Atomically precise alkynyl-protected Ag19Cu2 nanoclusters: synthesis, structure analysis, and electrocatalytic CO2 reduction application.

Carbon dioxide electroreduction over imidazolate ligands coordinated with Zn(II) center in ZIFs

Improving CO2 electroreduction over ZIF-derived carbon doped with Fe-N sites by an additional ammonia treatment

Reaction temperature is an important parameter to tune the selectivity and activity of electrochemical CO2 reduction reaction (CO2 RR) due to different thermodynamics of CO2 RR and competitive hydrogen evolution reaction (HER). In this work, temperature-dependent CO2 RR over Fe-N-C and Ni-N-C single-atom catalysts are investigated from 303 to 343 K. Increasing the reaction temperature improves and decreases CO Faradaic efficiency over Fe-N-C and Ni-N-C catalysts at high overpotentials, respectively. CO current density over Fe-N-C catalyst increases with temperature, then gets into a plateau at 323 K, finally reaches the maximum value of 185.8 mA cm-2 at 343 K. While CO current density over Ni-N-C catalyst achieves the maximum value of 252.5 mA cm-2 at 323 K, and then drops significantly to 202.9 mA cm-2 at 343 K. Temperature programmed desorption results and density functional theory calculations reveal that the difference of temperature-dependent variation on CO Faradaic efficiency and current density between Fe-N-C and Ni-N-C catalysts results from the varied adsorption strength of key reaction intermediates during CO2 RR.

Electrocatalytic CO2 reduction (CO2RR) has emerged as a promising approach for converting CO2 into valuable chemicals and fuels to achieve a sustainable carbon cycle. However, the development of efficient electrocatalysts with high current densities and superior product selectivity remains a significant challenge. In this study, we present the synthesis of a porous nitrogen-doped carbon nanosheet loaded with heterostructured Ni/Ni3ZnC0.7 nanoparticles through a facile hydrothermal-calcination method (Ni/Ni3ZnC0.7-NC). Remarkably, the Ni/Ni3ZnC0.7-NC catalyst exhibits outstanding performance towards CO2RR in an H-cell, demonstrating a high CO faradaic efficiency of 92.47% and a current density (jCO) of 15.77 mA cm-2 at 0.87 V vs. RHE. To further explore its potential industrial applications, we constructed a flow cell and a rechargeable Zn-CO2 flow cell utilizing the Ni/Ni3ZnC0.7-NC catalyst as the cathode. Impressively, not only does the Ni/Ni3ZnC0.7-NC catalyst achieve an industrial high current density of 254 mA cm-2 at a voltage of -1.19 V vs. RHE in the flow cell, but it also exhibits a maximum power density of 4.2 mW cm-2 at 22 mA cm-2 in the Zn-CO2 flow cell, while maintaining excellent rechargeability. Density functional theory (DFT) calculations indicate that Ni/Ni3ZnC0.7-NC possesses more spontaneous reaction pathways for CO2 reduction to CO, owing to its heterogeneous structure in contrast to Ni3ZnC0.7-NC and Ni-NC. Consequently, Ni/Ni3ZnC0.7-NC demonstrates accelerated CO2RR reaction kinetics, resulting in improved catalytic activity and selectivity for CO2RR.

Transition metal nitrogen carbon based single‐atom catalysts (SACs) have exhibited superior activity and selectivity for CO2 electroreduction to CO. A favorable local nitrogen coordination environment is key to construct efficient metal‐N moieties. Here, a facile plasma‐assisted and nitrogen vacancy (NV) induced coordinative reconstruction strategy is reported for this purpose. Under continuous plasma striking, the preformed pentagon pyrrolic N‐defects around Ni sites can be transformed to a stable pyridinic N dominant Ni‐N2 coordination structure with promoted kinetics toward the CO2‐to‐CO conversion. Both the CO selectivity and productivity increase markedly after the reconstruction, reaching a high CO Faradaic efficiency of 96% at mild overpotential of 590 mV and a large CO current density of 33 mA cm‐2 at 890 mV. X‐ray adsorption spectroscopy and density functional theory (DFT) calculations reveal this defective local N environment decreases the restraint on central Ni atoms and provides enough space to facilitate the adsorption and activation of CO2 molecule, leading to a reduced energy barrier for CO2 reduction.

