Patent Application
20150221445
This disclosure relates to photoelectrochemical fuel cells that utilize light and photoactive materials to achieve complete oxidation of carbon-containing fuels.
Commercially available fuel cells, which typically use heat or electrochemical overpotentials to overcome reaction barriers, are largely incompatible with C2 and higher fuels unless operating at near-combustion temperatures. This limitation is due at least in part to the production of easily adsorbed catalyst-poisoning intermediates. These catalyst-poisoning intermediates are difficult to oxidize, and can lower the activity of thermal catalysts. Examples of such commercially available fuel cells include high-temperature fuel cells (e.g., solid-oxide fuel cells) and proton-exchange fuel cells (e.g., direct methanol fuel cells).
In a first general aspect, electricity is generated by oxidizing a carbon-containing fuel in a photoelectrochemical fuel cell via a cyclic oxidation pathway to yield carbon dioxide and water, and collecting the electrons released via the cyclic oxidation pathway to yield a flow of electrons. The cyclic oxidation pathway includes a series of reactions including oxidation reactions, at least one of which is a photooxidation reaction.
In a second general aspect, a photoelectrochemical fuel cell includes an anode compartment including a photoanode and a carbon-containing fuel, a cathode compartment including a cathode, and a proton exchange membrane separating the anode compartment and the cathode compartment. The photoelectrochemical fuel cell is configured to oxidize the carbon-containing fuel via a cyclic oxidation pathway to yield carbon dioxide and water and to collect the electrons released via the cyclic oxidation pathway to yield a flow of electrons. The cyclic oxidation pathway includes a series of reactions including oxidation reactions, wherein the oxidation reactions include at least one photooxidation reaction.
Implementations of the first and second general aspects may include one or more of the following features.
The carbon-containing fuel is in the liquid phase or the gas phase. The carbon-containing fuel may be, for example, an organic fuel or a hydrocarbon. In some cases, the carbon-containing fuel is selected from the group consisting of hydrocarbons, alcohols, aldehydes, acetals, carboxylic acids, sugars, carbohydrates, ketones, esters, and ethers. The carbon-containing fuel may have at least two carbon atoms. The series of reactions may include one or more of: photooxidation of a first carboxylic acid to yield a carbocation, formation of an alcohol from the carbocation, oxidation of the alcohol to yield an aldehyde, formation of a geminal diol or acetal from the aldehyde, and oxidation of the geminal diol or acetal to yield a second carboxylic acid. In certain cases, the series of reactions includes one or two photooxidation reactions. The oxidation reactions include at least one dark oxidation, and may include at least one oxidation reaction catalyzed by a metal catalyst or a molecular catalyst.
In certain cases, oxidizing the carbon-containing fuel includes sequentially removing carbon atoms from the carbon-containing fuel and/or repeating the series of reactions at least once. The series of reactions may include breaking one C—C bond of the carbon-containing fuel and forming CO2. The carbon-containing fuel may be completely oxidized. Oxidizing the carbon-containing fuel via the cyclic oxidation pathway occurs under ambient thermal conditions, or in a temperature range between 0° C. and 100° C.
The thermodynamic efficiency of the conversion of the carbon-containing fuel to electricity exceeds the Carnot cycle limitation, or is at least 37%. The photoelectrochemical fuel cell may power a light source, and the light source may provide photons to the photoanode to initiate the at least one photooxidation reaction.
The photoanode may include a semiconductor. The semiconductor may have a valence band more positive than 1.5 eV versus a normal hydrogen electrode. In some cases, the photoanode includes a dark catalyst, such as a metal catalyst. For example, the photoanode may be a rotating ring disk electrode, with both a semiconductor and a metal catalyst. In certain cases, the photoelectrochemical fuel cell includes an additional anode proximate the photoanode, with the additional anode including a metal catalyst.
Advantages for the conversion of chemical energy to electrical energy via photoelectrochemical fuel cells as described herein can include high efficiency, quantum yields that can approach the number of electrons exchanged between the fuel and the anode, and the use of photogenerated deep valence band (VB) holes to oxidize kinetically stable carbon-containing fuels to protons and high energy organic radicals that then can undergo thermal electron transfer to the anode.
These general and specific aspects may be implemented using a device, system or method, or any combination of devices, systems, or methods. The details of one or more embodiments are set forth in the accompanying drawings and the description below. Other features, objects, and advantages will be apparent from the description and drawings, and from the claims.
Photoelectrochemical fuel cells (PECs) described herein utilize light and a photoanode including a semiconductor to achieve complete oxidation of carbon-containing fuels. Photon absorption by the semiconductor triggers electrochemical oxidation of the carbon-containing fuels, facilitating complete oxidation to final combustion products CO2 and H2O through a series of oxidation reactions in which photogenerated semiconductor valence band holes are utilized to activate carbon-hydrogen and carbon-carbon bonds with light rather than heat. By sequentially breaking down the oxidation process of carbon-containing fuels into smaller steps, electrons from the oxidation reactions can be collected using minimal photoenergy and without implementation of high temperature processes or electrochemical overpotential.
