Electrochemical CO2 Reduction to Formic Acid at Low Overpotential and with High Faradaic Efficiency on Carbon-Supported Bimetallic Pd–Pt Nanoparticles

Controlling Hydrogen Evolution and CO2 Reduction at Transition Metal Hydrides

ConspectusFuel-forming reactions such as the hydrogen evolution reaction (HER) and CO2 reduction (CO2R) are vital to transitioning to a carbon-neutral economy. The equivalent oxidation reactions are also important for efficient utilization in fuel cells. Metal hydride intermediates are common in these catalytic and electrocatalytic processes. Guiding metal hydride reactivity is important for achieving selective, kinetically fast, and low overpotential redox reactions. Our work has focused on understanding kinetic and thermodynamic aspects for controlling these reactive hydride species in an effort to design more selective electrocatalysts that operate at low overpotentials. Key to our research approach is understanding the free energy changes and rate of discrete steps of catalysis through the synthesis of proposed intermediates to independently investigate catalytic steps. Hydricity, the free energy of hydride dissociation, and how these values change with metal and ligand environment have informed catalyst design in the past few decades. We describe here how we have advanced upon these earlier studies.In our early studies we sought to understand solvent-dependent changes in hydricity for transition metal hydrides and how they impact the free energy for reduction of CO2 to formate (HCO2-). Additionally, we described how hydricity values can be applied to optimize HER and CO2R catalysis. This framework provides general guidelines for achieving selective CO2 reduction to formate without concomitant generation of H2. Kinetic information on steps in the proposed catalytic cycle of HER and CO2R catalysts were evaluated to identify potential rate-determining steps. As a second approach to achieve selective reduction for CO2, we explored two catalyst design strategies to kinetically inhibit HER using electrostatic (charged) and steric interactions. Hydricity values and other considerations for minimizing the free energy of proposed catalytic steps were also used to design an electrocatalyst for the interconversion between CO2 and HCO2- at low overpotentials. Further, we discuss our efforts to translate the CO2 hydrogenation activity of homogeneous catalysts to electrocatalysis.All of these catalytic systems operate with classical metal hydrides, where the electrons and proton are colocated on the metal center. However, classical metal hydrides all require very reducing potentials to generate sufficiently strong hydride donors for CO2 reduction. An analysis of metal hydride hydricity and reduction potentials shows that the strong correlation between reduction potential and hydricity is a general trend because the former is also highly correlated to pKa. However, formate dehydrogenase (FDH) generates a competent hydride donor at more mild potentials through bidirectional hydride transfer, where the proton and electrons of the hydride are not colocated. This bioinspired approach points to a promising new strategy for generating strong hydride donors at milder potentials and will surely open new avenues for using hydricity as a guide for addressing new and existing problems in catalysis.

Computational Study of Electrochemical CO2 Reduction on 2D Graphitic Carbon Nitride Supported Single-Atom (Al and P) Catalysts (SACs)

To mitigate the adverse effects of CO2 emissions, CO2 electroreduction to small organic products is a preferable solution and potential catalysts include the single-atom catalyst (SAC) which comprises individual atoms dispersed on 2D materials. Here, we used aluminum and phosphorus as the active sites for CO2 electroreductions by embedding them on the 2D graphitic carbon nitride (g-C3N4) nano-surface. The resulting M-C3N4 (M=Al and P) SACs were computationally studied for the CO2 electroreduction using density functional theory (DFT) and ab-initio molecular dynamics (AIMD) simulations. Computations showed that CO2 can be adsorbed to the active sites in forms of a frustrated Lewis pair (Al/N or P/N) or single atom Al or P. The adsorbed CO2 can be converted to various intermediates by gaining proton and electron (H++e-) pairs, a process simulated as electroreduction. While both SACs prefer to produce HCOOH with low potential determining steps (PDSs) and small overpotential values of 0.25 V and 0.08 V for Al-C3N4 and P-C3N4 respectively, to produce CH4, P-C3N4 exhibits a lower potential barrier of 0.9 eV than Al-C3N4 (1.07~1.17 eV).

Integrating Active Learning and DFT for Fast-Tracking Single-Atom Alloy Catalysts in CO2-to-Fuel Conversion

Electrocatalytic carbon dioxide reduction (CO2RR) technology enables the conversion of excessive CO2 into high-value fuels and chemicals, thereby mitigating atmospheric CO2 concentrations and addressing energy scarcity. Single-atom alloys (SAAs) possess the potential to enhance the CO2RR performance by full utilization of atoms and breaking linear scaling relationships. However, quickly screening high-performance metal portfolios of SAAs remains a formidable challenge. In this study, we proposed an active learning (AL) framework to screen high-performance catalysts for CO2RR to yield fuels such as CH4 and CH3OH. After four rounds of AL iterations, the ML model attained optimal prediction performance with the test set R2 of approximately 0.94 and successful prediction was achieved for the binding free energy of *CHO, *COH, *CO, and *H on 380 catalyst surfaces with an accuracy within 0.20 eV. Subsequent analysis of the SAA catalysts' activity, selectivity, and stability led to the discovery of eight previously unexplored SAA catalysts for CO2RR. Notably, the surface activity of Ti@Cu(100), Au@Pt(100), and Ag@Pt(100) shines prominently. Utilizing DFT calculations, we elucidated the complete reaction pathway of the CO2RR on the surfaces of these catalysts, confirming their high catalytic activity with limiting potentials of -0.11, -0.34, and -0.46 eV, respectively, which are significantly lower than those of pure Cu catalysts. The results showcase the exceptional predictive prowess of AL, providing a valuable reference for the design of CO2RR catalysts.

