METAL HOLLOW FIBER ELECTRODE
The invention is directed to a metal hollow fiber electrode, to a method of electrolyzing carbon dioxide in an aqueous electrochemical cell, to a method of converting carbon dioxide, to a method of preparing a metal hollow fiber, to a use of a metal hollow fiber electrode. The metal hollow fiber electrode comprises aggregated copper particles forming an interconnected three-dimensional porous structure, wherein said metal comprises copper.
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This application is a United States National Phase under 35 U.S.C. § 371 of International Application No. PCT/NL2016/050826, filed on Nov. 24, 2016, which claims the benefit of, and priority to, U.S. Provisional Application No. 62/259,089, filed on Nov. 24, 2015, both of which are hereby incorporated by reference in their entirety for all purposes.
The invention is directed to a metal hollow fiber electrode, to a method of electrolyzing carbon dioxide in an aqueous electrochemical cell, to a method of converting carbon dioxide, to a method of preparing a metal hollow fiber, to a use of a metal hollow fiber electrode.
The accumulation of carbon dioxide (CO2) in the atmosphere has already large impact on local climate conditions, and starts to affect human and natural life. Immediate measures must be taken to minimize carbon emissions. A promising way is to convert CO2 to useful chemicals by using electricity generated by renewable energy sources. Nevertheless, the development of an efficient and stable electrocatalyst that can reduce CO2 at high current densities, remains a challenge. In recent years, researchers achieved to produce CO at low potentials in aqueous solutions albeit with limited current densities or using noble metals [Lu et al, Nat. Commun. 2014, 5, 3242; Zhu et al, J. Am. Chem. Soc. 2014, 136, 16132-16135; Chen et al, J. Am. Chem. Soc. 2012, 134, 19969-19972]. Reasonable current densities at low overpotentials towards CO were achieved by using ionic liquids [Medina-Ramos et al., J. Am. Chem. Soc. 2014, 136, 8361-8367; Rosen et al., Science 2011, 334, 643-644]. However, considering ionic liquids, several issues remain challenging such as cost, recycling, stability and electrolyzer performance in practical applications. Aqueous conversion of CO2 is still more attractive if high selectivity towards CO2 over water splitting can be achieved at low overpotentials and high current densities.
Copper electrodes are well known for producing hydrocarbons from CO2 with variable onset potentials (˜0.5-0.7 V) depending on the preparation method. Generally, high potentials (˜0.8-1 V) are necessary to obtain reasonable faradaic efficiency (FE). Although less expensive and much more abundant than other CO evolving catalysts such as e.g. silver and gold, poor activity, selectivity and stability towards CO and formic acid have been reported for polycrystalline copper. Recently, Li et al. reported production of CO and formic acid with reasonable faradaic efficiency at low overpotentials on copper nanoparticles, when formed by electrochemical reduction of cuprous oxides [Li et al, J. Am. Chem. Soc. 2012, 134, 7231-7234]. At a potential of −0.5 V vs. RHE, a copper surface covered with nanoparticles delivered a CO2 reduction current density of 2.1 mA/cm2, with a faradaic efficiency of 35% and 32% for CO and formic acid respectively. Even though such a selectivity can be considered as low, this was the first publication showing CO and formic acid formation is feasible over copper electrodes. Additionally, production of energy dense products such as ethylene, ethanol and methane might require selective conversion to CO first, since CO is a common intermediate for nearly all hydrocarbons and oxygenates.
Inorganic hollow fibers are of potential significance for solid oxide fuel cells due to their high surface area to volume ratio, higher power outputs and lower fabrication costs, but utilization in room temperature solution based electrochemistry is quite rare. Several examples exist using nickel and carbon hollow fibers with dual functionality, where they served as both membrane for effluent purification and as cathode for proton and oxygen reductions, respectively. Recently, microtubular gas diffusion electrodes made of carbon nanotubes were proposed for tubular electrochemical reactor design [Gendel et al., Electrochem. Commun. 2014, 46, 44-47].
An objective of the invention is to overcome one or more disadvantages seen in the prior art.
A further objective of the invention is to provide an electrode that allows low pressure and room temperature electrolysis of CO2.
It was found that one or more of these objectives can be met, at least in part, by a metal hollow fiber electrode with a specific structure.
