Two-compartment Electrochemical Cell Research Articles

Electrochemical Surface Area Quantification, CO2 Reduction Performance, and Stability Studies of Unsupported Three-Dimensional Au Aerogels versus Carbon-Supported Au Nanoparticles.

The efficient scale-up of CO2-reduction technologies is a pivotal step to facilitate intermittent energy storage and for closing the carbon cycle. However, there is a need to minimize the occurrence of undesirable side reactions like H2 evolution and achieve selective production of value-added CO2-reduction products (CO and HCOO–) at as-high-as-possible current densities. Employing novel electrocatalysts such as unsupported metal aerogels, which possess a highly porous three-dimensional nanostructure, offers a plausible approach to realize this. In this study, we first quantify the electrochemical surface area of an Au aerogel (≈5 nm in web thickness) using the surface oxide-reduction and copper underpotential deposition methods. Subsequently, the aerogel is tested for its CO2-reduction performance in an in-house developed, two-compartment electrochemical cell. For comparison purposes, similar measurements are also performed on polycrystalline Au and a commercial catalyst consisting of Au nanoparticles supported on carbon black (Au/C). The Au aerogel exhibits a faradaic efficiency of ≈97% for CO production at ≈−0.48 VRHE, with a suppression of H2 production compared to Au/C that we ascribe to its larger Au-particle size. Finally, identical-location transmission electron microscopy of both nanomaterials before and after CO2-reduction reveals that, unlike Au/C, the aerogel network retains its nanoarchitecture at the potential of peak CO production.

Electrochemical degradation of methylene blue accompanied with the reduction of CO2 by using carbon nanotubes grown on carbon fiber electrodes

In this study, the degradation of Methylene Blue (MB) dye accompanied with the reduction of CO2 was performed in an electrochemical (EC) process by using carbon nanotubes grown on carbon fiber (CNTs/CFM) electrodes as the cathode and anode in a two-compartment electrochemical cell. The growth of CNTs on CFM via chemical vapor deposition led to the significant improvement in physicochemical properties of CNTs/CFM which were beneficial for the EC process. The effects of various operating parameters including supporting electrolytes (KHCO3 and H2SO4), initial concentration of MB (5, 10, 15 and 20 mg L− 1) and applied currents (10, 50 and 100 mA) on the degradation of MB were investigated. The results confirmed the vital influence of applied current and initial concentration of MB while the supporting electrolytes played a minor role in MB degradation. On the contrary, the influence of electrolytes in the performance of CO2 reduction was more significant on the production and selectivity of generated products. The optimal electrochemical system included 0.1 M KHCO3 as the electrolyte and an applied current of 50 mA in anodic cell and CO2 saturated solution in cathodic cell; such a system resulted in the EC degradation efficiency of 72% at the MB initial concentration of 10 mg L− 1 in the anodic cell and production of 4.7 mM cm− 2 CO, 67 mM cm− 2 H2, and 11.3 mg L− 1 oxalic acid in the cathodic cell corresponding to the Faradaic efficiencies of 28, 40 and 4%, respectively. The results of reusability test deduced that the stability of CNTs/CFM was still satisfactory after 4 runs. The results of this study demonstrated the good applicability of CNTs/CFM to be simultaneously used the electrodes for the EC oxidation of dye and the EC reduction of CO2 to obtain valuable compounds.

Bidirectional Light-Driven Ion Transport through Porphyrin Metal–Organic Framework-Based van der Waals Heterostructures via pH-Induced Band Alignment Inversion

Bidirectional Light-Driven Ion Transport through Porphyrin Metal–Organic Framework-Based van der Waals Heterostructures via pH-Induced Band Alignment Inversion

Constructing FeN4/graphitic nitrogen atomic interface for high-efficiency electrochemical CO2 reduction over a broad potential window

Summary Atomically dispersed Fe–N–C catalyst has shown great performance in the CO2-to-CO conversion, yet the high CO selectivity is achieved only within a rather narrow potential range, which cannot well meet the requirements for the following CO dimerization or hydrogenation. Here, we developed a hydrogen-pyrolysis etching strategy to precisely manipulate the uncoordinated N dopants in the Fe–N–C catalyst. This strategy could preferentially eliminate pyridinic and pyrrolic N atoms while leaving graphitic N. The resulting catalyst gave a CO faradaic efficiency (FECO) above 90% over a broad window from −0.3 to −0.8 V (versus RHE) (in particular, FECO > 97% at −0.6 V). In situ ATR-SEIRAS and first-principle calculations further revealed that this strategy can not only suppress the parasitic H2 generation but also promote CO2 activation and protonation with the assistance of co-adsorbed H2O on the “atomic interface” of “FeN4/graphitic N.”

