Dual Role of Pyridinic-N Doping in Carbon-Coated Ni Nanoparticles for Highly Efficient Electrochemical CO2 Reduction to CO over a Wide Potential Range

Synthesizing nickel single atom catalyst via SiO2 protection strategy for efficient CO2 electroreduction to CO in a wide potential range

At present, electrochemical CO2 reduction has been developed towards industrial current density, but the high faradaic efficiency at wide potential range or large current density is still an arduous task. Therefore, in this work, the highly exposed Ni single atoms (NiNCR-0.72) was synthesized through simple metal organic frameworks (MOFs)-derived method with SiO2 protection strategy. The obtained catalyst keeps CO faradaic efficiency (FECO) above 91 % under the wide potential range, and achieves a high FECO of 96.0 % and large CO partial current density of -206.8 mA cm-2 at -0.7 V in flow cell. The experimental results and theoretical calculation disclose that NiNCR-0.72 possesses the robust structure with rich mesopore and more highly exposed Ni-N active sites under SiO2 protection, which could facilitate CO2 transportation, lower energy barrier of CO2 reduction, and raise difficulty of hydrogen evolution reaction. The protection strategy is instructive to the synthesis of other MOFs-derived metal single atoms.

Recent advancements in carbon/metal-based nano-catalysts for the reduction of CO2 to value-added products

Due to the increased human activities in burning of fossil fuels and deforestation, the CO2 level in the atmosphere gets increased up to 415 ppm; although it is an essential component for plant growth, an increased level of CO2 in the atmosphere leads to global warming and catastrophic climate change. Various conventional methods are used to capture and utilize CO2, among that a feasible and eco-friendly technique for creating value-added products is the CO2RR. Photochemical, electrochemical, thermochemical, and biochemical approaches can be used to decrease the level of CO2 in the atmosphere. The introduction of nano-catalysts in the reduction process helps in the efficient conversion of CO2 with improved selectivity, increased efficiency, and also enhanced stability of the catalyst materials. Thus, in this mini-review of nano-catalysts, some of the products formed during the reduction process, like CH3OH, C2H5OH, CO, HCOOH, and CH4, are explained. Among different types of metal catalysts, carbonaceous, single-atom catalysts, and MOF based catalysts play a significant role in the CO2 RR process. The effects of the catalyst material on the surface area, composition, and structural alterations are covered in depth. To aid in the design and development of high-performance nano-catalysts for value-added products, the current state, difficulties, and future prospects are provided.

Bi-modified Cu-Based Catalysts for Acetylene Hydrogenation: Leveraging Dispersion and Hydrogen Spillover

Removing trace acetylene from the ethylene stream through selective hydrogenation is a crucial process in the production of polymer-grade ethylene. However, achieving high selectivity while maintaining high activity remains a significant challenge, especially for nonprecious metal catalysts. Herein, the trade-off between activity and selectivity is solved by synergizing enhanced dispersion and hydrogen spillover. Specifically, a bubbling method is proposed for preparing SiO2-supported copper and/or bismuth carbonate with high dispersion, which is then employed to synthesize highly dispersed Bi-modified CuxC-Cu catalyst. The catalyst displays outstanding catalytic performance for acetylene selective hydrogenation, achieving acetylene conversion of 100% and ethylene selectivity of 91.1% at 100 °C. The high activity originates from the enhanced dispersion, and the exceptional selectivity is due to the enhanced spillover capacity of active hydrogen from CuxC to Cu, which is promoted by the Bi addition. The results offer an avenue to design efficient catalysts for selective hydrogenation from nonprecious metals.

Guanine-derived carbon nanosheet encapsulated Ni nanoparticles for efficient CO2 electroreduction