Catalyst-Free Transformation of Carbon Dioxide to Small Organic Compounds in Water Microdroplets Nebulized by Different Gases

A straightforward nebulized spray system is designed to explore the hydrogenation of carbon dioxide (CO2) within water microdroplets surrounded by different gases such as carbon dioxide, nitrogen, oxygen, and compressed air. The collected droplets are analyzed using water-suppressed nuclear magnetic resonance (NMR). Formate anion (HCOO-), acetate anion (CH3COO-), ethylene glycol (HOCH2CH2OH), and methane (CH4) are detected when water is nebulized. This pattern persisted when the water is saturated with CO2, indicating that CO2 in the nebulizing gas triggers the formation of these small organics. In a pure CO2 atmosphere, the formate anion concentration is determined to be ≈70 µm, referenced to dimethyl sulfoxide, which has been introduced as an internal standard in the collected water droplets. This study highlights the power of water microdroplets to initiate unexpected chemistry for the transformation of CO2 to small organic compounds.

H-assisted CO2 dissociation on PdnPt(4-n)/In2O3 catalysts: a density functional theory study

CO2 hydrogenation into valuable chemical compounds can effectively address the issues of greenhouse gas emissions and energy scarcity. The activation and dissociation processes of CO2 are crucial for its reduction reactions, but the effects of *H adatoms on the C-O cleavage are still confusing. This study investigates the H-assisted CO2 dissociation pathways on the PdnPt(4-n)/In2O3 (n = 0-4) catalysts via DFT calculation. Initially, the adsorption properties of *H2, *COOH, and *HCOO species are calculated. Then, two H-assisted CO2 dissociation channels, i.e., *CO2 + *H → *COOH → *CO + *OH and *CO2 + *H → *HCOO → *CHO + *O, are studied. Results show that Pt and Pd promote the CO2 hydrogenation and C-O bond cleavage reactions, respectively. In comparison to CO2 direct dissociation, the COOH-mediated and HCOO-mediated channels facilitate and impede the C-O bond cleavage, respectively. Overall, the Pd3Pt/In2O3 constituent is suggested for the H-assisted CO2 dissociation reaction. The electronic effects of the PdnPt(4-n) bimetals adjust the stabilities of the intermediates and barriers of the elementary steps, and the interactions between PdnPt(4-n) and In2O3 provide extra sites for the adsorbates and reaction steps. This study reveals the effects of *H on the C-O bond dissociation processes and provides useful insight into designing PdPt/In2O3 catalysts for CO2 hydrogenation reactions.

Direct in-situ imaging of electrochemical corrosion of Pd-Pt core-shell electrocatalysts

Corrosion of electrocatalysts during electrochemical operations, such as low potential - high potential cyclic swapping, can cause significant performance degradation. However, the electrochemical corrosion dynamics, including structural changes, especially site and composition specific ones, and their correlation with electrochemical processes are hidden due to the insufficient spatial-temporal resolution characterization methods. Using electrochemical liquid cell transmission electron microscopy, we visualize the electrochemical corrosion of Pd@Pt core-shell octahedral nanoparticles towards a Pt nanoframe. The potential-dependent surface reconstruction during multiple continuous in-situ cyclic voltammetry with clear redox peaks is captured, revealing an etching and deposition process of Pd that results in internal Pd atoms being relocated to external surface, followed by subsequent preferential corrosion of Pt (111) terraces rather than the edges or corners, simultaneously capturing the structure evolution also allows to attribute the site-specific Pt and Pd atomic dynamics to individual oxidation and reduction events. This work provides profound insights into the surface reconstruction of nanoparticles during complex electrochemical processes.

Atomic/molecular layer deposition strategies for enhanced CO2 capture, utilisation and storage materials

Elevated levels of carbon dioxide (CO2) in the atmosphere and the diminishing reserves of fossil fuels have raised profound concerns regarding the resulting consequences of global climate change and the future supply of energy. Hence, the reduction and transformation of CO2 not only mitigates environmental pollution but also generates value-added chemicals, providing a dual remedy to address both energy and environmental challenges. Despite notable advancements, the low conversion efficiency of CO2 remains a major obstacle, largely attributed to its inert chemical nature. It is imperative to engineer catalysts/materials that exhibit high conversion efficiency, selectivity, and stability for CO2 transformation. With unparalleled precision at the atomic level, atomic layer deposition (ALD) and molecular layer deposition (MLD) methods utilize various strategies, including ultrathin modification, overcoating, interlayer coating, area-selective deposition, template-assisted deposition, and sacrificial-layer-assisted deposition, to synthesize numerous novel metal-based materials with diverse structures. These materials, functioning as active materials, passive materials or modifiers, have contributed to the enhancement of catalytic activity, selectivity, and stability, effectively addressing the challenges linked to CO2 transformation. Herein, this review focuses on ALD and MLD's role in fabricating materials for electro-, photo-, photoelectro-, and thermal catalytic CO2 reduction, CO2 capture and separation, and electrochemical CO2 sensing. Significant emphasis is dedicated to the ALD and MLD designed materials, their crucial role in enhancing performance, and exploring the relationship between their structures and catalytic activities for CO2 transformation. Finally, this comprehensive review presents the summary, challenges and prospects for ALD and MLD-designed materials for CO2 transformation.

Molecular Mechanism for Converting Carbon Dioxide Surrounding Water Microdroplets Containing 1,2,3-Triazole to Formic Acid

Spraying water microdroplets containing 1,2,3-triazole (Tz) has been found to effectively convert gas-phase carbon dioxide (CO2), but not predissolved CO2, into formic acid (FA). Herein, we elucidate the reaction mechanism at the molecular level through quantum chemistry calculations and ab initio molecular dynamics (AIMD) simulations. Computations suggest a multistep reaction mechanism that initiates from the adsorption of CO2 by Tz to form a CO2-Tz complex (named reactant complex (RC)). Then, the RC either is reduced by electrons that were generated at the air-liquid interface of the water microdroplet and then undergoes intramolecular proton transfer (PT) or switches the reduction and PT steps to form a [HCO2-(Tz-H)]- complex (named PC-). Subsequently, PC- undergoes reduction and the C-N bond dissociates to generate COOH- and [Tz-H]- (m/z = 69). COOH- easily converts to HCOOH and is captured at m/z = 45 in mass spectroscopy. Notably, the intramolecular PT step can be significantly lowered by the oriented electric field at the interface and a water-bridge mechanism. The mechanism is further confirmed by testing multiple azoles. The AIMD simulations reveal a novel proton transfer mechanism where water serves as a transporter and is shown to play an important role dynamically. Moreover, the transient •COOH captured by the experiment is proposed to be partly formed by the reaction with H•, pointing again to the importance of the air-water interface. This work provides valuable insight into the important mechanistic, kinetic, and dynamic features of converting gas-phase CO2 to valuable products by azoles or amines dissolved in water microdroplets.

