Formate Faradaic Efficiency Research Articles

High-selectivity electrochemical CO2 reduction to formate at low overpotential over Bi catalyst with hexagonal sheet structure

The electrochemical conversion of CO2 to formate still suffers from poor selectivity, low production rate, and high overpotential. In this study, a facile strategy is developed to obtain Bi catalysts with a hexagonal sheet structure on copper foil via the constant potential electrodeposition method. The electrocatalyst shows high activity for formate production from CO2 reduction, with the formate faradaic efficiency (FE) reaching nearly 100% at an overpotential of 0.65 V; a high production rate of 96.37 μmol· h−1·cm−2 is obtained, and the corresponding power consumption is as low as 3.64 kW·h·kg−1. The excellent catalytic ability is derived from the sharp edges and corner sites of the catalyst, as they provide numerous surface-active sites and increase the electrical conductivity and local electric field intensities of the surface electrode; thus, the electrochemically active surface area (ECSA) and the electron-donating ability of the Bi electrode are enhanced, while the competing hydrogen evolution reaction (HER) is significantly inhibited. Moreover, the Bi sheets show excellent stability in 24 h electrolysis, with a formate FE of ≥95.8% in aqueous 0.1 M KHCO3 solution. This work indicates that structural adjustment is a critical factor in enhancing the electrocatalytic performance of metallic Bi.

Electroreduction of Carbon Dioxide to Methane Enabled By Molybdenum Carbide Nanocatalyst

The electrochemical conversion of carbon dioxide to value-added products powered by renewable energies is potentially cost-effective and green method of synthesizing hydrocarbon fuels. It is also of significant interest as a strategy to reduce the concentration of atmospheric CO2 and close anthropogenic carbon cycle. An overarching challenge for this technology is developing an inexpensive and earth-abundant catalyst with high activity, stability and selectivity toward hydrocarbon fuels such as methane (CH4), methanol (CH3OH) and ethylene (C2H4). Among all type of heterogenous catalysts used for carbon dioxide reduction reaction, only copper-based catalysts have shown the ability to form hydrocarbon fuels. But they possess too low reaction rate and high overpotentials to justify their use for large-scale applications.Here, we are presenting an earth-abundant nanostructured molybdenum carbide nanoflakes (Mo2C NFs) as a highly effective catalyst for electrochemical CO2 reduction reaction. The Mo2C NFs were synthesized using a facile colloidal chemistry method followed by liquid exfoliation. The electrocatalytic performance of Mo2C NFs were carried out in a custom-made two-compartment three-electrode electrochemical cell using CO2 saturated water like buffer electrolyte and compared with Cu nanoparticles (Cu NPs) which is the conventional catalysts for electrochemical CO2 reduction reaction. The linear sweep voltammetry (LSV) results for Mo2C NFs and Cu NPs indicate that at the potential of -1.25 V vs RHE, a total current density of -138.2 mA/cm2 was obtained for Mo2C NFs while the Cu NPs show a total current density of -44.9 mA/cm2 at the same applied potential suggesting higher activity of Mo2C NFs. Our selectivity analysis, product formation faradaic efficiency (FE) measurements, show a CH4 formation onset potential of -0.45 V vs RHE for Mo2C NFs which is 500 mV less than that of Cu NPs (-0.95 V vs RHE) at identical experimental conditions. At this potential (-0.45 V vs RHE), a CH4 formation efficiency of 36.12% is recorded for Mo2C NFs that further reaches to its maximum to 51.73% at a potential of -0.65 V vs RHE while Cu NPs remain inactive for CH4 formation up to a potential of -0.95 V vs RHE, confirming higher CH4 formation selectivity of Mo2C NFs at low potentials. The results also indicate H2, CO and C2H4 production FEs of 7%, 36% and 2%, respectively, as the side products for Mo2C NFs at the potential of -0.65 V vs RHE. Moreover, our turnover frequency (TOF) calculation, actual CH4 formation activity per active site, exhibits a CH4 formation TOF of 0.4868 s-1 for Mo2C NFs that is approximately 500-times higher than Cu NFs (0.001 s-1) at the potential of -0.95 V vs RHE.We also performed different characterization methods such as X-Ray Diffraction (XRD), X-Ray Photoelectron Spectroscopy (XPS), Raman spectroscopy and Scanning Transmission Electron Microscopy (STEM) to determine the structural and electronic properties as well as the origin of this high catalytic activity of the Mo2C NFs. The highly active and inexpensive catalyst found by this study makes it a promising candidate for effective electrochemical reduction of CO2 to CH4 that can work with renewable energy resources such as solar or wind to address ever-increasing energy demands in a sustainable pathway.

