Efficient Electrochemical Reduction Research Articles

Selective and stable CO2 electroreduction at high rates via control of local H2O/CO2 ratio

Controlling the concentrations of H2O and CO2 at the reaction interface is crucial for achieving efficient electrochemical CO2 reduction. However, precise control of these variables during catalysis remains challenging, and the underlying mechanisms are not fully understood. Herein, guided by a multi-physics model, we demonstrate that tuning the local H2O/CO2 concentrations is achievable by thin polymer coatings on the catalyst surface. Beyond the often-explored hydrophobicity, polymer properties of gas permeability and water-uptake ability are even more critical for this purpose. With these insights, we achieve CO2 reduction on copper with Faradaic efficiency exceeding 87% towards multi-carbon products at a high current density of −2 A cm−2. Encouraging cathodic energy efficiency (>50%) is also observed at this high current density due to the substantially reduced cathodic potential. Additionally, we demonstrate stable CO2 reduction for over 150 h at practically relevant current densities owning to the robust reaction interface. Moreover, this strategy has been extended to membrane electrode assemblies and other catalysts for CO2 reduction. Our findings underscore the significance of fine-tuning the local H2O/CO2 balance for future CO2 reduction applications.

Efficient electrochemical reduction of nitrate to ammonia over metal-organic framework single-atom catalysts

The design and preparation of efficient catalysts for ammonia production under mild conditions is a desirable but highly challenging target. Here, we report a series of single-atom catalysts [M-SACs, M = Mn(II), Fe(III), Co(II), Ni(II), Cu(II), Zn(II), Mo(II)] derived from UiO-66 containing structural defects and their application to electrochemical reduction of nitrate (NO3-) to ammonia (NH3). Cu-SAC and Fe-SAC exhibit remarkable yield rates for NH3 production of 30.0 and 29.0 mg h−1 cm−2, respectively, with a high Faradaic efficiency (FENH3) of over 96% at −1.0 V versus the reversible hydrogen electrode. Importantly, their catalytic performance can be retained in various simulated wastewaters. Complementary experiments confirmed the nature of single-atom sites within these catalysts and the binding domains of NO3- in UiO-66-Cu. In situ spectroscopic techniques, coupled with density functional theory calculations confirm the strong binding of NO3- and the formation of reaction intermediates, thus facilitating the catalytic conversion to NH3.

Engineering CuMn Prussian blue analog-shelled nanobox for efficient electrochemical reduction of nitrate to ammonia

Engineering CuMn Prussian blue analog-shelled nanobox for efficient electrochemical reduction of nitrate to ammonia

Crystalline CdS/Amorphous Cd(OH)2 Composite for Electrochemical CO2 Reduction to CO in a Wide Potential Window.

Electrochemical CO2 reduction is a promising method for converting atmospheric CO2 into valuable low-carbon chemicals. In this study, a crystalline cadmium sulfide/amorphous cadmium hydroxide composite was successfully deposited on the carbon paper substrate surface by in-situ chemical bath deposition (named as c-CdS/a-Cd(OH)2/CP electrodes) for the efficient electrochemical CO2 reduction to produce CO. The c-CdS/a-Cd(OH)2/CP electrode exhibited high CO Faradaic efficiencies (>90 %) under a wide potential window of 1.0 V, with the highest value reaching ~100 % at the applied potential ranging from -2.16 V to -2.46 V vs. ferrocene/ferrocenium (Fc/Fc+), superior to the crystalline counterpart c-CdS/CP and c-CdS/c-Cd(OH)2@CP electrodes. Meanwhile, the CO partial current density reached up to 154.7 mA cm-2 at -2.76 V vs. Fc/Fc+ on the c-CdS/a-Cd(OH)2/CP electrode. The excellent performance of this electrode was mainly ascribed to its special three-dimensional structure and the introduction of a-Cd(OH)2. These structures could provide more active sites, accelerate the charge transfer, and enhance adsorption of *COOH intermediates, thereby improving the CO selectivity. Moreover, the electrolytes consisting of 1-butyl-3-methylimidazolium tetrafluoroborate and acetonitrile also enhanced the reaction kinetics of electrochemical CO2 reduction to CO.

