Synthesis of a N-doped mesoporous carbon as an efficient electrocatalyst for oxygen reduction

A facile and effective approach was developed for the preparation of mesoporous Fe-NC by pyrolyzing the mixture of FeCl₂, urea, (NH₄)₂MoO₇, phthalic anhydride and SBA-15, during which the in-situ formation of iron phthalocyanine is confirmed. The obtained catalyst exhibits high catalytic activity towards ORR, whose half-wave potential can be 53 mV more positive than that of commercial Pt/C catalyst. Besides, the catalyst also exhibits high selectivity of four electron path, along with excellent stability and methanol tolerance in alkaline media. Based on the characterization results, we suggest, the higher surface areas, highly porous structures induced by SBA-15 addition, as well as high graphitic N content should be the proper origins for its outstanding catalytic performance.     

S,N co-doped graphene quantum dots decorated TiO2 and supported with carbon for oxygen reduction reaction catalysis

Sluggish kinetics and catalyst instability in oxygen reduction reaction are the central issues in fuel cell and metal-air battery technologies. For that, highly active, stable, and low-cost non-platinum based electrocatalysts for oxygen reduction reaction are an immediate requirement in fuel cell and metal-air battery technologies. A new composite (S,N-GQD/TiO₂/C-800) is synthesized, made of sulfur (S) and nitrogen (N) co-doped graphene quantum dot (GQD) with TiO₂. This composite is supported on carbon on heating at 800 °C under N₂ atmosphere and is explored for oxygen reduction reaction (ORR) catalyst. The synthesized composite S,N-GQD/TiO₂/C-800, shows outstanding catalytic activity with an onset potential of 0.91 V and a half-wave potential of 0.82 V vs. RHE, an alkaline medium. The Tafel slope of the catalyst is 61 mV dec^?1. The catalyst is an excellent methanol tolerant and shows good stability in an alkaline medium. The excellent ORR activity of S,N-GQD/TiO₂/C-800 is ascribed to well-built interactivity between the S,N-GQD/TiO₂, and the carbon support. The unique structure offers advantages, with outstanding electrical conductivity, high surface area, and excellent charge transfer kinetics between the doped GQD and TiO₂ interface and subsequently from the carbon surface to the S,N-GQD/TiO_2.     

A dual ligand coordination strategy for synthesizing drum-like Co,N co-doped porous carbon electrocatalyst towards superior oxygen reduction and zinc-air batteries

We herein propose a dual ligand coordination strategy for deriving puissant non-noble metal electrocatalysts to substitute valuable platinum (Pt)-based materials toward oxygen reduction reaction (ORR), a key reaction in metal-air batteries and fuel cells. In brief, cobalt ions are firstly double-coordinated with proportionate 2-methylimidazole (2-MeIm) and benzimidazole (BIm) to obtain drum-like zeolitic imidazolate frameworks (D-ZIFs), which are then carbonized to output the final Co, N co-doped porous carbon (Co–N–PC_D) catalyst inheriting the drum-like morphology of D-ZIFs. The Co–N–PC_D is featured by mesopores and exhibits superb electrocatalytic behavior for ORR. Impressively, the half-wave potential of Co–N–PC_D catalysts is 0.886 V with finer methanol-tolerance and stability than those of commercial Pt/C. Additionally, a zinc-air battery assembled from the Co–N–PC_D displays an open-circuit voltage of 1.413 V, comparable to that of commercial Pt/C (1.455 V), suggesting the potentials of Co–N–PC_D in practical energy conversion devices.     

