Patent Application
20240093390
Electrochemical reduction of CO2 (CO2R) using renewable electricity offers an attractive approach to produce widely needed chemicals and feedstocks while mitigating greenhouse gas emissions (1, 2). Effort has been dedicated to developing catalysts that achieve high faradaic efficiency (FE) toward carbon monoxide and formate; and to promoting C—C coupling toward multi-carbon (C2+) products such as ethylene and ethanol (3-6). Lowering overpotentials of these reactions and increasing their productivity (current density) have been priorities for the field (7, 8).
Despite many recent advances, CO2R remains far from practical viability because strong local alkaline conditions are present (
2OH−(aq)+CO2(g)→CO32−(aq)+H2O(aq) (Eq. 1)
Carbonate formation imposes a limit of carbon utilization efficiency (ratio of CO2 converted to CO2R products to feeding CO2) that is prohibitively low (9). For every proton/electron pair transferred during the CO2R, one hydroxide is produced at the cathode, reacting with ½ CO2 to form carbonate. As a result, the maximum carbon efficiency is 50% for two-electron-transfer processes such as CO2 to CO.
For CO2R to more valuable C2+ products, the effect is even more acute: the carbon efficiency of CO2R to ethylene/ethanol is limited to 25% as six electrons are needed per CO2 reacted. In practice, due part to non-unity selectivity and use of alkaline electrolyte, the carbon efficiency is even lower than these best-case theoretical limits (
Low carbon efficiency results in a significant energy penalty: the cost to regenerate the lost CO2 (9, 16), which would otherwise be emitted again. Techno-economic analysis (TEA) of alkaline CO2 electrolyzers shows that achieving high carbon utilization with these systems incurs a severe energy penalty, with over 50% of input energy used to regenerate CO2 lost to carbonate (
CO2 electrolyzers employing neutral electrolyte produce a local alkaline environment under operating conditions, and thus also suffer from carbonate formation and crossover (17, 18). The problem of inefficient CO2 utilization in CO2R is central to the field and severely limits its prospects (9). While advances in FE and current density have been steady, the utilization challenge demands a new approach.
There are various challenges associated with carbon utilization in electrolytic systems. The present techniques address at least some of these challenges to achieve an enhanced carbon utilization in comparison to known techniques in the field.
CO2 electroreduction (CO2RR) is a promising route to convert the CO2 that is contained in emissions into valuable chemicals and fuels. However, carbon utilization—that can be understood as a ratio of converted CO2 to provided CO2— can remain low, typically below 2% when converting of CO2 into multicarbon products. This loss arises due to the consumption of CO2 by local hydroxide (OH−), to form carbonate in both alkaline and neutral electrolytes.
In a first aspect, the disclosure provides an electrolytic system for CO2 electroreduction into multicarbon (C2+) products, the system comprising:
With preference, the catholyte has an alkali metal ion concentration between 0.5 M and 4 M, or between 0.5 M and 3 M, preferably between 1 M and 2.5 M.
In particular, the disclosure provides an electrolytic system for CO2 electroreduction into multicarbon (C2+) products, the system comprising:
In fact, the electrolytic system for CO2 electroreduction into multicarbon (C2+) products can be also defined as comprising:
The present techniques involve performing CO2RR in an acidic medium while avoiding domination of the hydrogen evolution reaction (i.e., 2H++2e− →H2) due to high proton availability at these acidic conditions. More specifically, there is proposed a cation fixation strategy that can enhance the availability of the one or more alkali metal ions, in particular potassium cations, in the vicinity of electrochemically active sites, overcoming hydrogen evolution and enabling efficient CO2R in acidic media through improved CO2 adsorption and C—C coupling. Upon application of the cation fixation strategy, CO2RR can, for example, be achieved on copper in strong acid (pH<1) with a single pass carbon utilization of 77%, including a conversion toward C2+ products of 50% at 1.2 A/cm2.
With preference, the catholyte has a K+ concentration between 0.5 M and 4 M, or between 0.5 M and 3 M, preferably between 1 M and 2.5 M.
For example, in the cathode, the metal-based catalyst material and the cation-augmenting material are co-deposited as an active layer onto the substrate.
For example, in the cathode, the metal-based catalyst material is deposited as a catalyst layer onto the substrate, and the cation-augmenting material is deposited as a cation-augmenting layer onto the catalyst layer.
For example, in the cathode, the metal-based catalyst layer has a thickness of 300 nm as determined by scanning electron microscopy.
For example, in the cathode, the cation-augmenting layer has a thickness between 1.5 μm and 2 μm as determined by scanning electron microscopy.
For example, in the cathode, the metal-based catalyst material comprises or consists of copper and/or silver.
For example, in the cathode, the metal-based catalyst material is provided as a high-surface area material.
For example, in the cathode, the metal-based catalyst material is provided as nanoparticles.
For example, in the cathode, the cation-augmenting material comprises or consists of a cationic ionomer.
For example, in the cathode, the acidic group is —SO3H.
For example, in the cathode, the cation-augmenting material comprises a cationic perfluorosulfonic acid (PFSA) ionomer.
For example, in the cathode, the PFSA ionomer is composed of tetrafluoroethylene and sulfonyl fluoride vinyl ether.
For example, in the cathode, the cation-augmenting material further comprises carbon nanoparticles or graphite.
For example, in the cathode, the substrate is polytetrafluoroethylene (PTFE) that is configured for gas diffusion.
For example, the gas component further comprises N2.
For example, the catholyte is an acidic catholyte. With preference, the acidic catholyte has a bulk pH between 1 and 4 or the acidic catholyte has a bulk pH of at most 1.
For example, the acidic catholyte comprises at least one of phosphoric acid, sulfuric acid, and perchloric acid.
For example, the cation species further comprise caesium and/or sodium ions.
For example, the catholyte has a total concentration in phosphorous species is ranging between 0.8 M and 1.2 M.
For example, the electrolytic system further comprises a power source providing electric current at an applied current density between 100 mA·cm2 and 1.5 A·cm2.
For example, the electrolytic system further comprises a mass flow controller that is operatively connected to the reactant inlet to adjust an inlet gas flowrate. With preference, the inlet gas flowrate is between 1 and 50 sccm; more preferably, the inlet gas flowrate is between 3 and 10 sccm
For example, the electrolytic system further comprises a first peristaltic pump operatively connected to a first tube in fluid communication with the anolyte chamber to circulate the anolyte therein, and a second peristaltic pump operatively connected to a second tube in fluid communication with the catholyte chamber to circulate the catholyte therein. With preference, the catholyte and anolyte are circulated at a constant flowrate.
