The use of poly-cation oxides to lower the temperature of two-step thermochemical water splitting

We report the discovery of a new class of oxides - poly-cation oxides (PCOs) - that consist of multiple cations and can thermochemically split water in a two-step cycle to produce hydrogen (H-2) and oxygen (O-2). Specifically, we demonstrate H-2 yields of 10.1 +/- 0.5 mL-H-2 per g and 1.4 +/- 0.5 mL-H-2 per g from (FeMgCoNi)O-x (x approximate to 1.2) with thermal reduction temperatures of 1300 degrees C and 1100 degrees C, respectively, and also with background H-2 during the water splitting step. Remarkably, these capacities are mostly higher than those from measurements and thermodynamic analysis of state-of-the-art materials such as (substituted) ceria and spinel ferrites. Such high-performance two-step cycles 1100 degrees C are practically relevant for today's chemical infrastructure at large scale, which relies almost exclusively on thermochemical transformations in this temperature regime. It is likely that PCOs with complex cation compositions will offer new opportunities for both fundamental investigations of redox thermochemistry as well as scalable H-2 production using infrastructure-compatible chemical systems.113sciescopu

Substantial Oxygen Loss and Chemical Expansion in Lithium-Rich Layered Oxides at Moderate Delithiation

Delithiation of layered oxide electrodes triggers irreversible oxygen loss, one of the primary degradation modes in lithium-ion batteries. However, the delithiation-dependent mechanisms of oxygen loss remain poorly understood. Here, we investigate the oxygen nonstoichiometry in Li- and Mn-rich Li1.18-xNi0.21Mn0.53Co0.08O2-δ electrodes as a function of Li content by utilizing cycling protocols with long open-circuit voltage steps at varying states of charge. Surprisingly, we observe significant oxygen loss even at moderate delithiation, corresponding to 2.5, 4.0 and 7.6 mL O2 g-1 after resting at 135, 200, and 265 mAh g-1 (relative to the pristine material) for 100 h. Our observations suggest an intrinsic oxygen instability consistent with predictions of high equilibrium oxygen activity at intermediate potentials. From a mechanistic viewpoint, we show that cation disorder greatly lowers the oxygen vacancy formation energy by decreasing the coordination number of transition metals to certain oxygen ions. In addition, we observe a large chemical expansion coefficient with respect to oxygen nonstoichiometry, which is about three times greater than those of classical oxygen-deficient materials such as fluorite and perovskite oxides. Our work challenges the conventional wisdom that deep delithiation is a necessary condition for oxygen loss in layered oxide electrodes and highlights the importance of calendar aging for investigating oxygen stability

Calcination Heterogeneity in Li-rich Layered Oxides: a Systematic Study of Li2CO3 Particle Size

Li- and Mn-rich (LMR) layered oxide positive electrode materials exhibit high energy density and have earth abundant compositions relative to conventional Ni-, Mn-, and Co-oxides (NMCs). The lithiation of coprecipitated precursors is a key part of synthesis and offers opportunities for tuning the properties of LMR materials. Whereas the morphology of transition metal precursors has received substantial attention, that of Li sources has not. Using Li1.14Mn0.57Ni0.29O2 as a model system, in this work we establish a detailed understanding of LMR calcination pathways via in situ and ex situ diffraction, spectroscopy, microscopy and thermogravimetry. Our work shows that large Li2CO3 particle size modulates a previously misunderstood thermogravimetric feature present at the Li2CO3 melting point during layered oxide calcination and causes heterogeneity at larger length scales (inter-secondary particle) than previously reported (intra secondary particle). This work highlights the sensitivity of layered oxide calcination pathways to synthesis conditions and suggests design rules to minimize calcination heterogeneity in layered oxides beyond LMR

Electrochemical trapping of metastable Mn3+ ions for activation of MnO2 oxygen evolution catalysts.

Electrodeposited manganese oxide films are promising catalysts for promoting the oxygen evolution reaction (OER), especially in acidic solutions. The activity of these catalysts is known to be enhanced by the introduction of Mn3+ We present in situ electrochemical and X-ray absorption spectroscopic studies, which reveal that Mn3+ may be introduced into MnO2 by an electrochemically induced comproportionation reaction with Mn2+ and that Mn3+ persists in OER active films. Extended X-ray absorption fine structure (EXAFS) spectra of the Mn3+-activated films indicate a decrease in the Mn-O coordination number, and Raman microspectroscopy reveals the presence of distorted Mn-O environments. Computational studies show that Mn3+ is kinetically trapped in tetrahedral sites and in a fully oxidized structure, consistent with the reduction of coordination number observed in EXAFS. Although in a reduced state, computation shows that Mn3+ states are stabilized relative to those of oxygen and that the highest occupied molecular orbital (HOMO) is thus dominated by oxygen states. Furthermore, the Mn3+(Td) induces local strain on the oxide sublattice as observed in Raman spectra and results in a reduced gap between the HOMO and the lowest unoccupied molecular orbital (LUMO). The confluence of a reduced HOMO-LUMO gap and oxygen-based HOMO results in the facilitation of OER on the application of anodic potentials to the δ-MnO2 polymorph incorporating Mn3+ ions

Electrochemical trapping of metastable Mn3+ ions for activation of MnO2 oxygen evolution catalysts.

