Patent Application
20120193222
1. Technical Field
The present disclosure is directed to electrochemical reactors and, more particularly, to electrochemical reactors that operate on a carbonate cycle at extremely low temperatures (e.g., less than about 50° C.), wherein the electrochemical reactors have improved performance characteristics, allowing operation in as many as three (3) modes, namely as: (i) a room temperature carbonate fuel cell; (ii) an electrochemically assisted CO2 membrane separator; and (iii) a CO2 conversion device. The present disclosure further provides an electrocatalyst and, more specifically, an electrocatalyst having the ability to selectively form carbonate anions over hydroxide anions under fully humidified conditions.
2. Background Art
In general, the price of petroleum is rising and prices are volatile. As petroleum-derived materials are integrated into nearly every market in the world, cost and uncertainty in oil prices has a considerably negative impact, e.g., by lowering consumer and market confidence, curtailing investment, reducing manufacturing, etc. This has led researchers and the chemical process industry to search for alternative feedstocks from cheaper, and preferably domestic, feedstocks.
In general, the US and the UK are world leaders with respect to the production of methane (i.e., natural gas). With recent discoveries in both countries, in addition to the introduction of renewable biogas (e.g., a mixture of primarily methane, CH4, and carbon dioxide, CO2) to the market, the availability of methane is generally at an all-time high and its inflation-corrected cost has risen only approximately 10% over the past 30 years and is expected to decrease over the next several years (see, e.g., U.S. Energy Information Administration, 25th Anniversary of the 1973 Oil Embargo, available at http://www.eia.doe.gov/emeu/25opec/sld006.htm). This has made gas to liquid (“GTL”) conversion processes popular (see, e.g., Mokrani, T. et al., Gas Conversion to Liquid Fuels and Chemicals: The Methanol Route-Catalysis and Processes Development, Catalysis Reviews, 51, 1-145 (2009)). In GTL processes, methane is typically oxidized through steam reforming to syngas (i.e., CO+H2). With reference to
Though steam reforming is generally used in the industry and is well developed, it is expensive from a processing perspective for several reasons. First, industrial reactors for these processes are typically run in excess of about 700° C., which places stringent conditions on materials selection and requires high quality heat. Second, this reaction is strongly endothermic (e.g., ΔH about 200 kJ/mol), requiring a large amount of heat. From here, the conversion of syngas to methanol is both thermodynamically and kinetically favored and inexpensive from an industrial perspective. Thus, an interest exists for the discovery of a low temperature route to convert methane to syngas which would reduce industrial cost and provide a truly transformative technology for the processing of natural gas and biogas to higher order organics.
However, there are certain limitations for the thermochemical activation and conversion of methane. Upgrading of methane, the primary component of natural gas and biogas, to industrially relevant chemicals, e.g., methanol and other easily transportable liquid fuels, has been investigated over the last few decades. Despite research efforts into finding novel catalyst materials and reaction pathways, the activation of methane at low temperature, preferably approximately room temperature, has proven elusive and challenging. In conventional processes, the adsorption and thermochemical activation of methane is generally slow because: i) the C—H bond has a high dissociation energy (about 105 kcal/mole); ii) the C—H bond has low polarity; iii) the number of valence electrons and valence orbitals is the same, leaving no easily reactive lone pairs or empty orbitals; and iv) methane's tetrahedral structure has high steric hindrance (see, e.g., Zhidomirov, G. M. et al., Molecular Models of Active Sites of C1 and C2 hydrocarbon activation, Catalysis Today, 24, 383-387 (1995)). Thus, high temperatures or highly active catalysts are required.
In general, redox processes have been used to facilitate new reaction chemistries. Specifically, electrochemical reactions typically allow two control features that conventional heterogeneous processes do not: i) direct control of the surface free energy of the catalyst through the electrode potential, allowing the reaction rate and pathway selectivity to be dialed in, and ii) a non-direct reaction between precursors through complementary redox processes on two separate catalysts. This typically permits researchers to tailor the properties needed for each redox process independently, which allows for different reaction pathways depending on catalyst selection with identical precursors at the same reaction conditions while minimizing competition between alternate pathways. As such, this generally enables unique chemistries to occur that would not be possible in conventional systems.
Over the past several years, some electrochemical synthesis methods have received attention for the formation of high value products for the pharmaceutical and food industries. A first representative case is the hydrogenation of oils. For example, An et al. reported an electrochemical reactor with a proton exchange membrane, which utilized water as a hydrogen source for hydrogenation (see, e.g., An, W. et al., The Electrochemical Hydrogenation of Edible Oils in a Solid Polymer Electrolyte Reactor. I. Reactor Design and Operation, Journal of the American Oil Chemists' Society, 75, 917-925 (1998)). Also, the reactor was run at lower temperatures than traditional hydrogenation reactors and the product had higher cis-isomer selectivity, which has important implications for food products. A second representative case is the synthesis of caffeic acid derivatives, which are generally agriculturally and pharmacologically important as they have been shown to play a role in the infection defense mechanism of several plant species. Moghaddam and coworkers reported an electrochemical route for the formation of new caffeic acid derivatives that generally reduced the energy required for synthesis and eliminated the need for environmentally harmful reagents (see, e.g., Moghaddam, A. B. et al., A green method on the electro-organic synthesis of new caffeic acid derivates: Electrochemical properties and LC-ESI-MS analysis of products, Journal of Electroanalytical Chemistry, 601, 205-210 (2007)).
As such, electrochemical processes can be generally designed to control the adsorption and surface coverage of reactants and products, dictate reaction pathways and selectivity, reduce energy requirements for synthesis and lower operating temperatures compared with chemical routes.
Some of the electrochemical synthesis methods discussed above have been generally performed utilizing electrochemical devices. In general, electrochemical devices, such as, for example, fuel cells and batteries, are similar electrochemical devices that generate and/or store electrical energy. Fuel cells are typically different from batteries in that they generally consume reactant from an external source, which must be replenished. Thus, fuel cells are typically a thermodynamically open system.
Generally, a fuel cell is an electrochemical energy conversion device. Fuel cells typically produce electricity from fuel on the anode side and an oxidant on the cathode side. In general, the reactants flow into the cell, and react in the presence of an electrolyte. The reaction products typically flow out of it, while the electrolyte generally remains within it. Typically, fuel cells can operate virtually continuously as long as the necessary flows and the thermal balance is maintained.
Fuel cells are generally electrochemical cells in which a free energy change resulting from a fuel oxidation reaction is converted into electrical energy. Fuel cells are attractive electrical power sources due to their higher energy efficiency and environmental compatibility compared to, for example, the internal combustion engine. Some of the known fuel cells are those using a gaseous fuel (e.g., hydrogen) with a gaseous oxidant (e.g., pure oxygen or atmospheric oxygen), and those fuel cells using direct feed organic fuels such as methanol. Electrical energy from fuel cells may be produced for as long as the fuels, e.g., methanol or hydrogen, and oxidant, are supplied. Thus, an interest exists in the design of improved fuel cells to fill future energy needs.
The anion exchange membrane fuel cell (AEMFC) is a type of fuel cell that has been of interest in the industry due to its improved performance and characteristics. Several research groups have worked on the hydroxide exchange membrane fuel cell (HEMFC), an AEMFC implementing a hydroxide anion, as an energy conversion device (see, e.g., Varcoe, J. R. et al., Prospects for Alkaline Anion Exchange Membranes in Low Temperature Fuel Cells, Fuel Cells, (2005) 187; Park, J. et al., Performance of solid alkaline fuel cells employing anion-exchange membranes, Journal of Power Sources, 178 (2008) 620; Agel, E. et al., Characterization and use of anionic membranes for alkaline fuel cells, Journal of Power Sources, 101 (2001) 267; Yu, E. H. et al., Development of direct methanol alkaline fuel cells using anion exchange membranes, Journal of Power Sources, 137 (2004) 248; Yu, E. H. et al., Direct methanol alkaline fuel cell with catalysed metal mesh anodes, Electrochemistry Communications, 6 (2004) 361; Li, L. et al., Quaternized polyethersulfone cardo anion exchange membranes for direct methanol alkaline fuel cells, Journal of Membrane Science, 262 (2005)1; Slade, R C. T. et al., Investigations of conductivity in FEP-based radiation-grafted alkaline anion-exchange membranes, Solid State Ionics, 176 (2005) 585; Wu, Y. et al., Novel anion-exchange organic-inorganic hybrid membranes: Preparation and characterizations for potential use in fuel cells, Journal of Membrane Science, 321 (2008) 299; Varcoe, J. R. et al., Steady-State dc and Impedance Investigations of H2/02 Alkaline Membrane Fuel Cells with Commercial Pt/C, Ag/C, and Au/C Cathodes, J. Phys. Chem. B., 110 (2006) 21041; Xiong, Y. et al., Preparation and characterization of cross-linked quaternized poly(vinyl alcohol) membranes for anion exchange membrane fuel cells, Journal of Membrane Science, 311 (2008) 319; Hou, H. et al., Alkali doped polybenzimidazole membrane for high performance alkaline direct ethanol fuel cell, Journal of Power Sources, 182 (2008) 95; Wu, Y. et al., Free-standing anion-exchange PEO—Si02 hybrid membranes, Journal of Membrane Science, 307 (2008) 28; Wu, L. et al., Improving anion exchange membranes for DMAFCs by inter-crosslinking CPPO/BPPO blends, Journal of Membrane Science, 322 (2008) 286; Lu, S. et al., Alkaline polymer electrolyte fuel cells completely free from noble metal catalysts, PNAS, 105 (2008) 20611; Varcoe, J. R., Investigations of the ex situ ionic conductivities at 30° C. of metal-cation-free quaternary ammonium alkaline anion-exchange membranes in static atmospheres of different relative humilities, Phys. Chem. Chem. Phys., 9 (2007) 1479; Yanagi, H. et al., Anion Exchange Membrane and Ionomer for Alkaline Membrane Fuel Cells (AMFCs), ECS Transactions, 16 (2008) 257; Fujiwara, N. et al., Direct ethanol fuel cells using an anion exchange membrane, Journal of Power Sources, 185 (2008) 621; Sata, T. et al., Change of anion exchange membranes in an aqueous sodium hydroxide solution at high temperature, Journal of Membrane Science, 112 (1996) 161; Varcoe, J. R et al., An alkaline polymer electrochemical interface: a breakthrough in application of alkaline anion-exchange membranes in fuel cells, Chem. Commun., 13 (2006) 1428; and Torres, C. I. et al., Carbonate Species as OH-Carriers for Decreasing the pH Gradient between Cathode and Anode in Biological Fuel Cells, Environmental Science and Technology, 42 (2008) 8773). In general, the HEMFC is a modification of the traditional alkaline fuel cell (AFC), where the liquid potassium hydroxide electrolyte is replaced with a compact, solid polymer electrolyte, which simplifies cell design and construction and typically increases the intrinsic energy density of the device.