Bifunctional Nickel and Copper Electrocatalysts for CO2 Reduction and the Oxygen Evolution Reaction

In this study, a bifunctional electrocatalyst for CO2 reduction and the O2 evolution reaction (OER) was constructed from the electrodeposition of cuprous oxide (Cu2O) and Ni on a carbon substrate. Different Ni thicknesses on Cu2O were achieved by varying the time of chronopotentiometric deposition of Ni. Electrochemical CO2 reduction was carried out at -0.89 V and -1.89 V vs. RHE, and it was found that formate and CO were the two major products. Cu2O modified with a Ni overlayer with a thickness of ~700 nm resulted in the highest formate Faradaic efficiency of 18%, and Cu2O resulted in highest CO Faradaic efficiency of 7.9%. The enhanced Faradaic efficiency for formate is attributed to the synergistic effect between Ni and Cu2O due to maximized amounts of exposed bimetallic sites that facilitate CO2 reduction. The electrocatalyst also produces ~9 times more current density than previous studies using Ni-Cu2O electrocatalysts for the OER. The ability of the Ni-Cu2O thin films to catalyze both the OER and CO2 reduction allows them to be incorporated in the first demonstration of a two-electrode CO2 conversion device with a bifunctional catalyst.

Ultrasmall Bi nanoparticles confined in carbon nanosheets as highly active and durable catalysts for CO2 electroreduction

Developing high-efficient and durable electrocatalysts for CO2 electroreduction reaction (CO2RR) at high current density is attractive but still challenging. In this study, ultrasmall Bi nanoparticles confined in carbon nanosheets are successfully prepared by directly annealing of dipped carbon cloth. Benefited from the uniform distribution of the ultrasmall Bi nanoparticles and the in‐situ‐formed carbon nanosheets, the resultant self-supported electrocatalysts exhibit excellent CO2RR activity in a synergistic way. The optimized Bi-PVP/CC600 exhibits high formate faradaic efficiency of over 81 % at high current densities in a wide potential range. Impressively, high durability is also achieved for this binder-free electrode, with a remarkable high current density of 54 mA∙cm−2 for nearly 40 h. The high performance associated with this facile and potentially scalable synthetic method suggests the great application potential of ultrasmall Bi nanoparticles in CO2RR. The strategy developed herein is versatile and can be extended to prepare other high‐performance ultrasmall metal nanoparticles catalysts.

Monoclinic Scheelite Bismuth Vanadate Derived Bismuthene Nanosheets with Rapid Kinetics for Electrochemically Reducing Carbon Dioxide to Formate

AbstractElectrochemical reduction of CO2 to high‐value chemical feedstocks, such as formate, is one of the most promising ways to alleviate the greenhouse effect. Unfortunately, the exploration of electrocatalysts with high activity and selectivity over a wide potential window (especially low potential for high current density) still remains a grand challenge. In this study, the fabrication of bismuthene nanosheets using an in‐situ electrochemical transformation strategy of monoclinic scheelite BiVO4 flakes is demonstrated. Catalyzing the CO2 electroreduction in 1 m KHCO3 aqueous solution, the bismuthene nanosheets exhibit a dramatically high formate Faradaic efficiency (FE) of ≈97.4% and a very large current density of −105.4 mA cm−2 at −1.0 V versus reversible hydrogen electrode. Significantly, over a record wide potential window of 750 mV from the initial −0.65 V to the applied minimum −1.4 V, the formate FEs of the bismuthene nanosheets are always higher than 90%, outperforming state‐of‐the‐art electrocatalysts. Both experimental and theoretical investigations reveal that, in comparison with •COOH and H• intermediates, the bismuthene nanosheets preferentially promote fast reaction kinetics towards HCOO•, which eventually accelerates the production of formate.

Tracking structural evolution: operando regenerative CeOx/Bi interface structure for high-performance CO2 electroreduction.