Developing novel electrocatalysts for achieving high selectivity and faradaic efficiency in the carbon dioxide reduction reaction (CO2RR) poses a major challenge. In this study, a catalyst featuring a nitrogen-doped carbon shell-coated Ni nanoparticle structure is designed for efficient carbon dioxide (CO2) electroreduction to carbon monoxide (CO). The optimal Ni@NC-1000 catalyst exhibits remarkable CO faradaic efficiency (FECO) values exceeding 90% across a broad potential range of -0.55 to -0.9 V (vs. RHE), and attains the maximum FECO of 95.6% at -0.75 V (vs. RHE) in 0.5 M NaHCO3. This catalyst exhibits sustained carbon dioxide electroreduction activity with negligible decay after continuous electrolysis for 20 h. More encouragingly, a substantial current density of 200.3 mA cm-2 is achieved in a flow cell at -0.9 V (vs. RHE), reaching an industrial-level current density. In situ Fourier transform infrared spectroscopy and theoretical calculations demonstrate that its excellent catalytic performance is attributed to highly active pyrrolic nitrogen sites, promoting CO2 activation and significantly reducing the energy barrier for generating *COOH. To a considerable extent, this work presents an effective strategy for developing high-efficiency catalysts for electrochemical CO2 reduction across a wide potential window.

Self-Powered Electrochemical CO2 Conversion Enabled by a Multifunctional Carbon-Based Electrocatalyst and a Rechargeable Zn-Air Battery

Multifunctional electrocatalysts are required for diverse clean energy-related technologies (e.g., electrochemical CO2 reduction reaction (CO2RR) and metal-air batteries). Herein, a nitrogen and fluorine co-doped carbon nanotube (NFCNT) is reported to simultaneously achieve multifunctional catalytic activities for CO2RR, oxygen reduction reaction (ORR), and oxygen evolution reaction (OER). Theoretical calculations reveal that the superior multifunctional catalytic activities of NFCNT are attributed to the synergistic effect of nitrogen and fluorine co-doping to induce charge redistribution and decrease the energy barrier of rate-determining step for different electrocatalytic reactions. Furthermore, the rechargeable Zn-air battery (ZAB) with NFCNT electrode delivers a high peak power density of 230 mW cm-2 and superior durability over 100 cycles, outperforming the ZAB with Pt/C+RuO2 based electrodes. More importantly, a self-driven CO2 electrolysis unit powered by the as-assembled ZABs is developed, which achieves 80% CO Faraday efficiency and 60% total energy efficiency. This work provides a new insight into the exploration of highly efficient multifunctional carbon-based electrocatalysts for novel energy-related applications.

Constructing FeNiPt@C Trifunctional Catalyst by High Spin-Induced Water Oxidation Activity for Zn-Air Battery and Anion Exchange Membrane Water Electrolyzer

Developing cost-efficient trifunctional catalysts capable of facilitating hydrogen evolution reaction (HER), oxygen evolution reaction (OER), and oxygen reduction reaction (ORR) activity is essential for the progression of energy devices. Engineering these catalysts to optimize their active sites and integrate them into a cohesive system presents a significant challenge. This study introduces a nanoflower (NFs)-like carbon-encapsulated FeNiPt nanoalloy catalyst (FeNiPt@C NFs), synthesized by substituting Co2+ ions with high-spin Fe2+ ions in Hofmann-type metal-organic framework, followed by carbonization and pickling processes. The FeNiPt@C NFs catalyst, characterized by its nitrogen-doped carbon-encapsulated metal alloy structure and phase-segregated FeNiPt alloy with slight surface oxidization, exhibits excellent trifunctional catalytic performance. This is evidenced by its activities in HER (-25 mV at 10 mA cm-2), ORR (half-wave potential of 0.93 V), and OER (294 mV at 10 mA cm-2), with the enhanced water oxidation activity attributed to the high-spin state of the Fe element. Consequently, the Zn-air battery and anion exchange membrane water electrolyzer assembled by FeNiPt@C NFs catalyst demonstrate remarkable power density (168 mW cm-2) and industrial-scale current density (698 mA cm-2 at 1.85 V), respectively. This innovative integration of multifunctional catalytic sites paves the way for the advancement of sustainable energy systems.

