Efficient Oxygen Reduction Reaction Catalysts Research Articles

High-Activity Fe3 C as pH-Universal Electrocatalyst for Boosting Oxygen Reduction Reaction and Zinc-Air Battery.

Transition metal catalysts are regarded as one of promising alternatives to replace traditional Pt-based catalysts for oxygen reduction reaction (ORR). In this work, an efficient ORR catalyst is synthesized by confining Fe3 C nanoparticles into N, S co-doped porous carbon nanosheets (Fe3 C/N,S-CNS) via high-temperature pyrolysis, in which 5-sulfosalicylic acid (SSA) demonstrates as an ideal complexing agent for iron (ΙΙΙ) acetylacetonate while g-C3 N4 behaves as a nitrogen source. The influence of the pyrolysis temperature on the ORR performance is strictly examined in the controlled experiments. The obtained catalyst exhibits excellent ORR performance (E1/2 = 0.86V; Eonset = 0.98V) in alkaline electrolyte, coupled by exhibiting the superior catalytic activity and stability (E1/2 = 0.83V, Eonset = 0.95V) to Pt/C in acidic media. In parallel, its ORR mechanism is carefully illustrated by the density functional theory (DFT) calculations, especially the role of the incorporated Fe3 C played in the catalytic process. The catalyst-assembled Zn-air battery also exhibits a much higher power density (163mW cm-2 ) and ultralong cyclic stability in the charge-discharge test for 750h with a gap increase down to 20mV. This study provides some constructive insights for preparation of advanced ORR catalysts in green energy conversion units correlated systems.

Transforming bio-oil into nitrogen-doped hierarchical porous carbons: Excellent oxygen reduction electrocatalysts for Zn-air batteries

Nitrogen-doped hierarchical porous carbons are considered as important substitutes for noble metal electrocatalysts for the oxygen reduction reaction (ORR) in green-energy devices. Herein, bio-oil, the liquid product of biomass pyrolysis, and urea were used as the carbon and nitrogen source based on a simple MgO template approach. The obtained carbons exhibited a micro-/meso-/macroporous structure, considerable specific surface area and abundance of Graphitic and Pyridinic nitrogen species, leading to the excellent ORR catalytic performance with a half-wave potential of 0.84 V. The assembled primary Zn-air battery displayed a maximum power density of 239.36 mW cm−2 and consistent discharging stability. This work verified the great potential of bio-oil as a sustainable and low-cost precursor of advanced functional carbon materials and was conducive to the large-scale production of efficient carbon-based ORR catalysts.

Mesoporous LaFeO3 perovskite as an efficient and cost-effective oxygen reduction reaction catalyst in an air cathode microbial fuel cell

Microbial fuel cells (MFCs) produce renewable energy from organic matter in presence of electrochemically active anaerobes at the anode and cathode oxygen reduction reaction (ORR). The ORR catalyst is pivotal for power generation and the overall cost of constructing an MFC. This study uses a synthesized novel LaFeO3 perovskite electrocatalyst as an alternative to the conventional Pt/C cathode catalyst which enhances ORR activity in a pH-neutral electrolyte. LaFeO3-based air cathode MFC produced 726.43 mWm−2 of maximum power density and is comparable to the commercial Pt/C catalyst i.e., 775 mWm−2. Furthermore, it has achieved 2.3 mAm−2 of the maximum current density and 556 mV of voltage. The improved catalytic activity of the synthesized LaFeO3 is associated with a large number of active sites, oxygen vacancies, and its mesoporous nature. Therefore, LaFeO3perovskite was able to catalyze ORR with an electron transfer of 3.8 in a neutral medium which is close to superior indirect four-electron pathways. It is highlighted that LaFeO3 perovskite iscost-effectiveand has comparable electrochemical properties with Pt/C which is evident in its scale-up potential of MFCs for green energy synthesis.

