Patent Application
20230366109
Hydrogen has been proposed as an energy carrier in a sustainable energy future. When coupled with fuel cells, the energy content is converted on demand and with high efficiency into useful work, with water being the only emission generated at the point of use. However, hydrogen has a low volumetric energy density, a low flash-point, and lacks a wide infrastructure for its storage and transport. These challenges have caused a reconsideration of early commitments of national governments to hydrogen fuel cells. (Bullis, K. (2009). Q & A: Energy Secretary Steven Chu. In MIT Technology Review.) Thus, realization of the potential environmental benefits of fuel cell technology is likely to rely on the identification of viable solutions to hydrogen storage and delivery. Ammonia has recently been suggested as an ideal candidate to act as a hydrogen vessel. (Soloveichik, G. (2016). UCLA Luskin Conference Center, Los Angeles, CA: NH3 Fuel Association; Klerke, A. et al., (2008) Journal of Materials Chemistry 18, 2304-2310; Rouwenhorst, K. H. R. et al., (2019) Renewable & Sustainable Energy Reviews 114; and Lamb, K. E. et al., International Journal of Hydrogen Energy 44, 3580-3593.) Ammonia is lightweight, less flammable than hydrogen, easily liquefiable, commercially produced at high volume, and can make use of an existing transportation infrastructure. Furthermore, although most ammonia production today utilizes hydrogen derived from natural gas and hence contributes to green-house gas emissions, cycling between stored hydrogen in ammonia and retrieved hydrogen can, in principle, be done without producing additional emissions.
The retrieval of hydrogen stored in ammonia is described by the decomposition reaction:
This reaction is mildly endothermic at standard conditions, and under standard pressure it proceeds spontaneously at temperatures greater than 183° C. Achieving high conversion, however, requires high temperatures, typically above about 400° C., to overcome the twin challenges of thermodynamic limitations and kinetic barriers. Residual ammonia in the fuel stream resulting from incomplete conversion is, in turn, highly detrimental to polymer electrolyte membrane fuel cells, the catalysts of which can tolerate no more than ˜0.1 ppm NH3. (Uribe, F. A. et al., (2002). Journal of the Electrochemical Society 149, A293-A296; and Miyaoka, H. et al., (2018). International Journal of Hydrogen Energy 43, 14486-14492.) As an alternative to high temperature thermal decomposition, electrochemical decomposition of ammonia holds potential for production of high purity hydrogen at near ambient conditions and with high conversion rates. To date, electrocatalytic approaches, which have largely employed aqueous alkali electrolytes, have required high operating potentials, implying poor energy efficiency, and have suffered from catalyst deactivation over time. (Modisha, P. et al., (2016). International Journal of Electrochemical Science 11, 6627-6635; and Vitse, F. et al., (2005). Journal of Power Sources 142, 18-26.) Accordingly, innovations in ammonia-to-hydrogen conversion are required if ammonia is to provide hydrogen on demand and serve as a flexible energy delivery medium.
Electrochemical cells for the production of hydrogen from fuels and methods of operating the cells to produce hydrogen and electricity are provided. One embodiment of an electrochemical cell includes: a catalyst layer that includes a thermochemical conversion catalyst; an electrooxidation layer that includes a hydrogen oxidation catalyst adjacent to the catalyst layer; a proton conducting membrane that includes a solid acid electrolyte adjacent to the electrooxidation layer; a hydrogen evolution layer that includes a hydrogen evolution catalyst that is separated from the electrooxidation layer by the proton conducting membrane; and a circuit that provides a path for electrons generated in the electrooxidation layer to travel to the hydrogen evolution layer.
Illustrative embodiments of the invention will hereafter be described with reference to the accompanying drawings, wherein like numerals denote like elements.
Electrochemical cells for the production of hydrogen from fuels and methods of operating the cells to produce hydrogen and electricity are provided. The electrochemical cells are solid state cells that incorporate a thermochemical conversion catalyst and a hydrogen oxidation catalyst into the anode and utilize solid acid electrolytes. This hybrid thermal-electrochemical cell design integrates thermally driven chemical conversion of the starting fuel with electrochemical removal of the hydrogen from the conversion reaction zone. The cells are able to produce hydrogen that is free from residual fuel, do not require solvents or high operating pressures, and, because the cells are able to convert fuels internally, eliminate the need for an external cracker.
