WO2017069831A2

WO2017069831A2 - Fuel cell with improved electro catalyst

Info

Publication number: WO2017069831A2
Application number: PCT/US2016/045193
Authority: WO
Inventors: Siyu Ye; Dustin William H. BANHAM; Alan Patrick YOUNG; Shanna D. Knights
Original assignee: Ballard Power Systems Inc; Siemens VDO Electric Drives Inc; Ballard Material Products Inc
Current assignee: Ballard Power Systems Inc; Siemens VDO Electric Drives Inc; Avcarb Material Solutions Inc
Priority date: 2015-08-06
Filing date: 2016-08-02
Publication date: 2017-04-27
Anticipated expiration: 2018-02-06
Also published as: WO2017069831A3; WO2017069831A4

Abstract

A fuel cell with an improved electrocatalyst and a method of pretreating a fuel cell to improve the electrocatalyst are disclosed. The method of pretreating a fuel cell comprising an anode catalyst comprising platinum supported on a niobium-doped titanium oxide support comprises holding the fuel cell at a potential of about 1.45V for at least about 2 hours. In another embodiment the method of pretreating the fuel cell comprises potential cycling the fuel cell from about 1.0V to 1.4V for at least about 700 cycles, or for at least 2000 cycles or 3000 cycles. It was found that by pretreating the fuel cell as described above, the hydrogen reduction reaction electrocatalytic activity of the anode catalyst is maintained while the oxygen reduction reaction electrocatalytic activity is selectively reduced.

Description

FUEL CELL WITH IMPROVED ELECTRO CATALYST

BACKGROUND OF THE INVENTION

Field of the Invention

The present invention relates to membrane electrode assemblies with improved catalyst for use in fuel cells, and to methods of making thereof.

Description of the Related Art

Fuel cell systems are currently being developed for use as power supplies in numerous applications, such as automobiles and stationary power plants. Such systems offer promise of delivering power economically and with environmental and other benefits. To be commercially viable, however, fuel cell systems should exhibit adequate reliability in operation, even when the fuel cells are subjected to conditions outside their preferred operating ranges.

Fuel cells convert reactants, namely, fuel and oxidant, to generate electric power and reaction products. Polymer electrolyte membrane fuel cells ("PEM fuel cell") employ a membrane electrode assembly ("MEA"), which comprises a solid polymer electrolyte or ion-exchange membrane disposed between the two electrodes, namely a cathode and an anode. A catalyst typically induces the desired electrochemical reactions at the electrodes. Separator plates, or flow field plates for directing the reactants across one surface of each electrode substrate, are disposed on each side of the MEA.

In operation, the output voltage of an individual fuel cell under load is generally below one volt. Therefore, in order to provide greater output voltage, multiple cells are usually stacked together and are connected in series to create a higher voltage fuel cell stack. (End plate assemblies are placed at each end of the stack to hold the stack together and to compress the stack components together. Compressive force effects sealing and provides adequate electrical contact between various stack components.) Fuel cell stacks can then be further connected in series and/or parallel combinations to form larger arrays for delivering higher voltages and/or currents. Currently, the lifetime requirement for proton exchange membrane fuel cells (PEMFCs) for automotive applications is 5500 hours. One of the key challenges to meeting this requirement is the degradation associated with air/air startup/shutdown (SU/SD) of the fuel cell stack. During this event, a hydrogen/oxygen front forms at the anode. When this occurs, the oxygen reduction reaction (ORR) can occur at the anode, which increases the cathode potential to >1.4 V. These high potential excursions at the cathode lead to significant carbon corrosion and severe performance degradation.

To mitigate startup/shutdown degradation, two main approaches have traditionally been explored, specifically, the design of more stable materials, and system level mitigation strategies. In general, the first approach has primarily focused on designing durable, low surface area support structures (e.g., graphitic carbon or metal oxides) that can withstand these high potentials. Unfortunately, when loaded with platinum, these materials typically show lower performance compared to similarly Pt- loaded high surface area (lower durability) carbon supports. Due to this lack of suitable materials, the second approach is often relied upon, leading to higher system complexity and cost.

