Enhanced Oxygen Reduction Reaction Research Articles

Oxygen reduction reaction enhancement in microbial fuel cell cathode using cesium phosphomolybdate electrocatalyst

The sluggish oxygen reduction reaction (ORR) is a significant hurdle against microbial fuel cell (MFC) performance. Polyoxometalates are excellent candidates to address this challenge because they have the potential of improving ORR efficiency. Here, cesium phosphomolybdate was successfully synthesized through a facile solid-state method and then used as an ORR catalyst on a graphite cathode in an MFC with sodium acetate as the substrate in the anolyte. The CV analysis demonstrated that modified graphite has two ORR peaks at −0.25, −0.5 V vs Ag/AgCl and a higher current density than the bare graphite. The MFC equipped with the modified cathode displayed a maximum power density of 64.73 mW m−2, higher than that obtained for bare graphite (42 mW m−2). COD removal efficiencies in MFCs with modified and bare graphite cathodes were 86.4% and 85%, respectively which are similar to or better than what is published in the literature. This study offers an efficient ORR catalyst in MFC applications helping in using organic wastewater as a fuel by converting the chemical energy into electricity.

Tuning the charge distribution and crystal field of iron single atoms via iron oxide integration for enhanced oxygen reduction reaction in zinc-air batteries

Metal-air batteries face a great challenge in developing efficient and durable low-cost oxygen reduction reaction (ORR) electrocatalysts. Single-atom iron catalysts embedded into nitrogen doped carbon (Fe-N-C) have emerged as attractive materials for potential replacement of Pt in ORR, but their catalytic performance was limited by the symmetrical electronic structure distribution around the single-atom Fe site. Here, we report our findings in significantly enhancing the ORR performance of Fe-N-C by moderate Fe2O3 integration via the strong electronic interaction. Remarkably, the optimized catalyst (M−Fe2O3/FeSA@NC) exhibits excellent activity, durability and good tolerance to methanol, outperforming the benchmark Pt/C catalyst. When M−Fe2O3/FeSA@NC catalyst was used in a practical zinc-air battery assembly, peak power density of 155 mW cm−2 and specific capacity of 762 mA h gZn−1 were achieved and the battery assembly has shown superior cycling stability over a period of 200 h. More importantly, theoretical studies suggest that the introduction of Fe2O3 can evoke the crystal field alteration and electron redistribution on single Fe atoms, which can break the symmetric charge distribution of Fe-N4 and thereby optimize the corresponding adsorption energy of intermediates to promote the O2 reduction. This study provides a new pathway to promote the catalytic performance of single-atom catalysts.

Protection Against Absorption Passivation on Platinum by a Nitrogen-Doped Carbon Shell for Enhanced Oxygen Reduction Reaction.

In polymer electrolyte type fuel cells, the platinum-based catalysts are applied for the oxygen reduction reaction. However, the specific adsorption from the sulfo group in perfluorosulfonic acid ionomers has been considered to passivate the active sites of the platinum. Herein, we present platinum catalysts covered by an ultrathin two-dimensional nitrogen-doped carbon shell (CNx) layer to protect the platinum from the specific adsorption of perfluorosulfonic acid ionomers. Such coated catalysts were obtained by the facile polydopamine coating method, which is available to tune the thickness of the carbon shell by tuning the polymerization time. The coated catalysts that possess a CNx with a thickness of 1.5 nm demonstrated superior ORR activity and comparable oxygen diffusivity when compared to the commercial Pt/C. These results were supported by the changes in the electronic statements observed in the X-ray photoelectron spectroscopy (XPS) and CO stripping analyses. Furthermore, the oxygen coverage, CO displacement charge, and operando X-ray absorption spectroscopy (XAS) tests were employed to identify the protection effect of CNx in coated catalysts compared with the Pt/C catalysts. In summary, the CNx could not only suppress the oxide species generation but also prevent the specific adsorption of the sulfo group in the ionomer.

