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The innovated approach of the electrolyte system of this invention is the use of tailored electrolyte environments on each side of a hydrogen-permeable barrier for the respective electrode reactions, namely alkali for the cathode to take advantage of superior oxygen reduction kinetics, and acid for the anode to take advantage of its CO 2 -rejection properties.

Although alkaline fuel cells possess rapid oxygen reduction kinetics, and the use of non-noble metal catalysts is feasible, the electrolyte carbonation problem currently restricts alkaline fuel cells to the use of pure H 2 and O 2. The hybrid electrolyte system of this invention enables use of an alkaline environment for the cathode because the barrier between the cathode and the anode prevents carbonation from CO 2 produced at the anode. The dense phase separating the cathode from the anode permits only the transport of protons and, thus, the alkaline electrolyte never "sees" the anode and the fuel oxidation products produced there.

As a result, this hybrid electrolyte system permits the direct electro-oxidation of organic fuels or impure H 2 at the anode in an acidic environment, while the oxygen reduction reaction is accomplished in a basic electrolyte. The major challenge, of course, is the completely removed CO 2 and prevent the effects of carbonation. For both space and terrestrial applications, the use of alkaline fuel cells, thus, is presently limited to the use of pure H 2. The CO 2 removal problem, however, is greatly diminished in the hybrid fuel cell of this invention compared to alkaline fuel cells because only CO 2 removal from the oxidant is required, even if organic fuels or impure hydrogen are oxidized at the anode.

As shown, hybrid fuel cell 10 comprises a dense phase proton permeable material 17 separating acidic electrolyte-containing matrix layer 15 from basic electrolyte-containing matrix layer Adjacent to the face of basic electrolyte matrix layer 16 facing away from dense phase proton permeable material 17 is electrocatalyst layer 14, adjacent to which is porous gas diffusion cathode Similarly, adjacent to the face of acidic electrolyte matrix layer 15 facing away from dense phase proton permeable material 17 is electrocatalyst layer 13, adjacent to which is porous gas diffusion anode By the term "dense phase proton permeable material" we mean a material which is permeable to protons but impermeable to chemical species larger that atomic hydrogen.

In accordance with a preferred embodiment of this invention, said dense phase proton permeable material comprises a foil of a metal hydride. In accordance with a particularly embodiment of this invention, said dense phase proton permeable material comprises palladium hydride. In accordance with one preferred embodiment of this invention, the acid electrolyte is disposed within a matrix material. In accordance with one particularly preferred embodiment of this invention, the acid-containing matrix material comprises concentrated phosphoric acid in a silicon carbide matrix.

Similarly, the base electrolyte in accordance with one embodiment of this invention is also disposed within a matrix material. In accordance with one particularly preferred embodiment of this invention, said base-containing matrix comprises concentrated potassium hydroxide in a potassium hexatitanate matrix. It will be apparent to those skilled in the art that acids and bases which are normally employed in conventional fuel cells may be employed in the hybrid electrolyte system of this invention.

Although the problem of CO 2 removal is greatly diminished in a hybrid fuel cell employing the hybrid electrolyte system of this invention when compared to conventional alkaline fuel cells in that only CO 2 removal from the oxidant cathode side of the fuel cell is required. For example, if air is used as the oxidant gas, atmospheric CO 2 must be removed by way of scrubbers or other processes, or the effects of electrolyte carbonation must be overcome by circulating the alkaline electrolyte.

Although dense phase proton permeable material 17 prevents CO 2 produced at the anode from carbonating the alkali on the cathode side of dense proton permeable material 17, it does so at the cost of additional impedance due to the bulk of dense phase proton permeable material 17 and the two additional interfaces created between the acid and dense phase proton permeable material 17 and the base and dense phase proton permeable material 17, respectively. By activating the surfaces of dense phase proton permeable material 17 comprising, for example, palladium hydride with a catalytic material such as platinum, these interfacial impedances are significantly reduced so as to allow the passage of high current fluxes.