The HEMFC also offers several advantages over its acidic electrolyte counterpart, the proton exchange membrane fuel cell (PEMFC), including: (i) enhanced kinetics for both the oxygen reduction reaction (ORR) on non-Pt catalysts and hydrogen oxidation reactions (HOR) on Pt and non-Pt catalysts with less costly electrocatalysts, (ii) reduction in fuel crossover due to the suppression by the electroosmotic drag resulting from the anion transport from cathode to anode during operation, and (iii) lower cost membrane electrolytes (see, e.g., Kiros, Y. et al., Long-term hydrogen oxidation catalysts in alkaline fuel cells, Journal of Power Sources, 87 (2000) 101; Alcaide, F. et al., Hydrogen Oxidation Reaction in a Pt-Catalyzed Gas Diffusion Electrode in Alkaline Medium, J. Electrochem. Soc., 152 (2005) E319; Lasia, A., Hydrogen evolution/oxidation reactions on porous electrodes, Journal of Electroanalytical Chemistry, 454 (1998) 115; Zhang, J. et al., High catalytic activity of nanostructured Pd thin films electrochemically deposited on polycrystalline Pt and Au substrates towards electro-oxidation of methanol, Electrochemistry Communications, 9 (2008) 1298; Hernandez, J. et al., Methanol oxidation on gold nanoparticles in alkaline media: Unusual electrocatalytic activity, Electrochimica Acta, 52 (2006) 1662; and Tripkovic, A. et al., Methanol oxidation at platinum electrodes in alkaline solution: comparison between supported catalysts and model systems, Journal of Electroanalytical Chemistry, 572 (2004) 119; Erikson, H et al., Electrochimica Acta, 54, 7483 (2009); Markovic, N. M. et al., Oxygen Reduction on Platinum Low-Index Single-Crystal Surfaces in Alkaline Solution: Rotating Ring DiskPt(hkl) Studies, J. Phys. Chem., 100 (1996) 6715; Genies, L. et al., Electrochemical reduction of oxygen on platinum nanoparticles in alkaline media, Electrochimica Acta, 44 (1998) 1317; Anastasijevic, N. A. et al., Oxygen reduction on a ruthenium electrode in alkaline electrolytes, J. Electroanal. Chem., 199 (1986) 351; Demarconnay, L. et al., Electroreduction of dioxygen (ORR) in alkaline medium on Ag/C and Pt/C nanostructured catalysts—effect of the presence of methanol, Electrochimica Acta, 49 (2004) 4513; and Longo, J. M. et al., Pb2M207-x (M=Ru, Ir, Re)—Preparation and properties of oxygen deficient pyrochlores, Mat. Res. Bull., 4 (1969) 191; Hernandez, J. et al., Methanol oxidation on gold nanoparticles in alkaline media: Unusual electrocatalytic activity, Electrochimica Acta, 52 (2006) 1662; Tripkovic, A. et al., Methanol oxidation at platinum electrodes in alkaline solution: comparison between supported catalysts and model systems, Journal of Electroanalytical Chemistry, 572 (2004) 119; Narayanan, S. R. et al., Recent advances in PEM liquid-feed direct methanol fuel cells, Annu. Battery Conf. Appl. Adv., 11 (1996): 113; Cruickshank, J. et al., The degree and effect of methanol crossover in the direct methanol fuel cell, J. Power Sources, 70 (1998): 40-47; and Scott, K. et al., Performance of a direct methanol fuel cell, J. Appl. Electrochem., 28 (1998) 289).
Also, alcohol versions of HEMFCs can operate on pure fuel since water does not take part on the anode reaction, contrary to PEMFCs where the fuel must be diluted. In addition, water is produced at the anode and partially consumed at the cathode, potentially simplifying water management and preventing electrode flooding. Finally, there have also been promising reports of HEMFCs operating with hydrogen and alcohol fuels (see, e.g., Agel, E. et al, J. Power Sources, 101, 267 (2001); Li, L. et al., J. Membrane Sci., 262, 1 (2005); Hebrard, G. et al., Chem. Eng. J., 148, 132 (2009); Yu, E. et al., J. Power Sources, 137, 248 (2004); Wu, Y. et al., J. Membrane Sci., 307, 28 (2008); Xiong, Y. et al., J. Membrane Sci., 311, 319 (2008); and Yu, E. et al., Electrochem. Commun., 6, 361 (2004)).
However, the HEMFC has some troublesome technical limitations. State-of-the-art anion exchange membranes with nitrogen functionalities typically undergo a catalyzed degradation by hydroxide anions through nucleophilic attack and Hofmann elimination reactions (see, e.g., Varcoe, J. R. et al., Prospects for Alkaline Anion Exchange Membranes in Low Temperature Fuel Cells, Fuel Cells, 5 (2005) 187; Li, L. et al., Quaternized polyethersulfone Cardo anion exchange membranes for direct methanol alkaline fuel cells, Journal of Membrane Science, 262 (2005)1; Slade, R C. T. et al., Investigations of conductivity in FEP-based radiation-grafted alkaline anion-exchange membranes, Solid State Ionics, 176 (2005) 585; Wu, Y. et al., Novel anion-exchange organic-inorganic hybrid membranes: Preparation and characterizations for potential use in fuel cells, Journal of Membrane Science, 321 (2008) 299; Varcoe, J. R. et al., Steady-State dc and Impedance Investigations of H2/02 Alkaline Membrane Fuel Cells with Commercial Pt/C, Ag/C, and Au/C Cathodes, J. Phys. Chem. B., 110 (2006) 21041; and Xiong, Y. et al., Preparation and characterization of cross-linked quaternized poly(vinyl alcohol) membranes for anion exchange membrane fuel cells, Journal of Membrane Science, 311 (2008) 319). Moreover, the pH at the HEMFC cathode pH is typically in excess of 14. These are complex hurdles to overcome with current technologies and device chemistries as the purpose of the HEMFC cathode catalyst is to produce hydroxide as quickly as possible (i.e., high current) and the purpose of the electrolyte is to have both high solubility and high mobility of OH-ions in order to increase conductivity (i.e., low internal resistance). To address this fundamental limitation, an anionic charge-carrying species that will lower the localized pH at the electrocatalyst surface while maintaining high ionic conductivity would be highly advantageous.
Consequently, several researchers have started investigating AEMFCs that operate on the carbonate cycle (see, e.g., Lang, C. et al., Electrochem. Solid State, 9, A545 (2006); Adams, L. A. et al., ChemSusChem, 1, 79 (2008); Zhou, J. et al., J. Power Sources, 190, 285 (2009); and Vega, J. A. et al., Electrochimica Acta, 55, 1638 (2010)). Carbonate anions have long been used as a reliable charge-carrying species in a molten carbonate fuel cell (MCFC) (see, e.g., Selman, R. J., 5. Molten carbonate fuel cells (MCFCs), Energy, 11 (1986) 153; Maru, H. C. et al., Molten Carbonate Fuel Cell Product Design Improvement, prepared for US DOE/DARPA, Annual Report, DE-FC21-95MC31184; K. Jooh, Critical issues and future prospects for molten carbonate fuel cells, Journal of Power Sources, 61 (1996) 129; Dicks, A. L., Molten carbonate fuel cells, Current Opinion in Solid State and Materials Science, 8 (2004) 379; and Dicks, A. et al., Assessment of commercial prospects of molten carbonate fuel cells, Journal of Power Sources, 86 (2000) 316). Though the MCFC has shown promise as an efficient electrochemical power source, its high operating temperature (>650° C.) has increased system complexity, significantly elevating cost despite having non-noble metal electrocatalysts. In the MCFC, CO3−2 anions are fanned at the cathode by the electrochemical activation of oxygen on the electrocatalyst, where four electrons are accepted. The activated oxygen species then chemically reacts with strongly adsorbed carbon dioxide, forming the carbonate anion. This “direct pathway” is shown in Equations 1-2.
O2+4e−→2O−2 (1)
2O−2+2CO2→2CO3−2, E0=0.62 V (2)
However, in the presence of water, the carbonate anion is preferentially formed by chemical reaction of carbon dioxide with hydroxide anions. Specifically, carbonate anions should be produced at the cathode by the selective reduction of O2 and CO2 (Equation 5), instead of O2 and H2O (Equation 3). This “hydroxide pathway” is summarized in Equations 3-4.
O2+2H2O+4e−→4OH−, E0=0.40 V (3)
4OH−+2CO2→2CO3−2+2H2O, exothermic (4)
Equations 3 and 4 together yield the following:
O2+2CO2+4e−→2CO3−2 (5)
The direct pathway is typically preferred for the following reasons. First, the hydroxide pathway has a lower theoretical potential, leading to at least a 20% reduction in power when the device is active. For example, it has been shown that electrolyte degradation is suppressed in concentrated carbonate environments (see, e.g., Zhou, J. et al., J. Power Sources, 190, 285 (2009) and Vega, J. A. et al., J. Power Sources, 195, 7176 (2010)). Second, OH− is still present locally at the cathode catalyst operating on the hydroxide pathway. This means that the electrolyte adjacent to the catalyst will still be unstable and undergo degradation. Third, hydrogen oxidation has been shown to be kinetically favored with carbonate anions, compared to hydroxide anions (see, e.g., Vega, J. A. et al., J. Electrochem. Soc., 158, B349 (2011)). This could lead to improved long-term performance of a room temperature (e.g., from about 15° C. to about 40° C.) carbonate fuel cell (RTCFC), compared to the HEMFC. However, conventional catalysts, e.g., Pt/C, have a low selectivity towards CO2 adsorption and electrochemical carbonate formation due to their low surface alkalinity and wetting properties. Therefore, it is desired that electrocatalysts are implemented that preferentially operate through the “direct” pathway.
In the alkaline fuel cell (AFC), Equation 4 is the main obstacle regarding commercialization for terrestrial applications due to carbonate saturation and salting on the cathode catalyst. This is caused by the aqueous KOH electrolyte in the AFC, where K+ combines with free CO3−2 to form K2CO3, which has an extremely low solubility in water. However, there is substantially no evidence for carbonate salting in the HEMFC. This is expected as there are no free cations present in the HEMFC. Therefore, carbonate anions are freely transported through anion exchange membranes (see, e.g., Xiong, Y. et al., Preparation and characterization of cross-linked quaternized poly(vinyl alcohol) membranes for anion exchange membrane fuel cells, Journal of Membrane Science, 311 (2008) 319; Adams, L. A. et al., A Carbon Dioxide Tolerant Aqueous-Electrolyte-Free Anion-Exchange Membrane Alkaline Fuel Cell, ChemSusChem, 1 (2008) 79; and Lang, C. M. et al., High-Energy Density, Room-Temperature Carbonate Fuel Cell, Electrochemical and Solid State Letters, 9 (2006) A545). However, this suggests that in order for an electrochemical device operating on the carbonate cycle to perform effectively, it needs to be shown that carbonate anions can readily oxidize common fuels, which has not yet been shown at lower temperatures. The redox reactions for hydrogen and methanol with CO3−2 are shown below.
H2+CO3−2H2O+CO2+2e−, E0=−0.61 V (6)
CH3OH+3CO3−24CO2+2H2O+6e−, E0=−0.59 V (7)
In addition, several characteristics are typically necessary for an electrochemical catalyst to produce CO3−2 over OH−. High electrical conductivity and electrochemical activity are generally necessary to facilitate the electron transfer process and activate the oxygen double-bond. Also, the catalyst should show preferential surface adsorption of carbon dioxide over water. The selective electrochemical formation of carbonate may be accomplished by the use of; for example, alkaline earth-based pyrochlore oxides, A2B2O7-y. Introduction of alkaline earth metals on the “A” site can yield a pyrochlore catalyst with a high surface basicity, which can lead to the preferential adsorption of CO2 over H2O, since CO2 is a stronger Lewis acid compared to H2O, providing preferential adsorption through Lewis acid-base interactions. This preferential adsorption has been observed for the conversion of NO on CaO (see, e.g., Fliatoura, K. D. et al., J. Catal., 183, 323 (1999)). Therefore, Ca is a feasible candidate for the “A” site of the pyrochlore to attain a high surface basicity. In turn, the “B” site could be used to introduce metals with ORR activity in alkaline media. The introduction of ruthenium in the “B” site has resulted in lead ruthenate pyrochlore and has shown electrochemical activity towards the ORR (see, e.g., Prakash, J. et al., J. Electrochem. Soc., 146, 4145 (1999)). Therefore, a calcium ruthenate pyrochlore, Ca2Ru2O7-y, should have the desired high surface basicity along with ORR activity.
