Patent Publication Number: US-2018034064-A1

Title: Shape controlled palladium and palladium alloy nanoparticle catalyst

Description:
BACKGROUND 
     A unitized electrode assembly for a fuel cell includes an anode, a cathode and an electrolyte between the anode and cathode. In one example, hydrogen gas is fed to the anode, and air or pure oxygen is fed to the cathode. However, it is recognized that other types of fuels and oxidants can be used. At the anode, an anode catalyst causes the hydrogen molecules to split into protons (H + ) and electrons (e − ). The protons pass through the electrolyte to the cathode while the electrons travel through an external circuit to the cathode, resulting in production of electricity. At the cathode, a cathode catalyst causes the oxygen molecules to react with the protons and electrons from the anode to form water, which is removed from the system. 
     The anode catalyst and cathode catalyst commonly include platinum or a platinum alloy. 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 the oxygen-reducing cathode in order to improve the efficiency of the fuel cell. 
     SUMMARY 
     A unitized electrode assembly (UEA) for a fuel cell includes an anode electrode, a cathode electrode, an electrolyte and palladium catalytic nanoparticles. The electrolyte is positioned between the cathode electrode and the anode electrode. The palladium catalytic nanoparticles are positioned between the electrolyte and one of the anode electrode and the cathode electrode. The palladium catalytic nanoparticles have a {100} enriched structure. A majority of the surface area of the palladium catalytic nanoparticles is exposed to the UEA environment. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view of a fuel cell repeat unit having a catalyst layer. 
         FIG. 2  is an enlarged view of the catalyst layer of the fuel cell repeat unit of  FIG. 1 . 
         FIG. 3  is a transmission electron microscope (TEM) image of palladium nanoparticles having an enriched {100} structure. 
     
    
    
