Ultrathin Bismuth Nanosheets as a Highly Efficient CO2 Reduction Electrocatalyst

Growth and Electrochemical Study of Bismuth Nanodendrites as an Efficient Catalyst for CO2 Reduction

There is a growing interest in creating cost-effective catalysts for efficient electrochemical CO2 reduction to address pressing environmental issues and produce valuable products. A bimetallic ZnBi catalyst that enhances catalytic activity and stability toward the electrochemical reduction of CO2 is designed. It is based on bismuth nanodendrites grown using a facile, scalable, and low-cost method. The results have shown that the incorporation of bismuth can decrease the charge transfer resistance and facilitate CO2 reduction toward the formation of CO and formate. It was revealed that the ZnBi catalyst exhibited higher catalytic activity compared with that of the pure Zn catalyst for CO2 reduction, with a lower onset potential [-0.75 V vs a reversible hydrogen electrode (RHE) compared with -0.85 V vs RHE for Zn]. In situ electrochemical attenuated total internal reflection Fourier transform infrared spectroscopy was employed to study the reaction mechanism, showing the formation of CO and formate through the adsorbed *COO- intermediates. This study has demonstrated a new approach for the feasible synthesis of high-performance catalysts for large-scale electrochemical CO2 reduction.

Revealing the Intrinsic Restructuring of Bi2O3 Nanoparticles into Bi Nanosheets during Electrochemical CO2 Reduction

Bismuth is a catalyst material that selectively produces formate during the electrochemical reduction of CO2. While different synthesis strategies have been employed to create electrocatalysts with better performance, the restructuring of bismuth precatalysts during the reaction has also been previously reported. The mechanism behind the change has, however, remained unclear. Here, we show that Bi2O3 nanoparticles supported on Vulcan carbon intrinsically transform into stellated nanosheet aggregates upon exposure to an electrolyte. Liquid cell transmission electron microscopy observations first revealed the gradual restructuring of the nanoparticles into nanosheets in the presence of 0.1 M KHCO3 without an applied potential. Our experiments also associated the restructuring with solubility of bismuth in the electrolyte. While the consequent agglomerates were stable under moderate negative potentials (-0.3 VRHE), they dissolved over time at larger negative potentials (-0.4 and -0.5 VRHE). Operando Raman spectra collected during the reaction showed that under an applied potential, the oxide particles reduced to metallic bismuth, thereby confirming the metal as the working phase for producing formate. These results inform us about the working morphology of these electrocatalysts and their formation and degradation mechanisms.

Elaborate tree-like Cu-Ag clusters from green electrodeposition for efficiently electrocatalyzing CO2 conversion into syngas

The electrocatalytic carbon dioxide reduction (CO2RR) is one of the emerging technologies that can effectively transform carbon dioxide (CO2) into valuable products. Electrocatalysts deriving from green synthesis methods will significantly help to establish a new green carbon cycle. Herein, a green electrodeposition method without additional reducing agents was used to synthesize Cu-Ag bimetallic catalysts, and it is shown that the combination of Cu and Ag obviously affects the morphology of the Cu-Ag catalysts, resulting in the formation of elaborate tree-like Cu-Ag clusters. An as-deposited Cu-Ag/carbon fiber (Cu-Ag/CF) catalyst exhibits high activity, selectivity and stability toward the CO2RR; in particular, the elaborate dendritic Cu-Ag/CF can efficiently reduce CO2 to syngas with high selectivity (Faradaic efficiency (FE) > 95%) at a low onset potential (-0.5 V). This work provides a rational strategy to overcome the significantly different reaction capacities during the reduction of Ag+ and Cu2+, leading to the formation of a controlled morphology of Cu-Ag, which is favourable for the design and development of highly efficient Cu or Ag catalysts via green methods for electrocatalyzing the CO2RR.

