Patent Description:
The anode catalyst and cathode catalyst commonly include supported platinum atoms. Platinum is a high-cost precious metal. Much work has been conducted to reduce the platinum loading in the cathode in order to reduce manufacturing costs. Additionally, work has been conducted to improve the kinetics of oxygen reduction in platinum oxygen-reducing cathode and reduce losses in potential in order to improve the efficiency of the fuel cell.

Platinum monolayer electrocatalysts for O<NUM> reduction which contain fluorosulfonic acid polymers are disclosed in <NPL>.

A review of polymeric electrolyte membranes for methanol fuel cells is provided in <NPL>.

Platinum and palladium particles having various uses are disclosed in <CIT> and <CIT>.

<CIT> discloses an electrode ink which is useful in the formation of fuel cells and electrolytic cells.

It is an object of the present invention to provide a unitized electrode assembly with enhanced ion-conductivity within the ionomer.

This object is solved by an unitized electrode assembly according to claim <NUM>. Preferred embodiments are mentioned in the dependent claims.

A catalyst layer for use in a fuel cell includes catalytic nanoparticles and a perfluorosulfonic acid (PFSA) ionomer. The catalytic nanoparticles have a palladium or palladium alloy core and an atomically thin layer of platinum on an outer surface of the palladium or palladium alloy core. The PFSA ionomer has an equivalent weight equal to or greater than about <NUM>. A unitized electrode assembly is also described.

A catalyst layer comprising a perfluorosulfonic acid (PFSA) ionomer with an equivalent weight equal to or greater than about <NUM> and core-shell catalyst nanoparticles for use in a fuel cell is described herein. Core-shell catalyst nanoparticle structures are being investigated for use in the catalyst layers of fuel cells due to their enhanced activity towards oxygen reduction and their reduction in platinum usage. The core of the core-shell catalyst nanoparticles is formed of palladium or a palladium alloy and the shell is formed of platinum or a platinum alloy. Core-shell catalysts experience palladium dissolution and exchange into the membrane. Palladium can dissolve readily at potentials over <NUM> volts (V) (all potentials described herein are on hydrogen scale), and defects in the platinum shell will expose the palladium core during use in the fuel cell. The properties of the ionomer are changed when palladium ions exchange with protons into the ionomer surrounding the catalytic particles. As described further below, the ionomer of the current unitized electrode assembly (UEA) exhibited little to no change in properties when contaminated with palladium.

Fuel cells convert chemical energy to electrical energy using one or more fuel cell repeat units. <FIG> illustrates a perspective view of one example fuel cell repeat unit <NUM>, which includes unitized electrode assembly (UEA) <NUM> (having anode catalyst layer (CL) <NUM>, electrolyte <NUM>, cathode catalyst layer (CL) <NUM>, anode gas diffusion layer (GDL) <NUM>, and cathode gas diffusion layer (GDL) <NUM>), anode flow field <NUM> and cathode flow field <NUM>. Fuel cell repeat unit <NUM> can have coolant flow fields adjacent to anode flow field <NUM> and cathode flow field <NUM>. Coolant flow fields are not illustrated in <FIG>.

Anode GDL <NUM> faces anode flow field <NUM> and cathode GDL <NUM> faces cathode flow field <NUM>. Anode CL <NUM> is positioned between anode GDL <NUM> and electrolyte <NUM>, and cathode CL <NUM> is positioned between cathode GDL <NUM> and electrolyte <NUM>. Fuel cell repeat unit <NUM> will be described as receiving hydrogen fuel (i.e., hydrogen gas) and oxygen oxidant (i.e., oxygen gas or air). However, other fuels and oxidants may be used.

In operation, anode GDL <NUM> receives hydrogen gas (H<NUM>) by way of anode flow field <NUM>. Anode CL <NUM>, which contains a catalyst such as platinum, causes the hydrogen molecules to split into protons (H+) and electrons (e-). The protons and electrons travel to cathode CL <NUM>; the protons pass through electrolyte <NUM> to cathode CL <NUM>, while the electrons travel through external circuit <NUM>, resulting in a production of electrical power. Air or pure oxygen (O<NUM>) is supplied to cathode GDL <NUM> through cathode flow field <NUM>. At cathode CL <NUM>, oxygen molecules react with the protons and electrons from anode CL <NUM> to form water (H<NUM>O), which then exits fuel cell <NUM>, along with excess heat.

Electrolyte <NUM> is located between anode CL <NUM> and cathode CL <NUM>. Electrolyte <NUM> allows movement of protons and water but does not conduct electrons. Protons and water from anode CL <NUM> can move through electrolyte <NUM> to cathode CL <NUM>. In one example, electrolyte <NUM> is a perfluorosulfonic acid (PFSA)-containing polymer or ionomer, such as Nafion® by E. DuPont, USA. PFSA polymers are composed of fluorocarbon backbones with sulfonate groups attached to short fluorocarbon side chains. In another example, electrolyte <NUM> is a hydrocarbon based persulfonic acid.

