A comparison of single and double Co sites incorporated in N-doped graphene for the oxygen reduction reaction

Precise Design and Modification Engineering of Single-Atom Catalytic Materials for Oxygen Reduction

Due to their unique electronic and structural properties, single-atom catalytic materials (SACMs) hold great promise for the oxygen reduction reaction (ORR). Coordinating environmental and engineering strategies is the key to improving the ORR performance of SACMs. This review summarizes the latest research progress and breakthroughs of SACMs in the field of ORR catalysis. First, the research progress on the catalytic mechanism of SACMs acting on ORR is reviewed, including the latest research results on the origin of SACMs activity and the analysis of pre-adsorption mechanism. The study of the pre-adsorption mechanism is an important breakthrough direction to explore the origin of the high activity of SACMs and the practical and theoretical understanding of the catalytic process. Precise coordination environment modification, including in-plane, axial, and adjacent site modifications, can enhance the intrinsic catalytic activity of SACMs and promote the ORR process. Additionally, several engineering strategies are discussed, including multiple SACMs, high loading, and atomic site confinement. Multiple SACMs synergistically enhance catalytic activity and selectivity, while high loading can provide more active sites for catalytic reactions. Overall, this review provides important insights into the design of advanced catalysts for ORR.

Interface engineering of transition metal-nitrogen-carbon by graphdiyne for boosting the oxygen reduction/evolution reactions: A computational study

Exploring high-efficiency electrocatalysts to boost the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) is pivotal to the large-scale applications for clean and renewable energy technologies, such as fuel cells, water splitting, and metal-air batteries. Herein, by means of density functional theory (DFT) computations, we proposed a strategy to modulate the catalytic activity of transition metal-nitrogen-carbon catalysts through their interface engineering with graphdiyne (TMNC/GDY). Our results revealed that these hybrid structures exhibit good stability and excellent electrical conductivity. Especially, CoNC/GDY was identified as a promising bifunctional catalyst for ORR/OER with rather low overpotentials in acidic conditions according to the constant-potential energy analysis. Moreover, the volcano plots were established to describe the activity trend of the ORR/OER on TMNC/GDY using the adsorption strength of the oxygenated intermediates. Remarkably, the d-band center and charge transfer of the TM active sites can be utilized to correlate the ORR/OER catalytic activity and their electronic properties. Our findings not only suggested an ideal bifunctional oxygen electrocatalyst, but also provided a useful strategy to obtain highly efficient catalysts by interface engineering of two-dimensional heterostructures.

Surface Curvature Effect on Dual-Atom Site Oxygen Electrocatalysis

Improved oxygen electrocatalysis is crucial for the ever-growing energy demand. Metal-nitrogen-carbon (M-N-C) materials are promising candidates for catalysts. Their activity is tunable via varying electronic and geometric properties, such as porosity. Because of the difficulty in modeling porosity, M-N-Cs with variable surface curvature remained largely unexplored. In this work, we developed a realistic in-pore dual-atom site M-N-C model and applied density functional theory to investigate the surface curvature effect on oxygen reduction and evolution reactions. We show that surface curving tailors both scaling relations and energy barriers. Thus, we predict that adjusting the surface curvature can improve the catalytic activity toward mono- and bifunctional oxygen electrocatalysis.

Bioinspired and Bioderived Aqueous Electrocatalysis

The development of efficient and sustainable electrochemical systems able to provide clean-energy fuels and chemicals is one of the main current challenges of materials science and engineering. Over the last decades, significant advances have been made in the development of robust electrocatalysts for different reactions, with fundamental insights from both computational and experimental work. Some of the most promising systems in the literature are based on expensive and scarce platinum-group metals; however, natural enzymes show the highest per-site catalytic activities, while their active sites are based exclusively on earth-abundant metals. Additionally, natural biomass provides a valuable feedstock for producing advanced carbonaceous materials with porous hierarchical structures. Utilizing resources and design inspiration from nature can help create more sustainable and cost-effective strategies for manufacturing cost-effective, sustainable, and robust electrochemical materials and devices. This review spans from materials to device engineering; we initially discuss the design of carbon-based materials with bioinspired features (such as enzyme active sites), the utilization of biomass resources to construct tailored carbon materials, and their activity in aqueous electrocatalysis for water splitting, oxygen reduction, and CO2 reduction. We then delve in the applicability of bioinspired features in electrochemical devices, such as the engineering of bioinspired mass transport and electrode interfaces. Finally, we address remaining challenges, such as the stability of bioinspired active sites or the activity of metal-free carbon materials, and discuss new potential research directions that can open the gates to the implementation of bioinspired sustainable materials in electrochemical devices.