Surface modification strategy for constructing Fe-Nx species and FeF2/Fe3C nanoparticles co-anchored N, F co-doped carbon nanotubes for efficient oxygen reduction

Fe-N-C catalysts have served as ideal catalysts for oxygen reduction reaction (ORR), but the strong adsorption to ORR-relevant intermediates and less utilization of Fe-Nx sites endow those catalysts with inferior performance. Here, an efficient catalyst consisted of Fe-Nx species and FeF2/Fe3C nanoparticles co-anchored on N, F co-doped carbon nanotubes (Fe-NF-CNTs) has been constructed through a surface modification strategy. Sodium trifluoroacetate as functional linker modified on metal-organic framework surface promotes to form abundant and accessible active sites. FeF2/Fe3C nanoparticles and hydrogen fluoride generated in pyrolysis process facilitate to construct N, F co-doped CNTs with defects and holes. N, F co-doping modifies the adsorption characteristic of Fe-Nx sites, while CNTs improve the accessibility of active sites and the capability of material transport. Therefore, Fe-NF-CNTs exhibits outstanding activity (E1/2 = 0.85 V) and stabilization (ΔE = 6 mV, 10k cycles). Impressively, Fe-NF-CNTs delivers high power density (144 and 68 mW cm–2) and cycle stability (300 and 60 h) in liquid and flexible Zn-air batteries, respectively. This work provides a surface modification strategy to guide the tuning of electronic structure of efficient ORR catalysts designed for renewable green energy.

Ordered mesoporous carbon fiber bundles with high-density and accessible Fe-NX active sites as efficient ORR catalysts for Zn-air batteries

Fe-NX/C electrocatalysts have aroused extensive interest in accelerating sluggish oxygen reduction reaction (ORR) kinetics as potential alternatives to platinum catalysts in rechargeable Zn-air batteries (ZABs). However, the low density and poor accessibility of Fe-NX sites have severely restricted the electrocatalytic performance of Fe-NX/C. Herein, Fe, N co-doped ordered mesoporous carbon fiber bundles are prepared through a ligand-assisted strategy with nitrogen-rich 1,10-phenanthroline as space isolation agent. 1,10-Phenanthroline reveals a six-membered heterocyclic structure containing abundant nitrogen species to tightly coordinate with Fe ions, which is conducive to achieving high-density Fe-NX sites. Meanwhile, the adoption of SBA-15 as hard-templates enables the catalysts with highly ordered channels and large specific surface areas, improving the accessibility of Fe-NX sites. The optimal catalyst (PDA-Fe-900) demonstrates a positive half-wave potential of 0.84 V (vs. RHE) in alkaline solution, outperforming the commercial Pt/C (0.83 V). In addition, PDA-Fe-900 delivers comparable ORR performance to commercial Pt/C in acidic electrolyte. Impressively, when PDA-Fe-900 is employed as an air cathode, it achieves large power densities of 163.0 mW/cm2 in liquid-state ZAB and 116.6 mW/cm2 in the flexible solid-state ZAB. This work provides an efficient ligand-assisted pathway for fabricating catalysts with dense and accessible Fe-NX sites as high-performance ORR electrocatalysts for ZABs.

High-dispersion Co/N/C ultra-thin carbon nanosheets modified with trace Ce as efficient oxygen reduction reaction catalysts

The adjustment of the electronic structure of Co using Ce allows Ce–Co/N/C to show excellent ORR performance (E1/2 = 0.893 V).

First-principles study on the catalytic performance of transition metal atom-doped CrSe2 for the oxygen reduction reaction.

Since the oxygen reduction reaction (ORR) is the cathodic reaction of energy storage and conversion devices such as fuel cells and metal-air batteries, the search for catalysts with high-performance toward ORR has become the focus of attention. Transition metal dichalcogenides (TMDs) have the advantages of easy integration, inexpensive, harmless, good stability, and have vast application prospects in stabilizing the single atom. Hence, in this study, we investigated the feasibility of several 4d/5d single transition metals (TM = Rh, Pd, Ag, Ir, Pt, and Au) doped with CrSe2 for ORR electrocatalysis on the basis of density functional theory (DFT) calculations. Our results demonstrated that most of the TM-doped systems are stable, exhibiting metal conductivity, and can well activate the adsorbed O2. Interestingly, compared with end-on adsorption configuration, O2 is more likely to be adsorbed on the catalysts by a side-on adsorption configuration. Among all the candidate ORR catalysts, Pd-doped and Pt-doped CrSe2 have the best catalytic performance with a low overpotential of 0.43 and 0.50 V, respectively, making them good ORR candidate high performance catalysts. Our DFT work helps to understand the interaction between the O2 molecule and transition metal single atoms on CrSe2 supports and provides ideas for designing stable and efficient ORR catalysts.