The electrochemical cells can be used to produce hydrogen (H2) from a variety of hydrogen-containing fuels. Some embodiments of the cells are designed to produce hydrogen from ammonia at relatively low operating temperatures, including temperatures below 300° C. Other fuels that can be used include alcohols, such as methanol and ethanol, formic acid, and dimethyl ether. Further, hydrogen can be produced from hydrogenated hydrocarbons such as methylcyclohexane, perhydro-dibenzyl-toluene, propanol-2, and perhydro-N-ethylcarbazole, which “dehydrogenate” into toluene, dibenzyl-toluene, acetone, and N-ethylcarbazole, respectively.
One embodiment of an electrochemical cell is shown in
When a hydrogen atom-containing fuel is fed into a catalyst layer, the thermochemical conversion catalyst catalyzes a reaction that converts the fuel into H2 and one or more additional products. For example, when a hydrogen atom-containing fuel is fed into a catalyst layer that includes a thermal cracking catalyst, the catalyst catalyzes the decomposition (“cracking”) of the fuel into H2 and one or more additional decomposition products. If ammonia is used as a fuel, the thermochemical decomposition catalyst decomposes the ammonia into H2 and N2. The selection of the thermochemical conversion catalyst will depend upon the fuel being used. Examples of thermochemical conversion catalysts that can be used for the conversion of ammonia and other hydrogen atom-containing fuels include ruthenium (Ru), rhodium (Rh), iridium (Ir), nickel (Ni), platinum (Pt), palladium (Pd), gold (Au), silver (Ag), copper (Cu), zinc (Zn), and iron (Fe), and combinations thereof. Metal nitrides and metal carbides of transition metals, such as Fe, cobalt (Co), Ni, titanium (Ti), vanadium (V), manganese (Mn), and chromium (Cr) can also be used.
Examples of ruthenium-based catalysts for thermochemical decomposition of ammonia are described in Hill et al., International Journal of Hydrogen Energy 39, 7646-7654; Hill et al., Applied Catalysis B-Environmental 172, 129-135; Li et al., Nano Research 11, 4774-4785; Mukherjee et al., Applied Catalysis B-Environmental 226, 162-181; and Yin et al., Applied Catalysis a-General 277, 1-9, the contents of which are incorporated herein by reference for the purpose of providing additional examples of thermochemical decomposition catalysts.
In addition to the thermochemical conversion catalyst, the catalyst layer may include a support for the catalyst and/or a promoter. Examples of supports include carbon nanotubes (CNTs), activated carbon, and particles of metal/metalloid oxides, such as Al2O3, SiO2, TiO2, and ZrO2. The promoters are substances that are mixed with the thermochemical decomposition catalyst to improve the efficiency of the catalyst. They may do so by a variety of mechanisms. For example, promoters may prevent crystal sintering. Examples of promoters that can be used with ruthenium and other thermochemical conversion catalysts include alkali, alkaline earth, and rare earth metals, such as cesium (Cs), potassium (K), barium (B), sodium (Na), lithium (Li), cerium (Ce), and lanthanum (La).
The catalyst layer may include an electronically conductive component, such as particles of carbon, such as carbon black, metallic carbon nanotubes (CNTs), and graphite, and particles of metal, to provide, or enhance, electrical conductivity through the catalyst layer, wherein the thermochemical conversion catalyst may be coated on or dispersed with the electronically conductive component. In some embodiments, high electrical conductivity through the catalyst layer aids in the overall rate of hydrogen production.
For non-ammonia fuels, the thermochemical conversion catalyst will catalyze chemical reactions other than ammonia decomposition, and so it can be referred to more generally as a catalyst layer (see layer 6 of
CO+H2O→CO2+H2 (2)
This reaction is particularly important because carbon monoxide is often produced alongside hydrogen in the reforming reactions of carbon-based fuels, such as methanol, dimethyl ether, and formic acid. Therefore, the ability of the solid acid electrochemical cell to produce hydrogen directly from the carbon-based fuel, as well as the water-gas-shift reaction of any carbon monoxide produced in the steam reforming of such fuels, improves the overall efficiency of the hydrogen generation process.