Unveiling the structural evolution and working mechanism of catalysts under realistic operating conditions is crucial for the design of efficient electrocatalysts for CO2 electroreduction, yet remains highly challenging. Here, by virtue of operando structural measurements at multiscale levels, it is identified under CO2 electroreduction conditions that an as-prepared CeO2/BiOCl precatalyst gradually evolves into CeOx/Bi interface structure with enriched Ce3+ species, which serves as the real catalytically active phase. The derived CeOx/Bi interface structure compared to pure Bi counterpart delivers substantially enhanced performance with a formate Faradaic efficiency approaching 90% for 24 hours in a wide potential window. The formate Faradaic efficiency can be further increased by using isotope D2O instead of H2O. Density functional theory calculations suggest that the regenerative CeOx/Bi interfacial sites can not only promote water activation to increase local *H species for CO2 protonation appropriately, but also stabilize the key intermediate *OCHO in formate pathway.

Tuning Sn3O4 for CO2 reduction to formate with ultra-high current density

Abstract In light of environmental concerns and energy demand, significant progress has been made toward CO2 reduction with high activity and selectivity. Particular interest has been focused on liquid products because of the easier separation process and higher value comparing to traditional gaseous products. Formic acid (or formate) can be one of the most economically viable reduction products. Nevertheless, few catalysts can meet the requirement for commercial-viable production, especially the current density. Herein, a hierarchical-Sn3O4 nanosheet (H–Sn3O4 NS) electrocatalyst is synthesized and applied into a three-compartment CO2 flow cell electrolyzer with ultra-high current density. The as-synthesized H–Sn3O4 NS electrocatalyst enables electrochemical CO2 reduction to formate with the selectivity of 91.1% at −1.02 V versus reversible hydrogen electrode (RHE) as well as a partial current density of 421 mA cm−2, whose performance is among the best that has been reported for CO2 electrocatalysts. The superior performance is due to the three-dimensional (3D) hierarchical structure, which supplies a large electrochemical surface area (ECSA) to facilitate mass and charge transfer. And the stability study with in-situ Raman spectroscopy and XRD characterization exhibited that the Sn3O4 materials could still maintain its original oxidation state under the maximum formate Faradaic efficiency of CO2 reduction reaction (CO2RR). The further theoretical calculation shows that the catalyst surface is beneficial for the formation of OCHO* (the intermediate of formate) than COOH* (the intermediate of CO) or H* (the intermediate of H2), which is in favor of the high activity and selectivity for formate in the experiments.

Interface engineering of Mo8/Cu heterostructures toward highly selective electrochemical reduction of carbon dioxide into acetate

Electrocatalytic CO2 reduction reaction (CO2RR) is a promising pathway for storage of renewable electricity and converting CO2 into value-added products. It`s highly desired to obtain acetic acid via CO2RR since it`s an important chemical feedstock and high energy-density liquid fuel. However, developing highly efficient electrocatalysts for selective CO2RR toward acetate remains formidable challenge. We report an interface engineering strategy to modify copper nanocubes with polyoxometalate (POM) to generate Cu–O–Mo interface as active sites for CO2RR, achieving state-of-the-art activity with 48.68% acetate formation Faradaic efficiency and current density of ∼110 mA cm−2 at -1.13 V vs RHE. DFT calculations suggest the interface of Cu planes and polyoxometalate clusters with abundant Cu–O–Mo active sites promote the generation of *CH3 and successive coupling with CO2 insertion, showing a potential dependence of acetate production. This work provides a Cu–O–Mo interface model for the rational design of earth-abundant metal based electrocatalysts for CO2RR and other renewable energy conversions.

Uncovering the nature of electroactive sites in nano architectured dendritic Bi for highly efficient CO2 electroreduction to formate

Porous electrodes based on Bismuth dendrites (Biden) were fabricated using Dynamic Hydrogen Bubbling Templates (DHBT). Direct visualization of surface-active sites with the help of advanced transmission electron microscopic (TEM) imaging reveals the presence of abundant defect structures and high-index faceting, while selective decoration of low-index facets of Bi dendrites indicate that they are the most active Bi surface sites. The fabricated electrodes show high selectivity toward CO2 electroreduction to formate synthesis, with a 98% Faradaic Efficiency and a current density of 18.8 mA cm−2 (344 μmol cm−2 h–1) at –0.82 V vs RHE, which is only 600 mV more negative than the thermodynamic potential. Both the dendritic structure and CO2 electrolysis performances are remarkably stable over long electrolysis periods (∼15 h). Additionally, large production rates (1.63 mmol cm−2 h–1 or 95 mA cm−2 at –0.82 V vs RHE) without significant penalty on the formate Faradaic Efficiency (92 ± 4%) was achieved in high-pressure flow cell, where the CO2 flux is greatly enhanced compare to an H-cell. The work thus outlines an effective strategy for the development of dendritic materials with superior performance toward electroreduction of CO2.