Additionally, it has recently been demonstrated that Ca2Ru2O7-y showed very low resistivity at room temperature (see, e.g., Munenaka, T. et al., J. Phys. Soc. Japan, 75, 103801 (2006)). However, the high temperature and pressure synthesis conditions (about 600° C. and 150 MPa) produced large particles (about 100 μm) and yielded a low surface area, an undesirable property for a fuel cell catalyst. Therefore, it is desired to find a synthesis method which will yield a high surface area calcium ruthenate pyrochlore, followed by evaluation as a selective carbonate catalyst.
Thus, despite efforts to date, a need remains for enhanced electrochemical reactor systems and associated catalyst systems. In particular, alternatives to carbonate based fuel cells that currently operate above 400° C. (and more commonly at or above 650° C.) are desired, as are alternative CO2 conversion devices to address shortcomings of conventional CO2 conversion devices that currently operate at high pressure and elevated temperature, thereby making such devices very expensive to operate. Still further, catalyst systems are needed for use in anion exchange membrane fuel cells that offer highly desirable performance stability. Moreover, a need remains for a low temperature route to convert methane to syngas which reduces industrial cost and provides a transformative technology for the processing of natural gas and biogas to higher order organics. These and other needs are met by the systems, catalysts and methods of the present disclosure.
The present disclosure provides advantageous electrochemical reactors that operate on the carbonate cycle at extremely low temperatures (e.g., less than about 50° C.), thereby allowing operation in as many as three (3) modes, namely as: (i) a room temperature carbonate fuel cell; (ii) an electrochemically assisted CO2 membrane separator; and (iii) a CO2 conversion device. Thus, for all modes of operation, exemplary embodiments of the disclosed electrochemical device operate at (or relatively close to) room temperature and atmospheric pressure. Also, the materials requirements of the disclosed electrochemical reactors are not demanding and device sealing is not an issue. Accordingly, the present disclosure provides a low cost alternative to conventional technologies for any (or all) of the three applications/modes of operation noted above.
The present disclosure further provides an electrocatalyst with the ability to selectively form carbonate anions over hydroxide anions under fully humidified conditions. The ability of the disclosed electrocatalyst to catalyze formation of carbonate anions over hydroxide at room temperature offers many advantages, including much higher stability for next generation anion exchange membrane fuel cells.
Thus, the disclosed electrochemical reactor that is operational at (or relatively close to) room temperature (e.g., from about 15° C. to about 40° C.) provides at least two critical improvements over conventional HEMFC systems. First, the low pKa for the carbonate-bicarbonate equilibrium, Equation 8, will lead to reduced electrolyte degradation by significantly reducing the localized pH at the cathode.
CO3−2+H2OHCO3−+OH−, pKa=10.3 (8)
Second, the disclosed electrochemical device is able to act as a “carbon pump”, essentially purifying atmospheric CO2, which may then be stored, utilized in chemical processes and/or sequestered. Therefore, CO3−2 is an extremely promising replacement ion for OH− in low temperature electrochemical reactors and its use as the charge carrier in the disclosed carbonate fuel cell has the potential to provide enhanced performance and durability at lower cost than both the PEMFC and HEMFC with a net negative CO2 footprint.
The present disclosure provides for an electrochemical reactor including an anode electrically coupled to a cathode; an electrolyte in communication with the anode and the cathode; wherein the anode, cathode and the electrolyte are adapted to operate at a temperature of about 50° C. or less to: (i) produce carbonate anions at the cathode, and (ii) transport the carbonate anions from the cathode to the anode via the electrolyte.
The present disclosure also provides for an electrochemical reactor wherein the anode, cathode and the electrolyte are adapted to operate at about atmospheric pressure to produce and transport the carbonate anions. The present disclosure also provides for an electrochemical reactor wherein the carbonate anions are produced via the following equation:
O2+2CO2+4e−2CO3−2.
The present disclosure also provides for an electrochemical reactor wherein the electrolyte is a substantially solid, polymer electrolyte. The present disclosure also provides for an electrochemical reactor wherein the electrolyte is substantially non-electrically conducting, and includes functional groups that allow for the transport of ions through the functional groups.
The present disclosure also provides for an electrochemical reactor wherein when a fuel is fed to the anode, the fuel is oxidized by the carbonate anions, thereby yielding CO2 and water via the following equation:
2H2+2CO3−2→2CO2+2H2O+4e−.
The present disclosure also provides for an electrochemical reactor wherein the yielded CO2 is emitted from the anode or recycled to the cathode. The present disclosure also provides for an electrochemical reactor wherein the yielded CO2 is separated from the H2O via a separator. The present disclosure also provides for an electrochemical reactor wherein the fuel is hydrogen or alcohol. The present disclosure also provides for an electrochemical reactor further including a catalyst associated with the anode, the catalyst adapted to absorb the produced carbonate anions and oxidize an incoming anode feed.
The present disclosure also provides for an electrochemical reactor wherein the anode feed is oxidized to form dimethyl carbonate or formaldehyde. The present disclosure also provides for an electrochemical reactor wherein the anode, cathode and the electrolyte are adapted to operate at a temperature of about 15° C. to about 40° C. to produce and transport the carbonate anions. The present disclosure also provides for an electrochemical reactor further including a catalyst associated with the cathode, the catalyst adapted to selectively form carbonate anions over hydroxide anions under fully humidified conditions.
The present disclosure also provides for an electrochemical reactor wherein the catalyst preferentially absorbs CO2 over H2O, catalytically activates the O═O bond, and has high electronic conductivity. The present disclosure also provides for an electrochemical reactor wherein the catalyst is tri-functional and is a single compound. The present disclosure also provides for an electrochemical reactor wherein the catalyst is an alkaline earth pyrochlore.
The present disclosure also provides for an electrochemical reactor wherein the catalyst has a molecular structure of A2B2O7-y, and wherein the A and B sites may be individually controlled to tailor the catalytic properties of the catalyst and the oxygen vacancy (y) gives the catalyst conductivity. The present disclosure also provides for an electrochemical reactor wherein an alkaline earth metal is selected from the group consisting of Ca, Mg, Ba and Sr is at the A site.
The present disclosure also provides for an electrochemical reactor wherein a high activity oxygen reduction reaction catalyst in alkaline media is at the B site. The present disclosure also provides for an electrochemical reactor wherein the A and B sites take the form of single components. The present disclosure also provides for an electrochemical reactor wherein the A and B sites take the form of combined components.
The present disclosure also provides for an electrochemical reactor wherein the A site takes the form of a combination of Ca0.5 and Ba1.5. The present disclosure also provides for an electrochemical reactor wherein the B site takes the form of RuPt.
The present disclosure also provides for an electrocatalyst including a pyrochlore having a molecular structure of A2B2O7-y, wherein the A and B sites may be individually controlled to tailor the catalytic properties of a disclosed catalyst, and the oxygen vacancy gives the catalyst conductivity.
The present disclosure also provides for an electrocatalyst wherein the pyrochlore is an alkaline earth pyrochlore. The present disclosure also provides for an electrocatalyst wherein an alkaline earth metal is selected from the group consisting of Ca, Mg, Ba and Sr is at the A site.
The present disclosure also provides for an electrocatalyst wherein a high activity oxygen reduction reaction catalyst in alkaline media is at the B site. The present disclosure also provides for an electrocatalyst wherein the A and B sites take the form of single components. The present disclosure also provides for an electrocatalyst wherein the A and B sites take the form of combined components.
The present disclosure also provides for an electrocatalyst wherein the A site takes the form of a combination of Ca0.5 and Ba1.5. The present disclosure also provides for an electrocatalyst wherein the B site takes the form of RuPt. The present disclosure also provides for an electrocatalyst wherein the catalyst is Ca2Ru2O7-y. The present disclosure also provides for an electrocatalyst wherein the pyrochlore is Ca2Ru2O7-y. The present disclosure also provides for an electrocatalyst wherein the catalyst is Ca1.5Ba0.5PtRuO7-y. The present disclosure also provides for an electrocatalyst wherein the pyrochlore is Ca1.5Ba0.5PtRuO7-y.
The present disclosure also provides for a method of fabricating an electrochemical reactor, the method including: a. providing an anode electrically coupled to a cathode; and b. providing an electrolyte in communication with the anode and the cathode, wherein the anode, cathode and the electrolyte are adapted to operate at a temperature of about 50° C. or less to: (i) produce carbonate anions at the cathode, and (ii) transport the carbonate anions from the cathode to the anode via the electrolyte.
The present disclosure also provides for a method of fabricating an electrochemical reactor wherein the anode, cathode and the electrolyte are adapted to operate at about atmospheric pressure to produce and transport the carbonate anions.
The present disclosure also provides for a method of fabricating an electrochemical reactor further including providing a catalyst associated with the anode, the catalyst adapted to absorb the produced carbonate anions and oxidize an incoming anode feed. The present disclosure also provides for a method of fabricating an electrochemical reactor further including providing a catalyst associated with the cathode, the catalyst adapted to selectively form carbonate anions over hydroxide anions under fully humidified conditions.
The present disclosure also provides for an electrochemical reactor wherein the anode feed is oxidized to form syngas. The present disclosure also provides for an electrochemical reactor wherein the anode feed includes methane or a mixture of methane and carbon dioxide.
The present disclosure also provides for an electrochemical reactor wherein the catalyst is a co-precipitated transition metal oxide:ZrO2 electrocatalyst. The present disclosure also provides for an electrochemical reactor wherein the catalyst is selected from the group consisting of a co-precipitated NiO/ZrO2 composite catalyst, a co-precipitated CoO/ZrO2 composite catalyst and a co-precipitated MnO/ZrO2 composite catalyst.
Additional features, functions and benefits of the disclosed electrochemical reactors and electrochemical catalysts will be apparent from the detailed description which follows, particularly when read in conjunction with the accompanying figures. All references listed in this disclosure are hereby incorporated by reference in their entireties.
To assist those of ordinary skill in the art in making and using the disclosed systems and catalysts, reference is made to the accompanying figures, wherein:
a) and (b) are schematics depicting exemplary electrochemical reactors adapted to function as methane conversion devices according to the present disclosure;
a)-(d) are SEM images of the pyrochlore synthesized through Method 3;
a) and (b) are TPD plots of Pt/C and Ca2Ru2O7-y after exposure to humidified He or CO2;
The present disclosure provides advantageous electrochemical reactors. More particularly, the present disclosure provides for improved electrochemical reactors that operate on the carbonate cycle at extremely low temperatures (e.g., less than about 50° C.), thereby allowing operation in as many as three (3) modes, namely as: (i) a room temperature carbonate fuel cell; (ii) an electrochemically assisted CO2 membrane separator; and (iii) a CO2 conversion device. The present disclosure further provides an electrocatalyst with the ability to selectively form carbonate anions over hydroxide anions under fully humidified conditions. The disclosed systems/catalysts have wide ranging application, e.g., in connection with fuel cells, batteries, heterogeneous transesterification of oils for biodiesel, electrochemically-assisted carbon sequestration, reduction of nitrous oxides (e.g., in automotive pollution prevention), and water treatment and electrolysis.
