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.     

Phosphorus-doped hierarchical porous carbon as efficient metal-free electrocatalysts for oxygen reduction reaction

Recently, fuel cells and metal-air batteries have attracted extensive attentions. Researching and developing non-noble metal catalyst with high electrocatalytic activity and low cost is one of the important challenges for these energy storage and conversion devices. In this study, phosphorus doped hierarchical porous carbon (P-HPC) has been firstly synthesized via a hard template method. The prepared PHPC possesses a unique porous structure which consists of micropores, mesopores and macropores simultaneously. The electrocatalytic activity of the PHPC toward ORR in KOH solution has been studied and compared with the ordinary structured phosphorus doped carbon (PC) and the commercial Pt/C by means of rotating ring-disk electrode (RRDE) technique. The prepared PHPC exhibits an excellent electrocatalytic performance toward ORR in terms of the electrocatalytic activity, the reaction kinetics, the durability and the methanol tolerance. And the high electrocatalytic activity and durability of PHPC could be attributed to the special hierarchical porous structure. This research demonstrates that the rational design of the microstructures for catalyst plays significant roles in improving the catalytic activity for the ORR.     

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.     

Controllable synthesis of three-dimensional nitrogen-doped graphene as a high performance electrocatalyst for oxygen reduction reaction

Three-dimensional nitrogen-doped graphene (3D-NG@SiO₂) is prepared by pyrolyzing poly (o-phenylenediamine) (POPD) with high nitrogen content. POPD is prepared via an in situ chemical oxidation polymerization of o-phenylenediamine (OPD) in acetic acid with silica colloid as templates. The optimum parameter is OPD:SiO₂ = 1:2, pyrolysis @ 900 °C. SEM and TEM images show the wrinkled and porous graphene structures. Raman spectra indicate that 3D-NG@SiO₂ consists of 4–6 layers graphene. N₂ adsorption–desorption isotherms reveal that the pore size distributions mainly centralize at 5–10 nm. XRD illustrates the amorphous structure. XPS analysis shows that the nitrogen content is 3.6% and nitrogen mainly exists in the form of pyridinic nitrogen and pyrrolic nitrogen. The oxygen reduction reaction (ORR) performance investigated using a rotating disk electrode shows that the initial potential of 3D-NG@SiO₂ is 0.08 V (vs. Hg/HgO). The electron transfer number is 3.92 @ ?0.3 V (vs. Hg/HgO), indicating that 3D-NG@SiO₂ mainly occurs via a four-electron process. The oxygen reduction current density decreases by 21% after 60 h in the chronoamperometry test. The CVs manifests a current density loss of 0.16 mA cm^?2 after scanning for 5000 cycles. The high activity and durability indicate the promising potential of 3D-NG@SiO₂ as ORR catalysts.     

Medulla stachyuri-derived iron and nitrogen co- doped 2D porous carbon-flakes for highly efficient oxygen reduction electrocatalysis and supercapacitors

Iron and nitrogen co-doped two-dimensional (2D) porous carbon-flakes have been fabricated by using foam-like Medulla stachyuri (MS, the stem pith of tetrapanax papyrifer) as both carbon precursor and template and ammonium ferric citrate as iron and nitrogen precursor. The ammonium ferric citrate-impregnated foams are subsequently converted into iron and nitrogen co-doped 2D porous carbon-flakes by pyrolysis at high temperature in an inert atmosphere. The porous carbon-flakes fabricated at 900 °C (MS-Fe-900) possess high surface area (1140.9 m² g⁻¹) and effective Fe/N co-doping (0.22 at.% Fe and 2.02 at.% N). In comparison with Pt/C, MS-Fe-900 exhibits superior ORR activity (E₀ = 968 mV; E_1/2 = 830 mV vs RHE), preferable methanol/CO tolerance and better stability. Furthermore, the MS-Fe-900-based electrode presents high-rate performance (80.1% capacitance retention from 1 to 100 A g⁻¹), and good cycling stability for over 10000 cycles in 6 M KOH electrolyte. This work takes full advantage of the unique structure of biomass and provides a feasible approach to develop cost-efficient and high performance activated carbon materials for ORR electrocatalysis and supercapacitors.     