The H2 formed in the catalyst layer flows into the electrooxidation layer, while unused fuel and other conversion products exit the cell. The electrooxidation catalyst catalyzes the oxidation of the H2 to generate hydrogen ions (Hf) and electrons. Examples of hydrogen oxidation catalysts include noble metals, such as Pt and palladium (Pd). However, other catalysts, such as non-noble metal catalysts, including Ni, and bimetallic catalysts can also be used. Conductive supports, such as particles of carbon black and/or proton conducting metal phosphates, may also be included in the electrooxidation layer, wherein the hydrogen oxidation catalyst may be coated on or dispersed in the support material. It should be noted that, as used herein, the term “adjacent” does not require adjacent layers to be in direct physical contact; although in some embodiments, the layers are in direct physical contact. Adjacent layers may be next to one another but separated by intervening layers of material that do not alter or impede the respective layers from carrying out their intended functions and that do not significantly impede the flow of reactants through the electrochemical cell. For example, a thin layer of material may be inserted between the layers to provide structural integrity and/or to simplify cell assembly.
The hydrogen ions generated in the electrooxidation layer are selectively passed through the solid-state proton conducting membrane, while the electrons travel to the hydrogen evolution layer via a connecting circuit (e.g., wire). Examples of solid-state proton conducting membrane materials include solid acids having a stable superprotonic phase at the intended operating temperature of the electrochemical cell, for example, in the temperature range from 180° C. to 300° C. Superprotonic phases are characterized by orientationally-disordered acidic oxyanion groups and high protonic conductivities, including conductivities of 1×10'S cm−1 or greater. Metal phosphates and, in particular, cesium phosphates, such as cesium dihydrogen phosphate, are examples of solid acids that can have stable superprotonic phases. Descriptions of other proton conducting solid acids, including metal phosphates, can be found in Haile et al, Faraday discussions 134 (2007): 17-39 and in U.S. Pat. No. 8,202,663, the disclosures of which are incorporated herein for the purpose of providing additional specific examples of solid acids that can be used as solid-state electrolytes.
The proton conducting materials used in the proton conducting membranes can also be used as proton conducting support materials in the electrooxidation layer and the hydrogen evolution layer.
The hydrogen ions that flow through the proton conducting membrane and into the hydrogen evolution layer are catalytically reduced by the hydrogen evolution catalyst to form H2, which then passes out of the electrochemical cell. Examples of hydrogen evolution catalysts include noble metals, such as Pt and palladium. However, other catalysts, such as non-noble metal catalysts, including Ni, and bimetallic catalysts can also be used. Conductive supports, such as particles of carbon black and/or proton conducting metal phosphates, may also be included in the hydrogen evolution layer, wherein the hydrogen evolution catalyst may be coated on or dispersed in the support material.
This example illustrates a hybrid thermal-electrochemical approach to the ammonia conversion reaction at an intermediate temperature of 250° C., with the aim of simultaneously addressing the NH3 impurities in the hydrogen produced by high-temperature thermochemical decomposition and the low conversion efficiency of ambient temperature electrolysis. Cs-promoted Ru/CNT was employed as the thermochemical decomposition catalyst. A cell based on the proton conducting electrolyte, cesium dihydrogen phosphate (CDP), a solid acid compound that is non-reactive with NH3 was employed as the electrochemical component. The electrocatalyst was Pt, which has high tolerance to fuel impurities at the operation temperature of 250° C. (for example, up to 20% CO), suggesting electrochemical functionality even in the presence of residual NH3. By integrating the thermochemical decomposition with electrochemical removal of hydrogen from the reaction zone, thermodynamic limitations otherwise imposed by product accumulation were overcome.
The overall configuration of the ammonia decomposition cells is presented in
Using three distinct cells to assess reproducibility, leakage through the electrolyte membrane was first checked for by measuring the open circuit voltage (OCV) with dilute H2 supplied to the working electrode. The recorded voltages of 72, 73, and 73 mV are consistent with the value of 73 mV implied by the Nernst equation.
The agreement between the Nernst equation and measured values demonstrated not only the absence of gas leaks, but also the high ionic transference number of CDP. The electrochemical characteristics were then assessed under open circuit conditions by impedance spectroscopy. The measured ohmic losses of 0.24-0.26 Ωcm2 were comparable to the expected value of 0.25 Ωcm2 for the 50 μm thick electrolyte with conductivity of 2.0×10−2S/cm at 250° C.