Produced water softening using high-pH catholyte from brine electrolysis: reducing chemical transportation and environmental footprints

This study evaluates the benefits of using brine electrolysis for generating caustic soda (NaOH) and free chlorine for on-site produced water (PWs11Produced Water) treatment. A two-compartment electrochemical cell was shown to generate NaOH solutions (pH > 12, faradic efficiency 93%) and chlorine (faradic efficiency 32%) from a NaCl brine solution at a current density of 10 mA/cm2. The catholyte was used for softening field-collected PWs. The degree of Mg removal depends mostly on the catholyte/PW mixture pH with pH 11 achieving >90% removal. Ca removal is poor (<10%) due to low bicarbonate alkalinity in the PWs. Soda ash alone at a dose equivalent to the total hardness of the PWs helps CaCO3 precipitation and Ca removal (>90%). The combined treatment of the catholyte and a reduced quantity of soda ash achieves better or comparable Ca removal compared to the full stoichiometric amounts of soda ash alone. Ba and Sr removal patterns closely follow those of Ca, suggesting co-precipitation of these cations as the primary removal mechanism. Organic removal is negligible during chemical softening; however, activated carbon filtration achieves >90% of total organic carbon (TOC) removal in all PWs. A treatment scheme is proposed for field generation of caustic soda and chlorine from PW. The economic analysis demonstrates the significant cost-effectiveness of the approach compared to purchasing the NaOH.

Nitrogen-doped graphene supported copper nanoparticles for electrochemical reduction of CO2

Increasing CO2 concentration in the atmosphere causes a negative impact on the global climate. Utilization of CO2 into value-added chemical products by electrochemical reduction method has attracted great attention to reduce the CO2 emissions and achieve net-zero carbon footprints. Herein, we report a nanostructured electrocatalyst consisting of N-doped graphene (NGN) supported Cu nanoparticles (Cu NPs) with high catalytic activity for electrochemical CO2 reduction (ECR). The electrocatalyst was optimized for loading of Cu NPs on NGN. The physico-chemical properties of electrocatalysts were studied by SEM, TEM, Raman spectroscopy, XPS, etc. Characterization results show that the high loading of Cu (30 wt. %) increases the size of Cu NPs due to agglomeration of particles. ECR experiments were carried out in a two-compartment electrochemical cell. High performance liquid chromatography (HPLC) was employed to analyze the liquid products. Amongst all tested electrocatalysts, Cu20/NGN shows the highest activity for ECR in the entire potential range studied. It gives a total 54 % Faradaic efficiency at -1.0 V (vs. RHE) for the liquid products. The study also demonstrates that the electronic and structural properties of the electrode were improved by the addition of Cu NPs on NGN surface, which in turn enhanced the performance of the catalyst as confirmed by potential-controlled electrocatalysis.

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.

Electrochemical CO2 Reduction Activity of Reduced Copper Oxide Derived from Layered Copper Hydroxide with Different Interlayer Distance and Anion Species