The present disclosure further provides an electrochemical energy conversion reactor that operates at about room temperature on the “direct” carbonate pathway. In an exemplary device, oxygen and carbon dioxide are fed to the cathode, which are reduced to carbonate anions (Equations 1-2 above). CO3−2 then travels through the membrane electrolyte from the cathode to the anode. At the anode, the carbonate anions oxidize the fuel, (e.g., hydrogen or methanol (Equations 6-7)), yielding water and CO2. Electrons travel through an external circuit, both generating power and completing the electrochemical cell. Though this process is CO2 neutral, the anode effluent is high purity water and CO2, which can be easily separated using conventional methods. Both effluent species can be utilized to advantage, e.g., water for a myriad of applications, such as drinking, and CO2 can be utilized in chemical processing or sequestered. Combining this reactor with a CO2 consuming process or sequestration technology gives it the potential to provide power with a net negative CO2 footprint.
1. Room Temperature Carbonate Fuel Cell
With reference to
In exemplary embodiments, device 100 typically includes a catalyst 150 (e.g., a platinum based catalyst, etc., as described below) that is associated with the anode 103, and a catalyst 130 (e.g., Ca2Ru2O7-y, as discussed below) that is associated with the cathode 102. In general, catalyst 130 is adapted to selectively form carbonate anions over hydroxide anions under fully humidified conditions. Moreover, catalyst 130 preferentially absorbs CO2 over H2O, catalytically activates the O═O bond, and has high electronic conductivity. In exemplary embodiments, catalyst 130 is tri-functional and is a single compound. In one embodiment, catalyst 130 is an alkaline earth pyrochlore. Catalyst 130 may have a molecular structure of A2B2O7-y, wherein the A and B sites may be individually controlled to tailor the catalytic properties of the catalyst 130 and the oxygen vacancy (y) gives the catalyst 130 high electronic conductivity. Exemplary catalysts 130 are discussed in further detail below.
2. Electrochemically Assisted CO2 Membrane Separator
Now with reference to
2H2+2CO3−2→2H2O+2CO2+4e−.
This CO2 (106) can either be emitted, recycled to the cathode 102 or separated from the water (107) via a separator 108, i.e., a condenser, and stored/sequestered. This processing scheme makes the disclosed room temperature carbonate reactor, i.e., carbonate fuel cell 100, a combined power device as well as an electrochemically assisted membrane separator 108. Of note, an alcohol (e.g., methanol, ethanol, ethylene glycol, etc.) fuel can be used in the place of the H2 (105) reactant identified in
As noted above, device 100 typically includes a catalyst 150 that is associated with the anode 103, and a catalyst 130 that is associated with the cathode 102.
3. Low Temperature Methane Conversion Device or CO2 Conversion Device
An exemplary embodiment of an electrochemical reactor 100′ which acts as a methane conversion device is disclosed herein. In general, exemplary electrochemical reactor 100′ utilizes the carbonate anion cycle to convert methane (e.g., natural gas) and/or methane/CO2 mixtures from biogas to syngas at temperatures less than or about room temperature (e.g., at less than about 50° C.). With reference to
½O2+CO2+2e−→CO3−2 (9)
The carbonate anions (101′) travel from the cathode 102′ to the anode 103′ through a humidified anion-conducting polymer membrane electrolyte 104′ (e.g., similar to polymer electrolyte 104 discussed above). Next, the carbonate anions react with methane (115′) at the anode (103), producing syngas (118′), which is illustrated by Equation 10 below:
CH4+CO3−2→CO+2H2+CO2+2e− (10)
In exemplary embodiments, device 100′ includes a catalyst 150′ that is associated with the anode 103′ that adsorbs the carbonate anions 101′ produced at the cathode 102′ and oxidizes an incoming anode feed 125′ (e.g., methane and/or biogas) to produce syngas 118′ at low temperatures (e.g., at or below about 100° C., for example, at or below about 50° C. or about 40° C.). For example, catalyst 150′ may be a platinum based catalyst, or a composite material catalyst, such as a co-precipitated transition metal oxide:ZrO2 composite catalyst or the like as described below (e.g., a MO:ZrO2 electrocatalyst, such as a NiO/ZrO2, CoO:ZrO2 or a MnO:ZrO2 electrocatalyst, or a Co3O4:ZrO2 electrocatalyst or the like). Device 100′ also typically includes a catalyst 130′ (e.g., similar to cathode 130 discussed above) that is associated with the cathode 102′
One advantageous attribute for device 100′ is that the stoichiometry suggests an H2:CO ratio slightly higher than 1, which is generally desirable for Fischer-Tropsch reactions to higher order organics (see, e.g., Choudhary, V. et al., Catal. Let., 32, 391 (1995)). Another advantageous feature for the disclosed electrochemical reactor 100′ is the overall consumption of CO2 (106′), which may come from, e.g., the atmosphere, combustion waste streams and/or biogas.
With reference to
Thus, the total cell reaction for the room temperature electrochemical methane (or biogas) to syngas reactor/device 100′ may be depicted by Equation 11 as:
With respect to this total cell reaction, at least four items are notable. First, this process would not be possible in conventional systems due to the extremely high enthalpy of combustion of methane (115′) (e.g., about 800 kJ/mol). Second, only a small applied voltage (e.g., about 0.44 V) is necessary for this reaction to proceed at approximately 25° C., shown in Equation 11. For comparison, this is significantly lower than the voltage required for both water and CO2 electrolysis for which the theoretical voltages are both approximately 1.2 V, although typical operating voltages are much greater than about 2.5 V (see, e.g., Whipple, D. T. et al., Prospects of CO2 Utilization Via Direct Heterogeneous Electrochemical Reduction, The Journal of Physical Chemistry Letters, 1, 3451-3458 (2010)). Third, the cell voltage is positive in the exemplary process of the present disclosure, indicating that power is extracted from the cell, which is in contrast with other syngas synthesis methods. Fourth, in a conventional chemical system, CH4 (115′) is weakly adsorbed, leading to low surface coverage and limited interaction with the catalyst material and adsorbed precursors and/or intermediates. In exemplary electrochemical cell 100′, the positive surface potential relative to the potential of zero charge decreases the free energy, thereby giving the catalyst surface a more electron-withdrawing character. Thus, when gas phase CH4 (115′) approaches the catalyst of device 100′, partial electron transfer of a valence π electron from the C atom in methane (115′) to the catalyst is facilitated by a surface free energy shift, which increases the adsorption energy. This significantly increases the surface coverage of methane (115′) and creates opportunities for oxygen (117′) attack from CO3−2 (101′) due to weakened C—H bonding.
In another exemplary embodiment and as depicted in
4. Room Temperature Carbonate Electrocatalyst
According to the present disclosure, an advantageous catalyst is provided that: (i) preferentially adsorbs CO2 over H2O at very low temperatures (<50° C.); (ii) catalytically activates the O═O bond; and (iii) has high electronic conductivity. The disclosed catalyst is generally tri-functional in nature and is a single compound. In exemplary implementations, the catalyst is an alkaline earth pyrochlore. However, beyond the exemplary alkaline earth pyrochlore catalyst disclosed herein, it is further contemplated that alternative chemistries as well as composites of two or more materials could be used to simultaneously achieve the three (3) advantageous properties and/or functionalities described above.
Pyrochlores generally have the structure A2B2O7-y. The A and B sites may be individually controlled to tailor the catalytic properties of the disclosed catalyst, while the oxygen vacancy (y) gives the catalyst electronic conductivity. For example, alkaline earth metals (Ca, Mg, Ba and Sr) may be used at the ‘A’ site and high activity oxygen reduction reaction (ORR) catalysts in alkaline media (Ru, Pt, Ag, W) may be used at the ‘B’ site. In addition, the ‘A’ and/or ‘B’ sites may take the form of single components or combined components, e.g., the ‘A’ site could take the form of Ca0.5 and Ba1.5 and/or the ‘B’ site could take the form of RuPt. As will be readily apparent to persons skilled in the art, various combinations may be implemented at the ‘A’ and/or ‘B’ sites to achieve desired catalytic properties and performance.
An exemplary catalyst according to the present disclosure—Ca2Ru2O7-y—generally works well with even about 1% CO2 in the cathode stream. Mixed site materials, e.g., Ca1.5Ba0.5PtRuO7-y, are also possible and logical extensions of the exemplary catalysts disclosed herein.
The ability to adsorb CO2 over water is generally caused by the alkaline nature of the surface (CO2 is a stronger Lewis acid than water). This gives the catalyst its high selectivity for the carbonate pathway (Equation 4 above) over the hydroxide pathway (Equation 3 above). The hydroxide pathway is typically preferred on all other known catalysts.
5. Experimentation Protocols
The following testing protocols were implemented to demonstrate that: i) alkali earth oxides with the pyrochlore structure (A2B2O7) can selectively form carbonate anions in an alkaline electrochemical reactor; ii) operating the alkaline electrochemical reactor with carbonate anions reduces the degradation of state-of-the-art quaternary ammonium functionalized membranes compared with operation on the hydroxide cycle; and iii) H2 and methanol can be electrochemically oxidized on Pt electrocatalysts by CO3−2.
