Four-electron Oxygen Reduction Reaction Research Articles

Electrochemical membrane systems for remediating ammonium-containing wastewater via coupling capacitance adsorption and oxygen reduction

As the main nitrogen-containing pollutant of industrial and agricultural wastewater, ammonium (NH4+) is obtainable from the natural nitrogen cycle and industrial emission. Electrochemical recovering nitrogen (N) from nitrate-polluted wastewater to produce ammonia (NH3) is a promising route to mitigate pollution risks and create economic values. Since side reactions at the electrodes are inevitable, and the competition mechanism between electric double layer capacitance (EDLC) behavior and side reactions is still unclear. Herein, electrochemical selective NH4+ recovery from wastewater was achieved by coupling NH4+ adsorption and side oxygen reduction reaction (ORR) in a novel electrochemical membrane extraction system using Ti3C2TX-based gas-diffusion cathodes. Batch experiments showcase simultaneous NH4+ removal and recovery, with TiOX/TiC-C exhibiting the highest removal capability of 408 mgN g−1. In situ Raman spectroscopy revealed the critical role of the four-electron transfer ORR reaction in the conversion of NH4+ to NH3. Density functional theory (DFT) showed that excellent electron-donating ability for NH4+ of TiOX/TiC-C electrode (1.97 eV) facilitated NH4+ diffusion onto electrode interface. The study sheds light on the intricate interplay between NH4+ adsorption and ORR, highlighting the significance of tailored support and vacancy engineering in advancing wastewater treatment and resource recovery.

Insights into the Activity and Stability of Dual-Site Single Atom Catalysts for Oxygen Reduction Reaction Using a 2D Phthalocyanine-MOF

Hydrogen fuel cells have been considered as an effective strategy to shift fossil-fuel based economy to hydrogen economy. The performance of the fuel cells is often limited by the slow kinetics of the cathodic oxygen reduction reaction (ORR). Metal-organic frameworks (MOFs) based on 2D Phthalocyanine offer dual-site single atom catalyst (SAC) where M1 site typically amine coordinated node and M2 site is the phthalocyanine center (M1/M2 = Co, Cu, Ni). Here we conduct both computational studies and experimental measurements to elucidate the site selectivity and activity of two- and four-electron ORR process for pure (contains same element on both sites) and mixed (different M1 and M2) MOF system, and their overall stability. Using density functional theory (DFT), we find that regardless of site, Cobalt (Co) element shows better four-electron ORR activity than other elements in the mixed MOF system. Our integrated crystal orbital Hamilton population (ICOHP) calculations indicate that for pure MOF system, M1(d)-4N(2p) bonds are stronger than M2(d)-4N(2p), which raises d-band center of M2 sites closer to Fermi level, hence improves its activity towards four-electron ORR process. For mixed systems, M2 sites activity largely depends on the geometrical effects caused by M1 site elements. Our activity calculations reveal that Co (as M2) in the Ni-Co mixed MOF systems predicts lowest overpotential for ORR among all studied MOF system. Our dissolution mechanism and stability calculations show that Cu (as M1) causes stress/strain within the mixed MOF system denoting least stable among all studied during ORR process. In experiment, we found Co-based MOF, particularly Ni-Co demonstrates the highest kinetic activity and Cu is the least stable element, showing excellent agreement with computational studies. We hope this study will help in understanding the insight into the rational design of highly active and stable MOF for electrochemical processes. This research was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, Chemical Sciences, Geosciences, and Biosciences Division, Catalysis Science Program to the SUNCAT Center for Interface Science and Catalysis.

