Potential For Oxygen Reduction Reaction Research Articles

Synthesis of a Co-Nx type catalyst derived from the pyrolysis of a covalent triazine-based framework for oxygen reduction reaction

• A Co N C catalyst had been developed using an inexpensive and N -rich precursor. • The covalent triazine-based framework provided high density and evenly distributed M N C sites. • The HAT-CN-Co/C-800 catalyst showed the highest ORR activity close to Pt/C. The preparation of non-noble metal catalysts with performance in the oxygen reduction reaction (ORR) comparable or superior to platinum-based catalysts remains challenging. In the present study, hexaazatriphenylenehexacarbonitrile (HAT-CN), which is rich in both cyano groups and pyrazine units, was used as the precursor and then cyclotrimerized into a covalent triazine-based framework with a macrocyclic structure containing abundant complexation sites for transitional metal ions. Subsequently, a series of non-platinum Co N C type catalysts (HAT-CN-Co/C) were prepared, and then pyrolyzed at different temperatures (700 °C, 800 °C, and 900 °C). The pyrolysis treatment endowed the catalysts with activated Co-N x sites, large pore sizes and high specific surface areas, which can promote oxygen transfer and accelerate the ORR catalytic process. It is found that the catalyst (HAT-CN-Co/C-800) obtained from pyrolysis at 800 °C possessed the best ORR activity and followed the four- electrons transfer pathway; the onset potential of ORR was determined as 0.972 V vs reversible hydrogen electrode (RHE), very close to that of Pt/C (0.993 V vs RHE), and the half-wave potential (0.895 V vs RHE) was significantly higher than that of Pt/C (0.859 V vs RHE). Furthermore, the HAT-CN-Co/C-800 catalyst showed good durability (83.2 % current retention after 28000 s) and significant methanol tolerance with good application potential as a substitute for the platinum-based ORR catalyst in fuel cell.

Why Does Pt Shell Bearing Tensile Strain Still Have Superior Activity for the Oxygen Reduction Reaction?

Nanoparticles with a 55 nm Au core and a few monolayers of Pt shell (denoted as Au@Ptx ML, x = 1–4) are found to have superior activity toward the oxygen reduction reaction (ORR) compared to commercial Pt nanoparticles and polycrystalline Pt (pc-Pt) electrodes in 0.1 M HClO4. The half-wave potential for ORR at Au@Pt1 ML is ca. 20 mV positively shifted. With the decrease of the Pt shell thickness from 4 to 1 monolayer, both the extent of enhancement of the ORR activity and the adsorption strength of oxygen-containing species increase; the latter is confirmed by results from surface-enhanced Raman spectroscopy (SERS) and X-ray photon emission spectroscopy (XPS). Furthermore, SERS results also reveal that adsorbed CO on such catalysts displays a blue shift in Pt–CO and a red shift in C–O stretching frequencies compared to that at pc-Pt under otherwise identical conditions. Positive double layer effects induced by diminishing surface charge density, which leads to the less rigid first layer of water, are suggested to be the origin of the superior ORR performance of Au@Ptx ML to that of commercial Pt/C catalysts.

Bifunctional Catalytic Activity of γ-NiOOH toward Oxygen Reduction and Oxygen Evolution Reactions in Alkaline Solutions

Nickel oxyhydroxides (NiOOHs) are well-known for their superior activity toward oxygen evolution reaction (OER) in alkaline solutions. However, their activity toward oxygen reduction reaction (ORR) has been largely unexplored. There exist three NiOOH polymorphs: α-, β-, and γ-NiOOH, characterized by different interlayer spacing. Although still debated, γ-NiOOH with a large layer spacing has been indicated as the active phase for OER. Here, a highly crystalline γ-NiOOH was prepared in a carbon matrix by the in situ electrochemical transformation of nickel dithiooxamide Ni(dto) in 1 M KOH solution. The catalyst prepared in this way showed low overpotential not only for OER, but also for ORR in alkaline solutions. The onset potential for ORR is ~0.81 V vs. RHE, and the reaction proceeds via the 2e− transfer pathway. The high OER catalytic activity and relatively low ORR overpotential make this nanocomposite catalyst a good candidate for bifunctional OER/ORR catalyst, stable in alkaline solutions.

