Constant Electrode Potential Quantum Mechanical Study of CO2 Electrochemical Reduction Catalyzed by N-Doped Graphene

Doping and defect engineering in carbon-based electrocatalysts for enhanced electrochemical CO2 reduction: From 0D to 3D materials

The increasing atmospheric CO2 levels and the urgent need for sustainable energy solutions have driven research into electrochemical CO2 reduction. Carbon-based materials have received significant attention for their potential as electrocatalysts, yet their inert nature often limits their performance. Defect engineering and heteroatom doping have emerged as transformative approaches to overcome these limitations, enhancing both catalytic activity and Faradaic efficiency. This review systematically examines the role of these strategies across diverse carbon materials, including graphene, carbon nanotubes, carbon dots, and boron-doped diamond. Special attention is given to the incorporation of heteroatoms, such as nitrogen and boron, and the modulation of defect structures to optimize CO2 reduction pathways. By exploring the interplay between dopant type, defect density, and material dimensionality, we provide a comprehensive understanding of how tailored carbon-based electrocatalysts can drive advancements in sustainable electrochemical CO2 conversion. This work underscores the potential of defect-engineered and doped carbon materials to revolutionize the field of electrocatalysis, paving the way for innovative solutions to environmental and energy challenges.

Enhancing CO2 Electroreduction Precision to Ethylene and Ethanol: The Role of Additional Boron Catalytic Sites in Cu-Based Tandem Catalysts

The electrocatalytic conversion of carbon dioxide (CO2) into valuable multicarbon (C2+) compounds offers a promising approach to mitigate CO2 emissions and harness renewable energy. However, achieving precise selectivity for specific C2+ products, such as ethylene and ethanol, remains a formidable challenge. This study shows that incorporating elemental boron (B) into copper (Cu) catalysts provides additional adsorption sites for *CO intermediates, enhancing the selectivity of desirable C2+ products. Additionally, using a nickel single-atom catalyst (Ni-SAC) as a *CO source increases local *CO concentration and reduces the hydrogen evolution reaction. In situ experiments and density functional theory (DFT) calculations reveal that surface-bound boron units adsorb and convert *CO more efficiently, promoting ethylene production, while boron within the bulk phase of copper influences charge transfer, facilitating ethanol generation. In a neutral electrolyte, the bias current density for ethylene production using the B-O-Cu2@Ni-SAC0.05 hybrid catalyst exceeded 300 mA cm-2, and that for ethanol production with B-O-Cu5@Ni-SAC0.2 surpassed 250 mA cm-2. This study underscores that elemental doping in Cu-based catalysts not only alters charge and crystalline phase arrangements at Cu sites but also provides additional reduction sites for coupling reactions, enabling the efficient synthesis of distinct C2+ products.

Oxygen Vacancy-Driven Heterointerface Breaks the Linear-Scaling Relationship of Intermediates toward Electrocatalytic CO2 Reduction

Smart metal-metal oxide heterointerface construction holds promising potentials to endow an efficient electron redistribution for electrochemical CO2 reduction reaction (CO2RR). However, inhibited by the intrinsic linear-scaling relationship, the binding energies of competitive intermediates will simultaneously change due to the shifts of electronic energy level, making it difficult to exclusively tailor the binding energies to target intermediates and the final CO2RR performance. Nonetheless, creating specific adsorption sites selective for target intermediates probably breaks the linear-scaling relationship. To verify it, Ag nanoclusters were anchored onto oxygen vacancy-rich CeO2 nanorods (Ag/OV-CeO2) for CO2RR, and it was found that the oxygen vacancy-driven heterointerface could effectively promote CO2RR to CO across the entire potential window, where a maximum CO Faraday efficiency (FE) of 96.3% at -0.9 V and an impressively high CO FE of over 62.3% were achieved at a low overpotential of 390 mV within a flow cell. The experimental and computational results collectively suggested that the oxygen vacancy-driven heterointerfacial charge spillover conferred an optimal electronic structure of Ag and introduced additional adsorption sites exclusively recognizable for *COOH, which, beyond the linear-scaling relationship, enhanced the binding energy to *COOH without hindering *CO desorption, thus resulting in the efficient CO2RR to CO.

