Oxygen Reduction Reaction Half-wave Potential Research Articles

A superior unitary oxygen electrode with accelerated mass transfer and highly exposed active sites for rechargeable air-based batteries

Nitrogen doped carbon is a promising candidate for noble Pt and IrO2 to catalyze oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). However, its catalytic activity is principally restricted by the low density of exposed active sites. Herein, we design hierarchically structured oxygen electrodes with a hierarchical nitrogen doped carbon layer on macroporous carbon cloth (termed as CC@NCLs). The outer NCLs are well-controlled with the same chemical components and defect degrees, but different geometric structures. It is noted that only catalytic moieties adjacent to mesopores and macropores can be utilized. The multi-scaled CC@NCL possesses a large oxygen diffusion coefficient (4.97 × 10−3 cm2 s−1) and thereby a high utilization of catalytic sites. Resultingly, CC@NCL exhibits excellent catalytic activity for both ORR (half-wave potential of 0.855 V) and OER (1.580 V at 10 mA cm−2), and shows preferable activity and stability over noble Pt and IrO2 in zinc air flow batteries. This provides an insight for developing high density and accessible active sites in the practical energy device, including, but not limited to, metal-air battery, CO2 electrolyser and N2 electrolyser.

Enhanced activity towards oxygen electrocatalysis for rechargeable Zn-air batteries by alloying Fe and Co in N-doped carbon.

Large-scale application of rechargeable Zn-air batteries requires low-cost electrocatalysts for the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) as alternatives to noble metals. Herein, FeCo nanoparticles embedded in N-doped carbon (FeCo/N-C) were prepared by a two-step pyrolysis route. FeCo/N-C exhibits excellent activities toward both the ORR (half-wave potential of 0.84 V) and OER (overpotential of 345 mV at 10 mA cm-2), which are comparable to those of commercial Pt/C and RuO2, and by far exceeding their counterparts Fe/N-C and Co/N-C. Furthermore, the FeCo/N-C catalyst was evaluated in a rechargeable Zn-air battery for the full-cell test. The FeCo/N-C based battery is more durable with a smaller round-trip overpotential after 800 cycles than the battery using an expensive Pt/C + RuO2 mixture catalyst.

Constructing bifunctional Co/MoC@N-C catalyst via an in-situ encapsulation strategy for efficient oxygen electrocatalysis

The large-scale application of Zn-air battery requires the development of efficient, low-cost, and stable bifunctional electrocatalysts for the oxygen evolution reaction (OER) and oxygen reduction reaction (ORR). Herein, an electrocatalyst of a Co/MoC-nanoparticles embedded N-doped carbon (Co/[email protected]) was prepared via the in-situ encapsulation of Co2(CO)8 and Mo(CO)6 in ZIF-8 using ball-milling, followed by pyrolysis under an Ar atmosphere. When used as a bifunctional electrocatalyst, the as-prepared Co/[email protected] showed an ORR half-wave potential of 0.824 V in an alkaline medium and an overpotential of only 290 mV for the OER at 10 mA cm−2, which was 70 mV lower than RuO2. The Co/[email protected] was also evaluated as a cathode in a solid-state Zn-air battery and delivered a higher peak discharge power density and better cycling stability (over 1600 min) than Pt/C-RuO2. Most importantly, our work provides a new strategy to prepare bifunctional catalysts and promotes their applications in the energy conversion field.

Separation of Metal-N4 Units in Metal-Organic Framework for Preparation of M-Nx/C Catalyst with Dense Metal Sites.