Iron and Nickel Phthalocyanine‐Modified Nanocarbon Materials as Cathode Catalysts for Anion‐Exchange Membrane Fuel Cells and Zinc‐Air Batteries**

AbstractIron and nickel phthalocyanines along with different carbon supports, i. e., multi‐walled carbon nanotubes (MWCNT), graphene, carbide‐derived carbon, Vulcan carbon, and mesoporous carbon (MC, from Pajarito Powder, LLC), are used to prepare six bimetallic (Fe, Ni) N‐doped carbon‐based catalysts. The aim of this work is to investigate the electrocatalytic activity of bimetal phthalocyanine‐modified nanocarbon catalysts, e. g., the effect of carbon supports on the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER), including the anion‐exchange membrane fuel cell (AEMFC) and rechargeable zinc‐air battery (RZAB) configuration. The catalysts exhibit excellent electrocatalytic activity as exemplified by their half‐wave potential (E1/2) for ORR and the potential at which the OER current density reaches 10 mA cm−2 (Ej=10), but the best performing catalysts are FeNiN−MC (E1/2=0.88 V, Ej=10=1.58 V) and FeNiN−MWCNT (E1/2=0.87 V, Ej=10=1.59 V). In AEMFC analyses, FeNiN−MWCNT cathode provides peak power density (Pmax) of 406 mW cm−2, slightly higher than that of FeNiN−MC (Pmax=386 mW cm−2). Both catalysts exhibit a good RZAB performance (Pmax of 85 mW cm−2 for FeNiN−MWCNT). The assembled RZABs run stably for 48 h without any significant loss of performance.

Efficient functionalization of reduced graphene oxide by 3-(pyridin-2-yl)-1,2,4,5-tetrazine and design of hybrids with silver nanoparticles for electrocatalysis

The efficiency of graphene functionalization by 3-(pyridin-2-yl)-1,2,4,5-tetrazine using inverse demand Diels-Alder reaction was investigated in comparison with the symmetrical 3,6-bis(2-pyridyl)-1,2,4,5-tetrazine. Elemental compositions on the surface of functionalized graphene were analyzed by X-ray photoelectron spectroscopy (XPS), evidencing the higher amount of nitrogen when using the H-substituted tetrazine derivative. In a second step, silver nanoparticles were electrodeposited on these functionalized graphene sheets to enhance the electrocatalytic properties of graphene. The amount of silver determined by XPS increases with the amount of nitrogen introduced on graphene as it was intended for. The electrocatalytic properties have been assessed on the Oxygen Reduction Reaction (ORR), showing good performances in terms of ORR potential even with very small amounts of silver compared to previously reported data.

MOF-derived nitrogen-doped carbon-based trimetallic bifunctional catalysts for rechargeable zinc-air batteries

Zinc-air battery (ZAB) is a promising new metal-air energy system, but the large overpotentials of oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) around the air electrode lead to their poor energy efficiency. Herein, a novel bifunctional oxygen electrocatalyst is reported with the preparation of a zeolite imidazolate framework (ZIF-67) derived trimetallic composites decorated nitrogen-doped carbon, which consist of NiFe alloy and Co nanoparticles. The ZIF-derived porous N-doped carbon shell can speed up the mass transfer efficiency. Whereas the electronic effect between the formed NiFe alloy and Co nanoparticles, as well as the N-doped carbon framework can enrich the active centers and enhance the electrical conductivity. As a result, the NiFe-Co@NC-450 catalyst shows superior performance manifested as a small potential gap (ΔE = 0.857 V) between the overpotential at 10 mA cm−2 (E j=10) for OER (460 mV) and half-wave potential (E 1/2) for ORR (0.833 V). The liquid ZABs exhibit a high specific capacity reaching 798 mAh/gZn and a stable cycling performance at 10 mA cm−2 for more than 200 h. Meanwhile, the NiFe-Co@NC-450 based flexible ZABs also presents robust flexibility and stability. This study has certain implications for the development of economical, powerful and stable bifunctional catalysts for ZABs.