In the test cell, a 25 micron foil of palladium modified on both sides by electrochemically deposited platinum was used as the proton permeable material. The foil was hot-pressed to one side of a Nafion polymer membrane which serves as an acidic electrolyte. The other side of the proton exchange membrane was interfaced with a layer of electrocatalyst. The electrocatalyst was commercially obtained carbon supported-platinum. The test cell consisted of two compartments sealed from each other, an upper compartment open to air that could be filled with a liquid electrolyte of either acid or base, and a lower compartment with inlets and outlets for delivery of vapor phase fuel.

The upper compartment was used to contain the cathode, which, for these experiments, was a high surface area platinum gauze electrode which rested upon the teflon-base of the upper compartment. The constant electrolyte volume also fixed the current path length from the meniscus, where most of the oxygen reduction occurs, to the palladium barrier. The high surface area platinum electrocatalyst exposed to the fuel served as an anode. Current-voltage I-V curves were measured potentiostatically for both fuel cells.

Typical results are shown in FIG. The primary observation of the experiment was that the open circuit voltage OCV for the hybrid cell containing alkali in the upper compartment was invariably higher than for the cell containing acid, typically by 60 mV, indicating superior kinetics for the oxygen reduction reaction in base. At low fuel cell currents, the current-voltage performance for the base-containing cell was superior.

While in the foregoing specification this invention has been described in relation to certain preferred embodiments thereof, and many details have been set forth for purpose of illustration, it will be apparent to those skilled in the art that the invention is susceptible to additional embodiments and that certain of the details described herein can be varied considerably without departing from the basic principles of the invention.

Effective date : Year of fee payment : 4. A hybrid electrolyte system for fuel cells and other electrochemical reactors comprising an acid electrolyte, a base electrolyte, and a proton permeable dense phase separating the acid electrolyte from the base electrolyte. Description of Prior Art Electrochemical devices, such as fuel cells, comprise an electrolyte, an air or oxygen electrode, and a fuel electrode. The dispersion of platinum is especially fine for the Pt HGS 1—2 nm catalyst, as reflected in the blue inset and in the particle size distribution before the degradation test.

This well-defined particle growth is attributed to the defined pore size distribution within the mesoporous network. The insets highlight sub-regions in the micrograph. The yellow circles for the Pt HGS 1—2 nm material mark several particles which result from untypical, strong particle growth. Bars referring to the same particle diameter before filled and after degradation shaded are pictured next to each other. The strong particle growth is the most obvious occurring degradation process and the shape of the formed clusters in the insets with higher magnification indicates coalescence to be an important degradation mechanism.

The particle growth is also clearly reflected in the change in particle size distribution with a tailing towards larger particle sizes, which is frequently interpreted as evidence for agglomeration and coalescence in post-mortem TEM investigations of fuel cell catalysts. Particle growth is accompanied by detachment of particles from the Vulcan support, likely as a consequence of the corrosion of carbon in direct contact with platinum red circle , and dissolution of platinum particles blue arrow. Blue, red and green symbols mark dissolving, detached and coalescing platinum particles.

We hereafter refer to this type of particle growth mechanism as agglomeration due to migration, as it is often found in the literature [15]. The situation is slightly different for the platinum particles depicted by the 2nd green circle, where the particles are already in contact with each other from the very beginning and coalescence is immediately possible resulting in larger particles with reduced surface area. We hereafter refer to this type of mechanism as coalescence due to initial contact. In this case, coalescence is most likely controlled by surface diffusion processes of platinum atoms of the touching particles.

Green circles indicate examples for agglomeration of platinum nanoparticles. The particle size distributions before and after degradation cycles indicate both particle growth and dissolution to occur. Naturally the changes in microstructure are more severe at this later stage in the degradation process and the increase in inter-particle distances becomes even more obvious than after cycles. In fact, a large number of platinum particles decrease in size due to dissolution, which appears to be the more important degradation pathway regarding its relative contribution to surface area loss at this later stage in the degradation process.