Metal and nitrogen codoped carbon (M-Nx/C) materials with good metal dispersion in a carbon matrix have attracted great attention because they can efficiently catalyze various kinds of chemical/electrochemical reactions. However, the existing strategies for preparing M-Nx/C materials still face the challenge of metal site aggregation when the carbon skeleton in the precursor is substantially lost during pyrolysis. Herein, we propose a general strategy that the increase of metal site density and inhibition of metal aggregation could be realized by separating M-N4 units in metal-organic frameworks (MOFs). The metal sites can be well separated and distributed in carbon materials during the pyrolysis of a metal-coordinated block copolymer, one of the specific MOFs. The strategy can be widely applicable for the synthesis of M-Nx/C materials with dense metal sites, such as Fe-, Mn-, Ni-, and Co-Nx/C materials, which are potentially used as the efficient catalysts for various kinds of reactions. Taking Fe-Nx/C as a model oxygen reduction reaction (ORR) electrocatalyst, it shows the ORR half-wave potentials of 0.90 and 0.81 V vs RHE in alkaline and acidic electrolytes, respectively.

Amino functionalized carbon nanotubes supported CoNi@CoO–NiO core/shell nanoparticles as highly efficient bifunctional catalyst for rechargeable Zn-air batteries

Oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) are the core reaction processes of rechargeable Zn-air battery (ZAB) cathode. Therefore, exploring a bifunctional catalyst with excellent electrochemical performance, high durability, and low cost is essential for rechargeable ZAB. In this work, amino functionalized carbon nanotubes supported core/shell nanoparticles composed of CoNi alloy core and CoO–NiO shell (CoNi@CoO–NiO/NH2-CNTs-1) is synthesized through a simple and efficient hydrothermal reaction and calcination method, which shows higher ORR/OER bifunctional catalytic performance than the single metal-based catalyst, such as Ni@NiO/NH2-CNTs and Co@CoO/NH2-CNTs. The fabricated bimetallic alloy based catalyst CoNi@CoO–NiO/NH2-CNTs-3 with the optimized loading content of CoNi@CoO–NiO core/shell nanoparticles, presents the best bifunctional catalytic performance for ORR/OER. Experimental studies reveal that CoNi@CoO–NiO/NH2-CNTs-3 exhibits the onset potential of 0.956 V and 1.423 V vs. RHE for ORR and OER, respectively. It also exhibits a low overpotential of 377 mV to achieve a 10 mA cm−2 current density for OER, and positive half-wave potentials of 0.794 V for ORR. And the potential difference between half-wave potential of ORR (E1/2) and the potential at 10 mA cm−2 for OER (Ej10) is 0.813 V. In addition, when CoNi@CoO–NiO/NH2-CNTs-3 is used as an air electrode catalyst of rechargeable ZAB, its maximum power density and open circuit voltage (OCV) can reach 128.7 mW cm−2 and 1.458 V (The commercially available catalyst of Pt/C–RuO2 is 88.1 mW cm−2), which strongly demonstrates that the fabricated catalyst CoNi@CoO–NiO/NH2-CNTs-3 can be used as a highly efficient bifunctional catalyst for ZABs, and is expected to replace those expensive precious metal electrocatalysts to meet the growing demand for new energy devices.

Unveiling the Potential of an Fe Bis(terpyridine) Complex for Precise Development of an Fe-N-C Electrocatalyst to Promote the Oxygen Reduction Reaction.

Improvements in highly efficient precious-metal-free electrocatalysts for the oxygen reduction reaction (ORR) are extremely important but still a significant challenge. Herein, we report a novel catalyst design strategy integrating a bis(terpyridine) (hexadentate chelating ligand) with Fe which acts as nitrogen, a self-supporting carbon source, and a potent metal-ligand active site binding structure (Fe-btpy) and promotes the formation of Fe-Nx/C active sites, bypassing the complications induced during Fe-N-C catalyst synthesis. The resulting Fe-N/C(H,P) electrocatalyst shows a very high ORR onset (Eonset) and half-wave potential (E1/2) of 1.05 and 0.89 V (vs reversible hydrogen electrode), respectively, outperforming the commercial Pt/C catalyst in alkaline medium. Most importantly, the Fe-N/C(H,P) catalyst displays decent stability and remarkable methanol tolerance in comparison to the Pt/C catalyst. A fabricated rechargeable zinc-air battery with an Fe-N/C(H,P) cathode catalyst demonstrated an excellent peak power density of 225 mW cm-2 at a current density of 240 mA cm-2, in comparison to the Pt/C cathode catalyst. This work illuminates blueprints utilizing a new long-chain one-dimensional macromolecule that could be viable to produce Fe-N/C-based carbon electrocatalysts toward energy conversion applications.