3D Co3O4‐RuO2 Hollow Spheres with Abundant Interfaces as Advanced Trifunctional Electrocatalyst for Water‐Splitting and Flexible Zn–Air Battery

AbstractExploiting efficient and stable electrocatalysts with trifunctional catalytic activity toward hydrogen evolution reaction (HER), oxygen evolution reaction (OER), and oxygen reduction reaction (ORR) act has a crucial role with sustainable energy development. Therefore, this study fabricates Co3O4‐RuO2 hollow spheres using a facile and eco‐friendly solvothermal and low temperature oxidation procedure followed by ice water treatment (IW‐Co3O4‐RuO2‐HS). The specific hollow nanostructure could provide sufficient active sites and channels in the electrocatalytic procedure. Then, the IW‐Co3O4‐RuO2‐HS presents small overpotentials toward HER (40 mV@ 10 mA cm−2) and OER (250 mV@ 10 mA cm−2), and high half‐wave potential for ORR (E1/2@ 0.79 V). Remarkably, the IW‐Co3O4‐RuO2‐HS also presents superior catalytic performances toward water‐splitting and flexible rechargeable Zn–air batteries. Furthermore, the water electrolysis can be driven by sustainable energy, including solar, wind, thermal energy, and the assembled flexible rechargeable Zn–air battery. This study provides a valid path to synthesize multifunctional electrocatalysts on energy‐related devices.

Ultrasmall Co9S8 nanocrystals on Carbon Nanoplates for Efficient Bifunctional Oxygen Electrocatalysis

Electrochemical energy storage and conversion technologies based on electrocatalysis have been attracting more and more attention addressing increasing concerns on fossil fuel crisis and environmental deterioration. Fuel cells, zinc-air batteries, and water electrolyzer are believed to be promising candidates due to the environmental friendliness and high efficiency. These systems are associated with key reactions including oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). Due to slow kinetics of these reactions, efficient electrocatalysts, e.g., Pt for ORR and RuOx/IrOx for OER, are usually required to overcome the energy barrier in electrochemical reactions to increase the reaction rate. However, the most advanced electrocatalysts are still based on above-mentioned noble metals with high cost and scarcity, which inevitably retards the large-scale commercialization of these noble metal-based energy systems. It is of great significance to replace noble metal catalysts with earth-abundant, cost-effective, and highly efficient catalysts. Here, we reported the controlled synthesis of ultrafine Co9S8 nanocrystals embedded in N, S-codoped multilayer-assembled carbon nanoplates (Co9S8/NSCP) for highly efficient oxygen electrocatalysis. The bifunctional Co9S8/NSCP electrocatalyst displays a high half-wave potential for ORR, and a low overpotential for OER in 0.1M KOH at a current density of 10 mA cm -2, much better than those of single component counterparts (Co9S8 or carbon) and comparable to noble metal catalysts. The high performance of Co9S8/NSCP can be attributed to the rationally designed hierarchical architecture with nanosized Co9S8 nanocrystals, rich N, S-codopants, highly exposed surface area, and protective graphitic layers, providing abundant active sites with full utilization and stable carbon support towards fast catalytic kinetics and durability. This work will promote further research on the development of highly efficient and stable non-noble metal electrocatalysts for ORR and OER.

(Keynote) Interpretation of Oxygen Reduction Activity Volcano Correlations for MN4 Molecular Catalysts Compared to Those for Metallic Electrodes