Mesoporous CuCe dual-metal catalysts for efficient electrochemical reduction of CO2 to methane

In this manuscript, we report the successful synthesis of CuxCe1-xO2 bimetallic mesoporous oxides using a straightforward two-step hydrothermal method aided by CTAB, demonstrating exceptional selectivity for methane (CH4). We hypothesize that the superior CH4 selectivity of CuxCe1-xO2 can be attributed to the strong adsorption of ∗CO facilitated by the mesoporous structure, which effectively promotes the conversion of ∗CO to ∗CHO.

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.

Towards a zero liquid discharge process from brine treatment: Water recovery, nitrate electrochemical elimination and potential valorization of hydrogen and salts

This research addresses the issues related with treatment and valorization of brines and nitrate decontamination of surface and ground waters. The objective was to approximate to zero liquid discharge (ZLD) minimizing the environmental impact of brines of an electrodialysis reversal water treatment plant (EDRWTP) as an example. The innovative in flow process was developed from lab to pre-industrial scale and joined several main concepts: ion-exchange equilibrium for softening or demineralization of brines; reversed osmosis to recover suitable water and to enrich the waste in nitrate for efficient electrochemical reduction of NO3− to N2; valorization of subproducts by direct use or by precipitation; and assessment of the whole process by measuring in-line several parameters. The achieved softening was around 98 % and the recovered water from this current by reversed osmosis was 75 %. The brine of this step (25 %) contained around 1500 mg/L of nitrate and it was treated by electrochemical reduction with a Bi/Sn cathode providing a gas current of 60 % of initial nitrate reduced to N2, O2, H2O, NH3 and at least 97 % of H2. The aqueous current contained around 40 % of initial nitrate as ammonium and nitrite lower than 50 and 5 mg/L, respectively. Hypochlorite was added to this last current for oxidizing ammonium and nitrite to N2 and nitrate, respectively, being nitrate and ammonium lower than 50 and 5 mg/L, respectively. After the obtained water was demineralized and conducted to the EDRWTP inlet. The recovery of insoluble salts as calcium carbonate, reuse of saline solutions for the regeneration of process resins and the potential use of hydrogen generated as a by-product during the electrochemical reduction are other possible utilities.

Introduction of a Conductive Layer into Flood-Resistant Gas Diffusion Electrodes with Polymer Substrate for an Efficient Electrochemical CO2 Reduction with Copper Oxide.

Conversion of atmospheric carbon dioxide (CO2) into valuable feedstocks is a crucial technology, and electrochemical reduction of CO2 is a promising approach that can provide a useful source of ethylene (C2H4). Gas diffusion electrodes (GDEs) placed at the interface of the CO2 gas and electrolyte can achieve high current density through a sufficient supply of dissolved CO2 to the reaction site, making them indispensable in industrial applications. However, conventional GDEs with carbon substrate have suffered from electrolyte flooding and consequent loss of efficiency, posing an obstacle for practical application. While flood-resistant GDEs with hydrophobic polymer substrate have been proposed recently, only conductive materials can be employed as electrocatalysts because of their insulative properties, despite the high activities of oxide materials such as copper oxide. Here, we introduce an aluminum conductive layer in GDE with polymer substrate to enable the use of electrically resistive catalysts. Cuprous oxide (Cu2O) with silver particles was tested as a model material and has shown prolonged stability (>17 h) with high C2H4 Faraday efficiency (>50%) while suppressing flooding. A thorough characterization revealed that the conductive layer makes Cu2O an efficient electrocatalyst, even on the polymer substrate, by providing sufficient electrons through its conduction path. This research significantly expands the scope of electrode design by enabling the incorporation of a wide range of nonelectrically conductive materials on GDEs with polymer substrate.

Hydrophilic-hydrophobic Janus polybenzimidazole membrane electrode assemblies regulate hydrogen evolution for high efficient electrochemical CO2 reduction

The membrane Electrode Assemblies (MEAs) electrolyzers are the most attractive systems for the electrolytic conversion of CO2 into commodity chemicals and fuels at commercially relevant current densities. Suppressing the hydrogen evolution reaction (HER) is vital to the highly efficient electrochemical CO2 reduction reaction (CO2RR). However, little attention has been paid to the HER. Herein, polybenzimidazole (PBI) Janus membranes with significant hydrophobic/hydrophilic asymmetric surface wettability were constructed for regulating the HER in CO2RR. The HER was modulated by adjusting the microstructure and surface hydrophobicity of the membrane as well as the CO2 feeding method. The p-PBI-HCF MEA showed a significant suppression of HER and a 6-fold improvement in CO selectivity over p-PBI MEA. The p-PBI-HCF exhibited superior CO2RR performance in MEA compared to commercial membranes (FAA-3-50 and Nafion115). At 2.0–3.0 V, the CO Faraday efficiency (FECO) of the p-PBI-HCF MEA electrolyzer remained above 92%. The optimal energy efficiency range was between 2.0 and 2.4 V when the FECO was close to 100%. The p-PBI-HCF provided 92% FECO and a current density of 225 mA cm−2 at 3 V. This work provides guidelines for the development of viable PBI membranes and MEAs for CO2RR.

