Hard template derived N,S dual heteroatom doped ordered mesoporous carbon as an efficient electrocatalyst for oxygen reduction reaction

At present, a low-cost and efficient electrocatalyst is vital to conquering the sluggish oxygen reduction reaction (ORR) in fuel cells. In particular, N and S dual heteroatom doped mesoporous carbon (NSMC) catalysts are believed to be one of the best ORR catalyst options due to the distribution of nitrogen, sulfur sites. In this work, for NSMC synthesis we employed 2D Santa barbara amorphous (SBA-15) silica as support material and L-cysteine as N and S dual precursor. The optimal loading of NSMC-0.4, reveals the high concentration of defect sites (I_D/I_G = 0.99), pyridinic (21.41 at. %), graphitic-N (50.27 at. %), thiophene-S (77.16 at. %) sites on MC surface resulting in an improved ORR performance. The NSMC-0.4 showed more positive onset potential of 0.78 V vs. RHE, half-wave potential of 0.68 V, current density of 2.8 mA/cm², peroxide production of 81%, followed by two-electron reduction process and lower R_ct of 10 Ω/cm² in an alkaline electrolyte solution. However, NSMC-0.6 demonstrated the higher amount of peroxide selectivity (150%) due to the presence of a large quantity of pyrrolic-N sites. In addition, our work provide an excellent guide for the synthesis and design of NSMC for efficient peroxide production via an electrochemical synthesis route.     

Zn doped ZIF67-derived porous carbon framework as efficient bifunctional electrocatalyst for water splitting

The electrochemical splitting of water is considered to be an efficient and potential technique for producing clean hydrogen and oxygen. Although, there are lots of significant developments in composite of superior hydrogen evolution reaction (HER) or oxygen evolution reaction (OER) catalyst applied in water splitting currently, designing non-precious and low-cost bifunctional electrocatalysts with high performance is still an attractive challenging issue. In this article, we report a novel bifunctional electrocatalyst with cobalt-based nanoparticles (NPs) embedded in Zn-doped hierarchical porous three-dimension N-doped carbonization structure via an annealing process of metal organic frameworks (MOFs) connected by N-doped carbon nanotube (denoted as Co–Zn/PNC). This composite structure possesses the characteristics of more active sites, numerous mesopores and high conductivity. The resulting electrocatalyst (Co–Zn/PNC) can be used as both anode and cathode to roust the overall water splitting, getting a current density of 10 mA cm⁻² at a cell voltage of 1.63 V in 1.0 M KOH electrolyte.     

Nitrogen doped mesoporous carbon supported Pt electrocatalyst for oxygen reduction reaction in proton exchange membrane fuel cells

Nitrogen doped mesoporous carbons are employed as supports for efficient electrocatalysts for oxygen reduction reaction. Heteroatom doped carbons favour the adsorption and reduction of molecular oxygen on Pt sites. In the present work, nitrogen doped mesoporous carbons (NMCs) with variable nitrogen content were synthesized via colloidal silica assisted sol-gel process with Ludox-AS40 (40 wt% SiO₂) as hard template using melamine and phenol as nitrogen and carbon precursors, respectively. The NMC were used as supports to prepare Pt/NMC electrocatalysts. The physicochemical properties of these materials were studied by SEM, TEM, XRD, BET, TGA, Raman, XPS and FTIR. The surface areas of 11 wt% (NMC-1) and 6 wt% (NMC-2) nitrogen doped mesoporous carbons are 609 m² g^?1 and 736 m² g^?1, respectively. The estimated electrochemical surface areas for Pt/NMC-1 and Pt/NMC-2 are 73 m² g^?1 and 59 m² g^?1, respectively. It is found that Pt/NMC-1 has higher ORR activity with higher limiting current and 44 mV positive onset potential shift compared to Pt/NMC-2. Further, the fuel cell assembled with Pt/NMC-1 as cathode catalyst delivered 1.8 times higher power density than Pt/NMC-2. It is proposed that higher nitrogen content and large pyridinic nitrogen sites present in NMC-1 support are responsible for higher ORR activity of Pt/NMC-1 and high power density of the fuel cell using Pt/NMC-1 cathode electrocatalyst. The carbon support material with high pyridinic content promotes the Pt dispersion with particle size less than 2 nm.     