Exploring the Kinetics of Oxygen Reduction Reaction in Relation to Pitting Corrosion Resistance in Fe-Cr Alloys

Pitting corrosion, a major concern in materials science and engineering, threatens critical metallic structures and components. Its implications reach across daily life and industries such as energy, transportation, and infrastructure, potentially causing environmental harm, economic losses, and even loss of life. This complex process is influenced by factors including material composition, local environment, and mechanical stresses.1 Once initiated, pit propagation rates are largely dependent on the magnitude of the supporting cathodic current available from the oxygen reduction reaction (ORR) occurring on the external passive film.2 In Fe-Cr alloys, the ORR occurs on this film, undergoing a mixed diffusion and kinetic-controlled process. The number of electrons involved and the corrosion rate are determined by the thermodynamically stable state of the passive film.This research studies the ORR kinetics on FeCr alloys fabricated by arc-melting, using both computational and experimental methods. The rotating disc electrode technique was employed across various Fe:Cr alloys in alkaline and neutral electrolytes. The Koutecky-Levich analysis showed a dominant four-electron ORR pathway in all tested Fe-Cr alloys, with the ORR significantly inhibited by higher proportions of Cr2O3 in the film, suggesting a correlation with high chromium content. Experimental ORR overpotentials, when combined with theoretical potential-pH (Pourbaix) diagrams, helped discern the likely characteristics of the passive films on the alloys. It was deduced that the catalytic properties of potential passive films increased in the order Cr2O3 < Fe3O4 < Fe2O3 < FeCr2O4. These findings, excellently aligned with density functional theory (DFT) modelling, underscore the role of increased chromium levels in slowing pitting progression by suppressing the ORR.In summary, this research offers distinctive insights into the correlation between the kinetics of the ORR and the resistance to localized corrosion in Fe-Cr alloys, which enhances our understanding of the underlying mechanisms of pitting corrosion. Keywords FeCr alloy, pitting corrosion, ORR, rotating disc electrode, DFT Reference [1] B. Zhang, J. Wang, B. Wu, X. W. Guo, Y. J. Wang, D. Chen, Y. C. Zhang, K. Du, E. E. Oguzie, and X. L. Ma, Nat. Commun. (2018) 9, 2559.[2] G. T. Burstein, P. C. Pistorius, and S. P. Mattin, Corros. Sci. (1993) 35, 57.

In situ tuning of platinum 5d valence states for four-electron oxygen reduction

The oxygen reduction reaction (ORR) catalyzed by efficient and economical catalysts is critical for sustainable energy devices. Although the newly-emerging atomically dispersed platinum catalysts are highly attractive for maximizing atomic utilization, their catalytic selectivity and durability are severely limited by the inflexible valence transformation between Pt and supports. Here, we present a structure by anchoring Pt atoms onto valence-adjustable CuOx/Cu hybrid nanoparticle supports (Pt1-CuOx/Cu), in which the high-valence Cu (+2) in CuOx combined with zero-valent Cu (0) serves as a wide-range valence electron reservoir (0‒2e) to dynamically adjust the Pt 5d valence states during the ORR. In situ spectroscopic characterizations demonstrate that the dynamic evolution of the Pt 5d valence electron configurations could optimize the adsorption strength of *OOH intermediate and further accelerate the dissociation of O = O bonds for the four-electron ORR. As a result, the Pt1-CuOx/Cu catalysts deliver superior ORR performance with a significantly enhanced four-electron selectivity of over 97% and long-term durability.

Oxidation Evolution and Activity Origin of N-Doped Carbon in the Oxygen Reduction Reaction

N-doped carbon materials, with their applications as electrocatalysts for the oxygen reduction reaction (ORR), have been extensively studied. However, a negletcted fact is that the operating potential of the ORR is higher than the theoretical oxidation potential of carbon, possibly leading to the oxidation of carbon materials. Consequently, the influence of the structural oxidation evolution on ORR performance and the real active sites are not clear. In this study, we discover a two-step oxidation process of N-doped carbon during the ORR. The first oxidation process is caused by the applied potential and bubbling oxygen during the ORR, leading to the oxidative dissolution of N and the formation of abundant oxygen-containing functional groups. This oxidation process also converts the reaction path from the four-electron (4e) ORR to the two-electron (2e) ORR. Subsequently, the enhanced 2e ORR generates oxidative H2O2, which initiates the second stage of oxidation to some newly formed oxygen-containing functional groups, such as quinones to dicarboxyls, further diversifying the oxygen-containing functional groups and making carboxyl groups as the dominant species. We also reveal the synergistic effect of multiple oxygen-containing functional groups by providing additional opportunities to access active sites with optimized adsorption of OOH*, thus leading to high efficiency and durability in electrocatalytic H2O2 production.

