Chapter 1 Introduction of electrocatalysts for Zn–air batteries
1.4 Pyrochlore oxides as bi-functional electrocatalysts
Recently, pyrochlore oxides have been emerged as promising candidates for bi-functional electrocatalysts. The chemical composition of pyrochlore oxides is A2B2O7, which consists of A- and B-site cations and divalent oxygen anions.68 Pyrochlore oxide structure is cubic structure with space group of Fd-3m consisting eight formula units per one unit cell. They could be synthesized by the fluorite structure via eliminating one eighth of the anions. Thus, the composition of pyrochlore oxides can be indicated as A2B2O7–x with oxygen vacancies.69 There are two types of divalent oxygen ions in the pyrochlore lattice structure (Figure 5). One is the divalent oxygen ions at the 48f site, which are located at tetrahedral sites by bonding with A- and B-site cations. The other is the divalent oxygen ions at 8b sites, which are located at tetrahedral sites by bonding with four of A-site cations. Notably, the coordination contains two of the divalent oxygen ions at 8b sites with very short distance and six of the divalent oxygen ions at 48f sites with much longer distance. The A-site cations are located at 16d sites by coordinating with eight of divalent oxygen ions. Moreover, the B-site cations are located at 16c sites by coordinating with six of divalent oxygen ions.
15 Figure 5. Schematic of a unit cell of the pyrochlore oxide.
This part was reproduced from Brik M. G., Srivastava A. M., Tanaka I. Pyrochlore Structural Chemistry:
Predicting the Lattice Constant by the Ionic Radii and Electronegativities of the Constituting Ions. J.
Am. Ceram. Soc., 95, 1454-1460 (2012).
16 Pyrochlore oxides as electrocatalysts
In general, the chemical composition of pyrochlore oxides is A2B2O6O’1–x consisting of A- and B-site cations and divalent oxygen ions.70 The A-site cations are rare-earth elements, lead, bismuth and titanium. The B-site cations are transition and post-transition metals including ruthenium, iridium and lead. The pyrochlore oxide structures are consisted of two-sublattice including B2O6 and A2O’ structures (Figure 6).71 Notably, the framework of corner-shared BO6 structures with bent B-O-B bonds effectively provides pathway for high electrical conductivity. In terms of the divalent oxygen ions, the general type oxygen ions (O) bond with the A- and B-site cations, whereas that of the distinct oxygen ions (O’) only bond with A-site cations by forming corner-shared A4O’ tetrahedral structures.72-74 In addition, the d electron wave functions of B-site cations were controlled by the types of A-site cations because of A-B interactions in the pyrochlore oxides. Accordingly, the electronic conductivity of the pyrochlore oxides were determined, and they were classified into two groups; metallic pyrochlore oxides based upon the A-site cations of lead, bismuth and titanium; semiconducting pyrochlore oxides based upon the A-site cations of yttrium, neodymium and praseodymium.75-77 Interestingly, the pyrochlore oxides contain high oxygen vacancies based upon A2O’ sublattices with stable structure suggesting high structural flexibility.78 The nonstoichiometric values of x in A2B2O6O’1–x ranged from 0.5 to 1 which corresponds the A2B2O6.5 and A2B2O7, respectively. The types of A- and B-site cations determine the electrocatalytic activity of pyrochlore oxides. In particular, the B-site cations could be doped at A-sites, resulting in the formation of A2[B2–xAx]O7–y.79
The electrocatalytic activities of pyrochlore oxides have been intensively reported as bi-functional electrocatalysts for electrical energy storage systems. They are affected by various factors including the types of A- and B-site cations, oxygen vacancies, surface area and valence states of B-site cations.80, 81 The effect of actions of pyrochlore oxides on the electrocatalytic activities were revealed by investigating a series of the materials. Among them, the Pb2Ir2O7–x and PbxBi2–xRu2O7–y exhibited high ORR and OER activities with stable structure in alkaline solution.82 In this context, the iridium based pyrochlore oxides have been studied for investigating the effect of oxygen vacancies on the catalytic performance. Kortenaar et al. revealed the origin of enhancement of ORR and OER activities of Pb2[PbxIr2–x]O7–x and Nd3IrO7 based upon the oxygen deficiency.81 In addition, Prakash et al.
demonstrated the effect of doping of A-site cations at B-site and corresponding changes of specific surface area of Pb2Ru2O7–x.72 For example, partially doped Pb2[Pb0.33Ru1.67]O6.5 and Pb2[Pb0.2Ru1.8]O6.5
showed higher catalytic activity during ORR than that of stoichiometric Pb2Ru2O6.5. This result could be ascribed to the increase of specific surface area of the catalysts based upon the values of 55, 44 and 35 m2 g–1 for Pb2[Pb0.33Ru1.67]O6.5, Pb2[Pb0.2Ru1.8]O6.5 and Pb2[Pb0.33Ru1.67]O6.5, respectively.
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Recently, Nazar group reported the pyrochlore oxides as bi-functional electrocatalysts for Li–air batteries.78 They synthesized the Pb2[Ru1.73Pb0.27]O6.5 and Bi2[Ru1.53Bi0.47]O7−x by using a chemical precipitation method with high surface area, high oxygen nonstoichimetry and abundant defects. They showed significantly enhanced discharge capacities and lowered anodic overpotentials based upon the high active sites. Moreover, the prepared electrocatalysts effectively transfer the electrons with mixed oxidation states. Notably, the small amount of gold additives derived substantial improvement of electrochemical performance of the pyrochlore oxides. In order to further enhance the catalytic activity of pyrochlore oxides, they reported the mesoporous Pb2[Ru1.6Pb0.44]O6.5 with metallic conduction.83 It was synthesized by using liquid-crystal self-assembly followed via chemical oxidation (Figure 7a).
Figure 7b shows the disordered mesoporous structure of the pyrochlore oxides. The Pb2[Ru1.6Pb0.44]O6.5
with surface area of 155 m2 g–1 demonstrated higher electrochemical performance upon battery cycling than that of the Pb2[Ru1.7Pb0.3]O6.5 with surface area of 66 m2 g–1. These results suggest that the higher concentration of oxygen defects on the surface of pyrochlore oxides could improve the ORR and OER activities.
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Figure 6. Structure of the oxygen nonstoichiometric pyrochlore A2B2O7−x (x = 0.5; A = Pb, Bi; B = Ru), demonstrating oxygen vacancies and electron conduction paths via BO6 octahedral structure that provide metallic conductivity.
This part was reproduced from Oh S. H., Black R., Pomerantseva E., Lee J. H., Nazar L. F. Synthesis of a metallic mesoporous pyrochlore as a catalyst for lithium-O2 batteries. Nat. Chem., 4, 1004-10 (2012).
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Figure 7. (a) Schematic representation for the synthetic method of the mesoporous lead ruthenium pyrochlore oxide. (b) TEM image of the mesoporous lead ruthenium pyrochlore oxide. The inset figure indicates the SAED pattern of the nanocrystalline walls. The inset graph presents the pore size distribution (PSD) result of the surfactant-free mesoporous lead ruthenium pyrochlore oxide. (c) The first three cycles of discharge-charge curves for carbon in LiPF6/TEGDME (i) and the mesoporous lead ruthenium pyrochlore oxide (ii) at a current rate of 70 mA g−1.
This part was reproduced from Oh S. H., Black R., Pomerantseva E., Lee J. H., Nazar L. F. Synthesis of a metallic mesoporous pyrochlore as a catalyst for lithium-O2 batteries. Nat. Chem., 4, 1004-10 (2012).
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