Patent Application
20230099785
In at least one aspect, palladium hydride catalysts for electrocatalytic formate formation is provided.
Electrochemical carbon dioxide reduction reactions (CO2RR), are a promising method to effectively convert carbon dioxide into value-added fuel using renewable electrical energy, cannot only reduce the industrial carbon footprint, alleviating global greenhouse emissions but also utilize renewable and clean energy, reducing the demand for fossil fuels.1-5 However, traditional electrochemical CO2RR still faces many challenges, such as high overpotential6-10, low selectivity of products7-15, and competition from the hydrogen evolution reaction16. The general optimization in current field includes the utilization of gas diffusion electrode-based electrolyzer17, advanced electrocatalysts18, and the integration of electrocatalytic and biocatalytic cascade systems19. But the development of an efficient electrocatalyst is always the key point to the approach of CO2 electroreduction technology. The CO2RR products depend on the binding energy of intermediates (i.e., *COOH, *OCHO, and *H) to different catalyst surfaces20, which can be divided into four categories:
(1) hydrogen (H2) producing catalysts including Pt, Ni, Fe, etc.;21,22 (2) carbon monoxide (CO) producing catalysts including Au, Ag, Zn, as well as atomically dispersed metal on nitrogen-doped carboneous material (M—N—C), etc.18,23-30 (3) hydrocarbon (e.g., methane, methanol, ethanol, etc.) producing catalysts, in which Cu is the only functioning and most widely studied metal due to its near-ideal binding strength;31-39 (4) formate or formic acid (HCOO- or HCOOH) producing catalysts including Sn, In, Pb, Pd, etc.40-45 Among all the products from CO2RR, formic acid possesses the highest normalized price per electron (16.1 $/e-)46, indicating its high practical value.47-52 So far, three reaction pathways for HCOOH formation have been proposed, which proceed via *OCHO, *COOH, and *H intermediates.53-58
CO_2+[e−+H+(aq)]+*→*OCHO (1)
*OCHO+[e−+H+(aq)]→+HCOOH (2)
CO_2+[e−+H+(aq)]+*→*COOH (3)
*COOH+[e−+H+(aq)]→*+HCOOH (4)
*+[e−+H+(aq)]→*H (5)
*H+CO_2→*HCOO (6)
*HCOO+[e−+H+(aq)]→*+HCOOH (7)
where * indicates the vacant site on the catalysts surface or the adsorbed intermediates. For example, Koh et al. showed by theoretical calculations that the *OCHO pathway was more energetically favorable on bismuth surfaces.53 The whole process consists of two-electron and two-proton transfers, in which the first proton/electron transfer is usually regarded as the rate-determining step (RDS).
Tin (Sn) has been widely investigated since it is located near the top of the volcano plot using *OCHO binding energy as the descriptor for formate57, suggesting its near-optimal binding energy towards the formate production via the *OCHO pathway. A mesoporous SnO2 nanosheet catalyst has been reported to produce formate with a faradaic efficiency of 83% at −0.9 V (vs. RHE).59 SnO2 porous nanowires (Sn-pNWs) also show a faradaic efficiency of 80% at −0.8 V (vs. RHE).60 Similarly, bismuth (Bi) also favors the *OCHO pathway over the *COOH and *H pathways.61,62 Faradaic efficiency above 90% for formate has been reached using bismuth-based catalysts.63-66 Additionally, other metal-based catalysts have been reported in the literature for formate production from CO2, such as Indium (In)67, Cobalt (Co)68, Antimony (Sb)69, etc. However, a key drawback of these catalysts is the high overpotential required and thus low cathodic energy efficiency, rendering superfluous energy loss.21,70-72
In recent years, Pd-based materials have shown unique catalytic advantages in CO2RR: Pd can selectively reduce CO2 to formic acid at near-equilibrium potential.55,73,74 Several works have shown that metallic Pd was capable of exclusively reducing CO2 into HCOO- with high faradaic efficiency (≥95%) in the low-overpotential range (≤−200 mV vs. RHE), whereas a more negative potential (≤−500 mV) promoted the formation of unwanted CO and H2 by-products.55,75-77 A critical limitation, however, is the poor stability of Pd catalysts in CO2 reduction due to poisoning and deactivation of active sites from minor produced CO. Since the CO molecule has very strong adsorption energy on Pd surfaces (−1.36 eV on hollow fcc surface)78, once CO is produced or adsorbed, it cannot spontaneously