Boosting Oxygen Reduction for High-Efficiency H2 O2 Electrosynthesis on Oxygen-Coordinated CoNC Catalysts.

Atomically dispersed CoNC is a promising material for H2 O2 selective electrosynthesis via a two-electron oxygen reduction reaction. However, the performance of typical CoNC materials with routine CoN4 active center is insufficient and needs to be improved further. This can be done by fine-tuning its atomic coordination configuration. Here, a single-atom electrocatalyst (Co/NC) is reported that comprises a specifically penta-coordinated CoNC configuration (OCoN2 C2 ) with Co center coordinated by two nitrogen atoms, two carbon atoms, and one oxygen atom. Using a combination of theoretical predictions and experiments, it is confirmed that the unique atomic structure slightly increases the charge state of the cobalt center. This optimizes the adsorption energy towards *OOH intermediate, and therefore favors the two-electron ORR relevant for H2 O2 electrosynthesis. In neutral solution, the as-synthesized Co/NC exhibits a selectivity of over 90% over a potential ranging from 0.36 to 0.8 V, with a turnover frequency value of 11.48 s-1 ; thus outperforming the state-of-the-art carbon-based catalysts.

Nitrogen, sulfur co-doped carbon coated zinc sulfide for efficient hydrogen peroxide electrosynthesis.

Oxygen electroreduction (ORR) via a two-electron pathway is a promising alternative for hydrogen peroxide (H2O2) synthesis in small-scale applications. In this work, nitrogen and sulfur co-doped carbon coated zinc sulfide nanoparticles (ZnS@C) are synthesized using facile high-temperature annealing. In an alkaline electrolyte, the presence of ZnS suppresses the reduction of H2O2 during the ORR and contributes to high H2O2 selectivity (∼90%) over a wide potential range (0.40-0.80 V). Continuous generation of H2O2 is in turn achieved at an outstanding rate of 1.485 mol gcat.-1 h-1 with a faradaic efficiency of 93.7%.

Oxygen Coordination on Fe-N-C to Boost Oxygen Reduction Catalysis.

The coordination environments of iron (Fe) in Fe-N-C catalysts determine their intrinsic activities toward oxygen reduction reactions (ORR). The precise atomic-level regulation of the local coordination environments is thus of critical importance yet quite challenging to achieve. Here, atomically dispersed Fe-N-C catalyst with O-Fe-N2C2 moieties is thoroughly studied for ORR catalysis. Advanced synchrotron X-ray characterizations, along with theoretical modeling, explicitly unraveled the penta-coordinated nature of the Fe center in the catalytic domain and the energetically optimized ORR pathways on the well-tailored O-Fe-N2C2 moieties. The combined structure identification from both experiments and theory provides an opportunity to understand the role of the coordination environments in directing the catalytic activity of single-atom or single-site catalysts; not only the center metal atom but also the whole coordinating atoms participate in the catalytic cycle.

Nickel-Iron Nitride-Nickel Sulfide Composites for Oxygen Evolution Electrocatalysis.

Advance applications like water splitting system and rechargeable metal-air battery are highly dependent on efficient electrocatalyst for the oxygen evolution reaction (OER). Heterostructured materials, with a high active surface area and electron effect, accomplish enhanced catalytic performance. Here, a nitride-sulfide composite (FeNi3N-Ni3S2) has been prepared by a simple hydrothermal process coupled with nitridation. The prepared composite electrocatalyst FeNi3N-Ni3S2 possesses lower electron densities compared to those of FeNi3N and Ni3S2, lessening the activation energy (Ea) toward the OER. Consequently, the prepared FeNi3N-Ni3S2 exhibits excellent OER performance with a low overpotential (230 mV) and a small Tafel slope (38 mV dec-1). Highly stable FeNi3N-Ni3S2 composite delivers lower charging voltage and extended lifetime in rechargeable Zn-air battery, compared with IrO2.

Sandwich-like Catalyst-Carbon-Catalyst Trilayer Structure as a Compact 2D Host for Highly Stable Lithium-Sulfur Batteries.

Herein, we propose the construction of a sandwich-structured host filled with continuous 2D catalysis-conduction interfaces. This MoN-C-MoN trilayer architecture causes the strong conformal adsorption of S/Li2 Sx and its high-efficiency conversion on the two-sided nitride polar surfaces, which are supplied with high-flux electron transfer from the buried carbon interlayer. The 3D self-assembly of these 2D sandwich structures further reinforces the interconnection of conductive and catalytic networks. The maximized exposure of adsorptive/catalytic planes endows the MoN-C@S electrode with excellent cycling stability and high rate performance even under high S loading and low host surface area. The high conductivity of this trilayer texture does not compromise the capacity retention after the S content is increased. Such a job-synergistic mode between catalytic and conductive functions guarantees the homogeneous deposition of S/Li2 Sx , and avoids thick and devitalized accumulation (electrode passivation) even after high-rate and long-term cycling.

