Size-Dependent Nickel-Based Electrocatalysts for Selective CO2 Reduction

Closing the anthropogenic carbon cycle by converting CO₂ into reusable chemicals is an attractive solution to mitigate rising concentrations of CO₂ in the atmosphere. Herein, we prepared Ni metal catalysts ranging in size from single atoms to over 100 nm and distributed them across N-doped carbon substrates which were obtained from converted zeolitic imidazolate frameworks (ZIF). The results show variance in CO₂ reduction performance with variance in Ni metal size. Ni single atoms demonstrate a superior Faradaic efficiency (FE) for CO selectivity (ca. 97 % at −0.8 V vs. RHE), while results for 4.1 nm Ni nanoparticles are slightly lower (ca. 93 %). Further increase the Ni particle size to 37.2 nm allows the H₂ evolution reaction (HER) to compete with the CO₂ reduction reaction (CO₂RR). The FE towards CO production decreases to under 30 % and HER efficiency increase to over 70 %. These results show a size-dependent CO₂ reduction for various sizes of Ni metal catalysts.     

Understanding the Origin of Highly Selective CO2 Electroreduction to CO on Ni,N‐doped Carbon Catalysts

Ni,N‐doped carbon catalysts have shown promising catalytic performance for CO₂ electroreduction (CO₂R) to CO; this activity has often been attributed to the presence of nitrogen‐coordinated, single Ni atom active sites. However, experimentally confirming Ni?N bonding and correlating CO₂ reduction (CO₂R) activity to these species has remained a fundamental challenge. We synthesized polyacrylonitrile‐derived Ni,N‐doped carbon electrocatalysts (Ni‐PACN) with a range of pyrolysis temperatures and Ni loadings and correlated their electrochemical activity with extensive physiochemical characterization to rigorously address the origin of activity in these materials. We found that the CO₂R to CO partial current density increased with increased Ni content before plateauing at 2 wt % which suggests a dispersed Ni active site. These dispersed active sites were investigated by hard and soft X‐ray spectroscopy, which revealed that pyrrolic nitrogen ligands selectively bind Ni atoms in a distorted square‐planar geometry that strongly resembles the active sites of molecular metal–porphyrin catalysts.     

Understanding the Origin of Highly Selective CO2 Electroreduction to CO on Ni,N-doped Carbon Catalysts

Ni,N-doped carbon catalysts have shown promising catalytic performance for CO₂ electroreduction (CO₂R) to CO; this activity has often been attributed to the presence of nitrogen-coordinated, single Ni atom active sites. However, experimentally confirming Ni−N bonding and correlating CO₂ reduction (CO₂R) activity to these species has remained a fundamental challenge. We synthesized polyacrylonitrile-derived Ni,N-doped carbon electrocatalysts (Ni-PACN) with a range of pyrolysis temperatures and Ni loadings and correlated their electrochemical activity with extensive physiochemical characterization to rigorously address the origin of activity in these materials. We found that the CO₂R to CO partial current density increased with increased Ni content before plateauing at 2 wt % which suggests a dispersed Ni active site. These dispersed active sites were investigated by hard and soft X-ray spectroscopy, which revealed that pyrrolic nitrogen ligands selectively bind Ni atoms in a distorted square-planar geometry that strongly resembles the active sites of molecular metal–porphyrin catalysts.     

Effect of CO2 adsorbents on the Ni-based dual-function materials for CO2 capturing and in situ methanation

The present work studied the effect of different carbon dioxide (CO₂) adsorbents on Ni-based dual-function materials (DFMs) for the development of carbon capture and on-site utilization in a reactor at isothermal condition. The DFMs containing Ni functioning as a methanation catalyst with various CO₂ adsorbents (i.e., CaO, MgO, K₂CO₃, or Na₂CO₃) were prepared on γ-Al₂O₃ through sequential impregnation. The result indicated that Ni-Na₂CO₃/γ-Al₂O₃ had the highest methanation capacity (i.e., 0.1783 mmol/g) and efficiency (i.e., 71.09%) in the CO₂ adsorption–methanation test. The CO₂ uptake and the subsequent methanation capacity of the Ni-Na₂CO₃/γ-Al₂O₃ increased to more than 24 times and more than 17 times, respectively, compared to Ni/γ-Al₂O₃. The high methanation capacity was correlated to its highest amount of weak basic sites, substantial CO₂ capture capacity and capture/release efficiency, and reactivity to H₂ at a lower temperature, supported by CO₂-TPD, TGA analyses for adsorption or adsorption–desorption at the isothermal condition, and H₂-TPRea, respectively. A continuous cyclic CO₂ adsorption–methanation was performed by using the Ni-Na₂CO₃/γ-Al₂O₃ and Ni-CaO/γ-Al₂O₃, showing that the CO₂ adsorption capacity was stabilized from third cycle onward, whereas the methanation capacity was stabilized at all cycles, indicating the high stability of the DFMs for both CO₂ adsorption and subsequent methanation. This work demonstrated successful synthesis of the Ni-based, low-cost, and stable DFMs with the ability to produce methane via the direct capture of CO₂.     

