Effective Oxygen Reduction Reaction Catalyst Research Articles

Construction of core‐shell heterostructured MnO@Co/NC for enhancing the oxygen electrocatalytic performance

AbstractThe development of non‐precious metal electrocatalysts for efficient oxygen reduction reaction (ORR) is a significant research field vital for the progress of zinc−air battery technology. Here, we purposely constructed a core‐shell heterostructured electrocatalyst consisting of MnO nanotubes covered by Co nanoparticles anchored on N‐doped carbon substrate. MnO@Co/NC displayed superior ORR capability with a half‐wave potential of 0.85 V and almost no potential decay after 10,000 cycles CV test, outform that of commercial Pt/C catalysts. Employing MnO@Co/NC as the air electrode catalyst in primary Zn−air batteries leads to a top power density of 145 mW cm−2 and an enhanced specific capacity of 869 mAh g−1 as compared to commercial noble metal catalysts. Such superb ORR performance outcomes primarily stem from the synergistic effect between Co/NC shell and MnO core since the coupling structure not only regulates the inherent electronic structure but also accelerates electron transfer speed and boosts the active sites. Besides, the Co/NC outer layer can also help to improve the corrosion resistance of MnO@Co/NC in the electrolyte. This work demonstrated that the core‐shell heterostructured non‐precious catalysts open up a new avenue for obtaining sufficiently effective ORR catalysts in Zn−air batteries and other related applications.

Rational Design of HighPerformance M-N-C Single Atom Catalysts

High-performance electrocatalysts are required because the sluggish kinetics of Oxygen Reduction Reaction (ORR) at the cathode in either Fuel Cell (FC) or Metal-Air Battery (MAB). The current most effective ORR catalysts are noble Pt-group metals, whose high price and low abundance severely hamper the widespread application of FC and MAB. Dispersively non-noble transition metal coordinated with nitrogen atoms doped in carbon nanomaterials (M-N-C) Single Atom Catalysts (SACs) have been considered as the most promising catalysts for the ORR. Here, the effect of typical morphology on the activity and stability of SACs was discussed firstly. Moreover, the effect of metal type, coordination nitrogen number, heteroatom decoration on the central metal atom were discussed. Based on the rate determining step (RDS), we propose the ideal characters that high-performance SACs should possess.

Study of the electrocatalytic activity of silicon and nitrogen co-doped carbon towards oxygen reduction reaction

The development of highly efficient and cost effective non-precious metal (NPM) catalysts for fuel cells and metal-air batteries is challenging but of great significance. Although the research and development of non-noble metal and non-metal carbon catalysts for the oxygen reduction reaction (ORR) has gained a lot of attention recently, the electrocatalytic activity of tetravalent metalloids is a less investigated topic. Herein, an effective ORR catalyst based on silica and nitrogen co-doped multi-walled carbon nanotubes (Si,N-MWCNTs) was synthesized via a simple one-pot pyrolysis. The Si,N-MWCNTs catalyst demonstrated exceptional electrocatalytic activity towards the ORR in 0.1 M KOH, with a half-wave potential (E1/2) of 860 mV (vs. RHE). Furthermore, Si,N-MWCNTs displayed apparent stability and excellent tolerance to methanol compared to the commercially available platinum catalyst. Physical characterization was carried out by employing X-ray photoelectron spectroscopy and transmission electron microscopy, which confirmed the integration of both silicon and nitrogen into the carbon structure. These results clearly shed light on the significance of further study on tetravalent metalloids as cost effective and easily available dopants for the preparation of electrocatalysts.