Design and Construction of Artificial Biological Systems for One-Carbon Utilization

The third-generation (3G) biorefinery aims to use microbial cell factories or enzymatic systems to synthesize value-added chemicals from one-carbon (C1) sources, such as CO2, formate, and methanol, fueled by renewable energies like light and electricity. This promising technology represents an important step toward sustainable development, which can help address some of the most pressing environmental challenges faced by modern society. However, to establish processes competitive with the petroleum industry, it is crucial to determine the most viable pathways for C1 utilization and productivity and yield of the target products. In this review, we discuss the progresses that have been made in constructing artificial biological systems for 3G biorefineries in the last 10 years. Specifically, we highlight the representative works on the engineering of artificial autotrophic microorganisms, tandem enzymatic systems, and chemo-bio hybrid systems for C1 utilization. We also prospect the revolutionary impact of these developments on biotechnology. By harnessing the power of 3G biorefinery, scientists are establishing a new frontier that could potentially revolutionize our approach to industrial production and pave the way for a more sustainable future.

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.

Influence of Cations on HCOOH and CO Formation during CO2 Reduction on a PdMLPt(111) Electrode

Understanding the role of cations in the electrochemical CO2 reduction (CO2RR) process is of fundamental importance for practical application. In this work, we investigate how cations influence HCOOH and CO formation on PdMLPt(111) in pH 3 electrolytes. While only (a small amount of adsorbed) CO forms on PdMLPt(111) in the absence of metal cations, the onset potential of HCOOH and CO decreases with increasing cation concentrations. The cation effect is stronger on HCOOH formation than that on CO formation on PdMLPt(111). Density functional theory simulations indicate that cations facilitate both hydride formation and CO2 activation by polarizing the electronic density at the surface and stabilizing *CO2-. Although the upshift of the metal work function caused by high coverage of adsorbates limits hydride formation, the cation-induced electric field counterbalances this effect in the case of *H species, sustaining HCOOH production at mild negative potentials. Instead, at the high *CO coverages observed at very negative potentials, surface hydrides do not form, preventing the HCOOH route both in the absence and presence of cations. Our results open the way for a consistent evaluation of cationic electrolyte effects on both activity and selectivity in CO2RR on Pd-Pt catalysts.

Integrated CO2 capture and electrochemical upgradation: the underpinning mechanism and techno-chemical analysis

Coupling post-combustion CO2 capture with electrochemical utilization (CCU) is a quantum leap in renewable energy science since it eliminates the cost and energy involved in the transport and storage of CO2. However, the major challenges involved in industrial scale implementation are selecting an appropriate solvent/electrolyte for CO2 capture, modeling an appropriate infrastructure by coupling an electrolyser with a CO2 point source and a separator to isolate CO2 reduction reaction (CO2RR) products, and finally selection of an appropriate electrocatalyst. In this review, we highlight the major difficulties with detailed mechanistic interpretation in each step, to find out the underpinning mechanism involved in the integration of electrochemical CCU to achieve higher-value products. In the past decades, most of the studies dealt with individual parts of the integration process, i.e., either selecting a solvent for CO2 capture, designing an electrocatalyst, or choosing an ideal electrolyte. In this context, it is important to note that solvents such as monoethanolamine, bicarbonate, and ionic liquids are often used as electrolytes in CO2 capture media. Therefore, it is essential to fabricate a cost-effective electrolyser that should function as a reversible binder with CO2 and an electron pool capable of recovering the solvent to electrolyte reversibly. For example, reversible ionic liquids, which are non-ionic in their normal forms, but produce ionic forms after CO2 capture, can be further reverted back to their original non-ionic forms after CO2 release with almost 100% efficiency through the chemical or thermal modulations. This review also sheds light on a focused techno-economic evolution for converting the electrochemically integrated CCU process from a pilot-scale project to industrial-scale implementation. In brief, this review article will summarize a state-of-the-art argumentation of challenges and outcomes over the different segments involved in electrochemically integrated CCU to stimulate urgent progress in the field.

Smart manufacturing inspired approach to research, development, and scale-up of electrified chemical manufacturing systems

As renewable electricity becomes cost competitive with fossil fuel energy sources and environmental concerns increase, the transition to electrified chemical and fuel synthesis pathways becomes increasingly desirable. However, electrochemical systems have traditionally taken many decades to reach commercial scales. Difficulty in scaling up electrochemical synthesis processes comes primarily from difficulty in decoupling and controlling simultaneously the effects of intrinsic kinetics and charge, heat, and mass transport within electrochemical reactors. Tackling this issue efficiently requires a shift in research from an approach based on small datasets, to one where digitalization enables rapid collection and interpretation of large, well-parameterized datasets, using artificial intelligence (AI) and multi-scale modeling. In this perspective, we present an emerging research approach that is inspired by smart manufacturing (SM), to accelerate research, development, and scale-up of electrified chemical manufacturing processes. The value of this approach is demonstrated by its application toward the development of CO₂ electrolyzers.