The "Pull Effect" of a Hanging ZnII on Improving the Four-Electron Oxygen Reduction Selectivity with Co Porphyrin.

Due to the challenge of cleaving O-O bonds at single Co sites, mononuclear Co complexes typically show poor selectivity for the four-electron (4e-) oxygen reduction reaction (ORR). Herein, we report on selective 4e- ORR catalyzed by a Co porphyrin with a hanged ZnII ion. Inspired by Cu/Zn-superoxide dismutase, we designed and synthesized 1-CoZn with a hanging ZnII at the second sphere of a Co porphyrin. Complex 1-CoZn is much more effective than its Zn-lacking analogues to catalyze the 4e- ORR in neutral aqueous solutions, giving an electron number of 3.91 per O2 reduction. With spectroscopic studies, the hanging ZnII was demonstrated to be able to facilitate the electron transfer from CoII to O2, through an electronic "pull effect", to give CoIII-superoxo. Theoretical studies further suggested that this "pull effect" plays crucial roles in assisting O-O bond cleavage. This work is significant to present a new strategy of hanging a ZnII to improve O2 activation and O-O bond cleavage.

Screening of highly efficient electrocatalysts for hydrogen peroxide synthesis using single transition metal atoms embedded in carbon vacancy fullerene C60

The electrocatalytic two-electron oxygen reduction reaction (2e– ORR) synthesis of hydrogen peroxide (H2O2) is an eco-friendly process. In this study, density functional theory (DFT) and machine learning (ML) were utilized to systematically explore various single metal atoms anchored on mono-vacancy fullerene C60 (TM@C60) for H2O2 production. Among 19 single-atom catalysts (SACs), Sc@C60 and Y@C60 were identified for high the thermodynamic feasibility of H2O2 formation, exhibiting overpotentials of 0.01 V and 0.02 V, respectively. Moreover, competitive four-electron oxygen reduction reaction (4e– ORR) and hydrogen evolution reaction (HER) were significantly suppressed on these two catalysts. The formation of peroxide intermediates on the two screened SACs was identified as key to achieving high activity and selectivity for H2O2 formation. Analysis using ML methods also revealed that the balance between bond strengths and adsorption energies for O2 is crucial in regulating the performance of 2e– ORR. These results offer important insights for electrocatalytic H2O2 production.

B-site Doping Boosts the OER and ORR Performance of Double Perovskite Oxide as Air Cathode for Zinc-Air Batteries.

Double perovskite oxides are key players as electrocatalytic oxygen catalysts in alkaline media. In this study, we synthesized B-site doped NdBaCoaFe2-aO5+δ (a= 1.0, 1.4, 1.6, 1.8) electrocatalysts, systematically to probe their bifunctionality and assess their performance in zinc-air batteries as air cathodes. X-ray photoelectron spectroscopy analysis reveals a correlation between iron reduction and increased oxygen vacancy content, influencing electrocatalyst bifunctionality by lowering the work function. The electrocatalyst with highest cobalt content, NdBaCo1.8Fe0.2O5+δ exhibited a bifunctional index of 0.95 V, outperforming other synthesized electrocatalysts. Remarkably, NdBaCo1.8Fe0.2O5+δ, demonstrated facilitated charge transfer rate in oxygen evolution reaction with four-electron oxygen reduction reaction process. As an air cathode in a zinc-air battery, NdBaCo1.8Fe0.2O5+δ demonstrated superior performance characteristics, including maximum capacity of 428.27 mA h at 10 mA cm-2 discharge current density, highest peak power density of 64 mW cm-2, with an outstanding durability and stability. It exhibits lowest voltage gap change between charge and discharge even after 350 hours of cyclic operation with a rate capability of 87.14%.