In such experimentation, the selective electrochemical formation of carbonate anions may be accomplished using calcium-based alkaline earth oxide pyrochlores (Ca2Ru2O7, Ca2Pt2O7 and Ca2W2O7) for the reduction of O2 with CO2. These oxides were selected based on the high surface basicity of raw alkaline earth oxides (such as CaO), which have been previously utilized in several applications (see, e.g., Fliatoura, K. D. et al., Selective Catalytic Reduction of Nitric Oxide by Methane in the Presence of Oxygen over CaO Catalyst, Journal of Catalysis, 183 (1999) 323; Hess, C. et al., NO2 Storage and Reduction in Barium Oxide Supported on Magnesium Oxide Studied by in Situ Raman Spectroscopy, J. Phys. Chem. B., 107 (2003) 1982; Choudhary, V. R et al., Simultaneous Carbon Dioxide and Steam Reforming of Methane to Syngas over NiO—CaO Catalyst, Ind. Eng. Chem. Res., 35 (1996) 3934; Broqvist, P. et al., Toward a Realistic Description of NOx Storage in BaO: The Aspect of BaCO3, J. Phys. Chem. B., 109 (2005) 9613; Xie, S. et al., Catalytic Reactions of NO over 0-7 mol% Ba/MgO Catalysts: II. Reduction with CH4 and CO, Journal of Catalysis, 188 (1999) 32; Snis, A. et al., Catalytic Decomposition of N20 on CaO and MgO: Experiments and ab Initio Calculations, J. Phys. Chem. B., 102 (1998) 2555; Park, S. et al., Storage of NO2 on potassium oxide co-loaded with barium oxide for NOx storage and reduction (NSR) catalysts, Journal of Molecular Catalysis A: Chemical, 273 (2007) 64; Reddy, C. et al, Room-Temperature Conversion of Soybean Oil and Poultry Fat to Biodiesel Catalyzed by Nanocrystalline Calcium Oxides, Energy & Fuels, 20 (2006) 1310; Yan, S. et al., Supported CaO Catalysts Used in the Transesterification of Rapeseed Oil for the Purpose of Biodiesel Production, Energy & Fuels, 22 (2008) 646; Liu, X. et al., Transesterification of soybean oil to biodiesel using CaO as a solid base catalyst, Fuel, 87 (2008) 216; Gryglewicz, S., Alkaline-earth metal compounds as alcoholysis catalysts for ester oils synthesis, Applied Catalysis A: General, 192 (2000) 23; Dissanayake, D. et al., Oxidative Coupling of Methane over Oxide-Supported Barium Catalysts, Journal of Catalysis, 143 (1993) 286; Wang, Y. et al., Effective Catalysts for Conversion of Methane to Ethane and Ethylene Using Carbon Dioxide, Chemistry Letters, (1998) 1209; Wang, H. et al., CaO—ZrO2 Solid Solution: A Highly Stable Catalyst for the Synthesis of Dimethyl Carbonate from Propylene Carbonate and Methanol, Catalysis Letters, 105 (2005) 253; Liu, Z. et al., Effect of basic properties of MgO on the heterogeneous synthesis of flavanone, Applied Catalysis A: General, 302 (2006) 232; Wang, H. et al., Influence of preparation methods on the structure and performance of CaO—ZrO2 catalyst for the synthesis of dimethyl carbonate via transesterification, Journal of Molecular Catalysis A: Chemical, 258 (2006) 308; Breysse; E. et al., Addition of hydrogen sulfide to methyl acrylate over solid basic catalysts, Journal of Catalysis, 233 (2005) 288; Bhanage, B. et al., Synthesis of dimethyl carbonate and glycols from carbon dioxide, epoxides, and methanol using heterogeneous basic metal oxide catalysts with high activity and selectivity, Applied Catalysis A: General, 219 (2001) 259; Choudhary, V. et al., Epoxidation of styrene by TBHP to styrene oxide using barium oxide as a highly active/selective and reusable solid catalyst, Green Chemistry, 8 (2006) 689; Marino, F. et al., Supported base metal catalysts for the preferential oxidation of carbon monoxide in the presence of excess hydrogen (PROX), Applied Catalysis B: Environmental, 58 (20050 175; Jimenez, R. et al., Soot combustion with KlMgO as catalyst: II Effect of K-precursor, Applied Catalysis A: General, 314 (2006) 81; Tarnai, T. et al., Dichlorodifluoromethane Decomposition to CO2 with Simultaneous Halogen Fixation by Calcium Oxide Based Materials, Environ. Sci. Technol., 40 (2006) 823; McCaffrey, E. F. et al., Kinetic studies of the catalytic activity of alkaline earth oxides in 2-propanol decomposition, 1. Phys. Chem., 76 (1972) 3372; and Catlow, C. et al., Computation Studies of Solid Oxidation Catalysts, 1. Phys. Chem., 94 (1990) 7889) as well as the ability for Pt, Ru and W to electrochemically activate the O═O bond in alkaline media (see, e.g., Munenaka, T. et al., A Novel Pyrochlore Ruthenate: Ca2Ru207, Journal of the Physical Society of Japan, 75 (2006) 103801; Horowitz, H. S. et al., New oxide pyrochlores: A2[B2-xAx]07-y(A=Pb, Bi; B=Ru, Ir), Mat. Res. Bull., 16 (1981) 489; Horowitz, H. S. et al., Oxygen Electrocatalysis on Some Oxide Pyrochlores, Journal of the Electrochemical Society, 130 (1983) 1851; Widelov, A. et al., Electrochemical and Surface Spectroscopic Studies of Thin Films of Bismuth Ruthenium Oxide (Bi2Ru207), Journal of the Electrochemical Society, 143 (1996) 3504; Kohoul, A. et al., A Sol-Gel Route for the Synthesis of Bi2Ru207 Pyrochlore Oxide for Oxygen Reaction in Alkaline Medium, Journal of Solid State Chemistry, 161 (2001) 379; Prakash, J. et al., Investigations of ruthenium pyrochlores as bifunctional oxygen electrodes, Journal of Applied Electrochemistry, 29 (1999) 1463; and Prakash, J. et al., Kinetic Investigations of Oxygen Reduction and Evolution Reactions on Lead Ruthenate Catalysts, Journal of the Electrochemical Society, 146 (1999) 4145). The high surface basicity of the Ca2M2O7 pyrochlores was expected to lead to preferred adsorption of CO2 over H2O, which was observed by Fliatoura and co-workers for the conversion of NO on CaO (see, e.g., Fliatoura, K. D. et al., Selective Catalytic Reduction of Nitric Oxide by Methane in the Presence of Oxygen over CaO Catalyst, Journal of Catalysis, 183 (1999) 323). This preferred adsorption of CO2 was expected to encourage the reaction to occur preferentially through the “direct” pathway, yielding a high selectivity electrocatalyst.
At the anode (e.g., anode 103 of
Three methods were studied to synthesize pyrochlore-structured calcium ruthenium oxides. Method 1 involved high temperature solid-state reaction of oxide salts and both Method 2 and Method 3 were low temperature hydrothermal routes.
In Method 1, CaO (Reagent Grade) and RuO2 reaction precursors were used. RuO2 was synthesized in-house by heating RuCl3 (ReagentPlus) at about 800° C. in air for about six (6) hours. Calcium oxide and ruthenium oxide were mixed and ground with a mortar and pestle in approximately a 2:1 mol ratio (Ca:Ru). The mixed salts were formed into about 13 mm diameter pellets using a pellet dye and press (Carver, Inc). Subsequently, the pellet was heat treated in air to several temperatures between about 100° C. and 1100° C.
Method 2 was a modification of the hydrothermal route developed by Horowitz and coworkers for the synthesis of lead ruthenate pyrochlores (see, e.g., Horowitz, H. S. et al., Mater. Res. Bull., 16, 489 (1981)). Here, KOH (ACS reagent grade) was dissolved in 18MS) Millipore water to make about 1M, 0.1M and 0.01M solutions. Subsequently, CaO and RuCl3 were added in various Ca:Ru molar ratios and stirred vigorously for approximately 20 minutes. The solution was heated between about 55° C. and 95° C. while oxygen was bubbled for about 24-72 hours. The precipitate was filtered, washed with deionized water several times and dried overnight at about 80° C.
In Method 3, KMnO4 was used as a replacement for oxygen in the hydrothermal synthesis. Using KMnO4 allowed the solution to be refluxed at elevated temperatures while also providing a more strongly oxidizing environment than dissolved O2. About 1M KOH solutions was prepared using Millipore water. About 1 mM and 10 mM potassium permangante solutions were prepared by adding KMnO4 to the 1M KOH solutions. CaO and RuCl3 were solvated in approximately a 1:1 molar ratio and thoroughly mixed at about 80° C. for around 20 minutes. Then, the solution was heated to about 200° C. and maintained isothermally under reflux for about 12 to 120 hours. The resulting precipitate was filtered, washed with deionized water and dried overnight at about 80° C.
Products obtained by all synthesis methods were characterized using XRD. XRD patterns were obtained by scanning from about 10° to 90° (20) at a scan rate of approximately 2°/min in a Bruker D8 advance diffractometer (Cu—Kα radiation, λ=0.15418 nm). A Micromeritics ASAP 2020 system was used to collect N2 adsorption/desorption data at about 77 K. To remove adsorbed impurities prior to experimentation, the sample was degassed at about 200° C. and about 10 μmHg for approximately ten (10) hours. The BET specific surface area was calculated using the N2 adsorption isotherm between relative pressures (P/Po) of about 0.002 and about 0.05. SEM images were taken using a FEI Strata 400 s SEM.
Temperature programmed desorption (TPD) was performed by placing approximately 100 mg of the calcium ruthenate pyrochlore or platinum supported on carbon (10% Pt/C, BASF) in a Thermolyne 79300 tube furnace (ThermoFisher). All gases used were ultra high purity (Airgas). Prior to each experiment, the furnace tube (25 mL) was purged with 20 mL/min dry helium for about two (2) hours and the sample was pretreated by heating to about 500° C. Background TPD was obtained by heating from about 25° C. to 1000° C. at a rate of approximately 5° C./min while continuously purging with He. The effluent from the tube furnace was continuously analyzed with a QMS100 mass spectrometer (Stanford Research Systems). Afterwards, the sample was exposed to CO2 or humidified He for about two (2) hours followed by purging with dry He for about two (2) hours. Finally, the sample was heated from about 25° C. to 1000° C. at a rate of approximately 5° C./min while continuously purging with He and analyzing the effluent from the tube furnace using the QMS-100.
Fuel cell experiments were carried out with a Scribner 850e Fuel Cell Test Station. Humidified hydrogen and nitrogen were used as the anode gases, while humidified oxygen and carbon dioxide mixtures were used as the cathode feed. All gases were ultra high purity and obtained from Airgas. Experiments were carried out at a cell temperature of about 50° C. Catalyst inks were prepared by suspending in dimethylformamide (DMF) either commercial 10% Pt/C (BASF) or NiO nanoparticles synthesized using a room-temperature NaOH-induced precipitation method described elsewhere for the anode (see, e.g., Spinner, N. et al., Electrochim. Acta, 56, 5656 (2011)), and the Ca2Ru2O7-y pyrochlore prepared using synthesis Method 3, described above with respect to the cathode (e.g., cathode 102 of
Ex-situ electrochemical experiments were carried out by depositing a thin film of the Ca2Ru2O7-y pyrochlore on a 5 mm diameter glassy carbon (GC) rotating disk electrode (RDE). Thin-film working electrodes were prepared using a sonicated suspension of the Ca2Ru2O7-y pyrochlore in water (0.3 mg/mL). 20 μL of the suspension was placed on the GC electrode and dried, followed by the addition and drying of 20 μL of 100× diluted Nation DE520 dispersion. A platinum foil several times larger than the working electrode was used as the counter electrode. A saturated calomel electrode (SCE) was used as the reference electrode and all potentials are reported with respect to the SCE. The 1M KOH electrolyte was prepared by dissolving reagent grade KOH pellets in 18 MΩ Millipore deionized (DI) water.
Linear sweep voltammetry experiments were conducted in a custom-built three compartment jacketed glass cell (Adams & Chittenden Scientific Glass) with a Luggin capillary. The counter electrode was separated from the working electrode by a fitted glass separator. All electrodes were rinsed with DI water and fresh electrolyte before experimentation. Prior to experimentation, the electrolyte was purged with N2 for about one (1) hour, followed by about one (1) hour of O2 bubbling to ensure saturation. All experiments were thermostated at about 25±0.1° C. Polarization and cyclic voltammetry (CV) data was collected with an Autolab PGSTAT302N potentiostat. The RDE rotation speed was controlled with an AFMSRCE analytical rotator (Pine Instrument Company).
a. Carbonate Selective Oxygen Reduction Catalysts
The alkaline earth pyrochlores (AEPs) were prepared and characterized via a hydrothermal route with single phases of the raw alkaline earth oxide (CaO) and “B” metal oxide particles (see, e.g., Munenaka, T. et al., A Novel Pyrochlore Ruthenate: Ca2Ru207, Journal of the Physical Society of Japan, 75 (2006) 103801). The approximate elemental composition of the resulting catalysts was estimated using a cold cathode field emission scanning electron microscope with an integrated energy dispersive X-ray spectrometer (EDS). Phase identification and nanoparticle size was measured using XRD. Surface chemistry and bond formation analysis was determined by XPS.
The electrocatalytic activity of the resulting electrocatalysts, as well as their selectivity for the “direct” carbonate pathway, will be elucidated below in discussion of the experimentation results. The electrochemical experiments were executed in a custom-built three electrode electrochemical cell with a Luggin capillary. Here, the AEPs were deposited as a thin film electrode onto a 5 mm diameter glassy carbon disk-type working electrode. A Pt foil was used as the counter electrode and an Hg/HgO alkaline electrode was used as a reference. All electrochemical measurements were made with an Autolab PGSTAT302N potentiostat.