Promote electroreduction of CO2 via catalyst valence state manipulation by surface-capping ligand

Electrochemical CO2 reduction provides a potential means for synthesizing value-added chemicals over the near equilibrium potential regime, i.e., formate production on Pd-based catalysts. However, the activity of Pd catalysts has been largely plagued by the potential-depended deactivation pathways (e.g., [Formula: see text]-PdH to [Formula: see text]-PdH phase transition, CO poisoning), limiting the formate production to a narrow potential window of 0 V to -0.25 V vs. reversible hydrogen electrode (RHE). Herein, we discovered that the Pd surface capped with polyvinylpyrrolidone (PVP) ligand exhibits effective resistance to the potential-depended deactivations and can catalyze formate production at a much extended potential window (beyond -0.7 V vs. RHE) with significantly improved activity (~14-times enhancement at -0.4 V vs. RHE) compared to that of the pristine Pd surface. Combined results from physical and electrochemical characterizations, kinetic analysis, and first-principle simulations suggest that the PVP capping ligand can effectively stabilize the high-valence-state Pd species (Pdδ+) resulted from the catalyst synthesis and pretreatments, and these Pdδ+ species are responsible for the inhibited phase transition from [Formula: see text]-PdH to [Formula: see text]-PdH, and the suppression of CO and H2 formation. The present study confers a desired catalyst design principle, introducing positive charges into Pd-based electrocatalyst to enable efficient and stable CO2 to formate conversion.

Upcycling air pollutants to fuels and chemicals via electrochemical reduction technology

The intensification of fossil fuel usage results in significant air pollution levels. Efforts have been put into developing efficient technologies capable of converting air pollution into valuable products, including fuels and valuable chemicals (e.g., CO₂ to hydrocarbon and syngas and NO_x to ammonia). Among the strategic efforts to mitigate the excessive concentration of CO₂ and NO_x pollutants in the atmosphere, the electrochemical reduction technology of CO₂ (CO₂RR) and NO_x (NO_xRR) emerges as one of the most promising approaches. It is even more attractive if CO₂RR and NO_xRR are paired with renewables to store intermittent electricity in the form of chemical feedstocks. This review provides an overview of the electrochemical reduction process to convert CO₂ to C₁ and/or C₂₊ chemicals and NO_x to ammonia (NH₃) with a focus on electrocatalysts, electrolytes, electrolyzer, and catalytic reactor designs toward highly selective electrochemical conversion of the desired products. While the attempts in these aspects are enormous, economic consideration and environmental feasibility for actual implementation are not comprehensively provided. We discuss CO₂RR and NO_xRR from the life cycle and techno-economic analyses to perceive the feasibility of the current achievements. The remaining challenges associated with the industrial implementation of electrochemical CO₂ and NO_x reduction are additionally provided.

Investigation of Molecular Mechanism of Cobalt Porphyrin Catalyzed CO2 Electrochemical Reduction in Ionic Liquid by In-Situ SERS

This study explores the electrochemical reduction in CO₂ using room temperature ionic liquids as solvents or electrolytes, which can minimize the environmental impact of CO₂ emissions. To design effective CO₂ electrochemical systems, it is crucial to identify intermediate surface species and reaction products in situ. The study investigates the electrochemical reduction in CO₂ using a cobalt porphyrin molecular immobilized electrode in 1-n-butyl-3-methyl imidazolium tetrafluoroborate (BMI.BF4) room temperature ionic liquids, through in-situ surface-enhanced Raman spectroscopy (SERS) and electrochemical technique. The results show that the highest faradaic efficiency of CO produced from the electrochemical reduction in CO₂ can reach 98%. With the potential getting more negative, the faradaic efficiency of CO decreases while H₂ is produced as a competitive product. Besides, water protonates porphyrin macrocycle, producing pholorin as the key intermediate for the hydrogen evolution reaction, leading to the out-of-plane mode of the porphyrin molecule. Absorption of CO₂ by the ionic liquids leads to the formation of BMI·CO₂ adduct in BMI·BF₄ solution, causing vibration modes at 1100, 1457, and 1509 cm^-1. However, the key intermediate of CO2-· radical is not observed. The υ(CO) stretching mode of absorbed CO is affected by the electrochemical Stark effect, typical of CO chemisorbed on a top site.

Electrochemical organic reactions: A tutorial review

Although the core of electrochemistry involves simple oxidation and reduction reactions, it can be complicated in real electrochemical organic reactions. The principles used in electrochemical reactions have been derived using physical organic chemistry, which drives other organic/inorganic reactions. This review mainly comprises two themes: the first discusses the factors that help optimize an electrochemical reaction, including electrodes, supporting electrolytes, and electrochemical cell design, and the second outlines studies conducted in the field over a period of 10 years. Electrochemical reactions can be used as a versatile tool for synthetically important reactions by modifying the constant electrolysis current.

Recent trends of biotechnological production of polyhydroxyalkanoates from C1 carbon sources

Growing concerns over the use of limited fossil fuels and their negative impacts on the ecological niches have facilitated the exploration of alternative routes. The use of conventional plastic material also negatively impacts the environment. One such green alternative is polyhydroxyalkanoates, which are biodegradable, biocompatible, and environmentally friendly. Recently, researchers have focused on the utilization of waste gases particularly those belonging to C1 sources derived directly from industries and anthropogenic activities, such as carbon dioxide, methane, and methanol as the substrate for polyhydroxyalkanoates production. Consequently, several microorganisms have been exploited to utilize waste gases for their growth and biopolymer accumulation. Methylotrophs such as Methylobacterium organophilum produced highest amount of PHA up to 88% using CH₄ as the sole carbon source and 52-56% with CH₃OH. On the other hand Cupriavidus necator, produced 71-81% of PHA by utilizing CO and CO₂ as a substrate. The present review shows the potential of waste gas valorization as a promising solution for the sustainable production of polyhydroxyalkanoates. Key bottlenecks towards the usage of gaseous substrates obstructing their realization on a large scale and the possible technological solutions were also highlighted. Several strategies for PHA production using C1 gases through fermentation and metabolic engineering approaches are discussed. Microbes such as autotrophs, acetogens, and methanotrophs can produce PHA from CO₂, CO, and CH₄. Therefore, this article presents a vision of C1 gas into bioplastics are prospective strategies with promising potential application, and aspects related to the sustainability of the system.