Construct N-doped carbon anchored CoFe alloy nanoparticles with high content graphitic-N for electrocatalytic oxygen reduction

Oxygen reduction reaction (ORR) is an essential half-reaction in next-generation energy storage and conversion systems, such as metal-air batteries and fuel cells. However, its practical application is restricted by the slow intrinsic kinetics, and the high price and low storage of noble metal electrocatalysts. Herein, unique CoFe nanoparticles encapsulated in N-doped carbon (CoFe-NC-Z8-900) with high content graphite-N derived from CoFe-g-C3N4@ZIF-8 via stepwise pyrolysis is reported as effective ORR catalysts. The increase of graphitic nitrogen content can enhance both the electrical conductivity and the adsorption of oxygen-containing intermediates, resulting in improved catalytic performance. Fortunately, CoFe-NC-Z8-900 exhibits an exceptionally high half-wave potential (E1/2) of 0.914 V in a 0.1 M KOH solution. The excellent ORR electrocatalytic activity can be mainly attributed to the synergistic effect of the CoFe bimetal and the relatively high content of graphite-N. This study offers a unique method for creating powerful nitrogen-doped carbon coated metal nanoparticle electrocatalysts.

Towards superior metal phthalocyanine catalysts for electrochemical oxygen reduction: A comprehensive screening under experimental conditions

Metal phthalocyanines (MPcs) represent a promising class of single-atom catalysts with diverse structures, high thermal stability, and versatile chemical properties. In this work, we have computationally screened nineteen MPcs (metal M = V–Zn, Mo–Ag, and W–Pt ordered based on the atomic number) to assess their suitability as electrochemical oxygen reduction reaction (ORR) catalysts, with consideration of a broad range of experimental conditions, including the oxygen adsorption, temperature effects, partial pressure, solvation effects, pH effects, and demetallation decomposition. Using density functional theory calculations based on the hybrid functional, we find that the ORR behaviors on these MPcs fall into three categories, which are characterized by their distinct oxygen adsorption ability. We correctly predict the experimental ORR performance trend of common MPcs and forecast Rh−, Ir−, Ru−, and MnPc as the four most effective ORR catalysts, outperforming the state-of-the-art FePc in terms of catalytic activity and stability. This study highlights the crucial role of oxygen adsorption, predicts superior MPcs for ORR electrocatalysis, and offers detailed guidance on the optimal working conditions.

A Zn-based catalyst with high oxygen reduction activity and anti-poisoning property for stable seawater batteries.

Seawater batteries (SWBs) are a key part of the future underwater energy network for maritime safety and resource development due to their high safety, long lifespan, and eco-friendly nature. However, the complicated seawater composition and pollution, such as the S2-, usually poison the catalyst and lead to the degradation of the battery performance. Here, Zn single-atom catalysts (SACs) were demonstrated as effective oxygen reduction reaction catalysts with high anti-poisoning properties by density functional theory calculation and the Zn SACs anchoring on an N, P-doped carbon substrate (Zn-SAC@PNC) was synthesized by a one-pot strategy. Zinc active sites ensure the anti-poisoning property toward S2-, and N, P-doped carbon helps improve the activity. Therefore, Zn-SAC@PNC exhibits superior activity (E1/2: 0.87V, Tafel slope: 69.5 mV dec-1) compared with Pt/C and shows a lower decay rate of the voltage after discharge in lean-oxygen natural seawater. In the presence of S2-, Zn-SAC@PNC can still maintain its original catalytic activity, which ensures the stable operation of SWBs in the marine environment with sulfur-based pollutants. This study provides a new strategy to design and develop efficient cathode materials for SWBs.

Iron/iron carbide coupled with S, N co-doped porous carbon as effective oxygen reduction reaction catalyst for microbial fuel cells