Covalent hybrid of hemin and mesoporous carbon as a high performance electrocatalyst for oxygen reduction

A high performance hemin and mesoporous carbon hybrid electrocatalyst for the oxygen reduction reaction (ORR) is developed by using hemin as the Fe–N-containing precursor to control the chemistry of the metal and the chemical composition of the carbon surface. As a first step, Hemin is used as the Fe–N-containing precursor to prepare the Fe–N-doped mesoporous carbon (H-MC) via a nano-casting process by using sucrose as a carbon source and mesoporous silica as a hard template. Hemin is then used as the Fe–N₄-containing precursor to prepare H-MC supported hybrid catalyst. The Fe-doped and N-doped mesoporous carbons are also prepared and the catalytic properties of the prepared catalysts for ORR in alkaline media are investigated. The results show that as compared with the much more expensive Pt/C catalyst, the hybrid catalyst obtained in this work exhibits not only a higher onset potential, but also a higher current density.     

Nickel oxide nanoparticles grown on mesoporous carbon as an efficient electrocatalyst for urea electro-oxidation

Nickel oxide nanoparticles were formed on mesoporous carbon (NiO/MC) through precipitating nickel hydroxide species followed by post annealing treatment. The as-prepared electrocatalysts were physically characterized using XRD, Raman, EDX, SEM, TEM and HRTEM analysis techniques. Very fine nanoparticles were shown in TEM image and their chemical identity was proved in XRD pattern by three crystal indices of nickel oxide species as (111), (200) and (220). The electrocatalytic activity of NiO/MC electrocatalysts was investigated for oxidizing urea molecules in 0.5 M NaOH solution. Adding variable nickel oxide loading values during the electrocatalyst preparation method appreciably affected the obtained oxidation current density. Increasing the deposited oxide weight percentage would enhance the activity of formed nanocomposite to attain its optimum performance at the electrocatalyst containing 17.5 wt% NiO. The effect of altering urea concentration and scan rate during the oxidation process on the resultant electrocatalyst activity was studied. Chronoamperograms displayed improved steady state oxidation current density values after 1800 s revealing stable nanocatalysts. This promising electrocatalytic behavior of NiO/MC elects its application as an active component for urea electro-oxidation in many potential applications including hydrogen production, fuel cells and water purification.     

Nitrogen doped carbon fibers derived from carbonization of electrospun polyacrylonitrile as efficient metal-free HER electrocatalyst

Development of durable and efficient electrocatalyst for hydrogen evolution reaction (HER) is significantly important for forwarding the commercialization of water splitting technology. In this work, we report a facile synthesis of nitrogen doped carbon fibers derived from the carbonization of the electron-spun polyacrylonitrile (PAN) membrane at 800 °C (NCFs-800) as efficient and stable metal-free electrocatalyst for HER catalysis in both acidic and alkaline mediums. Ascribing to the homogenous nitrogen dopants in electrocatalyst, NCFs-800 requires only 114.3 mV and 198.6 mV vs. RHE to achieve current density of 10 mA cm⁻² in 0.5 M H₂SO₄ and 1 M KOH electrolytes, respectively. Moreover, the HER activity is well maintained after 2000 potential cycles indicating that NCFs-800 possesses high durability in both acidic and alkaline conditions due to the fibrous structure with high corrosion resistance. Our study offers new strategy to synthesize stable and efficient metal-free electrocatalyst, which could be extended to other heteroatom doped carbon electrocatalyst.     