The electrochemical stability and passivation resistance of the catalysts were investigated in N2-saturated 0.1 M KOH and 0.1 M Na2CO3/NaHCO3 aqueous electrolytes using cyclic voltammetry. Voltammograms were obtained by cycling the working electrode potential several times between about −0.8 and 0.62 V vs. NHE. The activity of the AEP catalysts toward the hydroxide pathway can be elucidated using rotating disk type electrodes (RDE) immersed in O2 saturated KOH solutions. The RDE system was ideal because of its well defined hydrodynamics, which are controlled by the electrode rotation rate. This allowed a subtraction of mass transfer effects from experimental data, yielding pure kinetic information. Here, slow scan, about 1 mV/s, voltammograms were collected at electrode rotation rates of approximately 400, 900, 1600 and 2500 RPM at about 25° C. between approximately −0.4 and 0.6 V vs. NHE. Due to the difficulty in determining the electrochemical carbonate formation in water, activity of the AEO electrocatalyst was also collected in O2/CO2 saturated acetonitrile electrolytes with 0.1 M NaHCO3/Na2CO3. In dry acetonitrile, reduction of O2 occurred exclusively with CO2 through the carbonate pathway.
b. Stability and Ionic Transport of Polymer Electrolytes
Several high ion exchange capacity (>1.0 meq/g) anion exchange membranes are commercially available. Six such membranes were selected for this experimentation: MA-3475 from Sybron Chemicals, AMI-7001 from Membranes International, Ralex AMH-PAD from Mega AS, Neosepta from Alstom, AMB-SS from Resin Tech, and Tokuyama A006 from Acta Nanotech. All of the listed commercially available films were delivered in their chloride form. Therefore, the first step in electrolyte preparation was to convert the membrane to either its hydroxide or carbonate form by ion exchange in either 0.1 M KOH or 0.1 M Na2CO3 for about 24 hours. Conversion to the hydroxide and carbonate forms was confirmed by FTIR. Then, the ion exchanged films were soaked in 1.0 M OH−,10−4 M OH− or 1.0M Na2CO3/1.0M NaHCO3. Film samples were extracted after about 1, 2, 5, 10, 20 and 30 days in order to observe the change in bonding over time. Active sites undergoing nucleophillic attack showed a decrease in the N—C bond stretch at around 1200 cm−1. It was further contemplated that because the nitrogen valence was lowered, a positive shift in wave number would also be observed. For films undergoing the Hoffman elimination, an increased C═C bond peak around 1650 cm−1 was expected.
As membrane degradation progressed, the ionic conductivity of the resulting electrolyte was expected to decrease due to the elimination of ion exchange sites within the polymer film. In this experimentation, the ionic conductivity of the polymer films was determined under fully humidified conditions by placing the membrane in a custom two compartment conductivity cell with fixed electrode areas and separation. The conductivity was determined by electrochemical impedance spectroscopy immediately following ion exchange and then at identical time intervals to the FTIR experiments.
With reference to
Now with reference to
c. Hydrogen and Methanol Oxidation with Carbonate Anions
The adsorption of carbonate anions was expected to proceed without obstacle on metallic surfaces; however, not all surfaces have the ability to readily facilitate C—O bond cleavage and show sufficiently high oxygen surface mobility. These are critical properties for methanol oxidation catalysts in the presence of carbonate and, thus, candidate catalysts were chosen with these as the primary design criteria: Pt, PtRu, Raney Ni and NiO. Further, the oxidation of H2 and CH3OH with CO3−2 on Pt was investigated. Pt was selected from the candidates due to its bonding, surface chemistry and electrochemical activity being the best understood and documented among the candidates, providing a reliable point for comparison. The experimental setup for electrochemical characterization of Pt is identical to the one that was used for the cathode studies. A polished 5 mm diameter Pt disk working electrode (Pine Instrument Company) was utilized. All anode characterization experiments were conducted in 0.1 M NaHCO3/0.1 M Na2CO3 and 10−4 M KOH/0.25M NaClO4 aqueous alkaline electrolytes thermostated at about 25° C. in the custom built three electrode cell. Hydrogen was introduced to the system by bubbling H2 across the working electrode and methanol (0.5 M) was directly mixed in the electrolyte. The data from both dilute hydroxide and concentrated carbonate solutions was combined to yield an accurate description of the oxidation activity with carbonate.
d. Room Temperature Carbonate Fuel Cell
To show that these studies lead to the development of an electrochemical reactor that operates with high efficiency and is at the very least competitive with current state-of-the-art HEMFC and PEMFC in terms of both performance and cost, a 5 cm2 cell was constructed and electrochemically characterized. Electrochemical measurements were conducted with a Scribner and Associates fuel cell test station with an 850E load box. Linear sweep polarization between the open circuit voltage (OCV) and about 0.2 V provided a baseline performance curve and high throughput technique to characterize these laboratory scale cells. Then, chronoamperometric experiments were used in order to obtain steady state polarization measurements at about 10 mV intervals between the OCV and about 0.2 V, which gave a more accurate representation of the true performance of the fuel cell under various loads. Chronoamperometry performed at about 0.6 V was used in order to observe the performance stability of the electrochemical reactor. Finally, electrochemical impedance spectroscopy was used in this experimentation in order to determine the Ohmic and charge transfer resistances for the electrolyte and electrocatalysts, respectively, between about 25 and 80° C.
e. Low Temperature Methane Conversion Device or Co2 Conversion Device
In testing the exemplary methane conversion devices or CO2 conversion devices, several device configurations will be implemented. For example and as similarly depicted in
In another proposed exemplary configuration (e.g., as similarly illustrated in
5. Experimental Results
With reference to the above-described experimental studies, experimentation results with respect to the disclosed electrochemical reactor device and electrocatalyst of the present disclosure have been obtained and are set forth in
a. Verification of the Carbonate Cycle
In this experimentation, analysis of the anode effluent was used to confirm operation on the carbonate cycle. There are three main routes through which CO2 may be present in the anode exhaust. First, diffusional crossover through the electrolyte could happen due to the CO2 concentration gradient between the cathode and anode. The extent of CO2 crossover through this path can be easily measured by imposing a concentration gradient between the two electrodes, maintaining the cell at open circuit voltage (OCV) and confirming the presence of CO2 at the anode. Second, during FC operation, the presence of CO2 at the cathode and its contact with the electrolyte can lead to the formation of carbonate anions by thermodynamic equilibrium between water, hydroxide, bicarbonate and carbonate, as was discussed with respect to Equations 3-4 above. This “indirect route” or “hydroxide route” for carbonate formation would be the primary source of carbonate anions for operation with a non-carbonate selective cathode catalyst. Third, carbonate can be electrochemically formed at the cathode by “direct route” electroreduction of O2 and CO2, as was discussed with respect to Equations 1-2 above. For both the direct and indirect pathways, the resulting carbonate anions carry the charge from cathode to anode where they oxidize H2 by Equation 6.
Recent work has suggested that conventional Pt catalysts can produce some CO3−2 anions through the direct pathway when CO2 is added to the AEMFC cathode feed (see, e.g., Vega, J. A. et al., Electrochim. Acta, 55, 1638 (2010); Unlu, M. et al., Electrochem. Solid State, 12, B27 (2009); and Lang, C. M. et al., Electrochem. Solid State, 9, A545 (2006)). However, Pt shows quite low selectivity for the direct carbonate pathway and much of the carbonate production with Pt catalysts occurs through the indirect route. This has been previously confirmed where partial carbonation of the membrane electrolyte occurs independent of operating conditions (see, e.g., Siroma, Z. et al., Electrochem. Soc., 158, B682 (2011); Kizewski, J. et al., ECS Trans., 33, 27 (2010); and Watanebe, S. et al., ECS Trans., 33, 1837 (2010)). The nature of these two pathways suggests that the quantity of CO2 measured in the anode effluent with a carbonate selective catalyst is notably higher than a cell operating with Pt at the cathode. Unfortunately, quantifying the contribution of each pathway has proven difficult and a reliable method to accomplish this is yet to be reported in the prior art, and is not attempted here.
Production of CO2 at the anode electrode was first confirmed by flowing the anode effluent through a 0.01 M calcium hydroxide solution while performing chronopotentiometric (CE) experiments at about 1 mA/cm2 for approximately two (2) hours using a Pt/C anode.
Ca(OH)2+CO2→CaCO3+H2O (12)
Before each experiment, the cell was flushed with reaction gases for about 30 minutes to ensure the anode compartment was free of ambient carbon dioxide. The cell was left at open circuit with H2 as the anode feed and O2/5% CO2 as the cathode feed. In this case, no CaCO3 precipitate was observed during the approximately two (2) hour experiment time, which indicated negligible CO2 diffusional crossover through the membrane.
Next, CE experiments using H2 as a fuel, coupled with CaCO3 precipitation, were performed. The following four experiments were performed, varying the exchanged membrane anion and the cathode feed: (1) membrane exchanged to OH−, O2 cathode feed and Pt/C as cathode catalyst; (2) membrane exchanged to CO32−, O2 cathode feed and Pt/C as cathode catalyst; (3) membrane exchanged to CO32−, O2/CO2 cathode feed and Pt/C as cathode catalyst; (4) membrane exchanged to CO32−, O2/CO2 cathode feed and Ca2Ru2O7-y as cathode catalyst. The variations of exchanged membrane anion and cathode feed for the four experiments is summarized in Table 1 below:
aAll feeds were humidified
bCO2 content was 5%
For Experiment 1, as expected, no CaCO3 precipitation was observed, demonstrating operation on the hydroxide cycle. During Experiment 2, initial precipitation of CaCO3 was observed. However, the precipitation slowed with time and completely stopped after approximately 30 minutes of operation and no further precipitation was observed for the remainder of the experiment. This demonstrated that the carbonate initially present in the membrane was able to carry the charge through the electrolyte and subsequently consumed at the anode through H2 oxidation (Equation 6). During this time, CO3−2 was slowly flushed and the membrane was exchanged to its hydroxide form while operating on the hydroxide cycle. For Experiments 3 and 4, CaCO3 precipitation was observed throughout the experiment, which confirmed the continuous production of carbonate. For these experiments, the amount of the charge carried by carbonate was estimated by drying and weighing the CaCO3 precipitate after each experiment.
For the CE experiments, the theoretical quantity of CO2 that should be formed at the anode assuming 100% operation on the carbonate cycle is described by Equation 13 below:
where i is the current, t is the time, n is the electron equivalence and F is Faraday's constant. Meanwhile, the number of moles of CO2 produced during the CE experiments can be calculated by Equation 14 below:
where MCaCO3 is the measured mass of precipitated CaCO3 and MWCaCO3 is its molecular weight. Finally, the selectivity for carbonate formation can be defined as the portion of charge carried by CO3−2 divided by the portion of charge carried by OH−, which can be calculated for both catalysts using Equation 15 below:
After each CE experiment, the CaCO3 precipitate was dried at about 100° C. overnight before its mass was measured. Table 2 below shows the results obtained with both Pt/C and Ca2Ru2O7-y cathode catalysts.
The results of Table 2 depict that more CO2 was evolved from the anode when Ca2Ru2O7-y was used as the cathode catalyst, which suggests the preferential formation of carbonate on this catalyst compared with Pt/C. The calculated selectivity for Pt/C was about 1.78, while for Ca2Ru2O7-y, the selectivity was about 7.33. This amounted to an approximately 4.1 times increase in carbonate selectivity using Ca2Ru2O7-y compared to Pt/C, which was most likely a product of increased adsorption of CO2 versus H2O on Ca2Ru2O2-y that was facilitated by its high surface basicity.
One obvious limitation to this selectivity calculation is in its inability to distinguish contributions of the direct and indirect pathway. This is particularly important for the Pt catalyst as it is likely that a large portion of the effluent CO2 was a product of the indirect route (see, e.g., Kizewski, J. et al., ECS Trans., 33, 27 (2010)). Thus, the selectivity gain in Table 2 is likely much lower than its true value. Despite this limitation, it is clear that a significantly larger percentage of CO3−2 was formed by the direct pathway when Ca2Ru2O7-y was employed at the cathode. However, the development of new experimental protocols that can deconvolute the contributions of the direct and indirect pathway would assist in truly quantifying this effect.