Also, modified catalysts have been used in the past to solve the problem of startup/shutdown degradation. For example, Genorio et al. (B. Genorio, R. Subbaraman, D. Strmcnik, et al. Angew. Chem. 2011, 123, 5582-86) proposed a method of preparing a hydrogen oxidation reaction (HOR) selective catalyst wherein the ORR is prevented from occurring at the anode, and as a result, the cathode potential is never forced above -1.0 V. This method is based on patterning a Pt catalyst using calix[4]arene molecules. These modified catalysts were able to selectively block the ORR without affecting the HOR activities and kinetics. However, this study was performed only using ex-situ methods (RDE - rotating disk electrode) rather than a fuel cell environment. 3M, working with ANL (R. Atanasoski. Project ID: FC006. 20142 and 2013 DOE Hydrogen and Fuel Cells Program Annual Merit Review, Washington DC), used a similar approach with calix molecules and other additives to limit the degradation due to SU/SD and cell reversals by modification of the anode, thus suppressing the ORR activity on the anode catalyst. Finally, Roberts et al. (J. Roberts, F. Berretta, H. Haas, et al. ECS Transactions, 2012, 50 (2) 711-721) proposed an approach in which oxide layers were added to the anode catalyst layer in order to be resistive in the presence of air and conductive in the presence of hydrogen.

While these initial studies were promising, there have not been many other reports on the successful synthesis of hydrogen oxidation reaction (HOR)- selective materials. This speaks to the extremely challenging nature of preparing catalysts that facilitate the HOR while preventing the ORR from occurring. The present invention addresses this need and provides associated benefits.

BRIEF SUMMARY

In brief, a method of pretreating a fuel cell is described, which comprises the steps of providing a fuel cell comprising an anode catalyst comprising platinum supported on a niobium-doped titanium oxide support and pretreating the anode catalyst by controlling the fuel cell potential to obtain a reduced electrocatalytic activity for oxygen reduction reaction, while not impacting the activity of platinum towards the hydrogen oxidation reaction.

In one embodiment, the step of controlling the fuel cell potential comprises potential cycling the fuel cell from about 1.0 V to 1.4 V for at least about 700 cycles. In some embodiments the method of pretreating the fuel cell comprises potential cycling the fuel cell from about 1.0 to 1.4 for at least about 2000 cycles, or for at least about 3600 cycles.

In another embodiment, the step of controlling the fuel cell potential comprises holding the fuel cell at a potential of about 1.45 V for at least about 2 hours.

A fuel cell is disclosed comprising an anode catalyst comprising platinum supported on a niobium-doped titanium oxide support, wherein the anode catalyst has been pretreated to obtain a reduced electrocatalytic activity for oxygen reduction reaction.

In one embodiment, the anode catalyst has been pretreated by potential cycling the fuel cell from about 1.0V to 1.4V for at least about 700 cycles. In some embodiments the anode catalyst has been pretreated by potential cycling the fuel cell from about 1.0V to 1.4V for at least about 2000 cycles or for at least about 3600 cycles. In another embodiment, the anode catalyst has been pretreated by holding the fuel cell at a potential of about 1.45 V for at least about 2 hours.

In some embodiments, the loading of platinum on the niobium-doped titanium oxide support is between about 10 wt% to about 50 wt%. In other embodiments, the loading of platinum on the niobium-doped titanium oxide support is preferably between about 10 wt% to about 20 wt%.

Furthermore, in some embodiments, the loading of platinum on the niobium-doped titanium oxide support is between about 0.01 mg Pt/cm² to about 0.4 mg Pt/cm².

The specific surface area of the niobium-doped titanium oxide support can be between about 10 m²/g to about 300 m²/g and in preferred embodiments, the specific surface area of the niobium-doped titanium oxide support is between about 40 m²/g to about 50 m²/g.

The anode catalyst can comprise carbon, graphitized carbon and/or graphite in the form of particles, fibers, nanoparticles, nanotubes, or nanofibers. The anode catalyst can further comprise a binder, for example an ionomer and/or a hydrophobic agent.

In some embodiments, the anode catalyst further comprises a water oxidation catalyst, such as ruthenium oxide or iridium oxide.

In some embodiments, the anode catalyst further comprises an additive to reduce membrane degradation, for example a radical scavenger, a membrane cross- linker, a hydrogen peroxide decomposition catalyst, a hydrogen peroxide stabilizer or a combination thereof.

These and other aspects of the invention will be evident in view of the present figures and the following detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 shows cyclic voltammetry (CV) plots obtained under N₂ during accelerated stress test (AST) cycling (1.0-1.4 V, 50 mV/s), with the CV plots obtained at a sweep rate of 20 mV/s in N₂ purged 0.1 M HC10₄ at room temperature. Figure 2(a) shows ORR and Figure 2(b) shows HOR obtained during AST cycling (1.0-1.4 V, 50 mV/s), with the CV plots obtained at a sweep rate of 20 mV/s in N₂ purged 0.1 M HC10₄ at room temperature.

Figure 3 shows the fraction of ORR and HOR activity retained for Pt/Nb-Ti0₂ and Pt/Graphitized C after AST cycling (1.0-1.4 V, 50 mV/s).