For example, the CEM is a membrane of perfluorinated sulfonic acid ionomer. With preference, the perfluorinated sulfonic acid ionomer comprises Nafion™ (commercial name of perfluoro(2-(2-sulfonylethoxy)propyl vinyl ether)-tetrafluoroethylene copolymer).
For example, the electrolytic system further comprises a counter electrode being provided in the anolyte chamber. With preference, the counter electrode comprising noble metals such as Pt, Ru, or Ir.
For example, the electrolytic system comprises a reference electrode being provided in the catholyte chamber. With preference, the reference electrode is an Ag/AgCl, Hg/HgSO4 or SCE reference electrode.
For example, the anolyte pH is equal to or less than the catholyte pH.
For example, the anolyte is of the same nature as the catholyte.
In a second aspect, there is provided a method for enhancing carbon utilization during CO2 electroreduction in an electrolytic system remarkable in that the electrolytic system is according to the first aspect, comprising an acidic catholyte and a cathode in contact with the acidic catholyte, and in that the method comprises increasing a local pH of the catholyte at a surface of the cathode.
For example, increasing the local pH of the catholyte at the surface of the cathode comprises operating the electrolytic system at a current density that results in a consumption rate of local H3O+ protons at the surface of the cathode being higher than mass transport of bulk H3O+ protons. In a first alternative, the acidic catholyte is advantageously a strong acid having a pH of at most 1; with preference, the current density is between 200 mA·cm2 and 1.5 A·cm2. In a second alternative, the acidic catholyte is advantageously an acid having a pH between 1 and 4; with preference, the current density is between 100 and 200 mA·cm2.
For example, increasing the local pH of the catholyte at the surface of the cathode comprises creating locally neutral or alkaline conditions; with preference, the local pH at the surface of the cathode is 7.
For example, increasing the local pH of the catholyte at the surface of the cathode comprises creating locally alkaline conditions, wherein the local pH at the surface of the cathode is above 7.
For example, increasing the local pH of the catholyte at the surface of the cathode comprises creating a local pH gradient from alkaline to acidic conditions from the surface of the cathode to a bulk of the catholyte. With preference, the local pH is between 8 and 10 at the surface of the cathode and the local pH is at most 6.5 within a distance of at least 30 μm from the surface of the cathode.
For example, increasing the local pH of the catholyte at the surface of the cathode comprises providing cation species at the surface of the cathode. In a first alternative, the cation species are preferably provided within the catholyte. More particularly, the catholyte comprises a cation donor that liberates the cation species. Even more particularly, the cation species is K+ and the cation donor is at least one of potassium chloride, potassium phosphate monobasic, potassium sulfate, potassium iodide, and potassium hydroxide. Most particularly, the catholyte has a K+ concentration between about 0.5 M and 5 M or between 0.5 M and 3 M. In a second alternative, providing the cation species at the surface of the cathode preferably comprises confining the cation species within a cation-augmenting layer of the cathode. More particularly, the cation-augmenting layer comprises an acidic group, for example —SO3H, exchanging protons with the cation species or the CAL comprises an ion conducting polymer, more preferably the ion conducting polymer is a cationic perfluorosulfonic acid (PFSA) ionomer, for example, the PFSA ionomer is composed of tetrafluoroethylene and sulfonyl fluoride vinyl ether.
For example, the acidic catholyte comprises at least one of phosphoric acid, sulfuric acid, and perchloric acid.
For example, the catholyte comprises between 0.1M and 1M of acid and 0.1M and 3M of cation donor. With preference, the catholyte comprises 1 M H3PO4 and 3M KCl.
For example, a Faradaic Efficiency (FE) of the CO2 electroreduction toward C2H4 is of at least 10% at a current density between 300 and 800 mA/cm2; with preference, the FE towards C2H4 is of 13% at the current density of 400 mA/cm2 and/or the FE toward C2H4 increases from around 10% with the catholyte of 1 M K+ to 26% with the catholyte of 3 M K+ at the current density of 1.2 A/cm2 and/or an overall CO2R selectivity is about 61±3% with a total FE towards C2+ products of about 40±2%. With preference, the method comprises feeding gaseous CO2 to the cathode at an inlet flowrate between 1 and 50 sccm; more preferably, the inlet flowrate is between 3 sccm and 10 sccm. For example, the cathode comprises copper and a PFSA ionomer and the catholyte is a strong acid having a pH of at most 1, and in that a single pass carbon utilization is of about 77%, including a conversion toward C2+ products of about 50% at an applied current density of 1.2 A/cm2 for the inlet flowrate of 3 sccm.
In a third aspect, the present disclosure provides a method for manufacturing an electrode being the cathode of the electrolytic system according to the first aspect, remarkable in that the electrode operates a CO2R reaction in an acidic electrolyte, the method comprising depositing a metal-based catalyst material and a cation-augmenting material onto a substrate.
For example, the metal-based catalyst material comprises or consists of copper, silver, or any alloys thereof.
For example, the metal-based catalyst material is provided as a high-surface area material. With preference, the metal-based catalyst material is provided as nanoparticles.
For example, the cation-augmenting material comprises an acidic group exchanging protons with cation species of the acidic electrolyte so as to increase a concentration of the cation species at a surface of the electrode. With preference, the acidic group is —SO3H.
For example, that the cation-augmenting material comprises or consists of a cationic ionomer. With preference, the cationic ionomer is a cationic perfluorosulfonic acid (PFSA) ionomer. More preferably, the PFSA ionomer is composed of tetrafluoroethylene and sulfonyl fluoride vinyl ether.
For example, the cation-augmenting material further comprises carbon nanoparticles or graphite.
For example, the substrate is polytetrafluoroethylene (PTFE) that is configured for gas diffusion.