A classical well-known reactivity index in electrocatalysis is the binding energy of key intermediates to the active sites. For example, this is well documented for the catalytic activity of metal electrodes for the oxygen reduction reaction (ORR)[1], alloys and metal oxides and less studied for molecular catalysts [2]. The activity at constant potential (log(i)E) plotted versus the M-O2 binding energy has the shape of a volcano. This is well documented in several papers, especially by the group of Norksov for metallic electrodes [1] and can be applied to many electrochemical reactions. This has been extended to MN4 molecular catalysts. In contrast to metallic electrodes MN4 molecular catalysts have discrete energy levels and the active site is the central metal, surrounded by an organic ligand with an MN4 central moiety. Essentially one of the key intermediates is the binding of the reacting O2 molecule to the active sites at the rate determining step as: [MN4]ad + O2(aq) + e- ↔ [RMN4O2 -]ad where MN4 is a surface confined macrocyclic complex or a pyrolyzed catalysts bearing a MN4 active moiety embedded in a graphene or graphitic structure. As the ORR reaction involves the transfer of several electrons ( 2, 2+2 or 4 electrons), several adsorbed intermediates can be involved. If the controlling step is the first step, it is expected that for the binding step when DGad = 0 a maximum activity should be observed and the partial coverage of adsorbed intermediate is = 0.5. On thermodynamic ground this corresponds to a thermoneutral condition for the best catalyst. In this work we have tested this hypothesis by studying ORR in alkaline media using several iron porphyrins and iron phthalocyanines as catalysts immobilized on graphite and carbon nanotubes. The trends in reactivity versus the Fe(III)/(II) redox potential for ORR describe a typical volcano correlation. However if the currents are divided by the θFe(II), the fraction of active sites calculated at E=-0.24 V vs SCE using the Nernst equation, log(i/ θFe(II),)E vs. E°Fe(III)/(II) gives a straight line of slope -0.120V/decade. The maximum corresponds to θFe(II) = 0.5 and occurs at a potential close to that used for comparing the activities. A similar behaviour is observed for the oxidation of hydrazine but the continuation of the straight line is observed at potentials more positive than that of the maximum. Again the maximum is observed at the potential chosen for comparison. Those catalysts having Eº’ Fe(III)/(II) >> θFe(II) higher than -0.56V (for hydrazine oxidation) are in the Fe(III) state as predicted by the Nernst equation. That oxidation state Fe(III) is inactive for both reactions studied as OH- ions are strongly bound to Fe(III), especially in alkaline media. So the falling of the activities in the strong binding side of the volcano can be attributed preferentially to a gradual decline in the number of Fe(II) active sites and not to gradual decrease of the fraction (1-θ ) of empty or available active sites due to occupancy by intermediates. Schmickler and Santos [3] have shown that some volcano correlations for HER on metals that are in an oxidized form, and then inactive for this reason.We conclude that even though there are similarities between the volcano correlations for metallic and molecular catalysts, there some differences in the interpretation of the activity maximum that might not correspond to a thermoneutral condition. Acknowledgements: This work was supported by ANID, Anillo Project ACT 192175 and Fondecyt, Project 1181037

Benzenehexol-based 2D MOF as high-performance electrocatalyst for oxygen reduction reaction

Two-dimensional (2D) conductive metal–organic frameworks (MOFs) were an emerging class of potential oxygen reduction reaction (ORR) electrocatalysts to replace platinum-containing electrode materials. Herein, by first-principles calculations, we systematically studied the structural stability, electronic properties and ORR catalytic activity of metal-hexahydroxybenzene nanosheets, M3(C6O6), M = 3d and 4d transition metals. Based on formation energy, phonon spectrum and ab-initio molecular dynamics analyses, eight monolayer M3(C6O6) (M = Cr, Mn, Fe, Co, Cu, Zn, Ru and Rh) show superior thermodynamic, kinetic and thermal stabilities. Further ORR catalytic activity evaluation shown Cu3(C6O6) and Rh3(C6O6) had promising applications in catalyzing ORR 4e- pathway, with limiting potentials 0.76 and 0.80 V, respectively. The linear relationship between integrated crystal orbital Hamilton population (ICOHP) and ΔGOH∗ explains the suitable adsorption energy of OH* on the catalyst, resulting in the excellent ORR activity of Cu3(C6O6) and Rh3(C6O6). Our study on M3(C6O6) provides references for the research of other 2D MOFs as ORR catalysts.