Towards highly-selective H2O2 photosynthesis: In-plane highly ordered carbon nitride nanorods with Ba atoms implantation

Graphitic carbon nitride (g-C3N4) shows great potential in photocatalytic H2O2 production. However, challenges arise from its low in-plane crystallinity and selectivity in two-electron oxygen reduction reaction (2e−-ORR), greatly limiting its H2O2 photosynthesis efficiency. Herein, we develop an ingenious strategy to simultaneously increase the in-plane crystallinity and induce the highly-selective 2e−-ORR by rationally designing barium (Ba) atom-implanted in-plane highly ordered g-C3N4 nanorods. The approach involves controllable synthesis of in-plane high crystallinity g-C3N4 nanorods with Ba implantation (BI-CN) using a BaCl2-mediated in-plane polymerization strategy. The unique Ba-N interaction induces the oriented polymerization of 3-s-triazine units to form well-arranged in-plane structures. Experimental and theoretical calculations clarify that the implanted Ba atoms function as positive charge centers, resulting in a Pauling-type O2 adsorption configuration. This minimizes O–O bond breaking energy, thus suppressing the four-electron oxygen reduction reaction (4e−-ORR) and facilitating a highly-selective 2e−-ORR pathway for efficient photocatalytic H2O2 production. Consequently, the optimized BI-CN3 photocatalyst exhibits an outstanding H2O2 production rate of as high as 353 μmol L−1 h−1, surpassing the pristine g-C3N4 by 6.1 times. This study concurrently optimizes the in-plane crystallinity and O2 adsorption sites of g-C3N4 photocatalysts for highly-selective H2O2 production, providing innovative insights for designing efficient photocatalysts with diverse applications.

Interfacial engineering of high-performance Fe2P2O7-based electrocatalysts for alkaline exchange membrane fuel cells

Fuel cells promise high energy density and energy conversion efficiency but are plagued by the scarcity of platinum for facilitating the oxygen reduction reaction (ORR). Herein, a facile synthesis of a heterojunction electrocatalyst of ZIF-8 derived carbon (Z8C) supported Fe2P2O7 (Fe2P2O7@Z8C) is reported. The electrocatalyst exhibited higher half-wave potential than the state-of-the-art Pt/C catalyst by more than 33 mV and achieved a peak power density of 152.47 mW cm−2, outperforming the Pt/C-driven cell by ca. 14.5 mW cm−2 in alkaline membrane electrode assemblies under similar operating conditions. X-ray photoelectron and X-ray absorption spectroscopic studies suggested a spontaneous electron redistribution across the heterojunction interface in Fe2P2O7@Z8C. Density functional theory calculations indicated that the electron redistribution may remarkably promote the intrinsic activity of Fe2P2O7@Z8C toward the four-electron ORR pathway. These findings offer an indispensable strategy for rational design of highly efficient and durable non-noble electrocatalysts.

In situ modulating coordination fields of single-atom cobalt catalyst for enhanced oxygen reduction reaction

Single-atom catalysts, especially those with metal−N4 moieties, hold great promise for facilitating the oxygen reduction reaction. However, the symmetrical distribution of electrons within the metal−N4 moiety results in unsatisfactory adsorption strength of intermediates, thereby limiting their performance improvements. Herein, we present atomically coordination-regulated Co single-atom catalysts that comprise a symmetry-broken Cl−Co−N4 moiety, which serves to break the symmetrical electron distribution. In situ characterizations reveal the dynamic evolution of the symmetry-broken Cl−Co−N4 moiety into a coordination-reduced Cl−Co−N2 structure, effectively optimizing the 3d electron filling of Co sites toward a reduced d-band electron occupancy (d5.8 → d5.28) under reaction conditions for a fast four-electron oxygen reduction reaction process. As a result, the coordination-regulated Co single-atom catalysts deliver a large half-potential of 0.93 V and a mass activity of 5480 A gmetal−1. Importantly, a Zn-air battery using the coordination-regulated Co single-atom catalysts as the cathode also exhibits a large power density and excellent stability.

