Bi 2 O 3 Nanosheets Grown on Multi‐Channel Carbon Matrix to Catalyze Efficient CO 2 Electroreduction to HCOOH

Local hydroxide ion enrichment at the inner surface of lacunaris perovskite nanotubes facilitates the oxygen evolution reaction

Numerous strategies have been devised to optimize the intrinsic activity of perovskite oxides for the oxygen evolution reaction (OER). However, conventional synthetic routes typically yield limited numbers of active sites and low mass activities. More critically, the sluggish mass transfer poses a huge challenge, particularly under high polarization conditions, which impedes the overall reaction kinetics. Herein, lacunaris La0.5Pr0.25Ba0.25Co0.8Ni0.2O3-δ nanotubes (LPBCN-NTs) were prepared via electrospinning and post-annealing, which exhibited a small overpotential of 358.8 mV at 10 mA cm-2 and a lower Tafel slope of 71.46 mV dec-1, superior to the values for the same stoichiometric LPBCN nanoparticles and solid nanofibers, state-of-the-art counterparts and commercial IrO2. Density functional theory calculations revealed that the surface oxygen vacancies in LPBCN-NTs significantly lowered the OH- adsorption energy, while finite element analysis indicated that the precisely constructed lacunaris NT structure enriched the OH- concentration at its inner surface by an order of magnitude, both of which collectively resulted in accelerated OER kinetics. This study clarifies the underlying mechanism of how the lacunaris nanotubular architecture and the surface oxygen vacancies of perovskite oxides affect heterocatalysis, which undoubtedly paves the way to handling the long-standing issues of sluggish mass transfer rates and poor intrinsic catalytic activity.

Hierachical Aerogel-Supported Cu-Sn-Ox Solid Solutions for Highly Selective CO2 Electroreduction and Zn-CO2 Batteries

The electrochemical CO2 reduction reaction (CO2RR) into high-value carbon compounds such as CO and HCOOH is a promising strategy for the utilization and conversion of emitted CO2. However, the selectivity of the CO2RR for HCOOH is typically less than 90% and operates within a narrow voltage range, which limits its practical application. Herein, we propose a novel heterostructural aerogel as a highly efficient electrocatalyst for CO2RR to HCOOH. This catalyst consists of Cu-Sn-Ox solid solutions embedded in a reduced graphene oxide matrix (Cu-Sn-Ox/rGO). The incorporation of Cu2+ into the SnO2 matrix enhances HCOOH production by improving the adsorption of the *OCHO intermediate and inhibiting H2 evolution, as confirmed by in situ measurements and computational studies. As a result, Cu-Sn-Ox/rGO achieves a remarkable Faradaic efficiency (FE) of up to 91.4% for HCOOH and maintains high selectivity over a broad operating voltage range (-0.8 to -1.1 V). Additionally, the assembled Zn-CO2 batteries demonstrated an excellent power density of 1.14 mW/cm2 and exceptional stability for over 25 h.

Amphipathic Surfactant on Reconstructed Bismuth Enables Industrial-Level Electroreduction of CO2 to Formate

Developing efficient electrocatalysts for selective formate production via the electrochemical CO2 reduction reaction (CO2RR) is challenged by high overpotential, a narrow potential window of high Faradaic efficiency (FEformate), and limited current density (Jformate). Herein, we report a hierarchical BiOBr (CT/h-BiOBr) with surface-anchored cetyltrimethylammonium bromide (CTAB) for formate-selective large-scale CO2RR electrocatalysis. CT/h-BiOBr achieves over 90% FEformate across a wide potential range (-0.5 to -1.1 V) and an industrial-level Jformate surpassing 100 mA·cm-2 at -0.7 V. In situ investigations uncover the reconstructed Bi(110) surface as the active phase, with CTAB playing a dual role: its hydrophobic alkyl chains create a CO2-enriching microenvironment, while its polar head groups fine-tune the electronic structure, fostering a highly active phase. This work provides valuable insights into the role of surfactants in electrocatalysis and guides the design of electrocatalysts for the large-scale CO2RR.

Advances in the Stability of Catalysts for Electroreduction of CO2 to Formic Acid

The electroreduction of CO2 to high-value products is a promising approach for achieving carbon neutrality. Among these products, formic acid stands out as having the most potential for industrialization due to its optimal economic value in terms of consumption and output. In recent years, the Faraday efficiency of formic acid from CO2 electroreduction has reached 90~100 %. However, this high selectivity cannot be maintained for extended periods under high currents to meet industrial requirements. This paper reviews excellent work from the perspective of catalyst stability, summarizing and discussing the performance of typical catalysts. Strategies for preparing stable and highly active catalysts are also briefly described. This review may offer a useful data reference and valuable guidance for the future design of long-stability catalysts.

Achieving high selectivity and activity of CO2 electroreduction to formate by in-situ synthesis of single atom Pb doped Cu catalysts

Exploring highly selective and stable electrocatalysts is of great significance for the electrochemical conversion of CO2 into fuel. Herein, a three-dimensional (3D) nanostructure catalyst was developed by doping Pb single-atom (PbSA) in-situ on carbon paper (PbSA100-Cu/CP) through a low-energy and economical method. The designed catalyst exhibited abundant active sites and was beneficial to CO2 adsorption, activation, and subsequent conversion to fuel. Interestingly, PbSA100-Cu/CP showed a prominent Faraday efficiency (FE) of 97 % at -0.9 V versus reversible hydrogen electrode (vs. RHE) and a high partial current density of 27.9 mA·cm-2 for formate. Also, the catalyst remained significantly stable for 60 h during the durability test. The reaction mechanism was investigated by density functional theory (DFT), demonstrating that the doping PbSA induced the electrons redistribution, promoted the formate generation, reduced the rate-determining step (RDS) energy barrier, and inhibited the hydrogen evolution reaction. The study aims to provide a new strategy for developing of single-atom catalysts with high selectivity and stability, which will help reduce environmental pressure and alleviate energy problems.