PtNi alloy nanoparticles grown in situ on nitrogen doped carbon for the efficient oxygen reduction reaction.

Currently, Pt based materials are still the most efficient oxygen reduction reaction (ORR) catalysts. However, their poor stability obstructs the commercial viability of fuel cells. To lower the reaction potential barrier and enhance the stability, we constructed alloy PtNi nanoparticles (NPs) with a Pt-rich surface supported on nitrogen-doped carbon (NC) via a simple one-step solvothermal method using easily accessible reagents. The synthesized PtNi/NC exhibits enhanced mass activity (MA), specific activity (SA), and positive onset potential compared with commercial Pt/C catalysts. Meanwhile, the half-wave potential shifted negatively to only 18 mV after 5000 cycles for PtNi/NC, indicating excellent stability. The enhanced ORR performance can be ascribed to the introduction of Ni into Pt optimizing the adsorption energy of Pt towards oxygen by adjusting the d band center of the Pt atom and stronger interaction between the metal NPs and support. Our work provides a potential synthesis strategy for developing a Pt-based catalyst with a low Pt loading and high ORR performance.

Coupling PtZn intermetallic and atomically dispersed cobalt towards efficient and stable oxygen reduction reaction catalysts

A strong metal–support interaction between PtZn intermetallic and CoNx sites is developed to boost the overall oxygen reduction performance.

Cu-based catalysts with the co-existence of single atoms and nanoparticles for basic electrocatalytic oxygen reduction reaction.

Developing efficient and stable oxygen reduction reaction (ORR) catalysts to replace the precious Pt/C is very important for the industrial application of proton-exchange membrane fuel cells. Herein, using bismuth-based metal-organic frameworks as the substrate to disperse copper ions, we prepared a catalyst containing both Cu single atoms and Cu nanoparticles (CuSACuNP/BiCN) by a pyrolysis method. In 0.1 M KOH electrolyte, the electrocatalytic ORR performance of CuSACuNP/BiCN was superior to that of commercial Pt/C. With a hierarchical porous architecture, CuSACuNP/BiCN displayed a half-wave potential of 0.86 V vs. RHE and a diffusion-limiting current density of 5.82 mA cm-2 with a four-electron transfer process. In addition, it was stable during a 12-hour durability test. This study provides guidance for the synthesis of advanced Cu-based nano-single-atom catalytic materials for ORR applications.

Tuning NiCo2O4 bifunctionality with nitrogen-doped graphene nanoribbons in oxygen electrocatalysis for zinc-air battery application

• Composites thermally prepared from NiCo 2 O 4 and nitrogen-doped graphene nanoribbons; • Tuning the NiCo 2 O 4 and N-doped graphene ratio tailors the bifunctional activity for ORR and OER; • NiCo 2 O 4 /GNRN-based catalysts as efficient bifunctional catalysts for air electrode of ZAB; • Presence of GNRN proved to be crucial for enhancing the catalysts bifunctionality. One of the major barriers to the widespread use and commercialization of zinc-air batteries (ZAB) is the use of inefficient and expensive bifunctional materials for oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) catalysis on the air electrode. Herein, we propose a highly durable ORR/OER bifunctional catalyst, which was thermally prepared using NiCo 2 O 4 blended with nitrogen-doped graphene nanoribbons (GNRN). We evaluated the activity and electrochemical stability of catalysts with different ratios of NiCo 2 O 4 and GNRN as well as the role of each component in the electrocatalytic process to find the most efficient catalyst for both ORR and OER. ORR results revealed that the NiCo 2 O 4 (80):GNRN(20) and NiCo 2 O 4 (90):GNRN(10) novel catalysts exhibited a half-wave potential of ca. 0.79 V RHE in a 4-electron pathway and were practically unaffected by the presence of methanol. With regard to OER, the unblended NiCo 2 O 4 catalyst exhibited the lowest potential required to reach 10 mA cm −2 (1.60 V RHE ), followed by NiCo 2 O 4 (90):GNRN(10) (1.62 V RHE ), and NiCo 2 O 4 (80):GNRN(20) (1.68 V RHE ); however, the presence of tuned amount of N-doped graphene proved to be crucial for enhancing charge transfer, stability, and the ORR/OER bifunctionality of catalysts. After applying NiCo 2 O 4 /GNRN materials and commercial Pt-Ru/C as catalysts in ZAB, the results showed that NiCo 2 O 4 (80):GNRN(20) and NiCo 2 O 4 (90):GNRN(10) exhibited maximum power densities of 78 and 69 mW cm -2 , respectively; these values were quite similar to that obtained for Pt-Ru/C (81 mW cm -2 ). The specific capacity for Pt-Ru/C-based ZAB was 707 mAh g Zn -1 , while both NiCo 2 O 4 /GNRN-based ZABs showed 702 mAh g Zn -1 (86% of the theoretical capacity). Moreover, long-term operation tests involving high current densities and charge-discharge ZAB cycling revealed that NiCo 2 O 4 /GNRN catalysts are clearly more suitable for use at ZAB air electrode than Pt-Ru/C and are as efficient as other non-precious metal catalysts of similar kind which have been reported in the literature.