For example, the step of depositing the catalyst material and the cation-augmenting material comprises depositing the metal-based catalyst material onto the substrate to form a catalyst layer; and depositing the cation-augmenting material onto the catalyst layer to form a cation-augmenting layer. With preference, the step of depositing the catalyst layer onto the substrate comprises sputtering a metal onto a surface of the substrate in a vacuum environment. More preferably, sputtering the metal is performed at a deposition rate of 1 Å/sec. With preference, the step of depositing the cation-augmenting layer onto the catalyst layer comprises spraying a cation-augmenting solution onto the catalyst layer, and the cation-augmenting solution comprising the cation-augmenting material; more preferably, the cation-augmenting solution further comprises methanol. For example, the catalyst layer has a thickness of 300 nm as determined by scanning electron microscopy. For example, the cation-augmenting layer has a thickness between 1.5 μm and about 2 μm as determined by scanning electron microscopy.
For example, the step of depositing the metal-based catalyst material and the cation-augmenting material comprises combining the metal-based catalyst material and the cation-augmenting material to form a mixture; and depositing the mixture onto the substrate to form an active layer. With preference, the step of combining the metal-based catalyst material and the cation-augmenting material comprises forming a homogeneous dispersion of metal nanoparticles and cationic ionomer. More preferably, the step of depositing the mixture comprises spraying the dispersion onto a surface of the substrate to coat the substrate with the active layer. Even more preferably, the spraying is performed in multiple sequences to form multiple active sub-layers.
For example, the active layer comprises a first sublayer having a thickness between 5 μm and 6 μm as determined by scanning electron microscopy, a second sublayer having a thickness between 1.5 μm and 2 μm as determined by scanning electron microscopy, and a third sublayer having a thickness between 1.5 μm and 2 μm as determined by scanning electron microscopy.
In accordance with a further aspect, there is provided an electrode for CO2 electroreduction in an acidic electrolyte comprising cation species, the electrode comprising: a substrate, a metal-based catalyst material, and a cation-augmenting material; wherein the cation-augmenting material comprises an acidic group exchanging protons with the cation species of the acidic electrolyte so as to increase a concentration of the cation species at a surface of the electrode.
In some implementations, the metal-based catalyst material and the cation-augmenting material are co-deposited as an active layer onto the substrate.
In some implementations, the active layer comprises multiple active sub-layers.
In some implementations, the active layer comprises: a first sublayer having a thickness between about 5 μm and about 6 μm, a second sublayer having a thickness between about 1.5 μm and 2 μm, and a third sublayer having a thickness between about 1.5 μm and about 2 μm.
In some implementations, the metal-based catalyst material is deposited as a catalyst layer onto the substrate, and the cation-augmenting material is deposited as a cation-augmenting layer onto the catalyst layer.
In some implementations, the metal-based catalyst layer has a thickness of about 300 nm.
In some implementations, the cation-augmenting layer has a thickness between about 1.5 μm and about 2 μm.
In some implementations, the metal-based catalyst material comprises or consists of copper and silver.
In some implementations, the metal-based catalyst material is provided as a high-surface area material.
In some implementations, the metal-based catalyst material is provided as nanoparticles.
In some implementations, the cation-augmenting material comprises or consists of a cationic ionomer.
In some implementations, the acidic group is —SO3H.
In some implementations, the cation-augmenting material comprises a cationic perfluorosulfonic acid (PFSA) ionomer.
In some implementations, the PFSA ionomer is composed of tetrafluoroethylene and sulfonyl fluoride vinyl ether.
In some implementations, the cation-augmenting material further comprises carbon nanoparticles or graphite.
In some implementations, the substrate is polytetrafluoroethylene (PTFE) that is configured for gas diffusion.
In accordance with a further aspect, there is provided a method for manufacturing an electrode that operates a CO2R reaction in an acidic electrolyte, the method comprising depositing a metal-based catalyst material and a cation-augmenting material onto a substrate.
In some implementations, the metal-based catalyst material comprises or consists of copper, silver, or any alloys thereof.
In some implementations, the metal-based catalyst material is provided as a high-surface area material.
In some implementations, the metal-based catalyst material is provided as nanoparticles.
In some implementations, the cation-augmenting material comprises an acidic group exchanging protons with cation species of the acidic electrolyte so as to increase a concentration of the cation species at a surface of the electrode
In some implementations, the acidic group is —SO3H.
In some implementations, the cation-augmenting material comprises or consists of a cationic ionomer.
In some implementations, the cationic ionomer is a cationic perfluorosulfonic acid (PFSA) ionomer.
In some implementations, the PFSA ionomer is composed of tetrafluoroethylene and sulfonyl fluoride vinyl ether.
In some implementations, the cation-augmenting material further comprises carbon nanoparticles or graphite.
In some implementations, the substrate is polytetrafluoroethylene (PTFE) that is configured for gas diffusion.
In some implementations, the step of depositing the catalyst material and the cation-augmenting material comprises depositing the metal-based catalyst material onto the substrate to form a catalyst layer; and depositing the cation-augmenting material onto the catalyst layer to form a cation-augmenting layer.
In some implementations, the step of depositing the catalyst layer onto the substrate comprises sputtering a metal onto a surface of the substrate in a vacuum environment.
In some implementations, sputtering the metal is performed at a deposition rate of 1 Å/sec.
In some implementations, the step of depositing the cation-augmenting layer onto the catalyst layer comprises spraying a cation-augmenting solution onto the catalyst layer, and the cation-augmenting solution comprising the cation-augmenting material.
In some implementations, the cation-augmenting solution further comprises methanol.
In some implementations, the catalyst layer has a thickness of about 300 nm.
In some implementations, the cation-augmenting layer has a thickness between about 1.5 μm and about 2 μm.
In some implementations, the step of depositing the metal-based catalyst material and the cation-augmenting material comprises: combining the metal-based catalyst material and the cation-augmenting material to form a mixture; and depositing the mixture onto the substrate to form an active layer.
In some implementations, the step of combining the metal-based catalyst material and the cation-augmenting material comprises forming a homogeneous dispersion of metal nanoparticles and cationic ionomer.
In some implementations, the step of depositing the mixture comprises spraying the dispersion onto a surface of the substrate to coat the substrate with the active layer.
In some implementations, the spraying is performed in multiple sequences to form multiple active sub-layers.
In some implementations, the active layer comprises a first sublayer having a thickness between about 5 μm and about 6 μm, a second sublayer having a thickness between about 1.5 μm and about 2 μm, and a third sublayer having a thickness between about 1.5 μm and about 2 μm.
In accordance with a further aspect, there is provided a use of the electrode as defined above for generating multicarbon products with an enhanced yield when in contact with an acidic catholyte having a bulk pH of at most 4.