Synthesis of MoS2 nanoparticles embedded,N, S co-doped mesoporous carbon via molten salt method as hydrogen evolution electrocatalyst under alkaline and neutral conditions

We developed a salt-template strategy to prepare MoS₂ nanoparticles (NPs) embedded, N, S co-doped carbons via the solid-state process. The addition of the inorganic salt played two main roles in the synthetic proceeding. First, the salts could be utilized as the templates to produce the mesopores, which could be removed by simple washing process. Second, the salts could promote the formation of MoS₂ NPs. The as-received electrocatalyst, K-G4.0T2.0Mo1.0, possessed high BET surface area of 446 m² g^?1, in addition to high double layer capacitance of 24.5 mF cm^?2 in the alkaline media. When evaluated as the electrocatalyst for hydrogen evolution reaction (HER), K-G4.0T2.0Mo1.0 demonstrated excellent performance in the alkaline and neutral medias. In details, K-G4.0T2.0Mo1.0 showed a low overpotential of 173 and 358 mV to afford 10 mA cm^?2 under alkaline and neutral conditions, respectively, as well as outstanding durability.     

Facile synthesis of boron and nitrogen-doped graphene as efficient electrocatalyst for the oxygen reduction reaction in alkaline media

Boron-doped graphene and nitrogen-doped graphene have been respectively synthesized by a facile thermal solid-state reaction of graphene oxide with boric acid and urea. The morphology and structure of the doped graphene have been characterized by the scanning electron microscopy, infrared spectroscopy, ultraviolet visible spectroscopy and X-ray photoelectron spectroscopy, while the electrocatalytic activity toward oxygen reduction reaction has been evaluated by the cyclic voltammetry. It has been shown that the morphology, structure, doping level and fashions of graphene could be finely tuned by the thermal treatment conditions, and which have substantial effects on the activity of oxygen reduction reaction. The boron-doped graphene and nitrogen-doped graphene calcined at 700 °C demonstrate excellent electrocatalytic oxygen reduction activities as the appropriate introduction of boron and nitrogen functional groups in graphene, which might be promising for low temperature fuel cell applications.     

N,S co-doped NiCo2O4@CoMoO4/NF hierarchical heterostructure as an efficient bifunctional electrocatalyst for overall water splitting

Electrocatalytic overall water splitting technology has received considerable attention in recent years. The fabrication of low-cost, earth-rich and potent bifunctional electrocatalysts is vital for hydrogen evolution (HER) and oxygen evolution reactions (OER). Herein, the N and S co-doped NiCo₂O₄@CoMoO₄ heterostructures (N, S–NCO@CMO₄₀₀) are fabricated by CVD and hydrothermal methods. N and S atoms as auxiliary active centers can increase the activity of Ni, Co and Mo atoms at the same time. Hierarchical heterostructures generate more interfaces to accelerate mass transfer and enlarge the electrochemical surface area, which greatly enhances the catalytic activity. The catalyst displays outstanding OER performance. The overpotentials of OER and HER are 165 and 100 mV at a current density of 10 mA cm^?2, respectively. More importantly, the N, S–NCO@CMO₄₀₀-based water splitting cell has a low voltage of 1.46 V at 10 mA cm^?2. Furthermore, the N, S–NCO@CMO₄₀₀ runs for 120 h in stable operation. This work provides new ideas for the design of hierarchical heterostructures with two-element incorporation.     

Porous carbonized egg white as efficient electrocatalyst for oxygen reduction reaction

Hierarchical porous carbonized egg white (EW) is synthesized and used as oxygen reduction reaction (ORR) catalyst. The typical EW that is carbonized at 650 °C (EW-650) possesses ultrahigh specific surface area of 1904 m² g^?1, large average pore diameter of 9.8 nm, high contents of doped heteroatoms (N, O and S), fairly high graphitization degree and dense defects, corresponding to dense active sites and excellent transportation of both mass and electrons. Therefore, the EW-650 shows higher activity than commercial Pt/C for ORR in both alkaline and acidic media even at higher mass loadings, with excellent cyclic stability. The effects of reaction temperature and electrolyte concentration on ORR activity are also studied. It is found that appropriate temperature and electrolyte concentration speed up ORR kinetics, ensure higher oxygen solubility and favor mass transportation.     