Revealing the synergistic effect of Ni single atoms and adjacent 3d metal doped Ni nanoparticles in electrocatalytic CO2 reduction

Herein, we report the successful fabrication of a series of transition metal doped Ni nanoparticles (NPs) coordinated with Ni single atoms in nitrogen-doped carbon nanotubes (denoted as Ni1+NPsM-NCNTs, M = Mn, Fe, Co, Cu and Zn; Ni1 = Ni single atom). X-ray absorption fine structure reveals the coexistence of Ni single atoms with Ni-N4 coordination and NiM NPs. When applied for electrocatalytic CO2RR, the Ni1+NPsM-NCNT compounds show the Faradaic efficiency of CO (FECO) with a volcano-like tendency of Mn < Fe ≈ Co < Zn < Cu, in which the Ni1+NPsCu-NCNT exhibits the highest FECO of 96.92%, a current density of 171.25 mA cm-2 and a sustainable stability over 24 hours at a current density of 100 mA cm-2, outperforming most reported examples in the literature. Detailed experiments and theoretical calculations reveal that for Ni1+NPsCu-NCNTs, the electron transfer from NiCu NPs to Ni single atoms strengthens the adsorption of *COOH intermediates. Moreover, the d-band center of Ni-N in Ni1+NPsCu-NCNT is upshifted, providing stronger binding with the reaction intermediates of *COOH, whereas the NiCu NPs increase the Gibbs free energy change of the Volmer step, suppressing the competitive HER.

Atomic Zn-Doping Induced Sabatier Optimum in NiZn0.03 Catalyst for CO2 Electroreduction at Industrial-Level Current Densities

The Sabatier principle defines the essential criteria for an ideal catalyst in heterogeneous catalysis, while reaching the Sabatier optimum is still challenging in catalyst design. Herein, an elegant strategy is described to reach the Sabatier optimum of Ni electrocatalyst in CO2 reduction reaction (CO2 RR) by atomically Zn doping. The incorporation of 3% Zn single atom into Ni lattice leads to the moderate degrade of d-band center via Ni-Zn electronic coupling, which balances the bonding strengths of *COOH and *CO, resulting in a relative low energy barrier for CO2 activation while not being substantially poisoned by CO. Consequently, NiZn0.03 /C exhibits unique catalytic activity (jCO >100 mA cm-2 at -0.6 V), wide potential range for selective CO production (FECO >90% from -0.65 to -1.15 V), and outstanding long-term stability (FECO >90% during 85 h electrolysis at -0.85 V). The results provide valuable insights for the rational fabrication of superior non-noble bimetallic electrocatalysts in CO2 electroreduction.

A N-doped carbon-supported In2O3 catalyst for highly efficient CO2 electroreduction to HCOOH

A novel In2O3@NC catalyst has been prepared and employed in CO2 electroreduction to HCOOH. The C and N species successfully improve the electronic structure of In2O3 and enhance the adsorption ability of CO2. The In2O3@NC catalyst exhibits a remarkably high FEHCOOH of 97.1%, jtotal of 190 mA cm-2, and stability for 60 h.

Optimizing copper nanoparticles with a carbon shell for enhanced electrochemical CO2 reduction to ethanol

The electrochemical reduction of carbon dioxide (CO2RR) holds great promise for sustainable energy utilization and combating global warming. However, progress has been impeded by challenges in developing stable electrocatalysts that can steer the reaction toward specific products. This study proposes a carbon shell coating protection strategy by an efficient and straightforward approach to prevent electrocatalyst reconstruction during the CO2RR. Utilizing a copper-based metal-organic framework as the precursor for the carbon shell, we synthesized carbon shell-coated electrocatalysts, denoted as Cu-x-y, through calcination in an N2 atmosphere (where x and y represent different calcination temperatures and atmospheres: N2, H2, and NH3). It was found that the faradaic efficiency of ethanol over the catalysts with a carbon shell could reach ∼67.8%. In addition, the catalyst could be stably used for more than 16 h, surpassing the performance of Cu-600-H2 and Cu-600-NH3. Control experiments and theoretical calculations revealed that the carbon shell and Cu-C bonds played a pivotal role in stabilizing the catalyst, tuning the electron environment around Cu atoms, and promoting the formation and coupling process of CO*, ultimately favoring the reaction pathway leading to ethanol formation. This carbon shell coating strategy is valuable for developing highly efficient and selective electrocatalysts for the CO2RR.

Facile synthesis of supported CuNi nano-clusters as an electrochemical CO2 reduction catalyst with broad potential range

A nitrogen-doped carbon-supported CuNi bimetallic nanocluster catalyst (CuNi-NC) was first synthesized via a facile ZIF-derived method. With a synergistic effect between Cu and Ni, the catalyst exhibited a maximum FECO of 96.3%. FECO is higher than 90% in a broad potential range of 600 mV, which was ascribed to the controllable pore size distribution. Density functional theory further demonstrated the preferred formation of *COOH in the catalytic process.

