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Electrocatalysis of the oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) was assessed for a series of Ni-substituted ferrites (NiyFe1-yOx, where y = 0.1 to 0.9) as expressed in porous, high-surface-area forms (ambigel and aerogel nanoarchitectures). We then correlate electrocatalytic activity with Ni:Fe stoichiometry as a function of surface area, crystallite size, and free volume. In order to ensure in-series comparisons, calcination at 350°C/air was necessary to crystallize the respective NiyFe1-yOx nanoarchitectures, which index to the inverse spinel structure for Fe-rich materials (y ≤ 0.33), rock salt for the most Ni-rich material (y = 0.9), and biphasic for intermediate stoichiometry (0.5 ≤ y ≤ 0.67). In the intermediate Ni:Fe stoichiometric range (0.33 ≤ y ≤ 0.67), the OER current density at 390 mV increases monotonically with increasing Ni content and increasing surface area, but with different working curves for ambigels versus aerogels. At a common stoichiometry within this range, ambigels and aerogels yield comparable OER performance, but do so by expressing larger crystallite size (ambigel) versus higher surface area (aerogel). Effective OER activity can be achieved without requiring supercritical-fluid extraction as long as moderately high surface area, porous materials can be prepared. We find improved OER performance (η decreases from 390 to 373 mV) for Ni0.67Fe0.33Ox aerogel heat-treated at 300°C/Ar, owing to an increase in crystallite size (2.7 to 4.1 nm). For the ORR, electrocatalytic activity favors Fe-rich NiyFe1-yOx materials; however, as the Ni-content increases beyond y = 0.5, a two-electron reduction pathway is still exhibited, demonstrating that bifunctional OER and ORR activity may be possible by choosing a nickel ferrite nanoarchitecture that provides high OER activity with sufficient ORR activity. Assessing the catalytic activity requires an appreciation of the multivariate interplay among Ni:Fe stoichiometry, surface area, crystallographic phase, and crystallite size. [ABSTRACT FROM AUTHOR]

Conventional Li-ion cathodes store charge by reversible intercalation of Li coupled to metal cation redox. There has been increasing interest in new materials capable of accommodating more than one Li per transition-metal center, thereby yielding higher charge storage capacities. We demonstrate here that the lithium-rich layered iron sulfide Li 2 FeS 2 as well as a new structural analogue, LiNaFeS 2 , reversibly store ≥1.5 electrons per formula unit and support extended cycling. Ex situ and operando structural and spectroscopic data indicate that delithiation results in reversible oxidation of Fe 2+ concurrent with an increase in the covalency of the Fe-S interactions, followed by reversible anion redox: 2 S 2- /(S 2 ) 2- . S K-edge spectroscopy unequivocally proves the contribution of the anions to the redox processes. The structural response to the oxidation processes is found to be different in Li 2 FeS 2 in contrast to that in LiNaFeS 2 , which we suggest is the cause for capacity fade in the early cycles of LiNaFeS 2 . The materials presented here have the added benefit of avoiding resource-sensitive transition metals such as Co and Ni. In contrast to Li-rich oxide materials that have been the subject of so much recent study and that suffer capacity fade and electrolyte degradation issues, the materials presented here operate within the stable potential window of the electrolyte, permitting a clearer understanding of the underlying processes.