Gram-scale synthesis of Ni-Zn diatomic sites catalyst for efficient electrochemical CO2 reduction

Diatomic catalysts inherit the advantages of single atom catalysts and use the adjacent metal-N sites for functional complementary and synergistic effects in electrochemical CO2 reduction (ECR). However, preparation of diatomic catalysts with high yield, Faradaic efficiency and stability together with low overpotential still remains challenging. In this work, we develop a nitrogen-doped carbon supported Ni-Zn diatomic catalyst, whose CO Faradaic efficiency of is higher than 92.0 % in the wide potential range of −0.57 ∼ −1.07 V vs RHE, and the maximum CO Faradaic efficiency is 97.5 % at −0.77 V vs RHE. And also, CN-NiZn performs an extraordinary stability for 30 h in the long-time durability test. More importantly, large-quantities (around 1.66 g) of Ni-Zn diatomic catalysts with remarkable catalytic performance can be prepared by this method. Synchrotron radiation X-ray absorption spectroscopy was tested to reflect the coordination environment and valence states around Ni and Zn atoms. Theoretical calculations further reveal that the synergistic effect between Ni and Zn binary metal sites largely reduces the energy barrier of the reaction to produce the intermediate *COOH, thus effectively increasing the selectivity of ECR. This work indicates that the introduction of another coordination metal can significantly affect the electronic structure of monatomic catalysts, thus improving the electrocatalytic activity and selectivity of ECR.

Achieving 78.2 % Faraday Efficiency for Electrochemical Ammonia Production Via Covalent Modification of CNTs with B4C

AbstractElectrochemical reduction of N2 to NH3 provides an alternative to the Haber‐Bosch process for sustainable NH3 production driven by renewable electricity. Here, we reported carbon nanotubes (CNTs) covalently modified with boron carbide (B4C) as a nonmetallic catalyst for efficient electrochemical nitrogen reduction reaction (NRR) under ambient conditions. The structure of the catalyst was characterized by transmission electron microscopy (TEM), X‐ray diffraction (XRD), elemental mapping, X‐ray photoelectron spectroscopy (XPS) and Raman spectroscopy. The catalyst held a superior selectivity for NRR with high Faraday efficiency of 78.2 % accompanying with NH3 yield rate of 14.0 μg mg−1cat. h−1 under the condition of 0.1 M Na2SO4 and −0.6 V vs. RHE. Electrochemical experiments including cyclic voltammetry, electrochemical impedance spectroscopy and Tafel polarization curves were performed to explain the best electrochemical properties of B4C/CNTs among the samples. This work demonstrates that the strategy of covalent modification plays an important role to improve the selectivity of electrochemical NRR catalyst, thus allowing the reactions to proceed more efficiently.

CuSnBi Catalyst Grown on Copper Foam by Co-Electrodeposition for Efficient Electrochemical Reduction of CO2 to Formate

Effective electrochemical reduction of carbon dioxide to formate under mild conditions helps mitigate the energy crisis but requires the use of high-performance catalysts. The addition of a third metal to the binary metal catalyst may further promote the electrochemical reduction of carbon dioxide to formate. Herein, we provided a co-electrodeposition method to grow CuSnBi catalysts on pretreated copper foam and discussed the effects of both pH value and molar ratio of metal ions (Cu2+, Sn2+, and Bi3+) in the electrodeposition solution on the electrocatalytic performance of CO2 to HCOO−. When the pH value of the electrodeposition solution was 8.5 and the molar ratio of Cu2+, Sn2+, and Bi3+ was 1:1:1, the electrode showed the highest FEHCOO− of 91.79% and the formate partial current density of 36.6 mA·cm−2 at −1.12 VRHE. Furthermore, the electrode kept stable for 20 h at −1.12 VRHE, and FEHCOO− was always beyond 85% during the electrolysis process, which is excellent compared to the previously reported ternary metal catalytic electrodes. This work highlights the vital impact of changes (pH value and molar ratio of metal ions) in electrodeposition liquid on catalytic electrodes and their catalytic performance, and refreshing the electrolyte is essential to maintain the activity and selectivity during the electrochemical reduction of CO2 to HCOO−.

