Chemical reactivity of graphene doped with 3d transition metals: nothing compares to a single vacancy

CONTEXT

Finding catalysts that do not rely on the use of expensive metals is one of the requirements to achieve sustainable production. The reactivity of graphene doped with 3d transition metals was studied. All dopants enhanced the reactivity of graphene and performed better than Stone-Wales defects and divacancies, but were inferior to monovacancies. For hydrogenation of doped-monovacancies, Sc, Ti, Cr, Co, and Ni induced more prominent reactivity on the carbon atoms. However, the metals were the most reactive center for V, Mn, and Fe-doped graphene. Cu and Zn turned the four neighboring carbon atoms into the preferred sites for hydrogenation. The addition of oxygen to doped graphene with Ti, V, Cr, Mn, Fe, Co, and Ni on a monovacancy revealed a more uniform pattern since the metal, preferred to react with oxygen. However, Sc induced a larger reactivity on the carbon atoms. The affinity of the 3d metal-doped graphene systems towards oxygen was inferior to that observed for single-vacancies. Therefore, vacancy engineering is the most favorable and least expensive method to enhance the reactivity of graphene.

METHODS

We applied Truhlar's M06-L method accompanied by the 6-31G* basis sets to perform periodic boundary conditions calculations as implemented in Gaussian 09. The ultrafine grid was employed and the unit cells were sampled employing 100 k-points. Results were visualized employing Gaussview 5.0.1.

One-step pyrolysis synthesis of ternary (P,S,N)-doped graphene as an efficient metal-free electrocatalyst for the oxygen reduction reaction

Graphene-based materials have been regarded recently as a promising substance for electrochemical energy conversion and storage devices owing to their unique structure and extraordinary properties. Herein, an enormously facile one-step pyrolysis approach is reported for the fabrication of ternary (P,S,N)-doped graphene, which is further investigated as an efficient metal-free electrocatalyst for the oxygen reduction reaction (ORR). Furthermore, optimized ternary-doped graphene can deliver excellent ORR catalytic activity that favors the four-electron ORR process and outstanding long-term durability (90.54% current retention after 20000 s which is far superior to that of commercial Pt/C) owing to the preferable synergetic coupling effect between P, S and N. Density functional theory (DFT) calculations were performed to reveal the synergetic coupling effect between doping elements in the ORR process. This work provides an extremely simple one-step pyrolysis method for the synthesis of P,S,N-doped graphene for electrochemical energy conversion and storage devices.

Heteroatom Codoped Graphene: The Importance of Nitrogen

Although graphene has exceptional properties, they are not enough to solve the extensive list of pressing world problems. The substitutional doping of graphene using heteroatoms is one of the preferred methods to adjust the physicochemical properties of graphene. Much effort has been made to dope graphene using a single dopant. However, in recent years, substantial efforts have been made to dope graphene using two or more dopants. This review summarizes all the hard work done to synthesize, characterize, and develop new technologies using codoped, tridoped, and quaternary doped graphene. First, I discuss a simple question that has a complicated answer: When can an atom be considered a dopant? Then, I briefly discuss the single atom doped graphene as a starting point for this review's primary objective: codoped or dual-doped graphene. I extend the discussion to include tridoped and quaternary doped graphene. I review most of the systems that have been synthesized or studied theoretically and the areas in which they have been used to develop new technologies. Finally, I discuss the challenges and prospects that will shape the future of this fascinating field. It will be shown that most of the graphene systems that have been reported involve the use of nitrogen, and much effort is needed to develop codoped graphene systems that do not rely on the stabilizing effects of nitrogen. I expect that this review will contribute to introducing more researchers to this fascinating field and enlarge the list of codoped graphene systems that have been synthesized.

Copper/Carbon Heterogenous Interfaces for Enhanced Selective Electrocatalytic Reduction of CO2 to Formate

Electrochemical reduction of CO₂ (CO₂ RR) to formate is a promising route to prepare value-added chemical. Developing low-cost and efficient electrocatalysts with high product selectivity is still a grand challenge. Herein, a novel Cu anchored on hollow carbon spheres catalysts (HCS/Cu-x, x represents the mass of CuCl₂ added in the system) is designed with controllable copper/carbon heterogenous interfaces. Rich copper/carbon heterogenous interfaces and hollow structure of optimized HCS/Cu-0.12 catalyst are beneficial to charge transmission. Compared with the CO₂ RR occurred in aqueous electrolyte over Cu-based catalyst that has been reported to date, it exhibits highest formate Faradaic efficiency (FE) of 82.4% with a current density of 26 mA cm^-2 and remarkable stability in a H-cell.

Copper- and Nitrogen-Codoped Graphene with Versatile Catalytic Performances for Fenton-Like Reactions and Oxygen Reduction Reaction

Copper- and nitrogen-codoped reduced graphene oxide material (Cu/N-rGO) was prepared with a hydrothermal method. Its versatile catalytic performances were demonstrated toward the oxidative degradation of rhodamine B (RhB) and oxygen reduction reaction (ORR). The Cu and N codoping of graphene enhanced not only its activation ability toward H2O2, but also its electrocatalytic ability for ORR. It was observed that the use of 3%Cu/N-rGO together with 40 mmol·L−1 H2O2 and 4 mmol·L−1 Na2CO3 could remove more than 94% of the added RhB (30 mg·L−1) in 20 min through a catalytic Fenton-like degradation. Quenching experiments and electron paramagnetic resonance (EPR) measurements indicated that the main reactive species generated in the catalytic oxidation process were surface-bound •OH. The modified graphene also showed good electrocatalytic activity for ORR reaction in alkaline media through a four-electron mechanism. On the electrode of Cu/N-rGO, the ORR reaction exhibited an onset potential of −0.1 V and a half-wave potential of −0.248 V, which were correspondingly close to those on a Pt/C electrode. In comparison with a Pt/C electrode, the 3%Cu/N-rGO electrode showed much greater tolerance to methanol. Such outstanding catalytic properties are attributed to the abundant active sites and the synergism between Cu and N in Cu/N-rGO.