As a novel energy device, microbial fuel cells (MFCs) have attracted much attention for their dual functions of electricity generation and sewage treatment. However, the sluggish oxygen reduction reaction (ORR) kinetic on the cathode have hindered the practical application of MFCs. In this work, metallic organic framework derived carbon framework co-doped by Fe, S, N tri-elements was used as alternative electrocatalyst to the conventional Pt/C cathode catalyst in pH-universal electrolytes. The amount of thiosemicarbazide from 0.3 to 3 g determined the surface chemical property, and therefore the ORR activity of FeSNC catalysts. The sulfur/nitrogen doping and Fe/Fe3C embedded in carbon shell was characterized by X-ray photoelectron spectroscopy and transmission electron microscopy. The synergy of iron salt and thiosemicarbazide contributed to the improvement of nitrogen and sulfur doping. Sulfur atoms were successfully doped into the carbon matrix and formed a certain amount of thiophene- and oxidized-sulfur. The optimal FeSNC-3 catalyst synthesized with 1.5 g of thiosemicarbazide exhibited the highest ORR activity with a positive half wave potential of 0.866 V in alkaline and 0.691 V (vs. Reversible Hydrogen Electrode) in neutral electrolyte, which both outperformed the commercial Pt/C catalyst. However, as the amount of thiosemicarbazide surpassed 1.5 g, the catalytic performance of FeSNC-4 was lowered, and this could be assigned to the decreased defects and low specific surface area. The excellent ORR performance in neutral medium urged FeSNC-3 as good cathode catalyst in single chambered MFC (SCMFC). It showed the highest maximum power density of 2126 ± 100 mW m−2, excellent output stability of 8.14% decline in 550 h, chemical oxygen demand removal of 90.7 ± 1.6% and coulombic efficiency of 12.5 ± 1.1%, all superior to those of benchmark SCMFC-Pt/C (1637 ± 35 mW m−2, 15.4%, 88.9 ± 0.9%, and 10.2 ± 1.1%). These outstanding results were associated to the large specific surface area and synergistic interaction of multiple active sites, like Fe/Fe3C, Fe–N4, pyridinic N, graphite N and thiophene-S.

Three-dimensional urchin-like N/S co-doped Mn-based metal-organic frameworks for efficiently enabling oxygen reduction reaction

The exploration of economical and effective non-noble metal catalysts is essential for oxygen reduction reaction (ORR) in energy devices. Recently, heteroatom-doped metal-organic frameworks (MOFs) have shown great potential in ORR due to their high efficiencies and low costs. Herein, three-dimensional (3D) urchin-like N/S co-doped Mn-based MOFs (U–N/S–Mn-MOFs) as an effective ORR catalyst have been successfully prepared by a facile one-step solvothermal methodology. The unique 3D urchin-like structure with a spherical interior and dendritic exterior may provide more catalytic sites and transport channels for ORR. Simultaneously, the doped N and S combined with Mn can promote oxygen adsorption and reduce the reaction energy barrier. These characteristics endow U–N/S–Mn-MOFs with high ORR performance. The present work provides a new opportunity for multiple heteroatom-doped MOFs to achieve high electrocatalytic performance.

Highly dispersed L12-Pt3Fe intermetallic particles supported on single atom Fe-Nx-Cy active sites for enhanced activity and durability towards oxygen reduction

Highly active and durable oxygen reduction reaction (ORR) catalysts with sufficient activity and stability of Pt are beneficial for the commercialization of proton exchange membrane fuel cells. Here we report an effective approach to prepare a composite catalyst comprising of ordered L12-Pt3Fe intermetallic nanoparticles interact with single atom Fe-Nx-Cy active sites. The addition of Fe and the confinement effect of hierarchical porous structure limit the growth of intermetallic particle size (around 2.5 nm). The ligand effect of the electron transfer from Fe to Pt and the synergistic interaction between L12-Pt3Fe and Fe-Nx-Cy work together to reduce oxygen intermediates adsorption and improve kinetics process. Experimentally, the L12-Pt3Fe/CFe-N-C catalyst shows high mass activity and specific activity at 1.010 A/mgPt and 1.166 mA/cm2, respectively, which are 5.8 and 5.1 times higher than those of commercial Pt/C (0.174 A/mgPt and 0.230 mA/cm2). Thanks to the more stable L12 structure, L12-Pt3Fe/CFe-N-C exhibits better durability (14 mV E1/2 loss of L12-Pt3Fe/CFe-N-C and 33 mV E1/2 loss of commercial Pt/C) after 30,000 cycles accelerated stress tests. The strategy to design and prepare small particle Pt-based intermetallic alloys coordinated with M-N-C active sites provides a new direction to obtain low-cost and easily prepared effective ORR catalysts.