The Impact of Electron Donating and Withdrawing Groups on Electrochemical Hydrogenolysis and Hydrogenation of Carbonyl Compounds

The hydrogenolysis or hydrodeoxygenation of a carbonyl group, where the C═O group is converted to a CH2 group, is of significant interest in a variety of fields. A challenge in electrochemically achieving hydrogenolysis of a carbonyl group with high selectivity is that electrochemical hydrogenation of a carbonyl group, which converts the C═O group to an alcohol group (CH-OH), is demonstrated not to be the initial step of hydrogenolysis. Instead, hydrogenation and hydrogenolysis occur in parallel, and they are competing reactions. This means that although both hydrogenolysis and hydrogenation require adding H atoms to the carbonyl group, they involve different intermediates formed on the electrode surface. Thus, revealing the difference in intermediates, transition states, and kinetic barriers for hydrogenolysis and hydrogenation pathways is the key to understanding and controlling hydrogenolysis/hydrogenation selectivity of carbonyl compounds. In this study, we aimed to identify features of reactant molecules that can affect their hydrogenolysis/hydrogenation selectivity on a Zn electrode that was previously shown to promote hydrogenolysis over hydrogenation. In particular, we examined the electrochemical reduction of para-substituted benzaldehyde compounds with substituent groups having different electron donating/withdrawing abilities. Our results show a strikingly systematic impact of the substituent group where a stronger electron-donating group promotes hydrogenolysis and a stronger electron-withdrawing group promotes hydrogenation. These experimental results are presented with computational results explaining the substituent effects on the thermodynamics and kinetics of electrochemical hydrogenolysis and hydrogenation pathways, which also provide critically needed information and insights into the transition states involved with these pathways.

Efficient conversion of propane in a microchannel reactor at ambient conditions

The oxidative dehydrogenation of propane, primarily sourced from shale gas, holds promise in meeting the surging global demand for propylene. However, this process necessitates high operating temperatures, which amplifies safety concerns in its application due to the use of mixed propane and oxygen. Moreover, these elevated temperatures may heighten the risk of overoxidation, leading to carbon dioxide formation. Here we introduce a microchannel reaction system designed for the oxidative dehydrogenation of propane within an aqueous environment, enabling highly selective and active propylene production at room temperature and ambient pressure with mitigated safety risks. A propylene selectivity of over 92% and production rate of 19.57 mmol mCu-2 h-1 are simultaneously achieved. This exceptional performance stems from the in situ creation of a highly active, oxygen-containing Cu catalytic surface for propane activation, and the enhanced propane transfer via an enlarged gas-liquid interfacial area and a reduced diffusion path by establishing a gas-liquid Taylor flow using a custom-made T-junction microdevice. This microchannel reaction system offers an appealing approach to accelerate gas-liquid-solid reactions limited by the solubility of gaseous reactant.

A theoretical descriptor for screening efficient NO reduction electrocatalysts from transition-metal atoms on N-doped BP monolayer

An electrochemical nitric oxide (NO) reduction reaction (NORR) is proposed as an attractive method for simultaneous realization of NO removal and ammonia (NH₃) synthesis. Here, the potentials of 29 transition-metal atoms anchored on the nitrogen-doped BP monolayer (MN₃/BP) as efficient NORR catalysts are systematically examined using first-principles calculations. Combining the adsorption Gibbs free energies of the N and OH species, a simple descriptor is constructed and a volcano plot of the NORR limiting potentials on the single atom catalysts (SACs) is established. Consequently, the MoN₃/BP and IrN₃/BP SACs are picked out as promising NORR electrocatalysts for NH₃ synthesis with the limiting potentials of -0.10 V and -0.06 V, respectively. Their corresponding rate constants are significantly larger than or close to that of the excellent Pt(111) surface. The electronic analysis shows that the Mo-4d or Ir-5d orbitals can be well hybridized with the NO-2p orbitals, sufficiently activating the adsorbed NO species. Particularly, the MoN₃/BP and IrN₃/BP SACs possess high thermal stabilities and can be easily synthesized by using MoCl₃ and IrCl₃ as precursors, respectively. This work not only offers a simple descriptor to efficiently design NORR electrocatalysts but also provides a comprehensive atomic understanding on the mechanism of NO-to-NH₃ conversion.

Examination of the Brønsted-Evans-Polanyi relationship for the hydrogen evolution reaction on transition metals based on constant electrode potential density functional theory

In the search for efficient and inexpensive electrocatalysts for the hydrogen evolution reaction (HER), the hydrogen binding energy is often used as a descriptor to represent the catalytic activity. The success of this approach relies on the Brønsted-Evans-Polanyi (BEP) relationship. In this study, we used constant electrode potential density functional theory calculations to examine this relationship. Eight fcc metals with a low hydrogen adsorption concentration of 1/9 were used as the model systems. We found that the HER kinetic barriers are indeed correlated to the . Both the s of the hollow site and less favourable top site correlate to the kinetic barriers; however, the correlation is better for the latter. This behaviour leads to a set of equations for estimating the HER kinetic barriers with improved accuracy that can be used to predict the HER performance of the materials with a low hydrogen adsorption concentration. This work demonstrates the importance of calculating the of a suitable adsorption site to establish good BEP relationships.

