Probing the Reaction Kinetics of the Charge Reactions of Nonaqueous Li–O₂ Batteries

Understanding the reaction mechanism of nonaqueous oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) is key to increase the low round-trip efficiency and power capability of rechargeable Li-air batteries. Here we show that the ORR kinetics are much faster than OER kinetics and OER occurs in two distinct stages upon Li-air battery charging. The first OER stage occurs at low overpotentials (<400 mV) with a slopping voltage profile, whose kinetics are relatively insensitive to charge rates and catalysts. This OER stage could be attributed to the delithiation of the outer part of Li₂O₂ forming lithium-deficient Li_2–<i>x</i>O₂, which is chemically disproportionate to evolve O₂. The second stage takes place at high overpotentials (400–1200 mV), whose kinetics are sensitive to discharge/charge rates and catalysts, which can be attributed to the oxidation of bulk Li₂O₂ particles. Our study provides insights into bridging current two schools of thought on the OER mechanism

Pt-Covered Multiwall Carbon Nanotubes for Oxygen Reduction in Fuel Cell Applications

Recently one-dimensonal (1-D) Pt nanostructures have shown greatly enhanced intrinsic oxygen reduction reaction (ORR) activity (ORR kinetic current normalized to Pt surface area) and/or improved durability relative to conventional supported Pt catalysts. In this study, we report a simple synthetic route to create Pt-covered multiwall carbon nanotubes (Pt NPs/MWNTs) as promising 1-D Pt nanostructured catalysts for ORR in proton exchange membrane fuel cells (PEMFCs). The average ORR intrinsic activity of Pt NPs/MWNTs is ∼0.95 mA/cm² Pt at 0.9 V_{<i>iR</i>-corrected} versus reversible hydrogen electrode (RHE), ∼3-fold higher than a commercial catalyst −46 wt % Pt/C (Tanaka Kikinzoku Kogyo) in 0.1 M HClO₄ at room temperature. More significantly, the mass activity of Pt NPs/MWNTs measured (∼0.48 A/mg_Pt at 0.9 V_{<i>iR</i>-corrected} vs RHE) is higher than other 1-D nanostructured catalysts and TKK catalysts. The enhanced intrinsic activity of 1-D Pt NPs/MWNTs could be attributed to the weak chemical adsorption energy of OH_ads-species on the surface Pt NPs covering MWNTs

Oxygen Point Defect Chemistry in Ruddlesden–Popper Oxides (La_1–<i>x</i>Sr_<i>x</i>)₂MO_4±δ (M = Co, Ni, Cu)

Stability of oxygen point defects in Ruddlesden–Popper oxides (La_1–<i>x</i>Sr_<i>x</i>)₂MO_4±δ (M = Co, Ni, Cu) is studied with density functional theory calculations to determine their stable sites, charge states, and energetics as functions of Sr content (<i>x</i>), transition metal (M), and defect concentration (δ). We demonstrate that the dominant O point defects can change between oxide interstitials, peroxide interstitials, and vacancies. In general, increasing <i>x</i> and atomic number of M stabilizes peroxide over oxide interstitials as well as vacancies over both peroxide and oxide interstitials; increasing δ destabilizes both oxide interstitials and vacancies but barely affects peroxide interstitials. We also demonstrate that the O 2p-band center is a powerful descriptor for these materials and correlates linearly with the formation energy of all defects. The trends of formation energy versus <i>x</i>, M, and δ and the correlation with O 2p-band center are explained in terms of oxidation chemistry and electronic structure

Site-Selective Deposition of Twinned Platinum Nanoparticles on TiSi₂ Nanonets by Atomic Layer Deposition and Their Oxygen Reduction Activities

