Metal-free Bifunctional Catalysts Research Articles

Size Control of Carbon Xerogel Spheres as Key Factor Governing the H2O2 Selectivity in Metal-Free Bifunctional Electro-Fenton Catalysts for Tetracycline Degradation.

Carbon xerogel spheres co-doped with nitrogen and eco-graphene were synthesized using a typical solvothermal method. The results indicate that the incorporation of eco-graphene enhances the electrochemical properties, such as the current density (JK) and the selectivity for the four transferred electrons (n). Additionally, nitrogen doping has a significant effect on the degradation efficiency, varying with the size of the carbon xerogel spheres, which could be attributed to the type of nitrogenous group doped in the carbon material. The degradation efficiency improved in the nanometric spheres (48.3% to 61.6%) but decreased in the micrometric-scale spheres (58.6% to 53.4%). This effect was attributed to the N-functional groups present in each sample, with N-CNS-5 exhibiting a higher percentage of graphitic nitrogen (35.7%) compared to N-CMS-5 (15.3%). These findings highlight the critical role of sphere size in determining the type of N-functional groups present in the sample. leading to enhanced degradation of pollutants as a result of the electro-Fenton process.

Carbon Gels-Green Graphene Composites as Metal-Free Bifunctional Electro-Fenton Catalysts.

The Electro-Fenton (EF) process has emerged as a promising technology for pollutant removal. However, the EF process requires the use of two catalysts: one acting as an electrocatalyst for the reduction of oxygen to H2O2 and another Fenton-type catalyst for the generation of ·OH radicals from H2O2. Thus, the search for materials with bifunctionality for both processes is required for a practical and real application of the EF process. Thus, in this work, bifunctional electrocatalysts were obtained via doping carbon microspheres with Eco-graphene, a form of graphene produced using eco-friendly methods. The incorporation of Eco-graphene offers numerous advantages to the catalysts, including enhanced conductivity, leading to more efficient electron transfer during the Electro-Fenton process. Additionally, the synthesis induced structural defects that serve as active sites, promoting the direct production of hydroxyl radicals via a 3-electron pathway. Furthermore, the spherical morphology of carbon xerogels enhances the accessibility of the reagents to the active sites. This combination of factors results in the effective degradation of Tetracycline (TTC) using metal-free catalysts in the Electro-Fenton process, achieving up to an impressive 83% degradation without requiring any other external or additional catalyst.

Ni–CoS2/WS2/MoS2 hollow cuboids as noble metal-free bifunctional catalysts for highly efficient photovoltaics and electrochemical hydrogen production

Transition metal compounds, especially NiS2, CoS2, WS2, and MoS2 as noble metal-free catalyst materials, are widely applied to replace Pt in the fields of dye-sensitized solar cells (DSSCs) and hydrogen evolution reactions (HER). Meanwhile, hollow structures have large specific surface area and their large void spaces can also effectively reduce ion migration resistance and diffusion length. In this work, Ni–CoS2/WS2 and Ni–CoS2/MoS2 hollow cuboids have been successfully synthesized and applied as promising bifunctional electrocatalysts in high-efficiency DSSCs and HERs. In detail, nickel-cobalt hydroxide cuboids were prepared and served as self-sacrificing templates to react with (NH4)2WS4 or (NH4)2MoS4 at room temperature, and then mixed with sulfur powder for annealing treatment. Besides, Ni–CoS2 hollow cuboids were synthesized as a reference. Profiting from the stable hollow structure, large specific surface area, abundant effective active sites exposed by WS2, and synergistic effect of multiple elements, Ni–CoS2/WS2 hollow cuboids manifested a preeminent performance. Compared with Pt (8.06%), the conversion efficiency of Ni–CoS2/WS2 hollow cuboids had reached 9.41% as counter electrode for DSSCs. As for HER, it exhibited the lowest overpotential of 72.4 mV@10 mA cm−2 and had the minimum Tafel slope (56.1 mV dec−1) compared with Ni–CoS2/MoS2 and Ni–CoS2.