Efficient CO2 Removal for Ultra‐Pure CO Production by Two Hybrid Ultramicroporous Materials

Removal of CO₂ from CO gas mixtures is a necessary but challenging step during production of ultra‐pure CO as processed from either steam reforming of hydrocarbons or CO₂ reduction. Herein, two hybrid ultramicroporous materials (HUMs), SIFSIX‐3‐Ni and TIFSIX‐2‐Cu‐i , which are known to exhibit strong affinity for CO₂, were examined with respect to their performance for this separation. The single‐gas CO sorption isotherms of these HUMs were measured for the first time and are indicative of weak affinity for CO and benchmark CO₂/CO selectivity (>4000 for SIFSIX‐3‐Ni ). This prompted us to conduct dynamic breakthrough experiments and compare performance with other porous materials. Ultra‐pure CO (99.99 %) was thereby obtained from CO gas mixtures containing both trace (1 %) and bulk (50 %) levels of CO₂ in a one‐step physisorption‐based separation process.     

Acetamide Electrosynthesis from CO2 and Nitrite in Water

Renewable electricity driven electrocatalytic CO₂ reduction reaction (CO₂RR) is a promising solution to carbon neutralization, which mainly generate simple carbon products. It is of great importance to produce more valuable C−N chemicals from CO₂ and nitrogen species. However, it is challenging to co-reduce CO₂ and NO₃⁻/NO₂⁻ to generate aldoxime an important intermediate in the electrocatalytic C−N coupling process. Herein, we report the successful electrochemical conversion of CO₂ and NO₂⁻ to acetamide for the first time over copper catalysts under alkaline condition through a gas diffusion electrode. Operando spectroelectrochemical characterizations and DFT calculations, suggest acetaldehyde and hydroxylamine identified as key intermediates undergo a nucleophilic addition reaction to produce acetaldoxime, which is then dehydrated to acetonitrile and followed by hydrolysis to give acetamide under highly local alkaline environment and electric field. Moreover, the above mechanism was successfully extended to the formation of phenylacetamide. This study provides a new strategy to synthesize highly valued amides from CO₂ and wastewater.     

A Biomimetic Nickel Complex with a Reduced CO2 Ligand Generated by Formate Deprotonation and Its Behaviour towards CO2

Reduced CO₂ species are key intermediates in a variety of natural and synthetic processes. In the majority of systems, however, they elude isolation or characterisation owing to high reactivity or limited accessibility (heterogeneous systems), and their formulations thus often remain uncertain or are based on calculations only. We herein report on a Ni?CO₂^2? complex that is unique in many ways. While its structural and electronic features help understand the CO₂‐bound state in Ni,Fe carbon monoxide dehydrogenases, its reactivity sheds light on how CO₂ can be converted into CO/CO₃^2? by nickel complexes. In addition, the complex was generated by a rare example of formate β‐deprotonation, a mechanistic step relevant to the nickel‐catalysed conversion of H_xCO_y^z? at electrodes and formate oxidation in formate dehydrogenases.     

CO2 methanation over nickel-based catalysts prepared by citric acid complexation method

Catalytic hydrogenation of carbon dioxide to methane can not only achieve the recycling of carbon resources, but also effectively meet the increasing demand for natural gas. In this paper, Ni-based catalysts on different supports including ZrO₂, CeO₂ and Al₂O₃ were synthesized using citric acid complexation method and their CO₂ methanation performances were tested. Among these catalysts, the Ni/ZrO₂ catalyst achieved the best CO₂ methanation activity. The catalysts were characterized by N₂-physisorption, XRD, H₂-TPR and H₂-TPD. The results indicate that the superiority of the Ni/ZrO₂ catalyst can be mainly ascribed to its not only high Ni dispersion but also high reduction degree. Since the reduction degree of Ni/Al₂O₃ is low, it exhibits poor activity. The preparation condition for the Ni/ZrO₂ catalyst was further optimized. The result shows that at molar ratio of citric acid to Ni ions of 3, the catalyst exhibits the best activity owing to the highest Ni dispersion, the largest Ni surface area, an appropriate metal-support interaction and the most moderate basic sites.     

