Strain-tunable electronic properties and optical properties of Hf2CO2 MXene

Two-dimensional materials have been extensively applied because of their unusual electronic, mechanical, and optical properties. In this paper, the electronic structure and optical properties of Hf₂CO₂ MXene under biaxial and uniaxial strains are investigated by the Heys-Scuseria-Ernzerhof (HSE06) method. Monolayer Hf₂CO₂ can sustain stress up to 6.453 N/M for biaxial strain and 3.072 N/M for uniaxial strain. Monolayer Hf₂CO₂ undergoes the transition from semiconductor to metal under −12% strain whether it is under biaxial or uniaxial strain. With the increasing biaxial compressive strain, the blue shift of Hf-d, O-p, and C-p orbitals in valence band maximum results in the metallization of monolayer Hf₂CO₂, while the red shift of Hf-d and O-p orbitals in conduction band minimum results in the metallization of monolayer Hf₂CO₂ with increasing uniaxial compressive strain. The analysis of optical properties indicates that uniaxial strain weakens the reflectivity and refractive index of monolayer Hf₂CO₂ in the visible-light range. In addition, the effective mass and the charge distribution under biaxial and uniaxial strains are also explored.     

Enhancing Local CO2 Adsorption by L-histidine Incorporation for Selective Formate Production Over the Wide Potential Window

Electrochemical carbon dioxide reduction reaction (CO₂RR) to produce valuable chemicals is a promising pathway to alleviate the energy crisis and global warming issues. However, simultaneously achieving high Faradaic efficiency (FE) and current densities of CO₂RR in a wide potential range remains as a huge challenge for practical implements. Herein, we demonstrate that incorporating bismuth-based (BH) catalysts with L-histidine, a common amino acid molecule of proteins, is an effective strategy to overcome the inherent trade-off between the activity and selectivity. Benefiting from the significantly enhanced CO₂ adsorption capability and promoted electron-rich nature by L-histidine integrity, the BH catalyst exhibits excellent FE_formate in the unprecedented wide potential windows (>90 % within −0.1–−1.8 V and >95 % within −0.2–−1.6 V versus reversible hydrogen electrode, RHE). Excellent CO₂RR performance can still be achieved under the low-concentration CO₂ feeding (e.g., 20 vol.%). Besides, an extremely low onset potential of −0.05 V_RHE (close to the theoretical thermodynamic potential of −0.02 V_RHE) was detected by in situ ultraviolet-visible (UV-Vis) measurements, together with stable operation over 50 h with preserved FE_formate of ≈95 % and high partial current density of 326.2 mA cm⁻² at −1.0 V_RHE.     

Understanding of Strain Effects in the Electrochemical Reduction of CO2: Using Pd Nanostructures as an Ideal Platform

Tuning the surface strain of heterogeneous catalysts represents a powerful strategy to engineer their catalytic properties by altering the electronic structures. However, a clear and systematic understanding of strain effect in electrochemical reduction of carbon dioxide is still lacking, which restricts the use of surface strain as a tool to optimize the performance of electrocatalysts. Herein, we demonstrate the strain effect in electrochemical reduction of CO₂ by using Pd octahedra and icosahedra with similar sizes as a well‐defined platform. The Pd icosahedra/C catalyst shows a maximum Faradaic efficiency for CO production of 91.1 % at −0.8 V versus reversible hydrogen electrode (vs. RHE), 1.7‐fold higher than the maximum Faradaic efficiency of Pd octahedra/C catalyst at −0.7 V (vs. RHE). The combination of molecular dynamic simulations and density functional theory calculations reveals that the tensile strain on the surface of icosahedra boosts the catalytic activity by shifting up the d ‐band center and thus strengthening the adsorption of key intermediate COOH*. This strain effect was further verified directly by the surface valence‐band photoemission spectra and electrochemical analysis.     