Challenges and Opportunities in Electrocatalytic CO2 Reduction to Chemicals and Fuels

The global temperature increase must be limited to below 1.5 °C to alleviate the worst effects of climate change. Electrocatalytic CO2 reduction (ECO2 R) to generate chemicals and feedstocks is considered one of the most promising technologies to cut CO2 emission at an industrial level. However, despite decades of studies, advances at the laboratory scale have not yet led to high industrial deployment rates. This Review discusses practical challenges in the industrial chain that hamper the scaling-up deployment of the ECO2 R technology. Faradaic efficiencies (FEs) of about 100 % and current densities above 200 mA cm-2 have been achieved for the ECO2 R to CO/HCOOH, and the stability of the electrolysis system has been prolonged to 2000 h. For ECO2 R to C2 H4 , the maximum FE is over 80 %, and the highest current density has reached the A cm-2 level. Thus, it is believed that ECO2 R may have reached the stage for scale-up. We aim to provide insights that can accelerate the development of the ECO2 R technology.

Surface Structure Engineering of PdAg Alloys with Boosted CO2 Electrochemical Reduction Performance

Converting carbon dioxide into high-value-added formic acid as a basic raw material for the chemical industry via an electrochemical process under ambient conditions not only alleviates greenhouse gas effects but also contributes to effective carbon cycles. Unfortunately, the most commonly used Pd-based catalysts can be easily poisoned by the in situ formed minor byproduct CO during the carbon dioxide reduction reaction (CRR) process. Herein, we report a facile method to synthesize highly uniformed PdAg alloys with tunable morphologies and electrocatalytic performance via a simple liquid synthesis approach. By tuning the molar ratio of the Ag⁺ and Pd²⁺ precursors, the morphologies, composition, and electrocatalytic activities of the obtained materials were well-regulated, which was characterized by TEM, XPS, XRD, as well as electrocatalytic measurements. The CRR results showed that the as-obtained Pd₃Ag exhibited the highest performance among the five samples, with a faradic efficient (FE) of 96% for formic acid at -0.2 V (vs. reference hydrogen electrode (RHE)) and superior stability without current density decrease. The enhanced ability to adsorb and activate CO₂ molecules, higher resistance to CO, and a faster electronic transfer speed resulting from the alloyed PdAg nanostructure worked together to make great contributions to the improvement of the CRR performance. These findings may provide a new feasible route toward the rational design and synthesis of alloy catalysts with high stability and selectivity for clean energy storage and conversion in the future.

Influence of Ag Metal Dispersion on the Catalyzed Reduction of CO2 into Chemical Fuels over Ag-ZrO2 Catalysts

Metal/metal oxide catalysts reveal unique CO₂ adsorption and hydrogenation properties in CO₂ electroreduction for the synthesis of chemical fuels. The dispersion of active components on the surface of metal oxide has unique quantum effects, significantly affecting the catalytic activity and selectivity. Catalyst models with 25, 50, and 75% Ag covering on ZrO₂, denoted as Ag₄/(ZrO₂)₉, Ag₈/(ZrO₂)₉, and Ag₁₂/(ZrO₂)₉, respectively, were developed and coupled with a detailed investigation of the electronic properties and electroreduction processes from CO₂ into different chemical fuels using density functional theory calculations. The dispersion of Ag can obviously tune the hybridization between the active site of the catalyst and the O atom of the intermediate species CH₃O^* derived from the reduction of CO₂, which can be expected as the key intermediate to lead the reduction path to differentiation of generation of CH₄ and CH₃OH. The weak hybridization between CH₃O^* and Ag₄/(ZrO₂)₉ and Ag₁₂/(ZrO₂)₉ favors the further reduction of CH₃O^* into CH₃OH. In stark contrast, the strong hybridization between CH₃O^* and Ag₈/(ZrO₂)₉ promotes the dissociation of the C-O bond of CH₃O^*, thus leading to the generation of CH₄. Results provide a fundamental understanding of the CO₂ reduction mechanism on the metal/metal oxide surface, favoring novel catalyst rational design and chemical fuel production.

Selective Reduction of CO2 on Ti2C(OH)2 MXene through Spontaneous Crossing of Transition States

Direct reduction of gas-phase CO₂ to renewable fuels and chemical feedstock without any external energy source or rare-metal catalyst is one of the foremost challenges. Here, using density functional theory and ab initio molecular dynamics (AIMD) simulations, we predict Ti₂C(OH)₂ MXene as an efficient electron-coupled proton donor exhibiting simultaneously high reactivity and selectivity for CO₂ reduction reaction (CRR) by yielding valuable chemicals, formate, and formic acid. This is caused by CO₂ spontaneously crossing the activation barrier involved in the formation of multiple intermediates. Metallic Ti₂C(OH)₂ contains easily donatable protons on the surface and high-energy electrons near the Fermi level that leads to its high reactivity. High selectivity arises from low activation barrier for CRR as predicted by proposed mechanistic interpretations. Furthermore, H vacancies generated during the product formation can be replenished by exposure to moisture, ensuring the uninterrupted formation of the products. Our study provides a single-step solution for CRR to valuable chemicals without necessitating the expensive electrochemical or low-efficiency photochemical cells and hence is of immense interest for recycling the carbon.

Energy conversion efficiency comparison of different aqueous and semi-aqueous CO2 electroreduction systems

An energy conversion efficiency index, that is independent of the anode reaction performance, is proposed for CO₂ reduction in aqueous and semi-aqueous systems. The energy conversion efficiency of CO₂ reduction under 107 typical conditions was calculated based on the derived formula. Notably, the resulting efficiency trends of the reduction products differed from their faradaic efficiency trends. When the products were CO, HCOOH, C₂H₄, and CH₄, the electrocatalysts with the higher energy conversion efficiencies were Au, Pd, Cu, and Pt, respectively. Based on the discussion on the overall energy conversion efficiency of all products, Pt should be a specific energetically advantageous catalyst for CO₂ reduction because the activation energy is negligibly small. Moreover, the energy conversion and faradaic efficiencies were discovered to not only depend on the electrocatalyst species, but also on the complexity of the reaction, including the number of reaction electrons. Our proposed method for evaluating the energy conversion efficiency of cathode reactions can potentially serve as a novel platform for comparing the CO₂ reduction efficiencies of different electroreduction systems.

