PULSED CURRENT CATALYZED GAS DIFFUSION ELECTRODES FOR HIGH RATE, EFFICIENT CO2 CONVERSION REACTORS

Abstract

An electro catalytic CO.sub.2 reduction method including forming a gas diffusion cathode including a porous layer and gas diffusion layer. The method includes electrocatalyzing the gas diffusion cathode by electrochemically depositing a CO.sub.2 reduction catalyst using a pulse current or pulse reverse current passed between the gas diffusion cathode and a counter electrode in a bath containing ions of the catalyst to balance nucleation/growth of the catalyst particles resulting in a more uniform deposition of catalyst particles of predominantly less than 20 nm. The electro catalyzed gas diffusion cathode is utilized in an electrochemical reactor along with an anode and voltage source connected to the cathode and anode to convert CO.sub.2 to another chemical (e.g., formic acid).

Claims

1. An electrocatalytic CO.sub.2 reduction method comprising: forming a gas diffusion cathode including a high surface area largely microporous layer on a low surface area gas diffusion layer, whereby the microporous layer is relatively hydrophilic compared to the relatively hydrophobic gas diffusion layer; electrocatalyzing the gas diffusion cathode by electrochemically depositing a CO.sub.2 reduction catalyst onto the microporous layer using a pulse current or pulse reverse current passed between the gas diffusion cathode and a counter electrode in a bath containing ions of the catalyst to balance nucleation/growth of the catalyst particles resulting in a more uniform deposition of catalyst particles of predominantly less than 50 nm onto the microporous layer; and employing the electrocatalyzed gas diffusion cathode in an electrochemical reactor along with an anode and voltage source connected to the cathode and anode to convert CO.sub.2 to another chemical.

2. The electrocatalytic CO.sub.2 reduction method of claim 1 in which the reduction catalyst is selected from the group including Pb, Hg, In, Sn, Cd, and Ti.

3. The electrocatalytic CO.sub.2 reduction method of claim 2 in which the other chemical is formic acid.

4. The electrocatalytic CO.sub.2 reduction method of claim 1 in which the reduction catalyst is Sn.

5. The electrocatalytic CO.sub.2 reduction method of claim 1 in which the low surface area gas diffusion layer is carbon paper.

6. The electrocatalytic CO.sub.2 reduction method of claim 1 in which the high surface area largely microporous layer includes conductive particles in a resin binder.

7. The electrocatalytic method of claim 1 where at least 50% of the catalyst particles are utilized for the electrochemical reduction reaction.

8. A method of making a gas diffusion cathode, the method comprising: applying a microporous layer to a macroporous layer to form a cathode; subjecting the cathode to an electrocatalyzation process including electrochemically depositing a reduction catalyst onto the microporous layer wherein catalyst particles are in contact with particles of the microporous layer which are in electrical continuity with the macroporous layer.

9. The method of claim 8 in which the macroporous layer is hydrophobic,

10. The method of claim 8 in which the microporous layer is partially hydrophobic and partially hydrophilic,

11. The method of claim 8 in which the macroporous layer includes carbon fiber substrate,

12. The method of claim 8 in which the microporous layer includes high surface area conductive particles and a resin binder,

13. The method of claim 8 in which the reduction catalyst is selected from the group including Pb, Hg, In, Sn, Cd, and Ti.

14. A method of making a gas diffusion cathode, the method comprising: applying high surface area carbon microporous layer to a macroporous carbon fiber substrate; subjecting the high surface area carbon microporous layer to an electrocatalyzation process including electrochemically depositing, using a pulse current or pulse reverse current, catalyst particles onto carbon particles of the carbon microporous layer and providing a proton conducting ionomer surface partially penetrating the carbon particles of the carbon microporous layer.

15. The method of claim 14 in which the electrocatalyzation process includes placing the carbon microporous layer in a bath including ions of the catalyst and connecting a power supply to the carbon fiber substrate and a counter electrode in the bath.

Description

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

[0025] Other objects, features and advantages will occur to those skilled in the art from the following description of a preferred embodiment and the accompanying drawings, in which:

[0026] FIG. 1 is a generalized pulse reverse waveform for electrochemical deposition;

[0027] FIG. 2 is a comparison of current densities and formate faradaic efficiencies obtained in accordance with the invention and as compared to prior art processes.