Fe3C cluster-promoted single-atom Fe, N doped carbon for oxygen-reduction reaction.

A key challenge in carrying out an efficient oxygen reduction reaction (ORR) is the design of a highly efficient electrocatalyst that must have fast kinetics, low cost and high stability for use in an energy-conversion device (e.g. metal-air batteries). Herein, we developed a platinum-free ORR electrocatalyst with a high surface area and pore volume via a molten salt method along with subsequent KOH activation. The activation treatment not only increases the surface area to 940.8 m2 g-1 by generating lots of pores, but also promotes the formation of uniform Fe3C nanoclusters within the atomic dispersed Fe-Nx carbon matrix in the final material (A-FeNC). A-FeNC displays excellent activity and long-term stability for the ORR in alkaline media, and shows a greater half-wave potential (0.85 V) and faster kinetics toward four-electron ORR as compared to those of 20 wt% Pt/C (0.83 V). As a cathode catalyst for the Zn-air battery, A-FeNC presents a peak power density of 102.2 mW cm-2, higher than that of the Pt/C constructed Zn-air battery (57.2 mW cm-2). The superior ORR catalytic performance of A-FeNC is ascribed to the increased exposure of active sites, active single-atom Fe-N-C centers, and enhancement by Fe3C nanoclusters.

Ordered mesoporous carbon assisted Fe-N-C for efficient oxygen reduction catalysis in both acidic and alkaline media.

Fe-N-C catalyst obtained by high temperature pyrolysis is one of the most promising electrocatalysts for non-precious metal oxygen reduction reaction (ORR). However, up to now, the lesser density of active sites results in a substantial performance gap between the Fe-N-C materials and the conventional Pt/C ORR catalysts. Herein, an N-doped mesoporous carbon is employed as the support for the dispersion of poly-m-phenylenediamine. With high specific surface areas of 1526 m2 g-1, the as-prepared Fe-N-C materials show the half-wave potential of 0.89 V and 0.79 V in 0.1 M KOH and 0.5 M H2SO4, respectively. Notably, the superior methanol tolerance, as well as excellent stability, makes our Fe-N-C materials as competitive candidates for oxygen electrochemical catalysis.

Mesoporous Ternary Nitrides of Earth-Abundant Metals as Oxygen Evolution Electrocatalyst.

As sustainable energy becomes a major concern for modern society, renewable and clean energy systems need highly active, stable, and low-cost catalysts for the oxygen evolution reaction (OER). Mesoporous materials offer an attractive route for generating efficient electrocatalysts with high mass transport capabilities. Herein, we report an efficient hard templating pathway to design and synthesize three-dimensional (3-D) mesoporous ternary nickel iron nitride (Ni3FeN). The as-synthesized electrocatalyst shows good OER performance in an alkaline solution with low overpotential (259 mV) and a small Tafel slope (54 mV dec-1), giving superior performance to IrO2 and RuO2 catalysts. The highly active contact area, the hierarchical porosity, and the synergistic effect of bimetal atoms contributed to the improved electrocatalytic performance toward OER. In a practical rechargeable Zn-air battery, mesoporous Ni3FeN is also shown to deliver a lower charging voltage and longer lifetime than RuO2. This work opens up a new promising approach to synthesize active OER electrocatalysts for energy-related devices.

Zirconium nitride catalysts surpass platinum for oxygen reduction.

Platinum (Pt)-based materials are important components of microelectronic sensors, anticancer drugs, automotive catalytic converters and electrochemical energy conversion devices1. Pt is currently the most common catalyst used for the oxygen reduction reaction (ORR) in devices such as fuel cells and metal-air batteries2,3, although a scalable use is restricted by the scarcity, cost and vulnerability to poisoning of Pt (refs 4-6). Here we show that nanoparticulate zirconium nitride (ZrN) can replace and even surpass Pt as a catalyst for ORR in alkaline environments. As-synthesized ZrN nanoparticles (NPs) exhibit a high oxygen reduction performance with the same activity as that of a widely used Pt-on-carbon (Pt/C) commercial catalyst. Both materials show the same half-wave potential (E1/2 = 0.80 V) and ZrN has a higher stability (ΔE1/2 = -3 mV) than the Pt/C catalyst (ΔE1/2 = -39 mV) after 1,000 ORR cycles in 0.1 M KOH. ZrN is also shown to deliver a greater power density and cyclability than Pt/C in a zinc-air battery. Replacement of Pt by ZrN is likely to reduce costs and promote the usage of electrochemical energy devices, and ZrN may also be useful in other catalytic systems.

Conductive Holey MoO2-Mo3N2 Heterojunctions as Job-Synergistic Cathode Host with Low Surface Area for High-Loading Li-S Batteries.