A robust electrocatalytic activity and stability of Pd electrocatalyst derived from carbon coating

Palladium (Pd) as an efficient anodic catalyst has been extensively investigated in direct formic acid fuel cells (DFAFCs); while, Pd catalyst is electrochemically unstable in acidic electrolyte resulting in low stability retarding the widespread application of DFAFCs. In this study, a new method is invented to prevent the Pd nanoparticles from rapid dissolution by carbon layer originated from the carbonization of glucose. Ascribing to the presence of carbon layer, Pd electrocatalyst demonstrates much higher stability in comparison with Pd electrocatalyst without carbon layer in the course of stability tests. Robust electrocatalytic activities toward formic acid and methanol/ethanol oxidation are observed for carbon-stabilized Pd electrocatalyst resulted from the higher content of metallic Pd atoms coming from the carbonization process, in which Pd (II) species are further reduced. Moreover, the fuel cell performance of carbon-stabilized Pd electrocatalyst reaches 90 mW cm_Pd⁻² measured with 1 M formic acid; while, power density of bare Pd electrocatalyst is only 74 mW cm_Pd⁻². This work highlights that carbon layer carbonized from glucose improves not only the stability of Pd electrocatalyst, but also the electrocatalytic activity.     

Introduction of a new active and stable cathode catalyst based on bimetal-organic frameworks/PPy-sheet for alkaline direct ethanol fuel cell

For the first time, the polypyrrole (PPy) with a sheet-like structure was synthesized by a high-efficiency and facile chemical reaction process. A new composite with the growth of bimetallic zeolitic imidazolate frameworks on polypyrrole sheet-like (BMZIF@PPy) was synthesized. Then, the BMZIF@PPy composite by different heat-treatment temperatures is applied to make oxygen reduction reaction (ORR) electrocatalysts. Electrochemical measurements perform to investigate the ORR properties in both acidic and alkaline media. The onset potential and the limiting current density for the Cobalt/Zinc-nanocarbon@polypyrrole pyrolysis at 800 °C (Co/Zn-NC@PPy-800) were 0.977 V_RHE and 4.99 mA cm^?2 in 0.1 M KOH and 0.85V_RHE and 5.48 mA cm^?2 in 0.5 M H₂SO₄. Finally, due to the good activity and stability in alkaline media, the Co/Zn-NC@PPy-800 electrocatalyst is used as the cathode in an alkaline direct ethanol fuel cell. The maximum power of the Co/Zn-NC@PPy-800 cathode catalyst was 77% higher than that of the commercial Pt/C electrocatalyst.     

Application of mesoporous carbon to counter electrode for dye-sensitized solar cells

The mesoporous carbons were prepared by the carbonation of the triblock copolymer F127/phloroglucinol-formaldehyde composite self-assembled in an acid medium and employed as the catalyst for triiodide reduction in dye-sensitized solar cells (DSCs). The characteristics of mesoporous carbon were analyzed by scanning electron microscopy, transmission electron microscopy, N₂ sorption measurement and X-ray diffraction. The mesoporous carbon with low crystallinity exhibited Brunauer-Emmett-Teller surface area of 400 m² g⁻¹, pore diameter of 6.8 nm and pore volume of 0.63 cm³ g⁻¹. The photovoltaic performances of DSCs with mesoporous carbon counter electrode were improved by increasing the carbon loading on counter electrode due to the charge-transfer resistance of mesoporous carbon counter electrode decreasing with the increase of the carbon loading. However, further carbon loading increase has no obvious effect on the photovoltaic performance of DSCs with carbon electrode when carbon loading exceeds 300 μg cm⁻². The overall conversion efficiency of 6.18% was obtained by DSCs composed of mesoporous carbon counter electrode with the carbon loading of 339 μg cm⁻². This value is comparable to that of DSCs with conventional platinum counter electrode.     