Hydrogen-Bonded Organic Framework Supporting Atomic Bi-N2O2 Sites for High-Efficiency Electrocatalytic CO2 Reduction

Single atomic catalysts (SACs) offer a superior platform for studying the structure-activity relationships during electrocatalytic CO2 reduction reaction (CO2RR). Yet challenges still exist to obtain well-defined and novel site configuration owing to the uncertainty of functional framework-derived SACs through calcination. Herein, a novel Bi-N2O2 site supported on the (1 1 0) plane of hydrogen-bonded organic framework (HOF) is reported directly for CO2RR. In flow cell, the target catalyst Bi1-HOF maintains a faradaic efficiency (FE) HCOOH of over 90 % at a wide potential window of 1.4 V. The corresponding partial current density ranges from 113.3 to 747.0 mA cm-2. And, Bi1-HOF exhibits a long-term stability of over 30 h under a successive potential-step test with a current density of 100-400 mA cm-2. Density function theory (DFT) calculations illustrate that the novel Bi-N2O2 site supported on the (1 1 0) plane of HOF effectively induces the oriented electron transfer from Bi center to CO2 molecule, reaching an enhanced CO2 activation and reduction. Besides, this study offers a versatile method to reach series of M-N2O2 sites with regulable metal centers via the same intercalation mechanism, broadening the platform for studying the structure-activity relationships during CO2RR.

Promoting hydrophilic cupric oxide electrochemical carbon dioxide reduction to methanol via interfacial engineering modulation

Copper-based catalysts have been extensively investigated in electrochemical carbon dioxide (CO2) reduction to promote carbon products generated by requiring multiple electron transfer. However, hydrophilic electrodes are unfavourable for CO2 mass transfer and preferentially hydrogen (H2) evolution in electrochemical CO2 reduction. In this paper, a hydrophilic cupric oxide (CuO) electrode with a grassy morphology was prepared. CuO-derived Cu was confirmed as the active site for electrochemical CO2 reduction through wettability modulation. To enhance the intrinsic catalytic activity, a metal-oxide heterogeneous interface was created by engineering modulation at the interface, involving the loading of palladium (Pd) on CuO (CuO/Pd). Both the electrochemically active area and the electron transfer rate were enhanced by Pd loading, and significantly the reduced work function further facilitated the electron transfer between the electrode surface and the electrolyte. Consequently, the CuO/Pd electrode exhibited excellent excellent performance in electrochemical CO2 reduction, achieving a 54 % Faraday efficiency at -0.65 V for methanol (CH3OH). The metal-oxide interfacial effect potentially improves the intrinsic catalytic activity of hydrophilic CuO electrodes in electrochemical CO2 reduction, providing a conducive pathway for optimizing hydrophilic oxide electrodes in this process.

Highly selective electrocatalytic reduction of CO2 to HCOOH over an in situ derived Ag-loaded Bi2O2CO3 electrocatalyst

The electrochemical reduction of CO2 to HCOOH is considered one of the most appealing routes to alleviate the energy crisis and close the anthropogenic CO2 cycle. However, it remains challenging to develop electrocatalysts with high activity and selectivity towards HCOOH in a wide potential window. In this regard, Ag/Bi2O2CO3 was prepared by an in situ electrochemical transformation from Ag/Bi2O3. The Ag/Bi2O2CO3 catalyst achieves a faradaic efficiency (FE) of over 90% for HCOOH in a wide potential window between -0.8 V and -1.3 V versus the reversible hydrogen electrode (RHE). Moreover, a maximum FE of 95.8% and a current density of 15.3 mA cm-2 were achieved at a low applied potential of -1.1 V. Density functional theory (DFT) calculations prove that the high catalytic activity of Ag/Bi2O2CO3 is ascribed to the fact that Ag can regulate the electronic structure of Bi, thus facilitating the adsorption of *OCHO and hindering the adsorption of *COOH. This work expands the in situ electrochemical derivatization strategy for the preparation of electrocatalysts.

Reconstructed Bismuth Oxide through in situ Carbonation by Carbonate-containing Electrolyte for Highly Active Electrocatalytic CO2 Reduction to Formate

The catalyst-reconstruction makes it challenging to clarify the practical active sites and unveil the actual reaction mechanism during the CO2 electroreduction reaction (CO2 RR). However, currently the impact of the electrolyte microenvironment in which the electrolyte is in contact with the catalyst is overlooked and might induce a chemical evolution, thus confusing the reconstruction process and mechanism. In this work, the carbonate adsorption properties of metal oxides were investigated, and the mechanism of how the electrolyte carbonate affect the chemical evolution of catalysts were discussed. Notably, Bi2 O3 with weak carbonate adsorption underwent a chemical reconstruction to form the Bi2 O2 CO3 /Bi2 O3 heterostructure. Furthermore, in situ and ex situ characterizations unveiled the formation mechanism of the heterostructure. The in situ formed Bi2 O2 CO3 /Bi2 O3 heterostructure with strong electron interaction served as the highly active structure for CO2 RR, achieving a formate Faradaic efficiency of 98.1 % at -0.8 Vvs RHE . Theoretical calculations demonstrate that the significantly tuned p-orbit electrons of the Bi sites in Bi2 O2 CO3 /Bi2 O3 optimized the adsorption of the intermediate and lowered the energy barrier for the formation of *OCHO. This work elucidates the mechanism of electrolyte microenvironment for affecting catalyst reconstruction, which contributes to the understanding of reconstruction process and clarification of the actual catalytic structure.