Polarization curves obtained under ammonia flow revealed excellent activity for ammonia decomposition,
The voltages obtained under open circuit conditions were 78±1 and 68±4 mV (as averaged across the three cells), for the respective ammonia partial pressures of 0.4 and 0.6 atm. Inverting the Nernst relationship, these voltages imply hydrogen partial pressures at the working electrode of 0.019 and 0.033 atm, respectively. From this, respective chemical ammonia-to-hydrogen conversion rates of 3.4±0.1 and 3.5±0.7% were computed at the two ammonia concentrations.
Away from open circuit conditions, the current rose under both ammonia and dilute hydrogen with a relatively low overall cell resistance, indicating rather moderate and similar overpotentials. Because the hydrogen partial pressures were similar between the three conditions (pH2=0.024, 0.019 and 0.033 atm, respectively, in dilute hydrogen, and at OCV in dilute and concentrated ammonia), the similarities in IV characteristics indicate that poisoning of the Pt electrocatalyst by unreacted NH3 was negligible. This was further corroborated by the impedance results, which indicate similar electrochemical reaction resistance for supply of dilute H2 and of NH3 under OCV conditions, with electrochemical reaction resistances ranging from 0.16 to 0.19 Ωcm2.
With increasing current and voltage, the IV curves deviated from linearity and from one another. In the case of dilute hydrogen, the IV curve plateaued relatively sharply at a current density corresponding to ˜90% of the limiting value, consistent with the supposition that depletion of hydrogen was responsible for the declining rate of increase in cell current density, and that H2O electrolysis did not occur under these conditions. Under ammonia, the IV curves followed a much more gradual change in slope. Substantially higher current densities were achieved using 0.6 rather than 0.4 atm pNH3. This behavior is consistent with electrochemical oxidation of ammonia being the source of the current. The resulting increase in current density, and hence hydrogen production rate, was, however, accompanied by a decrease in conversion efficiency,
In the absence of the thermal cracking catalyst layer,
It is shown here that by integrating electrochemical product removal with thermal decomposition of ammonia, it is possible to generate hydrogen at a substantially higher rate than by thermal decomposition alone. To put the present results into context, the hydrogen production rates achieved here are compared, on a catalyst-mass normalized basis, to those from conventional thermal-cracking experiments reported in the literature,
CDP powder was exposed to flowing humidified NH3 (pNH3=0.4 atm, pH2O=0.38 atm, balance N2) at a total gas flow rate of 50 sccm at 250° C. for 24 h. During the heating to the exposure condition, the gas supply was started after the sample reached a temperature of 150° C., and similarly on cooling the gas supply was stopped at this temperature. Diffraction patterns collected before and after NH3 exposure were identical.
The catalyst for the TCL was prepared following the polyol method in which ethylene glycol (Fisher Chemical, >95% purity) serves to reduce a metal salt precursor (RuCl3·4.5H2O, Alfa Aesar, 99.9% metals basis). (Kurihara, L. K. et al., Nanostructured Materials 5, 607-613.) The Ru loading on the multi-walled CNTs (NanoLab, >95% purity) was fixed at 60 wt. %, at which the ˜30 nm diameter CNTs were fully coated with Ru nanoparticles. Cesium promotion was achieved by dispersing the Ru/CNT into a 50 mM aqueous solution of CsNO3 (Alfa Aesar, 99.9%) with 1:1 molar ratio of Ru:Cs. The water was gently evaporated to induce precipitation of the nitrate. To promote uniformity, the powder was dispersed in ethanol and the solvent evaporation was repeated. Cells were fabricated using 53.4 mg of the Cs-promoted Ru/CNT material. The Ru crystallite size was 7 nm as determined by transmission electron microscopy imaging and X-ray powder diffraction. On the basis of thermogravimetric analysis, it can be concluded that crystalline CsNO3 obtained from the synthesis was decomposed to CsOH under H2, in a reaction that was apparently catalyzed by metallic Ru. As has been suggested in the literature, it is likely the decomposition process places CsOH in near proximity to the Ru. (Larichev, Y. V. et al., (2007). Journal of Physical Chemistry C 111, 9427-9436; and Aika, K.-i. (2017). Catalysis Today 286, 14-20.)