Two-Dimensional Metal Nanostructures: From Theoretical Understanding to Experiment

This paper reviews recent studies on the preparation of two-dimensional (2D) metal nanostructures, particularly nanosheets. As metal often exists in the high-symmetry crystal phase, such as face centered cubic structures, reducing the symmetry is often needed for the formation of low-dimensional nanostructures. Recent advances in characterization and theory allow for a deeper understanding of the formation of 2D nanostructures. This Review firstly describes the relevant theoretical framework to help the experimentalists understand chemical driving forces for the synthesis of 2D metal nanostructures, followed by examples on the shape control of different metals. Recent applications of 2D metal nanostructures, including catalysis, bioimaging, plasmonics, and sensing, are discussed. We end the Review with a summary and outlook of the challenges and opportunities in the design, synthesis, and application of 2D metal nanostructures.

Oxygenated Hydrocarbons from Catalytic Hydrogenation of Carbon Dioxide

Once fundamental difficulties such as active sites and selectivity are fully resolved, metal-free catalysts such as 3D graphene or carbon nanotubes (CNT) are very cost-effective substitutes for the expensive noble metals used for catalyzing CO2. A viable method for converting environmental wastes into useful energy storage or industrial wealth, and one which also addresses the environmental and energy problems brought on by emissions of CO2, is CO2 hydrogenation into hydrocarbon compounds. The creation of catalytic compounds and knowledge about the reaction mechanisms have received considerable attention. Numerous variables affect the catalytic process, including metal–support interaction, metal particle sizes, and promoters. CO2 hydrogenation into different hydrocarbon compounds like lower olefins, alcoholic composites, long-chain hydrocarbon composites, and fuels, in addition to other categories, have been explained in previous studies. With respect to catalyst design, photocatalytic activity, and the reaction mechanism, recent advances in obtaining oxygenated hydrocarbons from CO2 processing have been made both through experiments and through density functional theory (DFT) simulations. This review highlights the progress made in the use of three-dimensional (3D) nanomaterials and their compounds and methods for their synthesis in the process of hydrogenation of CO2. Recent advances in catalytic performance and the conversion mechanism for CO2 hydrogenation into hydrocarbons that have been made using both experiments and DFT simulations are also discussed. The development of 3D nanomaterials and metal catalysts supported on 3D nanomaterials is important for CO2 conversion because of their stability and the ability to continuously support the catalytic processes, in addition to the ability to reduce CO2 directly and hydrogenate it into oxygenated hydrocarbons.

Alloying strategies for tuning product selectivity during electrochemical CO2 reduction over Cu

Excessive reliance on fossil fuels has led to the release and accumulation of large quantities of CO₂ into the atmosphere which has raised serious concerns related to environmental pollution and global warming. One way to mitigate this problem is to electrochemically recycle CO₂ to value-added chemicals or fuels using electricity from renewable energy sources. Cu is the only metallic electrocatalyst that has been shown to produce a wide range of industrially important chemicals at appreciable rates. However, low product selectivity is a fundamental issue limiting commercial applications of electrochemical CO₂ reduction over Cu catalysts. Combining copper with other metals that actively contribute to the electrochemical CO₂ reduction reaction process can selectively facilitate generation of desirable products. Alloying Cu can alter surface binding strength through electronic and geometric effects, enhancing the availability of surface confined carbon species, and stabilising key reduction intermediates. As a result, significant research has been undertaken to design and fabricate copper-based alloy catalysts with structures that can enhance the selectivity of targeted products. In this article, progress with use of alloying strategies for development of Cu-alloy catalysts are reviewed. Challenges in achieving high selectivity and possible future directions for development of new copper-based alloy catalysts are considered.

Room Temperature Nanographene Production via CO2 Electrochemical Reduction on the Electrodeposited Bi on Sn Substrate

Electrochemical reduction of carbon dioxide (CO₂RR) to crystalline solid carbon at room temperature is challenging, but it is a providential CO₂ utilization route due to its indefinite storage and potential applications of its products in many advanced technologies. Here, room-temperature synthesis of polycrystalline nanographene was achieved by CO₂RR over the electrodeposited Bi on Sn substrate prepared with various bismuth concentrations (0.01 M, 0.05 M, and 0.1 M). The solid carbon products were solely produced on all the prepared electrodes at the applied potential -1.1 V vs. Ag/AgCl and were characterized as polycrystalline nanographene with an average domain size of ca. 3-4 nm. The morphology of the electrodeposited Bi/Sn electrocatalysts did not have much effect on the final structure of the solid carbon products formed but rather affected the CO₂ electroreduction activity. The optimized negative potential for the formation of nanographene products on the 0.05Bi/Sn was ca. -1.5 V vs. Ag/AgCl. Increasing the negative value of the applied potential accelerated the agglomeration of the highly reactive nascent Bi clusters in situ formed under the reaction conditions, which, as a consequence, resulted in a slight deviation of the product selectivity toward gaseous CO and H₂ evolution reaction. The Bi-graphene composites produced by this method show high potential as an additive for working electrode modification in electrochemical sensor-related applications.