Outstanding platinum group metal-free bifunctional catalysts for rechargeable zinc-air batteries

Developing highly active and durable catalysts for zinc-air batteries (ZAB) is critical for energy conversion and storage. Herein, we prepared Fe-N-C catalysts at a kilogram scale by the commercial VariPore™ method and the effect of synthesis conditions on the catalyst performance at ZAB air electrode was investigated. The results show the PA-450-HT exhibits excellent electrocatalytic activity toward oxygen reduction reaction (ORR) and it is the most suitable catalyst for primary ZAB with the galvanostatic polarization discharge peak power density of 149 mW cm−2, outperforming commercial Pt-Ru/C catalysts. Additionally, the NCB-600-HT catalyst displays the half-wave potential of 0.87 V vs. RHE for ORR and ∆E value of 0.81 V (indicating outstanding ORR and OER reversibility) and exhibits excellent charge-discharge cycling durability similar to NCB-550-LT around 160 h for the secondary ZAB. This work reports outstanding bifunctional Fe-N-C catalysts for rechargeable ZAB at mass production for the first time.

Anion-Modulated Platinum for High-Performance Multifunctional Electrocatalysis toward HER, HOR, and ORR.

SummaryEfficient electrocatalyst toward hydrogen evolution/oxidation reactions (HER/HOR) and oxygen reduction reaction (ORR) is desirable for water splitting, fuel cells, etc. Herein, we report an advanced platinum phosphide (PtP2) material with only 3.5 wt % Pt loading embedded in phosphorus and nitrogen dual-doped carbon (PNC) layer (PtP2@PNC). The obtained catalyst exhibits robust HER, HOR, and ORR performance. For the HER, a much low overpotential of 8 mV is required to achieve the current density of 10 mA cm−2 compared with Pt/C (22 mV). For the HOR, its mass activity (MA) at an overpotential of 40 mV is 2.3-fold over that of the Pt/C catalyst. Interestingly, PtP2@PNC also shows exceptional ORR MA which is 2.6 times higher than that of Pt/C and has robust stability in alkaline solutions. Undoubtedly, this work reveals that PtP2@PNC can be employed as nanocatalysts with an impressive catalytic activity and stability for broad applications in electrocatalysis.

Erratum: In Situ Activating Strategy to Significantly Boost Oxygen Electrocatalysis of Commercial Carbon Cloth for Flexible and Rechargeable Zn-Air Batteries.

[This corrects the article DOI: 10.1002/advs.201800760.].

Rapid microwave-assisted preparation of high-performance bifunctional Ni3Fe/Co-N-C for rechargeable Zn-air battery

Precious metal-free bifunctional catalysts offer scalable possibilities for achieving oxygen cathodes in rechargeable metal-air batteries. Literature thus far suggests that the oxygen reduction and evolution reactions (i.e ORR and OER) take place at different active sites. However, the design and preparation of oxygen electrocatalysts with high-performance is still a great challenge. This work develops a general approach to introduce a nanoparticle that offers oxygen evolution reaction sites onto an oxygen reduction reaction catalyst via rapid microwave treatment. The metal–organic frameworks (Co, ZIF-8) derived Co-N-C with high ORR performance is employed as a support to homogeneously disperse OER active Ni3Fe nanoparticles. Potentials of 1.54 V and 0.79 V have been achieved at current densities of 10 mA cm−2 for OER and 3 mA cm−2 toward ORR, respectively. The developed rechargeable Zn-air battery assembled with Ni3Fe/Co-N-C as an oxygen-electrode exhibits superior efficiency and robust durability, exceeding that of commercial Pt/C + IrO2. Furthermore, the satisfactory activity of our fabricated wristband shows the successfully practical application of flexible Zn-air batteries for wearable devices.