Oxygen reduction mediated by tetrapyridylalkylamine Cu(II) complexes with diimines

Oxygen reduction reaction (ORR) is important in artificial energy conversion systems such as fuel cells. However, it is kinetically sluggish. It is thereby highly desirable to develop ORR catalysts to improve the efficiency of energy conversion. In this work, three copper(II) complexes based on the hexadentate tetrapyridylalkylamine N,N,N’,N’-tetrakis(2-pyridyl-methyl)-1,4-butanediamine (tpbn), namely [Cu2(tpbn)(CH3OH)4(ClO4)2](ClO4)2 (Cu-tpbn), [Cu2(tpbn)(bpy)2](ClO4)4.2CH3COCH3 (Cu-tpbn-bpy, bpy = 2,2′-bipyridine), and [Cu2(tpbn)(Me2bpy)2](ClO4)4.3CH3CN (Cu-tpbn-Me2bpy, Me2bpy = 4,4′-dimethyl-2,2′-bipyridine), were prepared and their catalytic properties for ORR studied. All three complexes were homogeneous electrocatalysts for ORR in neutral phosphate buffer solutions. The ORR mediated by Cu-tpbn-Me2bpy had the most positive onset potential of 0.461 V vs RHE. In the applied potential window of -0.19 – 0.50 V vs RHE, Cu-tpbn boosted the stepwise four-electron ORR to mainly produce H2O, whereas Cu-tpbn-bpy and Cu-tpbn-Me2bpy mediated the ORR to mainly generate H2O2. The rate constants for the ORR to H2O promoted by Cu-tpbn, Cu-tpbn-bpy and Cu-tpbn-Me2bpy were 1.2 × 103 s−1, 3.2 × 10 s−1 and 7.8 × 10 s−1, respectively. The foot-of-the-wave analysis showed that the turnover frequencies for the reduction of O2 to H2O2 mediated by Cu-tpbn, Cu-tpbn-bpy and Cu-tpbn-Me2bpy were 1.5 × 104 s−1, 7.5 × 10 s−1 and 2.7 × 102 s−1, respectively. Meanwhile, complexes Cu-tpbn-bpy and Cu-tpbn-Me2bpy were more stable than Cu-tpbn. This work unveiled that the catalytic activity and the product selectivity of the ORR were greatly associated with the denticity, the steric effect and the electronic property of the organic ligands.

ZnMn2O4 spinel nanocrystals-decorated multi-walled carbon nanotubes for oxygen reduction: Experimental and theoretical studies on the strong coupling facilitated four-electron selectivity

In this work, we have successfully formulated a single-step hydrothermal synthesis of mixed transition metal oxide nanocrystals (ZnMn2O4) strongly coupled on carbon support (MWCNTs) that acts as an efficient electrocatalyst for the electrochemical oxygen reduction reaction (ORR). The results depict that the composite, MWCNTs@ZnMn2O4 shows better electrocatalytic activity as compared to the activity exhibited by its individual components MWCNTs and ZnMn2O4 combined, attributed to the strong coupling between the two components. Transmission electron images of the composite depict that ZnMn2O4 nanocrystals have been successfully integrated on MWCNTs resulting in high conductivity and activity. The composite, MWCNTs@ZnMn2O4 exhibits an onset potential of 0.9 V vs. RHE, 99 % four-electron ORR selectivity, and high stability. The high selectivity and stability are assigned due to the strong coupling of ZnMn2O4 to MWCNTs through Zn/Mn–O–C bonds which are proved from DFT studies. The presence of Zn/Mn–O–C bonds is verified from X-ray photoelectron spectroscopy.