Moreover, many clusters that were formed via coalescence are small compared to what would be expected on the basis of the amount of platinum particles initially present in that region, which indicates that also the formed clusters shrink as a consequence of dissolution. These observations are also reflected in the comparison of particle size distributions before and after degradation cycles.

While the number of medium sized particles has decreased, both the amount of larger and smaller particles has increased, confirming that not only coalescence, but also dissolution of platinum is taking place. It needs to be emphasized that considering the presence of dissolution it cannot be excluded that Ostwald ripening may contribute to the observed particle growth. Namely, in a real fuel cell electrode dissolved platinum ions could either precipitate in the ionomer or on larger particles within the extended 3D structure of the catalyst layer, besides being washed out with the exhaust water.

The catalyst on the grid is exposed to a large volume of electrolyte and thus the concentration of dissolved platinum species remains low, which makes platinum re-deposition less likely to occur. Only for cases in which high amounts of platinum dissolve in a very short period of time, the dissolved platinum concentration at the interface may be sufficient to observe re-deposition and Ostwald ripening in an IL-TEM experiment. The importance of the 3D structure of the catalyst layer for the observation of significant re-deposition as highlighted previously [16,40] , was recently confirmed by the investigation of catalyst layers with thicknesses of several micrometers by using IL-SEM [64].

As already seen from the macroscopic stability test, the Pt HGS 1—2 nm catalyst is the least stable of the three catalyst materials, which was attributed to a massive loss of the smallest platinum particles in the initial degradation stage based on an analysis of the CO-stripping features. An unambiguous identification of the underlying degradation mechanisms for the Pt HGS 1—2 nm material cannot be easily accomplished, because of the complex structure of the HGS support and the small particle size.

Indeed platinum dissolution possibly with successive re-deposition of some of the dissolved platinum appears as the most likely cause for the strong ECSA losses of the Pt HGS 1—2 nm material. In addition, the formation of large platinum particles with a size of about 10 nm highlights a minor contribution of particle growth to the overall degradation. However, coalescence is an at least as likely contributor to the observed particle growth. In particular, the fact that the Pt HGS 3—4 nm catalyst is prepared by sintering upon thermal treatment of the Pt HGS 1—2 nm catalyst, shows that coalescence is feasible and that enough platinum nanoparticles are initially in sufficient proximity to each other to allow agglomeration and coalescence.

Therefore it is reasonable to assume that it also plays an important role in the particle growth observed for the Pt HGS 1—2 nm material. Only a very slight increase in the number of the smallest particles and a concomitant minor decrease in the number of larger particles can be interpreted as an indication of dissolution. The particle size distribution before and after cycles depicts more clearly now that the number of small particles increases while the number of larger particles decreases, which is evidence for platinum dissolution to occur.

It also cannot be excluded that the detachment of platinum particles from the external surface of the HGS catalyst, which are not incorporated in the mesoporous network, may contribute to the modest decrease in total number of platinum particles. It is worth to mention in this context that first measurements in real fuel cells also indicate an improved stability in line with these findings [71].

The red rectangles in the micrographs mark regions, which are magnified on the right. Filled bars refer to the particle size before, shaded bars to the particle size after degradation. The results presented so far, can be summarized in the following. A structural breakdown of the carbon support is not playing a dominant role at room temperature for the three investigated catalysts. Nevertheless, the importance of such a mechanism may strongly increase with increasing temperature [54].

Platinum dissolution is an important degradation mechanism for all three investigated catalysts. The massive loss of smallest particles for the Pt HGS 1—2 nm catalyst indicates that dissolution is most severe for materials with very small platinum particle size.

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In contrast, Pt HGS 3—4 nm does not show any significant particle growth. Despite some similarities, there are distinct differences in the degradation behavior of the three materials. In order to understand how materials can be designed to mitigate degradation mechanisms and still maintain high ORR activities, it is important to analyze which properties are responsible for the observed differences in the degradation behavior. Particles can agglomerate due to migration on or shrinkage of the carbon support. When particles establish contact or are already in contact after synthesis, coalescence due to surface diffusion of platinum atoms during potential cycling leads to successive decrease in surface area.