Insights into the synergistic catalytic mechanism on the customized dual sites of an efficient ORR catalyst

A compelling PtCo/PC-2 catalyst has been developed and proved to be in double active sites to promote the ORR process along the associative pathway.

Influence of coordination structure of Fe-585DV/NxC4-x on the electrocatalytic performance of oxygen reduction reactions.

Fe-N-C material, known for its high efficiency, cost-effectiveness, and environmental friendliness, is a promising electrocatalyst in the field of the oxygen reduction reaction (ORR). However, the influence of defects and coordination structures on the catalytic performance of Fe-N-C has not been completely elucidated. In our present investigation, based on density functional theory, we take an Fe adsorbed graphene structure containing a 5-8-5 divacancy (585DV) defect as a research model and investigate the influence of the coordination number of N atoms around Fe (Fe-NxC(4-x)) on the ORR electrocatalyst behavior in alkaline conditions. We find that the Fe-N4 structure exhibits superior ORR catalytic performance than other N coordination structures Fe-NxC4-x (x = 0-3). We explore the reasons for the improved catalytic performance through electronic structure analysis and find that as the N coordination number in the Fe-NxC(4-x) structure increases, the magnetic moment of the Fe single atom decreases. This reduction is conducive to the ORR catalytic performance, indicating that a lower magnetic moment is more favorable for the catalytic process of the ORR within the Fe-NxC(4-x) structure. This study is of great significance for a deeper understanding of the structure-performance relationship in catalysis, as well as for the development of efficient ORR catalysts.

Transitional trimetallic alloy embedded polyacrylamide hydrogel derived nitrogen-doped carbon air-cathode for bioenergy generation in microbial fuel cell

Heterometallic carbons are popular materials for application in energy devices due to their high oxygen reduction reaction (ORR) activity and good catalytic stability. Particularly in microbial fuel cells (MFCs), Fe group metal alloys are efficient ORR catalysts. This work has embedded different metallic alloy nanoparticles in polyacrylamide hydrogel derived in-situ nitrogen-doped carbon. All the catalysts (Fe@NC, FeCo@NC, FeNi@NC, and FeCoNi@NC) are used as air cathodes and examined for power generation in single chamber MFCs. Trimetallic composite (FeCoNi@NC) performed better than Pt/C. FeCoNi@NC exhibited superior electrocatalytic activity with an oxygen reduction peak at 0.394 V (vs RHE) and a corresponding peak current density of −0.159 mA. FeCoNi@NC records maximum power output of 963.5 mW m−2 at 2483.2 mA m−2, 3.287 and 1.136 times higher than NC and 10 wt% Pt/C, respectively. This could be due to the uniformly distributed alloy nanoparticles, abundant pyridinic-N, and M−Nx sites that enhance the durability, charge transfer, and power generation in MFCs. These findings suggest that the trimetallic transition metal-carbon (MNC) electrodes could be promising ORR catalysts, justifying their application for energy devices.