Co2P Embedded into P and N Dual-Doped Carbon Nanotubes and Graphene as a Bifunctional Oxygen Electrocatalyst

The reasonable design of inexpensive and efficient bifunctional electrocatalyst towards oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) is still challenging for Zn-air batteries. Herein, a novel and effective design is proposed to fabricate Co2P nanoparticles encapsulated in doped carbon nanotubes and rGO sheets (Co2P/NPCNTs-rGO) through a direct self-catalyzed growth procedure. That is, cobalt phthalocyanine derivatives as the cobalt sources are not only employed to form Co2P nanoparticles, but also used to catalyze the formation of CNTs and rGO. Benefiting from the unique structure and composition, the Co2P/NPCNTs-rGO-800 shows significant OER (overpotential of 420 mV) and ORR (half-wave potential of 0.73 V) properties. Most importantly, the activity parameter of Co2P/NPCNTs-rGO-800 (ΔE=0.92 V) is comparable to that of RuO2(0.91 V) and lower than that of Pt/C (1.05 V), indicating that Co2P/NPCNTs-rGO-800 is of a good comprehensive ORR/OER bifunctional performance. This work provides an effective strategy for the synthesis of outstanding multifunctional catalysts for wider practical applications.

Deep-Breathing Honeycomb-like Co-Nx-C Nanopolyhedron Bifunctional Oxygen Electrocatalysts for Rechargeable Zn-Air Batteries.

SummaryMetal organic framework (MOF) derivatives have been extensively used as bifunctional oxygen electrocatalysts. However, the utilization of active sites is still not satisfactory owing to the sluggish mass transport within their narrow pore channels. Herein, interconnected macroporous channels were constructed inside MOFs-derived Co-Nx-C electrocatalyst to unblock the mass transfer barrier. The as-synthesized electrocatalyst exhibits a honeycomb-like morphology with highly exposed Co-Nx-C active sites on carbon frame. Owing to the interconnected ordered macropores throughout the electrocatalyst, these active sites can smoothly “exhale/inhale” reactants and products, enhancing the accessibility of active sites and the reaction kinetics. As a result, the honeycomb-like Co-Nx-C displayed a potential difference of 0.773 V between the oxygen evolution reaction potential at 10 mA cm−2 and the oxygen reduction reaction half-wave potential, much lower than that of bulk-Co-Nx-C (0.842 V). The rational modification on porosity makes such honeycomb-like MOF derivative an excellent bifunctional oxygen electrocatalyst in rechargeable Zn-air batteries.

Inner Sphere Electron Transfer Promotion on Homogeneously Dispersed Fe-Nx Centers for Energy-Efficient Oxygen Reduction Reaction.

The study reports the optimized incorporation of pyridinic nitrogen in nitrogen-doped carbon nanotubes (CNTs) to realize effective Fe-Nx centers throughout the framework. The study unveils nitrogen as a valuable asset to promote the homogeneous dispersion of Fe moieties throughout the CNT framework, which is a necessary component to institute uniform Fe-Nx centers. In addition, pyridinic nitrogen causes disruption in strongly delocalized π-electrons, which impart electron-withdrawing nature in the carbon matrix, resulting in an anodic shift in oxygen reduction reaction (ORR) onset potential (Eonset). The direct interaction of Fe-Nx with O2, as evidenced by poisoning and computational studies, ensures the preferential inner sphere electron transfer mechanism. Despite the alkaline medium, the outer sphere electron transfer mechanism was muted, with suppressed HO2- generation, preferential 4e- reduction pathways, and excellent cyclic stability. The study indicates the dependency of ORR half-wave potential on the electron transfer mechanism. The poisoning study unveils the direct involvement of Fe-Nx electroactive centers in facilitating ORR in alkaline medium. It further indicates a noticable increase (up to ∼25%) in peroxide generation-an unwanted ORR intermediate-and concomitant reduction in average electron transfer no. per oxygen molecule.

