Oxygen reduction reaction mechanism on nitrogen-doped graphene: A density functional theory study

Interfacial Engineering of Metal/Metal Oxide Heterojunctions toward Oxygen Reduction and Evolution Reactions

Oxygen reduction reactions (ORR) and oxygen evolution reactions (OER) are two very important electrochemical processes for renewable energy conversion and storage devices. Electrocatalysts are needed to accelerate their sluggish kinetics to improve energy conversion efficiencies. Hence, extensive efforts have been devoted to the development of OER and ORR electrocatalysts with high activity and stability as well as low cost. Among these developed electrocatalysts, metal/metal oxide heterostructures attract a great deal of research interest because their catalytic performances can be tuned by interface engineering. In this Review, the latest achievements in interface engineering of metal/metal oxides heterostructures toward ORR and OER are described. The effects of the metal/metal oxide interface on catalysis are first discussed. Then, the approaches for interface engineering are illustrated. The developments of interface engineering in OER and ORR catalysis as well as bifunctional electrocatalysis are further introduced. Lastly, a perspective for future development of interface engineering in metal/metal oxide for OER and ORR is discussed.

Highly efficient ethylene production via electrocatalytic hydrogenation of acetylene under mild conditions

Renewable energy-based electrocatalytic hydrogenation of acetylene to ethylene (E-HAE) under mild conditions is an attractive substitution to the conventional energy-intensive industrial process, but is challenging due to its low Faradaic efficiency caused by competitive hydrogen evolution reaction. Herein, we report a highly efficient and selective E-HAE process at room temperature and ambient pressure over the Cu catalyst. A high Faradaic efficiency of 83.2% for ethylene with a current density of 29 mA cm-2 is reached at -0.6 V vs. the reversible hydrogen electrode. In-situ spectroscopic characterizations combined with first-principles calculations reveal that electron transfer from the Cu surface to adsorbed acetylene induces preferential adsorption and hydrogenation of the acetylene over hydrogen formation, thus enabling a highly selective E-HAE process through the electron-coupled proton transfer mechanism. This work presents a feasible route for high-efficiency ethylene production from E-HAE.

Recent Advances in Electrocatalysts for Proton Exchange Membrane Fuel Cells and Alkaline Membrane Fuel Cells

The rapid progress of proton exchange membrane fuel cells (PEMFCs) and alkaline exchange membrane fuel cells (AMFCs) has boosted the hydrogen economy concept via diverse energy applications in the past decades. For a holistic understanding of the development status of PEMFCs and AMFCs, recent advancements in electrocatalyst design and catalyst layer optimization, along with cell performance in terms of activity and durability in PEMFCs and AMFCs, are summarized here. The activity, stability, and fuel cell performance of different types of electrocatalysts for both oxygen reduction reaction and hydrogen oxidation reaction are discussed and compared. Research directions on the further development of active, stable, and low-cost electrocatalysts to meet the ultimate commercialization of PEMFCs and AMFCs are also discussed.

Assessment of the Accuracy of Density Functionals for Calculating Oxygen Reduction Reaction on Nitrogen-Doped Graphene

Experimental studies of the oxygen reduction reaction (ORR) at nitrogen-doped graphene electrodes have reported a remarkably low overpotential, on the order of 0.5 V, similar to Pt-based electrodes. Theoretical calculations using density functional theory have lent support to this claim. However, other measurements have indicated that transition metal impurities are actually responsible for the ORR activity, thereby raising questions about the reliability of both the experiments and the calculations. To assess the accuracy of the theoretical calculations, various generalized gradient approximation (GGA), meta-GGA, and hybrid functionals are employed here and calibrated against high-level wave-function-based coupled-cluster calculations (CCSD(T)) of the overpotential as well as self-interaction corrected density functional calculations and published quantum Monte Carlo calculations of O adatom binding to graphene. The PBE0 and HSE06 hybrid functionals are found to give more accurate results than the GGA and meta-GGA functionals, as would be expected, and for a low dopant concentration, 3.1%, the overpotential is calculated to be 1.0 V. The GGA and meta-GGA functionals give a lower estimate by as much as 0.4 V. When the dopant concentration is doubled, the overpotential calculated with hybrid functionals decreases, while it increases in GGA functional calculations. The opposite trends result from different potential-determining steps, the *OOH species being of central importance in the hybrid functional calculations, while the reduction of *O determines the overpotential obtained in GGA and meta-GGA calculations. The results presented here are mainly based on calculations of periodic representations of the system, but a comparison is also made with molecular flake models that are found to give erratic results due to finite size effects and geometric distortions during energy minimization. The presence of the electrolyte has not been taken into account explicitly in the calculations presented here but is estimated to be important for definitive calculations of the overpotential.

