The Oxygen Reduction Activity of Nitrogen-doped Graphene

Graphite nitrogen, pyridine nitrogen and pyrrole nitrogen are the main nitrogen types in nitrogen-doped graphene materials. In order to investigate the mechanism of the oxygen reduction activity of nitrogen-doped graphene, several models of nitrogen-doped graphene with different nitrogen contents and different nitrogen types are developed. The nitrogen content is varied from 1.3 at% to 7.8 at%, and the adsorption energy is calculated according to the established models, then the band gaps are analyzed through the optimization results, so as to compare the magnitude of the conductivity. Finally, the oxygen reduction activity of graphite nitrogen-doped graphene (GNG) is found to be better than pyridine nitrogen-doped graphene (PDNG) and pyrrole nitrogen-doped graphene (PLNG) when the nitrogen content is lower than 2.6 at%, and the oxygen reduction activity of PDNG is the best when the nitrogen content was higher than 2.6 at%.

Design Principles of Nitrogen-doped Graphene Nanoribbons as Highly Effective Bifunctional Catalysts for Li - O2 Batteries

Li-O2 batteries are promising candidates in fields demanding high capacities like electric vehicles due to their superior theoretical energy density in contrast to lithium-ion batteries. However, oxygen reduction reaction (ORR)...

Growth Mechanism of Micro/Nano Metal Dendrites and Cumulative Strategies for Countering Its Impacts in Metal Ion Batteries: A Review

Metal-ion batteries are capable of delivering high energy density with a longer lifespan. However, they are subject to several issues limiting their utilization. One critical impediment is the budding and extension of solid protuberances on the anodic surface, which hinders the cell functionalities. These protuberances expand continuously during the cyclic processes, extending through the separator sheath and leading to electrical shorting. The progression of a protrusion relies on a number of in situ and ex situ factors that can be evaluated theoretically through modeling or via laboratory experimentation. However, it is essential to identify the dynamics and mechanism of protrusion outgrowth. This review article explores recent advances in alleviating metal dendrites in battery systems, specifically alkali metals. In detail, we address the challenges associated with battery breakdown, including the underlying mechanism of dendrite generation and swelling. We discuss the feasible solutions to mitigate the dendrites, as well as their pros and cons, highlighting future research directions. It is of great importance to analyze dendrite suppression within a pragmatic framework with synergy in order to discover a unique solution to ensure the viability of present (Li) and future-generation batteries (Na and K) for commercial use.

Comparative first principles-based molecular dynamics study of catalytic mechanism and reaction energetics of water oxidation reaction on 2D-surface

The study of the water-splitting process, which can proceed in 2e^- as well as 4e^- pathway, reveals that the process is entirely an uphill process, and the third step, that is, the oxooxo bond formation is the rate-determining step. The kinetic barrier of the oxygen evolution reaction (OER) on the 2D material catalysts in the presence of explicit solvents is scarcely studied. Here, we investigate the dynamics of the OER on the undoped graphene and the activation energy barrier of each step using first principles molecular dynamics simulations. Here we provide a detailed analysis of the kinetics of all the 4e^- transfer steps of OER on the graphene surface. We also compare the accuracy of one of the density functional theory (DFT) functionals and density functional based tight binding (DFTB) method in explaining the OER steps. The comparative study reveals that DFTB can be used for performing metadynamics simulations quipped with much less computational cost than DFT functionals. By both Perdew-Burke-Ernzerhof and DFTB methods, the third step is revealed to be the rate-determining step with an energy barrier of 21.19 ± 0.51 and 20.23 ± 0.20 kcal mol^-1 , respectively. DFTB gives an impression of being successful in predicting the energy barriers of OER in 4e- transfer pathway and comparable to the DFT method, and we would like to extend the use of DFTB for further studies with a sizable and complex system.

Three-Dimensional Graphene-Based Macrostructures for Electrocatalysis

Electrochemical energy storage and conversion is an effective strategy to relieve the increasing energy and environment crisis. The sluggish reaction kinetics in the related devices is one of the major obstacles for them to realize practical applications. More efforts should be devoted to searching for high-efficiency electrocatalysts and enhancing the electrocatalytic performance. 3D graphene macrostructures (3D GMs) are one kind of porous crystalline materials with 3D structures at both micro- and macro-scale. The unique structure can achieve large accessible surface area, expose many active sites, promote fast mass/electron transport, and provide wide room for further functional modification. All these features make them promising candidates for electrocatalysis. In this review, the authors focus on the latest progress of 3D GMs for electrocatalysis. First, the preparation methods of 3D GMs are introduced followed by the strategies for functional modifications. Then, their electrocatalytic performances are discussed in detail including monofunctional and bifunctional electrocatalysis. The electrocatalytic processes involve oxygen reduction reaction, oxygen evolution reaction, hydrogen evolution reaction, and carbon dioxide reduction reaction. Finally, the challenges and perspectives are presented to offer a guideline for the exploration of excellent 3D GM-based electrocatalysts.

