Synergistic Modulation at Atomically Dispersed Fe/Au Interface for Selective CO2 Electroreduction

High-Performance Overall Water Splitting Dominated by Direct Ligand-to-Cluster Photoexcitation in Metal-Organic Frameworks

Expanding the spectral response of photocatalysts to facilitate overall water splitting (OWS) represents an effective approach for improving solar spectrum utilization efficiency. However, the majority of single-phase photocatalysts designed for OWS primarily respond to the ultraviolet region, which accounts for a small proportion of sunlight. Herein, we present a versatile strategy to achieve broad visible-light-responsive OWS photocatalysis dominated by direct ligand-to-cluster charge transfer (LCCT) within metal-organic frameworks (MOFs). Three synthesized OWS MOFs, namely Fe2MCbz (M2+ = Mn2+, Co2+, Ni2+), exhibited intrinsic OWS capability without the requirement for extra photosensitizer or sacrificial agent or cocatalyst. Among these, Fe2NiCbz was identified as the superior performer, and when dispersed with polyacrylonitrile nanofibers using electrospinning technology, it achieved the highest OWS rates of 170.2 and 85.1 μmol g-1 h-1 for H2 and O2 evolution, surpassing all previously documented MOF-based photocatalysts. Experimental and theoretical analyses revealed that direct LCCT played a crucial role in enhancing the photocatalytic efficiency, with exceptional performance of Fe2NiCbz attributed to its well-optimized energy level structures and highly efficient charge transfer mechanism. This work not only sets a benchmark in OWS MOF photocatalysts but also paves the way for maximizing solar spectrum utilization, thereby advancing renewable hydrogen production strategy.

Quasi Fe MIL-53 nanozyme inducing ferroptosis and immunogenic cell death for cancer immunotherapy

Nanozymes offer diverse therapeutic potentials for cancer treatment which is dependent on the development of nanomaterials. Quasi-metal-organic framework is a class of metal-organic framework-derived nanomaterials with a transition state from metal-organic frameworks towards metal oxide featuring porous structure and high activity. Herein an iron-based quasi-metal-organic framework nanozyme Q-MIL-53(Fe) is reported via a controlled deligandation strategy, exhibiting enhanced peroxidase-/catalase-mimic activity and glutathione depletion capacity, whose underlying mechanisms are studied via density functional theory calculations. Q-MIL-53(Fe) demonstrates biocompatibility and superior antitumor efficacy compared to pristine MIL-53(Fe). It can activate antitumor immune response by inducing ferroptosis and immunogenic cell death, promoting dendritic cell maturation and T lymphocytes infiltration. Furthermore, a combination of Q-MIL-53(Fe) and programmed cell death protein 1 antibody amplifies cancer immunotherapy. This study validates the antitumor activity of quasi-metal-organic frameworks and its immunotherapy induction potential. It would broaden the application of quasi-metal-organic frameworks and open avenues for developing antitumor nanozymes.

Tailored Local Electronic Environment of Co-N4 Sites in Cobalt Phthalocyanines for Enhanced CO2 Reduction Reaction

Atomically dispersed Co-N4-based catalysts have been recently emerging as one of the most promising candidates for facilitating CO2 reduction reaction (CO2RR). The local electronic environment of Co-N4 sites in these catalysts is considered to play a critical role in adjusting the catalytic performance, the effort of which however is not yet clearly verified. Herein, a series of cobalt phthalocyanines with different peripheral substituents including unsubstituted phthalocyanine Co(II) (CoPc), 2,9,16,23-tetramethoxyphthalocyaninato Co(II) (CoPc-4OCH3), and 2,9,16,23-tetranitrophthalocyaninato Co(II) (CoPc-4NO2) are supported onto the surface of the multi-walled carbon nanotubes (CNTs), affording CoPc@CNTs, CoPc-4OCH3@CNTs, and CoPc-4NO2@CNTs. X-ray photoelectron spectroscopy and X-ray absorption near-edge structure measurements disclose the influence of the peripheral substituents on the local electronic structure of Co atoms in these three catalysts. Electrochemical tests indicate the higher CO2RR performance of CoPc-4OCH3@CNTs compared to CoPc@CNTs and CoPc-4NO2@CNTs as exemplified by the higher Faraday efficiency of CO, larger part current densities, and better stability displayed by CoPc-4OCH3@CNTs at the applied voltage range from -0.6 to -1.0 V versus RHE in both H-cell and flow cell. These results highlight the effect of the electron-donating -OCH3 substituent on the enhanced catalytic activity of CoPc-4OCH3@CNTs, which will help develop Co-N4-based catalysts with promising catalytic performance toward CO2RR.