Carbonized wood membrane decorated with AuPd alloy nanoparticles as an efficient self-supported electrode for electrocatalytic CO2 reduction

Unraveling the atomistic synergistic effects of nanoalloys on the electrocatalytic CO2 reduction reaction (eCO2RR), especially in the presence of copper, is of paramount importance. However, this endeavor encounters significant challenges due to the lack of the crystallographically determined atomic-level structure of appropriate monometallic and bimetallic analogues. Herein, we report a one-pot synthesis and structure characterization of a AuCu nanoalloy cluster catalyst, [Au15Cu4(DPPM)6Cl4(C≡CR)1]2+ (denoted as Au15Cu4). Single-crystal X-ray diffraction analysis reveals that Au15Cu4 comprises two interpenetrating incomplete, centered icosahedra (Au9Cu2 and Au8Cu3) and is protected by six DPPM, four halide, and one alkynyl ligand. The Au15Cu4 cluster and its closest monometal structural analogue, [Au18(DPPM)6Br4]2+ (denoted as Au18), as model systems, enable the elucidation of the atomistic synergistic effects of Au and Cu on eCO2RR. The results reveal that Au15Cu4 is an excellent eCO2RR catalyst in a gas diffusion electrode-based membrane electrode assembly (MEA) cell, exhibiting a high CO Faradaic efficiency (FECO) of >90%, and this efficiency is substantially higher than that of the undoped Au18 (FECO: 60% at -3.75 V). Au15Cu4 exhibits an industrial-level CO partial current density of up to -413 mA/cm2 at -3.75 V with the gas CO2-fed MEA, which is 2-fold higher than that of Au18. The density functional theory (DFT) calculations demonstrate that the synergistic effects are induced by Cu doping, where the exposed pair of AuCu dual sites was suggested for launching the eCO2RR process. Besides, DFT simulations reveal that these special dual sites synergistically coordinate a moderate shift in the d-state, thus enhancing its overall catalytic performance.

Unsaturated Ni single-atom catalysts (SACs), Ni-Nx (x=1,2,3), have been investigated to break the conventional Ni-N4 structural limitation and provide more unoccupied 3d orbitals for CO2 reduction reaction (CO2RR) intermediates adsorption, but their intrinsically low structural stability has seriously hindered their applications. Here, we developed a strategy by integrating Ni nanoclusters to stabilize unsaturated Ni-N3 atomic sites for efficient CO2 electroreduction to CO at industrial-level current. Density Functional Theory (DFT) calculations revealed that the incorporation of Ni nanocluster effectively stabilizes the unsaturated Ni-N3 atomic sites and modulates their electronic structure to enhance the adsorption of the key intermediate *COOH during CO2RR. Guided by these insights, we prepared an optimal composite catalyst, Ni6@Ni-N3, which features a Ni6N6 nanocluster surrounded by six Ni-N3 single atoms sites, through low-temperature pyrolysis. The morphology and coordinative structure of Ni6@Ni-N3 were confirmed by an aberration-corrected transmission electron microscope (AC-TEM) and X-ray absorption spectroscopy (XAS). As a result, Ni6@Ni-N3 demonstrated a remarkably high CO Faradaic efficiency (FECO) of 99.7 % and a turnover frequency (TOF) of 83984.2 h-1 at 500 mA cm-2 under -1.15 VRHE, much better than those of Ni-N4 with a lower FECO of 86 % at 100 mA cm-2 and a TOF of 39309.9 h-1under identical potential. XAS analyses of Ni6@Ni-N3 before and after long-term CO2RR testing confirmed the excellent stability of its coordinative environment. This work highlights a generalizable approach for stabilizing unsaturated single-atom catalysts, paving the way for their application in high-performance CO2RR.

Electrocatalytic reduction of carbon dioxide over reduced nanoporous zinc oxide

Harnessing in-situ oxidation of nanostructured mxene to modulate the electronic structure for improved selectivity for electrochemical carbon dioxide reduction

Edge-hosted single-atom sites provide asymmetric charge distribution compared with in-plane sites, showing higher intrinsic activity. However, traditional preparation methods for edge sites rely on creating mesopores by etching, making it difficult to obtain high-density edge sites. Herein, it is demonstrated that reducing the size of carbon support to the nanoscale by a nanocasting approach enables high-density edge-hosted single-atom sites, significantly enhancing the catalytic performance in electrochemical CO2 reduction. To achieve this, edge Ni sites on N-doped carbon (e-Ni/NC) with an average size of 26nm are prepared by calcination of ZIF-L confined within SBA-16. e-Ni/NC achieves a CO current density of 480.4mA cm-2 and a CO Faradaic efficiency over 90.0% from -0.6 to -1.6V, which are superior to the in-plane Ni sites on N-doped carbon. The Zn-CO2 battery with e-Ni/NC cathode possesses a maximum power density of 1.2mW cm-2 and shows stable performance in galvanostatic charge-discharge cycling.