Hybridized Graphene for Supercapacitors: Beyond the Limitation of Pure Graphene

Graphene-based supercapacitors have been attracting growing attention due to the predicted intrinsic high surface area, high electron mobility, and many other excellent properties of pristine graphene. However, experimentally, the state-of-the-art graphene electrodes face limitations such as low surface area, low electrical conductivity, and low capacitance, which greatly limit their electrochemical performances for supercapacitor applications. To tackle these issues, hybridizing graphene with other species (e.g., atom, cluster, nanostructure, etc.) to enlarge the surface area, enhance the electrical conductivity, and improve capacitance behaviors are strongly desired. In this review, different hybridization principles (spacers hybridization, conductors hybridization, heteroatoms doping, and pseudocapacitance hybridization) are discussed to provide fundamental guidance for hybridization approaches to solve these challenges. Recent progress in hybridized graphene for supercapacitors guided by the above principles are thereafter summarized, pushing the performance of hybridized graphene electrodes beyond the limitation of pure graphene materials. In addition, the current challenges of energy storage using hybridized graphene and their future directions are discussed.

Defect Engineering on Electrode Materials for Rechargeable Batteries

The reasonable design of electrode materials for rechargeable batteries plays an important role in promoting the development of renewable energy technology. With the in-depth understanding of the mechanisms underlying electrode reactions and the rapid development of advanced technology, the performance of batteries has significantly been optimized through the introduction of defect engineering on electrode materials. A large number of coordination unsaturated sites can be exposed by defect construction in electrode materials, which play a crucial role in electrochemical reactions. Herein, recent advances regarding defect engineering in electrode materials for rechargeable batteries are systematically summarized, with a special focus on the application of metal-ion batteries, lithium-sulfur batteries, and metal-air batteries. The defects can not only effectively promote ion diffusion and charge transfer but also provide more storage/adsorption/active sites for guest ions and intermediate species, thus improving the performance of batteries. Moreover, the existing challenges and future development prospects are forecast, and the electrode materials are further optimized through defect engineering to promote the development of the battery industry.

Boron and pyridinic nitrogen-doped graphene as potential catalysts for rechargeable non-aqueous sodium–air batteries

In this work, we performed density functional theory (DFT) analysis of nitrogen (N)- and boron (B)-doped graphene, and N,B-co-doped graphene as potential catalysts for rechargeable non-aqueous sodium–air batteries. Four steps of an NaO₂ growth and depletion mechanism model were implemented to study the effects of B- and N-doped and co-doped graphene on the reaction pathways, overpotentials, and equilibrium potentials. The DFT results revealed that two-boron- and three-nitrogen (pyridinic)-doped graphene exhibited plausible reaction pathways at the lowest overpotentials, especially during the charging process (approximately 200 mV), thus, significantly improving the oxygen reduction and oxidation reactions of pristine graphene. In addition, pyridinic nitrogen-doped graphene meaningfully increased the equilibrium potential by approximately 0.30 eV compared to the other graphene-based materials considered in this study. This detailed DFT study provides valuable data that can be used for the successful development of low-cost and efficient graphene-based catalysts for sodium–air battery systems operating with non-aqueous electrolyte.

We performed density functional theory analysis of heteroatom doped graphene as potential catalysts for rechargeable non-aqueous sodium–air batteries. Pyridinic nitrogen and boron doped graphene exhibited too low overpotential reaction pathways.

Recent advances in two-dimensional materials and their nanocomposites in sustainable energy conversion applications

Two-dimensional (2D) materials have a wide platform in research and expanding nano- and atomic-level applications. This study is motivated by the well-established 2D catalysts, which demonstrate high efficiency, selectivity and sustainability exceeding that of classical noble metal catalysts for the oxygen reduction reaction (ORR), oxygen evolution reaction (OER), and/or hydrogen evolution reaction (HER). Nowadays, the hydrogen evolution reaction (HER) in water electrolysis is crucial for the cost-efficient production of a pure hydrogen fuel. We will also discuss another important point related to electrochemical carbon dioxide and nitrogen reduction (ECR and N2RR) in detail. In this review, we mainly focused on the recent progress in the fuel cell technology based on 2D materials, including graphene, transition metal dichalcogenides, black phosphorus, MXenes, metal-organic frameworks, and metal oxide nanosheets. First, the basic attributes of the 2D materials were described, and their fuel cell mechanisms were also summarized. Finally, some effective methods for enhancing the performance of the fuel cells based on 2D materials were also discussed, and the opportunities and challenges of 2D material-based fuel cells at the commercial level were also provided. This review can provide new avenues for 2D materials with properties suitable for fuel cell technology development and related fields.