Modulated Sn Oxidation States over a Cu2O-Derived Substrate for Selective Electrochemical CO2 Reduction.

Pursuing high catalytic selectivity is challenging but paramount for an efficient and low-cost CO2 electrochemical reduction (CO2R). In this work, we demonstrate a significant correlation between the selectivity of CO2R to formate and the duration of tin (Sn) electrodeposition over a cuprous oxide (Cu2O)-derived substrate. A Sn electrodeposition time of 120 s led to a cathode with a formate Faradaic efficiency of around 81% at -1.1 V vs reversible hydrogen electrode (RHE), which was more than 37% higher than those of the Sn foil and the sample treated for 684 s. This result highlights the significant role of the interface between deposited Sn and the cuprous-derived substrate in determining the selectivity of CO2R. High-resolution X-ray photoelectron spectra revealed that the residual cuprous species at the Cu/Sn interfaces could stabilize Sn species in oxidation states of 2+ and 4+, a mixture of which is essential for a selective formate conversion. Such modulation effects likely arise from the moderate electronegativity of the cuprous species that is lower than that of Sn2+ but higher than that of Sn4+. Our work highlights the significant role of the substrate in the selectivity of the deposited catalyst and provides a new avenue to advance selective electrodes for CO2 electrochemical reduction.

Tuning the Product Selectivity of the Cu Hollow Fiber Gas Diffusion Electrode for Efficient CO2 Reduction to Formate by Controlled Surface Sn Electrodeposition.

The efficient CO2 electrochemical reduction reaction (CO2RR) relies not only on the development of selective/active catalysts but also on the advanced electrode configuration to solve the critical issue of poor CO2 mass transport and derived sluggish cathodic reaction kinetics. In this work, to achieve a favorable reaction rate and product selectivity, we designed and synthesized an asymmetric porous Cu hollow fiber gas diffusion electrode (HFGDE) with controlled Sn surface electrodeposition. The HFGDE derived from the optimal Sn electrodeposition condition exhibited a formate Faradaic efficiency (FE) of 78% and a current density of 88 mA cm-2 at -1.2 V versus reversible hydrogen electrode, which are more than 2 times higher than those from the pristine Cu HFGDE. The achieved performance outperformed most of the other Sn-based GDEs, indicating the creation of sufficient contact among CO2, electrolyte, and electrode catalyst through the design of the hollow fiber pore structure and catalytic active sites. The enhancement of formate production selectivity and the suppression of the hydrogen by-product were attributed to the optimized ratio of SnOx species on the electrode surface. The best performance was seen in the HFGDE with the highest Sn2+/Sn4+ (120 s deposition), likely due to the modulating effect of the Cu substrate via electron donation with Sn species. The selectivity control strategy developed in the asymmetric HFGDE provides an efficient and facile method to stimulate selective electrochemical reactions in which the gas-phase reactant with low solubility is involved.

Synergy effects on Sn-Cu alloy catalyst for efficient CO2 electroreduction to formate with high mass activity

To acquire the synergy effects between Sn and Cu for the jointly high Faradaic efficiency and current density, we develop a novel strategy to design the Sn-Cu alloy catalyst via a decorated co-electrodeposition method for CO2 electroreduction to formate. The Sn-Cu alloy shows high formate Faradaic efficiency of 82.3% ± 2.1% and total C1 products Faradaic efficiency of 90.0% ± 2.7% at −1.14 V vs. reversible hydrogen electrode (RHE). The current density and mass activity of formate reach as high as (79.0 ± 0.4) mA cm−2 and (1490.6 ± 7.5) mA mg−1 at −1.14 V vs. RHE. Theoretical calculations suggest that Sn-Cu alloy can obtain high Faradaic efficiency for CO2 electroreduction by suppressing the competitive hydrogen evolution reaction and that the formate formation follows the path of CO2 → HCOO* → HCOOH. The stepped (2 1 1) surface of Sn-Cu alloy is beneficial towards selective formate production.