Figure 4 shows CV plots obtained under N₂ during AST cycling (1.0-1.4 V, 50 mV/s), at a sweep rate of 20 mV/s in N₂ purged 0.1 M HCIO₄ at room temperature.

Figure 5 shows CV plots obtained under N₂ during AST cycling (1.0-1.5 V, 50 mV/s), obtained at a sweep rate of 20 mV/s in N₂ purged 0.1 M HCIO₄ at room temperature.

Figure 6 shows CV plots for the Pt/Nb-Ti0₂ catalyst at BOL, after 7740 cycles, and after 30 minutes at 0.1 V under H₂, obtained at a sweep rate of 20 mV/s in N₂ purged 0.1 M HCIO₄ at room temperature.

Figure 7(a) shows ORR activity and Figure 7(b) shows HOR activity, for the Pt/Nb-Ti0₂ catalyst at BOL, after 7740 cycles, and after 30 minutes at 0.1 V under H₂, with the CV plots obtained at a sweep rate of 20 mV/s in N₂ purged 0.1 M HCIO₄ at room temperature.

Figure 8 shows CV plots obtained under N₂ for an MEA with a catalyst loading of 0.1 mg/cm² Pt/NbTi0₂ at the anode and 0.4 mg/cm² Pt/C at the cathode, obtained at (1) BOL, (2) after 2 hours at 1.45 V, and (3) after 4 hours at 1.45 V.

Figure 9 shows the CV results for the NbTi0₂ support only.

Figures 10(a) and 10(b) show the CV plots obtained under N₂ in a 0.1 M HCIO₄ solution at room temperature before, and respectively, after cycling from 1.0 to 1.4 V and Figure 10(c) shows the subtracted ORR currents at different scan rates for the Pt/Nb-Ti0₂ catalyst after 5400 voltage cycles.

Figures 11(a) and 11(b) show the CV plots obtained at room temperature in 0.09 M H₂S0₄ at a scan rate of 20 mV/s under N₂, and respectively, 0₂ before the voltage cycling (BOL) and following voltage cycling from 1.0V to 1.4V DETAILED DESCRIPTION

In the following description, certain specific details are set forth in order to provide a thorough understanding of the various embodiments of the invention. However, one skilled in the art will understand that the invention may be practiced without these details. In other instances, well-known structures associated with fuel cells, fuel cell stacks, batteries and fuel cell systems have not been shown or described in detail to avoid unnecessarily obscuring descriptions of the embodiments of the invention.

Unless the context requires otherwise, throughout the specification and claims which follow, the word "comprise" and variations thereof, such as, "comprises" and "comprising" are to be construed in an open, inclusive sense, that is as "including, but not limited to."

In this application, the term "electrocatalytic activity" is meant as the hydrogen oxidation reaction or oxygen reduction reaction current generated at the surface of a catalyst as the electrochemical potential of the catalyst is changed.

Furthermore, the term "acid" is meant to include substances that donate a proton in a chemical reaction to a base. The term "acid derivative" is meant to include materials that behave similarly to acids such as acid salts, and acid esters, particularly lower alkyl esters containing from 1 to 4 carbon atoms.

In addition, the term "solutions" is meant to include homogeneous mixtures of one substance in another. Liquid solutions are optically clear because the particle size of the dissolved material is less than the wavelength of visible light. The term "dispersions" are meant to include non-homogeneous mixtures of one substance in another. Liquid dispersions are translucent and also include emulsions that are optically opaque because the particle size of the dispersed particle is greater than the wavelength of visible light. The dispersed material itself may be of such particle size or it may associate with itself or the dispersing medium forming micelles.

An electrochemical fuel cell includes a solid electrolyte interposed between an anode electrode and a cathode electrode, a cathode catalyst layer between the solid electrolyte and the cathode electrode, and an anode catalyst layer between the solid electrolyte and the anode electrode. Each of the anode and cathode catalyst layers typically comprise an anode catalyst and a cathode catalyst, respectively.

As discussed in the foregoing, fuel cells are often subjected to high potentials due to air/air startup/shutdowns, which lead to significant carbon corrosion and severe performance degradation. In particular, carbon supports of convention carbon-supported catalysts can suffer corrosion at the high end of potential cycles, which in turn (in the case of carbon-supported platinum) causes release of platinum particles and their consequent agglomeration. As agglomeration of platinum particles decreases the surface area of platinum that is available for catalytic activity, this results in decreased performance. Accordingly, conventional carbon-supported catalysts must be designed to account for high potentials and meet durability targets, and therefore require a certain minimum of catalyst loading. Furthermore, conventional carbon- or graphite-supported platinum catalysts do not exhibit a higher electrocatalytic activity for HOR than ORR.