Developing metal-nitrogen-doped carbon single-atom catalysts (M-NC SACs) with high loadings for the electrochemical CO2 reduction reaction (eCO2RR) remains challenging owing to the risk of metal aggregation. Herein, the study presents a facile strategy for synthesizing M-NC SACs using metal-chelating ligands, eliminating the need for additional processing steps. Specifically, using ethylenediaminetetraacetic acid as a strong metal-chelating ligand, the formation of Ni nanoparticles is effectively prevented and a high loading of ≈2.7 wt.% is achieved, leading to the development of high-loading Ni SACs. The resulting catalysts exhibit a high CO faradaic efficiency (FECO) of 96.6% and CO partial current density of -120.2 mA cm-2 and retain a FECO over 90% in a broad potential range of -0.4 to -0.9 V versus the reversible hydrogen electrode. Furthermore, theoretical calculations indicate that the asymmetric Ni-N3C1 local coordination structure within the catalyst reveals an optimal balance between *COOH formation and *CO desorption, which enhances the activity for eCO2RR to CO. This study offers an efficient strategy to suppress metal nanoparticle formation while simultaneously improving the metal loading in M-NC SACs.

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 NiSA -Nx -C) with different N coordination numbers have been fabricated by controlling the pyrolysis temperature. Significantly, the NiSA -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 NiSA -N3 -C and NiSA -N4 -C, in electrocatalytic CO2 reduction. Theoretical calculations reveal that the low N coordination number of single-atom Ni sites in NiSA -N2 -C is favorable to the formation of COOH* intermediate and thus accounts for its superior activity.

Developing efficient and stable electrocatalysts within a wide potential range is vital for the mature applications of the electrocatalytic CO2 reduction reaction (CO2RR) into value-added chemical products. Herein, we engineered a NC@Ni/C nano-composite featuring a core–shell structure of a pyridinic-N-rich carbon layer encapsulating Ni nanoparticles (NPs) as a highly effective electrocatalyst for CO2RR to CO over a wide potential range. The catalyst demonstrates a high CO Faradaic efficiency (FECO) of >90% in a wide potential range from −0.65 to −1.45 V [vs reversible hydrogen electrode (RHE)] with the maximum FECO of 97% at −1.05 V (vs RHE). Strikingly, it exhibits an excellent stability with a constant current density and a FECO > 95% for 92 h at −1.05 V (vs RHE). Structural studies and DFT calculations further reveal that pyridinic-N doping in the carbon shell of Ni NPs plays a dual role in promoting the CO2RR activity. It not only alleviates the mass transfer limitation of CO2 by enhancing the CO2 adsorption capacity, but it also lowers the reaction energy barrier of the *COOH formation rate-determining step with the electronic structure modulation by Ni. This work may shed more light on the seeking of practical catalysts for high-efficiency electrochemical CO2 reduction over a broad potential window.

Electrochemical CO2 reduction (CO2RR) represents an attractive strategy for achieving carbon neutrality. Nevertheless, developing efficient and inexpensive catalysts is still challenging. Herein, we report a yolk-shell catalyst consisting of Ni nanoparticles (NPs) confined in hierarchical nitrogen-doped carbon nanocages (Ni@hNCNC) for efficient CO2RR, which demonstrates high CO faradaic efficiency of 98% and impressive CO partial current density of 195 mA cm-2 at -1.4 V (versus reversible hydrogen electrode) in neutral electrolyte and 352 mA cm-2 at -1.1 V in alkaline electrolyte, outperforming the counterparts of Ni NPs supported on the outer surface of hNCNC (Ni/hNCNC) and confined in undoped hierarchical carbon nanocages. The combined experimental and theoretical studies reveal that the main active sites are the pyridinic N and Ni atoms at the interfaces of the Ni-hNCNC composite. The exceptional performance of Ni@hNCNC is attributed to the electronic interaction between N dopants and Ni NPs, the efficiently exposed interfacial active sites, as well as the confinement of hNCNC to keep a stable structure during CO2RR. This study provides an efficient route to enhance the electrocatalytic CO2-to-CO performance in neutral solution by confining a low-cost Ni catalyst in N-doped carbon nanocages.