Density functional theory study of active sites on nitrogen-doped graphene for oxygen reduction reaction

Oxygen reduction reaction (ORR) remains challenging due to its complexity and slow kinetics. In particular, Pt-based catalysts which possess outstanding ORR activity are limited in application with high cost and ease of poisoning. In recent years, nitrogen-doped graphene has been widely studied as a potential ORR catalyst for replacing Pt. However, the vague understanding of the reaction mechanism and active sites limits the potential ORR activity of nitrogen-doped graphene materials. Herein, density functional theory is used to study the reaction mechanism and active sites of nitrogen-doped graphene for ORR at the atomic level, focusing on explaining the important role of nitrogen species on ORR. The results reveal that graphitic N (GrN) doping is beneficial to improve the ORR performance of graphene, and dual-GrN-doped graphene can demonstrate the highest catalytic properties with the lowest barriers of ORR. These results provide a theoretical guide for designing catalysts with ideal ORR property, which puts forward a new approach to conceive brilliant catalysts related to energy conversion and environmental catalysis.

Overview on magnetically recyclable ferrite nanoparticles: synthesis and their applications in coupling and multicomponent reactions

Nanocatalysis is an emerging area of research that has attracted much attention over the past few years. It provides the advantages of both homogeneous as well as heterogeneous catalysis in terms of activity, selectivity, efficiency and reusability. Magnetically recoverable nanocatalysts provide a larger surface area for the chemical transformations where the organic groups can be anchored and lead to decrease in the reaction time, increase in the reaction output and improve the atom economy of the chemical reactions. Moreover, magnetic nanocatalysts provide a greener approach towards the chemical transformations and are easily recoverable by the aid of an external magnet for their reusability. This review aims to give an insight into the important work done in the field of magnetically recoverable nanocatalysts and their applications in carbon-carbon and carbon-heteroatom bond formation.

Carbon defects applied to potassium-ion batteries: a density functional theory investigation

Functionalized carbon nanomaterials are potential candidates for use as anode materials in potassium-ion batteries (PIBs). The inevitable defect sites in the architectures significantly affect the physicochemical properties of the carbon nanomaterials, thus defect engineering has recently become a vital research area for carbon-based electrodes. However, one of the major issues holding back its further development is the lack of a complete understanding of the effects accounting for the potassium (K) storage of different carbon defects, which have remained elusive. Owing to pressing research demands, the construction strategies, adsorption difficulties, and structure-activity relationships of the carbon defect-involved reaction centers for the K adsorption are systematically summarized using first principles calculations. Carbon defects affect the ability to trap K by affecting the geometry, charge distribution, and conductive behavior of the carbon surface. The results show that carbon doping with pyridinic-N, pyrrolic-N, and P defect sites tend to act as trapping K sites because of electron-deficient sites. However, graphite-N and sulfur doping are less capable of trapping K. In addition, it has been proved using calculations that the defects can inhibit the growth of the K dendrite. Finally, using the molten salt method, we prepared the undoped and nitrogen-doped carbon materials for comparison, verifying the results of the calculation.

Effect of silver electrode wetting state on oxygen reduction electrochemistry

Surface wettability plays an important role in heterogeneous electrocatalysis. Here we report a facile laser ablation strategy to directly modify the wettability of the silver catalyst surface and investigate its effect on oxygen reduction reaction (ORR). A broad range tuning of 2e^-/4e^- ORR pathways was achieved, with hydrophilic silver surfaces (contact angle (θ_w) 31.1°± 0.6°) showing high activity and selectivity towards 4e^- reduction of oxygen to water.

