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The electrocatalytic conversion of CO2 into value-added chemicals is a promising approach to realize a carbon-energy balance. However, low current density still limits the application of the CO2 electroreduction reaction (CO2 RR). Metal-organic frameworks (MOFs) are one class of promising alternatives for the CO2 RR due to their periodically arranged isolated metal active sites. However, the poor conductivity of traditional MOFs usually results in a low current density in CO2 RR. We have prepared conductive two-dimensional (2D) phthalocyanine-based MOF (NiPc-NiO4 ) nanosheets linked by nickel-catecholate, which can be employed as highly efficient electrocatalysts for the CO2 RR to CO. The obtained NiPc-NiO4 has a good conductivity and exhibited a very high selectivity of 98.4 % toward CO production and a large CO partial current density of 34.5 mA cm-2 , outperforming the reported MOF catalysts. This work highlights the potential of conductive crystalline frameworks in electrocatalysis.

Developing cost-effective and durable air-cathodes is crucial for improving metal-air batteries. Most reports of cathode formulation involve preparing bi-functional electrocatalysts from wet chemistry or solid-state synthesis, followed by pasting onto a substrate. In this work, the cathodes generated from electrochemical activation of normal carbon paper substrates were directly used in Zn-air batteries. The self-activated carbon paper substrate without any additional electrocatalysts exhibits an impressive cycling stability (more than 165 hours for 1,000 cycles) and a small discharge-charge voltage gap. After the activation, the maximum power density and electrochemical surface area were increased by over 40 and 1,920 times respectively. It is discovered that substrates after activation can be directly used as a cathode. The new method is scalable, inexpensive and produces near best in class performance. The mechanism behind this enhancement is due to the creation of oxygen functional groups within the cathode, which overcame slow kinetics, enhanced wettability and enabled optimum three-phase boundaries.

Zinc-anode-based batteries have been widely studied because of their low cost, high capacity, and high energy density. However, the formation of dendrites on the zinc anode during cycling severely ...

The special structure and composition are the important factors that determine the microwave absorption properties. In this study, the porous hollow carbon microsphere (PHCMS) is synthesized by the self-assembly technology, and ZnFe2O4 particles are synthesized inside the carbon sphere by in-situ preparation with taking advantage of the porous and hollow characteristics of the carbon sphere, which prepares ZnFe2O4@PHCMS composite material. The composite shows good performance in terms of minimum reflection loss and absorption bandwidth. The results show that the maximum adsorption capacity of the composite is −51.43 dB at 7.2 GHz. When the thickness is 4.8 mm, the effective absorption bandwidth of RL ≤ 10 dB electromagnetic wave is 3.52 GHz. Such enhanced electromagnetic wave absorption properties of ZnFe2O4@PHCMS are ascribed to the suitable impedance characteristic, the dipole polarization and interfacial polarization, the multiple Debye relaxation process and strong natural resonance, multiple reflection and scattering. This work provides an approach to design effective microwave absorbers having a unique structure to enhance the microwave absorption properties.

It is still a great challenge to achieve high selectivity of CH4 in CO2 electroreduction reactions (CO2 RR) because of the similar reduction potentials of possible products and the sluggish kinetics for CO2 activation. Stabilizing key reaction intermediates by single type of active sites supported on porous conductive material is crucial to achieve high selectivity for single product such as CH4 . Here, Cu2 O(111) quantum dots with an average size of 3.5 nm are in situ synthesized on a porous conductive copper-based metal-organic framework (CuHHTP), exhibiting high selectivity of 73 % towards CH4 with partial current density of 10.8 mA cm-2 at -1.4 V vs. RHE (reversible hydrogen electrode) in CO2 RR. Operando infrared spectroscopy and DFT calculations reveal that the key intermediates (such as *CH2 O and *OCH3 ) involved in the pathway of CH4 formation are stabilized by the single active Cu2 O(111) and hydrogen bonding, thus generating CH4 instead of CO.