Photoelectrochemical Reduction of Carbon Dioxide with a Copper Graphitic Carbon Nitride Photocathode

Research on the photoreduction of CO₂ often has been dominated by the use of sacrificial reducing agents. A pathway that avoids this problem would be the development of photocathodes for CO₂ reduction that could then be coupled to a photoanodic oxygen evolution reaction. Here, we present the use of copper-substituted graphitic carbon nitride (Cu−CN) on a fluorinated tin oxide (FTO) electrode for the photoelectrochemical two-electron reduction of CO₂ to CO as a major product (>95 %) and formic acid (<5 %). The results show that at a potential of −2.5 V versus Fc\Fc⁺ the CO₂ reduction activity of Cu−CN on FTO electrode improves by 25 % upon illumination by visible light with a faradaic efficiency of nearly 100 %. Independently, X-ray photoelectron spectroscopy conclusively shows a pronounced increase in the electrical conductivity of the Cu−CN upon white light illumination under vacuum and a contactless measuring configuration. This photo-assisted charge mobility is shown to play a key role in the increased reactivity and faradaic efficiency for the reduction of CO₂.     

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.     

Photo-thermal Cooperative Carbonylation of Ethanol with CO2 on Cu2O-SrTiCuO3-x

Carbonylation of ethanol with CO₂ as carbonyl source into value-added esters is of considerable significance and interest, while remains of great challenge due to the harsh conditions for activation of inert CO₂ in that the harsh conditions result in undesired activation of α-C−H and even cleavage of C−C bond in ethanol to deteriorate the specific activation of O−H bond. Herein, we propose a photo-thermal cooperative strategy for carbonylation of ethanol with CO₂, in which CO₂ is activated to reactive CO via photo-catalysis with the assistance of *H from thermally-catalyzed dissociation of alcoholic O−H bond. To achieve this proposal, an interfacial site and oxygen vacancy both abundant SrTiCuO_3-x supported Cu₂O (Cu₂O-SrTiCuO_3-x) has been designed. A production of up to 320 μmol g⁻¹ h⁻¹ for ethyl formate with a selectivity of 85.6 % to targeted alcoholic O−H activation has been afforded in photo-thermal assisted gas-solid process under 3.29 W cm⁻¹ of UV/Vis light irradiation (144 °C) and 0.2 MPa CO₂. In the photo-driven activation of CO₂ and following carbonylation, CO₂ activation energy decreases to 12.6 kJ mol⁻¹, and the cleavage of alcoholic α-C−H bond has been suppressed.     

In Situ Bismuth Nanosheet Assembly for Highly Selective Electrocatalytic CO2 Reduction to Formate

The reduction of carbon dioxide (CO₂) into value-added fuels using an electrochemical method has been regarded as a compelling sustainable energy conversion technology. However, high-performance electrocatalysts for CO₂ reduction reaction (CO₂RR) with high formate selectivity and good stability need to be improved. Earth-abundant Bi has been demonstrated to be active for CO₂RR to formate. Herein, we fabricated an extremely active and selective bismuth nanosheet (Bi-NSs) assembly via an in situ electrochemical transformation of (BiO)₂CO₃ nanostructures. The as-prepared material exhibits high activity and selectivity for CO₂RR to formate, with nearly 94% faradaic efficiency at −1.03 V (versus reversible hydrogen electrode (vs. RHE)) and stable selectivity (>90%) in a large potential window ranging from −0.83 to −1.18 V (vs. RHE) and excellent durability during 12 h continuous electrolysis. In addition, the Bi-NSs based CO₂RR/methanol oxidation reaction (CO₂RR/MOR) electrolytic system for overall CO₂ splitting was constructed, evidencing the feasibility of its practical implementation.     