High efficiency and selectivity from synergy: Bi nanoparticles embedded in nitrogen doped porous carbon for electrochemical reduction of CO2 to formate

Electrochemical reduction of CO2 (ECR) into formate is an efficient and environmentally friendly approach contributing to carbon-neutral cycle. However, conventional catalysts for ECR usually suffer from low product selectivity, inferior activity and poor durability. Herein, we highlight Bi nanoparticles embedded in nitrogen doped porous carbon (Bi@NPC) as a promising integrated catalyst for ECR. This catalyst demonstrates a remarkable formate faradaic efficiency of 92.0%, and more encouragingly, a high formate current density of 14.4 mA cm−2 at a low potential of −1.5 V (vs SCE) in 0.1 M KHCO3 solution, which is significantly superior to the individual Bi nanoparticles (Bi-NP). Besides, the Bi@NPC catalyst exhibits a robust durability for 20 h operation. The advanced electrocatalytic performance of the Bi@NPC catalyst is likely attributed to the ensemble effect of Bi nanoparticles and NPC matrix. This attractive route discloses a broad prospect for the development of a promising integrated catalyst for ECR.

Effect of Ionic Liquid Additives on the Product Selectivity and Activity in CO2 Electroreduction over Cu Catalysts

Electrochemical reduction of CO2 has been proposed as a promising way to attenuate the CO2 level in the atmosphere and consequently, mitigate the global warming. Furthermore, several valuable chemicals and fuels can be produced by CO2 electroreduction. Although copper catalysts are of interest due to their ability to produce hydrocarbons and alcohols, they produce a wide verity of products and need high overpotentialsin traditional aqueous electrolytes1. One of the methods to address these obstacles is to use ionic liquids (ILs) in the electrolyte. ILs are cation-anion couples which have unique properties such as high CO2 solubility. In CO2 electroreduction, ILs can play different roles such as co-catalyst, electrolyte, or CO2 absorbent. It has been reported that product selectivity and the reaction rate in CO2 electrocution is significantly impacted when using ionic liquids2, 3. Here, the effect of using ionic liquids in CO2 electroreduction on product selectivity and activity of Cu catalysts has been studied. For this purpose, Cu foils were first electropolished in 1M phosphoric acid for 300 sec. CO2 electroreduction was then performed on Cu catalysts for 30 mins at different potentials. According to the results, ionic liquids affected the product selectivity even in a diluted IL/water mixture. The results showed that adding 10 mM 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([BMIM][NTF2]) to the 0.1 M KHCO3 electrolyte leads to a 63% increase in the total current density at -1.1 V (vs RHE). A 50% increase in formate faradaic efficiency at -0.91 V was also detected when adding the IL to the electrolyte. Moreover, the faradaic efficiency of methane increased by 80% at -1.1 V (vs RHE) in the ionic liquid/water mixture. The results demonstrate how ILs influence the CO2 reduction on Cu catalysts. Hori, Y., Electrochemical CO2 reduction on metal electrodes. In Modern Aspects of Electrochemistry, Vayenas, C. G.; White, R. E.; Gamboa-Aldeco, M. E., Eds. Springer: New York, 2008; Vol. 42, pp 89-189.Mu, T.; Chen, Y., Conversion of CO2 to value-added products mediated by ionic liquids. Green Chemistry 2019.Zhou, F.; Liu, S.; Yang, B.; Wang, P.; Alshammari, A. S.; Deng, Y., Highly selective and stable electro-catalytic system with ionic liquids for the reduction of carbon dioxide to carbon monoxide. Electrochemistry Communications 2015, 55(0), 43-46.

Influence of Temperature on the Performance of Gas Diffusion Electrodes in the CO2 Reduction Reaction

AbstractA detailed investigation of the influence of operating temperature on the electrochemical reduction of CO2 to formate at tin oxide loaded gas diffusion electrodes (GDEs). Ambient pressure electrolysis is performed between 20 and 70 °C with a focus on maximizing current density and energy efficiency while maintaining an average formate faradaic efficiency of at least 80 %. The best performance is achieved at a temperature of 50 °C, which allows a current density of 1000 mA cm−2. Lower or higher temperatures both show an increased hydrogen evolution at said current density. Further investigation of CO2 transport limitation revealed a minimum at 50 °C, which is explained by the opposing influence of temperature on CO2 diffusion coefficients and solubility. This explanation is supported by an estimate of the current density at which hydrogen evolution starts to increase based on the flooded agglomerate model. Long‐term operation for 24 h also revealed an optimum temperature of 50 °C, which helps to suppress the increasing rate of hydrogen evolution and with that a mechanical degradation of the GDE.