Electrocatalysis of the oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) was assessed for a series of Ni-substituted ferrites (Ni y Fe 1-y Ox, where y = 0.1 to 0.9) as expressed in porous, high-surface-area forms (ambigel and aerogel nanoarchitectures). We then correlate electrocatalytic activity with Ni:Fe stoichiometry as a function of surface area, crystallite size, and free volume. In order to ensure in-series comparisons, calcination at 350 °C/air was necessary to crystallize the respective Ni y Fe 1-y Ox nanoarchitectures, which index to the inverse spinel structure for Fe-rich materials (y ≤ 0.33), rock salt for the most Ni-rich material (y = 0.9), and biphasic for intermediate stoichiometry (0.5 ≤ y ≤ 0.67). In the intermediate Ni:Fe stoichiometric range (0.33 ≤ y ≤ 0.67), the OER current density at 390 mV increases monotonically with increasing Ni content and increasing surface area, but with different working curves for ambigels versus aerogels. At a common stoichiometry within this range, ambigels and aerogels yield comparable OER performance, but do so by expressing larger crystallite size (ambigel) versus higher surface area (aerogel). Effective OER activity can be achieved without requiring supercritical-fluid extraction as long as moderately high surface area, porous materials can be prepared. We find improved OER performance (η decreases from 390 to 373 mV) for Ni 0.67 Fe 0.33 Ox aerogel heat-treated at 300 °C/Ar, owing to an increase in crystallite size (2.7 to 4.1 nm). For the ORR, electrocatalytic activity favors Fe-rich Ni y Fe 1-y Ox materials; however, as the Ni-content increases beyond y = 0.5, a two-electron reduction pathway is still exhibited, demonstrating that bifunctional OER and ORR activity may be possible by choosing a nickel ferrite nanoarchitecture that provides high OER activity with sufficient ORR activity. Assessing the catalytic activity requires an appreciation of the multivariate interplay among Ni:Fe stoichiometry, surface area, crystallographic phase, and crystallite size.

The high theoretical energy density of alloyed lithium and germanium (Li 15 Ge 4 ), 1384 mAh/g, makes germanium a promising anode material for lithium-ion batteries. However, common alloy anode architectures suffer from long-term instability upon repetitive charge-discharge cycles that arise from stress-induced degradation upon lithiation (volume expansion >300%). Here, we explore the use of the two-dimensional nanosheet structure of germanane to mitigate stress from high volume expansion and present a facile method for producing stable single-to-multisheet dispersions of pure germanane. Purity and degree of exfoliation were assessed with scanning electron microscopy, transmission electron microscopy, and Raman spectroscopy. We measured representative germanane battery electrodes to have a reversible Li-ion capacity of 1108 mAh/g when cycled between 0.1 and 2 V vs Li/Li + . These results indicate germanane anodes are capable of near-theoretical-maximum energy storage, perform well at high cycling rates, and can maintain capacity over 100 cycles.

The increasing interest in Na-ion batteries (NIBs) can be traced to sodium abundance, its low cost compared to lithium, and its intercalation chemistry being similar to that of lithium. We report that the electrochemical properties of a promising negative electrode material, Na 2 Ti 3 O 7 , are improved by exfoliating its layered structure and forming 2D nanoscale morphologies, nanoplatelets, and nanosheets. Exfoliation of Na 2 Ti 3 O 7 was carried out by controlling the amount of proton exchange for Na + and then proceeding with the intercalation of larger cations such as methylammonium and propylammonium. An optimized mixture of nanoplatelets and nanosheets exhibited the best electrochemical performance in terms of high capacities in the range of 100-150 mA h g -1 at high rates with stable cycling over several hundred cycles. These properties far exceed those of the corresponding bulk material, which is characterized by slow charge-storage kinetics and poor long-term stability. The results reported in this study demonstrate that charge-storage processes directed at 2D morphologies of surfaces and few layers of sheets are an exciting direction for improving the energy and power density of electrode materials for NIBs.

Charge storage devices with high energy density and enhanced rate capabilities are highly sought after in today's mobile world. Although several high-rate pseudocapacitive anode materials have been reported, cathode materials operating in a high potential range versus lithium metal are much less common. Here, we present a nanostructured version of the well-known cathode material, LiMn2O4. The reduction in lithium-ion diffusion lengths and improvement in rate capabilities is realized through a combination of nanocrystallinity and the formation of a 3-D porous framework. Materials were fabricated from nanoporous Mn3O4 films made by block copolymer templating of preformed nanocrystals. The nanoporous Mn3O4 was then converted via solid-state reaction with LiOH to nanoporous LixMn2O4 (1 < x < 2). The resulting films had a wall thickness of ∼15 nm, which is small enough to be impacted by inactive surface sites. As a consequence, capacity was reduced by about half compared to bulk LiMn2O4, but both charge and discharge kinetics as well as cycling stability were improved significantly. Kinetic analysis of the redox reactions was used to verify the pseudocapacitive mechanisms of charge storage and establish the feasibility of using nanoporous LixMn2O4 as a cathode in lithium-ion devices based on pseudocapacitive charge storage.