The non-precious materials with iron-pyrrolic nitrogen-carbon (Fe–N–C) moiety have attracted an increasing attention on the CO2 reduction reaction (CO2RR). However, the influence of iron valence state and environmental synergy is not still clear. Here, iron phthalocyanine (FePc) molecule with an intrinsic Fe–N4–C moiety is enough dispersed on graphene as the single-atom iron catalyst through a facile chemical method. The FePc-graphene composites with different FePc content and iron valence state are synthesized to investigate their catalysis for the CO2RR to CO. The onset overpotential of 190 mV and Faradaic efficiency of >90% may be achieved in the optimal composites. Experimental and calculational results prove the positive role and synergy of Fe(II)Pc with Fe(III)Pc and graphene. The formation of ∗COOH intermediate is confirmed to be the rate-limiting step. The theoretical calculation reveals that Fe(II)Pc should have higher activity than Fe(III)Pc, and Fe(II)Pc/FePc(III) dimer may be better than individual one. This work clearly exhibits the effect of Fe2+/3+- pyrrolic N4–C on the CO2RR.

Hydrogen peroxide is a widely used and important chemical in industry. A two-electron electrochemical oxygen reduction reaction (2e– ORR) is a clean and on-site method for H2O2 production. Here, we...

Electroreduction of CO2 (CO2RR) into value-added fuels is of significant importance but remains a big challenge because of poor selectivity, low current density, and large overpotential. Crystallin...

Electrochemical oxygen reduction offers an efficient and environmentally friendly route for hydrogen peroxide (H2O2) generation. This electrochemical process can produce H2O2 on-site under ambient conditions. High selectivity and activity two-electron oxygen reduction reaction (2e− ORR) catalysts are crucial for efficient electrochemical H2O2 synthesis. Here, metal/metal oxide, carbon material, and single-atom based 2e− ORR catalysts developed to produce H2O2 are reviewed. As the performance of the catalysts strongly depends on the pH values of electrolytes, pH effects and reaction mechanisms for the catalysts are further summarized to give a deep insight into this research area. This review would help us better understand the catalysts for electrochemical H2O2 synthesis and their rational design in the future.

Selective oxidation of aromatic alkanes by C–H activation is one of the key reactions in organic synthesis and the chemical industry. Activation of C–H bonds to obtain high-value added products under mild conditions by using sustainable carbocatalysts is of particular interest. Herein, we report a sustainable, green and template-free strategy towards the fabrication of N-doped and N/S codoped carbon nanosheets by metal-free carbonization of bioprecursors guanine and guanine sulfate. The formation of thin and N/S codoped carbon nanosheets was induced by multiple interactions of the nucleobases. Benefiting from the unique textural structure of the as-synthesized carbons, including ultrathin thickness, optimal porosity, and rich structural defects, and the synergistic coupling effect of multiple dopants, the carbon nanosheets show a high catalytic performance with 85% ethylbenzene conversion and 98% selectivity to acetophenone at 80 °C after 4 h reaction, which outperforms other equivalent benchmarks (e.g. 8.5 times higher conversion and 3.2 times higher selectivity than those of oxidized carbon nanotubes). Density functional theory simulations indicate that the oxidation of ethylbenzene is catalyzed by the synergistic effect of p-N/S and g-N/S catalysts via an OH radical mechanism. This N/S codoped strategy provides guidance for the design of carbon-based catalysts for the selective oxidation of other alkanes.

Birefringence of a crystal is an important optical property. For example, birefringence is crucial to realizing phase-matching condition for nonlinear optical crystals that are applied to laser technologies. However, it is difficult to analyze the origin of birefringence and then to tune it for practical applications. Here, we developed a method to apply the Bader charge analysis to the investigation of the birefringence of crystal on an atomic scale based on the density functional theory calculation. We calculated the birefringence of nonlinear optical crystals CsPbCO3F and KSrCO3F as a demonstration using this method. It is shown that [CO3]2– and F– make opposite contributions to the birefringence for both crystals. The birefringence contributions from cations are 22 and −29% for Pb and Cs, respectively, in the CsPbCO3F crystal, while the contributions from Sr and K is −11 and −4% to the birefringence of KSrCO3F, respectively. Using this Bader charge analysis method, we can obtain the individual ion’s c...