Catalytic CO oxidation reaction over N-substituted graphene nanoribbon with edge defects

Density functional theory calculations, including dispersion effects, are used to demonstrate how substitutional nitrogen atoms can improve the catalytic reactivity of graphene nanoribbons (GNR) with edge defects in the CO oxidation process. It is demonstrated that the addition of nitrogen impurities significantly enhances O₂ adsorption on GNR. Carbon atoms near the edges of defects are the most active sites for capturing O₂ molecules. The lower adsorption energy of CO relative to O₂ implies that the N-modified GNR is resistant to CO poisoning. The Eley-Rideal (E-R) mechanism has activation energies as low as 0.38 eV, making it the most energetically relevant pathway for the CO + O₂ reaction. The findings of this study might help in the design of catalysts for metal-free catalysis of CO oxidation.

Synergic effects between boron and nitrogen atoms in BN-codoped C59-n BN n fullerenes (n = 1-3) for metal-free reduction of greenhouse N2O gas

The geometries, electronic structures, and catalytic properties of BN-codoped fullerenes C59-n BN n (n = 1-3) are studied using first-principles computations. The results showed that BN-codoping can significantly modify the properties of C60 fullerene by breaking local charge neutrality and creating active sites. The codoping of B and N enhances the formation energy of fullerenes, indicating that the synergistic effects of these atoms helps to stabilize the C59-n BN n structures. The stepwise addition of N atoms around the B atom improves catalytic activities of C59-n BN n in N2O reduction. The reduction of N2O over C58BN and C57BN2 begins with its chemisorption on the B-C bond of the fullerene, followed by the concerted interaction of CO with N2O and the release of N2. The resulting OCO intermediate is subsequently transformed into a CO2 molecule, which is weakly adsorbed on the B atom of the fullerene. On the contrary, nitrogen-rich C56BN3 fullerene is found to decompose N2O into N2 and O* species without the requirement for activation energy. The CO molecule then removes the O* species with a low activation barrier. The activation barrier of the N2O reduction on C56BN3 fullerene is just 0.28 eV, which is lower than that of noble metals.

Oxygen Reduction Reaction on N-Doped Graphene: Effect of Positions and Scaling Relations of Adsorption Energies

The goal of this study is to provide insight into the mechanism of the oxygen reduction reaction (ORR) on N-doped graphene surfaces. Using density functional theory and a computational hydrogen electrode model, we studied the energetics of the ORR intermediates, the effect of the position of the reaction site, and the effect of the position of the N modification relative to the active site on model graphene surfaces containing one or two N atoms. We found that scaling relations can be derived for N-doped graphenes as well, but the multiplicity of the surface should be taken into account. On the basis of the scaling relations between intermediates OOH* and OH*, the minimal overpotential is 0.33 V. Analysis of the data showed that N atoms in the meta position usually decrease the adsorption energy, but those in the ortho position aid the adsorption. The outer position on the zigzag edge of the graphene sheet also promotes the adsorption of oxygenated species, while the inner position hinders it. Looking at the most effective active sites, our analysis shows that the minimal overpotential can be approached with various doping arrangements, which also explains the contradicting results in the literature. The dissociative pathway was also investigated, but we found only one possible active site; therefore, this pathway is not really viable. However, routes not preferred thermodynamically pose the possibility of breaking the theoretical limit of the overpotential of the associative pathway.

Fibrous-Structured Freestanding Electrodes for Oxygen Electrocatalysis

Electrocatalysts used for oxygen reduction and oxygen evolution reactions are critical materials in many renewable-energy devices, such as rechargeable metal-air batteries, regenerative fuel cells, and water-splitting systems. Compared with conventional electrodes made from catalyst powders, oxygen electrodes with a freestanding architecture are highly desirable because of their binder-free fabrication and effective elimination of catalyst agglomeration. Among all freestanding electrode structures that have been investigated so far, fibrous materials exhibit many unique advantages, such as a wide range of available fibers, low material and material-processing costs, large specific surface area, highly porous structure, and simplicity of fiber functionalization. Recent advances in the use of fibrous structures for freestanding electrocatalytic oxygen electrodes are summarized, including electrospun nanofibers, bacterial cellulose, cellulose fibrous structures, carbon clothes/papers, metal nanowires, and metal meshes. After detailed discussion of common techniques for oxygen electrode evaluation, freestanding electrode fabrication, and their electrocatalytic performance, current challenges and future prospects are also presented for future development.