In situ growing catalytic sites on 3D carbon fiber paper as self-standing bifunctional air electrodes for air-based flow batteries

Noble metal-free bifunctional catalysts are highly desirable for both oxygen reduction and evolution reactions in air-based flow batteries, but remain a big challenge. Herein, micropores dominated nitrogen and sulfur codoped carbon (NSC) film is directly growing on conductive carbon fiber paper (CFP) as self-standing air electrode through the electropolymerization of 2-amino-5-mercapto-1,3,4-thiadiazole and followed pyrolysis process. NSC film provides abundant active sites, while conductive CFP with 3D macro-networks offers affluent channels for electron and mass transfer. Benefit from the strong interaction between NSC and CFP, the resultant CFP@NSC displays highly catalytic activity for both oxygen reduction (comparable half-wave potential to commercial Pt/C) and oxygen evolution (lower overpotential than commercial IrO2), along with robust stability. As a proof of practical usage, zinc air flow battery with CFP@NSC displays lower charge voltage and higher discharge voltage, compared with commercial Pt/C and IrO2 coated CFP electrode. Importantly, zinc air flow battery also exhibits excellent cycling stability including nearly 100% retention of coulombic efficiency and energy efficiency over 3400 min, representing the most efficient bifunctional oxygen electrode reported so far.

Top-down synthesis of S-doped graphene nanosheets by electrochemical exfoliation of graphite: Metal-free bifunctional catalysts for oxygen reduction and evolution reactions

In this study, top-down synthesis of sulfur-doped graphene nanosheets (SDGNs) by simple electrochemical exfoliation was explored as a means of producing metal-free electrocatalysts for oxygen reduction and oxygen evolution reactions (ORR and OER, respectively). In a typical procedure, graphite foils were used to obtain bulk quantities of SDGN catalysts in the presence of thiosulfate as a sulfur source. Highly stable colloidal dispersions of SDGNs were obtained by applying a voltage of 15 V at an optimized Na2S2O3:H2SO4 molar ratio of 5 : 1 (denoted SDGN(5)). Physicochemical characterizations by Raman spectroscopy, transmission electron microscopy, X-ray diffraction, and X-ray photoelectron spectroscopy confirmed the existence of sulfur and its electronic/structural properties in graphene nanosheets. In alkaline media, SDGN(5)-modified electrodes were comparable or superior to pristine graphene and a benchmark commercial platinum-based electrodes in terms of stability, methanol tolerance, n values, and onset potential for ORR and OER. The specific capacitance (149.9 F g−1) of SDGN(5) supported its excellent ORR/OER performance and enhanced surface area.

Recent advances in precious metal-free bifunctional catalysts for electrochemical conversion systems

The gradually “depleting” fossil fuels calls for development sustainable and pollution free energy, this work summarize the recent progress in exploring bifunctional electrocatalysts toward several potential electrochemical conversion devices.

Reaction milling for scalable synthesis of N, P-codoped covalent organic polymers for metal-free bifunctional electrocatalysts

This study exploits an effective mechanochemical process (termed as reaction milling) to conduct Schiff-based coupling reaction with melamine and terephthalaldehyde for the synthesis of covalent organic polymer (RM-COP) as the carbon skeleton and the derivative phosphorus doped material (RM-COP-PA). Comparing with the tradition solvothermal method with reaction time of 3 days under 120 °C, the newly developed reaction milling method significantly shorten the reaction time of the synthesis to 3 h under room temperature as well as bypassing the usage for hazardous solvents. The space-time yield of the developed reaction milling method for synthesis of the bifunctional electrocatalytic precursor reaches 189 kg m−3 day−1. Significantly, the optimal products followed by further carbonization (RM-COP-PA-900) demonstrated excellent bifunctional electrocatalytic activities for an efficient ORR performance with similar commercialized Pt/C half-potential of 841 mV vs RHE as well as an IrO2-like OER activity with a potential of 1.69 V at 10 mA cm−2 in alkaline media, which is better than most metal-free bifunctional catalysts. Moreover, the obtained RM-COP-PA-900 exhibits much better durability and resistance to crossover effect even than the commercial 20 wt% Pt/C catalysts. Therefore, this work will open up a rapid, solvent-free and scalable approach for, but not limit to, highly efficient electrocatalysts.