In accordance with a further aspect, there is provided an electrolytic system for CO2 electroreduction, the system comprising the electrode as defined above, and an acidic electrolyte for contacting of the electrode.
In some implementations, the acidic electrolyte is an acid having a bulk pH of at most 4.
In accordance with a further aspect, there is provided an electrolytic system for CO2 electroreduction into multicarbon (C2+) products, the system comprising a cathode being the electrode as defined above, the cathode being provided in a catholyte chamber; an anode being provided in an anolyte chamber; an anolyte being contained in the anolyte chamber for contacting the anode; a catholyte being contained in the catholyte chamber for contacting the cathode; a cationic exchange membrane (CEM); a cathodic inlet in fluid communication with the cathode so as to feed the cathode with a gas component comprising CO2; and, a cathodic outlet in fluid communication the catholyte chamber so as to recover a product mixture comprising multicarbon (C2+) products.
In some implementations, the gas component further comprises N2.
In some implementations, the catholyte is an acidic catholyte.
In some implementations, the acidic catholyte has a bulk pH between 1 and 4, or of at most 1.
In some implementations, the acidic catholyte comprises at least one of phosphoric acid, sulfuric acid, and perchloric acid.
In some implementations, the cation species comprise potassium, caesium or sodium ions.
In some implementations, the cation species comprise potassium ions K+, and the acidic catholyte further comprises at least one of potassium chloride, potassium phosphate monobasic, potassium sulfate, potassium iodide, and potassium hydroxide.
In some implementations, the catholyte has a K+ concentration between about 0.5 M and 5 M, or between 0.5 M and 4 M, or between 0.5 M and 3 M.
In some implementations, the catholyte has a total concentration in phosphorous species is about 1M.
In some implementations, the system further comprises a power source providing electric current at an applied current density between 100 mA·cm2 and 1.5 A·cm2.
In some implementations, the system further comprises a mass flow controller that is operatively connected to the reactant inlet to adjust an inlet gas flowrate.
In some implementations, the inlet gas flowrate is between 1 and 50 sccm.
In some implementations, the inlet gas flowrate is between about 3 and about 10 sccm.
In some implementations, the system further comprises a first peristaltic pump operatively connected to a first tube in fluid communication with the anolyte chamber to circulate the anolyte therein, and a second peristaltic pump operatively connected to a second tube in fluid communication with the catholyte chamber to circulate the catholyte therein.
In some implementations, the catholyte and anolyte are circulated at a constant flowrate.
In some implementations, the CEM is a Nafion™ membrane.
In some implementations, the system further comprises a counter electrode being provided in the anolyte chamber, the counter electrode comprising noble metals, Pt, Ru, or Ir.
In some implementations, the system further comprises a reference electrode being provided in the catholyte chamber, the reference electrode being an Ag/AgCl, Hg/HgSO4 or SCE reference electrode.
In some implementations, the anolyte pH is equal to or less than the catholyte pH.
In some implementations, the anolyte is of the same nature as the catholyte.
In accordance with a further aspect, there is provided a method for enhancing carbon utilization during CO2 electroreduction in an electrolytic system comprising an acidic catholyte and a cathode in contact with the acidic catholyte, the method comprising increasing a local pH of the catholyte at a surface of the cathode.
In some implementations, increasing the local pH of the catholyte at the surface of the cathode comprises operating the electrolytic system at a current density that results in a consumption rate of local H3O+ protons at the surface of the cathode being higher than mass transport of bulk H3O+ protons.
In some implementations, the acidic catholyte is a strong acid having a pH of at most 1.
In some implementations, the current density is between 200 mA·cm2 and 1.5 A·cm2.
In some implementations, the acidic catholyte is an acid having a pH between 1 and 4.
In some implementations, the current density is between 100 and 200 mA·cm2
In some implementations, increasing the local pH of the catholyte at the surface of the cathode comprises creating locally neutral or alkaline conditions.
In some implementations, the local pH at the surface of the cathode is about 7.
In some implementations, increasing the local pH of the catholyte at the surface of the cathode comprises creating locally alkaline conditions.
In some implementations, the local pH at the surface of the cathode is above 7.
In some implementations, increasing the local pH of the catholyte at the surface of the cathode comprises creating a local pH gradient from alkaline to acidic conditions from the surface of the cathode to a bulk of the catholyte.
In some implementations, the local pH is between 8 and 10 at the surface of the cathode and the local pH is at most 6.5 within a distance of at least 30 μm from the surface of the cathode.
In some implementations, increasing the local pH of the catholyte at the surface of the cathode comprises providing cation species at the surface of the cathode.
In some implementations, the cation species are provided within the catholyte.
In some implementations, the catholyte comprises a cation donor that liberates the cation species.
In some implementations, the cation species is K+ and the cation donor is at least one of potassium chloride, potassium phosphate monobasic, potassium sulfate, potassium iodide, and potassium hydroxide.
In some implementations, the catholyte has a K+ concentration between about 0.5 M and 5 M.
In some implementations, providing the cation species at the surface of the cathode comprises confining the cation species within a cation-augmenting layer (CAL) of the cathode.
In some implementations, the CAL comprises an acidic group exchanging protons with the cation species.
In some implementations, the acidic group is —SO3H.
In some implementations, the CAL comprises an ion conducting polymer.
In some implementations, the ion conducting polymer is a cationic perfluorosulfonic acid (PFSA) ionomer.
In some implementations, the PFSA ionomer is composed of tetrafluoroethylene and sulfonyl fluoride vinyl ether.
In some implementations, the acidic catholyte comprises at least one of phosphoric acid, sulfuric acid, and perchloric acid.
In some implementations, the catholyte comprises between 0.1M and 1M of acid and 0.1M and 3M of cation donor.
In some implementations, the catholyte comprises 1 M H3PO4 and 3M KCl.
In some implementations, the cathode is the electrode as defined above.
In some implementations, a Faradaic Efficiency (FE) of the CO2 electroreduction toward C2H4 is of at least 10% at a current density between 300 and 800 mA/cm2.
In some implementations, the FE towards C2H4 is of 13% at the current density of 400 mA/cm2
In some implementations, the FE toward C2H4 increases from around 10% with the catholyte of 1 M K+ to 26% with the catholyte of 3 M K+ at the current density of 1.2 A/cm2.
In some implementations, an overall CO2R selectivity is about 61±3% with a total FE toward C2+ products of about 40±2%.