Grain Boundary Tailors the Local Chemical Environment on Iridium Surface for Alkaline Electrocatalytic Hydrogen Evolution

Even though grain boundaries (GBs) have been previously employed to increase the number of active catalytic sites or tune the binding energies of reaction intermediates for promoting electrocatalytic reactions, the effect of GBs on the tailoring of the local chemical environment on the catalyst surface has not been clarified thus far. In this study, a GBs-enriched iridium (GB-Ir) was synthesized and examined for the alkaline hydrogen evolution reaction (HER). Operando Raman spectroscopy and density functional theory (DFT) calculations revealed that a local acid-like environment with H3 O+ intermediates was created in the GBs region owing to the electron-enriched surface Ir atoms at the GBs. The H3 O+ intermediates lowered the energy barrier for water dissociation and provided enough hydrogen proton to promote the generation of hydrogen spillover from the sites at the GBs to the sites away from the GBs, thus synergistically enhancing the hydrogen evolution activity. Notably, the GB-Ir catalyst exhibited a high alkaline HER activity (10 mV @ 10 mA cm-2 , 20 mV dec-1 ). We believe that our findings will promote further research on GBs and the surface science of electrochemical reactions.

Anion-Mediated In Situ Reconstruction of the Bi2MoO6 Precatalyst for Enhanced Electrochemical CO2 Reduction over a Wide Potential Window

Electrochemical CO2 reduction reaction (eCO2RR) is a viable approach to achieve carbon neutrality. Bismuth-based electrocatalysts demonstrate exceptional selectivity in CO2-to-formate conversion, but their reconstruction mechanisms during the eCO2RR remain elusive. Herein, the reconstruction processes of bismuth molybdate (Bi2MoO6) nanoplates are elucidated during the eCO2RR. Operando and ex situ measurements reveal the in situ partial reduction of Bi2MoO6 to Bi metal, forming Bi@Bi2MoO6 at negative potentials. Meanwhile, CO32- ions in the electrolyte spontaneously exchange with MoO42- in Bi2MoO6. The obtained Bi@Bi2MoO6/Bi2O2CO3 delivers a formate Faradaic efficiency (FE) of 95.2% at -1.0 V. Notably, high formate FEs (>90%) are maintained within a wide 500 mV window. Although computational calculations indicate a higher energy barrier for *OCHO formation on Bi2O2CO3, the prevention of excessive reduction to metal Bi significantly enhances long-term stability. Furthermore, the CO32- ion exchange process occurs in various 2D Bi-containing precatalysts, which should be emphasized in further studies.

Active-site stabilized Bi metal-organic framework-based catalyst for highly active and selective electroreduction of CO2 to formate over a wide potential window

Bismuth-based materials have been validated to be a kind of effective electrocatalyst for electrocatalytic CO2 reduction (ECR) to formate (HCOO-). However, the established studies still encounter the problems of low current density, low selectivity, narrow potential window, and poor catalyst stability. Herein, a bismuth-terephthalate framework (Bi-BDC MOF) material was successfully synthesized. The optimized Bi-BDC-120 °C exhibited excellent activity, selectivity, and durability for formate production. At an operating potential of -1.1 V vs. RHE in 0.1 mol L-1 KHCO3 electrolyte, the ECR catalyzed by Bi-BDC-120 °C achieved a Faraday efficiency (FE) of 97.2% towards formate generation, and the total current density reached about 30 mA cm-2. The operating potential window with FEformate values > 95% ranged in -0.9 to -1.5 V vs. RHE. The density-functional theory (DFT) calculation demonstrated that the (001) crystalline planes of Bi-BDC are preferable for the adsorption of CO2 and the conversion of *OCHO intermediates, thus ultimately promoting the electrocatalytic production of formate. Although the MOF structure of Bi-BDC-120 °C was insufficiently stabilized, the FEformate could be maintained at around 90% after 36 h of ECR operation. The long-term durability for formate production was attributed to the fact that the in situ reconstructed Bi2O2CO3 could retain the Bi-O active sites in the structure. These results offer an opportunity to design CO2 reduction electrocatalysts with high activity and selectivity for potential applications.

Conceptual study on extraction of formic acid from the electrolyte after electroreduction of CO2: Desalination and dehydration using a high-silica chabazite zeolite membrane

Here, we propose a two-step pervaporation system with a high-silica CHA (chabazite) membrane, which has sufficient resistance to water and acid, to demonstrate the extraction and condensation of the formic acid formed by electroreduction of CO2. The kinetic diameters of water and formic acid are similar and smaller than the pore size of CHA, while the hydrated electrolyte ions (e.g., K+ and Cl-) are larger than the pore size of CHA. Consequently, the electrolyte ions are separated from the mixture of water and formic acid in the first desalination process, and then water molecules are easily removed from the mixture in the second dehydration process. From 300 ml of an approximately 3 wt% formic acid aqueous solution containing 0.5 M KCl, 10 ml of 18.2 wt% formic acid was obtained.

Zn-induced electron-rich Sn catalysts enable highly efficient CO2 electroreduction to formate

Renewable-energy-driven CO2 electroreduction provides a promising way to address the growing greenhouse effect issue and produce value-added chemicals. As one of the bulk chemicals, formic acid/formate has the highest revenue per mole of electrons among various products. However, the scaling up of CO2-to-formate for practical applications with high faradaic efficiency (FE) and current density is constrained by the difficulty of precisely reconciling the competing intermediates (*COOH and HCOO*). Herein, a Zn-induced electron-rich Sn electrocatalyst was reported for CO2-to-formate with high efficiency. The faradaic efficiency of formate (FEformate) could reach 96.6%, and FEformate > 90% was maintained at formate partial current density up to 625.4 mA cm-1. Detailed study indicated that catalyst reconstruction occurred during electrolysis. With appropriate electron accumulation, the electron-rich Sn catalyst could facilitate the adsorption and activation of CO2 molecules to form a intermediate and then promoted the carbon protonation of to yield a HCOO* intermediate. Afterwards, the HCOO* → HCOOH* proceeded via another proton-coupled electron transfer process, leading to high activity and selectivity for formate production.