A Three-Dimensional N/Co-Doped Macroporous Carbon Interweaving with Carbon Nanotubes As Excellent Bifunctional Catalysts for Zn-Air Batteries

Highly efficient noble metal-free bifunctional catalysts for expediting the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) in metal-air batteries or fuel cells are still challenging and imperative. In this work, we report a facile and scalable method for syntheizing three dimensional (3D) N/Co co-doped macroporous carbon interconnecting with in-situ growth carbon nanotubes (CNTs). The as-synthesized material exhibits great electrocatalytic performance for ORR with an onset potential of 0.901V vs.RHE as well as high limited current density of 4.83 mA/cm2 in alkaline electrolyte under a rotation speed of 1600 rpm at 5 mV/s. Furthermore, this 3D porous carbon also shows good electrocatalytic performance for OER in alkaline electrolyte. This high electrocatalytic performance is mianly attributed to its large specific surface area, high conductive CNTs and synergistic effect between Co active species and carbon framework. The result of a two-electrode Zn-air battery based on this carbon material achieves a peak density of 163 mW/cm2 at the voltage of 0.63 vindicates the great potential of the catalyst for battery application.

Three-Dimensional Macroporous Co-Embedded N-Doped Carbon Interweaving with Carbon Nanotubes as Excellent Bifunctional Catalysts for Zn-Air Batteries.

Highly efficient noble metal-free bifunctional catalysts for expediting the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) in metal-air batteries or fuel cells are still challenging and imperative. In this work, we report a facile and scalable method for syntheizing three-dimensional (3D) macroporous Co-embedded N-doped carbon interconnecting with in situ growth carbon nanotubes (CNTs). The as-synthesized material exhibits great electrocatalytic performance for ORR with an onset potential of 0.901 V vs RHE as well as a high limited current density of 4.83 mA/cm2 in an alkaline electrolyte under a rotation speed of 1600 rpm at 5 mV/s. Furthermore, this 3D porous carbon also shows good electrocatalytic performance for OER in an alkaline electrolyte. This high electrocatalytic performance is mainly attributed to its large specific surface area and highly conductive CNTs and the synergistic effect between Co-active species and the carbon framework. The result of a two-electrode Zn-air battery based on this carbon material achieves a peak density of 163 mW/cm2 at a voltage of 0.63 V, indicating the great potential of the catalyst for battery application.

Highly Porous N-Doped Graphitic Carbon from Bio-Waste as Bifunctional Electrocatalyst for Hybrid Na-Air Battery

Noble metal-free bifunctional catalysts are essential to develop high-performance and cost-effective metal-air batteries. In this work, low cost carbon-based bifunctional electrocatalysts have been synthesized by treating highly porous carbon (HPC), derived from waste tree leaves, with urea [CO(NH2)2]. The electrocatalytic oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) activities of the HPC, N-doped HPC (N-HPC) and S-doped HPC (S-HPC) have been investigated. The N-doped HPC showed excellent bifunctional (ORR/OER) activity in comparison to pristine HPC and S-doped HPC. The bifunctional N-doped HPC have been tested as air-cathode for sodium-air battery applications.

From biomass chitin to mesoporous nanosheets assembled loofa sponge-like N-doped carbon/g-C3N4 3D network architectures as ultralow-cost bifunctional oxygen catalysts

To accelerate the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) in the field of air-based energy technologies, efficient low-cost bifunctional catalysts are highly desired. In this paper, biomass chitin-derived N-doped carbon (C-Chitin)/graphitic C3N4 (g-C3N4) composites have been designed and fabricated via an ultralow-cost route including the low-temperature dissolution of biomass chitin and the subsequent carbonization. The C-Chitin/g-C3N4 composites exhibit well-designed loofa sponge-like three-dimensional (3D) network architectures assembled with massive interconnected ultrathin mesoporous nanosheets. Benefiting from the well-designed morphology and unique mesoporous structure with a high surface density of catalytic active sites, as well as the incorporation of N-doped carbon with g-C3N4, the C-Chitin/g-C3N4 composites exhibit pronounced ORR activity (4-electron pathway), superior OER activity (much lower onset potential and higher current density than Pt/C), excellent tolerance to methanol crossover and high durability toward both ORR and OER. Especially, the obtained C-Chitin/g-C3N4 composites with 20 wt % addition of g-C3N4 exhibit a potential gap of 0.90 V between the OER potential at a current density of 10 mA cm−2 and the ORR half wave potential, which is even higher than some highly active metal catalysts including noble-metals, thus holding great promise as low-cost metal-free bifunctional catalysts for efficient ORR and OER.