Reduced graphene oxide-supported Pd@Au bimetallic nano electrocatalyst for enhanced oxygen reduction reaction in alkaline media

Rational design and synthesis of core-shell bimetallic nanoparticles with tailored structural and functional properties is highly sought to realize clean and energy-efficient fuel cell systems. Herein, PdAu bimetallic nanoparticles (NPs) with core-shell morphology (Pd_Core–Au_Shell) were fabricated on the surface of reduced graphene oxide (RGO) support by a facile two-step protocol. In the first step, Pd_Core–Ag_Shell bimetallic NPs were synthesized on RGO support by reducing Pd²⁺and Ag⁺ ions with methyl ammonia borane (MeAB). Later, Pd_Core–Au_Shell bimetallic NPs were conveniently fabricated on RGO support via a galvanic replacement strategy involving sacrificial oxidation of metallic silver and reduction of gold ions. The resulting core/shell bimetallic NPs were characterized by X-ray diffraction (XRD), High-resolution transmission electron microscopy (HR-TEM), Energy dispersive X-ray spectroscopy (EDS), Fourier-Transform Infrared Spectroscopy (FT-IR) and cyclic voltrammetry (CV). The electrocatalytic performance of core/shell nanostructures for the room temperature oxygen reduction reaction (ORR) in alkaline media were systematically performed by CV. The electrode-area-normalized ORR activity of RGO-supported Pd_Core–Au_Shell NPs was higher than the corresponding commercially available carbon-supported Pt nanoparticles (Pt/C) at ?0.8 V vs Ag/AgCl (satd. KCl) (6.24 vs 5.34 mA cm^?2, respectively). Further, methanol-tolerant ORR activities of as-synthesized catalysts were also studied. The Au-on-Pd/RGO bimetallic NPs presented enhanced ORR activity both in presence and in the absence of methanol in comparison with a commercial Pt/C catalyst and as-synthesized Pd/RGO and Au/RGO catalysts. The enhanced catalytic activities of core/shell structures might be resulted owing to the optimized core/shell structure comprising of a small Pd core and a thin Au shell and synergistic effects offered by Pd and Au. The present synthesis protocol demonstrated for two-layer structure can be extended to multi-layered structures with desired functions and activities.     

Direct synthesis of nitrogen and phosphorus co-doped hierarchical porous carbon networks with biological materials as efficient electrocatalysts for oxygen reduction reaction

We described a process of the preparation of N, P co-doped hierarchical porous carbon by one-step pyrolysis of the chitosan/phytic acid (CS/PA) precursor without extra activation processes, and the nitrogen and phosphorus were successfully incorporated into the carbon framework. Experimentally, the best performance was identified with NPC-1000 which possessed the highest BET specific surface area of 1117.2 m² g^?1. This NPC-1000 showed a half-wave potential of 50 mV difference with commercial Pt/C, better tolerance to methanol and a superior stability comparable to commercial Pt/C catalyst. The results suggest that it is a simple, feasible, and economical route to synthesis of hierarchical porous carbon which can be used as metal-free catalysts for oxygen reduction.     

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.     

CeO2 overlapped with nitrogen-doped carbon layer anchoring Pt nanoparticles as an efficient electrocatalyst towards oxygen reduction reaction

To engineering high-efficient, sustainable and novel Pt-based composite system, a newly “Pt-oxide” based composites electrocatalyst of “CeO₂ overlapped with nitrogen-doped carbon layer anchoring Pt nanoparticles” (PtCeO₂@CN) has been fabricated. In comparison with Pt/C, the results exhibit that PtCeO₂@CN possesses a preferable methanol tolerance ability, superior stability (30000 s degradation: 35% for PtCeO₂@CN vs. 50% for Pt/C), and more positively the onset potential (16 mV) as well as half-wave potential (29 mV) towards oxygen reduction reaction. Further, the investigation shows that PtCeO₂@CN has a certain selectivity with quasi-four electron pathway (n = 3.2–3.3 e^?). This is attributed to the establishment of “nitrogen-doped carbon layer” structure, which heightens the conductivity of CeO₂, further promotes electron transfer between Pt and CeO₂, as well as strengthens the anchoring effect for Pt nanoparticles. Overall, this study would shed bright light to develop some effective Pt-oxide based composite electrocatalysts.     