Highly Dispersed Few-Nanometer Chlorine-Doped SnO2 Catalyst Embedded in a Polyaniline Matrix for Stable HCOO- Production in a Flow Cell

With the spread of alternative energy plants, electrolysis processes are becoming the protagonists of the future industrial generation. The technology readiness level for the electrochemical reduction of carbon dioxide is still low and is largely based on precious metal resources. In the present work, tin ions are anchored on a polyaniline matrix, via a sonochemical synthesis, forming a few atomic layers of chlorine-doped SnO₂ with a total loading of tin atom load of only 7 wt %. This catalyst is able to produce formate (HCOO^-) with great selectivity, exceeding 72% of Faradaic efficiency in the first hour of testing in 1 M KHCO₃ electrolyte, with a current density of more than 50 mA cm^-2 in a 2 M KHCO₃ electrolyte flow cell setup. Catalyst stability tests show a stable production of HCOO^- during 6 h of measurement, accumulating an overall TON_HCOO^- of more than 10,000 after 16 h of continuous formate production. This strategy is competitive in drastically reducing the amount of metal required for the overall catalysis.

Effects of the Catalyst Dynamic Changes and Influence of the Reaction Environment on the Performance of Electrochemical CO2 Reduction

Electrochemical reduction of carbon dioxide (CO₂ ) is substantially researched due to its potential for storing intermittent renewable electricity and simultaneously helping mitigating the pressing CO₂ emission concerns. The major challenge of electrochemical CO₂ reduction lies on having good controls of this reaction due to its complicated reaction networks and its unusual sensitivity to the dynamic changes of the catalyst structure (chemical states, compositions, facets and morphology, etc.), and to the non-catalyst components at the electrode/electrolyte interface, in another word the reaction environments. To date, a comprehensive analysis on the interplays between the above catalyst-dynamic-changes/reaction environments and the CO₂ reduction performance is rare, if not none. In this review, the catalyst dynamic changes observed during the catalysis are discussed based on the recent reports of electrochemical CO₂ reduction. Then, the above dynamic changes are correlated to their effects on the catalytic performance. The influences of the reaction environments on the performance of CO₂ reduction are also discussed. Finally, some perspectives on future investigations are offered with the aim of understanding the origins of the effects from the catalyst dynamic changes and the reaction environments, which will allow one to better control the CO₂ reduction toward the desired products.

CuBi electrocatalysts modulated to grow on derived copper foam for efficient CO2-to-formate conversion

Electrochemical reduction of CO₂ to fuels and chemicals is an effective way to reduce greenhouse gas emissions and alleviate the energy crisis, but the highly active catalysts necessary for this reaction under mild conditions are still rare. In this work, we grew CuBi bimetallic catalysts on derived copper foam substrates by co-electrodeposition, and then investigated the correlation between co-electrodeposition potential and electrochemical performance in CO₂-to-formate conversion. Results showed that the bimetallic catalyst formed at a low potential of - 0.6 V vs. AgCl/Ag electrode achieved the highest formate Faradaic efficiency (FE_formate) of 94.4% and a current density of 38.5 mA/cm² at a low potential of - 0.97 V vs. reversible hydrogen electrode (RHE). Moreover, a continuous-flow membrane electrode assembly reactor also enabled the catalyst to show better performance (a FE_formate of 98.3% at 56.6 mA/cm²) than a traditional H-type reaction cell. This work highlights the vital impact of co-electrodeposition potential on catalyst performance and provides a basis for the modulated growth of bimetallic catalysts on substrates. It also shows the possibility of preparing Bi-based catalysts with no obvious decrease in catalytic activity that have been partially replaced with more economic copper.