Elucidating the origin of catalytic activity of nitrogen-doped carbon coated nickel toward electrochemical reduction of CO2

Converting CO2 into valuable chemicals and fuels through clean and renewable energy electricity provides a way to achieve sustainable development for human societies. In this study, carbon coated nickel catalysts (Ni@NCT) were prepared by solvothermal and high-temperature pyrolysis methods. A series of Ni@NC-X catalysts were obtained by pickling with different kinds of acids for electrochemical CO2 reduction reaction (ECRR). The results show that Ni@NC-N treated with nitric acid has the highest selectivity but lower activity, Ni@NC-S treated with sulfuric acid has the lowest selectivity, and Ni@NC-Cl treated with hydrochloric acid shows the best activity and good selectivity. At -1.16 V, Ni@NC-Cl has a considerable CO yield of 472.9 μmol h-1 cm-2, which is significantly superior to Ni@NC-N (327.5), Ni@NC-S (295.6) and Ni@NC (270.8). The controlled experiments show that there is a synergistic effect between Ni and N. The chlorine adsorbed on the surface can promote the performance of ECRR. The poisoning experiments indicate that the contribution of surface Ni atoms to the ECRR is very small, and the increase of activity is mainly due to the nitrogen doped carbon coated Ni particles. The relationship between activity and selectivity of ECRR on different acid-washed catalysts was correlated by theoretical calculations for the first time, which is also in good agreement with the experimental results.

Rational Design of N-Doped Carbon-Coated Cobalt Nanoparticles for Highly Efficient and Durable Photothermal CO2 Conversion

Photothermal CO2 hydrogenation to high-value-added chemicals and fuels is an appealing approach to alleviate energy and environmental concerns. However, it still relies on the development of earth-abundant, efficient, and durable catalysts. Here, the design of N-doped carbon-coated Co nanoparticles (NPs), as a photothermal catalyst, synthesized through a two-step pyrolysis of Co-based ZIF-67 precursor, is reported. Consequently, the catalyst exhibits remarkable activity and stability for photothermal CO2 hydrogenation to CO with a 0.75 mol gcat -1 h-1 CO production rate under the full spectrum of light illumination. The high activity and durability of these Co NPs are mainly attributed to the synergy of the attuned size of Co NPs, the thickness of carbon layers, and the N doping species. Impressively, the experimental characterizations and theoretical simulations show that such a simple N-doped carbon coating strategy can effectively facilitate the desorption of generated CO and activation of reactants due to the strong photothermal effect. This work provides a simple and efficient route for the preparation of highly active and durable nonprecious metal catalysts for promising photothermal catalytic reactions.

Phase Engineering and Synchrotron-Based Study on Two-Dimensional Energy Nanomaterials

In recent years, there has been significant interest in the development of two-dimensional (2D) nanomaterials with unique physicochemical properties for various energy applications. These properties are often derived from the phase structures established through a range of physical and chemical design strategies. A concrete analysis of the phase structures and real reaction mechanisms of 2D energy nanomaterials requires advanced characterization methods that offer valuable information as much as possible. Here, we present a comprehensive review on the phase engineering of typical 2D nanomaterials with the focus of synchrotron radiation characterizations. In particular, the intrinsic defects, atomic doping, intercalation, and heterogeneous interfaces on 2D nanomaterials are introduced, together with their applications in energy-related fields. Among them, synchrotron-based multiple spectroscopic techniques are emphasized to reveal their intrinsic phases and structures. More importantly, various in situ methods are employed to provide deep insights into their structural evolutions under working conditions or reaction processes of 2D energy nanomaterials. Finally, conclusions and research perspectives on the future outlook for the further development of 2D energy nanomaterials and synchrotron radiation light sources and integrated techniques are discussed.

