The performance of metal oxides as redox materials is limited by their oxygen conductivity and thermochemical stability. Predicting these properties from the electronic structure can support the screening of advanced metal oxides and accelerate their development for clean energy applications. Specifically, reducible metal oxide catalysts and potential redox materials for the solar-thermochemical splitting of CO2and H2O via an isothermal redox cycle are examined. A volcano-type correlation is developed from available experimental data and density functional theory. It is found that the energy of the oxygen-vacancy formation at the most stable surfaces of TiO2, Ti2O3, Cu2O, ZnO, ZrO2, MoO3, Ag2O, CeO2, yttria-stabilized zirconia, and three perovskites scales with the Gibbs free energy of formation of the bulk oxides. Analogously, the experimental oxygen self-diffusion constants correlate with the transition-state energy of oxygen conduction. A simple descriptor is derived for rapid screening of oxygen-diffusion trends across a large set of metal oxide compositions. These general trends are rationalized with the electronic charge localized at the lattice oxygen and can be utilized to predict the surface activity, the free energy of complex bulk metal oxides, and their oxygen conductivity.
Colloidal quantum dot solar cells (CQDSCs) are attracting growing attention owing to significant improvements in efficiency. However, even the best depleted-heterojunction CQDSCs currently display open-circuit voltages (VOCs) at least 0.5 V below the voltage corresponding to the bandgap. We find that the tail of states in the conduction band of the metal oxide layer can limit the achievable device efficiency. By continuously tuning the zinc oxide conduction band position via magnesium doping, we probe this critical loss pathway in ZnO–PbSe CQDSCs and optimize the energetic position of the tail of states, thereby increasing both theVOC(from 408 mV to 608 mV) and the device efficiency.
An innovative and environmentally friendly battery chemistry is proposed for highpower applications. A carbon-coated ZnFe2O4nanoparticle-basedanode and a LiFePO4-multiwalled carbon nanotube-based cathode, bothaqueous processed with Na-carboxymethyl cellulose, are combined, for the first time,in a Li-ion full cell with exceptional electrochemical performance. Such novelbattery shows remarkable rate capabilities, delivering 50% of its nominalcapacity at currents corresponding to ≈20C (with respect to the limitingcathode). Furthermore, the pre-lithiation of the negative electrode offers thepossibility of tuning the cell potential and, therefore, achieving remarkablegravimetric energy and power density values of 202 Wh kg?1and 3.72W kg?1, respectively, in addition to grant a lithium reservoir. Thehigh reversibility of the system enables sustaining more than 10 000 cycles atelevated C-rates (≈10C with respect to the LiFePO4cathode), whileretaining up to 85% of its initial capacity.
Biophotovoltaics has emerged as a promising technology for generating renewableenergy because it relies on living organisms as inexpensive, self-repairing, andreadily available catalysts to produce electricity from an abundant resource:sunlight. The efficiency of biophotovoltaic cells, however, has remainedsignificantly lower than that achievable through synthetic materials. Here, aplatform is devised to harness the large power densities afforded by miniaturizedgeometries. To this effect, a soft-lithography approach is developed for thefabrication of microfluidic biophotovoltaic devices that do not require membranes ormediators.Synechocystis sp.PCC 6803 cells are injected and allowedto settle on the anode, permitting the physical proximity between cells and electroderequired for mediator-free operation. Power densities of above 100 mW m-2are demonstrated for a chlorophyll concentration of 100 μM under white light,which is a high value for biophotovoltaic devices without extrinsic supply ofadditional energy.