Electronic structure modulation of bismuth catalysts induced by sulfur and oxygen co-doping for promoting CO2 electroreduction

Carbon dioxide electroreduction into green fuels and value-added chemicals is an attractive method for the utilization of renewable energy to mitigate global warming. High-efficiency catalysts are necessary for mild and...

Integrated 3D Open Network of Interconnected Bismuthene Arrays for Energy-Efficient and Electrosynthesis-Assisted Electrocatalytic CO2 Reduction

Electrocatalytic CO₂ reduction reaction (CO₂ RR) toward formate production can be operated under mild conditions with high energy conversion efficiency while migrating the greenhouse effect. Herein, an integrated 3D open network of interconnected bismuthene arrays (3D Bi-ene-A/CM) is fabricated via in situ electrochemically topotactic transformation from BiOCOOH nanosheet arrays supported on the copper mesh. The resulted 3D Bi-ene-A/CM consists of 2D atomically thin metallic bismuthene (Bi-ene) in the form of an integrated array superstructure with a 3D interconnected and open network, which harvests the multiple structural advantages of both metallenes and self-supported electrodes for electrocatalysis. Such distinctive superstructure affords the maximized quantity and availability of the active sites with high intrinsic activity and superior charge and mass transfer capability, endowing the catalyst with good CO₂ RR performance for stable formate production with high Faradaic efficiency (≈90%) and current density (>300 mA cm^-2 ). Theoretical calculation verifies the superior intermediate stabilization of the dominant Bi plane during CO₂ RR. Moreover, by further coupling anodic methanol oxidation reaction, an exotic electrolytic system enables highly energy-efficient and value-added pair-electrosynthesis for concurrent formate production at both electrodes, achieving substantially improved electrochemical and economic efficiency and revealing the feasibility for practical implementation.

Enhanced Production of Formic Acid in Electrochemical CO2 Reduction over Pd-Doped BiOCl Nanosheets

Bismuth oxyhalides (BiOX, X = F, Cl, Br, I) are emerging energy materials because of their remarkable catalytic activity. The BiOX compounds usually have a tetragonal type crystal structure with unique layered morphology consisting of [X-Bi-O-Bi-X] sheets. Although the BiOX nanosheets exposed with {001} facets perform superior photoactivity, there is lack of understanding about their capability in the electrochemical CO₂ reduction reaction (CO₂RR). Herein, we adopt wet-chemical syntheses to make 2D BiOCl and Pd-doped BiOCl nanosheets for CO₂RR. In the results, formic acid is the only one kind of product converted from CO₂ along with H₂ gas from water reduction over both BiOCl and Pd-doped BiOCl nanosheets. By thorough analyses with ex situ and in situ spectroscopy, the results reflect that (1) metallic Bi⁰ atoms generated by the applied negative potentials serve as the catalytic sites for the hydrogen evolution reaction (HER) and CO₂RR and (2) the existence of doped Pd ions in the BiOCl structure reduces the barrier of charge transfer over the nanosheets, which enhances HER and CO₂RR activities. We believe that the observations are important references for making catalysts toward CO₂RR performance.

Two-dimensional materials for electrochemical CO2 reduction: materials, in situ/operando characterizations, and perspective

Electrochemical CO₂ reduction (CO₂ ECR) is an efficient approach to achieving eco-friendly energy generation and environmental sustainability. This approach is capable of lowering the CO₂ greenhouse gas concentration in the atmosphere while producing various valuable fuels and products. For catalytic CO₂ ECR, two-dimensional (2D) materials stand as promising catalyst candidates due to their superior electrical conductivity, abundant dangling bonds, and tremendous amounts of surface active sites. On the other hand, the investigations on fundamental reaction mechanisms in CO₂ ECR are highly demanded but usually require advanced in situ and operando multimodal characterizations. This review summarizes recent advances in the development, engineering, and structure-activity relationships of 2D materials for CO₂ ECR. Furthermore, we overview state-of-the-art in situ and operando characterization techniques, which are used to investigate the catalytic reaction mechanisms with the spatial resolution from the micron-scale to the atomic scale, and with the temporal resolution from femtoseconds to seconds. Finally, we conclude this review by outlining challenges and opportunities for future development in this field.