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.     

Nanoporous PdFe alloy as highly active and durable electrocatalyst for oxygen reduction reaction

Nanoporous PdFe (NP-PdFe) alloy with uniform structure size and controllable bimetallic ratio was easily fabricated by one-step mild dealloying from PdFeAl precursor alloy. NP-PdFe consisted of nanoscaled interconnected network skeleton with bicontinuous hollow channels extending in all three dimensions. Compared with NP-Pd and commercial Pt/C catalysts, the NP-PdFe exhibits superior electrocatalytic activity for oxygen-reduction reaction (ORR) with enhanced specific and mass activities. Electrocatalytic measurements indicated that NP-PdFe possesses higher catalytic durability than Pt/C with the less loss of ORR activity and electrochemical active surface area upon long term potential scan. NP-PdFe alloy also shows higher methanol tolerance relative to Pt/C catalyst. XPS and DFT calculations suggest that the downshift of Pd d-band center after alloying with Fe makes favorable reaction kinetics for ORR with decreased adsorption energy of O and OH on Pd surface.     

Spindle-shaped CeO2/biochar carbon with oxygen-vacancy as an effective and highly durable electrocatalyst for oxygen reduction reaction

Highly durable and active CeO₂ on biochar carbon (CeO₂/BC) derived from Spirulina platensis microalgae and synthesized by simple one-pot hydrothermal treatment and further activated through pyrolysis approach. A spindle-shaped morphology of CeO₂ with predominant (111) facet was evidently observed from X-ray diffraction patterns and electron microscopy images. The structural features such as high specific surface area, defect-rich carbon with N & P atoms, increased oxygen vacancy and π-electron transfer play an important role for the improved oxygen reduction reaction (ORR). The considerable amount of Ce³⁺ and higher proportion of pyridinic N and graphitic N species are substantially contributed to the superior ORR performance of CeO₂/BC700, which surpasses other similar catalysts and competing with Pt/C. Hence, the significant kinetic ORR parameters and extended stability (no loss after 5000 potential cycles) of the CeO₂/BC700 catalysts provides the promising insight to develop the rare-earth metal oxide nanostructures as a possible candidate for ORR in alkaline medium.     

One-step synthesis of carbon-encapsulated nickel phosphide nanoparticles with efficient bifunctional catalysis on oxygen evolution and reduction

As a promising and cost-efficient alternative to noble metal catalysts, transition metal phosphides (TMPs) show highly catalytic performance toward oxygen reduction and evolution reactions (ORR and OER). Mesoporous carbon-coated nickel phosphide (NiP) nanoparticles were successfully synthesized by thermal decomposition at 500 °C under N₂/H₂ (95:5) atmosphere. The NiP/C hybrid exhibits excellent OER/ORR activity. It can generate an OER current density of 10 mA cm^?2 at the overpotential of 0.26 V with a low Tafel slope of 43 mV dec^?1, and produce a limited ORR current density of 5.10 mA cm^?2 at 1600 rpm with a half-wave potential of 0.82 V via a 4-electron pathway. In addition, the OER/ORR catalytic currents remain considerable stable without significant loss for more than 25 h polarization. This work will open up a new avenue to design a bifunctional catalyst with a superior OER/ORR activity and stability, and this cost-efficient strategy will pave the way for the industrial application of the renewable energy technologies.     

Carbon-supported Pd-Co bimetallic nanoparticles as electrocatalysts for the oxygen reduction reaction