In some implementations, the method comprises feeding gaseous CO2 to the cathode at an inlet flowrate between 1 and 50 sccm.
In some implementations, the inlet flowrate is between 3 sccm and 10 sccm.
In some implementations, the cathode comprises copper and a PFSA ionomer and the catholyte is a strong acid having a pH of at most 1, and wherein a single pass carbon utilization is of about 77%, including a conversion toward C2+ products of about 50% at an applied current density of 1.2 A/cm2 for the inlet flowrate of 3 sccm.
Various implementations, features and aspects of the present techniques are described herein, including in the claims, figures and following description.
The figures describe various aspects and information regarding the techniques described and claimed herein.
For the disclosure, the following definitions are given:
The terms “comprising”, “comprises” and “comprised of” as used herein are synonymous with “including”, “includes” or “containing”, “contains”, and are inclusive or open-ended and do not exclude additional, non-recited members, elements or method steps. The terms “comprising”, “comprises” and “comprised of” also include the term “consisting of”.
The recitation of numerical ranges by endpoints includes all integer numbers and, where appropriate, fractions subsumed within that range (e.g., 1 to 5 can include 1, 2, 3, 4, 5 when referring to, for example, a number of elements, and can also include 1.5, 2, 2.75 and 3.80, when referring to, for example, measurements). The recitation of endpoints also includes the recited endpoint values themselves (e.g., from 1.0 to 5.0 includes both 1.0 and 5.0). Any numerical range recited herein is intended to include all sub-ranges subsumed therein.
The term “transition metal” refers to an element whose atom has a partially filled d sub-shell, or which can give rise to cations with an incomplete d sub-shell (IUPAC definition). According to this definition, the transition metals are Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Y, Zr, Nb, Mo, Tc, Ru, Rh, Pd, Ag, Cd, La, Hf, Ta, W, Re, Os, Ir, Pt, Au, Hg, Ac, Rf, Db, Sg, Bh, Hs, Mt, Ds, Rg, and Cn.
The metals Ga, In, Sn, TI, Pb and Bi are considered as “post-transition” metals.
The metals Au, Ag, Ru, Rh, Pd, Os, Ir and Pt show outstanding oxidation resistance and are considered “noble” metals. Other metals can be considered as “non-noble” metals.
The term “alkali metal” refers to an element classified as an element from group 1 of the periodic table of elements (or group IA), excluding hydrogen. According to this definition, the alkali metals are Li, Na, K, Rb, Cs and Fr.
The term “alkaline earth metal” refers to an element classified as an element from group 2 of the periodic table of elements (or group IIA). According to this definition, the alkaline earth metals are Be, Mg, Ca, Sr, Ba and Ra.
The term “rare earth elements” refer to the fifteen lanthanides, as well as scandium and yttrium. The 17 rare-earth elements are cerium (Ce), dysprosium (Dy), erbium (Er), europium (Eu), gadolinium (Gd), holmium (Ho), lanthanum (La), lutetium (Lu), neodymium (Nd), praseodymium (Pr), promethium (Pm), samarium (Sm), scandium (Sc), terbium (Tb), thulium (Tm), ytterbium (Yb), and yttrium (Y).
The present disclosure provides an electrolytic system for CO2 electroreduction into multicarbon (C2+) products, the system comprising:
With preference, the catholyte has an alkali metal ion concentration between 0.5 M and 4 M, or between 0.5 M and 3 M, preferably between 1 M and 2.5 M.
In particular, the present disclosure relates to the disclosure provides an electrolytic system for CO2 electroreduction into multicarbon (C2+) products, the system comprising:
CO2R in acidic media offers an avenue to reduce carbonate formation to near-zero, and thus also eliminate CO2 crossover (
There is provided herein a strategy to enhance carbon utilization by increasing a concentration of cation species at a surface of the electrode, so as to improve CO2 activation kinetics when the electrolyte is an acid, such as a strong acid (pH<1). This strategy can be referred to herein as a cation-augmenting strategy. Potassium ions are the cation species that have been tested herein. However, various cation species can be confined at the surface of the electrode, such as caesium and/or sodium as shown on
The Faradaic Efficiency (FE) of the CO2 electroreduction toward C2H4 in presence of potassium ions is of at least 10% at a current density between 300 and 800 mA/cm2. The FE towards C2H4 in presence of potassium ions is of 13% at the current density of 400 mA/cm2.
The FE of the CO2 electroreduction toward C2H4 in presence of Cs+ is 6% at the current density of 400 mA/cm2.
The FE of the CO2 electroreduction toward C2H4 in presence of Na+ is 12% at the current density of 400 mA/cm2.
The overall CO2R selectivity, measured at a current density of 1.2 A/cm2, in presence of potassium ions is 61±3% with a total FE towards C2+ products of 40±2%.
The overall CO2R selectivity, measured at a current density of 1.2 A/cm2, in presence of Cs+ is 56% with a total FE towards C2+ products of 36%.
The overall CO2R selectivity, measured at a current density of 1.2 A/cm2, in presence of Na+ is 55% with a total FE towards C2+ products of 30%.
The strategy—when applied on a high-surface-area copper (Cu) catalyst—can enable a single pass carbon efficiency (SPCE) of about 77%, thereby exceeding the theoretical limit in neutral and alkaline media. For example, the CO2R reaction can be operated to convert 50% of input CO2 to multicarbon (C2+) products at an applied current density of 1.2 A/cm2.
It should be noted that the local pH refers herein to a pH that varies according to a gradient within a 50 μm distance from the cathode surface, whereas the bulk pH refers to the pH of the bulk electrolyte. The local pH at the surface of the electrode (distance to electrode=0) can be referred to a surface pH. The electrode where CO2R is performed in the cathode, so the local pH variation can be found in the catholyte side of the system. Thus, the electrolyte can refer herein to the catholyte being an acid of pH of at most 4. For example, a high-concentration phosphate (total phosphorous species can be kept to between 0.8 M and 1.2 M, or to between 0.9 M and 1.1 M, for example to 1 M) can be used as electrolyte to keep a local pH at the cathode as close as possible to a bulk pH (26) of the electrolyte. In addition, the anolyte is selected to include enough protons to sustain the current density, and thus can have a pH at least equal to or less than the bulk pH of the catholyte. For example, the anolyte can be chosen to be the same as the catholyte.