[0028] FIG. 3 is a plot of total cell voltage and current density obtained using the flow reactor in a three electrode configuration using a baseline Sn-150 GDE cathode and Pt/H.sub.2 counter-electrode;

[0029] FIG. 4 is a plot of total cell voltage and formate faradaic efficiency obtained using the flow reactor in the two electrode configuration under galvanostatic control; conventional Sn-150 GDE cathode and Pt/H.sub.2 counter electrode;

[0030] FIG. 5 is a plot of total cell voltage at various applied currents over the course of an hour long electrolysis in the flow reactor; two-electrode configuration using a conventional Sn-150 GDE cathode and Pt/H.sub.2 counter electrode;

[0031] FIG. 6 is a side view of an example of a gas diffusion cathode in accordance with an example of the invention;

[0032] FIG. 7 is a schematic view of an electrochemical deposition cell in accordance with an example of the invention;

[0033] FIG. 8 is a view of the cathode of FIG. 7 after electrocatalyzation;

[0034] FIG. 9A is a view of a cathode structure according to the prior art methods such as spray-coating;

[0035] FIG. 9B is a view of the cathode structure when electrocatalyzation is used; and

[0036] FIG. 10 is a schematic view of an electrochemical reactor in accordance with an example of the invention.

DETAILED DESCRIPTION OF THE INVENTION

[0037] Aside from the preferred embodiment or embodiments disclosed below, this invention is capable of other embodiments and of being practiced or being carried out in various ways. Thus, it is to be understood that the invention is not limited in its application to the details of construction and the arrangements of components set forth in the following description or illustrated in the drawings. If only one embodiment is described herein, the claims hereof are not to be limited to that embodiment. Moreover, the claims hereof are not to be read restrictively unless there is clear and convincing evidence manifesting a certain exclusion, restriction, or disclaimer.

[0038] Featured is an electrodeposition based process to fabricate high-performance tin GDE electrocatalysts for the CO.sub.2 conversion to formate. A pulse/pulse reverse electric field, is preferably used as shown in FIG. 1 to improve electrocatalyst availability and activity. The waveform consists of a cathodic (forward) pulse followed by a first relaxation period (off-time) and an anodic (reverse) pulse and a second relaxation period (off-time). The cathodic peak current, cathodic on-time, anodic peak current, anodic on-time, the first relaxation-time and the second relaxation-time are individual variables for process control. In some cases only the first or second relaxation-time may be required. In other cases, no relaxation time may be required. In still other cases only the cathodic pulse and relaxation-time may be required. The initial data was evaluated based upon (1) the capability to deposit uniform, adherent tin particles on commercial GDE substrates; (2) the electrochemical performance (total current density, formate selectivity) of the electrodeposited GDEs in representative electroreactor hardware; and (3) scalability and economic feasibility of the GDE electrocatalysts on the scale of industrial electrical generation.

[0039] The present invention will be illustrated by the following example, which is intended to be illustrative and not limiting but could be embodiments of the process.

[0040] FIG. 2 shows measured current densities and formate selectivities for the GDEs prepared in the present study as well as selected literature values. See D. Kopljar, N. Wagner, E. Klemm. Chem Eng Technol 39(11): 2042 (2016) and H. Li, C. Oloman. J Appl Electrochem 35(10): 955 (2005) both incorporated herein by this reference. For both electrocatalyzed samples a higher absolute and partial formate current density was observed as compared both to the baseline (Sn-150) GDEs and to notable prior reports that utilized a GDL-type electrode and electrolytes of comparable ionic strengths. This is indicative of a break-in period where the current response of the GDE stabilizes until it attains a stable average value, consistent with previous literature reports of similar GDE type electrodes. See D. Kopljgar, A. Inan, P. Vindayer, N. Wagner, E. Klemm. Electrochemical reduction of CO2 to formate at high current density using gas diffusion electrodes. J Appl Electrochem 44(10): 1107 (2014) and E. Irtem, T. Andreu, A. Parra, M. D. Hernandez-Alonso, S. Garcia-Rodriguez, J. M. Riesco-Garcia, G. Penelas-Perez, J. R. Morante. J Mater Chem A 4: 13582 (2016) both incorporated herein by this reference. Electrolyte leakage was observed with several of the electrocatalyzed samples; the leaked electrolyte was collected and accounted for in all efficiency calculations. This flooding/leakage issue was largely absent in electrocatalyzed samples prepared from Triton-free plating electrolyte; however, these samples showed considerably poorer electrocatalytic performance. Thus, the composition of the deposition bath should be optimized to achieve a desirable balance of properties in the finished GDE. It is possible, for example, that the use of a lower Triton concentration in the plating bath in tandem with a higher ionomer MPL loading may provide optimal MPL (and thus catalyst) wetting while preventing electrolyte penetration into the CFS layer of the GDL.

