Construction of an autocatalytic reaction cycle in neutral medium for synthesis of life-sustaining sugars.

Autocatalytic mechanisms in carbon metabolism, such as the Calvin cycle, are responsible for the biological assimilation of CO2 to form organic compounds with complex structures, including sugars. Compounds that form C-C bonds with CO2 are regenerated in these autocatalytic reaction cycles, and the products are concurrently released. The formose reaction in basic aqueous solution has attracted attention as a nonbiological reaction involving an autocatalytic reaction cycle that non-enzymatically synthesizes sugars from the C1 compound formaldehyde. However, formaldehyde and sugars, which are the substrate and products of the formose reaction, respectively, are consumed in Cannizzaro reactions, particularly under basic aqueous conditions, which makes the formose reaction a fragile sugar-production system. Here, we constructed an autocatalytic reaction cycle for sugar synthesis under neutral conditions. We focused on the weak Brønsted basicity of oxometalate anions such as tungstates and molybdates as catalysts, thereby enabling the aldol reaction, retro-aldol reaction, and aldose-ketose transformation, which collectively constitute the autocatalytic reaction cycle. These bases acted on sugar molecules of substrates together with sodium ions of a Lewis acid to promote deprotonation under neutral conditions, which is the initiation step of the reactions forming an autocatalytic cycle, whereas the Cannizzaro reaction was inhibited. The autocatalytic reaction cycle established using this abiotic approach is a robust sugar production system. Furthermore, we found that the synthesized sugars work as energy storage substances that sustain microbial growth despite their absence in nature.

Self-organization in an Open Reaction Network for Developing High-function Organized Molecular Systems.

Self-organized molecular systems such as liposomes and supramolecules have attracted considerable attention due to their characteristic properties. An open reaction network (ORN) is another interesting candidate for such systems; however, no stabilization mechanism has been clarified. This work reveals, by computer simulation and experiments, that a network of irreversible processes such as an ORN can be stabilized by self-organization through a full balance between all the involved irreversible processes, thus forming a steady state. The formation of a steady state indicates that a large spontaneous order is formed; specifically, self-organization occurs. Computer simulations also reveal that such a steady state characteristically evolves toward a high-efficiency state through the development of highly ordered structures. These findings indicate that ORN provides a new method for developing high-function organized molecular systems, such as an efficient catalytic system in a composite of ORN and equilibrium molecular structures such as supramolecules and polymers.

Positive Feedback Mechanism to Increase the Charging Voltage of Li-O2 Batteries.

The large overpotential of nonaqueous Li-O2 batteries when charging causes low round-trip efficiency and decomposition of the electrode materials and electrolyte. The origins of this overpotential have been enthusiastically explored to date; however, a full understanding has not yet been reached because of the complexity of multistep reaction mechanisms. Here, we applied structural and electrochemical analysis techniques to investigate the reaction step that results in the increase of the overpotential when charging. Rietveld refinement of ex situ powder X-ray diffraction showed that a Li-deficient phase of Li2O2, Li2-xO2, formed when discharging and was present over the course of charging. The galvanostatic intermittent titration technique revealed that the rate-determining process in the first step of charging was a solid-solution type of delithiation. The chemical diffusion coefficient of Li+ ions in Li2-xO2, DLi, decreases as the cell voltage increases, which in turn leads to a decrease in the oxidation rate of Li2-xO2. Under galvanostatic conditions, the deceleration of oxidation induces further increase of the cell voltage; therefore, an intrinsic mechanism of positive feedback to increase the cell voltage occurs in the first step. The results demonstrate that the continuity of the first step can be extended by the suppression of changes in any of the elements of the positive feedback loop, i.e., the oxidation rate, cell voltage, or DLi.

Isotopic Depth Profiling of Discharge Products Identifies Reactive Interfaces in an Aprotic Li-O2 Battery with a Redox Mediator.

Prior to the practical application of rechargeable aprotic Li-O2 batteries, the high charging overpotentials of these devices (which inevitably cause irreversible parasitic reactions) must be addressed. The use of redox mediators (RMs) that oxidatively decompose the discharge product, Li2O2, is one promising solution to this problem. However, the mitigating effect of RMs is currently insufficient, and so it would be beneficial to clarify the Li2O2 reductive growth and oxidative decomposition mechanisms. In the present work, Nanoscale secondary ion mass spectrometry (Nano-SIMS) isotopic three-dimensional imaging and differential electrochemical mass spectrometry (DEMS) analyses of individual Li2O2 particles established that both growth and decomposition proceeded at the Li2O2/electrolyte interface in a system containing the Br-/Br3- redox couple as the RM. The results of this study also indicated that the degree of oxidative decomposition of Li2O2 was highly dependent on the cell voltage. These data show that increasing the RM reaction rate at the Li2O2/electrolyte interface is critical to improve the cycle life of Li-O2 batteries.

Dynamic Changes in Charge Transfer Resistances during Cycling of Aprotic Li-O2 Batteries.