Indium Cyanamide for Industrial-Grade CO2 Electroreduction to Formic Acid

Developing industrial-grade electroreduction of CO2 to produce formate (HCOO-)/formic acid (HCOOH) depends on highly active electrocatalysts. However, structural changes due to the inevitable self-reduction of catalysts result in severe long-term stability issues at industrial-grade current density. Herein, linear cyanamide anion ([NCN]2-)-constructed indium cyanamide nanoparticles (InNCN) were investigated for CO2 reduction to HCOO- with a Faradaic efficiency of up to 96% under a partial current density (jformate) of 250 mA cm-2. Bulk electrolysis at a jformate of 400 mA cm-2 requires only -0.72 VRHE applied potential with iR correction. It also achieves continuous production of pure HCOOH at ∼125 mA cm-2 for 160 h. The excellent activity and stability of InNCN are attributed to its unique structural features, including strongly σ-donating [NCN]2- ligands, the potential structural transformation of [N═C═N]2- and [N≡C-N]2-, and the open framework structure. This study affirms metal cyanamides as promising novel materials for electrocatalytic CO2 reduction, broadening the variety of CO2 reduction catalysts and the understanding of structure-activity relationships.

Engineering Under-Coordinated Active Sites with Tailored Chemical Microenvironments over Mosaic Bismuth Nanosheets for Selective CO2 Electroreduction to Formate

Selective electrochemical reduction of CO₂ into fuels or chemical feedstocks is a promising avenue to achieve carbon-neutral goal, but its development is severely limited by the lack of highly efficient electrocatalysts. Herein, cation-exchange strategy is combined with electrochemical self-reconstruction strategy to successfully develop diethylenetriamine-functionalized mosaic Bi nanosheets (mBi-DETA NSs) for selective electrocatalytic CO₂ reduction to formate, delivering a superior formate Faradaic efficiency of 96.87% at a low potential of -0.8 V_RHE . Mosaic nanosheet morphology of Bi can sufficiently expose the under-coordinated Bi active sites and promote the activation of CO₂ molecules to form the OCHO^- * intermediate. Moreover, in situ attenuated total reflectance infrared spectra further corroborate that surface chemical microenvironment modulation of mosaic Bi nanosheets via DETA functionalization can improve CO₂ adsorption on the catalyst surface and stabilize the key intermediate (OCHO^- *) due to the presence of amine groups, thus facilitate the CO₂ -to-HCOO^- reaction kinetics and promote formate formation.

Recent Development Strategies for Bismuth-Driven Materials in Sustainable Energy Systems and Environmental Restoration

Bismuth(Bi)-based materials have gained considerable attention in recent decades for use in a diverse range of sustainable energy and environmental applications due to their low toxicity and eco-friendliness. Bi materials are widely employed in electrochemical energy storage and conversion devices, exhibiting excellent catalytic and non-catalytic performance, as well as CO₂ /N₂ reduction and water treatment systems. A variety of Bi materials, including its oxides, chalcogenides, oxyhalides, bismuthates, and other composites, have been developed for understanding their physicochemical properties. In this review, a comprehensive overview of the properties of individual Bi material systems and their use in a range of applications is provided. This review highlights the implementation of novel strategies to modify Bi materials based on morphological and facet control, doping/defect inclusion, and composite/heterojunction formation. The factors affecting the development of different classes of Bi materials and how their control differs between individual Bi compounds are also described. In particular, the development process for these material systems, their mass production, and related challenges are considered. Thus, the key components in Bi compounds are compared in terms of their properties, design, and applications. Finally, the future potential and challenges associated with Bi complexes are presented as a pathway for new innovations.

Bionic Construction of Helical Bi2 O3 Microfibers for Highly Efficient CO2 Electroreduction

Helical Bi₂ O₃ microfibers (HBM) were prepared with the assistance of cotton template through a simple heating treatment in air. This twisted structure induced the lattice strains, enriched the oxygen vacancies of Bi₂ O₃ , and promoted the sufficient exposure of active sites simultaneously, thus performing outstanding activity and selectivity as catalyst for CO₂ electroreduction to formate. The faradaic efficiency (FE) of formate reached 100.4±1.9 % at -0.90 V vs. reversible hydrogen electrode (RHE) in an H-cell, and the partial current density was boosted to 226 mA cm^-2 with FE_formate of 96 % at -1.08 V vs. RHE in a flow cell. This work may open a new era for construction of metal oxide fibers by bionic strategy as high-performance electrocatalysts.

Driving up the Electrocatalytic Performance for Carbon Dioxide Conversion through Interface Tuning in Graphene Oxide-Bismuth Oxide Nanocomposites

The integration of graphene oxide (GO) into nanostructured Bi₂O₃ electrocatalysts for CO₂ reduction (CO₂RR) brings up remarkable improvements in terms of performance toward formic acid (HCOOH) production. The GO scaffold is able to facilitate electron transfers toward the active Bi₂O₃ phase, amending for the high metal oxide (MO) intrinsic electric resistance, resulting in activation of the CO₂ with smaller overpotential. Herein, the structure of the GO-MO nanocomposite is tailored according to two synthetic protocols, giving rise to two different nanostructures, one featuring reduced GO (rGO) supporting Bi@Bi₂O₃ core-shell nanoparticles (NP) and the other GO supporting fully oxidized Bi₂O₃ NP. The two structures differentiate in terms of electrocatalytic behavior, suggesting the importance of constructing a suitable interface between the nanocarbon and the MO, as well as between MO and metal.