Reconstructed Bismuth-Based Metal-Organic Framework Nanofibers for Selective CO2 -to-Formate Conversion: Morphology Engineering

Electrochemical reduction of carbon dioxide (ERCO₂ ) is an attractive and sustainable approach to close the carbon loop. Formic acid is a high-value and readily collectible liquid product. However, the current reaction selectivity remains unsatisfactory. In this study, the bismuth-containing metal-organic framework CAU-17, with morphological variants of hexagonal prisms (CAU-17-hp) and nanofibers (CAU-17-fiber), is prepared at room temperature through a wet-chemical approach and employed as the electrocatalyst for highly selective CO₂ -to-formate conversion. An H₃ BTC-mediated morphology reconstruction is systematically investigated and further used to build a CAU-17-fiber hierarchical structure. The as-prepared CAU-17-fiber_400 electrodes give the best electrocatalytic performance in selective and efficient formate production with FE_HCOO- of 96.4 % and j_COOH- of 20.4 mA cm^-2 at -0.9 V_RHE . This work provides a new mild approach for synthesis and morphology engineering of CAU-17 and demonstrates the efficacy of morphology engineering in regulating the accessible surface area and promoting the activity of MOF-based materials for ERCO₂ .

Electrochemical Production of Formic Acid from CO2 with Cetyltrimethylammonium Bromide-Assisted Copper-Based Catalysts

The electrochemical reduction of CO₂ (ERC) to valuable chemicals has attracted extensive attention. However, the relatively low selectivity and efficiency of the reaction remain challenges. In this study, Cu electrodes derived from Cu₂ O with predominant (111) facets are synthesized by cetyltrimethylammonium bromide-assisted preparation. The optimized electrode shows a high faradaic efficiency of 90 % for HCOOH obtained by ERC at -2.0 V (vs.SCE), which surpasses most reported Cu electrodes. Based on a comprehensive analysis of the relationship between the catalytic activity and the thickness of the Cu₂ O layer, the catalytic activity of the unit active site on the Cu₂ O-derived Cu electrodes is found to be higher than that on the blank Cu electrode. DFT calculations indicate that OCHO* would be produced preferentially over *COOH in the presence of cetyltrimethylammonium bromide (CTAB). This deduction is verified by testing of the effects of CTAB and KBr addition on HCOO^- selectivity.

Self-supported mesoscopic tin oxide nanofilms for electrocatalytic reduction of carbon dioxide to formate

Here we report a self-supported SnO₂ nanofilm prepared by a robust electrochemical process as an electrocatalyst for the CO₂ reduction reaction. The SnO₂ film had a large surface area originating from its nano-architecture and manifested high selectivity toward formate (over 60%), which resulted in CO₂-to-formate current density up to 33.66 mA cm^-2 that is among the state-of-the-art. We unveiled that the high performance of the SnO₂ nanofilm is attributable to the presence of a metastable oxide under reductive conditions in addition to the abovementioned advantages.

An industrial perspective on catalysts for low-temperature CO2 electrolysis

Electrochemical conversion of CO₂ to useful products at temperatures below 100 °C is nearing the commercial scale. Pilot units for CO₂ conversion to CO are already being tested. Units to convert CO₂ to formic acid are projected to reach pilot scale in the next year. Further, several investigators are starting to observe industrially relevant rates of the electrochemical conversion of CO₂ to ethanol and ethylene, with the hydrogen needed coming from water. In each case, Faradaic efficiencies of 80% or more and current densities above 200 mA cm^-2 can be reproducibly achieved. Here we describe the key advances in nanocatalysts that lead to the impressive performance, indicate where additional work is needed and provide benchmarks that others can use to compare their results.

Electrocatalysis for CO2 conversion: from fundamentals to value-added products

This timely and comprehensive review mainly summarizes advances in heterogeneous electroreduction of CO₂: from fundamentals to value-added products.

Efficient electrocatalytic CO2 conversion into formate with AlxBiyOz nanorods in a wide potential window

Novel kinds of AlxBiyOz nanorods have been developed for the CO2RR for the first time, which show high selectivity to formate under a wide potential window and exhibit a maximum faradaic efficiency to formate of 97.5% at −1.19 V vs. RHE.