Regulating the Coordination Environment of MOF-Templated Single-Atom Nickel Electrocatalysts for Boosting CO2 Reduction

The general synthesis and control of the coordination environment of single-atom catalysts (SACs) remains a great challenge. Herein, a general host–guest cooperative protection strategy has been developed to construct SACs by introducing polypyrrole (PPy) into a bimetallic metal–organic framework. As an example, the introduction of Mg²⁺ in MgNi-MOF-74 extends the distance between adjacent Ni atoms; the PPy guests serve as N source to stabilize the isolated Ni atoms during pyrolysis. As a result, a series of single-atom Ni catalysts (named Ni_SA-N_x-C) with different N coordination numbers have been fabricated by controlling the pyrolysis temperature. Significantly, the Ni_SA-N₂-C catalyst, with the lowest N coordination number, achieves high CO Faradaic efficiency (98 %) and turnover frequency (1622 h⁻¹), far superior to those of Ni_SA-N₃-C and Ni_SA-N₄-C, in electrocatalytic CO₂ reduction. Theoretical calculations reveal that the low N coordination number of single-atom Ni sites in Ni_SA-N₂-C is favorable to the formation of COOH* intermediate and thus accounts for its superior activity.     

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 (CO₂ER) to produce multicarbon (C₂₊) feedstocks (e.g., C₂H₄). 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 C₂H₄ 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 CO₂ER to C₂H₄.     

Non-Interacting Ni and Fe Dual-Atom Pair Sites in N-Doped Carbon Catalysts for Efficient Concentrating Solar-Driven Photothermal CO2 Reduction

Solar-to-chemical energy conversion under weak solar irradiation is generally difficult to meet the heat demand of CO₂ reduction. Herein, a new concentrated solar-driven photothermal system coupling a dual-metal single-atom catalyst (DSAC) with adjacent Ni−N₄ and Fe−N₄ pair sites is designed for boosting gas-solid CO₂ reduction with H₂O under simulated solar irradiation, even under ambient sunlight. As expected, the (Ni, Fe)−N−C DSAC exhibits a superior photothermal catalytic performance for CO₂ reduction to CO (86.16 μmol g⁻¹ h⁻¹), CH₄ (135.35 μmol g⁻¹ h⁻¹) and CH₃OH (59.81 μmol g⁻¹ h⁻¹), which are equivalent to 1.70-fold, 1.27-fold and 1.23-fold higher than those of the Fe−N−C catalyst, respectively. Based on theoretical simulations, the Fermi level and d-band center of Fe atom is efficiently regulated in non-interacting Ni and Fe dual-atom pair sites with electronic interaction through electron orbital hybridization on (Ni, Fe)−N−C DSAC. Crucially, the distance between adjacent Ni and Fe atoms of the Ni−N−N−Fe configuration means that the additional Ni atom as a new active site contributes to the main *COOH and *HCO₃ dissociation to optimize the corresponding energy barriers in the reaction process, leading to specific dual reaction pathways (COOH and HCO₃ pathways) for solar-driven photothermal CO₂ reduction to initial CO production.     

Electrochemical Conversion of CO2 to Syngas with Controllable CO/H2 Ratios over Co and Ni Single‐Atom Catalysts

The electrochemical CO₂ reduction reaction (CO₂RR) to yield synthesis gas (syngas, CO and H₂) has been considered as a promising method to realize the net reduction in CO₂ emission. However, it is challenging to balance the CO₂RR activity and the CO/H₂ ratio. To address this issue, nitrogen‐doped carbon supported single‐atom catalysts are designed as electrocatalysts to produce syngas from CO₂RR. While Co and Ni single‐atom catalysts are selective in producing H₂ and CO, respectively, electrocatalysts containing both Co and Ni show a high syngas evolution (total current >74 mA cm^?2) with CO/H₂ ratios (0.23–2.26) that are suitable for typical downstream thermochemical reactions. Density functional theory calculations provide insights into the key intermediates on Co and Ni single‐atom configurations for the H₂ and CO evolution. The results present a useful case on how non‐precious transition metal species can maintain high CO₂RR activity with tunable CO/H₂ ratios.     