desorb at cathodic conditions and thus deactivates the Pd surface. This fundamentally restricts Pd catalysts' further application. In 2015, Kanan and co-workers pointed out that even the formation of CO remained negligible at low overpotentials, the Pd surface was still poisoned and deactivated by CO accumulation over time, resulting in a rapid decrease of current and faradaic efficiency after the first few minutes or tens of minutes.55 A Similar trend was also observed in the study of Bao et al. in 2017 and Snyder et al. in 2019, which showed a complete deactivation in 10 min and 4 min, respectively.75,79 Although a brief exposure to air was able to remove surface-bound CO and partially restore the catalyst activity,55,80 this operation is not practical in industrial applications. So far, catalyst optimization is still the focus of the current field. Specifically, Sargent and co-workers reported that on high index Pd facets the CO2RR activity (˜18 mA cm−2) was increased 3-fold as compared to lower index facets (˜6 mA cm−2).80 With an excess Pd loading on the working electrode (˜83 mgPd/cm2), the reductive current density (22 mA cm−2) and faradic efficiency (˜97%) at −0.2 V vs RHE were maintained for up to 1 hour under a strong diffusion-limited condition. Similar activity improvement by high index facets was also observed on electrodeposited porous Pd.81 CO suppression at non-diffusion limited conditions was attempted by doping or alloying Palladium with other elements to downshift the d-band center of surface Pd atoms, which weakened the CO adsorption free energy. In 2018, Cai and co-workers reported a boron-doped Pd catalyst (Pd—B/C) that demonstrated improved HCOOH formation from CO2 as opposed to the undoped Pd catalyst. An enhanced CO tolerance was achieved with an 80% FE over 30 min and 55% FE over 3 hours.77,82 In addition to this, other literature showed that alloyed PdCu, PdNi, and PdCo displayed varying degrees of improved resistance to CO poisoning.79,83,84 Collectively, there is no Pd catalyst can well-balance the FE and stability—either high FE (95%) with low stability (10 min) or low FE (55%) with high stability (300 min). Therefore, a Pd-based catalyst with both faradaic efficiency, stability, as well as activity taken into account is needed for CO2RR.
Since Pd is capable of absorbing over 900 times its own volume of hydrogen at room temperature and atmospheric pressure,85,86 palladium hydride (PdHx) can be easily formed in α-phase (x<0.017) or β-phase (x>0.58), where x indicates the ratio of absorbed metallic H and Pd.87 In electrocatalysis, PdHx can be formed on the surface or subsurface of Pd at cathodic conditions88, promoting the electrochemical reduction process. For example, a permanent Pd hydride catalyst has shown potential for the electrochemical nitrogen reduction reaction.89 The engagement of PdHx facilitates the electrohydrogenation of CO255, and the formation of the *OCHO intermediate instead of *COOH as suggested by density functional theory (DFT) calculation.75 The selectivity towards formate for catalysts with varying morphology is determined by the formation and participation rate of the PdHx active phase, which the nanostructured surfaces with higher defect density can achieve more readily.76,81,90
Accordingly, there is a need for improved catalysts for reducing CO2 to formate.
In at least one aspect, a hydrogen-rich palladium hydride catalyst (PdH0.5/C) for HCOOH production via CO2RR with high faradaic efficiency at low overpotentials and high tolerance to CO poisoning is provided. The FE for formate on the PdH0.5/C catalyst was maintained above 90% over a 4-hour electrolysis at −0.4V in CO2-saturated 0.1 M KHCO3 electrolyte, which is about 15 times higher than that of a commercial Pd/C catalyst as a control. Meanwhile, the particle size and lattice hydrogen content of PdH0.5 was maintained throughout the electrolysis. Isotopic analysis demonstrated a direct participation of the lattice hydrogen in HCOO− formation and also supported a *H pathway as show in equation 5-7.53-55
In another aspect, a supported catalyst for reducing CO2 is provided. The supported catalyst includes a plurality of support particles; and a plurality of catalyst particles disposed over each support particle. Characteristically, the catalyst particles has formula PdHx/C wherein x is 0.3 to 0.7.