Surface states of dual-atom catalysts should be considered for analysis of electrocatalytic activity

Experimentally well-characterized dual-atom catalysts (DACs), where two adjacent metal atoms are stably anchored on carbon defects, have shown some clear advantages in electrocatalysis compared to conventional catalysts and emerging single-atom catalysts. However, most previous theoretical studies directly used a pristine dual-atom site to analyze the electrocatalytic activity of a DAC. Herein, by analyzing 8 homonuclear and 64 heteronuclear DACs structures with ab initio calculations, our derived surface Pourbaix diagrams show that the surface states of DACs generally differ from a pristine surface at electrocatalytic operating conditions. This phenomenon suggests that the surface state of a DAC should be considered before analyzing the catalytic activity in electrocatalysis, while the electrochemistry-driven pre-adsorbed molecules generated from the liquid phase may either change the electronic properties or even block the active site of DACs. Based on these results, we provide a critical comment to the catalyst community: before analyzing the electrocatalytic activity of a DAC, its surface state should be analyzed beforehand.

N-Doped CrS2 Monolayer as a Highly-Efficient Catalyst for Oxygen Reduction Reaction: A Computational Study

Searching for low-cost and highly-efficient oxygen reduction reaction (ORR) catalysts is crucial to the large-scale application of fuel cells. Herein, by means of density functional theory (DFT) computations, we proposed a new class of ORR catalysts by doping the CrS₂ monolayer with non-metal atoms (X@CrS₂, X = B, C, N, O, Si, P, Cl, As, Se, and Br). Our results revealed that most of the X@CrS₂ candidates exhibit negative formation energy and large binding energy, thus ensuring their high stability and offering great promise for experimental synthesis. Moreover, based on the computed free energy profiles, we predicted that N@CrS₂ exhibits the best ORR catalytic activity among all considered candidates due to its lowest overpotential (0.41 V), which is even lower than that of the state-of-the-art Pt catalyst (0.45 V). Remarkably, the excellent catalytic performance of N@CrS₂ for ORR can be ascribed to its optimal binding strength with the oxygenated intermediates, according to the computed linear scaling relationships and volcano plot, which can be well verified by the analysis of the p-band center as well as the charge transfer between oxygenated species and catalysts. Therefore, by carefully modulating the incorporated non-metal dopants, the CrS₂ monolayer can be utilized as a promising ORR catalyst, which may offer a new strategy to further develop eligible electrocatalysts in fuel cells.

Multilevel Computational Studies Reveal the Importance of Axial Ligand for Oxygen Reduction Reaction on Fe-N-C Materials

The systematic improvement of Fe-N-C materials for fuel cell applications has proven challenging, due in part to an incomplete atomistic understanding of the oxygen reduction reaction (ORR) under electrochemical conditions. Herein, a multilevel computational approach, which combines ab initio molecular dynamics simulations and constant potential density functional theory calculations, is used to assess proton-coupled electron transfer (PCET) processes and adsorption thermodynamics of key ORR intermediates. These calculations indicate that the potential-limiting step for ORR on Fe-N-C materials is the formation of the Fe^III-OOH intermediate. They also show that an active site model with a water molecule axially ligated to the iron center throughout the catalytic cycle produces results that are consistent with the experimental measurements. In particular, reliable prediction of the ORR onset potential and the Fe(III/II) redox potential associated with the conversion of Fe^III-OH to Fe^II and desorbed H₂O requires an axial H₂O co-adsorbed to the iron center. The observation of a five-coordinate rather than four-coordinate active site has significant implications for the thermodynamics and mechanism of ORR. These findings highlight the importance of solvent-substrate interactions and surface charge effects for understanding the PCET reaction mechanisms and transition-metal redox couples under realistic electrochemical conditions.