Electrochemically reduced copper oxide (EC-CuOx) has attracted attention as an electrocatalyst to obtain C2 products with high selectivity via electroreduction of CO2. EC-CuOx is typically formed by chemically induce dissolution of copper oxide and subsequent electororeduction to metalic copper during the electrochemical CO2 reduction reaction (CO2RR). It is well known that the electronic state and morphology of electrocatalysts have a strong correlation to the catalytic activity. Although the effects of electronic state for the electrocatalyst for CO2RR have been studied, the morphological effects of EC-CuOx during and after CO2RR have not been discussed sufficiently.In this study, we focused on layered copper hydroxides as a model electrocatalyst precursor to clarify the morphological effects. The interlayer distance of the layered copper hydroxide can be controlled through ion-exchange reactions while maintaining the morphology, crystallinity and crystal size in the lateral direction. The dissolution rate of copper ion from the host layer of layered copper hydroxide anticipated to differ due to the different interlayer distance of layered structures, leading to a difference in the accessibility of electrolyte. Based on the above hypothesis, three types of layered copper hydroxides with different interlayer distance were used as a model precursor for EC-CuOx.The layered acetate copper hydroxide (AcO-Cu2(OH)3) intercalation compound was synthesized according to a previously-reported procedure1. In order to prepare the layered structure with different interlayer distances and anion species, AcO-Cu2(OH)3 was dispersed in an aqueous solution of sodium dodecyl sulfate or nitric acid, and consequently the layered dodecyl sulfate copper hydroxide (DS-Cu2(OH)3) and layered nitrate copper hydroxide (NO3-Cu2(OH)3) intercalation compounds were obtained. CO2RR measurements were conducted in a two-compartment electrochemical cell separated by a Nafion® N117 membrane. The gaseous products were quantified by gas chromatography (GC; Shimadzu Corporation, Nexis GC-2030).SEM images show that after CO2RR, AcO-Cu2(OH)3 had needle-like morphology, while DS-Cu2(OH)3 and NO3-Cu2(OH)3 had dendritic and whisker-like morphology, respectively (Figure 1). The morphological changes during CO2RR were observed owing to the dissolution and electrodeposition of Cu-based compounds. The difference in morphology after CO2RR could be attributed to the difference of interlayer distances and anion species i.e. difference in accessibility of the inter layer. The Faradaic efficiency for ethylene production with DS-Cu2(OH)3 was ~60% at -1.4 V vs. RHE (Figure 2), which was twice higher than that for Cu(OH)2 particles (~30%). The results indicate that DS-Cu2(OH)3 with dendritic molphology possesses high selectivity for ethylene production. Acknowledgement DT thanks the partial support by The Public Foundation of Chubu Science and Technology Center. References S. Švarcová, M. Klementová, P. Bezdička, W. Śasocha, M. Dušek and D. Hradil, Cryst. Res. Technol., 46, 1051 (2011). Figure 1

Quantifying Electrocatalytic Reduction of CO2 on Twin Boundaries

Summary A protocol relying on the use of silver nanostructures with well-defined dimensions and morphologies (e.g., nanocubes and nanowires) has been developed to differentiate and quantitatively determine the atom-specific activities of different surface atoms toward catalyzing electrochemical CO2 reduction reaction (CO2RR). The atom-specific activity of the twin-boundary edges (TBEs) is comparable to (or slightly higher than) that of the single-crystal edges (SCEs) formed from the crossing of two {100} surfaces, and it is on the order of ∼16 attoampere/atom (aA/atom) toward electrochemical CO2RR. This value is more than two orders higher than the atom-specific activity of the {100} surface atoms, i.e., ∼0.1 aA/atom. The high catalytic activity of the TBEs, which represent a class of commonly existing surface defects in stable metal nanostructures with face-centered cubic lattices, is consistent with the DFT calculations, in which the bridge-type binding configuration of COOH∗ at the TBEs stabilizes this intermediate to promote overall CO2RR.

Electrochemical-assisted leaching of active materials from lithium ion batteries

The development of a circular economy for lithium-ion batteries (LIB) is essential to realize decarbonization and an electrified energy market. However, the current commercial operations that recover valuable constituents from LIBs require significant energy and reagents, which create toxic emissions and additional waste. We report an electrochemical-based method for leaching valuable metals from the active materials of mixed shredded LIBs. In this process, the use electrons, as green reagent, allows the use and regeneration of Fe2+ in low concentrations as substitute for hydrogen peroxide as a reducing agent. Leaching in a membrane separated two-compartment electrochemical cell contributes to decrease the acid requirements as H+ can be generated electrochemically. With this design, leaching efficiencies over 96% for the active metals (Li, Co, Mn, and Ni) were demonstrated at pulp densities up to 240 g/L. Copper present in the active material is electrowon and recovered separately. Preliminary cost analyses demonstrate ca. 80% reduction in energy and chemical costs as compared to traditional hydrometallurgical routes.