Facile synthesis of cobalt nanoparticles encapsulated in nitrogen-doped carbon nanotubes for use as a highly efficient bifunctional catalyst in rechargeable Zn-Air batteries

Abstract The oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) have a critical function as the cathode reactions in rechargeable Zn-air batteries. In this work, a facile strategy for the scalable fabrication of cobalt nanoparticles encapsulated in nitrogen-doped carbon nanotubes (Co@NC) was used. The experimental results and density functional theory calculations demonstrated that carbon nanotubes with a good graphitization degree were obtained by introducing a certain amount of cobalt species. The Co@NC-3 catalyst exhibited good catalytic performance, with a potential gap (ΔE) between the Ej=10 value of the OER and ORR half-wave potential (E1/2) of only 0.816 V, which is lower than that of many highly efficient bifunctional catalysts reported in the literature. Moreover, rechargeable Zn-air batteries based on the Co@NC-3 catalyst generated a high power density (163.4 mW∙cm−2) and specific capacity (795.5 mA h∙g−1 at 10 mA∙cm−2). The results confirm that Co@NC-3 could be used as a highly efficient bifunctional catalyst for renewable energy conversion devices.

(Invited) Boosting Non-Precious Metal Catalysts Toward the Oxygen Reduction Reaction through Ionic Liquid Modification

Finding cost effective catalysts for the oxygen reduction reaction (ORR) is one of the most overriding challenges in the field of electrochemistry [1]. Carbon-based materials emerge as one of the promising candidates due to their remarkable advantages of low cost, abundant structural variety, tailorable surface chemistry, and good conductivity. Tremendous efforts have been made to improve the performance of carbon-based materials for the ORR, mainly through modifying the inherent structures of carbon by doping heteroatoms (e.g., B, N, P) or combining with metal/metal oxides. We have demonstrated recently that modifying Pt-based catalysts by a minor amount of ionic liquid (IL, e.g., [BMIM][NTf2], [MTBD][NTf2]) could significantly boost their ORR activity [2-5]. This innovative strategy has also been successfully transferred to some non-precious metal Fe-N-C catalysts, on which an ionic liquid ([BMMIM][NTf2]) modification is found to boost both their activity and stability [6]. In the current contribution, we will present our latest progress toward boosting the emerging zeolitic imidazolate framework (ZIF)-derived carbons (ZDC) for the ORR using IL modification strategy. First we verified again the boosting effect of IL toward the ORR on the as-synthesized ZDC materials. The half wave potential of ORR on these ZDCs is positively shifted by up to 18 mV and their intrinsic kinetic current (@ 0.85 V) is increased by a factor of 2 after being modified with [BMMIM][NTf2]. At the same time, it is observed that the boosting effect from IL modification is highly sensitive to the exact loading amount of the IL. Exceeding a specific loading amount, however, leads to a mass transport related activity drop. Moreover, it is also unraveled that the IL takes effect by increasing electrochemically accessible surface area, as reflected by the significantly enhanced double layer capacitance of ZDCs after IL modification. These results again demonstrate the great promise of the IL modification strategy as a generic method to improve ORR catalysts, and at the same time are anticipated to form the basis for an unprecedented perspective in developing high-performing non-precious metal catalysts for low temperature fuel cell applications.