Facile fabrication of mesoporous carbon nanofibers with unique hierarchical nanoarchitecture for electrochemical hydrogen storage

A colloidal silica incorporated porous anodic aluminum oxide (AAO) was utilized as a dual-template to prepare mesoporous carbon nanofibers (MCNFs). Such a strategy is simple because it takes advantage of commercially available materials (i.e., colloidal silica and AAO) and the templates can be removed in one step. The as-prepared MCNF shows a hierarchical nanostructure consisting of open macroporous channel connected with large mesopores and micropores. As a result of the large surface area and unique hierarchical nanoarchitecture which facilitates fast mass and electron transport, the MCNF reveals a discharge capacity of 679 mA h g⁻¹ at 25 mA g⁻¹. This value is significantly greater than that (i.e., 394 mA h g⁻¹) observed for an ordered mesoporous carbon (OMC) with a similar specific surface area. Furthermore, at 3000 mA g⁻¹, the MCNF demonstrates a discharge capacity of 585 mA h g⁻¹, which is about twice that (i.e., 256 mA h g⁻¹) of the OMC.     

Novel in-situ P-doped metal-organic frameworks derived cobalt and heteroatoms co-doped carbon matrix as high-efficient electrocatalysts

Metal organic frameworks (MOFs) are considered as ideal templates for the synthesis of metal-heteroatom co-doped carbon materials. However, the tedious heteroatoms doping pathways hinders the maximizing of catalytic performances. Herein, we synthesize a series of high-efficient Co and N, P heteroatoms co-doped carbon-based composites by first constructing a novel in-situ P-doped MOF with novel larger N, P-containing ligands and 2-methylimidazole as mixed ligands, and then calcining these MOFs at high temperature. During the pyrolysis process, the generated gases derived from the thermal decomposition of organic ligands are liberated from inner of P-ZIF materials to make the Co–Co₂P@NC-P catalysts become loose and porous. When being used as electrode materials, the optimal Co–Co₂P@NC-P₃-700 catalyst exhibits excellent ORR and OER activity, the ORR performance is superior to the Pt/C catalysts, and the OER performance can be comparable with the commercial RuO₂ catalyst. Moreover, when applied in the assembled primary Zn-air battery, the performances of Co–Co₂P@NC-P₃-700 catalyst can outperform the commercial Pt/C catalysts, exhibiting a high peak power density, specific capacity and a long-term stability. Furthermore, the catalytic active sites of catalysts are carefully investigated in this work.     

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.     

Molten salt strategy to synthesize alkali metal-doped Co9S8 nanoparticles embedded,N, S co-doped mesoporous carbon as hydrogen evolution electrocatalyst

A molten salt strategy was proposed to prepare a series of electrocatalysts for hydrogen evolution reaction (HER) with salts as templates, which consisted of Co₉S₈ nanoparticles (NPs) and N, S co-doped mesoporous carbons. The porosities, heteroatoms contents, and crystalline structures of the final electrocatalysts were determined by the types of salts and the calcining temperatures. When KCl/NaCl, KCl/LiCl, and NaCl/CaCl₂ were adopted as the molten salts, Co₉S₈ NPs embedded, N, S co-doped carbons were obtained. However, when CaCl₂ and ZnCl₂ were used as the molten salts, Co₉S₈ could not be synthesized. The characterization results exhibited that alkali metal atoms could be introduced in the lattices of Co₉S₈. The combination of experimental and theoretical results revealed that Na and K atoms doping improved the electrocatalytic performance for HER. GTCo900-KCl/NaCl possessed the best HER activity, delivering a current density of 10 mA cm⁻² at 54 mV in acidic media, 142 mV in neutral media, and 103 mV in alkaline media.     

Polyethylene oxide-polypropylene oxide -polyethylene oxide derived porous carbon materials with different molecular weights as ORR catalyst in alkaline electrolytes

Polyethylene oxide (PEO)-polypropylene oxide (PPO)-polyethylene oxide block copolymer having different molecular weights are used as precursors of carbon materials to prepare Hollow -Derivatives carbon material as an electrocatalyst through block copolymer self-assembly. The composition and microstructure of the prepared catalysts are shown by Raman spectroscopy, X-ray diffraction (XRD), Test of nitrogen adsorption and desorption curves, High resolution transmission electron microscopy (HR-TEM) and scanning electron microscopy (HR-SEM). Oxygen was passed into alkaline electrolyte solution until the solution reached saturation state. With molecular weight increasing, the obtained sample gradually changed from block to hollow and spherical. When the molecular weight was 12600 g mol^?1, the evenly hollow carbon nanocages was acquired (C-12600). In O₂ saturated alkaline electrolyte (0.1 M KOH solution), C-12600's limited current density,half-wave potential and initial potential are 5.23 mA cm^?2@0.4 V, 0.72 V and 0.81 V, respectively. And most important is that half-wave potential and onset potential have barely change after 2000 cycles of cyclic voltammetry. As a result, the porous carbon materials exhibited excellent electrocatalytic activity while maintaining high stability in alkaline KOH solution.     