Ag Atom Induces Microstrain Environment around Cd Sites to Construct Diatomic Sites for Almost 100% CO2-to-CO Electroreduction

Deeply understanding how local microstrain environment around diatomic sites influences their electronic state and adsorption is crucial for improving electrochemical CO2 reduction (eCO2R) reaction; however, precise engineering of the atomic microstrain environment is challenging. Herein, we fabricate Ag-CdTMT electrocatalysts with AgN2S2-CdN2S2 diatomic sites by anchoring Ag to the nodes of CdTMT (TMT = 2,4,6-trimercaptotriazine anion) coordination polymers. The Ag-CdTMT catalysts achieve approximately 100% Faradaic efficiency for CO reduction with an industrial level current density (∼200 mA cm-2 in H-cell). The embedded Ag atoms induce the formation of Ag-Cd diatomic sites with local microstrain, stretching Cd-N/S bonds, and reinforcing electron localization at Cd sites. The microstrain engineering and adjacent Ag atoms synergistically reduced Cd 4d-C 2p antibonding orbital occupancy for intensifying *COOH adsorption as the rate-determining step. This study provides novel insights into customizing the electronic structure of diatomic sites through strain engineering.

Modulating the Structure and Composition of Single-Atom Electrocatalysts for CO2 reduction

Electrochemical CO2 reduction reaction (eCO2 RR) is a promising strategy to achieve carbon cycling by converting CO2 into value-added products under mild reaction conditions. Recently, single-atom catalysts (SACs) have shown enormous potential in eCO2 RR due to their high utilization of metal atoms and flexible coordination structures. In this work, the recent progress in SACs for eCO2 RR is outlined, with detailed discussions on the interaction between active sites and CO2 , especially the adsorption/activation behavior of CO2 and the effects of the electronic structure of SACs on eCO2 RR. Three perspectives form the starting point: 1) Important factors of SACs for eCO2 RR; 2) Typical SACs for eCO2 RR; 3) eCO2 RR toward valuable products. First, how different modification strategies can change the electronic structure of SACs to improve catalytic performance is discussed; Second, SACs with diverse supports and how supports assist active sites to undergo catalytic reaction are introduced; Finally, according to various valuable products from eCO2 RR, the reaction mechanism and measures which can be taken to improve the selectivity of eCO2 RR are discussed. Hopefully, this work can provide a comprehensive understanding of SACs for eCO2 RR and spark innovative design and modification ideas to develop highly efficient SACs for CO2 conversion to various valuable fuels/chemicals.

Cooperative adsorption of interfacial Ga-N dual-site in GaOOH@N-doped carbon nanotubes for enhanced electrocatalytic reduction of carbon dioxide

To reduce activation energy barrier and promote the kinetics of electrocatalytic CO2 reduction reaction (eCO2RR), the performance of CO2 adsorption and activation on electrocatalysts should be optimized. Here, GaOOH is successfully coupled with N-doped carbon nanotubes (NC) via a facile self-assembly-calcination process. The obtained GaOOH@N-doped carbon nanotubes (Ga-NC) display the best CO faradaic efficiency of 96.1 % at -0.6 V (vs. reversible hydrogen electrode). Control-experiment and characterization results suggest Ga-N dual-site in interface between GaOOH and NC shows cooperative adsorption of CO2. C atom in CO2 is adsorbed on N site while O atom in CO2 is adsorbed on Ga site. This cooperative adsorption efficiently promotes the CO2 adsorption and activation performance, as well as the breaking of CO bond due to opposite attraction from Ga-N dual-site. Moreover, in-situ Fourier transform infrared spectroscopy confirms decreased reaction barrier for formation of *CO2- and *COOH intermediates. This work inspires us to construct interfacial dual-site structure with cooperative adsorption property for promoting eCO2RR activity.