Further, operation on the carbonate cycle was also confirmed by constructing cells with Ca2Ru2O7-y at the cathode and a carbonate electrolyzing catalyst, NiO, at the anode. With reference to
4OH−(aq)→O2(g)+H2O+4e− (16)
2CO3 (aq)−2→O2(g)+2CO2(g)+4e− (17)
In turn, the increase in performance with the presence of CO2 at the cathode suggests enhanced kinetic performance and the preferential formation of carbonate.
The anode effluent for cells maintained at −2V was analyzed using a mass spectrometer to identify the gaseous species present. With reference to
b. Room Temperature Carbonate Fuel Cell Performance
Different ratios of oxygen to carbon dioxide were used in the cathode stream to observe the effect of CO2 concentration on RTCFC performance when Ca2Ru2O7-y was used as a cathode catalyst.
Further additions of CO2 above 10% caused the performance to gradually decrease, as can be seen in
c. Cyclic Voltammetry
Ex-situ CV was used to investigate the electrochemical stability and activity of the Ca2Ru2O7-y catalyst in the potential window relevant for the oxygen reduction reaction, −1.2 to 0.25V vs. SCE. With respect to
The working electrode was cycled approximately 300 times between about −1.2 and 0.3V to determine the electrochemical stability of the catalyst. Minimal changes in the electrochemical response were observed from the second to the 300th cycle. Also, the minimal change in current magnitude indicated negligible changes in physical or electrochemical aspects of the catalyst, i.e., surface roughening and electrochemically active area. These results suggest electrochemical stability of the catalyst over a wide potential window as well as chemical stability in alkaline media. A pressing limitation of state-of-the-art Pt catalysts is the loss of electrochemical activity during potential cycling due to Pt agglomeration or catalyst support corrosion (see, e.g., Shrestha, S. et al., Catal. Rev., In Press). However, the data depicted in
The activity of Ca2Ru2O7-y for oxygen reduction through the hydroxide and carbonate pathways was investigated using the RDE technique. With respect to
Both lines have a similar shape with a sharp onset at −0.2 V vs. SCE followed by a gradual increase in negative current towards the mass transport limiting current. This shape was reproducible over many experiments and may be attributed to low activity intermediates and/or surface adsorption/blocking, leading to complex behavior. However, when CO2 was added to the electrolyte, as represented by the dotted line in
The current response of a RDE is governed by the Koutecky-Levich equation, shown in Equation 18 below:
where i is the experimentally observed current, ik is the kinetic current and iL is the mass transport limited current. In turn, the kinetic current is described by the Butler-Volmer equation, shown in Equation 19 below, and is a function of the electrode potential:
log(i)k=log(io)+bE (19)
where io is the exchange current, E is the electrode potential and b is a constant dependent on temperature. Further, the kinetic current can be calculated from experimental RDE data using Equation 120 below:
where id is the mass transport limited current. In Equation 20, it is assumed that ohmic losses are negligible in the solid and electrolyte phases.
Turning now to
aη = overpotential
With reference to
Therefore, it appears that the carbonate ORR is favored compared the traditional hydroxide ORR on Ca2Ru2O7-y. Consequently, this could create a localized low alkalinity environment within a fuel cell, extending the membrane life and maintaining stable long term performance (see, e.g., Vega, J. A. et al., J. Power Sources, 195, 7176 (2010)). This result, combined with the improved kinetics for hydrogen oxidation with carbonate anions compared to hydroxide anions (see, e.g., Vega, J. A. et al., J. Electrochem. Soc., 158, B349 (2011)), could lead to improved performance of an AEMFC operating on the carbonate cycle, instead of the hydroxide cycle.
d. X-ray Diffraction
The solid state reaction of base oxide precursors is the most common route for the synthesis of various pyrochlores (see, e.g., Ashcroft; A. T et al., J. Phys. Chem., 97, 3355 (1993); Beck, N. K. et al., Fuel Cells, 6, 26 (2006); Konishi, T. et al., Top. Catal., 52, 896 (2009); Sellami, M. et al., J. Alloy Compd., 493, 91 (2010); Uno, M. et al., J. Alloy Compd., 400, 270 (2005); Zhang, F. et al., Mater. Lett., 60, 2773 (2006); and Koteswara, K. et al., Spectrochim. Acta Part A, 66, 646 (2007)). Thus, Method 1, discussed above, initiated investigations on the synthesis of Ca2Ru2O7-y.
With reference to
With further reference to
Turning now to
With further reference to
It has been previously determined that changes in the A:B cation ratio may lead to different crystal phases depending on which element is in excess (see, e.g., Wu, X. et al., J. Mater. Sci. Lett., 16, 1530 (1997)). Several Ca:Ru molar ratios were investigated in this study, including the following: 2:1, 1.5:1, 1:1, 1:1.5 and 1:2. For reactions with an excess of calcium, unreacted calcium oxide was readily removed by washing with deionized water. Excess calcium did not have an effect on the product obtained, whose XRD pattern was analogous to curve (c) in
In addition, variation of the precursor bath pH had an effect at lower levels of alkalinity. At approximately pH=13, the product showed a crystalline XRD reflection at about 29.5°. However, the peak intensity was considerably decreased, by approximately 50%, compared to the precipitate obtained at about pH=14. This suggests a decrease in the extent of reaction, since a secondary phase present in small proportions may exhibit a lower intensity or even escape detection (see, e.g., Aleshin, E. et al., J. Am. Ceram. Soc., 45, 18 (1962)). A further decrease in pH to about 11 or 12 yielded a completely amorphous precipitate, again with an XRD pattern analogous to curve (a) in
Now turning to
The results depicted in
With reference to
Still in reference to
The oxidizing environment strength also affected the extent of reaction and the crystallinity of the precipitate. A decrease in the permanganate concentration to 1 mM during reaction yielded a precipitate with a low degree of crystallization. Therefore, it can be understood that both high temperature and a strong oxidizing environment are needed for the synthesis of crystalline Ca2Ru2O7-y.
Turning now to
e. Microstructural Characterization
With respect to
Turning now to
where c is the BET constant that provides a measure of adsorbent-adsorbate interaction energy, P is the equilibrium pressure, Po is the saturation pressure at the temperature of adsorption, ν is the adsorbed gas quantity, and νm is the volume of an adsorbed N2 monolayer. Using Equation 21, the monolayer capacity (νm) was calculated to be around 40 cm3/g, which was very close to the volume adsorbed at 0.05 P/Po (39 cm3/g) validating our choice of 0.05 P/Po as the “Point B”. Thus, the calculated BET surface area of the Ca2Ru2O7-y pyrochlore was approximately 174 m2/g, a high surface area considering that it is unsupported, making it feasible for catalytic applications. It is also at least one order of magnitude higher compared to other pyrochlores found in the literature used for electrochemical applications (see, e.g., Konishi, T. et al., Top. Catal., 52, 896 (2009); Bang, H. J. et al., Electrochem. Commun., 2, 653 (2000); and Kahoul, A. et al., J. Solid State Chem., 161, 379 (2001)). The external surface area, determined from the linear region of t-plot using Harkins and Jura parameters (see, e.g., Harkins, W. D. et al., J. Am. Chem. Soc., 66, 1366 (1944)) was approximately 162 m2/g. This suggests a limited contribution of micropore area (12 m2/g) to the total surface area. However, Harkins and Jura derived their parameters on TiO2, and a standard more similar in surface properties to the synthesized Ca2Ru2O7-y may yield a more definitive determination of the relative micropore and the external surface areas.
f. Temperature Programmed Desorption
An essential characteristic of a carbonate selective electrocatalyst must be the preferential adsorption of carbon dioxide over water. TPD is a common method to identify molecules physically and chemically adsorbed on the surface of a catalyst and determine their adsorption energies (see, e.g., Punyawudho, K. et al., Langmuir, 27, 3138 (2011) and Punyawudho, K. et al., Langmuir, 27, 7524 (2011)). In general, molecules that desorb at a higher temperature have a higher desorption activation energy (Ea) or a lower adsorption Ea. Here, TPD was performed after exposure of Pt/C or Ca2Ru2O7-y to water or carbon dioxide.
With reference to
a) further shows a clear peak for both adsorbed H2O and CO2 on Pt/C. The H2O peak was observed at around 80° C., while the CO2 peak materialized at approximately 71° C. In addition, the H2O peak was broader than the CO2 peak, which indicates a higher quantity of adsorbed water of this catalyst compared to CO2. Thus, Pt catalysts preferentially adsorb water over carbon dioxide. In turn, this suggests that during fuel cell operation, Pt/C would favor Equation (3) over Equation. (5) and an AEMFC would operate primarily on the hydroxide cycle, rather than the carbonate cycle. This phenomenon has been observed in AMEFCs operating with Pt/C as the cathode catalyst and O2/CO2 mixtures in the cathode stream (see, e.g., Vega, J. A. et al., Electrochimica Acta, 55, 1638 (2010) and Unlu, M. et al., Electrochem. Solid State, 12, B27 (2009)) and has been discussed in detail above.
Turning now to
The reason for this preferential adsorption of CO2 in Ca2Ru2O7-y may be the presence of calcium, an earth alkaline metal, which gives this pyrochlore a high surface basicity. Since CO2 is a stronger Lewis acid, compared to H2O, this molecule is preferentially adsorbed due to acid-base interactions. This makes Ca2Ru2O7-y a promising initial candidate for a carbonate selective catalyst.
g. Low Temperature Methane Conversion Device or CO2 Conversion Device
By way of background, room temperature electrochemical reactors operating on the carbonate anion cycle utilizing polymer electrolyte membranes have been proposed as a response to the low chemical stability of commercial anion exchange membranes in the presence of OH-(see, e.g., Lang, C. M. et al., High-Energy Density, Room-Temperature Carbonate Fuel Cell, Electrochemical and Solid State Letters, 9, A545-A548 (2006) and Varcoe, J. R. et al., Prospects for Alkaline Anion-Exchange Membranes in Low Temperature Fuel Cells, Fuel Cells, 5, 187-200 (2005)). Since then, additional work has confirmed that carbonate-exchange membranes have high ion exchange capacity and are generally stable (see, e.g., Vega, J. A. et al., Effect of hydroxide and carbonate alkaline media on anion exchange membranes, Journal of Power Sources, 195, 7176-7180 (2010); Adams, L. A. et al., A Carbon Dioxide Tolerant Aqueous-Electrolyte-Free Anion-Exchange Membrane Alkaline Fuel Cell, ChemSusChem, 1, 79-81 (2008); Zhou, J. et al., Anionic polysulfone ionomers and membranes containing fluorenyl groups for anionic fuel cells, Journal of Power Sources, 190, 285-292 (2009); and Tones, C. I. et al., Carbonate Species as OH− Carriers for Decreasing the pH Gradient Between Cathode and Anode in Biological Fuel Cells, Environmental Science and Technology, 42, 8773-8777 (2008)). However, practical operation of room temperature carbonate devices requires the selective formation of carbonate (Equation 9) over OH− (Equation 22) at the cathode under fully humidified conditions.