Modelling of reaction and diffusion of species within a typical diffusion layer of 50 μm showed that, in a phosphoric acid (1 M, pH 1.05) electrolyte, the surface (distance to cathode=0) pH is similar to the bulk pH at current densities <200 mA/cm2 while becoming neutral and alkaline when current densities are further increased (
solutions at
temperatures from 273 K to 363 K”.
The locally alkaline conditions result from a consumption rate of local protons that exceeds mass transport of protons from bulk (27). Despite elevated pH at the surface of the cathode, pH decreases to acidic range within a short distance to the cathode. Even at a current density as high as 1 A/cm2, the local pH decreases to 6.3 (pKa, of carbonic acid) within 33 μm of the electrode. This confinement assures that any locally generated carbonate would be converted back to CO2 for ensuing reduction, avoiding carbonate crossover and the associated loss of reactant CO2. In comparison, similar conditions (local pH 6.3 at a distance to the cathode of 30 μm) are reached at much lower current densities (<200 mA/cm2) in electrolytes of pH 2-4 (
One operating parameter that can be controlled when adjusting the local pH at the surface of the cathode is the applied current density. To circumvent the kinetically more favourable HER in acid, CO2R was tested at current densities where the H3O′ mass-transport limitation occurs and H2O becomes the main proton donor at the cathode surface (19, 22). Modelling shows that the surface pH approaches neutrality when the current density reaches 100 mA/cm2 for electrolytes with pH 2-4 or above 200 mA/cm2 for electrolyte with pH 1 (
Another operating parameter that can be controlled when adjusting the local pH at the surface of the cathode is a concentration of the cation species at the surface of the electrode or proximal to this surface. The concentration of the cation species can be maintained higher proximal or at the surface of the electrode so as to establish local alkaline conditions that prevent domination of the HER over the CO2R. The term “proximal” used in relation to the surface of the electrode should be understood as within a distance of at most 30 μm, at most 20 μm or at most 10 μm from the surface of the electrode. When 0.5 M KCl was added into the phosphoric acid electrolyte (pH 1) in the middle of the reaction, a slight decrease in FE toward hydrogen was observed, and CH4 (FE 2.1±0.3%) was detected at a current density of 200 mA/cm2 (
To test this hypothesis, a Tafel analysis was carried out at different pH and the Tafel slope was found to decrease along with an increase in K+ concentration in the electrolytes (
The improvement of CO2R reactivity does not seem to come from suppression of HER in the presence of K+. Indeed, voltametric analyses in 1 M H3PO4 with various concentrations of K+ showed similar behaviour regardless of N2 or CO2 atmospheres (
CO2R product distribution in 1 M H3PO4 with different concentrations of K+ and current density held constant at the applied current density of 400 mA/cm2 (
It was sought to steer selectivity further toward C2H4 in view of its high value and broad application in the chemical sector (32). However, the solubility of K+ in aqueous electrolytes is limited while maintaining a low pH. In some implementations, a way to increase the concentration of cation species at the surface of the cathode can be to modify a surface of the cathode such that said surface is able to confine cation species. The cathode is said to include a cation-augmenting layer (CAL) comprising or consisting of a cation-augmenting material. The surface of the cathode can be functionalized with acidic groups that can exchange protons with the cation species of the catholyte. The cathode can include a catalyst material or catalyst layer such that the CAL is deposited onto the catalyst layer, or such that the cation-augmenting material is combined with the catalyst material in an active layer. For example, enrichment of cation species, such as potassium ions K+, at the Cu surface by the CAL can be performed. The cation-augmenting material can include an ion conduction polymer, such as a cationic ionomer, e.g., a cationic perfluorosulfonic acid (PFSA) ionomer composed of tetrafluoroethylene and sulfonyl fluoride vinyl ether. The acidic —SO3H group can exchange protons with K+ from the bulk electrolyte in a non-acidic local environment, sustaining a high K+ concentration at the catalyst surface (
The CAL-modified Cu showed a further increase of FE toward C2H4 to 13% and a much lower FE toward CH4 of <1% comparing to the bare Cu catalyst, while the remaining CO2R gaseous product was CO at a current density of 400 mA/cm2 in 1 M H3PO4 with 3 M KCl (
To improve CO2R productivity still further, the electrochemically active surface area of the electrode was increased by forming a Cu-NPs/PFSA composite material (
CO2R in acid enables CO2 electrolysis without carbonate formation and crossover, circumventing the CO2 utilization limit that is fundamental to neutral and alkaline systems, and permitting a carbon efficiency that is capable of increasing further in the direction of unity. To reduce energy demand of product separation from dilute streams (36), single-pass carbon efficiency (SPCE) toward the new theoretical limit was pursued.
By gradually reducing the flow rate of CO2 from 50 to 5 standard cubic centimeters per minute (sccm), the C2+ FE was improved to 49% (34% toward C2H4, 11% toward C2H5OH, and 4% toward C3H7OH). This combination of current density and selectivity results in a high overall C2+ productivity of 600 mA/cm2 (
By further lowing the flow rate of CO2 to 3 sccm, an SPCE of ˜77% for all the CO2R products, including ˜50% for C2+ products (
It should be noted that the flow rate values exemplified and claimed herein correspond to inlet flow rate values for feeding CO2 to a cathode at a laboratory scale for experimentation. One skilled in the art can understand that various of the tested parameters can be adapted to perform the described methods at a larger industrial scale, for example.
The cation augmentation takes CO2 electrolysis from high-pH neutral and alkaline electrolytes to pH<1 acidic environment. This work solves the carbonate regeneration and CO2 crossover challenge, and sets a new benchmark for carbon utilization and the viability of electrochemical CO2 conversion.