Water induced ultrathin Mo2C nanosheets with high-density grain boundaries for enhanced hydrogen evolution

Grain boundary controlling is an effective approach for manipulating the electronic structure of electrocatalysts to improve their hydrogen evolution reaction performance. However, probing the direct effect of grain boundaries as highly active catalytic hot spots is very challenging. Herein, we demonstrate a general water-assisted carbothermal reaction strategy for the construction of ultrathin Mo₂C nanosheets with high-density grain boundaries supported on N-doped graphene. The polycrystalline Mo₂C nanosheets are connected with N-doped graphene through Mo-C bonds, which affords an ultra-high density of active sites, giving excellent hydrogen evolution activity and superior electrocatalytic stability. Theoretical calculations reveal that the d_z² orbital energy level of Mo atoms is controlled by the MoC₃ pyramid configuration, which plays a vital role in governing the hydrogen evolution activity. The d_z² orbital energy level of metal atoms exhibits an intrinsic relationship with the catalyst activity and is regarded as a descriptor for predicting the hydrogen evolution activity.

Tailoring of Active Sites from Single to Dual Atom Sites for Highly Efficient Electrocatalysis

Single atom catalysts (SACs) have been attracting extensive attention in electrocatalysis because of their unusual structure and extreme atom utilization, but the low metal loading and unified single site induced scaling relations may limit their activity and practical application. Tailoring of active sites at the atomic level is a sensible approach to break the existing limits in SACs. In this review, SACs were first discussed regarding carbon or non-carbon supports. Then, five tailoring strategies were elaborated toward improving the electrocatalytic activity of SACs, namely strain engineering, spin-state tuning engineering, axial functionalization engineering, ligand engineering, and porosity engineering, so as to optimize the electronic state of active sites, tune d orbitals of transition metals, adjust adsorption strength of intermediates, enhance electron transfer, and elevate mass transport efficiency. Afterward, from the angle of inducing electron redistribution and optimizing the adsorption nature of active centers, the synergistic effect from adjacent atoms and recent advances in tailoring strategies on active sites with binuclear configuration which include simple, homonuclear, and heteronuclear dual atom catalysts (DACs) were summarized. Finally, a summary and some perspectives for achieving efficient and sustainable electrocatalysis were presented based on tailoring strategies, design of active sites, and in situ characterization.

Core-Shell Hierarchical Fe/Cu Bimetallic Fenton Catalyst with Improved Adsorption and Catalytic Performance for Congo Red Degradation

The preparation of heterogeneous Fenton catalysts with both adsorption and catalytic properties has become an effective strategy for the treatment of refractory organic wastewater. In this work, 4A-Fe@Cu bimetallic Fenton catalysts with a three-dimensional core-shell structure were prepared by a simple, template-free, and surfactant-free methodology and used in the adsorption and degradation of Congo red (CR). The results showed that the open three-dimensional network structure and the positive charge of the surface of the 4A-Fe@Cu catalyst could endow a high adsorption capacity for CR, reaching 432.9 mg/g. The good adsorption property of 4A-Fe@Cu for CR not only did not inactivate the catalytic site on 4A-Fe@Cu but also could promote the contact between CR and the active sites on the catalyst surface and accelerate the degradation process. The 4A-Fe@Cu bimetallic catalyst exhibited higher catalytic activity than monometallic 4A@Cu and/or 4A-Fe catalysts due to low work function value. The effects of different pH, H2O2 dosages, and catalyst dosages on the catalytic performance of 4A-Fe@Cu were explored. In the conditions of 7.2 mM H2O2, 2 g/L 4A-Fe@Cu, and 1 g/L CR solution, the degradation ratio of CR by 4A-Fe@Cu could reach 99.2% at pH 8. This strategy provided guidance to the design of high-performance Fenton-like catalysts with both adsorption and catalysis properties for dye wastewater treatment.

The Critical Role of Initial/Operando Oxygen Loading in General Bismuth-Based Catalysts for Electroreduction of Carbon Dioxide

Operando reconstruction of solid catalyst into a distinct active state frequently occurs during electrocatalytic processes. The correlation between initial and operando states, if ever existing, is critical for the understanding and precise design of a catalytic system. Inspired by recently established intermediate metallic state of Bi-based catalysts during electrocatalytic carbon dioxide reduction (CO₂RR), here we investigate a series of Bi oxide catalysts (Bi, Bi₂O₃, BiO₂) and demonstrate that the operando surface/subsurface oxygen loading, positively correlated to the initial oxygen content, plays a critical role in determining Bi-based CO₂RR performance. Higher initial oxygen loading indicates a better electrocatalytic efficiency. Further analysis shows that this conclusion generally applies to all Bi-based electrocatalysts reported up to date. Following this principle, cost-effective BiO₂ nanocrystals demonstrated the highest formate Faradaic efficiency (FE) and current density compared to Bi/Bi₂O₃, further allowing a pair-electrolysis system with 800 mA/cm² current density and an overall 175% FE for formate production.

Triple-Phase Interface Engineered Hierarchical Porous Electrode for CO2 Electroreduction to Formate

The aqueous electrochemical CO₂ reduction to valuable products is seen as one of the most promising candidates to achieve carbon neutrality yet still suffers from poor selectivity and lower current density. Highly efficient CO₂ reduction significantly relies on well-constructed electrode to realize efficient and stable triple-phase contact of CO₂ , electrolyte, and active sites. Herein, a triple-phase interface engineering approach featuring the combination of hierarchical porous morphology design and surface modification is presented. A hierarchical porous electrode is constructed by depositing bismuth nanosheet array on copper foam followed by trimethoxy (1H,1H,2H,2H-heptadecafluorodecyl) silane modification on the nanosheet surface. This electrode not only achieves highly selective and efficient CO₂ reduction performance with formate selectivity above 90% over wide potentials and a partial current density over -90 mA cm^-2 in H-cell but also maintains a superior stability during the long-term operation. It is demonstrated that this remarkable performance is attributed to the construction of efficient and stable triple-phase interface. Theoretical calculations also show that the modified surface optimizes the activation path by lowering thermodynamic barriers of the key intermediates *OCHO for the formation of formate during electrochemical CO₂ reduction.