     DETAILED DESCRIPTION 
     Palladium nanoparticles for use as a catalyst in a unitized electrode assembly (UEA) of a fuel cell are described herein. The palladium nanoparticles have a {100} enriched structure. Regular or non-shape controlled palladium is unstable in the UEA environment and has a lower oxygen reduction reaction (ORR) activity than platinum. However, palladium nanoparticles having a {100} enriched structure were unexpectedly found to have an activity comparable to carbon supported platinum catalysts. 
     Fuel cells convert chemical energy to electrical energy using one or more fuel cell repeat units.  FIG. 1  illustrates a perspective view of one example fuel cell repeat unit  10 , which includes unitized electrode assembly (UEA)  12  (having anode catalyst layer (CL)  14 , electrolyte  16 , cathode catalyst layer (CL)  18 , anode gas diffusion layer (GDL)  20  and cathode gas diffusion layer (GDL)  22 ), anode flow field  24  and cathode flow field  26 . Fuel cell repeat unit  10  can have coolant flow fields adjacent to anode flow field  24  and cathode flow field  26 . Coolant flow fields are not illustrated in  FIG. 1 . 
     Anode GDL  20  faces anode flow field  24  and cathode GDL  22  faces cathode flow field  26 . Anode CL  14  is positioned between anode GDL  20  and electrolyte  16 , and cathode CL  18  is positioned between cathode GDL  22  and electrolyte  16 . This assembly, once bonded together by known techniques, is known as unitized electrode assembly (UEA)  12 . In one example, fuel cell repeat unit  10  is a proton exchange membrane fuel cell (PEMFC) that uses hydrogen fuel (i.e., hydrogen gas) and oxygen oxidant (i.e., oxygen gas or air). It is recognized that fuel cell repeat unit  10  can use alternative fuels and/or oxidants. 
     In operation, anode GDL  20  receives hydrogen gas (H 2 ) by way of anode flow field  24 . Anode CL  14 , 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  18 ; the protons pass through electrolyte  16  to cathode CL  18 , while the electrons travel through external circuit  28 , resulting in a production of electrical power. Air or pure oxygen (O 2 ) is supplied to cathode GDL  22  through cathode flow field  26 . At cathode CL  18 , oxygen molecules react with the protons and electrons from anode CL  14  to form water (H 2 O), which then exits fuel cell  10 , along with excess heat. Electrolyte  16  is located between anode CL  14  and cathode CL  18 . 
     Electrolyte  16  allows movement of protons and water but does not conduct electrons. Protons and water from anode CL  14  can move through electrolyte  16  to cathode CL  18 . Electrolyte  16  can be a liquid, such as phosphoric acid, or a solid membrane, such as a perfluorosulfonic acid (PFSA)-containing polymer or ionomer. PFSA polymers are composed of fluorocarbon backbones with sulfonate groups attached to short fluorocarbon side chains. Example PFSA polymers include Nation® by E.I. DuPont, USA. Electrolyte  16  can be an absorption electrolyte or a non-absorption electrolyte. Absorption electrolytes include but are not limited to sulfuric acid and phosphoric acid. Non-absorption electrolytes include but are not limited to PFSA polymers and perchloric acid. 
     Anode CL  14  is adjacent to the anode side of electrolyte  16 . Anode CL  14  includes a catalyst, which promotes electrochemical oxidation of fuel (i.e., hydrogen). Example catalysts for anode CL  14  include carbon supported platinum atoms. Alternatively, anode CL  14  can include the palladium catalytic nanoparticles described below with respect to cathode CL  18 . 
     Cathode CL  18  is adjacent to the cathode side of electrolyte  16 , and opposite anode CL  14 . Cathode CL  18  includes a catalyst that promotes electrochemical reduction of oxidant (i.e., oxygen). As described further below, the catalyst includes palladium nanoparticles having an enhanced {100} structure. 
       FIG. 2  is an enlarged view of cathode CL  18  of  FIG. 1 , which includes catalyst  30  (having palladium catalytic nanoparticles  32  and catalyst support  34 ) and ionomer  36 . Ionomer  36  of cathode CL  18  contacts catalysts  30  to form a layer having palladium catalytic nanoparticles  32  finely dispersed throughout. Cathode CL  18  is a matrix of catalyst supports  34 , ionomer  36  and palladium catalytic nanoparticles  32 . The matrix allows electrons, protons, water and reactants to move through it. 
     Catalyst  30  of cathode CL  18  promotes electrochemical reduction of oxidant. As shown in  FIG. 2 , catalyst  30  includes palladium catalytic nanoparticles  32  supported by or on catalyst supports  34 . Catalyst supports  34  are electrically conductive supports, such as carbon black supports. 
     Palladium catalytic nanoparticles  32  are distributed on catalyst supports  34 . Palladium catalytic nanoparticles  32  are formed of palladium or a palladium alloy. The palladium alloy can be an alloy of palladium and at least one transition metal. Example transition metals include but are not limited to titanium, chromium, vanadium, manganese, iron, cobalt, nickel, copper, and zirconium. The palladium alloy can also be an alloy of palladium and at least one noble metal. Example noble meals include but are not limited to rhodium, iridium, platinum, and gold. Palladium catalytic nanoparticles  32  are used as the catalyst in cathode CL  18 , and the majority of the surfaces of palladium catalytic nanoparticles  32  are exposed to the environment of cathode CL  18  and UEA  12  of  FIG. 1 . That is, palladium catalytic nanoparticles  32  are exposed to the UEA environment in order to promote the electrochemical reduction of oxidant. 