Regulated Surface Electronic States of CuNi Nanoparticles through Metal-Support Interaction for Enhanced Electrocatalytic CO2 Reduction to Ethanol

Developing stable catalysts with higher selectivity and activity within a wide potential range is critical for efficiently converting CO2 to ethanol. Here, the carbon-encapsulated CuNi nanoparticles anchored on nitrogen-doped nanoporous graphene (CuNi@C/N-npG) composite are designedly prepared and display the excellent CO2 reduction performance with the higher ethanol Faradaic effiency (FEethanol  ≥ 60%) in a wide potential window (600 mV). The optimal cathodic energy efficiency (47.6%), Faradaic efficiency (84%), and selectivity (96.6%) are also obtained at -0.78 V versus reversible hydrogen electrode (RHE). Combining with the density functional theory (DFT) calculations, it is demonstrated that the stronger metal-support interaction (Ni-N-C) can regulate the surface electronic structure effectively, boosting the electron transfer and stabilizing the active sites (Cu0 -Cuδ+ ) on the surface of CuNi@C/N-npG, finally realizing the controllable transition of reaction intermediates. This work may guide the designs of electrocatalysts with highly catalytic performance for CO2 reduction to C2+ products.

Engineering iron carbide catalyst with aerophilic and electron-rich surface for improved electrochemical CO2 reduction

Highly efficient electrocatalyst for carbon dioxide reduction (CO₂RR) is desirable for converting CO₂ into carbon-based chemicals and reducing anthropogenic carbon emission. Regulating catalyst surface to improve the affinity for CO₂ and the capability of CO₂ activation is the key to high-efficiency CO₂RR. In this work, we develop an iron carbide catalyst encapsulated in nitrogenated carbon (SeN-Fe₃C) with an aerophilic and electron-rich surface by inducing preferential formation of pyridinic-N species and engineering more negatively charged Fe sites. The SeN-Fe₃C exhibits an excellent CO selectivity with a CO Faradaic efficiency (FE) of 92 % at -0.5 V (vs. RHE) and remarkably enhanced CO partial current density as compared to the N-Fe₃C catalyst. Our results demonstrate that Se doping reduces the Fe₃C particle size and improves the dispersion of Fe₃C on nitrogenated carbon. More importantly, the preferential formation of pyridinic-N species induced by Se doping endows the SeN-Fe₃C with an aerophilic surface and improves the affinity of the SeN-Fe₃C for CO₂. Density functional theory (DFT) calculations reveal that the electron-rich surface, which is caused by pyridinic N species and much more negatively charged Fe sites, leads to a high degree of polarization and activation of CO₂ molecule, thus conferring a remarkably improved CO₂RR activity on the SeN-Fe₃C catalyst.

Confinement of an alkaline environment for electrocatalytic CO2 reduction in acidic electrolytes

Acidic electrochemical CO₂ reduction reaction (CO₂RR) can minimize carbonate formation and eliminate CO₂ crossover, thereby improving long-term stability and enhancing single-pass carbon efficiency (SPCE). However, the kinetically favored hydrogen evolution reaction (HER) is generally predominant under acidic conditions. This paper describes the confinement of a local alkaline environment for efficient CO₂RR in a strongly acidic electrolyte through the manipulation of mass transfer processes in well-designed hollow-structured Ag@C electrocatalysts. A high faradaic efficiency of over 95% at a current density of 300 mA cm^-2 and an SPCE of 46.2% at a CO₂ flow rate of 2 standard cubic centimeters per minute are achieved in the acidic electrolyte, with enhanced stability compared to that under alkaline conditions. Computational modeling results reveal that the unique structure of Ag@C could regulate the diffusion process of OH^- and H⁺, confining a high-pH local reaction environment for the promoted activity. This work presents a promising route to engineer the microenvironment through the regulation of mass transport that permits the CO₂RR in acidic electrolytes with high performance.

Ultrathin Nitrogen-Doped Carbon Encapsulated Ni Nanoparticles for Highly Efficient Electrochemical CO2 Reduction and Aqueous Zn-CO2 Batteries