ZIF-L-Co@carbon fiber paper composite derived Co/Co3O4@C electrocatalyst for ORR in alkali/acidic media and overall seawater splitting

Green and clean energy technologies, including fuel cells, metal-air batteries, water splitting et al., are becoming more significant for our lives. Oxygen reduction reaction (ORR), hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) are key reaction processes for fuel cells, metal-air batteries and water splitting. Therefore, it is highly desirable to design a multifunctional catalyst, which owns catalytic performance under a widely applied range. Herein, we demonstrate a novel multifunctional catalyst (Co/Co₃O₄@C) by carbonizing a composite material constructed of zeolite imidazolate framework and carbon fiber paper (ZIF-L-Co@CP). It is a carbon-based material containing metallic Co and Co₃O₄ as a low-cost and effective catalyst toward the ORR and overall water splitting. For ORR, the Co/Co₃O₄@C catalyst shows high half-wave potential in both alkaline and acidic media, 0.823 V for 0.1 M KOH and 0.672 V for 0.1 M HClO₄. More importantly, it exhibits good catalytic activities of hydrogen and oxygen evolutions to perform overall splitting in actual seawater.     

Zeolotic imidazolate frameworks (ZIFs) derived porous carbon: A review from crystal growth & green synthesis to oxygen reduction reaction activity

Oxygen reduction reaction (ORR) has slow reaction rate that decrease the chemical conversion productivity in proton exchange membrane fuel cells (PEMFCs) which has to be improved. Noble metals such as Pt nanoparticles supported on carbon (Pt/C) was considered the most essential catalyst in ORR despite their limitations including being rare and expensive, CO poisoning etc. In the past few years, nitrogen doped carbons (N–C) or zeolitic imidazole framework (ZIFs) derived (M-N-C) including single or bimetallic metals take attention due to their outstanding properties such as high surface area, excellent electrical conductivity, cost effectiveness, thermal and chemical stability which were used either as catalyst or supports for noble metal nanoparticles to improve the sluggish ORR in PEMFC cathode. This review briefly outlines conventional crystal preparation and activation of porous carbons derived from ZIFs and their green synthesis methods, followed by modern synthesis methods of nanostructured MNP/MOF composites and recently their ORR activity evaluation in PEMFC. Particular attention was given to the porous carbon supports derived from two kind of frameworks such as ZIF-8 and ZIF-67 which are the most frequently reported ORR electrocatalysts and/or supports in the literature.     

Self-assembled mesoporous carbon sensitized with ceria nanoparticles as durable catalyst support for PEM fuel cell

Mesocarbon-ceria nanocomposite is proposed for developing highly durable catalyst for the application in fuel cells. Ordered arrays of the mesoporous channels with d spacing of ∼8 nm and wall thickness of ∼3 nm are fabricated through a self-assembly route between the phenolic oligomers and PEO-containing P123 block polymer combined with self-assembly of CeOH₂⁺ and the surfactant. As a result, the Pt-mesocarbon-ceria presents a high electrochemically active surface of 105 m²/g_Pt. It is also found that ceria has an appreciable influence on the performance of the fuel cell at low humidity due to the water retention of ceria nanoparticles. At 75 RH% humidity of 65 °C, single cell assembled with Pt-mesocarbon-ceria has performance better than that of the conventional Pt/C catalyst. The Pt-mesocarbon-ceria displays high resistance to corrosion because of radical scavenges of ceria. Under long period operation at open circuit voltage (OCV), the voltage of the fuel cell assembled with Pt-mesocarbon-ceria has a slight decay rate of 9.5 μV/min, in comparison to 28.5 μV/min of conventional Pt/C. After an OCV accelerated degradation of 2000 min, the electrochemically active surface of Pt-mesocarbon-ceria is 45%, much lower than 70% of Pt/C catalyst.     