Test and Determination Methods
Catalysts and Chemicals Preparation
All the chemicals used for electrolytes and catalyst synthesis, including phosphoric acid (85%), potassium chloride, potassium phosphate monobasic, potassium sulfate, potassium iodide, potassium hydroxide, sulfuric acid, perchloric acid, Aquivion (D79-25BS), Cu nanoparticles (25 nm), carbon nanoparticles and graphite, were purchased from Sigma Aldrich. Nafion™117 membrane (Nafion™ 117 indicates an extrusion-cast membrane with 1100 g/mol equivalent weight (EW) and 0.007 inches (7 thou) in thickness) and platinum mesh (grid aperture of 0.98×1.4 mm; purity 99.95%) were purchased from Fuel Cell Store. The polytetrafluoroethylene (PTFE) gas diffusion layer with 450 nm pore size was purchased from Beijing Zhongxingweiye Instrument Co., Ltd. Deionized water (18.2 MΩ) was used for all the electrolytes preparation. Cu and Ag were sputtered onto the PTFE substrate using pure Cu and Ag targets (>99.99%) in a vacuum environment (10−5˜10−6 Torr) in an Angstrom Nexdep sputtering system. The deposition rate was kept constant at 1 Å/sec. The thickness of the catalyst layer was kept constant for all the electrodes to 300 nm, as determined by SEM. Cation-augmenting layer (CAL) is a 2 μm-thick homogeneous blend of carbon NPs (50 nm, Vulcan XC-72R) and Aquivion. The CAL-modified Cu was prepared by spray coating the CAL solution dispersed in methanol onto a 300 nm-Cu sputtered hydrophobic PTFE substrates. CAL-modified Cu-NPs/PFSA was prepared by spray coating the following dispersion onto a PTFE substrate with 300 nm sputtered Cu in sequence: 6 μm-thick homogeneous blend of Cu nanoparticles and Aquivion, a 2 μm-thick homogeneous blend of C NPs and Aquivion, and a 2 μm-thick homogeneous blend of graphite flakes (325 mesh, <44 μm, 99%, Sigma Aldrich) and Aquivion.
The flow cell setup was composed of three chambers: anolyte chamber, catholyte chamber, and gas flow chamber. The size of the electrode exposed was 1 cm×1 cm. The cathode GDE of interest was clamped between catholyte chamber and gas diffusion chamber, with the substrate side facing the gas chamber and catalyst side (or CAL) facing the catholyte chamber. A Pt foil was employed in the anolyte chamber. The catholyte and anolyte chambers were separated by a cation exchange membrane (CEM, Nafion™117). The catholyte chamber contained an Ag/AgCl reference electrode (3M KCl).
Catholyte and anolyte were applied through separate silicone tubes that each connected to a peristaltic pump, offering a constant flow rate of approximately 10 mL/min. Electrolytes going through the pumps first entered each chamber from the bottom and exited from the top and flows back to their bulk electrolyte which forms a close cycle. For the gas supply, a digital mass flow controller (SmartTrack 100, Sierra) was connected to CO2/N2 gas cylinder to control the flow rate in gas flow chamber. The CO2 gas and N2 gas cylinders were purchased from Linde Gas.
All the electrochemical tests were carried out using an electrochemical workstation (Autolab PGSTAT302N) connected to a current booster (Metrohm Autolab, 10 A), except for the linear sweeping voltammetry (LSV) tests, which were performed via a CHI 660E potentiostat. The catholyte of pH 1 or lower was prepared using 1 M phosphoric acid as the base electrolyte, with the incorporation of different salts at various concentrations. The most frequently used salt was potassium chloride, with the highest concentration of 3 M (pH 0.57).
The CO2 reduction (CO2R) performance was tested in a flow cell assembly under galvanostatic mode. 1 M phosphate buffer solutions with different salts and concentrations were used as catholyte, and 1 M phosphoric acid was used as anolyte. Cu on PTFE (300 nm), CAL-modified Cu and Cu-NPs/PFSA were used as cathodes in different tests.
LSV was taken in the same flow cell setup that is used for performance evaluation and the electrolytes were saturated with N2 through continuous bubbling. Sputtered Cu on PTFE (300 nm), Ag/AgCl (3 M KCl), and a Pt foil were used as working, reference, and counter electrodes, respectively. The scan rate was kept constant at 50 mV/s. Phosphate was used as catholyte, in which the total phosphate concentration was kept constant as 1 M. 0.5 M H2SO4 was used as the anolyte. The cathode and anode chambers were separated by a CEM (Nafion™ 117). For LSVs of different pH, the total potassium concentration was kept as 2 M to sustain a high ion conductivity and achieve high current density. In detail, the catholyte of pH 1 was prepared using 1 M H3PO4 and 2 M KCl, and the pH was adjusted to around 1 (0.96) by a few drops of 5 M KOH. The catholyte of pH 2 was prepared using 0.5 M H3PO4, 0.5 M KH2PO4, and 1.5 M KCl, and the pH was adjusted to 1.94 through the addition of KOH. The catholyte of pH 3 was prepared using 0.1 M H3PO4, 0.9 M KH2PO4, and 1.1 M KCl, and the pH was adjusted to 2.94 through the addition of KOH. The catholyte of pH 4 was prepared using 1 M KH2PO4, and 1 M KCl, and the pH was adjusted to 3.96 by KOH. For the LSVs of different concentrations of potassium, KCl was added to 1 M H3PO4 electrolyte to supply the desired concentration of potassium. The pH of 0, 1, 2, and 3 M potassium were 0.85, 0.81, 0.70, and 0.57, respectively. All potentials were converted to RHE scale via the equation:
E(RHE)=E(Ag/AgCl)+0.059×pH+0.210+iR
CO2 crossover was measured at a constant current density of 400 mA/cm2 for 6 hours. For the neutral electrolyte, both anolyte and catholyte concentrations (25 mL for each) were 1 M KHCO3, which were saturated with CO2 prior to the experiment. For the acidic conditions, the anolyte (25 mL) was 0.5 M H2SO4, and the catholyte (25 mL) was 1 M phosphate buffer solution. 2 M of KCl was added to the catholyte to improve the ion conductivity. The CO2 flow rate was kept constant at 50 sccm using a mass flow controller (Alicat Scientific). The gas products collected from the anodic outlet were analyzed by a gas chromatography (PerkinElmer Clarus 680). The pH of catholyte and anolyte were monitored by a pH meter.
The gas products were collected from the gas outlet channel of the flow cell and injected into a gas chromatograph (PerkinElmer Clarus 680). The gas chromatograph was equipped with a thermal conductivity detector (TCD) for detection of H2, O2, N2 and CO signals and a flame ionization detector (FID) for the detection of CH4 and C2H4 signals. The gas chromatograph was composed of packed columns of Molecular Sieve 5A and Carboxen-1000 and employed Argon (Linde, 99.999%) as the carrier gas. For quantification, 1 mL of gas product was injected into the gas chromatograph, and the performance was evaluated as a function of current density, gas flow rate and gas products fraction.