Accordingly, in a first aspect, the invention is directed to a metal hollow fiber electrode, comprising aggregated cooper particles forming an interconnected three-dimensional porous structure, wherein said metal comprises copper.
It was surprisingly found that metal hollow fiber electrodes can be a potential candidate for low pressure and room temperature electrolysis of CO2, due to their excellent mass transport capabilities when used as gas diffuser and cathode. Not only the hydrogen evolution reaction is suppressed on these electrodes to levels not reached previously on copper surfaces, but also the overall CO2 reduction current density is unprecedentedly high at low potentials.
In accordance with the invention, the metal comprises copper and other metals may optionally be present. More preferably, the metal is copper.
The metal hollow fibers can typically have an inner diameter of 0.1-10 mm, such as 0.5-5 mm, or 0.7-3 mm. The outer diameter of the metal hollow fibers can be 0.1-10 mm, such as 0.5-5 mm, or 0.7-3 mm.
The fibers preferably comprise, or are composed of, sintered copper particles. Solid state sintering is the process of taking metal in the form of a powder and placing it into a mold or die. Once compacted into the mold the material is placed under a high heat for a long period of time. Under heat, bonding takes place between the porous aggregate particles and once cooled the powder has bonded to form a solid piece.
The copper particles in the metal hollow fiber electrode preferably have an average particle diameter of 0.1-10 μm, such as 0.3-5 μm, or 0.5-3 μm.
In a preferred embodiment, a porous outer layer of the hollow fiber is more dense than a porous inner layer of the hollow fiber, said outer layer preferably having a thickness in the range of 5-20 μm, such as 12-18 μm, or 10-15 μm.
The preparation of the metal hollow fibers i.e. nickel and stainless steel has been described in the literature previously [Meng et al, J. Alloy Compd. 2009, 470, 461-464; Luiten-Olieman et al, Scripta Mater. 2011, 65, 25-28]. The preparation of Cu hollow fiber, on the other hand, has not been reported to the best of our knowledge. The inventors adapted the method and prepared Cu hollow fibers by spinning a mixture containing copper particles, polymer and solvent. The mixture is suitably pressed through a spinneret into a coagulation bath. In this bath, non-solvent induced phase separation arrests the copper particles in the polymer matrix. By adding a bore-liquid during spinning, a hollow fiber is obtained. After thermal treatment, the polymer is decomposed and the copper particles are sintered together, resulting in hollow, porous copper oxide fibers. Hydrogenation of these precursor fibers at elevated temperatures was applied to obtain metallic copper fibers. Typical scanning electron microscope (SEM) images of the Cu hollow fibers are shown in
For monitoring the electrochemical activity of the fibers, linear sweep voltammetry experiments were performed in CO2 saturated electrolyte, while Ar or CO2 were purged through the fibers (
To confirm the catalytic activity, the faradaic efficiency of the major products was measured by varying the applied potential between −0.15 V and −0.55 V vs. RHE (
On that account, the formation of hydrocarbons still requires relatively large overpotentials on Cu hollow fiber, and direct CO2 reduction to energy dense products remains a challenge. Formation of CO at very low potentials implies a better stabilization of the key CO2— intermediate which is formed by the first electron transfer to adsorbed CO2. The remarkable activity of the rough copper electrodes prepared by oxidation of copper and subsequent reduction, namely oxide-derived copper, can be attributed to the metastable sites exist in grain boundaries. However, reasonable activities also achieved towards formic acid and CO with faradaic efficiency up to 45% at a potential of −0.5 V vs. RHE on copper nanofoams prepared by electrodeposition. Interestingly, the production of formic acid was controlled by changing thickness of the electrodeposited layer similar to the oxide-derived copper where CO2 reduction activity was a function of the parent oxide layer thickness. In addition, Reske et al. showed the reduction of CO2 can be significantly enhanced by decreasing the size of the copper nanoparticles which is correlated to the number of uncoordinated cites [Reske et al., J. Am. Chem. Soc. 2014, 136, 6978-69861]. By considering all these studies, the increase in the CO2 reduction activity on porous, foamy and nanoparticulate copper structures might be associated with the abundance of the low coordinated sites such as kinks, steps etc. which influence the binding energy of the key intermediate CO2—.