Performance of Paracoccus pantotrophus MA3 in heterotrophic nitrification-anaerobic denitrification using formic acid as a carbon source

Excess amount of nitrogen in wastewater has caused serious concerns, such as water eutrophication. Paracoccus pantotrophus MA3, a novel isolated strain of heterotrophic nitrification-anaerobic denitrification bacteria, was evaluated for nitrogen removal using formic acid as the sole carbon source. The results showed that the maximum ammonium removal efficiency was observed under the optimum conditions of 26.25 carbon to nitrogen ratio, 3.39% (v/v) inoculation amount, 34.64 °C temperature, and at 180 rpm shaking speed, respectively. In addition, quantitative real-time PCR technique analysis assured that the gene expression level of formate dehydrogenase, formate tetrahydrofolate ligase, 5,10-methylenetetrahydrofolate dehydrogenase, serine hydroxymethyltransferase, respiratory nitrate reductase beta subunit, L-glutamine synthetase, glutamate dehydrogenase, and glutamate synthase were up-regulated compared to the control group, and combined with nitrogen mass balance analysis to conclude that most of the ammonium was removed by assimilation. A small amount of nitrate and nearly no nitrite were accumulated during heterotrophic nitrification. MA3 exhibited significant denitrification potential under anaerobic conditions with a maximum nitrate removal rate of 4.39 mg/L/h, and the only gas produced was N₂. Additionally, 11.50 ± 0.06 mg/L/h of NH₄⁺-N removal rate from biogas slurry was achieved.

Electrochemical conversion of CO2 to value-added chemicals over bimetallic Pd-based nanostructures: Recent progress and emerging trends

Electrochemical conversion of CO₂ to fuels and chemicals as a sustainable solution for waste transformation has garnered tremendous interest to combat the fervent issue of the prevailing high atmospheric CO₂ concentration while contributing to the generation of sustainable energy. Monometallic palladium (Pd) has been shown promising in electrochemical CO₂ reduction, producing formate or CO depending on applied potentials. Recently, bimetallic Pd-based materials strived to fine-tune the binding affinity of key intermediates is a prominent strategy for the desired product formation from CO₂ reduction. Herein, the recent emerging trends on bimetallic Pd-based electrocatalysts are reviewed, including fundamentals of CO₂ electroreduction and material engineering of bimetallic Pd-electrocatalysts categorized by primary products. Modern analytical techniques on these novel electrocatalysts are also thoroughly studied to get insights into reaction mechanisms. Lastly, we deliberate over the challenges and prospects for Pd-based catalysts for electrochemical CO₂ conversion.

Tuning Reaction Pathways of Electrochemical Conversion of CO2 by Growing Pd Shells on Ag Nanocubes

The electrochemical carbon dioxide reduction reaction (CO₂RR) has been studied on Ag, Pd, Ag@Pd_1-2L nanocubes using a combination of in situ characterization and density functional theory calculations. By manipulating the deposition and diffusion rates of Pd atoms on Ag nanocubes, Ag@Pd core-shell nanocubes with a shell thickness of 1-2 atomic layers have been successfully synthesized for CO₂RR. Pd nanocubes produce CO with high selectivity due to the transformation of Pd to Pd hydride (PdH) during CO₂RR. In contrast, PdH formation becomes more difficult in Ag@Pd_1-2L core-shell nanocubes, which inhibits CO production from the *HOCO intermediate and thus tunes the reaction pathway toward HCOOH. Ag nanocubes exhibit high selectivity toward H₂, and there is no phase transition during CO₂RR. The results demonstrate that the CO₂RR reaction pathways can be manipulated through engineering the surface structure of Pd-based catalysts by allowing or preventing the formation of PdH.

The integration of bio-catalysis and electrocatalysis to produce fuels and chemicals from carbon dioxide

The dependence on fossil fuels has caused excessive emissions of greenhouse gases (GHGs), leading to climate changes and global warming. Even though the expansion of electricity generation will enable a wider use of electric vehicles, biotechnology represents an attractive route for producing high-density liquid transportation fuels that can reduce GHG emissions from jets, long-haul trucks and ships. Furthermore, to achieve immediate alleviation of the current environmental situation, besides reducing carbon footprint it is urgent to develop technologies that transform atmospheric CO₂ into fossil fuel replacements. The integration of bio-catalysis and electrocatalysis (bio-electrocatalysis) provides such a promising avenue to convert CO₂ into fuels and chemicals with high-chain lengths. Following an overview of different mechanisms that can be used for CO₂ fixation, we will discuss crucial factors for electrocatalysis with a special highlight on the improvement of electron-transfer kinetics, multi-dimensional electrocatalysts and their hybrids, electrolyser configurations, and the integration of electrocatalysis and bio-catalysis. Finally, we prospect key advantages and challenges of bio-electrocatalysis, and end with a discussion of future research directions.

Nano-crumples induced Sn-Bi bimetallic interface pattern with moderate electron bank for highly efficient CO2 electroreduction

CO₂ electroreduction reaction offers an attractive approach to global carbon neutrality. Industrial CO₂ electrolysis towards formate requires stepped-up current densities, which is limited by the difficulty of precisely reconciling the competing intermediates (COOH* and HCOO*). Herein, nano-crumples induced Sn-Bi bimetallic interface-rich materials are in situ designed by tailored electrodeposition under CO₂ electrolysis conditions, significantly expediting formate production. Compared with Sn-Bi bulk alloy and pure Sn, this Sn-Bi interface pattern delivers optimum upshift of Sn p-band center, accordingly the moderate valence electron depletion, which leads to weakened Sn-C hybridization of competing COOH* and suitable Sn-O hybridization of HCOO*. Superior partial current density up to 140 mA/cm² for formate is achieved. High Faradaic efficiency (>90%) is maintained at a wide potential window with a durability of 160 h. In this work, we elevate the interface design of highly active and stable materials for efficient CO₂ electroreduction.