Without being bound by theory, anode catalysts having selectively reduced electrocatalytic activity for the oxygen reduction reaction (ORR) while maintaining high electocatalytic activity for the hydrogen oxidation reaction (HOR) are desired because such anode catalysts may reduce or eliminate the anode potential difference between the inlet and outlet regions of the fuel cell that causes the high cathode potential during startup/shutdown, thereby increasing the number of air/air startup/shutdowns (and resulting potential cycles) that the fuel cell may undergo over its lifetime. In other words, such anode catalysts improve the overall durability of the fuel cell. Furthermore, loading restrictions imposed on conventional catalysts do not apply to such anode catalysts which may be selected to favour activity (that is, performance) over durability in the face of high potentials. Moreover, as such anode catalysts decrease potentials in startup/shutdown periods, various system control and valving strategies required in conventional systems to mitigate the effects of high potentials are rendered unnecessary, thereby allowing for less complexity (and therefore cost) in systems incorporating the catalysts of the invention.

The inventors have surprisingly discovered that by subjecting a fuel cell having an anode catalyst comprising platinum supported on a niobium-doped titanium oxide support to a pretreatment, the HOR electrocatalytic activity is maintained while the ORR electrocatalytic activity is selectively reduced. In one embodiment, the fuel cell is held at a potential of about 1.45V for at least about 2 hours. In another embodiment, the fuel cell potential cycled from about 1.0V to 1.4V for at least about 700 cycles, for example, for at least about 2000 cycles or for at least 3600 cycles.

Without being bound by theory, it is believed that the HOR and ORR mechanisms can proceed at different active sites on platinum. During the voltage cycling protocol, the titanium and/or niobium are likely dissolving and re-depositing (or the oxide is restructuring) onto the platinum surface, resulting in deactivation of any platinum site covered by titanium and/or niobium. Previous work in the literature has indicated that the minimum ensemble of platinum atoms required for maximum ORR activity is larger than that required for maximum HOR activity (Nat. Mater. 9 (2012) 998-1003). Thus, while the deposition of titanium and/or niobium greatly impacts the ORR, the coverage is not high enough to significantly impact the HOR (large ORR active platinum ensembles no longer exist but smaller HOR active ensembles remain).

The loading of platinum on the niobium-doped titanium oxide support for the anode catalyst may be any suitable range. For example, the range may be from about 10 wt% to about 50 wt%, more specifically, from about 10 wt% to about 20 wt%. Likewise, the specific surface area of the niobium-doped titanium oxide support may be any suitable range. For example, the range may be from about 10 m²/g to about 300 m²/g, more specifically, from about 40 m²/g about 50 m²/g.

In a membrane electrode assembly, the anode catalyst layer may include other constituents, such as additives to improve electrical conductivity and cohesion. In one embodiment, the anode catalyst layer may include carbon, graphitized carbon and/or graphite in various forms, such as particles, fibers, nanoparticles, nanotubes, and nanofibers. In another embodiment, the anode catalyst layer may include a binder, such as an ionomer and/or hydrophobic agent, or a water oxidation catalyst, such as RuOx and IrOx.

In some embodiments, the anode catalyst may also comprise an additive to reduce membrane degradation. For example, the additive may be a radical scavenger, a membrane cross-linker, a hydrogen peroxide decomposition catalyst, a hydrogen peroxide stabilizer or a combination thereof. Examples of radical scavengers include: hindered amines, hydroxylamines, arylamines, phenols, BHT, phosphites, benzofuranones, salicylic acid, azulenyl nitrones and derivatives thereof, tocopherols, DMPO, cyclic and acyclic nitrones, gold-chitosan nanocomposites, ascorbic acid and Mn²⁺. The additive can be a membrane cross-linker such as a multivalent cation. Examples of membrane cross-linkers include multivalent cations such as Al³⁺, Mn²⁺, Mg²⁺ and Zn²⁺. Examples of hydrogen peroxide decomposition catalysts include: organometallic Mn (II) or Mn (III) complexes, oxide catalysts, mixed oxide catalysts, and phosphites. Further, examples of hydrogen peroxide decomposition catalysts include: a salt, oxide or organometallic complex of Co, Fe, Cr, Mn, Cu, V, Ru, Pd, Ni, Mo, Sn and W. Examples of hydrogen peroxide stabilizers include stannic oxide, sodium aluminate, hydrous alumina, Zn²⁺ and sodium pyrophosphate or a precursor thereof.