On the other hand, contrary to polycrystalline copper and copper nanoparticles, formation of formic acid is relatively suppressed. Mechanistic information can be deduced for the two electron reduction of CO2 to CO by using a Tafel plot shown in
To test the stability of the Cu hollow fibers, 24 hours of continuous electrolysis was performed at an applied potential of −0.4 V vs. RHE (
An overview of the performance of different catalysts' as compared to our Cu hollow fiber (Cu hf) is shown in
In addition to the comparison in
Gas diffusion electrodes played an important role in fundamental electrocatalysis, however their mass production delayed due to economic and technical issues. The mature dry-wet spinning process allows mass production of organic hollow fibers that are already commercially available. Preparation of hollow metal fibers with diameters in the range of 100-500 μm, on the other hand, was developed recently which adapts a very similar method, implying a great potential of large scale production. Microtubular geometry has been deployed and investigated in solid oxide fuel cells for decades which could allow the adaptation of technologies developed such as stack design, sealing, current collection etc. Metal hollow fibers might provide cost effective and compact diffusion media and/or catalyst layer for gas diffusion electrodes which might also eliminate resistance associated with catalyst support interface. Furthermore, we believe there is plenty of room to increase the production rate by considering the controllability of the internal and external structure of the hollow fibers. The thickness of the active catalyst layer can be tuned by changing 3-D geometry, support material, porosity and/or precursor particle size, to further optimize the production rate.
The results reported herein highlight a new area to explore for the development of robust electrodes that can efficiently catalyze conversion of CO2 at high rates in aqueous media. Employing a simple, compact Cu hollow fiber as both gas diffuser and an electrode, leads to very high CO production rates which are comparable to what has been achieved by using noble metals. Selective formation of CO is observed with a maximum faradaic efficiency of 75% when high flow rates are used. The porous nature of the hollow fibers provides high surface area and correspondingly high geometric current densities for CO2 reduction ranging from 2 mA cm−2 to 17 mA cm−2 at moderate potentials (−0.3 to −0.5 V vs. RHE). The remarkable performance of the hollow fibers are attributed to defect-rich porous structure in addition to the excellent mass transport capabilities. In general, hollow fibers provide new possibilities to design practically relevant microtubular electrodes and electrochemical reactors where one or more reactants are present in the gas phase.
In a further aspect, the invention is directed to a method of electrolyzing carbon dioxide in an aqueous electrochemical cell comprising an anode and a cathode, wherein the cathode comprises one or more metal hollow fiber electrodes according to the invention, said method comprising
- applying a potential between said anode and cathode, and
- purging CO2 or a gas mixture comprising CO2 through the wall of the metal hollow fiber electrode.
Preferably, the method of the invention is performed in an aqueous environment.
The method can suitably be performed at a temperature in the range of 5-80° C., such as in the range of 10-30° C., more preferably in the range of 15-25° C.
In a further aspect, the invention is directed to a method of converting carbon dioxide into one or more selected from the group consisting of carbon monoxide, formic acid, a formate, methanol, acetaldehyde, methane, ethylene and ethane, comprising electrolyzing CO2 by a method according to the invention of electrolyzing carbon dioxide in an aqueous electrochemical cell comprising an anode and a cathode as described herein.
In a preferred embodiment, the carbon dioxide is converted into carbon monoxide.
In yet a further aspect, the invention is directed to a method of preparing a metal hollow fiber electrode according to the invention, comprising:
- spinning a mixture comprising copper particles, polymer and solvent together with a bore liquid to obtain hollow fibers;
- subjecting the hollow fibers to a thermal treatment such that copper particles are sintered together, thereby yielding hollow copper oxide fibers;
- hydrogenating the hollow copper oxide fibers.
The thermal treatment in this method preferably comprises subjecting the hollow fibers to a temperature in the range of 500-800° C., such as in the range of 550-700° C. This thermal treatment is preferably performed for a period of 1-6 hours, such as a period of 2-5 hours.
The hydrogenation preferably comprises subjecting the hollow copper oxide fibers to a temperature in the range of 200-400° C., such as in the range of 250-350° C. This hydrogenation is preferably performed for a period of 30-120 minutes, such as 45-90 minutes. Preferably the hydrogenation comprises subjecting the hollow copper oxide fibers to a flow of hydrogen in the concentration range of 0.1-100 vol. %, such as 5 vol. % in a balance gas.