Role of Pyridinic Nitrogen in the Mechanism of the Oxygen Reduction Reaction on Carbon Electrocatalysts

The introduction of pyridinic nitrogen (pyri-N) into carbon-based electrocatalysts for the oxygen reduction reaction is considered to create new active sites. Herein, the role of pyri-N in such catalysts was investigated from a mechanistic viewpoint using carbon black (CB)-supported pyri-N-containing molecules as model catalysts; the highest activity was observed for 1,10-phenanthroline/CB. X-ray photoemission spectroscopy showed that in acidic electrolytes, both pyri-N atoms of 1,10-phenanthroline could be protonated to form pyridinium ions (pyri-NH⁺ ). In O₂ -saturated electrolytes, one of the pyri-NH⁺ species was reduced to pyri-NH upon the application of a potential; no such reduction was observed in N₂ -saturated electrolytes. This behavior was ascribed to electrochemical reduction of pyri-NH⁺ occurring simultaneously with the thermal adsorption of O₂ , as supported by DFT calculations. According to these calculations, the coupled reduction was promoted by hydrophobic environments.

Alternative view of oxygen reduction on porous carbon electrocatalysts: the substance of complex oxygen-surface interactions

Electrochemical oxygen reduction reaction (ORR) is an important energy-related process requiring alternative catalysts to expensive platinum-based ones. Although recently some advancements in carbon catalysts have been reported, there is still a lack of understanding which surface features might enhance their efficiency for ORR. Through a detailed study of oxygen adsorption on carbon molecular sieves and using inelastic neutron scattering, we demonstrated here that the extent of oxygen adsorption/interactions with surface is an important parameter affecting ORR. It was found that both the strength of O₂ physical adsorption in small pores and its specific interactions with surface ether functionalities in the proximity of pores positively influence the ORR efficiency. We have shown that ultramicropores and hydrophobic surface rich in ether-based groups and/or electrons enhance ORR on carbon electrocatalysts and the performance parameters are similar to those measured on Pt/C with the number of electron transfer equal to 4.

Catalytic role of graphitic nitrogen atoms in the CO oxidation reaction over N-containing graphene: a first-principles mechanistic evaluation

The catalytic role of graphitic nitrogen atoms of a series of nitrogen-doped graphene surfaces is explored for low-temperature oxidation of CO using periodic DFT calculations.

Construction of a three-dimensional S,N co-doped ZIF-67 derivative assisted by PEDOT nanowires and its application in rechargeable Zn–air batteries

PEDOT nanowires were obtained before pyrolysis to inhibit structural collapse and anisotropic shrinkage of the ZIF-67 in the pyrolysis process, and S,N co-doping is realized at the same time. The synthesized Co/C@NS NWs exhibit excellent performance towards ORR and OER.

On the Origin of the Effect of pH in Oxygen Reduction Reaction for Nondoped and Edge-Type Quaternary N-Doped Metal-Free Carbon-Based Catalysts

Metal-free carbon-based catalysts have gained much attention during the last 15 years as an alternative toward the replacement of platinum-based catalysts for the oxygen reduction reaction (ORR). However, carbon-based catalysts only show promising catalytic activity in alkaline solution. Concurrently, the most optimized polymer electrolyte membrane fuel cells use proton exchange membranes. This means that the cathode electrode is surrounded by a protonic environment in which carbon materials show poor performance, with differences above 0.5 V in E_ONSET for nondoped carbon materials. Therefore, the search for highly active carbon-based catalysts is only possible if we first understand the origin of the poor electrocatalytic activity of this kind of catalysts in acidic conditions. We address this matter through a combined experimental and modeling study, which yields fundamental principles on the origin of the pH effects in ORR for carbon-based materials. This is relevant for the design of pH-independent metal-free carbon-based catalysts.