½O2+H2O+2e−→OH− (22)
An advantageous finding of the disclosed exemplary electrochemical reactors, as discussed above, is the design and discovery of the first and only catalyst—Ca2Ru2O7—that is shown to selectively form carbonate electrochemically at or about room temperature under fully humidified conditions (see, e.g., Vega, J. A. et al., Carbonate Selective Ca2Ru2O7-y Pyrochlore Enabling Room Temperature Carbonate Fuel Cells—Part I. Synthesis and Physical Characterization, J. Electrochem. Soc., In Press, DOI: 10.1149/2.028201jes; and Vega, J. A. et al., Carbonate Selective Ca2Ru2O7-y Pyrochlore Enabling Room Temperature Carbonate Fuel Cells—Part II. Verification of Carbonate Cycle and Electrochemical Performance, J. Electrochem. Soc., In Press, DOI: 10.1149/2.029201jes). Ca2Ru2O7 possesses a carbonate selectivity, shown by Equation 23 below, of approximately 7.33, suggesting that about 88% of the reacted O2 is converted to CO3−2. This compares favorably to Pt, whose selectivity in other preliminary experiments has been estimated to be less than about 0.5. The enhanced activity for Equation 9 over Equation 22 is also shown in
Another advantageous finding is that the hydrogen oxidation reaction is kinetically favored in carbonate media compared with hydroxide (see, e.g., Vega, J. A. et al., Hydrogen and Methanol Oxidation Reaction in Hydroxide and Carbonate Alkaline Media, Journal of the Electrochemical Society, 158, B349-B354 (2011)). It has been found that oxidation reactions with carbonate anions have low free energy intermediates generally due to the thermodynamic favorability of CO2 formation from CO3−2 on Pt. This is a positive result, indicating that CO3−2 may generally be an efficient oxygen donating species for electrochemically activating methane, as illustrated in Equation 10.
A further advantageous finding is that polymer membranes exchanged to the carbonate form are extremely durable. Five commercially available membranes were investigated and showed no measurable reduction in ionic conductivity or chemical degradation over a 30 day period. Generally, this is in contrast to hydroxide exchanged membranes, whose mechanical integrity was compromised and conductivity decreased by an approximate range of 6-27% over the same span (see, e.g., Vega, J. A. et al., Effect of hydroxide and carbonate alkaline media on anion exchange membranes, Journal of Power Sources, 195, 7176-7180 (2010)). The conductivity of CO3−2 through the polymer membranes is approximately 50% of that of OH−. However, it can typically be raised by preparing lower molecular weight polymer electrolytes that possess higher ion exchange capacity and allow enhanced mobility of the anion.
An alternative advantageous finding is that carbonate anions can be used at the anode as a high efficiency oxygen donator to oxidize incoming feeds other than hydrogen. Most notably, the preliminary data shows that a coprecipitated NiO/ZrO2 composite catalyst facilitates Equation 10, thereby electrochemically oxidizing methane to syngas at approximately 40° C. This coprecipitated catalyst is designed to satisfy several criteria (see, e.g., Spinner, N. et al., Effect of Nickel Oxide Synthesis Conditions On Its Physical properties and Electrocatalytic Oxidation of Methanol, Electrochimica Acta, 56, 5656 (2011)). One criteria is that the coprecipitated catalyst should have electrocatalytically active sites, thereby being electrically conductive. Another criteria is that the coprecipitated catalyst should have the ability to adsorb carbonate and transport it to the active sites. In exemplary embodiments of the present disclosure, NiO and ZrO2 were selected as candidates to fill these needs, respectively.
With reference to
In order to identify the product formed from this new oxidation reaction, room temperature electrochemical reactors (e.g., devices 100′ of
The anode effluent gas was analyzed using mass spectrometry. Turning now to
However, the exemplary embodiments of the CO2 conversion device 100′ are not limited to the disclosed anode 103′ electrode materials. In particular, electrocatalysts 150′ for the oxidation of methane to syngas generally are required to meet several criteria. First, the catalyst 150′ generally should have a methane active center, i.e., so that CH4 is both adsorbed and electrochemically activated on the surface. In addition, as CO3−2 is the charge-carrying/transfer species in the system, it typically needs to be adsorbed and have improved surface mobility. Surfaces with a slightly alkaline character generally facilitate carbonate adsorption through Lewis acid/base interactions while the high surface mobility will allow adsorbed CO3−2 and CH4 to intimately interact. Further, the molecular, not dissociative, adsorption of C—O containing species typically should be thermodynamically favored. This will not only ensure that methane will accept an oxygen atom from carbonate, it also generally ensures that the resulting carbon monoxide will not be further oxidized at low overpotentials, thereby providing a large operating window for the reactor.
Three exemplary catalyst materials that have demonstrated reactivity with methane, although at elevated temperatures, have improved electronic conductivity at or about room temperature, i.e., ideal for electrochemical applications, and the ability to adsorb short chain organics while having poor C—O bond cleavage activity are as follows: (i) NiO, which has been utilized to collect successful preliminary data; (ii) CoO; and (iii) MnO (see, e.g., Zafeirator, S. et al., Methanol oxidation over model cobalt catalysts: Influence of the cobalt oxidation state on the reactivity, Journal of Catalysis, 269, 309-317 (2010); Zhang, X. et al., Catalytic conversion of methane to methanol over Lanthanum-Cobalt-Oxide supported Molybdenum based catalysts, Prepr. Pap. Am. Chem. Soc., Div. Fuel Chem., 48, 837-838 (2003); Mann, R. S. et al., Oxidation of Methanol Over Manganese Dioxide-Molybdenum Trioxide Catalyst, Industrial and Engineering Chemistry Process Design and Development, 9, 43-46 (1970); and Samant, P. V. et al., Nickel-modified manganese oxide as an active electrocatalyst for oxidation of methanol in fuel cells, Journal of Power Sources, 79, 114-118 (1999)). On all three catalysts, the surface molecular adsorption of CO is preferred in their neat form with an M+2 (e.g., a transition metal “M”, such as, for example, M=Ni, Co, Mn) oxidation state. However, on all three catalysts, a transition to M+3 is typically required for the oxidation reaction. This reversible transition was observed for NiO, as illustrated in
On the other hand, these materials have generally not shown sufficient surface alkalinity to adsorb carbonate anions. One catalyst that has proven carbonate activity is ZrO2 and it is hypothesized that ZrO2 is able to facilitate the methane conversion reaction in preliminary data by providing a CO3−2 adsorption center, while NiO provides the methane active site (see, e.g., Jung, K. T. et al., An in Situ Infrared Study of Dimethyl Carbonate Synthesis From Carbon Dioxide and Methanol Over Zirconia, Journal of Catalysis, 204, 339-347 (2001)). This may suggest that all three transition metal oxide:ZrO2 electrocatalysts (e.g., MO:ZrO2) are generally active in converting methane to syngas. However, ZrO2 typically has a low electronic conductivity and large quantities may not be incorporated into the catalyst. Thus, to maximize the contact interface between the transition metal oxide (e.g., MO) and ZrO2, thereby increasing catalyst utilization and allowing for minimal inclusion of the non-conductive ZrO2, a coprecipitation route that was developed to synthesize NiO:ZrO2 composites may also be used to synthesize CoO:ZrO2 and MnO:ZrO2 (see, e.g., Spinner, N. S. et al., Effect of Nickel Oxide Synthesis Conditions On Its Physical Properties and Electrocatalytic Oxidation of Methanol, Electrochimica Acta, submitted (2011)).
6. Experimental Results Summary
As discussed in greater detail above, synthesis of Ca2Ru2O7 was investigated using both solid-state and hydrothermal methods. Heating of precursor oxide salts, CaO and RuO2, at temperatures up to approximately 1100° C. led to the formation of a perovskite phase. A low temperature O2 hydrothermal route led to the formation of a low crystallinity pyrochlore phase though the bulk of the precipitate was an amorphous material which consists mainly of RuO2. A third synthesis method was employed, using KMnO4 as an oxidizing agent. The permanganate hydrothermal synthesis led to the formation of a highly crystalline calcium ruthenate pyrochlore.
Using XRD, it was shown that the pyrochlore was thermally stable and the reaction had a high yield. Therefore, Ca2Ru2O7-y was successfully synthesized at moderate temperatures and low pressures. The material had a unique morphology and small particle size compared to other pyrochlores. Further, high surface area was obtained, likely due to the small particle size and the formation of nanocrystallites on the surface of the particles. TPD also showed the preferential adsorption of H2O versus CO2 in a Pt/C catalyst. However, CO2 was preferentially adsorbed in the Ca2Ru2O7-y pyrochlore, compared to H2O, making it a feasible candidate for a carbonate selective catalyst.
The experimentations discussed above show that pyrochlores can be obtained through low temperature, low pressure synthesis routes. The use of a strong oxidizing agent created the environment required to control and maintain the high ruthenium oxidations states necessary for the formation of the crystal. This creates a valuable synthesis method, allowing for high surface area materials with unique properties that would be beneficial for the use of high surface area pyrochlores as heterogeneous catalysts. The introduction of an alkaline earth metal in the structure allowed the tailoring and increase of the surface basicity. This characteristic led to the preferential adsorption of CO2 over H2O, an essential requirement for a carbonate selective catalyst.
Further, the experimentations discussed above explored the electrochemical activity of a Ca2Ru2O7-y pyrochlore and its selectivity towards carbonate formation. The presence of carbon dioxide at the anode was demonstrated by precipitation of CaCO3 from a Ca(OH)2 solution. The selectivity of Ca2Ru2O7-y), for carbonate formation was demonstrated to be considerably higher than Pt/C. Also, mass spectra of the anode effluent showed a considerable increase in the CO2 quantity when CO2 was present at the cathode, suggesting selective carbonate formation. Fuel cell experiments were performed with O2 or O2/CO2 on the cathode stream to confirm operation on the carbonate cycle. A considerable increase in performance was observed when CO2 was added to the cathode stream, specifically up to concentrations of 10%. However, further additions of CO2 were matched with gradual reduction in fuel cell performance. This was attributed to the high surface basicity of the pyrochlore combined with relatively low electrochemical activity, which causes disproportionate CO2 adsorption during reaction, hindering the O2 adsorption required for optimal performance by O2 site blocking. Thin-film electrodes in O2-saturated alkaline electrolytes were used to demonstrate its electrochemical stability within the oxygen reduction region. Addition of CO2 to the electrolyte caused an increase in current, suggesting preferential carbonate formation. Tafel plots showed higher kinetic performance when CO2 is present on the electrode surface.
The results of these experimentations depict the potential of a Ca2Ru2O7-y pyrochlore to electrochemically produce carbonate with high selectivity, instead of hydroxide, therefore enabling RTCFCs. This property can be attributed to the high surface basicity of this catalyst, which led to the preferential adsorption of CO2 instead of H2O. Further improvements may be attained by tailoring the catalyst competition to obtain optimal surface basicity and electrochemical activity. Also, optimization of MEA, as well as the use of a carbonate conducting ionomer, would significantly improve device performance.
Although the present disclosure has been described with reference to exemplary embodiments and implementations, it is to be understood that the present disclosure is neither limited by nor restricted to such exemplary embodiments and/or implementations. Rather, the present disclosure is susceptible to various modifications, enhancements and variations without departing from the spirit or scope of the present disclosure. Indeed, the present disclosure expressly encompasses such modifications, enhancements and variations as will be readily apparent to persons skilled in the art from the disclosure herein contained.
The present application claims priority to a provisional patent application entitled “Electrochemical Reactor for CO2 Conversion, Utilization and Associated Carbonate Electrocatalyst,” filed with the U.S. Patent and Trademark Office on Nov. 5, 2010, and assigned Ser. No. 61/410,614. The entire content of the foregoing provisional patent application is incorporated herein by reference.
The United States government may hold license and/or other rights in this invention as a result of financial support provided by governmental agencies in the development of aspects of the invention. Parts of this work were supported by a grant from the National Science Foundation, Grant No. CBET-1005303.
| Number | Date | Country | |
|---|---|---|---|
| 61410614 | Nov 2010 | US |