In yet a further aspect, the invention is directed to the use of a metal hollow fiber electrode according to the invention as cathode and/or gas diffuser.
Preparation of Cu Hollow FibersCommercial available copper powder (Skyspring nanomaterials, 99%) with particle size of 1-2 μm was used as catalyst precursor. N-methylpyrrolidone (NMP, 99.5 wt. %, Sigma Aldrich), Polyetherimide (PEI, Ultem 1000, General Electric) were used as solvent and polymer respectively. Copper powder (71.09 wt. %) was added to the NMP (22.14 wt. %) followed by stirring and ultrasonic treatment for 30 min. After addition of PEI (6.76 wt. %) this mixture was heated and kept at 50° C. and 60° C. for 30 minutes and 2 hours, respectively. Next, the solution is allowed to cool down by stirring overnight which is followed by degassing. Vacuum was applied for 90 min and the mixture was left overnight.
Spinning was carried out at room temperature (21±3° C.) using a stainless steel vessel, that was pressurized to 1 bar using nitrogen. The mixture was pressed through a spinneret (inner and outer diameters of 0.8 mm and 2.0 mm, respectively) into a coagulation bath containing tap water. Deionized water was pumped through the bore of the spinneret with a speed of 30 ml min−1 and the air gap was set to 1 cm.
After spinning the fibers were kept in the coagulation bath for 1 day to remove traces of NMP, followed by drying for 1 day. The green copper hollow fibers were thermally treated at 600° C. for 3 hours (heating rate and cooling rates: 1° C. min−1) in air to remove the PEI and subsequent sintering of the copper particles. The oxidized hollow fibers were reduced by hydrogenation at 280° C. for 1 hour (H2 in Argon: 4%, heating rate and cooling rate: 100° C./min). X-ray diffraction patterns were collected by using a Bruker D2 Phaser x-ray diffractometer, equipped with a Cu-Kα radiation source and operated at 30 kV and 10 mA (
All solutions were prepared and all glassware were cleaned by using deionized water (Millipore MilliQ, 18.2 MQ). Electrochemical CO2 reduction activity of Cu HF's was measured by using three electrode assembly in a glass cell at room temperature and pressures. A Princeton Applied Research versaSTAT 3 potentiostat was used to control the potentials. The counter electrode, Pt mesh, was separated by using a Nafion 112 membrane (Sigma Aldrich). An Ag/AgCl (3 M NaCl BAST) reference electrode was placed near the working electrode by using a Luggin capillary and all the potentials were converted to RHE scale afterwards. IR drops were measured before the electrolysis and compensated manually after the experiments. 4±0.5 cm long Cu HF are used as both working electrode and as gas diffuser. The fibers were sealed from the bottom by using epoxy glue and connected to gas inlet of the cell. The cathodic compartment is filled with 100 ml, 0.3 M KHCO3 (99.95%, Sigma Aldrich) and purged with CO2 at least 20 minutes before the experiments. During the electrolysis the CO2 is purged continuously through the fiber with 20 ml min−1 unless otherwise indicated, and sampled via gas chromatogram (GC) each 6 minutes. CO, CO2, H2 and hydrocarbons are separated with GC equipped with two different columns (ShinCarbon 2 m micropacked column and Rtx-1). A thermal conductivity detector (TCD) and flame ionization detector (FID) were used to perform the quantitative analysis of the gas phase products. The time needed to reach steady state concentration was approximately 10 minutes so all the reaction times were kept at least 20 minutes. A control experiment was conducted at −0.5 V vs. RHE under Argon atmosphere. No CO was detected that might associated with the organic residues remaining from polymers that is used during preparation of the hollow fibers. Liquid products formed during the electrolysis were analyzed by using High Performance Liquid Chromatography (HPLC) (Prominence HPLC, Shimadzu; Aminex HPX 87-H column, Biorad).