In some cases, PECs described herein target catalyst-poisoning intermediates with the combined action of co-localized photooxidation and dark oxidation. Where conventional fuel cells and combustion engines use heat or high overpotentials to overcome reaction barriers, the PEC photoanode uses light to overcome bond cleavage barriers and release the energy contained in the fuel under ambient thermal conditions (e.g., in a range between 0° C. and 100° C.). PECs described herein also demonstrate current multiplication, in which more than two electrons are driven through an external circuit per photon absorbed. This light-driven multiple electron oxidation can therefore operate at quantum efficiencies greater than two, boosting the efficiency of the PEC while operating under ambient thermal conditions.
PECs described herein include a photoanode, a cathode, and a proton exchange membrane positioned between the photoanode and the cathode. The photoanode includes one or more semiconductors capable of performing photooxidation. In some instances, the photoanode also includes one or more dark catalysts (e.g., metal catalysts, molecular catalysts, or the like) that facilitate electrochemical oxidation of a carbon-containing fuel without requiring absorption of a photon. In certain instances, the PEC includes a photoanode capable of performing light-driven oxidation and a second anode proximate the photoanode that includes one or more dark catalysts. Oxygen is reduced to water at the cathode.
Chemical mechanisms that occur in the PECs described herein include, but are not limited to: (1) multiple electron oxidation of carbon-containing fuels using multiple steps; (2) enhancement of internal photon to oxidative current quantum yields to above one on a semiconductor material (e.g., through the use of dark catalysts); and (3) a cyclical oxidation pathway for the complete oxidation of carbon-containing fuels. Conversion of the energy in the carbon-containing fuel to electricity includes current multiplication and overall thermodynamic efficiency exceeding Carnot cycle limitations.
Advantages for the conversion of chemical energy to electrical energy via PECs described herein can include: (1) quantum yields that can approach the number of electrons exchanged between the fuel and the anode (e.g., 8 for methane, 38 for hexane); and (2) the use of photogenerated deep valence band (VB) holes to oxidize kinetically stable carbon-containing fuels to protons and high energy organic radicals that then can undergo thermal electron transfer to the anode. In one example, a hydrocarbon fuel is oxidized by a hole from a photoexcited semiconductor in the photoanode. The singly oxidized hydrocarbon then fragments into a proton and an organic radical. The organic radical thermally injects an electron into the conduction band (CB) of the semiconductor. This produces another organic fragment which decomposes to form another proton and another high energy organic radical. While thermal deprotonation reactions form energetic organic species that are sufficiently reducing to inject electrons into the CB of the semiconductor, the semiconductor functions as an anode, harvesting the electrochemical energy directly from the carbon-containing fuel.
When a species in this series of electron transfers become too thermodynamically stable or kinetically slow to transfer electrons to the semiconductor CB, the photoexcited semiconductor holes will continue driving oxidation of a carbon-containing fuel. While running at ambient temperatures (e.g., temperatures between 0° C. and 100° C.) should inhibit or prevent the conversion of fuels to stable and highly carbonaceous materials (e.g., aromatics, coke, and the like), photogenerated holes deep in the VB of the semiconductor will decompose these stable materials to smaller, more reactive, and electrochemically active species should they form.
In one example, in the 38 electron electrochemical combustion of hexane to CO2 and H2O, 11 of the electrons for highly kinetically activated and thermodynamically demanding oxidation come from a UV irradiated photoanode (one photon process for each C—C bond and one photon process for one C—H bond per carbon), and the other 27 electrons are collected by dark oxidations of high energy organic radicals generated by the same photooxidations. Overall thermodynamic efficiency is calculated as maximum power output (38 electrons×1 eV) minus the minimum energy required for photoexcitation of the photoanode (11 electrons×2 eV indirect band gap) all divided by the hexane enthalpy (43 eV), or (38e−×1 eV-11e−×2.0 eV)/43 eV)×100%, or about 37%. If, however, 6 of the electrons come from a UV irradiated photoanode (one photon process for each C—C bond and one initial C—H bond) and the other 32 electrons are collected by dark oxidations of high energy organic radicals generated by the same photooxidations, overall thermodynamic efficiency is (38e−×1 eV-6e−×2.0 eV)/43 eV)×100%, or about 60%.