Indirect (hydrogen-driven) electrodeposition of porous silver onto a palladium membrane

Hydrogen permeation through a pure palladium film (25 μm thickness, optically dense) is employed to trigger electron transfer (hydrogen-driven) reactions at the external palladium | aqueous electrolyte interface of a two-compartment electrochemical cell. Two systems are investigated to demonstrate feasibility for (i) indirect hydrogen-mediated silver electrodeposition with externally applied potential and (ii) indirect hydrogen-mediated silver electrodeposition driven by external formic acid decomposition. In both cases, porous metal deposits form as observed by optical and electron microscopies. Processes are self-limited as metal deposition blocks the palladium surface and thereby slows down further hydrogen permeation. The proposed methods could be employed for a wider range of metals, and they could provide an alternative (non-electrochemical or indirect) procedure for metal removal or metal recovery processes or for indirect metal sensing.Graphical abstract

Efficacy of a two-compartment electrochemical flow cell introduced into a reagent-free UV/chlorine advanced oxidation process

As the advanced oxidation process of ultraviolet photolysis of chlorine (UV/chlorine) is more effective under an acidic condition than under a basic one, it requires reagent addition to keep the solution pH acidic and to supply free chlorine. In this study, a two-compartment electrochemical flow cell with an anion exchange membrane as a diaphragm was introduced into a UV/chlorine reactor for realizing a reagent-free operation using anodically produced protons and free chlorine. As a result, wastewater in the anodic compartment was successfully acidified by the protons generated through the water discharge at the anode, by the hydrolysis of electrochemically generated chlorine, and by the inhibition of the hydroxide ion supply from the cathodic compartment by the diaphragm. Furthermore, the electrochemical chlorine production was enhanced by increasing the chloride ion concentration in the anodic compartment through the electrodialysis effect. As the acidic effluent from the reactor was finally neutralized by hydroxide ions generated by the water discharge at the cathode, it was demonstrated that the proposed UV/electro-chlorine advanced oxidation process could work well without reagent addition. The electrical energy per order (EEO) of the process was estimated to be 15.7 kWh/(m3 order), which was comparable to that of the conventional advanced oxidation processes.

Preparation of nano-magnesium oxide from Indonesia local seawater bittern using the electrochemical method

In this study, nano-size MgO was synthesized from seawater and bittern obtained from local salt industry in Pamekasan, Madura, Indonesia, using electrochemical method. The main purpose was to study the effects of seawater pre-concentration and bittern dilution on the structure and morphology of the MgO nanoparticles produced. Electrochemical process was performed in a two-compartment electrochemical cell with a fixed potential of 18 V, for 4 h at ambient temperature, and without adjusting the pH. Solid Mg(OH)2 produced was converted into MgO by calcination treatment at 500 °C for 4 h, and then characterized using XRD and SEM technique. The characterization results revealed that spherical or cubic crystalline MgO were successfully produced, with the particle sizes in the range of 60–100 nm.

Cathodic hydrogen production by simultaneous oxidation of methyl red and 2,4-dichlorophenoxyacetate in aqueous solutions using PbO2, Sb-doped SnO2 and Si/BDD anodes. Part 2: hydrogen production.

In this work, results concerning hydrogen gas production during the oxidation of methyl red (MR) and sodium 2,4-dichlorophenoxyacetate (2,4-DNa), is presented, emphasizing not only the amount of hydrogen gas that was produced but also the kinetic and efficiency parameters involved in this process. For this purpose, a two-compartment electrochemical cell was used with a Nafion® membrane as separator in order to collect H2 without other chemical species (only with traces of water vapor). Under these experimental conditions, it was possible to guarantee the purity of the H2 collected. The electrochemical oxidation of MR and 2,4-DNa solutions was carried out by applying 30 mA cm−2 at 298 K, using different non-active anodes (Si/BDD, Pb/PbO2, or Sb-doped SnO2) and different cathodes (Pt mesh, 316-type stainless-steel, or Pt–10%Rh) in order to investigate the effect of the electrocatalytic materials and experimental conditions. Thus, the H2 produced was measured as a function of the electrolysis time and compared with the values estimated by Faraday's law. The results showed that the hydrogen production rate r(H2) is independent of the nature of the anodic material, although an important effect on the oxygen production was observed on the BDD anode by using sulfuric acid as supporting electrolyte. The effect was discussed through the formation of sulphate-oxidizing species (SO4−˙ and S2O82−) which interfere in the oxygen production step on BDD anodes. The use of different cathodes showed small changes in the hydrogen production rate r(H2), which were basically associated with the differences in hydrogen adsorption energy prior to its evolution. The results were discussed in light of the existing literature.

Cathodic hydrogen production by simultaneous oxidation of methyl red and 2,4-dichlorophenoxyacetate aqueous solutions using Pb/PbO2, Ti/Sb-doped SnO2 and Si/BDD anodes. Part 1: electrochemical oxidation.