In another aspect, a method for forming catalysts for reducing CO2 supported on a substrate particle is provided. The method includes steps of dispersing support particles into an organic solvent and dissolving a palladium-containing compound into the organic solvent to form a first reaction mixture at a first temperature. One or more surfactants are added to the first reaction mixture to form a second reaction mixture at a second temperature. The second reaction mixture is heated to a third temperature. A reducing agent is introduced into the second reaction mixture to form a third reaction mixture. The third reaction mixture is heated to a fourth temperature to form a supported catalyst. The supported catalyst includes a plurality of support particles and a plurality of catalyst particles disposed over each support particle. Characteristically, the catalyst particles has formula PdHx/C wherein x is 0.3 to 0.7.
In another aspect, an electrochemical cell for reducing CO2 is provided. The electrochemical cell includes an electrochemical cell chamber partitioned into a working compartment and a counter compartment. An ionomeric membrane separates the working compartment and the counter compartment. An electrolyte is disposed in the working compartment and the counter compartment. A working electrode is positioned in the working compartment. The working electrode includes an electrode support and a supported catalyst dispersed over a surface of the electrode support. The supported catalyst includes a plurality of support particles; and a plurality of catalyst particles disposed over each support particle. Characteristically, the catalyst particles has formula PdHx/C wherein x is 0.3 to 0.7. The electrochemical cell also includes a counter electrode disposed in the counter compartment. The electrochemical cell also includes and a CO2 source that introduces CO2 into the working compartment. A voltage source is configured to negatively bias the working electrode with respect to the counter electrode such that CO2 is reduced to formate.
The foregoing summary is illustrative only and is not intended to be in any way limiting. In addition to the illustrative aspects, embodiments, and features described above, further aspects, embodiments, and features will become apparent by reference to the drawings and the following detailed description.
The patent or application file contains at least one drawing executed in color. Copies of this patent or patent application publication with color drawing(s) will be provided by the Office upon request and payment of the necessary fee.
For a further understanding of the nature, objects, and advantages of the present disclosure, reference should be made to the following detailed description, read in conjunction with the following drawings, wherein like reference numerals denote like elements and wherein:
Reference will now be made in detail to presently preferred compositions, embodiments and methods of the present invention, which constitute the best modes of practicing the invention presently known to the inventors. The Figures are not necessarily to scale. However, it is to be understood that the disclosed embodiments are merely exemplary of the invention that may be embodied in various and alternative forms. Therefore, specific details disclosed herein are not to be interpreted as limiting, but merely as a representative basis for any aspect of the invention and/or as a representative basis for teaching one skilled in the art to variously employ the present invention.
Except in the examples, or where otherwise expressly indicated, all numerical quantities in this description indicating amounts of material or conditions of reaction and/or use are to be understood as modified by the word “about” in describing the broadest scope of the invention. Practice within the numerical limits stated is generally preferred. Also, unless expressly stated to the contrary: percent, “parts of,” and ratio values are by weight; the term “polymer” includes “oligomer,” “copolymer,” “terpolymer,” and the like; molecular weights provided for any polymers refers to weight average molecular weight unless otherwise indicated; the description of a group or class of materials as suitable or preferred for a given purpose in connection with the invention implies that mixtures of any two or more of the members of the group or class are equally suitable or preferred; description of constituents in chemical terms refers to the constituents at the time of addition to any combination specified in the description, and does not necessarily preclude chemical interactions among the constituents of a mixture once mixed; the first definition of an acronym or other abbreviation applies to all subsequent uses herein of the same abbreviation and applies mutatis mutandis to normal grammatical variations of the initially defined abbreviation; and, unless expressly stated to the contrary, measurement of a property is determined by the same technique as previously or later referenced for the same property.
It must also be noted that, as used in the specification and the appended claims, the singular form “a,” “an,” and “the” comprise plural referents unless the context clearly indicates otherwise. For example, reference to a component in the singular is intended to comprise a plurality of components.