Regulating the Electronic Structure of Cu-Nx Active Sites for Efficient and Durable Oxygen Reduction Catalysis to Improve Microbial Fuel Cell Performance.

The efficient and durable oxygen reduction reaction (ORR) catalyst is of great significance to boost power generation and pollutant degradation in microbial fuel cells (MFCs). Although transition metal-nitrogen-codoped carbon materials are an important class of ORR catalysts, copper-nitrogen-codoped carbon is not considered a suitable MFC cathode catalyst due to the insufficient performance and especially instability. Herein, we report a three-dimensional (3D) hierarchical porous copper, nitrogen, and boron codoped carbon (3DHP Cu-N/B-C) catalyst synthesized by the dual template method. The introduced B atom as an electron donor increases the electron density around the Cu-Nx active site, which significantly promotes the efficiency of the ORR process and stabilizes the active site by preventing demetallization. Thus, the 3DHP Cu-N/B-C catalyst exhibited excellent ORR performance with the half-wave potential of 0.83 V (vs reversible hydrogen electrode (RHE)) in a 0.1 M KOH electrolyte and 0.68 V (vs RHE) in a 50 mM PBS electrolyte. Meanwhile, 3DHP Cu-N/B-C had satisfactory stability with 94.16% current retention after 24 h of chronoamperometry test, which is better than that of 20% Pt/C. The MFCs using 3DHP Cu-N/B-C not only showed a maximum power density of up to 760.14 ± 19.03 mW m-2 but also operating durability of more than 50 days. Moreover, the 16S rDNA sequencing results presented that the 3DHP Cu-N/B-C catalyst had a positive effect on the microbial community of the MFC with more anaerobic electroactive bacteria in the anode biofilm and fewer aerobic bacteria in the cathode biofilm. This study provides a new approach for the development of Cu-based ORR electrocatalysts as well as guidance for the rational design of high-performance MFCs.

Optimal Pt-Au Alloying for Efficient and Stable Oxygen Reduction Reaction Catalysts.

Stabilization of cathode catalysts in hydrogen-fueled proton-exchange membrane fuel cells (PEMFCs) is paramount to their widespread commercialization. Targeting that aim, Pt-Au alloy catalysts with various compositions (Pt95Au5, Pt90Au10, and Pt80Au20) prepared by magnetron sputtering were investigated. The promising stability improvement of the Pt-Au catalyst, manifested in suppressed platinum dissolution with increasing Au content, was documented over an extended potential range up to 1.5 VRHE. On the other hand, at elevated concentrations, Au showed a detrimental effect on oxygen reduction reaction activity. A systematic study involving complementary characterization techniques, electrochemistry, and Monte Carlo simulations based on density functional theory data enabled us to gain a comprehensive understanding of the composition-activity-stability relationship to find optimal Pt-Au alloying for maintaining the activity of platinum and improving its resistance to dissolution. According to the results, Pt-Au alloy with 10% gold represent the most promising composition retaining the activity of monometallic Pt while suppressing Pt dissolution by 50% at the upper potential limit of 1.2 VRHE and by 20% at devastating 1.5 VRHE.

Graphene-Isolated Satellite Nanostructure Enhanced Raman Spectroscopy Reveals the Critical Role of Different Intermediates on the Oxygen Reduction Reaction

The development of efficient oxygen reduction reaction (ORR) catalysts is essential for the commercialization of fuel cells. Graphene can significantly improve the ORR performance of platinum-based catalysts, but the fundamental mechanism of such a promotional effect is unclear. Herein, the reaction processes of the ORR on graphene supported PtFe nanoalloys have been investigated using in situ enhanced Raman spectroscopy through the fabrication of graphene-isolated satellite nanostructures. The *OOH and *OH intermediates are identified and confirmed using isotopic substitution during the ORR on PtFe with or without graphene, respectively. Density functional theory (DFT) calculations and X-ray photoelectron spectroscopy further demonstrate such a difference result from the electronic effect of graphene. The graphene electronic effect promotes the formation of *OOH and the further conversion of *OH to the final product water, thus leading to the improved ORR performance of PtFe supported on graphene. This work sheds light on the molecular mechanism of the promotional role of graphene on the ORR and provides theoretical guidance for the rational design of ORR catalysts.