Carbon-supported Pd-Co bimetallic nanoparticle electrocatalysts of different Pd/Co atomic ratios were prepared by a modified polyol reduction. Electrocatalytic activities of the catalysts for the oxygen reduction reaction (ORR) have been investigated based on the porous rotating disk and disk-ring electrode techniques. As-prepared Pd-Co bimetallic nanoparticles evidence a single-phase fcc disordered structure, and the mean particle size is found to decrease with increase in Co content. A typical TEM image of the Pd₂Co/C catalyst, heat-treated at 500 °C, reveals a mean particle diameter is ca. 8.3 nm with a relatively narrow size distribution. For synthesized Pd-Co catalysts, the highest catalytic activity for the ORR, when supported on carbon (i.e., Pd-Co/C) was found for a Pd:Co atomic ratio of 2:1 and heat treatment at ca. 500 °C, corresponding to a Pd–Pd mean interatomic distance of ca. 0.273 nm. Kinetic analysis based on the rotating disk and disk-ring electrode measurements reveals that the ORR on Pd-Co/C catalysts undergoes a four-electron process in forming water. Because the Pd-Co/C catalyst is inactive for the adsorption and oxidation of methanol, it may function as a methanol-tolerant ORR catalyst in a direct methanol fuel cell.     

Nitrogen doped lotus stem carbon as electrocatalyst comparable to Pt/C for oxygen reduction reaction in alkaline media

The nitrogen doped carbon with high content of pyridine N and porous structure indicates high activity for oxygen reduction reaction (ORR). In this paper, nitrogen doped lotus stem carbon (N-LSC) with 6.3 at% of N (containing 52 at% of pyridine N) and porous structure is developed by using lotus stem as carbon source and dopamine hydrochloride as nitrogen source. The ORR activity, stability and methanol tolerance are characterized. The results show that the N-LSC has comparable activity to Pt/C, and much better methanol tolerance and stability than Pt/C. The porous structure and high content of pyridine N are believed to lead to the high ORR performances of the N-LSC.     

Preparation of iron and nitrogen co-doped carbon material Fe/N-CCM-T for oxygen reduction reaction

In this paper, iron and nitrogen co-doped carbon material with nanotube structure (Fe/N-C_CM-T) was synthesized by pyrolyzing a mixture of Fe salt, chitosan and melamine and displayed high electrocatalytic performance for oxygen reduction reaction (ORR). The structure of the Fe/N-C_CM-T was characterized and their ORR performance in alkaline media was investigated by linear sweep voltammetry, cyclic voltammetry and chronoamperometry. Fe/N-C_CM-T displayed better ORR performance than other carbon materials like Fe/N-C_C-800. The Fe/N-C_CM-800 with a large surface area (302.5 m²/g) especially exhibited the best ORR electrocatalytic performance among the prepared carbon materials, which was also proved by its similar Tafel slope (76 mV decade^?1) to Pt/C catalyst (74 mV decade^?1). Fe/N-C_CM-800 showed similar ORR activity as commercial Pt/C catalyst, but superior tolerance to methanol and stability. Such high ORR performance of the Fe/N-C_CM-T can be attributed to its nanotube structure, high specific surface area (SSA), high graphitic-N and pyridinic-N contents.     

Ultrasonic synthesis of nitrogen-doped carbon nanofibers as platinum catalyst support for oxygen reduction

Oxygen- and nitrogen-containing groups are successfully introduced onto the carbon nanofiber (CNF) surfaces by sonochemical treatment in mixed acids (concentrated sulfuric acid and nitric acid) and ammonia, respectively. Pt electrocatalysts supported on the acid-treated CNF (CNF-O) and ammonia-treated CNF (CNF-ON) are prepared and the effect of CNF surface functional groups on the electrocatalytic activities of supported catalysts for oxygen reduction reaction (ORR) is investigated. High resolution transmission electron microscopy reveals that Pt particles are uniformly dispersed on the two CNF supports and the CNF-ON supported Pt nanoparticles have a smaller average particle size and a more uniform particle size distribution. Cyclic voltammetric analysis shows the Pt/CNF-ON has a larger electrochemically active surface area than Pt/CNF-O. Rotating disk electrode measurements show that the Pt/CNF-ON exhibits a considerably higher electrocatalytic activity toward ORR as compared with Pt/CNF-O. It is believed that the good electrocatalytic activity of Pt/CNF-ON can be attributed to the smaller Pt particle size and more uniform particle size distribution, to the synergistic effect and the enhanced Pt-CNF-ON interaction, and to the unique structural and electronic properties of CNF-ON.     

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.