CO2 reduction to CH4 on Cu-doped phosphorene: a first-principles study

Optimizing the electrochemical carbon dioxide reduction reaction (CRR) to fuels is one of the most significant challenges in materials science and chemistry. Recently, single metal atom catalysts based on 2D materials have shown promise to improve the electroreduction performance of pristine 2D materials in the CRR. The physical origins of such performance enhancements are still poorly understood. Herein, we report the potential of a single Cu atom doped phosphorene catalyst for CO₂ electroreduction based on density functional theory (DFT) calculations. The doping sites (hollow, bridge, and on-top) of Cu on phosphorene are investigated first. Phosphorene with a Cu atom anchored on the hollow site is chosen for further study. The pathways for different CRR products, including HCOOH, CO, CH₃OH, and CH₄, are examined via constructing free energy diagrams and via comparing the limiting potentials. CH₄ is the most likely product after analysis of the adsorption energies and free energy pathways. Cu-Doped phosphorene in general shows improved CRR performance with lower limiting potential values. Cu doping leads to a decrease in the band gap value (about 0.2 eV), which is likely to be the physical origin of the CRR performance enhancement. Our study provides a novel promising CRR candidate catalyst based on phosphorene.

Density functional theory studies of transition metal carbides and nitrides as electrocatalysts

Transition metal carbides and nitrides are interesting non-precious materials that have been shown to replace or reduce the loading of precious metals for catalyzing several important electrochemical reactions. The purpose of this review is to summarize density functional theory (DFT) studies, describe reaction pathways, identify activity and selectivity descriptors, and present a future outlook in designing carbide and nitride catalysts for the hydrogen evolution reaction (HER), oxygen evolution reaction (OER), oxygen reduction reaction (ORR), nitrogen reduction reaction (N₂RR), CO₂ reduction reaction (CO₂RR) and alcohol oxidation reactions. This topic is of high interest to scientific communities working in the field of electrocatalysis and this review should provide theoretical guidance for the rational design of improved carbide and nitride electrocatalysts.

Synthesis and Applications of Nanostructured Hollow Transition Metal Chalcogenides

Nanostructures with well-defined structures and rich active sites occupy an important position for efficient energy storage and conversion. Recent studies have shown that a transition metal chalcogenide (TMC) has a unique structure, such as diverse structural morphology, excellent stability, high efficiency, etc., and is used in the fields of electrochemistry and catalysis. The nanohollow structure metal chalcogenide has broad application prospects due to the existence of a large number of active sites and a wide internal space, allowing a large number of ions and electrons to be transported. Summarizing synthetic strategies of nanostructured hollow transition metal sulfides (HTMC) and their applications in the field of energy storage and conversion is discussed here. Through some representative examples, the fabrication and properties of various hollow structures are analyzed, which prompt some emerging nanoengineering designs to be applied to transition metal chalcogenides. It is hoped that the construction of the HTMC will lead to a deeper understanding for the further exploration of energy storage and conversion.

Selective Activation of Propane Using Intermediates Generated during Water Oxidation

Electrochemical conversion of light alkanes to high-value oxygenates provides an attractive avenue for eco-friendly utilization of these hydrocarbons. However, such conversion under ambient conditions remains exceptionally challenging due to the high energy barrier of C-H bond cleavage. Herein, we investigated theoretically the partial oxidation of propane on a series of single atom alloys by using active intermediates generated during water oxidation as the oxidant. We show that by controlling the potential and pH, stable surface oxygen atoms can be maintained under water oxidation conditions. The free energy barrier for C-H bond cleavage by the surface oxygen can be as small as 0.54 eV, which can be surmounted easily at room temperature. Our calculations identified three promising surfaces as effective propane oxidation catalysts. Our complementary experiments demonstrated the partial oxidation of propane to acetone on Ni-doped Au surfaces. We also investigated computationally the steps leading to acetone formation. These studies show that the concept of exploiting intermediates generated in water oxidation as oxidants provides a fruitful strategy for electrocatalyst design to efficiently convert hydrocarbons into value-added chemicals.