Nitrogen-doped carbon nanotubes filled with Fe3C nanowires for efficient electrocatalytic oxygen reduction

It is of great significance to develop the highly active, durable and inexpensive oxygen reduction reaction (ORR) electrocatalysts for fuel cells and metal-air batteries. Nitrogen-doped carbon nanotube (NCNTs) was considered to be one of the most significant non-noble metal ORR electrocatalysts, but its catalytic activity has some shortcomings due to its unfavorable ORR intermediates adsorption characteristics. Herein, we report a new kind of nitrogen-doped carbon nanotubes, which have filled with Fe3C nanowires (NCNTs@Fe3C NWs), working as a stable and effective ORR catalyst. The half-wave potential of the obtained catalyst in a 0.1 M KOH solution is 0.88 V, which is significantly higher than the pristine NCNTs (0.79 V) and the commercial Pt/C (0.85 V). Compared with NCNTs, the improvement in ORR activity of NCNTs@Fe3C NWs can be attributed to the strain effect on the outermost carbon layer due to the high filling of Fe3C NWs. Most importantly, NCNTs@Fe3C NWs shows higher durability and better methanol tolerance than the commercial Pt/C catalyst. Density functional theory (DFT) calculation shows that the tensile strain can optimize the binding energy of ORR intermediates to a certain extent. This work provides new research ideas for the rational design of high-efficiency carbon-based ORR electrocatalysts through the direction of strain engineering.

Enhanced oxygen reduction of porous N-doped carbon nanosheets with graphitic N and defects obtained from coal-based graphene quantum dots

Metal-free carbon-based electrocatalysts for oxygen reduction reaction (ORR) have received considerable attention owing to their abundance and low cost. However, designing carbon-based electrocatalysts with excellent performance in a facile manner remains a challenge. In this work, porous nitrogen-doped carbon nanosheet (PNCNs) catalysts with high contents of graphitic N and defects were readily prepared via a simple self-doping method, in which coal-based graphene quantum dots (GQDs) were used as the carbon and nitrogen resource. Based on the results, PNCNs-900 (obtained at 900 °C) displayed good ORR performance, with a half-wave potential (E1/2) of 0.80 V. It also exhibited high stability and methanol tolerance compared with commercial Pt/C. Experimental investigations and theoretical calculations prove that the increased ORR electrocatalytic activity of PNCNs-900 is due to the synergistic effect of a large number of defects and graphitic N. Furthermore, its higher electrocatalytic activity for ORR also benefits from the porous lamellar structure, which facilitates the mass transfer process. This study offers a novel approach for developing effective ORR catalysts and for the high value-added utilization of coal resources.

Carbon materials functionalized by nitrogenous ligands for dual application in energy storage and production: Fuel cells and supercapacitors

Dual-purpose fuel cell/supercapacitor materials were made by covalent embedding of phenazin-iron adducts into various commercial carbon supports (Vulcan XC-72, Black Pearls 2000, Ketjen Black 600). The diazonium coupling reaction allows for controlled deposition of nitrogen functionalities to the carbon supports via the formation of the strong carbon-carbon bond between the nitrogenous ligand and carbon surface. The novel phenazin-functionalized carbon supports were prepared and fully characterized using thermogravimetric analysis, Raman spectroscopy, and BET pore size determination. Notably, the micro-and macrostructure and the porosity of the carbon support drastically affected the catalytic activity of the system in oxygen reduction reaction (ORR). The KB-phenazin-Fe-700 showed the highest activity for the ORR in both basic and acidic media. Interestingly, the heat treatment of the phenazin-based materials resulted in a significant increase in the ORR activity that is different from the trend we previously observed for terpyridine-based catalysts on Vulcan carbon that were demonstrating a significant decrease in the activity after heat treatment. Furthermore, the potential of the same materials to be used for energy storage was explored and their performance as supercapacitors was studied. The modification of the carbon surfaces by phenazin ligand shows significant improvement of the specific capacitance when examined in acidic media. Coordination of iron to the surface results in a further increase of the specific capacitance due to the additional Faradaic contributions. Drastic increases were seen in the capacitance when Fe was added to the BP-phenazin system. Interestingly, the heat treatment process was found to decrease the energy storage ability of the materials with a significant decrease of the specific capacitance. Thus, for phenazin-based materials, the heat treatment is a necessary step for the production of effective ORR catalysts but is undesirable for materials targeting mainly energy storage applications.