Anode CL <NUM> is adjacent to the anode side of electrolyte <NUM>. Anode CL <NUM> includes a catalyst that promotes electrochemical oxidation of fuel (i.e., hydrogen). Example catalysts for anode CL <NUM> include carbon supported platinum atoms and the core-shell catalyst nanoparticles described further below with respect to cathode CL <NUM>.

Cathode CL <NUM> is adjacent to the cathode side of electrolyte <NUM> and opposite anode CL <NUM>. Cathode CL <NUM> includes core-shell catalyst nanoparticles as described further below. The core-shell catalyst nanoparticles of cathode CL <NUM> promote electrochemical reduction of oxidant (i.e., oxygen). The core-shell catalyst nanoparticles have an enhanced activity towards oxygen reduction compared to previous carbon supported platinum catalysts. Further, the core-shell structure reduces platinum usage, and thus material costs because only a thin layer of platinum is used on the outer surface of the core-shell catalyst nanoparticles; the core comprises a lower cost metal such as palladium.

<FIG> is an enlarged view of cathode CL <NUM> of <FIG>, which includes catalyst <NUM> (having core-shell catalyst nanoparticles <NUM> and catalyst support <NUM>) and ionomer <NUM>. Ionomer <NUM> of cathode CL <NUM> contacts catalysts <NUM> to form a layer having core-shell nanoparticles <NUM> finely dispersed throughout. Cathode CL <NUM> is a matrix of catalyst supports <NUM>, ionomer <NUM> and core-shell catalyst nanoparticles <NUM>. The matrix allows electrons, protons, water and reactants to move through it. The catalyst support also increases the effective surface area.

Catalyst <NUM> of cathode CL <NUM> promotes electrochemical reduction of oxidant. As shown in <FIG>, catalyst <NUM> includes core-shell catalyst nanoparticles <NUM> supported by or on catalyst supports <NUM>. Catalyst supports <NUM> are electrically conductive supports, such as carbon black supports. Core-shell catalyst nanoparticles <NUM> are distributed on catalyst supports <NUM>. Core-shell catalyst nanoparticles <NUM> are nanoparticles. In one example, core-shell catalyst nanoparticles <NUM> have a diameter between about <NUM> and about <NUM>.

In cathode CL <NUM>, core-shell catalyst nanoparticles <NUM> promote the formation of water according to the oxidation reduction reaction: O<NUM> + <NUM>+ + 4e- → <NUM><NUM>O.

Ionomer <NUM> in cathode CL <NUM> connects electrolyte <NUM> to core-shell catalyst nanoparticles <NUM> on an ionic conductor level. As illustrated in <FIG>, ionomer <NUM> creates a scaffolding structure between catalyst supports <NUM> of catalyst <NUM>. Ionomer <NUM> creates a porous structure that enables gas to travel through cathode CL <NUM> and water to be removed from cathode CL <NUM>. Ionomer <NUM> also transfers protons from electrolyte <NUM> to active catalyst sites on core-shell catalyst nanoparticles <NUM>. Anode CL <NUM> can have the same structure as cathode CL <NUM>.

An enlarged cross-sectional view of core-shell catalyst nanoparticles <NUM> is shown in <FIG>. Core-shell catalyst nanoparticles <NUM> are formed of palladium core <NUM> and platinum shell <NUM>. Palladium core <NUM> is formed from palladium or a palladium alloy. Platinum shell <NUM> surrounds or encapsulates palladium core <NUM>. Platinum shell <NUM> is an atomically thin layer of platinum or platinum alloy atoms covering the outer surface of palladium core <NUM>. In one example, platinum shell <NUM> is a monolayer, bilayer or trilayer of platinum atoms. Although core-shell catalyst nanoparticles <NUM> are shown as being generally spherical in <FIG>, core-shell catalyst nanoparticles <NUM> may have any known shape. For example, core-shell catalyst nanoparticles <NUM> can have a cubo-octahedron shape.

The platinum atoms of platinum shell <NUM> cover or encapsulate substantially the entire outer surface of palladium core <NUM>. However, defects (i.e., pin holes) in platinum shell <NUM> will expose select portions of palladium core <NUM> to the surrounding environment. At the pH range of interest for UEA <NUM>, palladium is more soluble than platinum. Thus, defects in platinum shell <NUM> that expose palladium core <NUM> may result in palladium dissolution. Palladium dissolution can also be the result of movement of palladium to platinum shell <NUM> by diffusion or a site exchange.