One-dimensional single atom arrays on ferroelectric nanosheets for enhanced CO2 photoreduction

Single-atom catalysts show excellent catalytic performance because of their coordination environments and electronic configurations. However, controllable regulation of single-atom permutations still faces challenges. Herein, we demonstrate that a polarization electric field regulates single atom permutations and forms periodic one-dimensional Au single-atom arrays on ferroelectric Bi4Ti3O12 nanosheets. The Au single-atom arrays greatly lower the Gibbs free energy for CO2 conversion via Au-O=C=O-Au dual-site adsorption compared to that for Au-O=C=O single-site adsorption on Au isolated single atoms. Additionally, the Au single-atom arrays suppress the depolarization of Bi4Ti3O12, so it maintains a stronger driving force for separation and transfer of photogenerated charges. Thus, Bi4Ti3O12 with Au single-atom arrays exhibit an efficient CO production rate of 34.15 µmol·g-1·h-1, ∼18 times higher than that of pristine Bi4Ti3O12. More importantly, the polarization electric field proves to be a general tactic for the syntheses of one-dimensional Pt, Ag, Fe, Co and Ni single-atom arrays on the Bi4Ti3O12 surface.

Boosting the Proton-coupled Electron Transfer via Fe-P Atomic Pair for Enhanced Electrochemical CO2 Reduction

Single-atom catalysts exhibit superior CO2 -to-CO catalytic activity, but poor kinetics of proton-coupled electron transfer (PCET) steps still limit the overall performance toward the industrial scale. Here, we constructed a Fe-P atom paired catalyst onto nitrogen doped graphitic layer (Fe1 /PNG) to accelerate PCET step. Fe1 /PNG delivers an industrial CO current of 1 A with FECO over 90 % at 2.5 V in a membrane-electrode assembly, overperforming the CO current of Fe1 /NG by more than 300 %. We also decrypted the synergistic effects of the P atom in the Fe-P atom pair using operando techniques and density functional theory, revealing that the P atom provides additional adsorption sites for accelerating water dissociation, boosting the hydrogenation of CO2 , and enhancing the activity of CO2 reduction. This atom-pair catalytic strategy can modulate multiple reactants and intermediates to break through the inherent limitations of single-atom catalysts.

Structure-activity relationship of Cu-based catalysts for the highly efficient CO2 electrochemical reduction reaction

Electrocatalytic carbon dioxide reduction (CO₂RR) is a relatively feasible method to reduce the atmospheric concentration of CO₂. Although a series of metal-based catalysts have gained interest for CO₂RR, understanding the structure-activity relationship for Cu-based catalysts remains a great challenge. Herein, three Cu-based catalysts with different sizes and compositions (Cu@CNTs, Cu₄@CNTs, and CuNi₃@CNTs) were designed to explore this relationship by density functional theory (DFT). The calculation results show a higher degree of CO₂ molecule activation on CuNi₃@CNTs compared to that on Cu@CNTs and Cu₄@CNTs. The methane (CH₄) molecule is produced on both Cu@CNTs and CuNi₃@CNTs, while carbon monoxide (CO) is synthesized on Cu₄@CNTs. The Cu@CNTs showed higher activity for CH₄ production with a low overpotential value of 0.36 V compared to CuNi₃@CNTs (0.60 V), with *CHO formation considered the potential-determining step (PDS). The overpotential value was only 0.02 V for *CO formation on the Cu₄@CNTs, and *COOH formation was the PDS. The limiting potential difference analysis with the hydrogen evolution reaction (HER) indicated that the Cu@CNTs exhibited the highest selectivity of CH₄ among the three catalysts. Therefore, the sizes and compositions of Cu-based catalysts greatly influence CO₂RR activity and selectivity. This study provides an innovative insight into the theoretical explanation of the origin of the size and composition effects to inform the design of highly efficient electrocatalysts.

Localizing Tungsten Single Atoms around Tungsten Nitride Nanoparticles for Efficient Oxygen Reduction Electrocatalysis in Metal-Air Batteries

Combining isolated atomic active sites with those in nanoparticles for synergizing complex multistep catalysis is being actively pursued in the design of new electrocatalyst systems. However, these novel systems have been rarely studied due to the challenges with synthesis and analysis. Herein, a synergistically catalytic performance is demonstrated with a 0.89 V (vs reversible hydrogen electrode) onset potential in the four-step oxygen reduction reaction (ORR) by localizing tungsten single atoms around tungsten nitride nanoparticles confined into nitrogen-doped carbon (W SAs/WNNC). Through density functional theory calculations, it is shown that each of the active centers in the synergistic entity feature a specific potential-determining step in their respective reaction pathway that can be merged to optimize the intermediate steps involving scaling relations on individual active centers. Impressively, the W SAs/WNNC as the air cathode in all-solid-state Zn-air and Al-air batteries demonstrate competitive durability and reversibility, despite the acknowledged low activity of W-based catalyst toward the ORR.