A New Catalyst System for the Oxygen Reduction Reaction (ORR) at Fuel Cell Cathodes: P-Block Precious Group Metal-Free Tin and Nitrogen-Doped Carbon (SnNC) Catalyst

The sluggish kinetics of the Oxygen Reduction Reaction (ORR) and the consequently large amount of platinum catalyst required for cathode material in Proton Exchange Membrane Fuel Cell (PEMFC) is today’s primary hurdle for commercializing fuel cell technology in automotive application.1 A class of carbon-doped with metal and nitrogen electrocatalysts (labelled MNC) is promising substitute for platinum-group metals (PGMs) in oxygen reduction reaction (ORR) in PEMFC.2 Particular attention was placed on a specific family of MNC catalysts derived from 3d transition metal precursors with FeNC and CoNC being the most promising for PEMFC applications requiring high power density.3-5 In the present contribution, we report on a new MNC catalyst system, different by its chemical nature from all others catalysts previously reported as ORR-active in acidic medium. The highlights of this work are the successful design and synthesis of SnNC as a non-noble metal catalyst for the first time, and the finding that SnNC hosts Sn-based active sites that can achieve the same turnover frequency as Fe-based active sites in FeNC, and much higher turnover frequency than Co-based active sites in CoNC. FeNC and CoNC materials have, until now, been the state-of-art PGM-free catalysts for ORR in acidic medium. The present SnNC material shows also high and similar selectivity for the four-electron ORR pathway as FeNC, much higher than CoNC materials. The prepared SnNC catalysts meet and exceed the FeNC catalysts in terms of hydrogen-air fuel cell power density. The SnNC-NH3 catalysts displayed a 40-50% higher current density than FeNC-NH3 at cell voltages below 0.7 V. Added benefits include a Fenton-inactive character of Sn. Among other techniques, density functional theory and 119Sn Mössbauer spectroscopy were used in combination to investigate SnNC, providing for the first time insights on the structure of its active sites, their rate-determining step for ORR and selectivity for the four-electron reduction pathway. References Holton, O. T.; Stevenson, J. W., The Role of Platinum in Proton Exchange Membrane Fuel Cells. Platinum Metals Review 2013, 57 (4), 259-271.Luo, F.; Wagner, S.; Ju, W.; Primbs, M.; Li, S.; Wang, H.; Kramm, U. I.; Strasser, P., Kinetic Diagnostics and Synthetic Design of Platinum Group Metal-Free Electrocatalysts for the Oxygen Reduction Reaction Using Reactivity Maps and Site Utilization Descriptors. Journal of the American Chemical Society 2022, 144 (30), 13487-13498.Zitolo, A.; Ranjbar-Sahraie, N.; Mineva, T.; Li, J.; Jia, Q.; Stamatin, S.; Harrington, G. F.; Lyth, S. M.; Krtil, P.; Mukerjee, S.; Fonda, E.; Jaouen, F., Identification of catalytic sites in cobalt-nitrogen-carbon materials for the oxygen reduction reaction. Nature Communications 2017, 8 (1), 957.Zitolo, A.; Goellner, V.; Armel, V.; Sougrati, M.-T.; Mineva, T.; Stievano, L.; Fonda, E.; Jaouen, F., Identification of catalytic sites for oxygen reduction in iron- and nitrogen-doped graphene materials. Nature materials 2015, 14 (9), 937-942.Luo, F.; Roy, A.; Silvioli, L.; Cullen, D. A.; Zitolo, A.; Sougrati, M. T.; Oguz, I. C.; Mineva, T.; Teschner, D.; Wagner, S.; Wen, J.; Dionigi, F.; Kramm, U. I.; Rossmeisl, J.; Jaouen, F.; Strasser, P., P-block single-metal-site tin/nitrogen-doped carbon fuel cell cathode catalyst for oxygen reduction reaction. Nature materials 2020, 19 (11), 1215-1223.