Detailed formation process of Co@C catalysts and the influence of structural regulation on catalytic properties

Co@C is a novel class of catalysts with many structural advantages, such as highly dispersed active species, developed pore structure, and special encapsulated structure. Although considerable progress has been made in the development of new Co@C materials, research on the formation mechanism of these materials is lacking. Herein, the overall microcosmic structure of the Co@C catalyst was investigated by systematic characterization. Subsequently, a pseudo in situ method was employed to explore the detailed structure of the Co@C catalyst pyrolyzed at different temperatures. The special carbon environment of materials is essential for synthesizing materials during pyrolysis at high temperatures. Co ions were reduced to Co⁰ by the surrounding carbon atoms at a high temperature. In return, the surrounding carbon atoms were catalyzed by Co⁰ particles to form carbon nanotubes. However, with the obstruction of amorphous carbon atoms that are not in contact with Co⁰, the paths through which the carbon nanotubes move forward formed the porous structure of the catalyst, as well as the graphitic encapsulated structure. Further, the effects of pretreatment conditions on the structure and properties of the Co@C catalyst were studied systematically.

Mechanism of Oxygen Reduction Reaction in Alkaline Medium on Nitrogen-Doped Graphyne and Graphdiyne Families: A First Principles Study

Using extensive first principles protocols, a systematic investigation is performed to probe the oxygen reduction reaction (ORR) mechanism on nitrogen (N) doped graphynes (Gys, e. g. αGy, βGy, γGy and 6,6,12Gy) and graphdiyne (Gdy) in alkaline medium. We considered both associative and dissociative pathways, as well as two distinct intermediate forks for each of them depending on the first protonation site(s). Following the dissociative approach, the activation energy to form an O₂ dissociated configuration is found as a function of the distances migrated by the O atoms over the catalyst surface and the amount of charge transferred from the C atoms linked to N. N doped αGy and 6,6,12Gy emerged as the best electrocatalyst comparing both pathways having lowest overpotentials of 0.88 and 0.82 V, respectively. The rate-limiting steps for the two different intermediate routes are observed to be dependent on the first protonation site(s) and related to the desorption of the OH radical from the sp hybridized C atom site(s) linked to N. Hence, the OH adsorption energy is identified as a descriptor for the efficiency of the ORR for the considered systems. The stabilities of the ORR intermediates are further elaborated in terms of pH and electrode potential.

Catalytic Role of Adsorption of Electrolyte/Molecules as Functional Ligands on Two-Dimensional TM-N4 Monolayer Catalysts for the Electrocatalytic Nitrogen Reduction Reaction

Two-dimensional single-atom catalysts (2D SACs) have been widely studied on the nitrogen reduction reaction (NRR). The characteristics of 2D catalysts imply that both sides of the monolayer can be catalytic sites and adsorb electrolyte ions or molecules from solutions. Overstrong adsorption of electrolyte ions or molecules on both sides of the catalyst site will poison the catalyst, while the adsorbate on one side of the catalytic site will modify the activity and selectivity of the other side for NRR. Discovering the influence of adsorption of electrolyte ions or molecules as a functional ligand on catalyst performance on the NRR is crucial to improve NRR efficiency. Here, we report this work using the density functional theory (DFT) method to investigate adsorption of electrolyte ions or molecules as a functional ligand. Among all of the studied 18 functional ligands and 3 transition metals (TMs), the results showed that Ru&F, Ru&COOH, and Mo&H₂O combinations were screened as electrocatalysis systems with high activity and selectivity. Particularly, the Mo&H₂O combination possesses the highest activity with a low ΔG_MAX of 0.44 eV through the distal pathway. The superior catalytic performance of the Mo&H₂O combination is mainly attributed to the electron donation from the metal d orbital. Furthermore, the functional ligands can occupy the active sites and block the competing vigorous hydrogen evolution reaction. Our findings offer an effective and practical strategy to design the combination of the catalyst and electrolyte to improve electrocatalytic NRR efficiency.