In this work, the electrochemical oxidation of the Methyl Red (MR) dye and the herbicide sodium 2,4-dichlorophenoxyacetate (2,4-DNa) was investigated on Si/BDD, Pb/PbO2 and Ti/Sb-doped SnO2 anodes in aqueous acidic medium by applying 30 mA cm−2 at 298 K. The electrochemical experiments were carried out in a two-compartment electrochemical cell separated through a Nafion® membrane (417 type) in order to use two types of supporting electrolyte to measure the elimination of the organic compound, the hydrogen production and the amount of oxygen produced during the oxidation of the pollutants. Although the main goal of this study is to understand the relationship between both processes, the evaluation of the current efficiencies (η) is a key parameter to determine the anodic oxidative capacity to degrade the proposed pollutants. The results clearly showed that MR and 2,4-DNa can be oxidized on Si/BDD, Pb/PbO2 and Ti/Sb-doped SnO2 anodes; however, significant variations in the oxidation level and η are achieved. Thus, although the MR solutions were completely discolored in all cases, only on the Si/BDD anode was MR oxidized to carboxylic acids in less than 15 min of electrolysis time. On Pb/PbO2 and Ti/Sb-doped SnO2 electrodes, the discoloration was slower and the oxidation was quasi-completed, leaving in solution some organic by-products, such as 2-aminobenzoic acid and/or N,N′-dimethyl-p-phenylenediamine, in the fixed electrolysis time. The behavior observed during the elimination of 2,4-DNa is due to its difficulty in degrading the chlorine groups in its aromatic ring which makes 2,4-DNa a more stable molecule. In the first oxidation stage, 2,4-dichlorophenol (2,4-DP) is produced in all cases, but on Si/BDD, this intermediate is quickly consumed. From the polarization curves and Tafel analysis, a reaction scheme for the formation and consumption of 2,4-DP was proposed.

Time-resolved ATP measurements during vesicle respiration

In vitro synthesis of ATP catalyzed by the ATP-synthase requires membrane vesicles, in which the ATP-synthase is present within the bilayer membrane. Inverted vesicle prepared from Gram negative cells (e.g., Escherichia coli or Pseudomonas putida) can be readily obtained and used for in vitro ATP-synthesis. Up to now, quantification of ATP synthesized by membrane vesicles has been mostly analyzed via bioluminescence-based assays. Alternatively, vesicle respiration and the associated ATP level can be determined using biosensors, which not only provide high selectivity, but allow ATP measurements without the sample being illuminated. Here, we present a microbiosensor for ATP in combination with scanning electrochemical microscopy (SECM) using an innovative two-compartment electrochemical cell for the determination of ATP levels at E.coli or P. putida inverted vesicles. For a protein concentration of 22 mg/ml, a total amount of 0.29 ± 0.03 μM/μl ATP per vesicle was determined in case of E.coli; in turn, P. putida derived vesicles yielded 0.48 ± 0.02 μM/μl ATP per vesicle at a total protein concentration of 25.2 mg/ml. Inhibition experiments with Venturicidin A clearly revealed that the respiratory chain enzyme complex responsible for ATP generation is effectively involved.

Dismantling and electrochemical copper recovery from Waste Printed Circuit Boards in H2SO4–CuSO4–NaCl solutions

The worldwide growing of electrical and electronic equipment makes increasingly urgent to find environmentally friendly treatments for e-waste. In this paper, the attention has been focused on i) the eco-friendly dismantling of the electronic components from Waste Printed Circuit Boards and ii) recovering of pure metallic copper, which is the most abundant metal and one of the most valuable in Printed Circuit Boards. After an experimental optimization study, we found that a solution containing 0.5 M H2SO4, 0.4 M CuSO4, and 4 M NaCl can be successfully used to disassemble the electronic components from the boards by leaching of all exposed metals. Air was blown into the leaching solution in order to regenerate Cu2+ ions, which acts as the predominant oxidant specie. The key role in dismantling/leaching process is played by Cl− ion that stabilizes Cu+ through the chloro-complexes formation. The results show that, in this manner, the electronic components can be easier disassembled in undamaged state, allowing the efficient recycling and valorization of the base materials. The feasibility to recover electrochemically the copper from the solution resulting from dismantling/leaching tests was verified through a preliminary cyclic voltammetry study aimed to investigate the copper electrodeposition in sulphate-chloride solutions in the presence of other metal ions such as Ni2+, Fe2+, Zn2+, Pb2+, Sn2+. A two-compartment electrochemical cell operating in either galvanostatic or potentiostatic mode was employed to investigate simultaneous copper recovery and leaching solution regeneration. The results indicate that, in both modes, pure copper can be obtained after the removal of all surface impurities by dipping in acidic concentrated sodium chloride solution. The galvanostatic mode leads to deposit of higher quality, meanwhile the potentiostatic one determines a faster deposition and leaching solution regeneration.