As used herein, the term “about” means that the amount or value in question may be the specific value designated or some other value in its neighborhood. Generally, the term “about” denoting a certain value is intended to denote a range within +/−5% of the value. As one example, the phrase “about 100” denotes a range of 100+/−5, i.e., the range from 95 to 105. Generally, when the term “about” is used, it can be expected that similar results or effects according to the invention can be obtained within a range of +/−5% of the indicated value.
As used herein, the term “and/or” means that either all or only one of the elements of said group may be present. For example, “A and/or B” shall mean “only A, or only B, or both A and B.” In the case of “only A”, the term also covers the possibility that B is absent, i.e., “only A, but not B.”
It is also to be understood that this invention is not limited to the specific embodiments and methods described below, as specific components and/or conditions may, of course, vary. Furthermore, the terminology used herein is used only for the purpose of describing particular embodiments of the present invention and is not intended to be limiting in any way.
The term “comprising” is synonymous with “including,” “having,” “containing,” or “characterized by.” These terms are inclusive and open-ended and do not exclude additional, unrecited elements or method steps.
The phrase “consisting of” excludes any element, step, or ingredient not specified in the claim. When this phrase appears in a clause of the body of a claim, rather than immediately following the preamble, it limits only the element set forth in that clause; other elements are not excluded from the claim as a whole.
The phrase “consisting essentially of” limits the scope of a claim to the specified materials or steps, plus those that do not materially affect the basic and novel characteristic(s) of the claimed subject matter.
The phrase “composed of” means “including” or “consisting of” Typically, this phrase is used to denote that an object is formed from a material.
With respect to the terms “comprising,” “consisting of,” and “consisting essentially of,” where one of these three terms is used herein, the presently disclosed and claimed subject matter can include the use of either of the other two terms.
The term “one or more” means “at least one” and the term “at least one” means “one or more.” The terms “one or more” and “at least one” include “plurality” and “multiple” as a subset. In a refinement, “one or more” includes “two or more.”
The term “substantially,” “generally,” or “about” may be used herein to describe disclosed or claimed embodiments. The term “substantially” may modify a value or relative characteristic disclosed or claimed in the present disclosure. In such instances, “substantially” may signify that the value or relative characteristic it modifies is within ±0%, 0.1%, 0.5%, 1%, 2%, 3%, 4%, 5% or 10% of the value or relative characteristic.
It should also be appreciated that integer ranges explicitly include all intervening integers. For example, the integer range 1-10 explicitly includes 1, 2, 3, 4, 5, 6, 7, 8, 9, and 10. Similarly, the range 1 to 100 includes 1, 2, 3, 4 . . . 97, 98, 99, 100. Similarly, when any range is called for, intervening numbers that are increments of the difference between the upper limit and the lower limit divided by 10 can be taken as alternative upper or lower limits. For example, if the range is 1.1. to 2.1 the following numbers 1.2, 1.3, 1.4, 1.5, 1.6, 1.7, 1.8, 1.9, and 2.0 can be selected as lower or upper limits.
When referring to a numeral quantity, in a refinement, the term “less than” includes a lower non-included limit that is 5 percent of the number indicated after “less than.” For example, “less than 20” includes a lower non-included limit of 1 in a refinement. Therefore, this refinement of “less than 20” includes a range between 1 and 20. In another refinement, the term “less than” includes a lower non-included limit that is, in increasing order of preference, 20 percent, 10 percent, 5 percent, or 1 percent of the number indicated after “less than.”
In the examples set forth herein, concentrations, temperature, and reaction conditions (e.g., pressure, pH, flow rates, etc.) can be practiced with plus or minus 50 percent of the values indicated rounded to or truncated to two significant figures of the value provided in the examples. In a refinement, concentrations, temperature, and reaction conditions (e.g., pressure, pH, flow rates, etc.) can be practiced with plus or minus 30 percent of the values indicated rounded to or truncated to two significant figures of the value provided in the examples. In another refinement, concentrations, temperature, and reaction conditions (e.g., pressure, pH, flow rates, etc.) can be practiced with plus or minus 10 percent of the values indicated rounded to or truncated to two significant figures of the value provided in the examples.