MOF-Transformed In2O3-x@C Nanocorn Electrocatalyst for Efficient CO2 Reduction to HCOOH

For electrochemical CO₂ reduction to HCOOH, an ongoing challenge is to design energy efficient electrocatalysts that can deliver a high HCOOH current density (J_HCOOH) at a low overpotential. Indium oxide is good HCOOH production catalyst but with low conductivity. In this work, we report a unique corn design of In₂O_3-x@C nanocatalyst, wherein In₂O_3-x nanocube as the fine grains dispersed uniformly on the carbon nanorod cob, resulting in the enhanced conductivity. Excellent performance is achieved with 84% Faradaic efficiency (FE) and 11 mA cm^-2 J_HCOOH at a low potential of - 0.4 V versus RHE. At the current density of 100 mA cm^-2, the applied potential remained stable for more than 120 h with the FE above 90%. Density functional theory calculations reveal that the abundant oxygen vacancy in In₂O_3-x has exposed more In³⁺ sites with activated electroactivity, which facilitates the formation of HCOO* intermediate. Operando X-ray absorption spectroscopy also confirms In³⁺ as the active site and the key intermediate of HCOO* during the process of CO₂ reduction to HCOOH.

Metal-Organic Framework-Derived BiIn Bimetallic Oxide Nanoparticles Embedded in Carbon Networks for Efficient Electrochemical Reduction of CO2 to Formate

Bismuth-based catalysts exhibit excellent activity and selectivity for the electroreduction of carbon dioxide (CO₂). However, single-component bismuth-based catalysts are not satisfactory for the electrochemical reduction of CO₂ to formic acid, mainly due to their high hydrogen production, low electrical conductivity, and small catalytic current density. Herein, we used a coordination strategy to recombine Bi and In at the molecular level to form Bi/In bimetallic metal-organic frameworks (MOFs), which were then calcined to obtain MOF-derived Bi/In bimetallic oxide nanoparticles embedded in carbon networks. Thanks to the synergistic effect of bimetallic components, high specific surface area, suitable pore size distribution, and high electrical conductivity of the carbon network, the material exhibits excellent activity and selectivity for electroreduction of CO₂ to formate. In H-type electrolyzers, the formate Faradaic efficiency reaches 91% at -0.9 V (vs RHE) and does not decrease significantly within 48 h. In situ Fourier transform infrared spectroscopy confirms the reaction intermediates and reveals that CO₂ electroreduction is dominant by the *OCHO pathway.

Enhanced electrocatalytic reduction of CO2 to formate via doping Ce in Bi2O3 nanosheets

Formate is considered as the most economically viable product of the prevalent electrochemical CO₂ reduction (ECR) products. However, most of the catalysts for ECR to formate in aqueous solution often suffer from low activity and limited selectivity. Herein, we report a novel Ce-doped Bi₂O₃ nanosheet (NS) electrocatalyst by a facile solvothermal method for highly efficient ECR to formate. The 5.04% Ce-doped Bi₂O₃ NSs exhibited a current density of 37.4 mA cm^-2 for the production of formate with a high formate faradaic efficiency (FE) of 95.8% at -1.12 V. The formate FE was stably maintained at about 90% in a wide potential range from -0.82 to -1.22 V. More importantly, density functional theory (DFT) calculations revealed that Ce doping can lead to a significant synergistic effect, which promotes the formation and the adsorption of the OCHO* intermediate for ECR, while significantly inhibiting the hydrogen evolution reaction via depressing the formation of *H, thus helping achieve high current density and FE. This work provides an effective and promising strategy to develop efficient electrocatalysts with heteroatom doping and new insights for boosting ECR into formate.

Indium doped bismuth subcarbonate nanosheets for efficient electrochemical reduction of carbon dioxide to formate in a wide potential window

Electrochemical carbon dioxide (CO₂) reduction reaction (E-CO₂RR) to formate with high selectivity driven by renewable electricity is one of the most promising routes to carbon neutrality. Herein, we developed a novel indium (In)-doped bismuth subcarbonate (BOC) nanosheets (BOC-In-x NSs) through transformation of In-doped bismuth (Bi) nanoblocks (Bi-In-x NBs). The BOC-In-0.1 NSs achieved a maximum Faraday efficiency of formate (FE_formate) nearly 100% with high stability (22 h) and an appreciable average FE_formate of 93.5% in a wide potential window of 450 mV. The experimental and theoretical calculations indicate that the incorporation of In into BOC nanosheets enhanced the adsorption of CO₂ and the intermediates during the process of E-CO₂RR, and reduced the energy barrier for the formation of formate.

Heterostructured Bi-Cu2S nanocrystals for efficient CO2 electroreduction to formate

The electrochemical CO₂ reduction reaction (ECO₂RR) driven by renewable electricity holds promise to store intermittent energy in chemical bonds, while producing value-added chemicals and fuels sustainably. Unfortunately, it remains a grand challenge to simultaneously achieve a high faradaic efficiency (FE), a low overpotential, and a high current density of the ECO₂RR. Herein, we report the synthesis of heterostructured Bi-Cu₂S nanocrystals via a one-pot solution-phase method. The epitaxial growth of Cu₂S on Bi leads to abundant interfacial sites and the resultant heterostructured Bi-Cu₂S nanocrystals enable highly efficient ECO₂RR with a largely reduced overpotential (240 mV lower than that of Bi), a near-unity FE (>98%) for formate production, and a high partial current density (2.4- and 5.2-fold higher J_HCOO^- than Cu₂S and Bi at -1.0 V vs. reversible hydrogen electrode, RHE). Density functional theory (DFT) calculations show that the electron transfer from Bi to Cu₂S at the interface leads to the preferential stabilization of the formate-evolution intermediate (*OCHO).

