and the ORR proceeds via both the two-electron reduction and the four-electron reduction, while the latter is the main process. The actual performance of a single cell with the obtained CoTETA/C electrocatalyst was examined under a hydrogen-oxygen fuel cell system, and the maximal output power density reached 135 mW cm⁻² at 25 °C.     

Oxidized carbon/nano-SiC supported platinum nanoparticles as highly stable electrocatalyst for oxygen reduction reaction

Nano-SiC particles with derived carbon shells were prepared by an acid-etching method at room temperature. The mixture solutions of concentrated HF and HNO₃ were chosen to etch the nano-SiC particles, and an amorphous carbon shell absorbed by oxygen functional groups was formed on the SiC surface. The oxidized carbon/SiC (O-C/SiC) particles were used as supports for preparation of Pt electrocatalysts. The O-C/SiC supported Pt electrocatalysts showed a high catalytic activity and an excellent stability for oxygen reduction reaction. The improved stability can be ascribed to the anchoring effect of the carbon shell to Pt NPs and the high stability of nano-SiC core.