Interactions between Dopants in Dual-Doped Graphene Nanoribbons as Metal-Free Bifunctional Catalysts for Fuel Cell and Metal-Air Batteries

ABSTRACTDuel p-block element doped carbon nanomaterial is a new kind of metal-free bifunctional catalysts for fuel cells and metal-air batteries because of their low-cost and high efficiency compared to traditional noble metals and their alloys. To optimize co-doped catalysts, we studied the interactions of dopants on the doped graphene and their effect on oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). It is found that the interactions between N and X (P, B, S) occur within a distance of ∼0.5 nm, and beyond the distance their interactions are limited while the interactions between N and Cl take places beyond the distance of ∼0.5 nm.

Metal-Free Mesoporous Cathode for Lithium-Oxygen Batteries: Noble Carbons As Bifunctional Catalyst

The use of renewable energy is an urgent and serious request to ensure the energy supply in the 21st century.1 The rechargeable lithium-oxygen batteries (LOBs) are one of the most promising secondary batteries to be applied to smart grid and electronic vehicles which are electrified from mixed energy generated by renewable and traditional fossil fuels. However, LOBs are suffering from unoptimized combinations of catalysts and electrolytes: 1) carboneous materials, which are good oxygen reduction reaction (ORR) catalysts and support for other catalysts, are one of main causes of side-reactions associated with electrolyte decomposition and 2) from point of commercial viability, the price of LOBs should be affordable: We cannot use expensive noble metals.2-4 Here, we show that mesoporous nitrogen doped carbons (meso-NdCs) as efficient bifunctional catalysts which can process well both ORR and oxygen evolution reaction (OER).5 The meso-NdCs were synthesized by carbonization of all-organic ionic liquid and using SiO2 nanoparticles (14 nm of diameter) as a hard template. The electrochemical properties of meso-NdCs are characterized by galvanostatic charge-discharge curves. The formation/de-formation of lithium peroxide and other side products were checked by ex-situ X-ray diffractometer and ex-situ Fourier-transform infrared spectroscopy. As the results, the meso-NdCs shows good catalytic activities in both ORR and OER. Especially at OER, meso-NdCs show the recharge plateau at 3.7 V versus Li/Li+: the lowest overpotential as non-metal catalyst (0.7 V). Indeed, we found that the cycle property can be improved by synthetic condition due to higher conductivity enhanced by enrichment of atomic graphitic parts. Indeed, stability against active reduced oxygen species and fewer defects could give positive effects for this better electrochemical property. This study suggests that by changing the amount of dopant and the degree of graphitization, we can control both an electrochemical catalytic activity and the cycling property of the carboneous materials as possible catalysts in LOBs.We propose that meso-NdCs are the new candidate as affordable, efficient, metal-free bifunctional catalysts to be used in rechargeable energy storage devices. This is very important because several elements, such as sulfur and phosphor, are promising candidates to be doped in and improve the electrochemical catalytic properties of carbon, which is considered as an unfavorable catalyst for LOBs so far. S. Chu, A. Majumdar, Nature 2012, 488, 294-303.S. A. Freunberger, Y. Chen, Z. Peng, J. M. Griffin, L. J. Hardwick, F. Bardé, P. Novák, P. G. Bruce, J. Am. Chem. Soc. 2011, 133, 8040-8047J. Christensen, P. Albertus, R. S. Sanchez-Carrera, T. Lohmann, B. Kozinsky, R. Liedtke, J. Ahmed, A. Kojic, J. Electrochem. Soc. 2012, 159, R1-R30.Y.-C. Lu, B. M. Gallant, D. G. Kwabi, J. R. Harding, R. R. Mitchell, M. S. Whittingham, Y. Shao-Horn, Energy Environ. Sci. 2013, 6, 750-768.K. Sakaushi, T.-P. Fellinger, M. Antonietti, manuscript submitted.