Locking nitrogen dopants in porous carbons through in situ grafting strategy for enhanced oxygen reduction reactions

We report here the synthesis of N-doped porous carbon (PC) through a convenient melamine pre-grafting strategy. The melamine derivative with a monolayer structure was first grafted on the surface of the micropores of PC using a suitable diazotization reaction of melamine in an aqueous solution. After pyrolysis, PC grafting with melamine derivative was converted to the obtained N-doped PC. Meanwhile, more nitrogen dopants were grafted, and nitrogen functionalities were reconstructed to generate more graphitic N and pyridinic N atoms, in which the mesopores volume was consequently enlarged. Because of the unique structures mentioned above, the as-obtained N-doped PC exhibited enhanced oxygen reduction activity under alkaline compared with the N-doped PC from the standard physical mixture of PC and melamine. Besides, the obtained N-doped PC showed a comparable half-wave potential, remarkable durability, and good methanol tolerance compared with the commercial 20% Pt/C electrocatalyst. The results presented here could guide the development of new and improving metal-free nitrogen-doped porous carbon catalysts toward oxygen reduction reaction.

Fe/Co dual metal catalysts modulated by S-ligands for efficient acidic oxygen reduction in PEMFC.

Here, we report a conceptual strategy for introducing spatial sulfur (S)-bridge ligands to regulate the coordination environment of Fe-Co-N dual-metal centers (Spa-S-Fe,Co/NC). Benefiting from the electronic modulation, Spa-S-Fe,Co/NC catalyst showed remarkably enhanced oxygen reduction reaction (ORR) performance with a half-wave potential (E1/2) of 0.846 V and satisfactory long-term durability in acidic electrolyte. Combined experimental and theoretical studies revealed that the excellent acidic ORR activity with a remarkable stability observed for Spa-S-Fe,Co/NC is attributable to the optimal adsorption-desorption of ORR oxygenated intermediates achieved through charge-modulation of Fe-Co-N bimetallic centers by the spatial S-bridge ligands. These findings provide a unique perspective to regulate the local coordination environment of catalysts with dual-metal-centers to optimize their electrocatalytic performance.

Lignin-derived iron carbide/Mn, N, S-codoped carbon nanotubes as a high-efficiency catalyst for synergistically enhanced oxygen reduction reaction and rechargeable zinc-air battery

Design of efficient and durable oxygen reduction reaction (ORR) electrocatalysts still remains challenge in sustainable energy storage and conversion devices. To achieve sustainable development, it is of importance to prepare high-quality carbon-derived ORR catalysts from biomass. Herein, Fe5C2 nanoparticles (NPs) were facilely entrapped in Mn, N, S-codoped carbon nanotubes (Fe5C2/Mn, N, S-CNTs) by a one-step pyrolysis of the mixed lignin, metal precursors and dicyandiamide. The resulting Fe5C2/Mn, N, S-CNTs had open and tubular structures, which exhibited positive shifts in the onset potential (Eonset = 1.04 V) and high half-wave potential (E1/2 = 0.85 V), showing excellent ORR characteristics. Further, the typical catalyst-assembled Zn-air battery showed a high power density (153.19 mW cm−2) and good cycling performance as well as obvious cost advantage. The research provides some valuable insights for rational construction of low-cost and environmentally sustainable ORR catalysts in clean energy field, coupled by offering some valuable insights for reusing biomass wastes.

Insights into local coordination environment of main group metal-nitrogen-carbon catalysts for enhanced oxygen reduction reaction

For the 4e– oxygen reduction reaction (ORR) on main group metal-nitrogen-carbon catalysts, the effect of the local coordination environment of main group metals on ORR has rarely been reported. By performing density functional theory (DFT) calculations, we screen 18 MN4–xOx (x = 0–2, M = Mg, Ca, Sr, Al, Ga, In) candidates with different metal coordination environments and construct volcano curve between the adsorption free energy of OH* (ΔGOH*) and ORR overpotential, yielding MgN2O2 at the peak of the volcano with the overpotential as low as 0.55 V. The lower ORR overpotential on Mg coordinated with 2 N and 2 O atoms than other coordination environments is well explained by the decreased ΔGOH* because of more shifts of p-band position of Mg away from the Fermi energy level as well as the lower O highest occupied energy level when the number of the coordinated O atoms increases. Our DFT work provides theoretical guidance for engineering ORR performance of main group metal-nitrogen-carbon catalysts by modulating coordination environments.