First-principles study of rocksalt early transition-metal carbides as potential catalysts for Li-O2 batteries

A series of early transition-metal carbides (TMCs) in the NaCl structure have been constructed to compare the catalytic activity in Li-O2 batteries by first-principles calculations. The reasonable interfacial models of LixO2 (x = 4, 2, and 1) molecules adsorbed on early TMCs surfaces were used to simulate oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) processes. Taking overpotentials as a merit parameter of catalytic activity, more relationships between material properties relative to the adsorption/desorption behavior of active molecules and catalytic activity are constructed for early TMCs. The equilibrium and charging potentials used to calculate the OER overpotentials of early TMCs are inversely proportional to the adsorption energies of (Li2O)2 and LiO2, respectively. The ORR overpotentials are inversely proportional to the adsorption energies of (Li2O)2 and LiO2 for early TMCs, but the relationship between OER overpotentials and the adsorption energies of reactive intermediates is unclear. Additionally, the overpotentials of early TMCs for ORR and OER are proportional to the desorption energies of Li+ and O2, respectively. In general, both the adsorption energy of (Li2O)2/LiO2 and desorption energy of Li+/O2 are effective characterization parameters of catalytic activity. By providing the comprehensive valuable parameters on electrochemical performance to compare the catalytic activity of early TMCs and establishing more correlations between material properties relative to the adsorption/desorption behavior of active molecules with their catalytic activity, our investigation is helpful for knowing more about the catalytic process and beneficial to screen and design novel highly active catalysts for Li-O2 batteries.

A Review of Carbon-Composited Materials as Air-Electrode Bifunctional Electrocatalysts for Metal–Air Batteries

Metal–air batteries (MABs), particularly rechargeable MABs, have gained renewed interests as a potential energy storage/conversion solution due to their high specific energy, low cost, and safety. The development of MABs has, however, been considerably hampered by its relatively low rate capability and its lack of efficient and stable air catalysts in which the former stems mainly from the sluggish kinetics of the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) and the latter stems from the corrosion/oxidation of carbon materials in the presence of oxygen and high electrode potentials. In this review, various carbon-composited bifunctional electrocatalysts are reviewed to summarize progresses in the enhancement of ORR/OER and durability induced by the synergistic effects between carbon and other component(s). Catalyst mechanisms of the reaction processes and associated performance enhancements as well as technical challenges hindering commercialization are also analyzed. To facilitate further research and development, several research directions for overcoming these challenges are also proposed.

Functional and stability orientation synthesis of materials and structures in aprotic Li–O2batteries

This review presents the recent advances made in the functional and stability orientation synthesis of materials/structures for Li–O₂batteries.

Adsorption-energy-based activity descriptors for electrocatalysts in energy storage applications

Energy storage technologies, such as fuel cells, ammonia production and lithium–air batteries, are important strategies for addressing the global challenge of energy crisis and environmental pollution. Taking overpotential as a direct criterion, we illustrate in theory and experiment that the adsorption energies of charged species such as Li++e− and H++e− are a central parameter to describe catalytic activities related to electricity-in/electricity-out efficiencies. The essence of catalytic activity is revealed to relate with electronic coupling between catalysts and charged species. Based on adsorption energy, some activity descriptors such as d-band center, eg-electron number and charge-transfer capacity are further defined by electronic properties of catalysts that directly affect interaction between catalysts and charged species. The present review is helpful for understanding the catalytic mechanisms of these electrocatalytic reactions and developing accurate catalytic descriptors, which can be employed to screen high-activity catalysts in future high-throughput calculations and experiments.

Polyimide-coated carbon electrodes combined with redox mediators for superior Li-O2 cells with excellent cycling performance and decreased overpotential

We report an air electrode employing polyimide-coated carbon nanotubes (CNTs) combined with a redox mediator for Li-O₂ cells with enhanced electrochemical performance. The polyimide coating on the carbon surface suppresses unwanted side reactions, which decreases the amount of accumulated reaction products on the surface of the air electrode during cycling. The redox mediators lower the overpotential of the Li-O₂ cells because they can easily transfer electrons from the electrode to the reaction products. The low overpotential can also decrease the side reactions that activate at a high potential range. Specifically, the CsI redox mediator effectively interrupted dendrite growth on the Li anode during cycling due to the shielding effect of its Cs⁺ ions and acted as a redox mediator due to its I⁻ ions. LiNO₃ also facilitates the decrease in side reactions and the stabilization of the Li anode. The synergic effect of the polyimide coating and the electrolyte containing the LiNO₃/CsI redox mediator leads to a low overpotential and excellent cycling performance (over 250 cycles with a capacity of 1,500 mAh·g_electrode⁻¹).