Self-Polarization Triggered Multiple Polar Units Toward Electrochemical Reduction of CO2 to Ethanol with High Selectivity

Electrochemical conversion of CO₂ to highly valuable ethanol has been considered a intriguring strategy for carbon neutruality. However, the slow kinetics of coupling carbon-carbon (C−C) bonds, especially the low selectivity ethanol than ethylene in neutral conditions, is a significant challenge. Herein, the asymmetrical refinement structure with enhanced charge polarization is built in the vertically oriented bimetallic organic frameworks (NiCu-MOF) nanorod array with encapsulated Cu₂O (Cu₂O@MOF/CF), which can induce an intensive internal electric field to increase the C−C coupling for producing ethanol in neutral electrolyte. Particularly, when directly employed Cu₂O@MOF/CF as the self-supporting electrode, the ethanol faradaic efficiency (FE_ethanol) could reach maximum 44.3 % with an energy efficiency of 27 % at a low working-potential of −0.615 V versus the reversible hydrogen electrode (vs. RHE) using CO₂-saturated 0.5 M KHCO₃ as the electrolyte. Experimental and theoretical studies suggest that the polarization of atomically localized electric fields derived from the asymmetric electron distribution can tune the moderate adsorption of *CO to assist the C−C coupling and reduce the formation energy of H₂CCHO*-to-*OCHCH₃ for the generation of ethanol. Our research offers a reference for the design of highly active and selective electrocatalysts for reducing CO₂ to multicarbon chemicals.     

MXene-Regulated Metal-Oxide Interfaces with Modified Intermediate Configurations Realizing Nearly 100% CO2 Electrocatalytic Conversion

Electrocatalytic CO₂ reduction via renewable electricity provides a sustainable way to produce valued chemicals, while it suffers from low activity and selectivity. Herein, we constructed a novel catalyst with unique Ti₃C₂T_x MXene-regulated Ag−ZnO interfaces, undercoordinated surface sites, as well as mesoporous nanostructures. The designed Ag−ZnO/Ti₃C₂T_x catalyst achieves an outstanding CO₂ conversion performance of a nearly 100% CO Faraday efficiency with high partial current density of 22.59 mA cm⁻² at −0.87 V versus reversible hydrogen electrode. The electronic donation of Ag and up-shifted d-band center relative to Fermi level within MXene-regulated Ag−ZnO interfaces contributes the high selectivity of CO. The CO₂ conversion is highly correlated with the dominated linear-bonded CO intermediate confirmed by in situ infrared spectroscopy. This work enlightens the rational design of unique metal-oxide interfaces with the regulation of MXene for high-performance electrocatalysis beyond CO₂ reduction.     

Electrochemical properties of a gold electrode modified with a mixed monolayer

The electrochemical properties of a gold electrode modified with a mixed thiol monolayer containing both a polar and a non-polar head group have been investigated in aqueous Fe(CN)₆⁴⁻, flavin adenine dinucleotide (FAD) and ubiquinone-0 (2,3-dimethoxy-5-methyl-1,4-benzoquinone, UQ₀) solutions. The cyclic voltammetric current-potential (i-E) response of Fe(CN)₆⁴⁻ was found to be affected considerably by the polarity of the head group contained in the mixed monolayer assembly, as compared with those of FAD and UQ₀. It was also found that in the cases of UQ₀ and FAD the i-E responses for the modified electrode were affected by their own molecular size rather than the polarity of the mixed monolayer head group. Furthermore, compared with Fe(CN)₆⁴⁻ ion, these biologically related molecules are able to permeate readily into the well-organized and hydrophobic alkyl chains of the monolayer assembly. The voltammetric profile of UQ₀ was improved by the modification of aminoethanethiol, as compared with those of bare gold and the electrode modified with other polar thiols. Further, two different permeation paths of the electrode species into the mixed monolayer are suggested from the variation of the i-E response with the cycle of the potential scan.     

Selective CO2-to-Syngas Conversion Enabled by Bimetallic Gold/Zinc Sites in Partially Reduced Gold/Zinc Oxide Arrays

Electrocatalytic CO₂-to-syngas (gaseous mixture of CO and H₂) is a promising way to curb excessive CO₂ emission and the greenhouse gas effect. Herein, we present a bimetallic AuZn@ZnO (AuZn/ZnO) catalyst with high efficiency and durability for the electrocatalytic reduction of CO₂ and H₂O, which enables a high Faradaic efficiency of 66.4 % for CO and 26.5 % for H₂ and 3 h stability of CO₂-to-syngas at −0.9 V vs. the reversible hydrogen electrode (RHE). The CO/H₂ ratios show a wide range from 0.25 to 2.50 over a narrow potential window (−0.7 V to −1.1 V vs. RHE). In situ attenuated total reflection surface-enhanced infrared absorption spectroscopy combined with density functional theory calculations reveals that the bimetallic synergistic effect between Au and Zn sites lowers the activation energy barrier of CO₂ molecules and facilitates electronic transfer, further highlighting the potential to control CO/H₂ ratios for efficient syngas production using the coexisting Au sites and Zn sites.     