Li-S batteries have several advantages in terms of ultrahigh energy density and resource abundance. However, the insulating nature of S and Li2S, solubility and shuttle effects of lithium polysulfides (LiPSs), and slow interconversion between LiPSs and S/Li2S/Li2S2 are significant impediments to the commercialization of Li-S batteries. Exploration of the advanced S host skeleton simultaneously with high conductivity, adsorbability, and catalytic activity is highly desired. Herein, a heterojunction material with holey nanobelt morphology and low surface area (95 m2/g) is proposed as a compact cathode host to enable a conformal deposition of S/Li2S with homogeneous spatial distribution. The rich heterointerfaces between MoO2 and Mo3N2 nanodomains serve as job-synergistic trapping-conversion sites for polysulfides by combining the merits of conductive Mo3N2 and adsorptive MoO2. This non-carbon heterojunction substrate enables a high S loading of 75 wt % even under low surface area. The initial capacity of MoO2-Mo3N2@S reaches 1003 mAh/g with a small decay rate of 0.024% per cycle during 1000 cycles at 0.5 C. The long-term cyclability is preserved even under a high loading of 3.2 mg/cm2 with a reversible capacity of 451 mAh/g after 1000 cycles. The Li-ion diffusion coefficient for MoO2-Mo3N2@S is extremely high (up to 2.7 × 10-7 cm2/s) benefiting from LiPS conversion acceleration at heterojunctions. The affinity between LiPSs and heterojunction allows a dendrite-free Li plating at anode even after long-term cycling. Well-defined heterointerface design with job-sharing or job-synergic function appears to be a promising solution to high-performance Li-S batteries without the requirement of loose or high-surface-area carbon network structures.

Synergistic Effects between Atomically Dispersed Fe-N-C and C-S-C for the Oxygen Reduction Reaction in Acidic Media.

Various advanced catalysts based on sulfur-doped Fe/N/C materials have recently been designed for the oxygen reduction reaction (ORR); however, the enhanced activity is still controversial and usually attributed to differences in the surface area, improved conductivity, or uncertain synergistic effects. Herein, a sulfur-doped Fe/N/C catalyst (denoted as Fe/SNC) was obtained by a template-sacrificing method. The incorporated sulfur gives a thiophene-like structure (C-S-C), reduces the electron localization around the Fe centers, improves the interaction with oxygenated species, and therefore facilitates the complete 4 e- ORR in acidic solution. Owing to these synergistic effects, the Fe/SNC catalyst exhibits much better ORR activity than the sulfur-free variant (Fe/NC) in 0.5 m H2 SO4 .

Facile one-pot synthesis and application of nitrogen and sulfur-doped activated graphene in simultaneous electrochemical determination of hydroquinone and catechol.

Nitrogen (N) and sulfur (S) co-doped activated graphene (N,S-AGR) was prepared by the one-pot pyrolysis of a mixture of graphene oxide (GO), thiourea, and potassium hydroxide (KOH), where thiourea acts as the source of N and S dopants and KOH is the activator for porosity. N,S-AGR with a dopant abundance of 2.8 at% N and 2.3 at% S was then used as a high-activity electrocatalyst in the fabrication of an electrochemical sensor for simultaneous determination of dihydroxybenzene isomers, hydroquinone (HQ) and catechol (CC), in aqueous solution. Compared with the bare glassy carbon electrode (GCE), the electrodes modified with N,S-AGR showed enhanced electrochemical performance toward HQ and CC in both cyclic voltammetric (CV) and differential pulse voltammetric (DPV) measurements because of their enlarged surface area, enhanced electron-transfer rate and increased active sites. Compared with some recently reported electrochemical sensors based on graphene composites, the N,S-AGR modified electrode exhibits higher sensitivity, a much lower detection limit and a comparable linear range for the simultaneous determination of HQ and CC. Moreover, the proposed sensor is promising in practical application for the satisfactory recoveries obtained in real water sample analyses.

N-Doped Ordered Mesoporous Carbon Originated from a Green Biological Dye for Electrochemical Sensing and High-Pressure CO2 Storage.

Herein, a series of nitrogen-doped ordered mesoporous carbons (NOMCs) with tunable porous structure were synthesized via a hard-template method with a green biological dye as precursor, under various carbonization temperatures (700-1100 °C). Compared with the ordered mesoporous silica-modified and unmodified electrodes, the use of electrodes coated by NOMCs (NOMC-700-NOMC-1100) resulted in enhanced signals and well-resolved oxidation peaks in electrocatalytic sensing of catechol and hydroquinone isomers, attributable to NOMCs' open porous structures and increased edge-plane defect sites on the N-doped carbon skeleton. Electrochemical sensors using NOMC-1000-modified electrode were fabricated and proved feasible in tap water sample analyses. The NOMCs were also used as sorbents for high-pressure CO2 storage. The NOMC with the highest N content exhibits the best CO2 absorption capacities of 800.8 and 387.6 mg/g at 273 and 298 K (30 bar), respectively, which is better than those of other NOMC materials and some recently reported CO2 sorbents with well-ordered 3D porous structures. Moreover, this NOMC shows higher affinity for CO2 than for N2, a benefit of its higher nitrogen content in the porous carbon framework.