Bifunctional wood for electrocatalytic CO2 reduction to formate and electroanalytical detection of myricetin and cadmium (II)

In this work, we first discover the new bifunctional abilities of wood for highly selective electrocatalytic CO2 reduction to formate and highly sensitive electroanalytical detection of myricetin and cadmium (II). The highly developed microchannels in wood ensure most effective reactant and electrolyte transport, great facilitating the electrode reaction. The activated wood electrode displays excellent electrocatalytic CO2 reduction activity, attained the highest formate Faradaic Efficiency (FE) of 70.8% at −1.8 V with high current density of 53.8 mA cm−2 in aqueous solution, and displayed at least 24 h CO2 electrolysis durability. Furthermore, the wood is also first utilized as new electrode matrix for electroanalytical detection of drug (myricetin is as an example) and heavy metal ion (cadmium (II) is as an example). Compared with traditional glassy carbon electrode (GCE) and its derived modified electrodes, the wood electrode exhibits a wide linear detection range (0.01–50 μmol dm−3) and a low detection limit (0.002 μmol dm−3, S/N = 3) for myricetin, and displays a wide linear detection range (0.1–20 μg l−1) and a low detection limit (0.05 μg L−1, S/N = 3) for cadmium (II). These remarkable bifunctional performances of low cost wood electrode show great potential for large-scale using in CO2 electroreduction and electroanalytical detection.

The Effect of Pulse Electrodeposition of Bismuth on Electrochemical Reduction of Carbon Dioxide to Formate

Efficient electrochemical reduction of CO2 to formate is demonstrated by fabricating Bi nanoflake with pulse electrodeposition. The effect of pulse voltage on morphology of the Bi nanoflake and electrochemical property was investigated. The pulse voltage of 80 mVAg/AgCl with six pulse cycles successfully produces ultrathin Bi nanoflakes. The finite-element-based simulation shows that the two-dimensional geometry with large lateral dimensions and ultrathin thickness can localize a strong electric field at the corner site of the flakes, resulting in facilitating efficient reduction of CO2 to formate. As a result, formate Faradaic efficiency of the Bi nanoflakes showed over 90% at − 0.8 VRHE. Also, the formate production rate reached 69 µmol/cm2/h at − 0.8 VRHE. The pulse electrodeposition provides highly dense and uniform and tailored nanostructures in the large area without the requirements of high vacuum or high temperatures process.

Electrochemical Production of Formic Acid / Formate By CO2 Electrolysis Using High Performance Gas Diffusion Electrodes