     In cathode CL  18 , palladium nanoparticles  32  promote the formation of water according to the oxidation reduction reaction: O 2 +4H + +4e − →2H 2 O. Palladium catalytic nanoparticles  32  are only active when they are accessible to protons, electrons and the reactant. Ionomer  36  in cathode CL  18  connects electrolyte  16  to palladium catalytic nanoparticles  32  on an ionic conductor level. As illustrated in  FIG. 2 , ionomer  36  creates a scaffolding structure between catalyst supports  34  of catalyst  30 . Ionomer  36  creates a porous structure that enables gas to travel through cathode CL  18  and water to be removed from cathode CL  18 . Ionomer  36  also transfers protons from electrolyte  16  to active catalyst sites on palladium catalytic nanoparticles  32 . Anode CL  14  can have the same structure as cathode CL  18 . 
       FIG. 3  is a transmission electron microscope (TEM) image of palladium catalytic nanoparticles  32 . Palladium catalytic nanoparticles  32  have dimensions on the on the nanoscopic scale. In one example, palladium catalytic nanoparticles  32  have an edge length between about 2 nanometers and about 50 nanometers. In another example, palladium catalytic nanoparticles  32  have an edge length between about 3 nanometers and about 10 nanometers. 
     Palladium catalytic nanoparticles  32  are shape controlled to have a {100} enriched structure. Non-shape controlled palladium nanoparticles are typically cubo-octahedral in shape. At the particle size of interest (i.e., between 2 nanometers and 50 nanometers), a cubo-octahedral has at most about 10% to about 15% {100} surfaces. Palladium catalytic nanoparticles  32  contain a greater surface area of {100} surfaces compared to a cubo-octahedral nanoparticle. In one example, at least about 30% of the surface area of palladium catalytic nanoparticles  32  is bound by {100} surfaces. In another example, at least about 50% of the surface area of palladium catalytic nanoparticles  32  is bound by {100} surfaces. In a further example, at least about 70% of the surface area of palladium catalytic nanoparticles  32  is bound by {100} surfaces. 
     A cubic nanoparticle consists of six total surfaces, all of which are bound by {100} surfaces. Palladium catalytic nanoparticles  32  have a generally cubic shape. In one example, at least about 30% of the surfaces are bound by {100} surfaces. In another example, at least about 50% of the surfaces are bound by {100} surfaces. In a further example, at least about 70% of the surfaces are bound by {100} surfaces. 
     The activity of palladium nanoparticles is highly dependent on the facets or the surfaces of the nanoparticles. Regular or non-shape controlled palladium nanoparticles are susceptible to dissolution in the UEA environment. More specifically, non-shape controlled palladium is reactive at the potential cycling conditions of a typical fuel cell. During potential cycling, palladium oxidizes, dissolves and migrates away from cathode. The dissolved palladium reduces the ORR activity and may poison the electrolyte. 
     In contrast to non-shape controlled palladium nanoparticles, palladium catalytic nanoparticles  32  have a {100} enhanced structure. Palladium catalytic nanoparticles  32  are more active (i.e., have a higher ORR activity) than non-shape controlled palladium nanoparticles because of the increased number of {100} facets on palladium catalytic nanoparticles  32 . As described above, non-shape controlled palladium nanoparticles are typically cubo-octahedral, and contain a maximum of about 10% to about 15% {100} surfaces. In one example, palladium catalytic nanoparticles  32  show an ORR activity that is about four- to about six-times higher than that of non-shape controlled palladium nanoparticles. As discussed above, palladium catalytic nanoparticles  32  can be formed of a palladium alloy. Alloying palladium with at least one additional transition metal or noble metal will further enhance the ORR activity of palladium catalytic nanoparticles  32 . 
     The specific activity of palladium catalytic nanoparticles  32  is much greater than that of non-shape controlled palladium nanoparticles, and is comparable to or greater than that of carbon supported platinum catalysts. Platinum is a high cost noble metal. Palladium is less expensive than platinum. Using palladium catalytic nanoparticles  32  reduces the material costs of the UEA while achieving a comparable activity. 
     As illustrated in the following example, palladium catalytic nanoparticles  32  having {100} enhanced structures are more active than palladium octahedron nanoparticles and non-shape controlled palladium nanoparticles. Further, palladium catalytic nanoparticles  32  have an activity comparable to or greater than that of carbon supported platinum. 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. 
     EXAMPLE 
     Four electrodes were prepared. Electrode A contained carbon supported cubic palladium nanoparticles. The cubic palladium nanoparticles were shaped-controlled nanoparticles having essentially a total of six faces, each of which was bound by a {100} surface. 
     Electrode B contained carbon supported octahedron palladium nanoparticles. The octahedron palladium nanoparticles were shape-controlled nanoparticles having essentially a total of eight faces, each of which was bound by a {111} surface. 
     Electrode C contained carbon supported non-shape controlled palladium nanoparticles. As described above, typically, non-shape controlled palladium nanoparticles have a cubo-octahedral shape. The catalyst of electrode C was purchased from BASF SE of Ludwigshafen, Germany. 
     Electrode D contained carbon supported non-shape controlled platinum nanoparticles. The catalyst of electrode D was purchased from TKK of Japan. 
     Rotating disk electrode (RDE) experiments were conducted for each electrode in 0.1 M HClO 4  (a non-absorption electrolyte). The electrodes were rotated at 1600 rotations per minute (RPM). The specific activity was calculated at 0.9 volts (V) and normalized with respect to the electrochemical active area of the catalyst. The results of the experimental runs are presented in Table 1 below. 
     