Faradaic efficiency (%)=N×F×v×r/(i×Vm)
The liquid products were analyzed using 1H NMR spectroscopy (600 MHz Agilent DD2 NMR Spectrometer) with water suppression. Dimethyl sulfoxide (DMSO) was used as the reference standard and deuterium oxide (D2O) as the lock solvent. The Faradaic efficiency was calculated using the equation below:
Faradaic efficiency (%)=N×F×nproduct/Q
The single pass carbon efficiency (SPCE) of CO2 towards each product or a group of products was determined using this equation at 25° C., 1 atm:
SPC=(j×60 sec)/(N×F)÷(flow rate (L/min)×1 (min))/(24.05 (L/mol))
Where j is the partial current density of specific group of products from CO2 reduction, N is the electron transfer for every product molecule.
Scanning electron microscopy (SEM) was performed in a high-resolution scanning electron microscope (HR-SEM, Hitachi S-5200).
X-ray photoelectron spectroscopy (XPS) were carried out in an ECSA device (PHI 5700) with Al Kα X-ray energy source (1486.6 eV) for excitation. Prior to measurements, the catalysts were rinsed sequentially with 1 M H3PO4 and DI water to remove any potential residual salt from the surface.
Operando hard X-ray absorption spectroscopy measurements were conducted at 9BM beamline of the Advanced Photon Source (APS, Argonne National Laboratory, Lemont, Illinois). The data were processed by Athena and Artemis software incorporated into standard IFEFFIT package.
To assess the energy penalty and associated cost induced by CO2 crossover, two benchmark systems from literature were considered: neutral and alkaline CO2R electrolyzers. These neutral CO2R MEA and alkaline flow cell electrolyzers were considered specifically owing to their performance metrics—industrially relevant reaction rates (>100 mA/cm2), high FE towards C2+ products (i.e. an ethylene FE of >60%), and high full-cell energy efficiency (EE) towards C2+ products (i.e. an ethylene full-cell EE of >20%). It was postulated that the proximity of these performance metrics for each system being compared (alkaline, neutral electrolyzers) will help refine the effect of CO2 crossover and carbonate formation on the total energy requirement of producing C2+ products (i.e., ethylene). Table 1 summarizes the input parameters to the model for both systems, along with considerations of industrial costs. The majority of these input parameters were obtained from literature. The CO2R performance of both MEA electrolyzers and alkaline flow cell electrolyzers are still improving, thus it was considered optimistic performance metrics (current density, selectivity, and energy efficiency) towards a single C2 product for each system. The techno-economic model considers a production rate of 1 ton per day, with assumptions that H2 and O2 are the only by-products coming out of the cathodic and anodic streams, respectively. Detailed calculations of cost, along with the main assumptions made, for the capital, installation, operation, carbon regeneration (for alkaline flow cell), cathode separation (for both the alkaline flow cell and MEA electrolyzers), anode separation (for neutral MEA), can be found in previous work (see study of A. Ozden et al., entitled “Cascade CO2 electroreduction enables efficient carbonate-free production of ethylene”. Joule, 5, 706-719 (2021). Table 2 presents the cost breakdown of alkaline flow cell electrolyzers and neutral MEA CO2R electrolyzers. Comsol simulation modelling
A reaction-diffusion model was used to simulate the local pH using COMSOL Multiphysics software. All the interactions between species in the electrolyte (CO2, HCO3−, CO32−, H3PO4, H2PO4−, HPO42−, PO43−, OH−, H+ and H2O) were considered. Henry's law was used to calculate the CO2 concentration1, assuming that the CO2 fugacity is 1 bar.
CCO2,aq0=KH0CCO2,gas0
KH0 is the Henry's constant, which can be calculated by using the equation below, where T is the temperature.2
Due to the high concentration of the ions, the saturated concentration of CO2 in an electrolyte is corrected using the following equations—see study of S. Weisenberger et al., entitled “Estimation of gas solubilities in salt solutions at temperatures from 273 K to 363 K”. AlChE J. 42, 298-300 (1996).
h
G
=h
G,0
+h
T(T−298.15)
Cs is the molar concentration and Ks is the Sechenov's constant which can be estimated using equation above and Table 3.
The following homogeneous and heterogenous reactions in the model were considered, which are based on the previously published works (references 27 and 37-42). The heterogenous reactions (reference 43) (reactions 1-4) take place in the porous catalyst layer, and the homogenous reactions (reactions 5-11) occur in entire domain.
The bulk concentrations and pH values were measured experimentally and implemented in the model. The thickness of the diffusion layer was assumed to be 50 μm. The ion species transport is based on the reaction previously listed and follows the equation below. Ji is the molar flux. The species diffusion coefficients are listed in Table 4.
The heterogenous reactions were simulated by adding the electrochemical reaction rates to the equation as follow:
An average product distribution was assumed, where S is approximated to 40% and X+Y+Z to 60% (Table 5). A porosity of 60%, s, and length of the catalyst of 300 nm, Lcatalyst, layer was considered in the model.
The descriptions, examples, methods and materials presented in the claims and the specification are not to be construed as limiting but rather as illustrative only.
Meanings of technical and scientific terms used herein are to be commonly understood as by one of ordinary skill in the art to which the disclosure belongs, unless otherwise defined.
It is understood that whether the term “about” is used explicitly or not, every quantity given herein is meant to refer to an actual given value, and it is also meant to refer to the approximation to such given value that would reasonably be inferred based on the ordinary skill in the art, including approximations due to the experimental and/or measurement conditions for such given value. It is commonly accepted that a 10% precision measure is acceptable and encompasses the term “about”.
Although various implementations of the disclosure may be described in the context of a single embodiment, these implementations may also be provided separately or in any suitable combination. Conversely, although the disclosure may be described herein in the context of separate embodiments for clarity, the implementations of the techniques described herein may also be implemented in a single embodiment, unless incompatible.
Any publications, including patents, patent applications and articles, referenced or mentioned in this specification are herein incorporated in their entirety into the specification, to the same extent as if each individual publication was specifically and individually indicated to be incorporated herein. In addition, citation or identification of any reference in the description of some embodiments of the disclosure shall not be construed as an admission that such reference is available as prior art to the present disclosure.
The following references are incorporated herein by reference:
| Number | Date | Country | Kind |
|---|---|---|---|
| LU102678 | Mar 2021 | LU | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP2022/055570 | 3/4/2022 | WO |
| Number | Date | Country | |
|---|---|---|---|
| 63200393 | Mar 2021 | US |