Various electrolyte components have been investigated with the aim of improving the cycle life of lithium-oxygen (Li-O2) batteries. A tetraglyme-based electrolyte containing dual anions of Br- and NO3- is a promising electrolyte system in which the cell voltage during charging is reduced because of the redox-mediator function of the Br-/Br3- and NO2-/NO2 couples, while the Li-metal anode is protected by Li2O formed via the reaction between Li metal and NO3-. To maximize the potential of this system, the fundamental factors that limit the cycle life should be clarified. In the present work, we used nondestructive electrochemical impedance spectroscopy to analyze the temporal change of the charge transfer resistances during cycles of Li-O2 batteries with dual anions. The charge transfer resistance at the cathode was revealed to exhibit good correlation with the reduction of the discharge voltage. These results, combined with the results of electrode surface inspections, revealed that irreversible accumulation of insulating deposits such as Li2O2 and Li2CO3 on the cathode surface was a major cause of the short cycle life. Furthermore, the analyses of the time course of the solution resistance suggested that diminished reactivity between the redox mediators and Li2O2 was a critical factor that led to the irreversible accumulation of the less-reactive Li2O2 on the cathode and eventually to a shortened cycle life. These findings indicated that increasing the reactivity between Br3- and Li2O2 is essentially important for improving the cycle stability of Li-O2 batteries and the reactivity can be nondestructively assessed by tracking the dynamic changes in the solution resistance.

Periodic and chaotic oscillations of the electrochemical potential of p-Si in contact with an aqueous (CuSO4+HF) solution, caused by electroless Cu deposition.

Periodic and chaotic oscillations were observed for the potential of p-type Si(111) immersed in an aqueous (HF+CuSO(4)) solution, accompanied by electroless Cu deposition on p-Si. They were, to our knowledge, the first examples of open-circuit potential oscillations observed for semiconductor electrodes. The oscillations appeared only when the Cu deposit formed a continuous porous film composed of mutually connected submicrometer-sized particles. Besides, the Si surface was kept flat within the size less than 50 nm even after the prolonged oscillation for a few hours, though the Si surface should be etched considerably with HF for this time. A plausible model is proposed for the periodic oscillation, in which interestingly coupling of autocatalytic shift in the flat-band potential of Si (U(fb)) caused by the change in the coverage of the Si oxide and the connection and disconnection of the Cu film with the Si surface plays the key role. The appearance of the chaotic oscillation is also explained by taking into account an oscillation-coupled change in the HF or Cu(2+) concentration near the Si surface.

Potential oscillations in galvanostatic electrooxidation of formic acid on platinum: a mathematical modeling and simulation.

We have modeled temporal potential oscillations during the electrooxidation of formic acid on platinum on the basis of the experimental results obtained by time-resolved surface-enhanced infrared absorption spectroscopy (J. Phys. Chem. B 2005, 109, 23509). The model was constructed within the framework of the so-called dual-path mechanism; a direct path via a reactive intermediate and an indirect path via strongly bonded CO formed by dehydration of formic acid. The model differs from earlier ones in the intermediate in the direct path. The reactive intermediate in this model is formate, and the oxidation of formate to CO2 is rate-determining. The reaction rate of the latter process is represented by a second-order rate equation. Simulations using this model well reproduce the experimentally observed oscillation patterns and the temporal changes in the coverages of the adsorbed formate and CO. Most properties of the voltammetric behavior of formic acid, including the potential dependence of adsorbate coverages and a negative differential resistance, are also reproduced.

Potential oscillations in galvanostatic electrooxidation of formic acid on platinum: a time-resolved surface-enhanced infrared study.

The mechanism of temporal potential oscillations that occur during galvanostatic formic acid oxidation on a Pt electrode has been investigated by time-resolved surface-enhanced infrared absorption spectroscopy (SEIRAS). Carbon monoxide (CO) and formate were found to adsorb on the surface and change their coverages synchronously with the temporal potential oscillations. Isotopic solution exchange (from H13COOH to H12COOH) and potential step experiments revealed that the oxidation of formic acid proceeds dominantly through adsorbed formate and the decomposition of formate to CO2 is the rate-determining step of the reaction. Adsorbed CO blocks the adsorption of formate and also suppresses the decomposition of formate to CO2, which raises the potential to maintain the applied current. The oxidative removal of CO at a high limiting potential increases the coverage of formate and accelerates the decomposition of formate, resulting in a potential drop and leading to the formation of CO. This cycle repeats itself to give the sustained temporal potential oscillations. The oscillatory dynamics can be explained by using a nonlinear rate equation originally proposed to explain the decomposition of formate and acetate on transition metal surfaces in UHV.

Five current peaks in voltammograms for oxidations of formic acid, formaldehyde, and methanol on platinum.

We have found that five current peaks are present in the voltammograms in the positive and negative sweep directions for the oxidations of formic acid, formaldehyde, and methanol on Pt in the potential range of 0.05-1.8 V, although the experimental conditions for the peaks to appear are different. In particular, a current peak at ca. 0.6 V, the negative slope of which on the positive side is closely related to autocatalysis, inducing oscillation, has been observed even for methanol. We have clarified that the three substances produce very similar voltammograms at a very slow sweep rate, such as 0.1 mV/s, and show some of the same behaviors of the peaks in their voltammograms. All these facts support the idea that the electrochemical oxidation mechanisms for the three substances have the same dominating elementary reaction steps, which induce oscillation phenomena, although with different reaction and adsorption rate constants.