Ultrasmall NiFe layered double hydroxide strongly coupled on atomically dispersed FeCo-NC nanoflowers as efficient bifunctional catalyst for rechargeable Zn-air battery

An atomically dispersed FeCo-NC material with the 3D flower-like morphology was used as a unique substrate for the controllable deposition of ultrasmall NiFe layered double hydroxide nanodots (termed as NiFe-NDs) to simultaneously promote the sluggish kinetics of oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). The size-limiting growth of NiFe-NDs (~4.0 nm in diameter) was realized via the confinement of the 3D flower-like mesoporous structure and the rich N/O functionality of FeCo-NC. Benefiting from the distinctive structure with the simultaneously maximum exposure of both OER and ORR active sites, the NiFe-ND/FeCo-NC composite showed an ORR halfwave potential of 0.85 V and an OER potential of 1.66 V in 0.1 mol L KOH at 10.0 mA cm. In-situ Raman analysis suggested the activity of OER was derived from the Ni sites on NiFe-ND/FeCo-NC. Moreover, the NiFe-ND/FeCo-NC-assembled Zn-air battery (ZAB) exhibited a very small discharge- charge voltage gap of 0.87 V at 20 mA cm and robust cycling stability. Furthermore, the NiFe-ND/FeCo-NC composite was also applicable for fabricating all-solid-state ZAB to power wearable electronics with superior cycling stability under deformation. Our work could enlighten a new applicable branch of atomically dispersed metal-nitrogen-carbon materials as unique substrates for fabricating multifunctional electrocatalysts.

N–S-codoped mesoporous carbons from melamine-2-thenaldehyde polymers on carbon nanotubes for oxygen reduction and Zn-air batteries

Low-cost melamine and 2-thenaldehyde were assembled into N, S-enriched polymers supporting on commercially carbon nanotubes (CNTs) by one-pot synthesis method. The direct carbonization of this composite precursors harvested the metal-free carbons with synergistic doping of N and S atoms without post-treatment. Abundant large-pore mesopores were formed in the derived N–S–Cs with higher surface area. Higher graphitization degree of N–S–Cs endow it with the low resistance and fast electron conductivity. As a result, the N–S–Cs-900 exhibited as an efficient cathode catalyst for oxygen reduction reaction (ORR) especially in alkaline electrolyte, with positive ORR half-wave potential and higher limiting current density. In addition, the metal-free N–S–C materials also had better performance in Zn-air batteries with an initial voltage gap with 1.487 ​V. This work developed a low-cost synthesis strategy to obtain metal-free N–S–Cs with impressive ORR performance.

Prussian blue analogue derived Pd-Co composite bifunctional electrocatalyst for Zn–air battery

Abstract The development of economical and efficient bifunctional electrocatalyst is quite important for rechargeable zinc-air batteries (ZABs). Herein, a prussian blue analogue (Co3[Co(CN)6]2, PBA) derived PBA-Pd-Co/C composite electrocatalyst with good oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) electrocatalytic properties was developed via using bits of PdCl2 to modify carbonized Co3[Co(CN)6]2. The results show that as-prepared PBA-Pd-Co/C possesses good positive ORR half-wave potential (0.82 V), feasible OER potential (1.73 V) of 10 mA cm−2, minor ORR and OER Tafel slopes (53.61 and 57.07 mV dec−1), as well as high ZAB power density (138.8 mW cm−2) and good galvanostatic charge-discharge cycling stability. These good performances of PBA-Pd-Co/C are mainly ascribed to the good synergistic effect of Co and Pd nanoparticles (NPs) composited in PBA-derived nitrogen-enriched graphite carbon. This work would be supportive of expanding other low-cost and high-efficiency PBA-derived electrocatalysts in the future.

Porous Organic Polymer-Derived Fe2P@N,P-Codoped Porous Carbon as Efficient Electrocatalysts for pH Universal ORR.