Prussian Blue Analogue Derived Co3O4/CuO Nanoparticles as Effective Oxygen Reduction Reaction Catalyst for Magnesium-Air Battery

Developing efficient, durable, and cost-effective non-noble metal catalysts for oxygen reduction reaction (ORR) is necessary to promote the efficiency and performance of Mg-air batteries. Herein, the Co3O4/CuO nanoparticles were synthesized by a low-cost and simple approach using CuCo-based prussian blue analogue (PBA) as precursor of pyrolysis at different calcination temperatures. It was found that the Co3O4/CuO nanoparticles calcined at 600 °C (CCO-600) have relatively small size and superior ORR performance. The onset potential is 0.889 V and the diffusion limiting current density achieves 6.746 mA·cm−2, as well as prominent stability in 0.1 M KOH electrolyte. The electron transfer number of the CCO-600 is 3.89 under alkaline medium, which indicates that the reaction mechanism of ORR is dominated by 4 e process similar to commercial Pt. The primary Mg-air battery with the CCO-600 as the cathode catalyst has been assembled and possesses better discharge performance than the CuCo-based PBA. The open circuit voltage of CCO-600 arrives at 1.76 V and the energy density reaches 1895.95 mWh/g. This work provides an effective strategy to develop non-noble metal ORR catalyst for the application of metal-air batteries.

Performance and Stability Improvement of Nitrogen-Doped PtNi Catalyst and PBI-Functionalized Catalyst Support for the Oxygen Reduction Reaction in Proton Exchange Membrane Fuel Cells

PtM (M= 3d transition metals, Co, Ni, Fe, etc) alloys as promising ORR catalysts have been considered as effective electrocatalysts for oxygen reduction reaction (ORR) in polymer electrolyte membrane fuel cells (PEMFCs). However, the performance and the stability/durability of these PtM alloy catalysts are still needed to be further improved to meet the requirement for the practical application. Hereby, we propose a method that the particle size and structure of PtM alloy nanoparticles (NPs) can be controlled using both the pre-seed Pt nanoparticles and the functionalized carbon support (CCS) with different charge groups in the catalyst synthesis. Using such an approach, the catalyst utilization can be improved by utilizing the interaction between ionomer and catalyst surface. In addition, the stability can be enhanced by the functional groups on CCS. We have successfully synthesized the PtNiN catalysts supported on p-benzene-polybenzimidazole (p-PBI) functionalized CCS. The activity and stability of these new catalysts shows much better performance on both catalytic activity and stability. The performance and stability of the PtNiN catalyst, PtNiN/EC300-p-PBI, as well as the PtNiN/EC300 and PtNi/EC300 catalysts were carefully characterized in both rotating disc electrode (RDE) and membrane electrode assembly (MEA), which consequently shows that the PtNiN/EC300-p-PBI performs best among these three catalysts. The stability of the PtNiN intermetallic catalyst surpasses that of normal PtNi catalyst. It is worthwhile to point out that PtNiN/EC300-p-PBI has the significant improvement on the initial performance while keeping good stability. This work provides a novel idea and a promising approach on the development of effective ORR catalysts by engineering both crystalline structure of PtM alloy catalysts and physical property of CCS. Keywords: Pt, Pt alloy, PtNi, PtNiN, Carbon functionalization, N-doping, stability, durability, MEA, PEMFC.

Self-catalyzed growth of Zn/Co-N-C carbon nanotubes derived from metal-organic frameworks as efficient oxygen reduction catalysts for Zn-air battery