ZIF-8/PEDOT @ flexible carbon cloth electrode as highly efficient electrocatalyst for oxygen reduction reaction

Design and fabrication of highly efficient and low-cost oxygen reduction reaction (ORR) electrocatalysts is of paramount importance for practical applications. Herein, we proposed a cost-effective, metal-free catalyst based on ZIF-8 metal-organic framework nanoparticles/electro-polymerized poly(3,4-ethylenedioxythiophene) (PEDOT) film on the surface of flexible carbon cloth (CC) electrode (ZIF-8/PEDOT/CC) via a two-step procedure. For this purpose, worm-like PEDOT nanostructures were deposited on the surface of carbon fibers using a pulse electro-polymerization technique followed by facile growth of ZIF-8 polyhedra nanoparticles via a chemical bath deposition method. The ORR measurements in O₂-saturated KOH electrolyte solution using the modified CC electrode demonstrated that the prepared electrode exhibits remarkable electrocatalytic activity towards ORR with 8 times increase in the cathodic current density compared to bare CC (J = 0.13–1.1 mA/cm²) along with lower overpotential due to the synergetic effects between ZIF-8 nanoparticles as particularly porous nanostructure act as electrolyte reservoirs and highly conductive PEDOT film. The Kouteckey-Levich analysis for the ZIF-8/PEDOT-modified CC electrode revealed that the oxygen reduction reaction proceeds via a nearly four-electron pathway along with superior tolerance to methanol crossover as well as enhanced stability in alkaline solution compared to the gold standard commercial Pt catalyst.     

Waste wine mash-derived doped carbon materials as an efficient electrocatalyst for oxygen reduction reaction

Oxygen reduction reaction (ORR) is a core reaction of fuel cell and metal-air cell. In recent years, it has been a hot topic to study non-precious metal catalysts for ORR. Herein, we have used waste wine mash-derived carbon, melamine and ferric chloride to prepare a Fe- and N- co-doped carbon catalyst. The specific surface area of the catalyst is up to 1066.6 m² g⁻¹. And its wave potential is 15 mV higher than that of commercial Pt/C catalyst. The ORR on our catalyst followed a four-electron pathway; and it has high stability and high impressive immunity to methanol. After continuous oxygen reduction of 30,000s, the retention rate is 90%.     

Comparative investigation on the properties of carbon-supported cobalt-polypyrrole pyrolyzed at various conditions as electrocatalyst towards oxygen reduction reaction

A series of non-precious metal catalysts named as Co-PPy-TsOH/C towards oxygen reduction reaction (ORR) were synthesized by pyrolyzing carbon supported cobalt-polypyrrole at various temperatures for diverse durations. The catalytic activity of these catalysts was evaluated with electrochemical techniques of cyclic voltammetry, rotating disk electrode and rotating ring-disk electrode. Physicochemical techniques, such as XRD, TEM and XPS, were employed to characterize the structure/morphology of the catalysts in order to understand the effects of pyrolysis conditions on the ORR activity. The results showed that both pyrolysis temperature and the duration have essential effects on the structure/morphology as well as ORR activity of the Co-PPy-TsOH/C catalysts, pyrolyzing the precursor at 800 °C for 2 h is the optimal condition to synthesize the catalyst with the best ORR performance.     

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.     