A new porous organic polymer (CP-CMP) was designed and synthesized via the direct polymerization of pyrrole and hexakis(4-formyl-phenoxy)cyclotriphosphazene, skipping the tedious synthetic procedure of porphyrin-monomers containing special groups. This special porous organic polymer (POP) serves as an “all in one” precursor for C, N, P, and Fe. Direct carbonization of this special POP afforded Fe2P@N,P-codoped porous carbons with hierarchical pore structure and high graphitization. Finally, the optimal catalyst (CP-CMP-900) prepared by carbonization of CP-CMP at 900 °C exhibited high efficiency for oxygen electroreduction. Typically, CP-CMP-900 presented an oxygen reduction reaction half-wave potential (E1/2) of 0.85, 0.73, and 0.65 V, respectively, in alkaline, neutral, and acidic media, close to those of commercial Pt/C in the same electrolyte (0.843, 0.71, and 0.74 V). Furthermore, it also displayed excellent methanol immunity and long-time stability in various electrolytes better than commercial Pt/C (20%).

An alternate aqueous phase synthesis of the Pt3Co/C catalyst towards efficient oxygen reduction reaction

AbstractCarbon supported Pt-Co alloys are among the most promising electrocatalysts towards oxygen reduction reaction (ORR) for the application in low temperature fuel cells and beyond, thus their facile and green synthesis is highly demanded. Herein we initially report an alternate aqueous phase one-pot synthesis of such catalysts (containing nominally ca. 20 wt.% Pt) based on dimethylamine borane (DMAB) reduction. The as-obtained electrocatalyst (denoted as Pt3Co/C-DMAB) is compared with the ones obtained by NaBH4 and N2H4·H2O reduction (denoted as Pt3Co/C-NaBH4 and Pt3Co/C-N2H4·H2O, respectively) as well as a commercial Pt/C, in terms of the structure and electrocatalytic property. It turns out that Pt3Co/C-DMAB exhibits the highest ORR performance among all the tested samples in an O2-saturated 0.1 mol/L HClO4, with the mass activity (specific activity) ca. 4 (6) times as large as that for Pt/C. After 10000 cycles of the accelerated degradation test, the half-wave potential for ORR on Pt3Co/C-DMAB decreases only by 4 mV, in contrast to 24 mV for that on Pt/C. Pt3Co/C-NaBH4 or Pt3Co/C-N2H4·H2O shows a specific activity comparable to that for Pt3Co/C-DMAB, but a mass activity similar to that for Pt/C. ICP-AES, TEM, XRD and XPS characterizations indicate that Pt3Co nanoparticles are well-dispersed and alloyed with a mean particle size of ca. 3.4 plusmn; 0.4 nm, contributing to the prominent electrocatalytic performance of Pt3Co/C-DMAB. This simple aqueous synthetic route may provide an alternate opportunity for developing efficient practical electrocatalysts for ORR.

Covalent organic polymer-derived carbon nanotube-twined carbon nanospheres for efficient oxygen electroreduction and capacitance storage

Three-dimensional (3D) porphyrin-based covalent organic frameworks (COFs) supporting on carbon nanotubes (CNTs) were prepared by one-pot polymerization method. After annealing the prepared COF@CNT composites, CNT-twined nitrogen-doped carbon nanospheres (N-C@CNTs) were formed. N-C@CNTs-800 prepared at 800 °C demonstrated the efficient electrocatalytic performance for oxygen reduction reaction (ORR). The ORR half-wave potential of N-C@CNTs-800 reached about 0.87 V in KOH and 0.73 V in HClO4, comparable with the best non-precious metal catalysts (NPMCs). Moreover, the specific electric double-layer capacitance of N-C@CNTs-800 was about 250 F g−1, larger than many similar carbons reported previously. The special structure of COF precursors consisting of the flexible triangular pyramid-shaped tris(4-formylphenyl)amine and the rigid porphyrin units was favorable for the exposure of active N species on carbon surface, and also facilitated the fabrication of CNT-twined nitrogen-doped carbon nanospheres. The used CNTs clearly contributed to the enhanced graphitization degree and abundant porosities of the carbonized N-C@CNTs. Thermal conversion of COF@CNT composites provided a simple synthesis strategy for preparing robust carbon electrode materials for oxygen electroreduction and capacitance storage.