For all compounds expressed as an empirical chemical formula with a plurality of letters and numeric subscripts (e.g., CH2O), values of the subscripts can be plus or minus 50 percent of the values indicated rounded to or truncated to two significant figures. For example, if CH2O is indicated, a compound of formula C(0.8-1.2)H(1.6-2.4)O(0.8-1.2). In a refinement, values of the subscripts can be plus or minus 30 percent of the values indicated rounded to or truncated to two significant figures. In still another refinement, values of the subscripts can be plus or minus 20 percent of the values indicated rounded to or truncated to two significant figures.
“CO2RR” means carbon dioxide reduction reaction.
“FE” means faradaic efficiency.
“LSV” means linear sweep voltammetry.
In at least one aspect, the present invention is related to the electrochemical reduction of CO2. Electrochemical reduction of CO2 to formic acid is of great significance to renewable chemical synthesis and green energy storage. Palladium stands out from many potential electrocatalysts because of its unique capability of producing formate at low overpotential or even near-equilibrium potential. Inevitably produced CO, however, poisons and deactivates the surface of Pd, resulting in an insufficient operating life-time for conventional and even optimized Pd catalysts. Herein, we present a hydro-gen-rich Palladium Hydride catalyst (PdH0.5/C) derived from a one-step solvothermal synthesis. This catalyst showed a 93.1% faradaic efficiency for formate at −0.4 V (vs RHE). The working lifetime reached a record of 4 hours, which was ˜15 times longer than a commercial Pd catalyst and outperforming all previously reported Pd-based catalysts in electrosynthesis of formate from CO2. The high CO tolerance was attributed to the high selectivity towards formate with the presence of lattice hydrogen and the relatively weak CO adsorption strength on diverse active sites (i.e. kink, step, and terrace) of our catalyst. Isotopic analysis revealed a direct participation of lattice hydrogen in the protonation of the carbon atoms during formate formation. A detailed mechanism of the hydrogen transformation was proposed for both hydride and pure Pd catalysts.
With reference to
In some variations, the supported catalyst advantageously has a faradaic efficiently greater than 90% for formate at −0.4 V (vs. RHE) after 4 hours of initial operation. In some refinements, the supported catalyst advantageously has a faradaic efficiently greater than 80%, 85%, 90%, or 95% for formate at −0.4 V (vs. RHE) after 4 hours of initial operation
In another variation, the supported catalyst has a BET surface area from about 90 m2/g to 110 m2/g. In some refinements, the supported catalyst has a BET surface area of at least 75 m2/g, 80 m2/g, 85 m2/g, 90 m2/g, 95 m2/g, or 100 m2/g. In further refinements, the supported catalyst has a BET surface area of at least 150 m2/g, 130 m2/g, 125 m2/g, 120 m2/g, 110 m2/g, or 110 m2/g. In a refinement, the supported catalyst has a BET surface area greater than 100 m2/g.
Referring to
Typically, electrode support 38 is electrically conductive. In a refinement, the electrode support is composed of carbon.
Still referring to
In another embodiment, a method for reducing CO2 using the electrochemical cell of
In another embodiment, a method for forming the catalysts supported on substrate particles of
In step d), the second reaction mixture is heated to a third temperature, and then in step e), a reducing agent (e.g. LiBEt3H) is introduced into the second reaction mixture to form a third reaction mixture. The third reaction mixture is heated to a fourth temperature to form a supported catalyst comprising a plurality of support particles; and a plurality of catalyst particles disposed over each support particle, the catalyst particles having formula PdHx/C wherein x is 0.3 to 0.7. In some refinements, x is at least 0.1, 0.2, 0.3, 0.4, 0.45, or 0.48 and at most least 0.9, 0.8, 0.7, 0.6, 0.55, or 0.52. In a refinement, catalyst particles 14 are described by formula PdH0.5/C. Typically, the catalyst particles have an average particle diameter of about 1 to 10 nm. In a refinement, the catalyst particles have an average particle diameter of about 2.5 to 4 nm with an average of about 3.15. Typically, the first temperature is about room temperature (e.g., 20 to 25° C.), the second temperature (e.g., 80 to 120° C.) is greater than the first temperature, the third temperature (e.g., 130 to 170° C.) is greater than the second temperature, and the fourth temperature (e.g., 180 to 230° C.) is greater than the third temperature.