A Facile Route for Constructing Effective Cu-Nx Active Sites for Oxygen Reduction Reaction

The coordination number between copper and nitrogen in copper/nitrogen-based electrocatalysts is important for boosting the oxygen reduction reaction (ORR). However, it is difficult to control unsaturated copper/nitrogen constructions as well as to compare their ORR performances in similar carbon matrices in a simple yet efficient manner. In this study, we have easily attained two types of Cu⁺ -N₂ and Cu²⁺ -N₄ constructions simply by etching pyrolyzed Cu-doped zeolitic imidazolate framework nanoleaves (Cu-ZIF-L) with sulfuric acid or nitric acid, respectively. X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) spectra were recorded to further confirm the different copper/nitrogen constructions after the different acid treatments. Electrochemical studies have demonstrated that Cu⁺ -N₂ sites are more active in boosting the ORR performance than Cu²⁺ -N₄ sites. Furthermore, Cu-N/C-H₂ SO₄ , used as an air cathode in a zinc-air battery, exhibited excellent performance and stability.

An excellent alternative composite modifier for cathode catalysts prepared from bacterial cellulose doped with Cu and P and its utilization in microbial fuel cell

In this study, bacterial cellulose doped with phosphorus and copper via freeze-drying and high-temperature pyrolysis was used to prepare MFC cathode catalysts. After a series of characterization, the synthesized catalyst showed a three-dimensional network with a specific surface area of 580.09 m²/g. Due to the doping of Cu and P, more active sites were induced in the pores of bacterial cellulose and subsequently improved catalytic activity. The prepared catalyst was coated on the air cathode surface of the MFC to obtain the maximum output power and current density of 1177.31 mW/m² and 6.73 A/m², respectively, which were higher than those of Pt (1044.93 mW/m² and 6.02 A/m²). This work aimed to improve bioelectrical generation in MFC and find alternative commercial Pt catalysts.

Stable and Efficient Nitrogen-Containing Carbon-Based Electrocatalysts for Reactions in Energy-Conversion Systems

High activity and stability are crucial for the practical use of electrocatalysts in fuel cells, metal-air batteries, and water electrolysis, including the oxygen reduction reaction, hydrogen evolution reaction, oxygen evolution reaction, and oxidation reactions of formic acid and alcohols. Electrocatalysts based on nitrogen-containing carbon (N-C) materials show promise in catalyzing these reactions; however, there is no systematic review of strategies for the engineering of active and stable N-C-based electrocatalysts. Herein, a comprehensive comparison of recently reported N-C-based electrocatalysts regarding both electrocatalytic activity and long-term stability is presented. In the first part of this review, the relationships between the electrocatalytic reactions and selection of the element to modify the N-C-based materials are discussed. Afterwards, synthesis methods for N-C-based electrocatalysts are summarized, and strategies for the synthesis of highly stable N-C-based electrocatalysts are presented. Multiple tables containing data on crucial parameters for both electrocatalytic activity and stability are displayed in this review. Finally, constructing M-N_x moieties is proposed as the most promising engineering strategy for stable N-C-based electrocatalysts.

Experimental insight into the chemical corrosion mechanism of copper with an oil-in-water emulsion solution

Chemical corrosion mechanism of copper in an oil-in-water (O/W) emulsion is worthy of study since it would contribute to emulsion-lubrication in a metal-working process and for copper storage. The immersion experiments were carried out and the corrosion rates were measured using the weight-loss method. Surface morphology of the copper specimen was observed using a scanning electron microscope (SEM). The compositions of the corrosive residues were analyzed using an energy dispersive spectrometer (EDS) and an X-ray photoelectron spectrometer (XPS). It was found that the corrosion rate of copper in an emulsion linearly increases and the kinetics relationship could be deduced as D ₁ = 2.66 × 10^-3 t ^11.68 at room temperature (25 °C). After 1488 h of immersion time, the corrosion products on the copper surface were determined to be Cu₂O, CuO, Cu(OH)₂, CuCO₃ and Cu₂(OH)₂CuCO₃, which also changed the appearance of the emulsion. During adsorption, copper is more likely to coordinate with hydroxide, carboxylate or ester anions to generate copper compounds. The surfactants were consumed and the efficiency of emulsification characteristics was lost and finally, the O/W emulsion separated into two layers, which might hint the significance of introducing an inhibitor to protect the copper surfaces.

Enriched graphitic N in nitrogen-doped graphene as a superior metal-free electrocatalyst for the oxygen reduction reaction

The active center of N-G catalysts for ORR is confirmed to be related to the graphitic N, and the total N content in N-G catalysts is not the key factors to determine the ORR activity.