For many electrochemical reactions such as oxygen reduction, catalysts are of critical importance, as they are often necessary to reduce reaction overpotentials. To fulfill the promises held by catalysts, a well-defined charge transport pathway is indispensable. Presently, porous carbon is most commonly used for this purpose, the application of which has been recently recognized to be a potential source of concern. To meet this challenge, here we present the development of a catalyst system without the need for carbon. Instead, we focused on a conductive, two-dimensional material of a TiSi₂ nanonet, which is also of high surface area. As a proof-of-concept, we grew Pt nanoparticles onto TiSi₂ by atomic layer deposition. Surprisingly, the growth exhibited a unique selectivity, with Pt deposited only on the top/bottom surfaces of the nanonets at the nanoscale without mask or patterning. Pt {111} surfaces are preferably exposed as a result of a multiple-twinning effect. The materials showed great promise in catalyzing oxygen reduction reactions, which is one of the key challenges in both fuel cells and metal air batteries

Instability of Poly(ethylene oxide) upon Oxidation in Lithium–Air Batteries

The instability of aprotic and polymer electrolytes in Li–air batteries limits the development of these batteries for practical use. Here, we investigate the stability of an electrolyte based on poly­(ethylene oxide) (PEO), which has been used extensively for polymer Li-ion batteries, during discharge and charge of Li–O₂ batteries. We show that applying potentials greater than open circuit voltage (OCV, ∼3 V_Li), which is typically required for Li–O₂ battery charging, increases the rate of PEO auto-oxidation in an oxygenated environment, with and without prior discharge. Analysis on the rate of reaction, extent of oxidation, and the oxidation products allows us to propose that rate of spontaneous radical formation in PEO is accelerated at applied potentials greater than OCV. We also suggest that the phenomena described here will still occur in ether-based electrolytes at room temperature, albeit at a slower rate, and that this will prevent the use of such electrolytes for practical long-lived Li–air batteries. Therefore, PEO-based electrolytes are unsuitable for use in Li–air batteries

Orientation-Dependent Oxygen Evolution Activities of Rutile IrO₂ and RuO₂

The activities of the oxygen evolution reaction (OER) on IrO₂ and RuO₂ catalysts are among the highest known to date. However, the intrinsic OER activities of surfaces with defined crystallographic orientations are not well-established experimentally. Here we report that the (100) surface of IrO₂ and RuO₂ is more active in alkaline environments (pH 13) than the most thermodynamically stable (110) surface. The OER activity was correlated with the density of coordinatively undersaturated metal sites of each crystallographic facet. The surface-orientation-dependent activities can guide the design of nanoscale catalysts with increased activity for electrolyzers, metal-air batteries, and photoelectrochemical water splitting applications

Understanding the Chemical Stability of Polymers for Lithium–Air Batteries

Recent studies have shown that many aprotic electrolytes used in lithium–air batteries are not stable against superoxide and peroxide species formed upon discharge and charge. However, the stability of polymers often used as binders and as electrolytes is poorly understood. In this work, we select a number of polymers heavily used in the Li–air/Li-ion battery literature, and examine their stability, and the changes in molecular structure in the presence of commercial Li₂O₂. Of the polymers studied, poly­(acrylonitrile) (PAN), poly­(vinyl chloride) (PVC), poly­(vinylidene fluoride) (PVDF), poly­(vinylidene fluoride-<i>co</i>-hexafluoropropylene) (PVDF-HFP), and poly­(vinylpyrrolidone) (PVP) are reactive and unstable in the presence of Li₂O₂. The presence of the electrophilic nitrile group in PAN allows for nucleophilic attack by Li₂O₂ at the nitrile carbon, before further degradation of the polymer backbone. For the halogenated polymers, the presence of the electron-withdrawing halogens and adjacent α and β hydrogen atoms that become electron-deficient due to hyperconjugation makes PVC, PVDF, and PVDF-HFP undergo dehydrohalogenation reactions with Li₂O₂. PVP is also reactive, but with much slower kinetics. On the other hand, the polymers poly­(tetrafluoroethylene) (PTFE), Nafion, and poly­(methyl methacrylate) (PMMA) appear stable against nucleophilic Li₂O₂ attack. The lack of labile hydrogen atoms and the poor leaving nature of the fluoride group allow for the stability of PTFE and Nafion, while the methyl and methoxy functionalities in PMMA reduce the number of potential reaction pathways for Li₂O₂ attack in PMMA. Poly­(ethylene oxide) (PEO) appears relatively stable, but may undergo some cross-linking in the presence of Li₂O₂. Knowledge gained from this work will be essential in selecting and developing new polymers as stable binders and solid or gel electrolytes for lithium–air batteries