Electrolyte-controlled discharge product distribution of Na–O2 batteries: a combined computational and experimental study

Tuning the composition of discharge products is an important strategy to reduce charge potential, suppress side reactions, and improve the reversibility of metal–oxygen batteries.

Monolayer germanium monochalcogenides (GeS/GeSe) as cathode catalysts in nonaqueous Li–O2batteries

The possibility of using 2D-GeSe/GeS as a cathode catalyst for nonaqueous Li–O₂batteries is computationally confirmed.

Catalyst morphology matters for lithium-oxygen battery cathodes

The effectiveness of using catalyst nanoparticles to reduce the overpotential and energy efficiency of lithium-oxygen (or lithium-air) batteries (LOBs) is usually attributed to the inherent catalytic properties of individual nanoparticles. Here, we demonstrate that the morphology of the catalyst layer is equally important in maintaining integrity of the catalyst coating during product formation in LOBs. We demonstrate this by comparing the performance of smooth, conformal coated Mn₂O₃ catalyst nanoparticles prepared by electric field-assisted deposition, and more irregular coatings using conventional film assembly techniques both on three-dimensional mesh substrates. Smooth coatings lead to an improved overpotential of 50 mV during oxygen reduction and 130 mV during oxygen evolution in addition to a nearly 2X improvement in durability compared to the more irregular films. In situ electrochemical impedance spectroscopy combined with imaging studies elucidates a mechanism of morphology-directed deactivation of catalyst layers during charging and discharging that must be overcome at practical electrode scales to achieve cell-level performance targets in LOBs.

Facile Synthesis of Boron-Doped rGO as Cathode Material for High Energy Li-O2 Batteries

To improve the electrochemical performance of the high energy Li-O2 batteries, it is important to design and construct a suitable and effective oxygen-breathing cathode. Herein, a three-dimensional (3D) porous boron-doped reduction graphite oxide (B-rGO) material with a hierarchical structure has been prepared by a facile freeze-drying method. In this design, boric acid as the boron source helps to form the 3D porous structure, owing to its cross-linking and pore-forming function. This architecture facilitates the rapid oxygen diffusion and electrolyte penetration in the electrode. Meanwhile, the boron-oxygen functional groups linking to the carbon surface or edge serve as additional reaction sites to activate the ORR process. It is vital that boron atoms have been doped into the carbon lattices to greatly activate the electrons in the carbon π system, which is beneficial for fast charge under large current densities. Density functional theory calculation demonstrates that B-rGO exhibits much stronger interactions with Li5O6 clusters, so that B-rGO more effectively activates Li-O bonds to decompose Li2O2 during charge than rGO does. With B-rGO as a catalytic substrate, the Li-O2 battery achieves a high discharge capacity and excellent rate capability. Moreover, catalysts could be added into the B-rGO substrate to further lower the overpotential and enhance the cycling performance in future.

PEDOT:PSS as multi-functional composite material for enhanced Li-air-battery air electrodes

We propose PEDOT:PSS as a multi-functional composite material for an enhanced Li-air-battery air electrode. The PEDOT:PSS layer was coated on the surface of carbon (graphene) using simple method. A electrode containing PEDOT:PSS-coated graphene (PEDOT electrode) could be prepared without binder (such as PVDF) because of high adhesion of PEDOT:PSS. PEDOT electrode presented considerable discharge and charge capacity at all current densities. These results shows that PEDOT:PSS acts as a redox reaction matrix and conducting binder in the air electrode. Moreover, after cycling, the accumulation of reaction products due to side reaction in the electrode was significantly reduced through the use of PEDOT:PSS. This implies that PEDOT:PSS coating layer can suppress the undesirable side reactions between the carbon and electrolyte (and/or Li₂O₂), which causes enhanced Li-air cell cyclic performance.

NiFe-layered double hydroxides: a bifunctional O2 electrode catalyst for non-aqueous Li–O2 batteries

An improved Li–O₂ battery that exhibited a lower voltage gap and better cycle performance using NiFe-LDH. This result, although preliminary opens-up the possibilities of using LDHs as a promising catalyst for Li–O₂ battery systems.