Dislocation-Strained IrNi Alloy Nanoparticles Driven by Thermal Shock for the Hydrogen Evolution Reaction

Designing high-performance and low-cost electrocatalysts is crucial for the electrochemical production of hydrogen. Dislocation-strained IrNi nanoparticles loaded on a carbon nanotube sponge (DSIrNi@CNTS) driven by unsteady thermal shock in an extreme environment are reported here as a highly efficient hydrogen evolution reaction (HER) catalyst. Experimental results demonstrate that numerous dislocations are kinetically trapped in self-assembled IrNi nanoparticles due to the ultrafast quenching and different atomic radii, which can induce strain effects into the IrNi nanoparticles. Such strain-induced high-energy surface structures arising from bulk defects (dislocations), are more likely to be resistant to surface restructuring during catalysis. The catalyst exhibits outstanding HER activity with only 17 mV overpotential to achieve 10 mA cm^-2 in an alkaline electrolyte with fabulous stability, exceeding state-of-the-art Pt/C catalysts. These density functional theory results demonstrate that the electronic structure of as-synthesized IrNi nanostructure can be optimized by the strain effects induced by the dislocations, and the free energy of HER can be tuned toward the optimal region.

Fe(II)-Assisted one-pot synthesis of ultra-small core-shell Au-Pt nanoparticles as superior catalysts towards the HER and ORR

In this work, uniform ultra-small core-shell Au-Pt nanoparticles (denoted as USCS Au-Pt NPs) with Au-decorated Pt surfaces are successfully prepared by Fe(ii)-assisted one-pot co-reduction of Au(iii) ions and Pt(ii) ions in a citrate solution. The as-prepared USCS Au38.4@Au9.3Pt52.3 NPs have an average diameter of 2.3 ± 0.5 nm. It is found that the morphology, composition and size of Au-Pt NPs are highly dependent on the reaction conditions including the addition sequence of the precursors, and the concentrations of Fe(ii) ions, Au(iii) ions and Pt(ii) ions. In addition, USCS Au38.4@Au9.3Pt52.3-NP/C catalysts (USCS Au38.4@Au9.3Pt52.3 NPs loaded on the Vulcan XC-72R carbon black) exhibit excellent electrocatalytic performance towards the hydrogen evolution reaction (HER) and the oxygen reduction reaction (ORR) in acidic media due to the higher electrochemically active surface area (ECSA) and electronic effect between Pt and Au. For instance, USCS Au38.4@Au9.3Pt52.3-NP/C catalysts exhibited greatly enhanced HER activity in terms of overpotential (16 mV at a current density of -10 mA cm-2) and are better than commercial Pt/C catalysts (31 mV at a current density of -10 mA cm-2) reported in the literature thus far, to the best of our knowledge. Strikingly, their mass activity is about 13.1-fold higher than that of commercial Pt/C catalysts. Moreover, they also show an improved ORR activity, Eonset = 1.015 V and E1/2 = 0.896 V, which are positively shifted by nearly 28 mV and 21 mV than those of commercial Pt/C catalysts (0.987 V and 0.875 V), respectively. In addition, they also showed a higher kinetic current density (12.85 mA cm-2 at 0.85 V) and a better long-term durability. Our synthetic strategy presented here may be extended to the preparation of ultra-small Au-based bimetallic or multi-metallic NPs.

Mechanistic Insights into the Hydrogen Oxidation Reaction on PtNi Alloys in Alkaline Media: A First-Principles Investigation