Sr3Fe1.5Zn0.5O7-δ as Cathode for Protonic-Conducting Solid Oxide Fuel Cells with Enhanced Oxygen Reduction Reaction Activity

Perovskite oxides have attracted intensive attention for their good electrochemical performance and stability as electrodes for protonic ceramic fuel cells (PCFCs). However, operating under high temperatures and various atmospheres results in a large polarization resistance and sluggish oxygen reduction reaction (ORR) in cathodes. Here, Zn was successfully doped into Sr3Fe2O7-δ (SFO) Ruddlesden-Popper (RP) perovskite cathode and the Sr3Fe1.5Zn0.5O7-δ (SFZ05) shows a superior ORR activity. The polarization resistance of SFZ05 is 0.072 Ω·cm-2, which is 47% lower than that of the undoped SFO, and the maximum peak power density of the single-cell of NiO-BaZr0.1Ce0.7Y0.2O3-δ(BZCY)|BZCY|SFZ05 reaches 523.56 mW·cm-2 at 750℃, increased by 22% than SFO. The improvement is attributed to the formation of oxygen vacancies and the increase of proton conductivity. These results display that SFZ05 is a promising candidate for a highly active and stable cathode for PCFCs.

Coupling Single-Ni-Atom with Ni–Co Alloy Nanoparticle for Synergistically Enhanced Oxygen Reduction Reaction

Developing nonprecious metal-based oxygen reduction reaction (ORR) electrocatalysts with superior activity and durability is crucial for commercializing proton-exchange membrane (PEM) fuel cells. Herein, we report a metal-organic framework (MOF)-derived unique N-doped hollow carbon structure (NiCo/hNC), comprising of atomically dispersed single-Ni-atom (NiN4) and small NiCo alloy nanoparticles (NPs), for highly efficient and durable ORR catalysis in both alkaline and acidic electrolytes. Density functional theory (DFT) calculations reveal the strong coupling between NiN4 and NiCo NPs, favoring the direct 4e- transfer ORR process by lengthening the adsorbed O-O bond. Moreover, NiCo/hNC as a cathode electrode in PEM fuel cells delivered a stable performance. Our findings not only furnish the fundamental understanding of the structure-activity relationship but also shed light on designing advanced ORR catalysts.

The performance of heteroatom-doped carbon nanotubes synthesized via a hydrothermal method on the oxygen reduction reaction and specific capacitance

Due to the increasing demand for electrochemical energy storage, various novel electrode and catalysis materials for supercapacitors and rechargeable batteries have developed over the last decade. The structure and characteristics of these catalyst materials have a major effect on the device's performance. In order to lower the costs associated with electrochemical systems, electrochemical systems, metal-free catalysis materials can be employed. In this study, metal-free catalysts composed of nitrogen (N) and sulfur (S) dual-doped multi-walled carbon nanotubes were synthesized using a straightforward and cost-effective single-step hydrothermal method. Carbon nanotubes served as the carbon source, while l-cysteine amino acid and thiourea acted as doping elements. As a result of the physicochemical characterization, many defects and a porous structure were noted, along with the successful insertion of nitrogen and sulfur into the carbon nanotube was confirmed. According to the cyclic voltammetry tests for the dual-doped samples in alkaline conditions, the D-CNT2 catalyst exhibited onset potentials of -0.30 V higher than the -0.37 V observed for the D-CNT3 catalyst. This indicates enhanced oxygen–reduction reaction due to the synergistic effects of the heteroatoms in the structure and the presence of chemically active sites. Moreover, the outstanding specific capacitance of the D-CNT2 catalyst (214.12 F g-1 at scanning rates of 1 mV s-1) reflects the effective porosity of the proposed catalyst. These findings highlight the potential of N/S dual–doped carbon nanotubes for electrocatalytic applications, contributing to efficient energy conversion.