The probability of agglomeration and coalescence increases with decreasing distance between particles on the support, since shorter travelling is required to establish contact. As degradation proceeds, the distance between the particles will increase and the contribution of agglomeration and coalescence in the overall surface area loss should decrease over time. In fact, for the latter two catalysts, a large fraction of platinum nanoparticles are in contact already from the beginning and thus coalescence due to initial contact appears to be the likely cause for the massive particle growth of these materials.

A , C , E and G depict the catalysts before potential cycling, while B , D , F and H are micrographs of the identical locations after potential cycles between 0. A and B were reprinted from [52] with permission; Copyright Elsevier. G and H were kindly provided by Arenz and co-workers. It needs to be noted that highly graphitized supports are often used to prevent carbon corrosion however such supports quite commonly exhibit small specific surface areas. At the same time, high platinum loadings are often desirable to reduce the thickness of the catalyst layer in the fuel cell, and thus to reduce the mass transport limitations in the catalyst layer.

While such materials appear susceptible towards agglomeration and coalescence, they do show high carbon corrosion tolerance. The determination of the inter-particle distance along the surface of the support is not straightforward, as TEM only provides a 2D projection of a 3D reality. It should be noted that a similar equation was derived by Watanabe et al. The average inter-particle distance AID corresponds to the length a particle has to travel along the support surface to meet the next platinum particle. Additionally, three further catalyst materials are included.

Only for two materials, i. The calculated average inter-particle distances for these two particular catalysts are the largest among all depicted catalysts i. At these elevated temperatures, carbon corrosion played a much more dominant role, and the shrinkage of the carbon support led to a decrease of the inter-particle distances and to successive coalescence as a secondary degradation process. A shrinkage of the carbon support due to carbon corrosion with successive decrease in inter-particle distances and coalescence can be observed. The images were reprinted with permission from [49].

Copyright Elsevier. Figure A Dependence of the AID on platinum content for various platinum particle sizes, calculated for a A value of zero for the inter-particle distance in the model would correspond to a densely packed monolayer of platinum nanoparticles on the carbon surface, while negative values indicate that this monolayer would even be exceeded and further particles would be stacked on top. On the contrary, remarkably larger average inter-particle distances are offered by high surface area carbon supports HSA , e.

This indicates that carbon supports with a high specific surface area have the advantage that higher platinum loadings or smaller platinum particle sizes can be used without ending up in inter-particle distances below the critical value at which coalescence is expected. Apart from agglomeration and coalescence, it needs to be mentioned that small inter-particle distances may similarly enhance an Ostwald-ripening type of mechanism, because large concentrations of dissolved platinum can be expected in close proximity to other particles in regions with high particle density, acting as sites for redeposition.

However, while coalescence due to initial contact occurs most likely during the initial stage of catalyst degradation, redeposition of dissolved platinum requires a significant dissolution first, so it probably has a stronger contribution to the overall surface area loss at a later degradation stage, as the findings of Hodnik et al.

The Pt HGS 1—2 nm catalyst on the other side, which again has the same Pt content as the Pt HGS 3—4 nm catalyst along with the same carbon support, cannot reach a sufficient inter-particle separation due to the dispersion of the platinum into very small particles. The total number of platinum nanoparticles and thus their average distance is thus much smaller for the Pt HGS 1—2 nm compared to the Pt HGS 3—4 nm catalyst. Overall under potential cycling conditions between 0.

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Even though a distance of for instance 20 nm may appear as sufficiently large to prevent coalescence, it is crucial to understand that a certain fraction of nanoparticles will be in closer proximity, because the distance l is only an average value, which in reality corresponds to an inter-particle size distribution. In this case the fraction of smallest inter-particle distances i. The above mentioned regimes should be seen as a first guideline, however they require confirmation and refinement on the basis of larger data sets.