Climbing the Apex of the ORR Volcano Plot via Binuclear Site Construction: Electronic and Geometric Engineering.

Great enthusiasm in single-atom catalysts (SACs) for the oxygen reduction reaction (ORR) has been aroused by the discovery of M-NX as a promising ORR catalysis center. However, the performance of SACs lags far behind that of state-of-the-art Pt due to the unsatisfactory adsorption-desorption behaviors of the reported catalytic centers. To address this issue, rational manipulation of the active site configuration toward a well-managed energy level and geometric structure is urgently desired, yet still remains a challenge. Herein, we report a novel strategy to accomplish this task through the construction of an Fe-Co dual-atom centered site. A spontaneously absorbed electron-withdrawing OH ligand was proposed to act proactively as an energy level modifier to empower easy intermediate desorption, while the triangular Fe-Co-OH coordination facilitates O-O bond scission. Benefiting from these attributes, the as-constructed FeCoN5-OH site enables an ORR onset potential and half-wave potential of up to 1.02 and 0.86 V (vs RHE), respectively, with an intrinsic activity over 20 times higher than the single-atom FeN4 site. Our finding not only opens up a novel strategy to tailor the electronic structure of an atomic site toward boosted activity but also provides new insights into the fundamental understanding of diatomic sites for ORR electrocatalysis.

Surface-engineered cobalt nitride composite as efficient bifunctional oxygen electrocatalyst

Efficient and low-cost bifunctional catalysts for oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) are essential for the practical application of rechargeable metal–air batteries. In this work, we developed an efficient cobalt nitride hybrid bifunctional electrocatalyst, which consists of sulfur-doped and mildly oxidized Co5.47N nanoparticles supported on nitrogen-doped reduced graphene oxide sheet (O-S-Co5.47N@N-RGO). The composite exhibits good ORR-OER catalytic activity and excellent stability as well. It delivers an ORR half-wave potential of 0.82 V and an over-potential of 380 mV for OER at 10 mA cm−2 in 0.1 M KOH electrolyte. Density functional theory calculations indicate that the ORR activity of the composite might have originated from the Co-N4 site in the RGO sheet, whereas the surface Co sites on O-S-Co5.47N crystal are responsible for its OER activity. The facile preparation method and insight into the ORR-OER active sites reported in this study advances the development of high-performance bifunctional oxygen electrocatalyst.

Zinc Cobalt Sulfides on N, S Co-Doped Carbon Derived from Bimetallic-Organic Framework As an Advanced Electrocatalyst for Oxygen Reduction Reaction

Recently, the environmental problems caused by excess use of fossil fuels triggered intense research for efficient, inexpensive, and harmless energy conversion and storage devices such as fuel cells and rechargeable metal air batteries. In these applications, the oxygen reduction reaction (ORR) are a key electrochemical process for commercialization. Also, developing non-noble metal-based electrocatalysts with high electrochemical performance is very important in economic point of view. To address these issues, electrocatalysts based on nanocarbon frameworks such as metal organic frameworks (MOFs) and zeolitic imidazolate framework (ZIF) have been widely investigated as promising alternatives to Pt-based catalysts. In this work, we prepared bi-metallic ZIFs by coprecipitation of zinc and cobalt ions in the presence of 2-methylimidazole, followed by one-step sulfidation and carbonization to synthesize zinc cobalt sulfides supported on N, S co-doped porous carbon. The as-synthesized catalyst maintained inherent dodecahedral particle shape of ZIF. Furthermore, it exhibits outstanding ORR onset-potential (0.96 V vs. RHE) and half-wave potential (0.83 V vs. RHE), which is comparable to commercial Pt/C catalyst in alkaline electrolyte. Such outstanding electrochemical properties are mainly ascribed to the synergistic effect of the chemical composition, high electrochemical active surface area, and abundant active sites of the dodecahedral structure. These findings suggest that the transition metal sulfides on N, S co-doped porous carbon can be a promising alternative for the next generation of energy storage and conversion technologies.