Characterization of the Cu hollow fibers
Certified chemical analysis of the precursor copper powder is given in table 1 (taken from the datasheet provided by the company which supplied the copper powder, Skyspring Nanomaterials). The XRD pattern and SEM images of the copper powder are given in
An example for the reproducibility of the experiments is given in
Electrodeposition experiments were performed on Cu hollow fibers to determine the reactive zones. Nickel deposition was performed from solutions of nickel nitrate (50 mM Ni(NO3)2, 5 mA cm−2 for 900 s) while purging Ar at 20 ml min−1. The SEM images in
wherein Ii is the peak area of a photoelectron peak and Si is the relative sensitivity factor of the peak. The calculated amounts for the detected elements are given in table 3. The elemental scan Cu 2p XPS spectrum indicated the presence of Cu0 and/or Cu1+, but the peaks associated with Cu2+ in all the spectra were absent. Cu0 and Cu1+ ratio can be roughly estimated by using Cu LMM peaks (
Claims
1. A metal hollow fiber electrode, comprising aggregated copper particles forming an interconnected three-dimensional porous structure, wherein said metal comprises copper.
2. The metal hollow fiber electrode according to claim 1, wherein said metal is copper.
3. The metal hollow fiber electrode according to claim 1, wherein said fibers have an inner diameter of 0.1-10 mm.
4-5. (canceled)
6. The metal hollow fiber electrode according to claim 1, wherein said fibers have an outer diameter of 0.1-10 mm.
7-8. (canceled)
9. The metal hollow fiber electrode according to claim 1, wherein said fibers comprises or is composed of sintered copper particles.
10. The metal hollow fiber electrode according to claim 1, wherein said copper particles have an average particle diameter of 0.1-10 μm.
11-12. (canceled)
13. The metal hollow fiber electrode according to claim 1, wherein a porous outer layer of the hollow fiber is more dense than a porous inner layer of the hollow fiber.
14. The metal hollow fiber electrode according to claim 1, wherein said outer layer has a thickness in the range of 5-20 μm.
15-16. (canceled)
17. A method of electrolyzing carbon dioxide in an aqueous electrochemical cell comprising an anode and a cathode, wherein the cathode comprises one or more metal hollow fiber electrodes according to claim 1, said method comprising
- applying a potential between said anode and cathode, and
- purging CO2 or a gas mixture comprising CO2 through the wall of the metal hollow fiber electrode.
18. The method according to claim 17, wherein said method is performed in an aqueous environment.
19. The method according to claim 17, wherein said method is performed at a temperature in the range of 5-80° C.
20-21. (canceled)
22. A method of converting carbon dioxide into one or more selected from the group consisting of carbon monoxide, formic acid, a formate, methanol, acetaldehyde, methane, ethylene and ethane, comprising electrolyzing CO2 by a method according to claim 17.
23. The method according to claim 22, wherein carbon dioxide is converted into carbon monoxide.
24. A method of preparing a metal hollow fiber electrode according to claim 1, comprising
- spinning a mixture comprising copper particles, polymer and solvent together with a bore liquid to obtain hollow fibers;
- subjecting the hollow fibers to a thermal treatment such that copper particles are sintered together, thereby yielding hollow copper oxide fibers;
- hydrogenating the hollow copper oxide fibers.
25. The method according to claim 24, wherein said thermal treatment comprises subjecting the hollow fibers to a temperature of 500-800° C.
26. (canceled)
27. The method according to claim 24, wherein the hollow fibers are subjected to said thermal treatment for a period of 1-6 hours.
28. (canceled)
29. The method according to claim 24, wherein said hydrogenation comprises subjecting the hollow copper oxide fibers to a temperature of 200-400° C.
30. (canceled)
31. The method according to claim 24, wherein the hollow copper oxide fibers are hydrogenated for a period of 30-120 minutes.
32. (canceled)
33. The method according to claim 24, wherein the hollow copper oxide fibers are hydrogenated in a flow of hydrogen in the concentration range of 0-100 vol. %.
34. The method according to claim 24, wherein the hollow copper oxide fibers are hydrogenated in a flow of hydrogen in a concentration of 5 vol. % in a balance gas.
35. (canceled)
Type: Application
Filed: Nov 24, 2016
Publication Date: Sep 5, 2019
Applicant: Universiteit Twente (Enschede)
Inventors: Recep KAS (Enschede), Patrick DE WIT (Enschede), Nieck Edwin BENES (Enschede), Guido MUL (Enschede)
Application Number: 15/778,864