Electroreduction of CO2 toward High Current Density

Carbon dioxide (CO2) electroreduction offers an attractive pathway for converting CO2 to valuable fuels and chemicals. Despite the existence of some excellent electrocatalysts with superior selectivity for specific products, these reactions are conducted at low current densities ranging from several mA cm−2 to tens of mA cm−2, which are far from commercially desirable values. To extend the applications of CO2 electroreduction technology to an industrial scale, long-term operations under high current densities (over 200 mA cm−2) are desirable. In this paper, we review recent major advances toward higher current density in CO2 reduction, including: (1) innovations in electrocatalysts (engineering the morphology, modulating the electronic structure, increasing the active sites, etc.); (2) the design of electrolyzers (membrane electrode assemblies, flow cells, microchannel reactors, high-pressure cells, etc.); and (3) the influence of electrolytes (concentration, pH, anion and cation effects). Finally, we discuss the current challenges and perspectives for future development toward high current densities.

Electrocatalytic Reduction of CO2 in Water by a Palladium-Containing Metallopolymer

The palladium–salen complex was immobilized by electropolymerization onto a Pt disc electrode and applied as an electrocatalyst for the reduction of CO2 in an aqueous solution. Linear sweep voltammetry measurements and rotating disk experiments were carried out to study the electrochemical reduction of carbon dioxide. The onset overpotential for carbon dioxide reduction was approximately −0.22 V vs. NHE on the poly-Pd(salen) modified electrode. In addition, by combining the electrochemical study with a kinetic study, the rate-determining step of the electrochemical CO2 reduction reaction (CO2RR) was found to be the radial reduction of carbon dioxide to the CO adsorbed on the metal.

Intercalated Gold Nanoparticle in 2D Palladium Nanosheet Avoiding CO Poisoning for Formate Production under a Wide Potential Window

The electrochemical CO₂ reduction into formate acid over Pd-based catalysts under a wide potential window is a challenging task; CO poisoning commonly occurring on the vulnerable surface of Pd must be overcome. Herein, we designed a two-dimensional (2D) AuNP-in-PdNS electrocatalyst, in which the Au nanoparticles are intercalated in Pd nanosheets, for formate production under a wide potential window from -0.1 to -0.7 V versus a reversible hydrogen electrode. Based on the X-ray absorption spectra (XAS) characterizations, CO accumulation detection, and CO stripping voltammetry measurements, we observed that the intercalated Au nanoparticles could effectively avoid the CO formation and boost the formate production on the Pd nanosheet surface by regulating its electronic structure.

Emerging interstitial/substitutional modification of Pd-based nanomaterials with nonmetallic elements for electrocatalytic applications

Palladium (Pd)-based nanomaterials have been identified as potential candidates for various types of electrocatalytic reaction, but most of them typically exhibit unsatisfactory performances. Recently, extensive theoretical and experimental studies have demonstrated that the interstitial/substitutional modification of Pd-based nanomaterials with nonmetallic atoms (H, B, C, N, P, S) has a significant impact on their electronic structure and thus leads to the rapid development of one kind of promising catalyst for various electrochemical reactions. Considering the remarkable progress in this area, we highlight the most recent progress regarding the innovative synthesis and advanced characterization methods of nonmetallic atom-doped Pd-based nanomaterials and provide insights into their electrochemical applications. What's more, the unique structure- and component-dependent electrochemical performance and the underlying mechanisms are also discussed. Furthermore, a brief conclusion about the recent progress achieved in this field as well as future perspectives and challenges are provided.

Recent Progress in Two-Dimensional Materials for Electrocatalytic CO2 Reduction

Electrocatalytic CO2 reduction (ECR) is an attractive approach to convert atmospheric CO2 to value-added chemicals and fuels. However, this process is still hindered by sluggish CO2 reaction kinetics and the lack of efficient electrocatalysts. Therefore, new strategies for electrocatalyst design should be developed to solve these problems. Two-dimensional (2D) materials possess great potential in ECR because of their unique electronic and structural properties, excellent electrical conductivity, high atomic utilization and high specific surface area. In this review, we summarize the recent progress on 2D electrocatalysts applied in ECR. We first give a brief description of ECR fundamentals and then discuss in detail the development of different types of 2D electrocatalysts for ECR, including metal, graphene-based materials, transition metal dichalcogenides (TMDs), metal–organic frameworks (MOFs), metal oxide nanosheets and 2D materials incorporated with single atoms as single-atom catalysts (SACs). Metals, such as Ag, Cu, Au, Pt and Pd, graphene-based materials, metal-doped nitric carbide, TMDs and MOFs can mostly only produce CO with a Faradic efficiencies (FE) of 80~90%. Particularly, SACs can exhibit FEs of CO higher than 90%. Metal oxides and graphene-based materials can produce HCOOH, but the FEs are generally lower than that of CO. Only Cu-based materials can produce high carbon products such as C2H4 but they have low product selectivity. It was proposed that the design and synthesis of novel 2D materials for ECR should be based on thorough understanding of the reaction mechanism through combined theoretical prediction with experimental study, especially in situ characterization techniques. The gap between laboratory synthesis and large-scale production of 2D materials also needs to be closed for commercial applications.