Layered sodium titanium oxide, Na2Ti3O7, is synthesized by a solid-state reaction method as a potential anode for sodium-ion batteries. Through optimization of the electrolyte and binder, the microsized Na2Ti3O7 electrode delivers a reversible capacity of 188 mA h g(-1) in 1 M NaFSI/PC electrolyte at a current rate of 0.1C in a voltage range of 0.0-3.0 V, with sodium alginate as binder. The average Na storage voltage plateau is found at ca. 0.3 V vs. Na+/Na, in good agreement with a first-principles prediction of 0.35 V. The Na storage properties in Na2Ti3O7 are investigated from thermodynamic and kinetic aspects. By reducing particle size, the nanosized Na2Ti3O7 exhibits much higher capacity, but still with unsatisfied cyclic properties. The solid-state interphase layer on Na2Ti3O7 electrode is analyzed. A zero-current overpotential related to thermodynamic factors is observed for both nano- and microsized Na2Ti3O7. The electronic structure, Na+ ion transport and conductivity are investigated by the combination of first-principles calculation and electrochemical characterizations. On the basis of the vacancy-hopping mechanism, a quasi-3D energy favorable trajectory is proposed for Na2Ti3O7. The Na+ ions diffuse between the TiO6 octahedron layers with pretty low activation energy of 0.186 eV.
Li2MnO3 is the parent compound of the well-studied Li-rich Mn-based cathode materials xLi(2)MnO(3)center dot(1-x)LiMO2 for high-energy-density Li-ion batteries. Li2MnO3 has a very high theoretical capacity of 458 mA h g(-1) for extracting 2 Li. However, the delithiation and lithiation behaviors and the corresponding structure evolution mechanism in both Li2MnO3 and Li-rich Mn-based cathode materials are still not very clear. In this research, the atomic structures of Li2MnO3 before and after partial delithiation and relithiation are observed with spherical aberration-corrected scanning transmission electron microscopy (STEM). All atoms in Li2MnO3 can be visualized directly in annular bright-field images. It is confirmed accordingly that the lithium can be extracted from the LiMn2 planes and some manganese atoms can migrate into the Li layer after electrochemical delithiation. In addition, the manganese atoms can move reversibly in the (001) plane when ca. 18.6% lithium is extracted and 12.4% lithium is re-inserted. LiMnO2 domains are also observed in some areas in Li1.63MnO3 at the first cycle. As for the position and occupancy of oxygen, no significant difference is found between Li1.63MnO3 and Li2MnO
A new self-assembly platform for the fast and straightforward synthesis of bicontinuous, mesoporous TiO2 films is presented, based on the triblock terpolymer poly(isoprene-b-styrene-b-ethylene oxide). This new materials route allows the co-assembly of the metal oxide as a fully interconnected minority phase, which results in a highly porous photoanode with strong advantages over the state-of-the-art nanoparticle-based photoanodes employed in solid-state dye-sensitized solar cells. Devices fabricated through this triblock terpolymer route exhibit a high availability of sub-bandgap states distributed in a narrow and low enough energy band, which maximizes photoinduced charge generation from a state-of-the-art organic dye, C220. As a consequence, the co-assembled mesoporous metal oxide system outperformed the conventional nanoparticle-based electrodes fabricated and tested under the same conditions, exhibiting solar power-conversion efficiencies of over 5%.
A simple and scalable method to fabricate graphene-cellulose paper (GCP) membranes is reported; these membranes exhibit great advantages as freestanding and binder-free electrodes for flexible supercapacitors. The GCP electrode consists of a unique three-dimensional interwoven structure of graphene nanosheets and cellulose fibers and has excellent mechanical flexibility, good specific capacitance and power performance, and excellent cyclic stability. The electrical conductivity of the GCP membrane shows high stability with a decrease of only 6% after being bent 1000 times. This flexible GCP electrode has a high capacitance per geometric area of 81 mF cm(-2), which is equivalent to a gravimetric capacitance of 120 F g(-1) of graphene, and retains >99% capacitance over 5000 cycles. Several types of flexible GCP-based polymer supercapacitors with various architectures are assembled to meet the power-energy requirements of typical flexible or printable electronics. Under highly flexible conditions, the supercapacitors show a high capacitance per geometric area of 46 mF cm(-2) for the complete devices. All the results demonstrate that polymer supercapacitors made using GCP membranes are versatile and may be used for flexible and portable micropower devices.