[0041] Durability of the GDEs was assessed by performing duplicate electrolysis tests. The setup was rinsed with deionized water between trials, which may or may not have had an effect on the above-mentioned delamination of the catalyst from the GDL. The change in current density and faradaic efficiency observed for various samples over two electrolysis trials conducted on the same electrode, expressed as a percent change from the original value was determined. The two electrocatalyzed GDEs that exhibited the highest current densities observed (Sn-013 and Sn-014, at 388 and 328 mA cm.sup.2, respectively) both showed modest decreases in current density and formic acid efficiency in the repeated run, as compared to recent literature reports. However, sample Sn-016 exhibited a minimal decrease in both metrics over the course of the two trials, in line with the previous reports. The primary difference between samples Sn-013/-014 and Sn-016 is a modestly greater ionomer loading. Hence, again, optimization of the GDL pretreatment/conditioning may be required.

[0042] The W-cell electrochemical tests were run in either a semi-continuous or batch mode, depending on whether the flow was engaged. For scalability of the proposed CO.sub.2 conversion process and others under development, a family of continuous-flow electroreactors has been developed, as described in the methods section. This configuration utilizes a central liquid electrolyte channel with a GDE-type electrode on either side. The reactor can be operated in either a two- or three-electrode configuration, with the latter using a reference electrode port installed in the liquid channel. The small form factor of the reactor resulted in hindered performance when gaseous flow fields were used, so all tests were performed without flow fields. FIG. 3 shows representative current density and cell voltage data for the conventional GDE samples housed in the flow reactor using a three electrode configuration. Very high formate selectivities were achieved in these tests; the highest observed selectivity was 88%.

[0043] Further testing of the flow reactor was conducted using a two-electrode configuration, which is preferable for industrial-scale installations due to its comparatively simplicity, as long as reactor performance can be maintained in the absence of the additional control afforded by the reference electrode. FIG. 4 shows a plot of the total cell voltage as a function of the applied current density obtained under galvanostatic (current) control, which was found to be more stable than potentiostatic (voltage-controlled) operation. The corresponding selectivity to formate is also included for each data point, obtained from an hour long electrolysis. A selectivity above 70% was maintained up to 200 mA cm.sup.2, beyond which it began to drop appreciably. FIG. 5 shows the changes in reactor voltage over the course of an hour long electrolysis; i.e. the chronopotentiometry associated with FIG. 4. The instability of the reactor voltage at the highest current density (240 mA cm.sup.2) was persistent with repeated trials and is likely the source of lower faradaic efficiency reported in FIG. 4. These preliminary results show several enhancements over the existing literature. Comparison can be made to a 2016 study by Koplijar et al., utilizing the same experimental conditions (Sn catalyst, Pt/H.sub.2 counter, 1 M electrolyte), which reported noticeably inferior performance to that shown in FIG. 5: 80 mA cm.sup.2 at 2 V potential (relative overpotential of 250 mV) and 140 mA cm.sup.2 at 2.5 V potential (relative overpotential of 500 mV). These improvements are significant and will aid in improving the energy efficiency and hence techno-economics of the eCO.sub.2RR process.