Electrochemical Conversion of CO2 to Syngas with Controllable CO/H2 Ratios over Co and Ni Single-Atom Catalysts

The electrochemical CO₂ reduction reaction (CO₂RR) to yield synthesis gas (syngas, CO and H₂) has been considered as a promising method to realize the net reduction in CO₂ emission. However, it is challenging to balance the CO₂RR activity and the CO/H₂ ratio. To address this issue, nitrogen-doped carbon supported single-atom catalysts are designed as electrocatalysts to produce syngas from CO₂RR. While Co and Ni single-atom catalysts are selective in producing H₂ and CO, respectively, electrocatalysts containing both Co and Ni show a high syngas evolution (total current >74 mA cm⁻²) with CO/H₂ ratios (0.23–2.26) that are suitable for typical downstream thermochemical reactions. Density functional theory calculations provide insights into the key intermediates on Co and Ni single-atom configurations for the H₂ and CO evolution. The results present a useful case on how non-precious transition metal species can maintain high CO₂RR activity with tunable CO/H₂ ratios.     

Tailoring the Pore Size,Basicity, and Binding Energy of Mesoporous C3N5 for CO2 Capture and Conversion

We investigated the CO₂ adsorption and electrochemical conversion behavior of triazole-based C₃N₅ nanorods as a single matrix for consecutive CO₂ capture and conversion. The pore size, basicity, and binding energy were tailored to identify critical factors for consecutive CO₂ capture and conversion over carbon nitrides. Temperature-programmed desorption (TPD) analysis of CO₂ demonstrates that triazole-based C₃N₅ shows higher basicity and stronger CO₂ binding energy than g-C₃N₄. Triazole-based C₃N₅ nanorods with 6.1 nm mesopore channels exhibit better CO₂ adsorption than nanorods with 3.5 and 5.4 nm mesopore channels. C₃N₅ nanorods with wider mesopore channels are effective in increasing the current density as an electrocatalyst during the CO₂ reduction reaction. Triazole-based C₃N₅ nanorods with tailored pore sizes exhibit CO₂ adsorption abilities of 5.6–9.1 mmol/g at 0 °C and 30 bar. Their Faraday efficiencies for reducing CO₂ to CO are 14–38% at a potential of −0.8 V vs. RHE.     

Electrocatalytic CO2 Reduction with a Half‐Sandwich Cobalt Catalyst: Selectivity towards CO

We present herein a Cp*Co(III)‐half‐sandwich catalyst system for electrocatalytic CO₂ reduction in aqueous acetonitrile solution. In addition to an electron‐donating Cp* ligand (Cp*=pentamethylcyclopentadienyl), the catalyst featured a proton‐responsive pyridyl‐benzimidazole‐based N,N‐bidentate ligand. Owing to the presence of a relatively electron‐rich Co center, the reduced Co(I)‐state was made prone to activate the electrophilic carbon center of CO₂. At the same time, the proton‐responsive benzimidazole scaffold was susceptible to facilitate proton‐transfer during the subsequent reduction of CO₂. The above factors rendered the present catalyst active toward producing CO as the major product over the other potential 2e/2H⁺ reduced product HCOOH, in contrast to the only known similar half‐sandwich CpCo(III)‐based CO₂‐reduction catalysts which produced HCOOH selectively. The system exhibited a Faradaic efficiency (FE) of about 70% while the overpotential for CO production was found to be 0.78 V, as determined by controlled‐potential electrolysis.     

Highly Efficient Porous Carbon Electrocatalyst with Controllable N-Species Content for Selective CO2 Reduction

We report a straightforward strategy to design efficient N doped porous carbon (NPC) electrocatalyst that has a high concentration of easily accessible active sites for the CO₂ reduction reaction (CO₂RR). The NPC with large amounts of active N (pyridinic and graphitic N) and highly porous structure is prepared by using an oxygen-rich metal–organic framework (Zn-MOF-74) precursor. The amount of active N species can be tuned by optimizing the calcination temperature and time. Owing to the large pore sizes, the active sites are well exposed to electrolyte for CO₂RR. The NPC exhibits superior CO₂RR activity with a small onset potential of −0.35 V and a high faradaic efficiency (FE) of 98.4 % towards CO at −0.55 V vs. RHE, one of the highest values among NPC-based CO₂RR electrocatalysts. This work advances an effective and facile way towards highly active and cost-effective alternatives to noble-metal CO₂RR electrocatalysts for practical applications.     