Operando Converting BiOCl into Bi2O2(CO3)xCly for Efficient Electrocatalytic Reduction of Carbon Dioxide to Formate

Bismuth-based materials (e.g., metallic, oxides and subcarbonate) are emerged as promising electrocatalysts for converting CO₂ to formate. However, Bi^o-based electrocatalysts possess high overpotentials, while bismuth oxides and subcarbonate encounter stability issues. This work is designated to exemplify that the operando synthesis can be an effective means to enhance the stability of electrocatalysts under operando CO₂RR conditions. A synthetic approach is developed to electrochemically convert BiOCl into Cl-containing subcarbonate (Bi₂O₂(CO₃)_xCl_y) under operando CO₂RR conditions. The systematic operando spectroscopic studies depict that BiOCl is converted to Bi₂O₂(CO₃)_xCl_y via a cathodic potential-promoted anion-exchange process. The operando synthesized Bi₂O₂(CO₃)_xCl_y can tolerate - 1.0 V versus RHE, while for the wet-chemistry synthesized pure Bi₂O₂CO₃, the formation of metallic Bi^o occurs at - 0.6 V versus RHE. At - 0.8 V versus RHE, Bi₂O₂(CO₃)_xCl_y can readily attain a FE_HCOO- of 97.9%, much higher than that of the pure Bi₂O₂CO₃ (81.3%). DFT calculations indicate that differing from the pure Bi₂O₂CO₃-catalyzed CO₂RR, where formate is formed via a ^*OCHO intermediate step that requires a high energy input energy of 2.69 eV to proceed, the formation of HCOO^- over Bi₂O₂(CO₃)_xCl_y has proceeded via a ^*COOH intermediate step that only requires low energy input of 2.56 eV.

Synthesis of N-Doped Highly Graphitic Carbon Urchin-Like Hollow Structures Loaded with Single-Ni Atoms towards Efficient CO2 Electroreduction

The rational design of single-atom catalysts featuring excellent conductivity, highly accessible discrete active sites and favorable mass transfer is crucial for electrocatalysis but remains challenging. In this study, a reliable Ni-catalyzed and Ni-templated strategy is developed to synthesize a single-atom catalyst by transforming metallic Ni into single-Ni atoms anchored on hollow porous urchin-like (HPU) N-doped carbon (NC) (designated as Ni-NC(HPU)), which possesses high crystallinity and sufficient Ni-N₄ moiety (2.4 wt %). The unique hollow thorns on the surface, good conductivity and large external surface area facilitate electron/mass transfer and exposure of single-Ni sites. As a result, the Ni-NC(HPU) catalyst exhibits remarkable activity and high stability for CO₂ electroreduction. Moreover, this synthetic strategy can also be facilely extended to prepare distinct hollow porous architectures with similar components, such as the wire- and sphere-like ones.

Active and conductive layer stacked superlattices for highly selective CO2 electroreduction

Metal oxides are archetypal CO₂ reduction reaction electrocatalysts, yet inevitable self-reduction will enhance competitive hydrogen evolution and lower the CO₂ electroreduction selectivity. Herein, we propose a tangible superlattice model of alternating metal oxides and selenide sublayers in which electrons are rapidly exported through the conductive metal selenide layer to protect the active oxide layer from self-reduction. Taking BiCuSeO superlattices as a proof-of-concept, a comprehensive characterization reveals that the active [Bi₂O₂]²⁺ sublayers retain oxidation states rather than their self-reduced Bi metal during CO₂ electroreduction because of the rapid electron transfer through the conductive [Cu₂Se₂]^2- sublayer. Theoretical calculations uncover the high activity over [Bi₂O₂]²⁺ sublayers due to the overlaps between the Bi p orbitals and O p orbitals in the OCHO* intermediate, thus achieving over 90% formate selectivity in a wide potential range from −0.4 to −1.1 V. This work broadens the studying and improving of the CO₂ electroreduction properties of metal oxide systems.

It is important yet challenging to improve the interface contact between commonly used carbon conductive layer and metal oxide catalysts. Here the authors propose BiCuSeO with alternant active [Bi₂O₂]²⁺ sublayers and conductive [Cu₂Se₂]^2- sublayer within its superlattice as efficient catalyst for CO₂ electroreduction to formate.

Ionic liquid-based electrolytes for CO2 electroreduction and CO2 electroorganic transformation

CO₂ is an abundant and renewable C1 feedstock. Electrochemical transformation of CO₂ can integrate CO₂ fixation with renewable electricity storage, providing an avenue to close the anthropogenic carbon cycle. As a new type of green and chemically tailorable solvent, ionic liquids (ILs) have been proposed as highly promising alternatives for conventional electrolytes in electrochemical CO₂ conversion. This review summarizes major advances in the electrochemical transformation of CO₂ into value-added carbonic fuels and chemicals in IL-based media in the past several years. Both the direct CO₂ electroreduction (CO₂ER) and CO₂-involved electroorganic transformation (CO₂EOT) are discussed, focusing on the effect of electrocatalysts, IL components, reactor configurations and operating conditions on catalytic activity, selectivity and reusability. The reasons for the enhanced CO₂ conversion performance by ILs are also discussed, providing guidance for the rational design of novel IL-based electrochemical processes for CO₂ conversion. Finally, the critical challenges remaining in this research area and promising directions for future research are proposed.