Ionic liquid-derived FeCo alloys encapsulated in nitrogen-doped carbon framework as advanced bifunctional catalysts for rechargeable Zn-air batteries

Developing bifunctional ORR&OER electrocatalysts with high activity and stability is challenging for practical application of rechargeable Zinc air batteries. Herein, an ionic liquid strategy is proposed for construction of FeCo alloys encapsulated in nitrogen-doped carbon as bifunctional electrocatalyst for oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). By taking advantages of the unique properties such as solvent effect and low volatility, ionic liquid serves as solvent to disperse metal ions in the precursor preparation and nitrogen-containing carbon source during pyrolysis. The FeCo alloy nanoparticles encapsulated in the N-doped carbon matrix show a homogeneous distribution with average sizes of 50–100 nm. Such a core-shell structure endows the catalyst with both high ORR and OER activity and robust stability in alkaline environments. Especially, the half-wave potential (E 1/2 ) for ORR is 0.86 V and the overpotential (E over ) for OER to reach the 10 mA·cm −2 current density is only 280 mV. Furthermore, the obtained bifunctional electrocatalyst also displays high efficiency and great durability when applied in the rechargeable Zn-air battery. The specific capacity and energy density at the discharge current density of 10 mA·cm −2 reach up to 741.5 mAh·g −1 and 830.1 Wh kg −1 , respectively. It’s envisioned that this ultra-facile method would be applicable to synthesize highly efficient catalysts for other energy conversion systems. • The FeCo@NC electrocatalyst is prepared through a facile two-step method by employing the ionic liquid ([EMIM][DCA]). • The FeCo@NC electrocatalyst exhibits excellent ORR and OER activity and stability in alkaline environments. • The FeCo@NC based Zn-air battery displays high efficiency and good reversibility.

Non-Precious Metal-Doped Carbon Materials Derived From Porphyrin-Based Porous Organic Polymers for Oxygen Reduction Electrocatalysis.

The cathodic oxygen reduction reaction (ORR) is important in the development of renewable energy devices, to produce novel and non-precious metal catalysts with high electrocatalytic activity to reduce the consumption of non-renewable platinum (Pt) catalyst. In this work, we developed N-doped and Fe/N dual-doped porous carbons as catalysts for ORR simply by high-temperature pyrolysis of porphyrin-based conjugated microporous polymers (CMPs). By combination of heteroatom doping, highly porous structure and tubular morphology, the as-prepared carbon samples exhibited high electrocatalytic activity with 4-electron transfer mechanism, nearly close to the commercial Pt/C catalyst. In particular, among these samples, the Fe/N-CMP-1000 displayed a higher onset potential (0.95 eV), positive half-wave potential (0.85 eV) and limiting current density value (5.1 mA cm-2 ) as well as good durability and better methanol tolerance contrasting with Pt/C catalyst, suggesting that the as-prepared metal-free catalysts from porphyrin-based CMPs show great potential for ORR.

Harnessing Optimized Surface Reconstruction of Single-Atom Ni-Doped Ni-NiO/NC Precatalysts toward Robust H2O2 Production

Decentralized H2O2 production from a 2e– oxygen reduction reaction (ORR) pathway is appealing, wherein the rational design of electrocatalysts is the cornerstone rendering favorable catalytic activity/selectivity. Nevertheless, the real active moiety is elusive and the catalyst evolution during the reaction remains unexplored to date. Herein, we decipher in situ evolved surface chemistry of a Ni-based precatalyst (Ni SAC/Ni-NiO/NC-500) during the 2e– ORR process that results in a reconstructed catalyst (Ni SAC/Ni-NiO/NC-500-R). The phase transformation of the electrocatalyst is monitored by operando Raman spectroscopy and ex situ X-ray analysis. With the optimized catalyst in hand, a 2e– ORR potential of 0.78 V (at 1.0 mA cm–2) and a high selectivity of 95% in an alkaline electrolyte are harvested. More encouragingly, high-flux H2O2 is produced via photomediated electrosynthesis, enabling on-site degradation of organic pollutants with an impressive degradation rate approaching 100%.