Ni-doped Mn2O3 microspheres as highly efficient electrocatalyst for oxygen reduction reaction and Zn-air battery

Herein, Ni-doped Mn₂O₃ microspheres are successfully synthesized via the facile coprecipitation of metal ions and ammonium bicarbonates, followed by a heat treatment process. Ni-doped Mn₂O₃ exhibits outstanding catalytic performance toward the oxygen reduction reaction (ORR) in alkaline media with a half-wave potential of 0.801 V, limiting current density of 6.02 mA cm^?2 at 0.6 V vs. RHE, outstanding long-term durability, and strong tolerance to methanol. Furthermore, a Zn–air primary battery using Ni-doped Mn₂O₃ as an air cathode shows high open-circuit voltage of 1.52 V and high power density of 88.2 mW cm^?2, outperforming the commercial Pt/C cathode. The exceptional performance of the Ni-doped Mn₂O₃ microspheres is ascribed to the hierarchical structure, optimized particle size, and Ni incorporation into Mn₂O₃. The proposed synthesis strategy provides a new methodology for the design and fabrication of electrochemically active transition metal-doped materials as efficient electrocatalysts for a variety of energy storage and conversion reactions.     

NiCo alloy nanoparticles encapsulated in N-doped 3D porous carbon as efficient electrocatalysts for oxygen reduction reaction

A series of N-doped three dimensional porous carbons loaded with NiCo alloy nanoparticles (NiCo@NpCs) have been successfully fabricated by a template-assisted in-situ pyrolysis strategy and the ORR activities of different-concentration alloy supported N-doped carbons are systematically investigated. The optimized sample exhibits a positive half-wave potential of 0.78 V (vs. RHE), a high diffusion-limited current density (5.120 mA cm⁻²), and a high durability over 92%, which is superior to the Pt/C. The excellent activity and stability are mainly due to synergistic effect between carbon matrix and NiCo alloy. The N-doped porous carbon support with high surface area and good conductivity not only provides more active sites but also promotes electron transport and mass transfer process. Furthermore, the unique core-shelled structure NiCo@NpCs can effectively avoid the dissolution and corrosion of alloy particles and the surface peeling of particles during the catalyze reaction, which is beneficial to improving the activity and stability.     

Promising nature-based nitrogen-doped porous carbon nanomaterial derived from borassus flabellifer male inflorescence as superior metal-free electrocatalyst for oxygen reduction reaction

In this present study, novel hierarchical nitrogen-doped porous carbon for use as a metal-free oxygen reduction reaction (ORR) electrocatalyst is derived from borassus flabellifer male inflorescences by calcining at 1000 °C in an inert atmosphere using metal hydroxides as activating agent and melamine as nitrogen doping agent. The BET surface areas of the lithium-ion (Li-ion), potassium-ion (K-ion) and calcium-ion (Ca-ion) activated carbon are observed to be 824.02, 810.88 and 602.88 m² g^-1 respectively. Another interesting fact is that the total surface energy calculated by wicking method (73.2 mJ/m²), is found to be higher for Li-ion activated carbons. Among the prepared nitrogen-doped porous carbon, Li-ion activated system, showed an outstanding performance in ORR reaction in alkaline medium, thanks to its high surface area and notable surface activity. An incontrovertible of note that ORR half-wave potential of Li-ion activated nitrogen-doped carbon (0.90 V) is relatively higher in comparison to the commercial 20 wt % Pt/C catalyst (0.86 V). Inspite of overwhelming performance, the ORR reaction followed the preferred 4- electron transfer mechanism involving in the direct reduction pathway in all activated carbons. The ORR performance is also noticeably better and comparable to the best results in the literature based on biomass derived carbon catalysts.     

Hollow NH2-MIL-101@TA derived electrocatalyst for enhanced oxygen reduction reaction and oxygen evolution reaction

Non-precious metal-based electrocatalysts with excellent activity and stability are highly desired for the sluggish oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). Herein, a tannic acid (TA) etching strategy is used to inhibit the metal aggregation and achieve muti-metal doping. The hollow NH₂-MIL-101@TA derived Fe–N–C catalyst exhibits superior ORR catalytic activity with an E_1/2 of 0.872 V and a maximum output power density of 123.4 mW cm⁻² in Zn-air battery. Since TA can easily chelate with metal ions, Fe/Co–N–C and Fe/Ni–N–C are also synthesized. Fe/Ni–N–C manifests exceptional bifunctional activity with an E_j = 10 of 1.67 V and a potential gap of 0.833 V between E_j = 10 and E_1/2 in alkaline electrolyte, which is 45 mV smaller than Pt/C–IrO₂. The improvement of ORR and OER performance of the catalysts via the simple TA etching and chelation method provides a novel strategy for the design and synthesis of efficient electrocatalysts.     