[0044] In some embodiments, cathode 100, FIG. 6 includes a mixed hydrophobic/hydrophillc microporous layer 104 deposited onto an essentially hydrophobic gas diffusion layer 102. In one example, layer 102 is a carbon paper substrate coated with a polymer (e.g., Teflon) such that layer 102 is essentially hydrophobic. Layer 104 may include conductive (e.g., carbon) particles bound together with a polymer (e.g. Teflon) and penetrated with an ion conducting polymer (e.g. Nafion) such that layer 104 exhibits a mixed hydrophobic/hydrophilic character. See also U.S. Pat. No. 6,080,504 incorporated herein by this reference. Layer 104 may be high surface area carbon exhibiting mixed hydrophobic/hydrophilic by the incorporation of Teflon and Nafion. Alternatively, layer 104 need not incorporate Nafion and may be still exhibit hydrophobic/hydrophilic character by other means such as plasma discharge to partially oxidize the carbon in layer 104.

[0045] The electrocatalyst is deposited onto layer 104 of gas diffusion cathode 100 by electrodeposition. See U.S. Pat. No. 6,080,504 incorporated herein by this reference. Preferably, as shown in FIG. 7, a CO.sub.2 reduction catalyst (e.g., Pb, Hg, In, Sn, Cd, or Ti) is electrochemically deposited onto microporous layer 104 using a pulse current or pulse reverse current supplied by power supply 218, as previously discussed, passed between gas diffusion cathode 100 and a counter electrode 216 in a bath 214 containing ions of the catalyst. In this way, the nucleation/growth of the catalyst particles are balanced resulting in a more uniform deposition of catalyst particles of predominantly less than 20 nm in size onto the microporous layer 104. In one example, a PC forward only 1.1 A.sub.peak 0.2 ms on and 9.8 ms off times was used.

[0046] One preferred GDE generally includes a catalyst layer and a gas diffusion zone as the gas diffusion cathode 100, FIG. 8. The gas diffusion cathode 100 in turn may include a macroporous carbon fiber substrate (CFS) 102 with an applied microporous layer (MPL) 104. The CFS may include a carbon paper or cloth substrate rendered hydrophobic by the incorporation of TEFLON. The MPL may include high surface area carbon rendered with a mixed hydrophobic/hydrophilic character by the incorporation of TEFLON and NAFION. The NAFION may be applied by painting, spraying or floating the gas diffusion cathode of a solution of NAFION whereby the NAFION partially penetrates the microporous layer 104 and renders that portion of layer 106 hydrophilic. Care is taken not to allow the NAFION to penetrate the macroporous gas diffusion layer 102 as this layer should remain essentially hydrophobic. The application procedures of the NAFION generally result in a thin ionomer layer 106 on the surface of the gas diffusion cathode facing the electrolyte. Conductive plate 302 provides electrical contact from the macroporous carbon fiber substrate 102 side of the gas diffusion electrode 100 to the external circuit. Channel 302a provides a pathway for delivery of the CO2 reactant gas to the macroporous carbon fiber substrate side of the gas diffusion electrode 100.

[0047] In the conventional catalyzation process, the catalyst particles are incorporated within the ionomer that is applied to the MPL via painting or spraying leading to a partial ingress of a proton-conducting (typically hydrophilic) ionomer matrix. This mixed hydrophobic/hydrophilic character of the MPL 1) avoids liquid penetration through to the macroporous carbon fiber substrate layer 102 2) facilitating wetting at the electrolyte junction in order to establish a three-phase interface between the gaseous CO.sub.2 reactant and the solid catalyst supported on the electron-conducting carbon and the electrolyte. This three-phase interface concept is derived from fuel cell developments and is generally described by a flooded agglomerate model including spheres of ionomer-coated catalyst particles.

[0048] In an alternative procedure, the catalyst particle are applied to the high surface are carbon particles prior to their formation into a microporous layer 104 and applied to the macroporous carbon fiber substrate layer 102.