When a defined mass of platinum is dispersed on a given carbon support, decreasing the platinum particle size on a nanometer scale implicates i an alteration of electronic properties of platinum ii an increasing ECSA and iii decreasing inter-particle distances. This observation was discussed and investigated in more detail in several recent works [12,14] and indicates that an increase in initial mass activity for supported catalysts is optimized by aiming for very small particle sizes. The comparison indicates that besides the AID, the particle size has an outstanding impact on catalyst stability, which is, for instance, also supported by the findings of Shao-Horn and coworkers [14] or Makharia et al.

According to these studies especially catalysts with a large fraction of platinum particles smaller than 2 nm suffer from the most severe surface area losses.

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A typical explanation for this is that with decreasing size of the platinum particles the curvature of the particles increases and thus the surface energy rises, which can impact the dissolution thermodynamics according to the so-called Gibbs—Thomson effect. This is expected to result in a negative shift of the Nernst potential of platinum dissolution for these particles compared to bulk platinum [15]. Recent observations on the dissolution of polycrystalline platinum with an electrochemical flow cell and online detection of dissolved platinum in the electrolyte via inductively coupled plasma mass spectrometry ICP-MS [82] have resulted in three major conclusions: i platinum dissolution is a transient process that occurs only when the potential changes cause a substantial change in the platinum surface state, while at constant potential conditions no platinum dissolution was observed; ii platinum dissolution can be separated into anodic and cathodic dissolution; and iii the amount of anodically dissolved Pt is linked to low-coordinated surface atoms and the amount of cathodically dissolved platinum is linked to the extent of oxidation.

In the light of these observations the impact of particle size on platinum dissolution could be explained by the enhanced oxophilicity and thus increased oxide content at a given potential with decreasing particle size [11,14]. Additionally, the contribution of under-coordinated edge and corner sites to the overall surface area is rising for smaller particles.

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Still, more quantitative investigations will be necessary to resolve the effect of particle size on dissolution, while particularly taking into account the increase in ECSA. Namely, a larger ECSA naturally implicates that more platinum atoms are available for both the desired oxygen reduction reaction as well as the undesirable dissolution of platinum. Considering that significantly larger particles lead to low mass activities, an average particle size of about 3—4 nm, as often employed anyway in commercial systems, seems to be indeed a reasonable choice for fuel cell materials.

Moreover, a narrow particle size distribution, in order to neither waste platinum for unstable smaller particles nor for too large particles with low mass activity, are a precondition for the efficient utilization of platinum. Makharia et al. All catalysts were subjected to the same single-cell aging test, namely potential cycling between 0. Figure Impact of catalyst particle size and post-synthesis heat treatment on the normalized platinum surface area loss.

The cathode side of the membrane electrode assembly MEA is cycled between 0. The graph was reproduced from [81] with permission. Copyright The Electrochemical Society. Figure Impact of catalyst particle size and post-synthesis heat treatment on the normalized platinum surfa First, the findings of Makharia et al. Therefore, the stabilization due to thermal treatment cannot merely be attributed to an increase in average particle size, but also other aspects contribute to the effect of the thermal treatment.

However, an explanation for the origin of the improved stability upon thermal treatment was not provided. In this context, Shao-Horn et al. Since those smaller particles are expected to dissolve faster under degradation conditions, the improved stability of the thermally treated catalyst materials with a smaller fraction of small particles could be explained with the particle size effect, as described above, however, while taking the distribution into account.

However a further effect that contributes to the thermal-treatment effect, which is linked again to the inter-particle distance, was not considered so far.

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This is probably because particles in close proximity to each other or even in direct contact will preferentially already coalesce during the thermal treatment step. Therefore, the fraction of small inter-particle distances will decrease substantially and thus degradation due to agglomeration in general and coalescence due to initial contact in particular will be less likely to occur under fuel-cell operation conditions. Again, the non-treated catalyst shows significant coalescence among other degradation mechanisms, contrary to the thermally treated catalyst that shows no signs of coalescence during potential cycling at room temperature.

It is noteworthy that the effect of thermal treatment was recently also addressed by Stephens et al. This emphasizes that the thermal-treatment step to high temperatures is indeed a valuable option to improve the stability of the material, as long as the particle size can be kept in the desirable range for a high mass activity. This observation is in good agreement with literature reports that emphasize the positive effect of graphitization on the resistance against carbon corrosion [23,67,84].