Electroreduction of CO2 to High-Value Products Using Selenide Based Electrocatalysts

Electrochemical conversion of carbon dioxide (CO2) to useful chemicals presents both the potential for advantageous processes to the industry and for environmental benefits. Anthropogenic gas emissions has increased to a new level (413 ppm) in past few years. Concerns are high about impacts on climate due to high CO2 gas levels. Intergovernmental Panel on Climate Change (IPCC) report has indicated a strong risk of crisis as early as 2040 which includes worsening food shortages and wildfires, and a mass die-off of coral reefs.3 Fossil fuels has been predicted to last only for 100 years. Also the common renewable resources like wind energy or solar energy depends on the whim of weather. CO2 has been believed to cause the rise in temperature of earth’s atmosphere by 1°C. If we can use CO2 to produce energy then it can prove an everlasting energy source. Copper based catalysts are mostly active for the electrochemical reduction of CO2. Cu nanocrystals when used for CO2 electrochemical reduction give some C2 products which contain a C-C bond. Cu nanowires also reduced CO2 to CO with a faradaic efficiency of ∼50% at a moderate overpotential of 490 mV. Electrochemical reduction of CO2 if incorporated in the modern day energy resources can serve two purpose: make environment clean and provide energy too. In this work we have shown for the first time, electrocatalytic activity of Cu2Se for CO2 electroreduction to value-added chemicals. The Cu­2Se catalyst was synthesized hydrothermally and was dropcasted on Carbon Fiber paper substrate (CFP) using very low quantity of nafion as binder. The Cu2Se modified CFP served as the working electrode for CO2 electroreduction and the electrochemical measurement was measured in KHCO3 electrolyte under flowing CO2 using a two-compartment electrochemical cell. The electrocatalyst was characterized by XRD, XPS, SEM, TEM, and EDS for morphology, and elemental chemical compositions. It was observed that the reduction products could be controlled by tuning the reduction potential, and a highly selective formate production was obtained at a higher potential with a high total current density approximately 30 mA cm-2. C2 products such as ethanol and acetic acid were obtained as reduction products. The details of catalyst synthesis, characterization and activity will be presented in the meeting.

Carbon Monoxide-Tolerant Palladium-Based Electrocatalyst for Carbon Dioxide Reduction

The utilization of carbon dioxide (CO2) for the production of fuels and valuable chemicals has gained increasing attention as a strategy for solving both global energy and environmental issues.[1] Among the various proposed methods, the electrochemical reduction of CO2 using electricity powered by renewable resources is attractive, and numerous electrode materials capable of converting CO2 to carbon monoxide (CO), formate, methanol, methane, and other hydrocarbons have been identified in the past few decades. However, there still remain several fundamental challenges in the electrocatalytic reduction of CO2, such as high overpotential and low Faradaic efficiency due to the competitive hydrogen evolution reaction. Recently, it was reported that a palladium (Pd) nanoparticle electrode can reduce CO2 to formate with high energy efficiency (with low overpotential) [1]. However, the electrode was also reported to be deactivated by adsorption of CO which is produced as a byproduct. In this study, to suppress the deactivation by CO, we focused on a Pd nanoparticle electrode modified covered with copper (Cu) because the binding energy of CO on Cu was reported to be weaker than that on Pd [3]. In addition, it is known that Cu monolayer can be formed on Pd support by using underpotential deposition (UPD) [4]. Thus, we prepared a Pd nanoparticle electrode covered with Cu monolayer (Cu/Pd electrode) and examined its CO2reduction activity and tolerance to CO. A Pd nanoparticle electrode was prepared by loading homogeneous catalyst ink onto a fluorine-doped tin oxide (FTO) electrode. The ink was composed of a mixture of Pd-supporting carbon black powder, Nafion, and isopropanol. The Pd-supporting carbon black powder was purchased from Premetek. UPD of Cu was carried out by holding the electrode potential at 0.35 V vs. reversible hydrogen electrode (RHE) for 50 s in a mixed solution of 50 mM sulfuric acid (H2SO4) and 50 mM copper sulfate (CuSO4). Electrolyses were performed in a gas-tight two-compartment electrochemical cell with a piece of anion exchange membrane as the separator. An aqueous solution of 0.5 M sodium hydrogen carbonate (NaHCO3) saturated with CO2was used as an electrolyte. Figure 1 shows chronoamperograms for CO2 reduction of Pd and Cu/Pd electrodes measured at -0.15 V vs RHE. In the initial stage, the current density of a Cu/Pd electrode was smaller than that of a Pd electrode. This is plausible because the surface area of Pd exposed to the electrolyte decreased by deposition of Cu. However, the Cu/Pd electrode was active for CO2 reduction even after deposition of Cu and production of formate was confirmed by analysis of the electrolyte using gas chromatograph (GC). Notably, the reduction current of the Cu/Pd electrode maintained more steadily than that of the Pd electrode during long-term electrolysis, suggesting that the electrodeposited Cu monolayer improved the tolerance to CO. To examine the effect of Cu layer on CO adsorption, CO2 reduction activity in the presence of CO was also investigate. When CO was introduced into the electrolyte by bubbling, the current density of both Pd and Cu/Pd electrodes decreased, however the degree of deactivation of the Cu/Pd electrode was less than that of the Pd electrode. These results indicate that the covering of Pd nanoparticle with Cu monolayer improves its tolerance to CO without losing CO2reduction activity.