PtCo incorporated porous carbon nanofiber as a promising oxygen reduction electrocatalyst

Designing oxygen reduction reaction (ORR) catalysts with high activity and long durability is significant for the development of proton exchange membrane fuel cells. Herein, the optimized platinum nanowires are used as templates for inducing growth of cobalt-containing metal-organic framework, deriving uniform nanofibers. After the calcination, the metal ions are transferred into the nitrogen-rich porous carbon, and wrapped by the carbon skeleton to form the PtCo bimetal incorporated nanofibers as high-performance ORR electrocatalyst. The Pt₄Co@NC-900 catalyst yields high specific activity (1.37 mA cm⁻²) in comparison to Pt/C (0.38 mA cm⁻²). The mass activity (MA) of Pt₄Co@NC-900 catalyst is approximately 3.8-fold higher than that of the commercial Pt/C under acidic conditions. After the accelerated durability tests, the Pt₄Co@NC-900 catalyst presents only 16% loss in MA, while Pt/C catalyst retains 73.0% of the initial MA. The improved ORR performance can be ascribed to the synergistic interaction between Co and Pt.     

Design and facile one-pot synthesis of uniform PdAg cubic nanocages as efficient electrocatalyst for the oxygen reduction reaction

Shape-controlled synthesis of multicomponent metallic nano-alloy materials is of great significance for the design and preparation of high-efficiency electrocatalysts. In this work, a simplified one-pot synthetic strategy for the preparation of uniform three-dimensional (3D) PdAg cubic nanocages (PdAg–CNC) was developed. The morphology and structure of PdAg–CNC were characterized by transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD). We elucidate a series of reaction pathways for the formation of various morphologies (sphere, cubic nanocage and nanowire) of PdAg alloys by tuning the amount of glycine and controlling the Pd/Ag molar ratio. The PdAg–CNC shows enhanced electrocatalytic activity, good durability and excellent methanol tolerance in comparison with commercial Pd–C and Pt–C catalysts for the oxygen reduction reaction (ORR) in alkaline medium. The excellent ORR performance of the PdAg–CNC can be correlated to the Pd–Ag alloy formation and unique mesoporous nanocage structure. This work demonstrates a facile strategy for shape-controlled synthesis of multicomponent metallic nano-alloys materials and their catalytic application.     

Microstructure effect of carbon nanofibers on Pt/CNFs electrocatalyst for oxygen reduction

Pt nanoparticles supported on microstructure controllable carbon nanofibers (CNFs), i.e. platelet CNFs (p-CNFs), fish-bone CNFs (f-CNFs) and tubular CNFs (t-CNFs), are synthesized, and CNF microstructure effect on physio-chemical and oxygen reduction reaction (ORR) properties of Pt/CNFs is investigated. The physio-chemical properties of different Pt/CNFs electrocatalysts are characterized by high resolution transmission electron microscope (HRTEM), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). HRTEM results exhibit Pt nanoparticles are uniformly dispersed on CNF surface, and Pt/p-CNFs shows a smaller particle size compared with those other catalysts. XPS results reveal that CNF microstructure can influence the metal–support interaction, and Pt/p-CNFs have a higher binding energy compared with Pt/t-CNFs. From cyclic voltammetric studies, it is found that Pt/p-CNFs performed a higher ORR activity than Pt/f-CNFs and Pt/t-CNFs, which may be resulted from the smaller Pt particle size and the stronger metal–support interaction of Pt/p-CNFs. Furthermore, CNF microstructure can influence the reaction process. ORR on Pt/p-CNFs or Pt/f-CNFs is controlled by diffusion process, while on Pt/t-CNFs is surface reaction controlled.     