The promising alkaline anion exchange membrane fuel cell suffers from sluggish kinetics of the hydrogen oxidation reaction (HOR). However, the puzzling HOR mechanism hinders the further development of highly active catalysts in alkaline media. In this work, we conducted detailed first-principles calculations to acquire a deep understanding of the alkaline HOR mechanism on PtNi bulk alloys [Pt₃Ni(111), Pt₂Ni₂(111), and PtNi₃(111)] and its surface alloy [PtNi_surf(111)]. The full free energy profiles suggest that the HOR on PtNi alloys proceeds via the Tafel-Volmer mechanism, that is, the direct decomposition of H₂ into two adsorbed H, followed by its reaction with OH^- in the electrolyte, as the rate-determining step, to form H₂O. Therefore, the HOR activity of PtNi alloys is solely impacted by the adsorption of hydrogen, rather than hydroxyl species, though the oxophilicity is also enhanced by alloying Pt with Ni. Thermodynamically, a moderate H adsorption free energy, ΔG_H* ≈ 0.414 eV, is calculated to be an optimal candidate for the HOR at pH = 13. Alloying Pt with Ni can elevate the d-band center (ε_d), push the value of ΔG_H* closer to 0.414 eV, and thus lower the free energy barrier (E_a) of the rate-determining Volmer reaction, leading to the highest HOR activity of PtNi₃(111) among all considered PtNi alloys. This situation is further confirmed by both the microkinetic model and the Tafel plot, where PtNi₃(111) exhibits the highest reaction rate (r = 9.42 × 10³ s^-1 site^-1) and the largest exchange current density (i₀ = 1.42 mA cm^-2) for HOR in alkaline media. This work provides a fundamental understanding of the HOR mechanism and theoretical guidance for rational design of electrocatalysts for HOR in alkaline media.

Single-atom Rh/N-doped carbon electrocatalyst for formic acid oxidation

To meet the requirements of potential applications, it is of great importance to explore new catalysts for formic acid oxidation that have both ultra-high mass activity and CO resistance. Here, we successfully synthesize atomically dispersed Rh on N-doped carbon (SA-Rh/CN) and discover that SA-Rh/CN exhibits promising electrocatalytic properties for formic acid oxidation. The mass activity shows 28- and 67-fold enhancements compared with state-of-the-art Pd/C and Pt/C, respectively, despite the low activity of Rh/C. Interestingly, SA-Rh/CN exhibits greatly enhanced tolerance to CO poisoning, and Rh atoms in SA-Rh/CN resist sintering after long-term testing, resulting in excellent catalytic stability. Density functional theory calculations suggest that the formate route is more favourable on SA-Rh/CN. According to calculations, the high barrier to produce CO, together with the relatively unfavourable binding with CO, contribute to its CO tolerance.

N-Doped Graphene Supported on Metal-Iron Carbide as a Catalyst for the Oxygen Reduction Reaction: Density Functional Theory Study

The development of an efficient electrocatalyst for the oxygen reduction reaction (ORR) is essential for the commercialization of fuel-cell technologies. Iron carbide encapsulated in N-doped graphene (NG/Fe₃ C) has been recognized recently as a promising ORR catalyst. In this study, the stability and catalytic activity of N-doped graphene supported on metal-iron carbide (NG/M_Fe₃ C) toward the ORR are investigated by using DFT calculations. The NG/M_Fe₃ C heterostructure is modeled by substituting Fe atoms in the Fe₃ C substrate near the NG/Fe₃ C interface by metal atoms M (M=Cr-Mn, Co-Zn, Nb-Mo, Ta-W). The calculations show that the introduction of the metal atoms M alters the work function of the overlayer N-doped graphene, which is found to correlate with the binding strength of the ORR intermediates. The introduction of Ni or Co atoms at the interface improves the ORR activity of the NG/Fe₃ C and stabilizes the heterostructure. The ORR activity increases as the concentration of Ni or Co atoms near the interface increases, and the stable heterostructure is available in a wide range of substituted concentrations. These results suggest approaches to improve the ORR activity of NG/Fe₃ C catalysts.

Mixed Copper/Copper-Oxide Anchored Mesoporous Fullerene Nanohybrids as Superior Electrocatalysts toward Oxygen Reduction Reaction

Developing a highly active, stable, and efficient non-noble metal-free functional electrocatalyst to supplant the benchmark Pt/C-based catalysts in practical fuel cell applications remains a stupendous challenge. A rational strategy is developed to directly anchor highly active and dispersed copper (Cu) nanospecies on mesoporous fullerenes (referred to as Cu-MFC₆₀ ) toward enhancing oxygen reduction reaction (ORR) electrocatalysis. The preparation of Cu-MFC₆₀ involves i) the synthesis of ordered MFC₆₀ via the prevalent nanohard templating technique and ii) the postfunctionalization of MFC₆₀ with finely distributed Cu nanospecies through incipient wet impregnation. The concurrence of Cu and cuprous oxide nanoparticles in the as-developed Cu-MFC₆₀ samples through relevant material characterizations is affirmed. The optimized ORR catalyst, Cu(15%)-MFC₆₀ , exhibits superior electrocatalytic ORR characteristics with an onset potential of 0.860 vs reversible hydrogen electrode, diffusion-limiting current density (-5.183 mA cm^-2 ), improved stability, and tolerance to methanol crossover along with a high selectivity (four-electron transfer). This enhanced ORR performance can be attributed to the rapid mass transfer and abundant active sites owing to the synergistic coupling effects arising from the mixed copper nanospecies and the fullerene framework.