The mid/far-infrared (MFIR) nonlinear optical (NLO) materials are very important for the development of new laser sources through laser frequency conversion technologies. In this work, two new noncentrosymmetric chalcogenide compounds Ba10In6Zn7S10Se16 (S10Se16) and Ba10In6Zn7Se26 (Se26) were successfully synthesized for MFIR NLO materials. They are composed of T2 super tetrahedron and show excellent comprehensive NLO performance. Their NLO coefficients are approximately 0.7 and 0.8 times of AgGaS2 at 2.05 μm, and both have high laser induced damage threshold (LIDT) about 12.3 and 10.0 times of AgGaS2, respectively. Both compounds show consistent melting characteristics, which is beneficial for the growth of large-size crystals. The band gaps, NLO coefficients, and density of states (DOS) of the crystals are calculated by first principles simulations to study the origination of NLO response, which indicates that the NLO performance of Se26 is mainly derived from tetrahedrons and supertetrahedrons composed of In/Zn and Se.

Enough exposure of an active face is a key factor of nanocatalysis for sustainable energy conversion. Here, we exhibit the effect of axial coordination of organic metal complex molecules on the mor...

Mid- and far-infrared nonlinear optical (MFIR NLO) materials are important in modern laser technologies. However, it is very challenging to develop materials that can achieve a subtle balance between the key requirements, such as large NLO response, high laser-induced damage threshold (LIDT), wide IR transparency, and phase-matching. In this work, a new wide IR transparency (0.38–15.3 μm) NLO crystal Ba10In6Zn7S26 (SS26) is synthesized. Further, its composite system Ba10In6Zn7S26-nZnS is synthesized by eutectic reaction. In particular, Ba10In6Zn7S26-14ZnS (SS40) shows excellent balanced NLO performance that includes a large band gap of 3.05 eV, high LIDT (13.3 × AgGaS2), large second harmonic generation (SHG) response (2.1 × AgGaS2 at 2050 nm, 5.2 × KDP at 1064 nm), and wide optical transmission window (0.37–15.4 μm). Importantly, the phase-matching condition is realized for SS40 by interfaces formed between the crystal face (112) of matrix SS26 and the crystal face (111) of reinforcement cubic ZnS by top...

Electrocatalytic nitrogen reduction reaction (NRR) plays a vital role for next-generation electrochemical energy conversion technologies. However, the NRR kinetics is still limited by the sluggish hydrogenation process on noble-metal-free electrocatalyst. Herein, we report the rational design and synthesis of a hybrid catalyst with atomic iron sites anchored on a N,O-doped porous carbon (FeSA -NO-C) matrix of an inverse opal structure, leading to a remarkably high NH3 yield rate of 31.9 μg NH 3 h-1 mg-1 cat. and Faradaic efficiency of 11.8 % at -0.4 V for NRR electrocatalysis, outperformed almost all previously reported atomically dispersed metal-nitrogen-carbon catalysts. Theoretical calculations revealed that the observed high NRR catalytic activity for the FeSA -NO-C catalyst stemmed mainly from the optimized charge-transfer between the adjacent O and Fe atoms homogenously distributed on the porous carbon support, which could not only significantly facilitate the transportation of N2 and ions but also effectively decrease the binding energy between the isolated Fe atom and *N2 intermediate and the thermodynamic Gibbs free energy of the rate-determining step (*N2 → *NNH).