Typically, the palladium-containing compound is Pd(acac)2 and the surfactants are oleylamine and oleic acid. As set forth above, the support particles are carbon particles.
The following examples illustrate the various embodiments of the present invention. Those skilled in the art will recognize many variations that are within the spirit of the present invention and scope of the claims.
Palladium (II) acetylacetonate (Pd(acac)2, 99%, Sigma Aldrich), Benzyl ether ((C6H5CH2)2O, 98%, Sigma Aldrich), Oleylamine (70%, Sigma Aldrich), Oleic acid (≥99%, Sigma Aldrich), LiBEt3H (Li(C2H5)3BH, 1.0 M lithium triethylborohydride in THF), Potassium bicarbonate (KHCO3, 99.97%, Sigma Aldrich), Potassium carbonate (K2CO3, 99.995%, Sigma Aldrich), Deuterium oxide (D2O, 99.9%, Sigma Aldrich), 3-(Trimethylsilyl)-1-propanesulfonic acid sodium salt (DSS, 97%, Sigma Aldrich), AvCarb MGL190 (Fuel Cell Store).
The PdH0.5/C (20 wt % Pd) catalysts were synthesized through one-step solvothermal synthesis technique as shown in
Inductively coupled plasma mass spectrometry (ICP-MS) was used to determine the composition of PdH0.5/C catalyst. Aberration-corrected scanning transmission electron microscopy and energy dispersive X-ray spectroscopy (EDS) was performed using a JEOL Grand ARM300F were employed to characterize the morphology, lattice constant, and elemental distribution in the PdH0.5/C catalyst. Transmission electron microscopy images were acquired using a JEOL JEM-2800. The surface electronic structures were analyzed using X-ray photoelectron spectroscopy (XPS) from AXIS Supra by Kratos Analytical. The binding energies were calibrated with respect to the C is peak at 285 eV. Crystalline structures and hydride content of catalysts were determined using powder X-ray diffraction (XRD). Temperature programmed desorption (TPD) in Ar environment was employed to help to identify the hydride content in catalysts. The specific surface area of catalysts was measured using Brunauer-Emmett-Teller (BET) theory.
Electrochemical reduction of CO2 was conducted in a glass H-cell with a three-electrode system at room temperature. Counter and working compartments were separated by a Nafion 117 membrane, containing 20 mL and 30 mL of 0.1 M KHCO3 electrolyte (made with Millipore water), respectively. A carbon rod and an Ag/AgCl (3 M KCl, BASi) served as the counter electrode and the reference electrode, respectively. A hand-cut carbon paper (AvCarb MGL190) with a surface area of 1 cm2 was prepared as a working electrode. Carbon paper was pretreated with plasma and acid washing to modify the surface to be hydrophilic. Catalyst ink was prepared by dispersing 2.5 mg of Pd/C in 960 μL of isopropanol and 40 μL of 5 wt % Nafion isopropanol solutions with ultrasonication for 30 min. 100 μL of well-mixed ink was drop-casted onto the pretreated carbon paper electrode to meet a Pd loading of 50 μg/cm2, followed by drying in a vacuum oven at 60° C. overnight. CO2 was purged to both counter and working electrolyte with 30 sccm for 30 min until saturation prior to all electrochemical experiments. CO2 was then continually bubbled into the electrolyte with 30 sccm during all electrochemical experiments for continuous saturation.
The CO stripping method was used to analyze the affinity of CO on the Pd-based catalyst electrode as Pd readily adsorb CO. A monolayer of CO adsorbed on the Pd surface by purging CO into the 0.1 M HClO4 solution for 10 min while holding the potential at 0.05V (vs RHE), followed by introducing Ar for 10 min to passivate the surface and remove superfluous CO in the system. The monolayer of CO already deposited on the surface of Pd is then electrochemically oxidized by sweeping potential from 0.05V to 1.5V at a scan rate of 5 mV/s. The total charge of CO oxidation can be derived by integrating the area between peak curve and baseline, in which the baseline is obtained from CV in Ar-saturated 0.1 M HClO4 with the same scan rate. Electrochemically active surface area (ECSA) can be calculated by dividing the charge by the conversion factor (420 μC/cm2).