Designing cost-effective and high-performance carbon-based oxygen reduction reaction (ORR) electrocatalysts is crucial in the development of Zn-air batteries (ZABs). In this study, a facile one-pot synthesis approach is engineered to construct Zn/Co-N-C carbonaceous polyhedrons interconnected with self-catalyzed-grown carbon nanotubes (CNTs) from zeolitic imidazolium frameworks linked with graphene oxide nanosheets. The special N-doped three-dimensional (3D) carbon matrix allows manipulating the exposure of active sites and the synergistic interaction between metal nanoparticles and CNTs. The as-synthesized catalyst features impressive ORR activity in 0.1 mol L−1 KOH (E1/2 = 0.83 V) and 0.5 mol L−1 H2SO4 (E1/2 = 0.73 V), satisfactory cycling stability and methanol resistance comparable to those of the benchmark Pt/C catalyst (E1/2 = 0.80 V in 0.1 mol L−1 KOH, E1/2 = 0.75 V in 0.5 mol L−1 H2SO4). Furthermore, the as-established ZAB demonstrates a competitive peak power density (90 mW cm−2) and prominent long-term stability, which are better than those of devices based on the commercial Pt/C catalyst (82 mW cm−2). This work provides promising guidance for fabricating highly effective ORR catalysts with in situ formed CNTs, which can be applied in portable ZAB-related devices.

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.

Improving the Durability of Pt-Alloy Nanoparticles for the Oxygen Reduction Reaction in Acidic Media Via Encapsulation

The Oxygen Reduction Reaction (ORR) is the desired cathode reaction in Proton Exchange Membrane Fuel Cells (PEMFCs). Carbon-supported platinum alloy (PtM/C) nanoparticles such as PtCo/C are by far the most effective ORR catalysts in PEMFCs. However, the durability of PtM/C is not satisfying, partly owing to the dissolution of the second transition metal M. In this study, by conducting accelerated stress tests (ASTs) on a commercial PtCo/C catalyst in both a rotating disk electrode and a PEMFC, we identified that a significant fraction of the electrochemical surface area (ECSA) loss of the PtCo/C upon the AST was recoverable. This recoverable ECSA loss was tentatively ascribed to the poisoning of the Pt surface by the sulfate ions either from the H2SO4 electrolyte in a RDE or the Nafion in a PEMFC. We further demonstrated that this poisoning effect can be largely suppressed without compromising the activity by encapsulating PtCo/C into thin functionalized carbon overlayers that were tuned to be permeable to H+, O2, and H2O, but not to sulfate ions. The encapsulation may also alleviate the poisoning of ionomer during on the Pt surface during PEMFC operation. Acknowledgement This work was supported by Massachusetts Clean Energy Center Catalyst Award Program G00006319 and the US Department of Energy under award number DE-EE0008416 and DE-EE0008075. This research used beamline 8-ID (ISS) of the National Synchrotron Light Source II, a U.S. Department of Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by Brookhaven National Laboratory under Contract No. DE-SC0012704. The authors declare no competing financial interests. Vielstich, W.; Lamm, A.; Gasteiger, H. A., Handbook of fuel cells: fundamentals technology and applications. Wiley New York: 2003; Vol. 2.Chung, D. Y.; Jun, S. W.; Yoon, G.; Kwon, S. G.; Shin, D. Y.; Seo, P.; Yoo, J. M.; Shin, H.; Chung, Y. H.; Kim, H.; Mun, B. S.; Lee, K. S.; Lee, N. S.; Yoo, S. J.; Lim, D. H.; Kang, K.; Sung, Y. E.; Hyeon, T., Highly Durable and Active PtFe Nanocatalyst for Electrochemical Oxygen Reduction Reaction. J Am Chem Soc 2015, 137 (49), 15478-85.

A Gas-Phase Migration Strategy to Synthesize Atomically Dispersed Mn-N-C Catalysts for Zn-Air Batteries.

Mn and N codoped carbon materials are proposed as one of the most promising catalysts for the oxygen reduction reaction (ORR) but still confront a lot of challenges to replace Pt. Herein, a novel gas-phase migration strategy is developed for the scale synthesis of atomically dispersed Mn and N codoped carbon materials (g-SA-Mn) as highly effective ORR catalysts. Porous zeolitic imidazolate frameworks serve as the appropriate support for the trapping and anchoring of Mn-containing gaseous species and the synchronous high-temperature pyrolysis process results in the generation of atomically dispersed Mn-Nx active sites. Compared to the traditional liquid phase synthesis method, this unique strategy significantly increases the Mn loading and enables homogeneous dispersion of Mn atoms to promote the exposure of Mn-Nx active sites. The developed g-SA-Mn-900 catalyst exhibits excellent ORR performance in the alkaline media, including a high half-wave potential (0.90 V vs reversible hydrogen electrode), satisfactory durability, and good catalytic selectivity. In the practical application, the Zn-air battery assembled with g-SA-Mn-900 catalysts shows high power density and prominent durability during the discharge process, outperforming the commercial Pt/C benchmark. Such a gas-phase synthetic methodology offers an appealing and instructive guide for the logical synthesis of atomically dispersed catalysts.