       
         
           
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 RDE results in HClO 4   
               
            
           
           
               
               
               
            
               
                   
                   
                 Specific activity at 0.9 V 
               
               
                 Electrode 
                 Catalyst 
                 (mA/cm 2 ) 
               
               
                   
               
            
           
           
               
               
               
            
               
                 A 
                 Pd cube/C 
                 0.3 
               
               
                 B 
                 Pd octahedron/C 
                 0.03 
               
               
                 C 
                 Pd/C (Purchased) 
                 0.05 
               
               
                 D 
                 Pt/C (Purchased) 
                 0.24 
               
               
                   
               
            
           
         
       
     
     As illustrated in Table 1, non-shape controlled palladium nanoparticles (Electrode C) (i.e., having a maximum of about 10%-15% {100} surfaces) are less active than non-shape controlled platinum nanoparticles (Electrode D); palladium nanoparticles have an octahedron shape (Electrode B) (i.e., having about 0% {100} surfaces) are even less active. Palladium nanoparticles having a cubic shape (Electrode A) (i.e., having about 100% {100} surfaces) are more active than each of the other catalysts tested, including the non-shape controlled platinum nanoparticles. Comparing Electrodes A, B and C shows that increasing the percentage of {100} surfaces improves the specific activity. 
     RDE experiments were also conducted for an absorption electrolyte. Electrodes E, F and G were prepared according to Table 2 below. Electrode E contained the same catalyst as Electrode A (carbon supported cubic palladium nanoparticles), Electrode F contained the same catalyst as Electrode B (carbon supported octahedral palladium nanoparticles), and Electrode G contained the same catalyst as Electrode C (carbon supported non-shape controlled palladium). The electrodes were rotated at 1600 RMP in 0.1 M H 2 SO 4  solution that was saturated with O 2 . The specific activity was calculated at 0.85 V. The results of the experimental runs are presented in Table 2. 
     
       
         
           
               
             
               
                 TABLE 2 
               
             
            
               
                   
               
               
                 RDE results in H 2 SO 4   
               
            
           
           
               
               
               
            
               
                   
                   
                 Specific activity at 0.85 V 
               
               
                 Electrode 
                 Catalyst 
                 (mA/cm 2 ) 
               
               
                   
               
               
                 E 
                 Pd cube/C 
                 0.26 
               
               
                 F 
                 Pd octahedron/C 
                 0.02 
               
               
                 G 
                 Pd/C (Purchased) 
                 0.06 
               
               
                   
               
            
           
         
       
     
     As illustrated in Table 2, the cubic palladium (Electrode E) was more active than the octahedral palladium (Electrode F) and the non-shape controlled palladium (Electrode G). Comparing Electrode E (100% {100} surfaces) to Electrode F (0% {100} surfaces) and Electrode G (10%-15% {100} surfaces) shows that increasing the percentage of {100} surfaces improves the specific activity. Further, comparing Table 1 and Table 2 shows that cubic palladium nanoparticles have a higher activity than octahedral palladium nanoparticles and non-shape controlled palladium nanoparticles when used with either a non-absorption electrolyte or an absorption electrolyte. 
     Although the present invention has been described with reference to preferred embodiments, workers skilled in the art will recognize that changes may be made in form and detail without departing from the spirit and scope of the invention.