The liquid products derived from electrolysis at constant potentials for 1 hour in the 0.1 M KHCO3 electrolyte were quantified by a Bruker CRYO 500 MHz nuclear magnetic resonance (NMR) spectroscopy instrument. Deuterium oxide (D2O) and 3-(trimethylsilyl)-1-propanesulfonic acid sodium salt (DSS) were used as locking solvent and internal standard, respectively. The NMR test sample was the mixture of 500 μL of post-electrolysis electrolyte, 100 μL of D2O, and 100 μL of 6 mM DSS (made with Millipore water). A solvent suppression method was applied to restrain the signal from H2O for better accuracy. The 1H-NMR spectrum was processed and analyzed on Topspin 4.0.8 software.
The faradaic efficiency (FE) of products from CO2 reduction was calculated from the following equation:
FE=ZFVC/Q×100%
where Z is the number of electrons transferred to obtain 1 molecule of a specific product such as 2 for formate, F is the Faraday's constant (96485 C/mol), V is the total volume of catholyte in L, C is the concentration of catholyte measured from NMR analysis in mol/L. Q is the total charge passed during the bulk electrolysis in C.
X-ray photoelectron spectroscopy (XPS) was carried out to determine the oxidation state of PdH0.5/C. The XPS spectra of Pd 3d peak region for PdH0.5/C and commercial Pd/C are presented in
The crystal structure of PdH0.5/C, commercial Pd/C, and commercial Pd/C (H2) is determined by powder XRD as shown in
Cathodic linear sweep voltammetry (LSV) is recorded at 5 mV/s for PdH0.5/C and commercial Pd/C casted on a carbon paper electrode in Ar- and CO2-saturated 0.1 M KHCO3 electrolyte as shown in
To assess the CO poisoning process, CO gas was artificially introduced into the system in the middle of CO2 reduction process.55 As shown in
CO stripping was employed to further investigate the CO affinity on PdH0.5/C catalyst. As shown in
Long duration CO2RR stability tests for PdH0.5/C and commercial Pd/C in CO2-saturated 0.1 M KHCO3 at −0.4V are shown in
The TEM images and XPS spectrum (
A cycling performance study was performed to measure the recovery capability of a single PdH0.5/C electrode from CO poisoning in multiple cycles. Two methods are employed to remove the surface covered CO: exposing the electrode to air or applying a positive potential shock between each cycle. As shown in
From the literature, DTF calculation has been employed to explain the mechanism of CO2RR on Pd catalysts. It's generally accepted that *COOH is favored on bare Pd.82 A higher free energy for *COOH is observed on Pd with more hydride82, and *OCHO is formed far easier than *COOH75, indicating formate formation is favored on PdH. Formate becomes the predominant product of CO2RR on the Pd surface with full hydride coverage.75 Besides, hydride and *CO exhibit interdependent and interactive behavior. Hydride weakens *CO adsorption75,77,82, hampering CO poisoning and promoting the formation of formate. Similar results have been shown in the work of Kersten et al. using microkinetic models that α-PdH is poisoned by CO, while β-PdH isn't.111 *CO can also restrict hydride in turn, that the more *CO, the less hydride will be on the surface, resulting in more *CO adsorption79, which could be the explanation of the accelerating decreasing of current density observed in the stability test on our PdH0.5/C (
To elucidate the reaction pathway towards HCOO- formation on PdH0.5/C, the role of lattice hydrogen and surface adsorbed H* species were studied via isotopic analysis, wherein D2O (deuterium oxide) and K2CO3 were used to provide a protium (1H or hydrogen-1) free environment. The 1H-NMR employed here was able to quantitatively detect the produced HCOO- molecules, but was blind to the DCOO− counterparts. As shown in Table 2, after a 12-minute CA at −0.4 V in 0.1 M K2CO4 electrolyte, the commercial Pd/C catalysts produced 5.76 μmol HCOO- in H2O and non-detectable HCOO- in D2O. In the D2O system, the potential external 1H contamination from Nafion ionomer was calculated at 1.42×10−6 μmol HCOO-, assuming a rapid proton exchange between the Nafion ionomer (0.813 μmol 1H) and bulk D2O (30 mL, 3.3×106 μmol 2H). The zero HCOO− formation in D2O system further confirmed the negligible 1H contamination from Nafion and other cell components.