Activation of CO2 and CH4 on MgO surfaces: mechanistic insights from first-principles theory

One of the most challenging topics in heterogeneous catalysis is conversion of CH₄ to higher hydrocarbons. Direct conversion of CH₄ to ethylene can be achieved via the oxidative coupling of methane (OCM) reaction. Despite studies which have shown MgO to activate CH₄ and initiate the OCM reaction, its large-scale applications face a significant impediment due to formation of a byproduct, CO₂, and poisoning of the catalyst due to carbonate formation. In the present work, we address two aspects of the OCM reaction on MgO surfaces: carbonate formation on the surface of the catalyst, and (dissociative) adsorption of CH₄. We use first-principles density functional theoretical calculations to determine the energetics and underlying mechanisms of interaction of CO₂ and CH₄ with various surfaces of MgO: (100), (110), and (111) (both Mg- and O-terminations), and the seldom studied, hydroxylated (111) MgO surface with O-termination. We find that the strength of the interaction of CO₂ with MgO surfaces depends on several factors: their surface energies, coordination number of surface O atoms, and ability to donate electrons. However, the O-terminated (111) surface of MgO bucks all aforementioned factors, with only oxygen richness affecting its reactivity towards CO₂. The interaction of CH₄ with MgO surfaces depends primarily on the coordination number of the surface O atoms and the orientation of the CH₄ molecule with respect to the surface. Finally, we provide insights into (a) formation of surface carbonates, which is relevant to CO₂ capture and conversion, and (b) C-H bond activation on MgO surfaces, which is crucial for direct conversion of CH₄ to value-added chemicals.

How palladium inhibits CO poisoning during electrocatalytic formic acid oxidation and carbon dioxide reduction

Development of reversible and stable catalysts for the electrochemical reduction of CO₂ is of great interest. Here, we elucidate the atomistic details of how a palladium electrocatalyst inhibits CO poisoning during both formic acid oxidation to carbon dioxide and carbon dioxide reduction to formic acid. We compare results obtained with a platinum single-crystal electrode modified with and without a single monolayer of palladium. We combine (high-scan-rate) cyclic voltammetry with density functional theory to explain the absence of CO poisoning on the palladium-modified electrode. We show how the high formate coverage on the palladium-modified electrode protects the surface from poisoning during formic acid oxidation, and how the adsorption of CO precursor dictates the delayed poisoning during CO₂ reduction. The nature of the hydrogen adsorbed on the palladium-modified electrode is considerably different from platinum, supporting a model to explain the reversibility of this reaction. Our results help in designing catalysts for which CO poisoning needs to be avoided.

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.

The inchoate horizon of electrolyzer designs, membranes and catalysts towards highly efficient electrochemical reduction of CO2 to formic acid

This review explores the recent advances in CO2 reactor configurations, components, membranes and electrocatalysts for HCOOH generation and draw readers attention to construct the economic, scalable and energy efficient CO2R electrolyzers.

Surface Hydrides on Fe2P Electrocatalyst Reduce CO2 at Low Overpotential: Steering Selectivity to Ethylene Glycol

Development of efficient electrocatalysts for the CO₂ reduction reaction (CO₂RR) to multicarbon products has been constrained by high overpotentials and poor selectivity. Here, we introduce iron phosphide (Fe₂P) as an earth-abundant catalyst for the CO₂RR to mainly C₂-C₄ products with a total CO₂RR Faradaic efficiency of 53% at 0 V vs RHE. Carbon product selectivity is tuned in favor of ethylene glycol formation with increasing negative bias at the expense of C₃-C₄ products. Both Grand Canonical-DFT (GC-DFT) calculations and experiments reveal that *formate, not *CO, is the initial intermediate formed from surface phosphino-hydrides and that the latter form ionic hydrides at both surface phosphorus atoms (H@P_s) and P-reconstructed Fe₃ hollow sites (H@P*). Binding of these surface hydrides weakens with negative bias (reactivity increases), which accounts for both the shift to C₂ products over higher C-C coupling products and the increase in the H₂ evolution reaction (HER) rate. GC-DFT predicts that phosphino-hydrides convert *formate to *formaldehyde, the key intermediate for C-C coupling, whereas hydrogen atoms on Fe generate tightly bound *CO via sequential PCET reactions to H₂O. GC-DFT predicts the peak in CO₂RR current density near -0.1 V is due to a local maximum in the binding affinity of *formate and *formaldehyde at this bias, which together with the more labile C₂ product affinity, accounts for the shift to ethylene glycol and away from C₃-C₄ products. Consistent with these predictions, addition of exogenous CO is shown to block all carbon product formation and lower the HER rate. These results demonstrate that the formation of ionic hydrides and their binding affinity, as modulated by the applied potential, controls the carbon product distribution. This knowledge provides new insight into the influence of hydride speciation and applied bias on the chemical reaction mechanism of CO₂RR that is relevant to all transition metal phosphides.

Regulation of electrocatalytic activity by local microstructure: focusing on catalytic active zone

Traditional regulation methods of active sites have been successfully optimized the performance of electrocatalysts, but seem unable to achieve further breakthrough in the catalytic activity. Unlike the conventional viewpoint of focusing on single active site, the concept of local microstructure active zone is more comprehensive and new suits of methods to regulate reaction zone for electrocatalytic reactions are developed accordingly. The local microstructure active zone refers to the zone with high catalytic activity formed by the interaction between active atoms and neighboring coordination atoms as well as the surrounding environment. Instead of the traditional single active atom site, the active zone is more suitable for the actual electrochemical reaction process. According to this concept, the activity of the electrocatalysts can be coordinated by multiple active atoms. This strategy is beneficial to understand the relationship between material, structure and catalysis, which realizes the scientific design and synthesis of high performance electrocatalysts. This review provides the research progress of this strategy in electrocatalytic reactions, with the emphasis on their important applications in oxygen evolution reaction, urea oxidation reaction and carbon dioxide reduction.

Machine Learning in Screening High Performance Electrocatalysts for CO2 Reduction

Converting CO₂ into carbon-based fuels is promising for relieving the greenhouse gas effect and the energy crisis. However, the selectivity and efficiency of current electrocatalysts for CO₂ reductions are still not satisfactory. In this paper, the development of machine learning methods in screening CO₂ reduction electrocatalysts over the recent years is reviewed. Through high-throughput calculation of some key descriptors such as adsorption energies, d-band center, and coordination number by well-constructed machine learning models, the catalytic activity, optimal composition, active sites, and CO₂ reduction reaction pathway over various possible materials can be predicted and understood. Machine learning is now realized as a fast and low-cost method to effectively explore high performance electrocatalysts for CO₂ reduction.