Controlled Synthesis of a Vacancy‐Defect Single‐Atom Catalyst for Boosting CO2 Electroreduction

The reaction of precursors containing both nitrogen and oxygen atoms with Ni^II under 500 °C can generate a N/O mixing coordinated Ni‐N₃O single‐atom catalyst (SAC) in which the oxygen atom can be gradually removed under high temperature due to the weaker Ni?O interaction, resulting in a vacancy‐defect Ni‐N₃‐V SAC at Ni site under 800 °C. For the reaction of Ni^II with the precursor simply containing nitrogen atoms, only a no‐vacancy‐defect Ni‐N₄ SAC was obtained. Experimental and DFT calculations reveal that the presence of a vacancy‐defect in Ni‐N₃‐V SAC can dramatically boost the electrocatalytic activity for CO₂ reduction, with extremely high CO₂ reduction current density of 65 mA cm^?2 and high Faradaic efficiency over 90 % at ?0.9 V vs. RHE, as well as a record high turnover frequency of 1.35×10⁵ h^?1, much higher than those of Ni‐N₄ SAC, and being one of the best reported electrocatalysts for CO₂‐to‐CO conversion to date.     

A DFT study on mechanisms of CO2 coupling with propargylic alcohols using alkali carbonates

The mechanisms of CO₂ coupling with the propargylic alcohol using alkali carbonates M₂CO₃ (M = Li, Na, K, Cs) have been investigated by means of density functional theory calculations. The calculations reveal that the target product tetronic acid (TA) is yielded through two stages: (a) the formation of the α-alkylidene cyclic carbonate (αACC) intermediate via Cs₂CO₃-mediated carboxylative cyclization of the propargylic alcohol with CO₂, and (b) the conversion of the αACC intermediate with Cs₂CO₃ to produce the cesium salt of the TA. Since the overall kinetic barriers for the two stages are comparable and affordable, the excellent chemoselectivity to the TA should be primarily originated from the high thermodynamic stability of the cesium salt of the TA. Moreover, relative to the TA, the possibility to yield the by-product acyclic carbonate can be excluded due to the both kinetics and thermodynamic inferiority. This result is different from the organic base-mediated reaction. Alternatively, our calculations predict that CsHCO₃ together generated with the cesium salt of the TA might also be an available mediating reagent for the incorporation of CO₂ with the propargylic alcohol. Compared to other alkali carbonates M₂CO₃ (M = Li, Na, K), the stronger basicity of Cs₂CO₃ and the lower ionic potential of cesium ion can raise the effective concentration of the αACC intermediate, and thus the conversion of the αACC intermediate into the cesium salt of the TA can be achieved with high yield.     

Robust nickel silicate catalysts with high Ni loading for CO2 methanation

CO₂ is the main component of greenhouse gases and also an important carbon source. The hydrogenation of CO₂ to methane using Ni-based catalysts can not only alleviate CO₂ emissions but also obtain useful fuels. However, Ni-based catalysts face one major problem of the sintering of Ni nanoparticles in the process of CO₂ methanation. Thus, this work has synthesized a series of efficient and robust nickel silicate catalysts (NiPS−X) with different nickel content derived from nickel phyllosilicate by the hydrothermal method. It was found that the Ni loading plays a critical role in the structure and catalytic performance of the NiPS−X catalysts. The catalytic performance gradually increases with the increase of Ni loading. In particular, the highly dispersed NiPS-1.6 catalyst with a high Ni loading of 34.3 wt% could obtain the CO₂ conversion greater than 80%, and the methane selectivity was close to 100% for 48 h at 330 °C and the GHSV of 40,000 mL g⁻¹ h⁻¹. The excellent catalytic property can be assigned to the high dispersion of Ni nanoparticles and the strong interaction between the active component and the carrier, which is derived from a unique layered silicate structure with lots of nickel phyllosilicate and a large number of Lewis acid sites.     

Sulfur Changes the Electrochemical CO2 Reduction Pathway over Cu Electrocatalysts

Electrochemical CO₂ reduction to value-added chemicals or fuels offers a promising approach to reduce carbon emissions and alleviate energy shortage. Cu-based electrocatalysts have been widely reported as capable of reducing CO₂ to produce a variety of multicarbon products (e.g., ethylene and ethanol). In this work, we develop sulfur-doped Cu₂O electrocatalysts, which instead can electrochemically reduce CO₂ to almost exclusively formate. We show that a dynamic equilibrium of S exists at the Cu₂O-electrolyte interface, and S-doped Cu₂O undergoes in situ surface reconstruction to generate active S-adsorbed metallic Cu sites during the CO₂ reduction reaction (CO₂RR). Density functional theory (DFT) calculations together with in situ infrared absorption spectroscopy measurements show that the S-adsorbed metallic Cu surface can not only promote the formation of the *OCHO intermediate but also greatly suppress *H and *COOH adsorption, thus facilitating CO₂-to-formate conversion during the electrochemical CO₂RR.