Electrochemical Reactors for CO2 Conversion

Increasing risks from global warming impose an urgent need to develop technologically and economically feasible means to reduce CO2 content in the atmosphere. Carbon capture and utilization technologies and carbon markets have been established for this purpose. Electrocatalytic CO2 reduction reaction (CO2RR) presents a promising solution, fulfilling carbon-neutral goals and sustainable materials production. This review aims to elaborate on various components in CO2RR reactors and relevant industrial processing. First, major performance metrics are discussed, with requirements obtained from a techno-economic analysis. Detailed discussions then emphasize on (i) technical benefits and challenges regarding different reactor types, (ii) critical features in flow cell systems that enhance CO2 diffusion compared to conventional H-cells, (iii) electrolyte and its effect on liquid phase electrolyzers, (iv) catalysts for feasible products (carbon monoxide, formic acid and multi-carbons) and (v) strategies on flow channel and anode design as next steps. Finally, specific perspectives on CO2 feeds for the reactor and downstream purification techniques are annotated as part of the CO2RR industrial processing. Overall, we focus on the component and system aspects for the design of a CO2RR reactor, while pointing out challenges and opportunities to realize the ultimate goal of viable carbon capture and utilization technology.

Influence of the Chemical Compositions of Bismuth Oxyiodides on the Electroreduction of Carbon Dioxide to Formate

Bismuth oxyiodides with varying chemical compositions were fabricated to effectively electrochemical reduce CO₂ to formate. Bi₅ O₇ I and Bi₇ O₉ I₃ nanosheets assemble irregularly, and BiOI nanosheets form a sphere-like structure. Compared with BiOI and Bi₇ O₉ I₃ , Bi₅ O₇ I exhibits an excellent Faradaic efficiency of 89 % for formate production with the partial current density of 13.2 mA/cm² at -0.89 V vs. RHE owing to the elevated amounts of Bi metal sites reduced from Bi³⁺ during electrolysis. The partial current densities of formate on BiOI were higher than those on Bi₇ O₉ I₃ which is attributed to the higher iodine content. The synergistic effect of bismuth and iodine of bismuth oxyiodides is responsible for their electrocatalytic properties during CO₂ reduction in aqueous solutions.

Advances of 2D bismuth in energy sciences

Since graphene has been successfully exfoliated, two-dimensional (2D) materials constitute a vibrant research field and open vast perspectives in high-performance applications. Among them, bismuthene and 2D bismuth (Bi) are unique with superior properties to fabricate state-of-the-art energy saving, storage and conversion devices. The largest experimentally determined bulk gap, even larger than those of stanene and antimonene, allows 2D Bi to be the most promising candidate to construct room-temperature topological insulators. Moreover, 2D Bi exhibits cyclability for high-performance sodium-ion batteries, and the enlarged surface together with the good electrochemical activity renders it an efficient electrocatalyst for energy conversion. Also, the air-stability of 2D Bi is better than that of silicene, germanene, phosphorene and arsenene, which could enable more practical applications. This review aims to thoroughly explore the fundamentals of 2D Bi and its improved fabrication methods, in order to further bridge gaps between theoretical predictions and experimental achievements in its energy-related applications. We begin with an introduction of the status of 2D Bi in the 2D-material family, which is followed by descriptions of its intrinsic properties along with various fabrication methods. The vast implications of 2D Bi for high-performance devices can be envisioned to add a new pillar in energy sciences. In addition, in the context of recent pioneering studies on moiré superlattices of other 2D materials, we hope that the improved manipulation techniques of bismuthene, along with its unique properties, might even enable 2D Bi to play an important role in future energy-related twistronics.

Bi-Doped SnO Nanosheets Supported on Cu Foam for Electrochemical Reduction of CO2 to HCOOH

Design and fabrication of efficient electrocatalysts is essential for electrochemical reduction of carbon dioxide (CO₂). In this work, bismuth (Bi)-doped SnO nanosheets were grown on copper foam (Bi-SnO/Cu foam) by a one-step hydrothermal reaction method and applied for the electrochemical reduction of CO₂ to formic acid (HCOOH). The experimental results indicated that Bi doping stabilized the divalent tin (Sn²⁺) existing on the surface of the electrocatalyst, making it difficult to be reduced to metallic tin (Sn⁰) during the electrochemical reduction process. In addition, combining with density functional theory (DFT) calculations, it is found that Bi doping and electron transfer from the catalyst to the Cu foam substrate could enhance the adsorption of *OOCH intermediates. As such, the Bi-doped SnO electrocatalyst exhibited a superior faradaic efficiency of 93% at -1.7 V (vs Ag/AgCl) for the reduction of CO₂ to HCOOH, together with a current density of 12 mA cm^-2 and excellent stability in at least 30 h of operation.