[0049] The significance of the pulse/pulse reverse electrocatalyzation process is depicted in FIGS. 9A-9B. For conventional GDEs, the catalyst particles 110a, 110b, 110care depicted in FIG. 9A. In the case of application of the catalyst particles by painting or, spraying the ionomer containing the catalyst, the catalyst particles are generally large (i.e. >100 nm) and distributed uniformly through the ionomer matrix. Due to this uniform distribution, some of the catalyst particles 110a are not in contact with the carbon particles 104a of the MPL and consequently are not at the necessary three-phase interface and are not utilized in the electroreduction reaction. Additionally, some of the catalyst particles 110b may be in contact with both the ionomer 106 and the carbon particles of the microporous layer 104, but are not in contact with the reactant gas and consequently are not at the necessary three-phase interface and are not utilized in the electroreduction reaction. In the case of depositing the catalyst particles on the carbon particles and subsequently applying the microporous layer 104 to the macroporous carbon fiber substrate 102 followed by applying the ionomer by spraying, painting or floating, the catalyst particles are uniformly distributed through the microporous layer 104. Consequently, some of the catalyst particles 110b may be in contact with both the ionomer and the carbon particles of the microporous layer 104, but are not in contact with the reactant gas and consequently are not at the necessary three-phase interface and are not utilized in the electroreduction reaction. Additionally, some of the catalyst particles 110c may be in contact with carbon particles 104a of the microporous layer 104 that are not in contact with the ionomer and consequently are not at the necessary three-phase interface and are not utilized in the electroreduction reaction. As a first order estimate assuming the various catalyst particles are equally distributed across the four types of catalyst particles with only catalyst particles 110d positioned in the three-phase zone and consequently utilized, the utilization of the catalyst particles in the prior art is limited to approximately 25%.

[0050] In both catalyst application techniques presented above, the catalyst particles are generally non-uniform. Furthermore, in both catalyst application techniques presented above, some catalyst particles 110d are in contact with the ionomer penetrated micropore 106a, the carbon particle 104a and the gas filled micropore 101. The catalyst particles 110d are at the necessary three-phase interface and are consequently used in the electroreduction reaction.

[0051] In the case of pulse/pulse reverse electrocatalyzation as shown in FIG. 9B, the catalyst particles 110b and 110d are electrochemically deposited after application of the microporous layer 104 onto the macroporous carbon fiber substrate 102. In this manner, the catalyst particles 110b and 110d are only deposited on the gas diffusion electrode where accessibility exists both to the proton conducting ionomer 106 and to a carbon particle 104a with an electron conducting pathway through the microporous layer 104 to the macroporous carbon fiber substrate. 102. The resulting electrocatalyst particles necessarily consist of catalyst particles 110d residing in the three-phase interface and are thereby utilized in the electroreduction reaction. There are still catalyst particles 110b in contact with both the ionomer and the carbon particles of the microporous layer 104, but are not in contact with the reactant gas and consequently are not at the necessary three-phase interface and are not utilized in the electroreduction reaction. Unlike catalyst particles 110a and 110c of FIG. 9A, in FIG. 9B there are no catalyst particles which are not in the three-phase zone and not utilized. As a first order estimate assuming the various catalyst particles are equally distributed across the two types of catalyst particles with only catalyst particles 110d positioned in the three-phase zone and consequently utilized, the utilization of the catalyst particles in the subject invention is approximately 50%.

[0052] In addition, the catalyst particles of the subject invention are of approximate uniform particle size and less than approximately 50 nm in diameter resulting in a large surface area and thereby high electroreduction activity. Next, the electrocatalyzed gas diffusion cathode 100 may be employed in an electrochemical reactor 300, FIG. 10 along with an anode 114 and voltage source 304 connected to the cathode and the anode to convert CO.sub.2 to another chemical. See also U.S. Pat. No. 9,145,615 incorporated herein by this reference. In some embodiments, the reduction catalyst used is Sn which can be used to convert CO.sub.2 to a formic acid.

[0053] Although specific features of the invention are shown in some drawings and not in others, this is for convenience only as each feature may be combined with any or all of the other features in accordance with the invention. The words including, comprising, having, and with as used herein are to be interpreted broadly and comprehensively and are not limited to any physical interconnection. Moreover, any embodiments disclosed in the subject application are not to be taken as the only possible embodiments. Other embodiments will occur to those skilled in the art and are within the following claims.

[0054] In addition, any amendment presented during the prosecution of the patent application for this patent is not a disclaimer of any claim element presented in the application as filed: those skilled in the art cannot reasonably be expected to draft a claim that would literally encompass all possible equivalents, many equivalents will be unforeseeable at the time of the amendment and are beyond a fair interpretation of what is to be surrendered (if anything), the rationale underlying the amendment may bear no more than a tangential relation to many equivalents, and/or there are many other reasons the applicant can not be expected to describe certain insubstantial substitutes for any claim element amended.