A Polyoxometalate-Based Binder-Free Capacitive Deionization Electrode for Highly Efficient Sea Water Desalination

Capacitive deionization is a promising technique in sea water desalination. Compared with common electrodes, mixed capacitive-deionization electrodes exhibit better performance in sea water desalination because they integrate pseudocapacitance and electric double-layer capacitance in one system. Herein, a 3D binder-free mixed capacitive-deionization electrode was fabricated by direct electrodeposition of SiW₁₂O₄₀⁴⁻ and polyaniline on a 3D exfoliated graphite carrier. In this electrode, SiW₁₂O₄₀⁴⁻/polyaniline composite particles with a size of about 100–120 nm are dispersed homogenously on the 3D exfoliated graphite carrier. Its specific capacitance reaches 352 F g⁻¹ at 1 A g⁻¹. With increasing current from 1 to 20 A g⁻¹, the specific capacitance only decays by 32 %. When employed in sea water desalination, the performance of this mixed capacitive-deionization electrode is also excellent. At 1.2 V, the salt adsorption capacity of this mixed electrode reaches 23.1 mg g⁻¹ with a salt adsorption rate of 1.38 mg g⁻¹ min⁻¹ in 500 mg L⁻¹ NaCl. The performance of this electrode is well retained after 30 cycles. The excellent sea water desalination performance originates from the synergistic effect between SiW₁₂O₄₀⁴⁻ and polyaniline. This work has developed polyoxometalate as a new material for capacitive-deionization electrodes.     

Zeolites modified by CuCl for separating CO from gas mixtures containing CO2

Although zeolites such as NaY and 13X adsorb CO₂ much more than CO, the adsorption amount of CO₂ and CO can be reversed if the zeolites are modified with CuCl. When zeolite NaY or 13X is mixed with CuCl and heated, high CO adsorption selectivity and capacity can be obtained. Isotherms show the adsorbents have CO capacity much higher than CO₂. This is because CuCl has dispersed onto the surface of the zeolites to form a monolayer after the heat treatment and the monolayer dispersed CuCl can provide tremendous Cu(I) to selective adsorb CO and inhibit the CO₂ adsorption. The monolayer dispersion of CuCl is confirmed by XRD and EXAFS studies. The loading of CuCl on the zeolites has a threshold below which the CuCl forms monolayer after heating and crystalline phase of CuCl can not be detected by XRD. An adsorbent of CuCl/NaY with CuCl content closed to the monolayer capacity shows very high CO selective adsorbability for CO₂, N₂, H₂ and CH₄. At temperature higher than room temperature, the adsorbent has even better CO selectivity for CO₂. Using the adsorbent, a single-stage 4 beds PSA process, working at 70°C and 0.4 MPa to 0.013 MPa, can obtain CO product with purity >99.5% and yield >85%.     

Tuning C1/C2 Selectivity of CO2 Electrochemical Reduction over in-Situ Evolved CuO/SnO2 Heterostructure

Heterostructured oxides with versatile active sites, as a class of efficient catalysts for CO₂ electrochemical reduction (CO₂ER), are prone to undergo structure reconstruction under working conditions, thus bringing challenges to understanding the reaction mechanism and rationally designing catalysts. Herein, we for the first time elucidate the structural reconstruction of CuO/SnO₂ under electrochemical potentials and reveal the intrinsic relationship between CO₂ER product selectivity and the in situ evolved heterostructures. At −0.85 V_RHE, the CuO/SnO₂ evolves to Cu₂O/SnO₂ with high selectivity to HCOOH (Faradaic efficiency of 54.81 %). Mostly interestingly, it is reconstructed to Cu/SnO_2-x at −1.05 V_RHE with significantly improved Faradaic efficiency to ethanol of 39.8 %. In situ Raman spectra and density functional theory (DFT) calculations reveal that the synergetic absorption of *COOH and *CHOCO intermediates at the interface of Cu/SnO_2-x favors the formation of *CO and decreases the energy barrier of C−C coupling, leading to high selectivity to ethanol.     