Unlike direct methanol fuel cells (DMFCs) which produce undesirable organic by-products like CO that can poison catalysts, the series of oxidations at a photoanode consumes unwanted organic species and radicals until complete conversion to carbon dioxide (CO2), protons (H+), and energetic electrons (e−) is realized. As with a typical fuel cell, the protons are passed through a proton exchange membrane (PEM), and the electrons drive a load until they meet at the cathode to reduce oxygen (O2) to water (H2O). The maximum attainable voltage of the cell is the difference in potentials of the semiconductor CB and the oxygen reduction reaction at the cathode. For TiO2, this yields a PEC voltage up to about 1.5 volts.
A cyclical oxidation pathway includes a series of oxidation reactions that successively break C—C bonds in the carbon-containing fuel to achieve complete oxidation of the carbon-containing fuel in the absence of high temperatures and electrochemical overpotential. As used herein, a “cyclical oxidation pathway” generally refers to a series of reactions including oxidation reactions, some or all of which may be repeated, to completely oxidize a carbon-containing fuel. In some instances, carbon atoms are removed from a carbon-containing fuel sequentially, from a first end to a second of a C—C chain.
A cyclical oxidation pathway typically includes one or more light catalysts (e.g., semiconductors) capable of photooxidizing a carbon-containing fuel. In some cases, an oxidation pathway also includes one or more dark catalysts (e.g., metal catalysts or molecular catalysts) to catalyze oxidation of carbon-containing fuels. An oxidation pathway that results in oxidation of a carbon-containing fuel and formation of CO2 typically includes at least one photooxidation and may include one or more additional dark oxidations, with each oxidation typically transferring one or more electrons to the photoanode, or to another anode if present.
Suitable carbon-containing fuels include traditional fuel such as gasoline, diesel, and JP-8, each comprised of different mixtures of carbon-containing compounds that have chains of carbon-carbon bonds, or aromatic rings, as well as carbon-hydrogen bonds of the aliphatic or aromatic type. Other suitable carbon-containing fuels include alcohols, aldehydes, acetals, geminal diols, carboxylic acids, sugars, carbohydrates, ketones, esters, ethers, and the like. Oxidation reactions described herein may occur in the liquid or gaseous phase, with the transfer or presence of H2O or OH− suitable to achieve complete oxidation of the carbon-containing fuel to CO2. In some cases, oxidation reactions described herein occur in an aqueous medium. Reaction rates may be enhanced under acidic or basic conditions. The carbon-containing fuel may be present in a wide range of concentrations (e.g., 1 mM to 50 vol %).
Suitable photoanodes include a semiconductor capable of photooxidation of a carbon-containing fuel. Suitable semiconductors typically have a band gap between the valence band and the conduction in a range between 2 eV (e.g., Fe2O3) and 4 eV (e.g., SnO2). In some cases, the semiconductor has a valence band more positive than 1.5 eV versus a normal hydrogen electrode (NHE). A more positive (lower) conduction band indicates that the photocatalyst may be thermodynamically capable of collecting more electrons without the use of light, thus leading to greater quantum yields. The semiconductor may be present in the form of an oxide (e.g., TiO2, WO3, Fe2O3, SnO2, SrTiO3, ZnO), a sulfide (e.g., CuS, and Cu2S), a nitride (e.g., GaN, C3N4), a phosphide (e.g., GaP, GaInP, InP), and an arsenide (e.g., GaAs, BAs), or combinations thereof. In some cases, doping can lead to the formation of new lower bands below the original conduction band. If the conduction band is low enough, the semiconductor may be used to collect all of the electrons in the oxidation process, thus reducing complexity of the anode and reducing the light energy needed for excitation. Suitable dark catalysts include metal catalysts such as Pt, Au, Pd, Cu, Ru, and alloys and nanoparticles thereof. Suitable dark catalysts also include molecular catalysts such as Ni(P2N2)2, Ni-cyclam, and various transition metal oxidation catalysts.
In Reaction (1), photooxidation of the hydrocarbon fuel results in the formation of a carbocation and the release of two electrons. In Reaction (2), the carbocation formed in Reaction (1) forms an alcohol. In Reaction (3), the alcohol is oxidized to yield an aldehyde and two electrons. In Reaction (4), the aldehyde forms a geminal diol. In Reaction (5), the diol is oxidized to yield a carboxylic acid. In Reaction (6), photooxidation of the carboxylic acid yields CO2 and a carbocation with n+1 carbon atoms. Reaction (6) may include photocatalytic decarboxylation via the photo-Kolbe reaction. The carbocation from Reaction (6) may then form an alcohol via Reaction (2), and oxidation continues via cyclical repetition of Reactions (2)-(6) until formic acid (CH2O2) is produced via Reaction (5). Formic acid, having only one carbon atom, does not undergo Reaction (6), but rather decomposes in the presence of a catalyst (e.g., Ni(P2N2)2) to yield H2 and CO2. In one example, when n=4 and the hydrocarbon fuel is hexane, complete oxidation via the cyclical oxidation pathway includes absorption of 6 photons and yields 6 CO2, 7 H2O, and 38 electrons.