Tailoring CoNi Alloy-Embedded Carbon Nanofibers by Coaxial Electrospinning for an Enhanced Oxygen Reduction Reaction

A flexible CoNi@CNF electrochemical catalyst was developed using coaxial electrostatic spinning technology. The distribution and content of CoNi alloy nanoparticles on the surface of carbon fibers were adjusted by regulating the feed speed ratio of the outer and inner axes of coaxial electrostatic spinning. The results indicate that the content of the CoNi alloy distributed on the carbon fiber surface increased from 26.7 wt.% to 38.4 wt.% with an increase in the feed speed of the inner axis. However, the excessive precipitation of the CoNi alloy on the carbon fiber surface leads to the segregation of the internal CoNi alloy, which is unfavorable for the exposure of active sites during the electrolytic reaction. The best electrocatalytic performance of the composite was achieved when the rate of the outer axis feed speed was constant (3 mm/h) and the rate of the inner axis was 1.5 mm/h. The initial oxygen reduction potential and half-slope potential were 0.99 V and 0.92 V (VS RHE), respectively. The diffusion-limited current density was 6.31 mA/cm−2 and the current strength retention was 95.2% after the 20,000 s timed current test.

Facet effect of MnCo2O4 nanocrystals for enhanced oxygen reduction reaction in alkaline medium

Understanding the mechanism of the four-electron transfer oxygen reduction reaction (ORR) and the structure–activity relationship of spinel oxide remains a challenge. Two bimetallic manganese-cobalt spinel oxide supported on carbon nanotube (MnCo2O4/CNT) composite materials with different crystal planes exposed are synthesized in this paper by adding unequal amounts of ammonia during the hydrothermal process. Physical characterization results indicate that the exposed surfaces of the two materials (MnCo2O4/CNT-100, MnCo2O4/CNT-800) are (001) and (112), respectively. Electrochemical tests demonstrate that the MnCo2O4/CNT-800 composite material exposed to the (112) crystal plane exhibit higher ORR catalytic activity in alkaline electrolyte, with a half-wave potential of 0.79 V and a limiting current density of 5.15 mA cm−2. The outstanding activity is attributed to the greater specific surface area, the exposure of high-index crystal plane and the high-valence metal sites on the surface. Density functional theory (DFT) calculations reveal that the active sites on the (112) crystal plane have improved adsorption and ORR thermodynamics. This work provides a strategy for developing the advanced bimetallic spinel electrocatalysts by adjusting the exposed plane.

Efficient degradation of chlorinated phenolic compounds by Fe@Cu materials with enhanced oxygen reduction reaction

Purification of wastewater that contains residual chlorinated phenolic compounds is important for water conservation. Fe@Cu bimetallic materials have the capability of degrading chlorinated phenolic pollutants, but the processes of electron transfer and active species generation have not been verified directly. In this work, the Fe@Cu bimetallic material, of which copper exhibits a special surface, obtained by a facile displacement reaction, was applied for 2,4-dichlorophenol (2,4-DCP) degradation. The degradation efficiency of 2,4-DCP, which is highly related to H2O2 generation and •OH production, can reach 100% at 30 min with an Fe@Cu dosage of 2 g/L in ambient atmosphere. The porous shell structure of Fe@Cu provides convenient channels for iron ion release and electron transport and transformation. According to DFT (density functional theory) calculations, Cu2O (111) on the surface of particles and Cu (111) with Fe doping possess higher selectivity and catalytic activity for H2O2 in situ generation through the 2e− oxygen reduction reaction (ORR) than Cu (111). The ⋅OH, which was the following product of *OOH and H2O2, was a strategic radical for 2,4-DCP degradation. The coexistence of •OH and •H in the Fe-Cu degradation system was directly proven. The intermediate products of 2,4-DCP degradation were identified, and the degradation intermediates revealed that the degradation function for 2,4-DCP mainly resulted from the synergistic action of ⋅H and ⋅OH. The highly efficient self-catalyst Fe@Cu can be a promising material for chlorinated phenolic compound removal in wastewater purification.