A Biomass-Derived Carbon-Based Electrocatalyst for Efficient N2 Fixation to NH3 under Ambient Conditions

Currently, NH₃ production primarily depends on the Haber–Bosch process, which operates at elevated temperatures and pressures and leads to serious CO₂ emissions. Electrocatalytic N₂ reduction offers an environmentally benign approach for the sustainable synthesis of NH₃ under ambient conditions. This work reports the development of biomass-derived amorphous oxygen-doped carbon nanosheet (O−CN) using tannin as the precursor. As a metal-free electrocatalyst for N₂-to-NH₃ conversion, such O−CN shows high catalytic performances, achieving a large NH₃ yield of 20.15 μg h⁻¹ mg⁻¹_cat. and a high Faradic efficiency of 4.97 % at −0.6 V vs. reversible hydrogen electrode (RHE) in 0.1 m HCl at ambient conditions. Remarkably, it also exhibits high electrochemical selectivity and durability.     

Modulating the thermoelectric transport properties of the novel material Er2Te3 via strain

The rare-earth chalcogenide Er₂Te₃, characterized by its low lattice thermal conductivity, represents a highly promising and innovative thermoelectric material. However, there have been limited studies exploring its thermoelectric properties in depth. Additionally, it has been discovered that strain engineering is an effective method for enhancing thermoelectric properties, a technique successfully applied to relevant materials. In this study, we employed a first-principles approach in conjunction with the semi-classical Boltzmann transport theory to investigate the thermoelectric properties of Er₂Te₃ materials under −4% to 4% strain. The results indicate that applying compressive strain modulates thermoelectric properties more effectively than tensile strain for Er₂Te₃. Under strain modulation, the maximum power factor for both p-type and n-type Er₂Te₃ increases significantly, from 0.9 to 2.5 mW m⁻¹ K⁻² and from 14 to 18 mW m⁻¹ K⁻² at 300 K, respectively. Moreover, the figure of merit (ZT) for p-type and n-type Er₂Te₃ improves notably, from 0.15 to 0.25 and from 1.15 to 1.35, respectively, under −4% strain. Consequently, the thermoelectric properties of Er₂Te₃ materials can be significantly enhanced through strain application, with n-type Er₂Te₃ demonstrating substantial potential as a thermoelectric material.     

Enhanced Electrochemical Reduction of CO2 to CO on Ag/SnO2 by a Synergistic Effect of Morphology and Structural Defects

Silver (Ag)-based materials are considered to be promising materials for electrochemical reduction of CO₂ to produce CO, but the selectivity and efficiency of traditional polycrystalline Ag materials are insufficient; there still exists a great challenge to explore novel modified Ag based materials. Herein, a nanocomposite of Ag and SnO₂ (Ag/SnO₂) for efficient reduction of CO₂ to CO is reported. HRTEM and XRD patterns clearly demonstrated the lattice destruction of Ag and the amorphous SnO₂ in the Ag/SnO₂ nanocomposite. Electrochemical tests indicated the nanocomposite containing 15% SnO₂ possesses highest catalytic selectivity featured by a CO faradaic efficiency (FE) of 99.2% at −0.9 V versus reversible hydrogen electrode (vs RHE) and FE>90% for the CO product at a wide potential range from −0.8 V to −1.4 V vs RHE. Experimental characterization and analysis showed that the high catalytic performance is attributed to not only the branched morphology of Ag/SnO₂ nanocomposites (NCs), which endows the maximum exposure of active sites, but also the special adsorption capacity of abundant defect sites in the crystal for *COOH (the key intermediate of CO formation), which improves the intrinsic activity of the catalyst. But equally important, the existed SnO₂ also plays an important role in inhibiting hydrogen evolution reaction (HER) and anchoring defect sites. This work demonstrates the use of crystal defect engineering and synergy in composite to improve the efficiency of electrocatalytic CO₂ reduction reaction (CO₂RR).     