Furthermore, the platinum loading in the anode catalyst layer may range from about 0.01 mg Pt/cm² to about 0.4 mg Pt/cm² , and in preferred embodiments between 0.025 mg Pt/cm² to about 0.4 mg Pt/cm².

As previously mentioned, the anode and cathode catalyst layers may be applied to a GDL to form anode and cathode electrodes, or to a decal transfer sheet which is then decal transferred to a surface of the GDL or solid electrolyte, or applied directly to the surface of the solid electrolyte to form a catalyst-coated membrane (CCM). The electrodes or CCM can then be bonded with other components to form an MEA. Alternatively, the application of the catalyst layer on the desired substrate may occur at the same time the remaining MEA components are bonded together.

The present catalyst layers may be applied according to known methods. For example, the catalyst may be applied as a catalyst ink or slurry, or as a dry mixture. Catalyst inks may be applied using a variety of suitable techniques (e.g., hand and machine methods, including hand brushing, notch bar coating, fluid bearing die coating, wire-wound rod coating, fluid bearing coating, slot-fed knife coating, three-roll coating, screen-printing and decal transfer) to the surface of the solid electrolyte or GDL. Examples of dry deposition methods include electrostatic powder deposition techniques and decal transfer. Catalyst inks typically incorporate the catalysts and binder in a solvent/dispersant to form a solution, dispersion or colloidal mixture. Suitable solvents/dispersants include water, organic solvents such as alcohols and polar aprotic solvents (e.g., N-methylpyrrolidinone, dimethylsulfoxide, and N,N-dimethylacetamide), and mixtures thereof. Depending on the amount of water, one can distinguish water- based inks, wherein water forms the major part of the solvents used, from inks wherein organic solvents form the major part. Catalyst inks may further include surfactants and/or pore forming agents, if desired. Suitable pore formers include methyl cellulose, sublimating pore-forming agents such as durene, camphene, camphor and naphthalene, and pore-forming solvents that are immiscible with the catalyst ink solvent/dispersant, such as n-butyl acetate in polar aprotic solvent/dispersant systems.

The selection of additional components for the catalyst mixture and the choice of application method and GDL to which it is applied are not essential to the present invention, and will depend on the physical characteristics of the mixture and the substrate to which it will be applied, the application method and desired structure of the catalyst layer. Persons of ordinary skill in the art can readily select suitable catalyst mixtures and application methods for a given application.

EXAMPLES

HOR/ORR Data for Pt/NbTiO_? Catalyst

Three different accelerates stress tests (ASTs) were used to evaluate the Pt/NbTi02 catalyst: (1) 0.6-1.0 V, (2) 1.0-1.4 V, (3) 0.6-1.2 V. The main observations from these protocols are summarized below:

a) During AST cycling from 1.0 to 1.4 V, the Pt/Nb-Ti02 catalyst demonstrated an unexpected loss in activity towards the oxygen reduction reaction (ORR), while showing no change in the hydrogen underpotential deposition (HUPD) region.

b) This phenomenon is not observed when cycling is performed from 0.6 to

1.0 V or when cycling from 0.6 to 1.2 V. c) Cycling between 0.6 and 1.0 V is designed to stress the Pt catalyst (by continually oxidizing/reducing it), but is not aggressive towards to the support (NbTi0₂). Therefore, no change in the NbTi0₂ support was observed or expected during this AST protocol. However, the hydrogen underpotential deposition (HUPD) region was found to decrease, as expected, due to Pt dissolution.

d) When cycling from 0.6 to 1.2 V, the Pt is repeatedly oxidized and reduced, which accelerates Pt dissolution. Therefore, while a loss in ORR activity was observed when cycling between 0.6 and 1.2 V, it correlated with a decrease in the HUPD region, and was thus attributed to decreasing the Pt surface area.

As no change in the HUPD region was observed after cycling between

1.0 and 1.4 V despite a significant loss in ORR activity, the kinetics of the hydrogen oxidation reaction (HOR) were monitored. It was found that despite the greatly suppressed ORR activity, the HOR activity remained unchanged. Based on these results, it is believed that the Pt/NbTi02 catalyst (after being deactivated towards the ORR) could be an excellent anode catalyst. This is because, in the prior art arrangements, during startup/shutdown, 0₂ reduction can occur at the H₂ deprived regions of the anode and, as a result of the ORR occurring at the anode, the oxygen evolution reaction (OER) is forced to occur at the cathode, driving local potentials to very high values (> 1.5 V). In the present invention, by inhibiting the ORR at the anode, the corresponding OER at the cathode should be greatly suppressed, thus greatly reducing the extreme potentials experienced at the cathode.