Atomic Co─P Catalytic Pair Drives Efficient Electrochemical Nitrate Reduction to Ammonia

AbstractElectrochemically reducing nitrate (NO3−), a common water pollutant, to valuable ammonia (NH3) offers a green, sustainable, and decentralized route for ammonia synthesis. Electrochemical nitrate reduction reaction (NO3−RR) involves two crucial reaction steps: nitrate deoxygenation followed by nitrite hydrogenation; in particular, the nitrite hydrogenation reaction is the rate‐determining step (RDS) for NO3−RR. In this work, an atomically dispersed cobalt‐phosphorus (Co─P) catalytic pair (CP) with strong electronic coupling is reported. The Co site in Co─P CP effectively activates NO3−, while the P site in Co─P CP facilitates water dissociation to release H+, synergistically enhancing the thermodynamic and kinetic performance of electrochemical nitrate reduction to ammonia.

CuO-In2O3 collaboration in CO2 electroreduction: Enhanced activity and synergistic mechanism

The conversion of CO2 into high value–added products via electrocatalytic technology is one of the most attractive methods of addressing the ecological crisis and achieving carbon neutrality. In this study, we present CuO-In2O3 nanosheets with bifunctional active components for the efficient electrochemical reduction of CO2 to CO. The catalyst demonstrates a current density of 35.3 mA cm−2 with a high faradic efficiency of 97.3 % toward CO at − 0.6 V versus the reversible hydrogen electrode. In situ attenuated total reflection surface–enhanced infrared absorption spectroscopy and density functional theory calculations reveal that the synergistic catalysis of CuO and In2O3 rationally balances the binding energy to *COOH and *CO intermediates. It promotes the catalyst selectivity for CO. Furthermore, CuO and In2O3 play key roles as bifunctional active sites. In particular, CuO enhances the activity of CO2 and reduces the formation energy barrier of the reaction intermediates, whereas In2O3 suppresses the adsorption of *H and limits the hydrogen evolution reaction.

One‐step synthesis of ultra‐small amount of Pd‐Alloyed Zinc Nanosheets for efficient electrochemical CO2 reduction to CO

AbstractThe practicality of the electrochemical CO2 reduction technique depends on the development of cost‐effective, robust, and highly selective catalysts. To achieve this goal, we have engineered self‐supported 3D electrodes composed of Pd‐Zn nanosheets (NSs) for CO2 electrochemical reduction to CO with minimal Pd content. This innovative electrode with an increased surface area was created using an electrodeposition method employing a dynamic hydrogen bubble template. By precisely adjusting the Pd content, we improved the thickness, porosity, and surface area of the electrodes, resulting in a CO2‐to‐CO selectivity reaching as high as 88.5 %, with an average of at least 80 % sustained over 10 hours. This remarkable improved activity can be attributed to the synergistic effects of an appropriate Pd/Zn atomic ratio as well as to the large surface area of nanosheets structures with rich edge active sites. Furthermore, to get around the limitations of CO2 mass transfer, reactions were done at high pressures conditions ranging from 3 to 9.5 bar; this strategic approach yielded an outstanding partial current density of −304.6 mA cm−2 for CO. These noteworthy findings establish concepts for constructing effective and earth‐abundant CO‐producing electrocatalysts.

Spatio-Temporal Electrowetting and Reaction Monitoring in Microfluidic Gas Diffusion Electrode Elucidates Mass Transport Limitations.

The use of gas diffusion electrodes (GDEs) enables efficient electrochemical CO2 reduction and may be a viable technology in CO2 utilization after carbon capture. Understanding the spatio-temporal phenomena at the triple-phase boundary formed inside GDEs remains a challenge; yet it is critical to design and optimize industrial electrodes for gas-fed electrolyzers. Thus far, transport and reaction phenomena are not yet fully understood at the microscale, among other factors, due to a lack of experimental analysis methods for porous electrodes under operating conditions. In this work, a realistic microfluidic GDE surrogate is presented. Combined with fluorescence lifetime imaging microscopy (FLIM), the methodology allows monitoring of wetting and local pH, representing the dynamic (in)stability of the triple phase boundary in operando. Upon charging the electrode, immediate wetting leads to an initial flooding of the catalyst layer, followed by spatially oscillating pH changes. The micromodel presented gives an experimental insight into transport phenomena within porous electrodes, which is so far difficult to achieve. The methodology and proof of the spatio-temporal pH and wetting oscillations open new opportunities to further comprehend the relationship between gas diffusion electrode properties and electrical currents originating at a given surfacepotential.