The palladium ions from palladium core <NUM> may exchange with protons in ionomer <NUM>. Such an exchange changes the properties of ionomer <NUM>. Replacing protons with palladium ions reduces the conductivity of ionomer <NUM>. Displacement of protons may also change the transport properties and equilibrium concentrations of water and dissolved gases. Reduction in available proton concentration may lower the rate of the oxygen reduction reaction, and lead to forming a proton concentration gradient that will affect the overpotential of the fuel cell.

In cathode CL <NUM> of <FIG>, ionomer <NUM> is a PFSA polymer having an equivalent weight (EW) between <NUM> and <NUM>. In a still further example, ionomer <NUM> is a PFSA polymer having an EW between <NUM> and <NUM>. EW is the molecular weight that contains <NUM> mol of ionic groups and indicates the ionic content of the polymer. More specifically, a low EW ionomer has a high ionic content relative to a high EW ionomer, and is therefore more conductive. Despite the lower conductivity of PFSA ionomers with an EW equal to or greater than about <NUM>, experimental results unexpectedly showed that PFSA ionomers with an EW equal to or greater than about <NUM> had superior transport properties compared with lower EW ionomer when palladium is present.

In the rotating disk electrode example described further below, films were formed on rotating disk electrodes (RDE). The films included carbon supported platinum atoms and a PFSA ionomer. The films contained PFSA ionomers with different EWs.

Each RDE was then doped with various amounts of palladium to simulate palladium contamination. The experiments show that palladium ion contamination has little or no affect on the properties of the high EW ionomers, while having a large affect on the transport properties of the low EW ionomers. These results illustrate that palladium dissolution in cathode CL <NUM> will have less of an affect on the properties of ionomer <NUM> when ionomer <NUM> is a PFSA polymer having an EW greater than about <NUM> as compared to when ionomer <NUM> is a PFSA polymer having a lower EW, such as <NUM>.

Cathode CL <NUM> can be formed using many different techniques. Example fabrication method <NUM> is illustrated in <FIG> and includes making a catalyst ink (step <NUM>), mixing the catalyst ink (step <NUM>) and forming a catalyst layer (step <NUM>). A catalyst ink is formed in step <NUM> by mixing catalyst particles with an ionomer in liquid form (i.e., ionomer dissolved or dispersed in a solvent, such as isopropyl alcohol, and water). The ionomer is a PFSA ionomer having an EW equal to or greater than about <NUM>. In one example, the ionomer is Nafion® by E. DuPont, USA. As described above, the catalyst particles can be carbon supported core-shell catalyst nanoparticles.

Next, the catalyst ink is mixed using an aggressive mixing procedure to form a dispersion (step <NUM>). The mixing should be sufficient to ensure that the ionomer and the catalyst particles form a homogenous mixture. Example mixing techniques include ball milling, ultrasonication and shear mixing.

In step <NUM>, a catalyst layer is formed with the catalyst ink. In one example, the catalyst layer can be formed by a decal transfer process in which the catalyst layer is formed on a release film by application methods such as spraying or mayer-rod coating. The catalyst layer is then transferred onto the UEA by hot pressing. Example release films suitable for a decal transfer process include Teflon® and Kapton® by E. DuPont, USA, and Teflon® derivative substrates. In another example, the catalyst layer is formed by directly applying the catalyst ink onto the UEA. Example application methods for direct deposition include spraying and mayer-rod coating.

As illustrated in the following example, palladium has little to no affect on the properties of an ionomer having an EW greater than about <NUM>. Despite the lower conductivity of higher EW ionomers, these ionomers exhibit superior transport properties compared to lower EW ionomers. The following example is intended as an illustration only, since numerous modifications and variations within the scope of the present invention will be apparent to one skilled in the art.

A glassy carbon electrode was polished using <NUM> alumina, then rinsed and ultrasonicated to remove any trace of metal. An ink was made using <NUM> of <NUM>% platinum on Ketjen black carbon, <NUM> of Millipore water, <NUM>µL concentrated nitric acid, <NUM> of isopropanol, and the appropriate amount of ionomer from Table <NUM> that results in a <NUM>:<NUM> ratio of platinum to ionomer solids. The ink was ultrasonicated until well mixed and no individual particles could be seen. <NUM>µL of the ink was then deposited onto the clean glassy carbon electrode and dried under a lamp.

The electrode was then doped by soaking the electrode in a Pd(NO<NUM>)<NUM> solution. The electrode was soaked in <NUM> of the appropriate concentration of Pd(NO<NUM>)<NUM> in <NUM>. 1N H<NUM>SO<NUM> with oxygen bubbling for two hours. The Pd(NO<NUM>)<NUM> concentration was determined by calculating the percentage of ions that were required to exchange with the ionomer to achieve the palladium doping level of Table <NUM>. The doping level (in percent) was calculated according the following equation: (number of palladium cations*charge of palladium (+<NUM>)) / (number of protons in ionomer*charge of proton(+<NUM>))*<NUM>.