During AST cycling from 1.0-1.4 V, the charge in the PtO (platinum oxide) region is greatly suppressed while the charge in the HUPD region remains unchanged, as shown in Figure 1 which represents the current vs. potential (all potentials are reported vs. the reversible hydrogen electrode (RHE)). It is believed that this is due to a restructuring of the Nb-Ti0₂ support, as the support is continually oxidized during the AST.

As AST cycling progresses, the kinetics of the oxygen reduction reaction (ORR) occurring at the Pt/NbTi0₂ catalyst are suppressed as expressed by the RDE data illustrated in Figure 2 (a), while the hydrogen oxidation reaction (HOR) kinetics remain unaffected as expressed by the RDE data illustrated in Figure 2(b). This is even more clearly illustrated in Figure 3, which compares the relative ORR and HOR activities for the Pt/NbTi0₂ catalyst as a function of cycle number. The ORR activity at Pt/Graphitized C is also shown for comparison.

Based on these initial results, further work was performed to determine if the ORR could be further suppressed through additional cycles, or through a higher upper potential limit (UPL). Figure 4 shows the results of performing up to 7740 cycles. From 0-1080 cycles, there is a continuous decrease in the charge in the PtO region, while no difference in HUPD area is observed. However, beyond 3600 cycles, there is little change in the catalyst structure until 7740 cycles are reached, at which point there is a slight decrease in both the HUPD and PtO regions. (Note: Since both regions appear to decrease by a similar percentage, it is possible this apparent decrease is due to the formation of a bubble(s) on the surface of the electrode after 7740 cycles.)

After performing 7740 cycles to 1.4 V, the upper potential limit (UPL) was increased to 1.5 V, and 2700 additional cycles were performed (while maintaining a lower potential limit of 1.0 V). As can be seen in Figure 5, no further suppression of the PtO region was observed.

If the ORR inhibiting properties of the Pt/Nb-Ti0₂ catalyst are to be of use under real operating conditions, it is important that the Pt/Nb-Ti02 maintains its structure while under reducing conditions (as it will be used at the anode). Therefore, after performing 7740 cycles to 1.4 V, the potential was held at 0.1 V for 30 minutes under H₂ (Figure 6). Overall, the structure remained quite stable.

Figure 7(a) and Figure 7(b) show the ORR and HOR activity, respectively, of the Pt/Nb-Ti0₂ catalyst at BOL, after 7740 cycles, and after holding at 0.1 V under H₂ for 30 minutes. It is clear that after 7740 cycles, the ORR activity is much lower compared to the ORR activity at BOL. Importantly, this low ORR activity is maintained even after holding at 0.1 V under H2 for 30 minutes. Also of importance in Figure 7(b) is that the HOR activity remains unchanged throughout all of the testing protocols.

Finally, in-situ MEA testing of this catalyst was performed to determine whether the same effect would be induced after bringing the Pt/NbTi0₂ catalyst to high potentials. An MEA, consisting of 4 gsm Pt/graphitized C at the cathode, and 1 gsm Pt/NbTi0₂ at the anode, was prepared and evaluated. After obtaining a beginning of life (BOL) anode CV under N₂, the potential of the anode was brought to 1.45 V for 2 h. Following this, an anode CV under N₂ was obtained, clearly demonstrating the same decrease in charge under the PtO region as was observed during the rotating disc electrode (RDE) testing, which was previously correlated with a decrease in ORR activity (see Figures 1, 2 and 3). Additionally, as with the RDE testing, no change in the HUPD region was observed. Increasing the time of the potentiostatic experiment to 4 hours had no further impact on the PtNbTi0₂ catalyst (see Figure 8). Importantly, the CV plots following the 1.45 V potentiostatic experiments suggest that the same effect that was observed through RDE work can be replicated in-situ.

Further experiments were conducted to confirm that the results described above were accurate and to explain the underlying process on which the reduction in electrocatalytic activity for the oxygen reduction reaction (ORR) is based and the experiment results, as described here, are illustrated in Figures 9 to 11.

In order to understand if the change in the PtO region of the CV was due to degradation of the support, the NbTi0₂ alone (without Pt) was subjected to the same cycling protocol described above in relation to Figure 1. However, as can be seen in Figure 9, the support appears quite stable during this voltage cycling protocol, with only minor changes being observed in the CV obtained under N₂ following 5400 cycles from 1 to 1.4 V. This strongly suggests that the observed decrease in ORR activity is not due to significant oxidation of the NbTi0₂ support.