The covalent linking of acetylenes presents an important route for the fabrication of novel carbon-based scaffolds with potential utilities in a large variety of applications. Beyond that, the incorporation of metal atoms into the acetylenic scaffold could significantly improve its physical, chemical and electronic properties, but the synthesis of such metalated materials remains a challenge. Herein, we demonstrate a solvent-mediated strategy for tailoring conjugated acetylenic polymers from metal-free to copper-metallated diacetylenic linkages. The metalation extends light absorption and promotes charge transport in acetylenic polymers. As a result, the Cu-metallated acetylenic polymers on a photocathode exhibit a hydrogen-evolution photocurrent density of 7-70 mA cm−2 at 0.3 V vs. the reversible hydrogen electrode, which is superior to that of their metal-free counterpart. This work offers a feasible strategy to modulate the Cu-metalation of acetylenic polymers, which may inspire further studies in this field.

Electrochemical water splitting is a promising approach to generate ‘green’ hydrogen. The efficiency of this process relies on the effectiveness of the electrocatalysts used. The electro-kinetics of the hydrogen evolution reaction (HER) is highly pH dependent and conventional catalysts typically are expensive and rare platinum-based materials. The development of low-cost, multi-component electrocatalysts, where each of the components has a synergistic effect, can be an effective approach to improve kinetics. Herein, a series of transition metal (Fe, Mn, Co, and Ni)-modified molybdenum carbides in a nitrogen-doped carbon matrix (TM-Mo2C@NCF) are synthesised to maximise exposed active sites. Among them, Fe-Mo2C@NCF delivers the best-in-class HER performance over a wide range of electrolytes. Tafel slopes of 76, 109 and 110 mV dec−1 and overpotentials of 65, 130 and 129 mV at 10 mA cm−2 were obtained in 1.0 M KOH, 1.0 M phosphate buffer solution (PBS) and 0.5 M H2SO4, respectively. The computational study further indicates that the synergistic electronic modulation co-activated by Fe and N dopants in Fe-Mo2C@NCF can reduce the Gibbs free energy of H adsorption (ΔGH*) and render the Mo–Mo bridge site the most energetically favorable adsorption site for the H* intermediate, which contributes to an increased HER performance.

Aqueous zinc-ion batteries (ZIBs) are attracting considerable attention because of their low cost, high safety and abundant anode material resources. However, the major challenge faced by aqueous ZIBs is the lack of stable and high capacity cathode materials due to their complicated reaction mechanism and slow Zn-ion transport kinetics. This study reports a unique 3D ‘flower-like’ zinc cobaltite (ZnCo2O4-x) with enriched oxygen vacancies as a new cathode material for aqueous ZIBs. Computational calculations reveal that the presence of oxygen vacancies significantly enhances the electronic conductivity and accelerates Zn2+ diffusion by providing enlarged channels. The as-fabricated batteries present an impressive specific capacity of 148.3 mAh g−1 at the current density of 0.05 A g−1, high energy (2.8 Wh kg−1) and power densities (27.2 W kg−1) based on the whole device, which outperform most of the reported aqueous ZIBs. Moreover, a flexible solid-state pouch cell was demonstrated, which delivers an extremely stable capacity under bending states. This work demonstrates that the performance of Zn-ion storage can be effectively enhanced by tailoring the atomic structure of cathode materials, guiding the development of low-cost and eco-friendly energy storage materials.

Organic mechanochromic luminescent (ML) materials have attracted extensive interest due to their potential uses in displays, sensing, bioimaging and data storage. ML materials that distinctively respond to different mechanical stimuli are especially fascinating. A simple pyridinium-based white-light emitter (P1-PF6) exhibiting this sort of ML with room-temperature fluorescence-phosphorescence dual emission (rFPDE) was found to possess a 3.66% quantum yield. Interestingly, mechanical grinding induced phosphorescence disappearance owing to a collapse of the crystalline ordering. Grinding followed by adding a drop of ethanol resulted in an extraordinary tricolor emission switching between white, blue and pinkish orange. More interestingly, mechanical pressing induced phosphorescence enhancement, resulting in the emission color changing from white to pinkish orange. This novel phenomenon may be due to the fact that pressure facilitates a closer arrangement between adjacent molecules, thereby enhancing the intermolecular interactions. In sum, a very scarce example is herein reported. Moreover, because the pure organic pyridinium with rFPDE achieves reversible two-channel ML over a wide wavelength range, this material has potential applications in multi-dimensional sensors.