When using PdH0.5/C catalysts, the cathodic current carried at −0.4V v. s. RHE gradually dropped to zero in ˜70 min in K2CO3+H2O system (
*+[e−+H+(aq)]→*H (5)
*H+C02→*HCOO (6)
*HCOO+[e−+H+(aq)]→*+HCOOH (7)
As mentioned before, the hydrogen content in PdH0.5/C was well maintained in the 4-hour CA (
In contrast, pure Pd nanoparticles rely on the dynamic surface hydride formed at cathodic conditions.88 Kanan and co-workers suggested the formation of β-hydride (Pd—Hx, x˜0.7) on the commercial Pd surface, based on an early work for the electrochemical Pd hydride formation in a non-CO2RR environment.115 In fact, the nature of pure Pd nanoparticles in electrolysis remains ambiguous. In our opinion, the Pd nanoparticles in CO2RR tend to form a dynamic hydride surface that is closed to a-phase.116 The core of the Pd particles, particularly the big ones, remains as a pure Pd phase or hydrogen-poor phase. Consequently, the surface H* species have chemical potentials for HCOO- formation as well as diffusing into the bulk Pd. Therefore, the protonation of the CO2 molecule or associated intermediates is not as favorable as the PdH0.5/C catalysts. This explains their undermined stability and FE as shown in Table 1.
Looking beyond the CO tolerance improvement by lattice hydrogen, the design of CO-immune Pd-based electrocatalysts can realize an energy-efficient HCOO- production in real. Another interesting finding in this work is the 6.5% sub-peak in CO stripping (
In summary, we synthesized hydrogen-rich PdH0.5 nanoparticles with an average size of 3.15 nm monodispersed onto carbon black by using an undemanding solvothermal synthesis method. The existence of hydride in the Pd lattice remarkably expands the window of CO2RR and ameliorates the electrocatalytic CO2 reduction activity as well as stability by modifying the surface electronic structure and participating in the electrohydrogenation of CO2, exhibiting a 93.1% faradaic efficiency of formate for the 1st hour and remained above 90% faradaic efficiency of formate for 4 hours of CO2 electroreduction in 0.1 M KHCO3 at −0.4V, which stability is over 15 times better than commercial Pd/C. The TEM and XPS results of PdH0.5/C electrode before and after the stability test indicate the constant lattice parameter during CO2RR, bespeaking the significant role of hydride in electrolysis. CO poisoning occupies and deactivates the active catalytic surface, where step and kink sites on the nanoparticle surface of PdH0.5/C are found to bind CO weaker than the terrace site, triggering the high CO tolerance and superb CO2 reduction stability. The plateau current density after the stability test matches the kink or grain boundaries site ratio compared to initial current density with formate continuously produced, suggesting a potential immune site for CO poisoning on PdH0.5/C. Cycling performance with Air exposure and positive potential shock shows the capability of restoring the CO2 reduction activity by oxidizing surface adsorbed CO. The unchanged hydride content during the cycling performance with air exposure exhibits better stability performance than the reduced hydride content after cycling performance with positive potential shock, further demonstrating the hydride role of promoting CO2RR activity and stability. Isotopic analysis in D2O and K2CO3 reveals the participation pathway of hydride towards HCOO-, that hydride is able to form C—H bond in formate and is replenished from solution at negative potentials.
While exemplary embodiments are described above, it is not intended that these embodiments describe all possible forms of the invention. Rather, the words used in the specification are words of description rather than limitation, and it is understood that various changes may be made without departing from the spirit and scope of the invention. Additionally, the features of various implementing embodiments may be combined to form further embodiments of the invention.
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This application claims the benefit of U.S. provisional application Ser. No. 63/250,673 filed Sep. 30, 2021, the disclosure of which is hereby incorporated in its entirety by reference herein.
The invention was made with Government support DOE-EERE-BETO to Colorado State University under Contract No. EE0008923 awarded by the Department of Energy (DOE). The Government has certain rights to the invention.
| Number | Date | Country | |
|---|---|---|---|
| 63250673 | Sep 2021 | US |