It is known that, for some metal oxides, HOR selectivity can be achieved by changing the gas atmosphere (e.g. N₂ vs. 0₂) leading to a change in conductivity. Specifically, some catalyst supports demonstrate low conductivity at high potentials (in an oxidizing environment where the ORR occurs) and high conductivity at low potentials (in a reducing environment where the HOR occurs) which results in selectivity towards the HOR. To determine whether a similar mechanism could explain the results observed for the Nb-Ti0₂ catalyst in the present invention, a scan rate study was performed. Increasing the scan rate increases the charging current, which should exacerbate any resistive losses through the catalyst layer. If the catalyst support has lower conductivity at high potentials, this effect should be more pronounced at high scan rates. However, for any catalyst support, if scan rate is increased high enough, resistive limitations will eventually be reached and will be manifested as a distortion in the CV over the entire potential range (not just at high potentials). To help distinguish these two phenomena (resistance at high potentials vs. an overall resistance of the metal oxide support), scan rate studies were performed prior to voltage cycling from 1.0-1.4 V (before the catalyst demonstrated HOR selectivity) and after voltage cycling.

The scan rate results for the catalyst prior to voltage cycling from 1.0 to 1.4 V and after 5400 cycles of voltage cycling from 1.0 to 1.4 V are illustrated in Figure 10(a) and, respectively Figure 10(b). The currents in these figures are normalized to the current generated by the 5 mV/s scan to aid in comparing any changes in the PtO region. Fig 10 (a) clearly shows that as scan rate increases, the charge in the PtO region is decreased. Since this result is before any HOR selectivity is imparted to the catalyst, it is likely that the suppression of platinum oxidation is simply due to resistance through the catalyst layer. This is further confirmed by examining the HUPD region, which also shows a distortion at high scan rates.

In Figure 10 (b), it is again apparent that for the 5400 cycles sample, the PtO region is suppressed by increasing the scan rate. However, the relative suppression between the 0 cycles and 5400 cycles samples is quite similar, suggesting that the HOR selectivity of the 5400 cycles sample cannot be attributed to a low conductivity only at high potentials. Further support for this conclusion is obtained by comparing the subtracted ORR currents at different scan rates for the catalyst following 5400 cycles which is illustrated in Figure 10(c). These results prove that the scan rate had no impact on the ORR currents, which again indicates that another mechanism (and not the electronic conductivity of the catalyst) must be responsible for the HOR selectivity.

An alternative mechanism for HOR selectivity known in the prior art is based on the observation that the selective blocking/poisoning of Pt sites can also lead to HOR selectivity through eliminating sites for the ORR while still allowing the HOR to proceed. Therefore, it is believed that during the voltage cycling protocol employed in the present method, some dissolution/redeposition of Nb and/or Ti occurs onto the Pt surface. At low surface coverages, this process could lead to HOR selectivity for the same reasons as the calix[4] functionalization of Pt discussed in the background of invention.

In order to test this hypothesis, the same voltage cycling protocol was performed in H₂SO₄. Since H₂SO₄ is known to strongly adsorb to the surface of Pt[2, 3], it was expected that the apparent HOR selectivity would be significantly reduced in H₂S0₄ as the deposition of Nb and/or Ti onto the Pt surface would have to compete with strongly adsorbed bisulphate anions. 0.09 M H₂SO₄ was used to ensure the same proton concentration as the 0.1 M HC10₄ tests. The results of this analysis are shown in Figures 11(a) and 11(b).

After 5400 voltage cycles from 1.0 to 1.4 V, hardly any change in the

PtO region is observed in Fig. 11(a). As expected, the initial ORR activity obtained for the Pt/NbTi0₂ catalyst in 0.09 M H₂S0₄ was lower than that obtained in 0.1 M HC10₄ due to the strongly bound H₂S0₄ anion blocking the Pt active sites. Moreover, it is evident in Fig. 11(b) that very little change in ORR activity is observed following the voltage cycling protocol, in agreement with the relatively unchanged PtO region shown in Fig. 11(a). This is clearly a very different result than was observed in HCIO₄, and highlights that the mechanism for HOR selectivity in 0.1 M HCIO₄ is highly sensitive to anion adsorption. If the HOR selectivity in the present invention would be due to a change in conductivity, it would be unlikely impacted by anion adsorption. However, if some dissolution/redeposition of Nb and/or Ti onto the Pt surface was responsible for the observed HOR selectivity, it is highly probable that this mechanism would be impacted by the presence of strongly adsorbing anions such as HSO₄ ^".

It is therefore believed that during the voltage cycling described in the present invention some dissolution/redeposition of Nb or Ti could be occurring on the catalyst surface and, while the deposition of titanium and/or niobium greatly impacts the ORR, the coverage is not high enough to significantly impact the HOR.