One-step carbonization of ZIF-8 in Mn-containing ambience to prepare Mn,N co-doped porous carbon as efficient oxygen reduction reaction electrocatalyst

Economical and efficient non-noble metal catalysts should be developed practically, instead of commercial Pt/C for fuel cells. In this paper, manganese, nitrogen co-doped porous carbon (Mn–N–C) was synthesized to catalyze oxygen reduction reaction (ORR) through the one-step carbonization of ZIF-8 in the Mn-containing (MnCl₂) atmosphere. During the carbonization process, MnCl₂ gas was captured with ZIF-8 and then transformed into uniform Mn–N active sites distributed in the porous carbon materials. The Mn–N–C catalyst exhibited plentiful porous structures, large specific surface areas, high graphitization and conductivity, which contributed to the transfer and transport of charge and exposed more active sites. The Mn–N–C catalyst exhibited superior catalytic ability in alkaline and acidic solutions. Half-wave potential of the Mn–N–C could reach 0.88 and 0.73 V in 0.1 M KOH and 0.5 M H₂SO₄, respectively. In addition, the Mn–N–C catalyst showed a prominent stability after the stability test of 18,000 s. Excellent electrochemical performance and endurance make the Mn–N–C expect to be an effective ORR catalyst to build high-performance fuel cells.     

Metal-free mesoporous carbon with higher contents of active N and S codoping by template method for superior ORR efficiency to Pt/C

Carbon materials with more exposed N or S atoms result in higher activity for oxygen reduction reaction. In this paper, mesoporous carbon based materials with 5.3 at% of N and 7.1 at% of S are synthesized (defined as porous-S-N-C). The solid core mesoporous shell silica as template results in mesoporous structure of the porous-S-N-C which favors mass transfer and higher doping (both increase by ～20%) and exposing of active N and S atoms. The electrochemical characterization results show that the mesoporous structure and the S modification favor much to the N-C based catalysts in catalyzing ORR. Typically, the porous-S-N-C has a 3.99 electron transfer number at 0.4 V, and higher ORR efficiency, much better CO tolerance, much better methanol tolerance and much higher electrochemical stability than commercial Pt/C. This novel method of improving the contents of doped and exposed N and S atoms is imagined to be applied to preparation of other high-content-heteroatom doped materials.     

Synthesis of nitrogen-doped mesoporous carbon nanosheets for oxygen reduction electrocatalytic activity enhancement in acid and alkaline media

In the present work, nitrogen-doped mesoporous carbon nanosheets (NMCNs) are prepared and extensively investigated for the oxygen reduction reaction. Initially, by using dual templates, viz. graphene oxide and a cationic surfactant, silica films (thickness: 1.0 nm) are synthesized and characterized using transmission electron microscope (TEM), nitrogen adsorption-desorption (N₂ ad/des) measurements, and small-angle X-ray diffraction (SA-XRD).Morphology and structure of silica evolve along with graphene oxide concentration, while the co-operative assembly and the final structure are determined by the electrostatic interaction between the dual templates. The effect of silica template on the resultant NMCNs is investigated physicochemically by photoelectron spectroscopy (XPS), TEM, N₂ ad/des and electrochemically by cyclic voltammetry (CV), and linear sweep voltammetry (LSV).NMCNs are featured of a high content of nitrogen dopant, high specific surface area (SSA) and ultrathin thickness (1.5 nm), favoring catalysis and facilitating the mass transport of the reactive species. From the electrochemical tests, it is confirmed that NMCNs yield a high oxygen reduction reaction (ORR) electrocatalytic activity in acid and alkaline environment; this activity is similar or even better as compared with the one measured over carbon supported platinum commercial catalyst (40 wt % Pt/C).