Oxygen-induced degradation in AgBiS2 nanocrystal solar cells

AgBiS₂ nanocrystal solar cells are among the most sustainable emerging photovoltaic technologies. Their environmentally-friendly composition and low energy consumption during fabrication make them particularly attractive for future applications. However, much remains unknown about the stability of these devices, in particular under operational conditions. In this study, we explore the effects of oxygen and light on the stability of AgBiS₂ nanocrystal solar cells and identify its dependence on the charge extraction layers. Normally, the rate of oxygen-induced degradation of nanocrystals is related to their ligands, which determine the access sites by steric hindrance. We demonstrate that the ligands, commonly used in AgBiS₂ solar cells, also play a crucial chemical role in the oxidation process. Specifically, we show that the tetramethylammonium iodide ligands enable their oxidation, leading to the formation of bismuth oxide and silver sulphide. Additionally, the rate of oxidation is impacted by the presence of water, often present at the surface of the ZnO electron extraction layer. Moreover, the degradation of the organic hole extraction layer also impacts the overall device stability and the materials' photophysics. The understanding of these degradation processes is necessary for the development of mitigation strategies for future generations of more stable AgBiS₂ nanocrystal solar cells.

Polymeric carbon nitride supported Bi nanoparticles as highly efficient CO2 reduction electrocatalyst in a wide potential range

It is still a great challenge to develop electrocatalysts for CO₂ reduction with high product selectivity and energy conversion efficiency. In this work, Bi nanoparticles supported on polymeric carbon nitride (Bi/CN) have been prepared for CO₂ electrocatalytic conversion. Transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS) analyses confirm the existence of Bi₂O₃ on Bi particle surface, forming Bi/Bi₂O₃ nanoparticles. CN, as the support, has been found not only to improve the dispersibility of Bi/Bi₂O₃ nanoparticles, but also to enhance the CO₂ adsorption on Bi/CN surface owing to the existence of amino and cyano groups. The electronic structure of Bi/CN has been optimized by the interaction between CN and Bi: the electron transfer from Bi to CN results in electron-deficient Bi sites which stabilize CO₂^-, HCOO^- intermediates and accelerate the formation rate of HCOOH. As a result, the maximum Faradaic efficiency of HCOOH reaches 98% at -1.3 to -1.5 V versus reversible hydrogen electrode (vs. RHE) and remains over 91% in a wide potential window of about 500 mV (-1.1 ∼ -1.6 V vs. RHE). The as-obtained Bi/CN in this work shows superior performance to most of the previously reported Bi-based electrocatalysts.

A Multiscale Strategy to Construct Cobalt Nanoparticles Confined within Hierarchical Carbon Nanofibers for Efficient CO2 Electroreduction

The efficiency of CO₂ electroreduction has been largely limited by the activity of the catalysts as well as the three-phase interface. Herein, a multiscale strategy is proposed to synthesize hierarchical nanofibers covered by carbon nanotubes and embedded with cobalt nanoparticles (Co/CNT/HCNF). The confinement effect of carbon nanotubes can restrict the diameter of the cobalt particles down to several nanometers and prevent the easy corrosion of these nanoparticles. The three-dimensional carbon nanofibers, in size range of several hundred nanometers, improve the electrochemically active surface area, facilitate electron transfer, and accelerate CO₂ transportation. These cross-linked carbon nanofibers eventually form a freestanding Co/CNT/HCNF membrane of dozens of square centimeters. Consequently, Co/CNT/HCNF produces CO with 97% faradaic efficiency at only -0.4 V_RHE cathode potential in an H-type cell. From the regulation of catalyst nanostructure to the design of macrography devices, Co/CNT/HCNF membrane can be directly used as the gas-diffusion compartment in a flow cell device. Co/CNT/HCNF membrane generates CO with faradaic efficiencies higher than 90% and partial current densities greater than 300 mA cm^-2 for at least 100-h stability. This strategy provides a successful example of efficient catalysts for CO₂ electroreduction and also has the feasibility in other self-standing energy conversion devices.

Nitrogen-doped mesoporous carbon supported CuSb for electroreduction of CO2

The construction of an efficient catalyst for electrocatalytic reduction of CO₂ to high value-added fuels has received extensive attention. Herein, nitrogen-doped mesoporous carbon (NMC) was used to support CuSb to prepare a series of materials for electrocatalytic reduction of CO₂ to CH₄. The catalytic activity of the composites was significantly improved compared with that of Cu/NMC. In addition, the Cu content also influenced the activity of electrocatalytic CO₂ reduction reaction. Among the materials used, the CuSb/NMC-2 (Cu: 5.9 wt%, Sb: 0.49 wt%) catalyst exhibited the best performance for electrocatalytic CO₂ reduction, and the faradaic efficiency of CH₄ reached 35%, and the total faradaic efficiency of C1–C2 products reached 67%.

CuSb anchored onto nitrogen-doped mesoporous carbon (CuSb/NMC) were prepared for electroreduction of CO₂ to CH₄, C₂H₄ and CO.

Loading Single-Ni Atoms on Assembled Hollow N-Rich Carbon Plates for Efficient CO2 Electroreduction

The rational design of catalysts' spatial structure is vitally important to boost catalytic performance through exposing the active sites, enhancing the mass transfer, and confining the reactants. Herein, a dual-linker zeolitic tetrazolate framework-engaged strategy is developed to construct assembled hollow plates (AHP) of N-rich carbon (NC), which is loaded with single-Ni atoms to form a highly efficient electrocatalyst (designated as Ni-NC(AHP)). In the carbonization process, the thermally unstable linker (5-aminotetrazole) serves as the self-sacrificial template and the other linker (2-methylimidazole) mainly serves as the carbon and nitrogen source to form hollow NC matrix. The formed Ni-NC(AHP) catalyst possesses enhanced mesoporosity and more available surface area, thus promoting mass transport and affording abundant accessible single-Ni sites. These features contribute to remarkable performance for electrochemical CO₂ reduction with exceptionally high selectivity of nearly 100% towards CO in a wide potential range and dramatically enhanced CO partial current density.