While the present electrodes have been described for use in PEM fuel cells, it is anticipated that they may be useful in other fuel cells having an operating temperature below about 250°C. They are particularly suited for acid electrolyte fuel cells, including phosphoric acid, PEM and liquid feed fuel cells. It is also contemplated that the present method of pretreating the anode catalyst by controlling the fuel cell potential may also be useful for fuel cells having an anode catalyst comprising platinum supported on other metal oxides for example on metal oxides comprising ruthenium, iridium, tantalum.

All of the above U.S. patents, U.S. patent application publications, U.S. patent applications, foreign patents, foreign patent applications and non-patent publications referred to in this specification and/or listed in the Application Data Sheet, are incorporated herein by reference in their entirety.

While particular elements, embodiments and applications of the present invention have been shown and described, it will be understood, of course, that the invention is not limited thereto since modifications may be made by those skilled in the art, particularly in light of the foregoing teachings. It is therefore contemplated by the appended claims to cover such modifications that incorporate those features coming within the scope of the invention.

This application also claims the benefit of U.S. Provisional Patent Application No. 62/202,109, filed August 6, 2015, and is incorporated herein by reference in its entirety.

Claims

CLAIMS What is claimed is:

1. A method of pretreating a fuel cell, comprising the steps of: providing a fuel cell comprising an anode catalyst comprising platinum supported on a niobium-doped titanium oxide support; and

pretreating the anode catalyst by controlling a fuel cell potential to obtain a reduced electrocatalytic activity for oxygen reduction reaction.

2. The method of claim 1, wherein the step of controlling the fuel cell potential comprises potential cycling the fuel cell from about 1.0 V to 1.4 V for at least about 700 cycles.

3. The method of claim 2, further comprising potential cycling the fuel cell from about 1.0 V to 1.4 V for at least about 2000 cycles.

4. The method of claim 2, further comprising potential cycling the fuel cell from about 1.0 V to 1.4 V for at least about 3600 cycles.

5. The method of claim 1, wherein the step of controlling the fuel cell potential comprises holding the fuel cell at a potential of about 1.45 V for at least about 2 hours.

6. A fuel cell comprising an anode catalyst comprising platinum supported on a niobium-doped titanium oxide support, wherein the anode catalyst has been pretreated to obtain a reduced electrocatalytic activity for oxygen reduction reaction.

7. The fuel cell of claim 6, wherein the anode catalyst has been pretreated by potential cycling the fuel cell from about l .OV to 1.4V for at least about 700 cycles.

8. The fuel cell of claim 6, wherein the anode catalyst has been pretreated by potential cycling the fuel cell from about l .OV to 1.4V for at least about 2000 cycles.

9. The fuel cell of claim 6, wherein the anode catalyst has been pretreated by potential cycling the fuel cell from about l .OV to 1.4V for at least about 3600 cycles.

10. The fuel cell of claim 6, wherein the anode catalyst has been pretreated by holding the fuel cell at a potential of about 1.45V for at least about 2 hours.

11. The fuel cell of claim 6, wherein a loading of platinum on the niobium-doped titanium oxide support is between about 10 wt% to about 50 wt%.

12. The fuel cell of claim 6, wherein a loading of platinum on the niobium-doped titanium oxide support is between about 10 wt% to about 20 wt%.

13. The fuel cell of claim 6, wherein a loading of platinum on the niobium-doped titanium oxide support is between about 0.01 mg Pt/cm² to about 0.4 mg Pt/cm².

14. The fuel cell of claim 6, wherein a specific surface area of the niobium-doped titanium oxide support is between about 10 m²/g to about 300 m²/g.

15. The fuel cell of claim 6, wherein a specific surface area of the niobium-doped titanium oxide support is between about 40 m²/g to about 50 m²/g.

16. The fuel cell of claim 6, wherein the anode catalyst further comprises carbon, graphitized carbon and/or graphite in the form of particles, fibers, nanoparticles, nanotubes, or nanofibers.

17. The fuel cell of claim 6 wherein the anode catalyst further comprises a binder.

18. The fuel cell of claim 17 wherein the binder is an ionomer and/or a hydrophobic agent.

19. The fuel cell of claim 6 wherein the anode catalyst further comprises a water oxidation catalyst.

20. The fuel cell of claim 19 wherein the water oxidation catalyst is ruthenium oxide or iridium oxide.

21. The fuel cell of claim 6 wherein the anode catalyst further comprises an additive to reduce membrane degradation.

22. The fuel cell of claim 19 wherein the additive for reducing membrane degradation is a radical scavenger, a membrane cross-linker, a hydrogen peroxide decomposition catalyst, a hydrogen peroxide stabilizer or a combination thereof.