Electrochemical CO2 Reduction in Ionic Liquid Using Two Compartment Cell Separated with Proton-Conducting Membrane

Reduction of CO2 gas using natural energy resource to produce useful chemicals has been an important issue to reduce CO2 emission and to establish the sustainable society. Some of ionic liquid such as 1-ethylimidazoliumbis (trifluoromethylsulfonyl) imide (EMIM-TSFI) ionic liquid is known to absorb CO2 gas and thus considered as a medium for CO2 gas separation and transportation. In this study EMIM-TSFI was used for electrolyte solution in which CO2 gas is absorbed and electrochemically reduced to form, for example, methane. EMIM-TSFI is also an hydrophobic liquid and thus water content can be limited to be low level. This means that the sub-reaction of electrochemical CO2 reduction such as water decomposition can be suppressed. In our previous work, it was found that a proper water content suitable for efficient CO2 reduction to form methane was ca. 5000 ppm. In a traditional single cell, however, reaction of H2O decomposition to form proton on a counter electrode (CE) requires large overpotential due to low water content resulting in decomposition of ionic liquid. To solve this problem, we have developed a two-compartment electrochemical cell composed of two rooms one of which contains working electrode (WE) and ionic liquid and an another room contains CE and KCl aqueous solution. These tow rooms were separated with a Nafion membrane to allow proton transportation between two rooms. In a room of aqueous solution, water decomposition (oxygen evolution) reaction was proceeded on a Pt-CE. In a room of ionic liquid, CO2 gas was bubbled to be absorbed and cathodically reduced on Cu-WE to form methane with protons supplied via membrane. Voltammogram of Cu-WE in the ionic liquid was measured as a function of CO2 gas supply and water content in ionic liquid. The result confirmed that supplying CO2 gas and 5000 ppm water increased cathodic polarization current in a potential lower than –1.4 V vs Ag. Cathodic current also responded rapidly to changing the flow gas between Ar and CO2. Analysis of gas composition using a gas chromatography showed that the production ratio of CH4 and H2 depends on polarization potential and a major product was CH4 in the potential range of –1.4 ~ –1.5 V while H2 in the potential range lower than –1.6 V. Spatial arrangement of electrodes and membrane was an another important factor to affect the efficiency, i.e., the efficiency of methane formation increased with decreasing distance between WE and Nafion membrane, indicating that transportation of proton from membrane to WE via ionic liquid medium limited the reaction rate. Contact electrode assembly of Cu-mesh CE/Nafion membrane/ Ti-mesh CE was therefore constructed and revealed better conversion efficiency of CO2 reduction to methane without any sign of ionic liquid decomposition. On the Cu-CE and membrane, however, byproduct of black color was deposited after long time operation. The deposits was supposed to be originated from the silicone sealant used to assemble the electrochemical cell because the deposits contained Si but not carbon. Currently we are working on improvement and refining the electrolysis system for long and efficient operation.