Electrochemical Transformation of Facet-Controlled BiOI into Mesoporous Bismuth Nanosheets for Selective Electrocatalytic Reduction of CO2 to Formic Acid

Mesoporous bismuth nanosheets are prepared through electrochemical transformation of (100)-facet exposed BiOI. Theoretical modeling and calculations are used to simulate the in situ morphological transformation of BiOI into Bi. Mesoporous Bi nanosheets show superior electrochemical CO₂ reduction performance. A faradaic efficiency of 95.9 % at -0.77 V_RHE for the conversion of CO₂ into formic acid, is achieved for the mesoporous Bi nanosheet catalyst compared with 93.8 % at -0.87 V_RHE for the smooth Bi nanosheets. Tafel analysis and DFT calculations indicate that the electrochemical CO₂ reduction on mesoporous Bi nanosheets is kinetically faster with a higher resistance to H₂ generation than that on smooth Bi(001) nanosheets. The CO₂ -to-HCOOH pathway is preferred through formation of an *OCHO intermediate on the (012) and (001) planes of Bi. The mesoporous structure induces a more accessible interaction with CO₂ , which makes a predominant contribution to the enhanced performance compared with the subsequent CO₂ activation on different facets of Bi.

Electrochemical Reduction of CO2 to Formate on Easily Prepared Carbon-Supported Bi Nanoparticles

Herein, the electrochemical reduction of CO2 to formate on carbon-supported bismuth nanoparticles is reported. Carbon-supported Bi nanoparticles (about 10 nm in size) were synthesized using a simple, fast and scalable approach performed under room conditions. The so-prepared Bi electrocatalyst was characterized by different physicochemical techniques, including transmission electron microscopy, X-ray photoelectron spectroscopy, and X-ray diffraction and subsequently air-brushed on a carbon paper to prepare electrodes. These electrodes were characterized by scanning electron microscopy, energy-dispersive X-ray spectroscopy and also by cyclic voltammetry. Finally, CO2 electroreduction electrolyses were performed at different electrode potentials for 3 h. At the optimal electrode potential (-1.6 V vs AgCl/Ag), the concentration of formate was about 77 mM with a faradaic efficiency of 93 ± 2.5%. A 100% faradaic efficiency was found at a lower potential (-1.5 V vs AgCl/Ag) with a formate concentration of about 55 mM. In terms of stability, we observed that after about 70 h (in 3 h electrolysis experiments at different potentials), the electrode deactivates due to the gradual loss of metal as shown by SEM/EDX analyses of the deactivated electrodes.

Phosphomolybdic Acid-Assisted Growth of Ultrathin Bismuth Nanosheets for Enhanced Electrocatalytic Reduction of CO2 to Formate

Oxides containing two-dimensional metallic catalysts have shown enhanced catalytic activity, stability, and product selectivity. Porous three-dimensional structures maximize the accessibility of the active sites, thus enhancing the catalytic performance of the catalysts. By integrating these desirable features in a single catalyst, further improvement in catalytic activity and selectivity is expected. In this study, oxide-containing bismuth (Bi) nanosheets of about 4 nm thickness interconnected to form a porous three-dimensional structure were synthesized by electrodeposition in the presence of phosphomolybdic acid under hydrogen evolution conditions. These Bi nanosheets catalyze CO₂ reduction in a CO₂ -saturated 0.5 m NaHCO₃ solution to formate with a faradaic efficiency of 93±2 % at -0.86 V vs. RHE with a formate partial current density as high as 30 mA cm^-2 . The Tafel slope of about 78 mV dec^-1 suggests that the protonation of the adsorbed CO₂ ^.- is the rate-limiting step.