Elucidating the Mechanism of Simultaneous Activation of CH4 and CO2 Mediated by Single Group 10 Metal Anions in Gas Phase

Quantum chemistry calculations predict that besides the reported single metal anion Pt⁻, Ni⁻ can also mediate the co-conversion of CO₂ and CH₄ to form [CH₃−M(CO₂)−H]^– complex, followed by transformation to C−C coupling product [H₃CCOO−M−H]⁻ ( A ), hydrogenation products [H₃C−M−OCOH]⁻ ( B ) and [H₃C−M−COOH]⁻. For Pd⁻, a fourth product channel leading to PdCO₂⁻…CH₄ becomes more competitive. For Ni⁻, the feed order must be CO₂ first, as the weaker donor-acceptor interaction between Ni⁻ and CH₄ increases the C−H activation barrier, which is reduced by [Ni−CO₂]⁻. For Ni⁻/Pt⁻, the highly exothermic products A and B are similarly stable with submerged barrier that favors B . The smaller barrier difference between A and B for Ni⁻ suggests the C−C coupling product is more competitive in the presence of Ni⁻ than Pt⁻. The charge redistribution from M⁻ is the driving force for product B channel. This study adds our understanding of single atomic anions to activate CH₄ and CO₂ simultaneously.     

Rational Design of Biaxial Tensile Strain for Boosting Electronic and Ionic Conductivities of Na2MnSiO4 for Rechargeable Sodium-Ion Batteries

Using first-principles calculations, biaxial tensile (ϵ=2 and 4 %) and compressive (ϵ=−2 and −4 %) straining of Na₂MnSiO₄ lattices resulted into radial distance cut offs of 1.65 and 2 Å, respectively, in the first and second nearest neighbors shell from the center. The Si−O and Mn−O bonds with prominent probability density peaks validated structural stability. Wide-band gap of 2.35 (ϵ=0 %) and 2.54 eV (ϵ=−4 %), and narrow bandgap of 2.24 eV (ϵ=+4 %) estimated with stronger coupling of p–d σ bond than that of the p–d π bond, mainly contributed from the oxygen p-state and manganese d-state. Na⁺-ion diffusivity was found to be enhanced by three orders of magnitude as the applied biaxial strain changed from compressive to tensile. According to the findings, the rational design of biaxial strain would improve the ionic and electronic conductivity of Na₂MnSiO₄ cathode materials for advanced rechargeable sodium-ion batteries.     

Modulating the Reaction Configuration by Breaking the Structural Symmetry of Active Sites for Efficient Photocatalytic Reduction of Low-concentration CO2

Photocatalytic conversion of low-concentration CO₂ is considered as a promising way to simultaneously mitigate the environmental and energy issues. However, the weak CO₂ adsorption and tough CO₂ activation process seriously compromise the CO production, due to the chemical inertness of CO₂ molecule and the formed fragile metal-C/O bond. Herein, we designed and fabricated oxygen vacancy contained Co₃O₄ hollow nanoparticles on ordered macroporous N-doped carbon framework (Vo−HCo₃O₄/OMNC) towards photoreduction of low-concentration CO₂. In situ spectra and ab initio molecular dynamics simulations reveal that the constructed oxygen vacancy is able to break the local structural symmetry of Co−O−Co sites. The formation of asymmetric active site switches the CO₂ configuration from a single-site linear model to a multiple-sites bending one with a highly stable configuration, enhancing the binding and structural polarization of CO₂ molecules. As a result, Vo−HCo₃O₄/OMNC shows unprecedent activity in the photocatalytic conversion of low-concentration CO₂ (10 % CO₂/Ar) under laboratory light source or even natural sunlight, affording a syngas yield of 337.8 or 95.2 mmol g⁻¹ h⁻¹, respectively, with an apparent quantum yield up to 4.2 %.