Uncovering the Nature of Active Sites during Electrocatalytic Reactions by In Situ Synchrotron-Based Spectroscopic Techniques

Catalysts can effectively accelerate the reaction kinetics process and are recognized as the core to realize the conversion and supply of carbon-free energy. However, the active sites of catalysts, especially nanocatalysts, usually undergo dynamic structural evolution under realistic working conditions, which may be induced by various reaction effects such as the applied voltages, electrolytes, or adsorbed intermediates. Therefore, in-depth and systemic insights into the nature of the active sites involved under the working conditions are prerequisites for correlating structure-performance relationships. However, uncovering and identifying active sites under operation conditions are still formidable scientific and technical challenges, which are severely hindered by the complex physical and chemical processes occurring on the active sites. Meanwhile, complementary and important information could be missed by conducting only the conventionally employed ex situ microscopic and spectroscopic measurements. Accordingly, it is highly desirable for us to develop the ever-increasing in situ synchrotron-based techniques to identify the nature of active sites, which renders the rational design of functional catalysts achievable.In this Account, we elaborately highlight the substantial achievements in cutting-edge in situ X-ray spectroscopy (XAS) techniques by presenting several representative carbon-neutral electrocatalytic examples performed in our group to broadcast the principles and virtues of identifying the active sites and tracing intermediate species during electrocatalytic water splitting and electrocatalytic CO₂ reduction (ECR). Specifically, we believe that the interactions between the active sites and the support as well as the adsorption behaviors of intermediates are considered to be the important factors that govern the performance in the water splitting reaction. Meanwhile, the structural rearrangement of alloy catalysts driven by the cathodic potential significantly governs the activity and selectivity toward ECR. More importantly, the directions and suggestions for addressing the current limitations and pitfalls that we may encounter in the course of executing in situ experiments are also provided. Accordingly, it is necessary to use multiple in situ synchrotron-based techniques to obtain the comprehensive details. Furthermore, bridging the gap between the real energy devices and half-reactions could help us to approach the realistic mechanism. Beyond that, developing the rapid time resolution of in situ XAS will overcome the challenge of timescale mismatch to capture the faster structural kinetics of catalysts. Therefore, this Account is aimed to increase the awareness and appreciation of conducting in situ investigations on energy conversion reactions, which would be a guideline for us to explore catalytic scopes that remain challenging.

Identifying and tailoring C–N coupling site for efficient urea synthesis over diatomic Fe–Ni catalyst

Electrocatalytic urea synthesis emerged as the promising alternative of Haber–Bosch process and industrial urea synthetic protocol. Here, we report that a diatomic catalyst with bonded Fe–Ni pairs can significantly improve the efficiency of electrochemical urea synthesis. Compared with isolated diatomic and single-atom catalysts, the bonded Fe–Ni pairs act as the efficient sites for coordinated adsorption and activation of multiple reactants, enhancing the crucial C–N coupling thermodynamically and kinetically. The performance for urea synthesis up to an order of magnitude higher than those of single-atom and isolated diatomic electrocatalysts, a high urea yield rate of 20.2 mmol h⁻¹ g⁻¹ with corresponding Faradaic efficiency of 17.8% has been successfully achieved. A total Faradaic efficiency of about 100% for the formation of value-added urea, CO, and NH₃ was realized. This work presents an insight into synergistic catalysis towards sustainable urea synthesis via identifying and tailoring the atomic site configurations.

The direct electrocatalytic synthesis of urea via C–N coupling is of great significance. The authors report a diatomic catalyst with bonded Fe–Ni pairs to improve the efficiency of electrochemical urea synthesis from nitrate and CO₂.