RDE experiments were completed for clean, undoped films and doped films for three different EW ionomers, <NUM>, <NUM>, and <NUM>, as illustrated in Table <NUM>. In each experiment, a Ag/AgCl electrode was used as the reference electrode and platinum gauze was used as the counter electrode. Each experiment was run in <NUM>. 1N H<NUM>SO<NUM>, and oxygen was flowed in the cell for a minimum of <NUM> minutes before the experiment. Two cycles were recorded from <NUM> to <NUM> VRHE at <NUM> mV/s and <NUM> RPM. This was repeated for <NUM>, <NUM>, <NUM>, and <NUM> RPM.

<FIG> shows a Koutecky-Levich (KL) plot at <NUM> V for electrodes A-G defined above in Table <NUM>, where ω is the rotational speed in RPM and J is the current density in mA/cm<NUM>. The slope for all data samples is the same and is characteristic of transport through the liquid electrolyte. The experimental data for electrode F and electrode G are vertically displaced from the other data sets. Electrodes F and G were palladium doped electrodes containing EW <NUM> ionomer. For thin film RDE samples, a larger intercept with the vertical axis is indicative of larger kinetic losses or larger transport losses through the ionomer film covering the supported catalyst. Thus, palladium contamination has a large affect on the transport properties of an ionomer with an EW of <NUM> (i.e., electrode F and electrode G) while having little to no affect on the properties of ionomers with a higher EW, such as <NUM> (i.e., electrode D) or <NUM> (i.e., electrode B). Further, the change in vertical displacement of the <NUM> EW ionomer electrodes (i.e., electrode F and electrode G) scale with the level of doping. More specifically, electrode G, which had a <NUM>% palladium doping, is more displaced from the majority of the data points compared to electrode F, which had a <NUM>% palladium doping. Thus, a greater palladium contamination results in greater transport losses for low EW (i.e., having an EW of <NUM> or less) ionomers.

<FIG> at <NUM> V emphasizes diffusional losses through the ionomer film. The kinetic overpotential should be quite large at <NUM> V which should lead to an inherent kinetic current much larger than the limiting current for oxygen diffusion through the ionomer film.

<FIG> shows a KL plot for electrodes A-G at <NUM> mV. At this potential, kinetic losses relative to diffusional losses through the ionomer film are emphasized. Similar to the results in <FIG>, electrode F and electrode G are vertically displaced from the other electrodes. Again, the displacement appears to scale with the level of doping. The significant differences between electrodes F and G, which contained EW <NUM> ionomer, compared to electrodes B and D, which contained higher EW ionomers, at this potential suggest that replacement of protons in the EW <NUM> ionomer by palladium also affects the inherent kinetic rate of the oxygen reduction reaction.

To summarize, palladium contamination can change a number of properties of the ionomer, such as but not limited to, reduced conductivity, a change in transport properties and a reduction in the rate of oxygen reduction. <FIG> and <FIG> show that palladium causes high transport losses in an electrode or catalyst layer formed with an ionomer having an EW of <NUM>. In comparison, palladium has no negative impact on oxygen transport in an electrode formed with an ionomer having an EW equal to or greater than about <NUM>.

As discussed above, cathode CL <NUM> of <FIG> includes core-shell catalyst nanoparticles <NUM> having palladium core <NUM> and platinum shell <NUM>. Palladium dissolution can occur at potentials over <NUM> V so that palladium dissolution and exchange into ionomer <NUM> is a concern. As illustrated in the experimental data above, ionomer <NUM> having an EW equal to or greater than about <NUM> will experience little to no transport losses even after palladium contamination. Thus, contrary to recent research that has focused on creating lower EW ionomers in order to increase the conductivity of the ionomer, ionomer <NUM> is formed of a PFSA ionomer having an EW equal to or greater than about <NUM> in order to reduce the negative impacts of palladium contamination. In one example, ionomer <NUM> has an EW between <NUM> and <NUM>. In a still further example, ionomer <NUM> has an EW between <NUM> and <NUM>.

Claim 1:
An unitized electrode assembly (<NUM>) comprising:
an electrolyte (<NUM>);
a catalyst layer (<NUM>) on a first side of the electrolyte (<NUM>), the catalyst layer (<NUM>) comprising:
catalysts (<NUM>) having electrically conductive catalyst supports (<NUM>) supporting core-shell catalytic nanoparticles (<NUM>) having a palladium or palladium alloy core and a platinum or platinum alloy shell generally encapsulating the core; and
a perfluorosulfonic acid (PFSA) ionomer (<NUM>) contacting the catalysts (<NUM>),
characterized in that
the perfluorosulfonic acid ionomer (<NUM>) creating a scaffolding structure between the catalyst supports (<NUM>) of the catalysts (<NUM>) and the perfluorosulfonic acid ionomer (<NUM>) having an equivalent weight equal to or greater than <NUM>.