Non‐Covalent Interaction of Atomically Dispersed Cu and Zn Pair Sites for Efficient Oxygen Reduction Reaction

AbstractDual–single‐atom catalysts with synergistic effect of adjacent atomic metal sites show a great potential for oxygen reduction reaction (ORR). Herein, a dynamical synthetic strategy is demonstrated for the rational design of dual‐atom catalyst ((Zn, Cu)−NC) with non‐covalent Cu and Zn sites as nitrogen‐doped carbon as support. Owing to the non‐covalent interaction of Zn and Cu atomic pair sites, (Zn, Cu)−NC exhibits significant performances for ORR, surpassing the catalysts with individual Zn or Cu site. The theoretical calculations reveal that (Zn, Cu)−NC can highly activate the linear O2 molecule via the non‐covalent interaction between Zn and Cu pairs, providing the more effective overlap between the metal 3d orbitals and O 2p orbital. Therefore, the ORR activity is optimized with the improvement of the adsorption configuration and adsorption energy of O2. Further, both liquid and quasi‐solid zinc−air batteries with (Zn, Cu)−NC as air cathodes achieve remarkable energy density and stability. This research proposes a facile synthetic strategy to construct single‐atom catalysts and presents an insightful understanding of the non‐covalent interplay between heteronuclear metal atoms in dual‐atom catalysts.

Carbon dots bridge NiO and Mn2O3 as highly efficient bifunctional oxygen electrocatalysts for rechargeable zinc-air batteries

The development of high-efficiency, low-cost and durable bifunctional oxygen electrocatalysts is of great significance for the large-scale application of rechargeable zinc-air batteries. Herein, the carbon dots bridging nickel oxide and manganese trioxide through the oxygen-containing group of carbon dots (NiO-Mn2O3-CDs) for oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) is proposed. The optimal NiO-Mn2O3-CDs yields remarkable electrocatalytic activity with low overpotential (298 mV) to derive the 10 mA·cm−2 for OER and a high half wave potential (0.84 V vs. RHE) for ORR. The outstanding performances are attributed to the CDs covalently bridging Ni and Mn atoms, which reprogram the electronic structure of active sites and enhance charge transfer. Consequently, the rechargeable zinc-air battery test with NiO-Mn2O3-CDs as the air cathode deliver the excellent performances, including a high open voltage (1.51 V), a maximum power density (287 mW∙cm−2 @ 0.35 A∙cm−2) and robust cycling life, outperforming the commercial Pt/C + IrO2. This work presents an efficient strategy to construct the highly active and stability bifunctional air cathodes for electrochemical energy devices.

Zinc-assisted MgO template synthesis of porous carbon-supported Fe-Nx sites for efficient oxygen reduction reaction catalysis in Zn-air batteries

Atomically dispersed iron-nitrogen-carbon catalysts offer great potential in oxygen reduction reaction (ORR), yet the poor exposure and low density of Fe-Nx sites causes relatively low ORR activity. Herein, a zinc-assisted MgO template strategy is reported to construct porous carbon-supported Fe-N4 sites (Fe-N-C). Iron atoms surrounded by zinc species are converted to abundant Fe-N4 sites rather than Fe containing nanoparticles. Meanwhile, both the zinc species and the MgO template can effectively produce porous structure so as to increase the utilization of Fe-N4 sites. Fe-N-C achieves superior ORR performance and stability in alkaline medium. Theoretical calculations manifest that Fe-N4 sites can narrow the energy barrier for ORR. Moreover, finite element simulation exhibits the porous framework in Fe-N-C could significantly accelerate the diffusion of O2. Therefore, Fe-N-C provides a high peak power density and superior discharge ability toward Zn-air batteries.

Doubling Micropore of Carbon Skeleton via Regulating Molecular Structure of Carbohydrate for Oxygen Reduction Reaction

Functional carbon materials prepared from biomass-derived carbohydrates have attracted extensive interests due to their low cost and high catalytic potential for oxygen reduction reaction (ORR). In this work, the pyranose ring, which is the basic repeating unit in lignocellulosic biomass, can be easily converted into micropore-riched carbon skeleton via regulating the side chain C-containing group. In striking contrast to glucopyranose, the simple pyranose ring (xylopyranose) exhibits outstanding nitrogen fixation ability. But higher N content in the carbon skeleton can lead to a decrease in C content, which is detrimental to the graphitization and pore-forming of carbon product. Benefiting from abundant micropore structures (volume increases from 0.22 cm3 g−1 to 0.50 cm3 g−1), the electrochemical surface area of Glu-SSC-950 is much larger than that of Xyl-SSC-950. Concretely, the double layer capacitance of Glu-SSC-950 (14.9 mF cm−2) is more than twice that of Xyl-SSC-950 (6.4 mF cm−2). Furthermore, the Glu-SSC-950 displays a more positive half-wave potential (0.84 V) and higher peak power density of Al-air battery (467.8 mW cm−2) compared with Xyl-SSC-950 (0.76 V and 373.6 mW cm−2). In general, it compensates for the shortcomings of the screening of carbohydrate precursors for preparing ORR carbonaceous electrocatalysts.