The effect of the surface structure of the carbon supports for fuel cell catalysts was demonstrated by Reetz et al. In general micro- and mesopores in carbon support materials are believed to act as a kind of physical barrier [86]. The concept of using the support surface structure to separate the platinum particles can be extended to the extreme case of a 3D interconnected mesoporous network.

Here the platinum particles are confined to the mesoporous structure that provides various hosting sites. These hosting sites also play a crucial role during the heat-treatment step and the control of the particle growth via confinement of Pt in pores [71]. Overall, the Pt HGS 3—4 nm material can thus be considered as an extension of the graphitized catalysts offering not only a high degree of graphitization, but also a high specific surface area and stabilization by a confinement to the pores, which results in a highly active and stable catalyst.

The combination of the graphitization, the thermal-treatment effect and the pore-confinement opens up also an interesting option for highly stable and active Pt-alloy fuel cell catalysts.

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The disadvantage of a pronounced thermal-treatment step for standard supports, i. The extraordinary control of the particle growth in the HGS support can help to circumvent this issue and thus offers an excellent opportunity for the synthesis of a new class of high-temperature annealed platinum-alloy catalysts with high ECSA and stability. In particular identical location electron microscopy revealed how the material design can impact the degradation behavior of electrocatalysts under accelerated-aging conditions.

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Coalescence of not sufficiently well separated platinum nanoparticles on the support often plays a major role, especially at the initial stage of catalyst degradation. The average inter-particle distance AID , which depends on the surface area of the carbon support, the platinum content of the catalyst, the platinum particle size as well as the quality of the distribution of the platinum particles on the support, was derived as a simple quantity that aids to evaluate an eventual impact of coalescence.

In order to optimize carbon-supported catalysts for both, activity and mass activity, it is thus necessary to thoroughly consider all of these aspects. In particular the transformation of the catalyst material during long-term operation should already be taken into account at the stage of materials design. The details of the synthesis and characterization of the HGS support as well as the pore-confinement of the Pt nanoparticles can be found in our previous publication [71]. The electrochemical procedures are shortly summarized in the following, as they have been previously described more extensively [12,16,40,72,73].

All electrochemical measurements were performed in a three-compartment electrochemical Teflon cell, while using a rotating disk electrode RDE setup, a radiometer analytical rotation controller and a Gamry Reference potentiostat. However, all potentials were referenced to the reversible hydrogen electrode RHE , which was determined prior to every measurement.

The rotator, the potentiostat and the gas flow were automatically regulated by using a LabVIEW based software that was developed in-house [87].

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The working electrode was made of a Teflon tip with an embedded glassy carbon disc 5 mm diameter, 0. In the latter case the catalyst was pipetted on the grid after contacting. The standard electrolyte volume was about mL and all tests were performed at room temperature. Evaluating fuel cells Fuel cells have different strengths and weaknesses, depending on the intended use.

Fuel cells in spacecraft Hydrogen-oxygen fuel cells are used in spacecraft. Their strengths include: they have no moving parts to maintain they are small for the amount of electricity they produce the water they produce can be drunk Hydrogen-oxygen fuel cells must be supplied with hydrogen fuel and oxygen. Fuel cells in vehicles Some cars and buses contain hydrogen-oxygen fuel cells. Fuel cell vehicle Petrol or diesel vehicle Strengths Quiet in use, only waste product is water, fewer moving parts Petrol and diesel are easier to store, thousands of filling stations Weaknesses Hydrogen is more difficult to store, few filling stations Noisy in use, carbon dioxide is a waste product, many moving parts Hydrogen, diesel and petrol are all highly flammable fuels, but hydrogen is more difficult to store.

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Question Suggest a reason why the quietness of fuel cell vehicles may be a weakness. Reveal answer up. Quiet in use, only waste product is water, fewer moving parts. Petrol and diesel are easier to store, thousands of filling stations.