Reduced graphene oxide-supported palladium oxide-MOx for improving the performance of air-cathode microbial fuel cells: Influence of the Sn, Ce, Zn, and Fe precursors

Over the past few decades, microbial fuel cells (MFCs) have attracted significant interest within the realm of renewable energy technologies owing to their capability to generate electricity from low-grade substrates. However, their commercialization and scaling-up are impeded by significant obstacles, including high capital costs, limited durability, and sluggish oxygen reduction reaction (ORR). Here, we demonstrate an environmentally-friendly approach for developing highly efficient electrocatalysts consisting of palladium oxide (PdO) and different transition metal oxides (i.e., tin (IV) oxide (SnO2), ceric oxide (CeO2), iron (III) oxide (Fe2O3), and zinc oxide (ZnO)) embedded within reduced graphene oxide (rGO) framework for improving the efficiency of ORR in neutral medium and MFCs. Among the as-prepared electrocatalysts, the PdO–SnO2/rGO possesses the highest ORR electrochemical catalytic activity in a neutral medium (pH ∼ 7.2) following a four-electron ORR pathway, with a current density slightly lower than that of benchmark Pt/C electrocatalyst. The assessment of MFC performance is conducted by employing activated sludge as the inoculum and glucose as the sole electron donor in the anode chamber of single-chamber, air-cathode MFCs. Consistent with electrochemical performance, the maximum power output of an MFC equipped with PdO–SnO2/rGO as an air cathode is significantly higher than other tested MFCs, reaching 84.6 ± 2.8 mW m−2, which represents ∼ 91 % of the power output of an MFC with Pt/C air cathode. Our results reveal that the incorporation of PdO into transition metal reduced graphene oxide framework has the potential to facilitate their long-term application as air cathodes in MFCs that possess easy fabrication, relatively affordable cost, and outstanding electrochemical catalytic oxygen reduction capability, for enhancing electricity generation from wastewater.

MXene induced two-electron oxygen reduction of Pd for H2O2 generation

The production of H2O2 by oxygen reduction reaction (ORR) is low energy consumption and environmentally friendly. Pd/C is an effective electrocatalyst for oxygen reduction, while its selectivity is low due to a tendentious four-electron pathway ORR process. The limited reserves of palladium and inevitable corrosion issue of carbon support add extra obstacles toward practical application. In this work, Pd nanoparticles with 3–5 nm were synthesized on MXene sheets. Results indicated that Pd/MXene exhibited 3.5∼4.2 times ring current than Pd/C and much higher H2O2 selectivity. The H2O2 yield of Pd/MXene was measured to be 1.43 mol g−1 h−1. The excellence in electrochemistry was from the synergistic effect of Pd and MXene. The existence of MXene increases metallic Pd on the surface. This study on MXenes as supports of Pd nanoparticles for two-electron oxygen reduction provides extensive reference significance to H2O2 generation.

Cu-Alginate/Graphene Aerogels Derived Electrocatalyst for Oxygen Reduction Reaction

In this work, the Cu-alginate/graphene aerogels were prepared by using alginate, graphene and copper chloride as precursors by a freeze-drying process. Finally, N-doped carbon aerogels supported by Cu nanoparticles (NPs) (Cu/N-CAs) were obtained through annealing in the NH3 atmosphere. The morphology, microstructures, specific surface area, and pore size distribution were studied by SEM, XRD, and BET analysis. The results showed that a significant amount of Cu NPs were uniformly disseminated on the aerogels’ surface, and the catalysts’ specific surface area reached 141 m2/g. Electrochemical tests revealed good catalytic capabilities for the oxygen reduction reaction (ORR) of the as-obtained Cu/N-CAs. Compared to the commercial 20%Pt/C, the Cu/N-CAs exhibited comparable catalytic performance, superior catalytic stability and methanol resistance. The transfer of 3.94 electrons indicated that the Cu/N-CAs were undergoing a four-electron (4e-) ORR process.