Electrochemical Reduction of Carbon Dioxide on Graphene-Based Catalysts

The current environmental situation requires taking actions regarding processes for energy production, thus promoting renewable energies, which must be complemented with the development of routes to reduce pollution, such as the capture and storage of CO₂. Graphene materials have been chosen for their unique properties to be used either as electrocatalyst or as catalyst support (mainly for non-noble metals) that develop adequate efficiencies for this reaction. This review focuses on comparing experimental and theoretical results of the electrochemical reduction reaction of carbon dioxide (ECO₂RR) described in the scientific literature to establish a correlation between them. This work aims to establish the state of the art on the electrochemical reduction of carbon dioxide on graphene-based catalysts.

Noble-Metal-Free Doped Carbon Nanomaterial Electrocatalysts

Electrocatalytic processes, such as oxygen reduction reaction (ORR), oxygen evolution reaction (OER), hydrogen evolution reaction (HER) and carbon dioxide reduction reaction (CO₂ RR), play key roles in various sustainable energy storage and production devices and their optimization in an ecological manner is of paramount importance for mankind. In this inclusive Review, we aspire to set the scene on doped carbon-based nanomaterials and their hybrids as precious-metal alternative electrocatalysts for these critical reactions in order for the research community not only to stay up-to-date, but also to get inspired and keep pushing forward towards their practical application in energy conversion.

Oxygen induced promotion of electrochemical reduction of CO2 via co-electrolysis

Harnessing renewable electricity to drive the electrochemical reduction of CO₂ is being intensely studied for sustainable fuel production and as a means for energy storage. Copper is the only monometallic electrocatalyst capable of converting CO₂ to value-added products, e.g., hydrocarbons and oxygenates, but suffers from poor selectivity and mediocre activity. Multiple oxidative treatments have shown improvements in the performance of copper catalysts. However, the fundamental underpinning for such enhancement remains controversial. Here, we combine reactivity, in-situ surface-enhanced Raman spectroscopy, and computational investigations to demonstrate that the presence of surface hydroxyl species by co-electrolysis of CO₂ with low concentrations of O₂ can dramatically enhance the activity of copper catalyzed CO₂ electroreduction. Our results indicate that co-electrolysis of CO₂ with an oxidant is a promising strategy to introduce catalytically active species in electrocatalysis.

Improving CO2 Electrochemical Reduction to CO Using Space Confinement between Gold or Silver Nanoparticles

Developing electrocatalysts that are stable and efficient for CO₂ reduction is important for constructing a carbon-neutral energy cycle. New approaches are required to drive input electricity toward the desired CO₂ reduction reaction (CO₂RR) rather than the competitive hydrogen evolution reaction (HER). In this study, we have used quantum mechanics to demonstrate that the space confinement formed in the gaps of adjacent gold or silver nanoparticles can be used to improve the Faradaic efficiency of CO₂RR to CO. This behavior is due to the space confinement stabilizing *COOH, which is the key intermediate in the CO₂RR. However, space confinement has almost no effect on *H, which is the key intermediate in the HER. Possible experimental approaches for the preparation of this type of gold or silver electrocatalyst have been proposed.

A ball-milling synthesis of N-graphyne with controllable nitrogen doping sites for efficient electrocatalytic oxygen evolution and supercapacitors

Low-cost and efficient multifunctional electrodes play an important part in promoting the practical application of energy conversion and storage. Herein, we report the facile synthesis of N-graphyne, with a novel structure, by one-step ball milling of CaC2 and pyrazine. The accurate doping of nitrogen atoms at the controllable sites of the molecular skeleton of γ-graphyne was achieved using the nitrogenous precursor (pyrazine) as a reactant. Various techniques were adopted for the investigation of the composition, structure, and morphology of the obtained samples. The electrochemical measurements demonstrated that N-graphyne can serve as an excellent electrode material for both electrocatalysis and supercapacitors. As an electrocatalyst, N-graphyne exhibited an overpotential of 280 mV at 100 mA cm-2 and a Tafel slope of 122 mV dec-1 for the oxygen evolution reaction with highly stable morphology and electrocatalytic performance. As a supercapacitor electrode, N-graphyne showed a maximum capacitance of 235 F g-1 at 1 A g-1, and capacitance retention of 87% after 3000 cycles. The superior electrochemical performance of N-graphyne is due to the nitrogen heteroatomic defects, large electrochemical active surface areas and fast electron migration. Our studies provide a facile synthesis of novel N-graphyne with controllable doping sites and promote its potential applications in electrocatalysis and supercapacitors.