Although the cyclical oxidation pathway in
While the cyclical oxidation pathway in
Thermodynamically, as a deep hole oxidizes a carbon-containing fuel and produces a proton, a highly energetic radical is also produced. If the radical adsorbs onto the photoanode, it can undergo efficient electron transfer to form the cation by electron injection into the conduction band. Two electrons have been contributed to the overall current: one by photooxidation and one by a thermal chemical reaction. With suitable catalysts, the radical dissociates to another proton and a second radical species which then thermally injects additional electrons into the conduction band, further multiplying the quantum yield.
As described herein, a PEC directs a minimum number of photogenerated holes to have the maximum effect in the combined photochemical and thermal oxidation of the fuel. At high light intensities, it is expected that the concentration of photogenerated holes would be high, and the entire oxidation of the fuel would be photochemical, and thus inefficient. Under low light intensity, photocurrents can be multiplied by thermal electron transfer reactions downstream of the photoelectrochemical oxidation. These reactions will continue until a rate limiting chemical reaction prevents further electron transfer. The introduction of another photogenerated hole reactivates the slow step, and the reaction continues. Thus, higher quantum yields can be obtained at lower light intensities, such that lowering the power given to the light source driving the reaction increases the energy conversion efficiency of the carbon-containing fuel. Operation of a PEC as described with respect to
Complete oxidation of methane to carbon dioxide in a PEC is depicted in
Anode compartment 612 and cathode compartment 614 are separated by proton exchange membrane 622 to allow for transportation of protons between the fluid medium in the anode compartment 612 and the fluid medium in the cathode compartment 614. Metal catalyst particles (e.g., Au or Pt nanoparticles) may be disposed on photoanode 616 and cathode 618. During operation of device 600, light, carbon-containing fuel, and water are delivered to anode compartment 612. In some embodiments, light is provided by LED 610. In other embodiments, light is provided by an external light source. The carbon-containing fuel in the fluid medium in anode compartment 612 decomposes under current multiplication conditions as described herein. The protons pass through the proton exchange membrane 622. When device 600 includes LED 610, the electrons drive the load 608 and the LED. The protons and electrons reduce oxygen to water in the cathode compartment 614.
In some cases, photoanode 616 includes a semiconductor as well as one or more dark catalysts (e.g., metal catalysts, molecular catalysts, or a combination thereof) disposed on the photoanode. The photoanode may be in the form of an array (e.g., a wire array, a microwire array, a nanowire array, or the like), a planar structure, a cylindrical structure, or a rotating ring disk. In one example, shown in
In certain cases, a PEC includes a photoanode as well as an additional anode including one or more dark catalysts (e.g., metal catalysts, molecular catalysts, or a combination thereof). The photoanode and the additional anode may be positioned proximate each other (e.g., with a separation of 3 cm or less) to allow photooxidized fuel intermediates to diffuse to and be oxidized by the dark catalyst anode.
Cathode 620 may have a structure generally known in the art. In some cases, cathode 620 includes a metal catalyst to facilitate formation of H2O.
The following examples are provided to more fully illustrate some of the embodiments of the present disclosure. However, those of skill in the art should, in light of the present disclosure, appreciate that many changes can be made in the specific embodiments that are disclosed without departing from the spirit and scope of the invention.
Potential applications for a fuel cell designed in accordance with various embodiments include, but are not limited to the following: the transportation sector, where higher-efficiency engines may reduce consumer load on petroleum; the military sector, where omni-fuel and/or omni-vehicle engines may be used to streamline fuel transportation and engine scale-up; and small, portable devices, where small, high-efficiency electrical devices may be used for mobile or remote applications.
Although the invention is described above in terms of various exemplary embodiments and implementations, it should be understood that the various features, aspects and functionality described in one or more of the individual embodiments are not limited in their applicability to the particular embodiment with which they are described, but instead can be applied, alone or in various combinations, to one or more of the other embodiments of the invention, whether or not such embodiments are described and whether or not such features are presented as being a part of a described embodiment. Thus, the breadth and scope of the present invention should not be limited by any of the above-described exemplary embodiments.
This application claims priority to U.S. App. Ser. No. 61/670,062 entitled “Photoassisted High Efficiency Conversion of Liquid Fuels to Electricity” and filed on Jul. 10, 2012, which is incorporated by reference herein in its entirety.
This invention was made with government support under W911NF-09-02-0011 awarded by the Defense Advanced Research Projects Agency, an agency of the U.S. Department of Defense. The government has certain rights in the invention.
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/US2013/049976 | 7/10/2013 | WO | 00 |
| Number | Date | Country | |
|---|---|---|---|
| 61670062 | Jul 2012 | US |