Electrochemical CO₂ reduction reaction (CO₂ RR), powered by renewable electricity, has attracted great attention for producing high value-added fuels and chemicals, as well as feasibly mitigating CO₂ emission problem. Here, this work reports a facile hard template strategy to prepare the Ni@N-C catalyst with core-shell structure, where nickel nanoparticles (Ni NPs) are encapsulated by thin nitrogen-doped carbon shells (N-C shells). The Ni@N-C catalyst has demonstrated a promising industrial current density of 236.7 mA cm^-2 with the superb FE_CO of 97% at -1.1 V versus RHE. Moreover, Ni@N-C can drive the reversible Zn-CO₂ battery with the largest power density of 1.64 mW cm^-2 , and endure a tough cycling durability. These excellent performances are ascribed to the synergistic effect of Ni@N-C that Ni NPs can regulate the electronic microenvironment of N-doped carbon shells, which favor to enhance the CO₂ adsorption capacity and the electron transfer capacity. Density functional theory calculations prove that the binding configuration of N-C located on the top of Ni slabs (Top-Ni@N-C) is the most thermodynamically stable and possess a lowest thermodynamic barrier for the formation of COOH^* and the desorption of CO. This work may pioneer a new method on seeking high-efficiency and worthwhile electrocatalysts for CO₂ RR and Zn-CO₂ battery.

Ni Nanoclusters Anchored on Ni-N-C Sites for CO2 Electroreduction at High Current Densities

Transition metal catalyst-based electrocatalytic CO₂ reduction is a highly attractive approach to fulfill the renewable energy storage and a negative carbon cycle. However, it remains a great challenge for the earth-abundant VIII transition metal catalysts to achieve highly selective, active, and stable CO₂ electroreduction. Herein, bamboo-like carbon nanotubes that anchor both Ni nanoclusters and atomically dispersed Ni-N-C sites (NiNCNT) are developed for exclusive CO₂ conversion to CO at stable industry-relevant current densities. Through optimization of gas-liquid-catalyst interphases via hydrophobic modulation, NiNCNT exhibits as high as Faradaic efficiency (FE) of 99.3% for CO formation at a current density of -300 mA·cm^-2 (-0.35 V vs reversible hydrogen electrode (RHE)), and even an extremely high CO partial current density (j_CO) of -457 mA·cm^-2 corresponding to a CO FE of 91.4% at -0.48 V vs RHE. Such superior CO₂ electroreduction performance is ascribed to the enhanced electron transfer and local electron density of Ni 3d orbitals upon incorporation of Ni nanoclusters, which facilitates the formation of the COOH* intermediate.

Accelerated Transfer and Spillover of Carbon Monoxide through Tandem Catalysis for Kinetics-boosted Ethylene Electrosynthesis

Cu-based catalysts have been widely applied in electroreduction of carbon dioxide (CO₂ ER) to produce multicarbon (C₂₊ ) feedstocks (e.g., C₂ H₄ ). However, the high energy barriers for CO₂ activation on the Cu surface is a challenge for a high catalytic efficiency and product selectivity. Herein, we developed an in situ *CO generation and spillover strategy by engineering single Ni atoms on a pyridinic N-enriched carbon support with a sodalite (SOD) topology (Ni-SOD/NC) that acted as a donor to feed adjacent Cu nanoparticles (NPs) with *CO intermediate. As a result, a high C₂ H₄ selectivity of 62.5 % and an industrial-level current density of 160 mA cm^-2 at a low potential of -0.72 V were achieved. Our studies revealed that the isolated NiN₃ active sites with adjacent pyridinic N species facilitated the *CO desorption and the massive *CO intermediate released from Ni-SOD/NC then overflowed to Cu NPs surface to enrich the *CO coverage for improving the selectivity of CO₂ ER to C₂ H₄ .

Highly selective CO2 electroreduction to CO by the synergy between Ni-N-C and encapsulated Ni nanoparticles

Efficient catalysts are highly desirable for the selective electrochemical CO₂ reduction reaction (CO₂RR). Ni single-atom catalysts are known as promising CO₂RR catalysts, while Ni NPs are expected to catalyze the competing HER. In this work, we have modified the Ni NPs by encapsulating them into porous Ni-N-C nanosheets (Ni@Ni-N-C), to boost the synergy between Ni NPs and dispersed Ni-N species towards CO₂RR. The CO faradaic efficiency (FE_CO) reached 96.4% at -0.9 V and retained over 90% in a wide potential window. More importantly, FE_CO values of over 94% have been obtained from -50 to -170 mA cm^-2 with a peak FE_CO of 99% in a flow cell. Our work demonstrates that the surface modification of Ni NPs can inhibit the unexpected HER and activate the surface sites, offering a practical design strategy for CO₂RR catalysts.