Facile hydrothermal synthesis of 3D flower-like NiCo2O4/CeO2 composite as effective oxygen reduction reaction catalyst

Exploiting high-performance and cost-effective non-noble metal catalysts for oxygen reduction reaction (ORR) is still greatly important for energy storage devices such as metal-air batteries and fuel cells. In this work, CeO2, NiCo2O4, and NiCo2O4/CeO2 were synthetized by facile hydrothermal method and used as an ORR catalyst. The as-prepared CeO2, NiCo2O4, and NiCo2O4/CeO2 samples were measured by XRD, XPS, SEM, and TEM. Electrochemical characterization tests exhibit that the ORR activity of NiCo2O4 is evidently enhanced by blending with CeO2 nanoparticles. The NiCo2O4/CeO2 composite exhibits more superior electrocatalytic performances and stability in alkaline solution as compared to the pure CeO2 and NiCo2O4. The electrochemical performance of composite is very near to that of 20 wt% Pt/C. Owing to the synergistic interactions, NiCo2O4/CeO2 favors a four-electron pathway in ORR and exhibits a higher ORR activity than CeO2 and NiCo2O4. The outstanding performance confirms the NiCo2O4/CeO2 composite as a promising efficient ORR catalyst in metal-air batteries and fuel cells.

Plasma Treatment As an Effective Way of Modifying Carbon-Based Electrocatalysts for an Enhanced Oxygen Reduction Reaction

Oxygen reduction reaction (ORR) is pivotal in renewable energy technologies such as in fuel cells and metal-air batteries. Precious metal-free electrochemical ORR is a critical component in designing cost-effective electrochemical energy conversion devices. In this report we demonstrate an efficient surface modification of carbon nanotubes (CNT) through one-step oxygen plasma irradiation that induces doping and charge redistribution around the doped heteroatom oxygen that promotes ORR activity. The defect sites generated owing to oxygen dopant in CNTs were confirmed by Raman spectra, X-ray photoelectron spectroscopy (XPS) surface composition and CHNO elemental analysis. The O-doped CNTs were thoroughly characterized by Fourier transform infrared (FT-IR) spectroscopy, scanning electron microscopy (SEM) and high-resolution transmission electron microscopy (HR-TEM) measurements. Our research shows that plasma-treated single-walled carbon nanotubes (SWCNT) are more effective ORR catalyst compared to multi-walled carbon nanotubes (MWCNT) due to the inherent structure of SWCNT that can access more defects and surface functional groups than MWCNT.In the following study, plasma modification approach was implemented in the synthesis of a highly active robust Metal-Nitrogen-Carbon (M-N-C) catalysts through NH3-plasma pretreatment on high surface area black pearl carbon. Pre-treated FeNC/NH3-C contains Fe3C and Fe-Nx as catalytically active sites. XPS confirms the presence of a high surface concentration of pyrrolic to pyridinic nitrogen and ferrous sites on FeNC/NH3-C that promotes low oxygen reduction overpotential (0.80 V vs RHE in 0.5 M H2SO4 and 0.96 V vs RHE in 0.1 M KOH). Hence plasma activates the surface and incorporates nitrogen which forms stable Fe-N coordination for accelerating oxygen reduction. Importantly, FeNC/NH3-C shows lower peroxide yield at 0.6V/RHE (<20% in 0.5 M H2SO4 and <2 % in 0.1 M KOH) that can reduce the detrimental effect of H2O2 on the catalysts support and exhibits high stability, seen from thousands of potential cycling stability tests of FeNC/NH3-C in acid solutions. Thereby this plasma-based new synthetic approach can promote more active and stable non-precious metal catalysts.