Dual enhancement of Cu2S/CuO nanocomposites in N-Doped porous carbon for highly efficient electrochemical nitrate reduction to ammonia

Dual enhancement of Cu2S/CuO nanocomposites in N-Doped porous carbon for highly efficient electrochemical nitrate reduction to ammonia

Downsizing Porphyrin Covalent Organic Framework Particles Using Protected Precursors for Electrocatalytic CO2 Reduction.

Covalent organic frameworks (COFs) are promising electrocatalyst platforms owing to their designability, porosity, and stability. Recently, COFs with various chemical structures are developed as efficient electrochemical CO2 reduction catalysts. However, controlling the morphology of COF catalysts remains a challenge, which can limit their electrocatalytic performance. Especially, while porphyrin COFs show promising catalytic properties, their particle size is mostly large and uncontrolled because of the severe aggregation of crystallites. In this work, a new synthetic methodology for rationally downsized COF catalyst particles is reported, where a tritylated amine is employed as a novel protected precursor for COF synthesis. Trityl protection provides high solubility to a porphyrin precursor, while its deprotection proceeds in situ under typical COF synthesis conditions. Subsequent homogeneous nucleation and colloidal growth yield smaller COF particles than a conventional synthesis, owing to suppressed crystallite aggregation. The downsized COF particles exhibit superior catalytic performance in electrochemical CO2 reduction, with higher CO production rate and faradaic efficiency compared to conventional COF particles. The improved performance is attributed to the higher contact area with a conductive agent. This study reveals particle size as an important factor for the evaluation of COF electrocatalysts and provides a strategy to control it.

Au modified TiO2 nanowires prepared by photodeposition for selective and efficient electrochemical reduction of CO2

Electrocatalytic CO2 reduction (CO2ER) to produce formic acid (HCOOH) would be an attractive means for carbon neutralization. In order to realize efficient CO2ER and economical conversion of CO2, the applying of metal active sites on surface of semiconductor has emerged as a promising strategy. Herein, we report the preparation of alkaline modified TiO2 nanowires (NWs) as the substrate for Au photodeposition. The as-fabricated Au/TiO2 NWs exhibits high activity and selectivity towards HCOOH production compared with traditional catalysts. The key factors including deposition time, solution, and etc. And their effects on CO2ER performance have been systematically investigated. At −0.8 V vs RHE, the Faraday efficiency of the HCOOH can reach to 86.49 %, while the current density is 15.94 mA cm−2. In addition, Au/TiO2 NWs also exhibited good stability after 18 h electrocatalytic CO2RR under the voltage of −0.8 V vs RHE. Electrochemical tests indicated that Au/TiO2 NWs process favorable intrinsic electrochemical properties, high ESCA and accelerating charge transfer process make Au/TiO2 NWs obtain abundant active sites and kinetically favorable. This work provides a convenient fabrication strategy of Au/TiO2 based catalyst, which can guide the design and modification of gold for CO2ER into formic acid.

Covalent Organic Frameworks as Promising Platforms for Efficient Electrochemical Reduction of Carbon Dioxide: A Review

In current research, achieving carbon neutrality has become a primary focus through the utilization of various conversion technologies that transform carbon dioxide (CO2) into valuable chemicals or fuels. Covalent organic frameworks (COFs), as emerging crystalline organic polymers, offer distinct advantages in CO2 conversion compared to other materials. These advantages include controllable nanoscale pores, predefined functional units, editable framework structures, and rich conjugated systems. The unique characteristics of COFs make them highly promising electrocatalysts for CO2 conversion. This review provides a comprehensive overview of pioneering works and recent research on the utilization of COF‐based materials as electrocatalysts for electrochemical CO2 reduction reaction. This review offers a comprehensive analysis of the design principles for various reactive sites, skeleton structures, pore functionalities, 3D frameworks, morphologies, and composite materials of COFs, aiming to enhance electrocatalysis. Finally, this review presents some recommendations for material design, reaction mechanisms, and theoretical computations to enhance the understanding of reaction mechanisms further and facilitate the design of high‐performance COF‐based electrocatalysts.