Freestanding palladium nanonetworks electrocatalyst for oxygen reduction reaction in fuel cells

Still it's a main challenge to design of highly efficient electrocatalysts to reduce the high overpotential of the oxygen reduction reaction (ORR). The 1 dimensional (1D) palladium nanonetworks (Pd-Net) can be a promising alternative to platinum (Pt)-based electrocatalyst for ORR. In this study, the Pd-Net electrocatalysts have been synthesized via a simple wet-chemical method with the assistance of cetyltrimethylammonium bromide (CTAB) and zinc precursor. Further investigation indicates that the thickness of Pd-Net can be regulated by simply changing the molar ratio of CTAB and the 5 ± 0.1 nm is proven as an efficient ORR electrocatalyst without any support material. The freestanding 1D Pd-Net has shown 2.2 and 3.6-fold higher electrochemical surface area than that of commercially available Pt/C and homemade Pd nanoparticles (PdNPs) catalysts, respectively. As a result, it provides a higher density of ORR active sites and facilitated the electron transport. The Pd-Net catalyst shows 2.1 and 4.1 times higher mass activity and 1.3 and 3.1 higher specific activity at 0.85 V (vs. RHE) with better ORR kinetics than that of Pt/C and PdNPs, respectively. Additionally, the Pd-Net catalyst has been demonstrated a significant tolerance to the anodic fuels (i.e. methanol) and enhanced durability than the Pt/C and PdNPs catalysts for ORR.     

Fe2O3-encapsulated and Fe-Nx-containing hierarchical porous carbon spheres as efficient electrocatalyst for oxygen reduction reaction

It is highly desirable to develop high-efficiency non-precious electrocatalysts toward oxygen reduction reaction (ORR). In this work, Fe₂O₃-encapsulated and Fe-Nx-containing porous carbon spheres (Fe₂O₃/N-MCCS) with unique multi-cage structures and high specific surface area (1360 m² g^?1) are fabricated. The unique porous structure of Fe₂O₃/N-MCCS ensures fast transportation of oxygen during ORR. The combined effect of Fe₂O₃ nanoparticles and Fe-Nx configurations endows Fe₂O₃/N-MCCS (E_1/2 = 0.837 V vs. RHE) with superior ORR activity and methanol tolerance to Pt/C. And, Fe₂O₃/N-MCCS exhibits better stability than nitrogen-modified carbon. The characterization results of Fe₂O₃/N-MCCS after long-term test reveals its excellent structural stability. Impressively, zinc-air battery based on Fe₂O₃/N-MCCS showed a peak power density of 132.4 mW cm^?2 and a specific capacity of 797 mAh g^?1, respectively.     

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.     

Transition metal carbides coupled with nitrogen-doped carbon as efficient and stable Bi-functional catalysts for oxygen reduction reaction and hydrogen evolution reaction

Developing non-precious metal catalysts for oxygen reduction reaction (ORR) and hydrogen evolution reaction (HER) is crucial for proton exchange membrane fuel cell (PEMFC), metal-air batteries and water splitting. Here, we report a in-situ simple approach to synthesize ultra-small sized transition metal carbides (TMCs) nanoparticles coupled with nitrogen-doped carbon hybrids (TMCs/NC, including WC/NC, V₈C₇/NC and Mo₂C/NC). The TMCs/NC exhibit excellent ORR and HER performances in acidic electrolyte as bi-functional catalysts. The potential of WC/NC at the current density of 3.0 mA cm^?2 for ORR is 0.814 V (vs. reversible hydrogen electrode (RHE)), which is very close to Pt/C (0.827 V), making it one of the best TMCs based ORR catalysts in acidic electrolyte. Besides, the TMCs/NC exhibit excellent performances toward HER, the Mo₂C/NC only need an overpotential of 80 mV to drive the current density of 10 mA cm^?2, which is very close to Pt/C (37 mV), making it the competitive alternative candidate among the reported non-precious metal HER catalysts.