www.advenergymat.de www.MaterialsViews.com FULL PAPER Oxide Catalysts for Rechargeable High-Capacity Li–O2 Batteries Si Hyoung Oh and Linda F. Nazar * Propylene carbonate is reported to undergo Nano-crystalline mixed metal oxides with an expanded pyrochlore struc- ring opening via nucleophilic addition of − ture are synthesized by a chemical precipitation route in alkaline media. O2 on the CH2 group on the ring near the [7] The high concentration of surface active sites afforded by their high surface carbonyl moiety, for example. Such reac- tivity can lead not only to the early deple- area, intrinsically variable oxidation state and good electron transport lead tion of the electrolyte but to discharge to promising electrocatalytic properties for oxygen evolution in Li-O cells, 2 products other than Li2 O2 ; their specifi c − yielding rechargeable discharge capacities over 10 000 mAh g 1 and lowering nature depends on the electrolyte system anodic overpotentials signifi cantly. The discharge capacity of the Li-O2 cell that is deployed, and affects the kinetics of is increased further when a small amount of gold is deposited on the pyro- the subsequent charging process. The elec- trolyte system must also be robust enough chlore oxide, which serves as a more effi cient oxygen reduction catalyst. The to resist a highly oxidizing environment amount of catalyst necessary for oxygen evolution performance is reduced to during charge without decomposition. as little as 5 wt% by supporting the highly-divided pyrochlore oxide crystal- Even with an ideal electrolyte system lites on carbon using in-situ deposition methods. which produces Li2 O2 and no side products, another major issue is rechargeability: the high activation energy involved in oxygen 1. Introduction evolution from Li2 O2 on charge leads to a large anodic overpo- tential, which prompts the need for a highly effi cient catalyst at The rechargeable Li-O cell represents an emerging energy 2 high Li O loadings. This arises mainly due to the insolubility storage system which has promise as a result of its very high 2 2 of Li O and its apparently insulating nature. [ 1 ] Transition metal theoretical gravimetric energy density and ability to use oxygen 2 2 oxides [ 10 , 11 ] and noble metals[ 12–14 ] adopted from oxygen reduc- as a “fuel” [ 1 , 2 ] In practice, energy densities of over 3 kWh g − 1 are tion (ORR) or oxygen evolution (OER) catalysts of conventional expected, signifi cantly higher than that of conventional Li-ion fuel cells, metal-air batteries have been investigated. Although cells based on intercalation chemistry, which typically provide the understanding of metal oxide electrocatalysts in aqueous about 0.6 kWh g − 1 for layered oxides such as Li[Co Ni Mn ] 1/3 1/3 1/3 systems has been developed over decades,[ 15 ] this process is O coupled to a graphitic negative electrode. [ 1 ] Nonetheless, poor 2 just underway for aprotic Li-air cells, and the criteria for a good power capabilities and cycling stability are among the major ORR or OER catalyst are not yet established. drawbacks in the current incipient state of the technology. Pyrochlores are well known for their excellent electrocata- Progress has been hampered by a lack of understanding of the lytic activity in aqueous media, [ 16 ] but have not been investi- factors responsible for high capacity and effi cient oxygen evolu- gated as catalysts for Li-O batteries. Pyrochlore is a generic tion, as most studies carried out to characterize detailed aspects 2 term for materials having a chemical formula A B X Z δ of this battery system since it was fi rst reported in 1996[ 3 ] have 2 2 6 1- which crystallize in the cubic space group Fd-3m. [ 17 ] Some only been conducted in the last few years. Numerous inroads pyrochlore oxides with composition A B O O’ δ ( A = Pb have been made, however. It has been established that oxygen 2 2 6 1- or Bi, B = Ru or Ir) exhibit the characteristics of a metallic is catalytically reduced to an intermediate, lithium superoxide, [18–20] oxide, where single crystal conductivity is as high as 4.3 × which subsequently (chemically or electrochemically) reacts to − 10 3 S cm 1 at 300 K for Pb Ru O . [ 18 ] The pyrochlores can be form lithium peroxide during discharge. [ 4 , 5 ] The reaction and 2 2 6.5 viewed as a composite of two interwoven substructures, where subsequent decomposition of many common electrolyte sys- corner-shared metal-oxygen octahedra ( BO ) generate a cage- tems with the superoxide radical is now established as one of 6 like B O framework that provides a conduction path for the the most prominent problems that needs to be addressed. [ 6–9 ] 2 6 electrons, while the A element is linearly connected to form A-O’-A linkages with special oxygen atoms (O’ ) that create ′ corner-shared O A4 tetrahedra ( Figure 1 inset). Pyrochlores exhibit a highly fl exible stoichiometry and structure. The spe- S. H. Oh, Prof. L. F. Nazar cial oxygen can be partially or completely absent, resulting in University of Waterloo up to 7% oxygen vacancies in the lattice when δ = 0.5; or alter- Waterloo, ON, N2L 3G1, Canada E-mail: [email protected] natively, the lattice can be fully oxygen stuffed to give the com- position A 2 B2 O7 . Furthermore, a portion of the noble metal in DOI: 10.1002/aenm.201200018 the B -site can be replaced by the A -site cation resulting in an Adv. Energy Mater. 