Constructing Co/Fe-Nx dual-site catalyst based on Co0.72Fe0.28 alloy nanoparticles anchored on hollow hierarchical porous carbon framework for enhanced oxygen reduction reaction and ZABs

Hollow hierarchical porous nitrogen-doped carbon (HNC) derived from ZIF-8@ZIF-67 (ZIF: zeolitic-imidazolate framework) is a promising nanocomposite electrocatalyst rich in active sites. However, the single active species (Co-Nx) constrains its application. By introducing Fe in the precursor preparation, this work increased the active species of the catalyst to Co/Fe-Nx species and Co0.72Fe0.28 nanoparticles. Due to the advantages of abundant active sites and the synergy of multiple active species, Co,Fe-HNC-1 exhibited excellent oxygen reduction reaction (ORR) performance in 0.1 M KOH electrolyte with an onset potential (Eonset = 0.92 V vs. RHE) and a half-wave potential (E1/2 = 0.85 V vs. RHE) comparable to those of commercial Pt/C, as well as better long-term stability and methanol resistance. Meanwhile, Co,Fe-HNC-1 additionally demonstrated oxygen evolution reaction (OER) activity. Co,Fe-HNC-1 also performed well as an air cathode in a zinc-air battery (ZAB) with a peak power density of 85.00 mW cm-2 and outstanding stability with only 1.8 % loss after more than 200 galvanostatic cycles at 10 mA cm-2. This work provides a new perspective for the rational design and preparation of electrocatalysts based on HNC nanocomposites.

Enhanced Oxygen Reduction Reaction Performance by Adsorbed Water on Edge Sites.

Pt-based alloy nanoparticles have broad application prospects as cathode catalyst materials for proton-exchange membrane fuel cells (PEMFCs). Optimization of the oxygen adsorption energy is crucial to boost the performance of oxygen reduction catalysis. We successfully synthesized well-dispersed Pt1.2Ni tetrahedra and obtained the Pt1.2Ni/C catalyst adopting the one-pot synthetic protocol, which exhibits superb activity and good long-term stability for oxygen reduction reaction (ORR), achieving a mass activity of 1.53 A/mgPt at 0.90 VRHE, which is 12 times higher than that of commercial Pt/C. On combining X-ray photoelectron spectroscopy and density functional theory calculations, abundant water is adsorbed stably on the Pt1.2Ni alloy surface. We find that the intense interaction between the adsorbed O atom and adsorbed water can weaken the adsorption of oxygen, contributing to the ORR performance.

Ultrathin ternary PtNiGa nanowires for enhanced oxygen reduction reaction

Achieving high activity and durability for the oxygen reduction reaction (ORR) with an ultra-low amount of platinum is significant to promote the widespread application of proton exchange membrane fuel cells (PEMFCs). Here we report a new ultrathin (∼1 nm) ternary PtNiGa alloy nanowires (PtNiGa NWs) electrocatalyst, in which the presence of gallium (Ga) enhances the oxidation resistance of platinum (Pt) and nickel (Ni) and suppresses the dissolution of Ni. The mass and specific activities of PtNiGa NWs are about 11.2 and 7.6 times higher than those of commercial Pt/C catalysts for ORR. Moreover, the mass activity of PtNiGa/C NWs nanocatalyst decreased only by 12.8% and largely retained its electrochemical surface area (ECSA) after 10,000 potential cycles, compared with 38% loss of ECSA for commercial Pt/C catalyst. Therefore, this work provides a general guideline for preparing ternary alloy electrocatalysts and enhancing the activity and stability of the cathode ORR reaction of PEMFCs.

Ag nanoparticle-loaded to MnO2 with rich oxygen vacancies and Mn3+ for the synergistically enhanced oxygen reduction reaction

MnO2 is considered to be one of the most promising electrocatalysts for oxygen reduction reactions (ORR) in alkaline media and can be applied to various electrochemical energy conversion and storage devices. However, it is limited by the relatively slow kinetics of the cathodic electrochemical reactions. In addition, it is difficult to control the presence state of Ag during the modification of MnO2. To this end, an efficient ORR electrocatalyst of Ag nanoparticles supported by MnO2 nanorods was successfully synthesized by using NH3·H2O as a complexing agent to inhibit the Ag+ intercalating into the tunnels of MnO2. The half-wave potential (E1/2) and limiting current density (Jlim) of the obtained Ag/MnO2 electrocatalysts are 0.81 V and −5.6 mA cm−2, respectively, showing comparable ORR catalytic activity to commercial Pt/C catalysts. The excellent catalytic performances can be attributed to the presence of abundant oxygen vacancies and Mn3+ species on the MnO2 surface, as well as the synergistic effect between MnO2 substrates and Ag nanoparticles. Among them, oxygen vacancies enhances the adsorption of O2, Mn3+ facilitates the displacement of O22−/OH−, MnO2 inhibits the accumulation of peroxide species to improving the oxygen environment on the Ag surface and Ag accelerates the electron transfer in the whole process. This work provides a useful guide for the design of efficient Mn-based ORR electrocatalysts.