Enhancing oxygen reduction reaction by using metal-free nitrogen-doped carbon black as cathode catalysts in microbial fuel cells treating wastewater

Microbial fuel cells (MFCs) is promising to combat environmental pollution by converting organic waste to electricity. One critical problem for practical application of MFCs treating wastewater is sluggish oxygen reduction reaction (ORR) on cathode. This study focused on developing novel metal-free cost-effective cathodic catalysts to enhance power generation of MFCs. Specifically, carbon powder (Vulcan XC-72R) was modified with acid treatment and pyrazinamide (as nitrogen precursor), and subsequently pyrolyzed at different temperatures. For CN-X (X = 700-1000 °C) materials, chemical compositions (the doping contents of nitrogen species, oxygen-containing groups, and sulfur-containing groups) were altered with pyrolysis temperature. Linear sweep voltammetry showed that CN-800 exhibited the highest ORR activity, with an onset potential of 0.215 V and a half-wave potential of -0.096 V (vs. Ag/AgCl). Electrochemical measurements clearly presented an enhancement of ORR activity by treating carbon powder with sulfuric acid and nitrogen doping, which was well correlated with voltage output in single chamber MFCs (SCMFCs). On the other hand, for the nitrogen-doped cathode catalysts, the best performance in SCMFCs was directly related with the amount of pyridinic nitrogen species and total nitrogen amount. The MFC operated with CN-800 exhibited a maximum power density of 371 ± 3 mW/m² with the chemical oxygen demand (COD) removal of 77.2 ± 1.5% and coulombic efficiency (CE) of 8.6 ± 0.3%. Furthermore, the MFC with CN-800 exhibited an excellent stability over longer than 580 h of operation with 1.5% voltage reduction. CN-800 possessed comparable COD removal efficiency to conventional costly Pt/C, and exhibited distinct cost-effectiveness for MFC practical applications in wastewater treatment.

A non-traditional biomass-derived N, P, and S ternary self-doped 3D multichannel carbon ORR electrocatalyst

A synthetic non-traditional biomass-derived ORR catalyst is developed to overcome traditional shortcomings, that is, the existence of solid micron blocks and fewer heteroatoms.

Enhanced electrochemical oxygen evolution reaction activity on natural single-atom catalysts transition metal phthalocyanines: the substrate effect

Some TMPc are excellent OER electrocatalysts and photocatalysts with a single-wall nanotube (SWNT) substrate which can enhance their catalytic activity.

Understanding the roles of amorphous domains and oxygen-containing groups of nitrogen-doped carbon in oxygen reduction catalysis: toward superior activity

The secret to high ORR activity lies in tuning the oxygen functionalities and the amount of graphitic vs. amorphous domains in nitrogen-doped carbons.

A computational evaluation of MoS2-based materials for the electrocatalytic oxygen reduction reaction

The potential of modified MoS₂-based materials in oxygen reduction reaction (ORR) was evaluated by DFT calculations.

Covalent structured catalytic materials containing single-atom metal sites with controllable spatial and chemical properties: concept and application

Keep your distance! A simple and effective protocol for connecting macrocycle polymers creates a new and versatile class of highly stable single-site catalytic materials.

Charge-compensated co-doping of graphdiyne with boron and nitrogen to form metal-free electrocatalysts for the oxygen reduction reaction

B,N co-doped graphdiyne (GDY) could be an efficient metal-free electrocatalyst for the oxygen reduction reaction (ORR) with an overpotential of 0.57 V.

