Confusión de los conceptos de mínimo exento y gasto deducible:

While not as prolific as perovskites and spinels, pyrochlores represent another major oxide system of interest for a number of different applications, including oxygen catalysis. Pyrochlore catalysts were first identified by Dr. Horrowitz et al., who discovered a series of specific com- pounds (Bi2Ru2-xBixO7-yand Pb2Ru2-xPbxO7-y) and combinations thereof that exhibited remarkably

strong catalytic properties for ORR and OER.[95] Consequently, their uniquely tunable structure and catalytic properties continues to spur a consistent output of catalyst research.

Figure 2.7: Pyrochlore structure (A2B2O7). Saved from http://www.chemtube3d.com/solidstate/SS-

Pyrochlore.htm on 12/04/2017

Pyrochlore Structure

A pyrochlore is a ternary oxide with an ideal formula of A2B2O7, belonging to a family

of phases iso-structurally based on the (NaCa)(NbTa)O6F/OH mineral.[236] Figure S.7 shows an

illustration of a typical pyrochlore structure. There are around 150 distinct pyrochlore compounds, which are predominantly cubic and ionic in nature. The A-site cations (∼1 Å radii), which are generally larger than the B-site cations (∼0.6 Å radii), range from rare earth elements to elements with inert lone-pair electrons, while B-site cations range from transition metals to post-transition metals, with oxidation states ranging from+2 to +5.[236] The pyrochlore structure, much like per-

ovskites, has a high tolerance for disorder and defect formation, which includes oxygen vacancies and cation substitution, both of which have been found to affect several material properties.[67] The unit cell of an ideal pyrochlore structure contains 8 compounds (88 atoms) with a cubic space group of Fd3m, and a lattice constant ranging from ∼10 to ∼11 Å depending on the size of the cationic species, and the degree of disorder due to lattice distortion, cationic substitution, and defect concentration. Pyrochlores contain four crystallographically non-equivalent atomic sites, which includes 2 non-equivalent oxygen anionic sites. To distinguish the 2 anionic sites, many represent the formula as A2B2O6O0, illustrating O and O0 as non-equivalent sites. This represen-

tation is often convenient for non-stoichiometric pyrochlores containing oxygen vacancies, which are almost exclusively located at the O0sites within the sublattice.

Many describe the pyrochlore structure as an interpenetrating network of corner sharing BO6 octahedra and linear A2O0 chains. The structure has a network of 8-fold and 6-fold coor-

dinated oxygen anion polyhedral around each A and B-site, respectively. Others have described the structure as an anion deficient fluorite unit cell.[3] The A-site polyhedral is a scalenohedra (distorted cube), and contains 6 equally spaced O anions and 2 equally spaced O0 anions, which are slightly closer in distance to the A-cation, though the A-O0 and A-O distances are equal if the A-site polyhedra conforms to a cubic coordination. The smaller B-site cations generally have a trigonal antiprism coordination with the O anions, which are equally spaced from the central cations. The B-O-B bond angles are buckled at around 120-140° in contrast to the perovskite’s B-O-B angle of 180°, while the A2O0 is a linearly coordinated network to a tetrahedrally shared O0

atoms within the A2O0network.

To elaborate more on the pyrochlore structure, one previously inferred feature is the vari- ation in the coordination geometry around the A and B-sites depending on the placement of the O-sites within the crystal structure. The distribution of the O-sites is quantified by the x-parameter, which is limited between 0.3125 and 0.375. For instance, at 0.3125, the B-site polyhedra conforms to a perfect octahedral geometry while the A-site polyhedra are distorted to form hexagons of six

O anions, whose plane is perpendicular to the A-O0 axis. For x = 0.375, the A-site polyhedra have a perfect cubic coordination while the B-site polyhedra have a trigonal antiprism geometry. Consequently, both the A-site and B-sites cannot simultaneously have an octahedral coordination and a cubic coordination polyhedral.

Figure 2.8: Polarization curve comparison between Pb2[Ru1.42Pb4+0.58]O6.5 and 15% Pt on

carbon.[95]

Oxygen Catalysis

The most attractive pyrochlore feature is the ability to ostensibly tune a wide range of physical properties due to their high degree of structural distortion, defection formation, and cationic doping, underscoring their substantial application value. For instance, pyrochlores have been shown to be insulating, semiconducting, and even metallic in behavior. Other examples include pyrochlores that are strong refractories, and lanthanide-containing pyrochlores that are flu- orescent and phosphorescent. In the case of oxygen catalysis, a series of pyrochlores compounds ([Pb,Bi]2[Ru,Ir]2-x[Pb,Bi]O6O0y) have demonstrated strong catalytic activity, as well as high elec-

tronic and ionic conductivity, which are enabled by the conductive RuO6/IrO6 network, and high

oxygen vacancy (O0-sites) concentration, respectively.[277, 240, 190, 186, 144, 135, 95, 97, 60, 51, 48] Consequently, this became one of my major material systems of interest for combinatorial high-throughput electrochemical testing (See Chapter 6).

performed by Horowitz et al. in the early 1980s.[95, 97] He first reported Bi2Ru2-xBixO7-y and

Pb2Ru2-xPbxO7-yas active catalysts for ORR and OER in alkaline conditions.[95, 97], which were

synthesized using his patented co-precipitation synthesis method. Figure 5.8 shows a polarization curve of a Pb4+_{-doped Pb}

2[Ru1.42Pb4+0.58]O6.5, demonstrating slightly better performance com-

pared to a 15 wt.% Pt carbon black mixture.[95] Since then, many have sought to integrate py- rochlores into existing fuel cells systems, often citing Pb2Ru2-xPbxO7-y as being the most active

pyrochlore compound.[277, 190, 185] In more specific cases, researchers studied the effects of pyrochlore B-site substitution[277, 96], vacancy concentration[60], and pH[67] on the catalytic activity and mechanistic behaviors. For instance, Goodenough et al. indirectly demonstrated the effects of pH on the oxygen reaction pathways, inferred by the degree of surface protonation measured by the mean surface-charge density, which provided some evidence of changing active sites.[67] For composition, Zen et al. studied the effects of both pH and B-site doping composi- tion of Pb2Ru2-xPbxO7-yand Bi2Ru2-xBixO7-yon ORR and OER catalytic activity.[277] They found

an optimum concentration of x∼0.26 for the Pb2Ru2-xPbxO7-y, postulating the Ru-Ob-Pb bridging

oxygen as the active site under acidic conditions.[277] Although pyrochlores have been shown to suffer stability issues under aqueous acidic conditions, some have reported better chemical stability when bonded to Nafion, or ostensibly any polymer-based proton-exchange membrane.[277, 67]

In this study, we evaluated a Pb-Ir oxide composition spread system, which encompasses the Pb2Ir2-xPbxO7-y compound over a certain composition range, which has also been reported as

a fairly activite bifunctional catalyst.[115, 135, 111] Despite the greater economic incentive in evaluating Pb2Ru2-xPbxO7-y due to the lower cost of the constituent materials (Ru and Pb) and the

high scarcity of Ir, Pb2Ir2-xPbxO7-y, or more broadly a Pb-Ir oxide composition spread, became a

suitable sample for study using the available high-throughput electrochemical and characterization methods. Chapter 6 discusses the work done on this material system.