Uncovering the Enhancement Mechanism of the Oxygen Reduction Reaction on Perovskite/Ruddlesden–Popper Oxide Heterostructures (Nd,Sr)CoO3/(Nd,Sr)2CoO4 and (Nd,Sr)CoO3/(Nd,Sr)3Co2O7

Although the perovskite (Nd,Sr)CoO3 (NSC113)/Ruddlesden-Popper (R-P) oxide (Nd,Sr)2CoO4 (NSC214) heterostructure is reported to improve the oxygen reduction reaction (ORR) activity by 2-3 orders of magnitude, the enhancement mechanism remains unclear. For the first time, we conclude that there are two main factors that can enhance the ORR activity: (1) Oxygen adsorbed on such heterostructures would gain more electrons, promoting the oxygen adsorption. (2) The more distant rock-salt layers on the heterointerfaces can facilitate the insertion of interstitial oxygen and form a high-speed transport channel of interstitial oxygen. Moreover, the perovskite/double-layered R-P oxide heterostructure, which has not been reported yet, is predicted to have better ORR performance than the perovskite/single-layered R-P oxide heterostructure. Our work elucidates the ORR enhancement mechanism on perovskite/R-P oxide heterostructures from the atomic level, which is demonstrated by experiments and, thus, is very meaningful for the development of high-performance electrochemical devices.

Cobalt-doped porous carbon nanofibers with three-dimensional network structures as electrocatalysts for enhancing oxygen reduction reaction

In this study, we design and prepare the cobalt-doped porous carbon nanofibers (Co@PCNFs) as efficient electrocatalysts based on the electro-blow spinning method and carbonization processes. The porous Co@PCNFs has the three-dimensional porous carbon network structures as the fast electron transport channels and endow the materials with high specific surface area. The central metal Co atoms will be converted into metal nanoparticles with high efficient catalysis in the PCNFs after the carbonization treatment. And the carbonized product still inherits the nanometer size of the ZIF-67 (40–50 nm) precursor, which is more beneficial to expose more active sites. These advantages enable the prepared electrocatalyst to exhibit excellent oxygen reduction reaction (ORR) performance (onset potential of 0.91 V vs. RHE, half-wave potential of 0.857 V vs. RHE), which is very close to commercial Pt/C. More importantly, the obtained Co@PCNFs catalyst exhibited superior electrochemical stability and methanol tolerance over commercial Pt/C under alkaline conditions. The work will open up novel options for various fields such as high performance Zn-air batteries and fuel cells in the future.

Tuning Structural and Electronic Configuration of FeN4 via External S for Enhanced Oxygen Reduction Reaction

The Fe–N–C material represents an attractive oxygen reduction reaction electrocatalyst, and the FeN4 moiety has been identified as a very competitive catalytic active site. Fine tuning of the coordination structure of FeN4 has an essential impact on the catalytic performance. Herein, we construct a sulfur‐modified Fe–N–C catalyst with controllable local coordination environment, where the Fe is coordinated with four in‐plane N and an axial external S. The external S atom affects not only the electron distribution but also the spin state of Fe in the FeN4 active site. The appearance of higher valence states and spin states for Fe demonstrates the increase in unpaired electrons. With the above characteristics, the adsorption and desorption of the reactants at FeN4 active sites are optimized, thus promoting the oxygen reduction reaction activity. This work explores the key point in electronic configuration and coordination environment tuning of FeN4 through S doping and provides new insight into the construction of M–N–C‐based oxygen reduction reaction catalysts.