Mechanisms of Morphological Evolution of Li₂O₂ Particles during Electrochemical Growth

Li–O₂ batteries, wherein solid Li₂O₂ is formed at the porous air cathode during discharge, are candidates for high gravimetric energy (3212 Wh/kg_Li₂O₂) storage for electric vehicles. Understanding and controlling the nucleation and morphological evolution of Li₂O₂ particles upon discharge is key to achieving high volumetric energy densities. Scanning and transmission electron microscopy were used to characterize the discharge product formed in Li–O₂ batteries on electrodes composed of carpets of aligned carbon nanotubes. At low discharge rates, Li₂O₂ particles form first as stacked thin plates, ∼10 nm in thickness, which spontaneously splay so that secondary nucleation of new plates eventually leads to the development of a particle with a toroidal shape. Li₂O₂ crystallites have large (001) crystal faces consistent with the theoretical Wulff shape and appear to grow by a layer-by-layer mechanism. In contrast, at high discharge rates, copious nucleation of equiaxed Li₂O₂ particles precedes growth of discs and toroids

Oxygen Reduction Activity and Stability Trends of Bimetallic Pt_0.5M_0.5 Nanoparticle in Acid

Pt-transition-metal (PtM) alloy catalysts are widely used to catalyze oxygen reduction reaction (ORR) and CO oxidation. Here we report a systematic investigation of compositional, particle size, and catalytic activity changes of seven Pt_0.5M_0.5 having similar initial sizes and transition-metal content. We found that the extent of transition-metal dissolution from PtM nanoparticles increases when Pt is alloyed with more negative <i>V</i>_dissolve transition metals despite their strong alloy-formation energy, where <i>V</i>_dissolve or dissolution potential is the thermodynamic potential for transition-metal dissolution (M ⇔ M^<i>n</i>+ + <i>n</i> e^–) at pH 0. Decreased transition-metal dissolution from PtM nanoparticles is accompanied by decreased positive shifts in the onset voltage of CO oxidation from surface-chemistry-sensitive CO stripping after voltage cycling. Moreover, increasing the extent of transition-metal dissolution and decreasing <i>V</i>_dissolve was correlated with the ORR activity of PtM nanoparticles. Our work suggests that the dissolution potential of the transition-metal solute in PtM alloying catalysts might be used to design catalysts with enhanced ORR activity and stability

Synthesis and Activities of Rutile IrO₂ and RuO₂ Nanoparticles for Oxygen Evolution in Acid and Alkaline Solutions

The activities of the oxygen evolution reaction (OER) on iridium-oxide- and ruthenium-oxide-based catalysts are among the highest known to date. However, the OER activities of thermodynamically stable rutile iridium oxide (r-IrO₂) and rutile iridium oxide (r-RuO₂), normalized to catalyst mass or true surface area are not well-defined. Here we report a synthesis of r-IrO₂ and r-RuO₂ nanoparticles (NPs) of ∼6 nm, and examine their OER activities in acid and alkaline solutions. Both r-IrO₂ and r-RuO₂ NPs were highly active for OER, with r-RuO₂ exhibiting up to 10 A/g_oxide at 1.48 V versus reversible hydrogen electrode. When comparing the two, r-RuO₂ NPs were found to have slightly higher intrinsic and mass OER activities than r-IrO₂ in both acid and basic solutions. Interestingly, these oxide NPs showed higher stability under OER conditions than commercial Ru/C and Ir/C catalysts. Our study shows that these r-RuO₂ and r-IrO₂ NPs can serve as a benchmark in the development of active OER catalysts for electrolyzers, metal-air batteries, and photoelectrochemical water splitting applications