Exploring an efficient route to prepare novel graphene-based nanostructure of organic small molecule as metal single-atom catalyst still faces a great challenge due to the difficult synergy of complicated nanostructure for multiple catalytic steps. Herein, the iron phthalocyanine (FePc)-based nanostructures are synthesized via the graphene-assisted colloid chemical reaction for the first time. Compared to the commercial Pt/C for oxygen reduction reaction (ORR), the optimal FePc/graphene nanostructure exhibits not only more positive onset potential (Eonset, 1.000 vs 0.973 V) and half-wave potential (△E1/2 = 100 mV), but also better stability and durability in alkaline solution. The corresponding zinc-air battery also shows better the maximum power density. The systematical characterizations prove the axial coordination between FePc and graphene via Fe-O-C bond and the formation of Fe(II)Pc/Fe(III)Pc/graphene nanostructures. The comprehensive analysis and simulated calculation reveal the synergistic ORR catalysis of four- and five- coordinated Fe atoms in these samples. This work provides a new insight into the synthesis and application of metal-organic complex for metal single-atom catalysis.

A new non-centrosymmetric (NCS) polar sulfide PbGa2GeS6 has been synthesized by a self-fluxing method at a relatively low temperature of 550 °C with desirable properties for nonlinear optical (NLO) applications. Its structure is built by three types of infinite chains intersecting in three dimensions, where the NCS building units are tetragonal pyramids of PbS4 and tetrahedra of MS4 (M = Ga, Ga/Ge). Powder of PbGa2GeS6 exhibits phase-matchable (PM) second-order NLO activity with strong second harmonic generation (SHG) intensity of 0.5× AgGaS2 at the particle size of 150–210 μm, high laser-induced damage threshold (LIDT) of 5× AgGaS2, and a wide transmission range in infrared (IR) region (0.47–23 μm). First-principles calculations suggest that the macroscopic SHG response is originated from the cooperation of the lone pairs on Pb2+ and MS4 (M = Ga, Ga/Ge) tetrahedra. Considering its strong PM SHG response, high LIDT, wide IR transmission range, and relatively low synthesis temperature, PbGa2GeS6 should be ...

A new Mn inorganic coordination polymer, formulated as {(H3O)2[Mn3(SO4)3(μ3-OH)2(H2O)2]}n (1), was synthesized by a hydrothermal reaction and characterized magnetically. Structural analysis reveals that 1 features a MnII3-hydroxyl-sulfate layer which is extended into a 3D supramolecular framework through hydrogen bonds with interlayer hydronium ions. Magnetic measurements indicate that 1 exhibits homospin ferrimagnetic behavior with a (5/2, 10/2) spin topology and a 1/3 magnetization plateau. The spontaneous magnetization induced by ferrimagnetic behavior was observed. Single-crystal magnetic measurements indicate that the a-axis of 1 is the magnetic easy axis. Notably, 1 exhibits significant MCE with the maxiumu −ΔSm value of 14.58 J Kg−1 K−1 for ΔH = 8 T at 16 K. Moreover, the density functional theory (DFT) calculations were performed to explain the ferrimagnetic behavior.

In this work, we chose to explore the infrared (IR) nonlinear optical (NLO) materials centering on Cd and Ge elements, and finally synthesized a new IR NLO quaternary selenide BaCdGeSe4. Compound BaCdGeSe4 crystallizes in non-centrosymmetric (NCS) space group Fdd2 (No. 43), and it is built by parallel two-dimensional (2D) anionic [CdGeSe4]2- layers with the cation Ba2+ embedded between the layers. Under the 2050 ​nm laser excitation, compound BaCdGeSe4 appears type-I phase-matchable behavior and its second-harmonic generation intensities is about 0.5 times of AgGaS2.