In situ Self-Catalyzed Growth of Manganese-Embedded 3D Flakes-Coated Carbon Rod as an Efficient Oxygen-Reduction Reaction Catalyst of Zinc-Air Batteries.

The in situ self-catalyzed growth of manganese-embedded 3D flakes-coated carbon rods (GFC) as an efficient oxygen-reduction reaction (ORR) catalyst of Zinc-air batteries is described for the first time. By optimizing the amount of Mn in the precursor, a series of 3D graphene-like flakes-coated carbon rods were synthesized. GFC with a doping amount of Mn of 10 % (GFC-10) exhibits excellent ORR performance with an onset potential of 0.94 V (vs. reversible hydrogen electrode). The Zinc-air battery is constructed with GFC-10 as the cathode catalyst, and it exhibits a peak power density of 128.9 mW cm-2 and a cycling stability of 75 h at a current density of 10 mA cm-2 , which are superior to the commercial 20 wt% Pt/C-based Zinc-air battery. Interestingly, the introduction of Mn facilitates the self-catalyzed growth of carbon rods, and the change of Mn amount can effectively regulate the morphology of materials. The improved ORR performance of the catalyst is ascribed to the synergistic effect of unique hierarchical porous structure, high-charge transport capacity, abundant carbon defects/edges and Mn-Nx sites. This research provides a new avenue to fabricating highly active Mn-based electrocatalysts for renewable energy systems.

Nickel-doped lanthanum cerate nanomaterials as highly active electrocatalysts.

The efficient oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) catalyst materials are crucial in the energy research domain due to their tunability. Structural modification in perovskites such as lanthanum cerates (LaCeO3) upon doping at A or B sites significantly affects the surface activity and enhances the catalysis efficacy. Herein, B-site nickel-doped lanthanum cerate (LaCe1-xNixO3±δ) nanopowders were applied as ORR indicators in high-temperature electrochemical impedance spectroscopy for solid-oxide fuel cell (SOFC) tests and in cyclic voltammetric OER investigations in alkaline medium. The integration into SOFC applications, via solid-state EIS in a co-pressed three-layered cell with LCNiO as cathode, is investigated in an oxygen-methane environment and reveals augmented conductivity with temperatures of 700-850°C. The higher electrokinetic parameters-including diffusion coefficients, Do heterogeneous rate constant, ko, and peak current density for OER in KOH-methanol at a LCNiO-9-modified glassy carbon electrode-serve as robust gauges of catalytic performance. CV indicators and EIS conductivities of LaCe1-xNixO3±δ nanomaterials indicate promising potencies for electrocatalytic energy applications.

Asymmetric cobalt porphyrins for oxygen reduction reactions: Boosted catalytic activity by the use of triphenylamine

Asymmetric metalloporphyrins have been rarely investigated in oxygen reduction reaction (ORR), which is probably due to the remaining unclear structural effects on electrocatalytic performance and the low synthetic yields. A remarkable feature of these molecules is their inherent large dipole moments, which can benefit the charge separation and electron density modification. Here we synthesize two asymmetric molecules A-Cb-CoPor and A-TPA-CoPor by substituting one of the meso-positions with carbazole and triphenylamine (TPA) derivatives, respectively, to coat on carbon black as ORR electrocatalysts. Density functional theory calculations suggest the well separation of electron density on the cobalt porphyrins. The electrochemical performances of the composites are performed in acid with comparing to a symmetric Co porphyrin-coated catalyst CoPor1/C. A-TPA-CoPor/C attains better ORR activity and selectivity than A-Cb-CoPor/C due to the structural merits of TPA unit. However, an identical electron transfer number of 3.6 is obtained by A-TPA-CoPor/C and CoPor1/C, which is possibly a trade-off result of the use of TPA from its structural advantage and deleterious electron-donating ability. Impressively, both asymmetric porphyrin-based composites exhibit greater limiting current densities than CoPor1/C, which mainly originates from the reduced charge transfer resistance as suggested by the impedance measurements. Our work demonstrates the promise of the asymmetric molecular approach as a viable strategy for efficient ORR catalysts.