Electrochemical low-temperature conversion of CO2 is a promising way to reduce GHG emissions and to valorize CO2 as a carbon-source to produce basic chemicals and/or to store excess electricity from renewable resources. The most widely studied products for these purposes are formic acid/formate, carbon monoxide and methane. In contrast to carbon monoxide and methane, aqueous formic acid/formate solutions show advantages in terms of handling, storage, toxicity and safety. The stored energy can thereby be recovered either in direct formate or formic acid fuel cells (DFFC or DFAFC), or by upstream hydrogen production from dehydrogenation and subsequent use in a H2-fuel cell. Besides the energetic use, formic acid may also serve as precursor for the decentralized production of synthesis gas as it can be decomposed to hydrogen or carbon monoxide. Formatotrophic bacteria might also use formic acid as substrate in fermentation towards higher hydrocarbons like isobutene or propane.[1] CO2 + H2O + 2e- --> HCOO- + OH- φ0=-0.665 V (vs. SHE, pH=10) Two of the biggest problems to overcome in electrochemical CO2 reduction reaction (CO2RR) is its low energetic efficiency and the sluggish reaction kinetics, which strongly limits the achievable current density. The energetic efficiency not only suffers from a high overpotential, which might be greatly reduced by state-of-the-art electrocatalysts, but also by several sources of ohmic losses throughout the whole electrolysis cell. Thus, for industrial application these resistances are to minimize by choosing an appropriate reactor design, but also by optimizing the used electrodes and reaction conditions. In order to overcome the slow reaction rates caused by the low solubility of CO2 in aqueous media, gas diffusion electrodes (GDEs) have become the number one solution. Their highly porous structure provides a huge three-phase boundary between gaseous CO2, electrolyte and catalyst. As most groups use layered GDEs with a supporting gas diffusion layer as backbone and the active component bound on top by a hydrophobic binding agent (mostly PTFE or Nafion®), we want to emphasize single-layer GDEs (SLGDE) that comprise all of these three components in one homogenous layer. SLGDEs in CO2RR have already been proven to show one of the highest current densities in literature of 400 mA cm-2 while maintaining a faradaic efficiency of at least 75 %.[2] Optimizing such SLGDEs is one of our current subject of research. It includes multiple aspects, like the catalyst’s nature, dispersion and loading [3], electrode composition, including type of carbon, porosity and hydrophobicity, as well as the extent of electrolyte intrusion during the reaction. Besides the electrodes properties, reaction conditions are of upmost importance. This includes temperature, type of electrolyte and concentration, but also controlling the electrochemical stress, especially during start-up phase. The results shown in figure 1 demonstrate the importance of these parameters. Acetylene black based gas diffusion electrodes, loaded with SnOx nanoparticles were screened for their maximum current density (defined as minimum 80 % formate faradaic efficiency for 45 min), showing a temperature depended maximum in performance at 1000 mA cm-2. This maximum results from slow intrinsic reaction kinetics and little mass transport coefficients at lower temperatures and decreasing CO2 solubility at higher temperatures. Optimizing the used electrolyte and carefully starting-up the GDE helps to further raise the performance up to 1200 mA cm-2. A comparison with values from literature (figure 1, right side) illustrates the high performance of such SLGDEs. Further investigations need to focus on transferring these results into continuous mode of operation and stabilization of the three-phase boundary for industrial relevant lifetimes.[4] [1] www.eforfuel.eu [2] D. Kopljar, N. Wagner, E. Klemm, Chem. Eng. Technol., 39 (11), 2016, p. 2042-2050. [3] D. Kopljar, A. Inan, P. Vindayer, N. Wagner, E. Klemm, J. Appl. Electrochem., 44 (10), 2014, p. 1107-1116 [4] D. Kopljar, A. Inan, P. Vindayer, R. Scholz, N. Frangos, N. Wagner, E. Klemm, Chem. Ing. Techn., 87 (6), 2015, p. 855-859 The authors would like to thank the German BMWi (Bundesministerium für Wirtschaft und Energie - support code: 03ET1037B) for the financial support. Figure 1

Preparation of Metal Amalgam Electrodes and Their Selective Electrocatalytic CO2 Reduction for Formate Production

Electrochemical CO2 reduction to produce formate ions has studied for the sustainable carbon cycle. Mercury in the liquid state is known to be an active metallic component to selectively convert CO2 to formate ions, but it is not scalable to use as an electrode in electrochemical CO2 reduction. Therefore, scalable amalgam electrodes with different base metals are tested to produce formate by an electrochemical CO2 reduction. The amalgam electrodes are prepared by the electrodeposition of Hg on the pre-electrodeposited Pd, Au, Pt and Cu nanoparticles on the glassy carbon. The formate faradaic efficiency with the Pd, Au, Pt and Cu is lower than 25%, while the one with the respective metal amalgams is higher than 50%. Pd amalgam among the tested samples shows the highest formate faradic efficiency and current density. The formate faradaic efficiency is recorded 85% at −2.1 V vs SCE and the formate current density is −6.9 mA cm−2. It is concluded that Pd2Hg5 alloy on the Pd amalgam electrode is an active phase for formate production in the electrochemical CO2 reduction.

Nanoporous Ag-Sn derived from codeposited AgCl-SnO2 for the electrocatalytic reduction of CO2 with high formate selectivity

Nanoporous Ag-Sn was prepared by direct electroreduction of a codeposited AgCl-SnO2 mixture in 0.1 M HCl, and evaluated as an electrode catalyst for the reduction of CO2 in 0.5 M KHCO3. A volcano-type correlation between selectivity for the formate product and the atomic ratio of Ag to Sn in the nanoporous catalysts was revealed. It was found that the bimetallic catalyst with a Ag:Sn ratio of 3:2, mainly composed of the Ag4Sn alloy, showed excellent catalytic performance for the conversion of CO2 to formate. This catalyst delivered a current of about 10 mA cm−2 with a high formate faradaic efficiency of about 85% at −0.8 V vs. the reversible hydrogen electrode. Moreover, the catalytic activity remained reasonably stable during a 13.5-hour electrolysis.