Carbon materials based catalysts (CMCs) are extensively investigated to replace expensive noble metal catalysts (NMCs) for electrochemical oxygen reduction reaction (ORR). However, two issues are needed to be clarified for further development: ORR on CMCs produces more H2O2 via 2e– process than that for NMCs in acidic condition, and the active sites for ORR of CMCs are still under debate. H2O2 formation on NMCs was thought to be activated by O2 adsorption on metal surfaces. Contrarily, the results of present study indicate that an O2 molecule would approach the hydrogen site on CMCs to form an OOH– ion which subsequently reacts with H+ to form a H2O2. The calculated electrochemical potentials, kinetics, and X-ray photoelectron spectroscopy (XPS) binding energy support well the new mechanism. Moreover, we found that the active sites for ORR are actually dependent on specific ORR process and the working potential range. The present work provides important insights into ORR for electrochemical devices.

There still remains a great technological challenge for the development of advanced rechargeable batteries for future electric vehicles and for storage for a more renewable energy grid. In this paper a reliable and simple method for the preparation of three-dimensional (3D) hierarchical nanohybrids with tin sulfide (SnS) nanoparticles decorating nitrogen-doped carbon nanosheet networks (SnS/N-CNNs) as an anode material for SIBs is reported. The interconnected network structure, associated with its unique porous feature, endows the developed SnS/N-CNNs with enough straining space for mitigating the effect of volume expansion upon cycling, and also ensures highly favorable transport kinetics for both electrons and sodium ions. Accordingly, the SnS/N-CNNs exhibit an outstanding electrochemical performance with a high capacity and long-term cycling performance at a high mass loading, delivering high reversible capacity of 484 and 322 mA h g−1 at the current rate of 1.0 and 5.0 A g−1, respectively, for 1000 cycles, running at a mass loading of 1.5 mg cm−2. Capacities of 428 and 331 mA h g−1 were obtained at the current densities of 1 A g−1 at 2.8 and 4.3 mg cm−2 loading, respectively. The first principles theoretical calculations indicated that robust binding between SnS and nitrogen-doped CNNs was of great significance for maintaining the 3D network structure that achieves high capacity, high rate capability, and superior long cyclic stability even at a high mass loading.

Single-layered two-dimensional (2D) materials have become very attractive due to their novel electronic behavior after the discovery of graphene. Here, we report the electronic structure and thermoelectric properties of single-layered 2D selerium (selenene) and tellurium (tellurene) by using density functional theory calculation. Both selenene and tellurene show three-phonon limited thermal conductivity. The prominent anharmonic phonon scattering process for tellurene makes it show the lowest lattice thermal conductivity among 2D single elemental materials till now. Their special square unit cells give rise to a highly anisotropic electronic structure along the zigzag and the armchair direction. The large effective mass and Seebeck coefficient along the armchair direction suggest that the thermal performance is better than that of the zigzag direction. The effect of spin-orbit coupling increases the band gap and is found to be crucial for tellurene. These studies provide a way to tune the thermoelectric properties of selenene and tellurene in the future.

Metal-free catalysts, such as graphene/carbon nanostructures, are highly cost-effective to replace expensive noble metals for CO2 reduction if fundamental issues, such as active sites and selectivity, are clearly understood. Using both density functional theory (DFT) and ab initio molecular dynamic calculations, we show that the interplay of N-doping and curvature can effectively tune the activity and selectivity of graphene/carbon-nanotube (CNT) catalysts. The CO2 activation barrier can be optimized to 0.58 eV for graphitic-N doped graphene edges, compared with 1.3 eV in the un-doped counterpart. The graphene catalyst without curvature shows strong selectivity for CO/HCOOH production, whereas the (6, 0) CNT with a high degree of curvature is effective for both CH3OH and HCHO production. Curvature is also very influential to tune the overpotential for a given product, e.g. from 1.5 to 0.02 V for CO production and from 1.29 to 0.49 V for CH3OH production. Hence, the graphene/CNT nanostructures offer great scope and flexibility for effective tunning of catalyst efficiency and selectivity, as shown here for CO2 reduction.