Recent Advances of the Confinement Effects Boosting Electrochemical CO2 Reduction

Powered by clean and renewable energy, electrocatalytic CO₂ reduction reaction (CO₂ RR) to chemical feedstocks is an effective way to mitigate the greenhouse effect and artificially close the carbon cycle. However, the performance of electrocatalytic CO₂ RR was impeded by the strong thermodynamic stability of CO₂ molecules and the high susceptibility to hydrogen evolution reaction (HER) in aqueous phase systems. Moreover, the numerous reaction intermediates formed at very near potentials lead to poor selectivity of reaction products, further preventing the industrialization of CO₂ RR. Catalysis in confined space can enrich the reaction intermediates to improve their coverage at the active site, increase local pH to inhibit HER, and accelerate the mass transfer rate of reactants/products and subsequently facilitate CO₂ RR performance. Therefore, we summarize the research progress on the application of the confinement effects in the direction of CO₂ RR in theoretical and experimental directions. We first analyzed the mechanism of the confinement effect. Subsequently, the confinement effect was discussed in various forms, which can be characterized as an abnormal catalytic phenomenon due to the relative limitation of the reaction region. In specific, based on the physical structure of the catalyst, the confinement effect was divided in four categories: pore structure confinement, cavity structure confinement, active center confinement, and other confinement methods. Based on these discussions, we also have summarized the prospects and challenges in this field. This review aims to stimulate greater interests for the development of more efficient confined strategy for CO₂ RR in the future.

CO2 Conversion on N-Doped Carbon Catalysts via Thermo- and Electrocatalysis: Role of C–NOx Moieties

N-doped carbon (N–C) materials are increasingly popular in different electrochemical and catalytic applications. Due to the structural and stoichiometric diversity of these materials, however, the role of different functional moieties is still controversial. We have synthesized a set of N–C catalysts, with identical morphologies (∼27 nm pore size). By systematically changing the precursors, we have varied the amount and chemical nature of N-functions on the catalyst surface. The CO₂ reduction (CO₂R) properties of these catalysts were tested in both electrochemical (EC) and thermal catalytic (TC) experiments (i.e., CO₂ + H₂ reaction). CO was the major CO₂R product in all cases, while CH₄ appeared as a minor product. Importantly, the CO₂R activity changed with the chemical composition, and the activity trend was similar in the EC and TC scenarios. The activity was correlated with the amount of different N-functions, and a correlation was found for the −NO_x species. Interestingly, the amount of this species decreased radically during EC CO₂R, which was coupled with the performance decrease. The observations were rationalized by the adsorption/desorption properties of the samples, while theoretical insights indicated a similarity between the EC and TC paths.

Heteroatom-Doped Porous Carbon-Based Nanostructures for Electrochemical CO2 Reduction

The continual rise of the CO2 concentration in the Earth's atmosphere is the foremost reason for environmental concerns such as global warming, ocean acidification, rising sea levels, and the extinction of various species. The electrochemical CO2 reduction (CO2RR) is a promising green and efficient approach for converting CO2 to high-value-added products such as alcohols, acids, and chemicals. Developing efficient and low-cost electrocatalysts is the main barrier to scaling up CO2RR for large-scale applications. Heteroatom-doped porous carbon-based (HA-PCs) catalysts are deemed as green, efficient, low-cost, and durable electrocatalysts for the CO2RR due to their great physiochemical and catalytic merits (i.e., great surface area, electrical conductivity, rich electrical density, active sites, inferior H2 evolution activity, tailorable structures, and chemical-physical-thermal stability). They are also easily synthesized in a high yield from inexpensive and earth-abundant resources that meet sustainability and large-scale requirements. This review emphasizes the rational synthesis of HA-PCs for the CO2RR rooting from the engineering methods of HA-PCs to the effect of mono, binary, and ternary dopants (i.e., N, S, F, or B) on the CO2RR activity and durability. The effect of CO2 on the environment and human health, in addition to the recent advances in CO2RR fundamental pathways and mechanisms, are also discussed. Finally, the evolving challenges and future perspectives on the development of heteroatom-doped porous carbon-based nanocatalysts for the CO2RR are underlined.