Revealing the Orbital Interactions between Dissimilar Metal Sites during Oxygen Reduction Process.

A FeCo/DA@NC catalyst with the well-defined FeCoN6 moiety is customized through a novel and ultrafast Joule heating technique. This catalyst demonstrates superior oxygen reduction reaction activity and stability in an alkaline environment. The power density and charge-discharge cycling of znic-air batteries driven by FeCo/DA@NC also surpass those of Pt/C catalyst. The source of the excellent oxygen reduction reaction activity of FeCo/DA@NC originates from the significantly changed charge environment and 3d orbital spin state. These not only improve the bonding strength between active sites and oxygen-containing intermediates, but also provide spare reaction sites for oxygen-containing intermediates. Moreover, various in situ detection techniques reveal that the rate-determining step in the four-electron oxygen reduction reaction is *O2 protonation. This work provides strong support for the precise design and rapid preparation of bimetallic catalysts and opens up new ideas for understanding orbital interactions during oxygen reduction reactions.

Recycling sulfidic spent caustic streams using a sulfide/air fuel cell

It is urgent to develop a green and cost-efficient process to recycle the resources and energy from spent caustic streams. We develop an energy-free strategy to recover sulfur, caustic, and energy from spent caustic streams in the meantime. Two-electron route sulfide oxidation reaction and four-electron route oxygen reduction reaction are combined to run as an abiotic sulfide/air fuel cell. The feasibility of the sulfide/air fuel cell is proven. The lab-scale prototype can self-driven run at >6.5 mA cm−2 for 10 h, with a current efficiency of about 14 %. A sulfidic spent caustic stream is adopted to evaluate the real stream treatment performances. The cell can discharge at >2 mA cm−2 for 60 h, where the sulfide concentration of the sulfidic spent caustic stream is continuously decreased from 0.56 mol/L to 0.31 mol/L. Cost-efficient carbon-supported silver catalysts are synthesized as the cathode catalysts, which can be prepared from silver electroplating effluent.

Enhanced Four-Electron Selective Oxygen Reduction Reaction at Carbon Nanotubes-Supported Sulfonic Acid-Functionalized Copper Phthalocyanine.

In the present work, oxygen reduction reaction (ORR) is explored in an acidic medium with two different catalytic supports (multi-walled carbon nanotubes (MWCNTs) and nitrogen-doped multi-walled carbon nanotubes (NMWCNTs)) and two different catalysts (copper phthalocyanine (CuPc) and sulfonic acid functionalized CuPc (CuPc-SO3-)). The composite, NMWCNTs-CuPc-SO3- exhibits high ORR activity (assessed based on the onset potential (0.57 V vs. reversible hydrogen electrode) and Tafel slope) in comparison to the other composites. Rotating ring disc electrode (RRDE) studies demonstrate a highly selective four-electron ORR (less than 2.5% H2O2 formation) at the NMWCNTs-CuPc-SO3-. The synergistic effect of the catalyst support (NMWCNTs) and sulfonic acid functionalization of the catalyst (in CuPc-SO3-) increase the efficiency and selectivity of the ORR at the NMWCNTs-CuPc-SO3-. The catalyst activity of NMWCNTs-CuPc-SO3- has been compared with many reported materials and found to be better than several catalysts. NMWCNTs-CuPc-SO3- shows high tolerance for methanol and very small deviation in the onset potential (10 mV) between the linear sweep voltammetry responses recorded before and after 3000 cyclic voltammetry cycles, demonstrating exceptional durability. The high durability is attributed to the stabilization of CuPc-SO3- by the additional coordination with nitrogen (Cu-Nx) present on the surface of NMWCNTs.