Three cells, 0.75″ in diameter, were fabricated and evaluated for ammonia decomposition. The EL was comprised of Pt/carbon (20 wt. % Pt on carbon black, HiSPEC® 3000, Alfa Aesar) and CDP (SAFCell) in a 1:6 mass ratio, as described in previous works (in which this component served as a hydrogen oxidation electrode). (Papandrew, A. B. et al., (2011). Chemistry of Materials 23, 1659-1667; and Lim, D. K. et al., (2018) Electrochimica Acta 288, 12-19.) For both TCL and EL components, 25 mg was used, resulting in respective Ru and Pt loadings of 10.3-11.1 and 0.5 mg/cm2 over the active cell area of 1.34-1.45 cm2. For ease of fabrication, a layer of carbon fiber paper (Toray, TGP-H-030) was placed between the two catalytic layers. The CDP electrolyte layer was 50 mg in mass and fully densified to yield a thickness of 50 μm. The hydrogen evolution (counter) electrode had the same formulation as the electrocatalyst in the working electrode and resulted in an additional 0.5 mgpt/cm2 in the complete cell. For the purpose of assessing the role of individual components, two analogous additional cells were fabricated, the first in which the Ru-based TCL was omitted, and the second in which the Pt-based EL was omitted. In the latter case, 60 wt. % Ru/CNT, prepared as described above, was combined with CDP in a 1:6 mass ratio to serve as a direct ammonia oxidation electrocatalyst. Because of reactivity between CDP and most Cs salts, no additional promotor was applied.
Electrochemical measurements (BioLogic, SP-300) were performed at 250° C. at a scan rate of 10 mV/s. Gas streams supplied to the anode and cathode were humidified (with steam partial pressure, pH2O, of 0.38 atm) to prevent dehydration of the CDP electrolyte. Humidified hydrogen (pH2=0.62 atm) was supplied to the counter electrode, and either humidified ammonia at one of two concentrations (pNH3=0.4 or 0.6 atm) or dilute humidified hydrogen (pH2=0.024 atm), balanced by a mixture of Ar and Nz, was supplied to the working electrode. The total gas flow rates at both electrodes were 50 sccm (standard cubic centimeters per minute) for all conditions. These flow rates imply limiting current densities for the 0.4 atm and 0.6 atm NH3 fed cells of 3.0-3.2 A/cm2 and 4.5-4.8 A/cm2, respectively, based on the cell active areas and the hydrogen content of the supplied ammonia. Under supply of dilute Hz, the limiting currents were 247-267 mA/cm2. Faradaic efficiency measurements were performed under similar conditions, but with humidified N2 (pH2O=0.38 atm) supplied to the counter electrode so as to ensure detection of only electrochemically evolved hydrogen and avoid drift of a high baseline in the mass spectrometer (Thermostar Pfeiffer GSD 301 T2) used for evolved gas chemical analysis.
Polarization curves were obtained for a 12-cell solid acid hydrogen stack running on H2, methylcyclohexane (MCH), and methanol (MeOH) (
The word “illustrative” is used herein to mean serving as an example, instance, or illustration. Any aspect or design described herein as “illustrative” is not necessarily to be construed as preferred or advantageous over other aspects or designs. Further, for the purposes of this disclosure and unless otherwise specified, “a” or “an” can mean only one or can mean “one or more.” Both embodiments are covered.
The foregoing description of illustrative embodiments of the invention has been presented for purposes of illustration and of description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed, and modifications and variations are possible in light of the above teachings or may be acquired from practice of the invention. The embodiments were chosen and described in order to explain the principles of the invention and as practical applications of the invention to enable one skilled in the art to utilize the invention in various embodiments and with various modifications as suited to the particular use contemplated. It is intended that the scope of the invention be defined by the claims appended hereto and their equivalents.
The present application claims priority to U.S. provisional patent application No. 63/088,111 that was filed Oct. 6, 2020, the entire contents of which are incorporated herein by reference.
This invention was made with government support under C2017.0013//DE-AR0000813 awarded by the Department of Energy. The government has certain rights in the invention.
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/US21/53466 | 10/5/2021 | WO |
| Number | Date | Country | |
|---|---|---|---|
| 63088111 | Oct 2020 | US |