Encapsulation of Fe-CoP with P, N-co-doped porous carbon matrix as a multifunctional catalyst for wide electrochemical applications

Hydrogen evolution reaction (HER), oxygen evolution reaction (OER), and oxygen reduction reaction (ORR) serve as the most promising electrochemical processes for renewable energy. Developing multifunctional electrocatalysts with high performance for HER, OER and ORR still remains a challenge. Herein, we report a facile method to prepare P, N-co-doped porous carbon encapsulated Fe-CoP (A-Fe-CoP/CPN) electrocatalyst through freeze drying, carbonization, and KOH activation. The acquired A-Fe-CoP/CPN possesses abundant pores with large surface area, which provides sufficient defect-rich sites for HER, OER, and ORR, in alkaline or acidic electrolytes. The A-Fe-CoP/CPN shows low overpotential and good stability for HER and OER in alkaline electrolyte. Density functional theory (DFT) computations reveal that the addition of Fe greatly facilitates the rate-determining step and overall catalytic pathway for both HER and OER. A-Fe-CoP/CPN also exhibits low half-wave potential (E1/2) for ORR, which indicates an enhanced performance than that of the noble Pt/C catalyst. Zn-air battery equipped with the A-Fe-CoP/CPN catalyst exhibits a high energy density (849 mAh g−1) and a long-time stability at 5 mA cm−2. This finding not only offers a facile strategy to prepare porous carbon materials with defect-rich sites, but also provides guidance for developing multifunctional electrocatalysts for wide electrochemical reactions.

Covalent organic polymer derived N–doped carbon confined FeNi alloys as bifunctional oxygen electrocatalyst for rechargeable zinc-air battery

N–doped carbon confined FeNi alloys are promising candidate to noble Pt and IrO2 or RuO2 for oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) in rechargeable zinc-air batteries. However, it is difficult to control the distribution of transition metals in the precursor and electrocatalyst. Herein, we design a covalent organic polymer to realize the uniform distribution of metal, nitrogen and carbon precursors. The structure of the obtained electrocatalyst is FeNi nanoparticles coated with carbon shells dispersed on N-doped multilayer porous carbon (FeNi@NC). The resultant FeNi@NC delivered a half-wave potential for ORR of 0.878 V and a low potential of 1.59 V to achieve 10 mA cm−2 for OER, which surpasses commercial platinum/carbon and ruthenium dioxide. The outstanding bifunctional properties of FeNi@NC attribute to the synergistic coupling between N-doped carbon shells and dense FeNi nanoparticles. Moreover, the self-made zinc-air battery with FeNi@NC air-cathode displayed an excellent energy density of 137.7 mW cm−2 as well as cycling stability (100 h, 200 cycles).

Nanocarbon-Induced Etching Property of Semiconductor Surfaces: Testing Nanocarbon’s Catalytic Activity for Oxygen Reduction Reaction at a Single-Sheet Level

In the field of fuel cells, the microscopic understanding of the catalytic activity of nanocarbons or graphene-based materials for oxygen reduction reaction (ORR) is highly demanded. In the current study, a novel concept is presented to achieve it, which is a different approach from familiar electrochemical measurements. To prove this concept, we prepared two nanocarbon materials, whose ORR activities were tested and compared via macroscale cyclic and linear sweep voltammetry. Next, considering the electrochemical potential of ORR and the band-edge position of semiconductors, we chose single-crystalline Ge as the substrate on which the single-sheet nanocarbons were dispersed. We found that etched hollows formed under the loaded nanocarbons after immersing the nanocarbon/Ge sample into O2-containing water. The histogram analysis of the hollows represented the difference in catalytic activities to promote Ge etching between the used nanocarbons, and the corresponding trends agreed qualitatively with the electrochemical measurements.