The engineering of acetylenic carbon-rich nanostructures has great potential in many applications, such as nanoelectronics, chemical sensors, energy storage, and conversion, etc. Here we show the synthesis of acetylenic carbon-rich nanofibers via copper-surface-mediated Glaser polycondensation of 1,3,5-triethynylbenzene on a variety of conducting (e.g., copper, graphite, fluorine-doped tin oxide, and titanium) and non-conducting (e.g., Kapton, glass, and silicon dioxide) substrates. The obtained nanofibers (with optical bandgap of 2.51 eV) exhibit photocatalytic activity in photoelectrochemical cells, yielding saturated cathodic photocurrent of ca. 10 µA cm−2 (0.3–0 V vs. reversible hydrogen electrode). By incorporating thieno[3,2-b]thiophene units into the nanofibers, a redshift (ca. 100 nm) of light absorption edge and twofold of the photocurrent are achieved, rivalling those of state-of-the-art metal-free photocathodes (e.g., graphitic carbon nitride of 0.1–1 µA cm−2). This work highlights the promise of utilizing acetylenic carbon-rich materials as efficient and sustainable photocathodes for water reduction, While photoelectrochemical devices combine light-absorption with fuel and electricity generation, their implementation is hampered by high costs and low output. Here, the authors synthesized acetylene-rich carbon fibers by copper-mediated polymerization for high-activity, metal-free photocathodes.

International audience; Developing cost-effective and high-performance oxygen reduction reaction (ORR) catalysts is a fundamental issue in fuel cells and metal-air batteries. To this aim, carbon materials catalysts (CMCs) are extensively investigated, because of their performance comparable to noble-metal-based catalysts in alkaline solution. Yet, acidic solutions are desirable for an efficient proton exchange across Nafion membranes to yield high power density for commercial applications. However, the ORR performance of CMCs in acidic solutions is rather low, because of undesirable two-electron processes and OH radical formation. By using first-principles simulations, we elucidate the mechanisms and identify the active sites of 2e– ORR processes for indirect 4e– ORR. We provide evidence for the fact that nitrogen-doped Stone–Wales defects in graphene favor an indirect four-electron ORR upon H2O2 formation and reduction. The low ORR potential for metal-free CMCs is ascribed to H2O2 formation via hydrogen abstraction and the critical point for OH radical generation on transition-metal-based CMCs is 0.82 V. Moreover, we provide an insight into the indirect 4e– ORR, which serves as a guide for suppressing undesired 2e– ORR, avoiding OH production, and promoting direct four electron ORR on CMCs. These results disclose a new strategy for developing high-efficiency ORR on CMCs in acidic solutions.

It is of increasing importance to explore new low-cost and high-activity electrocatalysts for oxygen reduction reaction (ORR), which have had a substantial impact across a diverse range of energy conversion system, including various fuel cell and metal–air batteries. Although engineering carbon nanostructures have been widely explored as a candidate class of Pt-based ORR electrocatalysts owing to their proved high activity, outstanding stability, and ease of use, there still remains a daunting challenge to develop high activity metal-free electrocatalysts in pH-universal electrolyte system. Here, a reliable and controllable route amenable to prepare nitrogen-doped porous carbon (NPC) with high yields and exceptional quality is described. The as-prepared NPC shows advantages of high activity, high durability, and methanol-tolerant as an efficient pH-universal electrocatalyst for ORR, showing comparable or even better activity as compared with the commercial Pt/C catalysts not only in alkaline media but also in acidic and neutral electrolyte. Systematic electrochemical studies, combining with density functional theory calculation, demonstrate the unique nitrogen-doping species and favorable pores in the as-designed NPC synergistically contribute to the significantly improved catalytic activity in pH-universal medium. The present work potentially presents an important breakthrough in developing ORR electrocatalysts for various fuel cells.