PdFe Single-Atom Alloy Metallene for N2 Electroreduction

Single-atom alloys hold great promise for electrocatalytic nitrogen reduction reaction (NRR), while the comprehensive experimental/theoretical investigations of SAAs for the NRR are still missing. Herein, PdFe₁ single-atom alloy metallene, in which the Fe single atoms are confined on a Pd metallene support, is first developed as an effective and robust NRR electrocatalyst, delivering exceptional NRR performance with an NH₃ yield of 111.9 μg h^-1  mg^-1 , a Faradaic efficiency of 37.8 % at -0.2 V (RHE), as well as a long-term stability for 100 h electrolysis. In-depth mechanistic investigations by theoretical computations and operando X-ray absorption/Raman spectroscopy indentify Pd-coordinated Fe single atoms as active centers to enable efficient N₂ activation via N₂ -to-Fe σ-donation, reduced protonation energy barriers, suppressed hydrogen evolution and excellent thermodynamic stability, thus accounting for the high activity, selectivity and stability of PdFe₁ for the NRR.

Recent Advances in Interface Engineering for Electrocatalytic CO2 Reduction Reaction

Electrocatalytic CO₂ reduction reaction (CO₂RR) can store and transform the intermittent renewable energy in the form of chemical energy for industrial production of chemicals and fuels, which can dramatically reduce CO₂ emission and contribute to carbon-neutral cycle. Efficient electrocatalytic reduction of chemically inert CO₂ is challenging from thermodynamic and kinetic points of view. Therefore, low-cost, highly efficient, and readily available electrocatalysts have been the focus for promoting the conversion of CO₂. Very recently, interface engineering has been considered as a highly effective strategy to modulate the electrocatalytic performance through electronic and/or structural modulation, regulations of electron/proton/mass/intermediates, and the control of local reactant concentration, thereby achieving desirable reaction pathway, inhibiting competing hydrogen generation, breaking binding-energy scaling relations of intermediates, and promoting CO₂ mass transfer. In this review, we aim to provide a comprehensive overview of current developments in interface engineering for CO₂RR from both a theoretical and experimental standpoint, involving interfaces between metal and metal, metal and metal oxide, metal and nonmetal, metal oxide and metal oxide, organic molecules and inorganic materials, electrode and electrolyte, molecular catalysts and electrode, etc. Finally, the opportunities and challenges of interface engineering for CO₂RR are proposed.

Engineering Bismuth-Tin Interface in Bimetallic Aerogel with a 3D Porous Structure for Highly Selective Electrocatalytic CO2 Reduction to HCOOH

Electrochemical reduction of CO₂ (CO₂ RR) into valuable hydrocarbons is appealing in alleviating the excessive CO₂ level. We present the very first utilization of metallic bismuth-tin (Bi-Sn) aerogel for CO₂ RR with selective HCOOH production. A non-precious bimetallic aerogel of Bi-Sn is readily prepared at ambient temperature, which exhibits 3D morphology with interconnected channels, abundant interfaces and a hydrophilic surface. Superior to Bi and Sn, the Bi-Sn aerogel exposes more active sites and it has favorable mass transfer properties, which endow it with a high FE_HCOOH of 93.9 %. Moreover, the Bi-Sn aerogel achieves a FE_HCOOH of ca. 90 % that was maintained for 10 h in a flow battery. In situ ATR-FTIR measurements confirmed that the formation of *HCOO is the rate-determining step toward formic acid generation. DFT demonstrated the coexistence of Bi and Sn optimized the energy barrier for the production of HCOOH, thereby improving the catalytic activity.

Recent advances in single atom catalysts for the electrochemical carbon dioxide reduction reaction

The electrochemical carbon dioxide reduction reaction (CO2RR) offers a promising solution to mitigate carbon emission and at the same time generate valuable carbonaceous chemicals/fuels. Single atom catalysts (SACs) are encouraging to catalyze the electrochemical CO2RR due to the tunable electronic structure of the central metal atoms, which can regulate the adsorption energy of reactants and reaction intermediates. Moreover, SACs form a bridge between homogeneous and heterogeneous catalysts, providing an ideal platform to explore the reaction mechanism of electrochemical reactions. In this review, we first discuss the strategies for promoting the CO2RR performance, including suppression of the hydrogen evolution reaction (HER), generation of C1 products and formation of C2+ products. Then, we summarize the recent developments in regulating the structure of SACs toward the CO2RR based on the above aspects. Finally, several issues regarding the development of SACs for the CO2RR are raised and possible solutions are provided.

Three-dimensional porous copper-decorated bismuth-based nanofoam for boosting the electrochemical reduction of CO2 to formate

A Cu-decorated Bi/Bi₂O₃ nanofoam with a 3D porous network structure was assembled, which exhibits excellent electrocatalytic performance toward electrocatalytic CO₂ reduction owing to the optimized morphology and electronic structure.