Electrodeposited manganese oxide films are promising catalysts for promoting the oxygen evolution reaction (OER), especially in acidic solutions. The activity of these catalysts is known to be enhanced by the introduction of Mn3+ We present in situ electrochemical and X-ray absorption spectroscopic studies, which reveal that Mn3+ may be introduced into MnO2 by an electrochemically induced comproportionation reaction with Mn2+ and that Mn3+ persists in OER active films. Extended X-ray absorption fine structure (EXAFS) spectra of the Mn3+-activated films indicate a decrease in the Mn-O coordination number, and Raman microspectroscopy reveals the presence of distorted Mn-O environments. Computational studies show that Mn3+ is kinetically trapped in tetrahedral sites and in a fully oxidized structure, consistent with the reduction of coordination number observed in EXAFS. Although in a reduced state, computation shows that Mn3+ states are stabilized relative to those of oxygen and that the highest occupied molecular orbital (HOMO) is thus dominated by oxygen states. Furthermore, the Mn3+(Td) induces local strain on the oxide sublattice as observed in Raman spectra and results in a reduced gap between the HOMO and the lowest unoccupied molecular orbital (LUMO). The confluence of a reduced HOMO-LUMO gap and oxygen-based HOMO results in the facilitation of OER on the application of anodic potentials to the δ-MnO2 polymorph incorporating Mn3+ ions

Thermodynamic guiding principles of high-capacity phase transformation materials for splitting H2O and CO2 by thermochemical looping

Thermochemical looping splitting of water and carbon dioxide (CO2) with greenhouse-gas-free (GHG-free) energy has the potential to help address the Gt-scale GHG emissions challenge. Reaction thermodynamics largely contributes to the main bottlenecks of cost reduction for thermochemical looping water/CO2 splitting cycle. Here, we analyze thermodynamic driving forces in such cycles with two-phase ternary ferrites as model systems. We find that cation configurational entropy chiefly determines the change of partial molar entropy with oxygen stoichiometry. In addition, our phase diagram analysis accurately predicts the optimal Fe ratio for maximal water/CO2 splitting capacity in thermal reduction and in chemical reduction based cycles, underlining the significance of phase boundary positions. With chemical reduction, >10% CO2 conversion and high oxygen exchange capacity can both be achieved. Furthermore, our reduced Gibbs free energy model illustrates critical thermodynamic factors that influence the water/CO2 splitting capacity. Our research reveals the thermodynamic driving forces underlying the unconventional high-capacity Fe-poor ferrites, further explained via phase diagrams of Fe-Co-O, Fe-Ni-O and Fe-Mg-O. Future materials improvements can be guided by our reduced Gibbs free energy model.N

Selective Incorporation of Colloidal Nanocrystals in Nanopatterned SiO2 Layer for Nanocrystal Memory Device

CdSe colloidal nanocrystals with a size of similar to 5 nm were selectively incorporated in SiO2 nanopatterns formed by a self-assembled diblock copolymer patterning through a simple dip-coating process. The selective incorporation was achieved by capillary force, which drives the nanocrystals into the patterns during solvent evaporation in dip-coating. The capacitor structures of an Al-gate/atomic layer deposition-Al2O3 (27 nm)/CdSe (5 nm)/patterned SiO2 (25 nm)/p-Si substrate were fabricated to characterize the charging/discharging behavior for a memory device. The flatband voltage shift was observed by a charge transport between the gate and the nanocrystals. It demonstrates the colloidal nanocrystal application to a memory device through selective incorporation in regularly ordered nanopatterns by a simple dip-coating process

Beyond Constant Current: Origin of Pulse-Induced Activation in Phase-Transforming Battery Electrodes

Mechanistic understanding of phase transformation dynamics during battery charging and discharging is crucial toward rationally improving intercalation electrodes. Most studies focus on constant-current conditions. However, in real battery operation, such as in electric vehicles during discharge, the current is rarely constant. In this work we study current pulsing in LiXFePO4 (LFP), a model and technologically important phase-transforming electrode. A current-pulse activation effect has been observed in LFP, which decreases the overpotential by up to ∼70% after a short, high-rate pulse. This effect persists for hours or even days. Using scanning transmission X-ray microscopy and operando X-ray diffraction, we link this long-lived activation effect to a pulse-induced electrode homogenization on both the intra- and interparticle length scales, i.e., within and between particles. Many-particle phase-field simulations explain how such pulse-induced homogeneity contributes to the decreased electrode overpotential. Specifically, we correlate the extent and duration of this activation to lithium surface diffusivity and the magnitude of the current pulse. This work directly links the transient electrode-level electrochemistry to the underlying phase transformation and explains the critical effect of current pulses on phase separation, with significant implication on both battery round-trip efficiency and cycle life. More broadly, the mechanisms revealed here likely extend to other phase-separating electrodes, such as graphite

Metal-oxygen decoordination stabilizes anion redox in Li-rich oxides.

Reversible high-voltage redox chemistry is an essential component of many electrochemical technologies, from (electro)catalysts to lithium-ion batteries. Oxygen-anion redox has garnered intense interest for such applications, particularly lithium-ion batteries, as it offers substantial redox capacity at more than 4 V versus Li/Li+ in a variety of oxide materials. However, oxidation of oxygen is almost universally correlated with irreversible local structural transformations, voltage hysteresis and voltage fade, which currently preclude its widespread use. By comprehensively studying the Li2-xIr1-ySnyO3 model system, which exhibits tunable oxidation state and structural evolution with y upon cycling, we reveal that this structure-redox coupling arises from the local stabilization of short approximately 1.8 Å metal-oxygen π bonds and approximately 1.4 Å O-O dimers during oxygen redox, which occurs in Li2-xIr1-ySnyO3 through ligand-to-metal charge transfer. Crucially, formation of these oxidized oxygen species necessitates the decoordination of oxygen to a single covalent bonding partner through formation of vacancies at neighbouring cation sites, driving cation disorder. These insights establish a point-defect explanation for why anion redox often occurs alongside local structural disordering and voltage hysteresis during cycling. Our findings offer an explanation for the unique electrochemical properties of lithium-rich layered oxides, with implications generally for the design of materials employing oxygen redox chemistry