2012, 2, 903–910 © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim wileyonlinelibrary.com 903 www.advenergymat.de www.MaterialsViews.com 2. Results and Discussion 2.1. Synthesis and Characterization of Bismuth and Lead Pyrochlores The bismuth and lead ruthenate pyrochlores FULL PAPER were prepared by chemically oxidizing the metal (M = Bi or Pb, Ru) precursors with sodium hypochlorite in alkaline solution at low temperature (see the Experimental Section). This synthesis differs from the ammonia pre- cipitation route previously described, [ 19 ] and offers a greater degree of control over crystal- lite size. The increase in average oxidation state of ruthenium catalyzes the condensation of the hydroxo-bridges (-OH-) in the hydrox- ides to form oxo-bridges (M-O-M), resulting in the desired metal oxides as nano-crystals. The indexed X-ray diffraction (XRD) patterns of the synthesized materials (Figure 1 ) confi rm that pyrochlores are crystallized with cubic lattice parameters a = 10.446 Å for bismuth ruthe- Figure 1 . XRD patterns for a) lead ruthenium pyrochlore oxide (PbRO) and b) bismuth ruthe- nium oxide and a = 10.340 Å for lead ruthe- nium pyrochlore oxide (BiRO) synthesized via chemical oxidation. Inset shows the crystal = = nium oxide, in good accord with the previously structure of the pyrochlore A2 B2 O7– δ ( A Bi, Pb; B Ru). The B2 O6 framework consisting of ′ reported values. [ 19 ] The coherence length deter- corner-shared BO6 octahedra (blue) is superimposed by the A-O -A structure. Loosely bound special oxygen O ′ (red) atoms can be partially or totally absent, generating oxygen vacancies. mined from the XRD peak broadening in the [111] direction is close to 4 nm for both mate- expanded pyrochlore, A2 [B2- x A x ]O7– δ with x ranging from 0 to rials, which is very small, but not surprising based on the low tem- 1. [ 16 , 19 , 20 ] Although partial substitution leads to a decrease in perature synthesis. High resolution TEM imaging confi rmed the the electronic conductivity, [ 20 ] these modifi ed oxides, where presence of 4 ∼ 5 nm nanocrystallite domains which are coalesced B = Ru, Ir show very good performance as bifunctional cat- into larger polycrystalline agglomerates (Figure 2 a,b). For lead alysts for ORR/OER in the strong alkaline media used in ruthenium oxide (PbRO), crystallization was achieved at room Zn-air cells. [ 21 ] The catalytic capability is believed to originate temperature - the fi rst time this has been accomplished to our from the variable-valent characteristics of the B cations and knowledge - whereas bismuth ruthenate (BiRO) required a slightly the oxygen vacancies. [ 22 ] These can be greatly enhanced by elevated temperature (70 ° C) to initiate crystallization. The pyro- the cooperative effect from effi cient transport of electrons to the chlores showed an increase in the lattice parameter due to B -site reaction site and a high surface area that provides good mass substitution of noble metal with the post transition metal. [ 19 , 20 ] activity. Since the fundamental features of ORR and OER The well described linear relationship between the lattice param- processes in the aqueous and in the non-aqueous electrolytes eters (as determined above) and composition of extended pyro- share similarities, [ 23 ] the nature of the pyrochlores suggests chlores in the literature, [ 18 , 19 , 25 ] allow us to estimate formulae of = that these materials can be very interesting candidates as Bi2 [Ru1.53 Bi0.47 ]O7- δ (Bi/Ru 1.61) and Pb2 [Ru1.73 Pb0.27 ]O6.5 (Pb/ = Li-O2 catalysts. In contrast to fuel cell applications where only Ru 1.31) for PRO and BRO respectively. EDX analyses agree very the properties for oxygen reduction are important, the cata- lytic behavior for oxygen evolution is one of the major chal- lenges in rechargeable Li-air cells, because the electrochemical decomposition of the solid lithium peroxide product involves a large anodic polarization even at moderate current density at very high discharge capacities.[ 10 , 11 , 24 ] Lowering the anodic overpotential during charge is of prime importance in order to avoid carbon corrosion and to diminish electrolyte oxidation. We report here the characteristics of bismuth and lead ruthe- nium pyrochlore oxides as electrocatalysts for the non-aqueous Li-O2 cell.