Atomic site electrocatalysts for water splitting, oxygen reduction and selective oxidation

This review summarized the fabrication routes and characterization methods of atomic site electrocatalysts (ASCs) followed by their applications for water splitting, oxygen reduction and selective oxidation.

Confinement Catalysis with 2D Materials for Energy Conversion

The unique electronic and structural properties of 2D materials have triggered wide research interest in catalysis. The lattice of 2D materials and the interface between 2D covers and other substrates provide intriguing confinement environments for active sites, which has stimulated a rising area of "confinement catalysis with 2D materials." Fundamental understanding of confinement catalysis with 2D materials will favor the rational design of high-performance 2D nanocatalysts. Confinement catalysis with 2D materials has found extensive applications in energy-related reaction processes, especially in the conversion of small energy-related molecules such as O₂ , CH₄ , CO, CO₂ , H₂ O, and CH₃ OH. Two representative strategies, i.e., 2D lattice-confined single atoms and 2D cover-confined metals, have been applied to construct 2D confinement catalytic systems with superior catalytic activity and stability. Herein, the recent advances in the design, applications, and structure-performance analysis of two 2D confinement catalytic systems are summarized. The different routes for tuning the electronic states of 2D confinement catalysts are highlighted and perspectives on confinement catalysis with 2D materials toward energy conversion and utilization in the future are provided.

Strategies for computational design and discovery of two-dimensional transition-metal-free materials for electro-catalysis applications

In this perspective, we review two new strategies for computational design and discovery of two-dimensional (2D) transition-metal (TM) free electro-catalysts for the oxygen reduction reaction (ORR) and the nitrogen reduction reaction (NRR). The "2D binary compound" strategy for designing ORR electro-catalysts shows promising applications, which benefits from abundant intrinsic catalytic sites for the adsorption of reaction intermediates. And with the "activated B site" strategy for designing NRR electro-catalysts, several novel NRR electro-catalysts with extremely low limiting potential are developed. Computational-simulation-driven material design from a bottom-up method could not only provide rational strategies, but also accelerate the discovery of novel materials.

Investigating the Integrity of Graphene towards the Electrochemical Hydrogen Evolution Reaction (HER)

Mono-, few-, and multilayer graphene is explored towards the electrochemical Hydrogen Evolution Reaction (HER). Careful physicochemical characterisation is undertaken during electrochemical perturbation revealing that the integrity of graphene is structurally compromised. Electrochemical perturbation, in the form of electrochemical potential scanning (linear sweep voltammetry), as induced when exploring the HER using monolayer graphene, creates defects upon the basal plane surface that increases the coverage of edge plane sites/defects resulting in an increase in the electrochemical reversibility of the HER process. This process of improved HER performance occurs up to a threshold, where substantial break-up of the basal sheet occurs, after which the electrochemical response decreases; this is due to the destruction of the sheet integrity and lack of electrical conductive pathways. Importantly, the severity of these changes is structurally dependent on the graphene variant utilised. This work indicates that multilayer graphene has more potential as an electrochemical platform for the HER, rather than that of mono- and few-layer graphene. There is huge potential for this knowledge to be usefully exploited within the energy sector and beyond.

Catalytic trends of nitrogen doped carbon nanotubes for oxygen reduction reaction

Replacing the state-of-the-art fuel cell catalyst platinum for a cheaper and abundant alternative would make the hydrogen economy viable. Both nitrogen-doped graphene and nitrogen-doped carbon nanotubes (N-CNT) have been shown to be capable of acting as a metal-free catalyst for the oxygen reduction reaction (ORR). Until now, most of the research has been focused on the nitrogen doping and less on the structure of the nanotubes. Here, density functional theory calculations are used to calculate trends in ORR catalytic activity of graphitic-N-doped CNTs of different sizes and chirality of selected tubes between (4,0) and (20,10). This includes 13 armchair tubes, 17 zig-zag tubes and 42 chiral tubes, or 72 N-CNTs in total. 22 tubes are predicted to have a lower overpotential than the platinum catalyst and 46 tubes have lower overpotential than nitrogen doped graphene. The most active tubes are (14,7), (12,6), and (8,8), and display an overpotential of around 0.35 V, or 0.1 V lower overpotential than predicted on Pt(111) with the same level of theory.